Application of biomass fast pyrolysis part Ⅰ: Pyrolysis characteristics and products

摘要

In the present study, the thermal decomposition behavior of three common biowastes in Taiwan (cedar sawdust, coffee bean residue, and rice straw) upon fast pyrolysis was studied. Products were determined by gas chromatography-mass spectrometry. The composition of the resulting bio-oils was also quantified. TGA (thermogravimetric analysis) results indicate that thermal degradation of the biowaste samples occurs through four steps, namely, start of decomposition of extractives (＜490 K), hemicellulose decomposition (490-650 K), cellulose and lignin decomposition (650-780 K), and lignin decomposition (＞780 K). The temperature for thermochemical conversion was 700 K. Maximum rates of bio-oil production (51, 48, and 28 wt% for cedar sawdust, coffee bean residue, and rice straw, respectively) were observed when the flow rate of carbon dioxide was 30 L/min. Char production decreased with increased reaction temperature, and increased with the increase in flow rate of carbon dioxide. Hence, char production was highly correlated with fluidization of the fluid bed. Bio-oil contained two phases, namely, the oily phase from lignin and cellulose, and the aqueous phase from cellulose and hemicellulose. The water content of the oily phase was relatively low and consisted mainly of extractives and low- and high-molecular-weight lignins. Extractives included hexane-soluble compounds consisting mainly of hydrocarbons. The oily phase was composed of hexane-soluble aliphatic, aromatic, and polar fractions. The aqueous phases of bio-oils derived from the three biowaste samples had compositions that were very similar, and most contained significant amount of aromatics and oxygenated compounds such as car-boxylic acids, phenols, and ketones. When CO_2 was utilized as fluidization gas, the vented gas produced by pyrolysis of the three biowaste samples contained roughly 95.3 vol% CO_2, and biomass pyrolysis produced around 4.6 vol% CO_2. The vented gases had compositions that were similar, and consisted mostly of CO_2 (nearly 48.2-51.3 vol%) and CO (about 44.2 vol%).