7.2.4.6 Gasification-based combustion

Biomass gasification, that is the conversion of biomass to a low- or medium-heating-value gaseous fuel, generally involves two processes. The first of them, pyrolysis, releases volatile vapours (hydrocarbon gases, CO, CO₂, hydrogen, tars, and water vapour) from the fuel at temperatures below 600°C. Because biomass fuels tend to have more volatile components (70-86% on a dry basis) than coal (30%), pyrolysis plays a proportionally larger role in biomass gasification than in coal gasification.

The by-products of pyrolysis that are not vaporized are known as char, and consist mainly of fixed carbon and ash. In the second gasification process, char conversion, the carbon remaining after pyrolysis undergoes the classic gasification reaction (i.e. steam + carbon) and/or combustion (carbon + oxygen). It is this latter combustion reaction that provides the heat energy required to drive the pyrolysis and char gasification reactions. Due to its high reactivity (as compared to coal and other solid fuels), all of the biomass feed, including char, is normally converted to gasification products in a single pass through a gasifier system.

Figure 7.15 Biomass integrated gasification combined cycle (IGCC) system
(HRSG = Heat Recovery Steam Generator)

In the integrated gasification combined cycle (IGCC) system shown in figure 7.15, a high pressure, direct gasifier is depicted inside the dashed line. Gasification and production of a clean fuel gas makes a wide array of power options possible, including the use of advanced power systems with higher efficiencies than those obtained from steam turbines alone. At the low cost end of the spectrum, gasifiers coupled to a high-efficiency, simple-cycle gas turbine offer simplicity as well as efficiencies competitive to the Rankine cycle. A fluidized-bed biomass gasifier (which does not necessarily require steam) integrated with a high-efficiency gas turbine may be the best combination for simple cycle operation.