COMBUSTION control

Every combustion process in the process industries generates emissions. The science of emissions is extremely complex and it is notoriously unwise to over-simplify statements about the subject. However, when sulphur in the fuel burns in the presence of oxygen, SOx (oxides of sulphur) are produced, and when nitrogen in air is heated to the extreme temperatures found in the combustion flame, NOx (oxides of nitrogen) are generated. Furthermore, carbon in fuel results in carbon dioxide (CO2 the principal greenhouse gas) being produced. In circumstances where there is insufficient oxygen for all the carbon in the fuel to be completely burned, carbon mon-oxide is also produced.

All these emission create their own particular environmental problems from acid rain to ozone depletion. Investment in reducing such emissions could save on society's health costs, but there are more immediate issues.

Emissions legislation so far (see panel overleaf) has enforced the monitoring of emissions rather than the controlling of the processes whose emissions are monitored. This focuses industry's attention on monitors and the avoidance of prosecution for excess emissions, rather than on improving the control of the process itself.

BATNEEC OR CATNAP?

The result is the conflict defined neatly as BATNEEC versus CATNAP. Those who draft the legislation seek BATNEEC solutions (Best Available Technology Not Entailing Excessive Cost) while those hoping to avoid the worst consequences of non-compliance adopt the CATNAP approach (Cheapest Available Technology Narrowly Avoiding Prosecution).

The facts are that the same optimal combustion conditions which minimise emissions and benefit the environment also use the least fuel. From a commercial standpoint, the hard-nosed reasons for installing the most efficient combustion analysis system possible are to be found in the bottom line.

All combustion control is a trade-off between combustion efficiency and the control of emissions. Combustion is at its most efficient, and produces the least harmful emissions, at the point when fuel combustion is complete (and therefore carbon monoxide emissions are at their lowest) and the oxygen content of the flue above the flame is at the lowest level compatible with complete burning of the fuel.

For any given boiler and fuel, this point of optimum efficiency will occur at a given oxygen content of the flue gas. Adding more oxygen will ensure that combustion is complete but will also increase fuel consumption and use the excess heat to raise the temperature in the flue. Reducing the oxygen at the flame will produce incomplete combustion and an excess of CO.

Typical combustion analysis techniques identify the point of 'combustion breakthrough' (see Figure 1) by analysing the relationship between flue gas components, combustion efficiency and the fuel/air ratio above the flame. In oxygen analysis based systems, an oxygen analyser provides feedback on the oxygen content of the flue to a control system which varies the fuel input to the burner and the air available to the flame until the optimum oxygen content of the flue gas is reached.

In the search for greater precision in the control of the efficiency of combustion, some systems measure the carbon monoxide (CO) in the combustion gases as well as the oxygen. This is usually achieved using pellistor sensors. These were developed to measure the lower explosive limit (LEL) of gases, and while they provide excellent results at the percent level, their capability for low-level measurement, as required in combustion control, is limited by base line instability and cross-sensitivity to other gases commonly found in the combustion process.

The emergence of a moderately priced, yet fast and accurate combustibles measurement technology, just launched by Servomex, is set to create a completely new cost/benefit scenario in combustion control. Companies can now buy the instrumentation to measure not only the oxygen but also the concentrations of CO above a burner on a continuous basis. They can do this at a price comparable with that of oxygen-only measurement systems and obtain the reliability of oxygen analysis equipment with the degree of accuracy of advanced multi-gas control systems.

The key is a refined type of transducer based on thick-film calorimetry which makes it possible to measure combustibles in the range 0-500ppm. This technology is now available in a new combustibles analyser, the Servomex xendos 2700.

It is now possible to refine the approach to combustion analysis, providing a more sensitive measurement of the concentrations of combustibles, in addition to the measurement of oxygen. Moreover, the new instrument meets the key customer requirements of lower initial cost, minimal simple maintenance and more effective combustion control. Extensive trials have shown that the new technology achieves major fuel and money savings by comparison with oxygen-based systems, and greater reliability by comparison with in situ infra-red systems.

A key benefit of the Tfx 1750 transducer at the heart of the xendos 2700 analyser is that it is considerably less affected by cross interference from other background gases like water and carbon dioxide than existing devices.

This technology has the potential to break the deadlock between accountants and environmentalists, providing the bottom line and hope for the future as part of the same equation.

Will this be the beginning of a move towards accepting that the control of combustion is far more effective in benefitting the environment than simply monitoring emissions?