Effects of Feedstock Properties on the Performance of A Downdraft Gasifier

Abstract

Physicochemical properties of biomass feedstocks, such as composition, shape, size, moisture content, etc., have profound effects on the gasification process. The properties affect feedstock selection, sizing, transportation, and storage; gasification and syngas recovery, and residue or co-product processing. The extent of the effects of feedstock properties depend on gasfier type, operating conditions, and syngas quality product requirements. Fixed-bed downdraft gasifiers are widely used in small-scale biomass gasification facilities because of their simple and robust construction, easy and reliable operation, suitability with various feedstocks, high conversion rate, and production of relatively clean syngas containing low tar and particulate concentrations. A series of gasification tests using different biomass feedstocks, including hardwood chips, softwood chips, softwood sawdust, corn cubes, crude glycerol (a byproduct of biodiesel production), and switchgrass, were conducted under similar operating conditions using a pilot scale fixed-bed downdraft gasifier. The results show that downdraft gasifier is suitable for gasifying diverse feedstocks to produce good quality syngas with good low heating value and low tar and particle concentrations. There are no significant differences in gas composition, low-heating value, tar and particle concentrations among the different feedstocks used in the experiments. The syngas produced by the gasification process can be directly be used as fuel in internal combustion engines (ICE), however the physicochemical properties of feedstock such as shape, size, porosity, and the chemical contents, was found to affect the performance of the fixed-bed downdraft gasifier. Feedstocks with small sizes, low porosity, or containing highly compounds that can caramelize in high temperature could cause problems with bridging, lumping, collapsing, or clogging inside the reaction chamber and could cause the gasifier to fail. Hardwood chips mixed with 20% of liquid crude glycerol can be gasified well in the downdraft gasifier and produced syngas with significant higher CH4 content, good low-heating value and lower tar concentration than those of regular hardwood chips.