Study on the nitrogen transformation during the primary pyrolysis of sewage sludge by Py-GC/MS and Py-FTIR

The nitrogen transformation with attention to the intermediates and NO_x precursors has been investigated during the primary pyrolysis of sewage sludge by using Pyrolyzer-gas chromatography/mass spectrometry (Py-GC/MS) and Pyrolyzer-Fourier transform infrared spectrometry (Py-FTIR). A three-stage process of nitrogen transformation during the sewage sludge pyrolysis was suggested. The decomposition of labile protein and inorganic ammonium salt mainly occurred in the first stage (<300 °C), giving rise to a small amount of NH₃. In the second stage (300–600 °C), the macromolecular protein firstly cracked into small molecular amine compounds, and then went through deamination process, contributed to a large release of NH₃. In the third stage (600–900 °C), the amine compounds converted into nitriles, and generated a large amount of HCN, while the formation of NH₃ slowed down accordingly.     

Microwave-assisted methane decomposition over pyrolysis residue of sewage sludge for hydrogen production

The microwave-assisted methane decomposition over a pyrolysis residue of sewage sludge (PRSS), which acted as a microwave receptor and a low-cost catalyst without further activation, was investigated in a multimode microwave reactor. For comparison purpose, methane conversion (MC) over an activated carbon (AC) was also studied. The results indicate that PRSS is a better microwave receptor than AC. Under the same microwave power (MWP), MC over PRSS is markedly higher than that over AC, due to the remarkably higher Microwave heating (MWH) performance of PRSS. MWH of PRSS and AC is heavily influenced by atmosphere. Under the same MWP, the stable temperatures of the catalysts in hydrogen, nitrogen and methane atmosphere follow the sequence: T_nitrogen > T_hydrogen > T_methane. On the other hand, it was observed that nitrogen showed different effect on MC over PRSS and AC under MWH. Specifically, under the microwave-assisted methane decomposition reactions, the effect of nitrogen on MC over PRSS is not obvious, but it has remarkable effect on MC over AC. Additionally, a large number of molten beads were formed on the surface of the used PRSS by microwave irradiation. The composition and formation mechanism of the molten beads were also reported.     

Pyrolysis gasification of dried sewage sludge in a combined screw and rotary kiln gasifier

A pyrolysis gasifier, with carbonization and activation steps, was developed to convert dried sludge into activated char and gas fuel energy. To determine the optimal driving conditions, parametric investigations were conducted on the amount of steam input, pyrolysis gasifier temperature and moisture content in the dried sludge.     

Investigation on the decomposition of chemical compositions during hydrothermal conversion of dewatered sewage sludge

Hydrothermal conversion (HC) can be used to convert sewage sludge into fuel-like products. The investigation of biomass compositions conversion can facilitate the understanding of reaction pathways. HC of dewatered sewage sludge (DSS) is conducted in sub-/supercritical water with batch reactors. Hemicellulose has the highest conversion efficiency of 99.1 wt %, followed by crude protein, cellulose, lignin, and lipid/oil. The total gas and H₂ yields increase slowly from 200 to 300 °C, then sharply rise up from 350 to 450 °C. At 450 °C, the H₂ yield reaches to the maximum of 0.70 mol/kg organic matter. HC of DSS includes reactants degradation to intermediates and final products formation from intermediates. The water-soluble products (WSPs) are formed throughout the HC process, the oil-phase products (OPs) are mostly produced at low temperatures (250–350 °C), and char and gases are mainly generated at higher temperatures (above 350 °C).     

The conversion of sewage sludge into biochar reduces polycyclic aromatic hydrocarbon content and ecotoxicity but increases trace metal content

The presence of contaminants considerably restricts the application of sewage sludge for the fertilisation and reclamation of soils. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the environment and vary widely in sewage sludge depending on the input of industrial effluents. The objective of the study was the investigation whether the pyrolysis affect (reduces or adds) the total quantity of PAHs in sewage sludge-derived biochars and whether the pyrolysis changes the PAHs spectrum in terms of relative contributions of more hazardous components. Additionally, the trace metal content was determined before and after pyrolysis as well as the ecotoxicological parameters test towards plant (Lepidium sativum), bacteria (Vibrio fischeri) and crustacean (Daphnia magna). Sewage sludges conversion to biochar significantly reduced the content of PAHs (from 8- to 25-fold depending on pyrolysis temperature and kind of sludge). The exception was the content of naphthalene. Naphthalene was predominant in sewage sludge-derived biochars. However the concentration of the most hazardous 5- and 6-rings PAHs in sewage sludge-derived biochars was much lower compared to sewage sludge. The pyrolysis of sewage sludges caused also a significant reduction of their toxicity towards the test organisms. Only in the case of crustacean it was observed that the extracts from some biochars, obtained at higher temperatures (600 °C and 700 °C) were more toxic to D. magna than extracts from sewage sludge. In turn, after pyrolysis an increase was noted for trace metals content (Pb, Cd, Zn, Cu, Ni and Cr).     

The effects of Fe2O3 catalyst on the conversion of organic matter and bio-fuel production during pyrolysis of sewage sludge

In this paper, the pyrolysis treatment of sewage sludge is studied in a fixed bed reactor at temperatures range of 400–600 °C. Meanwhile, the catalytic effect of Fe₂O₃ on the characteristics of the resulting gases, bio-oil and bio-char are also investigated. The experimental results indicate that the yields of gases and bio-oil respectively increase from 8.69 wt% and 32.54 wt% to 11.62 wt% and 38.74 wt%, and the char yield decreases from 58.77 wt% to 49.64 wt% during Fe-embedded sewage sludge pyrolysis when Fe₂O₃ is added equal to 5% in the dried sewage sludge. Meanwhile, Fe₂O₃ promotes the CO and H₂ formation and inhibits the CH₄ formation, while it exhibits no significantly effect on the composition of the bio-oil. Moreover, the bio-oil should be direct combustion for power generation due it contains higher oxygenated hydrocarbons. In addition, the bio-char exhibits good desulfurization activity.     

Air gasification of dried sewage sludge in a two-stage gasifier. Part 2: Calcined dolomite as a bed material and effect of moisture content of dried sewage sludge for the hydrogen production and tar removal

In order to produce a clean producer gas, the air gasification of dried sewage sludge was conducted in a two-stage gasifier that consisted of a bubbling fluidized bed and a tar-cracking zone. The kind and amount of bed materials, the kind of additives in the upper-reactor, and the moisture content in the sewage sludge were selected as operating variables in order to investigate their effects on the development of the producer gas characteristics. In our experiments, the gasification of a dried sewage sludge sample containing 30 wt.% of moisture with a combination of calcined dolomite as the bed material and activated carbon in the tar-cracking zone removed the most tar and produced the highest hydrogen concentration. The total tar removal efficiency and the H₂ content in the producer gas from the sample noted above reached 88.4% and 32.1 vol.%, respectively. The LHVs of all the producer gases were high with values above 7 MJ Nm⁻³.     

Isoconversional kinetic analysis of co-combustion of sewage sludge with straw and coal

In this paper, the co-combustion behaviour of sewage sludge with straw and coal were carried out in a thermogravimetric analyzer under different atmospheres and at different heating rates (10, 20 and 40 K min⁻¹) in the temperature range from ambient temperature to 1000 °C. TG and DTG curves were analysed. The Flynn–Wall–Ozawa’s (FWO) and Kissinger–Akahira–Sunose’s (KAS) isoconversional methods were used to yield dependency of the activation energy of reduction process on the degree of conversion. The values of E_α were obtained. The results indicate that: with the increase of heating rate, the maximum weight loss rate of samples increase obviously. The activation energy is practically constant in the 0.2 ? α ? 0.9 range, with the average values of E_α = 137.27 and 132.38 kJ mol⁻¹ calculated by FWO and KAS methods, respectively.     

Enhanced hydrogen production in catalytic pyrolysis of sewage sludge by red mud: Thermogravimetric kinetic analysis and pyrolysis characteristics

The catalytic mechanism of red mud (RM) on the pyrolysis of sewage sludge was investigated. The thermogravimetric data were used to study the kinetic characteristics by using a discrete distributed activation energy model (DAEM) to clarify the effects of three main components (Fe₂O₃, Al₂O₃, SiO₂) in the RM on the pyrolysis of organic matters in sewage sludge. The modeling results showed that the pyrolysis of organic matters, especially at the higher temperature stage, was promoted by Fe₂O₃ and Al₂O₃ in the RM. Adding Fe₂O₃ or the RM alone could reduce the mean activation energy of sewage sludge pyrolysis by 13.9 and 20.1 kJ mol^?1, respectively. The modeling results were validated by pyrolysis experiments of raw sludge with different additives at 600, 700, 800, and 900 °C. The experimental results showed that the addition of Al₂O₃, Fe₂O₃ or the RM could produce more gas than the addition of SiO₂, especially at high temperatures. Fe₂O₃ and Al₂O₃ acted as catalysts in the tar decomposition by in-situ catalyzing the cracking of CC and CH bonds to produce more gases. Especially, Fe₂O₃ and Al₂O₃ increased the H₂ yield from sewage sludge pyrolysis at 700, 800, and 900 °C by 268.5 and 50.7%, 111.1 and 56.0%, 10.9 and 10.3%, respectively. The char obtained from pyrolysis of sewage sludge with the RM possessed magnetic property, which has various potential applications. The research indicates that the RM is an efficient catalyst in the pyrolysis of sewage sludge.     

Air gasification of high-ash sewage sludge for hydrogen production: Experimental,sensitivity and predictive analysis

In this work, air gasification of sewage sludge was conducted in a lab-scale bubbling fluidized bed gasifier. Further, the gasification process was modeled using artificial neural networks for the product gas composition with varying temperatures and equivalence ratios. Neural network-based prediction will help to predict the hydrogen production from product gas composition at various temperatures and equivalence ratios. The gasification efficiency and lower heating values were also established as a function of temperatures and equivalence ratios. The maximum H₂ and CO was recorded as 16.26 vol% and 33.55 vol%. Intraileally at ER 0.2 gas composition H₂, CO, and CH₄ show high concentrations of 20.56 vol%, 45.91 vol%, and 13.32 vol%, respectively. At the same time, CO₂ was lower as 20.20 vol% at ER 0.2. Therefore, optimum values are suggested for maximum H₂ and CO yield and lower concentration of CO₂ at ER 0.25 and temperature of 850 °C. A predictive model based on an Artificial Neural network is also developed to predict the hydrogen production from product gas composition at various temperatures and equivalence ratios. The network has been trained with different topologies to find the optimal structure for temperature and equivalence ratio. The obtained results showed that the regression coefficients for training, validation, and testing are 0.99999, 0.99998, and 0.99992, respectively, which clearly identifies the training efficiency of the trained model.     

Influence of FeCl3 and lime added to sludge on sludge–coal pyrolysis

Copyrolysis of sewage sludge–coal blends at different ratios (0:100, 10:90, 50:50 and 100:0) was investigated using a simultaneous thermogravimetry–mass spectrometry analyser. During copyrolysis three thermal decomposition stages were identified between 180 and 800 °C. From 180 °C to 385 °C, the process is dominated by the sludge pyrolysis. From 385 °C to 560 °C, the coal is pyrolysed with a lower fraction of the sludge. In the last stage, the coal pyrolysis occurs together with carbonate decomposition. In the operational conditions, copyrolysis occurs with some interactions, which are principally due to the fact that inorganic matter from sludge (mainly lime and FeCl₃) affects some secondary reactions. The composition of pyrolysis gas (H₂, CO₂, H₂O, light hydrocarbons, CH₃COOH, chlorinated hydrocarbons and HCl) depends on both the temperature and the influence of inorganic products added to sludge in the wastewater treatment plant. The addition of FeCl₃ with lime affects the process in two ways: 1) an increase of H₂ is produced at 488 °C due to lime action on water–gas shift reaction, and 2) an increase of HCl and chlorinated hydrocarbons at 470 °C is also produced. The kinetic parameters were determined by using the global reaction model for each one of two first consecutive reaction stages.     

Air gasification of dried sewage sludge in a two-stage gasifier: Part 1. The effects and reusability of additives on the removal of tar and hydrogen production

Air gasification of dried sewage sludge was conducted in a two-stage gasifier. In the experiments, natural occurring materials, such as natural zeolite, olivine and dolomite, as well as biomass-based and coal-based activated carbons, were applied to the upper reactor of a two-stage gasifier, while sand and calcined dolomite were used as the fluidized bed material in the lower reactor. The reusability of the spent coal-based activated carbon and spent calcined dolomite was also investigated. The combination of calcined dolomite as the bed material and coal-based activated carbon in the upper reactor produced the highest H₂ (28 vol.%) and CO (21 vol.%) contents. Furthermore, total amount of tar generated with the combination was 91% less than that generated with no additive in the upper reactor and sand in the lower reactor. The H₂ content and tar removal efficiencies in the experiments with the spent activated carbons and spent calcined dolomites were shown to be better than those without additives in the upper reactor.     

Investigation on gasification performance of sewage sludge using chemical looping gasification with iron ore oxygen carrier

Chemical looping gasification (CLG), with inherent characteristic of low pollutant emissions can realize the treatment and disposal principle of sewage sludge of harmlessness, reduction, stabilization and reutilization. This work attempted to investigate the feasibility of CLG of sewage sludge with hematite oxygen carrier. The role of hematite in CLG was firstly determined and it was used as gasification agent to provide the required oxygen for sludge conversion. Subsequently, the effect of various operating parameters on sludge CLG was discussed. An optimal mass ratio of sludge to hematite oxygen carrier was determined at 0.33 to achieve the high-efficient conversion of sludge. The reaction temperature of sludge gasification was set at 900 °C to avoid the excessive reduction of hematite. Suitable steam content was assigned to at 19.52%, where the sludge conversion attained the maximum. Ash deposited caused the decrease of specific surface area of hematite, but the accumulated mineral elements in sludge ash can improve the reactivity of oxygen carrier. Additionally, the ash accumulation also can prolong the gas mean residence time. Hence, the sludge conversion showed a slight uptrend with the cycle numbers. The phosphorus element of sludge was formed of CaH₂P₂O₇ and CaHPO₄ during the CLG. Although some sludge ash deposited on the surface of hematite particles, the new compounds were not observed in the reacted oxygen carrier. Additionally, sludge ash has a relatively high resistance trend for bed agglomeration and deposit formation due to the high content of Ca/Al elements. Consequently, the accumulation of sludge ash only reveals physical effects rather than chemical effects on the hematite particles. Thus, it is easy to separate the sludge ash from hematite particles according to the differences in density and size.     

煤与污泥混合物在不同温度下燃烧时SO_2排放特性研究

选取三种煤与污泥的混合物在不同温度下进行加热,分析其SO2排放特性,本研究中温度分别设置为700℃和1000℃,结果表明不同比例的污泥对混合物SO2排放有着重要影响;同时,不同温度对石炭煤的累积SO2排放量有着一定影响,在700℃下,累积SO2排放量为2000~5000 mL/m3,而在1000℃时,其累积排放量为2500~3500 mL/m3,另外,高温能够加快各混合物SO2产生速率。     

Influence of NaOH and Ni catalysts on hydrogen production from the supercritical water gasification of dewatered sewage sludge

Dewatered sewage sludge was treated with NaOH additive and Ni catalyst in supercritical water in a high-pressure autoclave to examine the effects of separate and combined NaOH additive and Ni catalyst on hydrogen generation. The effects of Ni/NaOH ratio on hydrogen production were also investigated to identify possible catalytic mechanism and interactions. NaOH and Ni, separately or in combination, improved the hydrogen production and hydrogen gasification efficiency. The addition of NaOH additive not only promoted the water–gas shift reaction, but also favored H₂ generation of Ni catalyst by capturing CO₂. The hydrogen yield of combined catalysts with different Ni/NaOH ratios was higher than the theoretical sum of hydrogen yield from the mixture by 10–33%. The largest hydrogen yield, of 4.8 mol per kilogram of organic matter, which was almost five times as much as without catalyst, was achieved with the addition of 3.33 wt% Ni and 1.67 wt% NaOH. The combined NaOH additive and Ni catalyst also improved the gasification of several other dewatered sewage sludges, increasing the hydrogen yield by four to twelve times that seen without catalyst. Combined NaOH additive and Ni catalyst are effective in dewatered sewage sludge gasification at low temperature.     

Thermophilic and mesophilic temperature phase anaerobic co-digestion (TPAcD) compared with single-stage co-digestion of sewage sludge and sugar beet pulp lixiviation

The performance of temperature phase anaerobic co-digestion (TPAcD) for sewage sludge and sugar beet pulp lixiviation (using the process of exchanging the digesting substrate between spatially separated thermophilic and mesophilic digesters) was tested and compared to both single-stage mesophilic and thermophilic anaerobic co-digestion. Two Hydraulic Retention Times (HRT) were studied in the thermophilic stage of anaerobic digestion in two temperature phases, maintaining the optimum time of the mesophilic stage at 10 days, obtained as such in single-stage anaerobic co-digestion. In this way, we obtained the advantages of both temperature regimes.Volatile solids removal efficiency from the TPAcD system depended on the sludge exchange rate, but fell within the 72.6–64.6% range. This was higher than the value of 46.8% obtained with single-stage thermophilic digestion and that of 40.5% obtained with mesophilic digestion. The specific methane yield was 424–468 ml CH₄ per gram of volatile solids removed, similar to that of single-stage mesophilic anaerobic digestion. The increase in microbial activity inside the reactor was directly proportional to the organic loading rate (OLR) (or inversely proportional to the HRT) and inversely proportional to the size of the microbial population in single-stage anaerobic co-digestion systems.     

城市污泥与玉米秸秆共热解及炭粉吸附特性研究

利用外热式固定床反应器,在400～700℃下对不同比例的城市污泥与玉米秸秆混合物进行共热解,研究热解条件对炭粉的产率、比表面积、孔径分布的影响。结果表明,随热解温度的提高和玉米秸秆量的增加,炭粉的产率逐渐减小,700℃时,纯污泥热解的炭粉得率是51.5%(以干原料计),添加45%的秸秆时,炭粉得率是41.75%;炭粉的比表面积随热解温度的提高而增大,随玉米秸秆量的增加而增大,700℃时,比表面积在50~80m2/g;纯污泥热解炭粉的孔径以中大孔为主,随玉米秸秆添加量的增大,炭粉的孔径分布由中大孔趋向中微孔。     

Bioelectrochemical enhancement of hydrogen and methane production from the anaerobic digestion of sewage sludge in single-chamber membrane-free microbial electrolysis cells

Batch tests were carried out to investigate the bioelectrochemical enhancement of hydrogen and methane production from the anaerobic digestion of sewage sludge in single-chamber membrane-free microbial electrolysis cells (MEC) and non-MECs. Hydrogen and methane were produced from the anaerobic digestion of sewage sludge in all reactors. Compared with controls, hydrogen production was enhanced 1.7–5.2-fold, and methane production 11.4–13.6-fold with Ti/Ru electrodes at applied voltages of 1.4 and 1.8 V, respectively. Most of hydrogen was produced in the first 5 days of digestion and most of methane was generated after 5 days. No oxygen was detected in the biogas and no hydrogen production was detected in the control test with water. The applied voltages can enhance the removal of suspended and volatile suspended solids, increase the transformation of soluble chemical oxygen demand, accelerate the conversion of volatile fatty acids and maintain an optimal pH range for methanogen growth.     

Air gasification of dried sewage sludge in a two-stage gasifier. Part 3: Application of olivine as a bed material and nickel coated distributor for the production of a clean hydrogen-rich producer gas

Air gasification of dried sewage sludge (DSS) was performed with olivine as the bed material and coal-based activated carbon as the tar-cracking additive in a two-stage gasifier to produce a hydrogen-rich producer gas with low tar content. Additionally, to investigate the possible regeneration of coal-based activated carbon, two different activating agents (carbon dioxide and steam) were applied. Finally, to reduce ammonia and tar contents, the performance of a Ni-coated distributor was examined. In the experiments, gasification at a steam to fuel ratio (STF) of 1.11 achieved a tar removal efficiency of 98% and a maximum hydrogen content of 34 vol.%. Furthermore, the activated carbon used for the gasification was less deactivated, compared to those used in the experiments without any reforming agent. When a Ni-coated distributor was applied, the ammonia content in the producer gas was significantly reduced to 11 ppm.     

The performance of a triple pressure level absorption cycle (TPLAC) with working fluids based on the absorbent DMEU and the refrigerants R22, R32, R124, R125, R134a and R152a

This paper compares the performance of a single-stage triple pressure level (TPL) absorption cycle with different refrigerant–absorbent pairs. Four HFC refrigerants namely: R32, R125, R134a and R152a which are alternative to HCFC, such as R22 and R124, in combination with the absorbent dimethylethylenurea (DMEU) were considered. The highest coefficient of performance (COP) and the lowest circulation ratio (f), were found as a function of the generator temperature for a given evaporating and cooling water temperatures. The sensitivity of the COP and f for evaporator and cooling water temperatures changes at the maximum COP for the best three working fluids were also examined. It was obtained that the preferable pair is R124–DMEU and among working fluids based on HFC the preferable pair is the R125–DMEU.