In Situ Catalytic Precracking of Ammonia for Fluidized Bed Coal/Ammonia Cofiring

In the context of carbon neutrality, ammonia has emerged as a promising carbon-free fuel alternative to coal, owing to its chemical stability, high hydrogen density (17.8%), and mature synthesis infrastructure. However, its low flammability and high nitrogen content pose challenges including combustion instability and elevated NOx emissions during the NH3 combustion. This study proposes a novel strategy to improve NH3 combustion by utilizing boiler flue gas heat for in situ ammonia precracking, thus enabling clean and efficient combustion of NH3. In this study, a series of catalytic materials were evaluated for NH3 cracking: Fe, Ni, or Ru supported on γ-Al2O3/honeycomb ceramics (HC) via impregnation, carbon-based materials (coal coke and petroleum coke), and natural mineral (hematite). Systematic activity tests were conducted across temperature range of 200–900 °C, investigating effects of active components, supports, temperature, and space velocity. For the more active Fe/γ-Al2O3, Ni/γ-Al2O3, and Ru/γ-Al2O3 packing materials, characterizations were performed to further analyze the changes in the physicochemical properties before and after the reaction. The results show that NH3 cracking strongly depends on temperature and catalytic materials, while gas hourly space velocity exhibits limited influence in industrial operational ranges (2000–4000 h–1). The Ru-based catalyst achieved >95% NH3 conversion at 550 °C (highest activity), Ni/γ-Al2O3 (nonprecious benchmark) reached 83.39% at 600 °C, with complete conversion by 650 °C, and natural hematite showed competitive activity (93.96% at 700 °C). Coal and petroleum coke also demonstrated catalytic activity for ammonia cracking. Kinetic analysis indicates the sequence of ammonia cracking activation energy hierarchy: empty bed > petroleum coke > coal coke > hematite ≈ Fe/Al2O3 > Ni/Al2O3 > Ru/Al2O3. This study establishes a catalyst selection map for waste heat utilization (200–900 °C), achieving 0–100% NH3 cracking. The results provide critical data support for catalyst selection in ammonia precracking-assisted fluidized bed coal/ammonia cofiring systems.