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Structural optimization of baffle internals for fast particle pyrolysis in a downer reactor using the discrete element method

Structural optimization of baffle internals for fast particle pyrolysis in a downer reactor using... The structural optimization of baffle internals for fast pyrolysis of coal with particulate mixing and heat transfer in a downer reactor using the discrete element method (DEM) has been investigated in this research. The pyrolysis terminal temperature at the exit of the downer reactor is not only decided by the volume-feeding-rate ratio of the coal to the sand, but also is affected by the inner structural design of the baffle internals in the downer reactor. As presented in the previous publication of the author, the inhibition from the baffle internals in a downer reactor can improve the particulate-mixing degree and heat carrier, and increase the mean residence time of the coal and heat-carrier particles in the downer reactor. The structure of the baffle internals in the downer reactor mentioned in this research can be optimized by the independently developed 3D soft-sphere model of the DEM programme of a 40-mm baffle length, a 30° baffle-slope angle and at least four baffles designed in the downer reactor, which is beneficial for the process design of coal pyrolysis with a heat carrier in the downer reactor. Received: 17 October 2020; Accepted: 26 December 2020 © The Author(s) 2021. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (http:// creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, 167 provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 168 | Clean Energy, 2021, Vol. 5, No. 2 Graphic abstract Particulate temperature Particulate mixing degree in distribution in downer reactor downer reactor 75° 60° 45° 30° 15° 0° 1.0 α : baffle 0.9 0° 15° 30° 0.8 45° 60° 75° 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 0123 45 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 I I m t / sec downer None internals Coal Baffle internals None internals Sand Baffle internals 0.0 0.2 0.4 0.6 0.8 1.0 1.2 010203040506070 α I ° Residence time / sec baffle Residence time distribution in Temperature increasing rate in downer reactor downer reactor Keywords: structural optimization; baffle internals; downer reactor; DEM; coal pyrolysis Rapidly heating the coal particles can increase the yield Introduction of the light liquid products and fine chemicals. The coal- The downer reactor is considered a high-efficiency chem- particle temperature at the exit of the downer reactor can ical reactor in the twenty-first century, which has a very be determined by the volume-feeding-rate ratio of the coal broad application prospect in energy conversion and to the heat-carrier particle, but the rapid heating of coal chemical engineering [1–4]. The characteristics of a downer particles is the most important factor during the coal- reactor can be summarized as follows: (i) it has a high flow pyrolysis process, which is affected by the rapid mixing rate of the solid phase [5–7]; (ii) the radial concentration of between the coal and the sand (a commonly used heat- the solid phase is uniformly distributed and the phenom- carrier particle). Thus, some internal baffles can be de- enon of the axial back mixing can be ignored [89 , ]; (iii) the signed and distributed in the downer reactor to enhance mixture ratio of the solid and gas is not limited, which is the mixing and heat transfer [19–22] between the pulv-er especially suitable for high-load solid particles [ 10 5, ]; (iv) ized coal, the sand and the circulating hot ash. The baffle low energy consumption in pneumatic conveying; (v) the internals can increase the MRT of particles in the downer mean residence time (MRT) of the solid particles is short reactor to ensure the terminal thermal balance tempera- and the residence time distribution is narrow [6, 7]. ture and also to improve the mixing of coal and hot ash. Based on these advantages, downer reactors can operate In this research, the internal design of the baffle has under some difficult process conditions, such as the rapid been optimized using the developed 3D soft-sphere model pyrolysis process of coal or biomass [11–14]. As mentioned of the discrete element method (DEM) to simulate the in previous research [15–18], a pyrolysis downer reactor is rapid pyrolysis of coal in the downer reactor. Increasing generally coupled with a fluidized-bed reactor. Pulverized the baffle length can raise the coal-pyrolysis temperature coal charged from a pot at the top of the downer reactor at the exit of the downer reactor. The baffle length on a is mixed with the hot ash or silica sand (heat carrier) dis- horizontal projection should be no less than the width of charged from the fluidized-bed reactor, which heats the the particle-feeding entrance. Increasing the baffle angle coal to release gaseous volatile substances. Structure of baffle internals in downer reactor M I real-time M I real-time ° –1 t I C.sec °C p, coal t I p, coal Optimization of baffle internals in downer reactor Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 Liu | 169 can raise the coal-pyrolysis temperature at the exit of the , rotating movement velocity vector ω , net force vector F i i downer reactor both by fixing the horizontal-projection , total moment vector M , particle temperature and the T length of the baffle and the actual baffle length. Increasing enthalpy value H . the baffle number in the downer reactor not only can in- The basic hypotheses of the DEM can be summarized crease the particle MRT in the downer reactor, but also can as follows: (i) the particles are assumed to be spherical; (ii) enlarge the contact space for thermal conduction through when collision or compression happens, a certain overlap particle contact, which can raise the particle-pyrolysis area between two particles is allowed, the size of which temperature at the exit of the downer reactor. The larger is small compared with the particle surface area. The al- the number of baffles in the downer reactor, the closer the gorithm structure of the DEM is composed of the status coal approaches thermal equilibrium. analysis, the kinematics analysis and the search algorithm for particle collisions. Fig. 1 illustrates the main elements of the developed algorithm of the DEM. 1 Mathematical modelling In this research, the motion of all particles in a downer The DEM [23, 24] is a Lagrangian method for calculating reactor is determined by the gravitational forG ce , the col- and recording the information of each particle in a granular lision force F and the forces from the gas phase—that is, system, including the space position vector x , phase pos- the buoyancy force F and the drag force F . The analysis b,i d,i ition vector ϕ , translational movement velocity vector v of the collision force between particles has been presented i i Fig. 1: Foundational principle of the developed DEM software package coupling with the particulate heat-transfer model Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 170 | Clean Energy, 2021, Vol. 5, No. 2 in previous research [25, 26]. The calculation method of the UNIT: mm buoyancy force and the drag force can be listed as shown in Equations (1)–(3). G = m g (1) i i Ä ä m 4 3 F = −ρ g = − πr ρ g b,i g g (2) ρ 3 i F = ζ · A · ρ u − v u − v g g g  d,i w,i i i    Re < 2  i  Re (3) 18.5 2 < Re < 500 ζ = 0.6 i  Re      5 0.44500 < Re < 2 × 10 Therefore, the forces and momentum balance of particle i can be calculated as Equation (4): F = F + F + F + G i c,i b,i d,i i (4) M = M + M i t,i r,i The enthalpy of the solid particle is assumed to be a func- tion of temperature. Thus, the particle temperature of the ith particle T is based on the enthalpy difference ΔH , i i which can be calculated using Equation (5): ΔH T = T + (5) i i,0 m ·c i p,i The net force acting on the particle is governed by Newton’s Second Law of Motion (Equation (6a)), which determines the translational movement of the particle. The par - ticle rotation is determined by Newton’s Law of Rotation (Equation (6b)): dv d x i i F = m a = m = m (a) i i i i i 2 dt dt (6) dω d ϕ i i M = I j = I = I (b) i i i i i 2 dt dt Heat transfer to the particle takes place because of par - ticle collisions or interactions with walls through thermal conduction, the interaction with the gas phase via thermal Fig. 2: Schemes of the structure and size of a downer reactor with baffle internals convection and the radiative heat transfer. AB 0.55 56 1.25 104 Viscosity of dry air at 101.33 kPa Coefficient of heat conduction of dry air Density of dry air at 101.33 kPa at 101.33kPa 0.50 96 Specific heat capacity of dry air 1.20 at 101.33kPa 0.45 88 0.40 80 44 1.15 0.35 72 0.30 1.10 0.25 56 1.05 48 0.20 32 400 600 800 1000 1200 400 600 800 1000 1200 Temperature t / °C Temperature t / °C Fig. 3: Density, viscosity, heat-conduction coefficient and heat capacity of the gas phase in a downer reactor at different temperatures under a con- stant pressure of 101.33 kPa –3 Density ρ / kg·m Viscosity µ / µPa·s –1 –1 Specific heat capacity c /kJ·kg ·K Coefficient of heat conduction –1 –1 λ / mW·m ·K Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 Liu | 171 The model of particulate thermal conduction uses the where d is the diameter of particle , in m; i k represents the i g –1 –1 following assumptions: (i) when two particles collide, the gas-phase thermal-conduction coefficient, in W·m ·K ; direction of the temperature gradient is perpendicular to Nu stands for the Nusselt number [2930 , ]; Re and Pr are their contact surface; (ii) thermal conduction takes places the Reynolds number of particle and the Pr i andtl number, in the area of overlap of two colliding particles; (iii) there respectively; c is the heat capacity of the gas phase, in p.g –10 –1 –1 is a gap with a thickness of 4× 10 m between the contact kJ·kg ·K ; μ is the gas-phase dynamic viscosity, in Pa·s; ρ g g –3 surfaces of two colliding particles [27 28 , ], through which stands for the density of the gas phase, in kg·m; u − u g i –1 the heat can be transferred; the thermal resistance of this means the gas velocity relative to the particle, in m·s . gap is calculated as follows: The radiative heat transfer between the particle and dgap R =  con the main zone of the downer reactor cannot be neglected k A g gap Å ãÅ ã 2 2 2 2 2 2 d +r −r d +r −r (7) ij j i ij j i because of the high temperature of the reactor surface. A = π r + r −  gap j j 2d 2d ij ij Kirchhoff’s law of thermal radiation can be used to calcu- late the radiative flux as follows: The heat flux can be also affected by the thermal resistance of the particle interior, which can be calculated as follows: 4 4 (12) Q = σ ζ A T − T S−B rad i i i b 1 1 1 R = − s,i 2πk r r p,i i,m i –8 (8) where σ is the Stefan-Boltzmann constant, 5.67 × 10 S–B –2 –4 W·m ·K ; ζ is the particle i emissivity, –; A is the heat- i i where k is the thermal-conduction coefficient of the solid, p 2 transfer area between particle i and the reactor bulk, in m ; –1 –1 in W·m ·K ; r presents the radius of the particle, in m; T is the particle i temperature, in K; is the bed temper T a- i b –1/3 r = 2 ·r . Therefore, the heat flux via conduction be- i, m i ture, which is assumed to be approximately equal to the tween two colliding particles with different temperatures wall temperature, in K. can be calculated as follows: ΔT The indicator for describing the mixing degree of the ij Q = (9) con R +Rcon+R s,i s,j coal particle and the heat carrier can employ the vari- The convective heat flux between the particle and the ance or standard deviation of the concentrations of the surrounding gas can be calculated from Newton’s law of key components (coal particles) in each position of the cooling, which is described as follows: particle-motion space. A  low value of standard deviation represents good mixing of the particles within the model (10) Q = h A T − T cov g i g i domain. The calculation method can be summarized as where h represents the coefficient of heat transfer through follows: convection between the particle and the surrounding gas, –2 –1 in W·m ·K ; A is the heat-transfer area between particle i and the surrounding gas, in m; T and T are the gas-phase g i temperature and the particle i temperature, respectively. The coefficient of thermal convection h can be calculated as follows: Nu·k h =  g  i  √  3 Nu = 2.0 + 0.6 Re · Pr (11) d ρg|ug−v | i i Re =  μ c μ p,g g Pr = kg Table 1: Parameter values of coal and silica sand in the model of mechanics and thermodynamics Particulate materials Coal Sand –3 Particle density ρ/kg·m 1250 2450 Particle diameter d /mm 5 5 9 10 Elastic modulus E/Pa 5.08 × 10 4.10 × 10 Poisson ratio v/– 0.28 0.22 Sliding friction coefficient μ /– 0.51 0.66 –5 –5 Rolling friction coefficient μ/m 5 × 10 5 × 10 Recovery coefficient e/– 0.85 0.90 Specific thermal capacity l.13 0.92 –1 –1 c /W·m ·K Thermal-conduction coeffi- 0.26 0.52 –1 –1 cient k /W·m ·K Fig. 4: Particulate mixing and temperature of coal and sand in a downer Blackness ζ /– 0.85 0.70 reactor simulated by the DEM software package Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 172 | Clean Energy, 2021, Vol. 5, No. 2 (i) The volume fraction (as concentration) x of coal downer reactor; N is the number of spatial samples in the coal,i particles (as key components) in spatial sample i at mixing zone of the downer reactor. The maximum and the any position of the mixing zone of the downer reactor minimum of the standard variance can be calculated by: can be calculated as ¯ ¯ σ = x · (1 − x ) V coal coal coal (13) » x = coal,i V +V (16) coal sand ¯ x ·(1−¯ x ) coal coal σ = (ii) It is w orth noting that the particle number in eac n h An index of mixing of the coal and the sand for real-time spatial sample at any position of the mixing zone analysis M can be defined as: real-time σ−σ M = of the downer reactor may be different, because the (17) real−time σ −σ 0 r volume of the spatial sample is fixed. Therefore, the The reactor internals can affect not only the coal- variance or standard variation of the volume fraction particle temperature at the exit of the downer reactor, of the whole reactor space can be of coal particles x coal,i but also the rate of change of the temperature of the calculated as particles within the reactor by the process of solid–solid heat transfer. The rate of particle-temperature change is influenced by the heat transfer as determined by the x = w · x (14) coal i coal,i i=1 temperature difference between the particle and its sur - rounding, the heat-transfer coefficient and the physical σ = w · x − x¯ chemical properties of the particulate material itself. (15) i coal,i coal i=1 The rate of change of the th particle temper i ature can be defined as follows: where w is the weight of sample ,i which can be calculated by w  = n /n; n is the particle number in the spatial sample ∂T (x ,t) T (x ,t+Δt)−T (x ,t−Δt) i 0 i 0 i 0 i i i (18) α = = lim ∂t 2·Δt i; n is the total particle number in the mixing zone of the Δt→0 Coal Sand (a) (b) (c) (d) (e) (f) (g) 0.5 cm 1.0 cm 1.5 cm 2.0 cm 4.0 cm 6.0 cm 8.0 cm 1.0 1.0 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 baffle,proj 0.2 0.2 0.0 cm 1.0 cm 2.0 cm 0.1 0.1 3.0 cm 4.0 cm 5.0 cm 0.0 0.0 0.00.5 1.01.5 2.02.5 t / sec l / d baffle,proj gap Fig. 5: Effect of the baffle length on the particulate-mixing degree of coal and sand simulated by the developed DEM software package M / – real-time M / – real-time Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 l / d baffle, proj gap Liu | 173 A B l / d baffle,proj gap t = 684.57 °C wall 100 0.0 700 0.5 600 500 1.0 1.5 400 2.0 2.5 1.6 1.4 100 100 1.2 1.0 0.8 0.6 2.0 .0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0.4 1.5 0.2 1.0 0.5 l / m 0.0 downer 0.0 CD t = 684.57 °C wall 0 300 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 l / d l / d baffle, proj gap baffle, proj gap Fig. 6: Effect of the baffle length of a downer reactor on the distribution of the coal-pyrolysis temperature along the height direction of the downer reactor simulated by the developed DEM software package 2 Simulation conditions of the downer reactor if no vertical baffle is installed and In order to research the influence of the baffle internals on the effect of the internal baffles on particulate mixing the particle mixing and heat transfer in the downer reactor, and heat transfer in the mixing zone of the downer re- the developed algorithms of a 3D DEM software package actor will be unclear. A  series of baffle internals are in- were used to establish the numerical-simulation system. stalled in the main zone of the traditional type of downer The correctness of the software package and the ration- reactor to form the new type of baffle downer reactor, as ality of the particle parameters have been validated in past shown in Fig. 2. The angle of the baffle with respect to the research [25, 26]. The shape and size of the downer reactor downer-reactor wall of the six baffles is 45°, the length with baffle internals are illustrated in Fig. 2, showing the of the six baffles is 70.70 mm and the interval between front and side views. the baffles along the vertical direction is 250mm. In this As described in Fig. 2, the downer reactor mainly con- study, the flow type and mixing behaviour of particulate sists of two hoppers, each containing a different kind of materials affected by the baffle internals in the downer granular material and a long vertical tube, which is the reactor can be investigated clearly. In addition, the design main zone for particulate mixing and heat transfer. In of the baffle internals has been optimized to strengthen particular, there are two vertical baffles at the exits of the the mixing and heat-transfer behaviour of the particles two hoppers—that is, the entrance of the mixing zone of using the developed algorithm of the 3D DEM software the downer reactor—to avoid disturbing the evaluation package. of the effect of baffle internals on the particulate mixing The initial conditions and basic assumptions for simu- and heat transfer in the downer reactor. The results of lating the particulate mixing and heat transfer in the particulate mixing and heat transfer will consider the downer reactor are as follows: (i) to simplify the simula- structure and shape of the entrance of the mixing zone tion, the gas phase in the downer reactor is considered l / m downer t / °C t / °C p, coal, terminal p, coal -1 α / °C·sec coal t / °C p, coal Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 174 | Clean Energy, 2021, Vol. 5, No. 2 Coal Sand (a) (b) (c) (d) (e) 15° 30° 45° 60° 75° 1.0 1.0 α : baffle 0.9 0.9 0° 15° 30° 0.8 0.8 45° 60° 75° 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0.0 t / sec α / ° baffle Fig. 7: Effect of the baffle angle on the particulate-mixing degree of coal and sand simulated by the developed DEM software package to be still dry air; the gas-phase density and viscosity are the developed DEM software package, as shown in Fig. 5. functions of temperature, as illustrated in Fig. 3; (ii) uni- l stands for the projection of baffle length l onto baffle, proj baffle form temperature on the inner surface of the downer the horizontal plane. α is the angle between the ver - baffle reactor is assumed during the calculation; (iii) at a tem- tical direction and the baffle-length direction, i.e. l = baffle perature of 0°Ϲ , the specific enthalpy of the material com- l /sinα . The dimensionless parameter l /d , baffle, proj baffle baffle, proj gap –1 ponents in downer reactor is 0 kJ·kg; (iv) the physical the horizontal-projection length of the baffle divided by properties of the coal and the sand have been listed in the gap width of the particle-feeding entrance (2.0 cm) Table 1 [31–33]. shown in Fig. 5, should be optimized to maximize the coal- pyrolysis conditions in the downer reactor. It can be concluded from Fig. 5 that l should be no baffle, pro less than the width of the particle-feeding entrance d , 3 Results and discussion gap which can mix the coal and the sand sufficiently. However, The granular-mixing and heat-transfer behaviour of the too large a baffle length cannot improve the mixing de- coal and the sand affected by internals in a downer re- gree of the coal and the sand further, but will increase the actor have been simulated by the developed DEM software particle-residence time unnecessarily. When the volume- package, which is shown in Fig. 4. The baffle internals can feeding-rate ratio of coal to sand is fixed at 1:4, the wall affect the mixing degree of the fuel particles and heat car - temperature is equal to 684.57°Ϲ under the adiabatic con- riers in the downer reactor, and the change in the final dition. The effect of the baffle length in the downer reactor coal-pyrolysis temperature and the particle-heating rate on the distribution of coal-particle temperatures along the indirectly. Therefore, optimization of the design of baffle height of the downer reactor simulated by the developed internals is important for the coal-pyrolysis process in a DEM software package is illustrated in Fig. 6. downer reactor. As shown in Fig. 6, the coal-temperature distribution in the downer reactor is remarkably affected by the baffle- length design. The improvement in the coal-temperature 3.1 Baffle length distribution is affected by the enhancement of the heat The effect of the baffle length on the mixing degree of transfer between the coal and the sand due to the effect the coal and the sand particles has been simulated by of the mixing degree of the coal and the sand, especially M / – real-time M / – real-time Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 baffle Liu | 175 baffle AB 75° 60° 45° 30° 15° 0° t = 684.57 °C 100 wall 600 500 1.6 1.4 100 100 1.2 1.0 0 0.8 0.6 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 50 0.4 0.2 l / m downer 0.0 t = 684.57 °C wall 0 300 010203040 50 60 70 80 90 010203040506070 α / ° α / ° baffle baffle Fig. 8: Effect of the baffle angle of a downer reactor on the distribution of the coal-pyrolysis temperature along the height direction of the downer reactor simulated by the developed DEM software package due to the horizontal length of the baffle ranging from It can be concluded from Fig. 7 that the mixing degree 0 to 1.0d . In addition, the MRT of coal particles in the increases with the slope angle of the baffle internals in the gap downer reactor will increase from 0.497 to 0.859 sec when downer reactor. When the volume-flow-rate ratio of coal to the dimensionless parameter l /d increases from 0 sand is fixed at 1:4, the wall temperature of the downer re- baffle, proj gap to 2.5. Therefore, the coal-particle-temperature distribu- actor can be calculated as 684.57°Ϲ. The effect of the slope tion along the height of the downer reactor and the coal- angle of the baffle internals on the coal-particle terminal pyrolysis temperature at the exit of the downer reactor temperature and the average particle-heating rate are il- are remarkably affected by the horizontal length of the lustrated in Fig. 8. baffle ranging from 0 to 1.0d , as shown in Fig. 6c. Most It can be concluded from Fig. 8 that the coal-particle gap importantly, the coal-heating rate can reach a maximum temperature increases with the particle distance from value when l is equal to d . Therefore, the hori- the entrance of the downer reactor. In addition, the baffle, proj gap zontal projection of the baffle length should be no less coal-particle terminal temperature increases with than the width of the particle-feeding entrance, which is the slope angle of the baffle internals in the downer a benefit for the fast pyrolysis of the coal in the downer reactor. The larger slope angle of the baffle internals reactor. makes the coal-particle terminal temperature closer to the wall temperature of the downer reactor. However, the slope angle of the baffle internals will increase 3.2 Baffle angle the residence time of coal particles in the downer re- The effect of the slope angle of the baffle internals on the actor. Therefore, the coal-particle-temperature heating particulate mixing of the coal and the sand in the downer in the downer reactor does not increase monotonically reactor can be simulated by the developed DEM software with the obstacle of the baffle internals. As is vividly package, as shown in Fig. 7. shown in Fig. 8, the coal-particle heating rate increases l / m downer t / °C t / °C p, coal, terminal p, coal -1 α / °C·sec coal t / °C p, coal Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 176 | Clean Energy, 2021, Vol. 5, No. 2 Coal Sand (a) (b) (c) (d) (e) (f) 1 baffle 2 baffles 3 baffles 4 baffles 5 baffles 6 baffles AB 1.0 1.0 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 Baffle number: 0.1 0.1 0 1 2 3 4 5 6 0.0 0.0 012345 012345 6 t / sec baffle Fig. 9: Effect of the baffle number on the particulate-mixing degree of coal and sand simulated by the developed DEM software package with the obstacle of the internals from 0° to 30° and the baffle-slope angle is 45°. The baffle interval is 5l . baffle, proj decreases after 30°. Therefore, a 30° slope angle of the The upper edge of the first top baffle is fixed at the position baffle internals is beneficial for the rapid pyrolysis of of 4l (20.0 cm) from the entrance of the sand hopper. baffle, proj coal under the condition of a fixed volume-flow-rate The effect of the baffle number on the particulate mixing ratio of coal to sand. of coal and sand in the downer reactor can be simulated by the developed DEM software package coupling the particu- late model of heat transfer, as shown in Fig. 9. 3.3 Baffle number It can be concluded from Fig. 9 that the mixing de- The number of baffles is one of the most important de- gree increases with the internal-baffle number in the sign parameters for the baffle-type downer reactor. The downer reactor. When the volume-flow-rate ratio of horizontal-projection length of the baffle is 2.5d and coal to sand is fixed at 1:4, the wall temperature of the gap M / – real-time M / – real-time Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 baffle Liu | 177 AB baffle t = 684.57 °C 100 wall 500 2 1.6 1.4 1.2 1.0 0 0.8 0.6 0.00.2 0.40.6 0.81.0 1.21.4 1.61.8 0.4 0.2 l / m 2 downer 0.0 CD t = 684.57 °C wall 0 300 012345 6 012345 6 n n baffle baffle Fig. 10: Effect of the baffle number of a downer reactor on the distribution of the coal-pyrolysis temperature along the height direction of the downer reactor simulated by the developed DEM software package downer reactor can be calculated as 684.57° . The Ϲ effect It can be concluded from Sections 3.1–3.3 that the design of the internal-baffle number on the coal-particle ter - of the internal baffles can affect the coal-particle-heating minal temperature and average particle-heating rate is rate in the downer reactor for the pyrolysis process. The illustrated in Fig. 10. optimized baffle internal design is that the baffle length is It can be concluded from Fig. 10 that the coal-particle 40 mm, the baffle-slope angle is 30° and the baffle number temperature increases with the particle distance from is no fewer than four. Under this condition, a higher coal- the entrance of the downer reactor. In addition, the particle-heating rate can be obtained in the downer re- coal-particle terminal temperature increases with the actor for the pyrolysis process. internal-baffle number in the downer reactor. A  large number of internal baffles makes the coal-particle ter - minal temperature closer to the wall temperature of the 4 Conclusions downer reactor. However, more internal baffles will in- crease the residence time of coal particles in the downer The structural optimization of baffle internals in a downer reactor, so the coal-particle-heating rate in the downer reactor has been discussed in this research. For one thing, reactor will not increase monotonically with the increase the coal-pyrolysis temperature at the exit of the downer re- in the internal-baffle number. As vividly shown in Fig. 10, actor can be determined by the volume-feeding-rate ratio the coal-particle-heating rate will not be raised when the of the coal to the sand. For another, it will be also affected number of internal baffles is larger than four. Therefore, by the structural design of the internals in the downer re- no fewer than four internal baffles should be installed in actor. It can be concluded from the simulation results that the downer reactor for the rapid pyrolysis of coal under the effect of the baffle internals not only can improve the the condition of a fixed volume-flow-rate ratio of coal mixing degree, but also can enlarge the MRT of particles to sand. in the downer reactor, which can make the fuel particles l / m downer t / °C t / °C p, coal, terminal p, coal –1 α /°C sec coal t / °C p, coal Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 178 | Clean Energy, 2021, Vol. 5, No. 2 [8] Wang  C, Li  C, Zhu  J. Axial solids flow structure in a high achieve the predicted pyrolysis temperature in the downer density gas–solids circulating fluidized bed downer. Powder reactor as soon as possible. The conclusions can be summar - Technology, 2015,272:153–164. ized as follows: [9] Wang  C, Zhu  J, Lan  X, et  al. Radial solids flow structure in high flux gas-solids circulating fluidized bed downers. Powder (i) Increasing the baffle length can raise the coal-pyrolysis Technology, 2016,301:848–857. temperature at the exit of the downer reactor, which is [10] Lian W, Pan X, Zheng S, et al. Mechanism analysis of the solids remarkable when the horizontal length of the baffle holdup variations in downer reactors based on volumetric ranges from 0 to 1.0d and is weakened when the hor- gap flux. Chemical Engineering Science, 2019,205:259–268. izontal length of the baffle increases further. It is ob- [11] Fushimi  C, Okuyama  S, Kobayashi  M, et  al. Pyrolysis of low- vious that the baffle-length design is a benefit for the rank coal with heat-carrying particles in a downer reactor. rapid pyrolysis of coal in the downer reactor, whose Fuel Process Technol, 2017,167:136–145. [12] Li X, Jin X, Wang M, et al. Effect of volatiles’ reaction on coking horizontal projection should be no less than the width of tar during pyrolysis of Naomaohu coal in a downer-bed re- of the particle-feeding entrance. actor. Fuel Process Technol, 2021,212:106623. (ii) Increasing the baffle angle can raise the coal-pyrolysis [13] Tian Y, Li J, Wei W, et  al. Parametric effect of biomass partial temperature at the exit of the downer reactor both by hydropyrolysis process in a downer reactor to co-produce fixing the horizontal-projection length of the baffle high-quality tar and syngas. Bioresour Technol, 2021,320:124401. and by fixing the actual length of the baffle. When the [14] Makkawi  Y, Yu  X, Ocone  R. Parametric analysis of biomass baffle angle is ~30°, a higher coal-particle-heating rate fast pyrolysis in a downer fluidized bed reactor. Renew Energ, 2019,143:1225–1234. can be achieved. [15] Liu B, Yang X, Song W, et al. 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Powder Technology, 2013,247:235–259. Clean Energy, 2021. doi:10.1093/ce/zkaa029 [7] Sachs  M, Friedle  M, Schmidt  J, et  al. Characterization of a [27] Rong D. DEM simulation of hydrodynamics, heat transfer and downer reactor for particle rounding. Powder Technology, combustion in fluidized beds. Ph.D.  thesis. Tokyo University of 2017,316:357–366. Agriculture and Technology, Japan, 2000. Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 Liu | 179 [28] Liu A, Liu S. Theoretical study on impact heat transfer between [31] González-Montellano C, Ramírez Á, Gallego E, et al. Validation particles in fluidized bed. Proceedings of the CSEE, 2003,23:161–165. and experimental calibration of 3D discrete element models [29] Kaneko  Y, Shiojima  T, Horio  M. DEM simulation of fluidized for the simulation of the discharge flow in silos. Chemical beds for gas-phase olefin polymerization. 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Structural optimization of baffle internals for fast particle pyrolysis in a downer reactor using the discrete element method

Clean Energy , Volume 5 (2) – Jun 1, 2021

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Abstract

The structural optimization of baffle internals for fast pyrolysis of coal with particulate mixing and heat transfer in a downer reactor using the discrete element method (DEM) has been investigated in this research. The pyrolysis terminal temperature at the exit of the downer reactor is not only decided by the volume-feeding-rate ratio of the coal to the sand, but also is affected by the inner structural design of the baffle internals in the downer reactor. As presented in the previous publication of the author, the inhibition from the baffle internals in a downer reactor can improve the particulate-mixing degree and heat carrier, and increase the mean residence time of the coal and heat-carrier particles in the downer reactor. The structure of the baffle internals in the downer reactor mentioned in this research can be optimized by the independently developed 3D soft-sphere model of the DEM programme of a 40-mm baffle length, a 30° baffle-slope angle and at least four baffles designed in the downer reactor, which is beneficial for the process design of coal pyrolysis with a heat carrier in the downer reactor. Received: 17 October 2020; Accepted: 26 December 2020 © The Author(s) 2021. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (http:// creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, 167 provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 168 | Clean Energy, 2021, Vol. 5, No. 2 Graphic abstract Particulate temperature Particulate mixing degree in distribution in downer reactor downer reactor 75° 60° 45° 30° 15° 0° 1.0 α : baffle 0.9 0° 15° 30° 0.8 45° 60° 75° 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 0123 45 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 I I m t / sec downer None internals Coal Baffle internals None internals Sand Baffle internals 0.0 0.2 0.4 0.6 0.8 1.0 1.2 010203040506070 α I ° Residence time / sec baffle Residence time distribution in Temperature increasing rate in downer reactor downer reactor Keywords: structural optimization; baffle internals; downer reactor; DEM; coal pyrolysis Rapidly heating the coal particles can increase the yield Introduction of the light liquid products and fine chemicals. The coal- The downer reactor is considered a high-efficiency chem- particle temperature at the exit of the downer reactor can ical reactor in the twenty-first century, which has a very be determined by the volume-feeding-rate ratio of the coal broad application prospect in energy conversion and to the heat-carrier particle, but the rapid heating of coal chemical engineering [1–4]. The characteristics of a downer particles is the most important factor during the coal- reactor can be summarized as follows: (i) it has a high flow pyrolysis process, which is affected by the rapid mixing rate of the solid phase [5–7]; (ii) the radial concentration of between the coal and the sand (a commonly used heat- the solid phase is uniformly distributed and the phenom- carrier particle). Thus, some internal baffles can be de- enon of the axial back mixing can be ignored [89 , ]; (iii) the signed and distributed in the downer reactor to enhance mixture ratio of the solid and gas is not limited, which is the mixing and heat transfer [19–22] between the pulv-er especially suitable for high-load solid particles [ 10 5, ]; (iv) ized coal, the sand and the circulating hot ash. The baffle low energy consumption in pneumatic conveying; (v) the internals can increase the MRT of particles in the downer mean residence time (MRT) of the solid particles is short reactor to ensure the terminal thermal balance tempera- and the residence time distribution is narrow [6, 7]. ture and also to improve the mixing of coal and hot ash. Based on these advantages, downer reactors can operate In this research, the internal design of the baffle has under some difficult process conditions, such as the rapid been optimized using the developed 3D soft-sphere model pyrolysis process of coal or biomass [11–14]. As mentioned of the discrete element method (DEM) to simulate the in previous research [15–18], a pyrolysis downer reactor is rapid pyrolysis of coal in the downer reactor. Increasing generally coupled with a fluidized-bed reactor. Pulverized the baffle length can raise the coal-pyrolysis temperature coal charged from a pot at the top of the downer reactor at the exit of the downer reactor. The baffle length on a is mixed with the hot ash or silica sand (heat carrier) dis- horizontal projection should be no less than the width of charged from the fluidized-bed reactor, which heats the the particle-feeding entrance. Increasing the baffle angle coal to release gaseous volatile substances. Structure of baffle internals in downer reactor M I real-time M I real-time ° –1 t I C.sec °C p, coal t I p, coal Optimization of baffle internals in downer reactor Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 Liu | 169 can raise the coal-pyrolysis temperature at the exit of the , rotating movement velocity vector ω , net force vector F i i downer reactor both by fixing the horizontal-projection , total moment vector M , particle temperature and the T length of the baffle and the actual baffle length. Increasing enthalpy value H . the baffle number in the downer reactor not only can in- The basic hypotheses of the DEM can be summarized crease the particle MRT in the downer reactor, but also can as follows: (i) the particles are assumed to be spherical; (ii) enlarge the contact space for thermal conduction through when collision or compression happens, a certain overlap particle contact, which can raise the particle-pyrolysis area between two particles is allowed, the size of which temperature at the exit of the downer reactor. The larger is small compared with the particle surface area. The al- the number of baffles in the downer reactor, the closer the gorithm structure of the DEM is composed of the status coal approaches thermal equilibrium. analysis, the kinematics analysis and the search algorithm for particle collisions. Fig. 1 illustrates the main elements of the developed algorithm of the DEM. 1 Mathematical modelling In this research, the motion of all particles in a downer The DEM [23, 24] is a Lagrangian method for calculating reactor is determined by the gravitational forG ce , the col- and recording the information of each particle in a granular lision force F and the forces from the gas phase—that is, system, including the space position vector x , phase pos- the buoyancy force F and the drag force F . The analysis b,i d,i ition vector ϕ , translational movement velocity vector v of the collision force between particles has been presented i i Fig. 1: Foundational principle of the developed DEM software package coupling with the particulate heat-transfer model Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 170 | Clean Energy, 2021, Vol. 5, No. 2 in previous research [25, 26]. The calculation method of the UNIT: mm buoyancy force and the drag force can be listed as shown in Equations (1)–(3). G = m g (1) i i Ä ä m 4 3 F = −ρ g = − πr ρ g b,i g g (2) ρ 3 i F = ζ · A · ρ u − v u − v g g g  d,i w,i i i    Re < 2  i  Re (3) 18.5 2 < Re < 500 ζ = 0.6 i  Re      5 0.44500 < Re < 2 × 10 Therefore, the forces and momentum balance of particle i can be calculated as Equation (4): F = F + F + F + G i c,i b,i d,i i (4) M = M + M i t,i r,i The enthalpy of the solid particle is assumed to be a func- tion of temperature. Thus, the particle temperature of the ith particle T is based on the enthalpy difference ΔH , i i which can be calculated using Equation (5): ΔH T = T + (5) i i,0 m ·c i p,i The net force acting on the particle is governed by Newton’s Second Law of Motion (Equation (6a)), which determines the translational movement of the particle. The par - ticle rotation is determined by Newton’s Law of Rotation (Equation (6b)): dv d x i i F = m a = m = m (a) i i i i i 2 dt dt (6) dω d ϕ i i M = I j = I = I (b) i i i i i 2 dt dt Heat transfer to the particle takes place because of par - ticle collisions or interactions with walls through thermal conduction, the interaction with the gas phase via thermal Fig. 2: Schemes of the structure and size of a downer reactor with baffle internals convection and the radiative heat transfer. AB 0.55 56 1.25 104 Viscosity of dry air at 101.33 kPa Coefficient of heat conduction of dry air Density of dry air at 101.33 kPa at 101.33kPa 0.50 96 Specific heat capacity of dry air 1.20 at 101.33kPa 0.45 88 0.40 80 44 1.15 0.35 72 0.30 1.10 0.25 56 1.05 48 0.20 32 400 600 800 1000 1200 400 600 800 1000 1200 Temperature t / °C Temperature t / °C Fig. 3: Density, viscosity, heat-conduction coefficient and heat capacity of the gas phase in a downer reactor at different temperatures under a con- stant pressure of 101.33 kPa –3 Density ρ / kg·m Viscosity µ / µPa·s –1 –1 Specific heat capacity c /kJ·kg ·K Coefficient of heat conduction –1 –1 λ / mW·m ·K Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 Liu | 171 The model of particulate thermal conduction uses the where d is the diameter of particle , in m; i k represents the i g –1 –1 following assumptions: (i) when two particles collide, the gas-phase thermal-conduction coefficient, in W·m ·K ; direction of the temperature gradient is perpendicular to Nu stands for the Nusselt number [2930 , ]; Re and Pr are their contact surface; (ii) thermal conduction takes places the Reynolds number of particle and the Pr i andtl number, in the area of overlap of two colliding particles; (iii) there respectively; c is the heat capacity of the gas phase, in p.g –10 –1 –1 is a gap with a thickness of 4× 10 m between the contact kJ·kg ·K ; μ is the gas-phase dynamic viscosity, in Pa·s; ρ g g –3 surfaces of two colliding particles [27 28 , ], through which stands for the density of the gas phase, in kg·m; u − u g i –1 the heat can be transferred; the thermal resistance of this means the gas velocity relative to the particle, in m·s . gap is calculated as follows: The radiative heat transfer between the particle and dgap R =  con the main zone of the downer reactor cannot be neglected k A g gap Å ãÅ ã 2 2 2 2 2 2 d +r −r d +r −r (7) ij j i ij j i because of the high temperature of the reactor surface. A = π r + r −  gap j j 2d 2d ij ij Kirchhoff’s law of thermal radiation can be used to calcu- late the radiative flux as follows: The heat flux can be also affected by the thermal resistance of the particle interior, which can be calculated as follows: 4 4 (12) Q = σ ζ A T − T S−B rad i i i b 1 1 1 R = − s,i 2πk r r p,i i,m i –8 (8) where σ is the Stefan-Boltzmann constant, 5.67 × 10 S–B –2 –4 W·m ·K ; ζ is the particle i emissivity, –; A is the heat- i i where k is the thermal-conduction coefficient of the solid, p 2 transfer area between particle i and the reactor bulk, in m ; –1 –1 in W·m ·K ; r presents the radius of the particle, in m; T is the particle i temperature, in K; is the bed temper T a- i b –1/3 r = 2 ·r . Therefore, the heat flux via conduction be- i, m i ture, which is assumed to be approximately equal to the tween two colliding particles with different temperatures wall temperature, in K. can be calculated as follows: ΔT The indicator for describing the mixing degree of the ij Q = (9) con R +Rcon+R s,i s,j coal particle and the heat carrier can employ the vari- The convective heat flux between the particle and the ance or standard deviation of the concentrations of the surrounding gas can be calculated from Newton’s law of key components (coal particles) in each position of the cooling, which is described as follows: particle-motion space. A  low value of standard deviation represents good mixing of the particles within the model (10) Q = h A T − T cov g i g i domain. The calculation method can be summarized as where h represents the coefficient of heat transfer through follows: convection between the particle and the surrounding gas, –2 –1 in W·m ·K ; A is the heat-transfer area between particle i and the surrounding gas, in m; T and T are the gas-phase g i temperature and the particle i temperature, respectively. The coefficient of thermal convection h can be calculated as follows: Nu·k h =  g  i  √  3 Nu = 2.0 + 0.6 Re · Pr (11) d ρg|ug−v | i i Re =  μ c μ p,g g Pr = kg Table 1: Parameter values of coal and silica sand in the model of mechanics and thermodynamics Particulate materials Coal Sand –3 Particle density ρ/kg·m 1250 2450 Particle diameter d /mm 5 5 9 10 Elastic modulus E/Pa 5.08 × 10 4.10 × 10 Poisson ratio v/– 0.28 0.22 Sliding friction coefficient μ /– 0.51 0.66 –5 –5 Rolling friction coefficient μ/m 5 × 10 5 × 10 Recovery coefficient e/– 0.85 0.90 Specific thermal capacity l.13 0.92 –1 –1 c /W·m ·K Thermal-conduction coeffi- 0.26 0.52 –1 –1 cient k /W·m ·K Fig. 4: Particulate mixing and temperature of coal and sand in a downer Blackness ζ /– 0.85 0.70 reactor simulated by the DEM software package Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 172 | Clean Energy, 2021, Vol. 5, No. 2 (i) The volume fraction (as concentration) x of coal downer reactor; N is the number of spatial samples in the coal,i particles (as key components) in spatial sample i at mixing zone of the downer reactor. The maximum and the any position of the mixing zone of the downer reactor minimum of the standard variance can be calculated by: can be calculated as ¯ ¯ σ = x · (1 − x ) V coal coal coal (13) » x = coal,i V +V (16) coal sand ¯ x ·(1−¯ x ) coal coal σ = (ii) It is w orth noting that the particle number in eac n h An index of mixing of the coal and the sand for real-time spatial sample at any position of the mixing zone analysis M can be defined as: real-time σ−σ M = of the downer reactor may be different, because the (17) real−time σ −σ 0 r volume of the spatial sample is fixed. Therefore, the The reactor internals can affect not only the coal- variance or standard variation of the volume fraction particle temperature at the exit of the downer reactor, of the whole reactor space can be of coal particles x coal,i but also the rate of change of the temperature of the calculated as particles within the reactor by the process of solid–solid heat transfer. The rate of particle-temperature change is influenced by the heat transfer as determined by the x = w · x (14) coal i coal,i i=1 temperature difference between the particle and its sur - rounding, the heat-transfer coefficient and the physical σ = w · x − x¯ chemical properties of the particulate material itself. (15) i coal,i coal i=1 The rate of change of the th particle temper i ature can be defined as follows: where w is the weight of sample ,i which can be calculated by w  = n /n; n is the particle number in the spatial sample ∂T (x ,t) T (x ,t+Δt)−T (x ,t−Δt) i 0 i 0 i 0 i i i (18) α = = lim ∂t 2·Δt i; n is the total particle number in the mixing zone of the Δt→0 Coal Sand (a) (b) (c) (d) (e) (f) (g) 0.5 cm 1.0 cm 1.5 cm 2.0 cm 4.0 cm 6.0 cm 8.0 cm 1.0 1.0 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 baffle,proj 0.2 0.2 0.0 cm 1.0 cm 2.0 cm 0.1 0.1 3.0 cm 4.0 cm 5.0 cm 0.0 0.0 0.00.5 1.01.5 2.02.5 t / sec l / d baffle,proj gap Fig. 5: Effect of the baffle length on the particulate-mixing degree of coal and sand simulated by the developed DEM software package M / – real-time M / – real-time Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 l / d baffle, proj gap Liu | 173 A B l / d baffle,proj gap t = 684.57 °C wall 100 0.0 700 0.5 600 500 1.0 1.5 400 2.0 2.5 1.6 1.4 100 100 1.2 1.0 0.8 0.6 2.0 .0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0.4 1.5 0.2 1.0 0.5 l / m 0.0 downer 0.0 CD t = 684.57 °C wall 0 300 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 l / d l / d baffle, proj gap baffle, proj gap Fig. 6: Effect of the baffle length of a downer reactor on the distribution of the coal-pyrolysis temperature along the height direction of the downer reactor simulated by the developed DEM software package 2 Simulation conditions of the downer reactor if no vertical baffle is installed and In order to research the influence of the baffle internals on the effect of the internal baffles on particulate mixing the particle mixing and heat transfer in the downer reactor, and heat transfer in the mixing zone of the downer re- the developed algorithms of a 3D DEM software package actor will be unclear. A  series of baffle internals are in- were used to establish the numerical-simulation system. stalled in the main zone of the traditional type of downer The correctness of the software package and the ration- reactor to form the new type of baffle downer reactor, as ality of the particle parameters have been validated in past shown in Fig. 2. The angle of the baffle with respect to the research [25, 26]. The shape and size of the downer reactor downer-reactor wall of the six baffles is 45°, the length with baffle internals are illustrated in Fig. 2, showing the of the six baffles is 70.70 mm and the interval between front and side views. the baffles along the vertical direction is 250mm. In this As described in Fig. 2, the downer reactor mainly con- study, the flow type and mixing behaviour of particulate sists of two hoppers, each containing a different kind of materials affected by the baffle internals in the downer granular material and a long vertical tube, which is the reactor can be investigated clearly. In addition, the design main zone for particulate mixing and heat transfer. In of the baffle internals has been optimized to strengthen particular, there are two vertical baffles at the exits of the the mixing and heat-transfer behaviour of the particles two hoppers—that is, the entrance of the mixing zone of using the developed algorithm of the 3D DEM software the downer reactor—to avoid disturbing the evaluation package. of the effect of baffle internals on the particulate mixing The initial conditions and basic assumptions for simu- and heat transfer in the downer reactor. The results of lating the particulate mixing and heat transfer in the particulate mixing and heat transfer will consider the downer reactor are as follows: (i) to simplify the simula- structure and shape of the entrance of the mixing zone tion, the gas phase in the downer reactor is considered l / m downer t / °C t / °C p, coal, terminal p, coal -1 α / °C·sec coal t / °C p, coal Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 174 | Clean Energy, 2021, Vol. 5, No. 2 Coal Sand (a) (b) (c) (d) (e) 15° 30° 45° 60° 75° 1.0 1.0 α : baffle 0.9 0.9 0° 15° 30° 0.8 0.8 45° 60° 75° 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0.0 t / sec α / ° baffle Fig. 7: Effect of the baffle angle on the particulate-mixing degree of coal and sand simulated by the developed DEM software package to be still dry air; the gas-phase density and viscosity are the developed DEM software package, as shown in Fig. 5. functions of temperature, as illustrated in Fig. 3; (ii) uni- l stands for the projection of baffle length l onto baffle, proj baffle form temperature on the inner surface of the downer the horizontal plane. α is the angle between the ver - baffle reactor is assumed during the calculation; (iii) at a tem- tical direction and the baffle-length direction, i.e. l = baffle perature of 0°Ϲ , the specific enthalpy of the material com- l /sinα . The dimensionless parameter l /d , baffle, proj baffle baffle, proj gap –1 ponents in downer reactor is 0 kJ·kg; (iv) the physical the horizontal-projection length of the baffle divided by properties of the coal and the sand have been listed in the gap width of the particle-feeding entrance (2.0 cm) Table 1 [31–33]. shown in Fig. 5, should be optimized to maximize the coal- pyrolysis conditions in the downer reactor. It can be concluded from Fig. 5 that l should be no baffle, pro less than the width of the particle-feeding entrance d , 3 Results and discussion gap which can mix the coal and the sand sufficiently. However, The granular-mixing and heat-transfer behaviour of the too large a baffle length cannot improve the mixing de- coal and the sand affected by internals in a downer re- gree of the coal and the sand further, but will increase the actor have been simulated by the developed DEM software particle-residence time unnecessarily. When the volume- package, which is shown in Fig. 4. The baffle internals can feeding-rate ratio of coal to sand is fixed at 1:4, the wall affect the mixing degree of the fuel particles and heat car - temperature is equal to 684.57°Ϲ under the adiabatic con- riers in the downer reactor, and the change in the final dition. The effect of the baffle length in the downer reactor coal-pyrolysis temperature and the particle-heating rate on the distribution of coal-particle temperatures along the indirectly. Therefore, optimization of the design of baffle height of the downer reactor simulated by the developed internals is important for the coal-pyrolysis process in a DEM software package is illustrated in Fig. 6. downer reactor. As shown in Fig. 6, the coal-temperature distribution in the downer reactor is remarkably affected by the baffle- length design. The improvement in the coal-temperature 3.1 Baffle length distribution is affected by the enhancement of the heat The effect of the baffle length on the mixing degree of transfer between the coal and the sand due to the effect the coal and the sand particles has been simulated by of the mixing degree of the coal and the sand, especially M / – real-time M / – real-time Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 baffle Liu | 175 baffle AB 75° 60° 45° 30° 15° 0° t = 684.57 °C 100 wall 600 500 1.6 1.4 100 100 1.2 1.0 0 0.8 0.6 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 50 0.4 0.2 l / m downer 0.0 t = 684.57 °C wall 0 300 010203040 50 60 70 80 90 010203040506070 α / ° α / ° baffle baffle Fig. 8: Effect of the baffle angle of a downer reactor on the distribution of the coal-pyrolysis temperature along the height direction of the downer reactor simulated by the developed DEM software package due to the horizontal length of the baffle ranging from It can be concluded from Fig. 7 that the mixing degree 0 to 1.0d . In addition, the MRT of coal particles in the increases with the slope angle of the baffle internals in the gap downer reactor will increase from 0.497 to 0.859 sec when downer reactor. When the volume-flow-rate ratio of coal to the dimensionless parameter l /d increases from 0 sand is fixed at 1:4, the wall temperature of the downer re- baffle, proj gap to 2.5. Therefore, the coal-particle-temperature distribu- actor can be calculated as 684.57°Ϲ. The effect of the slope tion along the height of the downer reactor and the coal- angle of the baffle internals on the coal-particle terminal pyrolysis temperature at the exit of the downer reactor temperature and the average particle-heating rate are il- are remarkably affected by the horizontal length of the lustrated in Fig. 8. baffle ranging from 0 to 1.0d , as shown in Fig. 6c. Most It can be concluded from Fig. 8 that the coal-particle gap importantly, the coal-heating rate can reach a maximum temperature increases with the particle distance from value when l is equal to d . Therefore, the hori- the entrance of the downer reactor. In addition, the baffle, proj gap zontal projection of the baffle length should be no less coal-particle terminal temperature increases with than the width of the particle-feeding entrance, which is the slope angle of the baffle internals in the downer a benefit for the fast pyrolysis of the coal in the downer reactor. The larger slope angle of the baffle internals reactor. makes the coal-particle terminal temperature closer to the wall temperature of the downer reactor. However, the slope angle of the baffle internals will increase 3.2 Baffle angle the residence time of coal particles in the downer re- The effect of the slope angle of the baffle internals on the actor. Therefore, the coal-particle-temperature heating particulate mixing of the coal and the sand in the downer in the downer reactor does not increase monotonically reactor can be simulated by the developed DEM software with the obstacle of the baffle internals. As is vividly package, as shown in Fig. 7. shown in Fig. 8, the coal-particle heating rate increases l / m downer t / °C t / °C p, coal, terminal p, coal -1 α / °C·sec coal t / °C p, coal Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 176 | Clean Energy, 2021, Vol. 5, No. 2 Coal Sand (a) (b) (c) (d) (e) (f) 1 baffle 2 baffles 3 baffles 4 baffles 5 baffles 6 baffles AB 1.0 1.0 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 Baffle number: 0.1 0.1 0 1 2 3 4 5 6 0.0 0.0 012345 012345 6 t / sec baffle Fig. 9: Effect of the baffle number on the particulate-mixing degree of coal and sand simulated by the developed DEM software package with the obstacle of the internals from 0° to 30° and the baffle-slope angle is 45°. The baffle interval is 5l . baffle, proj decreases after 30°. Therefore, a 30° slope angle of the The upper edge of the first top baffle is fixed at the position baffle internals is beneficial for the rapid pyrolysis of of 4l (20.0 cm) from the entrance of the sand hopper. baffle, proj coal under the condition of a fixed volume-flow-rate The effect of the baffle number on the particulate mixing ratio of coal to sand. of coal and sand in the downer reactor can be simulated by the developed DEM software package coupling the particu- late model of heat transfer, as shown in Fig. 9. 3.3 Baffle number It can be concluded from Fig. 9 that the mixing de- The number of baffles is one of the most important de- gree increases with the internal-baffle number in the sign parameters for the baffle-type downer reactor. The downer reactor. When the volume-flow-rate ratio of horizontal-projection length of the baffle is 2.5d and coal to sand is fixed at 1:4, the wall temperature of the gap M / – real-time M / – real-time Downloaded from https://academic.oup.com/ce/article/5/2/167/6220073 by DeepDyve user on 13 April 2021 baffle Liu | 177 AB baffle t = 684.57 °C 100 wall 500 2 1.6 1.4 1.2 1.0 0 0.8 0.6 0.00.2 0.40.6 0.81.0 1.21.4 1.61.8 0.4 0.2 l / m 2 downer 0.0 CD t = 684.57 °C wall 0 300 012345 6 012345 6 n n baffle baffle Fig. 10: Effect of the baffle number of a downer reactor on the distribution of the coal-pyrolysis temperature along the height direction of the downer reactor simulated by the developed DEM software package downer reactor can be calculated as 684.57° . The Ϲ effect It can be concluded from Sections 3.1–3.3 that the design of the internal-baffle number on the coal-particle ter - of the internal baffles can affect the coal-particle-heating minal temperature and average particle-heating rate is rate in the downer reactor for the pyrolysis process. The illustrated in Fig. 10. optimized baffle internal design is that the baffle length is It can be concluded from Fig. 10 that the coal-particle 40 mm, the baffle-slope angle is 30° and the baffle number temperature increases with the particle distance from is no fewer than four. Under this condition, a higher coal- the entrance of the downer reactor. In addition, the particle-heating rate can be obtained in the downer re- coal-particle terminal temperature increases with the actor for the pyrolysis process. internal-baffle number in the downer reactor. A  large number of internal baffles makes the coal-particle ter - minal temperature closer to the wall temperature of the 4 Conclusions downer reactor. However, more internal baffles will in- crease the residence time of coal particles in the downer The structural optimization of baffle internals in a downer reactor, so the coal-particle-heating rate in the downer reactor has been discussed in this research. For one thing, reactor will not increase monotonically with the increase the coal-pyrolysis temperature at the exit of the downer re- in the internal-baffle number. As vividly shown in Fig. 10, actor can be determined by the volume-feeding-rate ratio the coal-particle-heating rate will not be raised when the of the coal to the sand. For another, it will be also affected number of internal baffles is larger than four. Therefore, by the structural design of the internals in the downer re- no fewer than four internal baffles should be installed in actor. 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Journal

Clean EnergyOxford University Press

Published: Jun 1, 2021

References