Presentation Outlines

Thermochemical Conversion Technology - Biomass Gasification

Presentation Outlines
Introduction - Thermochemical conversion Technologies Gasification Overview Advantages of Biomass Gasification Biomass Gasification Upstream Processing Different gasification technologies Types of biomass gasifiers Updraft fixed bed gasifier Downdraft fixed bed gasifier Cross draft fixed bed gasifier Twin fire fixed bed gasifier Fluidized bed gasifiers Circulating Fluidized Bed Entrained flow biomass gasifiers Plasma gasifiers Gasification process Practical gasifier systems

Introduction - Thermochemical conversion technologies

Thermochemical conversion is a high- temperature chemical reforming process that breaks apart the bonds of organic matter and reforms these intermediates into: Solid fraction (biochar). Gas fraction (synthesis gas or syngas). Liquid fraction (highly oxygenated bio-oil).

Benefits of the thermochemical process: Small footprint. Efficient nutrient recovery. No gas emissions. Short reaction time. Capability of handling a variety of wastes and blends. Production of thermal energy is the main driver for this conversion route that has four broadways: Combustion. Pyrolysis. Gasification. Liquefaction.

Introduction - 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

Overview The Concept 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.

Overview The Concept 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.

Overview The Concept Difference between producer gas and syngas

Overview The Concept 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.

Overview The Concept Gasification Agent:
Three product gas qualities can be produced from gasification by varying the gasfiying agent; the method of operation; and the process operating conditions The main gasifying agent is typically air but oxygen/steam gasification and hydrogenation are also used. Catalytic steam gasification is another mode of operation that influences both the overall performance and efficiency. The three types of product gas have different calorific values (CV): Low CV: MJ/Nm3 using air and steam/air Medium CV: MJ/Nm3 using oxygen and steam High CV: 40MJ/Nm3 using hydrogen & hydrogenation

Overview Gasification Agent:
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. The disadvantage in using air is that the nitrogen introduced with the air dilutes the product gas, giving a net CV of 4-6MJ/Nm3 (compared with natural gas at 36MJ/Nm3). Gasification with oxygen gives a gas with a net CV of 10-15MJ/Nm3 and with steam, MJ/Nm3. It can be seen that while a range of product gas qualities can be produced, economic factors are a primary consideration. the reaction with air/oxygen, the reaction of carbon with steam (the water gas reaction) is endothermic, requiring heat to be transferred at temperatures around 700oC, which is difficult to achieve. Gasifiers self-sufficient in heat are termed autothermal and if they require heat, allothermal: autothermal processes are the most common.

Overview 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

Use of gasification for generation of fuels, chemicals and power:

The most important end uses practiced commercially or under research, can be summarized as follows: Close-coupled combustion (stoves, kilns, ovens, furnaces, dryers, boiler firing), External combustion for power: externally fired turbines, Stirling engines, steam engines, Thermo-photovoltaic cells, catalytic oxidation, and thermo-electric systems, Internal combustion (IC) diesel and Otto engines, Compressors, Hydrogen fuel production, Gas turbine internal combustion, Fuel cells: molten carbonate, solid oxide, proton exchange membrane, phosphoric acid, Chemical synthesis: methanol, ammonia, methane, Fischer-Tropsch liquids, other oxygenates.

Advantages of Biomass Gasification
Main 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.

Advantages of Biomass Gasification
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

Stages of Biomass Gasification System
Biomass gasification is completed in many stages that elaborated and explained with a flow diagram given below

Stages of Biomass Gasification System
Components of Biomass Gasification System Gasifier applications To fuel internal combustion (IC) engines for electric power generation, irrigation ,grain milling, sawing of timber etc–powergasifier To fuel external burners to produce heat for boilers, dryers, ovens, or kilns Heat gasifiers <1 MW

Biomass Gasification - Gasification process
Biomass gasification can be considered to include upstream processing, gasification and downstream processing

Biomass Gasification Process

Gasification - Basic Process Chemistry Schematic
The gasification of biomass takes place in four stages: Drying: water-vapour is driven off the biomass Pyrolysis: as the temperature increases the dry biomass decomposes into organic.vapours, gases, carbon (char) and tars. Combustion: some of the char and tars burn with oxygen from air to give heat and carbon dioxide. This heat enables the other stages of the gasification process to take place. Reduction: : water-vapour reacts with carbon, producing hydrogen, carbon monoxide and methane. Carbon dioxide reacts with carbon to produce more carbon monoxide.

Gasification process Main Stages of Gasification Process

Biomass Gasification: Upstream Processing
Feed materials pre-treatment The degree of pre-treatment of the biomass feedstock is dependent on the gasification technology used. The main problem areas are: drying: the biomass moisture content should be below 10-15% before gasification particle size: in most gasifiers gas has to pass through the biomass and the feed has to have both sufficient compressive strength to withstand the weight of the feed above and a size distribution to provide sufficient porosity for gas-flow through the feed. fractionation: the nitrogen and the alkali contents of the biomass are critical, as they are partially carried over into the gas stream. Small particles tend to contain less nitrogen and alkalis so that fractionation into fine and coarse particles helps to produce a gas with fewer impurities leaching: the nitrogen and alkali contents of the biomass can be reduced by prior leaching with water

Biomass drying (usually to <10% level) Drying of wood (from 50-60% moisture as-felled or using wood air- dried for two years) to give moisture content <20% requires the use of driers. The driers can be either directly-heated rotary driers using the flue gas, or indirectly-heated fluidised bed driers heated with steam. Waste heat of gasification can be used. Perforated bin dryers, band conveyer driers and rotary cascade dryers are used. The vapours emitted during drying contain a number of volatile organic compounds (VOCs), mainly terpenes, which require appropriate air pollution control systems.

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 Densification Low bulk density fuels present problems in the gasifiers and hence biomass is densified into pellets or briquettes

Feed material properties The characteristics of the biomass feedstock have a significant effect on the performance of the gasifier. Of particular importance are the following characteristics: Moisture content: fuel moisture above about 30% makes ignition difficult (20-30% is usually specified) and affects the calorific values (CV) of the product gas. Increasing the moisture content reduces the CV of the gas High moisture contents decrease the temperature in the oxidation zone, resulting in incomplete cracking of the hydrocarbons released from the pyrolysis zone. Increased levels of moisture convert CO into H2 by the water gas shift reaction and in turn the increased H2 produces more CH4 by direct hydrogenation. However the gains in H2 and CH4 do not compensate for the loss of CO.

Feed material properties Ash content: a high mineral-matter content can make gasification impossible. The oxidation temperature is often above the melting point of the biomass ash, leading to clinkering/slagging problems in the hearth and subsequently, feed blockages. Clinkering is a problem with ash contents above 5%, especially if the ash is high in alkali oxides and salts which produce eutectic mixtures with low melting points. Volatile compounds: the gasifier must be designed to destruct tars and the heavy hydrocarbons released during the pyrolysis stage. Particle size: the particle size of the feed depends on the hearth dimensions but is usually 10-20% of the hearth diameter or in the range 20-80mm. Large particles can form bridges that prevents the feed moving down, while smaller particles tend to clog the available air voidage or porosity of the bed. Both these problems lead to a high pressure drop and the subsequent shutdown of the gasifier.

Different gasification technologies

Biomass gasifiers Two principal types of gasifiers have emerged: fixed bed and fluidized bed. Fixed bed gasifiers are further classified into four types as updraft, downdraft, cross draft and twin-fire gasifier, depending on the flow of gas through the fuel bed. Updraft gasifier Downdraft gasifier Cross draft gasifier

Bubbling fluidized bed gasifier Circulating fluidized bed gasifier
Biomass gasifiers Main Classification of Gasifiers 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. Bubbling fluidized bed gasifier Circulating fluidized bed gasifier

Biomass gasifiers Other types of gasifiers
Several other categories of gasifier design have been immerged over the time to cater for different requirements. These include: Entrained flow (EF), Duel fluidized bed (Duel FB) and Plasma gasifiers.

Biomass gasifiers Main kinds of Reactors for Gasification
Updraft and Downdraft reactors have been developed since ~ They produce a low BTU Gas (~ 6000 KJ/m3) with tars. Actually the new systems use mainly fluidized beds and circulating fluidized beds….but they are often too complicated energy output < energy in put!

Biomass gasifiers Gasifier Types Advantages Disadvantages
Fixed/Moving bed, Updraft Mature for heat Small scale applications Can handle high moisture Simple, inexpensive process Exit gas temperature about 2500C Operates satisfactorily under pressure High carbon conversion efficiency Low dust levels in gas No carbon in ash High thermal efficiency Scale limitations Producer gas Slagging potential Large tar production Potential channelling Potentia l bridging Small feed size Potential clinkering Fixed/Moving bed, Downdraft Simple process Low particulates Only traces of tar in product gas Feed size limits Limits to scale up capacity Moisture sensitive Limited ash content allowable in feed Potential for bridging and clinkering

Biomass gasifiers Gasifier Types Advantages Disadvantages Fluid Bed
Large scale applications Direct/indirect heating Can produce syngas Flexible feed rate and composition High ash fuels acceptable Able to pressurize High CH4 in product gas High volumetric capacity Easy temperature control Higher particle loading Operating temperature limited by ash clinkering High product gas temperature High tar and fines content in gas Possibility of high C content in fly ash Circulating Fluid Bed Feed characteristics Flexible process Up to 8500C operating temperature Medium tar Corrosion and attrition problems Poor operational control using biomass

Biomass gasifiers Gasifier Types Advantages Disadvantages
Double Fluid Bed Oxygen not required High CH4 due to low bed temperature Temperature limit in the oxidiser More tar due to lower bed temperature Difficult to operate under pressure Entrained Flow Can be scaled Can produce syngas Very low in tar and CO2 ·flexible to feedstock 10000C exit gas temperature Large amount of carrier gas Higher particle loading Potentially high S/C, low in CH4 Extreme feedstock size reduction required Complex operational control Carbon loss with ash Ash slagging

Biomass gasifiers Capacity ranges of different gasifiers
Rough comparison of gasifier types with respect to their capacity ranges (capacity is in oven dry tons per day - odt/day).

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

Downdraft fixed bed gasifier
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

Cross draft fixed bed gasifier
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 gasifier
Has two defined reaction zones Drying, low-temp. Carbonization and cracking of gases occur in (upper) zone-1 Gasification of charcoal takes place in (lower) zone-2 Product gas is fairly clean and its temperature is 460 to 520oC 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

Circulating fluidized bed gasifier 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

Circulating Fluidized Bed

Circulating Fluidized Bed
Advantages of Gasification by fast Pyrolysis in a Circulating Fluidized Bed System product gas nearly free of nitrogen calorific value higher than 13 MJ/Nm³ very low tar content due to steam gasification gas quality is independent of water content in biomass feed now, the apparatus are compact……not enough! a wide range of feedstock can be gasified possibility to use a catalyst as bed material (regeneration of catalyst in combustion zone) to influence the gas composition and gasification kinetic in a more positive way But sometimes energy output < energy input!

Circulating Fluidized Bed
Circulating Fluidized Bed with CO2 Absorber

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. ( C) 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%

Gasification process Main Stages of 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. Step one: Heat biomass to a temperature below 600°C ( °C) in the near absence of air to drive off the easily vaporized or volatile materials - a process called pyrolysis Step one produces a mixture of CO, H2, CH4, and CO2 (the first 3 of which have energy value) 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).

The gases contain about 2/3 of the energy content of the fuel, which is lost if they are not captured (as is the case in the production of charcoal). If captured, the gases can be used for heating, cooking, generation of electricity, or for cogeneration of useful heat and electricity. Conversely, the gases can be shifted to consist almost entirely of H2, or can be used to synthesize methanol, dimethyl ether, or Fischer-Tropsch liquids (all of which are potential transportation fuels)

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.

Biomass to Syngas Conversion pathways
Gasification stages occurs at the same time in different parts of gasifier.

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.

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.

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.

In case of downdraft biomass gasifiers, the fuels oxidized are carbonized biomass plus vapours and gases formed in the pyrolysis zone; some non-condensable gases are also formed as a result of thermal cracking of tar vapours (coming from pyrolysis zone). In the reduction zone the products of complete oxidation (i.e., CO2, H2O, etc.) undergo reduction by the carbonized biomass. In fluidized bed gasifiers, because of mixing, separate reaction zones do not exist. Here all processes are taking place simultaneously throughout the reactor volume, although intensity may vary depending on the location.

Gasification process Chemical Reactions
During gasification of biomass, a number of chemical 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 combustible gaseous products. Some of the basic reactions of gasification: C + O2 = CO ,800 kJ/kg mole carbon C + H2O = H2 + CO – 131,400 kJ/kg mole CO2 + C = 2CO – 172,600 kJ/kg mole carbon CO + H2O = CO2 + H ,200 kJ/kg mole C + 2H2 = CH ,000 kJ/kg mole carbon

Gasification process Gasifier Efficiency
Performance of a gasifier is often expressed in terms of its efficiency, which can be defined in two different ways: cold gas efficiency and hot gas efficiency. The cold gas efficiency is used if the gas in used for running an internal combustion engine in which case it is cooled down to ambient temperature and tar vapors are removed from the gas. The cold gas efficiency is defined as where Vg = gas generation rate (m3/s) Cg = heating value of the gas (kJ/m3) Mb = biomass consumption rate (kg/s) Cb = calorific value of biomass (kJ/m3)

Gasification process Gasifier Efficiency
For thermal applications, the gas is not cooled before combustion and the sensible heat of the gas is also useful. The hot gas efficiency is used for such applications, which is defined as: The cold gas efficiency and hot has efficiency are approximately 70% and 75%, respectively. where Hsensible = CpVg(tg - ta) tg = gas temperature ta = ambient temperature

Gasification process Factors Affecting Rate of Gasification
Apart from the basic technology and design aspects of gasifiers, the rate of gasification is affected by several factors including the following. The size of the feeding material particles (small; large) and its distribution The shape of the particulates (powdery  lump) The structure of the material (porous  non-porous) Environment (Reactive: Air/Oxygen  Inert: Nitrogen/Argon) Flow of medium (Static  Continuous) Heating rate (Slow  Fast) Temperature (Low: < 500 C  High: > 500 C) Ash (Catalytic  Non-catalytic)

Composition (% Volume)
Gasification process Properties of Producer gas The composition of the producer gas is affected by the type of biomass material, technology and process environment. Fuel Gasification method Composition (% Volume) Heating Value (MJ/m3) CO H2 CH4 CO2 N2 Charcoal Downdraft 5 - 10 1 - 2 Updraft 30 19.7 - 3.6 46 5.98 Wood (10-20% MC) 2 - 3 Wheat straw pellets 4.50 Coconut husks 5.80 Coconut shells 7.20 Pressed sugarcane 5.30 Corn cobs 18.6 16.5 6.4 6.29 Paddy husks pellets 16.1 9.6 0.95 3.25 Cotton stalks cubed 15.7 11.7 3.4 4.32

Gasification process Properties of Producer gas
When gasification is performed in air, the maximum dilution of producer gas occurs due to the presence of nitrogen. About 50% of gas by volume is composed of noncombustible nitrogen. The heating value of producer gas is in the range of 4.5 to 5.5 MJ/m3 (note that the heating value of methane is about 20 MJ/m3). Therefore it may be beneficial to use pure oxygen instead of air for gasification. However the cost and availability of oxygen may be a limiting factor in this regard.

Gasification process Properties of Producer gas
The chemical reaction of gasification could be rewritten for a general biomass fuel with a composition represented by CxHyOz (neglecting nitrogen and sulfur) as In the above, the first three terms of the products of combustion (in the square bracket) represent fuel components in gaseous form. In this equation, the terms x, y and z are known for a given biomass material. The other coefficients are variable parameters that depend on properties of the fuel, gasifire technology, the operational conditions.

Gasification process Gas Cleaning
Removal of the contaminants, such as tar, which is considered to be the most cumbersome and problematic parameter, is required, for which several gas cleaning methods are available:

Gasification of biomass
Uncarbonized Biomass In case of gasification of un-carbonized biomass such as wood or biomass residues, tar, a complex and corrosive mixture of condensed liquid vapours, is produced along with the gas. Therefore use of producer gas obtained from un-carbonized biomass for power generation using an internal combustion engine requires elaborate gas cleaning for removing tar. Projects on such power generation have often been abandoned due to a variety of reasons, the most important being the problem created by tar in the gas. Worldwide, there are, however a number of units each of which has accumulated several thousand hours of operations.

Waste Agricultural Biomass Compared to wood, residues are more difficult to gasify. Except some residues (e.g. coconut shell), the residues have low bulk density and would present a problem of flow in gasifiers having throat. Some important residues (e.g. rice husk) have much higher ash content compared to wood (the highest ash content normally necessitates use of rotating grates). Gasification of rice husk has attracted a great deal of interest in recent years. The husk consumption is about 2 kg/kWh. Coconut shell is known to be a good fuel for gasification.

Waste Agricultural Biomass Gasification characteristics of several types of waste agricultural biomass materials: 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 cm, 65kgm3 Insignificant tar coconut 3.4 Slag on grate but no operational problem Com cobs 304 kg/m3 , MC = 11% 7.24 1.5 Excellent fuel. No slagging Com fodder Cubed, 390 kg/m3, MC= 11.9% 1.43 6.1 Sever slagging and bridging Cotton stalks Cubed, 259kg/m3 , MC= 20.6% 5 17.2 Severe slag formation Peat Briquettes, 555kg/m3, MC=13% _ Severe slagging Rice hulls Pelleted, 679 kg/m3, MC = 8.6% 4.32 14.9 Sugarcane Cut 2-5 cms, 52 kg/m3 Insignificant 1.6 Slag on hearthring. Bridging

Waste Agricultural Biomass Gasification characteristics of several types of waste agricultural biomass materials: Fuel Treatment, bulk density, moisture (MC) Tar (g/m3) Ash (%) Gasifier Experience Walnut shell Cracked, 337 kg/m3 , MC = 8% 6.24 1.1 downdraft Excellent fuel. No slagging Pelleted 14.5 1.0 Good fuel Wheat straw Cubed, 395 kg/m3 , MC = 9.6% _ 9.3 Severe slagging, bridging, Irregular gas production Wheat straw and corn stalks Cubed (50% mix), 199 kg/m3 , MC = 15% 7.4 Slagging Wood blocks 5 cm cube, 256 kg/m3 , MC = 5.4% 3.24 0.2 Excellent fuel Wood chips 166 kg/m3 , MC = 10.8% 6.26 Severe bridging and slagging

Practical gasifier systems
Small scale electricity generation 3.5 kW e fuel wood gasifier – IC engine system

Small scale electricity generation 9 kW e biomass gasifier – IC engine system

Industrial Thermal 1 MWth 2 MWth

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