reading / DOE docs / 5 DOE-EPRI 2001 cofiring Biomass w. Coal

Temperature (C)

Figure 5-3. Nitrogen and Total Fuel Volatile Evolution Patterns for Black Thunder Coal

Temperature (C)

Figure 5-4. Volatile Evolution Patterns for Caballo Rojo Coal

Temperature (C)

Figure 5-5. Volatile Evolution Patterns for Shoshone Coal

results from the fact that the boiler was originally designed for Illinois Basin coal and was converted to lower sulfur, lower Btu, lower efficiency PRB-western coal blends.

Table 5-3. Test Conditions at the Michigan City Cofiring Test Program

Figure 5-6. Impact of Cofiring on Cyclone Feeder Speeds at Michigan City Cofiring Tests

5.2.2.2. Efficiency Impacts

Efficiency impacts of cofiring are shown in Table 5-4. There is no apparent impact associated with cofiring, however, Caballo Rojo coal at 8424 Btu/lb was used in the baseline testing and Black Thunder coal was used during the cofiring tests. Additional calculations show that, if the PRB coal type is held constant, there may be an efficiency loss of ~0.3 - 0.5 percent when cofiring at 10 percent wood waste by mass (6.5 percent biomass on a heat input basis). It is significant, however, that cofiring did not cause an increase in unburned carbon in the flyash, cause an increase in excess O2 required, or cause an increase in air heater exit temperature.

The apparent increase in boiler efficiency associated with Test 2 and Test 5 is an artifact of low load testing. Under low load testing the air heater exit temperature declined dramatically, creating this phenomenon. The efficiency number is an economic number, limiting the economic value of biofuel. As PRB coal has demonstrated, low cost fuels having a modest negative impact on heat rate can still be economically desirable in coal-fired boilers.

Table 5-4. Efficiency Measurements During Cofiring Tests at Michigan City Generating Station

5.2.2.3. Temperature Impacts of Cofiring.

The economizer exit temperature and the air heater exit temperature were not impacted by cofiring. Similarly, the recirculated flue gas temperature was not impacted by cofiring. Figure 5-7 illustrates the impact of cofiring on calculated combustion/flame temperatures while Figure 5-8 illustrates the impact of cofiring on measured furnace exit gas temperature (FEGT). Note that there is an apparent slight rise in flame temperature potentially caused by the volatility of the biofuel and the switch in PRB coals. Note, also, that there is a decrease in measured FEGT when cofiring.

The phenomenon creating the drop in FEGT is typically that the highly volatile wood waste ignites rapidly in the exit of the scroll section of the cyclone barrel, helping to ignite the coal and thereby increasing the relative percentage of combustion occurring in the cyclone barrel. This decreases the combustion in the primary furnace, thereby reducing FEGT.

Figure 5-7. Estimated Flame Temperatures During Cofiring at Michigan City Generating Station

Cofiring Test

Figure 5-8. Furnace Exit Gas Temperatures Measured During Cofiring Tests at Michigan City Generating Station.

5.2.2.4. Environmental Impacts of Cofiring at Michigan City Generating Station

Cofiring at Michigan City Generating Station reduced SO2, NOx, and opacity emissions as shown in Table 5-5. The primary environmental impacts of consequence are impacts on NOx formation and on fossil CO2 mitigation. Figures 5-9 and 5-10 illustrate the impact of cofiring on NOx emissions, with Figure 5-10 focusing on FEGT influences.

Table 5-5. Airborne Emissions From Cofiring at Michigan City Generating Station

Test Number

Figure 5-9. Impact of Cofiring on NOx Emissions at Michigan City Generating Station

Furnace Exit Gas Temperature (F)

Figure 5-10. Impact of Cofiring on FEGT, and the Consequence for NOx Emissions

There was an apparent 9.5 percent decrease in NOx emissions caused by a 6.5 percent cofiring (heat input basis). This decrease was consistent with numerous other test experiences as presented previously. The impact of cofiring on NOx emissions can be directly attributed to fuel volatility, and its impact on the combustion process as shown in equation [17].

Where NOx is measured in lb NOx (as NO2)/106 Btu, FN is fuel nitrogen in lb/106 Btu, O2 is the percentage of oxygen in the gaseous combustion products measured at the economizer exit, and FEGT is furnace exit gas temperature, measured in oF. The r2 for this equation is 0.979. The probability of random occurrence for each variable is as follows: intercept, .0023; FN, .0007; O2, .2832; and FEGT,

.0001. Only the O2 term is weak. The wood waste reduced the fuel nitrogen content and, because of its volatility, reduced the FEGT. It is significant to note, however, that the reduction in FEGT did not cause any reduction in main steam or reheat steam temperature.

The tests at Michigan City demonstrated the potential of cofiring as an environmental tool, while also documenting some of the operational benefits and concerns. The potential shown from these tests led to a triburn demonstration at the Bailly Generating Station of NIPSCO.

5.3.THE TRIBURN DEMONSTRATION AT BAILLY GENERATING STATION

The Bailly Generating Station triburn project has been reported extensively in the literature (Hus and Tillman, 2000; Tillman and Hus, 2000; Tillman, 1999a; Tillman, 1999b; Tillman, 1999c). In this demonstration, NIPSCO sought to obtain the benefits of biomass cofiring while overcoming the efficiency penalty that has been common to most biomass cofiring programs.

To pursue this opportunity, NIPSCO worked within the framework of the EPRI-USDOE Cooperative Agreement and developed a long term demonstration firing biomass and petroleum coke with coal at its Bailly Generating Station boiler #7, a 160 MWe (net) cyclone boiler. The NIPSCO demonstration program—the triburn program—involved designing and constructing a fuel preparation and blending facility. It then involved extensive testing of firing clean urban wood waste—biomass—with coal, firing petroleum coke with coal, and firing various blends of urban wood waste and petroleum coke with coal. Results of the extensive testing program have shown that the triburn blends of biomass and petroleum coke with coal have accomplished the following: 1) increased boiler efficiency, 2) reduced fuel costs; and 3) reduced emissions of oxides of nitrogen (NOx), mercury, and fossil carbon dioxide (CO2). At the same time, the triburn program has not increased other emissions.

5.3.1. Overview of the Program

Bailly Generating Station Boiler #7 is a 160 MWe (net) cyclone boiler generating about 1.2x106 lb/hr of 2400 psig/1000oF/1000oF steam. It is typically fired with a blend of 70 percent Illinois Basin coal/30 percent Shoshone coal. It is equipped with a Pure Air scrubber for SO2 management. This boiler was