Volatile gases from biomass pyrolysis under conditions relevant for fluidized bed gasifiers

Abstract

The pyrolytic volatiles released from a converting biomass particle are investigated in this work through laboratorial fluidized bed experiments simulating conditions typical of large-scale gasifiers. Two types of wood (eucalyptus and pine) and two types of pellets (forest residues and wood) with particles of 6–8mm in diameter are fed over the hot bubbling bed at temperatures within 600–975°C. The resultant major pyrolytic products (char, soot, liquids and permanent gas) are collected to verify the overall mass balance, and the composition of the permanent gas is resolved in C3H8, C2H6, C2H4, CH4, CO2, CO, and H2. Primary pyrolysis of the parent fuel particles is essentially complete at 600°C and further increase of the temperature mainly leads to a progressive change in the composition of the volatile gas mixture. Although the gas release does not attain thermodynamic equilibrium under the conditions tested, our results show that the yields of CO2 and light hydrocarbons go through maxima as temperature increases to give rise to CO and H2 as the preferable species at high temperatures. As a whole, the gas composition evolves in such a way that the corresponding lower heating value steadily increases with temperature increase, from about 11MJ/kg at 600°C to above 17MJ/kg at 950°C. Furthermore, the yields of key gas species were found well correlated to each other (C2H4vs. CH4, CH4vs. CO and H2vs. CO), with the relation between the yields of H2 and CO being slightly dependent on the composition of fuel.