CHP replaces fired boilers;
reduces emissions, boosts efficiency
Smith College Energy Center
Northampton, Mass.
Smith College
Pacesetting Plants • Class of 2008-09
Combined Cycle Journal
second quarter 2009
Principal participants
Commercial operation:
September 2008
Engineer:
van Zelm, Heywood and Shadford
Environmental consultant:
Epsilon Associates, Inc.
Energy management consultant:
Carrier Strategic Partnerships Group
Controls integrator:
ABLE (Associated Boiler Line Equipment Co.)
Gas turbine:
Solar Turbines, Inc.
Heat-recovery steam generator:
Rentech Boiler Systems, Inc.
Packaged boiler/burner:
Rentech Boiler Systems, Inc., The Coen Co., Inc.
Power generation equipment is built to last. There are large numbers of steam and/or electric generating facilities at industrial plants and institutions still operating 40, 50, even 60 years or more after their installation.
But as these systems approach a half-century of service, sound as they may seem, there generally are good reasons for replacement – provided the thermal host has years of remaining life. Reasons often include escalating O&M costs, new emissions control regulations and increased host demand.
There are many legacy powerplants serving dedicated sites in the New England, Mid-Atlantic and Midwest regions. These were built during World War II or shortly thereafter when the U.S. supplied the world with manufactured goods, and education beyond high school became more of a necessity than a privilege.
Manufacturers and educators usually were able to think of better ways to spend money than on their power plants, and many operators spent entire careers nursing equipment to keep it in service. Budgets generally didn’t go much beyond fuel, water, salaries, some chemicals, welding rods, gaskets and packing materials.
For example, Smith College, home today to about 2,500 students, was founded in 1875, before Thomas A. Edison patented his incandescent electric light bulb. For more than 70 years, space heating and hot water were provided by boilers located in each campus building. A central heating plant and steam distribution system were installed in 1946, vaulting the institution into the modern world of energy. It wasn’t until last year’s major renovation and reconfiguration of the 1940s plant that the college began producing power onsite.
History
The first campus heating plant was equipped with three 40,000-lb/hr, field-erected, three-drum Edge Moor boilers designed to burn pulverized coal and oil. Coal was used only during the oil crisis of the late 1970s. Two 55,000-lb/hr, oil/gas-fired, packaged Keystone boilers (Erie City Iron Works, now Indeck Keystone Energy LLC) were added in the 1970s to accommodate campus growth.
The thermal distribution grid installed when the heating plant was built delivers steam to campus buildings from a 125-psig header serving all boilers. The thermal network is first-class and robust. A walk-through concrete tunnel that extends from the power plant to the main campus contains a 10-inch delivery main, an 8-inch backup delivery line, and a 6-inch condensate return line. Circuits to individual buildings are crawl-through to allow inspection and repair. Makeup is only about 10 percent, chief engineer Francis J. Raymond told Combined Cycle Journal , attesting to system condition and integrity.
At each building, two reducing valves in series knock down the distribution pressure to the nominal 10 psig required by steam radiators and by systems producing hot water for space heating and domestic use.
Collaboration
The five colleges in western Massachusetts’ Pioneer Valley, Mt. Holyoke, Smith, Amherst, Hampshire and UMass Amherst, often share resources. Knowledge of and experience with energy systems is an area of collaboration. Among the Pacesetting Plants profiled last year by
Combined Cycle Journal
were new combined heat and power (CHP) facilities at UMass Amherst and Amherst College.
Joe Camean, director of power and utility services, and Dave Chivoloni, lead mechanical engineer for van Zelm, Heywood and Shadford, Farmington, Conn., said that the energy needs of a new science and engineering facility (Ford Hall) is what stimulated rethinking of the existing power plant and how steam production could be both increased and accomplished more cost-effectively. CHP was a perfect fit.
Bob Lesko, manager of mechanical systems for the college, said that analysis of campus energy requirements showed the Centaur® 40 was a good match for the electrical load. Adding an unfired HRSG, he continued, would produce about 20,000 lb/hr of steam when the engine was operating at its 3.5-MW ISO rating – without burning any more fuel.
Smith selected van Zelm as the engineer for its CHP project and ordered the gas turbine (GT) from Solar Turbines, Inc., San Diego, and heat-recovery steam generator (HRSG) from Rentech Boiler Systems, Inc., Abilene, Texas – all three firms had played major roles at Amherst College. Also, Solar and Rentech had supplied equipment for the UMass Amherst CHP.
Engineering effort
Camean said that while installation of a relatively small GT and HRSG appears simple at first glance, the Smith project was made challenging by the need to install the new equipment in a cramped powerhouse. The solution: Demolish and remove the Edge Moor boilers and associated coal and ash handling equipment and refurbish the building (figure 1). Asbestos added to the complexity of that task.
Another major effort involved upgrading the building’s foundation. The original foundation was built on piles, and it required significant reinforcement to accommodate the dynamic load of the GT.
Also parts of the project were the installation of a new control room and the addition of a 65,000-lb/hr low-emissions packaged boiler to replace some capacity lost by removing the three Edge Moors. Rentech was the successful bidder for this unit as well. Available floor space was used quickly, and that dictated a HRSG of vertical design rather than a more conventional horizontal unit.
Centaur® 40 generates up to 3.5 MW at an ISO heat rate of 12,240 Btu/kWh. When operating at full load, it delivers 150,000 lb/hr of 820F exhaust gas to the unfired Rentech HRSG to produce nearly 20,000 lb/hr of 125-psig steam.
Outside the building, a screw-type gas compressor was provided in a sound-attenuated enclosure to boost 60-psig line pressure to the 200 psig required by the GT. A tank also was installed to store aqueous ammonia required by the selective catalytic reduction (SCR) system incorporated into the HRSG to minimize NOx emissions.
Chivoloni noted that the 125-psig saturated steam conditions were maintained, allowing use of the existing makeup and boiler-water treatment programs. Makeup is taken from the city main and softened. A polymer program is used to treat boiler water.
Raymond added that the feedwater system was standard for heating plants built during the war. Condensate returns via the steam tunnel to a storage tank. From there it is pumped to a surge tank on the floor above the firing level; makeup is piped to this vessel. The deaerator is next in the circuit with boiler-feed pumps taking suction from it.
Three feedpumps are provided, although only one normally is required, the chief engineer said. One is driven by a variable-frequency drive, two by single-wheel steam turbines. The latter, which exhaust to the deaerator, are used primarily in winter.
Existing stack serves all three fired boilers (new Rentech and the two existing Keystones). A new stack was installed on top of the HRSG to serve the CHP system.
HRSG extends from the main floor to the roof. The unfired boiler is characterized by two evaporator sections. Note how baffles direct exhaust gas through the second evaporator, which is arranged vertically.
GT, boiler details
The GT was sized to satisfy most, if not all, of the power required by the college. Utility backup is available during planned and forced outages as well as to supplement internal generation if required. Lesko said it didn’t make economic sense for the college to install a larger CHP system – one to more closely match the thermal load – and export power to the grid.
The Centaur® 40 has an 11-stage axial compressor and annular combustor equipped with 10 natural-gas fuel injectors (figure 2). Simple-cycle heat rate is 12,910 Btu/kWh. Compressor airfoils are coated with an inorganic aluminum formulation, turbine-section airfoils with platinum aluminide. Lube-oil cooler is of the finfan type. Generator delivers power directly to the Smith distribution network at 13.8 kV.
HRSG extends from the main floor of the powerhouse where the GT is located to the roof – passing through both the operating floor and the deck above it (figure 3). Two evaporator sections are incorporated into the unfired unit; first is just downstream of the GT on the main elevation.
First evaporator cools exhaust gas to the optimum reaction temperatures for CO and NOx capture. CO catalyst is located at the operating-floor level, and the SCR (selective catalytic reduction) for NOx control a few feet above that.
The second, and largest, evaporator is arranged vertically and supported by stiffeners under the upper deck (as shown in the side view). Note how baffles direct exhaust gas through that heat-transfer section. Final heat trap is the economizer, which connects to the stack near the building roof via a transition piece.
The new D-type packaged boiler (figure 4), designed to minimize the formation of airborne pollutants, burns natural gas or 0.5 percent sulfur fuel oil. Critical to emissions control are the following:
- A flue-gas recirculation system. It extracts some of the flue gas flowing through the economizer and injects it into the air inlet duct to reduce the amount of oxygen available for combustion (see side view and photo inset). This lowers peak flame temperature and minimizes the formation of so-called thermal NOx.
- A low-NOx burner from The Coen Co., Inc., Foster City, Calif., has multiple air and fuel zones to assure thorough mixing of fuel with a minimum amount of air.
Conservation initiative
Conversion of the legacy fired-boiler steam plant to a modern CHP facility with fired boilers relegated to a backup role was only one part of Smith’s energy-system upgrade program (figure 5). The second initiative focused on reducing energy consumption.
Raymond said that the Btu police did such a good job of eliminating waste that peak steam sendout on cold winter days dropped from about 100,000 lb/hr to 80,000. The new science and engineering center will add to current demand. Designed by van Zelm, it is LEED certified by the U.S. Green Building Council, Washington, D.C., to assure efficient energy use and minimum environmental impact.
New D-type packaged boiler features a low-NOx burner and flue-gas recirculation system (inset), enabling it to meet challenging emissions regulations.
Lesko said the energy conservation effort is ongoing. Carrier Corporation’s Building Systems & Services Group has proposed modifications to optimize the production and use of steam, chilled water and electricity. Gregory Hester, PE, LEED AP, Carrier’s regional engineering manager for the Northeast, said the suggested mods include the following:
- Increase thermal demand in the summer and swing months to improve CHP-system capacity factor. Replacement of a 1,000-ton screw-type electric chiller with a 700-ton absorption unit would be a major action item for achieving this objective.
- Install a 275-kW Carrier Microsteam® turbine to reduce boiler header pressure to 15 psig for the absorption unit. The turbine also would supply steam for (1) domestic water heating, (2) reheat and heating hot-water loads and (3) process use during the shoulder months – thereby eliminating throttling losses associated with the use of pressure reducing valves. The 8-inch standby line would be repurposed to distribute 15-psig steam.
- Retrofit several chilled-water systems for year-round operation to eliminate humidification and comfort issues.
Environmental protection
Reducing the power plant’s environmental impact was a primary consideration in project development, Camean said. Smith College was committed to a “green” future, and this included special requirements for the demolition and construction work associated with installing the new CHP system. For example, nearby residences and a nature walk had to be protected from noise and dust, and asbestos had to be removed, packaged appropriately and shipped offsite to a licensed disposal facility.
Assessment and mitigation of the power plant’s air-emissions impact was the responsibility of Epsilon Associates, Inc., Maynard, Mass., which had a long-term relationship with the college. Senior consultant Stephen H. Slocomb, PE, ran the project for Epsilon, including permitting.
Eliminating the coal option and switching the boilers from 2.2 percent to 0.5 percent sulfur fuel oil were big steps in improving air quality. Slocomb said the SCR is designed to reduce uncontrolled NOx emissions from the GT by 94 percent. Limit for gas firing is 0.14 lb NOx/MWh, or about 2.6 ppm. CO limit is 0.09 lb/MWh, or about 2.8 ppm.
Emissions limits for the new fired boiler also are demanding. Experience at other plants indicates that Rentech packaged steam generators equipped with flue-gas recirculation and low-NOx burners are capable of reliable operation while satisfying some of the nation’s most challenging environmental standards.
A continuous emissions monitoring system (CEMS) is not required on a plant of this size in Massachusetts (not considered a “major source”). However, annual state inspections are conducted to verify operation within prescribed limits.
Controls integration
Associated Boiler Line Equipment Co. (dba ABLE Co.), Milford, Conn., headed by Douglas Zuklie, PE, was responsible for controls integration on the Smith project. ABLE started as a power plant combustion-control service organization nearly 40 years ago and today serves the New England/New York/New Jersey area both as a manufacturer’s representative (Coen, for example) and as an independent control-system integrator.
Zuklie said ABLE’s Lee Stewart was the lead designer for the combustion control (fully metered, cross limited) and burner management systems serving the new fired boiler. He also was responsible for overseeing the design of the HRSG feedwater and SCR controls, balance-of-plant (BOP) control system, and the SCADA (supervisory control and data acquisition) system.
Seven Siemens 353 process automation controllers (Siemens Energy & Automation, Inc., Alpharetta, Ga.) are used for the fired-boiler combustion control system, an Allen Bradley SLC 505 (Rockwell Automation, Inc., Milwaukee) for the burner management system. The 353 controller’s Ethernet and Modbus communications enable it to function as an integral element in a plant control system. The SLC 500 family of small programmable controllers offers multiple processor choices, numerous power supply options and extensive I/O capacity.
The HRSG relies on the Compact-Logix family of programmable logic controllers from Allen Bradley/Rockwell. The AB Logix product line spans a range of options from a cost-effective micro-control solution to high-performance, expandable controllers for evolving application needs.
The BOP PLC, also a Compact-Logix, is used to bring signals into the SCADA that aren’t part of the other packaged control systems. They include steam-header flows, pump start/stop signals, steam and feedwater flows, and pressures.
The open-architecture SCADA system communicates with the controllers and field devices identified above and provides a central operator interface for plant operations. The SCADA system interfaces with all plant control systems, including those for the new packaged boiler, HRSG, turbine, gas compressor and switchgear, plus those for the existing boilers.
It was developed using Wonderware InTouch software and functions much like distributed control systems (DCS) used in large generating plants. InTouch is said to facilitate plant operations, control and optimization by use of enhanced graphics and the capability to deliver the right information to the right people at the right time. It supports trending and report generation as would a DCS.
Operating experience
Raymond seemed satisfied with the way the new CHP system “handled.” He said the GT was a learning experience for the plant staff, but was running well. The plant has a Solar long-term service agreement (LTSA), and operators certainly will learn more about their engine during scheduled maintenance outages. In-service monitoring of the unit by the OEM is an option for future consideration.
One pearl Raymond offered to others installing their first SCR was to be sure to specify stainless steel throughout the ammonia system – from the storage tank up to and including the spray nozzles. He said Smith had iron pipe from the storage tank to the ammonia skid (figure 6), and until the system was properly flushed, corrosion products periodically plugged some spray nozzles adversely impacting SCR performance.
Wintertime demand now is about 3 MW. At that load, the unfired HRSG delivers about 17,000 lb/hr of steam to the header, and the balance of the thermal requirement is met by the fired boilers; the new Rentech is dispatched first. When electrical demand drops below about 1,400 kW, Smith switches to utility power.
Once the new science building is in full operation, and Carrier’s proposed system optimization retrofits and upgrades are complete, the GT is expected to run year-round except for planned outages.