Effects of H2O on NO Emission during Oxy-coal Combustion with Wet

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Effects of H2O on NO emission during oxy-coal combustion with wet recycle Zhijun Sun, Sheng Su, Jun Xu, Kai Xu, Song Hu, Yi Wang, Long Jiang, Ningning Si, Yingbiao Zhou, Syed Shatir A. Syed-Hassan, Anchao Zhang, and Jun Xiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00897 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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Energy & Fuels

1

Effects

of

H2O

on

NO

Emission

during

Oxy-coal

2

Combustion with Wet Recycle

3

Zhijun Suna, b, Sheng Sua*, Jun Xua, Kai Xua, Song Hua, Yi Wanga, Long Jianga,

4

Ningning Sia, Yingbiao Zhoua, Syed Shatir A. Syed-Hassana, c, Anchao Zhangb, Jun

5

Xianga*

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a. State Key Laboratory of Coal Combustion，School of Energy and Power

7

Engineering, Huazhong University of Science and Technology, Wuhan 430074,

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China；

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b. School of Mechanical and Power Engineering，Henan Polytechnic University,

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Jiaozuo 454001, China

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c. Faculty of Chemical Engineering, Universiti Teknologi MARA, 40450 Shah Alam,

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Selangor, Malaysia

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*Corresponding author: Jun Xiang, Sheng Su

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Tel.: +86-27-87542417, Fax: +86-27-87545526

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E-mail address: [email protected]

17

[email protected]

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ACS Paragon Plus Environment

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ABSTRACT

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This study investigated the characteristics of NO emissions during oxy-coal

21

combustion with wet-recycle, especially in the presence of high H2O concentrations.

22

The oxy-combustion was carried out using two types of coal, namely Leiyang (LY)

23

anthracite and Zhundong (ZD) bituminous coals, inside a 24kW drop tube furnace

24

under different O2/CO2/H2O atmosphere. The results showed that the NO conversion

25

increased with decreasing CO2 concentration from 70% to 30% and decreased with

26

increasing H2O concentration from 10% to 40%. Under the experimental conditions

27

employed in this study, the fuel-N conversion for both LY and ZD coals under the

28

oxy-coal wet recycled combustion was lower than that under the oxy-coal dry

29

recycled combustion. The results indicated that H2O and CO2 showed a competitive

30

effect on NO emissions though both of them have positive effect on NO reduction. In

31

order to investigate the effects of H2O/CO2 on recycled NO, oxy-coal combustion

32

experiments were also performed with the initial addition of NO concentrations of

33

900 ppmv. The results showed that NO reduction ratio in O2/CO2 was lower than that

34

in O2/CO2/H2O for both coals under our experimental conditions. It is believed that

35

the results of this study can provide some fundamental information on the effect of

36

H2O/CO2 on the characteristics of NO emissions during oxy-coal combustion with

37

wet-recycle.

38

Keywords:

39

Oxy-coal Combustion, NO, O2/ CO2/ H2O, Reburn, Drop Tube Furnace

40


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1. INTRODUCTION

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Coal-fired power plants are among the major producers of CO2 emissions, a

43

primary cause of global warming 1. Coal is expected to be the dominant source of heat

44

and power production in the foreseeable future in many countries, especially China,

45

India, and Australia

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CO2 from coal combustion activities. Unfortunately, the removal of CO2 in the

47

traditional coal combustion process is ineffective and costly because of the low CO2

48

concentration (10%–15%) in flue gas

49

been proposed as an alternative because the flue gas mainly comprising CO2 and

50

steam is recycled, and a high concentration of CO2 can be attained. Many studies

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9-15

52

to capture CO2.

53

2, 3

. Many efforts have been done to minimize the emission of

4-8

. Oxy-coal (O2/CO2 recycle) combustion has

3, 6,

have demonstrated that oxy-coal combustion is one of the most efficient methods

Given that coal-fired power plants are major producers of pollutants, especially 16

54

the NOx

, the NOx emission characteristics should also be clarified in oxy-coal

55

combustion before this technology can be applied industrially 17, 18. Many studies have

56

investigated the NOx emissions under simulative oxy-coal combustion conditions in

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O2/CO2 atmospheres. Buhre et al.3, Chungen and Jinyue19, Normann et al. 14, Wall et

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al. 9, 13, 20, MacDowell et al. 21, Scheffknecht et al. 22, and Stanger et al. 11 all reported

59

that a larger amount of NOx emissions could be reduced in the O2/CO2 atmospheres

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than in that the traditional air combustion atmosphere, due to the thermal NOx was

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reduced greatly for N2 was replace by CO2. Our previous studies 23, 24 have also come

62

to the same conclusions. However, most of these studies were mainly performed in


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O2/CO2 atmospheres without considering the effects of H2O. It is known that the oxy-coal combustion can be classified into wet recycle and

64 65

dry recycle according to the position of the recycle for the flue gas4,

25

66

researchers suggested that wet recycle with a higher energy efficiency is more

67

appropriate for oxy-fuel combustion26, 27. In oxy-coal combustion with wet recycle,

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some studies showed that the steam volume concentration reaching up to 37 vol. % 28,

69

29

70

and it will be a major atmosphere in the combustion. Thus, the effects of the steam on

71

the NO emission in oxy-coal combustion with wet recycle should not be ignored.

. Some

. And Jiang et al.’s 30 studies showed that steam concentration can be as high as 45%

72

Moro et al. 31 studied the replacement of CO2 with 10% H2O in an entrained flow

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reactor, and found that the emissions of NOx were lower than that in the air

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combustion and oxy-coal combustion with dry recycle. Stadler et al.

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H2O could inhibit the oxidation of intermediates to form NO during oxy-coal

76

combustion. Stewart et al.

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oxy-fired 100 kW circulating fluidized bed combustor and found that the addition of

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8% vol. H2O decreased the NOx emissions by up to 44%. Álvarez et al. 34 showed that

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the NOx concentration was reduced by replacing CO2 with 5%, 10 % and 20% of H2O

80

in the oxy-coal combustion atmosphere, but found no relevant differences for the NOx

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emissions at different steam concentrations. All these studies indicate that the steam

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can also affect the NO emission remarkably, however the steam concentration in these

83

studies was mainly lower than 20% and the effects of the high concentration steam on

84

the NO emission is still unclear. Moreover, with the changing of steam concentration

33

32

showed that

studied the effects of steam on NOx formation in an


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in the wide range (0 - 45%), the H2O/CO2 mole ratio in the oxy-coal combustion

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would change from about 0 to 1, or even more, and the effects of H2O and CO2 on

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oxy-coal combustion respectively would change significantly. The major factor that

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affects the NO emission may be various in a wide range of H2O/CO2 mole ratio. In

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addition, it has been found that there are complex interactions between H2O and CO2

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on the char structures under oxy-coal combustion conditions in our previous studies35,

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36

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the NO emission is still unclear. Therefore, the co-effects of steam and CO2 on the NO

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emission in oxy-coal combustion with wet recycle should be further clarified

94

especially in a wide range of H2O/CO2 mole ratio.

, but whether there are interactions or not between the effects of H2O and CO2 on

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This study estimates the characteristics of NO emissions under oxy-coal

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combustion with wet recycle in a wide range of H2O/CO2 mole ratio in a drop tube

97

furnace (DTF). The NO emissions from coal combustion in air, O2/CO2, O2/CO2/Ar

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and O2/CO2/H2O were investigated at 1573 K. To examine the effects of H2O/CO2 on

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NO reburning process, the experiments were performed under NO initial

100

concentration of 900 ppmv (part-permillion by volume) in an O2/CO2/H2O

101

atmosphere with different H2O/CO2 molar ratios.

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2. EXPERIMENTS AND METHODS

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2.1. Fuel Samples

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Two Chinese coals with different ranks, namely, anthracite coal from Leiyang,

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Hunan Province (LY) and highly volatile bituminous coal from Zhundong, Xinjiang

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Province (ZD), were used in this study. The coal samples were crushed and sieved


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into particles with sizes of 74 µm to 105 µm. Table 1 analyzes the coal samples.

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2.2 Test Facility

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Figure 1 presents a schematic diagram of the experimental system, which mainly

110

includes a 24 kW drop tube furnace (DTF), a steam generator system, a coal and gas

111

supply system, and a flue gas monitoring system.

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The DTF is a vertical tube furnace with the temperature range between 1100 K to

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1700 K and heating rates of 104 K/s to 105 K/s. An alumina tube with a 2.2 m length

114

and 55 mm inner diameter was used as the DTF reactor. A U-tube manometer with a 6

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mm inside diameter was used to monitor the inside pressure of the reactor. The

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temperature in the furnace was measured using three alumina sheathed R-type

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thermocouples inserted into the wall. Moreover, the temperature profile in the reactor

118

was calibrated before experiment. A 1.5-meter-long type S thermocouple (a steel

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bushing with water-cooling) was used to calibrate the inside temperature profile of the

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reactor. The thermocouple was inserted into the DTF reactor from its two ends

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respectively. Nitrogen gas with two liters per minute (at room temperature) was used

122

during the calibration process. Figure 2 shows the steady temperature distributions

123

along the central axis of the reactor, demonstrating a good constant temperature zone

124

can be obtained in this experimental system.

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The steam generation systems used in our previous works36 included a furnace

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and a syringe pump. Deionized water was injected into the steam generator using a

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syringe pump. The gasified steam was supplied into the DTF reactor by the carrier gas

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(in the secondary oxidation stream). To convert all water into steam, the carrier gas


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was heated to 673 K by the pre-heat furnace. To prevent the steam from condensing,

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the gas pipeline was all coated with a heating jacket.

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The flow rates of the gas during the experiments were controlled by mass flow

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controllers, the gas was mixed before being fed into the furnace. A spiral feeder was

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used to control the coal feeding. Table 2 shows the feeder rate error.

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A flue gas analyzer (Nova plus, MRU Instruments Inc., Germany) was used to

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measure the concentrations of NO (Chemiluminescent Detector), CO (Non-Dispersive

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Infra-Red), and O2 (Chemiluminescent Detector). A paramagnetic oxygen analyzer

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(BA4000, MRU Instruments Inc., Germany) was used for the conditions with high

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concentration of O2. Table 2 shows the accuracy of the gas analyzer. To compare the

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NO emissions between the oxy-coal combustion with wet recycle and dry recycle,

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high-purity Argon was used as the balance gas in the outlet of the DTF to compensate

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for the condensed steam under the wet oxy-coal conditions.

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2.3 Experimental Conditions

143 144 145

Excess oxygen ratio ( α ) is an important factor that affects NO emissions. This factor is computed as following: α = O2 ar /O2 tr

(1)

146

where O2 ar and O2tr denote the actual and theoretical requirements of oxygen,

147

respectively. The oxygen concentration varies from 21 vol. % to 30 vol. %.

148

Coal burnout is another important factor to the credibility of experimental data.

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In this study, the fly ash was collected from the exhaust gases by a quartz fiber filter

150

and the coal burnout rate was computed by ash tracer method37. The major types of


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NOx are thermal and fuel NOx during coal combustion. However, generally, the

152

formation of thermal NOx is more than 1500℃38. Therefore, NO is mainly derived

153

from fuel-N for the experimental conditions. The conversion ratio from fuel-N to NO

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can be calculated as follows 18:

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CR = [C （NO）× V × P /（RT）] / [ M（fuel）× Y（N, fuel）/ M（N）] × 100%

(2)

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where CR is the conversion ratio of fuel-N to NO, %; C(NO) is the concentration of

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NO in the flue gas when that in the carrier gas is zero, ppmv; V is the volumetric flow

158

rate of gases, m3/min; P is the system pressure, Pa; R is the ideal gas constant,

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J/(mol•K); T is the absolute temperature of flue gas, K; M(fuel) is the mass flow rate of

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fuel, g/min; Y (N, fuel) is the mass fraction of nitrogen in the fuel; and M(N) is the

161

atomic weight of nitrogen.

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To investigate the effect of CO2/H2O on NO reburning process during the coal

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combustion, several experiments were also performed with a concentration of 900

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ppmv NO in O2/CO2/H2O atmospheres. After addition of NO to the inlet gas, the NO

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will be partly reduced by reburning with coal18. The reduction ratio of NO to N2

166

without NO addition, with NO addition and recycled NO can be computed by Eq. (3a),

167

Eq. (3b) and Eq. (3c), respectively.

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RR1=1-CR

(3a)

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RR2=1-NOC3/ (NOC1+ NOC2)×100%

(3b)

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RR3=(NOC1+NOC2-NOC3)/ NOC1×100%

(3c)

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where RR1，RR2，RR3，denote the NO reduction ratio without NO addition, with NO

172

addition and recycled NO, respectively; NOC1 denotes the initial NO concentration


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added to the inlet gas, NOC2 denotes the NO concentration in the exhaust gas without

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NO addition to the inlet gas; and NOC3 denotes the NO concentration in the exhaust

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gas when the initial NO is 900 ppmv.

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To minimize experimental error, a continuous online measurement was

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performed thrice for each condition. The calculated NO concentration is the average

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of thrice measurements. The calculated NO concentration is the average of three

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measurements, the error bar shows the maximum different from the average value.

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The experiments were operated at a constant temperature of 1573 K. Table 3 shows

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the experimental conditions. In this table, H2O/CO2 was the ratio of initial

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concentration for H2O and CO2, and it was just used as a reference condition to

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express the effect of H2O on NO emission more exactly. For each run in drop tube

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furnace, coal was entrained by 0.6 L/min primary gas and was fed at a rate of range

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0.24 to 0.64 g/min according to the variety of excess oxygen ratio and coal type. The

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secondary gas was introduced at a flow rate 1.4L/min that was heated and mixed in

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the steam generator. The average residence time of coal particle in the reactor is about

188

0.8s.

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3 RESULTS AND DISCUSSION

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3.1 Effects of Excess Oxygen Ratios on NO Emissions

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Figure 3 shows the changes in the conversion ratio from fuel-N to NO (CR) for

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the LY and ZD coals under the oxy-coal conditions with increasing the excess oxygen

193

ratio (α). The burnout rate for ZD coal is more than 95% and for LY coal is between

194

85% to 90%. The value is similar for each coal in all experimental conditions. It can


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be found that the CR under the air condition is higher than that under the oxy-coal

196

conditions for both coals. Moreover, under the oxy-coal conditions, the CR for both

197

coals increases along with α though the CR under these conditions is lower than that

198

under the air condition at the same α (1.3).

199

It can be also seen from Figure 3 that the ZD coal has a lower CR than the LY

200

coal under lower excess oxygen ratio conditions (α=1.1), and more fuel-N convert to

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NO for ZD coal than LY coal. This may be due to the ZD coal has a much higher

202

volatile releasing than the LY coal, and most NO can be reduced by the homogenous

203

reduction for the ZD coal. However, as α increase, the NO emission shows the

204

opposite trend compared with the lower α condition for both coals, and the CR for the

205

ZD coal is higher than that for the LY coal. Moreover, with the increasing of α, the

206

volatile releasing for the ZD coal is much faster than that for the LY coal, and more

207

fuel-N is released and oxidized to NO under a rich oxygen atmosphere.

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3.2 Effects of CO2 on NO Emissions

209

Figure 4 shows the effect of CO2 on CR for the LY and ZD coals. The burnout

210

rates for both coals are similar with that in section 3.1 conditions. The NO conversion

211

decreases along with the increasing mole fraction of CO2 for both coals. According to

212

our previous studies36, an increase in CO2 concentration can significantly enhance the

213

gasification reaction of CO2 with char to form more carbon monoxides and a large

214

amount of hydrocarbons are released at the beginning of coal combustion, all of these

215

will accelerate NO reduction. The possible NO reduction mechanism under high CO2

216

concentrations may include the following 39, 40:


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CO2+H→CO+OH

(R1)

218

CO2+CH→CO+HCO

(R2)

219

NO + CO→1/2N2+CO2

(R3)

220

2NO+4H→N2+2H2O

(R4)

221

NO+CHi→XN (HCN & NH3) +CO+CO2

(R5)

222

C+NO→N2+CO2 or CO

(R6)

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According to the above reactions, the homogeneous reduction of NO was mainly

224

induced by R1 to R5 at the beginning of the combustion, while the heterogeneous

225

reduction of NO was induced by R6 where NO was reduced at the char surface after

226

the devolatilization of coal.

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Figure 4 shows that the CO2 concentration has a significantly different effect on

228

the CR for both coals. Increasing the CO2 molar fraction from 30% to 70% decreased

229

the CR for the LY and ZD coals by 6% and 20%, respectively. The pyrolysis and

230

gasification rates of the higher rank coal were slower than those of the lower rank

231

coal41. And the ZD coal also has a higher hydrogen content than the LY coal. More H,

232

CHi radical groups, and CO will be produced for NO reduction according to R2 to

233

R5.

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3.3 Effects of H2O and H2O/CO2 on the NO Emissions

235

Increasing the H2O concentration will decrease the CO2 concentration in the H2O

236

and CO2 mixture when the total volume of the mixed gases remains unchanged. The

237

analyses in the section 3.2 reveal that decreasing the CO2 content will increase the CR.


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238

In the experimental conditions, the coal burnout rate is similar to the value mentioned

239

in the previous sections. Figure 5 shows the decrease in CR with different additions of

240

H2O. Increasing the H2O mole fraction will further decrease the CR for both coals.

241

Apart from the gasification reactions, the addition of H2O to the diluent gas produces

242

the following reactions 40:

243

O+H2→H+OH

(R7)

244

H2O→H+OH

(R8)

245

OH+CO→CO2+H

(R9)

246

OH+H2→H2O+ H

(R10)

247

NO+H→HNO

(R11)

248

CO+HNO→CO2+NH

(R12)

249

HNO+NH→N2+H2O

(R13)

250

R7 to R10 show that more OH and H groups will be produced as the H2O mole

251

fraction increases, but the OH groups will be consumed by the gasification of coal.

252

The NO will be reduced by R11 to R13.

253

Based on CO2 as diluent gas condition, Figure 6 shows the comparison of the

254

effects of the increase of H2O and the decrease of CO2 on the change of CR. As the

255

percentage of CO2 in the gas mixture decreases, the change of CR decreases for the

256

two types of coal. At different CO2 concentrations, the change of CR showed higher

257

values for the ZD coal type while the values of LY coal type are less. Regarding the

258

effect of water vapor, it is clear that the change of CR is directly proportional to the

259

water vapor concentration. In addition, ZD coal type showed high values of CR


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change at different CO2 concentrations compared to LY coal type.

261

Figure 7 shows the combined effect of H2O and CO2 on CR under different

262

H2O/CO2 molar ratios. The CR for each type of coal significantly differs as the

263

H2O/CO2 molar ratio increases. The CR in the O2/H2O/CO2 condition is lower than

264

that in the O2/CO2 condition for both coals. Under the O2/H2O/CO2 condition, the CR

265

for the LY coal decreased as the H2O/CO2 molar ratio changed from 0:70 to 30:40,

266

and then increased as the H2O/CO2 molar ratio increased to 40:30. Meanwhile, the CR

267

for the ZD coal reached its lowest level at a H2O/CO2 molar ratio of 10:60, and then

268

gradually increased along with the H2O/CO2 molar ratio.

269

According to our previous study36, H2O and CO2 show a competitive relationship

270

with coal char during their co-gasification. The formation of NO from fuel-N can be

271

reduced by the carbon hydrogen radicals. For the LY coal, as Arenillas et al.

272

reported that the fuel type NOx was mainly derived from coal char-N for a high order

273

coal. Increasing the H2O/CO2 molar ratio will promote the gasification reaction,

274

producing more CO, which will reduce the number of OH atomic groups according to

275

the reaction R9. Both of these reactions have a positive effect on NO reduction.

276

Therefore, the CR decreases as the H2O/CO2 molar ratio increases from 0:70(dry) to

277

30:40. More oxidation groups, including OH groups, will be formed as the H2O/CO2

278

molar ratio further increases. According to Le Cong and Dagaut

279

increase along with the H2O concentration because of the H2O with O group

280

decomposition. Therefore, high concentration OH radicals will increase the

281

consumption of CO, decrease the NO reduction, and increase the CR.


42

43

, the OH radicals

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Table 1 shows that the volatile, hydrogen, and oxygen content in the ZD coal are

283

much higher than that in the LY coal. Moreover, the ZD coal has a much easier

284

pyrolysis and gasification than the LY coal. Therefore, the NO formation and

285

reduction of the ZD coal significantly differ from those of the LY coal. Combining

286

Figures 6 and Figure 7, the lowest CR was obtained for ZD coal at 10% H2O/CO2

287

molar ratio, but the lowest CR was obtained for LY coal at 30% H2O/CO2 molar ratio.

288

As a lower order coal, the ZD coal mainly generates NO from the conversion of

289

volatile-N

290

molar ratio according to R1 and R7. Meanwhile, R8 and R9 show that the increasing

291

H2O addition will produce more mole fractions of H, O, and OH in the H2O condition

292

than in the CO2 condition

293

groups. At the same time, the O content of the ZD coal is higher than that of the LY

294

coal, thereby allowing the ZD coal to form OH groups. As shown in sections 3.2 and

295

3.3, the increasing H2O has a negative effect on NO formation for the ZD coal, while

296

the decreasing CO2 shows an opposite effect. Moreover, decreasing the CO2

297

concentration in H2O/CO2 has a much greater effect on NO formation than increasing

298

the H2O concentration. Therefore, the CR for the ZD coal increases along with the

299

H2O/CO2 mole fraction.

44

. The concentration of OH groups increases along with the H2O/CO2

45

, thereby increasing the CO consumption of the OH

300

An optimal H2O/CO2 molar ratio that corresponds to a minimum NO conversion

301

rate may exist. However, different coals have varying molar ratios. Specifically, the

302

LY coal has a 30:40 molar ratio, while the ZD coal has a 10:60 molar ratio. Such

303

differences may be explained by the H2/CO ratios, for the H2/CO ratios will change


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Energy & Fuels

304

for different coal gasification at different H2O/CO2. In sum, for the wet oxy-coal

305

combustion, different types of coal may show optimal H2O/CO2 ratios corresponding

306

to a minimized NO CR, but the corresponding mechanism requires verification by

307

further investigation.

308

3.4 Influence of Initial NO Concentration on NO Reduction

309

Reburning nitrogen oxides in oxy-coal combustion is an important method for 46-48

310

reducing NO

. In the wet oxy-coal combustion, apart from the influence of

311

H2O/CO2 on the conversion of fuel nitrogen to NO, the NO reduction in the recycled

312

flue gas may also may be affected by H2O/CO2.

313

Figure 8 shows the reduction ratios of NO without NO addition (RR1), with NO

314

addition (RR2) and recycled NO (RR3) at different H2O/CO2 molar ratios, respectively.

315

For the LY coal, the RR1 increases as the H2O/CO2 molar ratio changes from 0:70 to

316

30:40, and then decreases as the molar ratio increases from 30:40 to 40:30. With the

317

change of H2O/CO2 molar ratio, RR1 shows the opposite trend with the CR. The RR2

318

and RR3 showed the same trend as that without NO added conditions. For the ZD coal,

319

the maximum reduction ratio can be obtained at a 10:60 H2O/CO2 molar ratio for both

320

with and without NO conditions. The maximum recycled NO reduction ratio can be

321

achieved at a 10:60 H2O/CO2 molar ratio. Figure 8 also shows that the reduction

322

ratios with NO added condition for both coals are lower than that without NO added

323

conditions. According to sections 3.2 and 3.3, the mechanism of NO reduction can be

324

explained by the reaction R1 to R12, the reduction ratios showed the different trends

325

with the change of H2O/CO2 molar ratio for both coals. In general, the NO reduction


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

326

ratio will increase with the increase of the reactant (initial NO concentration), but in

327

fact, the results showed the contrary trend. This can be explained by considering that

328

the NO reduction ability (the reduction groups such as H, CHi, CO, etc. is limited) is

329

greatly reduced especially in the presence of excess oxygen. Therefore, the reduction

330

ratio with NO addition conditions is lower than that without NO added conditions at

331

experimental conditions.

332

4. CONCLUSIONS

333

The NO emission behavior in oxy-coal combustion with wet recycle conditions

334

was tested in a DTF at 1573 K. The effects of excess oxygen ratio, CO2, H2O,

335

H2O/CO2, and addition of initial NO on the NO emissions were studied. The

336

following results were obtained:

337

(1) Excess oxygen ratio is a main contributor of NO emissions. As the excess

338

oxygen ratio increases, more fuel-N will be converted to NO. The conversion ratio

339

under the oxy-coal conditions is lower than that under the air condition.

340

(2) A higher CO2 concentration contributes to a lower NO emission in oxy-coal

341

combustion for both LY and ZD coals. In oxy-coal with dry recycled combustion, the

342

NO conversion ratio increases as the CO2 concentration decreases. CO2 produces a

343

greater effect on the conversion of NO to N2 for the low ranking coal.

344

(3) The he NO conversion ratio for both LY and ZD coals under the oxy-coal with

345

wet recycled combustion is lower than that under the oxy-coal with dry recycled

346

combustion. The experimental results showed that H2O has a positive effect on NO

347

reduction. The NO further reduces as the H2O content increases under the


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Energy & Fuels

348

experimental conditions. H2O and CO2 show a competitive effect on NO formation

349

and reduction under the H2O/CO2 conditions.

350

(4) The reduction ratio decreases after the initial NO addition of 900 ppmv. The

351

H2O/CO2 molar ratio has no relevant effect on NO emission under the experimental

352

conditions.

353

ACKNOWLEDGMENTS

354

This work was sponsored by the National Natural Science Foundation of China (NO.

355

51576081, 51576086, 51576072, 51676064) and the National Key Research and

356

Development Program of China. Technical supports from the Analytical and Testing

357

Center of Huazhong University of Science and Technology are highly appreciated.

358


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359 360 361

References (1) Haszeldine, R. S. Carbon capture and storage: how green can black be? Science 2009, 325(5948), 1647-1652.

362

(2) Châtel-Pélage, F.; Varagani, R.; Pranda, P.; et al. Applications of oxygen for

363

NOx control and CO2 capture in coal-fired power plants. Therm Sci 2006, 10, (3),

364

119-142.

365

(3) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; et al. Oxy-fuel combustion

366

technology for coal-fired power generation. Prog Energy Combust Sci 2005, 31, (4),

367

283-307.

368

(4) Nakayama, S.; Noguchi, Y.; Kiga T.; et al. Pulverized coal combustion in

369

O2/CO2 mixtures on a power plant for CO2 Recovery. Energy Convers Manage 1992,

370

33, (5), 379-386.

371

(5) Wolsky, A. M.; Daniels, E. J.; Jody, B. J. Recovering CO2 from large−and

372

medium-size stationary combustors. J Air Waste Manage Assoc 1991, 41, (4),

373

449-454.

374

(6) Figueroa, J. D.; Fout, T.; Plasynski, S.; et al. Advances in CO2 capture

375

technology—the U.S. department of energy's carbon sequestration program. Int J

376

Greenh Gas Con 2008, 2, (1), 9-20.

377

(7) Liu, H.; Okazaki, K. Simultaneous easy CO2 recovery and drastic reduction of

378

SOx and NOx in O2/CO2 coal combustion with heat recirculation. Fuel 2003, 82, (11),

379

1427-1436.

380

(8) Liu, H.; Zailani, R.; Gibbs, B. M. Comparisons of pulverized coal combustion in


Page 18 of 36

Page 19 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

381 382

air and in mixtures of O2/CO2. Fuel 2005, 84, (7-8), 833-840. (9) Wall, T. F. Combustion processes for carbon capture. P Combust Inst 2007, 31,

383

(1), 31-47.

384

(10) Hu, Y.; Li, H.; Yan, J. Techno-economic evaluation of the evaporative gas

385

turbine cycle with different CO2 capture options. Appl Energ 2012, 89, (1), 303-314.

386

(11) Stanger, R.; Wall, T.; Spoerl, R.; et al. Oxyfuel combustion for CO2 capture in

387

power plants. Int J Greenh Gas Con 2015, 40, 55-125.

388

(12) Li, L.; Zhao, N.; Wei, W.; et al. A review of research progress on CO2 capture,

389

storage, and utilization in Chinese Academy of Sciences. Fuel 2013, 108, 112-130.

390

(13) Wall, T.; Liu, Y. H.; Spero, C.; et al. An overview on oxyfuel coal combustion -

391

state of the art research and technology development. Chem Eng Res Des 2009, 87,

392

(8), 1003-1016.

393

(14) Normann, F.; Andersson, K.; Leckner, B.; et al. Emission control of nitrogen

394

oxides in the oxy-fuel process. Prog Energy Combust Sci 2009, 35, (5), 385-397.

395

(15) Li, H.; Yan, J.; Anheden M. Impurity impacts on the purification process in

396

oxy-fuel combustion based CO2 capture and storage system. Appl Energ 2009, 86, (2),

397

202-213.

398

(16) Wang, C. A.; Du, Y.; Che, D. Study on N2O reduction with synthetic coal char

399

and high concentration CO during oxy-fuel combustion. P Combust Inst 2015, 35, (2),

400

2323-2330.

401

(17) Okazaki, K.; Ando, T. NOx reduction mechanism in coal combustion with

402

recycled CO2. Energy 1997, 22, (2-3), 207-215.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

403

(18) Shaddix, C. R.; Molina, A. Fundamental investigation of NOx formation during

404

oxy-fuel combustion of pulverized coal. P Combust Inst 2011, 33, (2), 1723-1730.

405

(19) Yin, C.; Yan, J. Oxy-fuel combustion of pulverized fuels: combustion

406

fundamentals and modeling. Appl Energ 2016, 162, 742-762.

407

(20) Wall, T.; Stanger, R.; Santos, S. The current state of oxy-fuel technology:

408

demonstrations and technical barriers. In 2nd Oxyfuel Combustion Conference,

409

Queensland, Australia, 2011.

410

(21) MacDowell, N.; Florin, N.; Buchard, A.; et al. An overview of CO2 capture

411

technologies. Energ Env Sci 2010, 3, (11), 1645-1669.

412

(22) Scheffknecht, G.; Al-Makhadmeh, L.; Schnell, U.; et al. Oxy-fuel coal

413

combustion-A review of the current state-of-the-art. Int J Greenh Gas Con 2011, 5S,

414

S16-S35.

415

(23) Wang, B.; Sun, L. S.; Su, S.; et al. A kinetic study of NO formation during

416

oxy-fuel combustion of pyridine. Appl Energ 2012, 92, 361-368.

417

(24) Sun, Z. J.; Su, S.; Ning, X.; et al. The investigation of NOx formation and

418

reduction during O2/CO2 combustion of raw coal and coal char. Energy Procedia

419

2015, 66, 69-72.

420

(25) Zhou, W.; Moyeda, D. Process evaluation of oxy-fuel combustion with flue gas

421

recycle in a conventional utility boiler. Energy Fuels 2010, 24, (3), 2162-2169.

422

(26) Chen, L.; Yong, S. Z.; Ghoniem, A. F. Oxy-fuel combustion of pulverized coal:

423

characterization, fundamentals, stabilization and CFD modeling. Prog Energy

424

Combust Sci 2012, 38, (2), 156-214.


Page 20 of 36

Page 21 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

425

(27) Toftegaard, M. B.; Brix, J.; Jensen, P. A.; et al. Oxy-fuel combustion of solid

426

fuels. Prog Energy Combust Sci 2010, 36, (5), 581-625.

427

(28) Mao, Z.; Zhang, L.; Zhu, X.; et al. Modeling of an oxy-coal flame under a

428

steam-rich atmosphere. Appl Energ 2016, 161, 112-123.

429

(29) Hecht, E. S.; Shaddix, C. R.; Geier, M.; et al. Effect of CO2 and steam

430

gasification reactions on the oxy-combustion of pulverized coal char. Combust Flame

431

2012, 159, (11), 3437-3447.

432

(30) Jiang, Z.; Duan, L.; Chen, X.; et al. Effect of water vapor on indirect sulfation

433

during oxy-fuel combustion. Energy Fuels 2013, 27, (3), 1506-1512.

434

(31) Moroń, W.; Rybak, W. NOx and SO2 emissions of coals, biomass and their

435

blends under different oxy-fuel atmospheres. Atmospheric Environment 2015, 116,

436

65-71.

437

(32) Stadler, H.; Christ, D.; Habermehl, M.; et al. Experimental investigation of NOx

438

emissions in oxycoal combustion. Fuel 2011, 90, (4), 1604-1611.

439

(33) Stewart, M. C.; Symonds, R. T.; Manovic, V.; et al. Effects of steam on the

440

sulfation of limestone and NOx formation in an air-and oxy-fired pilot-scale

441

circulating fluidized bed combustor. Fuel 2012, 92, (1), 107-115.

442

(34) Álvarez, L.; Riaza, J.; Gil, M. V.; et al. NO emissions in oxy-coal combustion

443

with the addition of steam in an entrained flow reactor. Greenh Gases 2011, 1, (2),

444

180-190.

445

(35) Xu, J.; Su, S.; Sun, Z. J.; et al. Effects of steam and CO2 on the characteristics of

446

chars during devolatilization in oxy-steam combustion process. Appl Energ 2016, 182,


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

447

20-28.

448

(36) Xu, J.; Su, S.; Sun, Z. J.; et al. Effects of H2O gasification reaction on the

449

characteristics of chars under oxy-fuel combustion conditions with wet recycle.

450

Energy Fuels 2016, 30, (11), 9071-9079.

451

(37) Ballantyne T R, Ashman P J, Mullinger P J. A new method for determining the

452

conversion of low-ash coals using synthetic ash as a tracer. Fuel, 2005, 84(14):

453

1980-1985.

454

(38) Normann, F.; Andersson, K.; Leckner, B. et al. High-temperature reduction of

455

nitrogen oxides in oxy-fuel combustion. Fuel 2008, 87, (17-18), 3579-3585.

456

(39) Watanabea, H.; Marumob, T.; Yamamotoa, J. I.; et al. The effect of CO2

457

reactivity on NOx reduction in O2/CO2 combustion. In 2nd Oxyfuel Combustion

458

Conference, Queensland, Australia, 2011.

459

(40) Wang, Z.; Zhou, Y.; Whiddon, R.; et al. Investigation of NO formation in

460

premixed adiabatic laminar flames of H2/CO syngas and air by saturated

461

laser-induced fluorescence and kinetic modeling. Combust Flame 2016, 164, 283-293.

462

(41) A. Cuadrat; Abad, A.; García-Labiano, F.; et al. Relevance of the coal rank on

463

the performance of the in situ gasification chemical-looping combustion. Chem Eng J.

464

2012, 195, 91-102.

465

(42) Arenillas, A.; Rubiera, F.; Pis, J. J. Nitric oxide reduction in coal combustion:

466

role of char surface complexes in heterogeneous reactions. Environ Sci Technol 2002,

467

36, (24), 5498-5503.

468

(43) Le Cong, T.; Dagaut, P., Experimental and detailed modeling study of the effect


Page 22 of 36

Page 23 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

469

of water vapor on the kinetics of combustion of hydrogen and natural gas, impact on

470

NOx. Energy Fuels 2009, 23, (2), 725-734.

471

(44) Gao, J. S. Coal gasification. Coal, Oil shale, Natural Bitumen, Heavy Oil and

472

Peat- Vol. I, 380. Eolss Publishers: Paris, UNESCO, 2009.

473

(45) Park, J.; Kim, S. G.; Lee, K. M.; et al. Chemical effect of diluents on flame

474

structure and NO emission characteristic in methane-air counterflow diffusion flame.

475

Int J Energy Res 2002，26，(13), 1141-1160.

476

(46) Dao, D. Q.; Gasnot, L.; Marschallek, K.; et al. Experimental study of NO

477

removal by gas reburning and selective noncatalytic reduction using ammonia in a

478

lab-scale reactor. Energy Fuels 2010, 24, (3), 1696-1703.

479

(47) Normann, F.; Andersson, K.; Johnsson, F.; et al. Reburning in oxy-fuel

480

combustion: a parametric study of the combustion chemistry. Ind Eng Chem Res 2010,

481

49, (19), 9088-9094.

482

(48) Su, S.; Xiang, J.; Sun, L. S.; et al. Numerical simulation of nitric oxide

483

destruction by gaseous fuel reburning in a single-burner furnace. P Combust Inst 2007,

484

31, (2), 2795-2803.


Energy & Fuels

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485

List of tables

486

Table 1 Proximate and Ultimate Analytical Results of Tested Coal Samples (on the

487

basis of air-dried weight).

488

Table 2 Accuracies of the Measurements Uncertainties in the Results.

489

Table 3 Experimental Conditions.

490


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Energy & Fuels

491

Table 1

492

Proximate and Ultimate Analytical Results of Tested Coal Samples (on the basis of

493

air-dried weight). Proximate analysis (wt. %)

494

1.

Ultimate analyses (wt. %)

Sample

Moisture

Ash

VM

FC

C

H

N

S

O1

LY

3.59

34.46

5.68

56.27

56.97

1.95

0.75

0.51

1.77

ZD

12.47

4.64

25.78

57.11

61.45

4.29

0.46

0.42

16.27

calculated by difference.

495


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

496 497

Page 26 of 36

Table 2 Accuracies of the Measurements Uncertainties in the Results. 498 Accuracies and uncertainties 499 Fuels flow rate (g/h)

Uncertainty = ±1 %

Flow rate of syringe pump( ml/min)

Accuracy=±0.005

Temperature (K)

Accuracy = ±1

CO (vol. %)1

Accuracy = ±0.2

NO (ppmv)1

Accuracy = ±5

O2 (%)1

Accuracy = ±0.2

O2 (%)2

Accuracy = ±1

500 501 502 503 504 505 506 507 508 509

1 2

. Flue gas analyzer (Nova Plus, MRU Instruments Inc., Germany) . Paramagnetic oxygen analyzer (BA4000, MRU Instruments Inc., Germany)


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Energy & Fuels

510 511

Table 3 Experimental Conditions. Conditions

Case 1

Case 2

Case 3

Case 4

Case 5

α

1.1-1.4

1.3

1.3

1.3

1.3

O2 (Vol. %)

30

30

30

21

30

CO2 (Vol. %)

70

/

/

/

/

N2 (Vol. %)

/

/

/

79

/

CO2/Ar (molar ratio)

/

30:40, 40:30,

/

/

/

10:60, 20:50,

/

10:60, 20:50,

50:20, 60:10, 70:0 H2O/CO2 (molar ratio)

/

/

30:40, 40:30 NO addition（（ppmv））

/

/

/

512 513


30:40, 40:30 /

0, 900

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

514

List of Figure Captions

515

Figure 1. Schematic diagram of experimental apparatus. 1. Particle feeding element,

516

2. Water cooling, 3. Corundum tube, 4. Flue gas exit, 5. Gas analysis, 6. Drier, 7.

517

Filtration, 8. Cyclone, 9. Balance gas, 10. Mass Flowmeter, 11. U-tube manometer, 12.

518

Collection probe, 13. Electrical heating and temperature control elements, 14.

519

De-ionized water,15. Steam generator, 16. Gas mixer, 17. Gas cylinders

520

Figure 2. Temperature profiles within the reactor.

521

Figure 3. Average CR for the LY and ZD coals with varying excess oxygen ratios

522

(α).

523

Figure 4. Effect of CO2 on NO conversion ratio for the LY and ZD coals at different

524

mole fractions.

525

Figure 5. Decreased NO conversion ratio with different H2O content.

526

Figure 6．The change of NO conversion ratio based on CO2 as diluent condition

527

versus the varieties of Ar/CO2 and H2O/CO2.

528

Figure 7. NO conversion ratio for the LY and ZD coals in the O2/CO2 and oxy-coal

529

combustion with wet recycle experiments with various H2O concentrations.

530

Figure 8. NO reduction ratios for (a) LY coal and (b) ZD coal combustion with

531

various H2O/CO2 molar ratios with and without an initial NO addition of 900 ppmv.

532


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Energy & Fuels

533 534 535 536 537 538 539

1. Particle feeding element, 2. Water cooling, 3. Corundum tube, 4. Flue gas exit, 5. Gas analysis, 6. Drier, 7. Filtration, 8. Cyclone, 9. Balance gas, 10. Mass Flowmeter, 11. U-tube manometer, 12. Collection probe, 13. Electrical heating and temperature control elements, 14. De-ionized water,15. Steam generator, 16. Gas mixer, 17. Gas cylinders Figure 1. Schematic diagram of experimental apparatus.

540


Energy & Fuels

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541 542

Figure 2. Temperature profiles within the reactor.

543 544 545 546


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Energy & Fuels

547 548

Figure 3. Average NO conversion ratio for the LY and ZD coals with varying

549

excess oxygen ratios (α).

550


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

551 552

Figure 4. Effect of CO2 on NO conversion ratio for the LY and ZD coals at

553

different mole fractions.

554


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Energy & Fuels

555 556

Figure 5. Decreased NO conversion ratio with different H2O content.

557


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

558 559

560

561 562 563

Figure 6．．The change of NO conversion ratio based on CO2 as diluent condition versus the varieties of Ar/CO2 and H2O/CO2.


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564 565

Figure 7. NO conversion ratio for the LY and ZD coals in the O2/CO2 and wet

566

oxy-coal combustion experiments with various H2O concentrations.

567


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

568

569 570

Figure 8. NO reduction ratios for (a) LY coal and (b) ZD coal combustion with

571

various H2O/CO2 molar ratios with and without an initial NO addition of 900

572

ppmv.


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Effects of H2O on NO Emission during Oxy-coal Combustion with Wet

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