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High-power steam plasma for heating the coal powders were developed, where the magnetron power at 915 MHz was available up to 75 kW. The steam plasma itself is an impedance load, which depends on physical conditions, including the microwave power. By monitoring the minimum reflected power, the optimum injection rate of the steam and the corresponding reflected power ratio in terms of the microwave power was found, showing that most of the microwave power is absorbed by the torch plasma with a minimal reflected wave-power of less than a few percent, once the plasma torch was ignited. Indonesian brown coal with high ash content is gasified by two microwave steam plasmas heating up the gas temperature in a reaction chamber of 1145 L in a swirl-type gasifier. With additional heating of synthetic gas from a partial oxidation, the inner temperature of the gasifier can reach to 1700 °C. The carbon conversion rate at the average chamber temperature of 1640 °C is almost 100%, ensuring a complete gasification of carbon in a low-grade coal. The cold gas efficiency is 84%, very high in a relatively small gasifier like the experiment here. The total calorific power of the synthetic gas is 500 kW. Therefore, this gasification system may serve as a moderately sized power plant due to its compactness and lightweight nature. A power plant utilizing low-grade coal would be useful in rural or sparsely populated areas without access to a national power grid.

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The concept of a water surface plasma source (WSPS) was proposed to directly interact plasmas with water for large area water treatment, which is the type of volume dielectric barrier discharge (vDBD) with plate-to-plate. One electrode is submerged in water, while the other is floated in air, which is covered with a dielectric material. The characteristics of the WSPS were investigated by using a complementary metal-oxidesemiconductor (CMOS) camera, voltage and current probes, and optical emission spectroscopy (OES). The electrochemical parameters of plasma-activated water (PAW, 2 L) after plasma treatment times of 3 min by the WSPS were analyzed by using a multiparameter meter. As results, the formation of the water wave due to plasma generation, caused by the effect of the induced polarization forces, was observed at the WSPS. By comparison with the tap water, the applied voltage of the distilled water required higher than 130% for stable operation of the WSPS due to lower electrical conductivity (EC). As gap distance between dielectric plate and water surface increased, the applied voltage increased. In addition, an increase of 2 mm in the water level from the lower electrode required an approximately 5% increase in applied voltages for the ignition and stable plasma generation of the WSPS. The dominant peaks that were for N 2 species system in the spectrum of plasmas at the WSPS were analyzed by using the OES. In the case of distilled water, the pH values decreased from 6.25 to 4.24 and the EC increased from 2.00 to 22.33 µS • cm by using multiparameter meter during plasma treatment, whereas in the case of tap water, the effects on the pH and EC were insignificant.

We report the generation of a high-power pure steam torch plasma operated by a 915-MHz microwave and its temperature measurements. Steam from a steam generator enters the discharge tube as a swirl gas at a temperature of ∼140°C. This steam becomes a plasma forming gas and produces a stable steam torch plasma. The torch volume is almost linearly proportional to the microwave power. The temperature of the torch flame was measured with an optical spectroscopy, analyzing the optical emissions of hydroxyl molecules ∼309 nm. The plasma torch showed two distinctive regions: 1) a bright, whitish region of a high-temperature zone and 2) a reddish, dimmer region of a relatively low-temperature zone. The maximum temperature in a microwave-driven region was measured to be ∼6720 K. We also investigated the plasma temperatures at the axial distances of plasma torch column.Index Terms-915 MHz, gas temperature, microwave plasma, steam plasma.

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