Energy recovery from sewage sludge using hydrothermal processing
Date of Issue2015
School of Civil and Environmental Engineering
Hydrothermal conversion technology represents a more sustainable technique to convert wet sewage sludge to biofuels since prior thermal drying process is eliminated. Previously, extensive studies have been devoted to supercritical water gasification of sewage sludge for hydrogen production. However, harsh supercritical water conditions could result in severe corrosion and high operating cost. Therefore, the present study has performed comprehensive investigations on direct energy recovery from dewatered sewage sludge (DSS) using hydrothermal processing under sub- and near-critical water. Hydrothermal carbonization (HTC) process has been successfully applied to convert DSS into hydrochars at 200 oC with prolonged residence time. Different carbonization times are adopted in order to produce hydrochars possessing better fuel properties. Uniform brown hydrochars with good grindability and nut-like smell are produced after HTC. The dehydration and decarboxylation reactions are found to be essential reaction pathways. Hydrochars possess a fuel ratio much higher than that of DSS due to higher fixed carbon and lower volatile matter. Specifically, 60% of nitrogen and sulfur can be removed while 88% of carbon is retained and higher heating values of hydrochars are 0.98-1.03 times of DSS, resulting in a cleaner solid fuel. Prolonging carbonization time could reduce the total content of surface oxygen containing functional groups and make hydrochars highly hydrophobic. On the other hand, HTC has altered patterns of main combustion decomposition from a single stage for DSS to two stages for hydrochars. The first order combustion reaction of DSS and hydrochars is verified and kinetic parameters are derived. Hydrochar obtained after 8 h of carbonization is a better fuel in terms of easier and more stable combustion. To shorten the HTC process, hydrothermal conversion of DSS within 20 min has been conducted under various sub- and near-critical water conditions. Effects of moisture content and CaO additive are also examined in near-critical water. Increasing temperature and pressure reduces energy recovery rate, however, more significant decarboxylation and dehydration reactions lead to production of solid fuel with high quality. The results suggest that decarboxylation reaction dominates hydrothermal conversion of DSS while intense dehydrogenation and methanation reactions initiate when approaching near-critical water. Mineralization of heteroatomic compounds and dissolution of metals or mineral elements are observed. Under higher temperature and pressure, except increased destruction of organic compounds, heavy metals or mineral elements are prone to be immobilized whereas dehalogenation is promoted. Higher moisture content favors decarboxylation and hydrolysis reactions. Nevertheless, it reduces H2 and CH4 yield. Introduction of CaO additive has some positive effects on H2 yield, hydrolysis of organic compounds, and immobilization of inorganic elements. At 380 oC and Ca/C molar ratio of 0.2, H2 yield increases almost 6-fold, resulting in 58% H2 and 26% CH4 in final fuel gas. However, CaO additive has detrimental effects on the calorific value of solid fraction. Systematical co-combustion of hydrochar with different coals has been studied towards efficient utilization of hydrochar in existing co-firing power plants. Hydrochar derived under 320 oC and 12.0 MPa (SHC-320) has been screened for co-combustion with different rank coals. SHC-320 reduces activation energy of blends and alters main combustion profiles. Meanwhile, blending SHC-320 may induce greater heat loss for higher rank coals, whereas higher portion (≥ 70%) of SHC-320 further improves ignition reactivity of high rank coal blends. Blending SHC-320 leads to higher heat loss to high rank coals in main co-combustion. In high temperature region, pre-exponential factor value increases with elevated Coal/SHC-320 ratios, resulting in more intense synergistic effects in blends. Under a low Coal/SHC-320 ratio (30:70), intense anti-synergistic effects occur in co-combustion with low or high rank coals. Due to distinct synergistic interactions, co-combustion with moderate rank coal achieves the best combustion performance and highest burn out efficiency among blends. The fate of nitrogen during hydrothermal conversion of DSS has been investigated and a novel system integrating hydrothermal deamination and air stripping has been successfully developed to effectively remove and recover nitrogen from DSS for the first time. The overall nitrogen transformation reactions and conversion pathways have been elucidated. In particular, three characteristic hydrothermal regimes contributing to deamination are identified. CaO has been identified as an effective and economical catalyst to accelerate hydrothermal deamination. It can not only accelerate deamination of stable protein-N, pyrrole-N, and pyridine-N, but also favor transformations of protein-N and quaternary-N to nitrile-N and pyridine-N, respectively. The coupled air stripping process demonstrates good performance to remove and recover ammonia from liquid fraction via ammonium sulfate. Based on an ideal scale-up, 51.93 kg N could be removed and 37.27 kg N would be recovered from 1 tonne dry mass of this DSS containing 60.12 kg N. To conclude, this study gives a better understanding on direct energy recovery from DSS using hydrothermal conversion under sub- and near-critical water. Under very low temperature, HTC could convert DSS to an alternative solid fuel from the perspectives of fuel characteristics and combustion performance. A holistic understanding of reaction chemistry contributes to the design of hydrothermal conversion of DSS under mild conditions. These findings also offer valuable insights into co-combustion scenario for sludge derived solid fuel with coals and effective strategy for N control during sustainable DSS management in future applications.
DRNTU::Engineering::Environmental engineering::Waste management