BSEE-JUL21-P20 Riello_Layout 1 01/06/2021 09:37 Page 20
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Burner manufacturers continue to develop and refine their designs to meet, and even exceed, NOx regulations. *Paul Sharp, Sales & Marketing Director of Riello, explains
applications there are two key burner technologies for specifiers to consider for mitigating NOx emissions – flue gas recirculation (FGR) and premix. Each is highly effective in reducing NOx emissions and the choice of technology will be determined by the particular requirements of the project.
E NOx formation
To put this into context, it’s worth briefly summarising how combustion results in NOx emissions. During combustion of any fossil fuel, carbon and hydrogen from the fuel 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 unburnt hydrocarbons (CxHy - smoke). The NOx component is a direct result of air containing around 79% nitrogen; in introducing sufficient oxygen for efficient combustion, a considerable amount of nitrogen is unavoidably introduced to the combustion chamber. There are three factors in the combustion process which determine NOx formation; 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 lowered and this is the focus of NOx-reducing technologies such as premix and FGR.
missions of nitrogen oxides (NOx) from combustion equipment are becoming an increasingly important environmental and regulatory consideration. In commercial and industrial
BOILERS & HOT WATER Complying with NOx regulations
Premix burners
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 the space available in the combustion chamber (diffusive flame). This is the most familiar arrangement. 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 the sock to the external side of combustion head where ignition occurs through spark generation and the flame is established.
This arrangement means 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. Additionally, 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.
A key consideration when specifying premix burners is the design of the combustion head, which can vary. The best design uses a seamless knitted wire for the ‘sock’ that allows its application around the support without any welded seams. Thus, critical situations to components caused from thermal stress, are eliminated. Moreover, this “sock” technology allows for an even distribution of the heat along the length of the combustion head, also providing precision of heat control. A combination of premixing the gas/air mixture with a higher excess combustion air level, and an even distribution of gas over the surface of the burner, serves to reduce the peak flame temperature and, therefore, thermal and prompt NOx.
20 BUILDING SERVICES & ENVIRONMENTAL ENGINEER JULY 2021
Premix packaged burners are currently available in outputs from 5kW to 3MW and have been seen to regularly reduce NOx emissions to as low as 30mg/kWh, and have even gone down to 20mg/kWh, in some independently witnessed site tests.
Flue gas recirculation (FGR)
Flue gas recirculation with pressure jet burners is a popular option for reducing NOx emissions. The technology has been available for some years but uptake in the UK has been slow because of the need for additional controls. As NOx regulations become tighter the use of the FGR becomes more attractive and financially viable.
As noted earlier, the principle of FGR is to reduce peak flame temperature, and therefore NOx, which is achieved by piping the relatively cool and inert flue gases back into the burner air supply system. The carbon dioxide in the flue gases also helps to inhibit combustion, thereby reducing peak flame temperature even further. 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 recirculated flue gas. The use of FGR with electronic cam control helps to deliver significant improvements in control as well as reductions in NOx emissions. Our company has developed a range of Monobloc (350 - 19,500 kW) and Dual-block (2,500 - 24,000 kW) burners based on flue gas recirculation (FGR) technology. These include new monobloc burners in which all components are integrated in a compact footprint, so as to make installation and maintenance extremely easy.
In a range of applications, the latest generation of FGR burners has been shown to provide ultra-low NOx emissions of <40mg/kWh.
Summary
Optimum burner selection is determined by a wide range of considerations and the choice of FGR or premix will depend on various other factors. The important thing is to be aware of the options and engage with appropriate experts to ensure the best solution.
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