Experiments and Simulation on Co-Combustion of Semicoke and Coal

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Experiments and simulation on co-combustion of semicoke and coal in a full-scale tangentially fired utility boiler Pengqian Wang, Chang'an Wang, Yongbo Du, Qinqin Feng, Zhichao Wang, Wei Yao, Jiali Liu, Jinping Zhang, and Defu Che Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04482 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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Experiments and simulation on co-combustion of semi-coke and coal in a full-scale tangentially fired utility boiler

Pengqian Wang1, Chang’an Wang1, Yongbo Du1, Qinqin Feng1, Zhichao Wang2, Wei Yao2, Jiali Liu2, Jinping Zhang1, and Defu Che1*

1State

Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China 2Xi’an

*To

Thermal Power Research Institute Co., LTD., Xi’an 710054, PR China

whom correspondence should be addressed. Tel: +86-29- 82665185; Fax: +86-29-82668703.

E-mail: [email protected] (D.F. Che).

ACS Paragon Plus Environment

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Abstract Co-firing semi-coke and coal in utility boilers is a promising approach to cleanly and efficiently utilize semi-coke, while the co-combustion characteristics have yet to be fully understood, including the effects of blending method and secondary air distribution mode on NOx emission and combustion efficiency. In the present work, both full-scale experiments and simulation were carried out to investigate the co-combustion characteristics of semi-coke and coal on a 300 MW coal-fired untility boiler with a focus on blending method and air distribution. The results indicated that co-firing semicoke would reduce the furnace temperature, and both NOx emission and incomplete combustion heat loss increased with the blending ratio of semi-coke. Hence, to limit the excessive deterioration of boiler performance, the blending ratio of semi-coke was recommended to be below 50% for in-furnace blending method and below 33% for out-furnace blending method. When the in-furnace blending method was employed, the semi-coke and coal were recommended to be fed from burners in alternate layers, in particular without semi-coke fed from the top and/or bottom burners. The NOx emission was proved to positively correlate with average height of semi-coke burners, while the burnout ratio increased at first and then decreased with the increasing average height. The in-furnace blending method with optimal burner allocation was proved to have advantage over out-furnace blending method. The secondary air distribution of pagoda down type could reduce NOx generation and unburned carbon due to the formation of the high temperature and strong reducing atmosphere within furnace. Key words: semi-coke, coal-fired boiler; co-combustion; blending method; air distribution


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1. Introduction In China, low-rank coal accounts for more than half of the proven coal reserves.1, 2 The low-rank coal can be utilized effectively and cleanly through the graded conversion approach, with the production of coal tar, coke oven gas, semi-coke, and so on.2-4 Semi-coke is a solid carbon-based product with low volatile content and high heat value, and widely used in the industries of metallurgy,5 chemistry,6 and adsorbent.7, 8 However, due to the booming development of Chinese coal chemical industry, the annual production of semi-coke has increased up to several hundred million tons, which far exceeds the consumption of these traditional markets.1, 9 The excessive amount of semi-coke takes up too much space, and causes the waste of resources and environmental pollution. Hence, new markets or fields are urgently needed for the large-scale consumption of semi-coke. The use of semicoke as a power fuel in the field of power generation can be a feasible solution, while the economic benefit of power plants can also be improved due to the low price of semi-coke powder. Previous studies focused on the utilization of semi-coke in circulating fluidized bed boiler.1, 10, 11 Considering the large output, existing pulverized coal fired boilers, whose capacity occupies the most fraction of utility boiler in China, can be another primary option for large-scale utilization of semicoke. Unfortunately, despite the improved specific surface area and enhanced reactivity of semi-coke due to the graded process, the ultra-low content of volatile matter, hydrogen (H), and oxygen (O) usually leads to ignition difficulty, unstable combustion and poor burnout.1, 12 Although the high heat value makes semi-coke be a promising power fuel, the NOx emissions will evidently increase due to the high char nitrogen fraction and combustion reaction temperature. Furthermore, impingement abrasion index of semi-coke is proved to be significantly increased, which would result in more wear


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of mills and burners. Based on the characteristics, co-firing semi-coke and coal in coal-fired boiler can be a promising strategy to utilize semi-coke cleanly and efficiently. The co-combustion of multiple fuels has been numerously investigated in laboratory scale,10,

13-16

pilot-scale,17,

18

and full scale

furnace,19-23 However, few studies are specific to the semi-coke produced by coal upgrading, especially the research on full-scale coal-fired boiler. The specific study on full-scale coal-fired boiler is necessary to provide the engineering guidance for the co-combustion. In fact, the characteristics and mechanism of co-combustion remain insufficiently understood, such as the effect of blending method on NOx emission and combustion efficiency. The blending methods can be classified into out-furnace blending method and in-furnace blending method.13 For the former, different fuels are mixed in mills, bunkers, or conveyors before being injected into furnace. While each fuel is fed into furnace from separate burners without prior mixing under the condition of in-furnace blending. Baek et al.21 numerically investigated the effect of blending method on coal blending combustion, and found that the co-combustion efficiency was sensitive to blending method but the NOx emission was not. The conclusion may be incomprehensive and differ for co-combustion of semicoke and coal. Moreover, the burner allocations of multiple fuels also have great influence on cocombustion of full-scale furnace. Zhang et al.20 numerically studied the coal blending combustion performance on a utility boiler, and found that the allocation of brown coal in either middle or top layers ensured a higher combustion efficiency. Li et al.24 investigated the co-combustion characteristics of biomass and coal on a wall-fired boiler, and found that lowering biomass inlet led to an enhanced burnout. However, the effects of burner allocation are still not comprehensively understood due to the limited burner allocations and different fuels in these works. Systematic analyses


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are in demand to optimize the burner allocations for co-combustion of semi-coke and bituminous coal and further reveal the effect of blending method. Besides, suitable distribution mode of secondary air can be beneficial to further reducing NOx emission and improving boiler performance under cocombustion condition. Nevertheless, most previous studies only focused on the influences of the excess air coefficient, over fire air rate and so on.25-27 Little literature studied the effects of secondary air distribution mode, and the effects on NOx emission and combustion efficiency under co-combustion condition remains unclear. The present work aims to explore the large-scale utilization of semi-coke on coal-fired utility boiler and optimize the co-combustion strategy. The full-scale experiments were firstly conducted on a 300 MW coal-fired boiler for co-firing semi-coke with bituminous coal, which was rarely reported. Based on the verification of those experiments, computational fluid dynamics technology was subsequently employed to further discuss detailed conditions in the full-scale furnace. The influences of blending ratio, burner allocation, and blending method on NOx emission and combustion efficiency were comprehensively investigated to provide the optimal operating scheme for co-firing the special fuel with coal. The correlations between boiler performance and average height of semi-coke burners were further analyzed. Eventually, the adjustment of secondary air distribution mode was conducted to further reduce NOx emission and unburned carbon. The work can provide engineering guidance for clean and large-scale utilization of semi-coke in power plant, and then promote the development of low-rank coal industry in China.

2. Full-scale experimental section The co-combustion tests were carried out on a 300 MW tangentially fired utility boiler. The


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schematic structure of the furnace and burners arrangement are illustrated in Fig. 1. The width, depth and height of the boiler are 14.05, 14.02 and 55.92 m, respectively. The steam of 266.7 kg·s-1 with 17.4 MPa and 814 K can be produced under boiler rated load (BRL). Totally 24 burners located in 6 layers were symmetrically arranged at the corners of furnace, and the secondary air nozzles are regularly distributed between the burners. The furnace equipped the deep-air-staging system, including closed-couple over fire air (CCOFA) and separate over fire air (SOFA). Figure 1(b) and (c) illustrate the horizontal and vertical arrangement of burners and nozzles. Apart from the secondary air in the bottom layer (AA), there are 5o deviation between the primary air and secondary air. The proximate analysis, ultimate analysis, and calorific values of the fuels are summarized in Table 1, in which Hami bituminous coal is abbreviated as HA. The semi-coke used in the study was produced by the lowtemperature pyrolysis of one bituminous coal, whose properties were also listed in the Table 1 as raw coal.

Fig. 1. Schematic (a) and burner arrangement (b-c) of the coal-fired boiler


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Energy & Fuels

Table 1. The properties of fuels Sample

Proximate analysis (wt.%)

Ultimate analysis (wt.%)

Qnet,ar

Mar

Aar

Var

FCar

Car

Har

Oar

Nar

Sar

(MJ/kg)

HA

14.20

7.41

28.80

55.81

64.00

3.23

10.21

0.68

0.23

23.92

Semi-coke

8.10

12.80

8.22

70.88

74.42

1.40

1.97

0.91

0.40

25.35

Raw coal

22.30

6.62

34.97

36.11

54.75

3.67

11.47

0.81

0.38

20.63

O, by difference; wt. %, mass faction; Qnet,ar, net calorific value; ar, as received basis Table 2. Full-scale experiment operation conditions Test

Exp T1

Exp T2

Exp T3

Exp T4

Exp T5

Exp T6

0

17%

33%

50%

50%

50%

3%

3%

3%

3%

3%

3%

Regular

Regular

Regular

Regular

Pagoda

Pagoda

type

down type

/

D

DE

CDE

CDE

CDE

37.97

31.27

25.55

20.02

20.50

19.57

/

6.31

12.60

20.21

20.28

19.59

65.83

63.33

61.39

62.78

63.17

60.17

209.45

219.73

233.31

241.11

239.05

231.77

Main steam temperature (K)

812.5

811.5

815.0

812.0

808.0

810.0

Main steam pressure (Mpa)

15.84

15.80

16.06

16.53

16.38

16.78

227.22

227.78

237.15

242.16

245.91

240.66

403.9

401.17

398.2

397.4

399.4

401.2

Semi-coke blending ratio Operating oxygen Secondary air distribution mode Burners for semi-coke Feeding rate of coal (kg·s-1) Feeding rate of semi-coke (kg·s-1) Feeding rate of primary air

(kg·s-1)

Feeding rate of secondary air

Main steam flow

(kg·s-1)

Exhaust gas temperature (K)

(kg·s-1)

The full-scale experiments were performed employing in-furnace blending method, and the burners for semi-coke were arranged in the middle of the burner zone. Six full-scale experimental cases were performed to investigate the effect of blending ratio and secondary air distribution mode on cocombustion. The operation conditions of the full-scale experiments are briefly summarized in Table 2. The semi-coke and HA bituminous coal were pulverized in different mills and injected into the furnace from separate layers. The pulverized fuels were sampled by isokinetic sampling system to measure the particle sizes. The furnace temperature was measured by infrared optical pyrometer, with four measurement points arranged in each measured cross-section. As shown in the Fig.1, the observation holes #1 and #4 of each measured cross-section were arranged in the front wall, and the others were


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arranged in the rear wall. The temperature of flu gas was measured by thermocouple arranged before air preheater. The NOx concentration and oxygen at furnace exit were measured by a flue gas analyzer (TESTO-350). The carbon content in fly ash was obtained by analysis of the fly ash collected at airpreheater exit.

3. Numerical section 3.1 Mesh generation The mesh system generated by structured hexahedral grids is depicted in Fig. 2a and b. The mesh was refined in the burner zone where the turbulence mixing and combustion reaction intensely occurred. The grid independence test was firstly performed, and three mesh systems with total grid number of 1,279,283, 1,754,374 and 2,228,998 were selected for the primary simulation. Figure 2c compared the furnace temperatures of the three mesh systems, and the mesh system with 1,754,374 cells was eventually employed in this study for the satisfactory accuracy and relatively short computing time.

Fig. 2. Mesh system and grid independence tests (a) mesh of the furnace (b) cross-section of the burner zone (c) furnace temperatures


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3.2 Numerical models CFD technology has been validated to be an effective and reliable tool to investigate co-combustion in full-scale furnace for special fuels, and the simulation work based on a validated CFD model can partly replace the expensive full-scale experiments.19 In the preset work, a series of numerical studies were performed on FLUENT (ANSYS, Inc., USA), serving as the supplementation to further investigate the co-combustion characteristics. Table 3 summarizes the models employed in the simulation. The emissivity and scattering factor of particles were set to 0.9 and 0.6, respectively.

28

Gas-phase interaction for the co-combustion can be represented as follows, and the detailed kinetic parameters of reactions (1) to (5) are available in reference.26

VolHA coal + 1.09O 2   0.90CO+ 2.12H 2 O+ 0.0431N 2

(R1)

Volsemi-coke + 1.64O 2   1.19CO+ 2.54H 2 O+ 0.01191N 2

(R2)

CO + 0.5O 2   CO 2

(R3)

CO H 2 O   CO 2  H 2

(R4)

H 2  0.5O 2   H 2O

(R5)

where VolHA coal and Volsemi-coke is the volatiles derived from the bituminous coal and semi-coke, respectively. The NOx formation was calculated based on the converged simulation results. Only thermal-NO and fuel-NO were taken into consideration, because prompt-NO could be neglected in coal fired boiler.25 The simulation models of thermal-NO and fuel-NO are also listed in Table 2. Fuel nitrogen (fuel-N) consists of volatile nitrogen (volatile-N) and char nitrogen (char-N). HCN and NH3 were considered as the main intermediates during the release of the volatile-N, and the ratio of HCN to NH3 was 9:1


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for bituminous coal and 99:1 for semi-coke.12 The BET surface area were 26260 m2/kg and 21570 m2/kg for chars of semi-coke and bituminous coal according to the experimental results. The conversion fraction of char-N was set to be 0.9 for both bituminous coal and semi-coke, and 30% of char-N was directly conveys to NO.28 The proportion of volatile-N was assumed to be 70% of the total fuel-N for bituminous coal.25 Based on ash balance in the upgrading process, volatile-N of semi-coke can be calculated to be 42% by the following equation. It was assumed that the reduction of fuel-N was totally due to the reduction of volatile-N during the production of semi-coke.

 SC  1  (1   RC )  SC

AarSC N arRC  AarRC N arSC

(6)

SC

where  SC , Aar and N ar are volatile-N proportion, ash content and nitrogen content of semiRC

coke, %;  RC , Aar

RC

and N ar are volatile-N proportion, ash content and nitrogen content of raw coal,

%. Table 3. Summary of simulation models. Item Model Turbulence

Item Model Realized k-ε model Standard wall functions29

Gas phase chemical reaction

Finite-Rate/Eddy-Dissipation model30

Particle-tracking model

Stochastic particle trajectory model31

Radiation

P-1 model28

Devolatilization

Two-competing-reactions model32

Char combustion

Kinetics/diffusion-limited model33

NOx formation

Extended Zeldovich mechanism34 De Soete mechanism35

3.3 Case condition Table 4 summarize the brief conditions of the simulation cases. Cases O1-O6 and cases I1-I6 were designed to study the influence of blending ratio and blending method on combustion efficiency and NOx emission. Cases I7-I17 were set up to optimize the burner allocation strategy for semi-coke, and


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Energy & Fuels

correlation analysis between boiler performance and average height of semi-coke burners was carried out. Furthermore, cases I18-I22 were conducted to optimize secondary air distribution mode for the co-combustion of semi-coke and bituminous coal. The feeding schemes of secondary air for different distribution modes were summarized in Table 5. Furthermore, the temperatures of primary air and secondary air were 336 K and 592 K, respectively. Excess air coefficient was 1.167 for all cases. The surface temperature of water wall was assumed to be 688 K (70 K higher than the water/steam temperature in tubes), and the emissivity was 0.6.36 The parameter for platen super-heaters were 778 K, and 0.46. Notably, two simulation method have been developed for out-furnace blending method. One is treat treated semi-coke and coal as separate fuels, and the other regards the blends of multiple fuels as a single fuel, whose properties are obtained by mass average. The former was employed in the present work due to the consideration of interaction effect and actual physical processes, and the comparisons of the two simulation method were illustrated in Fig. S1 (Supporting information) and Table S1 (Supporting information). Table 4. Detailed distributions fuel and air for simulation cases Case

Burners for

Blending ratio (%)

Blending method

Case O1(I1)

0

Out-furnace

/

Regular

Case O2

17

Out-furnace

A-D

Regular

Case O3

33

Out-furnace

A-D

Regular

Case O4

50

Out-furnace

A-D

Regular

Case O5

67

Out-furnace

A-D

Regular

Case O6

83

Out-furnace

A-D

Regular

Case I2

17

In-furnace

D

Regular

Case I3

33

In-furnace

DE

Regular

Case I4

50

In-furnace

CDE

Regular

Case I5

67

In-furnace

BCDE

Regular

Case I6

83

In-furnace

BCDEF

Regular

Case I7

50

In-furnace

ABC

Regular

Case I8

50

In-furnace

BCD

Regular

SC


Secondary air distribution mode

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Table 4. Continued Case

Burners for

Blending ratio (%)

Blending method


Case I9

50

In-furnace

DEF

Regular

Case I10

50

In-furnace

ACE

Regular

Case I11

50

In-furnace

BCE

Regular

Case I12

50

In-furnace

BDE

Regular

Case I13

50

In-furnace

BDF

Regular

Case I14

33

In-furnace

BD

Regular

Case I15

33

In-furnace

BE

Regular

Case I16

33

In-furnace

CD

Regular

Case I17

67

In-furnace

BCEF

Regular

Case I18

50

In-furnace

CDE

Pagoda type

Case I19

50

In-furnace

CDE

Pagoda down type

Case I20

50

In-furnace

CDE

Equal type

Case I21

50

In-furnace

CDE

Waist drum type

Case I22

50

In-furnace

CDE

Shrunk-middle type

SC

Table 5. Mass flow rate of air for cases with different secondary air distribution modes Mass flow rate in primary zone (kg·s-1) Cases

Type

primary air

secondary air AA

AB

BB

CC

CD

DD

EE

EF

FF

Case 4

Regular

62.78

39.5

10.0

21.2

30.9

10.0

21.2

30.9

10.0

21.2

Case 18

Pagoda type

63.17

43.6

36.8

23.5

23.2

22.1

14.1

13.7

11.0

7.0

Case 19

Pagoda down type

60.17

41.7

7.8

10.0

14.5

15.7

20.0

29.1

31.3

25.0

Case 20

Equal type

62.78

39.5

23.1

23.1

23.1

23.1

23.1

23.1

23.1

23.1

Case 21

Waist drum type

62.78

42.0

14.2

13.6

26.3

35.3

22.6

19.8

14.2

6.8

Case 22

Shrunk-middle type

62.78

41.2

27.8

17.7

16.1

13.9

8.9

19.4

27.8

22.2

4. Result and Discussion 4.1 Effect of blending ratio of semi-coke The effects of semi-coke blending ratio on NOx generation and combustion efficiency were experimentally and numerically investigated for in-furnace blending to estimate the achievable consumption capacity of semi-coke. The measured and predicted furnace temperatures of exp T3 are selected and displayed in Table 6 for each measured points , and the detailed data for the other fullscale experiments are summarized in the Table S2 (Supporting information). Moreover, the important


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indictors at furnace exit of experimental and numerical cases are also summarized in Table S3 (Supporting information). As shown in Table 6 and Table S2 (Supporting information), a good agreement was obtained between most of the calculated (Case I1-I4) and measured (Exp T1-T4) furnace temperatures. It can be found in Table S3 (Supporting information) that simulation results are also consistent with those of full-scale experiments in unburned carbon in fly ash (CFA) and NOx emission, and the differences are also within an acceptable range. Therefore, the employed models are applicable for this work to investigate the co-combustion characteristics. Table 6. The comparison of experimental and predicted furnace temperatures

Measurement points

Furnace temperature of Exp T3/Case I3 (K) #1

#2

#3

#4

Measured

Predicted

Measured

Predicted

Measured

Predicted

Measured

Predicted

I (10.49 m)

1431

1403

1408

1394

1437

1420

1486

1429

II (15.70 m)

1509

1619

1521

1514

1482

1593

1496

1553

III (17.80 m)

1679

1617

1710

1631

1663

1644

1683

1496

IV (27.78 m)

1506

1599

1481

1520

1503

1606

1518

1576

V (39.5 m)

1406

1392

/

/

/

/

1429

1408

Fig. 3. Experimental and Predicted distribution of temperature and NOx for in-furnace blending method (a) Temperature (b) NOx massflow

Figure 3 depicts the distribution of temperature and NOx generation along the height for the cases with various blending ratios of semi-coke. As shown in Fig. 3a, the furnace temperature increased fast


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in the burner zone and then declined. The calculated temperature distributions also showed similar trends with an initially fast increase in the primary zone, and a sharp decrease before reaching the location of SOFA near 27 m. When the separated over fire air was injected into furnace, a slight increase and then reduction of temperature were observed. Both measured and calculated temperature level trended to reduce with the increasing blending ratio of semi-coke, especially in the burner zone and burnout zone, which indicated that co-firing semi-coke in furnace would reduce furnace temperature. This can be explained on one hand by the delay of ignition, burnout, and heat release of semi-coke due to the low volatile content and on the other by the growth of total air feeding. Actually, co-firing more semi-coke in furnace would increase the total air feeding because of the high C/H of semi-coke, and then reduced the furnace temperature. Although the heat value of semi-coke was higher than that of bituminous coal, more cold air feeding and poor burnout characteristic of semi-coke played a more significant role in temperature reduction. Zhang et al.20 found that replacing brown coal with semi-coke would lower the furnace temperature in burner zone, which could be due to great differences between semi-coke and brown coal in heat value and moisture content. As shown in Fig. 3b, the overall trend of NO mass flow was upward along the height. The decrease of NO mass flow in the zone near 17.5 m and 23 m indicated that a certain portion of NO was reduced due to the low O2 concentration. The NO mass flows of these cases were roughly the same in bottom burner zone, but turned to show enlarged gaps after the middle burners (Layer C and D), which was the injection position of semi-coke. Similar to anthracite, semi-coke have more char-N due to the pyrolysis process, and thus is more likely to produce NOx because char-N is easier to generate NOx than volatile-N.12 Thus, the NOx generation increased with the blending ratio of semi-coke. More


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Energy & Fuels

details of NO distribution can be observed in Fig. 4. The highest mole fraction of NO was found in zone near the bottom burners for all cases. Although NO was continually produced with combustion, the dilution of secondary air led to lower NO concentration along the height. Furthermore, NO tended to accumulate centrally with fluctuant shape due to the alternating injection of the primary and secondary air. Once away from the SOFA, NO distributed more uniformly in flue gas, and the results reflected in furnace exit were consistent with Fig. 3a.

Fig. 4. NO distribution of cases with various semi-coke blending ratio for in-furnace blending, (a) 0 (case I1), (b) 17% (case I2), (c) 33% (case I3), (d) 50% (case I4), (e) 67% (case I5)

Figure 5 illustrated the unburned carbon in fly ash and NOx emission at furnace exit. The measured results showed that the NOx emission at furnace outlet increased fast with the blending ratio of semicoke from 152 mg·m-3 (only firing bituminous coal) to 229 mg·m-3 (50% semi-coke co-fired), and the growth was high up to 51%. Similarly, the carbon content in fly ash increased from 2.47% to 3.87%. According to the numerical study, the predicted parameters would increase to 266.35 mg·m-3 and 6.87% for 80% blending ratio. The higher blending ratio of semi-coke led to the lower temperature at furnace exit, and the maximum of temperature fall was 55 K. Reasonable and achievable blending ratio of semi-coke could balance reduction of boiler performance and maximum consumption of semi-coke.


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Therefore, the blending ratio of semi-coke is recommended to be below 50% for in-furnace blending method, and other operation adjustments need to be employed to further reduce NOx emission and improve combustion efficiency.

Fig. 5. Unburned carbon in fly ash and NOx emission of experiments and numerical prediction

The out-furnace blending cases were further studied via the verified numerical models. Although similar effect of blending ratio on co-combustion can be obtained for the two blending methods, the differences cannot be ignored. Figure 6 illustrates the temperature distributions and components concentration of out-furnace blending cases. Unlike in-furnace blending cases, temperature of outfurnace blending cases decreased remarkably once semi-coke was co-fired, while the temperature reduction was not obvious as blending ratio of semi-coke was further increased. The oxygen continually decreased to a low level in the primary zone with the fluctuant raises because of air feeding, and then increased rapidly with SOFA injection before continuous decline. The low level of O2 concentration before SOFA was beneficial to the reduction of NOx. More semi-coke co-fired in furnace led to higher O2 concentration in burnout zone, indicating the poorer burnout condition.


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Energy & Fuels

Fig. 6. Temperature and species concentrations along the height for out-furnace blending cases

It should be noted for out-furnace blending cases that NO mass flow still increased with height in the so-called reduction zone for NOx (the zone between CCOFA and SOFA), which was different from conventional coal-fired condition. Figure 7 illustrates that the mass flow of particles gradually increased to the peak in top burner zone because of fuel feeding and ash accumulation, but declined significantly in the reduction zone, which indicated that the fuels burned fiercely. The delay of char combustion from the burner zone to the reduction zone occurred due to the poor ignition and burnout characteristics of semi-coke. Therefore, the majority of fuel-N in semi-coke was released with NOx generation in the reduction zone. The generation of NO could exceed the reduction, causing the increasing mass flow of NO in this zone. Table 7 summarizes some indicators at furnace outlet for out-furnace blending cases. The temperature at furnace exit decreased slightly with the blending ratio of semi-coke, and the maximum relative deviation was approximately 4%, which had little influence on the downstream convective heat exchangers. However, the oxygen content tended to rise with the blending ratio of semi-coke. Unburned carbon in fly ash increased sharply from 2.65% to a high level (more than 4.94%) when


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semi-coke was fed into furnace. Moreover, NOx concentration increased evidently from 174.07 mg·m-3 (case O1) to 235.81 mg·m-3 (case O5). Considering combustion efficiency and NOx emission, co-firing 33% of semi-coke is acceptable and favorable for out-furnace blending method.

Fig. 7. Particles mass flow along the height for out-furnace blending Table 7. Some indicators at furnace outlet for out-furnace blending cases O2 concentration（

Carbon content in

%）

fly ash (%)

1220

3.19

2.65

99.47

174.07

Case O2

1190

3.23

5.13

98.68

177.41

Case O3

1192

3.30

4.94

98.65

182.58

Case O4

1183

3.35

5.45

98.44

211.30

Case O5

1171

3.54

5.94

98.3

235.81

Case O6

1170

3.65

6.21

98.17

247.11

Case

T (K)

Case O1

Burnout ratio (%)

NOx concentration (mg·m-3, 6% O2)

4.2 Effect of burner allocation for semi-coke The effect of burner allocation of semi-coke on boiler performance was investigated to explore a better feedstock strategy for in-furnace blending method. Cases I7-I9 were firstly designed, and the corresponding burners allocation of semi-coke were A B C, B C D, and D E F. Figure 8 illustrates the temperature and NO distribution of case 7, case 8, case 4 and case 9, and Fig. 11 depicts the corresponding contours. As shown in Fig. 10a and Fig. 11, when the height of semi-coke injection was


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Energy & Fuels

raised, the furnace temperature tended to decrease in the zone below the SOFA, especially in the top burners zone (E F) and the reduction zone. The reason could be the poorer burnout performance caused by the shorter residence time of semi-coke in furnace when semi-coke was injected from the higher layers.

Fig. 8. Temperature and NO massflow along the height for cases with various burner allocations (a) temperature (b) Mass flow of NO

As shown in Fig. 8b, NO massflow increased significantly in the zone where semi-coke was fed into furnace for all the four cases. In case I7, semi-coke was fed from burners in the bottom layers, and thus the most amount of NO in layer A were observed. Accordingly, the maximum mass flow of NO was found in the zones near layer C for case I8 and layer E for case I4. The combustion of semi-coke was inevitably postponed to the reduction zone when it was fed from burners in the three top layers (D E F). Hence, NO massflow of case I9 increased rapidly just in the reduction zone, and eventually exceeded that of the other cases. The sequence of NO mass flow after SOFA was: case I7 < case I8 < case I4 < case I9, which indicated that the lower height of semi-coke injection led to less generation of NO. More details about NO concentration can be found in Fig. 9. In the burner zone, a large quantity of NO was produced but gradually diluted or reduced along the height. Case I7 showed the highest


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NO concentration in the burner zone, but the least at furnace exit. Feeding semi-coke into the top burner zone would lead to the more generation of NOx due to the higher temperature, while fuel-N in semi-coke was more likely to be reduced to nitrogen if it was fed into the bottom burner zone with low temperature. In addition, the lower height of semi-coke burners would make NOx be generated earlier, prolonged its residence time in furnace, and led to more opportunities for NOx to be reduced by unburned carbon and the pulverized fuel fed from the higher burners, which was similar to fuelstaging.36 Hence, the combustion of semi-coke at low temperature and prolongation of NOx residence time in furnace could reduce NOx production, which together determined NOx emission for co-firing semi-coke with bituminous coal.

Fig. 9. Temperature and NO distribution of cases with different burner allocations


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Energy & Fuels

Figure 10 displays the particle tracks colored by particle char mass fraction of fuels with various burner allocations, which indicate the burnout condition of the two fuels. It can be found from the particles in furnace exit that the CFA of semi-coke decreased evidently with the increasing height of the semi-coke inlet. While the numbers of particles trapped by cold ash hopper sharply declined with the increasing height of the semi-coke burners, reflecting the decreasing bottom slag. The particle tracks of bituminous coal showed the similar trends. Given the significant difference of fix carbon content of the two fuels, more bottom slag was trapped when semi-coke was fed from the bottom burners, despite the better burnout condition for bituminous coal. The burnout conditions of the four cases can be also obtained from Table 8, in which carbon content in both bottom slag and fly ash are displayed. Unburned Carbon in bottom slag decreased with the increasing injection height of semicoke, while carbon content in fly ash presented a fluctuant growth. Although the least NOx emission and the lowest carbon content in fly ash could be achieved in case I7, the highest carbon content in bottom slag was also obtained, which led to the lowest burnout ratio. When the injection of semi-coke was raised from the bottom to the top burners, the residence time of semi-coke in furnace dropped by almost half from 10.6 s (case I7) to 5.5 s (case I9). The burnout ratio actually increased for great reduction of unburned carbon in bottom slag, although the unburned carbon in fly ash accordingly increased. The rise of injection position of semi-coke would certainly prolong the residence time of bituminous coal, but the effect on the hard-to-burn fuel is greater.20, 24, 36 The burnout ratio of blended fuels can be determined residence time and reaction temperature of hard-to-burn fuel, and carbon content in bottom slag in this study. Although the residence time of semi-coke would be prolonged by the low injection height of semi-coke, the low furnace temperature would weaken the combustion of


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semi-coke, form big char particles, make it more possibly drop to the bottom hopper, and thus increase unburned carbon in bottom slag, which led to the worse burnout condition. However, the ignition and combustion rate of semi-coke would be enhanced by feeding semi-coke from the top layers because of the high furnace temperature. Meanwhile, the burnout ratio of semi-coke would be reduced due to the shorter residence time. Therefore, the burnout ratio was also determined by reaction temperature and residence time of semi-coke for the co-combustion.

Fig. 10. Particle tracks colored by particle char mass fraction of fuels with various burner allocations

Zhang et al.20 found that the combustion of the lower-reactivity brown coal could be enhanced by feeding bituminous coal from adjacent layers in utility boiler. Here, a new strategy was designed as feeding the two fuels from discrete layers to further optimize the burner allocation for co-firing semicoke and coal. Since totally 6 layers of burners were equipped in the boiler, the burner allocations of


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Energy & Fuels

case I10 and case I13 were completely discontinuous for the two fuels with the bottom and top burners respectively included, while those of case I11 and case I12 were not entirely discrete. Similar to case I7, case I10 showed extremely high carbon content in bottom slag and low burnout ratio, which was also due to the semi-coke injection from burners in layer A. Moreover, the supply of semi-coke from the bottom burners would cause difficulty in the ignition of semi-coke especially under the condition of low-load operation. Hence, the bottom burners were unsuitable to feed semi-coke. Furthermore, the results of case I9 and case I13 indicated that feeding semi-coke from the top layers would lead to the most NOx generation, which should be also avoided. In conclusion, it is recommended for semi-coke to be fed from burners in the middle layers rather than the bottom or top layers. Table 8. The indictors in furnace exit for various fuel allocations Burner Case

allocation for semi-coke

T

Carbon content in

Carbon content

Burnout

NOx concentration

(K)

bottom slag (%)

in fly ash (%)

ratio (%)

(mg·m-3, 6% O2)

Case I7

ABC

1187

17.58

3.86

98.41

192.83

Case I8

BCD

1183

13.62

4.47

98.59

203.76

Case I4

CDE

1189

12.43

4.14

98.75

222.82

Case I9

DEF

1179

12.21

4.33

98.66

238.80

Case I10

ACE

1167

16.04

3.92

98.59

213.18

Case I11

BCE

1188

12.35

4.08

98.77

207.84

Case I12

BDE

1186

12.46

4.27

98.69

216.23

Case I13

BDF

1188

12.53

4.67

98.63

225.39

Eventually, the semi-coke was fed from layers B C E (case I11) and layers B D E (case I12), in which way the burners in the bottom and top layers were not arranged for feeding semi-coke. As shown in Table 10, the burnout ratio of case I11 was the highest among the eight cases, and the NOx emission was lower than most of the cases. Moreover, the carbon contents in both fly ash and bottom slag were within the acceptable range. Therefore, the burner allocation of case I11 can be regarded as the optimal strategy under the condition of 50% blending ratio of semi-coke. In summary, semi-coke and


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bituminous coal were recommended to be fed from burners in discrete layers, in particle without semicoke fed from the top and bottom burners. In order to further investigate the effect of burner allocation on NOx emission and combustion efficiency, totally 12 cases with blending ratios of 50% and 33% were systematically analyzed through introducing a new parameter. The parameter, namely average height of semi-coke burners (abbreviated to AHS, hereafter), was calculated by averaging the height of semi-coke injection, even though the semi-coke burners were in the discrete layers. The correlational analysis between AHS and two main indictors at furnace outlet was shown in Fig. 11. Under condition of 50% blending ratio, NOx concentration at furnace outlet could be approximatively judged to positively correlate with AHS, while the burnout ratio first increased and then decreased with AHS, which indicated that it was better to select burner allocation strategy before the inflection point to improve the boiler performance. The relationships could be attributed to the two factors, residence time and reaction temperature. Some cases with blending ratio of 33% were taken into consideration to verify this rule, and the similar phenomenon was obtained.

Fig. 11. The correlational analysis of NOx emission and burnout ratio with the average height of semi-coke burners, (a) 50% blending ratio of semi-coke (b) 33% blending ratio of semi-coke


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Energy & Fuels

4.3 Effect of blending method Figure 12 compared the temperature curves of in-furnace blending cases and out-furnace blending cases under various blending ratios. Although similar variations and trends can be found for the cases using two blending methods, significant deviation of temperature values can be distinguished. Except 50% blending ratio, temperatures of out-furnace blending cases were higher than those of in-furnace blending cases, which could be explained as follows. On one hand, feeding semi-coke from continuous burners in the middle layers probably improved the furnace temperature due to the high heat value of semi-coke and high combustion temperature. On the other hand, the competition of bituminous coal and semi-coke for oxygen would delay the ignition and burnout of semi-coke, and the influence could be greater for out-furnace blending. Furthermore, more unburned carbon in bottom slag was found for out-furnace blending cases, which was due to the arrangement of the bottom burners for feeding semicoke. The differences between the two blending methods tended to be narrowed with blending ratio of semi-coke, which could be attributed to the increasingly similar feeding strategy of semi-coke. As for 50% blending ratio of semi-coke, temperatures of three in-furnace blending cases were depicted in Fig. 12. The highest and lowest furnace temperature levels were respectively found in case I7 and case I9 under 50% blending ratio, and case I11 with the optimal burner allocation had the middle temperature level between case I7 and case I9. It can be observed from Fig. 12 that the temperature of case O4 was similar to that of case I11, which was higher than that of case I9, but lower than that of case I7 in the whole furnace zone. Hence, temperature distributions of out-furnace blending cases were between the highest and lowest temperature levels of in-furnace blending cases, but lower than those of cases with continuous burner allocation for semi-coke.


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Fig. 12. The comparison of temperature between the two blending methods

The NO distributions of cases using different blending methods are depicted in Fig. 13. The results showed that blending methods had significant influence on generation and reduction of NO. Because NO was more likely to be generated near the semi-coke burners, more NO was generated in the bottom burner zone for out-furnace blending cases than in-furnace blending cases, but then NO concentration gradually declined along the height. By contrast, when in-furnace blending method was employed, the zone of high NO concentration not only occupied the bottom of burner zone, but also extended to the middle burner zone, where semi-coke was fed into furnace. Hence, the more uniform zone of high NO concentration was formed in the center of burner zone. Similarly, NO was partially reduced as the flue gas entered the reduction zone beyond CCOFA. The burnout condition can be obtained in Fig. 14, in which the particle tracks colored by char mass fraction of the two blending method are compared. As shown in Fig. 14, more fuel particles were trapped by the cold ash hopper for out-furnace blending


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Energy & Fuels

method, and the number trend to increase with the blending ratio. When the in-furnace blending method was employed, the fuel particles were fed into the middle of the primary zone, and the char mass fraction of particles first increased and then sharply decreased in the primary and burnout zone.

Fig. 13. The comparison of NO distribution between two blending methods

Ultimately, the analyses of NOx emission and burnout ratio were conducted by totally 20 cases to optimize the blending methods for co-firing semi-coke with bituminous coal. Figure 15 demonstrates NOx emission and burnout ratio of cases with various blending ratios to compare the two blending methods, in which a globule represents one case. In-furnace blending cases with various burner allocations were also depicted in Fig. 15 for semi-coke blending ratio of 33%, 50% and 67%. It is evident for both of the blending methods that NOx emission increased with blending ratio of semi-


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coke, and burnout ratio decreased with blending ratio of semi-coke. However, the reductions were definitely different. Burnout ratio of out-furnace blending cases showed more reduction with blending ratio of semi-coke, and thus was lower than that of in-furnace blending cases. The differences in residence time, temperature, and carbon content in bottom slag should be responsible for this. When the blending ratio was increased up to 83%, the burnout ratios of the two blending methods tended to be roughly the same. Under condition of 50% blending ratio, NOx emission of out-furnace blending case was higher than that of the optimal in-furnace blending case, which could be clearly observed by the circles marked in Fig. 15. The same conclusion was drawn for 67% blending ratio of semi-coke. However, the limited data showed that out-furnace blending method led to the lower NOx emission when blending ratio of semi-coke is below 50%. On the whole, the in-furnace blending method with optimal burner allocation have advantage over out-furnace blending method under condition of 50% and 67% blending ratio of semi-coke. Although many other factors need to be taken into consideration in practical operation, such as the limit of the mills, the work can still provide beneficial guidance.

Fig. 14. The comparison of particle tracks between two blending methods


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Energy & Fuels

Fig. 15. The statistics analysis of NOx emission and burnout ratio for different blending methods

4.4 Effect of distribution mode of secondary air Suitable secondary air distribution mode can effectively improve the boiler performance for cofiring semi-coke with bituminous coal. In this section, five types of secondary air distribution were compared to seek for the optimal strategy for co-combustion of semi-coke and bituminous coal. Among those types, pagoda type and pagoda down type were investigated by both experimental and numerical studies. The detailed mass flow rates of different secondary airs have been summarized in Table 5, and the graphical representation is displayed in Fig. 16 for better understanding of the distribution modes of secondary air. The mass flow rate of the bottom secondary air (layer AA) remains roughly constant for various cases to hold the fuel particles. Figure 17 shows the temperature and oxygen distribution of cases with different secondary air distribution modes of pagoda type, pagoda down type, equal type, waist drum type and shrunk-middle type. Temperature of case I19 was the highest in the burner zone, and a huge ‘fireball’ was observed


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at the center of furnace, which would lead to more heat exchange of the water wall than other cases. In case I19, bituminous coal ignited and burned in the bottom burner zone with low oxygen concentration, and then a large quantity of air was fed into furnace near the semi-coke burners, which was helpful for the combustion of semi-coke. However, most oxygen was fed to the bottom zone in case I18, and consumed by bituminous coal, resulting in the ignition and burnout of semi-coke under low oxygen condition. Furthermore, the reduction of secondary air near semi-coke burners was unfavorable for the mixing of semi-coke and oxygen, and then worsen the burnout. When equal type was employed for secondary air in case I20, the highest temperature in furnace decreased, and the zone of high temperature tended to be closed to the walls, which would cause the slagging in water wall. More air was fed from the secondary air nozzles in middle layers in case I21, resulting in the corresponding temperature and oxygen distribution, which is contrast to case I22.

Fig. 16. The graphical representation for various distribution modes of secondary air


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Energy & Fuels

Fig. 17. Temperature and oxygen of cases with different distribution modes of secondary air

More details can be found in Fig. 18, in which the curves of temperature, oxygen and NO mass flow with different secondary air distribution modes were depicted. As shown in Fig. 18, the temperature of case I19 was much higher than those of other cases before the injection of separated over fire air, although a decrease occurred in the burnout zone. Because the supply of oxygen was delayed for pagoda down type, more strong reductive components were produced with combustion, such as HCN and NHi.12, 37 Hence, the high temperature and strong reducing atmosphere was created. According to our previous studies, the atmosphere with high temperature and strong reduction can reduce the NOx emission. 28, 37 Therefore, NO mass flow of case I19 was much lower than those of other cases within the whole furnace. The initial oxygen concentration of case I18 was abundant, and thus a large quantity of NOx was generated in the burner zone. However, a more reductive zone was created by the less


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oxygen supply in the top burner zone, and thus a decrease of NO mass flow occurred, which was similar to case I21. Under the condition of equal distribution of secondary air, NO was generated continuously and fast with fuel and air continuously fed, even in the reduction zone, resulting in the highest NO emission. In addition, a descending peak of NO mass flow can be found in the middle burner zone due to the less air supply in the middle layers. The detailed formation and decomposition rate of fuel-NO and thermal-NO can be found in Fig. 19, in which the molar reaction rates of NO for cases I18-22 are displayed. The reaction rate of thermal-NO was much lower than that of fuel-NO. Hence, although the thermal-NO reaction rate of pagoda down type show is higher than that of other modes, the fuel-NO reaction rate was the lowest. The generated NO was even reduced in the primary zone for pagoda down type to the high temperature and reducing atmosphere.

Fig. 18. Temperature and species concentrations along the height with different secondary air distribution modes

Table 9 summarizes some indictors at furnace exit for cases with various distribution modes of secondary air. Also, the deviations between experimental and numerical results was accepted, and the variations were consistent. According to the numerical results, case I20 exhibited the highest oxygen,


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Energy & Fuels

which indicated the bad burnout characteristic. Although waist drum type and shrunk-middle type could improve the burnout ratio, the NOx emission was also increased. The burnout ratio of case I19 was high up to 99.43%, which could be attributed to the increase of air supply near the semi-coke burners and the high furnace temperature. Fortunately, NOx emission in case I19 was also the lowest. Also, Exp T6 showed the better performance in NOx emission and combustion efficiency than Exp T4 and Exp T5. Hence, the pagoda down type for secondary air distribution is recommended for co-firing semi-coke with bituminous coal under 50 % blending ratio.

Fig. 19. Molar reaction rate of NO for cases with different distribution modes of secondary air (a) fuel-NO (b) thermal-NO Table 9. The indictors in furnace exit for cases with various distribution modes of secondary air Carbon content

Burnout ratio

NOx concentration

in fly ash (%)

(%)

(mg·m-3, 6% O2)

3.22

3.87

/

229

Pagoda type

3.08

3.55

/

241

Exp T6

Pagoda down type

2.98

1.98

/

210

Case I4

Regular

3.23

4.14

98.75

222.82

Case I18

Pagoda type

3.43

3.77

98.86

241.26

Case I19

Pagoda down type

3.25

1.42

99.43

217.17

Case I20

Equal type

4.66

5.46

98.27

316.17

Case I21

Waist drum type

3.34

2.60

99.11

254.85

Case I22

Shrunk-middle type

3.66

2.34

99.04

282.65

Case

Type

Oxygen (%)

Exp T4

Regular

Exp T5


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5. Conclusion Both full-scale experiments and numerical studies were conducted on a 300 MW tangentially fired utility boiler to explore the optimal strategy for co-firing semi-coke with bituminous coal. The effects of blending method and air distribution on NOx emission and combustion efficiency were mainly investigated. The results showed that it is feasible and adaptable for pulverized coal boilers to utilize semi-coke for co-combustion. The conclusions are summarized below: (1) Co-firing semi-coke in coal-fired boiler would reduce the furnace temperature. NOx emission and heat loss increased with the blending ratio of semi-coke. The blending ratio of semi-coke was recommended to be below 50% for in-furnace blending method and 33% for out-furnace blending method to limit excessive NOx emission and unburned carbon. (2) Considering NOx emission and combustion efficiency, the optimal strategy of burner allocation under 50% blending ratio was to feed semi-coke from B, C, and E layers for in-furnace blending method. Semi-coke and bituminous coal was recommended to be fed from burners in discrete layers, with avoiding semi-coke to be fed from the top and bottom burners. The NOx emission and burnout ratio was determined together by residence time of semi-coke, temperature, and carbon content in slag. Moreover, NOx concentration at furnace outlet was positive correlation with the average height of semi-coke burners, while the burning ratio first increased and then decreased with the increasing average height of semi-coke burners. (3) The in-furnace blending method with optimal burner allocation was proved to have advantages over out-furnace blending for co-firing semi-coke with bituminous coal. Furnace temperature distributions of out-furnace blending cases were between the highest and lowest temperature levels of


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Energy & Fuels

in-furnace blending cases, but lower than those of cases with continuous burner allocation for semicoke. (4) Among the five secondary air distribution modes, pagoda down type was recommended under 50% blending ratio due to the low NOx emission and high burnout ratio. The reason can be attributed to the formation of a high temperature and strong reducing atmosphere within furnace.

Supporting Information Two simulation method have been developed for out-furnace blending method. One is treat treated semi-coke and coal as separate fuels, and the other regards the blends of multiple fuels as a single fuel, whose properties are obtained by mass average. Comparison of gas temperature and NO massflow for different simulation methods are shown in Fig. S1, and the indicators at furnace outlet for two simulation methods are summarized in Table S1. Moreover, experimental (EXP T1-T4) and predicted (Case I1-I4) furnace temperature and parameters at furnace exits are compared in Table S2 and Table S3, respectively. Table S4 displayed the experimental results of BET surface areas for chars of semicoke and bituminous coal. Figure S2 to S4 are the results of particle trajectory to further understand the effect of blending ratio and secondary air distribution mode on burnout condition. Figure S5 to S9 are results of molar reaction rate of NO for better understanding of the effect of blending ratio, burner allocations and blending method on NOx formation and decomposition.

Acknowledgements The authors acknowledge financial support from the National Key R&D Program of China (2017YFB0602003).


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References 1. Yao, Y.; Zhu, J.; Lu, Q. Experimental study on nitrogen transformation in combustion of pulverized semi-coke preheated in a circulating fluidized bed. Energy Fuel 2015, 29, 3985-3991. 2. Xie, K.; Li, W.; Zhao, W., Coal chemical industry and its sustainable development in China☆. Energy 2010, 35 (11), 4349-4355. 3. Pei, P.; Wang, Q.; Wu, D., Application and research on Regenerative High Temperature Air Combustion technology on low-rank coal pyrolysis. Appl. Energy 2015, 66, 205-208. 4. Wang, C. A.; Wu, S.; Lv, Q.; Liu, X.; Chen, W.; Che, D., Study on correlations of coal chemical properties based on database of real-time data. Appl. Energy 2017, 204, 1115-1123. 5. Liu, G. Y.; Zhang, Y. T.; Cai, J. T.; Zhou, A. N.; Dang, Y. Q.; Qiu, J. S., A strategy for regulating the performance of DCFC with semi-coke fuel. Int. J. Hydrogen Energy 2018, 43 (15), 7465-7472. 6. You, Q.; Wu, S. Y.; Wu, Y. Q.; Huang, S.; Gao, J. S.; Shang, J. X.; Min, X. J.; Zheng, H. A., Product distributions and characterizations for integrated mild-liquefaction and carbonization of low rank coals. Fuel Process. Technol. 2017, 156, 54-61. 7. Gao, X.; Dai, Y.; Zhang, Y.; Fu, F., Effective adsorption of phenolic compound from aqueous solutions on activated semi coke. J. Phys. Chem. Solids. 2017, 102 (12), 142-150. 8. Li, Y.; Chen, J.; Sun, Y., Adsorption of multicomponent volatile organic compounds on semi-coke. Carbon 2008, 46 (6), 858-863. 9. Lyu, Q.; S. Z.; Zhu J., Wu, H.; Fan, Y. Experimental study on NO emissions from pulverized char under MILD combustion in an O2/CO2 atmosphere preheated by a circulating fluidized bed. Fuel Process. Technol. 2018, 176, 43-49. 10. Yang, Y.; Wang, Q.; Lu, X.; Li, J.; Liu, Z., Combustion behaviors and pollutant emission characteristics of low calorific oil shale and its semi-coke in a lab-scale fluidized bed combustor. Appl. Energy 2018, 211, 631-638. 11. Zhu, S.; Lyu, Q.; Zhu, J.; Wu, H.; Wu G. Effect of air distribution on NOx emissions of pulverized coal and char combustion preheated by a circulating fluidized bed. Energy Fuel 2018, 32, 7909-7915. 12. Glarborg, P.; Miller, J. A.; Ruscic, B.; Klippenstein, S. J., Modeling nitrogen chemistry in combustion. Prog. Energy Combust. Sci. 2018, 67, 31-68. 13. Lee, B.; Kim, S.; Song, J.; Chang, Y.; Jeon, C., Influence of coal blending methods on unburned carbon and NO emissions in a drop-tube furnace. Energy Fuel 2011, 25 (11), 5055-5062. 14. Yörük, C. R.; Meriste, T.; Sener, S.; Kuusik, R.; Trikkel, A., Thermogravimetric analysis and process simulation of oxy‐fuel combustion of blended fuels including oil shale, semicoke, and biomass. Int. J. Energy Res. 2018, 42, 2213-2224. 15. Liu, H. P.; Liang, W. X.; Qin, H.; Wang, Q., Synergy in co-combustion of oil shale semi-coke with torrefied cornstalk. Appl. Therm. Eng. 2016, 109, 653-662. 16. Lei, K.; Zhang, R.; Ye, B. Q.; Cao, J.; Liu, D., Study of sewage sludge/coal co-combustion by thermogravimetric analysis and single particle co-combustion Method. Energy Fuel 2018, 32 (5), 6300-6308. 17. Jurado, N.; Simms, N. J.; Anthony, E. J.; Oakey, J. E., Effect of co-firing coal and biomass blends on the gaseous environments and ash deposition during pilot-scale oxy-combustion trials. Fuel 2017, 197, 145-158.


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18. Fuller, A.; Maier, J.; Karampinis, E.; Kalivodova, J.; Grammelis, P.; Kakaras, E.; Scheffknecht, G.; Sciubba, E., Fly ash formation and characteristics from (co-)combustion of an herbaceous biomass and a greek lignite (low-rank coal) in a pulverized fuel pilot-scale test facility. Energies 2018, 11, 1581. 19. Tan, P.; Ma, L.; Xia, J.; Fang, Q. Y.; Zhang, C.; Chen, G., Co-firing sludge in a pulverized coalfired utility boiler: Combustion characteristics and economic impacts. Energy 2017, 119, 392-399. 20. Zhang, J.; Wang, Q.; Wei, Y.; Lian, Z., Numerical modeling and experimental Investigation on the use of brown coal and its beneficiated semicoke for coal blending combustion in a 600 MWe utility furnace. Energy Fuel 2015, 29, 1196-1209. 21. Baek, S. H.; Park, H. Y.; Ko, S. H., The effect of the coal blending method in a coal fired boiler on carbon in ash and NOx emission. Fuel 2014, 128 (14), 62-70. 22. Drosatosa, P.; Nikolopoulos, N.; Karampinis, E.; Grammelis, P.; Kakaras, E. Comparative investigation of a co-firing scheme in a lignite-fired boiler at very low thermal-load operation using either pre-dried lignite or biomass as supporting fuel. Fuel Process. Technol. 2018, 180, 140-154. 23. Ma, W.; Zhou, H.; Zhang, J.; Zhang, K.; Liu, D.; Zhou, C.; Cen, K., Behavior of slagging deposits during coal and biomass co-combustion in a 300 kW down-fired furnace. Energy Fuel 2018, 32 (4), 4399-4409. 24. Li, D.; Lv, Q.; Feng, Y.; Wang, C. A.; Liu, X.; Du, Y.; Zhong, J.; Che, D., Numerical study of co-firing biomass with lean coal under air–fuel and oxy-fuel conditions in a wall-fired utility boiler. Energy Fuel 2017, 31 (5), 5344-5351. 25. Zha, Q.; Li, D.; Wang, C. A.; Che, D., Numerical evaluation of heat transfer and NOx emissions under deep-air-staging conditions within a 600 MW e tangentially fired pulverized-coal boiler. Appl. Therm. Eng. 2017, 116, 170-181. 26. Liu, X.; Wang, C. A.; Lv, Q.; Zhu, T.; Li, D.; Du, Y.; Che, D., Effects of O2 feeding strategy and over-fire air configuration on oxy-fuel combustion characteristics in an opposed wall-fired utility boiler. Energy Fuel 2018, 32, 2479-2489. 27. Wang, J.; Fan, W.; Li, Y.; Xiao, M.; Wang, K.; Ren, P., The effect of air staged combustion on NOx emissions in dried lignite combustion. Energy 2012, 37 (1), 725-736. 28. Du, Y.; Wang, C. A.; Lv, Q.; Li, D.; Liu, H.; Che, D., CFD investigation on combustion and NOx emission characteristics in a 600 MW wall-fired boiler under high temperature and strong reducing atmosphere. Appl. Therm. Eng. 2017, 126, 407-418. 29. Launder; B., E.; Spalding; D., B., The numerical computation of turbulent flows. Comput. method appl. M. 2014, 3 (2), 269-289. 30. Peng, T.; Lun, M.; Fang, Q.; Cheng, Z.; Gang, C., Application of different combustion models for simulating the co-combustion of sludge with coal in a 100 MW tangentially coal-fired utility boiler. Energy Fuel 2016, 30, 1685-1692. 31. Ge, X.; Dong, J.; Fan, Z.; Zhang Z.;Shang X.; Hu, X.; Zhang, J. Numerical investigation of oxyfuel combustion in 700 oC-ultrasupercritical boiler. Fuel 2017, 207, 602-614. 32. Chen, S.; He, B.; He, D.; Cao, Y.; Ding G.; Liu X.; Duan, Z.; Zhang, X.; Song J.; Li, X. Numerical investigations on different tangential arrangements of burners for a 600 MW utility boiler. Energy 2017, 122, 287-300. 33. Drosatos, P.; Nikolopoulos, N.; Nikolopoulos, A.; Papapavlou, Ch.; Grammelis, P.; Kakaras E.


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Numerical examination of an operationally flexible lignite-fired boiler including its convective section using as supporting fuel pre-dried lignite. Fuel Process. Technol. 2017, 166, 237-257. 34. Hanson, R. K.; Salimian, S., Survey of Rate Constants in the N/H/O System. Springer US: 1984; p 361-421. 35. Soete, G. G. D., Overall reaction rates of NO and N2 formation from fuel nitrogen. Symp. (Int.) Combust. 1975, 15 (1), 1093-1102. 36. Du, Y.; Wang, C. a.; Wang, P.; Meng, Y.; Wang, Z.; Yao, W.; Che, D., Computational fluid dynamics investigation on the effect of co-firing semi-coke and bituminous coal in a 300 MW tangentially fired boiler. Proc IMechE Part A: J Power and Energy 2018, doi.org/10.1177/0957650918783923. 37. Bai, W.; Li, H.; Deng, L.; Liu, H.; Che, D., Air-Staged Combustion Characteristics of Pulverized Coal under High Temperature and Strong Reducing Atmosphere Conditions. Energy Fuel 2014, 28 (3), 1820–1828.


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Table Caption Table 1. The properties of fuels Table 2. Full-scale experiment operation conditions Table 3. Summary of simulation models. Table 4. Detailed distributions fuel and air for simulation cases Table 5. Mass flow rate of secondary air for cases with different secondary air distribution modes Table 6 The comparison of experimental and predicted furnace temperatures Table 7. Some indicators at furnace outlet for out-furnace blending cases Table 8. The indictors in furnace exit for various fuel allocations Table 9. The indictors in furnace exit for cases with various distribution modes of secondary air


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Table 1. The properties of fuels Proximate analysis (wt.%)

Sample

Ultimate analysis (wt.%)

Qnet,ar

Mar

Aar

Var

FCar

Car

Har

Oar

Nar

Sar

(MJ/kg)

HA

14.20

7.41

28.80

55.81

64.00

3.23

10.21

0.68

0.23

23.92

Semi-coke

8.10

12.80

8.22

70.88

74.42

1.40

1.97

0.91

0.40

25.35

Raw coal

22.30

6.62

34.97

36.11

54.75

3.67

11.47

0.81

0.38

20.63

O, by difference; wt. %, mass faction; Qnet,ar, net calorific value; ar, as received basis

Table 2. Full-scale experiment operation conditions Test

Exp T1

Exp T2

Exp T3

Exp T4

Exp T5

Exp T6

0

17%

33%

50%

50%

50%

3%

3%

3%

3%

3%

3%

Regular

Regular

Regular

Regular

Pagoda

Pagoda

type

down type

/

D

DE

CDE

CDE

CDE

37.97

31.27

25.55

20.02

20.50

19.57

/

6.31

12.60

20.21

20.28

19.59

65.83

63.33

61.39

62.78

63.17

60.17

209.45

219.73

233.31

241.11

239.05

231.77

Main steam temperature (K)

812.5

811.5

815.0

812.0

808.0

810.0

Main steam pressure (Mpa)

15.84

15.80

16.06

16.53

16.38

16.78

227.22

227.78

237.15

242.16

245.91

240.66

403.9

401.17

398.2

397.4

399.4

401.2

Semi-coke blending ratio Operating oxygen Secondary air distribution mode Burners for semi-coke Feeding rate of coal (kg·s-1) Feeding rate of semi-coke (kg·s-1) Feeding rate of primary air

(kg·s-1)

Feeding rate of secondary air

Main steam flow

(kg·s-1)

Exhaust gas temperature (K)

(kg·s-1)

Table 3. Summary of simulation models. Item Model Turbulence

Item Model Realized k-ε model Standard wall functions29

Gas phase chemical reaction

Finite-Rate/Eddy-Dissipation model30

Particle-tracking model

Stochastic particle trajectory model31

Radiation

P-1 model28

Devolatilization

Two-competing-reactions model32

Char combustion

Kinetics/diffusion-limited model33

NOx formation

Extended Zeldovich mechanism34 De Soete mechanism35


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Energy & Fuels

Table 4. Detailed distributions fuel and air for simulation cases Case

Burners for

Blending ratio (%)

Blending method

Case O1(I1)

0

Out-furnace

/

Regular

Case O2

17

Out-furnace

A-D

Regular

Case O3

33

Out-furnace

A-D

Regular

Case O4

50

Out-furnace

A-D

Regular

Case O5

67

Out-furnace

A-D

Regular

Case O6

83

Out-furnace

A-D

Regular

Case I2

17

In-furnace

D

Regular

Case I3

33

In-furnace

DE

Regular

Case I4

50

In-furnace

CDE

Regular

Case I5

67

In-furnace

BCDE

Regular

Case I6

83

In-furnace

BCDEF

Regular

Case I7

50

In-furnace

ABC

Regular

Case I8

50

In-furnace

BCD

Regular

Case I9

50

In-furnace

DEF

Regular

Case I10

50

In-furnace

ACE

Regular

Case I11

50

In-furnace

BCE

Regular

Case I12

50

In-furnace

BDE

Regular

Case I13

50

In-furnace

BDF

Regular

Case I14

33

In-furnace

BD

Regular

Case I15

33

In-furnace

BE

Regular

Case I16

33

In-furnace

CD

Regular

Case I17

67

In-furnace

BCEF

Regular

Case I18

50

In-furnace

CDE

Pagoda type

Case I19

50

In-furnace

CDE

Pagoda down type

Case I20

50

In-furnace

CDE

Equal type

Case I21

50

In-furnace

CDE

Waist drum type

Case I22

50

In-furnace

CDE

Shrunk-middle type

SC



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Table 5. Mass flow rate of air for cases with different secondary air distribution modes Mass flow rate in primary zone (kg·s-1) Cases

Type

secondary air

primary air

AA

AB

BB

CC

CD

DD

EE

EF

FF

Case 4

Regular

62.78

39.5

10.0

21.2

30.9

10.0

21.2

30.9

10.0

21.2

Case 18

Pagoda type

63.17

43.6

36.8

23.5

23.2

22.1

14.1

13.7

11.0

7.0

Case 19

Pagoda down type

60.17

41.7

7.8

10.0

14.5

15.7

20.0

29.1

31.3

25.0

Case 20

Equal type

62.78

39.5

23.1

23.1

23.1

23.1

23.1

23.1

23.1

23.1

Case 21

Waist drum type

62.78

42.0

14.2

13.6

26.3

35.3

22.6

19.8

14.2

6.8

Case 22

Shrunk-middle type

62.78

41.2

27.8

17.7

16.1

13.9

8.9

19.4

27.8

22.2

Table 6 The comparison of experimental and predicted furnace temperatures

Measurement points

Furnace temperature of Exp T3/Case I3 (K) #1

#2

#3

#4

Measured

Predicted

Measured

Predicted

Measured

Predicted

Measured

Predicted

I (10.49 m)

1431

1403

1408

1394

1437

1420

1486

1429

II (15.70 m)

1509

1619

1521

1514

1482

1593

1496

1553

III (17.80 m)

1679

1617

1710

1631

1663

1644

1683

1496

IV (27.78 m)

1506

1599

1481

1520

1503

1606

1518

1576

V (39.5 m)

1406

1392

/

/

/

/

1429

1408

Table 7. Some indicators at furnace outlet for out-furnace blending cases O2 concentration（

Carbon content in

%）

fly ash (%)

1220

3.19

2.65

99.47

174.07

Case O2

1190

3.23

5.13

98.68

177.41

Case O3

1192

3.30

4.94

98.65

182.58

Case O4

1183

3.35

5.45

98.44

211.30

Case O5

1171

3.54

5.94

98.3

235.81

Case O6

1170

3.65

6.21

98.17

247.11

Case

T (K)

Case O1

Burnout ratio (%)


NOx concentration (mg·m-3, 6% O2)

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Energy & Fuels

Table 8. The indictors in furnace exit for various fuel allocations Burner Case

allocation for semi-coke

T

Carbon content in

Carbon content

Burnout

NOx concentration

(K)

bottom slag (%)

in fly ash (%)

ratio (%)

(mg·m-3, 6% O2)

Case I7

ABC

1187

17.58

3.86

98.41

192.83

Case I8

BCD

1183

13.62

4.47

98.59

203.76

Case I4

CDE

1189

12.43

4.14

98.75

222.82

Case I9

DEF

1179

12.21

4.33

98.66

238.80

Case I10

ACE

1167

16.04

3.92

98.59

213.18

Case I11

BCE

1188

12.35

4.08

98.77

207.84

Case I12

BDE

1186

12.46

4.27

98.69

216.23

Case I13

BDF

1188

12.53

4.67

98.63

225.39

Table 9. The indictors in furnace exit for cases with various distribution modes of secondary air Carbon content

Burnout ratio

NOx concentration

in fly ash (%)

(%)

(mg·m-3, 6% O2)

3.22

3.87

/

229

Pagoda type

3.08

3.55

/

241

Exp T6

Pagoda down type

2.98

1.98

/

210

Case I4

Regular

3.23

4.14

98.75

222.82

Case I18

Pagoda type

3.43

3.77

98.86

241.26

Case I19

Pagoda down type

3.25

1.42

99.43

217.17

Case I20

Equal type

4.66

5.46

98.27

316.17

Case I21

Waist drum type

3.34

2.60

99.11

254.85

Case I22

Shrunk-middle type

3.66

2.34

99.04

282.65

Case

Type

Oxygen (%)

Exp T4

Regular

Exp T5


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Figure 1. Schematic (a) and burner arrangement (b-c) of the coal-fired boiler


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Energy & Fuels

Figure. 2. Mesh system and grid independence tests (a) mesh of the furnace (b) Cross-section of the burner zone (c) 199x98mm (300 x 300 DPI)


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Figure 3. Experimental and Predicted distribution of temperature and NOx for in-furnace blending method (a) Temperature (b) NOx massflow


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Energy & Fuels

Figure 4. NO distribution of cases with various semi-coke blending ratio for in-furnace blending, (a) 0 (case I1), (b) 17% (case I2), (c) 33% (case I3), (d) 50% (case I4), (e) 67% (case I5)


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Figure 5. Unburned carbon in fly ash and NOx emission of experiments and numerical prediction 239x178mm (300 x 300 DPI)


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Energy & Fuels

Figure 6. Temperature and species concentrations along the height for out-furnace blending cases 250x187mm (300 x 300 DPI)


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Figure 7. Particles mass flow along the height for out-furnace blending 209x181mm (300 x 300 DPI)


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Energy & Fuels

Figure 8. Temperature and NO massflow along the height for cases with various burner allocations (a) temperature (b) Mass flow of NO 171x69mm (300 x 300 DPI)


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Figure 9. Temperature and NO distribution of cases with different burner allocations


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Energy & Fuels

Figure 10. Particle tracks colored by particle char mass fraction of fuels with various burner allocations 177x124mm (300 x 300 DPI)


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Figure 11. The correlational analysis of NOx emission and burnout ratio with the average height of semicoke burners, (a) 50% blending ratio of semi-coke (b) 33% blending ratio of semi-coke 163x61mm (300 x 300 DPI)


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Energy & Fuels

Figure. 12. The comparison of temperature between the two blending methods 202x289mm (300 x 300 DPI)


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Figure 13. The comparison of NO distribution between two blending methods 185x60mm (300 x 300 DPI)


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Energy & Fuels

Figure 14. The comparison of particle tracks between two blending methods 189x122mm (300 x 300 DPI)


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Figure 15. The statistics analysis of NOx emission and burnout ratio for different blending methods 99x76mm (300 x 300 DPI)


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Energy & Fuels

Figure 16. The graphical representation for various distribution modes of secondary air 272x208mm (300 x 300 DPI)


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Figure 17. Temperature and oxygen of cases with different distribution modes of secondary air 138x104mm (299 x 299 DPI)


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Energy & Fuels

Figure 18. Temperature and species concentrations along the height with different secondary air distribution modes 272x208mm (300 x 300 DPI)


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Figure 19. Molar reaction rate of NO for cases with different distribution modes of secondary air (a) fuel-NO (b) thermal-NO 182x66mm (300 x 300 DPI)


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Experiments and Simulation on Co-Combustion of Semicoke and Coal

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