Many researching biomass energy, often promote agricultural wastes as a source of gasification fuel. Quite often, because these wastes are used as combustion fuels in boilers and furnaces, it is assumed, that these same fuels can be gasified if densified into pellets or briquettes. Some can, but many wastes have an uptake of minerals, such as silicon, and others, high levels of potassium and sodium. These three components of biomass make them very difficult fuels to use in high temperature gasification processes required to make a tar free gas for engines.
Probably the largest crop waste available is rice husks, and while these will burn, only the surface carbon surrounding the silica skeleton is consumed. The remaining silica core retains its structure, but once the surface carbon is consumed the oxidation stops, and the fire will slowly die. Only constant removal of hulls will maintain the fire or oxidation zone, making gas with a high tar. Other known crops with high silica content, are palm oil tree stems, and their palm nut shell, which resembles a miniature coconut. Coconut trees which are really a type of grass, have calcium oxalate crystals in the form of raphide bundles, but can be gasified in an appropriately designed gasifier. The biggest problem, is cutting these trees into small blocks as only tungsten blade saws can be used.
Bagasse from processed sugar cane can be a huge resource for gasification fuel, Usually the sugar mills use it in their own boilers, but emissions from these boilers are beginning to force mills to conform with EPA standards. Emissions from bagasse combustion is shown in the photographs, and compared to that from gasified fuel.
Many other seed crops have husks and outer cases that might be potential gasifiable fuel, but it will be necessary to eliminate those with silica. Only testing in a gasification process of known performance and operated by experienced staff can resolve these questions.
Updraught gasifiers can gasify most fuels with mineral content, but the resulting gas is best used in furnaces or boilers.
The above information is not presented as a complete summary of these agricultural wastes, but an indication of the problems one might expect when assessing these fuels for gasification, and that gas used in an engine.
|Single (flat) Rice Hull before gasification.|
|Single (curled) Rice Hull after complete combustion.|
|Outer surface of rice hull before gasification.|
|Outer surface of rice hull after gasification.|
Inner surface of rice hull before gasification.
Inner surface of rice hull after gasification.
|This traverse section of coconut tree stem shows the porous top section of the trunk, making it unsuitable for milling. Although it has low density, it can be used as gasifier fuel.|
|Calcium oxalate crystals|
|Calcium oxalate crystals|
|Bagasse stored as boiler fuel for next season's sugar mill start-up.|
|Note how fibrous texture prevents bagasse from becoming a free flowing fuel.|
|Fine waste residue that can be gasified, need a thermally bonded briquette similar to this one, obtained from Europe.|
|Boiler stack emissions using bagasse fuel.|
|Gasified fuel emissions are a cleaner option compared to combustion.|