Energy Vol. 10. No. IO. PP. 1151-l 157, 1985 Printed in Great Britain. 03~5442/85 $3.00 + .oo 0 1985 Pergamon F’res Ltd. THERMAL STORAGE-MASS ENHANCES WOODSTOVE COMBUSTION AND REDUCES POLLUTION K. R. PILCHER and A. J. GHAJAR School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078, U.S.A. (Received 28 November 1984; received for publication 18 March 1985) Abstract-Experiments were performed to study the effects of using a thermal storage-mass to enhance woodstove combustion-efficiency. The relation between thermal storage-mass spacing and combustion- zone temperature has been established. The results demonstrate that mass enhances combustion- efficiency. The stove may be operated at high temperatures that reduce pollution. Combustion-efficiency enhancement is inversely proportional to the mass spacing. The added mass moderates the frequency of room-temperature fluctuations without imposing a significant thermal lag, while providing a safety shield. However, use of the mass may cause a slight increase in energy consumption. I. INTRODUCTION Rising fuel costs have stimulated renewed interest in woodheating, as evidenced by the 1.5 x lo6 woodburning appliances that are now being sold each year.’ In 1980, wood accounted for approximately 2.5% of the total energy consumed in the U.S., an amount comparable to the contributions from either nuclear or hydroelectric sources.2 Between 0.25 and 0.5% of this energy was used for residential purposes. During the last decade, a radically new breed of stoves was introduced. Although varied in appearance and basic design, they all incorporated a tightly sealed combustion chamber with a well controlled air supply. They are referred to as airtight stoves. These modern, airtight stoves with efficiencies of 50-60%, excellent temperature regulation and the ability to maintain burn cycles up to 8 hours, have overcome many of the traditional discomforts previously associated with residential woodheating. However, these benefits are achieved by imposing a significant restriction on combustion- air supply. The fire is actually choked, lowering the combustion temperature and preventing complete combustion of the wood, especially its gaseous by-products. Operating the stove in this manner produces a cold fire. Correspondingly, if the air dampers are opened as wide as permissible, an oxygen-rich environment is provided which promotes a hot fire with higher combustion temperatures and less unburned material in the exhaust gases. Recent studies have shown that cold fire operation has several distinct disadvantages to hot fire operation.‘-3 One disadvantage is increased creosote accumulation in the exhaust stack. As wood undergoes pyrolysis, it primarily decomposes into charcoal, volatile gases, C02, and H20. The volatile gases account for 30-60% of the potential energy available from the wood. However, the gas-ignition temperature is between 593 and 704°C.2 A cold fire is less likely to yield these temperatures; hence, most of the gases escape unburned to the exhaust, where they cool and condense. The condensation coats the inside of the stack, then dries to a hard crust. This residue is creosote. Left unattended, the creosote accumulation will severely constrict the exhaust stack in a short period of time. The most important aspect of creosote formation is that it presents a potentially dangerous fire hazard. Creosote is highly combustible. When it ignites, flue-gas temperatures exceeding 1093°C occur. The intense heat can warp prefabricated flues and severely crack masonry chimneys, necessitating costly repair. In addition, a house fire may be caused by heat con- duction to the surrounding wood structure or burning exhaust debris falling on the roof. In 198 1, 22,000 fires and 800 deaths in the U.S. resulted from fires caused by woodburning appliances.4 Most of the fires caused by woodburning equipment start as chimney fires. A second disadvantage of a cold fire is excessive pollution. In the absence of adequate air for combustion, large quantities of CO and smoke particles are emitted. Studies conducted in Montana, Oregon, Colorado, Maine, and Vermont have emphasized the magnitude of 1151