Optimising calciner burners: Theoretical treatment of turbulent diffusion jets in cross flow

Abstract

For the cement industry one of the ways of reducing specific fuel consumption, which, is the major cost component in cement production, is the flame control. Controlling the flames in a precise manner results not only in specific fuel consumption savings, but also in maintaining longer refractory life and better quality. An understanding of the aerodynamic aspect of the flames is important in controlling the shape and length of the flame, achieving good fuel/air mixing and recirculation, which are essential in preventing a poor flame and inefficient combustion. Energy losses can occur as a result of incomplete combustion, which manifests itself as CO presence in exhaust gases due to poor fuel/air mixing, as well as long flames. Positioning of the flames in a symmetrical manner both in the rotary kiln and the secondary firing units/calciners is also essential for uniform distribution of heat transfer which results in better heat transfer between the material and hot gas, as well as prevention of flame impingement on the refractory. Flames encountered in cement manufacturing processes are of the turbulent diffusion type. Rotary kiln flame can be described as a long enclosed turbulent jet passing into a turbulent co-flowing stream constrained by a circular duct. The flames in the secondary firing units or calciners are mostly described as short semi-free turbulent diffusion jets in cross flow. In order to study the properties and behaviour of the auxiliary or calciner flames, which are short, impinging, semi-free, turbulent diffusion jet flames in cross flow, the theory and isothermal modelling of the turbulent jets injected at an angle into the main stream can be applied. An empirical equation was developed to describe the behaviour of the turbulent jets in cross flow which can be used to predict the position of the axis of such jets, which, are the flames in calciners or riser ducts of the cement kilns. By modifying the discharge conditions of the jet or mainstream properties, the flames can be positioned in the desired configuration for better fuel/air mixing and heat transfer conditions. Calciner burners can be designed for optimum flame configuration and combustion conditions in the calciners and the secondary firing units where the turbulent jets in cross flow theory is applicable. The empirical equation developed was used to predict the jet paths of the auxiliary firing flames . The acid/alkali physical modelling technique was also used to simulate the same auxiliary flames under same conditions to check the accuracy and validity of the theoretical method. Results showed that both the theoretical method and the experimental method agreed well with each other in determining the jet axis of the turbulent diffusion flames in cross flow