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Effects of Gas Staging on the NO Emission during O2/CO2 Combustion with High Oxygen Concentration in Circulating Fluidized Bed Xu Mingxin,†,‡ Li Shiyuan,*,† Li Wei,†,‡ and Lu Qinggang† †

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China

‡

ABSTRACT: Oxy-fuel circulating fluidized bed (oxy-CFB) combustion, which combines the advantages of oxy-fuel combustion with those of CFB technology, is one of the most promising technologies to capture CO2. In this paper, experiments were carried out in a 50 kWth CFB combustor to investigate the effects of gas staging on the formation and emission of NO in the oxy-fuel CFB combustor with high oxygen concentrations by analyzing the NO axial profiles along the combustor height at different conditions. The results show that the concentration of NO in the dense zone in oxy-fuel firing with high oxygen concentrations is slightly higher than that in air-firing, but the final emission of NO is lower in oxy-fuel combustion. With the secondary gas location rising, the emission of NO maximally drops from 171.2 mg/MJ to 31.9 mg/MJ in 50% O2/50% CO2 atmosphere. Gas staging is also an efficient method to reduce NO emission in oxy-fuel CFB combustion with a high oxygen concentration. The decrease of the primary/secondary gas ratio leads the dropping of NO emission maximally from 171.2 mg/MJ to 66.7 mg/MJ. However, the effects of gas staging lessen in oxy-fuel CFB combustion with a high oxygen concentration compared with that in traditional air combustion. In addition, oxygen staging could be more effective in restricting the formation of NO in the dense zone than gas staging in oxy-fuel CFB combustion with high oxygen concentration. excess air ratio, the prompt-NOx can also be disregarded.32 So what needs to be focused on is only the fuel-NOx that comes from the fuel nitrogen in oxy-fuel CFB combustion.33 The emission of NOx in conventional combustion has been extensively studied by many researchers. Defu et al.34 studied the emission characteristics of NOx in coal combustion with thermos-gravimetric analyzer. They found that the conversion of fuel nitrogen to NOx was higher at higher oxygen concentrations. Winter et al.35 summarized the main reaction pathways of the fuel nitrogen to NOx. Reidick et al.36 found that the most important zone for NO formation was the oxygen-rich region. Basu37 studied the influence of operating parameters on NOx emissions in air-firing systematically, and it was found that air-staging could be considered as one of the most effective measures to control the NOx in an air-firing CFB. Lyngfelt and Leckner38 did experiments to explore the influence of temperature and air staging on SO2 and NOx, and their results showed that the effect of air staging on NOx was more pronounced at high temperature (930 °C) than at 850 °C. However, there is technical limitation for air-staging in the air-firing combustor. The oxygen concentration of the primary air and the secondary air is fixed to 21%. Compared with traditional air-firing, it is more flexible for gas staging to be adopted in the oxy-fuel combustors. The oxygen concentration in the primary and secondary gas can be altered separately with wide ranges, which is called oxygen staging. For oxy-fuel combustion, Lupiáñez et al.39 have done experiments about the

1. INTRODUCTION Oxy-fuel combustion is considered one of the major options for capturing CO2 for both retrofitting existing fossil-fuel combustion systems and the green field power plants. The mixtures of pure oxygen and recycled flue gas, instead of air, are utilized for fuel combustion. Flue gas consisting mainly of CO2 and water is generated with high concentrations of CO2 ready for sequestration and storage. Many studies about pulverizedcoal (PC) oxy-fuel combustion have been done so far.1−8 In comparison with PC, there are many advantages for circulating fluidized bed (CFB) combustion, such as fuel flexibility, low NOx emission, and high in situ desulfurization efficiency. Nowadays benefiting from the development of CFB combustion, oxy-fuel circulating fluidized bed (oxy-CFB) combustion, which combines the advantages of oxy-fuel and CFB, draws more and more attention within the field. Currently, many institutions such as CanmetENERGY in Canada;9−11 Babcock and Wilcox,12 FOSTER WHEELER,13−15 and University of Utah in United States;16,17 Valmet18 and VTT technical research center in Finland;19,20 Chalmers University of Technology in Sweden;21,22 Czestochowa University of Technology in Poland;23,24 IET-CAS25,26 and Southeast University27−29 in China; etc. have been involved in the research on oxy-fuel CFB combustion. In the oxy-fuel combustor, as a result of the flue gas recycle, the emission characteristics of pollutants are different from those in traditional air-firing,30,31 especially for NOx. Due to the combination of the absence of nitrogen injected into the combustor with air and the usual operating temperatures in CFB combustors, the production of thermal-NOx is eliminated in oxy-fuel CFB firing mode, the same as air-firing mode. Because of the technical parameters in CFB combustion such as © 2015 American Chemical Society

Received: February 14, 2015 Revised: April 14, 2015 Published: April 15, 2015 3302

DOI: 10.1021/acs.energyfuels.5b00358 Energy Fuels 2015, 29, 3302−3311

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Energy & Fuels effect of gas-staging on pollutant emissions from fluidized bed oxy-firing, in which they varied only the secondary gas ratio, but oxygen staging was not considered. Duan et al.29 reported the combustion characteristics in a small circulating fluidized bed, in which they introduced the effects of oxygen staging briefly. The axial distribution of NO was not analyzed. Tan et al.26 have studied the effects of oxygen staging and excess oxygen on O2/ CO2 combustion, where they did not take the effects of some factors such as secondary gas location and atmosphere into consideration. In addition, as a result of the 0.1 MWth CFB combustor’s limitations, they did not elaborate the formation and destruction process of NO along the combustor height in oxy-fuel combustion with high oxygen concentrations, either. What is more, there are some other key points that are still unclear, such as the different conversion tendency of NO after formatting in the primary zone between air-firing and oxy-fuel firing, the effects of secondary gas location and portion on the formation of NO in the dense zone, the influence of oxygen staging on the concentration distribution of NO along the combustor with high oxygen concentration. In this paper, the study aims at increasing the knowledge about the effects of gas staging on the NO formation along the combustor height in an oxy-fuel circulating fluidized bed with a high oxygen concentration. The NO axial formation profile along the combustor was studied experimentally when the location, ratio, and oxygen concentration of the secondary gas were changed in air-firing mode and in O2/CO2 oxy-firing mode, respectively. Unlike the traditional studies in which the exhaust gases were the main measuring points, detailed analyses of the axial profile of NO along the combustor height in oxyfuel combustion are explored here in order to improve comprehension of the formation and destruction of NO in oxy-fuel CFB combustion with high oxygen concentration.

Figure 1. (a) Schematic diagram of the test rig. (b) Schematic diagram of gas sampling.

125 mm, 475 mm, 825 mm, 1525 mm, 2925 mm, 2225 mm, and 3100 mm along the combustor above the distributor. The oxygen concentration of flue gas is measured with a zirconia oxygen analyzer. Flue gases are sampled from four gas probes along the combustor, which are located at four different levels of 825 mm (named as FG1), 1525 mm (named as FG2), 2225 mm (named as FG3), and 2925 mm (named as FG4) above the distributor by a peristaltic pump equipped in the FTIR analyzer (GASMET DX4000), and the pumping rate is 3 L/min. The flue gas samples are filtrated by coarse filter and filter cartridge initially. Subsequently, the sampled flue gas, heated to 180 °C by heating tapes made of Teflon in order to avoid condensation, can be propelled into the sample gas chamber for analysis by means of FTIR. There is also another measuring point (named the Outlet), located at the outlet of the cyclone, where the FTIR analyzer (GASMET DX4000) can supervise the exhaust gas. The schematic diagram of the gas sampling system is shown in Figure 1b. 2.2. Fuel Characteristics. Datong bituminous coal was used in the experiments. The proximate and ultimate analyses which were based on the coal as received (ar) are listed in Table 1. Coal was sieved to sizes between 0.1 and 1 mm prior to being used. The 50% cut mean diameter (d50) is 0.29 mm, and the size distribution is shown in Figure 2. Silica sand (2.5 kg) with particle sizes between 0.1 and 0.5 mm was used as initial bed material. 2.3. Experimental Conditions. The experimental conditions are listed in Table 2. In all the experiments, the average temperatures in the combustion chamber were maintained at 850 °C. The overall oxygen concentration was 29% in condition 2 and 50% at condition 3 to condition 10. The coal feeding rate was 6.50 ± 0.3 kg/h in condition 3 to condition 10, and the rate was 2.73 ± 0.2 kg/h in condition 1 and condition 2. The excess oxygen ratio αo, which is defined in eq 1,26 was fixed at 1.15. The concentrations of pollutants concerned in the experiments were monitored by GASMET DX4000. The units of original values of the monitoring data were parts per million. For ease of comparison, all the fundamental values were arithmetically averaged originally, and then all the averages were converted to mg/MJ in this paper. The conversion results were all rounded up to one digit after the decimal point. The measurement errors for the pollutants were ±5%. All the final data have been listed in Table 3.

2. EXPERIMENTAL SECTION 2.1. 50 kWth oxy-fuel Circulating Fluidized Bed Combustor System. The investigation was carried out in a 50 kWth oxy-fuel circulating fluidized bed combustor. The experimental system consists of a combustor, a cyclone, a return-leg, a loop-seal, and a bag filter. The system is equipped with a fuel feed unit, a gas supply unit, a flue gas cooling unit, and a measurement and data acquisition unit. The schematic diagram of the experimental setup is given in Figure 1a. The circulating fluidized bed combustor, which is made of stainless steel, is 3250 mm in height and 100 mm in diameter. The combustor is furnished with three zones of external electric heaters. At the beginning of the tests, all the heaters worked in order to speed up the temperature rising. During the steady operation stage, the lowest heater was the only one that still ran for maintaining the temperature uniform. Coal is fed into the combustor via a screw feeder, which is located at the height of 275 mm above the distributor. The primary gas, which is abbreviated to PG in the following figures and tables, is led into the combustor through the distributor plate at the bottom of the riser. The secondary gas, which is abbreviated to SG in the following figures and tables, can be injected into the combustor from three different locations along the combustor height at 625 mm, 1000 mm, and 1700 mm above the distributor plate. In the experiments, the reactant gases such as O2, CO2, and air are supplied from bottles by mass flow controllers to simulate oxy-fuel combustion with completely clean recycled flue gas. The total oxygen concentration can range from 21% up to 50%. The total flows introduced into the combustor are held constantly. The pressure of the combustor can be kept positive during the experiments in order to eradicate the air leakage completely. The measurement unit consists of thermocouples, pressure sensors, flow meters, and gas analyzers. The temperatures and pressures are measured by thermocouples and pressure sensors located at heights of 3303

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Energy & Fuels Table 1. Proximate and Ultimate Analysis of the Datong Bituminous Coal LHV/MJ·kg−1

proximate analysis/%

ultimate analysis/%

Qnet,ar

FCar

Mar

Aar

Var

Car

Har

Oar

Sar

Nar

22.61

44.38

2.20

26.05

27.37

58.08

3.73

8.58

0.32

1.04

result of the transformation of combustion atmosphere from air to O2/CO2, and it is irrelevant to temperature changes. Figure 5 gives the effect of combustion atmosphere on NO axial concentration profile along the combustor. As is shown, the axial concentration of NO along the combustor height drops both in air-firing and in oxy-fuel firing. The FG1 that is located at the lower part of the combustor is the sampling point in which the concentration of NO is the highest. It means that NO can be destructed along the combustor both in air-firing and in oxy-fuel firing after it generally formed in the dense bed zone. However, the formation of NO at FG1 situated above the coal feed point are different, depending on different conditions. The formation of NO at FG1 is higher at condition 1 than that at condition 2. The coal feeding rates at condition 1 and condition 2 were maintained as 2.73 kg/h. In consequence, the amount of the fuel nitrogen introduced into the combustor is almost unvaried at both conditions. At condition 2 (29%O2/ 71%CO2), the presence of the much higher amount of CO2 can stimulate the CO2 gasification reaction with char in the dense bed zone, as follows:

Figure 2. Size distributions of Datong bituminous coal samples.

αo =

actual oxygen quantity supplied per unit fuel theoretical oxygen requirement per unit fuel

(1)

3. RESULTS AND DISCUSSIONS 3.1. Effect of Combustion Atmosphere on NO Emission. The effect of combustion atmosphere on temperature profile along the combustor height is given in Figure 3, in which (1) means condition 1. The labels have the same meaning in the other figures. Because of the variation of the superficial gas velocities, the pressure drops in the chamber are dissimilar, as given in Figure 4. Compared with condition 1, the gas velocity at condition 2 dropped from 2.50 to 1.86 m/s, as shown in Table 2. Meanwhile, the overall oxygen concentration went from 21% at condition 1 to 29% at condition 2. As a result, the coal feeding rate at condition 1 and condition 2 was kept at 2.73 ± 0.2 kg/h; the excess oxygen ratios also remained the same. As shown in Figure 3, the temperatures can be maintained approximately at nearly 850 °C under the conditions, depending on the combined adjustment of external electric heaters and cooling system on the return-leg. It means that the variation of NO in different combustion atmospheres is the

C + CO2 → 2CO

(2)

The reducing atmosphere is reinforced by CO in the dense bed zone, resulting in the lesser conversion from fuel nitrogen to NO although the oxygen concentration rises from 21% to 29%. Contrasting the results of condition 2 and condition 3, the formation of NO at FG1 is higher at condition 3 than at condition 2. It is likely attributable to the lower CO2 concentrations at condition 3. As the CO2 concentration falls, the CO2 gasification reaction declines and the concentration of CO drops at condition 3, weakening the reducing atmosphere. Meanwhile the higher oxygen concentration accelerates the combustion rate once the coal is fed into the riser,34,37 so that the formation of NO from fuel nitrogen rises under the 50% O2/50%CO2 condition. Nevertheless, the final emission of NO at Outlet is lower in oxy-fuel firing than that in air-firing. For further analysis, FG ΔNO|FGi(i+1) is defined in eq 3:

Table 2. Experimental Conditions condition

combustion atmosphere

1 2 3 4 5 6 7 8 9 10

air 29%O2/71%CO2 50% O2/ 50% CO2 50% O2/ 50% CO2 50% O2/ 50% CO2 50% O2/ 50% CO2 50% O2/ 50% CO2 50% O2/ 50% CO2 50% O2/ 50% CO2 50% O2/ 50% CO2

height of SG (mm)

PG/SG ratio

O2 % in PG

650 1000 1700 1000 1000 1000 1000

no secondary gas no secondary gas no secondary gas 60/40 60/40 60/40 80/20 50/50 60/40 60/40

21% 29% 50% 50% 50% 50% 50% 50% 40% 30%

3304

O2 % in SG

gas velocity (m/s)

50% 50% 50% 50% 50% 65% 80%

2.50 1.86 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 DOI: 10.1021/acs.energyfuels.5b00358 Energy Fuels 2015, 29, 3302−3311

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Energy & Fuels Table 3. Experimental Process Data condition temperature (°C)

CO2 (%)

CO (mg/MJ)

NO (mg/MJ)

T1 T2 T3 T4 T5 T6 T7 FG1 FG2 FG3 FG4 outlet FG1 FG2 FG3 FG4 outlet FG1 FG2 FG3 FG4 outlet

1

2

3

4

5

6

7

8

9

10

844.8 855.5 851.9 852.9 855.2 850.1 810.8 7.4 10.6 12.1 12.2 13.8 1505.1 1111.2 827.8 572.4 402.0 159.9 116.9 88.6 72.6 69.5

850.9 863.4 848.8 850.7 853.5 849.6 811.2 66.4 80.4 87.4 88.3 89.3 5605.2 3111.4 1728.0 1022.5 522.0 149.9 116.3 71.0 46.2 37.0

844.5 861.3 849.7 854.7 858.6 853.9 811.8 66.4 80.4 87.4 88.3 89.3 3048.0 1282.0 797.4 519.1 327.0 171.2 114.2 65.9 32.8 28.4

838.6 851.9 848.2 864.6 864.8 849.4 805.0 64.5 84.4 87.4 88.4 89.4 5253.6 2980.0 1411.6 862.6 386.1 145.5 101.5 56.3 30.4 25.7

830.5 845.0 869.5 878.3 873.9 876.3 833.0 61.8 77.7 87.6 88.6 89.5 6390.4 4548.4 1725.2 909.7 381.5 74.3 91.0 49.1 23.9 19.9

828.7 838.6 840.7 873.9 885.3 895.1 901.0 60.2 77.1 83.0 88.9 89.8 7858.8 5390.4 3920.5 1097.9 568.8 31.9 27.6 40.3 44.6 46.4

835.5 851.6 856.3 864.6 863.0 857.3 817.1 64.0 77.8 87.7 88.5 89.3 6103.1 4093.0 2195.8 1174.2 333.4 123.9 93.6 50.6 26.4 21.6

839.4 844.0 857.5 879.2 878.1 881.1 843.7 60.8 76.6 87.5 88.3 89.1 7501.0 4486.2 2118.2 1016.6 456.4 66.7 78.9 44.6 25.2 19.0

828.8 844.1 852.6 882.6 898.9 900.3 867.6 60.8 75.6 87.4 88.2 89.0 8102.7 4596.0 1525.5 1090.1 415.4 61.6 81.6 59.7 47.8 45.3

830.0 844.9 856.7 891.6 919.3 931.5 891.1 60.2 73.8 87.6 88.3 89.0 9253.6 5332.6 1568.4 1097.9 439.2 56.9 78.5 60.1 48.3 46.5

Figure 3. Effect of atmosphere on temperature profile along the combustor. i ΔNO|FG FG(i + 1) =

NO of FGi − NO of FG(i + 1)

= 1, 2, 3)

NO of FG1

Figure 4. Pressure distribution along the combustor.

× 100% (i (3)

2Cfas + NO → C′(N) + C(O)

(4)

C′(N) + NO → N2 + C(O)

(5)

where Cfas are the activated carbon molecules existing on the surface and pores of the char particles. The more surfaces of char particles are in the combustor, the more Cfas there is and the more NO can aggregate on, so that the degree of reaction reduction is greater. The key element for NO/char reaction is the amount of Cfas in the combustor. The other reduction path is the homogeneous reaction of NO/CO, as follows:41,42

where NO of FGi is the value of NO concentration at sampling point FGi, NO of FG(i+1) is the value of NO concentration at sampling point FG(i+1), and the units are mg/MJ. FG The ΔNO|FGi(i+1) can be considered as the percentage of NO i destruction along the combustor. The values of ΔNO|FG FG(i+1) along the combustor are given in Figure 6. As it is shown, i ΔNO|FG FG(i+1) at both Condition 2 and Condition 3 are apparently higher than that at Condition 1. It means that the decomposition of NO along the combustor in oxy-fuel firing is more than that in air-firing. There are two possible reaction paths for NO destruction. One is the heterogeneous reaction of NO by char, as follows:40,41

CO + NO → CO2 + (1/2)N2

(6)

in which the char surface would catalyze CO oxidation to CO2 by NO. For a homogeneous reaction, the key factors are the catalysts and the concentration of CO. 3305

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Energy & Fuels

Figure 7. CO emissions at Outlet in different atmospheres.

Figure 5. Effect of atmosphere on NO emission profile along the combustor.

concentration of NO at Outlet has a similar tendency at both i conditions. In detail, ΔNO|FG FG(i+1) is higher at condition 3 than that at condition 1, which means an increase of NO destruction along the combustor. It may be owing to the heterogeneous reaction of NO/char principally. Compared with condition 1, the same tendency of NO and CO at both conditions proves the crucial importance and influence of NO/char reaction in the destruction of NO in oxy-fuel combustion with high oxygen concentration. For condition 2 and condition 3, with the concentration of CO2 dropping and the concentration of O2 getting up, the intensity of the combustion reaction was strengthened, resulting in the decline of CO and the growth of coal feeding rate, which generated more chars accumulating in the combustor. The decay of CO may weaken the homogeneous reaction of NO/CO while the chars may upturn the heterogeneous reaction of NO/char. The reduction of NO for condition 3 is the synthetic effects of the above two aspects. The final combination is that the concentration of NO is lower at Outlet at condition 3 (50%O2/50%CO2). From what has been discussed above, due to the higher concentration of CO in the dense bed zone and more circulating chars in the combustor in oxy-fuel firing, particularly at a higher oxygen concentration, the reduction reaction of NO can take place with fewer difficulties through both heterogeneous reaction and homogeneous reaction,44,45 and the final emission level of NO at Outlet drops considerably in oxy-fuel firing. However, the results are limited to the qualitative analysis, and it cannot signify the priority of the NO/char reaction and NO/CO reaction, which needs to be investigated quantitatively and meticulously. FGi Meanwhile, another significant item is that ΔNO|FG (i+1) declines noticeably along the combustor both in air-firing and in oxy-fuel firing. Possible reasons are that (1) there is more excess oxygen in the upper part of the combustor than in the lower part, so the oxidizing atmosphere lessens the destruction of NO along the combustor, and (2) the concentration of the circulating chars drops at the upper part of the combustor, and the homogeneous reaction NO/CO becomes dominant instead of the NO/char reaction. Unfortunately, there are fewer chars as catalysts at the upper part of the combustor, the rate of homogeneous NO/CO reaction decreases, and then i ΔNO|FG FG(i+1) declines along the combustor height.

FG

Figure 6. ΔNO|FGi(i+1) along the combustor in different atmospheres.

For condition 1 and condition 2, the combustion efficiency calculated was 90.5% and 89.3%, respectively. Meanwhile, the coal feeding rates and excess oxygen ratios were kept the same in both conditions as a result of changes in gas velocities; the tendencies of temperature along the combustor were similar. From the above, the assumption that the chars circulating in the combustor in both conditions may be regarded as equal can be i established. So the reason why ΔNO|FG FG(i+1) is different at these conditions is the homogeneous reaction of NO/CO. As described previously, the concentration of CO in 29%O2/ 71%CO2 (condition 2) is higher than that in air-firing, as shown in Figure 7. Therefore, CO may enhance the reducing atmosphere in the combustor, causing the improvement of the NO/CO homogeneous reaction,28,39,43 causing the destruction of NO along the combustor height stepping up. Because of the limitation of the bench-scale combustor whose height was only 3 m and the shorter residence time in the tests, the emission of CO may be higher than some results based on pilot-scale systems.11 For condition 1 and condition 3, the concentration of CO in air-firing is more than that in 50%O2/50%CO2; as well, the 3306

DOI: 10.1021/acs.energyfuels.5b00358 Energy Fuels 2015, 29, 3302−3311

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Energy & Fuels 3.2. Effect of the Secondary Gas and Its Location on NO Emission. In 50%O2/50%CO2 combustion atmosphere, the effect of the secondary gas and its location on temperature profile along the combustor and the cyclone is given in Figure 8. The experiments are at condition 3, condition 4, condition 5,

zone falls visibly when the secondary gas injection location goes higher, as the concentration of NO at FG1 indicated in Figure 9. It seems that the primary gas zone has been transformed from a fuel-lean zone to a fuel-rich zone with the injection of the secondary gas rising. The location of the secondary gas injection determines the lengths of the primary gas zone and burn-out zone. An increase of the secondary gas injection height lengthens the primary gas zone together with the residence time of circulating particles, such as chars, in the primary gas zone. Thus, a heterogeneous reaction between NO and char in the fuel-rich zone could be strengthened. The formation of NO can be diminished effectually, hence the emissions of NO at the Outlet drop positively, as shown in Figure 9. However, the trend turns contrary when the secondary gas injection is at 1700 mm. It is the result of postcombustion. As shown formerly, when raising the secondary gas injection point to 1700 mm, the temperatures of the upper combustor and the cyclone rise to 900 °C. The higher temperatures and the higher location of chars that burned cause the increasing of NO emission.48 Another interesting point is the significant NO formation near the injection when the secondary gas was injected into the combustor, as given in Figure 8. It is associated with the oxidation of combustible species, mainly gases, coming from the fuel-rich dense zone. The excess oxygen in the secondary gas is able to hasten the oxidation of unreacted fuel nitrogen and the precursors such as HCN and NH3 coming from the primary gas zone. These unstable precursors will be oxidized to NO once the secondary gas is injected, as follows:34

Figure 8. Effect of SG and its location on temperature profile along the combustor.

and condition 6. It suggests that the temperatures can be maintained unvaryingly (about 850 °C) when the secondary gas injection is at 650 mm and 1000 mm above the distributor. However, when the secondary gas injection was at 1700 mm, the temperatures of the upper combustor and the cyclone rose continuously, nearly 70 °C higher than those at the lower part of the combustor. It seems that postcombustion, which is defined here as fuel or combustible gases burning in the upper part of the combustor and in the cyclone, occurring in condition 6, causes the upturning of the temperatures of flue gas. It is difficult to retain the combustion stability as well as the combustion efficiency under this condition.46,47 The effect of the location of the secondary gas on the NO concentration axial profile along the combustor is given in Figure 9. As can be seen, the formation of NO in the dense bed

HCN + 2.5O2 → 2NO + 2CO + H 2O

(7)

2NH3 + 2.5O2 → 2NO + 3H 2O

(8)

The reactions contribute to the formation of NO near the secondary gas injection point. Tarelho et al.49 found a similar tendency with bituminous coal in a bubbling fluidized bed in air-firing. Their results were similar to those that have been presented in this paper because of similar excess oxygen ratio in the secondary gas zone, which can be defined as eq 9. excess oxygen ratio in secondary zone actual oxygen quantity in secondary zone = total theoretical oxygen quantity

(9)

For example, the emission of NO increased nearly by 20% with the excess oxygen ratio in the secondary air zone being 0.44 in Tarelho’s experiments, while the emission of NO increased nearly by 22% with the excess oxygen ratio in the secondary gas zone being 0.45 in condition 4. 3.3. Effect of Primary/Secondary Gas Ratio on NO Emission. The effect of the primary/secondary gas ratio on the temperatures at the combustor and at the cyclone in a 50% O2/ 50% CO2 atmosphere is given in Figure 10. As can be seen, the temperatures at the combustor can be maintained constantly around 850 °C when the primary/secondary gas ratio is changed during different conditions. Then we can obtain the conclusion: the variations of NO emission at these four different conditions are not due to the change of the temperatures. The effect of the primary/secondary gas ratio on the NO axial concentration profile along the combustor is shown in Figure 11. As can be seen from the axial profile, with the PG/ SG ratio dwindling, the formation of NO at FG1 reduces.

Figure 9. Effect of SG and its location on NO emission profile along the combustor. 3307

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Energy & Fuels αo ‐ primary_zone =

actual oxygen quantity in primary zone total theoretical oxygen quantity (10)

The values of αo‑primary_zone corresponding to the primary/ secondary gas ratios are shown in Table 4. The relationship of αo‑primary_zone, NO emission, and CO emission at Outlet is given in Figure 12. Table 4. Correspondence between PG/SG Ratio and αo‑primary_zone condition

PG/SG ratio

αo‑primary_zone

3 7 5 8

100/0 80/20 60/40 50/50

1.15 0.92 0.69 0.58

Figure 10. Effect of PG/SG ratio on the temperature profiles along the combustor.

Figure 12. Effect of αo‑primary_zone on NO emission and CO emission at Outlet.

As is shown, the emission of NO at Outlet drops significantly as αo‑primary_zone falls, particularly when αo‑primary_zone drops to as low as 1. However, with the αo‑primary_zone reducing further, the decrease rate of NO begins to lessen. It is caused by the oxidation of the secondary gas. With the fraction of the secondary gas increasing, the oxidizing atmosphere is enhanced in the secondary gas zone, where the NO reduction reaction competes with the oxidation of fuel-N (including volatile nitrogen and char nitrogen) and NO precursors (HCN and NH3) to NO.51 Although the CO concentration grows gradually with the secondary gas ratio increase, which may be the result of the shorter residence time, the concentration of circulating particles, such as chars, falls at the upper part of the combustor, and both NO/CO and NO/char reactions cannot occur easily as a result of the loss of catalysts. Therefore, the conversion of fuel nitrogen to NO becomes dominant. So the rate of decrease of NO begins to lessen. If αo‑primary_zone falls further, the combustion fraction of the upper part of the combustor possesses more and the combustion efficiency drops gradually. With αo‑primary_zone decreasing unceasingly, the rate of increase of CO accelerates. As a consequence, the αo‑primary_zone does not have to be maintained at a very low level, nor does the PG/SG ratio. In other words, there may be a limitation of gas staging for NO

Figure 11. Effect of PG/SG on NO concentration profile along the combustor.

Although the oxidation of the secondary gas injection may be enhanced, the final NO emission falls apparently as the secondary gas rises, as shown in Table 3. Wang et al.50 found that one of the measures to sway the NO emission was altering the fuel nitrogen partition into char nitrogen, NH3, HCN, and N2. When the secondary gas was introduced into the combustor, the primary gas was moderated proportionally to keep the total amount of oxygen constant. The diminution of the primary gas can reinforce the reducing atmosphere in the primary gas zone. As a result, the coal in the primary gas zone may be pyrolized, but it cannot burn utterly. Both the CO concentration and the amount of unburned char increase considerably in the primary gas zone. The unburned chars can offer more surfaces for NO aggregation, and both the NO/CO and the NO/char reaction are intensified. The excess oxygen ratio in the primary gas zone αo‑primary_zone, which is defined as eq 10, can be used to label the degree of the reducing atmosphere in the primary gas zone. 3308

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residence time to sustain the heat balance. Temperatures at the upper part rises substantially, nearly 100 °C higher than that at the primary gas zone, as shown in Figure 14. The higher

emission in oxy-fuel combustion, especially with a high oxygen concentration, as a result of the oxidation of secondary gas at a high oxygen concentration and higher location of char burning. This effective range of the PG/SG ratio may depend on the actual parameters of different combustors. In these experiments, it is suitable for the ratio to be kept at 60/40 (αo‑primary_zone is 0.69). 3.4. Effect of Oxygen Staging on NO Emission. Compared with conventional air-firing, it is possible to change the oxygen concentration in the primary gas and the secondary gas separately. It is referred to as oxygen staging. In the following experiments, the oxygen concentration of the primary gas was changed to explore its influence on the formation of NO in the primary gas zone. The experiments were at condition 3, condition 5, condition 9, and condition 10. Though the oxygen concentration of the primary gas was changed, the total oxygen concentration was maintained at 50%. The effect of oxygen staging on the NO axial concentration profile is given in Figure 13. As can be seen,

Figure 14. Effect of O2 concentration in PG on temperature profiles along the combustor height.

temperatures in the combustor and the rising of char burning zone have negative effect on the final emission of NO, as mentioned previously. Duan et al.29 has reported a similar tendency of temperatures during oxygen staging. Compared with the results stated by Duan et al., the overall oxygen concentration rose to 50% in this paper and the oxygen concentration of the primary gas dived to 30%, which may restrict the formation of NO in the primary zone more effectively. Correspondingly, the highest secondary gas oxygen concentration was elevated to 80%, which means it is more difficult to keep the combustion stable. Nevertheless, if there were enough heat transfer surfaces in the upper part of the combustor to balance the temperatures, such as in the pilotscale or utility scale furnaces, the formation of NO in the secondary gas zone could be reduced considerably and the final emission of NO can be controlled effectively. For oxygen staging, the key factor may be the measures that need to be taken to realize the stable operation of the combustion system. If the combustor can keep steady, oxygen staging may hold great potential. Meanwhile, for flue gas recycling in the oxy-fuel combustion, NO, CO, and H2O existing in the flue gas may have significant effects on the fuel-nitrogen conversion in the combustor. This will be our next step to study the effects of oxygen staging on NO emissions in the new 1 MWth pilot-scale oxy-fuel CFB.52

Figure 13. Effect of O2 concentration in PG on NO concentration profile along the combustor height.

the formation of NO in the primary gas zone (FG1) decreases noticeably, which is lower than that at condition 3. When the oxygen concentration of the inlet stream fell, the concentration of NO at FG1 reduced apparently. With the oxygen concentration of the primary gas dropping, the reducing atmosphere in the primary gas zone is strengthened. The incomplete combustion of carbon and the CO2 gasification reaction36 cause the accumulation of char and CO, which provide the catalytic surfaces and reactants to promote the NO/char and NO/CO reduction reactions, as previously illustrated in sections 3.1 and 3.2. It also proves that the oxygen concentration of the primary gas has more significant influence on the formation of NO in the primary gas zone than the primary/secondary gas ratio. With the oxygen concentration of the primary gas decreasing, the combustion fraction of the secondary gas zone spreads noticeably, like the location of char burning being higher. The formation peak of NO near the location of secondary gas injection becomes more and more obvious, as a result of the combination of PG/SG ratio and the enrichment of oxygen in secondary gas. Unfortunately, because of the limitation of the combustor, there are not enough heat transfer surfaces and

4. CONCLUSIONS The effects of gas staging on the axial formation and emission of NO along the combustor with high oxygen concentrations in the circulating fluidized bed combustion were investigated in this paper. The following conclusions can be drawn from the investigation: NO can be destructed along the combustor in oxy-fuel firing after it forms in the dense zone, similar as that in air-firing. The formation of NO in the dense zone in oxy-fuel firing is slightly higher than that in air-firing, but the destruction degree of NO in oxy-fuel firing is stronger than that in air-firing, so that the 3309

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Energy & Fuels final emission of NO in oxy-fuel firing is less than that in airfiring. Raising the height of the secondary gas injection point is an effective way to reduce the formation of NO in the dense zone as well as the final emissions of NO in oxy-fuel firing, if the combustion efficiency and combustion stability can be preserved. The formation of NO in the dense zone can be restricted by gas staging. Lessening the primary/secondary gas ratio can regulate the formation and the final emission of NO, especially when the αo‑primary_zone drops to as low as 1. However, the effect will become negative gradually as a result of the oxidation of secondary gas at a high oxygen concentration and higher location of char burning. There may be a limitation of gasstaging on oxy-fuel CFB combustion with high oxygen concentration. Oxygen staging is a more effective and feasible way to reduce the formation of NO in oxy-fuel firing. There is a more significant effect on the inhibition of NO formation in the primary zone by oxygen staging than by gas staging through adjusting the primary/secondary gas ratio only.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-82543055. Fax:+86-10-82543119. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA07030200) and the External Cooperation (BIC), Chinese Academy of Sciences (Grant GJHZ201301).

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