New Type Energy||Renewable Energy

The rapid advances in technology and improved living standard of the society necessitate abundant use of fossil fuels which poses two major challenges to any nation. One is fast depletion of fossil fuel resources; the other is environmental pollution. The porous medium combustion (PMC) has proved to be one of the feasible options to tackle the aforesaid problems to a remarkable extent. PMC has interesting advantages compared with free flame combustion due to the higher burning rates, the increased power dynamic range, the extension of the lean flammability limits, and the low emissions of pollutants.

The fast depletion of fossil fuel resources and the environmental pollution are the major issues caused by the abundant use of fossil fuels. As the energy saved is more precious than the energy generated, the demand for various efficient and eco-friendly energy management schemes to be implemented in industrial commercial and domestic sectors become increasingly significant day by day. The porous medium combustion, hereafter abbreviated as PMC is proved to be one of the feasible options to tackle the aforesaid problems to a remarkable extent in both technical and economic perspectives. As an internally self-organized process of heat recuperation, combustion in porous media differs significantly from the homogeneous flames. This difference is attributed to the following main factors: (1) the highly developed inner surface of the solid porous medium results in efficient heat transfer between the combustible medium and the inert solid; (2) dispersion of the reactant flowing through a porous media increases effective diffusion and heat transfer between the phases. Thus an internal method of energy recuperation is activated. This process facilitates a combustion process that ensures stability in a wide range of reactant velocities, oxidant-fuel ratios, and power loads. As the combustion becomes more efficient the possibility of formation of NOx and CO becomes minimal. A porous material means a material with connected voids, where flow can easily penetrate through the medium.

Filtration combustion and excess enthalpy

Porous media combustion (PMC) pertains to the heterogeneous interaction between two different media, usually a solid and a gas. The stationary and the transient systems are the two major design approaches commonly employed in PMC. The first approach (stationary) is widely used in radiant burners and surface combustor-heaters due to high radiant emissivity of the solid. Here, the combustion zone is stabilized in the finite element of a porous matrix by the imposed boundary conditions and there is no flame propagation. The second approach(transient) to PMC leads to the ‘excess enthalpy’ flame theory where an unsteady reaction zone possesses the freedom to freely propagate as a combustion wave in the downstream or upstream direction due to positive and negative energy fluxes. This type of combustion within a porous inert media (PIM) is also known as filtration combustion.

Premixed and non-premixed combustion

PMC systems may work with a premixed flow (fuel and air been forced through one end), or, more rarely, with a non-premixed fuel flow that meets a back-diffusing air flow at the exit. Premixed porous burners consist of two sequential zones: the premix fuel/air stream first enters a fine-pore hot solid matrix (below the flame quenching size, for safety), where it is heated until it enters the second larger-pore hottest solid matrix, where alveolar flames stabilise themselves for a wide range of flow rates and air/fuel ratios. Depending on the application, a third section may be directly added to the burner, with a compact heat exchanger.

Infrared heating

Porous radiant burners are developed to utilize the benefits of infrared heating. Infrared heating is widely used by industry for manufacturing processes. The heat transfer rates for infrared heaters are determined by the temperature of the source and the absorption characteristics of the product being heated. Because the radiation output is proportional to the fourth power of the temperature, the temperature of the source dominates the performance of the device. This temperature depends on the emissivity of the radiating matrix, the rate of energy release of the reactants, and the heat transfer rates between the hot combustion products and the porous media. Another benefit of infrared heating is that it can provide much faster heating times than from convective heat transfer alone which relies on the temperature difference between the heated air and the surface of the product. These advantages applied to industrial applications can allow reduced oven lengths, increased conveyor speeds, and improved surface finish of products such as coated paper that might be sensitive to air currents associated with convective drying. While the radiative heat transfer rates can be high, the combustion temperatures of radiant burners are relatively low and the combustion is very efficient, allowing the emissions of NOx and CO to be low.

A review on applications on porous media combustion is being made. Yet many of the works have been left without citation due to page limitation but without any intention. More investigations have to be done on the application of PMC in gas turbines, IC engines, combustion of gaseous fuels with low heating value such as the producer gas, heat exchangers, district heating and room heating, bakery ovens and many other applications. The concept of PMC-based CHP is an excellent idea in terms of both energy efficiency and clean environment. Moreover, the recently introduced micro and meso-scale applications is a promising breakthrough in PMC technology, as this will result in miniaturized combustion systems for MEMS (micro electro mechanical system) applications. The need and wide scope for further research in all the aforementioned fields are quite obvious.