Does the use of hydrogen produce air pollutants such as nitrogen oxides?

In many current and expected future applications for hydrogen, no air pollution will be produced at the point of use. For example, hydrogen can be used in a fuel cell to generate electricity without any harmful emissions, emitting only pure water vapor. Because fuel cells operate via an electrochemical reaction instead of combustion, even high-temperature fuel cells (which can operate up to 1,000°C) do not create any harmful direct emissions. By providing a zero-emissions option for certain applications—for example, by shifting from fossil fuels and internal combustion engines to hydrogen and fuel cells in heavy trucks, trains, and ships—the use of hydrogen can improve local air quality.

On the other hand, nitrogen oxides (NOx) may be emitted in cases where hydrogen is combusted—such as in a turbine used to generate electricity or in industrial heating applications. NOx formation occurs when air (which contains nitrogen and oxygen) is exposed to very high temperatures (›1,500°C). Any type of high-temperature combustion (e.g., of diesel, gasoline, natural gas, hydrogen) can produce NOx, which include nitric oxide (NO) and nitrogen dioxide (NO₂), both hazardous air pollutants. Because hydrogen burns at higher temperatures than natural gas, combustion of pure hydrogen may result in comparably higher NOx emissions. However, this may be offset by the fact that hydrogen has a larger stable combustion temperature range, meaning a higher ratio of air to fuel can be used. This additional air effectively dilutes the hydrogen, which cools off the flame and results in lower-temperature combustion, thereby reducing the amount of NOx emissions produced. Current research has shown that hydrogen combustion via turbines can achieve comparable performance and NOx emissions to those of today's turbines running on natural gas.¹

Scenarios for achieving net-zero greenhouse gas emissions by 2050, as laid out in the U.S. National Clean Hydrogen Strategy and Roadmap,² involve some uses of hydrogen for combustion (e.g., for industrial heating and seasonal energy storage using turbines for power generation), but these would account for a small fraction of the 50 million metric tons of clean hydrogen projected to be used annually by 2050. And multiple mitigation strategies exist to prevent or reduce emissions of these pollutants when hydrogen is used for combustion. When NOx is produced, its release into the atmosphere can be reduced using existing and emerging technologies. For example, flue gas treatment approaches can convert harmful emissions into less harmful compounds, using processes similar to those in the catalytic converters currently integrated into gasoline- and diesel-powered vehicles.

More information on NOx emissions can be found in the presentation for HFTO's H2IQ Hour "Addressing NOx Emissions from Gas Turbines Fueled with Hydrogen."³