Integration of pyrolysis and anaerobic digestion – Use of aqueous liquor from digestate pyrolysis for biogas production
Graphical abstract
Introduction
The use of biomass as a renewable resource for energy and various biomaterials gains more and more attention. A currently vividly discussed use of biomass is the production of biochar, which is among other applications considered as soil amender and for carbon sequestration. Biochar is a charcoal-like material usually produced by pyrolysis of biomass, where organic material is transformed into a carbon-rich material under the influence of high temperatures and limited oxygen supply (Lehmann and Joseph, 2009). Using charcoal for improving soil fertility is an old technique that was already used by indigenous civilizations living in the amazon basin centuries ago (Woods and McCann, 1999). In this region archaeologists found untypical black soils that contained very high amounts of stable organic matter and high amounts of nutrients compared to the typically infertile clay soils of the tropics. This so called ‘terra preta’ was discovered to be of anthropogenic origin which was created by mixing soil with charcoal and residues of burned biomass (Glaser et al., 2001). Presumably the indigenous people of the amazon basin already knew how to produce charcoal by pyrolysis of organic material in clay furnaces (Basu, 2013). Today, this knowledge about the soil amending properties of highly carbonaceous materials is successively recovered.
Despite its ancient history, pyrolysis as a process and technology is still a subject of ongoing research and development. The goals are to improve the material properties of chars for use as soil amender or other material uses, to enable the use of complex feedstocks, to improve the energetic and economic efficiency, and to reduce the production and release of unwanted by-products (Manyà, 2012). One group of by-products with no direct use besides burning is the condensable fraction of the pyrolysis gas known as pyrolysis oil, biocrude 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 undistillable fraction (Mohan et al., 2006). However, these procedures require high amounts of organic solvents and increase the cost of the process.
An approach to utilize the aqueous phase of pyrolysis oil is to 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), biomethanation 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.
Section snippets
Origin and properties of pyrolysis liquor and anaerobic inoculum
The digestates used as feedstock for pyrolysis and as inoculum for anaerobic digestion were both obtained from an on-farm biogas plant (Hof Karp, Rastow, Germany). The biogas plant operates at mesophilic temperature at an organic load rate (volatile solids basis) of 5.72 kg m−3 d−1 feeding cow manure and maize at a ratio of 4:3. For pyrolysis, the raw digestate was dewatered on site by a press screw separator and belt dryer (70 °C, 8 h). A batch of 20 kg of this solid digestate was retrieved and
Inoculum quality, process start-up and inhibition
The samples’ methane production courses of both experimental runs are presented in Fig. 1. The two inoculum-only controls showed a low but steady gas production resulting in a total methane volume of 31.2 ± 1.1 mL (first run) and 16.6 ± 0.2 mL (second run). Thus, no signs of inoculum-based inhibition could be found, only a marginal methane potential was detected and the statistical accuracy was high. Therefore, it be can concluded that the inoculum provided good conditions for the fermentation
Conclusion
This study showed that aqueous pyrolysis liquors from pyrolysis of digestate can be converted into biogas without any additives and by an un-adapted inoculum. CODs initial concentrations up to 12 g L−1 were tolerated and CODs removal rates up to 63% and methane yields up to 220 L gCODs−1 were observed. Low temperature pyrolysis (330 °C and 430 °C) allows a more complete CODs and TOCs degradation than higher temperatures. All identified VOCs except cresol were degraded below the detection limit
Acknowledgements
The research conducted by the authors was supported by Grants commissioned from the German Federal Ministry of Research and Education to Project Management Jülich (PtJ). The authors would like to express their gratitude to Ulf Lüder for technical assistance, Laureen Herklotz for chemical analysis and the staff of APECS for cooperation and help. Furthermore, they like to thank Dr.-Ing. Joachim Brummack (Technical University of Dresden) for advice and Christian Karp (Hof Karp, Rastow) for
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