Reaction System and Experimental Procedure
The reaction system consisted of a CO2 steel cylinder, a reaction (pyrolysis) unit, a liquid collection unit, a gas treatment unit, and a product gas analysis unit. The CO2 steel cylinder was used to supply CO2 as a carrier gas for providing a pyrolysis environment. The flow-rate of CO2 was controlled at 75 mL min−1 (25°C) by an electronic flow-rate controller (KD-4000) and a readout (Brooks 5850E). In the reaction system, both conventional and microwave-assisted heating were performed. In the experiments with conventional heating, the reaction unit comprised a quartz reaction tube (45 mm i.d.) and a tube furnace. The bagasse (20 g) was placed in the quartz tube and the tube was situated in the tube furnace. The schematics of the pyrolysis reactors with conventional heating and microwave-assisted heating are shown in Figure 1. In the experiments with microwave-assisted heating, the reaction unit was made up of a quartz tube (36.5 mm i.d.), a K-type thermocouple, a chamber, a magnetron, and a power controller. The biomass was packed at the bottom of the reaction tube wherein the thermocouple was installed at the center. The thermocouple was shielded by an alumina tube to prevent microwave absorption by any metallic part of the thermocouple (Chen et al., 2008b). A watt-hour meter was installed in the reaction unit to measure the inlet electricity of the unit at a frequency of 1 Hz. The supplied power was recorded by a recorder (computer). It should be illustrated that only part of the supplied power was delivered to the solid for heating. Charcoal was used as the microwave absorber. The charcoal was also grinded and sieved to particle sizes of 40–100 mesh (=0.12–0.45 mm). The bagasse (10 g) was blended with charcoal (1 or 3 g) and the reaction temperature was fixed at 550°C; that is, the absorber (or charcoal) blending ratio is 0.1 or 0.3. The liquid collection unit was composed of a conical flask for bio-oil collection. In the gas treatment unit, a condenser (at 5°C) and a dryer wherein the commercial silica gel pellets were packed were used to treat the product gas. Considering the gas analysis unit, a gas chromatography (SRI 310C TCD), and a gas analyzer (Fuji ZRJF5Y23-AERYR-YKLYYCY-A) were utilized. The volumetric concentrations of CO, CO2, and CH4 were detected by the gas analyzer, while the H2 concentration was measured by the gas chromatography.