An ESP retrofit at a 1,400 MW coal-fired unit. Photo courtesy Southern Environmental Inc.

By Lindsay Morris, Associate Editor

Current regulations for controlling particulate matter (PM) have resulted from decades of research and have transformed over the years as methods of gathering data have improved. The first standards for PM were established by the U.S. Environmental Protection Agency (EPA) in 1971 under the Clean Air Act. However, the standards have become tighter over the years due to research on particulate size and its effects on health. In a time of regulatory reconstruction, an understanding of which technology to install in order to meet EPA's requirements is essential.

Regulations in the Mix

Most recently, the EPA's Maximum Achievable Control Technology (MACT) rules, which were released Feb. 23, introduced additional, more stringent requirements to new and existing boiler PM and other emission limits and requirements. Utility MACT specifically limits Hazardous Air Pollutants (HAPs).

Mitsubishi's High Efficiency System, installed at Chugoku Electric's 1000 MW Misumi plant in Japan, can reduce flue gas temperature and PM emissions with a heat extractor upstream of a dry ESP. Photo courtesy Mitsubishi Heavy Industries America, Inc.

"You wouldn't think of PM being a driver initially for Utility MACT, but it is," said Andrew Byers, associate vice president of Black & Veatch Environmental Services. That's because PM is not a type of HAP, but it is a surrogate pollutant for controlling HAPs. The surrogate comes into play for the 10 metallic HAPs the EPA has identified to control under Utility MACT. "If a power producer reaches a certain level of PM, that gives EPA confidence that they are also achieving control for all metallic HAPS," Byers said.

Todd Palmer, an environmental lawyer with Michael Best & Friedrich LLP, said the new PM emission standards contained in the Utility MACT rules are "difficult to meet because they are set so low, especially for new coal and biomass units." The standards' aggressiveness makes it difficult to obtain meaningful emission performance guarantees from vendors of new coal and biomass units, Palmer said. Some of his clients have been forced to change the design of new facilities to eliminate biomass as a fuel in favor of natural gas. The standards for existing coal and biomass sources, however, are more reasonable, he said.

Another regulatory program aimed at controlling PM emissions stems from the National Ambient Air Quality Standards (NAAQS). NAAQS define the concentration of a pollutant in ambient air that EPA deems to be protective of human health and the environment. Stationary sources of PM are regulated to ensure their emissions do not exceed these NAAQS (in EPA jargon the phrase is "cause an exceedance"). Over the last four decades, the PM NAAQS have become more stringent in terms of lower numeric standards, reduced averaging times and regulations imposed on smaller particle sizes. This, in turn, has required more stringent limitations on stationary sources of PM emissions.

PM Through the Years

In 1971, EPA promulgated the original PM NAAQS which focused on larger airborne PM referred to as "total suspended particulate." During the 1970s and early 1980s, research indicated that the severity of adverse health impacts due to PM emissions depended in part on particulate size. This is due to the ability of smaller particles to more effectively penetrate through a human being's upper respiratory tract. The relationship between particle size and human health effects is also due to the variance in chemical composition of small particles and their ability to remain airborne for long periods of time.

Based on this research, EPA revised the PM NAAQS in 1987 to regulate particles smaller than 10 micrometers in diameter (PM10). In 1997, EPA revised the NAAQS to also regulate particles smaller than 2.5 microns in diameter (PM2.5). In 2006, EPA reduced the PM2.5 NAAQS from the 1997 level of 65 µg/m3 to 35 µg/m3. At this point, the PM NAAQS are so low that stationary sources may require installation of more efficient baghouses or control the PM that forms as gases condense after emission from a stack, Palmer said.

Many parts of the United States have current ambient air quality that does not meet the new PM2.5 NAAQS. To address this situation, EPA promulgated the Clean Air Transport Rule (CATR) to reduce the level of PM2.5 pollution across large regions.

"PM2.5 is a rather unique pollutant in the sense that it can be emitted directly from a source, but can also be formed miles away when pollutants like SO₂ and NO_x chemically interact in the atmosphere," Palmer said. EPA created the CATR to force utilities to limit their PM2.5 precursor emissions (that is, SO₂ and NO_x) to reduce the downwind secondary formation of PM2.5. This is designed to help many parts of the country, primarily in the eastern U.S., achieve air quality that meets the NAAQS.

State Approaches to PM

While a host of federal regulations affect PM control, some states impose additional requirements. "The federal government creates minimal emission standards under the Clean Air Act that all states must adopt, but states can choose to be more restrictive," Palmer said. Many states regulate the opacity of stack emissions based on the assumption that visible emissions are an indicator of PM.

For example, a court recently interpreted the Wisconsin Prevention of Significant Deterioration (PSD) program as regulating opacity. "EPA has not considered opacity to be a pollutant subject to the burdensome requirements of the PSD (prevention of significant deterioration) program, nonetheless Wisconsin does," Palmer said. Thus, larger PM emitters in the state, such as utilities and industrial boilers, must undergo case-by-case opacity emission control analyses when building new sources or modifying existing units. These sources must use Best Available Control Technology (BACT) to control PM and PM precursor pollutants that might affect opacity from the stack.

Byers said that of all the current and proposed regulations driving PM control installations, Utility MACT is the leader, as many power generators will choose to install PM controls to meet many, if not all, of the proposed rule's emissions limits.

Control Technology

Two main drivers influence the amount of PM emitted from coal-fired boilers in power generation: the ash content in the coal and the type of boiler used, said Steve Francis, product engineering manager with Alstom Power's Environmental Control Systems. Conventional tangential-fired or wall-fired pulverized coal (PC) firing tend to emit higher PM levels than cyclone-fired boilers, but the ash from cyclone-fired boilers is finer and more difficult to collect. Adding limestone to a circulating fluid bed boiler can lower SO₂ emissions but increase the PM levels exiting the boiler. As far as control technologies go, the two main offerings targeting PM are electrostatic precipitators (ESP) and fabric filters, or baghouses.

An ESP uses high-voltage fields to apply electrical charges to particles moving through the field. This causes the charged particles to move toward an oppositely charged collection surface where they accumulate. ESPs are available in both dry and wet options. Forms of ESPs have been around since the early 1900s, operating on the four basic principles of charging, collecting, removing, and disposing of particles, said Francis.

Most ESPs are dry, but special circumstances can require wet ESP installations. More than 70 percent of existing coal-fired power plants have installed ESPs, according to a report by Environmental Health and Engineering. According to the EPA, an ESP can capture more than 99 percent of total PM and 80 to 95 percent of PM2.5. However, Francis said the latest ESP systems are able to remove up to 99.96 percent of all PM. Different coal compositions are the main determinant of how well the ESP is able to charge and collect particles.

"Some coal compositions create an ash compound that is difficult to charge," Francis said. Fortunately, the U.S. is home to coals that don't have as high of an ash content as coal fired in, say, India and other parts of the world where Alstom installs also technology.

Ghassem Manavi, assistant general manager of the Environmental Systems Division of Mitsubishi Heavy Industries (MHI), said conventional dry ESP is the most cost-effective method of PM control and allows for easy maintenance. A power producer may also consider installing a heat extractor with a dry ESP, which would allow plant heat efficiency to be increased, thereby recovering lost power. A heat extractor enhances PM removal performance of a particulate control device by lowering the temperature to condense SO3 on fly ash, Manavi said.

In addition to having some resistance to certain coal compositions, ESPs are not as effective at collecting fine particulate matter, that is, PM2.5. For this reason, Francis said industry in recent years has moved toward the compliance alternative, fabric filters (or baghouses).

Fabric filters can drive down PM levels more than dry ESP because the technology is not as affected by particulate characteristics. "Fabric filtration is able to handle coals of any resistivity," Francis said. Fabric filters are also "as efficient on small particle collection as they are to large particles."

However, fabric filters have a higher pressure drop than ESPs, which may result in a higher operational cost. Fabric filters also have a higher maintenance cost due to the filter media, which must be changed periodically.

"A fabric filter typically has a lower capital cost than an ESP, but a higher maintenance cost," Francis said. "You trade initial capital investment cost for maintenance costs."

A utility ESP in the Midwest undergoes an upgrade by Southern Environmental Inc. Photo courtesy Southern Environmental Inc.

A fabric filter collects PM on the surfaces of fabric bags. Most of the particles are captured on already collected particles that have formed a dust layer. The fabric material itself can capture particles that have penetrated the dust layers. According to EPA, a fabric filter on a coal-fired power plant can capture up to 99.9 percent of total particulate emissions and 99.0 to 99.8 percent of PM2.5. Thirty-five percent of coal-fired power plants in the U.S. have already installed fabric filters, according to Environmental Health and Engineering.

Emissions regulations in the proposed stages, like Utility MACT, have turned operators more toward fabric filter installations, said Phil Rader, business sales manager for Alstom Power Environmental Control Systems. "The emissions standards being imposed by Utility MACT are such that they require very fine particulate control at very high efficiencies." Because of this, Rader said, the market is headed "to the fabric filter arena."

Mick Chambers, director of precontract

operations for Southern Environmental, said each technology will "continue to have its place" even after the Utility MACT ruling. "What we've seen with the most recent release of the Utility MACT ruling earlier this spring is that the regulations have come out softer than the original release. That's opened the market up to a few more ESP solutions."

Ultimately, Chambers said, the manufacturers of both technologies, as well as end users, must conduct continued due diligence in order to produce the optimum solution. "Neither one technology nor the other is the panacea." Chambers said that once the Utility MACT Q&A period is over in November, there will likely be another "adjustment" to the regulations that may relax them further. "With that date in mind, it appears that 2012 is going to be a big year for environmental upgrade activity."

While most power generators usually choose one technology or the other, some power plant scenarios call for both. One example is an existing power plant that has been operating for many years. Here, the plant may have an ESP installed, in which the ash can be collected and sold as a form of revenue. The plant may also choose to install a polishing fabric filter to collect any excess PM emissions that were not captured through the ESP. The filter would also be a means to collect mercury emissions to meet the new Utility MACT standards, Francis said. Many plants are currently using this method of injecting sorbents into the gas stream to absorb the mercury onto the sorbent material for disposal at a classified landfill, he said.

A wet ESP can help not only with PM reduction but also removal of SO3 mist downstream of a wet FGD scrubber. If dry ESP and wet ESP are combined, extremely low PM levels can also be achieved. Francis said a plant with an existing dry ESP will occasionally install selective catalytic reduction (SCR) for NO_x control as well as flue gas desulfurization for SO₂ control. In this case, visible emissions from the plant could go up as a result of the additional conversion of SO₂ to SO3 in the SCR. In a few cases, wet ESP has to be installed in addition to wet FGD to collect the fine particulate and SO3 that may have escaped. Francis said wet ESPs have been used for years in other industries, but are more of a niche product in the power industry.

Additional Control Methods

Though not as commonly used for PM control as ESPs or fabric filters, wet scrubbers can offer removal efficiencies greater than 90 percent for PM10, according to a recent NESCAUM report. However, efficiencies for smaller particles are typically significantly lower.

Venturi scrubbers are the most commonly used type of scrubber for particulate collection. These devices send scrubbing liquid and flue gases accelerating through a converging section of duct into a narrow throat, then passing through the throat into a diverging section. In the throat, high gas velocity separates the scrubbing liquid into a cloud of fine droplets, which collect particles. Increased pressure drop across the venturi leads to better droplet formation and particle collection.

Another lesser-used form of PM control is the cyclone. Cyclones use centrifugal force to separate particulate from gas streams. A multiple cyclone (an array of a large number of small cyclones in parallel) boast overall mass removal efficiencies of 70 to 90 percent. However, cyclone collection efficiencies fall off rapidly with particle size, so the control of fine particulate (PM2.5) is limited. Typically, cyclone removal efficiencies will be 90 percent or greater for PM10, dropping about 70 percent for PM2.5, according to NESCAUM.

Multiple cyclones have no moving parts, but do require regular cleaning to avoid plugging, and preventive maintenance to avoid leaks which would disrupt flow patterns and thus lower collection efficiency.

Technology Specifics

Mitsubishi Heavy Industries offers a variety of ESPs and fabric filters and also manufactures a proprietary high efficiency system that can reduce flue gas temperature and PM emissions with a heat extractor upstream of a dry ESP. It has the capability of gaining more power (equivalent to approximately 5 MW for a 1,000 MW power plant) from the waste heat. The system also has high SO3 and mercury removal capabilities.

Alstom's newest emissions capture device, known as the NID, integrates an FGD into a fabric filter. "We see this product as being one of our dominant products to address the Clean Air Transport Rule and Utility MACT," Rader said.

One advantage of the NID is its ability to fit into a small area, allowing for the plant to explore future carbon capture and sequestration (CCS) possibilities. "We are trying to consume less plant area with our traditional pollution control systems, leaving more space for CCS," Francis said.

Southern Environmental has traditionally been an ESP company, enjoying "phenomenal growth" over the last 10 to 15 years, Chambers said. "We've gone from a $5 million company to bookings in excess of $100 million in one year." Southern Environmental attributes its success to several technology advances that they have developed in ESPs. In addition, the company established a long-term teaming agreement with FL Smidth in the fall of 2010 to develop turn-key fabric filter systems, turning its business into a multi-PM solutions enterprise.

While some businesses are already booming with PM control technology sales, Wes McKenzie, director of fabric filter technology for Southern Environmental, said that this is still a season where power generators are studying what is happening with regulations. "Generators have been waiting for the new regulations to be promulgated before moving forward." For this reason, he predicts that there will be a rush for PM equipment once regulations are finalized – both from the manufacturing and the installation side.

When Utility MACT with its expected large impact for PM control is finalized, power providers will likely begin buying more control equipment. For now, manufacturers continue to produce ESPs and fabric filters that provide lower PM emissions levels than ever before in preparation for the flood of finalized EPA regulations.

Diesel Generators and PM emissions

Under Tier 4 Interim(i) regulations, EPA requires that non-road diesel engines undergo a 90 percent reduction in diesel particulate matter (PM) and a 50 percent reduction in nitrogen oxide (NO_x) from Tier 3 regulations. Applications that will require Tier 4 certified generator sets in 2011 are non-emergency standby units, prime power applications, load management/peak shaving and electric power rental units. Exempt from Tier 4i regulations are stationary emergency units, which are permitted to remain at Tier 3 or Tier 2 levels.

Technologies equipped to reduce genset emissions are either internal or external to the diesel engine. Internal methods affect or modify combustion parameters. Internal methods include electronic engine controls, injection systems, adapting the physics of the combustion chamber, turbocharging and exhaust gas recirculation (EGR). Electronic controls are responsible for fuel quantity regulation, injection timing, turbocharger boost pressure and load, temperature and barometric pressure compensation.

Slowing the injection timing in conjunction with increasing the injection pressure is effective in reducing NO_x without significantly increasing PM or hydrocarbon (HC) production. Higher injection pressures also ameliorate fuel atomization and combustion chamber penetration, improving fuel economy while reducing PM. Turbocharging is used on most medium and large gensets to reduce emissions, increase power, and improve combustion efficiency. A turbocharger increases power output by boosting charge air density.

EGR operates by reducing NO_x through recycling part of the inert exhaust gases with incoming engine air, thus lowering combustion temperatures. Unfortunately, EGR also reduces power output and fuel efficiency and increases PM. These effects can be compensated for by improving optimizing and turbocharging technology.

There are many types of control technologies available to control diesel particulate matter from portable diesel engines, such as diesel oxidation catalysts, diesel particulate filters, fuel additives and alternative diesel fuels.

John Deere Power Systems released a product for units from 138 bkW to 224 bkW. The Tier 4i compliant unit is a build off of the company's Stage III A engine platform, which includes cooled exhaust gas recirculation (EGR) for NO_x control. The Tier 4i version has added an exhaust filter, resulting in a total reduction of PM by 90 percent from previous Tier 3 levels. The engine feature full-authority electronic controls, a 4-valve cylinder head, a high-pressure fuel system, single variable geometry or series turbocharging and an air-to-air aftercooling system.

John Deere elected not to use SCR in this unit, instead relying on the cooled EGR and exhaust filter approach. Brian Brown, manager of worldwide marketing support, said the single-fluid approach of cooled EGR means owners and operators won't have to incur the cost of diesel fuel plus the additional cost for diesel exhaust fluid required by SCR systems.

"For Interim Tier 4/Stage III B, we're not only looking at fuel economy; we're taking into consideration total fluid consumption," said Brown.—LM

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