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Major Constraints Associated with Gasifiers

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Biomass Gasification process has some common limitations and technology-specific limitations, some of which are given here.

Common Limitations

  • Gasification is a complex and sensitive process. There exists high level of disagreement about gasification among engineers, researchers, and manufacturers. Several manufacturers claim that their unit can be operated on all kinds of biomass. But it is a questionable fact as physical and chemical properties varies fuel to fuel.
  • Gasifiers require at least half an hour or more to start the process. Raw material is bulky and frequent refueling is often required for continuous running of the system. Handling residues such as ash, tarry condensates is time consuming and dirty work. Driving with producer gas fueled vehicles requires much more and frequent attention than gasoline or diesel fueled vehicles.
  • Getting the producer gas is not difficult, but obtaining in the proper state is the challenging task. The physical and chemical properties of producer gas such as energy content, gas composition and impurities vary time to time. All the gasifiers have fairly strict requirements for fuel size, moisture and ash content. Inadequate fuel preparation is an important cause of technical problems with gasifiers.
  • Gasifier is too often thought of as simple device that can generate a combustible gas from any biomass fuel. A hundred years of research has clearly shown that key to successful gasification is gasifier specifically designed for a particular type of fuel. Hence, biomass gasification technology requires hard work and tolerance.

 

Gasifiers - Technology Specific Limitations

Fixed Bed - Updraft fixed bed gasifiers

Major drawbacks are the high amounts of tar and pyrolysis products that occur because the pyrolysis gas does not pass the hearth zone and therefore is not combusted. This is of minor importance if the gas is used for direct heat applications in which the tar is simply burned. But when the gas is used for engines, extensive gas cleaning is required.

Fixed Bed - Downdraft fixed bed gasifiers

  • High amounts of ash and dust particles remain in the gas because the gas has to pass the oxidation zone, where it collects small ash particles
  • Fuel requirements are relatively strict; fuel must be uniformly sized from 4 to 10 cm so as not to block the throat and allow pyrolysis gases to flow downward and heat from the hearth zone to flow upward; therefore, pelletization or briquetting of is often necessary.
  • The moisture content of the biomass must be less than 25 percent (on a wet basis).
  • The relatively high temperature of the exit flue gas results in lower gasification efficiency.

Fluidized bed gasifiers

  • High tar and dust content of the producer gas could result in problems while using the gas in the engines.
  • High producer-gas temperatures, which leave alkali metals in the vapor state
  • Incomplete carbon burnout results in lesser energy output
  • Complex operation because of the need to control the supply of both air and solid fuel
  • Need for power consumption for the compression of the gas stream.

 

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