Flame front stability of low calorific fuel gas combustion with preheated air in a porous burner

Accepted Manuscript lame front stability of low calorific fuel gas combustion with preheated air in a porous burner

Guanqing Wang, Pengbo Tang, Yuan Li, Jiangrong Xu, Franz Durst PII:

S0360-5442(18)32500-3

DOI:

10.1016/j.energy.2018.12.128

Reference:

EGY 14381

To appear in:

Energy

Received Date:

28 May 2018

Accepted Date:

18 December 2018

Please cite this article as: Guanqing Wang, Pengbo Tang, Yuan Li, Jiangrong Xu, Franz Durst, lame front stability of low calorific fuel gas combustion with preheated air in a porous burner, Energy (2018), doi: 10.1016/j.energy.2018.12.128

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ACCEPTED MANUSCRIPT Flame front stability of low calorific fuel gas combustion with preheated air in a porous burner Guanqing Wanga,1, Pengbo Tanga, Yuan Lia, Jiangrong Xua, Franz Durstb a

b

Energy Institute, Hangzhou Dianzi University, Hangzhou, 310018, P.R. China

FMP Technology GmbH, Am Weichselgarten 34, D-91058, Erlangen, Germany

ABSTRACT: The flame front stabilization of low calorific fuel gas (LCFG) combustion with preheated air was investigated numerically in a burner filled with Al2O3 ceramic foams. A two-dimensional model was implemented to simulate the flame propagation process of methane–preheated air combustion at an extra-low equivalence ratio. In addition to the wall heat loss, the effects of the preheated air temperature were analyzed by focusing on the flame front inclination and propagation velocity. The flame front instability for room temperature air agreed well with the results of the corresponding experiments. The results show that an increase in the preheated air temperature helps to improve the flame stabilization, such as inhibiting flame front inclination, decreasing its propagation velocity, and increasing the maximum combustion temperature. A stable combustion flame can be realized for LCFG by preheating the air to a critical temperature, which decreases with increase in the equivalence ratio and decrease in the inlet velocity. The wall heat loss promotes the flame front inclination, whereas it has little effect on its propagation velocity for stable combustion. The results are conducive to clean combustion of LCFG in porous media, and helpful for the design and operation of porous burners. Keywords: Flame front stability; Preheated air; Porous media; Low calorific fuel gas; Porous burner



Corresponding author: Tel.: +86 571 86878543; fax: +86 571 86919032. E-mail address: [email protected]

ACCEPTED MANUSCRIPT Nomenclature A

pre-exponential factor (1/s)

W

molecular weight (kg/mol)

C1

permeability factor (-)

x

axial direction coordinate (m)

C2

inertial resistance factor (-)

Yi

local mass fraction of species i (-)

Cp

specific (J/kg·K)

Greek Symbols

diffusion coefficient of species i Dm,i

ε

porosity (-)

(m2/s) E

Activation energy (J/mol)

φ

equivalence ratio (-)

H0

lower heat value (J/kg)

λ

thermal conductivity (W/m·K)

h

heat loss coefficient (W/ m2·K)

λal

thermal conductivity of Al2O3 (W/m·K)

k

extinction coefficient ( 1/m)

λeff-s

effective thermal conductivity (W/m·K)

L

length of the burner (m)

λrad

radiative thermal conductivity (W/m·K)

M

gas molar mass (kg/mol)

λw

thermal conductivity of burner wall (W/m·K)

o

origin of coordinate (-)

μ

kinematic viscosity (Pa·s)

p

pressure (Pa)

ρ

density (kg/m3)

R

universal gas constant (J/mol·K)

σ

Stefan-Boltzmann constant (W/m2·K4)

r

radial direction coordinate (m)

ω

reaction rate (kmol/(m3·s))

T

temperature (K)

Subscripts

Tair

air temperature (K)

air

air

T0

ambient temperature (K)

g

gas

Tw

wall temperature of the burner (K)

f

flame propagation

t

time (s)

i

species

u

axial velocity (m/s)

in

inlet of the burner

uf

flame propagation velocity (mm/s)

s

solid

uin

inlet velocity of the gas (m/s)

w

wall of the burner

v

radial velocity(m/s)

0

ambient

1. Introduction Porous media combustion (PMC) has developed rapidly in the last few decades owing to its lower emissions and advantages over traditional combustion, such as the combustion of low calorific fuel gas (LCFG), higher burning rates, higher power intensity, and low emissions [1–3]. Its

ACCEPTED MANUSCRIPT advantages have been driving researchers to study various combustion modes in porous media. The modes of PMC are classified into steady and filtration combustion according to the flame stability [4, 5]. It can be operated in submerged and surface stabilized combustion depending on the flame position [6]. PMC are applied not only to the combustion of premixed (or non-premixed) fuel gas with air [7, 8], but also to the liquid fuel combustion [9], even further to the suspended solid fuel [10]. Various porous burners have been developed to industrial and household fields [11]. A comprehensive survey on combustion in porous media was summarized by Howell et al. [1] and Weinberg [12]. Developments in applications were reviewed by Mujeebu et al [13]. The application of PMC to LCFG combustion has recently attracted considerable attention because of its ultra-lean flammability limit [14–16]. Many researchers have already investigated extending the flammability limit in self-designed burners. Koester et al. [17] pointed out that a lean flammability limit was observed to extend down toφ = 0.1 with ignition temperatures on the order of 1000 ℃. Bingue et al. [18] obtained the lean flammability limit atφ = 0.25 for u = 0.12 m/s. Kennedy et al. [19] confirmed that the lean flammability limit can be further extended toφ = 0.2 for u = 0.25 m/s. Zheng et al. [20] reported that the lean flammability limit can be further extended toφ = 0.08 in a SiC foam burner at u = 0.3 m/s. More information on lean-burn applications in porous burners were outlined by Wood and Harris. [21]. The above studies have shown unequivocally the feasibility of LCFG combustion in porous burners. For LCFG combustion in a porous burner, studies on flame stabilization are necessary for the stable and safe operation of the burner [22–26], in addition to the emissions and super-adiabatic effects [2, 3, 18, 19, 26–30]. Most studies have involved investigation by using experimental [15–20, 22–26], analytical [30–33], and numerical methods 34–38]. Gao et al. [25] studied the flame stability of methane–air premixed in a two–layer porous burner with different porous

ACCEPTED MANUSCRIPT materials. Their results indicated that the flame stability limits expanded with increased foam conductivities but shrank with increased pore density. Zhdanok et al. [30] experimentally and theoretically investigated the flame propagation of methane in a particle packed burner. They accounted for the interaction of combustion wave with thermal wave, and observed a stable propagation of the combustion wave. Saveliev et al. [31] reported that a stable combustion wave can be obtained at a super-adiabatic temperature with a certain hydrogen concentration, and it becomes unstable and forms multi-dimensional cellular structures when the hydrogen concentration increases. Dobrego et al. [32] found that the inclination characteristic can be estimated in terms of the system length, the dimensionless wave velocity, and the inverse diameter of the porous media particles. A correlation of the inclination growth velocity was obtained. Bubnovich et al. [33] carried out analytical studies on the propagation of the combustion wave by assuming preheating, a combustion reaction, and a combustion products zone in a porous burner. Simple formulas were given to predict the combustion wave velocity and the reaction zone thickness. Barra et al. [34] reported that the combustion flame can be stabilized at the interface between the two-layer matrices. Shi et al. [35] studied numerically and theoretically the combustion wave, the maximum temperature of the combustion, and the flame instabilities of inclination and breakup for lean methane–air in a packed burner. Zheng et al. [36] investigated the flame inclination development process by two–dimensional double temperature models and confirmed that the final inclination angle of the front can be kept constant. Zhang et al. [37] found that the Lewis number has a significant influence on the breakup of the inclination front by numerical simulation. It is difficult for breakups of the inclination front to occur for a fuel gas with a large Lewis number, but easy for a fuel gas with a relatively small Lewis number. To obtain a stable flame in a porous burner, in most studies, the flame was stabilized at the

ACCEPTED MANUSCRIPT interface by using a two-layer structure of the porous media [13–15, 24–26]. Some workers chose a gradually changing structure (porosity or pores) to stabilize the flame at the different interfaces in a porous burner [16]. In some cases, the flame was even stabilized in a specific zone for a quasi-steady state by combing a periodic reciprocal flow style [5, 39, 40]. However, most of the investigations mentioned adopted the packed particle, seldom considered a ceramic foam [20, 41]. Here, the flame instability (e.g. inclination) is relatively more difficult to capture because the high porosity (85%) of the ceramic foam results in a much lower viscosity and inertia resistance compared with packed particle. Furthermore, the effect of the preheated air temperature on the flame front stability was ignored by many researchers. So far, there have been few reports on the effects of the preheated air temperature on flame front stability in a ceramic foam burner, especially for the extra LCFG. In this work, we numerically investigated the flame front stabilization of extra LCFG combustion with preheated air in a ceramic foam burner. By considering the effects of the preheated air temperature, a two–dimensional model was developed to investigate the flame front stability and the propagation process for the ultra lean combustion. In order to verify the numerical results, the flame front instability for room temperature air was compared with that of the experiments, and good agreement was observed. In addition to the wall heat loss, the effects of the preheated air temperature were investigated by focusing on the flame front inclination and propagation velocity. We found that an increase in the preheated air temperature helped to stabilize the flame for LCFG combustion. The wall heat loss promotes the flame front inclination, but has little effect on the propagation velocity for stable combustion. This study is helpful for the design and operation of porous burners. 2. Numerical methods

ACCEPTED MANUSCRIPT 2.1. Governing equations We first numerically investigate the flame front stabilization of the extra LCFG combustion with preheated air in a ceramic foam burner. We consider a ceramic foam burner the main part of which, filled with 20 ppi Al2O3 ceramic foam, was treated as the computational domain, as shown in Fig. 1. The origin o is set at the center of the starting position of the 20 ppi ceramic foam. x is set as the axial direction, and r is set as the radial direction. The pore structure distribution of the ceramic foam has high complexity, randomness, and uncertainty, which make it very difficult to establish its real three-dimensional structure model. At the same time, the average volume method is adopted since the effect of the pore structure can be ignored owing to the high porosity (up to 85%) of the Al2O3 ceramic foam. Therefore, a typical two-dimensional model was adopted to investigate the effect of the preheated air temperature on the flame instability in the ceramic foam. A similar model method was also used in a packed burner by other groups, such as Shi et al. [35], Zheng et al. [36], Zhang et al. [37], and so on. The volumetric heat transfer coefficient between gas and porous media is very high, and it is up to 105–107, or even to 109 [1, 42, 43]. This indicates that their temperature differences are relatively small, and the heat transfer process could be ignored [1, 24]. On the other hand, recent researchers considered solid energy equation during simulation. The gas temperature was still adopted to analyze flame stability as it was found to be higher than that of porous media in combustion zone [35–37]. Therefore, one temperature equation model was adopted in present work for saving computation cost. Some assumptions are made to simplify the numerical calculations: (1) The ceramic foams are treated as a continuous medium for its dodecahedron interconnected by the grid. (2) The ceramic foams are considered as non-catalytic, homogeneous, and optically thick.

ACCEPTED MANUSCRIPT (3) The gas radiation can be ignored, and the solid radiation is considered by the Rosseland diffusion equation [44]. (4) The gas flow in porous media is considered as laminar flow [45]. (5) Combustion reaction is assumed to obey the Arrhenius Law. The corresponding Lewis and Schmidt numbers are assumed to 1, respectively. Based on the above assumptions, the governing equations for mass, momentum, energy, and species transport are given as follows: Mass equation:

   g  t

     g u   0 .

(1)

Momentum equation in the axial direction:

   p   g u      g u  u       u   u  g u 2 .  t x C1 C2

(2)

Momentum equation in the radial direction:

   p   g v      g v  u       v   v  g v 2 .  t r C1 C2

(3)

Energy equation:    g C pgT      g C pg uT     eff T     i H 0iWi .  t i

(4)

Species transport equation:



   iYi     iuYi     gi Dm,iYi   iWi . t

(5)

The gas densities are calculated from the ideal gas equation of state:

p   g ( R M )T .

(6)

where C1 and C2 represent the permeability and the inertial resistance, respectively [46]. The Al2O3 ceramic foam density is  s  3950 kg/m3, and its specific heat is C ps  950 J/ (kg ·K) [47].

ACCEPTED MANUSCRIPT eff  s is the effective thermal conductivity, which is defined as eff  s  s  rad . s  al 1    3 is the thermal conductivity of ceramic foam including the porosity effects [48], and λal is the thermal conductivity of Al2O3. The radiation of the porous matrix rad  16 T 3 3k is obtained by assuming an optically thick region around the high–temperature combustion zone in the ceramic foam burner. 2.2. Boundary and initial conditions The boundary conditions are given as follows: At x  0 :

u  uin , v  0 , T  Tin  Tair , YCH 4  YCH 4 ,in , YO2  YO2 ,in , s

T x

x 0

  (T 4  T0 4 )

x 0

. (7)

At x  L : T u v T Yi   0,   0 , s x x r x x

xL

  (T 4  T0 4 )

xL

.

(8)

At r  0 : v  0,

T Yi  0. r r

(9)

At r  0.03 m:

u  v  0, w

T r

 h Tw  T0  .

(10)

r  0.03

The initial conditions are chosen as follows: At t  0 , for computational zone 0  x  L , and 0  r  0.03 : u  v  0 , Yi  0 , T  Tignitor  1200 K;

(11)

The subscript in refers to inlet of the burner. Tair is air temperature at inlet of the burner. The burner wall heat loss to the surroundings was considered in computations by the boundary conditions, and it is assumed to be proportional to the temperature difference

Tw  T0 

multiplying the effective

heat loss coefficient h (including radiation). The wall temperature Tw is considered as a variable, and was calculated from the equation (10) as the inner wall temperature was obtained iteratively by

ACCEPTED MANUSCRIPT solving the energy equation. T0 refers to the ambient temperature, which is assumed to be 300 K. For starting the combustion, the temperature of the computation at the initial moment Tignitor was specified 1200 K as the initial condition. 2.3. Chemical Reaction and Solution Methods Based on Hess’s law, the total combustion heat is equivalent no matter how many steps of the combustion reaction are used. And the present work aim is to investigate the combustion thermal effect (flame) rather than emissions. Then it is feasible to adopt a simple reaction mechanism. In addition, other researchers, e.g. Shi et al. [35], Zheng et al. [36], and Hashemi et al. [38] also adopted a simple reaction mechanism to investigate the behavior of the combustion flame. Their results show that it is reasonable to adopt the simple reaction mechanism. Therefore, a simple two-step reaction mechanism [49] is adopted to save the computation time in present work:

  CO+2H 2 O+5.64N 2 . CH 4 +1.5(O 2 +3.76 N 2 )  

(12a)

  CO 2 +1.88 N 2 . CO+0.5 (O 2 +3.76 N 2 )  

(12b)

In simulations, the computational domain was discretized into 175×15 grids. Utilizing Fluent software 14.5, the thermal parameters of the Al2O3 foam were imported by a user-defined program including the effects of temperature and porosity. A residual error of 10E–6 for gas and solid energy equations and 10 E–3 for other equations were taken as convergence criteria. 3. Results and Discussion 3.1. Flame inclination process The simulation flame front inclination process can be explicitly manifested by the reaction rate contours for operating conditions ofφ = 0.3, uin = 0.2 m/s, h = 80 W/ (m2K), and Tair = 300 K. As shown in Fig. 2, the position of the flame front propagates to the region between x = 0.04 m and x = 0.066 m at t = 400 s. At the same time, the flame front near the internal wall of the burner has begun

ACCEPTED MANUSCRIPT to incline downstream (Fig. 2a). With increasing combustion time, the flame front gradually moves downstream, and its inclination becomes more and more clear (Fig. 2b–d). It is found that the flame front length is gradually stretched in the axial direction because of the increase in the flame front inclination (Fig. 2b–d). The inclination mainly arises from the fluid dynamics mechanism due to the viscous and inertial resistance of the Al2O3 ceramic foam. On the other hand, owing to the heat loss of the burner wall, the fresh fuel gas near the internal wall needs to absorb more heat by flowing further downstream in order to reach its ignition temperature. Therefore, the flame inclination downstream is promoted by the wall heat loss. The effect of heat loss gradually increases as the flame propagates downstream. Thus, its inclination gradually increases as it propagates downstream, and its shape changes continually during the propagation. 3.2. Experimental verification of the flame instability 3.2.1. Experimental setup and procedures. In order to verify the validity of our numerical model, we carried out an experiment to study the propagation instability of the flame front. We adopted a ceramic foam burner consisting of four parts: a gas/fuel supply system, a flow measurement and control system, a data acquisition system, and a quartz tube burner filled with Al2O3 ceramic foam with porosity ε = 0.85, which is shown schematically in Fig. 3. The air is produced by an air compressor and stored in an air tank. Methane is supplied from a high-pressure bottle. Both are regulated and measured by mass flow controllers (Seven-star). The internal diameter of the burner is 60 mm. A piece of ceramic foam with 40 ppi is placed at the bottom of the burner for uniform flow and the remainder of the burner is filled uniformly with 20 ppi ceramic foams to eliminate the effects of the porous structures on the flame stabilization. The outer wall of the burner is exposed to the surroundings, and its heat loss including radiation is described by the effective heat loss coefficient h. It can be assumed to be 80 W/(m2K) according to the burner surroundings. The

ACCEPTED MANUSCRIPT temperature testing points are illustrated with an equal interval distance of 40 mm. Temperatures are measured by S–type thermocouples, which are inserted between the ceramic foam and the burner inner surface to avoid destroying the ceramic foam structure. They are connected to the date-logger Agilent 34970 to record the testing point temperatures on the computer. As the experiment begins, air is provided by the air compressor and stored in the air tank. It is delivered to the pre-mixer by the mass flow controller. Methane is supplied from the high-pressure methane bottle and mixes with the air in the pre-mixer. Combustion with equivalence ratioφ = 0.8 is initiated by the electric pulsing igniter downstream of the burner. Combustion flame gradually propagates upstream because the initiating equivalence ratio is high. When the flame propagates to the bottom interface between the 20 ppi and 40 ppi ceramic foams, the mixed fuel gas is adjusted to the desired mass flow rate, and the equivalence ratios are changed to their pre-determined values. Then, the combustion flame starts to propagate downstream. The flame propagation process is recorded every 400s with an S–PRIplus industrial camera with a vertical shooting angle. 3.2.2. Experimental flame front instability. The inclination process of the experimental flame front for room temperature air is shown in Fig. 4 as it propagates downstream under the particular operating conditions of Fig. 2. The luminous zone of each photograph taken by the camera is considered to be the flame front, which in fact is the maximum temperature zone. Within a short combustion time (t = 400 s), the flame front inclination begins to appear (Fig. 4a). The experimental flame front gradually propagates downstream with increasing the time, and its inclination becomes more and more clear (Fig. 4b–d). The flame inclination gradually increases as it propagates downstream, and its shape changes continually during the propagation. The flame inclination process, including the positions and the inclinations of the flame front, agrees well with the flame behaviors obtained from the numerical calculations (Fig. 2). This means that our numerical model

ACCEPTED MANUSCRIPT can efficiently describe the flame behavior of the ceramic foam burner. In order to analyze the flame front instability in detail, we further study the flame propagation velocities for room temperature air under different operating conditions, as shown in Fig. 5. It is obvious that the flame propagation velocities gradually decrease as the equivalence ratio increases. Compared with the results of other workers [22, 35, 36, 50], our propagation velocities are based on Al2O3 ceramic foam filled in the porous burner, whereas they used the alumina pellets. The propagation velocities of our numerical simulation agree well with our experimental results, as shown in Fig. 5. The numerical velocities are slightly higher than the experimental results obtained by Kennedy et al. [22] without considering the effect of the wall heat loss, and lower than the results obtained by Shi et al. [35], Zheng et al. [36] and Henneke et al. [50]. This is probably due to the different porous media and operating parameters used in their works. Even though some differences exist, propagation velocities obtained by the different groups are of the same order of magnitude. 3.3. Influence of Preheated Air on the Flame Front and Temperature In order to understand clearly the influence of preheated air on the flame front, we numerically plotted two-dimensional contours of the gas temperature, as shown in Fig. 6, which shows the flame shapes for different preheated air temperatures atφ = 0.3, uin = 0.2 m/s, h = 80 W/ (m2K), and t = 1600 s. The dense zones of the isotherm changes can be treated as the flame fronts, which are consistent with those in Fig. 2. For the preheated air temperature Tair = 300 K, the flame front propagates to the regions between x = 0.132 and 0.19 m when the combustion time t increases to 1600 s (Fig. 6a). The position agrees with that of the reaction rate in Fig. 2d. The flame front is stretched considerably, and its inclination is clear. With increase in the preheated air temperature, the flame front gradually moves upstream, and its inclination decreases (Fig. 6b–c). When the air is

ACCEPTED MANUSCRIPT preheated to 750 K, the flame front is located almost at the inlet of the porous burner, and its inclination near the internal wall becomes unclear (Fig. 6d). This is mainly caused by the following reasons. As the preheated air temperature increases, more enthalpy of fresh methane-air mixture will have. Then, it doesn’t need to absorb more heat from the ceramic foam downstream to reach its ignition point. This indicates ignition of methane-air is easier to be achieved, and the flame front position moves upstream. If the air is preheated with higher temperature, the methane-air will have more enthalpy, and its ignition will become easier. This results in forward movement of the flame front with increase in the preheated air temperature. Likewise, the flame inclination gradually decreases as the temperature of the preheated air increases. Fig. 6 also shows the combustion temperature profiles of the porous burner for different preheated air temperatures. It is obvious that the combustion temperature depends on the positions of the porous burner. The combustion temperature gradient is clear not only in the axial direction, but also in the radial direction because of the wall heat loss. The maximum combustion temperature zone (orange–red part) appears downstream of the flame front. For the preheated air temperature Tair = 300 K, the maximum temperature zone is stretched to a long-axial distance because of the large flame inclination (Fig. 6a). With increase in the preheated air temperature, the distribution zone of the maximum temperature gradually moves upstream and decreases (Fig. 6b–d). Next, the maximum combustion temperatures are shown as a function of the preheated air temperature in Fig. 7. When the preheated air temperature Tair = 300 K, maximum temperatures obtained both from numerical simulation and from the experiment agree well with that obtained by Kennedy et al. [22] with similar operating parameters. The relative error is about 2.8%, which is considered to be reasonable and acceptable. For different preheated air temperatures, the adiabatic temperatures are compared with the maximum temperatures of the numerical simulation. Both of

ACCEPTED MANUSCRIPT them gradually increase with the preheated air temperature increasing, and the latter increases more slowly. All adiabatic temperatures are lower than the results of the numerical simulation, especially for low temperature air. This means that super-adiabatic combustion occurs for LCFG combustion with the preheated air in a porous burner. The temperature ratio between simulation and adiabatic temperature can be defined as a super-adiabatic ratio. Obviously, when it is larger than 1, it means that super-adiabatic combustion occurs. It is clear that the super-adiabatic ratio gradually decreases with increase in the preheated air temperature. 3.4. Influence of Preheated Air on propagation velocity To understand further the influence of the preheated air temperature on the location of the flame front, the flame propagation velocities at different equivalence ratios were calculated for different preheated air temperatures, as shown in Fig. 8. It can be seen that the propagation velocity gradually decreases with increase in the preheated air temperature. When the preheated air temperature is fixed, the propagation velocity gradually decreases with increase in the equivalence ratio increasing. When Tair = 750 K, as the equivalence ratio increases to the critical value of 0.3 the propagation velocity decreases to zero, and the flame stops propagating. This indicates that the critical equivalence ratio associated with the zero propagation velocity gradually increases with decrease in the preheated air temperature. Similarly, for the same equivalence ratio, the propagation velocity can be decreased to zero by preheating the air to a critical temperature, which is obviously lower than the methane ignition temperature (923 K). If the equivalence ratio (or preheated air temperature) continues to increase under the corresponding operating conditions, the propagation velocity will become negative, and the flame front will begin to propagate upstream, which has already been verified by Kennedy et al. [22]. It is also found that the relationship of the reduction of propagation velocity with the equivalence ratio is complex, which deserves further study in future.

ACCEPTED MANUSCRIPT Fig. 9 shows the influence of the fuel gas inlet velocity on the flame propagation velocity for different preheated air temperatures. It demonstrates that the inlet velocity of the fuel gas has a large influence on the flame propagation velocity. With decrease in the inlet velocity of the fuel gas, the propagation velocity obviously decreases for different preheated air temperatures. For the same inlet velocity of the fuel gas, the flame propagation velocity gradually decreases with increase in the preheated air temperature. It is clear that the propagation velocity decreases to zero when the air temperature is preheated to a critical value (Tair = 700 K) under the corresponding operating conditions (uin = 0.1 m/s). Then the critical temperature decreases with decrease in the inlet velocity of the fuel gas. This indicates that a stable flame front can be obtained by preheating the air temperature to a critical value for extra LCFG combustion in the Al2O3 ceramic foam burner. The flame front stability is also demonstrated in Fig. 10. The temperature profiles on the axis of the burner are plotted as a function of the time under the corresponding operating conditions. The flame front positions, located upstream of the dashed line, completely coincide with each other for the different times. When the combustion time increases to 1600 s, the flame front is still located at the burner inlet, and does not propagate further downstream. From theoretical analysis, the flame propagation velocity can be defined as a function of adabatic combustion temperature [30]: 2

 u f   Tad 1      ut   Ts ,i

2

 4hvweff  s   2    g CPg u g 

(13)

where ut   C pg  g uin 1    C ps  s  is the thermal wave velocity, Tad is adiabatic combustion temperature rise, Ts ,i is temperature rise in combustion wave, and hvw is volumetric heat loss coefficient of burner to surrounding. Interestingly, for the ultra-lean combustion, compared with the term

 T

ad

Ts ,i  , the 2

intercept term value is very small as most of the combustion gas is air (4 hvweff  s    g CPg u g  ). 2

ACCEPTED MANUSCRIPT This is proved by the research results obtained by Bubnovich et al [33]. Then, the intercept can be ignored, and equation (13) becomes: 2

 u f   Tad 1      ut   Ts ,i

  

2

(14)

For operating conditions ofφ= 0.3, uin = 0.2 m/s, h = 80 W/ (m2K), and Tair = 750 K, the maximum temperatures for the adiabatic combustion (1419.7 K) and for the simulation (1471.2 K) can be found in Fig. 7. It is clear that the two temperatures are almost identical. According to equation (14), Tad  Tad  Tin is almost equal to Ts ,i  Tmax  Tin , and

1  u

ut   1 . Then 2

f

u f ut is nearly equal to zero, that is, u f tends to zero. Since the growth velocity of flame inclination amplitude X

is found to be proportional to flame propagation velocity, namely

X  0.4u f [32]. Then X  0 . Therefore, a stable combustion flame is obtained at Tair = 750 K under the operating conditions of φ= 0.3, uin = 0.2 m/s, and h = 80 W/ (m2K). 3.5. Influence of Wall Heat Loss on the Flame Front In order to elucidate the influence of the wall heat loss on the flame front inclination, the reaction rate contours of methane were adopted to show the flame fronts in Fig. 11 under the conditions ofφ= 0.17, uin = 0.2 m/s, Tair = 300 K, and t = 800 s. Overall, the flame front inclination is clear for the different wall heat loss conditions. Interestingly, when the wall heat loss h = 0 W/(m2K), the flame front inclination still takes place as the combustion time is about 800 s (Fig. 11a). Its inclination is much smaller than those with wall heat loss. This result further confirms that the start-up of flame front inclination is independent of the wall heat loss burner. With increase in the wall heat loss, the flame front is gradually stretched further, and the flame front inclination gradually increases (Fig. 11b–d). If the wall heat loss increases further, the inclination of the flame front will become faster. This indicates that the wall heat loss can accelerate the flame front inclination, which results in the enlongation of the flame front. Interestingly, it is found that the

ACCEPTED MANUSCRIPT flame fronts are located at almost the same position of x = 0.013 m for different wall heat losses ( using the middle point of the reaction zone as a refference). This indicates that the flame fronts propagate to the same position for different wall heat loss conditions. Hence the wall heat loss has little influence on the flame front propagation velocity. To understand further the effect of the wall heat loss, the flame front propagation velocities for different wall heat loss conditions are plotted as a function of the preheated air temperature atφ= 0.3 and uin = 0.2 m/s, as shown in Fig. 12. It is obvious that the flame propagation velocities for different wall heat loss conditions coincide with each other for the same operating parameters. From theoretical aspects, the relationship between wall heat loss and flame propagation velocity can be expressed by equation (13) [30]. The influence of the wall heat loss h is reflected by the volumetric heat loss hvw in the intercept 4hvweff  s

 C g

u g  . Interestingly, for ultra-lean 2

Pg

combustion, the value of intercept term is very small as it is analyzed in the third paragraph of section 3.4. Hence the intercept has little influence on the term of uf. This means that the wall heat loss has little influence on the flame propagation velocity for stable combustion. It is also reported by Henneke et al [50]. Through detailed analysis, it is found that the decrement of the propagation velocity gradually decreases with the preheated air temperature increasing. The variation trend gives a parabolic change. 5. Conclusions The flame front stabilization of extra LCFG combustion with preheated air was investigated numerically in an Al2O3 ceramic foam burner. The effects of the preheated air temperature were investigated by focusing on flame inclination and propagation. The effects of wall heat loss on the flame stability were discussed for different preheated air temperatures. The numerical flame instability agrees well with the results of the experiments. The results show that super-adiabatic

ACCEPTED MANUSCRIPT combustion is achieved for the extra LCFG combustion with preheated air in the ceramic foam burner. The maximum combustion temperature gradually increases while the super-adiabatic ratio gradually decreases with increase in the preheated air temperature. The flame front gradually moves upstream, and its propagation velocity and inclination gradually decrease as the preheated air temperature increases. A stable combustion flame can be realized by preheating the air to a critical temperature, which decreases with increase in the equivalence ratio and decrease in the inlet velocity. The flame front inclination gradually increases with increase in the wall heat loss. The wall heat loss promotes the flame inclination, but has little influence on the propagation velocity. This can provide a useful basis for the design and operation of porous burners, especially for the clean combustion of LCFG. Acknowledgements The authors would like to thank Mathias Etzold and Y. Han of FMP Technology GmbH for their support during the first author visit to their company. This work was supported by NSF of Zhejiang Province of China (Grant No. LY15E060007) and NSF of China (Grant No. 11574067). References [1] Howell JR, Hall MJ, Ellzey JL. Combustion of hydrocarbon fuels within porous inert media. Prog Energ Combust 1996; 22(2):121–45. [2] Keramiotis C, Seltzer B, Trimis D, Founti M. Porous burners for low emission combustion: An experimental investigation. Energy 2012; 45(1):213–19. [3] Ghorashi SA, Hashemi SA, Hashemi SM , Mollamahdi M. Experimental study on pollutant emissions in the novel combined porous-free flame burner. Energy 2018; 162 (1):517–25. [4] Bani S, Pan J, Tang A, Lu Q, Zhang Y. Numerical investigation of key parameters of the porous

media

157(15):969–78.

combustion

based

Micro-Thermophotovoltaic

system.

Energy

2018;

ACCEPTED MANUSCRIPT [5] Contarin F, Saveliev AV, Fridman AA, Kennedy LA. A reciprocal flow filtration combustor with embedded heat exchangers: numerical study. Int J Heat Mass Tran 2003; 46 (6):949–61. [6] Mujeebu MA, Abdullaha MZ, Mohamad AA. Development of energy efficient porous medium burners on surface and submerged combustion modes. Energy 2011; 36(8):5132–39. [7] Meadows J, Agrawal AK. Time-resolved PIV of lean premixed combustion without and with porous inert media for acoustic control. Combust Flame 2015; 162 (1):1063–77. [8] Kamal MM, Mohamad AA. Enhanced radiation output from foam burners operating with a nonpremixed flame. Combust Flame 2005; 140 (3):233–48. [9] Mujeebu MA, Abdullah MZ, Baka MZA, Monhamad AA, Abdullah MK. A review of investigations on liquid fuel combustion in porous inert media. Prog Energ Combust 2009; 35 (2):216–30. [10] Kayal TK, Chakravarty M. Combustion of suspended fine solid fuel in air inside inert porous medium: a heat transfer analysis. Int J Heat Mass Trans 2007;50(17–18):3359–65. [11] Trimis D, Durst F. Combustion in a porous medium- Advances and applications. Combust Sci Technol 1996; 121(1):153–68. [12] Weinberg F. Heat recirculating burners: principles and some recent developments. Combust Sci Technol 1996; 121(1–6):3–22. [13] Mujeebu MA, Abdullah MZ, Baka MZA, Monhamad AA, Muhad RMN, Abdullah MK. Combustion in porous media and its applications – A comprehensive survey. J Environ Manage 2009; 90(8):2287–312. [14] Huang R, Cheng LM, Qiu KZ, Zheng CH, Luo ZY. Low-calorific gas combustion in a two-layer porous burner. Energ Fuel 2016; 30(2):1364–74. [15] Al-attab KA, Ho JC, Zainal ZA. Experimental investigation of submerged flame in packed bed porous media burner fueled by low heating value producer gas. Exp Therm Fluid Sci 2015; 62:1–8. [16] Song FQ, Wen Z, Dong ZY, Wang EY, Liu XL. Ultra-low calorific gas combustion in a

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ACCEPTED MANUSCRIPT [40] Hoffmann JG, Echigo R, Yoshida H, Tada S. Experimental study on combustion in porous media with a reciprocating flow system. Combust Flame 1997; 111 (1):32–46. [41] Wang GQ, Luo D, Ding N, Huang XF, Xu JR. Two-dimensional combustion flame profiles in porous media with ultra low-calorific gases. J Chem Ind Eng (China) 2012; 63 (6):1893–901. [42] Fu X, Viskanta R, Gore JP. Measurement and correlation of volumetric heat transfer coefficient of cellular ceramics. Exp Therm Fluid Sci 1998; 17:285–93. [43] Viskanta R, Younis LB. Experimental determination of the volumetric heat transfer coefficient between stream of air and ceramic foam. Int J Heat Mass Tran 1993; 36(62):1425–34. [44] Bian BH. Calculation and analysis of radiation and heat transfer. High Education Publish House. Beijing, China; 1988:344–45 (In Chinese). [45] Kaviany M. Principles of heat transfer in porous media. Springer-Verlag, NewYork, US; 1991:48–50. [46] Garrido GI, Patcas FC, Lang S, Kraushaar-Czarnetzki B. Mass transfer and pressure drop in ceramic foams: A description for different pore sizes and porosities. Chem Eng Sci 2012; 63(21): 5202–17. [47] Pickenäcker O, Wawrzinek K, Trimis D, Pritzkow EC. Innovative ceramic materials for porous-medium burners II. Interceram: Int Ceram Rev 1999; 48 (6): 424–33. [48] Bodla KK, Murthy JY, Garimella SV. Resistance network–based thermal conductivity model for metal foams. Computational Materials Science 2010; 50(2):622–32. [49] Westbrook CK, Dryer FL. Simplified reaction mechanisms for the oxidation of hydrocarbon fuel in flames, Combust Sci Technol 1981; 27(1)31–43. [50] Henneke MR, Ellzey JL. Modeling of filtration combustion in a packed bed. Combust Flame 1999; 117(4):832-40.

ACCEPTED MANUSCRIPT

combustion zone

r Preheated Air + CH4

o

20 ppi Al2O3 Flue gas

x

Fig. 1. Computational domain of the physical model.

`radial distance r/m radial distance r/ mRadial distance r/ mdistance radial radial r/ m distance r/ m

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0.01

0.03 0

0

0.02

0.03

0.04

0.03 0

0.07

0.08

0.09

ω1/ (kmol/(m3s))

0.15 0.2 0.25 0.05 0.07x/m0.08 0.09 axial0.06 distance

0.3

0.35

0 0.01 0.02 0.05 0.03 0.1 0.04 b. t = 800 s

0.2 0.08 0.25 0.050.150.06 0.07 0.09 axial distance x/m

0.3

0.35

0 0.01 0.02 0.05 0.03 0.1 0.04 c. t = 1200 s

0.2 0.08 0.050.15 0.06 0.07 axial distance x/m

0.25 0.09

0.3

0.35

0.15 0.2 axial distance x/m

0.25

0.3

0.35

0

0

0.06

0.05 0.1 a. t = 400 s0.03 0.04 0.01 0.02

0.03

0.03

0.05

0

d.

0.05 t = 1600 s

0.1

Axial distance

x/m

Fig. 2. Simulation flame front inclination process during the propagation (φ = 0.3, uin = 0.2 m/s, Tair = 300 K, h = 80 W/ (m2K)).

ACCEPTED MANUSCRIPT

Computer

S style Thermocouple

350 mm

Quartz tube

20 ppi ceramic foam 40 ppi ceramic foam

Air tank Air compressor

Valve

Valve Premixer

Mass flow controller CH4 tank

Fig. 3. Schematic of the experimental setup.

ACCEPTED MANUSCRIPT

Flame inclination

t = 400 s

Flame inclination

Radial distance

r/m

a.

b.

t = 800 s

Flame inclination

c.

t = 1200 s

Flame inclination

d.

t = 1600 s

Axial distance

x/m

Fig. 4. Experimental flame front propagation instability (φ= 0.3, uin = 0.2 m/s, Tair = 300 K).

ACCEPTED MANUSCRIPT

Propagation velocity mm/s

0.35 0.30 uin=0.43 m/s

0.25 0.20 0.15 0.10

Our simulation, ceramic foam Our experiments, ceramic foam Simulation by Zheng et al, alumina pellets Experiment by Shi et al, alumina pellets Simulation by Shi et al, alumina pellets Experiment by Kennedy et al, alumina pellets Simulation by Henneke et al, alumina pellets uin=0.29 m/s hv=600 W/(m3.K) uin=0.4m/s

uin=0.2 m/s

h=30 W/(m2.K)

h =80 W/(m2.K) uin=0.29 m/s

0.05

uin=0.2 m/s

Tair=300 K

0.00

uin=0.25 m/s

0.1

0.2 0.3 Equivalence ratio φ

0.4

0.5

Fig. 5. Comparison of flame propagation velocities with those of other workers.

Frame 001  22 Apr 2018  title

ACCEPTED MANUSCRIPT

0.03 0.03

300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1470 T / K 400 500 600 700 800 Front 900 1000 1100 1200 1300 1400 1560 T/K 0

1300

1200 110040 1000 1300

900

700 1300

140 0

800 1400

1200

1000 1100600

00

14 1400 70

14

Radial distance r/m r/ m distance r/ m / m distance radial radial distance radial r/ m distance rradial radial distance r/ m

Front

00 T/K 0 0300 400 0.05 0.20.21200 1300 0.25 0.31502 0.35 500 600 0.1 700 900 1400 1430 0.05 0.1 800 0.15 0.151000 1100 0.25 0.3 0.35 a. Tair = 300 K axial distance x/m axial distance x/m Front 0.03 600

Front

14

500

0

0.03

1430

30

130

1400

0

1 2 0 1100 0

700 800

900

1000

400 500 600 700 800 900 1000 1100 1200 1300 1400 1450 0 0.05 0.1 0.15 0.2 0.25 0.3 Front b. Tair = 500 K axial distance x/m Front

T/K

0.35

600 1450

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130

1200

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900

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110 0

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500 600 0.1700 8000.15 900 1000 0.2 1100 1200 0.25 1300 1400 1470 0 300 4000.05 0.3 T / K0.35 c. Tair = 700 K axial distance X /m Front 0.03 600 14

0

0

70

d.

1400

0.05

1300

Tair = 750 K

1200

0.1

1100

100

0

900

800

0.15 0.2 axial distance x/m Axial distance x/m

700

0.25

0.3

0.35

Fig. 6. Effect of preheated air on flame front and temperature profiles (φ= 0.3, uin = 0.2 m/s, h = 80 W/(m2K), t = 1600 s).

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1400

uin=0.25 m/s,φ =0.3

1200

2.0 φ =0.3 uin=0.2 m/s h =80 W/(m2.K) t =1600 s

1000

400 200

1.6

1.2

800 600

Fig. 7.

2.4

Adiabatic combustion temperature Our simualting temperature Our Experimental temperature Experiment by Kennedy L.A. Superadiabatic ratio

300

400 500 600 700 Air preheated temperature T/K

Superadabatic ratio

Maximum temperature T/K

1600

0.8

0.4 800

Maximum temperatures and super-adiabatic ratios for different preheated air (φ= 0.3, uin = 0.2 m/s, h = 80 W/(m2K), t = 1600 s).

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Propagation velocity uf / mm/s

0.25

Tair=300 K Tair=500 K Tair=700 K

0.20

Tair=750 K uin=0.2 m/s

0.15

h =80 W/(m2.K)

0.10

0.05

0.00

the critical value 0.1

0.2

0.3

0.4

0.5

Equivalence ratio φ

Fig. 8. Influence of preheated air temperature on flame propagation velocity.

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Propagation velocity uf / mm / s

0.12

uin =0.1 m/s uin =0.2 m/s

0.10

uin= 0.3 m/s

0.08

φ =0.3 2

h =80 W/(m .K) 0.06 0.04 0.02 0.00 300

400

500

600

700

800

Air preheated temperature Tair / K

Fig.9. Influence of velocity on flame propagation velocity.

ACCEPTED MANUSCRIPT 1600

Axial temperature T/K

φ =0.3 uin =0.2 m/s

1400

h =80 W/(m2.K) Tair = 750 K

1200 1000

t =200 s t =400 s t =600 s t =800 s t =1000 s t =1200 s t =1400 s t =1600 s

800 Flow direction

600 400 0.00

0.05

0.10

0.15 0.20 0.25 Axial distance x/m

0.30

0.35

0.40

Fig. 10. Temperature profiles on the axis for 750 K preheated air.

Radial distance r/m Radial distance r/m distance r/m Radial distanceRadial r/m distance r/m Radial

ACCEPTED MANUSCRIPT 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

0.03 0

0

0.03 0

0.1

0.3

0.35

0 0.05 0.1 0.15 0.2 0.25 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 2K) b. h = 10 W/(m Axial distance x/m

0.3

0.35

0

0.3

0.35

0.3

0.35

0.05

0.005 0.01 2K) 0.015 c. h = 45 W/(m

0.1 0.02

0.03 0

0.15

0.2

0.25

a.0.005 h = 0 W/(m 0.012K)0.015 0.02 0.025 0.035 Axial0.03 distance x/m 0.04 0.045

0.03 0

0.05

ω1/ (kmol/(m3s))

0

0.05

0.1

d. h = 80 W/(m K) 2

0.15 0.2 0.025 0.03 0.035 Axial distance x/m

0.15

0.2

0.25 0.04 0.045

0.25

Axial distance x/m Axial distance

x/m

Fig. 11. Influence of wall heat loss on flame front (φ= 0.17, uin = 0.2 m/s, Tair = 300 K, t = 800 s).

ACCEPTED MANUSCRIPT Flame propagation velocities uf /mm / s

0.12

h =0 w/(m2.K) h =20 w/(m2.K) h =50 w/(m2.K) h =80 w/(m2.K)

φ =0.3 0.10

uin =0.2 m/s

0.08

0.06

0.04

0.02

0.00 300

400

500

600

700

800

Air preheat temperature Tair/ K

Fig.12. Influence of wall heat loss on flame propagation velocity.

ACCEPTED MANUSCRIPT ► Flame stability of LCFG combustion with preheated air was studied in a porous burner. ► A temperature rise of preheated air inhibits flame inclination and its propagation. ► A stable flame can be obtained by preheating the air to a critical temperature. ►Wall heat loss boosts flame inclination, but has little effect on propagation speed.