Controlling PM

Today’s emission control devices are capable of keeping particulate matter emissions at reasonable levels and in line with regulations, but the best option may vary with each project.
By Anna Austin | March 22, 2011

Biomass power plant project opponents commonly claim that particulate matter (PM) emissions cause lung ailments and other health detriments and use that  information to block the building of new facilities in their communities. What the developers of these plants, and authorities who permit them know, however, is that there are strict PM and other emissions limitations in place, and a number of options to help achieve these limits.

Another common misconception of project opponents is that emissions regulation enforcement is passive, when it is quite the opposite. The U.S. EPA and state air pollution control districts closely monitor all power plants to make sure that they are in compliance with these laws, in order to protect public health and preserve air quality. In February, two 15.5-megawatt wood-fired biomass power plants in California’s San Joaquin Valley were fined a combined total of nearly $850,000 for failing to comply with PM and other emission regulations on both federal and district levels. In addition to the fines, the plants had to immediately install new equipment, and will be closely monitored for the next two years while they complete several other compliance orders.


PM regulations currently in place are the result of decades of research and have changed over the years as methods of gathering relevant data have become more advanced. The first standards for PM were established by the EPA in 1971 under the Clean Air Act, and then were significantly revised in 1987 when the EPA changed the indicator of the standards to regulate inhalable particles smaller than or equal to 10 micrometers in diameter (PM10), or one-fourth of the size of a grain of table salt. Ten years later, they were changed again, setting separate standards for PM10 and fine particles (PM2.5) due to links with health problems and smaller microparticles. Twenty-four hour and annual standards were established for PM2.5, and PM10 has only a 24-hour standard due to insufficient evidence linking health problems to long-term exposure.

Most recently, the EPA’s Maximum Achievable Control Technology rules, which were released Feb. 23, made additional, tighter requirements to new and existing biomass boiler PM and other emission limits and requirements. (To learn more about the changes, see “MACT Madness” on page 30.)
So what might influence a biomass plant’s PM emissions and the type of control equipment that’s required? According to Paul Beauchemin, director and partner at Envirochem Services Inc., that depends on a number of factors.

PM Parameters and Equipment
The chemical composition of the feedstock, physical and operational parameters, the combustion process and the type of air pollution control equipment used all affect PM emissions, Beauchemin says. For example, fuels high in ash, such as agricultural biomass, will require more costly PM controls than low-ash fuels.

 The combustion of wood, which is commonly used for power generation, produces different types of particles, according to Andy Miller, EPA national program director of global change research. These include small pieces of unburned fuel, which tend to be the largest particles. “These are fairly rare in industrial systems, especially in power generation plants, and can easily be captured by electrostatic precipitators (ESPs), cyclones and fabric filters,” he says.

 There are also particles composed of the inorganic ash in wood, and these tend to be much smaller, on the order of a micrometer in diameter. “These are similar to the ash particles from coal plants, but do not have as much heavy metal as is the case for coal,” Miller says. “These are well-controlled using both fabric filters and ESPs.” 

The last type of particle is composed of condensed liquid or gas-phase organic matter that cools and forms small droplets. “These are also about a micrometer in diameter, but sometimes they are still in the gas phase when they get to the control system,” Miller says. “This material often condenses on the ESP or the filter, and is usually combustible, so that adds some operational difficulties to the system. There are ways around this problem, but the designers need to be aware of the issue, which they are for large systems like power generation plants.”

In general, Miller says the most effective PM controls are fabric filters, with cyclones being much less effective and electrostatic precipitators (ESPs) falling in between, but much closer to fabric filters than to cyclones. For larger wood-fired plants, Beauchemin says he believes the best control option is ESPs. “For other fuels, material balances on the other chemicals in the fuel such as chlorine and sulfur may determine that some form of scrubbing or other mitigation process is required.”

The final control level selected may depend on the evaluation of local and long-range impacts through the use of air shed analyses and dispersion modeling, Beauchemin adds. “There are now integrated controls appearing on the market that can manage PM, nitrous oxides (NOx), sulfur oxides and other gases in a single system,” he says. “Larger scale units typically have better controls than smaller units, usually because they can afford to, or are required by permit to have them. For example, small-scale industrial facilities are not currently fitted with ESPs, NOx reduction systems or air preheaters, all of which can affect the emissions.”

Carlos Garcia, senior air quality engineer at the San Joaquin Valley Air Pollution Control District in California says the best controls depend a lot on the specifics of the particular installation. “[The best controls] for new biomass plants are likely to be a series of control techniques, such as a set of cyclones or a multiclone to take out larger particles, followed by a scrubber, followed by a baghouse,” Garcia says.

All eight biomass plants in the San Joaquin Valley were equipped with baghouses or ESPs when they were constructed. “These controls, which have similar abilities to control particulate emissions by 90-plus percent, were considered best available control technology (BACT) when constructed, and are still generally thought of as very good control technologies,” Garcia says.

District rules require new biomass plants to control emissions with BACT, so their choices are limited, Garcia points out. “BACT is defined as the most effective control that has been achieved in practice anywhere in the world, at minimum. Also, anything that is more effective must be considered, and it will be required if it is also cost-effective.”

Garcia points out that the process of determining BACT is project-specific, and is quite complex and time consuming. “It would probably result in requiring the installation of something like the series of controls [cyclones-scrubber-baghouse],” he says.

Easing Fears of PMs

As those in the biomass power industry know, air quality permitting regulations are designed to prevent health problems from industrial facilities, including biomass plants, Garcia says. “We can’t speak for the rest of the country, but PM emissions from biomass plants in the San Joaquin Valley are well-controlled. All have either baghouses or ESPs, and newly proposed plants would be expected to be even better controlled as technology has improved since the existing plants were permitted.”

As for fears about PM in siting new plants, Garcia says it shouldn’t be an issue, at least not in the San Joaquin Valley. “Our permitting process would not allow us to issue a permit to a facility that is expected to cause a significant health risk to the public,” he says. “Of course, no one wants one in their backyard, just like they don’t want any industry in their backyard.”

To assure compliance, biomass plants in the San Joaquin Valley are equipped with continuous emissions monitors that record emissions of some pollutants (although generally not PM) on an ongoing basis, which can be accessed from the district offices. The plants are also required to hire a third party to test the pollution being emitted from their facilities, Garcia adds, including particulate matter emissions, and report that information to the district. “Permits also contain conditions of operation that are expected to assure compliance with permitted emission limits, and we regularly inspect these facilities, as we do all permitted facilities, to ascertain compliance with those permit conditions.”

When the district finds sources operating out of compliance, it issues Notices of Violation and requires them to come back into compliance. These facilities can be charged significant penalties for their period of noncompliance—the longer the periods of noncompliance, the higher the penalties, Garcia says.

He points out that some biomass plants operate for years without significant violations, and some have larger problems, such as the two recently fined in the San Joaquin Valley.

Miller says, generally speaking, biomass PMs don't pose any greater risk than coal emissions, but there will be differences between coal and wood, in that coal has more heavy metals and wood has more polycyclic aromatic hydrocarbons on a per unit energy input basis (pounds of emissions per million Btu of fuel input). “My view is that the public should be aware of these issues, and that all wood units should have effective controls,” he says. “It’s easy to look at small residential, commercial and industrial wood combustion units and think that a power plant will be the same, and will generate more [emissions] because it is bigger. However, it is always more cost-effective to have effective controls on larger units, simply because the cost of the control is a much smaller fraction of the total plant cost, and because large plants will have enough technical capability and people to make sure the systems are operating well.”

Pollution control equipment manufacturers such as Eisenmann Corp., which offers a wet ESP that provides PM, acid gas and sodium oxide (SOX) removal, are confident their equipment can meet boiler MACT emissions limits. “The developers who feel [that EPA's emissions limits] are too harsh will convert their boilers to fossil fuels, while others will put in the appropriate emissions controls to meet compliance and probably modify their feedstocks to meet the [return on investment] they are looking for,” says Bradley Ginger of Eisenmann. “We'll most likely see new projects that will be using different feedstocks. Either way, the market will adapt so the economics will work out.”

Some projects use separate systems to handle each pollutant, including PM, but that isn’t ideal, Ginger says. “Having multipollutant controls in place will give the developer the flexibility needed to stay in compliance and stay profitable.”

Author: Anna Austin
Associate Editor, Biomass Power & Thermal
(701) 738-4968