The use of a condensing economizer within a biomass combustion system is a potentially attractive heat recovery solution for the pulp and paper industry.

A condensing economizer is a flue gas-to-water heat exchanger that is designed to use the latent heat in hot flue gases to heat process water when installed just ahead of the flue gas stack. It is well-suited for large integrated operations where the available biomass fuel (wood-waste and effluent treatment sludge) can be particularly wet and for smaller mechanical pulping plants that operate biomass power boilers.

These types of plants are not typically able to meet their water heating needs with internally recovered heat. A typical mill in these categories would burn 5-10 GJ (giga Joules) of biomass per tonne of product. While each mill application is different, increasing the overall thermal efficiency of biomass combustion by 20% with one of these heat exchangers could reduce fossil fuel use by 1-2 GJ of fossil fuel per tonne of product. At $7/GJ for fossil fuel, these savings could amount to $7-$14 per tonne of product. Selecting materials from which to construct condensing economizers for installation within utility power boiler units is not trivial, considering that aqueous sulphuric acid (H2SO4) condensates are typically formed. The corrosivity of the aqueous H2SO4 condensate is governed in large part by the chemistry of the flue gas, including its particulate matter, which in turn is determined by the combustion conditions, fuel impurities, and the specific pollution control equipment installed upstream of the condensing economizer. It is also depends on the temperature of the condensate and the level of chloride ions present. Construction materials have ranged from carbon steel and stainless steel for near-condensing economizers installed in natural gas-fired heat recovery steam generation (HRSG) combined cycle units to Alloy 625 (UNS N06625) for a condensing heat extractor installed as part of a cyclic reheat system in an oil-fired utility boiler to PTFE-covered stainless steel for a condensing economizer installed in an oil-fired utility power boiler.

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