Valorization of CO2-Riched Gaseous to Formic Acid via Electrochemical Routes: Current Status and Perspectives

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Electrochemical Treatment of Wastewater by ElectroFenton, Photo-ElectroFenton, Pressurized- ElectroFenton and Pressurized Photo ElectroFenton: A First Comparison of these Innovative Routes

In the last few years increasing attention has been devoted to the utilization of electroFenton (EF) and EF based technologies for the treatment of wastewater polluted by recalcitrant organics. It has been shown that the performances of EF can be strongly improved using ultraviolet (UV) irradiation, e.g., by the photo-electroFenton (PEF) method, or pressurized air or oxygen, e.g., by the pressurized-electroFenton (PrEF) one. Although several studies were carried out on the degradation of many organic pollutants using EF, PEF or PrEF, a systematic comparison between PEF and PrEF was never reported as well as the possibility to couple the irradiation with pressurized air. In this study the performances of EF, PEF and PrEF were systematically compared using synthetic solutions of three model organic substrates (e. g., formic acid, oxalic acid and Acid Orange 7). In addition, the pressurized-photo-electroFenton (PrPEF) process was proposed for the first time

Catalytic hydrothermal liquefaction of municipal sludge in subcritical water

In the last decades, the dwindling of the fossil sources of energy coupled with the growth of energy demand and of waste production prompted the research in developing novel industrial technologies for renewable energy production and waste valorization. Hydrothermal liquefaction (HTL) is a good alternative to transform wet biomasses as microalgae, macroalgae, agricultural residues, food waste, and municipal sludge (MS) into value-added products with high efficiency and decreasing the amounts that has to be disposed of. HTL takes place in an aqueous environment, without the energy cost of drying the biomass, at 300-400°C and pressure of 10-40 MPa [1,2]. At these operative conditions, an integral degradation of wet biomass produces a bio-oil termed biocrude and other C-containing products i.e. a solid residue, a gaseous phase rich in CO2 and an aqueous phase with soluble organics. The development of the process to the industrial scale is hindered by many challenges related both to the heterogeneous nature of the raw material and the complexity of the phase behavior downstream of the process and the poor quality of the biocrude produced as fuel precursor. This work aims to investigate the potentiality of catalytic HTL to obtain a biocrude more competitive as fuel precursor. We have studied catalytic HTL of MS in a stirred AISI 316 high-pressure batch reactors at 325 °C and 30 min as reaction temperature and time using NiMo/Al2O3, CoMo/Al2O3 and activated carbon felt as catalysts and formic acid (FA) as liquid hydrogen donor. Optimized work-out procedures were used to separate and quantify the products with the aim to decrease the amount of not detected mass [3]. With adopted methods the formation of an hydrocarbon fraction (HC) recovered from the biocrude, was detected in the presence of the catalysts. This result indicates that tested catalysts promote the in-situ up-grading of the produced biocrude. Furthermore, the addition of FA as liquid hydrogen donor allowed us to achieve higher H/C and HHV of biocrude as it was possible to increase the biocrude yield at more than 50% with energy recovery approaching 100%. Collected results suggest that use of catalysts can increase the yield and quality of biocrude in the HTL of municipal sludge

Comparative investigation on the value-added products obtained from continuous and batch hydrothermal liquefaction of sewage sludge

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

Electrochemical Treatment of Synthetic Wastewaters Contaminated by Organic Pollutants at Ti4O7 Anode. Study of the Role of Operative Parameters by Experimental Results and Theoretical Modelling

In the last years, an increasing attention has been devoted to the utilization of anodic oxidation (AO) technologies for the treatment of wastewater polluted by recalcitrant organics. Recently, Ti4O7 was proposed as a promising anode for AO for the treatment of various organics. Here the potential utilization of commercial Ti4O7 anodes has been evaluated considering the electrochemical treatment of synthetic wastewater contaminated by three very different organic molecules (namely, oxalic acid, phenol and Acid Orange 7), all characterized by a very high resistance to AO. The performances of Ti4O7 were compared with that of two largely investigated anodes: Boron-doped diamond (BDD), which is probably the most effective electrode for AO, and an Ir-based anode which presents a relatively low cost. Moreover, the effect of various operative conditions (current density, mixing rate and initial concentration of the organic) was evaluated by both experimental studies and the adoption of a theoretical model previously developed for BDD anodes. It was shown that the performances of the process can be improved by a proper selection of operative conditions. Moreover, it was found that the proposed model can be effectively used to predict the effect of operative parameters at Ti4O7 anodes, thus helping the process optimization

Hydrothermal liquefaction of wet waste biomass: a practical assessment to achieve an integral valorization of municipal sludge

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\ub0C and 400\ub0C) 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

Hydrothermal liquefaction of wet biomass in batch reactors: Critical assessment of the role of operating parameters as a function of the nature of the feedstock

A scientometric analysis of articles published from 1986 to 2022 on batch hydrothermal liquefaction of microalgae, macroalgae, lignocellulosic biomass, sewage sludge and organic wastes in water was performed. We found that biocrude yield can be correlated with the kinetic severity factor (KSF) and scattering of experimental data increases in the supercritical region probably for uncertainty in the medium density affecting the kinetics of reactions involved in the process. The level of correlation and the yield increased when fast heating rates of the reactor, higher than 25 ◦C/min, were adopted. Energy recovery of biocrude obtained in fast heating experiments changes linearly with biocrude yield independently on the adopted biomass. From this analysis, it seems interesting to assess more systematically the role of KSF, nominal density of reaction medium and heating rate with costless not algal biomass

Hydrothermal co-liquefaction of sewage sludge and formic acid

The biocrude produced by non-catalytic hydrothermal liquefaction (HTL) has a too high heteroatom content to be used directly as a fuel [1-3]. Several studies found that different H2 initial pressures during HTL experiments were ineffective in improving the biocrude yield [4-6], nevertheless it was observed that higher initial pressures of H2 brought an improvement of the concentration of light compounds in the biocrude, decreasing the average molecular weight of biocrude oil. To date H2 is mainly generated from fossil sources and realization of a possible hydrogen economy from alternative sources presents many challenges. The use of a liquid hydrogen storage media could represent for the HTL process an alternative solution to avoid the mass transport resistance associated to gaseous H2, furthermore Biller and Ross [7] observed that the addition of selected organic acids had similar effects than the addition of molecular H2. According to these premises, we studied the influence of formic acid as hydrogen donor on the yield and quality of the biocrude produced by HTL of sewage sludge (SS). Formic acid was selected because it can decompose to hydrogen and carbon dioxide under mild conditions [8] and it can be produced by electrochemical reduction of carbon dioxide, an appealing process to capture atmospheric carbon dioxide under the form of a useful chemical [9]