Application of Underwater Discharge Shock Wave to Pretreatment for Enzymic Saccharification of Wood Flour

In recent years, biomass energy has been attracting attention as a stockable renewable energy. Bioethanol, a kind of biomass fuel, is produced from plants. Especially, the bioethanol production from woody biomass shows its importance because it does not compete with food problems. Woody bioethanol is produced by saccharifying, fermenting, and distilling cellulose fibers in wood. However, it is difficult to saccharify wood directly because cellulose has a large amount of hydrogen bonds and is physically and chemically robust. Among the saccharification methods, the enzymic saccharification is widely used owing to less hyperdecomposition than the other methods, whereas it requires a long reaction time. In the enzymic saccharification of wood, pretreatment is performed to increase the saccharification efficiency. Various pretreatments have been studied, but many of them requires improvement in terms of cost or effectiveness. We propose an underwater discharge shock wave treatment as a new pretreatment for wood saccharification as an alternative to the existing methods. The hydrogen bond between celluloses is broken by the shock wave, and the water molecules contact the cellulose surface more widely, thus, the saccharification efficiency of wood is expected to increase by expanding the action area of the enzyme.<br/><br/>The underwater discharge was generated between needle electrodes with a gap distance of 2 mm, placed in an acrylic container filled with cedar wood flour dispersed water. A pulsed high voltage was applied to one of the needle electrodes through a spark gap switch after charging a capacitor from a high-voltage DC power supply via a 4 MΩ resistor. The cedar wood flour was treated by the underwater discharge generated typically at an applied voltage of 15.5 kV, a repetition rate of 3 Hz, and the discharge energy of 2.3 J/pulse for 10 minutes. The diameter size of cedar wood flour was about 100 µm. The treatment effect was evaluated by Water Retention Value (WRV), which represents the amount of water molecules bound to cellulose inside the flour, and the glucose concentration obtained of enzymic saccharification of the treated wood flour. We also estimated the pressure of the underwater discharge shock wave.<br/><br/>The underwater discharge shock wave treatment as a pretreatment increased the enzymic saccharification efficiency of wood flour. The glucose concentration obtained after 1 day of enzyme saccharification with pretreatment increased up to 1.52 times as compared with the untreated case, and the WRV increased by 13%. It is reasonable to consider that the effect of underwater discharge shock wave treatment on the enzymic saccharification of wood flour depends on the shock wave intensity and the treatment time. The shock wave intensity was evaluated by the shock wave pressure, which was estimated from the shock wave velocity using Rankine-Hugoniot equation. The shock wave velocity was obtained from the spatio-temporal variation of the laser beam intensity transmitting through the liquid. The shock wave pressure was measured to be several MPa to 25 MPa depending on the discharge condition. The treatment time, in other words the number of discharge pulses for the treatment, was evaluated by varying the frequency of the underwater discharge. We found that the enzymic saccharification of wood flour by the present pretreatment was increased with increasing the discharge frequency and was not affected strongly by the shock wave pressure within the present experimental condition. These results suggest that the shock wave pressure intensity in this experiment exceeds the threshold value required for processing, and further shock wave intensity does not strongly improve the pretreatment effect. In contrast, the shock wave with intensity higher than the threshold gradually changes the structure of wood flour with treatment accumulation, and eventually improves the enzymic saccharification.