Mohammad R. Hajaligol

An experimental and modeling study of pressure effects on tar release by rapid pyrolysis of cellulose sheets in a screen heater

Abstract Effects of total external pressure (vacuum [0.0001 atm], 1.3 and 69 atm of helium), on tar, char, and gas yields from rapid pyrolysis of 101-μm-thick cellulose sheets were measured in a screen heater reactor, using holding times of 0, 2, or 5 s at final temperatures from 400° up to 1100°C. At a heating rate of 1000°C/s, increasing pressure from 0.0001 to 1.3 atm at > 750°C or from 1.3 to 69 atm at > 650°C, decreased tar yields and increased gas production. Tar yields at 0.0001, 1.3, and 69 atm, at this heating rate, were well correlated for a wide range of temperatures, by mathematically modeling tar release rates in terms of the kinetics of tar generation by cellulose thermal decomposition, tar destruction by secondary reactions, and tar transport by pressure-cognizant mass transfer. The data and analysis imply that increasing pressure above vacuum, enhances tar holdup within the cellulose by slowing intrasubstrate diffusion of tar, and assists tar escape from the cellulose surface by augmenting free convection of surrounding gas. Related conclusions are that, at a 1000°C/s heating rate, tar secondary reactions are significant within the fuel piece at pressures near and above 1 atm, and temperatures ≥ 650°–750°C, and external to the fuel piece, including on the heater screen, at 0.0001 atm and ≥ 800°C.