Several ferments yield a biofuel using wheat chaff as substrate. The focus here will be on butanoic acid. According to an old British patent [565.773], this can be a septic ferment using a mix of wild microbes plus one butanoic acid bacteria which benefits from the enzymes excreted by the others. One which converts lactic acid directly into butanoic acid is especially suitable.* Given time to develop, the endpoint will be mostly butanoic acid or its calcium salt. The ferment is shorter if much of the lignin in the material is first removed. [* US Pat 4138 498]
The butanoic acid can be isolated and made into di·propyl ketone (PPK), a clear liquid with a pleasant odour which is almost insoluble in water. For blending in gasoline this alone makes it superior to ethanol which tends to attract moisture; plus PPK has much greater energy density as well. ..{An interesting aside, though not really relevant here, is that PPK may be an extractant for ethanol, so it can replace distillation as a means of getting ethanol from a ferment. Very likely the PPK with ethanol dissolved in it could be jointly used as a gasoline additive, instead of ethanol alone.}
A major component of chaff is cellulose, a polymer that gives glucose units, each of which can yield one butanoic molecule plus two each of CO2 and H2. These co·products are one reason to choose this ferment. Since all the carbon a plant uses to grow is from CO2 out of the air, when an annual plant is fermented and releases CO2, the carbon in it came weeks earlier from the air. Call this ‘indirect air capture’ of CO2. It could be liquified and sent underground, earning ‘fermenter carbon credits’. Clearly, this is less expensive than any ‘direct air capture’ scheme, or ‘capture’ of CO2 from hot flue gases. Both those are ridiculously expensive and cost hundreds of millions $$$ to construct. Baffling that any government would be willing to put up the money for such idiocy, and yet some do.
Cereal Chaff as an example
The frost-free period in the Canadian prairies is mostly too short to allow cereals such as wheat or barley to be directly cut by a combine harvester. Instead, it is common practice to cut the stand early and put it into windrows to dry down faster. Then combine, using a small front unit to pick the windrow off the ground and feed it into the maw of the machine. The big expensive combine is a mobile thresher which retains the grain and strews chaff and straw on the ground.
But when cereal chaff has value and is also to be taken from the field, the harvest should change. Take the entire grain heads intact — cut when mature but far from dry — and air dry them before threshing. Barley and wheat are bearded with long awns, so much air space exists within a pile letting humid air to drain out and wind to enter, hastening the process.
Park the combine, as only a header is needed attached to a swather style ‘tractor’, with a belt conveyor or thrower sending heads into a wagon — simpler equipment, smaller engine, cheaper to acquire and less expense to run. Quicker and earlier field work as well. Being bearded, the heads interlock when lightly compacted and hold that shape. So a stack former or pendulum-type soft round baler could be used, or a ‘tall’ firm windrow made. The heads may then be left in the field several days before being gathered.
Thresh the heads weeks later when they are very dry, and press the chaff into pellets. Adding a minor amount of lye, soda, or sodium acetate prior to compaction aids lignin removal by the fermentor. Both thresher and pelleter could be run by hydraulic motors, the fluid coming from a storage tank kept up in pressure by an hydraulic pump directly attached to a windmill. Wind is nearly always present on these farmlands.
Processor
Delivered to the processor are chaff pellets which contain a small amount of chemical that has reacted with the lignin, also with the xylan to an extent, due to the heat generated in pellet formation. The acetyls on the xylan will now be sodium acetate or acetic acid. Hot water and squeezing, repeatedly perhaps, will remove solubles and about half of the lignin. The lignin could be absorbed on straw or waste paper, dried and compacted to serve as a solid fuel. Some processors might choose instead to grow fungi on this material in order to obtain an organic fertiliser having quite a bit of nitrogen.
The rest of the pellet provides the fermentation substrate for the mix of wild microbes plus one producing mainly butanoic acid. At the end of the ferment, some acetic acid will also likely be present. Being in dilute amounts, concentration and isolation are required to obtain them. Several means of doing it may be used; ..freeze concentration is one.
In future, engineered thermophilic bacteria which convert acetic acid into acetone will exist, its vapour escaping. Then the primary ferment of chaff may be to acetic acid rather than the butanoic. Acetone plus H2 in a one·step process gives methyl·isobutyl ketone [MiBK] to blend with gasoline. Many petroleum refineries do that now. But it is only one biogasoline possibility, and as dozens of processors would come to exist, they won’t all be using the same technology.
As instance, in making MiBK, the H2 is only there to saturate a created double bond, for which instead an alcohol will do. Or instead of acetone as the ketone, butanone works, it being arrived at readily from a 2·3,butanediol ferment [which also produces much CO2]. There are, of course, other possibilities.