sign in sign up
<<
Home , Particulates, Pollution prevention

Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998 Airborne Particulate : 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 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 pose a significant environmental risk. and 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 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 , investment in coal clean- improving 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. 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 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 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: 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 (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--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 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 (for particles 10 mm or less) is 99.9% or more of the inlet dust loading. ESPs are lower than that of a . 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 , 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 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 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, , 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 particulate control technolo- • Select optimal particulate removal devices gies discussed here. ESPs can handle very large such as ESPs and baghouses. volumetric flow rates at low pressure drops and can achieve very high efficiencies (99.9%). They Note are roughly equivalent in costs to fabric filters and are relatively inflexible to changes in pro- 1. However, controlling emissions of many heavy metals, such as cadmium, lead, and , that are cess operating conditions. Wet scrubbers can also present as trace elements in fuels is a difficult and achieve high efficiencies and have the major ad- largely unsolved problem. vantage that some gaseous can be re- moved simultaneously with the particulates. References and Sources However, they can only handle smaller gas flows 3 (up to 3,000 m /min), can be very costly to oper- Bounicore, Anthony J., and Wayne T. Davis, eds. 1992. ate (owing to a high pressure drop), and produce Engineering Manual. New York: Van a wet sludge that can present disposal problems Nostrand Reinhold. (Cooper and Alley 1986). For a higher Cooper, C. David, and F. C. Alley. 1986. Air Pollution flow rate and greater than 99% removal of PM, Control: A Design Approach. Prospect Heights, Ill.: ESPs and fabric filters are the equipment of Waveland Press. choice, with very little difference in costs. Croom, Miles L. 1993. “Effective Selection of Filter Dust.” Chemical Engineering (July). Recommendations Hanly, J., and Petchonka, J. 1993. “Equipment Selec- For effective PM control in industrial applica- tion for Solid Gas Separation.” Chemical Engineer- 10 ing (July). tion, the use of ESPs or baghouses is recom- mended. They should be operated at their design Henderson-Sellers, B. 1984. Pollution of Our . efficiencies. In the absence of a specific emissions Bristol: Adam Hilger. requirement, a maximum level of 50 milligrams Jechoutek, Karl G., S. Chattopadhya, R. Khan, F. Hill, per normal cubic meter (mg/Nm3) should be and C. Wardell. 1992. “Steam Coal for Power and achieved. Industry.” Industry and Energy Department Work- 238 PROJECT GUIDELINES: POLLUTANT CONTROL TECHNOLOGIES

Table 1. Advantages and Disadvantages of Particulate Control Technologies

Advantages Disadvantages Inertial or impingement (cyclone) separators • Low capital cost (approximately US$1/cu ft/min flow rate) • Relatively low overall particulate collection efficiencies, • Relative simplicity and few maintenance problems especially for particulate sizes below 10 mm • Relatively low operating pressure drop (for the degree of • Inability to handle sticky materials particulate removal obtained) in the range of approxi- mately 5–15 cm (2–6 inches) water column • Temperature and pressure limitations imposed only by the materials of construction used • Dry collection and disposal • Relatively small space requirements Wet scrubbers • No secondary dust sources • Potential water disposal/effluent treatment problem • Relatively small space requirement • Corrosion problems (more severe than with dry systems) • Ability to collect gases, as well as particulates (especially • Potentially objectionable steam plume opacity or droplet “sticky” ones) entrainment • Ability to handle high-temperature, high-humidity gas streams • Potentially high pressure drop—approximately 25 centi- • Low capital cost (if wastewater treatment system is not required) meters (10 inches) water column and horsepower require- • Insignificant pressure-drop concerns for processes where ments the gas stream is already at high pressure • Potential problem of solid buildup at the wet-dry inter- • High collection efficiency of fine particulates (albeit at face the expense of pressure drop) • Relatively high maintenance costs Electrostatic precipitators • Collection efficiencies of 99.9% or greater for coarse and • High capital cost—approximately US$160/square meter fine particulates at relatively low energy consumption ($15/square foot) of plate area • Dry collection and disposal of dust • High sensitivity to fluctuations in gas stream conditions • Low pressure drop—typically less than 1–2 cm (0.5 inch) (flow rates, temperature, particulate and gas composi- water column tion, and particulate loadings) • Continuous operation with minimum maintenance • Difficulties with the collection of particles with extremely • Relatively low operation costs high or low resistivity • Operation capability at high temperatures (up to 700oC, • Relatively large space requirement for installation or (1,300oF) and high pressure (up to 10 . • Explosion hazard when dealing with combustible gases or 150 pounds per square inch, psi) or under vacuum or particulates • Capability to handle relatively large gas-flow rates (on • Special precautionary requirements for safeguarding per- the order of 50,000 m3/min sonnel from high voltage during ESP maintenance by de- energizing equipment before work commencement • Production of by the negatively charged electrodes during gas ionization • Highly trained maintenance personnel required Fabric filter systems (baghouses) • Very high collection efficiency (99.9%) for both coarse • Requirement of costly refractory mineral or metallic fab- and fine particulates ric at temperatures in excess of 290oC (550oF) • Relative insensitivity to gas stream fluctuations and large • Need for fabric treatment to remove collected dust and changes in inlet dust loadings (for continuously cleaned reduce seepage of certain filters) • Relatively high maintenance requirements • Recirculation of filter outlet air • Explosion and fire hazard of certain dusts at concentra- • Dry recovery of collected material for subsequent pro- tion (~50 g/m3) in the presence of accidental spark or cessing and disposal flame, and fabric fire hazard in case of readily oxidizable • No corrosion problems dust collection • Simple maintenance, flammable dust collection in the ab- • Shortened fabric life at elevated temperatures and in the sence of high voltage presence of or alkaline particulate or gas constitu- • High collection efficiency of submicron and gas- ents ·Potential crusty caking or plugging of the fabric, or eous contaminants through the use of selected fibrous need for special additives due to hygroscopic materials, or granular filter aids moisture condensation, or tarry adhesive components • Various configurations and dimensions of filter collectors • Respiratory protection requirement for fabric replacement • Relatively simple operation • Medium pressure-drop requirements—typically in the range of 10–25 centimeters (4–10 inches) in water column

Source: Adapted from Bounicore and Davis 1992. Airborne Particulate Matter: Pollution Prevention and Control 239

ing Paper. Energy Series Paper No. 58. World Bank, Vatavuk, W. M. 1990. Estimating Costs of Air Pollution Washington, D.C. Control. Chelsea, Mich.: Lewis Publishers. Moore, T. 1994. “Hazardous Air Pollutants: Measur- World Bank. 1991. “: Efficiency and Environmen- ing in Micrograms.” EPRI Journal 19(1). tal Impact of Coal Use.” Report No. 8915-CHA. Stultz, S. C., and John B. Kitto, eds. 1992. Steam: Its China Department, Industry and Energy Division, Generation and Use. 40th ed. Barberton, Ohio: The Washington, D.C. Babcock & Wilcox Co.