Abstract: Co-firing ammonia is an effective way to reduce carbon emissions from coal-fired units. As increasing amounts of renewable energy are integrated into the grid, deep peak regulation of coal-fired units has become common, yet the compatibility of ammonia co-firing in coal-fired boilers under low loads requires further investigation. This study establishes a thermodynamic model for ammonia co-firing in a 600 MW supercritical coal-fired boiler under low loads (50%~30% rated capacity) and examines the impact of ammonia co-firing on heat transfer in the furnace and convective heating surfaces. Results indicate that under low loads, co-firing 0~50% ammonia has minimal influence on the furnace's volume heat load and sectional heat load, with the furnace outlet flue gas temperature rising by less than 10℃. By adjusting the flue gas distribution in the tail section, the main and reheated steam temperatures remain stable when co-firing 0~50% ammonia. As the ammonia ratio increases, the primary and secondary hot air temperatures experience a slight decline. Compared to pure coal combustion, co-firing 50% ammonia raises the exhaust flue gas temperature by 5.8~7.6℃, leading to a boiler efficiency reduction of approximately 0.5%~0.7%. These findings offer valuable insights for assessing the feasibility of coal-ammonia co-firing in coal-fired boilers under low-load conditions.