Revealing the heating value characteristics of sludge-based hydrochar in hydrothermal process: from perspective of hydrolysate

He Zhang; Gang Xue; Hong Chen; Xiang Li; Shanping Chen

doi:10.1016/j.watres.2021.117170

Revealing the heating value characteristics of sludge-based hydrochar in hydrothermal process: from perspective of hydrolysate

Water Res. 2021 Jun 15:198:117170. doi: 10.1016/j.watres.2021.117170. Epub 2021 Apr 21.

Authors

He Zhang¹, Gang Xue², Hong Chen³, Xiang Li⁴, Shanping Chen⁵

Affiliations

¹ College of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, China; School of Environment and Architecture, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, China.
² College of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, China; Shanghai institute of pollution control and ecological security, Shanghai 200092, China.
³ College of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, China. Electronic address: chenhong@dhu.edu.cn.
⁴ College of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, China.
⁵ Shanghai Institute for Design & Research on Environmental Engineering Co., Ltd; Shanghai Environmental Sanitation Engineering Design Institute Co., Ltd.

PMID: 33945948
DOI: 10.1016/j.watres.2021.117170

Abstract

Hydrothermal treatment (HT) is a promising method to convert sewage sludge to hydrochar biofuel. The heating value is directly correlated to the carbon content in hydrochar; however, the release of organic matter from sludge to hydrolysate and the transfer of the Maillard reaction products generated in the hydrolysate to the solid phase alter the carbon content in hydrochar. In this study, the relationship between hydrolysate and heating value of sludge-based hydrochar was presented, aiming to explain how the calorific value of hydrochar was affected by HT conditions. We adopted a direct combustion test to verify its clean combustion features. Hydrochar derived at 260 °C and residence time of 4 h (HC 260-4) exhibited the highest calorific value (HHV_daf = 26.23 MJ/kg) with an energy density of 1.43, and its fuel characteristics were similar to those of lignite. The increase in the HT temperature and residence time up to 260 °C and 4 h, respectively, was beneficial for enhancing HHV_daf. Conversely, further increase of the HT temperature to 300 °C and HT time to 6 h yielded a decrease in HHV_daf. Investigation of the underlying mechanism revealed that the protein and polysaccharide releasing from sludge to hydrolysate occurred the Maillard reaction (MR). The generated humic-like Maillard reaction product (MRP) was transferred to hydrochar, inducing an increase in the carbon content and calorific value and a decrease in the organic content of hydrolysate. As the carbohydrate content in the hydrolysate decreased, the MR was terminated, so no more MRP was transferred to hydrochar. At the same time, the protein was still continuously released at higher temperatures and longer residence times, yielding a decline in the HHV_daf. Moreover, clean energy utilization was verified from the reduced nitrogen content in hydrochar and lower CO and NO_x emission of HC 260-4 in the combustion test. After the HT, increased hydrophobicity and a lower fraction of bound water improved the dewaterability, which is of great significance for applying hydrochar as biofuel. The findings of this study provided a new perspective to explain the heating value generation of hydrochar and more direct evidence to assess its clean combustion properties, with promising perspectives for practical applications.

Keywords: clean combustion; heating value; hydrolysate; hydrothermal condition; sewage sludge.

MeSH terms

Carbon
Heating*
Nitrogen
Sewage*
Temperature

Substances

Sewage
Carbon
Nitrogen