State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering,College of Chemistry and Chemical Engineering,Ningxia University,Yinchuan 750021,China

Keywords:Chemical looping gasification Fuel reactor Gasification characteristics Fe2O3 oxygen carrier

ABSTRACT Chemical looping gasification(CLG)of Ningdong coal by using Fe2O3 as the oxygen carriers(OCs)was studied,and the gasification characteristics were obtained.A computation fluid dynamics (CFD) model based on Eulerian-–Lagrangian multiphase framework was established,and a numerical simulation the coal chemical looping gasification processes in fuel reactor(FR)was investigated.In addition,the heterogeneous reactions,homogeneous reactions and Fe2O3 oxygen carriers'reduction reactions were considered in the gasification process.The characteristics of gas flow and gasification in the FR were analyzed and it was found that the experiment results were consistent with the simulation values.The results show that when the O/C mole rate was 0.5:1,the gasification temperature was 900°C and the water vapor volume flow rate was 2.2 ml·min?1,the mole fraction of syngas reached a maximum value of the experimental result and simulation value were 71.5%and 70.2%,respectively.When the O/C mole rate was 0.5:1,the gasification temperature was 900°C,and the water vapor volume flow was 1.8 ml·min?1;the gasification efficiency reached the maximum value was 62.2%,and the maximum carbon conversion rate was 84.0%.

1.Introduction

Coal is a significant part of the world's fossil fuels,which is mainly used for combustion power generation.The carbon dioxide(CO2)emission from coal combustion is intensifying the process of global warming[1].Nowadays,CO2capture and separation technology has been widely concerned,but it will increase the energy consumption and reduce the total efficiency of the system[2].It is essential to develop the clean coal combustion and gasification technology to reduce the emission of CO2[3,4].Coal gasification technology is widely concerned for conversion to syngas(CO+H2).Chemical looping gasification(CLG)is also a novel technology for the production of syngas as coal conversion[5–7].The schematic of CLG process is showed in Fig.1;there are two fluidized bed reactors in the CLG system,they are air reactor(AR)and fuel reactor(FR),respectively[8,9].The CLG process employs the lattice oxygen[O]from oxygen carriers(OCs)to instead of molecular oxygen from air,and then coal is partially oxidized into syngas[10].Coal and oxidants feed into the FR,where the OCs supply [O]to generate syngas,the OCs transport the AR to recovered[O]by oxygen.In addition,the OCs play an important role in CLG process,the highly quality oxides not only can provide [O],but also can reduce the production of SOXand NOX[11–13].Several metal oxides have been considered as oxygen carriers for CLG,mainly including CuO,NiO,CdO,Mn2O3,Fe2O3and CoO and so on[14,15].The iron-based OCs perform well in CLG process,owing to its stronger chemical properties,high oxygen carrying capacity and mechanical strength than other metal oxides[16,17].Although great processes have been made in the study of the CLG process,the research on the coal as fuel is relatively limited.Guo et al.[18]reported the gasification performance of CLG using the coal as the fuel.The results show that the carbon conversion rate,gasification efficiency and syngas content reach maximum of 96.8%,84.3%and 66.9%at the temperature of 900°C,and the Ca-based oxides were used as the OCs in this process.Abad et al.[19]developed a theoretical model for CLC process in FR using the nature iron ore.They described the dynamics characteristic of gas flow and the reaction of OCs with gases which are released from coal.Huang et al.[20]analyzed that the reaction characteristics of nature iron ore in CLG with the biomass char,the results indicated that the iron ore as the OCs still had reaction activity after undergoing 20-cycles of redox experiments.Moreover,Zhang et al.[21]studied the reactivity,recyclability and consumption performance of OCs using natural pyrite as the OCs of coal CLC.The results showed that pyrite slag was an ideal candidate for ironbased OCs.

In addition,with the improvement of numerical methods and more advanced hardware technology,the computational fluid dynamics(CFD)is widely used to simulate the complex gas–solid flow,heat transfer and chemical reaction characteristics in CLG process.Jung et al.[22]and Deng et al.[23]simulated the CLC process in fluidized bed FR with NiO and CaSO4oxygen carriers,respectively.The results showed the 3D fluidized bed was simplified to 2D,which had an impact on the simulation results.Besides,Shuai et al.[24]and Kruggle et al.[25]performed the CFD simulation of FR using gaseous fuels.Wang et al.[26]and García et al.[27]carried out the numerical model of FR using the solid fuels.

Fe2O3was used as the OCs in coal CLG progress in the study,and the synthesis gas contents,carbon conversion and gasification efficiency were investigated.Meanwhile,the ANSYS FLUENT was applied to establish the coal CLG progress in FR.The characteristics of gas flow,temperature and each component distribution in the FR were examined with the changing of the parameters of O/C molar ratio,gasification temperature and water vapor flow.

2.Experimental Methods

2.1.Samples

The Fe2O3was used as OCs in this study.The raw Fe2O3(purchased chemicals,purity of 99.8%,density of 5240 kg·m?3)was made into particles with a diameter of 75 μm.In addition,the coal sample came from the coalfield of Ningdong,Ningxia,China.The coal was ground and sieved to a particle size of 150 μm.The proximate analysis and ultimate analysis of coal sample were shown in Table 1.The proximate analysis was determined by the Chinese National Standard GB/T 212-2008,and the ultimate analysis was determined by the Chinese National Standard GB/T 476-2008.

2.2.Experimental apparatus

The experimental apparatus is shown Fig.2.It composes of FR,gas distribution system,electric heating system,temperature control and gas sampling and analysis system.The coal particle gasification experiment was performed on the FR experimental device.The total height of the FR is 1000 mm,and the inner diameter is 60 mm.A porous gas distribution plate is located at the bottom of reactor to distribute gas and carry the OCs particles.The feed tube is arranged at 300 mm at bottom of the reactor with an aperture of 12 mm.The FR is equipped with an electric heating system and controlled the bed temperature for k-type thermocouples.The gas supply system is controlled by the mass flow controller(MFC,S4933),which is mainly controlled the flow of nitrogen and air.Water vapor is provided as a gasification agent by a constant flow pump(2PB,Beijing Xingkeda Tech.Inc.).The online flue gas analyzer (MRU Vario Plus)is used to analyze and measure the gasification products.

2.3.Experimental conditions

In order to study the coal gasification characteristics in the CLG process,the experimental scheme is shown in Table 2.The entire bed of FR was heated to 850 °C in air atmosphere before starting the experiment.The OCs of Fe2O3was placed on a gas distributor equipped with an asbestos screen,and the Ningdong coal was placed in a storage bin.Afterwards,the air was replaced by N2into the FR,and the gas flow rate was always 2 L·min?1,the purpose of which was to exclude the air in the bed and as a fluidizing gas.Then,the coal from storage bin fed into the FR when the temperature reached experimental setting value,and contents of gasification products measured by an online gas analyzer.

2.4.Experimental date evaluation

The O/C mole ratio in Table 2 is defined as:

Fig.1.Schematic of chemical looping gasification process.

where mocand mcoalare the molar mass of the OCs and coal respectively,g·mol?1.Rocis the oxygen donating capacity of OCs,g.φcoalis the oxygen needed to completely combust per unit molar mass of coal,g·mol?1.

As for the repeatability experimental tests of coal in fluidized bed,the gas relative composition(φi/%)in reaction process is calculated as:

where χi,ν are expressed as the volume fraction of species i,vol%,and synthesis gas flow rate,L·min?1respectively.andare represented the volume fractions of CO,CO2,CH4and H2in the flue gas,vol%respectively.

The gas yield(Fout/L ?min?1)in CLG process was evaluated following N2balance,which is calculated as:

Besides,synthesis gas content(φsyn/%),carbon conversion(XC/%)and gasification efficiency(η/%)in CLG process are calculated as following:

where ncrepresents the total number of moles containing carbon gas during the CLG process,mol.Qiis low thermal value for gas products,kJ·m?3.is low thermal value of coal,MJ·kg?1.Fiis the volume flow rate of gas component,L·min?1.i is present the gas compositions(CO,H2and CH4).

3.Numerical Model

3.1.Geometry and meshing

In this work,a three-dimensional model of the FR based on Cartesian coordinates was established.The structure diagram and grids wereshown in Fig.3.The schemes in the simulation were listed in Table 3.In addition,grid independence verification was performed on computational domains containing 360 k,720 k and 1190 k cells,respectively.

Table 1 Proximate analysis and ultimate analysis of Ningdong coal

Fig.2.Schematic illustration of experimental apparatus.

3.2.CFD model

3.2.1.Continuity equations

The continuity equations for gas and particle phases are given by[28]:

3.2.2.Momentum equations

For the gas phase:

Table 2 Experimental conditions in Ningdong coal CLG process.

In Eq.(14),μgis the gas-phase shear viscosity;μglis the gas-phase laminar viscosity,and μgtis the gas phase turbulent viscosity,and computed as a function of κ and ε:

where Cμis the constant,which is the 0.09.

Fig.3.Structure diagram and grids of the FR.

Table 3 Schemes of the simulations

The turbulence models equations for κ and ε can be expressed as:

where Πkand Πεrepresents the influence of the solid phase on the gas phase.Gκis the production of turbulent kinetic energy.The constants of C1=1.44 and C2=1.92.The turbulent Prandtl numbers for κ and ε of σk=1.0 and σε=1.3.

For the solid phase:

where ppis the solid phase pressure from the particle collisions,the solid particle-phase stress strain tensor.They are expressed as[30].

where,in Eq.(19),ξPis the solid particle-phase bulk viscosity.μpis the solid particle-phase shear viscosity.In Eq.(20),gois the radial distribution function.They have the following form:

where Θ is the granular temperature,which is proportional to the kinetic energy of the random motion of solid particles.The transport equation derived from kinetic theory takes the form:

where γP,ΓΘand φPrepresents the collisional dissipation of energy,the diffusion coefficient and the energy exchange between the gas phase and the particle phase,respectively.They are given by:

3.2.3.Energy equations

where H,λ and h(hgp=hpg)are the specific enthalpy,the mixture thermal conductivity and the transfer coefficient between phases,respectively.The third term on the right-hand is the heat transfer from the particle phase to the gas phase.

The heat transfer coefficient is related to the solid particle phase Nusselt number,Nupgiven by:

3.2.4.Species transport equations

where Jg,iis the diffusion flux of species i in the gas phase.Siis the net rate of production of species i.

Diffusion flux Jg,iis calculated by:

where Sctis the turbulent Schmidt number,and is set as 0.7,Di,mis the diffusion coefficient for species i in the mixtures.

3.3.Chemical reactions mechanism

In this work,a preliminary chemical reaction of coal was studied at the FR in CLG process.For the coal,the global reaction modeling was adapted with the de-volatilization reaction,and assumed to take place in a single reaction.The volatile released in the de-volatilization process and rapidly decomposes,and then the final mixture species were formed in a rapid volume reaction.Considering the complexity of model building,the main products of the volatilization were regard as CO,H2,CH4,CO2,H2O and Tar.It was described as flowing:

From the Eq.(34),it can be seen that the volatiles in coal particles were mainly considered four gases of CO,CO2,CH4and H2.In addition,considered the tar was the liquid phase mixture,it would be ignored in this simulation.The coefficient of a-e in Eq.(34) was calculated from the conservation of mass and elemental conservation based on the chemical analysis of Ningdong coal.The de-volatilization reaction of the entire coal can be expressed as:

The reaction mechanisms of Ningdong coal CLG are given in Table 4.The gasification reactions are R2 to R5,and the Fe2O3oxygen carrier's reduction reactions were R6 to R8.

3.4.Model setups

ANSYS FLUENT software was employed to simulate the CLG of Ningdong coal in the study.The SIMPLE was used the steady-state simulation,which was applied for the pressure–velocity coupling equations.The equations mentioned above were solved by a finite volume method,and a first-order upwind discretization was chosen for all solution.The radiation model was set to P1.The standard residual of the convergence of the solution was the default value,the residual of the continuity equation installed to 10?5,and the remaining equations were set to 10?4.The important boundary and simulation conditions could be referred from Table 5.

4.Results and Discussion

4.1.Experiment pathway analysis

The reaction pathway for the conversion of Fe2O3and Ningdong coal in CLG progress is described,as shown in Fig.4.As the coal CLG is an extremely complex process,the main reactions are considered in the FR and divided into three stages:coal pyrolysis and volatilization release,gasification reactions and oxygen carrier's reduction reactions.The coal pyrolyzed rapidly and volatiles released under high temperature after coal was fed into FR.The volatiles consist of H2O,CO2,CO,H2and CH4.Afterwards,a series of subsequent reactions(R2–R5)between volatiles and water vapor to generate the syngas were considered,then the chemical components and contents of syngas were improved and promoted by redox reactions between Fe2O3and volatiles reaction(R6–R8).The characterization analysis of samples was shown in Fig.5.Among which,Fig.5(a) and Fig.5(b)display the SEM photographs of the OCs of Fe2O3before the CLG experiments.It can be seen that the samples of Fe2O3show a polyhedral shape and the surface are smooth and flat,and there are large pores.Fig.5(c)and Fig.5(d)display the SEM photographs of the OCs particles after the CLG experiments.It can be seen that the crystal structure of OCs has changed,showing an irregular hexagonal structure and a rough surface.The granulation degree of OCs is enhanced,and the surface area is increased,thereby improving the reactivity of the OCs.Meanwhile,the OCs particles do not appear agglomeration or sintering.Fig.5(e)and Fig.5(f)display the XRD transform of the OCs particles in coal CLG process.It can be seen that OCs of Fe2O3are converted into Fe3O4after CLG,which is used as a basis for numerical simulation to establish the Fe2O3reduction reaction mechanism.Moreover,it has been reported that the iron-based oxygen carrier evolution process and pathway used in CLG and CLC,and verified by Parker et al.[34]and Chen et al.[35].

4.2.Simulation analysis

4.2.1.Grid independency

In this section,the grid independence study was performed to choose an adequate grids number,and the simulation computation was conducted by using the number of girds with 360 K (Case 1),720 K(Case 2)and 1190 K(Case 3).The simulation results at FR outlet were listed Table 6.The results indicated that the content of each component of in Case 1 was quite different compared with Case 2 and Case 3,so the Case1 to be ignored.The content of each component at outlet was less than 1%between Case 2 and Case 3,but the calculation time of Case 3 was 1.75 times that of Case 2.Hence,the Case 2 was used for the computational model.

Table 4 Kinetic parameters for important reactions in coal CLG

4.2.2.Model validation

In order to validate the numerical model,the simulation results of the temperature and the mole fraction of each component at reactor outlet were compared with the experimental results,as listed in Table 7.It shows a relatively good agreement for each component at outlet of FR.The difference of all components was less than 5%.The results show that the accuracy of the numerical simulation was acceptable.Therefore,the calculation model could be used to describe the coal CLG process.

4.2.3.Computation analysis

Fig.6 shows the contours of temperature and mole fraction of each species on a vertical mid-plane in the FR under the condition#2.Fig.6(a)shows the distribution of temperature in the FR.Coal,H2O and OCs are mixed vigorously when coal particles entered into the reactor.Volatile gas(CO,CO2,CH4,H2)is released by rapid pyrolysis of coal and a small amount of[O]react with volatile gas and coke.Meanwhile,the water–gas shift reaction(R4)takes place,too.These are exothermic reactions.The exothermic reactions result in higher temperature in the area near inlet.Subsequently,synthesis gas is formed by chemical reaction of char and oxidizing gas([O]comes from the OCs and H2O)at high temperature.The temperature decreases gradually with the gasification reaction(R2)and reduction reaction(R6,R8)going on,which are endothermic reactions.The temperature has a tendency to stabilize at the upper part of the FR.Fig.6(b)shows the distribution of the H2in the FR.The highest mole fraction of H2is at the top of the reactor.This is mainly due to the hydrogen produced by the gasification reaction(R2),water gas shift reaction(R4)and methane steam reforming reaction(R5).In addition,the mole fraction of H2fluctuates slightly in the middle of the FR,which is mainly due to the consumption of the reduction reaction(R8).Fig.6(c)shows the distribution of CO in the FR.It should be noted that the mole fraction of CO gradually increases and reaches a maximum value at the outlet of the FR.The content of COmainly comes from gasification reaction (R2) and methane steam reforming reaction(R5).Meanwhile,the reduction reaction(R6)and water–gas shift reaction(R4)consumes a small amount of CO to reduce its content.Fig.6(d)shows the distribution of CO2in FR.It can be described that the change of CO2content is relatively obvious.In the middle part,the content of CO2increases due to the water–gas shift reaction(R4)and the reduction reaction(R6,R7),then the content decreases due to the boudouard reaction (R3) consumption of CO2.Fig.6(e)shows the distribution of CH4,which is derived from coal pyrolysis(R1).The content of CH4is the highest at the inlet,and then gradually decreases due to the methane steam reforming(R5)and reduction reaction(R7)consumption of CH4.Fig.6(f)shows the distribution of H2O.With the reaction proceeding,the water vapor consumption increased,the content gradually decreases and reaches the minimum at the outlet of the FR.

Table 5 Boundary conditions and others simulation details

Fig.4.Reaction pathway of OCs of Fe2O3 in bituminous coal CLG process

Fig.7 shows the variation of components at different heights in FR.At 300 mm,the content of each component at the feed pipe shows a small fluctuation,mainly due to the release of volatile gases from the R1 reaction.With increase of the FR height,the reaction reaches equilibrium and the content of each component tends to be stable at 900 mm.

4.3.Effects of process parameters

4.3.1.O/C mole ratio

Fig.5.SEM photographs of OCs of Fe2O3 and XRD patterns of OCs before and after CLG(The JCPDS numbers of(e)and(f)are PDF#33-0664 and PDF#26-1136.)

Table 6 Species contents,temperature and calculation time in CLG process

Fig.8 shows the effects of different O/C mole ratios on gasification characteristics of Ningdong coal CLG.With the increase of the O/C mole ratio,the mole fraction of the syngas,carbon conversion rate and gasification efficiency increases first and then decreases at the outlet of the FR.With the O/C mole ratio raise from 0.2:1 to 0.5:1,the content of OCs of Fe2O3increases,which leads to an acceleration of thermal cycle and promotes the gasification reaction(R2).In addition,the Fe2O3oxygen carrier's reduction reactions(R6,R8)are enhanced,consuming theH2and CO generated by the gasification reaction(R2)and accelerating the forward reaction rate.The results show that the mole fraction of syngas,the carbon conversion rate and gasification efficiency increase.When the O/C mole ratio is raised from 0.5:1 to 0.8:1,the mass of the reaction bed augments to affect the gas–solid flow,resulting in a decrease in the rate of gasification reaction(R2).In addition,the[O]provided in the OCs of Fe2O3increases,and the syngas generated by the gasification reaction(R2)is oxidized to H2O and CO2.As a result,the mole fraction of the syngas,carbon conversion rate and gasification efficiency are decreased.At the O/C mole ratio of 0.5:1,the experimental result and simulation value of the syngas mole fraction reaches a maximum value of 70.1% and 68.3%,respectively.Meanwhile,the carbon conversion is 84.0%,and the gasification efficiency is 56.2%.

Table 7 Compared with experimental results and simulation values.

4.3.2.Water vapor volume flow

Fig.6.Contours of temperature and species mole fraction on a vertical mid-plane in the FR.

Fig.9 shows the effects of different water vapor volume flow on gasification characteristics of Ningdong coal CLG.With the increase of water vapor volume flow,the mole fraction of syngas increases first and then decreases,the carbon conversion rate decreases and the gasification efficiency increases at the outlet of the FR.When the water vapor volume flow is increased from 1.8 to 2.0 ml·min?1,the gas–solid contact is intensified in the FR,which is beneficial to the gasification reaction (R2) to generate syngas.However,the water–gas shift reaction(R4)is able to consume CO,thereby promoting the reduction reaction(R8),resulting in the mole fraction of syngas decreases slightly and the gasification efficiency increases.When the water vapor volume flow is increased from 2.0 to 2.2 ml·min?1,the fluidization velocity is accelerated,and the residence time of the gas is reduced in the FR,which is beneficial to the forward progress of each reaction,resulting in the molar fraction of syngas and gasification efficiency are increased.At the water vapor volume flow of 2.2 ml·min?1,the experimental results and simulation values of the syngas mole fraction reaches a maximum value of 71.5%and 70.2%,respectively.Meanwhile,the carbon conversion is 78.1%and the gasification efficiency is 62.2%.

4.3.3.Gasification temperature

Fig.10 shows the effects of different gasification temperature on gasification characteristics of Ningdong coal CLG.With the increase of gasification temperature,the syngas mole fraction,carbon conversion rate and gasification efficiency increases first and then decreases at the outlet of FR.When the gasification temperature is increased from 850 to 900°C,the gasification reaction rate(R2)elevates because it is an endothermic reaction,which promoted more char consumption and produces more H2and CO,resulting in the molar fraction of syngas,the carbon conversion rate and gasification efficiency are increased.When the gasification temperature is increased from 900 °C to 950 °C,the mole fraction of syngas,the carbon conversion rate and gasification efficiency are decreased,because the temperature increase is not conducive to the water–gas shift reaction(R4).In addition,the high temperature inhibits the reduction reactions(R6,R8),because of this reactions are exothermic.At the gasification temperature of 900 °C,the experimental results and simulation values of the mole fraction of syngas reaches a maximum value of 70.1% and 68.3%,respectively.Meanwhile,the carbon conversion is 84.0%,and the gasification efficiency is 56.2%.

5.Conclusions

In this study,a three-dimensional numerical model was investigated about the Ningdong coal CLG progress.The gas flow and gasification characteristics were obtained.The main conclusions as follow:

Fig.7.Variations the contents of each component with different heights in the FR.

Fig.8.Effect of O/C mole ratio on gasification characteristics.

(1) The optimum O/C mole ratio is 0.5:1.The high O/C mole ratio is beneficial for the gasification reaction(R2)to produce more syngas,but excessively high O/C mole ratio will improve the bed quality to affect the gas flow efficiency.

(2) The optimal water vapor volume flow rate is 2.2 ml·min?1.With the increase of water vapor volume flow rate,the gas flow characteristic enhances,resulting in the rate of gasification reaction(R2)improves.However,in order to ensure the normal fluidization and heat transfer of the particles,the volume flow of water vapor must be controlled within a certain range.

(3) The optimum gasification temperature is 900 °C.With the increase of the gasification temperature,the conversion rate of coal increases,thereby the rate of the gasification reaction(R2)increases,but excessively high temperature is not conducive to the water–gas shift reaction(R4)and Fe2O3oxygen carrier's reduction reactions(R6–R8).

Nomenclature

CDDragcoefficient

Di,mDiffusion coeffciient for species i in the mixture,m2·s?1

dpGranular diameter,m

GκProductionofturbulent kinetic energy

goRadial distribution function

Fig.9.Effect of water vapor volume flow on gasification characteristics.

H Specific enthalpy,J·kg?1

h Heat-transfer coefficient,W·m?2·K?1

JiDiffusion flux of species i,kg·m?2·s?1

m Mass source term,kg·m?3·s?1

Nu Nusselt number

P Solid phase pressure,Pa

Pr Prandtl number

SctTurbulent Schmidt number

SiNet rate of production of species i,kg·m?3·s?1

T Temperature,K

Y Mass fraction of species

α Volume fraction

β Interphase momentum exchange coefficient,kg·m?3·s?1

ΓΘDiffusion coefficient between the gas phase and the particle phase

γ Collisional dissipation of energy,W·m?3

ε Dissipation rate of turbulent kinetic energy,m?2·s?3

Θ Particle-phase pseudo-temperature,m2·s?2

κ Turbulent kinetic energy,m2·s?2

λ Thermal conductivity,W·m?2·K?1

μ Viscosity,kg·m?1·s?1

ξPParticle-phase bulk viscosity,kg·m?1·s?1

ρ Density,kg·m?3

ΠkInfluence of the particle phase on the solid phase,m2·s?3

ΠεInfluence of the particle phase on the gas phase,m?2·s?4

σεTurbulent Prandtl number for ε σkTurbulent Prandtl number for k

φPEnergy exchange between the gas phase and particle phase,W·m?3

Subscripts

g Gas phase

i ith species

j jth reaction

p Particle phase

t Turbulent flow

Acknowledgements

This study was supported by the Key Research and Development Program of Ningxia (2018 BCE01002),and the Discipline Project of Ningxia(NXYLXK2017A04).

Fig.10.Effect of gasification temperature on gasification characteristics.