The Study On The Problem Of Deposit And Corrosion On Heat-Transfer Surface During SDBF Combustion

With fossil fuels exhausting and environment pollution increasingly serious, Generating technology by straw direct combustion is becoming the focus of attention at home and abroad. But the direct combustion of straw in boiler causes a lot of problems to combustion system owing to its special form conditions and process making the composition and structure of Straw different from fossil energy, among them, the deposit and corrosive on the heat-transfer surface is most severe, which not only influences the transfer of heat in the heat-transfer surface, but also seriously corrodes the heat-transfer surface and impacts on the running of boiler. Currently, the deposits and corrosive on the heat-transfer surface caused by the direct combustion of straw has become a main obstacle of the development of generating technology by straw direct combustion.Although at home and abroad the study on deposits on the heat-transfer surface in boiler caused by straw combustion has made some progress, owing to the forming of deposits being a complicated physical chemistry process which comes down to furnace principle, fuel and chemistry and reaction dynamics of ash and slagging, multiphase hydrodynamics,heat and mass transferring,burning principle and technology,material and so on, in addition, which is affected by many factors such as the melting point,elements,viscidity of ash,thermal parameter of hearth,structure of combustion and air dynamical condition in furnace,running parameter of combustion and so on. At the time, current study means is also limited. As a result, the forming process and corrosion principle of deposits is still unclear and the problems caused by deposit still exist.In order to seek the Methods and measures of reducing Corrosion and deposit quantity, eliminate the harm caused by deposit and impel the development of generating technology by straw direct combustion, on the basis of consulting a great deal of correlative Literatures of home and abroad, the author designed a test device to simulation the formation of deposit on the heat-transfer surface caused by the direct combustion of straw, and studied and analyzed the influence of the component change of materials, the temperature change of hearth and the speed change of wind on the formation of deposits and corrosion. The result shows: the main cause that the quantity of deposit during straw combustion higher than that during lumber combustion is the quantity of alkali metals and Cl in straw is higher than that in lumber; The quantity of alkali metals separating from straw fuel increases along with the temperature of hearth rising, which is beneficial to the formation of the low melting point share-crystal by alkali metals and SiO₂; Wind speed influences on the aerodynamic field of hearth, smoke speed and direction, changes the velocity, direction and depositing location of fly ash particles, the upper wend speed is good to the separation of alkali metals and Cl from straw, and is easy to the formation of fouling. But when wend speed exceeds 12m/s, the ash in smoke including a lot of gas compositions has no chance to touch the heat-transfer surface and is directly given out, under the condition, the granule that have adhered to heat-transfer surface renewably return to smoke, as a result, the amount of deposits on the heat-transfer surface declines; The rate of deposits declines with the heat-transfer surface temperature enhancing, when the whole surface of deposits with viscosity is formed, the influence of heat-transfer surface temperature falls, and at last, it is eliminated with the amount of deposits increasing. Besides, with the heat-transfer surface temperature increasing, fouling is going to be appearing in deposits. The organization and structure change of straw has an important influence on the deposits. Compared to raw straw, after SDBF burns, the close-grained coke framework is formed which not only changes the rule of the formation of deposits but also declines the rate of deposits.In order to get to the bottom of process and mechanism of deposit formation on heat-transfer surface during straw combustion, the author first studied and analyzed separately the formation of deposit on the inner surface of barring fire loop and the surface of water tank, the results show: the deposits on the different surface in the hearth is different, the deposit on the inner surface of barring fire loop is made of fine particles and reflects white light, there are not Si,Al,Fe,Ti or other Refractory compounds found in its components, however the deposit on the surface of water tank is silver gray and made of coarse grains, in it, the content of Ca,Si,Ti,Mg and other Minerals is higher. According to above analysis, the author reasoned separately their formation process and mechanism and thought that the formation of deposits is caused by the configuration change and transportation of ash, which is also the depositing process of ash. The main mechanism of deposit on the inner surface of barring fire loop is condensation and chemical reaction and the deposit on the surface of water tank is formed by thermophoresis,particle impaction.To seek the mechanism of corrosion caused by deposit, the author analyzed the deposit shelled from the heat-transfer surface, the result shows that the process of corrosive is also the process of gradually transferring Fe to deposit by chemical reaction, which destroys the original protective film of the heat-transfer surface. During corrosion, chloride and sulphate play an important role, first, chloride goes . through oxide and reacts with Fe to make FeCl₃, then, sulphate reacts with FeCl₃ to make FeS, so that, the concentration of FeCl₃ reduces and the reactor is towards the direction of FeCl₃In order to have a clear understanding of harm caused by deposit and corrosive and to provide some reference to straw direct combustion of China, the paper first studied the influence of deposit on heat transfer efficiency of heat-transfer surface and corrosion on heat-transfer surface by test, then, discussed the influence of deposit on the running of boiler with the actual straw burning boiler. Research shows that the deposits formed by straw combustion more seriously inhibits heat transfer than that formed by limber combustion. Generally, when deposit is 3mm loose ash or 10mm slagging, heat transferred through the heat-transfer surface decreases 40%, however, when the thickness of deposit formed by straw combustion adds from 0mm to 0.56mm, heat transferred by the heat-transfer surface decreases 51%; a great deal of Cl who enriches in deposit erodes seriously heat-transfer surface, reduces the life time of heat-transfer surface; at the time, the deposit impacts on the running of the boiler safely.In view of the current various methods and measures that reduce deposit, this paper classified them according to their characteristic and analyzed and evaluated them separately, by this way, the author thought that these methods in use have their pros and cons, thus a suitable method must be chose from them to reduce deposit according to the actual situation and Principle...