Product Compositions from Sequential Biomass Pyrolysis and Gasification of Its Char Residue

Abstract

Sequential pyrolysis of biomass and gasification of the resultant char generate syngas. The Aspen Plus software was used to simulate a two-stage high-temperature cyclone pyrolysis and gasification of corn straw and rice husk. The effects of the pyrolyzer temperature (700°C–1,400°C) and of the gasification agent (air/steam) on the products of both pyrolysis and gasification, and on the gasifier temperature were assessed. Pyrolysis of biomass generates gases that are rich in H2 and CO, and solid residues amounting to 14%–35% of the feedstock mass, depending on the process temperature. High pyrolysis temperatures promote the H2 and CO formation. Burning half the mass of the pyrolysis gas ensures energy sufficiency in the pyrolyzer. The CO concentration in the gasification syngas increases with input O/C ratios increasing from 0.1 to 1, and decreases with O/C>1. Gasification in steam-enriched air initially promotes the formation of H2 and, to a lesser extent, enhances the CO concentrations in the output syngas as the amount of H2O increases; eventually they both reach maximum values and then decrease. In gasification processes, 30%–60% fractions of steam in air are recommended.