Lab-scale Investigation of Deposit-induced Chlorine Corrosion of Superheater Materials under Simulated Biomass-firing Conditions. Part 1: Exposure at 560 °C^†

Abstract

Deposit-induced chlorine corrosion was studied under well-controlled laboratory conditions, simulating the conditions in straw-fired boilers and boilers cofiring coal and straw. This was done by exposing pieces of superheater tube (TP 347H FG) covered with synthetic deposits of known Cl content to gas mixtures simulating straw-firing and cofiring of coal and straw, at 560°C (1040°F), for 3 days. The corroded specimens, and the reacted deposits, were studied in detail using a scanning electron microscope to determine the corrosion rate, investigate the chemistry and morphology of the corrosion attack, and study the sulfation behavior. Besides the gas compositions, various parameters were studied systematically. Most specimens suffered some internal attack, mostly by selective corrosion and in some cases by grain boundary attack. In all experiments with KCl and KCl-SiO 2 deposits, the corrosion products consisted of an oxide scale, containing oxides of Cr and Fe, and on top of that a characteristic mixed layer of iron oxide threads in a potassium sulfate matrix. However, the thickness and shape of this layer was found to be strongly dependent on the experimental conditions. An increase of the percentage of KCl in the deposit resulted in a more uniform and deeper internal corrosion attack. The presence of HCl in the flue gas did not seem to be essential for chlorine-induced corrosion to occur, when a deposit containing KCl was present, but it enhanced the corrosion rate. The degree of sulfation of KCl in the deposits after exposure was quantified by wet chemical analysis and was shown to be dependent on many parameters, including the SO 2 concentration in the gas flow, the concentration of KCl in the deposit, and the SiO 2 and KCl particle sizes in the deposit. No simple relation was observed between the degree of sulfation in the deposit and the depth of internal attack or the thickness of the oxide or mixed layer. Whereas SiO 2 particles were found to be chemically inert with respect to the flue gas and the corrosion attack, CaO particles reacted with HCl from the flue gas, and the resulting CaCl 2 played an important role in the corrosion mechanism. As a result, the corrosion rate was strongly enhanced when CaO was present in the deposit, instead of SiO 2 .

Porosity-Based Corrosion Model for Alkali Halide Ash Deposits during Biomass Co-firing

“…In order to preserve the integrity of the chlorides and other water-soluble compounds on the samples, post-test polishing was done without a lubricant 11,12 . The samples were then examined using an Olympus BX51M optical microscope and a Hitachi S-4700 scanning electron microscope (SEM), to measure the depth of corrosion and provide insight into the subsurface material degradation and structure of the corrosion layer.…”

“…It has been found that the corrosion process can be severe in air but can also be increased modestly by the presence of SO 2 which can cause intra-deposit sulfation of alkali chlorides releasing HCl or Cl 2 close to the metal surface 6,10,11 . Lith et al 12 showed that the degree of sulfation of KCl in the deposit in the presence of SO 2 does not necessarily influence corrosion rate. An increase in SO 2 has also been shown to decrease corrosion rate through conversion of KCl in the flue gas to the less corrosive K 2 SO 4 13,14 .…”

“…Equation 12 gives the oxide layer depth predictions. It is proposed here to determine the value of ε from experimental results.…”

Characteristics of Corrosion Related to Ash Deposition on Boiler Heating Surface during Cofiring of Coal and Biomass

“…e content of HCl and SO 2 is adjusted through a flow meter. ey are mixed well in the gas mixing device in order to simulate the flue gas atmosphere in the boiler superheater [13]. e high-temperature reaction part adopts a horizontal tube furnace electric heating system.…”

“…e component of deposits varies widely as the fuel characteristics, combustion conditions, and other factors. Lith et al [13] suggest that the corrosion in the process of cofiring of biomass and coal is mainly based on the alkali metals existed in deposits.…”