Patent ID: 12195407

MODES FOR CARRYING OUT THE INVENTION

In the invention, the raw fly ash powder that is to be subjected to the treatment for decreasing the amount of the unburned carbon stands for the fly ashes in general that generate in the facilities that burn coal, such as coal burning thermal plants. The raw fly ash powder, further, includes the fly ashes that generate from the facilities that burn, in combination with the coal, fuels other than the coal as well as inflammable wastes. Usually, the fly ash chiefly comprises silica (SiO2) and alumina (Al2O3) (these inorganic components account for 70 to 80% of the whole components) and, further, includes ferric oxide (Fe2O3), calcium oxide (CaO), magnesium oxide (MgO) and the like.

The fly ash contains the unburned carbon which is the half-burned carbon remainder, and its content amounts to about 15% by mass at the greatest. A large amount of the unburned carbon (hereinafter often described as LOI) causes a problem when the fly ash is used as a mixing material. For example, when the fly ash is mixed to a mortar or a concrete, it is highly probable that the unburned carbon floats on their surfaces and form darkened portions. It is, further, probable that the unburned carbon adsorbs chemicals such as chemical blending agents.

The invention decreases the amount of the unburned carbon by the heating by using the fluidized bed heating unit that has the medium-fluidized bed, and efficiently recovers the reformed fly ash that is obtained, i.e., efficiently recovers the fly ash that contains the unburned carbon in decreased amounts. A variety of methods have been known for measuring the amount of the unburned carbon. Examples are a method of detecting the CO2·CO gases generated by the combustion by using infrared rays, a method of measuring the ignition loss and estimating the amount of the unburned carbon from the amount of the ignition loss, a method based on the calculation of the amounts of Methylene Blue adsorption, a bulk specific gravity testing method, and a method of estimating the amount of the unburned carbon by the irradiation with microwaves. Any of the above methods can be employed in the present invention.

The amount of the unburned carbon is, hereinafter, often called LOI (loss on ignition).

FIG.1illustrates a process of the present invention using a fluidized bed heating unit.

InFIG.1, the fluidized bed heating unit generally designated at1has an upright cylindrical shape and is forming, upward from the lower side, a combustion chamber3, a medium-fluidized bed7(hereinafter called fluidized bed) partitioned from the combustion chamber3by a dispersion plate5, and a hollow head portion9.

The combustion chamber3is a region where a fuel such as hydrocarbon is burned by a burner11to form a high-temperature gas. Oxygen in an amount theoretically required for completely burning the fuel and a gas (usually, the air) containing oxygen in an amount for burning the unburned carbon contained in the raw fly ash powder, are fed into the combustion chamber3together with the fuel, and are burned by the burner11to form a high-temperature gas. The ascending current of the high-temperature gas forms the fluidized bed7that is heated up to a predetermined temperature and, further, heats, fluidizes and conveys the raw fly ash powder that is fed into the heating unit1. The high-temperature gas is hereinafter called fluidized gas.

In this embodiment, for example, it is desired to feed oxygen in an amount of 1.05 to 5.0 times as great as the amount of oxygen theoretically required for completely burning the fuel. If the amount thereof is less than 1.05 times, then oxygen is almost all consumed for burning the fuel and no oxygen is left for burning the unburned carbon.

From the standpoint of cost and safety in the invention, it is desired to use the air or nitrogen as the fluidized gas. In this case, the fluidized gas contains nitrogen in addition to containing the combustion gas (carbon dioxide, etc.) produced by the combustion of the fuel and excess of oxygen gas (surplus of oxygen gas) that was not consumed by the combustion.

InFIG.1, white arrows indicate the flow of the fluidized gas while black arrows indicate the flow of the fly ash powder fed into the heating unit1.

It is also allowable to produce the fluidized gas (high-temperature gas) by methods other than the method described above. For instance, a gas that is flown is heated by a method other than the combustion method, or a system is employed for heating the gas from the outer circumferential side of the heating unit by using an electric heater or flame (externally heating system).

The fluidized bed7is formed by a particulate medium13that is heated and fluidized. The raw fly ash is fed through a raw material feed port15formed in the lower part of the fluidized bed7. That is, the solid particulate medium held on the dispersion plate5is heated and floated by the fluidized gas to thereby form the fluidized bed7. Due to the fluidized gas, further, the raw fly ash powder is fluidized and conveyed. Here, the raw material feed port15has been formed in the lower part of the fluidized bed7, and hence the raw fly ash powder introduced through the raw material feed port15is heated by the fluidized gas and the fluidized bed7to a sufficient degree.

In the upper part of the fluidized bed7, there is formed a hollow head part9where no granular medium13is floating. Namely, the granular medium13that forms the fluidized bed7has been so set as will not to be discharged by the fluidized gas out of the heating unit1.

As the particulate medium13, there is preferably used a material whose chemical composition is close to that of the fly ash, such as a particulate oxide like particulate SiO2, Al2O3, Fe2O3or Cao, or a particulate material that contains these oxides as main components so that properties of the fly ash will not be deteriorated even in case the medium is mixed into the fly ash.

It is, further, necessary that the granular medium13has a particle diameter larger than that of the raw fly ash powder introduced through the raw material feed port15. This is because if the particle diameter is smaller than the particle diameter of the raw fly ash powder, then the particulate medium13, too, is discharged out of the unit accompanying the flow of the fluidized gas. That is, it means that no hollow head part7is formed. Usually, the raw fly ash powder has a particle diameter of not larger than about 300 μm at the greatest while the granular medium13has a particle diameter of, preferably, about 0.5 to about 5 mm.

In the invention, the raw fly ash powder introduced into the fluidized bed7is heated upon coming in contact with the particulate medium13that forms the fluidized bed7and with the fluidized gas. Here, the temperature of heating is a temperature at which the unburned carbon burns and is, usually, 600° C. to 1100° C. and, preferably, 750° C. to 1000° C. If the temperature of heating is low, it becomes difficult to remove the unburned carbon by combustion to a sufficient degree. If the temperature of heating is unnecessarily high, on the other hand, then the fly ash may melt. Therefore, the temperature of the fluidized gas is adjusted to be so high that the raw fly ash powder is heated at the above-mentioned temperature.

The temperature of heating the raw fly ash powder can be measured by inserting a thermocouple in the fluidized layer7(or in the hollow head portion9).

In the invention, the raw fly ash powder is heated by the fluidized gas of a high temperature that is fed as described above (and by the fluidized bed7), and the amount of the unburned carbon (LOI) deceases in the raw fly ash powder. The fly ash that is heated is then discharged together with the fluidized gas through the take-out port17formed in the head part of the heating unit1. That is, the rate of and the velocity of flow of the fluidized gas are so set that the raw fly ash powder introduced into the heating unit1is taken out in substantially the whole amount through the take-out port17. In effect, an equilibrium state is maintained by the amount of the raw fly ash powder thrown into the heating unit1and by the amount of the fly ash that is heated and taken out through the take-out port17. This, however, does not exclude such a case where a certain amount of the fly ash may stay in the heating unit1due to the structure or the like of the heating unit1.

If compared in terms of the mass, the mass of the fly ash after heated and taken out through the take-out port17would be smaller than the mass of the raw fly ash powder by at least the amount of the carbon that has burned out.

In order to take out the fly ash in its whole amount, the amount of the fluidized gas should be suitably set depending on the form and the like of the heating unit1that is used. Usually, however, the amount of the fluidized gas fed into the heating unit1(combustion chamber3) should be so set that a superficial velocity in the column is not less than 0.5 m/sec. Here, the superficial velocity in the column is a value obtained by dividing the amount of the fluidized gas (m3/sec) by the sectional area (m2) in the portion of a maximum inner diameter in the medium-fluidized bed heating unit. The amount of the fluidized gas is calculated by using values at the temperature of heating described above.

According to the present inventors, it was learned that if the superficial velocity in the column is set to be not less than 0.5 m/sec. and, preferably, not less than 0.6 m/sec., then the raw fly ash powder can be discharged in its whole amount out of the heating unit1; i.e., the raw fly ash powder, even if it is continuously fed, does not stay in the heating unit1but can be discharged continuously (see the results of experiments inFIG.2).

If the superficial velocity of the fluidized gas becomes too great in the column, then the hollow head portion9is not formed, and the granular medium13is discharged together with the fly ash through the take-out port17. To avoid such an inconvenience, it is desired that the superficial velocity of the fluidized gas in the column is set to be, usually, about 3 m/sec. at the greatest.

Here, in the invention, the fly ash powder (fly ash after heated) discharged through the take-out port17of the heating unit1is introduced by the fluidized gas into the air classifier21, and is classified into a fine powder and a coarse powder.

That is, in the step of heating the raw fly ash powder described above, the unburned granular carbon burns upon reacting with oxygen and extinguishes. However, the granular carbon that did not burn up, i.e., the unburned granular carbon, is discharged through the take-out port17together with the ash component and the fly ash powder after heated.

According to the study by the present inventors, relatively large particles of the unburned granular carbon may not burn to a sufficient degree under the above-mentioned heating conditions, and are likely to be discharged from the heating unit1without burned out. On the other hand, small particles of the unburned carbon mostly burn out under the above-mentioned conditions.

In the invention, therefore, the fly ash taken out from the heating unit1is introduced into the air classifier21and is separated into the fine powder and the coarse powder.

That is, from the fine powder, the unburned carbon is removed to a sufficient degree by the combustion. Therefore, the fine powder separated through the air classifier21is fed together with the fluidized gas to a dust collector23where the fluidized gas is discarded whereas the fine powder is recovered as the reformed fly ash.

The coarse powder, on the other hand, is separated from the fine powder since it is probable to contain much unburned granular carbon of large particle sizes that was not burned to a sufficient degree. The separated coarse powder is once fed into a storage silo25where the amount of the unburned carbon (LOI) is measured by an LOI measuring instrument27provided in the silo25. When the LOI is smaller than a preset threshold value (LOI≤ threshold value), the coarse powder is recovered as the reformed fly ash. When the LOI is in excess of the preset value (LOI>threshold value), the coarse powder is returned back to the heating unit1(fluidized bed7) where it is heated again to lower the LOI. Here, in case the LOI is equal to the threshold value (LOI=threshold value), the coarse powder may be returned back to the heating unit1to heat-treat it again, as a matter of course.

The threshold value should be suitably set depending upon the LOI required for the fly ash that is used as a mixing material for the cement and concrete. For instance, the threshold value is set to lie in a range of, preferably, 0.5 to 4% by mass and, specifically, 0.5 to 3% by mass.

The coarse powder recovered as the reformed fly ash from the temporary storage silo25has a low LOI. Therefore, the coarse powder can also be recovered being mixed with the reformed fly ash (fine powder) that is recovered from the above-mentioned dust collector23, as a matter of course.

So far, study has been continued in an effort to remove the unburned carbon of large particle sizes relying on the air classification by utilizing the principle that the unburned carbon contained in the unburned carbon-containing fly ash has large particle sizes. However, the unburned granular carbon has a small specific gravity and is hence much contained in the fine powder, too. In practice, therefore, the fly ash could not be substantially separated, relying on the air classification, into the course particles and the fine particles that contain the unburned carbon in different amounts. According to the present invention, on the other hand, the fly ash is once heated and is then separated by the air. Accordingly, the unburned carbon of large particle diameters is distributed toward side of coarse particles and is introduced again into the heating unit1depending on the LOI of coarse particles, making it possible to efficiently reduce the content of the unburned carbon.

In the invention described above, the classification point in the air classifier is set to be in a range of, desirably, 50 to 150 μm and, specifically, 100 to 150 μm. The ratio of the unburned carbon recovered as the coarse powder increases as the classification point becomes smaller. As the classification point becomes smaller, however, it becomes more probable that even those particles that do not have to be recovered as the coarse powder are also recovered. From the standpoint of efficient operation, therefore, it is desired that the classification point is set to lie in the above-mentioned range.

No specific limitation is imposed on the air classifier21, and there can be used, for example, an air stream classifier that utilizes a centrifugal force field, a classifier that utilizes a gravitational field or a classifier that utilizes an inertial force field.

No specific limitation is imposed, either, on the dust collector23that is used for collecting the fine powder classified through the air classifier21, and there can be used anyone of any type, such as an electric dust collector, a gravity type dust collector or a centrifugal force type dust collector.

As described already, further, the LOI measuring instrument27provided for the temporary storage silo25will execute the LOI measurement relying on any known method.

EXAMPLES

The invention will now be described by the following Experimental Examples.

Experiments

There was provided a raw fly ash powder having 10.0% by mass of LOI. An average particle diameter was 44 μm while a maximum particle diameter was about 300 μm.

The raw fly ash powder was fed into the heating unit1shown inFIG.1, and the fluidized gas (air) was fed at a superficial velocity in the column of 0.64 m/sec. and a heating temperature of 850° C. The raw fly ash powder was thus heat-treated and was separated into a coarse powder and a fine powder. The classification point was set to be roughly 50 μm. The fine powder and the coarse powder were recovered at a ratio of 63% by mass and 37% by mass, respectively, per the raw fly ash powder, and the sum of the fine powder and the coarse powder was 100% by mass.

The LOI in the fine powder was 1.1% by mass while the LOI in the coarse powder was 5.0% by mass. The LOI in the fine powder was low to a sufficient degree. Therefore, the fine powder could be readily used as the reformed fly ash. On the other hand, the LOI in the coarse powder was of a high level. Therefore, the coarse powder had to be heated again when it was to be used in many fields of applications.

The LOI was measured in compliance with the method of testing the loss on ignition stipulated under the JIS A6201.

By varying the superficial velocity of the fluidized gas in the column, furthermore, the raw fly ash powder was heat-treated to measure the superficial velocity in the column and the rate of the fly ash discharged from the heating unit1(rate of the fly ash to the raw fly ash powder that was fed). The results were as shown inFIG.2.

As will be learned from the results ofFIG.2, the raw fly ash powder can be almost all discharged out of the unit without staying therein provided the superficial velocity in the column is set to be not less than 0.5 m/sec. and, specifically, not less than 0.6 m/sec.

DESCRIPTION OF REFERENCE NUMERALS

1: heating unit3: combustion chamber5: dissipation plate7: fluidized bed9: hollow head part11: burner13: granular medium15: raw material feed port17: take-out port21: air classifier23: dust collector25: temporary storage silo27: LOI measuring instrument