I have been asked about this several times, so I figured I might as well post a reply. Recent news reports have Vinod Khosla making a new investment:
First, the obligatory hype we have all come to expect:
This technology could produce 50 billion gallons of ethanol from the world’s steel mills alone, turning the liability of carbon emissions into valuable fuels worth over $50 billion per year at very low costs and adding substantial value to the steel industry. The technology will also be a key contributor to the cellulosic biofuels business as it can convert syngas produced through gasification into ethanol.
But let’s strip out the hype and get right down to the fundamental problem with such approaches.
Natural gas forms carbon dioxide and water vapor when burned with a sufficient oxygen supply. The heat released is a bit less than 1,000 BTUs per standard cubic foot of natural gas. If the natural gas is burned in an oxygen-restricted atmosphere, syngas – CO and H2 – can be produced. This is a partial combustion, or oxidation (partial oxidation = POX). Syngas can be used to make numerous compounds, including many that can be used as fuels (like diesel via the Fischer-Tropsch reaction).
CO can be combusted to form CO2, which would have been the end product if the initial reactant hadn’t been oxygen-deficient. The amount of heat released from this is about 340 BTU/scf, or about a third of the heat release of burning natural gas. The bacterium in question converts CO to ethanol, which seems like a pretty nifty trick. And from a purely scientific point of view, it is pretty interesting.
How About the Efficiency?
But this is an incredibly inefficient means of energy production/waste disposal. CO as a gas can be combusted for energy, or it can undergo various chemical reactions to produce useful chemicals. But if you ferment it into ethanol, you have now essentially destroyed the vast majority of the heating value initially present. How? These fermentations take place in water, and bacteria are not able to tolerate very high ethanol concentrations. Therefore, you are going to have a very dilute ethanol product that will have to now undergo a very energy-intensive distillation step. How energy intensive? If the ethanol concentrations attained are typical of bacterial fermentations, then it will take as much energy to distill off the ethanol as is contained in the ethanol. (This is exactly why I have never liked the BRI process).
So, where does that leave us? We have essentially run around in a circle in which no net useful energy was extracted. Let’s say 340 BTUs of CO get fermented to 340 BTUs of ethanol, and then it takes 340 BTUs of natural gas to purify the ethanol. In effect, what we have is an input of 680 BTUs of CO plus natural gas to produce 340 BTUs of ethanol. How about instead of this kind of system, we use the CO in a gas-phase reaction – keeping the product away from water – and then use the natural gas we saved in the distillation step to run a compressed natural gas vehicle?
From an economic perspective, this scheme is a non-starter. It is an interesting bit of scientific research, and certainly worth funding, but you aren’t going to see any ethanol fermentation plants commercializing gaseous waste streams. After all, ethanol can be produced in a gas-phase reaction without the need for an inefficient fermentation. But then that would not be “bio-ethanol”, and therefore wouldn’t qualify for subsidies. These are the sorts of schemes that our subsidy system encourages. I am just waiting for someone to figure out how to ferment 1 BTU of gasoline into 0.9 BTUs of ethanol – which would then be eligible for a “bio-ethanol” subsidy. Then again, that’s not too terribly far off from what we are doing now with grain ethanol.
Technical Details (My Calcs)
From Perry’s Chemical Engineers’ Handbook, (McGraw Hill, New York, 1997).
Enthalpies of Combustion
Natural gas = 802.6 KJ/mole
CO = 283.0 KJ/mole
1 KJ = 0.948 BTU
Natural gas = 760.72 BTU/mole
1 mole natural gas at STP = 22.4 liters
1 cubic ft = 28.3 liters
So, 1 mole = 0.79 cubic feet
Then natural gas = 760.72 BTUs/0.79 cubic feet = 961 BTUs/scf
And CO = 340 BTUs/scf
Natural gas/CO = 2.8; therefore the full natural gas combustion delivers 2.8 times the energy of the CO combustion.