OCR Text |
Show TOTAL SYSTEM CONSIDERATION As aention before, any successful solid-fuel combustion system must be evaluated as an optimum combination of all physical, environmental, and economical factors. This is true for all complex modern systems such as aircraft, cars, ships, and computers. For example, a majority of power plants are now converted to burn ultra-fine coal particles because it is compatible with previous conversion to liquid fuel. This has resulted in major combustion and plant operation problems that have no simple solutions in sight. fuel particles produces vast quantity of submicron flyash. Combustion of fine The coating of chemically active micro-dust over the heat exchange system is a primary operational problem for power plants that has rarely being mentioned in the technical circle. Since the present design consideration is addressed to future power-plant design when cheap clean oil and coal are no longer economically available, the best solution in this case seems to be the flat-bed combustion with preshaped coal blocks. A simplified power plant system for burning a pre shaped slab of coal is shown in Fig. 3. Coal is first broken down into the size of coarse sand. The coal particles are then mixed with small percentage of lime, clay, and other necessary additives to improve combustion characteristic. The mixture is then placed on a long conveyer belt to form a uniformly thick belt of coal. The strip of coal is then punched with uniformly spaced venting holes. Both the size and the number density of holes are predetermined by the immediate requirement of combustion rate. The shaped coal belt then enters a drying and preheating oven to raise its temperature to 8000 C without any ambient air. The volatile components of coal may be extracted for the processing of side products or it may be diverted into the main combustion to be burnt as gas. The preheated char belt is then fed into the combustion chamber for controlled combustion. The temperature within the fuel bed is to be maintained uniform within 1000 ± 2000 C. Heat exchange between the solid fuel and the heat-exchange system is to be carried out by forced recirculation of hot fluegas. The recirculation rate is to be controlled by a computer based on power demand. At the same time, the combustion temperature is also controlled by the computer through regulating the O 2 or air supply-rate. Since the shaped-charge combustion process does not involve free fuel particles, the problem of micro-flyash is completely eliminated. Even for small private home heating system the pollution problem is several orders less than a typical oil-fired furnace. The air pollution level should approach that of a gasfired furnace. As an afterthought, a shaped-charge coal briquette is superior in every respect to wood which is presently popular for rural home-heating. The disposal of residual ash is also much easier, because the spent fuel retains its 5 |