There is a popular misconception that combustion optimization is primarily focused on reducing NOx (nitrogen oxide), and that it accomplishes this mostly by taking oxygen out of the boiler. This two-part misconception is wrong on both counts. In fact, the key to combustion optimization is improving the balanced mix of fuel and air, as well as attuning the temperature profile so that it will help achieve objectives.

NOx Isn’t Everything
While reducing NOx is an objective for many CombustionOpt® installations, it’s never the only objective, it’s often not the primary objective, and in many cases it’s not even an explicit goal. In fact, some CombustionOpt installations are configured to keep NOx at a setpoint, and in certain instances this setpoint is higher than average NOx prior to the optimizer installation. In cases where reducing NOx is an objective, this goal is often accomplished with average boiler O2 levels that are the same or higher than those experienced prior to installing the optimizer.

Combustion optimization is about improving the balanced of fuel and air and attaining the temperature profile best-suited to achieving objectives. While in most cases improving the fuel and air mix can reduce NOx, increase boiler efficiency, and reduce excess O2, the latter is not necessary to achieving these objectives. This is because NOx is created through local pockets of oxygen-rich combustion, while CO (carbon), slagging, and waterwall corrosion are created though local pockets of oxygen-deficient combustion. The reality of a non-optimized boiler is that both of these conditions occur simultaneously in different regions of the boiler. Even the most modern plant control systems are unable to avoid such conditions. 

Controlling CO Formation
CombustionOpt’s ability to avoid the reducing atmosphere conditions that occur with localized oxygen-deficient combustion is enhanced by its ability to control CO formation. This is particularly true for coals containing iron. Under good, balanced combustion conditions, the iron in the coal oxidizes into harmless Fe2O3 (iron [III] oxide).

The oxygen that’s used in CO formation means that the lower remaining oxygen is more likely to bond with iron in the coal to form the highly corrosive FeO (iron [II] oxide). Even with the same average excess O2, a non-optimized boiler will create both more CO and more FeO. This in turn increases efficiency losses through unburned carbon, waterwall wastage and FeO-induced corrosion.  Traditional control systems address CO formation by dialing up unnecessarily high levels of excess O2. CombustionOpt controls CO to a desired limit.

Improving Efficiency and Heat Rate without O2 Degradation
Interestingly, CombustionOpt can lower NOx, improve boiler efficiency, and reduce heat rate while not reducing average boiler O2 and in some cases even while increasing it. This is because CombustionOpt finds the control biases that overcome stratification and/or local pockets of poor fuel-air mixing, no matter its objectives. CombustionOpt can also improve heat rate while holding NOx constant, or increasing it (where there is no cost to increased NOx). While these results may fly in the face of traditional views about combustion, we believe that in many cases, CombustionOpt’s ability to reduce process variability and parasitic loads (e.g. fan and mill power) outweighs the increased O2 that is needed to keep NOx at a desired higher level than prior to optimization.  

What CombustionOpt does to maximize boiler efficiency and minimize heat rate tends to reduce NOx and/or reagent costs simply because better mixing of fuel and air will result in better combustion, fewer hot spots, and less heated excess air leaving the stack.

For units where reduced NOx is not an objective and water wall corrosion and/or slagging from poor mixing is a concern, CombustionOpt can be configured to minimize the variation across available O2 probes and constrain the minimum O2 levels at any individual probe. For units with more advanced instrumentation –such as a combined O2/CO grid – CombustionOpt can be particularly helpful, given CO’s value as an indication of conditions that promote waterwall wastage.

In summary, the takeaways from more than 15 years of experience with combustion optimization indicate that: 1) NOx is only one among many objectives; 2) that more balanced combustion and temperature profiles can improve boiler efficiency, heat rate, opacity, NOx/reagent reduction and other objectives without reducing and in some cases increasing average boiler O2; and 3) that the problems associated with poor fuel-air mixing, lack of CO control and deleterious temperature profiles can be addressed with combustion optimization technology.          

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