• Geoff Russell

Renewable gas

[N.B. Longer article on Grid Capacity, promised last week, has been delayed a little ... but is still coming]


I keep seeing news reports about "renewable gas". The term sounds like it came from the same marketing firm that gave the world "natural gas"; aka methane, a very potent climate change gas.


What is renewable gas? Graham Readfearn wrote about it recently in The Guardian; but didn't give much in the way of background and context. His article began:


The green gas nirvana touted by industry is hydrogen made using renewable energy and biomethane produced from organic waste – and it’s decades away

If he's right about it being decades away, and I think he is, then the other gas, methane, will continue to cover for the intermittency in wind and solar in our grid; with "firm up" being the popular euphemism.


Why will it be decades away?


I've given some calculations in a previous post about the land required if you make hydrogen with renewables. The technology is well understood, it's just intrinsically inefficient; meaning that the sheer volume of stuff required is massive when you want significant amounts of hydrogen. In that previous post, I considered Germany and the problem of replacing the gas coming from Russia in Nord Stream 1; you'd need some 18 millions of tonnes of PV panels to do that.


But the bigger prize is ...

Moving from gas to biomethane or hydrogen is hard, but what about the much bigger problem of replacing oil? Given that we (globally) are way behind in building factories to make batteries for EVs (and opening the extra mines to supply the required minerals), making synthetic fuels has been the target of plenty of research.


How close is that? Will we be able to cover the EV mineral shortfall by "simply" permuting biobased materials into good old-fashioned petro-chemicals? We can synthesise DNA and RNA into vaccines in bulk, so what's different about hydrocarbons?


Writing in a peer reviewed journal in 2020, one researching entrepreneur, Rob McGinnis, with reckoned:


... putting all of these advances together, it will be possible to offer renewable gasoline from DAC CO2-to-fuels within the next two years that is price competitive with fossil gasoline

Has anybody seen it? To be fair, McGinnis goes on to outline the challenges of scaling any such technology.


Let's sketch out the processes involved.


First, if you can make biomethane then you should be able to build any hydrocarbon; like jet fuel (kerosene) or octane and much else besides. The first of McGinnis's advances was a method of "upgrading" one kind of hydrocarbon into another.


But you wouldn't use organic waste to make biomethane, as Readfearn suggests. Just using waste would seriously limit the amount you could make. If you want to make synthetic fuels like biomethane and other hydrocarbons, and you want more than laboratory quantities, then you will need special purpose crops grown for the task.


But let's step back a few steps to explain hydrocarbons.


"Natural gas" is methane extracted from underground where it forms when plant or animal material decomposes. Most methane we use took millions of years to make, but you can make it in small amounts in your backyard with a badly constructed compost heap. Contrary to popular belief it has no smell. The smell of household natural gas is added artificially as a safety measure.


Decomposition involves microorganisms eating things and expelling waste, just like us; the waste can include methane when there is no oxygen around; which is common with things happening underground. The chemical formula for methane is CH4; meaning one carbon (C) bonded to four hydrogens (H). There are many ways of joining up hydrogen and carbon atoms into long chains. These are called, fairly obviously, hydrocarbons. Methane is a very simple one. It's like a chain with a single link. Octane is another hydrocarbon; it's a little longer; made from 8 carbons and 18 hydrogens. Kerosene is another, but it's a bunch of slightly different hydrocarbons with some common properties. Most jet aircraft are powered by kerosene.


So if methane starts off as dead plants, can't we cut out the middlemen (the millions of years and the microorganisms chomping it up) and just make it with freshly grown plants? Yes. And if we can make methane with plants, then can we make all hydrocarbons with plants? Yes. Call the result "renewable gas" and you can probably persuade a politician somewhere to donate to your retirement fund by funding your project to make it.


While it is true that we can certainly make methane and octane and any other hydrocarbon with plants, it's incredibly inefficient to do so; small details can trip up elephants.


Before going on we need yet another preparatory piece of background; an understanding of the scale of energy densities in the things we are talking about. Burn a teaspoon of sugar and you get about 70,000 joules of energy. A joule is a tiny unit so we often measure things in kilojoules or megajoules. 70,000 joules is 70 kilojoules. And a kilogram of sugar? Do the math, it's about 17.5 megajoules. Amazingly, burning a kilogram of just about any plant material gives you roughly the same amount of energy; after it is dried! Shovel leaves or logs into a boiler and all that matters is the (dry) weight.


If burning a litre of petrol releases some number of joules of heat, then how many joules are we prepared to put into making that litre of petrol in the first place? It takes very little energy to pump oil out of the ground. And, relative to the embodied energy, relatively little to refine it and ship it massive distances to petrol stations.


The basics of energy density

Here's a table of energy densities. The data comes from Wikipedia, but I've been selective. There are plenty of liquid hydrocarbon fossil fuels and they all have a similar energy density, so I've picked a few which are representative; similarly with the other categories. What matters in a post like this is the relative scale of things, not the details. We need to see the forest rather than the trees.


Look at the table and see what you notice.




Look for example, at Charcoal and Wood. Back in the days when wood powered everything you can see why people went to the trouble of making charcoal; because it is lighter to carry. So if you were cutting wood for energy, then you'd cut your trees, burn the wood slowly in the forest to form charcoal and then ship that rather than the wood. Note that the weight difference would be much higher than illustrated in the graph, because the data in the graph is for dry wood, not freshly cut wood, which has a high moisture content. The charcoal industry is still alive and well as a major global cause of deforestation in many poor countries; just as it was rich countries before coal.


Second, look at how bad batteries are compared to wood. There is 30 times more energy in a kilogram of wood than in a kilogram of Li-ion batteries; despite decades of development that have been poured into the latter. Raising the energy density of Li-Ion batteries comes with "spontaneous" combustion risks; something we don't see in wood. A graph of energy per cubic metre would look a little different because the same volume of battery is heavier than that volume of wood.


Carbon flows

We can get hydrocarbons fully formed from mines or we can get the carbon from plants and the hydrogen from water and combine them.


Once we have hydrocarbons, we can make plastics and fuels from them and then burn or bury these.


Burying can be deep or shallow. When it's deep, then decomposition, when it occurs won't involve oxygen and we'll get methane. Shallow burial is more likely to involve oxygen in the decomposition. Deep burial is usually called "Sequestration".


That gives us a bunch of different possible carbon flows.


  1. Mines/Burn/Sequester - carbon neutral

  2. Mines/Burn/Release - climate-damaging - BAD

  3. Mines/Plastic/Bury/Deep - carbon neutral

  4. Mines/Plastic/Bury/WithAir - climate-damaging - BAD

  5. Mines/Plastic/Bury/WithoutAir - climate-damaging - VERY BAD

  6. Plants/Burn/Sequester - draw-down-carbon - VERY GOOD

  7. Plants/Burn/Release - carbon neutral

  8. Plants/Plastic/Bury/Deep - draw down carbon - VERY GOOD

  9. Plants/Plastic/Bury/WithAir - carbon neutral

  10. Plants/Plastic/Bury/WithoutAir - climate-damaging - ?

When I talk about "Deep" burial of carbon, it may not actually be very deep, but it needs to be permanent. I've no idea how deep we need to bury plastic to make its burial permanent; but I'm sure that there are experts who do.


There are a couple of other starting points for flows that are also possible. The oceans have water and absorb carbon dioxide, so they have everything we need, alternatively we can get the carbon directly from the air and combine it with water for the hydrogen. This gives us another 10 flows, starting either with sea water or air+fresh water.


But each row represents a complex set of engineering problems; especially those that don't begin in mines!


Conversion efficiencies

My one-line carbon flow descriptions don't quite capture the complexity of the processes.


Consider a recent study of one part of the process. Starting with water and carbon dioxide, it was focused on the simultaneous splitting of both molecules and recombining; rather than on the production of the input streams. The end result was reported as using solar power to make kerosene. Note that the report doesn't say how efficient the process was. This is a constant problem with journalists reporting on science without any attempt to quantify or analyse. The general public hears these things and assumes problems are far closer to resolution than they are. For the next few days after such a story, you will hear people saying in casual conversation: "Oh yes, I heard they can produce jet fuel using solar power now." It's a bit like me saying I've begun walking to London and that being reported by a journalist without mention of the distance and oceans still to be crossed; on foot.


But back to the study. The process was in two parts. The first part created "syngas"; a mixture of hydrogen and carbon monoxide. The second part converts the syngas into kerosene using established technology.


The efficiency of the first part was 4.1 percent and the efficiency of the second part wasn't given, but is usually about 50 percent. That means for every 100 megajoules of solar energy input, you get 4.1 megajoules of syngas and use that to get about 2 megajoules worth of kerosene. Wait a minute, they started with carbon dioxide already supplied. If you had to get it from the ocean, or the air, the 2 percent efficiency is slashed again. I'd be pretty sure it would be negative! If you read the study, it's clear that the 13 authors are a very clever bunch. They didn't just dream up an idea, they made it happen.


What does it say about a problem when a group of 13 very clever people work so hard on a method that is so inefficient? It tells you that it's a really tough problem. It's much easier to manipulate words like "hydrocarbons" in a sentence than it is to manipulate the real thing.


Between 2014 and 2016 another very clever group at Google X embarked on a similar project; called Foghorn. It's task was to make hydrocarbons (which they called "Sea Fuel") from sea water. A recent recap of the failed project put the problem bluntly:


What the team realized early on, is that building the Sea Fuel wasn’t hard — making the Sea Fuel affordable was the real challenge.

Both the recent study and the Foghorn failed. Neither got anywhere even close to what was required.


If you look again at the energy density graph you can understand the beauty of chainsaws. Rather than burning a little bit of oil to keep warm, you can use it to cut down a tree and keep warm for a very long time ... until you run out of trees.


Using renewables to make synthetic fuels is plagued by a very simple problem; the amount of land and material you need to destroy to make a large amount of renewable energy means you can't afford to waste it. The efficiency of your synthetic fuel process is critical.


When we burn petrol in internal combustion engines, the process is extremely inefficient; most of the energy in the fuel is lost as heat rather than turned into motion. We don't care because being cheap is far more important than being efficient. With renewables, the amount of habitat occupied to harvest the energy is large and very few people care. "Cheap" is considered far more important than biodiversity protection.


What if ...

Wouldn't it be wonderful to have a super fuel; something that we could burn that would produce so much energy that we didn't have to worry about the inefficiencies of producing synthetic oil with it? It would be terrific to have a fuel that we could burn and not worry about using more gigajoules of energy to make synthetic oil than the oil was worth.


There are, as it happens, a couple of super fuels; uranium and thorium. Energy density in these fuels is measured in millions of megajoules per kilogram.







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