21.8) The influence of fuel parameters on the accuracy of result calculations

  As has already been mentioned, combustion gas analyzers do not generally measure the concentration of carbon dioxide, but calculate it from the measured oxygen concentration and the CO_2max parameter. On the basis of CO₂ concentration calculated in this way, the chimney loss, combustion efficiency and loss by incomplete combustion are calculated. Obviously, the fuel parameters (especially CO_2max) , have a fundamental influence on calculations of combustion processes. The following results calculated by combustion gas analyzers are affected by fuel parameters:

CO₂ content - depends on CO_2max

λ (excess air) - depends on CO_{2max ,} but is cancelled out in the calculation, as is explained below.

η and η* - depend on CO_2max , A1 and B

S_L chimney loss- depends on CO_2max , A1 and B

I_L loss by incomplete combustion - depends on CO_2max and α.

As can be seen from the formula, the value of the excess air factor does not depend on the fuel parameters. The calculated results of gas quantities (except CO₂) and the results of temperature measurements and power quantities do not depend on fuel parameters either. The undiluted gas concentrations is an exception to this rule. Since the excess air is a factor in this calculation, it will be affected by the type of fuel chosen. In general, the fuel parameters should not be underestimated and setting the wrong fuel will often produce very incorrect results. thsi is a great problem in cases where the fuel is not easy to define or a mixture of fuels is in use at load-dependent ratios. The classic case is an oil refinery, where the excess process gas is used for reheating and extra oil or natural gas are added to make up any shortfall. This is discussed under a separate section as process gas, a generic term for all forms of gas produced as by-products of a chemical process and used for heating purposes. Clearly, the fuel parameters stored in a flue gas analyzer are of very fundamental nature, and it is a shame that there is no international agreement about their values. This is mainly due to the historical fact that countries have different suppliers for fuels in general, but these differences are slowly disappearing, and a time will come when the commonest fuels are identical in all countries.