Thermochemical Conversion Behaviour of Different Biomass Feedstocks: Pyrolysis and Gasification

Abstract

Abstract:In this study, a bench-scale bubbling fluidized bed (BFB) gasifier and thermogravimetric analyzer (TGA) were applied for the determination of the thermochemical conversion reactivity of biomass fuels under both gasification and pyrolysis conditions. Six different biomass feedstocks, namely; straw pellet (SP), softwood pellet (WP), torrefied wood chips (TWC), pyrolysis char (PC), milled sunflower seed (MSS) and dried distillers' grains and solubles (DDGS) were investigated. TGA of biomass feedstocks were carried out under pyrolysis conditions at four different heating rates (2-15 °C/min). Raw data obtained from the experiments were used to calculate the kinetic parameters (A, Ea) of the samples by using two different models; Coats-Redfern and Isoconversional Method. TGA analysis showed that pyrolysis char was the only sample having decomposition temperature above 800 K since it was the prepyrolized sample before gasification. According to Derivative Thermogravimetric Analysis (DTG) profiles, two peaks and two shoulders at around 450-650 K were observed for DDGS whereas no peaks were detected for pyrolysis char as the indication of absence of volatiles/cellulosic components. It was seen that the highest devolatilization rates and devolatilization temperatures (associated mainly with cellulose decomposition) were obtained for softwood and torrefied wood samples, which had the least char yields among the other biomass feedstocks. It was seen that WP was more reactive for thermochemical conversion and less prone to agglomeration. Furthermore high ash content and agglomeration index of MSS were the potential drawbacks in front of its utilization via thermochemical conversion. During the air gasification of these feedstocks (except DDGS), the product syngas was characterized in terms of main gas composition, tar, and sulfur compounds. It was shown that the highest cold gas efficiency, carbon conversion and calorific value were obtained for the gasification of SP. On the other hand, SP had some drawbacks regarding its high agglomeration tendency and low deformation temperature. Among all feedstocks, gasification reactivity of MSS was found to be quite poor. MSS seemed to expose to pyrolization instead of gasification. WP and TWC were gasified with acceptable conversion values and efficiencies when compared with SP. It was understood that WP is the preferred choice for the thermochemical conversions.