Going negative

One concept near and dear to environmentalists is restoration.  Seeding of native species and regular burning is re-establishing the tallgrass prairies, rendered nearly extinct by the plow; Lake Erie provides excellent fishing; wolves once again roam Yellowstone.  Restoration does not just relate to nature; it also extends to human activities.  Organic farming in particular is intended to be a restorative, adding humus to depleted soils and re-establishing biological cycles broken by agrichemicals.

Restoration has been applied to many things, but so far there has been one major (and global) exception:  the atmosphere.  Not that air hasn't been the subject of cleanup and protection efforts; far from it.  The first controlled pollutants included sulfur, oxides of nitrogen, carbon monoxide, non-methane hydrocarbons and mercury.  The Montreal Protocol added ozone-depleting chemicals to this list, limiting the production and controlling the handling of halocarbons.  The Kyoto Protocol in turn attempted to regulate greenhouse gases, though its lack of comprehensive coverage and assent make it unlikely to succeed.

Human activities and the atmosphere

The various efforts and treaties aimed at the air have thus far limited their goals to stopping undesirable emissions.  None have yet been aimed at restoration, removing unnatural and unwanted constitutents from the air to return it to its historical state.  The rather obvious question is, why not?  One reason is that it's a mighty big job.  The atmosphere weighs roughly a ton per square foot of Earth's surface, about 5.3*10¹⁵ tons total; changing the composition by one part per million (by mass) means fiddling with 5.3 billion tons of material.  However you cut it, that's not something you do for a weekend project.

On the other hand, it is something we do whether we want to or not.  This is an intractable problem at the moment because the energy systems which run industrial civilization are built around the removal of carbonaceous fuels from the earth, burning them, and exhausting the combustion products to the atmosphere.  It's going to be difficult to change this, save for a minority of current technologies; the expense of capturing carbon from them (save for a few processes like IGCC fuel-gas scrubbers) will be large, and will have no other payoffs.

What could we do?

Anyone who's been reading this blog for very long knows I like to do what if's.  Since I'd hate to disappoint anyone on that account... what if our major energy systems either did not use carbon at all, or were easily and cheaply set up to be carbon negative?  That would certainly be a huge accomplishment; it would represent a capability for tuning the CO₂ content of the atmosphere to our specifications.

It also appears to be possible.  The thermochemical zinc process, which converts carbon and zinc oxide to carbon monoxide and metal, is so far as I know unique in that it accepts biomass as a carbon source and releases it without any contamination from e.g. nitrogen.  This pure carbon monoxide can be burned in a gas turbine, but it could also be used to run a solid-oxide fuel cell and kept nitrogen-free.  The product stream of pure carbon dioxide would be disposal-ready.

The chemistry

In principle, almost anything could be used as a carbon source; given the heats of reaction of zinc vs. other things, it appears best that this material be free of oxygen.  Carbon char from anaerobic pyrolysis of biomass or MSW would probably do.  This would not retain all the carbon of the input material, but the pyrolytic gas could be used as fuel along with the off-gas from zinc reduction.  For the sake of carbon sequestration, the exact route of the carbon through the process is irrelevant so long as it is captured.

High-temperature processes favor simpler molecular products.  If the biomass is considered to be carbohydrate with a general formula of (CH₂O)_n, the possible decompositions into simple molecules would include these:

CH₂O + Δ --> C + H₂O

CH₂O + Δ --> CO + H₂

2 CH₂O + Δ --> CH₄ + CO₂

For this purpose, the reaction conditions would be adjusted to yield as much carbon char as possible.  It may also be possible to use methane and heavier hydrocarbons as carbon sources; they would react to CO and H2 in the reduction step.  Water in any form is undesirable.  The need to purify and dry the off-gas may make it too expensive to use as a reducing agent, in which case only the char would be used in that role.

Both the pyrolysis off-gas and the zinc-reduction off-gas have considerable energy content.  After scrubbing, all the gas might be suitable for SOFC fuel.  The efficiency of conversion of the gas to electricity could reach 60%, or perhaps 70% if a steam-turbine bottoming cycle can be used. 

Magnitudes

The hypothetical solar-zinc plant in "miracle metal" consumed 308 tons of carbon per day; if supplied as carbohydrate, this would be 770 dry tons of biomass.  Configured as an all-electric plant and ignoring the efficiency gains from fuel cells, its output would be 3570 MWh/day or 149 MW average, yielding a biomass feed rate of 5.18 tons/day/MW.

Possible sources of biomass include corn stover, wheat and rice straw, waste wood and the organic fraction of MSW.  While it's easy to get figures for per-acre yields of grain, the production of byproduct materials is not so widely published.  Biomass crops are another matter.  Switchgrass has been studied as a crop for both livestock fodder and energy production, with published yields ranging from 7 to 14.7 dry tons/acre/year.

Assuming 10 tons/acre/year as a reasonable value and 30 million acres planted to switchgrass, the total annual yield would be 300 million dry tons/year (of which 120 million tons would be carbon).  If crop byproducts yielded a similar amount of stalks, cobs and straw, the total annual yield would be 600 million dry tons biomass (240 million tons carbon).  At a biomass consumption rate of 5.18 tons/day/MW for the solar-zinc plants, this would supply the carbon demand for 317 gigawatts of power production and sequester 0.24 billion tons of carbon (0.88 billion tons CO₂) per year.  Total US power consumption averages about 440 GW, so this system could satisfy about 72% of current US electric demand.  (That figure surprised me - it's pretty much the total electric demand supplied by fossil fuels.)  If used to supply energy for vehicles it could satisfy about 1.8 times the US's current demand of roughly 180 GW, rendering the entire US road transportation system carbon-negative and allowing plenty of room for expansion.

Net effects

If this system was used to replace all combustion-supplied electricity and all other energy consumption went carbon-neutral, US use alone could reduce atmospheric CO₂ concentrations by about 0.16 ppm/year by mass (about 0.11 ppm by volume).  If we assume the entire world uses the same system and world electric consumption is 4 times US consumption, the impact becomes 0.64 ppm/year (0.44 ppmv).  Increasing world electric production from thermochemical zinc plants also increases the potential atmospheric CO₂ reductions.  Use of metallic zinc as vehicle fuel would increase the rate of CO₂ reduction if it was recycled via the thermochemical pathway with sequestration, and would be carbon-neutral if it was reduced electrolytically using energy from solar, wind or nuclear.

Summary

Thermochemical reduction of zinc oxide to metal using biomass as the carbon source captures atmospheric carbon and can convert it to a stream ready for permanent disposal.  Possible biomass sources include biomass crops, byproducts from food crops, forestry waste and post-consumer waste materials.  At a biomass consumption rate of 600 million dry tons per year, the system would supply roughly 317 gigawatts.  This would supply 72% of US electric needs or 1.8 times US transportation energy needs, while sequestering 880 million tons of carbon dioxide every year.  World-wide use of biomass-fed thermochemical zinc systems could reduce atmospheric concentrations of carbon dioxide by about 0.44 parts per million (by volume) per year; acting alone, such systems could restore the global atmospheric CO₂ concentration to its pre-industrial level in about 225 years.  Expansion of thermochemical zinc systems and their biomass feeds to allow increased electric production would hasten this process.  Both the effluent CO₂ stream and the stored CO₂ could be returned to the atmosphere at any time should that be desirable.  This spells the end of air pollution and human control of global warming.

Restore the atmosphere?  Engineer the climate?  If we want to, we can do it.

Related items:  Zinc:  Miracle metal?  ¶ 7/07/2005 12:49:00 AM