Method and apparatus for treating exhaust gas

1. A method for treating exhaust gas, comprising the steps of:

measuring SO 3 concentration in the exhaust gas which contains at least SO 2 and SO 3 by using an SO 3 gas analyzer;

injecting fine particles into the exhaust gas after the step of measuring;

cooling the exhaust gas after the step of injecting, by recovering heat from the exhaust gas;

collecting fly ash from the cooled exhaust gas; and

removing at least SO 2 from the exhaust gas after the step of collecting, by absorbing the SO 2 with an absorbing solution,

wherein the step of injecting comprises controlling a injection amount of the fine particles so that a weight ratio of a sum of flying dust concentration A and fine particles concentration P to the SO 3 concentration S, as shown in an expression “{(A+P)/S)}”, becomes 2.0 or higher.

2. The method according to claim 1 , wherein the flying dust concentration A is calculated by using the following equation (1):

Flying dust concentration (g/Nm 3 )=(Amount of coal fed)×(Ash concentration in the coal)×(Ash residual factor)/(Exhaust gas flow rate)  (1)

wherein the amount of coal fed is a fed amount of coal for generating the exhaust gas containing at least SO 2 and SO 3 , the ash concentration in the coal is ash concentration contained in the coal, ash residual factor is a ratio of ash quantity contained in the exhaust gas before the step of injecting to ash quantity contained in the coal, and the exhaust gas flow rate is a flow rate of the exhaust gas containing at least SO 2 and SO 3 before the step of injecting.

3. The method according to claim 1 ,

wherein the method for treating exhaust gas is a method for treating exhaust gas fed continuously, and at least a part of the collected fly ash is used as the fine particles.

4. The method according to claim 1 ,

wherein the step of measuring further comprises measuring a temperature of the exhaust gas and an SO 2 concentration in the exhaust gas before the step of injecting;

an estimated SO 3 concentration is calculated based on the measured temperature and SO 2 concentration;

a difference between the estimated SO 3 concentration and the measured SO 3 concentration by the SO 3 gas analyzer is calculated; and

when the difference exceeds a predetermined range, a warning is issued.

5. An apparatus for treating exhaust gas, comprising:

an SO 3 gas analyzer for measuring an SO 3 concentration in the exhaust gas which contains at least SO 2 and SO 3 ;

a fine particles injection grid for injecting fine particles into the exhaust gas whose SO 3 concentration has been measured;

a heat exchanger for cooling the exhaust gas into which the fine particles have been injected, by recovering heat from the exhaust gas;

an electrostatic precipitator for collecting fly ash from the cooled exhaust gas;

an absorption tower for removing at least SO 2 from the exhaust gas from which the fly ash have been collected, through a gas-liquid contact between the exhaust gas and an absorbing solution; and

a fine particle injection controller for controlling an amount of the fine particles to be injected so that a weight ratio of a sum of flying dust concentration A and fine particles concentration P to the SO 3 concentration S, as shown in an expression “{(A+P)/S)}”, becomes 2.0 or higher.

6. The apparatus according to claim 5 , further comprising an outlet port for taking out a portion of the exhaust gas which contains at least SO 2 and SO 3 , the port being provided upstream of the fine particle injection grid for measuring the flying dust concentration.

7. The apparatus according to claim 5 , wherein the electrostatic precipitator and the fine particle injector are connected to each other to use at least a part of the fly ash collected by the electrostatic precipitator as the fine particles.

8. The apparatus according to claim 5 , further comprising:

a thermometer, which is provided upstream of the fine particle injection grid, for measuring a temperature of the exhaust gas before the fine particles are injected; and

an SO 2 gas analyzer, which is provided upstream of the fine particle injection grid, for measuring SO 2 concentration in the exhaust gas before the fine particles are injected,

wherein the fine particle injection controller is configured to calculate an estimated SO 3 concentration based on signals input by the thermometer and the SO 2 gas analyzer, configured to calculate a difference between the estimated SO 3 concentration and the measured SO 3 concentration based on a signal input by the SO 3 gas analyzer, and configured to issue a warning when the difference exceeds a predetermined range.