|Low Cost Charcoal Gasifiers for Rural Energy Supply (GTZ, 1994, 49 p.)|
Environmental concerns, especially with respect to a rising content of CO2 in the atmosphere, make the extensive use of fossil fuels in the future more and more suspect. But what is the alternative? Perhaps liquid hydrogen, produced with the help of photovoltaics in desert areas - but this is still speculative, like the complete energy scenario of the future is. What can be done in short terms to at least limit the increasing consumption of fossil fuels?
Energetic use of biomass is one of the options which are dicussed under the headline "renewable sources of energy". There is one basic advantage of big-energy, compared to fossil energy:
If fossil fuels are burnt (and heat production by combustion is the first step in nearly all technically relevant energy conversion processes), the carbon, stored in the coal or oil, reacts with the oxigen of the air and forms carbon dioxide. This is the reason for the steadily increasing CO2 content of the atmosphere, together with the reduction of CO2 sinks' represented by tropical rain forests. If harvested plant material is burnt, the combustion reaction is the same. But, the storage time of the involved carbon is much shorter. The combustion is the reverse process of the photosynthesis: The amount of carbon, assimilated during the growth period of the plant, is released as CO2. If the plant is replanted, the carbon cycle is closed. Thus the combustion of plants, if handled in a sustainable way, is CO2-neutral (apart from eventual CO2 production by fertilizer production or harvesting machinery etc.).
It is not increased consumption of wood which is meant by energetic use of biomass. It is the more effective use of plant materials by integration of waste products of agriculture and forestry in the product line. Bio-Energy can be the result of anerobic digestion (in biogas plants), it can be represented by plant oils, alcohols, and so on. Another technology of energetic use of biomass is gasification.
Gasification means thermochemical conversion of solid plant material into gaseous components. In a few phrases, a gasifier works as follows: Air is sucked through an already ignited charcoal bed. In a first reaction layer, the oxygen in the air is reacting with the hot charcoal, resulting in carbon dioxide (the typical combustion gas). The carbon dioxide, together with the water vapour of the fuel, is reacting with the adjacent hot charcoal layer, resulting in a mixture of gases, which contains around 28% of carbon monoxide, some 8% of hydrogen, a few percent of carbon dioxide, and around 58 % of nitrogen, the latter being an inert component of the air flow into the gasifier. Only about 36% volume of the gas mixture are inflammable components, and the heating value of the mixture (approx. 4000 - 5000 kJ/Nm³, Nm³ = norm cubic meter) is not very high. But, mixed with air, the gas can be ignited by the spark plug of a standard gasoline engine, serving as an engine fuel. Theoretically, the engine could run with 30 % power loss compared to gasoline operation, but in practice the losses are higher. As a rule of thumb, 5 kW shaft power can be expected per liter engine volume at 2500 revolutions, if producer gas (that's the name of it) is applied.
The basic chemical reactions of gasification take place in a charcoal bed. This does not mean that only charcoal can be gasified. If wood or any other dry plant matter is filled into the tank of a gasifier, the material is automatically dried and pyrolized due to the high temperatures of the combustion zone, before entering this zone. For more details, extensive literature is available [1,2].
Theoretically, any plant material with a moisture content of 5-30 % can be gasified, as the basic composition of carbon, hydrogen and oxigen is almost the same (on dry, ash free basis). That means that agricultural wastes like rice husks, straw, nut shells, and forestry residues Like branches, trunks, bark and so on are all potential gasifier fuels-but only theoretically. The physical properties of the fuel as well as the ash content are aspects which define if it is worth the effort to gasify or not.