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

Figure 4-2. Sawdust Being Transported Up the Walking Floor Unloader

Figure 4-3. Sawdust Being Screened on the Disc Screen

Figure 4-4. Sawdust Being Metered on the Weigh Belt Feeder

Figure 4-5. The Sawdust Injector in the Albright Boiler #3

Table 4-1. Typical Analyses of the Coal and Sawdust at Albright Generating Station

Cofiring at Albright Unit #3 was performed at levels up to 10 percent (mass basis, or 4.7 percent on a heat input basis). This is equivalent to firing 6 ton/hr of sawdust at full load operations. Excess O2 varied from 2.5 percent to 4.9 percent during the test program. There are three SOFA levels at Albright Unit #3, with each level having dampers with 0 – 100 percent open capability. During the testing, SOFA damper positions ranged from 15 percent open (dampers were opened only on one level) to 240 percent open (from a total of 300 percent).

Cofiring did not significantly impact either opacity or CO emissions, as is shown in Figures 4-7 and 4-8. These emissions also are an indication of carbon conversion efficiency. Note that there is a slight apparent upward trend in opacity largely as a result of one 4-hour period of time. For the vast majority of operations, opacity remained between 3 and 4 percent. It is useful to note that cofiring did not increase unburned carbon in flyash or bottom ash appreciably, also supporting the data in Figures 4-7 and 4-8.

Figure 4-7. Opacity Emissions (Percent) as a Function of Cofiring at Albright Generating Station.

Cofiring Percentage, Mass Basis

Figure 4-8. Influence of Cofiring on Carbon Monoxide Emissions at Albright Generating Station

The data presented in Figures 4-7 and 4-8 relates to normal full load (130 MWe net) operation. There was no evidence of increased opacity or CO emissions at part load operation.

SO2 emissions generally decreased as a function of fuel substitution. The inherent variability in the fuel quality results in some scatter in the data generated. These SO2 results are presented in Figures 4-9 and 4-10 on a ppmv basis.

Figure 4-9. SO2 Emissions as a Function of Cofiring at Albright Generating Station (mass basis)

Cofiring Percentage, Heat Input Basis

Figure 4-10. SO2 Emissions as a Function of Cofiring at Albright Generating Station (heat input basis)

The cofiring demonstration at Albright Generating Station has reduced NOx emissions as a function of cofiring level and the management of other known NOx – causing variables: excess O2 and SOFA usage. Figure 4-11 depicts the NOx reductions achieved. Note that the variability implied by the data results from the numerous variables employed to achieve NOx reduction.

Cofiring Percentage, Mass Basis

Figure 4-11. NOx Reduction from Cofiring at the Albright Generating Station

The data generated during the testing were converted into a single, robust, regression equation as shown below. The r2 for this equation is 0.873 with 68 data points, or 67 degrees of freedom.

NOx (lb/106 Btu) = 0.361 – 0.0043(W%) + 0.0217(O2%) – 0.00055(SOFA) [16]

Where, W% is sawdust percentage in the fuel on a mass basis, O2% is the percentage excess oxygen measured at the furnace exit, and SOFA is the total percentage of the three SOFA dampers expressed as percent open. Note that the range of W terms is 0 – 10, the range of O2% terms is 2.5 – 4.9, and the range of SOFA terms is 15 – 240. This may explain the difference in the coefficients.

Table 4-2 demonstrates the robustness of equation 16. It presents the probabilities that any given calculated value occurs randomly. Any probability <0.05 can be considered significant; any probability <0.01 can be considered highly significant.

Table 4-2. Probability That The Equation, or any Term, Occurred Randomly

Figure 4-12 compares the observed NOx emissions to those predicted by this equation. Note that there is some deviation from the trend predicted; the equation clearly lacks all explanatory terms. Note, also, that the equation explains the vast majority of the NOx reduction observed during the test program.

Figure 4-12. Comparison of Observed and Predicted NOx Emissions Based Upon Equation [16]

The NOx reduction observed was, therefore, consistent and substantial. To further understand the mechanisms of NOx reduction involved in cofiring at Albright Generating Station, fuel characterization experiments were conducted at Pennsylvania State University evaluating the pattern of nitrogen evolution from the sawdust and coal at Albright Generating Station (Johnson et. al., 2001a; Johnson et. al., 2001b). These experiments were patterned after work by Baxter et. al. (1996c) concerning the evolution of volatile nitrogen from coals. Both the sawdust and Albright coals were analyzed. The fuels were reacted in a drop tube reactor (DTR) at temperatures ranging from 400oC to 1700oC. The total

volatile matter evolution was measured, along with the total nitrogen evolved as volatile matter and the total carbon evolved as volatile matter. Figures 4-13 and 4-14 present the results for the sawdust; Figure 4-15 presents the results for the coal burned at Albright. Note that Figures 4-13 and 4-15 present the data in terms of DTR temperature. Figure 14 normalizes the volatile evolution from the sawdust to the total volatile evolution from the coal as another means of analysis.

Temperature (C)

Figure 4-13. Nitrogen Volatile Evolution from Sawdust as a Function of Temperature

Percent Volatile Matter Evolved

Figure 4-14. Nitrogen Volatile Evolution from Sawdust Normalized to Total Volatiles