MODELING OF METHANE GAS GENERATION AND EMISSIONS FROM LANDFILLS

This study was carried out to develop a model capable of predicting the generation and potential emission of methane gas into the environment. Ryerson Landfill Gas Model (RLFGM) was developed using the IPCC tier two model (revised guidelines), the most recent national assessment model (Brown et. al. 1999) fi'om UK and the model developed fi'om DEFRA, UK. This report outlines the findings of the study and its recommendations. The study was performed on detailed characterization of solid waste as RDO, MDO and SDO and by dividing these waste components into nine waste fi-actions. The waste fi’actions are described by the percentage of moisture content of the fi-action, the proportion of cellulose and hemi-cellulose, and the percentage of the degradability of the cellulose and hemi-cellulose fi-action. Methane generating potential (Lo) was calculated from DOC and DOCp value. The three methane generation rate constants (k) were used. Methane oxidation, methane correction factor, percentage of methane gas recovery, percentage of methane content in landfill gas were assumed based on comprehensive literature review. After the verification of the output of the RLFGM model with the LandGEM model it was found that the model is producing similar type of output graph as the LandGEM model but the rate of emissions of methane gas into the environment is lower in RLFGM model. It has been concluded that RLFGM model gives the realistic output with respect to individual landfill sites, taking into account of specific waste streams and deposition rates.


Introduction
The latest estimation of climate change found that the temperature of the earth has been increased by 0.6°C in last hundred years (IPCC 2001).According to the Intergovernmental Panel on Climate Change, the temperature will be increasing continuously and in the next 50-100 years the temperature will increase by 5-6 ° C (IPCC 2000).This high elevation in earth temperature will cause serious changes in climate and other environmental conditions all over the world.
Some of the signs of climate changes are currently found as in the increasing amounts of rainwater and number of tropical storms.It is strongly believed by researchers and scientists that one of the most important sources of greenhouse gas is MSW landfill gas.MSW landfill gas is primarily composed of methane and carbon dioxide.Methane is about 21 times stronger green house gas than carbon dioxide.
The use of landfills for MSW disposal is expected to increase in United States, Australia and many developing nations (Bogner, et al. 2000).About 55% of all MSW generated in U.S.A is currently being disposed in approximately 2,300 municipal landfills (EPA 1998a).The U.S.A generated approximately 220 million tons of MSW in 1998(EPA. 1998b).The estimated release of methane gas was approximately 9x10^ mg/year (Eklund, et al. 1998).
As the methane gas emission from landfills is causing serious harm to the environment of the world, currently more attention has been drawn in landfill gas recovery.It reduces greenhouse gas emissions and creates an alternative renewable source of energy through the systematic recovery and utilization of landfill gas (Alexander Klein, 2002).Methane gas provides a potential renewable power source (Pembina Institute, 2003).One cubic metre of waste has an energy value of 4 to 5 kilowatts (kwh) or 0.5 litres of heating oil (Thompson and Tanapat, 2004;Tanapat and Thompson, 2003;Tanapat et al, 2003).In Canada, methane gas recovery from landfill gases is one of the most cost effective means to reduce greenhouse gas emissions.
Currently forty-one landfills in Canada are capable to capture methane gas.This methane gas recovery results in an annual reduction of GHG (Greenhouse Gas) emissions of more than 7 megatonnes/year of carbon dioxide equivalents (Environment Canada, 2001).Approximately 70 percent of the captured gas in Canada is used for energy generation at 13 facilities and six of them generate electricity to sell to the grid (e.g., Keele St. Landfill in Toronto).Other seven facilities use the gas directly as an industrial process fuel (Environment Canada, 2001).
Many researches have been performed on landfill gas generation and emission to help to estimate the amount of generation and emission of landfill gases.This estimates help to develop the landfill gas recovery systems.This report focuses on developing a computer model to estimate the generation of methane gas and emission from landfills into the enviionment.

Related works
Landfill gas generation and emission into the environment is a very important issue today with respect to protect the environment of the current world.Many researches have been performed for the estimation of generation and emission of landfill gas in the past decades.For the pinpose of development of the RLFGM model ten computer models have been studied.Studied models are from US EPA, IPCC, Environmental Protection Agency UK, the model developed from Department of Environment Food and Rural Affairs (DEFRA), UK and some other models.
Among the studied models nine have been summarised in Appendix A (Tables 1 to 4).

Limitations of the model
• The RLFGM model was developed considering the landfills scenario in North America.It could be assumed that landfills in North America are well-managed as well as engineered landfills.It cannot be used for un-managed, non-engineered landfills.
• Lignin component was not considered in the simulation of the parameters DOC(x).DOCf).

Waste category used in the simulation of RLFGM model
The model was implemented by dividing the wastes into three degradability of waste category as slowly degradable, moderately degradable and rapidly degradable wastes.These three waste categories divided the waste streams into nine waste fractions as displayed in Appendix A, Table 5.The model characterizes the biodegradability of the rapidly, moderately and slowly degradable wastes by means of three main parameters as SDO (slowly degradable organic), MDO (moderately degradable organic) and RDO (rapidly degradable organic).

RLFGM conceptual model
RLFGM conceptual model has a modular structure, see Figure 1.The conceptual model is divided into four modules as below: i) Methane Oxidation; ii) Source; iii) Methane Utilization and iv) Methane emission into the environment; The modules are described below; i) Methane oxidation: RLFGM model was developed for a well-managed, engineered landfill in North America.According to IPCC (2000), the model used 0.1 as its oxidation factor (OX term in equation 2.4) for its implementation purpose.
ii) Source: The heart of the model is the source term.The source term simulates the generation of methane gas taking into account the following waste characteristics: • The waste breakdown, the mix of different municipal solid waste fractions; • The waste composition e.g. the proportion of rapidly degradable organic (paper, textiles, misc. combustible plus non-inert, composted putrescible), moderately degradable organic (ferrous and non ferrous metals), and slowly degradable organic (dense plastics, misc.combustible plus inert fines and glass).The waste composition is defined by the percentage of moisture content of the fraction, the proportion of cellulose and hemi-ceUulose, as well as the percentage of the degradability of the cellulose and hemi-cellulose fraction, see Appendix A, Table 6.
• Physical parameters of waste; • The biodegradability of the waste fractions (rapidly degradable organic, moderately degradable organic, slowly degradable organic), the rate of decay of waste fractions.The methanogenic degradation of carbon is simulated by dividing the different waste streams into three waste categories.These waste categories are divided into nine waste fractions (Appendix A, Table 5), as rapidly degradable organic (RDO -paper, textiles, misc.combustible (plus non inert), composted putrescible), moderately degradable organic (MDO -ferrous and non ferrous metals), and slowly degradable organic (SDO -dense plastics, misc.combustible (plus inert fines) and glass).The gas generation is then calculated using highly flexible multi-phases equations (Equations 2.1 to 2.3).Total degradable organic carbon that is dissimilated within each waste fraction (rapidly, moderately and slowly degradable organic) was calculated separately using the equation 2.  8) the methane correction factor for a managed SWDS should be 1.0.Values less than 1.0 may be adopted for developing or countries with unmanaged solid waste disposal sites.It is assumed that in North America all landfills are well-managed and engineered landfills.

Methane Emission in to ih e environment
For that reason RLFGM model used 1.0 as its methane correction factor value during its development.
iii) Methane Oxidation, OX: According to IPCC (2000) the oxidation factor for well-managed landfills at a national level should be 0.1.Considering that approach the model assumed 0.10 as its methane oxidation factor value for its implementation.
iv) Methane Content in LFG, F (percentage by volume): Typical landfill gas composition is shown in Appendix A, Table 9.The landfill gas composition is influenced by the decomposition of landfill solid waste.The decomposition of biodegradable waste in landfills gives rise to both methane gas and carbon dioxide in approximately equal quantity by volume.The mechanics of this process are governed by a number of different biochemically mediated reactions (AFRC, 1988).It could be said that the actual quantity of methane gas or carbon dioxide produced by decomposition will vary according to the dominant microbiological processes.In older uncapped sites, natural diffusion of air through the cover materials led to a greater degree of aerobic degradation, and thus the proportion of methane gas produced changed from 50:50.This reflected the increased carbon dioxide and reduced methane gas production.For an older un engineered not well-managed landfill the percentage of methane gas content in LFG may be of 30%.The RLFGM model used a methane gas content of 50% in LFG for its investigation purpose.For the purpose of developing the RLFGM model, a value of 0.5 for methane gas content in LFG has been accepted.
v) Degradable organic carbon (DOC) and fraction dissimilated (DOCf): The IPCC states that the fraction of the degradable organic carbon that actually degrades to release methane gas and carbon dioxide should be by default 0.77 (if lignin is excluded from the DOC) or between 0.5-0.6 (if lignin is included).But the LQM model from Department of Environment Food and Rural Affairs (DEFRA), UK used the equation 2.3 for the calculation of total degradable organic carbon that is dissimilated within each waste fraction (rapidly, moderately and slowly degradable organic).The model calculated it separately and used for its implementation.The RLFGM model used this approach for its study purpose.The methanogenic degradation of carbon was implemented by dividing the different waste streams into three waste categories (Appendix A,  1989).The model does not consider lignin component for its study purpose.Some researchers (Eleazer et al., 1997) found that the lignin degrades slowly under anaerobic conditions.Some other researchers have demonstrated that lignin has no methane gas generating potential because it is recalcitrant under the anaerobic conditions (Young and Frazer 1987).Based on these studies it was decided not to consider the lignin component for the investigation of the RLFGM model.

Results and Discussions
This section discusses the output provided by the model.Three steps have been adopted for the generation and emission of methane gas in RLFGM model.i) DOC(x).DOCfI DOC(x).DOCf value has been calculated using the equation 2.3 in the implementation of RLFGM model.It has been calculated for different waste streams category (Appendix A, Table 5).A sample model output is shown in Appendix A, iii) Methane gas generation and emission rates from landfills: The above equation 2.1 and equation 2.4 have been used for the generation and emission of methane gas from landfills.One set of municipal solid waste data (Appendix A, Table 6) has been used for the simulation of methane gas generation and emission from landfills.It has been assumed from current practice that 62% of the generated solid waste would be disposed into landfills and 72% of the landfill gas generated would be recovered (flared or utilized).The rate of methane gas generation and emission from landfill has been displayed in graphs (see Figure 2 and Figure 3).During the 30 years (from 1995 to 2025) simulation period from year 1998 to year 2007 methane gas generation rate was high.Methane gas emission rate was high from year 1998 to year 2011.Rate of methane gas generation and emission from landfill both have decreased with time after that period.The highest methane gas generation rate and emission rate were found in the year of

Conclusion
RLFGM model has been developed for the purpose of the generation and emission of methane gas from landfills.The model has been constructed using the help of IPCC tier two model (revised guidelines), the most recent national assessment model (Brown et. al. 1999)  • John Pichtel, Waste Management Practices -Municipal, Hazardous and Industrial, pp 321 -324.
• Land Quality Management Ltd. for the department of environment food and rural affairs, UK.
(Januaiy 2003).Report on "Methane emissions from landfill sites in the UK".
• Manley BJW, Wilson DC and Tillotson HS (1990b).National Assessment of Landfill Gas Production.Department of Energy ETSU Report B1192, Energy Technology Support Unit, Culham.
• United States Environmental Protection Agency (EPA), Office of research and development.
National Risk Management Research Laboratory (NRMRL), and Clean Air Technology Centre

Assumptions:
1.The model requires only input of a limited set of parameters.The input parameters are: • The composition of the waste and • The conditions of the SWDS.
2. The model does not need to determine the rate of decomposition of waste at SWDS.The IPCC guidelines contain the default values for the most of the data needed in the model.

Limitations:
1. IPCC default methodology provides estimates on potential methane gas emissions without incorporating any time factors.
2. The method will produce fairly good estimates of the yearly emissions if the yearly amounts as well as composition of waste disposed have been nearly constant for long periods.Increasing amounts of waste disposed will lead to an overestimation, and decreasing amounts correspondingly to underestimation of yearly emissions.
3. The uncertainties in the emission estimates produced by IPCC method are • Acute events associated with sudden drops in atmospheric pressure resulting in lateral migration e.g asphyxiation and acute health effects by exposure to VOCs.
• There is no biological oxidation of methane gas, dispersion, retardation or Other attenuation/reaction processes that reduce the concentration of any gas as it moves through the ground.This will result in an overly conservative approach.
4. The model is dependent on the following factors that affect biodégradation rates; • Age of waste, • Moisture content.
• Average annual placement rates are used 5.The model uses two adjustable variables namely Lo and k.

Limitation:
1.The model is impractical for use global scale where site-specific data are not available.M ethane gas em ission rate Metliane gas emission rate output was lower (Appendix A , Table 13) til an LandGEM output.
Methane gas em ission rate output found higher compared to RLFGM model output in tlie simulation (Appendix A, Table 13).

Figure 1
Figure 1 RLFGM conceptual model 3. The calculated total degradable organic carbon was then summed across all waste categories (RDO, MDO and SDO).The term D O C (x)D O C f does vary with time as a function of the mass of each individual waste category but does not vary significantly with cellulose and hemi-cellulose contents, moisture contents of individual waste category.RLFGM model adopted this equation and the concept for its implementation.The DO C (x)D O C f value was used to derive the specific methane generating potential, Lo for each waste fraction.Total methane generating potential was calculated by the addition of the previous years Lo value with the current year Lo value.It was accepted accounting the concepts that the waste already disposed into the landfill yet generating the methane gas.This methane generating potential value provides the input into equation 2.1mentioned, to obtain the value of the methane gas generated in each year, iii) Methane utilization: According to the current US EPA regulations under the Clean Air Act, many larger landfills require to collect and combust landfill gas.The options available are flaring the gas or installing a landfill gas recovery and utilization system.There are a number of environmental and economic benefits of recovering landfill gas.Gas recovery systems reduce landfill gas odor and migration of landfill gas, reduce the danger of explosion and fire.The recovered methane gas is also a very good source for the generation of electricity in the current world.According to UK's national assessment, it was estimated that in the year of 2002 at least 63% of the total landfill gas generated in UK was flared or utilized and it increased to approximately 72% by 2005.During the development of the RLFGM model it was assumed that 72% of methane gas would be recovered.iv) Methane gas emission from landfills: Methane gas emission from landfills is normally controlled by engineering measures, e.g. the installation of engineered barriers (cap and liner) and gas collection systems.RLFGM model determined methane gas emissions from landfills based on comprehensive literature review.The RLFGM model assumes that methane gas generated and not collected would be emitted through the landfill cap or liner at a steady state and would be released into the surface to the atmosphere.RLFGM model assumes that 72% of methane gas recovery for its development purpose.RLFGM model reduces the emission of methane gas through the cap by biological methane gas oxidation.The model allows the use of the IPCC (2000) methodology which states that the oxidation factor for a well-managed landfill should be 0.1 based on available information.RLFGM model assumes oxidation factor as 0.1 for its design purpose.Assuming 72% recovery of methane gas and oxidation factor as 0.1 the model uses the IPCC (2000) tier two model for the calculation of methane gas emission into the environment.The equation is shown below: Defining Equation: • Methane emitted in year t (Gg/yr) = [CHj generated in year t -R(t)].(l-OX)...................... (2generation rate constants, k: RLFGM model used three methane gas generation rate constants for RDO, MDO and SDO components accounting the approach developed for the Environmental Agency's GasSim model, UK.For methane generation rate constant, k the IPCC(2000)  proposed a single value of k as 0.05 per year corresponding to a half-life period of 15 years.Manely et al. (1990a; 1990b)  first used three rate constants for slowly degradable, moderately degradable, and rapidly degradable wastes, andBrown et al. (1999)  introduced three rate constants into the national assessment model, UK.The methane generation rate constants used byManely et al. (1990a; 1990b  ), Brown et al. (1999) )  and GasSim models are given in Appendix A, Table7.It was noted that in all cases, the half-life period of the slowly degradable organic is consistent with the IPCC (2000) value 0.05.The half-life period for rapidly and moderately degradable wastes was increased.It has been found from comprehensive literature review that it has been accounted to avoid immediate peaks coiresponding to short half-lives period in the simulation.ii) Methane Correction Factor, MCE: Unmanaged solid waste disposal sites (SWDS) produce less methane gas compared to managed SWDS.It is because of a larger fraction of waste decomposes aerobically in the top layers of unmanaged SWDS.According to IPCC (2000), (Appendix A, Table

Figure 2 Figure 3 Figure 4
Figure 2 Analysis of methane gas generation rate (Gg/yr) from landfill

2.1. RLFGM -Ryerson Landfill Gas Model
RLFGM model has been designed using the help of IPCC tier two model (revised guidelines), the most recent national assessment model(Brown et.al.1999)from UK and the model developed by LQM for the Department of Environment Food and Rural Affairs (DEFRA), UK.

Table 10
ii) Methane generating potential, Lo: The methane gas generating potential Lo has been calculated using the equation 2.2.It used DOC(x).DOCf values as input values.Calculated values were summed up for each degradability rate (RDO, MDO, SDO).During the implementation of RLFGM model it has been assumed that the solid waste already disposed into landfill yet producing the methane gas.So it added the Lo of the current year to the previous years.The Table 11 in Appendix A shows the sample output of Lo of the model simulation.
from UK and the LQM model developed from the Department of Environment Food and Rural Affairs (DEFRA), UK.The model was validated using the help of LandGEM model.The implementation was performed with some model-defined data sets as well as some user-defined data sets.The data sets used are methane gas generation rate constants, k for rapidly degradable organic, moderately degradable organic and slowly degradable organic, methane correction factor and methane oxidation.The user-defined data sets are yearly rate of solid waste disposal into landfills.Degradable organic carbon (DOC) and fraction dissimilated (DOCf) were calculated using the model input data and used for the calculation of methane generating http://www.sepa.org.uk/pdl7consultation/closed/2003/landfill/suidancelandfilleas.pdf)•EnvironmentAgency and ETSU (1999)."HELGA: Health and Environmental Risk Effects from Landfill Gas." (http://\\'vw.lqm.co.uk/do\vnloads/proiect profiles/w01-helsa.pdf).
potential (Lo).During the validation of the RLFGM model using the help of the LandGEM model it was found that the output graph for the methane gas emission from landfill resembled similar with the LandGEM model output but the value was smaller.The RLFGM model calculates the methane gas generation and emission rate considering three types of waste degradability fraction as rapidly degradable organic, moderately degradable organic and slowly degradable organic.The waste fractions are described by the moisture content, cellulose content, hemi-cellulose content of the waste fraction, degradability of the cellulose and hemi-cellulose of waste category.It was found that it gives the realistic output with respect to individual landfill site, taking into account of specific waste streams and deposition rates.It has been concluded as one of the greatest benefits of the RLFGM model.•EnvironmentAgency,UK(November 2002)."Guidance on the management of landfill gas".(• Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories (IPCC 2000a).(http://w\vw.ipcc-nggip.iges.or.ip/public/gp/english/'5 waste.pdf).• H. Lanier Hickman, Jr., Principles of Integrated Solid Waste Management, pp 420 -438.• Heijo Scharff, NV AfValzorg (October 2005)."Landfill gas production and emission on former landfills."North East South West Interreg IIIC.(http://www.sufalnet.net/get.php7f.588I.• HotRot composting system limited, England."Composting of food and putrescible wastes" (.httpi//yAv:vy±plrplsxstmS:Cojni/ç^^• Intergovernmental Panel on Climate Change (IPCC) (1997).Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories: Volume 3 reference manual.J.T. Houghton et al.,

Table 5
Different types of waste categories used for the analysis of the RLFGM model

Table 10 Calculated D O C ( x ) .D O C f values
based on data input in Table6.Measured unit Gg C/Gg waste

Table 11
Sample output of RLFGM model implementation for methane gas generating potential

Table 12
Parameters from RLFGM model used for the simulation of the LandGEM model forTable 13 Methane gas emission rates from RLFGM model and LandGEM model.

Model uses multilevel phases of equations for tlie generation and emission o f methane gas from landfills (equation 2.1 to 2.4) Model uses first order decay equation (A ppaidix A , Table 1) W aste Category Model uses three types o f waste components as rapidly degradable, moderately degradable and slow ly degradable organic making up nine waste fractions as paper and textiles, misc. combustible (plus non inert), composted putrescible, ferrous metal, non ferrous metal, dense plastics, misc. non combustible (plus inert fines) and glass. A ll waste components having tlie potential to generate methane gas have been used. Only rapidly degradable organic has been used for the simulation piu)>ose as moderately degradable and slow ly degradable organic component have very little possibility to generate methane gas. Methane gas generation rate constants Three methane gas generation rate constants were used as 0.116 (for rapidly degradable organic), 0.076 (for moderately degradable organic) and 0.046 (for slow ly degradable organic) One methane gas generation rate constant 0.116 year ' from RLFGM model was used for simulation considering the fact that primarily rapidly degradable organic components generate methane gas.
Table 14 Comparison between RLFGM and LandGEM model