calculate the load on the plant...then as per the efficiencies of the plant and generator design the generator.

I've worked in the power industry for 35 years, but I'm not sure what you mean. The rated capacity  (in megawatts) of a power plant is determined by the design and will be stated in the contract between the Owner and the EPC. So the rating of an existing plant is already determined. If you mean to ask, "how much capacity should you design a future plant for?" that depends on the market. What is the expected demand and how much you can sell? Forecasting power demand is a completely different subject from selecting the best plant design for the intended market. If you mean to ask, "what is the current capacity of a plant that has been operating for some time?" Then that's what we do at McHale Performance. We do performance testing and analysis.

Hello Dudley

I am currently working on my undergraduate project based on "Design a power production system from Anaerobic Digestion of municipal solid waste" whereby I intend to treat 200,000 tonnes of waste per year through Anaerobic Digestion and therefore use the biogas for power production. 

What I really want to find out is how can I determine the size of my prime mover (which is engine-generator set in this case) based on the amount of biogas generated per year from anaerobic digestion of the waste.

I do not necessarily mean to meet any specific demand but I am expecting to generate power for the plant as well as sell to national grid.

it depends on biogas production trend along the year. anyway, let us suppose it constant and yearly equal to 8040000 Sm3/yr.

annual fuel consumption (AFU) 8040000 Sm3/yr

fuel mass flow rate AFU/8040/3600=0.28 Sm3/s

biogas lower heating value=35MJ/Sm3 (assumed. it highly depends on the biogas)

input power to the engine=9.8 MW

depending on the thermal engine, its efficiency (mech pwr/fuel pwr) can change a lot. Let us assume 35%

engine size would be 9.8*0.35=3.43 MW

to get the output electrical power, you need to take into account the efficiency of the electric generator connected to the engine. Let us assume 90%

electrical generator size=3.43*0.9= 3.1 MW

the energy produced per year is eventually

3.1*8040 approx 25 TWh/year

this datum can be useful to estimate benefits from incentive mechanisms that are a positive cash flow (together with the electrical energy sold) and might help the feasibility of the overall biogas plant.

if production varies, efficiency map of the engine at partial load conditions needs to be taken into account. the preliminary sizing would require more attention ...

I have come across the equation below.

Rated engine power = (total biogas produced per year / hours of operation) x electrical efficiency x energy content of biogas

Source: Biogas Technology GmbH (S. Schulz, F. Garcia, F. Sonnek)

Can I now the difference between using LHV or energy content value ?