Wheat Straw Bio-refining. Part I: Optimization of the Microwave Radiation Process with Sulphuric Acid Pre-treatment

Background: This study investigates the microwave-assisted acid extraction of lignin from wheat straw. Methods: Two experimental design methods, the Central Composite Design (CCD) and the Response Surface Method (RSM) methods were used to select conditions and optimise the process. The parameters studied were: the stirring rate, H2SO4 concentration, microwave radiation power and residence time. Results: The optimum conditions found to achieve the maximum lignin yield (wt%) from wheat straw were: 0.46M H2SO4 under microwave radiation power of 602 W at 65% of stirring rate (3200rpm) and residence time of 39min. These conditions led to a delignification of 78.5% (by weight) of the parent lignin content, which accounted for 11.8wt% on dry biomass basis; this compares to a yield of 3.35wt% for the conventional H2SO4 pre-treatment method under similar conditions. Conclusion: This work is a first step to demonstrate the potential of innovative microwave methods to overcome the high cost of acid extraction of lignin from biomass waste.