Burning Hydrogen

Where is this "HEAT" coming from?

"The combustion of ammonia is represented by this equation: 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g) ΔH°rxn = -904.8 kJ "

https://www.clutchprep.com/chemistry/practice-problems/11187/the-combustion-of-ammonia-is-represented-by-this-equation-4nh3-g-160-5o2-g-160-8

https://phys.org/news/2019-11-catalysts-nitrogen-oxide-pollutants-temperatures.html

Bond energy. All atom-to-atom bonds have an energy value that is needed to break them. In a reaction, the new bonds have such values too. Energy is measurable, convertible among many forms, and never disappears.

Hydrogen by itself is not available on earth. We can get it from water, where it was already combined, It takes energy to break those H-O bonds but we get much of that back when we burn it (oxidize). These numbers are known, but conveniently ignored by those anxious to see hydrogen as an environmental savior. Its combustion doesn't make carbon dioxide and thus is supposed to slow down global warming, but HMM (How Much Matters). And the energy to break the bonds to get the hydrogen from water may come from combustion of carbon-containing molecules. Hydrogen may in fact come out OK after all is said and done, but make sure you hear it all before you're done.

<...may come from combustion of carbon-containing molecules...>

Ideally it comes from sunlight. That's the whole point. There is no point in making hydrogen by burning fossil fuels!

Hydrogen is an energy carrier, much like electricity.

https://en.wikipedia.org/wiki/Hydrogen_fuel

As of 2020, about 95% of hydrogen is produced by the gasification of fossil fuels, which produces CO₂ in someone else's backyard.

Old news.

Where is the trend-line going?

Seems to be heading for ammonia(NH₃), much easier to store and ship, more energy dense than liquid hydrogen, simple to make with hydrogen and nitrogen...but still expensive comparatively speaking to fossil fuels....

..." Ammonia has a higher energy density, at 12.7 MJ/L, than even liquid hydrogen, at 8.5 MJ/L. Liquid hydrogen has to be stored at cryogenic conditions of –253 °C, whereas ammonia can be stored at a much less energy-intensive –33 °C. And ammonia, though hazardous to handle, is much less flammable than hydrogen....

...By most estimates, green ammonia will cost two to four times as much to make as conventional ammonia. And some of the technologies needed to harness the molecule, such as ammonia-burning engines, are still experimental....

...Furthermore, thanks to a century of ammonia use in agriculture, a vast ammonia infrastructure already exists. Worldwide, some 180 million metric tons (t) of ammonia is produced annually, and 120 ports are equipped with ammonia terminals....

...Several much larger projects were announced last year. By far the most ambitious one is in Saudi Arabia. The $5 billion project is a partnership between the US company Air Products and Chemicals, the local firm ACWA Power, and NEOM, a developer building a carbon-free city in Saudi Arabia.

Slated for completion in 2025, the installation will sit on the Red Sea coast. Solar cells will harness the sun during the day, while turbines will capture nighttime winds to generate 4 GW of electricity for water electrolysis plants. The hydrogen will be fed into a traditional Haber-Bosch plant to produce 1.2 million t per year of ammonia—a large amount even by conventional standards.

Air Products will spend an additional $2 billion to set up a novel distribution scheme. It will ship the ammonia around the world to specialized plants installed at depots for buses and trucks fueled by hydrogen cells. These units will dissociate the ammonia to recover the hydrogen, enough for up to 15,000 trucks and buses in all."....

https://cen.acs.org/business/petrochemicals/ammonia-fuel-future/99/i8

One of the Holy Grails that catalysis scientists hope to eventually make Hydrogen fuel widely practical is not a catalysis procedure that splits water molecules into hydrogen and oxygen for energy storage. As the Wikipedia link subsection discusses, simple electrolysis of water is fairly efficient. Thus catalysis has little room to improve efficiencies. The primary complication for Hydrogen fuel is hydrogen storage and the energy needed to put Hydrogen into and out of storage.

The experts here might be able to say the combined mass of two H₂O water molecules is less than the combined mass of two H₂ molecules and one O₂ molecule.

E=MC²...that might explain where the energy comes from....assuming C does not change during the process ... and Einstein is right...!

Energy has to go IN to separate the H from the O, and the H-H and O-O bond energies are different from H2O, hence the mass difference. When it recombines in combustion the bonds are remade, but we still need the energy IN to separate. And we need to get THAT from nonfossil before we can claim an enviro benefit (another critical issue involving CO2 in oceans, too big to bring in here).

There are practical problems like storage which may be resolved, but no miracles in science. That's why so many people are uncomfortable with science, as we learn miracles in babyhood when nothing can be explained, and it gets reinforced in a make-believe childhood (look at Halloween) and relied on as adults to keep us functionally sane.

Maybe a gasoline/ammonia fuel mixture would work....?

"Can you mix gasoline and ammonia?

The use of ammonia with gasoline allowed knock-free operation with MBT spark timing at higher compression ratios and higher loads than could be obtained with the use of gasoline alone."

"An overall stoichiometric mixture of air, gaseous ammonia, and gasoline was metered into a single cylinder, variable compression ratio, supercharged cooperative fuel research (CFR) engine at varying ratios of gasoline to ammonia.

The engine was operated such that the combustion was knock-free with minimal roughness for all loads ranging from idle up to a maximum load in the supercharge regime.

For a given load, speed, and compression ratio, there was a range of ratios of gasoline to ammonia for which knock-free, smooth firing was obtained. This range was investigated at its rough limit and also at its maximum brake torque (MBT) knock limit.

If too much ammonia was used, then the engine fired with an excessive roughness. If too much gasoline was used, then knock-free combustion could not be obtained while the maximum brake torque spark timing was maintained.

Stoichiometric operation on gasoline alone is also presented, for comparison. It was found that a significant fraction of the gasoline used in spark ignition engines could be replaced with ammonia. Operation on about 100% gasoline was required at idle. However, a fuel mix comprising 70% ammonia/30% gasoline on an energy basis could be used at normally aspirated, wide open throttle.

Even greater ammonia to gasoline ratios were permitted for supercharged operation. The use of ammonia with gasoline allowed knock-free operation with MBT spark timing at higher compression ratios and higher loads than could be obtained with the use of gasoline alone."

https://www.researchgate.net/publication/228372050_The_Fuel_Mix_Limits_and_Efficiency_of_a_Stoichiometric_Ammonia_and_Gasoline_Dual_Fueled_Spark_Ignition_Engine

I wonder what they could get away with if it was a fuel injected type of fuel delivery system?