Quantitative Analysis of NOx Reduction in Oxy-Coal Combustion

The dried gas flowed through a flow meter and then was monitored for NO, CO, CO2, and O2 by a flue gas analyzer (MRU, Vario Plus). This flue gas analy...
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Quantitative Analysis of NOx Reduction in Oxy-Coal Combustion Yongchun Zhang, Jun Zhang,* Changdong Sheng, Yangxian Liu, Liang Zhao, and Qizhong Ding School of Energy and Environment, Southeast University, Nanjing, 210096, PR China ABSTRACT: The conversion ratios of fuel-N to NO were investigated during combustion of two Chinese bituminous coals in four different atmospheres (O2/N2, O2/Ar, O2/CO2, and O2/RFG) in an electrically heated drop-tube furnace at elevated temperatures (1273-1673 K). The oxygen concentration in the inlet gas was changed from 8% to 40%. The recycling ratio of flue gas ranged from 0.4 to 0.8. Through comparing the conversion ratios of fuel-N to NO among the four different atmospheres, the contributions of elimination of atmospheric nitrogen, high CO2 concentration in the furnace, and reduction of recycled-NOx to the reduction of total NOx in oxy-coal combustion were investigated. Results show that the contribution of recycled-NO reduction was dominant. It contributed 45-70% of the total reduction of NOx. It decreased with oxygen concentration in the inlet gas and increased with flue gas recycling ratio. The contribution of elimination of atmospheric nitrogen accounted for 12-25% and had a trend opposite that observed for the recycled-NO reduction. The contribution of the high CO2 concentration in the furnace accounted for 18-33%. It initially increased with oxygen concentration, and then decreased as the oxygen concentration reached and exceeded 21%. The influence of temperature on the contributions of elimination of atmospheric nitrogen, high CO2 concentration in the furnace, and reduction of recycled-NOx to the reduction of total NOx was complex. It depended on combustion conditions and the type of employed coals.

1. INTRODUCTION Recently, oxy-fuel (O2/CO2) combustion technology has received considerable attention as one of the most effective ways to capture CO2 from large coal-fired power plants.1-4 In this technology nearly pure O2 is used instead of air to oxidize coals to obtain high CO2 concentration in flue gas. So the temperature of pure oxygen combustion will be extremely high. To lower the flame temperature to withstanding temperature of the materials used in combustors, boilers, and gas turbines the O2 is mixed with recycled flue gases (RFG). The RFG mainly consist of CO2 and moisture. Therefore, the combustion environment is greatly different from that of conventional air combustion, which leads to significant differences between oxy- and air-combustion, including formation of emissions, flame and heat transfer characteristics, combustion efficiency, and so on.2,5-15 The behavior of NOx emission in oxy-fuel combustion has been studied in laboratory-scale and pilot-scale experiments.7,16-22 These studies indicate that NOx emission in oxy-fuel combustion is much lower than that in air combustion. Although the detailed mechanisms of NOx formation and reduction in oxy-combustion conditions are still unclear, it can be expected that the significant decrease in NOx emission is mainly attributed to the following: (a) NOx produced in combustion may be reduced because of the elimination of atmospheric nitrogen fixation to NO; (b) at high CO2 concentrations in the flame zone, the concentration of CO is increased and NOx reduction is enhanced by reaction with CO; (c) a part of recycled NOx is reduced in furnace. Determining the contribution of each effect to the decrease of total NOx in oxy-fuel combustion is helpful for understanding the NOx formation process and taking measures to control the NOx emission. However, the information on the contribution of these effects is rare. In the present work, coals are combusted in O2/N2, O2/Ar, O2/ CO2, and O2/RFG atmospheres, respectively. The objective is to r 2011 American Chemical Society

quantitatively determine the contribution of each effect to the reduction of total NOx in oxy-coal combustion with recycled flue gas. The influences of different operating conditions are investigated.

2. EXPERIMENTAL APPARATUS AND PROCEDURES The experiments were performed in a drop-tube furnace (Figure 1). The combustion chamber consisted of a cylindrical alumina tube (40 mm i.d., 50 mm o.d.) heated by silicon carbide rod elements (SiC); the heated part was about 700 mm long. Three Pt-Pt/Rh (S-type) thermocouples were placed at different positions along the axis of the tube. The isothermal part of the radiation zone of the furnace was about 400 mm long as shown in Figure 2. The feed gases were supplied from gas cylinders and regulated by mass flow controllers; they were mixed in a mixer. The gas mixture flowed through a flow meter and then was divided into two streams, one of which was used to entrain pulverized coal into the furnace. The other one was introduced into the furnace at the top of the alumina tube as oxidant. Pulverized coal was supplied continuously by a screw feeder and was introduced to the combustion zone through a water-cooled injector by the feed gas. Solid and gas samples were extracted from the furnace using a movable watercooled probe inserted from the bottom of the alumina tube. The cooled effluent gas passed through a glass-fiber filter to collect the solid particulate residue, and then was dried in a dryer for the removal of moisture. The dried gas flowed through a flow meter and then was monitored for NO, CO, CO2, and O2 by a flue gas analyzer (MRU, Vario Plus). This flue gas analyzer used NDIR detection for CO and CO2, and electrochemical sensors for NO and O2 measurements. The uncertainty of these measurements is (3% and the standard detection ranges for NO, O2, CO, and CO2 are 0-4000 ppm, 020.9%, 0-10.0%, and 0-100.0%, respectively. Two Chinese bituminous coals, Lijiata coal (LJT) and Jinjie coal (JJ), were used in the experiments of this work and their properties are given Received: November 30, 2010 Revised: January 19, 2011 Published: February 15, 2011 1146

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controlled by a flow meter. The flue gas recycling ratio refers to the ratio of the recycled flue gas flow rate to the total exhaust gas flow rate. The expected recycling ratio was realized by adjusting the recycled flue gas flow rate. It should be pointed out that the amount of oxygen exceeded significantly the stoichiometry in all cases due to the low coal feed rate. The data reported are the median values. The reliability of the data strongly depends on the stability of coal feed rate and the accuracy of the flue gas analyzer. To guarantee the accuracy of the coal feed rate in each case it was measured both at the beginning and at the end of the run. It was ensured that the two measuring values were close to each other and the average was adopted finally. So the uncertainty of the experimental data is mainly derived from the measurement error of the flue gas analyzer. All data points are included in the plots presented herein.

3. RESULTS AND DISCUSSION Figure 1. Schematic diagram of experimental apparatus. 1. Gas cylinders, 2. Mass flow controllers, 3. Gas mixer, 4(5,18,19). Flow meter, 6. Electric motor, 7. Pulverized coal container, 8. Water-cooled injector, 9. Thermocouples, 10. Silicon carbide rod, 11. Heat insulating material, 12. Temperature controlling system, 13. Alumina tube, 14. Sampling probe, 15. Filter, 16. Desiccator, 17(20). Vacuum pump, 21. Gas analyzer.

3.1. Calculation of the Nitrogen Conversion Ratio. The expression of pollutant emissions in terms of concentration (ppm) is inadequate if pollutant emissions are compared among different combustion atmospheres. To facilitate the comparison, the conversion ratio from fuel-N to NO (CR) is used in the present paper. It is defined as follows:

CR ¼ Nemission =Nfuel

Figure 2. Temperature profiles of alumina tube.

Table 1. Ultimate and Proximate Analyses of Coals ultimate analysis (wt %, ada)

a

proximate analysis (wt %, ada)

C

H

N

FC

M

ash

VM

LJT Coal

75.79

4.06

0.78

52.27

JJ Coal

66.80

4.48

0.94

50.40

9.99

6.80

30.94

7.79

11.21

30.60

ad = air-dry basis.

in Table 1. The coal feed rate was about 5 g/h. Coals were combusted in O2/N2, O2/Ar, O2/CO2, and O2/RFG atmospheres, respectively. The total inlet gas flow rate was fixed at 2500 mL/min in all the experiments. The entrained-flow rate was 200 mL/min. The oxygen concentrations in the inlet gas amounted to 8%, 15%, 21%, 30%, and 40%. The expected concentration was realized by changing the O2 and base gases (N2, Ar, or CO2) flow rates. When experiments of coal combustion in O2/RFG were performed, the expected O2 concentration was realized by changing the flow rates of O2 and balance gas CO2. The exhaust gas was divided into two streams before analysis, one of which was introduced back into the gas mixer by a vacuum pump as recycled flue gas. The other one was introduced into the flue gas analyzer for analysis. The flow rate of the recycled flue gas was

where Nemission is the nitrogen atom number of NO discarded into atmosphere and Nfuel is the nitrogen atom number of fuel-N. 3.2. Effect of O2 Concentration in Different Atmospheres. Figure 3 shows the effect of O2 concentration in inlet gas on CR during coal combustion in O2/N2, O2/Ar, O2/CO2, and O2/RFG atmospheres at 1673 K, respectively. The concentrations of O2 in the inlet gas were changed from 8% to 40%. At the same O2 concentration in the inlet gas, CR in different combustion atmospheres followed the sequence of O2/N2 > O2/Ar > O2/CO2 > O2/RFG. This result agrees with previous experiments.22,23 During LJT coal combustion in O2/N2 atmosphere, CR was in the range of 0.290.50. It was 4-19% higher than that in O2/Ar combustion process (0.28-0.42). CR in O2/CO2 combustion process was, on the average, 83% of that in O2/Ar combustion process (i.e., 0.240.34). It was 71-113% higher than that in O2/RFG combustion process (0.14-0.16). For JJ coal, CR in O2/N2 combustion process was in the range of 0.29-0.44. It was 12-26% higher than that in O2/Ar combustion process (0.26-0.35) and 38-57% higher than that in O2/CO2 combustion process (0.21-0.28). CR in O2/RFG combustion process was nearly half of that in O2/CO2 combustion process (0.09-0.13). In all cases CR increased with O2 concentration. Similar results were reported by Croiset and Thambimuthu.22 They observed a significant increase in the flame temperature with higher oxygen concentration in the feed gas and attributed the increase of NOx emission to the increase of temperature. But it is believed that the flame temperature was not changed greatly with oxygen concentration in our experiments because the coal feed rate was very low. In fact, the temperature of the burning particles will be increased significantly with higher oxygen concentration.14 This enhances the formation of thermal NOx and the conversion of fuel-N to NOx. On the other hand, it is generally accepted that NOx is reduced by hydrocarbon fragmentation (CHi), fuel-N (HCN, NHi), and char particles in coal combustion.24-28 These reducing components will be oxidized more with the increase of O2 concentration. All of these lead to the increase of NO emission. The CR in O2/RFG combustion is lowest in all cases (as shown in Figure 3) due to the contributions of the three effects mentioned 1147

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Figure 3. Changes in conversion ratio of fuel-N to NO with oxygen concentration during coal (LJT and JJ) combustion in different atmospheres; flue gas recycling ratio = 0.6, residence time = 2.1 s, and T = 1673 K.

Figure 4. Changes in contributions of separate effects to the CR decrease from coal combustion in O2/N2 to that in O2/RFG with oxygen concentration during coal (LJT and JJ) combustion; flue gas recycling ratio = 0.6, residence time = 2.1 s, T = 1673 K.

in the Introduction. To compare the contributions of these effects quantitatively, the contribution of the each effect to the total decrease of CR from coal combustion in O2/N2 to that in O2/ RFG is defined as follows: Contribution of the elimination of atmospheric nitrogen ¼ ðCR O2=N2 - CR O2=Ar Þ=ðCR O2=N2 - CR O2=RFG Þ Contribution of the high CO2 concentration ¼ ðCR O2=Ar - CR O2=CO2 Þ=ðCR O2=N2 - CR O2=RFG Þ Contribution of the reduction of recycled-NO ¼ ðCR O2=CO2 - CR O2=RFG Þ=ðCR O2=N2 - CR O2=RFG Þ where CRO2/N2, CRO2/Ar, CRO2/CO2, and CRO2/RFG denote CR

in O2/N2, O2/Ar, O2/CO2, and O2/RFG combustion processes, respectively. Figure 4 shows the contribution of each effect as a function of O2 concentration. For LJT coal combustion, the contribution of each effect is as follows: (a) for the elimination of atmospheric nitrogen fixation to NO, 5-23%, (b) for the high CO2 concentration in the furnace, 22-34%, and (c) for a reduction of recycled NO in the furnace, 50-67%. For JJ coal combustion, the contributions of the three effects account for 15-31%, 20-34%, and 49-60%, respectively. It appears that with the increase of O2 concentration in the inlet gas the contribution of effect (a) increases while that of effect (c) decreases. The contribution of effect (b) initially increases with O2 concentration, and then decreases as the O2 concentration reaches and exceeds 21%. Thermal NOx is mainly influenced by temperature and oxygen concentration. The temperature of burning particles is greatly 1148

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Figure 5. Changes in conversion ratio of fuel-N to NO with temperature during coal (LJT and JJ) combustion in different atmospheres; flue gas recycling ratio = 0.6, O2 concentration = 30%.

Figure 6. Changes in contributions of separate effects to the CR decrease from coal combustion in O2/N2 to that in O2/RFG with temperature during coal (LJT and JJ) combustion; flue gas recycling ratio = 0.6, O2 concentration = 30%.

increased with the oxygen concentration as mentioned before although the gas temperature in the reactor is not significantly affected. Thus, the formation of thermal NOx is enhanced both due to the higher oxygen concentration and the elevated particle temperature. The contribution of effect (a) increases with the oxygen concentration. On the other hand, the oxidation rates of hydrocarbon fragmentation (CHi), fuel-N (HCN, NHi), and char particles are enhanced due to the increase of oxygen concentration. Less recycled-NO is reduced in the furnace because of the decrease of these reducing components. Hence, the contribution of effect (c) decreases with the oxygen concentration. It is especially interesting to note that there is a maximum for both coals corresponding to the contribution of the high CO2 concentration for 21% oxygen concentration. High

CO2 concentration results in the increase of CO concentration, and NOx reduction by char is enhanced by reduction with CO. The heterogeneous reduction reaction may be accelerated by oxygen. And the accelerating effect reaches maximum at a certain oxygen concentration, e.g., 21%. 3.3. Effect of Temperature. Figure 5 shows the effects of temperature on CR during coal combustion in O2/N2, O2/CO2, and O2/RFG atmospheres, respectively. The temperature was changed from 1273 to 1673 K. It is expected that the formation of thermal NOx is negligible at low temperature. Thus, coal combustion in O2/Ar atmosphere was not investigated herein. For LJT coal combustion, CR increased by 11% in O2/N2 atmosphere while it decreased by 9% in O2/RFG atmosphere. There was not an obvious influence of temperature on CR in O2/ 1149

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Figure 7. Cchanges in conversion ratio of fuel-N to NO with recycling ratio during coal (LJT and JJ) combustion in O2/RFG atmosphere;, O2 concentration = 30%, T = 1673 K.

Figure 8. Changes in contributions of separate effects to the CR decrease from coal combustion in O2/N2 to that in O2/RFG with recycling ratio during coal (LJT and JJ) combustion; O2 concentration = 30%, T = 1673 K.

CO2 combustion process. For JJ coal combustion, CR increased with temperature for all the three cases examined, especially in O2/N2 combustion process. These results indicate that the influences of the temperature on CR are different in different combustion atmospheres and for different coals. Some factors associated with the effect of the temperature on NO reduction could be considered. On one hand, higher temperature accelerates the oxidation of fuel nitrogen components to NO. On the other hand, NO reduction by char is enhanced at higher temperature due to the increased rates of NO/CO/char reaction. Moreover, more amounts of volatiles are devolatilized at the higher temperature, and the NO reduction by volatiles may also be enhanced. Therefore the final effect of the temperature on NO reduction depends on combustion conditions and the type of employed coals.

Figure 6 shows the contribution of each effect to the total decrease of CR from coal combustion in O2/N2 to that in O2/ RFG as a function of temperature. The formation of thermal NOx below 1600 K can be regarded as negligible.29 So the contribution of the elimination of atmospheric nitrogen is not presented alone in Figure 6. It is considered together with the contribution of the increase of CO2 concentration in the furnace. During LJT coal combustion, the contribution of recycled-NO reduction increased initially as temperature increased from 1273 to 1373 K, and then decreased lightly with temperature, while it is opposite for the contributions of the other two effects. For JJ coal combustion, it appears that the contribution of recycled-NO reduction increased with temperature; accordingly the contributions of the other two effects decreased. These results indicate that the influence of temperature on the contributions of the 1150

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Energy & Fuels effects also depends on combustion conditions and the type of employed coals. 3.4. Effect of Recycled Flue Gas Ratio. The changes of CR with flue gas recycling ratio during coal combustion in O2/RFG atmosphere with an O2 concentration of 30% at 1673 K are shown in Figure 7. The CR for coal combustion in O2/N2, O2/ Ar, and O2/CO2 atmospheres with the same O2 concentration are also presented as comparisons. There is a lower value of CR for the higher recycling ratio. With the recycling ratio increased from 0.4 to 0.8, CR decreased from 0.24 to 0.11 for LJT coal combustion and from 0.17 to 0.08 for JJ coal combustion. The lowest CR values for LJT coal and JJ coal combustion in O2/RFG with a recycle ratio of 0.8 are only about one-fourth and one-fifth of that in O2/N2, respectively. These results are consistent with that reported by Hu et al.30 They studied the reduction efficiency of recycled-NO under flue gas recycling ratio of 0-0.4. The results showed that under the same equivalence ratio, there was lower reduction efficiency, i.e., higher CR, for the lower recycling ratio than for the higher recycling ratio. In fact, the recycled-NO concentration is increased and more amounts of recycled-NO are introduced into the furnace under higher flue gas recycling ratio conditions. So it can be expected that more amounts of recycledNO are reduced by volatiles and chars in the furnace. This is confirmed by Okazaki and Ando.31 Their results showed that recycled-NO reduction increased with recycled-NO concentration. Figure 8 shows the contribution of each effect to the total decrease of CR from coal combustion in O2/ N2 to that in O2/ RFG as a function of flue gas recycling ratio. During LJT coal combustion, when recycling ratio was increased from 0.4 to 0.8, the contribution of recycled-NO reduction to the total decrease of NO monotonously increased from 45% to 70%, and accordingly the contributions of the elimination of atmospheric nitrogen and the high CO2 concentration in the furnace decreased from 22% to 12% and from 33% to 18%, respectively. Similar varying tendencies were observed for JJ coal combustion. The contribution of recycled-NO reduction increased from 47% to 62% while the contributions of the other two effects decreased from 25% to 18% and from 28% to 20%, respectively. As analyzed above, more quantities of recycled-NO are reduced under higher recycling ratio conditions. This results in the increasing contribution of recycled-NO reduction to decrease of total NO.

4. CONCLUSION The low NOx emission in oxy-coal combustion is attributed to the elimination of atmospheric nitrogen, the high CO2 concentration in the furnace, and the reduction of recycled NOx. The focus of this study is on the influences of different operating conditions on the contribution of each of these effects to the reduction of total NOx in oxy-coal combustion. • The contributions of the elimination of atmospheric nitrogen, the high CO2 concentration in the furnace, and the reduction of recycled NO account for 12-25%, 18-33%, and 45-70%, respectively, when coal combustion was in O2/RFG atmosphere with an O2 concentration of 30% and the flue gas recycling ratio range of 0.4-0.8 at 1673 K. • The contribution of the recycled-NO reduction is dominant. It decreases with oxygen concentration in the inlet gas and increases with flue gas recycling ratio • The contribution of the elimination of atmospheric nitrogen has a trend opposite that observed for the recycled-NO reduction.

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• The contribution of the high CO2 concentration in the furnace initially increases with oxygen concentration, and then decreases as the oxygen concentration reaches and exceeds 21%. It decreases with flue gas recycling ratio. • The influence of temperature on the contribution of the effects is complex. It depends on combustion conditions and the type of employed coals.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We acknowledge the financial support of Projects of International Cooperation and Exchanges National Science Foundation of China under Project 507211460649, the Scientific Research Foundation of Graduate School of Southeast University, and the support of part of the present work by Opening Foundation of State Key Laboratory of Coal (FSKLCC1003) at Huazhong University of Science and Technology, Wuhan.

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