JP2008281297A - Method for removing sulfur in fluidized bed combustor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of effectively removing sulfur in combustion gas in a fluid bed furnace which effectively utilizes lime sludge generated in a paper mill. <P>SOLUTION: In the method of removing sulfur in the fluid bed combustion equipment, the lime sludge generated in slaking and causticizing processes as a white liquor manufacturing process for pulp digestion is charged in the fluid bed furnace. The lime sludge has a specific surface area 0.300 m<SP>2</SP>/g, and is charged in the fluid bed by mixing with sludge generated in an effluent treatment process. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a sulfur removal method in a fluidized bed combustion apparatus, and more particularly to a sulfur removal method using lime sludge as a desulfurization agent.

For the purpose of obtaining thermal energy, not only fossil fuels such as oil and coal, but recently, waste tires are also used as fuel because of the high calorific value among the effective use of waste. Combustion causes an increase in harmful sulfur oxides (SOx) such as sulfurous acid gas and sulfuric acid gas. Since these must be removed to a concentration that meets environmental standards and the combustion gas must be discharged, sulfur oxides are conventionally removed. The method is roughly divided into the following two methods. One is a method of desulfurization in the furnace in which a desulfurization agent is introduced into the furnace and the sulfur oxide is absorbed and removed. Usually, the desulfurization agent is limestone. Dolomite is used, and this method is mainly used in fluidized bed furnaces. Also, there is a flue gas desulfurization method in which a contact layer (tower) of a desulfurization agent and exhaust gas is provided in a part of the flue downstream of the furnace, and the sulfur oxide in the exhaust gas is removed by a neutralization reaction between the sulfur oxide and the desulfurization agent. In this method, sulfur oxide and limestone, which is a desulfurizing agent, are neutralized and changed to gypsum, so that they are effectively used.

In the in-furnace desulfurization method, the utilization rate of limestone or dolomite mainly composed of calcium used as a desulfurizing agent is low. For example, in a normal pressure bubbling fluidized bed boiler, a desulfurization rate of 90% or more is obtained. In this case, the utilization rate of limestone is about 15 to 25%, and in a circulating fluidized bed boiler, the utilization rate is said to be 25 to 35%. Therefore, in order to completely remove the sulfur content contained in the fuel and raw material, it is necessary to input a large amount of desulfurizing agent. This is because in conventional limestone, a desulfurization reaction occurs only on the surface and does not react to the inside of the particles, so that a lot of calcium remains unused. The use of a large amount of desulfurizing agent not only has the problem of increasing the amount of residue processed as ash and requiring energy for the treatment, but the addition of excessive desulfurizing agent also oxidizes the N content in the fuel with nitrogen. It is considered that the desulfurization agent suppresses the conversion to N ₂ O by converting it to a product, while promoting the conversion to NOx.

In order to solve such a problem, calcium oxide (CaO) or calcium hydroxide (Ca (OH)) is used as a desulfurization agent having a finer particle diameter, a larger proportion of available surface area, and excellent reactivity. _A method using fine particles such as ₂ ) has been studied. However, pulverization of limestone is costly, and there is a problem that it is discharged to the outside of the furnace along with the gas flow when it is charged.

In place of desulfurization agents such as limestone and dolomite, raw concrete sludge (Patent Document 1) and lime cake (Patent Document 2) discharged when refining the sugar solution of raw material beet in the sugar production process are substituted. Although it is effective in terms of effective use of waste, the use of the technology is limited because it is obtained at a specific place. Moreover, although a method (Patent Document 3) in which porous alumina is added as a catalyst together with a desulfurizing agent is also disclosed, the apparatus needs to be modified and is not practical.
JP-A-10-267221 JP 2005-98673 A JP 2004-82111 A

An object of this invention is to provide the removal method of the sulfur content in the combustion gas in a fluidized bed furnace which utilizes effectively the lime sludge generated in a paper mill.

As a result of intensive investigations to achieve the above object, the present inventors have found that lime sludge (hereinafter simply referred to as lime sludge) generated from a causticizing process that is a white liquor manufacturing process for pulp cooking is purchased lime. Therefore, the present invention has been completed by finding that it is effective as a desulfurization agent because of its large specific surface area and high reactivity.

According to the present invention, the following sulfur removal method and fluidized bed furnace desulfurization agent in a fluidized bed combustion apparatus are provided.
(1) A method for removing sulfur in a fluidized bed combustor in which lime sludge generated from a soaking / causticizing process, which is a white liquor manufacturing process for pulp cooking, is introduced into a fluidized bed furnace.
(2) The sulfur content removal method in the fluidized bed combustion apparatus according to (1), wherein the lime sludge has a specific surface area of 0.300 m ² / g or more.
(3) The method for removing sulfur content in a fluidized bed combustion apparatus according to any one of (1) and (2), wherein the lime sludge is mixed with sludge sludge generated from a wastewater treatment step and charged into a fluidized bed. .
(4) The fluidized bed combustion according to any one of (1) to (3), wherein the lime sludge is mixed with dregs generated from a green liquor clarification step which is a cooking chemical recovery step and charged into a fluidized bed furnace. Method for removing sulfur content in the apparatus.

According to the present invention, in a fluidized bed combustion apparatus, it is possible to provide a method for removing sulfur efficiently and inexpensively, reduce ash treatment costs by reducing the amount of lime input, and reduce heavy oil by reducing the amount of lime fired in the lime kiln. The amount used can be reduced.

According to the present invention, in a fluidized bed combustion apparatus, lime sludge is introduced into a furnace as a desulfurizing agent, and an oxidation reaction (combustion) or gasification using petroleum, coal, biomass, waste tires, waste plastics or the like as fuel or raw materials. The sulfur content in the gas generated in the furnace is removed by pyrolysis or the like.
The lime sludge used in the present invention is a calcium carbonate powder generated from a process of producing a white liquor (a mixture of sodium sulfide and sodium hydroxide) used when pulping wood chips, a so-called soaking / causticizing process. It is. That is, the waste liquid (black liquor) containing sodium and sulfur discharged from the pulping process is reduced and burned in the recovery boiler to recover inorganic solids called smelt containing sodium carbonate and sodium sulfide. By adding calcium oxide to the green liquor obtained by dissolving the smelt, the soaking reaction (1) and the causticizing reaction (2) proceed to produce a white liquor composed of sodium sulfide and sodium hydroxide. In the course of this series of reactions, calcium carbonate is produced, and the white powder that has been washed and dehydrated is lime sludge.
(1) CaO + H ₂ O → Ca (OH) ₂
(2) Na ₂ CO ₃ + Ca (OH) ₂ → 2NaOH + CaCO ₃

Lime sludge preferably has a specific surface area of 0.300 m ² / g or more.
A specific surface area of less than 0.300 m ² / g is not preferred because the reactivity with the sulfur content decreases and a sufficient desulfurization effect is obtained because the input amount increases and the amount of ash increases.
As a method for keeping the specific surface area of the lime sludge within the above range, it is effective to provide a sludge classification step before charging the lime sludge to remove the large particle size.

In the present invention, lime sludge is used as a desulfurizing agent in the fluidized bed furnace of the fluidized bed combustion apparatus, and the Ca content of the desulfurizing agent with respect to the sulfur content in the fuel and / or raw material is usually 0.5 to 4 (molar ratio). The desulfurization is performed in the furnace. When this molar ratio is lower than 0.5, sufficient reaction may not be obtained. On the other hand, when the ratio is higher than 4, a high reaction rate can be obtained, but the amount of residue generated may increase and the amount of ash treatment may increase. Therefore, it is preferable that the desulfurization is performed in the furnace by introducing the mixture in a ratio of preferably 1 to 3 (molar ratio).

The lime sludge is pumped in a slurry state (solid content concentration of about 40%) before being dehydrated in the causticizing process, and directly or mixed with sludge sludge. Or it conveys in the state (vacuum dehydration process about 70%) which was vacuum-dehydrated in the causticizing process, and mixes with direct or sludge sludge etc. Alternatively, it is blown in a state where moisture is further removed using a kiln exhaust gas or the like (solid content concentration of about 100%) and directly or mixed with sludge sludge and the like.

When the lime sludge is introduced into the fluidized bed combustion apparatus as a desulfurizing agent, it is preferably mixed with the sludge sludge generated from the waste water treatment process and introduced into the fluidized bed. By mixing with sludge sludge, powdery lime sludge reaches the bottom of the furnace without being scattered in the furnace and promotes the desulfurization effect by obtaining sufficient residence time.

When the lime sludge is introduced into the fluidized bed combustion apparatus as a desulfurizing agent, it is preferably mixed with the dregs generated from the green liquor clarification process, which is a cooking chemical recovery process, and then introduced into the fluidized bed. Dregs are settled and removed in a green liquor clarifier, then concentrated to a solids concentration of about 30% in a dehydration process such as a belt press, and incinerated in a fluidized bed combustor. By mixing the powdered lime sludge with the dregs, the lime sludge reaches the furnace low without scattering in the furnace, and the desulfurization effect is promoted by obtaining sufficient residence time.

The fluidized bed combustion apparatus used in the present invention is a state in which solid particles of an appropriate size are filled in a container, and when air is blown uniformly from the container, the particles float to a certain height and move around violently. . This layer of particles that moves violently is called a fluidized bed. It is a combustor that burns coal and generates steam, and the coal particles are instantly dried and ignited by the flow of solid particles. In order to burn efficiently with good mixing with air and a long residence time, it is a device that can burn even low-quality fuel that was difficult to burn with the conventional combustion method. In the present invention, an atmospheric fluidized bed combustion apparatus, a pressurized fluidized bed combustion apparatus, an external / internal circulation type, or a bubbling type may be used.
In the present invention, lime sludge can be used as it is as a desulfurization agent, but can be used as a desulfurization agent by mixing with the above-mentioned other components.

In the method of the present invention, the fuel species or raw materials to be combusted or gasified, thermally decomposed, and partially oxidized include those containing a sulfur component that generates sulfurous acid gas, sulfuric acid gas, and other sulfur oxides (SOx). For example, solid fuel such as coal, petroleum coke, oil sand and biomass, pseudo fluid fuel containing water or oil blended with coal, liquid fuel such as heavy oil, kerosene, alcohol mixture, LPG, LNG, gaseous fuel such as factory exhaust gas, waste, sludge, plastic, sludge, tire and other waste, or at least two types of mixtures selected from these are used.

In the method of the present invention, the reaction in various fluidized bed apparatuses such as a fluidized bed combustion boiler, a fluidized bed gasification furnace, a fluidized bed pyrolysis furnace, a fluidized bed partial gasification furnace and the like is preferably in the range of 500 to 2,000 ° C. However, in order to further increase the desulfurization efficiency, it is preferable to perform the reaction at a temperature of 600 to 1,000 ° C. from the viewpoint of improving the reaction rate.

Next, although an example and a comparative example show the present invention concretely, the present invention is not limited to the following example.

Example 1
Lime sludge containing Ca corresponding to a molar ratio (Ca / S) of 1.3 with respect to the S content contained in the waste tire 1.5% (including 20% of water after drum filter dehydration) and other fuels Table 2 shows the result of confirming the effect of desulfurization by introducing it into a fluidized bed combustion apparatus having the fuel configuration shown in Table 1 without mixing.

Example 2
Lime sludge obtained by mixing lime sludge containing Ca corresponding to a molar ratio (Ca / S) 1.2 with respect to S content contained in the waste tire 1.5% with waste sludge (from single lime sludge and waste sludge) Table 2 shows the result of confirming the effect of desulfurization by introducing into the fluidized bed combustion apparatus having the fuel configuration shown in Table 1.

Example 3
Lime sludge mixed with lime sludge and dregs containing Ca corresponding to a molar ratio (Ca / S) of 1.2 with respect to S content of 1.5% in the waste tire (total carry-in from single lime sludge and dregs Table 2 shows the result of confirming the effect of desulfurization by charging the fluidized bed combustor having the fuel composition shown in Table 1 into the fluidized bed combustion apparatus.

Comparative Example 1
Flow of fuel composition shown in Table 1 for purchased lime (powder with 0% water content) containing Ca corresponding to a molar ratio (Ca / S) of 1.8 to S contained in 1.5% of the waste tire Table 2 shows the result of the desulfurization effect that was introduced into the layer combustion apparatus.

As shown in Table 2, by mixing lime sludge with drainage sludge or dregs (molar ratio 1.2), it was more efficiently desulfurized than lime sludge alone (molar ratio 1.3). This is because the lime sludge thrown into the furnace reached the furnace bottom together with the sludge, and sufficient residence time was obtained.

As shown in Table 2, purchased lime requires a larger amount of input than lime sludge to obtain equivalent exhaust gas SOx because purchased lime has a smaller specific surface area than lime sludge. This is because the reactivity with the minute is poor. Moreover, since the purchased lime does not contain moisture, it is scattered in the furnace immediately after the addition, and is discharged out of the system without obtaining sufficient retention.

Claims (4)

A method for removing a sulfur content in a fluidized bed combustion apparatus, wherein lime sludge generated from a soaking / causticizing process, which is a white liquor manufacturing process for pulp cooking, is introduced into a fluidized bed furnace. ^{2. The} method for removing a sulfur content in a fluidized bed combustion apparatus according to claim 1, wherein the lime sludge has a specific surface area of 0.300 m ² / g or more. The method for removing a sulfur content in a fluidized bed combustion apparatus according to claim 1 or 2, wherein the lime sludge is mixed with sludge sludge generated from a wastewater treatment step and charged into a fluidized bed furnace. The fluidized bed combustion apparatus according to any one of claims 1 to 3, wherein the lime sludge is mixed with dregs generated from a green liquor clarification step, which is a cooking chemical recovery step, and charged into a fluidized bed furnace. How to remove sulfur.