Comment by littlestymaar | Hacker News Reader

Comment by littlestymaar | Hacker News Reader

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> one could use an oxygen concentrator to supply nearly pure oxygen as the input gas. With much less N2 available, production of nitrogen oxides would be reduced.

I suspected the amount of energy needed to concentrate oxygen would dwarf the amount of energy of the combustion so I did the calculation:

I took the following device[1]: 350W to produce up to 5L of Oxygen per minute.

5L of dioxygen means roughtly 6 gram or 0.18 mol per minute.

Pure carbon combustion produces 393 kJ or energy per dioxygen molecule, whereas dihydrogen produces 572kJ per dioxygen molecule.

Wood has roughtly as much Carbon as di-hydrogen (glucose, the basic block for cellulose is 6 carbon for 12 hydrogen, I'm voluntary ignoring the oxygen here since I want an upper bound), so when burning wood, for each of our dioxygen molecule we can take ~500kJ/mol as an approximation.

That means your device can only produce enough oxygen to sustain a fire generating around 1,5kW of power. (I was wrong)

So, even though my intuition was wrong, spending 350W of electricity to get less than 1,5kW of heat out of the wood you're burning doesn't sounds like a great deal.

You're much better off with a heat pump at that point…

> Or you could do this:

> https://en.m.wikipedia.org/wiki/Chemical_looping_combustion

I had never heard about that, this is so cool!

[1]:https://oxystore.fr/concentrateurs-d-oxygene-stationnaires/5...

There are at least one misconception here that I can see (I work in gasification technology).

You only need the pure oxygen for producing enough heat to pyrolysize the fuel (wood/biomass), once it's broken down into tar gas, you can feed it through your charcoal layer to further reduce it into syngas. The syngas can then be burned at a separate space with air at a lower temperature where nitric oxides aren't created.

Utilizing the residual heat in the exhaust gas (flue gas) is another means of feeding the pyrolysis phase. As you mentioned its also a great source of CO2 and H2O as a feedstock for producing CO and H2 from the hot charcoal.

What I'd like to see happen is usage of the "waste" oxygen produced while producing hydrogen from electrolysis of water. The oxygen could be used to produce syngas, and the hydrogen could be fed into this fuel stream or used separately for off hour electricity production.

There is at least one misconception here that I can see. You don't need pure oxygen to initiate pyrolysis, combustion with atmospheric air is suitable to initiate pyrolsyis as well as gasification systems that pyrolyze biomass without any oxygen/combustion using electric resistive elements eg powered by solar pnaels if you want to learn more about gasificatoin theres soem great resources in the internet

You're correct. But the whole initial conversation was based around not wanting to create NOx from usage of atmospheric air which contains Nitrogen.

As I've noted, I work in gasification technology.

What scale what feedstock?

To complete the analysis, you should (a) use the high temperature combustion heat to run a heat engine to power the oxygen concentrator, and (b) include the waste heat from the oxygen concentrator in the overall heat output of the device. Then you have a box with (wood, air) in and (combustion products, heat) out.

You could do (a) but I don't think there exists oxygen concentrator running on anything else than electricity on the market, so if you need to design you own custom oxygen concentrator and get it built, your CapEx are going to be ludicrous. Or you can make electricity from the heat then power a regular concentrator, that would be more reasonable but that's still another complex and costly machine to add to the system. that's the thing with thermodynamic machines, you can always increase the yield by increasing temperature and getting closer to the Carnot cycle, but every step is more costly than the previous one.

In the real world you always have CapEx + OpEx as additional input.

Regarding (b), here waste heat is hardly usable as it is low temperature and diffuse. You could still you the residual heat from the engine in (a) since it would still be hot enough, but the 350W dissipated by the concentrator can't realistically be collected.

1.5kW for a room heater isn’t bad.

In any case, I imagine that an industrial scale oxygen concentrator would be dramatically more efficient.

That's 1.5kW in exchange for 350W + the fuel. Whereas a heat pump gives you the same without the wood at all. (And a basic stove gives you the 1.5W for just wood without electricity nor complex and expensive machine).

An industrial scale thing may be more energy efficient, but why bother when you can just get rid of the NOx with catalysis instead…

Your average heat pump doesn’t burn wood, and wood is, in several respects, an excellent fuel. It’s renewable, it’s easy and safe to store, it’s dense, and it’s easy and safe to transport.

(Yes, on a large scale, it’s better for everyone to use heat pumps. But heat pumps plus wood for backup in unusually cold weather may also be a good choice.)

On an industrial scale, some form of nitrogen-free combustion may simplify CO2 capture. And, if a CO2-capturing plant needs to remove nitrogen from a gas stream, removing it from the intake stream instead of the exhaust stream may eliminate the need for catalytic reduction. Also, the intake gas is clean and cool, and there’s no risk of contamination the zeolite adsorption bed (or whatever gets used) with soot.

> Your average heat pump doesn’t burn wood, and wood is, in several respects, an excellent fuel. It’s renewable, it’s easy and safe to store, it’s dense, and it’s easy and safe to transport.

You don't understand: your heat pump roughly generate the same amount of heat without burning anything. Instead of feeding electricity to an oxygen concentrator, you feed it to your heat pump and tada! you have your heat without burning anything!

> But heat pumps plus wood for backup in unusually cold weather may also be a good choice.)

Sure, but in this case you just need a good stove, if that's only used occasionally then you don't care about a little bit of NOx.

> On an industrial scale, some form of nitrogen-free combustion may simplify CO2 capture.

Sure, but then it's a completely different topic, at least because a concentrator won't give you nitrogen-free gas, just 8 times less nitrogen, which may not be enough for the needs. CLC is probably better suited for that, or regular air distillation to get pure oxygen.

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