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Taking control of burner performance: modern pressure jet burners may incorporate a number of features that will help to optimise energy efficiency and minimise emissions. Bernard Dawson discusses some of the most important areas to consider.

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. A properly specified, well-designed and correctly commissioned burner will reduce emissions and deliver maximum efficiency. Electronic cam control, variable speed drives and oxygen trim can all play a key role in achieving the desired performance.

At the risk of 'teaching granny to suck eggs' it's worth reviewing the basics as an aid to understanding how to reduce combustion emissions. During combustion the main components of any fuel, carbon and hydrogen, react with the oxygen in the combustion air 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 un-burnt hydrocarbons (CxHy - smoke).

The first objective for maximising efficiency is to minimise the excess air required to maintain a clean, stable and reliable flame. Oxygen for combustion is provided by air; a very low cost oxygen supply but one which comprises only 21% oxygen - the remainder being predominantly nitrogen (N2) with traces of other gases such as CO2. Therefore 5 times the quantity of air is required for combustion than if pure oxygen could be used. The consequences of this high volume are considered below.

There is a theoretical optimum quantity of air that would result in all the carbon in the fuel burning to form CO2 and all the hydrogen burning to form water, with no excess of oxygen. This is referred to as the stoichiometric air level.

If the quantity of air is less than stoichiometric (a fuel-rich mixture) some of the fuel will only be partially burned. If the quantity of air is above stoichiometric (an airrich mixture) there will be residual oxygen in the flue gas after all the fuel is burned.

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.

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% excess air level can be maintained.

Carbon monoxide and unburnt hydrocarbons 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.

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.

Consequently, controlling the burner effectively will help to tackle issues with CO2, CO and CxHy. Typically, burners are supplied for on-off (single-stage), high/low (two-stage) or fully modulating control. Single stage control reduces capital costs but will usually result in higher running costs. Each time the burner starts the combustion chamber is purged with fresh air as a safety precaution, thus cooling the boiler prior to ignition and exposing it to increased thermal shock. Temperature control is also generally less stable with single-stage control.

Two-stage control is a better option as this provides a little more ability to respond to changing heat requirements. However, optimum performance will only be achieved with modulating control that is able to respond smoothly and efficiently to changing heating requirements. This may be achieved by mounting a controller on each boiler, or through a building management system (BMS). It is important to note that the BMS option typically senses the main temperature requirements from the main header serving all of the boilers, which can lead to overheating of individual boilers.

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 far less mechanical wear and tear such 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. 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.

Tackling NOx

As noted above, enhanced control will help to minimise CO2, CO and CxHy but minimising NOx requires other measures.

Essentially there are three factors contributing to the overall NOx emission, 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 air borne nitrogen reacts with oxygen; a process that is accelerated at higher temperatures..

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. These flue gases are relatively cool and inert so help to reduce the flame temperature. 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 air recirculation is possible at a cooler temperature - resulting in a cooler flame and lower NOx levels. EN test criteria are based on combustion chambers of this design--so all NOx figures given are based on three-pass chambers.

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/m3. 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/m3, 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.

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Title Annotation:Process, Controls & Plant
Author:Dawson, Bernard
Publication:Plant & Works Engineering
Date:Jul 1, 2013
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