Biomass gasification consists of two processes: a fast pyrolysis step and a slow rate limiting step of gasification. Due to the relative rates at which pyrolysis and gasification occur, these steps can be considered to occur in series and can thus be studied separately. Our previous work has shown that the pyrolysis variables (pressure, temperatures, heating rate, and residence time) affect the gasification rate of chars formed. In literature, gasification is usually performed in pure H2O or CO2. However, in an actual gasifier, the chars are exposed to a complex mixture of CO2, H2O, CO, H2 and CH4. This work aims to understand the effect of mixtures of CO2 and H2O on the gasification reactivity of switchgrass chars.
Switchgrass (sieved to 180-250 µm) was pyrolyzed in an entrained-flow reactor at different temperatures (600-1000 °C), pressures (5-20 bar) and residence times (3-30s). The heating rate during pyrolysis has a dramatic effect on the char morphology and other physical characteristics which are likely to affect gasification behavior. The entrained flow reactor used here can achieve heating rates which are close to large scale gasifiers (~103-104 K/s), and thus the chars generated in this work represent those likely to be formed in industrial gasifiers. Chars were characterized using several techniques: SEM, N2 and CO2 adsorption, ultimate analysis, ICP, XRD, and Raman spectroscopy. Pyrolysis gases were analyzed using micro-GC and tars were analyzed using GCxGC-TOFMS.
Switchgrass char formed in the pressurized entrained flow reactor (PEFR) was gasified in a pressurized thermo-gravimetric analyzer (PTGA) under transport-free conditions in the presence of CO2, H2O, CO, and H2. The Langmuir-Hinshelwood (L-H) kinetic rate expression was found to fit the rate data very well. As expected, the presence of CO in the gas-phase has an inhibiting effect on the gasification rate. A much greater inhibiting effect is observed due to the presence of H2 (syngas product gas) in the gas stream.
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