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?
When the topic of transport decarbonization comes up, all I hear is batteries and hydrogen. But what about renewable natural gas (RNG)?
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.
Does it apply to its liquified form too? Dec 9, 2021 at 1:42
1Liquefied natural gas is less likely to leak, but if it does leak, the effects are the same as with the gaseous form.– MarkDec 9, 2021 at 1:51
Liquefied natural gas is more likely to leak. You can liquify natural gas only if you make it very cold. The trouble is, it won't stay cold forever. Heat is slowly leaking into the liquid natural gas container, which makes the liquid gas due to evaporation. The pressure starts to build up. If you aren't prepared to consume the produced gaseous natural gas as quickly as it is being produced, the only solution to prevent a pressure explosion is to vent that gaseous natural gas into atmosphere. Thus, liquid natural gas slowly leaks, even in ideal conditions. Pressurized natural gas never does it. Dec 9, 2021 at 17:45
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.
But if the feed is plant-based (e.g. food waste, seaweed), it's net-zero Dec 10, 2021 at 0:05
Yes, it is. However, there is not enough plant-based feed available. Dec 10, 2021 at 18:32
...and besides, you could store the plants underground in anaerobic conditions to capture the carbon. You could also burn them directly for energy, capture the carbon dioxide, and store the carbon dioxide underground. However, if you create methane and sell it for use in mobile sources like cars, they will emit the carbon dioxide directly to atmosphere, with no possibility to capture it. Dec 10, 2021 at 18:33
Seaweed. 6 more to go... Dec 11, 2021 at 3:31
If it's a closed loop, no biggie. We need to make our emissions net-zero, not our atmospheric CO2 concentration Dec 11, 2021 at 3:35