Experimental And Numerical Research On Fly Ash Deposition Under CFB Oxy-fuel Combustion

The oxy-fuel combustion in circulating fluidized bed（CFB） is regarded as one of the most promising technologies for reducing the CO₂ emission. It combines many advantages of both fluid bed combustion and oxy-fuel combustion producing strong combination effect, therefore it becomes more competitive in the commercialization process. However, it is necessary to carry out fundamental research on some potential problems before it is applied to power plants in large-scale commercial applications. This paper focuses on ash deposition on the heating surfaces of heat exchangers in convection flue gas pass in CFB boiler under oxy-fuel combustion condition where both experimental and modeling research on that has been carried out.Firstly, the experimental research involving combustion and ash deposition for two typical coal samples in China（Jincheng Anthracite and Shuozhou bituminous） was carried out in a bench-scale fluidized-bed combustor（FBC）. Ash deposition characteristics under air, 21%O₂/79%CO₂ and 30%O₂/70%CO₂ combustion atmospheres were compared. The results show that oxy-fuel combustion in FBC has an obvious influence on ash deposition, and there is a distinctly higher mass concentration for PM10 in 30%O₂/70%CO₂ combustion condition, especially for fine particles which diameter is less than 0.3μm. It has a slightly great particle size distribution. It is concluded that the effect of oxy-fuel combustion on fly ash formation was one of possible reasons resulting in more serious ash deposi tion.Secondly, the effect of limestone addition on ash deposition under both oxy-fuel combustion and air conditions investigated for Jincheng Anthracite in the initial stage of the formation of ash deposit in FBC. The influence s of Ca/S molar ratio, the probe surface temperature and combustion atmospheres on ash deposition were investigated. The results show that adding limestone had a significant effect on the deposition characteristics of fly ash, there is a certain linear relationship between deposition propensity and alkali metal content in ash deposits and there is also an obvious linear relationship between deposition propensity and temperature differences, which is between the surface temperature of the probe and the flue gas temperature.Then based on the previous studies, SO₂ emission and removal for two coal samples in FBC under both air and 30%O₂ oxy-fuel combustion were investigated. The results show that: oxygen concentration（31-51%） has a significant influence on the release of SO₂ but the bed temperature ranging from 840 to 920℃ has no significant on that; For Anthracite, the maximum SO₂ removal efficiency appeared at 880-900 °C for both conditions, while for the Shuozhou bituminous coal, the maximum SO₂ removal efficiency appeared at 860 ℃.To further understand the mechanism of ash deposition, a complete set of hot test setup and measurement system was developed. The influences of the probe surface temperature, flue gas velocity and the probe dimension on particles deposition were investigated as well as the initial investigation on carbonate phenomenon of ash deposit. A simple method was devised to measure the effective thermal conductivity of ash deposit. The results show that fly ash deposition process is a dynamic process.The temperature of 400-770℃ has no significant effect on effective thermal conductivity of ash deposits. The factors of the probe surface temperature, flue gas velocity and the probe dimension have a significant effect on the ash deposition. Due to that obvious carbonate phenomenon of ash deposit observed, it is necessary to consider contribution of carbonation reaction to deposition mass. In the simulated flue gas atmosphere, the effective thermal conductivity of ash deposit from oxy-fuel combustion is higher approximately 0.02 W/m/K than that from air combustion.Finally, the commercial software Fluent14.5 was used to simulate the ash deposition in the hot test setup of ash deposition using an existing deposition model considering the relation among the elastic force, gravity, and van der Waals force. The influences of the surface temperature, flue gas velocity and atmosphere on ash deposition were simulated as well as some parameters from Fluent. The modeling result is compared with the previous experimental result. Si mulation results show that turbulent diffusion has a significant effect on particle trajectories rather than tracks number of particles. The distribution of deposited particles on the probe exhibits a ‘bimodal’ shape with low deposition mass in the center and obvious high deposition mass near the center of this probe. The probe surface temperature and flue gas composition had no significant effect on the deposition of particles but increasing gas velocity significantly reduces the deposition of particles. Generally, there is a certain gap between the simulation and experimental results, so deposition model needs further improvement.