| OCR Text |
Show local incomplete combustion due to the lack of O 2 and nonuniform combustion temperature. However, the stable ablative-combustion of large fuel blocks is a desirable form of combustion. This problem can be solved by reconstituting the pulverized coal into desirable shaped-charge fuel as in the case of a giant solid-fuel undt for a space rocket or the small shaped-charge of a bazooka. It should be noted that even for guns the explosives are in various forms of shaped charges instead of fine powders. Shaped charges are necessary to control the rate of combustion so that the resulting energy can be transferred to accelerate the bullet instead of deforming the gun barrel and the bullet. The control of the reaction rate of solid-fuel combustion is critical. It is ever more important when one considers the combustion process as part of an overall power system which involves: mi ning, processing, and transporting the fuel; generation, transmission and supply of power; and finally the environmental controls involving acid rain and waste disposal. That is, the combustion process must be one that is compatible with the total system. Any forced fitted system will always end up as a failure sooner or later with great waste of resource and human effort. This study has indicated that the combustion of solid fuels should be in the form of s haped charge as in the case of a space rocket. Ironically, shaped charge has been used for the combustion of coal since ancient time in Far Eastern countries. It is still used extensively in these countries for home cooking and heating. Coal is first pulverized into millimeter size and mixed with some lime and clay to serve as binder as well as absorbent for sulfur in the coal. The mixture is then cast into flat cylindrical briquettes measuring 6" in diameter and 4" in height, see Fig. 1. There are 12 holes, each measuring 5/8" in diameter, punched through the briquette. These briquettes are burnt in a thick walled brick oven. Combustion can be easily started by setting a small fire under the briquette. The key to proper operation of the stove is to get the whole briquette heated to a uniform temperature of 10000 C, which is judged by the radiating color of the briquette. The rate of heat production over a wide range is achieved by regulating the air flow rate driven by self induced draft. Under operation, there is always a tongue of blue flame over each venting hole, see Fig. 1. This, in general. indicates that complete combustion is achieved with no production of CO, NO x , and SO x gases. With proper care, the combustion is known to be surprisingly stable, complete, and safe. The stove is often used indoors without the need for an exhaust pipe as in the case of a kerosene stove which operates essentially on the same principle of controlled reaction rate and temperature. Due to intimate physical contact of the lime with the coal particles and the low combustion temperature, there is a complete capture of sulfur in the coal to form a stable calcium-sulfate compound. The ablative nature of combustion in the venting holes causes the effective reacting surface to increase as the combustion progresses. This is counterbalanced by the increasing diffusion path for the oxygen to reach the reacting surface and for the reacted 3 |
