CO2-pressurizing reactor for saccharification of Ca(OH)2-pretreated herbaceous lignocellulosics

Sugar platform for lignocellulosic feedstock is greatly expected to provide environmentally-friendly processes for production of fuels and chemicals, whereas breakthroughs could still be needed to overcome barriers for the commercialization. For fermentable sugar production from strategic herbaceous feedstock in Japan: rice straw and domestic cellulosic energy crops, we proposed a novel process with an alkali pretreatment, termed the CaCCO (Calcium Capturing by Carbonation) process; a slurry of Ca(OH)₂-pretreated feedstock is carbonated for pH neutralization to convert calcium ions to CaCO₃, precipitating in the vessel. This process omits a rinsing step of pretreated fibers so as to keep not only small fibrous particles but also liberated polysaccharides such as xylan and starch in the vessel, resulting in a higher yield of solubilized sugars after enzymatic saccharification compared to the corresponding process including a rinsing step. We, however, found that the pH in the vessel after CO₂-neutralization (~pH 6.5) was significantly far from the optimum pH of main enzymes needed for saccharification (~pH 5), which resulted in a low performance of the enzymes. Therefore, we attempted to further pressurize with CO₂ for lowering the pH and confirmed that the pH of the neutralized slurry could be reduced below 6.0 at 10 atm, increasing the saccharification ratio for both glucans and xylan. We also introduced a 100 L CO₂-pressurizing saccharification reactor for pilot scale demonstration tests. In this paper, the effectiveness of the CO₂-pressurizing saccharification is also discussed from the viewpoints of substrate concentrations and enzyme loadings.