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Developments in Sorbent Injection Technology for Sulfuric Acid Mist Emissions Control

Summary As the U.S. electric power industry faces increasingly stringent emission control regulations, the economic benefits of using integrated solutions requires due consideration of the impacts of applying a given set of process technologies for the sitespecific conditions of any given plant. The widespread application of selective and non-selective catalytic reduction (SCR and SNCR, respectively) for the control of nitrogen oxides (NOx) has resulted in attendant impacts on equipment and processes downstream of the economizer, leading to increased formation of sulfur trioxide (SO3) and subsequent potential emission of undesirably high concentrations of sulfuric acid mist. Moreover, the increased use of fuels or fuel blends with higher vanadium content (a catalyst for SO3 formation) has also contributed to the phenomenon. In the absence of direct federal regulatory measures and under some sets of circumstances, the effect can be minimal or even self-controlled, but for many mitigation has effectively become essential. With the pressure to address mercury and fine particulate emissions at the same time, utilities are seeking solutions cost-effective solutions with synergetic effects. The particular combinations of fuels burned, the combustion conditions, the selection and design of the NOx control technology, and the types of particulate collection and desulfurization techniques employed all contribute to the ultimate physical characteristics and chemical composition of the flue gas leaving the stack. Within the past three years, Chemical Lime has had several opportunities to work with industrial and utility customers on trials and demonstration tests of hydrated lime injection as a means of reducing flue gas SO3 concentration and/or opacity resulting from the formation of sulfuric acid mist. The nature of the tests have ranged from one extreme of being very short (several hours) “proof of concept” trials to extended demonstrations lasting a week or more that have been planned with the customer to explore potential longer term impacts and compatibility with other emission control systems, especially dry electrostatic precipitators (ESP). The specific results of the individual tests have usually been considered confidential as the tests have been exploratory in design or directed at potential performance of competing technological approaches at a given plant. It can generally be said, however, that the not-too-surprising primary conclusions are that, properly applied, calcium hydroxide as the strong chemical base in commercial hydrated lime is an effective sorbent for the capture of strongly acidic, gas-phase SO3, and that the removal is primarily a function of the feed stoichiometry and the residence time available for diffusion of the gas to the reactive sites on and within the lime sorbent. Several published studies