Pretreatment is a procedure used to disrupt the chemical bonds that hold cellulose, lignin and hemicellulose (three structural macromolecules in wood) together. The process exposes the cellulose polymers so they can be broken down into simple sugars. Those simple sugars serve as the feedstock molecules for a host chemical products including isobutanol.
It takes a good deal of energy to break the bonds holding lignin and cellulose together, which makes the pretreatment step relatively tricky and expensive. Consequently, NARA has evaluated multiple pretreatment options to help industry select the best protocol to convert forest residuals into chemical products.
The pretreatment protocols evaluated by NARA to date include sulfite-based pretreatments (SPROL and mild bisulfite), wet oxidation, wood milling and dilute acid. In 2014, NARA chose the mild bisulfite pretreatment as the protocol used to develop a wood-to biojet life cycle assessment and techno-economic analysis. The mild bisulfite pretreatment will also be used to generate 1000 gallons of biojet fuel in 2015.
NARA researchers used the dilute acid pretreatment protocol to rapidly screen Douglas-fir samples for levels of recalcitrance. The dilute acid pretreatment protocol is relatively simple and low cost and has been used extensively as a benchmark method to deconstruct cellulosic materials in grasses and hardwoods.
In a recent paper funded by NARA, researchers based at Washington State University-Tri Cities evaluated the dilute acid pretreatment method on forest residuals and comparatively clean wood chips. The paper provides detailed accounting for sugar yields and inhibitor concentrations culminating in a complete mass balance for pretreatment and enzyme hydrolysis.
First step was to evaluate the chemical composition of the samples. The researchers used the feedstock samples that were collected early in the NARA project for all pretreatment and fermentation experiments. Sample FS-01 consists of Douglas-fir pulp chips taken from a chip pile at the Weyerhaeuser Longview pulp mill. Sample FS-03 was collected from a roadside slash pile in Northwest Oregon and consists of 87% Douglas-fir, 6% hardwood and 7% other softwoods. Sample FS-10 was collected from a roadside slash pile in southwest Oregon and contains 64% Douglas-fir, 15% hardwood and 21% other softwood species.
Turns out that the FS-01 (pulp chips) had the highest cellulose content (44%) and the least amount of lignin (27.7%) compared to the two forest residual samples. The forest residual samples contained more bark (3.4%) compared to the pulpwood (1.4%). The additional bark in the forest residual samples would partially account for the lower cellulose and higher lignin values.
Dilute Acid conditions
The sweet spot for dilute acid pretreatment temperature exists between 135°C to 210°C. Pretreatment is ineffective below this temperature range, and simple sugars degrade above 210°C. The pretreatment temperature chosen for this study was 200°C. In addition, the reaction time was 30 minuets and 1% sulfuric acid (H2SO4) was added. The researchers’ intent was to set pretreatment conditions so that a maximum amount of glucose would be released during enzymatic hydrolysis from which they could compare results between the pulp chips and the forest residuals.
One of the parameters used to measure pretreatment effectiveness is glucose yield. Glucose is liberated from the cellulose fibers when specialized enzymes called cellulases are applied after pretreatment. If pretreatment is successful, then the wood’s cellulose is exposed and the cellulases are able to remove glucose. Glucose yields were 68%, 59% and 53% for FS-01 (pulp chips), FS-03 and FS-10 respectively. These yields are relatively low when compared to other pretreatment protocols, which can achieve yields up to 90%. When the solid material from dilute acid pretreatment was milled prior to enzymatic hydrolysis, then glucose yields improved to 86.6% 69.0% and 55.6% for FS-01 (pulp chips), FS-03 and FS-10 respectively. An interesting finding was that the commercially available hydrolysis enzyme mixture contained a small amount of primary sugars. These sugar amounts had to be subtracted from the total sugar yields or otherwise yields would be overstated by 5-6%.
High heat and extreme pH levels can change the structure of molecules. Often, these conditions will produce molecules, like organic acids and furfural from sugars and phenols from lignin, that inhibit enzyme function and fermentation. Up to 20% of the initial weight of wood material can be converted into inhibitory compounds depending on the treatment severity. Therefore, the authors evaluated the effect of pretreatment time, acid dosage and temperature on inhibitor levels. What they found is that inhibitor levels increased dramatically when sulfuric acid (H2SO4) levels increased above 2% or if the reaction temperature increased from 180°C to 200°C.
Potential wood characteristics leading to lower sugar yield
The results from this study showed that clean pulp chips responded more favorably to dilute acid pretreatment and hydrolysis than forest residuals. Factors such as high bark content, knots, ash content and bulk density characterized the forest residual samples and probably contributed to their lower sugar yields. The authors point out that effective pretreatment strategies for forest residuals will need to address these characteristics.
This paper presents a thorough evaluation of the dilute acid pretreatment procedure on forest residuals and provides a baseline to compare other pretreatment protocols.