Modern combustion steam-electric plants are designed to recover as much heat as economically feasible from the combustion products. As a part of the continuing effort by utilities to increase plant efficiency, extracting low quality heat from the flue gas stream prior to discharge through the stack to the environment has become economically attractive. “Economic feasibility” is strongly dependent on the cost of the fuel as well as quality of the heat recovered. The economic feasibility of deploying low-temperature economizers to cool flue gas from coal-fired steam-electric plants to a temperature well below the sulfuric acid mist dew point is not commonly practiced but could have a number of salutary effects on unit operations including reduction in fuel use, reduction in water, reduction in fly ash resistivity upstream of cold-side electrostatic precipitators and enhanced mercury oxidation/capture.
Using a theoretical 600 MW (nominal) coal fired facility an additional 30.8 MW of electrical output is available with the installation of a Low Temperature Economizer. This represents a 1% improvement in the plant heat rate with an attractive payback period. The components required for this heat recovery sub-system are readily available and the technology has matured to a point where uncertainties are minimized.
In addition to improving the operation of the plant, Low Temperature Economizer can reduce emissions of SOx, NOx, Hg, PM and CO2. In a difficult regulatory environment reducing emissions while increasing plant performance is extremely beneficial. Furthermore Low Temperature Economizer lowers the volume of scrubber water required.
Cooling the flue gas leaving the air heater below the acid mist dew point is not commonly practiced. The corrosion potential of the condensed sulfuric acid is a major materials selection/maintenance challenge as is the potential for gas-side fouling of the heat exchange surface with fly ash.
The Thermal Economy of a Circulating Medium and Low Temperature Waste Heat Recovery System of Industrial Flue Gas