Microwaves quickly heat fly ash. Figure 1 (a) shows the temperature change when 1.5 g of the mixture material is heated by a microwave electromagnetic field. The fly ash shows good microwave absorption regardless of the microwave irradiation conditions; however, electric field heating shows a higher gradient than that of magnetic field heating near 1000°C. This phenomenon is attributed to the improvement of the dielectric constants of the components of the mixture at high temperatures. Figure 1 (b) and (c) show the temperature dependence of the real and imaginary dielectric constants of the components of the mixture measured using the resonant perturbation method, respectively. Here, it is difficult to measure the dielectric constant using the coaxial transmission and resonance perturbation methods because M fly ash has a large dielectric loss. Thus, the composition of M fly ash is increased, and the microwave absorption dependence is measured. Figure 1 (c) shows that increasing the amount of M fly ash improves microwave absorption, and CaCO3 and NaCl barely absorb microwaves; this indicates that the unburned carbon remaining in M fly ash significantly contributes to microwave absorption.
Figure 2 (a) shows the carbon content of the mixture sintered at 1000°C. The mixture before microwave heating contains 10.2 mass% carbon; this is from both unburned carbon and the carbon derived from CaCO3. The mixture without water was heated to 1000°C with microwave irradiation (1 min), following which a decrease in carbon concentration was confirmed. This decrease in the carbon concentration can be attributed to the decomposition of CaCO3 and the combustion of carbon. An increase in the carbon concentration can be confirmed in the mixture to which water is added, after the same amount of heating. A gas that makes the lime water cloudy can be obtained when the sintered mixture to which water is added is pulverized again and heated at 1000°C in an Ar atmosphere, as shown in Fig. 2 (a). This is attributed to the burned carbon causing elements other than carbon to escape into the gas and suppress decomposition and combustion of CaCO3 and carbon, respectively. Figure 2 (b) shows the heating time dependence of the carbon content of the sintered body obtained by heating the mixture with water added at 1000°C. Both electric and field heating show high carbon content when the retention time is short; this is because CaCO3 decomposition and carbon combustion do not progress sufficiently owing to the short sintering time. The carbon concentration decreases with an increase in the heating retention time. This decrease in carbon concentration in the sintered body of the mixture is smaller in magnetic field heating than that in electric field heating.
Figure 3 (a) shows the XRD measurement results of the sintered body obtained by heating the mixture with water using a microwave electric field. The measurement result shows the peak of aragonite, which is a crystal of CaCO3. The peak of aragonite decreases compared to the peak of halite with an increase in the heat retention time. The XRD measurement results of the sintered body obtained by heating the mixture in which water is added using a microwave magnetic field are shown in Fig. 3 (b). In the microwave magnetic field, the peak of aragonite exhibits the same tendency as that in the electric field; however, it shows a higher peak than the sintered body heated by the electric field when the holding time is long.
The microwave magnetic field suppresses the generation of CO2 compared with that in the microwave electric field when the mixture is sintered. This tendency can also be confirmed from the results of the exhaust gas analysis. Figure 4 (a) shows the temperature change and exhaust gas analysis results versus time when the mixture is heated by a microwave electric field. Here, the exhaust gas analysis is conducted by operating an exhaust gas by Q-mass; the vertical axis is the ion current and the carrier gas is CO2 (0.4 l / min). The vertical axis is the ion current because the CO2 emission is directly observed without converting the vertical axis into partial pressure. As shown in Fig. 4(a), an increase in the gas with a mass number of 44, which corresponds to CO2, can be confirmed immediately after the start of microwave irradiation. Since an increase in CO gas can be confirmed, it is inferred that carbon combustion occurs. Further, the temperature of carbon is locally high because CO gas is generated only when carbon is burned at a high temperature, according to thermodynamics principle [32]. Figure 4 (b) shows the temperature change and exhaust gas analysis results versus time when the mixture is heated with a microwave magnetic field. As shown in Fig. 4(b), the gas with a mass number of 44 corresponding to CO2 increases after the mixture reaches 1000°C. In addition, the CO gas quickly returns to the baseline after the temperature of the mixture drops, whereas the CO2 gas emits a certain amount of gas. Thus, the temperature of carbon is relatively low when the microwave is turned off. In this region, hot carbon cools quickly when it came in contact with cold CaCO3, and CaCO3 remained cold. The results of this exhaust gas analysis indicates that the microwave magnetic field burns carbon at a higher temperature more selectively than that with the microwave electric field.
Volume reduction rate and rigidity are important parameters that determine the engineering utility value of the sintered body. Therefore, the effect of adding water on the volume reduction rate and rigidity of the fly ash sintered body is investigated. Figure 5 (a) shows the retention time dependence of the volume reduction rate of the mixture sintered at 1000°C. Here, the error bar adopts the standard deviation measured three times. Figure 5(a) confirms that the addition of water is effective for improving the volume reduction rate. In addition, magnetic field heating is more effective in reducing the volume of the mixture than electric field heating when the holding time is short. This implies that the addition of water and magnetic field heating reduce air bubbles in the sintered body. Figure 5 (b) shows the retention time dependence of the rigidity of the mixture sintered at 1000°C; the addition of water does not affect the rigidity of the sintered body. Further, the magnetic field heating is more effective for the rigidity of the mixture than electric field heating when the holding time is short; electric field heating is more effective than magnetic field heating when the holding time is long.
However, the question “Why does the addition of water suppress the decomposition of CaCO3?” remains unanswered. This phenomenon can be explained by considering that the high-speed heating property of microwaves completes sintering before water diffuses. Once the temperature has risen, the sintered body undergoes the process of decreasing temperature. The presence of water and CaO in the thermodynamically stable region of CaCO3 promotes the carbonation of the sintered body. This hypothesis explains that the addition of water and shortening of sintering time are effective for carbon storage. However, the exhaust gas analysis results indicate that the carbon combustion temperature drops when heated by a microwave magnetic field, and therefore, it is necessary to consider the cause.
It is necessary to discuss mesoscale thermodynamics to consider the current microwave chemistry. Thus far, several researchers have reported that microwave-heated materials behave differently than materials heated via conventional methods [33–36]. In recent years, research has clarified that the inhomogeneity of the material creates a mesoscale superheat point on the object to be heated [16–18]; further, a superheat point is observed in this system.
Figure 6 (a) shows the temperature change of the mixture with time and (b-e) show the measurement result of the narrow temperature distribution at points A and D in the microwave electric field and magnetic field heating; each pixel in Fig. 6 (b-e) is 2.7 µm. Figure 6 (b-c) show that the mixture heated by the microwave electric field has a superheat point the size of several hundred micrometers, and this superheat point is approximately 20–50°C higher than the surroundings and maintained for 5 min. The mixture heated by the microwave magnetic field has a superheat point the size of several micrometers or less, and it is approximately 50–100°C higher than the surroundings and is maintained for 5 min (Fig. 6 (d- e)). In the monochromatic thermometer measurement results shown in Fig. 6 (a), the microwave electric field and the magnetic field show almost the same behavior; however, the temperature distribution of the mixture differs significantly between the electric and magnetic fields.
CaCO3, which has low electrical conductivity, can be heated only by a microwave electric field. Microwave magnetic field heating heats the mixture without heating CaCO3. Further, components that can be heated by microwave electric and magnetic fields in the mixture are carbides. Therefore, the high-temperature points observed by magnetic field heating are likely to be carbides, and the low temperature points are likely to be CaCO3 and NaCl. Thus, the phenomenon can be explained well if it is considered that CaCO3 does not decompose because the region near the carbide is heated and sintered in the mixture.