硅酸盐学报

多孔介质燃烧具有燃烧效率高、污染物排放低和清洁利用低热值燃料的特点,对当前高温工业的节能降碳意义重大。针对高温多孔介质燃烧用氧化铝多孔陶瓷表面辐射效率低的问题,首先采用低温共沉淀法制备具有特殊花状结构的氧化铈(CeO₂)粉末,再结合浆料喷涂工艺在氧化铝多孔陶瓷表面形成高辐射、高催化活性的CeO₂涂层。结果表明:以NH₄HCO₃和Ce(NO₃)₃·6H₂O为原料,通过共沉淀法和500℃热处理能分别制备出具有花瓣结构的花状CeO₂前体和花状CeO₂。同普通CeO₂相比,花状CeO₂具有更大的比表面积、更高的表面活性氧比例。所制备的花状CeO₂涂层因具有发达的花瓣孔隙和微孔结构,能将红外发射率由0.52提升至0.71。相比于负载普通CeO₂涂层的多孔陶瓷燃烧器,负载花状CeO₂涂层多孔陶瓷燃烧器表面温度最高提升至1032.6℃,加热速率由32.38℃/min提升到41.81℃/min,CO排放仅为4.58 mg/m³,NO_x排放低至0.19 mg/m³,贫燃极限拓宽至3.5%(体积分数)。

Introduction Porous medium combustion has high efficiency, low pollutant emissions, and effective utilization of low-calorific-value fuels, which is crucial for energy conservation and carbon reduction in high-temperature industries. As a core component of porous medium burners, the porous medium material must withstand the high temperatures generated by fuel combustion and the corrosive effects of water vapor and excess oxygen in flue gases. Among various inorganic nonmetallic materials, Alumina porous ceramics are widely used as dielectric materials in high-temperature porous medium burners due to their excellent thermal stability and resistance to wet oxidation and corrosion. However, the relatively low thermal radiation coefficient(i.e., approximately 0.3) of alumina significantly reduces the heating efficiency of the burner. Therefore, enhancing the surface infrared emissivity of alumina porous ceramics is critical to improving the radiative heat transfer performance of alumina porous ceramic burners. While enhancing material radiation primarily involves constructing infrared radiation coatings on material surfaces, these methods often overlook the combustion stability of low-calorific-value fuels in porous medium burners. This study was to address this issue by firstly preparing flower-like CeO₂ catalysts via coprecipitation and subsequently developing alumina porous ceramics coated with flower-like CeO₂. The unique flower-like structure of the CeO₂ coating could enhance both radiative heat transfer and catalytic performance, thereby improving the overall radiation capability and heating efficiency of the porous medium burner. Methods To address the issue of low radiation efficiency on the surface of alumina porous ceramics used in high-temperature porous media combustion, this study adopted a systematic approach. NH₄HCO₃ and Ce(NO₃)₃·6H₂O were utilized as raw materials to synthesize flower-like CeO₂ precursor via a low-temperature co-precipitation method. These precursors were subsequently transformed into flower-like CeO₂ through heat treatment at 500 ℃. Alumina porous ceramics were fabricated by an organic foam impregnation technique. Alumina porous ceramic preforms were obtained after heat-treatment at 1300 ℃. Subsequently, α-Al₂O₃ powder and deionized water were mixed to prepare an impregnating slurry, which was applied to the preforms via vacuum impregnation. After drying, these preforms underwent further heat-treatment at 1500 ℃ to finalize the alumina porous ceramics. Commercially available ordinary CeO₂, synthesized flower-like CeO₂ powder, and flower-like CeO₂ precursor powder were sprayed onto the surfaces of the alumina porous ceramic skeletons. After heat-treatment at 500 ℃, three distinct CeO₂ samples CCA, FCA, and PFCA were produced. The effect of the flower-like CeO₂ coating on the combustion characteristics, lower flammability limit, and combustion temperature of alumina porous ceramics were evaluated by a porous media combustion platform, complemented by infrared thermal imaging(Fluke TiX 1060) and flue gas analysis(Testo 350). In addition, the surface element valence, specific surface area, and microstructure of the three types of CeO₂ were also characterized to elucidate the advantages of flower-like CeO₂ over ordinary CeO₂. Furthermore, the enhancement mechanism of infrared emissivity for flower-like CeO₂ was investigated. Results and discussion Flower-like CeO₂ precursor and flower-like CeO₂ are synthesized with NH₄HCO₃ and Ce(NO₃)₃·6H₂O as raw materials through a coprecipitation method and a subsequent heat-treatment at 500 ℃. Compared to conventional CeO₂, flower-like CeO₂ exhibits a significantly larger specific surface area and a higher proportion of surface reactive oxygen species. The prepared flower-like CeO₂ coating enhances infrared emissivity from 0.52 to 0.71 due to its well-developed petal and microporous structure. When applied to porous ceramic burners, this coating increases the surface temperature to 1032.6 ℃ and the heating rate from 32.38 ℃/min to 41.81 ℃/min, and reduces CO emissions to only 4.58 mg/m³. In addition, NO_x emissions are lowered to as low as 0.19 mg/m³, and the lean fuel limit is extended to 3.5%(in volume fraction). Conclusions The alumina porous ceramics with flower-like CeO₂ coatings were fabricated. The interface between the coating and the alumina skeleton exhibited the superior characteristics. The unique morphology of the flower-like CeO₂ coating, characterized by well-developed inter-petal pores and micropores formed during the decomposition of CeO₂ precursor, could enhance the infrared emissivity of the coating. This improvement in emissivity led to increased surface temperature and heating efficiency of the alumina porous media burner. The flower-like CeO₂ coating had a high specific surface area and a significant proportion of reactive oxygen species, which facilitated methane combustion within the alumina porous ceramics. Consequently, this could broaden the range for stable methane combustion and substantially reduce CO emissions in the flue gas.