Rheological Characterization of Bio-Oils from Pilot Scale Microwave Assisted Pyrolysis

Abstract

Renewable energy is gaining importance in satisfying environmental concerns and addressing economical concerns over fossil fuel usage. Lignocellulosic materials are the most abundant renewable resources on earth (Lynd et al., 2005). Energy can be obtained from biomass either biochemically or thermochemically. In the biochemical process, pretreatment of biomass is a necessary and the first step in opening up structure of the biomass cell wall to permit the access of enzymes to cellulose and hemicellulose. Pyrolysis, gasification, and combustion are the three main thermochemical processes to get energy from biomass. Combustion has a maximum efficiency of more than 30% (Yu et al., 2007). Because gasification offers higher efficiency compared to combustion, it has attracted a high level of interest (Bridgwater, 2004). According to Wornat et al (1994), the burning of bio-oils produced through the pyrolysis of biomass is more efficient. Bio-oil also offers advantages in storage and transport and in its versatility as an energy carrier and as a source of chemicals (Bridgwater, 2004). The thermochemical process can convert a low-carbohydrate or non-fermentable biomass to alcohol fuels, thus adding technological robustness to efforts to achieve the 30 x 30 goal. Pyrolysis is an endothermic reaction wherein thermal decomposition occurs in the absence of oxygen. It is always the first step in gasification and combustion, wherein partial or total oxidation of the substrate occurs. Gas is the main product (85%) in gasification, whereas biooil (70-80%) is the main product in most types of pyrolysis. The yield of pyrolysis products such as syngas/ producer gas (mixture of CO and H2), bio-oil, and bio-char (charcoal) would vary depending upon the pyrolysis methods (conventional, fast, vacuum, flash, and ultra), biomass characteristics (feedstock type, moisture content, particle size), and reaction parameters (rate of heating, temperature, and residence time). Bridgwater (2003) listed four essential features to get bio-oil from fast pyrolysis: very high heating rates (1000°C/s), high heat transfer rates (600-1000 W/cm2), short vapor residence times (typically <2 s), and rapid cooling of pyrolysis vapors and aerosols. Because the heart of a fast pyrolysis process is the reactor, during the last two decades several different reactor designs to meet the rapid heattransfer requirements have been explored. Achieving very high heating and heat transfer rates during pyrolysis usually require a finely ground biomass feed. Pyrolysis using microwave irradiation is one of the many ways of converting biomass into high value products and chemicals. Not only does microwave assisted pyrolysis (MAP) not require a high degree of grinding (e.g., large chunk of wood logs can be used) as required in