Carbonic Acid Retreatment of Biomass Page: 6 of 65
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scatter, but that the combined severity function did appear to offer a more predictive
capability than the regular severity function, which does not take into account the pH of the
system.
Task 4: Preliminary experiments using a 150 mL reactor were conducted using water and a
range of CO2 pressures and reaction temperatures to evaluate the reactor performance and to
determine when the pressure (and hence temperature) inside the reactor reaches steady-state.
This revealed optimal reaction conditions and will minimize variation between experimental
results generated from small (15 mL) and larger (150 mL) reactors. The data also provided an
accurate determination of the time required for the reactor to reach reaction temperature.
Task 5: The newly constructed 150 mL reactor was used in reactions that replicated the
conditions of the 15 mL reactor. 1.0 grams of aspen wood, 80 ml of de-ionized water were
reacted with and without CO2 at 800 psig. Reaction temperatures were 1800C, 2000C, and
2200C with reactions times of 8, 16, and 32 minutes. The reactor was preheated in a sand
bath set to a temperature 400C above reaction temperature for two minutes. This allowed the
reactor to quickly reach reaction temperature, as determined and reported in the previous
progress report. The 150ml reactor successfully delivered the expected 10-fold increase in
hydrolysate compared to the 15ml reactor. Results of the pH and UV analysis of the
hydrolysate were consistent with those yielded by the 15ml reactor.
Task 6: Inhibition tests measured the rate of sugar consumption by Saccharomyces cerevisiae
growing in batch culture of hydrolysate. It was found that inhibition of the yeast culture
increased with severity of pretreatment above a mid level severity. Below this severity, little
to no inhibition was observed. No difference was observed between the inhibition of
hydrolysates prepared either with or without the presence of CO2. To conduct the
experiments, serum vials were charged with 21g/L of sterile growth medium containing 20ml
of pretreatment hydrolysate. The vials were inoculated with 0.2ml of freshly grown cell broth
and incubated. Glucose concentrations over time were determined via glucose assay (Infinity
Glucose Reagent) and the HPAE when available.
Task 7: Enzyme digestibility tests measured enzymatic hydrolysis rates of pretreated solids
by cellulase enzymes (Novozyme 188 and Iogen cellulase). It was found that more severe
pretreatments enhanced enzymatic digestibility. The addition of pressurized CO2 to the
pretreatment system did not significantly increase enzymatic hydrolysis rates compared to
water-alone pretreatment. To conduct the experiments, s erum vials were charged with a pH
5.0 buffer, preservative, enzyme and pretreated solid sample estimated to have 2g/L cellulose
(calculated from dry weight of the solid residue) and incubated in a 400C shaker bath. Glucose
concentrations over time were determined via glucose assay and the HPAE when available.
Tasks 8+ 9: Pretreatment costs for carbonic acid pretreatment are driven by the high cost of
pretreatment reactors capable of containing the pressures used. This makes the cost of the
carbonic acid system highly sensitive to reactor volume and thus the concentration of solids in
the reactor. The cost of the reactor can be reduced by raising the concentration of the solids in
the pretreatment reactor, which in turn reduces the size of the pretreatment reactor and thus
diminishes the cost differential between dilute acid and carbonic acid equipment costs. If the
solids concentration is put very high, and equal to that used in the NREL model, equipment
costs become comparable to those for dilute sulfuric acid pretreatment. Cost of compressing
CO2 is relatively low compared to the equipment cost for the high pressure reactor vessel.4
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university, Baylor. Carbonic Acid Retreatment of Biomass, report, June 1, 2003; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc785336/m1/6/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.