| OCR Text |
Show the stack. Too little excess air results in fuel being unburned and also wasted up the stack. Some level of excess air is required to completely burn the fuel because of imperfect fuel/air mixing conditions in any commercial burner. Note from Figure 1 that the value of measured O2 increases as the excess air value is increased above the optimum. Also, the value of measured CO increases as the excess air value is decreased below the optimum. Depending on the specific combustion process, the optimum value of O2 is in the range of 1-3% and the optimum value of CO is in the range of 200-250 parts per million (ppm). Therefore, a combustion controller that provides measurements of O2 and CO in these ranges can ensure that the proper fuel/air ratio is maintained in the face of disturbances such as changes in fue~ heating value or deterioration of burners. In the low cost combustion controller application, both O2 and CO measurements are required. The reason for this is that some processes must be run slightly on the oxidizing side of stoichiometric to protect the manufactured product from the byproducts of fuel-rich combustion. Other processes must be run slightly on the reducing side to keep the product from oxidizing. Having both measurements allows the controller to run the process at maximum efficiency while maintaining the atmosphere that is appropriate for the manufactured product. The amount of improvement in efficiency (and reduction of fuel usage) that can be obtained for a particular process depends on two factors: the current fuel/air ratio at which the process is being operated, and the temperature of the exhaust gases going up the stack. In most processes, efficiency is lost because the fuel/air ratio is too lean (too much excess air). The effect of this type of loss is shown in Figure 2 in which the percent fuel required is plotted as a function of excess oxygen for several values of exhaust gas temperature. It is clear from this figure that the potential savings are greatest in the case of high-temperature processes. For example, a reduction of excess oxygen from 6% to 5% for a process with 15000F exhaust gases produces a fuel savings of over 5%. For a heating process consuming 50 MCF per year, this translates into a dollar savings of $12,500. Even if the fuel savings for this size process were only 2%, this would still result in a savings of $5,000 per year. The potential savings for processes that are run on the reducing side of stoichiometric are even more dramatic, since the efficiency curve drops off faster on that side rather than on the oxidizing side for an equal change in fuel/air ratio. By providing more accurate control of the fuel/air ratio in the combustion process, the controller will improve the quality and reduce the cost of the end product manufactured using the process. At present, it is not costeffective to measure the O2 and CO 214 concentrations in the vicinity of the product; as a result, the operators of the process select a fuel/air ratio based on trial-and-error (resulting in much scrap product) supplemented occasionally by sampling the combustion gases using portable analyzers. Of course, this approach does not automatically compensate for hour-to-hour variations in operating conditions (e.g., changes in product throughput rates, variation in BTU of fuels) that might cause variation in product quality beyond the allowable specification range. Some examples of this deterioration of product quality due to changing combustion conditions are as follows: 1) In the iron and steel industry, slab reheating furnaces are supposed to be run at about 1% excess oxygen to avoid tight scaling on the slabs. Deviations from this level now cause significant product wastage and rework. 2) In aluminum die cast operations, the melting furnace is supposed to be operated slightly on the oxidizing side. If the environment becomes too fuel-rich the aluminum can absorb hydrogen, which causes bubbles in the resulting castings. On the other hand, if the environment is too fuel-lean, the resulting oxidation of the aluminum causes wastage (up to 3-4% metal loss) and requires the use of additional fluxes to clean the excess dross. 3) Heat treating furnaces in the iron and steel and copper/bronze industries must be run at or near stoichiometric conditions to avoid oxidation and resultant wastage of the metals. A low cost combustion controller would allow significant product savings to be achieved, even in small processes. The final benefit of the low cost combustion controller is that its' improved measurements, diagnostics, and fail-safe control capabilities will ensure that the processes equipped with the controller are run more safely. Some examples of improved safety of operations are as follows: 1) At present, most controllers for small combustion processes are not capable of changing their fuel/air ratio set point as a function of operating condition (e.g., product throughput rate or steam flow rate). The new controller would be able to do this, calling for more air at low loads (to improve mixing) and less at high (to improve efficiency). This will help the turndown capabilities of the process significantly. 2) Existing combustion controllers have failure modes that are potentially unsafe; if, for example, the air flow controller fails and shuts the air damper. The new controller has a fail-safe design feature that allows the user to specify the default value of the control outputs if there is a controller failure. In this case, the user would specify that the fuel valve be closed and the air damper opened in case of a failure. 3) The self-diagnostic features of the new controller will detect internal problems |
