Comparing Greenhouse Gas Emissions from Various Methods of Organic Waste Disposal. Anna M. Brockway, Summer 2012

Transcription

1 Comparing Greenhouse Gas Emissions from Various Methods of Organic Waste Disposal I. Introduction Anna M. Brockway, Summer 2012 Finding and implementing the most sustainable method of organic waste disposal is an ongoing challenge that has the potential to impact waste processing practices throughout the United States and elsewhere. Defined as carbon-based matter with animal or plant origins, organic waste is a broad category that includes food remains, livestock manure, sewage sludge, and yard waste. It can be sourceseparated into fully organic waste streams, deliberately mixed with other materials prior to disposal (e.g., sand or ash in the case of manure), or found in municipal solid waste (MSW), which is the net output waste from residential, commercial, and some industrial sources in a community. The U.S. Environmental Protection Agency estimates that food and yard waste can each constitute approximately 13-14% of MSW (EPA 2010); the proportion of food waste might be even greater in some communities that explicitly require the diversion of yard waste or other materials. There are multiple options for organic waste disposal currently in use, including landfilling, incineration, composting, various forms of livestock manure management, and anaerobic digestion (AD). A thorough comparison of these disposal methods must take into account their environmental impacts and possible benefits, economic factors, as well as location-specific options and constraints. Focusing on the environmental considerations, this paper analyses the potential greenhouse gas (GHG) emissions impact of these various disposal options. The decomposition of organic waste produces a gas byproduct, which is typically composed of several greenhouse gases. Alternatives to landfills and incineration facilities have demonstrated direct and indirect environmental benefits; just one of these is their potential to reduce GHG emissions. This analysis considers direct GHG emissions from organic waste disposal methods and power conversion, as well as direct offsets of fossil fuel emissions, where applicable. It does not take into account indirect emissions (e.g., from transportation of feedstock or building materials) or avoided emissions (e.g., indirect offsets from avoiding more GHG-intensive disposal methods). These indirect impacts must be evaluated when considering the implementation of an organic waste disposal method on a specific site. They are left out here due to the diversity of possible projects and in an effort to minimize the complexity of the calculations. II. Technical Background: Greenhouse Gas Emissions from Organic Waste The decomposition of organic waste (i.e., feedstock) is a biological process that results in the emission of gas byproducts. Greenhouse gases (GHGs), such as methane (CH 4 ), carbon dioxide (CO 2 ), and nitrous oxide (N 2 O), constitute the bulk of these byproducts. Generally, methane and CO 2 make up 95% of gas emissions, while N 2 O and other components are less than 5 percent typically less than 1 or 2 percent of emitted gases (Rasi 2009, Jonsson 2003). The specific composition of any given gas byproducts is 1

2 determined by numerous variables, including the source and characteristics of the organic matter and the type of environment in which it decomposes. Global Warming Potential (GWP). The GWP of a greenhouse gas is a mass-based measure of how much heat the gas traps in the atmosphere relative to a standard, typically carbon dioxide. The average GWP of methane is 25 times that of CO 2 ; the GWP of N 2 O is 298 (IPCC 2007). These values are revised from the IPCC s earlier estimates of 21 and 310 for CO 2 and N 2 O, respectively. Aerobic and Anaerobic Decomposition. Aerobic decomposition of organic matter occurs when extra oxygen is available in the surrounding environment. This type of decomposition generally produces only carbon dioxide and water vapor (e.g., Equation 1). Anaerobic decomposition, which occurs in a closed system in the absence of oxygen, typically produces both carbon dioxide and methane (e.g., Equation 2). C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O (1) C 6 H 12 O 6 3CO 2 + 3CH 4 (2) To mitigate the impact of gas byproducts, methane can be combusted and converted into CO 2 prior to release, giving a 96 percent emissions reduction per repurposed carbon atom and an 89 percent emissions reduction per unit weight of gas. (The slight discrepancy here occurs because CO 2 has a higher mass than CH 4, and the GWP is mass-based.) This conversion occurs via the following chemical reaction: CH 4 + 2O 2 CO 2 + 2H 2 O (3) For the sake of simplicity, this analysis will assume that gas byproducts are entirely composed of methane and carbon dioxide. These two gases constitute a large majority of GHG emissions from organic waste decomposition, and the type of decomposition (aerobic vs. anaerobic) impacts mainly the relative methane/carbon dioxide ratio in the emitted gas. Therefore, while this assumption underestimates the GHG impact of organic waste disposal, it still allows for fruitful comparison across disposal methods. Furthermore, since the measurement error may be greater than the proportion of other GHGs, it is difficult to find accurate data on these gases. Methane is a potent GHG if released to the atmosphere. But methane gas also has high energy content and can be used in place of natural gas, offsetting the use of fossil fuels. If gas byproducts are produced via a method of organic waste disposal that collects and uses the gas for energy, gas with a high methane composition is considered good-quality due to its high potential for energy production. Methane Conversion Factor (MCF). The MCF is a measure that indicates the percentage (0-100%) of degradable (volatile) organic solids converted to methane, as compared to the theoretical maximum, which is typically referred to as B o in the literature (Mangino 2011). MCF values are typically used to characterize the methane-producing potential of different types of organic waste management systems. Chemical Oxygen Demand (COD). Any material with organic (i.e., carbon) content can, in theory, be oxidized to form CO 2. The amount of oxygen needed for this process is given by the COD and indicative of the amount of carbon present in the substrate. Organic waste also has the potential to generate a 2

3 certain amount of methane based on its carbon content. With conversion to methane, the carbon present in the substrate no longer demands oxygen to become carbon dioxide, so the theoretical maximum methane yield is a constant in terms of COD: 318 m 3 of CH 4 can be generated per ton 1 of COD removed (Wilkie 2011). Carbon to Nitrogen (C/N) Ratio. The C/N ratio of a particular feedstock is also indicative of its methane production potential, as the microbes that drive the digestion process need a certain amount of each for optimal growth. Typically, C/N ratios between 20 and 30 are best (Puyuelo 2011, Austin 2011). Different types of organic waste have different C/N ratios: livestock manure tends to be high in nitrogen (C/N ratios around 3-10), while food scraps often have higher carbon contents (C/N ratios around 15), and cellulosic organic wastes, such as yard waste, have very high carbon contents (typical C/N ratios from ) (Steffen 1998, Zhang 2007). Solids Content (TS and VS). Organic wastes contain water as well as some amount of total solids (TS), which include some amount of fixed solids (i.e., ash), and volatile solids (VS) that can decompose to generate gas byproducts. The amount of TS is generally reported as a percentage of the total feedstock. The amount of VS can be reported as a percentage of either the feedstock or of the TS; the relative magnitude of the VS to the TS value indicates which. Unless otherwise noted, the amount of VS is reported as a percentage of the total feedstock throughout this document. Feedstocks with high VS/TS ratios are considered to be high-energy (Scott 2004); decomposition is most effective when the ratio is at least 80% (Pine Island Farms 2008). Food waste has VS/TS ratios of 80-90% (Zhang 2005), while sewage sludge and livestock manure typically have VS/TS ratios of 70-80% (Gray 2008, Steffen 1998). Volatile Solids Destroyed (VSD). For all feedstocks, the degree of decomposition is usually given by the percentage of volatile solids that have been destroyed (VSD). The methane generation potential is not a constant in terms of VSD; rather, it depends on the specific composition (i.e., carbon content) of the VS. For example, the theoretical methane potential is 345, 544, and 907 m 3 /tons VSD respectively for pure carbohydrates, proteins, and lipids (Wilkie 2011). However, some combination of these is typically present in organic matter, and even if the specific composition is known, caution is required when interpreting these values. For example, lipids have a relatively high CH 4 production potential due to their high hydrogen-to-carbon ratios (Wilkie 2011), but in order for microbes to productively decompose organic wastes, the feedstock must also contain some nitrogen, as discussed above. The methane potential for actual VS decomposition is therefore lower, and may not be proportionate to, these theoretical estimates. The VSD potential tends to be higher for food waste (74-81%) than for wastewater sludge (57%) (Gray 2008). Co-Digestion. In some cases, the co-digestion of feedstocks may increase the rate of decomposition and biogas potential of the overall mixture by balancing the VS/TS and C/N ratios (McDonald 2008). Common feedstocks for co-digestion are food waste (high VS/TS ratios, high carbon content) and manure (lower VS/TS ratios, high nitrogen content) (Scott 2004, Austin 2011). Cellulosic organic wastes, such as wood and grass fibers, typically have very high carbon contents and are not easily degradable. 1 All tons referred to in this document are defined as standard U.S. short tons, equal to 2,000 lbs or 907 kg. 3

4 However, co-digestion of these wastes with manure can balance out the C/N ratio of the overall mixture and drastically increase biogas production (Ahn 2010). III. Methods of Organic Waste Disposal Life-cycle analyses of waste disposal methods, which typically consider indirect emissions from transportation and building materials as well as avoided emissions, demonstrate numerous environmental benefits of organic waste diversion from landfills and incinerators. These include reductions in pathogen content, odor, and greenhouse gas emissions. A study conducted by Börjesson and Berglund (2007) found that biogas-based heat led to 75-90% lower GHG emissions as compared to fossil-fuel based heat; the highest reductions were observed for manure and food waste-based biogas systems. The same study found that the use of biogas systems could lead to reductions in eutrophication and acidification consequences of nitrogen and phosphorous leakage into soils as well as air pollution, although the causes and mitigation efforts related to these processes are beyond the scope of the present work. Several studies have examined GHG emissions from different waste management methods. Mohareb et al. (2011) compared four treatment options for Toronto MSW (landfill with 75% gas capture, incineration, compost, and AD with an estimated 5% biogas leakage rate). The IPCC 2006 model used showed that anaerobic digestion had by far the lowest potential GHG emissions per ton of waste disposed. Captured landfill gas or biogas from organic waste decomposition can be used for fuel; once offset energy use is taken into account, AD was the only waste management process with net negative GHG emissions (Table 1). Table 1. IPCC 2006 GHG emissions results for different waste management options (Mohareb 2011). GHG Emissions (tons CO 2 e) Treatment Option* Gross Emissions (GE) GE Per Ton Disposed Emissions Offset Net Emissions (NE) NE Per Ton Disposed Landfill with 75% gas capture 348, , , Anaerobic Digestion** Incineration (Waste to Energy) 29, ,200 20, Composting 75, n/a 75, *Assuming 47% efficient methane-to-electricity conversion for gas captured from all options. **Assuming 5% gas leakage rate. DiStefano and Belenky (2009) performed a life-cycle analysis of GHG emissions comparing MSW disposal in landfills and with anaerobic digestion. Based on 2006 data, they estimated that full diversion of MSW from landfills to AD systems with electricity production throughout the United States would lead to an annual reduction in GHG emissions of 146 million tons of CO 2 eq per year (1.9% of total US GHG emissions), or 7.2 billion tons of CO 2 equivalents (tons CO 2 eq) in a 50-year period. Weitz et al. (2002) suggest that improvements in waste disposal methods between 1974 and 1997, including the diversification of waste processing options and increasing diversion of recyclables and organics from landfills, have prevented 57.3 million tons CO 2 eq from being released to the atmosphere. 4

5 The controlled anaerobic digestion of organic waste does result in some GHG emissions. However, since organic matter decomposes rapidly in nature, carbon dioxide emissions from decomposition are often considered to be biogenic (i.e., they would have been present even without technological intervention). For example, the California Climate Action Registry (CCAR), a non-profit organization that facilitates voluntary recording of GHG emissions, does not require CO 2 produced in a closed AD process to be reported when calculating a carbon footprint (CCAR 2011). Methane emissions from digester gas are considered anthropogenic and do need to be reported, but emissions are still biogenic if methane is converted to CO 2 prior to release (Arifian 2008). In fact, the CCAR s Organic Waste Digestion (OWD) Protocol allows institutions to claim carbon credits for organic waste that is diverted to anaerobic digesters, indicating the effectiveness of AD at mitigating GHG emissions (DuBuisson 2009, CCAR 2011), and potentially providing an additional economic incentive for sustainable waste disposal. Since waste disposal facilities process organic matter in different forms (e.g., with varying moisture contents), comparing the GHG emissions of organic matter by weight or volume across different disposal methods can be misleading. A more reliable comparison can be made by evaluating GHG emissions from a given amount of organic matter volatile solids, which are decomposable. For this reason, and unless otherwise noted, the units used to describe the gas emissions potential of organic waste in the remainder of this document will be m 3 gas/ton VS, where m 3 refers to standard cubic meters. A more detailed discussion of the various methods of waste disposal is found below. Landfills Organic waste that ends up in landfills decomposes to generate landfill gas (LFG) that is typically composed of about percent methane, percent CO 2, and less than 5 percent N 2 O and other gases (NCDENR 2010, Rasi 2009). The EPA standard value for estimating the methane content in landfill gas is 50 percent (EPA LMOP 2012). Landfills are a major source of greenhouse gases in the United States: in 2009, they were the third largest man-made source of methane, accounting for 17.1 percent of methane emissions (EPA LMOP 2011). Numerous factors influence the amount of LFG produced from organic waste decomposition, including temperature, moisture, the proportion of organic waste relative to non-degradable waste, the type of organic waste, and the amount of bacteria present (Hayes 2006, CRA 1995). Typically, organic waste in a landfill has a moisture content of approximately 25% (CRA 1995). Some landfills utilize bioreactor technology, which infuses additional moisture into the waste. This increases the rate of LFG production, but does not change the total amount of gas produced (RWC 2001). Although the relative proportion of the emitted gases varies with time (EPA 2005), the bulk of LFG generation typically occurs over a period of years from the time of waste disposal (RWC 2001, Samir 2011). The relative emission rates are not considered here; instead, emissions estimates are given for the entire year period. LFG emissions are typically reported by volume per weight of municipal solid waste (MSW). In order to compare landfill emissions to emissions from other disposal methods, the LFG generation potential per ton of volatile solids is required, so the amount of VS in MSW must be calculated. This calculation is one of the biggest challenges in determining a reliable value for potential LFG production. 5

6 Evaluating LFG Potential and VS Content of Various MSW Components The EPA has published values for the proportion of organic wastes in MSW (EPA 2010, Table 2, column 1). Various estimates for the solids content and LFG production potential of these wastes are in Table 2. Table 2. Estimates for VS content and amount of LFG produced from separated landfill matter. Type* TS VS VS/TS Moisture Retention Reported Calculated CH 4 (%) (%) (%) (%) (%) Time (y) (m 3 CH 4 /ton VS) (m 3 /ton VS) (%) Source*** Guyer 2002 Food Mohapp 2011 (13.9) ** Samir 2011 (Suthatip 2006) AVG Paper (28.5) Yard (13.4) Wood (6.4) Mohapp ** Samir 2011 (Suthatip 2006) ** Samir 2011 (Owens 1993) AVG Guyer Mohapp 2011 AVG Guyer Samir 2011 (Suthatip 2006) AVG *Percentages from EPA **Average value assumed. ***Original source in brackets, where applicable. The landfill gas potential per ton of VS (Table 3) can be calculated based on the average composition of MSW (Table 2), the VS content of different organic feedstocks (Table 2) and the reported landfill gas emissions potential per ton of waste (Table 3), using Equation 4. m 3 LFG mass VS = m 3 LFG per mass MSW %V paper %VS paper +(%V food %VS food ) + (%V yard %VS yard ) + (%V wood %VS wood ) (4) Table 3. Estimates for the amount of LFG produced from 1 ton of landfill waste. Type* TS (%) VS (%) VS/TS Landfill Gas Moisture Retention CH Reported Calculated 4 (%) (%) Time (y) (%) (m 3 /ton)* (m 3 /ton VS) Source** MSW RWC 2001 (Oweis 1988) CRA 1995 (Ham 1989) OW Filtrexx Intl 2009 MSW EPA LandGEM Std (EPA 2005) HW GE Energy 2004 OW optimal Hayes 2006 MSW <60 van Zanten 2000 MSW *** RWC 2001 Estimated Average *HW: household waste; MSW: municipal solid waste; OW: organic waste. **Original source in brackets. ***Theoretical max. 6

7 Since estimates of VS content in any given type of organic waste are few and vary widely (Table 2), values for the landfill gas potential derived in this manner are rough at best. Evaluating LFG Potential and VS Content of Overall MSW Few authors report VS content of MSW or LFG potential per ton of volatile solids; several sources are gathered in Table 4. Table 4. Estimates for volatile solids content and LFG produced from 1 ton of landfill waste. TS (%) VS (%) VS/TS Landfill Gas Moisture Retention CH 4 (%) (%) Time (y) Reported Calculated (%) (m 3 CH 4 /ton) (m 3 /ton VS) Source*** Samir 2011 (Chiemchaisri 2007) Samir ** (tmsw) 55 DiStefano 2009 (OWS 2003) ± 8.5 Mor Samir 2011 (Barlaz 2002) * Samir 2011 (Suthatip 2006) AVG *Average value used. **55% biodegradable VS, 65% VS. ***Original source in brackets, where applicable. Type MSW Suthatip et al. (2006, in Samir 2011) report a methane potential of m 3 /ton VS. At a methane composition of 50%, the total gas emissions would be m 3 LFG/ton VS. The reported volatile solids content of MSW is averaged from several sources to be approximately 28.78%. These numbers tend to be more consistent, and therefore more reliable, than the reported volatile solids content or gas generation potentials for different MSW components (Table 2). Dividing the EPA standard value for LFG potential (154.2 m 3 /ton MSW, Table 3) by the VS content of MSW yields m 3 LFG/ton VS. LFG Production Potential from Volatile Solids in MSW In conjunction with the results from Tables 3 and 4, the LFG potential is estimated to be m 3 /ton VS, with a methane content of 50%. This value will be used in later calculations. Incineration Waste combustion, attractive due to its potential to reduce waste volume and destroy pathogens, has been in use in the United States for close to 100 years. Today, incineration facilities typically divert recyclable items from the waste stream prior to combustion and house energy recovery systems to generate power, enabling them to reliably offset some use of fossil fuels (Weitz 2002, Clarke 2002). Waste that is ultimately incinerated may include organic as well as inorganic matter, making GHG emissions comparison to other organic waste disposal methods difficult. Complete waste drying and combustion can lead to 90% reduction of waste by volume and 75% reduction by weight (MSW Learning Tool 2008). This process also results in multiple outputs, including ash, volatile gases, particulate matter, acid gases, mercury, nitrogen oxides, CO 2, and water vapor. (It should be noted, however, that these are not all unique to incineration.) Technologies that aim to 7

8 control some of these emissions have successfully reduced the amount of toxins, including mercury and nitrogen oxides, in the output stream (Clarke 2002), but significant GHG emissions remain. One ton of incinerated waste can create 3.18 MW of power (MSW Learning Tool 2008) and release approximately tons of CO 2 (Johnke 2000). Composting Organic matter decomposes much more rapidly through composting than in landfills and produces a beneficial product. Large-scale composting can lead to significant estimated at over 90 percent reductions in greenhouse gas emissions as compared to landfilling, since exposure to oxygen results in a greater proportion of CO 2 created relative to methane. Small-scale composting is even more sustainable, largely because it does not generate GHG emissions from fuel transportation or electricity use, but the variety of projects make estimates difficult (GCC 2011). Agriculture and Livestock Manure Management Greenhouse gas emissions from livestock waste vary based on the type of livestock and the waste disposal method. Additionally, the rate of emission is directly correlated with the temperature and moisture content of the waste (EPA 1999, Key 2011, Mangino 2011, Steffen 1998). Common manure disposal methods include liquid-based systems, dry systems, and daily spreading on fields as fertilizer. In temperate climates (defined by the IPCC as C), dry systems and daily spreading on dairy farms result in relatively low greenhouse gas emissions: methane conversion factors (MCFs) for these largely-aerobic methods range from 0.5 to 4 (however, other environmental impacts such as nutrient runoff and odors may result). In contrast, anaerobic liquid-based systems, including slurries and manure lagoons, have MCFs around and 74-79, respectively (CCAR 2009, Zeeman 2003). The theoretical maxima for methane production are 435 m 3 /ton VS and 218 m 3 /ton VS for liquidbased digestion of swine and dairy cattle manure, respectively (Hashimoto 1984 and Morris 1976, in Mangino 2011). Liquid-based storage systems are more common on larger farms and are typically confined to dairy and swine farms (EPA 1999). Biogas produced through anaerobic decomposition whether from manure or food waste typically has a higher methane content than landfill gas, ranging from roughly percent methane (Rasi 2009, Jönsson 2003, Van Opstal 2006), with an estimated average of 60 percent (Key 2011). Methane emissions from livestock manure management are a major source of greenhouse gases, comprising ten percent of total methane emissions in the United States in Based on past trends, the EPA has predicted that the proportion of liquid-based storage systems, such as manure slurries or lagoons, will continue to increase, leading to even higher overall methane emissions (EPA 1999). Anaerobic Digestion Anaerobic digestion (AD) is a proven technology for reducing the volume of and pathogens in noncellulosic organic wastes, including food scraps, livestock waste, and sewage sludge. Digesters facilitate the controlled degradation of organic waste with naturally-occurring bacteria, performing this process 8

9 much more rapidly and efficiently than landfills (DiStefano 2009). Many types of anaerobic digester systems exist. They can be classified into thermophilic, mesophilic, and psychrophilic systems, which operate at temperatures of 50-60, 35-40, and C, respectively (Octaform 2011). Thermophilic and mesophilic digesters tend to be more efficient at organics-to-energy conversion, are much more common than psychrophilic digesters, and may be tailored to specific feedstocks (DiStefano 2009). Biogas generation potentials and methane composition are often very similar for thermophilic and mesophilic systems. In a digester, organic waste is contained in a closed space. It decomposes to yield biogas, which can be used for power generation, and digestate, which can be processed and used for fertilizer, animal bedding, or disposed of in a landfill. Biogas generation potentials vary for different AD feedstocks; they are often presented as the volume of gas that can be produced by a given mass of volatile solids. A summary of this data is found in the tables below. Data from sources labeled Actual was reported by operating pilot- or full-scale anaerobic digestion facilities, unless otherwise noted. Data from sources labeled Potential is taken from laboratory-scale studies, theoretical calculations, or sources where the origin of the data is unclear. When the biogas production potential was reported in units other than volume/weight VS, the calculation is explained either in the table or in a footnote. Numbers reported in units where reasonably accurate conversion is not available are italicized. Food Waste and Source-Separated Organics Food waste, which often ends up in landfills, can provide a valuable feedstock for biogas generation and lead to reductions in GHG emissions if it is separated and diverted to anaerobic digestion facilities. As an example, the City of Toronto currently operates an anaerobic digestion organics processing program that takes in source separated organic (SSO) material from residential and commercial sources, as well as city water to aid with processing and cleaning (Van Opstal 2006). The solid digestate produced by the AD system is used as a feedstock in composting, the liquid fraction is disposed of in the sewer system, and the biogas is flared. During its first six months of operation, the facility took in an average of tons of organic waste and produced 52,836 m 3 of biogas per week, with an average methane composition of 56% (Van Opstal 2006). In this time period, controlled anaerobic digestion and flaring therefore prevented the weekly emission of 29,588 m 3 methane on average, as it was first converted to the significantly less harmful CO 2. Anaerobic digestion facilities report biogas production potentials of approximately 100 m 3 /ton SSO, but estimates increase significantly when only volatile solids are considered. Based on this data, degradable food waste has the calculated potential to generate biogas at a rate of approximately 450 to 650 m 3 /ton VS at a methane content of approximately 60%, depending on the retention time, temperature, and AD system type. 9

10 Table 5. Estimates for the amount of biogas produced from 1 ton of degradable food waste. Type TS (%) VS (%) VS/TS Retention VSD Biogas (%) Time (d) (%) (m 3 CH /ton VS) 4 (%) Location / Source*** * 70 Anyang City, S. Korea Caddet SSO* * 60 Newmarket AD Arsova 2010 M * - Dufferin Organics M 4-5, , 37 C (a) 56 ± 9.4 Processing, Toronto Van Opstal 2006 SSO 10, 50 C AD Phased Solids , 50 C Pilot Zhang Pinetop-Lakeside, AZ Nantucket, MA Batch AD tests of organics , Delaware County, NY - collected from C Rapid City, SD actual facilities, Seiverville, TN Zhang Cobb County, GA T C Pilot AD, UC-Davis Zhang (b) - Aalborg Municipality Poulsen , 37 C (b) - Solid-Bed 2-Phase AD Cho 1995 M (c) (d) 64 East Bay Municipal SSO Gray (c) 60 Utility District T (d) (b) - CNBE Estimate CNBE 2012 M , 35 C 77 - Lab & Pilot Hybrid Zhang , 35 C 78 - Solid-Liquid ADs (Wang 2005) T 4 (e), 55 C MSW M 8 (f), 40 C Laboratory-scale study, Gibson Scott Veg Arthur 2010 (GATE and GTZ 2007) SSO Steffen 1998 (Nordberg 1997) M MSW , 37 C Laboratory-scale study, Rivard SSO Viswanath (b) - Laboratory-scale study, WRAP 2011 Estimated Average *Units: m 3 /ton SSO. **Methane only. ***Location indicated in separate cell and original source in brackets, when appropriate. (a) Calculated from biogas generation rate of 7,548 m 3 /day and weekly VS loading of 90.4 tons, given 5-day operating week. (b) Calculated assuming 60% methane content. (c) Calculated from m 3 CH 4 /ton TS given % CH 4 and % VS/TS. (d) Calculated from 481 m 3 CH 4 /ton TS given % CH 4 and % VS/TS. (e) Preceded by pasteurization at 70 C. (f) Preceded by thermal hydrolysis for 20 min. Actual Potential Thermophilic MSW Organics Livestock Manure Manure is typically a lower-energy feedstock than food waste, but its energy content depends on the type of livestock. The number of anaerobic digesters processing livestock manure in the United States is increasing, and numerous case studies demonstrate the GHG benefits of AD for manure management. One study of GHG emissions was conducted by Cornell researchers at seven dairy farms (525-1,800 cows) in western New York State. Five of the farms studied used their AD systems purely for manure 10

11 management, while two farms supplemented manure feedstock with food waste. On average, the farms saw annual greenhouse gas reductions of 2611 tons CO 2 eq from the anaerobic digesters (Pronto 2010). A life-cycle analysis comparing manure-only AD to a reference liquid-based manure storage system was conducted by Turnbull et al. (2003) at Langerwerf Dairy Farm, a 400-cow farm near Durham, CA. The study showed that the AD system reduced CO 2 emissions by 61% and CH 4 emissions by 90%, for a total greenhouse gas reduction of 79%, even when emissions from material production and transport, construction, operation, and demolition were included (Turnbull 2003). Electricity and heat generated by the AD system through a combined heat and power (CHP) unit contributed to farm operations, and the wastewater was used successfully as corn fertilizer. An AD system at Jordan Dairy Farms, a 300-cow farm in Rutland, MA, operates on both manure and food waste. The facility is estimated to process 9,125 wet tons of manure and 6,023 wet tons of sourceseparated organics per year, operating at 85 percent of the system potential. The AD system has a capacity of 300 kw and produces 2,240,000 kwh of energy per year, enough to power the farm and 300 homes (Jordan Dairy Farms 2012). It is estimated to emit a maximum of 3,200 tons of CO 2, and no methane, per year (Jordan Dairy Farms 2010). Table 6. Estimates for biogas produced from 1 ton of degradable cow manure (dairy unless otherwise noted). Type TS VS VS/TS Retention VSD Biogas CH C/N 4 (%) (%) (%) Time (d) (%) (m 3 /ton VS) (%) Location / Source* Langerwerf Dairy Turnbull ** Pine Island Farms Pine Island Farms 2008 M , 35 C AA Dairy , 35 C Gordondale Farms Martin , 10 C , 15 C 18.7 P Laboratory-scale, Masse , 10 C , 15 C (avg) T , 55 C *** - Manure/switchgrass mix, Ahn 2010 M , 35 C , 32.8 C Walsh 1988 (Price 1981) Arthur 2010 (GATE and GTZ 2007) P C Fulhage USDA Steffen 1998 (Brachtl 1998, Braun 1982, Thome-Kozmiensky 1995, Wellinger 1984) M , 37.2 C Walsh 1988 (Fannin 1982) T , 55 C , 55 C Beef manure, Walsh 1988 (Price 1981) Parameter for Eastern Europe, MNPRA 2010 M , 37 C McDonald 2008 T , 55 C Beef manure, Walsh 1988 (Fannin 1982) Theoretical Maximum 362.9*** - Mangino 2011 (Morris 1976) Estimated Average *Location indicated in separate cell and original source in brackets, when appropriate. **Calculated from manure inputs only. ***Calculated from m 3 CH 4 /ton VS assuming 60% CH 4 for cow biogas. Actual Potential 11

12 Based on this data, it is estimated that degradable cow manure generates biogas at a rate of approximately m 3 /ton VS with a methane content of roughly 60% through anaerobic digestion. Table 7. Estimates for biogas produced from 1 ton of degradable swine manure. TS VS VS/TS Retention VSD Biogas CH Type C/N 4 (%) (%) (%) Time (d) (%) (m 3 /ton VS) (%) Actual Potential T M , 55 C *** - 15, 60 C *** - 15, 37 C *** - 15, 45 C *** - Hansen 1998 Location / Source* Carroll s Westerman 2008 (Safley 1993) P Barham Westerman 2008 (Cheng 2004) M Malik 2009 (Lee 1970) 180, 10 C 33.9*** P , 15 C 165.6*** - 272, 10 C 47.9*** - Laboratory-scale, Masse , 15 C 265.8*** Steffen 1998 (Brachtl 1998, Braun 1982, Thome-Kozmiensky 1995, Wellinger 1984) Arthur 2010 (GATE and GTZ 2007) USDA 2007 P C Fulhage 1993 M , 37 C McDonald 2008 P C Westerman 2008 (Laboratory study, Safley 20 C ) For developing countries, MNPRA *** - Hansen 1998 T , 55 C *** - Manure/switchgrass mix, Ahn 2010 Theoretical Maximum 622.1*** - Mangino 2011 (Hashimoto 1984) Estimated Average *Location indicated in separate cell and original source in brackets, when appropriate. **Calculated from manure inputs only. ***Calculated from m 3 CH 4 /ton VS assuming 60% CH 4 for pig biogas. Based on this data, it is estimated that degradable pig manure generates biogas at a rate of roughly 350 m 3 /ton VS with a methane content of approximately 70% through anaerobic digestion. Table 8. Estimates for biogas produced from 1 ton of degradable chicken manure. Type TS VS VS/TS Retention VSD Biogas CH C/N 4 (%) (%) (%) Time (d) (%) (m 3 /ton VS) (%) Location / Source* T , 55 C ** - Manure/switchgrass mix, Ahn For developing countries, MNPRA 2010 P C Fulhage 1993 M , 37 C Poultry, McDonald USDA > Steffen 1998 (Brachtl 1998, Kuhn 1995) Estimated Average *Location indicated in separate cell and original source in brackets, when appropriate. **Calculated from m 3 CH 4 /ton VS assuming 65% CH 4 for chicken biogas. potential 12

13 Based on this data, it is estimated that degradable chicken manure generates biogas at a rate of roughly 350 m 3 /ton VS with a methane content of approximately 65% through anaerobic digestion. Wastewater and Sewage sludge In May 2009, Sylvis Environmental prepared an extensive report on sewage sludge disposal methods for the Canadian Council of Ministers of the Environment (Sylvis 2009). The report resulted in the Biosolids Emissions Assessment Model (BEAM), a computational tool which enables the evaluation of GHG emissions from sewage sludge subjected to various waste disposal practices. The BEAM model shows that alternatives to landfills are accompanied by significant reductions in GHG emissions; when offset energy use is factored in, anaerobic digestion can even result in net negative GHG emissions (Carpenter 2010). However, the best waste management system depends on the specific project characteristics. Since the moisture content of sewage sludge varies widely, estimates for potential biogas generation are typically given per dry weight of biosolids. GHG emissions from sludge drying must therefore be considered in a full comparison of disposal methods used for sewage sludge (Sylvis 2009). However, the biogas production potential of anaerobic digestion per ton of sludge volatile solids (Table 9) can still be compared to other feedstocks and methods. Table 9. Estimates for amount of biogas produced from 1 ton of degradable sewage sludge. Type TS VS VS/TS Retention VSD Biogas CH 4 (%) (%) (%) Time (d) (%) (m 3 /ton VSD) (m 3 /ton VS) (%) Location / Source** 10, 28 C , 28 C Kwame Nkrumah, , 28 C Ghana Arthur , 28 C (b) (a) - CNBE Estimate CNBE 2012 Actual Mesophilic Potential (b) 63 East Bay Municipal Utility District Gray , 36 C Pittsfield WWTP Pittsfield Hardwar, Uttarakhand Malik Aalborg Municipality Poulsen 2009 M , 36 C (c) 61 Deer Island WWTP MWRA Old ADs, Deer Island MWRA Hanjie 2010 (Rittmann 2000) Hanjie 2010 (Speece 2001) Arthur 2010 (GATE and GTZ 2007) M , 37 C McDonald Hanjie 2010 (Sato 2001) * Sylvis 2009 (WEF 1998) Walsh 1988 (Stafford 1980) Estimated Average *Average value assumed. **Location indicated in separate cell and original source in brackets, when appropriate. (a) Assuming 60% CH 4. (b) From 5 m 3 CH 4 /ton TS given 63% CH 4 and 77% VS/TS. (c) From 1053 m 3 /ton VSD given 65% VSD. 13

14 For sewage sludge treated in an anaerobic digester, the BEAM report estimates that volatile solids are destroyed by approximately 50-60% (Sylvis 2009). For every ton of volatile solids destroyed, biogas yield from sewage sludge averages 816 m 3 (WEF 1998, in Sylvis 2009). Assuming a VSD rate of 55%, biogas yield from one ton of volatile sewage sludge should therefore average 449 m 3. This is in line with many of the other estimates found in Table 9. Based on this data, it is estimated that degradable sewage sludge has the potential to generate biogas at a rate of approximately m 3 /ton VS with a methane content of approximately 60% when it undergoes anaerobic digestion. However, it should be noted that the presence of siloxanes in sewage sludge is a limiting factor for the productive use of this type of biogas for fuel (Dewil 2006). Some mitigation strategies exist, but a full analysis of these is beyond the scope of the present work. Yard and Garden Waste Plant-based organic matter provides a challenge for anaerobic digestion: even though the volatile solids content may be high, the consistency and C/N ratios of these substrates often inhibit their decomposition and biogas generation. Yard and garden waste is rarely a sole feedstock for digesters; if included, it is typically only in conjunction with one or more other feedstocks. Table 10. Estimates for amount of biogas produced from 1 ton of degradable yard waste. Type TS (%) VS (%) VS/TS Retention VSD Biogas CH C/N 4 (%) Time (d) (%) (m 3 /ton VS) (%) Location / Source* Yard Aalborg Mun. Poulsen 2009 Green , 50 C AD Phased 55 28, 50 C Solids Pilot Zhang 2005 Leaves, Pilot-Scale Wood, Grass Batch Digester Guyer 2002 Grass McDonald 2008 Leaves Wood Straw Garden Grass All Steffen 1998 (Brachtl 1998, Thome-Kozmiensky 1995) Estimated Average *Location indicated in separate cell and original source in brackets, when appropriate. Based on this data, it is estimated that degradable yard waste has the potential to generate biogas at a rate of approximately 350 m 3 /ton VS with a methane content of approximately 60% when it undergoes anaerobic digestion. 14

15 IV. Methods of Landfill Gas and Biogas Processing Landfill gas and biogas produced through the decomposition of organic waste can either be released into the atmosphere or collected. If captured, the gas can be either flared to convert methane into CO 2 or used to generate power. All common methods of gas processing reduce GHG emissions from the baseline scenario of uncontrolled release. However, methods that use gas to produce power have additional GHG emissions benefits, as generated heat and electricity can be used to offset the use of fossil fuels. Uncontrolled Release Landfills, compost heaps, and some types of agricultural waste management may release their generated biogas to the atmosphere without further processing. In this scenario, GHG emissions due to methane and CO 2 can be calculated directly from the amount of biogas produced and the relative proportion of the two gases using Equation 5. Where: CO 2 eq released = m 3 gas [(%CH 4 den CH4 GWP CH4 ) + (%CO 2 den CO2 )] (5) Variable Description Value/Units m 3 gas %CH 4 %CO 2 den CH4 den CO2 GWP CH4 Total volume of the emitted mixed gas Estimate of mixed gas that is methane Estimate of mixed gas that is carbon dioxide Density of CH 4 Density of CO 2 Global warming potential of CH 4 relative to CO 2 m 3 % % kg/m 3, tons/m kg/m 3, tons/m 3 25 This equation assumes that all gases emitted are either methane or CO 2. These two gases typically make up > 95% of emitted gases; the other greenhouse gas constituents (e.g. N 2 O) are much more potent and much more difficult to measure. This expression therefore underestimates the total greenhouse gas emissions. However, since different methods of organic waste disposal generally affect only the relative proportions of methane and CO 2 in gas byproducts, this approximation can still serve as a fruitful basis for comparing disposal methods. Gas Collection All anaerobic digesters and some landfills collect gas byproducts generated from organic waste decomposition for further processing. Anaerobic digesters generate and collect biogas in an enclosed environment. Therefore, it is reasonable to assume that modern, well-maintained AD systems capture 100% of their generated gas. Some landfills are also equipped with gas collection systems which seek to capture LFG and prevent fugitive emissions. The operating efficiency for these systems is generally between 60 and 85 percent, with an estimated average of 75 percent (EPA 1998). However, there is a lag time between organic matter deposition and gas collection; the United States Composting Council estimates that even if all landfills had conventional collection systems, between 1/4 and 1/3 of methane would still be emitted (USCC 2009). 15

16 Once landfill gas or biogas is collected, facilities have a number of options for gas post-processing. Flaring Many facilities opt to flare the collected gas, combusting methane to form CO 2 and thereby reducing greenhouse gas emissions. Flaring is not completely efficient; it is commonly assumed that 1% of the methane remains after flaring (USEPA 2007, in Sylvis 2009). However, Shah et al. (2011) recently demonstrated that actual flare efficiencies tend to be lower, and depend on factors such as gas composition, flare jet speed and diameter, and wind speed. Here, it is assumed that 4% of methane remains after flaring (Shah 2011, Willis 2012). This estimate is reasonable for gases with higher methane contents, while flaring lower-energy gases such as landfill gas is likely somewhat less efficient. The flared greenhouse gas emissions due to methane and carbon dioxide were calculated using Equation 6. CO 2 eq flared = m 3 gas %eff %CH 4 den CH mm CO2 mm CH4 + (0.04 GWP CH4 ) + %eff [%CO 2 den CO2 ] + (1 %eff) [(%CH 4 den CH4 GWP CH4 ) + (%CO 2 den CO2 )] (6) Where the first and second rows calculate the emissions due to collected methane and carbon dioxide, respectively, and the third row calculates the emissions due to released gas. In addition to the variables described for Equation 5, the following are found in Equation 6: Variable Description Value/Units mm CH4 mm CO2 %eff Molar mass of CH 4 Molar mass of CO 2 Efficiency of the gas collection system g/mol g/mol % Following the discussion above, %eff was set to 75% for landfill calculations and to 100% for anaerobic decomposition; this quantity assumes that the gas is produced in a controlled digestion process (i.e. in an anaerobic digester). Energy Production Using collected biogas to produce power can result in net-negative GHG emissions, due to the offsetting of fossil fuels, such as natural gas (Poulsen 2009). Biogas can be used to generate electrical or thermal power or both, using combined heat and power (CHP) conversion. Table 11. Theoretical biogas power production. Biogas CH 4 Content (%) Heating Value (Btu/m 3 ) Electricity Potential (kwh/m 3 ) , , , , The theoretical power production potential of biogas depends on its methane content. Pure methane gas has a heating value of 35,315 Btu/m 3 (1000 Btu/ft 3 ) (Bartok 2004), while the other gases found in biogas have negligible heating values. One watt-hour is equal to approximately Btu, and 1000 Btu is equal to watt-hours. 16

17 Electricity Production Methane-rich gases, including landfill gas, biogas, and natural gas, can be used to generate electricity with the aid of reciprocating engines, turbines, microturbines, sterling engines, or fuel cells. According to the EPA, separate power production is typically only about 33 percent efficient (EPA CHP 2012). The electricity produced from biogas can be estimated using Equation 7. kw separate = 0.33 m 3 35,315 Btu kwh gas %CH 4 m 3 CH Btu (7) Thermal Power Production Methane-rich gases can also be combusted in boilers or process heaters to generate thermal power. Conventional boiler efficiencies are around 75, 80, and 83 percent for boilers fired with biomass, natural gas, and coal, respectively. Separate heat production is typically percent efficient; an average 80 percent efficiency will be used here (EPA CHP 2012). The thermal power produced from biogas in conventional systems can be estimated using Equation 8. Btu separate = 0.8 m 3 35,315 Btu gas %CH 4 m 3 CH4 (8) Combined Heat and Power (CHP) Conversion (Cogeneration) CHP systems are able to capture waste heat from electricity production, using some of the lost efficiency of electrical power production to generate usable heat. They are a natural accompaniment to gas collection systems, as they can integrate with an existing facility and generate both thermal and electrical power on-site. Combined (co-) generation drastically increases power production, and the power conversion efficiency of CHP systems is much greater than the efficiency of producing equivalent amounts of electricity and thermal power separately (Equations 9-12) Efficiency of separate electricity and heat generation (EPA CHP 2008): %eff electricity = Power Output Energy Input (9) %eff heat = Net Useful Thermal Output Energy Input Total system efficiency of combined heat and power generation (EPA CHP 2008): Power Output + Net Useful Thermal Output %eff CHP = (11) Energy Input Effective electrical efficiency of combined heat and power generation (EPA CHP 2008): Power Output %eff elecchp = (12) Net Useful Thermal Output Energy Input Typical Boiler Efficiency The total system efficiency is typically used to compare CHP systems to separate electricity and heat production, while the effective electric efficiency can compare the relative electricity production benefits of a CHP unit. To consider both the thermal and electrical power benefits of a CHP, the former will be used. The EPA s Combined Heat and Power Partnership estimates that total CHP efficiencies (10) 17

18 typically range between 65 and 80 percent (Table 12); an estimate of 75 percent will be used here (EPA CHP 2012). Table 12. Typical CHP parameters (EPA CHP 2008). CHP Type Total System Effective Electric Power Capacity Electricity-to- Efficiency (%) Efficiency (%) Efficiency (%) (MW) Heat Ratio Steam Turbine Gas Turbine Microturbine Reciprocating Engine Fuel Cell The relative proportions of thermal and electrical power generated by a CHP can vary (EPA CHP 2008). In a CHP unit, as with separate heat and power generation, higher overall efficiencies are possible when the proportion of usable thermal output is higher (EPA CHP 2008). kw CHP = m 3 35,315 Btu kwh biogas %CH m 3 CH Btu Btu CHP = m 3 35,315 Btu biogas %CH m 3 CH4 (13) (14) We assume here that 60 percent of the usable power generated by a CHP unit is thermal, while 40 percent is electrical. The same proportion will be used for biogas input to separate heat and power generation systems. Emissions Offset Every unit of usable energy produced by a renewable energy source offsets some amount of greenhouse gases that would have been emitted by the use of fossil fuels. To estimate this offset, it is necessary to use the carbon emissions factor (CEF), which gives the amount of carbon equivalents emitted per unit of energy generated for a given fuel. The CEFs for different fuels can be weighted based on the types of energy used within a given region. Every unit of renewable energy generated in that area results in a carbon emissions offset equal to the weighted CEF (WCEF) (Table 13). Table 13. Emissions factors by fuel type and use. Sources: *ISO-NE **U.S. EIA ***EPA Type of Fuel Percent of Total CEF*** Massachusetts* New England* United States** (tons CO 2 e/kwh) Natural gas Oil Coal Nuclear Power Hydro Wind, Solar, Biomass Weighted CEF (tons CO 2 e/kwh)