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Show Unmixed Combustion for Efficient Heat and American Flame Research Committee Mass Transfer in Chemical Processing Systems 1997 Fall International Symposium INTRODUCTION Fire is the oldest and most familiar method of generating heat. Unfortunately, for many applications fire is also an inefficient means for transferring heat to a purpose. Firstly, fire produces a supply of hot gases at a very high temperature. Transferring that heat to a solid or liquid is an inherently low efficiency process, requiring a large surface area over which heat transfer occurs. For many industrial processes the cost of providing heat transfer surfaces to recover the heat is a major part of the total process cost. Secondly, it is often very difficult to utilize all, or even a significant fraction of the heat that is generated. This is because effective heat transfer requires a large temperature difference between the heat source and the item to be heated. As the item becomes warmer and the hot gases cooler the rate of heat transfer diminishes, and eventually a volume of gas is exhausted from the heating system carrying with it a large amount of "low quality" heat. Another disadvantage of fire deals with flammability limits. For any given fuel, a flame can be sustained only within a range of fuel/air mixtures. Furthermore, over much of that range oxidation is incomplete, leading to release of unburned fuel and products of incomplete combustion which can be serious pollutants. Thus, fire must be relatively intense in order to be self sustaining, yet to be useful, it is sometimes desirable to have a less intense, lower temperature source of heat that fire can provide. Despite these and many other shortcomings, very few satisfactory alternatives to fire have been exploited by mankind. Consider: Nuclear power has yet to be demonstrated in a format that is convenient for industrial, m u c h less domestic applications; Other forms of chemical energy release tend to be uneconomical or inconvenient except for very specialized applications such as "instant heat" packages used by athletes or thermite mixtures for welding; Catalytic combustor can overcome some of the disadvantages of fire, but the catalysts tend to be expensive and often are fuel specific; Electricity is a very effective means for supplying heat for a great variety of purposes, but the vast majority of electricity is produced indirectly by fire and so is a convenient, but thermodynamically inefficient means for delivering heat; Solar energy and fuel cells are both emerging as potential sources of both process heat and electric energy for the future, but both suffer from high capital costs, and solar energy is likely to be limited in application to domestic and smaller industrial operations. Unmixed Combustion! (UMC) is a process that, for many applications, promises to be as practical as fire but overcomes the disadvantage of poor heat transfer, while also showing promise for reducing emissions of unburned fuel and pollutants. Fairmont Hotel Chicago, Illinois September 21 -241997 Page 2 |