Emission of toxic and fire hazardous gases from open air coal stockpiles

When fresh coal is exposed to air it undergoes exothermic chemisorption of oxygen which is followed by formation of surface oxides and to some extent oxidation of the coal, resulting also in emission of various gases (the most prominent of which being CO₂, H₂O and CO). Large bituminous coals stockpiles that are stored for long periods may develop hot spots due to autogenous heating. In extreme cases spontaneous fires have been reported. These processes may raise environmental and economical problems to coal consumers, who transport and store large coal piles. Israel, at present, consumes more than five million tons annually in two utility plants, and the amount of consumed coal is expected to reach at least 11 million tons in the next decade. The imported coal is stored in large stockpiles for long periods under open air. The delayed use of the atmospheric stored coal increases the costs and may contribute to environmental problems. The initial temperature of the coal piles in Israel is relatively high, > 25 °C. Monitoring the temperatures and gases inside the stockpiles (up to 6 m depth) was carried out for more than 12 months. The maximum temperature measured in a hot spot was 172 °C at a depth of 1.2 m. Carbon monoxide, low molecular weight hydrocarbons (up to C₅), as well as molecular hydrogen, have been detected. The results of the monitoring and conclusions about how the different toxic and fire hazardous gases are distributed within the coal pile are discussed in detail. The behaviour of different potential coals (to be used in the utility plants in Israel) and their oxidation resistance (in the temperature range 50-95 °C) were studied in simulation experiments in the laboratory using small quantities of coal (1-100 g). It has been observed that emission of molecular hydrogen, carbon monoxide, low molecular weight hydrocarbons and olefins accompany the oxidation process.