Numerous existing Anaerobic Digestion (AD) plants throughout the UK often discard energy in the form of heat from the engine's heat jacket and the exhaust stack.

In a typical scenario, a 500 kW electric system tends to reject approximately 440 kW of heat for 20 hours a day, with some of this heat being employed to maintain the temperature within the tanks' jacket.

Carbon Focus Engineering (CFE) has collaborated with several AD schemes to optimize the utilization of rejected heat, thereby enhancing overall system efficiency and creating supplementary revenue streams.

Beyond direct applications such as heating buildings and facilitating drying processes, there exists an opportunity to incorporate an efficient Organic Rankine Cycle (ORC) generator for additional electricity generation.

For instance, a 500 kW AD system could potentially yield an extra 30 kW electric through an ORC generator harnessing the rejected heat.

Below are two case studies where CFE have utilised the rejected heat from AD plants. 

Case study 1: Norfolk

Challenge – Utilise 500 kW of rejected heat from an existing AD plant to serve a proposed Heston straw bale drier rejected from a local straw fired power station.

Solution – Divert rejected heat to a new underground heating mains serving a straw bale drier system.

Fuel type – Rejected waste heat from existing AD plant.

Heat use – Straw drying plant serving local power station.

Main components – mechanical pipework, heat exchangers, electrical & controls

Annual rejected heat mitigation – 3,200,000 kwh/yr.

Case study 2: Lincolnshire

Challenge – Utilise 500 kW of rejected heat from an existing AD plant to heat poultry facility approx. 350m away.

Solution – Divert rejected heat from AD plant engine room to a new underground heating mains terminating in a containerised heat station adjacent to the poultry facility. Distribute heat into the individual poultry houses via pumped circuits & =wall mounted finned pipe heat emitters.

Fuel type – Rejected waste heat from existing AD plant.

Heat use – Modern poultry farm

Main components – Mechanical pipework, heat exchangers, electrical & controls

Annual rejected heat mitigation – 2,800,000 kwh/yr.