• Energy Research

Abstract Hydrothermal liquefaction (HTL) of municipal sludge (MS) was performed at 350 °C for 30 min (subcritical water) and at 400 °C for 0 min (supercritical water) at fixed kinetic severity (LogR0 = 8.9) in static and stirred batch reactors to study the effect of the flow regime on the energy recovery (ER) of the process and on the quality of the products. With adopted experimental procedures it was possible to reduce to less than 10% the yield of lost organic compounds, termed volatiles (VT), and to collect and quantify a liquid hydrocarbon fraction (HC) separated from the biocrude (BC). The highest value of the HC yield, 25% w/w, was obtained in supercritical conditions. The C content of the solid residues (SR) and the H/C ratios of the BC increased when the stirred reactor was used. Quite interestingly in this research we have found, to the best of our knowledge for the first time, that the cumulative ER of the product phases was significantly higher than 100% thus indicating that HTL of MS can be energetically driven by renewable thermal energy, such as solar heat, offering a storage option for it.

Hydrothermal liquefaction of municipal sludge was conducted to perform a practical assessment to achieve an integral valorization of this wet waste biomass. Experiments were done to investigate the effects of sub- and supercritical water and of the fluid dynamic regime on the energy recovery of the process and on the quality of the products. Experimental runs were performed at two different temperatures (350°C and 400°C) changing reaction time in order to work at the same kinetic severity of the process (LogR0=8.9, calculated as in ref. [3]). An improvement of the C % in the solid residues and of the ration of H/C of the biocrude was obtained when stirred reactor was used both in sub- and supercritical water. An hydrocarbon fraction was separated from biocrude. The maximum value of the hydrocarbon yield was 25% and it was achieved in supercritical conditions. Moreover results showed that the cumulative energy recovery of the products was significantly higher than 100% suggesting that hydrothermal liquefaction of municipal sludge could be fed by renewable source of thermal energy.

Abstract Hydrothermal liquefaction (HTL) of municipal sludge (MS) was studied at 325 °C and 30 min at 10 and 30% w/w concentration of MS using formic acid (FA) as a green liquid hydrogen donor both in the absence and in the presence of heterogeneous catalysts. Pellets of commercial NiMo/Al2O3, CoMo/Al2O3 and felt of activated carbon ACF 1600 were used as catalysts. The addition of FA at high MS loading allowed to increase the yields in BC from 41 to 62% and its H/C ratio from 1.80 to 2.01 leading to energy recovery (ER) higher than 100%.When heterogeneous catalysts were used together with FA, a marked improvement of BC yields and quality was obtained at 10% MS loading. This behavior was not observed at 30% w/w concentration probably for the fouling of the catalyst surface. Experimental results obtained in this study indicates that FA can be an effective additive to improve BC yields and quality in the HTL of MS at high loading, that is a very promising result in the perspective of industrial utilization of the process.

Abstract Production of biocrude through hydrothermal liquefaction (HTL) of sewage sludge (SS) and Chlorella vulgaris was investigated. NiMo/Al2O3 (KF 851), CoMo/Al2O3 (KF 1022) and activated carbon felt, tested as catalysts at 598 K for 30 min, did not change the biocrude yield but significantly improved its quality. Quantitative sulfur removal and lower oxygen content were found in biocrude when KF 851 and KF 1022 were used with C. vulgaris. The same catalysts decreased O/C and S/C ratio of the biocrude from SS. The highest HHV (38.19 MJ/kg) and H/C ratio (1.65) and the lowest O/C (0.11) of the biocrude from SS were obtained with the cheaper activated carbon felt. A new product phase, probably related to the presence of microplastics in the sludge, was found when SS were converted with catalysts. SS constitute a promising and costless alternative feedstock to microalgae for the HTL processes.

Hydrothermal liquefaction (HTL) can be considered a promising route for the energy valorisation of waste sewage sludge (SS). However, not much information is available on continuous flow processing. In this study, the mixed SS was subjected to HTL at 350°C for 8 min in a continuous reactor with loadings of 10 wt% in the feed flow. The results show that the mass recovery reached 88%, with a biocrude yield of 30.8 wt% (3.9 wt% N content). The recovered biocrude yields are highly dependent on the selection of the recovery solvent for extraction, and dichloromethane can contribute an additional 3.1 wt% biocrude from aqueous phase, acetone can extract some pyrrole derivatives into the trapped phases. Comparable results were also achieved by performing batch reactions under the same conditions: a slightly higher biocrude yield (33.1 wt%) with an N content of 4.3 wt%. The higher N content observed in the biocrude from the batch process indicates that interactions and chelation between intermediates are enhanced during heating up and cooling period, which lead to more N-containing compounds.

The dehydrogenation of ethylbenzene to styrene was used as a model of an energy intensive endothermic process to assess the economic sustainability of the utilization of solar heat from a concentrating solar power (CSP) plant to decarbonize an industrial chemical processes.To this purpose a process configuration compatible with the hybridization with a CSP plant using a binary mixture NaNO3/KNO3, 60/40 w/w as heat transfer fluid (HTF) was selected.The adopted chemical reactor is a shell and tube bundle converter with 30000 tubes of 6 m length and 0.025 m inside diameter that approaches isothermal regime with a productivity of 103 kT/year of styrene if a flowrate of 200 kg/s of molten salt at 560 °C are fed to the shell. The residual enthalpy of the HTF leaving the dehydrogenation reactor was further injected in the process by vaporizing and pre-heating ethylbenzene and dilution water.A cash flow analysis of the hybridized plant was performed considering solar field of increasing size so that the required solar power of 45 MW can be supplied for longer period of the year. We found that a CSP plant of 70 collectors can decrease CO2 emissions of about 50 % with a rate of return on investment (ROROI) of 9.1 % for the solar field of the hybridized plant and can grant 410 k€/year of economic benefit arising from the methane and the lower emissions of CO2. This study demonstrates that solar heat can be used to decarbonize energy intensive endothermic chemical processes without economic penalty for the plant profitability.