Effect of Mineral Matter in Coal

Mineral matter in coal is one of the most important sources of problems in coal combustion, including fouling, slagging, corrosion, among others. Mineral matter transformation and slag formation are specific properties of coal that provide more information on the suitability for coal combustion or gasification. 

Advanced industrial and utility power systems typically use direct fired gas turbine engines. Using coal to directly power a gas turbine has yet to be accomplished commercially, primarily because the ash causes erosion of the blades and deposition on the blades. If the combustion products contain a significant fraction of molten ash particles, deposition on the tmbine blades occurs which blocks the flow path and degrades performance. If the ash particles are solid, erosion of the blades occurs which also degrades performance. In addition, mineral matter can cause corrosion of the blades. The size distribution, concentration, and composition of the ash, as well as the turbine design, determine the lifetime of the turbine blades. 

Mineral matter in coal, during combustion, transforms into fly ash, and results in the buildup of ash deposits on heat transfer surfaces in PC-fired boilers. The ash formation process determines the ash character, that is, its particle size distribution and variation in chemistry. There are two models that can be used to represent ash formation from mineral matter (1) coalescence of included mineral grains and (2) fragmentation of excluded mineral grains during combustion. 

When coal is burned, most of the mineral matter and trace elements generally form ash however, some minerals break down into gaseous compounds which go out the furnace s flue. Pyrite, for example, breaks down into the individual elements iron and sulfur. Each element then combines with oxygen to become, respectively, iron oxide and sulfur dioxide. Iron oxide, a heavy solid, becomes part of the ash and sulfur dioxide is emitted as part of the flue gas. Some trace elements also dissociate from their organic or mineral hosts when coal is burned and follow separate paths. Most become part of the ash, but a few of the more volatile elements, such as mercury and selenium, may be emitted in the flue gas. 

Coal that is relatively rich in iron-bearing minerals (such as pyrite or siderite) has a low fusion temperature while coal relatively rich in aluminum-bearing minerals (such as kaolinite or illite) tends to have a high fusion temperature. If an electric generating or heating plant is designed to bum 

---

Reid, W.T., "The Effect of Mineral Matter in Coal on Ash Behavior in Large Boiler Furnaces", Presentation to the ASME Committee on Corrosion and Deposits from Combustion Gases, September 29, 1971. 

---