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Combustion behavior in a dual-staging vortex rice husk combustor with snail entry
International Communications in Heat and Mass Transfer 35 (2008) 1134–1140
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Combustion behavior in a dual-staging vortex rice husk combustor with snail entry☆ S. Eiamsa-ard a,1, Y. Kaewkohkiat a,1, C. Thianpong b, P. Promvonge b,⁎ a b
Department of Mechanical Engineering, Faculty of Engineering, Mahanakorn University of Technology, Bangkok 10530, Thailand Department of Mechanical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 10520, Thailand
A R T I C L E
I N F O
Available online 22 July 2008 Keywords: Vortex-combustor Swirling flow Vortex flow Rice husk Combustion
A B S T R A C T The combustion characteristics of rice husk fuel in a dual-staging vortex-combustor (DSVC) are experimentally investigated. In the present work, the vortex flow is created by using a snail entrance mounted at the bottom of the combustor. The temperature distributions at selected locations inside the combustor, the flue gas emissions (CO, CO2, O2, NOx), and the combustion/thermal efficiency are monitored. Measurements are made at a constant rice husk feed rate of 0.25 kg/min with various excess airs (37%, 56%, 74% and 92%) and different secondary air injection fractions (λ = 0.0, 0.15 and 0.2), respectively. The combustion chamber is 1800 mm high and 300 mm in diameter (D) with a centered exhausted pipe while the middle chamber of the combustor is set to 0.5D. The smaller section at the middle chamber is introduced to split the chamber to be dual-staging chamber where a large central toroidal recirculation zone induced by swirl flow through the small section is generated in the top chamber. The experimental results reveal that the highest temperature inside the combustor is about 1000 °C whereas both the thermal and the combustion efficiency are 41.6% and 99.8% for 74% excess air without the secondary air injection (λ = 0.0). In addition, the emissions are CO2 = 8.1%, O2 = 9.3%, CO = 352 ppm, NOx = 294 ppm and small amount of fly ash. Therefore, the DSVC shows an excellent performance, low emissions, high stabilization and ease of operation in firing the rice husk. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The vortex-combustor (VC) concept is germinated from the basic understanding of the strongly swirling gas–solid flows and combustion in vortex chambers. It integrates many advantages of cyclone combustor, multistage combustor, swirl burner, pulverized coal fired combustor, and fluidized-bed combustor while eliminates most of their inherent disadvantages. In the combustion systems, the design of strong vortex-flows of air and fuel provides the large-scale effects on the entrainment and the decay, the heat and mass transfer, the flame stability and the pollutant abatement reactions. In the past decade, many investigations on the vortex-combustor have been attempted numerically and experimentally, concerning both hot and cold models. A hot model vortex-combustor was recently developed and investigated by Nieh and Fu [1,2] to help increase coal utilization in an efficient, clean manner. A vortex-combustor similar to Nieh and Fu [1] but using rice husk fuel instead was experimentally investigated by Promvonge and Silapabanleng [3]. Nieh et al. [4]
☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (P. Promvonge). 1 Tel./fax: +662 9883666x241. 0735-1933/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2008.05.010
measured the gas flow field, and the particle mass fluxes and the elutriation under different the swirl numbers and the secondary air fractions in a cold model vortexing fluidized-bed combustor. Zhang and Nieh [5,6] predicted the strongly swirling turbulent flow and the pulverized coal combustion in a novel vortex-combustor by using an algebraic Reynolds stress turbulence model (ASM). Their results showed the detail characteristics of the gas-particle flow and the combustion in terms of gas velocities, turbulence quantities, temperature, species concentrations, particle density, trajectories, burnout time and residence time. Zhang and Nieh [7] presented the numerical simulation and the measurement of the isothermal and reacting gas flow and the gas-particle slip motion in the VC. They found that the gas flow in the VC with a coaxial center tube and multiple air injection was characterized with the swirling, and recirculating features. Eiamsa-ard et al. [8] measured the temperature distributions and the flue gas emissions during the combustion of fine and normal rice husk fuels in the vortex-combustor. They observed that the fine rice husk fuel yields a significant effect on the temperature distributions inside and helps to reduce emissions and the fly ash elutriated. Numerical simulations of strongly swirling turbulent flows (axial and tangential velocities, pressure fields, and turbulence kinetic energy) in a VC were presented by Ridluan et al. [9]. A three-dimensional isothermal vortex-combustor flow using three first-order turbulence models: the standard k–ε turbulence model, Renormalized Group (RNG) k–ε model and Shear Stress Transport
S. Eiamsa-ard et al. / International Communications in Heat and Mass Transfer 35 (2008) 1134–1140
(SST) k–ω model; and a second-order turbulence model, Reynolds Stress Model (RSM) was conducted. They reported that the RSM is superior to the other turbulence models in capturing the swirl flow effect. Eiamsa-ard et al. [10] studied experimentally the combustion characteristics (temperature distribution, fly ash and emission gas) in a multi-staging VC by using rice husk fuel. The middle size of the combustor was replaced or changed from 1.0D (conventional VC) to 0.75D or 0.5D leading to multi-staging vortex inside the combustor. Their results showed that the mean temperature distribution in the VC with the middle chamber of 0.5D is higher than those with the middle chamber of 0.75D and 1.0D. In the present work, an effort is made to develop a newly dualstaging vortex-combustor (DSVC) for combustion of rice husk based on the conventional VC. The main aim of this work is to present a preliminary study of the combustion characteristics (CO, CO2, O2, and NOx), the combustion efficiency and the temperature distribution, in a DSVC burning rice husk. Effect of excess airs from 37 to 92%, and the secondary air injection fractions (λ), from 0.0 to 0.2, on flame stability, temperature variations in the combustor, emissions, ashes and smokes from the exhausted stack are studied and observed for a design guideline in order to improve the performance of the combustor. In the experiments, the swirl or the vortex flow is generated by using the snail entry placed at the bottom of the DSVC.
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2. Experimental setup 2.1. Material The rice husk particles were milled and sieved up to about 2.0 × 8.0mm size and stored in the laboratory “as received”. The arrangement of the experimental system of the combustor is depicted in Fig. 1. The combustor was a concentric cylindrical shape, made of steel with the cast-able refractory cement lining as the insulation, while the exhaust center pipe was made of stainless steel. The combustor was 1800 mm high and 300 mm inside diameter (D) with the exhausted center pipe of 78 mm in diameter. In the experiment, the snail was mounted at the bottom of the combustion chamber to create the vortex/swirl flow into the combustor. The detail and the sketch of a newly DS vortexcombustor are shown in Table 1 and Fig. 2, respectively. 2.2. Method As schematically shown in Fig. 1, the primary air was drawn into the premixed chamber (or the bottom chamber), together with the rice husk fuel. The entry air was tangentially injected into the bottom combustion chamber of the DSVC by using the snail-type swirl generator. The combustion occurred primarily in the first
Fig. 1. Experimental setup of DS vortex-combustor system.
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Table 1 Detail of DS vortex-combustor geometry Combustion chamber Chamber diameter (Dc) Top chamber diameter (Dt = 1.0Dc) Middle/throat chamber diameter (Dm = 0.5Dc) Bottom chamber diameter (Db = 1.0Dc) Centre/exhaust tube diameter (De = 0.26Dc) Chamber height (Hc = 6.0Dc) Top chamber height (Ht = 2.5Dc) Bottom/Throat chamber height (Hm = 1.0Dc) Bottom chamber height (Hb = 2.5Dc) Centre/exhaust tube height (He = 5.0Dc) Nozzle Number of inlet nozzle at bottom chamber (N) Number of inlet nozzle at throat chamber (N) Nozzle diameter (d) Nozzle height location Material Combustion chamber Centre/exhaust tube Nozzle
300 mm 300 mm 150 mm 300 mm 78 mm 1800 mm 750 mm 300 mm 750 mm 1500 mm 4 nozzles 2 nozzles 10 mm 100, 900 mm Steel Stainless steel Stainless steel
annular vortex chamber or the bottom combustion chamber. Then, the hot combustion gas spirally ascended to the throat ring/ constriction and then to the top chamber or second vortex chamber
before leaving through the exhaust center tube. Before entering the top chamber, the swirling combustion gas flow from the bottom was accelerated when passing through the throat ring which provided the increase in combustion rate. The highly swirling turbulent gas, behind the throat ring, could induce a central toroidal recirculation zone in the entry region of the second chamber that helps to increase the mixing rate between air and fuel. In addition, the combined effects of the centrifugal [11], the gravitational and the fluid drag forces would trapped fuel particles along the height of the combustor which prolonged residence time of husk particle in the chamber. As rice husk particles were burnt, they continually reduce in mass and size until completely burnt out. The majority of the ash particles became light or small enough to be entrained by the flue gas and exit the combustor as fly ash. The large ash particles were trapped in the bottom vortex chamber with high thermal storage capacity inherent and act like a thermal flywheel. The temperatures inside the chamber were monitored at various selected locations with Chromel-alumel (type K) thermocouples read-out with the data logger while the volumetric flow rates of the inlet air was measured by using calibrated orifice meters. The flue gas emissions at the exit were measured by a calibrated gas analyzer (Testo 350 MXL). All data collection was taken at steady state conditions.
Fig. 2. Configuration of DS vortex-combustor with snail entrance.
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3. Results and discussion
3.1. Temperature distributions in DSVC
The dual-staging vortex-combustor is originally designed to accommodate various types of biomass fuels as sources of heat energy. The DSVC responds significantly to the various fuels as expected. For rice husks, the excess air and the secondary air are varied to optimize the combustion characteristics (thermal efficiency, combustion efficiency, heat loss, gas emission, temperature distribution, and exit temperature) in the DSVC. The measurements of temperature are made at 13 axial locations, namely, x/D = 0.25, 0.50, 0.75, 1.25, 1.75, 2.25, 2.75 (throat chamber), 3.25, 3.75, 4.25, 4.75, 5.25, and 5.75, and at six radial stations, namely, r/R = 0.25, 0.40, 0.55, 0.70, 0.85 and 1.0, respectively. The radial and axial profiles of the temperature inside the DSVC for various excess airs and the secondary air injection fractions are presented in Figs. 3 and 4 respectively. The husk feed rate of 0.25 kg/min is kept constant throughout the experiment.
3.1.1. Influence of excess air Effect of excess airs on radial temperature variation in the DSVC is presented in Fig. 3a. In the figure, the excess air is adjusted to be 37%, 56%, 74%, and 92%, at a constant husk feed rate of 0.25 kg/min. The temperature profiles for all locations generally are nearly uniform except for the vicinity of the wall and the bottom chamber for all of the excess airs used. The higher combustion temperature degree at the top of the DSVC indicates a stronger intensity of turbulence flow and combustion, especially at the core area. The peak temperature area is visible in the middle region of the top of combustion zone. It is worth noting that the use of excess air at 74% provides slightly higher and better uniform temperature distributions in the chamber than that of others while the use of 56% excess air causes the lowest temperatures over the chamber.
Fig. 3. Radial temperature distributions in combustor with fuel feed rate of 0.25 kg/min for, (a) various excess airs at λ = 0 and (b) various λ values at 74% excess air.
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Fig. 4. Axial temperature distributions in combustor with fuel feed rate of 0.25 kg/min for, (a) various excess airs at λ = 0 and (b) various λ values at 74% excess air.
3.1.2. Influence of secondary air injection Effects of the secondary air injection on the radial temperature distributions inside the DSVC with the excess air of 74% are illustrated in Fig. 3b. It is visible that the temperature distributions show a nearly flat profile throughout the combustor and a high temperature area can be found in the core region of the chamber. The fraction of volumetric flow rate of the secondary air to that of the total (primary + secondary)
air, defined as λ = Q2/(Q1 + Q2), is an indicator of the strength of the vortex of the flow. The introduced secondary air injection provides the higher tangential velocity component (strong vortex) at downstream of the injection location. This means that the impact of the swirling phenomenon from using the air injection on the flow and the temperature fields become more pronounced as the value of secondary air is increased. In the figure, the use of the secondary air
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In Fig. 4a showing axial temperature variations in the combustor for various excess airs at λ = 0, a close examination reveals that only 37% excess air provides higher axial temperature distributions at the bottom than other excess airs. This indicates that better mixing and high combustion stability occurs throughout the core region at the bottom chamber for the lowest excess air value. On the other hand, at the top chamber, the 74% excess air case yields slightly higher combustion temperature than other cases especially at r/R = 0.25, 0.4, and 0.55. Effect of the secondary air fraction (λ) on the axial temperature distributions is clearly seen in Fig. 4b. In the figure, at the fraction of λ = 0.0, the results show the significant improvement and the temperature profiles are more uniform than those at λ = 0.15 and 0.2 with the highest temperature found at r/R = 0.4, 0.55, 0.7, and 0.8 in the top chamber. This means that the better mixing and more uniform combustion reactions in all regions can be achieved for this case because of higher temperatures obtained. It can be observed that flammability limits of rich and lean mixtures can be extended without secondary air injection. The main reason maybe come from the fact that the introduced secondary air injection causes lower strength of vortex flow created by the snail at the bottom leading to lower strength of the induced central toroidal recirculation zone in the top. Therefore, if a snail-type vortex generator is mounted at the bottom chamber the secondary air injection is not necessary to avoid lower strength of vortex flow inside due to combustion air splitting for the injection. 3.2. Gas emissions The flue gas emissions of CO, CO2, NOx and O2 for various excess air values are presented in Fig. 5. As can be seen in the figure, the emission of CO2 shows a gradual reduction for the increase in excess air while the values of O2 and CO tend to increase with the rise of excess air or high lean mixture. This can be attributed to higher rate of combustion over the top chamber (higher combustion temperature near the top chamber) due to efficient mixing between fuel particle and air in a central toroidal recirculation zone. The emission of NOx is seen to slightly increase for increasing the excess air. Nevertheless, the application of the swirling flow and constriction chamber leads to a small increase in NOx. 3.3. Efficiency and exit temperature
Fig. 5. Emissions of exhaust gases (O2, CO2, CO, and NOx).
fraction λ = 0.2, gives higher and more uniform temperature distributions at the bottom chamber than that of λ = 0.0 (without secondary air injection) and 0.15. The average temperature in the combustion chamber is about 800 to 900 °C. This can be attributed to the higher swirling intensity enhancement and the strong degree of turbulence of air and fuel particles in the combustor. This situation results in prolonging the duration of the combustion. On the other hand, only at the top chamber for the combustion air without secondary air (λ = 0), the temperature profiles for all locations generally are higher than λ = 0.15 and 0.2. This may be attributed to the more complete combustion in this zone due to stronger turbulence leading to better mixing between air and fuel particles, and then heat energy is released considerably leading to the highest temperature. It is interesting to note that the influence of the constriction chamber on the temperature profiles shows the significant improvement on the top chamber while little effect on rising temperature profile at the bottom chamber is seen. The temperature in the vicinity of the center pipe is higher than that of the chamber wall, in general. Maximum temperature of about 1000 °C can be obtained in the core region of the chamber while lower temperatures near the chamber wall.
Influences of the excess air on the exit combustion temperature, the heat loss, the thermal efficiency, and the combustion efficiency at a
Fig. 6. Exit temperature and thermal efficiency versus excess air.
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the test runs, the application of excess air leads to a significant improvement of temperature profiles and yields uniform combustion temperatures including the extension of flammability limits. Besides, the increased swirling intensity, generated by a snail at the combustor entrance, results in an abatement of particle elutriation, lower pollutant emissions and enlargement of recirculation region. The use of secondary air injection is found to be inefficient for the combustor with snail entrance because the split air causes lower strength of vortex flow inside. References
Fig. 7. Heat loss and combustion efficiency against excess air.
constant husk feed rate of 0.25 kg/min, are depicted in Figs. 6 and 7. In the figures, it is found that the exit temperature and thermal efficiency tends to increase with raising the excess air. In general, the use of higher excess air results in higher combustion efficiency than that of lower excess air for burning solid fuels. The exit temperature is found to be in a range from 572 °C and 632 °C while the combustion efficiency of the DSVC is between 99.7%, and 99.9%, depending on excess air values used. 4. Conclusions The measurements of the temperature distribution, flue gas emission, and thermal/combustion efficiency, have been conducted for the combustion of rice husk in a newly DS vortex-combustor. The husk combustion of 0.25 kg/min in the DSVC with the excess air of 74% provides better combustion efficiency, flame stabilization, low emission and reliable furnace condition than other excess air values. For all
[1] S. Nieh, T.T. Fu, Development of a non-slagging vortex combustor (VC) for space/ water heating applications, Proceeding of The 5th International Coal Conference, Pittsburgh, 1988, pp. 761–768. [2] S. Nieh, T.T. Fu, A non-slagging vortex combustor firing coal–water fuel for commercial heating applications, Proceeding of The 7th International Coal Conference, Pittsburgh, 1990, p. 223. [3] P. Promvonge, K. Silapabanleng, Experimental study of combustion characteristics in a rice husk fired vortex combustor, Proceedings of the 36th Intersociety Energy Conversion Engineering Conference, Savannah, Georgia, 2001, 2001-RE-17. [4] S. Nieh, G. Yang, A.Q. Zhu, C.S. Zhao, Measurements of gas-particle flows and elutriation of an 18 inch i.d. cold vortexing fluidized-bed combustion model, Powder Technology 69 (2) (1992) 139–146. [5] J. Zhang, S. Nieh, Mathematical model of strongly swirling gas-particle turbulent flow and pulverized coal combustion and its application to a vortex combustor: part II — application, Fuel and Energy 37 (3) (1996) 206. [6] J. Zhang, S. Nieh, Comprehensive modelling of pulverized coal combustion in a vortex combustor, Fuel 76 (2) (1997) 123–131. [7] J. Zhang, S. Nieh, Swirling, reacting, turbulent gas-particle flow in a vortex combustor, Powder Technology 112 (2000) 70–78. [8] S. Eiamsa-ard, P. Akkarakuntron, P. Promvonge, Effects of fuel particle sizes on combustion behaviors in a rice husk fired vortex combustor, The 2nd Regional Conference on Energy Technology towards a Clean Environment, Phuket, 2003, paper number 1-029-0. [9] A. Ridluan, S. Eiamsa-ard, P. Promvonge, Numerical simulation of 3-D turbulent isothermal flows in a vortex combustor, International Communication Heat and Mass Transfer 34 (7) (2007) 860–869. [10] S. Eiamsa-ard, Y. Kaewkaokiet, W. Lelaphatikul, C. Thianpong, P. Promvonge, Experimental investigation of combustion characteristics in a multi-staging vortex combustor firing rice husk, International Communication Heat and Mass Transfer 35 (2) (2008) 139–148. [11] A.K. Gupta, D.G. Lilley, Swirl Flow, Abacus Press, Tunbrige Wells, England, 1984.
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Combustion behavior in a dual-staging vortex rice husk combustor with snail entry☆ S. Eiamsa-ard a,1, Y. Kaewkohkiat a,1, C. Thianpong b, P. Promvonge b,⁎ a b
Department of Mechanical Engineering, Faculty of Engineering, Mahanakorn University of Technology, Bangkok 10530, Thailand Department of Mechanical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 10520, Thailand
A R T I C L E
I N F O
Available online 22 July 2008 Keywords: Vortex-combustor Swirling flow Vortex flow Rice husk Combustion
A B S T R A C T The combustion characteristics of rice husk fuel in a dual-staging vortex-combustor (DSVC) are experimentally investigated. In the present work, the vortex flow is created by using a snail entrance mounted at the bottom of the combustor. The temperature distributions at selected locations inside the combustor, the flue gas emissions (CO, CO2, O2, NOx), and the combustion/thermal efficiency are monitored. Measurements are made at a constant rice husk feed rate of 0.25 kg/min with various excess airs (37%, 56%, 74% and 92%) and different secondary air injection fractions (λ = 0.0, 0.15 and 0.2), respectively. The combustion chamber is 1800 mm high and 300 mm in diameter (D) with a centered exhausted pipe while the middle chamber of the combustor is set to 0.5D. The smaller section at the middle chamber is introduced to split the chamber to be dual-staging chamber where a large central toroidal recirculation zone induced by swirl flow through the small section is generated in the top chamber. The experimental results reveal that the highest temperature inside the combustor is about 1000 °C whereas both the thermal and the combustion efficiency are 41.6% and 99.8% for 74% excess air without the secondary air injection (λ = 0.0). In addition, the emissions are CO2 = 8.1%, O2 = 9.3%, CO = 352 ppm, NOx = 294 ppm and small amount of fly ash. Therefore, the DSVC shows an excellent performance, low emissions, high stabilization and ease of operation in firing the rice husk. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The vortex-combustor (VC) concept is germinated from the basic understanding of the strongly swirling gas–solid flows and combustion in vortex chambers. It integrates many advantages of cyclone combustor, multistage combustor, swirl burner, pulverized coal fired combustor, and fluidized-bed combustor while eliminates most of their inherent disadvantages. In the combustion systems, the design of strong vortex-flows of air and fuel provides the large-scale effects on the entrainment and the decay, the heat and mass transfer, the flame stability and the pollutant abatement reactions. In the past decade, many investigations on the vortex-combustor have been attempted numerically and experimentally, concerning both hot and cold models. A hot model vortex-combustor was recently developed and investigated by Nieh and Fu [1,2] to help increase coal utilization in an efficient, clean manner. A vortex-combustor similar to Nieh and Fu [1] but using rice husk fuel instead was experimentally investigated by Promvonge and Silapabanleng [3]. Nieh et al. [4]
☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (P. Promvonge). 1 Tel./fax: +662 9883666x241. 0735-1933/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2008.05.010
measured the gas flow field, and the particle mass fluxes and the elutriation under different the swirl numbers and the secondary air fractions in a cold model vortexing fluidized-bed combustor. Zhang and Nieh [5,6] predicted the strongly swirling turbulent flow and the pulverized coal combustion in a novel vortex-combustor by using an algebraic Reynolds stress turbulence model (ASM). Their results showed the detail characteristics of the gas-particle flow and the combustion in terms of gas velocities, turbulence quantities, temperature, species concentrations, particle density, trajectories, burnout time and residence time. Zhang and Nieh [7] presented the numerical simulation and the measurement of the isothermal and reacting gas flow and the gas-particle slip motion in the VC. They found that the gas flow in the VC with a coaxial center tube and multiple air injection was characterized with the swirling, and recirculating features. Eiamsa-ard et al. [8] measured the temperature distributions and the flue gas emissions during the combustion of fine and normal rice husk fuels in the vortex-combustor. They observed that the fine rice husk fuel yields a significant effect on the temperature distributions inside and helps to reduce emissions and the fly ash elutriated. Numerical simulations of strongly swirling turbulent flows (axial and tangential velocities, pressure fields, and turbulence kinetic energy) in a VC were presented by Ridluan et al. [9]. A three-dimensional isothermal vortex-combustor flow using three first-order turbulence models: the standard k–ε turbulence model, Renormalized Group (RNG) k–ε model and Shear Stress Transport
S. Eiamsa-ard et al. / International Communications in Heat and Mass Transfer 35 (2008) 1134–1140
(SST) k–ω model; and a second-order turbulence model, Reynolds Stress Model (RSM) was conducted. They reported that the RSM is superior to the other turbulence models in capturing the swirl flow effect. Eiamsa-ard et al. [10] studied experimentally the combustion characteristics (temperature distribution, fly ash and emission gas) in a multi-staging VC by using rice husk fuel. The middle size of the combustor was replaced or changed from 1.0D (conventional VC) to 0.75D or 0.5D leading to multi-staging vortex inside the combustor. Their results showed that the mean temperature distribution in the VC with the middle chamber of 0.5D is higher than those with the middle chamber of 0.75D and 1.0D. In the present work, an effort is made to develop a newly dualstaging vortex-combustor (DSVC) for combustion of rice husk based on the conventional VC. The main aim of this work is to present a preliminary study of the combustion characteristics (CO, CO2, O2, and NOx), the combustion efficiency and the temperature distribution, in a DSVC burning rice husk. Effect of excess airs from 37 to 92%, and the secondary air injection fractions (λ), from 0.0 to 0.2, on flame stability, temperature variations in the combustor, emissions, ashes and smokes from the exhausted stack are studied and observed for a design guideline in order to improve the performance of the combustor. In the experiments, the swirl or the vortex flow is generated by using the snail entry placed at the bottom of the DSVC.
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2. Experimental setup 2.1. Material The rice husk particles were milled and sieved up to about 2.0 × 8.0mm size and stored in the laboratory “as received”. The arrangement of the experimental system of the combustor is depicted in Fig. 1. The combustor was a concentric cylindrical shape, made of steel with the cast-able refractory cement lining as the insulation, while the exhaust center pipe was made of stainless steel. The combustor was 1800 mm high and 300 mm inside diameter (D) with the exhausted center pipe of 78 mm in diameter. In the experiment, the snail was mounted at the bottom of the combustion chamber to create the vortex/swirl flow into the combustor. The detail and the sketch of a newly DS vortexcombustor are shown in Table 1 and Fig. 2, respectively. 2.2. Method As schematically shown in Fig. 1, the primary air was drawn into the premixed chamber (or the bottom chamber), together with the rice husk fuel. The entry air was tangentially injected into the bottom combustion chamber of the DSVC by using the snail-type swirl generator. The combustion occurred primarily in the first
Fig. 1. Experimental setup of DS vortex-combustor system.
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Table 1 Detail of DS vortex-combustor geometry Combustion chamber Chamber diameter (Dc) Top chamber diameter (Dt = 1.0Dc) Middle/throat chamber diameter (Dm = 0.5Dc) Bottom chamber diameter (Db = 1.0Dc) Centre/exhaust tube diameter (De = 0.26Dc) Chamber height (Hc = 6.0Dc) Top chamber height (Ht = 2.5Dc) Bottom/Throat chamber height (Hm = 1.0Dc) Bottom chamber height (Hb = 2.5Dc) Centre/exhaust tube height (He = 5.0Dc) Nozzle Number of inlet nozzle at bottom chamber (N) Number of inlet nozzle at throat chamber (N) Nozzle diameter (d) Nozzle height location Material Combustion chamber Centre/exhaust tube Nozzle
300 mm 300 mm 150 mm 300 mm 78 mm 1800 mm 750 mm 300 mm 750 mm 1500 mm 4 nozzles 2 nozzles 10 mm 100, 900 mm Steel Stainless steel Stainless steel
annular vortex chamber or the bottom combustion chamber. Then, the hot combustion gas spirally ascended to the throat ring/ constriction and then to the top chamber or second vortex chamber
before leaving through the exhaust center tube. Before entering the top chamber, the swirling combustion gas flow from the bottom was accelerated when passing through the throat ring which provided the increase in combustion rate. The highly swirling turbulent gas, behind the throat ring, could induce a central toroidal recirculation zone in the entry region of the second chamber that helps to increase the mixing rate between air and fuel. In addition, the combined effects of the centrifugal [11], the gravitational and the fluid drag forces would trapped fuel particles along the height of the combustor which prolonged residence time of husk particle in the chamber. As rice husk particles were burnt, they continually reduce in mass and size until completely burnt out. The majority of the ash particles became light or small enough to be entrained by the flue gas and exit the combustor as fly ash. The large ash particles were trapped in the bottom vortex chamber with high thermal storage capacity inherent and act like a thermal flywheel. The temperatures inside the chamber were monitored at various selected locations with Chromel-alumel (type K) thermocouples read-out with the data logger while the volumetric flow rates of the inlet air was measured by using calibrated orifice meters. The flue gas emissions at the exit were measured by a calibrated gas analyzer (Testo 350 MXL). All data collection was taken at steady state conditions.
Fig. 2. Configuration of DS vortex-combustor with snail entrance.
S. Eiamsa-ard et al. / International Communications in Heat and Mass Transfer 35 (2008) 1134–1140
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3. Results and discussion
3.1. Temperature distributions in DSVC
The dual-staging vortex-combustor is originally designed to accommodate various types of biomass fuels as sources of heat energy. The DSVC responds significantly to the various fuels as expected. For rice husks, the excess air and the secondary air are varied to optimize the combustion characteristics (thermal efficiency, combustion efficiency, heat loss, gas emission, temperature distribution, and exit temperature) in the DSVC. The measurements of temperature are made at 13 axial locations, namely, x/D = 0.25, 0.50, 0.75, 1.25, 1.75, 2.25, 2.75 (throat chamber), 3.25, 3.75, 4.25, 4.75, 5.25, and 5.75, and at six radial stations, namely, r/R = 0.25, 0.40, 0.55, 0.70, 0.85 and 1.0, respectively. The radial and axial profiles of the temperature inside the DSVC for various excess airs and the secondary air injection fractions are presented in Figs. 3 and 4 respectively. The husk feed rate of 0.25 kg/min is kept constant throughout the experiment.
3.1.1. Influence of excess air Effect of excess airs on radial temperature variation in the DSVC is presented in Fig. 3a. In the figure, the excess air is adjusted to be 37%, 56%, 74%, and 92%, at a constant husk feed rate of 0.25 kg/min. The temperature profiles for all locations generally are nearly uniform except for the vicinity of the wall and the bottom chamber for all of the excess airs used. The higher combustion temperature degree at the top of the DSVC indicates a stronger intensity of turbulence flow and combustion, especially at the core area. The peak temperature area is visible in the middle region of the top of combustion zone. It is worth noting that the use of excess air at 74% provides slightly higher and better uniform temperature distributions in the chamber than that of others while the use of 56% excess air causes the lowest temperatures over the chamber.
Fig. 3. Radial temperature distributions in combustor with fuel feed rate of 0.25 kg/min for, (a) various excess airs at λ = 0 and (b) various λ values at 74% excess air.
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Fig. 4. Axial temperature distributions in combustor with fuel feed rate of 0.25 kg/min for, (a) various excess airs at λ = 0 and (b) various λ values at 74% excess air.
3.1.2. Influence of secondary air injection Effects of the secondary air injection on the radial temperature distributions inside the DSVC with the excess air of 74% are illustrated in Fig. 3b. It is visible that the temperature distributions show a nearly flat profile throughout the combustor and a high temperature area can be found in the core region of the chamber. The fraction of volumetric flow rate of the secondary air to that of the total (primary + secondary)
air, defined as λ = Q2/(Q1 + Q2), is an indicator of the strength of the vortex of the flow. The introduced secondary air injection provides the higher tangential velocity component (strong vortex) at downstream of the injection location. This means that the impact of the swirling phenomenon from using the air injection on the flow and the temperature fields become more pronounced as the value of secondary air is increased. In the figure, the use of the secondary air
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In Fig. 4a showing axial temperature variations in the combustor for various excess airs at λ = 0, a close examination reveals that only 37% excess air provides higher axial temperature distributions at the bottom than other excess airs. This indicates that better mixing and high combustion stability occurs throughout the core region at the bottom chamber for the lowest excess air value. On the other hand, at the top chamber, the 74% excess air case yields slightly higher combustion temperature than other cases especially at r/R = 0.25, 0.4, and 0.55. Effect of the secondary air fraction (λ) on the axial temperature distributions is clearly seen in Fig. 4b. In the figure, at the fraction of λ = 0.0, the results show the significant improvement and the temperature profiles are more uniform than those at λ = 0.15 and 0.2 with the highest temperature found at r/R = 0.4, 0.55, 0.7, and 0.8 in the top chamber. This means that the better mixing and more uniform combustion reactions in all regions can be achieved for this case because of higher temperatures obtained. It can be observed that flammability limits of rich and lean mixtures can be extended without secondary air injection. The main reason maybe come from the fact that the introduced secondary air injection causes lower strength of vortex flow created by the snail at the bottom leading to lower strength of the induced central toroidal recirculation zone in the top. Therefore, if a snail-type vortex generator is mounted at the bottom chamber the secondary air injection is not necessary to avoid lower strength of vortex flow inside due to combustion air splitting for the injection. 3.2. Gas emissions The flue gas emissions of CO, CO2, NOx and O2 for various excess air values are presented in Fig. 5. As can be seen in the figure, the emission of CO2 shows a gradual reduction for the increase in excess air while the values of O2 and CO tend to increase with the rise of excess air or high lean mixture. This can be attributed to higher rate of combustion over the top chamber (higher combustion temperature near the top chamber) due to efficient mixing between fuel particle and air in a central toroidal recirculation zone. The emission of NOx is seen to slightly increase for increasing the excess air. Nevertheless, the application of the swirling flow and constriction chamber leads to a small increase in NOx. 3.3. Efficiency and exit temperature
Fig. 5. Emissions of exhaust gases (O2, CO2, CO, and NOx).
fraction λ = 0.2, gives higher and more uniform temperature distributions at the bottom chamber than that of λ = 0.0 (without secondary air injection) and 0.15. The average temperature in the combustion chamber is about 800 to 900 °C. This can be attributed to the higher swirling intensity enhancement and the strong degree of turbulence of air and fuel particles in the combustor. This situation results in prolonging the duration of the combustion. On the other hand, only at the top chamber for the combustion air without secondary air (λ = 0), the temperature profiles for all locations generally are higher than λ = 0.15 and 0.2. This may be attributed to the more complete combustion in this zone due to stronger turbulence leading to better mixing between air and fuel particles, and then heat energy is released considerably leading to the highest temperature. It is interesting to note that the influence of the constriction chamber on the temperature profiles shows the significant improvement on the top chamber while little effect on rising temperature profile at the bottom chamber is seen. The temperature in the vicinity of the center pipe is higher than that of the chamber wall, in general. Maximum temperature of about 1000 °C can be obtained in the core region of the chamber while lower temperatures near the chamber wall.
Influences of the excess air on the exit combustion temperature, the heat loss, the thermal efficiency, and the combustion efficiency at a
Fig. 6. Exit temperature and thermal efficiency versus excess air.
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the test runs, the application of excess air leads to a significant improvement of temperature profiles and yields uniform combustion temperatures including the extension of flammability limits. Besides, the increased swirling intensity, generated by a snail at the combustor entrance, results in an abatement of particle elutriation, lower pollutant emissions and enlargement of recirculation region. The use of secondary air injection is found to be inefficient for the combustor with snail entrance because the split air causes lower strength of vortex flow inside. References
Fig. 7. Heat loss and combustion efficiency against excess air.
constant husk feed rate of 0.25 kg/min, are depicted in Figs. 6 and 7. In the figures, it is found that the exit temperature and thermal efficiency tends to increase with raising the excess air. In general, the use of higher excess air results in higher combustion efficiency than that of lower excess air for burning solid fuels. The exit temperature is found to be in a range from 572 °C and 632 °C while the combustion efficiency of the DSVC is between 99.7%, and 99.9%, depending on excess air values used. 4. Conclusions The measurements of the temperature distribution, flue gas emission, and thermal/combustion efficiency, have been conducted for the combustion of rice husk in a newly DS vortex-combustor. The husk combustion of 0.25 kg/min in the DSVC with the excess air of 74% provides better combustion efficiency, flame stabilization, low emission and reliable furnace condition than other excess air values. For all
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