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Repowering with Biomass

Waste fuels struggle despite coal’s decline.

Fortnightly Magazine - December 2010

other items in 1979. Over the years, two of the three units have burned combinations of just about everything—waste wood, railroad ties, discarded tires, natural gas, and petroleum coke. Currently, these two units use biomass as their primary fuel, but they also burn some coal. The third unit burns coal exclusively. “Our goal is to get all three units burning biomass 100 percent,” says David Donovan, Xcel’s manager of regulatory policy. In 2008, Xcel received approval from the Public Service Commission of Wisconsin to convert the plant’s remaining coal-fired unit to biomass, but the project must overcome some hurdles before it can be completed.

Sourcing of biomass material isn’t a problem, according to Donovan, as long as the utility remains competitive in what it is willing to pay. There is some history here: When Xcel decided to burn something other than just coal in 1979, it began looking for a low-price alternative fuel. At the time, the Wisconsin Department of Natural Resources was working to establish guidelines on how saw mills and paper mills could dispose of their waste products, such as bark and chips. “Since we had several of these customers in our service territory, it seemed like a good fit for us to use that material as a low-cost fuel and avoid a waste stream for them,” he explains. “We paid just slightly above the transportation cost to take that material off their hands.”

However, over the years, wood mulch and chips became increasingly attractive to the horticultural industry for landscaping. Consequently, demand for these products began rising, and about three years ago Xcel saw its biomass fuel prices increase substantially. “Eventually, we were paying the same price as we were for coal on a per-BTU basis,” Donovan says. Xcel was willing to pay the price, so its fuel supply continued. It also helped that Xcel uses lower quality, unused materials that generally get left behind in area forests following traditional harvests, such as treetops, logging slash, damaged trees, and cull trees.

To convert all three of its Bay Front plant units to 100 percent biomass, Xcel would need to make minor modifications to the third boiler, and also set up additional biomass receiving and handling facilities, plus install an external gasifier and an enhanced air quality control system.

While biomass is an effective fuel source, it isn’t as efficient as coal. As such, after conversions, utilities end up with power plants that provide fewer megawatts. The Bay Front plant produces about 76 MW (23 MW in each of the two combination biomass-coal units, and 30 MW in the 100 percent coal unit). If Xcel succeeds with the conversion to 100 percent biomass in all three units, output will be around 60 MW (20 MW in each unit).

However, Xcel says what will be lost in efficiency will be made up with environmental benefits. The company estimates the conversion will reduce nitrogen dioxide emissions by 60 percent, sulfur dioxides by 80 percent, and particulate matter by 80 percent.

Under the original timeline, the conversion was to begin in 2010 and