Numerical Modeling and Experimental Investigation on the Use of

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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 Jian Zhang,† Qunying Wang,‡ Yajuan Wei,‡ and Lian Zhang*,† †

Department of Chemical Engineering, Monash University, GPO Box 36, Clayton, Victoria 3800, Australia China Huadian Electric Power Research Institute, Xiyuan First Rd, Hangzhou 310030, China

‡

ABSTRACT: In this paper, coal blending combustion performance in a 600 MWe supercritical boiler has been investigated through a series of on-site measurements and computational fluid dynamic modellings. A bituminous coal was used as reference, which was blended with brown coal or brown coal semicoke in an indirect way, i.e., different coals being fed separately into the boiler without prior mixing in the mill. The coal blending combustion behavior was assessed upon varying brown coal injection location, brown coal blending ratio, air staging ratio, and the use of brown coal pyrolysis-derived semicoke. ANSYS FLUENT 15.0 was employed for numerical simulations. Upon a successful validation, the model was further used to predict the influence of individual variables. The main conclusions achieved include (1) the bituminous coal-designed mill can be used safely for brown coal, at the expense of an increased mass ratio of primary air to coal, a decreased temperature for the outlet of the mill, and an incomplete drying of brown coal. (2) The allocation of brown coal in the middle burner zone is the best option that ensures a relatively long residence time of bituminous coal particles in the boiler, as well as provides sufficient heat to bituminous coal particles for a quicker ignition and burnout. (3) The 50 wt % blending ratio for brown coal is optimum, striking a good balance between the feeding amounts of combustible hydrocarbons and inherent moisture into the burner. (4) The separate over fire air of 30% ratio is beneficial in reducing total NOx emission through increasing the unburnt char quantity in the NOx reduction zone. (5) The high heating value of beneficiated Victorian brown coal and its semicokes ensures a lower feeding rate for each of them to be used as a substitute fuel. The high volatile-O2 and char-O2 oxidation reactivity of these samples also ensured a strong heat provision from their flame to the bituminous coal allocated in both the bottom and top of the burner zones. (6) The use of brown coal and semicoke is in favor of the reduction in NOx, particularly fuel-NOx, due to the generally low nitrogen content within these samples, relative to bituminous coal.

1. INTRODUCTION A large proportion of electricity produced in the world is based on a pulverized coal fired utility boiler. With an increasing trend of the consumption and price of high-rank coal, the usage of low-rank coal such as brown coal and sub-bituminous coal has been receiving increased attention in the international energy market. For the power generation sector, the use of brown coal as a substitute fuel to blend with bituminous coal is the easiest and simplest way to reduce the operating cost. However, compared with bituminous coal, brown coal contains relatively abundant water and thus a low heating value. Its use in a combustion utility leads to various problems such as lowering plant net efficiency, requiring additional predrying system, varied ignition, and temperature profiles as well as altered pollutant emissions.1,2 All of these issues could be intensified when the blending of bituminous coal and brown coal occurs in a boiler. There are two practical ways for coal blending in an industrial furnace. One way, namely, direct blending, is to mix deferent coals directly in coal bunker, and the mixed fuel is fed through mills and burners into the boiler.3 Another way, namely, indirect blending, is to feed different coals separately into the specific burner layers in the boiler without prior mixing. The coal particles are supposed to be mixed in the furnace, rather than in the mill. The latter method is adopted for the power generators that require frequent change on coal © XXXX American Chemical Society

feedstock. However, it raises issues such as the performance of brown coal in a bituminous coaldesigned mill and mixing extent of different coal particles in the boiler. Table 1 summarizes the experimental works that have been conducted for coal direct blending combustion in the literature. In refs 3−5 the reduction in the emission of air-borne pollutants upon coal blending combustion in industrial boilers was studied, whereas in refs 6−9 are studies on reactivity and ash slagging of coal blends in lab-scale facilities. Note that all these research studies refer to the direct blending of coals. As far as the authors are aware, no research has been conducted regarding the indirect blending in an industrial boiler. Computational fluid dynamics (CFD) modeling has been extensively used in the assessment and optimization of combustion performance in the pulverized coal (pc)-fired boilers.10−14 In this study, the CFD modeling of a 600 MWe bituminous-coal-fired boiler was first developed to assess the combustion performance of bituminous coal mixed with brown coal in an indirect blending mode. For the validation of the CFD model, on-site measurement was conducted to collect the necessary data during the combustion of coal blend with a mass ratio of 30% and 50% for the brown coal. Upon a full validation, Received: October 13, 2014 Revised: January 2, 2015

A

DOI: 10.1021/ef502287c Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Literature Survey on Coal Blending Combustion Experimentsa no.

purpose

blended fuels

1

NOx emission reduction SO2 emission reduction Hg capture

bituminous and subbituminous coals high and low sulfur coals

blending combustion characteristics blending combustion characteristics ash slagging

brown and bituminous coals

thermogravimetric analysis

brown and bituminous coals

isothermal plug flow reactor (905−1400 °C)

Varied blending influences were found on particle density, volatile matter, char burnout rate and ash level.

8

coal samples with different ash fusion temperatures

drop tube furnace (1300 °C)

Ignition, slagging and NOx emission were studied for blending coals. No linear relationship were found between coal blend ratio and the above three issues.

9

2 3 4 5 6 a

test facility

main conclusion

500 MWe boiler tow boiler units with 138 MWth and 170 MWth 1 MWth coal-fired facility

ref

The changes of flame pattern and NOx emission were clarified by modeling for blending 40% sub-bituminous coal. Evaluations were performed about on- line blending control options. The capture of mercury was enhanced by combining necessary unburned carbon in fly ash with blend calcium. Volatile release was influenced by brown coal, while char burnout step dominated by high-rank coal.

3 4 5 6, 7

Note: all are for a direct blending of different coals prior to combustion. close-coupled overfire air (CCOFA) and separate overfire air (SOFA) are used to execute secondary air staging for NOx reduction. The percentages of secondary air for CCOFA or SOFA were fixed by the boiler operators at approximately 10 vol % and 30 vol %, respectively. The remaining secondary air is evenly distributed in the other nozzles. In the upper furnace, the platen superheater (six groups) and rear super heater (20 groups) sections were considered in the CFD modeling, considering their influence on the concentric gas flow and radiative heat-transfer in the furnace. The superheater areas in the boiler mesh grid were simplified to the actual external area exposed to flame radiation in the boiler. The other convection heat exchangers were not taken into account in this work, including reheaters, airpreheaters, and economizer. These facilities are located in the rear pass of a boiler and have negligible influence on heat transfer in the combustion zone. The simulation was commenced from the design case which uses a bituminous coal (namely, Datong in Table 2) fired with only five mills used, followed by varying cases with a maximum of three coals mixed and all the six mills on service, as summarized in Tables 2 and 3. It is noteworthy that case 2, case 4, case 5, and case 6 were also experimentally examined on-site. The overall excess O2 level was controlled at approximately 1.2. CFD modeling aims to assess the influence of three key variables, coal allocation in burner zone (cases 1−3), brown coal blending ratio (cases 2, 6, 7), and SOFA ratio (cases 2 and 8). The beneficiated semicokes mentioned in Table 3 were yielded from the pyrolysis of dried Victorian brown coal in a lab-scale drop tube furnace in N2 or CO2 at 400, 600, 800, and 1000 °C.17,18 A short residence time of ∼2 s was applied for mild pyrolysis. The beneficiated brown coal was created via drying and briquetting. For the CFD modeling, the energy input remains the same for all the cases. Consequently, the coal feeding rate was varied for different cases. 2.2. Properties and Kinetics of Coals and Brown Coal Semicokes Tested. As tabulated in Tables 4 and 5, all coal samples used in this study were tested for the properties and first-order Arrhenius kinetic data, which were measured in a thermogravimetric analyzer (TGA, Shimadzu, 60H) with a nonisothermal heating program.19 Huadian brown coal possesses the largest oxygen content and largest moisture content of 36.7 wt %. Its heating value is thus the lowest, 15.65 MJ/kg. The properties of dried Victorian brown coal and its updated semicokes are tabulated in Table 5. The heating value of beneficiated semicoke was improved remarkably upon the increase of pyrolysis temperature, while the volatile matter quantity in the resulting semicoke correspondingly was reduced from 56.1 wt % to 8.8 wt %. The heating values of semicokes are higher than the briquette which was produced through mere predrying. The moisture content in the beneficiated semicokes has the comparable level with that in bituminous coal, whereas the ash contents are no larger than 3.4 wt %, relative to 18.13 wt % in the design bituminous coal.

the CFD code developed was further used for the conduct of sensitivity analysis on the combustion performance for flame pattern, flue gas temperature, heat transfer in the convective zone, and NOx emission. Another purpose of this paper is toward the blending combustion of beneficiated Victorian brown coal and its semicokes in furnace. In Victoria, Australia, there is a large amount of brown coal resources. Through an appropriate beneficiation, such as drying or mild pyrolysis, the Victorian brown coal can be upgraded to an export grade with a comparable and even better quality than the standard thermal and coking coal.15,16 To date, no study has been done to address the use of upgraded brown coal as an alternative thermal coal in a bituminous coal-designed boiler. In light of this, the CFD prediction was extended to the virtual use of beneficiated Victorian brown coal and its semicokes in the 600 MWth plant.

2. MODELING METHODOLOGY 2.1. Boiler Configuration and Operating Conditions. The geometry of the simulated boiler is illustrated in Figure 1. It is a

Figure 1. Furnace geometry and the specification of burners and secondary air nozzles. tangentially pulverized coal-fired boiler with an installed capacity of 600 MWe, yielding 1913 t/h steam at 25.4 MPa and 571 °C when being operated at the boiler maximum continuous rate (BMCR). The height of furnace is 71.85 m, and its rectangular cross-section has a width of 17.696 m and a depth of 18.816 m. Totally 24 burners are arranged in six layers, namely A to F, with four burners installed at the four corners on each layer. Each layer is connected with one independent mill to provide coal. The secondary air is supplied though a series of nozzles located above and below the burners. Furthermore, B


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Table 2. Various Cases Studied for Design Bituminous Coal and the Blending of Bituminous Coal with Huadian Brown Coala

a

case

design case

case 1

case 2

case 3

case 4

case 5

case 6

case 7

case 8

electric power (MWe) blending ratio of brown coal total coal feed rate (t/h) total air supply (t/h) SOFA ratio, (vol %) Primary air for brown coal air temperature (°C) air-fuel ratio (kg/kg) Primary air for bituminous coal air temperature (°C) air-fuel ratio (kg/kg) secondary gas temperature (°C) Coal allocation in burners F layer E layer D layer C layer B layer A layer

600 0 247.2 2313.3 30

600 30% 265 2241.1 30

600 30% 265 2241.1 30

600 30% 265 2241.1 30

400 50% 190 1495.6 30

500 50% 238 1873.5 30

600 50% 285 2243.4 30

600 70% 307 2241.3 30

600 50% 265 2241.1 20

60 2.5

60 2.5

60 2.5

60 2.5

60 2.5

60 2.5

60 2.5

60 2.5

70 2.22 337

70 2.22 337

70 2.22 337

70 2.22 337

70 2.22 337

70 2.22 337

70 2.22 337

70 2.22 337

70 2.22 337

none Datong Datong Datong Datong Datong

Indo. Buli. Indo. Buli. Hua. Hua.

Indo. Buli. Hua. Hua. Indo. Bul.

Hua. Hua. Indo. Buli. Indo. Buli.

Buli. Hua. Hua. Hua. Buli. Buli.



Hua. Hua. Hua. Hua. Buli. Buli.

Hua. Hua. Hua. Hua. Buli. Buli.

Note: Indo. = Indonisian coal (bituminous), Buli. = Bulianhun coal (bituminous), Hua. = Huadian brown coal.

Table 3. Cases Studied for Victorian Brown Coal and Its Two Derivatives, Beneficiated Briquette, and Pyrolysed Semicokesa case electricity power (MW) blending ratio total coal feed rate (t/h) total air flow rate (t/h) Primary air for brown coal air temperature (°C) air-fuel ratio (kg/kg) Primary air for bituminous coal air temperature (°C) air-fuel ratio (kg/kg) secondary gas temperature (°C) Coal allocation in burners F layer E layer D layer C layer B layer A layer a

case 9

case 10

case 11

case 12

case 13

600 23.7% 243.2 2226

600 22.9% 240.7 2225

600 21% 231.8 2232.4

600 19.7% 226.1 2238.4

600 24.2% 245.3 2231

60 2.5

60 2.5

60 2.5

60 2.5

60 2.5

70 2.22 337

70 2.22 337

70 2.22 337

70 2.22 337

70 2.22 337

Indon. Buli. SC-400 °C SC-400 °C Indo. Buli.

Indo. Buli. SC-600 °C SC-600 °C Indo. Buli.

Indo. Buli. SC-800 °C SC-800 °C Indon. Buli.

Indo. Buli. SC-1000 °C SC-1000 °C Indo. Buli.

Indo. Buli. Briquette Briquette Indo. Buli.

Note: SC = semicoke. commenced at 455 °C, which is clearly higher than Victorian brown coal and its semicoke derivatives. 2.3. Submodels in CFD Modeling. The construction of grid is primarily important for a reliable and accurate calculation. A partition meshing method was applied to generate high quality mesh with hexahedral cells through the use of Gambit. The resulting mesh structure for the boiler is illustrated in Figure 4a. To reduce the artificial diffusion, a mesh with lines approximately along the swirl flow direction was created for the burner zone. A grid independence test was first conducted through comparing the gas velocity profiles for the mesh grids with different cell numbers. A total of 550 000 cells were chosen eventually because its results agreed well with that of 872 000 cells. Note that the total cell number used here is comparable with those reported for large-scale industrial boilers with traditional burner outfits in the literature.11,13,20,21 A total of 50 particle injection points were set up in each pulverizedcoal inlet, shown in Figure 4b. Ten groups of particle sizes were assumed based on Rosin-Rammler distribution.10 The average particle

Figure 2 shows the kinetic rates measured for coal pyrolysis and char-O2 oxidation rate. Both two brown coals (Huadian coal and dried Victorian brown coal19) bear very higher devolatilization reactivity, which is 102−106 faster than bituminous coals, as demonstrated in Figure 2a. Regarding char-O2 oxidation rates in Figure 2b, the Victorian brown coal and its two derivatives, beneficiated briquette and 1000 °C semicoke, have a similar reaction rate, which is around 10 times higher than the bituminous coal. The dynamics for the drying and devolatilization of Victorian brown coal and its two semicokes are further demonstrated in Figure 3, which was measured by heating a sample at a heating rate of 10 °C/min in air. In each curve shown in panel (a), the pyrolysis start and end points were determined by the content of volatiles in the respective sample. By taking the first order derivative for the curves, the ignition points for the three samples were further determined in panel (b). As can be seen, the ignition of raw brown coal and its 400 °C semicoke was commenced at 335−350 °C, relative to 440 °C for its 800 °C semicoke. As a reference, the ignition of Datong bituminous coal was C


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Energy & Fuels Table 4. Proximate and Ultimate Properties of Coal Tested by Experiment coal

Datong

Bulianhun

Indonesian

Huadian

coal type

bituminous

bituminous

bituminous

brown

Proximate analysis (wt %), air-dried (ad) moisture ash volatile matter fixed carbon moisture, wt %, as received (ar) Ultimate analysis (wt %), dried basis (db) carbon hydrogen nitrogen sulfur oxygen low heating value, (MJ/kg), ar (1) Pyrolysis rate pre-exponential coefficients (s−1) activation energy (kJ/mol) (2) Char oxidation rate pre-exponential coefficients (kg·m−2s−1 Pa−1) activation energy (kJ/mol)

11 18.13 22.15 48.72 11

4.31 12.62 27.95 55.12 13.8

9.25 3.55 41.84 45.36 18.5

8.3 3.42 39.16 49.12 36.7

64.2 3.99 9.85 0.84 0.74 21.850

69.33 4.24 1.17 0.87 11.2 22.120

73.41 5.19 1.39 0.18 15.92 22.490

68.38 4.55 0.92 0.24 22.19 15.650

5.910 × 107 180.00

1.123 × 105 85.31

2.111 × 105 88.02

4.115 × 1012 164.0

0.1070 117.00

5.220 × 10−3 87.11

8.990 × 10−5 57.59

8.03 × 10−4 70.261

Table 5. Properties of Victorian Brown Coal and Its Beneficiated Semicokes pyrolysis semicoke properties Approximate analysis, wt % moisture, ar ash, db volatile matter, db fixed carbon, db Ultimate analysis, wt % carbon, db hydrogen, db oxygen, db nitrogen, db sulfur, db low heating value, (MJ/kg), ar Char oxidation rate pre-exponential coefficients (kg m−2s−1Pa−1) activation energy (kJ/mol)

raw coal

beneficiated briquette

400 °C

600 °C

800 °C

1000 °C

∼65 3.0 50.1 46.9

9.3 8.9 45.2 45.9

6.6 1.5 56.1 42.4

6.4 0.4 48.4 51.2

7.9 3.1 30.1 66.8

9.6 3.4 8.8 87.8

65.9 4.7 25.3 0.6 0.5 7.2

63.5 4.4 22.31 0.48 0.41 21.32

66.7 3.8 27.24 0.47 0.29 21.96

68.1 4 26.69 0.58 0.23 22.73

78.7 2.7 14.49 0.78 0.23 25.6

88.9 1.1 5.65 0.72 0.23 27.57

0.0024 69.06

0.0038 73.12

diameter of coal was assumed 76 μm with a spread parameter of 4.41. A total of 12 000 particles were tracked via coupling modeling. Such a number was found to be good enough to create a similar temperature distribution with the number of 24 000. CFD modeling was performed using the commercial code ANSYS FLUENT 15.0.22 Eulerian description was employed to formulate gasphase time-averaged conservation equations for gaseous velocity, temperature, and species components. Realizable k-ε model was used for turbulent flow, taking into account viscous heating and full buoyancy effects. The coal particle trajectories were modeled by Discrete Random Walk model in a Lagrangian frame of reference, considering some factors such as random eddy lifetime, particle radiation interaction, and Saffman lift force. The discrete ordinate (DO) model was used to model the radiative heat transfer with the option of particle radiation interaction. The domain-based weighted-sum-of-gray-gases (WSGG) model was applied to determine the absorption coefficients of gas phase, which was refined further by considering the distinctive radiative properties of gases H2O and CO2 in bulk gas.23,24 The emissivity of burning coal particle surface was set as 0.85. In refs 25 and 26 the modeling methodology comparison was carried out about the radiation influence

0.0265 97.585

of soot in pulverized coal flame and gaseous fuel combustion. The results showed that particle radiation exclusively dominates the total radiation in a pulverized coal flame, whereas the effect of soot is insignificant and ignorable in most simulation cases. Herein soot radiative was not included. Four well-defined steps of pulverized coal combustion take place after the coal particles enter the furnace, i.e., drying, devolatilisation, volatile combustion, and char combustion. The surface and inherent moisture were both considered in brown coal.19 The surface moisture was assumingly released into the primary gas, as determined by the drying extent of brown coal in the mill (as shown in Table 7). The remaining inherent moisture was assumed to be released with volatiles together as the same rate. The measured data (seen in Tables 4 and 5) were used to depict the coal devolatilization rate. The finite-rate/eddydissipation model was used to express the coupling between turbulence and gaseous reactions. The process of char oxidation was described by the kinetics/diffusion-limited model (viz. single-film model) of Field.27 Regarding coal blending combustion, FLUENT 15.0 offers a platform for the users to set up several properties of individual coals within it. Regarding char oxidation reaction, only the char-O2 reaction was considered, whereas the endothermic C−CO2 D


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Figure 2. Comparison of the kinetics of various coal samples tested here. Panels (a) for coal pyrolysis rate and (b) for semicoke oxidation rate. Figure 3. Drying and devolatilization dynamics for Victorian brown coal and its semicoke samples, Panels (a) for accumulative conversion rate and (b) for derivative of the accumulative conversion rate.

and C−H2O reactions were neglected, due to the quite low partial pressures of CO2 and H2O in a conventional air-fired boiler. 2.4. Thermal Boundary Conditions. Because of the complexity of a real industrial boiler, its furnace wall was modeled by dividing into three different boundaries, e.g., water wall zone, radiative superheater (SH) zone, and convection tube zone. This classification is determined by heat transfer module and different tube-wall temperatures in furnace. In this study, the operating parameters for steam in different locations along the boiler are listed in Table 6. The tube-wall temperatures were determined by the average steam temperature and assuming an increased temperature difference (50−100 °C) between tube external surface and steam due to the heat conduction within steel tube. To get a reliable CFD modeling result, Park28 applied the coupling of a CFD model by ANSYS CFX and a 1-D steam-water-side model, which presents a heat-transfer calculation between coal flame and the steam side in the water wall, convective pass and economizer. However, the radiative exchanger, platen superheater in tangentially fired furnace, was assumed as a convective exchanger, oversimplifying the radiation modeling in furnace. The other researchers including AlAbbas21 and Tian29 used the heat sinks (heat source term in gas-phase temperature equation) in the fixed mesh to match the heat absorption values in the superheater and reheaters of a tower furnace. However, one has to be aware that such a method is only suitable for the heat exchange in a convective zone, not in a radiative zone. In the above literature for the simulation of coal combustion in a tower furnace,21,29 the outside surface of superheaters has very limited contact with the coal flame to receive any radiative heat. However, for a tangentially fired furnace, the radiative heat is supposed to be predominant for the platen superheaters at the top of the furnace.30 In light of this, the thermal boundary of platen and rear superheaters was assumed as a wall surface in this study, which bears a certain temperature and a proper emissivity 0.5 that is referenced from the emissivity data of

stainless steels, e.g., 0.54−0.63 (type 301)31 and 0.36−0.58 (type 304 at 525 °C after being exposed for 42 h).32 The furnace enclosure in the water wall zone was formed by a set of spiral tubes. The tube size and spacing were designed to provide a proper heat-transfer and flow pattern near the flame. Therefore, the effective radiative area of total fire-side tubes in water-wall zone is not like the simplified wall in the modeling geometry in Figure 1. The effective radiation surface area was estimated to be about 70% of the total tube surface due to tube spacing. The radiative heat flux absorbed by wall surface was estimated by the equation 1 in Fluent,22 in which fd is the diffusion fraction, εw is wall emissivity, qin is incident radiative heat flux, and Aw is radiative surface area.

Q r,w = fd εw qinA w

(1)

To accommodate the loss of radiative surface (Aw) of water wall, the εw was decreased by 70% reaching 0.35, which was used throughout this paper. The absorption heat quantity in superheaters can be expressed as

Q SH = msteam(hout − h in)

(2)

where msteam is the mass flow of steam, and h is steam enthalpy that was obtained from the h-p diagram of water and steam in ref 33. The desuperheating spray flux was considered to calculate the steam mass flow rate in each part of the superheater system. 2.5. NOx Formation Modeling. The NOx emissions were predicted by means of postprocessor computation. 34−42. The formation of thermal-NOx can be described by the extended Zeldovich’s mechanism.35,36 In this work, the required O and OH concentrations in flow field were determined by equilibrium and E


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Figure 4. Mesh grid system of furnace (a) and schematic diagram of simplified burner nozzle (b).

overcome such a problem is to increase the flow rate and/or temperature of preheated primary air. All these changes could alter the performance of the mill, particularly its outlet temperature and the drying extent of brown coal within it. Table 7 summarizes the experimentally measured results for the optimized performance of a bituminous-coal-designed roller

Table 6. Experimental Data Measured for the Temperature and Pressure of Steam in the Tubes of Water Wall and Superheaters (SH) items

design (BMCR)

Heated Steam Temperature, °C inlet of water-wall 414.0 tube outlet of water-wall 430.0 tube inlet of platen SH 439.0 outlet of platen SH 480.0 inlet of rear SH 469.0 outlet of rear SH 517.0 Final Main Steam Condition pressure, MPa 25.4 steam flux, t/h 1913.0 steam temperature, 571 °C Mass of Desuperheating Spray, t/h between platen and 48.0 rear SH between rear and 28.0 final SH

case 4 (400 MWe)

case 5 (500 MWe)

case 6 (600 MWe)

Table 7. Performance of Mill for the Feeding of Bituminous Coal and Huadian Brown Coal 418.0 463.0 452.0 506.0

427.0 468.0 459.0 504.0

426.0 459.0 457.0 501.0

21.4 1120.7 560

24.2 1446.3 560

24.2 1763.5 560

coal type bituminous coal

Huadian brown coal 59.0

30.7

10.66

21.0

23.2

11.23

partial-equilibrium models, respectively. Fuel-NOx calculation followed the De Soete’s model,37,38 in which the volatile-N first converts to intermediate HCN and NH3 and then form NO or N2. NO can be reduced by a heterogeneous reaction on char surface.39,40 In this study, 70% (volatile-N) and 30% (char-N) were chosen. Regarding bituminous coal, it was assumed that 99% of volatile-N converts to HCN, and the rest forms NH3.41 Instead, for brown coal, 85% of volatile-N is generally assumed to convert to HCN, and the rest for NH3.34 Furthermore, the conversion rate of volatile-N and char-N was set as 100% and 70%, respectively. This is to reflect the experimental observation that approximately 30% char-N was found to reside with unburnt carbon in fly ash.42

coal feed, t/h

hot air temp, o C

primary air temp, °C

moisture in dried coal, wt %

2

50.29

223.6

81.1

7.0

36

2.1 2.4

205.8 246.5 235.9 266.1

83.8 76.5 78.1 55.4

6.0 3.5

45 68

2.3

49.69 52.22 64.9 50.58

2.4 2.5 2.7 3.0

50.57 50.59 44.41 43.26

272.1 245.7 245.6 290.7

58.0 55.1 54.0 62.0

21.0 20.0 15.0

air-fuel ratio, kg/kg

wt % of coal dried

42.8 45.5 56.4

mill upon the use of Huadian brown coal. As can be seen, the primary air-fuel mass ratio has to be increased from 2 to 2.4 for bituminous coal to 2.3−3.0 for brown coal, whereas the outlet gas temperature was decreased from 75 to 85 °C for bituminous coal to 55−60 °C for brown coal. This is due to the large amount of moisture inherited in the brown coal. Although the inlet gas amount and temperature have been increased in the mill, the drying of brown coal is still incomplete at the outlet. For Huadian brown coal tested on site, approximately 42.8−56.4 wt % of its moisture was released in the mill, whereas the remaining moisture still resides within coal particles that are fed into the furnace. 3.2. Numerical Model validation. The CFD model established was first validated by the measured operating data, including the exhaust flue gas temperature at furnace exit, the amount of heat absorbed by steam in superheaters and gaseous compositions (e.g., O2, NOx) in flue gas, as shown in Table 8 and Figures 5 and 6. The experimental data listed in Table 8 were conducted for case 2 with 30 wt % brown coal

3. RESULTS AND DISCUSSION 3.1. Performance of Bituminous Coal-Designed Mill for the Grinding of Brown Coal. The bituminous-coaldesigned mill of a pulverized coal-fired plant has a specified outlet temperature of around 70−80 °C, so as to avoid the delay of coal ignition in the boiler. The feeding of brown coal could reduce the outlet temperature, due to the evaporation of its relatively abundant inherent moisture. One method to F


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and design data. For comparison, the other reported measurement data10,20,38,43,44 were tabulated here too, in which the flue gas temperature was found to be 1220−1440 °C for the furnaces in the same tangentially fired mode. Obviously, our prediction results are very close to the other reported experimental data too. They are even better than the predictions in the ref 44 and 38, where the modeling results are 100−200 °C lower than the respective experimental data. This is because a reasonable heat loss in the water wall zone was considered in our model, which was however missing in these two studies. Figure 5 presents the comparison of the amount of heat absorbed in platen and rear superheaters between the modeling results and experimental measurement. A nearly perfect agreement was confirmed for the platen superheater under the operation cases, case 4, 5 and 6, with different electrical power generation outputs, shown in Figure 5a. However, the prediction of rear superheaters carry an error of approximately 15% for underestimating the heat absorbed there, given in Figure 5b. The probable cause for this could be the fact that the convective heat transfer has a comparable weighing factor with the radiative heat flux in the rear superheater. The convective heat transfer is highly dependent on the detailed pipe configurations that were omitted and simplified in this paper. Establishing a detailed modeling for heat transfer is out of the scope of this paper. Therefore, no effort was made to further improve the accuracy of the model. Nevertheless, the modeling results for the rear superheater are still acceptable here, because the convective heat transfer only impacts the local flue-gas temperature nearby the convective exchangers, which exerts no/little influence on the modeling accuracy of flame temperature in furnace. The accuracy of the model was further witnessed by a satisfactory agreement for the oxygen and NOx concentrations in Figure 6. Note that the NOx concentrations refer to the values measured before the selective catalytic reduction (SCR). With the increase of power generation output, more coal is burned, and hence, the NOx emission increases. Upon the use of our fully validated model, the influences of individual variables on coal blending combustion were conducted either through modeling alone or the combination of modeling prediction and experimental measurement, as detailed below. 3.3. Influence of Brown Coal Allocation in the Burner Zone. This variable was first assessed for the blending of 30% brown coal (Huadian coal) and 70% bituminous coals (Bulianhun coal and Indonesian coal). The mass-weighted average temperature profiles along the furnace height were plotted in Figure 7, including design case (all bituminous coal), cases 1, 2, and 3 with the bottom two (A and B in Figure 1), middle two (C and D), and top two layer (E and F) burners applied for brown coal, respectively. The average flue gas temperatures above the SOFA zone of the case 2 and case 3 reach the rather similar pattern that matches the design case. However, allocating brown coal in the bottom two layer burners for case 1 resulted in a slight rise of about 60 °C for the temperatures in the SOFA zone. This can be explained by a shortened residence time for the bituminous coal particles when their travel was commenced from the middle zones, rather than from the bottom of the boiler. Its combustion was thus postponed and continued in the later stage in the boiler. Although such a temperature rise is quite small, it may cause an

Table 8. Comparison of Flue Gas Temperature at Furnace Exit Plane between Experimental (exp.) Data and CFD Modelling Results electric energy power, MWe 600 (this study, case 2)

firing style tangential

609

tangential

610 150

tangential tangential tangential

350

tangential

data style

average temperature at furnace exit, °C

ref

design

1367

exp. modeling exp./ modeling exp. exp. exp./ modeling exp./ modeling

1230−1340 1284 ∼1327/1329

10

1220−1270 ∼1427a 1227−1400/1127−1327

20 43 44

1257/1035

38

a

Note that the measurement location was below the furnace exit plane.43

Figure 5. Predicted and measured heat absorption amounts in (a) platen superheater and (b) rear superheater for cases 4, 5, and 6.

Figure 6. Comparison of modeling results of NOx and oxygen concentration in flue gas and measured data for cases 4, 5, and 6.

being used. The gas temperature was measured in the region at the exit plane of the furnace. As can be seen from Table 8, the mass-weighted average temperatures predicted by the established CFD model agree satisfactorily with both the measured G


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bituminous coal flame at the bottom can benefit the dewatering process of brown coal to shorten the preheating time. Moreover, although the cases 2 and 3 carry a difference in particle trajectory in Figure 8, the impact of such a difference was not observed in terms of flue gas temperature in Figure 7. In other words, the allocation of brown coal in either the middle or top zone of the burner is appreciable, whereas the bottom zone for brown coal should be avoided in practice. 3.4. Influence of Blending Ratio of Brown Coal. This variable was assessed secondly by comparing case 2 with case 6 and case 7 for the blending ratio of 30%, 50%, and 70% of Huadian brown coal, respectively. Note that the brown coal was allocated to the middle and/or top burners for these three cases, as mentioned in Table 2. However, the total coal feeding rate was increased remarkably upon the rise in the blending ratio of brown coal. Compared to the design case with a coal feeding rate of 247.2 t/h, the three cases for coal blending have a total feeding rate of 265, 285, and 307 t/h, respectively. The results in Figure 9 are for averaged flue gas temperature versus furnace height. In panel (a) one can see a similar temperature pattern for all the cases. In particular, the temperatures from the SOFA zone and above of the three blending cases match the design values very well. This is expected, as the coal combustion is finished here, and the total energy input is identical for all the cases. This is also desirable because the heat exchange and the parameters of steam in the radiative and convective zones remain unchanged when the combustion shifts from bituminous coal alone to its partial substitution by brown coal. In contrast, the temperature alteration in the burner zone is quite remarkable, as highlighted for three typical heights in panel (b). For the furnace height of 18.105 m referring to the bottom two layers of the burners for bituminous coal, the blending cases resulted in the increase of about 40−70 °C in the temperature. This reflects a quick ignition/oxidation of brown coal volatiles and the radiative heat feed from brown coal flame to bituminous coal. Among the three blending cases, 50% led to the highest temperature at this position, followed by 70% and 30% in a descending sequence. The discrepancy among these four cases is more obvious at the furnace height of 28.149 m referring to the second top layer of the burner for combustion of brown coal. The differences at these two heights should be attributed to the increased feeding rate of brown coal in the coal blending cases. On the one hand, an increased amount of combustible hydrocarbons is expected upon the increase of coal feeding rate, which in turn benefits the improvement of local gas temperatures near the burner vicinity. On the other hand, the absolute amount to be fed into the boiler is also increased at the elevated coal feeding rate. This in turn reduces the local gas temperature near the burner. This is clearly the case for the blending ratio of 70% for brown coal. The temperature distribution in longitudinal section and radial cross sections, as depicted in Figure 9c, further supports the double effect of the blending ratio of brown coal. For the case 6 with 50% brown coal fed through its middle burner zone, the isothermal area (in red color) is enlarged compared with 30% blend, due to more of the combustible volatiles entering the furnace upon the feeding of more brown coal. However, for the case 7 with blending 70% brown coal, the isothermal area for 1700 °C is absent, whereas the maximum temperature dropped to 1600 °C due to the latent heat absorption of its abundant moisture. It showed that 50% blend is optimum to benefit the combustion stability in the burner zone.

Figure 7. Average temperature profile for flue gas along the furnace height as a function of the allocation position for Huadian brown coal. The bottom two-layer burners are for case 1, middle two-layer burners for case 2, and top two-layer burners are for case 3.

overheating of the steam in superheaters and increase the fouling potential of coal ash upon the long-term operation. Comparison was further made for coal particle trajectories fed from one-corner burners between the three simulation cases, as illustrated in Figure 8. For the brown coal allocated in

Figure 8. Coal particle trajectories for the different allocation of brown coal burners in cases 1, 2, and 3. Different colors indicate the different coal burners.

the bottom two layer burners in case 1, a few of its particles from the bottom burners initially circulates around in the bottom and ash hopper of furnace, whereas more particles travel upward through the middle and top burners into the furnace. Since few of the bituminous coal particles take circulation in the bottom zone of the burner; their ignition and combustion thus occur in the later stage. This confirms the above hypothesis. Moreover, one has to be aware that the delayed combustion in bituminous coal may cause the rise in the content of unburned carbon in fly ash. This is another disadvantage of case 1 for the allocation of brown coal in the bottom burners. The advantage of case 2 and case 3 in enhancing the bituminous coal combustion is obvious. As can be seen, a larger fraction of the bituminous coal particles from bottom burners circulates around in the bottom zone of the boiler for a longer residence time. In addition to the fact the bituminous coal particles will pass the brown coal flame when they travel upward in the furnace, the combustion of bituminous coal is completed timely in the combustion zone, thereby ensuring a good heat balance without overheating in the SOFA zone. This allocation has another advantage that the H


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Table 9. Influence of Huadian Brown Coal Blending Ratio on the Combustion Characteristics, Predicted by CFD Modelling predicted items blending ratio of brown coal flue gas flux, t/h flue gas temperature at furnace exit, °C flue gas velocity at furnace exit, m/s heat transfer in platen superheater, MW NOx emission, ppm at 6% O2

design case

case 2

case 6

case 7

0 2485.5 1284.0

30% 2464.4 1288.0

50% 2482.7 1282.0

70% 2506.8 1285.0

15.7

15.9

16.0

16.2

96.5

96.1

94.0

92.3

188.8

172.5

156.0

the total NOx emission. In short, 50% blending of Huadian coal is appreciated to match the design case with less variation on heat transfer, stable combustion of coal in the burner zone, and lower NOx emission. 3.5. Influence of SOFA Staging Ratio on NOx Emission. Third, this variable was assessed for the blending of 30 wt % Huadian brown coal with bituminous coals by varying the fraction of SOFA from 20 vol % for case 2 to 30 vol % in case 8. The modeling results are demonstrated in forms of concentration contour of O2 and NO in flue gas, and temperature contour profile in Figure 10. Upon the increase

Figure 9. Flue gas temperature distribution (°C) as a function of Huadian brown coal blending ratio of 30% (case 2), 50% (case 6), and 70% (case 7). Panel (a) average temperature profiles along the whole furnace height, (b) flue gas temperature at three typical furnace height, and (c) temperature contours for different burner layers.

The influence of brown coal blending ratio on the other combustion characteristics are tabulated in Table 9. As can be seen, all these characteristics were changed remarkably upon substituting bituminous coal for brown coal. The amount of flue gas and its temperature at furnace exit in the 50% blending scenario are very close to the respective design values. However, its flue gas upward velocity at the furnace exit is slightly increased, whereas the heat transfer amount in platen superheater is slightly low. Increasing the brown coal blending ratio further worsens the results for these two variables. The more flue gas flow rate with a higher blending ratio is the primary reason for this. Furthermore, upon the blending of more brown coal, the total amount of nitrogen entering the boiler was reduced, therefore leading to a decreasing trend for

Figure 10. Contour distribution of O2 concentration, flue gas temperature, and NOxemission as a function of SOFA ratio for the blending of 30% Huadian brown coal. Panels (a) for 20 vol % SOFA (case 8) and (b) for 30 vol % (case 2).

of SOFA from 20% (panel a) to 30% (in panel b), the O2 concentration between the burner zone and SOFA was decreased, as expected. Therefore, the combustion of coal was elongated, and a wider distribution of low oxygen concentration (blue color for 2%) took place before the SOFA in panel b. Regarding flue gas temperature contour, coal flame front with a maximum temperature of 1700 and 1600 °C I


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reductive reaction. Such a reaction is clearly more significant for the 30% SOFA scenario, as demonstrated by the intensified negative rate for its fuel-NOx formed between the top burner and SOFA zone. 3.6. Assessment of Blending Combustion of Beneficiated Victorian Semichar. At last, effort was made to explore the potential for use of beneficiated Victorian brown coal and its pyrolysis derived semicokes as a substitute fuel in the bituminous coal-fired boiler. For such a purpose, the properties and kinetic data of beneficiated Victorian brown coal and its semicoke, listed in Table 5, were used instead of Huadian brown coal in FLUENT modeling. The modeling conditions are listed in Table 3. To reiterate, the study cases from 9 to 12 are for the test of semicokes generated from four different pyrolysis temperatures, 400−1000 °C, where case 13 was used to test the use of beneficiated briquette in place of semicoke. These new samples were fed through the middle two layer burners into the boiler, which is the same as in the above subsections for the use of Huadian brown coal. The heating value of Victorian brown coal was enhanced remarkably upon drying or mild pyrolysis. Increasing the pyrolysis temperature was also in favor of removing the volatiles and increasing the heating value of the remainders, thus leading to a continuous decrease in the feeding rate from Huadian brown coal through to the 1000 °C semicoke, as demonstrated in Table 10. The predicted NOx emission of Victorian beneficiated brown coal and semicoke shows similar values with the blending combustion of Huadian brown coal, reaching a lower NOx level of 169−188 ppm (6% O2) at the exit. Figures 12 and 13 depict the formation rates of both thermalNOx and fuel-NOx as a function of furnace height for the various cases studied in this subsection. For the thermal-NOx formation rate, its noticeable variation between different cases was observed at 21.889 m for the allocation of beneficiated brown coal/semicoke. Thermal NOx yielded from Huadian coal is the lowest, which is due to the lower temperature profile (to be explained in detail in Figure 14) at the burner zone caused by the evaporation of its intrinsic moisture. Looking at the semicokes samples alone in Figures 12 and 13, a clearly descending trend can be found for thermal NOxwith the sample varying from 400 °C semicoke through to 1000 °C semicoke. Interestingly, an opposite trend was observed for fuel-NOx at the same height (see panel b in Figure 13). Clearly, these two opposite trends counterbalanced each other and thus produced

(red color) remains similar between the two scenarios, due to the same amount of primary gas used for the entrainment of coal particles. However, the isothermal area for the temperature of 1500 °C in orange is remarkably longer and wider in the 30% SOFA than its 20% counterpart. This is due to the presence of insufficient oxygen between burner and SOFA zone, and thus coal combustion was extended to a relatively long duration. The NOx emission is affected significantly by the SOFA ratio. For the NOx contour formed at 20% SOFA ratio, its maximum concentration reached 250 ppm in the convective heat transfer zone, relative to only 220 ppm for the 30% SOFA ratio. This is closely linked with the strong reducing atmosphere, higher temperature, and lower O2 concentration formed between burner and SOFA for the larger SOFA ratio. All these conditions facilitated a reductive conversion of NOx back to N2 via Char + NO → N2.45 This is further demonstrated by Figure 11 for the formation rates of thermal NOx and fuel-NOx

Figure 11. Formation rates of thermal-NOx and fuel-NOx as a function of SOFA ratio for the blending of 30% Huadian brown coal.

in the two scenarios. As can be seen, the thermal-NOx is mainly generated near the upper burners with local higher temperature above 1523 °C, the formation rate of which is rather independent of the SOFA ratio. However, regarding the fuelNOx formation rate, it remains positive for the entire burner zone and turns negative slightly in the area from top burner to the bottom of SOFA zone, due to the above-mentioned

Table 10. CFD Prediction Results for the Feeding Rate and NOx Emission of Victorian Brown Coal Semicokes and Briquette to Match Case 2

J


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Figure 12. Thermal and fuel NOx formation rates along the furnace height for blending beneficiated Victorian brown coal or its semicokes with bituminous coal.

Figure 14. Flue gas temperature distributions for the blending of beneficiated Victorian brown coal or its semicokes. Panel (a) average temperature profile along furnace height, and (b) flue gas temperature at 18.105, 21.889, 24.727 m.

for these cases. Again, this suggests a stable heat transfer in the convective zone. The temperature differences in the burner zone were further amplified and detailed in panel (b) for three typical heights. Interestingly, the first height, 18.105 m for the burner assigned to bituminous coal was also affected significantly by the type of brown coal assigned to the middle zone. This is another direct indicator demonstrating the heat flux between different burner layers and different coals. The allocation of wet Huadian brown coal in the middle burner zone apparently absorbed a portion of the radiative heat from bituminous coal underneath it, for the sake of the evaporation of the abundant moisture within it. In contrast, a quick ignition and oxidation of beneficiated Victorian brown coal briquette enhanced the oxidation of bituminous coal, leading to an enhanced gas temperature in the bottom zone. This is apparently beneficial in improving the burnout of bituminous coal and NOx emission level as well. For the semicoke used, the low content of volatiles within them provided less heat feedback to bituminous coal. The thermal-NOx formation rate was further plotted versus flue gas temperature in Figure 15a. One can see that a strong relationship between these two variables, that is, the thermalNOx formation is exponentially increased upon the rise of flue gas temperature, particularly from 1500 °C onward. In particular, it took place principally at the furnace height of 24.727 m for the top of the burner zone where all the heats generated are eventually accumulated together. Regarding the formation rate of fuel-NOx derived from the inherent nitrogen

Figure 13. (a) Thermal and (b) fuel NOx formation rates for blending beneficiated Victorian brown coal or its semicokes with bituminous coal.

a similar NOx emission level for different semicokes, as shown in Table 10. As mentioned, the thermal-NOx emission is strongly linked with flue gas temperature. Figure 14a shows flue gas temperature profile for the various blending cases. Among the different cases, the obvious discrepancy was merely observed in the burner zones. Once leaving the burner zone and entering the SOFA area, all the flue gas temperatures remain identical K


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lower nitrogen content than bituminous coal, as evident in Tables 4 and 5. The predicted gas temperature distribution at the crosssection of burner C is further demonstrated in Figure 16 for the comparison of aforementioned different cases. Although the volatile matter in semicokes is partly or almost completely removed, their ignitions are rapidly accomplished, partly due to the heat feed from bituminous coal allocated to the next burner layers A and B. The ignition length of Huadian brown coal (case 2) is much longer than bituminous coal, due to the delay of moisture evaporation process. However, the ignition length of semicokes (cases 9−12) and briquette (case 13) is much shorter, due to a low volatile and/or moisture content with these beneficiated samples. As a result, the high temperature zone (above 1700 °C) caused by particle flame is very close to the burner, yielding a distance of only ∼3 m for all the semicokes, relative to ∼6.5 m for brown coal in case 2. This raises the concern regarding the damage on the burners upon the use of semicokes. However, the gas temperature in the burner local region is remarkably lower upon the use of semicokes. For instance, the local gas temperature was decreased from >1700 °C for design case to only 1400 °C for case 12. This apparently benefits the decrease on the propensity of ash slagging and deposition in the combustion zone.

4. CONCLUSIONS CFD simulations have been conducted to compare with on-site measurement of flue gas composition, temperature, and NOx emission in a 600 MWe supercritical boiler for the combustion of bituminous coal blended with brown coal. The blending of two different types of coals occurred inside the boiler, whereas they had no any prior contact outside. Upon a successful validation, the model was further employed to explore the influence of brown coal allocation, blending ratio, and brown coal type (i.e., brown coal produced in different places and its pyrolysis semicoke) on the boiler performance. The major conclusions achieved are drawn as follows. (1) Apart from flue gas temperature, the use of heat absorption quantity in platen superheater has also been proven effective in terms of validating the CFD simulation model for the large-scale industrial boiler. Good agreement has been achieved between the experimentally measured heat absorption quantity and the model prediction, upon the setting of an appropriate wall emissivity, tube temperature, and effective radiative area. (2) On-site retrofit experiments confirmed a stable operating of the bituminous coal-designed mill for the grinding of brown coal, at the expense of an increased primary air to coal mass ratio to ∼2.7; a lower primary air temperature of 55−60 °C (close to the requested minimum temperature for mill), and an incomplete drying for the wet brown coal. (3) Among three operating variables for coal blending combustion, the allocation of brown coal, brown coal blending ratio, and SOFA ratio. The allocation of brown coal in either middle or top layers in the burner zone ensured a long residence time for bituminous particles in the boiler. Its quick combustion also provided sufficient heat to bituminous coal for its quicker burnout when it passes brown coal flame zone. The effect of brown coal blending ratio is complex. The 50 wt % blending ratio for brown coal ensured a good balance between the feeding of combustible hydrocarbons and moisture into the boiler, thereby leading to insignificant change on flue gas

Figure 15. Influence of flue gas temperature (panel a), N content in coal (panel b), and oxygen level at furnace cross sections (panel c) on NOx formation.

in fuel, its relationship with nitrogen content in fuel is however rather insignificant, except the last point (0.92 wt % for nitrogen in Huandian brown coal) for the middle burner zones, as demonstrated in Figure 15b. Such an abnormal point is indeed caused by a slower combustion of Huadian brown coal which is rich in moisture. A slow combustion of coal particle ensured a high “unreacted” oxygen concentration around coal particle, which in turn facilitated the oxidation of nitrogen into NOx. A clear trend for the fuel NOxemitted versus the averaged oxygen content in the middle burner zone, as illustrated in Figure 15c, supports this hypothesis. Moreover, one can see a much higher fuel-NOx emitted from the bituminous coals tested here. This is mainly because brown coal usually has a L


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Figure 16. Gas temperature distributions at the cross-section of burner C-layer to analyze the combustion characteristics of Victorian semicokes and briquette. station, http://www.mpr.com/uploads/news/low-sulfer-coal-blending. pdf. (5) Gale, T. K.; Merritt, R. L. Coal blending, ash separation, ash reinjection, ash conditioning and other novel approaches to enhance Hg uptake by ash in coal-fired electric power stations. Library in U.S. Department of Energy; http://www.netl.doe.gov/File%20Library/ Research/Coal/ewr/mercury/41183_AQIV.pdf. (6) Rubiera, F.; Arenillas, A.; Arias, B.; Pis, J. J. Fuel Process. Technol. 2002, 77−78, 111−17. (7) Haykiri-Acma, H.; Ersoy-Mericboyu, A.; Kucukbayrak, S. Energy Sources 2010, 22 (4), 325−32. (8) Haas, J.; Tamura, M.; Weber, R. Fuel 2001, 80, 1317−23. (9) Qiu, J. R.; Li, F.; Zeng, H.; Yao, B.; Ma, Y. Combust. Sci. Technol. 2000, 157, 167−84. (10) Yin, C.; Caillat, S.; Harion, J.; Baudoin, B.; Perez, E. Fuel 2002, 81, 997−1006. (11) Belosevic, S.; Sijercic, M.; Tucakovic, D.; Crnomarkovic, N. Fuel 2008, 87, 3331−38. (12) Hashimoto, N.; Shirai, H. Energy 2014, 71, 399−413. (13) Modlinski, N. Fuel Process. Technol. 2010, 91, 1601−08. (14) Agraniotis, M.; Nikolopoulos, N.; Nikolopoulos, A.; Grammelis, P. Fuel 2010, 89, 3693−709. (15) Clarke, M. C. Low rank coal/lignite upgrading technologies. Coal Technologies; http://www.metts.com.au/low-rank-coaltechnologies.pdf. (16) Coal Project Brings Jobs and Investment to the Valley, Australia; http://www.thefreelibrary.com/ Australia+%3A+Coal+project+brings+jobs+and+investment+to+the+ Valley.-a0371506289. (17) Zhang, L.; Binner, E.; Qiao, Y.; Li, C. Z. Fuel 2010, 89, 2703− 2712. (18) Wijaya, N.; Zhang, L. Fuel 2012, 99, 217−225. (19) Zhang, J.; Prationo, W.; Zhang, L.; Zhang, Z. Energy Fuels 2013, 27, 4258−4269. (20) Vuthaluru, H. B.; Vuthaluru, R. Appl. Energy 2010, 87, 1418− 1426. (21) Al-Abbas, A. H.; Naser, J.; Dodds, D. Fuel 2012, 102, 646−65. (22) FLUENT User’s Guide, Version 15.0; ANSYS, Inc.: Canonsburg, PA, 2015. (23) Yin, C. G.; Johansen, L. C. R.; Rosendahl, L. A.; Kar, S. K. Energy Fuels 2010, 24, 6275−82. (24) Yin, C. G. Energy Fuels 2013, 27 (10), 6287−6294. (25) Andersson, K.; Johansson, R.; Johnsson, F. Int. J. Greenhouse Gas Control 2011, 55, 558−565.

temperature profile. The SOFA of 30% ratio is beneficial in reducing the thermal − NOx and thus the total NOx emission during coal blending combustion process. (4) The use of beneficiated Victorian brown coal and its pyrolysis derived semicokes ensured a lower feeding rate for them to be used as a substitute fuel. In addition, the high volatile-O2 and char-O2 oxidation reactivity of these samples led to a quick combustion of these samples that in turn provide heat feedback to bituminous coal for its quicker burnout. The use of brown coal semicoke favored low NOx emission, a similar level with those yielded from Huadian brown coal. The substitute of semicoke for black coal is potentially in favor of decreasing ash slagging and propensity in the coal burner vicinity.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: 61-3-9905-2592. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from Monash-China Huadian Electric Research Institute collaboration project and the innovation project (No. 20132011A05) of industrial science in Hangzhou, China. We are also grateful to two Monash graduates, Mr. Thant Zin and Mr. Luke David Rana, for their assistance with CFD modelling, and Ms. Niken Wijaya for conducting Victorian brown coal pyrolysis experiments.

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REFERENCES

(1) Li, C. Z. Advances in the Science of Victorian Brown Coal; Elsevier: Amsterdam, 2004. (2) Bosoaga, A.; Panoiu, N.; Mihaescu, L.; Backredy, R. I.; Ma, L.; Pourkashanian, M.; Williams, A. Fuel 2006, 85, 1591−1598. (3) Baek, S. H.; Park, H. Y.; Ko, S. H. Fuel 2014, 128, 62−70. (4) Russell, J. M.; Gillespie, M. B.; Gibson, W. C.; Bhamidipati, N. V.; Mahr, D. Considerations for low sulphur coal blending at B.L. England M


Article

Energy & Fuels (26) Johansson, R.; Leckner, B.; Andersson, K.; Johnsson, F. Int. J. Heat Mass Tansfer 2013, 65, 143−152. (27) Field, M. A.; Gill, D. W.; Morgan, B. B.; Hawksley, P. G. W. BCURA, Leatherhead, U.K., 1967; p 186. (28) Park, H. Y.; Faulkner, M.; Turrell, M. D.; Stopford, P. J.; Kang, D. S. Fuel 2010, 89, 2001−10. (29) Tian, Z. F.; Witt, P. J.; Schwarz, M. P.; Yang, W. Numerical modelling of brown coal combustion in a tangentially-fired furnace. 7th International Conference on CFD in the Minerals and Process Industries. CSIRO, Melbourne, Australia, December 9−11 2009. (30) Viskanta, R.; Menguc, M. P. Prog. Energy Combust. Sci. 1987, 13, 97−160. (31) Emissivity Coefficients of some common Materials. The Engineering Toolbox. http://www.engineeringtoolbox.com/ emissivity-coefficients-d_447.html. (32) Baukal, C. E. Heat Transfer in Industrial Combustion; CRC Press: Boca Raton, FL, 2000; p 125. (33) Franke, J.; Kral, R. Supercritical boiler technology for future market conditions. SIEMENS. http://www.energy.siemens.com/mx/ pool/hq/power-generation/power-plants/steam-power-plantsolutions/benson%20boiler/Supercritical_Boiler_Technology_for_ Future_Market_Conditions.pdf. (34) Van, D.; Lans, R. P.; Glarborg, P.; Dam-Johansen, K. Prog. Energy Combust. Sci. 1997, 23, 349−77. (35) Zeldovich, Y. B. Acta Phys. Chim. 1946, 21, 577. (36) Hill, S. C.; Douglas, S. L. Prog. Energy Combust. Sci. 2000, 26 (4−6), 417−58. (37) De Soete, G. G. Proceedings of the 15th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1975; pp 1093−1102. (38) Xu, M.; Azevedo, J. L. T.; Carvalho, M. G. Fuel 2000, 79, 1611− 1619. (39) Levy, J. M.; Chen, L. K.; Sarofim, A. F.; Beer, J. M. Proceedings of the 18th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1981. (40) Mao, J. X.; Mao, J. Q.; Zhao, S. M. Clean combustion of coal. Press of Science: Beijing, China, 1998; p 217. (41) Winter, F.; Wartha, C.; Loffler, G.; Hofbauer, H. 26th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 3325−3334. (42) Visona, S. P.; Stanmore, B. R. Combust. Flame 1996, 106, 207− 218. (43) Eaton, A. M. Prog. Energy Combust. Sci. 1999, 25, 387−436. (44) Gera, D. Combust. Sci. Technol. 2010, 172 (1), 35−69. (45) Choi, C. R.; Kim, C. N. Fuel 2009, 88, 1720−1731.

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