This thesis introduces a biorefinery process to fractionate lignocellulosics followed by treatment of the produced hydrolysate for microbial fermentation to acetone, butanol and ethanol (ABE). The process utilizes SO2-Ethanol-Water (SEW) fractionation technology and a ‘conditioning’ protocol to treat SEW spent liquor for ABE fermentation by Clostridia bacteria. It is found that SEW fractionation of spruce chips, mixed softwood biomass and Oil Palm Empty Fruit Bunch (OPEFB) at conditions of 12% SO2 in 55 v/v% ethanol-water, liquor-to-feedstock (L/F) ratio of 3 L kg-1, 150°C, 30 min, is suitable for industrial scale application. SEW fractionation is followed by pulp washing and ‘conditioning’ to detoxify the spent liquor and to increase its monosugars content. The ‘conditioning’ scheme in its basic form comprises of the consecutive steps of vacuum evaporation, steam stripping, liming and catalytic oxidation. ‘Conditioning’ successfully removes most ABE fermentation inhibitors for Clostridia. It also allows almost total recovery of the cooking chemicals (ethanol and SO2) leading to an economical and environmentally benign process. Levels of residual inhibitory dissolved lignin in the final conditioned liquors correspond to only about 10% of the original lignin in the respective feedstocks. However, these levels are still too high for microbial ABE fermentation and therefore additional treatment with anion exchange resins followed by 4-fold dilution is employed before ABE fermentation to reach dissolved lignin levels of approximately 1 g L-1 (tolerance limit for Clostridia). All the different feedstock-based hydrolysates that are produced after ‘conditioning’ are fermentable by Clostridia and ABE solvents (mostly butanol) are produced at satisfactory total concentration and yield. Hydrolysis of OPEFB fibers in particular is impaired (compared to spruce) due to their high ash/alkali metals content. Acidic leaching of this feedstock did not remove sufficient amounts of metal cations leading to only marginally improved hydrolysis. However, it is possible to improve hydrolysis of this feedstock by adding inorganic acids (nitric, phosphoric) in the fresh fractionation liquor at a level to provide the required nutrients for Clostridia. Finally, it is demonstrated that by introducing some small modifications to the basic SEW spent liquor ‘conditioning’ scheme and by performing nanofiltration instead of resins treatment it is possible to reach lower dissolved lignin levels (below 1 g L-1 upon 4-fold dilution) in the feed liquor for fermentation. Furthermore, it is possible to significantly improve the production of solvents and ABE fermentation yield; total solvents concentration increases from 7 to 11 g L-1, yield increases from 0.26 to 0.30 g g-1 sugars.
|Translated title of the contribution||Conditioning of SO2-ethanol-water (SEW) spent liquor from lignocellulosics for ABE fermentation to biofuels and chemicals|
|Publication status||Published - 2014|
|MoE publication type||G5 Doctoral dissertation (article)|
- ABE fermentation
- SO2-ethanol-water fractionation