The Effects of Feedstock Pre-treatment on the Fluidized Bed Gasification of Biomass

Abstract

Gasification is a promising technique for transforming solid biomass into a gas that can be used to produce renewable heat, power, fuels or chemicals. Biomass materials, such as forestry residues, can be high moisture, heterogeneous mixtures with low bulk density - properties that make them difficult to handle and convert. Consequently, this means that feedstock pre-treatment is usually necessary in order to facilitate its conversion by gasification. Pre-treatments methods, which include comminution, drying, pelletization, torrefaction, or carbonization will affect the properties of the biomass which will affect their gasification in a fluidized bed. The objective of this thesis was to determine how biomass pre-treatment can influence gasification in a fluidized bed. A single forestry residue was processed using five pre-treatment process levels: sieving (as a surrogate for comminution), drying (moisture content), pelletization, torrefaction, and carbonization. The fractions derived from these processes were gasified in a small pilot-scale air blown bubbling fluidized bed gasifier (feed rate 8 – 25 kg/h). The particle size and form had an impact on the gas composition, tar content, and cold gas efficiency of the gasification. Over the conditions tested, the finest fraction produced a gas with a H2/CO ratio of 0.36 – 0.47 containing 7 – 59 g/m3 tar (gravimetric) at a cold gas efficiency of 30 - 41%. The pellets on the other hand yielded a gas with a H2/CO ratio of 0.89 - 1.14, containing 3 – 37 g/m3 tar (gravimetric) at a cold gas efficiency of 41 – 60%. Drying, torrefaction and carbonization also had an impact on the gasification performance. Carbonization was able to reduce the yield of tar (as measured by gas chromatography) by more than 95% relative to the parent material. Finally, four different forestry residues were gasified in a large pilot-scale bubbling fluidized bed with air and steam-oxygen mixtures (feed rate 200 – 245 kg/h) in order to assess whether the comminution effect could be observed at the large scale. One feedstock with a significant portion of small particles showed the expected effects compared to the feed materials with large feed particles: lower H2/CO ratio, greater tar yield, lower cold gas efficiency while the other feed material containing a substantial amount of small particles did not show these effects.