Abstract
Problem statement: Rice straw has been treated with different chemical (acid, alkali) and physical (subcritical water, ultrasound) methods to convert lignocellulose material to sugar. In addition enzyme treatment of pretreated samples has been applied to improve the conversion of lignocellulose material in sugar. Approach: Sulfuric acid at concentration of 1-9% was applied for acid treatment. For alkali treatment was sodium hydroxide solution at concentration of 1-5% used. Subcritical water treatment carried out at 160°C (5 bar) and 200°C (15 bar) for 10 min. Ultrasound was applied as combination method after acid pretreatment. The condition during ultrasound treatment was 40 W at 50°C and 10 min. Finally the pretreated sample was fermented using Saccharomyces cerevisiae yeast and the amount of produced ethanol was measured. Results: Acid treatment at 121°C, 15 min is an effective pretreatment method for converting lignocellulose to sugar. Up to 21.45% sugar w/w could be measure after acid treatment. Combination of chemical pretreatment and subsequent enzyme treatment increased the sugar yield drastically. Up to 37 and 28% sugar w/w could be achieved for acid and alkali pretreated samples respectively. Subcritical Water (SCW) treatment method is an effective physical method. SCW treatment at 200°C and 10 min followed by enzyme treatment yielded up to 17% sugar w/w. Combination of acid pretreatment with ultrasonic before enzyme treatment increased the conversion of lignocelluloses to sugar. Sugar yield up to 44% w/w after combination of acid and ultrasonic pretreatment and subsequent enzyme treatment could be achieved. Fermentation of pretreated rice straw shown that after 3 days fermentation most of sugar (55-65%) will be converted to bioethanol. The remaining sugar could not be converted in ethanol even after 6 days fermentation. Under these conditions, the maximum ethanol of 1.69% (v/v) was obtained. Conclusion: The combination method of acid pretreatment combined with ultrasound and subsequent enzyme treatment result the highest conversion of lignocelluse in rice straw to sugar and consequently, highest ethanol concentration after 6 days fermentation with S. cerevisae yeast.
Highlights
Ethanol from renewable resources has been of interest in recent decades as an alternative fuel to the current fossil fuels
The pentoses are dominant in hemicelluloses which xylose is the most important sugar followed by arabinose and hexoses
The aim of this study is to investigate the effect of combined physical and chemical methods as pretreatment on conversion of lignocellulose in rice straw to sugar and fermentation of sugar to bioethanol using S. cerevisae yeast
Summary
Ethanol from renewable resources has been of interest in recent decades as an alternative fuel to the current fossil fuels. Lignocelluloses biomass like wood and agricultural crops residue, e.g., straw and sugarbeet pulp are potential raw materials for producing several high-value products like fuel ethanol and biodiesel. Up to 80% of the lignocelluloses is polysaccharides (Kaparaju et al, 2009) These renewable raw materials look promising for replacing environmentally unfriendly fossil hydrocarbon raw materials and creating ”green” products. Rice straw is a by-product of rice production and great bio resource. It is one of the abundant lignocellulosic waste materials in the world. Rice straw can potentially produce 205 billion liters bioethanol per year, which is about 5% of total of consumption. It is the largest amount from a single biomass feedstock. The carbohydrate of rice straw involves glucose 41-43.4%, xylose 14.8-20.2%, arabinose 2.7-4.5%, mannose 1.8%
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