How to Estimate Biogas from Anaerobic Digestion of Organic Solid Waste
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The methodology is, we consider useful, to those wishing to assess biogas potential and CO2 equivalent emissions.
STEP 1 - Estimate the biogas potential of the organic solid waste
The main determinant of the amount of biogas is the amount of carbon in the organic waste. When the waste degrades some of the carbon becomes part of the cellular material of the microbes (assimilated carbon) and the rest of the carbon forms methane and carbon dioxide (dissimilated carbon). The more anaerobic the process, the more of this carbon is converted to methane.
The amount of carbon is expressed in terms of the percentage of fresh weight. These percentages are:
- Paper and paperboard 40
- Textiles 40
- Wood and straw 30
- Garden and park waste (green waste) 17
- Food waste 15
(Bingemer and Crutzen, 1987; p. 2183)
Assume that recyclable materials like paper and paperboard and textiles are withdrawn, and that there is no wood and straw waste. The organic waste is comprised of green waste and food waste. Further assume that the waste is 50 per cent green waste and 50 per cent food waste. Then for each kilogram of fresh waste 0.085 kilograms of carbon from the green waste and 0.075 kilograms of carbon from the food waste are available to form biogas, a total of 0.16 kilograms of carbon.
There are two basic types of anaerobic bacteria, mesophilic and thermophilic, that function at different temperatures. The mesophilic range is 35-37oC and the thermophilic range is 55-57oC. Thermophilic processes are more expensive to get up to and maintain at operating temperature, so examples in the workbook assume that mesophilic bacteria are used and the temperature of the anaerobic digester is 36oC. However, thermophilic process have an advantage in that they partially sterilise residues and have a greater methane yield and thus should be examined as an option. The Atlas Group anaerobic digesters recently installed at the City of Stirling use a thermophilic process (see the case study in Appendix IV of the main Workbook).
The amount of carbon available for biogas formation can be calculated from the following equation:
Coe/Co = 0.014T + 0.28
where Coe is the amount of carbon available for biogas formation, Co is the total amount of carbon, and T is the temperature (Bingemer and Crutzen, 1987; p. 2181).
At the temperature of 36oC:
Coe/Co = 0.014 x 36 + 0.28
= 0.784
That is, 78.4 per cent of the carbon is available for biogas formation. We know that Co is equal to 0.16 kilograms, therefore:
Coe = 0.784 x 0.16
= 0.12544
Of the total amount of carbon per kilogram of the waste, 0.12544 kilograms of carbon is available to form biogas.
For the preliminary calculation, assuming that all of this carbon is converted to methane, and using the fact that the molecular weight of methane is 16 comprising 12 units of carbon and 4 units of hydrogen, then for each kilogram of waste:
Quantity of methane = 16/12 x 0.125
= 0.17 kilograms per kilogram of waste.
STEP 2 - Estimate the energy that could be produced and the value of the energy
Now the energy potential per m3 of methane is approximately 33,810 kJ per m3. This implies that the energy potential of 1 kilogram of methane is 50,312.5 kJ per kg using a conversion factor of 0.672 kg per m3.
The energy potential per kg of organic solid waste is:
= 0.17 x 50,312.5 kJ
= 8,553 kJ
The maximum energy potential of the methane produced per kg of the organic solid waste, based on the assumed composition is 8,553 kJ.
STEP 3 - Compare the value of the energy produced with an estimate of the project's costs
To convert 8,553 kJ to kilowatt hours (kWh) you need to divide by 3,600 because one kWh equals 3,600 kJ.
Number of kWh = 2.38
If the energy is sold at 7 cents per kWh, the energy per kg of waste is worth 16.7 cents. If the cost of the project is less than 16.7 cents (Australian) per kg of waste processed, this implies that the project is viable.
STEP 4 - Estimate the greenhouse benefit
If this waste were to be disposed to landfill, not all of the available carbon would be converted to methane. Assume that 50 per cent would be converted to methane. The greenhouse saving per kilogram of waste is 0.085 kg.
Multiply this by 21 to derive the CO2 equivalent emissions.
For each kilogram of waste the greenhouse saving is 1.785 kg of CO2 equivalent emissions. equivalent emissions. equivalent emissions.
STEP 5 - To find out more
Refer to Section B1 of the Workbook referenced here, and the case study on Brecht, Belgium.
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