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Show The effect of the differences in the fuel and characteristics can be minimized but not eliminated by appropriate design of the burner. The objective with gas firing is to increase the rate of fuel/air mixing and the emissivity simultaneously. These qualities are normally mutually incompatible but can be addressed by appropriate attention to primary air flowrate and burner nozzle design. To obtain the best performance tile burner should be designed to be specifically tailored to match the particular kiln. As well as the obvious parameters of burner design such as fuel type and liberation, the kiln process and size, the combustion air system and kiln aerodynamics should also be taken into account. If these factors are all carefully considered the performance of the burner will be optimized for the installation in question. The most effective method of achieving this requirement is to model the proposed burner within its kiln environment. This then enables the model burner to be optimized prior to mechanical design of the full size unit. 3.2.2 The Use of Modelling for Burner Design To ensure optimum burner design it is necessary to use both physical and mathematical modelling techniques to assess the kiln aerodynamics and flame characteristics. Acid/alkali modelling is used to investigate fuel/air mixing in the kiln. A physical model of the kiln and cooler is constructed to an appropriate scale in clear acrylic plastic. The actual scale to which the model is manufactured is dependent on a number of factors but dynamic similarity must be maintained. The fuel is represented by dilute caustic soda solution containing an indicator, whilst the combustion air is represented by dilute acid. The indicator becomes colourless at the boundary where the mixing is complete, thus the model flame envelope is defined by the coloured region. This flame length is corrected for the full size conditions. The aerodynamics of the full size system are reproduced on the physical model thus allowing a simulation of the burner jet fuel/air mixing characteristics, and hence the full size flame under different conditions. A mathematical model (21) is also used to predict the fuel burn out and determine the flame temperature and heat flux profile. By using acid/alkali and mathematical modelling it is possible to assess the performance of any burner in any kiln system. The effect of changes, such as burner design and fuel type, can be rapidly assessed wi thout a disruption to the operating plant. Burners designed using these techniques are thus uniquely matched to the kiln for which they are designed and the optimum kiln fuel consumption and output is ensured by low excess air operation, combined with minimal fuel waste due to incomplete combustion. A 1/50 scale plexiglass model was constructed of the kiln, including the firing hood and product coolers. The full size kiln was fitted with a number of small retention dams and these were also represented on the model, figure 11. The existing oil burner was modelled first and this showed that the fuel/air mixing was relatively poor, with a non-recirculatory flame, which tended to give a relatively long flame. The associated heat flux patterns were calculated using a symmetrical slice zone model, which calculates the burnout and heat transfer within 4 inch slices of the kiln, based on the |
