Biomass Co-Firing in Electric Utility Boilers

Background

Within the United States, several states have deregulated electric power. National deregulation has been proposed in Congress, but as yet has not been enacted. Contained within most of these deregulation acts is a requirement that electric companies who would sell to consumers within the state boundaries must have a percentage of the electric power generated from renewable energy sources. The purpose of this paper is to explain how gasification and "co-firing" of renewable biomass can fulfill this mandate and the co-current reduction in "greenhouse" gases.

Renewable energy or "green power" is useable energy, either steam or electricity, produced from the sun (photovoltaic), wind, geothermal heat or biomass. Biomass is plant material, either raw or processed. Biomass stores solar energy which can be converted to consumer useable energy.

As a renewable energy resource, biomass has several advantages when compared to the other renewable sources. First, biomass is widely available and an essentially untapped source of energy. Secondly, the capital cost of biomass conversion equipment is comparatively less than solar or wind. Third, the operation of a biomass conversion facility can be integrated into existed fossil fuel fed power generation stations. Fourth, biomass power is one of the most attractive options for addressing concerns over carbon dioxide because trees and other plants sequester or capture atmospheric carbon dioxide. The growth of plants and their conversion to energy as biomass fuels recycles atmospheric carbon. The result is no net addition of carbon dioxide into the atmosphere. Finally, the low sulfur content of most biomass materials means that emissions of sulfur dioxide gas (acid gas and resulting acid rain) are minimized.

To encourage the generation of power from renewable energy, the United States Department of Energy has funded or co-funded several demonstration projects involving the "co-blending" of biomass in utility power boilers. Although referred to within the industry as "co-firing", the processes employed have been the mixing or blending of the biomass feed with solid fossil fuel (coal) and the injection of the biomass into the boiler furnace. All of these efforts have involved coal fired boilers and have demonstrated that this method of "co-firing" does reduce air emissions from a coal boiler, but there are no continuously operating, commercial facilities.

The problems encountered when blending the biomass material with coal feed or separately injecting the biomass solid into the boiler are:

• The biomass material must be reduced in size to a consistency of saw dust.

• Much of the coal ash expelled from utility boilers is sold as an additive for concrete. This amendment to concrete is approved under an American Standard Testing Method (ASTM). This standard recognizes only "coal fly ash" as the additive material. If the biomass solid is fed into the boiler, the ash from the biomass contaminates the coal fly ash. The end result is that the ash can no longer be sold as a concrete amendment and must be disposed, turning a profitable sale into a disposal cost.

• The sponsored "co-firing" has been limited to wood only and to coal fired boilers. This method has not been attempted with agricultural by-products, energy crops or other biomass material nor in oil or gas fired boilers.

External Gasification and True "Co-Firing"

Another technology that has the ability to release the energy stored in biomass is gasification, which is the process of converting a solid into a gas. Employed in the utility industry, a gasifier, located externally to the power boiler furnace, can convert the solid biomass feed into a gas. The evolved gases can then be drawn or blown into a gas burner located in the wall of the boiler and burned in a fashion similar to a natural gas burner. This technology solves or eliminates the problems of the previously discussed technologies.

• Gasification based conversion of biomass can overcome the obstacle of performance in boilers other than coal fired, as the solid material never enters into the boiler.

• The external gasification of biomass permits a greater range of usage of biomass materials. Since the gasification reaction is controlled external to the operation of the power boiler, temperatures within the gasifier can be maintained independently.

• Gasification also retains and discharges the ash from the biomass external to the power boiler. This eliminates the co-mingling of ashes and allows the ongoing sales of coal ash.

Primenergy's Gasification Technology

Primenergy has licensed the most commercially proven biomass gasification technology in the world. With nineteen gasifiers operating world wide and with industrial operation for over eighteen years, this patented gasification technology converts over four hundred thousand tons of biomass into energy every year.

At our Tulsa location, we have a full scale, fully instrumented gasification demonstration plant. To date, we have successfully demonstrated this gasification technology to sustain the energy conversion without the addition of supplemental fossil fuel on twenty-one different types of biomass, proving fuel flexibility. Our process engineering staff can design a gasification system that will provide a reliable, cost effective method of green energy generation. Through proactive project management, we can procure, construct and commission a renewable energy generator that will operate in parallel with existing equipment on schedule and within budget.

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