Zinc: Miracle metal?

Research at ETH Zürich (News release) have implemented a pilot-scale thermochemical process to capture solar energy using a zinc cycle and a supply of free carbon (diagram) (hat tip: Green Car Congress).  This process converts zinc oxide (ZnO) and carbon to metallic zinc and carbon monoxide; the zinc can be used to make either hydrogen or electricity, and the ZnO product completes the cycle.  The carbon source can be anything from coal to biomass.  The researchers claim 30% conversion efficiency in the pilot plant, with full-scale plants expected to hit 50%-60%.

The adjective "impressive" is, I believe, an understatement.

When I read this, I was initially troubled by the use of carbon (coal!) in what was represented as a solar energy system (bait and switch).  However, the researchers claim that the amount of carbon required to reduce the zinc is reduced to 1/5 of that required by pure chemical processes and that biomass can be used as the carbon source.  It's also obvious that the off-gas (CO) retains much of the energy from the original carbon fuel.  The big questions are:

1. Is this true?
2. How much energy is retained?
3. What are the follow-on possibilities?

Is it true?

This one starts with the chemistry:  specifically, heats of formation.  (Elements have zero heat of formation by definition.)

Table 1:  Heats of formation
Compound	ΔH, gram     calories/mol
ZnO (from zinc solid)	-84670
ZnO (from zinc gas)	-115940
CO	-25400
CO2	-93960
H2O (liquid)	-70600

A process with the inputs of elemental carbon, zinc oxide and heat and the outputs of zinc gas and carbon monoxide has the following energy balance:

ZnO + C + 90540 cal/mol -> Zn (gas) + CO

The energy supplied by heat vs. the total is 90540/115940 = 0.78, or 78% (the remaining 22% comes from the oxidation of carbon to CO).  This is close enough to the author's claim of 4/5 to count the latter as correct.  However, it is not the whole truth; out of the chemical energy output of the process, 61% comes from the input carbon and only 39% from the thermochemical additions.  But that's misleading too; the outputs are not equivalent.

How much energy is retained?

The reaction inputs include carbon with a heat of combustion of 93960 cal/mole (plus the thermal energy required to bring the inputs up to reaction temperature, but which are not quantified here).  The outputs include:

• Zinc metal, with 84670 cal/mol heat of combustion (solid).
• Carbon monoxide, with 68560 cal/mol heat of combustion to CO₂.
• Sensible and latent heat of the reaction products:  20420 cal/mol heat of condensation for the zinc (difference between heat of formation of ZnO from liquid vs. gas) and 7 cal/mol°K specific heat for the CO.  (I have no figures for the specific heat of zinc gas or liquid.)

The chemical energy of the outputs (at room temperature) is 153230 cal/mol, while the chemical energy of the input carbon is 93960 cal/mol.  The thermal process has increased the total energy substantially, but the carbon input still accounts for 61% of the total input energy (chemical and thermal).

A conservative estimate of the sensible and latent heat of the products, assuming the zinc is condensed to liquid and the CO is cooled to 200 C, is 20420 + (7 * 1000) = 27420 cal/mol.  Perhaps 25% of this could be converted to work via a steam turbine, so the net output would be 6855 cal/mol (7.97 Wh/mol); this is about 7.6% of the input solar energy.  This would retain the full chemical energy of the products for other purposes, but this is not the only option.

What are the follow-on possibilities?

The zinc can be used in one of three basic ways, all of which convert it back to zinc oxide:

1. Use in a stationary zinc-air fuel cell to make electricity for the grid.
2. Use in a vehicular zinc-air fuel cell for as a motor-fuel replacement.
3. Reaction with water to make H₂.

Examining these possibilities in turn:

Stationary fuel cell:  The carbon monoxide is surplus in this process, and could be burned in a combined-cycle powerplant.  At 50% efficiency, the CO would yield 34280 cal/mol or 39.8 Wh/mol electric output; this electricity would be added to the 7.97 Wh/mol from the thermal output, for a total of 47.8 Wh/mol from the heat and off-gas.

The real surprise is the output from the zinc-air process.  At 1.65 volts per cell and 100% coulomb efficiency (ha!), a Zn-air cell will deliver 91.4 watt-hours per mole of metallic zinc (about 78600 cal/mole).  The total output is 139.2 Wh/mol.  An IGCC powerplant burning carbon (coal) at 40% efficiency would only yield 37580 cal/mol (43.7 Wh/mol).  The total useful energy output is more than tripled.

Mobile fuel cell:  There are already projects to run vehicles such as buses on zinc-air fuel cells.  If these could be moved down to cars, the results could be quite impressive; a vehicle using 250 Wh/mile would require only 179 grams of zinc (2.74 moles) per mile.  Zinc is a reasonably dense metal at 7.14 g/cc; solid zinc would yield about 40 miles to the liter, or upwards of 150 miles per gallon (powdered forms would not be quite so energy-dense).  The carbon monoxide would also be surplus in this scenario.

The actual available energy (electricity) from a Zn-air fuel cell is several times as great as what can be obtained from the same chemical input of gasoline to an internal combustion engine.  The metallic zinc contains about 90% as much energy as the input carbon, and it can be converted to motion with very high efficiency.  It appears likely that a solar-mediated zinc reduction process using coal could power 3.5 times as many vehicle-miles as a conversion of coal to liquid fuel.

Reaction with water to make hydrogen:  This one is interesting, because the efficiency of conversion is relatively high (83% conversion of zinc metal to hydrogen).  But what do you do with the hydrogen?  Among possibilities like the manufacture of ammonia for fertilizer, you could use it to convert half of the carbon monoxide (CO) off-gas to methanol (CH₃OH).  Each mole of reactants would yield 19.9 Wh/mol of electricity and 1/2 mole (16 grams) of methanol; methanol has 173.6 kcal/mol heat of combustion (86.8 kcal/mol of reactants).

Methanol is an excellent motor fuel, powering a great many racing vehicles and mixing well with gasoline.  At a density of 0.79, it packs about 55% of the energy of the same volume of gasoline.  A vehicle achieving 30 MPG of gasoline could be expected to get about 16.5 MPG on methanol; each mile of travel would require about 5.66 moles of methanol.

Suppose that a solar power system is set up on a square kilometer of land in a sunny locale.  It receives an average of 5400 Wh/m^2 per day, or 5.4 GWh/day over the whole array.  It converts this thermal energy to zinc with an efficiency of 50%, plus the carbon inputs and outputs.  How much does it process each day, and what could it do?

At 50% capture and 90540 cal/mol captured thermal energy, the process would consume 25.7 million moles of ZnO and carbon (308 tons carbon) plus 5.4 GWh of heat (4.65*10¹² cal); it would produce 25.7 million moles of metallic zinc and 25.7 million moles of carbon monoxide.  Byproduct electricity from the thermal output comes to 204 MWh.  Where it goes from there depends on the process options chosen:

Table 2:  Process outputs per day
Process option	Electric output	Chemical output	Vehicle miles powered
All electric	3570 MWh	n/a	n/a
MeOH motor fuel	716 MWh	275,000 gallons MeOH	4.54 million
Zn fuel cell vehicle	1230 MWh	1680 tons zinc	9.38 million

Of the three options, the last is by far the most impressive.  At an average daily consumption of 24 kWh, the electric output of such a plant could power 51,000 homes; the zinc output would be sufficient to drive approximately 188,000 vehicles 50 miles a day.  A second such plant devoted to electricity would power an additional 149,000 homes.  Total carbon consumption would be 616 metric tons per day, or about 7.2 pounds per household per day.  (Much of this carbon could probably be obtained from the organic and plastic content in the residential trash, and the rest from biomass.)  That is a very big win.  A car getting 30 MPG driven 50 miles per day would consume 8.7 pounds of carbon in its gasoline alone; producing 24 kWh from carbon at 40% efficiency would release another 14.5 pounds.

A bedroom suburb with .25 acres per house (including streets and parks) would have 2560 homes per square mile, or 988 homes per square kilometer.  Two square kilometers of solar-zinc plants could power 190 square kilometers of such suburb (188,000 homes), with one vehicle each; this is about 1% of the total suburban land area devoted to power production.  This is clearly something people would accept.

Zinc cars?  Getting more than three times as much useful energy out of a ton of coal, and cutting carbon emissions accordingly?  Where do I sign up?

UPDATE 2005-Jul-1:  Two little bits of fallout from this come to mind.  Okay, three:

1. 25.7 million moles of zinc per day could yield 25.7 million moles (51.4 tons) of hydrogen.  This hydrogen could fix about 240 tons of nitrogen to make about 291 tons of ammonia.  Every day.  At 86 kg/ha/year nitrogen requirement for crops, one day's operation of the plant in nitrogen-fixing mode would yield sufficient nitrate to fertilize 2790 hectares (almost 28 square kilometers) of farmland for a full year.  If you can't make the green algae trick work, here's a backup that only requires a carbon supply like rice straw or corn cobs to drive the process.
2. There is no requirement that the off-gas be dumped to the atmosphere.  If the carbon monoxide was burned in e.g. a solid-oxide fuel cell, the product CO2 could be captured and compressed to liquid with relative ease; it could be fully sequestered at low cost.  If the input carbon came from the atmosphere, such as biomass or municipal solid waste (which is largely cellulose), the system could become carbon negative.
3. Solar heat is good, but this process could be driven by anything which supplies heat at a sufficiently high temperature.  If an HTGR could be operated at perhaps 1300°C (maybe during off-peak hours, cranking the power output down and allowing the tempeature to climb), nuclear heat could keep such a process running 24/7 and during the winter in northern climes.  This would permit carbon-negative waste disposal regardless of night or clouds.

It looks like it's going to be harder and harder to keep technological civilization down, oil or no oil.

Edit 2005-Jul-01:  Corrected entry in Table 2; erroneous figure 204 MWh changed to 716 MWh.

Related items:  Fertilize this!, Going negative.  ¶ 6/29/2005 09:03:00 PM