Coal-fired power plants (CFPP) are a significant resource for flexibility regulation of power systems containing a high proportion of renewable energy (RE). This paper presents an extended study of the technology for co-firing coal and ammonia, which can decarbonize CFPP. The excess RE produces green hydrogen, converted into green ammonia for CFPP combustion. Moreover, this system has energy storage features that can cope with short-term fluctuations and store and utilize RE seasonally. In this paper, this system’s complex coupled thermal and electrical rocesses are simplified and characterized as a mixed integer linear programming (MILP) model for the first time. Furthermore, the operation of the green ammonia co-firing system is simulated at two time scales of five days and a full year. Finally, sensitivity and efficiency analyses are performed. The results shows that as the maximum co-firing ratio of ammonia and coal increases, the total operation cost (TOC), carbon emission (CE), and RE curtailment shows a decreasing trend. Therefore, the ratio of ammonia-coal co-firing is recommended to be 25% to compromise the TOC and exhaust gas emission. In this situation, the TOC and CE will decrease by 8.59% and 12.74%, respectively, compared with the scenario without co-firing technology. The overall efficiency of green ammonia synthesis and combustion for generation is about 13.92%.