Aqueous Phase Recirculation Research Articles

Hydrothermal liquefaction of composite household waste to biocrude: the effect of liquefaction solvents on product yield and quality.

The hydrothermal liquefaction (HTL) of composite household waste (CHW) was investigated at different temperatures in the range of 240-360 °C, residence times in the range of 30-90 min, and co-solvent ratios of 2-8 ml/g, by utilising ethanol, glycerol, and produced aqueous phase as liquefaction solvents. Maximum biocrude yield of 46.19% was obtained at 340 °C and 75 min, with aqueous phase recirculation ratio (RR) of 5 ml/g. The chemical solvents such as glycerol and ethanol yielded a biocrude percentage of 45.18% and 42.16% at a ratio of 6 ml/g and 8 ml/g, respectively, for 340 °C and 75 min. The usage of co-solvents as hydrothermal medium increased the biocrude yield by 35.30% and decreased the formation of solid residue and gaseous products by 19.82% and 18.74% respectively. Also, the solid residue and biocrude obtained from co-solvent HTL possessed higher carbon and hydrogen content, thus having a H/C ratio and HHV that is 1.01 and 1.23 times higher than that of water as hydrothermal medium. Among the co-solvents, HTL with aqueous phase recirculation resulted in higher carbon and energy recovery percentages of 9.36% and 9.78% for solid residue and 52.09% and 56.75% for biocrude respectively. Further qualitatively, co-solvent HTL in the presence of obtained aqueous phase yielded 33.43% higher fraction of hydrocarbons than the pure water HTL and 7.70-17.01% higher hydrocarbons when compared with ethanol and glycerol HTL respectively. Nitrogen containing compounds, such as phenols and furfurals, for biocrudes obtained from all HTL processes, were found to be present in the range of 8.30-14.40%.

Coupling of struvite crystallization and aqueous phase recirculation for hydrochar upgrading and nitrogen recovery during hydrothermal carbonization of sewage sludge

Recycling of aqueous phase (AP) as a by-product after hydrothermal carbonization (HTC) of sewage sludge (SS) has been of interest. The combination of magnesium ammonium phosphate (MAP) or the so-called struvite crystallization and aqueous phase (AP) recirculation has great potential for resource recovery and hydrochar enhancement. In this study, both the aqueous phase of HTC after MAP recovery of NH4+-N (AP-MAP) and the untreated aqueous phase of HTC (AP-HTC) were reused for HTC of fresh SS, and both aqueous phases were recycled four times. The effects of the two AP cycles on the properties of AP and hydrochar at 200, 230, and 260 °C were studied, and the effect of temperature on the two AP cycles was similar. The hydrochar produced by the AP-MAP cycle had lower nitrogen content than that of the AP-HTC cycle due to the low ammonia nitrogen (NH4+-N) content, and the combustion performance was improved. MAP recovery reduces the accumulation of NH4+-N in the AP cycle and MAP is also a high-quality fertilizer. Therefore, the combination of MAP recovery and AP recycling provides a feasible technical approach for resource utilization, eutrophic AP treatment, and production of high-quality hydrochar in the HTC process of SS.

Phycocyanin to biocrude via the integration of isothermal/fast hydrothermal liquefaction and aqueous phase recirculation: Reaction products and process analyses

Aqueous phase (AP) recirculation is an efficient and economically feasible approach to handle the post wastewater from hydrothermal liquefaction (HTL). In this study, isothermal (300 °C, 30 min) and fast HTL (set-point temperature of 500 °C, 40 s) of phycocyanin with different mixing ratios (6:0, 4:2, 3:3, 2:4, 0:6) of water and AP products were conducted. The optimum mixing ratios which obtained the highest biocrude yield were further selected for multiple AP recirculation experiments and process analysis. The results show that with the optimum mixing ratio of 2:4, isothermal and fast HTL produced the highest biocrude yields (24.2 wt% and 30.0 wt%) after the 2nd and 1st recycles, respectively. Nitrogen heterocycles from Maillard reaction and amides from acylation reaction were the major compounds in biocrude and AP, which contributed to the biocrude generation and increased the nitrogen content of biocrude. COD and NH3-N contents in HTL wastewater increased while TP contents decreased with the consecutive recirculation of AP. With AP recirculation for three times, nearly half of the wastewater discharges could be avoided. Consequently, AP recirculation is a promising method to enhance the biocrude production and alleviate the environmental impacts of AP discharges.

The influence of key reactions during hydrothermal carbonization of sewage sludge on aqueous phase properties: A review

Hydrothermal carbonization (HTC) emerges as a promising technology to reduce the volume of activated sewage sludge (SS) and generate solid products as fuels. During HTC, extracellular polymeric substance and cell wall degrade to form low molecular compounds, which can further carry out hydrolysis, dehydration, polymerization, etc. The temperature has a notable impact on the HTC, which ultimately changes the composition of the aqueous phase. A large amount of the aqueous phase remains a by-product containing high amounts of organic and inorganic compounds. Herein, the HTC of SS process and aqueous phase treatment were discussed, especially in industrial applications within China. This review depicts the influence of aqueous phase properties on the hydrochar properties of SS, especially using the aqueous phase recirculation process. The mechanism during HTC of SS has been discussed, especially Maillard reaction, which would significantly influence the aqueous phase and hydrochar. The systematic information on the current research, new applications and new perspectives in this field to promote its further development were provided.

Co-hydrothermal carbonization of cellulose, hemicellulose, and protein with aqueous phase recirculation: Insight into the reaction mechanisms on hydrochar formation

Co-hydrothermal carbonization (Co-HTC) coupled with aqueous phase (AP) recirculation has huge potential to improve hydrochar production. In this study, three model compounds, namely wheat straw cellulose (cellulose, Ce), xylan (hemicellulose, He), and soya-protein (protein, Pr), were processed by HTC individually or Co-HTC in combination, with or without AP recirculation. Hydrochar formation behavior was investigated through analysis of hydrochar and AP by elemental analysis, X-ray photoelectron spectroscopy (XPS), Pyrolyzer-gas chromatography-mass spectrometry (Py-GC-MS), GC-MS, and a series of other characterizations. Elemental compositions reveal that both Co-HTC and AP recirculation promoted hydrochar formation and reduction of oxygen in hydrochar, and the combination promoted these effects. Furthermore, XPS and Py-GC-MS results show that the building blocks of hydrochar were oxygenated compounds from Ce and He such as furans and furfurals, and nitrogen-containing compounds from Pr such as amino acids and N-heterocycles. Subsequently, hydrochar formation mechanisms were proposed by results from GC-MS analysis of AP and the above observations. The major mechanisms account for enhanced hydrochar formation during Co-HTC with AP recirculation were Maillard reaction occurred between amino acids and saccharides, catalytical effects (mainly on Maillard reaction) from acids in the recycling AP, and the repolymerization or condensation of accumulated compounds in AP.

A novel strategy to simultaneously enhance bio-oil yield and nutrient recovery in sequential hydrothermal liquefaction of high protein microalgae

Advances in sequential hydrothermal liquefaction (SEQ-HTL, or two-step HTL) is limited by its low bio-oil production and enormous aqueous phase (AP) byproduct.The objective of this study was to integrate AP recirculation and fungal-microalgal cultivation with the SEQ-HTL to simultaneously improve bio-oil yield, energy recovery, and nutrient recovery. Specifically, AP from the first step (SI-AP) of SEQ-HTL was used to culture microalgae and fungi for nutrient recovery, and AP from the second step (SII-AP) was recycled four times as reaction solvent to enhance bio-oil yield. Interestingly, the SI-AP showed excellent biodegradability, and the complementary cultivation of fungi and algae has tremendous potential to recover nutrients. For example, 0.96 g/L fungi biomass and 87.71% chemical oxygen demand (COD) removal rate were obtained by fungi cultivation in SI-AP, and 1.35 g/L biomass and 63.80% ammonia nitrogen (NH4-N) removal rate were received by microalgae cultivation in SI-AP. The difference of dissolved organic matter in SI-AP and the AP from conventional HTL (HTL-AP, inhibited microalgae significantly) pointed out unsaturated substances and lignins/carboxylic-rich alicyclic molecules substances might be the primary factor inhibiting microalgae growth. During SII-AP recirculation, the yields of bio-oil (SII-Oil) and hydrochar (SII-Char) increased from 16.00% to 28.65% and 2.93% to 5.66%. 32.96–53.08% C and 15.93–24.00% N in SII-AP were recovered and transferred into SII-Oil and SII-Char. A series of characterizations showed that the increased bio-oil formation was mainly the transformation of amines, glycerol, alcohols, acetic acid, and amino acid fragments from the AP into the oil phase via Maillard reaction, Michael addition reaction and acylation. Overall, the novel strategy could improve the economic feasibility of processing high-protein microalgae biomass using the SEQ-HTL technique and is promising to the industrial applications of microalgae.

Co-liquefaction of Chlorella and soybean straw for production of bio-crude: Effects of reusing aqueous phase as the reaction medium

Co-liquefaction was combined with hydrothermal liquefaction (HTL) aqueous phase (AP) recirculation to improve the practicality of HTL process. The Chlorella powder (CL), soybean straw (SS), and their mixture (CS) with ratio 1:1 were processed at 300 °C for 20 min, and the AP was recirculated four times. The yield of CS bio-crude was increased (from 24.28% to 31.83%) by co-liquefaction, but remained stable during AP recirculation. By contrast, the yields were increased for CL bio-crude (from 32.40% to 41.19%), SS hydrochar (from 19.55% to 30.88%), and CS hydrochar (from 9.42% to 14.76%) by recirculation. The elemental analysis, chemical composition analysis, functionality analysis, thermogravimetric analysis, and verification experiments (HTL with model AP components) show the N-containing compounds (e.g., amines) in AP were converted into amides (acylation) for CL bio-crude, into N-heterocycles (Maillard reactions) for CS hydrochar, and into Mannich bases for SS hydrochar, which contributed to the increased yield and N content (from 7.27% to 8.82% for CL bio-crude). Furthermore, the O content of CS bio-crude was decreased (from 15.31% to 12.52%) by recirculation, resulted from the conversion of N-heterocyclic ketones into pyrazine derivates. The decreased O content and comprehensive combustibility index (from 0.306 to 0.177) of CS bio-crude indicate the great potential of this craft combination.

Co-Hydrothermal Carbonization of Cellulose, Hemicellulose, and Protein with Aqueous Phase Recirculation: Insight into the Reaction Mechanisms on Hydrochar Formation

Co-Hydrothermal Carbonization of Cellulose, Hemicellulose, and Protein with Aqueous Phase Recirculation: Insight into the Reaction Mechanisms on Hydrochar Formation

Hydrotreating of bio-crude obtained from hydrothermal liquefaction of biopulp: effects of aqueous phase recirculation on the hydrotreated oil

Aqueous phase recirculation increased the bio-crude yield and energy recovery along with promoting the production of N-heterocyclic compounds that lead to harsher required hydrotreating conditions.

Investigation of aqueous phase recirculation on co-hydrothermal carbonization of sewage sludge and lignite: Hydrochar properties and heavy metal chemical speciation

Excessive process water remained an important challenge in hydrothermal carbonization (HTC). In this work, co-HTC of sewage sludge (SS) and lignite (LIG) along with the aqueous phase recirculation (APC) was employed to explore the feasibility of producing clean solid fuels at 230 °C. Chemical speciation and concentration of heavy metals (HMs) in hydrochars were evaluated, as well as fuel properties. The results showed that the co-HTC with LIG was beneficial for the decline of total concentrations of Cu, Cr, Ni, Mn, and Zn. Furthermore, APC further decreased the risk of Ni without expanding the pollution of HMs. Compared with the hydrochar produced from SS, the coalification degree, higher heating values (5.61─6.19 MJ/kg), and combustion behaviour of the hydrochar derived from co-HTC were upgraded and improved. The yield of hydrochar was enhanced via APC but several fuel properties changed slightly. These findings offered a viable technology that could lower the output of wastewater and realize the energy utilization of SS.

Effects of aqueous phase recirculation on product yields and quality from hydrothermal liquefaction of rice straw

Aqueous phase (AP) recirculation is a promising process intensification strategy to improve the yield and quality of the products and cost efficiency of the hydrothermal liquefaction (HTL) process by replacing the fresh water used in the experiments. The results demonstrate that AP recirculation in the HTL of rice straw decreases the bio-crude yield from 32.6 wt% to 9.1 wt% after the third recycle, while enhancing the bio-char yield up to 64.1 wt%. The bio-crude and bio-char show improved carbon and hydrogen content with AP recirculation. The decrease in selectivity to aliphatic hydrocarbons in the bio-crude and bio-char, coupled with increase in H2 content in the gaseous phase, suggests the prevalence of dehydrogenation reactions. The bio-char achieved better thermal stability, water retention and cation exchange capacity with AP recirculation. There was a significant accumulation of K+, Ca2+ and Cl– with a concomitant decrease in silicates, sulfate and phosphate in the AP.

Bio-Crude Production Improvement during Hydrothermal Liquefaction of Biopulp by Simultaneous Application of Alkali Catalysts and Aqueous Phase Recirculation

This study focuses on the valorization of the organic fraction of municipal solid waste (biopulp) by hydrothermal liquefaction. Thereby, homogeneous alkali catalysts (KOH, NaOH, K2CO3, and Na2CO3) and a residual aqueous phase recirculation methodology were mutually employed to enhance the bio-crude yield and energy efficiency of a sub-critical hydrothermal conversion (350 °C, 15–20 Mpa, 15 min). Interestingly, single recirculation of the concentrated aqueous phase positively increased the bio-crude yield in all cases, while the higher heating value (HHV) of the bio-crudes slightly dropped. Compared to the non-catalytic experiment, K2CO3 and Na2CO3 effectively increased the bio-crude yield by 14 and 7.3%, respectively. However, KOH and NaOH showed a negative variation in the bio-crude yield. The highest bio-crude yield (37.5 wt.%) and energy recovery (ER) (59.4%) were achieved when K2CO3 and concentrated aqueous phase recirculation were simultaneously applied to the process. The inorganics distribution results obtained by ICP reveal the tendency of the alkali elements to settle into the aqueous phase, which, if recovered, can potentially boost the circularity of the HTL process. Therefore, wise selection of the alkali catalyst along with aqueous phase recirculation assists hydrothermal liquefaction in green biofuel production and environmentally friendly valorization of biopulp.

Hydrothermal liquefaction of pre-treated municipal solid waste (biopulp) with recirculation of concentrated aqueous phase

This study investigates the integration of a biomass mechanical pretreatment technology and hydrothermal liquefaction for the valorization of biopulp; the organic fraction of municipal solid waste. A preliminary screening of the hydrothermal liquefaction conditions was carried out to investigate the impact of temperature (350 and 400 °C) and the presence of alkali catalyst (K2CO3) during the hydrothermal conversion of biopulp. Sub-critical conditions and the presence of the catalyst were noticed to have a significant positive impact on the energy recovery of the bio-crude. Therefore, the 350 °C-catalytic experiment was deemed as the reference test. To increase the energy recovery in the biocrude, recirculation of the aqueous phase was further investigated. In this case, the obtained aqueous phase was fractionated into a distillate and a concentrate phase. By recirculating the concentrate phase in four consecutive cycles, the bio-crude production yield and energy recovery increased to 49.3% and 84.3%, respectively. Additionally, evaporation of the aqueous phase revealed to have a positive impact on the removal of nitrous compounds (mostly in the form of ammonia) during the fractionation process. In conclusion, the recirculation-assisted HTL of biopulp is an energy efficient process to valorize the organic matter of municipal solid waste.

Co-hydrothermal Carbonization of Water Hyacinth and Sewage Sludge: Effects of Aqueous Phase Recirculation on the Characteristics of Hydrochar

Co-hydrothermal carbonization (Co-HTC) of sewage sludge (SS) and lignocellulosic biomass has been reported as a promising technology for SS pretreatment, in terms of sterilization, deodorization, a...

Ethanol addition during aqueous phase recirculation for further improving bio-oil yield and quality

The increase of nitrogen content caused by the re-polymerization of nitrogenous compounds remains an important challenge during aqueous phase recirculation in the water solvent. In this work, aqueous phase recirculation was conducted in the ethanol-water solvent to evaluate the feasibility of ethanol addition in inhabiting the re-polymerization of nitrogenous compounds. The present study provides a comprehensive comparison of yield and quality between bio-oils derived from aqueous phase recirculation in the water solvent and ethanol-water solvent. The results showed that the addition of ethanol in aqueous phase recirculation displayed higher promotion in bio-oil yield (7%) compared with water solvent (3.55%). The energy recovery efficiency of bio-oil from ethanol-water solvent reached 87.43% after recirculation for three times, while that from the water solvent was 74.61%. The elemental analysis and gas chromatography-mass spectrometry analysis indicated that ethanol addition could inhibit the re-polymerization of nitrogenous compounds and thus decrease the nitrogen content in bio-oil. This resulted in lower NO and HCN emission peaks during the combustion process. Besides, the addition of ethanol increased the content of light molecular weight fractions and decreased the coke formation, which contributed to complete combustion and resulted in lower CO emission peaks. Overall, this work provides a feasible way for further improving the bio-oil yield and fuel quality through ethanol addition in solvent during aqueous phase recirculation.

Bio-Crude Production through Aqueous Phase Recycling of Hydrothermal Liquefaction of Sewage Sludge

Hydrothermal liquefaction (HTL) is a promising technology for the production of bio-crude. However, some unresolved issues still exist within HTL, which need to be resolved before its promotion on a commercial scale. The management of the aqueous phase is one of the leading challenges related to HTL. In this study, the sewage sludge has been liquefied at 350 °C with and without catalyst (K2CO3). Subsequently, aqueous phase recycling was applied to investigate the effect of recycling on bio-crude properties. Obtained results showed that the energy recovery in the form of bio-crude increased by 50% via aqueous phase recirculation, whereas nitrogen content in the bio-crude was approximately doubled after eight rounds of recycling. GCMS characterization of the aqueous phase indicated acetic acid as a major water-soluble compound, which employed as a catalyst (0.56 M), and resulted in a negligible increase in bio-crude yield. ICP-AES highlighted that the majority of the inorganics were transferred to the solid phase, while the higher accumulation of potassium and sodium was found in the aqueous phase via successive rounds of recycling.

Aqueous phase recirculation during hydrothermal carbonization of microalgae and soybean straw: A comparison study

Aqueous phase (AP) is the wastewater (byproduct) produced during hydrothermal treatment of biomass. This study aims to investigate the effect of AP recirculation on hydrochar from different types of biomass. Therefore, Chlorella (CL) and soybean straw (SS) were hydrothermally carbonized (HTC) at 220 °C for 240 min with AP recirculation for four cycles, respectively. The yield of hydrochar was increased from 20.5% to 26.7% for CL and from 47.7% to 54.7% for SS. More carbon distribution in hydrochar was observed, which increased the energy recovery from biomass by HTC by 10%–12%. Most fuel properties such as elemental compositions of the hydrochar were only slightly influenced by the AP recirculation. Furthermore, AP was used for algal cultivation, and the maximum biomass density of ~1.2 (inoculum optical density 680 nm of ~ 0.2) was obtained in CL AP without recirculation and in SS AP with three times of recirculation, respectively.

Hydrothermal Liquefaction of Enteromorpha prolifera for Bio-Crude Production: An Experimental Study on Aqueous Phase Recirculation

Abstract. In this study, catalytic hydrothermal liquefaction (HTL) of with aqueous phase recirculation (APR) was tested in order to improve bio-crude yield and reduce catalyst dosage. Bio-crude yields of 17.0% and 18.5% were obtained at 300°C in 30 min of non-catalytic HTL and catalytic HTL with 5.0% NaOH, respectively. Determination of alkali metals showed that 76.7% of the Na was recovered in the aqueous phase, indicating that APR provided the possibility for catalyst recycling. Bio-crude yields were increased to 20.0% and 22.7% when performing APR for non-catalytic and catalytic HTL, respectively. To investigate the effects of recycled catalyst and volatile aqueous products (VAPs), a different APR experiment with VAPs removal for catalytic HTL was conducted, and a bio-crude yield of 17.8% was achieved. It was concluded that bio-crude yield increased with APR, mainly due to the effects of VAPs. Keywords: Alkali metal, Aqueous phase recirculation, Hydrothermal liquefaction.

Effects of aqueous phase recirculation in hydrothermal carbonization of sweet potato waste

Aqueous phase recirculation was investigated in hydrothermal carbonization of sweet potato waste at 220 °C for 60 min. The result showed that the aqueous phase reuse significantly increased the hydrochar yield. The lower H/C and O/C ratios indicated that decarboxylation reaction was promoted. The CC vibration of the benzene backbone became intense, suggesting the occurrence of aromatization and polymerization reactions. Thus, the carbon content and HHV were improved. After recirculation, hydrochar showed a decrease in combustion ignition temperature whereas an increase in pyrolysis initial decomposition temperature. The burnout temperatures in combustion and terminated temperature in pyrolysis both showed an increase trend. The hydrochars obtained from the recirculation step possessed lower emissions of NOX or SO2 than that from reference step. The pyrolysis emission result showed that more high thermal stability components were formed during recirculation step. Overall, aqueous phase recirculation was a feasible way to improve hydrothermal carbonization process.

Modelling Continuous Process for Precipitated Calcium Carbonate Production from Oil Shale Ash

Oil shale ash has been described as an alternative calcium source for both CO2 absorption and to produce precipitated calcium carbonate (PCC)-type filler. The current study focuses on the feasibility of the technologies for upgrading the alkaline mineral waste on the basis of mathematical models which define the operating parameters and simulate the leaching of water-soluble waste ash components and the crystallisation of solid products in the multi-phase ash leachates–flue gas system. Proposed models using the Aspen Plus® platform was implemented and calculated both the design and operating parameters for leaching, carbonisation and aqueous phase recirculation. The accuracy of proposed PCC production (R2>93%) model was confirmed by directly comparing the results of process simulations with experimental data.