Experimental Study on Nitrogen Transformation in Combustion of

Article pubs.acs.org/EF

Experimental Study on Nitrogen Transformation in Combustion of Pulverized Semi-coke Preheated in a Circulating Fluidized Bed Yao Yao,† Jian-guo Zhu,*,‡ and Qing-gang Lu‡ †

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

‡

ABSTRACT: A series of pulverized semi-coke combustion experiments were carried out to investigate the characteristics of fuelN transformation and the effect of operating conditions in a circulating fluidized bed (CFB) and a down-fire combustor (DFC) on fuel-N transformation. The fuel-N transformation takes place in two stages: first, the fuel-N in pulverized semi-coke is partly converted to N2 and NH3 in a circulating fluidized bed; second, the fuel-N in preheated semi-coke and NH3 is converted to NOx or N2 in a down-fire combustor. The fuel-N in the preheated semi-coke and the NH3 concentration in the syngas both decreased with increasing λCFB in the preheating process. The distribution of the secondary air determined the paths of NH3 conversion in the reducing zone. The air staging of the tertiary air promoted NH3, which was generated for fuel-N, converting to N2 in a complete combustion process. With this process, the NOx emissions were effectively limited. The lowest NO emission from combustion of pulverized semi-coke was 50 mg/m3 (about 60 mg/m3 NOx) (at 6 vol % free O2), which satisfied the emission standard of air pollutants for thermal power plants in China.

1. INTRODUCTION Low-rank coal accounts for about 46% of whole coal resources in China. Low-rank coal pyrolysis technology is an effective route for high-efficiency usage of low-rank coal. Pulverized semi-coke as a byproduct from the pyrolysis of low-rank coal is characterized by low volatile content, high ignition point, and difficulty in complete burning out.1,2 Typical semi-coke combustion processes solve these problems through raising the combustion temperature and prolonging the residence time. However, increasing combustion temperatures promotes the NOx formation. With the ever-increasing environmental pressure, it is a critical issue to dispose of the semi-coke in a clean and effective way. Ouyang et al.3 applied a new technology to pulverized anthracite for clean and effective combustion. The new combustion technique included dissecting the traditional combustion process into two parts: the fuel was first preheated in a circulating fluidized bed (CFB) and then burned in a down-fired combustor (DFC). The advantageous characteristics of the technology include low NOx emissions and good combustion efficiency. The combustion characteristics of pulverized anthracite after preheating in the CFB were studied. The effects of operating conditions in the CFB and DFC on NOx emissions were also previously investigated. As reported, the NO x emissions decreased with increasing the air equivalence ratio in the CFB (λCFB) or decreasing the air equivalence ratio in the reducing zone in the DFC (λRZ).4 However, the characteristics of fuel-N transformation in the preheating and combustion process have not been investigated, and the effects of operation conditions on fuel-N transformation have not been evaluated under different operation conditions. Therefore, the formation route of fuel-N to NOx is investigated in a pulverized semi-coke preheating and combustion system. The effects of operation conditions including the preheating process, reducing zone, and complete © 2015 American Chemical Society

combustion process on fuel-N transformation and NOx emissions were studied.

2. EXPERIMENTAL SECTION 2.1. Experimental Setup. The pulverized semi-coke preheating and combustion system is shown in Figure 1. This system consists of three parts: a CFB, a DFC, and an auxiliary system. The CFB riser is 90 mm in diameter and 1500 mm high. The primary air, which accounted for 15%−40% of the stoichiometric air requirement, was supplied from the bottom of the CFB as a fluidizing and oxidizing agent. The feeding pipe connects with the bottom of the CFB riser. High-temperature gases (called syngas) containing CO, H2, and CH4 were produced in the CFB. The fine particles escaping the cyclone (called preheated semi-coke) were carried over by the syngas flow into the DFC to achieve efficient clean combustion. The preheated semi-coke and syngas flow into the DFC, 3 m high and 0.26 m in diameter, through the nozzles shown in Figure 2. The secondary air was supplied from the top through different nozzles shown in Figure 2. Nozzle a is a center spraying nozzle of the secondary air, and nozzle b is a ring-shaped nozzle (total of 4 spouts). The tube ring diameter of the ring-shaped tube is 120 mm, and the diameter of the ring-shaped spray pipe is 11 mm. The flow rates at nozzles a and b are controlled by flowmeters. The tertiary air was introduced at a position 600 and 1000 mm below the nozzle to ensure complete combustion. A reducing zone was formed between the secondary air and tertiary air ports in order to control the NOx formation. The sampling points are set as follows: (1) The port at the outlet of the CFB is for syngas and preheated semi-coke. (2) The port 1500 mm above the bottom of the CFB riser is for particles in the preheating process. (3) Five ports located at 100, 400, 900, 1400, and 2400 mm along the DFC below the nozzle are for flue gas and residues. (4) The port at the outlet of a bag filter is for sampling fly ash. Four Ni−Cr/Ni-Si thermocouples in the CFB and five Pt/Pt−Rh Received: April 12, 2015 Revised: May 25, 2015 Published: May 26, 2015 3985

DOI: 10.1021/acs.energyfuels.5b00791 Energy Fuels 2015, 29, 3985−3991

Article

Energy & Fuels

Figure 3. Size distribution of the pulverized semi-coke. Mastersizer 2000 laser analyzer. The size range of pulverized semi-coke is 0−0.5 mm, and the d50 is 103 μm. The nitrogen functional groups in the pulverized semi-coke were measured by X-ray photoelectron spectroscopy (XPS). The result is shown in Figure 4, where Raw is the tested peak-value curve; N-Q, N-

Figure 1. Pulverized semi-coke preheating and combustion system. (1) Air compressor; (2) Liquefied petroleum gas; (3) Screw feeder; (4) Electric heater; (5) Primary air pipe; (6) CFB; (7) Sampling nozzle (1500 mm); (8) Sampling nozzle; (9) Secondary air; (10) Sampling nozzle (100 mm); (11) Sampling nozzle (400 mm); (12) Sampling nozzle (900 mm); (13) Sampling nozzle (1400 mm); (14) Sampling nozzle (2400 mm); (15) Tertiary air (at 600 mm); (16) Tertiary air (at 1000 mm); (17) DFC.

Figure 4. XPS graphs and peak splits of pulverized semi-coke. X, and N-6 are the results of peak splits. Nitrogen in the pulverized semi-coke exists mainly as N-Q, N-6, and N-X. Specifically, N-6 is pyridine nitrogen mainly appearing in the margins of aromatic structure units; N-Q is quaternary nitrogen mainly appearing in the interior of aromatic structure units; N-X is nitrogen oxides in which the nitrogen atoms in pyridine directly connects with oxygen atoms.5−8 The peak value of N-Q is higher versus N-6 or N-X. Since the pulverized semi-coke was a byproduct from the pyrolysis of Shenmu coal, during the pyrolysis, the branched chains in molecular structure were destroyed, and the pulverized semi-coke underwent polycondensation, which increased the aromatic degree and led to the enrichment of more nitrogen inside the aromatic rings.9 As a result, the existing form of nitrogen was stabilized, and thus, the release of nitrogen from the pulverized semi-coke was more difficult than from the original coal.10,11 2.3. Experimental Conditions. The experimental conditions are listed in Table 2. Case 1, case 2, and case 6 were used to investigate the

Figure 2. Arrangement of the secondary air nozzles.

thermocouples in the DFC are used to measure the temperature. The syngas produced in the CFB and other gases generated in the DFC are measured by a Gasmet FTIR DX-4000 analyzer. The O2 concentration along the DFC is measured by the KM9106. 2.2. Pulverized Semi-coke Characteristics. The pulverized semi-coke used in the experiments was crushed from Shenmu semicoke. The proximate and ultimate analysis results are shown in Table 1. Figure 3 shows the size distribution of pulverized semi-coke. The size distribution of pulverized semi-coke was obtained using a Malvern

Table 1. Proximate and Ultimate Analysis of Pulverized Semi-coke lower heating value MJ·kg−1

proximate analysis wt %

ultimate analysis wt %

Qnet,ar

Mar

Aar

Vdaf

FCar

Car

Har

Oar

Nar

Sar

23.32

14.6

11.36

7.31

66.72

68.31

0.85

4.01

0.58

0.3

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Energy & Fuels Table 2. Experimental Conditions parameter

case 1

case 2

feed rate (kg/h) λCFB λRZ flow ratio (nozzle A:B) λ (at 600 mm) λ (at 1000 mm)

4.19

4.34

4.34

case 3

4.34

case 4

4.34

case 5

case 6 4.45

0.38 0.68 0:1

0.55 0.74 1:0

0.55 0.74 0.75:0.25

0.55 0.74 0.5:0.5

0.55 0.74 0.75:0.25

0.67 0.83 0:1

1.3

1.13

1.13

1.13

0.96

0.90

1.3

1.13

1.13

1.13

1.13

1.13

effect of the air equivalence ratio in the CFB (λCFB) on the fuel-N conversion in the preheating process. Case 2, case 3, and case 4 were used to study the nitrogen transformation in the reduction zone. Case 3 and case 5 were used to investigate the effect of the tertiary air on NOx formation in the complete combustion process. Based on the ash balance of the preheating process, the conversion rate of each component in preheated semi-coke was calculated:12 CN = 1 −

A1 × X 2 A 2 × X1

Figure 5. Temperature distributions in the CFB in case 1, 2, and 6.

Figure 7 shows the relationship between nitrogen conversion and carbon conversion. The results indicated that the nitrogen conversion increased with an increase in the carbon conversion, indicating that the nitrogen transformation is sensitive to the carbon conversion. Due to the low volatile content in pulverized semi-coke, the nitrogen in pulverized semi-coke is mainly char-N. Therefore, when carbon conversion increased, the nitrogen was released and the nitrogen conversion increased. The nitrogen-containing compounds in the syngas at the outlet of CFB were sampled and analyzed. The results showed that only NH3 was detected in the syngas. Similar results were obtained in a previous study.3 Figure 8 shows the changes in NH3 concentration in the syngas along with λCFB. As λCFB increased from 0.38 to 0.67, the NH3 concentration decreased from 898 to 31 mg/m3, which was consistent with a previous report.14 As reported, N-5 was more likely to be converted to HCN in pyrolysis below 700 °C, while, above 700 °C, the generation of NH3 would be accelerated.14−16 As shown in Figure 4, there was no N-5 in the pulverized semi-coke, and the preheating temperatures in case 1, 2, and 6 were all higher than 700 °C. Thus, the nitrogen in the pulverized semi-coke was not converted to HCN in the preheating process. A previous study showed that N-Q was converted to NH3 in a reducing atmosphere.17 Particles from 1500 mm above the bottom of the CFB riser were sampled and measured by XPS. Figure 9 shows the distributions of nitrogen functional groups for different λCFB. The results indicated the increasing λCFB decreased the counts value of N-Q. When λCFB increased from 0.38 to 0.67, the counts value of N-Q decreased from 320 to 240, which lowered the NH3 concentration in the syngas. 3.2. Effects of the Secondary Air Distribution in Reducing Zone on the Nitrogen Conversion. The temperature of the preheated semi-coke and syngas was higher than 800 °C, which is higher than the self-ignition point, and the combustion process was stable. The maximum combustion temperature in the DFC was 1050 °C. Nitrogen sources entering the DFC included NH3 in the syngas and fuel-N in preheated semi-coke. The conversion of NH3 was different from that of fuel-N in preheated semi-coke. To study the difference between NH3 conversion and fuel-N conversion in the reducing zone, two kinds of secondary air nozzles were designed, taking into consideration the NH3

(1)

where A1 and A2 are the ash content in the pulverized semi-coke and in the preheated semi-coke, respectively; X1 and X2 are the content of component X in the pulverized semi-coke and in the preheated semicoke, respectively. The air equivalence ratio in the CFB, The air equivalence ratio in the reducing zone, and the excess air ratio in this system were defined as follows, respectively:3

λCFB = λRZ =

λ=

APR A STOI

(2)

APR + A SE A STOI

(3)

APR + A SE + A TE A STOI

(4)

APR is the primary air flow supplied to the CFB (m /h), ASE is the secondary air flow supplied to the DFC (m3/h), ATE is the tertiary air flow supplied to the DFC for complete combustion (m3/h), and ASTOI is the air flow for stoichiometric combustion (m3/h). 3

3. RESULTS AND DISCUSSION 3.1. Effects of Preheating on Nitrogen Conversion. Figure 5 shows the temperature distributions in the CFB in case 1, 2, and 6. The temperature profile indicated that the λCFB had almost no influence on the temperature profile in the CFB. The preheating process was energy self-sustaining, and the heat releasing from partial combustion of the pulverized semi-coke could maintain the preheating temperature at 900 °C. The preheated semi-coke was sampled and analyzed at the outlet of CFB. The results of proximate and ultimate analysis are listed in Table 3. According to eq 1, the conversion of carbon and nitrogen were calculated in different cases. Figure 6 shows the fuel-N conversion for different λCFB values in the CFB. The results showed that increasing λCFB could promote the fuel-N conversion. When λCFB increased from 0.38 to 0.67, the nitrogen conversion in the preheating process increased from 60.78% to 85.01%. This result was consistent with a previous report.13 3987


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Energy & Fuels Table 3. Proximate and Ultimate Analysis of Preheated Semi-cokes lower heating value MJ·kg−1 Case 1 Case 2 Case 6



Qnet,ar

Mar

Aar

Vdaf

FCar

Car

Har

Oar

Nar

Sar

22.74 19.71 17.39

0.93 0.83 0.85

30.47 42.35 49.64

4.08 2.68 2.13

64.44 54.14 47.38

65.51 54.63 47.69

0.74 0.29 0.21

0.99 0.75 0.55

0.61 0.40 0.38

0.74 0.75 0.68

Figure 8. NH3 concentration in the syngas with λCFB.

Figure 6. Fuel-N conversion rate of pulverized semi-coke with λCFB.

Figure 9. Distribution of nitrogen functional groups with λCFB. Figure 7. Fuel-N conversion rate with carbon conversion rate.

oxidization was faster than fuel-N oxidization. Nozzle a is set at the center of the DFC, while nozzle b is set at the four nozzles surrounding the fuel spray pipe. The secondary air is completely sprayed inside from nozzle a in case 2; the flow ratio of nozzle a to nozzle b is 0.75:0.25 in case 3 and 0.5:0.5 in case 4. The temperature profiles in the DFC under the three conditions are shown in Figure 10. Since the amounts of the preheated fuel and the secondary air are the same, the temperatures in the DFC at a distance of 100 mm from the top are basically similar. When the secondary air entered through different nozzles, the resulting mixing effects were also different, and the temperature distributions in the DFC after the distance of 900 mm were also changed modestly. Previous studies indicated that thermal NO was generated above 1500 °C, and prompt NO was nearly ignored in the

Figure 10. Temperature distributions in the DFC in case 2, 3, and 4.

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Energy & Fuels combustion.18,19 Therefore, NO in the DFC was mainly transformed by the fuel-N and NH3. Figure 11 shows the

Figure 12. Profiles of O2 concentration along the axis of the DFC in case 2, 3, and 4. Figure 11. Profiles of NO concentration along the axis of the DFC in case 2, 3, and 4.

atmosphere, limits the formation of NO at the reduced zone in the DFC. Figure 13 shows the changes in NH3 concentration in the DFC. The curve of NH3 concentration in case 2 was the

profiles of NO concentration along the axis of the DFC under different conditions of the secondary air nozzles. The results indicated that the curve of NO concentration in case 2 was different from those in conditions case 3 and 4. NO concentration was zero at the outlet of the CFB due to the reducing atmosphere, and the peak of NO concentration appeared at a position 100 mm below the nozzle in case 2. These results are basically consistent with a previous report.3 Ouyang et al.3 investigated the combustion of anthracite in a similar system. The NO concentration went up to a maximum at a position 400 mm below the nozzle. The NOx might be transformed by the precursors, such as NH3 and HCN. The residues at the position 100 mm below the nozzles were sampled and analyzed. The proximate and ultimate analysis results are shown in Table 4. Based on the ash balance of preheated semi-coke and residues 100 mm below the nozzle, the released fuel-N was about 300 mg, and the nitrogen of NO was 1100 mg. Therefore, NH3 in syngas was a main contribution to NOx in the position 100 mm below the nozzle. Figure 12 shows the profiles of O2 concentration along the centerline of the DFC for different secondary air nozzles. The results showed that the peak of O2 concentration appeared at 100 mm below the nozzle in case 2, which was the same position for maximum NO concentration. These results indicated that this zone of 0−100 mm was under a local oxidized atmosphere, and part of NH3 in the syngas was oxidized to NO, leading to the rapid formation of NO. The secondary air was uniformly distributed in case 3 and 4, which avoided the appearance of local oxidized zones. The O2 concentration is 0 in the reducing zone avoiding the formation of NO.20,21 Clearly, efficient mixing between the secondary air and preheated fuel, and elimination of the local oxidized

Figure 13. Profiles of NH3 concentration along the axis of the DFC in case 2, 3, and 4.

highest. The results of previous studies showed that the uniform distribution of O2 could promote the reduction of NH3.22 In case 2, the local oxidized atmosphere delayed the NH3 reduction, and NH3 was accumulated along the DFC. The inlet of tertiary air converted part of fuel-N into NH3, and the NH 3 concentration increased slightly. Along with the progression of combustion, part of NH3 was converted into NO. Due to accumulation of NH3 along the DFC, the concentration of NO emissions in case 2 is obviously higher than that in case 3 and case 4.

Table 4. Proximate and Ultimate Analysis of Residues lower heating value MJ·kg−1



Qnet,ar

Mar

Aar

Vdaf

FCar

Car

Har

Oar

Nar

Sar

19.38

0.78

43.45

2.27

53.5

53.78

0.2

0.72

0.38

0.68

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Energy & Fuels

from 87.5 to 130 mg/m3. The combustion efficiencies3 in case 3 and case 5 were 98.01% and 97.74%, respectively.

In comparison, the NH3 was reduced effectively and no NO was formed within 0−900 mm from the DFC in case 3 or case 4. After the inlet of tertiary air at 600 mm from the DFC, the peak of NO appears at 1400 mm. These results are consistent with previous reports.20,21 3.3. Effects of the Tertiary Air Distribution on the Conversion of Nitrogen in the Preheated Fuel. To study the effects of the tertiary air on the conversion of nitrogen in the preheated fuel, the tertiary air was directly inputted at 600 mm in case 3, but at both 600 and 1000 mm in case 5. The flow ratio of the tertiary air at 600 and 1000 mm is 0.56:0.44. Figure 14 shows the profiles of NO concentration along the axis of the DFC under different conditions of the tertiary air.

4. CONCLUSIONS In this paper, the effects of operating conditions in a CFB and DFC on nitrogen transformation from pulverized semi-coke combustion preheated in a CFB were investigated. The conclusions can be summarized as follows: (1) Under the same preheating temperature, the fuel-N in the preheated semi-coke and the NH3 concentration in the syngas both decreased with increasing λCFB. When λCFB increased from 0.38 to 0.67, the NH3 concentration in the syngas decreased from 898 to 31 mg/m3, while the fuel-N in the preheated semi-coke decreased from 39.22% to 14.99%, reducing the fuel-N entering the DFC. (2) The maximum temperature was 1050 °C in the combustion process, limiting the thermal NOx generation. The peak of NO concentration at 100 mm below the nozzle was transformed by NH3, which corresponds to the maximum O2 concentration. Appropriate distribution of the secondary air can efficiently avoid the appearance of the local oxidized zone, promoting the NH3 reduction and reducing the accumulation of NH3 along the DFC, effectively. (3) When the tertiary air was introduced at 600 mm below the nozzle, the NH3 and fuel-N in residues had been oxidized to NOx. When the tertiary air ports were set up 600 and 1000 mm below the nozzle, the additional reducing zone was formed between 600 mm and 1000 mm below the nozzle, and part of fuel-N released in this zone would be converted to N2 instead of NO, which decreased final NOx generation in the complete combustion process. (4) The lowest NO emission is 50 mg/m3 (NOx 60 mg/m3) (at 6 vol % free O2), which is lower than the standard of atmospheric pollutants emissions for thermal power plants (GB13223-2011).

Figure 14. Profiles of NO concentration along the axis of the DFC in case 3 and 5.

The results indicated that the NO emission was 104 mg/m3 in case 3, which is reduced to 50 mg/m3 in case 5. The curve of NH3 concentration shown in Figure 12 showed that the NH3 concentration increased slightly with introducing the tertiary air at 600 mm below the nozzle, indicating that part of fuel-N was converted to NH3. The tertiary air was introduced at both 600 and 1000 mm below the nozzle in case 5. The additional reducing zone was formed between 600 mm and 1000 mm below the nozzle, and fuel-N released in this zone would be converted to N2 instead of NO, which decreased final NOx generation in the complete combustion process. Notably, the NOx emission was 60 mg/m3 (at 6 vol % free O2) in case 5 without SCR or SNCR, which is quite lower than the standard atmospheric pollutant emission limit for thermal power plants23 (100 mg/m3 in GB13223-2011). The theoretical NOx from the tested fuel was 2206 mg/m3 (at 6 vol % free O2), and the experimental NOx emission was 60 mg/m3 (at 6 vol % free O2). The comparison results showed that 97.3% of the theoretical NOx was reduced in the experiments. The CO emission was 87.5 mg/m3 in case 3, and the CO emission increased to 130 mg/m3 in case 5. All the tertiary air was introduced at 600 mm below the nozzle in case 3. The tertiary air was introduced at both 600 mm and 1000 mm below the nozzle in case 5. The λ at 600 mm below the nozzle in case 3 and 5 were 1.13 and 0.96. The λ at 1000 mm below the nozzle in case 5 was 1.13. The complete combustion process was shortened in case 5 which led to the increased CO emission. Since the complete combustion process was shortened in case 5, and the CO emission was increased

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+86 10 82543139. 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. XDA07030100).

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REFERENCES

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Experimental Study on Nitrogen Transformation in Combustion of

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