Power Engineering

Scrubber myths and realities

Scrubber myths and realities

By the Wet and Dry Gaseous Scrubber Divisions of the Institute of Clean Air Companies Inc. (ICAC)

Don`t let common misperceptions about flue gas desulfurization systems bias a realistic appraisal of this capable control technology

Commonly known as scrubbers, flue gas desulfurization (FGD) systems are a highly efficient and reliable means of removing SO2 as well as particulate matter, hydrochloric acid and other air toxics. Scrubbers, which have been used for 25 years, are not only commercially proven, but are the standard by which new technology is judged. This standard, however, continues to rise as scrubber cost-effectiveness, reliability, waste recycling and efficiency has steadily improved over the last several years.

The last decade has brought tremendous improvement in process chemistry, a better understanding of materials, simplified designs and other technological enhancements. All of these have increased scrubbers? reliability and efficiency, while decreasing costs. Once considered a technology to be avoided, these advances have renewed FGD as an important SO2 control option.

Some people may, however, be unaware of the improvements that 25 years of continuous refinements have brought. Further, the current global debate over the best way to meet the demand for reduced SO2 emissions, and the debate in the U.S. regarding implementation of the Title IV, Phase II provisions of the Clean Air Act Amendments (CAAA) of 1990, both contain numerous misconceptions about scrubbers. To set the record straight, ICAC?s wet and dry FGD divisions have written this paper on common scrubber myths and realities.

Reliability

Myth: Scrubbers are unreliable.

Reality: The utility industry?s own operating data, confirmed by Environmental Protection Agency (EPA) studies, show that scrubber systems rank among the most reliable of all power plant equipment, with average availability approaching 100 percent.

Early scrubber systems did have problems and some were fairly unreliable, with availabilities as low as 85 percent in the United States before 1980. Scrubber components often suffered from plugging and scaling, and material failures were frequently responsible for unplanned outages.

By simplifying process configurations, understanding process chemistry better, selecting better materials, using redundant equipment in critical areas and, to a great extent, applying practical utility construction experience in power plant operation, much higher scrubber reliability has been attained. Recent experience at individual utilities confirms that wet FGD systems now are very reliable, with availability over 99 percent in many cases, and often close to 100 percent.

Indeed, the North American Electric Reliability Council concluded that wet FGD systems have a minimal impact on plant performance contributing, on average, only .31 percent to plant unavailability. Interestingly, the task force found no statistically significant difference in unit availability between retrofit FGD systems and systems that were part of a new plant?s design.

Scrubbers are operating at more than 250 U.S. power plants with a total electric generating capacity of approximately 80,000 MW. This amounts to close to 24 percent of the total U.S. coal-fired capacity. To comply with Phase I of the CAAA, wet FGD systems are being added to more than 16,000 MW of capacity.

Wet FGD systems also are widely used abroad, with installed global capacity (at the end of 1993) climbing to 164,000 MW and another 90,000 MW in the construction or design phase. Its widespread use around the world also has generated valuable operating data on high-technology innovations and cost-savings that are available to U.S. utilities. Furthermore, the majority of FGD systems in Europe are retrofit, and this experience has led to a new generation of advanced wet FGD designs that are even more reliable.

Removal Efficiency

Myth: Scrubbers do not achieve high enough SO2 removal efficiencies for today?s regulatory environment.

Reality: Scrubbers are very efficient air pollution control devices, and can remove greater than 95 percent of the SO2 from power plant stack emissions. This high efficiency allows scrubbers to trap from tens to hundreds of tons of SO2 per day at individual power plants, and (if desired) over-control SO2 emissions, thereby generating marketable emission allowances.

Operating data prove that scrubbers can reliably remove 95 percent or more of the SO2 from stack emissions. Agreement on this point is not limited to scrubber manufacturers, but extends to utilities, the EPA, the Department of Energy, and the U.S. Senate Committee on Environment and Public Works which concluded that scrubbers are OreliableO and Overy efficient.O

In fact, SO2 removal efficiencies often are as high as 98 percent to 99 percent. Scrubbers with advanced designs routinely meet targeted efficiencies of 95 percent. Many scrubbers currently retrofitted to comply with the Clean Air Act?s acid rain provisions will remove more SO2 than required, thus generating marketable emission allowances.

The use of recently developed additives, such as dibasic acid, formic acid and magnesium compounds improves SO2 removal efficiencies, especially for high-sulfur coal. Organic buffering agents, for example, allow removal efficiencies of 97 percent or more. Such high efficiencies allow utilities burning high sulfur coals to reduce SO2 emissions to levels well below the limits set by federal regulations.

While the previous information pertains to wet scrubbers, dry scrubbers also are quite efficient. Spray dryers often achieve greater than 90 percent SO2 removal on coals of 1 percent to 2 percent sulfur.

Cost

Myth: Scrubbers cost too much.

Reality: Scrubber costs have been greatly exaggerated. Numerous studies show that scrubbers are often one of the cheapest SO2 control options to install and operate, in either new or retrofit high-sulfur coal applications, particularly when revenue from byproducts and emission allowances is considered.

Technological advances, increased experience and flexible regulations have contributed to significant declines in the capital and operating costs of scrubbers relative to historic values. Innovations which characterize nearly all new wet FGD systems in the United States have reduced costs by more than 30 percent. These include installing larger (as well as fewer) absorber modules, eliminating flue gas reheat components, incorporating additives in the process design, fitting higher velocity absorbers and alternative duct work designs, installing absorbers in the base of a new chimney and reducing re-agent preparation costs.

Three caveats must be considered when analyzing true FGD costs. Since retrofit applications vary, it is impossible to specify a single cost figure. Site-specific factors such as space and access limitations, major modifications to existing equipment (e.g., ductwork and stack) and the operating condition of the units, all affect retrofit costs. Second, a market exists for SO2 reductions which allows a source to earn money from excess SO2 reductions. Third, the total scope of an FGD system installation (not just the FGD vendor?s scope) must be the point of comparison. As a result of these caveats, the cost figures to follow are for comparative and informational purposes only.

Typical examples of independent estimates of capital costs for wet FGD retrofits are $130-170/kW and $205/kW. Some rule-of-thumb capital cost estimates for FGD retrofits have centered on $150/kW, with one EPA estimate of capital costs as low as $118/kW.

More useful than these estimates, however, are the actual average capital costs (full-scope) of Phase I FGD retrofits. These have been reported as $317/kW, $270/kW, $216/kW, $204/kW, $160/kW and $123/kW for different units.

Average operating and maintenance costs for scrubbers in the United States, exclusive of capital recovery, are .142 cents/kWh (1.42 mills/kWh). With a capital cost assumption of $150/kW, total costs for FGD have been estimated at .52 cents/kWh (5.20 mills/kWh). FGD costs would therefore represent approximately 7 percent of current consumer electricity costs (typically, 8 cents/kWh) and add about $10 per ton of coal fired. (This increase in electrical rates is about one-half that associated with pre-1990 wet FGD.) If commercial grade gypsum is produced and sold, at $5/ton (dry), it would produce revenue and reduce disposal costs (typically $8/ton), reducing the annual operating cost in our example by .077 cents/kWh (.77 mills/kWh).

Estimates of the cost effectiveness of FGD have centered on $400/ton of SO2 removed, although actual costs have been half that value in some cases.

The development of a market of SO2 allowances?an allowance being the right to emit 1 ton of SO2?is further improving scrubber economics. Sales of excess allowances can help utilities reduce net scrubber operating costs significantly.

Furthermore, novel financing schemes have been devised which reduce or eliminate the capital costs borne by the utility. One example is the Obuild-own-operateO approach under which the FGD vendor designs, finances, builds, owns, maintains and operates the FGD system. Under any financing scheme, it usually makes sense financially to keep the vendor?s scope broad to avoid unnecessary engineering costs, obtain full-scope services (thus simplifying performance management and reducing such costs), obtain performance guarantees and obtain first-hand technical expertise.

Intense vendor competition has helped to lower scrubber costs, and Japan?s and Germany?s need for scrubbers has created a competitive international market with those countries? manufacturers vying for projects with U.S. manufacturers. In addition, many different scrubber systems are commercially available, and these processes offer a variety of economic profiles (waste product handling characteristics, removal efficiencies, energy penalties, complexity, site requirements and maintenance needs).

In particular, recent developments in this country and overseas show that dry scrubbers are attractive alternatives to wet scrubbers in certain situations (e.g., small to medium capacity boilers). Because of lower capital costs (at the expense of somewhat higher operating costs), dry scrubbers can make economic sense for low- to medium-sulfur coal-burning plants. Overall, the effect of this technical diversity is to give utilities the flexibility to select the most suitable and cost-effective system for each situation.

Energy consumption and global warming

Myth: Scrubbers contribute to global warming by reducing plant efficiency by 3 percent to 5 percent.

Reality: Modern scrubbers reduce plant efficiency by less than 1 percent.

The current generation of wet scrubbers (which use technological advancements in chimney design, construction materials, regenerative heaters and additives to enhance pollutant removal efficiencies) consumes less than 1 percent of total plant energy, and dry scrubbers consume even less. As a result, the total increase in CO2 emissions attributable to increased scrubber use in compliance with the Clean Air Act would likely be only about 1/10 of 1 percent of the total electric utility CO2 emissions forecast for that time period under a high emissions scenario.

Advances in design and technology have greatly improved a given scrubber?s energy efficiency. In fact, some novel scrubber designs employ heat exchangers which use waste heat from stack gases and actually increase power plant efficiency. Scrubbers with condensing heat exchangers can recover up to 4 percent additional energy, offsetting the scrubbers? parasitic power loss while reducing toxic metal emissions such as mercury.

Waste disposal

Myth: Scrubbers cause an unmanageable sludge disposal problem.

Reality: Scrubbers used to be routinely dismissed as simply turning an air pollution control problem into a solid waste problem because of the sludge they produce. The additional amount of scrubber system waste (which is not considered hazardous) is typically about the same or less than the ash produced by a coal-fired plant without a scrubber system. Scrubbers, according to the EPA, are highly efficient and, as a result, produce less waste than many alternative technologies. For example, the total waste volume from a gypsum-producing scrubber system would be only about 60 percent of that from a fluidized bed combustion system. Sludge waste can be mixed with fly ash to aid in fly ash disposal or recycled into marketable products such as gypsum.

Recent FGD projects in the United States and Canada have called for gypsum production, thus avoiding sludge disposal issues altogether. An individual FGD system can produce hundreds of tons per day of wallboard-quality gypsum. In fact, one Phase I wet FGD installation is producing enough gypsum in a year to supply 18,750 new homes with wallboard. In addition to building materials, gypsum can be used for a variety of civil engineering applications from road construction, landscaping and reef blocks to agricultural applications such as soil conditioners, nutritional sulfur and fertilizer absorption enhancers.

Most residues from dry scrubbers are composed of a mixture of fly ash and spent sorbent, although some systems remove the fly ash before the gases are cleaned to give separate residue streams. Part of the byproduct is recycled and mixed with fresh lime slurry to enhance sorbent utilization. These alternatives reduce or eliminate waste handling problems by producing a dry waste product which also have building material, civil engineering and agricultural applications. It too can be mixed with water and fly ash to produce a disposable fixed product.

Finally, there are a variety of FGD systems, each generating a different type of waste. Therefore, an FGD system user can select the system best suited to local, site-specific waste disposal requirements.

Other pollutants

Myth: Scrubbers remove only SO2.

Reality: Scrubbers are capable of removing significant quantities of particulate matter and chlorine gas. In fact, cost and benefit estimates for scrubber use have often failed to account sufficiently for their particulate removal abilities, which are especially significant in light of the intensified focus on hazardous air pollutants (many of which are particulates) provided by Title III of the CAAA. Indeed, many experts advise utilities that scrubbers are the best OinsuranceO against future air toxics rules. Switching to lower sulfur coal may increase emissions of several air toxics likely to be regulated in the near future. In response to a lawsuit, EPA recently agreed to complete a health affects study of utility air toxics emissions by November of this year, and if warranted, promulgate regulations limiting these emissions by November 15, 2000.

Unlike newer coal-fired power plants that incorporate highly efficient particulate control devices, some older units affected by acid rain legislation have less efficient particulate emission controls. Independent industry experts have concluded that retrofitted SO2 scrubbers can remove a high percentage of particulate matter from flue gas. While precise particulate removals depend on site-specific factors, the overall effect of installing scrubbers to comply with CAAA acid rain provisions would be to reduce utility particulate emissions, most of which are in the 10 micron size range. In addition, scrubbers can remove nearly all the hydrochloric acid (HCl) from stack emissions. This is important as some coals are high in chlorides.

The great potential for particulate emission reductions as a byproduct of scrubber retrofits is illustrated by German experience. Scrubber retrofits at 70 utility sites in Germany yielded an 82 percent reduction in power plant particulate emissions.

New developments

Myth: Scrubbers are an old technology.

Reality: In just the past few years numerous technical enhancements to scrubbers have occurred in such areas as reliability, cost, waste prevention and energy consumption. These enhancements allow users to exploit the Clean Air Act Amendments? market-based compliance options.

The air pollution control business is highly competitive. It is also international in scope and the latest technology enhancements flow easily across national borders. Increasingly, the U.S. government?s clean air policy is market-based, allowing regulated industries to pick the most cost-effective compliance options. At the same time, the utility industry?the scrubber vendors? traditional best customer?is itself undergoing tremendous change which, among other things, has made it more aggressive, competitive and cost-conscious.

To compete in the present business and regulatory climate, FGD systems must reflect the latest cost-effectiveness enhancements. As discussed previously, by the testimony of users, government agencies, independent experts and vendors alike, today?s FGD system designs must incorporate innovations which lower costs, increase reliability, increase SO2 removal efficiency, reduce power consumption, and avoid solid waste problems.

In addition, research and development, demonstration and full-scale testing are ongoing. Examples include a process demonstrated for cement kilns that may have important utility applications and an ammonia-based FGD process which, in a pilot-test, achieved 99+ percent SO2 removal efficiencies with less than 3 parts per million ammonia slip, an ammonium sulfate product purity of 99.5 percent, and expected annual levelized costs that will decrease if the sulfur content of coal increases. END

Editor?s note: Any questions or inquiries may be directed to Michael Wax or Jeffery Smith at ICAC headquarters 1707 L St. NW, Suite 570 Washington D.C., 20036 (202-457-0911).

ICAC members

The Institute of Clean Air Companies (ICAC) is a national association of companies that supply stationary source air pollution monitoring and control systems, equipment and services. It was formed in 1960 (under the name IGCI) as a nonprofit corporation to promote the industry and encourage improvement of engineering and technical standards.

The Institute?s mission is to assure a strong and workable air quality policy that promotes public health, environmental quality and industrial progress. As the primary representative of the air pollution control industry, the Institute seeks to evaluate and respond to regulatory initiatives and establish technical standards to the benefit of all citizens.

Click here to enlarge image

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Scrubber basics

All scrubber systems rely on a chemical reaction with a sorbent to remove SO2 from flue gases. According to the most common classification, scrubber systems are either OwetO or Odry.O In the more-common wet FGD process, flue gases containing SO2 are contacted with a sorbent liquid. Uptake of SO2 by the sorbent results in the formation of a wet solid byproduct. In a dry FGD process, particles of an alkaline sorbent are injected into a SO2-containing flue gas, producing a dry solid byproduct.

Regenerable and non-regenerable systems

Scrubbers also can be classified as regenerable and non-regenerable based on the way the sorbent is treated after it has taken up SO2. In non-regenerable systems, the SO2 is permanently bound by the alkaline sorbent and must be disposed of as a waste or sold as a byproduct, e.g., gypsum. In regenerable systems the SO2 is released in a regeneration process following the sorption process, and the regenerated sorbent is recycled to the SO2 scrubbing step. Recovered SO2 from the regeneration process may be further processed to obtain sulfuric acid, elemental sulfur or liquid SO2 The majority of FGD systems in operation today are non-regenerable, and since the mid-1980s, most FGD systems installed worldwide produce gypsum.

Wet FGD

In a wet scrubber, a liquid sorbent is sprayed into the flue gas in an absorber vessel. Most wet FGD systems (Figure 1) use alkaline slurries of limestone or slaked lime as sorbents. Sulfur oxides react with the sorbent to form calcium sulfite and calcium sulfate. Oxidation results in a gypsum byproduct that can be sold.

Dry FGD

Dry scrubber systems can be grouped into three categories: spray dryers, circulating spray dryers and dry injection systems. All of these efficiently combine SO2 and particulate control, avoid total water saturation of the flue gas, and provide a dry, free-flowing waste product. (As noted previously, the elimination of any liquid waste is the key characteristic differentiating dry scrubbers from wet scrubbers.)

In a spray dryer, a slurry of alkaline reagent, typically lime, is atomized into a cloud of very fine droplets, which mix intimately with the hot flue gas. The heat of the gas evaporates the moisture from the slurry droplets, while the alkaline reagent simultaneously absorbs acid gases (primarily SO2) from the flue gas. The resulting dry material, along with flyash, is collected in a downstream particulate control device, typically an electrostatic precipitator or fabric filter. In some cases, a portion of the dry material is recycled into the lime slurry mixture. Figure 2 shows a lime spray dryer that is typically installed on industrial and utility boilers, as well as on municipal and hazardous waste incinerators.

A circulating dry scrubber uses an entrained fluidized bed reactor for contacting the reagent, usually hydrated lime, with sulfur dioxide- and particulate-laden flue gas (Figure 3). The intensive gas-solid mixing which occurs in the reactor promotes the reaction of sulfur oxides (SO) in the flue gas with the dry lime particles. The mixture of reaction products (calcium sulfite/sulfate), unreacted lime and flyash is carried to a downstream particulate collector, in which it is separated from the gas stream. Part of the dry waste product is removed for disposal, but a majority is retained to be mixed with fresh calcium hydroxide for use in the reactor. Water spray is introduced into the fluidized bed separately to enhance performance (for maximum SO2 capture with minimum lime utilization) by optimizing the surface moisture content of the lime.

Dry injection systems involve the injection of a dry sorbent (normally lime or limestone) into the flue gas in the upper reaches of the boiler, or in the ductwork following the boiler (Figure 4). SOs react directly with the dry sorbent, which again is collected in a downstream particulate control device. Because a separate scrubber vessel is not needed, capital costs are minimized. Low capital costs are somewhat offset by lower reagent utilization, which result in higher operating costs for equivalent SO2 removal rates. Dry injection systems are generally applied when lower removal efficiencies are required, or on small plants where the capital cost for other scrubber types may not be justified.

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