the fuel, which means no soot (smoke), carbon monoxide, or unburned hydrocarbons. In many applications, the limit on increasing an existing burner’s firing rate is the combustion air capacity. While it is usually easy to get more fuel flow by increasing the size of the fuel injector holes, it may not be very easy to significantly increase the combustion air capacity. Since hydrogen requires less combustion air (see Figure 2 ), it may be possible to increase a burner’s firing capacity less expensively using hydrogen than by alternative methods such as increasing the combustion air fan capacity. Potential challenges There are many potential challenges of using high hydrogen fuels. Hydrogen has a considerably higher flame speed, which makes it much more susceptible to flashback in a premix burner. This means the turndown (ratio of highest to lowest firing rate) in premix burners may be significantly reduced with high hydrogen fuels. Hydrogen embrittlement is another fuel delivery system concern, which can be handled with using appropriate materials selections. Noise could increase due to higher exit velocities for high hydrogen fuels. However, noise can usually be mitigated with proper fuel injector design and muffler selection. The higher adiabatic flame temperature may increase thermal NOx. This can be mitigated by increased furnace gas recirculation, a well-known NOx reduction technique, which is possible because of hydrogen’s wider flammability limits. Design considerations Valve packings and seals need to be considered because of the higher likelihood of leakage. Welding piping joints may be preferred to minimise leaks. Since hydrogen embrittlement is a problem with carbon steel, stainless steel piping should be used. A more conventional fuel like natural gas may be used at startup before switching to hydrogen. This typically occurs when hydrogen is produced by SMR. Burners designed to fire on hydrogen start up initially on natural gas until the SMR process is established and hydrogen is being produced (see Figure 3 ). This means burners may need to be capable of firing on both natural gas and high hydrogen fuels. Since the combustion characteristics between hydrogen and other fuels can be significantly different, this could complicate
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Wet ue gas products
Combustion air required
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H content, balance CH
atmospheric temperature and pressure is H 2 . In that state, it is a low density gas that can easily leak from piping systems. Hydrogen has many unique properties compared to common hydrocarbon fuels (see Table 1 ). It has a very high heating value on a mass basis but a very low heating value on a volume basis because of its low density. Hydrogen’s low volumetric heating value means much higher volumetric flow rates for a given heat input, compared to other common fuels. Higher volumetric flows mean higher fuel gas pressure. Hydrogen has a higher flame speed and ignition temperature compared to many common fuels. It has a very low minimum ignition energy so it is easily ignited with a minimal spark. This is a good characteristic when ignition is desired but less desirable when trying to prevent ignition, as static electricity, for example, could easily ignite H 2 . Hydrogen has a relatively high adiabatic flame temperature and wide flammability limits compared to other fuels. It also requires considerably less combustion air, per unit firing rate, and generates fewer combustion products (see Figure 2 ). Potential advantages An important potential advantage of using pure hydrogen as a fuel is the combustion products do not contain CO 2 . However, if hydrogen is generated by conventional SMR, then CO 2 is a by-product of the production process. If hydrogen can be produced using renewable energy, it is possible to minimise or even eliminate CO 2 generation depending on the process. Another emissions benefit is the absence of carbon in Figure 2 Combustion air required and wet flue gases produced for blends of H 2 in CH 4 (15% excess air assumed)
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