Enhanced and environment-friendly chemical looping gasification of crop straw using red mud as a sinter-resistant oxygen carrier

Red mud-based perovskite oxygen carrier preparation for chemical looping gasification of municipal sludge

Chemical looping gasification (CLG) is a promising technology that utilizes lattice oxygen for partial oxidation of solid organic waste to produce high-quality syngas. The utilization of low-cost and high-performance oxygen carriers (OCs) is important for the success of this technology. The red mud from aluminum production was mixed with calcium and manganese oxides to prepare CaMn0.5Fe0.5O3-δ perovskite OCs. The comparative redox tests were carried out to analyze the reactivity using a thermogravimetric analyzer (TGA). Multiple cycle CLG experiments were conducted on a wet municipal sludge in a lab-scale fluidized bed to produce the hydrogen-rich gas. The results showed that the CaMn0.5Fe0.5O3-δ-washed demonstrated higher oxygen transfer capacity and better cycling stability with a maximum weight loss of 7.3096 %. After the 5th cycle in CLG, the syngas obtained using CaMn0.5Fe0.5O3-δ-washed maintained a H2 volume fraction exceeding 40%. However, an increase in CO2 production was also observed, which could be due to the catalytic effect of MnO in the OC on the steam-reforming reaction. The XRD curves showed that fresh CaMn0.5Fe0.5O3-δ-washed exhibited prominent diffraction peaks characteristic of perovskite. It was observed that after undergoing 5 cycles, the presence of iron calcium silicate structures containing Mn became evident due to the attachment of sludge ash, leading to the increased impurities on the surface of OCs with a decrease in the specific surface area. Additionally, some of the reacted OC particles exhibited a hollow structure, facilitating the fluidization. This preliminary study provides the basis for the improvement of the OC performance in sludge gasification.

A review of metallurgical slags as catalysts in advanced oxidation processes for removal of refractory organic pollutants in wastewater

With the rapid growth of the metallurgical industry, there is a significant increase in the production of metallurgical slags. The waste slags pose significant challenges for their disposal because of complex compositions, low utilization rates, and environmental toxicity. One promising approach is to utilize metallurgical slags as catalysts for treatment of refractory organic pollutants in wastewater through advanced oxidation processes (AOPs), achieving the objective of "treating waste with waste". This work provides a literature review of the source, production, and chemical composition of metallurgical slags, including steel slag, copper slag, electrolytic manganese residue, and red mud. It emphasizes the modification methods of metallurgical slags as catalysts and the application in AOPs for degradation of refractory organic pollutants. The reaction conditions, catalytic performance, and degradation mechanisms of organic pollutants using metallurgical slags are summarized. Studies have proved the feasibility of using metallurgical slags as catalysts for removing various pollutants by AOPs. The catalytic performance was significantly influenced by slags-derived catalysts, catalyst modification, and process factors. Future research should focus on addressing the safety and stability of catalysts, developing green and efficient modification methods, enhancing degradation efficiency, and implementing large-scale treatment of real wastewater. This work offers insights into the resource utilization of metallurgical slags and pollutant degradation in wastewater.

Dechlorination and fuel gas generation in chemical looping conversion of waste PVC over inherently Na/Ca/K-containing bauxite residue-based oxygen carriers

The inert atmosphere in chemical looping (CL) technology can considerably inhibit the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans during the thermal treatment of polyvinyl chloride plastic (PVC) waste. In this study, PVC was innovatively converted to dechlorinated fuel gas via CL gasification under a high reaction temperature (RT) and the inert atmosphere by applying an unmodified bauxite residue (BR) as both a dechlorination agent and oxygen carrier. The dechlorination efficiency reached 49.98% at an oxygen ratio of only 0.1. Furthermore, a moderate RT (750 °C in this study) and an increased oxygen ratio enhanced the dechlorination effect. The highest dechlorination efficiency (92.12%) was achieved at an oxygen ratio of 0.6. Iron oxides in BR improved the generation of syngas from CL reactions. The yields of the effective gases (CH4, H2, and CO) increased by 57.13% to 0.121 Nm3/kg with an increase in oxygen ratio from 0 to 0.6. A high RT improved the production of the effective gases (an 809.39% increase to 0.344 Nm3/kg from 600 to 900 °C). Energy-dispersive spectroscopy and X-ray diffraction were used to study the mechanism, and formation of NaCl and Fe3O4 was observed on the reacted BR, indicating the successful adsorption of Cl and its capability as an oxygen carrier. Therefore, BR eliminated Cl in situ and enhanced the generation of value-added syngas, thereby achieving efficient PVC conversion.

Biomass waste as a clean reductant for iron recovery of iron tailings by magnetization roasting

The magnetization roasting with coal as primary reductants adds cost and causes environmental pollution. Therefore, it is of great importance to investigate the biomass application as a reductant for magnetization roasting to recover iron from low-utilization iron tailings for emission mitigation and green utilization. This study systematically investigated the impact of biomass (pyrolysis gas from agricultural and forestry waste) as a reductant on the conversion of iron tailings to magnetite in magnetization roasting. Additionally, the thermal decomposition of biomass, phase transformation and microstructure evolution of iron tailings were analyzed by TG, XRD, BET, and other methods to elucidate the conversion mechanism for facilitating magnetized hematite in iron tailings with biomass-derived gas. The results showed that woody biomass was a more appropriate reductant for magnetization roasting; 650 °C was the optimal temperature for the complete transformation of hematite to magnetite by reduction roasting with biomass waste. Through magnetic separation, the concentrate with an iron grade of 62.04% and iron recovery of 95.29% was obtained, and the saturation magnetization was enhanced from 0.60 emu/g to 58.03 emu/g of iron tailings. During the magnetization roasting, CO and H₂ generated from biomass reduced the hematite in tailings particles from interior to exterior, forming a loose structure with rich microfissures, facilitating the subsequent separation operations. This study offers a novel reference for applying biomass to exploit hematite minerals and shows the potential of biomass for energy savings and emission reduction in the utilization of iron tailing resources.

Effect of microwave pretreatment on catalytic gasification of spirit-based distillers' grains to hydrogen-rich syngas

Steam gasification of spirit-based distillers' grains (SDGs) was performed in a fixed-bed reactor under different microwave pretreatment (MWP) approaches with or without addition of red mud (RM). The effects of MWP on the gasification rate, total gas yield, H₂/CO, and gasification mechanism of action were investigated. The results showed that RM could enhance the effect of MWP. The gasification rate, total gas yield and H₂/CO increased by 21.29%, 8.23% and 16.08%, respectively. In addition, RM and MWP had a synergistic effect on the catalytic gasification reaction. This was because MWP disrupted the complete ordered surface structure of the SDGs, allowing a large number of inherent alkali and alkaline earth metal ions to dissolve onto the surface and combine with the catalytically active material in RM to form a uniformly dispersed bimetallic catalyst. The catalytic mechanism consisted of an active-site catalytic mechanism and a bimetallic synergistic catalytic mechanism. Therefore, the combination of MWP and SDGs steam gasification is a promising, clean, efficient hydrogen-rich synthesis gas technology.

Production of a bio-magnetic adsorbent via co-pyrolysis of pine wood waste and red mud

The efficient reduction of accumulated waste biomass and red mud by converting them into a value-added magnetic adsorbent is both difficult and tempting in terms of sustainability. This study focused on investigating the reaction mechanism of co-pyrolysis of different biomasses, including pine wood, cellulose, and lignin, with red mud at 500, 650, and 800 °C, and the comprehensive characterizations of the produced bio-magnetic particles. The performance of biomass and red mud based magnetic adsorbents is also evaluated, and their primary adsorption mechanisms for organic pollutants are revealed by using different organic model compounds. The samples produced at 800 °C showed the best performance. For example, the sample prepared using red mud and pine wood at 800 °C showed the highest adsorption capacity of ibuprofen, which was 21.01 mg/g at ∼pH 4.5, indicating strong π stacking interactions as the dominant adsorption mechanism. When compared to lignin-rich biomass, adsorbents composed of cellulose-rich biomass showed greater adsorption efficacy. The findings show that co-pyrolysis of biomass with red mud can reduce waste while also producing a flexible adsorbent that is magnetically separable and effective at absorbing different organic contaminants from water.

Performance of electronic waste based mixed metal oxide as novel oxygen carriers for chemical looping co-combustion of high ash coal and rice straw

Electronic waste (e-waste) is one of the major pollutants accumulated due to its huge demand and short lifespan. Hence, it is essential to reuse and extract the value added components from e-waste. In this context, firstly, a printed circuit board (PCB) is used to produce calorific valuable gases by pyrolysis and gasification reactions. Secondly, the resultant residue of PCB is combusted to extract metals such as iron, copper, nickel etc. as oxygen carriers for the chemical looping combustion (CLC) process. CLC is an emerging and appealing technology for producing rich CO₂ that can be directly sent for sequestration. In the present study, a detailed investigation is performed to ensure the reactivity of the e-waste based metal oxide with high ash coal, rice straw and their blends in the CLC process. CO₂ yield, gas conversion, and char conversion are evaluated to assess the performance of the co-combustion based CLC process. It is found that 90.9% CO₂ yield, 94.1% gas conversion and 93.2% char conversion can be obtained using the blends of coal and rice straw in the first cycle of the CLC operation. Further, a reduction of 5% to 7% of these parameters is evaluated at the end of the third consecutive cycle of CLC operations. The interaction between coal and rice straw is further studied by evaluating their synergistic effects, char-oxygen carrier interaction and kinetic parameters using a thermogravimetric analyzer under N₂ and CO₂ atmosphere. The co-combustion process has reduced the activation energy by 13.4% at 800-1000 °C under CO₂ atmosphere.

Biogas Upgrading via Cyclic CO2 Adsorption: Application of Highly Regenerable PEI@nano-Al2O3 Adsorbents with Anti-Urea Properties

Solid amine adsorbents are among the most promising CO₂ adsorption technologies for biogas upgrading due to their high selectivity toward CO₂, low energy consumption, and easy regeneration. However, in most cases, these adsorbents undergo severe chemical inactivation due to urea formation when regenerated under a realistic CO₂ atmosphere. Herein, we demonstrated a facile and efficient synthesis route, involving the synthesis of nano-Al₂O₃ support derived from coal fly ash with a CO₂ flow as the precipitant and the preparation of polyethylenimine (PEI)-impregnated Al₂O₃-supported adsorbent. The optimal 55%PEI@2%Al₂O₃ adsorbent showed a high CO₂ uptake of 139 mg·g^-1 owing to the superior pore structure of synthesized nano-Al₂O₃ support and exhibited stable cyclic stability with a mere 0.29% decay per cycle even under the realistic regenerated CO₂ atmosphere. The stabilizing mechanism of PEI@nano-Al₂O₃ adsorbent was systematically demonstrated, namely, the cross-linking reaction between the amidogen of a PEI molecule and nano-Al₂O₃ support, owing to the abundant Lewis acid sites of nano-Al₂O₃. This cross-linking process promoted the conversion of primary amines into secondary amines in the PEI molecule and thus significantly enhanced the cyclic stability of PEI@nano-Al₂O₃ adsorbents by markedly inhibiting the formation of urea compounds. Therefore, this facile and efficient strategy for PEI@nano-Al₂O₃ adsorbents with anti-urea properties, which can avoid active amine content dilution from PEI chemical modification, is promising for practical biogas upgrading and various CO₂ separation processes.