Flue-gas oxygen

The reader can easily determine how a low-signal selector works for the fuel flow controller. It would compare the signals from the steam pressure and the air flow. A flue gas oxygen analyzer should be installed to continuously monitor or even trim the air flow. 

Close control of the amount of excess air is possible by the use of oxygen trim control equipment. Such equipment will control the flue gas oxygen content within the range of 2.0-3.0 per cent as compared to the normal 3.0-5.0 per cent. The improvement in boiler plant efficiency is of the order of 1.0-2.0 per cent. 

Of the major components of flue gas, oxygen and water vapor influence the reactions of the oxides considerably, hut carbon dioxide does not. Under irradiation, NO is oxidized by reactions with O, OH, and H02 radicals. The resultant N02 is oxidized to HN03 by reaction with OH radicals. S02 is similarly oxidized to H2S04 by reactions with O and OH. The products can also be converted to aerosols and collected in electrostatic precipitators. 

Impurities - There are two key impurities in the flue gas oxygen and oxides of sulfur. 

Stack gas sampling is also used to monitor the performance of the furnace—denoting oxidizing or reducing conditions. For instance, flue gas oxygen concentration can be tied to safety interlocks and the... 

Temperature C) 20.000 VM-bulb T emperature CQ 17.953 73.203 Flue Gas Oxygen Mole Fractioo DOOO... 

PiesBure (ItPa) 101.3000 Ory-butb Temperature fC) 25 0000 423.0000 1 Flue Gas Oxygen Mass Fraelkin 1 01891... 

Tempetaniie CC) 2DOOOO Temperaiiire PC) 17.S527 529557 Flue Gas Oxygen Mole Fraction 1 01701... 

Mass Flow Rate Dry Basis (kgih) 15GD.DQQQ 1556.4400 Flue Gas Oxygen Mole Fraction 0.17K... 

The primary objectives, in the operation of a fired heater are to avoid excessive heat density in the firebox, maintain a small negative pressure below the bottom row of convective tubes, and obtain complete combustion of the fuel in the firebox with minimum oxygen in the flue gas. Regardless of the flue-gas oxygen content, however, the point of absolute combustion is the combustion air rate that maximizes process-side heat absorption for a given amount of fuel. 

Excess air (process heaters), 318, 322-325 combustion benefits, 318 flue gas oxygen, 324 flame appearance, 324-325 fin tube damage, 325 sealing skin leaks, 325... 

A similar approach can be applied to the measurement of flue gas oxygen. If the air-to-fuel ratio falls below the stoichiometric requirement then the oxygen analyser will indicate zero - no matter how bad the problem. In Chapter 10 we show how incorporating a measurement of carbon monoxide (CO) can apparently extend the range into negative values of oxygen content. 

For example, consider the result of a step test performed to obtain the process dynamics for flue gas oxygen control. The purpose of the test is to obtain the process dynamics of flue... 

Of course we could automatically switch tuning with mode changes, but the air-to-fuel ratio scheme already proposed resolves the problem. With this ratio in place the result of testing in swing mode will be the same as that in baseload mode. In swing mode, where the header pressure control adjusts the duty, the air flow will be maintained in proportion and the change in flue gas oxygen will be unaffected. 

Monitoring Efficiency. Continuous monitoring of flue-gas temperatures and flue-gas oxygen content by a regularly calibrated recorder or indicator and by periodic checks on combustibles in the refuse will indicate if original efficiencies are being maintained. If conditions vary from the established performance base, corrective adjustments or maintenance steps should be taken. 

The fuel-air mixtures are combusted in extent of reaction models. Complete combustion to water vapor and carbon dioxide is assumed, with no residual carbon monoxide remaining. The model can be implicitly kept valid by lower bounding the flue gas oxygen content so complete combustion is maintained. 

The point of absolute combustion in terms of either maximum heater outlet temperature or minimum fuel fired for a given outlet temperature would correspond to the same flue-gas oxygen content for a given furnace at the same moment in time. However, we must also note that this flue-gas oxygen content that corresponded to the point of absolute combustion on, say, heater 3304-A on Tuesday, January 2,... 

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