(397c) Subcritical Water Upgrading of Coal Refuse with Food Waste

The goal of this research was to improve the fuel characteristics of coal refuse through co-hydrothermal carbonization (co-HTC) with food waste. Coal refuse, also known as coal waste and gob, are by-products resulted from coal mining. Coal processing plants reject 30-60% materials from the mined coal during the cleaning procedure and it contains 40-70% energy compared to coal. However, toxic elements (sulfur, heavy metals, and ash) present in coal refuse possess serious environmental threat and that is why it is not possible to combust coal refuse without prior treatment. Co-HTC of coal refuse along with food waste can be an efficient option to utilize this huge amount of available fuel resource. Co-HTC is a thermochemical process, where two feedstocks are treated at high temperature (180-280 °C) and corresponding vapor pressure (5-60 bars) for a short period of time. Coal refuse from local coal mine and food wastes from Ohio University Dining Hall were co-hydrothermally treated at three different temperatures (180, 230, and 280 °C) for 30 min. The acidic medium created by the hydrothermal carbonization of food waste catalyzed the upgrading of coal refuse. The elemental carbon (%) increased to a maximum value of 49.4 % for co-HTC solids from 18.3 % for raw coal refuses. Additionally, the ash percentage decreased from 66.4 % to 27.4 % for co-HTC solids. Moreover, the van-Krevelen diagram showed that the co-HTC solids have fuel characteristics similar to bituminous and sub-bituminous coals. The total sulfur concentration also decreased to less than 1.4 % from 9.9 %. The raw coal refuse samples and treated samples were further analyzed using the standard test method (ASTM D2492-02) and an induced coupled plasma optical emission spectroscopy (ICP-OES) to determine sulfate sulfur and pyritic sulfur concentration, respectively. The organic sulfur was calculated by subtracting the summation of sulfate and pyritic sulfur from the total sulfur concentration. The study also evaluated the distribution of sulfur among the co-HTC products. Later, an economic analysis was performed to check the economic feasibility of large scale hydrochar production. The economic study revealed that the production cost of co-HTC solids to be $39 per ton in the basis of a 110 MWe power plant.