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As NOx regulations become tighter, it makes sense to take advantage of technologies such as flue gas recirculation in pressure jet burners. Bernard Dawson of Riello Burners explains.
PRESSURE JET BURNERS BSEE Designing for low NOx W
uRiello has introduced low NOx ‘blue flame’ versions to all its RS range of burners.
hen selecting a pressure jet
burner, ensuring it will provide the required heating capacity is clearly
a vital criterion, as is ensuring it will help to minimise NOx and other emissions.
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. 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.
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, so that they absorb heat from the flame, resulting in lower peak flame temperatures. FGR burners need to be used in conjunction with servomotors to control the volume of air and gas required for correct combustion. Such systems require 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. 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.
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
Electronic Cam Burner Control
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.
www.rielloburners.co.uk
There are 3 levels of NOX emissions according to CE Certification
Gas (EN676) uLevel 1: Less than or equal to 170mg/KWh uLevel 2: Less than or equal to 120mg/KWh uLevel 3: Less than or equal to 80mg/KWh
Oil (EN267) uLevel 1: Less than or equal to 250mg/KWh uLevel 2: Less than or equal to 185mg/KWh uLevel 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 as net fuel CV.
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www.bsee.co.uk
COMMERCIAL & INDUSTRIAL
BUILDING SERVICES & ENVIRONMENTAL ENGINEER APRIL 2017 17
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