Impact of Flue Gas Recirculation on Distributions of Temperature and Solids Concentration Inside a Large-Scale Supercritical CFB Boiler

The purpose of this work was to determine the impact of a flue gas recirculation (FGR) on the furnace temperature and the solids suspension in a 966MW th supercritical circulating fluidized bed (CFB) boiler. A performance test on a 1296t/h supercritical CFB boiler was carried out for 80% unit load at high level of bed pressure (ca. 7.0kPa) and also under excess air ratio in the range of 1.211.30. Furthermore, the recirculation flue gas mass flow (20.5 mn ·s) and the ratio of secondary air to the primary air (0.11-0.33) were considered as experimental variables. Measurement data for FGR test run was compared with data from the others tests. Whereas flue gas recirculation into CFB furnace, the vertical temperature profile had quasi-linear character as opposed to air staging test runs. A significant difference (ca. 560 times) in the suspension density between the grid zone and the upper part of the furnace was observed.The results obtained from this work remain in agreement with the experimental data for the CFB boilers. INTRODUCTION Circulating fluidized bed boilers are commonly used to facilitate high-efficiency air-firing of fossil fuels with biomass in heat and power generation industry. The distributions of temperature and solids concentration are key parameters for proper operating CFB boilers. It is well known that, these parameters directly affect on bed hydrodynamics(1,2), combustion(3), the heat transfer(4,5) and pollutant emissions (6,7) in fluidized bed combustors. The most commonly method used to an equalization of temperature furnace profile is flue gas recirculation (FGR). In the case of oxy-fuel combustion technology, an external flue gas recirculation is the most widespread option for controlling of the adiabatic flame temperature (8). Temperature is a very important parameter which can cause some problems of uncontrollable slag formation and fouling of the heating surfaces. However, FGR can play a negative role in a relation to flame stability, combustion efficiency and content of unburned carbon in the bottom ash. Main advantage of oxy-fuel combustion technology with flue gas recirculation is a possibility to reduce CO2 and NOx emissions from coal-fired fluidized bed combustion. More information and a number of review articles about oxy-fuel technology is described by Buhre et. al (9). In this work, the objective of this study was an assessment of effect of FGR on the temperature and suspension density profiles inside the furnace chamber at the two excess air ratio levels. Measurements were carried out for three representative conditions, one with and others without flue gas recirculation into combustion chamber. All test runs were performed on 966MW th supercritical CFB boiler, fueled with Polish bituminous coal. Combustion process based on flue gas recirculation into the circulating fluidized bed boiler was easy to implement for control of the heat transfer conditions inside the furnace. The multi-pollutant control during combustion process with FGR was not considered in the present paper. DESCRIPTION OF THE TEST FACILITY A schematic of the supercritical CFB facility is shown in Figure 1. The CFB unit is designed to generate 1296t/h of steam, at 27.5MPa and 560°C with feedwater temperature of 289.7°C. Unlike typical second generation circulating fluidized bed combustion systems, CFB unit is equipped with a vertically tubed BENSON evaporator, which is a new supercritical steam technology. Detailed information with respect to the supercritical steam generation technology is precisely described by (10). Besides, the boiler consists of the following systems: (i) furnace chamber; (ii) solids separators; (iii) INTREX integrated heat exchangers; (iv) low temperature flue gas heat recovery system. The furnace cross section dimensions are 27.6m  10.6m, deep and wide respectively. The supercritical CFB boiler has a height 48m and a thermal capacity of 966MWth. Figure 1. Schematic layout of utility supercritical CFB boiler (SH – superheater, RH reheater). The bottom part up to 9.0m height, the walls are covered with refractory lining. On the bottom of combustion chamber is a fluidization grid with nozzles of primary air. Secondary air nozzles are at three levels above the grid in the two sidewalls. INTREXTM-RH II INTREXTM-SH IV SH III SH III Water/Steam