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The effect of limestone on SO2 and NOX emissions of pulverized coal combustion preheated by circulating fluidized bed
Fuel 120 (2014) 116–121
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The effect of limestone on SO2 and NOX emissions of pulverized coal combustion preheated by circulating fluidized bed Ziqu Ouyang a,b, Jianguo Zhu a,⇑, Qinggang Lu a, Yao Yao a,b, Jingzhang Liu a a b
Institute of Engineering Thermophysics, Chinese Academy of Science, Beijing 100190, China University of Chinese Academy of Science, Beijing 100049, China
h i g h l i g h t s Proposing a method of controlling SO2 and NOX emissions from pulverized coal combustion. Analyzing the effects of limestone on SO2 and NOX emissions. Investigating coal sulfur and nitrogen transformation mechanisms in the system. Comparing the combustion and emission characteristics of different coal types.
a r t i c l e
i n f o
Article history: Received 6 November 2013 Received in revised form 6 December 2013 Accepted 9 December 2013 Available online 18 December 2013 Keywords: Pulverized coal combustion Limestone SO2 NOX
a b s t r a c t A series of pulverized coal combustion experiments were carried out in order to investigate effects of limestone addition on SO2 and NOX emissions. The process takes place in two stages: first, the coal is preheated in a circulating fluidized bed with limestone addition; second, combustion of the resulting fuel gas and preheated char particles takes place in a down-fired combustor under air staging conditions. Four types of coal were used in the experiments including lignite, bituminous, anthracite, and semi-coke. Results showed that SO2 emission decreases after the addition of limestone to the circulation fluidized bed for all types of coal due to the reaction between H2S and CaO. The addition of limestone to the circulating fluidized bed was observed to have little effect on NOX emissions for low-volatile-content samples (anthracite and semi-coke), where as high-volatile-content coals (lignite and bituminous) displayed significant decreases in NOX emissions after the addition of limestone in this technique. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction A new technique has been recently proposed and adopted for partial pyrolysis, gasification, and combustion processes in circulating fluidized beds using low air equivalence ratios, whereby preheated pulverized coal at a temperature of nearly 900 °C is pneumatically transported into a down-fired combustor with staged air to achieve a uniform temperature profile and low NOX emission levels [1,2]. The combustion characteristics and NOX formation mechanism of this process have already been thoroughly investigated [2]. Advantageous features of this technology include high combustion efficiency, good flame stability, and low NOX emission; despite these characteristics, an effective method for controlling SO2 emissions from this technique is still required. During the preheating process, coal is partially gasified within the circulating fluidized bed to produce high-temperature coal gas and coal char, which is then fired in a down-fired combustor. ⇑ Corresponding author. Tel.: +86 10 82543139; fax: +86 10 82543119. E-mail address: [email protected] (J. Zhu). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.12.013
The sulfur contained in coal reacts in the reducing atmosphere, forming mainly H2S [3]; therefore, the removal of H2S prior to combustion of the high-temperature coal gas would effectively result in the reduction of SO2 emissions. Calcium-based sorbents such as limestone, dolomite, or scallopderived materials are usually introduced into the gasifier during the coal gasification process to capture and remove H2S in the coal gas [4–8]. Several advantages exist with respect to the use of calcium-based sorbents for coal gas desulfurization at high temperatures, such as primarily higher thermal efficiency, abundant availability, and low sorbent cost. At present, limestone is the primary calcium-based sorbent added to the fluidized bed gasifier in current industrial practices [4]. In this experimental study, coal preheating was performed in a circulating fluidized bed at temperatures conducive to desulfurization reactions; thus, addition of calcium-based sorbents to the circulating fluidized bed would be a suitable strategy for desulfurization. Some studies [9–13] have also revealed that the addition of Ca is, to some extent, effective in the conversion of nitrogenous organic materials to molecular N2 during coal
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pyrolysis and gasification. Therefore, the pulverized coal preheating technology as described above, incorporating desulfurization techniques in the circulating fluidized bed, can be expected to realize lower SO2 and NOX emissions in pulverized coal combustion. The objective of this work is to investigate the effect of limestone on SO2 and NOX emissions in a pulverized coal preheating and combustion system. The effects of calcium-based sorbent addition on preheating performance in the circulating fluidized bed and combustion characteristics in the down-fired combustor are also analyzed.
2. Experiment 2.1. Test rig The test rig diagram shown in Fig. 1 is composed of a circulating fluidized bed (CFB), a down-fired combustor (DFC), and an auxiliary system [2]. A horizontal tube with a diameter of 48 mm and length of 500 mm is used to guide the preheated pulverized coal from the CFB to the DFC. The riser of the CFB is 90 mm in diameter and 1500 mm in height. The feeding port is located 240 mm above the air distributor, through which pulverized coal and limestone are introduced into the CFB by two separated screw feeders. The air, defined as primary air, is supplied to the CFB, and contains about 10–30% of the theoretical air. The primary air fluidizes the bed materials and provides oxygen for partial coal pyrolysis, gasification, and combustion in order to achieve and maintain a bed temperature of nearly 900 °C. The gas at the outlet of the CFB is defined as high-temperature coal gas and the solid particles are defined as preheated coal particles. Preheated coal particles and high-temperature coal gas enter a nozzle 260 mm in diameter and 3000 mm in height at the top center of the DFC. Secondary air is supplied to the nozzle at a velocity of 18 m/s to provide oxygen for preheated coal combustion.
Tertiary air is supplied to the DFC at positions 1200 mm below the nozzle to provide extra oxygen for complete combustion. There are eight thermocouples in the test facility: three Ni–Cr/ Ni–Si thermocouples in the CFB, and five Pt/Pt–Rh thermocouples in the DFC. Eight sampling ports are set as follows: one is placed at the CFB outlet for sampling preheated coal particles and hightemperature coal gas; one is at the bag filter outlet for sampling fly ash, and; the other six ports are 100, 400, 900, 1400, 2400, and 3000 mm below the nozzle. All gas samples were dried and filtered before entering individual online analyzers. High-temperature coal gas compositions were measured at the above ports using a MAIHAK S710 analyzer, a Kane–May KM9106 analyzer, and a Gasmet FTIR DX-4000 analyzer, respectively; H2S concentrations in the high-temperature coal gas were measured using the iodometry method, and other gases in the DFC are measured using a Gasmet FTIR DX-4000 analyzer. 2.2. Fuel characteristics The materials chosen for this study included four different types of pulverized coal with a volatile content ranging between 6.74– 27.46%, and one type of limestone with 54.87% CaO content. Table 1 shows the ultimate analysis and proximate characteristics of these materials. The diameters of the pulverized coals obtained were smaller than 0.355 mm, with a mean particle diameter d50 of 82 lm. Quartz sand with a diameter ranging from 0.1 to 0.5 mm was added to the CFB as bed material. Limestone with a diameter ranging from 0.1 to 0.5 mm was chosen as calcium-based sorbent. 2.3. Description of operating conditions Table 2 provides the operating conditions used for the experiments. The air equivalence ratio in the CFB (kCFB) and the air equivalence ratio in the reducing zone of the DFC (kRZ) are described by the following expressions:
kCFB ¼ 9
AI ; Astoi
kRZ ¼
AI þ AII ; Astoi
k¼
AI þ AII þ AIII Astoi
ð1Þ
Secondary air
where AI, AII, and AIII are the primary, secondary, and tertiary air flows, respectively, Astoi is the air flow in stoichiometric combustion for pulverized coal, and k is the excess air ratio. 4
8
5
Tertiary air 6
Table 1 The analysis of coals and limestone.
10 7
Lignite
Primary air 13
3
14
12
2
11
1
1 air compressor, 2 liquefied petroleum gas, 3 electricity heater, 4 coal screw feeder, 5 limestone screw feeder, 6 riser, 7 U-valve, 8 cyclone, 9 sampling port, 10 DFC, 11 water tank, 12 water cooler, 13 bag filter, 14 gas analyzer. Fig. 1. Schematic diagram of the test rig.
Bituminous
Anthracite
Semi-coke
Ultimate analysis (wt%, air dry) Carbon 54.52 Hydrogen 3.08 Oxygen 11.54 Nitrogen 0.73 Sulfur 0.85
58.28 3.74 8.61 1.04 0.32
82.08 3.13 1.87 1.18 0.70
76.82 1.41 3.99 0.76 0.38
Proximate analysis (wt%, air dry) Moisture 15.78 Ash 13.52 Volatile matter 27.16 Fixed carbon 43.56 Low heating value (MJ/kg) 20.24
1.87 26.14 27.46 44.53 22.70
2.40 8.64 6.74 82.22 31.04
1.20 15.46 8.20 75.14 27.06
Limestone composition (wt%) CaO SiO2 Al2O3 Fe2O3 K2O TiO2 SrO Cr2O3
54.87 2.06 0.81 0.53 0.30 0.07 0.41 0.04
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Table 2 Operating conditions of the experiments. Conditions
E1
E2
E3
E4
E5
E6
E7
E8
Coal type Coal feed rate (kg/h) Primary air flow (m3/h) kCFB Preheating temperature (°C) Limestone feed rate (g/h) Ca/S molar ratio Secondary air flow (m3/h) kRZ Tertiary air flow (m3/h) Tertiary air-flow point (mm) k
Lignite 5.3 7.5 0.3 900 0 0 10 0.7 14.5 1200 1.3
Lignite 5.3 7.5 0.3 900 270 2 10 0.7 14.5 1200 1.3
Bituminous 3.7 6.0 0.3 900 0 0 7.5 0.7 11.5 1200 1.3
Bituminous 3.7 6.0 0.3 900 76 2 7.5 0.7 11.5 1200 1.3
Anthracite 3.5 8.5 0.3 900 0 0 11 0.7 17.0 1200 1.3
Anthracite 3.5 8.5 0.3 900 160 2 11 0.7 17.0 1200 1.3
Semi-coke 3.7 8.0 0.3 900 0 0 10 0.7 15.5 1200 1.3
Semi-coke 3.7 8.0 0.3 900 88 2 10 0.7 15.5 1200 1.3
In the experiments, a preheating temperature of 900 °C, kCFB = 0.3, kRZ = 0.7, and k = 1.3 were used as basic operating conditions for the four types of coal. To investigate the effects of limestone addition, two different operating conditions were employed for one type of coal; one experiment in which no limestone was added into the CFB, and one in which limestone with a Ca/S molar ratio of 2 was employed. 3. Results and discussion 3.1. Effect of limestone on preheating performance and combustion characteristics The temperatures in the CFB and the DFC for experiments E5 and E6 (operating conditions listed in Table 2) are shown in Fig. 2. No significant temperature change in the CFB and DFC was observed after the addition of limestone. Similar results were also found for the lignite, bituminous, and semi-coke coals. It can be concluded that limestone addition has almost no influence on preheating performance or combustion characteristics. The main compositions of high-temperature coal gas in all experiments were also analyzed and are shown in Table 3. The concentrations of CO2, CO, H2, and N2 remained nearly the same regardless of limestone addition. The absence of O2, NOX, N2O,
o
Temperature ( C)
900
600
300
E5 E6
0 0
500
1000
1500
2000
1200
o
Temperature ( C)
Height of the CFB riser (mm)
and SO2 was apparent in the high-temperature coal gas as a result of the strong reducing atmosphere, while the coal nitrogen and sulfur components were mainly converted into NH3, N2, and H2S, respectively, in the preheating process. Upon limestone introduction into the CFB, a sharp decrease in the H2S concentration could be observed for all four types of coal, indicating the effective limestone-mediated removal of sulfur during the preheating process. NH3 concentrations were found to decrease with the introduction of limestone into the CFB for lignite and bituminous samples. However, no changes in NH3 concentrations were noted for anthracite and semi-coke before and after limestone addition. This variation is likely attributed to the differences in coal rank. Wu [14] found that addition of calcium remarkably promotes the conversion of coal–nitrogen into N2 from demineralized low-rank coal; however, such an effect is much slighter for high-rank demineralized coal. The chemical analysis and physical characteristics of preheated coal particles are shown in Table 4. The limestone addition has almost no influence on coal characteristics of preheated coal particles, as the ultimate and proximate analysis have no change before and after limestone addition. In addition, coal nitrogen and sulfur content in the preheated coal particles have also no change before and after limestone addition, indicating that limestone has no contribution to the release of coal nitrogen and sulfur in the preheating process. The addition of limestone had almost no influence on O2 concentrations as evidenced by the profiles along the DFC axis as shown in Fig. 3. Concentrations of O2 were nearly same before and after limestone addition. The concentration curves were similar for lignite, bituminous, anthracite, and semi-coke. In all the experiments, tertiary air is supplied to the DFC at a position 1200 mm from the nozzle, and O2 concentrations at 400 and 900 mm along the axis are nearly zero. A reducing atmosphere, favorable to the reduction of nitrogen oxides and SO2, is formed between the secondary and tertiary air ports. The combustion efficiencies determined for each experiment indicated that limestone had no effect for the four types of coal examined. The combustion efficiencies were 99.6%, 99.0%, 95.4%, and 96.7% for lignite, bituminous, anthracite, and semi-coke samples, respectively, which are relatively higher than those for some other combustion technologies in similar scale test facilities [15–17].
800
3.2. Effect of limestone on SO2 emission 400
E5 E6
0 0
500
1000
1500
2000
Distance frome the nozzle in the DFC (mm) Fig. 2. CFB and DFC temperatures during E5 and E6.
2500
The limestone added into the CFB functions primarily as a sorbent to decrease SO2 emissions resulting from the expected reactions between limestone and sulfur-containing gases. The effects of limestone on SO2 formation and emission are demonstrated in Fig 4. While SO2 emissions were observed to decrease after the addition of limestone for all the coal types in this study,
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Z. Ouyang et al. / Fuel 120 (2014) 116–121 Table 3 Compositions of high-temperature coal gas. Coal gas composition
E1
E2
E3
E4
E5
E6
E7
E8
CO (%) H2 (%) CO2 (%) CH4 (%) N2 (%) NH3 (mg/Nm3) H2S (mg/Nm3)
6.59 16.60 12.77 0.74 63.0 426 600
6.62 16.21 12.96 0.79 63.3 232 50
6.72 13.32 12.03 1.59 65.94 778 389
6.92 13.09 12.10 1.63 66.15 463 105
4.64 9.89 12.85 0.25 71.66 815 350
4.56 9.00 12.76 0.40 71.76 795 110
7.36 15.59 12.13 2.06 62.89 643 260
7.83 15.63 12.01 2.40 63.66 632 90
Note: concentrations of O2, NO, N2O, NO2, and HCN are zero in the high-temperature coal gas.
Table 4 The analysis of preheated coal particles. E2
E3
E4
E5
E6
E7
E8
Ultimate analysis (wt%, air dry) Carbon 55.01 Hydrogen 1.07 Oxygen 2.34 Nitrogen 0.57 Sulfur 0.41
E1
54.98 1.12 2.34 0.56 0.39
59.80 1.20 1.67 0.92 0.33
59.2 1.19 1.70 0.91 0.31
78.09 1.17 1.68 1.51 0.92
77.61 1.09 1.66 1.49 0.91
62.30 1.02 0.46 0.65 0.62
63.02 1.08 0.47 0.63 0.60
Proximate analysis (wt%, air dry) Moisture 1.64 Ash 36.18 Volatile matter 7.02 Fixed carbon 55.16
1.63 35.94 6.97 55.46
0.88 34.90 6.39 57.83
0.87 34.91 6.37 57.85
0.80 16.24 2.78 80.18
0.71 16.30 2.69 80.30
1.97 32.98 5.84 59.41
2.01 33.00 5.62 59.37
10 E7
E7
1000
E8
E8
5 500 0
0
E5 E6
6 0 E3 12
E4
6
E5
SO 2 concentration (mg/m3)
O 2 concentration (%)
12
1800
E6
900 0 E3
1200
E4
600
0
0
E1
E1 12
1400
E2
E2
700
6
0
0 0
1000
2000
0
3000
Distance from the nozzle (mm) Fig. 3. O2 concentration profiles as a function of nozzle distance along the DFC axis.
the absolute values differed between the coal samples; for E1, E3, E5, and E7, SO2 emissions were measured as 1159, 500, 1368 and 557 mg/m3 (@ 6% O2), respectively, at the outlet of the DFC. With limestone addition, SO2 emissions decreased to 795, 177, 799, and 275 mg/m3 (@ 6% O2) for E2, E4, E6, and E8, respectively. In this study, sulfur removal efficiency (SR) is defined as follows:
1000
2000
3000
Distance from the nozzle (mm) Fig. 4. SO2 concentration profiles as a function of nozzle distance along the DFC axis.
SR ¼
Eb Ea 100 Eb
ð2Þ
where Eb and Ea refer to SO2 emission without and with desulfurization, respectively. The sulfur removal efficiencies in this study were 31%, 65%, 42%, and 51%, for lignite, bituminous, anthracite, and semi-coke,
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respectively. The SR values differed among the various types of coal under the same operating conditions. The variation of SO2 emissions before and after limestone addition is in accord with the variation of H2S in the high-temperature coal gas. It is demonstrated that the diminution of H2S is contribute to the reduction of SO2 emissions. As limestone is introduced into the CFB, it can be inferred that the main desulfurization reactions are those between H2S and limestone. Studies [18–20] on the H2S sulfidation reactivity of calcium sorbents in thermo balances and small differential reactors have been previously reported, with results indicating that the reaction temperature, Ca/S molar ratio, residence time, and particle size have great influence on sulfur removal efficiency. Adánez [21] studied H2S sulfidation processes involving limestone in an entrained bed reactor at a temperature of 900 °C, and Ca/S molar ratio of 2, finding that the sulfur removal efficiencies range from 30% to 60%, in good agreement with results obtained in the present study. As mentioned above, H2S concentrations in high-temperature coal gas decrease as limestone is added. The direct sulfidation reaction of limestone in the CFB may occur as follows [22]:
CaCO3 þ H2 S CaS þ CO2 þ H2 O
ðR:1Þ
The thermodynamic equilibrium of the reaction is given by the following equation [23]:
PCO2 PH2 O 13212 ¼ 4:66 1012 exp T P H2 S
ð3Þ
Under calcining conditions, the sulfidation of calcined sorbents occurs as follows:
CaCO3 CaO þ CO2
ðR:2Þ
CaO þ H2 S CaS þ H2 O
ðR:3Þ
The thermodynamic equilibrium of these reactions is given by the following equation [23]:
K sc ¼
P H2 O 7262 ¼ 1:127 exp T P H2 S
12000 Q
Q = Quartz, SiO2 C = Calcium oxide, CaO
10000
Intensity (cps)
The effects of limestone on NOX formation and emission were next compared, as shown in Fig 6; experimental results indicated that the sorbent effects were quite different for the different types of coal. The NOX emissions of lignite, bituminous, anthracite, and semi-coke before limestone addition were 161, 331, 114, and 266 mg/m3 (@ 6% O2), and after limestone addition, NOX emissions of lignite, bituminous, anthracite, and semi-coke changed to 108,
ð4Þ
In the present experiments, the CFB temperature was maintained at 900 °C. CaCO3 can be decomposed into CaO and CO2 as the decomposition of CaCO3 to CaO and CO2 begins around 700 °C. Studies [19] have also shown that the reaction of H2S with CaO is much faster than that with CaCO3; thus, the reaction between H2S and CaO may be the main sulfidation reaction.
8000
6000
3.3. Effect of limestone on NOX emission
C
1000
E7 E8
500
0 600
NO X concentration (mg/m3)
K ds ¼
X-ray diffraction analysis of the CFB bottom ash was performed to determine the nature of the sulfidation reactions, and is shown in Fig. 5. SiO2 was identified as the main phase in the bottom slag sample, and CaO was also found to be present; however, CaCO3, the main component of limestone, could not be found in the sample, which demonstrates that most of the limestone decomposes to CaO and CO2 in the CFB. Therefore, the main sulfidation reaction is between R.2 and R.3. Furthermore, peaks attributed to primary sulfidation product CaS were not detected in the bottom ash sample. The reason for this may be because quartz sand accounts for 95% of the slag, and the amount of CaS is too small, making its detection by XRD analysis difficult. But it is confirmed that there must be CaS exist in the bottom ash, according to some related researches [3–5]. Although the amount of CaS in the bottom ash is small, it is a relatively unstable product and disruptive to the environment. Usually, the treatment method is to put CaS into a high temperature oxygen rich reactor to form CaSO4 [24–26]. The limestone particle diameters ranged from 0.1 to 0.5 mm, most of which could be captured by the cyclone. Therefore, almost no CaCO3 or CaO should enter the DFC. According to Fig. 4, SO2 concentration profiles in the DFC were similar before and after limestone addition, with the only differences observed at each sampling port corresponding to a decrease in SO2 concentrations after limestone addition. From this, it can be inferred that desulfurization occurs mainly in the CFB, and that no sulfur removal reactions occur in the DFC.
E5 E6
300
0 600
E3 E4
300
0
E1
400
E2
4000 C
C
2000
200
Q Q Q
0
0 10
0
20
30
40
50
60
70
1000
2000
3000
Distance from the nozzle (mm)
2 theta (deg.) Fig. 5. XRD pattern of the CFB bottom slag.
Fig. 6. NOX concentration profiles as a function of nozzle distance along the DFC axis.
Z. Ouyang et al. / Fuel 120 (2014) 116–121
206, 96, and 260 mg/m3 (@ 6% O2). The addition of limestone appeared to have little effect on NOX formation for anthracite and semi-coke; however, NOX emissions decreased after limestone addition for both lignite and bituminous coals. The effects of limestone on NOX emissions are similar with that on NH3 concentration in high-temperature coal gas. As noted from the high-temperature coal gas compositions provided in Table 3, NH3 formation decreased significantly with limestone addition for both lignite and bituminous coals, and decreased only slightly for anthracite and semi-coke. During the preheating process, coal nitrogen is converted mainly into N2 and NH3 [2]. As discussed in Table 4, there was no change in the coal nitrogen ratio before and after limestone addition, thus indicating that the decrease in NH3 formation results in the increase in N2 formation. For lignite and bituminous coals, limestone aids in increasing the formation of N2, meaning that a reduced amount of coal nitrogen enters the DFC, thus, NOX emission of the combustion decreases. It is suggested from these observations that the effect of limestone addition on NOX emission depends strongly on the type of coal. The effect of limestone is obvious for low-rank coal, whereas for high-rank coal and semicoke, the effect is less significant. There are two reasons for such differing effects: one is the dispersion of the added limestone, and the other is the reactivity of coal nitrogen. The calcium added to lignite and bituminous coal is expected to exist in the ionexchangeable form, allowing the limestone to be reduced to highly dispersed particles because of the high oxygen and carboxyl contents of low-rank coals [27]. In contrast, the calcium added to anthracite and semi-coke likely exists on the surfaces of the coal particles, and can only be reduced to larger sized particles due to lack of COOH groups in the coal [10]. More finely dispersed calcium is characterized by better mobility and higher reactivity, which has a greater effect on N2 formation. With regard to the reactivity of coal nitrogen, carbon reactivity is known to decrease with an increase in the coal rank. A similar trend also exists for nitrogen in coal, and the lower reactivity of coal nitrogen may also contribute to the lower reduced effects of limestone added to anthracite and semi-coke. 4. Conclusions The effects of limestone addition on SO2 and NOX emissions, preheating performance, and pulverized coal combustion characteristics of lignite, bituminous, anthracite, and semi-coke were investigated, and the following conclusions can be summarized: 1. Limestone is observed to have almost no effect on preheating performance or combustion characteristics. There are no changes apparent in CFB and DFC temperatures after the limestone is introduced into the CFB. Furthermore, compositions of the high-temperature coal gas also remain unchanged after limestone addition, with the exception of NH3 and H2S. Upon introducing limestone into the CFB, a sharp decrease in H2S concentration can be observed for all four types of coal. The effect of limestone on NH3 formation is also observed to be strongly dependent on the coal type. 2. SO2 emission decreases after limestone addition for all coal types examined. Sulfur removal efficiencies for lignite, bituminous, anthracite, and semi-coke are 31%, 64%, 42%, and 51%, respectively. The desulfurization reactions occur in the CFB, and the main sulfidation reaction occurring is identified as that between H2S and CaO. 3. There is little effect of limestone addition on NOX formation for anthracite and semi-coke, whereas for lignite and bituminous, NOX emission decreases after limestone addition. It is
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concluded that during the preheating process, limestone increases the conversion of nitrogen to N2 for low-rank coals; in contrast, limestone has little effect on N2 formation for high-rank coals.
Acknowledgement Financial support of this work by National Natural Science Foundation of China (51006103) is gratefully acknowledged. References [1] Wang J, Zhu JG, Lü QG. Experimental study on the combustion characteristics and NOX emissions of pulverized anthracite preheated by circulating fluidized bed. J Therm Sci 2011;20:355–61. [2] Ouyang ZQ, Zhu JG, Lu QG. Experimental study on preheating and combustion characteristics of pulverized anthracite coal. Fuel 2013;113:122–7. [3] Labiano FG, Adánez J, Abad A, Diego LF, Gayán P. Effect of pressure on the sulfidation of calcined calcium-based sorbents. Energy Fuel 2004;18:761–9. [4] Abad A, Adánez J, Labiano FG, Diego LF, Gayán P. Hot coal–gas desulfurization with calcium-based sorbents in a pressurized moving-bed reactor. Energy Fuel 2004;18:1543–54. [5] Katalambula H, Bawagan A, Takeda S. Mineral attachment to calcium-based sorbent particles during in situ desulfurization in coal gasification processes. Fuel Process Technol 2001;73:75–93. [6] Squires AM, Graff RA, Pell M. Desulfurization of fuels with calcined dolomite – introduction and first kinetic results. Chem Eng Prog Symp Ser 1971;67:23–4. [7] Freund H. Intrinsic global rate constant for the high-temperature reaction of CaO with H2S. Ind Eng Chem Fundam 1984;23:338–41. [8] Abbasian J, Rehamt A, Leppin D, Banerjee DD. Desulfurization of fuels with calcium-based sorbents. Fuel Process Technol 1990;5:1–15. [9] Tsubouchi N, Ohtsuka Y. Nitrogen release during high temperature pyrolysis of coals and catalytic role of calcium in N2 formation. Fuel 2002;81:2335–42. [10] Mori H, Asami K, Ohtsuka Y. Role of iron catalyst in fate of fuel nitrogen during coal pyrolysis. Energy Fuel 1996;10:1022–7. [11] Ohtsuka Y, Wu Z, Edward F. Effect of alkali and alkaline earth on nitrogen release during temperature programmed pyrolysis of coal. Fuel 1997;76:1361–7. [12] Wu ZH, Sugimoto Y, Kawashima H. Catalytic nitrogen release during a fixedbed pyrolysis of model coals containing pyrrolic or pyridinic nitrogen. Fuel 2001;80:251–4. [13] Wang J, Morishita K, Takarada T. High-temperature interactions between coal char and mixtures of calcium oxide, quartz, and kaolinite. Energy Fuel 2001;15:1145–52. [14] Wu ZH, Sugimoto Y, Kawashima H. Effect of demineralization and catalyst addition on N2 formation during coal pyrolysis and on char gasification. Fuel 2003;82:2057–64. [15] Cloke M, Lester E, Thompson AW. Combustion characteristics of coals using a drop-tube furnace. Fuel 2002;81:727–35. [16] Suda T, Takafuji M, Hirata T, Yoshino M, Sato J. A study of combustion behavior of pulverized coal in high-temperature air. Proc Combust Inst 2002;29:503–9. [17] 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. Proc Combust Inst 2007;31:2779–85. [18] Efthimiadis EA, Sotirchos SV. Sulfidation of limestone-derived calcines. Ind Eng Chem Res 1992;31:2311–21. [19] Fenouil LA, Lynn S. Study of calcium-based sorbents for high-temperature H2S removal. 3. Comparison of calcium-based sorbents for coal gas desulfurization. Ind Eng Chem Res 1995;34:2343–8. [20] Yrjas P, Lisa K, Hupa M. Limestone and dolomite as sulfur absorbents under pressurized gasification conditions. Fuel 1996;75:89–95. [21] Adánez J, Labiano FG, Diego LF, Fierro V. H2S removal in entrained flow reactors by injection of Ca-based sorbents at high temperatures. Energy Fuel 1998;12:726–33. [22] Meng XM, Jong WD, Pal R, Verkooijen AHM. In bed and down stream hot gas desulphurization during solid fuel gasification: a review. Fuel Process Technol 2010;91:964–81. [23] Barin I. Thermochemical data of pure substances. Weinheim, Germany: VCH; 1989. [24] Qiu K, Anthony EJ, Jia L. Oxidation of sulfided limestone under the conditions of pressurized fluidized bed combustion. Fuel 2001;80:549–58. [25] Anthony EJ, Jia L, Qiu K. CaS oxidation by reaction with CO2 and H2O. Energy Fuel 2003;17:363–8. [26] Ozawa S, Morita Y, Huang L, Matsuda H, Hasatani M. Oxidation of coal char/ CaS mixture at high temperature. Energy Fuel 2000;14:138–41. [27] Wu Z, Sugimoto Y, Kawashima H. The influence of mineral matter and catalyst on nitrogen release during slow pyrolysis of coal and related material: a comparative study. Energy Fuel 2002;16:451–6.
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The effect of limestone on SO2 and NOX emissions of pulverized coal combustion preheated by circulating fluidized bed Ziqu Ouyang a,b, Jianguo Zhu a,⇑, Qinggang Lu a, Yao Yao a,b, Jingzhang Liu a a b
Institute of Engineering Thermophysics, Chinese Academy of Science, Beijing 100190, China University of Chinese Academy of Science, Beijing 100049, China
h i g h l i g h t s Proposing a method of controlling SO2 and NOX emissions from pulverized coal combustion. Analyzing the effects of limestone on SO2 and NOX emissions. Investigating coal sulfur and nitrogen transformation mechanisms in the system. Comparing the combustion and emission characteristics of different coal types.
a r t i c l e
i n f o
Article history: Received 6 November 2013 Received in revised form 6 December 2013 Accepted 9 December 2013 Available online 18 December 2013 Keywords: Pulverized coal combustion Limestone SO2 NOX
a b s t r a c t A series of pulverized coal combustion experiments were carried out in order to investigate effects of limestone addition on SO2 and NOX emissions. The process takes place in two stages: first, the coal is preheated in a circulating fluidized bed with limestone addition; second, combustion of the resulting fuel gas and preheated char particles takes place in a down-fired combustor under air staging conditions. Four types of coal were used in the experiments including lignite, bituminous, anthracite, and semi-coke. Results showed that SO2 emission decreases after the addition of limestone to the circulation fluidized bed for all types of coal due to the reaction between H2S and CaO. The addition of limestone to the circulating fluidized bed was observed to have little effect on NOX emissions for low-volatile-content samples (anthracite and semi-coke), where as high-volatile-content coals (lignite and bituminous) displayed significant decreases in NOX emissions after the addition of limestone in this technique. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction A new technique has been recently proposed and adopted for partial pyrolysis, gasification, and combustion processes in circulating fluidized beds using low air equivalence ratios, whereby preheated pulverized coal at a temperature of nearly 900 °C is pneumatically transported into a down-fired combustor with staged air to achieve a uniform temperature profile and low NOX emission levels [1,2]. The combustion characteristics and NOX formation mechanism of this process have already been thoroughly investigated [2]. Advantageous features of this technology include high combustion efficiency, good flame stability, and low NOX emission; despite these characteristics, an effective method for controlling SO2 emissions from this technique is still required. During the preheating process, coal is partially gasified within the circulating fluidized bed to produce high-temperature coal gas and coal char, which is then fired in a down-fired combustor. ⇑ Corresponding author. Tel.: +86 10 82543139; fax: +86 10 82543119. E-mail address: [email protected] (J. Zhu). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.12.013
The sulfur contained in coal reacts in the reducing atmosphere, forming mainly H2S [3]; therefore, the removal of H2S prior to combustion of the high-temperature coal gas would effectively result in the reduction of SO2 emissions. Calcium-based sorbents such as limestone, dolomite, or scallopderived materials are usually introduced into the gasifier during the coal gasification process to capture and remove H2S in the coal gas [4–8]. Several advantages exist with respect to the use of calcium-based sorbents for coal gas desulfurization at high temperatures, such as primarily higher thermal efficiency, abundant availability, and low sorbent cost. At present, limestone is the primary calcium-based sorbent added to the fluidized bed gasifier in current industrial practices [4]. In this experimental study, coal preheating was performed in a circulating fluidized bed at temperatures conducive to desulfurization reactions; thus, addition of calcium-based sorbents to the circulating fluidized bed would be a suitable strategy for desulfurization. Some studies [9–13] have also revealed that the addition of Ca is, to some extent, effective in the conversion of nitrogenous organic materials to molecular N2 during coal
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pyrolysis and gasification. Therefore, the pulverized coal preheating technology as described above, incorporating desulfurization techniques in the circulating fluidized bed, can be expected to realize lower SO2 and NOX emissions in pulverized coal combustion. The objective of this work is to investigate the effect of limestone on SO2 and NOX emissions in a pulverized coal preheating and combustion system. The effects of calcium-based sorbent addition on preheating performance in the circulating fluidized bed and combustion characteristics in the down-fired combustor are also analyzed.
2. Experiment 2.1. Test rig The test rig diagram shown in Fig. 1 is composed of a circulating fluidized bed (CFB), a down-fired combustor (DFC), and an auxiliary system [2]. A horizontal tube with a diameter of 48 mm and length of 500 mm is used to guide the preheated pulverized coal from the CFB to the DFC. The riser of the CFB is 90 mm in diameter and 1500 mm in height. The feeding port is located 240 mm above the air distributor, through which pulverized coal and limestone are introduced into the CFB by two separated screw feeders. The air, defined as primary air, is supplied to the CFB, and contains about 10–30% of the theoretical air. The primary air fluidizes the bed materials and provides oxygen for partial coal pyrolysis, gasification, and combustion in order to achieve and maintain a bed temperature of nearly 900 °C. The gas at the outlet of the CFB is defined as high-temperature coal gas and the solid particles are defined as preheated coal particles. Preheated coal particles and high-temperature coal gas enter a nozzle 260 mm in diameter and 3000 mm in height at the top center of the DFC. Secondary air is supplied to the nozzle at a velocity of 18 m/s to provide oxygen for preheated coal combustion.
Tertiary air is supplied to the DFC at positions 1200 mm below the nozzle to provide extra oxygen for complete combustion. There are eight thermocouples in the test facility: three Ni–Cr/ Ni–Si thermocouples in the CFB, and five Pt/Pt–Rh thermocouples in the DFC. Eight sampling ports are set as follows: one is placed at the CFB outlet for sampling preheated coal particles and hightemperature coal gas; one is at the bag filter outlet for sampling fly ash, and; the other six ports are 100, 400, 900, 1400, 2400, and 3000 mm below the nozzle. All gas samples were dried and filtered before entering individual online analyzers. High-temperature coal gas compositions were measured at the above ports using a MAIHAK S710 analyzer, a Kane–May KM9106 analyzer, and a Gasmet FTIR DX-4000 analyzer, respectively; H2S concentrations in the high-temperature coal gas were measured using the iodometry method, and other gases in the DFC are measured using a Gasmet FTIR DX-4000 analyzer. 2.2. Fuel characteristics The materials chosen for this study included four different types of pulverized coal with a volatile content ranging between 6.74– 27.46%, and one type of limestone with 54.87% CaO content. Table 1 shows the ultimate analysis and proximate characteristics of these materials. The diameters of the pulverized coals obtained were smaller than 0.355 mm, with a mean particle diameter d50 of 82 lm. Quartz sand with a diameter ranging from 0.1 to 0.5 mm was added to the CFB as bed material. Limestone with a diameter ranging from 0.1 to 0.5 mm was chosen as calcium-based sorbent. 2.3. Description of operating conditions Table 2 provides the operating conditions used for the experiments. The air equivalence ratio in the CFB (kCFB) and the air equivalence ratio in the reducing zone of the DFC (kRZ) are described by the following expressions:
kCFB ¼ 9
AI ; Astoi
kRZ ¼
AI þ AII ; Astoi
k¼
AI þ AII þ AIII Astoi
ð1Þ
Secondary air
where AI, AII, and AIII are the primary, secondary, and tertiary air flows, respectively, Astoi is the air flow in stoichiometric combustion for pulverized coal, and k is the excess air ratio. 4
8
5
Tertiary air 6
Table 1 The analysis of coals and limestone.
10 7
Lignite
Primary air 13
3
14
12
2
11
1
1 air compressor, 2 liquefied petroleum gas, 3 electricity heater, 4 coal screw feeder, 5 limestone screw feeder, 6 riser, 7 U-valve, 8 cyclone, 9 sampling port, 10 DFC, 11 water tank, 12 water cooler, 13 bag filter, 14 gas analyzer. Fig. 1. Schematic diagram of the test rig.
Bituminous
Anthracite
Semi-coke
Ultimate analysis (wt%, air dry) Carbon 54.52 Hydrogen 3.08 Oxygen 11.54 Nitrogen 0.73 Sulfur 0.85
58.28 3.74 8.61 1.04 0.32
82.08 3.13 1.87 1.18 0.70
76.82 1.41 3.99 0.76 0.38
Proximate analysis (wt%, air dry) Moisture 15.78 Ash 13.52 Volatile matter 27.16 Fixed carbon 43.56 Low heating value (MJ/kg) 20.24
1.87 26.14 27.46 44.53 22.70
2.40 8.64 6.74 82.22 31.04
1.20 15.46 8.20 75.14 27.06
Limestone composition (wt%) CaO SiO2 Al2O3 Fe2O3 K2O TiO2 SrO Cr2O3
54.87 2.06 0.81 0.53 0.30 0.07 0.41 0.04
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Table 2 Operating conditions of the experiments. Conditions
E1
E2
E3
E4
E5
E6
E7
E8
Coal type Coal feed rate (kg/h) Primary air flow (m3/h) kCFB Preheating temperature (°C) Limestone feed rate (g/h) Ca/S molar ratio Secondary air flow (m3/h) kRZ Tertiary air flow (m3/h) Tertiary air-flow point (mm) k
Lignite 5.3 7.5 0.3 900 0 0 10 0.7 14.5 1200 1.3
Lignite 5.3 7.5 0.3 900 270 2 10 0.7 14.5 1200 1.3
Bituminous 3.7 6.0 0.3 900 0 0 7.5 0.7 11.5 1200 1.3
Bituminous 3.7 6.0 0.3 900 76 2 7.5 0.7 11.5 1200 1.3
Anthracite 3.5 8.5 0.3 900 0 0 11 0.7 17.0 1200 1.3
Anthracite 3.5 8.5 0.3 900 160 2 11 0.7 17.0 1200 1.3
Semi-coke 3.7 8.0 0.3 900 0 0 10 0.7 15.5 1200 1.3
Semi-coke 3.7 8.0 0.3 900 88 2 10 0.7 15.5 1200 1.3
In the experiments, a preheating temperature of 900 °C, kCFB = 0.3, kRZ = 0.7, and k = 1.3 were used as basic operating conditions for the four types of coal. To investigate the effects of limestone addition, two different operating conditions were employed for one type of coal; one experiment in which no limestone was added into the CFB, and one in which limestone with a Ca/S molar ratio of 2 was employed. 3. Results and discussion 3.1. Effect of limestone on preheating performance and combustion characteristics The temperatures in the CFB and the DFC for experiments E5 and E6 (operating conditions listed in Table 2) are shown in Fig. 2. No significant temperature change in the CFB and DFC was observed after the addition of limestone. Similar results were also found for the lignite, bituminous, and semi-coke coals. It can be concluded that limestone addition has almost no influence on preheating performance or combustion characteristics. The main compositions of high-temperature coal gas in all experiments were also analyzed and are shown in Table 3. The concentrations of CO2, CO, H2, and N2 remained nearly the same regardless of limestone addition. The absence of O2, NOX, N2O,
o
Temperature ( C)
900
600
300
E5 E6
0 0
500
1000
1500
2000
1200
o
Temperature ( C)
Height of the CFB riser (mm)
and SO2 was apparent in the high-temperature coal gas as a result of the strong reducing atmosphere, while the coal nitrogen and sulfur components were mainly converted into NH3, N2, and H2S, respectively, in the preheating process. Upon limestone introduction into the CFB, a sharp decrease in the H2S concentration could be observed for all four types of coal, indicating the effective limestone-mediated removal of sulfur during the preheating process. NH3 concentrations were found to decrease with the introduction of limestone into the CFB for lignite and bituminous samples. However, no changes in NH3 concentrations were noted for anthracite and semi-coke before and after limestone addition. This variation is likely attributed to the differences in coal rank. Wu [14] found that addition of calcium remarkably promotes the conversion of coal–nitrogen into N2 from demineralized low-rank coal; however, such an effect is much slighter for high-rank demineralized coal. The chemical analysis and physical characteristics of preheated coal particles are shown in Table 4. The limestone addition has almost no influence on coal characteristics of preheated coal particles, as the ultimate and proximate analysis have no change before and after limestone addition. In addition, coal nitrogen and sulfur content in the preheated coal particles have also no change before and after limestone addition, indicating that limestone has no contribution to the release of coal nitrogen and sulfur in the preheating process. The addition of limestone had almost no influence on O2 concentrations as evidenced by the profiles along the DFC axis as shown in Fig. 3. Concentrations of O2 were nearly same before and after limestone addition. The concentration curves were similar for lignite, bituminous, anthracite, and semi-coke. In all the experiments, tertiary air is supplied to the DFC at a position 1200 mm from the nozzle, and O2 concentrations at 400 and 900 mm along the axis are nearly zero. A reducing atmosphere, favorable to the reduction of nitrogen oxides and SO2, is formed between the secondary and tertiary air ports. The combustion efficiencies determined for each experiment indicated that limestone had no effect for the four types of coal examined. The combustion efficiencies were 99.6%, 99.0%, 95.4%, and 96.7% for lignite, bituminous, anthracite, and semi-coke samples, respectively, which are relatively higher than those for some other combustion technologies in similar scale test facilities [15–17].
800
3.2. Effect of limestone on SO2 emission 400
E5 E6
0 0
500
1000
1500
2000
Distance frome the nozzle in the DFC (mm) Fig. 2. CFB and DFC temperatures during E5 and E6.
2500
The limestone added into the CFB functions primarily as a sorbent to decrease SO2 emissions resulting from the expected reactions between limestone and sulfur-containing gases. The effects of limestone on SO2 formation and emission are demonstrated in Fig 4. While SO2 emissions were observed to decrease after the addition of limestone for all the coal types in this study,
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Z. Ouyang et al. / Fuel 120 (2014) 116–121 Table 3 Compositions of high-temperature coal gas. Coal gas composition
E1
E2
E3
E4
E5
E6
E7
E8
CO (%) H2 (%) CO2 (%) CH4 (%) N2 (%) NH3 (mg/Nm3) H2S (mg/Nm3)
6.59 16.60 12.77 0.74 63.0 426 600
6.62 16.21 12.96 0.79 63.3 232 50
6.72 13.32 12.03 1.59 65.94 778 389
6.92 13.09 12.10 1.63 66.15 463 105
4.64 9.89 12.85 0.25 71.66 815 350
4.56 9.00 12.76 0.40 71.76 795 110
7.36 15.59 12.13 2.06 62.89 643 260
7.83 15.63 12.01 2.40 63.66 632 90
Note: concentrations of O2, NO, N2O, NO2, and HCN are zero in the high-temperature coal gas.
Table 4 The analysis of preheated coal particles. E2
E3
E4
E5
E6
E7
E8
Ultimate analysis (wt%, air dry) Carbon 55.01 Hydrogen 1.07 Oxygen 2.34 Nitrogen 0.57 Sulfur 0.41
E1
54.98 1.12 2.34 0.56 0.39
59.80 1.20 1.67 0.92 0.33
59.2 1.19 1.70 0.91 0.31
78.09 1.17 1.68 1.51 0.92
77.61 1.09 1.66 1.49 0.91
62.30 1.02 0.46 0.65 0.62
63.02 1.08 0.47 0.63 0.60
Proximate analysis (wt%, air dry) Moisture 1.64 Ash 36.18 Volatile matter 7.02 Fixed carbon 55.16
1.63 35.94 6.97 55.46
0.88 34.90 6.39 57.83
0.87 34.91 6.37 57.85
0.80 16.24 2.78 80.18
0.71 16.30 2.69 80.30
1.97 32.98 5.84 59.41
2.01 33.00 5.62 59.37
10 E7
E7
1000
E8
E8
5 500 0
0
E5 E6
6 0 E3 12
E4
6
E5
SO 2 concentration (mg/m3)
O 2 concentration (%)
12
1800
E6
900 0 E3
1200
E4
600
0
0
E1
E1 12
1400
E2
E2
700
6
0
0 0
1000
2000
0
3000
Distance from the nozzle (mm) Fig. 3. O2 concentration profiles as a function of nozzle distance along the DFC axis.
the absolute values differed between the coal samples; for E1, E3, E5, and E7, SO2 emissions were measured as 1159, 500, 1368 and 557 mg/m3 (@ 6% O2), respectively, at the outlet of the DFC. With limestone addition, SO2 emissions decreased to 795, 177, 799, and 275 mg/m3 (@ 6% O2) for E2, E4, E6, and E8, respectively. In this study, sulfur removal efficiency (SR) is defined as follows:
1000
2000
3000
Distance from the nozzle (mm) Fig. 4. SO2 concentration profiles as a function of nozzle distance along the DFC axis.
SR ¼
Eb Ea 100 Eb
ð2Þ
where Eb and Ea refer to SO2 emission without and with desulfurization, respectively. The sulfur removal efficiencies in this study were 31%, 65%, 42%, and 51%, for lignite, bituminous, anthracite, and semi-coke,
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respectively. The SR values differed among the various types of coal under the same operating conditions. The variation of SO2 emissions before and after limestone addition is in accord with the variation of H2S in the high-temperature coal gas. It is demonstrated that the diminution of H2S is contribute to the reduction of SO2 emissions. As limestone is introduced into the CFB, it can be inferred that the main desulfurization reactions are those between H2S and limestone. Studies [18–20] on the H2S sulfidation reactivity of calcium sorbents in thermo balances and small differential reactors have been previously reported, with results indicating that the reaction temperature, Ca/S molar ratio, residence time, and particle size have great influence on sulfur removal efficiency. Adánez [21] studied H2S sulfidation processes involving limestone in an entrained bed reactor at a temperature of 900 °C, and Ca/S molar ratio of 2, finding that the sulfur removal efficiencies range from 30% to 60%, in good agreement with results obtained in the present study. As mentioned above, H2S concentrations in high-temperature coal gas decrease as limestone is added. The direct sulfidation reaction of limestone in the CFB may occur as follows [22]:
CaCO3 þ H2 S CaS þ CO2 þ H2 O
ðR:1Þ
The thermodynamic equilibrium of the reaction is given by the following equation [23]:
PCO2 PH2 O 13212 ¼ 4:66 1012 exp T P H2 S
ð3Þ
Under calcining conditions, the sulfidation of calcined sorbents occurs as follows:
CaCO3 CaO þ CO2
ðR:2Þ
CaO þ H2 S CaS þ H2 O
ðR:3Þ
The thermodynamic equilibrium of these reactions is given by the following equation [23]:
K sc ¼
P H2 O 7262 ¼ 1:127 exp T P H2 S
12000 Q
Q = Quartz, SiO2 C = Calcium oxide, CaO
10000
Intensity (cps)
The effects of limestone on NOX formation and emission were next compared, as shown in Fig 6; experimental results indicated that the sorbent effects were quite different for the different types of coal. The NOX emissions of lignite, bituminous, anthracite, and semi-coke before limestone addition were 161, 331, 114, and 266 mg/m3 (@ 6% O2), and after limestone addition, NOX emissions of lignite, bituminous, anthracite, and semi-coke changed to 108,
ð4Þ
In the present experiments, the CFB temperature was maintained at 900 °C. CaCO3 can be decomposed into CaO and CO2 as the decomposition of CaCO3 to CaO and CO2 begins around 700 °C. Studies [19] have also shown that the reaction of H2S with CaO is much faster than that with CaCO3; thus, the reaction between H2S and CaO may be the main sulfidation reaction.
8000
6000
3.3. Effect of limestone on NOX emission
C
1000
E7 E8
500
0 600
NO X concentration (mg/m3)
K ds ¼
X-ray diffraction analysis of the CFB bottom ash was performed to determine the nature of the sulfidation reactions, and is shown in Fig. 5. SiO2 was identified as the main phase in the bottom slag sample, and CaO was also found to be present; however, CaCO3, the main component of limestone, could not be found in the sample, which demonstrates that most of the limestone decomposes to CaO and CO2 in the CFB. Therefore, the main sulfidation reaction is between R.2 and R.3. Furthermore, peaks attributed to primary sulfidation product CaS were not detected in the bottom ash sample. The reason for this may be because quartz sand accounts for 95% of the slag, and the amount of CaS is too small, making its detection by XRD analysis difficult. But it is confirmed that there must be CaS exist in the bottom ash, according to some related researches [3–5]. Although the amount of CaS in the bottom ash is small, it is a relatively unstable product and disruptive to the environment. Usually, the treatment method is to put CaS into a high temperature oxygen rich reactor to form CaSO4 [24–26]. The limestone particle diameters ranged from 0.1 to 0.5 mm, most of which could be captured by the cyclone. Therefore, almost no CaCO3 or CaO should enter the DFC. According to Fig. 4, SO2 concentration profiles in the DFC were similar before and after limestone addition, with the only differences observed at each sampling port corresponding to a decrease in SO2 concentrations after limestone addition. From this, it can be inferred that desulfurization occurs mainly in the CFB, and that no sulfur removal reactions occur in the DFC.
E5 E6
300
0 600
E3 E4
300
0
E1
400
E2
4000 C
C
2000
200
Q Q Q
0
0 10
0
20
30
40
50
60
70
1000
2000
3000
Distance from the nozzle (mm)
2 theta (deg.) Fig. 5. XRD pattern of the CFB bottom slag.
Fig. 6. NOX concentration profiles as a function of nozzle distance along the DFC axis.
Z. Ouyang et al. / Fuel 120 (2014) 116–121
206, 96, and 260 mg/m3 (@ 6% O2). The addition of limestone appeared to have little effect on NOX formation for anthracite and semi-coke; however, NOX emissions decreased after limestone addition for both lignite and bituminous coals. The effects of limestone on NOX emissions are similar with that on NH3 concentration in high-temperature coal gas. As noted from the high-temperature coal gas compositions provided in Table 3, NH3 formation decreased significantly with limestone addition for both lignite and bituminous coals, and decreased only slightly for anthracite and semi-coke. During the preheating process, coal nitrogen is converted mainly into N2 and NH3 [2]. As discussed in Table 4, there was no change in the coal nitrogen ratio before and after limestone addition, thus indicating that the decrease in NH3 formation results in the increase in N2 formation. For lignite and bituminous coals, limestone aids in increasing the formation of N2, meaning that a reduced amount of coal nitrogen enters the DFC, thus, NOX emission of the combustion decreases. It is suggested from these observations that the effect of limestone addition on NOX emission depends strongly on the type of coal. The effect of limestone is obvious for low-rank coal, whereas for high-rank coal and semicoke, the effect is less significant. There are two reasons for such differing effects: one is the dispersion of the added limestone, and the other is the reactivity of coal nitrogen. The calcium added to lignite and bituminous coal is expected to exist in the ionexchangeable form, allowing the limestone to be reduced to highly dispersed particles because of the high oxygen and carboxyl contents of low-rank coals [27]. In contrast, the calcium added to anthracite and semi-coke likely exists on the surfaces of the coal particles, and can only be reduced to larger sized particles due to lack of COOH groups in the coal [10]. More finely dispersed calcium is characterized by better mobility and higher reactivity, which has a greater effect on N2 formation. With regard to the reactivity of coal nitrogen, carbon reactivity is known to decrease with an increase in the coal rank. A similar trend also exists for nitrogen in coal, and the lower reactivity of coal nitrogen may also contribute to the lower reduced effects of limestone added to anthracite and semi-coke. 4. Conclusions The effects of limestone addition on SO2 and NOX emissions, preheating performance, and pulverized coal combustion characteristics of lignite, bituminous, anthracite, and semi-coke were investigated, and the following conclusions can be summarized: 1. Limestone is observed to have almost no effect on preheating performance or combustion characteristics. There are no changes apparent in CFB and DFC temperatures after the limestone is introduced into the CFB. Furthermore, compositions of the high-temperature coal gas also remain unchanged after limestone addition, with the exception of NH3 and H2S. Upon introducing limestone into the CFB, a sharp decrease in H2S concentration can be observed for all four types of coal. The effect of limestone on NH3 formation is also observed to be strongly dependent on the coal type. 2. SO2 emission decreases after limestone addition for all coal types examined. Sulfur removal efficiencies for lignite, bituminous, anthracite, and semi-coke are 31%, 64%, 42%, and 51%, respectively. The desulfurization reactions occur in the CFB, and the main sulfidation reaction occurring is identified as that between H2S and CaO. 3. There is little effect of limestone addition on NOX formation for anthracite and semi-coke, whereas for lignite and bituminous, NOX emission decreases after limestone addition. It is
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concluded that during the preheating process, limestone increases the conversion of nitrogen to N2 for low-rank coals; in contrast, limestone has little effect on N2 formation for high-rank coals.
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