Thermodynamic investigation on gasification performance of sewage sludge-derived hydrochar: Effect of hydrothermal carbonization

Compared with the conventional thermal drying process, hydrothermal carbonization (HTC) can reduce the energy cost of water removal from sewage sludge prior to its steam gasification. However, less attention is paid on the interactions between HTC and gasification. In this study, the thermodynamic evaluation on hydrochar gasification performance under different operating conditions including HTC duration (τ), HTC temperature (T_HTC), gasification temperature (T_g), and steam/hydrochar mass ratio (S/C ratio) is performed. Two indicators including carbon conversion rate (CC) and cold gas efficiency (CGE) are used to assess the gasification performance. The results show that elevating both gasification temperature and S/C ratio can enhance the H₂ production, which also result in the increase of CC and CGE. The content and gasification activity of fixed carbon increase under moderate HTC duration and temperature, favoring the H₂ formation despite of the apparent loss of volatiles species in the hydrochar. Longer HTC duration or higher HTC temperature declines the H₂ production due to the sharp reduction of carboxyl and hydroxyl groups, weakening water gas reaction and on-site reforming reaction of tar occurred on the hydrochar surface. In terms of the values of CC = 93.9% and CGE = 64.38%, the optimum HTC conditions of τ = 30min and T_HTC = 200 °C can be determined. The data provided here favor guiding HTC treatment of sewage sludge targeting gasification and thus promoting the development of this promising waste-to-energy technology.     

Kinetics of hydrothermal carbonization (HTC) of soft rush

Hydrothermal carbonization of soft rush (Juncus effusus) has been studied in the temperature range of 453–513 K. Citric acid at the concentration of 0.2 kg m⁻³ did not have a significant catalytic effect. A kinetic model is suggested for substrate hydrolysis and the formation of hydrochar as well as byproducts remaining dissolved. Specifically, increasing hydrochar yield at higher substrate concentration can be described with a reaction order of 1.53 for hydrochar formation from intermediate products.     

A review of the current knowledge and challenges of hydrothermal carbonization for biomass conversion

Greenhouse gases emitted from the excessive use of fossil fuels are threatening the environment, and thus alternative resources like biomass are being considered as a replacement. Biomass with high moisture content is better treated by hydrothermal carbonization method than any other process to generate biofuel. Research on this method on a lab scale has progressed recently. However, due to the complex reaction mechanisms and operational barriers, more improvements are required to make it a commercial technology. This paper aims to review the development of hydrothermal carbonization with a focus on the practical aspects of the process. Many references have been reviewed critically to provide a well-structured source for improving this process. After providing information about the biomass structure and general knowledge of hydrothermal carbonization, the challenges faced in attempts to improve the process have been identified as lack of valid kinetic and heat transfer models and insufficient data on continuous and large-scale reactors. Useful and practical suggestions have been presented to tackle all these challenges.     

Influence of phosphorus on ash fouling deposition of hydrothermal carbonization sewage sludge fuel via drop tube furnace combustion experiments

Phosphorus effect on ash fouling deposition produced during combustion process of sewage sludge solid fuel is a very important factor. Previous studies have only focused on decrease of the ash melting temperature and increase of slagging and sintering by phosphorus content. Therefore, research regarding combustion fouling formation and its effect on temperature reduction of deposit surface by phosphorus content is insufficient. Ash fouling is an important factor, because ash in the combustion boiler process deposits on the surface of heat exchanger and interferes with heat exchange efficiency. In particular, temperature reduction of heat exchanger surface via fouling should be considered together with fouling deposition, because this is related to the heat exchanger efficiency. Synthetic ash, phosphorus vaporization, and drop tube furnace experiments were performed to investigate effect of phosphorus on ash fouling formation and temperature reduction of deposit surface under combustion condition. Phosphorus was highly reactive and reacted with ash minerals to produce mineral phosphate, which promoted ash fouling deposition during the combustion experiments. In contrast, the occurrence of sintering on deposited fouling resulted in formation of a large hollow structure, which alleviated the temperature reduction on the deposit surface. Phosphorus content had a substantial correlation with fouling deposition behavior and influenced reduction in the surface temperature of the heat exchanger, because it led to generating low temperature mineral phases.     

A novel approach for prediction of mass yield and higher calorific value of hydrothermal carbonization by a robust multilinear model and regression trees

This study shows a mathematical and statistical analysis to generate models based on multiple linear regression (MLR) and regression trees (RT) that allow a reliable prediction of the Mass Yield (MY) and the Higher Heating Value (HHV) of the final solid product obtained by Hydrothermal Carbonization, called hydrochar. MLR models were obtained for lignocellulosic and non-lignocellulosic biomass using a set of experimental data with more than 500 points collected from the literature. A new approach based on dimensionless groups of variables that describe the composition of biomass and operational conditions was used. The analysis for each equation indicated that the MY depends on the process conditions and the biomass composition, which is proportional to the Polarity Index (IP) and Reactive Index (IR) values. On the other hand, the severity factor (log Ro) and the initial calorific value (HHVo) were the main factors for the HHV, but also the raw biomass composition (IP and H/C ratio) had an opposite and equal significant effect. For these equations, the results indicated an adjusted R² (R²_a) of about 0.90 and an average RMSE of 6% and 1.7 MJ/kg for MY and HHV, respectively.Besides, explanatory variables were analyzed by their Relative Importance for the RT models. The severity factor (65%) and the IR (18%) were the most decisive variable in the MY prediction. The R² and RMSE were 0.73 and 2%, respectively. For HHV, the variables with the most significant impact were the HHVo (33%), the log Ro (24%), and the IP (22%). In this case, the R² and RMSE were 0.87 and 0.68 MJ/kg, respectively. Therefore, the model equations obtained are a powerful tool to predict the mass yield and the energetic value of the hydrochar before developing an experimental study.     

Investigation of hydrothermal carbonization and chemical activation process conditions on hydrogen storage in loblolly pine-derived superactivated hydrochars

While the challenge of storing hydrogen in inexpensive and renewable adsorbents is relentlessly pursued by researchers all over the world, application of hydrochar derived from biomass is also gaining attention as it can be subsequently chemically activated using activating agents like KOH in order to tailor the development of favorable porosity. However, the synergistic effect of hydrothermal carbonization (HTC) process conditions as well as KOH activating conditions on the development of surface morphology is required to be assessed with the application of such porous superactivated hydrochars in hydrogen storage application. In this study, highly porous superactivated hydrochars were fabricated from inexpensive and abundant loblolly pine. Loblolly pine was hydrothermally carbonized at 180 °C, 220 °C and 260 °C and the hydrochars were then activated at different experimental conditions of 700 °C, 800 °C and 900 °C using solid KOH to loblolly pine hydrochar ratio of 2:1, 3:1 and 4:1 to produce superactivated hydrochars. Superactivated hydrochars as well as loblolly pine and its corresponding hydrochars underwent physicochemical analysis as well as surface morphology analysis by SEM and nitrogen adsorption isotherms at 77 K in order to investigate the effect on BET, pore volume, and pore size distribution due to various process conditions. The superactivated hydrochars were then analyzed to quantify total hydrogen storage capacity of these materials at 77 K and up to pressure of 55 bar. Porosity of superactivated hydrochars were as high as 3666 m²/g of BET specific surface area (SSA), total pore volume of 1.56 cm³/g and micropore volume of 1.32 cm³/g with the hydrogen storage capacity of 10.2 wt% at 77 K and 55 bar. It was conclusive from principal component analysis that higher HTC temperature with moderate activation condition demonstrated favorability in developing porous superactivated hydrochars for hydrogen storage applications.     

Chemical evaluation of chars produced by thermochemical conversion (gasification,pyrolysis and hydrothermal carbonization) of agro-industrial biomass on a commercial scale

Technologies for agro-industrial feedstock utilization such as pyrolysis, gasification and hydrothermal carbonization at industrial scale develop rapidly. The thermochemically converted biomasses of these production technologies have fundamentally different properties controlled by the production technology. This is reflected by general properties such as pH or elemental composition. The ¹³C NMR spectroscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy and black carbon results confirmed these observations showing that hydrochars have lower proportions of aromatic compounds than biochars (less stable) but are rich in functional groups (higher cation exchange capacity) than biochars. Analyses of pollutants indicate that polycyclic aromatic hydrocarbons as well as dioxin contents of most samples were under the threshold values recommended by International Biochar Initiative and European Biochar Certificate. In conclusion, biochars and hydrochars are entirely different from each other and these materials will probably have a complementary reaction in a soil environment.     

Hydrothermal carbonization of microalgae II. Fatty acid,char, and algal nutrient products

A process for isolation of three products (fatty acids, chars and nutrient-rich aqueous phases) from the hydrothermal carbonization of microalgae is described. Fatty acid products derived from hydrolysis of fatty acid ester groups in the microalgae were obtained in high yield and were found to be principally adsorbed onto the char also created in the process. With the highest lipid-containing microalga investigated, 92% of the fatty acids isolated were obtained by solvent extraction of the char product, with the remaining 8% obtained by extraction of the acidified filtrate. Obtaining the fatty acids principally by a solid–liquid extraction eliminates potential emulsification and phase separation problems commonly encountered in liquid–liquid extractions. The aqueous phase was investigated as a nutrient amendment to algal growth media, and a 20-fold dilution of the concentrate supported algal growth to a level of about half that of the optimal nutrient growth medium. Uses for the extracted char other than as a solid fuel are also discussed. Results of these studies indicate that fatty acids derived from hydrothermal carbonization of microalgae hold great promise for the production of liquid biofuels.     

Highly crystalline lithium-manganese spinel prepared by a hydrothermal process with co-solvent

Lithium-manganese spinel is prepared by a hydrothermal process that uses ethanol as the co-solvent. The crystallinity, particle morphology and electrochemical performance of the spinel are examined and compared with those obtained without the co-solvent. The amount of co-solvent and reaction time are adjusted to control the properties. The addition of ethanol leads to uniform particle size and shape, as well as higher crystallinity, than for spinel prepared in pure water. The co-solvent also reduces the time required for synthesis. A prolonged reaction time is effective in obtaining high-purity Li-Mn spinel in pure water but more impurities form after a long reaction time in an ethanol-added solvent. A mechanism for this process is suggested. A report is given of the electrochemical performance of Li-Mn spinel, including the capacity, rate capability and cyclability, as well as the effects of the co-solvent on these properties.     

我国市政污泥处理处置现状研究

随着我国生态文明建设进程日益加快,污泥处理处置行业也得到快速发展。我国市政污泥处理处置技术呈现多样化发展趋势,焚烧、厌氧发酵、好氧堆肥、热解炭化等方式尤为常见。统计分析了国内385个典型污泥处理处置项目的处理规模、各技术占比、投资、运行成本等指标,并针对各污泥处理工艺碳排放情况进行综述,以期为未来污泥处理处置技术发展方向提供一定参考。数据显示,截至2022年3月,我国污泥项目建设总规模达4851.4万t/a,污泥无害化理论处置率达73.5%。污泥处理处置项目以焚烧、厌氧消化、好氧堆肥为主,分别占比65.41%、15.55%、9.61%。对污泥碳排放水平、处理成本、能源回收效率、资源化利用水平等因素的综合分析表明,污泥厌氧消化是当前较具潜力的处理技术。     

Growth of single-crystal α-MnO2 nanotubes prepared by a hydrothermal route and their electrochemical properties

Single-crystal α-MnO₂ nanotubes are synthesized by a facile hydrothermal method without the assistance of a template, a surfactant and heat-treatment. The single-crystal α-MnO₂ nanotube electrode possesses a high specific capacitance with a good power capability. The excellent pseudo-capacitive properties are attributed to a nanotubular microstructure and a large tunnel cavity in the α-MnO₂ crystal structure. Single-crystal α-MnO₂ nanotubes with good electrochemical performance can be a promising candidate as supercapacitor materials.     

Hydrothermal treatment of black liquor for energy and phenolic platform molecules recovery in a pulp mill

The aim of this study was to study the conversion of black liquor under hydrothermal conditions and its integration in a pulp mill. Three sulfur-free black liquors produced from caustic soda cooking of prehydrolyzed softwood, prehydrolyzed hardwood and non prehydrolyzed hardwood chips were converted. Experiments were performed in a batch reactor, for temperature between 250 °C and 310 °C, and for holding time between 5 and 120 min. Three phases were formed: an aqueous phase containing monomeric phenolic compounds and the sodium cations, a biocrude containing most of the carbon, and a small amount of gas. The combustion of the biocrude could allow an energy recovery for the mill of up to 70%. The main monomeric compounds identified in aqueous phase were phenol, catechol, guaiacol and syringol, with a total yield up to 28 g kg⁻¹ of dry BL, at 250 °C. Among them, guaiacol was the major product. Sodium recovery was 97 %, slightly better than typical kraft recovery value, and compatible with causticizing. Finally, results obtained with a kraft softwood lignin were compared to those obtained with softwood black liquor. Results show that biocrude yields were greater with black liquor, whereas platform molecules production was higher with lignin. Presence of carbohydrates derivatives in black liquor is identified as a major parameter for biocrude production as it would favor bonding between phenolic species.     

Optimizing external voltage for enhanced energy recovery from sludge fermentation liquid in microbial electrolysis cell

Waste activated sludge (WAS), which is rich in organic substances, provides an energy resource. To recover hydrogen from the organic wastes, microbial electrolysis cell may be used as an efficient device. Since different extra applied voltages have significant effects on the efficiency of microbial electrolysis cell, this paper explores different extra applied voltages (0.6 V–1.2 V) affecting the utilization of sludge fermentation liquid (SFL) that is treated with synchronous double-frequency (28 + 40 kHz) and alkali coupling 72-bacth mesothermal anaerobic fermentation (35 °C). It is found that 0.8 V was the optimum extra applied voltage. With this voltage, the highest energy recovery efficiency will be 169 ± 1% and the peak of soluble chemical oxygen demand (SCOD) removal efficiency can be found at 51.4 ± 0.6%; Coulombic efficiency is 98.9 ± 1.0%. The order of complex matter consumption is found to be HAc > HPr > nHBu > nHVa > total carbohydrates > protein. The processing methods of synchronous double-frequency, alkaline, coupling with anaerobic fermentation are feasible for microbial electrolysis cell to transform large amount of waste activated sludge into energy.     

Coupling of hydrothermal pretreatment and supercritical water gasification of sewage sludge for hydrogen production

In this study, hydrothermal pretreatment and supercritical water gasification were coupled to form a combined process for the treatment of dewatered sludge for hydrogen production. First, the effects of varying hydrothermal pretreatment conditions on the transformation of organic matter in sludge were studied. Results showed that about 31% of the carbon in sludge was transferred into liquid products at 250 °C for 60 min, which were considered to be the optimal pretreatment conditions considering both the hydrothermal pretreatment effects and the energy consumption requirements. The organic matter components were determined, showing that 87% of the carbohydrate components in sludge were transformed during the process of hydrothermal pretreatment, with 49% of crude proteins and 62% of humus remaining in the solid phase products. During the subsequent process of supercritical water gasification, AlCl₃, KOH, K₂CO₃ and CaO were selected as catalysts. Compared with directly catalyzed supercritical water gasification of sludge, the integrated process was found to improve H₂ selectivity, H₂ yield and energy recovery. Moreover, the use of AlCl₃ as a catalyst showed the highest H₂ yield and energy recovery. The H₂ yield and the energy recovery increased by 45.1% and 13.2%, respectively.     

An approach of auxiliary carbohydrate source on stabilized biohythane production and energy recovery by two-stage anaerobic process from swine manure

In this study, a two-stage biohythane production system was used to treat swine manure to solve the high Chemical Oxygen Demand (COD) concentration and verify the total energy recovery between the two-stage and a traditional single-stage system. Experiments were carried out in single-stage methane production, two-stage biohythane production in long Hydraulic Retention Time (HRT), and short HRT. The COD removal efficiency and energy recovery were finally compared between single-stage (CH₄ fermenter) and two-stage (H₂+ CH₄ fermenter) systems. The results showed that the methane production rate of 53.2 ± 2.7 mL/d.L, the COD removal efficiency of 29.6 ± 5.8%, and total energy recovery of 2.9 ± 0.1 kJ/L.d was obtained in the single-stage of methane production system with HRT 11.08 d, pH 7, and temperature 55 °C, respectively. In the two-stage of hydrogen and methane productions system, the hydrogen production rate of 1.8 ± 0.7 mL/d.L, the methane production rate of 65.7 ± 2.5 mL/d.L, the COD removal rate was 97.8 ± 1.7%, and the total energy recovery of 3.6 ± 0.1 kJ/L.d was obtained and stabilized when the sugary wastewater content gradually reduced to 0%. This study shows that the methane production rate increases 20%, COD removal efficiency increases to 97.8 ± 1.7%, and total energy recovery increases 30%. At the same time, the single-stage (CH₄ fermenter) switched to a two-stage (H₂+ CH₄ fermenter) system. The two-stage anaerobic biohythane production system successfully treated the high organic swine manure and obtained a higher energy recovery against the traditional single-stage of the biomethane production system.     

Novel nanorod Ti0·7Ir0·3O2 prepared by facile hydrothermal process: A promising non-carbon support for Pt in PEMFCs

Late transition metal doped TiO₂ has been exploited for generating efficient catalyst support by enhancing electrical conductivity and modifying properties of TiO₂. The Ti_0·7Ir_0·3O₂ nanorod (NRs), a novel catalyst support for Pt nanoparticles, was prepared for the first time via single-step hydrothermal process at low temperature using IrCl₃·3H₂O and TiCl₄ as starting materials. We found that the Ti_0·7Ir_0·3O₂ NRs with 70–80 nm in length and 25–30 nm in width is successful prepared at 210 °C for 12 h without utilizing surfactants or stabilizers. In addition, the Ti_0·7Ir_0·3O₂ NRs was presented principally as a single-phase solid with the TiO₂ is in the rutile form with high crystallinity without using further treatment after synthesis. More importantly, we found that the Ti_0·7Ir_0·3O₂ NRs possesses high electrical conductivity (0.028 S cm⁻¹) dealing the intrinsically non-conducted drawback of TiO₂. The Pt nanoparticles were then deposited on the support of Ti_0·7Ir_0·3O₂ NRs via chemical reduction method. The properties of 20 wt % Pt/Ti_0·7Ir_0·3O₂ NRs electrocatalyst were characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), the cyclic voltammetry (CV). The uniformly distributed small Pt nanoparticles (3–4 nm diameter) were well adhered to the Ti_0·7Ir_0·3O₂ NRs. The electrochemically active surface area (ECSA) of 20 wt % Pt/Ti_0·7Ir_0·3O₂ NRs was higher than that of the commercial 20 wt % Pt/C (E-TEK) due to the small size and good dispersion of Pt nanoparticles on the surface of Ti_0·7Ir_0·3O₂ NRs. Moreover, the ECSA value of the Pt/Ti_0·7Ir_0·3O₂ NRs retained up to 88% after 2000 cycles of cyclic voltammetry, suggesting the high stability of catalyst resulted from strong metal support interaction (SMSI) of Titania-based materials with the noble metals. More importantly, the onset potential of Pt/Ti_0·7Ir_0·3O₂ NRs catalyst towards oxygen reduction reaction is more positive (∼80 mV) compared to commercial Pt/C, indicating the high catalytic activity of the Pt/Ti_0·7Ir_0·3O₂ NRs catalyst. The results of this research suggested that novel Ti_0·7Ir_0·3O₂ NRs could be applied as promising robust non-carbon support for Pt. This research also creates a preliminary step for investigating systematically promising Iridium doped Titania materials.     

Cascaded production of biogas and hydrochar from wheat straw: Energetic potential and recovery of carbon and plant nutrients

The increasing interest in biogas production has brought notable attention to lignocellulosic wastes as a promising and yet unexploited feedstock. As these materials are usually highly recalcitrant the energetic efficiency of biogas production, however, is comparatively low. With the aim to overcome this drawback, a novel cascaded approach was investigated that combines anaerobic digestion with hydrothermal carbonization (HTC). The latter is used to convert the digestate into a carbon-rich product termed hydrochar. An energetic evaluation of this cascaded treatment shows that the energy recovery can be nearly doubled compared to single anaerobic digestion.Furthermore, systematic HTC experiments with both fresh and digested wheat straw and with reaction temperatures of 190 °C, 210 °C, 230 °C, and 250 °C revealed an effect of reaction temperature on carbon, nitrogen, and phosphorus concentration in the final hydrochar. Carbon, nitrogen and phosphorus are primarily retained in the hydrochar, which could favor its use as soil ameliorant instead of an energy carrier.     

Simulated process integration of wastewater electrooxidation with recuperated micro gas turbine for energy recovery

Electrooxidation (EO) is a promising wastewater treatment technology that is plagued with high energy consumption. This study simulates the energy recovery potential from EO by process integration with recuperated micro gas turbine (RMGT). Gainful utilization of hydrogen produced at cathode is key to it. Percent recoverable energy (PRE) from EO would be strongly influenced by the cathodic current efficiency (CCE) and interelectrode potential (IP) and was likely to vary from a high of 46% to a low of 14%. Simulations suggested that process integration with RMGT could potentially recover 72.4% of this energy – 30.7% as electricity and 41.7% as thermal energy in the form of hot water at 353 K. This translated to 6.3%–9.7% electricity saved in EO besides recovered thermal energy. Energy dynamics was found to be highly sensitive to recuperator performance of the RMGT. Better recuperator performance made energy recovery through process integration more electricity driven at the cost of recovered heat and total energy recovery.     

Maximize the operating profit of a SWRO-PRO integrated process for optimal water production and energy recovery

Pressure retarded osmosis (PRO) is a promising technology to reduce the specific energy consumption and the operating expenditure of a seawater reverse osmosis (SWRO) plant. In this study, a simple analytical PRO model is developed to predict the PRO performance as the dilution of draw solutions occurs. The model can predict the PRO performance with a high accuracy without carrying out complicated integrations and experiments. The operating profit of SWRO-PRO is also studied by calculating the profit generated for every m³ of seawater entering the process because maximizing the operating profit is the uttermost objective of the SWRO-PRO process. Based on the PRO analytical model, the operating profit and the dynamics of the SWRO-PRO process, a strategy has been proposed to maximize the operating profit of the SWRO-PRO process while maintaining the highest power density of the PRO membranes. This study proves that integration of SWRO with PRO can (1) push the SWRO to a higher recovery and maintain its high profitability, (2) effectively reduce the specific energy consumption of desalination by up to 35% and (3) increase the operating profit up to 100%.     

Sewage sludge-to-energy approaches based on anaerobic digestion and pyrolysis: Brief overview and energy efficiency assessment

Energy recovery from sewage sludge offers an opportunity for sustainable management of sewage sludge and energy. Anaerobic digestion and pyrolysis are among the most promising processes applicable for sewage sludge-to-energy conversion. Anaerobic digestion of sewage sludge forms methane-rich biogas, which can be utilized as fuel to offset heat and electricity consumption of the wastewater treatment sector. However, the digestion process has the limitation that it cannot sufficiently extract the energy in sewage sludge. The digested sludge is still energy profitable in that it contains considerable organic matter, but poor in biodegradability. Sludge pyrolysis is an innovative process that can convert both raw and digested sludge into useful bioenergy in the form of oil and gas, forming biochar as a byproduct that is environmentally resistant and holds potential for carbon sequestration and soil conditioning. It is expectable that sludge pyrolysis would step into practical deployment in the near future.This paper presents a brief overview of anaerobic digestion and pyrolysis in the application to bioenergy production from sewage sludge. An assessment of energy conversion efficiency of two parallel sludge-to-energy pathways is also presented. One pathway relies on an exclusive pyrolysis process (fed with raw sludge) while the other is based on anaerobic digestion followed by pyrolysis (fed with the digested sludge). The pathway via the combination of anaerobic digestion and pyrolysis could achieve higher energy efficiency compared to the pathway employing the pyrolysis alone.