Numerical Simulation of a New Porous Medium Burner with ... - MDPI

processes Article

Numerical Simulation of a New Porous Medium Burner with Two Sections and Double Decks Zhenzhen Jia, Qing Ye *, Haizhen Wang, He Li and Shiliang Shi School of Resource, Environment and Safety Engineering, Hunan University of Science and Technology, Xiangtan 411201, China; [email protected] (Z.J.); [email protected] (H.W.); [email protected] (H.L.); [email protected] (S.S.) * Correspondence: [email protected]; Tel.: +86-0731-5829-0040 Received: 11 July 2018; Accepted: 26 September 2018; Published: 6 October 2018

Abstract: Porous medium burners are characterized by high efficiency and good stability. In this study, a new burner was proposed based on the combustion mechanism of the methane-air mixture in the porous medium and the preheating effect. The new burner is a two-section and double-deck porous medium with gas inlets at both ends. A mathematical model for the gas mixture combustion in the porous medium was established. The combustion performance of the burner was simulated under different equivalence ratios and inlet velocities of premixed gas. The methane combustion degree, as well as the temperature and pressure distribution, was estimated. In addition, the concentrations of emissions of NOx for different equivalence ratios were investigated. The results show that the new burner can not only realize sufficient combustion but also save energy. Furthermore, the emission concentration of NOx is very low. This study provides new insights into the industrial development and application of porous medium combustion devices. Keywords: two sections and double decks; porous medium; combustion; equivalence ratio; gas velocity in inlet; NOx

1. Introduction Nowadays, a lot of low-grade combustible gases are not effectively used. Direct emission of these gases will result in severe energy dissipation and environment pollution. Porous media have many advantages such as good storage, conduction and radiation of heat, and explosion suppression, etc. These features help the porous medium to achieve super-adiabatic combustion [1,2]. Porous medium combustion technology presents a new type of combustion. Compared with traditional combustion technology, it has a larger combustion limit, higher combustion efficiency, and lower pollutant emission [3–6]. Therefore, porous medium combustion technology is suitable for the utilization of low-calorific-value gases such as low-concentration gas, blast furnace gas, biomass gas, and landfill gas. Based on the chemical kinetic model of the Gibbs minimum free energy theory, Slimane et al. established a mathematical model to simulate the combustion of hydrogen sulphide in porous medium. The composition of combustion products and combustion limit were numerically determined [7]. The combustion of the propane-air mixture was comprehensively investigated by using the ceramic material of honeycomb foam as the porous medium [8]. The temperature of the gas was much higher than that of the adiabatic flame, which was attributed to the internal heat recirculation. Hayashi simulated gas mixture combustion in a porous medium using a three-dimensional mathematical model [9]. The results showed that the flame tended to spread to the combustion zone with lowest air proportion. Hayashi suggested that the pore size of the porous medium should be properly reduced to avoid tempering. Kotani et al. proposed a two-layer porous medium burner [10]. The results showed that the space structure mutation of the junction plane of two different porous media can stabilize the Processes 2018, 6, 185; doi:10.3390/pr6100185

www.mdpi.com/journal/processes

Processes 2018, 6, 185

2 of 18

main body of the flame in the flame support layer. Pereira et al. studied the energy focus-ability of a gas mixture in a burner with two-layer porous media [11]. The results showed that the excess temperature is a function of the modified Lewis number, the porosity of the porous medium, and the gas thermal conductivity ratio of solid to gas. Mital et al. concluded that the radiant heat efficiency in the outlet of the burner can reach 30% [12]. According to the one-step chemical reaction of the methane-air mixture, the one-dimensional concentration and flame velocity field were calculated, and the temperature distribution of the flame under different optical thicknesses was obtained [13]. Based on the multistep reaction model, the gas-solid energy equation was established, and the combustion reaction was simulated [14]. Also, the thermal conductivity, volume heat transfer coefficient, and outlet boundary condition of the porous medium were studied. The influences of the combustion equivalence ratio of the gas mixture and steady airflow velocity on the combustion efficiency were experimentally studied by Wang Enyu [15]. The results showed that the combustion and flow of gas can be improved by using a gradient structure. The characteristics of combustion, pollutant emission, heat diffusion, and resistance in gradually varied porous media (GVPM) were studied, and a GVPM gas-fired boiler was designed [16]. The results revealed that the GVPM burner has the merits of lower NOx and CO emission, higher heat diffusion, and lower resistance. A “volume mean transport equation” of premixed combustible gas flow in isotropic porous medium was proposed by Du [17,18]. Using this equation, he simulated the super-adiabatic combustion of the gas flow in a porous medium burner and found that the reciprocating flow of premixed combustible gas in porous medium has greater advantages than the single-direction flow. The effect of the characteristic parameters of the porous medium on the combustion temperature and pressure can be found in the literature [19–21]. The industrial applications and numerical simulation of premixed gas combustion in a porous medium were studied in the literature [22]. The results showed that a square burner can achieve a stable combustion of premixed gases, but it is more demanding in terms of working conditions, and there is a problem of uneven combustion. Circular burners are recommended for industrial applications. The excess enthalpy flames stabilized in a radial multichannel as a model of a cylindrical radial-flow porous medium burner were experimentally and theoretically studied [23]. The result showed that the multichannel flames had excess combustion velocities due to heat recirculation via the channel walls. The stabilization diagram of premixed methane-air flames in finite porous medium under a uniform ambient temperature was numerically studied [24]. The results showed that both stable and unstable solutions, for upper and lower flames, either exist on the surface or submerged in the porous matrix. The conversion of nitric oxide inside a porous medium was investigated in the literature [25]. The effects of equivalence ratio and flow velocity on the flame stabilization were investigated. At the same time, the NOx and TFN (total fixed nitrogen) conversion ratios and temperature profiles along the burner were obtained. The flame propagation and stationary mechanism in single/double-layer porous medium burners were studied by Zhao [26]. The results showed that the double-layer porous medium burner can realize stable standing in the vicinity of the material interface, which can effectively prevent tempering and stripping. It was also found that the burner with multiple sections or double layers would more easily achieve the above objectives than the burner with a single direction or single layer. The combustion characteristics of gaseous fuels with low calorific value in a two-layer porous burner were numerically simulated [27]. The results showed that OH and O radicals are most actively involved in combustion. OH-involved elementary reactions are significantly faster than O-involved elementary reactions. NO is the only NOx species which is significantly presented in the exhaust gases. The concentrations of NO2 and N2 O never exceeded 1 mg/m3 . The performance characteristics of premixed inert porous burners were numerically simulated [28]. The results indicated that the flow, temperature, and concentration fields of species appear to be approximately one-dimensional. Compared with the traditional burner, the chemical reaction zone is extended in the porous medium burner, while the gas temperature gradient near the flame zone is reduced. Premixed natural gas combustion in porous inert media was numerically studied [29]. The results showed that the radiation of the solid and the heat exchange


3 of 18

between gas and solid are the two main factors impacting the combustion performance of porous inert media burners. Based on the wide application of porous medium burners with a two-section structure (“i.e., ordinary burner”) at present and the advantages of the porous medium burner, the combination of porous media is optimized in this paper. Finally, a new two-section and double-deck porous medium burner with gas inlets at both ends (“i.e., new burner”) is proposed and designed, and the combustion effects of the two kinds of burners are numerically simulated. It is expected that it is simpler and more effective to realize regenerative heat from the combustion zone to the preheating zone, which can effectively solve the problem of low temperature in the vicinity of the burner inlet. 2. Mechanism of Gas Combustion in a Porous Medium The physical-chemical reaction process of gas combustion is very complicated. Free radicals, ions, electrons molecules, and other transient/intermediate products will be produced during combustion. The flame features of the combustion reaction of CH4 -air can be reflected by study of the oxidation reaction mechanism of CH4 [30]. Experimental study on the combustion reaction of N2 and CH4 showed that the oxidation process of CH4 includes a series of free radicals involved in the reaction. According to the element composition of the gas mixture, ions and molecules containing C, H, O, and N are the vast majority of the intermediate products and transient products. These products, especially free radicals, become the activation center in the reaction process, which greatly promotes the continuation, acceleration, and termination of the reaction. The conversion of methane to hydrogen in a porous medium reactor was investigated using fuel with equivalence ratios ranging from 1.5 to 5. The wave velocity, peak combustion temperature, flame structure, volumetric heat release, wave thickness, and hydrogen yield were obtained. The parameters affecting these characteristics included inlet velocity, equivalence ratio, and the thermal conductivity, and the specific heat of the porous medium [31–35]. Generally, when the temperature is high, the oxidation of CH4 can be simplified to CH4 →CH3 →CH2 O→CO→CO2 . A continuous combustion reaction increases the temperature, which in turn accelerates the combustion; as a result, the temperature rapidly rises. In this process, hot molecules, ions, and electrons are sharply generated. They carry a huge impulse, momentum, and kinetic energy, and collide with each other. So, the N2 molecules are bound to be impacted by these active particles, and then be decomposed and involved in the oxidation of CH4 , which also explains why the pollutants produced by gas combustion in porous media are NOx mostly, in addition to CO2 . The design of the new burner fully considers the material structure and the outstanding regenerative properties which can promote chain break and reduce the activation energy in gas combustion reactions. The combustion efficiency is increased, and the combustion of lean gas is promoted. 3. Methodology 3.1. Numerical Model At present, burners with two-section structures have been widely used, as shown in Figure 1. This burner is composed of two porous media with different porosities. The smaller-pored medium is regarded as the preheating zone, and the other is the combustion zone. As illustrated in Figure 2, the new burner is divided into two sections: the preheating zone and the combustion zone. Our previous research suggested an optimal ratio of 1.2:1 for the burner. The exterior of the new burner is a cylinder with a 0.025 m radius and 0.22 m length. The lengths of the porous media of the preheating and combustion zones are 0.12 m and 0.1 m, respectively. The new burner consists of double decks, both of which are composed of two porous media with different porosities. The material is silicon carbide. The media with different pores of the inner and outer deck are staggered, and the small-pore medium is used as the preheating zone of the combustion; the other is the main combustion zone. The cross-sectional radius of the inner deck is 0.0125 m. The gases simultaneously enter into the


4 of 18

Processes 2018, 6, x FOR PEER REVIEW preheating zone the small-pore Processes 2018, 6, x of FOR PEER REVIEW

4 of 19

medium from two ends of the new burner. In addition to convection 4 of 19 heating, the premixed gas can be heated by the solid heat conduction and heat radiation from the heat radiation from the surrounding combustion zone with the big-pore medium. The heat reflux surrounding combustion with the big-pore medium. The will medium. further the reflux heat heatfurther radiation from the thezone surrounding combustion zone withheat thereflux big-pore The heat will enhance heat storage effect, and the combustion stability is thusenhance improved. The storage effect, and the combustion stability is thus improved. The contact area between the small-pore will further enhance the heat storage effect, and the combustion stability is thus improved. The contact area between the small-pore preheating zone and the big-pore combustion zone of the new preheating zone and the big-pore combustion zone of the new burner is larger. Therefore, the fully contact between the small-pore preheating and the big-pore zonecompletes of the new burner is area larger. Therefore, the fully premixed gas zone will be burned in a flowcombustion process, which premixed gas will be burned in a flow process, which completes the chemical reaction and releases burner is larger. Therefore, the fully premixed gas will be burned in a flow process, which completes the chemical reaction and releases energy. The equivalence ratio of premixed gas in the burner inlet energy. The equivalence ratio of premixed gas in the burner inlet was set to 0.65, the corresponding the chemical reaction and releases energy. The equivalence ratio of premixed gas in the burner inlet was set to 0.65, the corresponding volume ratio of methane/air was 6%, the inlet flow velocity was volume of methane/air was 6%, the inlet flow velocity waswas 0.856%, m/s, and theflow temperature of wasm/s, setratio to 0.65, the corresponding volume ratio of300 methane/air the inlet velocity was 0.85 and the temperature of premixed gas was K. premixed K. 0.85 m/s,gas andwas the 300 temperature of premixed gas was 300 K.

the small pore medium the small pore medium

the big pore medium the big pore medium

Figure 1. The ordinary burner. Figure Figure1.1.The Theordinary ordinaryburner. burner.

Figure 2. 2. Sectional Sectional view burner. 1, premixed gasgas inlet; 2, exhaust gas Figure view of of the thephysical physicalmodel modelofofthe thenew new burner. 1, premixed inlet; 2, exhaust outlet; 3, premixed gas inlet; 4, exhaust gas outlet; 5, premixed gas inlet; 6, exhaust gas outlet. Figure 2. Sectional view of the physical model of the new burner. 1, premixed gas inlet; 2, exhaust gas outlet; 3, premixed gas inlet; 4, exhaust gas outlet; 5, premixed gas inlet; 6, exhaust gas outlet. gas outlet; 3, premixed gas inlet; 4, exhaust gas outlet; 5, premixed gas inlet; 6, exhaust gas outlet.

Meshing enables the discretization of the geometry into small units of simple shapes, referred to as Meshing enables the discretization of the geometry into small units of simple shapes, referred to mesh elements. Becausethe thediscretization new burner has double decks of a nested type,of the unstructured mesh canto Meshing enables the geometry into units shapes, referred as mesh elements. Because the new burnerofhas double decks ofsmall a nested type,simple the unstructured mesh adapt to the structural requirements. partition method wasofthe T meshtype, and the element form mesh was as adapt mesh elements. Because the new The burner double decks a nested can to the structural requirements. Thehas partition method was the T mesh and unstructured the element form the regular tetrahedral element. Due to the obvious symmetry of the model, the axisymmetric model canthe adapt to thetetrahedral structural requirements. The partition wasof thethe T mesh and element form was regular element. Due to the obviousmethod symmetry model, thethe axisymmetric was adopted in the mesh. To element. ensure the accuracy of the calculation, the mesh spacing was 0.1 mm; was the regular tetrahedral Due to the obvious symmetry of the model, the axisymmetric model was adopted in the mesh. To ensure the accuracy of the calculation, the mesh spacing was 0.1 finally, the mesh with 16,028 nodes was selected. According to the research theory [36], the reaction model was adopted the mesh. ensure theselected. accuracy of the calculation, the mesh spacing was 0.1 mm; finally, the meshinwith 16,028To nodes was According to the research theory [36], the mm; finally, the mesh selected. distribution According toofthe research theoryburners. [36], the reaction mechanism has with little 16,028 effect nodes on the was temperature porous medium reaction mechanism has little effect on the temperature distribution of porous medium burners. Therefore, a simple one-step reaction mechanism was adopted in the numerical model, namely, Therefore, a simple one-step reaction mechanism was adopted in the numerical model, namely, CH4+2(O2+3.76N2) = CO2+2H2O+7.52N2 CH4+2(O2+3.76N2) = CO2+2H2O+7.52N2


5 of 18

mechanism has little effect on the temperature distribution of porous medium burners. Therefore, a simple one-step reaction mechanism was adopted in the numerical model, namely, CH4 + 2(O2 + 3.76N2 ) = CO2 + 2H2 O + 7.52N2 3.2. Fundamentals Because the structure of the porous medium burner is simple, the pores of the porous material are small, and the vortex is also small, it is assumed that the turbulence effect increases the flame front thickness and heat transfer and that the flame propagation is one-dimensional. The chemical source can be calculated by the Arrhenius formula. To simplify the calculation, the following assumptions were made: 1 gravity effect is ignored, and the gradient of each physical quantity is zero in the cross

The section of the circular cylinder. 2

The porous medium is considered a homogeneous optical material and dispersion structure. 3

The outer wall of the burner is adiabatic and nonslip, whereas the inner wall is set to have grey body radiation. The heat process between different parts is an unsteady-state process. 4

Gas phase radiation is neglected. 5

The potential high-temperature solid catalysis effect is ignored. 6

The flame surface is at the coordinate origin of the X axis during steady combustion; the flame front is located near the interface of the two-deck porous medium. 7

The premixed methane-air gas is regarded as an ideal gas. Based on the above assumptions, the balance equations are as follows: (1) Continuity equation: ∂(ρε) ∂(ερui ) + =0 (1) ∂t ∂xi where ρ is the density of the premixed gas, ε is the porosity of the porous medium, and ui is the velocity vector. (2) Momentum conservation equation: ∂(ερu j ) ∂t

− ∂(∂xεP) j

+

+

∂u ερu j ui − ∂x∂ ρεν ∂x j = i i h i µε2 ∂u ∂ui ρεν ∂x − 23 ∂x k δij − KP u j

∂ ∂xi

∂ ∂xi

j

(2)

k

where v is the viscous resistance coefficient, µ is the inertia resistance coefficient, and K p is the permeability of the porous medium. (3) Component conservation equation: ∂ ρ g Yn + ∇ · ρ g ui Yn = −∇ · ρ g Yn Vi + ωn Wn ∂t

(3)

where Yn is the mass fraction of the nth component, Vi is the diffusion velocity, ui is the velocity with respect to a stationary coordinate system, Wn is the molecular weight of the nth component, and ωn is the molar formation rate of the nth component. (4) Gas energy equation: ∂(ερ g Cg Tg ) ∂t

+ ∇ · εu ρ g Cg Tg + ρ = hv Ts − Tg + ∇ · ελ g ∆Tg − ε∑ hn ρYn (ui − u) + ε(τu) + εQ

(4)

n

where Q is the heat efficiency of the chemical reaction, λ g is the thermal conductivity of the premixed gas, hν is the volumetric convective heat transfer coefficient, hn is the molar enthalpy of formation of


6 of 18

the nth component, Cg is the specific heat capacity of the mixed gas at constant pressure, and Tg is the gas phase temperature. Because the gas content is low, the one-step chemical reaction mechanism of methane/air is used. The gas viscosity is significantly related to the temperature but is almost independent of the pressure. When the temperature Tg is less than 2000 K, the gas viscosity can be calculated by the Sutherland formula. (5) Solid state energy equation: ∂[(1 − ε)ρs Cs Ts ] = ∇ · (λse ∆Ts ) + hv Tg − Ts ∂t

(5)

where λse = λce + λre is the effective heat transfer coefficient of the solid porous medium, λce is the effective thermal conductivity of the porous medium, and λre is the heat transfer coefficient; β s is the radiation attenuation coefficient of the porous medium, β s = m(1 − ε)/d p , and the coefficient m reduces with the increase in the average pore size of the porous medium, but increases with the increase in porosity. According to the porosity of the porous medium and the results of the experiment, m is 4, ρs is the density of the porous medium, Ts is the porous medium temperature, and Cs is the heat capacity at constant pressure. (6) State equation of ideal gas: WP ρg = (6) RTg where W is the average molecular weight of the mixed gas, and R is the gas constant. Without consideration of the rich combustion condition, the single-step chemical reaction mechanism of methane/air is used. (7) Determination of solid radiation parameters: The radiation absorption coefficient of the foam ceramic material is 0.4 in the literature [34]. Based on the literature data, the radiation scattering rate of foam ceramics is defined as a function of porosity: ω = 0.251 + 0.635ϕ (7) The radiation scattering rate can be calculated by the following formula [35]: ω=

σs σs = β α + σs

(8)

After conversion, it can be concluded that the calculation formulae of the scattering coefficient and absorption coefficient of the foam ceramic are as follows: σs = ωm(1 − ε)/d p

(9)

α = ωm(1 − ε)(1 − ω )/d p

(10)

The mechanism of methane oxidation (GRI1.2) is used in the chemical reaction, which consists of 32 components and 177 basic elements; the mechanism has been verified by many scholars. The thermal physical properties of the porous media are assumed to be constant [37]. 3.3. Boundary Condition The energy and component conservation equations are boundary value problems, and the relevant boundary conditions are required for the solution. The inlet of the gas mixture was set as the velocity inlet, and the inlet temperature and ambient temperature of the premixed mixture were


7 of 18

300 K. The external wall heat loss includes two parts: heat transfer to the environment and thermal radiation. The specific settings are as follows: Inlet : Tg = T0 = 300 K, YCH4 = YCH4 ,in , YO2 = YO2 ,in , Ug = Ug,in Outlet :

∂Yn ∂x

=

∂Tg ∂x

(11)

= 0, Pout = 0.1 MPa

At the burner outlet, there are large radiation heat losses; the outlet boundary condition for a given solid temperature is as follows: λs

∂Ts = −ε T σ( Ts − T0 ) ∂x

(12)

where ε T is the surface radiation coefficient of the porous medium. 3.4. Numerical Scheme The equations were discretized and solved using Fluent software. When the energy equation of the porous medium is solved, only the single-temperature model can be used in the software. However, the high-temperature heat in the flame zone of a porous medium burner will be transferred to a solid medium. At the same time, the solid medium can accept the energy of convection, heat conduction, and heat radiation. Therefore, two energy equations need to be used. To ensure good convergence of the calculation results, sub-relaxation iteration was adopted. The residual error of the energy equation was set to 1.0 × 10−6 , and the residual error of all other variables was set to 1.0 × 10−5 . When the initialization was finished, the initial velocity of the whole flow field was set to the flow velocity of the premixed gas in the inlet, and steady-state iteration was carried out. When chemical reactions were calculated, to ensure the stability of the calculation results and accelerate the convergence, a high-temperature heat source for the ignition needed to be set up, so the temperature in the calculation area was initially set to 1600 K. The same setting also was found in the literature [28]. Eight turbulence models were set up in Fluent software according to the different situations of turbulent fluid flow. The standard k–ε model was used to simulate the combustion in this study. To obtain the characteristics of premixed gas combustion in a porous medium with different equivalence ratios, the equivalence ratios simulated were 0.85, 0.75, 0.45, and 0.35, with inlet flow velocities of 1.15 m/s, 1 m/s, 0.7 m/s and 0.55 m/s, respectively. Parameters of the porous medium burner are listed in Table 1. Table 1. Physical parameters of the porous media.

Length (m) Pore diameter (PPcm) Porosity Thermal conductivity (W/m·k) Radiation attenuation coefficient (m−1 ) Scattering albedo

Medium with Small Pores (Upstream)

Medium with Big Pores (Downstream)

0.12 25.6 0.835 0.2 1707 0.8

0.1 3.9 0.87 0.1 257 0.8

4. Results and Discussion 4.1. Analysis of the Effect of the Premixed Gas Combustion Figure 3 shows the stable temperature of the premixed gas combustion in the ordinary burner and the new burner when the equivalence ratio is 0.65 and inlet flow velocity is 0.85 m/s. In Figure 3a, the temperature change is not obvious in the small-pore-medium zone, indicating that the combustion has not been fully developed. A lot of the premixed gas is burned at the interface of the porous medium, and the stationary flame front is formed in the big-pore-medium zone. When the equivalence ratio is

4.1.Analysis of the Effect of the Premixed Gas Combustion Figure 3 shows the stable temperature of the premixed gas combustion in the ordinary burner and the new burner when the equivalence ratio is 0.65 and inlet flow velocity is 0.85 m/s. In Figure Processes 2018, 6, 185 8 of 18 3a, the temperature change is not obvious in the small-pore-medium zone, indicating that the combustion has not been fully developed. A lot of the premixed gas is burned at the interface of the porous medium, the stationary front formed in the big-pore-medium When the 0.65 and the flowand velocity in the inletflame is 0.85 m/s,isthe temperature peak value in the zone. ordinary burner equivalence ratio is K, 0.65 and the flow velocity in the inletisisabout 0.85 m/s, temperature peak value in reaches about 1900 and the temperature in the outlet 1850 the K; this result is consistent with the ordinary burner reaches about 1900 K, and the temperature in the outlet is about 1850 K; this previous work in the literature [19–21]. There is a difference between the results in this simulation and result is consistent with previous work in the literature [19–21]. Therereaction is a difference the literature; the reason for this difference is that a three-dimensional model isbetween used in the this results in this is simulation and the the reason for this is that a three-dimensional study, which different from theliterature; two-dimensional model [21].difference In addition, the simulation results are reaction is used in this study, which is different from the two-dimensional model [21]. In sensitivemodel to the model. addition, thethe simulation results are to the model. Under same conditions, thesensitive temperature peak of the new burner is up to approximately 2600 K, Under the same conditions, the temperature peak of the newtemperature burner is uppeak to approximately and the temperature in the outlet zone is higher than 2400 K. The is higher than 2600 K, and the temperature in the outlet zone is higher than 2400 K. The temperature is higher the corresponding maximum temperature of the ordinary burner, which indicates thatpeak premixed gas than the corresponding maximum temperature of the ordinary whichand indicates that combustion in the new burner is carried out more thoroughly, and theburner, heat storage heat transfer premixed gas combustion in the new is carried heat storage capacity of the porous medium in theburner new burner canout be more betterthoroughly, realized. Inand the the ordinary burner, and heat transfer capacity of the porous medium in the new burner can be better realized. In the the temperature in the outlet zone is slightly lower than that in the inner zone. The porous medium ordinary the temperature intransfers the outlet zoneofisheat slightly lower than that in the inner zone.inThe with highburner, temperature in the outlet a part upstream; therefore, the temperature the porous medium with high temperature in the outlet transfers a part of heat upstream; therefore, the outlet will decrease. Among the released heat from premixed gas combustion in the porous medium, temperature in part the outlet will decrease. Among the released from premixed combustion in a considerable is from high-temperature radiation of theheat porous medium; thegas radiation heat can the porous medium, a considerable part is from high-temperature radiation of the porous medium; directly heat materials, and has high heat transfer efficiency. the radiation heat can directly heat materials, and has high heat transfer efficiency. Temperature

Temperature

1900.00 1792.86 1685.71 1578.57 1471.43 1364.29 1257.14 1150.00 1042.86 935.71 828.57 721.43 614.29 507.14 400.00

2750.43 2589.03 2427.63 2266.23 2104.83 1943.43 1782.03 1620.63 1459.23 1297.83 1136.43 975.03 813.63 652.23 490.83

(a)

(b)

Figure 3.Temperature 3. Temperaturecontours contoursininthe theordinary ordinaryburner burnerand andthe thenew newburner burnerwhen when the the equivalence equivalence Figure ratioisis0.65 0.65and andthe theinlet inletflow flow velocity velocity is is 0.85 0.85 m/s. m/s.(a) (a)Temperature Temperaturecontours contoursininthe theordinary ordinaryburner; burner; ratio (b)Temperature Temperature contours contours in in the the new new burner. burner. (b)

Figure 44 illustrates illustrates the the gas gas concentration concentration contours contours in in the the two two kinds kinds of of burners burners after after steady steady Figure combustion. The gas mass fractions of the axis outlets in the two kinds of burners stabilize at 0.002. combustion. The gas mass fractions of the axis outlets in the two kinds of burners stabilize at 0.002. Theadvantages advantagesofofpremixed premixedcombustion combustion a porous medium fully reflected; in consequence, The in in a porous medium are are fully reflected; in consequence, the Processes 2018,of 6, gaseous x FOR PEERfuels REVIEW 9 of 19 the efficient use can be achieved using the new burner. efficient use of gaseous fuels can be achieved using the new burner. 02

00 0.

0. 0

4

0.030

At a certain flow velocity, the new burner can obtain a higher combustion temperature and Level CH efficiency, because the contact area between the preheating zone and the higher combustion 8 0.030 0.006 0.018 0.014 combustion zone larger in the new burner. 0.010 transfer effect of the reverse heat reflux, 0.022 Thus, the heat 7 is 0.026 0 .0 20 6 0.022 heat conduction, and radiation heat in the combustion zone is more significant, and the reaction 5 0.018 proceeds more 4fully. 0.030 0.014 06 0 .0 0. 0

10

2

2

0.006

1

0.002

0.018

0.0 26

0 .0

02

06

0.010

0. 0

3

14 0.0

0.002

2

0.006

1

0.002

0.0 3

0.010

0.0 0

2

10

0.014

3

0. 0

0.018

4

14

1

5

0 .0

27

0.0 3

0.022

31

0.027

6

0. 0

7

0 .0

02

0.031

0 0.

8

1

Level CH4

0.002

(a)

(b) Figure CH4 concentration contours the ordinary burner andnew the burner new burner the Figure 4. CH4 4.concentration contours in theinordinary burner and the whenwhen the equivalence equivalence ratio is 0.65 and the inlet flow velocity is 0.85m/s. (a) CH4 concentration contours in the ratio is 0.65 and the inlet flow velocity is 0.85m/s. (a) CH4 concentration contours in the ordinary ordinary burner; (b) CH4 concentration contours in the new burner. burner; (b) CH4 concentration contours in the new burner.

4.1.1. Influence of the Equivalence Ratio of Premixed Gas on Combustion Degree Gas concentration is often described using the concentration equivalence ratio. The greater the ratio, the higher the proportion of methane in the mixture. Figures 5 and 6 show the temperature contours of gas mixtures with equivalence ratios of 0.75 and 0.85, respectively, during combustion in the ordinary and new burners. Figures 7 and 8 illustrate the gas concentration contours

0.

0 .0

0.022 0.018 0.018 0.014 0.014 0.010 0.010 0.006 0.006 0.002 0.002

0. 0.0 01 1 0 0

0. 0.0 03 3 1 1

0.0 0.002 02


2 2 00 00 0. 0.

6 5 5 4 4 3 3 2 2 1 1

9 of 18

(b) (b) Figure 4. CH4 concentration contours in the ordinary burner and the new burner when the At a certain flow velocity, thecontours new burner canordinary obtain a higher andthe higher Figure 4. CH 4 concentration in the burner combustion and the newtemperature burner when equivalence ratio is 0.65 and the inlet flow velocity is 0.85m/s. (a) CH4 concentration contours in the equivalence ratio isbecause 0.65 andthe the contact inlet flow velocity is 0.85m/s. (a) CH4 concentration in the zone combustion efficiency, area between the preheating zone and thecontours combustion ordinary burner; (b) CH4 concentration contours in the new burner. ordinary burner; (b) CH4 concentration contours in the new is larger in the new burner. Thus, the heat transfer effect ofburner. the reverse heat reflux, heat conduction,

and radiation heat in the combustion zone is more significant, and the reaction proceeds more fully. 4.1.1. 4.1.1. Influence Influence of of the the Equivalence Equivalence Ratio Ratio of of Premixed Premixed Gas Gas on on Combustion Combustion Degree Degree 4.1.1.Gas Influence of the Equivalence Ratio of Premixed Gas on Combustion Degree Gas concentration concentration is is often often described described using using the the concentration concentration equivalence equivalence ratio. ratio. The The greater greater the the ratio, the higher the proportion of methane in the mixture. Figures 5 and 6 show the temperature is often described using the mixture. concentration equivalence ratio.the Thetemperature greater the ratio,Gas the concentration higher the proportion of methane in the Figures 5 and 6 show contours gas with ratios of and respectively, during combustion in ratio, theof higher the proportion of methane in the mixture. Figures 5 and 6 show the temperature contours of gas mixtures mixtures with equivalence equivalence ratios of 0.75 0.75 and 0.85, 0.85, respectively, during combustion in the ordinary and new burners. Figures 7 and 8 illustrate the gas concentration contours contours of gas and mixtures equivalence ratios 0.758and 0.85, respectively, combustion in the the ordinary newwith burners. Figures 7 of and illustrate the gas during concentration contours correspondingly. ordinary and new burners. Figures 7 and 8 illustrate the gas concentration contours correspondingly. correspondingly. Temperature Temperature 2100.00

Temperature Temperature 2766.82 2602.53 2766.82 2438.24 2602.53 2273.95 2438.24 2109.67 2273.95 1945.38 2109.67 1781.09 1945.38 1616.80 1781.09 1452.51 1616.80 1288.23 1452.51 1123.94 1288.23 959.649 1123.94 795.361 959.649 631.072 795.361 466.784 631.072 466.784

1978.57 2100.00 1857.14 1978.57 1735.71 1857.14 1614.29 1735.71 1492.86 1614.29 1371.43 1492.86 1250.00 1371.43 1128.57 1250.00 1007.14 1128.57 885.71 1007.14 764.29 885.71 642.86 764.29 521.43 642.86 400.00 521.43 400.00

(a) (a)

(b) (b)

Figure 5. Temperature contours in the ordinary burner and the new burner when the equivalence Figure 5. Temperature contours in the ordinary burner and the new burner when the equivalence ratio is 0.75 and the inlet flow velocity is 0.85 m/s. m/s. (a) (a)Temperature Temperature contours contours in in the the ordinary ordinary burner; burner; ratio is 0.75 and the inlet flow velocity is 0.85 0.85 m/s. (a) Temperature contours in the ordinary burner; (b) Temperature contours in the new burner. (b) Temperature Temperature contours contours in in the the new new burner. burner. Temperature Temperature 2136.97 2014.37 2136.97 1891.78 2014.37 1769.19 1891.78 1646.59 1769.19 1524.00 1646.59 1401.41 1524.00 1278.81 1401.41 1156.22 1278.81 1033.63 1156.22 911.03 1033.63 788.44 911.03 665.85 788.44 543.25 665.85 420.66 543.25 420.66

Temperature Temperature 2800.00 2628.57 2800.00 2457.14 2628.57 2285.71 2457.14 2114.29 2285.71 1942.86 2114.29 1771.43 1942.86 1600.00 1771.43 1428.57 1600.00 1257.14 1428.57 1085.71 1257.14 914.29 1085.71 742.86 914.29 571.43 742.86 400.00 571.43 400.00

(a) (a)

(b) (b)

Figure 6. Temperature contours in the ordinary burner and the new burner when the equivalence Figure 6. Temperature contours in the ordinary burner and the new burner when the equivalence (a) Temperature Temperature contours contours in in the the ordinary ordinary burner; burner; ratio is 0.85 and the inlet flow velocity is 0.85 m/s. m/s. (a) ratio is 0.85 and the inlet flow velocity is 0.85 m/s. (a) Temperature contours in the ordinary10burner; 2018, 6, x contours FOR PEER REVIEW of 19 Temperature in (b)Processes Temperature contours in the the new new burner. burner. (b) Temperature contours in the new burner.

0.031

7

0.027

6

0.023

5

0.019

4

0.015

3 2

0.012 0.008

1

0.004

0.031 0.023 0 .0 27 0.038

0.004 0.008 0.035

3 02 0. 7 02 0.

8

0.008

0.035

0.035

15

9

0 .0

Level CH4 10 0.038

0 .0

0.019 8 0 0 .0

0.027 0.015 08 00.0 .004

31

0.0

0.015 0.012

19

0.004

(a)

3

7 0. 01 0.40 2

10

1

0 .0

0. 03

0.003

0.0 0

1

0.007 21 0 .0

3

0.010 0.007

0.0 0

0.014

3 2

0.003

0.007

0.003

0.017

4

31

5

0. 0

0.021

4

0.024

6

10

.02 4 0

7

0 .0

03 0.

0.027

1

0.031

8

2 0.0

9

0 0 . . 034 02 7

Level CH4 10 0.034

(b) Figure CH4 concentration contours the ordinary burner andnew the burner new burner the Figure 7. CH4 7.concentration contours in theinordinary burner and the whenwhen the equivalence equivalence ratio is 0.75 and the inlet flow velocity is 0.85 m/s. (a) CH4 concentration contours in the ratio is 0.75 and the inlet flow velocity is 0.85 m/s. (a) CH4 concentration contours in the ordinary ordinary burner; (b) CH4 concentration contours in the new burner. burner; (b) CH4 concentration contours in the new burner. Level CH4 10

0.034

9

0.031

8

0.027

7

0.024

6

0.021

5

0.017

0.003

31 0. 0

0.007

4

0.003

2 0.0

1

1

0.010 0.007

2 0.0

3 2

(b) Figure 7. CH4 concentration contours in the ordinary burner and the new burner when the equivalence ratio is 0.75 and the inlet flow velocity is 0.85 m/s. (a) CH4 concentration contours in the Processes 2018, 6, 185

10 of 18

ordinary burner; (b) CH4 concentration contours in the new burner. Level CH4 10

0.034

9

0.031

8

0.027

7

0.024

6

0.021

5 4

0.017 0.014

3

0.010

2

0.007

1

0.003

(a) Level CH4 10

0.038

9

0.035

8

0.031

7

0.027

6

0.023

5

0.019

4

0.015

3

0.012

2

0.008

1

0.004

(b) Figureconcentration 8. CH4 concentration contours the ordinary burner the new burner when Figure 8. CH contours in the in ordinary burner and and the new burner when thethe equivalence 4 equivalence ratio is 0.85 and the inlet flow velocity is 0.85 m/s. (a) CH4 concentration contours in the ratio is 0.85 and the inlet flow velocity is 0.85 m/s. (a) CH4 concentration contours in the ordinary ordinary burner; (b) CH4 concentration contours in the new burner. burner; (b) CH4 concentration contours in the new burner.

When the equivalence ratio is high and the excess air coefficient is small, it is hard to complete

Whenthe the equivalence ratio is two highequivalence and the excess air location coefficient isflame small, it is hard to gas complete the combustion. Under these ratios, the of the surface and the mixture combustion consumption areratios, similar.the When the equivalence ratio is 0.85,and the the inner combustion. Under these two equivalence location of the flame surface gas mixture Processes 2018, 6, x FOR PEER REVIEW 11 of 19 Processes 2018, 6, x FOR PEER REVIEW 19 combustion consumption are similar. When the equivalence ratio is 0.85, the inner temperature11ofofthe new burner can reach 2700 K. However, the2700 outlet is not lesstemperature than 2000 K,iswhich is higher temperature of the new burner can reach K.temperature However, the outlet not less than temperature of the new burner canordinary reach 2700 K. However, the outlet temperature isshows not less than than the temperature peak in the burner under the same conditions. This the 2000 K, which is higher than the temperature peak in the ordinary burner under thethat same 2000 K, which is higher than the temperature peak in the ordinary burner under the same combustion This degree and the temperature in the new burner and the new burner’s advantage conditions. shows that the combustion degree andare thehigher, temperature in the new burner are conditions. This shows that the combustion degree and the temperature in the new burner are of saving energy is very obvious. higher, and the new burner’s advantage of saving energy is very obvious. higher, and the new10burner’s advantage of saving energy is very obvious. Figures and show the the temperature contours of gas gas mixture combustion in in the the two two kinds kinds of of Figures 99 and 10 show temperature contours of mixture combustion Figures 9 and 10 show the temperature contours of gas mixture combustion in the two kinds of burners with 0.45 andand 0.35.0.35. Figures 11 and are 12 the are gas the concentration contours burners with equivalence equivalenceratios ratiosof of 0.45 Figures 1112and gas concentration burners with equivalence ratios of 0.45 and 0.35. Figures 11 and 12 are the gas concentration of gas mixture combustion in the two kinds of burners. contours of gas mixture combustion in the two kinds of burners. contours of gas mixture combustion in the two kinds of burners. Temperature

Temperature

1746.39 Temperature 1649.84 1746.39 1553.29 1649.84 1456.73 1553.29 1360.18 1456.73 1263.63 1360.18 1167.08 1263.63 1070.52 1167.08 973.97 1070.52 877.42 973.97 780.87 877.42 684.31 780.87 587.76 684.31 491.21 587.76 394.66 491.21 394.66

2400.00 Temperature 2257.14 2400.00 2114.29 2257.14 1971.43 2114.29 1828.57 1971.43 1685.71 1828.57 1542.86 1685.71 1400.00 1542.86 1257.14 1400.00 1114.29 1257.14 971.43 1114.29 828.57 971.43 685.71 828.57 542.86 685.71 400.00 542.86 400.00

(a) (a)

(b) (b)

Figure 9. Temperature contours in the ordinary burner and the new burner when the equivalence Figure 9. 9. Temperature Temperature contours contours in in the the ordinary ordinary burner burner and and the the new new burner burner when when the the equivalence equivalence Figure ratio is 0.45 and the inlet flow velocity is 0.85 m/s. (a) Temperature contours in the ordinary burner; ratio is is 0.45 0.45 and and the the inlet inlet flow flow velocity velocity is is 0.85 0.85 m/s. m/s.(a) (a)Temperature Temperaturecontours contoursin inthe theordinary ordinaryburner; burner; ratio (b) Temperature contours in the new burner. (b) Temperature Temperature contours in the new burner. burner. (b) Temperature

Temperature

1678.88 Temperature 1580.25 1678.88 1481.63 1580.25 1383.00 1481.63 1284.37 1383.00 1185.75 1284.37 1087.12 1185.75 988.49 1087.12 889.87 988.49 791.24 889.87 692.61 791.24 593.98 692.61 593.98 593.98 495.36 593.98 396.73 495.36 396.73

2200.00 Temperature 2071.43 2200.00 1942.86 2071.43 1814.29 1942.86 1685.71 1814.29 1557.14 1685.71 1428.57 1557.14 1300.00 1428.57 1171.43 1300.00 1042.86 1171.43 914.29 1042.86 785.71 914.29 657.14 785.71 528.57 657.14 400.00 528.57 400.00

(a) (a)

(b) (b)

Level Level 10 10 9 9 8 8 7

CH4 CH4 0.019 0.019 0.017 0.017 0.015 0.015 0.013

0 .0 0 . 013 13

0.0 0.0 02 02

Figure 10. 10. Temperature Temperature contours contours in in the the ordinary ordinary burner burner and and the the new new burner burner when when the the equivalence equivalence Figure Figure 10. Temperature contours in the ordinary burner and the new burner when the equivalence ratio is is 0.35 0.35 and and the the inlet inlet flow flow velocity velocity is is 0.85 m/s. m/s.(a) (a)Temperature Temperaturecontours contoursin inthe theordinary ordinaryburner; burner; ratio ratio is 0.35 and the inlet flow velocity is 0.85 m/s. (a) Temperature contours in the ordinary burner; (b) Temperature contours in the new burner. (b) Temperature burner. (b) Temperature contours in the new burner.

0.004 15 0.00.004 0.015

0.011 0.017 0.011 0.017

0.00 0 0.0090.0 6 .004 0 0 0.009 6 0.0.0040.008

692.61 593.98 593.98 495.36 396.73

914.29 785.71 657.14 528.57 400.00

(a)

(b)

8

0.015

7

0.013

6

0.011

5

0.009

4

0.008

3 2

0.006 0.004

1

0.002

0 .0

13

0.004 0.015

0.011 0.017

0.00 0 0.009 6 .004 0.0 0.008 11

0.010

0.017

0.017

9

0.0 0

Level CH4 10 0.019

2

Figure 10. Temperature contours in the ordinary burner and the new burner when the equivalence ratio6,is185 0.35 and the inlet flow velocity is 0.85 m/s. (a) Temperature contours in the ordinary burner; 11 of 18 Processes 2018, (b) Temperature contours in the new burner.

1 0.0

5

0.017

0.002

0.015 11 0.0 9 0.006 0 0.0 0.004

(a)

2 1

0.004 0.002

0.0

0.006

11

4

3

0.0

00

0.008

0 .0

02

0.

0.009

4

00. .00 0.00094 17

0.011

5

0.019 0.017

0.013

6

1

0.0 0 00.0.0062 0.008 13

0.015

7

1 0.008 0 .0

0.002 0.004

8

0.017

0.00069 0.0

0.019 0.017

9 01 15 0. 0.0

10 9

19

Level CH4

(b) Figure 11. 4CH 4 concentration contours the ordinary newwhen burner when the Figure 11. CH concentration contours in theinordinary burner burner and the and new the burner the equivalence equivalence ratio is 0.45 and the inlet flow velocity is 0.85 m/s. (a) CH 4 concentration contours in the ratio is 0.45 and the inlet flow velocity is 0.85 m/s. (a) CH4 concentration contours in the ordinary Processes 2018, 6, x FOR PEER REVIEW 12 of 19 ordinary burner; (b) CH4 concentration the new burner. burner; (b) CH contours incontours the newinburner. 4 concentration Level CH4

0.007

4

0.006

3

0.004

2

0.003

1

0.001

0. 01

1

3

0.008

5

00 0.

0.010

6

0.0 0.004 01 0.006 0.008 0.011

0.007 0.010

4

7

01 0.

0.011

3

8

00 0.

10 0.014 9 0.013

0.014.013 0 0.006

0.007

0.003

0.0 0.01113

0.003

(a) Level CH4

0.006

3

0.004

2 1

0.003 0.001

01

0.007

4

0.0

0.008

5

07

08 3 0 . 0 . 00 0

13

6

0 .0

14

0. 0

0.010

0 . 0 0 . 011 04

7

03 001 0.0 0.

0.

00

0.0 1

4

0.011

7

6

8

0 .0

0. 00

0.014 0.013

0.001

10 9

(b) Figure 4 concentration contours in ordinary the ordinary new when burner the Figure 12.12. CHCH contours in the burnerburner and theand newthe burner thewhen equivalence 4 concentration equivalence ratio 0.35 and inlet flow velocity 0.85CH m/s. (a) CH4 concentration contours in the ratio is 0.35 and theis inlet flowthe velocity is 0.85 m/s.is (a) contours in the ordinary 4 concentration ordinary (b) CH4 concentration in burner. the new burner. burner; (b) burner; CH4 concentration contours contours in the new

With ratio, the the combustion combustionflame flameisisoriented oriented towards Withthe thereduction reductionofofthe theequivalence equivalence ratio, towards thethe rearrear and becomes is reflected reflectedin inboth bothkinds kindsofofburners burners and is consistent and becomescloser closertotothe theoutlet outlet position; position; this is and is consistent with thelaws lawsfrom fromthe theliterature literature [19–21,30]. [19–21,30]. The results is also proved. with the The reliability reliabilityofofthe thesimulation simulation results is also proved. makingthis thiscomparison, comparison, itit can can be be concluded concluded that gas combustion is is ByBy making that when whenlow-concentration low-concentration gas combustion carried out in the ordinary burner, the combustion flame is oriented towards the rear and dispersed. carried out in the ordinary burner, the combustion flame is oriented towards the rear and dispersed. At a low equivalence ratio, the outlet temperature of the new burner is not lower than 2000 K, and the temperature peak of the new burner is up to 2400 K (equivalence ratio 0.45) or 2200 K (equivalence ratio 0.35); the new burner not only has a higher temperature, but also has better stability of the flame surface.


12 of 18

At a low equivalence ratio, the outlet temperature of the new burner is not lower than 2000 K, and the temperature peak of the new burner is up to 2400 K (equivalence ratio 0.45) or 2200 K (equivalence ratio 0.35); the new burner not only has a higher temperature, but also has better stability of the flame surface. 4.1.2. Influence of the Inlet Flow Velocity of Premixed Gas on Combustion Intensity To analyze the inner temperature distribution of the burner and the gas temperature of the outlet at different mass flow velocities, the same equivalence ratio was set up to investigate the influence of the inlet flow velocity of premixed gas on the combustion state. Although the porous medium has the characteristics of high temperature resistance and corrosion resistance, the excessive temperature of the material may cause damage to the bearing capacity, affecting the working ability of the burner and threatening production. On the contrary, if the premixed gas temperature is too low, it is difficult to achieve effective heating for the material. Numerical simulation can predict the adjustment range of the inlet flow velocity of the new burner. According to the related literature [22], the combustion intensity should not be higher than 1500 kw/m2 (namely, when the equivalence ratio is 0.65, the corresponding inlet flow velocity is about 0.85 m/s), so the four flow velocities near to the flow velocity in the literature were calculated and compared. Figures 13–16 are the inner temperature distributions of the two kinds of burners when the equivalence ratio is 0.65 and the inlet flow velocity is 0.65 m/s, 0.7 m/s, 0.95 m/s, or 1.15 m/s, respectively. Figures 17–20 show the gas concentration contours of the two Processes 2018, 6, x FOR PEER REVIEW 13 of 19 Processes 2018, 6, x FOR PEER REVIEW 13 of 19 kinds of2018, burners regarding these four flow velocities, respectively. Processes 6, x FOR PEER REVIEW 13 of 19 Temperature Temperature 1914.61 1806.86 Temperature 1914.61 1699.11 1806.86 1914.61 1591.36 1699.11 1806.86 1483.61 1591.36 1699.11 1375.86 1483.61 1591.36 1268.11 1375.86 1483.61 1160.36 1268.11 1375.86 1052.61 1160.36 1268.11 944.86 1052.61 1160.36 837.10 944.86 1052.61 729.35 837.10 944.86 621.60 729.35 837.10 513.85 621.60 729.35 406.10 513.85 621.60 406.10 513.85 406.10

(a) (a) (a)

Temperature Temperature 2274.95 1957.29 Temperature 2274.95 1839.63 1957.29 2274.95 1721.98 1839.63 1957.29 1604.32 1721.98 1839.63 1486.66 1604.32 1721.98 1369.01 1486.66 1604.32 1251.35 1369.01 1486.66 1133.70 1251.35 1369.01 1016.04 1133.70 1251.35 898.38 1016.04 1133.70 780.73 898.38 1016.04 663.07 780.73 898.38 545.42 663.07 780.73 427.76 545.42 663.07 427.76 545.42 427.76

(b) (b) (b)

Figure 13. Temperature contours in the ordinary burner and the new burner when the equivalence Figure 13. contours in ordinary burner and burner the Figure 13. Temperature Temperature contours in the the and the the new new burnerinwhen when the equivalence equivalence equivalence ratio is 0.65 and the inlet contours flow velocity is ordinary 0.65 m/s. burner (a) Temperature contours the ordinary burner; ratio is 0.65 and the inlet flow velocity is 0.65 m/s. (a) Temperature contours in the ordinary burner; ratio velocity 0.65 m/s. (a) Temperature contours in the ordinary burner; ratio is 0.65 and the inlet flow is m/s. (a) Temperature contours in the ordinary burner; (b) Temperature contours within the new burner. (b) Temperature contours contours within within the the new new burner. burner. (b) Temperature the new burner. Temperature Temperature 1846.47 1743.24 Temperature 1846.47 1640.02 1743.24 1846.47 1536.80 1640.02 1743.24 1433.57 1536.80 1640.02 1330.35 1433.57 1536.80 1227.12 1330.35 1433.57 1123.90 1227.12 1330.35 1020.68 1123.90 1227.12 917.45 1020.68 1123.90 814.23 917.45 1020.68 711.01 814.23 917.45 607.78 711.01 814.23 504.56 607.78 711.01 401.34 504.56 607.78 401.34 504.56 401.34

(a) (a) (a)


(b) (b) (b)

Figure Figure 14. 14. Temperature Temperature contours contours in in the the ordinary ordinary burner burner and and the the new new burner burner when when the the equivalence equivalence equivalence Figure 14. Temperature contours in the ordinary burner and the new burner when the equivalence ratio is 0.65 and the inlet flow velocity is 0.7 m/s. (a) Temperature contours in the ordinary burner; (b) ratio is is 0.65 0.65and andthe theinlet inletflow flowvelocity velocityisis0.7 0.7 m/s. Temperature contours in the ordinary burner; ratio m/s. (a) (a) Temperature contours in the ordinary burner; (b) ratio is 0.65 and the inlet flow velocity is 0.7 m/s. (a) Temperature contours in the ordinary burner; (b) Temperature contours in the new burner. (b) Temperature contours in the burner. Temperature contours in the newnew burner. Temperature contours in the new burner. Temperature Temperature 1384.84 Temperature 1312.39 1384.84 1239.93 1312.39 1384.84 1167.48 1239.93 1312.39 1095.02 1167.48 1239.93 1022.57 1095.02 1167.48 950.11 1022.57 1095.02 877.66 950.11 1022.57 732.75 877.66 950.11 660.30 732.75 877.66 587.84 660.30 732.75 515.39 587.84 660.30 442.93 515.39 587.84 370.48 442.93 515.39 370.48 442.93 370.48

(a) (a) (a)


(b) (b) (b)

Figure 15. 15. Temperature Temperature contours contoursin in the the ordinary ordinary burner burnerand andthe the new new burner burner when when the the equivalence equivalence Figure Figure 15. Temperature contours in the ordinary burner and the new burner when the equivalence Figure 15. Temperature contours in the ordinary burner and the new burner when the equivalence ratio is 0.65 and the inlet flow velocity is 0.95 m/s. (a) Temperature contours in the ordinary burner; ratio is 0.65 and the inlet flow velocity is 0.95 m/s. (a) Temperature contours in the ordinary burner; ratio is 0.65 and the inlet flow velocity is 0.95 m/s. (a) Temperature contours in the ordinary burner; ratio is 0.65 and the inlet flow velocity is 0.95 m/s. (a) Temperature contours in the ordinary burner; (b)Temperature Temperature contours contours in in the the new new burner. (b) (b) Temperature contours in the new burner. (b) Temperature contours in the new burner. Temperature Temperature 1396.72 Temperature 1323.47 1396.72 1250.23 1323.47 1396.72 1176.98 1250.23 1323.47 1103.74 1176.98 1250.23 1030.49 1103.74 1176.98 957.25 1030.49 1103.74 884.00 957.25 1030.49 810.75 884.00 957.25 737.51 810.75 884.00 664.26 737.51 810.75 591.02 664.26 737.51

Temperature Temperature 2606.76 2455.69 Temperature 2606.76 2304.62 2455.69 2606.76 2153.54 2304.62 2455.69 2002.47 2153.54 2304.62 1851.39 2002.47 2153.54 1700.32 1851.39 2002.47 1549.25 1700.32 1851.39 1398.17 1549.25 1700.32 1247.10 1398.17 1549.25 1096.02 1247.10 1398.17 944.95 1096.02 1247.10

587.84 515.39 442.93 370.48

1095.02 1022.57 950.11 877.66 732.75 660.30 587.84 515.39 442.93 370.48

951.91 1866.88 799.42 1714.39 1561.89 646.92 1409.40 494.43 1256.90 1104.41 951.91 799.42 646.92 494.43

(a)

(b)

(a) (b) when the equivalence Figure 15. Temperature contours in the ordinary burner and the new burner ratio is 0.65 and the inlet flow velocity is 0.95 m/s. (a) Temperature contours in the theequivalence ordinary burner;13 of 18 Processes 2018,Figure 6, 185 15. Temperature contours in the ordinary burner and the new burner when (b) Temperature in the burner. ratio is 0.65contours and the inlet flownew velocity is 0.95 m/s. (a) Temperature contours in the ordinary burner; (b) Temperature contours in the new burner. Temperature

Temperature 2606.76 Temperature

Temperature 2455.69

1396.72 1323.47 1250.23 1176.98 1103.74 1030.49 957.25 884.00 810.75 737.51 664.26 591.02 517.77 444.53 371.28

2606.76 2304.62 2455.69 2153.54 2304.62 2002.47 2153.54 1851.39 2002.47 1700.32 1851.39 1549.25 1700.32 1549.25 1398.17 1398.17 1247.10 1247.10 1096.02 1096.02 944.95 944.95 793.88 793.88 642.80 642.80 491.73 491.73

(a) (a)

(b) (b)

Figure 16. Temperature contours in the ordinary burner and the new burner when the equivalence

Figure 16. 16. Temperature Temperature contours contours in in the the ordinary ordinary burner burner and and the the new new burner burner when when the the equivalence equivalence Figure ratio is 0.65 and the inlet flow velocity is 1.15 m/s. (a) Temperature contours in the ordinary burner; ratio is 0.65 and the inlet flow velocity is 1.15 m/s. (a) Temperature contours in the ordinary burner; ratio is (b) 0.65Temperature and the inlet flowinvelocity 1.15 m/s. (a) Temperature contours in the ordinary burner; contours the new is burner. (b) Temperature Temperature contours in the new burner. (b)

4 3

0.009

5 0.0152 0.006 0.003 4 0.0121 0.003 3 0.0092018, 6, x FOR PEER REVIEW Processes

0.006

21 8 0.0 0.01 0.009

Level CH44 0.003

0.015 0.012

3 2

0.009 0.006

1

0.003

(a)

(a)

00..000 033 0.003 0.003

.00099 233 00.0 .02 00.0 02299 . 0 . 0 0 0.026 .0110.026 00.0 22

0.006 0.006

00. . 0022 11

5 4

06

066 .000 99 00. 022 00. .0

0.021 0.018

00.0.01122

7 6

.02299 0.012 0.012 00.0 .00066 00.0

0 .0

14 of 19

0.006

0.003

10 0.029 9 0.026 8 0.023

06

00.0.0003 3

1

6

0 .0

00.0.0 0033

0.006

0.0

2 0 .0

0.0 0.02329

0.015

2

0.003

21 188 0.0 0.01 0.009

0.029 0.015

26

00. . 0022 11

0.018

0 .0

00. . 0011 0.018 0.018 55

6

0.015 0.012

0. 0.015 01 2

0.018

0.0215

0. 0.015 01 2

0.0236

7

0.003

0.018

0.029

0.015

3

8

0.015

0.0 0.02112

0.0 2

0.021

3

0.0267

0.0 2

9

0.0 0.02112

6

0.023

00 0.

0.0298

6

0.026

10

00 0.

0.029

9

6 3 00 0. .02 0.026 0

10

Level CH4

6 3 00 0. .02 0.026 0

Level CH4

0.0 0.02329 0.029 0.029

(b) CH44 concentration contours the ordinary burner andnew the burner new burner the Figure Figure 17. CH17. contours in theinordinary burner and the whenwhen the equivalence 4 concentration equivalence ratio is 0.65 and the inlet flow velocity is 0.65 m/s. (a) CH 44 concentration contours in the ratio is 0.65 and the inlet flow velocity is 0.65 m/s. (a) CH4 concentration contours in the ordinary ordinary burner; (b) CH44 concentration contours in the new burner. burner; (b) CH4 concentration contours in the new burner. Level CH44 10 0.029 9 0.026 8 7

0.023 0.021

6 5

0.018 0.015

4 3

0.012 0.009

2 1

0.006 0.003

0.006 0.006 0.021 0.021 00..002 200

00033 00..0 00..000 0.01122 .0223 0.0 00.0 099 .00.015 00.0 188 10.015 3

0.029 0.029

.02266 00.0 0.015 02211 0.0 0.015 1122 0 .0 00..0

0.023 0.023

0.003 00...0 00000663

.01188 00.0

01122 00..00..000099 0

99 .0000.006 0.006 00.0

.00033 00.0

1

0.003

00.0.022300.0.01 3 155

0.012 0.009 0.006

00.0.0 2299

4 3 2

0.012 02266 0.012 00..0 0.026 0.026 0.029 .0220.029 00.0 11

0.026 0.026

00. . 0000 33

0.018 0.015

99 022 00. .0001188 00. . 99 000 00. .0

6 5

.01188 00.0 0.003 0.003

0.003 0.003

0.023 0.021

99 000 00. .0

8 7

00. . 0000 33

(a) Level CH44 10 0.029 9 0.026

(b)

99999

03322229 9999999

00.0.0 0000 33223300000000 30000

0.03229999 0.03229999

0.03229999 0.03229999

9999 9999 2222 033 00. .0

55

98989 999999 222299 033322 00.0.0.0 9 29999999 00.0.03333222222299999999 00.0.0

0.03229999 0.03229999 0.03229999 0.03229999

0.03229999 0.03229999

0.03230000 0.03230000

77 66

00000 33000 03322 00..0 00..003 32233 000000 00

0.03229999 0.03229999

Level CH44 Level CH 10 0.03230000 0.03230000 10 0.03230000 99 0.03230000 0.03230000 88 0.03230000

00.0.030302.0.033223300 23300000 000000 000

CH44 concentration contours the ordinary burner andnew the burner new burner the Figure Figure 18. CH18. contours in theinordinary burner and the whenwhen the equivalence 4 concentration equivalence ratio is 0.65 and the inlet flow velocity is 0.75 m/s. (a) CH 44 concentration contours in the ratio is 0.65 and the inlet flow velocity is 0.75 m/s. (a) CH4 concentration contours in the ordinary ordinary burner; (b) CH44 concentration contours in the new burner. burner; (b) CH4 concentration contours in the new burner.

0000 0

1396.72 1323.47 1250.23 1176.98 1103.74 1030.49 957.25 884.00 810.75 737.51 664.26 591.02 517.77 444.53 371.28

2

0.006

1

0.003

(b)

0.009 0.006 0.006 0.003

1

0.003

9999

99

0.0

32

0.0 3

29

22

9

0.032

2999

9

0.032 2

0.03229999

99

0.0 3

99

23

00

0 3000 0.032

00 0.0 0.03230000 3230000

00 23

0.0 3

00

0.020.02 3 3

32 21

21 0 .0 1 2 0 0.

0.029 0.029

0.015 0.012 0.012 0.009

15 of 19 15 of 19

(a)

06 0.00.018 06 18 0 . 0 .0 0.0 0.00 03 150.021 0.0 0.0 03 150.021

0.0 0.0 03 03

54 43

9

0.021 0.018 0.018 0.015

9

76 65

99

0.026 0.023 0.023 0.021

00.03229998 .0322 9999

0

0.029 0.029

09 09 0.0 0.0

98 87

3000

0.003

0.003

03 03 0.06 0.06 0 0 0.0 0.0 0.0105.015

CH4 CH4 0.029 0.029 0.026

26 26 0.0 0.0

Level Level 10 10 9

0.032

00

1 0.03229998 Processes 2018, 6, x FOR PEER REVIEW Processes 2018, 6, x FOR PEER REVIEW

29

0.03229998

32

0.03229999

2

2999

0.03229999

3

0

0.032

0.03229999

4

000

0 0.

5

323

989 9999 2229 0332 00..0

0.03229999

9 2999 0.0332229999 0.0

6

0 .0

0.03230000

0.03229999

0.03229999

7

32 0.0

00

0.03230000

0.03229999

23

8

00 300

3 0.0

0.03230000 0.03230000

0000

CH4

10 9

3 0.032

Level

0.0302.03230 3000 000 0

Figure 18. CH4 concentration contours in the ordinary burner and the new burner when the equivalence ratio is 0.65 and the inlet flow velocity is 0.75 m/s. (a) CH4 concentration contours in the Processes 2018, 6, 185 14 of 18 ordinary burner; (b) CH4 concentration contours in the new burner.

12 0 .0 2 1 0 0.

(b) (b)

0.0 0. 32 032 30 30 00 00 00 00 .03 .03 22 22 99 99 99 99

0.0 0.0 06 06

0.0 0.0 32 32 29 29 99 99 9 9

0.0 0.0 32 32 29 29 99 99 9 9 0. 0. 03 03 23 23 00 00 00 00

0.0302.03 300203000 0 0

0.03229999 00.03229999 .0302.0 2939292999 0. 0. 9 90 03 03 . 03 0 . 0 23 23 00 00.00 0.0 23 323 00 03022 322 00 00 0 0 .0 99 99 32.0 293929290099 00 99 99 9 9

0. 0. 02 02 6 6

0.006 0.003 0.003

0. 0. 00 00 3 3

21 1

0.0209.02 9

0.012 0.009 0.009 0.006

0.0 0.0 26 26

43 32

0. 0. 01 01 5 5

0.018 0.015 0.015 0.012

0.0003.003

65 54

0.003 0.003 29 0.0 29 0 . 0

0.0290.018 0.0290.018

00.0.0120801.0.01281 0.0206.026

0.023 0.021 0.021 0.018

(a) (a)

9 9 02 2 0. 0.0 5 5 01 1 0. 030.003 0 0. 0.0

87 76

0.03229999

0.03229999

0.021 0.021

Level CH4 Level CH4 10 0.029 10 9 0.029 0.026 98 0.026 0.023

9 299929999 0.0302.0329 9 99 9 29 99 32 322 0.0 0.0 0.0302.03 299292999 9 9

0.03229999

0.032 29 0.0 0.0322 9 9 9 9 0 99 9 9 9 322 .0 9 99322999 0.0 0.0939292 2 9 9 2 9999 3 9 322 0 9 999 0 . 229 9 3 0 0.

0.03229999 0.03229999

1

00 00 00 00 23 23 03 3 0. 0.20999299999 0.0302.032

0.03229999 0.03229999

00 00 00 00 23 23 03 3 0. 0.0

32 21

0.0 323 0.0 0000 323 000 0

9 299929999 0.0302.032

0.03229999 0.03229999

0.03230000 0 .0 322 0.03230000 0 . 0 9999 322 999 9

9 99 99 29 99 32 322 0.0 0.0

54 43

0 00 00 30 00 32 323 0 .0 0 .0

0.03229999 0.03229999

0.03230000

0.03230000

00 00 00 00 23 23 03 3 0. 0.0

76 65

0 00 00 30 00 32 323 0.0 0.0

00 00 00 00 23 23 03 3 0. 0.0

Level CH4 Level CH4 10 0.03230000 10 0.03230000 9 98 0.03230000 87 0.03230000 0.03229999

0.0302.03 300203000 0 0

Figure 19. CH4 concentration contours in the ordinary burner and the new burner when the Figure 19. contours in the ordinary and(a)the new burner when the equivalence Figure 19. CH CH concentration contours invelocity the ordinary burner and the new burner when the 4 4concentration equivalence ratio is 0.65 and the inlet flow is burner 0.95 m/s. CH 4 concentration contours in the ratio is 0.65 and the inlet flow velocity is 0.95 m/s. (a) CH concentration contours in the ordinary equivalence ratio is 0.65 and the inlet flow velocity is 0.95 m/s. (a) CH 4 concentration contours in the 4 ordinary burner; (b) CH4 concentration contours in the new burner. burner; (b) CH4 concentration contourscontours in the new ordinary burner; (b) CH4 concentration in burner. the new burner.

(b) (b)

Figure 20. CH CH concentration contours in ordinary the ordinary new burner when the Figure 20. contours in the burnerburner and theand new the burner when the equivalence 4 4concentration Figure 4 concentration contours in the ordinary burner and the new burner when the equivalence ratio is 0.65 thevelocity inlet flow velocity 1.15 m/s. (a) CH4 concentration in the ratio is 20. 0.65CH and the inletand flow is 1.15 m/s.is(a) CH contours contours in the ordinary 4 concentration equivalence ratioconcentration is 0.65 and thecontours inlet flow velocity is 1.15 m/s. (a) CH4 concentration contours in the burner; (b) CH in the new burner. ordinary burner; (b) CH 4 concentration contours in the new burner. 4 ordinary burner; (b) CH4 concentration contours in the new burner.

As can can be in in thethe vicinity of the burner inletinlet at low As be seen seen from fromFigures Figures13–16, 13–16,the thetemperature temperature vicinity of the burner at flow low As can be seen from Figures 13–16, the temperature in the vicinity of the burner inlet at low velocity is higher than that at high flow velocity. The combustion reaction begins earlier when the flow velocity is higher than that at high flow velocity. The combustion reaction begins earlier when flow velocity is higher than that at high flow velocity. The combustion reaction begins earlier when flow velocity is low. The main reason for time the flow velocity is low. The main reason forthis thisisisthat thatdue dueto tothe thelow low flow flow velocity, velocity, the contact time the flow velocity is low. The main reason for this is that due to the low flow velocity, the contact time between the gas and the porous medium is prolonged, and the preheating effect is more durable. between the gas and is prolonged, andThe the reason preheating effectis isthat more durable. The temperature nearthe theporous outletmedium of the burner is higher. for this due to the The temperature near the outlet of the burner is higher. The reason for this is that due to gas the complete reaction in the porous medium burner, the mass flow velocity of the premixed complete reaction in the porous mass flow velocity of of thegaspremixed gas increases with the increase of the medium inlet flowburner, velocity,the i.e., the reaction amount in unit time increases with the increase of the inlet flow velocity, i.e., the reaction amount of gas in unit time increases, which produces more heat. Figures 13–16 show that the outlet temperature is 1800–2000 K, increases, which produces more heat. Figures 13–16 show that the outlet temperature is 1800–2000 K,


15 of 18

NOX concentration /ppm

between the gas and the porous medium is prolonged, and the preheating effect is more durable. The temperature near theREVIEW outlet of the burner is higher. The reason for this is that due to the complete Processes 2018, 6, x FOR PEER 16 of 19 reaction in the porous medium burner, the mass flow velocity of the premixed gas increases with the increase of the inletpropagation flow velocity, velocity, i.e., the reaction amount gas in unit time increases, maximum flame the flame willof move toward the outletwhich and produces produce more heat. Figures 13–16 show that the outlet temperature is 1800–2000 K, according to the combustion extinguishment. In the flame stabilization zone, each equivalence ratio correspondshigh to a temperature the porous This temperature is acceptable practical velocity rangeresistance that can of stabilize flamemedium. in the support layer. With range the increase of the for equivalence industrial applications. Atzone the same time, at velocity of 0.65~1.15 temperature peak of the ratio, the flame stability increases. Asthe the premixed gas has m/s, beenthe effectively preheated, new burner is always higheristhan that of the ordinary With burner. flame propagation velocity significantly improved. the increase of the equivalence ratio, the thezone inlet becomes flow velocity of the premixed gas is less than the minimum flame propagation flameWhen stability wider. velocity, the flame will move inlet at and produce tempering, but premixed gas can begas, burned As can be seen from thetoward Figuresthe 17–20, the lower inlet flow velocity of the premixed the in the porous If the inlet flow velocity therealize premixed gas iscombustion more than the maximum flame premixed gas medium. with the equivalence ratio of 0.65ofcan effective in the two kinds of propagation the flameconsumption will move toward theisoutlet andWhen produce extinguishment. burners, andvelocity, the combustion degree similar. thecombustion inlet flow velocity reaches In them/s flame zone, each equivalence ratio corresponds to a velocitycontour range that can that stabilize 0.95 andstabilization 1.15 m/s, the temperature cloud figure and gas concentration show gas flame in the support layer. With the increaseburner of the equivalence ratio, the flame stability zone combustion is not effective in the ordinary (the concentration experiences almost no increases. change); As the premixed gas burner, has been preheated, the moves flame propagation velocity however, in the new theeffectively high-temperature flame toward the rear with is thesignificantly increase in improved.gas With theflow, increase the equivalence ratio, the flameover stability wider. premixed mass the of temperature peak is maintained 2500 zone K, thebecomes attenuation of the gas As can begradient seen from the Figures 17–20, at the lower inlet flow velocity of the premixed gas, concentration is normal, and the combustion state is good. the premixed gas with the equivalence ratio of 0.65 can realize effective combustion in the two kinds 4.2. NOXEmission Characteristics Different Equivalence in the Newthe Burner of burners, and the combustionofconsumption degree isRatios similar. When inlet flow velocity reaches 0.95 m/s and 1.15 m/s, the temperature cloud figure and gas concentration contour show that gas When the combustion reaction occurs, the nitrogen in the air/fuel is heated and decomposed combustion is not effective in the ordinary burner (the concentration experiences almost no change); under the oxidizing conditions of combustion, and toxic nitrogen oxides are then produced. Its however, in the new burner, the high-temperature flame moves toward the rear with the increase in compositions are mainly NO and NO2, which are collectively called NOx. The harm from emission premixed gas mass flow, the temperature peak is maintained over 2500 K, the attenuation of the gas of NOx cannot be ignored, and many countries have strict regulations concerning their emissions concentration gradient is normal, and the combustion state is good. [38]. For this portion of the study, the flow velocity of the premixed gas in the new burner was fixed at Figures 21 is the schematic diagram of NORatios x emission, which depicts the NOx emission 4.2.0.65m/s. NOX Emission Characteristics of Different Equivalence in the New Burner states under equivalence ratios of 0.35, 0.45, 0.65, 0.75, and 0.85, respectively. When reaction the nitrogen in the air/fuel isthe heated and decomposed As canthe be combustion seen from Figure 21,occurs, in the porous medium combustion, emission of pollutant under the oxidizing of combustion, and toxic oxides are then produced. NO x is very low. Whenconditions the equivalence ratio of premixed gas nitrogen is 0.65 and inlet flow velocity is 0.85 Its compositions are mainly NO and NO , which are collectively called NO . The harm from emission x 2 burner outlet is only 1 ppm approximately. When the m/s, the NOx concentration in the new of NOx cannot be ignored, and many countries haveof strict concerning emissions [38]. equivalence ratio is 0.85, the highest concentration NOregulations x in the ordinary burnertheir is about 2.3 ppm. For this NO portion the study, the flow on velocity of the premixed gas in the new burner was fixed at Because x hasof a strong dependence temperature, with the increase of mass flow velocity of the 0.65m/s. Figure 21 is the schematic diagram of NO emission, which depicts the NO emission states x premixed gas, the combustion temperature becomes higher and higher, and thex NOx emission under equivalence of 0.35, 0.45, 0.65, 0.75, and 0.85, respectively. consequently showsratios a trend of increase.

2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0

0.35 0.45 0.65 0.75 0.85

0

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 Axial distance /m

0.2

0.22

Figure 21. NOxx emissions emissions schematic schematic diagram diagram for for the the new new burner burner at at different different equivalence ratios.

The NOx emission for an equivalence ratio of 0.45 is lower than that for an equivalence ratio of 0.35, which shows that NOx emission will decrease with the increase in the complete reaction degree(the emission concentration is lower than the minimum standard (100 ppm) in Beijing, China). However, the total emission amount of NOx will show an upward trend with the increase in the equivalence ratio. The reason for this is that with the increase in the equivalence ratio, the mass


16 of 18

As can be seen from Figure 21, in the porous medium combustion, the emission of pollutant NOx is very low. When the equivalence ratio of premixed gas is 0.65 and inlet flow velocity is 0.85 m/s, the NOx concentration in the new burner outlet is only 1 ppm approximately. When the equivalence ratio is 0.85, the highest concentration of NOx in the ordinary burner is about 2.3 ppm. Because NOx has a strong dependence on temperature, with the increase of mass flow velocity of the premixed gas, the combustion temperature becomes higher and higher, and the NOx emission consequently shows a trend of increase. The NOx emission for an equivalence ratio of 0.45 is lower than that for an equivalence ratio of 0.35, which shows that NOx emission will decrease with the increase in the complete reaction degree(the emission concentration is lower than the minimum standard (100 ppm) in Beijing, China). However, the total emission amount of NOx will show an upward trend with the increase in the equivalence ratio. The reason for this is that with the increase in the equivalence ratio, the mass flow velocity of the combustible gas increases, the incomplete reaction degree increases, the combustion temperature is higher, and the NOx relatively increases. However, the emission is still lower than that from free combustion. 5. Conclusions Based on the combustion mechanism of methane-air premixed gas in a porous medium, a new burner was proposed. A mathematical model for the premixed gas combustion in porous medium was established. The combustion performance for different equivalence ratios and premixed gas velocities in inlets was simulated. The methane combustion degree and distribution of temperature and pressure in the porous medium were analyzed. In addition, the concentrations of NOx emission for different equivalence ratios were studied. The following results were obtained: (1)

(2)

(3)

(4) (5)

(6)

(7)

Compared with the ordinary porous medium burner, the new burner can gather heat more effectively, thereby promoting gas combustion. As a result, the gas temperature in the new burner is much higher than that in the ordinary one. At higher flow velocity, the short preheating time results in poor reaction of the premixed gas in the porous medium of the ordinary burner. On the contrary, under the condition of high flow velocity, the gas concentration contour gradually fades along the gas flow direction in the new burner. The combustion state is good, and the combustion heat is large. The change of flow velocity of the premixed gas in the inlet exerts an important influence on the combustion in the porous medium. In the case of low flow velocity, the temperature in the inlet increases significantly. In contrast, the temperature in the outlet increases dramatically in the case of high flow velocity. The inlet flow velocity should be set within a reasonable range to ensure the safety and normal operation of the burner. The emission concentration of major pollutants (NOX ) from the new burner is much lower than the minimum standard ((100 ppm) in Beijing, China). The radiation attenuation coefficient of the small-pore medium in the new burner increases, but the radiation attenuation coefficient of the big-pore medium in the new burner remains unchanged, which increases the stable operating range of the burner. The thermal conductivity coefficient of the small-pore medium in the new burner increases, but the thermal conductivity coefficient of the big-pore medium in the new burner remains unchanged or reduces slightly, which increases the temperature peak and the stable operating range. The aperture of the porous medium should not be too large. A reasonable range of aperture size can maintain the stability of flame front and obtain the ideal temperature. Compared with the traditional burner, the new porous medium burner has advantages in improving combustion efficiency, expanding flammability limits, and improving environmental friendliness. Therefore, the application prospects of the new porous medium burner are very broad. The influences of various factors on the combustion process of the new porous medium


17 of 18

burner were simulated by CFD (computational fluid dynamics) and the basic characteristics were mastered. The numerical results can provide a theoretical basis and data support for the design of the new porous medium burner. If the design and manufacturing process of the new porous medium burner can be further strengthened, better application value will surely be attained. Author Contributions: All the authors designed research, performed research, analyzed data, wrote and revised the paper, all authors contributed equally. Funding: This research was funded by the National Natural Science Foundation Project of China, grant number (51704111, 51374003, 51774135 and 51604110). Conflicts of Interest: The authors declare no conflicts of interest.

References 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14.

15. 16. 17. 18. 19.

Zhang, Y.; Lu, C.; Li, T. A Practical Countercurrent Fluid Catalytic Cracking Regenerator Model for in Situ Operation Optimization. AICHE J. 2012, 58, 2810–2819. [CrossRef] Jia, Z.; Feng, T. Gas Explosion Characteristics and Its Control Measures in Closed Fire Zone. Comput. Modell. New Technol. 2014, 18, 21–25. Kamal, M.M.; Mohamad, A.A. Combustion in Porous Media. Proc. Inst. Mech. Eng. Part A J. Power Energy 2006, 220, 487–508. [CrossRef] Valerio, G.; Rajnish, N.S.; Robert, R.R. Premixed Combustion of Methane-Air Mixture Stabilized over Porous Medium: A 2-D Numerical Study. Chem. Eng. Sci 2016, 152, 591–605. Mujeebu, M.A.; Abdullah, M.Z.; Bakar, M.A.; Mohamad, A.A.; Abdullah, M.K. Applications of Porous Media Combustion Technology Review. Appl. Energy 2009, 86, 1365–1375. [CrossRef] Wood, S.; Harris, A.T. Porous Burners for Lean-Burn Applications. Prog. Energy Combust. Sci. 2008, 34, 667–684. [CrossRef] Slimane, R.B.; Lau, F.S.; Khinkis, M.; Bingue, J.P.; Saveliev, A.V.; Kennedy, L.A. Conversion of Hydrogen Sulphide to Hydrogen by Super Adiabatic Partial Oxidation: Thermodynamic Consideration. Int. J. Hydrogen Energy 2004, 29, 1471–1477. [CrossRef] Min, D.K.; Shin, E.D. Laminar Premixed Flame Stabilized inside a Honeycomb Ceramic. Int. J. Heat Mass Transf. 1991, 34, 341–356. Hayashi, T.C.; Malico, I.; Pereira, J.C.F. Three-dimensional Modeling of Two-Layer Porous Burner for Household Application. Comput. Struct. 2004, 82, 1543–1550. [CrossRef] Kotani, Y.; Takeno, T. An Experimental Study of Stability and Combustion Characteristics of an Excess Enthalpy Flame. Symp. (Int.) Combust. 1982, 19, 1503–1509. [CrossRef] Pereira, F.M.; Oliveira, A.A.M. Analysis of the Combustion with Excess Enthalpy in Porous Media. In Proceeding of the European Combustion Meeting, Orléans, France, 2003; pp. 388–392. Mital, R.; Gore, J.P.; Viskanta, R. A Study of the Structure of Submerged Reaction Zones in Porous Ceramics Radiant Burners. Combust. Flame 1997, 111, 175–184. [CrossRef] Sathe, S.B.; Kulkarni, M.R.; Peck, R.E.; Tong, T.W. An Experimental and Theoretical Study of Porous Radiant Burner Performance. Symp. (Int.) Combust. 1990, 23, 1011–1018. [CrossRef] Chen, Y.K.; Matthews, R.D.; Howell, J.R. The Effect of Radiation on the Structure of Premixed Flame within a Highly Porous Inert Medium; American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD: Boston, MA, USA, 1987; pp. 35–41. Wang, E.; Chen, L.; Wu, J. Application and Existing Problems of Porous Ceramics in the Combustion Field. Foshan Ceram. 2005, 100, 35–39. Chu, J. Study and Development of a Gradually-varied Porous Media Burner. Master’s Thesis, Zhejiang University, Hangzhou, China, 2005. Du, L.; Xie, M. Numerical Investigation on Effects of Porous Media in Premixed Combustion. J. Dalian Univ. Technol. 2004, 1, 70–75. Du, L. Investigation on Super-adiabatic Combustion of Lean Premixed Gases in Porous Media. Ph.D. Thesis, Dalian University of Technology, Dalian, China, 2003. Liu, H. Experimental Study and Numerical Simulation of Premixed Gas Combustion in Porous Media. Ph.D. Thesis, Northeastern University, Shengyang, China, 2010.


20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

18 of 18

Liu, H.; Dong, S.; Xing, N.; Li, B. Numerical Simulation of the Influence of Porous Medium Characteristics on Combustion Temperature and Pressure. Appl. Energy Technol. 2009, 10, 31–37. Barra, A.J.; Diepvens, G.; Ellzey, J.L.; Henneke, M.R. Numerical Study of the Effects of Material Properties on Flame Stabilization in a Porous Burner. Combust. Flame 2003, 134, 369–379. [CrossRef] Wang, H. Industrial Application and Numerical Simulation of Premixed Gas Combustion in Porous Media. Master’s Thesis, Northeastern University, Shengyang, China, 2009. Lee, D.K.; Noh, D.S. Experimental and Theoretical Study of Excess Enthalpy Flames Stabilized in a Radial Multi-Channel as a Model Cylindrical Porous Medium Burner. Combust. Flame 2016, 170, 79–90. [CrossRef] Mendes, M.A.A.; Pereira, J.M.C.; Pereira, J.C.F. A Numerical Study of the Stability of One-Dimensional Laminar Premixed Flame in Inert Porous Media. Combust. Flame 2008, 153, 525–539. [CrossRef] Afsharvahid, S.; Ashman, P.J.; Dally, B.B. Investigation of NOx Conversion Characteristics in a Porous Medium. Combust. Flame 2008, 152, 604–615. [CrossRef] Zhao, P. Study on Premixed Combustion in Inert Porous Media. Ph.D. Thesis, University of Science and Technology of China, Hefei, China, 2007. Huang, R. Combustion Characteristics of Multi-Species Low Calorific Gaseous Fuels in a Two-Layer Porous Burner. Master’s Thesis, Zhejiang University, Hangzhou, China, 2016. Dong, S. Preliminary Experimental and Numerical Study of Premixed Combustion in Porous Inert Media. Master’s Thesis, Northeastern University, Shengyang, China, 2008. Zheng, Z. Numerical Study of Natural Gas Premixed Combustion in Porous Inert Media. Master’s Thesis, Shanghai Jiaotong University, Shanghai, China, 2007. Jia, Z.; Lin, B.; Ye, Q. Analysis of Influencing Factors and Flame Acceleration Mechanism of Gas Explosion Propagation in Tube. Min. Eng. Res. 2009, 24, 57–62. Gao, Z.; Cui, G.; Zuo, D. Discussion on the Reaction Mechanism of Carbon Combustion. J. Shandong Polytech. Univ. 2002, 16, 39–42. Yang, S.; Tao, W. Heat Transfer, 3rd ed.; Higher Education Press: Beijing, China, 1998. Han, Z. Fuel and Combustion; Metallurgical Industry Press: Beijing, China, 2007. Wang, P. Theoretical and Experimental Study on Gas Thermal Counter Current Oxidation of Low Concentration Methane in Coal Mine. Ph.D. Thesis, Central South University, Changsha, China, 2012. Dhamrat, R.S.; Ellzey, J.L. Numerical and Experimental Study of the Conversion of Methane to Hydrogen in a Porous Media Reactor. Combust. Flame 2006, 144, 698–709. [CrossRef] Mohammadi, I.; Hossainpour, S. The Effects of Chemical Kinetics and Wall Temperature Performance of Porous Media Burners. Heat Mass Transf. 2013, 49, 869–877. [CrossRef] Younis, L.B.; Viskanta, R. Experimental Determination of the Volumetric Heat Transfer Coefficient between Stream of Air and Ceramic Foam. Int. J. Heat Mass Transf. 1993, 36, 1425–1434. [CrossRef] Li, Z.; Cheng, Y. Fire Engineering; China University of Mining and Technology Press: Xuzhou, China, 2002. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Numerical Simulation of a New Porous Medium Burner with ... - MDPI

Numerical Simulation of a New Porous Medium Burner with ... - MDPI

Suggest Documents

Numerical Simulation on Effect of Porous Medium on ...

numerical simulation of high pressure burner with partially ... - Dialnet

NUMERICAL INVESTIGATION ON A SWIRL BURNER WITH ...

Asymptotic and numerical analysis of a porous medium model for ...

A simulation of suspension flow in an artificial porous medium.

Simulation of thermallyâenhanced combustion in a porous medium ...

Numerical simulation of saltwater upconing in a porous ... - Google Sites

numerical simulation of unsaturated flow in porous media using a ...

Testing a new lignite prototype burner by experiment and numerical ...

Numerical Simulation: Field Scale Fluid Injection to a Porous ... - Comsol

darcian porous medium

Description of the porous medium

Numerical blow-up for the porous medium equation with a source

Comprehensive Numerical Simulation of Filling and ... - MDPI

Numerical Simulation of Non-normal Wind Load on Porous Fences

numerical modelling of isothermal burner jets - CFD

Combustion Fronts in a Porous Medium with Two Layers

Porous medium equation with absorption and a nonlinear ... - CiteSeerX

Radial transport in a porous medium with Dirichlet ... - Springer Link

A Direct Numerical Simulation-Based Analysis of Entropy ... - MDPI

Numerical Simulation and Experimental Study of a Simplified ... - MDPI

Numerical Investigation of a MILD Combustion Burner: Analysis of ...

Numerical Investigation of a MILD Combustion Burner: Analysis of ...

Numerical Simulation of a Novel Sensing Approach Based on ... - MDPI

Numerical Simulation of a New Porous Medium Burner with ... - MDPI