Biogas is a gaseous mixture generated during anearobic digestion processes using waste water, solid waste (e.g. at landfills), organic waste, and other sources of biomass. Biogas can be upgraded to a level compatible with natural gas (‘green gas’) by cleaning (removal of H2S, ammonia and some hydrocarbons from the biogas) and by increasing its methane share (by removing the CO2). The resulting green gas can subsequently be delivered to the natural gas distribution grids. In developing countries, biogas could be an interesting energy option, in particular for those countries that rely heavily on traditional biomass for their energy needs.
Biogas is a gaseous mixture generated during anearobic digestion processes using waste water, solid waste (e.g. at landfills), organic waste, e.g. animal manure, and other sources of biomass (Welink et al., 2007). Anaerobic digestion is the biological degradation of biomass in oxygen-free conditions. In the absence of oxygen, anaerobic bacteria will ferment biodegradable matter into methane (40-70%), carbon dioxide (30-60%), hydrogen (0-1%) and hydrogen sulfide (0-3%), a mixture called biogas. Biogas is formed solely through the activity of bacteria. Although the process itself generates heat, additional heat is required to maintain the ideal process temperature of at least 35°C. In comparison, the methane component of natural gas could amount to over 80%. In nature, biogas is generated at the bottom of stagnated ponds, lakes, swamps or in the digestive system of animals (Jepma et al., 2006).
Biogas can be produced on a very small scale for household use, mainly for cooking and water heating or on larger industrial scale, where it can either be burnt in power generation devices for on-site (co)generation, or upgraded to natural gas standards for injection into the natural gas network as biomethane or for use directly as gaseous biofuel in gas engine-based captive fleets such as buses.
The feedstock, e.g. animal dung or sewage, is converted to a slurry with up to 95% water, and – for small-scale applications – fed into a purpose-built digester. Digesters come in many forms and sizes, which may range from 1 m3 for a small household unit to some 10 m3 for a typical farm plant and more than 1,000 m3 for a large installation (Larkin et al., 2004). Biogas production in such cases can be both continuous and in batches with digestion taking place for a period from ten days to a few weeks.
A small domestic biogas system will typically consist of the following components:
- Manure collection: raw, liquid, slurry, semi-solid and solid manure can all be used for biogas production.
- Anaerobic digester: The digester is the component of the manure management system that optimizes naturally occurring anaerobic bacteria to decompose and treat the manure while producing biogas.
- Effluent storage: The products of the anaerobic digestion of manure in digesters are biogas and effluent. The effluent is a stabilized organic solution that has value as a fertilizer and other potential uses. Waste storage facilities are required to store treated effluent because the nutrients in the effluent cannot be applied to land and crops year round.
- Gas handling: piping; gas pump or blower; gas meter; pressure regulator; and condensate drain(s).
- Gas use: a cooker or boiler (EPA, 2010).
For applications on a larger scale, feedstocks such as sewage sludge from waste water treatment plants, wet agricultural residues and the organic fraction of municipal solid waste (MSW) can be collected and used. Biogas can be used for all applications designed for natural gas, given a certain upgrading of its quality (IEA Bioenergy Task 37, 2005).
Upgrading can be done to a level compatible with natural gas (‘green gas’) by cleaning (removal of H2S, ammonia and some hydrocarbons from the biogas) and by increasing the methane share (by removing the CO2). (Welink et al., 2007).
Small scale applications
Small scale biogas for household use is a simple, low-cost, low-maintenance technology, which has been used for decades across the developing world. Such small-scale applications are mostly implemented through programmes supported by governments. In such cases, it usually concerns rural areas and communities without connection to the grid. Although some cattle would be needed to feed the digester (about seven) and water needs to be available as well, other requirements are rather low.
Data on biomass use is often hard to access and difficult to evaluate because of the diversity in consumption patterns, differences in units of measurement, the lack of regular surveys and the variation in heat content of the different types of biomass.
The switch to biogas in cooking is not without challenges. According to the IEA, with an increase in income, households do not simply switch from one fuel to another. The use of multiple fuels in parallel may enhance energy security compared to reliance on a single fuel or technology. Besides, traditional food preparation processes are not easily being overhauled because of taste preferences and the familiarity of cooking with traditional technologies. Nevertheless, in the long run and on a regional scale, households in countries that become more wealthy are generally projected to shift from cooking exclusively with biomass to using more efficient technologies, amongst which biogas can be one option (IEA, 2006, 2008).
Currently, low per-capita incomes and a lack of awareness of the benefits of more sustainable fuels provide an important barrier. Therefore, financing investments in biogas installations, especially in least developed countries, is a problem. Hence, financing programmes and additional incentives are clearly needed to deal with this general reluctance among the target group. In India, for example, many of the biogas plants are concentrated on wealthier farms with a relatively large number of cattle (Boyle, 2004).
Industrial applications are designed to process large amounts of feedstock into biogas, which requires a well-developed logistical system for feedstock collection and effluent disposal. Because of costs associated with feedstock collection, the viability of such plants depends on the availability of very cheap or free feedstock such as sewage sludge, manure, agricultural residues or organic fractions of municipal solid waste. Decentralized farm-size units are increasing productivity by supplementing their feedstock with agricultural residues or crops (IEA Bioenergy, 2009).
Both small and large scale anaerobic digestion is a well established commercial technology. Today, the highest degree of market maturity can be found in the area of municipal sludge treatment, industrial wastewater purification and treatment of agricultural wastes (GTZ, 2009). Improvements still need to be achieved in the use of contaminated feedstock, where biomass pre-treatment and separation processes are needed to remove contaminants which may end up in the digestate, making it unsuitable as fertilizer and difficult to dispose of (IEA Bioenergy, 2009).
There are regional differences in the application of biogas technologies, depending on the local situation and infrastructure available. In rural Asia, mostly small-scale biodigesters are used, that generate enough energy for farmers to become self-sustaining. According to the Dutch development organisation SNV (personal communication), which manages a small-scale biogas installations programme in Vietnam, an average Vietnamese farmer’s family need about five pigs and two cows in order to produce enough biogas for cooking. In general, in developing countries, biogas is mostly used for cooking, heating and lighting with a strong emphasis on the former two.
At present, China is the biggest biogas producer in the world, with around 18 million farm households using biogas and about 3,500 medium to large-scale digester units (DEFRA 2007). The use of the technology in municipal wastewater treatment has increasingly been deployed in Asia (India in particular) and Latin America. Agricultural biogas plants in developing countries are usually promoted as part of the solution to energy and environmental issues, in particular where liquid manure from agriculture causes severe water pollution (GTZ, 1999). Large biogas for domestic use programs have been rolled out in several developing countries, notably in Nepal, where around 150.000 biogas installations have been build over ten years (Bajgain & Shakya, 2005). A number of biogas-based CDM projects exist.
In Europe, on the other hand, demand for biogas comes mostly from the power generation and industry sectors (grid-connected). Although a significant potential exists for using biogas for electricity generation, further improvements are yet to be realised for a large-scale introduction. In the UK, for example, in the mid-1990s the total installed capacity remained under 1 MW. The first large-scale plant in the UK was commissioned in 2002 and built on the basis of a German/Danish design. It uses 146,000 tonnes of slurry per year from 28 farms, together with wastes from food processors, to supply the heat input for a generating capacity of 1.43 MW. The total efficiency in such large-scale applications depends on how the generated heat is used in the process. In Europe, a typical biogas plant has an average capacity of 250 to 300 kW with a minimum recommended capacity of 200 to 250 kW. The electricity delivered by the plant to the grid provides an extra source of income. In the whole of Europe, 5.9 Mtoe of biogas were produced in 2007 (Eurobserv’ER, 2008).
Because biogas can make a positive contribution to multiple goals in government programmes, it has the potential to increasingly become one of the most efficient and economical sources of renewable fuel with anaerobic digestion an economically viable technology for both small-scale rural applications in developing countries and for a range of scales in the developed world (IEA Bioenergy, Task 37, 2005). Therefore significant growth is expected in the coming years.
Both small and large scale biogas applications offer several direct and indirect benefits
Small scale applications
- Smoke-free and ash-free kitchen, so women and their children are no longer prone to respiratory infections;
- Women are spared the burden of gathering firewood;
Environmental and health benefits
- Keeping manure and waste in a confined area and processing themin the digester reduces the amount of pollutants in the immediate environment and increases sanitation;
- Households no longer need to extract wood for cooking, which can reduce deforestation levels where people heavily rely on woodfuel;
- The sludge remaining after digestion is a good fertilizer, increasing land productivity (and farm incomes).
- The release of methane is avoided thus contributing to climate mitigation. A single, small scale biodigester reduces between 3 and 5 tCO2-eq./year,
- Buying (fossil) fuel resources (e.g. kerosene, LPG, charcoal or fuel wood) is no longer needed
- Switching from traditional biomass resources (e.g., in developing countries) or fossil fuels (e.g. in industrialised countries) to biogas fired generation capacity improves security of energy supply (locally as well as nationally or regionally) as the feedstock can mostly be acquired locally
Figure 1 shows the benefits of a domestic biodigester in Cambodia, based on an analysis in the frame of Cambodia’s National Biodigester Programme.
Possible negative aspects of the biogas installations are the possible reduction in soil fertility since animal dung is now used as feedstock for the biogas installation instead of for fertilisation. This aspect can be addressed by using the bioslurry that remains as a side-product of the biogas production process for soil fertilisation. Another potential problem is related to the possible build-up of pathogens (worms, protozoa and some fatal bacteria such as salmonella) in the biogas system. A study carried out for biogas systems in Nepal has shown that some pathogens were present in the bio-slurry. Studies have been undertaken to explore whether the biogas systems could enhance the breeding of mosquitoes. However, no direct relation was found between biogas production and mosquito breeding (Netherlands Ministry of Foreign Affairs, 2007).
Industrial scale digesters also offer a number of benefits
- Biogas can contribute to replace fossil fuels, thus reducing the emission of GHGs and other harmful emissions;
- By tapping biogas in a biogas plant and using it as a source of energy, harmful effects of methane on the biosphere are reduced;
- Industrial estates can, by processing their waste in a biogas plant, fulfill legal obligations of waste disposal while at the same time, generate energy for production processes, lighting or heating;
- Municipalities can use biogas technology to solve problems in public waste disposal and waste water treatment (GTZ, 1999);
- It’s a natural waste treatment process;
- Requires less land then anaerobic composting;
- Reduces disposed waste volume and weight to be landfilled;
- It generates high quality renewable fuel proven to be useful in a number of end-use applications
- It significantly reduces GHG emissions
- It maximizes recycling benefits
- Considering the whole life-cycle, it is more cost-effective then other waste treatment options (IEA Bioenergy, Task 37, 2005).
Green gas or biogas offers several sustainable development benefits since it is a clean and GHG-neutral source of energy. Most of the biogas has a methane component of 50 to 60%, a CO2 component of 35 to 50%, and a relatively small amount of hydrogensulfide (H2S) and ammonia. In comparison, the methane component of natural gas could amount to over 80%. Applying the CDM to biodigester type of projects or programmes has proven, to be problematic for various reasons. First, a single biodigester reduces between 3 and 5 tCO2-eq./year, which makes bundling of project activities necessary in order to be able to cover transaction costs. Second, although small-scale methodologies for the accounting of GHG emission reductions have been applied to biogas projects in Nepal (see below), they have proven cumbersome to apply in the context of biodigester programmes and unpractical.
For calculation of these GHG emission reductions, it is recommended to apply the approved methodologies for thermal energy production with or without electricity, thermal energy for the user project (large scale activities) which has been developed under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html.
The major cost categories for a biodigester are (GTZ, 1999):
- manufacturing or acquisition costs (production costs): all expenses and lost income which are necessary for the erection of the plant
- operation and maintenance costs (running costs): acquisition and handling of the substrate (feedstock), if not acquired externally, feeding and operating of the plant; supervision, maintenance and repair of the plant; storage and disposal of the slurry; gas distribution and utilization;
The production costs of biogas plants are determined by the following factors:
- purchasing costs or opportunity costs for land which is needed for the biogas plant and slurry storage;
- model of the biogas plant;
- size and dimensioning of the biogas unit
- amount and prices of material
- labor input and wages
- the degree of participation of the future biogas user and his opportunity costs for labor.
A rough estimate of costs of a simple, unheated biogas plant, including all essential installations but not including land, is between 50-75 US$ per m3 capacity. 35 - 40% of the total costs are for the digester (GTZ, 1999). The Biogas for Africa initiative estimates the cost of a small household unit somewhat higher at 600-800 eur per unit (Biogas for Africa, 2007). For small scale applications in developing countries, the farmer typically contributes to financing the digester with payback periods depending on the price of otherwise purchased firewood/kerosene, as the digester has zero fuel costs. Only water and dung or leafy biomass material need to be collected.
For larger plants producing electricity from biogas, a rough estimate of capital costs of a digester and an engine of 0.3-10MW is between 3500 and 5500 US$/kWe (IEA Bioenergy, 2009).
[This information is kindly provided by the UNEP Risoe Centre Carbon Markets Group.]
The following CDM methdologies are applied for biomass gasification projects: AMS-I.D.: Grid connected renewable electricity generation, AMS-III.I.: Avoidance of methane production in wastewater treatment through replacement of anaerobic systems by aerobic systems, AMS-III.H.: Methane recovery in wastewater treatment, AMS-I.A.: Electricity generation by the user and AMS-I.C.: Thermal energy production with or without electricity for small-scale projects.
As of March 2011, there are 14 biomass gasification projects in the CDM pipeline, out of which 8 are registered and for 1 project CERs have been issued.
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