Joe Klover of Jansen Combustion and Boiler Technologies recently asked a question regarding an insitu oxygen (O2) probe application. Their client operates several coal-fired stoker boilers. An illustration of a stoker boiler can be seen on this University of Missouri link. On each boiler they use a single O2 probe (pictured below) for fuel-air control and the probe is located downstream of the induced draft (ID) fan. Joe encouraged the client to move the probe closer to the furnace due to air infiltration in the back pass and ID fan. The concern is that air in-leakage can lead to inaccurate furnace O2 measurement and boiler performance issues. The boiler operator said they tried the probes in locations closer to the furnace, but it "fouled" within months and they feel a clean stream downstream of the ID fan is a better option. Joe asked if we are aware of conditions in a coal-fired stoker application having a negative impact on the functionality of an O2 instrument?
I responded that Rosemount agrees with Joe’s recommendation to relocate the O2 probe to reduce potential air infiltration and boiler performance issues. Assuming that the fuel is typical bituminous stoker coal, there is no reason our probe should not work perfectly when installed downstream of the boiler furnace exit and generating tubes. We have dozens of O2 probes in these exact stoker boiler applications. For most stoker applications the snubber diffuser is effective at filtering the ash without fouling issues. If the flue gas temperature and ash chemistry plugs the snubber diffuser too quickly, then a ceramic or Hastelloy diffuser would provide a better option. Click here to see the different diffusers options and also to read about the new plugged diffuser diagnostic and other features of our latest Model 6888Xi operator interface.
Rosemount Analytical 6888 O2 Transmitter Probe and 6888Xi Local Operator Interface
In reply to Jim McLelland:
Hi Jim,
That's a great question. There is more coal fly ash upstream and the fly ash will abrade the stainless steel probe over time. Rosemount offers two options to increase probe life; an abrasion resistant probe and abrasion shields. Customers can also provide their own abrasion protection. The cost of abrasion protection and replacing probes is typically minor compared to damaging the boiler from operating at improper air-fuel ratios. Therefore we would not recommend installing the probes downstream of the ID fan to reduce abrasion.
An accurate measurement of furnace exhaust excess O2 level is critical to understanding the furnace air-fuel ratio. Some of the potential boiler losses associated with operating the furnace too fuel-rich includes delayed heat release, high furnace exit gas temperatures (FEGT) and fuel-rich corrosive gases which can cause generator tubes erosion, corrosion, and excessive fouling and slagging. These impacts can result in lost availability due to tube leaks or slag and clinker build-up, lower efficiency and increased emissions like opacity, CO and hazardous air pollutants (HAPs). On the other hand operating too fuel-lean, i.e. with excessive combustion air, reduces efficiency and can increase emissions of oxides of nitrogen (NOx), carry-over of particulate fly ash, as well as increased fan auxiliary power consumption and air pollution control device throughput, and last but not least, it can limit boiler output.
One advantage of stoker boilers is that the coal is relatively large, typically 50% between ¼” and 2” mesh size. The coal burns on the grate and results in lower ash carryover than pulverized coal-fired boilers where coal is predominantly burned in suspension. We have thousands of probes and decades of experience in pulverized coal boiler applications too. Incidentally for those applications, the Hasteloy or ceramic diffuser is always recommended to increase time between filter maintenance. These insitu O2 probes have proven to be a highly accurate and reliable in these harsh gas environments. Ultimately the value of an accurate furnace O2 measurement far outweighs the lifetime cost of probe maintenance, repair and operation.
Thanks for the question and for raising this important issue.
Best regards,
Neil