論文

In an MHD generator using a frozen inert gas plasma (FIP), the availability of a frozen argon plasma, the influence of plasma uniformity at the generator inlet on the performance, and the feasibility of a large-scale generator are numerically examined by r-theta two-dimensional imulation. The FIP is produced by pre-ionizing inert gas without an alkali metal seed at the generator inlet, then the ionization degree of the plasma is kept almost constant in the whole of the channel because of considerable slow recombination of the inert gas just like frozen reaction plasma. It is found that not only helium but also argon frozen plasma MHD generation is realized, although highly accurate control of inlet onization degree is necessary for argon. It is important to reduce the nonuniformity of plasma properties at the generator inlet in order to raise the maximum enthalpy extraction ratio. Even for the large-scale generator with 1000MW thermal input, the ionization degree is kept almost constant in the whole of the channel and the high performance is obtainable. This result is extremely attractive for the FIP MHD generator.

Thermal performance in the experiments and three-dimensional numerical simulations for a high temperature air combustion boiler where fuel can be efficiently combusted by high temperature preheated air (800C-1000C) is examined. The boiler can burn not only natural gas but also low calorific gas (e.g. full gasification gas obtained from coal or wastes). In the boiler, four regenerative burners are installed. This boiler has new features that not only air but also gasification gas is heated up to 900C, and combination of burners is switched every 15 seconds where two burners are used as inlets of fuel and air and the other two burners are used as outlets of exhaust gas. Natural gas and syngas obtained from coal are burned. The NOx emission for each fuel is less than 50ppm. The heat transfer of three-dimensional calculation is predicted higher than that of experiment.

A new concept boiler where fuel can be efficiently combusted by high temperature preheated air is proposed and experimental demonstration is done. This boiler is suitable for low BTU gas derived from gasification process of coal and wastes with no dioxin emission. This boiler is characterized by the following features; uniform heat flux field, augmentation of heat transfer, reduction of combustion noise level, suppression of NOx emission and compactness. Preliminary experiments using natural gas as a fuel demonstrate above distinct performance as well as good agreement with three-dimensional numerical simulation.

A new concept compact boiler using high temperature air is proposed and experimental demonstration is done. In the boiler, high temperature air combustion is realized. Fuel can be efficiently burned by high temperature preheated air of about 800C-1000C under low oxygen content ( < 5% ). The temperature distribution in the furnace is very uniform and there is no hot spot, so that low NOx emission ( < 40ppm) and relatively high average temperature in the furnace can be achieved. Thermal efficiency of more than 90% is accomplished including waste heat recovery of exhaust gas. This boiler is useful to partial load operation, because the response of thermal output is linear. This boiler is also suitable for low calorific value gas derived from gasification process of coal and wastes with no dioxin emission. Simulated low calorific value gas made from natural gas diluted with argon can be burned with low NOx emission.

MHD electrical power generation with frozen helium plasma (FHP) is examined numerically. The FHP can be initiated by pre-ionized helium without the alkali metal seed at the generator inlet. Since the three-body recombination coefficient of helium ions is low at the electron temperature above 5000K, the ionization degree can be kept almost constant in the entire region of the generator channel. The r-theta two-dimensional numerical results show that the performance of the FHP MHD generator is comparable to that of the seeded plasma MHD generator, if the additional power consumed to pre-ionization is ignored. In the FHP MHD generator, the ionization degree at the inlet should be controlled precisely, as well as the seed fraction in the seeded plasma MHD generator. Under the adequate inlet ionization degree for sustaining the FHP plasma, the plasma maintains the uniform structure. On the other hand, a slightly excess ionization degree causes the strong Lorentz force in the upstream region of the generator, then the generator performance is deteriorated.

The performance of dynamic subgrid-scale models is numerically examined in the large eddy simulation (LES) of rotating homogeneous turbulences in comparison with the corresponding filtered data of the direct numerical simulation (DNS). The examined dynamic SGS models are: the dynamic Smagorinsky model (DSM), the dynamic mixed model (DMM), the dynamic Clark model (DCM), and the dynamic two-parameter Clark model (DTCM). All models are mathematically reformulated in a rotating frame from the corresponding expressions in an inertial frame. It is shown that the DSM and the DMM are not consistent with the constraint of asymptotic material frame indifference, but the DCM and the DTCM are consistent. All models except the DSM show similar decays of the grid-scale turbulent energies both in non-rotating and in rotating frames; they agree well with the DNS in the non-rotating case, but they are slightly less dissipative than the DNS in the rotating case. The DSM underestimated the GS energy dissipation in the non-rotating case, though there is no major difference from other models in the rotating case. However, the DSM in a rotating frame, which takes a different form from that in an inertial frame, leads to an unphysical fluctuating decay for a homogeneous turbulence suddenly submitted to a rotation.

The feasibility of an MHD generator with frozen inert gas plasma is examined numerically. The frozen inert gas plasma is produced by pre-ionizing helium of inert gas without alkali metal seed at the generator inlet. Since the three-body recombination coefficient of helium ions is low at the electron temperature more than 5000K, the ionization degree of inert gas is kept almost constant in the entire region of the generator channel and the relation between the electron temperature and the electron number density deviates from the Saha equilibrium. In a small scale disk shaped generator, the enthalpy extraction ratio of 22.7% and the isentropic efficiency of 54.8% are obtainable for the load resistance of 3.0Omega and magnetic flux density of 4T under the pre-ionization degree of 4.79x10^{-5}. The auxiliary power to sustain the pre-ionized plasma corresponds to about 2% of the thermal input. For high magnetic flux density, the generator performance can be improved as well as a conventional seeded plasma MHD generator.

A new concept of gasification power generation systems fueled by coal, RDF and various kinds of wastes using high temperature air is proposed. Cycle analyses of these IGCC and boiler systems are done. The calculation shows that heating value of syngas almost doubles when we increase the gasification air temperature from 25C to 1000C. More than 45% thermal efficiency is obtainable both for coal and RDF by employing the new high temperature air blown IGCC system. This IGCC system is applicable to various wastes including low grade wastes such as sludge.