I really don\’t believe this number

I\’m a great believer in hydrogen as the transport fuel of the future. But I still don\’t believe this number:

Biomass – trees, plants and other waste vegetable matter – is an abundant and rapidly renewable source of starch and sugars, that is nowadays used to produce biofuels. Exploiting biomass to produce sugar, and turning that sugar into hydrogen, could lead a change in global energy production.

In 2011, the US consumed 134bn gallons (507bn litres) of gasoline, but \”with our technology, just 700m pounds [317,500 tonnes] of biomass would be enough to replace the whole yearly [gasoline] production,\” says Zhang. The last official assessments estimate the availability of crop residues for biomass in the US to be about 157m tonnes per year.

No, I\’ve not examined it. But there seem to be too few zeros on that biomass number.

507 billion litres is, close enough, 507 million tonnes of gasoline. I\’m really very unconvinced that you can replace 507 million tonnes of gasoline with the hydrogen produced from 317,500 tonnes of biomass. Perfectly willing to be proven wrong but it really just doesn\’t sound in the slightest bit likely to me.

Anyone know enough chemistry to be able to grind through the possibilities?

Assume that biomass (wheat straw, say) is equal to grain production from a field. Wheat yields are four tonne a hectare these days? Thus we would get 317,500 tonnes of biomass from 80,000 hectares. 800km2.

Land area of US is 9 million km2. We can grow the fuel to replace gasoline on 0.008% of the US?

I\’m afraid I just don\’t believe it.

27 comments on “I really don\’t believe this number

  1. You only get about 1 tonne/acre of straw these days, due to plant breeding of shorter stemmed varieties (less likely to get flattened by heavy rain), and the use of plant growth regulators. Even in the old days before all that you might only have been getting 2 tonne/acre.

    And in the US the yields per acre are far lower than the UK (it being a far more extensive production system) so tonnes of straw/acre there would be even lower than here.

  2. Tim, you’re doing this Fermi estimation all wrong – wandering out to the land area just adds an extra step (and thus error) to the calculation.

    507b litres ~400m tonnes gasoline. To replace that with 317k tonnes of biomass would require each tonne of biomass to replace over 1,000 tonnes of gasoline.

    Even if we ignore the losses in converting the biomass, it would need to start out with that much greater energy density than an already very energy dense material. It doesn’t – the energy density of gasoline is more than double that of wood or grain.

    Either they are out by a factor of 3,000+, or they’re amping up DECC’s trick of assuming a large reduction in demand by the time their target is to be reached.

  3. *continued

    However, given they’re talking not about burning the whole plant, but microbial production of H2 from the biomass, I think we can assume relatively low capture of the energy available – so call it a round factor of 10,000 too small in the estimate for biomass available. That requires 80% of the US land area by your estimate, which sounds more likely.

    Of course, they could always be using the DECC trick of factoring reductions of demand into the analysis of they pet new technology, but that 700m tonnes is awfully close to the “projections find that by 2030 the total biomass resources …available… will be close to 680m”, so I’d guess they just did the old trick of plucking the most convenient number to hand.

  4. “turning that sugar into hydrogen”: does anyone know what they plan to do with the carbon?

  5. That’s the trouble with these new-fangled decimals. It’s so easy to get your sums wrong by a factor of 10,000 or so.

    Bring back the Babylonian number system, say I.

  6. “507 billion litres is, close enough, 507 million tonnes of gasoline”
    - Actually, petrol is lighter than water, so it’s not the usual one-to-one volume-to-weight conversion. The conversion is somewhere between 0.71 and 0.77, depending on the ambient temperature and on any additives or variations in formula. 0.73 is generally quoted as the average. Thus your 507bn litres weighs 370m tonnes.
    Nevertheless it’s still out by several orders of magnitude compared to the biomass figure given.

  7. The chemical energy content of gasoline is about 132 MJ/US gal. The chemical energy content of hydrogen is about 120 MJ/kg. So they are looking for about 147mn tonnes of hydrogen. Zhang’s process has a hydrogen/starch mass ratio of about 14% (some of the hydrogen produced by the reaction comes from water), so they would need about a billion tonnes of starch. How much biomass it would take to get it depends on the nature of the biomass and how much you value ethanol as a by-product, but let’s say three times as much. So the article is wrong by a factor of about 10,000.

    dearieme: The starch reaction makes hydrogen and carbon dioxide. I suppose they could in principle capture the carbon dioxide, but if they just let it go the process would be carbon neutral over the cycle.

  8. “Nasa used it to power its space shuttles” (hydrogen)
    Can’t say I’m a chemist or a physicist, let alone a space cadet, but isn’t hydrogen used in rocket engines because the combustion product has the highest specific impulse due to having the least density? Why it would be the working fluid of choice for a thermal nuclear rocket, despite not being the energy source? Can’t say I can hack the math but I think the Isp is in inverse proportion to the density, isn’t it? And it’s being phased out in favour of kerosene & rubber in the latest generation of orbital lifters because advances in engine design mean they can avoid the problems of handling large volumes of liquid H2.
    So using the Space Shuttle to advocate hydrogen as a fuel is a bit like advocating two-stroke to run your central heating because it’s used in Lambrettas

  9. This is one of the many reasons why SI is a better system than US Customary (note that the Guardian has had to provide the conversions itself, hence the square brackets).

    In Zhang’s original, it’s far less obvious to a listener that his numbers are bullshit, because the conversions are difficult.

    In SI, it takes 3 seconds to realise it’s at least three orders of magnitude out (with the fact that it’s more like four requiring the harder work that’s been done in this thread).

  10. Good point. (Liquid) Hydrogen is used for rocket fuel because it has a high energy/mass density. That’s pretty well irrelevant for a car, where a high-pressure hydrogen tank would weigh much more than the hydrogen in it. Alternatively, Zhang et al are suggesting an “on-board starch-H2 converter and starch container” which would again weigh more than the fossil fuels they replace.

  11. John B>

    “This is one of the many reasons why SI is a better system than US Customary ”

    Say rather, this is one of the many situations in which SI is the better system. That’s by no means always the case. Both Metric/SI and Imperial/US Standard have their advantages and disadvantages. As a general rule, any situation where you’d pull out a calculator, you want to be working in the decimal system, and any situation where you’re doing things in your head, you want to be working in base-twelve/sixteen, with the attendant factor-advantages.

    I can’t agree with you that this one is more obvious in SI, though. Perhaps to you. I have no trouble seeing that millions of pounds is of a different order to billions of gallons, to the extent that volume and mass are comparable.

  12. Paul B>

    ” a high-pressure hydrogen tank would weigh much more than the hydrogen in it.”

    Compressed hydrogen at 700PSI still weighs next to nothing – ~50-60g/l – so you’re not wrong on the low density. You might be rather surprised and impressed by the weight of some of the Kevlar balloon tanks people have been working on, though. Still not going to weigh less than the contents, but the same order of magnitude at least.

  13. I’m not a great believer in hydrogen, and to compare a space ship with a car is ridiculous.

    You can’t get more energy density than gasoline, because both the H and the C burn, it’s liquid at 1 bar so you can use a plastic bag if you like, you get your O free from the atmosphere. That’s on land, in a car.

    Conversely, there’s no O in space so you need to carry it (pressurised containers add weight, etc) and the calculations of the most efficient fuel / weight will change entirely.

  14. “By inspection” as the maths exams used to say, I agree that this is BS.

    Consideration of the energy density* of hydrocarbon fuels suggests there ain’t no get out of jail free card like this.

    * Such as, how many kJ you get from a Kg of petrol as opposed to almost anything else except fissionable Uranium.

    And no I haven’t worked the numbers as PaulB has, this is as I said “by inspection”, ie it’s obvious.

  15. @Dave
    But H2 is still the smallest molecule. Which means it diffuses through anything. Even metal tank walls. You can lighten the material the tank’s made out of but that will inevitably increase the diffusion rate. Not a problem with a balloon because there’s plenty of opportunity to vent the tank enclosure. A hydrogen build up in a vehicle with hydrogen’s low ignition point? It was a hydrogen explosion blew the roof off that Japanese reactor.
    For the same safety reason, it’d be worth knowing the shut down lag-time on Zhangboy’s starch converter. At ambient pressure there’s not much energy in H2/litre so the volume throughput would have to considerable. You really don’t want to be losing the compressor(?) & still have umpteen litres of hydrogen backing up on you. It all sounds a bit like those wartime charcoal powered vehicles to me. Better than nothing but…

  16. I can’t understand why anyone would want to stick with Imperial units. Yes, evil European conspiracy, metric, and all that. Other than that.

    Imperial is a bogged up mess of customary bodging, and the whole idea that some virtue exists in converting foot pounds per chain into acre yards per baker’s dozen is ridiculous. Really.

  17. @ian
    “I can’t understand why anyone would want to stick with Imperial units”

    Strange the world does, then. Go and actually measure things. The counter heights in your kitchen. The available thicknesses of plywood. Lengths of wood in a woodyard. Door heights. Drawer widths. Amazing how often they are in or very close to imperial measurements. And that’s here as well.
    “some virtue exists in converting foot pounds per chain into acre yards per baker’s dozen”
    But people rarely do that. If it’s weight in pounds, it’s to compare with something else, weighed in pounds. And if you want an ounce, just keep dividing in half four times. Half of a foot is 6″. Third of a foot 4″ Width of your thumb 1″.
    The old measures are based on what ordinary people actually do & use. You know they still use feet in France? Try buying a shed. They’re dimensioned in pieds.
    It’s metric that’s bloody awkward.

  18. The standard sizes are grandfathered in, yes, but 25 or 20mm ply would make sense anyway.

    The problem for imperial comes when you’ve got to use the arbitrary units from different scales, which are unrelated to each other in any way; mass, length, etc, all with different multipliers. 16 oz to the pound, 14 pounds to the stone. Yards, chains, furlongs, on and on the complexity goes. 1760 yards to the mile. It’s just antiquated bumfoolery.

    Our number system is base 10, and SI relate the different units to one another, instead of them being the average weight of a mediaeval rooster, or how many sword lengths a plough travels between Whitsunday and harvest time.

    Honestly, I really can’t fathom why anyone won’t use metric.

    The width of my thumb is 20mm, by the way.

  19. Ian, frog plumers use pouces. (thumbs to you. = one inch)
    But you measure it not by the width of your thumb but the length from the last knuckle.
    Do try to keep up without getting your measures 90 degrees out or your sums 10,000 times out.

  20. Ian.
    They’re grandfathered into metric because they were grandfathered into imperial. They’re good working sizes.

    “The problem for imperial comes when you’ve got to use the arbitrary units from different scales, which are unrelated to each other in any way;”
    Yeah. But who does that every day? Only university graduates. Who gives a f**K if it’s hard for them? That’s they went to uni for.

    What’s so clever about decimal, anyway? We use doudecimal for hours with no problem. Base 60 for minutes & angles.
    I blame it all on the French.

    (But not Napoleon. Who got a law passed said – although the official French measurements were metric, no Frenchman could be prosecuted for using the old system.)

  21. The energy density of hydrogen is actually about 3 times that of petrol – it’s the difficulty in dealing with compared to petrol that makes it a bugger. However, my main reason for posting was to link to xkcd.

  22. BIS-

    The problem for imperial comes when you’ve got to use the arbitrary units from different scales, which are unrelated to each other in any way;”
    Yeah. But who does that every day? Only university graduates. Who gives a f**K if it’s hard for them? That’s they went to uni for.

    Well, engineers. I’d have thought you’d think of that. I mean, ordinary ones. Like the time I worked out how fast a particular water heater ought to be heating up, on site, in my head. Or the time I worked out for the benefit of a less experienced cow-orker the approximate air pressure on the door of a large air handling unit.

    Which were both a lot easier because I didn’t need lots of conversion factors or multiplying by paradoxical numbers like 14 and 16 instead of the metric ease of shifting a decimal point.

    How many inches in a mile? Mmm,1760x3x12. How many gallons in a tank of W,H,D expressed in inches? No fucking idea.

    For people who don’t need to do calculations, it doesn’t matter what you use. The old dear in the screw-on hat buying a quarter of ham doesn’t care. She can buy it in Megalithic Yards for all it matters.

    But if you actually need to choose which system is more ergonomic, metric wins by a country mile.

  23. Ian
    I’m not against metric units if they make things easier. if. But I’m not against any system, makes things easier. You want to work out thermal capacities use metric. Somebody wants to cook, ounces, cups & tbsps are better.
    I work in metric & imperial. Simultaneously some of the time. If what’s already there was made in imperial, it’s asking for trouble to try matching it with metric conversions. Used to work in Troy & grams at one time, as well.
    Tell you something, though. You see one of these big order of magnitude errors, people make a ass of themselves with. You can guarantee the ass*le works in metric. Can’t do simple arithmetic without a calculator. Anyone works in imperial is used to doing figures in their head. So you’ve a good idea what the answer should look like before you get there.
    Tank in inches? Gallon’s roughly 275cu” Plumbers remember things like that.

  24. Incidentally, going back to the original topic, Are we sure Zhangboy isn’t reinventing the wheel, here? Getting a high energy fuel from carbohydrates has been done before. About 6000 years at a guess. Alcohol. Being done currently with ‘biofuels’. And aren’t we’re going to run into the same problem again. Feedstocks. Now theoretically you can get alcohol (or methane for that matter) out of any vegetable organics. But to run the process economically, so your getting more energy out than your putting in, you need to be working with organics with a high energy density. Those starches & sugars. Which are the plants we eat. And aren’t biofuels being blamed for high food prices?

  25. Rule of thumb: It takes 3 to 4 tons of biomass (wood, woodchip, other plants to produce as much heat from combustion as you would get from a ton of hydrocarbon (coal, oil (including biofuels), methane and other hydrocarbons), give or take a bit.

    Of course the energy you get per ton of biomass will depend on its moisture content and the energy content per ton of hydrocarbon varies by type but are mostly in the same ball park around 45GJ/ton.

  26. Regarding the metric system and the supposed advantages of adding more bases to make things simpler… there’s a reason the European standard building unit is 600 mm. I say this as a firm imperialist and frogophobe to the core – up with Metric! Down with feet, inches and all the bloody rest!

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