Some coal plants achieve lower emissions rates than are allowed in their permits. For instance, the Mount Tom coal plant in Holyoke, MA, has turned in stack test data that indicate the plant’s SO2 emission rate is 0.066 lb/mmbtu, lower than any rate reported in the BACT clearinghouse database. The data also indicate a total PM10 emission rate of 0.0059 lb/mmbtu, meaning that yearly emissions of PM10 from this 147 MW facility are around 39 tons per year if the facility operates continuously. In contrast, the 35 MW Palmer Renewable Energy biomass plant to be built in nearby Springfield, MA, will have a total PM10 emission rate of 0.019 lb/mmbtu and will emit 42.4 tons of PM10 per year.
Does co-firing biomass reduce emissions?
Co-firing biomass is sometimes seen as a way to reduce emissions at coal plants, particularly of sulfur dioxide, because wood generally has a lower sulfur content than coal (an exception is construction and demolition debris, which can have relatively high sulfur content probably due in some part to contamination by gypsum wallboard waste).
While replacing a significant amount of coal with wood would reduce sulfur emissions, the effect on other pollutants is not straightforward. Test-firing data from the Killen coal plant in Ohio, which proposes to co-fire 5% wood by heat content (amounting to over 8% by mass) shows that adding even this small amount of wood to the fuel stream produces dramatic increases in emissions of carbon monoxide and hazardous air pollutants.
The following data are from the Killen plant’s technical support document for the air permit application to co-fire biomass. Metals emissions show some small increases and decreases with addition of biomass, and there are small percentage decreases in emissions of hydrochloric acid and hydrofluoric acid, but the key thing to notice is that replacing 5% of the boiler’s heat input with biomass produces no improvement in criteria pollutant emissions with the exception of a small decrease in lead emissions. It does, however produce large percentage increases in carbon monoxide, volatile organic compounds (VOCs), and certain organic hazardous air pollutants (HAPs).
“PTE” means “potential to emit”, which standardizes emissions by assuming the boiler operates at full capacity 365 days of the year. All emissions are expressed as tons per year (“tpy”).
The Killen plant will co-fire about 180,000 tons of wood per year, or the equivalent of the wood that could be produced by clearcutting about 2,050 acres of Ohio’s forests per year (we use U.S. Forest Service data to estimate standing biomass). Increasing the amount of wood co-fired to a level that would actually decrease sulfur emissions would require significantly increasing forest harvesting, and would meanwhile have unacceptably large effects on emissions of other pollutants. Co-firing biomass should not be allowed as a way for coal plants to avoid installing modern pollution control equipment, which is vastly more effective at reducing sulfur emissions than changes in fuel.
Biomass and the Clean Air Act
Large-scale biomass burners are subject to Clean Air Act regulation and permitting like other emitters, with one crucial difference. The Clean Air Act requires “Prevention of Significant Deterioration” (PSD) review for air quality when a new “major source” facility is built or when an existing facility undergoes major modification in an area that is either classed as in attainment, or unclassifiable, with regard to the National Ambient Air Quality Standards (NAAQS). PSD review requires installation of the Best Available Control Technology (BACT, a standard that takes into consideration the cost of air pollution controls, and allows rejection of some controls as too expensive), air quality analysis, an additional impacts analysis, and public involvement. However, while the threshold for triggering PSD review for a fossil-fueled boiler 250 mmbtu/hr or greater is emission of 100 tons of a criteria pollutant, the threshold for triggering PSD review for a biomass burning facility is 250 tons. This means that a facility burning biomass can emit two and a half times the amount of pollution that a fossil-fueled facility emits before it is subjected to a similar level of scrutiny and emissions control.
Biomass, Hazardous Air Pollutants, and EPA’s “boiler rule”
To reduce the amount of hazardous air pollutants emitted by commercial and industrial boilers, EPA sets limits for certain pollutants under the “boiler rule”, which is part of the Clean Air Act. The boiler rule is especially significant for biomass burners because although the boiler rule regulates fossil-fueled boilers only up to 25 MW in size, it regulates all biomass boilers, no matter how large.
The boiler rule sets the maximum allowable emission limits for just five pollutants, although there are 187 different HAPs recognized by EPA. Setting emissions limits for each one would be difficult, so instead, EPA regulates the majority of pollutants by dividing them into classes and using proxies to estimate how well their emissions are controlled. Under the boiler rule, particulate matter (PM) serves as a proxy for metallic HAPs like arsenic and lead, and carbon monoxide (CO) is regulated as a proxy for organic HAPs like formaldehyde and acrolein. The boiler rule regulates dioxins/furans, mercury, and hydrochloric acid (HCl) directly, with HCl also acting as a proxy for other acid gases (like hydrogen fluoride).
To control emissions of these pollutants, EPA sets maximum emissions rates for different kinds of boilers and different fuels. These emission rates are based on the “best performing” (lowest emitting) units that already exist. To set emissions rates for existing boilers, EPA essentially averages the best performing 12% of boilers, using their emission rates as the standard that all existing units are supposed to achieve. For new as-yet-unbuilt boilers, EPA expects those to achieve the same emission rate as the single best performer (lowest emitter) that already exists. These standards are referred to as Maximum Achievable Control Technology (MACT), and are set for coal, biomass, gas, and other liquid fuels boilers.
EPA further identifies two categories of emission sources under the boiler rule – “major” and “area” sources. A major source is one which emits 25 tons or more of total HAPs, and 10 tons or more of any single HAP (for biomass, the HAP usually emitted in the greatest quantities is hydrochloric acid, or HCl). An “area” source is one which emits HAPs below these thresholds.
Since the emission rules for major sources are considerably more stringent than for area sources, there is a great incentive for facilities to characterize themselves as area sources. Below is a summary of the allowable emission rates for major and area sources. Emission rates for biomass under the major source rule are the same as for coal, except for emission rates of carbon monoxide and dioxins/furans, which are generally higher than for coal.
Boiler rule emission rates for major sources.
Boiler rule emission rates for “area” sources.
From the range of heat input values in the above table, we can see that EPA had in mind regulating relatively small boilers with the area source rule. A 10 mmbtu/hr boiler is representative of the size being installed in some school districts, as it is capable of heating more than one building. A 30 mmbtu boiler is the size used by a small sawmill. However, many of the extremely large boilers being built currently are classified as “area” sources. For instance, the Gainesville Renewable Energy Center, a 116 MW (gross; 100 MW net) biomass burner with a 1,359 mmbtu boiler, is considered to be an “area” source, because the project claims it will emit less than 25 tons of hazardous air pollutants (the exact value in the permit is 24.7 tons of HAPs). This massive project, which will burn well over a million tons of wood a year, will therefore not be held to the stricter standards of the “major” source boiler rule.
An important thing to notice about the area source rule is that while EPA set limits for mercury and carbon monoxide for existing and new coal boilers, to which it adds the regulation of PM for new coal boilers, new biomass boilers are only required to meet PM standards, and there are no standards at all for existing biomass boilers.
Information on air pollutants
Nitrogen dioxide (NO2) is the indicator species for the NOx group of gases, which includes nitrous acid and nitric acid. It primarily forms when fuels are burned at high temperatures. These acidic gases directly impact respiratory health, and also contribute to formation of ozone and condensable particulate matter. Nationwide, the majority of NO2 is from the transportation sector, but utilities and other sources of combustion account for about 34% of total emissions.
As of January 2010, EPA set a new 1-hour standard for NO2 of 100 ppb in ambient air, and retained the annual average pollution standard of 53 ppb.
A principle component of smog, ground level ozone is formed when nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide (CO), and methane react, energized by UV light. The main sources of NOx and VOCs are industrial facilities, electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents. As a highly reactive oxidant gas, ozone aggravates the airways, causing respiratory distress and exacerbating asthma. It also damages vegetation and is increasingly recognized as a threat to forest health.
EPA has proposed revising its eight-hour standard for ozone from 0.075 ppm to 0.06 – 0.07 ppm, acknowledging that the ozone standards set in 2008 were not as protective as recommended by EPA’s panel of science advisors, the Clean Air Scientific Advisory Committee (CASAC). EPA has also proposed a new “seasonal secondary standard” for ozone exposure that represents cumulative exposure during peak ozone season.
Sulfur dioxide (SO2)
Sulfur dioxide (SO2) exposure causes breathing difficulty for people with asthma, and is also implicated in regional haze and acid rain formation. A recent EPA risk assessment for SO2 concludes that definite health risks to asthmatics occur at concentrations significantly lower than the current 24-hour health standard for SO2. The document further notes that “over 20 million people in the U.S. have asthma, and therefore, exposure to SO2 likely represents a significant health issue.” The main sources of SO2 are fossil fuel combustion at power plants and industrial facilities. Along with its direct effects, SO2 also contributes to the formation of fine particulate matter. EPA concluded that a new SO2 standard with a 1-hour averaging time would be more protective. As of June 2, 2010, EPA strengthened the NAAQS for SO2 by adding a 1-hour standard set at 75 ppb.
Fine particle emissions arise from both direct ash emissions from combustion at energy plants, but also form from emissions of sulfur dioxide, nitrogen oxides, ammonia, and volatile organic compounds. Particulate air pollution has long been known to be associated with increased cardiopulmonary symptoms, asthma attacks, days lost from work due to respiratory disease, emergency room visits, hospitalization rates, and mortality. Two size classes are recognized in regulatory schemes: PM10 and PM2.5, with the numeric value referring to the particle size in microns (a micron is one millionth of a meter). There is no current health standard for PM10; EPA’s 24-hour and annual exposure standards for PM2.5 are 35 micrograms per cubic meter (µg/m3) and 15 µg/m3. EPA’s most recent risk assessment for PM acknowledges that the current standards are insufficiently protective and indicates that the agency will be lowering the National Ambient Air Quality Standards (NAAQS) for PM2.5 once more.
The classes of particulate matter classed as “black carbon” are implicated in a recent study as having up to 60% of the climate warming effect of CO2, by both creating “brown clouds” and darkening and thus increasing the heat absorption of snow and ice in polar regions. Controlling soot emissions and thus lessening albedo effects may thus be an even faster way to mitigate sea ice melting than controlling greenhouse gas emissions. A recent UN report found that controlling black carbon emissions and ozone could dramatically reduce global warming and improve human health.
Lead exposure primarily occurs from paint that has not been remediated. Lead exposure in children is linked to a variety of developmental and neurological problems. A recent study concluded that
“long-term trends in population exposure to gasoline lead were found to be remarkably consistent with subsequent changes in violent crime and unwed pregnancy. Long-term trends in paint and gasoline lead exposure are also strongly associated with subsequent trends in murder rates going back to 1900. The findings on violent crime and unwed pregnancy are consistent with published data describing the relationship between IQ and social behavior. The findings with respect to violent crime are also consistent with studies indicating that children with higher bone lead tend to display more aggressive and delinquent behavior. This analysis demonstrates that widespread exposure to lead is likely to have profound implications for a wide array of socially undesirable outcomes.”
EPA recently dropped the NAAQS for lead from 1.5 µg/m3 to 0.15 µg/m3.
Carbon monoxide (CO)
Carbon monoxide is a product of incomplete combustion that when inhaled, interferes with oxygen absorption in the blood. Emissions of CO from biomass boilers generally increase with fuel moisture; “good combustion practices” are frequently cited as the best control for CO emissions. Carbon monoxide can accumulate in closed spaces and could be a problem in the vicinity of improperly ventilated combustion sources. Carbon monoxide is treated under EPA’s “boiler rule” as a proxy for certain organic toxics that are assumed to increase as CO emissions increase.
Hazardous air pollutants
Hazardous air pollutants (HAPs) is the group name for 187 compounds which are known to have highly harmful health or environmental effects. The list includes metals like chromium, lead, and mercury, as well as compounds like dioxins (products of combustion that are widely considered to be among the most toxic chemicals known), benzene (a constituent of gasoline) and formaldehyde. EPA has classified two HAPs as human carcinogens (arsenic and the hexavalent form of chromium, CrVI) and four as probable human carcinogens (cadmium, lead, dioxins/furans, and nickel). All of these HAPs, as well as others, can be emitted in significant amounts by biomass energy facilities that burn “urban wood” as fuel, which contains lead-painted wood, wood treated with copper chromium arsenate, and non-wood materials that exacerbate dioxin/furan formation. Monitoring for these pollutants is rare, but emission levels can be high in the vicinity of specific emitters.
Considered a human carcinogen by EPA, arsenic is highly toxic, and is a principle component of copper-chromium-arsenate (CCA) mixture that was used for pressure-treating lumber. Facilities that proposed to burn waste wood generally rely on visual sorting techniques to remove arsenic-containing pressure-treated wood from the CDD that it burns. However, such detection can be difficult, as noted by the Massachusetts Department of Environmental Protection website, which states
“You can usually recognize pressure treated wood by its greenish tint, especially on the cut end, and staple-sized slits that line the wood. However, the greenish tint fades with time, and not all pressure treated wood has the slits”.
Chromium is also a constituent of pressure-treated wood, and is toxic, particularly in the hexavalent form (CR VI). EPA’s website states: “The respiratory tract is the major target organ for chromium (VI) toxicity, for acute (short-term) and chronic (long-term) inhalation exposures. Shortness of breath, coughing, and wheezing were reported from a case of acute exposure to chromium (VI), while perforations and ulcerations of the septum, bronchitis, decreased pulmonary function, pneumonia, and other respiratory effects have been noted from chronic exposure. Human studies have clearly established that inhaled chromium (VI) is a human carcinogen, resulting in an increased risk of lung cancer. Animal studies have shown hexavalent chromium to cause lung tumors via inhalation exposure.” EPA’s conversion constant for the proportion of total chromium from biomass burning that is emitted in the hexavalent form is 56%.
Mercury is a significant and dangerous contaminant that damages neurological development and other organ functions. It accumulates up food chains, presenting the greatest threat to humans and fish-eating birds like loons. Mercury is transported in the atmosphere but a significant amount from a point source can be deposited nearby, contaminating soils and water bodies. Biomass burning can emit surprisingly high amounts of mercury; for instance, the 21.5 MW Hu Honua facility planned in Hawaii would emit about 10 lb of mercury per year. This emissions rate is about 0.053 lb/kWh, more than 21 times the 0.0025 lb/kWh emissions rate at the Mount Tom coal plant in Holyoke, Massachusetts. To be sure, the rate at the coal plant is this low because the facility has installed expensive emissions control equipment – equipment that biomass developers refuse to install because it is not “cost effective”.
Dioxins/furans are “persistent, bioaccumulative, and toxic” (PBT) compounds that are created as by-products of chemical manufacturing, and also from combustion. Dioxin/furans are known to affect hormone levels and functions, as well as affecting fetal development, the immune system, and reproduction. They are toxic at levels that already exist in the environment. EPA states: “Because dioxins are widely distributed throughout the environment in low concentrations, are persistent and bioaccumulated, most people have detectable levels of dioxins in their tissues. These levels, in the low parts per trillion, have accumulated over a lifetime and will persist for years, even if no additional exposure were to occur. This background exposure is likely to result in an increased risk of cancer and is uncomfortably close to levels that can cause subtle adverse non-cancer effects in animals and humans.”