Method for controlling NOx concentration in exhaust gas in combustion facility using pulverized coal

Abstract

A method for controlling an NOx concentration in an exhaust gas in a combustion facility by: measuring a reaction velocity k.sub.i of each of a plurality of chars, each corresponding to a plurality of types of pulverized coals; determining a relationship between the NOx concentration in the exhaust gas and the reaction velocity k.sub.i for each of the chars; (iii) blending the plurality of the types of the pulverized coal, wherein a blending ratio of the plurality of the types of the pulverized coal is determined by using, as an index, a reaction velocity k.sub.blend of the char of the blended pulverized coal, which corresponds to a target NOx concentration or below, on the basis of the relationship; and supplying the blended pulverized coal to the combustion facility as the fuel of the combustion facility.

Claims

1. A method for controlling an NOx concentration in an exhaust gas in a combustion facility that uses a pulverized coal as a fuel, comprising, in the following order: measuring a reaction velocity of each of a plurality of chars represented by k.sub.i and corresponding to a plurality of types of pulverized coals; determining a relationship between the NOx concentration in the exhaust gas and the reaction velocity k.sub.i for the each of the chars in the plurality of chars; blending the plurality of the types of the pulverized coal, to obtain a blended pulverized coal, wherein a blending ratio of the plurality of the types of the pulverized coal is determined by using, as an index, a reaction velocity of the char of the blended pulverized coal, which is represented by k.sub.blend and which corresponds to a target NOx concentration or below, on the basis of the relationship; and supplying the blended pulverized coal to the combustion facility as the fuel of the combustion facility wherein the reaction velocity of the each of the plurality of the chars, k.sub.i, is determined by: drawing a curve of a time change of weight loss for the each of the plurality of the types of the pulverized coals under the condition of a plurality of temperatures by using a thermal balance; and dividing inclination of a tangent of the curve by a measured partial pressure of oxygen, thereby determining the reaction velocity of the each of the plurality of the chars, k.sub.i at the respective temperatures.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph schematically illustrating a method of measuring a reaction velocity of char and determining a reaction frequency factor of char from the Arrhenius plot which has been drawn on the basis of the result of the measurement, in one embodiment of the present invention.

(2) FIG. 2 is a graph illustrating a relationship between the reaction frequency factor of char, which has been determined in the above described embodiment, and a necessary amount of ammonia water to be used.

(3) FIG. 3 is a schematic block diagram illustrating a cement manufacturing facility to which the above described embodiment has been applied.

(4) FIG. 4A is a graph illustrating a relationship between a nitrogen mass ratio of the pulverized coal and an NOx concentration in an exhaust gas.

(5) FIG. 4B is a graph illustrating a relationship between a fuel ratio of the pulverized coal and the NOx concentration in the exhaust gas.

(6) FIG. 5 is a graph illustrating a relationship between the reaction frequency factor of char of the pulverized coal and the NOx concentration in the exhaust gas, which has been determined in the present embodiment.

EMBODIMENTS OF THE INVENTION

(7) One embodiment will be described below in which a method according to the present invention for controlling an NOx concentration in an exhaust gas in a combustion facility using pulverized coal, on the basis of an experimental example that has been conducted by the present inventors, has been applied to the control of the concentration of NOx originating in the combustion of the pulverized coal in a calciner 12 in the cement manufacturing facility illustrated in FIG. 3.

(8) Firstly, as illustrated in FIG. 1, a reaction velocity k of char corresponding to each of a plurality of types of pulverized coals which were used in the above described calciner 12 was measured with the use of a thermal balance.

(9) Incidentally, the used thermal balance is an infrared differential type differential thermal balance TG8120 made by Rigaku Corporation, and a weight of a sample was measured with an electronic balance XS105DU made by Mettler-Toledo International Inc. In addition, a powder was used of which the 50% cumulative diameter was 10 to 40 m when measured with a laser diffraction method, as the sample which were used for the evaluation of combustion properties.

(10) The mass reductions of samples were measured according to a process of: raising the temperature of a predetermined amount of the sample of the pulverized coal in an atmosphere of nitrogen gas at a rate of 15 K/s; after the temperature has reached a predetermined temperature, holding the temperature until the change of the weight loss due to the thermal decomposition of a volatile component contained in the above described sample became sufficiently small (for 1 to 4 minutes); then switching the atmosphere of nitrogen gas to an atmosphere containing oxygen; and holding the sample at a plurality of constant temperatures (K).

(11) Incidentally, the plurality of the constant temperatures and the amounts of the samples when the mass reductions of the above described samples were measured were 500 C. (1.5 mg), 550 C. (1.0 mg), 600 C. (0.5 mg), 650 C. (0.2 mg) and 700 C. (0.1 mg), respectively.

(12) At this time, a time change (reaction rate change dX/dt) of the weight loss due to the oxidization of the char in the above described thermal balance can be expressed by the following expression (2).
dX/dt=k(1X)n P.sub.O2m(2)

(13) Here, k represents a reaction velocity (1/s.Math.Pa) of char, X represents a reaction rate of char, P.sub.O2 represents a partial pressure (Pa) of oxygen, and m and n represent orders of the reaction.

(14) If the above described orders of the reaction have been determined to be n=0 and m=1 (which are determined by another prediction experiment), k (dX/dt)/P.sub.O2, and accordingly a value obtained by dividing the inclinations of tangents in the time change of the weight loss in the cases of unburned ratios of 0.3, 0.5 and 0.7 (though in the figure, the case of 0.5 is illustrated) by the measured partial pressure of oxygen shall be the reaction velocity k of char at the temperature T.

(15) Next, the above described measurement and the like were conducted at each of the above described temperatures (K), and the Arrhenius plot was drawn which sets 1/T (1/K) for the horizontal axis and sets the reaction velocity k of char for the vertical axis. Then, the above described activation energy E was determined from the inclination of the Arrhenius plot and the reaction frequency factor A of char was determined from the intercept of the vertical axis, for each of the unburned ratios 0.3, 0.5 and 0.7 (though in the figure, the case of 0.5 is illustrated). Then, the average value of the reaction frequency factors A of char, which had been determined for each of the unburned ratios 0.3, 0.5 and 0.7, was determined to be the frequency factor A of the measured sample.

(16) Subsequently, a relationship was determined between the reaction frequency factor A of char of the pulverized coal which was actually used in the calciner 12 and the NOx concentration in the exhaust gas discharged at that time. FIG. 5 is a graph illustrating this relationship. It is understood that there is a strong correlationship between the frequency factor and the NOx concentration.

(17) In addition, it is understood that in order to control the NOx concentration to 500 ppm or below which is an allowable range in the operation of the above described calciner 12, the pulverized coal may be blended so that the reaction frequency factor A of char becomes 15 or more.

(18) Furthermore, when ammonia water or the like is finally added to a gas including an exhaust gas to be discharged from the rotary kiln 1, the amount of ammonia water or the like to be used can be predicted as is illustrated in FIG. 2.

(19) Thus, the method for controlling the NOx concentration in the exhaust gas in the calciner 12 which has the above described structure and uses the pulverized coal therein can easily control the NOx concentration in the exhaust gas to be discharged from the calciner 12 to such a value as to be within an allowable range in the operation of the calciner 12, by blending a plurality of types of the pulverized coals which are used in the calciner 12 so that the reaction frequency factor of char of the blend becomes the value of the reaction frequency factor A of char at which the NOx concentration becomes the target value or below, on the basis of the relationship between the reaction frequency factor A of char and the NOx concentration, which is illustrated in FIG. 5. Incidentally, the reaction velocity k of char and the reaction frequency factor A after the blending can be practically determined without problems by the weighted average of each value of the blended pulverized coals in consideration of the blend ratio, in addition to actual measurement.

(20) In addition, the method can prevent also such an operational problem from occurring that the quality is lowered and a quantity of production is reduced, which originates in excessively lowering the temperature in the rotary kiln 1 so as to lower the NOx concentration in the exhaust gas that is discharged from the exhaust line 9, by controlling the NOx concentration in the calciner 12 to be within the allowable range in the operation. In addition, it also becomes possible to reduce the amount of ammonia water which is finally added to a gas including the exhaust gas from the rotary kiln 1, in the exhaust line 9.

INDUSTRIAL APPLICABILITY

(21) The method of controlling an NOx concentration in the exhaust gas to be discharged from a fuel facility which uses a pulverized coal as a fuel, on the basis of the properties of the pulverized coal beforehand, can easily control the NOx concentration to or below a regulation value according to the Air Pollution Control Law and the like, and can also reduce an amount of the denitrifying agent or the like to be used, which is necessary for the control.

EXPLANATION OF REFERENCE NUMERALS

(22) 12 calciner (combustion facility)