Potassium, chlorine, and sulfur in ash, particles, deposits, and corrosion during wood combustion in a circulating fluidized-bed boiler

The effect of the addition of chlorine and/or sulfur to the fuel on fly ash composition, deposit formation, and superheater corrosion has been studied during biomass combustion in a circulating fluidized-bed boiler. The chlorine (HCl (aq)) and sulfur (SO2 (g)) were added in proportions of relevance for the potassium chemistry. The composition of the bottom and the fly ashes was analyzed. Gas and particle measurements were performed downstream of the cyclone before the convection pass and the flue gas composition was recorded in the stack with a series of standard instruments and an FTIR analyzer. At the position downstream of the cyclone, a deposit probe was situated, simulating a superheater tube. Deposits on the probe and initial corrosion were examined. It is concluded that addition of sulfur and chlorine increases the formation of submicron particles leading to deposition of potassium sulfate and chloride. The results compare well with earlier work based on laboratory-scale experiments concerning effects of chlorine and sulfur on potassium chemistry

Effect of water-vapor-induced Cr vaporization on the oxidation of austenitic stainless steels at 700 and 900 degrees C - Influence of Cr/Fe ratio in alloy and Ce additions

The oxidation of 153MA, 310 and 353MA austenitic stainless steels was investigated at 700\ub0C and 310 and 353MA at 900\ub0C in O 2 and in O 2 + 40% H 2 O. 153MA was not studied at 900\ub0C because it suffered excessive corrosion. The influence of gas velocity was studied. The oxidized samples were investigated by grazing angle X-ray diffraction, scanning electron microscopy/energy dispersive analysis by X-rays Auger-electron spectroscopy, and glow discharge optical emission spectroscopy. In the presence of water vapor, gas velocity strongly influenced oxidation. This effect is attributed to chromium evaporation in the form of CrO 2 (OH) 2 . Exposure in O 2 + 40% H 2 O at 700\ub0C using high flow rates caused breakaway corrosion on all alloys. At 900\ub0C, only the 310 and 353MA alloys were investigated. In O 2 + H 2 O environment, both alloys showed a mass loss at low flow rates due to chromium evaporation. At high flow rates, alloy 310 suffered breakaway corrosion while 353MA did not. The corrosion products consisted of a relatively thin Cr-rich (Cr,Fe) 2 O 3 oxide plus thick, iron-rich oxide islands. The greater corrosion resistance of 310 and 353MA steels in environments where chromium evaporation is a factor is attributed to the high Cr/Fe ratio. In contrast, the presence of Ce and Si in the MA grade steels appears to have little beneficial effect on breakaway corrosion triggered by Cr evaporation. \ua9 2004 The Electrochemical Society. All rights reserved

Effect of water-vapor-induced Cr vaporization on the oxidation of austenitic stainless steels at 700 and 900 degrees C - Influence of Cr/Fe ratio in alloy and Ce additions

The oxidation of 153MA, 310 and 353MA austenitic stainless steels was investigated at 700\ub0C and 310 and 353MA at 900\ub0C in O 2 and in O 2 + 40% H 2 O. 153MA was not studied at 900\ub0C because it suffered excessive corrosion. The influence of gas velocity was studied. The oxidized samples were investigated by grazing angle X-ray diffraction, scanning electron microscopy/energy dispersive analysis by X-rays Auger-electron spectroscopy, and glow discharge optical emission spectroscopy. In the presence of water vapor, gas velocity strongly influenced oxidation. This effect is attributed to chromium evaporation in the form of CrO 2 (OH) 2 . Exposure in O 2 + 40% H 2 O at 700\ub0C using high flow rates caused breakaway corrosion on all alloys. At 900\ub0C, only the 310 and 353MA alloys were investigated. In O 2 + H 2 O environment, both alloys showed a mass loss at low flow rates due to chromium evaporation. At high flow rates, alloy 310 suffered breakaway corrosion while 353MA did not. The corrosion products consisted of a relatively thin Cr-rich (Cr,Fe) 2 O 3 oxide plus thick, iron-rich oxide islands. The greater corrosion resistance of 310 and 353MA steels in environments where chromium evaporation is a factor is attributed to the high Cr/Fe ratio. In contrast, the presence of Ce and Si in the MA grade steels appears to have little beneficial effect on breakaway corrosion triggered by Cr evaporation. \ua9 2004 The Electrochemical Society. All rights reserved

KCl induced corrosion of a 304-type austenitic stainless steel at 600 degrees C; The role of potassium

The influence of KCl on the oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel at 600 degrees C in 5% O-2 and in 5% O-2 + 40%H2O is investigated in the laboratory. The samples are coated with 0.1 mg/cm(2) KCl prior to exposure. Exposure time is 1-168 h. Uncoated samples are exposed for reference. The oxidized samples are analyzed by ESEM/EDX, XRD and AES. The results show that small additions of potassium chloride strongly accelerate high temperature corrosion, the oxide thickness being up to two orders of magnitude greater after exposure in the presence of KCl. The rapid corrosion is initiated by the formation of potassium chromate through the reaction of KCl with the protective oxide. Chromate formation is a sink for chromium in the oxide and leads to a loss of its protective properties. The resulting rapidly growing scale consists of an outer hematite layer with embedded K2CrO4 particles and an inner layer consisting of spinel oxide, (Fe,Cr,Ni)(3)O-4. Little or no chlorine is found in the scale or at the scale/metal interface

A pilot plant study of the effect of alkali salts on initial stages of the high temperature corrosion of alloy 304L

Alloy 304L was exposed for between 15 min to 12 hr in the 12MW CFB research boiler at the Chalmers university of technology using an air-cooled probe. The base fuel consisted of a mixture of 67% wood chips and 33% pellets. In addition to the base fuel experiment, a number of exposures were performed where S and Cl was added to the fuel in the form Of SO2(g) and HCl(aq) in order to control the flue gas chemistry in the superheater region. After the exposures the samples were analysed by ESEM/EDX, XRD and SAM. Burning a mixture of woodchips/pellets without adding sulphur or chlorine results in the formation of K2SO4 deposits on the corrosion probes. When HCl is added to the fuel KCl deposits form. The simultaneous addition of HCl and SO, results in a deposit consisting of a mixture of KCl and K2SO4. In all environments studied an oxide in the 100nm range forms. With time, the oxide becomes covered by ash deposits. After exposure to the biomass flue gas environment, the oxide is enriched in K, especially the outer part. Chlorine is not present in the oxide even when the KCl(s) forms on the surface. It is suggested that potassium chromate formation occurs by the reaction of potassium chloride with chromium oxide

Chromic acid evaporation upon exposure of Cr2O3(S) to H2O(g) and O-2(g) - mechanism from first principles

Density functional theory is employed to address the mechanism for chromic acid desorption from a Cr2O3 surface. The reaction path involves the 110 Cr2O3 surface, which is subject to initial oxidation to produce Cr(IV) surface sites. Effects of subsequent further oxidation, hydration and hydrolysis are described. Molecular oxygen exposure has the formation of a monolayer of Cr(VI) groups with 506 kJ/mol exothermicity. An intermediate molecularly chemisorbed peroxide species on a reactive five-coordinated surface Cr site is characterized. Competition between O-2 and H2O for the five-coordinated Cr site is quantified. The Cr(VI) compound is subject to hydrolysis to form free Cr(VI)02(OH)2(g) and surface hydroxides whereby an additional 33 kJ/mol is released. A net 44 kJ/mol endothermicity for the formation Cr(VI)O-2(OH)(2)(g) is in qualitative agreement with the experimentally obtained 60 kJ/mol

KCl-Induced High Temperature Corrosion of the Austenitic Fe-Cr-Ni Alloys 304L and Sanicro 28 at 600 \ub0C

The influence of KCl(s) on the high temperature oxidation of the austenitic alloys 304L and Sanicro 28 at 600 \ub0C in O2 + H2O environment is reported. 0.10 mg/cm2 KCl(s) was added before exposure. The samples are investigated by grazing angle XRD, SEM/EDX, and AES. In the absence of KCl, both alloys show protective behaviour in dry O2. In O2 + H2O environment, alloy 304L suffers local breakaway corrosion while Sanicro 28 still shows protective behaviour. The oxidation of both alloys is strongly accelerated by KCl. KCl reacts with chromium in the normally protective corundum-type oxide, forming K2CrO4. This depletes the scale in chromia and leads to the formation of a non-protective, iron-rich scale. The significance of KCl-induced corrosion in real applications is discussed and the oxidation behaviour of the two steels is compared

A pilot plant study of the effect of alkali salts on initial stages of the high temperature corrosion of alloy 304L

Alloy 304L was exposed for between 15 min to 12 hr in the 12MW CFB research boiler at the Chalmers university of technology using an air-cooled probe. The base fuel consisted of a mixture of 67% wood chips and 33% pellets. In addition to the base fuel experiment, a number of exposures were performed where S and Cl was added to the fuel in the form Of SO2(g) and HCl(aq) in order to control the flue gas chemistry in the superheater region. After the exposures the samples were analysed by ESEM/EDX, XRD and SAM. Burning a mixture of woodchips/pellets without adding sulphur or chlorine results in the formation of K2SO4 deposits on the corrosion probes. When HCl is added to the fuel KCl deposits form. The simultaneous addition of HCl and SO, results in a deposit consisting of a mixture of KCl and K2SO4. In all environments studied an oxide in the 100nm range forms. With time, the oxide becomes covered by ash deposits. After exposure to the biomass flue gas environment, the oxide is enriched in K, especially the outer part. Chlorine is not present in the oxide even when the KCl(s) forms on the surface. It is suggested that potassium chromate formation occurs by the reaction of potassium chloride with chromium oxide

KCl-Induced High Temperature Corrosion of the Austenitic Fe-Cr-Ni Alloys 304L and Sanicro 28 at 600 \ub0C

The influence of KCl(s) on the high temperature oxidation of the austenitic alloys 304L and Sanicro 28 at 600 \ub0C in O2 + H2O environment is reported. 0.10 mg/cm2 KCl(s) was added before exposure. The samples are investigated by grazing angle XRD, SEM/EDX, and AES. In the absence of KCl, both alloys show protective behaviour in dry O2. In O2 + H2O environment, alloy 304L suffers local breakaway corrosion while Sanicro 28 still shows protective behaviour. The oxidation of both alloys is strongly accelerated by KCl. KCl reacts with chromium in the normally protective corundum-type oxide, forming K2CrO4. This depletes the scale in chromia and leads to the formation of a non-protective, iron-rich scale. The significance of KCl-induced corrosion in real applications is discussed and the oxidation behaviour of the two steels is compared