Cause and cure for high volatile coal and corrossive gases at TXI, Midlothian Plant

The plant has raw materials, which are high in pyritic sulfur. The coal mill uses the preheater exhaust gases, which have elevated amounts of SO₂. The coal being used is highly volatile. Therefore the coal mill bag filter had few occurrences of smoldered bags causing potentially unsafe conditions. This problem was solved by implementing some operational changes like reducing the mill exit temperature based on dewatering curve of coal and making the system more inert. To achieve this water had to be added into the system. When the mill exit temperature was reduced, the operating temperatures were below the Sulfuric and hydrochloric acid dew points. Because of this corrosive acid stream in the gas flow, the bag filter started corroding. En route to solving these issues, the plant neutralized the acid by adding the raw meal dust and changing the bag filter into a stainless steel construction. Furthermore, the requirement to spray water in the system was removed by adding a heat exchanger to the coal mill inlet. Also, there were some design changes made to the coal mill bag filter, which helped in stable operation and extended bag life in the bag filter. This paper discusses these issues and how these problems were solved. This paper would be of beneficial use for other plants, which have to deal with high volatile coal and highly corrosive gases