JP6719154B2 - Low calcium fluidized bed coal ash solidification method and solidified body - Google Patents

Description

The present invention relates to a technique for recycling low-calcium fluidized-bed coal ash having a low content of calcium (CaO) among coal ash (fluidized-bed coal ash) discharged from a fluidized-bed boiler, and more particularly, to low-calcium fluidized-bed coal ash. The present invention relates to a solidification technique for adding a solidified geopolymer.

Coal ash is a residue generated when a coal-fired power plant burns coal to generate electric energy. The total amount of coal ash generated in 2013 was 12.89 million tons (the breakdown is 9.93 million tons for the electric power industry and 2.96 million tons for the general industry). It increased by 210,000 tons from the previous year. On the other hand, the effective amount of coal ash in 2013 was 12.49 million tons. The breakdown is that the cement sector accounts for a high level of 67% (83.9 million tons) of effective use. In other fields, it is used in a wide range of applications including cement admixtures, artificial ultralight aggregates, road materials, landfill materials, and embankment materials.

However, cement production has been declining in recent years, and it is difficult to expect a significant increase in production in the future. In addition, it is clear that the conditions for collection will worsen in the future due to competition between municipal waste and incinerator ash from sewage sludge. Regarding ash disposal, it is becoming difficult to secure a final disposal site, and there are some movements in the local government to impose industrial waste tax. Therefore, in order to cope with the future increase in coal ash, the development of effective utilization methods other than cement raw materials has become an important issue, and it is particularly expected to be used as a civil engineering material with a large potential for large-scale utilization. There is.

There are two types of coal combustion methods: a pulverized coal combustion method (PC method) and a circulating fluidized bed combustion method (CFB method). In the case of the PC method, the combustion temperature reaches 1500° C., so the coal ash melts. Of these, 85 to 95% are suspended in combustion gas to form spherical particles, which are collected by an electrostatic precipitator (fly ash), and the remaining 5 to 15% are clinker ash. Further, in the PC system, NO _x and SO _x are exhausted from the chimney after being processed by a flue gas denitration/desulfurization device so as to be below a required standard. The CaO content of fly ash is 5% or less. On the other hand, in the case of the CFB method, the combustion temperature is 1000° C. or lower (800 to 900° C.), NO _x is not generated, and the coal ash is not melted, so that it does not vitrify and is angular. It becomes a fixed form. In the CFB method, a desulfurization method of any one of the limestone-gypsum method, the magnesium hydroxide method and the activated carbon method is used in order to prevent the sulfur content contained in coal from being converted into SO _x in the combustion process. When the limestone-gypsum method or activated carbon method is used, the coal ash contains a large amount of CaO components (15 to 30%), but in the case of the magnesium hydroxide method, coal ash with a low CaO content (5% or less) is used. Is discharged.

In recent years, thermal power plants with a power generation output of less than 11.25 million kW (hereinafter referred to as "small-scale thermal power plants") are backed by tight power supply and demand, rising electricity prices, power system reforms, and the arrival of power generation facility renewal. ) Installation projects and plans are rapidly increasing. Coal and natural gas are candidates for fossil fuels used in thermal power plants, but in small-scale thermal power plants, natural gas accounts for a large proportion of the fuel price in the power generation cost, making it a large-scale thermal power plant. Compared to the above, it is relatively inefficient, resulting in relatively high fuel costs, and is susceptible to price fluctuations. Furthermore, it has a huge infrastructure for liquefying natural gas in producing countries and transporting it to domestic demand sites. Since investment is required and it is difficult for small and medium-sized customers to procure their own, and because of the restrictions on the points where natural gas can be supplied, it is considered that coal is likely to be adopted. Comparing the PC method and the CFB method, the former has relatively high thermal efficiency, but the biomass fuel requires a high-quality fuel, while the latter is a solid fuel, the It is possible to adopt various fuels such as high- to low-grade ones such as physical fuels, homogeneous and non-homogeneous ones, and there are also cases where multiple types of these are mixed and burned. Until now, the PC method has been the mainstream, but the CFB method has advantages in terms of measures against global warming and recycling of waste, since various fuel types such as biomass and waste fuel can be used by burning or mixed burning. Therefore, it can be assumed that the CFB method will increase more and more in the case of small-scale thermal power plants. Also, the magnesium hydroxide method is simpler and less expensive than the other two methods, so it is widely used in small-scale thermal power plants. Therefore, it is expected that the emission amount of low-calcium fluidized bed coal ash having a low CaO content will increase in the future.

On the other hand, the method of causing a polycondensation reaction to generate a monolith (solidified body of geopolymer) by the stimulation of an alkaline solution is used in industries such as blast furnace slag, fly ash, municipal waste incinerated ash molten slag and sewage sludge incinerated ash molten slag. It has attracted much attention as a new effective use method of waste. It has been confirmed that fly ash and high-calcium fluidized-bed coal ash having a high CaO content can be recycled as active fillers for producing a solidified geopolymer (Non-Patent Documents 1 and 2). However, there have been no studies on the solidification method of low-calcium fluidized bed coal ash having a low CaO content, including whether it can be used for producing a solidified geopolymer.

P.T.Fernando, et al.: Alkali-activated binders: A review Part 1. Historical background, terminology, reaction mechanisms and hydration products, Construction and Building Materials, Vol.22, pp.1305-1314, 2008. T. Kihara, et al.: Consolidation of pushed fluidized bed combustion ash (PF-Ash) by the geopolymer technique at ambient temperature, Science for New Technology of Silicate Ceramics, CIMTEC 2002, pp.163-168, 2003.

Based on the above background, the problem to be solved by the present invention is to try to utilize low calcium fluidized bed coal ash with a low CaO content in the production of a solidified geopolymer, and to develop a new low calcium fluidized bed coal ash. To provide recycling technology.

According to the present invention, "a low-calcium fluidized bed coal ash solidifying method, in which low-calcium fluidized-bed coal ash is added to an active filler and an alkaline solution and kneaded, and cured to solidify" and "active filler and alkaline solution are low in A solidified product of low calcium fluidized bed coal ash, which is obtained by adding and kneading calcium fluidized bed coal ash and curing, is provided.

Alkaline solution (usually JIS No. 1 water sodium silicate aqueous solution or JIS No. 1 water sodium silicate aqueous solution and mixed aqueous solution of caustic soda) and an active filler excellent in the ability to elute metal ions, for example, amorphous metakaolin , When mixed with at least one of glassy slag such as blast furnace slag, municipal waste/sewage sludge incinerator ash molten slag, and glassy fly ash, a polycondensation reaction occurs and a solidified geopolymer can be produced. .. As described above, high calcium fluidized bed coal ash having a high CaO content is capable of producing a solidified geopolymer, and is considered to be one type of active filler. On the other hand, powder (hereinafter referred to as "inert filler") having a poor ability to elute metal ions in an alkaline solution, such as quartz and red mud, does not undergo a polycondensation reaction and cannot be solidified.

As will be described later, experiments by the present inventors have revealed that low-calcium fluidized bed coal ash can be classified as an inert filler. Therefore, in the present invention, a solidified geopolymer is produced by substituting (substituting) a part of the low-calcium fluidized bed coal ash with an active filler, thereby solidifying the low-calcium fluidized-bed coal ash, that is, a low calcium fluidized bed. It was possible to recycle the bed coal ash into a solidified geopolymer.

According to the present invention, it is possible to recycle low-calcium fluidized bed coal ash into a geopolymer solidified body, and the low-calcium fluidized bed coal ash can be effectively used.

5 shows the measurement results of bending strength and compressive strength of a solidified product of low-calcium fluidized bed coal ash (geopolymer solidified product to which low-calcium fluidized bed coal ash is added) according to an example of the present invention.

The low-calcium fluidized bed coal ash is a fluidized bed coal ash having a low CaO content produced by the circulating fluidized bed combustion method (CFB method) as described above. In many cases of the CFB method, low-sulfur coal is burned or a desulfurization method of the magnesium hydroxide method is adopted, so no limestone is added. Therefore, the chemical composition of the fluidized bed coal ash is similar to that of pulverized coal combustion system (PC system) coal ash (fly ash), and the content of CaO is small.

On the other hand, recently, not only coal, but also biomass and waste plastic fuel (RPF) tend to be mixed with coal and burned, and two types of fluidized bed coal ash, so-called "exclusive ash" and "mixed ash" are discharged. ing. As mentioned above, the standard material for recycling coal ash is the cement raw material. Since the burnt ash has the same chemical composition as PC-based fly ash, it can be used as a cement raw material. However, the mixed ash is not suitable as a cement raw material because it contains a large amount of alumina. In the present invention, both the exclusively burned ash and the mixed burned ash can be used as low-calcium fluidized bed coal ash for producing a solidified geopolymer. That is, in the present invention, as the low-calcium fluidized bed coal ash, a powdery material obtained by burning coal alone or a mixture of coal and waste in a fluidized bed at 1000° C. or lower can be used. The CaO content of the low-calcium fluidized bed coal ash used in the present invention is 10% by mass or less, typically 5% by mass or less.

As the active filler, one containing at least one of blast furnace slag powder, fly ash, high calcium fluidized bed coal ash, municipal solid waste incinerated ash molten slag powder, metakaolin and sewage sludge incinerated ash molten slag powder can be used. .. As the alkaline solution, an aqueous solution of sodium silicate or potassium silicate or a mixed aqueous solution of sodium silicate or potassium silicate and sodium hydroxide or potassium hydroxide can be used.

In the present invention, the blending ratio of the active filler and the low-calcium fluidized bed coal ash, and the solid content concentration and usage amount of the alkaline solution (hereinafter, collectively referred to as “solidification conditions”) mainly produce a solidified product. It affects the fluidity at the stage of curing (before curing) and the performance of the solidified product (product). Therefore, these solidification conditions should be appropriately determined in consideration of the fluidity required at the stage of manufacturing the solidified product (before curing) and the performance required for the solidified product (product) according to the manufacturing conditions of the product, the application, etc. Good.

An attempt was made to produce a solidified geopolymer by using specially burned ash and mixed ash as low-calcium fluidized bed coal ash (Experiments I to VI below). The quality of the burned coal varies depending on the timing of discharge of the coal ash, so the quality of the coal ash differs. Table 1 shows the chemical components and physical properties of the three types of low-calcium fluidized bed coal ash (N11, N12, N12A) and blast furnace slag (BFS) used in this example. In the following description, the three types of low-calcium fluidized bed coal ash and blast furnace slag are collectively referred to as "filler".

The CaO content of the mixed ash is higher than that of the combustion ash (10% or less), but the CaO of the mixed ash mostly comes from sources other than coal. However, the CaO content of the mixed ash is 10 mass% or less, which is far less than the CaO content of the blast furnace slag.

<Experiment I>
The mixed burned ash (N11) and the exclusively burned ash (N12, N12A) were individually mixed with an alkali solution at a predetermined liquid-solid ratio ((mass of alkali solution/mass of filler)) to make a solidified geopolymer. The alkaline solutions used were the following two types, solution 1 and solution 0.
Liquid No. 1: A commercially available JIS No. 1 water sodium silicate liquid (commonly called No. 1 water glass) was diluted with water to have a specific gravity of 1.27.
Solution No. 0: Solution No. 1 and a caustic soda aqueous solution having a molar concentration of 10 M were mixed at a volume ratio of 3:1 (specific gravity 1.31).

Alkaline solution that is usually used for producing solidified geopolymer is No. 0 solution, but its cost is high, and it may cause inconvenience in storage/measurement and wastewater treatment of caustic soda solution in concrete manufacturing plant. In this example, an experiment using the No. 1 solution was also conducted. The mixed ash (N11) foams after kneading with No. 0 liquid, so after the foaming stopped, the mixture was stirred again to remove the foam. The foaming time was 20 to 40 minutes, and the re-stirring was performed 1 hour after the kneading. It is considered that the foaming is due to the fact that metallic aluminum derived from waste fuel produces hydrogen gas in an alkaline environment such as caustic soda contained in the mixed ash. When another filler and an alkaline solution were used, foaming did not occur, so that re-stirring for removing bubbles was not performed.

Immediately after kneading or re-stirring, it is driven into a 20×20×80 mm three-column prismatic metal mold frame, sealed with a wrap, and subjected to high temperature curing at 80° C. or 60° C. to a certain degree of curing (approximately 3 hours Later), the mold was removed, and curing was continued until a predetermined time. Bending strength was measured by a three-point method at the age of 2 days. Further, the time from the kneading to the start of curing, that is, the so-called pot life was measured. As a measuring method, at the room temperature of 20±3° C., the smoothed sample surface is pierced with a laboratory microspatel, and the time until the indentation does not show any liquid and the indentation remains clearly is measured. , The pot life.

The results of Experiment I are shown in Tables 2 and 3.

As shown in Tables 2 and 3, the pot life is shorter when the solution No. 1 is used as the alkaline solution with the co-fired ash than the solution No. 0. The pot life of the geopolymer using mixed ash and No. 1 solution is only about 20 minutes. The flexural strength of the solidified geopolymer is 3 MPa or less when the mixed ash or the exclusively burned ash is used alone as the filler, and is 15 MPa or less when converted to the compressive strength, which makes it difficult to produce a practical geopolymer solidified body. is there. According to these results, the low calcium fluidized bed coal ash having a low CaO content can be classified as an inert filler.

<Experiment II>
A part of the mixed ash (N11) was replaced with blast furnace slag fine powder (BFS) and mixed with an alkaline solution to prepare a solidified geopolymer. The performance of the produced geopolymer solidified product is shown in Table 4. The methods for measuring the bending strength and the pot life are the same as in Experiment I.

As shown in Table 4, when the No. 0 solution was used as the alkaline solution, the pot life was shortened from 5 hours to 35 minutes when the BFS substitution rate was increased from 0% to 25%. However, it is observed that the strength was significantly increased by the addition of BFS. If the substitution ratio of BFS was the same, when using solution No. 0, the bending strength was higher than that of solution No. 1, and a geopolymer solidified body having a bending strength of 6 MPa or more could be produced by high temperature curing at 80°C (compression). When converted into strength, it is 30 MPa or more).

<Experiment III>
A part of the exclusively baked ash (N12, N12A) was replaced with blast furnace slag fine powder (BFS) and mixed with an alkaline solution to prepare a solidified geopolymer body. The performance of the produced solidified geopolymer is shown in Tables 5 and 6. The methods for measuring the bending strength and the pot life are the same as in Experiment I.

As shown in Table 5, the geopolymer obtained by substituting 25% of the exclusively burned ash (N12) with BFS and using No. 0 solution had a bending strength of 9.63 MPa after curing at 80° C. for 6 hours. When 50% of BFS was added, the flexural strength of the test body aged at 80° C. for 6 hours was 8.21 MPa even with the solution No. 1. In other words, it can be said that by adding BFS, it is possible to produce a common concrete equivalent product having a compressive strength of 40 MPa or more with the exclusively burned ash (N12). Further, even if the burnt ash (N12) and BFS are used together, the pot life is 1 hour or more, and it can be said that there is no problem in the work of driving into the mold.

In addition, as shown in Table 6, when the second sample N12A of burnt ash and BFS are used together, if the replacement ratio of BFS is 50% and 25%, the pot life is 2 hours 50 minutes and 10 hours, respectively. Since it is half, it is considered that there is no problem in the production work of the solidified geopolymer.

As shown in Table 5 and Table 6, the higher the substitution rate of BFS or the higher the curing temperature, the higher the bending strength of the solidified geopolymer. When the substitution rate of BFS is 50%, even if the No. 1 solution is used, high temperature curing at 60°C or higher can produce a solidified geopolymer having a bending strength of 6.0 MPa or more, that is, a compressive strength of 30 MPa or more. Admitted.

<Experiment VI>
A solidified geopolymer (geopolymer mortar) was prepared by using a combination of exclusively baked ash (N12) and blast furnace slag fine powder (BFS), and the performance of room temperature curing was measured. The mortar formulation is shown in Table 7. Toyoura sand was used as sand (fine aggregate).

The size of the test piece was 4×4×16 cm, and three prism-shaped test pieces were prepared at 20° C. and 0% R.S. After being cured in a H-curing tank up to 28 days of age, the bending strength was measured by a three-point method. The bending strength was the average value of three test pieces. A compression test was performed using the folded pieces after the bending test, and the average value of the six folded pieces was taken as the compressive strength.

FIG. 1 shows the measurement results of bending strength and compressive strength. From the figure, it was confirmed that when the burned ash and BFS were used together, a solidified geopolymer (geopolymer mortar) having a compressive strength of 28 MPa or more could be produced even at room temperature curing, and the higher the substitution rate of BFS, the greater the strength. .. It was also found that the compressive strength is 5 times or more the bending strength.

The following findings were obtained by summarizing the results of the above Experiments I to VI.
a) Low-calcium fluidized bed coal ash having a low CaO content is an inert filler regardless of mixed ash and exclusively ash, and a monolith solidified by mixing with an alkaline solution does not have the strength required for practical use.
b) By substituting a part of the low-calcium fluidized bed coal ash with blast furnace slag fine powder which is an active filler, a geopolymer solidified body having a compressive strength of 30 MPa or more can be produced under both high temperature curing and normal temperature curing.
c) In order to prepare a geopolymer solidified body having a compressive strength of 30 MPa or more, the substitution rate (mixing rate) of the blast furnace slag fine powder was 25% or more when No. 0 solution was used, and No. 1 solution was used. In this case, 50% or more is preferable.
d) In the case of high temperature curing, it is necessary to set the curing temperature to 60°C or higher. Further, the curing time may be set within 12 hours.
e) When replacing 50% or less of exclusively burned ash with blast furnace slag fine powder, the pot life of the mixture of filler and alkaline solution is about 80 minutes, but when replacing 25% of mixed burned ash with blast furnace slag fine powder Has a pot life of only 35 minutes.
f) When mixed ash and No. 0 liquid are used, the mixture of filler and alkaline solution foams within 20 to 40 minutes after kneading, regardless of whether blast furnace slag fine powder is replaced or not, but No. 1 It does not foam when a liquid is used. A porous lightweight solidified body can be produced by utilizing the foaming property of the mixed ash of No. 0 liquid.

Claims (2)

A method of solidifying low calcium fluidized bed coal ash, which comprises adding low calcium fluidized bed coal ash to an active filler and an alkaline solution, kneading, and curing to solidify .
The low-calcium fluidized bed coal ash is a powdery material obtained by burning coal alone or a mixture of coal and waste in a fluidized bed, and having a CaO content of 10 mass% or less,
The active filler is at least one of blast furnace slag powder, fly ash, high calcium fluidized bed coal ash (content of CaO is more than 10% by mass), municipal solid waste incinerated ash molten slag powder, metakaolin and sewage sludge incinerated ash molten slag powder. It includes one type,
A method for solidifying low-calcium fluidized bed coal ash, wherein the alkaline solution is an aqueous solution of sodium silicate or potassium silicate or a mixed aqueous solution of sodium silicate or potassium silicate and sodium hydroxide or potassium hydroxide . Kneaded by adding low calcium fluidized bed coal ash to the active filler and alkaline solution, obtained by curing, a solidified product of low calcium fluidized bed coal ash ,
The low-calcium fluidized bed coal ash is a powdery material obtained by burning coal alone or a mixture of coal and waste in a fluidized bed, and having a CaO content of 10 mass% or less,
The active filler is at least one of blast furnace slag powder, fly ash, high calcium fluidized bed coal ash (content of CaO is more than 10% by mass), municipal solid waste incinerated ash molten slag powder, metakaolin and sewage sludge incinerated ash molten slag powder. It includes one type,
A solidified product of low-calcium fluidized bed coal ash, wherein the alkaline solution is an aqueous solution of sodium silicate or potassium silicate, or a mixed aqueous solution of sodium silicate or potassium silicate and sodium hydroxide or potassium hydroxide .

Images