Pyrolysis of Solid Digestate from Sewage Sludge and Lignocellulosic Biomass: Kinetic and Thermodynamic Analysis, Characterization of Biochar

Abstract

This study investigates the pyrolysis behavior and reaction kinetics of two different types of solid digestates from: (i) sewage sludge and (ii) a mixture of sewage sludge and lignocellulosic biomass—Typha latifolia plant. Thermogravimetric data in the temperature range 25–800 °C were analyzed using Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose kinetic methods, and the thermodynamic parameters (ΔH, ΔG, and ΔS) were also determined. Biochars were characterized using different chemical methods (FTIR, SEM–EDS, XRD, heavy metal, and nutrient analysis) and tested as soil enhancers using a germination test. Finally, their potential for biosorption of NH4+, PO43−, Cu2+, and Cd2+ ions was studied. Kinetic and thermodynamic parameters revealed a complex degradation mechanism of digestates, as they showed higher activation energies than undigested materials. Values for sewage sludge digestate were between 57 and 351 kJ/mol, and for digestate composed of sewage sludge and T. latifolia between 62 and 401 kJ/mol. Characterizations of biochars revealed high nutrient content and promising potential for further use. The advantage of biochar obtained from a digestate mixture of sewage sludge and lignocellulosic biomass is the lower content of heavy metals. Biosorption tests showed low biosorption capacity of digestate-derived biochars and their modifications for NH4+ and PO43− ions, but high biosorption capacity for Cu2+ and Cd2+ ions. Modification with KOH was more efficient than modification with HCl. The digestate-derived biochars exhibited excellent performance in germination tests, especially at concentrations between 6 and 10 wt.%.

Investigation of Pyrolysis Behavior of Sewage Sludge by Thermogravimetric Analysis Coupled with Fourier Transform Infrared Spectrometry Using Different Heating Rates

“…However, recently A. Petrovic et al investigated the pyrolysis kinetics of sewage sludge and biomass using 15, 30 and 100 • C/min heating rates. They compared the results using the Kissinger-Akira-Sunose and Flynn-Wall-Ozawa models, and concluded that the two models had good correlation in the case of sewage sludge raw material; however, due to the composition of other raw materials, a weaker correlation was found [29].…”

Improving Lignocellulosic and Non-Lignocellulosic Biomass Characteristics through Torrefaction Process

“…Hence, due to lower gas inflow to the European Union (EU) and increased demand for coal from China, as a result of higher electricity consumption, the prices for natural gas and coal began to rise [1]. On this account and on account of satisfying the environmental requirements set out in the European Green Deal [2], studying the potential and characteristics of renewable energy sources (RES), such as wind, solar, geothermal, marine energy, and hydropower, became necessary [3,4].…”

Optimization of pyrolysis regime for chicken manure treatment and biochar production

“…If nitrogen (N) is present in the substrate in organic form, then the N content during pyrolysis decreases because of volatile organic matter loss. Amine groups in substrates at low pyrolysis temperature gradually transform into pyridine-like compounds, with increased surface basicity at higher temperatures (>600 C) (Huang et al, 2017;Petrovic ˇet al, 2021). If potassium (K) is presented in water-soluble form, it can be partially lost by vaporization, as well as incorporation of K into the silicate structure, which is expected to be much less bioavailable.…”

“…On one hand, the tars created from thermal biomass C-decomposition impede the continuity of pores at low temperatures, and these pores become increasingly accessible as the temperatures increase and the tar components are volatilized. On the other hand, some processing conditions, such as high temperature, result in ash fusion or sintering (Huang et al, 2017;Petrovic ˇet al, 2021).…”

“…Physical transformations that occur with organic compounds in the pyrolysis process are temperature dependent groups (Ding et al, 2017; He et al, 2018; Li & Chen, 2018; Petrovič et al, 2021; Song & Guo, 2012). Thus, at relatively low temperatures (<200°C), organic matter loses free and chemically bound water, and with further temperature increases, small organic molecules are volatilized (Li & Jiang, 2017).…”