A lot of words have been written about the prospect of the USA and the world running short of oil and natural gas. Together, these supply much of the energy used in the USA for transportation, fertilizer and home heating. Word is that North American gas production has already peaked, and world production of oil is predicted to peak as early as this Thanksgiving. (Coal, which supplies half the electricity used in the United States
, shows no immediate prospect of shortages.)
Economic expansion depends on greater yield from whatever inputs are available. The energy intensity of the US economy has been falling steadily since the 1970's, yielding more and more economic output per unit of energy; still, greater efficiency can be overcome by falling supply. The alternative is to convert something else to suit. American society is no stranger to this phenomenon; in the past 229 years, US society has seen a number of transformations in its use of energy for various purposes:
- Coal replaces firewood as the primary source of space heat.
- Steam replaces draft animals for rail transport power.
- Kerosene replaces wax and tallow as the primary source of light.
- Incandescent electric lights supplant kerosene.
- Internal combustion replaces draft animals for road transport.
- Natural gas and fuel oil supplant coal as the primary source of space heat.
- Diesel engines replace steam for rail transport power.
- Fluorescent lamps supplant incandescent.
Each transformation either created a resource out of a material which had not been used widely or at all before (coal, petroleum, natural gas) or greatly increased the efficiency of use (fluorescent lighting, diesel locomotives vs. oil-fired steam). Each time, some resource was leveraged to yield more benefit.
The USA's energy supplies come mostly from fossil sources (counting nuclear as fossil), with hydropower being by far the biggest renewable contributor at 7% of electric production. The other two readily accessible and renewable energy supplies, wind and solar, contribute relatively little. Surprisingly, per the EIA
waste provides twice as much electricity as wind, and wood almost four times as much. Obviously there are resources which are not being leveraged to best advantage.
One of the principles of life is that one organism's waste is some other organism's resource. Our energy systems don't follow this; the vast majority of our energy comes from once-through handling of a single supply with the products dumped to landfills or the atmosphere, and the closest we come is with cogeneration systems which use effluent heat from power production as space heat or industrial process heat. Is it possible to do better? Looking at the electricity production statistics
and then at municipal solid waste production
, it looks like there are possibilities we might be ignoring.
So what do we have?
Lots and lots, both resources that are being somewhat underused and resources which are almost entirely ignored. In the "somewhat underused" category, we have the fuel which is burned for industrial process heat and space heat; if half of these uses were adapted to cogeneration, the additional power would amount to tens of gigawatts (perhaps 70+ GW, or almost 1/6 of current generation). But these draw from existing fuels which are rising in price and shrinking in availability; boosting efficiency can get us by for a while, but we're eventually going to have to use something else. Replacing a few percent of current inputs (a la ethanol) isn't nearly ambitious enough; ideally, it would be something we can leverage to power most of our industry and transport.
The solar-zinc process has lots of leverage; it turns a fairly small amount of carbon and roughly the same amount of solar heat into two chemical fuels, one of which (carbon monoxide) is good for gas-turbine fuel and some industrial uses, and the other (zinc) which is both portable and can be turned into either electricity or hydrogen with very high efficiency. Another virtue of the process is that it doesn't appear to care where its carbon comes from so long as it is mostly pure (coke, charcoal or even coal) by the time it gets to the input hopper. Within those limits it looks like a great many things will do as carbon sources.
A great many things are made of carbon. The ideal sources would be generated in large quantity, mostly dumped as waste products, and renewable. Biomass may not be the best of fuels, but it does make pretty good charcoal. If we needed enough biomass to get on the order of 240 million tons of carbon per year (per "Going negative"
), where could we get it?
Coming from town
Let's start with waste. The USA disposes of 237 million tons of municipal solid waste every year, or more than 1500 pounds per capita. A large fraction of this is either biomass or textiles. According to Dr. Debra Reinhart of UCF
, the various biomass components and their moisture contents
are as follows:
|| Dry mass,
% of total
|Total % dry biomass:
|Total dry biomass in MSW,
This may be a serious underestimate. It appears that some 160 million tons/annum of urban wood waste
is uncounted or partially counted in the above (perhaps because it is designated construction waste or yard waste rather than MSW). If even half of this could be captured as biomass, the impact would be very large.
This suggests that something between 120 and 280 million dry tons/year of biomass can come from cities. What else is being thrown away?
Out of the woods
One heck of a lot of wood waste is created in the forest products industry. The national total total is amazing: 178 million mettric tons/year from timber harvesting
with 86 million tons unused, and a whopping potential 110 million tons/year from thinning in national forests
What's the total which could be captured (either currently unused or diverted from their current use)? Heck if I know, but 200 million tons per year seems reasonable.
Off the back 40
Many plants are grown for fruit, seed or tubers but create a great deal of other plant matter as well. In zero-till farming this material can be problematic, as it insulates and prevents the earth from warming as desired in the spring and delays the start of growth. It is desirable to remove this excess matter, but what to do with it?
The stalks and such left over from corn (maize) is called "corn stover". The productivity of corn stover is considerable; at a harvest rate of 170 bushels/acre and allowing 1 ton/acre for ground cover, the remaining matter amounts to 3.0 dry tons/acre
. (The 2004 maize harvest was approximately 11.8 billion bu over ~80 million acres, for approximately 150 bu/ac; the corresponding production of surplus stover would be roughly 2.5 dry tons/acre.) Even if corn was reduced from 80 million acres to 60 million, corn stover could provide 150 million tons/year of dry biomass.
What could grow on 20 million idled acres? Switchgrass and Miscanthus have been advanced as biomass crops; they could be planted on buffer zones between fields and waterways to capture nitrogen in runoff and help prevent erosion. Projections of yield are variable, but 10 tons/acre appears reasonable based on some searches. 20 million acres at 10 dry tons/acre would yield 200 million dry tons.
This does not exhaust the list; crops other than maize yield stalks and straw, some of which needs to be burned or otherwise removed to eliminate pests. All of this matter is potential biomass feedstock.
Summing up: 120-280 million tons/year from cities, 200 million tons/year from forests, and 350 million tons/year from current and former maize acreage indicates a potential biomass harvest of 670-830 million dry tons per year. This is sufficient to supply the requirements projected in Going negative
. The next question: What should we do with it?
How not to get leverage
Levers can work for you or against you, by either making superior or inferior use of an input in limited supply. One example of a lever which can be disadvantageous is fermentation of carbohydrates to make ethanol. Ethanol's chemical formula is C2
O; yeasts make it from carbohydrates with a general chemical formula of CH2
O, and emit CO2
as a byproduct. If this is the only reaction going on, it balances like this:
O -> C2
O + CO2
One third of the carbon and roughly half the total mass (44 AMU out of 90) is lost as carbon dioxide in the fermentation process. It seems likely that some advocates of ethanol forget this little detail, and it throws their calculations way off. I recall a claim that 300 million tons/year of biomass would create enough ethanol to replace US gasoline consumption. After fermentation this would only yield 153 million tons/year (46.5 billion gallons) of ethanol; this is the energy equivalent of 32.6 billion gallons of gasoline, which is roughly 1/4 of annual US motor gasoline consumption. This claim is clearly false; even without allowing for the smaller energy content of ethanol it would still take upwards of 900 million tons of fermented biomass to replace gasoline, and still more to replace diesel, jet fuel and other uses of petroleum. Replacement of petroleum with ethanol made from near-term renewable biomass stocks is clearly not possible.
Forget ethanol; convert to carbon
If the purpose of the biomass collection is to produce carbon for reduction of metal, it must be pryolized. Pyrolysis produces an off-gas which contains most of the hydrogen and nitrogen and some of the carbon; the maximum recovery achievable under batch conditions using partial combustion for heat is about 30%. It may be possible to increase this yield using external heating rather than partial combustion, but the system would no longer be simple.
The feasible production of carbon from biomass appears to be 210 to 250 million tons per year. This carbon would be dry, sterile and inert, and thus could be stored easily for later use. This carbon could be fed to a thermochemical zinc reduction process, powered either by solar heat or by excess electricity from wind power.
If there was 210 to 250 million short tons per year of carbon available, it could be used to produce between 1.14 billion and 1.36 billion tons of metallic zinc per year (from zinc oxide) 
if the byproduct was carbon monoxide. If the carbon was fully oxidized to CO2
in the reduction process these amounts would be doubled to between 2.28 and 2.72 billion tons of zinc, but any production of power or chemicals from the carbon byproduct would be lost.
Where the rubber meets the road
Using Electric Fuel's figures 
, 1.14-1.36 billion tons of zinc could produce between 966 million and 1.15 billion megawatt-hours (9.66e14 WH to 1.15e15 WH) per year. This is an average power between 110 GW and 132 GW. My previous calculation
(somewhat generous) for the amount of power actually delivered to the wheels by vehicles in the USA was around 107 GW average for gasoline vehicles alone and 183 GW including trucks and other diesels; it appears that this amount of zinc could easily replace all gasoline used in the USA, and if we allow for some efficiencies of electric propulsion it could replace the rest of the motor fuel too (give or take a bit). Any extra required could be supplied by regeneration of zinc metal via electrolysis using power from wind, nuclear or any other source of electricity (preferably carbon-free). In the CO-byproduct scenario, an efficiency of 39.8 Wh/mol of CO creates an additional
632-752 million megawatt-hours (
6.32e14-7.52e14 WH) per year, or 72-86 GW of electricity. That's about 32-38% of the amount generated by coal in the USA, or roughly the production from natural gas 
. (This does not include any energy produced from the pyrolysis off-gas, which may or may not be combustible.)
It appears that a process which uses biomass to produce carbon which is then used to drive a zinc cycle for zinc-air fuel cells could replace all petroleum-based motor fuel used in the USA, and all of the natural gas burned for electric generation as well. No process for turning biomass into ethanol could accomplish anywhere near as much for the same inputs, and no alcohol process can use wind power to generate the same product. Even allowing for rather poor efficiency of zinc-air fuel cells, the zinc route gets much better leverage out of limited inputs.
UPDATE: Figures for energy from CO byproduct corrected, old figures struck out.
You find you get what you need
Zinc: Miracle metal?
A pound of carbon at molecular weight 12 can reduce zinc oxide to produce 5.448 pounds of zinc at molecular weight 65.35, with 2.67 pounds of carbon monoxide as a byproduct. If the carbon is fully oxydized to CO2
, the amount of zinc reduced doubles to 10.896 pounds. (back
Electric fuel implies an average cell potential of 1.139 volts
(17400 WH / (325 AH * 47 cells)), producing 219.8 kJ/mol or 423.6 WH/lb of zinc. (back