The effect of torrefaction on the chemistry of fast-pyrolysis bio-oil
Highlights
► Torrefaction pretreatment alters the thermo-chemical properties of Loblolly pine. ► Pyrolysis bio-oil from torrefied biomass features reduced oxygen-to-carbon ratio. ► Pyrolysis bio-oil from torrefied biomass features reduced water content. ► Pyrolysis bio-oil from torrefied biomass has highly concentrated lignin compounds.
Introduction
Thermochemical conversion of lignocellulosic biomass has received increasing attention as a strategy to produce biofuels from renewable resources. Fast pyrolysis, one of the promising thermochemical approaches, rapidly converts organic biomass into bio-oil, bio-char, and syngas in the absence of oxygen. With a residence time of less than two seconds, this simple and low-cost technology can transform various feedstocks such as agricultural and forest residues into value-added biofuels and chemicals. In addition, biofuels derived from renewable biomass contributes a positive effect on greenhouse gas emission as compared with petroleum-based transportation fuels (Reijnders, 2006). Because of these advantages, fast pyrolysis is thought to have great potential as an avenue for replacing petroleum-based fuels and chemicals.
Although fast pyrolysis has been studied over the last few decades, several unfavorable properties of its liquid product hinder its application to the production of value-added fuels and chemicals (Czernik and Bridgwater, 2004). First, owing to its high oxygen content (∼40%) (Mohan et al., 2006), pyrolysis bio-oil has a relatively high chemical reactivity. For example, bio-oil components such as acids and aldehydes can engage in several types of reactions to form esters and other oligomers, resulting in higher molecular weights (Diebold, 2002). Therefore, increased viscosity and phase separation often occur to the bio-oil during storage. Second, the bio-oil is also plagued by high acidity (pH ∼2) and volatility, which also lead to numerous application challenges. In short, fresh bio-oil is not suitable for direct application without stabilization and reduction of oxygen concentration in it.
Several upgrading technologies have been proposed to reduce the oxygen content of bio-oil, usually involving catalytic hydrotreating and hydrocracking (Elliott and Hart, 2009, Sharma and Bakhshi, 1993). Unfortunately, these approaches require high hydrogen consumption and their overall yield of high quality bio-oil is low (Elliott, 2007). To reduce high hydrogen demand during catalytic upgrading, fast pyrolysis on biomass having low oxygen content is a promising alternative as it should produce bio-oil with low oxygen content.
A potential method of reducing the oxygen content of biomass feedstock is through torrefaction. A mild pyrolysis process, torrefaction, occurs at 200–350 °C and is valued as an effective way to improve the thermochemical quality of biomass in its application with coal co-combustion (Prins et al., 2006a, Zwart et al., 2006, Couhert et al., 2009). Most importantly, this mild thermal pretreatment reduces the oxygen content significantly by losing water, carbon dioxide, and carbon monoxide (Yan et al., 2009). In addition, the removal of acidic components like acetic acid originating from acetoxy groups in the xylan side chain has been reported (Yan et al., 2010, Prins et al., 2006b, Bourgois and Guyonnet, 1988). As another advantage, the specific grinding energy of torrefied pine chips can be reduced to 30 kW h/t compared to 260 kW h/t of untreated pine chips (Phanphanich and Mani, 2011). It is also reported that torrefaction can reduce the size distribution of particles, where evenly sized wood particles can be utilized for the process (Repellin et al., 2010). All these advantages are potentially beneficial to the production and quality of pyrolysis bio-oil.
To the best of our knowledge, torrefaction as a pretreatment technology for fast pyrolysis to enhance bio-oil properties has not yet been reported. The work presented here puts forward an idea of integrated torrefaction and fast pyrolysis processes. It is expected that under suitable combinations of torrefaction and pyrolysis conditions, pyrolysis bio-oil with enhanced physical and chemical properties can be produced from thermally treated biomass. The objective of this study, therefore, was focused on comparing fast pyrolysis of torrefied wood with that of non-treated wood by examining the change of the bio-oil chemistry.
Section snippets
Torrefied biomass production
Loblolly pine chips (15 by 6 mm) containing bark were used as the raw material for torrefaction. A pilot-scale torrefaction system at North Carolina State University was operated to produce torrefied wood at three different severities, which were produced at 270, 300, and 330 °C for 2.5 min respectively and referred as LP-TA, LP-TB and LP-TC in this study. The raw wood without torrefaction was named LP-Raw and the torrefaction severity increases in the series from LP-TA to LP-TC. The severity of
Torrefied wood characterization: compositional, proximate, and ultimate analyses
Loblolly pine derived torrefied wood is considered to have a similar composition as its original wood with respect to cellulose, hemicellulose, and lignin, but the amount of each component is expected to be different depending on the torrefaction conditions. To investigate the effect of torrefaction on the composition of loblolly pine, klason lignin and sugar analysis were performed on the torrefied wood. However, klason lignin and acid soluble lignin are named in Table 1 as acid insoluble
Conclusions
Pyrolysis bio-oils produced from torrefied loblolly pine feature reduced oxygen-to-carbon ratios and water content, which are beneficial for downstream processes such as hydrotreating and hydrocracking. However, these reductions are realized with the penalty of bio-oil yield due to the increased thermal resistance of torrefied biomass at the same pyrolysis conditions. Based on the GC/MS analysis, the chemical composition of bio-oils produced from torrefied biomass shows a high quantity of
Acknowledgements
Authors would like to thank Chris Hopkins at North Carolina State University for torrefaction operation to produce torrefied wood and Steve Kelley for his valuable comments during the work. This work is supported by the National Science Foundation under Grant No. 0832498.
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