Traditionally, measures designed to reduce local- removal of sulfur in the feed; use of appropriate ized ground-level concentrations of sulfur oxides combustion technologies; and emissions control
(SOx) used high-level dispersion. Although these technologies such as sorbent injection and flue measures reduced localized health impacts, it is gas desulfurization (FGD). now realized that sulfur compounds travel long distances in the upper atmosphere and can cause Choice of Fuel damage far from the original source. Therefore the objective must be to reduce total emissions. Since sulfur emissions are proportional to the
The extent to which SOx emissions harm hu- sulfur content of the fuel, an effective means of man health depends primarily on ground-level reducing SOx emissions is to burn low-sulfur fuel ambient concentrations, the number of people such as natural gas, low-sulfur oil, or low-sulfur exposed, and the duration of exposure. Source coal. Natural gas has the added advantage of location can affect these parameters; thus, plant emitting no particulate matter when burned. siting is a critical factor in any SOx management strategy. Fuel Cleaning The human health impacts of concern are short-term exposure to sulfur dioxide (SO2) con- The most significant option for reducing the sul- centrations above 1,000 micrograms per cubic fur content of fuel is called beneficiation. Up to meter, measured as a 10-minute average. Prior- 70% of the sulfur in high-sulfur coal is in pyritic ity therefore must be given to limiting exposures or mineral sulfate form, not chemically bonded to peak concentrations. Industrial sources of sul- to the coal. Coal beneficiation can remove 50% fur oxides should have emergency management of pyritic sulfur and 20–30% of total sulfur. (It is plans that can be implemented when concentra- not effective in removing organic sulfur.) tions reach predetermined levels. Emergency Beneficiation also removes ash responsible for management plans may include actions such as particulate emissions. This approach may in some using alternative low-sulfur fuels. cases be cost-effective in controlling emissions of Traditionally, ground-level ambient concentra- sulfur oxides, but it may generate large quanti- tions of sulfur dioxide were reduced by emitting ties of solid waste and acid wastewaters that must gases through tall stacks. Since this method does be properly treated and disposed of. not address the problem of long-range transport Sulfur in oil can be removed through chemi- and deposition of sulfur and merely disperses the cal desulfurization processes, but this is not a pollutant, reliance on this strategy is no longer widely used commercial technology outside the recommended. Stack height should be designed in petroleum industry. accordance with good engineering practice (see, for example, United States, 40 CFR, Part 50, 100(ii). Selection of Technology and Modifications
Approaches for Limiting Emissions Processes using fluidized-bed combustion (FBC) reduce air emissions of sulfur oxides. A lime or
The principal approaches to controlling SOx emis- dolomite bed in the combustion chamber absorbs sions include use of low-sulfur fuel; reduction or the sulfur oxides that are generated.
258 Sulfur Oxides: Pollution Prevention and Control 259
Emissions Control Technologies Table 1. Comparison of SOx Emissions Control Systems The two major emissions control methods are Percent SOx Capital cost sorbent injection and flue gas desulfurization: System reduction ($/kilowatt)
• Sorbent injection involves adding an alkali com- Sorbent injection 30–70 50–100 pound to the coal combustion gases for reac- Dry flue gas desulfurization 70–90 80–170 tion with the sulfur dioxide. Typical calcium Wet flue gas sulfurization >90 80–150 sorbents include lime and variants of lime. Sodium-based compounds are also used. Sor- Source: Kataoka 1992. bent injection processes remove 30–60% of sulfur oxide emissions. surrogate monitoring. Continuous stack monitor- • Flue gas desulfurization may be carried out us- ing (CSM) involves sophisticated equipment that ing either of two basic FGD systems: regener- requires trained operators and careful mainte- able and throwaway. Both methods may nance. Spot sampling is performed by drawing include wet or dry processes. Currently, more gas samples from the stack at regular intervals. than 90% of utility FGD systems use a wet Surrogate monitoring uses operating parameters throwaway system process. such as fuel sulfur content. Throwaway systems use inexpensive scrub- bing mediums that are cheaper to replace than Recommendations to regenerate. Regenerable systems use expen- sive sorbents that are recovered by stripping sul- The traditional method of SOx dispersion through fur oxides from the scrubbing medium. These high stacks is not recommended, since it does not produce useful by-products, including sulfur, reduce total SOx loads in the environment. Natu- sulfuric acid, and gypsum. Regenerable FGDs ral gas is the preferred fuel in areas where it is generally have higher capital costs than throw- readily available and economical to use. Meth- away systems but lower waste disposal require- ods of reducing SOx generation, such as fuel ments and costs. cleaning systems and combustion modifications, In wet FGD processes, flue gases are scrubbed should be examined. Implementation of these in a liquid or liquid/solid slurry of lime or lime- methods may avoid the need for FGD systems. stone. Wet processes are highly efficient and can Where possible and commercially feasible, pref- achieve SOx removal of 90% or more. With dry erence should be given to dry SOx removal sys- scrubbing, solid sorbents capture the sulfur ox- tems over wet systems. ides. Dry systems have 70–90% sulfur oxide re- moval efficiencies and often have lower capital References and Sources and operating costs, lower energy and water re- quirements, and lower maintenance require- Cooper, C. David, and F. C. Alley. 1986. Air Pollution ments, in addition to which there is no need to Control: A Design Approach. Prospect Heights, Ill.: handle sludge. However, the economics of the Waveland Press. wet and dry (including “semidry” spray ab- sorber) FGD processes vary considerably from Godish, Thad. 1991. Air Quality. Chelsea, Mich.: Lewis Publishers. site to site. Wet processes are available for pro- ducing gypsum as a by product. Kataoka, S. 1992. “Coal Burning Plant and Emission Table 1 compares removal efficiencies and Control Technologies.” Technical Note. World Bank, China Country Department, Washington, D.C. capital costs of systems for controlling SOx emis- sions. Stern, C., R. Boubel, D. Turner, and D. Fox. 1984. Fun- damentals of Air Pollution. Orlando, Fla: Academic Monitoring Press. Stultz, S. C., and John B. Kitto, eds. 1992. Steam: Its
The three types of SOx monitoring systems are Generation and Use. 40th ed. Barberton, Ohio: The continuous stack monitoring, spot sampling, and Babcock & Wilcox Co. 260 PROJECT GUIDELINES: POLLUTANT CONTROL TECHNOLOGIES
United States. CFR (Code of Federal Regulations). Wash- World Bank. 1992. “Steam Coal for Power and Indus- ington, D.C.: Government Printing Office. try, Issues and Scenarios.” Energy Series Working Paper No. 58. Industry and Energy Department. Vatavuk, W. 1990. Estimating Costs of Air Pollution Con- Washington, D.C. trol. Chelsea, Mich.: Lewis Publishers.