High-temperature corrosion phenomena in waste-to-energy boilers

Waste-to-Energy (WTE) technology is an essential part of sustainable waste management. It generates electricity by combusting municipal solid waste (MSW) under controlled conditions. There are over 600 WTE facilities globally combusting over 170 million tons of MSW annually. However, high temperature corrosion of boiler tubes remains an operational and economic problem for the WTE industry.;Past research work concentrated on reducing corrosion in WTE boilers either by improving the process conditions in the boiler, or by developing alloys that can withstand better the relatively high chlorine concentration in the combustion gases (400--600 ppm HCl).;This research examined the corrosion mechanisms in WTE boilers by conducting laboratory tests under conditions that simulated the WTE environment. The controlling variables in the tests were based on the analysis of data provided by over fifty U.S. WTE facilities in response to a corrosion survey that was distributed by the Waste-to-Energy Research and Technology Council, an academic-industry organization headquartered at Columba University. This study also explored the feasibility of a novel procedure that aimed to reducing the hydrogen chloride concentration in the combustion gases flowing through the boiler.;The research effort included (1) an in-depth analysis of the survey on high temperature corrosion; (2) laboratory tests that, for the first time simulated the large temperature gradients encountered across the wall of WTE heat exchanging metal surfaces and clarified the mechanism and kinetics of chlorine induced corrosion and the effect of hydrogen chloride gas concentrations on corrosion rates; and (3) laboratory tests on the sequestration of chlorine in the WTE process gas by means of injecting chemicals into the furnace.;The corrosion survey showed that the most common waterwall tubing was low-carbon steel SA178A (>99%Fe) cladded with Inconel 625 (58%Ni-20-23%Cr-8-10%Mo) applied. Low-carbon, intermediate-chrome alloys SA213 T11 (Fe-1.05%Cr-0.08%C) and T22 (Fe-2.21%Cr-0.1%C) were mostly used as superheater tubing.;In the experimental corrosion tests, the stainless steel alloy NSSER-4 (Fe-17.3%Cr-13.1%Ni-2.5%Si) that is produced by a Japanese company showed excellent corrosion resistance. Although NSSER-4 is not available in the U.S. steel industry now, it is highly recommended for superheater tubing where higher metal temperatures are required. The low-carbon, intermediate-chrome steel SA213 T11 exhibited acceptable corrosion resistance at metal temperatures up to 540°C. The low carbon steel SA178A had the worst corrosion resistance among all alloys tested.;Increasing the HCl concentration in the synthetic gas flow through the experimental apparatus increased the corrosion rates of test coupons. The HCl effect was amplified with increasing metal temperature. In addition, the presence of HCl promoted the formation of sulfate salts and increased the corrosion. The results of the chemical rate test showed that the overall reaction process of alloy SA178A during the 100-hour test followed the parabolic time dependence which was often found in high temperature oxidations. The overall apparent activation energy of alloy SA178A within the 100-hour test was 178kJ/mol which was determined from multiple tests. The calculated activation energy of alloy SA178A after a single 100-hour test was 149kJ/mol, which was close to its overall activation energy showing that the 100 hours of exposure was suitable for the corrosion test. From the comparison of activation energies of three test materials, it was inferred that the corrosion of the low-carbon steel, SA178A was more kinetically controlled while the stainless steel, NSSER-4 was more diffusion controlled.;The injection of calcium hydroxide slurry droplets, in order to react with HCl/Cl2 and thus lower the effective chlorine concentration in the gas, was shown to reduce appreciably the corrosion rate of the metal coupons in the test chamber. The observed reduction of overall mass loss ranged from 0.3--18% for three different metal alloys. These accelerated corrosion tests were conducted at metal temperatures of 700°C, that is appreciably higher than the temperature gradient tests (450--580°C). Even under these conditions the stainless steel alloy, NSSER-4, exhibited vastly superior performance to the steel alloys that are commonly used in the WTE industry. Its overall mass loss per surface area was 64 times lower than SA178A and 70 times lower than SA213 T11.