Chapter 13
Power Plant Emissions: An Overview John N. Armor Air Products & Chemicals, Inc., 7201 Hamilton Boulevard, Allentown,PA18195
Most people probably do not appreciate the magnitude of the problem with regard to stack gas emissionsfrompower plants. The combustion of fossil fuels is conducted in the presence of excess air and results in huge volumes of emissions of low levels of NOx, SOx, CO, and particulates. Although emitted at low concentrations [ppm levels], the number of sources and the total volume of the emissions are large and thus contribute significantly to atmospheric pollution. The primary gaseous pollutants generatedfrompower plants are NOx and SOx. The combustion of coal generally converts about 95% of the sulfur content to SO2, about 1 or 2 % to SO3, and the remainder to solid sulfur compounds in the ash; US coals contain~0.5to 5. % sulfur. Current commercial technologies used to control SO2 emissionsfromcoal-fired boilers include wet scrubbing of the flue gas with a lime or a limestone slurrry, spray drying of the lime slurry in the flue gas, and several less prevelant processes that produce elemental sulfur or a concentrated stream of SO2 which may be further processed to sulfuric acid. Some of the wet scrubbing processes produce a commercial grade of gypsum. Commercial methods of controlling NOx emissionsfromcoal-fired boilers include combustion modifications, natural gas reburning, selective catalytic reduction [SCR], and selective non-catalytic reduction [SNCR] (1,2). The method of choice is usually determined by regulatory requirements and process economics. NOx control at power plants by SCR is discussed in the overview chapter on "NOx Removal" The thermal methods of NOx removal are not discussed in this book. Norman Kaplan of the US Environmental Protection Agency led the sessions on catalytic approaches to power plant emissions. He summarized the papers in this area as follows: "Speakers in the session cited experience with selective catalytic reduction (SCR) for control of NOx emissions in Japan and Germany. Systems were categorized as high dust, low dust, and tail end. Applications of the high dust configuration predominate in foreign applications. The advantages and disadvantages of catalyst types (e.g., plate, honeycomb, various compositions) were contrasted based on some German experience with power plant applications. Some data indicated acceptable
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catalyst activity (80-100% of initial level) for periods of >18,000 hours of operation. Systems are usually designed to emit less than 1 ppm of unreacted ammonia when installed, and about 5 ppm at the end of the useful catalyst life. The SNOX process, of Danish origin, for simultaneous catalytic NOx reduction and S0 control by catalytic conversion to sulfuric acid was described as having the capability for >95% control of both pollutants, with fewer problems caused by unreacted ammonia. The advantages of a family of catalysts for various applications including coal-fired boilers, nitric acid plants, and refinery heaters were cited with indication of commercial experience. Most of the catalytic processes described above employ vanadium pentoxide/titanium dioxide catalysts with various modifiers. Another process being developed in Germany for reducing NOx at low temperature using an activated carbon pseudo catalyst was also described and compared to the more prevalent V205/TÎ02 processes. R. A. Dalla Betta, et al (Catalytica, Inc.) reported on New Catalytic Combustion Technology for Very Low Emissions Gas Turbines. They described their new process for catalytic methane combustion in gas turbines by burning methane at about 1300°C with a 20 milli-second residence time at 10 atmosphere pressure. Here the levels of NOx will be less than 10 ppm. The metal monolith support provides rapid transfer of heat and thus the catalyst avoids over-heating beyond 1300°C. Apparently the equilibrium between Pd and PdO helps limit the catalyst temperature to 1300°C. They obtain less than 1 ppm of NOx and less than 2 ppm of CO at 11 atmospheres with 10,000 liter/minute of air using a 2" diameter catalyst. They are working with several turbine companies to evaluate the technology." Several of these presentations are included in this section on Power Plant Emissions. 2
References 1. Armor, J. N. Appl. Catal. B, 1992, 1, 221-256. 2. Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369-532. RECEIVED
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