JPS6053808B2 - Combustion method in black liquor recovery boiler - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a combustion method for a black liquor recovery boiler that burns black liquor discharged as waste liquor from the process of manufacturing kraft pulp and recovers its heat and the chemicals it contains.

In paper mills, kraft pulp is made by scaling small pieces of wood using a fish decomposition solution containing caustic soda and sodium sulfide. As a result of the fish scaling, the cellulose in the wood becomes pulp, and the remaining resin and chemicals are called black liquor. It is discharged as waste liquid.

The black liquor recovery boiler burns the black liquor to recover the combustion heat as steam, and also separates and recovers the chemicals consumed for pulping fish scales from the resin. In this black liquor recovery boiler, as mentioned above, the black liquor contains a large amount of sulfides such as soda sulfide, so a large amount of sulfur oxides (hereinafter abbreviated as SOx) are generated as the black liquor is burned. and contributes to air pollution. Additionally, black liquor is flame retardant and cannot be used as a heat source for a boiler as it is, so it must be dried. That is, black liquor is generally injected into the furnace at a concentration of 55-65% (resin and chemicals 55-65%, moisture 45-35%), but the calorific value in this state is 1700-2200Kca1/9 Before combustion, it is necessary to dry it thoroughly using radiant heat from inside the furnace or high-temperature combustion gas. There are two drying methods: a floating drying method and a wall drying method. Of these, the floating drying method involves spraying a relatively small amount of black liquor upwards and drying it by suspending it in the combustion gas from the furnace. .
In addition, in the wall drying method, the black liquor is made into relatively coarse particles and is injected almost horizontally to make it adhere to the furnace wall one after another.The radiant heat from the furnace and high-temperature combustion gas are used to dry the black liquor, and the adhesion is made to a certain thickness ( (for example, 30 to 50 cm), and when drying is completed, it falls naturally due to its own weight.
In the floating drying method, the black liquor is made into fine particles, so the surface area for the particles to exchange heat is large, and the drying effect is great within a single period of time, but the time that the particles remain suspended in the combustion gas is several seconds. However, this has the disadvantage that drying tends to be insufficient and particles often carry over into the combustion gas. On the other hand, in the wall drying method, the heat exchange surface area of black liquor particles is small, but the thickness of the black liquor layer increases after adhering to the wall surface.
It takes about several tens of minutes for the product to finally fall under its own weight, allowing sufficient drying time, and from a comprehensive evaluation it can be said that the wall drying method is superior to the floating drying method. The black liquor dried by any of the drying methods described above becomes a combustible material called char, and this char is deposited on the bottom of the furnace to form a combustible layer called cherbet. When this charbet burns, the resin component radiates heat and becomes combustion gas, while the non-flammable chemicals melt and form a smelt.
The liquid becomes a liquid with a temperature of 1000°C, is separated from the resin, and is collected and transported to the next process. In the above-mentioned black liquor drying process and chartrette combustion process, high concentrations of SOx of 200 to 2000 ppm are emitted from sulfides in chemicals. For this reason, it was necessary to install expensive flue gas desulfurization equipment from the perspective of preventing air pollution. The purpose of the present invention is to provide a combustion method for a black liquor recovery boiler that eliminates the above-mentioned drawbacks of the prior art, performs efficient black liquor drying, reduces the generation of SOx, and does not require a flue gas desulfurization device. be.

In short, the present invention maintains the temperature of the chamber above a certain temperature in a black liquor recovery boiler, volatilizes the sodium compound, reacts and fixes SOx in the combustion gas with this sodium compound, and reduces SOx in the exhaust gas. , the wall drying method was further improved to dry black liquor more efficiently.

Embodiments of the present invention will be described below with reference to the drawings.

In FIG. 1, black liquor is sprayed into the furnace 6 by a black liquor injection pipe 7. The black liquor particles are sprayed in a substantially horizontal direction across the inside of the furnace 6, but in some cases, the range of the black liquor adhesion layer 8 may be expanded by swinging the spray direction up and down. And for the same reason! Depending on the situation, it may be swung in the left-right direction, or may be rotated about the axis of the black liquor jet 7 as the case may be. Reference numeral 5 indicates a screen pipe provided in the upper part of the furnace 6, 4 indicates a superheater, 1 indicates a boiler, 2 indicates an air cylinder provided in the boiler 1, and 3 indicates a water cylinder.

The diameter of the sprayed particles varies slightly depending on the black liquor concentration, temperature, viscosity, pressure, black liquor spray method, and nozzle shape, but if the particles are too small, the drying effect will be large. It is easy to carry over on the rising air current of combustion gas, and if the particles are coarse, even if a wall drying method is used, the drying effect until reaching the wall is poor, and therefore the particles have low stickiness and do not form a black liquor adhesion layer. 8 will fall to the bottom of the furnace 6 without forming any. In this regard, the inventor has discovered that when the particle size of the black liquor is about 1 to 5 particles, adhesion to the wall surface and drying of the particles are very good. This particle size is 1 to 5?
When black liquor is injected into the furnace 6 within the range of 9 is formed. Actual measurements of the temperature of a chartreuse that has been operated satisfactorily with the particle size described above show that it is in the range of 900 to 1200°C, and within this range, sodium compounds such as Na2O are sufficiently volatilized and mixed into the exhaust gas, producing SOx. For example, by the reaction with SO2 according to the following formula, it becomes mirabilite, which is fixed in the smelt and discharged from the smelt spout 10. Through this reaction, the concentration of SOx in the exhaust gas could be drastically reduced as shown in FIG. 3, and according to actual values, the amount of SOx generated could be reduced to 2 to 5 ppm, which was 1110 to 11100 compared to the conventional value.

According to the present invention, it is possible to effectively dry black liquor and obtain a good quality charbet, and it is also possible to significantly reduce the SOx concentration in the combustion exhaust gas, and there is no need to install an expensive desulfurization device.

[Brief explanation of drawings]

Fig. 1 is a schematic side cross-sectional view of a black liquor recovery boiler in which the method of the present invention is implemented, Fig. 2 is a plan view of the furnace of the black liquor recovery boiler shown in Fig. 1, and Fig. 3 shows the effects of the present invention. 3 is a graph showing chart temperature and SOx concentration. 1...Boiler, 2...Air and water cylinder, 3...
... Water barrel, 4 ... Superheater, 5 ... Screen pipe, 6 ... Furnace, 7 ... Black liquor injection pipe, 8 ... . . . Black liquor adhesion layer, 9 . . . Charbet, 10 . . . Smelt spout.

Claims (1)

[Claims] 1 In a black liquor recovery boiler in which the black liquor injected into the furnace dries and falls under its own weight to form a charbet, the temperature of the charbet is maintained at 900°C or higher to volatilize sodium compounds, which cause combustion. A combustion method for a black liquor recovery boiler characterized by reducing sulfur oxides in combustion gas by reaction-fixing sulfur oxides in the gas.