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.
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.
Water Res. 2021 Jun 15;198:117170. doi: 10.1016/j.watres.2021.117170. Epub 2021 Apr 21.
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 = 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. Conversely, further increase of the HT temperature to 300 °C and HT time to 6 h yielded a decrease in HHV. 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. Moreover, clean energy utilization was verified from the reduced nitrogen content in hydrochar and lower CO and NO 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.
水热处理（HT）是将污水污泥转化为水热炭生物燃料的一种很有前途的方法。热值与水热炭中的碳含量直接相关；然而，污泥中的有机物向水解液的释放以及在水解液中生成的美拉德反应产物向固相的转移改变了水热炭中的碳含量。在这项研究中，提出了污泥基水热炭的水解液与热值之间的关系，旨在解释水热炭的热值如何受到 HT 条件的影响。我们采用直接燃烧试验来验证其清洁燃烧特性。在 260°C 和 4 小时的停留时间下获得的水热炭（HC 260-4）表现出最高的热值（HHV=26.23MJ/kg），能量密度为 1.43，其燃料特性类似于褐煤。分别将 HT 温度和停留时间提高到 260°C 和 4 小时，有利于提高 HHV。相反，进一步将 HT 温度提高到 300°C 和 HT 时间提高到 6 小时会降低 HHV。对潜在机制的研究表明，蛋白质和多糖从污泥中释放到水解液中会发生美拉德反应（MR）。生成的类腐殖质美拉德反应产物（MRP）转移到水热炭中，导致水热炭的碳含量和热值增加，而水解液中的有机含量降低。由于水解液中的碳水化合物含量减少，MR 终止，因此没有更多的 MRP 转移到水热炭中。同时，在较高温度和较长停留时间下，蛋白质仍在不断释放，导致 HHV 下降。此外，在燃烧试验中，水热炭中氮含量的降低和 HC 260-4 中 CO 和 NO 排放量的降低证明了清洁能源的利用。经过 HT 处理后，疏水性增加和结合水比例降低改善了脱水性能，这对于将水热炭用作生物燃料具有重要意义。本研究的结果提供了一种新的视角来解释水热炭的热值生成，并提供了更直接的证据来评估其清洁燃烧特性，具有实际应用的广阔前景。
