In recent years, attempts for reducing carbon dioxide (CO2) gas, which is a main cause of the global warming phenomenon, have been made worldwide and in all industries.
The cement industry, together with the electric power industry, the steel industry, and the like, is one of the industries which discharge a large amount of CO2 gas. The discharge amount of CO2 gas of the cement industry is about 4% of the total amount of CO2 gas discharged in Japan. For this reason, the reduction in the amount of CO2 gas discharged in the cement industry greatly contributes to the reduction in the total amount of CO2 gas discharged in Japan.
FIG. 12 shows a common cement manufacturing facility in the cement industry. In the figure, reference numeral 1 denotes a rotary kiln (cement kiln) for burning a cement material.
Two sets of preheaters 3 for preheating the cement material are provided in parallel at the kiln inlet part 2 on the left side of the rotary kiln 1 in the figure. Further, a main burner 5 for heating the inside of the rotary kiln 1 is provided in the kiln outlet part on the right side of the rotary kiln 1 in the figure. Note that reference numeral 6 in the figure denotes a clinker cooler for cooling the burned cement clinker.
Here, each of the preheaters 3 is configured by a plurality of stages of cyclones arranged in series in the vertical direction. As the cement material fed to the uppermost cyclone from a feed line 4 is successively dropped to the lower cyclones, the cement material is preheated by the high-temperature exhaust gas fed from the rotary kiln 1 and ascending from the bottom part of the preheater 3. Further, the preheated cement material is extracted from the second cyclone from the bottom part of the preheater 3, so as to be sent to a calciner 7. In the calciner 7, the cement material is heated and calcined by a burner 7a, and then introduced from the lowermost cyclone into the kiln inlet part 2 of the rotary kiln 1 via a transfer pipe 3a. 
On the other hand, an exhaust gas pipe 3b for feeding the combustion exhaust gas discharged from the rotary kiln 1 to the lowermost cyclone is provided at the kiln inlet part 2. The exhaust gas sent to the lowermost cyclone is successively sent to the upper stage cyclones so as to preheat the cement material. Finally, the exhaust gas is exhausted from the upper part of the uppermost cyclone by an exhaust fan 9 via an exhaust line 8.
In the cement manufacturing facility having the above described configuration, cement clinker is manufactured in such a manner that limestone (CaCO3) contained as a main raw material in the cement material is first preheated by the preheater 3, then calcined in the calciner 7 and the lowermost cyclone of the preheater 3, and thereafter burned in the rotary kiln 1 in high temperature atmosphere at about 1450° C.
In the calcination process, the chemical reaction as represented by the formula: CaCO3→CaO+CO2↑ is caused so as to generate CO2 gas (generation of CO2 gas resulting from the raw material). The concentration of CO2 gas resulting from the raw material is theoretically 100%. Further, fossil fuel is combusted in the main burner 5 in order to maintain the atmosphere in the rotary kiln 1 at a high temperature. As a result, CO2 gas is also generated by the combustion of fossil fuel (generation of CO2 gas resulting from the fuel). Here, much N2 gas in the combustion air is contained in the exhaust gas discharged from the main burner 5. Thus, the concentration of CO2 gas contained in the exhaust gas and resulting from the fuel is as low as about 15%.
As a result, the high-concentration CO2 gas resulting from the raw material and the low-concentration CO2 gas resulting from the fuel mixedly exist in the exhaust gas discharged from the cement kiln. Thus, in spite of the fact that the discharge amount of CO2 is large, there is a problem that the CO2 gas has a concentration of about 30 to 35% and hence is difficult to be recovered.
On the other hand, as the CO2 gas recovery methods which are being developed at present, there are methods based on a fluid recovery system, a membrane separation system, a solid adsorption system, and the like. However, the methods have a problem that the cost for recovering CO2 gas is still very high.
Further, as a method to prevent the global warming due to CO2 discharged from the cement manufacturing facility, a method is also proposed in which CO2 discharged at a low concentration from the discharge source is separately recovered so as to be condensed up to a concentration of about 100%, and is then liquefied so as to be stored in the ground. However, in this method, the cost for separating and recovering CO2 is high, and hence this method is not realized for the same reason as that for the above described methods.
On the other hand, Patent Literature 1 described below proposes an indirect heating type limestone burning furnace including: a burning zone in which, while limestone (CaCO3) filled in a heat transfer pipe made of a refractory material is moved, the limestone is indirectly burned by high temperature gas (1000° C. to 1300° C.) introduced from a combustion furnace, so as to be decomposed to quicklime (CaO) and carbon dioxide gas (CO2 gas); a cooling zone in which the high-temperature quicklime is cooled by circularly using the generated carbon dioxide gas; and a preheating zone in which the limestone is preheated by the high-temperature carbon dioxide gas generated in the burning zone and by the circulated carbon dioxide gas heated to a high temperature by cooling the quicklime.
According to the heating type limestone burning furnace, limestone is indirectly burned without being in direct contact with the high-temperature combustion gas. This enables high purity quicklime to be obtained regardless of the fuel used. Further, the concentration of the CO2 gas in the heat transfer pipe filled with the limestone is about 100%. Thus, the carbon dioxide gas generated at the time of burning the limestone can be recovered at a high concentration.
However, as shown in FIG. 13, the temperature, at which the calcination reaction of limestone occurs, is rapidly increased as the concentration of CO2 gas is increased in the atmosphere of the heat transfer pipe. As a result, when the concentration of CO2 gas becomes close to 100% (equivalent to the partial pressure of 1 atm under the atmospheric pressure (1 atm)), the temperature exceeds 860° C.
For this reason, when after limestone is calcined by the prior art based on the indirect heating type limestone burning furnace, cement clinker is attempted to be manufactured by adding the other cement materials, such as clay containing SiO2, Al2O3, and Fe2O3 to the calcined limestone, the heat transfer pipe needs to be indirectly heated by high-temperature gas having a temperature of 1000° C. to 1300° C. This results in a problem that the cost for manufacturing the cement clinker is increased. Further, since the heat transfer pipe is indirectly heated, there is also a problem that, when fossil fuel is burned so as to obtain the high-temperature gas, a large amount of CO2 gas is, on the contrary, generated by the combustion of the fossil fuel.