| Comparing with the traditional premixed, free-flame characterized combustion, the premixed combustion in porous media is a new-type, clean and effective technology. It can be used to implement steady combustion with fuel of low or extremely low calorific value. The porous media burner has advantages of stable combustion, higher flame speed, wider lean flammability limit and less pollutants emission. It also has superiorities on increasing combustion efficiency, extending the flammability limit, saving energy, protecting entironment and dealing with sorts of wastes. It can be widely used in various fields, such as metallurgy, chemical process, energy, building materials, food processing, household and so on. It is a completely different, novel and unique mode of combustion.This study is a foundational and applied research on combustion, flow, and heat transfer in porous media. The main content of this study is about premixed combustion of gas fuel in porous media burner. Experiments and numerical simulations are carried out for the finnal goal of practical application of the porous media burner.Experiments:1. An experimental platform was designed and built by our group. The experiments of premixed gas combustion in porous media are carried out by adding nitrogen in liquefied gas. In experiments, the combustion characteristics in different cross-sections of porous media are compared and, structure of porous media burner is improved according to the state of combustion. The improved burner, diameter of which is 100mm, can support the stable combustion under the gas consumption of 1.122 m3·h-1 and the firing rate of 1345kW·m-2. Due to the features of selected porous media, saying, the numbers of the big pore is 10 PPI (Pores Per Inch), and the small pore is 40～50 PPI, conclusions are made that, a long-term stable combustion can be supported by the maximum firing rate of 1200kW·m-2, and the radiating temperature of porous media under this condition is no higher than 1200℃.2. To compare with the free flame combustion under the similar conditions, an industrial experiment is also carried out. The combustion characteristics in porous media burner are investigated, including the propagation and stability of premixed flame, the combustion efficiency and the pollutant emission in porous media. The reference values of operational parameters are provided for industrial applications of the porous media burner, and the results of subsequent numerical simulation are validated by the results of experiment.Numerical simulations:1. A one-dimensional laminar flow model of the premixed combustion is built, and the industrial gas with combustible components, that is, blast furnace gas is used as the research target. The simplified reaction mechanisms of blast furnace gas are obtained by analyzing the sensitivity coefficient of temperature with respect to the reaction-rate, and the effect of primary constituents of blast furnace gas on reaction-rate. The time consuming on calculating the details kinetics of blast furnace gas is about 18 seconds, while that on calculating the simplified kinetics is less than one second. The later is absolutely superior than the former one from the aspect of calculation. Compared the results of detailed reaction mechanisms with simplified reaction mechanisms, the concentration differences of the primary constituents are less than 0.4%, and the simplified reaction mechanisms are validated.2. Using the simplified chemical reaction mechanism, the temperature and constituent profiles of combustion in porous media burner are compared with those of mathane/air combustion in traditional burner. For a given condition, the temperature difference at the outlet of porous burner is about 300K higher than that of traditional burner. During the combustion process, the concentration of CO in porous burner is about half of that in traditional burner. Superiorities of porous media burner are fully demonstrated by these comparisions.3. The two-dimensional, two-section combustion model of porous media burner is set up, the methane/air mixture combusting in porous media is simulated numerically by the software FLUENT. The user defined codes of C language are used to extend the ability of FLUENT and enable two-dimensional distributions of temperature and velocity to be obtained. Eeffects of the equivalence ratio, the extinction coefficient and the thermal conductivity of porous media on these distribustions are also investigated. The results show that the profile of pressure is not only affected by the equivalence ratio, the extinction coefficient and the thermal conductivity of porous media, but also closely related to the temperature and the combustion area. The variations of extinction coefficient lead to the great changes of the combustion area, while profiles of temperature and pressure are changed responsely. The peak value of temperature corresponds to the variation of pressure gradient.4. Comparing the simulation results of speed limit with the results of experiments and Barra’s simulation, it shows that our simulated values are higher than experimental values, and are lower than pre-published value. The reasons are:heat loss is neglected in simulation, and the expression of heat dispersion, which impacted speed limits in stable combustion greatly, is different in this research with that in literature.5. The effects of equivalence ratio, radiative extinction coefficient and thermal conductivity of the porous media on speed limit are investigated. The results show that:the stable flame stays upstream while inlet veloctiy is minimum, on the contrary, it stays in the downstream zone while inlet velocity is maximum; the stable operating range enlarges and its maximum and minimum shifts to the larger values as the equivalence ratio increases; compared with the reference case, the table operating range increases to 0.47 m·s-1 if the downstream conductivity is 5 times of that in reference case and the upstream conductivity keeps the same. In practices, radiative extinction coefficient in the upstream section is desired to be larger in order to obtain a larger stabilization range. |
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