The Ergosphere
Saturday, December 03, 2016
 

Fast, cheap, good biofuels: Can we have all 3?

Converting biomass (lignocellulose) to high-quality liquid fuels has been a huge, costly headache since the first log was retorted to make wood alcohol.  Acid hydrolysis of cellulose is costly and leaves lots of sulfate to deal with.  Enzyme hydrolysis is also costly in materials and typically slow.  Various types of pyrolysis have their good and bad points, but while some products like "liquid smoke" flavoring are never going to go out of style while people still eat barbecue, none appear to have ever made it as a source of commodity energy fuels.

That may have just changed.  Even more interesting, the crucial advance hasn't come from a chemist or an agronomist, but astrophysicist Frank Shu.

Dr. Shu's second career may wind up being of world-changing importance to mankind.  As part of the company which he founded, Astron Solutions Corporation, he has re-thought the process of pyrolysis.  Rather than heating biomass in a stream of hot gas, his advance is to use a molten salt as the heat-transfer medium.  The volumetric heat capacity of salt is hundreds or thousands of times as great as thin hot gas, which radically increases the speed of the pyrolysis reaction.  It also allows the separation of the heat-generation and pyrolysis steps (not unlike processes such as chemical looping combustion).  Last, because hot salt can be supplied separately from the stream of off-gas from the pyrolysis step and even driven counter-flow, large molecules can be thermally cracked to lighter gases and coke rather than escaping as troublesome tars.

The (seminal, I think) paper from Astron Solutions Corporation is publicly available.  Pay particular attention to Table 2 on page 3.  The thermal characteristics of the molten salt medium yield an off-gas which is effectively a clean syngas needing no further processing before use in synthesis.  (It's also a HOT syngas, which may be significant.)

That synthesis is a matter of choice.  There are a host of available catalysts which each favor different products.  However, there are vendors of turnkey methanol synthesis plants which take streams of CO, CO2 and H2 and produce CH3OH.  Methanol is a room-temperature liquid which makes a good motor fuel, and is also a basis for further synthesis (e.g. Mobil's Methanol-To-Gasoline process).  That seems hard to beat for a first cut.

Analyzing Table 2 in more detail, I get this:

Bio-product
Amount (kg)
Heating value
(MJ/kg)
Moles
Char/tar
6.00
26.46?
N/A
Water (H2O)
11.00
0.00
611.1
Carbon Dioxide (CO2)
0.10
0.00
2.27
Methane (CH4)
0.74
55.50
46.25
Carbon monoxide (CO)
1.48
10.11
52.86
Hydrogen (H2)
0.68
145.80
340

Of these products, water is waste (though it can be used to wash residual salt out of char) and methane is essentially unreactive under reducing conditions.

One's eye is drawn to the last entry.  Hydrogen isn't much by mass but it makes up the whopping majority of the molar quantity of the products after water.  If a catalyst reacted all the CO and CO2 with hydrogen to make MeOH (it would make about 1.76 kg), there would still be 227 moles of it left.  This suggests that about 2x as much additional carbon could be converted to CO and reacted to MeOH.  This CO could come from e.g. gasification of the char fines in the salt mixture.

If 90% of the remaining hydrogen could be converted to MeOH, it would consume 2.45 kg of carbon and make an additional 6.54 kg of MeOH for a total of 8.30 kg.  Finding a way to recycle the methane and convert it as well would add another 1.48 kg for a total of 9.78 kg.  This is a liquid fuel yield of almost 49 wt% from wood (albeit 50% oxygen by weight).  It would also leave about 3.5 kg of char, almost 100% of it carbon.

If this yield can be produced from lignocellulose in general, a not-unreasonable 500 million tpy of biomass would produce over 240 million tpy of methanol and 87.5 million tpy of stable fixed carbon char.  At a density of 0.79 this comes to over 80 billion gallons of MeOH, some 5.2 million barrels per day.  If 80% of LDV motor fuel can be replaced by electricity via PHEVs, that would do for the remainder with plenty left over.

The remaining carbon char, if buried, would sequester 87.5 million tpy of carbon (some 320 million tpy of CO2-equivalent).  Char reportedly holds soil nutrients and increases water retention, improving fertility.

These are not the only possibilities.  If air-separation plants can be made small, cheap and reliable enough, the excess hydrogen could instead be reacted to produce ammonia.  I've read brief claims about catalysts which are fairly specific for ethylene as a product, rather than methanol.  Both are liquid at reasonable temperatures under relatively moderate pressure, so are easily shipped.

Motor fuel, plastics, fertilizer, a third of a billion tons of CO2 sucked OUT of the air... when can we get started? 
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