A university on the eastern seaboard includes 14 million square feet of research and administrative space. It has a 35-MW peak electrical load for a total of around 240 Gwh/year as well as needing 1.2 million klbs./year of steam and 40 million ton-hours/year of chilled water. Previously, campus heating relied on coal-fired boilers that released about 65,000 tons of CO2 per year. Management decided to switch to on-site combined heat and power (CHP) to help reduce CO2 emissions to 30% below 1990 levels.
The new CHP plant can fulfill all electricity requirements; however, the steam side presented a problem due to two factors:
- Steam production is decoupled from a lake-based cooling system; and
- Steam load varies from a peak of 380,000 pounds in the winter down to 55,000 pounds in the summer.
The core of the CHP plant consists of two 14-MW Solar Titan 130 dual-fuel combustion turbines. Natural gas is fed from a three-mile high-pressure gas line. This line was added to connect the school directly to the interstate gas transportation system to deliver 15,000 decatherms per day of natural gas. As a backup, the facility has 700,000 gallons of on-site fuel oil storage.
The engineer responsible for the project selected heat recovery steam generators (HRSG) from Rentech Boiler Systems due to their efficiency and ability to extract plenty of heat from the exhaust gas. The dual-pressure system produces steam at 400 psi and 600ºF and sends it to the high-pressure steam header for the back-pressure steam turbine generator. The low-pressure section feeds directly into the campus steam distribution system. The pressure floats with the distribution system pressure. The HRSGs have a modular design that enabled them to be shipped in sections and assembled on-site. Each HRSG generates 58,000 pounds of steam per hour.
As part of the project, the university upgraded its 50-MVA substation to 78 MVA to accommodate the additional generation. The campus generates and distributes at 12.2 KV, so the power from the project is fed directly into the secondary bus at the substation.
As well as reducing CO2 significantly, the college benefits by generating most of its own power. Previously, it purchased 80% of its electricity from the grid. Now, it generates 80% of its own. It could generate 100%, but the variation in steam load makes this option less economical and efficient than continuing to purchase a portion of its power from the grid.
Since each turbine/HRSG combination produces 58,000 pounds of steam per hour, only one is needed during the summer. During a power emergency in the summer months, or if electricity costs increase, both units can be brought online. During spring and fall, one turbine/HRSG operates full time, with the second coming on when needed. During the winter, both operate continuously. The university also installed an air-cooled steam condenser to smooth out system pressure and match the demand.
“The new plant allowed the university to cut CO2 emissions by 20% below its 1980 level and by 30% below its 1990 level, despite the square footage increasing by more than 40%,” said the facility engineer in charge of the project.