What is biogas?
Biogas is produced by the digestion of biomass, manure and/or biosolids under anaerobic (without oxygen) conditions. Biogas contains 60 to 80 percent methane, 20 to 40 percent carbon dioxide, and trace amounts of other components such as hydrogen sulphide, ammonia, and water vapour. This high methane content makes biogas an excellent source of renewable energy to replace natural gas and other fossil fuels. The methane in the biogas can be separated, purified and combusted to run a generator producing electricity and heat, or it can be burned as a fuel in a boiler for direct heat. Biologically produced methane is also a potential sustainable source of hydrogen for use in fuel cell technologies.
Potential agricultural feedstocks for biogas production include manure (hog, dairy, beef and poultry) and biomass from energy crops (maize, perennial grasses, sorghum and winter rye). |
Did you know?
Bio-gas was first used for heating bath water in Assyria during the 10th century BC. In the 17th century, it was discovered that the decaying organic material produced flammable gases and that there was a direct connection between the amount of organic material and how much gas was produced. The first anaerobic digester was built India in 1859 to provide light to an isolated leper colony. The first commercial use of biogas was in England in 1895 when it was recovered from sewage systems and used to fuel street lamps. In the 1930’s, research led to the discovery of the bacteria responsible and conditions required for anaerobic digestion. In 1978, Cornell University built the first plug flow digester that was able to digest the manure from sixty cows. |
| Feedstocks are introduced to the anaerobic digester, producing biogas and a residual biomaterial, which can be used as a fertilizer and soil amendment. There are four broad categories of biogas generation technologies, defined by their scale and the feedstock they are produced from: |
1. On-farm digesters using only manure and/or energy crops produced;
2. Centralized digesters using manure, energy crops and/or industrial organics from a wide range of sources;
3. Municipal sewage treatment digesters using municipal biosolids as a primary feedstock;
4. Waste water treatment systems used by food and beverage processing companies where biogas is produced as a by-product and either flared off or co-fired into boilers to produce steam.
Canada has many large farms. These agricultural operations produce a significant amount of organic waste in the form of manure and animal remains which can contaminate groundwater and produce unwanted odours. Handling such large amounts of organic waste in an environmentally friendly manner is a challenge, and using anaerobic digesters is a promising option to tackle this challenge.
The need to find renewable ways to produce heat and electricity in Canada is also driving interest in the development of biogas. Biogas has the potential to co-generate heat and electricity at the farm-level, be exported to nearby users and communities or sold back to the electrical grid. Canadian Biogas production is currently at 8038 TJ, and expected to increase.
Ontario is the main province leading the development of biogas in Canada. In Ontario, federal and provincial incentives for green power generation amount to approximately 6.5 cents/kwh. The Ontario government also offers the $9 million “Biogas Systems Financial Assistance Program” to help farmers and agri-food businesses develop and build generating systems that produce clean energy, reduce electricity costs and contribute to local economies. The program can cover costs of carrying out a feasibility study and eligible construction and implementation costs up to a maximum of $400,000 for each anaerobic digester system.
Energy feedstocks for biogas production
There are four ingredients needed to produce biogas, namely: organic matter, bacteria, anaerobic conditions and heat. The organic matter acts as the food source for the bacteria, with methane produced during digestion of the biomass. Potential sources of organic matter for farm-based biogas production include manure, crop wastes and residues, and dedicated energy crops. Each cubic meter of biogas produces approximately 23.2 MJ of energy, but this value can vary from 20-25 MJ/m3 depending on the methane content of the gas produced.
Livestock manure
Canadian livestock currently produce an estimated 320 million tonnes of manure every year. Of this total, 52% is produced by beef cattle, followed by dairy cows (19%), hogs (16%), calves (7%), poultry (3%), horses (3%) and sheep (less than 1%).
Livestock manure contains organic solids including fats, carbohydrates, proteins and nutrients that provide food and energy for the growth and reproduction of anaerobic bacteria. Most livestock manure (particularly from swine and poultry) contains appreciable amounts of nitrogen, which is converted to ammonia in the digester. Biogas yields differ between manure from different animals and can range from 19 to 96 m3 per tonne of manure. The biogas yield from dairy manure is on the low end, between 25 and 32 m3 per tonne of fresh mass. This is due to the low organic dry matter (ODM) content of the material and because most of the energy-rich substances have already been digested by the animals. The relatively slow digestion of manure is the main reason the use of manure is not economical in many applications unless incentives are provided for green power generation, GHG mitigation or odour reduction. Digestion of low cost co-substrates such as food processing wastes along with manure can help lower biogas production costs to make the systems more economical. |
Biogas yield potential of livestock manures
| Manure source |
Solids (%) |
Biogas yield (m³/t) |
| Beef cattle |
8 - 12 |
19 - 46 |
| Hog |
9 - 11 |
28 - 46 |
| Dairy |
12 |
25 - 32 |
| Poultry |
25 - 27 |
69 - 96 | |
| In eastern Canada where relatively strong green power incentives are available in Ontario, energy crops may become economically viable to grow for biogas production in the future. They have been the primary feedstock for the rapid growth of biogas digesters in Germany, where biogas now exceeds wind power as a means to produce green power. |
Corn silage
In Europe, silage corn is the most common energy crop grown for methane production. Methane yield per hectare is influenced by the yield and digestibility of the variety of corn, moisture content at harvest and the fineness of the silage chop. Late ripening corn varieties make better use of the solar radiation in the growing season to produce more biomass than medium or early ripening corn varieties. Maximum methane yield per hectare is achieved from selecting corn varieties that have both high yield and high digestibility. In Europe, corn cultivars are already ranked for biogas production per ha. Corn yields in Eastern Canada average approximately 8.4 t/ha (at 15.5% moisture content). This yield can be doubled when silage is harvested and the whole plant biomass is used, with silage yields in eastern Canada approximately 13 oven dry tonnes per hectare. Corn seeds are also planted at higher density for silage compared with grain, another reason why silage yields are higher. Because of whole plant harvest, corn silage captures considerably more energy per hectare than grain corn. When transforming silage yields into energy, corn silage produces 500 m3 biogas per ODT, equivalent to approximately 151 GJ/ha. In Germany and France, new more productive corn varieties are now being tested which incorporate genes from sub-tropical maize cultivars that stay vegetative for much of the growing season. 'Giant maize' varieties are being developed specifically for biogas production with very high dry matter, high starch content and low lignin content. These new silage maize varieties harvested for year round supply of biomass for digesters, can further improve the energy production potential capacity of corn silage relative to grain corn production. |
Graph Img |
Sorghum
Sorghum and sweet sorghum are now viewed in Europe as important new energy crops for biogas. They may be able to produce as much or more energy than corn silage. Sweet sorghum is especially suited for biogas production because of the large content of sucrose in the stem. However, the major advantage of sorghum compared with corn is that it is more drought tolerant and can grow better on lower quality soils with lower levels of fertilizer. Depending on the cultivar selected it can also be harvested twice during the growing season. The high yields and high water use efficiency of sorghum may make it an important biogas energy crop for North America in the future. Research is required to optimize sorghum and sweet sorghum as energy crops for biogas production in Canada. |
Did you know?
African slaves first introduced sorghum, which then was known as "Guinea corn," into North America in the early part of the 17th century. A cane-like plant with a high sugar content, sweet sorghum has been widely cultivated in the U.S. since the 1850s, primarily for forage, silage, and sugar production. By the early 1900's, the U.S. produced 20 million gallons of sweet sorghum syrup annually. Making syrup from sorghum (as from sugar cane) is heavily labor intensive and production fell drastically following World War II with the declining availability of farm labor. However, sorghum is still produced for forage and silage in the Great Plains (Texas, Kansas, and Nebraska) where insufficient rainfall and high temperature make corn production unprofitable. |
Perennial grasses
Cool season perennial grasses are commonly used as co-substrates for biogas digesters in Europe. These are typically used in digesters as grass silage at 65% moisture content. In the future, warm-season grasses that have been selected for improved digestibility for livestock feeding such as big bluestem and switchgrass may also be viable feedstock for biogas production. These can also produce higher biogas yields per hectare than cool season grasses in southern zones of eastern Canada. In general, perennial grasses are characterised by a moderate to high yield and a low energy input to produce the crop, low cost of production and low nutrient requirements. Silage yields obtained would be expected to be 9 ODT per hectare and 400 m3 biogas per tonne, equivalent to approximately 84 GJ of energy per hectare.
Potential biogas yield of selected energy crops in Eastern Canada
|
Crop |
Feedstock yield (t/ha) |
Biogas yield
(m3/t) |
Biogas production
(m3/ha) |
Energy production
(GJ/ha) |
|
Corn / sorghum silage |
13 |
500 |
6500 |
151 |
|
Perennial grasses |
9 |
400 |
3600 |
84 |
|
Winter rye cover crop |
5 |
400 |
2000 |
46 |
Winter Rye
Winter Rye is a common winter cover crop, sown after cash crops are harvested in the fall. It is adapted to a wide range of conditions across North America and seed is inexpensive. Winter Rye is the most winter-hardy cereal crop available and is the first to break dormancy in the spring. Rye is easy to establish and is characterized as being the best cereal for absorbing unused soil nitrogen. In the spring it can produce a significant amount of biomass in the spring, yielding approximately 5 ODT per hectare and 400 m3 biogas per tonne, equivalent to 46 GJ per hectare. It may be viable in southern Ontario as a rotation crop planted after corn silage harvest and harvested prior to soybean planting in the subsequent year. In this way it would have a limited land cost associated with its production and make an excellent winter cover crop. In western Canada, winter rye could be sown as a full season energy crop for biogas production as is done in Germany.
Economics of biogas production
Anaerobic digesters produce biogas for electricity, heat and transportation applications. They also have a number of useful by-products such as decontaminated bio-solids (originating from the manure and biomass) suitable for use as fertilizer or bedding, along with waste heat and treated water from farm sewage. The digestion process allows farmers to comply with environmental safeguards around manure and water management. It also eliminates much of the odour resulting from livestock operations. However, these by-products such as waste heat, treated water, odour reduction and environmental benefits are considered non-market goods and can be difficult to price. The waste heat can only be used in close proximity to the generator and its economic value varies greatly depending upon how it is used. Some applications include providing heat for on-farm processing requirements and greenhouses, or the drying of solid waste for bedding. The value of odour reduction is especially difficult to estimate. Large livestock farming operations near populated areas would be good candidates for biogas installations for odour reduction.
Costs
| The capital cost of a complete anaerobic digestion system includes expenses for the digester itself, engine-generator and switchgear, feedstock collection, gas handling equipment, effluent separation, storage equipment, engineering costs, permitting fees, and labor. A system that is both well engineered and maintained has the capacity to remain functional for at least 20 years. At present, it is difficult to estimate the capital costs involved in a Canadian anaerobic digester system with any degree of precision, due to a lack of an installed base for comparison, the large variety of designs available, and the degree of customization required for each installation. |
Practical tip.
The AgSTAR FarmWare 3.0 software is a computerized decision support program that assesses whether or not a methane production, capture, and utilization system can be integrated into your farm’s existing or planned manure management system. FarmWare estimates how much the system will cost and the financial benefits that may be gained by producing energy for on-farm use or sale or both.
http://www.epa.gov/agstar/resources/handbook.html |
| The costs of establishing and running a methane digester are also greatly dependent on the specific type and size of the digester. The size of the digester will mainly be determined by the average daily amount of feedstock produced. Generally for on-farm anaerobic digester systems, the capital cost is estimated to be about $50-75 per m3 of capacity. This rough approximation can be lower with larger scale on-farm systems and should be considered to have a plus or minus 30% range to incorporate variances. To reduce capital costs, digesters may be built with local construction materials to local specifications. |
Operating costs for the anaerobic digester include the acquisition of raw materials, water for mixing materials, feeding and operation of the plant, preventative and on-going maintenance, supervision, storage and disposal of the slurry, gas distribution and utilization, and administration. Anticipated operation and maintenance costs of an unfamiliar technology may be a factor discouraging many operators from implementing anaerobic digestion projects. In reality, however, these costs tend to be modest. Once installed, the cost of operation and maintenance on the system is heavily dependent on the quality of the equipment used and the dedication of the operator in keeping the system running as well as possible. A good rule of thumb is that annual operation and maintenance costs are approximately 5% of the initial capital cost of the system.
Revenues
Revenues from anaerobic digestion come from the sale or use of the end product of biogas (for electricity and/or heat) and by-products like the solid sewage for utilization as a fertilizer. Savings can also be had from reduced on-farm use of natural gas and propane, and bedding. Farmers may also generate additional revenues by adding off-farm waste materials such as food processing wastes as co-feedstocks, as industry currently pays to dispose of these materials. However this may require special permits which may be difficult to acquire.
Biogas is generally used to generate electricity and/or heat. The most efficient way is to generate electricity and heat in a combined heat and power generator with a typical efficiency of 90% (30% electricity and 60% heat). Generally, biogas systems generate more energy than can be used on-farm. Farm energy demands are not static and constant and there are peaks in energy demand that biogas systems may not be responsive enough to supply. Therefore, connecting to the grid is recommended. The grid acts as a large battery with the biogas system putting energy in and the farm drawing energy out. Net metering is a system where the energy generator (the farmer) pays the electricity distributor only for the net amount of electricity required over the year. When a farmer produces more energy then required it is possible to register as an energy generator and the farmer receives the base price as a producer of energy. The revenues and rules for being an electricity generator in Canada differ between each province.
Some biogas systems are designed exclusively to combust biogas for heat. In addition to heating the digester, buildings or hot water, it is sometimes used to heat, dry or process agricultural products and for heating greenhouses. Finding a use for surplus heat can be a significant contributor to making biogas systems economical. Since heat losses are important when steam or hot water is pipelined, the heat generally needs to be used in close proximity to the digester.
Biogas can also be used as a transport fuel. Biogas, if compressed for use as an alternative transportation fuel in light and heavy duty vehicles, can use the same technology currently being employed in compressed natural gas vehicles. The biogas first needs to be enriched to achieve methane content of 97%, equivalent to an energy content of 36.6 MJ per m3 which is comparable to a litre of petrol. One m3 of biogas produces 0.55-0.68 m3 of enriched biogas.
A byproduct of the anaerobic process is the production of composted fertilizer from the biomass of the original feedstock, which can be applied directly to fields and is less costly than chemical fertilizers. Once applied, the compost will be more predictable in nutrient release, allowing the farmer to reduce commercial fertilizer use. Other co-products from various digester designs include fiber, struvite and heat. Struvite is a slow-release, crystallized solid that is rich in nutrients, including nitrogen and phosphorus, and therefore suitable as a fertilizer. The fiber can also be used for lower-value products such as organic soil amendments and animal bedding, or for higher-value products such as a growing medium for plants by greenhouses and horticultural nurseries.
Budgets for biogas digester of various scales
Mallon and Weersink (2007) estimated the construction and operation costs and revenue of 4 different-sized on-farm biogas digesters ranging from less than 100kW to larger than 300kW with total electricity outputs from 800,000 to 3,200,000 kWh annually. Using Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) Agricultural Anaerobic Digestion Calculation Spreadsheet (AADCS) the anaerobic digestion inputs, outputs, cost and revenues were estimated and used to conduct a financial analysis. The results from their study suggest that anaerobic digestion systems smaller than 300 kilo-watts are not financially feasible under the assumptions used and revenues offered by Ontario’s Standard Offer Contract to purchase renewable energy. However, incorporating off-farm organic material improved financial feasibility by increasing biogas production and offering potential tipping fee revenues.
| CASE STUDY 1: Financial Feasibility of 4 sizes of on-farm Biogas Digesters in Ontario (Mallon and Weersink, 2007) |
| Digester Assumptions and Capital Costs |
Digester Size |
|
|
<100kW |
100-200kW |
200-300kW |
>300kW |
| Standard Offer Electricity Price |
|
|
|
|
| Real Non-Peak Electricity Price ($/kWh) |
0.11 |
0.11 |
0.11 |
0.11 |
| Real Peak Electricity Price ($/kWh) |
0.145 |
0.145 |
0.145 |
0.145 |
| Farm Inputs |
|
|
|
|
| Livestock Manure (Metric tonnes/year) |
16,606 |
33,258 |
49,864 |
66,470 |
| Anaerobic Digester Yearly Operation Time |
|
|
|
|
| Non-Peak Operating Time (Hours/year) |
6,000 |
6,000 |
6,000 |
6,000 |
| Peak Operating Time (Hours/year) |
2,000 |
2,000 |
2,000 |
2,000 |
| Total Operating Time (Hours/year) |
8,000 |
8,000 |
8,000 |
8,000 |
| Anaerobic Digestion Output |
|
|
|
|
| Non-Peak Electricity Output (kWh/year) |
599,133 |
1,199,926 |
1,799,059 |
2,398,192 |
| Peak Electricity Output (kWh/year) |
199,711 |
399,975 |
599,686 |
799,397 |
| Total Electricity Output (kWh/year) |
798,844 |
1,599,901 |
2,398,745 |
3,197,589 |
| Anaerobic Digester Capital Cost |
|
|
|
|
| Size of AD System (kW) |
99.9 |
199.9 |
299.9 |
399.9 |
| Per kW Capital Cost ($/kW) |
$5,740 |
$5,096 |
$4,452 |
$3,477 |
| Total Capital Cost ($) |
$573,426 |
$1,018,690 |
$1,335,155 |
$1,390,452 |
| Additional Parameters |
|
|
|
|
| Real Discount Rate (%) |
8 |
8 |
8 |
8 |
| Investment Period (Years) |
10 |
10 |
10 |
10 |
| Income Tax Rate |
30 |
30 |
30 |
30 |
| Reduction in Organic Material (%) |
5 |
5 |
5 |
5 |
| Organic Material Application Cost ($/metric tonne) |
2 |
2 |
2 |
2 |
| Financial Analysis |
|
|
|
|
| Revenue |
|
|
|
|
| 6000 Hours Non-Peak Electricity Revenue |
$65,905 |
$131,992 |
$197,896 |
$263,801 |
| 2000 Hours Peak Electricity Revenue |
$28,958 |
$57,996 |
$86,955 |
$115,913 |
| Total Electricity Revenue |
$94,863 |
$189,988 |
$284,851 |
$379,714 |
| Manure Reduction Revenue |
$1,661 |
$3,326 |
$4,986 |
$6,647 |
| Total Revenue |
$96,523 |
$193,314 |
$289,837 |
$386,361 |
| Expense |
|
|
|
|
| Anaerobic Digester Annual Operation and Maintenance Costs |
$33,838 |
$67,770 |
$101,608 |
$135,446 |
| Total Expenses |
$33,838 |
$67,770 |
$101,608 |
$135,446 |
| Annual Net Revenue Before Tax |
$62,685 |
$125,544 |
$188,229 |
$250,915 |
| Income Tax and Depreciation |
|
|
|
|
| Depreciation |
$28,659 |
$50,958 |
$66,749 |
$68,531 |
| Tax Savings |
$8,598 |
$15,287 |
$20,025 |
$20,559 |
| Annual Net Revenue After Tax |
$71,283 |
$140,831 |
$208,254 |
$271,474 |
| Financial Measures |
|
|
|
|
| Payback Period (Years) |
9.1 |
8.2 |
7.3 |
5.8 |
| Simple Rate of Return (%) |
11.0% |
12.2% |
13.8% |
17.2% |
| Net Present Value ($) |
-$131,519 |
-$155,268 |
-$72,844 |
$225,289 |
| Internal Rate of Return (%) |
1.9% |
4.0% |
6.6% |
11.9% |
Environmental impacts associated with biogas production and use
Agricultural Sustainability
The high levels of manure produced from intensive livestock operations has become a serious threat to water and soil contamination. Manure can contain high levels of pathogens such as E. coli that can contaminate groundwater and are potentially hazardous to human health. Manure can also contain high levels of nutrients, particularly phosphorus and nitrogen, and other pollutants that once introduced to water, can have severe impacts on ecosystem health and function. In addition, conventional manure disposal produces strong odours that may impact air quality in surrounding areas. The combination of community pressure and increasingly restrictive environmental standards is motivating farmers to seek alternative ways to manage manure.
Problems with nitrate pollution in groundwater from fertilizer application and waste disposal from intensive livestock operations can be mitigated through the use of anaerobic digestion. During the anaerobic digestion process, manure is composted at high temperatures. Organic nitrogen is mineralized and odour production is minimized, leading to more available nutrients in the compost than in the original energy feedstock. Upon field application, the compost will release nutrients in a more slow and steady manner, allowing the farmer to reduce commercial fertilizer use through the growing season. However, as with all fertilization regimes, if crops are not available for uptake, application of fertilizer may increase nutrient loss. Longer storage of the compost or use of cover crops to hold the nutrients can be strategies to alleviate water contamination. The high level of heat achieved in the digester also ensures that the compost produced is relatively free of disease, germs and weed seeds and has a reduced potential to contaminate water. This is of particular importance to organic farming systems. The compost produced is easy to handle, protects against insects, fungi, bacterial diseases, and soil erosion. The digested material is also easier to agitate, pump and move through small distribution pipes used in an applicator.
The cultivation of annual crops for biogas such as corn silage can cause soil erosion and often require high amounts of production inputs. It is important to grow these crops in rotation as intensive cropping leads to soil compaction, soil nutrient and organic matter depletion, and decreased crop yields. Maintenance of soil organic matter is generally favourable with energy crops for biogas as the carbon rich effluent can be returned to the field. The main risk is soil erosion. Leaving approximately 30% residue cover on the soil is recommended to reduce erosion. This may be difficult to achieve under corn silage harvest. Interseedings of ryegrass grown in corn harvested for silage may help significantly improve soil cover. Winter rye seeded after corn silage harvest may also considerably reduce erosion potential while providing an excellent biogas crop. The cultivation of warm-season perennial grasses also offers environmental benefits due to their low-input requirements and deep rooting structure, adding organic matter to the soil, protecting soil from erosion, biodiversity enhancement and reducing the use of chemical pesticides and fertilizers.
Energy production versus consumption
The energy output to input ratio for biofuels is the total amount of energy contained within the end product (the output), relative to the total amount of energy used in the production and processing of the crop (the input). The higher the ratio value, the higher the sustainability of the energy production system. The energy output to input ratio of biogas is approximately 6:1, which is high compared to other bioenergies such as biodiesel and bioethanol that have energy ratios between 1.25:1 and 3.43:1, respectively. This is because whole plant biomass is used as the feedstock and the gas production process is largely biological requiring limited outside energy inputs from fossil fuels to turn plant matter into biogas.
Greenhouse gases
The use of biogas reduces CO2 emissions by displacing the use of fossil fuels. The reduction depends on the type of feedstock for biogas and which fossil fuel is substituted. Manure is considered as a waste product so no greenhouse gasses are associated with its production, however there are 39.44 kg CO2e/GJ of emissions produced during feedstock collection, feedstock transport, electricity generation and digestate transportation and spreading. When comparing this to conventional power generation sources however, the utilization of manure biogas can reduce GHG emissions by 87%, 86% and 68% by substituting coal, oil and natural gas respectively. |
Graph Img |
In addition, the decomposition of manure in the field or in lagoons and holding tanks emits significant amounts of GHG’s. Methane emissions from manure depend on the type of animal, the amount of manure produced, the digestibility and quality of the animal feed, the condition of the digestive tract, and the quality of the feed consumed. The manure handling method (liquid versus solid storage) and environmental conditions such as temperature and moisture also affect methane emissions. Liquid manure management systems such as ponds, lagoons, and holding tanks lead to anaerobic conditions and can create very high methane emissions, while solid manure emits little or no methane. Capturing uncontrolled methane emissions and composting manure through anaerobic digestion greatly reduces the emission of these gases to the atmosphere. These systems are not completely efficient with some losses from the digester itself (due to leakages) and from methane trapped in the manure, released once the compost is applied. However, even though there are losses, capturing methane emissions from livestock manure can present a significant reduction of overall Canadian GHG emissions. Substituting compost for conventional fertilizers can further reduce GHG emissions by eliminating the intensive energy use required for the manufacture of chemical fertilizers.
The mitigation of greenhouse gases due to biogas production and utilization is hard to calculate because it depends on many issues. The biggest environmental benefits occur when liquid manure is used to produce biogas because no greenhouse gas emissions are allocated to the production of the feedstock and the methane emissions from manure management systems are mitigated. Liquid manure based biogas digestors have also been identified as one of Canada’s lower cost greenhouse gas mitigation strategies from a renewable energy technology.
Risks
While biogas is a very promising energy resource, the non-methane components of biogas (hydrogen sulfide, carbon dioxide, and water vapor) tend to inhibit methane production and, with the exception of the water vapor, are harmful to humans and/or the environment. In addition, the methane itself represents a serious danger, as it is odorless, colorless, and difficult to detect. Methane is also highly explosive if allowed to come into contact with atmospheric air at proportions of 6 to 15 percent methane. For these reasons, it is recommended that buildings be well ventilated; motors, wiring, and lights should be explosion-proof; flame arrestors should be used on gas lines; and alarms and gas-detection devices should be used in areas where the risk of leakage exists.
Processing: From Feedstock to End Product
| Farmers are familiar with anaerobic digestion through the existing practice of using manure lagoons. Lagoons are lined earthen pits where waste and used water are collected and digested. Bacteria digest the waste and produce gases. Unfortunately, the open air format of lagoons allows methane to leak, which prevents the gas from being recovered for energy. The offensive smelling gases, which are commonly produced, are also be permeated into the air near the facility. |
Did you know?
Anaerobic digestion is a process that occurs in nature. Swamp gas, which contains methane, is produced from the anaerobic degradation of wetland vegetation that has settled to the bottom of the wetland. |
An advanced type of anaerobic digestion is the enclosed waste management system or mechanical digester. These digesters manage organic waste by recycling nutrients, treating waste and providing odour control. The method requires less space than the traditional lagoons and uses the organic resources at a greater degree of efficiency because no methane leakages occur. Enclosed anaerobic digesters can be a heated tank or an enclosed lagoon that excludes oxygen from the process. This allows bacteria to break down the waste at an accelerated rate.
The anaerobic digester is an airtight tank with heating coils and, in some systems, a mechanical mixer. As the feedstock is fed into the digester, it is heated to a temperature of 35°C or higher, depending on the type of digester. At this temperature, bacteria produce biogas; which occurs in a two-step process. The first step involves a group of anaerobic bacteria (acid formers) that produce organic acids from the initial organic degradation. The second step involves a group of bacteria (methane formers) that break down the organic acids and produce methane in the process. As the gas is produced, it rises above the feedstock to the top of the digester where it is collected into the piping system. Many different digesters are available, and new advancements in anaerobic treatment are evolving as interest in the technology increases. The digesters vary in terms of length of time required for treating the feedstock, the size, configuration and complexity of the reaction vessel, and the operating temperature.
Biogas is not typically produced at the time or in the quantity needed to satisfy the conversion system load that it serves. As such, storage systems have been employed to smooth out variations in gas production, gas quality and gas consumption. The storage component also acts as a reservoir, allowing downstream equipment to operate at a constant pressure. The choice in the final means for utilization of biogas impacts the design and equipment requirements for biogas processing and storage. The biogas may be applied in direct combustion systems (boilers, turbines, or fuel cells) for producing space heating, water heating, drying, absorption cooling, and steam production. The gas used directly in gas turbines and fuel cells may produce electricity. An alternative choice in biogas conversion is the use of stationary or mobile internal combustion engines which may result in shaft horsepower, cogeneration of electricity, and/or for transportation of vehicles. A final opportunity exists for sale of the biogas through injection into a natural gas pipeline. Biogas can be used readily in all applications designed for natural gas if cleaned of impurities.
References
Agriculture Environmental Renewal Canada (AERC) Inc. 2007. Pages on Canadian Sorghum hybrids. Accessed July, 2007 http://www.aerc.ca
Agrotechnological Research Institute (ATO-DLO). 2003. Final Report FAIR 5-CT97-3701 “Switchgrass” : Switchgrass (Panicum virgatum L.) as an alternative energy crop in Europe. Co-ordinated by Dr. Ir. H.W. Elbersen. Accessed July, 2007 http://www.switchgrass.nl/pdf/Sw_FinalRep_full2.pdf
Ah-You, K., Suleiman, M., Jaworski, J. 2000. Biotechnology and Cleaner Production in Canada. Life Sciences Branch Industry Canada.
Alberta Agriculture and Food, 2007. Biogas Energy Potential in Alberta, PDF online at:
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