May 17, 2016

Optimal Boiler Combustion Control


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).

Figure1goodFlue 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.

Figure2The 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.