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Two-dimensional experimental study of superadiabatic combustion in a packed bed burner Huaming Dai, Baiquan Lin, Kaige Ji, and Yidu Hong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01046 • Publication Date (Web): 06 Aug 2015 Downloaded from http://pubs.acs.org on August 7, 2015
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Two-dimensional experimental study of superadiabatic combustion in a packed bed burner Huaming Daia,b*, Baiquan Lina,b, Kaige Jic, Yidu Honga,b (aSchool of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China; bState Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou 221116, China; cSchool of medical technology, Xuzhou medical college, Xuzhou 221004, China)
ABSTRACT：High-efficiency use of low-concentration coal mine methane is favorable for energy saving and reduction of greenhouse gas emissions. During the porous media combustion, abundant methane at extremely low concentrations will be used not through stable combustion, but through superadiabatic combustion, which transmits flames to the downstream. To directly study the superadiabatic combustion of low-concentration methane, we set two-dimensional (2D) temperature measuring points in the burners, extended the temperature measurements via a radial basis function (RBF) to the whole burner, and plotted a 2D temperature distribution. Then the effects of working conditions, pellet diameter, and burner length on the superadiabatic combustion of lowconcentration methane were investigated. The results show that unstable phenomena such as flame rupture and inclining occurred during the combustion wave propagation in the porous medium. The effects of the heat dissipation through burner walls and the heat dissipation from the outlet on the 2D temperature distribution provide a basis for validation of heat dissipation coefficient in numerical modeling. When the equivalence ratio was 0.35, the increase of flow velocity (from 25 to 45 cm/s) accelerated the propagation velocity of the combustion wave from 0.10 to 0.25 mm/s, but the peak temperature of the flames was almost unchanged (about 1425 K). When the flow velocity of methane was 30 cm/s, the increase of equivalence ratio (from 0.25 to 0.35) reduced the propagation velocity of combustion wave from 0.22 to 0.14 mm/s, and significantly raised the peak temperature of flames from 1125 to 1375 K. With a smaller pellet diameter, the increase of pellet diameter is less favorable for acceleration of combustion wave propagation. When the burner length was prolonged from 20.8 cm and 25.6 cm at 4.8-cm interval, the propagation velocity of combustion waves declined by 11.4% and 7.3%, which shows that the prolonging of length was less favorable for deceleration of combustion wave propagation with a larger burner length. KEYWORDS：porous media; two-dimensional temperature distribution; low-concentration coal mine methane; combustion wave
1. INTRODUCTION Coal is a major energy source in China and accounts for about 70% of primary-energy consumption1. Since a great deal of methane is trapped in coal, the release of methane during coal mining may cause many accidents, such as methane explosion, coal and methane outburst, and methane combustion2. Thus, methane drainage, a practical method to extract the methane from coal, is necessary during coal mining, which guarantees mining safety. Along with the increase of coal production, the quantity of methane drainage is also growing annually, but the utilization rate of methane is still very low3. The quantity of methane drainage in China grew from 1.9 billion m3 in 2004 to 15.6 billion m3 in 2013, but the utilization rate only increased from 31.2% to 42.3%, and nearly 60% of methane was directly drained out as exhaust gas. Since the major component of coal mine methane (CMM) is methane, the emissions will cause resource waste and enhance the greenhouse effect. The low concentrations and large concentration/flow fluctuation make a large
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part of CMM unavailable. As a typical low calorific gas, the high-efficiency utilization of CMM is unachievable with the existing techniques. Compared with the traditional free flame combustion technology, the porous media combustion technology is superior with a broader lean burn limit range, higher efficiency, less emission, and faster flame speed4,5, which are favorable for combustion and utilization of low-concentration CMM. During the porous media combustion of low calorific gas, superadiabatic combustion will occur when the peak temperature in the combustion area exceeds the temperature of the adiabatic flames. At this moment, the heat generated during the porous medium backheating is more abundant than the heat dissipated from the burner5,6. The main burners used in superadiabatic combustion include one-section or two-section porous medium burners. During the lean combustion in a one-section porous medium, the flames will be stabilized in the porous medium under specific working conditions. This is an adiabatic status. When the inlet velocity was unchanged to keep the equivalence ratio smaller than that under this stable working condition, the combustion will be changed to the superadiabatic status. As reported, during the methane combustion in 5.6-mmdiameter alumina pellets, when the flow velocity is 0.25 m/s, the combustions with equivalence ratios < 0.45 are all superadiabatic7. During lean combustion in two-section porous medium at an equivalence ratio of 0.6 - 0.65, the superadiabatic and subadiabatic statuses occur simultaneously with the increase of inlet velocity, and the flame can be stabilized on the interface within a specific range of velocity6. During experiments and simulation of methane combustion in a two-section porous medium, the lean burn limit of stable methane combustion is 0.41, and thus at an equivalence ratio < 0.41, the superadiabatic combustion occurs when the combustion wave propagates to the downstream8. Stable propagation of combustion waves occurs during superadiabatic combustion with an equivalence ratio < 0.15, which proves that superadiabatic combustion of methane can be achieved at low equivalence ratios9. After the drained CMM and ventilation air methane were mixed, the methane concentration was improved to about 3% (Figure 1610). Thus, research on superadiabatic combustion is practically meaningful for utilization of low-concentration CMM. The research of porous media combustion is mainly based on experiments and numerical simulation, and during experiments, thermocouples are used to measure the temperatures in the porous burners, usually in a single direction11. When thermocouples were installed outside the circumference of the porous ceramics, the 1D wall temperature distributions under different working conditions could be studied12. When a thermocouple was inserted at 27 mm away from the walls in a porous medium, the effects of installation of heat exchange tubes on the 1D temperature distributions in the burner were investigated13. When the equivalence ratio decreased from 0.394 to 0.286, the changes of temperature distribution at different time periods were investigated13. Thermocouples were installed at the burner's center to investigate the axial temperature distributions in different porous media14-16. At first, only 1D numerical simulation was used, and with the subsequent development of modeling, two-dimensional (2D) simulation was gradually promoted later. As reported, simplified reaction kinetics was combined with a 1D model for porous media combustion to study the propagation of flame in a porous radiant burner17. When a 1D porous medium model was added with detailed methane reaction kinetics (Gri 1.2), the difference in the reaction routes of rich burn and lean burn was numerically studied7. The axial productions of CO and H2 in rich burn were determined, and the effects of working conditions on the combustion wave propagation were investigated in a single temperature model and a double temperature model7. In a 2D numerical model, the effects of porous medium parameters on the axial temperature distribution,
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volumetric heat transfer coefficient, and gas velocity were investigated, and the 2D temperature distributions were discussed18. The temperature data measured by a 1D model and a 2D model were mutually validated, and the combustion status in the burner was photographed and compared with the 2D temperature distribution, and the inclining process of the flame in the burner was also investigated19, 20. The 2D temperature distributions during methane combustion were investigated on a 2D numerical model under different working conditions, and the results were compared with a 1D model21. The simulated results were 7.3% different in temperature and 2.7% different in the moving velocity of combustion waves21. Thus, the 2D temperature distributions in burners have been extensively studied via numerical simulation22, but rarely via experimental methods4,5,23. Since the 3D temperature distribution is difficult to be obtained with limited measuring points, the objective of this study is to experimentally investigate the 2D temperature distribution basing on the 2D axisymmetric assumption of the burner during superadiabatic combustion of lowconcentration CMM in a porous medium. Thus, 2D measuring points were set in porous medium burners, and then the measured 2D temperatures were extended via interpolation to the whole burners. During the experiments, alumina pellets were used as porous medium units, because the pellets could form discrete packed beds which are favorable for the location of 2D measuring points compared with the rigid ceramic foam or honeycomb ceramic5,11. In the experiments, the working conditions of low-concentration CMM were changed to study the combustion characteristic. Meanwhile, the effects of burner length (20.8, 25.6, 30.4 cm) and pellet diameter (6, 9, 13 mm) on the 2D temperature distributions during superadiabatic combustion were explored. Then the changes of combustion wave propagation with the burner length or pellet diameter were determined.
2. EXPERIMENT 2.1 Experimental material Alumina pellets (composition reported in reference24) with diameters of 6, 9, 13 mm were put in the burner randomly without any extrusion to form a porous medium region (Figure 1). The diameters of the pellet (dp), corresponding to the mean pore diameter of the ceramic foam (df), are the characteristic lengths and can be used to determine the radiation coefficient and the effective heat transfer coefficient in the packed bed25,26,38. The radiative heat transfer in porous medium can be simplified by Rosseland approximation. For the alumina pellets, the effective thermal conductivity of radiation is krad -p =
hv, p =
6λg (1 − ε p ) dp
2
32σ d pε pTp3 9(1 − ε p )
, and the volumetric heat exchange coefficient is
(2 + 1.1Rep0.6 Pr1/3 ) , where ε p is the porosity of packed bed, σ is the Stefan-
Boltzmann constant, the Prandtl number is Pr =
number is Rep =
μc p , μ is the viscosity of the gas, the Reynolds λg
ρg vg d p , vg is the velocity of the gas25,26; For the ceramic foam, the effective μ
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thermal conductivity of radiation is krad -f =
αf =
4.4(1 − ε f ) df
16σ Tf 3 3α f
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, where the radiation extinction coefficient is
, and the volumetric heat exchange coefficient is hv, f =
λg df
2
(CRe f m ) , where ε f is
the porosity of ceramic foam, C and m are the empirical coefficients corresponding to special ceramic foam structure, the Reynolds number is Re f =
ρg vg d f μ
36.
The mass density ( ρ p ) of the alumina pellets can be considered as constant: ρ p =3987 kg/m3. However, the specific heat ( C p ) and thermal conductivity ( λ p )of the pellets are related to the pellet 2 3 temperature ( Tp )26: C p =29.567+2.61177· Tp −0.00171· Tp +3.382×10−7· Tp J/(kgK), and λ p
2 =−0.21844539+0.00174653· Tp +8.2266×10−8· Tp W/(mK). The discrete structure makes the
pellets unstable in the burners. Thus, ceramic foams (Figure 1) with fixed shape and pore size = 60 PPI (pore per inch) were placed as support at the lower part of the burner (Figure 2). These ceramic foams, with the thickness of 50 mm, could also prevent flashback and disperse the gas uniformly across the cross-section of the burner28.
6 mm
9 mm
13 mm
60 PPI
Figure 1. Images of alumina pellets and ceramic foam with different sizes
The data about the porosity of the pellet-formed porous medium cavities as well as the 60 PPI ceramic foams are listed in Table 114. Clearly, the porosity gradually rises with the increase of pellet diameter. According to the calculation method of pressure loss19, the gas permeability in the packed bed increases with the pellet diameter. Thus, the pellet diameter reflects the particularity of packed bed and the study of burners with different pellet’s diameters contributes to the porous material selection during the burner design. It is also found that the porosity of the pellet-formed porous medium region is obviously smaller than that of the ceramic foams from Table 1. Table 1. Porosity of different porous media Alumina pellets/ceramic foam
Porosity
6 mm
0.43
9mm
0.46
13mm
0.52
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60 PPI
0.83
The low-concentration CMM used in the experiments was made via uniformly mixing the high concentration methane and air. The high concentration methane (Luling Coal Mine in the North Anhui Mining Area) was stored in high pressure tanks. Gas chromatographic analysis shows that the main combustible component in the high concentration methane gas is methane, and other components were ignored owing to the very small concentrations29. Thus, the equivalence ratio (φ) of methane/air mixture, which is a parameter exhibiting the ratio of experimental and stoichiometric fuel-oxygen mass ratio, is used to represent the concentration of combustible substances in the CMM29.
ϕ=
(mF / mO )exp (mF / mO ) sto
where mF is the mass of fuel, mO is the mass of oxygen, the subscript of exp represents the experimental fuel-oxygen mass ratio, the subscript of sto represents the stoichiometric fuel-oxygen mass ratio. 2.2 Experimental setup The experimental setup (schematic diagram in Figure 2) consists of five parts: a mixing and formulating system, a burner system, a data collection system, an ignition system, and a gas analysis system. The key components are introduced below. 1) Air tank: Its inlet was connected to an air compressor, and the outlet to a pressure stabilizer. The air tank (capacity 0.6 m3) can significantly reduce the gas pressure and flow fluctuation in the air compressor, and guarantees the precision of methane concentration formulation. 2) Flowmeter: This target-type flowmeter with digital display function can monitor pressure and flow rate simultaneously. The flow rates in the air route and the methane route are largely different. In order to improve the accuracy of the flowmeter, the range and precision were set as 0−80 m3/h and 1.5% (air route) and 0−3 m3/h and 1.5% (methane route). 3) An igniter, which can reignite the fuel gas rapidly (response time
