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?
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 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.
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:
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).
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.)
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.)
This points to things that bear investigation:
 See US Patent 6,790,317 (back)
 CRC Handbook of Chemistry and Physics. (back)
 e.g. the GE LM2500+. Some GE simple-cycle gas turbines are rated at 40%. (back)
 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)
 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)
 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)
Zinc: miracle metal?
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