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Rumplestiltskin, in the famous fairy-tale, has the knack of spinning straw into gold.
We can’t do that–but we are learning to spin straw into something just about as valuable: biofuel.
Sure, you can make ethanol out of corn or wheat, but in a hungry world, wouldn’t it be better to keep our food crops for food and find another source of plant material to use as a biofuel feedstock?
That’s where straw comes in.
It’s not like there’s any shortage of it: between 1994 and 2003, the three prairie provinces alone produced, on average, 37.347 million tonnes of straw annually.
There’s a huge amount of energy tied up in that straw, if only we can get at it. (Which is not to stay we could use all of that straw to produce ethanol and other biofuels: some straw needs to be left on the field after harvest to help prevent soil erosion and maintain nutrient levels, and quite a lot of it already has its uses, as feed and bedding for cattle.)
Still, according to a recent study, the prairie provinces could conceivably offer up more than 15 million tonnes of cereal straw every year as a feedstock for biofuel production.
Harvesting it isn’t a problem: many farmers already harvest straw, rolling it up into those giant round bales that dot the landscape every fall. And as straw becomes more valuable as an energy source, you can expect to see more development of single-pass harvesting equipment that will allow farmers to harvest straw and grain at the same time, while ensuring enough straw and stubble remains to maintain soil nutrient levels, conserve moisture and prevent erosion.
Eliminating that second pass with the baler to pick up straw will also go a long way toward making it cost-effective to harvest.
So, how do you spin straw into ethanol?
What makes cereal straw a potential energy source is lingocellulose, which makes up plant cell walls: it’s sort of like reinforced concrete, with hemicellulose as the concrete, lignin as the water-resistant sealant, and cellulose microfibrils as reinforcing steel bars.
The energy is bound up in the cellulose (especially the hemicellulose), which has to be freed from the inert lignin and then converted into sugars (a process called, naturally enough, saccharification), which can then be fermented into ethanol.
The process can be broken down into four major steps.
In the pretreatment stage, the straw is cleaned, milled, mixed with a mineral acid solution, then sent to the dangerous-sounding “steam explosion reactor.” There it is heated to between 180 and 220 C for anywhere from half a minute to five minutes, then suddenly (explosively!) cooled by the sudden release of the reactor pressure. This reduces the sizes of solid particles, changes the consistency from a damp fiber to sludge, and most importantly breaks down the walls of the plant cells, allowing hemicellulose to escape into the syrupy mixture.
The solids, which consist primarily of lignin and cellulose, are washed and conditioned (so they won’t be toxic to microbes), then mixed with the syrup to produce a slurry. Enzymes are added to the slurry to thin it and begin saccharifying the cellulose.
Once that process is complete, a biocatalyst (though not brewer’s yeast; it can’t metabolize some of the sugars produced) is added to begin the fermentation of the resulting sugars into ethanol.
The ethanol is stripped out of the liquid (beer, really, though you wouldn’t want to drink it), distilled, and then passed through a molecular sieve to remove the last of the water.
The solids, which consist largely of lignin, are recovered from the stillage with a centrifuge, and can then be burned to generate steam for the overall process.
An alternative way to turn straw into biofuel, called fast pyrolysis, doesn’t rely on fermentation: instead, it involves heating the plant material rapidly in the absence of air to between 400 and 600 C. This causes it to decompose into a liquid “bio-oil” which can then be refined.
None of this is mature technology yet, but all over the world, researchers are working on it.
Spin straw into gold? Impossible.
Spin straw into liquid gold? Now you’re talking!
1 comments
The process sounds a little like thermal depolymerization, another way of making fuel from garbage (except in the case of TDP, it’s literally garbage that’s used). TDP is apparently tunable, and so can produce a variety of fuels, so conceivably you could use it for diesel also.
The nice thing about both processes is that, even though by burning the eventual fuel and putting CO2 into the atmosphere, you’re not really unbalancing the carbon cycle like fossil fuels.