Bury or Burn North America MSW? LCAs Provide Answers for Climate

Sep 24, 2010 - Environmental Science & Technology 2018 Article ASAP ... Life-Cycle Assessment Harmonization and Soil Science Ranking Results on ...
0 downloads 0 Views 416KB Size
Download PDF
Environ. Sci. Technol. 2010, 44, 7944–7949

Bury or Burn North America MSW? LCAs Provide Answers for Climate Impacts & Carbon Neutral Power Potential JEFFREY MORRIS* Sound Resource Management Group, 2217 60th Lane NW, Olympia, Washington 98502

Received February 16, 2010. Revised manuscript received September 3, 2010. Accepted September 13, 2010.

This study uses life cycle assessment (LCA) to compare climate impacts of landfill (LF) and waste-to-energy (WTE) for disposal of municipal solid waste (MSW). To avoid possibly arbitrary assumptions about landfill gas (LFG) capture rates, the study develops a crossover function for LFG capture that indicates the capture rate at which LF and WTE breakeven for climate impacts. Above the crossover rate LF is better for the climate; below WTE is superior. Base case and sensitivity analyses show how this crossover rate is affected by waste composition, electricity conversion efficiency, heat capture, scrap metal recovery, greenhouse gas (GHG) intensity of displaced power, and LCA time horizon. In general, crossover rates are in the 50% to 70% range. Notable exceptions include much higher crossover when WTE has high heat recovery, and much lower crossover for low carbon displaced power. The study also compares GHG emissions for electricity generated by WTE, captured LF methane, coal and natural gas, and concludes that none are carbon neutral. Further, the study tentatively suggests that MSW is a particularly carbon intensive fuel due to GHGs avoidable when readily recyclable materials in MSW are used in manufacturing new products rather than used to generate electricity.

Introduction What should a North American community do with municipal solid waste (MSW) that is not currently reduced, reused, recycled, or composted? Disposal practices have not changed significantly over the past ten years (1). However, there is increased interest in mass burn waste-to-energy (WTE) combustion prompted by belief that MSW is a carbon neutral energy source and that landfills contribute to climate change through releases of methane (CH4), a greenhouse gas (GHG) much more damaging to the climate than carbon dioxide (CO2) (2). Several recent studies have suggested that WTE is preferable to landfill (LF) (3-6). At the same time, Christensen et al. (5) show landfill preferable to WTE in situations of lower WTE waste heat recovery and higher LF methane capture. In addition, Kaplan et al. (3) fail to account for climate benefits from LF sequestration of biogenic carbon, and also base conclusions on MSW fossil carbon levels below those in many North American communities. * Corresponding author [email protected]. 7944

9

phone:

(360)

867-1033;

e-mail:

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 20, 2010

Furthermore, U.S. EPA and Environment Canada have developed models for evaluating waste management GHG emissions (7, 8). These models show LF preferable to WTE, especially given medium-high methane capture rates and typical MSW fossil carbon content, even when electricity is mostly powered by coal. Given the multicriteria complexity of the WTE versus LF choice, the purpose of this study is to use life cycle assessment (LCA) to clearly lay out the landfill and combustion facility performance factors, waste characteristics, and methodological issues that determine whether WTE or LF is better for the climate and whether either can generate carbon neutral electricity. This should at least clarify these two dimensions of this complex MSW management decision. LCA Modeling Scope. The functional unit for the LCA comparison of WTE and LF on climate impacts is one metric ton (Mg) of MSW shipped from a transfer facility to LF or WTE for disposal. The life cycle inventory encompasses GHG emissions that occur as a result of transporting MSW from the transfer station to the disposal facility, handling MSW at the disposal facility, burying or burning MSW, generating electricity and heat, and transport and disposal of combustion ash. The system boundary is expanded to include GHG reductions from power and heat displaced by energy from MSW disposal facilities. Because North American LF and WTE facilities often do not recover waste heat from power generation, GHG reductions from utilization of heat are treated in the sensitivity analysis. To account for differences between WTE and LF in the fate of biogenic carbon in products and packaging materials in MSW, the system boundary is also expanded to include the climate benefits from continued storage in anaerobic LF of a portion of previously sequestered biogenic carbon. The functional unit for the LCA comparison of the GHG releases from MSW, natural gas and coal used for power production is the amount of fuel required to produce one kilowatt hour (kWh). This LCA system boundary includes GHG emissions from production, as well as combustion, of the fuels. GHG emissions from construction of capital and operating equipment are not included in either inventory. GHG emissions per Mg disposed or per kWh generated are relatively insignificant (9). MSW composition is based on three different levels of North American geographic specificitysa city: Seattle (WA); a metropolitan area: Metro Vancouver (BC); and a state: Massachusetts. GHG Impacts for Biogenic Carbon. The distinction between fossil and biogenic carbon in wastes is important for evaluating climate impacts when waste materials are buried or burned. Combustion of wastes such as plastics containing fossil carbon generates climate changing CO2, just as burning fossil fuels does. Emissions of methane from LF decomposition of biogenic carbon also count as GHGs (2), (10). In contrast, CO2 emissions from combustion of biogenic materials do not count as GHGs as long as new plant and tree growth resequester an equivalent amount of carbon from the atmosphere over the LCA’s time frame. Climate neutrality for biogenic carbon emissions is typically assumed for the 100-year time period often used in LCAs. However, recent evidence suggests that it may take much longer than a hundred years for new tree growth to resequester the carbon that is lost when trees are harvested and their wood products and residues eventually burned (11). In this case, a portion of tree-based biogenic carbon released during combustion 10.1021/es100529f

 2010 American Chemical Society

Published on Web 09/24/2010

FIGURE 1. Comparison of methane cumulative generation for high and low k. should be counted as a GHG emission, even in a 100-year LCA. The difficulty is estimating what that exact portion is. Hence, biogenic carbon that is not resequestered in the 100 years following combustion of MSW or landfill methane is not considered here in calculating GHG emissions for WTE or LF. What is considered in the GHG accounting here is that significant portions of landfilled biogenic carbon do not decompose under anaerobic conditions (7), (10). Rather, portions of previously sequestered carbon that have been stored in paper, cardboard, wood products, leaves, grass clippings, and other discarded materials remain sequestered when these materials are landfilled. Thus, landfills serve as a carbon sink for climate GHG calculations, as discussed and supported by IPCC (10); US EPA (7); and Environment Canada’s Waste GHG Calculator (12). For power generation, release of previously sequestered carbon that is LF storable is counted as a GHG emission for WTE, following the approach discussed in Searchinger et al. (13) for harvested biomass used as fuel. Modeling Details for LF GHG Emissions and Energy Generation. Methane Generation Potential. Consider a megagram of MSW landfilled in period 0. According to U.S. EPA’s Landfill Gas Emissions Model (LandGEM) (14), q(t) ) CH4 generation at time t (m3 Mg-1t-1) can be characterized by the declining exponential (first order decomposition rate) function: q(t) ) kLoe-kt

(1)

where k ) generation rate per period, Lo ) potential CH4 generation capacity (m3 Mg-1), and t ) time. According to EPA (14), the parameter k is dependent on moisture content, pH and temperature of the waste mass, as well as availability of nutrients for microorganisms that break down waste in an anaerobic environment to form CH4 and CO2. Unlike k, which is influenced by local LF site conditions as well as waste composition, Lo depends only on composition, with higher proportions of cellulosic material, for example, resulting in higher Lo. Cumulative CH4, Q(T), generated through time Tsfor example, 20 or 100 yearssfrom one Mg MSW landfilled in period 0 is given by the following: Q(T) ) Lo

∫

T

0

ke-ktdt ) Lo(1 - e-kT)

(2)

Figure 1 shows cumulative methane generation as a percent of total lifetime generation for two values of k, 0.02 and 0.35. The former is the LandGEM default for an arid area landfill. The latter could portray the generation rate for a landfill sited where precipitation is very high. Note that high k implies that 99% of potential methane generation occurs in less than 15 years, while low k implies that less than 32% occurs within 20 years and only 87% within 100 years.

Lo ) 88, 91, and 97 for Seattle, Metro Vancouver, and Massachusetts, respectively (Tables S1, S2, and S3 of Supporting Information (SI)). Biogenic Carbon Sequestration. Sequestered biogenic carbon is between 0.37 and 0.47 Mg of CO2 equivalents (CO2e) per Mg MSW for the three localities; while total biogenic carbon is between 0.68 and 0.78 Mg CO2e (Tables S1-S3 of SI). Sequestered biogenic carbon, thus, accounts for 55% to 60% of biogenic carbon, consistent with IPCC’s estimate that at least 50% of biogenic carbon in landfilled wastes remains sequestered (10). Methane Capture and Oxidation. In modern landfills much of the methane generated from biogenic wastes is not emitted to the atmosphere, but is captured via landfill gas (LFG) collection systems and flared or used to generate electricity. Flaring or energy production converts captured methane to CO2, which does not count as GHG because its source is biogenic carbon. A portion of the noncaptured (fugitive) methane is oxidized to CO2 as it rises through the landfill and landfill cover materials, and does not count as GHG. Base case oxidation rate for fugitive methane is assumed to be 15% (3). Because there may be disagreement regarding estimated or potential LFG capture rates, even for a newly constructed landfill, the approach used here is to portray net landfill GHG emissions as a function of the LFG capture rate. Note that using eq 1 it is possible to calculate a weighted average LFG capture rate even when capture varies over timese.g., capture may begin some months after initial deposition of waste in the landfill cell, or capture may cease while methane is still being generated. For example, Kaplan et al. (3) assumed 100% venting for either the first 2 or 4 years, high capture rates for the next 65 years, and 100% venting after that. These two profiles yield weighted average capture over 100 years of 64% and 61%, respectively, for low k; and only 49% and 24%, respectively, for high k. Electricity Generation from Captured Methane and GHG Offsets from Displaced Power. Methane captured for electricity generation can be combusted on-site in a reciprocating engine. Given methane’s heating value of 37.7 MJm-3, electricity generation is 4.1 kWhm-3 at 39% efficiency for the reciprocating engine generator (15). GHG displacement by electricity generated from LFG methane depends on the displaced power plant’s fuel and technology. Electricity from a low carbon source would emit few GHGs. Electricity generated from a combined cycle natural gas turbine with a 40% efficiency rating emits 0.54 kg CO2ekWh-1. Electricity generated via conventional coalfired steam electric power technology with a 32% efficiency rating emits 1.09 kg CO2ekWh-1. These latter two estimates include GHGs emitted during extraction, processing and distribution of the fuel, as well as combustion GHGs (16) (17). Given this range of GHG emissions for power generation, there is considerable discussion regarding the GHG offset that should be credited to landfills and WTE facilities generating power from MSW (9). This discussion involves whether the GHG offset should be calculated from base load average emissions, peaking or marginal, consumption average, or the build (i.e., planning) margin emissions for new facilities needed to satisfy future demand. For its WARM calculator, EPA uses U.S. average emissions from fossil-powered electricity generation, which are close to coal-fired GHGs due to the coal heavy fossil fuel mix used in US power plants (7). By contrast, BC Hydro’s production average is only 0.03 kg CO2ekWh-1 due to the preponderance of hydro power (9). Similarly, Seattle City Light’s production average is 0.02 kg CO2ekWh-1 (18). In Massachusetts, marginal peaking power is supplied by natural gas, as it is at times in Metro Vancouver. VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

7945

FIGURE 2. Metro Vancouver MSW net LF GHG emissions as a function of LFG capture efficiency. Rather than attempting to resolve this debate, this study uses natural gas powered electricity for base case offset, and provides sensitivity results for both the carbon neutral and the coal power ends of the offset spectrum. Carbon neutral power marks the hypothetical lower limit for low carbon power. Carbon Emissions from MSW Hauling and Landfill Operations. GHGs from hauling MSW amount to 0.036 kg CO2eMg-1km-1 (6). Climate changing emissions from landfill operating equipment and materials are estimated at 40 kg CO2eMg-1 (6, 7). Base case MSW transport distances (round trip) are assumed to be 600 km to an arid area landfill and 75 km to a high precipitation area landfill. These distances represent a situation that often occurs in western North America, where population centers are in high precipitation areas and MSW is shipped relatively long distances to landfill disposal in an arid region. On the basis of these distance parameters, GHG emissions from transport and landfill operations are 62 and 43 kg CO2eMg-1, respectively, for the distant and close by landfills. Net LF Carbon Emissions as a Function of LFG Capture Rate. For both low and high k landfills, Figure 2 shows net GHG emissions as a linear function of the LFG capture rate. The function accounts for emissions of fugitive methane over the 100 years following burial of one Mg MSW, emissions from hauling MSW and from landfill operations, and offsets for biogenic carbon sequestration and displaced naturalgas-fired power generation. As expected, higher capture rates result in lower potential climate impacts. Figure 2 shows the breakeven gas capture rate at which the landfill becomes climate neutral. Below breakeven capture the landfill is a net GHG emitter; above breakeven the landfill is a carbon sink. The climate breakeven capture efficiencies shown in Figure 2 are 62% for high k and 59% for low k landfills. Metro Vancouver MSW is the basis for Figure 2 because its disposed MSW has biogenic carbon content, biogenic carbon storage, and methane generation potential that all fall in between those parameters for Seattle and Massachusetts. Climate breakeven LFG capture for Seattle is 64% for high k, and 62% for low k. Corresponding figures for Massachusetts are 60% and 57%, respectively. Modeling Details for WTE Combustion GHG Emissions and Energy Generation. Fossil and Biogenic Carbon Emissions from Waste Combustion. WTE combustion GHGs depend on MSW fossil carbon content. MSW waste composition data indicate that fossil carbon amounts to 0.41, 0.51, and 0.43 Mg CO2eMg-1 MSW, respectively, for Seattle, Metro Vancouver and Massachusetts (Tables S1-S3 of SI). Biogenic carbon emissions from WTE are not counted as a GHG, following conventional LCA practice. Biogenic carbon released from MSW combustion is 0.68, 0.71, and 0.78 Mg CO2eMg-1 MSW, respectively, for Seattle, Metro Vancouver and Massachusetts (Tables S1-S3 of SI). Thus, fossil carbon amounts to between 36% and 42% of total carbon in MSW for these three localities. Analysis by Metro Vancouver’s Air 7946

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 20, 2010

Quality Group determined that fossil carbon in their combusted MSW was within this range as well (19). By contrast, Kaplan et al. (3) estimated 23% fossil carbon for their analysis. Electricity Generation from MSW WTE Combustion and GHG Offsets from Displaced Power. The amount of energy available for conversion to electricity in a mass burn WTE facility depends in part on MSW heating value, conversion efficiency, and parasitic power needs. Here net (gross less parasitic) conversion efficiency is assumed to be 19%, the base case conversion efficiency used in other studiesse.g., Kaplan et al. (3). MSW composition and estimated heating values for each waste component indicate that average heating values for the three localities are between 11 800 and 13 100 MJMg-1. Metro Vancouver’s heating value of 12 900 MJMg-1 falls between Seattle and Massachusetts. At 19% efficiency and 91% average system availability for generating power, base case net electricity generation for Metro Vancouver MSW is 620 kWhMg-1. At 0, 0.54, and 1.09 kg CO2ekWh-1 for carbon neutral, natural-gas-fired, and coal-fired power, respectively, GHG offsets for power displaced by WTE amount to 0, 0.33, and 0.68 MgCO2eMg-1. GHG Offsets for Ferrous Metals Recovered from Combustion Ash. WTE facilities reportedly can recover up to 90% of ferrous scrap through sorting MSW prior to combustion and running combustion ash over magnets. GHG emissions are 1.3 MgCO2eMg-1 lower for steel produced using recycled ferrous scrap instead of iron ore and other virgin raw materials. 1.2 Mg ferrous scrap are needed to produce one Mg new steel (20). Ferrous scrap for the three localities is 2% to 6% of disposed MSW. Assuming 5% composition and a 90% recovery rate at the WTE facility, ferrous recycling yields a 0.05 MgCO2eMg-1 offset. Aluminum scrap can also be recovered from WTE combustion ash. However, this is uncommon in North American facilities. Hence, WTE GHG benefits from scrap aluminum recovery are discussed in the sensitivity analysis, assuming that 2 kg marketable aluminum scrap can be recovered per Mg combusted. Producing aluminum from recycled aluminum rather than bauxite and other virgin raw materials saves 10 MgCO2eMg-1 aluminum scrap (20). Carbon Emissions from MSW Hauling, WTE Facility Operations and Ash Hauling. Climate changing emissions from WTE facility operating equipment and materials are approximately 30 kg CO2eMg-1 (7). Base case assumed transport distance (round trip) to WTE is 50 km, with ash hauling to an ash landfill also assumed as 50 km. Ash amounts to 18% by weight of combusted MSW (20). Thus, GHG emissions from transport and WTE facility operations are 32 kg CO2eMg-1. Net WTE GHG Emissions. On the basis of these GHG emissions and offsets estimates for WTE, net GHG emissions for a Metro Vancouver mass burn WTE facility would be 0.49, 0.16, or -0.19 MgCO2eMg-1MSW, depending on whether the displaced electricity is carbon neutral, natural-gas-fired or coal-fired. For Seattle MSW, which has a lower heating value than Metro Vancouver MSW, the corresponding figures are 0.49, 0.18, and -0.13 MgCO2eMg-1MSW. Massachusetts heating value is similar to Metro Vancouver, thus its net GHG emissions are similar. Heat Recovery from WTE or LF Electricity Generation. Combusting MSW or LFG to generate electricity produces heat that can be recovered to displace heat energy provided by fossil-fuel-fired furnaces and boilers. Because heat recovery at disposal sites is relatively uncommon in North America, its impact on GHG emissions is covered in the sensitivity analysis. Recovered heat displaces 2.2 kg CO2e

TABLE 1. Sensitivity of Landfill Methane Capture Crossover Level Metro Vancouver low k

FIGURE 3. Metro Vancouver MSW net LF GHG emissions minus net WTE GHG emissions.

FIGURE 4. Metro Vancouver MSW net LF GHG emissions minus net WTE GHG emissionss20-year perspective. from production and 90% efficient combustion of a cubic meter of natural gas.

Results and Discussion LF versus WTE GHG EmissionssCrossover LFG Capture Rates. Given net GHG emissions for WTE, as well as the linear relationship between LF net GHG emissions and gas capture efficiency shown in Figure 2, one can plot the difference between net emissions for LF versus WTE as a linear function of landfill gas capture rates. Figure 3 shows this functional relationship for both low and high k landfills for Metro Vancouver MSW and displacement of naturalgas-fired power. The figure indicates that landfill disposal has lower net GHG emissions for low k LFs with LFG capture above 47%, and for high k with capture above 51%. The LFG capture rate at which burning and burying have equal GHG emissions is termed the crossover rate. Above this crossover rate, LF has lower GHG emissions than WTE. Below the crossover rate, WTE is better for the climate. Seattle and Massachusetts crossover rates are higher than Metro Vancouver, mainly due to Seattle and Massachusetts MSW having lower fossil carbon content, which results in lower WTE fossil CO2 emissions. Nevertheless, the crossover LFG captures rates for both Seattle and Massachusetts are no higher than 58% for high k and 55% for low k LFs. The crossover LFG capture rate shown in Figure 3 is sensitive to displaced power. If displaced power is carbon neutral, then LF is better than WTE for climate change as long as LFG capture is above 19% and 30% for low and high k, respectively. If displaced power is generated by coal, then the crossover capture rate is 65% for both low and high k. Impact of Time Frame on LF GHG Emissions. The longer methane is in the atmosphere the greater the proportion of it that is oxidized. Thus, global warming potential (GWP) for methane is 25 over 100 years and 72 over 20 years subsequent to release (2). Figure 4 compared with Figure 3 shows substantial sensitivity for high k LFs to the LCA time frame and more

base case scenario best case scenarios LF WTE worst case scenarios LF WTE displaced electricity scenarios carbon neutral coal

Seattle

Massachusetts

high k low k high k low k high k

47%

51%

55% 58%

52%

56%

42% 86

46% 85

52% 53% 91 89

49% 89

50% 87

57% 38

62% 43

65% 69% 46 50

62% 43

67% 47

19% 65

30% 65

35% 42% 70 70

32% 68

40% 68

moderate sensitivity for low k. Figure 4 indicates that low k LF disposal is better than WTE when methane capture exceeds 57%. High k LF methane capture has to exceed 82% to have less damaging climate impacts than WTE. Thus, when the LCA time perspective is shortened to 20 years, low k crossover rate goes up ten percentage points, while high k shoots up over thirty points. The basis for this dramatic difference in sensitivity to time frame was portrayed in Figure 1. Almost 100% of methane generation occurs within 20 years in a high k landfill, compared with just 32% for low k. Over 20 years methane’s multiplier is 2.9× higher than over 100 years; however, cumulative methane generation for low k is nearly 65% lower over 20 versus 100 years. For high k cumulative methane is approximately the same over both time frames. Summary for Sensitivity of LF versus WTE Methane Capture Crossover. Table 1 shows sensitivity analyses for crossover methane capture rates for best and worst case LF and WTE scenarios. For comparison, it summarizes crossover rates for the base case. In addition, the table shows sensitivities for the hypothetical carbon neutral and coal ends of the displaced electricity spectrum to compare against the natural gas offset used in the base case. LF best case scenario entails 80% combined heat and power (CHP) recovery of potential energy in captured methane, and 20% oxidation of fugitive methane before it reaches the landfill surface. Best case scenario for WTE assumes 80% CHP recovery of energy in MSW, and recovery of aluminum in addition to ferrous metals. Worst case LF scenario assumes flaring of captured methane (i.e., no energy recovery), and 10% oxidation of fugitive methane. Worst case WTE involves 15% net energy recovery as electricity, no recovery of waste heat from electricity generation, and no recovery of metals. As indicated by sensitivity results shown in Table 1, LF methane capture crossover level is most sensitive to the level of WTE waste heat recovery. Aluminum recovery only accounts for a few points of the change in crossover level between base case and WTE best case scenarios. If a WTE facility is able to achieve very high usage and minimal distribution heat losses for heat energy, then the methane capture crossover rate goes up by 30 or more percentage points to levels of 85% and above. The caveat here is the feasibility of high year-round heat energy uses that are located so that heat energy losses from distribution are exceedingly low. The 75% recovery and use of waste heat from electricity generation that yields the 80% CHP recovery efficiency for the WTE best case would be quite unusual in North America. For example, Metro Vancouver’s current WTE facility provides heat energy to a nearby industrial user and electricity to BC Hydro. It attains waste heat recovery of 30% and CHP utilization under 50% (9). VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

7947

FIGURE 5. Comparison of GHG emissions per kWh for electricity from natural gas, coal and Metro Vancouver MSW. Substantial sensitivity in the opposite direction is indicated for displacement of low carbon electric power that is produced by suppliers such as BC Hydro and Seattle City Light. In this case, the LF methane capture crossover level drops by 15 to 20 points. For displacement of coal-fired power the crossover level goes up about 15 points from base case crossover levels. A final caveat involves high LFG capture rates. Several recent studies have indicated annual LFG capture rates of 85% or higher are possible (21-24). The open question is whether a landfill can maintain these high capture rates throughout the entire period of time when methane is being generated. For the high k landfill, this may not be much of an issue, because most methane generation occurs within a short number of years after MSW burial. At the same time, capture rates may be substantially lower during some years when a landfill cell is receiving MSW, which could make attainment of a high average capture rate problematic even for a high k landfill (24). GHG Emissions for Electrical Power Generation Options. One can use the emissions data presented here to compare climate impacts of using MSW for electricity generation against using natural gas or coal. To calculate climate impacts from the fuel used for power generation, the conventional LCA methodology is to inventory GHG emissions from the fuel’s combustion, and add GHG emissions from the “upstream” or “pre-combustion” portion of the fuel’s life cyclesi.e., extraction and refining of the energy resource into fuel, and transportation of fuel to the power plant. Energy intensities of different fuels are normalized by dividing GHG emissions by kWh generated. This was the methodology followed in Kaplan et al. (3). However, that study failed to take into account the difference in CO2 emissions between WTE and landfill gas-to-energy (LFGTE) due to the carbon sink provided by landfills through long-term storage of biogenic carbon. Not counting the release of sequestered biogenic carbon when MSW is used as a fuel for WTE is an accounting error similar to those discussed by Searchinger et al. (13). Life cycle GHG emissions for natural gas and coal power are 0.54 and 1.09 kgCO2ekWh-1, respectively, including upstream emissions. For base case WTE net conversion efficiency of 19%, Metro Vancouver MSW fossil carbon release is 0.87 kgCO2ekWh-1. For base case LF methane conversion efficiency of 39%, and assuming 75% LFG capture, low k LF GHG emission is 1.41 kgCO2ekWh-1. Long-term biogenic carbon storage for Metro Vancouver’s landfilled MSW is 417 kgMg-1. Release of this stored carbon yields WTE emission of 1.47 kgCO2ekWh-1. Figure 5 shows these results. Both LF and WTE emissions are above natural gas and coal, and none of the four fuels provide carbon neutral power. WTE emissions are lower if LCA system boundaries are expanded to include offsets for recovering scrap metals from WTE bottom ash. Recovering 90% of ferrous offsets WTE emissions by 0.08 kgCO2ekWh-1, assuming 5% of disposed 7948

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 20, 2010

MSW is ferrous metals. Assuming optimistic recovery of 50% for aluminum in bottom ash and 0.35% aluminum in MSW, recovering aluminum scrap from bottom ash offsets WTE emissions by 0.03 kgCO2ekWh-1. These offsets reduce WTE emissions per kWh by 7.5% to 1.36 kgCO2ekWh-1. Waste heat recovery also could change per kWh GHG rankings for WTE versus LF. It would not change WTE’s ranking versus the fossil fuels because their waste heat from power generation can also be recovered. Furthermore, very high recovery of WTE waste heat can not lower its per kWh GHGs to natural gas levels even if there is no heat recovery from natural-gas fired power generation. A final caveat for Figure 5 comparisons is that GHG emissions for LF and WTE assume a zero upstream burden when MSW is used as fuel. However, studies show that recycling MSW materials into new products substantially reduces energy usage and GHG emissions compared with products manufactured from virgin raw materials (25, 26). For example, upstream GHG emissions of 1.6 MgCO2eMg-1 are avoided when just the readily recyclable products and packaging in Metro Vancouver’s MSW are manufactured into new products (Table S1 of SI). These upstream avoidable emissions might be viewed as the upstream burden and opportunity cost of using MSW for fuel rather than recycling. Under this concept for the upstream burden of using MSW for fuel, GHG emissions per kWh for WTE would nearly triple compared with Figure 5 and LFGTE emissions would increase almost six times. The upstream avoidable emissions per kWh for LFGTE are much larger than for WTE because it takes more MSW input to produce electricity via LFGTE. Recall that LFGTE recovers energy only from the anaerobically degradable portion of biogenic carbon in MSW.

Acknowledgments The author thanks Kate Bailey, Gretchen Brewer, Eric Lombardi, Michelle Morris, and four anonymous referees for suggestions on drafts of this study that improved the analysis and discussion appreciably. Errors in the study remain the responsibility of the author.

Supporting Information Available MSW composition for Seattle, Metro Vancouver, and Massachusetts, as well as energy and carbon content, methane generation potential, biogenic carbon storage in landfill, and the upstream avoidable carbon footprint for readily recyclable MSW materials are available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) U.S. Environmental Protection Agency. Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and Figures for 2008; EPA-530-F-009-021; EPA; Washington, DC, 2009. (2) Intergovernmental Panel on Climate Change (IPCC), Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change; Cambridge University Press: New York, 2007. (3) Kaplan, P. O.; Decarolis, J.; Thorneloe, S. Is it better to burn or bury waste for clean electricity generation? Environ. Sci. Technol. 2009, 43 (6), 1711–1717, DOI 10.1021/es802395e. (4) Gentil, E.; Clavreul, J.; Christensen, T. H. Global warming factor of municipal solid waste management in Europe. Waste Manage. Res. 2009, 27, 850–860, DOI 10.1177/0734242 × 09350659. (5) Christensen, T. H.; Simion, F.; Tonini, D.; Moller, J. Global warming factors modeled for 40 generic waste management scenarios. Waste Manage. Res. 2009, 27, 871–884, DOI 10.1177/ 0734242 × 09350333. (6) Thorneloe, S. A.; Weitz, K.; Jambeck, J. Application of the US decision support tool for materials and waste management. Waste Manage. 2007, 27, 1006–1020, DOI 10.1016/j.wasman. 2007.02.024.

(7) U.S. Environmental Protection Agency. Solid Waste Management and Greenhouse GasessA Life-Cycle Assessment of Emissions and Sinks, 3rd ed.; EPA: Washington, DC, 2006. (8) ICF Consulting. Determination of the Impact of Waste Management Activities on Greenhouse Gas Emissions, 2005 Update-Final Report; prepared for,Environment Canada and Natural Resources Canada, ICF Consulting: Toronto, ON, 2005. (9) Sheltair Group. Environmental Life Cycle Assessment of Solid Waste Management: Evaluation of Two Waste Disposal Scenarios for the Metro Vancouver Region; prepared for, Metro Vancouver, Sheltair Group: Vancouver, BC, 2008. (10) Intergovernmental Panel on Climate Change. Climate Change 2007: Mitigation, Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change; Cambridge University Press: New York, 2007. (11) US Federal RegistersPart III Environmental Protection Agency, 40 CFR Chapter 1 Proposed Endangerment or Cause and Contribute Findings for Greenhouse Gases Under Section 202(A) of the Clean Air Act; Proposed Rule. 2009, 74(78), Friday April 24, 2009/Proposed Rules, page 18899. (12) Environment Canada’s Waste GHG Calculator discussed, and a contact for obtaining the Excel- based model pro vided, at http://www.ec.gc.ca/drgd-wrmd/default.asp?lang) En&n)6BFF0449-1. (13) Searchinger, T. D.; Hamburg, S. P.; Melillo, J.; Chameides, W.; Havlik, P.; Kammen, D. M.; Likens, G. E.; Lubowski, R. N.; Obersteiner, M.; Oppenheimer, M.; Robertson, G. P.; Schlesinger, W. H.; Tilman, G. D. Fixing a Critical Climate Accounting Error. Science. 2009, 326, 527–528, DOI 10.1126/science.1178797. (14) U.S. Environmental Protection Agency. Landfill Gas Emissions Model (LandGEM) Version 3.02 User’s Guide; EPA-600/R-05/ 047, EPA: Washington, DC, 2005. (15) National Renewable Energy Laboratory. Power Technologies Energy Data Book, 4th ed.; NREL/TP-620-39728, NREL: Golden, CO: 2006.

(16) Carnegie Mellon University Green Design Institute’s Economic Input-Output Life Cycle Assessment (EIO-LCA) model, at http:// www.eiolca.net. (17) U.S. Energy Information Administration, http://www.eia.doe. gov/oiaf/1605/coefficients.html. (18) Seattle City Light generation profile at http://www.seattle.gov/ light/FuelMix/. (19) AECOM Canada Ltd. Management of Municipal Solid Waste in Metro Vancouver - A Comparative Analysis of Options for Management of Waste After Recycling; prepared for, Metro Vancouver, AECOM: Burnaby, BC, 2009. (20) ResearchTriangleInstitute.MunicipalSolidWasteLife-CycleDatabase; prepared for EPA; RTI: Research Triangle Park, NC, 2003. (21) SCS Engineers. Current MSW Industry Position and State-of-the Practice on LFG Collection Efficiency, Methane Oxidation, and Carbon Sequestration in Landfills; prepared for Solid Waste Industry for Climate Solutions, SCS: Sacramento, CA, 2007. (22) Michels, M.; Hamblin, G. M. LFG Collection Efficiency: The Fine Tuning of Landfill Gas. MSW Management, 2008, March. (23) Solid Waste Association of North America. Landfill Gas Collection System Efficiencies. MSW Management, 2008, July-August. (24) Spokas, K.; Bogner, J.; Chanton, J. P.; Morcet, M.; Aran, C.; Graff, C.; Moreau-Le Golvan, Y.; Hebe, I. Methane mass balance at three landfill sites: What is the efficiency of capture by gas collection systems? Waste Manage. 2006, 26 (5), 516–525, DOI10.1016/j.wasman.2005.07.021. (25) Morris, J. Recycling versus incineration: an energy conservation analysis. J. Hazard. Mater. 1996, 47 (1-3), 277–293, DOI10.1016/ 0304-3894(95)00116-6. (26) Morris, J. Comparative LCAs for curbside recycling versus either landfilling or incineration with energy recovery. Int. J. Life Cycle Assess. 2005, 10 (4), 273–284, DOI10-1065/lca2004.09.180.10.

ES100529F

VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

7949