Effect of silica additive on the high-temperature fireside tube corrosion during the air-firing and oxy-firing of lignite (Xinjiang coal) - Characteristics of bulk and cross-sectional surfaces for the tubes

- The pure CO2 caused the oxidation of Cr and Fe.
- The use of silica additive is effective in alleviating the diffusion rate of oxygen.
- Formation of augite by reactions between free oxides in ash and carbon steel surface.
- Speciation of S varies with ash deposit and tube material.
- A slightly accelerated outward diffusion of Cr by the ash deposit.

Detailed speciation analysis has been conducted on the corroded tubes coated with two different ash deposits collected from the combustion of a lignite, namely Xinjiang coal mixed with and without external silica in both air- and oxy-firing modes. The exposure conditions are of temperature of 650 °C, 50 h and the use of different flue gases (i.e. pure CO2, air, air-firing flue gas and oxy-firing flue gas). Apart from the lab-based XRD used for the characterisation of the top surface, synchrotron XANES was employed to determine the oxidation states of Fe, S and Cr on both top and cross-sectional surfaces of the corroded tubes. The results indicate that, irrespective of the Cr content, the tubes exposed to pure CO2 underwent oxidation, which is most likely due to the reaction CO2 + M = CO + MO where M stands for Fe or Cr. Cr in the Cr-bearing tubes were oxidised more rapidly than Fe in the pure CO2. The combined use of silica additive and Cr-bearing tubes with a minimal Cr content of 2 wt% (i.e. T23) is beneficial in inhibiting the tube surface oxidation as well the penetration of oxygen and sulphur. Except alkali sulphates, the free oxides in ash deposit can also trigger the reactions for the formation of new species such as augite ((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6) on the carbon steel tube surface. The oxidation states of sulphur on the tube surface is highly tube-specific and dependent on ash deposit composition as well, with the abundance of pyrite (FeS2) on the carbon steel tube coated with the raw coal ash deposits, whilst the enrichment of troilite (FeS) upon the coating of ash deposits derived from the combustion of coal mixed with silica additive, due to the shortage of free sulphur in ash deposit and a preferential scavenging of Fe into chromite. For the Cr-bearing tubes, sulphate is the only S-bearing species due to the inhibited inward diffusion of S by the Cr-oxide layer, irrespective of ash deposit type. In addition, spatial organisation of the oxidation state of Cr suggests a slightly accelerated outward diffusion of Cr by the ash coating, thereby leading to a dense Cr-O layer that is highly protective on the tube top outer surface.