When the topic of transport decarbonization comes up, all I hear is batteries and hydrogen. But what about renewable natural gas (RNG)?

Question

When the topic of transport decarbonization comes up, all I hear is batteries and hydrogen. But what about renewable natural gas (RNG)? The milage is better, less energy loss, as I understand. You can refill it in an instant (unlike recharging your Tesla). Plus, you can make your own fuel by decomposing your food waste and poo in your anaerobical digester in the backyard, it's also an advantage. What's the catch?

Answer 1

Leakage.

Hydrogen isn't a greenhouse gas. If it escapes into the atmosphere, no big deal: it'll either combine with oxygen to produce water, or it'll reach the outer atmosphere and escape. Either way, no global warming effect.

Methane is a rather potent greenhouse gas, second only to carbon dioxide (it has a stronger effect, but doesn't last as long -- and it combines with atmospheric oxygen to produce carbon dioxide). Any leaks during production or use will make global warming worse, not better.

Answer 2

Renewable natural gas ain't happening, ever, at the scales required.

We are using massive amounts of mobile energy as gasoline and diesel. The only feasible way to replace that is a generation mix of onshore wind, offshore wind, solar, nuclear, hydropower and maybe little (but very little) natural gas should hydropower and nuclear be incapable of meeting the momentary demand in the electric grid. About the only feasible way to store that energy in mobile sources is batteries. Hydrogen has very poor electricity-to-hydrogen-back-to-electricity-again efficiency (about 38% as opposed to over 90% of batteries), electrolysis cells are expensive, fuel cells are expensive and have a short lifetime, and storage of hydrogen is difficult due to it having very poor volumetric energy density. Therefore, the total cost of a hydrogen solution would probably be higher than what the market expects from the gasoline and diesel replacement technology. Batteries, on the other hand, have been demonstrated to be economically viable, and carmakers are adopting batteries as the definite solution.

The problem of renewable natural gas is that it releases carbon. Where do you get it if you create "renewable natural gas"? From carbon dioxide, naturally, combined with hydrogen produced using electrolysis. Combusting fossil fuels to get carbon dioxide is counterproductive. Atmospheric carbon dioxide capture is too expensive due to the low concentration of carbon dioxide in the atmosphere. If you capture carbon dioxide from office spaces where the concentration is higher (about double that of general atmospheric concentration), you make the economics of carbon dioxide capture from air more favorable but then you run into the problem that there aren't enough humans to breathe enough carbon dioxide. So about the only way to get sustainable carbon dioxide is from combustion of biofuels. The trouble is, we don't have enough biofuels to create enough carbon dioxide. Not only that, but if carbon dioxide of biofuels is captured from stationary sources like forestry industry plants, you could directly store that carbon dioxide underground (BioCCS) as opposed to creating "renewable natural gas" for combustion in cars where it would release its carbon dioxide back to the atmosphere. So claiming the product is "renewable" is silly because there's a far better place for that carbon -- under ground, not in atmosphere. Thus, so-called "renewable natural gas" contributes to the global warming problem due to atmospheric carbon dioxide.

Thus, the only solutions feasible are solutions that emit zero carbon dioxide. Those are basically hydrogen and batteries. It seems batteries are winning.