- Email: [email protected]
Rice husk combustion characteristics in a rectangular fluidized-bed combustor with triple pairs of chevron-shaped discrete ribbed walls
Case Studies in Thermal Engineering 14 (2019) 100511
Contents lists available at ScienceDirect
Case Studies in Thermal Engineering journal homepage: www.elsevier.com/locate/csite
Rice husk combustion characteristics in a rectangular fluidized-bed combustor with triple pairs of chevron-shaped discrete ribbed walls
T
Suriya Chokphoemphuna, Smith Eiamsa-ardb, Pongjet Promvongec, Varesa Chuwattanakulc,∗ a
Rajamangala University of Technology Isan Sakonnakhon Campus, Sakonnakhon, Thailand Faculty of Engineering, Mahanakorn University of Technology, Bangkok, Thailand c Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok, Thailand b
A R T IC LE I N F O
ABS TRA CT
Keywords: Combustion Rectangular fluidized bed combustor Ribbed wall Vortex
An experimental combustion work using the rice husk as a fuel was performed in a rectangular fluidized bed combustor with triple pairs of chevron-shaped discrete ribbed walls. The effect of percent excess air (EA = 40%–70%) on temperature distribution and gas emissions in a rectangular fluidized-bed combustor on combustion behaviors (temperature distributions inside the bed, exhaust gas emissions and combustion efficiency) is reported. The free-board had a rectangular shape which modified by increasing the chamber size from 300 × 100 mm2 to 600 × 200 mm2 or two times of the bottom chamber and 2400 mm in height. Triple pairs of rib configurations, namely, 30° chevron-shaped discrete ribbed walls were introduced and placed in the bottom part of the combustion chamber to generate counter-rotating vortices in the chamber. The results show that the fluidized bed combustor with 30° chevron-shaped discrete ribbed walls at 40% percent excess air provides the higher temperature than other of 861 °C. Among the studied excess air percentages, the highest combustion efficiency of 99.2% is obtained at EA = 60%. In addition, combustion at EA = 60% shows the lowest emissions CO, CO2, O2 and NOx of 236.8%Vol, 2.55 ppm, 13.53 %Vol, 110.2 ppm, respectively.
1. Introduction Biomass is one of the main sustainable energy resources due to its abundance and high potential in energy production. In Thailand, rice husk is one of the most abundant agricultural residues which can be used as a biomass material for an alternative energy production. Several attempts have been made to improve biomass combustion. For decades, the rice husk combustion systems for many engineering purposes have been extensively studied. In general, rice husk can be efficiently converted to energy via combustion in a vortex combustor, cyclone combustor and fluidized bed combustor. Fluidized bed technology has been widely applied to coal combustion and waste incineration. This kind of combustion system offers the advantages of low pollutant emissions, high combustion efficiency which helps to reduce the elutriation of fine particles. Recent investigations on developed combustors for enhancing the combustion efficiency with low emission [1–8] using biomass materials or coals as a fuel were performed. Rozainee et al. [9] studied the optimum fluidizing velocity during the combustion of rice husk in a bench-scale fluidized bed combustor for obtaining the low carbon ash in the amorphous form. They found that the optimum
∗
Corresponding author. E-mail addresses: [email protected] (S. Chokphoemphun), [email protected] (S. Eiamsa-ard), [email protected] (P. Promvonge), [email protected] (V. Chuwattanakul). https://doi.org/10.1016/j.csite.2019.100511 Received 4 June 2019; Received in revised form 25 July 2019; Accepted 1 August 2019 Available online 01 August 2019 2214-157X/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
fluidizing velocity was approximately 3.3 Umf as the mixing of rice husk with the bed was good with a high degree of penetration into the sand bed. Eiamsa-ard et al. [10] reported the influences of equivalence ratio (Φ = 0.8, 1.0 and 1.2) and secondary air ratio (λ = 0.0, 0.15 and 0.25) on combustion characteristics (temperature distribution, fly ash and gas emission) in a multi-staging vortex combustor. The variable size of middle section of multi-staging vortex combustor was adjusted from 1.0D to 0.75D and 0.5D as desired. The experimental results showed that the mean temperature in the multi-staging vortex combustor with middle chamber size of 0.5D was higher than those of the ones with 0.75D and 1.0D. At the highest temperature of 1176 °C, the O2, CO2 and CO emissions from cyclone collector were 2.5%, 17.3% and 270 ppm, respectively. Kuprianov et al. [11] presented results on co-firing of ‘asreceived’ sugar cane bagasse and rice husk in a conical fluidized-bed combustor using silica sand as the bed material. It was found that the CO emission decreased as excess air increased and combustion efficiency was up to 96%. Rozainee et al. [12] used computational fluid dynamics to investigate the trajectories and residence time of burning rice husk particles in the fluidised bed combustor (FBC) at different secondary air flow rates. Their results offered significant insights into the trajectory and mass loss history of the rice husk particle combustion. Eiamsa-ard et al. [13] studied the temperature distributions inside the combustor, the flue gas emissions (CO, CO2, O2, NOx) behaviors in a dual-staging vortex-combustor with rice husk as the fuel. The vortex flow was generated with a snail entrance placed at the bottom combustor. Their results indicated that the highest temperature inside the combustor was 1000 °C whereas both the thermal and the combustion efficiency respectively, were 41.6% and 99.8% for 74% excess air without the secondary air injection (λ = 0.0). Kuprianov et al. [14] examined the burning of rice husk in a swirling fluidized-bed combustor. Radial and axial profiles of temperature and gas concentrations (O2, CO and NO) characteristics were reported. Their result showed that with increasing the fuel-moisture content, the emission of NO from the combustor apparently reduced, while the emission of CO was adjusted at a quite low-level due to the effects of secondary air. Kaewkohkiat et al. [15] studied the axial/radial temperature distributions, gas emissions and combustion efficiency in a fluidized bed combustor (FBC) with wavy surfaces in five modules of converging-diverging nozzles. Rice husk was combusted at various percent excess airs (EA = 15–75%). They found that the emissions of flue gases were found to be CO = 308 ppm and NOx = 298 ppm, at EA = 75%. The combustion efficiency of FBC with wavy surface wall increased up to 92% and 86% as compared to those of the one without wavy surface wall. Eiamsa-ard et al. [16] examined the influence of wavy-ribbed surface chamber on combustion temperature/efficiency and exhaust gas behavior in a fluidized bed combustor (FBC). The five wavy-rib pairs were mounted with the chamber bottom for inducing longitudinal vortex flow. It was found that a FBC with wavy-ribbed surface chamber gave high combustion efficiencies for all test runs especially at the percent excess airs of EA = 75%. Somjun and Chinsuwan [17] studied the overall heat transfer between circulating fluidized beds and the ribbed membrane fins of a water wall tube. In the tested, three isosceles triangular cross section prisms were used as ribs and mounted transversely on the fins. As compared to typical membrane fins, the ribbed membrane fins gave higher heat transfer coefficients and the cluster solid fractions. Bed with various modified chamber walls such as multi-staging chamber [10], dual-staging vortex chamber [11], wavy-ribbed surface chamber [15,16], wavy-rib pairs [17] were used in the fluidized bed combustors or circulating fluidized beds for enhancing heat transfer/combustion efficiencies. The present work proposes a novel modified chamber installed with triple pairs of chevronshaped discrete ribbed walls to generate counter-rotating vortices. This work aims to investigate combustion behaviors including temperature distributions inside the bed, exhaust gas emissions and combustion efficiency. The present work deals with a preliminary study of combustion behaviors inside a rectangular fluidized-bed combustor with chevron-shaped discrete ribbed walls. The triple pairs of chevron-shaped discrete ribbed walls were located at the bottom of the fluidized-bed combustor for generating the longitudinal vortex in the bed. The influence of percent excess air (40%, 50%, 60% and 70%) on combustion behaviors (temperature distributions inside the bed, exhaust gas emissions (O2, CO2, CO and NOx) and combustion efficiency) was investigated. A fuel mass flow rate was kept constant at 8.5 kg/hr throughout the experiments. 2. Combustion foundation The amount of air used in combustion processes is expressed in terms of the percent excess air, EA, which can be written as
%EA =
(moles of air )actual − (moles of air )stoichiometric × 100% (moles of air )stoichiometric
(1)
Stoichiometric or theoretical air is the moles (for a batch system) or molar flow rate (for a continuous system) of air needed for complete combustion of all fuel fed to the combustor. 3. Experimental setup A schematic diagram of the experimental apparatus system used in this study is depicted in Figs. 1–3. The total height of a rectangular fluidized-bed combustor with diffuser chamber was 2400 mm. The free-board was modified by increasing the chamber size from 300 × 100 mm2 to 600 × 200 mm2 or two times of the bottom chamber. The rectangular fluidized-bed combustor with ribbed surface wall was installed as the bottom part of combustor chamber. Triple pairs of rib configurations, namely, chevronshaped discrete ribs with attack angle 30° were employed as vortex generators. Fluidized bed combustor was made of stainless steel with a thickness of 6 mm. The outer wall of the chamber was insulated with 40 mm thick brick and covered by 1 mm thick galvanized steel. A 10 hp blower was used for supplying air flow in the chamber. A set of orifice meter (ANSI/API 2530) and control valve was installed for measuring and adjusting the flow rates. A perforated distributor plate was placed at the bottom of the combustion chamber. The perforated distributor plate was made of stainless steel and had a diameter of 62 mm. LPG supply was used for pre2
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
Fig. 1. Dimensional details of rectangular fluidized-bed combustor with diffuser chamber.
Fig. 2. Schematic diagram of rectangular fluidized-bed combustor and facility.
Fig. 3. Schematic diagram of fluidized-bed combustor with triple pairs of chevron-shaped discrete ribbed walls and chevron-shaped ribs (θ = 30°, h = 108 mm and H = 206 mm).
3
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
heating the temperature of combustion air in the bed. A hopper with a screw feeder driven by 1/2 hp motor was used to contain and feed rice husk. A series of K-type thermocouple probes connecting to the data logger were utilized to measure the temperatures in the chamber, Twelve K-type thermocouple probes were located along the rectangular fluidized-bed combustor for measuring the radial temperature profiles. The temperature probes were installed on the wall of the combustor at 12 stations at X = 200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800, 2000, 2200 and 2400 mm above the distributor. A flue gas analyzer (Testo 350 MXL) was used for measuring gas emission and a cyclone was for capturing the fly ash that elutriates from the bed. Combustion was commenced by heating up the rectangular fluidized-bed combustor with LPG torch inserted at the bottom chamber. The preheating took about 20–30 min for the chamber to raise its temperature to be about 500 °C to 550 °C. Air was drawn by the blower and then discharged into the premixed bed (or the bottom chamber), together with milled rice husk particles possessing particle size up to 2.0 × 8.0 mm2. Rice husk particles were fed through a screw feeder into the premixed bottom chamber with constant feed rate of 8.5 kg/hr. When the temperature in the bed reached 700 °C to 750 °C, preheating by LPG was ceased. The combustor was operated with temperature up to 1000 °C. The tests were conducted at four different excess air percentages, EA = 40%, 50%, 60% and 70%, respectively. During tests, the inlet air from distributor plate was directed through the bottom chamber and the combustion occurs primarily in the bottom chamber. Then, hot combustion gas spirally ascended to the ribbed surface chamber and then to the top chamber before leaving through the exhaust tube. During the combustion rice husk particles were transformed into ash particles which were light and small enough to be entrained by the flue gas and exited the combustor as fly ash. Large ash particles were trapped in the bottom chamber with high thermal storage capacity inherent and acted like a thermal flywheel. For all tests, the following variables were measured at steady conditions: (1) air flow rates (percent excess air, EA = 40%, 50%, 60% and 70%), (2) temperatures along the rectangular fluidized-bed combustor, and (3) emissions of CO, CO2, O2, NOx. 4. Results and discussion Fig. 4 shows the temperature profiles in in various parts of rectangular fluidized-bed combustor (combustion chamber, diffuser and freeboard) for different percent excess air (EA) of 40%, 50%, 60% and 70%. The effect of excess air percentage (EA) on temperatures in fluidized-bed combustors with three pairs of rib is also presented Table 1. The high combustion temperature indicates the high efficient combustion due to the good mixing between air and fuel particles. This is attributed to a strong turbulence intensity caused by longitudinal vortex and re-circulating flows [15–17] which were generated by triple pairs of chevron-shaped discrete ribbed walls. For all excess air percentages, temperature profiles were nearly flat throughout the mixing and combustion chambers. The temperatures in the combustion chamber are higher than those in the diffuser and freeboard chambers. Apparently, temperatures in mixing chamber and combustion chamber (bottom chamber) decreased with increasing excess air ratio (EA). The temperature drop can be attributed to the cooling effect by excess air fed to the combustor. For the investigated range, the maximum temperature in the combustor with three pairs of rib of 861 °C was achieved at EA of 40%. At high percent excess air (EA = 60% and 70%), it is found
Fig. 4. Radial temperature profiles in a rectangular fluidized-bed combustor at different excess air percentages. 4
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
Table 1 Combustion temperature inside a fluidized-bed combustor with multiple chevron-shaped ribbed walls. Percent excess air (EA: %)
40 50 60 70
Mean combustion temperatures (°C) Mixing chamber
Combustion chamber
Diffuser
Freeboard
792 698 750 719
861 811 798 789
520 518 520 547
447 498 511 521
Fig. 5. Emission gas results in a rectangular fluidized-bed combustor at different excess air percentages. 5
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
104
Eff(%)
102 100 98 96 94 35
40
45
50
55
60
65
70
75
EA(%) Fig. 6. Combustion efficiency results in FBC with chevron-shaped ribbed walls at different excess air percentages.
that temperature in the mixing and combustion chambers are lower than those found at low percent excess air (EA = 40% and 50%) due to high speed of the air flow. However, the opposite trend is found at the top bed. The temperature peaks are found in the combustion chamber, for all cases. The effects of excess air percentage (EA) on emissions of exhaust gases (O2, CO, CO2 and NOx) and combustion efficiency are presented. Fig. 5 shows the exhaust gas emissions from rectangular fluidized-bed combustor at various excess air percentages (EA = 40%, 50%, 60% and 70%). Among the studied excess air percentages, combustion at EA = 50% gave the lowest emissions CO, CO2, O2 components but the highest emission of NOX which is comparable to those at EA = 60% and 70%. Evidently, the emissions of O2, CO2 and NOx gradually increase while the emission of CO decrease with the increase in excess air percentage. The reduction of the emission of CO indicates that the combustion is more completed. Combustion efficiency tends to increase gradually with raising excess air percentage. Fig. 6 presents the effect of the chevron-shaped discrete ribbed walls on combustion efficiency. The calculation of combustion efficiency is based on unburned carbon in the fly-ash. The combustion efficiency tends to decrease with increasing percent excess air. However, the maximum combustion efficiency is obtained at EA = 50%. The combustion efficiencies at EA = 40%, 50%, 60% and 70% are found to be 94.7%, 95.8%, 94.1% and 94.3%, respectively. The high combustion efficiencies can be attributed to high rate of combustion caused by the efficient mixing between fuel and combustion air in rectangular fluidized-bed combustor with triple pairs of chevron-shaped discrete ribbed walls [15–17]. The highly turbulent gas beyond the triple pairs of chevron-shaped discrete ribbed walls generated a central recirculation zone in bottom chamber that helps to improve mixing between air and fuel particles and to prolong residence time of husk particles in the chamber. In addition, the combined effects of centrifugal, gravitational and fluid drag forces gives rise to fuel particle trapping along the height of the combustor. 5. Conclusion The temperature variations, exhaust gas emissions and combustion efficiency in a rectangular fluidized-bed combustor at different percent excess airs (EA = 40%–70%) were experimentally studied. The FBC with triple pairs of chevron-shaped discrete ribbed walls yields high combustion efficiency, flame stabilization, and low emission. Among the studied excess air percentages, the highest combustion efficiency of 99.2% is obtained at EA = 60%. In addition, combustion at EA = 60% gave the emissions CO, CO2, O2 and NOx of 236.8%Vol, 2.55 ppm, 13.53 %Vol, 110.2 ppm, respectively. Conflicts of interest The authors declare there are no conflicts of interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.csite.2019.100511. References [1] H.M. Abdelmotalib, M.A.M. Youssef, A.A. Hassan, S.B. Youn, I.T. Im, Influence of the specularity coefficient on hydrodynamics and heat transfer in a conical fluidized bed combustor, Int. Commun. Heat Mass Transf. 75 (2016) 169–176.
6
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
[2] G. Chen, G. Du, W. Ma, B. Yan, Z. Wang, W. Gao, Production of amorphous rice husk ash in a 500 kW fluidized bed combustor, Fuel 144 (2015) 214–221. [3] S. Cao, F. Duan, L. Zhang, C. Chyang, C.Y. Yang, Application of response surface methodology to determine effects of operational conditions on in-bed combustion fraction in vortexing fluidized-bed combustor using different fuels, Energy 139 (2017) 862–870. [4] K. Sirisomboon, P. Charernporn, Effects of air staging on emission characteristics in a conical fluidized-bed combustor firing with sunflower shells, J. Energy Inst. 90 (2017) 316–323. [5] L.H. Zhang, C.S. Chyang, F. Duan, P.W. Li, S.Y. Chen, Comparison of the thermal behaviors and pollutant emissions of pelletized bamboo combustion in a fluidized bed combustor at different secondary gas injection modes, Energy 116 (2016) 306–316. [6] M. Gaba, H. Kumar, S.K. Mohapatra, Modeling of bubbling fluidized bed combustor based on biomass & co-firing, Mater. Today: Proceedings 4 (2017) 1615–1625. [7] A. Ridluan, S. Eiamsa-ard, P. Promvonge, Numerical simulation of 3D turbulent isothermal flow in a vortex combustor, Int. Commun. Heat Mass Transf. 34 (2007) 860–869. [8] M.M. Farid, H.J. Jeong, K.H. Kim, J. Lee, D. Kim, J. Hwang, Numerical investigation of particle transport hydrodynamics and coal combustion in an industrialscale circulating fluidized bed combustor: effects of coal feeder positions and coal feeding rates, Fuel 192 (2017) 187–200. [9] M. Rozainee, S.P. Ngo, A.A. Salema, K.G. Tan, M. Ariffin, Z.N. Zainura, Effect of fluidising velocity on the combustion of rice husk in a bench-scale fluidised bed combustor for the production of amorphous rice husk ash, Bioresour. Technol. 99 (2008) 703–713. [10] S. Eiamsa-ard, Y. Kaewkohkiat, W. Lelaphatikul, C. Thianpong, P. Promvonge, Experimental investigation of combustion characteristics in a multi-staging vortex combustor firing rice husk, Int. Commun. Heat Mass Transf. 35 (2008) 139–148. [11] V.I. Kuprianov, K. Janvijitsakul, W. Permchart, Co-firing of sugar cane bagasse with rice husk in a conical fluidized-bed combustor, Fuel 85 (2006) 434–442. [12] M. Rozainee, S.P. Ngo, A.A. Salema, K.G. Tan, Computational fluid dynamics modeling of rice husk combustion in a fluidised bed combustor, Powder Technol. 203 (2010) 331–347. [13] S. Eiamsa-ard, Y. Kaewkohkiat, C. Thianpong, P. Promvonge, Combustion behavior in a dual-staging vortex rice husk combustor with snail entry, Int. Commun. Heat Mass Transf. 35 (2008) 1134–1140. [14] V.I. Kuprianov, R. Kaewklum, K. Sirisomboon, P. Arromdee, S. Chakritthakul, Combustion and emission characteristics of a swirling fluidized-bed combustor burning moisturized rice husk, Appl. Energy 87 (2010) 2899–2906. [15] Kaewkohkiat, Y., Eiamsa-Ard, S., Wongcharee, K., Thungsotanon, D., Promvonge, P., Combustion of rice husk in a fluidized bed combustor with wavy-surfaced chambers, Proceeding of International Conference on Power Engineering-09 (ICOPE-09), November 16-20, 2009, Kobe, Japan. [16] Eiamsa-ard, S., Wongcharee, K., Chokphoemphun, S., Chuwattanakul, V., Promvonge, P., Investigation on rice husk combustion in a fluidized bed with longitudinal vortex generators, IOP Conference Series: Earth and Environmental Science 265(1), Paper Number 012008 (2019), 10th International Conference on Combustion, Incineration/Pyrolysis, Emission and Climate Change, 18-21 December 2018, Bangkok Thailand. [17] J. Somjun, A. Chinsuwan, Effect of transverse rib on heat transfer between circulating fluidized bed and membrane fins of water wall membrane tubes, Powder Technol. 332 (2018) 178–187.
7
Contents lists available at ScienceDirect
Case Studies in Thermal Engineering journal homepage: www.elsevier.com/locate/csite
Rice husk combustion characteristics in a rectangular fluidized-bed combustor with triple pairs of chevron-shaped discrete ribbed walls
T
Suriya Chokphoemphuna, Smith Eiamsa-ardb, Pongjet Promvongec, Varesa Chuwattanakulc,∗ a
Rajamangala University of Technology Isan Sakonnakhon Campus, Sakonnakhon, Thailand Faculty of Engineering, Mahanakorn University of Technology, Bangkok, Thailand c Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok, Thailand b
A R T IC LE I N F O
ABS TRA CT
Keywords: Combustion Rectangular fluidized bed combustor Ribbed wall Vortex
An experimental combustion work using the rice husk as a fuel was performed in a rectangular fluidized bed combustor with triple pairs of chevron-shaped discrete ribbed walls. The effect of percent excess air (EA = 40%–70%) on temperature distribution and gas emissions in a rectangular fluidized-bed combustor on combustion behaviors (temperature distributions inside the bed, exhaust gas emissions and combustion efficiency) is reported. The free-board had a rectangular shape which modified by increasing the chamber size from 300 × 100 mm2 to 600 × 200 mm2 or two times of the bottom chamber and 2400 mm in height. Triple pairs of rib configurations, namely, 30° chevron-shaped discrete ribbed walls were introduced and placed in the bottom part of the combustion chamber to generate counter-rotating vortices in the chamber. The results show that the fluidized bed combustor with 30° chevron-shaped discrete ribbed walls at 40% percent excess air provides the higher temperature than other of 861 °C. Among the studied excess air percentages, the highest combustion efficiency of 99.2% is obtained at EA = 60%. In addition, combustion at EA = 60% shows the lowest emissions CO, CO2, O2 and NOx of 236.8%Vol, 2.55 ppm, 13.53 %Vol, 110.2 ppm, respectively.
1. Introduction Biomass is one of the main sustainable energy resources due to its abundance and high potential in energy production. In Thailand, rice husk is one of the most abundant agricultural residues which can be used as a biomass material for an alternative energy production. Several attempts have been made to improve biomass combustion. For decades, the rice husk combustion systems for many engineering purposes have been extensively studied. In general, rice husk can be efficiently converted to energy via combustion in a vortex combustor, cyclone combustor and fluidized bed combustor. Fluidized bed technology has been widely applied to coal combustion and waste incineration. This kind of combustion system offers the advantages of low pollutant emissions, high combustion efficiency which helps to reduce the elutriation of fine particles. Recent investigations on developed combustors for enhancing the combustion efficiency with low emission [1–8] using biomass materials or coals as a fuel were performed. Rozainee et al. [9] studied the optimum fluidizing velocity during the combustion of rice husk in a bench-scale fluidized bed combustor for obtaining the low carbon ash in the amorphous form. They found that the optimum
∗
Corresponding author. E-mail addresses: [email protected] (S. Chokphoemphun), [email protected] (S. Eiamsa-ard), [email protected] (P. Promvonge), [email protected] (V. Chuwattanakul). https://doi.org/10.1016/j.csite.2019.100511 Received 4 June 2019; Received in revised form 25 July 2019; Accepted 1 August 2019 Available online 01 August 2019 2214-157X/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
fluidizing velocity was approximately 3.3 Umf as the mixing of rice husk with the bed was good with a high degree of penetration into the sand bed. Eiamsa-ard et al. [10] reported the influences of equivalence ratio (Φ = 0.8, 1.0 and 1.2) and secondary air ratio (λ = 0.0, 0.15 and 0.25) on combustion characteristics (temperature distribution, fly ash and gas emission) in a multi-staging vortex combustor. The variable size of middle section of multi-staging vortex combustor was adjusted from 1.0D to 0.75D and 0.5D as desired. The experimental results showed that the mean temperature in the multi-staging vortex combustor with middle chamber size of 0.5D was higher than those of the ones with 0.75D and 1.0D. At the highest temperature of 1176 °C, the O2, CO2 and CO emissions from cyclone collector were 2.5%, 17.3% and 270 ppm, respectively. Kuprianov et al. [11] presented results on co-firing of ‘asreceived’ sugar cane bagasse and rice husk in a conical fluidized-bed combustor using silica sand as the bed material. It was found that the CO emission decreased as excess air increased and combustion efficiency was up to 96%. Rozainee et al. [12] used computational fluid dynamics to investigate the trajectories and residence time of burning rice husk particles in the fluidised bed combustor (FBC) at different secondary air flow rates. Their results offered significant insights into the trajectory and mass loss history of the rice husk particle combustion. Eiamsa-ard et al. [13] studied the temperature distributions inside the combustor, the flue gas emissions (CO, CO2, O2, NOx) behaviors in a dual-staging vortex-combustor with rice husk as the fuel. The vortex flow was generated with a snail entrance placed at the bottom combustor. Their results indicated that the highest temperature inside the combustor was 1000 °C whereas both the thermal and the combustion efficiency respectively, were 41.6% and 99.8% for 74% excess air without the secondary air injection (λ = 0.0). Kuprianov et al. [14] examined the burning of rice husk in a swirling fluidized-bed combustor. Radial and axial profiles of temperature and gas concentrations (O2, CO and NO) characteristics were reported. Their result showed that with increasing the fuel-moisture content, the emission of NO from the combustor apparently reduced, while the emission of CO was adjusted at a quite low-level due to the effects of secondary air. Kaewkohkiat et al. [15] studied the axial/radial temperature distributions, gas emissions and combustion efficiency in a fluidized bed combustor (FBC) with wavy surfaces in five modules of converging-diverging nozzles. Rice husk was combusted at various percent excess airs (EA = 15–75%). They found that the emissions of flue gases were found to be CO = 308 ppm and NOx = 298 ppm, at EA = 75%. The combustion efficiency of FBC with wavy surface wall increased up to 92% and 86% as compared to those of the one without wavy surface wall. Eiamsa-ard et al. [16] examined the influence of wavy-ribbed surface chamber on combustion temperature/efficiency and exhaust gas behavior in a fluidized bed combustor (FBC). The five wavy-rib pairs were mounted with the chamber bottom for inducing longitudinal vortex flow. It was found that a FBC with wavy-ribbed surface chamber gave high combustion efficiencies for all test runs especially at the percent excess airs of EA = 75%. Somjun and Chinsuwan [17] studied the overall heat transfer between circulating fluidized beds and the ribbed membrane fins of a water wall tube. In the tested, three isosceles triangular cross section prisms were used as ribs and mounted transversely on the fins. As compared to typical membrane fins, the ribbed membrane fins gave higher heat transfer coefficients and the cluster solid fractions. Bed with various modified chamber walls such as multi-staging chamber [10], dual-staging vortex chamber [11], wavy-ribbed surface chamber [15,16], wavy-rib pairs [17] were used in the fluidized bed combustors or circulating fluidized beds for enhancing heat transfer/combustion efficiencies. The present work proposes a novel modified chamber installed with triple pairs of chevronshaped discrete ribbed walls to generate counter-rotating vortices. This work aims to investigate combustion behaviors including temperature distributions inside the bed, exhaust gas emissions and combustion efficiency. The present work deals with a preliminary study of combustion behaviors inside a rectangular fluidized-bed combustor with chevron-shaped discrete ribbed walls. The triple pairs of chevron-shaped discrete ribbed walls were located at the bottom of the fluidized-bed combustor for generating the longitudinal vortex in the bed. The influence of percent excess air (40%, 50%, 60% and 70%) on combustion behaviors (temperature distributions inside the bed, exhaust gas emissions (O2, CO2, CO and NOx) and combustion efficiency) was investigated. A fuel mass flow rate was kept constant at 8.5 kg/hr throughout the experiments. 2. Combustion foundation The amount of air used in combustion processes is expressed in terms of the percent excess air, EA, which can be written as
%EA =
(moles of air )actual − (moles of air )stoichiometric × 100% (moles of air )stoichiometric
(1)
Stoichiometric or theoretical air is the moles (for a batch system) or molar flow rate (for a continuous system) of air needed for complete combustion of all fuel fed to the combustor. 3. Experimental setup A schematic diagram of the experimental apparatus system used in this study is depicted in Figs. 1–3. The total height of a rectangular fluidized-bed combustor with diffuser chamber was 2400 mm. The free-board was modified by increasing the chamber size from 300 × 100 mm2 to 600 × 200 mm2 or two times of the bottom chamber. The rectangular fluidized-bed combustor with ribbed surface wall was installed as the bottom part of combustor chamber. Triple pairs of rib configurations, namely, chevronshaped discrete ribs with attack angle 30° were employed as vortex generators. Fluidized bed combustor was made of stainless steel with a thickness of 6 mm. The outer wall of the chamber was insulated with 40 mm thick brick and covered by 1 mm thick galvanized steel. A 10 hp blower was used for supplying air flow in the chamber. A set of orifice meter (ANSI/API 2530) and control valve was installed for measuring and adjusting the flow rates. A perforated distributor plate was placed at the bottom of the combustion chamber. The perforated distributor plate was made of stainless steel and had a diameter of 62 mm. LPG supply was used for pre2
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
Fig. 1. Dimensional details of rectangular fluidized-bed combustor with diffuser chamber.
Fig. 2. Schematic diagram of rectangular fluidized-bed combustor and facility.
Fig. 3. Schematic diagram of fluidized-bed combustor with triple pairs of chevron-shaped discrete ribbed walls and chevron-shaped ribs (θ = 30°, h = 108 mm and H = 206 mm).
3
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
heating the temperature of combustion air in the bed. A hopper with a screw feeder driven by 1/2 hp motor was used to contain and feed rice husk. A series of K-type thermocouple probes connecting to the data logger were utilized to measure the temperatures in the chamber, Twelve K-type thermocouple probes were located along the rectangular fluidized-bed combustor for measuring the radial temperature profiles. The temperature probes were installed on the wall of the combustor at 12 stations at X = 200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800, 2000, 2200 and 2400 mm above the distributor. A flue gas analyzer (Testo 350 MXL) was used for measuring gas emission and a cyclone was for capturing the fly ash that elutriates from the bed. Combustion was commenced by heating up the rectangular fluidized-bed combustor with LPG torch inserted at the bottom chamber. The preheating took about 20–30 min for the chamber to raise its temperature to be about 500 °C to 550 °C. Air was drawn by the blower and then discharged into the premixed bed (or the bottom chamber), together with milled rice husk particles possessing particle size up to 2.0 × 8.0 mm2. Rice husk particles were fed through a screw feeder into the premixed bottom chamber with constant feed rate of 8.5 kg/hr. When the temperature in the bed reached 700 °C to 750 °C, preheating by LPG was ceased. The combustor was operated with temperature up to 1000 °C. The tests were conducted at four different excess air percentages, EA = 40%, 50%, 60% and 70%, respectively. During tests, the inlet air from distributor plate was directed through the bottom chamber and the combustion occurs primarily in the bottom chamber. Then, hot combustion gas spirally ascended to the ribbed surface chamber and then to the top chamber before leaving through the exhaust tube. During the combustion rice husk particles were transformed into ash particles which were light and small enough to be entrained by the flue gas and exited the combustor as fly ash. Large ash particles were trapped in the bottom chamber with high thermal storage capacity inherent and acted like a thermal flywheel. For all tests, the following variables were measured at steady conditions: (1) air flow rates (percent excess air, EA = 40%, 50%, 60% and 70%), (2) temperatures along the rectangular fluidized-bed combustor, and (3) emissions of CO, CO2, O2, NOx. 4. Results and discussion Fig. 4 shows the temperature profiles in in various parts of rectangular fluidized-bed combustor (combustion chamber, diffuser and freeboard) for different percent excess air (EA) of 40%, 50%, 60% and 70%. The effect of excess air percentage (EA) on temperatures in fluidized-bed combustors with three pairs of rib is also presented Table 1. The high combustion temperature indicates the high efficient combustion due to the good mixing between air and fuel particles. This is attributed to a strong turbulence intensity caused by longitudinal vortex and re-circulating flows [15–17] which were generated by triple pairs of chevron-shaped discrete ribbed walls. For all excess air percentages, temperature profiles were nearly flat throughout the mixing and combustion chambers. The temperatures in the combustion chamber are higher than those in the diffuser and freeboard chambers. Apparently, temperatures in mixing chamber and combustion chamber (bottom chamber) decreased with increasing excess air ratio (EA). The temperature drop can be attributed to the cooling effect by excess air fed to the combustor. For the investigated range, the maximum temperature in the combustor with three pairs of rib of 861 °C was achieved at EA of 40%. At high percent excess air (EA = 60% and 70%), it is found
Fig. 4. Radial temperature profiles in a rectangular fluidized-bed combustor at different excess air percentages. 4
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
Table 1 Combustion temperature inside a fluidized-bed combustor with multiple chevron-shaped ribbed walls. Percent excess air (EA: %)
40 50 60 70
Mean combustion temperatures (°C) Mixing chamber
Combustion chamber
Diffuser
Freeboard
792 698 750 719
861 811 798 789
520 518 520 547
447 498 511 521
Fig. 5. Emission gas results in a rectangular fluidized-bed combustor at different excess air percentages. 5
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
104
Eff(%)
102 100 98 96 94 35
40
45
50
55
60
65
70
75
EA(%) Fig. 6. Combustion efficiency results in FBC with chevron-shaped ribbed walls at different excess air percentages.
that temperature in the mixing and combustion chambers are lower than those found at low percent excess air (EA = 40% and 50%) due to high speed of the air flow. However, the opposite trend is found at the top bed. The temperature peaks are found in the combustion chamber, for all cases. The effects of excess air percentage (EA) on emissions of exhaust gases (O2, CO, CO2 and NOx) and combustion efficiency are presented. Fig. 5 shows the exhaust gas emissions from rectangular fluidized-bed combustor at various excess air percentages (EA = 40%, 50%, 60% and 70%). Among the studied excess air percentages, combustion at EA = 50% gave the lowest emissions CO, CO2, O2 components but the highest emission of NOX which is comparable to those at EA = 60% and 70%. Evidently, the emissions of O2, CO2 and NOx gradually increase while the emission of CO decrease with the increase in excess air percentage. The reduction of the emission of CO indicates that the combustion is more completed. Combustion efficiency tends to increase gradually with raising excess air percentage. Fig. 6 presents the effect of the chevron-shaped discrete ribbed walls on combustion efficiency. The calculation of combustion efficiency is based on unburned carbon in the fly-ash. The combustion efficiency tends to decrease with increasing percent excess air. However, the maximum combustion efficiency is obtained at EA = 50%. The combustion efficiencies at EA = 40%, 50%, 60% and 70% are found to be 94.7%, 95.8%, 94.1% and 94.3%, respectively. The high combustion efficiencies can be attributed to high rate of combustion caused by the efficient mixing between fuel and combustion air in rectangular fluidized-bed combustor with triple pairs of chevron-shaped discrete ribbed walls [15–17]. The highly turbulent gas beyond the triple pairs of chevron-shaped discrete ribbed walls generated a central recirculation zone in bottom chamber that helps to improve mixing between air and fuel particles and to prolong residence time of husk particles in the chamber. In addition, the combined effects of centrifugal, gravitational and fluid drag forces gives rise to fuel particle trapping along the height of the combustor. 5. Conclusion The temperature variations, exhaust gas emissions and combustion efficiency in a rectangular fluidized-bed combustor at different percent excess airs (EA = 40%–70%) were experimentally studied. The FBC with triple pairs of chevron-shaped discrete ribbed walls yields high combustion efficiency, flame stabilization, and low emission. Among the studied excess air percentages, the highest combustion efficiency of 99.2% is obtained at EA = 60%. In addition, combustion at EA = 60% gave the emissions CO, CO2, O2 and NOx of 236.8%Vol, 2.55 ppm, 13.53 %Vol, 110.2 ppm, respectively. Conflicts of interest The authors declare there are no conflicts of interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.csite.2019.100511. References [1] H.M. Abdelmotalib, M.A.M. Youssef, A.A. Hassan, S.B. Youn, I.T. Im, Influence of the specularity coefficient on hydrodynamics and heat transfer in a conical fluidized bed combustor, Int. Commun. Heat Mass Transf. 75 (2016) 169–176.
6
Case Studies in Thermal Engineering 14 (2019) 100511
S. Chokphoemphun, et al.
[2] G. Chen, G. Du, W. Ma, B. Yan, Z. Wang, W. Gao, Production of amorphous rice husk ash in a 500 kW fluidized bed combustor, Fuel 144 (2015) 214–221. [3] S. Cao, F. Duan, L. Zhang, C. Chyang, C.Y. Yang, Application of response surface methodology to determine effects of operational conditions on in-bed combustion fraction in vortexing fluidized-bed combustor using different fuels, Energy 139 (2017) 862–870. [4] K. Sirisomboon, P. Charernporn, Effects of air staging on emission characteristics in a conical fluidized-bed combustor firing with sunflower shells, J. Energy Inst. 90 (2017) 316–323. [5] L.H. Zhang, C.S. Chyang, F. Duan, P.W. Li, S.Y. Chen, Comparison of the thermal behaviors and pollutant emissions of pelletized bamboo combustion in a fluidized bed combustor at different secondary gas injection modes, Energy 116 (2016) 306–316. [6] M. Gaba, H. Kumar, S.K. Mohapatra, Modeling of bubbling fluidized bed combustor based on biomass & co-firing, Mater. Today: Proceedings 4 (2017) 1615–1625. [7] A. Ridluan, S. Eiamsa-ard, P. Promvonge, Numerical simulation of 3D turbulent isothermal flow in a vortex combustor, Int. Commun. Heat Mass Transf. 34 (2007) 860–869. [8] M.M. Farid, H.J. Jeong, K.H. Kim, J. Lee, D. Kim, J. Hwang, Numerical investigation of particle transport hydrodynamics and coal combustion in an industrialscale circulating fluidized bed combustor: effects of coal feeder positions and coal feeding rates, Fuel 192 (2017) 187–200. [9] M. Rozainee, S.P. Ngo, A.A. Salema, K.G. Tan, M. Ariffin, Z.N. Zainura, Effect of fluidising velocity on the combustion of rice husk in a bench-scale fluidised bed combustor for the production of amorphous rice husk ash, Bioresour. Technol. 99 (2008) 703–713. [10] S. Eiamsa-ard, Y. Kaewkohkiat, W. Lelaphatikul, C. Thianpong, P. Promvonge, Experimental investigation of combustion characteristics in a multi-staging vortex combustor firing rice husk, Int. Commun. Heat Mass Transf. 35 (2008) 139–148. [11] V.I. Kuprianov, K. Janvijitsakul, W. Permchart, Co-firing of sugar cane bagasse with rice husk in a conical fluidized-bed combustor, Fuel 85 (2006) 434–442. [12] M. Rozainee, S.P. Ngo, A.A. Salema, K.G. Tan, Computational fluid dynamics modeling of rice husk combustion in a fluidised bed combustor, Powder Technol. 203 (2010) 331–347. [13] S. Eiamsa-ard, Y. Kaewkohkiat, C. Thianpong, P. Promvonge, Combustion behavior in a dual-staging vortex rice husk combustor with snail entry, Int. Commun. Heat Mass Transf. 35 (2008) 1134–1140. [14] V.I. Kuprianov, R. Kaewklum, K. Sirisomboon, P. Arromdee, S. Chakritthakul, Combustion and emission characteristics of a swirling fluidized-bed combustor burning moisturized rice husk, Appl. Energy 87 (2010) 2899–2906. [15] Kaewkohkiat, Y., Eiamsa-Ard, S., Wongcharee, K., Thungsotanon, D., Promvonge, P., Combustion of rice husk in a fluidized bed combustor with wavy-surfaced chambers, Proceeding of International Conference on Power Engineering-09 (ICOPE-09), November 16-20, 2009, Kobe, Japan. [16] Eiamsa-ard, S., Wongcharee, K., Chokphoemphun, S., Chuwattanakul, V., Promvonge, P., Investigation on rice husk combustion in a fluidized bed with longitudinal vortex generators, IOP Conference Series: Earth and Environmental Science 265(1), Paper Number 012008 (2019), 10th International Conference on Combustion, Incineration/Pyrolysis, Emission and Climate Change, 18-21 December 2018, Bangkok Thailand. [17] J. Somjun, A. Chinsuwan, Effect of transverse rib on heat transfer between circulating fluidized bed and membrane fins of water wall membrane tubes, Powder Technol. 332 (2018) 178–187.
7