Study On The Deposit And Corrosion Problem On Heating Surface During Straw Combustion

Biomass energy as an important renewable energy, with low sulfur, low ash, high volatile and zero emission of CO2characteristics, has a wide range of application. Among various biomass utilization technologies, combustion for electricity is most developed and widely applied because of its low cost and high reliability. But as the high content of alkali metal and low ash melting point of biomass fuel, the deposition and corrosion problem on the heating surface of heat exchanger and furnace is serious. The deposition and corrosion interact with each other in the combustion process. Generally speaking, the heating surface sediments is not only affect the heat transfer, but also the key factor leading to high-temperature corrosion, therefore, deposition corrosion problem has already become a threat on the long-term operation of the biomass boiler. Based on the independently designed biomass powder swirl burner system and the deposition and corrosion problems in the engineering cases, we investigated the formation process and mechanism of deposition corrosion on the heating surface in this paper.First of all, based on the test requirements we designed a new biomass powder swirl burner system. The test device can satisfy the powder fuel combustion conditions, realizing the efficient and stable combustion of different biomass species under the controllable excess air coefficient, and produce typical stable flows of flue gas. According to the characteristics of different biomass fuel, we choose the herb fuel straw, cotton and woodiness fuel sawdust, and installed three coupons in the furnace along the flow direction. The experiment studied the influence of different factors on deposition, such as furnace position.deposition time, fuel type and sampling sheet position and so on.Secondly, in view of the deposition and corrosion problems on the high temperature heating surface in220t/h biomass boiler (Zhanjiang, Guangdong), through sampling, testing and other investigations we found that:the platen superheater stable sedimentary has typical inner, middle and outer layer structure and component distribution. The component of inner part is affected by the high temperature corrosion; the middle layer has a high content of alkali metal salt, quartz sand and inert bed material impurity; the outer layer has a lower content of KC1because of the higher temperature, and based on sulphate with high melting point, bed material particles and other silicon aluminum acid salt with high melting point. At the same time, the decrease of the KCl in the outer layer, reduced the local melting phenomenon and the adhesion property, thus declined the sediment’s ability to catch particles in the flue gas. The high temperature heating surface corrosion in biomass boiler is mainly caused by the melting of alkali-chloride in the sediment. Chloride is not consumed but played as catalyst in the corrosion process, which leads to severe corrosion.In order to reduce the heating surface deposition in the engineering, we used inert inhibitors to restrain the process of deposition in this study. Experimental data showed that adding the inhibitor with higher melting point inert material at700-800℃can remit the sintering of the sediments on the biomass boiler heating surface in a certain extent. The experiment proved that changing the sedimentary structure and sintered properties can restrain the formation of densification sintering sediment. But as a result of temperature limitations of the inhibitor, studies showed that the inhibitors played a promoting role on the sediment at1000℃. In the engineering operation of the biological fluidized bed boiler, the inhibitor did not show obvious effects, so further study for industrial application will be needed.