Combined heat and power (CHP) or cogeneration plants generate both electricity and heat from the same fuel source thereby increasing efficiency and making use of the waste heat that results from generating electricity. At District Energy St. Paul, the CHP boiler is fueled primarily by locally sourced tree residuals with natural gas as a secondary fuel.
The CHP biomass boiler generates steam at high temperature and pressure (called “superheated”). This superheated steam is used to drive the rotating blades inside of a turbine. The turbine rotor spins at nearly 100 revolutions per second. The mechanical energy output from the turbine drives a shaft that connects the turbine and the generator. As the shaft spins the generator, the rotation in a magnetic field produces electricity. The mechanical energy output from the turbine is converted by the generator to electrical energy and produces an electric current. The turbine can generate up to 37 megawatts of electricity. 25 megawatts are sold to the grid through the local electric utility.
After driving the turbine, the high-pressure steam loses some of its energy and is reduced to low-pressure steam, which is captured and used to heat water for the District Energy hot water network. CHP was integrated into the system in 2003 to increase the efficiency of the Saint Paul district heating system, to utilize a local and renewable fuel source, and to provide an environmentally sound energy source for our heating customers and the local electric provider.
Operations Information
- Simultaneously produces 55 megawatts of heat and up to 25 megawatts of electricity
- 25 megawatts of this renewable electricity are supplied to the local electric utility, enough for 20,000 homes
- Excess thermal energy heats enough water for approximately 40 to 50% of District Energy’s heating needs
- Utilizes approximately 240,000 tons of urban tree residuals (biomass) annually including the wood residuals produced through Emerald Ash Borer management efforts.