A tale of one city’s response to the California energy debacle
2005 case study from Combined Cycle Journal
It didn’t take the Corona City Council long to answer the wakeup call provided by the California energy crisis. A resolution passed in April 2001 established a municipally owned electric, natural gas, telephone, and telecommunications utility. One of the first activities of the new utility, which is part of the city’s Department of Water & Power (CDWP), was the development of an agreement with Southern California Edison Co. of Rosemead allowing Corona to provide retail electric service as an energy services provider (ESP).
In its role as an ESP, the municipal utility buys and sells energy on behalf of the city’s more than 1,000 electric accounts and more than 50 Corona businesses. These businesses contracted with the city for energy services before the California Public Utilities Commission formally suspended “direct access” Sept. 20, 2001. This means that Corona cannot now offer energy services contracts to any business that was not receiving direct access service prior to the commission’s action. Expectations are that the restriction on the utility’s business development activities will not be permanent.
One of CDWP’s responsibilities under the City Council’s landmark resolution was to develop municipally owned power generation resources to ensure reliable energy supply at a predictable price. Construction of the Clearwater cogeneration project adjacent to the city’s wastewater treatment plant was approved in December 2001 (figure 1).
Corona, assisted by CH2M Hill’s offices in Santa Ana, Calif., and Denver, showed its resourcefulness in project development and detailed design by integrating the city’s first powerplant — a 31-MW combined-cycle cogen facility — with existing wastewater treatment facilities and a new biosolids treatment process, which eliminates the need to ship truckloads of sludge out of Corona daily. Thermal treatment produces solids acceptable to a nearby landfill while reducing the waste volume significantly.
“Essential” public facilities, such as those for processing municipal wastewater, receive favorable treatment in air emissions permitting. Integrating the new generating plant into the wastewater treatment facility, says CDWP assistant general manager George Hanson, facilitated its permitting and earned the project some emissions credits.
CW — cooling water system, DSH — desuperheater, EF — exhaust flow, lb/hr, F — temperature, deg, HP — high-pressure, P — pressure, psia, SF — steam flow, lb/hr, W —- water flow, lb/hr
David Cheever, project engineering manager for CH2M Hill of Englewood, Colo., architect/engineer for the cogen facility and biosolids improvements, says the combined cycle is integrated into the wastewater treatment system in several ways, including:
- A special coil in the heat-recovery steam generator (HRSG), manufactured by Rentech Boiler Systems Inc., recovers low-grade heat just ahead of the stack and transfers it to digesters to promote biological activity. Heat had been provided previously by a small, fired hot-water boiler.
- Some gas-turbine (GT) exhaust gas is withdrawn from the HRSG just downstream of the selective catalytic reduction (SCR) system installed to reduce NOx emissions (nominal temperature of the gas at this point is about 500F). It is used to preheat combustion air for the new sludge dryer serving the wastewater treatment plant. After the preheating step, the cooled GT exhaust is returned to the boiler where an induced-draft fan discharges the gas up the stack.
- It supplies all the electricity required on site for wastewater treatment and power production.
CDWP resource manager Yarek Lehr, P.E., has overall responsibility for the new cogen plant. He came to Corona from the California ISO (independent system operator), experience that will prove particularly valuable if Corona is able in the future to sell power into the grid on an economic-dispatch basis. Lehr says the plant is designed for base-load service but initially will operate 12-16 hours per day, five or six days a week, depending on market conditions.
The 23-MW LM2500 package at Clearwater is the first of its type installed in the U.S., according to Andrew Morton, who manages General Electric’s Aero Energy business in the West. The basic GT has not changed, he says, but the package features major improvements over that offered previously by the company. Essentially, it reflects best practices from both the marine and land-based packages to accomplish:
- Improve reliability and availability.
- Reduce installation time and cost.
- Facilitate maintenance.
The specific features that enable these benefits include:
- Smart transmitters replace switches formerly used, thereby providing the capability for predictive monitoring/maintenance.
- Top engine removal for faster change-out.
- Direct-drive fans in the positive-pressure vent system, replacing less-reliable belt-driven equipment.
- Fuel gas valve with an integral driver, eliminating the need for wiring to connect the valve to a remote driver — a source of problems in the past.
- Alarm and shutdown philosophy integrated into the new vibration system is conducive to higher reliability.
- On-skid controls package, factory tested prior to shipment, reduces the number of interconnects by at least 70 percent, dramatically reducing field wiring and the chance of failure.
The GT discharges more than 500,000 lb/hr of 970F exhaust gas into the Rentech HRSG (figure 2). Maximum steaming capability is nearly 65,000 lb/hr at 832 psig/938F. No duct burners are provided.
The boiler manufacturer had responsibility for integration of the SCR and the CO/oxidation catalyst systems. SCR catalyst was sourced from Cormetech Inc. of Durham, N.C.; CO catalyst from EmeraChem LLC of Knoxville. Emissions guarantees are 2 ppm NOx (by volume, dry), 6 ppm CO (by volume, dry), and 4 ppm (by volume) ammonia slip. The reagent is 19 percent aqueous ammonia.
Plant construction was done by general contractor ARB Inc. of Lake Forest, Calif., which was assigned both the powerplant and biosolids projects. ARB project manager Wayne McAlpin, an industry veteran with more than 40 years of experience who normally works on much larger projects, says the Corona job was not without its challenges. They include the complexities associated with working on a very small site, and the foundation work, which was made difficult by the sequence of activities and pre-existing infrastructure associated with the wastewater treatment plant.
The combined-cycle unit is confined to an area of less than an acre, and there is very little laydown space nearby. Boiler erection was simplified by maximum shop assembly and shipment of the unit to the site by truck in seven or eight major pieces. Key lifts of boiler components took considerable planningand required coordination of two cranes. Harris and Associates of Concord, Calif., was the city’s construction manager, providing oversight of powerplant, biosolids, and sludge dryer construction activities. It also provided and/or supervised construction QA/QC. At least one contractor mentioned that Harris helped to develop and nurture a spirit of cooperation among all participants that was above and beyond industry standards and key to the successful completion of a challenging project.
Don Ries, who handled site coordination work for CDWP, points out that the heat exchanger (figure 5) installed to preheat combustion air for the biosolids dryer with hot gas from the HRSG may seem unnecessary at first glance, but direct use of oxygen-rich GT exhaust for that purpose would have promoted sludge combustion. Natural gas and digester gas (previously flared) are burned to dry the sludge to about five percent moisture. The EPA Class A pellets produced, about equivalent to subbituminous coal in heating value, will be landfilled at least initially. Extracting some GT exhaust from the boiler to preheat the dryer combustion air penalizes electric production by only a few kilowatts, according to Cheever.
Steam from the HRSG drives a nominal 8-MW turbine built by Shin Nippon of Japan and supplied exclusively in the U.S. by Maxi International of Glendale, Calif. The 12-stage turbine was received fully assembled; the reduction gear, which reduces the 6200-rpm turbine output to 1800 rpm for the generator, was packaged separately.
The turbine exhausts to a surface condenser, which receives cooling water from a two-cell tower operating at about 3.3 cycles of concentration on tertiary water from the adjacent treatment facility. A quick look at the water-cycle diagram reveals that city water is the source of boiler and evaporative- cooler makeup, and GT NOx control water, following treatment in reverse osmosis and EDI systems arranged in series.
Calpine Power Services of Houston is currently commissioning Clearwater and will operate the plant under a three-year contract before turning over the keys to the city. Engineers from the company’s Folsom office participated in a design review before construction began to insure that “lessons learned” by the nation’s largest independent power producer were incorporated into the combinedcycle facility.
Plant manager Mike Horn heads the Calpine operating team, which includes four operators and one I&C technician. He was brought on board in April 2004, when equipment began arriving.