Abstract: Under the dual-carbon goal, biomass has become the most direct and effective approach to achieve large-scale carbon reduction and low-carbon energy transition. However, the combustion of biomass produces a significant amount of “fuel-derived ammonia”, which easily leads to air preheater fouling, low-temperature corrosion, and excessive ammonia emissions. The migration and transformation laws, as well as the core mechanisms of fuel ammonia in the tail flue, remain unclear and lack systematic in-situ verification. In-situ measurements of gaseous ammonia were conducted at multiple typical positions along the tail flue (including the inlet and outlet of the air preheater and desulfurization tower) on a 130 t/h biomass circulating fluidized bed (CFB) boiler without external ammonia injection. Combined with systematic characterization and analysis of crystalline deposits, biomass ash, desulfurization slurry components and flue gas HCl measurement, the study systematically investigated the effects of furnace outlet oxygen volume fractionon the generation, transformation, and multiphase migration characteristics of fuel ammonia. The results demonstrate that: furnace outlet oxygen volume fractionplays a key factor in controlling fuel ammonia generation. At a strong reducing atmosphere with low oxygen (mean furnace outlet O₂ of 1.02%), the ammonia mass concentration at the air preheater inlet reached up to 35 mg/m³; conversely, under a high oxygen condition (mean furnace outlet O₂ of 3.51%), it decreased significantly to 5.26 mg/m³, which is merely 15.6% of that under the low-oxygen condition (converted to the 6% reference oxygen concentration, the ammonia concentration fell from 25.27 mg/m³ to 4.51 mg/m³, representing a 17.9% reduction). During the cooling process in the air preheater (with temperature decreasing from 200 °C to 115 °C), gaseous fuel ammonia preferentially undergoes homogeneous reaction with HCl to form ammonium chloride (NH₄Cl), which crystallizes rapidly at the cold end of the air preheater. The deposits on the tail flue wall are predominantly high-purity NH₄Cl crystal phase, free from by-products such as ammonium bisulfate that commonly exist in coal-fired boilers. This NH₄Cl-dominated deposition is the main cause of fouling, blockage and corrosion at the air preheater’s cold end. With elevated oxygen volume fraction enhancing gaseous HCl liberation, and biomass ash supplying abundant CaO together with porous aluminosilicate frameworks to form adequate adsorption sites for NH₄Cl aerosols, the occurrence of ammonia in ash exhibits a typical characteristic of “oxidation-promoted adsorption”. As the oxygen volume fraction increases from 1.02% to 3.51%, the \mathrmNH_4^+ mass fraction in ash increased from 1.59 mg/g to 2.12 mg/g—both values far exceed the national standard limit of ≤0.21 mg/g for ammonium ion in ash. This severe exceedance greatly restricts the resource utilization of ash. After long-term circulation, the desulfurization slurry forms a stable ammonium ion buffer system. The gas-liquid mass transfer in the tower is regulated by the ammonium ion equilibrium of the slurry. Under conditions of low inlet gaseous ammonia concentration, the \mathrmNH_4^+ enriched in the slurry is prone to reverse desorption into gaseous ammonia, leading to an increase instead of a decrease in outlet ammonia emissions. This study reveals the ammonia migration mechanisms of NH₄Cl-dominated deposition and ash adsorption contradictions unique to biomass CFB boilers. The findings provide an important theoretical basis and engineering guidance for clean combustion of biomass, coordinated control of ammonia emissions, pollutant emission reduction, and safe and stable operation of boiler equipment.