Pyrolysis Of Sewage Sludge Research Articles

High temperature pyrolysis of sewage sludge: Synergistic effects of co-pyrolysis with rice straw and composite plastics wastes

The synergistic effect of co-pyrolysis for the mixtures of sewage sludge (SS), rice straw (RS) and composite plastic wastes (PE&PP) has been investigated with the aim of high gas yields and hydrogen generation. In this context, the applicability of high temperature pyrolysis technology was evaluated at both rotary type batch reactor and rotating screw type continuous reactor operating under the industrially relevant conditions. Gas yields have been improved at increasing the operating temperatures up to 850 °C. The gas yield of 75.06 % with its H2 portion of 32.35 % has been achieved as the best results for continuous pyrolysis unit at pilot scale. Secondary reactions such as steam de-alkylation, steam cracking, water gas shift and so on were regarded as being responsible for the high gas yields when the primary products of pyrolysis, i.e. condensable vapors, were much exposed to the hot zones inside the reactor. The dehydration of organic components involving the loss of water may supply steam to create a reactive pyrolysis atmosphere. While the pyrolysis of sewage sludge and rice straw favors CO and CO2 formation due to the high oxygen contents of these feedstocks, more methane and light olefins were produced when the plastics were added to the feedstock blends. This was ascribed to the initially generated radicals from the decomposition of sewage sludge and/or rice straw that might promote the scission reactions of plastics towards volatiles. It was shown that high temperature pyrolysis can be applied to a variety of organic wastes such as rice straw, sewage sludge and waste plastics. Conversion of wastes to hydrogen fosters the circular economy by putting the disposal costs down and creates a new route on renewable hydrogen generation.

Enhancement of sulfur control by conditioners during sewage sludge pyrolysis: Focusing on screening and in-situ sulfur fixation mechanism based on model compounds

Utilizing conditioners is effective to reduce S-containing gases during sludge pyrolysis. Revealing the relationship between conditioners of different prices and multiple S control factors under similar conditions is key for efficient screening. However, a detailed performance comparison of conditioners remains unavailable. This study, for the first time, comprehensively compared the S control effect of six typical conditioners, and the model compounds of organic-S were used to reveal the in-situ S fixation mechanism of the conditioners. Results show that the total release of gas-S (especially H2S) decreased by 34.4∼54.2% after the addition of CaO, Fe2O3, and FeCl3. These conditioners (especially Fe2O3) can enhance the stepwise oxidation of easily decomposed organic-S (especially aliphatic-S) into stable sulfoxide-S and sulfone-S. Additionally, sulfide-S and sulfate-S can be generated via the capture of S-containing intermediates (i.e., ·SH) by conditioners (especially CaO). The presence of Cl can react with inorganic sulfide-S or organic-S via replacement, leading to the re-release of H2S. Among all conditioners, CaO, followed by Fe2O3 and FeCl3, showed the best comprehensive performance, especially in terms of price and effective H2S control temperature range. This work can guide the selection of conditioners for the synergistic control of S-containing gases in the integrated treatment of sludge.

Optimization of Pyrolysis Parameters by Design of Experiment for the Production of Biochar from Sewage Sludge

Sewage sludge management is a key concern in today’s world. Improper disposal can lead to various environmental issues including air, water and soil pollution. Among the available technologies, thermal treatments, particularly pyrolysis, are gaining interest for their ability to reduce sewage sludge volume and to recover materials and energy from it. This study explored the influence of some relevant parameters in the thermal pyrolysis process. The design of experiment, named central composite design, was accounted to optimize temperature, heating rate and residence time in order to maximize the biochar yield and its CO2 adsorption capacity. A two-factor interaction model provided a satisfactory interpretation of the results. Within the studied ranges, maximum values of 47.8 wt% and 0.514 mol CO2/kg were obtained for the yield and CO2 adsorption capacity, respectively. Two significant experiments were repeated in a different pyrolysis system highlighting how other factors (e.g., reactor geometry, granulometry, etc.) can influence the quantity and the quality of produced biochar. The biochar obtained under the best pyrolysis conditions was characterized by a surface area of 124 m2/g and an ash content of 61 wt%. Lastly, the theoretical energy balance showed that the drying process is the main energy-intensive step in the pyrolysis of sewage sludge.

Simultaneous regulation of nitrogen, sulfur and carbon using biochar during sewage sludge pyrolysis

Due to the complex nature of sewage sludge (SS), how to control the emission of precursor pollutants during its pyrolysis remains a challenge. In this work, the evolution of N, S and C during SS pyrolysis was investigated and biochars were employed to regulate their distribution in the resulting products. The release of N-containing products followed a descending order of NH3>HCN > NO > NO2 and the biochar addition resulted in a reduction of 40.7–54.3 % for these volatiles. The nitrile-N and pyrrolic-N species decreased, while the quaternary-N in residue increased significantly. The emission of S-containing products showed a descending order of COS > SO2>CH3SH > H2S > CS2 and the co-pyrolysis with biochar showed a reduction of 40.6–65.7 % for these products. The relative contents of thiophene-S and sulfoxide-S species increased, while the content of sulfone-S decreased. Biochar incorporation also resulted in a reduction of 67.2 % and 47.4 % for CO and CO2, respectively. Both Raman and XPS analysis indicated an increase in the graphitization degree and a decrease in structural defects.

Insights into the Synergistic Effect and Inhibition Mechanism of Composite Conditioner on Sulfur-Containing Gases during Sewage Sludge Pyrolysis.

Sewage sludge odorous gas release is a key barrier to resource utilization, and conditioners can mitigate the release of sulfur-containing gases. The gas release characteristics and sulfur compound distribution in pyrolysis products under both single and composite conditioning strategies of CaO, Fe2O3, and FeCl3 were investigated. This study focused on the inhibition mechanisms of these conditioners on sulfur-containing gas emissions and compared the theoretical and experimental sulfur content in the products to evaluate the potential synergistic effects of the composite conditioners. The findings indicated that at 650 °C, CaO, Fe2O3, and FeCl3 inhibited H2S release by 35.8%, 23.2%, and 9.1%, respectively. Notably, the composite of CaO with FeCl3 at temperatures ranging from 350 to 450 °C and the combination of Fe2O3 with FeCl3 at 650 °C were found to exert synergistic suppression on H2S emissions. The strongly alkaline CaO inhibited the metathesis reaction between HCl, a decomposition product of FeCl3, and the sulfur-containing compounds within the sewage sludge, thereby exerting a synergistic suppression on the emission of H2S. Conversely, at temperatures exceeding 550 °C, the formation of Ca-Fe compounds, such as FeCa2O4, appeared to diminish the sulfur-fixing capacity of the conditioners, resulting in increased H2S emissions. For instance, the combination of CaO and FeCl3 at 450 °C was found to synergistically reduce H2S emissions by 56.3%, while the combination of CaO and Fe2O3 at 650 °C synergistically enhances the release of H2S by 23.6%. The insights gained from this study are instrumental in optimizing the pyrolysis of sewage sludge, aiming to minimize its environmental footprint and enhance the efficiency of resource recovery.

Study on the effect of conditioner on NOx precursor control behavior from sewage sludge pyrolysis: Focusing on conditioner assessments and in-situ fixation mechanism

The nitrogen transformation during sludge pyrolysis is affected by the dewater conditioner. However, the comparative analysis of the conditioner under identical pyrolysis conditions has been previously absent. In this study, Ca-, Fe- and Al-based conditioners were selected as the representatives. A comprehensive evaluation considering the cost of the conditioners and the product characteristics was conducted. Additionally, the in-situ fixation mechanism of the conditioner on nitrogen-containing gas was concurrently revealed. Among the six conditioners, CaO and AlCl3 were identified as the top performers, ranking first and second, respectively. Furthermore, Fe/Ca-based conditioners reduced NH3 and HCN release by 1.5 ∼ 5.53 % and 0 ∼ 1.55 %, respectively, by facilitating the conversion of amine-N to a more stable form in condensable fraction. Fe promoted volatile amine-N cyclization, while Ca encouraged its dehydrogenation. Both Fe/Ca-based conditioners increased 7.5 ∼ 14.8 % nitrogen retention in char, by inhibiting the decomposition of protein-N. Al-based conditioners had little effect on NH3 and HCN, but contributed to 2.3 ∼ 2.8 % production of stabilized nitrogen in char. The introduction of Cl in Fe/Ca/Al chloride conditioners would promote the decomposition of inorganic ammonium salts to produce NH3 at 30 ∼ 185 °C. And Cl also reacted with volatiles through electrophilic substitution reaction, leading to the formation of halogenated hydrocarbons in condensable fraction and the release of more NH3, HCN, and HNCO at 30 ∼ 465 °C. The findings of this study provide a detailed comparative analysis of various conditioners under uniform conditions and reveal the in-situ fixation mechanism of nitrogen-containing gas. This will provide guidance for the sludge conditioning-dewatering-drying integrated treatment and disposal.

Energy recovery from syngas and pyrolysis wastewaters with anaerobic mixed cultures

The anaerobic digestion of aqueous condensate from fast pyrolysis is a promising technology for enhancing carbon and energy recovery from waste. Syngas, another pyrolysis product, could be integrated as a co-substrate to improve process efficiency. However, limited knowledge exists on the co-fermentation of pyrolysis syngas and aqueous condensate by anaerobic cultures and the effects of substrate toxicity. This work investigates the ability of mesophilic and thermophilic anaerobic mixed cultures to co-ferment syngas and the aqueous condensate from either sewage sludge or polyethylene plastics pyrolysis in semi-batch bottle fermentations. It identifies inhibitory concentrations for carboxydotrophic and methanogenic reactions, examines specific component removal and assesses energy recovery potential. The results show successful co-fermentation of syngas and aqueous condensate components like phenols and N-heterocycles. However, the characteristics and load of the aqueous condensates affected process performance and product formation. The toxicity, likely resulting from the synergistic effect of multiple toxicants, depended on the PACs’ composition. At 37 °C, concentrations of 15.6 gCOD/gVSS and 7.8 gCOD/gVSS of sewage sludge-derived aqueous condensate inhibited by 50% carboxydotrophic and methanogenic activity, respectively. At 55 °C, loads between 3.9 and 6.8 gCOD/gVSS inhibited by 50% both reactions. Polyethylene plastics condensate showed higher toxicity, with 2.8 gCOD/gVSS and 0.3 gCOD/gVSS at 37 °C decreasing carboxydotrophic and methanogenic rates by 50%. At 55 °C, 0.3 gCOD/gVSS inhibited by 50% CO uptake rates and methanogenesis. Increasing PAC loads reduced methane production and promoted short-chain carboxylates formation. The recalcitrant components in sewage sludge condensate hindered e-mol recovery, while plastics condensate showed high e-mol recoveries despite the stronger toxicity. Even with challenges posed by substrate toxicity and composition variations, the successful conversion of syngas and aqueous condensates highlights the potential of this technology in advancing carbon and energy recovery from anthropogenic waste streams.Graphical

Transformation and Mitigation of Tar and Related Secondary Pollutants during Sewage Sludge Pyrolysis

Sewage sludge has long been perceived as the bottleneck of wastewater treatment plants in China, restraining the healthy development of sewage treatment for decades. In recent years, pyrolysis as a promising multifunctional platform has attracted increasing interest for converting sludge into valuable resources. However, the generation and presence of pyrolysis tar, one of the key by-products during sludge pyrolysis, limit the wide application of pyrolysis product resources. The efficient and selective conversion of tar is complicated by the inherent complexity of sludge and various pollutants (e.g., N-, S-, and Cl-containing organic pollutants, heavy metals) in pyrolysis products, which may either migrate into tar or be released into the environment, complicating the in-depth treatment of tar and posing environmental risks. This review systematically examines the transformation and migration of tar and related secondary pollutants during sludge pyrolysis in order to optimize the pyrolysis process for resource recovery. We provide an overview of the research progress on tar generation, transformation, and secondary pollutants during pyrolysis; discuss potential control strategies for pollution abatement; and highlight the importance of understanding tar transformation during pyrolysis. Additionally, we offer insights into future development trends and research hotpots in this field. This review aims to deliver valuable information on the mechanism of tar formation, the conversion pathways of secondary pollutants, and corresponding control strategies, thus guiding the design and optimization of sludge pyrolysis processes to achieve higher efficiency and selectivity, with minimal environmental pollution.

Sewage sludge pyrolysis ‘kills two birds with one stone’: Biochar synergies with persulfate for pollutants removal and energy recovery

The disposal and resource utilization of sewage sludge (SS) have always been significant challenges for environmental protection. This study employed straightforward pyrolysis to prepare iron-containing sludge biochar (SBC) used as a catalyst and to recover bio-oil used as fuel energy. The results indicated that SBC-700 could effectively activate persulfate (PS) to remove 97.2% of 2,4-dichlorophenol (2,4-DCP) within 60 min. Benefiting from the appropriate iron content, oxygen-containing functional groups and defective structures provide abundant active sites. Meanwhile, SBC-700 exhibits good stability and reusability in cyclic tests and can be easily recovered by magnetic separation. The role of non-radicals is emphasized in the SBC-700/PS system, and in particular, single linear oxygen (1O2) is proposed to be the dominant reactive oxygen. The bio-oil, a byproduct of pyrolysis, exhibits a higher heating value (HHV) of about 30 MJ/kg, with H/C and O/C ratios comparable to those of biodiesel. The energy recovery rate of the SS pyrolysis system was calculated at 80.5% with a lower input cost. In conclusion, this investigation offers a low-energy consumption and sustainable strategy for the resource utilization of SS while simultaneously degrading contaminants.

Sustainable valorisation of sewage sludge via carbon dioxide-assisted pyrolysis

The escalating volume of sewage sludge (SS) generated poses challenges in disposal, given its potential harm to the environment and human health. This study explored sustainable solutions for SS management with a focus on energy recovery. Employing CO2-assisted pyrolysis, we converted SS into flammable gases (H2 and CO; syngas). Single-stage pyrolysis of SS in a CO2 conditions demonstrated that CO2 enhances flammable gas production (especially CO) through gas phase reactions (GPRs) with volatile matter (VM) at temperatures ≥520 °C. Specifically, the CO2 partially oxidized the VM released from SS and concurrently underwent reduction into CO. To enhance the syngas production at temperatures ≤520 °C, multi-stage pyrolysis setup with additional heat energy and a Ni/Al2O3 catalyst were utilized. These configurations significantly increased flammable gas production, particularly CO, at temperatures ≤520 °C. Indeed, the flammable gas yield in the catalytic pyrolysis of SS increased from 200.3 mmol under N2 conditions to 219.2 mmol under CO2 conditions, representing a 4.4-fold increase compared to single-stage pyrolysis under CO2 conditions (50.0 mmol). By integrating a water-gas-shift reaction, the flammable gases produced from CO2-assisted catalytic pyrolysis were expected to have the potential to generate revenue of US$4.04 billion. These findings highlight the effectiveness of employing CO2 in SS pyrolysis as a sustainable and effective approach for treating and valorising SS into valuable energy resources.

Effect of polyethylene terephthalate plastics on nitrogen, sulphur and chlorine contaminants via sludge pyrolysis and Pb/Cu adsorption properties of biochar

ABSTRACT Pyrolysis is an effective process for disposing of municipal sewage sludge (SS). Plastics can affect the SS pyrolysis behaviour and pyrolysis products due to their low ash and high hydrocarbon ratio. The secondary pollutants from the pyrolysis process may also be affected. Therefore, polyethylene terephthalate (PET), a typical plastic, was chosen to investigate the release characteristics of pollutants containing nitrogen, sulphur, and chlorine via SS pyrolysis, and the changes of biochar to adsorb two typical heavy metals, Pb and Cu. The pyrolysis of PET plastics facilitates the migration of N toward solid and liquid-phase products, S and Cl to the gas-phase products via pyrolysis. Oxygenated compounds of pyrolytic volatiles decreased from 38.18% to 28.43%, concurrently promoting the formation of phenolic compounds. The co-pyrolysis improved the quality of biochar and the ability to adsorb Pb and Cu. This systematic study can provide some support for the further improvement of SS pyrolysis technology, and will also be beneficial for subsequent applications.

Investigation on pyrolysis characteristics and kinetics of sewage sludge with different heat-mass transfer rates

The pyrolysis process of sewage sludge (SS) in fluidized bed has the characteristics of high speed heat-mass transfer rate, which challenges the reliability of thermal analysis methods. In this study, TGA and fast-reaction thermogravimetric analysis (FR-TGA) were used to investigate the pyrolysis characteristics of SS. The apparent kinetic parameters of pyrolysis at constant heating rate (β) and excellent heat-mass transfer rate were obtained, respectively. Resuls shows that, the pyrolysis of SS under both conditions showed multi-step reaction characteristics. The apparent activation energy (Eα) of pyrolysis of SS determined by FR-TGA was significantly lower than that determined by TGA. With the increase in temperature of the FR-TGA reactor, E decreased further. The excellent heat-mass transfer rate could improve the reactivity during SS pyrolysis. In addition, FR-TGA experiments showed that moisture could promote the pyrolysis of organic components in SS and further reduce Eα. Based on these conclusions, a kinetic model of SS pyrolysis under excellent heat-mass transfer rate was established. This study can serve as a reference for predicting the pyrolysis behavior of SS in a fluidized bed reactor.

A kinetics study of microwave pyrolysis of sewage sludge and corn stalk mixture

AbstractFast pyrolysis of sewage sludge (SS), corn stalk (CS), and their mixture under both direct and indirect microwave heating was carried out in a prototype microwave thermogravimetric reactor at high heating rates, 179 and 203 K/min, respectively. It was found that the samples could be heated up rapidly by direct microwave heating, while there is a temperature lag of the sample to the heated reactor wall in indirect heating. The Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS) methods were used to analyze the thermogravimetric data of SS, CS, and their mixture under direct microwave heating. When the FWO method was used, the average activation energies of SS, CS, and their mixture were obtained as 6.0, 18.9, and 39.8 kJ/mol, respectively. When the KAS method was used, the average activation energies of CS and mixture were 12.6 and 31.6 kJ/mol, respectively. Those values are much lower than reported under conventional heating, and likely result from the interaction of microwave irradiation and the biomass samples. The reaction kinetics of directly microwave‐heated biomass pyrolysis from this study provide essential data for modelling, scaling, and designing the microwave‐assisted in‐situ catalytic pyrolysis reactors with mixed biomass and catalyst particles.

Enhanced generation of aromatic-enriched bio-oil from sewage sludge pyrolysis using zeolite-based catalysts under H2-rich conditions

Sewage sludge is solid waste resulting from wastewater treatment processes, and its improper management adversely affects the environment and human health. This study aims to convert sewage sludge into valuable aromatics, including benzene, toluene, ethylbenzene, and xylenes, or BTEX, via the catalytic pyrolysis using mono- and bimetal-modified protonated Zeolite Socony Mobil–5, or HZSM-5, catalysts under nitrogen, methane and methane decomposition conditions. In the methane medium, HZSM-5 yielded more BTEX (20.91 wt%) compared to those obtained using protonated Y zeolite (HY) (17.24 wt%) and protonated beta zeolite (Hβ) (11.70 wt%) owing to its proper acidity, higher shape selectivity, and higher methane activation. When HZSM-5 is used, the BTEX yield increased considerably in the methane decomposition environment (27 wt%), 1.29 and 1.46 times higher than those produced under methane and nitrogen, respectively. This increase is attributed to the dual effects of the surplus in situ hydrogen (H2) and direct involvement of methane, which resulted in accelerated hydrocracking, hydrodeoxygenation, and co-aromatization reactions. Under methane decomposition conditions, the use of the Ni/HZSM-5, Mo/HZSM-5, and Ni-Mo/HZSM-5 catalysts led to 1.23-, 1.30-, and 1.66-fold increases in BTEX production, respectively, than that of the parent HZSM-5, owing to the dual effects of acidity enhanced by the newly formed Lewis acid sites, increased oxygen vacancies, and improved physical properties of the catalysts. Hence, the catalytic pyrolysis of sewage sludge using mono- and bimetal-modified HZSM-5 catalysts in a H2-rich environment proposed in this study is expected to be an efficient and sustainable method for converting undesirable materials into valuable biofuels.

Sewage sludge biochar as a sustainable and water-safe substrate additive for extensive green roofs

Article describes the application of nutrient-rich sewage sludge (SS) biochar as an innovative additive to extensive green roof substrate at application rates of 10 (SB10) and 20% (v/v) (SB20) and its long-term impact on discharge quality. In addition to a routine analysis of pH, EC, TSS, and COD in the discharge, concentrations of nutrients (total nitrogen and total phosphorus) and metals (Cd, Cu, Fe, Mn, Pb, and Zn) were used to monitor leaching from the experimental green plots. SS biochar improves the water-holding capacity and air-filled porosity of the developed substrates SB10 and SB20, where the chemical and physical parameters meet the minimum requirements for multi-layer extensive roof substrate. The addition of biochar didn't substantially increase the concentrations of nutrients in discharge compared to the conventional extensive substrate, even after 30 months. Results suggested that biochar tends to release nutrients gradually when leached out by rain and may therefore serve as a source of P and other micronutrients. Although an increase in the concentrations of metals (Cu, Pb, and Zn) in the SB10 and SB20 substrates was observed, no significant differences in metal leaching were found. Metal concentrations in the aboveground parts of Sedum plants indicated that the pyrolysis of SS transforms water-extractable forms of metals into stable forms, thereby significantly reducing their bioavailability to Sedum species and any direct ecotoxicological risk. Based on our results, we assert that SS biochar could be used as a sustainable, valuable and water-safe component of extensive substrates for green roofs.

Chemical insight into thermal degradation, chemical kinetics, and thermodynamics behavior of sewage sludge through thermogravimetry and Fourier transform infrared analysis

Pyrolysis emerges as a promising path for sewage sludge (SS) management and value creation by producing compounds with enhanced utility. This study focuses on a comprehensive analysis of the thermal decomposition of SS using thermogravimetric analysis (TGA) and Fourier transform infrared spectrometry (FTIR). The TGA and FTIR were employed to evaluate the pyrolysis behavior, kinetics, and characteristics during pyrolysis of SS. The process revealed three distinct stages: 1st stage ranged between 30–185 °C with the removal of moisture, 2nd stage saw devolatilization of SS in the range of 185–595 °C, and the 3rd stage was mainly the formation of char in the temperature range of 595–900 °C. Pyrolysis kinetics, reaction mechanisms, and thermodynamic properties were extensively investigated at three different heating rates of 10, 20, and 30 °C /min. It was observed that with the temperature rise, the decomposition rate also increased rapidly, notably for the heating rate of 30 °C/min. Using Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, Starink, and Tang methods, activation energies for SS pyrolysis were estimated for α in the range of 0.2–0.85, and the average value for that range was found to be 84.72, 92.29, 91.99, and 91.76 kJ/mol, respectively for different iso-conversional methods. The chemical kinetics analysis through Coats-Redfern methods identified that 3-D diffusion-reaction mechanisms have the highest coefficient of determination among the other models. Criado z-master plot emphasizes the prevalence of Avrami-Erofeev (A4) as the bestsuited reaction model in combination with α, providing a detailed examination that illuminates the complex processes and mechanisms inherent in the pyrolysis of sewage sludge.

Life cycle assessment of sewage sludge treatment: Comparison of pyrolysis with traditional methods in two Swedish municipalities

To achieve a closed nutrient cycle and more sustainable food production, enhanced nutrient recycling in the agri-food system is a necessity. Pyrolysis is an emerging technology to recycle the nutrient content of sewage sludge. The produced biochar can be used to reduce the need for mineral fertilizers; in addition, pyrolysis can also handle potential pollutants such as microplastics and pathogens present in sewage sludge. In this research, a life cycle assessment (LCA) was carried out to determine the environmental impact of sewage sludge pyrolysis as an alternative to current practices of two different cases of sewage sludge treatment in two municipalities in Sweden. The results indicated that avoiding mineral fertilizer production by using biochar has a significant influence on the environmental benefits. Furthermore, it showed that an integrated system of anaerobic digestion followed by pyrolysis could perform as the most environmental-friendly option for sewage sludge treatment with a lower risk of transferring pollution to the soil.

Hydrogen rich syngas production through sewage sludge pyrolysis: A comprehensive experimental investigation and performance optimisation using statistical analysis

Bioenergy is anticipated to play a significant role in the United Kingdom’s Net Zero 2050 scenario. This study aims to explore the possibility of producing hydrogen-rich syngas using sewage sludge from a wastewater treatment plant located in England, United Kingdom. The primary objective of this study is to experimentally produce hydrogen-rich syngas from sewage sludge through pre-treatment, drying, and pyrolysis. Furthermore, statistical methods have been employed to optimise the performance of the pyrolyser. The individual desirability scores for lower heating value (LHV) and cold gas efficiency (CGE) were estimated to be 0.83902 and 0.85307, respectively. Combining these scores, the overall desirability of the model reached 0.8460, indicating favourable predictive performance. The optimal operational conditions are reported to be a feed rate of 3.0488 revolutions per minute (rpm) and an operational temperature of 800°C. Under these conditions, the highest calculated CGE of 66.31% and the peak LHV value of 18.36 MJ/ m^3 were achieved.

Application of untreated versus pyrolysed sewage sludge in agriculture: A life cycle assessment

Recycling of phosphorus (P) from waste streams such as sewage sludge to agriculture is essential in order to ensure future food security. Land application of sewage sludge is controversial due to its content of pollutants, such as heavy metals and toxic organic compounds. Pyrolysis is a technology that can eliminate harmful contaminants while enabling the recycling of P in the sludge. Organic pollutants are degraded during pyrolysis, while most of the P remains and approximately 40 % of the carbon is retained in a stable form in the biochar. When performing pyrolysis at a temperature of around 800 °C or more, cadmium is evaporated and can be separated from the biochar. Electricity-driven pyrolysis facilitates this by means of easier temperature control. The aim of the present study was to undertake a Life Cycle Assessment (LCA) to evaluate the environmental impacts of high-temperature electricity-driven pyrolysis of sewage sludge, including the separation of cadmium, and field application of sludge biochar as compared with sewage sludge storage and field application. The results showed that pyrolysis can offer a more climate-friendly solution due to avoided greenhouse gas emissions from the sludge and to carbon sequestration of the biochar. However, field application of untreated sludge resulted in a higher application of nitrogen (N) and more plant-available P. Agricultural modelling tools indicated that this produces higher crop yields than biochar application. In the LCA model, higher crop yields lead to savings in land use and water consumption as the higher yields can replace other crop production. The lower plant availability of biochar P implies a possible risk of lower yields when the P application is restricted based on total P, and a risk of greater P loss with soil erosion to the environment. Overall, the present study stresses the importance of including the long-term agronomic impacts of sludge and biochar field application in LCAs, and highlights the trade-offs that need to be considered in decision-making regarding the implementation of pyrolysis technology.

Mechanistic elucidation and tuning pathway of environmentally persistent free radicals during sewage sludge pyrolysis

Pyrolysis is a promising technology for the treatment of vast amounts of sewage sludge due to its lower emission of secondary pollutants compared with that of combustion and landfilling. However, the generation of environmentally persistent free radicals (EPFRs, emerging pollutants) due to the high temperature and oxygen deficiency of the pyrolytic atmosphere is a difficult issue. In this study, we systematically investigated the pyrolytic panorama and correlated the pyrolytic parameters to EPFR formation using electron spin resonance (ESR) spectrometry and in situ thermogravimetry–Fourier transform infrared–mass spectrometry (TG–FTIR–MS). The results showed that the EPFR concentration in biochar increased from 8.2×1018 spins g−1 at 300 °C to 16.3 ×1018 spins g−1 at 400 °C but decreased to 0.6 ×1018 spins g−1 at 600 °C, indicating that the majority of EPFRs decomposed at 600 °C during pyrolysis. Notably, the N species and ash in the sludge were also found to reduce the formation of EPFRs during sludge pyrolysis. Aromatic compounds, including carboxyl, hydroxyl, and aldehyde groups, are the main precursors of EPFRs based on the correlation of EPFR concentrations and intermediates determined via TG–FTIR–MS. The formation pathway of EPFRs in sludge char involves three processes, namely, the adsorption of aromatic compounds, elimination of water, and electron transfer from aromatic compounds to char. The proposed formation mechanism of EFPRs during sludge pyrolysis not only broadens the understanding of the pollutant formation during sludge pyrolysis but also provides a strategy for managing emerging pollutants during pyrolysis.