Abstract
This paper presents the effect of mass recirculation on the structure of steady state flame propagation in a uniform cloud of volatile organic particles. Based on large Zeldovich number or high reaction rate of combustion, asymptotic analysis is used. This one-dimensional laminar flame structure is divided into three zones. The first zone is a preheat zone in which the rate of chemical reaction is small, and evaporation plays a significant role in temperature and mass distribution. The second zone is a reaction zone in which convection and vaporization rates of particles are small. The third zone is a convection zone in which diffusive terms in the conservation equations are negligible. In this model, it is assumed that particles vaporize to yield a gaseous fuel; also, a massive exhaust gas recirculates to the preheat zone where it mixes with primary particles and gases either to dilute the combustion air and to generate relatively low flame temperature. At last, the flame characteristics for different equivalence ratios of the solid mixture are studied by considering the effects of radiation and mass recirculation. Radiations from reaction zone to the mixture of particles and gases existed in the preheat zone increase flame temperature and burning velocity. As the hot exhaust gases are recirculated to the inlet combustion air, the combustion air is diluted; thus, flame temperature and consequently NO x emission are decreased. DOI: http://dx.doi.org/10.5755/j01.mech.19.4.5042
Highlights
Energy efficiency and clean combustion are two main challenging issues in recent fuel utilization
The results indicated that increasing air preheat results in increased combustion efficiency and reduced NOx emission, whereas decreasing the oxygen content of the combustion air leads to large reduction in combustion efficiency, accompanied with a slight decrease or increase in NOx
A source term is needed to be considered in the preheat zone, leading to increase in flame velocity, and an additional sink term neglected in this study is required to be taken into account in the flame zone
Summary
Energy efficiency and clean combustion are two main challenging issues in recent fuel utilization. Flameless Oxidation (FLOX), known as High Temperature Air Combustion - (HTAC), or Moderate and Intensive Low oxygen Dilution (MILD) combustion is a promising combustion technology among various techniques [1, 2] capable of accomplishing high efficiency and low emissions It is based on delayed mixing of fuel and oxidizer and high flue gas recirculation in the flame zone [3]. From viewpoints of the environment and fuel cost reduction, small scale biomass CHP plants are in demand, especially waste-fueled system, which are simple to operate and maintenance with high thermal efficiency similar to oil fired units To meet these requirements, Stirling engine CHP systems combined with simplified biomass combustion process has been developed in which powder of less than 500 μm is mainly used, and a combustion chamber length of 3m is applied [17]. Diffusion caused by pressure gradient is negligible and all external forces including gravitational effects are assumed to be negligible
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