Corrosion behaviour of boiler tube materials during combustion of fuels containing Zn and Pb

Many power plants burning challenging fuels such as waste-derived fuels experience failures of the superheaters and/or increased waterwall corrosion due to aggressive fuel components already at low temperatures. To minimize corrosion problems in waste-fired boilers, the steam temperature is currently kept at a relatively low level which drastically limits power production efficiency.

The elements found in deposits of waste and waste-derived fuels burning boilers that are most frequently associated with high-temperature corrosion are: Cl, S, and there are also indications of Br; alkali metals, mainly K and Na, and heavy metals such as Pb and Zn. The low steam pressure and temperature in waste-fired boilers also influence the temperature of the waterwall steel which is nowadays kept in the range of 300 °C - 400 °C. Alkali chloride (KCl, NaCl) induced high-temperature corrosion has not been reported to be particularly relevant at such low material temperatures, but the presence of Zn and Pb compounds in the deposits have been found to induce corrosion already in the 300 °C - 400 °C temperature range. Upon combustion, Zn and Pb may react with Cl and S to form chlorides and sulphates in the flue gases. These specific heavy metal compounds are of special concern due to the formation of low melting salt mixtures. These low melting, gaseous or solid compounds are entrained in the flue gases and may stick or condense on colder surfaces of furnace walls and superheaters when passing the convective parts of the boiler, thereby forming an aggressive deposit. A deposit rich in heavy metal (Zn, Pb) chlorides and sulphates increases the risk for corrosion which can be additionally enhanced by the presence of a molten phase.

The objective of this study was to obtain better insight into high-temperature corrosion induced by Zn and Pb and to estimate the behaviour and resistance of some boiler superheater and waterwall materials in environments rich in those heavy metals, including at increased temperatures of materials. Therefore, extensive laboratory, bench-scale and full-scale tests were carried out. The results from these tests may be directly made use of in practical applications, for example for screening steels from the materials selection, as well as in the development of corrosion preventing tools by finding corrosion initiating triggers and understanding their effect on high-temperature corrosion.

The laboratory study covered steel exposure tests with pure ZnCl₂, ZnO, PbCl₂ and PbO as well as with a number of salt mixtures: ZnCl₂-K₂SO₄, PbCl₂-K₂SO₄, PbCl₂-KCl and PbCl₂-ZnCl₂-KCl. It was shown that pure PbCl₂ starts to be aggressive to the low-alloy steel (10CrMo9-10) and also to the stainless steels (AISI 347) already at temperatures around 350 °C, below the melting temperature of PbCl₂ which is 501 °C. The protective Cr₂O₃ on the AISI 347 was destroyed due to PbCrO₄ formation. The exposures to ZnCl₂ showed an increased oxide layer growth on the 10CrMo9-10 already at 350 °C, but negligible oxide layer growth on the AISI 347 up to 450 °C. Above 350 °C, the fast evaporation of ZnCl₂ suppressed the growth of the oxide layer.

The tests with ZnCl₂- and PbCl₂-containing mixtures (ZnCl₂-K₂SO₄, PbCl₂-K₂SO_4, PbCl₂-KCl and PbCl₂-ZnCl₂-KCl) showed that the ZnCl₂-containing mixture (PbCl₂-ZnCl₂-KCl) was more aggressive and active at lower temperatures than the PbCl₂-KCl mixture. It suggests, therefore, that ZnCl₂ is more likely to cause problems at lower material temperatures, while PbCl₂ is more stable and is expected to be problematic at both waterwall and superheater temperatures. At 400 °C, the highest corrosion rates on both test materials were observed when both PbCl₂ and ZnCl₂ were present in the salt. The PbCl₂-ZnCl₂-KCl mixture contained the highest fraction of melt out of all tested salt mixtures but the corrosiveness of this mixture was not the highest at all test temperatures. Thus, the amount of melt does not necessary decide the extent of corrosion. At 500 °C and above the corrosion caused by all three mixtures containing PbCl₂ was significant and both steels were damaged to a similar degree. The results from the tests with the mixtures containing 5 wt-% PbCl₂ were similar to the results from the tests with pure PbCl₂ showing its extremely corrosive character.

ZnO was shown not to be corrosive to the low-alloy steel (10CrMo9-10) and nor to the stainless steel (AISI 347) at 550 °C. The oxide layer thickness was comparable to the test with no salt present. However, tests with PbO at 550 °C caused a noticeable oxide layer growth on 10CrMo9-10 and fairly low on AISI 347.

To better understand the fate of Zn and its effect on high-temperature corrosion specifically in waste-wood fired fluidized bed boilers, high-temperature corrosion/deposit probe tests were performed in a 30 kW_th bubbling fluidized-bed reactor by firing wood pellets doped with ZnCl₂ to simulate waste wood. Specific issues of interest in this study included the general impact of firing waste wood containing higher amounts of Zn and Cl and the evaluation of the role of ZnCl₂ in high-temperature corrosion. The tests showed that the presence of ZnCl₂ had a clear impact on high-temperature corrosion of low-alloy steel. When compared to the combustion of pure wood pellet, corrosion increased at temperatures above 450 °C (probe cooling temperature). The K₂ZnCl₄ which was found in the deposit was concluded to be the main corrosive agent.

During the planning stage of further experiments there were strong indications of bromide induced high-temperature corrosion of the waterwalls. In consequence, a measurement campaign in a BFB co-combusting SRF was performed to determine the occurrence of corrosive Cl-, Br-, Zn- and Pb-compounds in the fuel, in the furnace vapours and in the waterwall deposits. The relative corrosiveness of chlorides and bromides was further established by means of laboratory experiments. A ZnBr₂-K₂SO₄ salt mixture was tested and compared with a corresponding ZnCl₂-K₂SO₄ salt mixture. The mixture with ZnBr₂ was found to be more aggressive at 400 °C in oxidising conditions than the corresponding mixture with ZnCl₂. A measurement campaign showed that vapours in the furnace were enriched with Cl and small amounts of Br, Zn and Pb. The chemical thermodynamic calculations indicated that possible forms of those compounds at the waterwall deposit temperatures (400 °C) were Na-, K-bromides and chlorides and Zn- and Pb-sulphides or sulphates in reducing and oxidizing conditions, respectively. The thermodynamic calculations correlated with the deposit analysis.