Effects of Temperature Gradient on Ash Deposit Aging and Heat Exchanger Corrosion

Boilers firing biomass and/or waste-derived fuel have in general lower electrical efficiencies compared to coal-fired boilers. The root cause for the lower electrical efficiency lies in the high-temperature corrosion of the boiler heat exchanger tubes. The high corrosion rate in biomass and waste-fired boilers is tied to the ash composition of those fuels. The ash from the fuel forms deposits on the heat exchanger tubes of the boilers. The ash deposit composition and the heat exchanger material temperature are key parameters affecting the high-temperature corrosion. In order to minimize the corrosion risk, the final steam temperature, ergo, the material temperature is kept at a lower temperature in boilers combusting fuels with challenging ash compositions, which results in lower efficiency.

This work concentrates on ash deposit aging mechanisms as well as on high-temperature corrosion occurring in industrially relevant temperature gradients. The aging mechanisms were studied in laboratory scale and mathematical modeling was applied to validate the aging mechanisms observed in the laboratory experiments. The laboratory work revolved around synthetic alkali halide-alkali sulfate mixtures, relevant for biomass firing boilers and for black liquor recovery boilers. In addition, the behavior and interactions of PbCl₂ with alkali salts, relevant for waste-firing, in a temperature gradient were studied. The high-temperature corrosion of P235GH carbon steel and 10CrMo9-10 low alloyed steel was studied.

The main experimental equipment was a laboratory-scale temperature gradient probe. The probe is air-cooled and during exposure, it is inserted into a hot tube furnace, resulting in a temperature gradient from the furnace air to the probe. The probe houses two sample rings, which are covered with a deposit material. In addition, an entrained flow reactor (EFR) was used to form synthetic ash deposits on an air-cooled probe and to study the aging of the deposits. With both the temperature gradient furnace and the EFR, after exposure, the sample rings and the deposits were cut for a cross-section and analyzed using Scanning electron microscopy/Energy-dispersive X-ray spectroscopy (SEM/EDX).

Ash deposit aging was observed to occur both in the liquid and in the gas phase. Liquid phase sintering is known from earlier studies to affect the deposit density and removability. In addition to the densification of the deposits, the results from the laboratory experiments show that liquid phase sintering in a temperature gradient results in local species enrichment within ash deposits. The enrichment occurs when the liquid phase migrates within the deposits, filling pores and solidifying in lower temperatures. In addition, temperature gradient zone melting phenomenon was observed to occur within the synthetic ash deposits. The phenomenon results in a species migration within a liquid phase due to a temperature gradient induced difference in the melt composition, i.e. concentration gradient within the liquid phase. The practical implications of the temperature gradient zone melting phenomenon are still unclear and further research on the topic is suggested.

Gas phase migration of alkali halides was observed to occur within the synthetic ash deposits when exposed to temperature gradients. The phenomenon was concluded to be due to the concentration gradient of alkali halides within the gas phase. The concentration gradient is induced by the temperature gradient and the temperature dependence of the partial pressures of the alkali halides. The partial pressure is higher in higher temperatures, which leads to diffusion of alkali halides from the higher temperature region to the lower temperature region. The mechanism was confirmed by modeling.

PbCl₂ was observed to migrate to and within ash deposits and to interact with alkali salts. PbCl₂ together with NaCl resulted in a formation of a eutectic melt but did not form new compounds. PbCl₂ and Na₂SO₄ reacted and formed caracolite (Na₃Pb₂(SO₄)₃Cl) and NaCl. PbCl₂ reacted with both KCl and K₂SO₄ and formed K-Pb-Cl species. In addition, PbCl₂ and K₂SO₄ were observed to react and form a caracolite type compound (K₃Pb₂(SO₄)₃Cl), which so far has not been corroborated to exist in the literature.

The corrosion mechanisms induced by alkali chlorides and alkali bromides were similar in nature and in line with the literature. The halides react with the steel and form metal halides, which have the potential to form low melting mixtures with alkali halides and/or lead chloride species. With alkali halides, fast corrosion rates were observed with steel temperature at 500 °C, while the steel temperature of 400 °C or lower resulted in slow corrosion. When lead chloride species were present in the deposit, a steel temperature of 400 °C led to catastrophic corrosion. Pure PbCl2 was observed to be more corrosive than K-Pb-Cl species. 

In light of the results of this thesis, ash deposit aging has the potential to affect the corrosive properties of ash deposits. In addition, the deposit density is likely to be affected, resulting in harder deposits, which are challenging to remove. This work shows that temperature gradients induce ash deposit aging mechanisms, which can drastically alter the deposit properties. Indications of the aging mechanisms have been reported before. However, this work makes an effort to systematically study the said effects and provide a broader understanding of their detailed nature and the relevance to boiler operation and design.