the coking properties of coal at elevated pressures. - Argonne ...
the coking properties of coal at elevated pressures. - Argonne ... the coking properties of coal at elevated pressures. - Argonne ...
60 50 aJ > .z 40 rn - 2 30 aJ L 3 g 20 0 R3 M 10 60 al > .r 2 40 7 E al a, L c.’ 3 a 2 20 zs 0 1 2 3 4 5 6 Ca/S Molar Ratio Figure 3. Relative SO Capture, Sorbent Injected With the Staged Air Internal Mo$e With Heat Extraction in the Radiant Zone. ,/so2 (ca/s) Figure 4. The Impact of Load and Rad ant Zone Cooling on Sulfur Capture 162
Tests have been carried out to determine the influence of burner zone stoichiometry on sulfur capture in the internally staged mode. Provided the burner zone Stoichiometry does not rise above 80% the sulfur capture appears almost to be independent of burner zone stoichiometry, However as the staging air is reduced to a minimum and the burner zone becomes fuel lean the sulfur capture is reduced. these experiments this reduction is probably caused by a reduction in sorbent velo- city and because of the increase in peak flame temperatures as the burner zone stoichiometry increases. In the internal staging mode thermal environment has a very significant impact upon sulfur capture. This is illustrated by the data presented in Figure 4 which shows the sulfur capture in both the first two zones as a function of the product of the calcium to sulfur ratio and the square root of the sulfur dioxide concentration in that zone. Three conditions are shown: high load with and without radiant zone cooling and low load without cooling. Reducing the load will lower temperatures and increase residence times. At high load cooling the radiant zone dramatically increases the sorbent reactivity. At low load reactivity in the radiant zone is less than that with cooling at high load but the reactivity in the post flame section is similar to the high load, cooled case. External Staging The purpose of the external staging tests was to determine whether sulfur dioxide emissions could be reduced by adding limestone under reducing conditions and then burning the fuel completely by the addition of second stage air downstream. This requires that the majority of the sulfur captured under reducing conditions be retained by the sorbent as the fuel burns out. Equilibrium calculations indicate that under fuel rich conditions hydrogen sulfide is the dominant sulfur species while measurements in the fuel rich region indicate that sulfur dioxide, hydrogen sulfide and carbonyl sulfide all are present. Sulfur dioxide concentrations decrease and hydrogen sulfide concentrations increase as the primary zone stoichiometry decreases. Thus, the sorbent may react with any of three sulfur species. Initial reaction rates for the reaction of H2S and COS with CaO have been measured (7, 8) and are similar. The data from two different external staging experiments are shown in Figures 5 and6. In one experiment sorbent was added with the coal (location 1) and measurements of sulfur species at the exit of the rich zone (port 2) were made with and without sorbent. Figure 5 shows the percent capture of SO2, COS and H2S as a function of first stage stoichiometric ratio. It can be seen that all three species were captured. The data in Figure 6 are from an experiment comparing calcium utilization firing with two fuels, coal and propane doped with H2S to give the same sulfur con- tent as the coal. (location 2) and the staging air was added in the post flame cavity (location b). SO2 was measured with and without sorbent for both fuels at sample port 3 (exit of furnace). Total calcium utilization as a function of first stage stoichiometry is shown in Figure 6. Measurements indicate that as much as 50 percent of the input coal remains as solid at the lower first zone stoichiometries. The data for coal presented in Figure 6 has been plotted as a function of the actual gas phase stoichiometry. decreasing gas phase stoichiometric ratio for coal but increased for propane doped with H2S. It should be noted that the data shown in Figure 6 represent the sum of sulfur species capture under reducing conditions in the first zone, retention of sulfur during burnout and the sulfur capture under oxidizing conditions in the second stage. 4. CONCLUSIONS The sorbent was added at the base of the post flame section It can be seen that the ultimate sulfur capture decreased with An investigation has been carried out in a bench scale facility to determine 163 In
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60<br />
50<br />
aJ ><br />
.z 40<br />
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2 30<br />
aJ<br />
L<br />
3<br />
g 20<br />
0 R3<br />
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a, L<br />
c.’ 3<br />
a<br />
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1 2 3 4 5 6<br />
Ca/S Molar R<strong>at</strong>io<br />
Figure 3. Rel<strong>at</strong>ive SO Capture, Sorbent Injected With <strong>the</strong> Staged Air<br />
Internal Mo$e With He<strong>at</strong> Extraction in <strong>the</strong> Radiant Zone.<br />
,/so2 (ca/s)<br />
Figure 4. The Impact <strong>of</strong> Load and Rad ant Zone Cooling on Sulfur Capture<br />
162
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