Composting Hydrochar-OFMSW Digestate Mixtures: Design of Bioreactors and Preliminary Experimental Results

Abstract

An increasing number of industrial plants integrate the anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) with a subsequent composting phase. To improve the plant productivity, a fraction of OFMSW digestate can be converted into a carbonaceous material, called hydrocar (HC), through Hydrothermal Carbonization (HTC), and then composted together with the OFMSW digestate itself, to produce “hydrochar co-compost”. The aim of this paper is to present the design and assembly of batch bioreactors, built in-house to investigate the co-composting process of OFMSW digestate and its HC, and to provide some preliminary results. The OFMSW digestate from an industrial plant was carbonized at 200 °C for 3 h in a 2 L HTC reactor, to produce wet HC after filtration. The ratio of OFMSW digestate and green waste (1:1) used as bulking medium was reproduced in four bioreactors with an increasing percentage of HC substituting the OFMSW digestate (0, 25, 50, 75%). The bioreactors managed to effectively compost the solid wet biomasses in a wet environment with temperature and oxygen control, while measuring online the oxygen consumption and thus the dynamic respirometric index (DRI). The DRI24,max measured with AIR-nl solid respirometer (standardized offline measurement) started from values above 800 mg O2 kgVS−1 h−1 before composting and dropped at the end of the process to values in the range 124–340 mg O2 kgVS−1 h−1 for the four mixes, well below the recommended limit of 500 mg O2 kgVS−1 h−1 for high-quality compost stability. These offline DRI values were confirmed by the online DRI measurements. This research is part of the international C2Land Project funded by the European Institute of Innovation and Technology Climate Knowledge and Innovation Community (EIT Climate-KIC), which is greatly acknowledged.

Feasibility of Coupling Anaerobic Digestion and Hydrothermal Carbonization: Analyzing Thermal Demand

“…The solid product derived from HTC, also known as hydrochar, has been demonstrated to attain benefits when added as a supplement in anaerobic digestion and composting, similar to those reported by biochar obtained from pyrolysis processes [37][38][39][40][41][42]. Other reports have also demonstrated that char may act as an active compound, improving nutrient management in crops [43,44].…”

Thermal Analysis and Kinetic Modeling of Pyrolysis and Oxidation of Hydrochars

“…After some mathematical steps [35], and after imposing 1 − ∆v/(∆v ∞ ) = Φ(E, T) ∼ = 0.58, the representative equation of the Miura-Maki model is given by Equation (13).…”

“…This was found in the range of α 0.1-0.8 for pyrolysis and 0.1-0.9 for oxidation. Then, for the valid range of α, E and k 0 were determined graphically using the schemes in Figure 7; E is given by the slope of each line for each α, while k 0 is obtained from the intercept and by applying Equation (13). Under pyrolysis, the fitting is satisfactory, with a R 2 always higher than 0.95, except for one outlier at α = 0.7 for the 220 • C hydrochar.…”

“…Due to its potential, HTC is a mindful tool to treat biomass waste, recover materials, and provide an energy source. Hydrochar can be used as solid fuel and integrating HTC with other processes (like gasification, pyrolysis, anaerobic digestion [12], and composting [13]) can face shortcomings related to the single-stage, and improve the overall process efficiency. For example, HTC as a pre-treatment can affect the pyrolysis performance [1].…”

Influence of Hydrothermal Carbonization on Catalytic Fast Pyrolysis of Agricultural Biomass

“…Moreover, the process of hydrothermal carbonization also removes a part of the inorganic fraction of biomass [54,55]. Hydrothermal carbonization also has an influence on the morphology of the hydrochars [56,57], and, as a consequence, makes the latter a good precursor for the production of activated carbon [58,59] and also for direct application to soil or compost [60,61]. Furthermore, positive influence of the HTC process on biomass grindability should not be overlooked, as particle size is an important aspect of pyrolysis [62][63][64].…”