Mercury removal from coal by leaching with sulfur dioxide

Mercury from coal-fired utilities has been identified as one of the most hazardous air pollutants and the greatest potential public health concern. Furthermore, it has a tendency to bio-accumulate in the food chain. Mercury is present in coal in concentration well below 1 ppm; however, the large tonnages of coal consumed for electric power generation represent a significant source of mercury vapor entering the environment. There are various technologies available to control the emission of mercury from coal-fired power plants. Among them activated carbon injection into the flue gas stream has been studied for many years and is considered to be the "standard technology" at this time to control the mercury emissions from flue gas. In this research, it is proposed to develop a diverse technology beyond the "standard technology" of activated carbon injection. This technology is based on pre-combustion treatment of solid coal to remove its mercury content by a unique leaching method using SO2 and O 2. The overall objective of this study is to explore this new technology and determine its technical feasibility to be used for a commercial process. The minute amounts of mercury in Pittsburgh No.8 Coal (0.177 ppm) and Illinoi No.6 Coal (0.216 ppm) was removed by flowing a gas stream containing 10% oxygen and 1000 ppm sulfur dioxide into a coal slurry at 30 ml/s. A total of 50g of 35 x 65 mesh coal was leached every time in 500 ml solution for 3 hours. The variables were temperature (50 to 80°C), initial solution pH (1.5--5.7), Sulfur dioxide and oxygen gas concentrations. It was found that the mercury removal percentages increased with increase in temperature and decrease in pH. Removal percentages of as high as 92.09 were achieved in Pittsburgh No.8 coal where as percentage removals of about 99.98% were achieved in Illinoi No.6 Coal. The pyrite removal percentages were much lower than those of mercury. Mercury removal was high enough to consider the application of this technology to a commercial process.