There's an estimated 1.3 billion tons of unused (waste) biomass produced in the USA every year. <Dr. Evil voice> 1.3 billion tons. </Dr. Evil voice> Sounds like a lot, doesn't it? Should be a great energy supply, right?
Maybe, maybe not. It depends how it is used.
The buzz today is all about cellulosic ethanol. Ethanol is touted because it is miscible with gasoline and can be used by some vehicles in concentrations up to 85% (E-85). There is a large agribusiness lobby behind ethanol, which claims it as the route to energy independence. Is it?
Iogen is a biotech company which makes enzymes for the hydrolysis of cellulose to sugars; these sugars are then available for yeasts to ferment into ethanol. Iogen claims a net yield of 330 liters (87 gallons) of ethanol per dry ton of biomass. This is an energy efficiency of roughly 48%; the balance of the energy is in lignin which is not converted to sugars (and typically burned to distill the ethanol) or used by the yeast for their own metabolism.
A gallon of ethanol has energy equivalent to roughly 2/3 of a gallon of gasoline, so Iogen's process turns a ton of biomass (roughly 15.2 million BTU) to the equivalent of about 58 gallons of gasoline (about 7.3 million BTU). 1.3 billion tons of biomass would yield 75 billion gallons-equivalent, about 54% of US gasoline consumption or about 38% of total US motor fuel consumption last year. Due to other uses of petroleum, it represents an even smaller fraction of total demand.
This is clearly not going to get us to independence; even with complete use of the entire 1.3 billion tons, losing about a quarter of petroleum supply would put us right back where we are now. But are there other ways to use this biomass surplus which would get us there?
Someone I won't name has been bugging me to write about direct-carbon fuel cells (DCFC's). These things are lab test articles and not ready for production, but they are remarkable nevertheless. The efficiency of any energy conversion machine is limited by the increase in entropy during its operation; entropy can only be carried away as waste heat, and so limits the possible conversion of chemical energy to electricity. DCFC's convert oxygen and carbon directly to carbon dioxide, which has approximately the same entropy as the reactants. Their theoretical efficiency is accordingly very high, and researchers claim 80% at practical current densities (100 mA/cm2).
It's easier to convert biomass to carbon than to ethanol; the production of charcoal is older than recorded history. Simple processes can convert biomass to 28-30% of its dry weight of charcoal, of which perhaps 25% of the original mass is carbon. 1.3 billion tons could produce 325 million tons of carbon, plus pyrolysis products. How far could we get with that?
A mole of carbon burns to yield 93960 calories of energy; that's 7830 calories/gram, or 32.8 kJ/g. A ton of biomass at 16 GJ converted to charcoal yields about 8.2 GJ of carbon as charcoal (51%) and the balance (49%) as chemical energy and heat in the pyrolysis gases. The carbonization of 1.3 billion (metric) tons of biomass would produce 1.02e19 joules (9.7 quadrillion BTU) of energy in addition to the charcoal. This is 1.6 times as much as all the natural gas burned for electricity in 2004; if it could be converted to electricity at 45% efficiency, it would make 4.4 quads of electricity, or an average power of 145 GW. That's more than 30% of total US electric consumption in 2004. Alternative possibilities include conversion of the pyrolysis gas to syngas followed by F-T synthesis to produce liquid fuels and other products. As a SWAG, perhaps 15% of the pyrolyzed mass might be converted to hydrocarbons; that would come to 146 million tons, roughly 42 billion gallons (1 billion barrels) of liquid at the density of diesel fuel. A half-ton of hydrocarbon per capita should meet US needs for plastics and other chemicals.
1.3 billion tons of biomass converted to carbon at 25% efficiency yields 325 million tons (325 teragrams) of carbon. At 7830 calories/gram, this represents 2.54e18 calories (1.06e19 J, 10.1 quadrillion BTU) of energy. At an efficiency of 80%, DCFC's could convert this to roughly 8 quads of electricity. This is an average power of 267 GW. But what's that compared to motor fuel demand?
US gasoline consumption in 2004 was up to 139 billion gallons; at 126,000 BTU/gallon, this comes to 17.5 quads of raw energy. But gasoline vehicles are inefficient; at 16% efficiency, only 2.8 quads of this gets to the wheels. Diesel vehicles are better. The 60 billion gallons of distillate oil consumed in 2004 contained 8.7 quads at 145,000 BTU/gallon; converted to work at 35% efficiency, it would deliver 3.0 quads to the wheels. The total of 5.8 quads is about 73% of the energy available from the carbon, allowing a surplus for other uses.
The one liability is that the DCFC cycle cannot use energy from sources other than biomass; if productivity runs low, there's the potential for a crisis. This is why I still like the thermal zinc process (driven by solar or any other heat source). Its direct path is not as efficient as the DCFC system (93960 calories of carbon yields 84670 calories of zinc, which produces 52500 calories of electricity - about 56% throughput to the DCFC's 80%) but it produces more net energy (via the carbon monoxide, the output from a mole of carbon includes another 68330 calories of chemical energy), more useful byproducts, and zinc can also be regenerated using electricity from any source. Zinc also allows nearly complete carbon capture even when the energy is used in mobile applications.
So, what CAN you do with 1.3 billion tons? The answer, I think, is "enough."
Further reading: Direct Carbon Conversion Workshop presentations.
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