Decomposition behaviors of bamboo, chinquapin (hardwood), and Japan cedar (softwood) in hotcompressed water (HCW) were examined using a HCW flow type reactor to achieve the maximum utilization of plant biomass. HCW was passed through the reactor (3.6 mL) loaded with the biomass sample powder (177-250 µm) at a flow rate of 10 mL/min under 9.8 MPa. The temperature of HCW was maintained at 180 °C for 20 min, after which it was raised to about 285 °C at a rate of 5 °C /min and maintained at 285 °C for about 7 min. By these operations, free sugars, some of lignin and most of hemicellulose, were first solubilized in water and flowed out during the initial 20 min flow of HCW. Cellulose started to decompose when the temperature of HCW was over 230 °C. The orders of elution of these components were common for three biomass samples, although the amount of products obtained during the heat-up period up to 180 °C and that of residue which finally remained in the reactor differed with the samples.
Background: Lignocellulosic biomass such as wood is an attractive material for fuel ethanol production. Pretreatment technologies that increase the digestibility of cellulose and hemicellulose in the lignocellulosic biomass have a major influence on the cost of the subsequent enzymatic hydrolysis and ethanol fermentation processes. Pretreatments without chemicals such as acids, bases or organic solvents are less effective for an enzymatic hydrolysis process than those with chemicals, but they have a less negative effect on the environment.
The noncatalytic decomposition characteristics of cellulose in near-critical water were examined by heating a sealed reactor in which the cellulose and water were charged in a salt bath kept at 305, 355, or 405 °C. Cellulose was rapidly decomposed to water solubles (WS), and the WS was further decomposed after the WS yield reached nearly 80%. The heating time giving the maximum WS yield was shortened to under 15 s by increasing the treatment temperature to over 355 °C. In the WS formation process, hydrolysis preferentially occurred, and the glucose yield reached 40% by the treatment for 15 s in the bath kept at 355 °C. On entering the second decomposition process, the WS was converted to gaseous products and methanol-soluble products, and charlike solid products were formed from the liquid phase. The hydrolysate of cellulose obtained in the WS formation process was subjected to a fermentation test, and the formed glucose was confirmed to be converted to ethanol.
Cellulose was hydrolyzed with hot-compressed water (HCW; ∼310 °C, 9.8 MPa) using an HCW-flow reactor. HCW was continuously delivered into a reactor charged with cellulose. The effluent
from the reactor was cooled and separated into a water-soluble fraction (WS) and a water-insoluble fraction which deposited after cooling (DP). Cellulose started to decompose into WS
and DP when HCW was delivered above 230 °C. The main components of the WS were hexose
and oligosaccharides ranging from a dimer to a pentamer, and the DP consisted of polysaccharides
ranging from a hexamer to an eicosamer or more polymerized saccharides. When HCW was
delivered at 295 °C, nearly all of the cellulose decomposed after a 12-min flow of HCW, and 81
wt % of WS and 18 wt % of DP were obtained. The formation rates of WS and DP increased as
the temperature of HCW increased. However, the composition of WS and DP barely changed
with the HCW temperature under around 280 °C, which corresponds to the softening point of
the cellulose. When the HCW temperature was further increased, the depolymerization of the
products proceeded. The decomposition rate of cellulose itself was not affected by the flow rate
of HCW, but the depolymerization of the products was suppressed when the flow rate of the
HCW was increased.
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