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Research Highlights
Researchers Compare Options for Maximizing Energy Recovery from WastePodcastsListen to or download the podcast to learn more about how NRMRL research helps to support the protection of human health and the environment.   In 2007, Americans generated about 250 million tons of waste. Of this amount, 54 percent was buried and 13 percent was burned. Energy can be recovered from burying or burning waste. Communities across the United States are looking at ways to maximize resources and energy conservation in response to EPA’s Resource Conservation Challenge. Most of the attention to date has been on increasing materials recovery. However, for discards management, is it better to burn or bury waste? Both options can generate energy, but which is better in maximizing energy recovery while minimizing greenhouse gas emissions? This question was answered through a recent publication in Environmental Science & Technology (ES&T)* by researchers at the National Risk Management Research Laboratory in the Air Pollution Prevention and Control Division (APPCD). Background
The National Risk Management Research Laboratory Air Pollution Prevention and Control Vision recently published a report answering the question: Is more energy recovered from burying or burning waste?
The use of municipal solid waste to generate electricity through landfill-gas-to-energy (LFGTE) and waste-to-energy (WTE) projects represents roughly 14 percent of U.S. nonhydro-renewable electricity generation. Although various aspects of LFGTE and WTE have been analyzed in the literature, this paper was the first to present a comprehensive set of life-cycle emission factors per unit of electricity generated for these energy recovery options. While methane from landfills results from the anaerobic breakdown of biogenic materials, the energy derived from waste-to-energy results from the combustion of both biogenic and fossil materials. Most large landfills capture the gas generated from the decomposition of biodegradable waste. However, current regulations do not require gas capture until five years after waste placement. Even the best controlled landfills leak methane, which is 21 times more potent than carbon dioxide (CO2) emissions. For waste combustion, 100 percent of U.S. facilities recover energy. Stringent air pollution controls at waste-to-energy plants help to minimize criteria pollutants and toxic emissions. When waste is burned, CO2, rather than landfill gas, is generated. Landfill gas is typically 50 percent methane and 50 percent carbon dioxide. Modeling FrameworkUsing EPA’s decision support tool (DST) for Sustainable Materials And Residuals managemenT (SMART), a comprehensive life-cycle analysis was conducted comparing burning and burying waste. The SMART-DST has a number of process models for collecting, sorting, processing, transporting, burning, and burying waste. For this analysis, only the combustion and landfill process models were used. The functional unit was 1 ton of municipal solid waste, including nonhazardous solid waste, generated in residential, commercial, institutional, and industrial sectors. Each process model can track 32 life-cycle parameters, including energy consumption, total greenhouse (and other) gases, particulate matter, water pollutants, and solid wastes. The ES&T publication provides results comparing energy, greenhouse gas emissions, and criteria pollutants. The process models are described in more detail through background information documents and other peer-reviewed publications. Modeling landfills is much more difficult than modeling waste combustion. To address the uncertainties, a sensitivity analysis was conducted looking at a range of values for landfill gas capture and control, using recent data from EPA’s update of landfill gas emission factors. In addition, sensitivity analysis included other key variables: the efficiency of the waste-to-energy plant, timing of landfill gas capture and control, oxidation rate, and waste composition. Total emissions for buried waste are compared to total emissions from burning waste, including ash disposal. For the landfill-gas-to-energy (bury) method, the total life-cycle inventory landfill emissions generated were the sum of emissions from (1) on-site preparation, operation, and closure operations, (2) the decay of waste under anaerobic conditions, (3) equipment used during landfill operations and gas management operations, (4) production of diesel required to operate site vehicles, and (5) the treatment of leachate. Total landfill gas was calculated using a given time frame of 100 years and the empirical gas yield from each individual waste component. For the waste-to-energy (burn) method, the total life-cycle inventory emissions are those associated with (1) the combustion of waste, (2) the production and use of limestone in the control technologies (scrubbers), and (3) the disposal of ash in a landfill. Emission concentrations of criteria and metal pollutants were estimated using the limits on municipal solid waste combustors set by federal regulations. The life-cycle inventory model outputs the total megawatt hour of electricity and emissions that are generated per unit mass of each waste item. The electricity output is a function of the quantity, energy, and moisture content of the individual waste stream items. A lifetime of 20 years and a system efficiency of 19 percent (18,000 kW h) were assumed, along with other factors. ConclusionsOn the basis of this research, the waste-to-energy (burn) option is estimated to be capable of producing one order of magnitude more electricity than the bury method from the same mass of waste for the situations evaluated in the study. In addition, there are significant differences in emissions on a mass per unit of energy between the two. Even under the most optimistic assumptions for landfill gas capture and control, the net life-cycle environmental trade-offs is two to six times the amount of greenhouse gases compared to burning waste. When comparing electricity per ton of municipal waste, waste-to-energy per ton kWh generated is on the average six to eleven times more efficient at recovering energy from waste than landfills. If we want to achieve greater greenhouse gas reduction and maximize energy conservation, then communities should shift toward burning waste (with energy recovery) versus landfilling waste. *For complete details and results of the study, see the Environmental Science & Technology journal article, “Is It Better to Burn or Bury Waste for Clean Electricity Generation?” Cited this year by the Wall Street Journal, the ES&T article has also been used by Congress in informing policy for the Energy Bill. In the top ten list of the most accessed publications for ES&T in 2009, this paper was listed as number one. NRMRL researchers in APPCD are working on several additional journal publications regarding greenhouse gas (GHG) trends and evaluation of zero waste goals. A major focus of next year’s research will be using the SMART-DST to identify tipping points to maximize energy efficiency and GHG reductions for materials and discards management. (Thanks to Susan Thorneloe, co-author of the ES&T article, who contributed technical information for this report.) ContactJane Ice, NRMRL Office of Public Affairs (513) 569-7311
Hot Off the Presses—NRMRL PublicationsJournal Articles Butler, B.A., B.S. Caruso, and J.F. Ranville. (2009) “Reactive Transport Modeling of Remedial Scenarios to Predict Cadmium, Copper, and Zinc in North Fork of Clear Creek, Colorado.” Submitted to: Remediation. John Wiley & Sons, Ltd., Indianapolis, IN, 19(4):101–119. Ippolito, J.A., K.G. Scheckel, and K.A. Barbarick. (2009) “Selenium Adsorption to Aluminum-Based Water Treatment Residuals.” Journal of Colloid and Interface Science. Elsevier, Amsterdam, Holland, 338(1):48–55. Kinsey, J.S., P. Karther, and Y. Dong. (2009) “Evaluation of Methods for Physical Characterization of the Fine Particle Emissions from Two Residential Wood Combustion Appliances.” Atmospheric Environment. Elsevier Science Ltd, New York, NY, 43(32):4959–4967. Lombi, E., K.G. Scheckel, J. Pallon, A. Carey, Y.G. Zhu, and A.A. Meharg. (2009) “Speciation and Distribution of Arsenic and Localization of Nutrients in Rice Grains.” New Phytologist. Wiley InterScience, Silver Spring, MD, 184(1):193–201. Shanks, O.C., C.A. Kelty, M. Sivaganesan, M. Varma, and R.A. Haugland. (2009) “Quantitative PCR for genetic markers of human fecal pollution.” Applied and Environmental Microbiology. American Society for Microbiology, Washington, DC, 75(17):5507–5513. Conference Proceedings Liu, X., M.A. Mason, Z. Guo, K.A. Krebs, and N. Roache. (2009) “Gypsum wallboard as a sink for formaldehyde.” In Proceedings, 2009 Healthy Buildings International Conference, Syracuse, NY, September 13–17, 2009. International Society of Indoor Air Quality and Climate (ISIAQ), Santa Cruz, CA. Marr, D., X. Liu, R.B. Mosley, M.A. Mason, B. Pagnani, and C. Whitfield. (2009) “Impact of simulated climate and building features on the penetration of toxic gases from the ambient into the indoor environment.” In Proceedings, 2009 Healthy Buildings International Conference, Syracuse, NY, September 13–17, 2009. International Society of Indoor Air Quality and Climate (ISIAQ), Santa Cruz, CA. Zhang, J., M.A. Mason, A. Hodgson, B. Guo, K. Krebs, A. Barry, and B. Peters. (2009) “An inter-laboratory comparison study of the ANSI/BIFMA standard test method M7.1 for furniture.” In Proceedings, 2009 Healthy Buildings International Conference, Syracuse, NY, September 13–17, 2009. International Society of Indoor Air Quality and Climate (ISIAQ), Santa Cruz, CA. EPA Published Reports U.S. EPA. (2009) Chen, A.S., B.J. Yates, W. Condit, and L. Wang. "Arsenic Removal from Drinking Water by Coagulation/Filtration U.S. EPA Demonstration Project at City of Three Forks, MT, Final Performance Evaluation Report." (EPA/600/R-09/113). U.S. EPA. (2009) Hansen, V.E. “Development and Evaluation of Sustainability Criteria for Land Revitalization.” (EPA/600/R-09/093). Book Chapter Santo Domingo J. and E.W. Rice. (2009) “Detection of Microbial Water Quality Indicators and Fecal Waterborne Pathogens in Environmental Waters: A Review of Methods, Applications, and Limitations.” Chapter 12 in Encyclopedia of Analytical Chemistry, Section on Environment: Water and Waste. John Wiley & Sons, Ltd., Indianapolis, IN. Scheckel, K.G., R.L. Chaney, N.T. Basta, abd J.A. Ryan. (2009) “Advances in Assessing Bioavailability of Metal(Loid)s in Contaminated Soils.” Chapter 1 in Advances in Agronomy. Elsevier BV, Amsterdam, Netherlands, 104:1–52.
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