Gasification of Kentucky bluegrass (Poa pratensis l.) straw in a farm-scale reactor☆

Abstract

A novel gasification reactor was designed for conversion of grass straw to synthesis gas. Our design goal was to improve synthetic gas yield and thermal stability at a scale suitable for on-farm use at a cost similar to that of a combine harvester. The reactor that was constructed and tested in this study follows the newly emerging design technique whereby the endothermic pyrolysis or gasification and exothermic char combustion co-exist in the same reactor. It operates in a dual mode where straw gasification occurs in the annulus of an outer tube and an inner (draft) tube. Our trials established that the dual-mode operation could be performed without material flow problems. Sustained tests demonstrated reactor stability at gasification temperatures up to 650 °C and successful gasification of Kentucky bluegrass straw utilizing combustion heat from the inner tube. Calculated equivalence ratios of combustion in the inner tube ranged from 0.3 to 0.78 indicating fuel lean combustion of residual char without slagging. Carbon conversion ranged between 35.4 and 44.8%. Energy recovery, estimated as the ratio of the heat of combustion of the gas to that of the dry-ash-free feedstock, ranged from 14.7% to 30.92%. The estimated heating value for the synthesis gas ranged from 1.27 to 2.85 MJ m−3. Although these conversion parameters are low, a proof of the design concept was established. They can be improved with little modification by increasing the residence time in the draft tube and complete isolation of the gaseous products of combustion and the gasification. More tests are required to evaluate the economic feasibility of the farm-scale unit.