Biochar characteristics and early applications in anaerobic digestion-a review
Introduction
The sustainability of the current energy model is considered a key aspect for the development of society. As a result, an intense research activity is observed worldwide to support renewable sources to generate electric and thermal power with limited emissions. In this context, the intermediate conversion of solid biomass into liquid or gaseous products, through biochemical [1] or thermochemical [2] paths, is a reliable approach to increase energy density. Recently, the interest in waste to biogas conversion has been growing leading especially toward the design of highly integrated system concepts [3], mainly concerning the use of unconventional input materials. Thus, to improve the conversion yields, different kinds of pretreatment have been considered [4,5], such as the mechanical [6], chemical, hydrothermal and microwave/ultrasonic [7] ones. An interesting way to improve these system conversion efficiency is represented by adding external substrates and ecofriendly species. The positive effects of co-digestion on biogas yield are reported Table 1. Interesting results have been shown for poultry residues in municipal wastewater plants [8], manure and pre-treated lignocellulosic biomass [9], food wastes in municipal solid-liquid wastes [10,11], agroindustrial and slaughterhouse wastes [[12], [13], [14]].
It is clear that the anaerobic digestion (AD) process is one of the most favorable solutions with high energy value referring to different kinds of waste [22,23]. However, the digestion process of these substances, although environmentally friendly, is characterized by a low methane yield and may present some process instabilities [24,25]. Different issues, such as the very high biodegradability of some food waste, or the microorganisms dynamic may limit the process efficiency and inhibit severely the methanogenesis reactions. As an example, a high Inoculum-to-Substrate Ratio (ISR) is usually required in these cases to limit the acidification reactions [26]. While many and different technologies are available for the conversion of biomass to energy, rather low efficiency and a significant production of by-products still is a critical issue. The integration of different processes could be a noteworthy strategic approach for the development of a sustainable resources management strategy [27] to exploit synergistic effects in terms of energy recovery and residues production. This integration could extend the range of usable feedstock to biologically recalcitrant substrates (paper, woody materials, etc). This would allow for the use of a wider spectrum of biomasses as well as the use of refuse derived organic material toward the development of a sustainable circular economy. As an example, biochar has been widely proved to be effective for its capacity to absorb contaminants from the antibiotics residues [28], oily substances [29], pesticides [30] and metal ions [31,32] dispersed in water. In fact, in AD, the addition of supporting media to immobilize microorganisms [33] is also used to overcome some of the issues presented previously. Supporting materials have in fact a porous superficial providing an ideal substrate for the microorganisms to adhere, thereby enhancing the digestibility of municipal solid waste and shortening the digestion startup time [34]. To date, several works have been presented considering different supporting materials [[35], [36], [37]] to enhance methanogenesis. Biochar is then the perfect product in order to reach that aim. In fact, recent ideas for biochar utilization focus on the integration of thermochemical processes with anaerobic digestion to maximize the overall system efficiency both from the economic and environmental points of view. As schematically reported in Fig. 1, coupling thermochemical (TH) processes such as pyrolysis, gasification and hydrothermal carbonization (HTC) with anaerobic digestion (AD) [38] has been increasingly considered [39,40]. To date, several works have been presented in literature [[41], [42], [43], [44], [45]]. In particular, the behavior of pyrolytic and HTC biochars in AD [[46], [47], [48]] are currently under investigation for their effects on mild ammonia inhibition, support of archaeal bacteria growth and reduction of acids conditions [41,43]. In fact, several well-known issues regarding substrate induced inhibitors and direct inhibitors, can be partially solved and optimized by using biochar [49,50]. Nevertheless, the biochar application in AD is still not understood fully, and thus there is a need for the development of innovative research strategies with specific reference to the modeling of the fundamental processes. Firstly, in order to evaluate the main process interaction drivers, a complete biochar characterization will be carried out. The biochar characteristics such as composition, morphology and the pore distribution vary in fact according to the selected feedstock materials and process parameters. Literature experimental data are key to evaluate the biochar influence on the AD and to provide the necessary data and information for further applications. The influence of the characteristics abovementioned can be then investigated with respect to the standard AD process output. This step is crucial in defining the performance enhancement in terms of process efficiency that the biochar specifically can make to traditional processes. This increase can therefore be achieved either by purchasing biochar from specific production plants, thus opening a biochar market and, on the other side, to the possibility of implementing highly integrated solutions in smart grids. Thermochemical and biochemical processes operating in the same direction can then reduce conversion inefficiencies, such as by integrating not only products but energy fluxes as well. As a matter of fact, that application can enable a scale down of the existing technologies by means of interesting hybrid thermochemical-biochemical configurations. Then, given the wide interest in coupling technologies, biochar fundamental characteristics and early results of recent applications of using such materials in AD processes are presented and discussed in this paper.
Section snippets
Biochar characterization
Biochar is obtained by heating up residues of biomass with null or very low oxygen concentration [51]: equivalent ratios are typically lower than 0.25 [52] and temperature is in the range of 180–950 °C, preventing biomass combustion and promoting the formation of biochar. The main applications concern its use as soil amendment [53], for waste management, carbon storage [54] and lastly as a fuel. Over the years, biochar has been applied to other processes for its low production costs, taking
Biochar in AD: case studies
The biochar specific characteristics are of paramount importance for the processes integration design criteria. The specific biochar analysis has not been performed so far from that perspective. SSA, PV EC, CEC are still marginally used, and the authors comment that such analyses should be considered as routine for the next studies. Regressions proposed for the anaerobic digestion data analysis are reviewed first. To date, with different levels of complexity, many models have been proposed to
Conclusions
An analysis on biochar effects in coupling thermochemical and biochemical processes has been done with an analysis of literature available data and experimental evaluation of the behavior of on a specific substrate. The main obtained result are the following:
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The biochar has been identified as an effective key to improve system efficiency
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Different types of biochar have been analyzed in terms of their characteristics referring to different processes;
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The slow pyrolysis is the best one by surface
Acknowledgments
The authors would like to acknowledge Mr.Giulio Frezza for his valuable contribution in this paper. Moreover, the author Fábio Codignole Luz would like to thank CSF/CAPES (BEX-11965/13-4) – Brazilian Federal Agency for Support and Evaluation of Graduate Education within the Ministry of Education of Brazil.
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