Sulfur trioxide conditioning system control algorithm

A method for determining a most effective injection rate for SO3 into a flue gas resulting from a coal burned in a boiler, the method comprising:

• 1, obtaining proximate and ultimate analyses of the coal being burned in the boiler and ash mineral analysis for the coal,
• 2, determining an average temperature of the flue gas entering an electrostatic precipitator (ESP) from plant instrumentation, wherein the ESP is powered with multiple electrical fields,
• 3, estimating SO3 background level in the flue gas using correlation relating flue gas SO3 to coal type and coal sulfur content,
• 4, obtaining a value of a current applied to the ESP from controls for each transformer-rectifier set that is powering the precipitator,
• 5, determining an effective fly ash resistivity level in the ESP using a correlation that relates fly ash resistivity to ESP current density for each electrical field, averaging the results to produce the effective resistivity of the fly ash collected in the ESP, and if this resistivity is not close to, or lower than, the optimum range, proceeding with step 6;
• otherwise, going to step 10,
• 6, using a correlation relating fly ash composition and flue gas temperature and SO3 concentration to the fly ash resistivity to determine a flue gas SO3 concentration needed to produce the optimum fly ash resistivity,
• 7, subtracting the background SO3 from the needed SO3 concentration from step 6 to determine the amount of SO3 that must be added to the flue gas to produce the optimum fly ash resistivity,
• 8, sending a signal to the controls that operate an SO3 conditioning system to increase the amount of SO3 injected into the flue gas by the additional amount determined in step 7,
• 9, repeating steps 4 and 5,
• 10, a, if the indicated ash resistivity is equal to or less than the optimum resistivity, decreasing the rate of injection by x percent where x is between 5 and 25, or
• b, if the indicated ash resistivity is greater than the optimum resistivity, increasing the rate of injection by x percent where x is between 5 and 25,
• 11, repeating step 10 until the indicated fly ash resistivity passes through the optimum resistivity point, setting the rate of injection at a point in the range bounded by the levels calculated in the last two interactions, and then
• 12, every y minutes, where y is number between 5 and 30, restarting the process beginning at step 2.