TECHNOLOGY
Alternative Heat Recovery Method Based on Methane Reforming Thermochemical recuperator could provide means of using high-temperature heat to raise chemical energy of fuel rather than exchanging heat with air Catalytic reforming of m e t h a n e holds promise as a chemical alter native to conventional heat recov ery in a number of industrial appli cations. Reforming can provide a means for using high-temperature heat to increase the chemical ener gy of a fuel rather than exchanging heat with an air stream. The method has been proposed by a group at the Institute of Gas Technology headed by Donald K. Fleming, director of industrial en ergy utilization research at IGT. A device employing the technique, called a thermochemical recupera tor (TCR), would be particularly suit ed to systems that exhaust hightemperature heat but don't produce either electricity or shaft power— glassmaking and blast furnaces, for example. The basic idea behind a TCR is that heat would be absorbed by the entering fuel to change the fuel chemically to a state of higher chem ical energy. The increase in the chemical energy of the fuel is equiv alent to the heat absorbed and pro
vides more heat at the flame tem peratures of combustion. In conven tional heat exchange processes, the entering air is passed through a re cuperator or a regenerator, some times in elaborate heat-exchange networks. With the TCR, methane, for ex ample, might be reformed by reac tion with water to carbon monox ide and hydrogen. In the ideal case, 1 cu ft of methane, providing 912 Btu on combustion, could yield 1 cu ft of carbon monoxide providing 322 Btu, and 3 cu ft of hydrogen providing 825 Btu. The reforming products have a potential heating value 26% higher than the original methane. The reforming reaction oc curs above 1250 °F and has been well studied and documented over the years. It is routinely used— usually over nickel catalysts—in many plants. The higher the tem perature the better the conversion. An advantage expected from a TCR is low-pressure operation. Con ventional reforming frequently oc curs at pressures of 300 psig and up. The TCR, says Fleming, would require only enough pressure to feed the burners of the furnace. In the usual recuperators and regenerators, very high temperatures are desired. But a TCR is expected to work at temperatures as low as 1300 °F, al though a convenient minimum is 1500 °F. A TCR doesn't depend on high-temperature surfaces as radia-
Fleming: low-pressure operation tion heat sources to transfer heat. It uses available off-gases from the fur nace exhaust as the heat source. Fleming notes that the idea is not unique with IGT. It apparently first surfaced in the Soviet Union a dec ade ago but was never developed. Presumably, in an era of cheap nat ural gas, there was little incentive to develop it further. But that is changing now, and IGT believes that the idea is well worth developing to see if it is as promising as it appears to be. There are some problems with the scheme, however. One involves side
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reactions: For instance, the effect of the water gas shift reaction—where carbon monoxide and water react to form hydrogen and carbon di oxide—must be taken into account. Some of the carbon monoxide pro duced during reforming will be de graded. The net effect is a higher volume of hydrogen, which has a lower heating value. Another potential problem is soot formation from carbon produced in the reactions. In addition to soot, side reaction products also decrease the net heating value of the fuel. Fleming believes that these and oth er problems can be overcome in a development program. Soot forma tion, for example, might be over come by using an appropriate steam/ methane ratio of about 1.5 to 1. To illustrate the TCR system, Fleming and his associates have per formed an analysis assuming a glassmelting operation processing 250 tons of glass per day. The TCR re covers heat from the melting fur nace exhaust gases. This heat drives the reforming reaction and thereby increases the chemical energy of the fuel. There is enough remaining heat in the exhaust from the TCR to preheat furnace air to 1500 °F. The residual heat is used in a waste heat boiler to produce steam for the re former. Since the waste heat boiler exhausts heat at about 700 °F, it's possible to utilize this low-temper ature heat for other applications. In terms of the energy originally present in the fuel, the glass-melting operation with the TCR is conserva tively projected to be about 62% ef ficient. That would be accomplished with heat-recovery temperatures 1000 °F lower than any known con ventional system. In turn, this would permit less expensive materials of construction and other benefits. No TCR has actually been built. However, engineering evaluations indicate that the system would be 40% or more efficient than conven tional recuperation practices and 30% more efficient than the present state-of-the-art commercial recuper ation. When compared with the very-high-temperature recuperators now in development, the TCR ap pears to be about 16% more effi cient. Joseph Haggin, Chicago
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