| OCR Text |
Show can amount to 10 to 60% of the fuel consumption. When oxygen enriched air is used in combustion, the required volume of air is reduced when compared with conventional combustion and therefore the volume of combustion products decreases and the flame temperature increases. Further, if the exhaust gas volume decreases, the stack losses are reduced. For example, in the combustion of natural gas, if the air is completely replaced by oxygen and stack gases are at 2200°F, the stack losses are reduced from 62% to about 22%. Oxy-fuel combustion is characterized by higher flame temperatures and higher burning velocities which generally influence burner design. High noise levels are produced in large burners because of high combustion intensities and special burners must be designed to overcome this problem. However, these special burners can be developed. Air can be enriched in oxygen by standard separation techniques such as cryogenic, pressure swing absorption or membrane separation. While oxygen enrichment has been shown to be technically feasible, the ultimate consideration in its use will be its economic feasibility. The economic feasibility of oxygen enrichment needs to be evaluated for each application but it would appear that its use would be more cost effective in higher temperature processes. Other than economic considerations, the only other barrier to the use of oxygen enrichment in industrial combustion processes could be environmental. Because of the increased flame temperatures and increased oxygen availability and because nitrogen will be available in the enriched air unless pure oxygen is used, large amounts of NOx could be produced. The NOx concentration in furnace stack gas is very sensitive to the time-temperature history in the combustion zone~ Ryd er3 has presented NOx data as a function of glass furnace bridge-wall temperature which shows an approximately two-fold increase in NOx concentration for every 200°F rise in bridgewall temperature. The air/fuel ratio, temperature of the air and the degree of oxygen enrichment appear to have the most important influences on NOx emissions. It is therefore important to know the concentrations of NOx in the stack gas when using oxygen enriched air because of regulatory compliance. There are two categories of existing regulations that impact industrial NOx generation, the National Ambient Air Quality Standards (NAAQS) and the stationary New Source Performance Standards(NSPS). The NAAQS for N02 was set in 1971 as required by the Clean Air Act of 1970 at 100 micrograms per cubic meter. The NSPS for steam generators with thermal inputs of greater than 73 MW (250 million BTU/hr) are 0.6, 0.3 and 0.2 lbs N02:/million BTU for bituminous coal, oil and gas respectively. These standards may be subject to review as needed to protect the public health and new or revised NSPS for steam generators of less than 73 MW and gas turbines and reciprocating engines are being considered. While 188 there are no present NSPS that cover industrial furnaces, it is possible that ultimately regulations may be adopted to cover these sources. Furthermore, states and local air quality districts could establish more stringent emission limits than the federal standards. It would therefore be prudent to at least be aware of the NOx emissions from industrial furnaces and how these emissions would increase with the widespread use of preheated air and preheated oxygen enriched air. The objectives of the work were to determine the maximum flame temperatures using oxygen enriched (OE) and preheated oxygen enriched (POE) air so that burner design modifications could be considered; to determine the maximum (equilibrium) levels of pollutant concentrations in OE and POE combustion systems; and to predict actual levels of NOx that could be expected in typical industrial furnaces that use OE and POE air with a view to formulating future R&D activities for possible additional emissions control. CHEMICAL EQUILIBRIUM THERMODYNAMIC ANALYSIS In a wide variety of industrial combustion processes, the fuel and oxidant are usually injected separately into the combustion chamber. The fuel-oxidant reactions are rapid and are usually controlled by hydrodynamic (turbulent mixing) considerations. On the other hand, the reactions that produce nitric oxide which occur over the entire flame front, are slow in comparison and are controlled by the chemical kinetics which are sensitive to temperature. A complete model for the turbulent reacting flow that occurs in an industrial combustion process does not exist. All currently acceptable models involve emperical correlations for the interaction between turbulent mixing and kinetics. In most of the research work, the chemistry of nitric oxide formation is conducted by uncoupling the kinetics from the hydrodynamics. While this is acceptable in determining rate constants, it does not accurately predict actual NOx emissions from combustion processes where the kinetics and hydrodynamics are coupled. In the computer code developed on the basis of chemical equilibrium thermodynamics, the basic assumption is that a sufficiently large time is permitted for reactions between different chemical species to occur so that the results are essentially independent of kinetics or hydrodynamics. The limitation or disadvantage of equilibrium thermodynamic treatment is that in real processes the transport of the reacting species and the rate at which they react are time dependent. In the present study, the objective was to determine the adiabatic flame temperatures and the equilibrium composition of the products for the combustion of all types of gaseous, liquid and solid fuels with POE air with oxygen concentrations varying from 21% (atmospheric) to 50% oxygen. The program takes into account the |
