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Biomass Gasification
Dr. Akepati S. Reddy
School of Energy and Environment
Thapar University, Patiala
INDIA
Gasification
• Thermo-chemical process of converting solid biomass into syngas or
producer gas (N2 differentiates)
– Reacting biomass with gasifying agents (air, oxygen, steam, etc.) at
temperatures >700 °C
• 2nd generation route for biomass and waste utilization
– Developed in 1800s to produce town gas for lighting and cooking)
– Since 1920s, used in blast furnaces and synthetic chemical production
– During World War II, used to produce transportation fuels
– Currently used for heating (micro-gasifier biomass cook-stoves!), bio-
power generation, and hydrogen, bio-fuels and chemicals production
• Gasification is advantageous over combustion
– Wide range of feedstocks and low value feedstocks can be used
– Through syngas cleaning air pollution emissions can be reduced
– GHG emissions (biochar and CO2 sequestering) can be reduced
– Fuel efficiency (gas turbines and fuel cells use) can be increased
– Oxygen removal as CO2 and H2O from biomass (oxygenated) fuels is
possible (fuels combusted at higher temp. and in fuel cells possible)
– Provides easy to handle and efficient fuels
Component Producer gas Syngas
CO 18-22% 35-40%
Hydrogen 13-19% 20-40%
Methane 1-5% 0-15%
Heavier hydrocarbons 0.2-0.4% ---
Carbon dioxide 9-12% 25-35%
Nitrogen 45-55% 2-5%
Water vapour 4% Variable
Heating value 4.5 – 6 MJ/Nm3 5.3-12.6 MJ/Nm3
Syngas (synthesis gas):
Produced by gasification using oxygen and/or steam as gasification agent
Syngas is a mixture of CO and H2 - also has CO2, CH4, C2H4, N2, H2O and tar -
small amounts of H2S and Carbonyl sulfide, ammonia, etc. can also be there
After cleaning, it can be used to produce synthetic natural gas (SNG), H2, NH3,
Methanol, and synthetic hydrocarbon fuels (synthetic petroleum, synthetic diesel)
Can be used as fuel in IGCC power plants (integrated gasification combined cycle)
Called as producer gas if non-combustibles gases (N2 & CO2) are significant
Producer gas: produced by gasification using air as gasification agent
Biomass Gasification
Different from coal gasification
– Biomass is more reactive and has higher volatile content
– Gasification occurs at lower temperature
– Has lower sulfur content and higher alkali (sodium and
potassium) content – can cause slagging and fouling problems
– Product gas requires cleaning to remove tar and other
contaminants (alkali compounds)
Biomass gasification can be considered to include upstream
processing, gasification and downstream processing
Biomass Gasification: Upstream Processing
Particle size reduction, Drying and Densification
Biomass particle size reduction
• Smaller particles facilitate faster heat transfer rates and gasification
• Smaller particles result in more CH4, C2H4 and CO and lesser CO2
• Hammer mills, knife mills and tub grinders (small mobile hammer
mills) and screens are used
• Energy demands of particle size reduction depends on a) Initial
particle size, b) moisture content, c) Screen size, d) mill Properties
Biomass drying (usually to <10% level)
• Waste heat of gasification can be used
• Perforated bin dryers, band conveyer driers and rotary cascade
dryers are used
Densification
• Low bulk density fuels present problems in the gasifiers and hence
biomass is densified into pellets or briquettes
Biomass gasification: Gasifiers
Fixed bed gasifiers Fluidized bed gasifiers Other types of gasifiers
Updraft gasifiers Bubbling fluidized bed
gasifier
Entrained flow gasifiers
Down draft gasifiers Circulating fluidized
bed gasifier
Plasma gasifiers
Cross draft gasifiers Dual fluidized bed
gasifier
Twin fire gasifiers
Updraft fixed bed gasifier
• Biomass feed moves downwards and gasifying
agents move upward counter-current to
biomass through the fixed bed of biomass
• Ash is removed either as dry ash or as slag
(slagging gasifiers - temperatures are greater
than the ash fusion temperature)
• Has well defined drying, pyrolysis, reduction
and combustion zones
• Excessive tar in the product gas – uncarbonized
biomass is gasified (tar is a complex and
corrosive mixture of condensed liquid vapours)
• Has higher thermal efficiency
– Combustion takes place at the gasifier bed
bottom and hot gases pass through reduction,
pyrolysis and drying zones of the bed
– Product gas exits from top at lower temp. (500C)
• Biomass throughput is low for these gasifiers
• Good for fuels with high mechanical strength
and non-caking (permeable bed forming) fuels
Drying zone
Pyrolysis Zone
Reduction Zone
Combustion Zone
Ash Zone
Biomass
Air
Producer
gas
Downdraft fixed bed gasifiers
• Similar to updraft gasifier – but gasification
agent flows co-current to fuel (downwards -
down draft gasifier)
• Has well defined drying, pyrolysis, combustion
and reduction zones
• Tar levels in the product gas are lower
– Carbonized biomass is gasified and pyrolytic
gases pass through combustion - reduction zones
– Tar constituents of the pyrolytic gas, while
passing through combustion and reduction
zones, are consumed
• Overall efficiencies are lower - Product gas exits
from the bottom at higher temp. (800C)
• Quick startup (20 to 30 minutes) is a positive
feature
• High moisture and high ash content in the
biomass offer problems
• Preferred over updraft gasifiers for burning the
gas in IC engines
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
Cross draft fixed bed gasifiers
• Ash bin, combustion and reduction
zones are separated
• High operating temp., high exit gas
temp., high gas velocity and good load
following abilities
• Startup is even faster than that of the
downdraft gasifier
• Operates well on dry air and dry fuel
• CO2 reduction is poor
Twin fire fixed bed biomass gasifiers
• Has two defined reaction zones
• Drying, low-temp. Carbonization and cracking of gases
ccur in (upper) zone-1
• Gasification of charcoal takes place in (lower) zone-2
• Product gas is fairly clean and its temp. is 460 to 520 o C
• Gasification occurs at -30 mbar pressure
• Advantages of updraft and downdraft gasifiers are
combined here
Fluidized bed gasifiers
• Silica or alumina is used as the bed medium
– These materials have high specific heat and are stable at higher
temperatures
• Two types: Dry ash gasifiers and agglomerating gasifiers
– Biomass is fed from the bottom and ash is removed as dry ash or as
defluidized heavy agglomerates
– Temp. is lower in dry ash gasifiers than in agglomerating gasifiers
• Gasifying/fluidizing agents fluidize the bed - product gas is taken out
from top through cyclone separator (for particulate removal)
• Catalysts may be added to the fluidizing agents or to the fluidized
medium
• Fluidization enhances heat transfer, and increases reaction rates
and conversion efficiencies
– Throughput is higher than that of fixed bed but lower than that for
entrained bed gasifier
• Fluidization tolerates wide variations in fuel types and fuel
characteristics – biomass forming corrosive ash can be gasified
Fluidized bed gasifiers
Circulating fluidized bed gasifiers
• Fluidizing agents move the solids and ungasified particles along the
product gas
• An attached cyclone separator separates the solids and recirculates
back to the gasifier bed
Duel fluidized bed gasifier
• Includes (bubbling/circulating) fluidized bed gasification reactor
(fluidized by steam) and circulating fluidized bed combustion
reactor (fluidized by air)
• Biomass undergoes endothermic reaction to produce producer gas
(H2, CH4 and CO; and CO2 and H2O) in the gasification reactor
• Combustion reactor is used to heat the bed material and circulate
to the gasification reactor (supplies heat for endothermic
gasification reactions)
• Combustion of char (in bed material) from gasification reactor
occurs in the combustion reactor and heats the bed material
• Cyclones are used in the separation and circulation of bed material
Biomass
Air
Producer Gas
Fluidized
Bed
Air
Riser
Biomass
Cyclone
Return
Leg
Producer Gas
Entrained flow biomass gasifiers
• This gasifier is a vertical, cylindrical vessel
• Dry pulverized solid fuel is combusted with
oxygen in dense cloud of fine biomass
particles in the gasifier top
• Syngas exits from the bottom of the gasifier
– it is routed through a cyclone (and a water
scrubber) for the removal of fines
• Operates at high temp. (1000-1800C) and
high pressure (30-70 bars)
• Because of high temp. and pressure
throughput is high and because of high
temp. tar and methane are not present in
the product gas
• Oxygen requirement is higher than that for
other types of gasifiers
• Thermal efficiency is somewhat lower (the
gas must be cooled before it can be cleaned
with existing technology)
Plasma gasifiers
• Municipal solid waste, biomedical waste, organic waste, hazardous
wastes are gasified
• Process of conversion of organic matter into synthetic gas and slag
slag using plasma torch powered electric arc
• The torch ionizes gas and catalyzes organic matter into synthetic
gas and solid waste – slag
– Strong electric current under high voltage through the electrodes (vary
from copper/tungston to hanium/ zirconium) forms an electric arc of
2,200 to 13,900 °C that ionizes pressurized inert gas (argon)
– Waste is heated, melted and finally vaporized (dissociation and
separation of complex molecules into atoms occur -plasma pyrolysis
• Destruction of biomass is clean and clean alloyed slag from which
metals can be recovered is produced
– High temp. prevents formation of toxic compounds (furans, dioxins,
NOx, SOx, etc.)
– Conversion rate of plasma gasification is >99%
Thermo Gravimetric Analysis (TGA)
• Measures amount and rate of weight loss of a material (biomass)
with temperature or time in controlled (nitrogen) environments
• Applications include composition analysis, process simulation and
kinetic studies
• Thermal degradation occurs in 3 stages during TGA
– Dehydration – (free and bound) water loss occurs – occurs at <125C
temperature
– Active pyrolysis – occurs at 125-500C – loss of hemicellulose, cellulose
and part of lignin occurs
– Passive pyrolysis – occurs at >500C – slow and continuous loss of
residual lignin occurs
• Temperature ranges for the 3 stages and kinetic parameters of
degradation depends on
– Rate of heat transfer
– Composition of the biomass
– Degree of the oxidizing environment
15-25%
Biomass (CH1.4O0.6)
linear polymer of D-glucose linked with β
1-4 linkages
branched polymer of both C5 and C6
sugars
randomly constructed and highly aromatic
cross-linked macromolecule
38-50%
23-32%
Chemistry of Gasification
• Gas/vapour phase (syngas/producer gas)
– CO, H2, CH4, CO2 and H2O
– Light hydrocarbons (such as ethane and propane)
– Heavier hydrocarbons (like tars, that condense at 250-300)
– Undesirable gases like H2S and HCl
– Inert gases like nitrogen
• Composition depends on the biomass treated and the
operational conditions of the gasification process
• Low heating value (LHV) of the syngas is 4-13 MJ/Nm3
– LHV depends on the feedstock, the gasification technology, and
the gasifier operational conditions
• Solid phase (char)
– Mixture of unconverted organic fraction (largely carbon) and ash
– Amount of carbon depends on gasification technology and
operating conditions
– Amount of ash depends on the biomass being treated
– LHV of the char is 25-30 MJ/kg
Chemistry of Gasification
• Principal gasification reactions are endothermic
– Energy required of these reactions is obtained by the oxidation
of part of the biomass
– Partial oxidation of carbon carried out in the presence of a
gasifying agent (air, oxygen, steam, carbon dioxide)
• Allothermal or autothermal gasification processes
– Autothermal: gasifier is internally heated (by partial combustion)
– Allothermal: energy for gasification is supplied externally
• Autothermal process is considered to include
– Exothermic oxidation (combustion) and endothermic drying,
pyrolysis and reduction stages
– Tar decomposition is often considered as an additional stage
• Well defined zoning of stages is found in fixed bed gasifiers
– In the fluidized and circulating fluidized bed gasifiers zoning of
the stages is missing
– In the dual fluidized bed gasifiers the combustion is separated
into a second reactor
Chemistry of Gasification
Oxidation
• Supplies the required thermal energy and maintains temperature
• Occurs in the limited or controlled oxygen conditions (1/5 to 1/3rd
of the stoichiometric requirement)
• Only char and hydrogen of the syngas are believed to participate in
the oxidation reactions
C + O2 → CO2 (char combustion: - 394 kJ/mol)
C + ½ O2 → CO (Partial oxidation of char: - 111 kJ/mol)
H2 + ½ O2 → H2O (Hydrogen combustion: - 242 kJ/mol)
• Combustion product is a gas mixture of CO, CO2 and H2O
• If air is used in the oxidation then the gas mixture will have N2
Drying
• Endothermic and it is considered as complete if 150°C biomass
temperature is reached
• The released water vapour can involve in the reactions of other
gasification stages
Chemistry of Gasification
Pyrolysis
• Endothermic reactions and take place in the range of 250-700°C –
Thermochemical decomposition of the biomass matrix occurs –
Chemical bonds are cracked and smaller size molecules are formed
– Solid, liquid and gaseous fractions are formed
Biomass → H2 + CO + CO2 + CH4 + H2O + tar + Char (endothermic)
• Solid fraction is 5-10% for fluidized bed gasifiers and 20-25% for
fixed bed gasifiers – Has high carbon content (char) and inert
materials of the biomass as ash – LHV is higher
• Liquid fraction (called tars) varies with the type of gasifier – as low
as 1% for downdraft gasifiers, 1-5% for bubbling fluidized bed
gasifiers and 10-20% for updraft grasifiers – tars include complex
organic substances and are condensable at relatively lower
temperature (250-350°C)
• Gaseous fraction (pyrolysis gas) is 70-90% - mixture of gases (H2,
CO, CO2, light hydrocrabons – methane and upto C3 hydrcarbons;
and minor constituents like aicd and inert gases) non-condensable
at room temperature
Chemistry of Gasification
Reduction
• All products of pyrolysis and oxidation (also of drying) are involved
– The gas mixture react with char and final syngas is formed
C + CO2 ↔ 2CO (Boudouard reaction: + 172 kJ/mol)
C + H2O ↔ CO + H2 (Char reforming reaction: + 131 kJ/mol)
CO + H2O ↔ CO2 + H2 (water gas shift reaction: - 41 kJ/mol)
C + 2H2 ↔ CH4 (methanation reaction: - 75 kJ/mol)
• Overall the reduction reactions are endothermic, and the products
and the reactants are in chemical equilibrium
– Endothermic reactions are favoured when temperature increases, and
exothermic reactions at lower temperatures
– Temperature at which reduction reactions take place determine the
syngas composition
• Higher temperatures increase oxidation of char and reduce the tar
formation, but increase the risk of ash sintering
• Reduction temperature is key parameter in determining the
characteristics of the syngas and of the solid residue
Stages of Gasification
Steam Reforming
• Produces H2 and CO from hydrocarbons in a reformer device
– Steam reacts with the hydrocarbon (methane) at high temperature
(700-1100°C) in the presence of a metal (nickel) based catalyst
CH4 + H2O ⇌ CO + 3 H2 (strongly endothermic: +206 kJ/mole)
• The produced CO can be converted into H2 by LT (sweet) gas-shift
reaction, in the presence of a copper or iron catalyst
CO + H2O ⇌ CO2 + H2 (mildly exothermic: -41 kJ/mol).
• Steam Methane Reforming (SMR) and Steam Reforming of Non-
methane Hydrocarbons (NMHC)
• Steam reforming of NMHC at lower temp. with lower steam supply
first to H2 + CO and finally to CH4, CO2 and H2 through methanation
and water-gas shift reactions
CnHm + n H2O ↔ (n + m/2) H2 + n CO (steam reforming)
CO + 3 H2 ↔ CH4 + H2O (methanation)
CO + H2O ↔ H2 + CO2 (water-gas shift)
• Steam reforming of gaseous hydrocarbons is seen as a potential
way to provide fuel for fuel cells
Water-Gas Shifting (WGS reaction)
• Increasing or adjusting hydrogen/carbon monoxide (H2/CO) ratio of
scrubbed syngas is possible through water-gas shifting
– Syngas is passed through a multi-stage, fixed-bed reactor containing
shift catalysts at 400 - 900°F (pressure does not influence)
– CO + H2O ↔ H2 + CO2 (mildly exothermic: -41 kJ/mol)
– High moisture content shifts the equilibrium to right
– Gas is reheated to 30-50°F above saturation temperature prior to
feeding into the WGS to avoid catalyst damage by condensation
• WGS reaction is either a sour shift or a sweet shift reaction (after
removal of sulfur from the gas)
• Sour shifting uses cobalt-molybdenum catalyst
– Can convert organic sulfur (like corbonyl sulfide, COS) into H2S
• Sweet shifting is two types:
– High Temp. (HT) sweet shifting at 550 - 900°F using Chromium/copper
promoted iron-based catalysts
– Low temp. (LT) sweet shifting at 400 - 500°F using copper-zinc-
aluminum catalyst – used to reduce CO content to <1%
• The catalysts are extremely sensitive to sulfur and chloride poisoning
(hence not used in coal gasification applications)
Operating Conditions of Gasifiers
Equivalence ratio (ER)
• Ratio of actual air flow to the stoichiometric air flow required for
the biomass combustion
• Indicates the extent of partial combustion and controls the
gasification temperature (higher the flow greater will be the
temperature)
• Increasing ER from 0.2 to 0.45 increased gas yield, but decreased
LHV, and the percentages of H2, CO, CH4, C2H2, and tar
• Effect of ER on the composition of product gas is influenced by
temperature and S/B (steam to biomass) ratio
SV: Superficial velocity (m/sec.)
• Air flow rate to the gasifier cross-sectional area (m3/m2.sec.)
• Affect residence time of the gasifier – shorter time decreases the
extent of biomass conversion
Operating Conditions of Gasifiers
Steam to biomass ratio (S/B ratio):
• Gasification, where steam is used as the gasifying agent, and
temperature of gasification is >750-800C (catalysts can however
lower the temperature)
• Higher S/B ratio results in higher biomass conversion efficiency and
in reduced tar formation (is steam reforming responsible ?)
• With increased steam (partial pressure) water gas, water gas
shifting and methane reforming reactions are favoured and,
consequently, H2 production is increased
• (C + H2O → CO + H2; CO + H2O → CO2 + H2; CH4 + H2O → CO + 3H2)
– For S/B ratio >2.7, the gas composition do not change significantly
– For S/B ratio 0.0-1.35 yields of CH4, C2H4 and CO2 increase and CO
yield decreases
– For S/B ratio 1.35 – 2.7 yields of CO and CH4 decrease and yields of
CO2 and H2 increase
• Since the steam temp. is usually lower than the gasification temp.,
S/B ratio beyond a limit lowers the gasifier bed temp. and
negatively affect the gasification process
Operating Conditions of Gasifiers
H/C ratio:
• Ratio of Hydrogen (of biomass, moisture and steam) and Carbon (of
biomass)
• Increase of H/C ratio from 1.6 to 2.2 was found increasing H2 content
and LHV of gas, and decreasing tar content from 18 to 2 g/nm3
Temperature of the gasifying agent
• Increased temperature is found to increase the gas LHV and to
decrease tar and soot content
• A pre-heater for the gasifying agent can be of use
Temperature profile of gasification
• Composition and properties of the gas are influenced by temperature
– The content and the constituent gas ratios are also affected by pressure
• Increasing temperature increases gas yield and overall energy
content of the gas (increase of H2 % is mainly responsible)
– Temp. increase from 700 to 800C increases gas yield, H2 content, HHV,
energy efficiency, cabon conversion efficiency
– Temp. Increase from 700-800C decreases the CH4, CO and CO2 contents
Operating Conditions of Gasifiers
Temperature profile of gasification
• Between 750 and 950C, H2 content increased from 31% to 45%, CH4
and CO contents remained fairly constant and CO2 content decreased
• At >750-800C, H2 production increases but CH4 production decreases
• At >850-900C, steam reforming and Boudouard reactions
predominate and increase the CO content
CH4 + H2O ⇌ CO + 3 H2 (strongly endothermic: +206 kJ/mole)
C + CO2 ↔ 2CO (Boudouard reaction; predominates at >850-900C)
• High temperatures favour destruction and reforming of tar
• With air as gasifying agent H2 and CO content of gas increases with
temperature from 700 to 900C and CH4 and CO2 decreases
– CO/CO2 ratio linearly increases with temperature (0.85 at 700 and 2.7 at
900) – predominance of boudouard reaction at higher temperatures is
considered responsible
Downstream Processing
• Components of product gas that need addressing
– Tar
– Particulates (unconverted biomass, ash, char and bed material)
– Alkali compounds, and nitrogen, sulfur and chlorine containing
compounds
– Water vapour
• Adjustment of the composition of the product gas and/or ratio of
H2 to CO and tar removal
– Water-gas shift reactions
– Reforming reactions
• Removal of the undesirable constituents (particulates, tar, moisture
and carbon dioxide)
– Cyclone separation
– Wet scrubbing
– Wet electrostatic precipitation
– Barrier filters
• Cooling of the product gas and moisture removal
Downstream Processing
Removal of particulates
• Cyclones (and multiclones), Wet scrubbers, Barrier filters and ESPs
are used to remove particulates
• Cyclone separators remove larger particles (>5 μ) with little
pressure drop
– Cut diameter (increasing cyclone size increases cut diameter)
– Multiclones increase the separation efficiency
• Wet scrubbers are used with product gas at temperature <100C
– Venturi scrubbers have 2.5 to 25 Kpa headloss
– Scrubbers remove 1 micron particles with >95% efficiency
• Effectiveness of the ESP will depend on the particulate resistivity
and sulfur and alkali content
• Barrier filters: metal or ceramic porous candle filters; bag filters and
packed bed filters
– Candle filters are good for hot gas cleaning
– Bag filters operate at temperatures about 350C
– Packed bed filters use ceramic spheres/saw dust
Downstream Processing
Alkali compounds
• Alkali compounds expected are CaO, K2O, P2O5, MgO, Na2O, SiO3
and SO3
• Alkali compounds vapourize at >700°C and condense at <650°C
temperature, stick to surfaces and cause corrosion
• Alkali salts inactivate catalysts
• For alkali compounds removal, cool the gas and pass through
barrier filters
• Removal of alkali compounds from hot gases is needed – bauxite
filters at 650-725°C can remove Na and K compounds
Downstream Processing
Nitrogen (0.5 to 3% in the biomass)
• Nitrogen is converted to ammonia (60-65%), molecular nitrogen
and HCN (very little)
• Combustion of ammonia forms NOx
• Wet scrubbing can remove ammonia
• Ammonia can be destructed at higher temperature using dolomite,
nickel based catalysts and iron based catalysts
Sulfur (<0.5% in biomass)
• Sulfur is converted to H2S and SO2
• Sulfur can irreversibly inactivate catalysts (in methanol synthesis)
• Fuel cells and tar (hydrocarbon) conversion catalysts are sensitive to
sulfur and use syngas with low levels of sulfur
• SO2 can be removed by scrubbing
• Lime stone, dolomite or CaO can be used to remove SO2 and H2S
• Cleaning at 600-900°C and regeneration of the sorbents is the best
way for H2S removal from hot gases
Tar handling and management
• Tar is a generic term - refers to condensable organic compounds of
the product gas (condensable at >25°C !)
– Sticky (and highly viscous) brownish to black strongly smelling liquid
– Tar aerosols are <1 μ size
• Compounds of tar can be categorized as
– Mixed oxygenates
– Phenolic eathers
– Alkyl phenolics
– Heterocyclic ethers
– PAHs (lighter PAHs – 2 or3 rings, and heavier PAH - ≥4 rings)
• Composition of tar depends on
– Biomass fuel particle size and size distribution
– Elemental composition
– Mineral and water content of the fuel
• Composition is influenced by
– reactor design and by the gasification agent
– Rate of heating and the maximum temp reached
– The surroundign gaseous atmosphere
Tar handling and management
• Composition of tar shifts towards mixed oxygenates and larger PAH
with the increasing gasification reaction severity
– Reaction severity increases with Increasing temperature, flow of
oxidizing agents and with the increasing amount of catalyst
• Combustion systems can work with relatively high amounts of tar
• Tar beyond certain level can poison catalysts (deposition of tar can
occur on catalysts)
• Tar content of the product gas is reduced through primary
techniques within the gasifier
– Design of the gasifier (use of staged gasification systems) – well
designed downdraft gasifiers
– Operating conditions of the gasifier
– Use of catalysts (in-bed catalysts)
– High temperature gasification (entrained gasification
• Tar is removed from product gas through downstream processing
(secondary removal techniques)
– Wet and hot gas cleaning (Barrier filters and Cyclone separators are
not good for tar removal)
– Destruct and reform tar
Tar handling and management
Gasifier design
• Spatial separation of pyrolysis step from the following oxidation and
reduction steps
Designs and operating conditions of the gasifier
• Gasifier design, ER (equivalence ratio), Gasifying agent (type) used,
Gasification agent to biomass ratio, S/B (steam to biomass) ratio,
Gasification temperature profile, etc. influence tar levels in the gas
– Higher the ER, S/B and temperature lesser will be the tar content
– Higher superficial velocity (>0.7 m/sec.) can result in higher tar levels
for downdraft gasifiers
– Secondary injection of air/O2 can lower the tar content
In-bed catalysts
• Dolamite, limestone, olivine, alkali carbonates, nickel bound
catalysts, metal oxide catalysts, zeolite, char, etc.
• In-bed catalysts, through enhancing reforming reactions, reduce the
tar levels and increase overall carbon conversion efficiencies
Tar handling and management
Wet and hot gas cleaning
• Water (venturi) scrubbing for condensation removal of tar (can
reduce tar level to <20-40 mg/Nm3)
– Exit gas temperature of the scrubber may be 35-60°C
• Wet cleaning requires reduced temperature of product gas
(sensible heat loss problem)
• Using a liquid other than water for the scrubbing can be considered
• Scrubbing liquid may require treatment prior to disposal
• ESP wet scrubber can remove tar at temperature >150°C
Tar handling and management
Reforming for tar removal and increased H2 production
• Cracking of tar at higher (>850°C) temp. without using any catalyst
– Results in increased hydrogen production
– Can reduce the gasification efficiency, can causes material problems
and can produce soot
• High temperature and catalysts provide favourable conditions for
reforming and shift reactions
– Increasing temperature (from 650-850°C) increases H2 content,
decreases CH4, CO2, and C2 yield, but CO content remains same
– Steam/air in the presence of catalyst destructs tar and form CO, H2,
CO2 and CH4
– Supply/Use of air to the reformer decreases the product’s HHV
through partial combustion and dilution of the product gas
Tar handling and management
• Decreasing WHSV (from 10.7/hr to 2.7/hr) increases H2 and H2/CO
and decreases CH4 (H2 increased by 49%, H2/CO increased to 3.32
and CH4 decreased by 50%)
– WHSV (Weight Hourly Space Velocity): Ratio of biomass flow rate to
the catalyst mass in the reactor
• Temperature, space time, catalyst particle size, and gas composition
affect the tar conversion efficiencies
– Space time: Mass of catalyst per unit flow rate of product gas
Tar handling and management
Catalysts are two types: Mineral catalysts and Synthetic catalysts
Mineral catalysts
• Calcined rocks (dolomite, magnesite and calcite), olivine, clay
materials and ion ores
– Calcined rocks are best but prove erosive in fluidized bed gasifiers
– Olivine is less effective than dolomite (calcined rocks)
– Clay materials do not withstand high temperatures (800-900°C)
– Iron ores are rapidly deactivated
Synthetic catalysts
• Fluid Catalytic Cracking (FCC) catalysts, char, alkali metal based
catalysts, activated alumina, transition metal (Pt, Zr, Rh, Ru, Fe and
Ni) based catalysts
– Transition metal based catalysts are expensive
– FCC catalysts deactivate quite rapidly
– Alkali metal based catalysts can agglomerate in the bed

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biomass gasification

  • 1. Biomass Gasification Dr. Akepati S. Reddy School of Energy and Environment Thapar University, Patiala INDIA
  • 2. Gasification • Thermo-chemical process of converting solid biomass into syngas or producer gas (N2 differentiates) – Reacting biomass with gasifying agents (air, oxygen, steam, etc.) at temperatures >700 °C • 2nd generation route for biomass and waste utilization – Developed in 1800s to produce town gas for lighting and cooking) – Since 1920s, used in blast furnaces and synthetic chemical production – During World War II, used to produce transportation fuels – Currently used for heating (micro-gasifier biomass cook-stoves!), bio- power generation, and hydrogen, bio-fuels and chemicals production • Gasification is advantageous over combustion – Wide range of feedstocks and low value feedstocks can be used – Through syngas cleaning air pollution emissions can be reduced – GHG emissions (biochar and CO2 sequestering) can be reduced – Fuel efficiency (gas turbines and fuel cells use) can be increased – Oxygen removal as CO2 and H2O from biomass (oxygenated) fuels is possible (fuels combusted at higher temp. and in fuel cells possible) – Provides easy to handle and efficient fuels
  • 3. Component Producer gas Syngas CO 18-22% 35-40% Hydrogen 13-19% 20-40% Methane 1-5% 0-15% Heavier hydrocarbons 0.2-0.4% --- Carbon dioxide 9-12% 25-35% Nitrogen 45-55% 2-5% Water vapour 4% Variable Heating value 4.5 – 6 MJ/Nm3 5.3-12.6 MJ/Nm3 Syngas (synthesis gas): Produced by gasification using oxygen and/or steam as gasification agent Syngas is a mixture of CO and H2 - also has CO2, CH4, C2H4, N2, H2O and tar - small amounts of H2S and Carbonyl sulfide, ammonia, etc. can also be there After cleaning, it can be used to produce synthetic natural gas (SNG), H2, NH3, Methanol, and synthetic hydrocarbon fuels (synthetic petroleum, synthetic diesel) Can be used as fuel in IGCC power plants (integrated gasification combined cycle) Called as producer gas if non-combustibles gases (N2 & CO2) are significant Producer gas: produced by gasification using air as gasification agent
  • 4.
  • 5. Biomass Gasification Different from coal gasification – Biomass is more reactive and has higher volatile content – Gasification occurs at lower temperature – Has lower sulfur content and higher alkali (sodium and potassium) content – can cause slagging and fouling problems – Product gas requires cleaning to remove tar and other contaminants (alkali compounds) Biomass gasification can be considered to include upstream processing, gasification and downstream processing
  • 6. Biomass Gasification: Upstream Processing Particle size reduction, Drying and Densification Biomass particle size reduction • Smaller particles facilitate faster heat transfer rates and gasification • Smaller particles result in more CH4, C2H4 and CO and lesser CO2 • Hammer mills, knife mills and tub grinders (small mobile hammer mills) and screens are used • Energy demands of particle size reduction depends on a) Initial particle size, b) moisture content, c) Screen size, d) mill Properties Biomass drying (usually to <10% level) • Waste heat of gasification can be used • Perforated bin dryers, band conveyer driers and rotary cascade dryers are used Densification • Low bulk density fuels present problems in the gasifiers and hence biomass is densified into pellets or briquettes
  • 7. Biomass gasification: Gasifiers Fixed bed gasifiers Fluidized bed gasifiers Other types of gasifiers Updraft gasifiers Bubbling fluidized bed gasifier Entrained flow gasifiers Down draft gasifiers Circulating fluidized bed gasifier Plasma gasifiers Cross draft gasifiers Dual fluidized bed gasifier Twin fire gasifiers
  • 8. Updraft fixed bed gasifier • Biomass feed moves downwards and gasifying agents move upward counter-current to biomass through the fixed bed of biomass • Ash is removed either as dry ash or as slag (slagging gasifiers - temperatures are greater than the ash fusion temperature) • Has well defined drying, pyrolysis, reduction and combustion zones • Excessive tar in the product gas – uncarbonized biomass is gasified (tar is a complex and corrosive mixture of condensed liquid vapours) • Has higher thermal efficiency – Combustion takes place at the gasifier bed bottom and hot gases pass through reduction, pyrolysis and drying zones of the bed – Product gas exits from top at lower temp. (500C) • Biomass throughput is low for these gasifiers • Good for fuels with high mechanical strength and non-caking (permeable bed forming) fuels Drying zone Pyrolysis Zone Reduction Zone Combustion Zone Ash Zone Biomass Air Producer gas
  • 9. Downdraft fixed bed gasifiers • Similar to updraft gasifier – but gasification agent flows co-current to fuel (downwards - down draft gasifier) • Has well defined drying, pyrolysis, combustion and reduction zones • Tar levels in the product gas are lower – Carbonized biomass is gasified and pyrolytic gases pass through combustion - reduction zones – Tar constituents of the pyrolytic gas, while passing through combustion and reduction zones, are consumed • Overall efficiencies are lower - Product gas exits from the bottom at higher temp. (800C) • Quick startup (20 to 30 minutes) is a positive feature • High moisture and high ash content in the biomass offer problems • Preferred over updraft gasifiers for burning the gas in IC engines Biomass Drying zone Pyrolysis Zone Combustion Zone Ash Zone Air Reduction Zone Producer gas Air
  • 10. Reduction Zone Combustion Zone Ash Zone Air Drying zone Pyrolysis Zone Producer gas Biomass Cross draft fixed bed gasifiers • Ash bin, combustion and reduction zones are separated • High operating temp., high exit gas temp., high gas velocity and good load following abilities • Startup is even faster than that of the downdraft gasifier • Operates well on dry air and dry fuel • CO2 reduction is poor Twin fire fixed bed biomass gasifiers • Has two defined reaction zones • Drying, low-temp. Carbonization and cracking of gases ccur in (upper) zone-1 • Gasification of charcoal takes place in (lower) zone-2 • Product gas is fairly clean and its temp. is 460 to 520 o C • Gasification occurs at -30 mbar pressure • Advantages of updraft and downdraft gasifiers are combined here
  • 11. Fluidized bed gasifiers • Silica or alumina is used as the bed medium – These materials have high specific heat and are stable at higher temperatures • Two types: Dry ash gasifiers and agglomerating gasifiers – Biomass is fed from the bottom and ash is removed as dry ash or as defluidized heavy agglomerates – Temp. is lower in dry ash gasifiers than in agglomerating gasifiers • Gasifying/fluidizing agents fluidize the bed - product gas is taken out from top through cyclone separator (for particulate removal) • Catalysts may be added to the fluidizing agents or to the fluidized medium • Fluidization enhances heat transfer, and increases reaction rates and conversion efficiencies – Throughput is higher than that of fixed bed but lower than that for entrained bed gasifier • Fluidization tolerates wide variations in fuel types and fuel characteristics – biomass forming corrosive ash can be gasified
  • 12. Fluidized bed gasifiers Circulating fluidized bed gasifiers • Fluidizing agents move the solids and ungasified particles along the product gas • An attached cyclone separator separates the solids and recirculates back to the gasifier bed Duel fluidized bed gasifier • Includes (bubbling/circulating) fluidized bed gasification reactor (fluidized by steam) and circulating fluidized bed combustion reactor (fluidized by air) • Biomass undergoes endothermic reaction to produce producer gas (H2, CH4 and CO; and CO2 and H2O) in the gasification reactor • Combustion reactor is used to heat the bed material and circulate to the gasification reactor (supplies heat for endothermic gasification reactions) • Combustion of char (in bed material) from gasification reactor occurs in the combustion reactor and heats the bed material • Cyclones are used in the separation and circulation of bed material
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  • 15. Entrained flow biomass gasifiers • This gasifier is a vertical, cylindrical vessel • Dry pulverized solid fuel is combusted with oxygen in dense cloud of fine biomass particles in the gasifier top • Syngas exits from the bottom of the gasifier – it is routed through a cyclone (and a water scrubber) for the removal of fines • Operates at high temp. (1000-1800C) and high pressure (30-70 bars) • Because of high temp. and pressure throughput is high and because of high temp. tar and methane are not present in the product gas • Oxygen requirement is higher than that for other types of gasifiers • Thermal efficiency is somewhat lower (the gas must be cooled before it can be cleaned with existing technology)
  • 16. Plasma gasifiers • Municipal solid waste, biomedical waste, organic waste, hazardous wastes are gasified • Process of conversion of organic matter into synthetic gas and slag slag using plasma torch powered electric arc • The torch ionizes gas and catalyzes organic matter into synthetic gas and solid waste – slag – Strong electric current under high voltage through the electrodes (vary from copper/tungston to hanium/ zirconium) forms an electric arc of 2,200 to 13,900 °C that ionizes pressurized inert gas (argon) – Waste is heated, melted and finally vaporized (dissociation and separation of complex molecules into atoms occur -plasma pyrolysis • Destruction of biomass is clean and clean alloyed slag from which metals can be recovered is produced – High temp. prevents formation of toxic compounds (furans, dioxins, NOx, SOx, etc.) – Conversion rate of plasma gasification is >99%
  • 17. Thermo Gravimetric Analysis (TGA) • Measures amount and rate of weight loss of a material (biomass) with temperature or time in controlled (nitrogen) environments • Applications include composition analysis, process simulation and kinetic studies • Thermal degradation occurs in 3 stages during TGA – Dehydration – (free and bound) water loss occurs – occurs at <125C temperature – Active pyrolysis – occurs at 125-500C – loss of hemicellulose, cellulose and part of lignin occurs – Passive pyrolysis – occurs at >500C – slow and continuous loss of residual lignin occurs • Temperature ranges for the 3 stages and kinetic parameters of degradation depends on – Rate of heat transfer – Composition of the biomass – Degree of the oxidizing environment
  • 18. 15-25% Biomass (CH1.4O0.6) linear polymer of D-glucose linked with β 1-4 linkages branched polymer of both C5 and C6 sugars randomly constructed and highly aromatic cross-linked macromolecule 38-50% 23-32%
  • 19. Chemistry of Gasification • Gas/vapour phase (syngas/producer gas) – CO, H2, CH4, CO2 and H2O – Light hydrocarbons (such as ethane and propane) – Heavier hydrocarbons (like tars, that condense at 250-300) – Undesirable gases like H2S and HCl – Inert gases like nitrogen • Composition depends on the biomass treated and the operational conditions of the gasification process • Low heating value (LHV) of the syngas is 4-13 MJ/Nm3 – LHV depends on the feedstock, the gasification technology, and the gasifier operational conditions • Solid phase (char) – Mixture of unconverted organic fraction (largely carbon) and ash – Amount of carbon depends on gasification technology and operating conditions – Amount of ash depends on the biomass being treated – LHV of the char is 25-30 MJ/kg
  • 20. Chemistry of Gasification • Principal gasification reactions are endothermic – Energy required of these reactions is obtained by the oxidation of part of the biomass – Partial oxidation of carbon carried out in the presence of a gasifying agent (air, oxygen, steam, carbon dioxide) • Allothermal or autothermal gasification processes – Autothermal: gasifier is internally heated (by partial combustion) – Allothermal: energy for gasification is supplied externally • Autothermal process is considered to include – Exothermic oxidation (combustion) and endothermic drying, pyrolysis and reduction stages – Tar decomposition is often considered as an additional stage • Well defined zoning of stages is found in fixed bed gasifiers – In the fluidized and circulating fluidized bed gasifiers zoning of the stages is missing – In the dual fluidized bed gasifiers the combustion is separated into a second reactor
  • 21. Chemistry of Gasification Oxidation • Supplies the required thermal energy and maintains temperature • Occurs in the limited or controlled oxygen conditions (1/5 to 1/3rd of the stoichiometric requirement) • Only char and hydrogen of the syngas are believed to participate in the oxidation reactions C + O2 → CO2 (char combustion: - 394 kJ/mol) C + ½ O2 → CO (Partial oxidation of char: - 111 kJ/mol) H2 + ½ O2 → H2O (Hydrogen combustion: - 242 kJ/mol) • Combustion product is a gas mixture of CO, CO2 and H2O • If air is used in the oxidation then the gas mixture will have N2 Drying • Endothermic and it is considered as complete if 150°C biomass temperature is reached • The released water vapour can involve in the reactions of other gasification stages
  • 22. Chemistry of Gasification Pyrolysis • Endothermic reactions and take place in the range of 250-700°C – Thermochemical decomposition of the biomass matrix occurs – Chemical bonds are cracked and smaller size molecules are formed – Solid, liquid and gaseous fractions are formed Biomass → H2 + CO + CO2 + CH4 + H2O + tar + Char (endothermic) • Solid fraction is 5-10% for fluidized bed gasifiers and 20-25% for fixed bed gasifiers – Has high carbon content (char) and inert materials of the biomass as ash – LHV is higher • Liquid fraction (called tars) varies with the type of gasifier – as low as 1% for downdraft gasifiers, 1-5% for bubbling fluidized bed gasifiers and 10-20% for updraft grasifiers – tars include complex organic substances and are condensable at relatively lower temperature (250-350°C) • Gaseous fraction (pyrolysis gas) is 70-90% - mixture of gases (H2, CO, CO2, light hydrocrabons – methane and upto C3 hydrcarbons; and minor constituents like aicd and inert gases) non-condensable at room temperature
  • 23. Chemistry of Gasification Reduction • All products of pyrolysis and oxidation (also of drying) are involved – The gas mixture react with char and final syngas is formed C + CO2 ↔ 2CO (Boudouard reaction: + 172 kJ/mol) C + H2O ↔ CO + H2 (Char reforming reaction: + 131 kJ/mol) CO + H2O ↔ CO2 + H2 (water gas shift reaction: - 41 kJ/mol) C + 2H2 ↔ CH4 (methanation reaction: - 75 kJ/mol) • Overall the reduction reactions are endothermic, and the products and the reactants are in chemical equilibrium – Endothermic reactions are favoured when temperature increases, and exothermic reactions at lower temperatures – Temperature at which reduction reactions take place determine the syngas composition • Higher temperatures increase oxidation of char and reduce the tar formation, but increase the risk of ash sintering • Reduction temperature is key parameter in determining the characteristics of the syngas and of the solid residue
  • 25. Steam Reforming • Produces H2 and CO from hydrocarbons in a reformer device – Steam reacts with the hydrocarbon (methane) at high temperature (700-1100°C) in the presence of a metal (nickel) based catalyst CH4 + H2O ⇌ CO + 3 H2 (strongly endothermic: +206 kJ/mole) • The produced CO can be converted into H2 by LT (sweet) gas-shift reaction, in the presence of a copper or iron catalyst CO + H2O ⇌ CO2 + H2 (mildly exothermic: -41 kJ/mol). • Steam Methane Reforming (SMR) and Steam Reforming of Non- methane Hydrocarbons (NMHC) • Steam reforming of NMHC at lower temp. with lower steam supply first to H2 + CO and finally to CH4, CO2 and H2 through methanation and water-gas shift reactions CnHm + n H2O ↔ (n + m/2) H2 + n CO (steam reforming) CO + 3 H2 ↔ CH4 + H2O (methanation) CO + H2O ↔ H2 + CO2 (water-gas shift) • Steam reforming of gaseous hydrocarbons is seen as a potential way to provide fuel for fuel cells
  • 26. Water-Gas Shifting (WGS reaction) • Increasing or adjusting hydrogen/carbon monoxide (H2/CO) ratio of scrubbed syngas is possible through water-gas shifting – Syngas is passed through a multi-stage, fixed-bed reactor containing shift catalysts at 400 - 900°F (pressure does not influence) – CO + H2O ↔ H2 + CO2 (mildly exothermic: -41 kJ/mol) – High moisture content shifts the equilibrium to right – Gas is reheated to 30-50°F above saturation temperature prior to feeding into the WGS to avoid catalyst damage by condensation • WGS reaction is either a sour shift or a sweet shift reaction (after removal of sulfur from the gas) • Sour shifting uses cobalt-molybdenum catalyst – Can convert organic sulfur (like corbonyl sulfide, COS) into H2S • Sweet shifting is two types: – High Temp. (HT) sweet shifting at 550 - 900°F using Chromium/copper promoted iron-based catalysts – Low temp. (LT) sweet shifting at 400 - 500°F using copper-zinc- aluminum catalyst – used to reduce CO content to <1% • The catalysts are extremely sensitive to sulfur and chloride poisoning (hence not used in coal gasification applications)
  • 27. Operating Conditions of Gasifiers Equivalence ratio (ER) • Ratio of actual air flow to the stoichiometric air flow required for the biomass combustion • Indicates the extent of partial combustion and controls the gasification temperature (higher the flow greater will be the temperature) • Increasing ER from 0.2 to 0.45 increased gas yield, but decreased LHV, and the percentages of H2, CO, CH4, C2H2, and tar • Effect of ER on the composition of product gas is influenced by temperature and S/B (steam to biomass) ratio SV: Superficial velocity (m/sec.) • Air flow rate to the gasifier cross-sectional area (m3/m2.sec.) • Affect residence time of the gasifier – shorter time decreases the extent of biomass conversion
  • 28. Operating Conditions of Gasifiers Steam to biomass ratio (S/B ratio): • Gasification, where steam is used as the gasifying agent, and temperature of gasification is >750-800C (catalysts can however lower the temperature) • Higher S/B ratio results in higher biomass conversion efficiency and in reduced tar formation (is steam reforming responsible ?) • With increased steam (partial pressure) water gas, water gas shifting and methane reforming reactions are favoured and, consequently, H2 production is increased • (C + H2O → CO + H2; CO + H2O → CO2 + H2; CH4 + H2O → CO + 3H2) – For S/B ratio >2.7, the gas composition do not change significantly – For S/B ratio 0.0-1.35 yields of CH4, C2H4 and CO2 increase and CO yield decreases – For S/B ratio 1.35 – 2.7 yields of CO and CH4 decrease and yields of CO2 and H2 increase • Since the steam temp. is usually lower than the gasification temp., S/B ratio beyond a limit lowers the gasifier bed temp. and negatively affect the gasification process
  • 29. Operating Conditions of Gasifiers H/C ratio: • Ratio of Hydrogen (of biomass, moisture and steam) and Carbon (of biomass) • Increase of H/C ratio from 1.6 to 2.2 was found increasing H2 content and LHV of gas, and decreasing tar content from 18 to 2 g/nm3 Temperature of the gasifying agent • Increased temperature is found to increase the gas LHV and to decrease tar and soot content • A pre-heater for the gasifying agent can be of use Temperature profile of gasification • Composition and properties of the gas are influenced by temperature – The content and the constituent gas ratios are also affected by pressure • Increasing temperature increases gas yield and overall energy content of the gas (increase of H2 % is mainly responsible) – Temp. increase from 700 to 800C increases gas yield, H2 content, HHV, energy efficiency, cabon conversion efficiency – Temp. Increase from 700-800C decreases the CH4, CO and CO2 contents
  • 30. Operating Conditions of Gasifiers Temperature profile of gasification • Between 750 and 950C, H2 content increased from 31% to 45%, CH4 and CO contents remained fairly constant and CO2 content decreased • At >750-800C, H2 production increases but CH4 production decreases • At >850-900C, steam reforming and Boudouard reactions predominate and increase the CO content CH4 + H2O ⇌ CO + 3 H2 (strongly endothermic: +206 kJ/mole) C + CO2 ↔ 2CO (Boudouard reaction; predominates at >850-900C) • High temperatures favour destruction and reforming of tar • With air as gasifying agent H2 and CO content of gas increases with temperature from 700 to 900C and CH4 and CO2 decreases – CO/CO2 ratio linearly increases with temperature (0.85 at 700 and 2.7 at 900) – predominance of boudouard reaction at higher temperatures is considered responsible
  • 31. Downstream Processing • Components of product gas that need addressing – Tar – Particulates (unconverted biomass, ash, char and bed material) – Alkali compounds, and nitrogen, sulfur and chlorine containing compounds – Water vapour • Adjustment of the composition of the product gas and/or ratio of H2 to CO and tar removal – Water-gas shift reactions – Reforming reactions • Removal of the undesirable constituents (particulates, tar, moisture and carbon dioxide) – Cyclone separation – Wet scrubbing – Wet electrostatic precipitation – Barrier filters • Cooling of the product gas and moisture removal
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  • 35. Downstream Processing Removal of particulates • Cyclones (and multiclones), Wet scrubbers, Barrier filters and ESPs are used to remove particulates • Cyclone separators remove larger particles (>5 μ) with little pressure drop – Cut diameter (increasing cyclone size increases cut diameter) – Multiclones increase the separation efficiency • Wet scrubbers are used with product gas at temperature <100C – Venturi scrubbers have 2.5 to 25 Kpa headloss – Scrubbers remove 1 micron particles with >95% efficiency • Effectiveness of the ESP will depend on the particulate resistivity and sulfur and alkali content • Barrier filters: metal or ceramic porous candle filters; bag filters and packed bed filters – Candle filters are good for hot gas cleaning – Bag filters operate at temperatures about 350C – Packed bed filters use ceramic spheres/saw dust
  • 36. Downstream Processing Alkali compounds • Alkali compounds expected are CaO, K2O, P2O5, MgO, Na2O, SiO3 and SO3 • Alkali compounds vapourize at >700°C and condense at <650°C temperature, stick to surfaces and cause corrosion • Alkali salts inactivate catalysts • For alkali compounds removal, cool the gas and pass through barrier filters • Removal of alkali compounds from hot gases is needed – bauxite filters at 650-725°C can remove Na and K compounds
  • 37. Downstream Processing Nitrogen (0.5 to 3% in the biomass) • Nitrogen is converted to ammonia (60-65%), molecular nitrogen and HCN (very little) • Combustion of ammonia forms NOx • Wet scrubbing can remove ammonia • Ammonia can be destructed at higher temperature using dolomite, nickel based catalysts and iron based catalysts Sulfur (<0.5% in biomass) • Sulfur is converted to H2S and SO2 • Sulfur can irreversibly inactivate catalysts (in methanol synthesis) • Fuel cells and tar (hydrocarbon) conversion catalysts are sensitive to sulfur and use syngas with low levels of sulfur • SO2 can be removed by scrubbing • Lime stone, dolomite or CaO can be used to remove SO2 and H2S • Cleaning at 600-900°C and regeneration of the sorbents is the best way for H2S removal from hot gases
  • 38. Tar handling and management • Tar is a generic term - refers to condensable organic compounds of the product gas (condensable at >25°C !) – Sticky (and highly viscous) brownish to black strongly smelling liquid – Tar aerosols are <1 μ size • Compounds of tar can be categorized as – Mixed oxygenates – Phenolic eathers – Alkyl phenolics – Heterocyclic ethers – PAHs (lighter PAHs – 2 or3 rings, and heavier PAH - ≥4 rings) • Composition of tar depends on – Biomass fuel particle size and size distribution – Elemental composition – Mineral and water content of the fuel • Composition is influenced by – reactor design and by the gasification agent – Rate of heating and the maximum temp reached – The surroundign gaseous atmosphere
  • 39. Tar handling and management • Composition of tar shifts towards mixed oxygenates and larger PAH with the increasing gasification reaction severity – Reaction severity increases with Increasing temperature, flow of oxidizing agents and with the increasing amount of catalyst • Combustion systems can work with relatively high amounts of tar • Tar beyond certain level can poison catalysts (deposition of tar can occur on catalysts) • Tar content of the product gas is reduced through primary techniques within the gasifier – Design of the gasifier (use of staged gasification systems) – well designed downdraft gasifiers – Operating conditions of the gasifier – Use of catalysts (in-bed catalysts) – High temperature gasification (entrained gasification • Tar is removed from product gas through downstream processing (secondary removal techniques) – Wet and hot gas cleaning (Barrier filters and Cyclone separators are not good for tar removal) – Destruct and reform tar
  • 40. Tar handling and management Gasifier design • Spatial separation of pyrolysis step from the following oxidation and reduction steps Designs and operating conditions of the gasifier • Gasifier design, ER (equivalence ratio), Gasifying agent (type) used, Gasification agent to biomass ratio, S/B (steam to biomass) ratio, Gasification temperature profile, etc. influence tar levels in the gas – Higher the ER, S/B and temperature lesser will be the tar content – Higher superficial velocity (>0.7 m/sec.) can result in higher tar levels for downdraft gasifiers – Secondary injection of air/O2 can lower the tar content In-bed catalysts • Dolamite, limestone, olivine, alkali carbonates, nickel bound catalysts, metal oxide catalysts, zeolite, char, etc. • In-bed catalysts, through enhancing reforming reactions, reduce the tar levels and increase overall carbon conversion efficiencies
  • 41. Tar handling and management Wet and hot gas cleaning • Water (venturi) scrubbing for condensation removal of tar (can reduce tar level to <20-40 mg/Nm3) – Exit gas temperature of the scrubber may be 35-60°C • Wet cleaning requires reduced temperature of product gas (sensible heat loss problem) • Using a liquid other than water for the scrubbing can be considered • Scrubbing liquid may require treatment prior to disposal • ESP wet scrubber can remove tar at temperature >150°C
  • 42. Tar handling and management Reforming for tar removal and increased H2 production • Cracking of tar at higher (>850°C) temp. without using any catalyst – Results in increased hydrogen production – Can reduce the gasification efficiency, can causes material problems and can produce soot • High temperature and catalysts provide favourable conditions for reforming and shift reactions – Increasing temperature (from 650-850°C) increases H2 content, decreases CH4, CO2, and C2 yield, but CO content remains same – Steam/air in the presence of catalyst destructs tar and form CO, H2, CO2 and CH4 – Supply/Use of air to the reformer decreases the product’s HHV through partial combustion and dilution of the product gas
  • 43. Tar handling and management • Decreasing WHSV (from 10.7/hr to 2.7/hr) increases H2 and H2/CO and decreases CH4 (H2 increased by 49%, H2/CO increased to 3.32 and CH4 decreased by 50%) – WHSV (Weight Hourly Space Velocity): Ratio of biomass flow rate to the catalyst mass in the reactor • Temperature, space time, catalyst particle size, and gas composition affect the tar conversion efficiencies – Space time: Mass of catalyst per unit flow rate of product gas
  • 44. Tar handling and management Catalysts are two types: Mineral catalysts and Synthetic catalysts Mineral catalysts • Calcined rocks (dolomite, magnesite and calcite), olivine, clay materials and ion ores – Calcined rocks are best but prove erosive in fluidized bed gasifiers – Olivine is less effective than dolomite (calcined rocks) – Clay materials do not withstand high temperatures (800-900°C) – Iron ores are rapidly deactivated Synthetic catalysts • Fluid Catalytic Cracking (FCC) catalysts, char, alkali metal based catalysts, activated alumina, transition metal (Pt, Zr, Rh, Ru, Fe and Ni) based catalysts – Transition metal based catalysts are expensive – FCC catalysts deactivate quite rapidly – Alkali metal based catalysts can agglomerate in the bed