Geospatial green ammonia co-firing in China’s coal power fleets to avoid CO2 emissions lock-in

China's 600 gigawatts (GW) of post-2010 coal power capacity locks in over 2 gigatons (Gt) of annual CO₂ emissions. Green ammonia co-firing offers a viable decarbonization pathway, yet spatial mismatch between ammonia production hubs and coal power plants remains a barrier. Here, we develop a spatially explicit framework covering 3958 islanded green ammonia production hubs to evaluate coal power units' spatial retrofit outcomes under two distinct co-firing strategies: local sourcing and geospatially optimized allocation. Results show that despite green ammonia subsidies, high fuel utilization costs persist for the local sourcing strategy, thereby limiting cumulative energy-related CO₂ reductions to 1.9-4.2 Gt (2020-2060) depending on climate policy stringency. In contrast, a geospatially optimized allocation strategy leveraging key transport corridors (e.g., Inner Mongolia to Hebei, Shandong, and Shanxi) delivers cumulative CO₂ abatement of 7.8-15.2 Gt, cuts cumulative CO₂ capture demand by 1.6-6.8 Gt, and lowers cumulative system costs between 2020 and 2060, relative to the local sourcing strategy.