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Environmental Impacts of Biomass for Electricity

Water Use | Air Emissions | Land Use | Life-cycle Global Warming Emissions

Biomass power plants share some similarities with fossil fuel power plants: both involve the combustion of a feedstock to generate electricity. Thus, biomass plants raise similar, but not identical, concerns about air emissions and water use as fossil fuel plants. However, the feedstock of biomass plants can be sustainable produced, while fossil fuels are non-renewable.

Sources of biomass resources for producing electricity are diverse, including energy crops (like switchgrass), agricultural waste, manure, forest products and waste, and urban waste. Both the type of feedstock and the manner in which it is developed and harvested significantly affect land use and life-cycle global warming emissions impacts of producing power from biomass.  

Note: This section does not focus on biofuels for vehicles. For more information about biofuels and the UCS Clean Vehicles program, see Cleaner Biofuels: Displacing Conventional Gasoline.

Water Use

Biomass power plants require approximately the same amount of water for cooling as coal power plants, but actual water withdrawals and consumption depends on the facility’s cooling technology. For biomass plants with once-through cooling systems—which take water from nearby sources, circulate it through the plants cooling system, and then discharge it—water withdrawals range between 20,000 and 50,000 gallons per megawatt-hour with consumption of 300 gallons per megawatt-hour. Biomass facilities that use wet-recirculating cooling systems—which reuse cooling water in a second cycle rather than immediately discharging it—withdraw between 500 and 900 gallons per megawatt-hour and consume approximately 480 gallons per megawatt-hour [1]. 

Approximately 75% of existing biomass plants that require cooling use wet-recirculating technology, while 25% of plants use once-through cooling technology. In either case, when withdrawn cooling water is returned to its source, it is much warmer than when it was withdrawn, which often has a negative impact on plant and animal life. As in all thermal plants, this impact must be closely monitored. Dry-cooling systems do not withdraw or consume any water, but the tradeoffs to these water savings are higher costs and lower efficiencies—meaning more fuel is needed per unit of electricity. (For more information, see How it Works: Water for Power Plant Cooling.)

Water is also needed to produce some biomass feedstocks. While some feedstock sources—such as agricultural, forest, and urban waste—require no additional water, others—such as energy crops—can be very water intensive. Different energy crops vary in terms of how much water they require. Miscanthus, one type of perennial grass, requires a large amount of water, while switchgrass, another perennial grass, generally requires much less. Water use efficiency of a given crop depends on a number of factors, including soil quality and temperature [2].  

In regions with sufficient rainfall where irrigation is not required, water use for producing energy crops may be less of a concern. However, even in water-rich areas, the increased cultivation of energy crops may harm regional water quality as a result of soil tillage and nutrient runoff. Such water quality impacts can be managed through proper harvesting techniques. Many of these same issues arise in the cultivation of energy crops for biofuels.

Air Emissions

Burning biomass to produce electricity can impact air quality. The level of air emissions associated with biomass power plants varies depending on the feedstock, combustion technology, and types of installed pollution controls, but the most common pollutants include nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide, and particulate matter. The table below compares air emissions from different types of biomass, coal, and natural gas power facilities with pollution control equipment. In general, biomass facilities emit less SO2 and mercury (a neurotoxin) than coal.

Nitrogen oxides from biomass are lower than those from coal but higher than natural gas. NOx emissions causes ground-level ozone, or smog, which can burn lung tissue and can make people more susceptible to asthma, bronchitis, and other chronic respiratory diseases. Like SO2, NOx also contributes to acid rain and the formations of harmful particulate matter. Biomass power plants also emit high levels of particulates (soot and ash) and carbon monoxide. Readily available technologies, such as fluidized bed or gasification systems, and electrostatic precipitators, can help reduce NOx, CO, and particulate emissions associated with biomass power.

Direct Air Emissions from Biomass, Coal and Natural Gas Power Plants, by Boiler Type

 

Enlarge table

Note: UCS does not consider waste-to-energy plants that burn raw municipal waste to be a sustainable form of biomass. Waste-to-energy plants emit high levels of air pollution, including toxic metals, chlorinated compounds, and plastics.

Land Use

Land use impacts from biomass power production are driven primarily by the type of feedstock: either a waste stream or an energy crop that is grown specifically for generating electricity. Because waste streams are only secondarily available as a result of another activity that would have otherwise occurred—such as logging or farming—there is no marginal increase in land use. However, if not collected properly, using agriculture and forest waste streams for biomass power could lead to land or habitat degradation.

Important safeguards and best practices for removal are needed to ensure that sufficient crop residues are left behind to improve soil carbon storage, maintain nutrient levels, and prevent erosion. Similarly, harvesting of forest waste products can be done sustainably, but proper forest management practices need to be followed to ensure that wildlife habitat is not destroyed and the forest remains healthy.

Impacts associated with the use of energy crops depends greatly on whether the planting leads to land use change or displaced food production. If energy crops are planted on a large scale and displace food production, then new lands may need to be cleared to maintain food supplies. As a result, this could potentially change U.S. or global land use patterns and lead to habitat destruction or increases in food prices [3].  However, it is possible to sustainably increase agricultural efficiency and reduce the land required for food production while also improving soil health, erosion, and eutrophication. Doing so could free up land for energy crops while minimizing food displacement and other land use changes. 

 Energy crops present many of the same environmental challenges as food crops, and therefore the same principles of sustainable agriculture apply: crop rotation, integrated pest management, and proper soil husbandry to prevent soil erosion. Many energy crops use less fertilizer and pesticides than typical food crops, and perennial grasses do not require annual tilling and planting. These crops can even be advantageous for some farmers; alternating the planting of food and energy crops can help stabilize the soil and provide supplemental farm income [4].  

Life-cycle Global Warming Emissions

There are global warming emissions associated with growing and harvesting biomass feedstock, transporting feedstock to the power plant, and burning or gasifying the feedstock. Transportation and combustion emissions are roughly equivalent for all types of biomass. However, global warming emissions from the sourcing of biomass feedstock vary widely. It was once commonly thought that biomass had net zero global warming emissions, because the growing biomass absorbed an equal amount of carbon as the amount released through combustion. It is now understood that some biomass feedstock sources are associated with substantial global warming emissions. Thus, it is important to distinguish between biomass resources that are beneficial in reducing net carbon emissions, those that have an ambiguous impact, and those that increase net emissions.

Beneficial biomass resources include energy crops that do not compete with food crops for land, portions of crop residues such as wheat straw or corn stover, sustainably-harvested wood and forest residues, and clean municipal and industrial wastes. The use of organic waste products for biomass energy is especially beneficial. When organic waste is disposed of in a landfill, it decomposes and releases methane, a potent global warming gas. Thus, diverting these wastes for electricity production reduces landfill volume and reduces methane emissions [5].

Harmful biomass resources and practices add net carbon to the atmosphere by either directly or indirectly decreasing the overall amount of carbon stored in plants and soils. Such practices include clearing forests, savannas, or grasslands to grow energy crops, and displacing food production for bioenergy production that ultimately leads to the clearing of carbon-rich ecosystems elsewhere to grow food.

For marginal biomass resources, the net carbon impact depends on the circumstances. For example, if grasslands are plowed up or forests cut down to make way for switchgrass farms, there will be an increase in net carbon emissions. This is because grasslands and forests contain large stores of carbon, and total carbon storage increases each year as these ecosystems mature. There could also be a net increase in global warming emissions associated with planting switchgrass on productive agricultural land. On a global level, as food crops are replaced with energy crops, the price of food increases, which gives farmers the incentive to clear more grasslands and forests to make way for food production. Thus, even if switchgrass does not directly displace grasslands and forest, the effect could be indirect [6].  However, plants like switchgrass can have zero or net negative emissions if they are planted in degraded or abandoned agricultural land. Research has shown that switchgrass, when planted in diverse mixtures with other perennial grasses and legumes, can help store carbon in degraded soils [7].

Forest feedstock is another example of a marginal biomass resource. The use of forest products for biomass feedstock can have net zero global warming emissions if forest managers harvest in a sustainable manner and replant with fast-growing tree species. However, even when following best practices, forest regeneration will not occur instantly, so there can be a long lag-time before the biomass resource achieves carbon neutrality [8].  

 Due to all of these factors, the range for estimates for lifecycle global warming emissions of biomass energy is wide. Excluding global warming emissions from land use changes, most estimates are between 0.04 and 0.2 pounds of CO2 equivalent per kilowatt-hour [9]. To put this into context, estimates of life-cycle global warming emissions for natural gas-generated electricity are between 0.6 and 2 pounds of carbon dioxide equivalent per kilowatt-hour and estimates for coal-generated electricity are 1.4 and 3.6 pounds of carbon dioxide equivalent per kilowatt-hour [10].

References:

[1] Macknick, et al. 2011. A Review of Operational Water Consumption and Withdrawal Factors for Electricity Generating Technologies. Golden, CO: National Renewable Energy Laboratory.

[2] Clifton-Brown, J.C.; Lewandowski, I. 2000. Water Use Efficiency and Biomass Partitioning of Three Different Miscanthus Genotypes with Limited and Unlimited Water Supply. Annals of Botany, 86(1): 191-200. 

Kiniry, J.R.; Lynd, Lee; Greene, Nathanel; Johnson, Mari-Vaugh. V.; Casler, Michael; Laser, Mark S. 2008. Biofuels And Water Use: Comparison Of Maize And Switchgrass And General Perspectives. In New Research on Biofuels. J. H. Wright and D. A. Evans, eds.

[3] Seachinger et al. 2008. Use of U.S. Croplands for Biofuels Increases Greenhouse Gases Through Emissions from Land-Use Change. Science 29 February 2008: 319 (5867), 1238-1240.

Wise et al. 2009. Implications of Limiting CO2 Concentrations for Land Use and Energy. Science 29 May 2009: 324 (5931), 1183-1186.

[4] Zegada-Lizarazum, Walter; Monti, Andrea. 2011. Energy crops in rotation. A review. Biomass and Bioenergy, 35. 12-25. Online at http://www.sciencedirect.com/science/article/pii/S0961953410002588

[5] Spath, Pamela; Mann, Margaret. 2004. Biomass Power and Conventional Fossil Systems with and without CO2 Sequestration – Comparing the Energy Balance , Greenhouse Gas Emissions and Economics Biomass Power and Conventional Fossil Systems with and without CO2 Sequestration. Golden, CO: National Renewable Energy Laboratory.

[6] Seachinger et al. 2008. Use of U.S. Croplands for Biofuels Increases Greenhouse Gases Through Emissions from Land-Use Change.

[7] Farione, et al. Land Clearing and the Biofuel Carbon Debt. Science 29 February 2008: 319 (5867), 1235-1238.

[8] Manomet Center for Conservation Sciences. 2010. Biomass Sustainability and Carbon Policy Study.

[9, 10] IPCC, 2011: IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation. Prepared by Working Group III of the Intergovernmental Panel on Climate Change [O. Edenhofer, R. Pichs-Madruga, Y. Sokona, K. Seyboth, P. Matschoss, S. Kadner, T. Zwickel, P. Eickemeier, G. Hansen, S. Schlömer, C. von Stechow (eds)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1075 pp. (Chapter 2 & 9).

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