This document discusses the torrefaction process for various types of biomass including hardwood, softwood, miscanthus, and rape straw. The torrefaction was conducted using a thermogravimetric analyzer to measure mass losses at temperatures ranging from 200-300°C. Chemical analysis including slow ashing, pyrolysis, and PY-GC-MS was used to analyze the effects of torrefaction on the biomass components. The results found that torrefaction primarily affected the hemicellulose component and improved the energy density of the biomass. The biomass varieties with a good balance of moderate/high hemicellulose and fixed carbon content saw the best increase in energy density from torrefaction.
1. The Torrefaction Of Biomass
Name : William Helliker,
Sun: 050768688,
Supervisors: Dr. D. J. Nowakowski,
Prof. A. V. Bridgwater
1
2. Abstract
This report looks into the torrefaction process, over a range of torrefaction
temperatures (200o
C-300o
C) for a variety of biomass: hard wood (Beech
Wood), soft wood (Willow), a herbaceous energy crop (Miscanthus) and an
agricultural waste (Rape Straw). The torrefaction process was carried out
using a Thermogravimetric analyser, to measure mass losses that occur during
the process. Samples of each biomass were produced for torrefaction
temperatures of 200, 225, 250, 275 and 300 centigrade. Ashing and pyrolysis
was conducted using the TGA apparatus, to compare volatiles content and
char content (fixed carbon and ash) in all samples. In addition PY-GC-MS
apparatus was also used to determine the volatiles released from the
torrefaction process, this allowed for the analysis of which lignocellulosic
components were affected by the torrefaction process. H,C,N analysis was
used to allow calculations and estimations of the heating values and energy
improvements of the fuels, after the torrefaction process. This report finds that
the torrefaction majorly affected the hemicellulose component, as well as a
smaller affect on the lignin and cellulose of the biomass. Thermal Degradation
of the hemicellulose caused an improvement in energy density seen within the
torrifeied biomass, and hence the level of hemicellulose present in biomass
was a key factor in determining how well the torrefaction process improved the
biomass. In addition the fixed carbon content was very important to the energy
retained in the biomass. The Biomass varieties that yielded the best energy
density increase had a good balance between moderate/high levels of
hemicellulose and fixed carbon.
2
3. Table of contents
1 Introduction...................................................................................................................5
2 Background and Review of Related Literature.............................................................6
2.1 The Chemistry of Biomass.........................................................................................6
2.1.1 Lignocellulose.....................................................................................................7
2.1.1.1 Cellulose......................................................................................................8
2.1.1.2 Hemicellulose............................................................................................11
2.1.1.3 Lignin.........................................................................................................14
2.1.2 Extractives.........................................................................................................15
2.2.1 pyrolysis............................................................................................................16
2.2.1.1 Fast Pyrolysis ...........................................................................................18
2.2.1.2 Slow pyrolysis............................................................................................19
2.2.2 Gasification.......................................................................................................21
2.2.3 Combustion of Biomass....................................................................................23
2.3 Torrefaction of Biomass......................................................................................25
2.3.1 The Torrefaction Process..............................................................................25
2.3.2 The Result & Gains of Torrefaction.............................................................30
2.3.3 The Economic Potential of the Torrefaction Process...................................32
3 Experiments and Planning..........................................................................................38
3.1 Sample Preparation..............................................................................................38
3.2 High Temperature Pyrolysis of Feed Stocks.......................................................39
3.3 Slow Ashing of Samples......................................................................................40
3.4 Pyrolysis of Prepared Samples & Feed stocks.....................................................41
3.5 H,C,N Analysis....................................................................................................41
3.6 PY-GC-MS of Feed Stocks..................................................................................42
4 Results and Discussion...............................................................................................43
4.1 Comparison of Biomass Feed Stocks..................................................................43
4.2 The Effects of Torrefaction on Mass and Energy of the Biomass.......................47
Figure 4.2.4: A Van Krevelen diagram, showing the raw biomass and torrified
(300oC) biomass in comparison to some other known fuels.....................................54
4.3 Chemical Analysis of the Torrefaction process...................................................55
4.3.1 Slow Ashing of the Raw and Torrified Biomass Samples............................55
4.3.2 Pyrolysis of the Torrified Biomass Samples.................................................60
4.3.3 PY-GC-MS Analysis of the Volatiles Produced During Torrefaction.........62
4.3.3.1Volatiles Produced During the Torrefaction of Beech Wood.....................65
4.3.3.2Volatiles Produced During the Torrefaction of Rapestraw.........................67
4.3.3.3Volatiles Produced During the Torrefaction of Green Miscanthus............68
4.3.3.4Volatiles Produced During the Torrefaction of Willow.............................69
.......................................................................................................................................79
9 Appendices .................................................................................................................79
9.1 Appendix A..........................................................................................................80
9.2 Appendix B..........................................................................................................82
Dry Basis Calculations:.........................................................................................82
Biomass:Green MiscanthusGreen Miscanthus (dry basis) Moisture (Wa)4.07-
Calculated mass loss (at 105oC) from 900oC pyrolysis of feed stock Volatiles
(Va)78.8982.24remaining =100-(Cha+Wa)Char (Cha)17.0317.76Mass remaining
after 900oC pyrolysis of biomass (in Nitrogen atmosphere)Fixed Carbon (Cfixa)-
11.80char - ash = Fixed CarbonAsh (Aa)5.725.96Mass remaining after slow
3
4. ashing of BiomassAshing Moisture Content4.03 Calculated mass loss (at 105oC)
during slow ashing of Feed stock...........................................................................82
Higher Heating Value Calculations:......................................................................83
Mass & Energy Yield and Energy Density Calculations:.........................................83
9.3 Appendix C .........................................................................................................84
...................................................................................................................................85
9.4Appendix D...........................................................................................................86
.......................................................................................................................................90
5 Conclusions................................................................................................................70
6 Recommendations ......................................................................................................72
7Glossary ......................................................................................................................74
8 References...................................................................................................................76
9 Appendicies.....................................................................................................................74
9.1 Appendix A..............................................................................................................75
9.2 Appendix B..............................................................................................................77
9.3 Appendix C..............................................................................................................81
9.4 Appendix D..............................................................................................................83
4
5. 1 Introduction
There is a realisation that the natural resources around the globe, such as
coal, oil and gas, are slowly depleting. These slowly depleting resources are
key feed stocks in the production of many different fuels and chemicals.
Therefore there has been an increasing necessity to look for alternatives feed
stocks to replace them.
One alternative for consideration Is Biomass, the use of plants and crops
grown specifically for use as the base for bio-fuels or bio-chemicals. Crops
such as short rotation woody crops, or herbaceous energy crops, are grown
specifically for use as a biomass resource[1]
, although there is also research
into agricultural wastes such as a few various types of straw.
Biomass as a fuel has several undesirable properties, it has a low calorific, a
high moisture content, due to its hygroscopic nature, and smokes when
combusted[2,3]
It is because of this poorer calorific value/lower energy density
that fuel biomass treatments, such as pyrolysis, have been used to increase
the energy density of the fuel. For examples fast pyrolysis of biomass to form
bio-oils, one advantage of converting the biomass to a bio-oil is its easier to
transport and store, and there is greater potential for use as a fuel or a base for
chemical production.[4]
This report will focus on the treatment of biomass making use of a mild form of
pyrolysis, known as torrefaction. The report aims to look at what effect this
process has on different forms of biomass. With references to any beneficial
results achieved, and the actual affect the process has on the biomass.
5
6. 2 Background and Review of Related Literature
2.1 The Chemistry of Biomass
Short rotation woody crops (SRWC): short rotation woody crop is the term
used to describe woody biomass that is fast growing, and suitable for use in
dedicated feedstock supply systems.[Error: Reference source not found]
The SRWC are
grown on a sustainable basis and are harvested on a rotation of around 3
years in the uk. Woody crops include hard woods and softwoods[Error: Reference source
not found]
Hardwoods (deciduous species) are trees classified as angiosperms;[Error: Reference
source not found,5]
they have the ability to resprout from stumps, a process known as
coppicing, which reduces their production costs relative to softwoods.[Error:
Reference source not found]
There are many other advantages of hard woods in processing including high
density for many species; the presence of hemicellulose high in xylan, which
can be removed relatively easily; low ash content (particularly silica relative to
softwood and herbaceous crops); hardwood lignin has a lower degree of
polymerisation than softwood and contains a greater methoxyl content[Error:
Reference source not found]
Softwoods (Coniferous species) are trees classified as gymnosperms[Error:
Reference source not found,Error: Reference source not found]
, softwood is generally faster growing, but
their carbohydrate is not as accessible for chemical processing as the
carbohydrates in hardwood.[Error: Reference source not found]
Also softwoods tend to have
a higher cellulose and higher lignin content than that of hardwoods.[Error: Reference
source not found]
Since softwoods are frequently used in construction lumber and
pulpwood, they are more readily available as waste material in the form of
logging and manufacturing residues.[Error: Reference source not found]
Herbacious energy crops (HEC): herbaceous energy crops are plants that
have little or no woody tissue, the above ground growth of these plants usually
lasts for only a single growing season. [Error: Reference source not found]
However
herbaceous crops include both annuals and perennials, Annuals die at the end
6
7. of each growing season, and hence must be replanted in spring, where as
Perennials die back each year in temperate climates, but establish themselves
each spring from rootstock.[Error: Reference source not found]
Recent development work with the herbaceous energy crops has focused on
grasses due to their higher yields of lignocellulose. There are two types of
these grasses either thick stemmed of thin stemmed. [Error: Reference source not found]
Thick stemmed grasses are generally seen as more labour intensive and may
require more specialist equipment, hence there is a preference to thin
stemmed grasses as they can be harvested with conventional hay equipment.
[Error: Reference source not found]
2.1.1 Lignocellulose
In general Biomass is comprised of three major constituents which form the
lignocellulose material that makes up the plant. [Error: Reference source not found]
It consists
of variable amounts of cellulose, hemicellulose, and Lignin, the ratio of each
component within the plant varies dependant if it is Hardwoods (from
deciduous trees), softwood (from coniferous trees), or herbaceous material
(from plants and agricultural crops) [Error: Reference source not found]
For woody biomasses about half is cellulose, a quarter hemicellulose and
extractives, and a quarter lignin.[6]
but this is subject to change dependant on
the type of biomass. Table 2.1.1.0.1 shows various types of biomass and their
approximate compositions based on a weight %
Table 2.1.1.0.1 Organic components of lignocellulosic crops (dry basis)[Error: Reference source not found]
Feed Stock
Cellulose (wt
%)
Hemicellulose
(wt%)
Lignin (wt%) Other (wt%)
Bagasse 35 25 20 20
Corn Stover 53 15 16 16
Corncobs 32 44 13 11
Wheat Straw 38 36 16 10
Wheat Chaff 38 36 16 11
S.R.W.C. 50 23 22 5
H.E.C. 45 30 15 10
Waste Paper 76 13 11 0
(Sources: Bull S.R. 1991, “The U.S. Department of Energy Biofuels Research Program” in
Energy Sources, Vol. 13 pp. 433-442; Wayman M. and Parcekh S., “Biotechnology of Biomass
7
8. Conversion: Fuels and Chemicals from Renewable Resources”,Philadelphia: Open university
Press)
2.1.1.1 Cellulose
Cellulose is a “homopolysaccharide of glucose, and is an important constituent
within the plant”[Error: Reference source not found]
. The base constituent of the cellulose
polymer is cellobiose[Error: Reference source not found,7]
, the repeating unit of cellobiose is
composed of two glucose anhydride molecules[Error: Reference source not found,Error: Reference
source not found]
. The number of glucose molecules within a cellulose chain is known
as the degree of polymerization. The average degree of polymerisation for
native cellulose is in the order of 10,000 “chemical pulping” can reduce this to
range between 500-2000. [Error: Reference source not found]
It is these long polymer chains
forming the cellulose that give the lignocellulosic biomass material much of its
tough fibrous nature.
Figure 2.1.1.1.1 shows the previously mention cellobiose unit, composed of the
two repeating Glucose Anhydride molecules. whilst figure 2.1.1.1.2 shows the
chemical structure of the cellulose with intra- and interchain hydrogen-bonded
bridging[8]
Figure 2.1.1.1.1 The Conformational Structure of the cellulbiose[Error: Reference source not found,Error: Reference
source not found]
8
9. Figure 2.1.1.1.2: The intra- and inter-chain hydrogen bonded bridging of the cellubiose
unit[Error: Reference source not found]
Cellulose pyrolysis: the generally accepted model for the pyrolysis of the
cellulose is represented by the Broido-Shafizadeh model, the mechanism was
originally suggested by Broido[9]
, was later simplified by Bradbury, Sakai and
Shafizadeh[10]
shown below in figure 2.1.1.1.2
Figu
re 2.1.1.1.2 Broido-Shafizadeh model of cellulose pyrolysis[11]
Although the model seems to over simplify the nature of the chemical reactions
and physical interactions taking place it is still considered a good model to
represent the kinetic process of cellulose pyrolysis.
As suggested by Varhegyi et al.[Error: Reference source not found]
This is because one
stage in the model may well represent a group of reactions working in parallel
or even the slowest (rate determining) step in a reaction sequence.
Varhegyi et al. also states that at around 250o
C the reaction rate is slow and
yields both char and gasses equally, “as the temperature increases, the lower
branch of the reaction scheme becomes dominant.”[Error: Reference source not found]
This
Kc
Char & Gases
K1
Cellulose Active Cellulose
Volatiles “tars”
Kv
9
10. agrees with the findings of Bergman and his colleagues[12]
that the
Depolymerisation of cellulose does occur at low temperature, but “at higher
temperature, this process takes place faster and to a larger extent.”[Error: Reference
source not found]
Also the main volatiles formed varies with the torrefaction conditions
but “Higher temperatures lead to increased formation of these
components.”[Error: Reference source not found]
Mohan et al. states cellulose degradation
occurs between 240-350 o
C, and produces anhydrocellulose and levglucosan.
[Error: Reference source not found]
These findings concur with the concept that the hemicellulose is the more
volatile component of the lignincellulosic biomass material, decomposing at
lower temperatures and being responsible for the majority of the volatiles and
gases released at the lower temperatures of torrefaction. Where as the
cellulose does not significantly depolymerise without the higher temperature
range of the torrefaction process, which could be why there is a further
increase in volatiles at these higher temperature ranges (>250o
C).
Cellulose Use in the plant: In terms of physical properties cellulose creates
the tough, stringy, fibrous structure of woody biomass[Error: Reference source not found]
. It is
this property that makes the biomass hard to grind up and mill into smaller
particles for use in applications like fluidised bed gasification.
The cellulose is grouped in bundles as shown in figure1.1.3.3. The cellulose
bundles are held together mainly through the hemicellulose, and partially
through the lignin
10
11. Figure 1.1.1.3 Typical Plant Cell Wall Arrangement [13]
2.1.1.2 Hemicellulose
Hemicellulose consists of a large number of heteropolysaccharides built from
six carbon sugars, hexoses (D-glucose, D-mannose, and D-galactose), five
carbon sugars, pentoses (D-xylose, and L-arabinose,) and 4-O-methyl
Glucuronic acid [Error: Reference source not found,Error: Reference source not found,Error: Reference source not
found,14,15]
Figure 1.1.2.1 shows the chemical structure of these monomers that make up
the hemicellulose biomass material.
a) b) c)
d) e) f)
Figure 1.1.2.1 Sugar monomer components of hemicellulose found in wood, (a)b-D-Glucose,
(b)B-D-Mannose, (c)B-D-Galactose, (d)B-D-Xylose (e)B-L-Arabinose (f)4-O-Methylglucuronic
acid[Error: Reference source not found]
There are also some amounts of other sugars, deoxyhexoses [Error: Reference source not
found,Error: Reference source not found]
.
The primary Hemicellulose components are formed from the sugars in figure
1.1.2.1, these are galactoglucomannans(glucomannan) and
glucuronoxylan(xylan). [Error: Reference source not found, Error: Reference source not found]
These components and their chemical structure within hemicellulose can be
seen in figures 1.1.2.2 and 1.1.2.3. Showing the major hardwood and major
softwood hemicellulose respectively
11
12. Figure 1.1.2.2: Major hardwood hemicellulose[Error: Reference source not found]
Figure 1.1.2.3 Major softwood hemicellulose[Error: Reference source not found]
Softwood hemicellulose have more mannose and galactose units but have less
xylose units, meaning galactoglucomannan(mannan) is the major softwood
hemicellulose.[Error: Reference source not found, Error: Reference source not found, Error: Reference source not found]
as
well as having less acetylated hydroxyl groups than hardwood.[Error: Reference source not
found]
Where as hard woods are high in glucuronoxylan(xylan), formed from the
xylose sugar monomer[Error: Reference source not found, Error: Reference source not found]
The chemical and thermal stability of hemicellulose is lower than that of
cellulose causing it to decompose at lower temperatures[Error: Reference source not
found,Error: Reference source not found]
. Hemicellulose decompose over the temperature
range 130-194o
C (with the majority of the weight loss occurring at
12
13. temperatures >180o
C)[16]
This is believed to be due to the lack of crystallinity
and lower degree of polymerisation, which is only 100-200, compared to
Cellulose’s in the order of 10,000 degree of polymerisation for woody
biomass[Error: Reference source not found,Error: Reference source not found]
.
Hemicellulose use in the plant: As shown in figure 1.1.1.3 hemicellulose
works to hold together the cellulose bundles within the lingnocellulose biomass
structure [Error: Reference source not found]
. Bergman [Error: Reference source not found]
also states that
the hemicellulose (as well as the lignin) is responsible for the “tenacity” of the
biomass. Hence this is why the torrefaction process works to weaken the
biomasses strength and resilience.
In addition to the hemicellulose destruction, torrefaction causes the previously
mentioned depolymerisation of the cellulose (the component responsible for
the biomasses fibrous nature) which adds to further weaken the biomass and
reduce its fiberous structure [Error: Reference source not found]
, increasing its ability to be
processed further.
13
14. 2.1.1.3 Lignin
Lignin is a phenylpropane based polymer, it is the largest non carbohydrate
faction found within the lignocellulose.[Error: Reference source not found]
It is constructed of
three monomers(as shown in figure 2.1.1.3.1): coniferyl alcohol, sinapyl
alcohol, and coumaryl alcohol, each of which has an aromatic ring with
different substituents[Error: Reference source not found,Error: Reference source not found]
. The functional
groups associated with lignin (eg alderhyde groups, methoxy groups, alcoholic
hydroxyl groups) result in highly reactive molecules.[Error: Reference source not found]
A) B) C)
OH
OH
O O
CH3CH3
OH
O
CH3OH
OH
OH
Figure 2.1.1.3.1: The monomers that form lignin; (A) P-Coumaryl alcohol,
(B)Coniferyl alcohol, (C) Sinapyl alcohol [Error: Reference source not found]
It is the lignin component in the plant that classifies it as ‘woody’[Error: Reference source
not found]
. The lignin component works with the hemicellulose to form a sheath
that surrounds the cellulosic portion of the lignocellulosic biomass.[Error: Reference
source not found]
The lignin is an amorphous cross-linked resin, cementing between
the woody cellulose fibre bundles and causing stiffening within the fibres[17]
.
This effect most likely results from covalent linking between ligning and
polysaccharides, which enhances the bond strength between the cellulose
fibers[Error: Reference source not found]
. Unlike Cellulose, lignin cannot be depolymerised to
its original monomers.[Error: Reference source not found]
Lignin is also designed to protect
the lignincellulose against insect attack.[Error: Reference source not found]
14
15. 2.1.2 Extractives
Within the biomass there are a range of organic extractives, which can be
removed with the use of polar and non polar solvents.[Error: Reference source not found]
The
extractives generally come from the cell wall of the biomass, the chemicals
include fats, fatty acids, waxes, alkaloids, phenols, proteins, terpenes, steroids,
simple sugars, pectins and many other chemicals and essential oils. [Error: Reference
source not found,18]
In general softwoods will have a higher extractive content than
hardwoods.[Error: Reference source not found]
Within the softwood Pinene is one of the most
common terpenes. (figure 2.1.2.1, structure A) [Error: Reference source not found]
“Polymeric
esters of gallic-acid are usually associated with sugars”(Figure 2.1.2.1,
structure B) [Error: Reference source not found]
conidendrin another example of an extractive
is found in spruce and hemlock(figure 2.1.2.1, structure C) [Error: Reference source not found]
and the extract Pinosylvin is an example of a polyphenol (Figure 2.1.2.1,
structure D) [Error: Reference source not found]
A) B)
OH
OH
H
H
OH
OH
OH
O
OH
C) D)
O
OH
OH
O
CH3
O
OCH3
CH3
CH3
CH3
CH3
CH3
O
OH
Figure 2.1.2.1: Chemical structure of some wood extractives (A)Pinene,
(B)Gallic Acid, (C)Pineresinol, (D)Pinosylvin
15
16. 2.2 Thermal Conversion Processes of Biomass
There are several different biomass conversion processes. Some of the
processes, such as combustion and co-combustion, aim to get energy directly
out of the biomass. The other processes tend to focus on creation of an
improved fuel/product, converting the biomass into a more useful product
depending on the application. In general the energy density of the fuel is
improved, along with other properties that are tailored to the desired
application of the product. For example fast pyrolysis is used to produce high
yields of bio-oil which has applications in chemical production and automotive
bio-fuels, where as gasification is used to produce a gas that can be used to
raise steam to create power, and drive gas turbines.
2.2.1 pyrolysis
Pyrolysis is a complicated series of thermally driven chemical reactions that
decompose organic compounds with the biomass.[Error: Reference source not found]
Pyrolysis occurs when biomass is heated in an inert atmosphere, (an absence
of oxygen) the aim is to produce a mixture of solid char, condensable liquids,
and gases.[Error: Reference source not found]
When the biomass is heated, like it is in pyrolysis, the biomass particle can
shrink. It has been found that the reduction in volume can be between 45-70%
though this is dependant on the type of biomass.[19]
It has also been found that
the shrinkage affects the pyrolysis products. There is a trend to increase the
production of tars and reduce the production of the light hydrocarbon volatiles.
[20]
It is believed that initial shrinkage forms cracks on the surface of the
biomass particle which allows heat to much more quickly and easily penetrate
the biomass.[Error: Reference source not found]
Cracks enhance the pyrolysis affect reducing
the thermal conversion time21
Pyrolysis proceeds at medium-high temperatures, and affects different
components of the biomass at varying temperatures dependant on the type of
plant material. Hemicellulose begins to pyrolyze at temperatures between 225
and 325 o
C, and lignin pyrolysis is initiated between 250 and 500 o
C [Error: Reference
source not found]
16
17. The resulting product composition contains various volatile organic and
inorganic compounds, the types and amounts depend on the type of biomass
processed, and the heating rate used.[Error: Reference source not found]
Some pyrolysis products include carbon monoxide, carbon dioxide, methane,
hydrogen and some light hydrocarbons, all of which are given off as a gas.
Some “vapour products with higher molecular weight can also be condensed to
a tarry liquid.”[Error: Reference source not found]
Table 2.2.1.0.1 Typical product yields (dry wood basis) obtained by different modes of
pyrolysis and wood thermal treatment[22]
Mode Conditions Liquid Char Gas
Fast
Moderate temperature, around 500C, short
hot vapour residents time ~ 1 sec
75% 2% 13%
Intermediate
Moderate temperature, around 500C,
moderate hot vapour residents time ~ 10-20
sec
0% 20% 30%
Slow
low temperature, around 400C, long vapour
residents time
30% 35% 35%
Gasification
high temperature, around 800C, long vapour
residents time
5% 10% 85%
(source: Bridgwater A.V., “Biomass Pyrolysis”, IEA Bioenergy Update 27, International Energy
Agency, Biomass and Bioenergy)
17
18. 2.2.1.1 Fast Pyrolysis
Fast Pyrolysis is utilized to optimize the production of condensable organic
vapors(bio-oils) from the biomass, with minimum production of gases and char.
[Error: Reference source not found]
The advantage of producing a liquid bio-oil is that it is
easier to transport and store as well as having the potential to be used as a
fuel or a base for chemical production. [Error: Reference source not found]
With the potential
to hopefully replace its fossil fuel derived counter part. In addition Bio-oils are
counted as being CO2/GHG neutral, with relatively insignificant SOx emissions
due to the minimal sulfur in plant/wood biomass. [Error: Reference source not found]
Bio-fuels
also produce 50% less NOx emmisions than diesel oil when used in a gas
turbine. [Error: Reference source not found]
Pyrolysis is defined as the thermal decomposition occurring in the absence of
oxygen[Error: Reference source not found]
Fast pyrolysis has a residence time of, at most, a
few seconds.[23]
Due to this short residence time the higher temperatures used
in fast pyrolysis are reached prior to the decomposition of the biomass. [Error:
Reference source not found]
These higher temperatures cause a tendency towards a
mechanism involving depolymerization, as appose to what is believed to be a
melt[Error: Reference source not found]
. Under the faster heating conditions and higher
temperatures the regular dehydration reactions that occur to form char become
less dominant and instead Organic volatiles are formed by cracking reactions,
[Error: Reference source not found]
due to this it is key to the process to bring the biomass
particle to the process temperature as quickly as possible, minimizing its
exposure to the lower temperatures that cause the formation of char.[Error: Reference
source not found,Error: Reference source not found]
The fast pyrolysis reaction temperature is
normally around 500o
C, with vapour phase temperature of 400-450o
C[Error:
Reference source not found]
The residences time as previously stated is under 2 seconds.
[Error: Reference source not found]
Rapid removal and cooling of the pyrolysis vapors form
bio-oil,[Error: Reference source not found,Error: Reference source not found]
and prevents further cracking
of the desired product caused by the char, which acts as a cracking catalyst.
[Error: Reference source not found]
The desired product from this process is the bio-oil, which
can be generated in yields up to 75%(wt) of the feed, on a dry feed basis. [Error:
Reference source not found]
18
19. The process of fast pyrolysis process itself requires around 15% of the energy
stored in the feed.[Error: Reference source not found]
The process creates char and gases
which can be used to supply this required energy, In this way process effluent
is minimized to just ash and flue gases. [Error: Reference source not found]
Some of the main chemicals from bio oil formed from fast pyrolysis include
levoglucosan (formed by the Pyrolysis of the cellulose in the absence of a
catalysts), hydroxyacetalderhyde, organic acids, and furfurals There are many
other oxygenated compounds, and they vary greatly depending on the
temperature, heating rate, biomass and reactor used. Water should be
removed from the biomass to minimize the water content of the bio-oil product
[Error: Reference source not found, Error: Reference source not found,Error: Reference source not found,24,25]
2.2.1.2 Slow pyrolysis
As previously mentioned pyrolysis is a series of thermally driven chemical
reactions that occur in the absences of oxygen. Slow pyrolysis conditions tend
to emphasise char production most likely due to the long residence times. In
general slow pyrolysis can be achieved with lower process temperatures
ranging from 400-800o
C[26]
and longer residence times.[27]
Research has shown that temperature from 500-600o
C seem to maximise the
production of volatiles/liquids[Error: Reference source not found,28]
where as past these
temperatures the liquid yield seems to decrease marginally but there is a
noticeable decrease in the char production as more gases are produced.[Error:
Reference source not found,Error: Reference source not found]
Research also shows that during the slow pyrolysis there are three main zones
that form within the particle. The outer most zone allows for char formation, this
zone will slowly deepen/thicken and inhibit heat transfer further into the
particle.[Error: Reference source not found]
There is another zone beyond this char layer
where the heat has penetrated deep enough and the pyrolysis reactions are
taking place.[Error: Reference source not found]
Then finally the inner zone where the
pyrolysis hasn’t occurred. [Error: Reference source not found]
19
20. It is also shown that increasing particle size with Slow pyrolysis leads to a
decrease in gas formation, this could likely be due to the char layer formed
reducing the heat transfer coefficient into the particle[Error: Reference source not found, 29]
20
21. 2.2.2 Gasification
Gasification is the high temperature (>750o
C) conversion of solid, carbon
based fuels into flammable gas mixtures. This gas, sometimes called produce
gas, consists of carbon monoxide, hydrogen, methane, nitrogen, carbon
dioxide and smaller quantities of longer chained hydrocarbons. [Error: Reference source
not found]
The process is designed to produce a low to medium energy fuel gas, or
can also be used to produce synthesis gases for the manufacture of chemicals
or hydrogen. [30]
The fuel gas produced from biomass tends to have a relatively
low heating value 5-10(MJ/Nm3
) [31]
which stems from the low energy density of
the biomass, wood for example has an energy density of around 18MJ/Nm3
. [32]
This energy density is not idea for gasification fuels, and instead it is
suggested that gasifiers running at temperatures slightly above 900o
C should
use fuels that have a LHV of around 23MJ/Nm3
.[Error: Reference source not found]
The overall process is endothermic, the heat required may be provided from
an external source to the gasifier, or alternatively it can be provided by
internally burning part of the fuel entering the gasifier.[Error: Reference source not found]
The high volatile content of biomass (70-90 wt%) compared to coal (typically
30-40 wt%),along with the high reactivity of its char, gives biomass good
potential gasification fuel [Error: Reference source not found]
. The main problem with biomass
though is its thermal instability, which can cause the formation of condensable
tars.[Error: Reference source not found]
Tars will cause problems in the down stream
equipment, ‘such as choking and blockage’[Error: Reference source not found]
Due to this problem there have been issues with the cost and convenience of
biomass gasification, which in turn have limited its application to special
situations and niche markets. [Error: Reference source not found]
21
22. Heating and Drying Pyrolysis Gas-Solid Reactions
Gas-Phase
Reactions
Thermal front penetrates Porosity increases
Figure 2.2.2.1 The four main stages of gasification.[Error: Reference source not found]
The gasification process has four main stages; shown above in figure 2.2.2.1
Drying, Pyrolysis (devolatilisation), Volatile Combustion, and finally Char
Gasification and Gas Reforming. [Error: Reference source not found]
The figure above shows
the several steps that occur during the gasification of biomass..[Error: Reference source
not found]
22
Volatile gasses:
CO, CO2, H2, H2O,
light hydrocarbons
and tar
Exothermic
reactions
Endothermic
reactions
CO + H2OCO2 +H2
CO+ 3H2CH4 +H2O
23. 2.2.3 Combustion of Biomass
Right up to the early 20th
century most of industrialised society relied on the
combustion of biomass and related thermal processes for heating, cooking,
chemical production, creating charcoal, generating steam and hence also the
generation of mechanical and electrical power.[33]
Combustion of wood based biomass can be represented by the oxidation of
the empirical formula of cellulose[Error: Reference source not found]
(C6H10O5)n:
(C6H10O5)n + 6nO2 6nCO2 + 5nH2O
This formula represents the complete combustion of the wood based biomass.
With complete combustion only CO2 and water are formed, and the reactant is
completely consumed producing radiant and thermal energy. [Error: Reference source not
found]
This complete combustion ideal is very hard to achieve. Much research
has been carried out in the field of solid fuel combustion to understand the true
mechanism and kinetics of the process.
“An empirical view of combustion involves the evaporation of high energy
volatiles such as terpenes, which burn in the gas phase with flaming
combustion” [Error: Reference source not found]
the lignocellulose components under the
influence of high temperatures decompose to form pyrolysis products, the
cellulose components are converted to combustible (and some non
combustible volatiles), which are then burnt in the gas phase with flaming
combustion. [Error: Reference source not found]
The lignin contributes to the residual char,
which burns at a lower rate by surface oxidation, represented by glowing
combustion. [Error: Reference source not found]
In industry one of the largest wood fuelled power stations is located in
Burlington, Vermont. It generates 50MW of net production, and requires an
equivalent dry wood rate of 925 t/day With a thermal energy consumption of
13MJ of fuel per kWh produced, and a thermal efficiency of around 27.5% [34]
If this is compared to other power stations, which make use of fossil fuels, the
overall thermal efficiencies at such power stations range from 28-40% and an
average thermal energy consumption of 11MJ of fuel per kWh produced. [Error:
Reference source not found]
Which clearly shows the potential for fuel savings that
23
25. 2.3 Torrefaction of Biomass
2.3.1 The Torrefaction Process
Biomass as a fuel has several undesirable properties, it has a low
calorific/heating value, a high moisture content, it is hygroscopic in nature and
smokes when combusted[Error: Reference source not found,Error: Reference source not found]
These
unwanted properties are vastly improved with the torrefaction process.
Torrefaction is a process for the thermal treatment of biomass. The process
heats biomass in an inert atmosphere up to a maximum temperature of
300o
C[Error: Reference source not found,35]
past this recommended limit (300o
C) pyrolysis
processes start[36]
. Torrefaction may also be referred to as mild pyrolysis.
Torrefaction is generally seen as a pre-treatment step, the aim of which is to
improve the properties of the biomass. Torrified biomass has similar
characteristics to that of low grade coal in terms of the van Krevelen coal rank
parameter,[37]
and could be used as a substitute for charcoal in a range of
applications[Error: Reference source not found]
There are other possible uses of torrefaction as a pre treatment.
Untreated biomass will retain its fibrous nature due to the cellulose present in
the lignincellulosic material the biomass is made from. This means the
biomass will not be easily reduced into small spherical particles, (which would
be necessary for use in a fluidised bed gasifier or fluidised bed pyrolysis
reactor) hence the size reduction process is expensive. The torrefaction
process, particularly at higher temperatures (250-300o
C) will depolymerise the
cellulose within the biomass.[Error: Reference source not found]
This gives the biomass many
improved properties. One such property is the biomass becomes much easier
to grind/cut into smaller particles. The heat treated biomass has improved
grind-ability because the long cellulose chains have been depolymerised,[Error:
Reference source not found]
resulting in an increase in cellulose crystalinity[38]
The
torrefaction of woody biomass can reduce the power consumption of the size
reduction process by 50%-85% depending on the conditions used[Error: Reference
source not found]
. In addition “because of the shortening of the fibres through
torrefaction, the particles resulting from the size reduction process become
more spherical” [Error: Reference source not found]
25
26. The torrefaction process also weakens the tenacious nature of the biomass at
all temperature ranges because of the degradation of the hemicellulose which
is responsible for holding together the cellulose bundles.[Error: Reference source not found]
Bergman et al. also showed the higher the temperatures used in the
torrefaction process the better the ability of the biomass to be mechanically
processed[Error: Reference source not found]
. This leads to the conclusion that torrefaction
has Ideal use as a pre treatment, in particular for use in a fluidised bed
biomass processes such as gasification or possibly in certain types of pyrolysis
processes, because of the improved physical properties of the torrified
biomass. There may also be considerations for use in suspension firing
combustors, though this will be limited to co-combustion with coal in such
applications because the suspension fired combustors are generally in a size
range of over 100MW. [Error: Reference source not found]
Due to this scale the possibility of
using torrified biomass as the only feed becomes unlikely, the collection and
transport of the required quantity of biomass will be expensive and may not
allow for a reasonable net energy gain. The torrefaction processing on such a
large scale would also require more research.
In addition to the improved physical properties, the torrified biomass has
improved chemical properties. These improved chemical properties could
potentially be utilised effectively in applications within gasification.
Some of the disadvantages of wood biomass for use in gasification include the
low energy density of wood, typically 18MJ/kg and its high moisture content as
a result of its hygroscopic character, which even after drying are around 10wt%
[39]
It has also been found that higher gasification efficiencies can be achieved
by using fuels with lower O/C ratios.[40]
Woody biomass has been found to have
around 50 wt% carbon and 45 wt% oxygen[41]
, compared to torrified biomass
that has around 60-64wt% carbon and 30-33wt% oxygen.[Error: Reference source not found]
As previously mentioned the decreased O/C ratio would give the torrified
biomass better potential for use in a gasifier due to the improved efficiency
over the woody biomass. Although torrefaction is an improvement on the
woody biomass the O/C ratio is still greater than that of charcoal (82 wt%
carbon, 12wt% oxygen[Error: Reference source not found]
) or coal, which has around 60–85
26
27. wt% carbon and 5–20 wt% oxygen.[Error: Reference source not found]
The Van krevelen
scale can be used to compare this relation ship between biomass and coal and
their relative O/C ratios as shown below in figure 2.3.2.1.
Figure 2.3.2.1 Van Krevelen diagram showing various fuels’ relative energy
densities.[Error: Reference source not found]
The decreasing O/C ratio represents an increase in the energy density of the
fuel on the scale. The higher biomass O/C ratios would have a negative effect
on the efficiency of a gasifier running on biomass. Another problem with the
wood biomass feedstock is its optimum gasification temperature is rather low
caused by its high O/C ratio this means that the wood is generally over-
oxidized in conventional gasifiers leading to thermodynamic losses.[Error: Reference
source not found]
Research shows that generally the optimum gasification
temperature for woody biomass is around 782o
C(for biomass with average
composition CH1.4O0.6)[Error: Reference source not found]
which is reflected in figure 2.3.2.2
27
28. FIGURE 2.3.2.2: Gasification temperatures for fuels of varying O/C and H/C ratios[Error: Reference
source not found]
This could be where the torrefaction process could help, the torrefaction
process will lower the O/C ratio of the biomass[Error: Reference source not found,42]
and
reduce the moisture content to around 0-3%[Error: Reference source not found]
Hence it is
possible to reduce the thermodynamic losses by prior thermal pre-treatment,
[Error: Reference source not found]
which should in turn improve the efficiency of gasification
of the fuel at higher temperatures. Research also shows higher temperatures
of torrefaction (250-300o
C) achieve more favourable results for gasification
feed stocks in a shorter time span.[Error: Reference source not found]
Process gases and volatiles released:
As the biomass is heated it undergoes a series of physical and chemical
processes. Firstly the biomass dries out with the removal of moisture from the
biomass. At temperatures over 160o
C further water removal occurs, the water
is formed from chemical reactions, and removed through a thermo-
condensation process[Error: Reference source not found]
at this temperature range carbon
dioxide gas is also formed. Between 180 and 270o
C hemicellulose
degrades[Error: Reference source not found]
and from temperatures over 250O
C the cellulose
28
29. depolymerises to a much greater degree than previously.[Error: Reference source not found]
There is little mention of the torrefaction process having any significant affect
on the lignin within the biomass, hence it is reasonable to assume that the mild
pyrolysis (torrefaction) process does not reach high enough temperatures to
significantly breakdown this part of the lignocellulosic biomass material.
Although there is some mention of lignin undergoing thermal cross linking[Error:
Reference source not found]
which would most likely disrupt the previously formed
covalent linking between the ligning and polysaccharides[Error: Reference source not found]
,
this disruption may well further weaken the bond strength between the
cellulose fibers, if it were to occur.
The main volatiles produced from the torrefaction process consists of Water,
acetic acid, methanol, carbon monoxide, carbon dioxide, and furan derivatives.
[Error: Reference source not found]
In addition to these products there are also some phenols
produced.[Error: Reference source not found]
Mamleev et al.[43]
shows that low-molecular
acids play a significant catalytic role in the products produced during pyrolysis.
This is expanded on in a proposal by Géradin et al.[Error: Reference source not found]
who
proposes that Acid degradation catalysed by the formation of acetic acid during
the hemicellulose degredation is the key step in the formation of furfural,
alderhydes and other volatile by-products.
At the higher temperature scale of the torrefaction process, with temperatures
in excess of 270o
C, there is more formation of phenols as well as cresols and
other heavier products.[Error: Reference source not found]
It is likely due to the process
beginning to become more like pyrolysis in its nature at this high temperature
range, causing increased breakdown of the cellulose and even possibly to a
lesser extent the lignin.
The relative yields of the volatiles depend on the conditions of the torrefaction
process and the type of biomass used. Whether the biomass is a hard wood or
soft wood can depict whether the hemicellulose is xylan or mannan based. [Error:
Reference source not found,Error: Reference source not found,Error: Reference source not found]
Hence It is expected
that the type of woody biomass used will have a big effect on the volatiles
released, because of research[Error: Reference source not found,44]
that shows hemicellulose
to be the main reactant in the torrefaction process.
29
30. 2.3.2 The Result & Gains of Torrefaction
Torrefied biomass has been found to retain between 70-95% of its original
mass.[Error: Reference source not found,Error: Reference source not found,45]
depending on the type of
biomass, the temperatures it was exposed to and the time it was exposed to
that temperature. The product from the torrefaction process is dry and
hydrophobic,[Error: Reference source not found]
having an equilibrium moisture reduction of
about 73%[46]
The torrefied biomass will also have an increased energy density
relative to the original biomass.[Error: Reference source not found]
The moisture content of
the torrified biomass is below that of the dried biomass feed, dropping from
around 10% to between 0-3%[Error: Reference source not found]
and the heating value has
increased by 5-25%[Error: Reference source not found,Error: Reference source not found]
The energy
retained in the biomass can range between 80-97% (and most commonly is
around 90%)once again dependant on the temperature ranges and biomass
used. [Error: Reference source not found,Error: Reference source not found]
The energy remaining in the
torrified sample could be calculated using a method developed by S.W.
Channiala & P.P. Parikh that can accurately predicted by the higher heating
value (HHV) of a fuel, for a range of different fuels.[47]
The experimentally
derived formulae calculates this value from the fuel composition with an
average absolute error of 1.45% [Error: Reference source not found]
The equation is shown
below:
Higher heating Value equation (1)[Error: Reference source not found]
HHV = 0.3491C + 1.1783H + 0.1005S -0.1034O – 0.0151N – 0.0211A
(Where:C=carbon weight%, H=hydrogen weight%, S=Sulphur weight%,
O=Oxygen weight%, N=nitrogen weight% and A=ash weight%)
Another HHV correlation produced by Friedl et al.[48]
calculated Linear
regression models to predict a HHV of a biomass fuel from its elemental
composition. The models were calculated by ordinary least squares regression
(OLS) and by partial least squares regression (PLS).
HHV(OLS model) = 1.87C2 − 144C − 2820H + 63.8C
×H + 129N + 20147
30
31. HHV(PLS model) = 5.22C2 − 319C − 1647H + 38.6C
×H + 133N + 21028 (5)
After analyzing the OLS and PLS models and results a final model for the
prediction of the HHV of biomass from elemental data was produced
Higher heating value equation (2):
HHV = 3.55C2
− 232C − 2230H +51.2CH + 131N + 20, 600
(Where: C=carbon weight%, H=hydrogen weight%, O=Oxygen weight% and
N=nitrogen weight%)
In the equation coefficients were rounded to three significant digits. The model
claims to give a standard error of calibration of 337 kJ/kg. [Error: Reference source not found]
The carbon content of the solid torrified biomass product shows a trend to
increases with higher temperatures of torrefaction, and longer resident times.
[Error: Reference source not found]
whilst the hydrogen and oxygen content deceases.[Error:
Reference source not found]
This is what causes the increased energy density of the
product. The decrease in the O/C ratio after the heat treatment[Error: Reference source not
found]
could be due to the degradation of the hemicellulose to volatile by-
products, and the thermal cross linking of the lignin[Error: Reference source not found]
, but it
is suggested by Géradin et al. [Error: Reference source not found]
that these aren’t the sole
factors contributing to the decrease in the O/C ratio, and on analysis believes
that there is some char formation during the torrefaction process, despite this
having been well documented occurring at temperatures above that of the
torrefaction process(300-450o
C).
31
32. 2.3.3 The Economic Potential of the Torrefaction Process
The torrefaction process increases the energy density of a fuel; the biomass
will lose mass, but retain higher amounts of its energy relative to the mass lost.
Figure 2.3.3.1 shows how the process can lead to this energy densification.
Figure 2.3.3.1: the effect of torrefaction on the energy density of biomass[49]
Increasing a fuels energy density can save costs on transportation and
storage, because it allows for storing and transporting higher amounts of
energy within the same volume. F. Felfi et al.[Error: Reference source not found]
carried out a
range of torrefaction experiments on wood briquettes, table 2.3.3.1 show some
results from the torrefaction process carried out over a range of temperatures
and residence times.
Table 2.3.3.1: Energy and Weight yields of wood briquettes after the
torrefaction process
Temperature(o
C) Time (hr) Weight yield (%) Energy Yield (%)
220
0.5 94 97
1 90 94
1.5 72 77
250
0.5 74 80
1 65 72
1.5 60 67
270
0.5 56 65
1 54 62
1.5 43 50
(source: Felfi F.F., Luengo C.A., Beaton P.A. & Suarez J.A. ’Wood Briquette Torrefeaction’ J.
Energy For Sustainable Development, Vol. 9, No 3, (2005) pp. 19-22)
Gas &
volatiles
Torrefaction
Biomass
Torrified
Biomass
Mass
Energy
0.1
1 0.9
1
0.3
0.7
Energy densification (E/kg) = 1 x ( / ) = 1.30.9 0.7
32
33. The data shows that the energy loss is noticeably less than the mass loss, and
hence that energy densification has been achieved. If torrified biomass was to
then under go pelletisation the energy density of the fuel could be further
increased. Giving further reductions to storage and transportation cost.
Kiel et al. also looked into pelletisation, and the preliminary findings indicated
that the pelletisation process was easy, and had a low energy input
requirement, and the quality of the pellet was dependant heavily on the
conditions used in the process, but was generally very good. [Error: Reference source not
found]
Pellets formed from torrified biomass retained their hydrophobic nature. In
addition further increases in energy density and heating value were seen.
Table 2.3.3.2 shows a comparison between the treated and untreated biomass
fuels.
Table 2.3.3.2: property comparison between treated and raw biomass[Error:
Reference source not found]
Properties Unit Wood
Torrified
biomass
Wood pellets
Torrified
biomass
pellets
Low High Low High
Lower Heating
value
(dry basis)
MJ/kg 17.7 20.4 17.7 17.7 20.4 22.7
Mass density Kg/m3
550 230 500 650 750 850
Energy density GJ/m3
5.8 4.6 7.8 10.5 14.9 18.4
(source: Felfi F.F., Luengo C.A., Beaton P.A. & Suarez J.A. ’Wood Briquette Torrefaction’ J.
Energy For Sustainable Development, Vol. 9, No 3, (2005) pp. 19-22)
As table 2.3.3.2 shows there is a marked increase in energy density through
pelletisation and torrefaction. Combining the processes show an increase in
energy density of 250-300% with the product reaching a respectable LHV of up
to 22.7 MJ/kg. [Error: Reference source not found]
This kind of improvement in the fuels
properties should noticeably improve the economics of transportation and fuel
storage.
33
34. As well as the experimental work previously mentioned kiel et al. & ECN have
looked into a possible process for the production of torrified biomass pellets.
The process shown in figure 2.3.3.2 makes use of the volatiles released from
the torrefaction process as a fuel to assist the torrefaction & pelletisation
process. The volatiles are separated from the inert gas used in torrefaction and
mixed with air and a utility fuel, the fuels are then combusted producing a flue
gas. The heat produced in the form of a flue gas is then used to drive the
torrefaction process itself, with waste heat being used to dry the biomass feed
prior to torrefaction. Producing the pellets in this way gives high energy
efficiency. (over 90%)[Error: Reference source not found]
Figure 2.3.3.2: Torrified biomass pellet production flow diagram [Error: Reference source
not found,50]
Kiel et al. have also produced a cost comparison between this proposed
process, and pelletisation of the biomass without the torrefaction step. Figure
2.3.3.3 shows a capital investment comparison between the two processes.
34
35. Figure 2.3.3.3: Total Capital Investment Comparison between Torrified
Pelletisation and Conventional Pelletisation[Error: Reference source not
found]
Figure 2.3.3.3 clearly shows that all though the extra torrefaction process
accounts for nearly 3million euros of total capital investment (half the cost of
the conventional process) savings on storage and processing costs make the
overall impact of the process less expensive than might have otherwise been
expected.[Error: Reference source not found]
The combined torrefaction and pelletisation
(T&P / TOP) process is shown to only cost around an extra 20%-25% capital
cost. [Error: Reference source not found]
When the saving on transportation costs are also
considered the TOP process has been shown to have favorable economics
over conventional pelletisation, for certain applications [Error: Reference source not found]
, as
reflected in figure 2.3.3.4
35
Sawdust
0.7 €/GJ
TOP Process
(South Africa)
2.0 €/GJ
Logistics
< 2.0 €/GJ
Sawdust
0.7 €/GJ
Conventional
Pelletisation
(South Africa)
2.3 €/GJ
Logistics
2.9 €/GJ
Co-firing coal fired
power stations or
entrained flow gasifiers
North-west Europe
< 4.7 €/GJ
5.9 €/GJ
36. Figure 2.3.3.4: economic comparison of the TOP process and the
conventional biomass pelletisation process[Error: Reference source not
found]
Summary of Torrified Biomass Properties:
• Hydrophobic Nature and lower moisture content[Error: Reference source not
found,Error: Reference source not found,Error: Reference source not found,Error: Reference source not found]
• Higher calorific value achieved, compare to the untreated
Biomass[Error: Reference source not found,Error: Reference source not found]
• Less smoke when burnt[51]
• Suitable for various applications as a fuel (co-combustion &
gasification)[Error: Reference source not found,Error: Reference source not found,Error: Reference
source not found,Error: Reference source not found,Error: Reference source not found,52]
• Can substitute charcoal/wood briquettes/biomass for use as a
domestic fuel[Error: Reference source not found,Error: Reference source not found]
• Increased energy density, leading to improved storage and
transportation cost efficiencies over untreated biomass[Error: Reference
source not found,Error: Reference source not found,Error: Reference source not found,Error: Reference source
not found,Error: Reference source not found,Error: Reference source not found,Error: Reference source not
found,Error: Reference source not found]
• Pelletisation further improves torrified biomasses properties[Error:
Reference source not found]
• Significantly improved/easier cutting and grinding ability over
untorrified biomass, leading to reduced energy costs for
processing.[Error: Reference source not found,Error: Reference source not found,Error: Reference
source not found]
36
38. 3 Experiments and Planning
In this report four different types of biomass will be tested on, Beech wood, a
hard wood, Willow, a soft wood, Green Miscanthus, an energy crop and Rape
straw, an agricultural waste.
3.1 Sample Preparation
Each one of the four feed stocks will be a homogenous sample of particles
ranging in size from 0.15mm to 0.25 mm. The raw feed stocks will then
undergo torrefaction sample preparation in a thermogravimetric analyser; the
Pyris 1 TGA apparatus will torrify the four different feed stocks in an inert
nitrogen atmosphere, at torrefaction temperatures: 200o
C, 225o
C, 250o
C,
275o
C and 300o
C. The high-precision balance will be able to accurately
measure the mass losses of each sample as the torrefaction process takes
place. The torrefaction samples will be prepared in this way, to record the
mass losses for comparison between feed stocks.
The heating rate on the thermal gravitational machine will be as follows:
[I] The raw biomass feedstock will enter the reactor which will be around
50o
C, The feedstock will be held at this temperature for five minutes.
[II] From here the biomass will be heated up in the reactors miniature
electric oven to a temperature of 105o
C at a heating rate of 10o
C/minute
[III] The TGA will hold at 105o
C for 5minutes. This stage removes the
moisture from the biomass, and allows the moisture content of the
feedstock to be accurately measured, for each of the torrified samples.
[IV] The sample is then heated from 105 o
C up to the desired torrefaction
temperature. The heating rate is once again 10o
C/min.
38
39. [V] Once the desired torrefaction temperature is reached the
thermogravimetric analyser will hold at the desired temperature for
15minutes.
[VI] Once the sample has been torrified the reactor will be cooled down to
50o
C so when the sample is removed it does not undergo any
combustion reactions.
Figure 3.1.1 shows a graphical representation of the heating profile.
Figure 3.1.1: torrefaction heating rate applied to the biomass feed stocks
3.2 High Temperature Pyrolysis of Feed Stocks
As well as carrying out the torrefaction process on the raw feed stocks the
TGA will be used to carry out feedstock pyrolysis in a nitrogen atmosphere up
to a temperature of 900o
C. The standard BERG pyrolysis method will be used
for the pyrolysis of the feed stocks. This will allow the accurate measurement
of the char present in each of the feed stocks, and also allow the water content
of the raw feed stocks to be measured. After calculating the char and water
content of the feed stocks the volatile content can also be worked out by
Heating Rate Profile
0 10 20 30 40 50 60 70
Time (min)
Torrefaction
Temperature
105o
C
39
I
II
III
IV
V
VI
Temperature
(o
C)
40. difference, giving the over all composition of the feed stocks. The heating rates
applied in this experiment will be exactly the same as those used in the
torrefaction process, and the hold times will also be the same. The difference
will be the peak temperature reached will be 900o
C as appose to the range
used in torrefaction (200-300o
C)
3.3 Slow Ashing of Samples
Slow ashing will be conducted using the Thermogravimetric analyser, with the
prepared set of samples being slowly heated in air. The heating rate will be
5o
C/min, but follow the standard BERG heating profile used for the torrefaction
of the feed stocks. The reactor will be held at 50o
C for 5minutes, then heated
up to 105o
C at 5o
C/min, held at 105o
C for 5 minutes, then further heated
(5o
C/min) up to a peak temperature of 575o
C. The peak temperature is then
held for 15minutes, before the sample is cooled back down to 50o
C. The peak
temperature is set to 575o
C because past this temperature the inorganics that
form the ash may begin to combust affecting the mass loss of the samples.
The samples will consist of the raw biomass, 200o
C, 225o
C, 250o
C, 275o
C &
300o
C across the four varieties of biomass. By combustion of all the volatiles
and fixed carbon in an air atmosphere it will be possible to accurately measure
the inorganics/ash content present in the char of all the samples, and hence
calculate the fixed carbon in each of the biomass sample sets. The derivative
of the slow ashing mass loss profile (d mass/d time) will give the rate of mass
loss over the temperature range 50-575o
C, this should allow the identification
of the regions/temperatures where the volatiles are combusted, and where the
char is combusted, as mentioned by klass [Error: Reference source not found]
hopefully, with
clear distinction between these two regions, it will be possible to see how the
balance between relative volatile content and char content (weight %) is shifted
with the torrefaction process.
40
41. 3.4 Pyrolysis of Prepared Samples & Feed stocks
Once again the TGA can be utilised with the standard BERG pyrolysis method
to pyrolyse the sample sets. The heating rate will be 10o
C/min. in a nitrogen
atmosphere and the heating profile will once again follow the profile used for
torrefaction, the exception is this time the peak temperature reached will be
600o
C, to ensure each torrified and raw feed stocks are pyrolysed. This will
allow a comparison between feed stocks after torrefaction, and analyse how
the torrefaction process affects the biomass with respect to volatile content.
Pyrolysis should show the torrified biomass contain less volatiles than the raw
biomass and increasing torrefaction temperatures further reduce volatile
content.
3.5 H,C,N Analysis
H,C,N analysis will be conducted on the raw biomass feed stocks as well as
the prepared torrified samples of each feedstock. Analysis will find the
elemental chemical make up of the biomass and torrified biomass, allowing the
carbon content, hydrogen content, and nitrogen content to be found as a
weight percentage of the sample. The oxygen content is assumed to constitute
the remainder of the biomass, and shall be calculated by difference.
This elemental analysis will allow calculations to be carried out to work out the
energy content of the samples. The equations mentioned in section 2.3.2 can
be utilised with the ash content derived from slow ashing, to calculate the
Higher Heating Value (HHV) of the samples. This will allows a comparison of
the feed stocks and how torrefaction affects the heating value of the feedstock
over the torrefaction temperature ranges proposed in section 3.1.
In addition to the higher heating value, atomic ratios of O/C and H/C can be
calculated by converting the weight percentage values to a mole basis. The
atomic ratios are then able to be used to compare the samples to each other
and to existing fuels on a Van Krevelen diagram
41
42. 3.6 PY-GC-MS of Feed Stocks
A CDS-2000 pyroprobe and CDS AS-2500 auto sampler coupled to a gas
chromatograph mass spectrometer will be utilized to recreate the effect of
pyrolysis and torrefaction on the feed stocks, allowing for the analysis of the
volatiles that are released from both processes, and a comparison between
them.
The samples will be 1mg and 3mg of the feed stocks for the pyrolysis runs and
torrefaction runs respectively. The sample is loaded into a quartz tube and
placed in the centre of an inductive heated coil. The samples are heated with a
heating rate of 3000o
C/s up to a temperature of 300o
C which is held for 45
seconds for torrefaction. For replication of pyrolysis conditions the samples is
heated to 600o
C and held for 15 seconds. The difference in residence time is
such that adequate amounts of volatiles are released for measurement.
The gas chromatograph will use a split ratio of 1:25 to separate the molecules,
the injector and detector temperatures were both at 280o
C and the carrier
velocity was at 38cm/s. Separation was achieved using a semi polar column
DB-1701 60m by 25 µ m, an a film thickness of 0.025 µ m, with a chemical
composition 14% cyanopropyl-phenyl 86%dimethylpolysiloxane, which
produces a different selectivity to (phenyl) dimethylsiloxane phases because of
the function cyano groups. In order to give good separation the temperature
progam started at 45o
C for 4minutes, followed by a 4o
C/minute heating rate to
240o
C and 39o
C/minute up to 280o
C
A Perkin Elmer MS GOLD (UK) electron impact mass spectrometer set at 70
eV will produce the mass spectra. The mass range from 28-300 m/z will be
scanned, and data processed by Perkin Elmer Turbo mass spectrometer
version 6.0. The Perkin Elmer NIST98 computer library will be used to identify
the compounds
42
43. 4 Results and Discussion
4.1 Comparison of Biomass Feed Stocks
Thanks to the experimentation as described in section 3 it is possible to
compile a table of the feed stocks and their relative component compositions.
Table 4.1.1: The composition of the raw feed stocks on a dry basis
Biomass:
Beech Wood
(Dry Basis)
Willow
(Dry Basis)
Green
Miscanthus
(Dry Basis)
Rape Straw
(Dry Basis)
Moisture %
(Wa
)
- - - -
Volatiles %
(Va
)
85.88 77.7 82.24 89.64
Char % (Cha
) 14.12 22.3 17.76 10.36
Fixed Carbon
% (Cfix
a
)
13.12 17.55 11.8 3.28
Ash % (Aa
) 1 4.75 5.96 7.08
Table 4.1.1 shows that willow, the softwood, has the highest char content and
the highest levels of fixed carbon, beech wood the hard wood has the lowest
ash content and also a very high level of fixed. The herbaceous energy crop
green miscanthus is not quite as rich in fixed carbon, but higher levels of
inorganics give a larger ash content than the woody biomasses. Rapestraw is
shown to have the lowest levels of char and fixed carbon, yet the highest levels
of ash.
Because of this rape straw is shown to have the highest volatile content, and
as such it could be expected that rape straw would be most affected by the
torrefaction process, until Figure 4.1.1 is considered. Where it is shown the
volatile mass losses of rape straw happen towards the later stages of the high
temperature pyrolysis process.
43
44. As mentioned Figure 4.1.1 shows the rape straw to contain the lowest amount
of char. This is interesting when considering the energy that will remain in the
biomass after torrefaction is directly affected by the fixed carbon. It would be
expected that the higher the levels of fixed carbon (and hence char) a
feedstock has, the better it will retain its energy when the more volatile
components of the biomass are removed. Hence according to table 4.1.1 and
figure 4.1.1 Willow contains the highest amount of char and fixed carbon and
therefore could be most likely to retain the highest levels of energy after
torrefaction.
Figure 4.1.1: The mass loss of the biomass feed stocks during high
temperature (900o
C) pyrolysis
The rate of mass loss of each of the feed stocks, relative to the temperature,
should also be considered during the high temperature pyrolysis. This will
allow better analysis of how each of the feed stocks will be affected by
torrefaction. this can be seen in figure 4.1.2
44
Biomass Feedstocks Pyrolysis Mass Loss Comparison
0
20
40
60
80
100
120
0 50 100 150 200 250
Time (m)
Weight(%)
Beech Wood
Willow
Rape Straw
Green Miscanthus
45. Figure 4.1.2: High temperature (900o
C) pyrolysis feedstock analysis,
comparing rate of mass loss to pyrolysis temperature
Figure 4.1.2 shows beech wood loses less weight over the torrefaction
temperature range. It may be assumed this is due to relatively lower levels of
hemicellulose and higher ratios of lignin and cellulose present in woody
biomass when compared to agricultural wastes, grasses and other herbaceous
energy crops.[Error: Reference source not found]
From figure 4.1.2 it would be expect that torrefaction will give the biggest mass
loss to green miscanthus, followed by rape straw then willow. Once again most
likely due to the higher levels of hemicellulose relative to those present in
beech wood. This mass loss relation may mean that the torrefaction process
gives hemicellulose rich feed stocks a much more noticeable increase in
heating value. Also note the shape of beech wood and willow peaks and the
way they shift to the right, possibly contributable to higher lignin contents.
Figure 4.1.3 shows lignin degrades at a higher temp range, and as stated by
Y. Yu et al.[Error: Reference source not found]
lignin is responsible for woody character of
biomass, which supports the higher content of lignin within the woody
biomasses.
45
Comparison of Mass Loss Rates of The Biomass Feedstocks.
-10
-8
-6
-4
-2
0
2
100 150 200 250 300 350 400 450 500 550 600
Temperature (C)
rateofmassloss(d%wt./dt.)
Beech Wood
Green Miscanthus
Willow
Rape Straw
46. The cellulose peak at around 320o
C in figure 4.1.3 could also contribute to the
shape of the willow and beech wood peaks in figure 4.1.2. hence the
combination of cellulose and lignin degradation around 350-400o
C explains the
large secondary peaks seen in beech wood and willow in figure 4.1.2, all of
which supports the idea that the woody feed stocks will show least mass loss
from the torrefaction process due to their relatively lower levels of
hemicellulose.
Although it is Expected that the beech wood mass will remain largely the least
affected by the process this may change towards the higher temperatures of
torrefaction where it is claimed pyrolysis reactions begin to occur.[Error: Reference
source not found]
Pyrolysis of the Lignocellulose Components
-25
-20
-15
-10
-5
0
100 200 300 400 500 600
Temperature (C)
rateofmassloss(d%wt/dt)
Cellulose
Hemicellulose
Lignin
Figure 4.1.3: A comparison between the lignocellulose components pyrolysis
temperatures (lignocellulose components derived from a beech
wood feedstock)
46
47. 4.2 The Effects of Torrefaction on Mass and Energy of the
Biomass
As expected the hemicellulose rich herbaceous energy crop and agricultural
waste showed the greatest mass losses (see appendix A for exact details)
retaining less than 60% of their original mass. The beech wood showed the
least mass loss overall.
Table 4.2.1 shows the relative atomic ratio of the biomasses constituent
elements and the higher heating value calculated using these ratios. The two
equations mentioned in section 2.3.2; equation 1 by S.W. Channiala & P.P.
Parikh [Error: Reference source not found]
and equation 2 by Friedl et al.[Error: Reference source not found]
were used to calculate the higher heating values HHV1 and HHV2
respectively. The equation proposed by Friedl et al. Only considered the CHN
(and by difference oxygen) chemical make up, which could lead to some errors
in values for feed stocks with higher inorganic content being unaccounted for.
The equation proposed by S.W. Channiala & P.P. Parikh accounts for the
inorganics(ash) and other constituents such as sulphur, nitrogen, hydrogen,
carbon, in both cases the oxygen was calculated by difference could well
cause some slight errors and anomalies in the results.
The trends seemed to be a constant increase in the carbon wt% and decrease
in the Hydrogen and Oxygen. Thus an increase in HHV. (as shown graphically
in van krevelen diagram fig.4.2.3) the decrease of Hydrogen in the torrified
samples was inconsistent at times. Which most likely caused errors because of
the nature of the equations, in addition the assumption of Oxygen by difference
would further compound any slight error in the hydrogen content, leading to
some samples not following the trend of increased HHV.
47
49. A comparison between the raw biomass HHV and the relative increase in HHV
achieved through the torrefaction temperature range showed some evidence
that the high hemicellulose containing green miscanthus showed the biggest
increase in energy relative to the raw biomass, (as shown in figure 4.2.1 and
4.2.2.) On the other hand the increase in higher heating value seen in the
willow was greater than that of rape straw, yet rape straw was shown to have a
larger mass loss (appendix A). This could show that the mass losses, and
hemicellulose content of the biomass is not the only deciding factor in the
increase in HHV, and it is quite likely the low char/fixed carbon content shown
in rapestraw also affected the relatively low HHV increase. Though the
comparison is not fully conclusive due to the anomalies witnessed in the rape
straw runs, this may stem from the low char content and high ash content
which could have had an adverse affect on the HHV equations used.
Effect of Torrefact on Increasing The Energy of Biomass
-4
-2
0
2
4
6
8
10
12
14
16
200 210 220 230 240 250 260 270 280 290 300
Torrefaction Temperature (C)
IncreaseinEnergyrelativetotherawbiomass(%)
Green Miscanthus
Willow
Rape Straw
Beech Wood
Figure 4.2.1: Relative increase in HHV(equation 1 used) of the feed stocks,
over the torrefaction temperature range
49
50. Effect of Torrefact on Increasing The Energy of Biomass
-2
0
2
4
6
8
10
12
14
200 210 220 230 240 250 260 270 280 290 300
Torrefaction Temperature (C)
IncreaseinEnergyrelativetotherawbiomass(%)
Green Miscanthus
Willow
Rape Straw
Beech Wood
Figure 4.2.2: Relative increase in HHV(equation 2 used) of the feed stocks,
over the torrefaction temperature range
The mass and energy yields of the feed stocks, across the range of
torrefaction temperatures, were also analysed. The results, shown in table
4.2.2 show a trend of a decreasing mass yield with increasing torrefaction
temperatures this trend was mimicked by the energy yield, but to a lesser
extent, such that there was a noticeable retention in energy yield greater than
that of mass yield with the higher temperatures of torrefaction.
50
51. Table 4.2.2: The mass and energy yields of the four different biomass feed
stocks, seen over the range of torrefaction temperatures
Mass
yield
Energy yield
{1}
Energy yield
{2}
Beech Wood 200 94.9 94.5 94.7
225 93.7 94.9 96.2
250 89.8 92.9 95.5
275 80.6 85.3 89.7
300 69.2 74.5 80.3
Willow 200 91.0 94.2 92.6
225 88.3 92.1 90.9
250 83.3 88.7 87.3
275 75.8 83.1 82.5
300 66.8 76.2 75.3
grn.
Miscanthus
200 98.3 99.8 99.5
225 87.4 89.9 90.7
250 81.3 87.1 85.6
275 71.5 79.3 78.1
300 58.3 66.2 65.9
Rape Straw 200 92.5 89.8 91.5
225 89.9 90.9 90.7
250 83.9 87.7 86.5
275 74.2 80.9 78.8
300 59.2 66.7 65.2
Table 4.2.2 is interesting because although green miscanthus and rape straw
were shown to have good mass losses this did not relate to a great energy
yield, probably due to the speculated relatively high levels of hemicellulose.
Although they had high mass loss because of the high levels of hemicellulose,
and Green Miscanthus even had one of the biggest marked improvent in HHV
(relative to the raw biomass), the high levels of hemicellulose meant it was
hard to retain high levels of energy due to subsequently lower levels of char
and fixed carbon. So the large mass losses of over 40% didn’t allow for high
levels of energy to be retained unlike beech wood and willow. Beech wood and
willow were shown to retain the higher levels of energy content, and this is
attributable to the woody nature of their lignocellulose being richer in lignin and
hence richer in fixed carbon.
Although the beech wood retained the highest levels of energy the torrefaction
process relatively increased the higher heating value of the beech wood least.
(fig.4.2.1 & fig.4.2.2)
51
52. On the basis of these results it is expected that willow and possibly other soft
woods as well as green miscanthus and possibly other HEC, will gain the most
benefit from the torrefaction process. This is most likely due to the balance of
hemicellulose and lignin, such that there is a large enough mass loss to show
a significantly marked improvement in HHV, at the same time as having the
right ratio of lignocellulose components and fixed carbon to allow for the
retention of energy. This is shown to some degree by looking at the energy
densification of the biomass feed stocks in question, shown in Table 4.2.3.
Table 4.2.3: The comparison of energy densification caused by torrefaction
HHV eq.1 HHv eq. 2
Torrefaction
Temp. (o
C)
Energy
Densification
Energy
Densification
Beech Wood 200 1.00 1.00
225 1.01 1.03
250 1.03 1.06
275 1.06 1.11
300 1.08 1.16
Willow 200 1.04 1.02
225 1.04 1.03
250 1.06 1.05
275 1.10 1.09
300 1.14 1.13
grn.
Miscanthus
200 1.01 1.01
225 1.03 1.04
250 1.07 1.05
275 1.11 1.09
300 1.13 1.13
Rape Straw 200 0.97 0.99
225 1.01 1.01
250 1.05 1.03
275 1.09 1.06
300 1.13 1.10
The energy densification of the biomasses are close, and show that higher
temperature torrefaction is beneficial for all the biomasses experimented on,
with a tendency to have the optimum effect on biomasses with higher fixed
carbon contents and possibly relatively medium/high hemicellulose contents.
52
53. The Van Kreveln Diagram compares the atomic ratios of Oxygen Hydrogen
and Carbon within a fuel, this allows for a relative comparison of the fuel to that
of existing fuels. There is also a trend of increasing heating value with
decreasing atomic O/C and H/C ratios. Table 4.2.4 shows these relative ratios
and how they decrease with the increasing torrefaction temperatures, leading
to an increase in heating value as shown in Figure 4.2.3.
Table 4.2.4: The effect of torrefaction on the atomic O/C and H/C ratios
Torrefaction Temperatures ( C )
Raw 200 225 250 275 300
Beech Wood
Atomic O/C ratio 0.71 0.70 0.68 0.66 0.63 0.60
Atomic H/C ratio 1.47 1.43 1.43 1.44 1.40 1.32
Willow
Atomic O/C ratio 0.64 0.61 0.60 0.56 0.51 0.44
Atomic H/C ratio 1.37 1.40 1.38 1.33 1.22 1.13
Green
Miscanthus
Atomic O/C ratio 0.68 0.65 0.60 0.57 0.50 0.43
Atomic H/C ratio 1.51 1.46 1.34 1.41 1.31 1.13
Rape Straw
Atomic O/C ratio 0.67 0.66 0.61 0.55 0.56 0.47
Atomic H/C ratio 1.43 1.43 1.35 1.31 1.38 1.23
Figure 4.2.3: Van Krevelen diagram showing raw and torrified biomass
samples’ atomic O/C and H/C ratio compared to existing fuels
Van Krevelen Diagram
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80
Atomic O/C ratio
Beech wood
Willow
Green Miscanthus
Rape Straw
atomic H/C ratio
Biomass
Peat
Incre
asin
g
torre
facti
on
temp
eratu
re
lignite
Coal
Anthra
cite
In
cr
ea
si
ng
H
ea
tin
g
va
lu
e
53
54. The van krevelen diagram in fig.4.2.3 much more clearly conveys how Willow
and Green Miscanthus benefitted the most from the torrefaction. Figure 4.2.3
also shows how the high torrefaction temperatures cause the torrified biomass
to become more like peat.
Figure 4.2.3 shows the beech wood being least affected by the torrefaction
process, something that is also clearly shown in the Van krevelen diagram
figure 4.2.4 where the raw biomass, high temperature torrified biomass and
other known compounds have been marked.
Figure 4.2.4: A Van Krevelen diagram, showing the raw biomass and
torrified (300oC) biomass in comparison to some other known
fuels
Van Krevelen Diagram
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80
Atomic O/C ratio
AtomicH/Cratio
Beech wood
Willow
Green Miscanthus
Rape Straw
Raw Biomass
Rape Straw
Beech wood
Willow
Green Miscanthus
Torrified Biomass (300o
C)
In
cr
ea
si
ng
H
ea
tin
g
va
lu
e
Charcoal
Lignite
Bituminious coal
Anthracite
Other Known Fuels
54
55. 4.3 Chemical Analysis of the Torrefaction process
4.3.1 Slow Ashing of the Raw and Torrified Biomass Samples
The data from the slow ashing of the different biomass varieties over the range
of the torrefaction temperatures can be manipulated to give the combustion
rate of mass loss against temperature. Figure 4.3.1.1 shows the beech wood
mass loss rate against the combustion temperature. It can be clearly seen that
there are two main peaks that have formed for all the beech wood samples,
raw and torrified. The trend shows that as the biomass progresses through the
torrefaction temperatures these peaks change. The first peak narrows and
deepens with the increase of torrefaction temperature, and represents the
volatiles that are being combusted, as explained by klass.[Error: Reference source not found]
As the first peak progresses through the range of torrefaction temperature the
volatiles released between 220-300o
C dramatically reduce, this nicely
represents the reduction in hemicellulose volatiles that remain in the biomass.
The first peak narrows and this most likely represents that most of the volatiles
that burn at the lower temperatures are being produced by the cellulose
especially if the 300o
C torrified sample is compare to cellulose peak shown in
figure 4.1.3
Mass loss Rates of raw and heat treated Beechwood
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
150 200 250 300 350 400 450 500 550
Temp (C)
Rateofmassloss(d%wt/dt)
Raw
200
225
250
275
300
55
56. Figure 4.3.1.1: The slow ashing mass loss rate of beech wood during
combustion
The second peak in figure 4.3.1.1 is representing the relative char percentage
of each of the samples, as more volatiles are removed during the higher
torrefaction temperatures the samples contains more char relative to the total
mass of the sample. This causes the gradual increase in peak size in both the
higher torrefaction temperature volatile peaks and the char peaks. These
increasing peaks also reflect how the energy density of the biomass increases
as there is more char and fixed carbon present relative to the overall mass of
the product.
The trend of an increasing fixed carbon peak, and narrowing volatile peak was
reflected in all of the slow ashing experiments. Showing the hemicellulose was
decomposing and the volatile components were being released. When looking
at the willow sample though there was an interesting difference with the high
temperature torrefaction profile. At torrefaction temperature of 300o
C the peak
dramatically dropped in size, this shows that more of the volatiles in the
biomass had been removed, not just the hemicellulose components but most
likely cellulose and possibly even some lignin decomposition had taken place.
Py-GC-MS Analysis of the products released at this higher 300o
C torrefaction
temperature will allow the evaluation of which biomass components are being
affected.
56
57. Mass loss Rates of raw and heat treated Willow
-8
-7
-6
-5
-4
-3
-2
-1
0
1
150 200 250 300 350 400 450 500 550
Temp (C)
Rateofmassloss(d%wt/dt)
Raw
200
225
250
275
300
Figure 4.3.1.2: The slow ashing mass loss rate of Willow during combustion
It is mentioned in the literature[Error: Reference source not found]
that as the torrefaction
process begins to approach the 300o
C the reaction becomes more like
pyrolysis in nature. This may be due to the cellulose composition in the soft
wood (galactoglucomannan based[Error: Reference source not found,Error: Reference source not found,Error:
Reference source not found]
) decomposing at a lower temperature and more readily,
meaning some pyrolysis reactions occur at lower temperatures due to the
willows lignocellulose composition, hence why the volatile peak in figure
4.3.1.2 drops dramatically at 300o
C torrefaction temperature.
This same anomaly was seen in the HEC Green Miscanthus, as shown in
figure 4.3.1.3. Once again, likely due to the plants lignocellulose composition,
past 250o
C torrefaction temperature the volatile peak of the combustion profile
began to reduce. This indicates more than the hemicellulose was
decomposing, and volatiles from the cellulose and possibly even some
components of the lignin were being removed in the higher temperature
torrefaction process. This will be looked at more in section 4.3.2. Once again
like in willow the green miscanthus biomass could have began to undergo
pyrolysis reactions, but this time at a lower temperature range than seen in the
willow. This may be what gave both willow and green miscanthus slightly
57
58. superior (relative) increases in heating value. And also their relative positions
on the Van Krevelen diagram
Mass loss Rates of raw and heat treated Green Miscanthus
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
100 150 200 250 300 350 400 450 500 550 600
Temp (C)
Rateofmassloss(d%wt/dt)
RAW
200
225
250
275
300
Figure 4.3.1.3: The slow ashing mass loss rate of Green Miscanthus during
combustion
58
59. When the rape straw was examined the rape straw’s profile was most similar
to that of the beech wood. The increasing torrefaction temperature caused a
narrowing and deepening of the volatile peak as shown below in figure 4.3.4.
Once again less volatiles were combusted over the lower temperature range,
showing the more unstable hemicellulose volatiles were being removed during
the increasing torrefaction temperatures.
Mass loss Rates of raw and heat treated Rape Straw
-8
-7
-6
-5
-4
-3
-2
-1
0
150 200 250 300 350 400 450 500 550
Temp (C)
Rateofmassloss(d%wt/dt)
Raw
200
225
250
275
300
Figure 4.3.1.4: The slow ashing mass loss rate of Rape straw during
combustion
One way the Rape Straw’s rate of combustion profile was quite different was
the small/negligible peak of fixed carbon seen during the slow-ashing. This
stems from a very low amount of fixed carbon, especially compared to the
other varities of biomass tested, something that is reflected in Table 4.1.1.
This low amount of fixed carbon is quite likely to be responsible for the slightly
poor energy yield shown by the rape straw seen in table 4.2.3, despite the
large mass loss observed.
59
60. 4.3.2 Pyrolysis of the Torrified Biomass Samples
The pyrolysis of the torrified biomasses allows further analysis of the volatiles
removed during torrefaction, with regards to the different types of biomass over
the range of torrefaction temperatures. Manipulating the data to give the mass
loss rate during pyrolysis clearly shows the trends and differences between the
varities of biomass and the range of torrefaction temperatures. The rate of
mass loss curves are shown in Appendix C.
The Beech Wood curve (Appendix C, figure C1) shows that the rate of volatiles
being lost between 200-340o
C decreases with increasing torrefaction
temperature, this reflects what was seen in the slow ashing experiments, the
more unstable hemicellulose polymers are decomposing more readily with the
increasing torrefaction temperatrues. The narrowing and deepening of the
peak also indicates that in the case of beech wood, the cellulose and lignin are
likely to be hardly affected.
The Willow curve (Appendix C, figure C2) reflects a similar trend as the beech
wood; a decrease in volatiles being produced at the lower pyrolysis
temperatures as the torrefaction temperature of the sample is increased. This
is representative of the decrease in hemicellulose remaining in the product as
the torrefaction temperatures increase. There is also a noticeable drop in the
peak depth between the 275o
C and 300o
C torrefaction samples. This may
represent the suspected cellulose degredation has began to take place as the
torrefaction process becomes more like pyrolysis in nature, though this cant be
confirmed without PY-GC-MS analysis
60
61. The Green Miscanthus curve (Appendix C, figure C3) is very similar to that of
willow. The curve has the same trends that have been shown before (the
degredation of hemicellulose increasing with torrefaction temperature)
represented by less volatiles being produced in the lower temperatures as the
samples are treated with higher torrefaction temperatures. And once again
there may well be some cellulose degradation also taking place, as there is a
drop in peak depth (rate of volatiles released during pyrolysis) at the higher
torrefaction temperatures. This will be investigated further by the mass spectra
of the volatiles released.
The Rape Straw curve (Appendix C, figure C4) has an unusual rate of mass
loss profile. In the previous curves there was a clear narrowing of the peak,
and the lower torrefaction temperature curves had a clear step between 200-
320o
C, (particularly visible with beech wood, fig C1) indicating volatiles are
being released from hemicellulose degradation, then as the temperature
increases the cellulose (and possibly partial lignin) degradation peak was
seen.
This stepped peak was not seen in the rate of mass loss curve for rape straw
pyrolysis. This could indicate that there are lower levels of cellulose and/or
lignin relatively, hence why there is no clear stepped peak. This makes it hard
to tell if there is any cellulose degradation, as the drop in the 300o
C torrefaction
sample peak could just represent that there is less volatiles remaining in the
product, due to low levels of cellulose and lignin relative to the high levels of
inorganics present within the rape straw. Once again the PY-GC-MS analysis
of the volatiles produced during torrefaction should indicate what is happening
to the biomass during torrefaction.
61
62. 4.3.3 PY-GC-MS Analysis of the Volatiles Produced During
Torrefaction
Table 4.3.3.1 shows the major volatile peaks Identified from the torrefection
process, the table gives each compounds details along with which type of
biomass sample it was released from. Chemicals that represent Major peaks
from any sample are marked in bold
Table 4.3.3.1: Analysis of the Volatiles Released During Torrefaction
Molecule
Molecular
Weight
Formula Structure
Volatile Released
From:
Acetic acid
anhydride, with
formic acid
88 C3H4O3
CH3 O
O
CH3
O Beech Wood
Willow
Green Miscanthus
Rape Straw
acetic acid methylester 74 C3H6O3
CH3
O
CH3
O
Beech Wood
Rape Straw
1,2-ethandiol,
monoacetate
104 C4H8O3
CH3
OO
OH Beech Wood
Green Miscanthus
Rape Straw
2-oxo,methylester,
propanoic acid
102 C4H6O3
CH3 O
O
CH3
O
Beech Wood
Green Miscanthus
Rape Straw
acetic acid anhydride 102 C4H6O3
CH3
OO O
CH3
Beech Wood
3-Furaldehyde 96 C5H4O2
O
O
Beech Wood
Willow
Green Miscanthus
Rape Straw
2-Furan methanol 98 C5H4O2
O
OH
Beech Wood
Willow
Green Miscanthus
Rape Straw
1-(acetyloxy)-2-
propanone
116 C5H8O3
CH3
OO
CH3
O
Beech Wood
Willow
Green Miscanthus
Rape Straw
2-cyclopentene-1,4-
dione
96 C5H4O2
O O Beech Wood
Willow
Green Miscanthus
Rape Straw
2-hydroxy-2-
cyclopenten-1-one
98 C5H6O2
O
OH
Beech Wood
Rape Straw
62
63. 4-hydroxy-butanoic
acid
104 C4H8O3
OH
O
OH
Green Miscanthus
Rape Straw
5-methyl-2-
furancarboxaldehyde
110 C6H6O2
O
O
CH3
Beech Wood
Willow
Green Miscanthus
(s)-5-hydroxymethyl-
2[5H]-furanone
114 C5H6O3
O
O
OH
Green Miscanthus
Rape Straw
benzylalcohol 108 C7H8O
OH
Willow
phenol 94 C6H6O
OH
Willow
methyl- β -D-
ribopyranoside
164 C6H9O5
O
OH
OH OH
O
CH3
Beech Wood
1,6:3,4-dianhydro-2-
deoxy- β -D-ribo-
hexopyranose
128 C5H8O3
O
O
O
Willow
propionaldehyde 58 C3H6O CH3
H
O Beech Wood
Willow
Green Miscanthus
Rape Straw
guaiacol 128 C7H8O2
O
OH
CH3
Beech Wood
Green Miscanthus
2,5-dimethyl-4-
hydroxy-3(2H)-
furanone
128 C6H8O3
O
OH O
CH3CH3
Willow
Green Miscanthus
1,6:3,4-dianhydro-2-O-
acetyl- β -D-
Galactopyranose
186 C8H10O5
O
O
O
O CH3
O
Willow
pentanal 86 C5H10O
O
CH3
Beech Wood
Willow
Green Miscanthus
Rape Straw
63
64. 2,3-dihydro-3,5-
dihydroxy-6-methyl-
4H-pyran-4-one
144 C6H8O4
O CH3
OH
O
OH
Green Miscanthus
3-methyl butanal 86 C5H10O
O
CH3
CH3
Green Miscanthus
2-methoxy-4-
vinylphenol
153 C9H9O2
OH
O
CH3
CH2
Beech Wood
Willow
Green Miscanthus
levoglucosan (1,6-
anhydro- β -D-
glucopyranose);
162 C6H10O5
O
O
OH
OH
OH
Willow
2,6-dimethoxy-4-
(2propenyl)-phenol
194 C11H14O3
O
CH3
O
CH3
OH
CH2
Willow
1-(4-hydroxy-3,5-
dimethoxyphenyl)-
ethanone
268 C16H12O4
O
CH3
O
CH3
OH
O CH3
Willow
2-hydroxy-
phenylmethylester
benzoic acid
228 C14H12O3
OH
O
O
Willow
2-methoxy-5-(1-
propenyl)
164 C10H12O2
OH
O
CH3 CH3
Beech Wood
3-hydroxy-4-methoxy
mandelic acid
198 C9H10O5
O
CH3
OH
OH
O
OH
Beech Wood
4,5-dimethyl-4-hexen-
3-one
126 C8H14O
CH3
CH3 CH3
O CH3
Green Miscanthus
64
65. Analysis of the volatiles released during torrefaction showed some of the
products formed were common among all the different feedstocks, particularly
at the lower retention times that were analysed. There was clear production of
low molecular acids, caused by hemicellulose degredation.[Error: Reference source not
found]
The low molecular, acids are believed to be responsible for the formation
of furfural, alderhydes and other volatile by-products, as they play an
important catalytic role in the acid degredation reactions that form these
products.[Error: Reference source not found,Error: Reference source not found]
The acetic acid, 3-Furaldehyde and 2-Furan methanol compounds and their
commonality among the varieties of biomass tested seemed to agree with the
literature, that torrefaction causes degradation of hemicellulose as the major
lignocellulosic component affected by the process , [Error: Reference source not found,Error:
Reference source not found,Error: Reference source not found,Error: Reference source not found,Error: Reference source not found,44]
and the acetic acid formed by the hemicellulose degredation has some role in
the production of furan and furfural products.
4.3.3.1 Volatiles Produced During the Torrefaction of Beech
Wood
When analysing the peaks of the PY-GC-MS torrefaction data for beech wood
there were not only signs of hemicellulose degredation but also some very
slight cellulose degredation and lignin degredation.
There are several small peaks [figure 4.3.3.1] which indicate the compounds:
3-hydroxy-4-methoxy mandelic acid, 2-methoxy-5-(1-propenyl), 2-methoxy-4-
vinylphenol and guaiacol, these compounds represent lignin degredation
having taken place. There was also a slightly bigger methyl- β -D-
ribopyranoside peak, indicating the degredation of cellulose.
These peaks showed that although there was some degredation of cellulose
and lignin during torrefaction, for beech wood it was only a very small amount.
This is what was expected, especially with the evidence provided by
figure4.1.3 which showed cellulose degredation barely started by 300o
C and
65
66. the lignin degredation was a lot less than that of hemicellulose and cellulose.
The mass loss profiles (appendix A) also indicated that beech wood was the
least affected by the torrefaction process, and hence possibly contained lower
levels of hemicellulose and less volatile cellulose/lignin components. The
amount and types of volatiles collected from the mass spectra back up this
belief.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18 1920
5 . 0 1 0 . 0 1 5 . 0 2 0 . 0 2 5 . 0 3 0 . 0 3 5 . 0 4 0 . 0 4 5 . 0 5 0 . 0 5 5 . 0 6 0 . 0 6 5 . 0 7 0 . 0 T im e ( m in )
0
1 0 0
%
Figure 4.3.3.1.1: PY-GC-MS Chromatogram for Beech wood under
torrefaction conditions (300o
C)
The main peaks identified in the mass spectrum are as follows:
1: Acetic acid anhydride, with formic acid; 2: acetic acid methylester 3: 2-oxo,methylester, propanoic acid;
4: 1,2-ethandiol, mono acetate; 5: acetic acid anhydride; 6: 3-Furaldehyde; 7: 2-Furan methanol;
8: 1-acetyloxy-2-propanone; 9: 2-cyclopentene-1,4-dione; 10: 2-hydroxy-2-cyclopenten-1-one;
11: 5-methyl-2-furancarboxaldehyde; 12:methyl- β -D-ribopyranoside; 13: propionaldehyde; 14:unknown;
15:Guaiacol; 16: unknown; 17: pentanal; 18: 2-methoxy-4-vinylphenol; 19:2-methoxy-5-(1-propenyl);
20:3-hydroxy-4-methoxy mandelic acid
66
