Optimizing excess air

The term optimizing excess air, as we have seen, does not refer to operation at some arbitrary oxygen level instead, consider the following in relation to your heater, and you will be optimizing excess air  

A typical air preheater will reduce the fuel required to liberate a given amount of heat by 10 percent. The debit for this improved thermal efficiency is a higher flame temperature, and the possibility of overheating the radiant section. There is a clear advantage to fit an air preheater to a furnace when the firebox is running below a maximum firebox temperature. Three types of air preheaters are in common use  

The wheel type is subject to air leaks across the mechanical seals. 

This leakage is identified by increased oxygen content in the flue gas, low flue-gas outlet temperature, and a greater temperature loss in the flue than rise in the air temperature across the preheater, as shown in Fig. 24.9. 

In general, air preheater leaks are a problem because they 

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Therefore, what is the optimal excess air or 2% The basis is to achieve complete combustion. For reliability considerations, optimal 2% should be determined with a safety margin on top of the minimum excess air when burners are in good condition. The safety margin depends on specific technology, design, and conditions for each heater as well as measurement. Figure 5.10 is commonly used to explain qualitatively the existence of optimal excess air. 

Cold reheat gas (sulfur recovery), 116 Combination head, 63 Combination tower, 35, 38, 71,83-89 bottoms screen, 35, 38 overhead condenser, 82 delayed coking process, 83-89 explosion-proof trays, 84-85 energy savings, 85-86 coke drum cycles, 86-89 coke drum yields, 88-89 Combustion air supply (process heaters), 317—325 trimming burner operation, 318 excess air benefits, 318 optimizing heater draft, 318— 321 insufficient air, 321-322 optimizing excess air, 322-325 Combustion chamber, 315 Composition instability (distillation tower), 381-382 temperature controller, 381-382 condensing capacity, 382... 

Since NO production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore, many new designs incorporate DENOX units that employ catalytic methods to reduce the NO limit. Platinum-containing monolithic catalysts are used (36). Each catalyst performs optimally for a specific temperature range, and most of them work properly around 400°C. 

While process design and equipment specification are usually performed prior to the implementation of the process, optimization of operating conditions is carried out monthly, weekly, daily, hourly, or even eveiy minute. Optimization of plant operations determines the set points for each unit at the temperatures, pressures, and flow rates that are the best in some sense. For example, the selection of the percentage of excess air in a process heater is quite critical and involves a balance on the fuel-air ratio to assure complete combustion and at the same time make the maximum use of the Heating potential of the fuel. Typical day-to-day optimization in a plant minimizes steam consumption or cooling water consumption, optimizes the reflux ratio in a distillation column, or allocates raw materials on an economic basis [Latour, Hydro Proc., 58(6), 73, 1979, and Hydro. Proc., 58(7), 219, 1979]. 

The characterization of PIC (products of incomplete combustion) from the combustion of wood treated with pentachlorophenol (penta) is more widely documented in the open literature than creosote alone. However, both products are similar in chemical composition and likely result in comparable forms and concentrations of PIC. Literature reported studies on the combustion of these chemicals and wood treated by them, and the PIC generated are based upon optimal conditions. Optimal conditions are defined as those in which the fuel burns at the designed heat release rate with nominally 160% excess air and a low level (< 100 ppm) of carbon monoxide (CO) emissions in combustion (flue) gases. 

Several methods have been investigated to find correlations between physical properties of fuel gas mixtures and the excess air ratio to optimize the combustion procedure. In spite of the varying composition of natural gas it is said to be possible to control a heater system by measurements of the dynamic viscosity of the gas [7]. One explanation could be the correlation between Wobbe number and viscosity With increasing Wobbe numbers the viscosity decreases, and if the Wobbe number of a gas is known, the excess air ratio can be adjusted, resulting in an open loop control. 

Many of the conservation measures require detailed process analysis plus optimization. For example, the efficient firing of fuel (category 1) is extremely important in all applications. For any rate of fuel combustion, a theoretical quantity of air (for complete combustion to carbon dioxide and water vapor) exists under which the most efficient combustion occurs. Reduction of the amount of air available leads to incomplete combustion and a rapid decrease in efficiency. In addition, carbon particles may be formed that can lead to accelerated fouling of heater tube surfaces. To allow for small variations in fuel composition and flow rate and in the air flow rates that inevitably occur in industrial practice, it is usually desirable to aim for operation with a small amount of excess air, say 5 to 10 percent, above the theoretical amount for complete combustion. Too much excess air, however, leads to increased sensible heat losses through the stack gas. 

As shown in Figure 2.2, the radiation and wall losses of a boiler are relatively constant, and most of the heat losses occur through the stack. Under air-deficient operations, unburned fuel leaves, and under air-excess conditions, heat is lost as the unused Oz and its accompanying nitrogen are heated up and then discharged into the atmosphere. The goal of optimization is to keep the total losses to a minimum. This is accomplished by minimizing both excess air and the stack temperatures. 

Boiler losses can be plotted as a function of excess air (top). The minimum of the total loss curve of a boiler is the point where optimized operation is maintained (bottom). Most efficient operation of a boiler occurs when the amount of excess air in the stack balances the losses of unburned fuel. 

It is usually desirable to operate the combustor at certain optimal temperature ranges in order to control the emission of NO and SO2 or to obtain the maximum combustion efficiency. This can probably be accomplished through the control of excess air flow rate. 

At present, no completely satisfactory and universally applicable control technique is available, but several approaches (e.g., flue gas recirculation, low excess air flring, and staged combustion) appear very promising. A better understanding of the chemistry of pollutant formation should lead ultimately to optimization of these and other new techniques for total control of NO emissions. 

Consider a burner where the fuel/air ratio is kept at its optimal value to achieve the highest efficiency of combustion. Excess fuel or air will reduce the efficiency. The optimal fuel/air ratio is maintained through a ratio control mechanism (Section 21.5). The control system is shown in Figure 22.3a. 

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