Sizing Biogas Plant
From the analysis carried out across an entire year on Homer we know that a 1 MW biogas plant is required to match the demand. Therefore, from this we can make the assumption that there must be an input into the engine of 3 MW. This is because most engines operate at an efficiency of roughly a third. With this information and the calorific value and density of methane the amount of methane being fed into the engine can be calculated. Biogas is comprised of 2/3 methane and 1/3 carbon dioxide, the total biogas input into the engine is calculated by dividing the methane input into the engine by 2/3.
Calorific value of Methane = 55 MJ/kg
Density of Methane = 0.716 kg/m^3
Input into the engine = 3 MW = 3 MJ/s
Methane input into the engine = 0.0545 kg/s
Methane input into the engine = 0.076 m^3/s
Biogas input into the engine = 0.114 m^3/s
The biogas system will be designed with a digester which will constantly produce biogas unless undergoing maintenance, this gas will feed into a storage tank that will be capable of storing of up to 4 days’ worth of biogas produced. The volume of the required storage tank necessary for this is calculated by working back from the yearly analyse carried out on Homer. From the results produced in Homer a day in January, April and July were selected. The days selected are 1/1/07, 1/4/07 and 1/07/07. The hours of operation for the Mackie’s plant were 9:00 am to 17:00 pm. From these days the volume of biogas was calculated using the below method:
The above data is taken from the Homer analyse for the entire year and will be used to measure the volume of storage required for the 1/1/07. This data alongside the biogas input to the engine 0.114 /s and the maximum output from the engine 1 MW will be used to calculate the values in the table below:
Biogas Averaged Hourly Production:
The addition of biogas input in hour 1 and hour 2 then averaged.
Biogas Max/Biogas Hourly Production:
The maximum 1 MW output of the system divided by the previously calculated averaged hourly production.
Use the number calculated from this to find the hourly production in /s by dividing the maximum biogas input to the engine 0.114 /s by the previously found number.
Volume:
Biogas Max/ Biogas Hourly (/s) x 3600 seconds = volume
Total volume is the sum of the values produced during the hours of operation
The same method for calculating the volume is repeated for the remaining two days.
The storage tank volume required to store 4 days of biogas production can be found by taking the 3 total volumes and averaging them, then using the averaged volume to calculate the averaged input into the storage tank in /s. The daily averaged input can be found by multiplying the biogas input into the storage tank by the number of seconds in a day. Once one day’s production has been calculated in 4 days production is easily calculated by multiplying it by 4. The storage tank volume will be 255 . The gas storage tank volume is flexible as it can be pressurized and reduce its volume significantly, however this is more costly.
Digester
The digester type that will be adopted for the system will be a fixed dome digester for numerous reasons, it offers a long life of up to 20 years, is underground so protected from damage, saves space, protected from outside and seasonal temperature fluctuations. Fixed dome digester equations:
Volume Digester = 0.65 x Volume Plant
Volume Gas Storage = 0.35 x Volume Plant
Volume Plant = Volume Digester + Volume Gas Storage
Volume Plant = = 728.6 m^3
Volume Digester = 728.6 – 255 = 473.6 m^3
Volume Digester = 475 m^3
References
Renewable energy for ice cream dairy | Mackie’s of Scotland. [online] Mackies.co.uk. Available at: https://www.mackies.co.uk/our-farm/our-environment/making-electricity.html [Accessed 23 March 2019].
Engineeringtoolbox.com. (2019). Methane - Density and Specific Weight. Available at: https://www.engineeringtoolbox.com/methane-density-specific-weight-temperature-pressure-d_2020.html.