Concept and verification of the “C-configuration” temperature distribution of circulating fluidized bed boilers with deep peak shaving

Circulating fluidized bed boilers are crucial equipment for the efficient and clean combustion of coal, and their deep peak shaving is a key technology for building a new power system and establishing low-carbon industrial parks under the “dual carbon” goal. Under low-load conditions, both low main steam temperature and high NOx emissions are the main technical difficulties to realize deep peak shaving for circulating fluidized bed boilers. This paper reforms the traditional technical feature where the temperature distribution from the bottom to the top is monotonically decreasing with low-load combustion of circulating fluidized bed boiler, innovatively proposing a “C-configuration” temperature distribution concept for low-load combustion. The basic characteristics are that there exist two high temperature zones, separately at the bottom of the furnace for keeping the hot bed layer and at the top of the furnace for raising main steam temperature and de-NOx efficiency with SNCR, and one low temperature zone at the middle, thereby forming the “C-configuration” temperature distribution curve, to change the ratio of vaporization heat to superheating heat to improve main steam temperature. Coupling preheating modified fuel into the middle of the circulating fluidized bed, the combustion of the circulating fluidized bed under low-load conditions will be transformed from normal fluidizing combustion only with coal particles at the bottom to a novel state with bottom fluidized combustion and upper space combustion of preheating modified fuel. A new system with three-stage controlling NOx formation is established, including self-denitrification in the preheating, homogeneous reduction, and heterogeneous reduction of NOx with preheating modified fuel, and an increasing de-NOx efficiency of SNCR resulted from a rise in reaction temperature. Based on the “C-configuration” temperature distribution concept and thermal calculation program, the 240 t/h high-temperature and high-pressure circulating fluidized bed boiler was retrofitted. The results of this investigation showed that when the boiler load is 50%, the temperature curve exhibits the basic characteristics of “C-configuration”, and with the further decrease of boiler loads, the tendency becomes more obvious. As a result, compared to traditional combustion, the main steam temperature coupling preheating modified fuel combustion has elevated 22°C, and the direct ultra-low NOx emissions were achieved with the load range of 35%–70%. The boiler load regulation depth has been reduced from 50% to 20%, realizing the deep peak shaving and finishing the industrial verification of the “C-configuration” concept. The results would provide good support for the development and application of deep flexible peaking regulation technology for circulating fluidized bed boilers, giving an advanced and feasible scheme for suppressing the fluctuation of renewable energy and thereby promoting the increase of renewable energy accommodation ratios.