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Fixed Bed Gasifiers

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The fixed bed gasification system consists of a reactor/gasifier with a gas cooling and cleaning system. The fixed bed gasifier has a bed of solid fuel particles through which the gasifying media and gas move either up or down. It is the simplest type of gasifier consisting of usually a cylindrical space for fuel feeding unit, an ash removal unit and a gas exit. In the fixed bed gasifier, the fuel bed moves slowly down the reactor as the gasification occurs. The fixed bed gasifiers are of simple construction and generally operate with high carbon conversion, long solid residence time, low gas velocity and low ash carry over. In fixed bed gasifiers, tar removal used to be a major problem, however recent progress in thermal and catalytic conversion of tar has given credible options.

fixed bed gasifier

Fixed bed gasifiers are the most commonly ones. These are in turn available under two important categories depending on the direction of the gas flow through the reactor:

  • Updraft gasifiers
  • Downdraft gasifiers

Updraft – mainly done for coal gasification; Indian gasifiers are mainly downdraft. For updraft, the tar formation will be much higher (10 times or more) than for the downdraft. In India, most gasifiers used are of the downdraft type.

 

 

Advantages of downdraft over updraft

Downdraft – gas with less tar, but with some particulates, so producer gas more suitable for use in gas engines Updraft – more suitable with some thermal applications

Updraft Gasifiers

Fuel flexibility is the main feature of updraft multifuel gasifiers. These gasifiers can operate on either coal or biomass and fuel switching does not require any changes in the reactor. Updraft gasifiers tolerate higher ash content, higher moisture content and greater size variation in fuel as compared to downdraft gasifiers.

In updraft gasifiers, gas is drawn out of the gasifier from the top of the fuel bed while the gasification reactions take place near the bottom. As the producer gas passes through the fuel bed, it picks-up volatile matter (tars) and moisture from the fuel. Therefore, the gas from the updraft gasifier contains condensable volatiles. The design and operation of the gasifiers is such that the gas comes out at 200-400 C temperature. At this temperature, most of the volatile hydrocarbons are in vapor form, which add to the energy content of the gas. It is most appropriate to utilize updraft gasifiers in close-coupled-hot gas mode for direct heating applications. However, if the application warrants, the scrubbing of gas to remove the volatiles/tars is also carried out.

Downdraft Gasifiers

Downdraft gasifiers are fuel specific. Downdraft wood gasifiers can operate on wood like biomass materials and biomass briquettes with a minimum bulk density of 250 kg/m3 and ash content of less than 5%. In downdraft gasifiers, gas is drawn from the bottom of the reactor while the hottest reaction zone is in the middle.

The volatile matter in the fuel gets cracked within the reactor and therefore the output gas is almost tar-free. However, the gas, as it comes out of the reactor, contains small amounts of ash and soot. The gas comes out of the gasifier at 250-450° C. This gas can also be used either in hot condition (after preliminary cleaning) or in cold-clean condition (after appropriate gas clean-up arrangement). The gas from the downdraft gasifiers can be cleaned to very high purity such that it can be used in IC engines or for direct heating applications where purity of gas is a critical requirement.

 

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