Towards Engineered Hydrochars: Application of Artificial Neural Networks in the Hydrothermal Carbonization of Sewage Sludge

Theodoros N Kapetanakis,Ioannis O Vardiambasis,Ramazan Keyikoglu,Alireza Khataee,Antonios I Konstantaras,Dimitrios Kalderis,Toshiki Tsubota,Duy Anh Khuong,Trinh Kieu Trang,Christos D Nikolopoulos

doi:10.3390/en14113000

Abstract

Sewage sludge hydrochars (SSHs), which are produced by hydrothermal carbonization (HTC), offer a high calorific value to be applied as a biofuel. However, HTC is a complex processand the properties of the resulting product depend heavily on the process conditions and feedstock composition. In this work, we have applied artificial neural networks (ANNs) to contribute to the production of tailored SSHs for a specific application and with optimum properties. We collected data from the published literature covering the years 2014–2021, which was then fed into different ANN models where the input data (HTC temperature, process time, and the elemental content of hydrochars) were used to predict output parameters (higher heating value, (HHV) and solid yield (%)). The proposed ANN models were successful in accurately predicting both HHV and contents of C and H. While the model NN1 (based on C, H, O content) exhibited HHV predicting performance with R2 = 0.974, another model, NN2, was also able to predict HHV with R2 = 0.936 using only C and H as input. Moreover, the inverse model of NN3 (based on H, O content, and HHV) could predict C content with an R2 of 0.939.

Highlights

Hydrothermal carbonization (HTC) of biomass is the thermochemical processing in an aqueous environment, in the temperatures range of 100–374 ◦C under autogenous pressure
Treatment duration usually lies in the range of 1–24 h and the two main products are, the solid hydrochar and the residual wastewater
The reactions involved in the conversion of biomass to hydrochar have been thoroughly reviewed in the literature [1,2,3]

Summary

Introduction

Hydrothermal carbonization (HTC) of biomass (including bio-waste) is the thermochemical processing in an aqueous environment, in the temperatures range of 100–374 ◦C under autogenous pressure. At these conditions, water has a higher diffusivity and lower viscosity and surface tension, it can penetrate more efficiently into dry and hydrophobic materials. Obtaining two high-added value products from HTC of sewage sludge has been shown to be one of the sustainable future strategies for sewage sludge management [7] It appears that the disadvantage of high ash content has been successfully dealt with through the co-hydrothermal carbonization of sewage sludge with low ash lignocellulosic biomass [8,9]

Methods

Findings

Discussion

Conclusion