Wood is composed of large sugar compounds (cellulose and hemicellulose), lignin, and ash (minerals). The amount of these components varies, and poplar’s relatively high sugar and low lignin content makes it a good candidate for conversion.
The conversion process requires smaller sugars, like glucose and xylose. To extract these sugars, the poplar chips are pretreated.
In the lab, poplar chips are treated with sulfur dioxide (SO2) gas. The material is then dropped into a steam gun (a high-pressure vessel). Once sealed, the steam gun hits the chips with pressurized steam for a few minutes. This step is called steam explosion. Afterwards, researchers remove a poplar slurry mixture and separate it into solids and liquids. At this point, much or all of the sugar molecules are still too large. To cut the sugar molecules, enzymes are used that break down the sugars into glucose and xylose. This is called enzymatic hydrolysis.
At the commercial-scale facility, the process runs continuously and employs acid along with steam and pressure. Leftover biomass residue can be burned for steam and electricity.
The sugars from pretreatment (glucose, xylose) are fermented into organic acid by microorganisms.
AHB used an efficient fermentation process with naturally occurring acetogenic bacteria instead of yeast. This produces acetic acid as an intermediate to ethanol and prevents loss to CO2. In most industrial and commercial fermentation systems, yeast is used to convert sugars into alcohol, which loses about half of the sugars as the greenhouse gas CO2.
The acid is converted to an ester [esterification] and reacted with hydrogen to produce alcohol – ethanol [hydrogenation].
The alcohol is separated into water and hydrocarbons in a catalytic reactor [dehydration]. Those hydrocarbons are chemically combined into longer molecules [oligomerization/polymerization] and ultimately become drop-in biofuels.