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The effect of steam explosion pretreatment parameters on softwood delignification efficiency Cullis, Ian Frank

Abstract

Current environmental problems have led to concerns about the condition of the environment and the effect of automobile emissions on global warming. Recent work has determined that there are other alternate forms of energy, which are cleaner burning, and can be produced from renewable resources. Some alternative sources of energy include the combustion of hydrogen in the fuel cell and ethanol. Ethanol fuel has shown significant promise due largely to its ability to be used in internal combustion engines that are currently running gasoline or gasoline ethanol blends. Previous work has shown that steam explosion is a viable pretreatment technique for the conversion of biomass to ethanol, and that medium severity conditions are the best compromise between hemicellulose recovery and cellulose digestibility. To further develop this pretreatment strategy the effect of feedstock variation (moisture content and chip size) on the steam explosion process was examined, and the effect of the substrate on subsequent fractionation and hydrolysis were evaluated. Additionally, the effect of a secondary treatment regime, post steam explosion particle size reduction, was examined for its effect on fractionation and hydrolysis efficiency. In the next series of experiments the effect of EDTA chelation, stabilization (DTMPA, Sodium Silicate, and Magnesium Sulphate), consistency, and alternative delignification techniques (oxygen and wet oxidation) were examined. The techniques, which showed an improvement in delignification efficiency, were then used to optimize chemical loading and temperature during peroxide fractionation. The manipulation of feedstock conditions (moisture content and chip size) caused noticeable variations in the properties and effectiveness of further stages of the bioconversion process. Increased chip size caused an increase in the solid recovery, increasing from 62 to 82%, with concurrent increases in the prehydrolysate sugar recovery (7.5%). Increased recovery is the result of decreased relative severity of steam treatment as chip size increases. Decreased severity affects the overall process by decreasing the recalcitrance of lignin and therefore increasing the efficacy of peroxide fractionation, which removed 16% more lignin from the largest chip size. Similarly increased initial moisture content appeared to reduce the relative severity of the treatment, prehydrolysate sugars (mainly glucose and mannose), and solid recovery. Both increasing chip size and moisture content results in a substrate that performs better in peroxide delignification and enzymatic hydrolysis. Furthermore, post steam-explosion refining solubilized more of the glucose and mannose present in the prehydrolysate, resulting in a decrease in the solid recovery, while concurrently increasing prehydrolysate sugar recovery. The resulting solid substrate was more effectively delignified, resulting in a decrease in the residual lignin of 4.6% in the largest chip size. Improvements in the peroxide delignification process previously optimized by Yang et al. (2002) were achieved by altering substrate consistency, and the addition of peroxide stabilizers and chelants. Increasing consistency from 2 to 10% resulted in a decrease in the residual lignin content from 5.4% to less than 3% respectively. Additionally, stabilization (DTMPA) reduced the residual lignin content, after optimization a 40% decrease in lignin content is achievable, while maintaining glucose yield from enzymatic hydrolysis. Therefore, by optimizing the conditions it was possible to reduce the chemical (peroxide) loading by greater than 40%.

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