Wood Is Tough to Burn
Biomass fuels, especially wood fuels characterized by widely different constituents and moisture contents, are more efficiently burned in a circulating fluidized bed (CFB) boiler than in a conventional stoker furnace. A CFB boiler also makes stack gas emissions easier to control and treat, which is an important consideration for a biomass plant located in or near inhabited areas. However, CFB boilers have presented unique challenges for their owners. CFB boilers may be able to efficiently handle a wider range of wood fuels, but the circulating of solid materials such as fuel ash and sand also increases erosion and corrosion of the combustor and heat transfer tubes.
One plant that has experienced these problems is the 25-MW Rio Bravo Rocklin Power Station (Rocklin), jointly owned by subsidiaries of Constellation Energy and North American Power Group Ltd. Rocklin is located on a 50-acre industrial site about 3 miles northeast of Roseville, Calif. (Figure 1).
1. Rockin’ Rocklin. The 25-MW Rio Bravo Rocklin biomass-fired power station, located in central California, began commercial operation 20 years ago. The plant burns a combination of urban wood and forest wood wastes. This conveyor moves fuel from the storage yard to the boiler. Courtesy: Constellation Energy
Rocklin uses a CFB system supplied by Combustion Engineering (now Alstom Power) when the plant entered commercial service in 1989. Their system was based on the Lurgi CFB design that was popular at the time. The 28-MW steam turbine was supplied by MHI, and Brush provided the generator.
Sourcing wood fuels from waste wood sources is always a challenge. At Rocklin, a diverse group of fuel suppliers provided approximately 240,000 tons of urban wood waste, in-forest brush and clearing, and other wood-related products each year. Since Rocklin began commercial operation the plant has operated with an average capacity factor of around 77%, but ongoing operations problems and equipment failures caused many forced outages and started the red ink flowing several years ago (Figure 2). A highly prized Standard Offer No. 4 PPA with Pacific Gas & Electric (PG&E) provides the plant’s sole revenue stream (ignoring production tax credits and other unique California biomass incentives), so any outages instantly hit the plant’s bottom line.
2. Ready for a change. Rocklin struggled for years to stay economic to operate when boiler and other plant problems put downward pressure on the plant’s capacity factor. Source: Constellation Energy
Unfortunately, Rocklin stuttered through several years of poor performance without making much improvement in its operating numbers. Many reliability improvement programs were implemented, but without much success. Each year the owners would invest in repairing a particular problem but have little to show for their investment as key operating metrics continued to slide. Most problems were highly interrelated, so solving problems in series never seemed to show progress. A more holistic solution that simultaneously resolved all the nagging problems was necessary. A compounding problem was that employees were frustrated with their inability to improve the plant’s reliability, and employee turnover was rising.
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