Gas to Power Journal
the latest news on natural gas power generation, plants and generators
Nov. 13, 2015
Technology and Innovation
Drew Gilpin Faust, the president of Harvard University, in 2008 committed her institution to a 30 percent reduction in greenhouse gas (GHG) emissions by 2016. Adding an 8MW gas-fired cogeneration plant, designed by Burns & McDonnel, to the 110-year-old Blackstone steam plant helped do the trick.
When Faust made her pledge, she left it to the university’s energy and facilities team to work out the details. Any emissions-reduction efforts, however, had to be cost-effective, and they couldn’t compromise the reliability of critical systems and services. At the time, 52 percent of the Harvard’s GHGs came from electricity production and 33 percent from the campus’ 110-year-old Blackstone steam and one-time electricity producing plant.
The university decided to adopt a two-pronged strategy: upgrade its steam plant and augment it with natural-gas fired power to reduce emissions while providing a reliable source of electricity.
“We are the sole source of energy for about 200 buildings on the campus,” Harvard University project manager Doug Schmidt, P.E., said, “so losing thermal energy in the winter is something you can’t do.”
A 5MW back-pressure steam turbine fulfilled some needs, but the university wanted to further increase its energy efficiency and reduce power purchases. And since steam demand varies with the weather, the steam turbine lies idle during the summer, thus limiting its usefulness.
Small-scale CHP added to the old steam plant
The university hired engineering firm Burns & McDonnell to consider different-sized turbines and heat recover steam generators (HRSG) at various locations on the campus, running dispatch-simulation models against thermal loads.
“The addition of 8MW of combined heat and power (CHP) capacity showed economic and environmental benefits, as well as improvements in reliability,” Burns & McDonnell senior project manager Kurt Koenig said.
After researching several GHG-reducing options, the team selected a Taurus 70 gas turbine from Solar Turbines and an HRSG from RENTECH Boiler Systems of Abilene, Texas. As well as meeting the university’s energy needs, this slashed emissions by 8,000 to 12,000 metric tons of carbon dioxide equivalent per year. Both the 110-year-old Blackstone steam plant and the new, flexible CHP operate primarily to meet the campus thermal load, with any electric power as a side benefit. This is why, according to Schmidt, using a larger turbine would have been less efficient. Although it could produce more power during peak demand times, it would run fewer hours, he said, stressing that the Solar Taurus 70 seemed to be the best fit since the university could operate it year-round.
“You can size a turbine big enough so we are getting lots of megawatts during very cold weather, but nothing at all when it is warm because we get below the minimum operating point on it,” he noted. “We would get megawatts, but not megawatt hours.”
The decision for RENTECH Boilers to build the HRSG, Schmidt said, was based on a competitive bid. “They had the right price and could make the delivery,” he said. “We assessed each HRSG in a heat-balance model as well, and RENTECH had the efficiency we needed.”
Squeezing in a vertical HRSG
Fitting new equipment into the site’s existing footprint posed a challenge. Although floor space was available in the plant, it wasn’t in the right place for a typical combined-cycle configuration. The Taurus 70 was to be located in one hall of the building, next to the steam turbine, while the HRSG would be located in the same room as the gas-fired boilers. Consequently, exhaust would go from the GT in one room to the HRSG in another. The steam would then return to the turbine room to power the steam turbine and, ultimately, join the header providing the campus steam. The HRSG also had to fit into a space originally designed to hold a gas-fired boiler.
Suppliers and client worked together to adapt the new equipment to the existing building configuration, which offered more vertical space (height) than horizontal space (footprint). In the case of the HRSG from RENTECH, the system had to be constructed in two sections: one with the superheater and evaporator, and the other with the economizer and SCR. The small size of these sections would allow them to be moved into the building individually and then be joined together. That meant crews wouldn’t have to stick-build the system onsite.
Commenting on his team’s role in the university’s Blackstone project, Schmidt pointed to the old “know thyself” axiom. “What’s important to us here is to know your load,” he said. “We were blessed that someone had the foresight to put in an automated metering system so we have hourly history on steam and electrical production and campus demands. Knowing what you are trying to work toward makes it easier to work in that direction.”