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Show 1 FIRING ALTERNATIVE FUELS IN A UTILITY POWER PLANT Mr. Andrew Dobrzanski, Fuels Manager Monroe Power Plant And, formerly Performance Manager River Rouge Power Plant DTE Energy ABSTRACT The 651 MWe coal-fired River Rouge Power Plant (RRPP) of Detroit Edison, a subsidiary of DTE Energy, was constructed in the late 1950's at a location near integrated steel mills equipped with coke oven batteries and blast furnaces. The plant is also located adjacent to an oxygen pipeline. The power plant includes several boilers and turbine generators, with the tangentially-fired Unit #2 at 260 MWe of capacity and the wall-fired Unit #3 at 280 MWe of capacity. In an effort to provide customers with least-cost power, and to minimize the environmental consequences of power generation, RRPP fires coke oven gas (COG) when available and will soon fire blast furnace gas (BFG) from the adjacent steel mill complex. The coke batteries are owned and operated by Energy Services, the non-regulated business units of DTE Energy. The units also fire a small amount of natural gas for flame stabilization. This paper describes the present and planned use of alternative fuels at RRPP, focusing on the COG. It also considers the BFG program, and several additional opportunity fuels. It also considers the potential for oxygen-enhanced combustion at RRPP, a process now under consideration. The paper focuses upon the characteristics of the alternative fuels, the current and potential methods of firing these fuels, the typical frequencies and percentages of fuel supplied by COG and BFG, issues of potential combustion control, and the consequences of their use. Consequences include improved pulverizer performance and coal combustion, reduced airborne emissions, reduced ash management issues, and related mechanical systems considerations. INTRODUCTION The River Rouge Power Plant (RRPP) is a 651 MWe (net) generating station built in the 1950's to serve the increasing loads of Detroit Edison/DTE Energy, the largest single electric utility in Michigan. This plant, located in the industrial River Rouge city, is located in close proximity to steel mills, cement kilns, and major elements of the automobile industry. It is located on the Rouge River, immediately adjacent to a coke oven battery owned by the non-regulated portion of DTE Energy. It is also located immediately adjacent to an oxygen pipeline that serves the industry of this locale. The plant, shown in Figure 1, consists of one tangentially-fired boiler and one wall fired boiler. Both are equipped with split furnaces-with one furnace devoted to the raising of main steam and the other furnace raising reheat steam. 2 Figure 1. The River Rouge Power Plant of DTE Energy The River Rouge power plant was originally designed to fire bituminous coal. When Powder River Basin (PRB) subbituminous coal became available to Midwestern utilities at very favorable costs, RRPP began blending PRB coal with its traditional eastern bituminous coal. Today that blend is 15 - 25 percent eastern bituminous coal and 75 - 85 percent southern PRB coal from the Antelope and Black Thunder mines located on the 117 mile "Joint Line" of the Burlington Northern Santa Fe (BNSF) and Union Pacific (UP) railroads. In addition to these coals, RRPP fires coke oven gas (COG) from the coke batteries when it is available, and it fires some natural gas for flame stabilization. PRB coals from Wyoming are the base fuels, and the typical composition of these coals can be seen from Table 1. The mid-sulfur and high-sulfur eastern bituminous coals blended with the PRB coals are typically 12,600 - 13,000 Btu/lb, 32 - 37% volatile matter (as-received basis), 1 - 2.4% sulfur (up to 3.7 lb SO2/106 Btu), and 5.4 - 11.8% ash. 3 Table 1. Typical Composition of PRB Coals Burned At River Rouge Power Plant THE OPPORTUNITY FUELS PROGRAM Opportunity fuels have played a significant role in numerous power plants across the United States, and this use is well documented (see, for example, Dobrzanski, 2004; Tillman and Harding, 2004; Harding, 2008). They are locally available and can address technical, economic, and environmental issues. For DTE and particularly RRPP, opportunity fuels and unconventional firing strategies have long been considered and adopted as appropriate. Today they are increasingly considered for economic and environmental reasons as follows: • Fuel cost is 67 - 75% of the cost of generating electricity, and reducing fuel costs brings savings to consumers • Reduced fuel costs increases the position of the unit on the dispatch ladder, increasing the amount of generation at that unit and increasing the economic desirability of the unit • Alternate fuels of reasonable quality can help meet the new and coming EPA [and equivalent state] regulations with respect to NOx, Hg, and other constituents; among the coming regulations are the Cross State Air Pollution Regulations (CSPAR) and others 4 • Some alternate fuels can help address CO2 issues, sustainability issues (renewable resources) in a readily dispatchable manner OPPORTUNITY FUELS FOR RIVER ROUGE POWER PLANT Coke Oven Gas is the first of several opportunity fuels tested and used at RRPP. It is currently in use today, when it is available. Because availability is not consistent or constant,it can only be used as an inexpensive source of Btu's. Any environmental benefits are happenstance at this time. Additional opportunity fuels under consideration include blast furnace gas (BFG), dried paint solids, and dried biosolids (dried wastewater treatment sludges). There is also the possibility of firing one or more of the RRPP boilers with supplementary oxygen. More COG is the first and highest priority. A pipeline exists from the coke ovens on Zug Island to the power plant and, when the gas is available it is used. There are other claims on the gas supply as well. The COG is a highly desirable gas, as is shown in Table 2. Its use reduces the SO2 emissions, and facilitates the use of higher sulfur coal. It is a highly concentrated form of energy at 14,854 Btu./lb or 590 Btu/ft3. It also reduces the ash loading on the precipitator. Table 2. Chemical Composition of Coke Oven Gas (dry basis) 5 Because of the configuration of the boiler, COG is introduced below the coal burners in the wall fired unit (Unit #3). The burners are old (see Figure 2) and not optimized for the needs of modern combustion systems. Further, the burners are warped and in need of repair/replacement if more COG is to be burned on a consistent basis. The low impact of COG on NOx emissions in RRPP Unit #3 is illustrated in Figure 3, from Reaction Engineering International and their modeling of Unit #3. Figure 2. Burner for Coke Oven Gas However there are concepts to pipe BFG, this more dilute energy source to the power plant. In burning the BFG, the burners shown in Figure 2 may be used. However there is also the consideration that BFG will be available on a 24 hr-7 day/wk basis. Under that condition the BFG may be fired over the top of the coal burners in Unit #3, and over the top of the fireball in Unit #2, if it proves desirable as a NOx control strategy. Like COG, BFG has environmental benefits, particularly if fired over the top of the coal. These include: • Reducing NOx emissions • Reducing SO2 emissions, broadening available coals for use (no sulfur in the fuel) • Reducing furnace exit gas temperatures (FEGT) somewhat • Reducing particulate emissions • Reducing HAPS emissions At the same time it may prove to be a low cost fuel for RRPP-an essential consideration in the current economy. The COG burners were also designed for BFG. Currently the gaseous product from the blast furnace-effectively a coal gasifier of simple design-is not burned at RRPP. BFG has a typical composition as follows (from Kitto and Stultz, eds. 2005): H2: 2.4% CH4: 0.1% CO: 23.3% CO2: 14.4% N2: 56.4% H2O: 3.4% Btu/ft3: 83.8 6 Figure 3. The Upward NOx Mass Flow Profile for River Rouge Power Plant Unit #3 (from Reaction Engineering International) Opportunity fuels available to RRPP also include solid materials: dried paint solids and dried biosolids (dried wastewater treatment sludge) are among the options proposed. The dried paint solids are currently used; however only ~10,000 tons are available annually and this is but a very small portion of the fuel used at the plant. Dried biosolids have more potential if the logistics can be resolved. Currently they are incinerated by the city of Detroit, in some very old and inefficient equipment. Unless burned they must be landfilled. Consequently they are a low cost potential fuel. The key is drying the material before it is brought to the RRPP units. 7 Biosolids present a unique set of problems. The biosolids are substantially different from the coals being burned-or even the premium biomass fuels such as sawdust and other wood wastes. While the moisture will be determined by plant specifications, other characteristics are as follows: Proximate analysis (dry basis) Volatile matter: 51% Fixed carbon: 7% Ash: 42% Ultimate analysis (partial, dry basis) Carbon: 30.1% Hydrogen: 5.7% Nitrogen: 2.5% Sulfur: 1.4% Chlorine: ~0.5% Oxygen (by difference): ~18% Calorific value: 7700 Btu/lb (dry basis) This is a highly volatile fuel that has significant pollution potential. The fuel nitrogen can contribute to NOx formation while the sulfur concentration is ~3.6 lb SO2/106 Btu. The ash will become particulate matter to be captured in the precipitator. The chlorine contributes to the formation of KCl and NaCl resulting in corrosion in the boiler-and HCl resulting in cold end corrosion. Previous studies have shown that there is a strong eutectic effect from blending coal and sewage sludge in a boiler; this depresses ash fusion temperatures and increases the potential for slagging and fouling. This is a very low cost fuel-with good reason. It may or may not be used at RRPP. OTHER OPPORTUNITY FUEL PROGRAMS AT DTE ENERGY River Rouge Power Plant is not the only DTE Energy facility using or considering alternative fuels to reduce power generation costs. St. Clair Power Plant regularly fires waste oil from across the DTE system. This program has been very successful. St. Clair also tested firing wood chips with coal, processing the wood chips with coal in the EL pulverizers. It was an unsuccessful program and was abandoned. Monroe Power Plant has just completed short term testing of firing petroleum coke with coal in very large, supercritical, wall fired pulverized coal boilers. This was a successful test program which may lead to additional test programs with this and other opportunity fuels. CONCLUSIONS At DTE Energy, as illustrated by the River Rouge programs, alternate fuels must stand on their own economically and environmentally. Alternate fuels must significantly reduce fuel 8 costs and achieve favorable environmental impacts when fairly and objectively assessed. While these fuels must achieve fuel cost savings, they can not do so at the expense of operating and maintenance budgets. A low cost fuel that causes forced outages to address slagging and fouling or corrosion problems is not favored. The fuel price and the total fuel cost must both be favorable. This also includes capital investments. The increased use of coke oven gas (COG) has proven itself both economically and environmentally. Further increases in its use will be beneficial. Regular use of blast furnace gas can occur only when the capital is invested to make it possible and profitable. When that day comes, BFG can become a very large part of the fuel strategy. Dried paint solids are already a part of the fuel strategy; however there is an insufficient quantity to make a substantial contribution. Dried biosolids pose significant operating risks and operating costs. Dried biosolids represent the potential problems with alternative fuels in an electric utility setting. All of these studies, particularly at RRPP, illustrate the fact that alternate fuels may become part of fuels and combustion strategies for all utilities, depending upon local situations and local economics. When they fit into localized economic and environmental solutions, they can be very productive. Otherwise they are not prudent options. REFERENCES Dobrzanski, A. 2004. Opportunity Fuels: a Plant Perspective. Electric Power Conference, Chicago, IL. Harding, N.S. 2008. Characteristics of Alternative Fuels. in Combustion Engineering Issues for Solid Fuel Systems (Miller, B.G. and D.A. Tillman, eds.). Academic Press. Burlington, MA. Kitto, J.B. and S.C. Stultz. 2005. Steam: Its Generation and Use. 41st Ed. Babcock & Wilcox. Barberton, OH. Tillman, D.A. and N.S. Harding. 2004. Fuels of Opportunity: Characteristics and Uses in Combustion Systems. Elsevier. Amsderdam. |