- Email: [email protected]
Numerical investigation of oxy-coal combustion to evaluate burner and combustor design concepts
Energy 29 (2004) 1285–1296 www.elsevier.com/locate/energy
Numerical investigation of oxy-coal combustion to evaluate burner and combustor design concepts E.H. Chui , A.J. Majeski, M.A. Douglas, Y. Tan, K.V. Thambimuthu CANMET Energy Technology Centre, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ont., Canada K1A 1M1
Abstract Significant progress has been made in both experimental investigations and numerical modelling of oxy-fuel combustion for CO2 capture purposes at the CANMET Vertical Combustor Research Facility. Detailed in-flame measurements have revealed insights into flow field development and pollutant formation characteristics over a wide range of operating conditions using natural gas and coals. A numerical modelling capability has also been developed in parallel and validated by in-flame data. This study marks the first use by CANMET of this numerical modelling expertise to develop design ideas before expensive and time-consuming experimental work is done. Its focus is on evaluating burner and combustor design concepts for oxy-coal combustion when air is substituted with oxygen in the recycled flue gas mode. Model results indicate that a new burner design approach can potentially reduce NOx at furnace exit by over 70% with respect to the existing design, while providing significant improvements in overall flame characteristics. Also, the numerical study produces quantitative evidence in support of enlarging the present combustor to an inner diameter of 1 m in order to minimize wall effects, which become important when trying to expand the flame volume so as to improve oxygen management within the flame. The first set of experimental results collected from the new burner–combustor combination validates the predicted improvements in NOx reduction and combustion performance. # 2004 Published by Elsevier Ltd.
1. Introduction The capture and sequestration of CO2 emissions from fossil fuel power plants can conceptually be achieved via oxy-fuel combustion, which reduces or completely eliminates nitrogen from the feed gas to the combustor and hence, produces a flue gas with high CO2 concentration [1]. Since 1994, CANMET Energy Technology Centre (CETC) has been active in the research
Corresponding author. Tel.: +1-613-943-1774; fax: +1-613-992-9335. E-mail address: [email protected] (E.H. Chui).
0360-5442/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.energy.2004.03.102
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and development of oxy-fuel combustion technologies [2–6] on behalf of a consortium represented by power utilities, equipment manufacturers, gas suppliers, the US Department of Energy, and two levels of the Canadian Government. The goal is to develop viable and optimal techniques for the design or retrofit of fossil fuel power plants in order to recover CO2 for utilisation in enhanced oil recovery (EOR) and coal bed methane (CBM) projects or for sequestration. The research and development approach to oxy-fuel combustion at CETC has two main components: pilot-scale experimental testing and numerical modelling. Douglas et al. [7] describe the latest set-up of the CANMET Vertical Combustor Research Facility (VCRF), the pilot-scale test rig dedicated to CO2 capture studies and Tan et al. [8] provide a summary of all the experiments completed to date. Parallel to the experimental investigations, CETC has also been developing and testing a computational fluid dynamic (CFD) modelling tool to facilitate future experimental investigations and subsequent scale-up studies of industrial size units for oxy-fuel combustion. The first CFD investigation [9] demonstrates the potential of the model but fails to establish its accuracy due to a lack of in-flame measurement data in the near burner region of the combustor. Since then a new section has been added to the VCRF combustor to facilitate measurement collection in the near-burner region. The latest numerical study [10] shows that the model can accurately predict the flame and NOx characteristics of oxy-coal combustion in both O2 enriched air (OEA) and recycled flue gas (RFG) operating modes. Also, the model results provide valuable insights on the impact on combustion performance due to combustion medium, burner swirl and burner configuration. Chui et al. [10] determine that further improvements can potentially be made to the two burners already in operation at the VCRF. These burners, though providing stable flames for satisfactory in-flame data collection under various oxy-fuel combustion modes, have not been optimised for NOx reduction through oxygen management and staging techniques. NOx reduction is an important consideration (although the goal of oxy-fuel combustion is to produce a flue gas stream for use or sequestration) because the trace amount of NO in the flue gas stream tends to accumulate in the vent portion of the product recovery process, which involves compression, dehydration and refrigeration of the flue gas stream prior to pumping it into a reservoir. In other words, the NO generated from oxy-fuel combustion for CO2 capture purposes will typically be discharged to the atmosphere and should be minimized. The purpose of this work is to use the validated numerical tool to evaluate burner and combustor design concepts prior to making any equipment modifications for future experimental studies.
2. Description of the VCRF combustion system The focus of the present study is on the combustion system of the VCRF. It is a cylindrical, down-fired vertical combustor (8.3 m long with an inner diameter, ID, of 0.61 m). It has a rated firing capacity of up to 0.3 MWth and can operate on solid, gaseous and liquid fuels. The combustor is lined with refractory to conserve heat in order to provide a realistic time–temperature history for burning coal. The two burners (A and B) shown in Fig. 1 were designed at CETC for the combustor. Both units are single-register, swirl-stabilized burners with the primary stream being admitted to an annulus located inside the swirling secondary stream. The primary
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Fig. 1. Schematics of two current burner configurations for the VCRF.
stream carries the dry pulverized coal into the combustor and is composed of air or CO2 from a bulk storage vessel. Small amounts of O2 can be added to the primary stream to simulate flue gas conditions. The secondary stream can comprise any combination of recycled flue gas, ambient air, carbon dioxide, nitrogen, and oxygen from bulk storage vessels. The amount of oxygen mixed into the secondary stream is generally restricted to less than 28% (dry volume basis) for safety reasons. Most test scenarios require some pure oxygen be admitted directly to the combustor. In the current designs, oxygen is injected through a series of holes arranged in a circle located either inside the primary stream (burner A), or between the primary and secondary streams (burner B) as shown in Fig. 1. 2.1. Performance of burners A and B Both burners provide stable flames in air and various oxy-coal modes of combustion. Table 1 summarises their performance with respect to NOx production in two test cases using a low sulphur western Canadian sub-bituminous coal. Since NOx production has proved in the past to be an excellent proxy for the convergence of experimental and numerical findings, the good agreement between measured and predicted values shown in Table 1 establishes the accuracy of Table 1 Measured and predicted NOx for burners A and B in two test cases Test condition
Baseline air casea Recycled flue gas modeb Combustor exit After recycle a
Burner A
Burner B
Measured (ng/J)
Predicted
Measured (ng/J)
Predicted
228
199
108
121
225 144
171 110
229 149
217 141
Mass flow rates (kg/h): coal feed (36.3), primary air (63.9), secondary air (205). Mass flow rates (kg/h): coal feed (35.5), primary stream CO2 (67.4), recycled flue gas stream (86.7), O2 input (59.3). b
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the numerical tool. Table 1 also shows that burner B has reduced NOx production, relative to burner A, in the air case but not the RFG case. In fact, in RFG mode the NOx level at the combustor exit (i.e. before the recycle stream is removed) is either similar to, or even higher than that in the baseline air case for both burners. This observation is somewhat counter-intuitive because the RFG mode is generally expected to decrease NOx formation due to the absence of molecular nitrogen (i.e. no thermal or prompt NOx). However, when oxy-coal burners are adapted from air combustion principles, NOx can actually increase because the fuel NOx mechanism is very sensitive to the method of admitting oxygen to the flame envelope.
3. Focus of the present study Based on the results in Table 1 and Ref. [10], the CFD model has been shown to provide realistic predictions of flame and NOx characteristics in both air and oxy-coal combustion modes. The focus of the current study is to use the numerical tool to investigate burner design concepts for the purpose of improving O2 management within the combustor and for reducing NOx formation via improved staging. The conditions for the RFG cases shown in Table 1 are typical to the experiments performed in the VCRF, where 35% (by mass) of the combustion products exiting the combustor are recycled into the furnace via the secondary stream. The same conditions are used in the current study. 3.1. Scope of investigation Two versions of new burner design concepts were developed. Each design was incorporated into a computational grid that also contained the details of the combustor. Simulations of oxycoal combustion and NOx formation were performed to determine the flame and NOx characteristics for each design. In addition to the existing combustor chamber of the VCRF, two different combustor sizes were also investigated using the model to quantify wall effects. 3.2. Modelling approach Oxy-coal combustion is modelled using a combined Eulerian and Lagrangian approach to treat respectively the fluid and particle phases. Equations representing turbulent fluid flow, heat transfer, volatile matter combustion, species transport, and NOx formation are solved over a computational grid with a large number of grid cells (about 425,000 in this study), which collectively represent the configuration of a given burner and combustion chamber combination. The interactions between the coal particle and fluid phases are calculated by tracking the motion, particle heat-up, devolatilization, and char burning histories of the coal particles statistically representative of every size group from the point of injection to the exit of the furnace. This modelling approach has been successfully applied to simulate coal combustion in air and high O2/CO2 atmospheres with good accuracy (see Refs. [10,11] for further details) and is executed using a customized version of commercial CFD software, CFX-TASCflow, jointly developed by CETC and ANSYS Canada Limited. The devolatilization and char combustion kinetics of the
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chosen sub-bituminous coal, input into the model, were based on the characterization study performed in a special reactor by the International Flame Research Foundation (IFRF) [12].
4. Discussion of results CETC is in the process of protecting the new burner design concepts and therefore, cannot yet reveal the details of the new burners. Only the global performance of the latest burner designs is included in the following discussion. 4.1. Burner C1—level 1 improvement Fig. 2 shows the contrast in temperature distribution over a cross-section of the vertical combustor between burner B (the latest existing design), and the new conceptual burner C1. The
Fig. 2. Temperature distribution on furnace cross-section for burners B and C1.
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temperature plot, covering a 4.75 m long region starting from the end of the quarl section, indicates that the flame from burner C1 is more central, axisymmetric and does not attach to one side of the furnace. Burner C1 also facilitates the creation of a large, distinct fuel-rich zone in the central core of the combustor with a low O2 concentration (<3 vol.% dry as shown in Fig. 3). As coal from the primary gas stream passes through this fuel-rich core in the middle of the furnace, fuel-bound nitrogen released during devolatilization does not transform to NO. Fig. 4 shows a significant drop in NO throughout the furnace and especially along the centreline when burner C1 is used. At the exit of the modelled furnace, the NO level is 55 ng/J (295 ppm average), as opposed to 217 ng/J (1160 ppm average) when burner B is used. With burner C1, the mean CO level at axial distance 5 m from quarl outlet is 361 ppm, which is lower than with burner B (725 ppm), indicating that burner C1 promotes better overall mixing of fuel and oxidant in addition to lowering NOx production. However, it should also be ascertained if the existing size of the combustor (0.61 m ID) is too small to properly evaluate the performance of the burner design concepts. To gain insights on wall effects, burner C1 is numerically modelled in a much larger 1.5 m ID chamber, and a new simulation is performed with identical input conditions to the burner. The flame changes shape from being central, axisymmetric and steady as in Fig. 2 to long and lazy, and impinges on the furnace wall (Fig. 5). This increase in furnace diameter is perhaps too much for the size of burner C1. In a less confining environment, the momentum of the flame is greatly reduced and
Fig. 3. Oxygen distribution on furnace cross-section and centreline for burners B and C1.
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Fig. 4. NO distribution on furnace cross-section and centreline for burners B and C1.
the burner can no longer generate a favourable flow field for proper mixing of fuel and oxidant. Then a stacked chamber (1.0 m ID in the first 1.2 m; 0.61 m ID for the remaining portion) is chosen to test burner C1. The intent is to minimize the wall effects in the near burner region without unduly reducing the flame momentum. Fig. 5 (middle picture) shows the resulting temperature distribution for C1 in the stacked furnace. This flame still attaches to the furnace wall, indicating that the flame characteristics from burner C1 may not be as desirable as shown in Figs. 2–4 when wall effects in the near burner region are reduced. A second level of design refinements is required to develop a more robust burner concept. 4.2. Burner C2—level 2 improvement In essence, the design of burner C2 is a modified version of C1 with features less sensitive to wall effects. With C2 the flame no longer attaches to the wall of the larger-diameter, stacked furv nace as seen in Fig. 5. Also, the peak flame temperature has dropped (from 1820 C in C1 to v 1620 C in C2) and the furnace cross-section has a fairly uniform temperature distribution, suggesting improved mixing. The CO plot (Fig. 6) further substantiates better mixing of fuel and oxidant with burner C2. The average CO level at 5 m downstream of the quarl outlet is 24 ppm, two orders of magnitude less than 2540 ppm, the average CO level at the same location when
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Fig. 5. Cross-sectional temperature distribution for burners C1 and C2 in 1.0 and 1.5 m ID furnaces.
burner C1 is used. Fig. 7 shows that burner C1 still succeeds in generating a distinct fuel-rich zone in the near burner region of the 1.0 m ID stacked furnace (as in the existing 0.61 m ID unit, Fig. 3), with a low O2 (fuel-rich) central core to inhibit NOx formation. However, in using burner C1, the high O2 streams near the wall in the larger stacked combustor do not migrate as quickly and uniformly (as in the smaller existing unit) to the central core downstream of the furnace to effectively react with the volatile matter and CO. This is the basis for the high CO level observed even at about 5 m from the burner exit. In contrast, burner C2 creates a less stratified distribution of O2 in the near burner region (Fig. 7) and yet maintains a low O2, fuel-rich region in the central core to reduce NOx. As a result of these improvements, both CO and NOx can be minimized. Fig. 8 compares the distribution of NO between burners C1 and C2. At the furnace exit, C2 yields 62 ng/J of NO (328 ppm average), somewhat higher than that from using C1 (40 ng/J of NO, 214 ppm average) but with much better combustion characteristics (i.e. much lower CO emissions).
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Fig. 6. CO distribution on ‘stacked’ furnace cross-section and centreline for burners C1 and C2 (note logarithmic scale).
4.3. Comparison with measurements Based on the findings of the numerical investigation, burner C2 was built and the combustor was enlarged to 1.0 m ID in the near burner region as stated above. The first set of commissioning runs was completed in early 2003 to test primarily the technical viability of this new burner– combustor combination. The experimental results indicate that stable flames can be achieved with burner C2 mounted on the stacked furnace with input and output conditions staying fairly constant over each test. Table 2 shows the operating conditions covered in the commissioning runs and the corresponding flue gas analyses. Although the test conditions between the experimental and numerical investigations are similar (Table 2), none of the experimental test cases has a set of operating conditions perfectly matching the numerical study due to some minor difficulties experienced in setting the various flow rates into the unit. Generally, this first set of commissioning runs confirms that NOx production drops significantly with the new burner–combustor combination (226–800 ppm versus 1230 ppm with burner B and old combustor) while combustion characteristics remain good (24–73 ppm CO at exit). In fact, the lowest measured NOx at furnace exit (226 ppm average) was registered when the experimental operating conditions were closest to those assumed for the numerical study, providing a
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Fig. 7. O2 distribution on ‘stacked’ furnace cross-section and centreline for burners C1 and C2.
preliminary validation of the predicted performance of burner C2 (328 ppm of NO under similar operating conditions).
5. Summary remarks Two versions of a new burner design concept were evaluated for oxy-coal combustion in recycled flue gas mode using a CFD modelling expertise developed at CANMET. The first burner, denoted C1, was found to provide better flame characteristics and lower NOx (75% less) than the existing burner B used in previous work. However, the performance of C1 deteriorated when the furnace size was increased, indicating that either burner C1 had a strong sensitivity to wall effects or that the current combustor might be too small to properly evaluate such a burner design concept. Burner C2 was developed based on a refinement of C1 and it was found to be able to retain its low NOx (62 ng/J at furnace exit versus 217 ng/J for burner B) and low CO (24 ppm at 5 m from burner versus 725 ppm for burner B) characteristics even in a larger ‘stacked’ furnace (1 m ID for the first 1.2 m; 0.61 m ID for the rest). Overall, the numerical tool was found to be an efficient and cost effective tool for evaluating such burner and combustor design concepts before committing to the cost of fabricating and testing prototypes.
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Fig. 8. NO distribution on ‘stacked’ furnace cross-section and centreline for burners C1 and C2.
Table 2 Operating conditions and flue gas analyses for the experimental and numerical investigations of burner C2–stacked furnace combination in recycled flue gas mode Numerical study Coal feed (kg/h) Primary stream CO2 (kg/h) Recycled flue gas (kg/h) Oxygen input (kg/h) Flue gas analysis O2 (vol.% dry) CO (ppm, dry) NOx (ppm, dry) CO2 (vol.% dry by balance)
Measurements from commissioning runs
35.5 67.4 86.7 59.3
35.7–36.7 65.0–79.9 58.3–112.1 50.6–55.5
2.0 24 328 98.0
1.9–3.4 24–73 226–800 96.6–98.1
The VCRF has recently been retrofitted to accommodate the new 1.0 m ID combustor barrel extension. A new burner based on the design of burner C2 has been fabricated and installed for evaluation. The experimental results obtained during the commissioning phase confirm the predicted performance of this new burner–combustor combination.
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Acknowledgements This work has received generous support from the Panel on Energy Research and Development (PERD) of the Federal Government of Canada and the CANMET CO2 consortium.
References [1] Payne R, Chen SL, Wolsky AM, Richter WF. CO2 recovery via coal combustion in mixtures of oxygen and recycled flue gas. Combust Sci Techol 1989;67:1–16. [2] Croiset E, Thambimuthu KV. Coal combustion with flue gas recirculation for CO2 recovery. Proceedings of the Fourth International Conference on Greenhouse Gas Control Technologies (GHGT-4), Interlaken, Switzerland. 1998, p. 581. [3] Croiset E, Thambimuthu KV, Palmer AD. A novel strategy for greenhouse gas abatement in a coal-fired power plant. Proceedings of the Combustion Canada ‘99 Conference, Calgary, Canada, May. 1999. [4] Croiset E, Thambimuthu KV, Palmer AD. Coal combustion in O2/CO2 mixtures. Can J Chem Eng 2000;78:402–7. [5] Tan Y, Douglas MA, Thambimuthu KV. CO2 capture using oxygen enhanced combustion strategies for natural gas power plants. Fuel 2002;81(8):1007–16. [6] Thambimuthu KV, Croiset E. Enriched oxygen coal-fired combustion. Proceedings of the Advanced Coal-Based Power and Environmental Systems ‘98 Conference, Morgantown, USA, July. 1998. [7] Douglas MA, Chui EH, Tan Y, Lee GK, Croiset E, Thambimuthu KV. Oxy-fuel combustion at the CANMET vertical combustor research facility. Proceeding of the First National Conference on Carbon Sequestration, Washington, DC, 14–17 May. 2001. [8] Tan Y, Douglas MA, Chui EH. A review of experimental findings in oxy-fuel combustion at the CANMET vertical combustor research facility. American Flame Research Committee Spring Meeting, Ottawa, Canada, 8–10 May. 2002. [9] Chui EH, Croiset E, Thambimuthu KV. Simulation of coal combustion in high CO2 and O2 environment. Proceedings of the Fifth International Conference on GreenHouse Gas Control Technologies (GHGT-5), Cairns, Australia, August. 2000. [10] Chui EH, Douglas MA, Tan Y. Modeling of oxy-fuel combustion for a western Canadian sub-bituminous coal. Fuel 2003;82(10):1201–10. [11] Chui EH, Hughes PMJ. Validation of NOx and NOx precursor predictions in coal flames. Combust Sci Technol 1996;119:51–75. [12] Morgan ME, Dekker JSA. Characterisation of the combustion performance of a suite of pulverized coals. Report on the CC1 Trials. International Flame Research Foundation (IFRF), F188/a/4; 1988.
Numerical investigation of oxy-coal combustion to evaluate burner and combustor design concepts E.H. Chui , A.J. Majeski, M.A. Douglas, Y. Tan, K.V. Thambimuthu CANMET Energy Technology Centre, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ont., Canada K1A 1M1
Abstract Significant progress has been made in both experimental investigations and numerical modelling of oxy-fuel combustion for CO2 capture purposes at the CANMET Vertical Combustor Research Facility. Detailed in-flame measurements have revealed insights into flow field development and pollutant formation characteristics over a wide range of operating conditions using natural gas and coals. A numerical modelling capability has also been developed in parallel and validated by in-flame data. This study marks the first use by CANMET of this numerical modelling expertise to develop design ideas before expensive and time-consuming experimental work is done. Its focus is on evaluating burner and combustor design concepts for oxy-coal combustion when air is substituted with oxygen in the recycled flue gas mode. Model results indicate that a new burner design approach can potentially reduce NOx at furnace exit by over 70% with respect to the existing design, while providing significant improvements in overall flame characteristics. Also, the numerical study produces quantitative evidence in support of enlarging the present combustor to an inner diameter of 1 m in order to minimize wall effects, which become important when trying to expand the flame volume so as to improve oxygen management within the flame. The first set of experimental results collected from the new burner–combustor combination validates the predicted improvements in NOx reduction and combustion performance. # 2004 Published by Elsevier Ltd.
1. Introduction The capture and sequestration of CO2 emissions from fossil fuel power plants can conceptually be achieved via oxy-fuel combustion, which reduces or completely eliminates nitrogen from the feed gas to the combustor and hence, produces a flue gas with high CO2 concentration [1]. Since 1994, CANMET Energy Technology Centre (CETC) has been active in the research
Corresponding author. Tel.: +1-613-943-1774; fax: +1-613-992-9335. E-mail address: [email protected] (E.H. Chui).
0360-5442/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.energy.2004.03.102
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and development of oxy-fuel combustion technologies [2–6] on behalf of a consortium represented by power utilities, equipment manufacturers, gas suppliers, the US Department of Energy, and two levels of the Canadian Government. The goal is to develop viable and optimal techniques for the design or retrofit of fossil fuel power plants in order to recover CO2 for utilisation in enhanced oil recovery (EOR) and coal bed methane (CBM) projects or for sequestration. The research and development approach to oxy-fuel combustion at CETC has two main components: pilot-scale experimental testing and numerical modelling. Douglas et al. [7] describe the latest set-up of the CANMET Vertical Combustor Research Facility (VCRF), the pilot-scale test rig dedicated to CO2 capture studies and Tan et al. [8] provide a summary of all the experiments completed to date. Parallel to the experimental investigations, CETC has also been developing and testing a computational fluid dynamic (CFD) modelling tool to facilitate future experimental investigations and subsequent scale-up studies of industrial size units for oxy-fuel combustion. The first CFD investigation [9] demonstrates the potential of the model but fails to establish its accuracy due to a lack of in-flame measurement data in the near burner region of the combustor. Since then a new section has been added to the VCRF combustor to facilitate measurement collection in the near-burner region. The latest numerical study [10] shows that the model can accurately predict the flame and NOx characteristics of oxy-coal combustion in both O2 enriched air (OEA) and recycled flue gas (RFG) operating modes. Also, the model results provide valuable insights on the impact on combustion performance due to combustion medium, burner swirl and burner configuration. Chui et al. [10] determine that further improvements can potentially be made to the two burners already in operation at the VCRF. These burners, though providing stable flames for satisfactory in-flame data collection under various oxy-fuel combustion modes, have not been optimised for NOx reduction through oxygen management and staging techniques. NOx reduction is an important consideration (although the goal of oxy-fuel combustion is to produce a flue gas stream for use or sequestration) because the trace amount of NO in the flue gas stream tends to accumulate in the vent portion of the product recovery process, which involves compression, dehydration and refrigeration of the flue gas stream prior to pumping it into a reservoir. In other words, the NO generated from oxy-fuel combustion for CO2 capture purposes will typically be discharged to the atmosphere and should be minimized. The purpose of this work is to use the validated numerical tool to evaluate burner and combustor design concepts prior to making any equipment modifications for future experimental studies.
2. Description of the VCRF combustion system The focus of the present study is on the combustion system of the VCRF. It is a cylindrical, down-fired vertical combustor (8.3 m long with an inner diameter, ID, of 0.61 m). It has a rated firing capacity of up to 0.3 MWth and can operate on solid, gaseous and liquid fuels. The combustor is lined with refractory to conserve heat in order to provide a realistic time–temperature history for burning coal. The two burners (A and B) shown in Fig. 1 were designed at CETC for the combustor. Both units are single-register, swirl-stabilized burners with the primary stream being admitted to an annulus located inside the swirling secondary stream. The primary
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Fig. 1. Schematics of two current burner configurations for the VCRF.
stream carries the dry pulverized coal into the combustor and is composed of air or CO2 from a bulk storage vessel. Small amounts of O2 can be added to the primary stream to simulate flue gas conditions. The secondary stream can comprise any combination of recycled flue gas, ambient air, carbon dioxide, nitrogen, and oxygen from bulk storage vessels. The amount of oxygen mixed into the secondary stream is generally restricted to less than 28% (dry volume basis) for safety reasons. Most test scenarios require some pure oxygen be admitted directly to the combustor. In the current designs, oxygen is injected through a series of holes arranged in a circle located either inside the primary stream (burner A), or between the primary and secondary streams (burner B) as shown in Fig. 1. 2.1. Performance of burners A and B Both burners provide stable flames in air and various oxy-coal modes of combustion. Table 1 summarises their performance with respect to NOx production in two test cases using a low sulphur western Canadian sub-bituminous coal. Since NOx production has proved in the past to be an excellent proxy for the convergence of experimental and numerical findings, the good agreement between measured and predicted values shown in Table 1 establishes the accuracy of Table 1 Measured and predicted NOx for burners A and B in two test cases Test condition
Baseline air casea Recycled flue gas modeb Combustor exit After recycle a
Burner A
Burner B
Measured (ng/J)
Predicted
Measured (ng/J)
Predicted
228
199
108
121
225 144
171 110
229 149
217 141
Mass flow rates (kg/h): coal feed (36.3), primary air (63.9), secondary air (205). Mass flow rates (kg/h): coal feed (35.5), primary stream CO2 (67.4), recycled flue gas stream (86.7), O2 input (59.3). b
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the numerical tool. Table 1 also shows that burner B has reduced NOx production, relative to burner A, in the air case but not the RFG case. In fact, in RFG mode the NOx level at the combustor exit (i.e. before the recycle stream is removed) is either similar to, or even higher than that in the baseline air case for both burners. This observation is somewhat counter-intuitive because the RFG mode is generally expected to decrease NOx formation due to the absence of molecular nitrogen (i.e. no thermal or prompt NOx). However, when oxy-coal burners are adapted from air combustion principles, NOx can actually increase because the fuel NOx mechanism is very sensitive to the method of admitting oxygen to the flame envelope.
3. Focus of the present study Based on the results in Table 1 and Ref. [10], the CFD model has been shown to provide realistic predictions of flame and NOx characteristics in both air and oxy-coal combustion modes. The focus of the current study is to use the numerical tool to investigate burner design concepts for the purpose of improving O2 management within the combustor and for reducing NOx formation via improved staging. The conditions for the RFG cases shown in Table 1 are typical to the experiments performed in the VCRF, where 35% (by mass) of the combustion products exiting the combustor are recycled into the furnace via the secondary stream. The same conditions are used in the current study. 3.1. Scope of investigation Two versions of new burner design concepts were developed. Each design was incorporated into a computational grid that also contained the details of the combustor. Simulations of oxycoal combustion and NOx formation were performed to determine the flame and NOx characteristics for each design. In addition to the existing combustor chamber of the VCRF, two different combustor sizes were also investigated using the model to quantify wall effects. 3.2. Modelling approach Oxy-coal combustion is modelled using a combined Eulerian and Lagrangian approach to treat respectively the fluid and particle phases. Equations representing turbulent fluid flow, heat transfer, volatile matter combustion, species transport, and NOx formation are solved over a computational grid with a large number of grid cells (about 425,000 in this study), which collectively represent the configuration of a given burner and combustion chamber combination. The interactions between the coal particle and fluid phases are calculated by tracking the motion, particle heat-up, devolatilization, and char burning histories of the coal particles statistically representative of every size group from the point of injection to the exit of the furnace. This modelling approach has been successfully applied to simulate coal combustion in air and high O2/CO2 atmospheres with good accuracy (see Refs. [10,11] for further details) and is executed using a customized version of commercial CFD software, CFX-TASCflow, jointly developed by CETC and ANSYS Canada Limited. The devolatilization and char combustion kinetics of the
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chosen sub-bituminous coal, input into the model, were based on the characterization study performed in a special reactor by the International Flame Research Foundation (IFRF) [12].
4. Discussion of results CETC is in the process of protecting the new burner design concepts and therefore, cannot yet reveal the details of the new burners. Only the global performance of the latest burner designs is included in the following discussion. 4.1. Burner C1—level 1 improvement Fig. 2 shows the contrast in temperature distribution over a cross-section of the vertical combustor between burner B (the latest existing design), and the new conceptual burner C1. The
Fig. 2. Temperature distribution on furnace cross-section for burners B and C1.
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temperature plot, covering a 4.75 m long region starting from the end of the quarl section, indicates that the flame from burner C1 is more central, axisymmetric and does not attach to one side of the furnace. Burner C1 also facilitates the creation of a large, distinct fuel-rich zone in the central core of the combustor with a low O2 concentration (<3 vol.% dry as shown in Fig. 3). As coal from the primary gas stream passes through this fuel-rich core in the middle of the furnace, fuel-bound nitrogen released during devolatilization does not transform to NO. Fig. 4 shows a significant drop in NO throughout the furnace and especially along the centreline when burner C1 is used. At the exit of the modelled furnace, the NO level is 55 ng/J (295 ppm average), as opposed to 217 ng/J (1160 ppm average) when burner B is used. With burner C1, the mean CO level at axial distance 5 m from quarl outlet is 361 ppm, which is lower than with burner B (725 ppm), indicating that burner C1 promotes better overall mixing of fuel and oxidant in addition to lowering NOx production. However, it should also be ascertained if the existing size of the combustor (0.61 m ID) is too small to properly evaluate the performance of the burner design concepts. To gain insights on wall effects, burner C1 is numerically modelled in a much larger 1.5 m ID chamber, and a new simulation is performed with identical input conditions to the burner. The flame changes shape from being central, axisymmetric and steady as in Fig. 2 to long and lazy, and impinges on the furnace wall (Fig. 5). This increase in furnace diameter is perhaps too much for the size of burner C1. In a less confining environment, the momentum of the flame is greatly reduced and
Fig. 3. Oxygen distribution on furnace cross-section and centreline for burners B and C1.
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Fig. 4. NO distribution on furnace cross-section and centreline for burners B and C1.
the burner can no longer generate a favourable flow field for proper mixing of fuel and oxidant. Then a stacked chamber (1.0 m ID in the first 1.2 m; 0.61 m ID for the remaining portion) is chosen to test burner C1. The intent is to minimize the wall effects in the near burner region without unduly reducing the flame momentum. Fig. 5 (middle picture) shows the resulting temperature distribution for C1 in the stacked furnace. This flame still attaches to the furnace wall, indicating that the flame characteristics from burner C1 may not be as desirable as shown in Figs. 2–4 when wall effects in the near burner region are reduced. A second level of design refinements is required to develop a more robust burner concept. 4.2. Burner C2—level 2 improvement In essence, the design of burner C2 is a modified version of C1 with features less sensitive to wall effects. With C2 the flame no longer attaches to the wall of the larger-diameter, stacked furv nace as seen in Fig. 5. Also, the peak flame temperature has dropped (from 1820 C in C1 to v 1620 C in C2) and the furnace cross-section has a fairly uniform temperature distribution, suggesting improved mixing. The CO plot (Fig. 6) further substantiates better mixing of fuel and oxidant with burner C2. The average CO level at 5 m downstream of the quarl outlet is 24 ppm, two orders of magnitude less than 2540 ppm, the average CO level at the same location when
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Fig. 5. Cross-sectional temperature distribution for burners C1 and C2 in 1.0 and 1.5 m ID furnaces.
burner C1 is used. Fig. 7 shows that burner C1 still succeeds in generating a distinct fuel-rich zone in the near burner region of the 1.0 m ID stacked furnace (as in the existing 0.61 m ID unit, Fig. 3), with a low O2 (fuel-rich) central core to inhibit NOx formation. However, in using burner C1, the high O2 streams near the wall in the larger stacked combustor do not migrate as quickly and uniformly (as in the smaller existing unit) to the central core downstream of the furnace to effectively react with the volatile matter and CO. This is the basis for the high CO level observed even at about 5 m from the burner exit. In contrast, burner C2 creates a less stratified distribution of O2 in the near burner region (Fig. 7) and yet maintains a low O2, fuel-rich region in the central core to reduce NOx. As a result of these improvements, both CO and NOx can be minimized. Fig. 8 compares the distribution of NO between burners C1 and C2. At the furnace exit, C2 yields 62 ng/J of NO (328 ppm average), somewhat higher than that from using C1 (40 ng/J of NO, 214 ppm average) but with much better combustion characteristics (i.e. much lower CO emissions).
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Fig. 6. CO distribution on ‘stacked’ furnace cross-section and centreline for burners C1 and C2 (note logarithmic scale).
4.3. Comparison with measurements Based on the findings of the numerical investigation, burner C2 was built and the combustor was enlarged to 1.0 m ID in the near burner region as stated above. The first set of commissioning runs was completed in early 2003 to test primarily the technical viability of this new burner– combustor combination. The experimental results indicate that stable flames can be achieved with burner C2 mounted on the stacked furnace with input and output conditions staying fairly constant over each test. Table 2 shows the operating conditions covered in the commissioning runs and the corresponding flue gas analyses. Although the test conditions between the experimental and numerical investigations are similar (Table 2), none of the experimental test cases has a set of operating conditions perfectly matching the numerical study due to some minor difficulties experienced in setting the various flow rates into the unit. Generally, this first set of commissioning runs confirms that NOx production drops significantly with the new burner–combustor combination (226–800 ppm versus 1230 ppm with burner B and old combustor) while combustion characteristics remain good (24–73 ppm CO at exit). In fact, the lowest measured NOx at furnace exit (226 ppm average) was registered when the experimental operating conditions were closest to those assumed for the numerical study, providing a
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Fig. 7. O2 distribution on ‘stacked’ furnace cross-section and centreline for burners C1 and C2.
preliminary validation of the predicted performance of burner C2 (328 ppm of NO under similar operating conditions).
5. Summary remarks Two versions of a new burner design concept were evaluated for oxy-coal combustion in recycled flue gas mode using a CFD modelling expertise developed at CANMET. The first burner, denoted C1, was found to provide better flame characteristics and lower NOx (75% less) than the existing burner B used in previous work. However, the performance of C1 deteriorated when the furnace size was increased, indicating that either burner C1 had a strong sensitivity to wall effects or that the current combustor might be too small to properly evaluate such a burner design concept. Burner C2 was developed based on a refinement of C1 and it was found to be able to retain its low NOx (62 ng/J at furnace exit versus 217 ng/J for burner B) and low CO (24 ppm at 5 m from burner versus 725 ppm for burner B) characteristics even in a larger ‘stacked’ furnace (1 m ID for the first 1.2 m; 0.61 m ID for the rest). Overall, the numerical tool was found to be an efficient and cost effective tool for evaluating such burner and combustor design concepts before committing to the cost of fabricating and testing prototypes.
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Fig. 8. NO distribution on ‘stacked’ furnace cross-section and centreline for burners C1 and C2.
Table 2 Operating conditions and flue gas analyses for the experimental and numerical investigations of burner C2–stacked furnace combination in recycled flue gas mode Numerical study Coal feed (kg/h) Primary stream CO2 (kg/h) Recycled flue gas (kg/h) Oxygen input (kg/h) Flue gas analysis O2 (vol.% dry) CO (ppm, dry) NOx (ppm, dry) CO2 (vol.% dry by balance)
Measurements from commissioning runs
35.5 67.4 86.7 59.3
35.7–36.7 65.0–79.9 58.3–112.1 50.6–55.5
2.0 24 328 98.0
1.9–3.4 24–73 226–800 96.6–98.1
The VCRF has recently been retrofitted to accommodate the new 1.0 m ID combustor barrel extension. A new burner based on the design of burner C2 has been fabricated and installed for evaluation. The experimental results obtained during the commissioning phase confirm the predicted performance of this new burner–combustor combination.
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Acknowledgements This work has received generous support from the Panel on Energy Research and Development (PERD) of the Federal Government of Canada and the CANMET CO2 consortium.
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