Combustion of Predried Brown Coal in a Tangentially Fired Furnace

Article pubs.acs.org/EF

Combustion of Predried Brown Coal in a Tangentially Fired Furnace under Different Operating Conditions∥ Zhao F. Tian,† Peter J. Witt,*,‡ M. Philip Schwarz,‡ and William Yang§ †

School of Mechanical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia Mathematics, Informatics and Statistics, and §Process Science and Engineering, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Clayton South, Victoria 3169, Australia

‡

ABSTRACT: A major challenge in the use of brown coal in Victoria, Australia, for power generation is its high moisture content, which results in high greenhouse gas emissions. Predrying technologies for coal are one option being considered for use in browncoal-fired power plants in Victoria to reduce greenhouse gas emissions. Using a validated computational fluid dynamics (CFD) model, this study investigates the combustion of predried brown coal in a 375 MW tangentially fired furnace that was designed for raw or non-predried brown coal. Different operating arrangements of the fuel gas for the predried coal with various moisture contents are proposed and assessed. When predried brown coal is burned with 55% or lower moisture content, the CFD results indicate that additional gas needs to be added to the fuel gas to maintain the original mass flow rate and reduce the heat flux in the furnace. Arrangements with additional air or recirculated flue gas added to the fuel gas are proposed and tested for predried coal with 55, 45, 35, 25, and 15% moisture contents. The increase in the furnace temperature for the recirculated flue gas arrangement is smaller than that of the additional air arrangement, and stack loss is expected to be smaller than for additional air cases. The temperature distributions and wall heat fluxes in the boiler for the recirculated flue gas cases are similar to that of the raw coal combustion case. Therefore, use of recirculated flue gas is proposed as an option for future operation of the furnace with predried coal. Nevertheless, further studies are essential to understand the technical and financial feasibility of using flue gas to maintain the fuel gas flow rate.

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INTRODUCTION The state of Victoria, Australia, has one of the largest brown coal resources in the world. The main use of Victorian brown coal is the generation of electricity through direct combustion in power plants in the Latrobe Valley region, producing over 85% of the electricity supply for Victoria.1 It is expected that brown coal will continue to be the main energy source for Victoria in the foreseeable future.2 Nevertheless, one challenge in the continuing use of Victorian brown coal for power generation is the high greenhouse gas emissions partially because of its high moisture content, up to 70% by weight.3 To better understand the impact that high moisture content in Victorian brown coal has on greenhouse gas emissions, the heating, drying, and combustion processes of raw brown coal (wet coal as mined) in a brown-coal-fired boiler are shown in Figure 1. This type of tangentially fired boiler is widely used in Latrobe Valley power plants. In current operation, raw brown coal without any predrying step is fed into the drying shaft of the boiler system and then pulverized in the mill. In the drying shaft and mill, raw coal is heated and dried by hot furnace gas that is extracted from offtakes. Pulverized coal, fuel gas, and carrier gas enter the furnace through inert burners and main burners. Herein, fuel gas includes furnace gas, tramp air, and water vapor evaporated from the raw coal during the drying process. A large volume of water vapor (about 20% by weight of flue gas) passes through the boiler and is emitted through the stack. A large amount of latent heat, which is derived from high-grade heat in the furnace, is wasted with the water vapor emission. Furthermore, significant amounts of electrical power are required for mill and fan operation to sustain the high mass and volume flows of coal and flue gas because of the high © 2011 American Chemical Society

Figure 1. Heating and drying processes of raw brown coal in the tangentially fired boilers in Latrobe Valley.

moisture content. All of these factors contribute to lower boiler efficiency and, thus, a higher level of CO2 emission per unit of Received: September 30, 2011 Revised: December 13, 2011 Published: December 23, 2011

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dx.doi.org/10.1021/ef2014887 | Energy Fuels 2012, 26, 1044−1053

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Department of Primary Industries of Victoria commissioned CSIRO under its ETIS Programme to carry out a series of investigations using both CFD and experiments. CFD was selected as the tool to evaluate furnace performance because of its ability to provide detailed flow, temperatures, heat fluxes, and flame information for unfamiliar fuel types.19 Early work20 applying CFD to coal combustion in tangentially fired furnaces was limited to structured grids with about 20 000 cells; this substantially limited the resolution and accuracy of the models. Increased computing power enabled modeling of more complex furnaces,21 such as wall-mounted swirl burners,19 as well as predictions of NOx emissions and ash-related problems. For example, recent work22 used a CFD model to identify the optimal location where an additive could be injected into a corner fired furnace to reduce ash fouling. The present paper reports on the application of a previously validated CFD model3 for simulating combustion of predried coal in a furnace that was designed for raw brown coal. The main objectives of the study were to understand how operating conditions can be changed to maximize the level of predried coal usage without or with minimum modifications to the existing furnaces and boilers. Three operating arrangements of fuel gas in the mill system for predried coal combustion are proposed. Details of the operating arrangements are given in the next section. The effects of the operating arrangements and moisture contents of predried coal on the performance of the furnace, such as furnace gas temperature, incident wall heat flux on the water wall, furnace gas exit temperature (FGET), and flue gas components, are compared and analyzed on the basis of the CFD results.

energy generated than that of natural gas or high-rank blackcoal-fired plants. For example, as reported in a previous paper,3 the CO2 emission per unit of electricity produced by TRUenergy’s brown-coal-fired power plant in the Latrobe Valley is 1.42 tons of CO2 per MWh,4 58% higher than that of a typical Australian black-coal-fired power plant, about 0.9 tons of CO2 per MWh.5 Several predrying technologies, such as steam fluidized bed drying (SFBD) with waste heat utilization (WTA) (e.g., the systems reported in refs 6 and 7) or mechanical thermal expression (MTE8,9), are being considered for use by Latrobe Valley power plants to reduce the greenhouse gas emission of browncoal-fired generation. Kakaras et al.10 describe three brown coal predrying technologies, MTE, WTA7 (SFBD), and tubular dryers, and, on the basis of their simulations, identified MTE as providing the greatest increase in thermal efficiency. Note that these predrying processes are different from the drying process in the drying shaft and mill system. In predrying processes, raw coal is dried using more energy-efficient processes, such as using low-grade waste heat, before the coal is put into the boiler. This means that water removed from raw coal in the predrying process does not flow into the boiler system. Less high-grade heat from the furnace is required to heat and dry the predried coal in the drying shaft and mill system. The boiler efficiency is therefore improved, and the greenhouse gas emission is reduced correspondingly. Recent studies at Monash University estimate that these predrying technologies may reduce the CO2 emission by up to 30% for Victorian brown coal power plants.11,12 A similar study13 on a Greek 339 MW lignite-fired power plant calculated that a 7.8% reduction in specific CO2 emissions was possible when co-firing 30% WTA predried coal. Three important test cases of predried brown coal combustion in utility boilers have been reported recently. In the first case, a set of fluidized-bed coal dryers has been employed to predry the brown coal at a 576 MW boiler at the Coal Creek power plant in the U.S.A.14 The moisture content of the brown coal is reduced from about 36.8 to 28.6% using waste heat rejected from the boiler. The reduction of the flue gas flow rate, NOx and SOx emissions, and an increase in boiler efficiency were observed in the tests. Calculated CO2 mass emission rates for the test are approximately 3.8% lower than the wet coal combustion case. In the second case, co-firing of predried brown coal firing with raw brown coal in a 75 MW Greek boiler has been studied both experimentally and numerically.15 The experimental measurements were conducted in a boiler co-firing raw coal (58% moisture content) with a small fraction of predried coal (12% moisture content), 6% by thermal share. No clear effects of cofiring on the boiler operation and performance were observed because of the low predried coal shares. A computational fluid dynamics (CFD) model16 was used to simulate combustion of the raw coal with higher co-firing thermal shares up to 20%. The numerical results showed an increase in the furnace temperature, total heat flux on the water wall, and a clear increase in total fuel burnout when co-firing predried coal in higher shares.15 The third case is the RWE Niederaußem demonstration plant17,18 that uses WTA7 drying technology to produce 110 tons h−1 of dry lignite from 210 tons h−1 of raw coal. The plant has been in operation since early 2009, and on the basis of its performance, it is estimated that CO2 emissions from a 1000 MW power plant could be reduced by up to 1 million tons per year without a reduction in electrical power output. To better understand the future use of predried brown coal in the existing boilers in Latrobe Valley power plants, the

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CFD MODEL DESCRIPTION

A CFD model of unit No. 3 at TRUenergy’s Yallourn power plant has been developed based on the commercial CFD code, ANSYS CFX 12.0.26 Gas flow in the furnace is modeled by the Eulerian continuum approach, where transport equations for the gas are solved for mass conservation, momentum, enthalpy, and composition. To allow for the prediction of CO, the gas phase is composed of CO, CO2, N2, O2, H2O, and volatiles, with chemical reactions between these species modeled using the eddy dissipation model. Turbulence is modeled using the standard k−ε turbulence model. This was chosen after testing a number of turbulence models for coal combustion in a non-swirl burner,23 gas−particle flows24 through rectangular jets inclined to a cross-flow, and brown coal combustion in a furnace.25 The second case is representative of air and coal particle flows through the slot burners used in the tangentially fired furnace. A Lagrangian particle-tracking approach is used to predict the behavior of coal particles in the furnace. A total of 42 600 sample particles are tracked and interacted through two-way coupling to the gas phase, and a stochastic model is used to account for gas-phase turbulent effects on the particles. A single first-order coal combustion model23 is used to model coal devolatilization and char combustion. Radiation heat transfer is modeled using a discrete transfer model, with particle emissivity being a function of char burnout. The flow domain included in the model is from the ash hopper to past the economizer exit; to improve burner predictions, a short section of ducting upstream of the burners is also included. A previously reported3 mesh independence study showed that a mesh of 950 000 nodes constructed from hexahedral and tetrahedral elements was adequate to discretize the flow domain. Geometry for the convective pass is included in the model. However, it was not possible to resolve individual tubes in the tube bundles; therefore, the effect of these was included by sink terms for thermal energy and momentum. In our previous work,3,25 the CFD model used here was validated by comparing the FGET, concentration of flue gas components, total boiler heat supply, and wall incident heat fluxes to power plant 1045


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Table 1. Simulated Cases for Different Operation Conditions

a

case number

1

2

3

4

5

coal moisture content after predrying (%) additional gas added to the fuel gas

66a

60 n/a

55

55

45

7

8

9

10

12

13

35 25 air at 15 °C

6

15

55

45 35 25 flue gas at 200 °C

11

15

Raw brown coal without any predrying process.

measurements. Additionally, validation of submodels for gas−particle flows,24 coal and volatile combustion,23 and turbulence23,24 was performed on simpler flows, where more detailed measurements could be taken under controlled operating conditions. Readers requiring further information on the CFD model used in this work are referred to the detailed description of the model and current furnace operating conditions with wet brown coal that are given in our previous studies.3,25 Operating Conditions for Predried Brown Coal. This unit includes eight firing groups. At full load, six firing groups are in operation in the current plant and two firing groups are out-of-service with no coal flows. In the present modeling efforts, firing groups 2 and 6 are out-of-service. For the raw coal combustion (66% moisture content), the fuel gas flow in each mill is approximately 73 kg s−1. When predried brown coal is fed to the mills, less furnace gas is required from the offtakes than during raw coal operation because of the lower moisture content of the predried coal. Less water vapor is expected to be evaporated from the predried coal in the mill and duct system than that of the raw coal case. As a result, the total mass flow rate of fuel gas is expected to be reduced for predried coal cases. To convey pulverized particles to burners, it is necessary to maintain the original mass flow rates of fuel gas (i.e., 73 kg s−1). A lower fuel gas velocity may cause deposition of larger coal particles and, consequently, a blockage in the mill and duct system. Lower velocities may also reduce the burner jet penetration distance into the furnace, altering the combustion behavior. Three operating arrangements of fuel gas in the mill and duct systems for the predried coal with different moisture contents are proposed and investigated in the current study (Table 1). A base case (case 1 in Table 1) that combusts raw brown coal (66% moisture content) under current operating conditions in TRUenergy’s Yallourn power plant is used to compare to predried coal cases. For the first arrangement (cases 2 and 3 in Table 1), no additional gas is added to the mill system for predried coal cases. The mass flow rate of fuel gas with 60 and 55% moisture content predried coal are less than that of the raw coal case (66% moisture content), because of less furnace gas being required to dry the coal and less water vapor being evaporated from the coal. For the second arrangement (cases 4−8 in Table 1), additional air is added to the mill system to bring the mass flow rate of fuel gas up to 73 kg s−1. The temperature of additional air is 15 °C. For the third arrangement (cases 9−13 in Table 1), flue gas from the exit of the air heaters or electrostatic precipitators is recirculated and added to the mill system to maintain the same fuel gas mass flow rate of 73 kg s−1. The flue gas has a temperature of 200 °C. Combustion air mass flow rate and its distribution within the burner system are the same for the raw coal and the predried coal combustion cases. The combustion air flow for each in-operation firing group is 55 kg s−1; additionally, 20 kg s−1 of air flows through each of the out-ofservice firing groups. The total combustion air flow is 370 kg s−1 and, at the inlet to the furnace, has a temperature of 310 °C based on power plant data.3 Predried coal with different moisture contents, namely, 60, 55, 45, 35, 25, and 15% by weight after a predrying process, is considered in this study. As less heat is required in the mill and duct system (assuming the coal is predried separately) for drying and heating the predried coal than is required for wet coal; the total thermal input to the boiler system and predried coal flow rate are reduced accordingly (Figure 2). The aim is to maintain the same thermal and, hence, electrical output from the unit. The predried coal mass flow rate, ṁ pdc, before the mill system is calculated by the following equation:

Figure 2. Schematic model of the furnace and mill duct system for predried coal cases. where MCpdc is the moisture content (by weight) of predried coal, MCwetc is the moisture content of raw coal (66% by weight), ṁ wetc is the mass flow rate of wet coal (66% moisture content), with a value of 156.6 kg s−1, ṁ pdc is the mass flow rate of predried coal fed to mill systems, hw−v is the heat required to heat the water at 15 °C to vapor at fuel gas temperature in inlets (170 °C), and GDSE is the gross dry specific energy of the coal (26.2 MJ kg−1). Table 2 lists the calculated coal mass flow rates for different moisture contents after the predrying process. Note that these values are total mass flow rates for six in-operation firing groups.

Table 2. Coal Mass Flow Rates of Predried Coal to Mills moisture content after predrying (wt %) mass flow rate of predried coal before mill (kg s−1)

45

35

25

15

126.2

108.3

84.0

68.4

57.5

49.6

ṁ frg C frgTfrg + ṁ taC taTta + ṁ pdcC pdcTpdc + ṁ aaCaaTaa ≈ ṁ fueg C fuegTfueg + L w (MPfueg.vapṁ fueg − MPfrd.vapṁ frg ) + ṁ pfcC pfcTpfc

(2)

where ṁ is the mass flow rate, C is the specific heat, T is the temperature (K), and MP is the mass percentage of a gas component (%), with subscript frg denoting the furnace gas, subscript ta denoting the tramp air, subscript fueg denoting the fuel gas, subscript vap denoting the vapor, subscript aa denoting the additional air, subscript pdc denoting

(ṁ wetc MC wetc − ṁ pdc MCpdc)h w−v GDSE

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Additional Air Cases. The following heat balance equation is used to calculate the required mass flow rate of furnace gas extracted from the gas offtakes in the additional air cases:

ṁ pdc(1 − MCpdc) = ṁ wetc(1 − MC wetc) −

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(1) 1046


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Table 3. Calculated Mass Flow Rates of Furnace Gas and Additional Air or Flue Gas for Each Case moisture content after predrying or blending (wt %) case number total hot furnace gas required (kg s−1) total fuel gas flow rate (kg s−1) case number total hot furnace gas required (kg s−1) total additional air for fuel gas (kg s−1) case number total hot furnace gas required (kg s−1) total additional flue gas for fuel gas (kg s−1) a

66a

60

55

No Added Gas into Fuel Gas 1 2 3 258 210 166 438 370 312 Additional Air (15 °C) Added into Fuel Gas 4 174 114 Flue Gas (200 °C) Added into Fuel Gas 9 162 130

45

35

25

15

5 141 173

6 145 169

7 151 164

8 154 160

10 108 206

11 71 257

12 44 293

13 25 319

Wet brown coal without any predrying process.

Figure 3. Predicted wall incident heat flux: (a) case 1, (b) case 2, and (c) case 3. the predried coal, and subscript pfc denoting the pulverized coal. Lw is the latent heat. Heat loss in the drying shaft and mill system is small and neglected. The energy associated with mill work is also neglected. An iterative method is used for the calculation of fuel gas composition, furnace gas temperature, and mass flow rate: (1) Guess a furnace

gas temperature Tfrg and compositions, i.e., MPfrg.CO2, MPfrg.O2, MPfrg.N2, and MPfrg.vap. Here, the values for the wet coal combustion case are used as an initial guess. (2) Cfrg can be determined from the initial furnace gas temperature and composition. (3) Equation 2 and other mass conservation equations are used to calculate the fuel gas 1047


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Figure 4. Predicted gas temperature on a vertical midplane: (a) case 1, (b) case 2, and (c) case 3.

Table 4. Summarized Results for Different Operation Conditions with No Additional Combustion Gas case number

FGET (°C)

total heat flux on furnace wall (MW)

maximum temperature in furnace (°C)

fuel gas velocity (m s−1)

1 2 3

1069 1074 1116

235 250 270

1514 1567 1644

19.2 16.6 14.5

MPfueg.CO2, MPfueg.O2, MPfueg.N2, and MPfueg.vap, looping back to step 3 until satisfactory convergence is achieved. Here, the convergence criteria are 5 °C for the furnace temperature and 1% for specific heat. Recirculated Flue Gas Cases. A similar approach is used for the recirculated flue gas cases. Here, the heat balance equation is

ṁ frg C frgTfrg + ṁ ta C taTta + ṁ pdcC pdcTpdc + ṁ af Caf Taf ≈ ṁ fueg C fuegTfueg + L w (MPfueg.vapṁ fueg − MPfrd.vapṁ frg − MPaf.vapṁ af ) + ṁ pfcC pfcTpfc

compositions MPfueg.CO2, MPfueg.O2, MPfueg.N2, and MPfueg.vap. These values are used as inlet boundary conditions in the CFD model. (4) Run the CFD model and obtain predicted results. (5) Compare the new CFD values of the furnace gas temperature Tfrg and compositions MPfrg.CO2, MPfrg.O2, MPfrg.N2, and MPfrg.vap to the original estimate and, if there is a substantial difference, use the new values to recalculate

(3)

where subscript af denotes additional flue gas. Again, the iterative method is used to calculate values of furnace gas temperature Tfrg and compositions MPfrg.CO2, MPfrg.O2, MPfrg.N2, and MPfrg.vap. The convergence criteria are 5 °C for the furnace temperature and 1% for specific heat.

Table 5. Summarized Results with Additional Air or Flue Gas for Different Amounts of Coal Drying moisture content after predrying or blending (wt %)

66a

55

45

case number FGET (°C) flue gas temperature (°C) flue gas O2 (wt %) flue gas H2O (wt %) flue gas CO2 (wt %) highest temperature in furnace (°C)

1a 1069 373 3.8 19.4 19.4 1514

Additional Air (15 °C) Added into Fuel Gas 4 5 1026 1010 322 305 7.6 9.6 11.4 7.7 16.3 14.8 1651 1751

case number FGET (°C) flue gas temperature (°C) flue gas O2 (wt %) flue gas H2O (wt %) flue gas CO2 (wt %) highest temperature in furnace (°C)

1a 1069 373 3.8 19.4 19.4 1514

Flue Gas (200 °C) Added into Fuel Gas 9 10 1010 999 350 359 4.9 5.45 13.3 9.9 19.0 19.0 1535 1541

a

35

25

15

6 989 276 10 7.2 14.3 1759

7 975 254 10.3 7.0 13.9 1766

8 964 237 10.6 6.8 13.7 1765

11 989 361 6.0 7.5 19.0 1560

12 981 358 6.6 5.9 19.0 1580

13 975 354 7.0 4.6 19.0 1603

Raw brown coal without any predrying process. 1048


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Figure 5. Predicted gas temperature at the upper main burner plane: (a) case 1, (b) case 9, (c) case 4, (d) case 10, (e) case 5, (f) case 11, (g) case 6, (h) case 12, (i) case 7, (j) case 13, and (k) case 8. This iterative method is found to be stable and generally converged in four or five iterations. Table 3 gives the calculated mass flow rates of furnace gas and additional air or flue gas for each of the different cases.

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increase in the gas temperature in the area near the burner outlets that could affect the burners. Table 4 summarizes the predicted FGET, the integrated incident heat flux to furnace walls, and the maximum temperature in the furnace for cases 1−3. FGET is the average gas temperature at the midplane of the gas offtakes (shown in Figure 4a), which is located just below the first superheater tube bank. Generally, the values of FGET, the total heat flux to furnace walls, and the maximum temperature in the furnace increase when the moisture content decreases. This is consistent with the numerical prediction by Agraniotis et al.15 The higher FGET and wall heat flux of predried coal cases could be attributed to the faster ignition of coal particles, higher particle temperature, and longer residence time of coal particles. Binner et al.11 showed that drying the brown coal to equilibrium moisture content (10−15%) and removing the coal moisture prior to combustion would cause ignition to be faster, particle temperature to be higher, and burnout to be faster. The predicted FGET of case 3 with 55% moisture content is 1106 °C, which is 56 °C higher than the designed FGET of 1050 °C, for the moderate fouling Yallourn coal fired in the boiler. The high FGET may cause severe fouling problems in the convective

RESULTS AND DISCUSSION

Predried Coal Combustion without Additional Air/ Recirculated Flue Gas. In this study, combustion of predried coal with moisture contents of 60 and 55% without any additional air or recirculated flue gas added to the fuel gas was investigated and results are compared to raw coal combustion (66% moisture content). Figure 3 compares contour plots of the incident heat flux on furnace walls for cases 1−3. As shown in panels a and b of Figure 3, when the moisture content of the fuel decreases from 66 to 60%, a slightly higher level of incident radiation is found on the region between two main burners on the north wall and south wall. The area of high heat incident zone increases further when the moisture content of fuel is decreased to 55%. Plotted in Figure 4 is the predicted temperature on a central vertical plane in the boiler for cases 1−3. From these temperature plots, reducing the moisture results in areas where the gas temperature exceeds 1500 °C. Also evident in Figure 4 is an 1049


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Figure 6. Predicted gas temperature at the vertical midplane: (a) case 9, (b) case 4, (c) case 11, (d) case 6, (e) case 13, and (f) case 8.

parts, especially the superheater tubes and reheater tubes immediately downstream of the furnace exit. The high incident heat flux on the wall shown in Figure 3c may cause fouling problems to occur on the water walls of the furnace during long periods of operation. Furthermore, it is necessary to maintain the original mass flow rates of fuel gas to convey pulverized particles to burners. As shown in Table 4, the fuel gas velocity of case 3 is 14.5 m s−1 and is much lower than in the current operation of case 1. Such low fuel gas velocity may cause deposition of larger coal particles in the ducting system. Results in this section show that heat transfer to water walls in the furnace changes for different cases. Total heat flux to water walls increases 6.3% for case 2 and 15% for case 3. The distribution of heat transfer in convective parts, such as superheaters, reheaters, and economizer, is therefore expected to change accordingly. Corresponding changes of operation and modifications of the boiler system are then likely to be required. For the predried coal combustion cases in the current study,

heat transfer in the convective passes is unknown and has been assumed to be the same as that of the raw coal case, because the main focus of the current study is on coal combustion and heat transfer in the furnace. Nevertheless, as pointed out by Park et al.,27 CFD techniques alone cannot capture the steam water side boiler performance, such as thermal efficiency, and steam pressures and temperatures that vary with changes to the operating conditions. To remedy this difficulty, Park et al.27 combined a CFD model of a furnace with a one-dimensional (1D) steam water side model to simulate the effects of different operating conditions on boiler efficiency, pollutant formation, and combustion efficiency. There is a need to combine the CFD furnace model and a boiler side simulator to successfully study the redistribution of heat transfer within the boiler and total boiler efficiency. This approach should be considered in future work. Predried Coal Combustion with Additional Air/ Recirculated Flue Gas. In Latrobe Valley power plants, brown-coal-fired boilers were designed with a large heat-transfer 1050


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Figure 7. Predicted wall incident heat flux: (a) case 9, (b) case 4, (c) case 11, (d) case 6, (e) case 13, and (f) case 8.

surface area to extract useful energy, because the high moisture content of raw brown coals leads to low flame and low flue gas temperatures.28 Brown-coal-fired boilers designed for raw brown coal are normally about twice the size of black-coal-fired boilers of similar power output.28 For predried brown coal, to maintain similar operation to the current furnace operation, it is important to maintain sufficient flow and thermal content in the furnace gas from the furnace, so that the convection section can operate as designed.29 Moreover, as mentioned in the previous

section, it is necessary to maintain the original mass flow rates of fuel gas to convey pulverized particles to burners. Therefore, when the moisture content of predried brown coal is 55% or less, additional gas needs to be added to the fuel gas to maintain the original mass flow rate and reduce heat flux to furnace walls. Two arrangements with either additional air or recirculated flue gas added to the fuel gas are tested for predried coal with 55, 45, 35, 25, and 15% moisture contents. 1051


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coal is milled at a low temperature before drying in a fluidized bed and then directly injected through dedicated burners. The above problems of adding air to the fuel gas can be overcome by recirculating flue gas into the fuel gas. Because the flue gas temperature is high (about 200 °C), stack loss is expected to be smaller than for additional air. The increase in the furnace temperature for the recirculated flue gas arrangement is smaller than that of the additional air cases. Finally, the temperature distributions and wall heat fluxes in the boiler for the recirculated flue gas cases are similar to that of the raw coal combustion case. Therefore, the arrangement of recirculated flue gas may be the preferred option for power plants operating with predried brown coal. Further studies are essential to understand the technical and financial feasibility of using flue gas to maintain the fuel gas flow rate. As showed in Table 3, a large amount of flue gas is required when the moisture content of predried coal is 45% or less. The capital cost, operating cost, space constraint, and safety issues still need to be carefully studied.

Gas temperatures predicted by the model for each of the base case and cases 4−13 on a horizontal plane through the upper main burner are plotted in Figure 5. For recirculated flue gas cases, the high-temperature zones (dark red zones in Figure 5) increase slightly in area as drier coal is used. For additional air cases, the high-temperature zone increases in area as the moisture content of the coal is decreased from 66% (Figure 5a) to 35% (Figure 5g). This is because, as the coal moisture content decreases, less furnace gas is required to heat and dry the coal in the mill system. More additional air is required to maintain the mass flow rate of fuel gas. As shown in Table 3, about 173 kg s−1 of additional air is required for the 45% moisture content case (case 5) and 169 kg s−1 of additional air for the 35% moisture content case (case 6), much higher than 114 kg s−1 of additional air for the case of 55% moisture content (case 4). With higher additional air in the fuel gas, higher oxygen content in the fuel gas increases mixing of volatiles and char with oxygen, leading to faster ignition and higher temperatures in the furnace. Higher temperatures are especially apparent near the burner exit area of case 5 (Figure 5e) and case 6 (Figure 5g) but less so for case 4 (Figure 5c). This is confirmed by the highest furnace temperature for additional air cases shown in Table 5. The highest furnace temperatures are predicted for additional air cases with 15−45% moisture content and are nearly 1770 °C. For cases with recirculated flue gas (Table 5), the highest furnace temperatures for cases with 15−45% moisture contents are mostly lower than 1600 °C, because there is less oxygen content in the fuel gas and, thus, a lower mixing rate for fuel and oxygen in the area of the burner exit. Figure 6 shows the predicted temperature profiles on a central vertical plane in the boiler for selected cases. It shows that most combustion occurs in the main burner area. Higher temperatures are found in the furnace for additional air cases than recirculated flue gas cases. It is worth noting that flue gas temperatures after the economizer for additional air cases are much lower than those of recirculated flue gas cases. This is due to the predried coal combusting faster in the furnace for the additional air cases because of better mixing with oxygen than in the flue gas cases and, hence, releases more heat in the furnace. Heat flux on the furnace walls is plotted in Figure 7 and also shows much higher heat flux for the additional air cases than that of recirculated flue gas cases. These higher heat fluxes may increase fouling on the furnace walls. For the recirculated flue gas cases, the peak heat fluxes are slightly lower than in the raw coal case but there is an increase in heat flux in the area between the lower burners. Also, as shown in Figure 5, combustion occurs earlier in the burner cavity and heat fluxes in the burner cavity are also higher than for the raw coal case. Generally, for the additional air arrangement, the flame temperature in the furnace is much higher, approaching 1770 °C, than the recirculated flue gas cases and raw coal case, which are typically 1515 °C. The high furnace temperature may result in severe slagging problems on the water walls. Furthermore, a large amount of additional air at ambient temperature could slightly reduce the total boiler efficiency because stack losses may be increased. Therefore, extra preheating systems for the additional air may be required. Finally, adding air in the drying shaft may be dangerous because it will increase the oxygen content in the fuel gas and may lead to early reaction or combustion of fuel in the dying shaft and mill, causing overheating of the mill system. To avoid mixing air and dried coal, an alternative process has been developed by RWE,7 where

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CONCLUSION AND FUTURE WORK

A CFD model of a 375 MW tangentially fired furnace at TRUenergy’s Yallourn power plant has been used to investigate predried brown coal combustion in a furnace that is designed for raw brown coal. The effects of different arrangements for fuel gas in the mill system and coal moisture contents on the flame and heat transfer in the furnace are studied. It is found that, when predried coal is burned, fuel gas in the mill and duct system is reduced as less furnace gas is extracted and less water vapor is evaporated from the coal particles. This may lead to large particle deposition and blockage in the mill and duct system. The FGET is increased significantly when burning predried coal with 55% or less moisture content. This may cause severe fouling problems in the convective parts. The high incident heat flux on the walls may also significantly increase fouling problems on the water walls of the furnace. It is recommended that additional gas should be added to fuel gas in the mill system to overcome these problems when the moisture content of the predried coal is 55% or less. Arrangements with additional air or recirculated flue gas added to the fuel gas are proposed and tested for predried coal with 55% or less moisture content. It is found that additional air arrangements have a high flame temperature and are likely to have lower boiler efficiency. Also adding air in the drying shaft may present safety issues for the mill system. For the recirculated flue gas arrangement, the increase in the furnace temperature is smaller than that of the additional air arrangement and stack loss is expected to be smaller than for additional air cases. The temperature distributions and wall heat fluxes in the boiler for the recirculated flue gas cases are similar to those of the raw coal combustion case. Therefore, the arrangement of recirculated flue gas is the preferred option for the power plant for future operation of the furnace with predried coal. Further studies are required to understand the technical and financial feasibility of using flue gas to maintain the fuel gas flow rate. Finally, it is recommended that further work be undertaken to combine the CFD furnace model and a boiler side simulator to successfully study the redistribution of heat transfer within the boiler and total boiler efficiency. 1052


Energy & Fuels

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Article

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +61-3-95458500. Fax: +61-3-95628919. E-mail: [email protected]. ∥

Presented at the 8th APCSEET held in Adelaide, Australia, July 10−13, 2011, and organized by the Centre for Energy Technology at University of Adelaide.

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ACKNOWLEDGMENTS The financial support provided by the Victorian Government Department of Primary Industries under the ETIS Program is gratefully acknowledged. The authors also thank Steve Pascoe and Yorrick Nicholson of TRUenergy’s Yallourn power plant for providing plant data for the CFD simulation and discussion about the predicted results.

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REFERENCES

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Combustion of Predried Brown Coal in a Tangentially Fired Furnace

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