Potential production of electricity from biogas generated in a sanitary landfill

Quetzalli Aguilar-Virgen¹, Paul Taboada-González², Sara Ojeda-Benítez³

¹ Industrial Engineer, M.Sc. Industrial Engineering, Ph.D. Science, Universidad Autónoma de Baja California. Research Professor, Instituto Tecnologico de Culiacan, Mexico. qaguilar@itculiacan.edu.mx

² Electrical Engineering, M.Sc. Industrial Engineering, Ph.D. Science, Universidad Autónoma de Baja California. Research Professor, Instituto Tecnologico de Culiacan, Mexico. ptaboada@itculiacan.edu.mx

³ B.Sc., in Science of education, M.Sc. Engineering, Ph.D. in Science. Research Group on Solid Waste, belongs to the National System of Researchers since 1998, Coordinator of the Environment Department at the Institute of Engineering, Universidad Autónoma de Baja California. Mexico. sara.ojedabenitez@uabc.edu.mx

ABSTRACT

Energy forms the cornerstone of almost every economic, social and cultural sector in modern societies. Energy is regarded as an irreplaceable ingredient in such societies' industrial development. The aim of this research was to estimate the generation of biogas in the city of Ensenada's sanitary landfill to ascertain the amount of energy which could be generated from the solid waste being disposed of. Biogas estimates were conducted in two stages: a waste characterisation study followed by implementing the regulations proposed by SCS Engineers (SCS Engineers, 2009) regarding the Mexican biogas model (version 2.0). The results showed that a large quantity of organic matter (around 70%) is a key element in anaerobic degradation of waste. As to energy generation, it is believed that a full 1.90 MW capacity will be reached in 2019. Such energy could increase Ensenada's current electricity generation capacity by 3.46% and provide 60% of the energy needed for street lighting, thereby leading to USD $1.423 million in savings.

Keywords: energy generation, municipal solid waste, biogas, renewable energy

Received: October 8th 2010 Accepted: October 26th 2011

Introduction

The role of energy in modern societies is central in almost all economic sectors and social and cultural activities as well. Energy is considered an irreplaceable ingredient for economic societies and industrial progress (Alazraque-Cherni, 2008).

Energy has mostly been generated in Mexico by fossil fuels. However, this has brought negative consequences for the environment by releasing greenhouse gases (GHGs), so that the use of alternative energy is an appropriate solution today. One of these energies could be the biogas generated by municipal solid waste (MSW) deposited in sanitary landfills.

It is widely recognised that biogas from landfills is a significant potential source of energy. Early research in this area began in the USA at the beginning of the 1970s; many countries have initiated programmes for encouragin the use of gas from landfills since the 1980s (Lawson, 1989).

Fundamental aspects concerning landfill biogas

A sanitary landfill is usually conceptualised as being a biochemical reactor; waste and water are thus the main inputs, while gas and leachate are the main products (Machado et al., 2009). Landfill gas is the biological result of the anaerobic decomposition of organic materials (Machado et al., 2009; Marshall, 2007); if it is not captured and harnessed, it can become a potent source of GHGs and a major contributor to climate change (Marshall, 2007; EPA, 1996).

The main constituents present in landfill biogas are methane (CH₄) and carbon dioxide (CO₂), but landfill gas is commonly saturated by water vapour and presents small quantities of nonmethane organic components and various other trace compounds (Machado et al., 2009; Marshall, 2007).

Methane can be exploited for generating electricity or used as fuel in transportation. Some of the advantages of biogas use include the mitigation of GHG emissions and fire risk reduction and explosion in landfills (Marshall, 2007; Garg et al., 2006; Christophersen et al., 2001; EPA, 1996).

Landfills represent the third largest source of anthropogenic methane emissions worldwide, constituting around 13% or more of global methane emissions (Zhang et al., 2008; Kumar et al., 2004).

Methane contributes 21 times that of CO₂ to the greenhouse effect, due to its stronger molar absorption coefficient regarding infrared radiation and longer residence time in the atmosphere (Batool and Chuadhry, 2008; Christophersen et al., 2001). This is important because implementing any policy aimed at reducing methane would have an immediate impact on GHG emission and the benefits of the initiatives could appear in the short- to medium-term (Batool and Chuadhry, 2008).

Landfill gas is generated by degrading the biodegradable fraction and is influenced by physical-chemical composition of waste and environmental variables. Many factors interfere with the generation of methane from sanitary landfills, but the most important ones include the total amount of organic matter disposed of, its age and moisture content, compaction techniques used, temperature and type of waste, particle size and nutrients (Kong, 2008; Kumar et al., 2004). Optimum methane production conditions are 50% to 60% moisture content, 40°C temperature, small particle size and a neutral pH (Kong, 2008).

From the above, it is important to have information regarding the generation and composition of waste to assist in selecting and operating equipment for the treatment and handling of waste, as well as energy recovery facilities and resources (Chang and Davila, 2008; Zeng et al., 2005).

Modelling methane production

Some empirical, stoichiometric and biochemical models have been developed for predicting the rate of methane production in sanitary landfills (Aronica et al., 2009; Chiemchaisri and Visvanathan, 2008; Meraz et al., 2008; Garg et al., 2006).

Empirical models are particularly dependent on local conditions. Some basic models describe fermentation using microbial reaction sequences, beginning with a period of aerobic degradation and continuing through the various stages of anaerobic degradation, such as hydrolysis, acetogenesis and methanogenesis. While this class of model inherits its certainty regarding basic science, it also suffers from a lack of reliable data related to mi-crobial activity and tends to be complex, often demanding large computational requirements. Since no rigorous and functional model of methane production in sanitary landfills has been advanced to date, empirical equations represent the most common practice (Meraz et al., 2008).

The specialised literature describes several models which are used to predict methane production.

The Tier 3 method involves gas extraction from one or more extraction wells in all cells and measure the response regarding the resulting pressure in a number of monitoring probes, at different depths and distances from the extraction well (Walter, 2003).

The estimate in the IPCC method depends on the categories of waste, the degradable organic carbon fraction and CH₄ gas in a landfill (Machado et al., 2009; Chiemchaisri and Visvanathan, 2008).

Flow is estimated in the closed-chamber flow method on CH₄ concentration changes related to time spent in the chamber and measured by a camera 60 min after it was placed on the soil surface (Chiemchaisri and Visvanathan, 2008);

The EPA model uses a first order degradation equation and is based on two fundamental parameters: L₀, being methane generation potential (m³CH₄/Mg of MSW) and k, the constant rate of methane generation (year^-1) (Machado et al., 2009, Garg et al., 2006).

The Mexican biogas model method uses a first order decay equation which assumes that maximum biogas generation is reached after a period of time before methane generation. This model requires that a user enter specific data, such as the opening year, the year of closure, annual disposal rates, average annual precipitation and collection system efficiency. The model automatically provides values for k and L₀ (SCS Engineers, 2009).

The last two models consider that methane generation potential (L₀) is related to waste composition, generation rate constant (k) depending on many site-specific parameters, such as moisture content, temperature, waste composition, oxidation reduction potential, alkalinity and pH, density and trash particle size (SCS Engineers, 2009; Garg et al., 2006).

This research was aimed at estimating biogas generation in the Ensenada sanitary landfill, to ascertain the amount of energy that could be generated from the decomposing solid waste being dumped at the disposal site.

Methodology

The study was carried out in the city of Ensenada in the state of Baja California, located in north-western Mexico; this city is located at latitude 31°52' N, longitude 116°36' W. Average annual rainfall is 250 mm; the city has a Mediterranean-like climate, having a mild temperature almost all year round and rainfall during the winter. Ensenada had 260,075 residents in 2005, according to 2005 data published by the Mexican Institute of Statistics, Geography and Data Processing (NISGDP).

Biogas estimates were made in two stages: a waste characterisation study and implementing the regulations proposed by SCS Engineers (SCS Engineers, 2009) regarding the Mexican biogas model (version 2.0).

Characterisation study

A characterisation study was carried out during two seasons representing Ensenada's climate over five days in a row (Monday to Friday) in February and June 2009. The residues analysed were dumped in the landfill by the municipality-run waste collection trucks. Since the waste is collected once a week in each route, it was considered that the samples were representative of generation during one week. Samples weighing around 260 kg per day were taken, this being more than the amount proposed in Mexican standard NMX-AA-015-1985, but in line with other studies (Zeng et al., 2005; Chung and Poon, 2001).

The samples taken were classified, weighed and recorded. For the record, components were quantified based on the methodology proposed by Mexican standard NMX-AA-022-1985. The components were grouped into 14 categories proposed by SCS engineers (SCS Engineers, 2009). These were: food scraps, paper and cardboard, yard trimmings, wood, rubber, leather, bones and straw, textiles, toilet paper, other organic products, disposable diapers, metals, construction and demolition material, glass and ceramics, plastics and other non-organic materials.

Data regarding Ensenada collection trucks' daily weight recorded on the sampling dates, the number of inhabitants and MINITAB 14.1 software were used for determining the per capita generation rate (98% confidence interval, using Student's t-test).

Biogas estimation

An initial step involved analysing how the site operated, so observations were made during June, August and September 2009. Interviews were held with the sanitary landfill's general manager and operations manager. The information obtained in situ by observations and interviews concerned the amount of waste annually disposed of in the landfill, the landfill's opening and closing years, estimated annual disposal growth, average landfill depth, landfill fires, percentage of waste area with daily cover (intermediate and final), the percentage of waste area having a clay/geomembrane, waste compaction, leached outcrops on the surface of the landfill and waste composition. This information was used in the model described below.

The Mexican biogas model (version 2.0) developed by SCS engineers (SCS Engineers, 2009) was used for estimating biogas generation in the Ensenada sanitary landfill. The model estimates the landfill biogas generation rate in a given year using the following first-order decay equation (see equation 1), modified from the US EPA's LandGEM (version 3.02, 2005):

where Q_LFG = maximum expected flow rate (m³/yr), i = 1 year time increment, n = (year of the calculation) - (initial year of waste acceptance), j = 0.1 year time increment, k = methane generation rate (1/yr), L₀ potential methane generation capacity (m³/Mg), M_i = mass of solid waste disposed in the i^th year (Mg), t_ij = age of the j^th section of waste mass M_i disposed in the i^th year (decimal years), MCF = methane correction factor, F = fire adjustment factor.

Results and Discussion

Regarding characterisation, 10 samples of around 260 kg per day were taken (2,511.35 kg in all). An Excel spreadsheet proposed by the SCS engineers (SCS Engineers, 2009) was used for calculating biogas recovery by applying a first order decay equation, similar to that providing maximum power plant capacity.

HSW composition in Ensenada

From the 2,511.35 kg weighed, 1,379.66 kg were obtained in February and 1,131.69 kg in June (winter and summer, respectively). Table 1 shows average HSW composition for the two seasons studied in Ensenada; 0.968 ± 0.208 kg per person per day was estimated regarding per capita HSW generation.

It can be seen that the percentage of organic components in waste composition was 68.57% and 30.30% in inorganic components. Research carried out in different places around Mexico has reported different organic waste percentages. Ojeda-Benitez et al., (2003) showed organic waste in Mexicali as being 64.78%. Gómez et al., (2008) reported that organic waste in Chihuahua accounted for 62.00%. As to Guadalajara, Bernache-Pérez et al., (2001) reported that 63.50% of the total amount of waste generated was organic waste. Buenrostro (2001) reported 68.10% organic waste for Morelia. These amounts were slightly lower than those reported in the present investigation, where organic waste decreased by an average of 4%.

Waste composition in other countries has been found to be different to that of the present research. Organic waste in Portugal has accounted for 27.40%, paper 20.30%, plastic 18%, glass 6%, metals 5% and textiles 3.8% (Gomes et al., 2008). It has been reported that waste composition in Missouri, USA, was about 41% for paper, 21% for organics, 16% for plastics, 6% for metals, 3% for glass and 13% for other waste (Zeng et al., 2005). Waste composition in Cyprus has been reported as being 24% for paper, 5% for plastics, 39% for food scraps, 14% for yard trimmings, 1.5% for glass and 2% for metals (Eleftheriou 2007). Organic waste composition in all these studies was less than that reported in Ensenada.

Estimating biogas in the Ensenada sanitary landfill

The results of in situ research and interviews with the sanitary landfill operation manager were as follows. Annual MSW disposal in 2005 was 132,055 Mg. Sanitary landfill operations began in 2004 and the site will close in 2018. No fires have been reported in the disposal site. The average depth of each cell is 15 m. A lower geomembrane liner is placed before discarding waste. Discarded waste is covered daily and only two cells have been closed so far, having a final covering. Waste is compacted by up to 75% and no leachate release was observed.

The annual estimated increase of waste disposal in Baja California was 5% according to the 2008 figures reported by the Secretariat of the Environment and Natural Resources (SENR).

The model automatically assigned values for k and L₀ according to their level of degradation from information about waste composition. The results for k were: 0.10 fast-decay (FD), 0.05 medium fast decay (MFD), 0.20 medium slow decay (MSD) and 0.10 slow-decay (SD). The results in L₀ were: 69 FD, 149 MFD, 214 MSD and 202 SD.

Assuming a median 50% CH₄ and 50% CO₂ biogas composition and that biogas capture began in 2009 with the k and L₀ values described above, estimated potential biogas recovery would reach its peak during the year of closure (2019) - 1,152 m³/hr (see Figure 1), 1.90 MW maximum capacity (see Figure 2). Afterwards, it would decrease by around 0.10 MW per year. This is because the model assumed that maximum generation would normally occur in the closure year or in the next year, and biogas generation would go down exponentially as the organic fraction of food scraps became consumed (SCS Engineers 2009).

Comparing the results with other studies (SCS Engineers, 2007), the maximum power generation in the closed Ensenada landfill would be around 65% lower than that reported in the present investigation. However, this approximation was obtained using the Mexican biogas model (version 1.0) where k and L₀ values were based on waste composition provided by the municipality from a study conducted in 2002.

A study conducted in 2005 by SCS Engineers in the Chihuahua sanitary landfill, Mexico, used the SCS model, a modified version of the EPA Model. Waste composition was compared using typical US rates, such data determining the values of k and L₀. Using this model and a mid-range scenario, maximum electric power generation was estimated to be 3.30 MW in 2009, rising to 4.30 MW in 2013 and then dropping to 2.6 MW in 2019 (SCS Engineers, 2005a). Another study by SCS Engineers in 2005 in the sanitary Queretaro landfill, Mexico, used the same model as in Chihuahua and typical US proportions. Biogas recovery in this study was 3.20 MW in 2009, rising to 5.40 MW in 2016 and dropping to 3.80 MW in 2019 (SCS Engineers, 2005b).

Both studies showed similar amounts of estimated biogas recovery. This may have been due to the Chihuahua sanitary landfill began operation in 1994 and its closure will be in 2013; organic composition is 68% and the amount of waste is around 390,000 Mg annually. Queretaro sanitary landfill began operation in 1996 and its closure will be in 2015; organic composition is 62% and the amount of waste is about 300,000 Mg annually.

The above results were higher than those obtained in the Ensenada sanitary landfill where estimated maximum electric power generation was 1.90 MW and annual amount of waste was about 132,055 Mg, although the present study's organic composition (68.57%) was similar to that reported in Chihuahua and Queretaro. Other typical rates were used in these studies when comparing waste composition, but there was a marked difference between percentages. For example, food waste in the USA was 11.50%, contrary to 36.00% in Mexico, or paper waste was 26.60% in the USA and 15.70% in Mexico. These percentages affected k and L₀ values which form the core of the model. The Mexican biogas model was not modelled, so reliable comparison cannot be made. A study of the SIMEPRODESO sanitary landfill in Monterrey, Nuevo Leon, used the "USEPA E-PLUS" model which is different from the previous ones and also used a different methodology for evaluating landfill gas recovery (LFG Consult, 2007). The results from that study are thus not comparable with those reported here. Criteria must thus be standardised to be able to make valid comparisons when evaluating bio-gas recovery.

Conclusions

Knowledge percentage organic waste is one of the key parameters for more accurately estimating methane generation and waste used to generate energy. This study has thus shown that Ensenada landfill waste had around 70% organic component.

The electrical energy that could be obtained from using the biogas generated from the local sanitary landfill would provide 3.46% of Ensenada's installed electricity generation capacity in 2004. As the amount of methane decreases in landfill biogas, this will reduce the biogas' calorific value and electric motor generator efficiency. If 81% alternative internal combustion engine overall efficiency can be achieved and 66% biogas capture system efficiency, then 11,080.80 MW/h could be generated of the 19,179.75 MW/h used by the city in 2007, according to the Baja California Statistical Yearbook, 2008 edition. That is, they cover 60% of the energy used for the lighting service in streets, squares, parks and public gardens in the city of Ensenada. Considering a rate of 13 pesos (MXP) per US dollar (USD), this would account for a saving of around USD$ 1.423 million.

If biogas recovery strategies and use were put into practice this would lead to an important reduction in CO₂ emission equivalent (CO₂e) which would no longer be released into the atmosphere. These emissions would account for around 747,060 metric tons during 2009-2022. Such volume would be worth USD$ 8.2 million on the carbon market, having an average baseline cost of USD$ 11.00 per metric ton of CO₂e reduced (Flores et al., 2008).

The next step would be to carry out other studies using a similar methodology to identify similarities and correlate aspects for creating new know-how. This would help in the use of solid waste as an energy source and achieving sustainability.

Acknowledgements

References

Alazraque-Cherni, J., Renewable Energy for Rural Sustainability in Developing Countries., Bulletin of Science Technology Society, Vol. 28, No. 2, 2008, pp. 105-114.

Aronica, S., Bonanno, A., Piazza, V., Pignato, L., Trapani, S., Estimation of biogas produced by the landfill of Palermo, applying a Gaussian model., Waste Management, Vol. 29, No. 1, 2009, pp. 233-239.

Batool, S.A., Chuadhry, M.N., The impact of municipal solid waste treatment methods on greenhouse gas emissions in Lahore, Pakistan., Waste Management, Vol. 29, No. 1, 2008, pp. 63-69.

Bernache-Perez, G., Sanchez-Colon, S., Garmendia, A.M., Davila-Villarreal, A., Sanchez-Salazar, M.E., Solid waste characterisation study in the Guadalajara Metropolitan Zone, Mexico., Waste Management Research, Vol. 19, No. 5, 2001, pp. 413-424.

Buenrostro Delgado, O., Los residuos sólidos municipales. Perspectivas desde la investigación multidisciplinaria., Universitaria (Ed), México, 2001, pp. 87-91.

Chang, N., Davila, E., Municipal solid waste characterizations and management strategies for the Lower Rio Grande Valley, Texas., Waste Management, Vol. 28, No. 5, 2008, pp. 776-794.

Chiemchaisri, C., Visvanathan, C., Greenhouse gas emission potential of the municipal solid waste disposal sites in Thailand., Journal of the Air & Waste Management Association (1995), Vol. 58, No. 5, 2008, pp. 629-35.

Christophersen, M., Kjeldsen, P., Holst, H., Chanton, J., Lateral gas transport in soil adjacent to an old landfill: factors governing emissions and methane oxidation., Waste Management Research, Vol. 19, No. 6, 2001, pp. 595-612.

Chung, S., Poon, C., Characterisation of municipal solid waste and its recyclable contents of Guangzhou., Waste Management Research, Vol. 19, No. 6, 2001, pp. 473-485.

Eleftheriou, P., Energy from waste: a possible alternative energy source for large size municipalities., Waste Management Research, Vol. 25, No. 5, 2007, pp. 483-486.

EPA, Turning a Liability into an Asset: A Landfill Gasto-Energy Project Development Handbook: Environmental Protection Agency, United States of America, 1996. http://www.epa.gov/landfill/res/pdf/handbook.pdf Acceso: 04/08.

Flores, R., Muñoz Ledo, R., Flores, B., Cano, K., Estimación de la generación de energía a partir de biomasa para proyectos del programa de mecanismo de desarrollo limpio., Revista Mexicana de Ingeniería Química, Vol. 7, No. 1, 2008, pp. 35-39.

Garg, A., Achari, G., Joshi, R.C., A model to estimate the methane generation rate constant in sanitary landfills using fuzzy synthetic evaluation., Waste Management Research, Vol. 24, No. 4, 2006, pp. 363-375.

Gomes, A., Matos, M., Carvalho, I., Separate collection of the biodegradable fraction of MSW: An economic assessment., Waste Management, Vol. 28, No. 10, 2008, pp. 1711-1719.

Gomez, G., Meneses, M., Ballinas, L., Castells, F., Characterization of urban solid waste in Chihuahua, Mexico., Waste Management, Vol. 28, No. 12, 2008, pp. 2465-2471.

Kong, I.C., Microbial characteristics associated with six different organic wastes undergoing anaerobic decomposition in batch vial conditions., Waste Management Research, Vol. 26, No. 3, 2008, pp. 261-266.

Kumar, S., Mondal, A., Gaikwad, S., Devotta, S., Singh, R., Qualitative assessment of methane emission inventory from municipal solid waste disposal sites: a case study., Atmospheric Environment, Vol. 38, No. 29, 2004, pp. 4921-4929.

Lawson, P., Landfill, Microbiology and Research: An Introduction to the Workshop., Landfill Microbiology: R & D Workshop. Harwell. England, 1989, pp. 1-9.

LFG Consult, Case studies of CDM - Landfill Gas Projects Monterrey, Mexico (Benlesa), 2007, siteresources.worldbank.org/INTLACREGTOPURBDEV/Resources/840343-1178120035287/ModelMonterrey.pdf. Acceso: 06/08.

Machado, S.L., Carvalho, M.F., Gourc, J., Vilar, O.M., Nascimento, J.C.D., Methane generation in tropical landfills: Simplified methods and field results., Waste Management, Vol. 29, No. 1, 2009, pp. 153-161.

Marshall, A., Growing bigger., Waste Management World, Vol. 8, No. 2, 2007.

Meraz, L., Aranda, C., Domínguez, A., Producción de metano en relleno sanitario: Un Modelo Cinético Fractal., Memorias del XVI Congreso Nacional de Ingeniería Sanitaria y Ciencias Ambientales: La sustentabilidad en las grandes ciudades, AIDIS-FEMISCA, México, 2008, pp. 1-7.

NMX-AA-022-1985 Norma Mexicana NMX-AA-022-1985: Secretaria de Comercio y Fomento Industrial. Protección al Ambiente-Contaminación del Suelo-Residuos Sólidos Municipales-Selección y cuantificación de Subproductos, 1985, pp. 1-7.

Ojeda-Benitez, S., Armijo-de Vega, C., Ramírez-Barreto, M.E., Characterization and quantification of household solid wastes in a Mexican city., Resources, Conservation and Recycling, Vol. 39, No. 3, 2003, pp. 211-222.

SCS Engineers, Manual de Usuario Modelo Mexicano de Biogás Versión 2.0, 2009, http://www.epa.gov/lmop/int/users_manual_mexico_lfg_model_v2_2009.pdf. Acceso: 09/09.

SCS Engineers, Informe de la prueba de extracción y estudio de pre-factibilidad para recuperación y utilización de biogás en el relleno sanitario de Ensenada, México, Preparado por Landfill Methane Outreach Program, EPA-USA, 2007, pp. 29 -38.

SCS Engineers, Estudio de prefactibilidad para la recuperación y utilización en el relleno sanitario de Chihuahua, Chihuahua, México, 2005a, www.bancomundial.org.ar/lfg/archivos/PrefeasibilityStudies/Spanish_Portuguese/Chihuahua_PreFeasibility_Study_Spanish.pdf. Acceso: 06/08.

SCS Engineers, Estudio de prefactibilidad para la recuperación de biogás y producción de energía en el relleno sanitario de Querétaro, Querétaro, México, 2005b, http://www.bancomundial.org.ar/lfg/archivos/PrefeasibilityStudies/Spanish_Portuguese/Chihuahua_PreFeasibility_Study_Spanish.pdf. Acceso: 06/08.

SEMARNAT, Compendio de Estadísticas Ambientales 2008, 2008, http://app1.semarnat.gob.mx/dgeia/cd_compendio08/compendio_2008/03_residuos1.html. Acceso: 03/09.

Walter, G.R., Fatal flaws in measuring landfill gas generation rates by empirical well testing., Journal of the Air & Waste Management Association (1995), Vol. 53, No. 4, 2003, pp. 461-468.

Zeng, Y., Trauth, K.M., Peyton, R.L., Banerji, S.K., Characterization of solid waste disposed at Columbia Sanitary Landfill in Missouri., Waste Management Research, Vol. 23, No. 1, 2005, pp. 62-71.

Zhang, H., He, P., Shao, L., Methane emissions from MSW landfill with sandy soil covers under leachate recirculation and subsurface irrigation., Atmospheric Environment, Vol. 42, No. 22, 2008, pp. 5579-5588.

The authors wish to thank CONACYT for the funding provided for this study.