| | Biogas | Volatile solids | Methane yield | Co-digestion | Feedstock quantity | Energy to electricity | Feedstocks and energy | More information | References
Biogas technology has become an attractive treatment option for agricultural, food processing and municipal organic wastes for pollution control and energy recovery (Chen et al. 2008). The organic wastes are described as feedstocks for anaerobic digestion (AD). Almost all organic matter (OM) in the feedstocks can be converted to biogas through AD; however, some materials produce more biogas than others. The energy potential is influenced by the amount of biogas produced, the concentration of methane in the biogas and the amount of the feedstock available.
Biogas
Biogas is a mixture of gas produced by AD. Primarily, biogas is methane (CH4), 55-75%, carbon dioxide (CO2), 44-24%, and trace amounts of water vapour and other gases (Alberta 2011). Methane is the prime component of natural gas so scrubbed biogas can be a valuable fuel source.
Volatile Solids
The volatile solids (VS) analysis determines the amount of OM in the feedstock. Feedstocks containing more than 60 or 70% VS on a dry matter basis are good candidates for AD. The non-volatile solids, or ash content, of a feedstock takes up valuable digester volume and will not contribute to biogas production (Hamilton 2012).
OM with high energy contents produces more biogas than OM with lower energy contents.
Methane Yield
Feedstock analysis can be performed to measure the biochemical methane potential, or methane yield. The biogas yield from a feedstock is not the same as the methane yield. Each feedstock or combination of feedstocks create different methane yields.
The methane yield is generally expressed as a function of the VS (normalized L/kg VS), and it can be calculated and expressed as a volume of gas per unit of feedstock (m3/1000 kg of feedstock), Example 1.
Generally speaking feedstocks rich with VS create higher volumes of methane than feedstocks with lower VS.
The energy, joules, created by the methane produced can be calculated using the lower heating value of methane (36 MJ/m3), and the methane yield (m3/1000 kg feedstock). The energy potentials of feedstocks in Alberta are shown in Table 1.
Example 1. Methane yield converted to energy
Feedstock analysis provides the total solids, VS, as well as the methane yield expressed as a function of the VS of the raw feedstock. Using this information the potential energy can be calculated.
| Methane yield: | 312 N L/kg VS |
| VS of the sample: | 95 kg/1000 kg feedstock |
| Methane yield as m3 | = VS * CH4 *efficiency* conversion
= 95 * 312 * 0.9 * 1/1000
= 27 m3/1000 kg of feedstock |
| Energy from CH4 | = CH4 * lower heating value
= 27 * 36
= 966 MJ/1000 kg of feedstock |
The energy calculated is not equivalent to the electricity that could be generated from the methane produced through AD.
Co-digestion
Co-digestion for AD means more than one feedstock is used at a time to produce biogas. Co-digestion is used to increase the methane yield from low yielding feedstocks.
Care must be taken to select compatible feedstocks that enhance methane yields and avoid materials that may inhibit biogas and methane production. Agricultural feedstocks have successfully been co-digested with restaurant biowastes, food processing and crop residues (EPA 2012).
The quantity, availability, and cost of co-digestion feedstocks are important factors to consider. Other factors to consider include: regulations and permitting; digester capacity; mixing of the feedstocks; and nutrient balance.
Feedstock Quantity
Valuable feedstocks for AD are dispersed throughout Alberta. Supermarket fresh fruit and vegetables and restaurant prep and plate wastes are available throughout Alberta, directly proportional to the population density. Manure is another potential feedstock that is available throughout the province yet highly variable due to animal density. In areas of Alberta, meat processing, as well as food, ethanol, oilseeds, and brewery processing produce potential feedstocks. The approximate distribution of the feedstocks is shown in Table 2.
Other feedstocks in Alberta have valuable OM however the quantities of the feedstocks are variable. For example, spoiled or excess straw and silage (greenfeed) may be available for AD in some years in various parts of the province. Waxed cardboard is another valuable feedstock however the supply is ever changing due to packaging trends.
In some cases it may not be possible to collect and use all feedstocks, for example some manure is generated in pasture situations and is therefore not collected therefore it wouldn’t be used to generate energy.
Energy to Electricity
The power can be calculated using the feedstock quantity available and the energy created by the accumulated CH4 (Vik 2003), Example 2.
Example 2. Power potential
A watt (W) is the rate of energy conversion or transferred which is equivalent to a joule (J) per second. Knowing the amount of feedstock available for AD and the energy potential the energy as a W can be calculated.
| Energy from CH4 | = 966 MJ/1000 kg of feedstock
(calculated in Example 1) |
| Feedstock amount: | = 271000 kg/year
= 0.0086 kg/second |
| Energy | = 966 * 0.0086
= 8300 W |
Alberta has 16 municipal districts or cities with the potential to create more than 50 MW of energy.
This energy potential does not take into account the efficiencies of a power generating unit. An example of a power generating unit is a combined heat and power unit which is a reciprocating gas engine that uses the gas, CH4, to drive a crank shaft. The crank shaft turns an alternator to produce electricity and heat is released during the gas combustion process. The heat can be recovered during cogeneration in order to maximize the energy conversion of the system. The electricity efficiency of a quality combined heat and power unit suitable for biogas is on average 40% with an additional 45% of the energy recoverable as thermal energy (Clarke Energy 2013). Approximately 15% of the input energy is typically lost.
Feedstocks and Energy
The potential energy generated from the feedstocks can be seen in a series of maps produced by Alberta Agriculture and Rural Development. The energy is shown per municipal district or city. An additional factsheet for each map is also available describing the feedstock qualities as well as the methane potential.
Energy Opportunities:
More Information
Increasing Anaerobic Digester Performance with Codigestion.
References
- Alberta Agriculture and Rural Development. 2011. Biogas energy potential in Alberta. Agdex 768-3. Edmonton: Alberta, Alberta Agriculture and Rural Development.
- Chen, Y., J. Cheng, K. Creamer. 2008. Inhibition of anaerobic digestion process: A review. Bioresource Technology 99: 4044-4064.
- Clarke Energy. 2013. CHP efficiency for biogas. (Accessed: 2013).
- EPA. 2012. Increasing anaerobic digester performance with codigestion. AgStar Report. United States Environmental Protection Agency.
- Hamilton, D.W. 2012. Anaerobic digestion of animal manures: methane production potential of waste materials. BAE-1762. Oklahoma: Oklahoma Cooperative Extension Service.
- Vik, T.E. 2003. Anaerobic digester methane to energy a statewide assessment. MCM. No. W0937-920459. Neenah, Wisconsin: McMahon Associates, Inc
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