OCR Text |
Show There m a y be other circumstances where P E M techniques are useful without the previously discussed requirements. These are situations where the emission is only required to be predicted for a period given a previous value of the emission. These situations occur in places when there is a C E M shared amongst several different units and therefore the C E M is cycled between the different units. Therefore, the emission measurement is only available for one unit for example 5 minutes out of every 15 minutes if the instrument is shared between three separate systems. Another situation where this occurs is for analyses such as chlorides where the analyzer m a y be a batch style analyzer that m ay by its physical nature be only able to deliver a reading every 20 minutes. In both these situations the P E M technique does not need to predict accurately from just the process data but can also use the previous data values as part of the relationship in addition to the process data. This is far less demanding because the P E M system needs only to predict h o w the process data is causing the emissions signals to deviate from the last known result. This technique is referred to as a data fill-in and it can be used in many more situations than the complete prediction. The fill-in will generally be of minimal error at the start of the prediction interval and the error will increase to obtain a maximum value just before it once again enters the actual reading interval. By comparing the last predicted value with the first actual reading the maximum prediction error is continually obvious to the user. PREDICTION OF EMISSIONS Currently, PEM methods appear to be most suitable towards applications involving the monitoring of N O x , C O , C 0 2 and in some situations S02. They are also applicable to most situations where the fuel is natural gas or oil. Fill-in P E M methods m a y expand the scope of these applications areas to include other emissions and other fuels. NOx Prediction NOx refers to the combined amounts of NO and N02, which is predominantly in the form of NO. There are several differentiable mechanisms for the formation of N O x. The first mechanism, termed prompt NOx, is associated with the fuel characteristics whereby nitrogen reacts with hydrocarbon free radicals to form hydrogen cyanide ( H C N ) which can be oxidized to N O x at relatively low temperatures. This is a minor mechanism in most situations. The second mechanism creates what is termed fuel NOx and refers to where the nitrogen in the fuel is transformed to N O x . For fuels where the fuel N O x mechanism is significant, parametric prediction m a y be less suitable unless the fuel nitrogen levels are small, consistent or continuously known. When firing natural gas, fuel N O x is not a major N O x formation mechanism, because the nitrogen in natural gas is low and what is there m a y be chemically bound similarly to that of air. |