Biomass Cofiring: Another Way to Clean Your Coal

Larry Eisenstat, Andrew Weinstein, and Steven Wellner

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Demand for renewable power is burgeoning as state governments (and maybe soon the U.S. federal government) impose increasingly rigorous environmental and procurement standards on the energy industry. Surprisingly, biomass cofiring has yet to attract much attention, even though it could help many utilities meet their renewable portfolio requirements, reduce carbon emissions, and solve other regional environmental problems. U.S. developers, investors, and regulators should consider including cofiring as part of the energy mix going forward.

Biomass cofiring is neither a new nor an experimental technology; it is a tested and proven one, vetted by private industry and government agencies both in the U.S. and abroad. Cofiring involves replacing a portion of a coal-fired generating plant’s coal supply with biomass, which is then combusted with the remaining coal to generate electricity. With relatively straightforward, plant-specific modifications — typically involving alterations to fuel-processing, storage, and delivery systems — cofiring can be deployed in nearly all types of coal-fired boilers, including stokers, fluidized beds, pulverized coal boilers, and cyclones.

Once deployed, biomass cofiring can contribute anywhere from 2% to 30% of the total heat input needed for generation, thereby replacing a sizable portion of the typical coal supply.

For that reason alone, cofiring biomass in U.S. coal-fired boilers should be viewed today as a serious option for reducing carbon emissions and as a strategy for meeting a renewable portfolio standard. That rationale was behind FirstEnergy’s decision to go even further and convert two units at its coal-fired R.E. Burger Plant to 100% biomass (see sidebar).

FirstEnergy to Convert Coal-Fired Burger Plant to Biomass

Akron-based FirstEnergy Corp. announced in April that it will convert two units at its coal-fired R.E. Burger Plant in Shadyside, Ohio, to burn biomass (Figure 1). Ohio’s renewable portfolio standard requires that 25% of the electricity sold in the state come from renewable forms of energy by 2025. Today, 86% of the state’s power is produced from coal.

1. Converting to biomass. FirstEnergy Corp. recently announced that Units 4 and 5 at the coal-fired R.E. Burger Plant will be retrofitted to combust 100% biomass. Courtesy: FirstEnergy Corp.

Units 4 and 5 at Burger have been designated for the $200 million retrofit scheduled for completion by 2013. Combined, these two units are rated at 312 MW. FirstEnergy expects these two units to burn biomass grown specifically as a fuel source, such as corn stalks, wheat, or grass. FirstEnergy will conduct test burns of varying amounts of the biomass fuel leading up to the conversion.

Improved environmental performance is certainly one of the big attractions of cofiring biomass fuels, especially in utility-scale projects. Biomass generally has lower sulfur content than typical coals consumed today, so SO₂ emissions will decrease for an equivalent energy content added to the furnace. NO_x reductions also have been noted in operating plants because the nitrogen content of biomass fuel is usually lower than the nitrogen content found in an equivalent amount of coal, and adding biomass can also reduce flame temperatures, leading to lower levels of thermal NO_x.

A closer look at typical biomass fuel properties reveals further fundamental fuel differences with coal. Biomass fuels are typically more volatile than coal fuels and have higher oxygen content. Also, wood fuels in particular have very little ash content, typically on the order of 1% or less. For the typical utility-scale power plant, each of these fuel qualities is a big improvement over burning coal, especially the ash advantage, as there is less ash to handle and send to a landfill.

There are downsides to biomass fuels. Some forms of biomass, such as some straws and grasses, have a higher percentage of potassium and chlorine than coal, which can lead to increased slagging, fouling, and corrosion of furnace tubing.

— Dr. Robert Peltier, PE

Although in the late 1980s through the 1990s there were many successful field tests at utility installations in the U.S. with support from the National Renewable Energy Laboratory, there has been comparatively little new work over the past decade.

The European Union, on the other hand, has been very actively engaging in retrofitting many central station coal-fired plants to cofire a variety of biomass and waste products, and the number of installations is steadily rising. The world’s largest cofiring project is at the Drax Power Station, in North Yorkshire, UK. The 4,000-MW power station is producing 10%, or 400 MW, from biomass cofiring with a goal of reaching 12.5% by mid-2010. Drax is also developing two other 300-MW biomass projects in the UK that, together with Drax, will produce 15% of the renewable energy in the UK.

Although wood historically has been the most common cofiring fuel, any number of other biomass fuels can be used, with the options typically driven by proximity to local supplies. Thus, even though regions such as the Northeast, Southeast, and Midwest might have the largest potential native wood stocks, other regions have their own potential fuel stocks. Also, the type of available biomass fuels will help define the best of the cofiring technology options for a particular plant (see "Three Cofiring Options," next page).

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