By Neil Widmer, Business Development Manager, Emerson Process Management
Recently, Bob Sabin of Emerson Process Management presented an article on “4 Key Measurements for Boiler Control Performance” in Flow Control. The purpose of boiler control is to achieve safe and reliable operation and optimized performance with respect to output, operational cost, and by-product emissions. His article emphasizes how the foundation of optimal control rests in the quality of measurement and actuation field devices as shown in Figure 1 below. The four keys of boiler control are drum level, fuel flow, air flow, and flue gas oxygen (O2).
Flue gas O2 measurement was suggested to be the most critical parameter for maintaining boiler safety, maximizing thermal efficiency, and minimizing emissions. With insufficient combustion air, indicated by low O2, incomplete combustion can occur and generate hazardous air pollutants and fuel conversion efficiency losses. On the other hand, excess combustion air, indicated by high O2, reduces thermal efficiency, can limit output and increase emissions of nitrogen oxides pollutants. Non-optimal O2 operation can also increase fouling and slagging, corrosion, erosion, thermal degradation, and other boiler reliability and availability losses.
An accurate and fast response time O2 measurement is ideal to support optimal combustion control. The industry standard O2 measurement technique is with a zirconia oxide (ZrO2) sensor. O2 sensors are housed in probes that can be inserted directly into high-temperature flue gas to provide a continuous and near instantaneous response to flue gas conditions. These probes are called in situ combustion O2 probes. Probes should be located close to the furnace exit to improve response time and, for induced draft boilers, to avoid air in-leakage that can occur at duct joints, air preheaters, air pollution control devices, and fans.
The O2 probe location is selected to measure a representative average O2 level exiting the furnace. Due to stratification of fuel and air within a burner and from burner to burner, a single measurement may not always provide adequate indication of the average furnace exit O2 levels. (See Figure 2) In highly stratified multi-burner facilities, multiple O2 probes can be used. Some large power generation boilers may have 20-24 O2 probes per boiler. However, on a single-burner industrial or commercial boiler, one O2 sensor may be sufficient as long as burner stratification is not an issue. Best practice however is to install a redundant O2, especially when O2 measurement is used to “trip” or shutdown the boiler.
With the importance of the flue gas O2 measurement, O2 probe reliability and accuracy is critical. Emerson’s Rosemount in situ O2 probes have set the standard in reliability and today’s products feature automatic calibration and other diagnostics to ensure reliability and accuracy for optimal boiler combustion control. More information on this technology can be found HERE.
To improve boiler control, it’s important to have a good base of instrumentation and actuations devices. The full article in Flow Control can be viewed HERE.
Hi and welcome to Analytic Expert. I’m Neil Widmer. Recently on our Emerson Exchange 365 site, I’ve fielded some interesting questions from customers and I thought the answers could be useful to you. So here are some application solutions that may save you money and improve your boiler operations.
An engineer from JANSEN, a combustion and boiler engineering service company in the Western U.S., recently asked a question regarding a Rosemount O2 probe application. Their client operates several stoker coal-fired boilers. On each boiler they use a single 6888 O2 probe located downstream of the ID fan for fuel-air control. The service company encouraged the client to move the probe closer to the furnace due to air infiltration in the back passes. The concern is that air in-leakage can lead to inaccurate furnace excess oxygen 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. The engineering service company asked if we are aware of conditions in a stoker coal fired application having a negative impact on the functionality of an oxygen instrument.
I responded that Rosemount agrees with the service company’s recommendation to move the probe to reduce boiler performance issues. In our experience, the probe should work very well upstream of the ID fan. Assuming that the fuel is typical bituminous stoker coal, there is no reason that our probe should not work perfectly when installed closer to the combustion process. Our probe can be located close to the boiler furnace exit and is often installed immediately downstream of the economizer convective pass. We have dozens of our probes in these exact stoker applications and they work very well.
For most of these stoker applications, the snubber diffuser works fine without ash fouling issues. If the flue gas chemistry plugs the snubber diffuser too quickly, then a ceramic or Hastelloy diffuser would be another option. Click HERE to learn more about the different diffuser options. I also mentioned that our latest model 6888A O2 system has a plugged diffuser diagnostic option which could help with predictive maintenance.
Another question came in asking about particulate or erosion due to ash content as a result of moving the probe closer to the point of combustion. 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, or an abrasive shield which covers and protects the probe. 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 include 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) and carry-over of particulate fly ash, as well as increase 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 carry-over 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. For these applications, the Hastelloy or ceramic diffuser is always recommended to increase time between filter maintenance. 6888 O2 probes with these diffuser options have proven to be 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.
Now it’s your turn. Do you have any questions on boiler operation I can help with?