High-Efficiency Gasification of Low-Grade Coal by Microwave Steam Plasma

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 degrees C. The carbon conversion rate at the average chamber temperature of 1640 degrees C is almost 100%, ensuring a complete gasification of carbon in a low-grade coal. The cold gas efficiency is 8496, 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.