Staged combustion properties for pulverized coals at high temperature

Combustion and Flame 158 (2011) 2261–2271

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Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e

Staged combustion properties for pulverized coals at high temperature Masayuki Taniguchi ⇑, Yuki Kamikawa, Tetsuma Tatsumi, Kenji Yamamoto Plant Analysis Unit, Department of Coal Science Research, Hitachi Research Laboratory, Hitachi, Ltd., 7-1-1 Omika-cho, Hitachi-shi, Ibaraki-ken 319-1292, Japan

a r t i c l e

i n f o

Article history: Received 9 December 2010 Received in revised form 4 March 2011 Accepted 4 April 2011 Available online 27 April 2011 Keywords: Low-NOx combustion Pulverized coal Staged combustion Drop-tube furnace Hydrocarbon formation reaction

a b s t r a c t Staged combustion properties for pulverized coals have been investigated by using a new-concept droptube furnace. Two high-temperature electric furnaces were connected in series. Coal was burnt under fuel-rich conditions in the first furnace, then, staged air was supplied at the connection between the two furnaces. Reaction temperature (1800–2100 K) and time (1–2 s) were similar to those used in actual boilers. When coal was burnt at the same stoichiometric ratio as in actual boilers, similar combustion performance values as for actual boilers were obtained regarding NOx emission and carbon in ash. The most important factor for low NOx combustion was to raise the combustion temperature above the present range (1800–2100 K) in the fuel-rich zone. The NOx emission was significantly increased with decrease of burning temperature in the fuel-rich zone when the temperature was lower than 1800 K. But, NOx emission was cut to around 100–150 ppm, for sub-bituminous coal and hv-bituminous coal, in the latest commercial plants by forming this high-temperature fuel-rich region in the boilers. If the temperature and stoichiometric ratio could be set to the most suitable conditions, and, burning gas and air were mixed well, it would be possible to lower NOx emission to 30–60 ppm (6% O2). The most important NOx reduction reaction in the fuel-rich zone was the NOx reduction by hydrocarbons. The hydrocarbon formation rate in the flame was varied with coal properties and combustion conditions. The NOx was easily reduced when coals which easily formed hydrocarbons were used, or, when burning conditions which easily formed hydrocarbons were chosen. Effects of burning temperature and stoichiometric ratio on NOx emission were reproduced by the previously proposed reaction model. When solid fuel was used, plant performance values varied with fuel properties. The proposed drop-tube furnace system was also found to be a useful analysis technique to evaluate the difference in combustion performance due to the fuel properties. Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction We previously investigated the influence of hydrocarbon formation reactions on the NOx reduction reaction for pulverized coal combustion under fuel-rich conditions [1] in which we measured the NOx, NH3, HCN, hydrocarbon and coal burnout (combustion efficiency) for air and oxy-fuel combustion by using a drop-tube furnace (DTF). The NOx concentrations under fuel-rich conditions are strongly influenced by gas phase stoichiometric ratio, SRgas [1,2]. Ignition conditions are also influenced by SRgas [3]. NOx reduction rate by hydrocarbons is strongly influenced by the SRgas, when the SRgas is low and burning temperature is high. The hydrocarbons are mainly generated by reactions of tar formed by coal pyrolysis. The reaction of large molecular weight hydrocarbons which have multiple aromatic rings is very important to analyze

⇑ Corresponding author. Address: Plant Analysis Unit, Department of Coal Science Research, Hitachi Research Laboratory, Hitachi, Ltd., 832-2 Horiguchi, Hitachinakashi, Ibaraki-ken 312-0034, Japan. Fax: +81 29 276 5783. E-mail address: [email protected] (M. Taniguchi).

the detailed reaction mechanism of hydrocarbons for coal combustion. We proposed a NOx reaction model [1] based on these results. We obtained our experimental data when the burning temperature was 1373–1673 K [1]. However, the combustion temperatures of the latest boilers are usually higher; for example, the maximum flame temperature had been reported to reach 1800– 2100 K [4,5]. In the present paper, we improved the DTF system to obtain NOx and coal burnout properties under the same burning temperature and reaction time as in the latest boilers and we verified the previous reaction model under high-temperature staged combustion conditions. Staged-combustion conditions have been widely used for lowNOx combustion technology [5–9]. Pulverized coal is burnt in a burner zone initially under fuel-rich conditions. NOx is reduced in the burner zone (fuel-rich zone). Then, the coal is oxidized downstream under fuel-lean conditions by adding staged-air. Previously, we analyzed the reaction in the burner zone (fuel-rich zone) [1]. In the present study, we added an electric furnace which simulated the fuel-lean zone reaction. Two high-temperature electric furnaces were connected in series. Staged air was supplied at the connection between the two electric furnaces. This structure

0010-2180/$ - see front matter Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2011.04.005

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Nomenclature a b Cs E k n [OH] [OHeq] PO2 PCO2 PH2O R

constant (ppm) constant (K) constant (1/kg m s) activation energy (kJ/mol) frequency factor constant (–) concentration of OH (mol/m3) equilibrium concentration of OH (mol/m3) partial pressure of oxygen (Pa) partial pressure of carbon dioxide (Pa) partial pressure of water (Pa) gas constant (kJ/mol K)

was able to burn coal at high temperatures. We compared the results of the actual boiler with the results obtained by using the DTF system. We considered the important factors for low-NOx combustion in actual boiler systems. The low NOx burner had been developed first in 1980s. These burners reduced NOx by forming a high-temperature fuel-rich region in the near field of the burner [10,11]. We investigated how high temperature was effective for NOx reduction. The demands to cut NOx emissions continue to increase but the further reductions in NOx by combustion modifications, such as burner staging, have become difficult. By 2016, in the European Union, NOx emission will have to be dropped to 200 mg NO2/m3 N (6% O2); this is around 100 ppm in electric plants with generating capacity larger than 500 MWe [8]. In China, the required cut will be to 200–300 mg NO2/m3 N for super critical steam generators (larger than 600 MWe) [12]. In Japan, NOx had been reduced to around 150 ppm (6% O2) for hv-bituminous coals in the beginning of 2000s [13,14]. NOx emission was to cut around 50% for latest technology [5]. So, in the present paper, we also considered how NOx could be decreased under the ideal condition, based on the feasible boiler size.

2. Experimental 2.1. Structure of the proposed DTF system A schematic drawing of the proposed DTF for staged combustion is shown in Fig. 1. An outline drawing of the actual boiler

Pfuel Rsoot r S S0 SRgas SRb SRf T [THC]

U

partial pressure of fuel (–) formation rate of soot (kg/m3 s) constant (–) total surface area of coal or char (m2/kg-total gas) total surface area of coal or char (m2/m3-total gas) gas phase stoichiometric ratio (–) stoichiometric ratio in fuel-rich zone (–) stoichiometric ratio in fuel-lean zone (–) temperature (K) concentration of total hydrocarbon (ppm, as CH4) equivalence ratio (–)

for staged combustion is also shown. We call this DTF a high-temperature tandem-type staged DTF. In the DTF, two high-temperature electric furnaces were connected in series. Gasification reaction rate has been previously studied using a pressurized high-temperature DTF [15,16]. We modified this DTF to allow use at atmospheric pressure, and we used this design for the basic construction of our two electric furnaces. Coal and air (air for the burner zone) were supplied from the upper part of the electric furnace in the upper section. The nozzle from which coal and air were supplied was described previously [17]. The coal supply rate was around 0.1 kg/h. Coal was burnt under fuel-rich conditions in the upper electric furnace. Staged air was supplied at the connection of the two electric furnaces. The total amount of air (air for burner zone + staged air) was 0.96 m3 N/h. Four annular-shaped carbon heaters were installed in each drop-tube furnace and their temperature could be independently controlled up to around 2100 K. An axial gas temperature distribution could be formed in one heating zone. Reaction tubes had parts made of alumina and stainless steel. The stainless steel tube was used for the connection between the upper and the lower electric furnace. Staged air was injected the connection point. It is important to inject the staged air without leakage. Stainless steel tube was used the injecting part of staged air to get tight sealing. The flue gas is cooled naturally in the connection part. The composition of the flue gas and the carbon in ash were measured at the exit of the lower electric furnace. Burning gas was cooled at the exit of the lower DTF by injecting water. After cooling, char and ash were separated from the flue gas using a filter, and carbon in ash was measured. The analysis methods of the char

Fig. 1. Schematic drawings showing the proposed staged DTF (high-temperature tandem-type staged DTF) and staged combustion for an actual boiler.

M. Taniguchi et al. / Combustion and Flame 158 (2011) 2261–2271

and ash and gas composition were shown previously [1,17]. The number of electric heaters in operation was changed when reaction time was changed. We checked that set the reaction time and temperature by measuring thermal NOx when only air was supplied to the furnace. The two DTFs are the same structure, dimensions and ability. Disassembly is possible for the upper DTF. Only the lower DTF was used, when reactions in the burner zone were examined. For actual boilers, stoichiometric ratio of the burner zone is usually regulated for NOx reduction. However, furnace wall is easily corroded by H2S, if the stoichiometric ratio is too low. This corrosion is prevented by the impingement air near the furnace wall. Local stoichiometric ratio near the wall is increased by the impingement air. For example, side stream air injection system

Fig. 2. An example of temperature profile in the actual boiler. Solid line is calculated temperature along the centerline of the actual furnace, reported by Choi and Kim [4], and, dashed line is the set temperature of the DTF.

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(SSAP) was developed to form the impingement air [18]. We did not include the impingement air for the DTF system. Staged air (oxidizer) ports are installed in upper part of the furnaces. Coal is oxidized by the staged air. This area was called the staged combustion zone (fuel-lean zone). The construction of the DTF system was compared with an actual boiler system. The section of the upper electric furnace was equivalent to the burner zone. The section of the lower electric furnace was equivalent to the staged combustion zone. Standard experimental conditions are also shown in Fig. 1. Averaged stoichiometric ratio, temperature and residence time for the staged DTF system were representative operating conditions of the latest actual systems [5,13,14]. The maximum temperature in the burner zone was around 1800–2100 K [4,5]. The temperature in the staged combustion zone was lower than that in the burner zone to prevent the formation of thermal NOx [5]. Figure 2 shows an example of calculated temperature distribution along the centerline of an actual furnace [4]. The temperature of the DTF was set based on the calculated results [4]. Stoichiometric ratio was around 1.15–1.2 in the staged combustion zone, and around 0.8 in the burner zone [19]. Residence times in the two zones were around 1–2 s [19]. These combustion conditions were basic design conditions to reduce NOx emission and to increase combustion efficiency. However, other conditions, such as mixing of air and coal and heat transfer, also influenced combustion performance. The effects of basic chemical conditions, such as stoichiometric ratio, temperature and residence time, could be obtained independently by using the staged DTF system. A detailed structure of the DTF system is shown in Fig. 3. Axial temperature distributions at the centerline of the furnaces are also shown, when air was supplied. The main bodies of the electric furnaces were put in stainless steel casings. Because the heaters were made of carbon, they were deteriorated when oxygen, water and carbon dioxide were present in the casings, so nitrogen was supplied in the casings during operation. Before operation, the inside of the casings was deaerated to less than 30 Pa by a vacuum pump. Reaction tubes made of alumina (50 mm inner diameter; 2500 mm length) and stainless steel were installed in the

Fig. 3. Detailed structure of the high-temperature tandem-type staged DTF system and axial temperature profiles in it.

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centerline of the casings. Alumina tubes were used where the tubes were heated and stainless steel tubes where they were not. Coal and air were supplied into the tubes. In order to raise the temperature of the electric furnaces, it was important to prevent leakage of oxygen, water and carbon dioxide from the burning gas and atmosphere to the heater zones. Stainless steel tubes were used for connection parts of the alumina tubes and casings, to get tight sealing. It was impossible to get tight sealing if the temperatures of the connection sections were high. Gas in these sections was cooled to less than 800 K. Therefore, the lengths of the alumina tubes were longer than the length of the actual heating zone. Staged air should be supplied to the fuel-rich burning gas for staged combustion but, these joint sections also could not be tightly sealed because of the high temperature. Burning temperature should be decreased to prevent deterioration of the heaters. We gave priority in the design of the DTF system to increase burning temperature to that in actual systems. Therefore, two electric furnaces were connected in series. The staged air was supplied at the connection section of the two electric furnaces.

It is necessary to keep a fixed pressure in the reaction tube to supply coal and air stably. Pressures in the reaction tubes and casings were measured continuously during experiments. An inverter controlled gas pump was installed downstream from the furnace and its displacement volume was fine-tuned to control the pressures. Examples of axial temperature distributions in the reaction tubes are shown in Fig. 3. Air (flow rate, 0.96 m3 N) was supplied to the furnace. The uniform heating zone lengths were around 800 mm. The difference between the set heater temperature and the measured gas temperature was less than ±20 K. When coals were supplied, the difference was ±50 K. Photographs of the high-temperature tandem-type staged DTF system are shown in Fig. 4. During the experiment, we checked fluctuations of measured O2 and NOx concentrations, and, the pressure in the furnace and the casing were small. The gas compositions were measured for a 20-min period and the mean was obtained. Char was collected simultaneously with the gas composition measurements and the ash content of the collected char was analyzed.

Fig. 4. Photos of the high-temperature tandem-type staged DTF system.

Table 1 Analyses of studied coals. Coal

Coal type

Higher calorific value (MJ/kg, dry)

Water (wt.%, as fired)

Volatile matter

Fixed carbon (wt.%, dry)

Ash

C (wt.%, dry)

H (wt.%, dry)

O (wt.%, dry)

N (wt.%, dry)

S (wt.%, dry)

A B C D E F G H I J

Sub-bituminous Lignite Sub-bituminous Sub-bituminous Hv-bituminous Hv-bituminous Hv-bituminous Hv-bituminous Petroleum coke Antracite

26.9 26.5 – 27.2 32.4 29.7 30.3 29.6 36.8 26.3

2.8 13.8 – 10.6 1.7 2.5 4.3 2.3 8.2 1.8

44.2 36.8 41.6 42.5 37.6 32.5 31.1 26.3 11.8 7.6

41.2 35.2 42.7 49.0 55.1 53.2 54.0 60.9 85.9 67.1

14.6 28.0 15.7 8.5 7.3 14.3 14.9 12.8 2.5 25.3

76.3 70.7 74.1 69.1 83.6 83.4 81.0 84.9 86.3 90.6

6.4 4.9 6.2 5.4 5.5 5.4 5.8 5.5 3.9 3.5

15.3 23.1 17.1 23.8 7.7 8.8 10.7 7.2 1.3 3.5

1.3 1.0 1.8 1.1 1.6 1.9 1.8 1.9 2.9 1.4

0.7 0.3 0.8 0.6 1.5 0.5 0.7 0.5 5.7 1.0

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Fig. 5. Particle diameter distributions of studied coals.

2.2. Fuel properties studied The properties of the fuels studied are shown in Table 1. Lignite, sub-bituminous coals, hv-bituminous coals, anthracite and petroleum coke were examined. Examples of their particle diameter distributions are shown in Fig. 5. Representative coal properties used in an actual system [13] are also shown. The diameters of most coals used for the present study were close to that used with an actual system. However, the diameter of coal B was fine and the diameter of coal A was coarse to have upper and lower extremes. 3. Results and discussion 3.1. NOx and carbon in ash properties under standard experimental conditions NOx concentration and carbon in ash at the furnace exit were investigated under standard experimental conditions (fuel-rich zone, stoichiometric ratio = 0.8, temperature = 1873 K; fuel-lean zone, stoichiometric ratio = 1.2, temperature = 1673 K). Results are shown in Fig. 6. One sub-bituminous coal (Coal A) and three hv-bituminous coals (Coals E, F and G) were studied. NOx concentration and carbon in ash varied with coal properties, even though the experimental conditions were the same. NOx concentration and carbon in ash were lower for sub-bituminous coal. Differences for the analyses of Coals E, F and G were slight, but the combustion performances varied with the coals. Carbon in ash of Coal A increased, because its particle diameter was coarse. Mass fraction of large particles (diameter is larger than 150 lm) was much larger than that used for actual systems. These large particles caused carbon in ash to increase. If the mass fraction of large particles is equal to that used for actual systems, carbon in ash becomes the half. For comparison, NOx emission and carbon in ash for the latest actual boilers are also shown [13,14]. The combustion performances were obtained with sub-bituminous and hv-bituminous coals. The combustion performances such as NOx and carbon in ash were influenced by reactions in the furnace. The present results obtained with the proposed DTF system were close to those of the actual systems. The performance values for coal-fired power plants are easily changed by coal properties. It is important to grasp combustion performance for the power plants before their construction. By using the proposed DTF system, the influences of the coal properties for the actual systems can be predicted experimentally. This is a comparatively low-priced and simple method, and it requires a comparatively short time and small amounts of coal.

Fig. 6. Relationship between NOx emission and carbon in ash at the furnace exit for various kinds of coals. The results were compared with the proposed hightemperature tandem-type staged DTF and actual boilers [13,14].

3.2. NOx reaction model Previously, we proposed a NOx reaction model which can estimate NOx concentration in fuel-rich conditions [1]. The main reaction scheme is shown in Fig. 7. The main feature of the model is the gas phase reduction of NOx by hydrocarbons. Ox(OH) is an important oxidative species. We thought that concentrations of hydrocarbons and Ox(OH)) strongly depend on SRgas. According to the scheme, NOx is mainly reduced by hydrocarbons to form XN (NH3, HCN). Most of the XN (NH3, HCN) forms NOx again. However, the remaining XN reacts with NOx to form N2. Reactions used for this study are listed in Table 2. (H/C)VM in Table 2 is molar ratio of H and C in volatile matter. (H/C)char in Table 1 is molar ratio of H and C in char. Example of (H/C)VM and (H/C)char values were described elsewhere [17]. The steady state assumption was used for reactions of XNradical. For R3 and R4, ratio of the reaction rate (R3/R4) is important. Concentration of OH was calculated by using equilibrium concentration and gas temperature. The equation proposed by Bose and Wendt [20] was used for this calculation.

½OH ¼ 1:3  104 expð13000=TÞ  ½OHeq 

ð1Þ

Calculation methods [1,3,17,21] and the NOx reaction model [1] were as described previously. The pyrolysis model (function of devolatilized fraction) used was described previously [3]. The form of the pyrolysis model was simple; however, it could provide almost the same pyrolysis rates as those of detailed models such as Flashchain [22,23]. The reaction parameters are listed in Tables 3 and 4. Table 3 shows parameters for the solid phase reactions for coal F (hvbituminous coal). The rate constant of R5 was estimated by the char combustion experiments [17]. Char was burnt in N2–O2–NO mixtures. O2 in the surrounding gas was varied from 2 to 9 vol.%. NO in the surrounding gas was varied from 0 to 512 ppm. The rate constants were estimated by difference of the NO concentration between inlet and exit value. For some coals, the values of kR5 were varied with surrounding oxygen concentration. Value of c and g in Fig. 7 were also varied with coal properties. For coal F, c was 0.2 and g was 0.07.

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Fig. 7. Reaction scheme of NOx formation and reduction.

Table 2 Reactions used in this study. No.

Reaction

R1 R2 R3 R4 R5 R6 R9 R10 R11 R12 Estimation of total hydrocarbon concentration

Pyrolysis of volatile matter See Ref. [3] XN + OH ? XNradical d[XN]/dt = kR2 exp (ER2/RT) [XN][OH] XNradical + OH ? NOx d[XNradical]/dt = kR3 exp (ER3/RT) [XNradical][OH] XNradical + NOx ? N2 d[XNradical]/dt = kR4 exp (ER4/RT)[XNradical][NOx] NOx + Char ? N2 d[NOx]/dt = kR5 exp (ER5/RT) S0 [NOx]n NOx + THC ? N2 + XN d[NOx]/dt = kR6 exp (ER6/RT) [NOx][THC] Extended Zeldovich mechanism C + 0.5 O2 ? CO dC/dt = kR10 exp (ER10/RT) S POn2 C + CO2 ? 2CO dC/dt = kR11 exp (ER11/RT) S PnCO2 C + H2O ? CO + H2 dC/dt = kR12 exp (ER12/RT) S PnH2O XTHC = kTHC ((H/C)VM R1 + (H/C)char (R10 + R11 + R12)) (1/SRgas)n exp (ETHC/RT)

Form

Table 3 Parameters of solid phase reactions for coal F. Reaction

k

E

n

R5 (mol-NO/m3-total gas s) R10 (kg-carbon/kg-total gas s) R11 (kg-carbon/kg-total gas s) R12 (kg-carbon/kg-total gas s) XTHC (mole fraction, as CH4)

kR5 = 0.865 + 10.8XO2 ((m3/mol)0.9 m s1) 7.99e4 (kg/m2 Pa s) 1.46e3 (kg/m2 Pa s) 1.46e2 (kg/m2 Pa s) 3.15e9 (kg-total gas s/kg-carbon)

28 (kJ/mol) 66 (kJ/mol) 154 (kJ/mol) 154 (kJ/mol) 195 (kJ/mol)

0.9 1 1 1 13

Table 4 shows parameters of gas phase reactions. O2, H2, CO and CO2 concentration in the drop-tube furnace were measured under various temperature and stoichiometric ratio. Concentrations of these species were able to be similar by equilibrium values. 3.3. Factors affecting low-NOx combustion Effects of burning temperature on NOx and carbon in ash were investigated by varying the furnace temperature of the proposed DTF system. Experimental and calculated results are shown in Fig. 8. Some results of coal burnout are summarized in Table 5. Burning temperature in the fuel-rich (burner) zone strongly influenced NOx concentration at the furnace exit.

Sets i–iv of Fig. 8 were the results of coal G. Part of the experiment results of coal F (set v) were shown for comparison. Stoichiometric ratios of fuel-rich and fuel-lean zone were the same, 0.8

Table 4 Parameters of gas phase reactions. Reaction R2 R3 R4 R6

(mol/m3 s) (mol/m3 s) (mol/m3 s) (mol/m3 s)

k

E

4.68e6 (m3/mol s) 5.35e10 (m3/mol s) 1.5e12 (m3/mol s) 1.64e15 (m3/mol s)

43.2 (kJ/mol) 168 (kJ/mol) 251 (kJ/mol) 372 (kJ/mol)

M. Taniguchi et al. / Combustion and Flame 158 (2011) 2261–2271

and 1.2, respectively. When the temperature of the fuel-rich zone was high (1873 K; labeled as set iv), the amount of NOx suddenly increased to nearly 1000 ppm in the early stage of combustion, then, decreased rapidly downstream. NOx concentration became low at the exit of the fuel-rich zone. NOx increased a little after staged air was mixed with the burning gas. The NOx emission was around 150 ppm (6% O2) when stoichiometric ratio of the fuel-rich zone, SRb was 0.8. Similar features were obtained for experiment results of coal F (set v), when temperature of the fuel-rich zone was high. The technology called in-flame NOx reduction was used for the low-NOx combustion technology for actual systems from the end of the 1980s [10,11]. NOx was reduced in the high-temperature fuel-rich region formed downstream of the burners. Ignition performances were accelerated by the low-NOx burner [24,25], and the high-temperature fuel-rich region was easily formed. The NOx increased rapidly in the burner neighborhood; however, NOx rapidly decreased downstream [10,11]. The sets iv

Fig. 8. Axial profiles of NOx and coal burnout in the DTF when furnace temperature in the fuel-rich zone was high (sets iv, v, vii, viii), and low (sets i, ii, iii, vi).

Table 5 Coal burnout of the furnace exit. Stoichiometric ratios of the fuel-rich zone (SRb) and fuel-lean zone (SRf) were 0.8 and 1.2, respectively. Temperature Fuel-rich (K)

Fuel-lean (K)

1873 1873 1673 1573 1373

1673 1673 1673 1573 1373

Experimental or calculated

Coal E (wt.%)

Coal F (wt.%)

Coal G (wt.%)

Experimental Calculated Calculated Calculated Calculated

99.4

99.4

99.2 99.19 99.08 98.59 96.58

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and v results of Fig. 8 reproduced this characteristic of low-NOx combustion. Figure 8 results of sets i–iii were obtained when the temperature of the burner zone was low, 1373–1673 K. NOx also increased rapidly in the early stage of combustion. However, the amount of NOx reduction downstream became small when burning temperature decreased. The NOx emission rose to around 400 ppm which was larger than in actual systems. When temperature of the fuelrich zone rose to 1873 K from 1673 K, NOx reduction performance changed much. According to the present results, increasing the burning temperature in the fuel-rich zone strongly contributed to low-NOx emission combustion. The effect of temperature in the fuel-rich zone on NOx concentration at the furnace exit (fuel-lean zone) was examined by using different coals under different stoichiometric conditions. Figure 8 experimental results of sets vi–viii were obtained for Coal E. Stoichiometric ratios of the fuel-rich zone (SRb) were low; i.e., 0.58–0.7. Temperature in the fuel-rich zone of set vi was low, and temperatures of sets vii and viii were high. Those in the fuellean zone for sets vi–viii were the same. NOx concentration at furnace exit increased when temperature in the fuel-rich zone was decreased. The coal burnout at the fuel-rich zone exit increased when the furnace temperature in the fuel-rich zone increased. This result depended on gasification reaction having been accelerated. Acceleration of gasification reactions by high temperature increased coal burnout. Around 90% of the char was gasified in the fuel-rich zone, when the temperature was 1873 K. Acceleration of gasification reactions was important for low-NOx combustion. Heterogeneous reaction rate constants varied with coal properties. Figure 9 shows effect of heterogeneous reaction rate constants on NOx and coal burnout for coal F. Stoichiometric ratios of the fuel-rich zone (SRb) and fuel-lean zone (SRf) were 0.8 and 1.2, respectively. Furnace temperatures in the fuel-rich zone and fuellean zone were 1873 K and 1673 K, respectively. Set i was a calculated result when all reaction rate constants were the same as those of coal G. Calculated coal burnout at fuel-rich zone exit was lower than experimental result. Calculated NOx concentration at both fuel-rich and fuel-lean zone exit was higher than experimental result. Set ii was a calculated result when gasification rate constants used were estimated by fitting to the coal burnout at fuel-rich zone exit. Gasification reaction rates were 1.8 times larger than those of coal G, while other heterogeneous reaction rate constants were the same. When the calculated result of the coal burnout of the fuel-rich zone exit became close to an experimental result, the calculated NOx concentration also became close to experimental results. Gasification reaction rates influenced NOx concentration. We estimated the heterogeneous reaction rates for each coal by fitting to the experimental data on NOx and coal burnout at fuel-rich and fuel-lean zone exits. NOx reduction phenomena were examined in detail when burning temperature was high. We previously proposed gas phase stoichiometric ratio SRgas as an index to examine NOx reduction phenomena in fuel-rich flames [1]. Figure 10 shows the relation between SRgas and NOx concentration for various coal properties and burning temperature. When SRgas was low (especially, SRgas < 0.85), NOx concentration was easily controlled by SRgas. The plotted data show the effect of burning temperature for Coals D and H. The NOx concentration was hardly influenced by the temperature, when SRgas was the same. The NOx concentrations are also shown for Coals E and F when burning temperature was 1873 K. When burning temperature was high, NOx decreased the same as when combustion temperature was low (<1673 K), if the SRgas was low. The effects of coal properties were small for Coals C, F, G, H, I and J. However, NOx concentrations were high for two, Coals D and E. When SRgas was lower than 0.8, the NOx con-

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We previously examined the role of hydrocarbon reactions for NOx reduction [1]. In this study, relationships between SRgas and hydrocarbon concentration were examined for Coals D and G. The results are shown in Fig. 12. When SRgas and burning temperature were the same, NOx concentration of Coal G was lower than that of Coal D. Hydrocarbon concentration of Coal G was larger than that of Coal D. The coal which easily formed hydrocarbons easily lowered the NOx concentration. The relationship between equivalence ratio and formation rate of soot has been examined for heavy oil combustion [26–28] and an empirical formula (2) was proposed.

Rsoot ¼ Cspfuel Ur expðE=RTÞ

ð2Þ

Here, Rsoot is the formation rate of soot; Cs and r are constants; pfuel is partial pressure of the fuel; U is equivalence ratio, and it is a reciprocal of stoichiometric ratio; T is temperature; and E is activation energy. We assumed that hydrocarbon concentration in coal flames could be expressed by a similar empirical formula, given as:

½THC ¼ að1=SRgasÞn expðb=TÞ Fig. 9. Effect of gasification rate constants on axial profiles of NOx and coal burnout of coal F. Symbols are experimental, and lines are calculated results. Stoichiometric ratios of the fuel-rich zone (SRb) and fuel-lean zone (SRf) were 0.8 and 1.2, respectively. Furnace temperatures in the fuel-rich zone and fuel-lean zone were 1873 K and 1673 K, respectively. Set i was a calculated result when all reaction rate constants were the same as those of coal G. Set ii was a calculated result when rate constants of gasification reactions were 1.8 times larger than those of coal G.

Fig. 10. Relationship between gas phase stoichiometric ratio (SRgas) and NOx concentration at the fuel-rich zone exit.

centrations became equal to or less than 100 ppm for all coals. It was important to lower SRgas as much as possible for low-NOx emission combustion. SRgas values varied with coal burnout in the fuel-rich flames, even if the operating condition (SRb) was the same. Figure 11 shows the relation between operating condition (SRb) and SRgas at the fuel-rich zone exit. When burning temperature was low, coal burnout in the fuel-rich zone was low because the gasification reaction rates were low. Therefore, it was difficult to decrease SRgas. SRb should be reduced to 0.6 (1673 K), or 0.4 (1373 K) to get NOx reduction reaction condition (SRgas around 0.8 or less) in the fuel-rich zone. These SRb values were considerably lower than the standard condition (SRb around 0.8). The amount of carbon in ash easily increased at the furnace exit for these operating conditions. The general these conclusions were recognized by first scaling up of low-NOx burners in the 1980s [10,11]. In the present study, these phenomena were also observed by the drop-tube furnace experiments.

ð3Þ

where [THC] is total hydrocarbon concentration; and a, b and n are constants. Results calculated by Eq. (2) are also shown in Fig. 12. Agreement between calculated and experimental results was good

Fig. 11. Relationship between stoichiometric ratio in the fuel-rich zone (SRb) and the gas phase stoichiometric ratio (SRgas) at the fuel-rich zone exit for various furnace temperatures.

Fig. 12. Relationship between gas phase stoichiometric ratio (SRgas) and total hydrocarbon concentration for coals D (M) and G (s) when the furnace temperature was 1373 K.

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if a was varied for the different coals; a of Coal G was five times larger than that of Coal D. n values were 14 for Coals D and G. The sensitivity toward temperature has been shown elsewhere [1]. Relations between hydrocarbon concentration and NOx concentration are shown in Fig. 13 for fuel-rich flames. Hydrocarbon concentrations were calculated by Eq. (2). The hydrocarbon concentration at 1573 K was the revised value when the temperature was 1673 K at the same SRgas. The correlations between hydrocarbon concentration and NOx concentration were good for fuel-rich conditions. Hydrocarbon concentration for oxy-fuel combustion has been reported to decrease to about 1/5 for Coal G [1]. Results for oxy-fuel combustion are also shown in Fig. 13. The NOx concentrations were almost the same for air and oxy-fuel combustion when hydrocarbon concentrations were the same. 3.4. Effects of stoichiometric ratio and temperature of fuel-rich zone It has been shown effective to control SRb for low-NOx emission combustion [8]. Figure 14 shows the effect of SRb on NOx concentration when temperature in the fuel-rich zone was 1873 K and that in the fuel-lean zone was 1673 K. Stoichiometric ratio of the fuel-lean zone SRf was 1.2. NOx concentrations at both the furnace exit (the fuel-lean zone exit) and at the fuel-rich zone exit are shown. NOx concentrations at the furnace exit were examined first. SRb was varied from 0.6 to 1.0. NOx decreased monotonously when SRb was decreased. We compared the NOx concentrations at the fuel-rich zone exit and the furnace exit. NOx concentration at former was larger than

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that at the latter when SRb was larger than 0.8. NOx was also reduced in the fuel-lean zone under these operating conditions. NOx is chemically reduced by a heterogeneous reaction (NOx reduction at the char surface) [17]. When SRb was lower than 0.8, NOx was formed in the fuel-lean zone. Two kinds of reactions occur for regeneration of NOx in the fuel-lean zone: the heterogeneous oxidation of char nitrogen; and the gas phase oxidation of NH3 and HCN formed in the fuel-rich zone. Char, NH3 and HCN concentrations increase when SRb is reduced [17]. Therefore, we saw that regeneration rates of NOx in the fuel-lean zone increased. NOx was also regenerated in the fuel-lean zone when SRb was large. However, the regeneration rate was lower than the rate of heterogeneous NOx reduction because char, NH3 and HCN concentrations were low. Calculated results are also shown in Fig. 14. The NOx reaction model was previously verified for fuel-rich condition when the burning temperature was lower than 1673 K [1]. In the present study, the reaction model was verified for high-temperature and staged combustion conditions. In the experiments of Fig. 14, SRb was changed for a constant temperature condition. However, burning temperature varies with SRb for actual systems. We examined how the NOx concentration varied with SRb, when burning temperature changed with the change of SRb with the adiabatic flame temperature. Figure 15a shows the relation between SRb and burning temperature. At first, two cases were examined when burning temperature was low (CASE I) and high (CASE II). For actual systems, burning temperature in the fuel-rich zone (burner zone) varies with temperature of the combustion air, water content of coal, and structures of the burners and furnaces. The relation between SRb and NOx concentration is shown in Fig. 15b. SRf and burning temperature in the fuel-lean zone were constant, 1673 K and 1.2. The influence of SRb varied with temperature. The difference was clear when SRb was low. In CASE I (low temperature), NOx concentration rose when SRb was low. NOx concentration showed a minimum value when SRb was around 0.7. In CASE II (high temperature), NOx concentration fell when SRb was lower than 0.7. Low-NOx emission

Fig. 13. Relationship between total hydrocarbon concentration and NOx concentration at the fuel-rich zone exit. The total hydrocarbon concentrations were calculated by using Eq. (3), the calculated concentrations were revised value when the furnace temperature was 1673 K.

Fig. 14. Relationship between stoichiometric ratio of fuel-rich zone (SRb) and NOx concentration at the furnace exit.

Fig. 15. Relationships between stoichiometric ratio of the fuel-rich zone (SRb) and NOx concentration at the furnace exit (fuel-lean zone). (a) Set furnace temperature in the fuel-rich zone for the experiment. (b) NOx concentration at the furnace exit when furnace temperature was low (CASE I), high (CASE II) and extremely low (CASE III).

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combustion became possible under the low-SRb conditions. It was important to consider the effect of burning temperature in the fuel-rich zone, in order to evaluate the NOx performances for actual systems. Burning temperature of CASE III was extremely low. Burning temperature is easy to lower when small experimental equipments are used. In CASE III (extremely low temperature), low-NOx condition could not be found. The influence of SRb was different from CASE I and II. For actual systems, NOx emission is usually adjusted by controlling burner zone stoichiometric ratio. In this case, it should be noted that burning temperature is also varied with stoichiometric ratio. The NOx emission varies with not only the stoichiometric ratio but also the temperature in the burner zone. The burning temperature changes by the design condition of the furnaces and coal properties. The relationship between burner zone stoichiometric ratio and NOx emission are different by burning temperatures. Figure 16 shows examples of the effect of burning temperature on coal burnout, NH3 and HCN concentrations at the fuel-rich zone exit. Coal D was used for these experiments. SRb was 0.73–0.78. The coal burnout increased monotonously with temperature. Regeneration of NOx from char nitrogen in the fuel-lean zone decreased with temperature. NH3 and HCN concentrations decreased with temperature. Regeneration of NOx from NH3 and HCN could be reduced when temperature was high, because concentrations NH3 and HCN became small. The concentration was low when the temperature was larger than 1800 K. We examined how much NOx and carbon in ash could be reduced by controlling the stoichiometric ratio of fuel-rich zone of the high-temperature tandem-type staged DTF system. Results are shown in Fig. 17. Burning temperature in the fuel-rich zone was 1873 K. NOx could be cut to 30–35 ppm (6% O2) for lignite (Coal B). NOx could be cut to 60–70 ppm (6% O2) for hv-bituminous coals (Coals E and F). Carbon in ash amounts were lower than 5 wt.% for these conditions. Usually, carbon in ash is expected to drop below 5 wt.% [14]. NOx could be cut to 35–50 ppm under some operating conditions for hv-bituminous coals, but, the carbon in ash amount became large for these conditions.

Fig. 17. Experimental results for the lowest NOx at the furnace exit (fuel-lean zone) and carbon in ash for lignite (Coal B) and hv-bituminous coal (Coals E and F).

Fig. 18. Relationships between temperature of fuel-lean zone and NOx increase and carbon in ash at the furnace exit for coal A. Temperature of fuel-rich zone was 1873 K. Stoichiometric ratios of the fuel-rich zone (SRb) and fuel-lean zone (SRf) were 0.8 and 1.2, respectively.

3.5. Effects of temperature of fuel-lean zone Carbon in ash amount could be reduced when burning temperature in the fuel-lean zone was increased. However, thermal

Fig. 16. Effect of furnace temperature of fuel-rich zone on coal burnout and concentration of NH3 + HCN at the fuel-rich zone exit. Stoichiometric ratio of the fuel-rich zone (SRb) was 0.73–0.78.

NOx is increased [5]. Effects of burning temperature in the fuellean zone on NOx and carbon in ash were examined. Results are shown in Fig. 18. Temperature in the fuel-rich zone was 1873 K. Coal A was used. These NOx increases were defined as thermal NOx. Formation of thermal NOx was also calculated by using the reaction rate of Zeldovich NOx [29]. Thermal NOx began to increase when burning temperature in the fuel-lean zone was above 1773 K. The relation between the burning temperature and NOx increase were the same as is characteristic of Zeldovich NOx. The effects of burning temperature in the furl-rich and fuellean zones are summarized in Fig. 19. The calculated temperature distributions of NOx concentration were uniform for fuel-rich and fuel-lean zones. Some experimental results are also shown. SRb was 0.7 and SRf was 1.2. When the burning temperature in the fuel-rich zone was less than 1800 K, the NOx concentration at the furnace exit was strongly influenced by the temperature in the fuel-rich zone. Burning temperature in the fuel-rich zone (burner zone) should be kept higher than 1800 K for low-NOx combustion. The effect of the temperature became small when the temperature was larger than 1900 K. The burning temperature in the fuel-lean zone did not influence the NOx concentration when the temperature was lower than 1800 K. However, NOx rose suddenly when the temperature in the fuel-lean zone was above this temperature.

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reactions by hydrocarbons. The hydrocarbon formation rate in the flame varied with coal properties and combustion conditions (for example, air combustion and oxy-fuel combustion). NOx was easily reduced when coals which easily formed hydrocarbons were used or burning conditions which easily formed hydrocarbons were chosen.

References

Fig. 19. Effects of furnace temperature of (a) fuel-rich zone and (b) fuel lean-zone on NOx concentrations at the furnace exit (fuel-lean zone). Solid lines are calculated results for Coal E, and symbols are experimental results for Coal F. Stoichiometric ratios of the fuel-rich zone (SRb) and fuel-lean zone (SRf) were 0.7 and 1.2, respectively.

4. Conclusions Staged combustion properties for pulverized coal combustion were investigated using a new-concept drop-tube furnace. Two high-temperature electric furnaces were connected in series. Coal was burnt at the fuel-rich condition in the first furnace; then, staged air was supplied at the connection between the two electric furnaces. The main findings are summarized below. (1) Similar combustion performances values to those of actual boilers were obtained for NOx emission and carbon in ash, when coal was burnt at the same stoichiometric ratio, burning temperature and residence time conditions. The proposed high-temperature tandem-type staged drop-tube furnace system provided a useful analysis technique to evaluate the difference in combustion performance according to fuel properties for actual systems. (2) The most important factor for low-NOx combustion was to raise combustion temperature in the fuel-rich zone to 1800–2100 K. The NOx emission increased with decreasing burning temperature in the fuel-rich zone when the temperature was lower than 1800 K. If the temperature and stoichiometric ratio could be set at the most suitable conditions, and, burning gas and air were mixed well, it was possible to reduce NOx emission to 30–60 ppm (6% O2). (3) The important NOx reduction reaction in the high-temperature fuel-rich zone was the gas-phase NOx reduction

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