10.2 The actual limits of gasification technologies

In this report, a low cost charcoal gasifier for power applications in the range of 2 - 10 kW is presented and discussed. This gasifier seems to be appropriate for applications in rural areas of developing countries. But, this does not mean that gasification technology is already available to an extent that could contribute significantly to solve the problems listed above. The fact that, at the given state of the art, only charcoal can be recommended as fuel for small gasifiers (and, as well, that the use of charcoal cannot be recommended for large gasifier units!) indicates the actual limits of this technology. Charcoal production is bound (or should be bound) to forest management, and charcoal gasifiers should be used within close reach of forest areas. Even within this limitation, there is enough room for a substantial number of sites for gasifier units.

With respect to the regional energy consumption patterns however, the impact of small gasifier-engine-systems is rather marginal. If charcoal production is established, the major part of it will be consumed as cooking fuel. The additional demand for gasifier fuel is limited by site-specific considerations (is a gasifier suitable for a certain application), by economic considerations (is it significantly cheaper than competing energy supply systems) and by the still important aspect of acceptance of a reduced operational comfort.

Considering all these limits, an uncontrollable dissemination of small gasifier plants to an extent that results in an additional stress on natural resources appears not to be realistic.

An aspect which is not yet sufficiently studied is the use of commercially low-value charcoal for gasification: Observations in Argentina as well as in Malaysia have shown that a certain percentage of the charcoal, produced in local kilns, is of a physical dimension (particle size 1-2 cm) which is not desired by domestic consumers but well suited for gasifier application. A classification of kiln charcoal into "cooking fuel" and "gasifier fuel" may result in an increased efficiency of charcoal use without much additional demand for wood to be carbonized.

But certainly, the gasification of unused biomass residues, especially from agriculture, for the purpose of gaining mechanical and electrical energy in rural areas could considerably enlarge the contribution of biomass energy within a national energy scenario. Using uncharred biomass in gasifiers is easier when the biomass fuel is already available in a shape and size which demand no further treatment. This is for instance the case with nutshells, corn cobs, rice husks, saw dust and wood shavings.

Especially rice husks and saw dust/wood shavings are raw materials which are widely spread but hardly used. Gasifiers for rice husks have been intensively investigated in South East Asia and India for many years. Gasifiers for sawdust are practically nonexistent at the moment, even though the technology was still being employed in Germany a few years ago. In many countries of the Third World, however, there is a high demand in mechanical and electrical power for small saw-mills (10-20 kW power output), which could be met by stationary gasifiers (application for driving machines as well as power supply for small settlements). But, all existing gasifiers for agricultural and forstry residues share the basic disadvantage of producing a tar loaden gas, which has to be cleaned in a special gas cleaning train before entering the engine. This process is not always effective and, in most cases, results in condensates which require a final treatment before being released to a drainage system. This final treatment is neglected in practice. The design of gasifiers for uncharred biomass, producing a ,,tar free gas" (at least in defined quality) within the thermochemical process (and not by a cleaning process) is a demand which is not yet solved.

Gasifiers for heat applications are technically much less sophisticated, as tar loaden gases can be burnt with excess air in the burner. Obtaining process heat for industrial drying plants by means of gasification systems could contribute considerably to the protection of natural resources. The efficiency of gas producers is about twice as high as that of a simple furnace. The potential savings in firewood have to be estimated to be rather high (e.g. for large tea-drying plants: several thousand tons of wood per year and plant).

Co-generation of heat and power, based on biomass fuels, is a technology of increasing importance for industrial applications.