In mass-burning in refractory-walled furnaces, the waste-heat boiler, located downstream of the furnace, receives heat from the flue gases. Older HRIs tend to be refractory-walled designs, and their steam production is usually limited to 1.5 to 1.8 lb of steam per pound of MSW burned, assuming that the heating value of MSW is 4400 Btu/lb. In older furnaces, the larger the furnace, the lower the surface-to-volume ratio, because less surface exists to cool the flame. These units need higher quantities of combustion air to prevent overheating the wall, which results in slagging and deterioration. Approximately 50 to 60% of the heat generated in the combustion process can be recovered from such systems.
Source: C.O. Velzy and R.S. Hechlinger, 1987, Incineration, Section 7.4 in Mark's standard handbook for mechanical engineers, 9th ed., edited by T. Baumeister and E.A. Avallone (New York: McGraw-Hill).
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