Transition from steam plant to cogen facility key to controlling energy costs
Amherst Cogeneration Plant
Amherst, Mass.
Amherst College
Combined Cycle Journal
2007 fourth quarter
High school students looking for a rewarding educational experience at the college level, and who value “small” and “quiet,” might consider Amherst College. The private, non-profit college has only 1,400 students and is located on the quiet side of Amherst in a “green” environment. When things get too quiet, you can walk to the center of this small city and rub elbows with students from four other area campuses: Hampshire, Smith, and Mt. Holyoke colleges and the University of Massachusetts.
Size really doesn’t matter much when it comes to energy. All campuses need electricity, steam or hot water, and chilled water–it’s just a question of how much. Energy demand at UMass, with 30,000 students and faculty, peaks at about 20 megawatt and 380,000 lb/hr of steam. In contrast, Amherst’s peaks are approximately 4 MW and 47,000 lb/hr of steam.
Spiraling energy costs have stung campuses of all sizes. An analysis probably would find that the cost of energy per student is significantly higher in small colleges than in large ones–as the numbers above indicate. Many colleges and universities have grown rapidly in the last decade and have relatively new buildings of more energy-efficient design.
The time is now for institutions both large and small to transition from old steam plants to state-of-the-art cogeneration facilities–combined heat and power. Energy cost per student and pollutant emissions will drop dramatically, and reliability of energy supply will improve.
At least three of these five colleges recently have installed, or are in the process of installing, cogeneration plants. UMass’ new central plant is characterized by its innovative architecture. Amherst’s is far more modest; the existing powerhouse merely was expanded to accommodate a new combined-cycle plant. Smith, with double the student body of Amherst, is holding down the cost of improvement by shoehorning a gas turbine (GT) and heat-recovery steam generator (HRSG) into its existing physical plant.
Saturn 20 , the heart of Amherst College’s cogen system, is configured to operate on natural gas or oil.
What cogen technology is optimal?
The first step in planning a cogen installation is to chart actual campus thermal and electrical demand over the last year, identify steps that can be taken to reduce consumption cost-effectively, and project energy demand over the next decade. For example, UMass conducted a campus energy management program in parallel with plant design that was so effective in reducing demand, one of three standby boilers that had been ordered was cancelled during construction.
Amherst College capital projects managers Peter Root and Aaron Hayden told editors that an in-house review of energy use and the price of electricity and oil in late 2004 showed that CHP–something they had con-sidered for years, but couldn’t justify financially–finally made economic sense. Amherst investment rules required that the cogen plant pay for itself within seven years.
The engineering firm of van Zelm, Heywood and Shadford of Farmington, Conn., was hired in 2005 to conduct an independent assessment of energy use, the pros and cons of alternative cogen options, and expected internal rate of return. Joseph Camean, PE, van Zelm’s director of power and utility services, says the project began with a “walk-around” to see the physical assets and review data.
History
Root and Hayden said the first central plant was built in 1923 as a coal-fired facility, which was typical of the time. Nearly 50 years later–1972 to be exact–a new plant was constructed to house two 40,000-lb/hr resid-fired boilers from Babcock & Wilcox Co. of Barberton, Ohio, and other utilities equipment. The boilers, designed for 250-psig service but operated at half that, were modified three years later to burn gas as well as oil.
The campus steam system serves two million square feet of building space. Boiler steam was reduced to 30 psig by a pressure reducing valve (PRV) for distribution through the looped header. It is used for space heating, making hot water, and, after reduction to 15 psig, powering three 400-ton absorption chillers in campus buildings.
Another 400-ton chiller located in the chiller hall adjacent to the boil-er room operates on 115-psig steam off the main distribution header. Absorption chillers help balance electrical and thermal demand in summer. Electric centrifugal chillers also are installed.
Projections of future demand
The campus thermal requirement was not expected to increase significantly over the coming decade. Primary reason was the replacement of some old buildings with new and the upgrade of others to reduce demand.
Electrical demand, by contrast, was projected to grow by about two percent per year. An additional 15 percent step increase in demand was expected to accommodate a new academic building. This translates to a projected peak demand of slightly more than 3,800 kW in 2009 and almost 4,750 kW in 2014. Before the cogen plant went into service, all electricity was provided via two 13.8-kV feeders from Western Massachusetts Electric Co. (WMECo).
Due diligence
van Zelm evaluated three basic cogen options:
- Addition of a high-pressure (HP) boiler and single- or multi-stage backpressure steam turbine.
- Gas turbine and HRSG.
- Reciprocating engine, supplementary-fired HRSG, and steam turbine/generator.
Camean said combining the first and third options to produce a total electrical output of 2,950 kW most closely matched Amherst’s electrical demand. Also, it offered optimum economic performance compared to the alternatives. All options offered campus-wide standby power in the event of a prolonged utility outage and varying degrees of primary and backup steam capacity.
Regarding thermal energy, 7.1 million Btu/hr of heat would be recovered from both the engine exhaust and jacket cooling system by the HRSG. Steam for the new turbine would come from the existing boilers following their modification to produce steam at 230 psig. The backpressure unit would exhaust at 30 psig for direct injection into the campus network, thereby eliminating the throttling losses of the PRV set-up.
Camean did not embrace GT-based generation initially because he believed that the commercially available machines with significant land-based experience were either too small or too large to fit Amherst’s load profile. One of the problems in using a large machine, he said, is efficiency drops off quickly at partial load. So “going large” didn’t make sense unless power would be exported to the utility, and that was considered impractical.
However, Root and Hayden thought recips offered a poor ratio of thermal to electrical energy, and when the engine being considered for Amherst was associated with reliability issues, the pendulum swung in favor of a GT-based system.
With preliminary plant design and budgets (capital and operating) roughed out and estimates of future energy prices done, it was time to prepare a proposal justifying the cogen project to the Amherst administration. That task fell on the shoulders of Jim Brassord, director of facilities and associate treasurer for campus services. Even good ideas must be “sold,” and Brassord was equal to the task. He put his reputation on the line and got the approvals to proceed. Then it was up to Root and Hayden to get the design finalized and project built and commissioned. Teamwork is vital to success.
HRSG from RENTECH produces 8,100 lb/hr of 200-psig steam with its duct burner off, more than twice that with it on. Duct-burner fuel-supply manifold is shown in A, side elevation is shown in B, evaporator end view in C.
Equipment details
An obvious choice was Solar Turbines, Inc.’s 1.2-megawatt Saturn® 20 (San Diego) because of the company’s depth of industry experience with the engine and its ability to operate reliably on both gas and distillate (Fig. 1). To increase both electricity production and efficiency, the Saturn 20 was configured in a 1×1 combined-cycle arrangement with a 500-kW Elliott Co. (Jeannette, Pa.) backpressure steam turbine (Fig 2). The thermodynamic link between the two turbines is the HRSG (Fig. 3) from RENTECH Boiler Systems Inc., of Abilene, Texas.
The Saturn package is a relatively standard offering, complete with lube-oil system, controls package, etc. The single-shaft unit has an eight-stage axial compressor and annular combustion chamber with a dozen fuel injectors. Solar’s DLE combustion system, SoLoNOx®, is not offered on the Saturn. Simple-cycle heat rate for the machine is 14,000 Btu/kWh.
The RENTECH HRSG produces 8,100 lb/hr of 200-psig steam with the Coen Company, Inc., (Foster City, Calif.) duct burner off. In winter with full firing, it’s supposed to generate 18,000 lb/hr, but can go higher. An SCR containing Cormetech, Inc., (Durham, N.C.) catalyst holds NOx emissions to 3 ppm with 2-ppm ammonia slip. Ammonia is stored onsite in bottles to minimize any perceived safety risk. Amherst volunteered to restrict total annual emissions of NOx from the site to 49 tons, SOx to 99 tons.
Gas turbine and HRSG arrangement accommodates building layout.
The HRSG (Fig. 4) is arranged at right angles to the GT to accommodate the building layout. The steam it generates is piped to a header shared by the two existing B&W boilers, which were modified to produce steam at 200 psig (Fig. 5). That header originally was designed to accommodate three boilers, but the third unit never was installed so it was large enough to serve the HRSG as well. The single-stage Elliott steamer runs on 200-psig steam and discharges at 30 psig to the campus steam network.
The cogen plant was built by general contractor Abington Group of Portsmouth, N.H. Project manager Tim Davidson told the editors that the plant is arranged to run in parallel with the grid. Root and Hayden say that the plant can be disconnected from WMECo and run in the “island mode” should grid problems arise. But they don’t expect to use that feature. Amherst College has not been without electricity for more than an hour since 1965.
Plant operations
Cogen plant superintendent Jeff Isabelle has an operations staff of five with one operator per shift. Plant operators require engineer licenses in Massachusetts when electricity is generated; a “fireman” certification suffices for steam plants. This meant that Isabelle’s operators had to upgrade their papers before the new system went into operation.
The plan is to operate the GT at full load when needed, and system economics are based on running 49 weeks annually. Meeting expectations is critical to the success of the new plant. The gap between electrical demand and generation must be purchased, as must gas for the GT and new boiler. This is a tall order for a small operation like Amherst considering today’s volatile energy market and knowing that mistakes can trigger higher-than-expected utility demand charges for a full year.
Operating cost also is impacted by throttling 200-psig steam to the 30 psig required by the campus system through PRVs instead of the steam turbine. To improve the reliability and flexibility of pressure reduction when the steamer is out of service, the old PRV located outside the powerhouse was removed and the system shown in Fig. 6 installed inside the plant.
Two PRV reducing circuits come off the boiler steam header. The smaller pipe, or low-flow line, has its PRV set to open first if the turbine is taken out of service. If that steam flow is not sufficient to meet demand, the PRV in the high-flow line below opens.
Reliability of steam supply is critical to the health and welfare of students and campus personnel. An emergency diesel/generator is dedicated to run the feed pumps and fuel pumps for the B&W boilers should power be lost temporarily.
To minimize parasitic power demand, the three boiler feedwater pumps are equipped with variable-speed drives. They sense flow based on pressure and turn pumps on and off as needed.
End note
The value of any economic analysis depends on the accuracy of the information used in decision-making. To illustrate how volatile the energy markets are, Camean pointed to fuel-cost data used in the cogeneration feasibility study van Zelm did for Amherst College in spring 2005.
He smiled when he reflected on the fact that the “best information available” indicated the average winter cost of No. 6 fuel oil for Amherst’s B&W boilers–$8/million Btu at the time–would escalate at 0.5% above inflation for the next 10 years. The price was double that in the immediate area within two years.
The best information available on the price of natural gas was $9.20/million Btu in summer and $11 in winter, also escalating at 0.5 percent above inflation for the next 10 years. That produced another smile from Camean.
For Amherst, of course, the rapid run-up in fuel cost made the decision to install a cogen plant even more important.