3. 21st Century Energy Challenges:
• to meet the growing energy demand for
transportation, heating and industrial processes
• to provide raw materials for chemical industries
(in sustainable ways with minimal environmental
impact)
Solutions:
• Efficiency improvement in existing technologies
• Alternative fuels and renewables
• Net negative emissions technologies (NETs)
• Carbon capture utilization and storage (CCUS)
3
6. Biomass refers to the mass of living organisms, including
plants, animals, and microorganisms, or, from a
biochemical perspective, cellulose, lignin, sugars, fats,
and proteins.
Biomass is plant or animal material used for energy
production (electricity or heat), or in various industrial
processes as raw substance for a range of products.
What is biomass ?
Biomass is biological or organic material
derived from living, or recently living organisms
including plants, animals, and microorganisms.
Properties:
• Solid carbon-based fuel (like coal):H:C ~1.5,
O:C ~1 containing
• Metals, S, N , minor elements come from soil
• High moisture (>30%)
• Low energy density (<10 MJ/kg wet basis)
• Diffuse, expensive to harvest, ship
• Annual cycle: biomass available only at
harvest time, may need to be stored
6
7. Total solar energy that the earth stores in plants through photosynthesis: 2200 EJ/year
Global energy demand: 500 EJ/year
• Out of 2200 EJ, 300 EJ/year is currently being exploited by human, of which approximately 230 EJ/year is
used for food, animal feed, fiber, and energy and the 70 EJ/year is lost during harvest or burnt in
anthropogenic field fires (Ref: Pour, 2019)
• Any projection of bioenergy potential higher than 250 EJ/year (40-50% of global primary energy demand)
exceeds the biophysical limits
• Natural upper limit of harvestable bioenergy is further constrained by technical, economic, environmental,
and social complications
(exa = 1018)
7
8. Biomass sources
8
• Forestry crops and residues- firewood, wood pellets, and
wood chips
• Agricultural crops and residues— corn, soybeans, sugar
cane, switchgrass, woody plants, and algae, and crop and
food processing residues, energy crops
• Biogenic materials in municipal solid waste— paper, cotton,
and wool products, and food, yard, and wood wastes, leaf
litter
• Animal residues, manure, and human sewage, dead animals
• Industrial and mill residues- wastes from food, dairy,
textiles, and sugar industry; lumber and furniture mill
sawdust and waste, black liquor from pulp and paper mills
10. Ref: The National Energy Education Project
Molecular mass (g/mole):
H2O = 18; CO2 = 12x1 + 16x2 = 44
Glucose (C6H12O6) = (12x6 + 1x12 + 16x6 = 180)
10
11. Biomass composition
Cellulose: (C6H10O5)n, n ranges from
500-10000
Hemicellulose: (C5H8O4)m ; m
ranges from 50-200
Lignin: heterogeneous
and varies from
species to species;
approx. formula for
aspen wood:
(C31H34O11)n
11
12. • Biomass, also termed as,
lignocellulosic biomass
- Mainly obtained from plants
- Constitute more than 80% of the
total biomass
12
14. 14
Proximate analysis of a fuel provides the percentage of the material that burns in a gaseous state (volatile matter), in the
solid state (fixed carbon), and the percentage of inorganic waste material (ash).
Biomass is heated under various conditions for variable amounts of time to determine moisture, volatile matter, fixed
carbon, and ash yield.
Ultimate analysis determines the carbon, hydrogen, nitrogen, and sulfur in the material, as found in the gaseous products
of its complete combustion, the determination of ash in the material as a whole, and the estimation of oxygen by
difference.
16. Van Krevelen Plot
Van Krevelen diagrams characterize source rock organic matter or coal or biomass on a plot of atomic O/C versus
atomic H/C from elemental analysis
16
19. • Combustion: the material is in an oxygen-rich atmosphere, at a very high operating temperature, with
heat as the targeted output.
• Gasification takes place in an oxygen-lean atmosphere, with a high operating temperature, and gaseous
products being the main target (syngas production in most cases).
• Hydrothermal liquefaction occurs in a non-oxidative atmosphere, where biomass is fed into a unit as an
aqueous slurry at lower temperatures, and bio-crude in liquid form is the product.
• Pyrolysis is conducted usually at 400-600°C in the absence of oxygen, and produces gases, bio-oil,
and char; it is one of the first steps in gasification and combustion.
19
22. Pyrolysis
Pyro = heat
Lysis = break down
22
Process Schematic
Pyrolysis is thermal breakdown
of big molecules into smaller
molecules. It is a complex
process involving numerous
reactions over a range of
temperatures 300-700°C, often
in absence of oxygen.
23. Types of Pyrolysis
23
Slow pyrolysis – Fixed bed reactor; Batch reactor
Fast pyrolysis – Bubbling Fluidized bed reactor
Flash pyrolysis – Circulating Fluidized bed; Entrained Flow reactor
24. Small particles offer negligible resistance to internal heat transfer and their
temperature can be assumed to be uniform during pyrolysis. Pyrolysis of a
large biomass particle is a complex process and involves following steps:
• Transfer of heat to the surface of the particle from its surrounding
usually by convection and radiation
• Conduction of heat through the carbonized layer of the particle
• Carbonization of the virgin biomass over a range of temperature inside
the particle
• Diffusion of the volatile products from inside to the surface of the
particles, and
• Transfer of the volatile products from the surface of the particle to the
surrounding inert gas.
24
(Radiation,
Convection)
(Diffusion)
Mechanism
25. Pyrolysis process control parameters
Important pyrolysis process control parameters include:
• Heating rate (length of heating and intensity),
• Prevailing temperature and pressure
• The presence of ambient atmosphere
• The chemical composition of the fuel (e.g., the biomass resource),
• Physical properties of the fuel (e.g. particle size, density),
• Residence time and the existence of catalysts.
These parameters can be regulated by selection among different reactor types and heat transfer modes
25
Variation in products with temperature,
residence time, and heating rate
26. 26
Based on the movement of solids through the reactor during pyrolysis:
• No solid movement through the reactor during pyrolysis (Batch reactors)
• Moving bed (Shaft furnaces)
• Movement caused by mechanical forces (e.g. rotary kiln, rotating screw etc.)
• Movement caused by fluid flow (e.g., fluidized bed, entrained bed etc.)
Types of reactors
Rotating cone Bubbling fluidized bed Recirculating fluidized bed Suction based
Reactor
type
Mode of heat
transfer
Fluidized
bed
90% conduction;
9% convection;
1% radiation
Circulating
fluidized
bed
80% conduction;
19% convection;
1% radiation
Entrained
flow
4% conduction;
95% convection;
1% radiation
27. 27
Numerical 1: A biomass sample composition on mass basis (ultimate analysis on as received basis) is as follows:
C: 37.5 (%),
H: 8.8 %,
O: 27.6 %,
N: 0.30 %,
S: 0.20 %,
Moisture: 20.5% and
Ash: 5.1%.
Find the molecular formula of the biomass on i) as received
basis and ii) on dry and ash free basis.
Atomic weights:
C: 12; H: 1; O: 16; N: 14; S: 32;
H2O: 18; Ash: 56
Basis: 100 g biomass Dry and ash free basis
(As received basis)
(Dry and ash free basis)
CxHyOzNaSb
Find x, y, z, a, b
C: 37.5 g = 37.5/12 = 3.125 moles
H: 8.8 g = 8.8/1 = 8.8 moles
O: 27.6 g = 27.6/16 = 1.725 moles
N: 0.30 g = 0.3/14 = 0.02142 moles
S: 0.20 g = 0.20/32 =0.00625 moles
Moisture (H2O): 20.5 g = 20.5/18 = 1.139 moles
Ash: 5.1 g = 5.1/56 =0.091 moles
O from moisture (H2O) = 1.139 moles
H from moisture (H2O) = 2*1.139 = 2.278 moles
C3.125H11.078O2.864N0.02142 S0.00625 Ash0.091
28. 28
Bio-oil / Pyrolysis Oil
• For bio-oil, fast pyrolysis is the most preferred process and the fluidized bed reactor is the most preferred set-up
• Can convert 65–75% of the dry original fuel in liquid
• Pyrolysis oil or bio-oil is a liquid, typically dark red-brown to almost black
29. 29
• Energy density of bio-oil is lower than
fossil oil because of higher water and
oxygen contents
• Quality of bio-oil from lignocellulose is too
poor for a direct use as transportation fuel
• Upgrading of crude bio-oil (reduce oxygen
content and increase the hydrogen
content): through distillation and/or
catalytic hydrotreatment
30. 30
Numerical 2: On slow pyrolysis, forest wood produced 35% bio-oil, 40% char and 25% gas. Molecular formula of
the char is CH0.56O0.28N0.013 and of bio-oil is CH1.47O0.36N0.005. Gas composition is as follows: H2:20 %, CO2:36 %,
CO:25 % and CH4:19 %. Determine the percentage of carbon converted to bio-oil.
Assume basis: 100 g wood: 35 g bio-oil, 40 g char, 25 g gas
32. Gasification
• Gasification is thermochemical decomposition of organic material in a limited oxygen atmosphere (20 to
40% of stoichiometric value) to obtain producer gas as main product; though some liquids and tars,
charcoal and mineral matter (ash or slag) are also formed as byproducts.
• It adds value to low value biomass by converting them to marketable fuels and products.
• Gasification agent is the oxidant or an oxygen carrier for the gasification process e.g. atmospheric air,
pure oxygen, steam, CO2, metal oxides
• Composition of the producer gas is dependent on the type of feedstock, gasification process, gasification
agent, gasification temperature, and catalysts.
32
Air,
Steam,
CO2, O2
+ BIOMASS
CO, H2, CO2, H2O, CH4, C2H4
Unconverted tars
Syngas or Synthesis gas: CO, H2 mixture
Producer gas
33. 33
Equivalence ratio (ER) is defined as the ratio of actual air-
fuel ratio and stoichiometric air-fuel ratio. ER is thus the net
effect of airflow rate, feed consumption rate and the run
duration.
ER =
Actual air volume supplied per kg of biomass
Stoichiometric air volume per kg of biomass
The stoichiometric air/fuel ratio (m3/kg of biomass) at
normal conditions:
SR = 0.0889*(C + 0.375*S) + 0.265*H - 0.0333*O
where C, H, S, and O are the respective dry ash free mass
percentages of carbon, hydrogen, sulfur, and oxygen in the
biomass feed. These values can be found from the ultimate
analysis of the feed.
34. 34
Shen et al. 2017, Sustainable Energy Fuels
• Processes taking place in the drying,
pyrolysis and reduction zones are driven
by heat from the combustion zone
• Dry biomass enters the pyrolysis zone
from the drying zone
• Pyrolysis converts the dried biomass into
char, tar vapor, water vapor and non-
condensable gases
• In the reduction/gasification zone, the
products of complete oxidation (i.e. CO2,
H2O, etc.) undergo reduction by the
carbonized biomass/char
• In fluidized bed gasifiers, because of
mixing, separate reaction zones do not
exist, and all processes take place
simultaneously throughout the reactor
volume, although intensity may vary
depending on the location
Process mechanism
35. 35
Chemical Reactions
• During gasification of biomass, a number of homogeneous and heterogeneous reactions take place.
• There is a combination of number of primary reactions as well as secondary reactions (in which the products of the
primary reactions also take part), resulting in combustible gaseous products.
• Some basic reactions of gasification:
Gasification reactions are reversible. The direction of the reaction and its conversion are subjected to the constraints of
thermodynamic equilibrium and reaction kinetics.
36. 36
Types of gasifiers
• Fixed bed: Updraft, Downdraft, Cross draft depending on the flow of gas through the bed (moving bed)
• Fluidized bed: Bubbling, Circulating
• Entrained flow
37. 37
Updraft
The solid and the gas circulate in opposite directions. The solid descends slowly and the gasifying agents (air and oxygen
and steam) circulate in an upward direction. When the biomass descends, it is heated by the gas stream until it reaches
the combustion zone where the maximum temperature is reached, suffering a subsequent cooling prior to the discharge
of the ash. A fairly polluted gas is obtained since the low temperatures of the gases (250–500 °C) do not allow the
decomposition of oils, tars and gases formed (phenols, ammonia and H2S)
Downdraft
The solid and the gas circulate in the same direction inside the gasifier. The biomass, which is introduced through the
upper part, is subjected to a progressive increase in temperature, drying at the beginning and pyrolysis below. This
temperature pattern is originated due to the high temperatures generated in the lower part of the reactor, through the
partial combustion of the products that get there (gases, tars and coal). A fairly cleaner gas with lower tar contents is
produced.
Crossdraft
In this case, the oxidizing agent is introduced through one side of the reactor, the synthetized gas leaves the diametrically
opposite side. This gasifier has certain advantages over the previous ones since it has lower starting times, it can operate
with dry and wet fuels, and the temperature of the gas obtained is relatively high, so that the composition of the gas
produced contains small amounts of H2 and CH4, but higher tar contents.
38. 38
Biomass
Air
Producer Gas
Fluidized
Bed
Air
Riser
Biomass
Cyclone
Return
Leg
Producer Gas
Bubbling Circulating
Fixed bed gasifiers are simpler, less expensive, and
produce a lower heat content producer gas; preferred
for lower capacities. Fluidized bed gasifiers are more
complicate, more expensive, but produce a syngas
with a higher heating value; preferred for higher
throughput.
(a) EF gasifier (b) Duel FB gasifier (c) Plasma gasifier
Slag
Plasma
torch
Syngas
Biomass
Syngas
Slag
Oxygen
Steam
Biomass
Gasifier
Steam
Combustor
Air
Flue gas
Other gasifier types
39. 39
Bubbling fluidized bed:
The bed material (which could be a mixture of inert particles such as sand along with finely ground biomass) rests on a
distributor plate (either perforated or porous type) through which the fluidizing medium (e.g. air) is passed at a velocity
of about five times that of minimum fluidization velocity. Typical temperature in the bed is about 700–900 °C. The feed,
which is finely grained biomass, is introduced just above the distributor plate. The biomass first undergoes pyrolysis in
the hot bed above the distributor to form char and gaseous products due to devolatilization. The char particles are
lifted along with fluidizing air and undergo gasification in relatively upper portions of the bed. Due to contact with high
temperature bed, the high molecular weight tar compounds formed are cracked; thus, reducing the net tar content of
the producer gas to less than 1–3 g/Nm3.
Circulating fluidized bed:
They are an extension of the concept of bubbling bed fluidization. In this case, the velocity of the fluidizing air is much
higher than the terminal settling velocity of the bed material. Thus, the entire bed material (biomass + inert material
such as sand) is lifted by the fluidizing air. The exhaust of the gasifier is a relatively lean mixture of solids and gas. This
exhaust is admitted into a cyclone separator where solids are disengaged from the gas and are returned to the bed
through a down comer pipe.
40. 40
Air and Steam/Air: Low calorific value gas ( 4 – 6 MJ/m3)
O2 and Steam: Medium calorific value gas ( 12 – 18 MJ/m3)
Composition of the producer gas is dependent on the type of gasification process, feedstock, gasification agent,
gasification temperature, and catalysts.
41. 41
The lower heating value (MJ/m3) of producer gas is calculated using the LHVs and mole fractions (yi) of CO, H2 and CH4
LHVgas = yH2
∗10.7426 + yCO∗12.59852 + yCH4
∗35.8226
Gasifier Efficiency
• For engine applications, gas is often cooled. Cold gas efficiency is used for such applications, defined as the ratio of
energy content of producer gas to the energy content of the biomass
Cold gas efficiency (%) =
(Volumetric flowrate∗LHV)producer gas
(Consumption rate∗LHV)biomass
∗100
• For thermal applications, the gas is not cooled before combustion and the sensible heat of the gas is also useful.
The thermal or hot gas efficiency is used for such applications, which is defined as:
Hot gas efficiency (%) =
Sensible heatpg + (Volumetric flowrate∗LHV)𝑝𝑔
(Consumption rate∗LHV)biomass
∗100
𝑆𝑒𝑛𝑠𝑖𝑏𝑙𝑒 ℎ𝑒𝑎𝑡pg = mpg∗Cppg∗(Tpg−Tref/atm)
42. 42
Numerical 1: Dry Bagasse (C: 45%; H: 15%; O: 35%; N: 0.4%; S: 0.1%; Ash: 4.5%) is processed at a rate of 5 kg/h in a fixed
bed gasifier in presence of air at 10 kg/h. Find the ER, and cold and hot gas efficiencies of the gasifier, if the gas composition
is as follows: CO: 20%, H2: 10%, CO2: 15%; N2: 50%; CH4: 5%. Producer gas comes out at 500 C at flow rate of 12 kg/h.
Assume density of air as 1.2 kg/m3, density of gas as 1 kg/m3,
Cp of gas as 1 KJ/kgK, LHV of bagasse as 15 MJ/kg.
LHVgas(MJ/m3) = yH2
∗ 10.7426 + yCO∗12.59852 + yCH4
∗35.8226
Cold gas efficiency (%) =
(Volumetric flowrate∗LHV)producer gas
(Consumption rate∗LHV)biomass
∗100
Hot gas efficiency (%) =
Sensible heatpg + (Volumetric flowrate∗LHV)𝑝𝑔
(Consumption rate∗LHV)biomass
∗100
𝑆𝑒𝑛𝑠𝑖𝑏𝑙𝑒 ℎ𝑒𝑎𝑡pg = mpg∗Cppg∗(Tpg−Tref/atm)
ER =
Actual air volume supplied per kg of biomass
Stoichiometric air volume per kg of biomass
SR = 0.0889*(C + 0.375*S) + 0.265*H - 0.0333*O
where C, H, S, and O are the respective dry ash free percentages
SR = 7.13
ER = 0.233
LHV of pg = 5.385 MJ/m3
Sensible heat of pg = 5.7 MJ
HE = 93.76 %
CGE = 86.16 %
43. 43
Biomass Fuels,
Wastes
Combustion
Turdine
Gas
Cleanup
Fuels &
Chemicals
Electric
Power
Generator
Heat Recovery
Steam Generator
Steam
Recovered
Solids
Steam
Steam
Turbine
Generator
Electric
Power
Synthesis Gas
Conversion
Particulate
Removal
Shift
Reactor
Particulate
Sulfur Byproduct
Air
Compressor
Air
Separator
Air
Oxygen
Gasifier
Exhaust
Fuels
&
Chemicals
Power
Generation
Process schematic with applications
Removal of tar, which is the most
problematic parameter, increases the
overall cost of gasification set-up.
Different cleaning methods can be
employed for removing tar from
producer gas:
- Wet or wet-dry scrubbing
- Catalytic reforming
- Thermal cracking
44. 44
Fuel
Treatment, bulk density, moisture
(MC)
Tar (g/m3)
Ash
(%)
Gasifier Experience
Coconut
shell
Crushed (1-4 cm), 435kg/m3
MC =11.8%
3 0.8 downdraft
Excellent fuel. No slag
formation
Coconut
husks
Pieces 2 - 5 cm, 65kgm3 Insignificant 3.4 downdraft
Slag on grate but no
operational problem
Com cobs 304 kg/m3 , MC = 11% 7.24 1.5 downdraft
Excellent fuel. No
slagging
Com fodder Cubed, 390 kg/m3, MC= 11.9% 1.43 6.1 downdraft
Sever slagging and
bridging
Cotton
stalks
Cubed, 259kg/m3 , MC= 20.6% 5 17.2 downdraft Severe slag formation
Peat Briquettes, 555kg/m3, MC=13% _ _ downdraft Severe slagging
Rice hulls Pelleted, 679 kg/m3, MC = 8.6% 4.32 14.9 downdraft Severe slagging
Sugarcane Cut 2-5 cms, 52 kg/m3 Insignificant 1.6 downdraft
Slag on hearth ring.
Bridging
Gasification characteristics of different types of waste agricultural biomass
47. 47
Economics
▪ Biomass Pellets HHV = 20 MJ/kg; LPG HHV = 50 MJ/kg; Ratio = 2.5
▪1 LPG cylinder (14 kg LPG, cost Rs. 800) energy equivalent to 70 kg pellets (14x2.5 = 35/0.5 gasifier
efficiency 50%).
▪ Targeted Pellets Cost: Rs. 5/kg for 1 TPD capacity
▪ Approx. Cost of Rs. 7/kg pellets (Rs. 500 for 70 kg) make it economical than LPG on O&M cost
▪ Gasifier (10 kg/h) Cost: 1.5 lakhs
▪ For a kitchen consuming 3 LPG cylinders a day, 1 cylinder could be substituted saving approx. Rs.
300/day
▪ Payback period: less than 2 years
Sonal K Thengane, IIT Roorkee, IAH-302
