The gasification process

Because of the limitations of state-of-the-art biomass technology, a quest to improve the efficiency and range of applications has been underway for several decades.

The point of departure was the recognition that the combustion process actually comprises several separate thermal processes which, if conducted in a controlled manner, may considerably improve the result. These processes are:

Drying, where free moisture and cell-bound water are removed from the biomass by evaporation. These processes should ideally take place at a temperature of up to about 160ºC using waste heat from the conversion process.

Pyrolysis, where volatile gases are released from the dry biomass at temperatures ranging up to about 700ºC. These gases are non-condensable vapours (e.g. methane, carbon-monoxide) and condensable vapours (various tar compounds) and the residuum from this process will be mainly activated carbon.

Reduction, where the activated carbon reacts with water vapour and carbon dioxide to form combustible gases such as hydrogen and carbon oxide. The reduction (or gasification) process is carried out in the temperature ranging up to about 1100ºC.

Oxidation, where part of the carbon is burned to provide heat for the previusly described processes.

The updraft gasification

In the updraft gasifier, moist biomass fuel is fed at the top and descends though gases rising through the reactor.

In the upper zone a drying process occurs, below which pyrolysis is taking place. Following this, the material passes through a reduction zone (gasification) and in the zone above the grate an oxidation process is carried out (combustion). To supply air for the combustion process and steam for the gasification process, moist hot air is supplied at the bottom of the reactor. Combustible gas at a low temperature (because of the evaporation of moisture in the drying zone) is discharged at the top of the reactor, and inert ash from the heat-generating combustion process is extracted from the reactor bottom through a water lock.