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Low Cost Raw Coal or “There are ROCKs in my coal”

image credit: Photo by Rod Hatt - Petrified wood mixed with coal.
Rod Hatt's picture
Chief, Coal Combustion, Inc.

Having worked with coal quality and combustion issues in the utility industry for over 40 years I have operated Coal Combustion, Inc. for over 17 years providing sound unbiased consulting and...

  • Member since 2005
  • 13 items added with 6,500 views
  • Mar 22, 2023

Raw or run of mine coal is the lowest cost per ton coal on the market.  Coal washing removes rocks and pyrites, FeS2, from coal, and makes a more consistent product, but comes at a cost.  Today’s coal buyer looks to provide the lowest cost heat to a boiler, and using our 1880’s vintage ASTM/ISO standard type coal testing you only get average quality of coal.  Coal may on average meet a given SPEC, but with raw coal or coals where rocks from the mining process can get mixed in may have some portions of low ash with pockets or portions of high ash.  Power plants mostly have problems with coal quality fluxuations or changes, not average quality coal.  We do not properly measure coal fluxuations when we get one analyses for a large (<4,000 tons) shipment.  Coal with the same average look the same on paper.  Coal with high standard deviations of properties like CV, ash, sulfur, and moisture, have all caused derates and shutdowns of coal plants.  The mines and buyers do business; the plant can suffer due to swings in coal quality.  Many of the slag outages I have reviewed had root causes of high ash due to “out of seam dilution”, OSD, or rock mixed in with coal during mining.  These rocks (and pyrites) are not always consistent, but on average the shipment meets the average coal spec.   This suggests improving our coal testing to include looking at these fluxuations.  I have tried for over 40 years to solve slag at power plants, and this is an issue I have not been able to change.  The boiler tells us something is wrong, but the coal testing does not show it.

Matt Chester's picture
Matt Chester on Mar 22, 2023

 I have tried for over 40 years to solve slag at power plants, and this is an issue I have not been able to change.  The boiler tells us something is wrong, but the coal testing does not show it.

If you were able to solve for this, what sort of efficiency / cost gain would you expect to come from it? Enough to justify digging deeper into finding it? 

Rod Hatt's picture
Rod Hatt on Mar 23, 2023

Hi Matt,   Slag can shut down a boiler relatively fast, it take days to recover from, and it happens more often when load demand is high, or you shut down just when you are needed on the grid.  There are some efficiency gains possible with consistent fuel, the real money/savings is not slagging.  

Slag is molten rocks fouling boiler fireside
Slag is molten rock sticking to boiler tubes, Photo by Rod Hatt


Rod Hatt's picture
Thank Rod for the Post!
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