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Study of optimal pulverized coal concentration in a four-wall tangentially fired furnace
Applied Energy 88 (2011) 1164–1168
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Study of optimal pulverized coal concentration in a four-wall tangentially fired furnace Houzhang Tan ⇑, Yanqing Niu, Xuebin Wang, Tongmo Xu, Shien Hui State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, 710049, Shaanxi, China
a r t i c l e
i n f o
Article history: Received 30 March 2010 Received in revised form 24 August 2010 Accepted 28 September 2010 Available online 1 November 2010 Keywords: Pulverized coal concentration Temperature Fly ash Slag NOx
a b s t r a c t The effect of fuel lean/rich conditions (1:1, 1:2, 1:3, 1:4, 1:5 and 1:6) on the furnace core temperatures, carbon in fly ash and slag and NOx emissions was investigated in a 1 MW four-wall tangentially horizontal bias fired furnace for Yibin anthracite and Shenmu bituminous, respectively. Results shown that furnace core temperatures increased at first and then decreased along the height of the furnace when anthracite burned. The furnace core temperature at the height of primary air nozzles was the highest when the bituminous lean/rich varied from 1:1 to 1:3, and its trend was similar to the anthracite when the bituminous lean/rich was changed from 1:4 to 1:6. The ignition of anthracite required a heating stage, while bituminous could timely ignite due to high volatile. However, when the bituminous lean/rich was too low resulting in the relative lack of oxygen, it still needed a heating stage. With increased coal concentration, the furnace core temperatures in the primary air section went up firstly and then down, but the carbon in fly ash and slag showed adverse behavior. This was due to the high coal concentration corresponding to high volatile concentration leading to the timely ignition and burnout, causing higher furnace core temperature in the primary air section and decreased carbon in fly ash and slag. Corresponding to the highest furnace core temperature in the primary air section and the lowest carbon in fly ash and slag, the optimal pulverized coal concentration of anthracite and bituminous was 0.796–0.810 kg coal/kg air and 0.586–0.607 kg coal/kg air, respectively. In addition, with increased pulverized coal concentration, the NOx emissions reduced quickly with a slight decrease in the range of the optimal pulverized coal concentration. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The widely applied corner tangential coal-fired boilers in modern thermal power plants are with well-defined degree of flame propagation in the furnace, good flame stability and low NOx emissions. However, there are some inevitable disadvantages such as fouling and slagging in the burner zone and upper furnace, heat imbalance and gas temperature deviation in horizontal flue [1– 5]. Especially the heat imbalance in furnace results in high heat load in the middle of the furnace wall and relatively low heat load in the corners. With low heat load in the burner zone, insufficient jet entrainment leads to ignition delay, low burn-out rate and even extinguishing when low-quality coals are used as fuel in the boiler. Therefore, corner tangentially firing is more suitable for bituminous coal and better ignition characteristics lean coal. While the W-shaped flame which can entrainment the highest temperature flue gas inside the furnace is favorable for the combustion of anthracite [6].
⇑ Corresponding author. E-mail address: [email protected] (H. Tan). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.09.026
In four-wall tangentially fired furnace, the burners were arranged on the regions with maximum wall heat flux, i.e. in the center of the furnace walls. It not only takes advantage of the strong gas–powder mixture, the upstream high-temperature flue gas assisting ignition of the downstream pulverized coal (PC) and high burn-out rate, but also absorbs the advantages of the W-shaped flame which can entrainment the high-temperature flue gas inside the furnace. In addition, the high-temperature flue gas of the fourwall tangentially can scour directly the flame root of the burner nozzles, which is not possible for the corner tangentially firing and the W-shaped flame. Due to the jet entrainment at the center of the furnace wall and no jet entrainment at the corner in the four-wall tangentially fired boilers, the wall temperatures of the entire burner region tend to average. This can effectively prevents slagging, high temperature corrosion and tube explosion. In addition, compared with corner tangentially firing, the maximum wall temperature difference of the four-wall tangentially firing is only 24.75% for anthracite and 58.7% for bituminous [7]. Also the actual tangential circle diameter of the four-wall tangentially firing is only 3 times of the illusional tangential circle diameter (anthracite), while that of corner tangentially firing is 6–8 times [8].
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Meanwhile, with increasing of environmental problems from combustion of fossil fuels, air stage and reburning technology used to reduce NOx emissions have attracted worldwide attention [9– 15]. Some new burners such as horizontal bias burners have also emerged [16,17]. That not only reduces NOx, but also improves the ignition and burnout of low-quality coals. Although some aspects of four-wall tangentially horizontal bias firing have been studied, the optimal pulverized coal concentration (PCC) or coal/air ratio was uncertain. On that condition, the fuels in the boiler could be timely ignited and completely combusted. The main objective in present work was therefore to investigate the effect of fuel lean/rich on the ignition, burnout and NOx emissions in a four-wall tangentially horizontal bias fired furnace and further obtain the optimal fuel lean/rich or the optimal PCC. 2. Experiment 2.1. Test facility Fig. 1 is the scheme of the test facility of 1 MW (thermal power) PC furnace. It is a four-wall tangentially horizontal bias fired furnace, which is 4810 mm in height, 800 mm in width and 650 mm in depth. It is composed of air supply system, burner system and measuring system. The air supplied by a 75 kW dual-stage centrifugal blower passes through air preheater first and into the horizontal bias burners as primary air, secondary air and over fired air. Meanwhile, primary air is divided into two streams by a plate added in the PC pipe, one is for a rich coal stream and another is for a lean coal stream fed by micro-spiral powder machine with an accuracy of 3.5%. In addition, the secondary air is separated into upper and lower streams. Based on that section heat load and excessive air coefficient was 1.763 MW/m2 and 1.2, respectively, the parameters of the burners
Table 1 Parameters of burners. Parameter
Lower secondary air
Primary air
Upper secondary air
OFA
Vent area (mm2) Air temperature (°C) Air ratio (%) Wind velocity of anthracite (m/s) Wind velocity of bituminous (m/s)
1429 100 28.1 17.87
2 478 80 23.93 22.1
805 100 23.93 26.94
805 100 23.94 26.94
20.19
23.93
30.48
30.48
and the PCC of different fuel lean/rich are listed in Tables 1 and 2. The structure and layout of the burners in the furnace has been described in detail elsewhere [8,16]. In addition, the PCC referred in paper is the concentration of the rich coal stream. The measurement points of furnace core temperatures and NOx are deployed in the left wall, and the sampling hole of fly ash is laid in the back wall (Fig. 2). The heights of the temperature measurement points are 1000 mm, 1270 mm, 1540 mm, 1810 mm, 2080 mm, 2550 mm, 3020 mm and NOx measurement point is 3340 mm. Note that the elevation of the primary air injection nozzles is just 1000 mm. The furnace temperatures along the height were measured by water-cooled platinum–rhodium thermocouples. Fly ash and slag samples could be taken from the sampling port and the exit of the furnace using water-cooled stainless probes. In addition, NOx concentration was measured using on-line analyzer. 2.2. Fuels The fuels used in tests were Yibin anthracite and Shenmu bituminous, which are the representative anthracite and bituminous in China. They were from the powder separator of Yinbin power plant and Huaneng coal company-owned power plant in Shenmu. The proximate analysis of the fuels is shown in Table 3. 3. Results and discussion The effect of fuel lean/rich on furnace core temperature (FCT), carbon in fly ash and slag and NOx emissions was investigated with the fuel lean/rich ratio of 1:1, 1:2, 1:3, 1:4, 1:5 and 1:6, respectively. 3.1. Effect of PCC on FCT
Fig. 1. Scheme of the test facility: (1) micro-spiral powder machine (rich); (2) micro-spiral powder machine (lean); (3) lower secondary air; (4) primary air; (5) upper secondary air; (6) OFA; (7) slag sampler; (8) outlet of economizer; (9) inlet of economizer; (10) blower; (11) fan; (12) chimney and (13) furnace wall.
The effect of PCC on FCT is illustrated in Fig. 3. It can be seen in Fig. 3a that the FCT increased and then dropped along the height of the furnace when anthracite burned. The combustion became more and more intense along the flue gas until the highest temperature, after which most coal was burnout and the oxygen was relatively low leading to the temperature drop. The relation of FCT and PCC is shown in Fig. 3b when bituminous burned. The temperature in the primary air nozzles was the maximum when the fuel lean/rich was between 1:1 and 1:3, which means that bituminous can ignite timely once PC injected into the furnace, and then FCT decreased along the height of furnace. However, when the fuel lean/rich was between 1:4 and 1:6, maximum temperature shifted to the second or third temperature measurement point, which shows the timely ignition of coal and maximum temperature moved back when the PCC was so high resulting in the lack of oxygen. The ignition of bituminous was more timely than anthracite. The bituminous could ignited timely in the distance between the
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Table 2 PCC and coal velocity in different fuel lean/rich. Lean/rich
1:1 1:2 1:3 1:4 1:5 1:6
Anthracite
Bituminous
Concentration (kg coal/kg air)
Velocity (kg/min)
Concentration (kg coal/kg air)
Velocity (kg/min)
Rich
Lean
Rich
Lean
Rich
Lean
Rich
Lean
0.483 0.642 0.723 0.772 0.805 0.827
0.483 0.324 0.242 0.195 0.162 0.136
0.298 0.396 0.446 0.476 0.496 0.510
0.298 0.200 0.149 0.120 0.100 0.084
0.373 0.498 0.558 0.599 0.624 0.641
0.373 0.249 0.185 0.159 0.126 0.105
0.214 0.286 0.320 0.344 0.358 0.368
0.214 0.143 0.106 0.086 0.072 0.060
exit of nozzles and the center of the furnace, but the anthracite needed a long residence time in furnace. Meanwhile, it was found that both anthracite and bituminous the FCT in the primary air section were different with different PPC. When the temperature of the primary air section was high, the ignition was timely and the combustion reaction was intense. Conversely, when the concentration of volatile was not enough, the ignition timely was poor and the temperature in the primary air section was low. It could be seen from Table 4 that with increased PPC the FCT in the primary air section went down after a first peak. With
increased PPC, the concentration of volatile increased and the ignition was more timely. However, when the PPC was so high resulting in high concentration of volatile and lower oxygen, the ignition was delayed, which led to the low FCT in the primary air section. The effect of the PCC on the FCT in the primary air section is illustrated in Fig. 4. It can be seen that the optimal PCC of anthracite and bituminous was 0.810 kg coal/kg air and 0.607 kg coal/kg air. The optimal PCC is defined that the maximum FCT in the primary air section. 3.2. Effect of PCC on carbon in fly ash and slag It can be seen from Fig. 5 that with increased PCC, carbon in fly ash and slag decreased due to timely ignition and complete combustion. However, once the coal concentration was high, the relative oxygen concentration dropped resulting in ignition delay
Fig. 2. The scheme of measurement points: (1) burners; (2) measurement points of temperatures; (3) NOx measurement point; (4) measurement point of fly ash and (5) horizontal flue.
Table 3 Proximate analysis of the fuels. Fuel
Wad
Aad
Vad
Cad
Qha.ad
Anthracite Bituminous
1.41 4.53
37.29 8.82
9.85 35.55
51.45 51.1
20,562 kJ/kg 28,725 kJ/kg
Fig. 3. FCT vs. PCC: (a) anthracite and (b) bituminous.
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H. Tan et al. / Applied Energy 88 (2011) 1164–1168 Table 4 FCT of primary air section with different PCC. Anthracite
Bituminous
Concentration (kg coal/kg air)
Temperature (°C)
Concentration (kg coal/kg air)
Temperature (°C)
0.483 0.642 0.723 0.772 0.805 0.827
860 875 900 950 1075 1050
0.373 0.498 0.558 0.599 0.624 0.641
1435.5 1434.3 1453.7 1478.9 1464.7 1412.6
Fig. 6. NOx vs. PCC.
3.3. Effect of PCC on NOx
Fig. 4. Temperature of primary air section vs. PCC.
Seen from Fig. 6, with increased PCC, NOx reduced quickly with a gentle decrease nearby the optimal PCC. In rich coal stream, the poor oxygen content inhibited the formation of NOx, and in lean stream, NOx formation was limited because of the low temperature and less volatile nitrogen. Approaching the optimal PCC, the FCT was maximum and the emission of thermal-NOx was increased leading to the relative slight decrease of NOx. With further increase of PCC, the relative insufficient oxygen concentration resulted in ignition delay and incomplete combustion further causing low furnace temperature that inhibited formation of NOx.
4. Conclusion
Fig. 5. Carbon in fly ash and slag vs. PCC.
and incomplete combustion, which further caused the increased carbon in fly ash and slag. As an indicator of burn-out rate, high content of residue carbon in fly ash and slag reflected that the ignition and combustion efficiency was poor, and vice versa. Considering this aspect alone, the optimal PCC of anthracite and bituminous was 0.796 kg coal/kg air and 0.586 kg coal/kg air, respectively. Synthetically considering the effect of PCC on ignition timely and burn-out rate, the optimal PCC of anthracite and bituminous was suggested to be controlled between 0.796–0.810 kg coal/kg air and 0.586–0.607 kg coal/kg air in four-wall tangentially horizontal bias fired furnace.
The effect of fuel lean/rich (1:1, 1:2, 1:3, 1:4, 1:5 and 1:6) on the ignition, burnout and NOx emissions was investigated in a 1 MW four-wall tangentially horizontal bias fired furnace for Yibin anthracite and Shenmu bituminous. FCT went up first and then down along the height of the furnace when anthracite burned. While FCT at the height of primary air nozzles was the highest when the bituminous lean/rich was between 1:1 and 1:3, and the furnace core temperature also went up first and then down when the bituminous lean/rich was between 1:4 and 1:6. The ignition of anthracite needed a heating stage, while bituminous could timely ignite due to high volatile. Nevertheless, when the bituminous lean/rich was too low, it still needed a heating stage due to the high volatile concentration resulting in the relatively lack of oxygen. With increased PCC, FCT in the primary air section increased firstly and then decreased. The carbon in fly ash and slag were just adverse, it went down firstly and then increased. With increased PCC the concentration of volatile increased, which promoted the timely ignition and complete combustion causing increased FCT in the primary air section and decreased carbon in fly ash and slag. However, when the PCC was too high resulting in so high concentration of volatile and relative poor oxygen, the ignition was delayed leading to the relatively low FCT in the primary air section and higher carbon in fly ash and slag. Corresponding to the maximum FCT in the primary air section and the lowest carbon in fly ash and slag, the optimal PCC of anthracite and bituminous was between 0.796–0.810 kg coal/kg air and 0.586–0.607 kg coal/kg air in four-wall tangentially horizontal bias fired furnace. In addition, with increased PCC, NOx concentration reduced quickly with a slight decrease nearby the optimal PCC.
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Acknowledgement The present work was supported by the National Nature Science Foundation of China (No. 50976086).
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Study of optimal pulverized coal concentration in a four-wall tangentially fired furnace Houzhang Tan ⇑, Yanqing Niu, Xuebin Wang, Tongmo Xu, Shien Hui State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, 710049, Shaanxi, China
a r t i c l e
i n f o
Article history: Received 30 March 2010 Received in revised form 24 August 2010 Accepted 28 September 2010 Available online 1 November 2010 Keywords: Pulverized coal concentration Temperature Fly ash Slag NOx
a b s t r a c t The effect of fuel lean/rich conditions (1:1, 1:2, 1:3, 1:4, 1:5 and 1:6) on the furnace core temperatures, carbon in fly ash and slag and NOx emissions was investigated in a 1 MW four-wall tangentially horizontal bias fired furnace for Yibin anthracite and Shenmu bituminous, respectively. Results shown that furnace core temperatures increased at first and then decreased along the height of the furnace when anthracite burned. The furnace core temperature at the height of primary air nozzles was the highest when the bituminous lean/rich varied from 1:1 to 1:3, and its trend was similar to the anthracite when the bituminous lean/rich was changed from 1:4 to 1:6. The ignition of anthracite required a heating stage, while bituminous could timely ignite due to high volatile. However, when the bituminous lean/rich was too low resulting in the relative lack of oxygen, it still needed a heating stage. With increased coal concentration, the furnace core temperatures in the primary air section went up firstly and then down, but the carbon in fly ash and slag showed adverse behavior. This was due to the high coal concentration corresponding to high volatile concentration leading to the timely ignition and burnout, causing higher furnace core temperature in the primary air section and decreased carbon in fly ash and slag. Corresponding to the highest furnace core temperature in the primary air section and the lowest carbon in fly ash and slag, the optimal pulverized coal concentration of anthracite and bituminous was 0.796–0.810 kg coal/kg air and 0.586–0.607 kg coal/kg air, respectively. In addition, with increased pulverized coal concentration, the NOx emissions reduced quickly with a slight decrease in the range of the optimal pulverized coal concentration. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The widely applied corner tangential coal-fired boilers in modern thermal power plants are with well-defined degree of flame propagation in the furnace, good flame stability and low NOx emissions. However, there are some inevitable disadvantages such as fouling and slagging in the burner zone and upper furnace, heat imbalance and gas temperature deviation in horizontal flue [1– 5]. Especially the heat imbalance in furnace results in high heat load in the middle of the furnace wall and relatively low heat load in the corners. With low heat load in the burner zone, insufficient jet entrainment leads to ignition delay, low burn-out rate and even extinguishing when low-quality coals are used as fuel in the boiler. Therefore, corner tangentially firing is more suitable for bituminous coal and better ignition characteristics lean coal. While the W-shaped flame which can entrainment the highest temperature flue gas inside the furnace is favorable for the combustion of anthracite [6].
⇑ Corresponding author. E-mail address: [email protected] (H. Tan). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.09.026
In four-wall tangentially fired furnace, the burners were arranged on the regions with maximum wall heat flux, i.e. in the center of the furnace walls. It not only takes advantage of the strong gas–powder mixture, the upstream high-temperature flue gas assisting ignition of the downstream pulverized coal (PC) and high burn-out rate, but also absorbs the advantages of the W-shaped flame which can entrainment the high-temperature flue gas inside the furnace. In addition, the high-temperature flue gas of the fourwall tangentially can scour directly the flame root of the burner nozzles, which is not possible for the corner tangentially firing and the W-shaped flame. Due to the jet entrainment at the center of the furnace wall and no jet entrainment at the corner in the four-wall tangentially fired boilers, the wall temperatures of the entire burner region tend to average. This can effectively prevents slagging, high temperature corrosion and tube explosion. In addition, compared with corner tangentially firing, the maximum wall temperature difference of the four-wall tangentially firing is only 24.75% for anthracite and 58.7% for bituminous [7]. Also the actual tangential circle diameter of the four-wall tangentially firing is only 3 times of the illusional tangential circle diameter (anthracite), while that of corner tangentially firing is 6–8 times [8].
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Meanwhile, with increasing of environmental problems from combustion of fossil fuels, air stage and reburning technology used to reduce NOx emissions have attracted worldwide attention [9– 15]. Some new burners such as horizontal bias burners have also emerged [16,17]. That not only reduces NOx, but also improves the ignition and burnout of low-quality coals. Although some aspects of four-wall tangentially horizontal bias firing have been studied, the optimal pulverized coal concentration (PCC) or coal/air ratio was uncertain. On that condition, the fuels in the boiler could be timely ignited and completely combusted. The main objective in present work was therefore to investigate the effect of fuel lean/rich on the ignition, burnout and NOx emissions in a four-wall tangentially horizontal bias fired furnace and further obtain the optimal fuel lean/rich or the optimal PCC. 2. Experiment 2.1. Test facility Fig. 1 is the scheme of the test facility of 1 MW (thermal power) PC furnace. It is a four-wall tangentially horizontal bias fired furnace, which is 4810 mm in height, 800 mm in width and 650 mm in depth. It is composed of air supply system, burner system and measuring system. The air supplied by a 75 kW dual-stage centrifugal blower passes through air preheater first and into the horizontal bias burners as primary air, secondary air and over fired air. Meanwhile, primary air is divided into two streams by a plate added in the PC pipe, one is for a rich coal stream and another is for a lean coal stream fed by micro-spiral powder machine with an accuracy of 3.5%. In addition, the secondary air is separated into upper and lower streams. Based on that section heat load and excessive air coefficient was 1.763 MW/m2 and 1.2, respectively, the parameters of the burners
Table 1 Parameters of burners. Parameter
Lower secondary air
Primary air
Upper secondary air
OFA
Vent area (mm2) Air temperature (°C) Air ratio (%) Wind velocity of anthracite (m/s) Wind velocity of bituminous (m/s)
1429 100 28.1 17.87
2 478 80 23.93 22.1
805 100 23.93 26.94
805 100 23.94 26.94
20.19
23.93
30.48
30.48
and the PCC of different fuel lean/rich are listed in Tables 1 and 2. The structure and layout of the burners in the furnace has been described in detail elsewhere [8,16]. In addition, the PCC referred in paper is the concentration of the rich coal stream. The measurement points of furnace core temperatures and NOx are deployed in the left wall, and the sampling hole of fly ash is laid in the back wall (Fig. 2). The heights of the temperature measurement points are 1000 mm, 1270 mm, 1540 mm, 1810 mm, 2080 mm, 2550 mm, 3020 mm and NOx measurement point is 3340 mm. Note that the elevation of the primary air injection nozzles is just 1000 mm. The furnace temperatures along the height were measured by water-cooled platinum–rhodium thermocouples. Fly ash and slag samples could be taken from the sampling port and the exit of the furnace using water-cooled stainless probes. In addition, NOx concentration was measured using on-line analyzer. 2.2. Fuels The fuels used in tests were Yibin anthracite and Shenmu bituminous, which are the representative anthracite and bituminous in China. They were from the powder separator of Yinbin power plant and Huaneng coal company-owned power plant in Shenmu. The proximate analysis of the fuels is shown in Table 3. 3. Results and discussion The effect of fuel lean/rich on furnace core temperature (FCT), carbon in fly ash and slag and NOx emissions was investigated with the fuel lean/rich ratio of 1:1, 1:2, 1:3, 1:4, 1:5 and 1:6, respectively. 3.1. Effect of PCC on FCT
Fig. 1. Scheme of the test facility: (1) micro-spiral powder machine (rich); (2) micro-spiral powder machine (lean); (3) lower secondary air; (4) primary air; (5) upper secondary air; (6) OFA; (7) slag sampler; (8) outlet of economizer; (9) inlet of economizer; (10) blower; (11) fan; (12) chimney and (13) furnace wall.
The effect of PCC on FCT is illustrated in Fig. 3. It can be seen in Fig. 3a that the FCT increased and then dropped along the height of the furnace when anthracite burned. The combustion became more and more intense along the flue gas until the highest temperature, after which most coal was burnout and the oxygen was relatively low leading to the temperature drop. The relation of FCT and PCC is shown in Fig. 3b when bituminous burned. The temperature in the primary air nozzles was the maximum when the fuel lean/rich was between 1:1 and 1:3, which means that bituminous can ignite timely once PC injected into the furnace, and then FCT decreased along the height of furnace. However, when the fuel lean/rich was between 1:4 and 1:6, maximum temperature shifted to the second or third temperature measurement point, which shows the timely ignition of coal and maximum temperature moved back when the PCC was so high resulting in the lack of oxygen. The ignition of bituminous was more timely than anthracite. The bituminous could ignited timely in the distance between the
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Table 2 PCC and coal velocity in different fuel lean/rich. Lean/rich
1:1 1:2 1:3 1:4 1:5 1:6
Anthracite
Bituminous
Concentration (kg coal/kg air)
Velocity (kg/min)
Concentration (kg coal/kg air)
Velocity (kg/min)
Rich
Lean
Rich
Lean
Rich
Lean
Rich
Lean
0.483 0.642 0.723 0.772 0.805 0.827
0.483 0.324 0.242 0.195 0.162 0.136
0.298 0.396 0.446 0.476 0.496 0.510
0.298 0.200 0.149 0.120 0.100 0.084
0.373 0.498 0.558 0.599 0.624 0.641
0.373 0.249 0.185 0.159 0.126 0.105
0.214 0.286 0.320 0.344 0.358 0.368
0.214 0.143 0.106 0.086 0.072 0.060
exit of nozzles and the center of the furnace, but the anthracite needed a long residence time in furnace. Meanwhile, it was found that both anthracite and bituminous the FCT in the primary air section were different with different PPC. When the temperature of the primary air section was high, the ignition was timely and the combustion reaction was intense. Conversely, when the concentration of volatile was not enough, the ignition timely was poor and the temperature in the primary air section was low. It could be seen from Table 4 that with increased PPC the FCT in the primary air section went down after a first peak. With
increased PPC, the concentration of volatile increased and the ignition was more timely. However, when the PPC was so high resulting in high concentration of volatile and lower oxygen, the ignition was delayed, which led to the low FCT in the primary air section. The effect of the PCC on the FCT in the primary air section is illustrated in Fig. 4. It can be seen that the optimal PCC of anthracite and bituminous was 0.810 kg coal/kg air and 0.607 kg coal/kg air. The optimal PCC is defined that the maximum FCT in the primary air section. 3.2. Effect of PCC on carbon in fly ash and slag It can be seen from Fig. 5 that with increased PCC, carbon in fly ash and slag decreased due to timely ignition and complete combustion. However, once the coal concentration was high, the relative oxygen concentration dropped resulting in ignition delay
Fig. 2. The scheme of measurement points: (1) burners; (2) measurement points of temperatures; (3) NOx measurement point; (4) measurement point of fly ash and (5) horizontal flue.
Table 3 Proximate analysis of the fuels. Fuel
Wad
Aad
Vad
Cad
Qha.ad
Anthracite Bituminous
1.41 4.53
37.29 8.82
9.85 35.55
51.45 51.1
20,562 kJ/kg 28,725 kJ/kg
Fig. 3. FCT vs. PCC: (a) anthracite and (b) bituminous.
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Bituminous
Concentration (kg coal/kg air)
Temperature (°C)
Concentration (kg coal/kg air)
Temperature (°C)
0.483 0.642 0.723 0.772 0.805 0.827
860 875 900 950 1075 1050
0.373 0.498 0.558 0.599 0.624 0.641
1435.5 1434.3 1453.7 1478.9 1464.7 1412.6
Fig. 6. NOx vs. PCC.
3.3. Effect of PCC on NOx
Fig. 4. Temperature of primary air section vs. PCC.
Seen from Fig. 6, with increased PCC, NOx reduced quickly with a gentle decrease nearby the optimal PCC. In rich coal stream, the poor oxygen content inhibited the formation of NOx, and in lean stream, NOx formation was limited because of the low temperature and less volatile nitrogen. Approaching the optimal PCC, the FCT was maximum and the emission of thermal-NOx was increased leading to the relative slight decrease of NOx. With further increase of PCC, the relative insufficient oxygen concentration resulted in ignition delay and incomplete combustion further causing low furnace temperature that inhibited formation of NOx.
4. Conclusion
Fig. 5. Carbon in fly ash and slag vs. PCC.
and incomplete combustion, which further caused the increased carbon in fly ash and slag. As an indicator of burn-out rate, high content of residue carbon in fly ash and slag reflected that the ignition and combustion efficiency was poor, and vice versa. Considering this aspect alone, the optimal PCC of anthracite and bituminous was 0.796 kg coal/kg air and 0.586 kg coal/kg air, respectively. Synthetically considering the effect of PCC on ignition timely and burn-out rate, the optimal PCC of anthracite and bituminous was suggested to be controlled between 0.796–0.810 kg coal/kg air and 0.586–0.607 kg coal/kg air in four-wall tangentially horizontal bias fired furnace.
The effect of fuel lean/rich (1:1, 1:2, 1:3, 1:4, 1:5 and 1:6) on the ignition, burnout and NOx emissions was investigated in a 1 MW four-wall tangentially horizontal bias fired furnace for Yibin anthracite and Shenmu bituminous. FCT went up first and then down along the height of the furnace when anthracite burned. While FCT at the height of primary air nozzles was the highest when the bituminous lean/rich was between 1:1 and 1:3, and the furnace core temperature also went up first and then down when the bituminous lean/rich was between 1:4 and 1:6. The ignition of anthracite needed a heating stage, while bituminous could timely ignite due to high volatile. Nevertheless, when the bituminous lean/rich was too low, it still needed a heating stage due to the high volatile concentration resulting in the relatively lack of oxygen. With increased PCC, FCT in the primary air section increased firstly and then decreased. The carbon in fly ash and slag were just adverse, it went down firstly and then increased. With increased PCC the concentration of volatile increased, which promoted the timely ignition and complete combustion causing increased FCT in the primary air section and decreased carbon in fly ash and slag. However, when the PCC was too high resulting in so high concentration of volatile and relative poor oxygen, the ignition was delayed leading to the relatively low FCT in the primary air section and higher carbon in fly ash and slag. Corresponding to the maximum FCT in the primary air section and the lowest carbon in fly ash and slag, the optimal PCC of anthracite and bituminous was between 0.796–0.810 kg coal/kg air and 0.586–0.607 kg coal/kg air in four-wall tangentially horizontal bias fired furnace. In addition, with increased PCC, NOx concentration reduced quickly with a slight decrease nearby the optimal PCC.
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Acknowledgement The present work was supported by the National Nature Science Foundation of China (No. 50976086).
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