Synergistic effect of combined hydrothermal carbonization of Fenton's reagent and biomass enhances the adsorption and combustion characteristics of sludge towards eco-friendly and efficient sludge treatment

Pre-drying limitedly affected the yield, fuel properties, pyrolysis and combusion behavior of sewage sludge hydrochar

While pre-drying of sewage sludge prior to hydrothermal carbonization is rarely practiced, various pre-drying methods have been performed in literature at lab-scale for convenient solid-to-liquid ratio adjustment. This has created a barrier for comparing hydrochar quality between different studies. Given pre-drying can destroy the floc structure of sewage sludge, we hypothesize that pre-drying may promote the hydrolysis step during hydrothermal carbonization process, resulting in improved hydrochar quality with low nitrogen content. In the current study, the influence of different pre-drying methods (freeze-dry, air-dry and vacuum-dry at 70 °C and 105 °C) on the subsequent hydrothermal carbonization of sewage sludge at 220 °C was assessed in terms of sewage sludge and hydrochar's chemical composition, fuel properties, pyrolysis and combustion behavior, as well as the characterization of the liquid phase. The results indicate that although pre-drying impacts sewage sludge's chemical composition, pyrolysis and combustion behavior, no significant differences exist in the yield, chemical composition, fuel properties, and pyrolysis and combustion behavior of the hydrochar. Therefore, the use of pre-drying would not affect the hydrothermal carbonization process of sewage sludge, and a comparison can be made on hydrochar quality between different studies with or without pre-drying.

Comparative analysis of adding cotton straw and corn stover to improve the combustion performance of municipal sludge

To address issues of high water content and low calorific value during combustion of municipal sludge, we added water-absorbent, easy-to-burn agricultural waste to improve the overall combustion performance. Cotton straw or corn stover were added to the sludge and mixed at high-speed to compare their capacities for improving combustion performance. Scanning Electron Microscopy (SEM) revealed that cotton straw or corn stover attached to the surface of the municipal sludge particles after blending, while analysis of thermogravimetric curves and activation energies of the blends showed that combustion and exhaustion rates increased significantly when 40% cotton straw or corn stover were blended into the sludge. Using the quadrilateral cut-ring boiler as a prototype, the mix of sludge with cotton straw or corn stover was simulated, and FLUENT software was used to obtain the temperature and pollutant emissions of the boiler. Sludge blended with cotton straw or corn stover increased furnace temperature and reduced SO2 and NO emissions, while that with cotton straw burned at higher temperatures with lower SO2 and NO emissions. Overall, the CO content of sludge combustion was lower when blended with proportions of cotton straw or corn stover under 50%. The findings of this study lay a theoretical foundation for treatment of municipal sludge according to local conditions.

One-step synthesis of iron and nitrogen co-doped porous biochar for efficient removal of tetracycline from water: Adsorption performance and fixed-bed column

Here, Fe/N co-doped porous biochars (FeNKBCs) were obtained by grinding corncob, CH3COOK, FeCl3·6H2O, and C3H6N6 via one-step synthesis and were applied to remove antibiotics from wastewater. Notably, CH3COOK had an excellent porous activation ability. The developed nanotubular structure of Fe1N2KBC had a high pore volume (Vtotal) (1.2131 cm3/g) and specific surface areas (SSA) (2083.54 m2/g), which showed outstanding sorption abilities for TC (764.35 mg/g), OTC (560.82 mg/g), SMX (291.45 mg/g), and SMT (354.65 mg/g). The adsorption process of TC was controlled by chemisorption. Moreover, Fe1N2KBC has an excellent dynamic adsorption performance (620.14 mg/g) in a fixed-bed column. The properties of SSA, Vtotal, and the content of graphite N and Fe-N were positively correlated with TC adsorption capacity. The high performance of TC removal was related to π-π stacking, pore-filling, hydrogen bond, and electrostatic interaction. Fe1N2KBC possessed stable sorption amounts in pH 2-12 and actual water, and well reuse performance. The results of this work present an effective preparation method of Fe/N porous biochar for TC-contaminated water remediation.

Co-hydrothermal carbonization of sludge and food waste for hydrochar valorization: Effect of mutual interaction on sulfur transformation

Co-hydrothermal carbonization of sludge and food waste is a promising method for hydrochar valorization. The sulfur content and form of hydrochar are the key parameters that determine its further utilization. However, the effect of the chemical composition of food waste on sulfur redistribution remains unknown. Herein, the sulfur transformation behavior during the co-hydrothermal carbonization of sludge and model compounds (cellulose, starch, xylan, and palmitic acid) of food waste was investigated, with focus on the detailed reaction pathways from inorganic-S/organic-S media in aqueous to hydrochar. The added model compounds, particularly the starch and xylan, increased the sulfur retention ratio from 41.0 to 44.7- 49.2 % in hydrochar. Among them, starch and xylan can react with aliphatic-S in aqueous via cyclization and oxidization to form the thiophene-S/aromatic-S and sulfone-S and can react with SO42--S to form sulfone-S via sulfonate reaction. These formed organic-S can polymerize with hydrolyzed intermediates (i.e., 5 hydroxymethyl-furfural, glucose, and xylose) from model compounds to transform into hydrochar. Cellulose enhanced the formation of sulfone-S in hydrochar via the reactions between the water-insoluble partial hydrolysate and SO42- in the aqueous. Additionally, palmitic acid hydrolysate provided an acidic environment that facilitated the polymerization of thiophene-S/aromatic-S from aqueous to hydrochar. Generally, the chemical composition of food waste largely affects the redistribution behavior of sulfur during co-hydrothermal carbonization, and this occurs primarily due to the differences in the hydrolysate and degree of hydrolysis for various model compounds. The results can provide guidance for preparing sludge-based hydrochar possessing different sulfur content and species, that can be used as clean fuel or carbon material.

Co-pyrolysis of peanut shell with municipal sludge: reaction mechanism, product distribution, and synergy

Biomass/sludge co-pyrolysis contributes to the high-efficiency resource utilization, harmless treatment, and reduction in volume of sludge. Due to the complexity of co-pyrolysis reaction, it is essential to evaluate the thermodynamic behavior, synergy, and reaction mechanism of this process to make it commercially viable. In this work, the pyrolysis properties, thermodynamic analysis and product distribution of municipal sludge (MS), peanut shell (PS), and their blends with various sludge mass ratios (SMRs) were investigated by a thermogravimetric analyzer and a fixed bed reactor. There was a considerable synergy existing in the process of PS/MS co-pyrolysis, and the synergy occurred mainly at the devolatilization phase, accelerating the mixture pyrolysis. When the conversion rate α was less than 0.7, the apparent activation energy decreased continuously with SMR at the same α; however, it increased dramatically with SMR when α was greater than 0.7. Reactants and reaction stages greatly affected the kinetic mechanism of fuel pyrolysis, and this finding was beneficial for the numerical simulation of mixture pyrolysis. Based on the conclusions and precision of this work, the mass ratio of PS to MS was recommended to be 6:4, which had the strongest synergy, with a gas yield of 26.69 wt.% at 600°C and a lower heating value (LHV) of pyrolysis gas of 14.89 MJ/Nm3.

Co-hydrothermal carbonization of pine residual sawdust and non-dewatered sewage sludge - effect of reaction conditions on hydrochar characteristics

Waste valorization is mandatory to develop and consolidate a circular bioeconomy. It is necessary to search for appropriate processes to add value to different wastes by utilizing them as feedstocks to provide energy, chemicals, and materials. For instance, hydrothermal carbonization (HTC) is an alternative thermochemical process that has been suggested for waste valorization aiming at hydrochar production. Thus, this study proposed the Co-HTC of pine residual sawdust (PRS) with non-dewatered sewage sludge (SS) - two wastes largely produced in sawmills and wastewater treatment plants, respectively - without adding extra water. The influence of temperature (180, 215, and 250 °C), reaction time (1, 2, and 3 h), and PRS/SS mass ratio (1/30, 1/20, and 1/10) on the yield and characteristics of the hydrochar were evaluated. The hydrochars obtained at 250 °C had the best coalification degree, showing the highest fuel ratio, high heating value (HHV), surface area, and N, P, and K retention, although presenting the lowest yields. Conversely, hydrochar functional groups were generally reduced by increasing Co-HTC temperatures. Regarding the Co-HTC effluent, it presented acidic pH (3.66-4.39) and high COD values (6.2-17.3 g·L^-1). In general, this new approach could be a promising alternative to conventional HTC, in which a high amount of extra water is required. Besides, the Co-HTC process can be an option for managing lignocellulosic wastes and sewage sludges while producing hydrochar. This carbonaceous material has the potential for several applications, and its production is a step towards a circular bioeconomy.

A review on hydrothermal carbonization of potential biomass wastes, characterization and environmental applications of hydrochar, and biorefinery perspectives of the process

It is imperative to search for appropriate processes to convert wastes into energy, chemicals, and materials to establish a circular bio-economy toward sustainable development. Concerning waste biomass valorization, hydrothermal carbonization (HTC) is a promising route given its advantages over other thermochemical processes. From that perspective, this article reviewed the HTC of potential biomass wastes, the characterization and environmental utilization of hydrochar, and the biorefinery potential of this process. Crop and forestry residues and sewage sludge are two categories of biomass wastes (lignocellulosic and non-lignocellulosic, respectively) readily available for HTC or even co-hydrothermal carbonization (Co-HTC). The temperature, reaction time, and solid-to-liquid ratio utilized in HTC/Co-HTC of those biomass wastes were reported to range from 140 to 370 °C, 0.05 to 48 h, and 1/47 to 1/1, respectively, providing hydrochar yields of up to 94 % according to the process conditions. Hydrochar characterization by different techniques to determine its physicochemical properties is crucial to defining the best applications for this material. In the environmental field, hydrochar might be suitable for removing pollutants from aqueous systems, ameliorating soils, adsorbing atmospheric pollutants, working as an energy carrier, and performing carbon sequestration. But this material could also be employed in other areas (e.g., catalysis). Regarding the effluent from HTC/Co-HTC, this byproduct has the potential for serving as feedstock in other processes, such as anaerobic digestion and microalgae cultivation. These opportunities have aroused the industry interest in HTC since 2010, and the number of industrial-scale HTC plants and patent document applications has increased. The hydrochar patents are concentrated in China (77.6 %), the United States (10.6 %), the Republic of Korea (3.5 %), and Germany (3.5 %). Therefore, considering the possibilities of converting their product (hydrochar) and byproduct (effluent) into energy, chemicals, and materials, HTC or Co-HTC could work as the first step of a biorefinery. And this approach would completely agree with circular bioeconomy principles.