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

https://doi.org/10.1016/j.enconman.2023.116658Get rights and content

Highlights

  • The properties of digestate-derived chars were examined.

  • Temperature increment led in reduced ion charges but increased pH and surface area.

  • Biochar-500 °C was the best choice for improvement of soil fertility in long term.

  • Biochar-700 °C exhibited the optimal surface area for soil remediation.

  • High-temperature biochar had appropriate properties for optimization of digestion.

Abstract

An industrial digestate was tested under pyrolysis to validate the influence of various temperatures on the potential of waste to provide biochars capable to improve soil productivity and enhance the efficiency of anaerobic digestion. The thermochemical conversion of digestate and the related kinetics were also examined by thermogravimetry and compared to those of biomass.

Digestate provided greater proportions of biochar (41.3–66.5 %wt.) than biomass (15.5–48.6 %wt.) at all temperatures, although cellulose and hemicellulose were less thermally stable than in the untreated biomass as indicated by the low activation energies of 150–180 kJ/mol and 83.1–88.2 kJ/mol, respectively compared to those of the polysaccharides in biomass. The behaviour of digestate during conversion and the quantity of generated biochar were both unaffected by the heating rate. Biochar yields however decreased from 55 to 41.3 %wt. in the isothermal tests with the increment of pyrolysis temperature from 300 to 700 °C. The soil amelioration ability of digestate was almost unaffected by pyrolysis at temperatures below 400 °C. Biochar produced at 500 °C possessed the greatest potential to improve soil fertility in the long-term due to its high alkalinity (pH 11.46) and modest cation exchange capacity (CEC) (72.2 cmol kg−1). The high-temperature biochar with slightly higher pH (11.51) and significantly larger surface area (116.1 m2/g) was the optimal choice for soil amelioration. Compared to the other biochars, the one produced at the highest temperature was also the best anaerobic digestion agent because of the higher alkalinity (pH 11.51), better minerals content (P, K, Ca, Al) and larger than 100 m2/g surface area, all properties that are desirable for the improvement of stability, methane productivity and mitigation of CO2, H2S and free-NH3 in a digestion system.

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].

References (77)

  • B. Biswas et al.

    Pyrolysis of azolla, sargassum tenerrimum and water hyacinth for production of bio-oil

    Bioresour Technol

    (2017)
  • C.-Y. Hung et al.

    Characterization of biochar prepared from biogas digestate

    Waste Manag

    (2017)
  • M. Stefaniuk et al.

    Characterization of biochars produced from residues from biogas production

    J Anal Appl Pyrol

    (2015)
  • N.A. Qambrani et al.

    Biochar properties and eco-friendly applications for climate change mitigation, waste management, and wastewater treatment: a review

    Renew Sustain Energy Rev

    (2017)
  • R.K. Garlapalli et al.

    Pyrolysis of hydrochar from digestate: effect of hydrothermal carbonization and pyrolysis temperatures on pyrochar formation

    Bioresour Technol

    (2016)
  • P. Basinas et al.

    Dry anaerobic digestion of the fine particle fraction of mechanically-sorted organic fraction of municipal solid waste in laboratory and pilot reactor

    Waste Manag

    (2021)
  • Y. Chen et al.

    Inhibition of anaerobic digestion process: a review

    Bioresour Technol

    (2008)
  • F.C. Luz et al.

    Ampelodesmos mauritanicus pyrolysis biochar in anaerobic digestion process: evaluation of the biogas yield

    Energy

    (2018)
  • F. et al.

    Biochar alleviates combined stress of ammonium and acids by firstly enriching Methanosaeta and then Methanosarcina

    Water Res

    (2016)
  • N.M.S. Sunyoto et al.

    Effect of biochar addition on hydrogen and methane production in two-phase anaerobic digestion of aqueous carbohydrates food waste

    Bioresour Technol

    (2016)
  • M.O. Fagbohungbe et al.

    The challenges of anaerobic digestion and the role of biochar in optimizing anaerobic digestion

    Waste Manag

    (2017)
  • P.J. van Soest et al.

    Methods for dietary fiber, neutral detergent fiber, and nonstarch polysaccharides in relation to animal nutrition

    J Dairy Sci

    (1991)
  • C.A. Takaya et al.

    Phosphate and ammonium sorption capacity of biochar and hydrochar from different wastes

    Chemosphere

    (2016)
  • G. Pantoleontos et al.

    A global optimization study on the devolatilization kinetics of coal, biomass and waste fuels

    Fuel Process Technol

    (2009)
  • L. Sorum et al.

    Pyrolysis characteristics and kinetics of municipal solid wastes

    Fuel

    (2001)
  • R. Isemin et al.

    Application of torrefaction for recycling bio-waste formed during anaerobic digestion

    Fuel

    (2019)
  • W. Liao et al.

    Effect of different biomass species and pyrolysis temperatures on heavy metal adsorption, stability and economy of biochar

    Ind Crop Prod

    (2022)
  • M.P. McHenry

    Agricultural bio-char production, renewable energy generation and farm carbon sequestration in Western Australia: certainty, uncertainty and risk

    Agric Ecosyst Environ

    (2009)
  • C. Torri et al.

    Biochar enables anaerobic digestion of aqueous phase from intermediate pyrolysis of biomass

    Bioresour Technol

    (2014)
  • Y. Wei et al.

    Thermal characterization and pyrolysis of digestate for phenol production

    Fuel

    (2018)
  • X. Gómez et al.

    An evaluation of stability by thermogravimetric analysis of digestate obtained from different biowastes

    J Hazard Mater

    (2007)
  • J. Neumann et al.

    Production and characterization of a new quality pyrolysis oil, char and syngas from digestate - introducing the thermo-catalytic reforming process

    J Anal Appl Pyrol

    (2015)
  • C.E. Efika et al.

    Influence of heating rates on the products of high-temperature pyrolysis of waste wood pellets and biomass model compounds

    Waste Manag

    (2018)
  • Y. Zhai et al.

    Study on the co-pyrolysis of oil shale and corn stalk: pyrolysis characteristics, kinetic and gaseous product analysis

    J Anal Appl Pyrol

    (2022)
  • M. Hu et al.

    Thermogravimetric study on pyrolysis kinetics of Chlorella pyrenoidosa and bloom-forming cyanobacteria

    Bioresour Technol

    (2015)
  • F. Ma et al.

    Influence of the co-fungal treatment with two white rot fungi on the lignocellulosic degradation and thermogravimetry of corn stover

    Process Biochem

    (2011)
  • F. Ma et al.

    Thermogravimetric study and kinetic analysis of fungal pretreated corn stover using the distributed activation energy model

    Bioresour Technol

    (2013)
  • M. Otero et al.

    Digestion of cattle manure: thermogravimetric kinetic analysis for the evaluation of organic matter conversion

    Bioresour Technol

    (2011)
  • Cited by (11)

    View all citing articles on Scopus
    View full text