Summary

Combined heat and power generation can be split into closed thermal cycles processes and open processes. In closed thermal cycles processes the combustion process and the power generation are physically separated by a heat transfer from hot combustion gas to a process medium used in a secondary cycle. Due to the separation between the fuel and engine, the engine is exclusively in contact with a clean process medium without undesired elements in the fuel and flue gas such as fly-ash particles cannot cause damage to the engine.

Therefore closed thermal cycles are well suited for solid fuels, and are widely applied for power production from biomass and municipal solid waste. Open thermal cycles are commonly applied for gaseous and liquid fuels used in internal combustion engines and gas turbines. The fuel is burned directly inside an internal combustion engine. The use of solid biomass in internal combustion engines is technically not feasible. Hence only the application of solid biomass in a combined heat and power plan with closed thermal cycles will be described in the following technology description.

Steam plants operated in CHP mode are mainly feasible for the production of process heat, which enables long operation periods. Steam engines can be operated in single- or multistage mode reaching similar efficiencies than small turbnines. Large steam turbine plants are operated with water tube boilers and superheaters, thus making high steam parameters and the use of multistage turbines is possible. As an alternative to conventional steam plants organic Rankine cycle (ORC) plants are also available. These can operate at lower temperatures, so that a combustion plant with a thermal oil boiler can be applied instead of a costly steam boiler.

For power production through biomass combustion steam turbines and steam engines are available as proven technology. While steam engines are available with capacities ranging from about 25 kWel to 1,500 kWel, steam turbines cover range from 500 kWel to 100 MWel. As an alternative to conventional steam plants in the range of 400 kWel to 1,500 kWel, organic Rankine Cycle (ORC) plants are also available.

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