|El Nashaar, Hossien|
Submitted to: Biomass and Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2006
Publication Date: 10/18/2006
Citation: Boateng, A.A., Banowetz, G.M., Steiner, J.J., Barton, T.F., Taylor, D.G., Hicks, K.B., El Nashaar, H., Sethi, V.K. 2007. Gasification of kentucky bluegrass (poa pratensis i.) straw in a farm-scale reactor. Biomass and Bioenergy, 31:153-161. Interpretive Summary: Grass seed production is a large agro-industry in certain parts of the country e.g., in the Pacific Northwest. The residual straw poses a disposal problem due to lack of local markets, high cost of transportation to long distance locations and restrictions on open-heap burning. The need to use the straw for supplemental energy at the farm is becoming increasingly attractive given the current high prices of imported petroleum. One proposed conversion technology is to produce an energy carrier such as synthetic gas by "thermal gasification" (heating the straw in a special vessel to convert part of it into a combustible gas) of the straw which can then be used to generate electricity or necessary heat. However, the existing gasification technologies have, in the past, been plagued with "slagging" (ash accumulation) problems. To successfully convert straw into energy there is the need to explore new technologies. We constructed and tested a new gasifier that has the potential to overcome some of these technological problems and which can be scaled for on-farm use and thereby address the transportation cost. Our tests demonstrated “proof of concept” in that the gasifier produced synthesis gas from Kentucky bluegrass straw without any slagging. Although further refinements of the unit are required to increase synthesis gas yield, we conclude that modest design changes will enable the construction of a unit suitable for on-farm energy production from straw.
Technical Abstract: A novel gasification reactor was designed for conversion of agricultural grass straw biomass to synthesis gas. Previous efforts utilizing conventional downdraft and updraft gasifiers faced challenges that demanded newly designed technologies. Our design goal was to overcome synthetic gas yield and thermal stability problems at a scale suitable for on-farm use at a cost similar to that of a tractor or a combine harvester. The newly developed reactor operates in a dual mode process whereby straw gasification, an endothermic reaction, takes place in an annulus of an outer tube and an inner (draft) tube. Char remaining after the gasification plus supplemental fuel are oxidized with air (combustion) within the draft tube to generate the energy needed for the endothermic gasification in the outer tube. Heat is transferred from the inner tube to the annular gasification section with the aid of steel balls that are pneumatically conveyed by the combustion products. Our trials established that the two-mode process operation could be successfully carried out, produce synthesis gas and prevent breaching problems. Sustained tests indicated consistent reactor stability at gasification temperatures up to 650 degree C which enabled successful gasification of Kentucky bluegrass straw by heat from the inner tube produced by combustion at equivalence ratios in the 0.22-0.61 range without slagging of the residual char. On the gasification side, carbon conversion ranged from 35.4 - 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 between 14.7% and 30.92%. The estimated heating value for the synthesis gas produced during trials ranged from 1.27 to 2.85 MJ m-3. Although these conversion parameters are low, they indicate a proof of the design concept. They can be improved with little modification including increasing the residence time in the draft tube, along with a complete isolation of the combustion and the gasification products. More tests are required to evaluate the economic feasibility of the farm-scale unit.