Study On Desulfurization Characteristics Of Double Circulation And Multi-stage Water Film Tower

With the fast developing of economy and continued increasing consumption of coal, wet flue gas desulfurization (WFGD) acts an important role in large-scale coal-fired boilers'SO₂ emission reduction as a mainly used flue gas desulfurization (FGD) technic, while the medium to small scale coal-fired boilers'flue gas desulfurization situation are unoptimistic and total SO₂ emission amount can not be belittled because of the boilers'load changing frequently, easy scaling and corrode, and lower desulfurization efficicency of integrative devices for dust removal and desulphurization. Through the modification of the WFGD technology or developing the new style desulfurzation reactor, improving the flow field and contacting mass transfer process between gas and liquid, increasing desulfurization efficicency, this paper provide the technological support for medium to small scale coal-fired boilers.The designing of double circulation and multi-stage water film tower in this paper bases on the calcium based desulfuration reaction dynamics, uses double circulation model and pH piecewise control, multi-stage water film produced by double cone and stream guidance weir substitited for the muti-layer sprayers inside the spray tower. In order to achieve the structural parameters of the combination of double cone and stream guidance weir which have the optimum flow field and contacting mass transfer inside the tower, three flow field simulation softwares: Autocad, Gambit and Fluent are used to orthogonal simulate and optimize the components inside the tower in this paper. The reactor uses double circulation running mode, the lower slurry pH was adopted of lower part of reactor in favor of the calcium carbonate's solubility, while the higher slurry pH was adopted of the upper part of reactor in favor of the absorption of sulfur dioxide. The reactor designed coordinated the contradictory between the limestone utilization and desulfurization efficiency. The designed tower has the advantages of running stabilization, high desulfurization efficiency, none scaling and corrosion.Through the orthogonal experiment and analysis of integrated equilibrium method, the experiment achieved the most favorable running parameters of room temperature state, the gas speed of empty tower is 3.0 m/s, the slurry pH of pretreatment circulation and absorption circulation are 4.8-5.0 and 5.9-6.1 respectively, the liquid versus gas ratio (L/G) of pretreatment circulation and absorption circulation are both 20 L/m³. The desulfurization efficiency is 98.2%, the limestone utilization is 95.2%, and the calcium versus sulfur ratio (Ca/S) is 1.03 when the system running at the optimized situation. Absorption part of inside the reactor appeared soft scale when the slurry pH of pretreatment circulation and absorption circulation lower to 4.0 and 5.4-5.6 respectively. Large numbers of hard scale were arisen both upper and lower part inside the reactor when the slurry pH of pretreatment circulation and absorption circulation both are 7.0. This paper reached an important conclusion through the cold state desulfurization experiment, that is, the designed double circulation and multi-stage water film tower not only find an effective way to treat low SO₂ concentration, but also breakthrough the restriction of traditional desulfurization tower to treat high SO₂ concentration, outlet SO₂ concentration could still meet the discharge standard when the inlet SO₂ concentration higher than 4500 mg/m3.Desulfurization efficiency and stability are improved markedly, the slurry condition that limestone's solution needed made better, the limestone utilization increased effectively, and scale inside the tower was inhibited when additives are added. The desulfurization efficiency and limestone utilization were uplifted 19.6%, 14.32%, 8.11% and 11%, 8%, 5% when the compound additives added compared with none additives'enhancement, MgSO4 enhancement and adipic acid enhancement. The experimental results show that synergistic effect exists when compound additive of adipic acid and MgSO4 is added. The desulfurization efficiency and limestone utilization were uplifted 19.6%, 14.32%, 8.11% and 11%, 8%, 5% respectively when the compound additives added compared with none additives'enhancement, MgSO4 enhancement and adipic acid enhancement.Through the hot-moisture exchanging experiment, the paper grasped the results of outlet flue gas state effected by inlet flue gas temperature, moisture and slurry temperature. The results showed that the outlet flue gas state effected by slurry temperature is greater than effected by inlet flue gas state, the flue gas temperature and moisture effected by slurry of upper part is greater than slurry of lower part. The paper established the mathematical model of lower part using the function of lsqnonlin which belongs to optimization toolbox. The model not only reflects the relationship of inlet and outlet flue gas states, be used to direct the upper part hot and moisture exchange experiment objectively, but also be use to adjusts and controls the desulfurization system's running effectively.Through the orthogonal experiment and analysis of integrated equilibrium method, the experiment achieved the most favorable running parameters of hot state flue gas, the gas speed of empty tower is 3.0 m/s, the inlet flue gas temperature is 105℃,the slurry pH and L/G of pretreatment circulation and absorption circulation are 4.5,6.0 and 18 L/m3, 15 L/m3. When the system running at the optimized parameters, the desulfurization efficiency is 88.3%, the limestone utilization is 82.2%, and the calcium versus sulfur ratio (Ca/S) is 1.07. The inlet of reactor easily appears scale when the slurry pH of pretreatment circulation is 6.0. The influence of slurry L/G to desulfurization efficiency shows that desulfurization efficiency effected by absorption slurry L/G is greater than pretreatment slurry L/G.Eviews software was used to establish the mathematical model between the desulfurization efficiency and six running parameters in this paper. The mathematical model contributes to comprehend the reactor's desulfurization characteristics, instruct the reactor's actual application and running parameters'adjustment from the angle of theory. The P value, significance, R2 and R 2 value, akaike info criterion, schwarz criterion, and etc. were used to evaluate the model's significance test, and the paper obtained the optimized model as follows: 426.44T/Y2-2778.24/Y=4.69V+0.45LGD+0.87LGU+2.67PHD+4.57PHU-100All the evaluating indicators of the model are lie in accurate level, the forecast precision is high, and there is no difference between the forecast value and test value. The model not only can help to understand the desufurization characteristics of reactor, but also can direct the double circulation and multi-stage water film tower's application and adjust the running parameters.