Water Wall Tube Corrosion Question Chlorine/Sulfur in Coal
Question from plant:
...We also use a calcium and magnesium additive to help the coal ash to be more friable. My question is what does the calcium and magnesium do once bonding with the high chlorine in IB Coal at 2,800 a 3,000 deg F range? The reason I ask is in 9 months time we lost over 90,000th of an inch wall thickness in the hot corners on our supercritical unit.
Answer from Rod
Several things going on. Calcium tends to lower fusion and make more slag, I have seen MgO help some situations, but not consistently. I'm not a big fan of additives because they mask underlying issues for one.
I feel you could have one of those underlying issues. You may have sulfide attack from large unburnt pyrite (FeS2) particles. Same with coal, smaller particle size and more oxygen improve combustion of sulfides. Because pyrites are 4 times denser, the same size particle weighs 4 times more, this tends to allow it to reach the wall more often than the coal. Both tend to have poor combustion once they hit the wall, unfortunately the pyrite can be molten and is the leading cause of wall slag in higher sulfur (ILB) coals. Convection pass ash deposits tend to correlate more with sodium and calcium levels in the coal. So high velocity also hurts this situation. High primary air flow can both increase particle velocity and particle size. Low oxygen levels in the lower furnace help NOx levels, but hurt the oxidation of the iron sulfides. This is one main differences between low and high sulfur coal. Low sulfur coal is easier to low NOx because there are low pyrite levels, high sulfur tends to increase slag and corrosion due to higher pyrite levels.
With regards to chlorine corrosion. Unless you have specific metallurgical evidence of chloride or chlorine attack, it is most likely a sulfide attack. A recent paper surveyed plant using higher chlorine coal and found little if any evidence of chlorine issues on boiler walls.
Paper on Operation Considerations When Burning High Chlorine Coal
Adding scrubbers to a plant does not mean you can burn high sulfur coal, only remove SO2. The slags and corrosion from the increased pyrite levels cause significant boiler issues. These get worse in reducing atmospheres (high CO), which is preferred by low NOx firing, particularly in the lower and burner zones of the boiler. Mill performance and operation is key to minimizing these issues, as is adding air to lower furnace. Adding metal overlay generally requires covering the whole furnace over time, so is expensive.
Please consider attending one of our seminars where we cover coal quality from the seam to the stack or inviting me to the plant as a trainer/consultant. Regards, Rod
Thermal Energy Expert ?? Consultant - Software Development
9 年Well I first would recommend a detailed coal and ash analysis. Then everything would be more easily understood. High chlorine coal however will not be easy to handle with supercritical boiler superheaters.
New Beginnings with Wide Open Doors!
9 年very good information
Chief Technical Officer at Coal Combustion Inc
9 年Thanks Conyers, That represents an almost 40% increase in sulfur. If corrosion is an exponential rather than linear function you can see the issues.
Plant Manager
9 年I have CFB boilers. We experienced Sulfide attack on our SH1 tube section (Tube Shields) with a slight increase in the average Sulfur content (from 0.8% to 1.1%). The SH1 tubes run horizontally from the front wall to the rear wall. We install tube shields to control the Ash Erosion. The tube shields were 304 SS. They worked well. Then the shields would only last about 3 months. They would be gone or severely eroded. We tried different materials (higher chrome) with no good results. The analysis of the remaining tube shield indicated sulfur attack. And looking back at our Sulfur content in the coal, it increased from around 08% to 1.1% (over 6 or so years- changes in contract language). We ended up installing a refractory shield to protect this tube sections.