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Mathematical modelling of semi-anthracite combustion in a single burner furnace
Fuel 82 (2003) 2069–2073 www.fuelfirst.com
Mathematical modelling of semi-anthracite combustion in a single burner furnaceq P.L. Stephenson* Innogy PLC, Windmill Hill Business Park, Whitehill Way, Swindon, Wiltshire SN5 6PB, UK Received 29 October 2002; revised 21 January 2003; accepted 28 January 2003; available online 12 June 2003
Abstract As part of an investigation into combustion behaviour at Aberthaw Power Station, experiments have been conducted using Innogy’s 0.5 MW Combustion Test Facility (CTF) at Didcot A. Most tests involved combustion, but a few were for isothermal conditions and included velocity measurements. Both isothermal and combusting flows have been calculated using a commercial computational fluid dynamics (CFD) program called Star-CD. The isothermal solutions have been validated against a simpler analytical solution and against rig measurements. The solutions with combustion and radiation have been validated qualitatively against the behaviour observed during rig tests and also quantitatively against incident heat flux measurements. The basis of the CFD for both the isothermal and combusting cases is described, together with the validation that has been achieved. q 2003 Elsevier Ltd. All rights reserved. Keywords: Computational fluid dynamics; Coal combustion; Anthracite
1. Introduction Innogy’s Aberthaw Power Station burns semi-anthracite and therefore has a downshot fired furnace. In this design, the furnace has a number of burners firing downwards from the burner arch. Primary and tertiary air (PA and TA) enter at the burner, with the PA conveying the pulverised fuel. The burners are unswirled. Further air (the secondary air or SA), enters from the furnace side walls at a distance below the burners. To increase understanding of the behaviour of such furnaces and thereby improve burner performance, tests have been made on Innogy’s Combustion Test Facility (CTF). To increase understanding of the results of these tests and increase Innogy’s capability in computational fluid dynamics (CFD), this computer modelling technique has been applied to them. Innogy’s CTF consists of a combustion chamber into which a single burner is fitted (Fig. 1). Measurements can be taken through a number of ports along the sides of the combustion chamber. For the tests considered here, a scaled version of the Aberthaw burner was used. In addition, the SA * Tel.: þ44-1793-896249; fax: þ44-1793-896251. E-mail address: [email protected] (P.L. Stephenson). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 0016-2361/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00158-3
inlets at Aberthaw were simulated by adding inlet air ducts through two of the measurement ports on the sides of the combustion chamber. A cross section through the outlet from the single geometrically-scaled version of an Aberthaw burner is shown in Fig. 2. A single burner consists of two roughly oval ducts carrying PA and fuel; each of these ducts is surrounded by another duct carrying TA. The combustion chamber is 0.8 £ 0.8 m2 in crosssection and the effective diameter of the burner is 143 mm. The tests discussed here were conducted using a scaled version of an Aberthaw burner and with a typical South Wales semi-anthracite coal. Tests were made with both isothermal and combusting conditions. Velocity measurements were made in the combustion chamber during isothermal tests, and incident wall heat fluxes were measured in the combusting cases.
2. CFD approach used The CFD modelling used the commercial Computational Fluid Dynamics program, Star-CD. The coal combustion model in Star-CD has already been applied to high volatile bituminous coals [1,2]. This is thought to be the first application of Star-CD to semi-anthracite combustion.
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P.L. Stephenson / Fuel 82 (2003) 2069–2073
Fig. 1. General arrangement of Innogy’s combustion test facility.
An unstructured non-orthogonal hexahedral mesh with about 185,000 cells was used (Fig. 3). The mesh represents the actual geometry of the combustion chamber, burner, measurement ports used as SA inlets and the convergent section downstream of the combustion chamber. The model also contained an artificial straight duct length at exit, to move the exit boundary condition plane well away from the region in which combustion occurred. A standard high Reynolds number version of the k-e turbulence model was used. A Lagrangian method was adopted for particle tracking, based on mean gas velocities, with an Eulerian approach for the gas-phase. Radiation heat transfer was calculated using the discrete transfer method [3]. As this was the first attempt to model semi-anthracite, some fairly simple models were selected for coal combustion. A fixed rate devolatilisation model was used. This meant that the devolatilisation rate is zero below a specified initial devolatilisation temperature. Above that temperature,
Fig. 2. Cross-section through outlet from a single geometrically-scaled Aberthaw burner.
devolatilisation occurs at a specified fixed rate until completion. Char combustion was calculated using the model of Field et al. [4]. This involves combining a chemical kinetic and an oxygen diffusion limit. The chemical kinetic limit is found from an Arrenhius
Fig. 3. CFD mesh—overall view and detail near burners.
P.L. Stephenson / Fuel 82 (2003) 2069–2073
expression and assumed first order oxygen kinetics. The gas combustion was calculated using a variant of Spalding’s conserved scalars approach (‘mixed is burnt’). With this approach, the gas combustion rate is assumed to be infinitely fast so that, at any point in space, there can be either fuel (volatile gas) or oxygen but never both. The pf was assumed to enter at two locations in each PA duct. All size fractions were injected at each of the four initial locations. The predictions with coal combustion correspond to tests using a typical South Wales semi-anthracite coal. The input data is summarised in Table 1. The data in Table 1 were obtained as follows. 1. CTF inlet conditions and the proximate and ultimate analysis of the coal and its CV were taken from test data. The moisture content is low because the coal was premilled and therefore pre-dried. 2. The initial pulverised fuel size distribution was based on sieving data. The coal mass was assumed to be equally divided between the six assumed sizes, which had mean diameters of 5, 10, 20, 50, 65 and 145 mm. 3. The high temperature devolatilisation yield was based on drop tube furnace (DTF) measurements. 4. The minimum temperature for devolatilisation was given a representative value for semi-anthracite. In the absence of other information, the overall time for devolatilisation was put equal to the default value in Star-CD, namely 83 ms. It was assumed that, once the appropriate temperature is reached, devolatilisation occurs sufficiently rapidly for the predictions to be insensitive to the exact devolatilisation rate. 5. Char kinetic data. This is based on a typical activation energy for semi-anthracite. The pre-exponential constant was derived from a separate analysis of DTF char refiring tests. Table 1 Summary of coal data used in the computer modelling Coal
South Wales semi-anthracite
Ash (as fired) (wt%) Moisture (as fired) (wt%) Proximate volatile matter (as fired) (wt%) C (daf) (wt%) H (daf) (wt%) N (daf) (wt%) Gross CV (MJ/kg) (as fired) Ratio of high temperature to proximate VM Initial temperature for devolatilisation (K) Devolatilisation time (s) Pre-exponential factor for char combustion kg m22 s21 (Nm22)21 Activation energy for char combustion (J/kmol)
24.72 0.96 12.50
2071
3. Predictions for isothermal flow Certain difficulties were encountered with initial attempts at predicting the isothermal flow. Firstly, there was a tendency for the solution to become very asymmetric. This was largely resolved by progressively refining the mesh in the near burner region. Even then, there were still problems in obtaining a fully-converged solution. A few transient calculations were made and suggested that the flow is inherently transient, with small fluctuations occurring in the main jet. The predictions presented here represent the best approach to steady state convergence that could be obtained. The predictions for isothermal flow showed that, away from the burner, the air flow is similar to that from a single non-swirling jet. Close to the burner, there is a more complex flow pattern, where the PA and TA flows from the two separate burner outlets mix to form a single jet. The isothermal predictions have been validated in two ways, namely comparison with an approximate analytical solution, and comparison with some measured velocities. The approximate analytical solution involved the calculation of an equivalent round jet with initial diameter and velocity chosen to give the same mass and momentum fluxes as for the actual burner geometry at outlet from the burner. The centreline velocity was then calculated using the expression for a free turbulent round jet given by Field et al. [4] and recommended by Rhine and Tucker [5]. Comparison between centre line velocities from this solution and from the CFD predictions is shown in Fig. 4. In the central region of the plot, the agreement is good. Near the burner is the core region of an equivalent single jet where the expression of Field et al. is not applicable. The centre line velocity from the CFD solution is zero at the burner; this is because the centreline is actually midway between the two burner outlets (Fig. 2). The predicted centre line velocity increases as the various burner flows mix to form a single jet. In the far region of the plot in Fig. 2, the CFD solution and analytical solution slowly diverge. One possible explanation for this is that the SA enters at 805 mm from the burner, and this is expected to increase the spread of the main jet and lower its velocity.
88.12 4.14 1.39 27.29 1.50 800 0.0833 s 6.0 £ 1024 64.9 £ 106
Fig. 4. Comparison with simple analytical solution—centre line velocities in isothermal flow.
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P.L. Stephenson / Fuel 82 (2003) 2069–2073
Also, the predictions in Fig. 2 are for the chamber centre line. As the jet is slightly asymmetric, the jet centre line slowly diverges from the chamber centre line. Therefore, the axial velocity on the chamber centre line will become slightly less that the value on the jet centre line. A comparison between measured velocities and those from CFD is given in Fig. 5 for ports 2 and 5 (305 and 1055 mm, respectively, from burner). The predictions agree well with measurements except for locations close to the jet centreline. If the actual jet was fluctuating slightly (as suggested by the CFD modelling), then the jet centre line would be fluctuating and it would be difficult to obtain accurate measurements in this region. The predictions in Fig. 5 show a slight asymmetry which is typical of all the isothermal solutions. Also, the predictions for port 5 suggest a slight recirculation towards the combustion chamber walls; this is not surprising at this location is a short distance down stream of the SA ports.
4. Predictions with coal combustion Fig. 5. Comparison with measurements for isothermal flow. Port 2 and 5 are at 305 and 1055 mm from burner. CFD: prediction; rh, lh: measurement via port on right hand or left hand side of combustion chamber, respectively.
Fig. 6. Predicted particle tracks with combustion.
Predictions of particle tracks are shown in Fig. 6. These show that the coal particles initially travel in a straight line along the furnace centre line. Once they reach the same axial location as the SA entry ports, they diverge in both the vertical and horizontal planes. The corresponding predictions of gas temperature are shown in Fig. 7. This shows that combustion occurs after the SA ports, with a complex flame structure. The temperature predictions for the vertical plane show that combustion is occurring in three horizontal regions, namely near the combustion chamber centre line and near the floor
Fig. 7. Temperature contours (K) on vertical central plane.
P.L. Stephenson / Fuel 82 (2003) 2069–2073
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the predicted incident fluxes in Fig. 8. The highest incident wall heat fluxes were measured at around 2.3 –3.8 m from the burner. As the combustion chamber is about 4.2 m long, this implies that most of the combustion was occurring about half to three quarters of distance along the chamber. The predicted incident fluxes are in reasonable agreement with measurement, which provides quantitative evidence that the predicted axial location of the flame is correct.
5. Conclusions Fig. 8. Comparison between measured and predicted incident heat fluxes. CFD: predicted values, r340 etc: measured values from various test runs for notionally similar conditions.
and ceiling. Corresponding predictions on a horizontal plane showed that combustion occurs only in a single region on the chamber centre line, and not near the combustion chamber side walls. This difference between vertical and horizontal planes is presumably a consequence of the SA ducts being arranged horizontally; this is the main geometric difference between the two planes. The predictions show that the two jets of SA act in a way similar to that of a flame stabiliser. These predictions are in qualitative agreement with what was observed during the CTF tests. The experimental observations included 1. Between the burner (PA/TA inlets) and the SA inlets, there was no combustion, and the pf could be observed in a fairly narrow jet near the furnace centreline. 2. After the SA inlets, the pf spread out over a large proportion of the combustion chamber and combustion occurred. 3. Significant amounts of slagging occurred on the combustion chamber floor and ceiling. This is consistent with the predictions of some of the combustion occurring close to the wall and ceiling. Incident radiation fluxes were measured through the measurement ports on the combustion chamber side wall for a number of tests. These measured values are compared with
Isothermal CFD solutions for Innogy’s CTF have been successfully compared with measurements and a simplified analytical solution Further CFD solutions have been obtained with coal combustion and radiation. Results are in qualitative agreement with experimental observations, and are in good agreement with measured incident heat fluxes.
Acknowledgements The CTF measurements reported here were obtained by Joanne Fagan.
References [1] O’Connor M. The effects of coal quality on NOx emissions and carbon burnout in pulverised coal-fired utility burners. ETSU Rep COAL 1999;R153. [2] Ghobadian A, Lee F, Stephenson PL. Validation of the coal combustion capability in the Star-CD code, I Mech E Seminar on CFD—technical developments and future trends, London; 13–14 December 1999. [3] Lockwood FC, Shah NG. A new radiation solution method for incorporation in general combustion prediction procedures. 18th Symp (Int) Combustion 1981;1405– 14. [4] Field MA, Gill DW, Morgan BB, Hawksley PGW. The combustion of pulverised coal, BCURA; 1967. [5] Rhine JM, Tucker RJ. Modelling of gas-fired furnaces and boilers. New York: McGraw-Hill; 1991.
Mathematical modelling of semi-anthracite combustion in a single burner furnaceq P.L. Stephenson* Innogy PLC, Windmill Hill Business Park, Whitehill Way, Swindon, Wiltshire SN5 6PB, UK Received 29 October 2002; revised 21 January 2003; accepted 28 January 2003; available online 12 June 2003
Abstract As part of an investigation into combustion behaviour at Aberthaw Power Station, experiments have been conducted using Innogy’s 0.5 MW Combustion Test Facility (CTF) at Didcot A. Most tests involved combustion, but a few were for isothermal conditions and included velocity measurements. Both isothermal and combusting flows have been calculated using a commercial computational fluid dynamics (CFD) program called Star-CD. The isothermal solutions have been validated against a simpler analytical solution and against rig measurements. The solutions with combustion and radiation have been validated qualitatively against the behaviour observed during rig tests and also quantitatively against incident heat flux measurements. The basis of the CFD for both the isothermal and combusting cases is described, together with the validation that has been achieved. q 2003 Elsevier Ltd. All rights reserved. Keywords: Computational fluid dynamics; Coal combustion; Anthracite
1. Introduction Innogy’s Aberthaw Power Station burns semi-anthracite and therefore has a downshot fired furnace. In this design, the furnace has a number of burners firing downwards from the burner arch. Primary and tertiary air (PA and TA) enter at the burner, with the PA conveying the pulverised fuel. The burners are unswirled. Further air (the secondary air or SA), enters from the furnace side walls at a distance below the burners. To increase understanding of the behaviour of such furnaces and thereby improve burner performance, tests have been made on Innogy’s Combustion Test Facility (CTF). To increase understanding of the results of these tests and increase Innogy’s capability in computational fluid dynamics (CFD), this computer modelling technique has been applied to them. Innogy’s CTF consists of a combustion chamber into which a single burner is fitted (Fig. 1). Measurements can be taken through a number of ports along the sides of the combustion chamber. For the tests considered here, a scaled version of the Aberthaw burner was used. In addition, the SA * Tel.: þ44-1793-896249; fax: þ44-1793-896251. E-mail address: [email protected] (P.L. Stephenson). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 0016-2361/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00158-3
inlets at Aberthaw were simulated by adding inlet air ducts through two of the measurement ports on the sides of the combustion chamber. A cross section through the outlet from the single geometrically-scaled version of an Aberthaw burner is shown in Fig. 2. A single burner consists of two roughly oval ducts carrying PA and fuel; each of these ducts is surrounded by another duct carrying TA. The combustion chamber is 0.8 £ 0.8 m2 in crosssection and the effective diameter of the burner is 143 mm. The tests discussed here were conducted using a scaled version of an Aberthaw burner and with a typical South Wales semi-anthracite coal. Tests were made with both isothermal and combusting conditions. Velocity measurements were made in the combustion chamber during isothermal tests, and incident wall heat fluxes were measured in the combusting cases.
2. CFD approach used The CFD modelling used the commercial Computational Fluid Dynamics program, Star-CD. The coal combustion model in Star-CD has already been applied to high volatile bituminous coals [1,2]. This is thought to be the first application of Star-CD to semi-anthracite combustion.
2070
P.L. Stephenson / Fuel 82 (2003) 2069–2073
Fig. 1. General arrangement of Innogy’s combustion test facility.
An unstructured non-orthogonal hexahedral mesh with about 185,000 cells was used (Fig. 3). The mesh represents the actual geometry of the combustion chamber, burner, measurement ports used as SA inlets and the convergent section downstream of the combustion chamber. The model also contained an artificial straight duct length at exit, to move the exit boundary condition plane well away from the region in which combustion occurred. A standard high Reynolds number version of the k-e turbulence model was used. A Lagrangian method was adopted for particle tracking, based on mean gas velocities, with an Eulerian approach for the gas-phase. Radiation heat transfer was calculated using the discrete transfer method [3]. As this was the first attempt to model semi-anthracite, some fairly simple models were selected for coal combustion. A fixed rate devolatilisation model was used. This meant that the devolatilisation rate is zero below a specified initial devolatilisation temperature. Above that temperature,
Fig. 2. Cross-section through outlet from a single geometrically-scaled Aberthaw burner.
devolatilisation occurs at a specified fixed rate until completion. Char combustion was calculated using the model of Field et al. [4]. This involves combining a chemical kinetic and an oxygen diffusion limit. The chemical kinetic limit is found from an Arrenhius
Fig. 3. CFD mesh—overall view and detail near burners.
P.L. Stephenson / Fuel 82 (2003) 2069–2073
expression and assumed first order oxygen kinetics. The gas combustion was calculated using a variant of Spalding’s conserved scalars approach (‘mixed is burnt’). With this approach, the gas combustion rate is assumed to be infinitely fast so that, at any point in space, there can be either fuel (volatile gas) or oxygen but never both. The pf was assumed to enter at two locations in each PA duct. All size fractions were injected at each of the four initial locations. The predictions with coal combustion correspond to tests using a typical South Wales semi-anthracite coal. The input data is summarised in Table 1. The data in Table 1 were obtained as follows. 1. CTF inlet conditions and the proximate and ultimate analysis of the coal and its CV were taken from test data. The moisture content is low because the coal was premilled and therefore pre-dried. 2. The initial pulverised fuel size distribution was based on sieving data. The coal mass was assumed to be equally divided between the six assumed sizes, which had mean diameters of 5, 10, 20, 50, 65 and 145 mm. 3. The high temperature devolatilisation yield was based on drop tube furnace (DTF) measurements. 4. The minimum temperature for devolatilisation was given a representative value for semi-anthracite. In the absence of other information, the overall time for devolatilisation was put equal to the default value in Star-CD, namely 83 ms. It was assumed that, once the appropriate temperature is reached, devolatilisation occurs sufficiently rapidly for the predictions to be insensitive to the exact devolatilisation rate. 5. Char kinetic data. This is based on a typical activation energy for semi-anthracite. The pre-exponential constant was derived from a separate analysis of DTF char refiring tests. Table 1 Summary of coal data used in the computer modelling Coal
South Wales semi-anthracite
Ash (as fired) (wt%) Moisture (as fired) (wt%) Proximate volatile matter (as fired) (wt%) C (daf) (wt%) H (daf) (wt%) N (daf) (wt%) Gross CV (MJ/kg) (as fired) Ratio of high temperature to proximate VM Initial temperature for devolatilisation (K) Devolatilisation time (s) Pre-exponential factor for char combustion kg m22 s21 (Nm22)21 Activation energy for char combustion (J/kmol)
24.72 0.96 12.50
2071
3. Predictions for isothermal flow Certain difficulties were encountered with initial attempts at predicting the isothermal flow. Firstly, there was a tendency for the solution to become very asymmetric. This was largely resolved by progressively refining the mesh in the near burner region. Even then, there were still problems in obtaining a fully-converged solution. A few transient calculations were made and suggested that the flow is inherently transient, with small fluctuations occurring in the main jet. The predictions presented here represent the best approach to steady state convergence that could be obtained. The predictions for isothermal flow showed that, away from the burner, the air flow is similar to that from a single non-swirling jet. Close to the burner, there is a more complex flow pattern, where the PA and TA flows from the two separate burner outlets mix to form a single jet. The isothermal predictions have been validated in two ways, namely comparison with an approximate analytical solution, and comparison with some measured velocities. The approximate analytical solution involved the calculation of an equivalent round jet with initial diameter and velocity chosen to give the same mass and momentum fluxes as for the actual burner geometry at outlet from the burner. The centreline velocity was then calculated using the expression for a free turbulent round jet given by Field et al. [4] and recommended by Rhine and Tucker [5]. Comparison between centre line velocities from this solution and from the CFD predictions is shown in Fig. 4. In the central region of the plot, the agreement is good. Near the burner is the core region of an equivalent single jet where the expression of Field et al. is not applicable. The centre line velocity from the CFD solution is zero at the burner; this is because the centreline is actually midway between the two burner outlets (Fig. 2). The predicted centre line velocity increases as the various burner flows mix to form a single jet. In the far region of the plot in Fig. 2, the CFD solution and analytical solution slowly diverge. One possible explanation for this is that the SA enters at 805 mm from the burner, and this is expected to increase the spread of the main jet and lower its velocity.
88.12 4.14 1.39 27.29 1.50 800 0.0833 s 6.0 £ 1024 64.9 £ 106
Fig. 4. Comparison with simple analytical solution—centre line velocities in isothermal flow.
2072
P.L. Stephenson / Fuel 82 (2003) 2069–2073
Also, the predictions in Fig. 2 are for the chamber centre line. As the jet is slightly asymmetric, the jet centre line slowly diverges from the chamber centre line. Therefore, the axial velocity on the chamber centre line will become slightly less that the value on the jet centre line. A comparison between measured velocities and those from CFD is given in Fig. 5 for ports 2 and 5 (305 and 1055 mm, respectively, from burner). The predictions agree well with measurements except for locations close to the jet centreline. If the actual jet was fluctuating slightly (as suggested by the CFD modelling), then the jet centre line would be fluctuating and it would be difficult to obtain accurate measurements in this region. The predictions in Fig. 5 show a slight asymmetry which is typical of all the isothermal solutions. Also, the predictions for port 5 suggest a slight recirculation towards the combustion chamber walls; this is not surprising at this location is a short distance down stream of the SA ports.
4. Predictions with coal combustion Fig. 5. Comparison with measurements for isothermal flow. Port 2 and 5 are at 305 and 1055 mm from burner. CFD: prediction; rh, lh: measurement via port on right hand or left hand side of combustion chamber, respectively.
Fig. 6. Predicted particle tracks with combustion.
Predictions of particle tracks are shown in Fig. 6. These show that the coal particles initially travel in a straight line along the furnace centre line. Once they reach the same axial location as the SA entry ports, they diverge in both the vertical and horizontal planes. The corresponding predictions of gas temperature are shown in Fig. 7. This shows that combustion occurs after the SA ports, with a complex flame structure. The temperature predictions for the vertical plane show that combustion is occurring in three horizontal regions, namely near the combustion chamber centre line and near the floor
Fig. 7. Temperature contours (K) on vertical central plane.
P.L. Stephenson / Fuel 82 (2003) 2069–2073
2073
the predicted incident fluxes in Fig. 8. The highest incident wall heat fluxes were measured at around 2.3 –3.8 m from the burner. As the combustion chamber is about 4.2 m long, this implies that most of the combustion was occurring about half to three quarters of distance along the chamber. The predicted incident fluxes are in reasonable agreement with measurement, which provides quantitative evidence that the predicted axial location of the flame is correct.
5. Conclusions Fig. 8. Comparison between measured and predicted incident heat fluxes. CFD: predicted values, r340 etc: measured values from various test runs for notionally similar conditions.
and ceiling. Corresponding predictions on a horizontal plane showed that combustion occurs only in a single region on the chamber centre line, and not near the combustion chamber side walls. This difference between vertical and horizontal planes is presumably a consequence of the SA ducts being arranged horizontally; this is the main geometric difference between the two planes. The predictions show that the two jets of SA act in a way similar to that of a flame stabiliser. These predictions are in qualitative agreement with what was observed during the CTF tests. The experimental observations included 1. Between the burner (PA/TA inlets) and the SA inlets, there was no combustion, and the pf could be observed in a fairly narrow jet near the furnace centreline. 2. After the SA inlets, the pf spread out over a large proportion of the combustion chamber and combustion occurred. 3. Significant amounts of slagging occurred on the combustion chamber floor and ceiling. This is consistent with the predictions of some of the combustion occurring close to the wall and ceiling. Incident radiation fluxes were measured through the measurement ports on the combustion chamber side wall for a number of tests. These measured values are compared with
Isothermal CFD solutions for Innogy’s CTF have been successfully compared with measurements and a simplified analytical solution Further CFD solutions have been obtained with coal combustion and radiation. Results are in qualitative agreement with experimental observations, and are in good agreement with measured incident heat fluxes.
Acknowledgements The CTF measurements reported here were obtained by Joanne Fagan.
References [1] O’Connor M. The effects of coal quality on NOx emissions and carbon burnout in pulverised coal-fired utility burners. ETSU Rep COAL 1999;R153. [2] Ghobadian A, Lee F, Stephenson PL. Validation of the coal combustion capability in the Star-CD code, I Mech E Seminar on CFD—technical developments and future trends, London; 13–14 December 1999. [3] Lockwood FC, Shah NG. A new radiation solution method for incorporation in general combustion prediction procedures. 18th Symp (Int) Combustion 1981;1405– 14. [4] Field MA, Gill DW, Morgan BB, Hawksley PGW. The combustion of pulverised coal, BCURA; 1967. [5] Rhine JM, Tucker RJ. Modelling of gas-fired furnaces and boilers. New York: McGraw-Hill; 1991.