Research On Acid-Grey Coupling Mechanism And Dew Point Dynamic Characteristics Of Coal-fired Boiler Flue Gas

At present, the coal-fired boiler unit is facing a serious task of energy conservation and emissions reduction, reduce exhaust temperature of coal-fired boiler unit can effectively improve the efficiency of coal utilization, so the deeply cooling waste heat utilization system will be used widely, but the system performance will be affected by the low temperature corrosion, i.e. flue gas acid dew point. The exhaust flue gas temperature needs to be higher than the dew point temperature based on the traditional design principles, which has large difference with the actual data of some units, the dew point temperature can be 40℃ to 50℃ higher than the actual exhaust temperature, which means the existing computational formula of the dew point temperature has a big deviation with the actual situation and loses guiding significances for boiler design, operation and waste heat utilization, so to determine the dew point of flue gas temperature is a problem to be solved.Coal-fired boiler heating surface affected by many factors of low temperature corrosion, such as the H₂SO₄ steam condensation when the flue gas temperature reaches the acid dew point temperature, at the same time there is a large amount of fly ash in flue gas. The coupling effect of acid and dust is relatively complex, which has a large effect to the corrosion. So the low temperature corrosion mechanism research involves condensable gas condensation, heat and mass transfer, fluid mechanics, materials science, physics, chemistry and other disciplines, but there has been no satisfactory explanation, which greatly restrict the use efficiency of low temperature waste heat recovery system.This article conducts theory analysis for low temperature corrosion phenomena and mechanism from the perspective of Chemical thermodynamics and Heat and mass transfer theory. Analysis of the theoretical results re-coupling deposition effect obtains the coal-fired boiler flue gas low temperature corrosion mechanism and affecting factors. Analysis of the dynamic characters of the flue gas acid dew point and engineering acid dew point corrosion put forward the safety design concept and design principles, corrosion temperature and other new concepts. Low temperature corrosion is studied by the power plant flue gas atmosphere low temperature corrosion experiment and deeply cooling experiment, and analyzed by the SEM, XRF, XRD and particle size analyzer, etc. Low temperature corrosion and fly ash properties are studied by the simulation experiment of flue gas atmosphere, and orthogonalization analysis is carried out on the experimental data. According to the researches mentioned above propose intervention of control or adjust the acid dew point. Combined with the theoretical analysis and experimental results, some conclusions with theoretical meaning and engineering value were obtained. The main research contents as follows:（1） Low temperature corrosion mechanism was analyzed by thermodynamics, and the Gibbs phase law, the condensation of the H₂SO₄ steam and phase equilibrium method. Through thermodynamic phase equilibrium theory, the thermodynamics acid dew point is deduced which is related to the flue gas volume, the volume of SO2, the conversion of SO2 to SO3, SCR and so on. In the process of boiler operation the thermodynamics of acid dew point is a dynamic variable. The critical radius of H₂SO₄ steam increases first and then decreases with the temperature and reduction of sulfuric acid solution concentration. When the flue has small gas super-cooling degree, the droplet diameter of H₂SO₄ is less than the critical radius and the condensation process is difficult to continue. Two conditions for H₂SO₄ steam condensation was obtained, first the steam has certain super-cooling degree, and secondly condensation nuclei was in the flue gas. The H₂SO₄ steam condensation is inevitable if the flue gas waste heat deep utilization system of the coal-fired power plant boiler meet the two conditions.（2） The design concepts of the coal-fired boiler deeply cooling waste heat utilization corrosion safety area and corrosion surplus temperature are proposed. Low temperature heating surface corrosion safety design concept and principles are established. The dynamic characteristics of the thermodynamics acid dew point and the engineering acid dew point are related to the sulfur content, moisture, ash content and boiler load. The corrosion safety area was between the engineering acid dew point and the thermodynamics acid dew point. The flue gas supercooled degree less than the corrosion residual temperature is the corrosion safety design principle to ensure the safety and economy of low temperature heating surface.（3） The H₂SO₄ steam condensation heat and mass transfer are analyzed. The Collburn-hougen model is used to obtain the heat transfer mechanism of the ash deposits and the H₂SO₄ condensation acid coupling effect and the calculation formula of the compound heat transfer coefficient h. Due to the ash deposits the temperature of the condensate film rise and the compound heat transfer coefficient h reduced. It can be concluded that the ash deposits is the prerequisite for the corrosion safety area through the analysis of h. The mechanism and the macro performance of the coupling effect are obtained by the analysis of the H₂SO₄ steam pressure changes on the condensate film. The corrosion safety area corresponds to the unsaturated coupling effect, the process started from the thermodynamics acid dew point and ended at the engineering acid dew point. When the coupling effect saturated, the metal pipe is corroded by the sulfuric acid through the ash deposits layer, which cause serious low temperature corrosion and the heat transfer performance deterioration Nu number fell sharply. While acid corrosion is prevented by the corrosion products, the increases again and the h reduced, then enter into a new stage of dynamic balance and the Nu number unchanged.（4） The low temperature corrosion experiment is designed and completed. The scanning electron microscope （SEM）, energy spectrum analysis and X-ray diffraction （XRD） experiment are used for the analysis of the ash aggregation and composition influence on the ash and sulphuric coupling effect. The effects samples obtained from the outer wall of experiment tube under the different tube wall temperature. It can be concluded from the SEM figures that the diameter of the ash deposits particles is significantly larger and the shape become irregular. Some small particles conglomerated to form the larger particles. The ash deposits format on outer wall and the coupling effect happens when the wall temperature below 80℃. When the coupling effect saturated, the serious corrosion occurred, which is the combination of chemical and electrochemical corrosion process. When the wall temperature reduced to 41～47℃, the corrosion phenomenon is more serious which including the oxygen corrosion, electrochemical corrosion and chemical corrosion.（5） To complete the cooling experiment of low temperature corrosion and fly ash properties when the power plant boiler temperature is deeply cooled. The gas temperature reduce from 127℃ to 90℃ by adjusting the loading on the low-pressure economizer. By the experiment results of gray sample and electrostatic precipitator ash pump ash aggregation, moisture content, particle size distribution, composition, viscous force and so on to obtain the effect of deeply cooling to the ash position and acid coupling and the electrostatic precipitator dust collection efficiency. It’s observed from the flue ash and pump ash SEM scans that the larger particles fly ash samples are made up by the micron particles and sub-micron particles with the temperature decrease. This situation shows that appropriate low temperature is benefit for electrostatic precipitator collecting. The ash sample moisture content and the H₂SO₄ condensate volume of each electric field increased with the temperature decrease, and decreased with the ash diameter reduction. Assuming that the hopper ash particles have the same density, we calculate the quantity of particles less than 10μm increase 9.45 times in hopper#1, and the quantity of particles less than 1μm increase 44.36% in hopper #4. The same law also existed in other hoppers. The micron and submicron particle collection efficiency was increased significantly for all of the above data. It has important practical significance for ultra-low emissions engineering. The composition of the ash samples obtained using XRF and XRD analysis. The analysis results show the sulphur alkali ratio increase, the acid alkali ratio decrease and the viscous force decrease with the temperature decrease. This shows that the proper low temperature is beneficial for dust collection. The viscous force is 34.74 mg/cm2 at 117℃, and increase to 56.43 mg/cm2 at 95 ℃. The surface viscosity increases with the temperature reduction, which indicates that the agglomeration phenomenon is likely to occur under low temperatures. The ash sample viscous force shows a decreased trend with the flue gas flow under the same gas temperature, the viscous force reduce with the particle diameter, and then more difficult to be collected.（6） The simulated flue gas atmosphere cooling experiment is conducted to determine the effect of deeply cooled low-pressure economizers on the low temperature corrosion mechanism and the fly ash properties, the experimental data is orthogonal processed. The range analysis results of the moisture content of fly ash shows that the moisture content of the flue gas is the most important factor, the H₂SO₄ steam content in flue gas and the flue gas temperature are the secondary factors and the effect of fly ash concentration is minimal. The range analysis results of the viscous force of the fly ash shows that the moisture content of the flue gas is the most important factor, the flue gas temperature and the H₂SO₄ steam content in flue gas are the secondary factors and the effect of fly ash concentration is minimal. The results of variance analysis of the moisture content of fly ash shows that the moisture content of the flue gas is the particularly significant factor, the significant level is 0.01. The flue gas temperature is significant factor, the significant level is 0.05, and the significant level of the H₂SO₄ steam content in flue gas is 0.1. The fly ash concentration has no significant effect on the moisture content. The results of the viscous force show that the moisture content of the flue gas is particularly significant factor, significant level is 0.05. The flue gas temperature is significant factor which significant level is 0.1. The H₂SO₄ steam concentration and the fly ash concentration have no significant effect on the viscous force. Due to the deviation SC caused by the various factors is significantly bigger than the deviation Se caused by the error, the random error of this experiment is small. The error caused by inaccurate measurement would not affect the accuracy of experimental results and the conclusion of the experiment has high credibility. The above conclusions show that the moisture content, the H₂SO₄ steam content in the flue gas has significant effect on the moisture content and the viscous force of the ash. It indirectly shows that two factors have significant effects on low temperature corrosion and electrostatic precipitator dust. All that can be regarded as the experimental basis for the control and adjust the dynamic acid dew point and change the properties of fly ash.