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Biomass Gasification
Introduction
Biomass gasification is a process of incomplete combustion of biomass resulting in production of combustible gases consisting of a mixture of Carbon monoxide (CO), Hydrogen (H2) and traces of Methane (CH4), which is called producer gas. Gasification is a two-stage reaction consisting of oxidation and reduction processes. These processes occur under sub-stoichiometric conditions of air with biomass. The first part of sub-stoichiometric oxidation leads to the loss of volatiles from biomass and is exothermic; it results in peak temperatures of 1400 to 1500 K and generation of gaseous products like carbon monoxide, hydrogen in some proportions and carbon dioxide and water vapor which in turn are reduced in part to carbon monoxide and hydrogen by the hot bed of charcoal generated during the process of gasification. Reduction reaction is an endothermic reaction to generate combustible products like CO, H2 and CH4.
The production of generator gas (producer gas) called gasification, is partial combustion of solid fuel (biomass) and takes place at temperatures of about 1000 C. The combustion products from complete combustion of biomass generally contain nitrogen, water vapor, carbon dioxide and surplus of oxygen. However in gasification where there is a surplus of solid fuel (incomplete combustion) the products of combustion are combustible gases like Carbon monoxide (CO), Hydrogen (H2) and traces of Methane and nonuseful products like tar and dust. The production of these gases is by reaction of water vapor and carbon dioxide through a glowing layer of charcoal. Thus the key to gasifier design is to create conditions such that the biomass is reduced to charcoal and, the charcoal is then converted at suitable temperature to produce CO and H2.
Types of Gasifiers:
Since there is an interaction of air or oxygen and biomass in the gasifier, they are classified according to the way air or oxygen is introduced in it. Design of gasifier depends upon type of fuel used and whether gasifier is portable or stationary. Gas producers are classified according to how the air blast is introduced in the fuel column. History of gasification reveals several designs of gasifiers. The most commonly built gasifiers are classied as:
- Updraft gas producer
- Downdraft gas producer
- Twin-fire gas producer
- Crossdraft gas producer
- Other gas producer
The Process of Gasification
Four distinct processes take place in a gasifier as the fuel makes its way to gasification.
a) Drying of fuelb) Pyrolysis – a process in which tar and other volatiles are driven offc) Combustiond) Reduction
Though there is a considerable overlap of the processes, each can be assumed to occupy a separate zone where fundamentally different chemical and thermal reactions take place. The biomass gasification technology package consists of a fuel and ash handling system, gasification system - reactor, gas cooling and cleaning system. There are also auxiliary systems namely, the water treatment plant to meet the requirements of industry and pollution control board. The prime mover for power generation consists of either a diesel engine or a spark ignited engine coupled to an alternator. In the case of thermal system, the end use device is a standard industrial burner.
The Process of Biomass Gasification
Source: Biomass gasification – State of the art description.
http://www.gasificationguide.eu/gsg_uploads/documenten/Gasification_Guide_D08_State_of_the_Art_Description_V09e.pdf
Advantages of Biomass Gasification
- Clean combustion,
- Compact burning equipment,
- High thermal efficiency a
- A good degree of control.
- Provides energy security
- Generates local employment in rural sector
- In locations where biomass is already available at reasonable low prices (e.g. rice mills) or in industries using fuel wood, gasifier systems offer definite economic advantages. Biomass gasification technology is also environment-friendly, because of the firewood savings and reduction in CO2 emissions.
Disadvantages
- Gasification is quite complex and sensitive process.
- Fuel is bulky and frequent refueling is often required for continuous running of the system.
- Handling residues such as ash, tarry condensates is a time consuming and dirty work.
- Getting the producer gas is not difficult, but obtaining in the proper state is the challenging task.
Applications
- Thermal applications: cooking, water boiling, steam generation, drying etc.
- Motive power applications: Using producer gas as a fuel in IC engines for applications such as water pumping
- Electricity generation: Using producer gas in dual-fuel mode in diesel engines/as the only fuel in spark ignition engines/in gas turbines.
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