Characterisation of hydrochar produced by hydrothermal carbonisation of organic sludge

Owing to increasing environmental concerns and concepts of sustainable development, sludge management has become an important issue recently. The energy efficient and economic feasible measure for sludge management is urgently required and hydrothermal carbonisation (HTC) has been regarded as one promising method. In this study hydrothermal carbonisation was used to convert organic sludge (OS) into the solid carbonaceous product (hydrochar) under various reaction temperature (T=180-240 °C) and residence time (t=60-240 min). The obtained hydrochar was analysed by elemental analysis, proximate analysis, SEM, FTIR, and BET to investigate the effect of reaction parameters on the hydrochar properties. Results are followed by discussions on potential applications of the obtained hydrochar. Obvious reduction of H/C and O/C ratio after HTC was observed based on the Van Krevelen diagram and FTIR analysis, revealing that dehydration and decarboxylation occur during the HTC process. The reaction temperature was regarded as the dominant parameter affecting hydrochar characteristic compared to residence time. The severity factor (log Ro) at the range from 4.13 to 5.90 showed good linear relationship with the ash content, volatile matter (VM), N content and H/C ratio of hydrochar, indicating that the severity factor could serve as a useful indicator analysing the hydrochar property. Although the surface area and porosity were improved by HTC, subsequent modification and activation should be needed to broaden the application field. Owing to the characteristic of high ash content, hydrochar derived from organic sludge might not be feasible for fuel use. Instead, the precursor of the activated carbon and low-cost adsorbent might be the potential applications, while further works is needed to verify the feasibility.