Pyrolysis Of Sewage Sludge Research Articles

Prediction of char yield and nitrogen fixation rate from pyrolysis of sewage sludge based on machine learning

Based on sewage sludge composition and pyrolysis processing conditions, machine learning was employed as a tool to predict the nitrogen fixation rate and char yield of sewage sludge pyrolysis. Multiple linear regression, decision tree, support vector machine, random forest, and gradient boosting tree methods were used in the research to model the collected data, and the model results were further discussed. The required data set is sorted by the existing articles, and the missing values are filled by k-Nearest Neighbors method. Predicting the char yield and nitrogen fixation rate by considering the ultimate and proximate composition of the sewage sludge can give good results. The results showed the gradient boosting tree method is the most accurate in predicting nitrogen fixation rate and char yield, with the coefficient of determination for char yield and nitrogen fixation rate reaching 0.864 and 0.860, respectively. Fusion of existing models by voting or stacking methods revealed promising further improvement in prediction accuracy, but it was limited. In further analysis, using the mean decrease impurity and SHapely Additive exPlanations methods to more accurately evaluate the feature importance. For example, HTT was the most important feature affecting char yield and nitrogen fixation rate, especially when predicting the nitrogen fixation rate, with a feature importance of 0.625. This study also quantified the effects of different features on char yield and nitrogen fixation rate using partial dependence and individual condition expectation plots to provide a reference for the utilization and research of sewage sludge pyrolysis.

Syngas generation from different types of sewage sludge using microwave-assisted pyrolysis with silicon carbide as the absorbent

In this study, the pyrolysis of sewage sludge was explored through microwave-assisted pyrolysis. Three kinds of sludge (primary sludge, waste-activated sludge, and digested sludge) from a sewage treatment process were used. All three kinds of sewage sludge had a low microwave absorption capacity; therefore, an absorber was added to enable microwave-assisted pyrolysis. By using silicon carbide as the heating element, it was possible to increase the temperature within a short time by applying microwaves. During the microwave-assisted pyrolysis of sewage sludges, the amount of gas generated and the H2 and CO fraction of the produced gas increased as temperature increased. The pyrolysis of waste-activated sludge produced the greatest quantity of gas. However, the primary sludge produced the highest amount of syngas in terms of H2 and CO, which indicate the high-quality of the syngas.

An Extensive Review and Comparison of Modern Biomass Reactors Torrefaction vs. Biomass Pyrolizers—Part 2

The depletion of fossil fuels has led to a search for new methods of fuel and chemical production from biomass. One of the methods of converting biomass into valuable products is the process of pyrolysis. This process has been extensively researched in recent years due to the rising prices of energy and chemicals. This work contains basic information on the pyrolysis process concerning the individual components present in the biomass and the types of biomass used in the pyrolysis process. Particular attention was paid to sewage sludge, the management of which is a big challenge. The influence of the most important process parameters (temperature, heating rate, residence time of the solid and vapor, reaction atmosphere) on the pyrolysis products (char, oil, and gas) was presented. The paper presents an overview of the reactors used in the pyrolysis process, from slow to fast pyrolysis, together with their efficiency, advantages, and disadvantages. The analysis of the application of other thermochemical processes for producing the energy used in the process of pyrolysis and in the drying of the biomass was carried out. Two industrial-scale installations for the pyrolysis of sewage sludge were presented.

Key element course-tracked copyrolysis of sewage sludge and biomass for resource recovery and pollution control through kinetic and thermodynamic insights

Ameliorating sewage sludge (SS) pyrolysis process with biomass increasingly becomes an effective strategy to utilize SS-entraining resources and depress pollutant evolution, while real-time track of SS and biomass copyrolysis as well associating thermochemical reactions in aspects of kinetics and thermodynamics is yet to be unveiled. Here a combinative thermogravimetry combined with Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry (TG-FTIR-GC/MS) pyrolysis system, lab-scale furnace and theoretical calculation were adopted to develop a key element course-tracked copyrolysis of SS and corn stalk (CS) for simultaneous accommodation of profitable products and derived pollutants. The formation of alkanes and alkene with small carbon number (below 4) was promoted during copyrolysis, while those with heavy carbons (over 5) were contributed by the inherent organic fraction of SS. The release of those nitrogen (N)-/sulfur-/chlorine-involving inorganic pollutants was depressed with the increase in the CS addition. The increased release of CO2 and H2O as well carboxylic volatiles with a high CS addition evidenced the promoted dehydroxylation and decarboxylation. Consequently, the speciation of N and phosphorus (P) and the bioavailability of toxic metals (TMs) were regulated with the increasing CS addition, that is, enhancements of proteins and protein-like N transformation to the other nitrogen species, P within orthophosphate diesters to orthophosphate monoesters, and TM reactivity-dependent bioavailability depression. A low addition (below 20 %) was prone to maintain a congruent mechanism, while the total Gibbs energy tended to decrease with an increasing addition and pyrolysis temperature, leading to a converged pressure-constant heat capacity (∼1430 J/K) at 700 °C. Those thermochemical reactions occurring over sewage sludge ash matrix were pivoted with biomass addition, which can be adopted as a useful strategy for an efficient, cleaner and sustainable mining of waste resources.

Process design and optimization on self-sustaining pyrolysis and carbonization of municipal sewage sludge

Pyrolysis can realize the reduction and resource utilization of municipal sewage sludge (MSS). In this paper, a self-sustaining pyrolysis process is designed for municipal sewage sludge, and the process flow is simulated by Aspen plus software. By changing the initial moisture content of sludge, moisture content after drying, pyrolysis temperature and air supply in the incinerator, the possibility of achieving energy self-balance in the system is analysed. The simulation results show that by adjusting the parameters of the system, this process can realize the energy self-balance of sludge drying and pyrolysis treatment. Considering the system’s energy loss, the dry basis calorific value of sludge should not be less than 10 MJ/kg. The higher the initial moisture content of sludge, the more external energy input the system needs. It is recommended to dehydrate sludge mechanically to about 60 % before entering the system. When the pyrolysis temperature is increased, the amount of oil and gas produced by sludge pyrolysis increases, and it is easier to achieve self-balance of system energy. But the higher the pyrolysis temperature, the greater the energy consumption required. In practice, it is suggested that the pyrolysis temperature is about 400 °C. The moisture content of dried sludge has little effect on the energy self-balance of the system, and it is recommended to be about 30 %. The air supply volume of the incinerator mainly affects the flue gas outlet temperature and flue gas volume, but has little effect on the energy balance of the system.

Sustainability assessment of alternative waste-to-energy technologies for the management of sewage sludge

The management of sewage sludge still represents a challenge in the EU sustainability plan for biowaste. Although there are consolidated alternatives for the valorization of sewage sludge (incineration, pyrolysis and gasification), technical issues related to heavy metals and other pollutants are not sufficiently understood considering the whole waste-to-energy process. In addition, societal-economic and environmental aspects are usually not included in the evaluation of these conversion technologies. In this study, we propose an integrated assessment from a sustainability perspective to evaluate the valorization of sewage sludge by thermal conversion, comparing different alternatives based on existing Waste-to-energy (WtE) technologies. The results provide an insightful vision on the challenges to manage the sewage sludge disposal and to transform the obtained waste into new raw materials with added value. In addition, the evaluation of the WtE alternatives shows that they face important challenges preventing their application, being the gasification the best performing technology according to the sustainability assessment. Finally, a decentralized scheme based on sewage sludge gasification is further evaluated using real data from wastewater treatment plants in Andalusia.

Comparative life-cycle energy and environmental analysis of sewage sludge and biomass co-pyrolysis for biofuel and biochar production

To address the gaps in current research on sewage sludge (SS) pyrolysis, this study examined the feasibility of the environmental and energy demands of co-pyrolysis as an SS treatment process. The analyses included a cradle-to-gate life cycle assessment, carbon emission and credit assessment, energy recovery calculations, and sensitivity analysis. Sawdust (SD) and wheat straw (WS) were chosen as co-feeds for the co-pyrolysis scenarios; four mixing ratios (20, 40, 60, and 80 wt%) were examined for each co-feed. The functional unit was 1000 kg of bio-oil. Mixing the co-feed biomass with SS reduced the overall environmental burden by 58–83 %; WS/SS co-pyrolysis performed better than SD/SS in this respect. All examined scenarios reduced the global warming potential (GWP) and achieved a net positive energy balance; SD/SS co-pyrolysis was more desirable owing to high energy recovery. The additional gas turbine operation generated a process energy demand of 23–224 %. GWP and terrestrial ecotoxicity potential are indicators that are significantly affected by co-feed material transportation. Sensitivity analysis revealed that the transportation of feedstock and pyrolysis products should be prioritised to limit the environmental burden of the process. Co-pyrolysis is an eco-friendly SS treatment method, and its economic viability should be further investigated.

Co-pyrolysis of sewage sludge and poplar sawdust under controlled low-oxygen conditions: Biochar properties and heavy metals behavior

Biochar production by pyrolysis of sewage sludge (SS) is one of effective ways to realize resource recovery and harmless treatment of SS. However, biochar prepared by direct pyrolysis of SS has a lower specific surface area and higher heavy metal content. This study was aimed to evaluate the effect of low-oxygen and addition of poplar sawdust (PS) on sludge-derived biochar. The properties of biochar and heavy metals behavior were systematically characterized. The presence of 10% O2 during SS pyrolysis had a significant effect on the specific surface area of the pyrolytic biochar and the chemical species of heavy metals. In comparison with the pyrolytic biochar under N2 atmosphere, the yield of co-pyrolytic biochar from 10% O2 decreased by 6.7% at 350 °C and 8.3% at 650 °C, while it had the highest fixed carbon content of 19.6%. Moreover, the addition of PS also increased the specific surface area of pyrolysis biochar to 26.8 m2 g−1 at 650 °C. With the increase of pyrolysis temperature, heavy metals including Zn, Cr, and Cu in SS were transformed from the exchangeable and reducible states to oxidizable states and residue states. Especially, the presence of 10% O2 can also promote the formation of oxidizable states of heavy metals in biochar. Based on risk assessment code analysis, the ecotoxicity of heavy metals in biochar with the presence of O2 was evidently lowered. Therefore, a low oxygen atmosphere and co-pyrolysis of sludge and biomass could be effective ways to improve the quality of sludge-based biochar.

Pyrolysis of sludge briquettes for the preparation of cylindrical-shaped biochar and comparison between CO2 and steam activation

Sludge briquettes were prepared through briquetting (under the pressure of 25 MPa) of half-dried sewage sludge with moisture content of 30 wt%, and the cylindrical-shaped biochar (CSB) was obtained by subsequent pyrolysis (600 °C) and CO2/steam activation (650–950 °C). Briquetting process showed great influence on the pyrolysis of sewage sludge. The temperature range for the organic decomposition increased from 150 to 500 °C for the sludge without briquetting to 150–700 °C for the sludge briquette. The mechanical performance of CSBs was greatly influenced by activation temperature. The axial compressive strength of the CO2/steam activated CSBs were in the range of 17.8–22.2 kg·cm‑2 at the activation temperature of 650 °C and 800 °C, but dropped markedly when the activation temperature increased to 950 °C. CSBs were negatively charged and rich in oxygen/nitrogen-containing functional groups like –OH, –COOH, amines and amides, and the specific surface area of the optimal steam and CO2-activated CSB were 123.2 and 113.5 m2·g‑1, respectively. The prepared CSBs were applied in methylene blue (MB) adsorption, and the maximum adsorption capacity of 513.81 and 419.29 mg·g‑1 could be achieved by the CO2 and steam-activated CSB, respectively. Both physical and chemical adsorptions played important role in the MB adsorption. Finally, the regeneration test showed that optimal samples of the CO2 and steam-activated CSB still retained around 90 % of its original adsorption capacity after three cycles of adsorption-regeneration tests, but the mechanical performance dropped by 58.1 % and 47.8 %, respectively.

Study on the multiple roles of CaO on nitrogen evolution mechanism of protein inside sewage sludge pyrolysis

The aim of this study was to clarify the multiple influence mechanisms through which Ca influences the emission of NOx precursors (NH3 and HCN) during sewage sludge pyrolysis and to establish a scientific correlation between the mechanisms and temperatures. In this study, the effects of CaO on pyrolysis kinetics and NOx precursor emission temperature of deashed sludge were estimated using Thermogravimetry-mass spectrometry. Based on this, the NH3 and HCN evolution pathways were investigated at these nitrogen emission temperatures by employing sludge protein model compounds in a self-designed all-quartz pyrolysis system. Results showed that the addition of CaO reduced the activation energy of sludge pyrolysis by approximately 0–40 kJ/mol. CaO promoted the retention of nitrogen in char and the conversion of nitrogen into gas phase while hampering its migration to tar. The Ca-fixed nitrogen in char reacted with NH3/HCN to form CaCxNy, such as CaCN2 and CaN2. Notably, results revealed the overwhelming effects of temperature on nitrogen evolution. Specifically, CaO was found to affect only the dominant reaction at the target temperature. At lower temperatures (255–350 °C), CaO promoted the conversion of protein-N to NH3 and inhibited nitrile-N decomposition to HCN in char. In contrast, at comparatively higher temperatures (490–750 °C), CaO inhibited the production of NH3 by promoting the hydrogenation of heterocyclic-N in char and the dimerization of amino acid molecules in tar, while CaO increased HCN release by promoting the dehydrogenation of amine-N in tar and ring opening of heterocyclic-N to form nitrile-N and subsequently HCN. These results would guide NOx control for obtaining energy from nitrogen-rich solid waste for industrial applications and the targeted regulation of NH3 (carbon-free hydrogen-rich fuels) utilization.

Comparative life cycle assessment of co-pyrolysing sewage sludge and wastewater-grown microalgae for biofuel production

Using available data, we analysed the biofuel production via pyrolysis of sewage sludge (SS) and microalgae grown in wastewater, either alone or at mass ratios of 1:1, 1:2, and 2:1. The life cycle assessment (LCA), global warming potential (GWP), energy recovery, and economic feasibility were compared. GWP was 36–44% lower for the co-pyrolysis scenarios than for SS alone. For energy recovery from pyrolysis gases, a gas turbine was more effective than gas heating, compensating for up to 28% of the pyrolysis energy requirement for all five scenarios. The drying process accounted for 69−88% of the total energy consumption, indicating that the dewatering process should be improved to maximise sustainability and profitability. All scenarios were profitable; the 1:1 mixture gave the highest net profit (9% more than that of SS). To conclude, co-pyrolysis of SS and microalgae is environmentally and financially viable for biowaste management in municipal wastewater treatment plants.

Identifying the fate of nitrogenous species during sewage sludge pyrolysis via in-situ tracing of protein-sludge inherent components interactions

The effect of interactions between different components in sewage sludge on the thermochemical transformation of nitrogenous species is usually neglected, which is important to explain the generation mechanism of some key nitrogenous by-products. Here, we investigated the distribution, form, and chemical properties of the products from sludge-extracted protein (PR) under different pyrolysis scenarios using several in-situ probe techniques, to elucidate the critical role of typical sludge organics/inorganics on the evolution of nitrogenous intermediates and by-products. The results suggested that Ca/Fe/Si/Al-containing inorganics significantly affected the pyrolytic behavior of PR and the thermal transformation of nitrogenous species, while sludge organics, including humic acids and polysaccharides, had limited effects on the temperature-dependent evolution of nitrogenous species in PR. Among them, calcium oxide catalyzed the ring-opening reaction of heterocyclic-N with aromatic-like structures, resulting in a 21.1 %–68.8 % reduction in nitrogen fixation efficiency in the char. At lower temperatures (350–450 °C), calcium oxide caused more nitrogen to be transferred to the gas/tar phases in the form of NH3 and heterocyclic-N, and it also enhanced the conversion of nitrile-N → HCN → NO at temperatures above 450 °C. In contrast, polyferric salts inhibited the devolatilization of mono-heterocyclic-N and enhanced the thermal stability of poly-heterocyclic-N, resulting in a maximum increase of 18.5 mg·g−1 of nitrogen content in the char, while reducing the release of NH3 and HCN by 71.1 % and 32.0 %. This work elucidated the interaction between PR and inherent components in sludge, providing key information for the control of nitrogenous volatiles and NOx.

Effect of carrier gas change during sewage sludge or sewage sludge and willow pyrolysis on ecotoxicity of biochar-amended soil

Different pyrolysis conditions determine the properties of the biochar. The properties of biochar may affect directly or indirectly their influence on living organisms. The aim of this study was to determine the toxicity of biochar obtained under different conditions (temperature: 500 or 700 °C, carrier gas: N2 or CO2, feedstock: sewage sludge or sewage sludge/biomass mixture) after adding to the soil in long-term pot experiment (180 days). Biochars were added to the podzolic loamy sand at a 2% (w/w) dose. Samples were collected at the beginning of the experiment and after 30, 90 and 180 days. The bacteria Aliivibrio fischeri (luminescence inhibition – Microtox), the plant Lepidium sativum (root growth and germination inhibition test – Phytotoxkit F), and the invertebrate Folsomia candida (mortality and reproduction inhibition test – Collembolan test) were used as the test organisms. In the long-term perspective for most tests, changing the carrier gas from N2 to CO2 resulted in reduced toxicity of the biochar. A particularly beneficial effect of changing the gas to CO2 was observed for the solid-phase test with L. sativum. The CO2 during pyrolysis had the least beneficial effect on toxicity towards A. fischeri.

A review on adsorption efficiency of widely adopted adsorbents used in Defluoridation of aqueous media

The review article provides an insight on various adsorbents and their efficiency in fluorideremoval fromdrinking and waste water. The capacity of fluoride removal of adsorbent is dependent on pH, contact time,temperature, surface area of adsorbent and initial concentration of fluoride solution etc. In comparison tothe traditional natural methods, the synthetic and nanostructure-based adsorbents, exhibited higherperformance and efficiency of defluorination which provides a sustainable approach to safe drinking waterquality. The present paper deals with some widely adopted natural adsorbent, synthetic and nanostructurebasedadsorbents. The natural adsorbent includes, thermally treated lateritic soil, modified natural magnetiteore (Fe3O4) with aluminum and lanthanum ions, lateritic soils, Titanium hydroxide, limestone (LS) andaluminium hydroxide, sedimentation with calcium and aluminium. The Synthetic adsorbent includes,Surface-functionalized polyurethane foam (SPUF); Schwertmannite; Carbonaceous material from pyrolysisof sewage sludge; Chitosan. And the effective adsorbent nano scale structure includes, nanoscale aluminiumoxide hydroxide (AlOOH), Magnetic iron oxide/aluminium hydroxide composite, sulfatedoped Fe3O4/Al2O3 nanoparticles, nano-scale aluminum oxide hydroxide (nano-AlOOH), Graphene Nanoparticlesadsorbent, Al2O3/Bio-TiO2 nano composite (ABN), Green synthesized Fe3O4/Al2O3nanoparticles with coatedpolyurethane foams etc. The activated alumina is most conventional fluoride adsorbing material. However,activated alumina performs well in acidic environment with complex regeneration issue. Although othertraditional adsorbents remove fluoride from water but they have lower efficiency, which restrict theirapplication. In case of synthetic and nano structured based adsorbents, due to increased surface area, nontoxicnature, limited solubility in water and high adsorption efficiency, these methods are widely accepted andproven environmentally suitable.

Catalytic reforming of sewage sludge pyrolysis products over the self-derived char

The catalytic pyrolysis of sewage sludge (SS) over the self-derived char was performed in a lab-scale fixed-bed reactor. The effect of char and catalytic temperature on the product distribution and the evolution of organic nitrogen species (ONSs) in bio-oil were analyzed. Results showed that although the bio-oil yield with char reforming was comparable to the no catalyst case, much aliphatic and less ONSs content were found in the bio-oil, indicating that the bio-oil quality was improved with char reforming. The bio-oil and gas yields were almost constant at low reforming temperature (<500 °C), while the bio-oil yield reduced slightly and the gas yield increased at high reforming temperature (>500 °C). The highest aliphatic (19.78 %) and lowest ONSs (58.30 %) in the bio-oil could be obtained at the reforming temperature of 500 °C. Moreover, at 500 °C, the highest increment of H2 and CO could be found, increased by 49.05 % and 25.36 %, respectively. The ash and demineralization char (d-char) as a catalyst was conducted to investigate the char reforming mechanism. It was found that the synergistic effect of inherent minerals and specific pore structures in char contributed to the recovery of high-quality bio-oil.

In Situ Catalytic Pyrolysis of Municipal Sewage Sludge under Calcined Copper Slag: Thermokinetic Analysis and Real-Time Monitoring of Evolved Gases

Copper slag (CS) is a solid waste from the copper pyrometallurgical process. Calcined CS is rich in Fe2O3 and Fe3O4 and has micro amounts of CaO, MgO, Al2O3, ZnO, and CuO, which can be used as catalysts in the thermochemical conversion of municipal sewage sludge (SS). This work aimed to study the in situ catalytic pyrolysis of SS under Calcined CS, including thermal degradation characteristics, pyrolysis kinetics, thermodynamics, and evolution of gaseous products. The results showed that devolatilization of organic substances in SS mainly happened at 150–600 °C. The addition of Calcined CS caused the thermal decomposition rate of SS to accelerate and the mass loss to increase. The kinetics of SS pyrolysis with and without Calcined CS were calculated by deconvolution, the Friedman method, and the generalized master plots method, and the results showed that Calcined CS led to a decrease in the apparent activation energy (Eα) and pre-exponential factor (A), while there was no significant influence on the pyrolysis reaction mechanism (f(α)). Online monitoring of SS pyrolysis gases revealed that Calcined CS promoted the formation of CO2, CO, CH4, NH3, C═O, and C═C. However, it had a negative impact on C–O. This study provides insights into coprocessing of the two solid wastes and promotes integrated resource recycling of CS and SS.

Effects of quicklime conditioning on the volatile reforming and tar elimination performance of sewage sludge pyrochar

The pyrochar of sewage sludge can be exploited for catalytic tar reforming since the developed pore structure and high metal content. However, Fe in the raw sludge tends to form Fe-S and Fe-P during pyrolysis, which inhibits catalytic activity of pyrochar. Here, the wet sludge was conditioned with quicklime, and the pyrochar was used to reform sewage sludge pyrolysis volatiles. The results showed that H2 generation from pyrochar reforming increased by 55.2% after conditioning. In addition, the tar elimination achieved the highest ratio (79.6%) compared with unconditioned char and quicklime, while PAHs and N-compounds were concentrated in tar with the relative content 46.9% and 37.1% due to the significant removal of other tar compounds such as oxygenates. During pyrolysis, a proportion of added CaO captured P and S anions which favored the formation of Ca2Fe2O5 and Fe-O, while the remaining CaO (21.9%) was highly dispersed on pyrochar. This promoted both the catalytic tar cracking effects and the CO2 absorption of the char. These findings provide a novel method to produce H2-rich gas from sewage sludge pyrolysis with less tar yield.

Changes in the pore structure of modified sludge-activated carbon and its effect on the adsorption characteristics of CO2 under high pressure

Although sludge pyrolysis has been widely studied in sludge disposal, the utilization of sludge pyrolysis products is still challenging. In this study, a sludge-activated carbon adsorbent with CO2 adsorption capacity was prepared by pyrolysis and activation of municipal sewage sludge, and then the activated carbon was modified with hydrofluoric acid (HF), aminopropyl triethoxysilane-diethanol monoisopropanolamine (A-D) and sodium hydroxide (NaOH) to increase CO2 adsorption capacity of activated carbon. By characterizing the microscopic morphology, surface functional groups, and pore characteristics of the sludge-activated carbon before and after modification, and measuring the CO2 adsorption capacity under different pressures, the modification mechanism and high-pressure adsorption characteristics of the sludge-activated carbon under different modification conditions were studied. The results show that HF and NaOH modification can increase the specific surface area and pore volume while reducing the pore size, and the effect of alkali modification is more significant. Amine modification reduces its specific surface area, pore volume and pore size due to the loading of amino groups. Acid modification reduced the saturated adsorption capacity of sludge-activated carbon, and both amine modification and alkali modification increased the saturated adsorption capacity of sludge-activated carbon. The adsorption-promoting pressure thresholds of HF, NaOH, and A-D were determined to be 0.95 MPa, 2.55 MPa, and 3.85 MPa, respectively. When the equilibrium pressure exceeds the adsorption-promoting pressure threshold, the three modification methods will no longer promote the CO2 adsorption of sludge-activated carbon. Moreover, when the adsorption equilibrium pressure is greater than 0.95 MPa, increasing the adsorption equilibrium pressure no longer causes a significant change in the change rate of adsorption capacity of the sludge-activated carbon.

Medium-Low Temperature Conditions Induce the Formation of Environmentally Persistent Free Radicals in Microplastics with Conjugated Aromatic-Ring Structures during Sewage Sludge Pyrolysis.

Medium-low temperature pyrolysis is an effective method of retaining active components in sludge char. However, we found that incomplete cracking reactions resulted in residues of microplastics (MPs) remaining in the char; moreover, high levels of environmentally persistent free radicals (EPFRs) were detected in these MPs. Here, we investigated the temperature-dependent variations in the char-volatile products derived from sludge and MPs under different pyrolysis scenarios using multiple in situ probe coupling techniques and electron paramagnetic resonance spectroscopy, thereby identifying the sources of EPFRs and elucidating the corresponding formation-conversion mechanisms. The temperature was the key factor in the formation of EPFRs; in particular, in the 350-450 °C range, the abundance of EPFRs increased exponentially. Reactive EPFR readily formed in MPs with conjugated aromatic-ring structures (polyethylene terephthalate and polystyrene) at a temperature above 350 °C; EPFR concentrations were 5-17 times higher than those found in other types of polymers, and these radicals exhibited half-lives of more than 90 days. The EPFR formation mechanism could be summarized as solid-solid/solid-gas interfacial interactions between the polymers and the intermediate products from sludge pyrolysis (at 160-350 °C) and the homolytic cleavage-proton transfer occurring in the polymers themselves under the dual action of thermal induction and acid sites (at 350-450 °C). Based on the understanding of the evolution of EPFRs, temperature regulation and sludge components conditioning may be effective approaches to inhibit the formation of EPFRs in MPs, constituting reliable strategies to diminish the environmental risk associated with the byproducts of sludge pyrolysis.

Pyrolysis of sewage sludge under conditions relevant to applied smouldering combustion

Pyrolysis of sewage sludge under conditions relevant to applied smouldering combustion was carried out in this study to investigate the influences of gas flow rate, oxidative atmosphere, and inert porous medium involvement on the properties of products. The experiments were carried out at 300–600 °C under atmospheres of N2, 5% O2/95% N2, 10% O2/90% N2, and 15% O2/85% N2, with Darcy flow rates of 1.0 and 3.5 cm/s, respectively, with dried sewage sludge loaded individually or as a mixture with sand. As a result, both the increment of gas flow rate and involvement of sand leaded to lower yields of char and higher yields of bio-oil and gas under N2 at temperature of ≤500 °C, due to the enhanced efficiency of pyrolysis reaction and gas transportation. However, when temperature increased to 600 °C, the influencing trends on product distributions changed due to the mechanisms of secondary cracking reaction and volatile-char interaction. The involvement of oxygen in fraction of ≤15 vol% at temperatures of 400–500 °C would lead to the intense decreasing yields of char and bio-oil, and increasing yield of the gaseous (dominated by CO2 and CO), due to the involved oxidation reaction during pyrolysis. Both increment of temperature and oxygen fraction would lead to the delay of ignition and the increase of activation energy of the produced char, except for that of char produced at 400 °C under 5% O2/95% N2, whose calculated activation energy was lower and volatile content was higher compared to that of char produced from pyrolysis at 400 °C under N2. The bio-oil from pyrolysis under N2 was dominated by aliphatic acids, phenols, steroids, amides, and indoles, etc., and the involvement of partial oxidation would lead to the weakened formation of aromatics, phenols, and S/Cl/F-containing compounds in bio-oil.