The heat of combustion from a flame at the centre of a pre-mixed cloud of gas and air produces a pressure wave which is sufficient to provide the source of ignition to the adjacent mixture (like a diesel engine) and so the combustion front proceeds at the speed of sound, or greater. The diffusion flame may well obtain sufficient temperature to crack the fuel before it is combusted. Below are the four latest issues. High hydrogen is ideal for these burners because of the wide flammable range of hydrogen in the lean zone and the high flame speed for stability of the rich jet. To achieve low NOx, one of the favourite procedures is to reduce the combustion air to all burners (making the flames sub-stoichiometric) and removing all the fuel lance internals from the last row of burners, thus supplying the “secondary” air at the end of the combustion chamber. Your plant is likely to be legally obliged to control the NOx emissions. The key is to slow down the rate at which the fuel and air mix. Phasing over from pure methane to high hydrogen could be achieved with new jets/nozzles, but this procedure would be suited to a specific change of fuel and not just to gradual changes in fuel composition. The secondary air is delivered outside the diffusion flame package, with low turbulence to minimise mixing. So, because changing to low NOx will probably require new burners, it would make sound economic sense to choose a burner design which is also suitable for high hydrogen. Unlike the flat flame wall burners above, many boiler/heater/furnace/kiln low-NOx burner designs separate the combustion air into two parts – the primary air and the secondary air. So, we cannot reduce the oxygen partial pressure in these burners without modification to the air and associated control systems. Furthermore, the control system fitted on a boiler is often sufficiently well instrumented that air:fuel ratios can be continuously controlled allowing gas composition changes without notice. This calculation is very similar. Nozzle mix flat flame burners should not have any significant problem as the hydrogen content of the gas is increased, save for the choice of nozzle material. Mike Menzies asks what effect does injected hydrogen have on furnace, flame and exhaust in natural gas combustion plant? Power station burners tend also to be suited to low NOx and high hydrogen. Adiabatic flame temperature hydrogen, methane, propane and octane - in Kelvin . (The Coanda effect results in a flow of gases, where these gases follow a curved surface because that curved surface has no gaseous molecules adhering to collide with – and therefore modify the direction of – the flow stream.). They may be suitable both for high hydrogen as well as low NOx. There are many other matters to consider. Burners can be designed for any gas composition with hydrogen. Sponsored Links . The flame propagates at the flame speed so the gases have to be injected at a velocity greater than the flame speed, otherwise the flame could “flash back” and burn in the burner mixing chamber. There are many ways of accommodating high hydrogen fuel gases whilst still keeping the flame cool enough to minimise NOx formation. We had hydrogen in towns gas throughout the middle of the last century and the researchers are investigating replacing methane in the gas main with hydrogen, at least in part to begin with. So, the public may see a change in the stack plume. Modulation of 2, 3, or 4:1 may be suitable with a methane gas but not for pure hydrogen. But in your neighbourhood and your nation you should be morally obliged to prepare for hydrogen. Retrieved 2008-01-27. Adapting these burners for low NOx will often involve separating the flame into a rich and lean section where both parts of the flame have a lower flame temperature because of being away from the stoichiometric region. There will then be free carbon which will burn with a yellow flame. Fitting flame detection on all, say 500 burners, is likely to be blocked by financial considerations. Process stability would suggest that modulation, even if it is only 2:1, would be beneficial. it, like methane is not poisonous, (just asphyxiating and explosive); it has quite a high spontaneous ignition temperature (SIT) of 650, it has very wide flammability limits (3–70% H, it burns to water vapour, thus eliminating CO. it burns with a much higher flame speed (300 cm/s) than methane (30 cm/s), thus stabilising the flame. The higher flame speed increases the flame temperature locally, which generates NOx. We offer readers a flexible range of subscription options and you are certain to find one that suits your needs. The refractory has an emissivity near 1, so radiation to the product is achieved. This sleeve or cloud of exhaust products slows the combustion because the gas has now got to diffuse out through the cloud, and the oxygen has to diffuse inward. The higher flame speed increases the flame temperature locally, which generates NOx. But ‘Replace’ is being investigated with respect to the carbon in the National Grid gas supply. It is just a little bit more difficult if the composition changes excessively. THE world is rising to the exciting challenge of controlling CO2 emissions. So the burner manufacturer has to design a burner to give a flame which will minimise the production of NOx. The combustion rate can be reduced by diluting combustion air with exhaust gases as a method of slowing down the flame to reduce the temperature, but many installations have natural draft or induced draft premix burners where chamber negative pressure and gas aspirators entrain fresh ambient air into the premixer and thence to the burner quarl (prefired refractory burner block). In these power station burners there can be several other fuels. Premix burners therefore have a limited turn-down range and are designed for a particular flame speed. Choosing one of the iron and nickel family of catalysts, together with heat, steam, methane and oxygen, a large proportion of hydrogen is made, together with CO2. This review has looked at the development of burner design to take account of emission regulations, specifically NOx. We just have to control it. This is the 11th article in a series discussing the challenges and opportunities of the hydrogen economy, developed in partnership with IChemE’s Clean Energy Special Interest Group. Burner manufacturers have designs for both premix and nozzle mix flat flame burners, and for both induced draft and forced draft. It’s not unknown for a boiler burner to have five different fuels, though one or two of these fuels may merely be a method of disposing waste or vent gas from another process. A conversation with the experts: watch the recordings of our previous webinars and sign-up to attend future online webcasts. Coke oven and blast furnace gases are common, but they do have a significant influence on the original design of the burner. This gives rise to a diffusion flame. This is a list of flame temperatures for various common fuels. The hydrogen is usually made from natural gas, by steam methane reforming (SMR) without capturing the CO₂, and in sufficient quantity for the process. Regenerators or recuperators are used to recover heat from high temperature furnace exhaust gases. So even if you can get the same heat out of a burner, you may need to put in more combustion air. During heatup after a shutdown, rather than trying to modulate the burners, gentle heat input is often achieved by lighting rows or individual burners on/off. It is likely that one of the fuels may be “plant gas” shared by energy consumers throughout the plant. The gas will be delivered in the centre with the primary air, and pilot flame or igniter (and often flame detection equipment). This design will need a further chamber for the final combustion (probably flameless combustion) before the exhaust gases pass into the convection section. The Wobbe Index is a measurement of the ability of a gas to deliver heat through a certain size jet, but this may require a different amount of air to achieve it. In these cases the furnace can be designed for the gases to wipe the crown of the furnace transferring heat by convection to refractory. Adiabatic flame temperatures for hydrogen, methane, propane and octane - and others - with oxygen or air as oxidizers in a constant pressure adiabatic reaction: So the burner manufacturer has to design a burner to give a flame which will minimise the production of NOx. The amount that is absorbed is also re-radiated or emitted. So how does the hydrogen content affect the NOx emissions? Adiabatic flame temperatures for common gases are provided for air and oxygen. The general public is increasingly aware of global warming, and remedies such as Reduce, Reuse, Recycle. You do not have to be a chemical engineer to join IChemE. For instance, when we put high levels of hydrogen in the fuel, the exhaust could turn white with condensing steam droplets as it cools. Hence this gas main composition changes from that of natural gas to include hydrogen, typically up to 30%. Hence the name “diffusion flame”. What effect does the hydrogen composition have on the furnace, the flame, and the exhaust? These boiler burners are generally well suited to utilising high hydrogen gases in a nozzle mix arrangement. This does not look as well controlled as a sharp blue flame, so low-NOx burners, involving diffusion flames rather than deflagration, are only slowly being recognised by the operators. There are many ways of accommodating high hydrogen fuel gases whilst still keeping the flame cool enough to minimise NOx formation. A diffusion flame is where the incoming gas is surrounded by the products of combustion where the gas and oxygen in the air have reacted. The radiation from the flame appears less because the flame is dissipated throughout the combustion chamber where the light intensity is high – white hot. Many operators will remember bright yellow flames from heavy fuel oil or vacuum residue oil which had less acceptable exhaust composition. The flame which one hopes never to see in a furnace is a detonation flame. So wherever the gas and oxygen meet they will react and give off more heat. These small, wall-hugging burners use convection to transfer heat of the flame to the refractory using the Coanda effect or by nozzle design. But undoubtedly, hydrogen is our friend in the furnace. © 2020 Institution of Chemical Engineers, Figure 1: Diffusion flame (left) and deflagration flame (right), Figure 2: Small wall hugging burners use convection to transfer heat of the flame to the refractory using the Coanda effect, then radiation to the process tubes, Figure 3: Roof-mounted burners with different fuels.
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