(519b) Biorefinery: Liquefaction of Pyrolysis Char to Biofuel

Lignocellulosic biomass is the most abundant renewable material [1] in the world with coverage of 95% of all plant biomass [2] which makes it a feasible feedstock for chemical conversion to biofuels. For successful substitution of fossil fuel there are still obstacles to overcome. State of the art technologies of the petrochemical and chemical industry are not directly applicable for converting biomass to biofuels. This is due to the fact that biomass is a highly oxygenated raw material. Behrendt et al. specify the chemical composition of dry wood as CH_1.4O_0.7 [3] figuring out nearly one atom of oxygen per carbon atom, underlining the indispensable need of hydrogen for converting lignocellulosic biomass to biofuels. By these means simple and cheap pre-treatment is needed to eliminate oxygen in the feedstock. Liquid-phase pyrolysis is a feasible route for thermo-chemical elimination of oxygen from biomass. Products of liquid-phase pyrolysis are (1) non-condensable gases, (2) water, (3) pyrolysis oil of poor combustion properties and (4) biochar with an amount of 40wt% on dry biomass feed basis. The formation of liquid-phase pyrolysis products was investigated and reported [4], [5].

Biochar is a major product of pyrolysis processes, such as torrefaction and hydrothermal carbonization [6], [7]. Biochar from liquid-phase pyrolysis mainly consists of biogenous carbon with an approximate amount of 99 wt% [4]. This makes biochar a considerable feedstock for direct liquefaction in order to produce biofuels. In principle the direct liquefaction of biochar may well base on the experience from fossil coal liquefaction with hydrogen and/or hydrogen donor solvents at elevated temperature and pressure. The products of direct coal liquefaction are highly aromatic and suitable for usage as high-octane gasoline or feedstock for aromatic chemicals [8].

During coal liquefaction, while thermal decomposition ruptures the coal bonds, hydrogen from the gaseous phase or from a hydrogen donor solvent stabilizes the fragments and eliminates heteroatoms like oxygen. Stabilization ensures the breakdown of macromolecules into smaller hydrocarbon fractions. At low hydrogen availability fragments are not stabilized and will undergo re-polymerization reactions to form tar [9].

Liquefaction of biochar was investigated in a stirred 450ml batch reactor between 370°C and 425°C. The feed product was obtained from pilot scale liquid-phase pyrolysis from BDI (OMV refinery, Vienna). Biochar was extracted with n-hexane, dried and milled to <200microns. For each experiment 30g of dried biochar batches, mixed with tetralin, were prepared. The mixture was fed to the reactor and pressurized with 50bar of hydrogen. Temperature was elevated from ambient to final operation temperature with a heating rate of 15 °C/min. In order to distinguish between the contribution of different process phases to conversion and product formation the heating period was investigated separately prior to investigation of the isothermal phase. Reaction time for the isothermal stage was limited to 30min. Experiments were conducted with continuous hydrogen supply at two hydrogen pressure levels. Upon completion of the experiment the heating jacket was removed and the reactor was cooled with compressed air. The gas amount as well as the gas composition were determined when reactor pressure was relieved.

The lumped parameters gas (G), oil (O), asphaltenes (A), pre-asphaltenes (PA) and residue (R) were chosen for evaluation of the progress of liquefaction. The parameters were calculated from the results of sequential extraction of filtered liquefaction residues after each experiment. The liquid product phase was subjected to GC-MS, GC-FID, and elemental Analysis as well as size exclusion chromatography (SEC) in order to generate data for product quality, product formation, donor-solvent consumption as well as product size distribution.

Experiments with hydrogen donor at 425°C resulted in biochar conversion (C) of 84% and an oil yield of 72% after 30min of isothermal reaction, with the initial heating phase contributing to biochar conversion (C) to an extent of 37%. Biochar is converted into oil (O), gas (G), pre-asphaltenes (PA) and asphaltenes (A). Pre-asphaltenes and asphaltenes are still high molecular intermediates, formed during conversion of biochar [8], [10]. Liquefaction of biochar at greater extent starts at 415°C.

[1]       X. Zhao, K. Cheng, and D. Liu, “Organosolv pretreatment of lignocellulosic biomass for enzymatic hydrolysis.,” Applied microbiology and biotechnology, vol. 82, no. 5, pp. 815–27, Apr. 2009.

[2]       R. Rinaldi and F. Schüth, “Design of solid catalysts for the conversion of biomass,” Energy & Environmental Science, vol. 2, no. 6, p. 610, 2009.

[3]       F. Behrendt, Y. Neubauer, M. Oevermann, B. Wilmes, and N. Zobel, “Direct Liquefaction of Biomass,” Chemical Engineering & Technology, vol. 31, no. 5, pp. 667–677, May 2008.

[4]       N. Schwaiger, R. Feiner, K. Zahel, A. Pieber, V. Witek, P. Pucher, E. Ahn, P. Wilhelm, B. Chernev, H. Schröttner, and M. Siebenhofer, “Liquid and Solid Products from Liquid-Phase Pyrolysis of Softwood,” BioEnergy Research, vol. 4, no. 4, pp. 294–302, Jun. 2011.

[5]       N. Schwaiger, V. Witek, R. Feiner, H. Pucher, K. Zahel, A. Pieber, P. Pucher, E. Ahn, B. Chernev, H. Schroettner, P. Wilhelm, and M. Siebenhofer, “Formation of liquid and solid products from liquid phase pyrolysis.,” Bioresource technology, vol. 124, pp. 90–94, Nov. 2012.

[6]       D. Ciolkosz and R. Wallace, “A review of torrefaction for bioenergy feedstock production,” Biofuels, Bioproducts and Biorefining, vol. 5, pp. 317–329, 2011.

[7]       A. Funke and F. Ziegler, “Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering,” Biofuels, bioproducts and biorefining, vol. 4, no. 2, pp. 160–177, 2010.

[8]       T. Kaneko, F. Derbyshire, E. Makino, D. Gray, and M. Tamura, “Coal Liquefaction,” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH, 2005, pp. 1–83.

[9]       M. W. Haenel, “Catalysis in Direct Coal Liquefaction,” in Handbook of Heterogeneous Catalysis, Wiley-VCH Verlag GmbH, 2008, pp. 3023–3036.

[10]    X. Li, H. Hu, S. Zhu, S. Hu, B. Wu, and M. Meng, “Kinetics of coal liquefaction during heating-up and isothermal stages,” Fuel, vol. 87, pp. 508–513, Apr. 2008.