This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Fuel 107 (2013) Contents lists available at SciVerse ScienceDirect Fuel journal homepage: Numerical simulation of brown coal combustion in a 550 MW tangentially-fired furnace under different operating conditions Audai Hussein Al-Abbas a, Jamal Naser b,, Emad Kamil Hussein a a Foundation of Technical Education, Al-Musaib Technical College, Babylon, Iraq b Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia highlights " CFD modeling study was performed for the combustion of the brown coal in a large-scale tangentially-fired furnace. " Performance of the boiler under ten different operating conditions was investigated. " The temperature distributions were better when the turned off burners are set in the opposite direction. " The results showed improvements on the combustion characteristics in comparison with the standard operating case. article info abstract Article history: Received 9 September 2012 Received in revised form 13 November 2012 Accepted 14 November 2012 Available online 8 December 2012 Keywords: Coal combustion Victorian brown coal Emissions CFD In the present paper, a computational fluid dynamics (CFD) modeling study was performed for the combustion of the brown coal in a large-scale tangentially-fired furnace (550 MW) under different operating conditions. The AVL Fire CFD code has been used to model the combustion processes. The mathematical models of coal combustion with the appropriate kinetic parameters were written and incorporated to the code as user defined functions. These models consist of pulverised coal (PC) devolatilization, char burnout, and heat and mass transfer. The simulation of the PC combustion was carried out using multi-step reaction chemistry mechanisms. The level of confidence of this numerical model was based on the previous validations of the lignite combustion in a lab-scale furnace, as well as the validation parameters of the present furnace at the standard existing conditions in terms of temperature values and species concentrations. Performance of the boiler under ten different operating conditions was investigated. The strategy of operation schemes for the first six combustion scenarios were based on the change of the out-of-service (turned off) burners under full load operation, while the rest cases were carried out at 20% lower and 20% higher loads than the standard operating conditions. The validated model was used to perform the following investigation parameters: furnace gas temperatures, species concentrations (O 2, CO and CO 2 ), velocity distributions, and char consumption. The predictions demonstrated that there are good temperature distributions in the furnace when the turned off burners are set in the opposite direction under full load operation. For higher aerodynamic effect, the numerical results showed improvements on the combustion characteristics in terms of species concentrations and char burnout rates in comparison with the standard operating case. The findings of this study provide good information to optimize the operations of the utility tangentially coal-fired boiler with less emission. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The brown coal combustion in the tangentially-fired power plants is the main source of electricity production in Victoria/Australia [1 4]. Although the abundance, low-cost, high reactivity, and low sulfur content of the brown coal [5] the major disadvantages of this source of energy are its contribution to the greenhouse gases Corresponding author. Tel.: address: Jnaser@swin.edu.au (J. Naser). (GHGs) emissions and a high moisture content which is equal to about wt% [6]. However, In order to obey the new environmental and political legislation against global warming, it is necessary to find out a cost-effective solution to cut pollution. The understanding of the brown coal reactivity and behavior under different operating conditions is required to design clean and efficient brown coal combustion systems. Computational fluid dynamics (CFD) modeling studies can comprehensively provide a wide range of information for optimizing and improving the combustion characteristics and boiler performance that can reduce the cost of time-consuming experimental tests. The flame structure, /$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.

3 A.H. Al-Abbas et al. / Fuel 107 (2013) temperatures distributions, chemical species concentrations, nitric oxides, radiative heat flux, etc., under different firing conditions can be carried out using the appropriate CFD code [7 10]. The work programme covered by this paper achieves the development and validation of the computational tool applied to the combustion system which indicates the practical relevance and the long term viability for such a tool. These developments allowed the complete simulation of commercial large-scale power plants as an effective alternative to expensive experimental tests. In general, after the devolatilization process of the coal particles finishes, the char combustion subsequently starts in the firing system. This combustion process has been considered the dominant factor constraining several reaction parameters such as the total burnout time, unburned carbon level, and radiation from burning char particles [11 16]. A better understanding of the effects of these parameters on the combustion characteristics and boiler heat transfer under different operating conditions enables engineers to optimize the applications of both the existing and new coal power plants. Ash deposition and slag formation from burning pulverized coal particles are significant parameters in the thermal boiler design, particularly on the heat transfer surfaces such as radiation zone (water wall furnace) and convection zone (superheater, reheater, and economizer) [8]. Therefore, it is necessary to understand how these parameters vary when the rates of pulverised coal (PC) and feed oxidizer gases are changed in the furnace inlets compared to the existing operating conditions. However, the full knowledge of the effects of these intrinsic parameters on the combustion characteristics and power plant efficiency is still not completely available. Tian et al. [17] developed a CFD model to investigate the coal combustion in a 375 MW tangentially-fired utility boiler, using ANSYS CFX 12.0 code. Due to the CFD code limitations regarding the multi-step reaction of hydrocarbon released, the authors utilized carbon monoxide reaction with oxygen to improve the model predictions. After validating the CFD model with the measured data obtained from the Yallourn power plant, then they used the validated model to examine the flame shape, temperature distribution, and wall incident heat flux. The numerical investigations were done under different combustion air distribution scenarios and at different out-of-service firing groups. The predictions showed that there is a significant difference in the combustion characteristics in the furnace, and these differences completely dependent upon which burner groups are turned off under full load operation. A validated 3D numerical model of the lignite combustion in a 300 MW large-scale utility boiler under the standard firing conditions was used by Karampinis et al. [18] to perform a numerical investigation for lignite coal and biomass particles at several co-firing scenarios. The numerical findings showed that the burnout rates of lignite char coal decreased in the hopper under co-firing conditions. While for biomass fuel, the small diameter of particles resulted in an increase in the char burnout for all burner levels used. The authors concluded on two main concepts for the potential operation schemes. The first was to mix the biomass particles with the lignite stream to ensure a better biomass size reduction. And the second concept was dependent on the fact that the injection system of the biomass particles should be placed between the lower and upper main burner to optimize the char burnout in the boiler. Up to date, in the field of numerical simulation on the commercial large-scale facility firing unit, there has been unfortunately little research work conducted on the brown coal combustion. Therefore, the objective of this study is to simulate the brown coal combustion in a large-scale tangentially-fired furnace under several operating conditions. A computational fluid dynamics (CFD) code, AVL Fire version , was used to model and analyze ten different combustion environments. The investigated combustion cases were dependent on the scenarios of change in the mass flow rates and distribution ratios for the PC, mill gases and air (i.e. 20% lower and 20% higher than the standard combustion conditions). In addition, the operation schemes of turned off (out-of-service) burners were also examined under full load operation. The species concentrations (O 2,CO 2, and CO), furnace gas temperature distributions, velocity fields, and char consumption obtained for all combustion cases were compared. The combustion characteristics and boiler performance are considerably depended upon the turbulent mixing conditions, residence times of combustibles, and aerodynamic effects used in this numerical study. 2. Geometry and operating conditions of the boiler Loy Yang A power plant located in the state of Victoria in Australia was selected to use in the present numerical investigations. The present tangentially-fired furnace is composed of eight mill-duct systems, which include six separate burners for each duct system. On each side of the furnace there are two firing groups, including three inert burners in the upper location and three main burners in the lower location of the furnace zone, as well as many secondary air ducts for each group. The top bottom sequence arrangement of each firing group on the furnace wall can be found in the previous numerical studies [19,20]. In the convection zone, there are eight sources of heat sink located above the hot gas off take (HGOT) pipes. The arrangement of the convection tubes in the boiler are economiser, reheater 1a, superheater 2, reheater 1b, superheater 3, reheater 2, superheater 4, superheater 1, and their heat absorption values are 100.2, , , , , 98.48, 52.27, MW, respectively. The heat absorption value of water wall is MW, and its location is beneath HGOT pipes. The effect of these heat sink sources are considered and taken into account of the calculation as a cell selection in the computational domain. Around 430 kg/s of steam flow is produced in this power unit under full load operation at 16.8 MPa and 540 C. The mesh generation with the geometric description of the CFD model used for the boiler is shown in Fig. 1a. The geometric dimensions of the furnace are taken from the drawings of the power plant, and CAD tool is employed to construct the 3D geometric model. The dimensions of the simulated furnace were m (height), m (width), and m (depth). Under the standard operating conditions, the mills 1, 2, 5, 6, and 8 are in service, while the rest mills (3, 4, and 7) are out-of-service, which presented for the combustion case 1, as illustrated in Table 1. Table 1 presents the operation scheme of turned off (out-of-service) burners under full load operation (cases 1 6) and 20% lower (cases 7 and 8) and 20% higher (cases 9 and 10) than the standard operating conditions. Around 81.3 kg/s of PC and kg/s of gas mixture are passed through both the inert and main burners of the furnace at different flow distribution ratios under the standard operating conditions (as specified in cases 1 6). More detailed information about the validated results of the CFD model against the power plant data and boundary conditions used in the standard combustion conditions were given in the recent published paper of Al-Abbas et al. [20]. Table 2 shows the mass flow rates (kg/s) of air for the standard firing case and investigated combustion cases at each secondary air duct. The overall number of vapor and PC burners was 48, while 18 of the total burners were practically out of service for all combustion cases examined. Regarding the burner configuration, each burner ports 1, 3, 5, and 7 were inclined by 24 with the perpendicular line to the furnace face, while each the remaining burner ports 2, 4, 6, and 8 were inclined by 30. In the most types of the tangentiallyfired furnaces/boilers, this configuration of the burner set up was typically used in order to improve flame stability inside the

4 690 A.H. Al-Abbas et al. / Fuel 107 (2013) Mathematical models and numerical description Fig. 1a. CFD geometrical model of unit 1 at Loy Yang A power plant. furnace, as schematically plotted in Fig. 1b. The mass flow rates of both the PC and mill gases (recycled flue gases) through the main and inert burners for the standard, 20% lower, and 20% higher combustion scenarios are presented in Table 3. Numerical modeling of the pulverized Victorian brown coal under different combustion conditions were carried out using a computational fluid dynamics (CFD), AVL Fire code [21]. The CFD code was used to solve the Eulerian partial differential equations (PDEs) [22] for mass, momentum, enthalpy, a number of species mass fractions, and turbulent fields in the gas phase flow. The Discrete Droplet Method (DDM) [23] was employed solving ordinary differential equations for the solid phase flow. The subroutines required for the devolatilization and char burnout, trajectories, convection and radiation heat transfer between particles and gases were written and incorporated into the CFD code as sub-models of source terms of PDEs. The standard k e model is used modeling the turbulent flow calculation. This model has been demonstrated a sufficient accuracy in the near-burner region of previous modeling simulation of lignite coal combustion [24]. The mixing rate of species is controlled using the eddy-breakup (EBU) turbulent combustion model in this coal reaction process. The proximate and ultimate properties of the brown coal particle are listed in Table 4. Regarding the heat transfer models, the discrete transfer radiation method (DTRM) has been used because of its ability for a better prediction in participating media, especially in furnaces. The chemistry reaction mechanisms for both the volatile matters and char coal are conducted. In this numerical study, methane is considered as a devolatilized hydrocarbon fuel because there is no essential differences can be distinguished between methane and the gases produced from devolatilization processes of the coal particles. However, the multi-step reaction mechanisms are carried out in this numerical calculation. The main reactions of the coal combustion model can be, however, expressed in three homogeneous and three heterogeneous chemical reactions, as follows: The chemical equations for devolatilized methane burned with oxygen, in a three-step reaction, are given in below: CH 4 þ O 2! CO þ H 2 þ H 2 O þ heat CO þ H 2 O CO 2 þ H 2 O 2 þ 2H 2 2H 2 O ð1þ ð2þ ð3þ Table 1 The operation scheme of turned off (out-of-service) burners under full load operation (cases 1 6) and 20% lower (cases 7 and 8) and 20% higher (cases 9 and 10) than the standard operating conditions. Combustion case no Burners turned off 3, 4, and 7 3,4, and 6 3, 4, and 8 1, 3, and 4 3, 5, and 7 2, 4, and 6 3, 4, and 7 3, 5, and 7 3, 4, and 7 3, 5, and 7 Combustion scenarios Standard operating conditions Lower by 20% Higher by 20% Table 2 The mass flow rates (kg/s) of air for the standard case and investigated combustion scenarios at each secondary air duct. Secondary air duct Distribution ratio (%) Air standard 20% Lower 20% Higher Mass flow (kg/s) Mass flow (kg/s) Mass flow (kg/s) Upper inert Upper intermediate inert Lower intermediate inert Lower inert Upper main Upper core Intermediate main Intermediate core Lower main Lower core Total

5 A.H. Al-Abbas et al. / Fuel 107 (2013) Fig. 1b. The schematic representation of the burners configurations. Table 3 Mass flow rates of both the PC and mill gases through the main and inert burners for the standard, 20% lower, and 20% higher combustion scenarios. Burner duct Mass flow rates (kg/s) of PC for combustion cases Mass flow rates (kg/s) of mill gases for combustion cases Standard 20% Lower 20% Higher Standard 20% Lower 20% Higher Upper inert Intermediate inert Lower inert Upper main Intermediate main Lower main Total While the chemical equations for char burned with oxygen, carbon dioxide, and water vapor are written as follows: C char þ 1=2O 2! CO þ heat C char þ CO 2! 2CO C char þ H 2 O! CO þ H 2 The heats of combustion of the above-mentioned chemical equations for methane and residual char can be found elsewhere [22]. This three-step chemical reaction scheme is used in this study based on the good prediction results against the measured data of the previous simulation studies of the coal combustion [20,22,25]. Regarding the numerical description, the solutions of the mathematical equations of the heat transfer, coal combustion, and turbulence were conducted under transient mode. The numerical results were run up to 48,000 time-steps, averaging the results over the final 8000 time-steps which were reached to the stable quasi steady state. The convergence limitation for all variables was attained with less than The standard SIMPLE algorithm ð4þ ð5þ ð6þ was employed solving the combination between the velocity and pressure in the Navier stokes equations. For the gas solid two phase flow, a Lagrangian/Eulerian approach was utilized. Around 50,000 particles were used in the present large-scale furnace simulation. Further detailed information in regards to the mathematical models and numerical description used in this study can be found elsewhere [20,22,24,25]. 4. Results and discussion 4.1. Temperature distribution Fig. 2 presents the effects of different operating conditions on the flame shape in the combustion zone at the 1650 K iso-surfaces for all combustion cases examined. The effects of different turned off burners operations under full load are presented in panels (a f), while in panels (g j) the effects of mass flow rates and distribution ratios for the brown coal particles and mill gases are investigated. That was done by using the validated CFD model [20] under two different combustion scenarios: 20% lower (cases

6 692 A.H. Al-Abbas et al. / Fuel 107 (2013) Table 4 The proximate and ultimate properties of the brown coal particle. Proximate analysis (%wt, db) Ultimate analysis (%wt, db) Specific energy (MJ/ kg) Moisture content (M.C.) (%wt, ar) Fixed carbon Ash content Volatile content C H N S Minerals and inorgs O Gross dry Net wet Fig. 2. Iso-surfaces of the combustion flames at 1650 K: (a) case 1, (b) case 2, (c) case 3, (d) case 4, (e) case 5, (f) case 6, (g) case 7, (h) case 8, ((i) case 9, (j) case and 8) and 20% higher (cases 9 and 10) than the standard combustion case, as shown in Tables 1 and 2. This iso-surface temperature can clearly show the flame distributions in the furnace and determines, as a result, the boiler performance under different combustion conditions. The operation scheme of the combustion cases used in this study was basically dependent on the inclined angles of each burner ports, as schematically plotted in Fig. 1b. However, this strategy of testing is used to improve the turbulent mixing of PC with the feed oxidizer gases in the tangentially-fired furnace which essentially dependents on the location of the central vortex in this type of furnaces. This improvement on the combustion conditions can bring several benefits to the combustion characteristics and reduces the fouling and slagging problems on the surfaces of the heat exchanges. Regarding the cases investigated under the strategy of turned off burners (cases 1 6 as specified in Table 1), it can be seen that the flame temperature distribution of case 2 showed a clear tendency toward the out-of-service burners (eastern water wall side in the furnace). This feature might be caused due to the movement of the central vortex toward the turned off burners. A good distribution of the flame was basically achieved in cases 3, 5 and 6 because of turning off the opposite burners. In panels (g) and (h), the flames are concentrated in the near-burner region due to the reduction in the mass flow rates of combustibles. Whereas the combustion flames are uniformly distributed in the burner and water wall regions for both cases 9 and 10 due to the higher aerodynamic effects of the PC, mill gases, and air adopted in these two cases. Fig. 3 presents the temperature distributions at the cross-section cut (X Y plane) of the lower intermediate main (LIM) burner for six combustion cases (cases 1 6 as specified in Table 1). In these combustion cases, the central vortexes of the flames are highly affected by the turned off burners adopted in this study. In cases 1 4, there was movement toward the adjacent turned off burners, this can be clearly seen in the next subsection of the velocity distribution. This may lead to a significant effect on the temperature field symmetry and, as a result, on the heat transfer distribution in the furnace. In contrast, in cases 5 and 6, the central vortexes are approximately concentrated in the central point of the

7 A.H. Al-Abbas et al. / Fuel 107 (2013) Fig. 3. Temperature distributions on the lower intermediate main burner at the cross-section cuts (X Y plane) for six combustion cases: (a) case 1, (b) case 2, (c) case 3, (d) case 4, (e) case 5, (f) case 6. Fig. 4a. Temperature distributions for combustion cases (1, 7, and 9) along the central line of the furnace in the burners and water wall regions. Fig. 4b. Temperature distributions for combustion cases (5, 8, and 10) along the central line of the furnace in the burners and water wall regions. furnace. This can lead to a good turbulent mixing between the PC and feed oxidizer gases and also helps to improve the stabilization of the flame position. Therefore, it can be concluded that the predicted results showed that the turned off burners in the opposite direction are better than those in the adjacent direction under full load operation. The influence of changing the mass flow rates of pulverized coal particles and feed oxidizer gases (recycled flue gases and air) on the temperature distribution along the Z-axis (X and Y = 0.0) of the furnace in the regions of burners and water tube wall is shown in Figs. 4a and 4b, compared with the standard combustion conditions. In panel a of Figs. 4a and 4b, the numerical results presented for combustion cases 1, 7, and 9, where the burner numbers 3, 4, and 7 were out-of-service (see Table 1). While in panel b of the same figure, the predictions were for combustion cases 5, 8, and 10, where the burner numbers 3, 5, and 7 were out-of-service.

8 694 A.H. Al-Abbas et al. / Fuel 107 (2013) Fig. 5. Gas velocity vector on the upper intermediate inert of the secondary air duct for six combustion cases: (a) case 1, (b) case 2, (c) case 3, (d) case 4, (e) case 5, (f) case 6. The predictions showed that the effects of combustibles (fuel/ gases) ratios, decreasing or increasing, have similar trends on temperature distributions with the standard operating conditions. In general, in both panels a and b, there were two maximum temperature values in the main burner and inert burner regions. This is because of the higher amounts of coal particles and oxygen available in these regions of the furnace. Based on the fuel/gases ratio adopted in this study, the combustion scenario (cases 9 and 10) with 20% higher than the standard operating conditions showed approximately the same temperature values in the burner region (Z = m). But, there was a slight decrease in the temperature profiles of the last combustion cases in the water tube wall region. This can be explained due to the fact of the heat absorption by the water wall (sink source), as well as the additional cooling from injecting extra cold air in the secondary air ducts. On the other hand, in cases 7 and 8, the temperature profiles were lower than the standard combustion conditions due to the reduction in the aerodynamic flows in the combustion zone. It is interesting to note that the average values of gas temperatures, in Figs. 4a and 4b, were , , and K for combustion cases 1, 7, and 9, respectively and , , and K for combustion cases 5, 8, and 10, respectively. Compared to cases 7 and 8, the average gas temperatures of combustion cases 9 and 10 were more close to those of cases that work under the standard operating conditions. For maintaining the heat transfer performance of the boiler with less emission, cases 9 and 10 can achieve that purpose, as will be shown in the subsections of species concentrations and char consumption. In these last two cases, the tangential distributions of the gas velocity vectors (central vortexes) were approximately close to the central zone of the furnace compared to the other combustion cases. As a result, this good tangential circulation of the gases led to improve the flame distribution, as seen in Fig. 2. In contrast, the central vortex in the case 2 was away from the central position of the furnace and skews toward the furnace wall. The aerodynamic effects, applying in cases 7 and 9, on the mean gas velocity along the centerline of the furnace in the regions of burners and water wall (Z = m) are showed in Fig. 6, compared to the combustion case 1 that works under the standard operating conditions. As reportedly mentioned in Table 3, the values of the mass flow rates in the inert burners are approximately double those in the main burners in all combustion scenarios adopted in this study Velocity distribution Fig. 5 presents the cross-section cuts of the gas velocity vectors on the upper intermediate inert (UII) of the secondary air duct for the combustion cases (cases 1 6 as specified in Table 1) that work under the standard operating conditions. With reference to Fig. 3, cases 5 and 6 showed good flow field distributions in the furnace. Fig. 6. Mean velocity (m/s) along the centreline of the furnace in the region of the burners for three combustion cases (1, 7, and 9).

9 A.H. Al-Abbas et al. / Fuel 107 (2013) Fig. 7. Oxygen mass fraction (kg/kg) along the hopper and burner regions of the furnace at the mid cut (X Z plane) for three combustion cases: (a) case 1, (b) case 7, (c) case 9. This gives a clear explanation why the mean gas velocity is high in the point Z = 14.0 m, which represents the level of inert burners in the furnace, and it is low in the main burners region. However, in Fig. 6, the maximum values of the mean gas velocity were 25.27, 22.59, and m/s for combustion cases 1, 7, and 9, respectively. In the hot gas off takes (HGOTs) region, the reduction in the velocity is evident for all combustion cases examined Species concentrations In Fig. 7, the distributions of O 2 mass fraction (kg/kg) are presented along the hopper and burner regions of the furnace at the mid cut (X Z plane) for three combustion scenarios 1, 7, and 9. In the zones of main and inert burners, there was a clear consumption in the oxygen concentrations for all cases investigated compared to Fig. 8. Carbon dioxide mass fraction (kg/kg) along the height of the furnace at the mid cut (X Z plane) for three combustion cases: (a) case 1, (b) case 7, (c) case 9.

10 696 A.H. Al-Abbas et al. / Fuel 107 (2013) Fig. 9. Carbon monoxide mass fraction (kg/kg) along the height of the furnace at the mid cut (X Z plane) for three combustion cases: (a) case 1, (b) case 7, (c) case 9. the other regions in the furnace. This reduction in O 2 was accompanied by high temperature values due to the intense combustion reaction occurred in this region, as earlier shown in Fig. 4a. The lower temperature profile observed in the combustion case 7 was connected with the higher O 2 content noticed in Fig. 7, and vice versa for the standard combustion case. Compared to cases 1 and 7, the higher O 2 concentration was observed in the hopper zone of the combustion case 9 because of the higher contents of oxygen entering through the burners and secondary air ducts, i.e. 20% higher than the standard combustion conditions. Therefore, it is important to mention here that these O 2 concentrations are strongly dependent on the aerodynamics and reaction conditions. Fig. 8 shows the distributions of carbon dioxide (CO 2 ) mass fraction along the height of the furnace at the mid cut (X Z plane) for three combustion cases 1, 7, and 9. The difference in the CO 2 concentrations between the standard (reference) combustion case and two investigated cases are evident, and it is comparable in this study. In Fig. 8, it can clearly see the reduction in the CO 2 for both combustion cases 7 and 9 compared to the conventional case. The reasons for this decrease can be explained as follows: in case 7 the load is decreased by reducing the PC, mill gases, and air flow rates by 20% for each inlet port burner in comparison with the conventional combustion conditions, and this can essentially increase the residence time of coal particles in the combustion zone, and thereby improve the coal reaction with oxidizers. This decrease of the CO 2 concentrations was also observed in the similar numerical study of Belosevic et al. [26]. They used a comprehensive threedimensional mathematical model and CFD code to predict the complex processes in two-phase turbulent reactive flows of pulverized coal particles within a large-scale utility boiler under different operating conditions. Similarly, Agraniotis et al. [27] numerically investigated the co-combustion of brown coal in large scale boilers under different operational conditions. In regards to the reduction of CO 2 observed in the combustion case 9, it can be said that the reduction happened due to the higher O 2 content available in the furnace which led to efficiently enhance the Fig. 10. (a) Mass fraction (MF) of carbon dioxide for combustion cases (2, 3, 5, 6, 9, and 10) along the central line of the furnace in the burner regions; (b) mass fraction of carbon monoxide for combustion cases (2, 3, 5, 6, 9, and 10) along the central line of the furnace in the burner regions.

11 A.H. Al-Abbas et al. / Fuel 107 (2013) reaction processes, as shown in Eqs. (1) and (4). However, the concentrations of carbon dioxide at the furnace exit were equal to 18.84, 16.65, and (wt.%) for combustion cases 1, 7, and 9, respectively. As mentioned previously, the combustion case 9 can provide a good opportunity to reduce emissions and maintain the heat transfer performance of the boiler compared to the existing combustion case. In contrast to the lower CO 2 concentrations observed in cases 7 and 9, the latter two combustion cases showed a slight increase in the carbon monoxide (CO) concentrations in the combustion zone relative to the standard firing case, as presented in Fig. 9. This was likely due to the thermal dissociating mechanism adopted in this study that led to an increase in the reactive processes between the main (CO 2 and H 2 O) and intermediate (H 2 and CO) chemical species. In addition, the three-step reaction mechanisms used in the heterogeneous reaction of char with the oxygen, carbon dioxide, and water vapor (see Eqs. (4) (6)) have a significant effect on the CO concentrations in the hot furnace zone. Fig. 10a and b presents the distributions of mass fraction of CO 2 and CO respectively along the central line (X and Y = 0.0) of the furnace in the region of combustion zone (burners region) for different combustion cases: 2, 3, 5, 6, 9, and 10. Both figures show that there are two peak values of CO 2 and CO concentrations for all combustion cases examined. The maximum values were firstly close to the main burners (PC burners), while the second max. values were in the inert burner region. These two values resulted from the availability of the higher O 2 concentrations in these regions. In the case 6, the concentrations of CO 2 and CO were somewhat lower than those of the other combustion cases. As mentioned earlier, this improvement on the emission reduction might be due to the good mixing condition adopted in this combustion case Char consumption In general, the numerical simulation of brown coal combustion in a tangentially-fired furnace is considered a complex process because of the higher water content in this type of low-rank coal (>60% water content in the raw coal). After the evaporation process from coal particles in the hot gas off take tubes (recirculation process), the thermal decomposition and the sequence burnout of the released volatile matter and char coal are started in the mouth of the furnace. As previously noted in the lignite coal combustion within the lab-scale furnace and in the present furnace as well, the reaction of the released hydrocarbon (volatile matter) with oxidizers was started and totally finished in vicinity to the burner region. And this resulted in an increase in the gas temperature. Therefore, the char consumption is the main interesting point that could be used to distinguish among the examined combustion cases in this numerical investigation. Fig. 11 shows the char content of brown coal particles (%) along the height of the furnace for three combustion cases 1, 7, and 9. A clear difference can be seen in the char burnout for these examined combustion cases. This is basically due to the turbulent mixing conditions, residence times of coal particles, and aerodynamic effects. Compared to the existing combustion case, cases 7 and 9 showed a considerable increase in the char consumption. The reason of this is that in the case 7 the higher residence time of coal particles in the furnace zone gave a sufficient time to burn much char coal, whereas the higher aerodynamics and oxygen content in the case 9 led to efficiently improved mixing and reaction conditions. This is of a great importance to reduce the slagging and fouling problems on the heat transfer surfaces. This interesting Fig. 11. Char content of the pulverized coal particles (%) of three combustion cases: (a) case 1, (b) case 7, (c) case 9.

12 698 A.H. Al-Abbas et al. / Fuel 107 (2013) result on the char burnout can lead to an increase in the combustion efficiency under these new firing conditions. 5. Conclusion A 3D CFD model of a 550 MW tangentially-fired furnace at Loy Yang A power plant has been developed. The validated model was used to investigate the combustion characteristics and boiler performance under different firing conditions. The combustion scenarios of this work were based on the effect of the mass flow rates and distribution ratios for the PC, mill gases, and air in the combustion zone compared to the reference scenario. Ten different combustion cases were investigated. The first six combustion cases were based on the change of the out-of-service burners under full load operation, while the rest cases were carried out at 20% lower and 20% higher than the standard operating conditions, as specified in Table 1. Temperature distributions, velocity distributions, species concentrations, and char consumption are compared for all examined combustion cases. The numerical results showed that the turned off burners in the opposite direction are better than those in the adjacent direction under full load operation, and this can help to provide an improved distribution of the flames in the furnace. When the higher aerodynamic effects of the PC and feed oxidizer gases were used, the combustion flames uniformly distributed in the furnace zone. The predictions showed that there is a clear reduction in the CO 2 concentrations and char coal under the suggested operating combustion conditions in comparison with the reference case. A slight increase in the carbon monoxide concentrations was evident under the high and low load operations. These predicted results were associated with the thermal chemistry mechanism adopted in this study. Based on the abovementioned findings, the combustion characteristics and boiler performance can be optimized for less emission. This is also important to reduce the slagging and fouling phenomena on the heat transfer surfaces of the furnace. Acknowledgement The financial support from the Ministry of Higher Education and Scientific Research in the Iraqi government ( gov.iq) is gratefully appreciated. References [1] Ahmed S, Hart J, Nikolov J, Solnordal C, Yang W, Naser J. The effect of jet velocity ratio on aerodynamics of a rectangular slot-burner in the presence of cross-flow. Exp Thermal Fluid Sci 2007;32(2): [2] Hart JT, Naser JA, Witt PJ. Aerodynamics of an isolated slot-burner from a tangentially-fired boiler. Appl Math Model 2009;33(9): [3] Ahmed S, Naser J. Numerical investigation to assess the possibility of utilizing a new type of mechanically thermally dewatered (MTE) coal in existing tangentially-fired furnaces. 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