Pressure influence on pyrolysis product properties of raw and torrefied Miscanthus: Role of particle structure

Abstract

Compared to coal, few studies were performed on biomass pyrolysis at elevated pressures, high heating rates, and moderate temperatures, the conditions usually prevailing in commercial fluidized bed gasifiers. In this work, Miscanthus is selected as one of the most promising biomass energy crops which is characterized by high annual yield and energy ratio. The effect of pressure on the product yields, gas composition, and char structure for pyrolysis of pulverized raw and torrefied Miscanthus was investigated in a drop tube reactor. Torrefaction was carried out at 250°C for 30min, while pyrolysis of raw and torrefied Miscanthus was performed at 800°C in a pressure range of 1–30bar. The gases were analyzed using a micro-GC, while the chars were characterized using proximate and ultimate analysis, porosity and BET surface area measurements, and Scanning Electron Microscopy (SEM). It was found that the pyrolysis pressure significantly affects the yields and properties of products, and that torrefaction significantly changes properties and pyrolysis behavior of Miscanthus. The torrefied Miscanthus was found to have higher char yields and char porosities at all the investigated pressures, while the raw Miscanthus was found to have higher gas yields, and higher liquid yields at high pressures. The char yield of both raw and torrefied Miscanthus increases till 10bar and did not show any further changes at higher pyrolysis pressures. The yield of CO2 increased with pyrolysis pressure for both raw and torrefied Miscanthus, while the yield of the high molecular weight gases like C2H4 and C2H6 decrease with increasing pyrolysis pressure and totally disappear at 30bar. Hydrogen represented the major difference in the gas yield trend for raw and torrefied Miscanthus. A non-monotonous trend with pressure was observed for the liquid product yield of raw Miscanthus, while a decreasing trend with asymptote-like nature took place for torrefied Miscanthus. The different pyrolysis behavior of raw and torrefied Miscanthus evidenced in the liquid product yield trend, char porosity and SEM images might be attributed to the force balance on the particles’ structure which led to extensive fragmentation occurrence in the 10–15bar range for raw Miscanthus chars, and its absence in the whole pressure range for torrefied Miscanthus chars, a hypothesis that is to be confirmed in future research work. The results obtained in this study are important inputs for modeling industrial fluidized bed gasifiers working on Miscanthus.