Updraft Gasifiers

Fuel flexibility is the main feature of Updraft Multifuel Gasifeirs. 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.

Selection Criteria
The selection of Updraft or Downdraft Gasifier would mainly depend upon the available fuel and the requirements of the application.

Downdraft wood Gasifier requires wood chips, biomass briquettes or wood like agro-residues (such as stems, stalks). If fuel flexibility is required and need for fuel switching (say from biomass to coal or from one biomass to another) is anticipated, the updraft gasifier would be suitable.

For gasification of coal, updraft gasifier is suitable. Downdraft gasifier cannot accept coal. For direct heating applications, such as firing in kiln, furnaces, boilers, etc, updraft gasifier would make a better choice. However, if the contaminants in fuel gas are likely to affect the end-product quality, the choice of downdraft gasifier makes better sense. There are situations where effect on end product quality is critical, but fuel flexibility is also an important consideration. In such situations, updraft gasifier with gas scrubbing arrangement may be selected.

For running IC engines, normally downdraft gasifiers are selected. However, for large installations where flexibility may be additional requirement, updraft gasifiers with extensive gas clean-up system could be an option.

In addition to the above general criteria, there would be site specific requirements, which need to be analyzed in detail while making the final selection.