Energy Fuels 2010, 24, 6411–6416 Published on Web 11/29/2010
: DOI:10.1021/ef101116s
Investigation of Nitrogen Release during Coal Pyrolysis in an Oxy-fuel Combustion Process John M. Sowa, Kolbein K. Kolste, and Thomas H. Fletcher* Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602-0002, United States Received August 19, 2010. Revised Manuscript Received October 27, 2010
Three coals were studied in a flat flame burner (FFB) to determine nitrogen release during pyrolysis as a function of the temperature from 1600 to 1900 K in an oxy-fuel environment. The oxy-fuel environment was created by replacing the background N2 with CO2 in the FFB experiments. Both types of experiments were conducted under fuel-rich conditions, with no O2 present in the postflame gases. The oxy-fuel results were compared to previous results from FFB experiments in a N2 atmosphere. A bituminous coal showed no difference in mass and nitrogen release between the two environments. The sub-bituminous coal exhibited higher mass and nitrogen release in the oxy-fuel (enhanced CO2) environment. The higher mass and nitrogen release in the oxy-fuel condition implies that another reaction is occurring with the subbituminous coal for these conditions, which is likely CO2-char gasification.
at the same post-flame oxygen concentration, the devolatilization is similar. The different gas properties in oxycombustion might have some effect on the nitrogen release and reduction rate. Borrego and Alvarez5 reported that chars from high- and low-volatile bituminous coals combusted in a TGA burned slower in a CO2 environment than in a N2 environment. Hu et al.6 reported the likely influence of CO2 gasification at high reaction temperatures (1250-1500 °C), low oxygen levels, and low-rank sub-bituminous coal. Rathnam et al.7 showed increased volatile yields in CO2 compared to N2 at 1673 K, inferring possible CO2 gasification during devolatilization. If gasification occurs during pyrolysis in oxycombustion systems, more nitrogen would be released and, subsequently, reduced by low-NOx burners, accounting for some of the observed difference in the NOx concentration. The purpose of this study was to identify any differences in single-particle nitrogen release during pyrolysis between airfired and oxy-fuel environments.
Introduction Increasingly stringent regulations from the government have led to the consideration of new technologies for coal-fired power plants. One of these technologies is oxycombustion or oxy-fuel for short. In this process, oxygen is separated from the nitrogen in the air and fed directly to the boiler. To reduce flame temperatures, the flue gas is partially recycled. The remaining flue gas is a concentrated CO2/H2O stream that can be separated easily and followed by CO2 sequestration. During pilot-scale oxy-combustion tests, NOx was reduced by as much as 50%.1 In fact, some tests have shown low enough NOx levels to eliminate using selective catalytic reduction (SCR) at the end of the process.2 Elimination of the SCR unit would result in significant cost reduction. The mechanism of this enhanced NOx reduction in oxycombustion is being debated. Some contest that recycling the flue gas leads to reduced NOx when it passes through the fuelrich zone in the burner.3 The NOx reduction could also be a result of increased nitrogen release during pyrolysis because of a different environment for ignition and devolatilization. Molina and Shaddix studied ignition times for both traditional N2- and CO2-rich combustion atmospheres at 1250 K.4 They found that the ignition time is longer in CO2 atmospheres for the same oxygen concentration. They concluded that the difference in heat capacity and diffusivity between CO2 and N2 was responsible for this increase in time and that,
Experimental Section Coal Sample and Preparation. Three coals (Illinois #6, Pittsburgh #8, and Black Thunder) were used in this pyrolysis study. The coals were ground and sieved to the 45-75 μm diameter size range. The average mass mean particle diameter was measured using a LS Series Coulter Counter. Each coal was run through the machine 8 times, and the measured diameters were averaged to obtain a mass mean diameter for each coal. Mass mean diameters were 70 μm for the Illinois #6 coal, 80 μm for the Black Thunder coal, and 50 μm for the Pittsburgh #8 coal.
*To whom correspondence should be addressed. E-mail: tom_fletcher@ byu.edu. (1) Buhre, B. J. P.; Elliot, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Oxy-fuel combustion technology for coal-fired power generation. Prog. Energy Combust. Sci. 2005, 31, 283–307. (2) Sangras, R.; Ch^atel-Pelage, F.; Pranda, P.; Farzan, H.; Vecci, S. J.; Lu, Y.; Chen, S.; Rostam-Abadi, M.; Bose, A. C. Oxycombustion process in pulverized coal-fired boilers: A promising technology for CO2 capture. Proceedings of the 29th Technical Conference on Coal Utilization and Fuel Systems; Clearwater, FL, April 18-22, 2004. (3) Okazaki, K.; Ando, T. NOx reduction mechanism in coal combustion with recycled CO2. Energy 1997, 22, 207–215. (4) Molina, A.; Shaddix, C. R. Effect of O2/CO2-firing on coal particle ignition. Proceedings of the Annual International Pittsburgh Coal Conference; Pittsburgh, PA, Sept 12-15, 2005; pp 268/1-268/9. r 2010 American Chemical Society
(5) Borrego, A. G.; Alvarez, D. Comparison of chars obtained under oxy-fuel and conventional pulverized coal combustion atmospheres. Energy Fuels 2007, 21 (6), 3171–3179. (6) Hu, S.; Zeng, D.; Mackrory, A. J.; Sayre, A. N.; Sarv, H. Effects of CO2 on char oxidation and ignition during oxy-coal combustion. Proceedings of the 26th International Pittsburgh Coal Conference; Pittsburgh, PA, 2009. (7) Rathnam, R. K.; Elliott, L. K.; Wall, T. F.; Liu, Y.; Moghtaderi, B. Differences in reactivity of pulverized coal in air (O2/N2) and oxy-fuel (O2/N2) conditions. Fuel Process. Technol. 2009, 90, 797–802. (8) Ma, J. Soot formation during coal pyrolysis. Ph.D. Dissertation, Department of Chemical Engineering, Brigham Young University, Provo, UT, 1996.
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: DOI:10.1021/ef101116s
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Figure 1. Schematic of the FFB system.
Apparatus and Operation. The flat flame burner (FFB) used by Ma8,9 and Zhang10,11 was employed in this study with several modifications. A schematic diagram of the FFB is presented in Figure 1. Coal particles were fed by a syringe-type particle feeder at a rate of ∼1 g/h. The particles were entrained in a nitrogen stream and carried up through the center of the burner surface. Particles then traveled up the centerline of the reactor, where they underwent pyrolysis shortly after passing through the flame. The resulting pyrolysis products were collected by a variableheight, water-cooled nitrogen-quenched probe. A virtual impactor and cyclone aerodynamically separated the char from the polyaromatic hydrocarbons and light gases. The fuel used for these experiments was CO-stabilized with H2 because the broad flammability limits of CO allow for stable flames between 1100 and 2000 K, as explained by Zhang.10,11 The gas temperature in the FFB was changed by adjusting the flow of CO, H2, O2, and N2 or CO2 to the burner. Oxycombustion settings used CO2 as the diluent gas, while traditional combustion settings used N2. Both settings used N2 as the carrier gas to entrain the coal. Because the flow of carrier gas was 0.0367 standard liters per minute (slpm), it was considered negligible compared to the 30 slpm of other gases flowing to the burner. Gas temperature profiles were measured with a type B thermocouple, coated with SiO2 to prevent catalytic reactions on the bead. All thermocouple measurements were corrected for radiation. Actual corrected centerline peak temperatures for the oxyfuel conditions studied were 1580, 1699, 1789, and 1896 K. The 1896 K setting caused some of the burner tubes to clog after a short period, creating channeling and changing the temperature profile. To remedy this problem, a profile with a corrected peak of 1909 K was obtained by operating the burner with inverted settings. The inverted settings on the FFB are created when the fuel feed and the oxidizer feed are switched, so that the oxidizer runs through the fuel tubes and the fuels runs through the honeycomb. This procedure was effective at preventing clogging.
Figure 2. Centerline gas temperature profiles for all conditions (a) with N2 as the background gas and (b) with CO2 as the background gas.
Temperature profiles were also collected for N2-diluted pyrolysis conditions in the same manner as explained for the CO2diluted conditions. The corrected centerline peak temperatures for these conditions were 1546, 1628, 1712, and 1886 K, respectively. Only the Black Thunder and Illinois #6 coals were pyrolyzed in the N2 environment in this study. While the peak temperatures were not exact matches to the CO2-diluted conditions, they allowed for a point of reference at similar positions. Gas temperature profiles were collected for all four conditions at locations of 0-5 in. above the burner along the reactor centerline. Figure 2 shows the centerline temperature profile for each condition. Enough sample was collected for one replicate of analyses, except for Illinois #6 at 1800 K CO2 and Black Thunder at 1900 K CO2, which had two replicates of analyses performed, and the results were averaged. Product Characterization. Coal pyrolysis experiments were conducted at each of the four temperature conditions with each of the three coals, collecting approximately 600 mg of char from each coal at each condition. Ultimate analysis and ash elemental analysis by inductively coupled plasma (ICP) were performed by Huffman Laboratories in Golden, CO. The proximate analyses were performed in house following the procedures outlined by the American Society for Testing Materials (ASTM). Table 1 shows the ultimate and proximate analyses for these coals.
(9) Ma, J.; Fletcher, T. H.; Webb, B. W. Conversion of coal tar to soot during coal pyrolysis in a post-flame environment. Symp. (Int.) Combust., [Proc.] 1996, 3161–3167. (10) Zhang, H. Nitrogen evolution and soot formation during secondary pyrolysis. Ph.D. Dissertation, Department of Chemical Engineering, Brigham Young University, Provo, UT, 2001. (11) Zhang, H.; Fletcher, T. Nitrogen transformations during secondary coal pyrolysis. Energy Fuels 2001, 15, 1512–1522.
Data Analysis Mass and Nitrogen Release. The mass release was calculated using Si, Al, and Ti in the ash as tracers. Equation 1 6412
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Table 1. Ultimate and Proximate Analyses of Coalsa coal
ash (wt %, dry)
volatile matter (wt %, daf)
C (wt %, daf)
H (wt %, daf)
N (wt %, daf)
Ob (wt %, daf)
S (wt %, daf)
Illinois #6 Pittsburgh #8 Black Thunder
8.08 9.71 6.25
51.6 45.9 50.2
75.08 81.6 71.19
5.55 5.79 5.3
1.28 1.33 1.02
15.33 7.23 21.91
2.77 4.05 0.58
a
daf = dry and ash-free basis. b Calculated by difference.
gives the formula for finding mass release based on the titanium trace element. Here, MRdaf is the mass release on a dry and ash-free basis, xTi, coal is the titanium fraction in the coal, xTi,char is the titanium fraction in the char, and xTi,ash is the titanium fraction in the ash. xTi, coal 1xTi, char ð1Þ MRdaf ¼ xTi, coal 1xTi, ash The nitrogen release was calculated using eq 2, where xN,daf char is the mass fraction of nitrogen in the char, xN,daf coal is the mass fraction of nitrogen in the coal, and MRdaf is the mass release calculated on a dry and ash-free basis. The mass and nitrogen release were calculated at each reactor condition for the three coals. ! xN, daf char ð2Þ % N release ðdafÞ ¼ 1 - ð1 - MRdaf Þ xN, daf coal
Figure 3. Predictions of mass release for the Black Thunder coal in the 1600 K gas condition with either N2 or CO2 as the background gas.
Chemical Percolation Devolatilization (CPD) Model. The CPD model, developed by Fletcher et al.,12 was used to predict both nitrogen and mass release. The CPD model has been quite successful in predicting coal devolatilization behavior over a large range of coal types, heating rates, temperatures, and pressures. The CPD model uses a generalized formulation for chemical structure based on solid-state nuclear magnetic resonance (NMR) data along with the elemental composition to predict mass release, gas speciation, and nitrogen partitioning. In the absence of NMR data, a correlation for chemical structure parameters13 was used to estimate parameters based on ultimate and proximate analyses. A transient particle energy equation was also solved to obtain the particle temperature as a function of time based on the measured gas temperature and velocity profile in a manner similar to that by Zhang and Fletcher.11 The original version of the CPD model was developed to run in a N2 environment, but the CPD code was modified in this study to account for gas property differences between CO2 and N2. Figure 3 demonstrates that the gas property differences had little effect on the predicted overall mass release at complete pyrolysis at 2 in. in the 1600 K gas condition in the FFB for the Black Thunder coal.
surface, corresponding to a residence time of 15 ms. These data are compared to similar, previous experiments conducted on the Illinois #6 and Black Thunder coals in the FFB system by Zhang and Fletcher11 with a N2 background. The solid lines in these figures represent predictions performed using the CPD model12 in a N2 background. Zhang reported a mass mean diameter of 60 μm for the Illinois #6 and Black Thunder coals.10 Zhang’s data are represented in Figures 4 and 6 by the open symbols. The mass release for the Illinois #6 coal during pyrolysis remained relatively constant after 1700 K, while the nitrogen release increased slightly with the temperature throughout the temperature range studied. Little difference was seen in either the mass or nitrogen release data between the N2 and oxy-fuel experiments for this coal. The CPD model predictions for the Illinois #6 coal were in good agreement with both the mass and nitrogen release data. The mass and nitrogen release for the Pittsburgh #8 coal did not show much change with the temperature in these experiments, within the scatter of the data. The lack of change in the mass and nitrogen release at these temperatures was computed accurately for the Pittsburgh #8 coal by the CPD model. However, the CPD model predictions of both mass and nitrogen release were slightly below (5-8% daf) the data for the Pittsburgh #8 coal. Prediction of volatile yields for Pittsburgh #8 bituminous coal have generally been quite accurate (within 5% daf).12 The fact that the CPD model predictions of volatile yields in N2 are close to those measured in CO2 is not a conclusive result but seems consistent with the agreement measured for the Illinois #6 bituminous coal between the two environments. The mass and nitrogen release data for the Black Thunder coal were higher in the oxy-fuel case than in Zhang’s experiments (Figure 6). The difference in nitrogen release between the two conditions (Figure 6b) was greater than the observed difference in mass release (Figure 6a). Figure 7
Results and Discussion Mass and nitrogen release data obtained in the oxy-fuel environment in the FFB for Illinois #6, Pittsburgh #8, and Black Thunder coals are shown in Figures 4-6. All of these data were collected at a location of 1 in. above the FFB (12) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Solum, M. S.; Grant, D. M. A chemical percolation model for devolatilization: 3. Direct use of carbon-13 NMR data to predict effects of coal type. Energy Fuels 1992, 6, 414–431. (13) Genetti, D.; Fletcher, T. H.; Pugmire, R. J. Development and application of a correlation of 13C NMR chemical structure analyses of coal based on elemental composition and volatile matter content. Energy Fuels 1999, 13, 60–68.
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Figure 4. (a) Mass release and (b) nitrogen release for the Illinois #6 coal.
Figure 5. (a) Mass release and (b) nitrogen release for the Pittsburgh #8 coal.
Figure 6. (a) Mass release and (b) nitrogen release for the Black Thunder coal.
comparisons at other temperatures were reported by Sowa.14 Because the coal was not fully pyrolyzed, additional pyrolysis experiments were conducted on the Black Thunder coal in the FFB in a N2 background at sampling locations of 1 and 2 in. for the four previously mentioned temperature conditions. The new data are also shown as a function of time for the 1600 K case in Figure 8, and the results of all eight experiments are shown in Figure 9. Examination of the data in Figures 8 and 9 shows that the mass and nitrogen release data in the oxy-fuel experiments at
shows a comparison of the nitrogen/carbon (N/C) ratios in the chars from the two environments. Higher N/C ratios mean that the nitrogen is released more slowly than carbon. The lower N/C ratios observed from the pyrolysis in the CO2 environment indicate that the nitrogen is being released more rapidly than in the N2 environment (similar to the observations in Figure 6). The CPD model predictions indicated that the Black Thunder coal was not fully pyrolyzed at the residence time corresponding to the 1 in. location in the FFB experiments, in either the oxy-fuel or N2 background experiments. Figure 8 shows the mass and nitrogen release predictions as a function of time for the 1600 K condition, along with Zhang’s data at a similar gas temperature condition. Similar time-dependent
(14) Sowa, J. M. Studies of coal nitrogen release chemistry for oxyfuel combustion and chemical additives. M.S. Thesis, Department of Chemical Engineering, Brigham Young University, Provo, UT, 2009.
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the 1 in. location were higher than observed in the corresponding temperatures in the N2 background. It is interesting that the mass and nitrogen release data in the 2 in. N2 environment matched the corresponding data at the 1 in. location in the oxy-fuel condition. As indicated in Figure 9, the CPD model predictions lag the time-dependent mass and nitrogen release data until the very highest temperature (1900 K). At the 1900 K temperature condition, complete devolatilization
was predicted at the 1 in. location. The increased mass and nitrogen release data for the Black Thunder coal in the oxyfuel condition (in comparison to the N2 background) were not observed in the Illinois #6 coal data or in the corresponding predictions for both bituminous coals. The increased mass and nitrogen release observed in the oxy-fuel condition for the Black Thunder coal may be due the heterogeneous gasification reaction of CO2 with the char at these temperatures. The CO2 concentration is high in these experiments, with high temperatures, which could conceivably permit some early gasification behavior. Possible CO2char reactions in oxy-fuel conditions have been suggested in the literature.6,7 A possible explanation for the increased nitrogen release is that CO2 attacks the weak sites in the char matrix at these high temperatures and short residence times and that the nitrogen in the aromatic ring represents a weak bond. Unfortunately, reliable experiments were not conducted at the 2 in. location for the oxy-fuel conditions, which would have helped to confirm the early gasification behavior. An alternate explanation may be that the difference in gas properties of N2 and CO2 causes the coal to lose its mass faster in the CO2 environment. Murphy and Molina used visual observations of single coal particle combustion to show that the different gas properties change the time to ignition and the duration of volatile combustion.4 However, the difference in gas properties should have affected the Illinois #6 coal in the present experiments in a manner similar to the Black Thunder coal. Therefore, the difference in gas properties did not seem to explain the mass release data for the two different ranks of coals. The nitrogen release data for the Black Thunder coal in the oxy-fuel environment were scattered but higher on average than in either set of data obtained at the 1 in. location in a N2 background (i.e., from Zhang’s data10 or from this work). Surprisingly, the nitrogen release data obtained at the 2 in. location in the N2 environment showed good agreement with the data obtained at the 1 in. location in the oxy-fuel condition. Additional nitrogen release data at longer residence times in the oxy-fuel environment are recommended to support the hypothesis that a heterogeneous reaction occurs early for low-rank coals under oxy-fuel conditions.
Figure 7. Comparison of N/C ratios in chars from CO2 and N2 environments.
Figure 8. Time-dependent mass and nitrogen release in the 1600 K gas condition in the FFB for Black Thunder coal with N2 as the background gas. Data shown from Zhang10,11 were obtained at a slightly cooler temperature condition.
Conclusion Three coals were pyrolyzed at high heating rates at different temperatures (1500-1900 K) to compare volatile yields and
Figure 9. (a) Mass release and (b) nitrogen release for the Black Thunder coal obtained in this work in the FFB. The solid line is the CPD prediction at the 1 in. location.
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nitrogen release behavior under oxy-fuel conditions versus the behavior under air-fired conditions (i.e., with a N2 background). Two bituminous coals and one sub-bituminous coal were examined. Measured mass and nitrogen release data were the same in both the N2 background condition and the oxy-fuel condition for the Illinois #6 bituminous coal during pyrolysis at temperatures ranging from 1500 to 1900 K. Although no direct comparison of measurements in the two environments was possible for the other bituminous coal, the volatile yields in the CO2 environment matched CPD model predictions from the N2 environment within 5-8%. The subbituminous coal exhibited increased mass and nitrogen release under oxy-fuel conditions at these high temperatures at a
15 ms residence time. Because the effects of the oxy-fuel environment were coal-dependent, gas property differences between oxy-fuel conditions and air-fired conditions did not appear to explain the data. These data, therefore, suggest that early gasification of the low-rank coal may be important at the high temperatures and high CO2 concentrations that existed in these experiments. Acknowledgment. The authors thank James Kendall, Brady Burgener, and Sam Goodrich for helping with the experiments. Funding was supplied by the Department of Energy (DOE)/ University Coal Research (UCR) program under DE-PS2605NT42244-03.
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