Role of coal maceral composition in reducing sulfur dioxide and oxides of nitrogen emissions from pulverized coal flames

Coal Science J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved.

1815

R o l e o f Coal Macera! C o m p o s i t i o n in R e d u c i n g Sulfur D i o x i d e and O x i d e s o f N i t r o g e n E m i s s i o n s from P u l v e r i z e d Coal F l a m e s S. Rajan a and J.K. Raghavan b aDepartment of Mechanical Engineering and Energy Processes, Southern Illinois University at Carbondale, Carbondale, Illinois 62901-6603 USA bBattelle Columbus Laboratories, Columbus, Ohio USA 1. INTRODUCTION With the wide variety of coals of different petrographic origin currently in use, a primary concern of large scale utility and industrial size coal fired plants is the acid rain precursor emissions such as oxides of nitrogen and sulfur dioxide. These emissions are influenced by fuel nitrogen and fuel sulfur content as well as combustion temperatures and combustion histories. Recent investigations have shown that the volatiles evolution and char combustion patterns are dependent not only on coal chemical composition, but also on its maceral composition. Thus, it is reasonable to expect that through a combination of effects, controlled by the petrographic distribution of fuel sulfur and nitrogen as well as the individual reactivities of the coal macerals themselves, the production histories of the oxides of sulfur and nitrogen and their final emission levels will be influenced by the coal petrographic composition. The present paper investigates these interactions. 2. EXPERIMENTAL COMBUSTION EQUIPMENT AND PROCEDURE

A number of coals were selected from the Pennsylvania State University Coal Bank, which had as far as possible the same total sulfur content and were of the same rank (high volatile bituminous). The variation in the total sulfur content of the coals was less than 6 percent. Simultaneously, it was desired to maintain the fuel sulfur content as constant as possible among the selected coals. Of the seven coals selected, the nitrogen content of three of the coals varied less than 2% while the maximum variation was 20% from the mean. The coals were also selected to provide fi wide distribution in the maceral composition so that the parametric effect of these macerals could be investigated. For comparison purposes, it is desirable to minimize the specific influence of a particular furnace design such as flow patterns, turbulence characteristics, etc. Hence, a Meker type burner capable of sustaining an unaugmented coal-dust-air flame was selected as the test apparatus over other laboratory devices. The design of the Meker burner is similar to that used by other workers (1, 2). A schematic of the test apparatus is shown in Figure 1. The test coal, ground to <325 mesh, is fed from a fluidized bed coal dust feeder and a stable coal-dustair flame is established at the mouth of the 6.3 cm diameter burner. Gas and particulate samples were extracted at different heights of the coal flame, using a 0.46 cm, i.e., S.S. water cooled probe. Samples from the probe were carried by 0.625 cm o.d. heated Teflon tubing through in-line filters for particulate collection. The gas samples from the flame were analyzed for CO, CO2, 02, SO2 and NO x concentrations. The flame temperatures at the different positions in the flame at specified heights from the stabilizing S.S. screen, were measured using a 0.127 mm grounded Pt/Pt 13% Rh thermocouple.

1816

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Table 1 shows the organic maceral compositions of the coals tested. This data was supplied with the coals when purchased from the Penn State Coal Bank and was not measured in this investigation. The liptinitic macerals contain a variety of sub-maceral groups and are generally richer in hydrogen content than other maceral groups. The hydrogen content of vitrinites is intermediate between that of the liptinites and the inertinite macerals. Oxygen contents of the vitrinites are often higher than that of liptinite. The trends in hydrogen content also reflect the volatile matter content of the maceral groups. These compositional differences influence the combustion and emissions data of the coals. Table 1 Maceral Composition of Constant Sulfur Coals (dry basis) Sample I.D. # PSOC 828 PSOC 672 PSOC 832 PSOC 796 PSOC 831 PSOC 671 PSOC 1109

Vitrinite (wt.%) 54.2 72.2 51.1 89.1 75.6 75.8 19.1

Inertinite (wt.%) 22.3 12.8 20.9 2.7 15.2 7.3 2.8

Liptinite (wt.%) 15.9 0.5 18.6 2.9 4.7 0.7 49.6

Mineral Matter (wt.%) 7.6 14.5 9.3 5.2 4.6 16.2 28.5

To evaluate the influence of maceral composition, the test conditions for the various coals were kept the same. To secure a stable flame at the burner mouth without oxygen enrichment, it was necessary to use ~ fuel rich mixture with a whole coal equivalence ratio of 3.25. The corresponding coal concentration varied from about 322 to 364 mg/liter of air. The cold gas velocity was maintained fairly constant at about 10 cms/sec. 3.1 Influence of Maceral Composition or Flame Temperature Profiles Figure 2 shows the flame temperature profiles for three of the coals tested. PSOC 796 has the highest vitrinite content of 89.1% and nominal amounts of inertinite and liptinite. The coal with the highest liptinite content was PSOC 1109, and it has the lowest vitrinite content,

1817 while PSOC 828 had the highest inertinite content of the coals tested. The individual temperature profiles are all highest near the flame stabilizer grid and gradually decrease at further distances downstream due to cooling and gas dispersion effects. Also, the volatiles release and combustion are most pronounced near the burner mouth. Distinct differences may be observed in the flame temperature profiles of these coals specifically between PSOC 828 and 796 on the one hand, and 1109 on the other. Although the vitrinite contents are different, both PSOC 828 and 796 exhibit similar temperature profiles with a maximum value of around 2000~ near the burner mouth. For the first 0.5 cm, the profiles are identical; however, the temperature profile of PSOC 828, which contains a higher fraction of liptinites is higher in the 0.5 to 1.5 cm region. It was also observed that at about 3.0 cm, the temperature from the high vitrinite 796 coal was beginning to drop off more sharply. In contrast, the very high liptinite coal PSOC 1109 yielded much lower temperatures throughout the flame, which remained fairly steady and did not drop off so rapidly as compared to the other two coals. As is generally accepted, liptinites being high hydrogen content fuels are necessary for good combustion. However, the temperature profiles indicate that too high a liptinite content may not be desirable to achieve high flame temperatures, and liptinite contents of about 5 to 10% may be the optimum value for good combustion performance. 3.2 Effect of Macerals on Combustion Characteristics Figures 3-5 show the gaseous emissions profiles for the high vitrinite, high inertinite and high liptinite flames. Peak carbon dioxide levels for PSOC 796 and 828 are of the same order of magnitude, consistent with the temperature profiles discussed above; however, oxygen utilization for high vitrinite PSOC 796 is lower at further distances downstream. Also carbon monoxide levels are somewhat lower. For the high liptinite coal PSOC 1109, carbon dioxide levels were the lowest as seen from Figure 5, being on the order of 6%. PSOC 828 also had the highest amount of volatiles, and as observed from Figures 3 and 4, the peak carbon dioxide levels are reached closer to the burner grid, in the devolatilization region. This data suggests that a mixture of vitrinite, liptinite and inertinite in the right proportion, is more to be desired in coals in order to obtain peak combustion performance, and good carbon burnout. 3.3 Effect of Maceral Composition on Oxides of Nitrogen and Sulfur Dioxide Emissions Figure 6 and 7 highlight the important differences resulting from maceral composition on the sulfur dioxide and oxides of nitrogen measured from the high vitrinite and high liptinite flames. Although the total sulfur content of the flames is the same, the combustion conditions as described above, result in somewhat different concentrations of these emissions from the flames. Even though the temperature is lower, PSOC 1109 produced somewhat higher sulfur dioxide levels than high vitrinite PSOC 796, Figures 6 and 7. Also the evolution profiles can be seen to be quite different. Peak values of sulfur dioxide are reached close to the end of the devolatilization region, followed by a reduction of sulfur dioxide values. This is considered to be brought about by a char reduction mechanism in the case of PSOC 796, which produces fi porous char unlike that of PSOC 1109. The data further suggests that the sulfur distribution in PSOC 1109 may be highly concentrated in the volatiles fraction of the coal similar to that of 796, but without the benefit of the char reduction mechanism. While the oxides of nitrogen profiles are similar to the sulfur dioxide profiles, PSOC 1109 exhibited higher levels of fuel nitrogen conversion than PSOC 796, even though the latter coal had slightly higher fuel nitrogen and higher gas temperatures. This suggests that in high liptinite coals, fuel bound nitrogen escapes the coal particle much easier and is converted to its oxides more readily than in high vitrinite coals. High vitrinite coals possibly retain much of the fuel nitrogen in the char mass and depend on the char burnout for complete oxidation.

1818 REFERENCES 1. Milne, T.A. and Beachey, J.E., "The Microstructure of Pulverized Coal-Air Flame 1. Stabilization on Small Burners and Direct Sampling Techniques," Combustion Science and Technology, Vol. 16, 123-128, 1977. 2. Altenkirch, R.A., Peck, R.E. and Chen, S.L., "The Appearance of Nitric Oxide and Cyanide in One-Dimensional Coal Dust/Oxidizer Flames," Combustion Science and Technology, Vol. 20, 49-58, 1979. 24

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