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.

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.

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, NOx emissions are lowered to as low as 0.19 mg/m³, and the lean fuel limit is extended to 3.5% (in volume fraction).

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.