Understanding the Business of Coal - Linear vs Exponential Relationships

In some electric markets it is prudent to look at pushing existing coal plants to steaming limits. Some units have 2-4% or more additional steam capacity. If you push a plant to say two percent over capacity, you will get a 2% increase in electricity/revenue. This is a linear relationship.

The issue is that some important operational and maintenance issues at the plant including slag and tube erosion are exponential functions with respect to velocity. 

Wall and Furnace Slag

My “Splat Factor” calculation for wall slag uses the kinetic energy equation which uses velocity squared. KE = m/2 x Vel2.

This means that if the velocity from the mill pipes increases 2% due to load increase, the slagging potential as indicated by the Slat Factor, increases 4%.

Tube Erosion

Particle erosion (fly ash) of steel tubes looks to be ever a higher power law factor in relationship to velocity, typically estimated at 2.9. (We use 3.0 to simplify math)   This means that a 2% increase in load potentially increases super heater/reheater and convection pass tube erosion about 6%

Cost Issues

The financial impact in the increase in revenue due to increase in load is easy to estimate. The issue is that the cost of slag and tube erosion is not as easy to estimate, particularly on an annual basis. Forced outages and tube leaks due to slag are expensive. Please keep in mind that when the plant is pushed to its limits, it is less able to handle swings in coal quality. Higher ash and sulfur, along with lower CV due to higher ash/moisture or both all impact pulverizer performance and wall slags. Then add decreased life of convection pass due to particle erosion, or the resultant tube leaks due to localized high velocity (due to ash deposit for example).

This is just a reality check for those who want to squeeze just a little more out of a unit.