A Novel Method of Waste Heat Recovery from High Temperature Furnaces Arvind Atreya, Department of Mechanical Engineering, University of Michigan ABSTRACT One of the largest heat losses in high temperature furnaces is the loss of flue gas enthalpy. Currently, up to 60% of the heating value of natural gas (or any other fuel) used in high temperature furnaces is lost via the flue gases. Here the benefits and technology of re- circulating the hot flue gases back into the furnace to avoid the large heat loss and supplementing the flue gases with oxygen to maintain the desired oxygen concentration are discussed. The energy-savings benefits are derived from two factors: (i) recirculating the hot flue gases back into the furnace, and (ii) a reduction in the mass of flue gases due to the use of oxygen. In addition to these energy benefits, environmental benefits are derived from a reduction in NOx production. In fact, flue gas recirculation (FGR) is a common method used to control NOx in engines and furnaces. The simple heat recovery device described in this paper can: (i) economically capture greater than 50% of the energy lost via the exhaust gases, (ii) provide a long trouble-free operational life of the heat recovery device at a significantly reduced installation cost, (iii) provide a method to control NOx produced by the furnace by FGR recirculation, (iv) increase furnace gas radiation and hence productivity, (v) can be profitably employed even with the high temperature recuperator, (vi) can be profitably used with 100% oxygen furnaces, and (vii) will work with both batch and continuous furnaces. Introduction Many high-temperature industrial production processes (for metals, glass, etc.) consume large quantities of fuel and produce large amounts of high temperature exhaust gases that are often wasted. While numerous types of equipment have been developed to recover and reuse this waste heat, they are expensive, require maintenance and have drawbacks. Most waste heat recovery methods transfer heat from the high temperature effluent stream to a lower temperature input stream. The most widely used waste-heat recovery devices are recuperators and regenerators. Recuperators are heat exchangers that recover heat from hot exhaust gases and transfer it to the incoming cold combustion air across tubes or plates. These tubes and plates get fouled and damaged by the high-temperature gases over time. Fouling requires periodic cleaning of the heat transfer surfaces and to prevent damage, the flue gases are often diluted by cold air before passing through the recuperator. Thus, only a fraction of the exhaust enthalpy is recovered. Typically, only 5% to 30% savings are possible (DOE, 2001). Regenerative heat exchangers, on the other hand, are basically rechargeable heat storage devices. Essentially, an insulated container absorbs and stores large amounts of thermal energy that can be later used to heat combustion air passing through it. Clearly, regenerators operate intermittently and high- temperature heat storage material (such as ceramic balls) must be used. The difficulty is that the intermittent operation requires maintenance and ceramic balls need to be periodically cleaned to reduce pressure drop across the regenerator. Both these heat recovery methods increase the combustion temperature and hence the production of NOx and other pollutants. 6-10 © 2007 ACEEE Summer Study on Energy Efficiency in Industry