Impact of CO2 on biomass pyrolysis, nitrogen partitioning, and char combustion in a drop tube furnace

Abstract

In this study, pyrolysis of sawdust and pinewood (120–250μm) was conducted in a drop-tube furnace (DTF) at temperatures of 900, 1100, 1300°C and residence times of 50–600ms in both CO2 and N2 atmospheres. The samples are fed at a rate of 5–10g/h in a gentle flow of nitrogen (1L/min) to ensure laminar flow. A silica tracer method has been developed to accurately determine the high temperature volatile matter yields. The elemental analysis of chars collected allowed the study of the release of nitrogen. BET surface area, scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses were also carried out to study the chars produced. Burnout tests were conducted at 1100°C, an O2 concentration of 5%v/v in N2 and CO2 respectively, using the chars produced at the same temperature and a residence time of 200ms. In nitrogen, the maximum volatile yield achieved was 97wt% while in CO2, the maximum volatile yield was over 99wt% for residence times above 200ms, indicating virtually complete gasification of the char. These are the highest reported volatile matter yields for biomass obtained using a DTF. The release of nitrogen into the volatile phase is proportional to the yield of volatiles both for air and oxy-fuel conditions. SEM images revealed higher porosities of the DTF CO2 chars than those of N2, being consistent with their higher BET surface areas. Faster char burnout was obtained for oxy-fuel firing attributable to the CO2/char gasification reactions. The results will be useful for modeling dedicated oxy-biomass firing and co-firing systems.