Sensible heat recovery efficiency

Viergutz (2009) reports that sulfur burning acid plants with acid-sensible-heat-to-steam recovery systems recover more that 90% of their heat in a useful form, mostly 

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Pelletizing grates possess a multiplicity of windboxes divided into major groupings to allow recovery of sensible heat (e.g. air from the cooling section may be used for drying or combustion purposes). This improves fuel efficiency and adds flexibility to the processing steps. 

In cases where heat recovery was practiced, the overall thermal efficiency was assumed to be 507o. The major heat loss was the hot flue gases, but other losses included sensible heat plus the unburned fixed carbon in the ash, and radiation losses from the incinerator unit. Figure 1 shows a summary of the mass and energy balances for a metric ton refuse input to the incinerator. Some auxiliary fuel consumption was assumed (based on discussions with system designers and the actual operating experience of users) for startup, temperature control, and pilot burners in the secondary combustion chambers. 

In all thermal recovery processes, tar sand is subjected to high processing temperatures, about 450-550°C for pyrolysis, and the residual coked sand is further heated to about 550-650°C during the combustion step. At these conditions, an acceptable thermal efficiency can only be obtained if a significant portion of the sensible heat in the spent sand is recovered and introduced back into the process. Almost all the processes in Table I provide for heat recovery from spent sand before it is discarded. 

An economizer (another type of heat recovery unit) is a boiler component which preheats incoming feedwater from its supplied temperature, utilizing sensible heat from the boiler outlet flue gas being exhausted from the unit. As in the air-heater principle, raising this inlet feedwater temperature (Fig. 6.12) increases the efficiency of the unit by eUminating the use of additional fuel for this operation. 

Acid plants (especially sulfur burning plants) are now often built with acid-sensible-heat-to-steam energy recovery systems. These significantly increase acidmaking energy efficiency. 

Fischer, J.C. (1988) High efficiency sensible and latent heat exchange media with selected transfer for a total energy recovery wheel. US Patent 4,759,053. 

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