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Ramping up the burner's performance

Control features on some modern pressure jet burners will help to optimise energy efficiency and minimise emissions. Bernard Dawson explains how to exploit them
When specifying a pressure jet burner, ensuring that it will provide the required heating capacity is a given. However, it's also important to address other vital criteria, such as emissions and energy efficiency. Electronic cam control, variable speed drives and oxygen trim can all play a key role in achieving the desired performance.

The first objective is to minimise the excess air required to maintain a clean, stable and reliable flame. The higher the level of excess combustion air, the lower the overall boiler efficiency due to 1) a reduction in the flame temperature, lowering radiated heat transfer from flame to boiler walls, and 2) higher gas volumes passing through the boiler, increasing the flue gas velocity, reducing gas residence time, and thereby reducing the convected heat transfer to the boiler walls. These result in a greater percentage of the potential heat from the fuel being lost up the flue.

Emissions cut virtually to zero

In practice there is always a minimum level of excess air required to ensure that combustion is completed and clean. Good burner head design will mean operation with only approximately 15 per cent excess air can be maintained.

Carbon monoxide (CO) and unburnt hydrocarbons (CxHy) result from incomplete combustion, indicating either the burner is incorrectly set or the design is not suitable for the fuel and/or the boiler. These emissions should be reduced to virtually zero by good burner combustion head design and correct commissioning. Consequently, controlling the burner effectively will help to tackle issues with CO and CxHy.

Typically, burners are supplied for on-off (single-stage), high/low (two-stage) or fully modulating control. Of these, optimum performance will only be achieved with modulating control that is able to respond smoothly and efficiently to changing heating requirements.

Modulating control uses a servomotor to control the volume of air and gas required for correct combustion. Such systems may use an electro-mechanical cam with a single servomotor controlling the air and fuel flow rates via a mechanism of cams and linkages, or an electronic cam control system that has separate servomotors for both air and fuel control. A potential problem with an electro-mechanical cam 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, the other controlling fuel flow, there is no mechanical wear and tear so precise control is maintained and burner efficiency remains consistent.

For further efficiency improvements, electronic cam burner control can be combined with a variable speed drive (VSD) and oxygen trim. Rather than adjusting an air damper to reduce air flow, 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 requires an oxygen sensor to be placed in the flue system to monitor excess air levels. As part of burner commissioning, certain parameters are set for the sensor in relation to burner operation. If these parameters are exceeded the burner controller will automatically reconfigure the settings of the electronic servomotors to compensate. This ensures optimum combustion and emissions at all times.

As the oxygen required for combustion is provided by air, and as air contains around 79 per cent nitrogen, a considerable amount of nitrogen is introduced to the combustion chamber, resulting in emissions of oxides of nitrogen (NOx).

Essentially there are three factors contributing to overall NOx emissions, known as Fuel NOx, Prompt NOx and Thermal NOx. Fuel NOx emission is related to the nitrogen contained within the fuel, which is higher in heavy oils and coal. Prompt NOx emission is formed in the very early stages of combustion when highly charged unstable molecules such as C, CH, CH2 and partially oxidised molecules (dissociated radicals) such as CO, HO, CHO interact with the nitrogen in the combustion air resulting in the formation of NOx. This reaction is more pronounced with higher flame temperatures. Thermal NOx results when the airborne nitrogen reacts with oxygen; a process that is accelerated at higher temperatures.

Both Prompt and Thermal NOx emissions are reduced if the flame temperature is reduced.

One way to reduce the flame temperature is to use external flue gas recirculation, where exhaust gases from the boiler flue are piped into the burner head. This system is effective, though the capital, installation and maintenance costs are increased.

Another option is internal flue gas recirculation, where recirculating air from within the combustion chamber is used to cool the flame. The air and fuel mixture within the burner head produces a particular shaped flame that creates recirculation of the flue gases at its root. 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 the design of the combustion chamber. For example, the hot return flue gases in a reverse flame chamber increase the flame temperature, thereby limiting NOx reduction.

In contrast, a three-pass combustion chamber is ideal for NOx reduction, as the gases exit the chamber at the rear, so that internal recirculation is possible at a cooler temperature - resulting in a cooler flame and lower NOx levels.

A further influencing factor on NOx emission is the level of heat release within the chamber - the burner firing capacity divided by the combustion chamber volume - usually quantified in MW/cu m. As the combustion heat release increases so too does the NOx emission. For best NOx emission levels the figure should be no more than 1.0-1.3 MW/cu m, with appropriate length and diameter ratios. This emphasises the importance of matching the burner and the boiler for optimum performance.

Clearly, then, it is important to take all of these considerations into account when specifying pressure jet burners. Understanding the issues and how they are influenced by various factors is the first step in ensuring they are addressed. The second step is to work with specialists who can help guide you to the best solution.

// The author is technical director of Riello //
10 September 2013

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