| Summary: | Because of widespread availability, low cost, sustainability, and potential supply
far greater than that of food crops, lignocellulosic biomass is one of the most promising
feedstocks for producing biofuels through fermentation processes. Among lignocellulose
choices, poplar wood is appealing because of high energy potential, above-average
carbon mitigation potential, fast growth, and high yields. Lignocellulose structural
features limit accessibility of enzymes or microorganisms. To overcome these
limitations, pretreatment is required. Among several choices of pretreatment, lime
pretreatment is preferred because lime is the cheapest alkali, safest to handle, easy to
recover, and compatible with oxidants.
The main effect of lime pretreatment is to degrade lignin, which occurs with
good carbohydrate preservation and is enhanced with oxidants. Among several choices
of oxidant, oxygen and air are preferred because of low cost and widespread availability.
This study systematically assesses the effects of lime pretreatment on poplar
wood using four different modes: long-term oxidative, long-term non-oxidative, short-term
constant pressure, and short-term varying pressure. Long-term pretreatments use
temperatures between 25 and 65° C, air if oxidant is used, and last several weeks. Short-term
pretreatments use temperatures between 110 and 180° C, pressurized oxygen, and
last several minutes to hours.
Pretreatment was assessed on the basis of 3-day enzymatic digestibility using
enzyme loadings of 15 FPU/g glucan in raw biomass. The results were used to
recommend pretreatment conditions based on highest overall yield of glucan (after
combined pretreatment and enzymatic hydrolysis) for each pretreatment mode.
For each pretreatment mode, kinetic models for delignification and carbohydrates
degradation were obtained and used to determine the conditions (temperature, pressure,
and time) that maximize glucan preservation subjected to a target lignin yield. This study
led to conclude that the most robust, and selective mode of lime pretreatment is varying
pressure.
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