Pollution Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998 Airborne Particulate Matter: Pollution Prevention and Control Airborne particulate matter (PM) emissions can Fuel Cleaning be minimized by pollution prevention and emis- sion control measures. Prevention, which is fre- Reduction of ash by fuel cleaning reduces the quently more cost-effective than control, should generation of PM emissions. Physical cleaning of be emphasized. Special attention should be given coal through washing and beneficiation can re- to pollution abatement measures in areas where duce its ash and sulfur content, provided that care toxics associated with particulate emissions may is taken in handling the large quantities of solid pose a significant environmental risk. and liquid wastes that are generated by the clean- ing process. An alternative to coal cleaning is the Approaches to Pollution Prevention co-firing of coal with higher and lower ash con- tent. In addition to reduced particulate emissions, Management low-ash coal also contributes to better boiler performance and reduced boiler maintenance Measures such as improved process design, op- costs and downtime, thereby recovering some eration, maintenance, housekeeping, and other of the coal cleaning costs. For example, for a management practices can reduce emissions. By project in East Asia, investment in coal clean- improving combustion efficiency, the amount of ing had an internal rate of return of 26% (World products of incomplete combustion (PICs), a Bank 1991). component of particulate matter, can be signifi- cantly reduced. Proper fuel-firing practices and Choice of Technology and Processes combustion zone configuration, along with an adequate amount of excess air, can achieve lower The use of more efficient technologies or process PICs. changes can reduce PIC emissions. Advanced coal combustion technologies such as coal gas- Choice of Fuel ification and fluidized-bed combustion are ex- amples of cleaner processes that may lower PICs Atmospheric particulate emissions can be re- by approximately 10%. Enclosed coal crushers duced by choosing cleaner fuels. Natural gas and grinders emit lower PM. used as fuel emits negligible amounts of particu- late matter. Oil-based processes also emit signifi- Approaches to Emission Control cantly fewer particulates than coal-fired combustion processes. Low-ash fossil fuels con- A variety of particulate removal technologies, tain less noncombustible, ash-forming mineral with different physical and economic character- matter and thus generate lower levels of particu- istics, are available. late emissions. Lighter distillate oil–based com- Inertial or impingement separators rely on the bustion results in lower levels of particulate inertial properties of the particles to separate emissions than heavier residual oils. However, them from the carrier gas stream. Inertial sepa- the choice of fuel is usually influenced by eco- rators are primarily used for the collection of nomic as well as environmental considerations. medium-size and coarse particles. They include 235 236 PROJECT GUIDELINES: POLLUTANT CONTROL TECHNOLOGIES settling chambers and centrifugal cyclones Their efficiency in removing toxic metals such as (straight-through, or the more frequently used arsenic, cadmium, chromium, lead, and nickel is reverse-flow cyclones). Cyclones are low-cost, greater than 99% (Moore 1994).1 They also have low-maintenance centrifugal collectors that are the potential to enhance the capture of sulfur di- typically used to remove particulates in the size oxide (SO2) in installations downstream of sor- range of 10–100 microns (mm); see Henderson- bent injection and dry-scrubbing systems (Stultz Sellers (1984). The fine-dust-removal efficiency and Kitto 1992). They typically add 1–2% to the of cyclones is typically below 70%, whereas elec- capital cost of new power plants. trostatic precipitators (ESPs) and baghouses can Wet scrubbers rely on a liquid spray to remove have removal efficiencies of 99.9% or more. Cy- dust particles from a gas stream. They are pri- clones are therefore often used as a primary stage marily used to remove gaseous emissions, with before other PM removal mechanisms. They typi- particulate control a secondary function. The cally cost about US$35 per cubic meter/minute major types are venturi scrubbers, jet (fume) flow rate (m3/min), or US$1 per cubic foot/ scrubbers, and spray towers or chambers. Ven- minute (cu. ft/min); see Cooper and Alley (1986). turi scrubbers consume large quantities of scrub- Electrostatic precipitators (ESPs) remove par- bing liquid (such as water) and electric power ticles by using an electrostatic field to attract the and incur high pressure drops. Jet or fume scrub- particles onto the electrodes. Collection efficien- bers rely on the kinetic energy of the liquid cies for well-designed, well-operated, and well- stream. The typical removal efficiency of a jet or maintained systems are typically in the order of fume scrubber (for particles 10 mm or less) is 99.9% or more of the inlet dust loading. ESPs are lower than that of a venturi scrubber. Spray tow- especially efficient in collecting fine particulates ers can handle larger gas flows with minimal and can also capture trace emissions of some toxic pressure drop and are therefore often used as metals with an efficiency of 99% (Moore 1994). precoolers. Because wet scrubbers may contrib- They are less sensitive to maximum temperatures ute to corrosion, removal of water from the ef- than are fabric filters, and they operate with a fluent gas of the scrubbers may be necessary. very low pressure drop. Their consumption of Another consideration is that wet scrubbing re- electricity is similar to that of fabric filters (see sults in a liquid effluent. Wet-scrubbing technol- Table 1). ESP performance is affected by fly-ash ogy is used where the contaminant cannot be loading, the resistance of fly ash, and the sulfur removed easily in a dry form, soluble gases and content of the fuel. Lower sulfur concentrations wettable particles are present, and the contami- in the flue gas can lead to a decrease in collection nant will undergo some subsequent wet process efficiency (Stultz and Kitto 1992). ESPs have been (such as recovery, wet separation or settling, or used for the recovery of process materials such neutralization). Gas flow rates range from 20 to as cement, as well as for pollution control. They 3,000 m3/min. Gas flow rates of approximately typically add 1–2% to the capital cost. of a new 2,000 m3/min. may have a corresponding pres- industrial plant. sure drop of 25 cm water column (Bounicore and Filters and dust collectors (baghouses) collect dust Davis 1992). by passing flue gases through a fabric that acts as a filter. The most commonly used is the bag Equipment Selection filter, or baghouse. The various types of filter media include woven fabric, needled felt, plas- The selection of PM emissions control equipment tic, ceramic, and metal (Croom 1993). The oper- is influenced by environmental, economic, and ating temperature of the baghouse gas influences engineering factors: the choice of fabric. Accumulated particles are Environmental factors include (a) the impact of removed by mechanical shaking, reversal of the control technology on ambient air quality; (b) the gas flow, or a stream of high-pressure air. Fab- contribution of the pollution control system to ric filters are efficient (99.9% removal) for both the volume and characteristics of wastewater and high and low concentrations of particles but are solid waste generation; and (c) maximum allow- suitable only for dry and free-flowing particles. able emissions requirements. Airborne Particulate Matter: Pollution Prevention and Control 237 Economic factors include (a) the capital cost of For gases containing soluble toxics and where the control technology; (b) the operating and the gas flow rate is less than 3,000 m3/min, wet maintenance costs of the technology; and (c) the scrubbers may be used. Cyclones and mechani- expected lifetime and salvage value of the equip- cal separators should be used only as precleaning ment. devices upstream of a baghouse or an ESP. Engineering factors include (a) contaminant characteristics such as physical and chemical Key Issues for Pollution Prevention properties—concentration, particulate shape, size and Control Planning distribution, chemical reactivity, corrosivity, abrasiveness, and toxicity; (b) gas stream charac- The principal methods for controlling the release teristics such as volume flow rate, dust loading, of particulate matter are summarized here. temperature, pressure, humidity, composition, viscosity, density, reactivity, combustibility, • Identify measures for improving operating corrosivity, and toxicity; and (c) design and per- and management practices. formance characteristics of the control system • Consider alternative fuels such as gas instead such as pressure drop, reliability, dependability, of coal. compliance with utility and maintenance require- • Consider fuel-cleaning options such as coal ments, and temperature limitations, as well as washing, which can reduce ash content by up size, weight, and fractional efficiency curves for to 40%. particulates and mass transfer or contaminant • Consider alternative production processes and destruction capability for gases or vapors. technologies, such as fluidized bed combus- Table 1 presents the principal advantages and tion, that result in reduced PM emissions. disadvantages of the