One group of the pyrolysis by-products with no direct use besides burning is the condensable fraction of the pyrolysis gas that is referred to as pyrolysis oil

One group of the pyrolysis by-products with no direct use besides burning is the condensable fraction of the pyrolysis gas that is referred to as pyrolysis oil, bio-crude or bio-oil. It is a dark-brown, free-flowing liquor with a distinctive odor that consists of a complex mixture of up to 400 organic compounds (Evans and Milne, 1987; Huber et al., 2006). It is a potential feedstock for the production of energy, bio-fuels and chemicals. However, because of the wide range of components and its pronounced toxicity, thermal or catalytic upgrading is necessary to meet the high requirements for fuel and chemical production (Mohan et al., 2006; Cordella et al., 2012). Common treatment methods for bio-oil that are proposed in the literature focus on solvent separation to obtain fractions with similar polarities and to concentrate the un-distillable fraction (Mohan et al., 2006). However, these procedures require high amounts of organic solvents and increase the cost of the process convert it into a fuel by anaerobic digestion. The principal suitability of an anaerobic treatment has been reported previously for bio-oils from pyrolysis of wood (Andreoni et al., 1990), for the aqueous phase from pyrolysis of corn stalks (Torri and Fabbri, 2014) as well as for bio-oil from flash pyrolysis of wood (Willner et al., 2004). Similar materials that are susceptible to anaerobic digestion are process liquors from coal gasification (Cross et al., 1982) and hydrothermal carbonization of maize silage (Wirth and Mumme, 2013). Treating these process liquors by anaerobic digestion was generally found to reduce large parts of their organic fractions including hazardous compounds such as phenol. Integration of anaerobic digestion and pyrolysis offers further potentially synergistic combinations including use of digestate as feedstock for pyrolysis (Inyang et al., 2010), bio-methanation of syngas (Guiot et al., 2011) or use of biochars as additive in anaerobic digestion to overcome inhibition problems (Mumme et al., 2014; Torri and Fabbri, 2014). With the intention to further investigate the concept of integrated anaerobic digestion and pyrolysis, the overall aim of this study was to determine to which extent aqueous liquors from pyrolysis of digestate can be used as feedstock for biogas production. Further objectives were to characterize potential inhibitory effects on anaerobic digestion, to determine the efficiency with respect to COD reduction and methane production, to describe the impact of pyrolysis temperature on the degradability of the aqueous liquor, and to determine the removal rates for selected organic compounds.

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