Probing of operation economy for coal-fired unit under low loads with two fixed boundary conditions

Coal-fired power plants are forced to participate in deep peak shaving of power grid. As load of peak shaving unit further decreases to 30 % or less, operation economy of coal-fired unit at ultra low load has become one of the main focuses of concern after unit flexibility. Excessive air coefficient, as an important technical parameter affecting economic operation of unit, can be enhanced to maintain combustion stability at low loads under stationary fuel boundary condition (SFBC) and stationary steam parameters boundary condition (SSBC). To obtain energy consumption characteristics of 660 MW supercritical unit during steady-state processes of SFBC and SSBC, the model of influence of excess air coefficient on overall performance and main parameters of unit is further developed. Subsequently, quantitative evaluation and analysis of unit performance on variable excess air coefficient is executed under modes of SFBC and SSBC. Influence mechanism of excess air coefficient on operation parameters and energy consumption characteristics of unit is revealed and clarified. Pre-eminently, evolution mechanism for heat rate and relative internal efficiency of steam turbine, steam temperature and coal consumption rate is given quantitatively and firstly. Results demonstrates that, SSBC mode is more economical for unit running with excess air coefficient lower than 1.3. With excess air coefficient higher than 1.4, economic difference between two operation modes is small. However, load as low as 30 % boiler maximum continuous rating (BMCR), SFBC mode shows its advantages, which is more suitable for peak shaving of ultra low load. Maximum difference in coal consumption under two operation modes is up to 0.5 g/kWh corresponding optimal operating range of excess air coefficient at 30 % BMCR. This study is expected to provide a detailed reference for upgrading operation economy of coal-fired power plants during low load peak shaving.