Researchers at Tohoku University in Japan have identified a way to reduce harmful NO emissions produced by ammonia fuel during combustion. The process involves swirling the gas with air as part of the power generation process. The finding helps advance research into ammonia as an alternative to carbon-based fuels for cars, airplanes, and power generation facilities.

A paper on their work is published in the journal Combustion Theory and Modelling.

Left: Vortex structures of swirling flows. Right: Mixing pattern of two ammonia streams with the air stream near the swirlers, located at the bottom of the figure. Colors indicate ammonia mole fraction distribution, with higher amounts of ammonia molecules in red, and lower amounts in blue. Copyright : Taylor & Francis

This study simulated bluff body stabilized non-premixed turbulent ammonia (NH₃)/air flames using swirling flows. Although the space and time-averaged emission (STAE) of NO was found to decrease with increase in the global equivalence ratio (global φ) under stoichiometric and rich conditions, similar to the behaviour of premixed flames, the local NO concentration distribution within the combustor was heterogeneous, unlike the almost uniform NO distribution in premixed flames.

Zones of high NO concentration were identified near the combustor wall boundaries, whereas there was almost nil NO in the combustor centre, irrespective of the global φ. The localized NO concentrations in non-premixed flames were shown to depend on the local φ within the combustor and the model reproduced the NO emission characteristics of premixed flames in terms of φ.

This study also found that the effect of pressure on NO emissions is significant irrespective of the combustion type, because reducing the OH radical concentration in NH₃/air flame at high pressure limits the NO generation. However, the STAE of NO of non-premixed flames were slightly higher than those of premixed flames, especially under rich conditions, possibly owing to zones of elevated NO concentrations near the wall boundaries.

The introduction of an additional NH₃ stream external to the air stream (by splitting the original NH3 flow) may mitigate this effect by creating an almost uniform φ distribution within the combustor and thus a homogeneous NO distribution. The STAE of NO was minimized (and equalled that of a premixed flame) by applying a volumetric flow ratio between the innermost and outermost NH3 flows of 0.6:0.4.

—Somarathne et al.

Ammonia (NH₃) is a compound that contains one nitrogen and three hydrogen atoms. It is under investigation as an alternative fuel source for several reasons. While it contains a great deal of hydrogen, it is less expensive and less flammable than pure hydrogen, making it safer to transport. Production plants already exist because ammonia is widely used in fertilizers.

So far, ammonia has been considered as a fuel when blended with gasoline, diesel, hydrogen and methane fuels to reduce the proportion of carbon-based fuels and their emissions that contribute to climate change. Developing ammonia as a pure fuel source remains a challenge, in part because relatively high levels of harmful nitric oxide (NO) emissions are produced during the combustion process.

Nitric oxide is known to be harmful to human health, contributes to ozone depletion, and when it reacts with other compounds, contributes to acid rain and atmospheric warming.

A team from the Institute of Fluid Science at Tohoku University used supercomputers to run large eddy simulations to analyze how ammonia fuel behaves under different combustion conditions, and to see if it is theoretically possible to reduce nitric oxide emissions.

Specifically, they analyzed what happened when ammonia was swirled together with air inside a theoretical combustion chamber under different pressures. They compared the results with those of ammonia and air premixed before entering the combustion chamber, which is known to produce fewer nitric oxide emissions at high fuel to air ratio conditions.

They found that swirling not one, but two streams of ammonia gas with one stream of air reduced nitric oxide emissions to levels on par with premixed processes. Making the volume of the two ammonia streams between the innermost and outermost swirlers uneven—60% and 40% of the total injected fuel, respectively—led to a more even distribution of fuel and air throughout the combustion chamber, which produced lower nitric oxide emissions.

This study simulated bluff body stabilized non-premixed turbulent ammonia (NH₃)/air flames using swirling flows. Although the space and time-averaged emission (STAE) of NO was found to decrease with increase in the global equivalence ratio (global φ) under stoichiometric and rich conditions, similar to the behaviour of premixed flames, the local NO concentration distribution within the combustor was heterogeneous, unlike the almost uniform NO distribution in premixed flames.

Zones of high NO concentration were identified near the combustor wall boundaries, whereas there was almost nil NO in the combustor centre, irrespective of the global φ. The localized NO concentrations in non-premixed flames were shown to depend on the local φ within the combustor and the model reproduced the NO emission characteristics of premixed flames in terms of φ.

This study also found that the effect of pressure on NO emissions is significant irrespective of the combustion type, because reducing the OH radical concentration in NH₃/air flame at high pressure limits the NO generation. However, the STAE of NO of non-premixed flames were slightly higher than those of premixed flames, especially under rich conditions, possibly owing to zones of elevated NO concentrations near the wall boundaries.

The introduction of an additional NH₃ stream external to the air stream (by splitting the original NH₃ flow) may mitigate this effect by creating an almost uniform φ distribution within the combustor and thus a homogeneous NO distribution. The STAE of NO was minimized (and equalled that of a premixed flame) by applying a volumetric flow ratio between the innermost and outermost NH₃ flows of 0.6:0.4.

—Somarathne et al.

Still, emission levels were higher than the Japanese environmental regulations for gas power turbines. The researchers plan to next test if injecting air downstream of the combustor further reduces emissions.

This research was supported by the Council for Science, Technology and Innovation (CSTI) and by the Cross-ministerial Strategic Innovation Promotion Program (SIP) Energy Carriers,’ (funded by the Japan Science and Technology Agency (JST)).

Resources

• Kapuruge Don Kunkuma Amila Somarathne, Sophie Colson, Akihiro Hayakawa and Hideaki Kobayashi (2018) “Modelling of ammonia/air non-premixed turbulent swirling flames in a gas turbine-like combustor at various pressures” Combustion Theory and Modelling doi: 10.1080/13647830.2018.1468035