Pressure jet burners: Achieving potential

It stands to reason that a properly specified, well designed and correctly commissioned burner will deliver efficient performance while reducing emissions. However, it is also important to recognise the importance of the control aspects in achieving the desired performance. Electronic cam control, variable speed drives and oxygen trim – all of which are available with modern burner designs – can help to improve performance.

Before going into the detail, it's worth reviewing the ‘combustion basics’. During combustion the carbon and hydrogen in the fuel react with oxygen to form carbon dioxide (CO2) and water (H2O). Other emissions of concern are oxides of nitrogen (NOx), oxides of sulphur (SOx), carbon monoxide (CO) and unburnt hydrocarbons (CxHy - smoke).

SOx emissions are entirely related to the level of sulphur in the fuel. Most common fuels now have a low sulphur content so SOx emissions are relatively insignificant.

The oxygen for combustion is provided by air and there is a theoretical optimum quantity of air (the stoichiometric air level) that will result in all the carbon burning to form CO2 and all the hydrogen burning to form water. The way the burner is controlled will influence the combustion air levels, so that controlling the burner effectively will help to tackle issues with CO2, CO and CxHy.

Burners are typically available with on-off (single-stage), high/low (two-stage) or fully modulating control and it is the latter that provides the highest levels of control and efficiency.

Modulating control uses either an electro-mechanical or electronic cam to control the air and fuel flow rates. A potential problem with an electro-mechanical cam, which uses a single servomotor, is that over a period of time the mechanical linkage system may experience ‘slippage’ due to wear – resulting in a lack of precision that reduces burner efficiency and performance.

As electronic cam burner control uses two servomotors, one controlling air flow and the other controlling fuel flow, there is less mechanical wear and tear, so that the precision of the control – and therefore the efficiency of the burner - remains consistent.

For further efficiency improvements, electronic cam burner control can be combined with a variable speed drive (VSD) and oxygen trim. VSD controls the fan motor speed in relation to the burner operation, potentially resulting in significant electrical energy savings and reduction in noise emission as the fan motor speed is reduced.

Oxygen trim control monitors the excess air levels in the flue gas; if the required O2 level deviates the burner controller will continuously adjust the settings to ensure optimum combustion and emissions at all times.

Of course, it’s also important to minimise NOx emissions and this is achieved by controlling the flame temperature, as the formation of NOx during combustion is accelerated at higher temperatures. One method is to use external flue gas recirculation where relatively cool and inert exhaust gases are piped to the burner head to reduce the flame temperature. This is effective but increases capital, installation and maintenance costs.

Another option is internal flue gas recirculation, using recirculating air from within the combustion chamber to cool the flame. The pattern of the flame that is produced tends to have a larger diameter and requires a larger combustion chamber diameter to be effective.

NOx levels are also influenced by various aspects of the boiler’s combustion chamber design and the level of heat release within the chamber. The selection of boiler, and matching the burner and the boiler for optimum performance, are therefore also very important.

Given the complexities of achieving maximum energy performance with minimum emissions there is a strong case for calling on the expertise of specialists in this area to help guide you to the best solution.