Low-temperature co-pyrolysis behaviours and kinetics of oily sludge: effect of agricultural biomass

Characteristics of heavy metal migration during pyrolysis of typical oily wastes and environmental risk assessment of pyrolysis residues

Solid-phase residues from pyrolysis of oily wastes (OS) are widely used due to their rich pore structure and strong adsorption capacity. In this study, pyrolysis residues (OS-P) were obtained from the pyrolysis treatment of four typical OS in Karamay, Xinjiang. The results indicate that the crystalline substances in OS-P mainly were SiO2, BaSO4, and graphite. The heavy metals of OS-P were higher than that of OS in the following order: Zn > Cu > Ni > Cr > Pb > Cd. The results of the improvement of Community Bureau of Reference (BCR) sequential extraction showed that the proportion of Cu, Ni and Cr in OS1-P in the residual fraction was higher than that of the other three OS. The residual fraction of Cu, Ni, and Cr in OS1-P increased from 16.0 %, 30.0 %, and 11.0 % to 66.1 %, 81.9 %, and 89.2 %, respectively. After pyrolysis treatment, the leaching concentration of heavy metals in the residue was reduced. Referring to the requirements for heavy metal control limits (GB 4284-2018), all heavy metals in OS-P showed low risk. Their potential ecological risk indices were 4.11, 3.13, 4.87 and 5.35, respectively, indicating that the potential ecological hazards of heavy metals from OS-P were slight. There was no significant effect on the histopathological changes of kidney, lung, liver, ovary and testis of mice, showing that the rational use of OS-P in production will not produce toxic effects on target animals. Based on risk assessment and safety evaluation, the application of OS-P is controllable, safe and reliable for resource utilization.

Auxiliary effect of CO2 on pyrolysis of oily sludge

The efficient recovery and conversion of energy in oily sludge has great prospects. In this article, the main objective is to investigate the impact of the addition of CO₂ during the pyrolysis of oily sludge on energy recovery and conversion by thermogravimetric analysis (TGA) and compare the effect with the traditional pyrolysis effect from the perspective of thermal conversion behavior, products composition, and kinetics analysis. The results of the experiment showed that in the CO₂ atmosphere, the main weight loss temperature of oily sludge was mainly concentrated in the range of 300-500 °C, which is lower than the reaction temperature range of traditional pyrolysis. The yields of CO and H₂ in the products have been greatly improved, and the highest proportion in the gas products can reach 19.29% and 22.38%, respectively. The E_a (activation energy) values of oily sludge were determined to be in the range of 40-120 kJ·mol^-1 with the conversion between 0.2 and 0.8 via DAEM, KAS, Starink, and FWO methods, respectively, in which the FWO method has shown the strongest adaptability. The results of this study provide reference values for practical engineering applications.

Novel conditioner for efficient dewaterability and modification of oily sludge with high water content

The oily sludge with high water content (OS) was dewatered, modified, and converted into solid fuel by a novel chemical conditioner (OSO-101). The effect of OSO-101 dosage on the dewaterability of OS was studied, showing that OSO-101 dosage of 15% (wt.) could achieve the best dewaterability efficiency of OS (98.18%). Meanwhile, compared with some conventional conditioners, OSO-101 developed by our team was more effective in improving OS dewaterability efficiency. And OSO-101 may have free radical reaction, polar reaction, and redox reaction with petroleum hydrocarbons in OS, thereby polymerizing and forming condensed solid structures. The calorific value change of OS after conditioning, heavy metal content, and dioxin content of fly ash leached from incinerated product were measured for resource analysis and environmental assessment. Results showed that the resultant OS fuel blocks had extremely low content of heavy metals, dioxins, and other toxic and hazardous substances leached from fly ash. And this process did not require secondary treatment and fully met environmental protection emission standards. Additionally, OSO-101 had certain economic rationality and could effectively recover the calorific value contained in OS. This research is expected to provide new insights for efficient dewaterability and modification of OS, as well as subsequent resource utilization and harmless treatment, bringing potential environmental and economic benefits.

Catalytic co-pyrolysis of oil sludge and biomass over ZSM-5 for production of aromatic platform chemicals

Hazardous oil sludge (OS) poses a great challenge to the environment, whereas conventional treatment methods (i.e., incineration or pyrolysis-incineration) are relatively less value-added and will bring about air pollution problems. To realize the high-value utilization of OS, catalytic co-pyrolysis with waste biomass to produce platform chemicals was studied using TG-FTIR and Py (pyrolyzer)-GC/MS methods. Results showed that for the non-catalytic co-pyrolysis of RH (rice husk) and OS, the main synergy on weight loss was the greatly lowered initial pyrolysis temperature of RH (for ∼55 °C) at the lower temperatures and the reduced weight loss ratio of OS (∼10-18 wt%) within the higher temperature range. ZSM-5 catalyst promoted the degradation of OS and RH mixtures at < 150 °C, yet showed minor effects on their weight loss at higher temperatures. The oxygenated and aliphatic compounds from non-catalytic co-pyrolysis were efficiently converted, resulting in an increased relative yield of aromatics to the highest of 46% and an elevated selectivity to BTX (as high as 60%). Despite the relatively short carbon chain length of OS components, ZSM-5 was proved effective to activate the OS pyrolysis products, thus enhancing the further aromatization reactions with biomass pyrolysis intermediates. This study provides a novel method for value-added co-utilization of hazardous OS waste and abundant biomass waste, and thus is beneficial to producing renewable chemicals while reducing the environment pollutant.

Conversion of plastic waste into fuel oil using zeolite catalysts in a bench-scale pyrolysis reactor

Catalytic pyrolysis of mixed plastic waste to fuel oil experiment was tested with ZSM-5 zeolite (commercial and synthesized) catalysts along with other catalysts.

Effect of additives on the distribution of three-phase products of oily sludge subjected to microwave pyrolysis

This study aimed to explore the influence of activated carbon, oily sludge pyrolysis residue, and biochar and their contents on the distribution of three-phase products of oily sludge subjected to microwave pyrolysis. A microwave reaction system, refinery gas analyzer, and chromatography-mass spectrometry were used to carry out the experiment and analyze the results. The results showed that all three additives reduced the yield of solid products and increased the yield of gas products. With an increase in the additive content, the volatile matter and moisture content in the pyrolysis residue greatly reduced. The content of CH₄ and H₂ in the pyrolysis gas increased with an increase in the additive content. When the amount of activated carbon was 20%, the H₂ content reached a maximum (39.7%), and when the amount of biochar was 20%, the CH₄ content reached a maximum (44.5%). All three additives increased the content of small molecules in the pyrolysis oil; when 10% activated carbon was added, the oil recovery rate reached up to 78.5%. The results of this study can guide the industrial application of microwave pyrolysis oily sludge.

The sludge-based adsorbent from oily sludge and sawdust: preparation and optimization

Sludge-based adsorbent (S-AB) converted by oily sludge can make full use of the precious resource. In this paper, oily sludge and discarded sawdust are used to prepare adsorbent through chemical activation. The adsorbent prepared is used to adsorb raw petroleum. Firstly, the most reasonable chemical activator ZnCl₂ is ascertained through parallel comparative experiments. The characterization results of N₂-adsorption are consistent with adsorption experiment results, which shows that higher mesopore surface area and volume are benefitted by the adsorption process. Secondly, the optimization of preparation technology is investigated through orthogonal experiments after parallel comparative experiments. The adsorption capacity of S-AB-ZnCl₂ is stronger when the preparation conditions are as follows: an activation temperature of 550°C, an activation time of 3.5 h, a solid-liquid ratio of 1:1.5, a sludge-sawdust ratio of 1:0.5 and the heating rate of 15°C/min. The maximum quantity adsorbed Q₀ = 434.78 mg/g, calculated through the Langmuir adsorption isothermal models, of S-AB-ZnCl₂ prepared under optimized condition is higher than that before optimization. In addition, the most reasonable kinetics fits were of the second-order model.

Products distribution and interaction mechanism during co-pyrolysis of rice husk and oily sludge by experiments and reaction force field simulation

In this work, the co-pyrolysis behavior of rice husk (RH) and oily sludge (OS) was investigated by combining experiments and simulation. The thermogravimetric-derivative thermogravimetric (TG-DTG) and Reaction force field (ReaxFF MD) results indicate that synergetic effects exist in co-pyrolysis. Compared with the single component pyrolysis, the activation energy of RH and OS in co-pyrolysis was decreased by 15.97% and 17.14% shown by kinetic analysis, respectively. The Pyrolysis-gas chromatography/mass spectrometry (PY-GC/MS) experiments, and simulation products analysis reveal that more bio-oil and molecules with low molecular weight were produced during the co-pyrolysis process. The synergetic effect mechanism was studied by detecting the variation of free radical intermediates. The results show that hydroxyl radicals from RH pyrolysis reduced cracking temperature of OS, and the hydrogen radicals from OS pyrolysis increased the degree of ring-splitting of RH. The study explores an approach to identify the synergetic effect and reveal the mechanism of co-pyrolysis.

A critical review on energy recovery and non-hazardous disposal of oily sludge from petroleum industry by pyrolysis

This review systematically reports the pyrolysis of oily sludge (OS) from petroleum industry in regards to its dual features of the energy recovery potential and the environmental risks. The petroleum hydrocarbons are the nonbiodegradable fractions in OS that possess hazardous properties, i.e. ignitability and toxicity. Besides, complicated hazardous elements (i.e. N, S and Cl) and heavy metals inherently existing in OS further aggravate the environmental risks. However, the high oil content and heating value of OS contribute to its huge energy resource potential. Considering the energy demand and the environmental pressure, the ultimate purposes of the OS management are to enhance the oil recovery efficiency to minimize the oil content as well as to stabilize the hazardous elements and heavy metals into the solid residue. Among various OS management technologies, pyrolysis is the most suitable approach to reach both targets. In this review paper, the pyrolysis principle, the kinetics and the product distribution in three-phases are discussed firstly. Then the effects of operating parameters of the pyrolysis process on the quality and the application potential of the three-phase products, as well as the hazardous element distribution are discussed. To further solve the dominant concerns, such as the oil content in the solid residue, the pyrolytic oil quality and the migration of hazardous elements and heavy metals, the potentials of the catalytic pyrolysis and the co-pyrolysis with additives are also summarized. Also, the typical pyrolysis reactors are then presented. From the perspective of the energy efficiency and the non-hazardous disposal, the integrated technology combining the pyrolysis and the combustion for the OS management is recommended. Finally, the remaining challenges of OS pyrolysis encountered in the research and the industrial application are discussed and the related outlooks are itemized.

Enhancing anaerobic degradation of oily sludge using subcritical hydrothermal pretreatment

AIMS

Oily sludge is a kind of mixture that is extremely harmful to the environment. Anaerobic digestion (AD) is a commonly used method for biodegrading oily sludge. However, the AD treatment cycle is usually long and inefficient. Here, we developed an approach to improve the degradation rate of oily sludge by integrating subcritical hydrothermal pretreatment (SHP) and AD.

METHODS AND RESULTS

First, using SHP, the hydrocarbon compounds with long carbon chains that make up oil sludge were decomposed into hydrocarbons with short carbon chains, which are conducive to microbial decomposition and transformation. Then, AD was performed using a variety of temperature and solid-liquid ratio parameters. The results showed that the degradation ratio of oily sludge was higher when SHP was combined with AD than when no pre-treatment was performed. Optimal degradation was reached by performing SHP to obtain CHS8, then performing AD at 30°C using a 1:5 solid-liquid ratio. Under these conditions, maximum degradation ratios of 69·00% of TOC, 59·02% of COD, 44·68% of ammonia and 54·24% of oil content were reached.

CONCLUSIONS

In conclusion, after SHP with 8% dilute sulphuric acid, most of the macromolecular hydrocarbons in the oily sludge were converted into smaller molecules, which facilitated subsequent microbial decomposition. The results showed that this combination of SHP and AD processes promotes more efficient degradation than a conventional single AD process without any hydrothermal pretreatment.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our experiments provide technical support for enhancing the rapid degradation of oily sludge.

Role of coke-bounded environmentally persistent free radicals in phenanthrene degradation by hydrogen peroxide

Emission of polycyclic aromatic hydrocarbons (PAHs) is accompanied with the discharge of carbonaceous particles during the coke production. To degrade the adsorbed PAHs, hydrogen peroxide (H₂O₂) was applied as an oxidising agent, which might be activated by the inherent environmentally persistent free radicals (EPFRs) on coke particles. The transformation of phenanthrene (PHE), selected as model molecule, was achieved in H₂O₂/coke particle system without the addition of additional activating agent. This process consumed the particle-bounded EPFRs, inducing the decreasing of spin density from 1.92 × 10¹⁸ to 4.4 × 10¹⁷ spins g^-1 in 30 min of reaction time. Electron paramagnetic resonance (EPR) technique coupled with spin-trapping agent 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) was used to probe the potential formation of reactive oxygen species. A higher capture [[Formula: see text]] concentration was observed with larger decreases in EPFRs concentration, indicating that EPFRs were the main contributor to the formation of [Formula: see text]. The obtained results suggested that the activation of H₂O₂ by EPFRs on coke particles resulted in the generation of hydroxyl radical ([Formula: see text]), which then back-reacted with adsorbed PHE. The finding of this study shed light on a new remediation technology for toxic carbonaceous byproducts discharged during the coke production.

Experimental study on catalytic pyrolysis of oil sludge under mild temperature

The pyrolysis performance of oil sludge (OS) was studied using a thermal gravimetric analysis apparatus and a tube furnace reactor. The oil recovery rate of OS pyrolysis showed a rapid growth trend at 450 °C. Moreover, the co-pyrolysis experiments of the OS and catalysts, including walnut shells, Fe₂O₃, K₂CO₃, polyvinyl chloride (PVC), and OS pyrolysis char with addition ratios of 5, 7, and 9 wt%, respectively, were conducted in a tube furnace reactor at 450 °C. The experiments demonstrated that all catalysts increased the oil recovery rate, but the optimal addition ratios differed. The pyrolysis chars produced above 450 °C had a well-developed pore structure, and the catalytic pyrolysis of OS at 450 °C could increase the yield of pyrolysis oil and reduce the potential ecological risk of heavy metals in the pyrolysis char. Therefore, catalytic pyrolysis is an inexpensive and highly efficient approach for treating solid waste.

Dewatering and low-temperature pyrolysis of oily sludge in the presence of various agricultural biomasses

Pyrolysis is potentially an effective treatment of waste oil residues for recovery of petroleum hydrocarbons, and the addition of biomass is expected to improve its dewatering and pyrolysis behavior. In this study, the dewatering and low-temperature co-pyrolysis of oil-containing sludge in the presence of various agricultural biomasses, such as rice husk, walnut shell, sawdust, and apricot shell, were explored. As a result, the water content gradually decreases with the increase of biomass addition within 0-1.0 wt % in original oily sludge. Comparatively, the dewatering efficiency of sludge in the presence of four types of biomasses follows the order of apricot shell > walnut shell > rice husk > sawdust. On the other hand, rice husk and sawdust are relatively more efficient in the recovery of petroleum hydrocarbons compared with walnut shell and apricot shell. The recovery efficiency generally increased with the increase in the biomass content in the range of 0-0.2 wt %, then exhibited a gradually decreasing trend with the increase in the biomass content from 0.2 to 1.0 wt %. The results suggest that optimum amount of biomass plays an important role in the recovery efficiency. In addition, the addition of biomass (such as rice husk) also promotes the formation of C_xH_y and CO, increasing the calorific value of pyrolysis residue, and controlled the pollution components of the exhaust gas discharged from residue incineration. The present work implies that biomass as addictive holds great potential in the industrial dewatering and pyrolysis of oil-containing sludge.