Originally built in 1916 on the south banks of Lake Superior’s Chequamegon Bay, the Xcel Energy Bay Front power plant made history as the first investor-owned utility plant to generate electricity using woody biomass in December 1979. Today it is one of the nation’s longest running facilities using wood as a significant portion of its feedstock. Xcel Energy is moving forward with its goal of reducing Bay Front’s coal consumption and ultimately becoming 100% biomass fired.
Six years ago, when Plant Manager David Fulweber came to Bay Front, the facility burned 70,000 tons of coal that year. That number had dropped to 18,900 tons of coal last year, while biomass consumption was near record levels, totaling just more than 253,000 tons. That is only slightly lower than the record 253,548 tons set in 2009.
Two of Bay Front’s three B&W boilers are stoker grate boilers, which are now generally 100% wood-fired. Each produces the steam to drive General Electric turbines and generators. Each boiler line currently is generating 12 MW of biomass-fired power.
A larger B&W cyclone boiler, now used mainly for peak loads, can burn natural gas or coal only. Due to a reduction in load demand in northern Wisconsin, it has not been used very much this year.
This boiler was scheduled for conversion to biomass gasification technology until a second phase feasibility study completed in 2010 indicated unacceptable conversion costs at that time. Future plans for that boiler hinge on the outcome of EPA’s EGU MACT regulations for boilers over 25 MW.
Bay Front is rated for up to 74 MW, though its base load is normally set between 40 to 45 MW.
Fulweber estimates that 60 to 70% of that power generating load today is fueled by woody biomass, which is composed of mill residues, used railroad ties and wood chips from logging operations. The latter has overtaken mill residues as the largest component in the mix due to the economy and resultant decline in wood products production in the region.
Wood Supply
The plant consumes 700 to 1,000 tons of woody biomass daily, drawn from a 75 mile radius around Ashland. Loggers supply about two-thirds of that volume.
That wood comes from a portion of the tops, branches and non-merchantable stems that would previously have been left as debris on the forest floor. Thus officials here note they complement rather than compete with paper mills for the pulpwood logs by creating an additional market for that unused wood. Biomass harvesting guidelines in Wisconsin require that a portion of the slash be left on site to provide nutrients for soil regeneration.
Bay Front is also permitted to burn used railroad ties, which is a better environmental solution for disposal than landfilling it, the manager notes. Ties are an excellent feedstock capable of generating higher energy levels—6,500-7,000 BTUs compared to raw wood’s 5,000 BTUs.
Railroad ties do not qualify under the current biomass definition in the latest EPA MACT regulations, however. So unless something changes, Bay Front will have to discontinue their use in 2014 when the final MACT regulation takes effect.
Originally, it was environmental concerns that led the railroads to contact Bay Front about utilizing their ties for boiler fuel many years ago. Previously, the railroads had just piled the used ones to the side of the tracks when they were replaced. Then regulators said they had to be moved. One railroad company estimated their initial volume of wood available that could be supplied to boilers such as Bay Front was about 4-6 million tons.
Emissions Upgrades
A $1.2 million upgrade is ongoing at the wood yard. The project is designed to recapture fugitive dust. This will significantly reduce wood dust around the plant site.
Hoods and fans are being installed at several transfer points on the wood yard’s United Conveyor belt conveyors. All are already covered belts, so the main issue is the transfer points when material flow separates or changes directions.
The hoods and fans will work in tandem with a pneumatic vacuuming system to capture the dust, send it to the baghouse silo before it is returned to the fuel storage bin and used to fire the boiler. Air Cure Inc., based in Minneapolis, Minn., is supplying the dust collection system and handling the installation.
A stilling shed will be built over the top of the truck dump. Fulweber explains, “When the trucks dump, the dust that is kicked up by that operation will be collected in this same system.”
Last summer a 400 HP Jeffrey Rader wood hog replaced a 600 HP wood hog. As anticipated the smaller unit significantly reduces the dust resulting from grinding chips into smaller particles. It also improves grinding efficiency.
Emissions from both wood-fired boilers are cleaned via electro-fluidized beds (EFB) installed in the late ’80s. These particulate control devices will be replaced with fabric filter baghouses (as further described below).
A 2008 upgrade at the wood-fired boilers targeted reductions in nitrous oxide (NOx) emissions as the plant prepared to meet a 2009 deadline for the Clean Air Inter-state Rule (CAIR). Steag Energy Services (formerly Energy Systems Associates), Carnegie, Pa., was project contractor. The vendor provided operator training, startup commissioning, fine-tuning and systems testing.
Combustion efficiencies to each boiler included the addition of an overfire air system to divert air away from the main combustion point, thereby lowering oxygen levels and reducing NOx conversion in the main flame zone. A flue-gas recirculation system provides low oxygen air to stabilize conditions at the grate. Further NOx reductions are achieved because of a multi-zone urea selected non-catalytic reduction (SNCR) system with auto-retract injectors.
As a result of these improvements, the plant reduced its NOx emissions level by 60-70%.
As stated above, with EPA’s latest boiler MACT rules slated to take effect in 2014, Fulweber says the installation of two new baghouses will probably be the next emissions reduction project Bay Front undertakes. If the proposed project is approved, he expects the engineering will be completed toward the end of the year and construction will start in early January for a March 2014 completion.
Wood Yard
There are now 17 suppliers feeding the facility, most of which are logging contractors. Two vendors deliver other fuels such as coal and tire derived fuel. All of the biomass chips received are green, as is sawmill bark and sawdust. Moisture content of the feedstock averages 45%.
Approximately 40-45 trucks arrive at the wood yard daily, maintaining a steady flow of wood chips around the clock.
Those with walking floor vans unload directly onto the chip pile. The rest go to an ’80s vintage truck dump system where only the trailer is tipped and emptied into a live bottom hopper. United Conveyor belt conveyors empty the hopper and feed the material under a 4 ft. wide Eriez magnet to remove tramp metals before it enters a rotary classifier.
Oversized wood is routed to the 400 HP Jeffrey Rader hog. Throughput at the hog is 50 tons per hour.
Undersized chips are directed to the hog discharge conveyor where a diversion gate directs chips to a Clark 450-ton storage bin. If the bin is full, the gate opens to send material to a stock out chute where chips are piled on the ground, then transferred to the outdoor woodpile.
Wood flows from the bin storage 24 hours per day, seven days a week. A John Deere front-end loader feeds chips into a conveyor system as needed during the week and on weekends when incoming trucks are fewer.
The power plant stores ~7,000 tons on the woodpile, which is slightly less than a two-week supply. The pile is rotated every three or four months to avoid chip decomposition and prevent hot spots that can turn into chip fires.
A bank of screw augers controls the feed rates as wood enters a blower line feeding both stoker boilers. Feed rate averages about 15 tons per hour, though it will vary depending on certain parameters such as moisture content. Chips are blown to the back of the boiler where they land on 25 ft. moving grates. Boiler temperatures are about 1,750° F.
Even though these boilers can combust coal or biomass fuel, managers have found that co-firing the two fuels presents numerous problems with ash fouling and slagging. Thus, both boilers are operating on biomass feedstock only.
A small amount of tire derived fuel, which is a finer, lighter crude produced from recycled tires, is added to the wood chips conveyor just before entering the boiler.
Twelve Synfab process cameras positioned throughout the wood system provide visual remote monitoring for the four-man operations team, who are responsible for running the plant. Two cameras give the operators a bird’s eye view into the boilers so they can monitor how the fuel is burning and make needed adjustments if there is a fuel combustion issue. Both B&W boilers are operated by electro-mechanical controls. Future plans call for converting them to computerized-controls. The cyclone boiler’s controls are already 90% computerized.
Each boiler generates 650psi steam that turns the turbines—two 22 MW General Electric and a 32 MW Allis Chalmers. Together these generators produce 13,800 volts of electricity. Eastbound transmission lines are stepped up to 88,000 volts. Voltage is stepped up to 115,000 volts for West and southbound transmission lines.
A United Conveyor vacuum transport system removes ash from all boilers and stores it in a common ash silo. Since 2002, Xcel Energy has partnered with Beneficial Reuse Management, Chicago, to utilize this fly ash and bottom ash in a number of applications. Because the material is very dense and stable, it compacts well and is used as the base for projects like pads under feedbags for farmers, or a base layer for pavement, parking lots and other construction projects.
“If we can ever get to 100% wood fuel it would change the characteristics of the ash such that it could qualify as a soil amendment to add nutrients back to the soil,” Fulweber says. Because of the intermittent operation of the third boiler, there can be some coal ash mixed into the byproduct. This can downgrade the ash to a category three instead of the higher valued category two.