Concentrated Sulfuric Acid Hydrolysis of Hardwood Aspen and Softwood Pine for Bioethanol Production

Abstract

Bioethanol production from lignocellulosic biomass has been targeted as an alternative solution to the existing dependence on fossil fuels in the transportation sector. However, the recalcitrant nature of lignocelluloses has been a challenge to the hydrolytic processes and hence commercialization.

This study has investigated the feasibility of the concentrated sulfuric acid hydrolysis (CSAH) process for bioethanol production from wood-based lignocelluloses. This is because the process enjoys high sugar yields and low sugar degradation products and inhibitors due to application of low temperatures. The major focus was the decrystallization stage of a two-stage CSAH process. One softwood (scots pine) and one hardwood (aspen) were used as representative lignocellulosic biomass feedstocks.

The multi-variable effect of the major operating variables at the decrystallization stage (acid concentration, temperature and time) and their influence on sugar yields were investigated in a two-stage CSAH process. By applying statistical modeling, the effect of decrystallization reaction conditions on the hydrolysis products (sugars), degradation products and other inhibitors was simulated. The reaction temperature and acid concentration were found to have more influence on sugar yields compared to the reaction time for both aspen and pine. The interaction between temperature and acid concentration was found to be the most important for both species. More observation showed that the sugar degradation products were much more influenced by the decrystallization temperature, for both aspen and pine. The empirical models prediction showed that optimum total sugar yields of 56 g / 100 g d.w (74% theoretical) for aspen and 64 g / 100 g d.w (91% theoretical) for pine could be achieved at 39.6 oC; 60 minutes and 66.4 wt.%. and 38 oC; 60 minutes and 70 wt.% respectively.

The applicability of the generalized severity parameter (ROH) for concentrated acid processes was also investigated. As an initial effort in the area of concentrated acids application; the decrystallization stage of a two-stage CSAH process was successfully modeled by the generalized severity parameter (ROH) with a time-independent rate constant. ROH combines the operating variables (temperature, acid concentration and time) in one single reaction ordinate to describe the extent of decrystallization. By describing the monosaccharide yield as a function of ROH, the pseudo-kinetic parameters of ROH for concentrated sulfuric acid hydrolysis were estimated from experimental data and non-linear regression. As anticipated, the decrystallization severity demand for cellulose conversion to maximum glucose yields was higher as compared to the hemicelluloses conversions to their respective monosugars for both aspen and pine. The fairly good correlation between the experimental values and the model predictions showed that the generalized severity parameter is a suitable tool for describing the extent of reaction at the decrystallization stage, and to predict sugar yields as a function of decrystallization conditions of a two-stage CSAH process within the investigated range.

The quality (fermentability) of hydrolyzates derived from two-stage concentrated sulfuric acid hydrolysis of Scots pine and trembling aspen was also investigated. Two sets of hydrolyzates were produced and analyzed quantitatively before being tested qualitatively by an anaerobic fermentation using Saccharomyces cerevisiae ATCC 96581. The first set of the original hydrolyzates from each wood species was produced directly from the two-stage CSAH process at different decrystallization conditions (severities). In an effort to increase the sugar concentration and simulate more industrially relevant process conditions, portions of each of the original hydrolyzates were concentrated by vacuum evaporation. Complete glucose consumption was observed for all the original hydrolyzates, with no signs of inhibition and the ethanol yields ranging from 68% to 90% of theoretical. Fermentation of concentrated hydrolyzates of aspen produced at mild and moderate severities showed a significant lag phase associated with their relatively high furfural content of approximately 2 g/L. No lag phase was apparent for aspen at high severity and pine hydrolyzates at all severities. The results also showed that increased furfural concentration increased the lag time, consequently increasing the fermentation time and decreasing the volumetric ethanol productivity. However, the furfural concentration had no effect on the maximum ethanol production and yield at the concentration levels experienced in this study. The effects of HMF, acetic acid, formic acid and levulinic acid in the concentrated hydrolyzates were insignificant, presumably due to the relatively low concentrations of these compounds. The ethanol yields from concentrated hydrolyzates ranged from 87% to 103% of theoretical. Furfural was singled out as the most important fermentation inhibitor in concentrated sulfuric acid hydrolyzates from wood.

In summary, the potential of the concentrated sulfuric acid process for ethanol production was revealed by the characteristic hydrolyzates which showed high sugar yields and low content of degradation products and inhibitors. Furthermore, the readily fermentable hydrolyzates from aspen and pine revealed that this process can produce clean streams of sugar solutions from various lignocellulosic feedstocks for ethanol production by S. cerevisiae ATCC 96581.