By Brian K. Schimmoller,
When Avista Corp. (then known as Washington Water Power) brought the Kettle Falls Generating Station in northeastern Washington state into operation 20 years ago to meet rising demand for power, the company was also motivated by the environmentally advantageous prospect of eliminating the widespread use of “wigwam burners” to get rid of wood waste. The 46 MW hog-fuel fired plant provided an opportunity to convert waste wood products into valuable electricity in an environmentally responsible manner. Since then, the plant has been a reliable source of renewable energy in the Northwest, and recently, a unique gas turbine-based feedwater heater repowering project has boosted plant output and improved efficiency.
The subcritical natural circulation grate boiler at Kettle Falls produces 415,000 lb/hr steam at 1500 psi and 950 F at the superheater outlet, feeding a condensing single-flow steam turbine. As originally designed, five feedwater extraction ports off the steam turbine pre-heat the boiler feedwater to increase efficiency. The top feedwater heater had a long history of being out of service, hampering plant output and efficiency. Although Kettle Falls determined that the out-of-service heater did not cause any significant operational problems with the steam turbine, the plant kept looking for an effective way to raise the feedwater heater temperature to the boiler’s inlet design conditions.
At POWER-GEN International 1997, Thomas Dempsey, Avista acting chief generation engineer, and Steve Wenke, Avista chief generation engineer on temporary assignment, started thinking about ways to incorporate gas turbines into existing cycles, particularly after seeing the improved performance of small industrial turbines. Several papers presented at the conference discussed unique re-powering applications somewhat similar to the Kettle Falls application. At Kettle Falls, the concept was to install a small gas turbine and heat recovery steam generator, with water supplied from the main unit’s condensate system returned as steam to the top feedwater heater at the appropriate design conditions. Detailed performance modeling by Scientech confirmed the viability of the scheme, and Kettle Falls ultimately purchased a 7 MW Taurus 70 gas turbine from Solar Turbines that matched the heat input requirements.
Dempsey could not find a similar repowering project for comparison purposes, which caused some apprehension, but Avista was confident in the technical analysis. “Also, the gas turbine and boiler are not as dependant on one another as in other repowering schemes,” said Dempsey. “If the gas turbine trips off line, the main unit’s boiler simply sees it as a reduction in feedwater inlet temperature, which the control system automatically compensates for in the wood fuel feed rate. Additionally, the gas turbine has a simple cycle bypass stack which enables it to operate when the hog fuel boiler is down for maintenance.”
In the repowered configuration, which began operation in summer 2002, a 25,000 lb/hr bleed off of the condensate discharge pump header is fed to the HRSG. Steam from the HRSG is then sent to the existing top feedwater heater at the steam turbine extraction design pressure and temperature. Fuel for the Taurus turbine is supplied from an existing natural gas line used for boiler light-off.
Although Kettle Falls has made great strides in improving input fuel quality and consistency via blending and fuel procurement strategies, moisture variations from 23-56% can complicate operation. “A few years ago, 30-40% derates were common when moisture levels spiked,” said Kettle Falls Plant Manager Dean Hull. “Fortunately, when the plant was designed in the 1980s, the engineers left some room for growth. While the fire side of the unit struggled at times to reach rated output, we had extra capacity on the water/steam side. This provided the cushion needed to retrofit the gas turbine and HRSG into the steam cycle.”
The incremental heat rate of the gas turbine combined cycle addition is 8500-9000 Btu/kWh (higher heating value basis), which is not on par with an F-class combined cycle, but is quite good for a 7 MW gas turbine. While it is not easy to calculate the exact overall heat rate for the wood fired plant due to the difficulty associated with measuring fuel quality, experience over time has clearly shown a reduction in the wood supply rate to the boiler.
The feedwater heater repowering project has also had a valuable impact on plant output. “When the plant is boiler limited and can’t push through any more fuel, the gas turbine combined cycle provides a 5-6% increase in output, or about 3 MW. If the main unit is not boiler limited due to fuel quality, the gas turbine provides for a reduction in the amount of wood required to achieve full load,” said Dempsey. Reaching the plant’s full rated output is readily achieved today without loading the fuel conveyors to 110-115% of capacity.
For a relatively small power plant, Kettle Falls sports an excellent availability record: 88.5% when including major turbine inspection outages, and 96% when considering just forced outages and normal maintenance outages. This accomplishment is more significant when one considers that the plant was originally designed to operate six months out of the year and now essentially runs year-round. “The addition of the gas turbine and HRSG has had no effect on availability,” said Hull, “and the repowering scheme didn’t cost a dime for manpower.”
The Kettle Falls facility operates with a total staff of only 30: five control room operators, five auxiliary operators, eight fuel equipment operators, three mechanics, two control and instrument technicians, and various support and administrative personnel. Interestingly, the plant operates with limited supervision. “Bodies cost money, especially for a facility only putting out about 60 MW,” said Hull. “People understand their responsibilities to the job here, they want those responsibilities, and they’re willing to work hard to fulfill those responsibilities.”
Hull is particularly proud of the sense of employee ownership at Kettle Falls. “Avista employees perform all maintenance, with limited outside contractor support, and no operator is insignificant. A bulldozer operator can have the biggest impact on plant output. If the bulldozer operator sees a lot of char coming out of the boiler with the ash, the bulldozer operator can communicate with the control room operator to make some changes and optimize plant efficiency.”