|Engines for Biogas (GTZ, 1988)|
|4. Biogas and its Properties as a Fuel for Internal Combustion Engines|
The fuel consumption of equipment using biogas is often specified in m³n/h or m³n/kWh, i.e. standard cubic meters per hour or per kilowatt hour (sic) respectively. The standard cubic meter (m³ n) means a volume of 1 cubic meter of gas under standard conditions, i.e. at a temperature of 0 °C (273 K) and a pressure of 1013 mbar. The consumption of biogas in actual volume will differ from these data according to the actual conditions of the biogas as fed to the equipment (motor, burner, etc.) in terms of
-composition, i.e. CH4 content.
The determination of the actual volumetric consumption of an engine operating on biogas fuel is of utmost importance for the dimensioning of biogas plant, engine, mixing device and other equipment. A difference of 50% between actual volumetric consumption and specified consumption of a biogas engine can easily occur and could result in poor performance of the engine if not considered.
Using the diagrams Figs 4.1 and 4.2 the consumption of the specific biogas can easily be found:
Check how the fuel consumption fc is specified.
-If in m³n/h, continue with step 2.
-If in m³/h without biogas specification assume a calorific value of Hu = 20 000 kJ/ m³ .
-If as specific fuel consumption at rated conditions use fc = sfc· P (in m³/h). (Equ. 4.10)
-If only the efficiency h is specified use fc = 1/h·p·1/Hu· 3600 (in m³/h). (Equ. 4.11)
-If no information is given use Equ. 4.11 with h= 0.3 for dual fuel and larger Otto gas engines and h = 0.25 for smaller Otto gas engines as well as Hu = 20 000 kJ/m³.
Determine the calorific value of the biogas used for specification of the equipment by the manufacturer.
- If consumption is specified by engine supplier in kJ/h, use this
value and continue further below in step 4.
- If calorific value of biogas is specified in kWh/m³ n transform this figure by multiplying by 3600 to obtain it in kJ/m³ n.
- If biogas is specified by its CH4 content in Vol % use diagrams in Figs. 4.1 and 4.2 to obtain the calorific value in kJ/m³ n.
Determine the required energy flow (calorific consumption) of the engine at rated performance in kJ/h by multiplying the specified consumption rate at standard conditions in m³ n/h with the calorific value of the biogas in kJ/m³ n, as-specified by the engine supplier (energy consumption = specified volumetric consumption x calorific value of biogas).
Determine the actual calorific value of your specific biogas in kJ/m³ using the procedure explained in Chapter 4.2.
Determine how much of your specific biogas will be consumed by the engine in m³/h by dividing the energy consumption (Step 3) by the calorific value of your specific biogas (Step 4):
Manufacturer's engine specification:
- power rating P = 20 kW
- fuel consumption at rated power fc = 10 m³ n/h
- biogas used 70% CH4, 30% CO2
Specification of biogas from your plant (see Chapter 4.2)
Hu = 18 900 kJ/m³
No calculation needed as the fuel consumption is specified.
From diagram Fig. 4.2 calorific value of biogas used in specification of manufacturer:
Hu,n = 25200 kJ/m³ n (at standard conditions).
Energy consumption (flow) of the engine at rated power
Calorific value of your specific biogas from plant (see Chapter 4.2)
Hu = 18 900 kJ/m³.
Actual biogas consumption fc of engine at rated power
The volumetric fuel consumption in this case would be 32% higher than specified by the manufacturer at standard ("n") conditions, which demonstrates that the above calculation should not be dispensed with.