Effects of Various Reactive Gas Atmospheres on the Properties of Bio-Oils Produced Using Microwave Pyrolysis

Fast pyrolysis of lignocellulosic biomass produces organic liquids (bio-oil), biochar, water, and noncondensable gases. The noncondensable gas component typically contains syngas (H-2, CO, and CO2) as well as small hydrocarbons (CH4, C2H6, and C3H8). To understand the influence of reactive gas in various pyrolysis processes, we have employed a laboratory scale microwave reactor and performed pyrolysis of switchgrass under varying gaseous atmospheres and characterized the bio-oils obtained. The batch (100 g of biomass) microwave pyrolysis was performed at 900-1000 W over the course of 7 min in the presence of a microwave absorber (10 g of activated charcoal). The products formed were quantified and the bio-oils were characterized by GC-MS, elemental analysis, Karl Fischer and TAN titrations, bomb calorimetry, and C-13 NMR spectroscopy. Pyrolysis experiments performed under a N-2 atmosphere were used as the control and then compared to experiments performed under various reactive gases (CO, H-2, and CH4) and a model pyrolysis gas mixture ("PyGas"). The use of a CO atmosphere had a negligible effect on the quantity and quality of bio-oils produced, whereas the use of H-2, CH4, and PyGas atmospheres each provided more deoxygenated products (i.e., BTEX, naphthalenes, etc.) and lower oxygen content. The use of different particle sizes also displayed a pronounced effect on the product distribution and the composition of the bio-oils obtained.