Reactivity of Biomass and Peat Chars Formed and Gasified at Different Conditions

Abstract

Process analyses and design of gasifiers requires knowledge about the reaction rates between char and gas mixtures. A very wide range of reactivities, using steam as oxidant, have been observed for biomass chars of different origin. For example, reaction rates of tested straw fuels differ by a factor of about 25, which corresponds to a temperature difference of 130^°C. The investigated fuels include wood, straw and peat. The reactivities of chars formed from highly volatile fuels are much dependent upon forming conditions. Influences of different pressures during char formation (pyrolysis) from vacuum to 1 MPa, different tar contents in the gas environment and different heating rates are shown. The general conclusion is made that a high reactivity of the char is favoured by process conditions where the volatiles are rapidly removed from the char. The reactivity of highly reactive char is reduced by freshly formed pyrolysis gas and this effect can be seen through all the course of conversion. It is also shown that the moisture content of the fuel in fast pyrolysis increases the reactivity. Peat chars, judged from reactivity measurements, seem to consist of two types of materials which typically show different dependence upon reactor conditions. The reaction rate of a more reactive part of the peat char is dependent upon the pressure during its formation and also on the steam pressure during gasification, while the low-reactive part is insensitive to these variations of reactor conditions. The effect from demineralization of a peat char may suggest that these different behaviours are related to catalytic effects. Influences of elevated pressure of steam-nitrogen on the reaction rates of chars from wood and peat are also shown. Reaction rates were measured by TGA at steam partial pressures from 0.073 to 2.2 MPa. Some data are also given for the heat of pyrolysis of wood.