Designing for low NOx

Published:  08 September, 2017

With the tightening of NOx regulations there are clear advantages to utilising the latest technologies to reduce emissions. Bernard Dawson of Riello Burners considers options such as flue gas recirculation in pressure jet burners.

When selecting a pressure jet burner, ensuring it will provide the required heating capacity is clearly a vital criterion. However, it’s also important to address other vital criteria, such as emissions and energy efficiency. Flue gas recirculation, electronic cam control and variable speed drives can all play a key role in achieving the desired performance.

Emissions of nitrogen oxides (NOx), in particular, are becoming an increasingly important consideration from both an environmental and regulatory point of view. The optimum solution will therefore include NOx emissions in the key specification criteria alongside reliability, safety and efficiency.

In identifying the best approach, it’s important to be aware of the chemistry of the combustion process. 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).

Also, as air contains around 79% nitrogen, a considerable amount of nitrogen is introduced to the combustion chamber in the combustion air, potentially resulting in emissions of oxides of nitrogen (NOx). In fact, there are three ‘classes’ of NOx; Fuel NOx, Prompt NOx and Thermal NOx.

Fuel NOx is related to the nitrogen content of the fuel, which is higher in heavy oils and coal. Prompt NOx is formed in the very early stages of combustion when highly charged unstable molecules interact with the nitrogen in the combustion air. This reaction is more pronounced with higher flame temperatures. Thermal NOx results when the airborne nitrogen reacts with oxygen; a process that is also accelerated at higher temperatures.

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

Flue gas recirculation

Flue gas recirculation (FGR) is a proven method for reducing NOx levels but historically has not been widely used in the UK as it requires additional controls. However, with tighter NOx regulations it is a very effective way to meet these stricter requirements and FGR burners are now available with outputs from 500kW to 32MW, to provide ultra-low NOx emissions as low as <40mg/kWh.

The principle of FGR is that a percentage of flue gases are piped back into the burner air supply system to reduce the flame temperature. The recirculated flue gases are relatively cool and inert, and with the inhibiting presence of CO2 they reduce peak flame temperatures.

Optimising efficiency

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. Systems that do not incorporate FGR may use an electro-mechanical cam burner control or an electronic cam burner control.

Electro-mechanical cam burner control uses a single servomotor to control the air and fuel flow rates via a mechanism of cams and linkages. 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.

FGR burners need to be used in conjunction with electronic cam control of both the combustion air/fuel ratio via separate servomotors and a further separate servomotor to control the flow of the re-circulated flue gas.

The fact that electronic cam control uses separate servomotors to control air flow and fuel flow means there is no mechanical wear and tear so precise control is maintained and burner efficiency remains consistent.

Thus, the use of FGR with electronic cam control helps to deliver significant improvements in energy efficiency as well as reductions in NOx emissions.

For further overall efficiency improvements, electronic cam burner control of FGR burners can be combined with a variable speed drive (VSD) to reduce the fan’s electrical consumption. 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.

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 the level of NOx reduction that is achieved. 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 and the best NOx emission levels are achieved by careful matching of the burner and boiler.

Clearly, then, it is important to take all of these considerations into account when selecting pressure jet burners to meet low NOx requirements. 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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There are 3 levels of NOX emissions according to CE Certification

Gas (EN676)

• Level 1: Less than or equal to 170mg/kWh

• Level 2: Less than or equal to 120mg/kWh

• Level 3: Less than or equal to 80mg/kWh

Oil (EN267)

• Level 1: Less than or equal to 250mg/kWh

• Level 2: Less than or equal to 185mg/kWh

• Level 3: Less than or equal to 120mg/kWh.

To gain CE certification burners must satisfy level 1.

For any burner to be classed as ‘Low NOx’ (BLU) it must meet level 3.

These NOx emission limits are referred to on a net fuel CV basis.

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