Pyrolysis of the anaerobic digestion solid by-product: Characterization of digestate decomposition and screening of the biochar use as soil amendment and as additive in anaerobic digestion
Graphical abstract
Introduction
Anaerobic digestion (AD) constitutes a competent and environmental-friendly process for production of energy. During AD an organic material is converted through a complicate microbial fermentation process occurring in anaerobic conditions into a gaseous fuel, namely biogas and a by-product (digestate) containing a relatively small amount of solids between 3.5 and 13 % [1]. In recent years, AD has attracted significant attention and a number of biogas plants has been constructed leading to production of digestate quantities that cannot be depleted at a local level. It is indicative that more than 10.000 t/year of digestate containing 10 %wt. of solids can be produced by merely a single 500 kW biogas plant [2]. The amount of unexploited digestate together with the logistics, specifying that its transportation becomes economically unfavourable for distances longer than 10 km [2], suggest that novel exploitation alternatives should be sought for the AD solid by-product.
Digestate has been used in agriculture and horticulture as an organic fertilizer and soil conditioner [3]. However, due to the diverse materials frequently utilized in a digester as feedstock digestates contain varying proportions of macro- and micro- nutrients, which causes instabilities in their fertilization capability [4]. Besides, a direct disposal of AD by-product on land increases the risk for pathogens, odours and greenhouse gases release [5], whereas the potential presence of non-stabilized organic matter in the material enhances the microbial activity in soil, which results in oxygen depletion and nitrogen immobilization having adverse effects on digestate’s fertilization value [6]. To mitigate the instability of the waste and minimize the effect of the contained pathogenic microflora on the environment a number of regulations have been established imposing a complementary stabilization and neutralization before its land deposition. A post-digestion treatment of digestate, however, has negative consequences on the economics of the overall AD.
Pyrolysis is the high-temperature thermochemical process that converts under non-oxidizing conditions a material to a carbonaceous solid (biochar), bio-oil, and non-condensable gas and could provide an excellent pathway for digestate neutralization and exploitation [7]. From the pyrolysis products, biochar can be used in several environmental applications, syngas is converted into heat or electricity (combined heat and power, CHP) and bio-oil can be exploited as a fuel or added in petroleum refinery products [8]. Earlier studies [9], [10] demonstrated that utilizing pyrolysis for the treatment of a digestate and valorising the gas and liquid side products increased the overall efficiency of an AD plant by 42 % in terms of electricity production. Distribution and physicochemical properties of the pyrolysis products depend on the applied thermal conditions and the properties of feedstock [11]. Thermogravimetric analysis has been broadly employed to analyse the phenomena occurring during the degradation of a variety of materials including agricultural and aquatic biomass and wastes over a wide temperature region, frequently between atmospheric to 900 °C, taking into account the thermal stability and conversion of the individual components of the material [11], [12], [13]. A systematic investigation of the degradation behaviour of a digestate via thermogravimetry, therefore, is required for a better determination of the conditions that could lead to the generation of a great quantity of biochar with desired characteristics.
Currently, pyrolysis has been employed for the treatment of digestate with the aim to produce biochar for soil applications [8]. Temperature is amongst the pyrolysis conditions that significantly influence biochar properties and define its ability for soil revamping [12], [15]. In principle, a biochar with an outstanding soil amendment potential is produced at high temperatures. However, there are cases of biomass materials that deviations from this behaviour were observed. For instance, pyrolysis of lignocellulose at temperatures above a threshold of 700 °C had a negative effect on the soil amelioration ability of the produced biochar due to a loss of micro-porosity [16], [17]. Digestate materials have different physicochemical properties than lignocellulosic biomass and hence pyrolysis temperature might have a different impact on the characteristics of produced chars. Physicochemical properties of digestate chars generated at various temperatures were determined in some earlier investigations. Stefaniuk and Oleszczuk [14] focused on the pyrolysis of three different types of digestate at temperatures between 400 and 800 °C and concluded that many of biochar properties including the pH, electric conductivity, aromaticity, ash, contained macro- and micro-nutrients, polarity etc. depended mostly on the pyrolysis temperature. Similar were the results of Garlapalli et al. [18] who prepared chars at similar temperatures (400–800 °C) in order to make comparison of their properties with those of biochars prepared by hydrothermal carbonization (180–260 °C). Despite that the impact of pyrolysis temperature on the physicochemical properties of digestate-derived chars has been examined to some extent the comparison to the effect of temperature on biomass chars remains unclear. Besides, the relationship between pyrolysis temperature and functionality of a digestate char as soil amendment has not been considered yet.
AD systems frequently experience instabilities and low methane yields especially when heterogeneous, poor-quality and low-energy substrates such as municipal solid waste and sewage sludge are converted [19], [20]. Materials with porous morphology and surface arrangement that allow microorganisms’ adhesion like biochar could improve the immobilization and activity of bacteria and archaea confronting some of those challenges [21]. In particular, biochars originate from biomass provide enough space inside pores for growth of bacterial colonies and concurrently participate in electron transfer among bacterial species [22]. As a result, conversion of substrate improves and the start-up period of an AD system is shortened. Lü et al. [23] who added fruitwood biochars produced at 800–900 °C in a digestion system noted that biomass-derived char also assists the AD by acting as a sorbent removing secondary AD inhibitors. In addition, Sunyoto et al. [24] stated that a biomass biochar such as the one produced from pine sawdust (650 °C) promotes the digestion stability functioning as buffer in an AD system [25]. From those reports, it is evident that a biomass-derived char shows a great potential to improve the efficiency of an AD system. A biochar prepared from digestate might also be a competent additive to assist the digestion process, however only limited information is available about the application of this type of biochar in an AD system. Moreover, the research on the influence of pyrolysis temperature on the ability of a digestate-derived char to catalyse the AD is relatively poor.
In the present study, a typical digestate produced from a mesophilic agricultural biogas plant was subjected to pyrolysis at various temperatures ranging from the vicinity of torrefaction of 300 °C to 700 °C. The aim of the work was to: (i) elucidate the details of the degradation of the AD solid by-product and make comparison with the thermochemical conversion of biomass, (ii) identify the effect of temperature on the properties of digestate-derived chars and correlate it with the case of biomass chars (iii) provide projections of the effect of pyrolysis temperature on biochar capability to improve soil productivity, (iv) evaluate the impact of digestate-derived char on the AD process and elucidate the role of the biochar production temperature. The results are expected to provide useful information on digestate management and the recovery of materials for generation of biofertilizer and AD catalysts particularly for the biogas production industry.
Section snippets
Sample collection
The feedstock used for the production of biochar was a digestate obtained from the first stage of a mesophilic agricultural biogas plant (Pustějov II-Zemspol Studénka Ltd) operating on wet mode at 40 °C with corn silage and small proportion of cattle slurry. The digestate was dried at 105 °C under inert conditions (N2) until constant weight for the removal of moisture and then milled and sieved to obtain a particle size fraction between 100 and 300 μm that used for the tests.
Thermogravimetric tests
The
Pyrolysis product distribution at various pyrolysis temperatures
The mass distribution of biochar, bio-oil and gas for the pyrolysis of digestate at a number of temperatures appear in Fig. 2. Char production at 300 °C was at 66.5 %wt., whereas the proportions for bio-oil and gas were smaller (29.6 and 3.9 %wt., respectively). At a low temperature such as the 300 °C decomposition of digestate might be expected to be limited however the amount of gas and mainly bio-oil summed up to a 33.6 %wt. indicating a significant progress of process. For a given
Conclusions
The solid digestate from a mesophilic biomass-fed biogas plant operating at a low solid mode was subjected to dynamic and isothermal pyrolysis at various temperatures to examine the devolatilization behaviour of the material and determine the impact of pyrolysis temperature on the ability of the produced biochar to improve the soil productivity and efficiency of AD. Digestate decomposed over four stages instead of three existing in biomass degradation. Cellulose and hemicellulose had lower
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic under the projects ERDF “Institute of Environmental Technology – Excellent Research” [No. CZ.02.1.01/0.0/0.0/16_019/0000853], “COOPERATION” [No. CZ.02.1.01/0.0/0.0/17_049/0008419] and “Large Research Infrastructure ENREGAT” [No. LM2018098].
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