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CO-MILLING GREEN WOOD CHIPS AT

ALABAMA POWER COMPANY’S PLANT GADSDEN UNIT 2

Doug Boylan

Keith Roberts

Bill Zemo

Tom Johnson

Biomass co-firing is the burning of biomass together with coal in an existing coal-fired power plant. Burning biomass is generally considered carbon neutral, and the co-firing technology may represent one of the lowest cost options for generating renewable energy in the Southeast United States.

Therefore Southern Company has been conducting a series of performance and operational tests on several co-firing technologies and fuels. This report describes tests on green wood chips processed from whole and from approximately needle free pine trees. Partners with Southern Company in this project include the USDA Forest Service, National Forests in Alabama, Forest Products Lab and Southern Research Station; Auburn University, Forest Products Development Center; Precision Husky; CAWACO RC&D; Forest Based Economic Development Services;, Center of Economic Development, University of Alabama; Reynolds Wood Products; and Temple Inland. The goal was to determine what level of processing of whole trees is required to successfully co-fire the chips in a boiler, and to evaluate the benefits and costs of wood processing compared with the effect of the fuel on the power plant.

Green wood chips were successfully co-fired in blends with coal between 8% and 15% wood by weight. Of the input fuel energy, 2.8% and 5.5% was from wood, and

represented about 2.0 MW to 3.8 MW biomass power.

Sulfur emissions were reduced, and particulates were no higher with co-firing. There are questions yet to be resolved regarding the effect of co-firing on carbon monoxide. The measured impact on NOx emissions was small.

With 10% co-firing, boiler efficiency was about the same as coal alone, while there was a slight reduction in efficiency with 15% wood. This result was unexpected because of the high moisture content of the wood chips (50%-67%). However, higher moisture losses were offset or nearly offset by reduced dry gas losses up the stack.

There were operational problems which will require further assessment. High moisture of the wood coal mixtures resulted in low mill temperatures and high mill bowl under pressures. Consequently, operation at 15% wood co-firing with these fairly coarse and very wet wood chips resulted in a 5 % load derate. However, the most difficult problem was unit operating stability. The coal wood mixtures had much lower fuel energy to volume ratio than the baseline coal. The issues appeared to have been related to the installation of new automatic controls. Stability issues were corrected in subsequent tests by tuning the fuel-steam pressure automatic controls.

Forest managers often need to clear whole trees from their lands, either for thinning, for removal of diseased trees, to reduce fire hazard, or to replace the type of trees growing there. This harvesting can result in large quantities of wood that often find no market for producing either lumber or paper mill chips.

Recently consideration has been given to using whole trees as fuel, particularly for

generating electricity. If a market as fuel could be found for the trees, it could offer economic return to the forest owners. Since combustion of the wood would result in essentially no net CO2 addition to the atmosphere and based on a renewable resource, that energy so produced could be considered renewable. This could offer benefits to the

power generating company.

A project was initiated by Southern Company, USDA Forest Service, Forest Based

Economic Development Services, University of Alabama, and Auburn University to investigate the effect of wood processing on co-milling forest harvested materials in a coal fired power plant. The goal of this project is to evaluate the overall costs and benefits of harvesting forest trees for use as fuel to make renewable energy in existing coal fired power plants. Auburn University and the Forestry Department assessed the harvesting and processing of the wood. Precision Husky Corporation of Leeds, AL designed and constructed a wood chipper for this project to make relatively fine green wood chips at commercial scale rates. Pine trees were harvested from a plot in the Talladega National Forest, and were chipped in a number of ways to provide five types of fuels. Southern Company and Alabama Power evaluated the most economical means of generating electricity using these wood chips with the technology of co-firing.

In co-firing, a percentage of biomass is introduced as fuel into an existing coal fired boiler, often directly blended with the coal itself. The high capital cost of building a stand-alone power plant is avoided, and the technology offers the potential for high efficiency. It is a renewable technology most likely to directly displace coal.

The units, however, are not designed for this mixture of materials. The properties of biomass are quite different from those of coal. The wood density and heating value of

the mixture are both less than in coal. Wood has a tendency to break into fibers than can

affect material handling. Overall, the extent to which the fuels differ can impact the operations and performance of a power plant. Extensive studies have been conducted by

Southern Company and others to quantify the costs and benefits related to co-firing various materials. Generally the drier and the finer the wood fuel, the easier it has been

to co-fire.

Wood chips from forestry applications, however, would have larger size, higher moisture, and also include pine needles. These chips could be reduced to fine size and low moisture, but such processing is expensive. Therefore, minimizing the amount of processing would help to reduce final product costs, but the amount of required processing is not well known. The amount of size reduction required for co-firing, the effect of high moisture wood, and the maximum amount a typical coal-fired power plant

could co-fire required research.

Therefore, as part of the forestry project, Southern Company and Alabama Power conducted co-firing tests of several wood chip grind sizes and compositions (i.e. with and without needles). About 1100 tons of wood chips from the Talladega National Forest Shoal Creek District were delivered to Plant Gadsden, an Alabama Power Company pulverized coal plant located in Gadsden, Alabama, The plant is served by two nominally 60 MWe Combustion Engineering tangentially fired units.