BSEE A FGR
Emissions of nitrogen oxides (NOx) from combustion equipment are becoming an increasingly important consideration from both an environmental and regulatory point of view. Bernard Dawson, technical director at Riello, explains how NOx levels from burners can be reduced
Flue gas recirculation (FGR) is a proven method for reducing NOx levels in pressure jet burners 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
As air contains around 79 per cent nitrogen, a considerable amount of nitrogen is introduced to the combustion chamber of boilers and other combustion equipment, potentially leading to formation of NOx. This NOx formation is exacerbated by high flame temperatures. The principle of FGR is that a percentage of flue gases, which are relatively cool and inert, are piped back into the burner air supply system to reduce the peak flame temperature. Further cooling is achieved by the inhibiting presence of carbon dioxide. 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
s NOx regulations continue to be tightened through legislation it becomes increasingly important for
engineers and building operators to make use of the latest burner technologies to reduce emissions. In the case of achieving low NOx levels, there are two key technologies to understand – flue gas recirculation and premix.
the flow of the re-circulated flue gas. The use of FGR with electronic cam control helps to deliver significant improvements in energy efficiency as well as reductions in NOx emissions. FGR burners are now available with outputs from 500kW to 32MW, to provide ultra-low NOx emissions as low as <40mg/kWh. To put this figure into context, EN 676 requires burners to achieve 80 mg/kWh or less to be classified as ‘low NOx’.
Premix
There are occasions where premix burner technology is an appropriate alternative to FGR and the main differences between traditional pressure jet burners and premix burners relate to the ways in which the air and gas is mixed, and in the technology of the combustion head.
In traditional pressure jet burners, the gas and the combustion air are mixed at the point of ignition on the end of the combustion head. In premix burners the air-fuel mixture is created upstream of the combustion head – which comprises a metallic cylinder that has multiple gas ports on its surface and is covered with a woven wire gauze (known as a ‘sock’). The air-fuel mixture flows inside the combustion head through precision-located ports and sock to the external side of the combustion head. Here, ignition occurs through spark generation to an electrode and the flame is established. This means that the flame develops only on the external surface of the combustion head, with combustion taking place very close to the metallic fibre of the blast tube. The result is
extremely compact flame geometry, so that its effective length is practically the same as the combustion head itself. The flame diameter is directly related to the burner firing rate and is more compact than a diffusive flame. The geometry of the gas ports has been designed specifically to ensure that throughout the modulation range the speed of the air/gas mixture is sufficient to avoid burn-back. Premix technology also offers a much lower risk of flame impingement, due to the reduced likelihood of contact of the flame with the combustion chamber walls. Also, the high combustion intensity developed on the premix burner head means that a greater heat output is achieved over a much smaller combustion head dimension, when compared to other burner technologies. In certain applications this enables the combustion chamber dimensions to be greatly reduced, resulting in lower manufacturing costs. Even on large burners up to 3MW, we have seen NOx emissions of lower than 30mg/kWh and, on some independently witnessed site tests, even down to less than 20mg/kWh.
Horses for courses
Selecting the most appropriate technology to achieve low NOx is very much a case of horses for courses, as each project will have particular requirements. The important thing is to be aware of the options available and make a selection in the light of specific project criteria.
uRiello RX premix burner 2
www.rielloburners.co.uk
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uRiello RS pressure jet burner with FGR
BOILERS & HOT WATER Reducing NOx with the right technologies
‘
Selecting the
most appropriate technology to achieve low NOx is very much a case of horses for courses, as each project will have particular requirements
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COMMERCIAL & INDUSTRIAL
16 BUILDING SERVICES & ENVIRONMENTAL ENGINEER FEBRUARY 2019
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