Biomass gasification

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Biomass gasification is a process of converting solid biomass fuel into gaseous combustible form (called producer gas) by means of partial oxidation carried out in a reactor called gasifier at elevated temperatures. Application of gasifier assumed significant importance and attention during world war II due to scarcity of petroleum products. By the end of world war in 1942 more than million vehicles were in operation using gasifiers throughout the world. The energy crisis in 1970s brought renewed in gasification. The Technology was perceived as a relatively cheaper option in developing countries having sufficient sustainable biomass for small scale industrial as well as power generation applications.

Principle of gasification:

Biomass gasification is a process of converting solid biomass fuel into gaseous combustible form (called producer gas) through a sequence of complex thermo chemical reactions. In the first stage partial combustion of biomass to produce gas and char occurs along with generation of heat. This heat is utilized in drying of biomass to evaporate its moisture as well as for pyrolysis reactions to bring out volatile matter and provide heat energy necessary for further endothermic reduction reactions to produce producer gas which mainly consist of mixture of combustible gases such as CO ( carbon monoxide), hydrogen (H2), traces of methane(CH4) and other hydrocarbon. Normally air is used as gasifying agent, however use of oxygen can produce rich higher calorific value gas but due to cost implipications is not usually preferred.

Gasifier Reactor Types:

Biomass gasifiers can be classified as:

  • Dense phase reactors and
  • Lean phase reactors

Dense phase reactors are classified further as follows:

i) Updraft or counter current gasifier:

The up draft gasifier consists of a fixed bed of carbonaceous fuel (e.g. coal or biomass) through which the "gasification agent" (steam, oxygen and/or air) flows in counter-current configuration. The ash is either removed dry or as a slag. The slagging gasifiers require a higher ratio of steam and oxygen to carbon in order to reach temperatures higher than the ash fusion temperature. The nature of the gasifier means that the fuel must have high mechanical strength and must be non-caking so that it will form a permeable bed, although recent developments have reduced these restrictions to some extent. The throughput for this type of gasifier is relatively low. Thermal efficiency is high as the gas exit temperatures are relatively low. However, this means that tar and methane production is significant at typical operation temperatures, so product gas must be extensively cleaned before use or recycled to the reactor.

ii) Downdraft or cocurrent gasifier:

The co-current fixed or bed down draft gasifier is similar to the counter-current type, but the gasification agent gas flows in co-current configuration with the fuel (downwards, hence the name "down draft gasifier"). Heat needs to be added to the upper part of the bed, either by combusting small amounts of the fuel or from external heat sources. The produced gas leaves the gasifier at a high temperature, and most of this heat is often transferred to the gasification agent added in the top of the bed, resulting in an energy efficiency on level with the counter-current type. Since all tars must pass through a hot bed of char in this configuration, tar levels are much lower than the counter-current type.

Cross draft gasifier:

In cross draft gasifier the air enters from one side of gasifier reactor and gas is taken out from other side. Cross draft gasifier have very few applications and can hardly be credited with any advantage beyond a good permeability of the bed.

Biomass Gasification is a new derivative of the 200 year old coal gasification technology. Its popularity stems from the recent interest in decentralized electricity generation in developing countries. The rise in the price of fossil fuels coupled with the difficulties of building large scale energy infrastructures to transmit and distribute centralized fuels like natural gas and grid electricity has provided impetus to this technology.