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A group of hardhat-clad utility officials and journalists braced themselves against a bitter December breeze on Friday as they watched a metal chute spill a stream of wood chips onto a conveyor of jet black coal.

Churning along at a rate of 1,000 tons per hour, the coal was headed up to the giant silos of Salt River Project’s Coronado Generating Station. The power plant sits among rolling hills of pinyon- juniper and shrubby grasses just north of St. Johns in eastern Arizona. Its two generating units produce up to 773 megawatts of power that help feed the electricity demand of SRP’s more than 1 million customers in the Valley.

The wood chips are part of a giant experiment for the utility to see if it would be economically and technologically feasible to burn a small percentage of biomass along with the coal.

The reason goes back to the source of the wood, in the forests around Flagstaff. The chips are finely ground ponderosa pine needles, bark, branches and trunks -- collectively called biomass -- that were the byproduct of forest thinning operations in the fall.

Removing a certain proportion of the trees benefits the ponderosa pine ecosystem, helps more moisture soak into soils and, most importantly for SRP, reduces the risk of severe wildfires that could torch the forest and trigger large amounts of sediment to flow into nearby water sources. The same water sources that SRP taps to deliver 800,000 acre feet, or 260 trillion gallons of water annually to municipal, urban and agricultural water users.

The goal of burning biomass is to generate a steady and relatively lucrative demand for this forest thinning byproduct, which usually has little value to loggers who bid on the Forest Service’s contracts. The entrance of another steady buyer of the product will hopefully spur more investment from companies either here or elsewhere to speed up forest thinning operations, said Bruce Hallin, director of water supply for SRP.

In short, the aim is to improve the economics of forest thinning via a coal-fired power plant, Hallin said.

Biomass struggles

While a final evaluation of the experimental burn won’t be available until early next year, three weeks in, the signs are positive, said Dan Bevier, the manager at Coronado Generating Station.

“I haven't seen a show-stopper," Bevier said. "There are technical issues we're trying to resolve but right now I feel pretty confident. I'm not sure what level we could burn at but I think we could do it."

It will take some tweaking to allow the plant to permanently accommodate biomass. The woody product is harder to grind up into same consistency as the dust-like coal that is fed into the boiler, for example. The wood chips also need a special weighing and delivery system to ensure the right coal-to-biomass mixture goes into the power plant.

On top of that, it’s going to be a challenge to obtain a steady supply of biomass, Hallin said. To meet a 2 percent to 5 percent mixture of biomass to coal, which are the ratios SRP is testing, the utility would need the equivalent of between 28,000 tons and 70,000 bone dry tons of biomass annually at full load.

There may or may not be enough biomass to feed all of SRP’s needs on the east side of the state, which would be the most economical source, said Brad Worsley, president of the biomass power plant Novo Power. Though he now has more than enough biomass to meet his 28 megawatt plant’s 240,000 tons-per-year demand, Worsley struggled in the past to secure a sustainable supply of wood chips.

He supports SRP’s project but said the real biomass bottleneck is on the west side of the state’s ponderosa pine forests, near Flagstaff, Worsley said.

“I would argue they would be better off to duplicate a facility like my facility in Flagstaff to resolve this biomass reality,” Worsley said. He estimated startup costs for a plant like his would average $56 million to $84 million.

While the benefit to northern Arizona’s forests will help protect SRP’s water infrastructure investments, burning biomass doesn’t bring any sort of upfront economic advantage to the utility. Throughout the experiment, the costs to process and transport a ton of biomass have been about double those per ton of coal, Hallin said. Natural gas and solar are cheaper than biomass as well, he said.

Nor does the biomass make much of a difference environmentally, according to initial data. The plant has seen very small increases in nitrogen oxide emissions while emissions of particulates and atmosphere-warming carbon dioxide haven’t been much different, Bevier said.

In fact emissions from burning biomass can actually be worse than those from burning coal. A 2014 analysis by the Partnership for Policy Integrity found that biomass plants can emit more than 150 percent of the nitrogen oxides and more than 190 percent the particulate matter of coal as well as more than 50 percent more carbon dioxide than coal.

Eyes on the forest

Considering the challenges biomass poses, SRP officials said it comes down to potential forest restoration benefits that are driving their effort forward. So far, large-scale thinning of northern Arizona’s ponderosa pine forest has fallen short of expectations. The Four Forest Restoration Initiative, or 4FRI, set an ambitious goal of thinning 50,000 acres per year over 10 years, starting in 2010. But the major contractor on the project, Good Earth Power, has underperformed, thinning just 8,300 acres since September 2013 when it should have been cutting trees at a rate of 45,000 acres per year by now. Since 2010, Forest Service data show all logging companies, including Good Earth, have thinned 95,000 acres over 4FRI’s 2.4 million-acre footprint.

For comparison, the 2 percent to 5 percent biomass to coal mixture that SRP is testing would create demand for the biomass from 5,000 to 10,000 acres per year, Hallin said.

For their part, Forest Service officials said they appreciate SRP’s effort to contribute to forest restoration and said the Coronado test burn will provide valuable data to help the agency determine the economic feasibility and energy value of biomass as a fuel.

Looking out from the conveyor belt outside the SRP plant, the few piles of biomass were dwarfed by mountains of coal stretching along the eastern horizon. While the biomass isn’t much compared to the fossil fuels powering the facility, SRP officials appear truly excited about the potential of the process.

“We’re looking at accomplishing multiple goals,” Hallin said.

Emery Cowan can be reached at (928) 556-2250 or


Environment, Health and Science Reporter

Emery Cowan writes about science, health and the environment for the Arizona Daily Sun, covering everything from forest restoration to endangered species recovery efforts.

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