21.2 ) Calculating the concentration of carbon dioxide

The volume concentration of carbon dioxide, expressed in [% vol], is not usually obtained by direct measurement, but is calculated on the basis of measured oxygen concentration and the CO_2max parameter, characteristic for the given fuel.

Formula 1 shows the formula according to which the analyser calculates volume concentration of CO₂:

Some flue gas analysers are fitted with an infrared sensor for carbon dioxide, but this practice is not yet widespread. The only real deficiency in the calculation of carbon dioxide is that there is a slight but noticeable cross-sensitivity of the oxygen sensor for carbon dioxide. This effect can be compensated electronically, but it is dependent on a correct setting of CO_2max to be effective.

Setting a fuel incorrectly will hence lead to very false readings. This also means that attempts to check the zero point of the oxygen sensor require that a special fuel be programmed with a CO_2max value of zero. If this is not done, then the sensor will show a rest value of around 0.4% even when the oxygen concentration is zero. An analyser which does not show this error will also not display oxygen in the presence of carbon dioxide correctly, since it is not compensating for the cross-sensitivity.

One of the problems often encountered when measuring CO₂ is flue gas recirculation. Here, a proportion of the flue gas is returned to the air inlet to ensure complete combustion of the carbon monoxide or hydrocarbons. This technology was common in the automobile branch years ago before the introduction of the catalytic converter. This is not the best way to reduce emissions. It is only used to reduce a poor level of pollution under a certain border line, and is not really an attempt to address the problem of pollution production. Flue gas recirculation will also produce a higher than expected level of CO₂ and is not really suitable for calculating systems. It is mostly used to reduce the levels of NO_x in the gas outlet. Here, quite a small amount of recirculation can have a significant effect on NO_x due to cooling of the combustion process

The carbon dioxide value has taken on a new meaning since the Kyoto Protocol became reality. Carbon dioxide was earlier a measure of how well a combustion process was functioning, now it is recognised as a pollutant in its own right and one of the causes of global warming. Carbon dioxide will be unavoidable, so long as we are dependent on fossil fuels, but all attempts to measure the quantities produced are a start towards a future reduction. This is also one of the arguments for the increasing attention paid to renewable fuel sources. The carbon dioxide produced during combustion would be produced anyway during the decomposition of the material, and has been absorbed from the atmosphere in the first place. This makes the net balance of carbon dioxide production zero. The argument becomes slightly flawed when talking about the combustion of larger trees and other slow growing plants. There is evidence that these store a quantity of atmospheric carbon dioxide, over and above what they require for growth and the production of cellulose and other sugars. This carbon dioxide is then released during burning. True, it was absorbed from the atmosphere to start with, but it does still contribute excesively to the global production of carbon dioxide. The argument is that it was absorbed a long time ago, so is not really a part of our present carbon dioxide cycle. When talking of biomass combustion, these are generally fast-growing plants that are only one or two years old.