Evolution of Cleaner Solid Fuel Combustion

By Christopher Long
Principal Scientist, Gradient
Peter Valberg
Principal, Gradient

Although uncommon in developed countries, solid fuels—including wood, charcoal, coal,A dung, and crop residues—are burned domestically by billions of people across the world for space heating, lighting, and cooking. For example, it is estimated that, as of 2010, approximately 41% of the world’s households (approximately 2.8 billion people) rely mainly on solid fuels for cooking.1 A comprehensive assessment of respiratory risks from household air pollution recently concluded that the health of one in three people worldwide is at risk because of exposure to emissions from traditional household solid fuel combustion.2

To provide some perspective on their relative air exposure impacts, we have compared exposure of people to traditional household solid fuel combustion emissions (e.g., smoke from domestic burning of biomass material or coal) to those of coal-fired power plants (CFPPs). We used data from published papers and reports and tabulated levels of people’s exposure to common air pollutants from these two different combustion sources. The coal-fired power plants used for the study were based in the U.S. due to the larger amount of air modeling data available, but were older and had more limited emissions controls than modern state-of-the-art plants. As a result, these plants are useful for representing the air exposure impacts of plants that might be built in developing countries today, including those built without international support for meeting high-efficiency, low-emissions standards.

A review of available literature has shown that traditional stoves for cooking and heating are a much more inefficient and dangerous means of energy utilization compared to modern electricity services.

A review of available literature has shown that traditional stoves for cooking and heating are a much more inefficient and dangerous means of energy utilization compared to modern electricity services.

For our comparisons, we focused on air exposure concentrations rather than emissions because possible exposure via breathing cannot be characterized based solely on emissions data (e.g., tons per year), but rather needs to be assessed through examining concentrations of pollutants [masses of pollutants per unit volume of air—e.g., micrograms per cubic meter (μg/m3)] that can potentially be inhaled. In addition, we used an alternative metric of exposure, namely intake fraction (iF), to supplement this analysis.

ASSESSMENT OF REPORTED AIR EXPOSURE LEVELS

Traditional Household Solid Fuels Contribute to Complex Indoor Air Pollution

Over the past several decades, numerous studies have investigated the air pollution generated by traditional household solid fuel combustion for space heating, lighting, and cooking in developing countries.1,3 It is now well established that, throughout much of the world, indoor burning of solid fuels (e.g., wood, charcoal, coal, dung, and crop residues) by inefficient, often insufficiently vented, combustion devices (e.g., ovens, stoves, or fireplaces) leads to highly elevated exposures to household air pollutants. This is due to the poor combustion efficiency of the combustion devices and the elevated nature of the emissions; moreover, they are often released directly into living areas.3 Smoke from traditional household solid fuel combustion commonly contains a range of incomplete combustion products, including both fine and coarse particulate matter (e.g., PM2.5, PM10), carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), and a variety of organic air pollutants (e.g., formaldehyde, 1,3-butadiene, benzene, acetaldehyde, acrolein, phenols, pyrene, benzopyrene, benzo(a)pyrene, dibenzopyrenes, dibenzocarbazoles, and cresols).2 In a typical solid fuel stove, approximately 6–20% of solid fuel mass is converted into toxic emissions, with such factors as the fuel type and moisture content, stove technology, and stove operation influencing the amount and relative composition of the pollution mixture.1

Even though the mixture of pollutants arising from traditional household solid fuel combustion is complex, most measurement studies have focused on characterizing breathing-zone exposure levels of two surrogate species in solid fuel smoke, namely PM and CO, which are the main products of incomplete combustion and are considered to pose the greatest health risks.2 Table 1, adapted from Naeher et al.,3 summarizes indoor air measurements of PM2.5 and CO associated with traditional household solid fuel combustion. As shown in the table, PM2.5 exposure levels have been consistently reported to be in the range of hundreds to thousands of micrograms per cubic meter (μg/m3); likewise, CO exposure levels as high as hundreds to greater than 1000 milligrams per cubic meter (mg/m3) have been measured. Consistent with these data, a more recent study of 163 households in two rural Chinese counties reported geometric mean indoor PM2.5 concentrations of 276 μg/m3 (combinations of different plant materials, including wood, tobacco stems, and corncobs), 327 μg/m3 (wood), 144 μg/m3 (smoky coal), and 96 μg/m3 (smokeless coal) for homes using a variety of different fuel types and stove configurations (vented, unvented, portable, fire pit, mixed ventilation stove).4

Table1

Air Modeling of CFPP Emissions Predicts Substantially Lower Air Quality Impacts

In comparison to traditional solid fuels, CFPP emissions are associated with far lower ground-level ambient exposure levels of both PM2.5 and CO.B Table 2 provides a summary of model-predicted ground-level PM2.5 and CO concentrations from publicly available studies of the ambient air quality impacts of U.S. CFPPs. We relied on model-predicted concentrations rather than measurement data because air-monitoring data are not specific to power plant emissions and include contributions from a variety of other common anthropogenic, natural, and distant air pollution sources. As indicated in Table 2, all but one of the studies we identified reflect modeled air quality impacts for groups of U.S. CFPPs in the same general vicinity; thus, these data encompass air quality impacts higher than what would be the case for a single newer U.S. power plant. Moreover, the majority of the modeled plants are older CFPPs that lack the clean coal technologies characteristic of newer and retrofitted CFPPs. With respect to PM, these studies generally accounted for both primary PM2.5 emissions and secondary atmospheric formation of sulfate and nitrate particles from gaseous SO2 and NOx emissions, respectively. With respect to CO, we identified just a single modeling study that predicted the CO air quality impacts of emissions from CFPPs (as well as a number of natural gas power plants).5 Most likely because CO emissions from U.S. CFPPs are low and not considered to pose significant air quality problems or public health impacts, CO has not received as much attention as PM2.5 in studies of the air quality impacts of power plants.6

The annual average ambient PM2.5 and CO concentrations in Table 2 are far below the comparable daily-average PM2.5 and CO indoor exposure levels associated with traditional household solid fuel combustion in Table 1.C For PM2.5, several studies7–9 of groups of older, grandfathered U.S. CFPPs predicted annual average concentrations of less than 1 μg/m3 for maximally impacted locations, as compared to daily average PM2.5 exposure levels of hundreds to thousands of μg/m3 inside homes with traditional solid fuel combustion. For CO, the single modeled estimate that we identified for 2002 county-average CO impacts from three CFPPs/units (plus 18 gas-fired power plants/units) in the San Antonio, TX, metropolitan area5 is over 10,000 times lower than the lowest CO exposure levels we found for traditional household solid fuel combustion (Table 1).

Table2

INTAKE FRACTION AS AN ALTERNATIVE COMPARISON TOOL

Defining Intake Fraction

The iF is a well-established metric in the exposure assessment and public health fields for quantifying the emission-to-intake relationship, in large part because iFs facilitate comparisons of the exposure implications of various emission sources. Intake fraction can be defined simply as the fraction of material emitted into the air from a given source that is actually inhaled; however, Bennett et al.10 provided a more thorough definition of iF as “the integrated incremental intake of a pollutant, summed over all exposed individuals, and occurring over a given exposure time, released from a specified source or sources, per unit of pollutant emitted.” It is generally reported as a unitless value, as expressed in the following equation10:

Long Equation

iF thus sums pollutant intake over two measures—population size and time duration—and incorporates a variety of factors related to the emission scenario and exposure conditions. These factors include chemical properties of the contaminant, emissions locations (e.g., release height, indoor versus outdoor, proximity to people), environmental conditions (climate, meteorology, land use), human receptor locations and activities, and population characteristics. Intake fractions can be based on both modeling results and measurements.

Relative Intake Fractions for Traditional Solid Fuel Combustion Versus CFPPs

We identified just a single study11 that reported iFs for both types of PM combustion emissions. Smith (1993) estimated iFs ranging from approximately one to two one-thousandths for PM emissions from traditional solid fuel combustion in biomass cookstoves versus substantially lower iFs of one one-millionth for a U.S. CFPP and 10 one-millionths for a CFPP in a least developed country (LDC) based on the assumption of a greater population density. In other words, these results indicate that about one one-thousandth of what is released from traditional household solid fuel burning is inhaled, while only about one one-millionth of what is released from a U.S. CFPP is inhaled.

These differences demonstrate the critical role of the proximity of the emission source to people in determining its exposure potential; whereas CFPP emissions are typically released from tall stacks often far from heavily populated areas, emissions from traditional household solid fuel combustion in developing countries are often released directly into poorly ventilated indoor spaces (e.g., kitchens), where they can remain trapped for extended periods of time in direct proximity of people. Smith12 has subsequently emphasized the concept that the “place makes the poison”.

More recent iF estimates for PM emissions from both traditional household solid fuel combustion and CFPPs confirm that iF differences between these sources span several orders of magnitude.9,13,14 For example, Levy et al.9 estimated a slightly smaller iF for primary PM2.5 emissions from seven older northern Georgia (U.S.) CFPPs (0.0000006), and even smaller iFs for secondary sulfates and nitrates (0.0000002 and 0.00000006). In contrast, Grieshop et al.14 estimated iFs of 0.0013 and 0.00024 for unvented and outdoor-vented cookstoves, respectively.

CONCLUSIONS

We found that measured PM2.5 and CO concentrations inside homes burning traditional solid fuels are thousands of times greater than even the high-end estimates of ground-level ambient exposure levels from U.S. coal-fired power plant stack emissions. Even if a low-efficiency coal-fired power plant with no emissions controls were employed—a likely scenario in areas where traditional solid fuels are combusted and in the absence of international support for efficiency and environmental upgrades—order-of-magnitude differ-
ences would likely be observed compared to traditional solid fuel combustion. Moreover, we saw similar, supporting results using an alternative comparison approach based on intake fractions. Overall, these conclusions point to traditional household solid fuel combustion being a significantly greater source of air pollution exposures of health concern. The basic difference is that coal-fired power plants burn coal much more efficiently and completely—and exhaust their emissions from tall stacks rather than in direct proximity to people. Overall, as compared to traditional household solid fuel combustion, which represents an inefficient, high-emission form of fuel utilization, modern coal-fired power plants (and even older ones with more limited air pollution controls) represent a more sophisticated, cleaner approach to getting the maximum energy out of solid fuel with significantly reduced impacts on the air that humans breathe.

NOTES

A.  Although coal is used both for traditional household solid fuel combustion and for electricity generation at modern power plants, coal handling and combustion conditions for the two situations are quite different. When used as a traditional household solid fuel, large chunks of often lower-quality coal are directly burned under uncontrolled combustion conditions, such that combustion is inefficient and incomplete. In contrast, modern power plants often burn higher-quality coal, usually pulverized and mixed with air, under efficient and controlled conditions, resulting in nearly complete combustion of coal organics.

B.  Actual personal exposures to ambient-derived pollutants can often be significantly lower than ambient (outdoor) air exposure levels. This is largely because people in countries such as the U.S. spend the majority (~90%) of their time indoors where the infiltration process can result in significantly reduced concentrations indoors compared to the corresponding ambient levels outdoors.

C.  Although expressed for different averaging periods, the annual average PM2.5 and CO concentrations shown in Table 2 (for power plants) should be compared to the daily average PM2.5 and CO concentrations in Table 1 (for traditional household solid fuel combustion), which would occur repeatedly on a daily basis. That is, the daily average PM2.5 and CO concentrations in Table 1 can be assumed to be representative of long-term average (e.g., annual average) exposure levels given the daily occurrence of solid fuel combustion for cooking, heating, and lighting.

ACKNOWLEDGEMENTS

This article was commissioned by Peabody Energy; it reflects the professional opinions of the authors and the writing is solely that of the authors.

REFERENCES

  1. Smith, K.R., Bruce, N., Balakrishnan, K., Adair-Rohani, H., Balmes, J., Chafe, Z., … HAP CRA Risk Expert Group. (2014). Millions dead: How do we know and what does it mean? Methods used in the comparative risk assessment of household air pollution. Annual Review of Public Health, 35, 185–206.
  2. Gordon, S.B., Bruce, N.G., Grigg, J., Hibberd, P.L., Kurmi, O.P., Lam, K.B., … Martin, W.J., II. (2014). Respiratory risks from household air pollution in low and middle income countries. The Lancet Respiratory Medicine, epub ahead of print.
  3. Naeher, L.P., Brauer, M., Lipsett, M., Zelikoff, J.T., Simpson, C.D., Koenig, J.Q., & Smith, K.R. (2007). Woodsmoke health effects: A review. Inhalation Toxicology, 19, 67–106.
  4. Hu, W., Downward, G.S., Reiss, B., Xu, J., Bassig, B.A., Hosgood, H.D. III, & Lan, Q. (2014). Personal and indoor PM2.5 exposure from burning solid fuels in vented and unvented stoves in a rural region of China with a high incidence of lung cancer. Environmental Science & Technology, 48, 8456–8464.
  5. Perkins, J., Heilbrun, L., Symanski, E., Coker, A., & Eggleston, K. (2009). A study to evaluate the health effects of air pollution in Bexar County with a focus on local coal and gas fired power plants. CPS Energy, www.cpsenergy.com/files/Health_Study_FullReport.pdf
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  14. Grieshop, A.P., Marshall, J.D., & Kandlikar, M. (2011). Health and climate benefits of cookstove replacement options. Energy Policy, 39, 7530–7542.

The authors can be reached at clong@gradientcorp.com and pvalberg@gradientcorp.com

The content in Cornerstone does not necessarily reflect the views of the World Coal Association or its members.

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