Combustion Characteristics of Hydrochar and Pyrochar Derived from Digested Sewage Sludge

Pablo J Arauzo,Javier Ábrego,Andrea Kruse,Zebin Cao,Maciej P Olszewski,María Atienza-Martínez

doi:10.3390/en13164164

Pablo J Arauzo, Javier Ábrego + Show 4 more

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https://doi.org/10.3390/en13164164

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Abstract

In this paper, hydrochars and pyrochars were produced at 260 °C under different residence times (2 and 4 h) using anaerobic digested sewage sludge (SSL) as initial feedstock. The effect of reaction time on the fuel properties of hydrochars and pyrochars was evaluated. Moreover, the combustion kinetics of raw SSL and the derived pyrochars and hydrochars without coal blending were determined at two different air flows (20 and 90 mL/min) and compared. In the same conditions, the yield of hydrochar was significantly lower than that of pyrochar, confirming the different reaction pathways followed in each process. The results showed hydrochars have lower carbon recovery and energy yield than pyrochars, making the latter more suitable for energy purposes. The thermogravimetric combustion study showed that both thermochemical treatments increased the ignition temperature but decreased the burnout temperature, which results in higher stability during handling and storage. However, raw SSL is better for combustion than hydrochar according to the combustibility index. In addition, the kinetic study showed that the activation energy of the combustion of biochars, especially pyrochar, is lower than that of raw SSL, which is advantageous for their combustion.

Highlights

The production of sewage sludge (SSL) as a by-product in wastewater treatment plants is still increasing due to human and industrial activity
While in hydrothermal carbonization (HTC) the conversion of the raw SSL into hydrochar is caused by complex reaction mechanisms [13,23,24], the conversion of SSL to pyrochar in a fixed bed reactor is limited by the particle size and by the diffusion mechanism [25]
This study shows the conversion of digested SSL into biochar by two different thermochemical treatments: HTC and pyrolysis

Summary

Introduction

The production of sewage sludge (SSL) as a by-product in wastewater treatment plants is still increasing due to human and industrial activity. More than 8 million tons (dry basis) of sewage sludge from urban wastewater are generated annually in the 27 European Union (EU) countries, of which approximately 20% corresponds to Germany [1]. The huge volume of SSL might contain high levels of pollutants (i.e., heavy metals, hormones) [2,3], so it is necessary to find an environmentally sustainable solution to this increasing social problem. The current environmental legislation in the EU applicable to SSL is becoming increasingly strict regarding its land application, which has so far been the most common disposal method for this residue.

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