Curing the oil crisis:- Starch or Sugar based Ethanol versus Cellulosic Ethanol
September 26th, 2007
Ethanol as a replacement for petroleum
Back in my undergrad degree, I did some research into the viability of using resources from Australia’s large sugar cane industry as a feedstock for ethanol production.
My research was completed about 10 years ago. I found that for Australia to fulfill its own oil demands with Sugar Cane based ethanol it would need to have the entire arable land mass of Australia under cane and that the cost of production would be around 60c per litre, or about US 2.26 per Gallon.
The Australian retail price of petrol back then was around the 60c mark. It was clearly an impractical and uneconomic proposition.
Replacing Oil with Ethanol?
I went on to spend some time with the Sugar Industry and I found along the way that really my calculations were entirely flawed.
Firstly, I’d assumed that the feedstock for my theoretical ethanol production would be something called Molasses. Molasses is relatively cheap and abundant by-product of sugar production and is often used to produce rum. It contains perhaps 5% of the available sugar produced from the processing of Sugar Cane - with the remainder generally converted into relatively high value crystal sugar (the type we use in our coffee).
Two things have changed since then - the world price of sugar has precipitously declined and the world price of oil has sky-rocketed - so the economics have changed. I thought I’d take this opportunity to cast a backwards glance and go over my figures once again.
Brazil can switch between ethanol and sugar production almost instantaneously!
Brazil (the largest producer of sugar cane in the world) has over the last 10 years or so built the capability to easily tailor their production to either crystal sugar or ethanol - basically if the oil price is high, Brazil starts producing ethanol.
Due to the enormous size of the Brazillian industry the resultant shortfall in crystal sugar tends to drive up the world sugar price - so much so that over the last 5 years or so we’ve started to see the world sugar price (WSP) follow the peaks and troughs of the oil price.
Already, without even realising it, Sugar has become an agro-industrial energy crop. The same thing has happened with corn as well - in recent years the price of maize has doubled in much the same way as the price of oil has.
This is a very different playing field to that I investigated 10 years ago - ethanol production from sugarcane is now starting to become an economic proposition of some merit.
A friend of mine in the south of India has secured the rights to build an ethanol factory in a state called Kerala, so I’ve been speaking to some experts here in Australia on his behalf about the new state-of-the-art in ethanol production.
Kerala has a rather large Coconut industry, and one of the interesting possibilities we are looking at is cellulosic ethanol, which has recently been featured in this great article from wired magazine.
This is still a technology in its infancy, but things are looking very promising. Cellulose is the stuff that makes plants stand up - it’s tough, it contains a great deal of Carbon and it’s not easy to break down.
For all our evolutionary complexity, we as humans haven’t yet developed the ability to utilise cellulose as an energy source in our diet, although that’s not the case for other organisms.
Ruminants like cows, goats and camels are capable of supporting vast populations of bacteria in their gut which digest the cellulose and produce by-products called (again, simplifying) short chain fatty acids which can then be digested by the animal as an energy source.
Termites go a step further - their guts apparently produce these enzymes in-situ without the need for a symbiotic colony of gut bacteria.
In recent years there has been a veritable explosion in the number of companies (and governments) investing in this particular technology.
What are the expected yields of alcohol per tonne of Cellulose?
It’s very hard to find definitive numbers here though that allow us to start looking at the potential economics of using cellulose from plants as a ethanol feedstock.
One article about the economics of cellulosic ethanol production states that the theoretical yield of ethanol per tonne of biomass is 114 gallons (431 litres), but in practice the real (achieved) yield is closer to 70 gallons (264 litres). Unfortunately that particular article doesn’t express ethanol production as percent dry biomass (aka dry matter) so it’s hard to compare it with other studies.
Another article I found goes into a bit more detail and compares potential cellulosic ethanol production between four feedstocks - corn stover (another name for corn stalks), alfalfa stems (lucerne), sugarcane bagasse (the dry fibre left over after sugar extraction, usually burnt as a waste material) and Oak Wood. This particular study expresses Alcohol production as litres of ethanol per tonne of dry matter - a much better measure.
|Conversion efficiency (%)|
These figures tend to correlate well with those given in the first reference - that expected production of ethanol per tonne dry biomass is going to be somewhere in the order of 220-280 litres of ethanol. It also correlates fairly well with a ‘tool’ provided by the US Department of energy which allows you to calculate the theoretical yield of cellulosic ethanol based upon the composition of a biomass feedstock.
So.. down to some economics..
Let’s take a reference (underestimate) of dry matter % sugarcane biomass as ~14%. So, for every 1000kg of sugarcane, we’ll assume we’re left with 140kg of dry cellulosic materials after extraction of the sugar. Assuming 260 litres per tonne biomass, that means that we’re going to expect to produce around 36.4 litres of cellulosic ethanol per tonne of harvested sugarcane.
Now the important part - Sugarcane uses what’s called C4 photosynthesis - meaning it’s extremely effective at producing biomass from sunlight - biomass yields are very high and the plant is extremely fast growing. A typical yield of sugarcane per hectare in Australia would be ~ 70 to 150 tonnes cane per hectare per annum. We’ll take the midpoint - 100 tonnes.
This means that from cellulosic sources, we’d be expecting to produce around 3640 litres (961 gallons) per hectare of cane from a material that is otherwise considered a waste product.
What about if we used the sugar too?
Now lets look at the sugar content.
Fleay et al (2006) states that (one tonne of) “sugar yields 0.385 tonnes of anhydrous ethanol” and the specific gravity of ethanol is somewhere between 79% and 81% that of water, meaning we can expect the yield of anhydrous alcohol per tonne of sugar to be (1/.8*385)=481 litres.
In general the CCS of cane (amount of sugar in a tonne of cane expressed as percent biomass) is somewhere between 10 and 14% - and that doesn’t include the sugars available in molasses. Let’s assume that we just macerate the cane, extract the juice and ferment that (a much less energy intensive process than extracting the sugar) - we’ll make a relatively broad assumption that we’ll have around 140kg of sugars available per tonne of cane, which, at our theoretical yield of 100 tonnes per hectare equates to around 14,000 kg sugar per hectare available for fermentation and conversion to ethanol. At 481 litres per tonne of sugar, we’ll be left with around 6735 litres of ethanol per hectare.
So.. we’re left with the following figures.. from a ‘typical’ cane field with 14% fibre, 14% available sugar and 100 tonnes per hectare..
Yield of Cellulosic Anhydrous Ethanol per hectare - 3640 litres
Yield of Fermented Anhydrous Ethanol per hectare - 6735 litres
Total yield of Anhydrous Ethanol per hectare - 10,375 litres
Could Ethanol totally replace our petroleum usage?
Would this be enough to satisfy Australia’s Oil needs?
Let’s assume an annual cane yield of 40 million tonnes, or roughly 400,000 hectares under cultivation - this equates to around 4 150 000 000 litres, or 4.15 billion litres of ethanol per annum (assuming one crop cycle per year)
The energy density of ethanol is only around 70% of that of petroleum so this would equate to around 2 905 000 000 litres (2905 megalitres) of petroleum (although ethanol does have a higher octane rating, which means it burns more efficiently, so this is probably a slight underestimate). Current petroleum consumption in Australia is around 20,000 megalitres per annum, leaving a shortfall of over 17,000 megalitres per annum.
WOW! If Australia was to convert our entire sugar industry to the production of ethanol, we’d only be able to satisfy around 14% of our demand for automobile fuel (not including diesel consumption which is another 4000 megalitres). Keep in mind Australia has only 20 million people.
The scale is enormous - in QLD you can basically drive for 2000 uninterrupted kilometres and all you’ll see growing is sugarcane.
If we wanted to gross up our cane production to satisfy our demand for foreign oil (for automotive use only), we’d need to have around 2,857,000 hectares under sugar cane - the entire cropable landmass of Queensland (approx 20% of Australia’s land mass) is just below that figure. Forget about eating - we’d need to crop our whole state just to satisfy the thirst of the nation’s automobiles. That’s quite amazing and I feel the figures for a country with a much higher population density like the US would be even more compelling - these are interesting figures.
This reinforces a few things to me:-
- The massive scale of our oil consumption is easier to comprehend when it is expressed as biomass equivalent production (the amount of oil beneath the earth was phenomenal)..
- That burning that finite resource to fuel inefficient vehicles rather than using it for higher value industrial purposes for which no other economic feedstock exists (plastics, medicines etc) is perhaps not the best long term use of an amazing natural resource.
- That folks are going to look back at this period of history and wish we had done things a bit differently.
What are your thoughts? Where do you think the future lies? I’d be keen to hear your ideas / opinions.
THE AUTHOR: Matthew holds degrees in Agricultural Science and Computer Engineering. Matt has extensive experience in the Sugar Industry worldwide and has a strong interest in Agricultural Mechanisation, Economic Modelling, Agronomics, Alternative energy, Comms Engineering / Remote Area Comms and Entrepreneurial Start-Ups. Matt is available to discuss these topics, and would welcome contact from anyone interested in discussing their business.