When a power plant switches from designed fuel to PRB coal, they will meet various challenges including PRB transportation, materials handling and storage, and combustion issues. The major problem they may face is severe slagging and fouling on the heating surface. The main causes of these problems could be attributed to past steam plant design or lousy plant operation. However, the properties of PRB coal reveal that it is the coal itself is the root cause. PRB coal has large pyrite particles that may impact the furnace wall before they completely combust. PRB coal's clay minerals that contain significant amounts of iron, calcium, sodium or potassium cause the coal to have low melting temperatures. Furthermore, interaction of pyrite, clays and alkalis with aluminosilicates form low viscosity melts.
Deposits are formed when the ash in the flue gas is at a temperature above its melting point. The laboratory term for the initial melting point is the initial deformation temperature. Typical values of initial deformation temperature for PRB coal are in the 2100F to 2200F range. With typical gas temperatures of 2500F in the furnace, the ash is in a semi-molten condition and when it comes in contact with furnace water walls that are at a relatively cooler temperature, a deposit is formed.
The majority of the ash passes through the boiler and is collected in the ash removal equipment (electrostatic precipitator or bag house). However, even a small portion collecting on the heat exchange surfaces can have a dramatic impact on plant performance. Not only is there an insulating effect from the deposit, but there is also a change in the emissivity of the surface. Surface emissivity is critical to heat transfer in the furnace area since most of the heat transfer is from radiant heat, as explained with the law of radiative heat transfer: