Role of ZnCl2 in High-Temperature Corrosion in a Bench-Scale Fluidized Bed Firing Simulated Waste Wood Pellets

The development and improvement of the handling of waste wood toward higher economical and environmental standards, such as use to produce heat and electricity, has increased in many countries. Waste wood originates mainly from construction and demolition activities and commercial and industrial sources. However, it is known that waste wood usually contains substantial concentrations of heavy metals, particularly elevated amounts of Zn and Pb. In this paper, the importance of Zn in the corrosion process was investigated. To better understand the fate of Zn and its role and effect on high-temperature corrosion of heat exchanger tubes in waste-wood-fired fluidized-bed boilers, high-temperature corrosion/deposit probe tests have been performed. Corrosion tests were carried out in a 30 kWth bubbling fluidized-bed (BFB) reactor. Tests were performed using pure wood pellets as a reference and wood pellets doped with similar to 0.5 wt % ZnCl2. Two steels were selected for the tests: a low-alloy steel (10CrMo9-10) and an austenitic stainless steel (Sanicro 28). After corrosion/deposit probe exposures of 7 h, the material test rings were analyzed using scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) to determine the oxide layer thickness and the elemental composition of the oxide scale and deposit. The results showed that combustion of wood pellets doped with ZnCl2 had a detrimental effect on low-alloy steel when compared to the firing of untreated pellets. A noticeably thicker oxide layer was formed on the low-alloy rings when the test ring temperatures were >500 degrees C. The oxide layer was mainly composed of non-protective, mixed iron oxides covered with a thin layer of KCl-rich deposit, with significant amounts of ZnO and most likely K2ZnCl4 on the top of the scale. The corrosion rate on the high-grade steel seemed to be negligible under the tested conditions and exposure times. The results showed a very low rate of deposit buildup during the 7 h exposures of the corrosion/deposit probe in the tests with the simulated waste wood. The deposited ash was rich in corrosive KCl.