Power to the People at Dapp

The Dapp Power plant is visible across the prairie from a long distance. Located halfway between Edmonton and Slave Lake, Alberta, the plant turns woody biomass into electricity for the consuming public. It’s winter when I visit, and it’s a far cry from the plant’s original home in balmy California.

Twenty-three full-time employees operate and maintain the Dapp Power plant 24/7, not counting the biomass transport crew and other trucking contractors.

This power plant has an interesting history, having had various owners and several upgrades. It was first built in California, where it produced power from forestry waste for about eight years. It was purchased and moved to its current location, a region with extensive peat resources, in 1997, with a plan of producing power from peat. The moisture content of the peat created difficulties in the boiler, so a gas-fired rotary dryer was later installed to dry the fuel prior to burning. However, the energy inefficiency of having to dry the fuel made the plant uneconomical to run, and it was shut down in early 2000.

After changing hands a couple of times but remaining idle for several years, Calgary-based Verdant Energy purchased half ownership of the plant. It converted the boiler to a bubbling fluidized bed style, and the plant was restarted with woody biomass as the fuel of choice. Verdant gained full ownership in 2007. Finally, in 2008, the current owner, New York-based Fortistar, purchased the 17-MW biomass plant, bringing its renewable portfolio to a total of 310 MW of power from six biomass plants and 50 landfill gas plants in the U.S. and Canada.

A shaker screen separates clinkers and rocks from sand released from the bubbling fluidized bed boiler.

“We probably go through approximately 180,000 green tonnes/year of woody biomass,” says plant manager Fred Stock. That quantity of biomass comes from several sources. “We get forestry waste from a sawmill and from a pulp mill, both owned by West Fraser, in Slave Lake. We also bring in landfill-diverted wood from our Biofuels Division at Northlands Landfill in Edmonton. And we also have several other sources of landfill-diverted wood from the Edmonton area,” he says.

One of these other sources is a wood exchange with a composting operation at another Edmonton landfill. For every tonne of clean wood biomass that’s sent to the composter, Dapp receives 1.5 tonnes of wood that’s unsuitable for composting but fine for burning. The plant also receives biomass from the Enoch First Nations from “an old pile of wood that they’ve had on their site for a number of years,” says Stock. And additional biomass is coming from the deconstruction of old buildings, camps, and pallets from a large energy corporation in the Fort McMurray, Alberta, area.

The goal is to have biomass coming in from many sources, says Stock, but the main challenge is transportation. “Trucking and fuel are probably the two highest costs,” he notes. The biomass is trucked in, in 53-foot walking floor trailers, as it is generated, so deliveries vary in frequency depending on the season and type of material.

Generating Power
When trucks arrive, they are weighed, and the biomass is dumped onto a shaker table with disc screens that remove oversize material. “Any oversize material over three inches is hogged,” says Stock. “Material that’s under three inches normally falls though our disc screen and it’s just accepted without being hogged.” Magnets remove much of the nails and other metal from the landfill-diverted wood, although the Doppstadt shredder used by the Biofuels Division removes a lot of the metal from that material source, says Stock. “Once it’s been ground and we remove probably about 90% of the metal, we store it on site.”

The sand is reinjected into the boiler while the impurities are discharged outside.

Following Dapp’s fuel storage management plan, Stock keeps about a two-months supply of fuel at the power plant, stored in piles outdoors. The oldest fuel is always burned first. “There’s enough moisture that, if you turn it over within two months, normally you don’t have a fire issue [from self-heating],” he explains.

Two reclaimers start the process that feeds biomass to the furnace. “They’re good for approximately six hours of run time on each, so we alternate them: while we’re running on one, we’re filling the other one,” explains Stock. A Deere loader fills each reclaimer, which feeds the fuel through another disc screen, more magnets, and into a fuel metering bin at the boiler house. The disc screen at this point removes frozen lumps of fuel and anything oversize that slipped through the first screens; those are re-hogged. From the metering bin, three feed systems send biomass to the Babcox & Wilcox furnace. The plant operator controls the reclaimer and infeed speeds from a central control room, regulating the fuel flow.

Plant manager Fred Stock, a power engineer, has worked at biomass power and cogeneration plants for more than 30 years, mostly in British Columbia.

Originally a circulating fluidized bed boiler, the furnace was converted to a bubbling bed to reduce erosion from sand circulation and improve biomass combustion, says Stock. “They both have a sand bed in the furnace, which is basically your mass of heat. In the circulating style, the sand is actually blown up to the top of the boiler, collected in hoppers, and reinjected into the furnace,” he explains. In the bubbling bed, the sand stays in the furnace, and air is bubbled upwards through it. “It’s a lot more efficient and there’s a lot less maintenance cost to it,” adds Stock. The sand bed operates at 1750°F (954°C), essentially gasifying the biomass.

Rocks, impurities, and slag, which are heavier than sand, fall below the air nozzles at the bottom of the bubbling bed. Periodically, one of three outlets at the bottom of the furnace opens, dropping these items and some of the sand onto a vibrating screen. The sand falls through and is returned to the boiler, whereas the impurities pass to a chute that takes them to a disposal pile outside the boiler room. Wood ash is mixed with a small amount of water to prevent it from becoming airborne and then drops onto an ash pile.

The furnace heats a boiler, generating steam, which turns a Turbodyne turbine, creating the power, explains Stock. The system is virtually closed, with the steam condensed and returned to the boiler as water. The power is sold to TransAlta, a power generator and wholesale electricity marketer headquartered in Calgary, and then to Alberta customers.

Byproducts
“All the ash that we generate basically goes to local farmers,” says Stock. They usually apply it in the fall or early spring to increase soil pH. Dapp may or may not charge a loading fee, and providing it to local growers keeps it out of the landfill, notes Stock.

The Dresser-Rand turbogenerator converts mechanical energy from the Turbodyne turbine into electricity for the grid.
Disc screens and magnets remove oversize biomass and ferrous metal, respectively.  The wood is re-hogged, and the iron is recycled.

Emissions are controlled by a Clyde Bergemann electrostatic precipitator that Stock says was totally rebuilt in fall 2009. “We have an online emissions monitoring control system, which is reportable to Alberta Environment monthly. It’s a regulation in Alberta that you have an online monitoring control system, and everything is recorded as either one-hour averages or six-minute averages, depending on which particular parameter they require,” he explains. If emissions exceed the limits for any reason, the plant must take corrective action and report to Alberta Environment within 24 hours, notes Stock.

A side benefit to burning landfill-diverted wood is that the Alberta provincial government has a carbon credit system in place that rewards this re-use. It involves keeping track of how much wood is diverted from landfill, as well as third-party verification and auditing of the records. Stock says the goal is to burn mainly landfill-diverted wood, both for the carbon credits and because it is dryer than the hog fuel that comes from forestry residue. “This past year [2010], we burned approximately 40% landfill-diverted wood,” he notes.