The Powder River Coal (PRB) is classified as sub-bituminous ranked coal. Unlike higher ranked bituminous and anthracite coals, which have tight pore structures that limit the amount of inherent moisture they could hold, low rank coals like PRB coal have this looser pore structure and additional moisture retention capacity. PRB coal also has more oxygen chemically bonded to the coal.
PRB coal has many different properties from the commonly burned bituminous or anthracite coal. These differences include lower heating value, lower sulfur, higher volatile, lower fusion temperature, etc. PRB coal’s lower heating value is resulted from its higher moisture and oxygen content. PRB coal has low sulfur nature because it was exposed to fresh water rather than seawater. PRB coal’s high volatile nature is the consequence of its abundant oxygenated compounds like CO. PRB coal’s low fusion temperature is mainly caused by its high Cao content. The following table shows the significant difference in the calcium levels between Illinois Bituminous and Wyoming Sub-bituminous coals.
|Illinois Bituminous||Wyoming Sub-bituminous|
|Heating value (Btu/lb)||12,770||8,683|
|Ash (%, dry)||17.4||6.6|
|Sulfur (%, dry)||4.2||0.4|
In addition, PRB coal has low inherent ash content, high SO3 level and medium HGI index. Its low inherent ash has large non-rock or organically bound calcium and sodium fraction. Sulfur Trioxide SO3 is high due to capture of sulfur by calcium in ash. This can bias the percent ash compared to actual amount of fly ash and bottom ash produced. The HGI of PRB coal is near mid fifties