NO X MODELING IN POROUS MEDIA COMBUSTION BURNER

Abstract

The modern society is driven by the need of manufacturing, transportation, heating and electricity. Combustion of fossil fuels has met approximately 90% of these energy needs. However, the steadily increasing use of fossil fuel has threatened the sustainability of life on the earth, since combustion of fossil fuels leads toy ncrease~the emission levels in the atmosphere at an alarming rate. Hence, to reduce the pollution level, there is always a demand for development and use of modem efficient technology for combustion of fossil fuel. Moreover, the present trend of depletion of fossil fuel reserves demands an efficient, cost effective combustion device, so as to minimize losses and conserve energy. In recent times, many researchers have developed many newer methods for efficient combustion of fossil fuel. Porous media combustion (PMC) is one such effective method, which can increase the system efficiency as well as minimize environmental pollution. This technology is entirely different from conventional combustion, which is characterized by a free flame, this reaction zone and high temperature gradients. It has got several advantages over conventional combustion systems. In conventional combustion device, the entire combustion takes in the gaseous environment, whereas in porous media, combustion takes place in a 3-D solid porous matrix having interconnected pores. Compared to conventional combustion device, the combustion efficiency of porous medium combustion is reasonably high. This increase in efficiency is the outcome of better transfer through solid to solid conduction and high radiation from the heated solid surface. Many experimental investigations have been carried out to get better insight of the phenomena of combustion in a porous medium combustion burner. Work has also been carried out to study the phenomena numerically. It has been observed from the literature review that very small amount of work has been carried out for modeling of two zone porous media combustion burner. In the present work the porous medium combustion burner has been modeled mathematically and the model is to analyze the combustion behaviour of such a burner. 111 Overview of the Thesis 1'W vG y~ Two types of burners -_ -: , be,,considered in the present study, viz, rectangular and cylindrical. In these burners two zones have been considered, the preheat zone L, <_ x _< L2 and the combustion zone L2 <_ x <_ L3 . In these zones, the porous material is same, however the porosity of the material is different. The porosity in preheat zone is less than that in the combustion zone. The outline of the present thesis is as follows Chapter 1 is introductory in nature and it contains a brief description of porous media combustion burner, characteristics of combustion in porous media, the materials for the porous media combustion and NO formation mechanism in combustion. In Chapter 2, a review of literature is presented. In Chapter 3, the governing equations of mass, momentum, energy and species are written for gas phase, while energy equation is written for solid phase. The appropriate boundary conditions relevant to the problem are prescribed. In this chapter, brief outline relevant to numerical solvers (finite volume method, simple algorithm) is also given. In Chapter 4, model for porous media rectangular burner is considered for investigation. The temperature distributions within gas and solid porous material, species distributions, and NO emissions from the burner , _5 determined for various values of thermal load, equivalence ratio, porosity and thermal conductivity of the porous material. It is observed that for the same thermal load, the peak gas and solid temperature does not vary much. The variation in temperature is of the order of 2.5% for 25% variation in the equivalence ratio. This is because, the equivalence ratio was varied, for fixed thermal load, by keeping the mass flow rate of fuel fixed while varying the air flow rate. Thus for the same energy input, the temperature do not vary much. As the thermal load increases, the peak temperatures also increase. - Also, the combustion region moves downstream with increase in load. For increasing thermal load, the mass flow rate of fuel is increased with corresponding increase in the air flow rate. With increased mass flow rate the region for stable combustion moves downstream. It is observed that with increase in porosity, the peak temperaturof solid and gas decrease and the combustion region moves downstream. For the same load, increasing porosity result0in an increased convective heat transfer area leading to increased heat iv transfer from the gas to the solid. This results in lower gas temperature. Also with increased porosity the resistance to flow through the porous media decreases resulting in the downstream movement of the stable combustion zone. With increase in thermal conductivity of the solid porous material, the peak gas and solid temperatures decrease and the combustion zone moves upstream. As thermal conductivity of the solid porous matrix increases, the heat conduction rate within porous material increases, leading to higher feed back of thermal energy from the solid porous material to the unburned fuel air mixture. Thus, the combustion zone moves upstream. The concentration of oxygen and methane remains constant in the preheating zone and decreases to zero in the combustion zone. The concentration becomes zero at a distance from the entrance which decreases as the equivalence ratio increases. For a fixed load, as explained earlier, the equivalence ratio is varied by decreasing the mass flow rate of air. As air mass flow rate increases, the combustion zones moves upstream where the concentration of fuel and air species decreases to zero. Results have also been obtained for NO X emissions for thermal load, equivalence ratio, solid thermal conductivity and porosity of the porous material. The results differ by less than 4% when compared with the experimental results :- . [56]. In Chapter 5, two dimensional model was developed for rectangular porous media burner. The momentum equation in axial and radial directions as alsoincorporated-in the model. The distributions of temperature, mass fraction of fuel and oxidizer and the,,NOX emission are obtained for various loads, equivalence ratio, solid thermal conductivity and porosity of the porous material. The computed results have been observed to be in agreement with the experimental values [56]. In Chapter 6, the two dimensional model developed in Chapter 5 was solved for rectangular burner by taking into account the heat loss to the walls of the burner and also considering the variation of viscosity of gas with temperature. The results from the one- dimensional model and two-dimensional models without the variation of viscosity with temperature were compared. These results have been compared with the experimental results. It was found that computed results are within 2% of the results obtained from experimental investigation. In Chapter 7, a 2-D model for cylindrical porous media burner is considered for investigation. Two-dimensional conservation equations for mass, momentum, energy and v chemical species with appropriate boundary conditions were solved. The finite volume method was used to solve the model for methane combustion with air. Results are presented for temperature distribution, mass fraction and NO emission, to assess the effect of thermal load, equivalence ratio, porosity, thermal conductivity. In Chapter 8, we have extended the work of Chapter 7 by considering the heat loss at the wall of the burner and the variation of viscosity of gas with temperature. Numerical investigation has been done for the effect of thermal load, equivalence ratio, porosity and thermal conductivity on temperature profile, mass fraction and NO formation. It has been found that an increase in thermal load at a fixed equivalence ratio the peak temperature increases and combustion region moves downstream. An increase the thermal conductivity at a given thermal load, the peak temperature decreases and combustion region moves upstream. An increas the porosity at a fixed thermal load, the peak temperature decreases and combustion region moves downstream. These results have been compared with the experimental results. It was found that computed results are within 3% of the results obtained from experimental investigation [10]. In Chapter 9, the conclusions and future scope are presented.