Mass Balance

The principle of a mass balance is that the total mass of the materials entering a process must equal the total leaving the process. The same principle applies to the energy balance and to the balance of chemical elements. The advantage of anaerobic digestion as an enclosed process is that it is possible to account for all the materials, energy and element movements.

The important feature of food waste is its moisture content; household kitchen waste includes 770 kg of water for every tonne of waste and this water must be accounted for in the mass balance.

The biogas plant transforms 74% of the dry matter into biogas, leaving a digestate with a dry matter content of only 7%, which becomes liquid fertiliser.

The stoichometric combustion of 1 m 3 of biogas @ 60% CH 4 , 40% CO 2 & 1000 ppm H 2 S is:

600 litres CH 4 + 400 litres CO 2 +1 litre H 2 S + 1201.5 litres O 2

= 1000 litres CO 2 + 1201 litres H 2 O + 1 litre SO 2 + 21.4 MJ energy.

The figures for the CHP unit assume stoichometric combustion, although there will inevitably be excess air.

The difference between the gross and net energy figures is the amount used by the process - electricity to drive the shredders, pumps and mixers, and heat for the pasteurisation and digestion processes.