Down Draft 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 critical requirement.

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.