High hydrogen fuels impact pollution emissions differently than most hydrocarbon fuels. Since there is no carbon in H2, any of the pollutants containing carbon are eliminated compared to hydrocarbon fuels. These pollutants include for example, carbon monoxide, carbon dioxide, and soot which in some processes may be a source of particulate emissions that could cause opacity concerns.
On the other hand, hydrogen burns hotter than many hydrocarbon fuels. Thermal NOx is highly temperature dependent, so increasing the flame temperature would normally increase NOx. However, there are some mitigating factors that can reduce and possibly eliminate that effect. It is well known that recycling flue gases into a flame can reduce NOx by making the average flame temperature lower and the temperature profile more uniform which minimizes hot spots. There is a limit to how much flue gas can be recirculated into a flame before stability becomes an issue. Because hydrogen has a very wide flammable range and is highly reactive, more furnace gas recirculation (FuGR) is possible with hydrogen which helps reduce NOx. Kikuchi et al. experimentally showed that retrofitting hydrogen to an existing industrial burner doubled the NOx, but that by increasing the forced draft combustion air velocity to increase FuGR reduced NOx levels comparable to the hydrocarbon case.
A third type of pollutant that could be impacted by high hydrogen combustion is noise. Because of hydrogen’s high flame speed, higher fuel exit velocities are needed to prevent flashback in premix burners. Those higher velocities typically generate more jet noise, although there are well-established techniques such as mufflers to reduce combustion noise.
This paper will discuss some of the important differences using hydrogen compared to hydrocarbon fuels for combustion. It will focus specifically on the impact on pollution emissions with high hydrogen fuels.
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