Grass power revisited
Something I never did when I was analyzing zinc cycles was to look at the conversion of biomass to carbon. There are substantial mass and energy losses associated with the carbonization process, and that energy has to go somewhere. The author of the paper on carbonization suggested that the pyrolysis gas might be used to run an engine if it was of sufficient quality. What are the possibilities?
All that's outgassed is not lost
Heiko Gerhauser gave an energy figure of 17.4 million BTU per metric ton (presumably for dried Miscanthus); this is 18.3 GJ/tonne. If Miscanthus is 4% ash and the carbon yield from the remainder is 28%1, the charcoal produced would contain 26.9% of the original mass as carbon and the remnant 4% ash (total 30.9%). According to my Rubber Bible2, the heat of formation of carbon dioxide from oxygen and graphite is -93960 cal/mol. 269 kg of carbon is 22,400 moles and would yield 8.81 GJ when burned; the difference, 9.49 GJ/tonne, is released during the carbonization process. Some of this would be heat and some would be as chemical energy in the off-gas. (I know I'm not accounting for every component of the char; better computations welcomed as I've no time now to do them myself.)
This is a very large amount of energy. Further, much of the gas is produced from solid and thus is an expansion of volume over the biomass. This expansion is perfect for driving a gas turbine (requires no work from the compressor). If the carbonizers are built in-line with the gas path of a gas turbine of 38.6% efficiency3, the pyrolysis process would produce 3.66 GJ/tonne (1020 kWh/tonne) of electricity. A secondary steam cycle powered by the turbine's exhaust heat and running at 28% efficiency4 would yield another 450 kWh/tonne, for a total of 1470 kWh/tonne (55.8% efficiency overall). In this process, roughly 30% of the carbon in the biomass is exhausted to the atmosphere as CO2.
The solid stream
The solid product of this process is char, rather high in ash content (Miscanthus contains between 1.5% and 4.5% ash, so a carbonization process which converts 28% of the organic fraction to carbon would create charcoal containing between 5.2% and 14.4% ash). This char is a possible substitute for coal in coal-fired plants (if they can handle the ash), or it could be used in something like the thermochemical zinc cycle. Unlike the raw biomass, it is not easily biodegradeable and can be stored indefinitely.
Possibilities for zinc
If the zinc cycle was used, the 22,400 moles of carbon would produce 22,400 moles of CO and another 22,400 moles of metallic zinc. CO is poisonous but stable, and could be stored in old gas wells or used for chemical synthesis. 22,400 moles of CO at 68560 cal/mol yields 6.43 GJ (1780 kWh), of which 60% (1070 kWh) might be recovered using something like solid-oxide fuel cells. If used in a fuel cell rather than a gas turbine or other heat engine, it would be relatively simple to capture and sequester the CO2.
Zinc oxide has a heat of formation of 84670 cal/mol, so an Electric Fuel-style cell operating at 62% efficiency would be able to take 22400 moles of zinc and squeeze 4.92 GJ (1370 kWh) out of it. The total product for this process:
- As much as 2540 kWh/tonne of electricity between the carbonization system and the carbon monoxide utilization.
- Up to 986 kg/tonne of CO2 captured and made ready for sequestration.
- A further 1370 kWh/tonne of high-density, mobile, noise-, pollution- and carbon-free energy (as metallic zinc) made available for any purpose. If used to replace gasoline burned in 20%-efficient engines, this much zinc could replace roughly 185 gallons of gasoline per tonne of biomass5.
Direct use of the biomass (17.4 mmBTU/tonne) in an IGCC plant at 55.8% efficiency would yield only 2840 kWh/tonne (none ready for mobile uses); if burned along with coal in a powerplant with a heat rate of 10200 BTU/kWh, it would produce a mere 1710 kWh/tonne (ditto).
Land use requirements
Returning to September's scribblings, the 2001 electric demand of Illinois was 92,358 million kilowatt-hours. Satisfying this demand using the electricity generated from the carbonization process and the zinc-production offgas (2540 kWh/tonne) would require 36.4 million tons of biomass. If it could be grown at 15 short tons/acre, the production of 2.68 million acres would suffice to supply the state's electric needs with no use of coal or nuclear. If used in passenger vehicles, the zinc produced would be able to displace a further 6.73 billion gallons of gasoline (about 5% of US consumption and 30% more than Illinois' 2004 gasoline consumption6).
At least for the state of Illinois (and probably many others), this has the potential to replace ALL electricity and ALL gasoline with 100% renewable energy (which can sequester rather than release carbon, no less) in one fell swoop. This sounds almost too good to be true. (Then there is the fact that carbon monoxide can be steam reformed to hydrogen [CO + H2O -> CO2 + H2], and the hydrogen combined with more carbon monoxide to make synthesis gas from which almost anything that comes out of a chemical plant can be made... and already is.)
Profitability
If the biomass crop gets $50/short ton, the cost for the carbon feed to this set of cycles is 1.41¢/kWh (ignoring the possibilities for higher-value products tapped off the carbonization process or produced as syngas). With current corn yields of roughly 150 bushels/acre and prices of roughly $2.50/bushel ($375/acre, minus fertilizer and cultivation), a farmer harvesting 15 tons/acre of Miscanthus or even 10 tons/acre of switchgrass would be in an enviable position. (At $50/ton, 2.5 tons/acre of corn stover would yield $125/acre; this would be more than enough to make most farmers profitable. We're throwing away money.)
Further work
This points to things that bear investigation:
- Suitability of carbonizers as producers of fuel gas for gas turbines.
- Ash-fusion characteristics and other behavior of biomass-derived char.
- Composition of biomass carbonizer off-gas (byproducts of carbonization process).
- Zinc recovery efficiency from biomass char-fed smelters.
- Efficiency and durability of solid-oxide fuel cells on carbon monoxide fuel.
- Small-scale CO2 purification for carbon sequestration.
Footnotes:
[1] See US Patent 6,790,317 (back)
[2] CRC Handbook of Chemistry and Physics. (back)
[3] e.g. the GE LM2500+. Some GE simple-cycle gas turbines are rated at 40%. (back)
[4] The average heat rate for steam turbine powerplants is less than 10300 BTU/kWh, corresponding to a thermal efficiency greater than 33%. 28% seems realistic. (back)
[5] If 1370 kWh of electricity is equivalent to 5 times its raw energy as gasoline at 20460 BTU/lbm LHV and 6.167 lbm/gallon, it would displace 185 gallons of gasoline (equivalent to about 278 gallons of ethanol at 84,000 BTU/gallon). Direct use of biomass to make ethanol might yield a bit more per tonne, but it wouldn't produce anything else. (back)
[6] Per the EIA, Illinois used 14,184,500 gallons/day of gasoline in 2004. This is 5.19 billion gallons for the (366-day) year. (back)
Related posts:
Zinc: miracle metal?
I would suggest that you also try to examine thermal depolymerization as well as anhydrous pyrolysis in order to produce liquid fuels instead of charcoal. Granted it isn't a carbon sink like your process but it does allow us to maintain our existing infrastructure. Capital costs are always on my mind.
Still, it remains the case that there are better methods for extracting energy from biomass than fermentation, and better means of storing portable energy than hydrogen. Good work.
The issue with thermal depolymerization and its ilk is that you're right back to liquid fuels. If you can make the same amount of energy either as hydrocarbons or metallic zinc, you'll need 3 times as much input energy for your 20%-efficient gasoline engine as you would need for your 62%-efficient zinc-air fuel cell.
The zinc process has the further advantage that a substantial amount of energy in its products is added from the input heat, and it takes much less land area to capture a gigawatt from e.g. solar heat than it does to fix it as biomass.
This would be the cost for the carbon feed only and does not include any of the capital costs associated with this process (i.e. the solar-zinc reformer, the combined cylce plants and solid oxide fuel cells etc.) which would add to the price of energy from this cycle (i.e. energy cost = fuel costs + levelized costs for capital & interest on loans).
Great post though. I love to see you work through these numbers. When I read "this has the potential to replace ALL electricity and ALL gasoline with 100% renewable energy (which can sequester rather than release carbon, no less) in one fell swoop" I get pretty excited. This solar-biomass-zinc cycle seems very promising. Keep it up...
Ditto the solar-zinc scheme. I'll bet that, in practice, solar heat wouldn't be used too much for that except in the south. Instead, it would be a combination of pure smelting (burning more char) or arc furnaces powered by surplus wind energy; the CO product is a way to store wind energy for later use.
$50/ton is a guesstimate of what biomass might be worth, deliberately high to see what it would do to power prices. As it turns out, not much. $50/ton and even 10 tons/acre would be $500/acre revenue; a farmer getting $2.50/bu for corn at 150 bu/ac is only getting $375/acre, and there's a lot of inputs required for annual crops that perennial grasses don't need. Fuel could be the way the American farmer stays in business, if we manage things well enough to actually make do on it - inefficient methods like cellulosic ethanol for wasteful internal combustion engines will produce much less useful energy per ton.
Very exciting, though. Keep up the great work!
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