Clyde Bergemann, Inc. Atlanta (USA): Information - Knowledge Base - Buring PRB Coal - Challenges of Burning PRB Coal

Challenges of Burning PRB Coal

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:

PRB contains high levels of calcium oxide and magnesium oxide, which are major sources of the reflective property of PRB ash deposits. Calcium oxide can be in the 24% range for PRB coal compared to 1 to 2 % for eastern coals. Also, magnesium oxide is in the 5% range for PRB coal compared to about 1% for eastern coals. The normal emissivity of a boiler tube with a coating of iron oxide is from 0.85 to 0.89. However, a slag deposit can cut this value to 0.5, and thus have a significant effect on the amount of heat absorbed by the furnace. From the radiant heat transfer equation it can be seen that this reduction in emissivity (or increase in reflectivity) would reduce heat transfer by almost half. This reduction of heat absorbed in the furnace results in higher furnace exit gas temperatures. Less heat is absorbed by the water walls and therefore more heat must be absorbed by the superheater and reheater sections in the convection passes to maintain full load steam conditions. It is often the case that this upset in boiler balance is enough to reduce the capacity of boilers. The higher furnace exit gas temperatures usually result in higher stack gas temperatures and therefore lower boiler efficiency. A reduction in boiler efficiency increases heat rate and therefore increases the cost of generation. The elevated temperatures of the gas leaving the furnace often results in ash entering the convection passes that is above its fusion temperature. Thus this ash will deposit on the superheat and reheat surfaces and further reduce the ability of the furnace to make steam conditions. High temperatures that exist in the furnace for longer time periods also result in more production of thermal NOx.