21.6) Calculating combustion parameters

The most important parameter is the amount of heat convected by combustion gases to the environment - the so-called chimney loss (stack loss) S_L. Chimney loss is calculated on the basic of an empirical formula known as Siegert's formula:

where:

S_L -chimney loss - the percentage of heat produced in combustion process which is convected with the combustion gases.

T_gas - flue gas temperature

T_amb -the temperature of the boiler inlet air (it is assumed by the analyser to be the ambient temperature)

CO₂ -the calculated (on the basis of oxygen concentration and CO_2max) amount of CO₂ in combustion gases, expressed in [% vol ]

A1, B - factors characteristic for a given fuel type (see Table 6)

Based on the calculated chimney loss the analyser estimates the efficiency of the combustion process η (this should not be confused with total boiler efficiency )

where: η - combustion efficiency

The above formula assumes that the only quantity decreasing combustion efficiency is stack loss. Thus, it omits incomplete combustion losses, radiation losses etc. Such a simplification is a result of the inability to measure the size of other losses with the gas analyser. Because of this gross simplification in the formula above it should be remembered that the efficiency calculated in this way can not be treated as precise.

However, efficiency calculated like this is very convenient as a comparable parameter when regulating the furnace.

The formula, though simplified, reflects precisely the tendencies of efficiency change, thus it is possible to observe whether the efficiency increases or decreases. It is sufficient information for the regulation process.

It is possible to take into account the efficiency reduction caused by incomplete combustion. This loss is represented by a quantity called the loss by incomplete combustion I_L. It determines the percentage of energy loss caused by the presence of flammable gases (in this case mainly CO) in the combustion gases. There will be some formation of methane in the flue gases, which will also add to the level of incomplete combustion. In most cases, however, this will be a negligible amount and can generally be ignored. The loss caused by incomplete combustion is calculated on the basis of the measured CO concentration in the combustion gases according to the following formula:

where:

  CO, CO₂ - volume concentrations of CO and CO₂ in the combustion gases

α - the factor specific for a given fuel

Calculating I_L enables correction of the previously calculated (formula 8) combustion efficiency. Then the so-called corrected efficiency η* is calculated:

The last combustion parameter calculated by instruments is the excess air factor λ. This factor expresses how many times the amount of air supplied to the boiler is larger than the minimum amount which is theoretically necessary to burn the fuel completely. The system calculates the λ factor on the basis of the known CO_2max value for the given fuel and the calculated concentration of CO₂ in the combustion gases using the formula:

The above formula may be transformed into the form: