Characteristics Of Porous Media Combustor With External Swiss-roll

The exhaust gas in the industrial processes contains little grade fuels (bases is methane). At such low concentration, it is difficult to convert this methane gas mixture to usable forms of energy with conventional technologies and is generally emitted into air. Although its concentration is very low, the discharge amount is huge. So large amounts of methane are discharged into the atmosphere with the exhaust gas, and it not only produces wasted energy resource but also adds undesirable pollutants to the atmosphere. Methane is also a greenhouse gas and its global warming potential is 21 times higher than that of CO2. The development of low concentration exhaust gas direct combustion technology can not only save energy, but it is important that it can reduce discharge. Therefore, it is very significant to explore sustain combustion of lean fuel for controlling environment pollution and using energy. Base the theory of excess enthalpy combustion, a new type porous media combustor with external Swiss-roll (abbreviated as SRPC) taking advantage of Swiss-roll combustor and porous combustor for combusting the low concentration fuel was proposed in the paper. For designing the very low concentration methane reactors, the high temperature oxidation characteristic of ultra-lean methane was theoretically investigated. The design method of the SRPC was studied and the combustion characteristics of lean gas mixture in the SRPC were carried out by not only experiments under cold state and thermal state, but also numerical simulations in this paper. Further, the SRPC used for conversion of natural gas to hydrogen under super adiabatic rich combustion is put forward in the paper and the kinetics simulation and the primary experiment was carried out.(1) For exploring the method of sustain combustion of lean fuel, the present methods of reducing the lean flammability limits of fuel were summed up and classified. It was concluded that raising the combustion temperature and putting catalyst are relatively effective methods. The method of rising the combustion temperature was based the idea of excess enthalpy combustion which can realize regenerative reheat cycle and recycle the combustion heat by heat interchanger and porous media heat accumulator. If the level of combustion temperature is raised to a certain extents, it can realize sustain combustion. The method of putting catalyst is to reduce the activation energy of the fuel, improve the ignition condition and extend lean flammability limits. Or the two combined combustion method is employed to reduce the lean flammability limits fatherly for the dispose and heat utilization of the exhaust gas. Based on the above analysis, a new type porous media combustor with external Swiss-roll bearing both advantages of Swiss-roll combustor and porous combustor was proposed in the paper.(2) For designing the very low concentration methane reactors, the high temperature oxidation characteristic of ultra-lean methane is theoretically investigated by the numerical simulations of methane oxidation in a perfectly stirred reactor PSR and plug flow reactor PFR with detailed reaction kinetics GRI 3.0. The effect of the process parameters such as methane concentration, residence time, preheating temperature, and heat loss of the oxidizer is explored. Results show that these parameters have important effect the lean methane oxidation. First, due to the low heat release of lean methane per unit volume, the temperature increase of the mixture is very low and the chemical reaction increases which cause the complete oxidation to be difficult. Therefore, in order to increase the oxidation speed of low concentration methane, it is necessary to warm up the lean methane mixtures to a certain temperature for increasing the oxidation speed. On the other hand, because the heat loss speeds of the complete oxidation of the mixture per unit volume is directly proportional to the oxidation temperature. Moreover, the heat release speed decreases with the decrease of the methane concentration. If the heat loss speed is faster than the heat release speed, the reaction temperature would decrease and it causes the reaction to stop. The crucial methods to control the heat loss speed are to strengthen heat preservation, reduce the contact area and heat transfer coefficient between the reactor and the environment and adopt heat recycle to reduce the heat loss of the exhaust and warm the lean methane mixture. The ability of low concentration methane oxidization is decided by gas velocity. The results provide some guidance for the design of ultra low concentration methane oxidation device.(3) The experimental study on the flow and combustion characteristics of uni-directional direct current porous media combustor without Swiss-roll structure was carried out. The effect of flow rate and pore density (PPI) on pressure drop were tested.Results show the resistance to flow increases with decrease of pore density and the increase of flow velocity. The mechanism of resistance to flow in foams ceramic and beds made of spherical particles was analyzed. It draws a conclusion that the resistance to flow in the two types of porous media is equal in the condition of the same surface area and same porosity. Based on the conclusion, the structure model of foam ceramic was established and the functional relations between PPI and particle diameter was created. From the functional relations and classical Ergun model for beds made of spherical particles, the resistance to flow in foams ceramic is calculated with two parameters (PPI, Porosity) provided by the manufacturer. The proposed model was successfully validated by experimental results, which fully confirm the validity of the model. The effect of flow rate and the number of channels on pressure drop were tested. The formulas for calculating permeability resistance and inertia resistance coefficient was obtained and it can provide parameters for simulation of foam ceramic porous media combustion. The experiments of gas mixture combustion in the porous with different materials and pore size were carried out.(4) The rectangle Swiss-roll heat exchange channels was made by split joint of many armor plate and cold state experiment was performed on it. The effects of flow rate and the number of channels on pressure drop were tested. Results showed that the pressure in the channels of the combustor increased with the increase of the distance from the inlet of gas. The local resistances caused by bend angle affected on-way resistance very much. The total pressure drop increased with the increase of the number of bidirectional countercurrent channels and flow rate. In the range of the experiment, the correlation of total pressure drop and the flow rate showed nonlinear relationship between line and parabola. Based on the formulas of line resistance and local resistance, the flow resistance of Swiss-roll combustor was described mathematically by synthetical resistance coefficient. A pressure drop formula was acquired by regression of experimental data. The calculation results form this formula agreed well with the experimental data. It can provide reference to the design and operation of the burner. Meanwhile, the method can also be widely applied to researches on the flow resistance of multi-elbow structures used in the electrical, water conservancy and chemical industries. The experiment results can provide necessary control parameters for the optimization design of the Swiss-roll structure, the preliminary selection of blower, first ignition and safe operation in the thermal state. Meanwhile, it can provide a test method for determining the sealing property of the combustor.(5) According to the problem of prototype sample, rectangle Swiss roll combustor, spiral-plate heat exchanger with internal alundum tube in the process of manufacture and experiment, the design proposal and manufacture method were continuously optimized. Finally an optimal method of design and manufacture was obtained. Based on the method, a SRPC was manufactured and some combustion experiments research were carried out. The temperature profiles and pollutant emission were measured in the startup process of preheating by ignition, and at various working parameters including fuel concentration and gas flow rate. The influences of the major parameters on the combustion characteristics of SRPC are analyzed. The experiments results shows the optimal design method met the challenge of convenient processing, leak tightness and so on. In the situation of stable combustion, the SRPC has higher efficiency wide range of adjustment and better flame stabilization than the combustor without Swiss-roll structure in the condition of the same methane concentration of premixed gas. For the actual project account, the optimal design method of unit combustor was proposed.(6) The SRPC used for conversion of natural gas to hydrogen under super adiabatic rich combustion is put forward, which can solve the problems of can solve the problems of flame drift, heat preservation, product cooling, and low transform efficiency. Due to its simple and compact structure, it is attractive for distributing hydrogen production system and solving the transportation and storage problems of hydrogen. The kinetics investigation on the hydrogen production from rich combustion of methane was carried out. The simulating results show that the key to increase hydrogen production is to increase residual time, raise the reaction temperation, and choose appropriate equivalence ratios. The primary rich combustion experiment was investigated over the methane-air mixture with equivalence ratio range of 1.5 to 5 and total flow range of 60 to 120 L/min. The combustion characteristics of auto-thermal reforming were studied. The experimental results show that it can realize the self-maintenance combustion in a certain extent and come to self-stabilization fast in the fuel-rich conditions. The preheating effects of Swiss-roll structure and the heat storage of porous media can raise the react temperature to more than 1600K, which realizes superadiabatic rich combustion.(7) The progress of gas mixture combustion in the SRPC was simplified reasonably, and physical models and mathematical model of the SRPC were built. Then two-dimensional numerical simulation of the SRPC for methane/air mixture combustion was conducted. And the veracity and feasibility of numeral combustion model could be verified by comparing the simulation and experimental results. The influence rules of different inlet velocity of flow and methane concentration on combustion characteristics. The simulation results were in qualitative agreement with experiment.