In the power industry, one often hears people use the words “erosion” and “corrosion” as if they were interchangeable. But these terms describe two very different processes, and two separate water wall challenges that power plants deal with on an ongoing basis. When it comes to optimization, it’s important to understand these distinctions since SootOpt® and CombustionOpt® address each issue in different ways.
Erosion refers to the physical wear that occurs as the result of either hot gasses or boiler cleaning media: steam, compressed air or water. SootOpt helps to avoid erosion from cleaning through fewer cleaning actions. Water wall erosion is a major cause of tube ruptures, the number one contribution to forced outages and the associated impacts on commercial availability. Where water is used as a cleaning medium, SootOpt also helps avoid the water wall and boiler tube cracks that occur through thermal stress, which compromises the strength and integrity of the metal and also causes tube ruptures.
Corrosion refers to a chemical process that compromises the structural integrity of steel used in the water wall and embedded tubes. It primarily occurs as a result of the reducing atmosphere from low-NOx (i.e. deeply staged) firing. In this situation, oxygen that would normally bond with iron is in such short supply (because it is used in the formation of CO). The highly corrosive FEO forms instead, causing water wall wastage, tube ruptures and the same type of unpredictable forced outages as the erosion process described earlier.
Avoiding water wall corrosion with optimization
CombustionOpt can help avoid or minimize reducing atmospheres though better mixing of fuel and air and less CO formation. However, a single measurement of CO in the stack (typically in tens or hundreds of parts-per-million [PPM]) is not a perfect proxy for the tens of thousands of PPMs of CO that form in local oxygen-starved regions of the furnace under low-NOx firing conditions. CombustionOpt’s adaptive neural network models resolve the cause-effect relationships between individual damper movements and a single measurement of stack CO, which is why it not only reduces NOx and improves boiler efficiency, but also avoids local oxygen-deficient conditions that minimize reducing atmospheres and the resulting water wall corrosion.
Combining technologies adds greater value
Instrumentation providing higher resolution of in-furnace stoichiometry (the technical term for the ratio of fuel-to-oxygen) can help CombustionOpt obtain even greater NOx reduction and boiler efficiency while avoiding adverse combustion conditions (i.e. the sub-stoichiometric conditions that create reducing atmospheres) that cause water wall corrosion.
One example is the use of on-line indicators of loss-on-ignition (LOI) or carbon-in-ash. Both of these signal local oxygen-deficient combustion conditions in the furnace. They provide a direct objective that helps fly ash be sold as an input to cement production and avoid the high costs of land-filling fly ash. NeuCo has commercially demonstrated this extension of CombustionOpt, and is further substantiating the benefits as part of its $16M DOE Round 2 Clean Coal Power Initiative project at the NRG-Texas Limestone plant.
Another example is Zolo Technologies tunable laser diode-based ZoloBOSS™ system, which provides multiple in-furnace measurement paths for O2, CO, and temperatures. CombustionOpt can leverage these measurements to provide finer-grained optimization that is better informed by local combustion conditions. The result is even greater NOx and boiler efficiency benefits and more effective avoidance of the local oxygen-deficient conditions that cause the corrosion and associated water wall corrosion described above. NeuCo and Zolo are working together with joint customers at several sites to demonstrate these benefits, with potentially enormous implications for the industry.
The in-furnace temperature measurements provided by the ZoloBOSS system can also be used by SootOpt, where local in-furnace temperature measurements can inform SootOpt of ash deposition and slag buildup. NeuCo and Zolo Technologies are also working with customers to demonstrate the benefits associated with this new combination of complementary technologies.
