Experimental study on preheating and combustion characteristics of pulverized anthracite coal

Paper from the AFRC 2013 conference titled Experimental study on preheating and combustion characteristics of pulverized anthracite coal by Jianguo Zhu

Anthracite, with low volatile content, has difficulties in ignition, stabilization, and burn out, especially under conditions of low boiler load. A new technique for preheating pulverized anthracite by a circulating fluidized bed was adopted. Experiments on preheated anthracite combustion were carried out in a down-fired combustor. Process of anthracite preheating, combustion characteristics, and NOX emissions were studied. The results show that preheated anthracite has a smaller mean size with a larger specific surface area. There is a uniform temperature profile along the axis of the down-fired combustor, and the combustion efficiency is 96.5%. The NOX emission in the exhaust is 256 mg/Nm3 (@ 6% O2) and the conversion ratio of fuel-N to NOX is 24.3%.

1 Experimental study on preheating and combustion characteristics of pulverized anthracite coal JianguoZhu*, Ziqu Ouyang, Qinggang Lu Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China Abstract: Anthracite, with low volatile content, has difficulties in ignition, stabilization, and burn out, especially under conditions of low boiler load. A new technique for preheating pulverized anthracite by a circulating fluidized bed was adopted. Experiments on preheated anthracite combustion were carried out in a down-fired combustor. Process of anthracite preheating, combustion characteristics, and NOX emissions were studied. The results show that preheated anthracite has a smaller mean size with a larger specific surface area. There is a uniform temperature profile along the axis of the down-fired combustor, and the combustion efficiency is 96.5%. The NOX emission in the exhaust is 256 mg/Nm3 (@ 6% O2) and the conversion ratio of fuel-N to NOX is 24.3%. Keywords: pulverized anthracite; preheating; combustion characteristic; NOX emission 1. INTRODUCTION In some thermal power plants, anthracite is widely used as fuel supplied to the boiler. However, it is difficult to ignite and burn out due to its low volatile and high fixed carbon content, especially under conditions of low boiler load. Therefore, stabilizing anthracite combustion and improving combustion efficiency have been a focus of research for a long time [1-3]. At present, some methods for boiler design and operation to solve these problems include optimizing burners, increasing combustion temperature, and prolonging residence time. One such method involves tangential combustion with an optimized burner to preheat anthracite using flue gas recirculation and a W-shaped flame to prolong residence time by a complicated configuration. However, NOX emissions in tangentially fired boilers and W-shaped flame boilers are high, generally 850-1300 mg/Nm3 (@ 6% O2) due to high combustion temperature [4-6]. Moreover, both tangentially fired boilers and W-shaped flame boilers have difficulties in stabilizing combustion, especially at low boiler load. A new technique for preheating pulverized anthracite by a circulating fluidized bed was adopted. Anthracite preheated to a temperature of nearly 900°C was pneumatically transported into a down-fired combustor to achieve highly efficient and stable combustion. A bench-scale rig of pulverized anthracite combustion was built, and a series of experiments were carried out. This paper outlines the process of anthracite preheating, the combustion characteristics, and the NOX emissions from this system. 2. EXPERIMENT 2.1 Test rig The test rig diagram shown in Fig. 1 is composed of a circulating fluidized bed which is abbreviated to CFB, a down-fired combustor which is abbreviated to DFC, and an auxiliary system. A horizontal tube 48 mm in diameter and 500 mm in length is used to guide the preheated anthracite from the CFB to the DFC. 2 1 air compressor, 2 liquefied petroleum gas, 3 electricity heater, 4 screw feeder, 5 riser, 6 U-valve, 7 cyclone, 8 sampling port, 9 DFC, 10 water tank, 11 water cooler, 12 bag filter, 13 gas analyzer. Fig. 1. Schematic diagram of the test rig. The riser of the CFB is 90 mm in diameter and 1500 mm in height. The coal feeding port is 240 mm above the air distributor on the riser, and the air, defined as primary air, is supplied to the CFB with about 10%-30% of theoretical air. The primary air fluidizes the bed materials and provides oxygen for partial pyrolysis, gasification, and combustion of anthracite, to achieve and maintain a bed temperature of nearly 900°C. Due to the strong reducing atmosphere throughout the CFB, the gas outflow the CFB is mainly comprised of N2, CO2, CO, CH4, and H2; this is similar to the composition of coal gas. Therefore, the gas at the outlet of the CFB is defined as high-temperature coal gas. In this system, the function of the CFB is to preheat pulverized anthracite by consuming a small amount of air. Preheated anthracite and high-temperature coal gas enter a nozzle at the top center of the DFC with 260 mm in diameter and 3000 mm in height. Secondary air at room temperature is supplied to the nozzle at a velocity of 18 m/s to provide oxygen for preheated anthracite combustion. Tertiary air, still at room temperature, is supplied to the DFC at a position 600 mm below the nozzle to provide extra oxygen for complete combustion. A reducing atmosphere, favorable to the reduction of nitrogen oxides, is present between the secondary air port and the tertiary air port. 2.2 Coal characteristics Raw anthracite coal from China (proximate and ultimate analyses listed in Table 1) was used in the experiments to investigate preheating, combustion, and NOX emissions. The diameter of the raw anthracite is smaller than 0.355 mm, with mean particle diameter, d50 = 82 μm. Quartz sand with a diameter ranging from 0.1 to 0.5 mm was added to the CFB as bed material. Table 1. Proximate and ultimate analysis of the raw anthracite. Items Data Proximate analysis (wt %) 2 1 3 4 5 7 6 10 11 12 9 13 8 Primary air Tertiary air Secondary air 3 Moisture a 3.40 Volatile matter b 7.58 Fixed carbon a 81.38 Ash a 8.55 Ultimate analysis b (wt %) Carbon 92.27 Hydrogen 3.52 Oxygen 1.92 Nitrogen 1.33 Sulfur 0.79 Low heating value a (MJ/kg) 30.70 Note: a as received; b dry ash free basis 2.3 Experimental conditions Experimental conditions are listed in Table 2. Table 2. Experimental conditions. Items Unit Data Pulverized anthracite feed rate kg/h 3.0 The primary air flow Nm3/h 7.5 The primary air temperature °C 20 CFB  - 0.3 The secondary air flow Nm3/h 8 The primary air temperature °C 20 RZ  - 0.6 The tertiary air flow Nm3/h 16 The primary air temperature °C 20  - 1.3 The air equivalence ratio in the CFB is defined as CFB  , and the air equivalence ratio in the reducing zone of the DFC is defined as RZ  : I CFB stoi A A   ; I II RZ stoi A A A    ; I II III stoi A A A A     (1) I A is the primary air flow, II A is the secondary air flow, III A is the tertiary air flow, stoi A is the air flow in stoichiometric combustion for pulverized coal, and is the excess air ratio. In the experiment, CFB  is set to 0.3, i.e., about 30% of theoretical air is supplied to the CFB. RZ  is set to 0.6, i.e., about 60% of theoretical air is supplied to the DFC ranging from the nozzle to 600 mm below the nozzle. The excess air ratio is 1.3 4 and the coal feed rate is 3.0 kg/h. 3. RESULTS AND DISCUSSIONS 3.1 Preheating process in the circulating fluidized bed The temperature variation with time in the CFB is shown in Fig. 2. It is obvious that preheated anthracite with a temperature of 880°C can be obtained steadily and continuously by partial pyrolysis, gasification, and combustion of anthracite in a low air equivalence ratio in the CFB. Fig. 2. Temperature variations with time in the CFB. The particle size distribution of the preheated anthracite is tested by a Malvern Mastersizer 2000 laser analyzer, and the diameter of the preheated anthracite is less than 100 μm, with a mean particle diameter, d50 = 20 μm, as shown in Fig. 3. Compared with the raw anthracite, the mean diameter of the preheated anthracite is significantly decreased. There are two reasons for this sharp decrease in particle size. One reason is the particle fragmentation due to thermal swelling, release of volatiles, char combustion, and strong mixing and friction between coal particles and bed materials [7]. The other reason is lager size preheated anthracite particles are captured by the cyclone, and only small size preheated anthracite particles can escape from the cyclone and enter the DFC. Fig. 3. Size distributions of preheated anthracite and raw anthracite. The pore structures of raw anthracite and preheated anthracite are shown in Fig. 4. It can be seen that the fraction of pores 0 10 20 30 40 50 60 70 80 90 100110120130 0 200 400 600 800 1000 Temperature (oC) Time (min) 100 mm 500 mm outlet 1 10 100 1000 0 20 40 60 80 100 Preheated anthracite Raw anthracite Cumulative volume (%) Size (m) 5 with a pore width less than 2 nm is significantly higher in the preheated anthracite than that of raw anthracite. As a result, the pore volume increases from 0.02 to 0.07 cm3/g and the specific surface area increases from 5.25 to 111.9 m2/g. The sharp increase of specific surface area and the pore volume after preheating is due to the high preheating temperature, fast heating fate and coal particle fragmentation [8-10]. The increasing of pore volume and specific surface area helps to accelerate the combustion velocity and stabilize the flame. Fig. 4. Distributions of pore volume and specific surface area of raw anthracite and preheated anthracite. The surface morphology of raw anthracite and preheated anthracite is analyzed using Scanning Electron Microscopy and the pictures are presented in Fig. 5.The surface of the raw anthracite is smooth with a dense texture, and barely any pores are observed. However, the surface of the preheated anthracite is rough with a well-developed pore structure, like a honeycomb, which verified that a number of pores formed in the preheating process. Raw anthracite 1 10 100 0.0 0.5 1.0 1.5 2.0 0 2 4 6 8 pore volume Raw anthracite Preheated anthracite particles Pore width (nm) pore volume 0 5 10 15 20 25 0 1 2 3 Specific surface area (m2/g) Pore volume (cm3/g/10-3) specific surface area specific surface area 6 Preheated anthracite Fig. 5. SEM structures of raw anthracite and preheated anthracite. To evaluate the preheating process, the proximate and ultimate analysis of preheated anthracite, as well as the remaining fraction of each component, is given in Table 3.The remaining fraction of each component (e.g., the volatile remaining fraction) can be calculated as PA RC V RC PA , V A R V A    (2) V R is the remaining fraction of volatile matter in preheated anthracite, PA V is the volatile content in preheated anthracite, RC A is the ash content in raw anthracite, RC V is the volatile content in raw coal, and PA A is the ash content in preheated anthracite. The remaining fraction of the volatile is 22.8%, which means 77.2% of the volatile has been released and transformed to high-temperature coal gases in the CFB, such as H2 and CH4. The remaining fraction of fixed carbon is 57.1%, which means 42.9% of fixed carbon has been transformed to CO2, CO, and other C-containing components. Nearly 28.3% of the nitrogen in the raw anthracite has been released and transformed to HCN, NH3, or N2 in the strong reducing atmosphere. Table 3 also gives the composition of high-temperature coal gas. The concentrations of CO and H2 are 7.23% and 8.44%, and the concentrations of NH3 and HCN are 704 and 8 mg/Nm3, respectively. There is no O2, NOX or N2O due to the strong reducing atmosphere in the CFB. It can be inferred that coal-nitrogen released mainly converts into N2 and NH3 during the preheating process, which helps to reduce NOX emissions in this system. The cold dry-basis heat value of high-temperature coal gas is 2.05 MJ/Nm3, and the heat value of preheated anthracite is 26.71MJ/kg, 13.9% lower than that of raw anthracite. Table 3. The analysis of preheated anthracite and high-temperature coal gas. Preheated anthracite analysis Data Component remaining fraction (%) Ultimate analysis Carbon 75.78 55.8 Hydrogen 0.93 18.0 Oxygen 1.18 38.1 Nitrogen 1.4 71.7 Sulfur 0.95 82.0 Proximate analysis a Moisture 5.48 7 Ash 14.3 Volatile matter 2.54 22.8 Fixed carbon 77.7 57.1 Low heating value (MJ/kg) 26.71 52.0 Coal gas composition Unit Data CO % 7.23 CO2 % 14.69 H2 % 8.44 CH4 % 0.61 O2 % 0 NO mg/Nm3 0 NO2 mg/Nm3 0 N2O mg/Nm3 0 NH3 mg/Nm3 704 HCN mg/Nm3 8 QL MJ/Nm3 2.05 3.2 Combustion characteristics in the DFC The temperature profile along the axis of the DFC, shown in Fig.6, is uniform. The maximum temperature difference is about 210°C, far below than that of ordinary combustion of pulverized coal [11, 12]. The temperature at the nozzle of the DFC is 880°C and the maximum flame temperature is 1213°C at a position 100 mm below the nozzle. The residence time of pulverized anthracite in the range of 0-100 mm below the nozzle is about 0.05s. Therefore, the preheated anthracite fueled into the combustor has a fast combustion and heat release rate, due to the temperature of preheated anthracite being higher than the ignition temperature. There is no ignition problem in the down-fired combustor. Fig. 6. Temperature profile along the axis of the DFC. The combustion efficiency can be defined as follows [16]: Combustion Efficiency = {[Coal Heating Values] − [Heating Values of Unburned carbon, Hydrogen, and CO gas generation]} / [Coal Heating Values]. In the experiment, the combustion efficiency is 96.5%, higher than that in similar test facilities for anthracite combustion reported in some of the literature [17-19]. Higher combustion efficiency can be expected by increasing the maximum combustion temperature or prolonging the residence time. 3.3 NOX formation mechanism in the DFC 600 900 1200 1500 3000 2000 1000 0 Distance from the nozzle (mm) Temperature (oC) 8 Thermal-NOX is usually generated under the condition of combustion temperature higher than 1500°C. In this experiment, thermal-NOX cannot be formed as the maximum combustion temperature is 1213°C. The fuel-NOX, transformed by precursors such as HCN and NH3, is a main contribution to nitrogen oxides in this system. HCN concentrations along the axis of DFC are shown in Fig. 7(a).The maximum concentration of HCN is 17mg/Nm3 at a position 100 mm below the nozzle, from the release of preheated anthracite-N. However, HCN, as unstable species, can be further converted into other species, such as NOX in an oxidizing atmosphere or N2 in a reducing atmosphere [20, 21]. NH3 concentrations along the axis of the down-fired combustor are shown in Fig. 7(b). NH3 has an initial concentration of 708 mg/Nm3 from the CFB, and it decreases sharply to nearly zero at a position 100 mm below the nozzle. The injection of NH3 into flue gas, commonly used in power plants, is an effective method of reducing nitrogen oxides [22, 23]. In the reducing zone in the range of 0-600 mm below the nozzle, NH3 may take part in some reactions of NO reduction. N2O concentrations along the axis of the down-fired combustor are shown in Fig. 7(c), with a maximum concentration of 40 mg/Nm3 at a position 100mm below the nozzle, and decreasing to nearly zero at the outlet. As is well known, N2O decreases with increasing furnace temperature. At a temperature of 1000°C, more than 90% of N2O is decomposed [24]. In the experiments, the maximum temperature of the DFC is 1213°C, so most of the N2O is thermally decomposed. NO concentrations along the axis of the down-fired combustor are shown in Fig. 7(d). NO concentration is zero at the outlet of the CFB due to the strong reducing atmosphere, and it goes up to a maximum at a position 400 mm below the nozzle because of the fast release and transformation of preheated anthracite-N and then decreases continuously due to the occurrence of homogeneous and heterogeneous reduction reactions. The NO emission is 240 mg/Nm3 in the exhaust. NO2 concentrations along the axis of the down-fired combustor are shown in Fig. 7(e). The initial concentration is zero at the outlet of CFB and reaches a maximum at a position 100 mm below the nozzle. As the tertiary air is injected into the DFC, the concentration of NO2 decreases continuously and NO2 emission is almost negligible in the exhaust. Lu [16] studied pulverized coal combustion and NOX emissions in high temperature air combustion, and the NO2 emission is also zero. The NOX (NOX=NO+NO2) emission is 256 mg/Nm3 (@ 6% O2). It can be inferred that this system could reduce NOX emission for anthracite coal combustion. Considering the preheating process, combustion characteristics, and the NO formation mechanism, the reason for the low NOX emission can be explained as follows: 1) Both high-temperature coal gas and NH3 produced in CFB are efficient at promoting homogeneous reductions. 2) The preheated anthracite has a larger specific surface area and is efficient at promoting heterogeneous reduction of nitrogen oxides. 3) The maximum combustion temperature is lower than 1500°C to avoid thermal-NOX, and air-staging technology is used with a reducing zone established in the down-fired combustor. The conversion ratio of fuel-N to NOX is defined as the measured value of NOX in the exhaust from DFC / the calculated value with all fuel-N to NOX. In this experiment, the conversion ratio of fuel-N to NOX is 24.3%. 9 Fig. 7. Concentrations of different gases along the axis of the DFC. 4. CONCLUSIONS In this paper, a process of anthracite preheating, combustion, and NOX formation is described and analyzed. The following conclusions can be obtained: 1. Preheated anthracite with a temperature of 880°C can be obtained steadily and continuously by partial pyrolysis, gasification, and combustion of anthracite at a low air equivalence ratio in the CFB. 2. Compared with raw anthracite, the mean size of preheated anthracite decreases while the specific surface area increases. 3. Preheated anthracite combustion exhibits a uniform temperature profile along the axis of the DFC, and the combustion efficiency is 96.5%. 4. NOX emission is 256 mg/Nm3 (@ 6% O2) with the conversion ratio of fuel-N to NOX of 24.3%, and the reason for the low NOX emissions has been explained. ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of the National Natural Science Foundation of China (51006103). REFERENCES [1] Fan WD, Lin ZC, Li YY, Kuang JG, Zhang MC. Effect of air-staging on anthracite combustion and NOX formation. Energ fuel 2009; 23: 111-20. [2] Wagner R, Mühlen HJ. Effect of a catalyst on combustion of char and anthracite. Fuel 1989; 68: 251-3. [3] Stephenson PL. Mathematical modeling of semi-anthracite combustion in a single burner furnace. Fuel 2003; 82: 2069-73. [4] Li ZQ, Ren F, Zhang J, Zhang XH, Chen ZC, Chen LZ. Influence of vent air valve opening on combustion characteristics of a down-fired pulverized-coal 300 MWe utility boiler. Fuel 2007; 86: 2457-62. [5] Kurose R, Ikeda M, Makino H, Kimoto M, Miyazaki T. Pulverized coal combustion characteristics of high-fuel-ratio coals. Fuel 2004; 83: 1777-85. [6] Fan WD, Lin ZC, Li YY, Li Y. Effect of temperature on NO release during the combustion of coals with different ranks. Energ Fuel 2010; 24: 1573-83. 0 1000 2000 3000 0 20 40 0 200 400 0 20 40 0 400 800 0 10 20 (a) Distance from nozzle (mm) NO2 NO Gas concentration (mg/Nm3) N2O (d) (e) (c) (b) NH3 HCN 10 [7] Lee SH, Kim SD, Lee DH. Particle size reduction of anthracite coals during devolatilization in a thermobalance reactor. Fuel 2002; 81: 1633-9. [8] Feng B, Bhatia SK. Variation of the pore structure of coal chars during gasification. Carbon 2003; 41: 507-23. [9] Zhang JW, Sun SZ, Yang JC, Hu XD, Qin YK. Structure development of coal chars under high temperature reducing conditions. Proceedings of the CSEE 2008; 28: 1-8. [10] Alonso MJG, Borrego AG, Alvarez D, Parra JB, Menéndez R. Influence of pyrolysis temperature on char optical texture and reactivity. J Anal Appl Pyrolysis 2001; 58: 887-909. [11] Visona SP, Stanmore BR. Modeling nitric oxide formation in a drop tube furnace burning pulverized coal. Combust Flame 1999; 118: 61-75. [12] Li S, Xu T, Hui S, Zhou QL, Tan HZ. Optimization of air staging in a 1 MW tangentially fired pulverized coal furnace. Fuel Process Technol 2009; 90: 99-106. [13] Turns SR. An introduction to combustion concepts and applications. Boston: WCB/McGraw-Hill; 2000. [14] Mon E, Amundson NR. Diffusion and reaction in a stagnant boundary layer about a carbon particle. 2. An extension. Ind Eng Chem Fundam 1978; 17: 313-21. [15] Fu WB, Zhang BL, Zheng SM. A relationship between the kinetic parameters of char combustion and the coal's properties. Combust flame 1997; 109: 587-98. [16] Lu QG, Zhu JG, Niu TY, Song GL, Na YJ. Pulverized coal combustion and NOX emissions in high temperature air from circulating fluidized bed. Fuel Process Technol 2008; 89: 1186-92. [17] Suda T, Takafuji M, Hirata T, Yoshino M, Sato J. A study of combustion behavior of pulverized coal in high-temperature air. P Combust Inst 2002; 29: 503-9. [18] Zhang H, Yue GX, Lu JF, Jia Z, Mao JX, Fujimon T, et al. Development of high temperature air combustion technology in pulverized fossil fuel fired boilers. P Combus Inst 2007; 31: 2779-85. [19] Kiga T, Yoshikawa K, Sakai M, Mochida Susumu. Characteristics of pulverized coal combustion in high-temperature preheated air. J Propul Power 2000; 16: 601-5. [20] Mitchell JW, Tarbell JM. A kinetic model of nitric oxide formation during pulverized coal combustion. AICHE J 1982; 28: 305-15 [21] Leichtnam JN. Schwartz D. Gadiou R. The behavior of fuel-nitrogen during fast pyrolysis of polyamide at high temperature. J anal apple pyrol 2000; 55: 255-68. [22] Ha XH, Wei XL, Schnell U. Detailed modeling of hybrid reburn / SNCR processes for NOX reduction in coal-fired furnaces. Combust flame 2003; 132: 374-86. [23] Ljungdahl B, Larfeldt J. Optimised NH3 injection in CFB boiler. Powder technol 2001; 120: 55-62. [24] Houser TJ, Mccarville ME, Gu ZY. Nitric oxide formation from fuel-nitrogen model compound combustion. Fuel 1988; 67: 642-9.