Thermochemical conversion of wood to biofuel via a fast pyrolysis pathway is regarded as a promising alternative for producing biofuels. This process involves a quick thermal degradation of wood in the absence of air at a temperature of approximately 530OC with a short residence time of less than 1 second in the pyrolysis unit. Pyrolysis bio-oil produced has advantages over raw biomass as an energy carrier because bio-oil has a higher volumetric energy density and is more efficient to transport. Despite its potential, one of the major drawbacks of this approach for production of biofuels is the process energy intensity. Drying and size reduction of wood are major contributors to energy consumption and the development of a two stage process that involves a torrefaction pretreatment step prior to pyrolysis was investigated as an approach to minimize the energy consumption associated with the size reduction step.
Torrefaction, often referred to as mild pyrolysis, tends to enhance bio-oil properties by reducing water content, minimizing acidity, and increasing heating value. The impacts that different torrefaction temperatures have on the environment has been investigated by carrying out environmental life cycle assessment (LCA), and the two stage processes will be compared to the one stage pyrolysis.
The effect of torrefaction severity on composition and yield of pyrolysis bio-oil was included using data from the works of Westerhof et al (2012)3,Zheng et al (2012) 4, and Jones et al (2009) 1 while the work of Phanphanich et al (2011) 2was used for the effect of torrefaction on size reduction energy requirements. Data obtained from process simulation using Aspen Plus has served as inputs for the LCA that was be carried out using the LCA software SimaPro 8.0.
Our work will aim to look at how a torrefaction pretreatment step as well as how torrefaction severity as a result of different torrefaction temperatures will impact GHG emissions associated with the bio-oil production system. Previous results have shown that the cost of producing pyrolysis bio-oil from pine using a two-step torrefaction-pyrolysis process is reduced compared to one-step pyrolysis alone, mostly due to avoidance of high electricity costs for size reduction. Our LCA research will determine whether this same trend is observed for greenhouse gas emissions and cumulative energy demand over the lifecycle assuming pine harvesting, transport, and processing in the Southeast of the United States.
1 Susanne B Jones, Corinne Valkenburg, Christie W Walton, Douglas C Elliott, Johnathan E Holladay, Don J Stevens, Christopher Kinchin, and Stefan Czernik, Production of Gasoline and Diesel from Biomass Via Fast Pyrolysis, Hydrotreating and Hydrocracking: A Design CasePacific Northwest National Laboratory Richland, WA, 2009).
2 Manunya Phanphanich, and Sudhagar Mani, 'Impact of Torrefaction on the Grindability and Fuel Characteristics of Forest Biomass', Bioresource technology,102 (2011), 1246-53.
3 Roel J. M. Westerhof, D. Wim F. Brilman, Manuel Garcia-Perez, Zhouhong Wang, Stijn R. G. Oudenhoven, and Sascha R. A. Kersten, 'Stepwise Fast Pyrolysis of Pine Wood', Energy & Fuels,26 (2012), 7263-73.
4 Anqing Zheng, Zengli Zhao, Sheng Chang, Zhen Huang, Fang He, and Haibin Li, 'Effect of Torrefaction Temperature on Product Distribution from Two-Staged Pyrolysis of Biomass', Energy & Fuels, 26 (2012), 2968-74.
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