I firmly believe we should be aggressively researching the potential of cellulosic ethanol. This was after all the topic of my graduate school work at Texas A&M. But I think the hype has gotten way out of touch with reality at this point in time. There is a reason that nobody today is making money with cellulosic ethanol. It is quite possible that they never will, and in this essay I will discuss the reasons for that.
I did an interview with a major publication last week on the topic of cellulosic ethanol. I won’t divulge any details, because I don’t want to leak out anything before it is published. But during the course of the interview, I made the point that one of the challenges is in securing a large and steady local supply of biomass to run through the plant. This was one of the points I made in my essay Cellulosic Ethanol vs. Biomass Gasification. Then I was asked about just how much biomass it would take to support a cellulosic ethanol plant. So I decided to do a little calculation.
Iogen, probably the closest to commercializing cellulosic ethanol, has reported that the theoretical yield of biomass to ethanol is 114 gallons of ethanol per ton of biomass. However, what they actually achieve in practice is 70 gallons per ton. Let’s consider a typical mid-sized 50 million gallon per year ethanol plant. Using Iogen’s demonstrated yields, the biomass requirement would be 50 million/70 = 714,286 tons of biomass per year. According to Dr. Bruce Marcot, an ecologist at the USDA Forest Service the average Douglas fir yields about 1660 lbs of pulp (90% of the tree’s weight). So, to run a mid-sized cellulosic ethanol facility would require the equivalent of 714,286 tons * 2000 lbs/ton /(1660) or 860,585 Douglas firs PER YEAR. That’s a lot of biomass, and it puts into perspective the issue of a declining EROEI as biomass must be secured from farther afield.
What does this mean? Even if one achieved the theoretical limit of 114 gal/ton, it is still going to be very difficult to grow enough biomass to keep the plant going. Furthermore, consider the conversion penalty that is being paid when compared to corn ethanol. Let’s presume for a moment that the conversion for corn is the same as for cellulosic ethanol. There are 56 lbs in a bushel of corn. In our example above, it took 714,286 tons to run the 50 million gallon per year facility. This much biomass is equivalent to 714,286 tons * 2000 lbs/ton * 1 bushel/56 lbs = 25.51 million bushels of corn. In a conventional corn ethanol plant, that much corn would produce about 25.51 * 2.8 = 71.43 million gallons, or 43% more than we would get from the same amount of biomass in a cellulosic ethanol plant.
On an energy equivalent basis, converting 860,585 Douglas firs to ethanol will displace 0.02% of our annual gasoline supply on a gross basis (not counting the fossil fuel inputs to produce and process the ethanol). If you look at the USDA reports on corn ethanol, they say that to produce 75,000 BTUs of ethanol the fermentation/distillation requirement is 50,000 BTUs on average. That is from actual plant surveys as reported in their 2004 report. However, that is for solutions that are 15-20% ethanol. Cellulosic ethanol produces a crude product that is only 4% alcohol, meaning it will take quite a bit more than 50,000 BTUs to separate it out. I can tell you from experience that once you get down to 3% alcohol, it is classified as a waste stream and sent to wastewater treatment.
Future cellulosic ethanol plants are envisioned as being supplied by something like switchgrass or miscanthus. Will they yield more or less biomass per acre than corn? According to Questions & Answers about Miscanthus:
Over large areas, under typical agricultural practices, an average of about 8t/ha (3t/acre dry weight) may be expected at harvest-time.
That means our 50 million gallon ethanol plant, displacing 0.02% of our annual gasoline demand, would require 714,286/3 = 238,000 acres. To displace 50% of our current gasoline consumption of 140 billion gallons per year would take 70 billion/0.65 (this is for the lower energy content of ethanol) * 238,000/50 million, for a total acreage requirement of 513 million acres. This is about 13% of the land area of the United States; land which is presumably being currently used. This is also about 7 times the land area currently utilized for corn production.
A similar story yesterday came out that echoed this theme:
Some excerpts from this story:
As many as 100 million acres of cropland and pastures would have to be dedicated to cultivating biomass fuels like switchgrass to support a national goal of 25 percent renewable energy use by 2025, a University of Tennessee study says.
Of course the reason cellulosic ethanol is so attractive is that the payoff would be huge, as the story explains:
But the rewards could be great. The study projects $700 billion in new economic activity including: a $180 billion growth in net farm income over the next 20 years; creation of 5.1 million jobs to support renewable energy enterprises; and government savings of more than $15 billion in crop subsidies.
The bottom line is that it is going to take enormous swaths of land to supply these cellulosic ethanol plants, and it is questionable whether a farmed source of biomass can be counted on to run the facilities. Better to locate cellulosic ethanol facilities close to a massive source of waste biomass – say a very large municipal dump in which paper is sorted out, a paper mill, or some other consistent source of large volume biomass. If you then use the unconverted waste biomass for process heat, you could end up with a workable process.
I certainly don’t advocate giving up on cellulosic ethanol, but we do need to approach this with a realistic and sober outlook. Men once desired to turn lead into gold. That was ultimately a futile quest (unless you want to try something like a nuclear reaction), but with cellulosic ethanol there is much more at stake. My impression is that many people in our government are basing energy policy decisions on the presumption that cellulosic ethanol is a done deal. My advice would be to have several backup plans.