Mechanism Study On Deposit Build-up And Corrosion Of Heating Surfaces In Biomass-fired Boiler

Biomass direct combustion for electricity is the most widely applied and developed biomass conversion and utilization technology, but due to the high content of alkali metals, it will cause deposition and high temperature corrosion of heating surfaces in boiler during biomass combustion, thus reduces the efficiency and influences the long-time safe operation of the boiler. Alkali metals problem especially deposition and high temperature corrosion is urgently need to be solved. The research focused on the ash deposition and high temperature corrosion of heating surfaces in biomass-fired boilers, combined engineering practice with experimental study, explored the mechanism of deposition build-up and high temperature corrosion.In an attempt to investigate the dynamic process of ash deposition build-up on heating surfaces of biomass furnaces, a modified sampling system was set up at a75t/h biomass-fired circulating fluidized bed boiler in east china. Deposition sampling intervals from2hour to1month were accomplished. Results suggest that the difference in temperature has influence on the earlier stage(2-37h) of deposit formation which is dominant by the condensation of KCl and capture of small particles (<5μm) consist mainly of Ca and s, while the influence weakens as the deposit grows when the inertial impaction of bed material particle (SiO2) larger than20μm becomes the major factor of deposition build-up. The main components of the deposit after1month are K2Ca(SO4)2, SiO2, it is unexpected that little KCl is found. Deposition of bed material plays an important role in the process of deposit build-up. The discrepancy between the long-time deposit and deposit from real-time sampling indicates that complicated physic and chemical reactions have taken place in deposit during its long-time development.Grate boiler is typical for layer burning technology of biomass, whose deposit mechanism is much different from CFB boiler. Aiming at the serious problem of deposition on the back-end heating surface and abnormities of fly ash from a biomass-fired grate boiler in china, which had affected normal operation of the plant, the characteristic and mechanism of deposit from water wall, high temperature superheater, low temperature superheater and economizer was analyzed. The micromorphology characteristic and element analysis of the deposits was conducted, the inorganic components of the deposits were analyzed, and influencing factors during the buildup of deposit including boiler design, operating conditions, fuel and fly ash were discussed. Results show that the high content of K, Ca, Cl and S in the fuels directly results in the high content of KCl and K3Na(SO4)2, which is the main cause of the serious deposition problem. Fuel of the boiler is secondary processed, containing lots of additive which will complicate the combustion property, and lack of secondary air makes the fly ash problem worse.Burning biomass fuel of high K and Cl will always cause serious high temperature corrosion problem in deposit, KCl in the inner layer close to the tube is regarded as the main cause for corrosion. Many biomass power plants choose high-grade steel for high temperature heating surfaces in face of corrosion problem. Resistance to high temperature chlorination and oxidation corrosion of6common boiler steel used in china:20G,12Cr1MoVG, Super304H, SUS316, TP347H, HR3C was compared, the corrosion rate was calculated by the mass gain of unit surface area, the resistance to corrosion was HR3C>Super304H>TP347H> SUS316>12Cr1MoVG>20G according to the mass gain result. The micromorphology characteristic and element analysis of the surfaces after corrosion was analyzed by SEM-EDS, the inorganic components of corrosion products were analyzed by X-ray diffraction(XRD), a possible reaction equation was speculated, and the differences between chlorination and oxidation corrosion were discussed briefly; the influence of contact condition between KCl salt and steel surface on corrosion was also investigated by spraying KCl on steel sample surfaces or bury it in KCl salt directly.