Water Gasification Of Sewage Sludge Research Articles

Thermodynamic and environmental analysis of integrated supercritical water gasification of sewage sludge for power and hydrogen production

Supercritical water gasification is a promising process for treating sewage sludge (SS) to obtain hydrogen and electrical energy. Here, a novel thermodynamic equilibrium model for supercritical water gasification of sewage sludge was developed and the thermodynamic and environmental effects of operating parameters on the system were investigated. The sensitivity analysis revealed that the optimum reaction condition for the system was: reaction temperature of 750 °C, preheated water to SS ratio of 1, and SS concentration of 25 %. Under this condition, the system energy efficiency and exergy efficiency were 44.74 % and 46.51 %, respectively, with H2 production of 103.19 kg/h and power generation of 2526.2 kW. The energy and exergy efficiencies could be improved by reducing the preheated water to SS ratio and increasing the reaction temperature and SS concentration. The heat exchangers and the heater were the key units causing exergy losses, accounting for 40.3 % and 19.5 % of the total exergy losses, respectively. When utilizing carbon capture and storage (CCS), the global warming potential (GWP) value was only 0.31 kg CO2-eq under the optimal operating parameters, which was 61.44 % less than without CCS.

Migration of nitrogen and phosphorus during supercritical water gasification of sewage sludge: Effect of alkaline additives

The effect of alkaline additives (KOH, NaOH, Ca(OH)2, NaCO3 and K2CO3) on syngas yield, phosphorus (P) and nitrogen (N) migration during supercritical water gasification (SCWG) of sewage sludge were investigated. The alkaline additives elevated the syngas yield by affecting the steam reforming reaction, water-gas shift reaction and methanation reaction. In addition, alkaline additives enriched P and N into the aqueous phase by promoting the dissolution of phosphide (AlP, CaP, and FeP) and the decomposition of nitrogenous organic matter and hydrolysis of nitrile in sludge. The results showed that K2CO3 exhibited the best catalytic effect in these five alkaline additives. In the presence of 0.5 g K2CO3 (5%), the syngas yield was up to 18.2 mol/kg, as well as, 78.2% of P and 80.2% of N were dissolved in the aqueous phase. Moreover, the effects of different K2CO3 additions (5%–11%) on the syngas yield and the migration of N and P were discussed.

Hydrogen production optimization from sewage sludge supercritical gasification process using machine learning methods integrated with genetic algorithm

Hydrogen production from the supercritical water gasification (SCWG) of sewage sludge (SS) is a sustainable and efficient process. However, the challenging and intricate task for the experimental technique is to find out the correlation between proximate, ultimate analysis and gasification conditions with H2 production. This process is complicated, expensive and requires many experimental techniques. To accurately predict and analyze the effect of input parameters on SCWG of SS process economically, an efficient model must be developed. The novelty of this study includes the consideration of four different machine learning (ML) methods integrated with Genetic Algorithm for the prediction, analysis, and evaluation of Hydrogen yield from the supercritical water gasification of sewage sludge. The ML methods included Support Vector Machine, Ensembled Tree, Gaussian Process Regression, and Artificial Neural Network. The results suggests that GPR is favored for predicting Hydrogen yield (Coefficient of determination (R2) = 0.997, Root Mean Square Error (RMSE) = 0.093, and is highly recommended for dealing with complex variable-target correlation. On the other hand, the performance of Support Vector Machine (SVM) was poor with R2 = 0.761 and RMSE = 2.479. The R2 and RMSE for Ensembled Tree (ET) and Artificial Neural Network (ANN) was 0.994, 0.560 and 0.943, 1.521 respectively. The partial dependence plot shows that temperature, moisture content and pressure are among the effective parameters of SCWG. Furthermore, optimization techniques such as genetic algorithms are incorporated to optimize H2 production by tuning the ML hyperparameters. Additionally, a Graphical User Interface was developed by utilizing the optimized GPR model for ease in computing H2 yield.The optimum ML method integrated with GA will be beneficial for researcher to predict the H2 yield for the experimental work.

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.

Effect of alkali additives on desulfurization of syngas during supercritical water gasification of sewage sludge.

Amount of H2S and SO2 were generated during the SCWG of sewage sludge. It is essential to reduce the sulfur concentration in syngas for depth utilization of syngas from SCWG of sewage sludge. The syngas desulfurization ability of five additives (KOH, K2CO3, NaOH, Na2CO3, AC) were tested and the result indicated that K2CO3 had the best syngas desulfurization effect while KOH could significantly promote the yield of syngas at 450°C and 4% loading. Increasing KOH and K2CO3 loading to 12% could reduce around 90% of sulfur in syngas comparing to no additives. The XPS analysis results indicated that alkali additives promoted the cyclization and oxidation of unstable sulfur compounds in raw sludge, which can convert it into stable sulfur compounds such as thiophene, sulfone and sulfate. The sulfur in liquid was mainly in the forms of sulfate, and the effect of alkali and AC additive on sulfur in liquid was relatively weak.

Sulfur conversion and distribution during supercritical water gasification of sewage sludge

In order to study the transformation and distribution of sulfur element during supercritical water gasification (SCWG) of sewage sludge, sulfur contained products in different phase were detected in batch reactor. The XPS analysis showed that organic sulfur was the main part of sulfur compounds in sewage sludge. On the other hand, inorganic sulfur was dominated by SiS2, and SiS2 was subsequently converted into SiO2 and H2S during SCWG. The increase of temperature promoted H2S and SO2 yield but reduced their concentration. The addition of KOH could reduce the yield and concentration of H2S and SO2. The minimum concentration of H2S and SO2 was observed at 450 °C with KOH addition, which was 1313.94 ppm and 698.03 ppm, respectively. Sulfur compound in liquid was mainly found in the form of sulfate and its concentration decreased with increasing temperature.

Suppression of tar and char formation in supercritical water gasification of sewage sludge by additive addition

This study explored the feasibility of char and tar formation inhibition during supercritical water gasification of sewage sludge (SS) by additive addition. Experiments were conducted in autoclave with 5 wt% additives at 400 °C for 30 min. The non-additive gasification of SS resulted in a higher char yield (12.6%) and tar yield (16.4%). In contrast, the five additives reduced the char yield (3.4–11.2%), the inhibition of char yield by additives was in the order of NaOH > K2CO3 > H2O2 > acetic acid > NiCl2. The inhibition of tar formation was limited, tar yield were 13.3–18.8% with additives. Fourier-transform infrared spectroscopy was used to determine the functional groups of char/tar, and it was observed that the spectra of char were more similar to those of hydrochar obtained in a low-temperature experiment. Model compounds of potential precursors was also tested to study the mechanism of action of additives, the results reveal that additives have different effects on char/tar formation from various components, the inhibitory effects of additives on the yield of char from humus and tar from lignin were limited. Finally, the effects of additives on gasification were also studied. The addition of additives will have an impact on the hydrogen yield and gasification efficiency, which also needs to be considered when use additive to reduce the by-products yield.

Alkali-catalyzed supercritical water gasification of sewage sludge: effect of liquid residue reuse as homogenous catalyst

Conventional supercritical water gasification (SCWG) of sewage sludge produces liquid residues with organic pollutants, heavy metals and others. In order to reduce liquid residues released into the environment, this study investigated the reuse of liquid residues as feed water in subsequent SCWG. We also evaluated the effect of wastewater reuse on syngas production and fate of heavy metal. The experiment was performed in a 500-ml-capacity Hastelloy reactor at 400 °C and above 25 Mpa for 60 min with a 4:1 ratio of 10% dry wt. of solid (mixture of sludge and empty fruit bunch) and 90% wt. of liquid feed. The use of recycled liquid increased the concentration of K and Na in the solution from 26 mg/L and 91 mg/L to 49 mg/L and 290 mg/L, respectively. Subsequently, syngas production increased from 9.4 to 14.1 mol/kg organic matter. In addition, H2 production increased by 30%. The increase in syngas generation could be attributed to the alkaline salts which act as homogenous catalysts and to the organic matter in the liquid residue. In addition, the total mass of As, Cr, Cd, Cu, Pb and Zn in the liquid residues reduced from 57.36 to 47.34 µg. Thus, experimental results imply that the recycling of liquid residues in SCWG will not only improve the syngas production efficiency of the system, but also enable the accumulation of heavy metals in the solid residue to prevent pollution risk.

Supercritical water gasification of sewage sludge for power generation– thermodynamic study on auto-thermal operation using Aspen Plus

Sewage sludge conversion through supercritical water gasification presents a viable pathway for waste management and power generation which would allow the waste treatment process to be incentivized economically rather than by policy aspiration. In this conceptual study, two system configurations were developed using Aspen Plus. In the first configuration, limitation on the preheating of dewatered sewage sludge was imposed to prevent biomass degradation and tar formation which may lead to plugging in heat exchanger. The second configuration processed sewage sludge and the constraints on feed preheating was not imposed due to the higher water content of feed stream. Supplementary liquid feedstock was used to achieve auto-thermal operation in the second configuration. The supplementary liquid feedstock considered in this study included (i) waste cooking oil, (ii) heavy oil and (iii) crude glycerol, which are underutilized commercially. Feed concentration required in order to operate the gasifier auto-thermally in the temperature range of 500–700 °C were analyzed using sewage sludge data from various sources. Feed concentration required for auto-thermal operation was found to be significantly dependent on the heating value of sewage sludge and minimal lower heating value of about 12.63 MJ kg−1 was recommended in the first configuration as mechanical dewatering in real operation may only thickened the sludge up to 25% dry matter content. Higher system efficiency was achieved at higher gasifier temperature and feed concentration, up to the concentration that achieved auto-thermal operation. Further increment on feed concentration would reduce the system efficiency as the surplus syngas was not collected. The use of supplementary liquid fuel in the second configuration enabled the treatment of sewage sludge with lower energy content and more optimal feed preheating allowed the system to reach up to 21.9% in energy efficiency, markedly higher than first configuration which achieved up to 18.8%.

Supercritical water gasification of sewage sludge by continuous flow tubular reactor: A pilot scale study

• The pilot scale, continuous flow tubular reactor was designed, specially produced. • Effects of temperature, feed concentration, and KOH catalyst were studied. • Temperature, sludge concentration and KOH had great effect on the system yield. • SCWG is a promising and efficient technology for the gasification of wet biomass. • Unlike conventional gasification methods, no hazardous emissions. Treatment and disposal of sewage sludge constitute one of the major problems of wastewater treatment plants due to high water content and more stringent environmental regulations. Supercritical water gasification (SCWG) technology is accepted as a promising method for sustainable sludge disposal because of the elimination of need for costly water reduction and drying processes before disposal by conventional methods. The aim of this study is to determine the effect of temperature (450–650 °C), solid matter content (1–2%) and catalyst addition (0.5–2% KOH) on supercritical gasification of sewage sludge in a continuous-flow pilot scale tubular reactor. The results indicate that the gasification efficiency is generally temperature dependent. Furthermore, catalyst addition improves the gasification efficiency at high solids content. The produced gas contains 60% of H 2 and 22% of CH 4 at experimentally determined optimal conditions (650 °C, 2% solid matter content, 2% KOH). The resulting gas contains H 2 S and CO below detection limits and there is no need for additional treatment. Consequently, SCWG technology provides complete decomposition of organic matter in a short time, clean gas formation with higher energy content, and volumetric reduction compared to conventional methods.

Supercritical water gasification of sewage sludge and combined cycle for H2 and power production – A thermodynamic study

An integrated system of supercritical water gasification (SCWG) and combined cycle has been developed for H2 production and power generation. Sewage sludge and lignite coal were selected as raw material in this simulation. The effects of feed concentration (10–30 wt%) and lignite coal addition (0–50 wt%) on syngas yield and H2 yield were also investigated in the temperature range of 500 °C–700 °C. Several heat exchangers were considered in the proposed integrated system to minimize energy loss. High pressure syngas was expanded by using turbo-expander to produce electricity, resulting in the improvement of the total efficiency. The results showed that the minimum feed concentrations of 14.25 wt%, 18.75 wt%, and 25.50 wt% were required to achieve self-sufficient energy at 500 °C, 600 °C, and 700 °C, respectively. However, the lower feed concentration and higher temperature were preferable for syngas production. The highest syngas and H2 yield were obtained at 700 °C and 10 wt% feed concentration. The SCWG could produce 178.08 kg syngas from 100 kg feed and 9.06 kg H2 were obtained after H2 separation. The total power generation from turbo-expander and combined cycle module was 48.37 kW. By combining SCWG and combined cycle, the total efficiency could reach 63.48%. It worth mentioning that the addition of lignite coal could help reduce the minimum feed concentration to achieve autothermal condition, but did not have significant improvement on H2 production.

Hydrogen production and phosphorus recovery via supercritical water gasification of sewage sludge in a batch reactor

In this study, gasification of sewage sludge in supercritical water using a batch reactor was investigated. The effects of temperature, retention time, and the oxidation coefficient on gas composition, gas yield, total organic carbon removal efficiency (XTOC), gasification efficiency (GE), carbon gasification efficiency (CE), and phosphorus release rate (Xp) were investigated. The experimental results indicated that the yields for hydrogen, methane, and carbon dioxide increased with the increase in temperature from 380 °C to 460 °C. A maximum hydrogen molar fraction of 55.72% and a yield of 19.86 mol/kg were obtained at 460 °C and 27 MPa after 6 min. The GE, CE, XTOC, and Xp also increased with the increase in temperature. An extension of the retention time promoted the gasification of sludge, thereby resulting in an increase in the hydrogen and methane molar fraction, yield, GE, CE, XTOC, and Xp. Under the conditions of 420 °C and 27 MPa after 6 min, with an increase in the oxidation coefficient from 1.5 to 2.5, the oxidation reaction became dominant in the supercritical water. Hydrogen and methane were converted to carbon dioxide and water with an excess of hydrogen peroxide, which resulted in a lower hydrogen yield. However, the decomposition of organic compounds in the sludge was promoted with the addition of hydrogen peroxide, thereby resulting in an increase in the GE, CE, XTOC, and Xp. When the oxidation coefficient reached 2.5, a maximum GE of 131.6% and Xp of 98.74% were obtained.

Polycyclic aromatic hydrocarbon formation during the gasification of sewage sludge in sub- and supercritical water: Effect of reaction parameters and reaction pathways

The formation of polycyclic aromatic hydrocarbon is a widespread issue during the supercritical water gasification of sewage sludge, which directly reduces the gasification efficiency and restricts the technology practical application. The changes of the concentrations and forms as well as the synthesis rate of polycyclic aromatic hydrocarbons in the residues from supercritical water gasification of dewatered sewage sludge were investigated to understand influence factors and the reaction pathways. Results showed that the increase of reaction temperature during the heating period favours directly concentration of polycyclic aromatic hydrocarbon (especially higher-molecular-weight), especially when it raise above 300 °C. Lower heating and cooling rate essentially extend the total reaction time. Higher polycyclic aromatic hydrocarbon concentration and higher number of rings were generally promoted by lower heating and cooling rate, longer reaction time and higher reaction temperature. The lower-molecular-weight polycyclic aromatic hydrocarbons can be directly generated through the decomposition of aromatic-containing compounds in sewage sludge, as well as 3-ring and 4-ring polycyclic aromatic hydrocarbons can be formed by aromatization of steroids. Possible mechanisms of reaction pathways of supercritical water gasification of sewage sludge were also proposed.

Supercritical water gasification of sewage sludge in continuous reactor

In this study, a process for the continuous recovery of phosphorus and generation of gas from sewage sludge is investigated for the first time using supercritical water gasification (SCWG). A continuous reactor was employed and experiments were conducted by varying the temperature (500–600 °C) and residence time (5–60 s) while fixing the pressure at 25 MPa. The behavior of phosphorus during the SCWG process was studied. The effect of the temperature and time on the composition of the product gas was also investigated. A model of the reaction kinetics for the SCWG of sewage sludge was developed. The organic phosphorus (OP) was rapidly converted into inorganic phosphorus (IP) within a short residence time of 10 s. The gaseous products were mainly composed of H2, CO2, and CH4. The reaction followed first order kinetics, and the model was found to fit the experimental data well.

Supercritical water gasification of sewage sludge: Gas production and phosphorus recovery

In this study, the feasibility of the gasification of dewatered sewage sludge in supercritical water (SCW) for energy recovery combined with P-recovery from the solid residue generated in this process was investigated. SCWG temperature (400°C, 500°C, 600°C) and residence time (15min, 30min, 60min) were varied to investigate their effects on gas production and the P recovery by acid leaching. The results show that the dry gas composition for this uncatalyzed gasification of sewage sludge in SCW mainly comprised of CO2, CO, CH4, H2, and some C2–C3 compounds. Higher temperatures and longer residence times favored the production of H2 and CH4. After SCWG, more than 95% of the P could be recovered from the solid residue by leaching with acids. SCWG combined with acid leaching seems an effective method for both energy recovery and high P recovery from sewage sludge.

Risk Assessment of PAHs in Solid Residues from Supercritical Water Gasification of Sewage Sludge

In this study, the contents of polycyclic aromatic hydrocarbons (PAHs) in sewage sludge (SS) and solid residue (SR) from supercritical water gasification of SS were analyzed and the pollution degree and human health risk with PAHs in SS and SR samples were determined. SR samples were collected from different reaction condition, according to different reaction temperature, time and water content. The results show that the contents of PAHs in SR samples were increased after gasification reaction, ranging from1541 to 8234.8μg/kg, and that the PAHs with 4-6 rings are major pollutants in the SR samples. Based on Nemero Index P, the classification evaluation shows that the SR samples almost exceed the light polluted. While, the health risk assessment according to Benzo [pyrene total potency equivalents present that the health risks from PAHs are acceptable for children and adults using the SR sample for agricultural land.

Supercritical water gasification of sewage sludge using bench-scale batch reactor: advantages and drawbacks

Sewage sludge, a byproduct of municipal wastewater treatment, was gasified by supercritical water using a bench-scale batch reactor. Configuration of bench-scale batch reactor and operation procedures are discussed in detail. Experience and challenges that arose during the experiment are also shared. Using the bench-scale reactor under the condition of 600 °C, 23 MPa, and a reaction time of 60 min without catalyst presence, a total gas yield of 9.8 mol/(kg-sewage sludge) was obtained. Furthermore, investigations on operational parameters were conducted. Extension of reaction time up to 60 min increased the gasification, reaching a plateau thereafter. Investigation on pressure indicated the superiority of supercritical pressure. The addition of Ni as a catalyst also promoted gasification, although inorganic salts and char seemed to cover the catalyst surface. With regard to the prospect of future operation at a municipal waste water treatment plant, the effect of operational parameters on heavy metal concentration in the liquid phase is also discussed.

Pa-216 超臨界水ガス化による下水汚泥からのリンの回収(2.技術,ポスターセッション1)

Pa-216 超臨界水ガス化による下水汚泥からのリンの回収(2.技術,ポスターセッション1)

Quantitative analysis of polycyclic aromatic hydrocarbons in solid residues from supercritical water gasification of wet sewage sludge

Hydrogen gas has been successfully produced from sewage sludge by supercritical water gasification. Therefore, whether toxic compounds are synthesized in the supercritical water gasification of sewage sludge should be clarified because of the complex pollutants in sludge. In the current study, the content and forms of polycyclic aromatic hydrocarbons (PAHs) in the solid residue from the supercritical water gasification of wet sludge was investigated using a high-pressure autoclave. The process parameters, such as reaction temperature (400–455°C), reaction time (0–60min), dry matter content (5.6–23.8wt%) in the feedstock, and heating period, were varied to investigate their effects on the PAH content and forms. The results showed that PAHs are generated during supercritical water gasification and that high reaction temperature, long reaction time, and low dry matter content favor the formation of PAHs in the solid residue, mainly involving 4-ring PAHs. Moreover, the amount of total PAHs in the solid residue did not exceed the allowable limits of soil quality assessment for exhibition (B class) in China and meets the Canadian soil quality for commercial land use. In addition, the mechanism of PAH formation during supercritical water gasification was discussed in this paper.

Quantitative evaluation of heavy metals in solid residues from sub- and super-critical water gasification of sewage sludge

Solid residues (SRs) are important byproducts of sub- and super-critical water gasification of sewage sludge (SS). In this study, the quantitative evaluation of heavy metals (HMs) in SRs, compared with SS, is applied in terms of potential ecological risks, pollution levels, and both bioavailability and eco-toxicity. The results show the bioavailability and eco-toxicity of HMs in SRs decrease, although the total concentration of HMs increased, particularly in the bioavailable fraction of Cu, which decreased nearly 97%. The geo-accumulation and potential ecological risk index indicated that the gasification process increased contamination by two levels (to the maximum), while the overall risk was in keeping with SS. However, based on the risk assessment code, each tested HM exhibited lower environmental risk after gasification, especially for Cd, which drastically dropped from 66.67 (very high risk) in SS to 0.71 (no risk) in SRs, with a reaction temperature of 375°C for 60min.