Study of Gasifiers

1. STUDY OF
GASIFIERS
Dr. Ajay Singh Lodhi
Assistant Professor
College of Agriculture, Balaghat
Jawahar Lal Krishi Vishwa Vidyalaya, Jabalpur (M.P.)

2. BIOMASS GASIFICATION
 The gasification is a thermo-chemical process that
converts any carbon-containing material into a
combustible gas by supplying a restricted amount of
oxygen.
 In case of biomass feedstock, this gas is known as wood
gas, producer gas or syngas, which composed primarily
of carbon monoxide and hydrogen as fuels, together
with small amount of methane.
 It will also contain other compounds, such as sulfur and
nitrogen oxides, depending on the chemical composition
of the fuel.
 Under typical gasification conditions, oxygen levels are
restricted to less than 30% of that required for complete
combustion.

3.  Raw producer is not an end product, but requires
further processing.
 Gasification adds value to low- or negative-value
feedstocks by converting them to marketable fuels and
products.
 In utilization of gases from biomass gasification, it is
important to understand that gas specifications are
different for the various applications.
 Furthermore, the composition of the gasification gas is
very dependent on the type of gasification process,
gasification agent and the gasification temperature.
 Based on the general composition and the typical
applications, two main types of gasification gas can be
distinguished as producer gas and syngas.

4. DIFFERENCE BETWEEN PRODUCER GAS AND SYNGAS
Low
temperature
gasification
(800 – 1000 C)
Producer Gas
CO, H2, CH4, CXHY
Syngas
CO, H2
High temperature
(1200 – 1400 C)
or catalytic
gasification
BIOMASS
Thermal
cracking
or
reforming
EndUseApplication

5. Difference between producer gas and syngas
 Producer gas is generated in the low temperature gasification
process (< 1000°C) and contains CO, H2, CH4, CxHy, aliphatic
hydrocarbons, benzene, toluene, and tars (besides CO2, H2O,
and N2 in case of gasification in air).
 H2 and CO typically contain only ~50% of the energy in the
gas, while the remainder is in CH4 and higher (aromatic)
HCs.
 Syngas is produced by high temperature (above 1200°C) or
catalytic gasification.
 Under these conditions the biomass is completely converted
into H2 and CO (besides CO2, H2O, and N2 in case of
gasification in air).
 Syngas is chemically similar to that derived from fossil
sources.
 This gas can also be made from producer gas by heating the
thermal cracking or catalytic reforming.

6. GASIFICATION AGENTS
 The oxidant for the gasification process can be either
atmospheric air or pure oxygen.
 Air gasification of biomass produces a low calorific value
(or Low Mega-Joule) gas, which contains about 50%
nitrogen and can fuel engines and furnaces.
 Gasification of biomass with pure oxygen results in a
medium calorific value (or Medium Mega-Joule) gas,
free of nitrogen.
 These system also offer faster reaction rates than air
gasification, but has the disadvantage of additional
capital costs associated with the oxygen plant.

7. GASIFIER END-USES
 Syngas or producer gas can be burned to create heat,
steam, or electricity.
 It can be converted to methane and fed into a natural
gas distribution system.
 Syngas can also be converted to methanol, ethanol, and
other chemicals or liquid fuels.
 Methanol produced through gasification can be further
refined into biodiesel with addition of vegetable oils or
animal fats.
 Use of gasification for generation of fuels, chemicals and
power.

8. ADVANTAGES OF BIOMASS GASIFICATION
 Produces a more convenient easily controllable form of
cleaner fuel for both thermal energy and electricity
generation, and provides a mean to reduce or remove
conventional fossil fuels.
 Gasification gives biomass the flexibility to fuel a wide range
of electricity generation systems: gas turbines, fuel cells, and
reciprocating engines.
 A wide variety of biomass materials can be gasified, many of
which would be difficult to burn otherwise.
 Gasification offers one means of processing waste fuels, many
of which can be problematic.
 Gasification has the potential of reducing emission of
pollutants and greenhouse gases per unit energy output.
 Projected process efficiencies are higher than the direct
combustion systems and comparable with fossil systems.

9. BIOMASS GASIFIERS
 Two principal types of gasifiers have emerged: fixed
bed and fluidized bed.
 Fixed bed gasifiers are further classified into three types
as updraft, downdraft and cross draft, depending on the
flow of gas through the fuel bed.
Drying zone
Pyrolysis Zone
Reduction Zone
Combustion Zone
Ash Zone
Biomass
Air
Producer
gas
Biomass
Drying zone
Pyrolysis Zone
Combustion
Zone
Ash Zone
Air
Reduction
Zone
Producer
gas
Air
Reduction
Zone
Combustion
Zone
Ash Zone
Air
Drying
zone
Pyrolysis
Zone
Producer
gas
Biomass
(a) Updraft gasifier (b) Down draft gasifier (c) Cross draft gasifier

10.  The processes taking place in the drying, pyrolysis and
reduction zones are driven by heat transferred from the
combustion zone (which is also called as the oxidation or
hearth zone).
 In the drying zone, moisture in biomass evaporates.
 In case of updraft gasifier this moisture leaves along with gas
at the top.
 In case of downdraft gasifier the moisture passes thorough
the reduction and combustion zones and participates in
certain chemical reactions.
 Essentially dry biomass enters the pyrolysis zone from
the drying zone.
 Pyrolysis converts the dried biomass into char, tar
vapour, water vapour and non-condensable gases.

11.  The vapours and non-condensable gases leave the
gasifier at the top in case of updraft gasifier.
 In case of downdraft gasifiers these pass through the
combustion zone and undergo further reactions.
 The char produced in the pyrolysis zone is around 20%
of the original biomass by weight and passes through
combustion and reduction zones.
 In the combustion zone, oxygen supplied for gasification
first comes in contact with the fuel.
 In case of updraft gasifier this fuel is carbonized
biomass, which can be regarded as consisting of mostly
carbon and ash.

12. GASIFICATION PROCESS
 Most gasification processes include several overlapping
steps.
 Among these steps, main two stages could be recognized
which a solid biomass fuel is thermo- chemically
converted into a Low- or Medium-MJ gas.
 In the first reaction, pyrolysis, the volatile components
of the fuel are vaporized at temperatures below 600°C
by a set of complex reactions. The volatile vapours
include hydrocarbon gases, H2, CO, CO2, tar, and water
vapor.
 In the second stage, char conversion, the carbon
remaining after pyrolysis undergoes the classic
gasification reaction (i.e. steam + carbon) and/or
combustion (carbon + oxygen).

13.  The combustion reaction provides the heat energy required to
drive the two stages of gasification reactions: pyrolysis and
char conversion.
 Because biomass fuels tend to have more volatile components
(70-85% on a dry basis) than coal (30%), pyrolysis plays a
larger role in biomass gasification than in coal gasification.
Heat
Biomass Pyrolysis
Char
Conversion
Combustion
Vapors(syngas)
GasTurbine
PowerCycles
Char
Vapors
(syngas)
Char
&
Ash Ash & Exhaust
Gases
Heat
Char&Ash
Heat/Power
Application

14. GASIFICATION PROCESS: REACTOR ZONES
 A fixed bed gasifier can be regarded as consisting of
four different zones: Drying zone, Pyrolysis zone,
Reduction zone and Combustion zone in which
different chemical and physical processes take place.
Drying zone
Pyrolysis Zone
Reduction Zone
Combustion Zone
Ash Zone
Biomass
Air
Producer
gas

15. UPWARD DRAFT OR COUNTER-CURRENT GASIFIER
 This one is oldest and simplest
type of gasifier. In this type of
gasifier, the air enters at the
bottom. The producer gas is drawn
off at the top.
 Near the grate at the bottom
combustion reaction occurs, above
that reduction reaction occurs.
 In the upper part of the gasifier
heating and pyrolysis of the
feedstock occurs as a result of heat
transfer by forced convention and
radiation from the lower zones.

16.  Tars and volatile produce produced during the reaction
will leave along with the syn gas at the top of the
gasifier. Which will be later separated by use of cyclone
and candle filter. The resulting gas is rich in
hydrocarbons (tars) and is suitable only for direct
heating purposes in industrial furnaces. If it is to be
used for electricity generation by I.C. engines, it has to
be cleaned thoroughly.
 The major advantages of this type of gasifier are its
simplicity, high charcoal burn out and internal heat
exchange leading to low temperature of exit gas and
high equipment efficiency. This gasifier can work with
several kind of feedstock ranging from Coal to Biomass.
 Inlet of coal can be decided based on the type of
gasification process selected to be used in this gasifier.

17. DOWNDRAFT OR CO-CURRENT GASIFIER
 In downdraught or co-current gasifiers air
enters at the combustion zone as sown in
the Figure. The producer gas leaves near
the bottom of the gasifier.
 The purpose of this type of gasifier is to
convert the tar (produced in the pyrolysis)
to gaseous products by complete thermal
cracking. This is not possible in an
updraught-type gasifier.
 The essential characteristic of this type of
gasifier is to draw tars (given off in the
pyrolysis zone) through the combustion
zone. They are broken down or burned in
the combustion zone. As a result, the
energy they contain is usefully released.
The mixture of gases in the exit stream is
relatively clean. The arrangement of the
combustion or hearth zone is thus a critical
element in a downdraught gasifier.

18.  In most downdraught gasifiers, the internal diameter
is reduced in the combustion zone to create a throat.
This is frequently made of replaceable ceramic
material. Air inlet nozzles are commonly set in a lining
round the throat to distribute air as uniformly as
possible.
 This type of gasifier is most commonly used for engine
applications due to its ability to produce a relatively
clean gas. A disadvantage of this type of gasifier is
that slagging or sintering of ash may occur due to the
concentrated oxidation (combustion) zone. Rotating
ash grates or similar mechanisms can solve this
problem. The gasifier efficiency is less than in an up-
draught gasifier due to the higher temperature.

19.  Main disadvantage is that downdraft gasifier cannot be
operated with range of different feed-stocks. Low
density feedstock gives rise to flow problems and
excessive pressure drop. High ash content coal also
gives more problem with this kind of gasifier than
updraft gasifier.
 Other disadvantage is it gives lower efficiency, since
there is no provision internal exchange compare to
updraft gasifier. The product stream also has low
calorific value.

20.  The flow of air and gas is across
the gasifier in a cross draft
gasifier as shown in Figure. It
operates at very high
temperatures. It confines its
combustion and reduction zones
by using a small diameter air
inlet nozzle. Water cooling of the
cast iron or steel tuyere is
essential due to the high
temperature. This type of
gasifier responds most rapidly to
changes in gas production due to
the short path-length for the
gasification reactions.
CROSS DRAFT GASIFIER
Reduction
Zone
Combustion
Zone
Ash Zone
Air
Drying
zone
Pyrolysis
Zone
Producer
gas
Biomass

21.  Ash formed due to the high temperature falls to the
bottom but does not hinder operation. The high exit
temperature of the gases and low CO2 reduction results
in poor quality of the gas with low efficiency. The fuel in
the hopper behaves as a heat shield against the radiant
heat. When operated with charcoal, the gasifier does not
need to be refractory lined. Cross-draft gasifiers have
very few applications due to their poor efficiency.
 Start up time (5-10 minutes) is much faster than that of
downdraft and updraft units. The relativey higher
temperature in cross draft gas producer has an obvious
effect on exit gas composition such as high carbon
monoxide and low hydrogen and methane content when
dry fuel such as charcoal is used. Cross draft gasifier
operates well on dry air blast and dry fuel.

22.  The fluidized bed gasifiers are categorized into two types
as bubbling fluidized bed and circulating fluidized
bed.
 Fixed bed gasifiers are typically simpler, less expensive,
and produce a lower heat content producer gas.
Fluidized bed gasifiers are more complicate, more
expensive, but produce a syngas with a higher heating
value.
Biomass
Air
Producer Gas
Fluidized
Bed
Air
Riser
Biomass
Cyclone
Return
Leg
Producer Gas
Bubbling
fluidized bed
gasifier
Circulating
fluidized bed
gasifier