Flue Gas Condensation Heat Recovery for Industrial Boilers

This paper summarizes the study of the energy conservation potential of recovering the waste heat from condensing the water vapor in the flue gas from industrial boilers and other combustion devices. The study was conducted by KVB, Inc. for the Industrial Utilization Research Sub-Division of the Gas Research Institute (GRI)*. Direct contact condensation heat recovery is the process of passing a water flue (or dryer exhaust) gas through a cold water stream resulting in a cold flue gas exit temperature ~100°F and producing a hot water stream at approximately 130°F (+10°F). Since the flue gas is cooled to an exit temperature below the dew point, a considerable amount of latent heat of condensation plus sensible heat is recovered resulting in a potential total efficiency increase of 10 to 15 percent. This process can provide the largest increase in boiler efficiency of any retrofit equipment available. This paper presents methods for estimating efficiency increase as a function of boiler operating conditions. The major limitation of this heat recovery technique is the lower temperature level (~130°F) of the heat produced. It is very important to match the low level heat output with the plant requirement to fully utilize the recovered heat. Typical applications that utilize this type of low level heat include (but are not limited to) the following: makeup feedwater preheat, hot water for processing (i.e., pulp and paper, textile washing, food processing and wash down), space heating, etc. Installations with this type of low level heat requirement that operate more than 4,000 hrs/yr can typically have payback periods of less than two years. Two other types of equipment that employ flue gas condensation heat recovery are also briefly reviewed. These are (1) direct contact combustion (submerged combustion), a process that provides 100 percent of its heat output as hot water of 180°F or less by direct contact of the hot burner gas (1500-2000°F) with the cold water resulting in flue gas temperatures of ~100°F and operating efficiency of 95 to 97 percent, (2) indirect contact condensation heat recovery, similar to a standard tubular economizer with corrosion resistant construction of either glass or teflon, to allow for a cold flue gas exit temperature and condensation. All three types of equipment result in decreased pollution by virtue of the 10-15 percent efficiency increase resulting in a significant decrease in the yearly fuel use. In addition, when fuel oil is burned containing sulfur, a significant reduction in the SOx emissions occurs due to the scrubbing action of the heat exchanger. The total energy saving potential from the widespread national application of this heat recovery technique is 605 x 10^12 Btu/hr representing approximately $3 billion/yr at a fuel cost of $5/MBtu.