Journal of Energy Resources Technology. Received August 21, 2014;
Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME SIMULATION OF OXY- FUEL COMBUSTION OF HEAVY OIL FUEL IN A MODEL FURNACE ed Co py ot Ma nu sc rip tN M. A. Habib, third author 1 King Fahd University of Petroleum & Minerals Professor, Mechanical Engineering Department KFUPM, Dhahran-31261, Saudi Arabia Office: Bldg-22, Room-121-B Phone: 966-13-860 4467 Fax: 966-13-860 2949 e-mail: [email protected] ite d Rached Ben-Manosur, first author King Fahd University of Petroleum & Minerals Assistant Professor, Mechanical Engineering Department KFUPM, Dhahran-31261, Saudi Arabia Office: Bldg-22, Room-121.1 Phone: 966-13-860 2014 Fax: 966-13-860 2949 e-mail: [email protected] Ac ce pt ed Pervez Ahmed, second author King Fahd University of Petroleum & Minerals Research Engineer, KACST-Technology Innovation Center on Carbon Capture and Sequestration KFUPM, Dhahran-31261, Saudi Arabia Office: Bldg-15, Room-5114 Phone: 966-13-860 7869 Fax: 966-13-860 2266 e-mail: [email protected] 1 Corresponding Author/ Prof. M. A. Habib, Tel: +966 13 860 4467, Email: [email protected] Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME ABSTRACT The present study aims at investigating the characteristics of oxy-combustion of heavy oil liquid fuel in a down-fired model furnace. Non-premixed PDF mixture model was used to simulate the combustion characteristics and turbulence chemistry. The validation of the present model was performed against the experimental data and is found to be in good agreement. The results depict that the oxy-combustion of ite d liquid fuels results in lower soot. It is observed that the soot formation is reduced when N2 in air- ed combustion is replaced by O2 in oxy-combustion. However, it increases as the amount of oxygen in oxycombustion increases. Replacing nitrogen in the air combustion by carbon dioxide in oxyfuel combustion tN ot Keywords: Model furnace, liquid fuels, oxy-combustion, CFD Co py tends to reduce the temperature levels in the upstream sections of the combustion chamber. rip 1. INTRODUCTION nu sc Significant efforts in the combustion process have been made in the past to Ma obtain high thermal efficiencies along with a reduction in NOx emissions [1]. Many researchers [2-6] have reviewed the aspects of atomization and vaporization of liquid pt ed fuel droplets along with spray combustion with emphasis on combustion engines and ce gas-turbine combustor applications. These aspects provide a method of instant Ac vaporization and mixing of liquid fuels with oxidizer that significantly affects the combustion rate. Spraying liquid fuels during the combustion process greatly affects the ignition, heat release rate, exhaust emissions and pollutant formations [7]. Many [8-11] modeling efforts of liquid atomization and spray combustion have been attempted during the past few years and satisfactory level of agreement between the experimental Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME data and numerical simulations in flame structure and soot predictions are successfully obtained. Givler and Abraham [5] reviewed the vaporization and combustion of paraffin fuel droplets from n-pentane to n-dodecane under supercritical pressures and ite d temperatures. Assumptions in modeling the vaporization and combustion and their ed relative importance were also discussed. Their work indicates that a vaporizing droplet Co py reach the critical state for ambient pressures and ambient temperatures greater than approximately twice the fuel’s critical pressure and critical temperature respectively tN ot where as combusting droplets reach this state when ambient pressure is approximately rip 2.5 times that of the fuel’s critical pressure. Aoki et al [12] studied numerically the spray nu sc combustion of petroleum coke and heavy oil in a swirling fired-type furnace for soot emissions in exhaust gas and formation of H2S at the bottom of the furnace. They found Ma satisfactory agreement between the numerical simulations and the experimental data. ed They also found that with increase in the ratio of tangential air to the secondary air, the pt unburned ratio decreases. They also showed that the tangential air introduced from the ce bottom of the furnace results in a reduction in the H2S formation rate. Watanabe et al Ac [10] studied numerically spray combustion of liquid kerosene oil including soot models. They found that the temperature without soot radiation model was higher than that of the experimental data. However, including soot model reduced the differences considerably as soot has significant effect on radiative heat transfer. Moreover, mole Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME fraction of NO in the exhaust including soot model is in good agreement with the experimental data. Syngas combustion under lean conditions in a supercharged dual engine using a constructed syngas chemical kinetic mechanism was studied by Azimov et al [13]. ite d Various syngas combinations of initial H2 concentration were examined to determine ed the total heat of reaction during the reaction propagation. Optimum results were Co py obtained in below 0.8 equivalence ratio for almost all types of syngas under study. In their experiments conducted by Roby et al [14], they demonstrated that the tN ot vaporization of lean, pre-vaporized premixed liquid fuel in atmospheric and high rip pressure combustor rigs incorporated with swirl-stablilized burners present similar nu sc operation to that obtained for burning natural gas. Extended lean operation was observed for the liquid fuels as a result of the wider lean flammability range for liquid Ma fuel compared with natural gas. Moreover, LPP systems were able to produce low NOx ed and low CO emissions without auto-ignition and flashback. Derudi and Rota [15, 16] pt carried out mild combustion experiments of different liquid fuels. They found that ce combustion characteristics are more influenced by the physical state of the fuel than by Ac its chemical composition indicating that the mild combustion burners can lower soot formation allowing the use of wide range of liquid fuels. Saario et al [17] simulated a cylindrical laboratory furnace for heavy oil fuel combustion using two different turbulence models, k-Ɛ and Reynolds stress model (RSM). It was found that the standard k-Ɛ model was not able to predict the swirling flow field satisfactorily whereas the RSM Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME model significantly improved the predictions with some discrepancies near the vicinity of the furnace. Combustion and characteristics of liquid fuel in engines were reported [18, 19]. In this work, the effect of different parameters on combustion emissions were analyzed. ite d In their work of large eddy simulations of an evaporating two-phase flow in a ed burner, Sanjose et al [20] demonstrated that simplified injection methods are Co py appropriate for the simulations of real-time combustion geometries. The extent of spray formation of fuels such as gasoline, n-pentane, n-butanol, ethanol and iso-octane at tN ot different temperatures and pressures were investigated. The influence of hydrogen rip additions on hydrocarbon emissions were reported [21]. Their results indicated that the nu sc lean burn limit of methane–air turbulent combustion was improved with hydrogen addition. The extent of spray formation of fuels such as gasoline, n-pentane, n-butanol, Ma ethanol and iso-octane at different temperatures and pressures were investigated by ed The extent of spray formation of fuels such as gasoline, n-pentane, n-butanol, ethanol pt and iso-octane at different temperatures and pressures were investigated by Aleiferis et ce al [22] using phase Doppler and laser diffraction techniques. Jager and Kohne [23] Ac showed that developing burner systems for light fuel oil can reduce NOx emissions compared to conventional burners. They also modeled the phenomenon using CFX software for optimization using RNG k-epsilon turbulence model. Cerea et al [24] investigated the combustion of liquid bio-fuels using a dual nozzle laboratory scale burner. Their investigations showed that dual nozzle burners can lower NOx and soot Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME concentrations indicating the possible use of wide range of low-BTU liquid fuels. For practical applications in computation fluid dynamics (CFD) codes, the simplified model for radiative heating, describing the average droplet absorption efficiency factor, appears to be the most useful both from the point of view of accuracy and CPU efficiency [4]. The computation of spray dynamics in terms of the equations of fluid ite d mechanics and droplet ballistics is now possible by use of CFD codes incorporating spray ed modeling [2]. Van Blarigan et al [25] conducted an experimental investigation of Co py methane fuel oxy-combustion in a variable compression ratio, spark-ignited piston engine. Their results indicated that an optimum oxygen concentration at which fuel- nu sc rip conditions with 29% oxygen content. tN ot conversion efficiency is maximized exists when operating under oxy-combustion There is no enough work conducted in the past on the combustion Ma characteristics of liquid fuels using CFD packages and there is no previous work for oxy- ed fuel combustion of heavy oil fuels. Therefore, the present study aims at investigating the pt oxy-combustion characteristics of heavy oil fuel in a down fired furnace in order to ce better understand the fluid dynamics of the process. Moreover, the best operating Ac O2/CO2 compositions for oxy-combustion of heavy oil fuel that are required to obtain the characteristics similar to air-fuel combustion are also investigated. 2. FURNACE MODEL Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME The furnace model used for the present study is shown in Figure 1. It is a down fired cylindrical furnace. Due to symmetry the present furnace is modeled as 2D axissymmetric to reduce computational and maintain accurate predictions. The height and diameter of the furnace are 2.4m and 0.6m respectively. The first half of the furnace is blanketed with a refractory lining around which a thick ceramic layer is laid. An air ite d assisted atomizer burner gun is used to inject fuel into the furnace from the top while ed the secondary air is supplied through a double concentric configuration around the Co py burner gun. 3. MATHEMATICAL MODELING tN ot Turbulent combustion modeling involves a wide range of coupled problems such rip as chemical reaction scheme, two or three phase system and radiative heat transfer in nu sc addition to fluid mechanical properties. The model is governed by Navier-Stokes equations, species and energy transport equations with incorporated Reynolds and Ma Favre averaging [26]. Probability Density Function (PDF) approach is recommended for ed better accuracy compared to Eddy Dissipation Model (EDM) [27]. Mixture Fraction ‘f’ is pt defined as mass fraction of primary (fuel) stream: ce sY fu − Yox + Yox ,0 sY fu ,1 + Yox ,0 Ac f = where, Yfu mass fraction of fuel Yox mass fraction of oxidizer subscript ‘0’ oxidant stream Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms (1) Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME subscript ‘1’ fuel stream and s is the stoichiometric oxygen to fuel ratio. Probability Density function, which can be defined as the fraction of time p ( f )∆f =lim 1 ∑τ i T i (2) ed T →∞ ite d spent by the fluid in the vicinity of state ‘f’, is given by equation (2) as follows: Co py where, ‘T’ is the time scale and τi is the amount of time that ‘f’ spends in the Δf band. Favre (density weighted) average of any other scalar quantities, like temperature ot and species, can be calculated by integrating the product of the scalar & PDF [28]. A PDF tN table for a desired composition of fuel and oxidizer stream is generated by performing nu sc rip the chemistry calculations and co-relating the gas variables with the mixture fraction. Ma Discrete Ordinate (DO) model is used to solve radiative heat transfer in the reactor, with 5 flow iterations for each iteration of radiation. Absorption coefficient of pt ed the gas mixture is determined by domain based weighted sum-of-gray-gas model. The ce Euler-Lagrange approach is utilized to solve the discreet phase model. The fluid phase is Ac treated as continuum, while the dispersed phase is solved by tracking the particles/droplets through the calculated flow field, where both the phases can exchange mass, momentum and energy. The trajectory of discrete particle is predicted by integrating the force balance on the particle in the Lagrangian reference frame. Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME du p dt = FD (u − u p ) + gx (ρ p − ρ ) + Fx ρp (3) where, FD(u – up) is the drag force and Fx is the force arising due to the pressure gradient along the fluid. The dispersion of droplets is calculated through Stochastic tracking (Discrete Random Walk) model, which solves the interaction of the droplets ed ite d with a succession of discrete stylized fluid phase turbulent eddies. Co py The heat and mass transfer for discrete phase are solved by incorporating inert heating Law is implied: rip tN dTp m p c p = hAp (T∞ − Tp )+ ε p Apσ (θ R4 − Tp4 ) dt ot three Laws. When droplet temperature is less than the vaporization temperature, the (4) nu sc where, the heat transfer coefficient is calculated using the correlation of Ranz & k∞ 2 + 0.6 Re1/d 2 Pr1/ 3 dD (5) pt ed h = Ma Marshall [29]. ce Droplet Vaporization Law is applied when droplet temperature is above the Ac vaporization temperature but below the boiling point. = N v κ c ( Cv , D − Cv , ∞ ) (6) Droplet Boiling Law is applied to predict the convective boiling of the droplet when the temperature of the droplet has reached the boiling point. Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME −r dmD = hAD (T∞ − TD ) + ε D ADσ (TR4 − TD4 ) dt (7) The effect of soot concentration on the radiation absorption coefficient has been taken into account by determining the absorption coefficient for soot. The overall ite d absorption coefficient was defined as the sum of the absorption coefficients of the ed radiating gas and soot. The soot formation and consumption are governed by the Co py following transport equation which was solved for the soot mass fraction. (8) ot → µ ∂ ( ρY s ) + ∇.( ρν Y s ) = ∇.( t ∇Y s ) + R s , net ∂t σs tN where Ys is the soot mass fraction, σx is the turbulent Prandtl number for soot rip transport and Rs,net is the net rate of soot generation [kg/m3s]. Rs,net is calculated nu sc according to R= R s , gen − R s ,ox s , net (9) pt ed R s= C s Pf Φ r e − ERT , gen Ma The rate of soot formation was based on a simple empirical rate [17] ce where Cs and r are constants, Pf is the fuel partial pressure, and φ is the fuel Ac equivalence ratio. The constants Cs and r were set to 1.5 /(kg.m.s) and 3, respectively. Soot formation occurs only when φ is greater than the incipient soot limit and a value above which soot formation becomes negligible. The soot oxidation can be limited either by soot concentration or oxygen concentration, and thereby the effective oxidation rate is the minimum of the following equations Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME R s ,ox ,1 = A ρY s ε (10) k Y Y sν s ε ) R s ,ox ,2 = A ρ ( ox )( ν s Y sν s +Y f ν f k (11) ite d where A is a constant, Yx is the stoichiometric oxygen to soot mass ratio and νf is the stoichiometric oxygen to fuel mass ratio. The definition of the turbulent mixing time Co py ed scale k/3 and the model constant A proposed in the above soot combustion rate expression are somewhat speculative. ot 3.1 Numerical solution tN A finite-volume based commercial CFD-code was used to solve the governing nu sc rip equations. The time-averaged equations for the conservation of mass, momentum, fuel mixture fraction and its variance, and enthalpy were solved along with transport Ma equations for k and Ɛ. The pressure field was calculated from the continuity equation ed using the SIMPLE algorithm. These partial differential equations are discretized and pt approximated by algebraic equations for finite number of volumes in the domain [28]. ce Grid independence test has been carried out on three different grids. The mass fraction Ac of O2 at a radial distance of 20 mm from the entrance of the boiler has been presented in. It is found that the grid with 9526 nodes is unable to capture the distribution accurately. Grids with 15889 and 22795 nodes present results that are in close proximity. Grids with more than 25000 nodes are also tested but resulted in no obvious advantage but rather increased the computational effort and time. Therefore, an Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME optimal grid of 22795 nodes has been used throughout the study. The difference between grid size 15889 and 22795 is less than 1 %. 4 RESULTS AND DISCUSSIONS ite d The results of air-combustion modeling of heavy oil fuel are discussed in this ed section along with the validation and comparison of air-combustion and oxy-combustion Co py characteristics of heavy oil liquid fuel. Combustion characteristics of oxy-combustion of heavy oil liquid fuel in a model furnace having a vertical cylindrical combustion chamber ot down fired were calculated and are presented in this section. The fuel has 85% Carbon, tN 11.2 % hydrogen in addition to small amounts of nitrogen sulfur and ash. Excess air of rip 15 % was used. The influence of replacing nitrogen by carbon dioxide on temperature nu sc distributions, evaporation rates and species concentrations are presented. As well, the Ma influence of the oxygen content in the oxidizer (the mixture of the oxygen and carbon dioxide) on these parameters are presented and discussed. ed 4.1 Model Validation ce pt The validation for the present combustion model was performed against the Ac experimental results presented by Saario et al [17]. In this work, experimental data for a combustor fired by an industry-type swirl burner for which the initial conditions of the spray have been characterized. The combustion data include measurements of gas species concentrations (O2, CO, CO2 and NOx) at several locations along the furnace. The combustion chamber is cylindrical in shape. Its axis is vertical and it is down-fired. The Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME burner consists of a central gun and a secondary air supply in a conventional doubleconcentric configuration, terminating in a refractory quarl. Swirl is imparted to the secondary stream. Figure 2 shows the molar distribution of O2 and CO2 at three different radial positions of 20mm, 300mm and 600mm from the entrance of the furnace. The figure indicates that the numerical results obtained using the current developed model ite d is in good agreement with the experimental data. Due to lack of experimental data for ed temperature profiles of liquid fuels combustion the present temperature contours are Co py compared with the work done by Saario et al [17] and are found to match well with their work. tN ot 4.2 General features of characteristics of oxy-fuel combustion rip Figure 3 presents the contours of the velocity vector in the boiler furnace for nu sc oxy-fuel combustion of heavy oil fuel. The velocity vector contour reveals that there are Ma two recirculation regions created downstream of the sudden expansion as a result of created adverse pressure gradient and are expected to improve mixing and enhance ed combustion. Due to the expansion of the swirled flow in the furnace regions, reversed ce pt flows are formed. When the jet of gases eject out of the nozzle into the furnace, two Ac different regions are distinguished. One is due to the sudden expansion of the gases that creates recirculation in the corners due to entrainment of oxygen. These are called corner recirculation zones (CRZ). The CRZ occupies almost the whole width of the furnace. The second is when the secondary flow hits the walls, thus, a second recirculation zone (SRZ) below the secondary flow zone is created downstream. A Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME results also indicate that similar evaporation rates to that of air combustion can be ite d achieved at increased oxygen content in the oxidizer gas. ed ACKNOWLEDGMENT Co py The authors wish to acknowledge the support received from King Fahd University of Petroleum and Minerals (KFUPM) and SABIC for funding this work through project No. Ac ce pt ed Ma nu sc rip tN ot SB 121002. Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME REFERENCES [1] G.-M. Choi and M. Katsuki, "Chemical kinetic study on the reduction of nitric oxide in highly preheated air combustion," Proceedings of the Combustion Institute, vol. 29, pp. 1165-1171, 2002. [2] X. Jiang, G. Siamas, K. Jagus, and T. Karayiannis, "Physical modelling and advanced simulations of gas–liquid two-phase jet flows in atomization and sprays," Progress in Energy and combustion Science, vol. 36, pp. 131-167, 2010. ed ite d [3] W. A. Sirignano, "Fuel droplet vaporization and spray combustion theory," Progress in Energy and combustion Science, vol. 9, pp. 291-322, 1983. Co py [4] S. S. Sazhin, "Advanced models of fuel droplet heating and evaporation," Progress in Energy and combustion Science, vol. 32, pp. 162-214, 2006. ot [5] S. D. Givler and J. Abraham, "Supercritical droplet vaporization and combustion studies," Progress in Energy and combustion Science, vol. 22, pp. 1-28, 1996. nu sc rip tN [6] O. Askari, H. Metghalchi, S. K. Hannani, A. Moghaddas, R. Ebrahimi and H. Hemmati, Fundamental Study of Spray and Partially Premixed Combustion of Methane/Air Mixture, J. Energy Resour. Technol. 135, 021001 (2012) (9 pages); Paper No: JERT-12-1093; doi:10.1115/1.4007911 Ma [7] Y. Wei, K. Wang. W. Wang, S. Liu and Y. Yang, Contribution Ratio Study of Fuel Alcohol and Gasoline on the Alcohol and Hydrocarbon Emissions of a Gasohol Engine, J. Energy Resour. Technol. 136, 022201 (2013) (7 pages); Paper No: JERT-12-1219; doi:10.1115/1.4024716 ce pt ed [8] L. Zhang and S.-C. Kong, "Modeling of multi-component fuel vaporization and combustion for gasoline and diesel spray," Chemical Engineering Science, vol. 64, pp. 3688-3696, 2009. Ac [9] J. Broukal and J. Hájek, "Validation of an effervescent spray model with secondary atomization and its application to modeling of a large-scale furnace," Applied Thermal Engineering, vol. 31, pp. 2153-2164, 2011. [10] H. Watanabe, Y. Suwa, Y. Matsushita, Y. Morozumi, H. Aoki, S. Tanno, and T. Miura, "Spray combustion simulation including soot and NO formation," Energy conversion and management, vol. 48, pp. 2077-2089, 2007. Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME the furnace. It can be seen that the extent of flame is shortened dramatically due to high swirl number of 1.1 otherwise with low swirl number the flame would have propagated to an extended length. This can be verified by the velocity contour presented in Fig. 7a. 4.3 Oxy-fuel combustion characteristics ite d Figure 8 presents the temperature contours of oxy-heavy oil and oxy-heavy oil ed fuel combustion. Three cases of oxy-heavy oil fuel combustion namely OF_23, OF_30 Co py and OF_35 are presented. OF_23 is similar to the air-heavy oil fuel case except that the N2 in air is replaced by CO2. OF_30 and OF_35 represent the oxy-heavy oil fuel cases tN ot with enriched oxygen i.e. 30% and 35% O2 respectively. It is important to mention that rip the contours are presented on different scales and the corresponding colors at the nu sc bottom highlights the temperature variation. It is observed that the replacement of N2 (in air) by CO2 (in oxy-heavy oil fuel) in case of OF_23 reduces the maximum Ma temperature obtained from 1836K in air-case to 1400K in OF_23 case. This may be ed attributed to the higher specific heat of CO2 compared to N2. Specific heat of CO2 is pt around 7 % higher than that of N2. At higher temperatures, the specific heat capacity of ce CO2 increases resulting in a reduced combustion temperature. It can also be observed Ac that by replacing CO2 with N2 in case of OF_23 shifts the maximum temperature zone towards the walls of the furnace. Moreover, the zone of maximum temperature is also increased. In order to explain this phenomenon, the velocities at three different radial sections are plotted in Fig. 9. It is clear from these figures that the velocity is higher in case of air-fuel case compared to oxy-combustion cases. This may be attributed to the Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME higher molecular weight of CO2 (44) compared to N2 (28). The higher densities in oxycombustion cases compared to air-combustion case, shown in Fig. 10a, along the axial distance also adds to the justification of reduced velocities. Furthermore, the high velocities increases the turbulent viscosity, as shown in Fig. 10b, that helps in better mixing of fuel with the oxidizer. Therefore, due to lower turbulent viscosity in OF_23 ite d case the maximum flame temperature zone is dispersed and is moved towards the side ed walls. As we increase the oxygen content at the inlet as in case of OF_30 and OF_35 the Co py zone of high temperature region is reduced. However, the maximum temperature obtained increases as we move from OF_23 case to OF_35 case. About 1440K, 1800K tN ot and 1988K temperatures are obtained in OF_23, OF_30 and OF_35 case respectively. An rip increase of approximately 27% is seen in the maximum temperature obtained when the nu sc oxygen percentage is increased from 23% to 35%. Due to increase oxygen content it is expected that the fuel is completed burned with high heat release rates thereby Ma increasing the maximum temperature. On the other hand, the increase in oxygen ed content tends to reduce the exhaust gas temperatures. The specific heat is shown in Fig. pt 10c and indicate that the multiple ρCp is higher for the oxy-fuel combustion cases. This is Ac combustion. ce expected to result in delay in ignition in the oxy-fuel cases in comparison to air-fuel Figure 11a presents comparison of temperature profiles along the axis of the furnace for the air and oxy-combustion cases. It is observed that the temperature is reduced when N2 in air combustion is replaced by CO2 in oxy-combustion case. However, the temperature levels increase with increased O2 in the oxy-combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME cases. These results confirm the temperature contours explained above. Moreover, from these temperature profiles one can depict that in order to maintain the temperature levels of air-combustion case, the amount of oxygen has to be slightly higher in oxy-combustion cases,. It is clear from these plots that for the case of OF_30 i.e. 30% O2 (remaining CO2) similar temperature profiles of air-combustion are obtained. ite d It is also revealed from these plots the region of maximum temperature is shifted away ed from the inlet when N2 in air is replaced by O2 in oxy-combustion cases. This may be Co py attributed to the higher molecular weight of CO2 (44) compared to N2 (28) that tends to reduce the velocities. In order to further explain the delay in combustion, the turbulent tN ot intensity profiles are presented in Fig. 11b and 8c. It can be seen that air-combustion rip case has the highest turbulent intensity compared to oxy-combustion cases. The higher nu sc turbulent intensity ensures better mixing whereas lower turbulent intensity causes a delay in mixing that leads to a shift in the maximum temperature zones. Ma Figure 12 presents the mass fraction of products of combustion along the center ed line of the furnace. High mass fraction of CO is observed near to the inlet region of the pt furnace where the availability of oxygen is low. However, it becomes almost negligible ce as one moves downstream. At downstream sections, the CO formed in the furnace gets Ac converted to CO2 due to enough supply of oxygen that reaches through the secondary recirculation zone where CO is completely oxidized to CO2 and its mass fraction along the center line increases. Since 15% excess air is supplied to the furnace traces of oxygen can still be found even after combustion. This excess amount is supplied in order to make sure that enough oxygen is supplied for the complete combustion of the fuel. It Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME is obvious that the formation of H2O starts once the fuel is burnt in the reaction. In addition, as there was a small amount of sulfur already present in the heavy oil fuel, very small traces of H2S formed during the reaction can be observed along the center line near to the inlet region. However, it tends to become negligible as one moves to the downstream sections. ite d The contours of mass fractions for the air combustion case are shown in Fig. 13 ed and confirm those of the axial distributions. The figure also indicates absence of O2 in Co py the vicinity of the centerline at the upstream region where CO and sulfur exist. As one moves downstream, CO and sulfur are burnt with diffused oxygen and they almost tN ot vanish close to exit of the furnace. rip Contours of mass fraction of soot are shown in Fig. 14 for air-combustion and nu sc three different oxy-combustion cases. It is important to mention that the contours are presented on same scales and the corresponding colors at the bottom highlights the Ma soot mass fraction variation. It is seen from these contours that the soot concentration ed starts with a low value close to the inlet in the vicinity of the fuel injector. Downstream pt of the location of the flame lift-off, it increases and reaches a maximum value where the ce mixture is rich and small amount of oxygen is available. As one moves towards the Ac leading edge of the flame, the soot concentration decreases as it is oxidized by the secondary air that diffuse into the flame. It is known that there are competing processes between soot formation and soot oxidation depending on the local equivalence ratio and temperature (Kitamura et al, [30]). It is observed that the air-combustion predicts the highest soot formation. It is reduced by approximately 1000 times when N2 is Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME replaced by CO2 in oxy-combustion. However, as we increase the oxygen content in the oxy-combustion cases, soot mass fraction also increases. It can be seen that, in air combustion and OF_23 cases, the soot mass fraction region is bigger but its size decreases as the amount of oxygen in OF_30 and OF_35 is increased. The maximum local soot concentration was found to be 7.2E-02 g/Nm3 air, ite d 1.06E-04 for OF-23 case, 1.55E-03 for OF-30 case, 6.8E-03 for OF-35 case in case of air- ed fuel combustion where as in oxy-fuel combustion. Soot formation starts increasing from Co py the inlet section where there is no availability of enough oxygen as can be seen from the O2 contours. Downstream of this point, it becomes zero since all the soot formed is tN ot oxidized due to oxygen recirculating. It is believed that soot formation and soot rip oxidation occur simultaneously depending upon the amount of available oxygen and the nu sc local temperature. The main disadvantage of sulfur is the risk of corrosion by sulfuric acid formed during and after combustion, and condensation in cool parts of the Ma chimney or stack, air pre-heater and economizer. ed Figure 15 presents CO concentrations for the case of air and three cases of pt oxyfuel combustion. It is believed that the emissions of CO are influenced by the ce temperature in the furnace reactions and post-flame zones. CO increases with increase Ac in temperature. This can be verified by the CO mass fraction contours presented in Fig. 15. It is observed that the temperature also increases as the amount of oxygen in the oxy-combustion cases increases and so the concentration of CO. As the flame temperature decreases, NO, decrease while CO and hydrocarbons tend to increase. Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Since the latter two emissions are indicative of combustion efficiency and stability, decreasing flame temperature can lead to both decreased efficiency and blowout limits. Radiation from a non-luminous flame is due primarily to carbon dioxide and water vapor, i.e., the products of complete combustion. The amount of heat radiated to the walls of the combustor is the product of flame emissivity and temperature raised to ite d the fourth power. As flame temperature increases, the cooling requirements for the ed liner also increase. This is not a problem for gaseous fuels since emissivity is usually Co py considerably low, and regardless of gaseous flame temperature, the radiant loading to the combustor wall will not be as great as is the case with distillate. Nevertheless, it may tN ot be desirable to alter the liner cooling to maintain acceptable metal temperature rip gradients on the liner. It may be important to note that molar concentrations of CO 8.9 nu sc mol/m3 OF-35, 9.1 mol/m3 OF-30, 8.1 mol/m3 OF-23, 3.04 mol/m3 AF-23. In the hydrocarbon fuel combustion, intermediate reaction species such as O, CO, OH, H, etc., Ma are normally formed. The oxidation of CO is one of the slowest reactions. Since the ed combustion process inside a gas turbine combustor must take place rapidly, any pt decrease in reaction rate may lead to decreased combustion efficiency, i.e., CO in the ce exhaust and the potential for high CO emissions exists. As this ratio H/O increases, Ac flammability limits and reaction rates are enhanced and the propensity for CO emissions decreases. 4.4 Fuel Evaporation Evaporation rates are presented in Figure 16a and Figure 16b at two radial locations of 20mm and 320mm for the case of air combustion and three cases of oxy- Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME fuel combustion. As shown from Fig. 16a, the oxy-fuel case exhibits the highest rate of evaporation. As O2% is increased, the evaporation rates are reduced and become close to the air case. At the downstream section, fuel is completely evaporated for the air case and for the O2 enriched cases. The delay in evaporation rate is only shown for the oxy-fuel case with 21% oxygen. ed ite d 5 CONCLUSIONS Co py The characteristics of oxy-combustion of liquid fuels such as heavy oil fuel in a down-fired model furnace have been studied. Non-premixed PDF mixture model was ot used to simulate the combustion characteristics and turbulence chemistry. The tN validation of the present model was performed against the experimental data and is rip found to be in good agreement. The results indicate that the oxy-combustion of liquid nu sc fuels results in lower soot. Evaporation rates are shown to be high in the oxy-fuel Ma combustion case. Oxygen enriching the oxy-fuel cases provide similar evaporation rates as the air case. It is observed that the soot formation is decreased when N2 in air- ed combustion is replaced by O2 in oxy-combustion. However, it increases as the amount of ce pt oxygen in oxy-combustion increases. The results also show that production of carbon Ac monoxide and sulfur depends significantly on the oxygen content in the oxidizer gas. The results also shows that replacing nitrogen in the air combustion by carbon dioxide in oxy-fuel combustion tends to reduce the temperature levels in the upstream sections of the combustion chamber. This was attributed to the high thermal capacity of combustion gases in the case of oxy-fuel combustion leading to delay in ignition. The Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms ed Ma nu sc rip tN ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Ac ce pt Fig. 9 Radial velocity distributions at two axial locations (a) X = 20mm (b) X = 600mm for air and oxy-fuel combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME REFERENCES [1] G.-M. Choi and M. Katsuki, "Chemical kinetic study on the reduction of nitric oxide in highly preheated air combustion," Proceedings of the Combustion Institute, vol. 29, pp. 1165-1171, 2002. [2] X. Jiang, G. Siamas, K. Jagus, and T. Karayiannis, "Physical modelling and advanced simulations of gas–liquid two-phase jet flows in atomization and sprays," Progress in Energy and combustion Science, vol. 36, pp. 131-167, 2010. ed ite d [3] W. A. 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Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Figure Captions List Line diagram of the model furnace in the present study Fig. 2 Comparison of present calculations and experimental data [17] Fig. 3 Contour of velocity (m/s) vector for oxy-fuel combustion of heavy oil fuel Fig. 4 Contour of turbulent intensity (%) for oxy-combustion of heavy oil fuel Fig. 5 The instantaneous view of the droplet diameter (m) distribution inside the furnace Fig. 6 Contours of Evaporation of heavy oil fuel droplet diameter (m) in oxyfuel combustion Fig. 7 (a) Velocity and (b) temperature contours for oxy-heavy oil fuel combustion Fig. 8 Temperature contours of air and oxy-fuel combustion Fig. 9 Radial velocity distributions at two axial locations (a) X = 20mm (b) X = 600mm for air and oxy-fuel combustion Fig. 10 Axial distributions of (a) Density (b) Turbulent viscosity and (c) Specific heat for air and oxy-fuel combustion Fig. 11 Axial distributions of (a) temperature and (b) Turbulent intensity for air and oxy-fuel combustion Fig. 12 Axial distributions of species concentrations of (a) CO (b) CO2 (c) H2S (d) S (e) O2 for air and oxy-fuel combustion ce Ac Fig. 13 pt ed Ma nu sc rip tN ot Co py ed ite d Fig. 1 Contours of species concentrations of (a) O2 (b) CO2 (c) CO and (d) Soot for air combustion Fig. 14 Contours of soot concentrations for air and oxy-fuel combustion Fig. 15 Contours of CO concentrations for air and oxy-fuel combustion Fig. 16 Radial distributions of evaporation rate for air and oxy-fuel combustion at different axial locations (a) X=20mm (b) X=300mm from the inlet Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Co py ed ite d FIGURES Ac ce pt ed Ma nu sc rip tN ot Fig. 1 Line diagram of the model furnace in the present study Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Ac ce pt ed Ma nu sc rip tN ot Fig. 2 Comparison of present calculations and experimental data [17] Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Ac ce pt ed Ma nu sc rip tN ot Fig. 3 Contour of velocity (m/s) vector for oxy-fuel combustion of heavy oil fuel Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Ac ce pt ed Ma nu sc rip tN ot Fig. 4 Contour of turbulent intensity (%) for oxy-combustion of heavy oil fuel Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Ac ce pt ed Ma nu sc rip tN ot Fig. 5 The instantaneous view of the droplet diameter (m) distribution inside the furnace Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Ac ce pt ed Ma nu sc rip tN ot Fig. 6 Contours of Evaporation of heavy oil fuel droplet diameter (m) in oxy-fuel combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Ac ce pt ed Ma nu sc rip tN Fig 7 (a) Velocity and (b) temperature contours for oxy-heavy oil fuel combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Ac ce pt ed Ma nu sc rip tN ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Fig. 8 Temperature contours of air and oxy-fuel combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms ed Ma nu sc rip tN ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Ac ce pt Fig. 9 Radial velocity distributions at two axial locations (a) X = 20mm (b) X = 600mm for air and oxy-fuel combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Ac ce pt ed Ma nu sc rip tN ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Ac ce pt ed Ma nu sc rip tN ot Co py ed ite d Fig 10 Axial distributions of (a) Density (b) Turbulent viscosity and (c) Specific heat for air and oxy-fuel combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms ed Ma nu sc rip tN ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Ac ce pt Fig 11 Axial distributions of (a) Temperature and (b) Turbulent intensity for air and oxyfuel combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Ac ce pt ed Ma nu sc rip tN ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Fig 12 Axial distributions of species concentrations of (a) CO (b) CO2 (c) H2S (d) S (e) O2 for air and oxy-fuel combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Ac ce pt ed Ma nu sc rip tN ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Fig. 13 Contours of species concentrations of (a) O2 (b) CO2 (c) CO and (d) Soot for air combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Ac ce pt ed Ma nu sc rip tN ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Fig. 14 Contours of soot concentrations for air and oxy-fuel combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Ac ce pt ed Ma nu sc rip tN ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Fig. 15 Contours of CO concentrations for air and oxy-fuel combustion Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms Ac ce pt ed Ma nu sc rip tN ot Co py ed ite d Journal of Energy Resources Technology. Received August 21, 2014; Accepted manuscript posted November 7, 2014. doi:10.1115/1.4029007 Copyright (c) 2014 by ASME Fig. 16 Radial distributions of evaporation rate for air and oxy-fuel combustion at different axial locations (a) X=20mm (b) X=300mm from the inlet Downloaded From: http://asmedigitalcollection.asme.org/ on 11/14/2014 Terms of Use: http://asme.org/terms
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