Sludge Gasification Research Articles

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.

CO2-assisted catalytic municipal sludge for carbonaceous biofuel via sub- and supercritical water gasification

This study uses CO2 as a reaction gas (99.999%) during the sub- (360 °C) and supercritical water gasification (380 °C–440 °C) of municipal sludge to realize the synergistic regeneration of CO2 and sludge into biofuels. The results showed that the reaction efficiency of sludge under CO2 atmosphere could be enhanced by adjusting the reaction parameters to increase the yield of biogas and bio-oil. Temperature was the primary driving force to promote the gasification reaction between CO2 and sludge. With increasing temperature, CO2 catalyzed the decomposition of fatty acids to form the biogas. An increase in the gasification temperature enhanced the reformation reaction between CO2 and the biogas, thus facilitating the generation of carbonaceous gases (CO, CH4, and C2–C4). Besides, CO2-assisted catalysis primarily facilitated the decomposition of organic matter by increasing the H+ concentration of reaction system. According to the elements distribution, more carbon was transferred to the biogas, and the oxygen content in the bio-oil was reduced in the presence of CO2. Correspondingly, the lower heating value of biogas and bio-oil increased to 30.18 MJ/kg and 6.32 MJ/Nm3. Using CO2 as a reaction gas has the potential to optimize the quality of sludge-derived biofuels and convert CO2 into carbonaceous fuels.

Effect of ash on the performance of iron-based oxygen carrier in the chemical looping gasification of municipal sludge

As sludge ash is rich in alkali metals, the oxygen carrier (OC) performance in the multi-cycle chemical looping gasification (CLG) of dried sludge is affected by simultaneously cycling and accompanying ash. Fe-based OC was used in a lab-scale fluidized-bed reactor to investigate the multi-cycle CLG of dried sludge and to study the effect of accompanied ash on OC particle. The synthesis gas, carbon conversion rate, and reaction rate were monitored over several cycles. Physicochemical characterization of the OC was accomplished to determine its effective components and the alkali metal deposition from ash. The results showed that as the number of cycles increased, the mole fractions of H2 and CO2 in synthesis gas decreased and increased, respectively, both eventually stabilizing. The carbon conversion rate reached its peak in fifth cycle. The gasification rate of sludge reached its maximum value in the 14th cycle, slowing down and then stabilizing thereafter. The OC was loaded with more alkali metal during the cycle, whose activity was enhanced at the beginning. However, once the cycle number became higher than 25, ash spheroidization and sintering occurred on its surface. Accordingly, the OCs particle size and mass became a little larger for the accumulation of unreacted lattice oxygen.

Steam-oxygen gasification of sewage sludge: Reduction of tar, H2S and COS with limestone as bed additive

Biogenic residue conversion to fuel and chemicals via fluidized bed gasification and synthesis is a promising pathway allowing biogenic residues to replace fossil carbon sources. This study focuses on sewage sludge gasification with the application of limestone as bed additive for tar, H2S and COS reduction. Experiments were conducted in a 20 kW fuel input bubbling fluidized bed facility with steam and oxygen as gasification agents. The syngas composition, considering the concentrations of H2, CO, CO2, CH4, CxHy, H2S, COS, NH3 and tar species, was determined for different limestone to fuel ratios. The results show that the produced syngas has high H2 and CO concentrations of up to 0.37 m3 m−3 and 0.18 m3 m−3 respectively and is thus suitable for synthesis. Tar species from sewage sludge gasification are characterized by various methods and differ from those from wood gasification. It was found that a small additive ratio of ~0.06 kg kg−1 (limestone to fuel ratio) is sufficient to reduce heavy tar species up to 75%, and with higher additive ratios, light tar species can also be partially reduced. Moreover, H2S and COS capture is demonstrated, with H2S and COS concentrations decreasing by 40%–65% with increasing limestone additive ratio. The ammonia (NH3) concentration was not affected by the additive. This study concludes that limestone addition is an effective and low cost measure to mitigate tar related facility problems and to reduce the downstream tar and sulfur cleaning efforts.

Experimental study on gasification of oil sludge with steam and its char characteristic

Hydrogen can be prepared by oil sludge (OS) gasification with steam, which is of great significance for industrial hazardous waste treatment and resource conservation. The gasification performance was studied by a tube furnace reactor. The OS gasification was carried out at different temperatures (600, 700, 800 and 900 °C) and with different steam to OS ratio (SOS) (0.1:1, 0.3:1, 0.5:1). During the gasification process, hydrogen production first increased and then decreased, and hydrogen production was faster in 5–15 min. The yield of hydrogen of OS gasification reached the maximum when the SOS was 0.3:1 at 800 °C. The highest hydrogen yield per unit mass OS was 48.50 mL min−1 g−1. After gasification, the char yield was high, generally more than 50%. It was necessary to treat the char and incineration was an effective solution for low carbon fuels. Thus particle size distribution, incineration thermogravimetric analysis and heavy metal leaching concentrations analysis were carried out. The results showed that the average particle size of char ranged from 85 to 120 µm. The char incineration process could be divided into three stages: water evaporation, the precipitation and combustion of volatiles, and the combustion of fixed carbon and heavy components. After OS gasification at 800 °C, the leaching concentrations of typical heavy metals (As, Cr, Cu, Ni, Pb and Zn) were all up to the standard. Therefore, OS gasification combined with char incineration was an effective approach for the utilization of solid waste, which can recover hydrogen energy and reduce environmental risks.

Sewage Sludge Gasification in a Fluidized Bed: Experimental Investigation and Modeling

Fluidized bed gasification is a promising process technology to manage the growing amount of sewage sludge (SS) requiring disposal. Two samples of SS, produced in different seasons of the year by a municipal wastewater treatment plant, were subjected to gasification at 850 °C in a bench-scale fluidized bed reactor using, as a gasification agent, a nitrogen/air mixture at different values of oxygen/fuel equivalence ratio (ER = 0.1–0.2). The starting materials and the output streams (syngas, tar, and solid residues) were thoroughly characterized. The fate of specific SS constituents and the characteristics of bottom ashes were addressed, so contributing to the problem of a proper SS management approach in the context of the circular economy. Computer-aided simulations were also performed, which allowed us to predict the composition of the syngas from SS gasification under operating conditions different from those experimentally investigated (i.e., reactor temperature and ER).

Co-hydrothermal gasification of microbial sludge and algae Kappaphycus alvarezii for bio-hydrogen production: Study on aqueous phase reforming

In this study, wastewater obtained from a sewage treatment plant was treated successively by using microbial consortium and macroalgae Kappaphycus alvarezii to generate microbial sludge and algal biomass. The production of green fuel was carried out via co-gasification of microbial sludge and macroalgae Kappaphycus alvarezii for a duration of 60 min, feedstock to solvent ratio (5 to 20 g of feedstock in 200 mL), sludge to algae ratio (ranging from 1:1 to 3:1) and temperature (300–400 °C) respectively. Maximum bio-hydrogen yield was 36.1% and methane yield was 38.4% at a temperature of 360 °C at a feedstock to solvent ratio of 15:200 g/mL and sludge to algae ratio of 2:1 individually. The liquid by product of co-gasification process was later subjected to photocatalytic reforming, resulted in an enhanced hydrogen composition of 61.25%.

Syngas Production from Two-Stage Sorption-Enhanced Steam Gasification of Sewage Sludge over Bifunctional Ni-Ca Catalyst

To further enhance the syngas production, the bifunctional Ni-CaO/Al2O3 catalyst was introduced into the two-stage sorption-enhanced catalytic steam gasification of sewage sludge (SS) in this study...

Effects of Lewis acid on catalyzing gasification of sewage sludge and model compounds in supercritical water

In this work, we investigated how different types and concentrations of Lewis acids effect gas yield and composition from supercritical water gasification (SCWG) of dewatered sewage sludge (DSS). Furthermore, the catalytic mechanism was investigated using AlCl3 as a representative Lewis acid catalyst. Results showed that non-catalytic gasification of DSS produced a hydrogen yield of 0.13 mol/kg organic matter (OM) and total gas yield of 4.82 mol/kg OM; while the addition of 10 wt% Lewis acid resulted in a significantly improved H2 and total gas yield of 0.77–7.76 mol/kg OM and 7.57–22.88 mol/kg OM, respectively. In addition, the Lewis acids tested herein depicted the following activity trend: AlCl3＞FeCl3＞NiCl2＞ZnCl2. The required temperature for generating hydrogen from SCWG of DSS was greatly reduced when AlCl3 was present during the heating process. Furthermore, AlCl3 significantly increased the hydrogen yield from SCWG of the model compounds. Finally, AlCl3 catalyzed SCWG of guaiacol and glycerol showed the best hydrogen selectivity.

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.

Experimental study on kinetic characteristics of oil sludge gasification

AbstractOil sludge (OS) is a typical dangerous waste in petroleum industry, which also contains abundant energy. The purpose of this study was to recover resources from OS gasification. To figure out the gasification process, a thermal analyzer was used to carry out OS gasification experiments under steam/nitrogen atmosphere with different heating rates. Results showed that the OS gasification reaction could be divided into five stages: free water volatilization, the escape of light components, the cracking of heavy components and steam reforming, the charring process and water gas shift (WGS) reactions, and inorganic mineral decomposition. The activation energy (E), pre‐exponential factor (A), enthalpy change (ΔH), Gibbs free energy change (ΔG), and entropy change (ΔS) of three main reaction stages (Stages 2–4) were analyzed through kinetic and thermodynamic analysis. From Stage 2 to Stage 4, E, ΔH, and ΔS decreased from 118.80 kJ·mol−1, 113.92 kJ·mol−1, and −4.09 J·(K mol)−1 to 13.19 kJ·mol−1, 5.92 kJ·mol−1, and −234.06 J·(K mol)−1, respectively. Nevertheless, ΔG increased from 116.01 to 210.04 kJ·mol−1. From these parameters, it could be found that the decrease of E was beneficial to the reaction. However, the values of ΔG were always positive and the reaction could not happen spontaneously.

Experimental Parameter Study on Synthesis Gas Production by Steam-Oxygen Fluidized Bed Gasification of Sewage Sludge

The conversion of biogenic residues to fuels and chemicals via gasification and synthesis processes is a promising pathway to replace fossil carbon. In this study, the focus is set on sewage sludge gasification for syngas production. Experiments were carried out in a 20 kW fuel input bubbling fluidized bed facility with steam and oxygen as gasification agent. In-situ produced sewage sludge ash was used as bed material. The sensitivity of the key operation parameters gasifier temperature, oxygen ratio, steam to carbon ratio, and the space velocity on the syngas composition (H2, CO, CO2, CH4, CxHy, H2S, COS, NH3, and tars) was determined. The results show that the produced syngas has high H2 and CO concentrations of up to 0.37 m3 m−3 and 0.18 m3 m−3, respectively, and is thus suitable for synthesis of fuels and chemicals. By adjusting the steam to carbon ratio, the syngas’ H2 to CO ratio can be purposely tailored by the water gas shift reaction for various synthesis products, e.g., synthetic natural gas (H2/CO = 3) or Fischer–Tropsch products (H2/CO = 2). Also, the composition and yields of fly ash and bed ash are presented. Through the gasification process, the cadmium and mercury contents of the bed ash were drastically reduced. The ash is suitable as secondary raw material for phosphorous or phosphate fertilizer production. Overall, a broad database was generated that can be used for process simulation and process design.

Investigation of the nitrogen migration characteristics in sewage sludge during chemical looping gasification

Increasing attention has been given to the control of nitrogen pollutants during the process of using sewage sludge. In this paper, the gasification characteristics and the law of nitrogen migration in the process of chemical looping gasification (CLG) of sewage sludge were explored. Copper slag calcined at 1100 °C (1100CS), FeAl (Fe2O3+Al2O3), NiFeAl (NiFe2O4+Al2O3), and NiAl (NiO + Al2O3) were used as oxygen carrier. The addition of an oxygen carrier could increase the carbon conversion rate and decrease the low heating value. NOx precursors (HCN and NH3) were the main nitrogen-containing gas pollutants (∼40%). The sum yield of char nitrogen and tar nitrogen was only ∼10%. Nitrile nitrogen, heterocyclic nitrogen and amide nitrogen were the main nitrogenous compounds in tar nitrogen. Increasing the oxidative activity of the oxygen carrier could significantly promote the oxidative transformation of N2 from the NOx precursors, tar nitrogen, char nitrogen. Additionally, the migration of fuel nitrogen includes synchronous autothermal pyrolysis (stage Ⅰ) and oxidative pyrolysis of the three-phase product under the action of [O] (stage Ⅱ). Finally, this research realizes the energy utilization of sludge and reduces the release of nitrogen pollutants.

Supercritical water gasification and partial oxidation of municipal sewage sludge: An experimental and thermodynamic study

Supercritical gasification (SCWG) and supercritical partial oxidation (SCWPO) technologies have emerged as preferred means of converting wet biomass to hydrogen-rich gases. We experimentally investigated the effects of moisture content, pressure and oxidation coefficient (n) on mole fraction, yield, gasification efficiency and energy recovery of gaseous products from SCWG or SCWPO of municipal sewage sludge, as well as on the carbon and nitrogen contents in liquid products. Potential of sludge for producing gaseous products was thermodynamically analyzed by an Aspen Plus model. The results show that 87 wt%, 25 MPa and n = 0 were optimum conditions for sludge gasification. Sludge with 87 wt% moisture content was pumpable at 75 °C, and further increasing the moisture content decreased the heating value and energy recovery of gaseous products. Pressure played little role in both the experimental and equilibrium gas yields. Highest mole fractions and yields of H2 and CH4 were achieved at n = 0.

Exploring the sustainable production of ammonia by recycling N and H in biological residues: Evolution of fuel-N during glutamic acid gasification

In this work, the recycling of nitrogen and hydrogen from nitrogen-rich (N-rich) biological residues via autothermal gasification has been proposed as a process suitable for the sustainable production of ammonia (NH3). Two N-rich biological residues, sewage sludge and meat and bone meal, were used and analyzed in this work and glutamic acid was selected as the model compound of the protein-fuel-N in these residues. Glutamic acid gasification experiments were carried out in order to study the effect of temperature (800–900 °C) and steam-to-carbon ratio (0.5–1.0 g g−1) on the conversion of fuel-N into the most typical N-containing gasification products: NH3, molecular nitrogen (N2), hydrogen cyanide (HCN), nitrogen monoxide (NO), tar-N and char-N. Sewage sludge and meat and bone meal were also gasified under selected operating conditions with the main aim of assessing the NH3 production. The most abundant N-containing compounds obtained in the gasification of glutamic acid were NH3 (35–51% over fuel-N) and N2 (45–63% over fuel-N). The highest conversion of fuel-N to NH3–N in the glutamic acid gasification experiments (51%) was obtained at the lowest temperature (800 °C) and the lowest S/C ratio (0.5 g g−1). The increase in the temperature caused a decrease in the yield of NH3, as a consequence of its decomposition into N2. A similar fuel-N distribution was found when sewage sludge and meat and bone meal were gasified, obtaining joint yields of HCN–N, NO–N, tar-N and char-N lower than 5%, and being NH3–N (30–67%) and N2–N (28–68%) the majority products. The yields of NH3–N obtained from glutamic acid (51%), sewage sludge (30%), and meat and bone meal (67%) under the same gasification operating conditions were significantly different. These differences were attributed to the catalytic effect of the metals present in these residues and point to the need to optimize the operating conditions specifically for each residue. In summary, gasification of sewage sludge and meat and bone meal may be able to produce around 10% of the NH3 produced annually in Europe and between 102 and 262 GJ·ton−1 NH3 thanks to the combustion of the syngas generated.

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.

Chemical looping gasification of sewage sludge using copper slag modified by NiO as an oxygen carrier

Abstract Chemical looping gasification (CLG) provides a novel approach to dispose the sewage sludge. In order to improve the reactivity of the calcined copper slag, NiO modification is considered as one of the good solutions. The copper slag calcined at 1100 °C doped with 20 wt% NiO (Ni20-CS) was used as an oxygen carrier (OC) in sludge CLG in the work. The modification of NiO can evidently enhance the reactivity of copper slag to promote the sludge conversion, especially for sludge char conversion. The carbon conversion and valid gas yield (Vg) increase from 67.02% and 0.23 m3·kg−1 using the original OC to 78.34% and 0.29 m3·kg−1 using the Ni20-CS OC, respectively. The increase of equivalent coefficient (Ω) facilitates the sludge conversion and a suitable Ω value is determined at 0.47 to obtain the highest valid gas yield (0.29 m3·kg−1). A suitable steam content is assigned at 27.22% to obtain the maximum carbon conversion of 87.09%, where an acceptable LHV of 12.63 MJ·m−3 and Vg of 0.39 m3·kg−1 are obtained. Although the reactivity of Ni20-CS OC gradually decreases with the increase in cycle numbers because of the generation of NiFe2O4-δ species, the deposition of sludge ash containing many metallic elements is beneficial to the sludge conversion. As a result, the carbon conversion shows a slight uptrend with the increase of cycle numbers in sludge CLG. It indicates that the Ni20-CS sample is a good OC for sludge CLG.

Effects of Process Parameters on Sulfur Migration and H2S Generation during Supercritical Water Gasification of Sludge

The generation of sulfur-containing pollution products affects the quality of biofuels obtained from the supercritical water gasification (SCWG) of sludge. This study investigates the effects of the gasification temperature, moisture content, and reaction atmosphere on the evolution of sulfur-containing compounds. The results showed that temperature was the key parameter causing the migration of sulfur from sludge to biogas and liquid products. The sludge decomposition reaction was dominated by ionic reactions at 360 °C, while the decomposition of organic matter was converted to free radical reactions as the temperature increased from 380 °C to 440 °C. The mercaptan and thioether contents of the bio-oil decreased to 0.3% at 440 °C. Correspondingly, the concentration of H2S increased from 6.7 ppm to 38.0 ppm. The decomposition of organic sulfur with an unstable structure (S-H bond and S-C bond) was the main cause of the increase in the content of H2S. Additionally, the solubility and oxidation properties of supercritical water were extremely strong. Some sulfur-containing organic compounds were converted into SO42- via hydrolysis and oxidation reactions, forming sulfate crystals with heavy metals in the bio-char, which aided in achieving the synergistic immobilization of sulfur and heavy metals.

Two-Stage Gasification of Sewage Sludge for Enhanced Hydrogen Production: Alkaline Pyrolysis Coupled with Catalytic Reforming Using Waste-Supported Ni Catalysts

Sewage sludge is a potential source of H2-rich fuel via a catalytic thermo-chemical reaction, which can facilitate the economical disposal of this hazardous waste. In this study, dry sludge is subj...

Thermodynamic analysis of hydrogen production via supercritical water gasification of coal, sewage sludge, microalga, and sawdust

This study investigates the thermodynamic equilibrium analysis of the supercritical water gasification (SCWG) involving several typical feedstocks (coal, sewage sludge, microalga, and sawdust). The effects of various parameters including feed concentrations, temperatures, and pressures are analyzed. It is observed that temperature and feed concentration play a determining role in the yield of hydrogen, while the effect of pressure is very limited. Results show that the feed concentration of 15–20 wt% is optimal for hydrogen production. Furthermore, the effects of composition (hydrogen to carbon ratio and oxygen to carbon ratio) of the feedstock on the yield of product gases are investigated, which is useful for screening potential feedstock for SCWG. The results show that maximum H2 and CH4 molar yields are achieved at a low O/C and a high H/C ratio.