Effects And Mechanisms Of Flue Gas Desulphurization With Enhancers

Coal with a high sulfur content (3-4%) is commonly the most important consumption resource in China, leading to a large number of SO2 emissions. The development of technology on the Flue Gas Desulphurization (FGD) is particularly meaningful. The SO2 removal efficiency and limestone utilization of conventional wet FGD using CaCO3 as sorbent can be improved by addition of various enhancers, and the operating costs can be reduced. In this paper, the enhancement of organic acids on wet FGD with limestone suspensions was studied in two laboratory-scale desulphurization reactors with continuous feed of gas, and the mechanisms were also analyzed on the basis of the theory of reaction engineering.Some endeavors were taken as below:1. A laboratory-scale bubbling reactor was established to investigate the enhancement of adipic acid on wet FGD under different conditions. The desulphurization rate was improved effectively by adding adipic acid, increased about 6% at 100 ppm (pH=5). Increasing the slurry concentration of adipic acid above 400 ppm gave only a slightly higher desulphurization rate. The effect of temperature on the desulphurization was also determined. The results showed that the desulphurization rate was not change significantly with temperature after adding adipic acid. The relationship of desulphurization rate with operation conditions for adipic acid concentration between ranges of 50ppm to 800ppm was established as follow:η= 1-(270.6-0.208wα+0.0273T-8.97y)exp(-1.87 pH), and the equation was in good agreement with the experimental results.2. Absorption process at atmospheric pressure was modeled using Aspen-plus, and the equilibrium concentrations of ions were calculated using this model. The results showed that equilibrium concentration of free calcium in the slurry increased in the present of enhancers, which promoted the desulphurization reaction. And the equilibrium concentration of free calcium kept constant (saturation value) when ddition of limestone was above a critical value (about 0.2%). Solid products were characterised by Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS) and X-ray Diffraction (XRD). The similar result with Aspen-plus simulation was obtained that CaCO3 was not wrapped up by calcium sulfate crystals.3. A spray tower of 0.35m in diameter and lm in height was set up, aiming at establishing a laboratory evaluation system for industrial operating conditions. Effects of three different organic acids (adipic acid, citric acid and AGS), pH buffer capacity of adipic acid, and limestone utilization were experimental investigated in this system. The results showed that desulphurization performance was as follow:adipic acid>AGS>citric acid. Adipic acid could buffer pH well, and limestone utilization was enhanced 15% when the additive concentration was 400 ppm. The effects of CaCO3 particle size and chloride ion concentration were also investigated.4. A detailed mass transfer model for FGD in the spray tower with enhancer was modeled based on the two-film theory. The model parameter (total gas mass transfer coefficient, KGa) was obtained. The total gas mass transfer coefficient was enhanced significantly when the additive concentration was above 400 ppm, and relative enhancement factor could reach 1.29(at pH=5).With these all above results, more cost-effective enhancer and desulfurization process can be further developed.