State variable control, deficiency

One of the major questions in control system design is the selection of process measurements. An important deficiency of state variable control is that measurements or estimates of all state variables are required. Usually only a few of the states can be monitored instantaneously, because of sensor cost or time delays caused by the need for chemical analysis. Distillation... 

Since the temperature of the flame falls with either an exeess or a deficiency of air, it is not a particularly good controlled variable. The most universally used indication of combustion efficiency is a measurement of oxygen content in the combustion products. The amount of excess air required to ensure complete combustion depends on the nature of the fuel. Natural gas, for example, can be burned effieiently with 8 to 10 percent excess air (1.6 to 2 percent excess oxygen), while oil requires 10 to 15 percent excess air (2 to 3 percent excess oxygen) and eoal, 18 to 25 percent excess air (3.5 to 5 percent excess oxygen). The reasons for the differences are the relative state of the fuel and the amount of noncombustibles present. 

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