The selective catalytic reduction system has become ubiquitous throughout the world of power plants. Emission control requirements are ever-more stringent, and the cost of excursions is becoming increasingly high. The key to staying under the regulators’ radar is precisely controlling the ammonia injected into the boiler. A new control strategy does precisely that.
Selective catalytic reduction (SCR) is the industry’s method of choice for removing the majority of nitrogen oxides (NOx) from flue gas, and it is a very effective method. The basic process used is the chemical reduction of NOx to nitrogen (N2) and water (H2O) by the reaction of NOx and ammonia (NH3) within a catalyst bed. The reaction itself occurs in a reactor chamber with a catalyst bed made up of replaceable catalyst modules and an NH3 injection system. Ammonia is injected into the flue gas before it reaches the catalyst. In short, the SCR system is quite simple, as there are no moving parts.
Ammonia Calculations
What isn’t simple is determining how much ammonia to use. The amount of ammonia introduced into the reactor is the critical control variable. If less than the amount required to reduce the NOx content to desired levels is injected, the reduction requirements will not be met. If too much ammonia is injected, then the phenomenon of ammonia slip occurs, where costly unreacted ammonia is exhausted to the stack. In some regions of the U.S., the amount of ammonia slip is also part of a plant’s operating permit restrictions. Tight control of the outlet NOx content is, therefore, a major concern for all plant operators.
The core requirement for achieving tight control of NOx is a robust control system that can nimbly respond to either a load or a NOx reduction rate setpoint change. It is well documented that the amount of NH3 injected into the SCR should be directly proportional to the amount of NOx entering the reactor. As a result, the SCR industry has developed a more-or-less standard method of modulating NH3 flow, as shown in Figure 1.
1. The standard. Most coal-fired plants use a more-or-less standard trim controller to determine NH3 demand to control NOx production. Source: Tim Leopold
In essence, this control scheme calculates the unit’s NH3 requirement for a given NOx reduction. The outlet NOx is then compared with a setpoint derived from some function of the product of the required reduction and the measured inlet NOx. This calculation varies in complexity from manufacturer to manufacturer and for different applications, but all are generally variations on this same control scheme.
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