Study On The Composite Fluidization Of Swirl-Once Through Flow And Uniformity In The CFB-FGD

Much attention is being paid to SO₂ from coal-fired power plants by our government. One of the main tasks during "the eleventh five-year plan" is to reduce the total discharge of SO₂. The industry of flue gas desulfurization has large market and expanded space. The CFB-FGD is widely applied with its advantage. CFB-FGD has been used in 300MW power plant which was largest in CFB-FGD technology. But disadvantages of CFB-FGD are obvious, such as easy deviation of flow field, ununiformity of flow especially when the CFB-FGD is applied in large-scale power plant. The ununiformity of flow influences the ununiformity of temperature field. They both lead to lower efficiency of desulfurization. The phenomenon of solid adhering to the wall makes the system difficult to operate stabilization in a long period. The diameter of the CFB-FGD tower has been larger than 10 meter. One of the important tasks is to improve the uniformity of the CFB-FGD tower.Ways to improve uniformity of CFB-FGD are discussed in this thesis. Composite fluidization of swirl-once through flow is put forward to. It adds swirl flow under the once-through fluidized of venturi. A valid way to improve uniformity is composite fluidization of straight-once through flow. The characteristics of flow under composite fluidization are studied in cold gas experimental apparatus. The characteristics of gas-solid two-phase flow and uniformity are studied based on gas flow. Reasons of the composite fluidization to improve uniformity are summarized. The composite fluidization was applied in a 145MW power plant. The distribution of the flow and temperature with uniformity is studied in this power plant. The effect of desulfurization as emphasis is studied, too.Firstly, gas flow field is investigated under composite fluidization. The composite fluidization increases the tangent momentum when it is compared to once-through fluidization. Its turbulivity and shearing stress are intense, too. The resistance when swirl blades installed in the diffuser of venture is proved to be 15%～46% larger than that of blades installed in throat. But the maximum tangent velocity is smaller when blades were installed in the throat of venturi. The effects of blades angle to the resistance, swirl intensity and flow are much more obvious than those of blades number. Six blades are better diversion mode. k-εmodel is used to simulate the gas flow, which can better reflect the flow regulation. Flow field symmetry of composite fluidization in section is much better than that of once through fluidization, the position of uniformity can be earlier got. The reasons are intense turbulence, higher shearing stress, larger turbulent transporting ability. Swirl intension decays with exponential regulation. The gas flow can reach uniformity more quickly when there are smaller blades angle. From the resistance, the flow regulation and the uniformity, the effects of composite fluidization are better than those of once through fluidization. The better effect can be got when blades installed in throat. Blades angle of 42°with which installed in throat is the best structure.Secondly, the gas-solid flow and uniformity experimental apparatus was constructed based on the gas flow study. The best blades structure was adopted. The two-fluid model was used to simulate the flow and uniformity in CFB-FGD tower. The swirl flow produced by the composite fluidization strengthens the turbulence, mixture and the turbulent transporting ability. The composite fluidization almost removes the deflexion of flow. Flow in the CFB-FGD tower is uniformity. The asymmetry coefficient of the composite fluidization is 22% lower than that of once through fluidization. The uniformity can be got more quickly. The uniformity becomes lower when the fluidization velocity is increased. The resistence of the composite fluidization is 12%～30% higher than once through fluidization. But high slip velocity of the composite fluidization is good for high mass transfer. The maximum particles residence time of the composite fluidization is 1.23～1.31 times higher than that of the once through fluidization. The particles concentration and the inner recycle in the tower are higher. The uniformity of concentration is better than once through fluidization.Thirdly, the composite fluidizaton was used in a CFB-FGD tower of 145MW power plant. The flue gas in the middle of the tower flows upward. The surplus flue gas flows downward near the wall. The resistance of composite fluidization is 200～300 Pa higher than once through fluidization, which is not more than 12% compared to the once through fluidization. And its better flow and temperature uniformity are testified. As to the solid concentration, the composite fluidization has merit list below: the solid concentration being higher, solid which leaves the tower and the concentration at the entrance of ESP being lower because the concentration of desulfurization ash in the exit of CFB-FGD tower is 1.1～1.2 times higher than the entrance of CFB-FGD tower. CaSO₃ is the main desulfurization product, the amount of CaSO₄ is a little.The utilization rate of Ca is high in the composition fluidization. At the same time, the CFB-FGD has high desulfurization efficiency of 93.6% when Ca/S=1.35, AAST=11.6℃. The desulfurization efficiency increased with Ca/S being increased and AAST being decerased. When circulating expense and the effect of desulfurization, the recommended value is Ca/S=1.3 and AAST=10～15℃.The phenomenon of solid adhering to the wall is awakened because of the high concentration ash recycle, the scour effect to the wall with higher tangent velocity , better uniformity and higher heatand mass transfer. So the CFB-2144FGD system can be operated stabilization in a long period.From the experiment, simulation and engineering, the composite fluidization of swirl-once through flow improves the flow uniformity, weakens the effect of solid adhering to the wall, have high desulfurization efficiency. So it suits the request of the CFB-FGD. And as a new technology, it has better foreground.