Servicios de terceros

Benefits and concerns from using biomass feedstocks for energy purposes have been discussed in many studies across the United States and abroad. Presented here is a

literature review on economic, environmental, and social benefits and concerns in relation with the energies and the feedstocks examined in this study.

3.3.1 Electric power generation

Biomass can be directly fired in dedicated boilers. However, “co-firing biomass and coal has technical, economical, and environmental advantages over the other options”

(Demirbas, 2003, p. 1). Hughes (2000) argues that “co-firing in existing coal-fired power plants makes it possible to achieve much better efficiency in converting biomass fuel into electric power, compared to the typical practice in the existing boilers that fire 100% wood-derived wastes as fuels” (p. 458). Biomass co-firing is applicable to most coal- fired boilers used for power generation. Typically, biomass fuels (e.g., wood wastes, short-rotation woody crops, agricultural wastes, short-rotation herbaceous crops, etc) utilized in co-firing are modest in heat content (e.g., 4000-5000 cal/g) and low in sulfur (Tillman, 2000). The woody resources are low in nitrogen and ash content while the agricultural resources can have high nitrogen and ash contents. Tillman (2000) argues that “these fuels can be co-fired at 10-25% (mass basis) without seriously impacting the heat release characteristics of most boilers” (p. 1). Because of the characteristics of biomass resources, co-firing biomass with coal helps reduce the total emissions of NOx,

SO2 and CO2 per unit of energy produced compared to coal fired alone (see for example,

Tillman, 2000; Hughes, 2000; Mann & Spath, 2001). In addition, co-firing involves the use of existing coal-fired units to combust together a combination of biomass and coal. Boylan et al. (2000) note that “the use of existing facilities reduces the capital

addition, the lower investment reduces the level of economic risk” (p. 411) attracting more investors.

Some biomass co-firing cases are discussed in the following sections. 3.3.2 Feedstock availability and costs

The local availability and cost of biomass is a principal factor in determining the

feasibility of co-firing at a specific site. Optimal sites for co-firing are those areas where there is enough available biomass fuel to easily support the level of co-firing and where the cost of the resource is less than that of coal. As Southeastern Regional Biomass Energy Program (SERBEP) reported in 1995, “studies by the Electric Power Research Institute (EPRI) have indicated that co-firing with biomass at levels up to 15 percent can be economical when the difference in costs between coal and wood is in the range of $0.25 to $0.40 per million BTU. However, when coal costs $1.00 to $1.50 per million BTU, it is difficult for biomass to compete” (ODOE, 2005, p. 33).

3.3.3 Co-firing

Interest in co-firing biomass with coal in existing power plants is growing largely due to the need to improve air emissions from coal-burning facilities as well as to diversify fuel supplies in attempt to reduce the dependence on foreign oil. Many cases of biomass co- firing have been tested around the nation. For example, in 1992, wood waste was successfully co-fired with coal in a 100 MW pulverized coal power plant at Georgia Power Company’s Hammond Unit 1 (King et al., 1998). Tree trimmings (as wood waste) and sawdust were used in the test. The percentage of wood in the boiler fuel

averaged 11.5% by weight, or 6.5% by heat input (King et al., 1998). The test results showed 14% wood loading (by weight) represented the maximum wood percentage without load reduction from the unit (King et al., 1998). Boiler efficiencies changed little during the wood co-firing process whereas NOx emissions remained the same compared

to normal coal firing. “Wood wastes were pre-ground before delivery to the plant, and the wood and coal were mixed at the plant before being delivered to the pulverizer and boiler” (King et al., 1998, p. 243).

Several short-term tests of co-firing switchgrass with coal were conducted in 1998 at the Alabama Power Company’s Plant Gadsden located in Gadsden, Alabama. Results indicated that switchgrass was successfully co-fired with coal, in some cases up to 10% of the energy input from switchgrass. Nearly 4.5 MW of renewable energy was produced by the co-firing system. Measuring the boiler efficiency indicated that it was about 0.3% to 1.0% less efficient than coal fired alone case, which was due to higher dry gas losses associated with introducing cold transport air into the furnace (Zemo et al., 2002). Emissions of sulfur dioxide and mercury were lower with switchgrass co-firing than with coal fired alone option. No change in NOx was reported compared with coal fired alone. These short-term tests raised some questions regarding long-term effects of switchgrass co-firing. One of the issues that need to be addressed is to determine the long-term effect of switchgrass co-firing on slagging and fouling. Analysis of switchgrass co-firing showed the ash to contain high percentages of alkali metal, especially potassium, which could be a problem for fouling back pass tubes.

Pennsylvania Electric Company conducted wood co-firing tests at the Shawville plant in Johnstown in 1995. Two boilers participated in the test: one 138 MWe wall-fired and one 190 MWe tangentially-fired pulverized coal boilers. The 3% biomass input was selected for co-firing test. Different fuels were involved in the test: a reference coal and biomass in the form of mill waste sawdust, utility right-of-way tree trimmings, and hybrid poplar. Although biofuels were processed before being mixed with coal grinding equipment, “tree trimmings and hybrid poplar, with longer, stringier fibers, proved to be more difficult to handle during fuel preparation and blending operations than sawdust. Only small amounts of hybrid poplar were fired because of the inability to successfully handle the fuel during operations” (King et al., 1998). The test results revealed two important issues: a) tree trimmings and hybrid poplar were more difficult to handle during fuel preparation and blending operations than sawdust, as they have longer and stringier fibers: and b) the boilers could not achieve their normal full capacity.

Specifically, the 138 MW boiler lost 8 to10 MW of capacity due to feeder limitations, and the 190 MW boiler lost 15 MW of capacity due to significant reductions in mill outlet temperatures (King et al., 1998). For both units, the 3% weight biofuel blend behaved like wet coal. Penelec concluded that wood fuel should be fed separately from pulverized coal (Prinzing et al., 1996).

Madison Gas & Electric (MG&E) was the first utility in the U.S. to undertake a large-scale co-firing of herbaceous energy crops with coal. In 1996, the company co- fired switchgrass in a 50 MW wall-fired, pulverized coal boiler. A 5-day test used a 10% switchgrass/ 90% coal (on a heat basis) combination. The test showed that “sulfur

dioxide emissions were largely unchanged, nitrogen dioxide emissions decreased 12%, and opacity (a measure of visible smoke) was reduced 50% compared to burning 100% coal. Post co-fire inspections of the boilers indicated no slagging or other detrimental effects” (King et al., 1998).