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To make the calculation of the carbon monoxide concentration in combustion gases independent of excess air with which the combustion process is conducted, the idea of "undiluted" carbon monoxide COundil was introduced (it is also called the CO concentration calculated for 0% O2 ). The value of COundil is calculated according to the formula below:
where:
CO - volume concentration CO[ppm]
λ - excess air number
Other gases can also be displayed as undiluted values. The great value of this value is that it defeats all attempts to disguise a high level of pollution by increasing the airflow and hence diluting the waste gases. Undiluted carbon monoxide is a good measure of the "true" pollution value of a burner. It shows how high the concentrtion would be if the burner was working at full efficiency. It is also useful in those cases where it is not possible to measure the gas concentrations before a gas dilution takes place. This is often unavoidable and the undiluted carbon monoxide is the only way to see what is really happening in the burner unit. The dilution of the flue gases is not necessarily an attempt to disguise something, but simply a part of some processes. Heated gas is often needed in industry, and often an oxidising atmosphere is essential, just as a reducing atmosphere can also be required. A reducing atmosphere would naturally be rich in carbon monoxide, whilst an oxidising atmosphere should contain as little as possible to prevent undesirable reactions.As can be seen, the undiluted carbon monoxide is dependent on the measurement of oxygen concentration and the knowledge of the correct fuel type. This is not always so easy, especially where non-standard or mixed fuels are used. Nevertheless, a reasonable approximation must be made to calculate the undiluted carbon monoxide with any form of accuracy. The undiluted carbon monoxide is a value that is often quoted and may be requested on official forms.
Care must be taken with this measurement should the airflow be reduced to a level producing an oxygen concentration below the official value for excess air. This can happen in certain systems with very slow control systems. Here, there will be no adjustment for the level of oxygen and the measured volumetric concentration will be shown. If needed, the result may be converted by hand afterwards. This is, nevertheless a potentially dangerous situation with the burner operating in a manner that will possibly cause the production of large quantities of unburnt products such as carbon monoxide, methane and hydrogen. Such situations should be avoided whenever possible, unless required by a particular process as discussed above.
Undiluted carbon monoxide becomes difficult to measure in systems operating on high levels of dilution due to the many inaccuracies which will all add up. This is particularly true when only trace levels of the gas can be measured. For this reason, all attempts must be made to find a measuring point as close as practically possible to the burner.
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