Air Quality Implications of Using Ammonia as a Renewable Fuel: How Low Can NO_x Emissions Go?

Abstract

In addition to their lifecycle carbon emissions, another important issue with decarbonized energy pathways is their air quality, water, or land use implications. This paper considers the air quality issue for ammonia combustion. When directly combusting ammonia, reactions of its N atom with atmospheric oxygen lead to NO x emissions that are O­(103) ppm, 2 orders of magnitude higher than EPA limits or the amount emitted by current natural-gas-fired technologies. In order to provide guidance to policymakers and technologists on what is fundamentally possible, this Perspective analyzes the fundamental minimum NO x emissions that can be produced from ammonia combustion. The analysis shows that it is possible to achieve quite low NO x emission levels of O(10) ppm, but these designs differ markedly from those used in today’s lean, premixed combustion systems.

Direct Numerical Simulation of a Reacting Turbulent Hydrogen/Ammonia/Nitrogen Jet in an Air Crossflow at 5 Bar

“…This suggests working with H 2 concentrations in fuel mixtures lower than the present one. Further reduction of NO can be achieved by increasing the nominal pressure [27]. This will be the aim of future work.…”

“…Numerical studies suggest that optimizing the equivalence ratio in the first (rich) combustion stage is essential for achieving low emissions at the outlet [26]. Furthermore, chemical kinetics investigations demonstrated that using a staged combustion (consisting of a rich stage, followed by a relaxation stage before the final lean stage) coupled with high-pressure conditions (>20 bar), it is fundamentally possible to achieve NO x emissions as low as in natural gas turbines [27].…”

Ammonia Can be Currently Considered as one of the Best Green Energy Ally

“…Recently, the Toyota Corporation presented an ammonia engine suitable for passenger cars developed in collaboration with the Chinese car manufacturer GAC [54]. Although environmental concerns may arise regarding NOx levels during ammonia combustion, the technology is expected to meet regulation standards by performing suitable changes in combustion equivalence ratio, combustion chamber pressure, and applying emission control strategies [55].…”

“…Gubbi et al [55] performed a numerical study, evaluating the NO emission expected from ammonia combustion by changing combustor pressure and residence time. These authors indicated that current NOx regulation limits can be complied with, by changing operating parameters and allowing higher residence time in the combustor.…”

The Colors of Ammonia

“…The use of ammonia in the power sector is driven by the fact that ammonia is a “carbon-free fuel” as it contains only nitrogen and hydrogen . When ammonia is thermally combusted or electrically converted there is no resulting carbon dioxide, although NOx must be managed . The use of ammonia in the agriculture sector is for fertilizer production. , Ammonia is also a widely used nitrogen-based building block for many other chemicals (e.g., nylons, refrigerants, explosives, poly­urethanes).…”