OCR Text |
Show mixed with the combustion air stream, or may be injected through a lance on the underside of the flame, or may be used in burners designed especially for high oxygen concentrations. The choice of a particular technique can have significant impact on the magnitude of cost savings. The choice generally depends upon the type and the size of the furnace, the operating benefits, and the capital costs. Installing burners that are designed to use oxygen instead of air generally results in the greatest operating cost savings. However, the capital costs related to new burner systems are higher than those for the other simpler techniques of using oxygen. New cost-effective natural gas burner systems which use high concentrations of oxygen for combustion in industrial applications are being developed at Air Products and Chemicals jointly with the Gas Research Institute. The objective of the project is to develop, evaluate and field test simple, low cost burner systems that have high turndown, low emissions of pollutants, long life, and reliable controls for minimizing waste of fuel and oxygen. In the first phase of this project, six off-the-shelf air-fuel burners were evaluated with air and with varying levels of oxygen enrichment. This paper reports on some of the results of these evaluations. BACKGROUND The use of oxygen enrichment to achieve high flame temperature is well documented. Figure 1 shows the calculated adiabatic flame temperatures for stoichiometric combustion of natural gas with cold air containing varying concentrations of oxygen. The calculations show that the adiabatic flame temperature increases from 3550°F with air to over 5000°F with pure oxygen. Higher adiabatic flame temperatures are a direct result of eliminating greater quantities of nitrogen and other gases inert to the combustion process. It should be pointed out that calculated adiabatic flame temperatures are rarely if ever achieved in practice. Actual flame temperature in any specific application is governed by many factors such as the degree of mixing between the fuel and the oxygen, the cooling effect of mixing with recirculating furnace gases, the heat loss from the flame, the chemistry of combustion including the pyrolysis of the fuel, the firing rate, excess air levels, and the furnace temperature. Consequently, the true flame temperatures with or without oxygen enrichment are extremely difficult to measure directly. Eliminating nitrogen serves to reduce the energy escaping in the flue gases and to increase the heat available to the process. 166 Therefore the thermal efficiency is increased. Figure 2 shows the improvement in thermal efficiency due solely to nitrogen elimination, at several levels of oxygen enrichment. The figure generally understates what can be achieved in practice. The effects of oxygen enrichment are better calculated by taking into account the type and the size of the furnace, the gas flow patterns, the type of burner, the characteristics of the product, the excess air levels, the composition of the flue gases, etc. in addition to the elimination of nitrogen. The use of oxygen enrichment reduces the volume of flue gases by up to a factor of ten. Not only is the amount of energy necessary to achieve the desired production rate reduced, but so is the volume of flue gases per unit of energy. Such dramatic reductions in flue gas volumes resulting from high oxygen enrichment levels, require careful analysis of convective heat transfer, furnace temperature uniformity, load penetration, etc. Figure 3 shows that oxygen enrichment increases the velocity of flame propagation and broadens the flammability limits of natural gas. These two factors can be advantageous for increasing flame stability. However, these factors may also lead to short, intense flames unsuited for many industrial applications. There exists a great body of literature on NOx emissions from industrial combustion processes. In general, the more intense the flame, the higher are the NOx emissions. However, enrichment does not necessarily lead to increase in NOx emissions. NOx levels depend on the particulars of the enrichment technique. But the possibility of increasing NOx with high enrichment levels has always been of concern. Other enrichment related concerns include survival of burners under potentially excessive temperature levels and compatibility of various materials of construction with oxygen enriched atmospheres. In summary, the technical objective of developing new burner systems for high levels of oxygen enrichment is to minimize the undesirable effects of enrichment such as short flame lengths, skewed heat release patterns, potentially excessive temperature levels, material compatibility, NOx generation, while maximizing the desirable effects such as increased fuel efficiency, increased productivity, reduced total emissions, and reduced production costs. The rest of the paper deals with the experimental study of effects of enrichment on the performance of two off-the-shelf air-fuel burners. The burners were characterized for temperature profiles, heat transfer rates, furnace temperature |