Anthropogenic Methane: Emission Sources and Mitigation Options

Kazi Bayzid Kabir1,2, Syeda Zohra Halim1

1Department of Chemical Engineering, Bangladesh University of Engineering and Technology (BUET), Dhaka-1000, Bangladesh

2Department of Chemical Engineering, Monash University, Clayton, VIC-3800, Australia

Email Addresses: bayzid.kazi@monash.edu (KBK); zohrahalim@che.buet.ac.bd (SZH)

Research Notes: Received 12 December 2010; received in revised form 10 January 2011; accepted 17 January 2011

ABSTRACT

Recent times have shown an increased awareness on climate changes and global warming. Rapid rise of temperature of the earth’s surface and atmosphere are partly due to continual increase in emission of greenhouse gases such as carbon dioxide, methane, nitrous oxide etc. This report focuses on methane as a greenhouse gas. It first identifies the various sources of methane emissions. Anthropogenic emission of methane comes from sources such as coal mining and the natural gas and petroleum sector; animal, human and industrial wastes, landfills, agricultural cultivation as well as biomass burning. The paper also discusses the various techniques that are available to reduce the emission of methane, particularly with reference to developing countries like Bangladesh. Such mitigation techniques involve modified methods of cultivation and usage of improved plant varieties, proper livestock management, landfill developments and better wastewater handling. Reduction of methane emission can not only reduce the greenhouse effect, but its collection and utilization can also replace the fossil fuels to some extent.

INTRODUCTION

Climate change is defined as long-term significant change in the average weather, including temperature, amount of precipitation, days of sunlight, and other relevant variables that can be measured at any given site, of a region or the earth as a whole. Climate change may occur because of internal changes within the climate system (e.g. glaciation, ocean variability, etc.), in the interaction between its components,  because of changes in external forcing either for natural reasons (e.g. increase in solar irradiance, volcanic dust, etc.) or because of human activities (Crowley, 2000; IPCC, 1995; Keller, 2004; Robock, 1978; Stott et al., 2001; van Geel et al., 1999). Intergovernmental Panel on Climate Change defines climate change as any change in the weather due to natural variability or human activity; United Nations Framework Convention on Climate Change (UNFCCC) usage of climate change refers to changes that is attributed directly or indirectly to human activity and in addition to natural climate variability observed over comparable time periods (IPCC, 2007a).

Global warming refers to increase in earth’s average temperature due to greenhouse effect. Water vapor (H2O) is the most important GHG; other important gases are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFCs). Without the GHGs the earth would be uninhabitable as these gases warm the earth’s atmosphere by trapping infra-red emissions from the surface; the average temperature of the earth (14°C) could be as low as -19°C without this effect (Le Treut et al., 2007). However, the problem lies with the anthropogenic global warming (AGW) caused by the enhanced greenhouse effect due to various human activities.

In the last 250 years, the atmospheric concentrations of CO2, CH4 and N2O have increased approximately by 36, 148 and 18%, respectively (Fig. 1). Fossil fuel burning, natural gas distribution, landfilling, activities related to agriculture, deforestation etc. are causing increased concentration of these gases in the atmosphere. The net radiative forcing has increased by 0.5-2.5 W/m2 since the start of the industrial era (about 1750), and leading to warming of the earth’s atmosphere (Fig. 2).

Over the last 100 years, mean surface temperature of the earth has risen by 0.74°C±0.18°C. However, this rate of warming has not been steady; the rate of warming over the last 50 years has almost doubled than that was in last 100 years. There has been substantial increase in heavy precipitation events, drought and intense tropical cyclone activities since 1970 (Trenberth et al., 2007). Global ocean temperature has risen by 0.10°C in the last 40 years. For the 20th century rise in sea level was 1.7±0.5 mm/yr due the volume expansion as well as melting of the ice cap; for the period 1993 to 2003 this rate was much higher; 3.1±0.7 mm/yr (Bindoff et al., 2007).

Bangladesh shares relatively low GHG emission: 0.36% of global GHG emission (WRI, 2005). However, Bangladesh is one of the countries more susceptible to climate change caused by global warming. Estimates by different models predict a sea level rise of 0.18 to 0.59 m by the end of the century (Meehl et al., 2007). Such increase would displace many residents of the Ganges delta from their home and livelihood. A 1-m rise in the sea level could submerge 11.5 percent of the land of Bangladesh (Philander, 2008). Again, higher sea levels could increase seasonal flooding by decreasing the drainage rate. Increased salinity, increased incidence of heat-related illness could be some other possibilities.

So it is of great importance for us to identify the GHG sources and minimize their emission. In addition to that significant amount of monetary gain can be realized by participating in the carbon trading market. The global carbon market has increased from $145 million to 2.3 billion since 2009 (CFU, 2010). Currently, there are few ongoing carbon trading projects in Bangladesh. However, possibilities and opportunities for better economic benefits are still there (Aziz and Chowdhury, 2009).

Use of nuclear and renewable energy sources (e.g. hydroelectricity, wind, solar etc.), switching from coal to gas, improved electricity supply and distribution system, use of more fuel efficient vehicles, improved rice cultivation and livestock and manure management, afforestation, reforestation, reduced deforestation, carbon capture and storage, and better waste management can mitigate GHG emission and subsequent impacts on the global environment (IPCC, 2007b).

This paper focuses on one particular GHG (methane, CH4) in Bangladesh context. Formation and emission of CH4, with respect to Bangladesh, are discussed here. Recommendations have been made regarding CH4 emission mitigation and subsequent reduction of the associated problems.

METHANE AS A GHG

Methane is one of the important trace gases in the atmosphere. It has an atmospheric life of about 12 years. It is a GHG with a high global warming potential (GWP) of 72 (averaged over 20 years) or 25 (averaged over 100 years) relative to CO2 (Forster et al., 2007). 70% of the global CH4 emissions are from anthropogenic sources and 30% from the natural sources (El-Fadel and Massoud, 2001). A recent study shows that methane emission rose by 27 million tons after nearly a decade of little or no increase (ESRL, 2008).

SOURCES OF METHANE EMISSION

The total amount of methane emission into the atmosphere by various sources globally are estimated to be approximately 500±100 Tg/yr (El-Fadel and Massoud, 2001). Emissions can occur naturally from oceans, fresh waters and even from insect activities, but the largest natural contributor is the wetlands. Any particular site in the wetland acts as a source of methane depending on the relative aerobic and anaerobic condition. Methanogenic bacteria work under anaerobic conditions to produce methane. If aerobic conditions prevail above the anaerobic environment, the methane may be oxidized as it moves up through this region. If wetlands are drained for agricultural purposes, then prevailing aerobic conditions can allow the site to act as a sink for methane (Amstel and Swart, 1994).

Anthropogenic emissions of methane can occur from the use of fossil fuels or can be generated from biogenic origin. Emissions from fossil fuel include those that are released from coal mining and from exploration of natural gas and petroleum. Of the biogenic sources, enteric fermentation that occurs in the stomach of ruminants as they digest high-cellulose food is one of the largest contributors to methane emission. Lerner et al. (1988) showed that as the number of herds of domesticated animals increased worldwide, so did the methane generation. According to this study more than 5000kg/km2/yr of methane was released from ruminants in Bangladesh.

Animal, human and industrial wastes and landfills are large sources of methane emission. The amount released depends on the type of waste, surrounding temperature and time.

Similar to the wetlands, flooded rice fields act as anthropogenic sources of methane due to prevailing anaerobic conditions. Information about the amount of methane released from paddies vary widely since it depends on the method adopted for cultivation, the climate, soil type and the amount and type of fertilizers applied (Amstel and Swart, 1994).

The other large source of methane emission is biomass burning. Burning fuel wood can produce large amount of methane due to incomplete combustion. Initially, when a fire is started, pyrolysis begins which generates fuel gases, including methane. In the next smouldering phase of the burning, charcoal is produced and methane is not oxidized and is eventually released into the atmosphere (Amstel and Swart, 1994).

National Methane Inventory of Bangladesh

Major sources of the methane emission in Bangladesh are
natural gas sector (natural gas venting and flaring, transmission and distribution losses) agriculture and livestock sector biomass burning landfills: municipal and industrial solid wastes, and industrial and municipal wastewater.

Ahmed et al. (1996) estimated greenhouse gas emissions in Bangladesh for the year 1990. According to the study, methane emissions from flooded rice cultivation were in between 257 and 622 Gg/yr. Methane emissions from livestock sector, gas sector and landfills were 453, 5.92 and 74 Gg/yr respectively.

METHANE EMISSION MANAGEMENT

Managing Rice Cultivation

Emission from rice cultivation can be significantly reduced without any detrimental effect on quality or yield. Intermittent irrigation can be effective compared to continuous flooding in reducing CH4 emission (Smith and Conen, 2004). However, availability of irrigation water is a major constraint for this practice.

CH4 emissions from rice varieties depend on plant physiology, gas diffusivity of the plant tissue and release of oxygen into the root environment. Studies among 10 high-yielding rice varieties showed that the differences in emission among them can be upto fourfold and the variety with highest yield produced lowest CH4 among them (Satpathy et al., 1998). Similar differences in emission had also been observed by some other studies (Huang et al., 1997; Shin and Yun, 2000). However extensive study on the seasonal variability, effect of soil textures, irrigation and fertilizer management is needed.

Straw and other organic residue additions in rice field are highly potent for methane emission. Composting before use has found to reduce emission by 58-63% while anaerobic digestion reduces emission by 10-16% (Wassmann et al., 2000). However, feeding agricultural residues to anaerobic digesters for biogas production and using discharge from the digester as fertilizer is now a common practice in many countries across Europe (IEA, 2008).

Using the chemical fertilizer only have a mitigation effect of 22-78% with a median of 46% (Zheng et al., 2000). Inhibitory effect of sulfate in CH4 formation causes 10-67% reduction in methane emission when ammonium sulfate is used instead of urea (Wassmann et al., 2000). However, all these studies on chemical fertilizers had excluded the release of the GHG encountered in their production.

Livestock Management

Livestock are one of the major sources of methane emission since Bangladesh has one of the highest-density livestock population in the world (Fao, 2005; Lerner et al., 1988). So far there are three possible options for reducing CH4 emission from this source: improved feeding practices (e.g. replacing forages with concentrates, addition of oil/oilseeds to the diet, improving pasture quality, optimizing protein intake), use of specific dietary additives for suppressing methanogenesis, and longer-term changes in farm management and animal breeding (Smith et al., 2007); none of these solutions are currently feasible for Bangladesh due to various socio-economic factors.

Animal manures can release significant amount of CH4 when stored and applied in the fields as fertilizer. Composting manures can reduce CH4 emission but may increase N2O formation. Retrieval of CH4 as a renewable energy from anaerobic digestion of manure is now widely used throughout the country (Islam et al., 2006; Islam et al., 2008). The digester residue has higher fertilizer value than the feedstock (Sørensen, 2004), while the produced CH4 replaces traditional fossil fuel and thereby reducing GHG emission to the atmosphere.

Landfill Management

All landfill sites in Bangladeshi cities are non-regulated. Significant amount of CH4 produced from anaerobic decomposition of organic fraction of municipal solid wastes (OFMSW) are lost to the atmosphere.

There are about 1000 landfill biogas plants around the world (Themelis and Ulloa, 2007). Landfill gas extraction systems use vertical wells or horizontal collectors. With appropriate extraction systems more than 90% of the produced gas can be utilized (Spokas et al., 2006). The recovered gas has applications similar to biogas and is being used for variety of purposes (Themelis and Ulloa, 2005).

Total solid waste generation in urban areas of Bangladesh was estimated to be 13,330 tonnes/day which is equivalent to 2.19 million tonnes of CO2-equivalnet GHG emission per year (Enayetullah and Hashmi, 2006). Large portions (79% in Dhaka city) of the generated wastes are organic. Therefore, landfilling sites are suitable for landfill biogas capture and utilization. Gas collected from the landfill sites can have applications similar to manure biogas.

Managing Industrial Wastewater

Wastewater discharged from the industries in Bangladesh is hardly treated before being discharged to the surface water (Karn and Harada, 2001). Like other waste treatment techniques wastewater can be treated under both aerobic and anaerobic conditions. Industries prefer anaerobic digestion in lagoons because of lower operating costs. These lagoon produce significant amount of CH4. Now-a-days, developed countries treat the industrial or municipal wastes in a central anaerobic treatment plants by converting the organic content to methane and using the gas as a source of energy or flaring where methane recovery is not feasible (El-Fadel and Massoud, 2001). The methane burned in the flares produce CO2 which has lower global warming potential (GWP) compared to CH4. Construction of centralized treatment plants in Bangladesh can therefore be effective for methane emission reduction from wastewater and could serve as a potential energy source.

CONCLUSION

Methane emissions from different anthropogenic sources are briefly discussed here with emphasis on Bangladesh. Manipulating traditional rice cultivation techniques, better agricultural and manure management, use of sanitary landfill sites and proper wastewater treatment can significantly decrease CH4 emission. In addition, extraction and recovery of methane and using the recovered gas as energy source can replace fossil fuels and therefore can also reduce the GHG emission associated with the production, processing, transportation and utilization of fossil fuels.

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Kabir, K. B. and Halim, S. Z., Antropogenic Methane: Emission Sources and Mitigation Options, ChE Thoughts 2 (1), 16-22, 2011.

 

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