A Study of Flameless Combustion Behavior of Pulverized Coal Preheated by Circulating Fluidized Bed

Paper from the AFRC 2015 conference titled A Study of Flameless Combustion Behavior of Pulverized Coal Preheated by Circulating Fluidized Bed.

A new technology based on pulverized coal preheated by CFB was proposed, which could achieve stable flameless combustion, high combustion efficiency, and low NOX emission of pulverized coal. Experiments were conducted on a bench-scale rig of pulverized coal combustion preheated by CFB to study the flameless combustion behavior. Typical coals were used in the experiments for comparison and to investigate the fuel flexibility of the technology. The results show that the fuel flexibility is well for bituminous coal, anthracite and Semi-coke. The flameless combustion is very stable and the temperature of the camber is uniform for the three coals. What's more, the average combustion temperature is 1200°C, which is much lower than that of traditional combustion technology. Fuel preheating is also effective to improve combustion efficiency and decrease NOX emissions especially for anthracite. The combustion efficiencies are 99%, 94.1% and 92.3% for bituminous coal, anthracite and Semi-coke. The NOX emissions are 352.34 mg/m3, 232.30 mg/m3 and 458.35 mg/m3 for bituminous coal, anthracite and Semi-coke.

A Study of Flameless Combustion Behavior of Pulverized Coal Preheated by Circulating Fluidized Bed Ziqu Ouyang*, Jianguo Zhu, Qinggang LU Institute of Engineering Thermophysics, Chinese Academy of Sciences 11 Beisihuanxi Road, Beijing 100190, People's Republic of China *T: 86-10-82543055; F: 86-10-82543119; E: [e-mail] gaseous fuels. Suda [7] and Zhang [8] studied the flameless combustion behavior of coal and confirmed the possibility of the realization of pulverized coal flameless combustion. Stadler [9] studied the NOX formation mechanism of pulverized coal flameless combustion at different atmosphere, especially the effects of coal char combustion and gasification on NOX emission. South China University of Technology [10] successfully achieved flameless combustion with biomass fuels. Abstract- A new technology based on pulverized coal preheated by CFB was proposed, which could achieve stable flameless combustion, high combustion efficiency, and low NO X emission of pulverized coal. Experiments were conducted on a bench-scale rig of pulverized coal combustion preheated by CFB to study the flameless combustion behavior. Typical coals were used in the experiments for comparison and to investigate the fuel flexibility of the technology. The results show that the fuel flexibility is well for bituminous coal, anthracite and Semi-coke. The flameless combustion is very stable and the temperature of the camber is uniform for the three coals. What's more, the average combustion temperature is 1200°C, which is much lower than that of traditional combustion technology. Fuel preheating is also effective to improve combustion efficiency and decrease NOX emissions especially for anthracite. The combustion efficiencies are 99%, 94.1% and 92.3% for bituminous coal, anthracite and Semi-coke. The NO X emissions are 352.34 mg/m3, 232.30 mg/m3 and 458.35 mg/m3 for bituminous coal, anthracite and Semi-coke. In traditional flameless combustion system of gas and liquid fuels, recuperative or regenerative heat exchangers are used to heat up combustible air, but the implementation of heat exchanger in burning solid fuels has not successfully fulfilled. In a coal or other fossil fuel fired boiler, it is difficult to apply the regenerator since the ash-containing flue gas at such a high temperature can easily introduce slagging and fouling [8]. Additionally, due to material constraints, temperature of fuel gas at the inlet of heat exchanger cannot be too high. Thus, it required huge amount of heat exchangers to heat up the combustible air. Consequently, alternative approach is needed. Keywords- preheating; pulverized coal; flameless combustion; NOX emission A new technology based on pulverized coal preheated by CFB is proposed. The temperature of pulverized coal can be heated up to above 800°C in a CFB and then the high temperature preheated coal is supplied to a combustor for flameless combustion. The technology is expected to achieve good fuel flexibility, high combustion efficiency, and low NOX emission for pulverized coal combustion. I. INTRODUCTION Recently, a combustion technology named flameless combustion has been developed. It is one of the promising techniques proposed to control pollutant emissions and improve combustion efficiency from combustion plant. Flameless combustion is a kind of combustion mode of diluted reactants in a low-oxygen atmosphere and temperature of the combustion air always preheated up to 1000°C, therefore is also called "moderate & intense low oxygen dilution" (MILD) or "high temperature air combustion" (HTAC) [1-3]. It is characterized by no obvious flame front and uniform temperature profiles. These requisites lead to high combustion efficiency and good control of thermal peaks and hot spots, lowering NOX emissions [4]. Compared with traditional combustion, in flameless combustion heat utilization efficiency can be increased by more than 30% and the NOX emissions can be reduced more than 70% [4]. What's more, flameless combustion also has broad fuel flexibility [5]. The purpose of this article is to investigate the flameless combustion behavior and NOX formation mechanism of different preheated coals. Hence, a test facility is built and series experiments with typical coals are carried out. II. EXPERRIMENTAL A. Test facility The test rig diagram is shown in Fig. 1. It is composed of a circulating fluidized bed, a down-fired combustor and an auxiliary system. A horizontal tube with 48 mm in diameter and 500 mm in length is used to guide preheated anthracite from CFB to the down-fired combustor. In the past few years, flameless combustion has been successfully achieved by gas, liquid and solid fuels and extensive research has been conducted. Wunning [2], Cavigolo [6] and Tsuji [4] studied the flameless combustion behavior of The riser of CFB is 90 mm in diameter and 1500 mm in height. The coal feeding port is 240 mm above the air 1 distributor on the riser, and the air, defined as primary air, is supplied to CFB with about 10% ~ 30% of theoretical air. The primary air fluidizes bed materials and provides oxygen for partial pyrolysis, gasification, and combustion of pulverized coal. The temperature of the bed materials can be maintained and pulverized coal can be preheated up to 800°C. By adjusting operating conditions of CFB and choosing appropriate diameter of pulverized coal, most of the high temperature preheated pulverized coal can escape from the capture of cyclone and goes into the down-fired combustor. Due to strong reducing atmosphere exist everywhere in CFB, the gas outflow CFB is mainly N2, CO2, CO, CH4 and H2, so it can be named as high temperature coal gas. Therefore, the preheated fuel flow goes into the down-fired combustor is composed of high temperature preheated pulverized coal and high temperature coal gas. B. Coal characteristics & experiment conditions Table 1 shows the proximate and ultimate analysis of coals used in the experiment. Three types of coal from high-volatile bituminous (DT) to anthracite (YQ) and semi-coke (SM) were used to demonstrate the effect of coal type on the combustion behavior. The size of the coal is in the range of 0~0.355 mm with mean particles size d50=82 µm. Quartz sand with the diameter ranging from 0.1 mm to 0.5 mm was added into CFB as the bed material. In the experiment, the sampling of gas is started after confirming that the combustion is stable and the exhaust gas composition does not change with time. Table 2 shows the experimental conditions. CFB is the air equivalence ratio in CFB and defined as: The primary air flow / theoretical air for pulverized coal combustion. The down-fired combustor is 260 mm in diameter and 3000 mm in height. Preheated pulverized coal and high temperature coal gas together enter a nozzle at the top center of the down-fired combustor. Secondary air with room temperature is supplied to the down-fired combustor also through the nozzle with a velocity of 18 m/s, to provide oxygen for preheated fuel combustion. Tertiary air, still at room temperature, is supplied to the down-fired combustor at the position of 600 mm below the nozzle, to provide extra oxygen for complete combustion. In the range of secondary air port to tertiary air port, it is a reducing atmosphere for decreasing nitrogen oxides formation. RE is the air equivalence ratio in the reducing zone of the down-fired combustor and defined as: (The primary air flow + The secondary air flow) / theoretical air for pulverized coal combustion.  is the excess air ratio of the experimental system and defined as: (The primary air flow + The secondary air flow+ The tertiary air flow) / theoretical air for pulverized coal combustion. In the experiments, CFB is set to 0.25, meaning about 25% of theoretical air supplied to CFB. The temperature of preheated fuel which is also the temperature at the outlet of CFB is set to 850°C in the three cases by adjusting the coal feed rate and the primary air flow. RE is set to 0.6, meaning about 60% of theoretical air supplied to the down-fired combustor in the range of 0 ~ 600 mm below the nozzle. The excess air ratio is set to 1.3 There are 8 thermocouples in the test facility, 3 Ni-Cr/NiSi thermocouples in CFB and 5 Pt/Pt-Rh thermocouples in the down-fired combustor. Seven sampling ports are set: one is at the outlet of CFB for sampling preheated pulverized coal and high temperature coal gas; one is at the outlet of a bag filter for sampling fly ash, and the other six ports are 100 mm, 400 mm, 900 mm, 1400 mm, 2400 mm and 3000 mm below the nozzle. All the gas samples are dried and filtered before they enter individual online analyzers. Gas in the down-fired combustor is measured by Gasmet FTIR DX-4000 analyzer. TABLE I. 8 Analysis Coal type DT YQ SM Moisture 1.9 2.4 1.2 Ash 26.1 8.6 15.5 Volatile 27.5 6.7 8.2 Fixed carbon 44.5 82.2 75.1 Carbon 58.3 82.1 76.8 Hydrogen 3.7 3.1 1.4 Nitrogen 1.0 1.2 0.8 Sulfur 0.3 0.7 0.4 Oxygen 8.6 1.9 4.0 Heating value (kJ/kg) 22700 31040 27060 Proximate (wt %) 7 4 PROXIMATE AND ULTIMATE ANALYSIS OF THE COAL (AIR DRY) 5 9 6 3 1 2 12 11 13 Ultimate (wt %) 10 1 air compressor, 2 liquefied petroleum gas, 3 electricity heater, 4 screw feeder, 5 riser, 6 U-valve, 7 cyclone, 8 sampling port, 9 down-fired combustor, 10 water tank, 11 water cooler, 12 bag filter, 13 gas analyzer. Figure 1. Diagram of the test rig 2 EXPERIMENTAL CONDITIONS 1600 Case 1 Case 2 Case 3 Coal type DT YQ SM Coal feed rate (kg/h) 5.72 3.61 4.12 The primary air flow (Nm3/h) 7 7 7 CFB 0.24 0.24 0.25 Temperature of preheated fuel (°C) 850 850 850 The secondary air flow (Nm3/h) 10 10 10 RE 0.6 0.6 0.6 The tertiary air flow (Nm3/h) 21 12 20  1.28 1.30 1.29 III. Furance temperature (oC) TABLE II. B 1200 C 800 A: Conventional flame B: High-temperature flame C: Flameless D: Non-reaction A 400 0 D 20 16 12 8 4 0 Concentration of O2 (%) Figure 2. Realization conditions of flameless combustion The idea to realize flameless combustion in this experiment system is as follows: RESULTS AND DISCUSSION A. The realization conditions of flameless combustion In traditional combustion technology, the combustion atmosphere is air or oxygen-enriched air. Hence, the fuel molecules will meet with a sufficient number of oxygen molecules in a small space near the burner, and in this region the oxygen is enough for the fuel to be oxidized completely. As a result, the flame size is small, high-temperature region appears, and the NOX emission is high. On the Contrary, in flameless combustion, the number of oxygen molecules in the region near the burner is significantly less than that in the traditional combustion. In this region, only a small amount of fuel molecules react with oxygen molecules. Most of the fuel molecules diffuse into the furnace space together with the hightemperature-speed air jet and then react with oxygen molecules. Therefore, the concentration of oxygen near the burner is a key factor to decide whether the flameless combustion occurs or not. High temperature preheated fuel flow can be generated in CFB with low air equivalence ratio. The temperature of preheated fuel is higher than 800°C, and the oxygen concentration is bout zero. The preheated fuel flow injected in to the down-fired combustor and mixed with the high speed secondary air flow. As the mount of preheated fuel flow and the secondary air flow is substantially same, oxygen concentration in the mixture is lower than 15%, and the temperature of the mixture is high than 800°C. The two realization conditions of flameless combustion mentioned above are satisfied, so the flameless combustion will occur in the down-fired combustor. B. Operating characteristics of CFB Three thermocouples were used to measure the temperature in the CFB. One is at the outlet of the CFB for measuring high temperature preheated fuel and the other two are at the position of 100mm and 550 mm above the air distribution plate. Wunning [2] studied the realization conditions of flameless combustion, as shown in Fig. 2. According to the furnace temperature and the oxygen concentration, the reaction zone can be divided into a conventional flame zone, a hightemperature flame zone, a flameless reaction zone, and a nonreaction zone. To make the reaction in the zone C, two conditions must be met: The temperatures variation with time in CFB is shown in Fig. 3. The temperature is stable in CFB, it is obvious that high temperature preheated fuel with the temperature of 850°C can be achieved steadily and continuously by partial pyrolysis, gasification, and combustion of pulverized coal in the three cases. The temperature distribution is uniform in CFB, and the maximum temperature difference is only about 50°C. 1) The temperature of furnace or reactants is sufficiently high, always higher than 800°C. Composition of the high temperature coal gas at the outlet of CFB is analyzed, and N2, CO, CO2, H2, CH4 and NH3 are the mainly components while the concentration of O2 is zero. The physical and chemical properties of high temperature preheated pulverized coal is also analyzed in the experiment. It is found that preheated pulverized coal is substantially free of volatiles, so the preheated coal particles is mainly composed by the fixed carbon and ash. The gas adsorption method, with N2 as the adsorptive at 77 K, is applied to analyze the pore structure of preheated pulverized coal. In the three cases, compared with the raw coal, pore volume and specific surface area of the preheated pulverized coal increase significantly. As the pore volume and the specific surface area increases, it is beneficial to accelerate the combustion velocity and stabilize the flame. 2) The concentration of oxygen is low, always lower than 15%. In order to meet the two conditions, the existing implementation ways can be summarized as [11-13]: 1) Increase the temperature of reactants immediately by air or fuel preheating technology together with high speed jet entrainment and recirculation. 2) Reduce the oxygen concentration by high temperature low oxygen combustion technology, high speed air jet method, flue gas circulation or a specific reactor for the sufficiently mixture of fuel, oxidant, and inert reaction products. 3 15 1000 DT YQ SM DT Concentration of O2 (%) 800 600 o Temperature ( C) 1000 SM 800 10 5 600 0 1000 YQ 0 500 1000 1500 2000 2500 3000 Distance from the nozzle (mm) 800 100 mm on the riser 550 mm on the riser outlet of CFB 600 0 Figure 5. O2 concentration along the axis of the down-fired combustor The temperatures variations with time in the down-fired combustor of different coals are shown in Fig. 6. The temperature is steady and no obvious change can be observed. The down-fired combustor has good fuel flexibility after the coal preheated in CFB. 20 40 60 80 100 120 140 160 180 Time (min) Figure 3. Temperature varations with time in CFB Combustion efficiency can be defined as: {[Coal Heating Values]-[Heating Values of Unburned carbon, Hydrogen and CO gases generation]} / [Coal Heating Values] C. Combustion behavior in the down-fired combustor The temperature profile along the axis of the down-fired combustor is shown in Fig. 4, and O2 concentration along the axis of the down-fired combustor is shown in Fig. 5. The temperature at the nozzle of the down-fired combustor is 850°C and O2 concentration is lower than 15%, which is meet the realization conditions of flameless combustion. Hence, the combustion of preheated fuel in the down-fired combustor is flameless combustion. The temperature profile in the downfired combustor is uniform, and the maximum temperature difference is about 300°C, which is according with the characteristic of flameless combustion. For the three coals, the maximum temperature is 1230°C at the position of 100 mm below the nozzle, no thermal peak or hot spot exist. In the range of 0~100 mm bellow the nozzle the temperature increases significantly from 850°C to 1250°C, so the preheated fuel has a fast combustion and heat release rate in the downfired combustor. Combustion performance is improved after preheating. Mass fraction of combustible substance and CO concentration is analyzed in the experiments, and the combustion efficiency in the three cases is 99.2% for DT coal, 94.1% for YQ coal, and 92.3% for SM coal. The combustion efficiency is high, especially for anthracite, higher than that in the same size test facilities for anthracite combustion reported in some literatures [7, 8]. Higher combustion efficiency can be expected if increase the combustion temperature. DT 1200 800 400 0 SM Temperature ( C) 1200 Distance from the nozzle (mm) 0 500 o DT YQ SM 1000 800 400 0 YQ 1200 1500 800 2000 2500 400 100 mm below the nozzle 400 mm 900 mm 1400 mm 2400 mm 400 0 600 800 1000 1200 0 1400 o Temperature ( C) 20 40 60 80 100 120 140 160 180 Time (min) Figure 6. Temperature varations with time in the down-fired combustor Figure 4. Temperature profile along the axis of the down-fired combustor 4 D. NOX emission NOX emission from pulverized coal combustion is mainly prompt-NOX, thermal-NOX and fuel-NOX. Prompt-NOX generated in the premixed combustion and it is always negligible. Thermal-NOX usually generated under the conditions of the combustion temperature higher than 1500°C. In this experiment the highest combustion temperature is 1230°C, thermal-NOX can be ignored. NOX emission in preheated fuel combustion is mainly from fuel nitrogen. 800 3 Concentration of NH3 (mg/Nm ) 600 500 400 300 200 For the fuel NOX formation, there are two paths: (1) a part of nitrogen contained in volatile is devolatilized and transformed into NOX; (2) a part of the remained nitrogen in char is transformed into NOX. Most of the volatile has already released during the preheating process, so NOX generated in down-fired combustor is mainly from nitrogen in char. 100 0 0 500 1000 1500 2000 2500 3000 Distance from the nozzle (mm) Figure 8. NH3 concentration along the axis of the down-fired combustor HCN and NH3 are two important precursors in the formation of NOX. In the reducing zone NH3 may take part in some reaction of NO reduction. HCN concentration along the axis of the down-fired combustor is shown in Fig. 7. In the three cases, the maximum concentration of HCN is all at the position of 100 mm below the nozzle, mainly from the release of char-nitrogen and the reaction is as follows: N+CH i → HCN+H 2 DT YQ SM 700 NO+NH i  N2+ (R. 2) The trend of N2O concentration variation along the axis of the down-fired combustor, shown in Fig. 9, is similar in the three cases. (R. 1) The maximum concentration of N2O is at the position of 100 mm below the nozzle, also mainly release of char- nitrogen. The N2O emission is about zero in the exhaust. As well known, more than 90% of the N2O will decompose when the temperature up to1000°C. In the experiment, the maximum temperature in the down-fired combustor is about 1230°C, so most of the N2O is thermally decomposed. HCN is an instable species, can be further converted into other species. In oxidizing atmosphere HCN will converted into NOX while in reducing atmosphere it will converted into N2 [14-15]. The HCN concentration is about zero at the outlet of the down-fired combustor. NH3 concentration along the axis of the down-fired combustor is shown in Fig. 8. NO concentration profiles along the axis of the down-fired combustor, shown in Fig. 10, are different in the three cases. For anthracite and bituminous, NO concentrations increase rapidly after injected into the DFC, reach a maximum, and then decrease continuously along the axis of the down-fired combustor. For semi-cock, NO concentrations increase rapidly after injected into the down-fired combustor, reach a maximum, and then keep almost invariant. It indicated that the formation and reduction mechanism of NO in the down-fired combustor has an important relationship with coal type. NH3 has an initial concentration of 780 mg/Nm3, 704 mg/Nm3, and 670 mg/Nm3 in the preheated fuel flow of DT coal, YQ coal, and SM coal, respectively. Then the concentration decreases continuously along the axis of the down-fired combustor. The injection of NH3 to the flue gas, widely used in power plants, is an effective method to reduce nitrogen oxides [16]. 50 Concentration of N2O (mg/Nm ) DT YQ SM 3 Concentration of HCN (mg/Nm ) 40 3 30 20 10 0 0 500 1000 1500 2000 2500 40 30 20 10 0 3000 DT YQ SM 0 500 1000 1500 2000 2500 3000 Distance from the nozzle (mm) Distance from the nozzle (mm) Figure 9. N2O concentration along the axis of the down-fired combustor Figure 7. HCN concentration along the axis of the down-fired combustor 5 3 Concentration of NO (mg/Nm ) 600 2) In flameless combustion no thermal peaks or hot spot exist and the concentration of O2 is lower than conventional combustion, which are beneficial for decreasing NOX emission. 500 3) Air-staging technology is applied with a reducing zone established in the down-fired combustor. 400 300 IV. 200 100 Tertiary air injection 0 0 500 1000 1500 2000 2500 Distanc from the nozzle (mm) Experiments are carried out on a bench-scale rig of pulverized coal flameless combustion preheated by a circulating fluidized bed. Pulverized coal preheating process in CFB, flameless combustion behavior in the down-fired combustor, and NOX emission are described and analyzed. The following conclusions can be obtained. DT SM YQ 3000 1) Preheated fuel flow with temperature of 850°C can be obtained steadily and continuously by pulverized coal combustion in low excess air ratio in CFB. Figure 10. NO concentration along the axis of the down-fired combustor In the three cases, NO concentration increases sharply in the range of 0~400 mm below the nozzle due to the oxidation of char-nitrogen and NH3from CFB. 2) The experiment system has good fuel flexibility. Flameless combustion can be obtained for anthracite, bituminous and semi-coke, and in the down-fried combustor it shows a steady and uniform temperature profiles. Combustion efficiency for anthracite, bituminous and semi-coke is 94.1%, 99.2%, and 92.3%. For anthracite and bituminous the NO concentration decreases in the range of 400 ～ 3000 mm due to some homogeneous and heterogeneous reduction reactions. Except for the reaction of R. 2, main paths of homogeneous reactions of NO reduction in reducing zone are [17]: NO+CH i  HCN+ NO+CO+  CO 2 +N 2 + NO+HCN  N 2 + CONCLUSIONS 3) NOX formation mechanism is studied and NOX emission for anthracite, bituminous and semi-coke is 232 mg/Nm3 (6%O2, dry), 352 mg/m3 (6%O2, dry), and 458 mg/Nm3 (6%O2, dry). (R. 3) ACKNOWLEDGMENT (HEADING 5) The authors gratefully acknowledge the support of the Presidential Foundation of Institute of Engineering Thermophysics, Chinese Academy of Sciences (Y554023Z11). (R. 4) (R. 5) REFERENCES In the oxidation zone, effects of homogeneous reaction become weak as the amount of CHi and CO decrease. Hence, heterogeneous reaction is the main reason making the concentration of NO decrease in the range of 600~3000 mm below the nozzle. 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