The fundamental assumption that infuses most discussions of biomass for energy is that it is renewable and sustainable. But the actual historical evidence is that societies do not sustainably renew their forests, and they can scarcely keep up with their food demands without using fertilizers that are either synthesized from fossil methane, or dug from ancient guano deposits.
Since the 17th century, when Europe's Renaissance, China's Ming Dynasty, and India's Mogul Emperors still existed, the global population has grown nearly fifteen-fold. It has muliplied by about six in just the last hundred years.
The Industrial Revolution in Britain occurred when the discovery and application of coal suddenly released industry from its dependence upon the forests which had been steadily cut down since the time of the Romans.
It is difficult to imagine that a field, forest, or peat moorland entirely devoted to photosynthetic capture of solar energy will be able to provide significantly more biofuel energy than when we harvest the entire growth, dry it, and burn it for fuel.
So how much land would we need to devote to biofuels, to replace even one tenth of our petroleum consumption?
Lord Kelvin's estimate of the capacity of vegetation to capture solar energy in Britain or Germany was two tons per square metre per thousand years.
This is equal to two thousand tons per square kilometre (a million square metres) per year.
Energy content of wood fuel
(air dry, 20% moisture) =about 15 GJ/tonne (gigaJoules per thousand kilograms)
but one Kwh = 3.6 MJ (megaJoules).
and one gigawatt-year is 8760 gigawatt-hours,
so two thousand tons of vegetation, optimistically, gives perhaps 30,000 GJ,
One gigawatt-hour is a million Kwh, i.e. 3,600 GJ
one gigawatt-year is 8760x3600 = approximately 31.5 million GJ
Which comes out to the accuracy of the available figures at just more than a thousand square kilometres for a gigawatt-year of production per year.
Divide by 2.6 to convert to sq. miles: 384 sq miles, probably 390 to 400, which agrees fairly well with the estimate in the next paragraph.
It is known that coppiced willow, which produces "withes" that were traditionally used for basketmaking, is quite a good way to obtain annual harvests of wood. For fuel use, the wood on a given tree is harvested at three year intervals. So the annual harvest is taken from one third of the entire coppice.
It is said to yield three to ten times as much woody mass per acre per year as Red Oak or Eastern Cottonwood, and 53% of the energy per pound of Red Oak. So it's probably the most productive form of cellulose/lignin biofuel. Such a technology is many times as efficient as alcohol from maize or oil from soybeans. It is also less abusive to wildlife.
The UK Forestry Commission, short rotation coppice webpage,
estimates that a power station needs one square km. of coppice, per MW of power capacity, if water is sufficiently plentiful.
But it does not say what the expected capacity factor is.
Worse, even if we could convert every scrap of cellulose and lignin that grew in our energy plantations to monosaccharides, and then ferment those to ethanol, we have to sacrifice at least a third of the carbon, to feed the yeast.
C6H12O6 = 2CO2+2C2H5OH
Ethanol is not as energy-dense as gasoline.
The high heating values (HHV) in British thermal units per US gallon (!) are
On the other hand, processes to convert wood chips to ethanol are said to have capacity of about 50 gallons per ton of wood chips. On various websites, e.g. New Phytologist , we find that total cellulose and lignin production can be as good as 5 to 18 tons per hectare, per annum.
Consider just the substitution of ethanol for gasoline in a city of a million automobiles, driving an average 15,000 miles per year per vehicle at an economical 30 mpg of gasoline. That consumes 500 million gallons of it. If ethanol engines can be made 35% more efficient in their use of ethanol (higher compression engines) then 500 million gallons will require 10 million tons of biomass, or at the intermediate value of 10 tons per hectare, a million hectares. A square mile is just less than 260 hectares, so our city would need 3,600 square miles of coppice to feed its automobiles. That's a square with sides 60 miles long. At 18 tons/ha, half that area would almost do. At 5 tons/ha, twice as much.
Or, after converting various units like megajoules, hectares, gallons and whatnot, it turns out that the energy per square mile at 50 gallons per ton of biomass averages 260 to 590 kilowatt-hours per hour of the year. One quarter to one half a megawatt per square mile. But even so, to replace the energy of a 1000 MW generating plant, capacity factor 90%, we'd need 1800 to 3600 square miles of coppice.
It has been reckoned that the entire English county of Kent would have to be covered in such coppice, in order to have an annual harvest of biofuel energy to match the single 1050 MW nuclear generator Dungeness B (Dungeness is in Kent).
My own attempt, using two optimistic sources for the energy information, was that it takes from 410 to 1100 square miles, to produce the equivalent of a gigawatt-year of heat energy in a year. That's before you convert it to electricity or vehicle fuel.
It has also been reckoned that if all the forests in the USA were harvested in a sustainable way for fuel, they could scarcely meet 10% of our energy demands.
New Phytologist also says that
On a wider scale, it has been suggested that 20–30% of all agricultural land in Europe could be used for energy crop production by 2030 (EEA, 2006), potentially producing energy equivalent to 15% of EU25 primary energy demand.This is bad news! It implies that if all agricultural land in Europe were devoted to EU25 primary energy demand, there would still be 25% to 50% of that demand unmet.(There is a manifest error in terminology, the article has a value of 1.7 PW h-1 petaWatts per hour, presumably they mean petaWatt.hrs, which are millions of kWh)
The growing of grain to ferment into alcohol for vehicle consumption is a process that has been described as
"letting people starve, so we can feed automobiles."
Put it another way, how many square miles of Brazil's tropical rainforest needs to be replaced by sugar cane to replace Brazil's oil imports with their E-85 motor fuel?
Besides that, a gallon of gasoline contains 1.48 times as much energy as a gallon of ethanol.
And, of course bio-engineering, Genetic Modification(GM) is anathema to some of the extreme "Back to Nature" wing of environmentalists.
Do not forget: Nature wants you to Die