Assessment of the suitability of various types of biomass as gasifier fuel
Charcoal
Because good quality charocal contains almost no tars it is a feasible fuel for all types of gasifiers. Good gasifier charcoal is low in mineral matter and does not crumble or disintegrate easily.
The major disadvantages are the relatively high cost of charcoal, which reduces its competitiveness as compared to liquid fuel, and the energy losses which occur during charcoal manufacture (up to 70% of the energy originally present in the wood may be lost). This latter factor may be of special importance for those developing countries which already suffer from an insufficient biomass energy base to cater for their domestic energy requirements.
Experience has shown that most types of wood as well as some agricultural residues (e. g. coconut shell) can provide first class gasification charcoal.
Wood
Most wood species have ash contents below two percent and are therefore suitable fuels fox fixed bed gasifiers
Because of the high volatile content of wood, updraught systems produce a tar-containing gas suitable mainly for direct burning. Cleaning of the gas to make it suitable for engines is rather difficult and capital and labour intensive. Downdraught systems can be designed to deliver a virtually tar-free product gas in a certain capacity range when fuelled by wood blocks or wood chips of low moisture content. After passing through a relatively simple cleanup train the gas can be used in internal combustion engines.
Sawdust
Most currently available downdraught gasifiers axe not suitable for unpelletized sawdust. Problems encountered axe: excessive tar production, inadmissible pressure drop and lack of bunkerflow.
Fluidized bed gasifiers can accommodate small sawdust particles and produce burner quality gas. For use in engines, a fairly elaborate clean-up system is necessary.
Peat
The biggest problems in gasification of peat is encountered with its high moisture content and often also with its fairly high ash content. Updraught gasifiers fuelled with sod peat of approximately 30 — 40% moisture content have been installed in Finland fox district heating purposes and small downdraught gasifiers fuelled with fairly dry peat-pellets have been successfully tested in gas-engine applications (25). During the Second World War a lot of transport vehicles were converted to wood or peat gas operation, both in Finland and Sweden.
Agricultural residues
In principle, developing countries have a wide range of agricultural residues available for gasification.
In practice, however, experience with most types of waste is extremely limited. Coconut shells (10) and maize cobs (39) axe the best documented and seem unlikely to create serious problems in fixed bed gasifiers. Coconut husks (35) axe reported to present bridging problems in the bunker section, but the material can be gasified when mixed with a certain quantity of wood. Most cereal straws have ash contents above ten per cent and present slagging problems in downdraught gasifiers (18). Rice husks can have ash contents of 20 percent and above and this is probably the most difficult fuel available. Research into downdraught gasifier designs fox this material is continuing (21) while published information indicates that Italian up-draught gasifiers have been powering small rice mills for decades (5). The system seems to have been revived in China, where a number of updraught gasifiers axe reported to be in operation (28).
It is possible to gasify most types of agricultural waste in pre-war design updraught gasifiers. However, the capital, maintenance and labour costs, and the environmental consequences (disposal of tarry condensates) involved in cleaning the gas, prevent engine applications
under most circumstances. Downdraught equipment is cheaper to install and operate and creates fewer environmental difficulties, but at present technology is inadequate to handle agricultural residues (with the possible exception of maize cobs and coconut shells) without installing expensive (and partly unproven) additional devices.
Even for coconut shells and maize cobs, the information available is based on a limited number of operating hours and must be further verified under prolonged (say 10000 hours) tests in practical conditions. Fluidized bed gasifiers show great promise in gasifying a number of "difficult" agricultural wastes. Currently, only semi-commercial installations are available and operating experience is extremely limited. It is for this reason that no immediate application in developing countries is foreseen.