Metal tube corrosion in WTE facilities is generally categorized as high-temperature corrosion. It is a major operating problem of WTE facilities because it results in downtime and periodic shutdowns in plants and accounts for a significant fraction of the total operating cost of WTE plants. In WTE boilers, higher steam temperature in the superheater represents higher energy recovery, but the accompanied higher metal temperature of the superheater tubes also leads to higher corrosion rates. The trade-off indeed involves not only the economic perspective but also the environmental and social perspectives. Therefore, it is critical for WTE operators to find out what are the best operating parameters and the most cost-effective protection methods of boiler tubes. Many efforts have been put forth in WTE industry to combat the corrosion problem either by improving the process conditions in the boiler thus extending the lifespan of boiler tubes, or by developing alloys that can withstand better the relatively high chlorine atmospheres.
Past research work on understanding the high temperature corrosion mechanisms has identified important factors that affect the corrosion rate. In addition, there have been many laboratory tests that have sought to classify the effects of these corrosion factors where metal coupons were subject to corrosion at constant furnace temperature. However, these tests may not be useful in forecasting long term and synergistic effects of various corrosion factors on the tube life. In particular, some dynamic factors such as fluctuations in the flue gas temperature and thermal gradient between the gas and metal surface that are difficult to reproduce and control in laboratory tests have been found to be accountable for the breakdown of the protective oxide scales on the metal surface and the resultant increased corrosion rate due to such breakdown. Additionally, the thermal gradient between the gas and metal surface has been observed to strongly influence the deposition behavior and rates of formation of molten salts that contain corrosive compounds resulting from WTE combustion.
This research aimed to examine the corrosion mechanisms in WTE boilers by conducting laboratory tests under conditions that simulate the WTE environment. In order to elucidate the synergistic effects of these corrosion factors, the author developed an apparatus that can maintain a thermal gradient between a representative WTE combustion gas and test samples that are maintained at representative waterwall and superheater temperatures. All the controlling variables in the tests were referred to the analysis results of a corrosion survey that was formerly distributed to 3 WTE companies in the U.S. Three commercial steels, NSSER-4 (Nisshin Steel, Inc., Japan), SA213-T11, and SA178A were tested under prescribed corrosive environment for 100 hours. It is well known that SA213-T11 and SA178A have been widely used as the base metal in the manufacture of superheater tubes. NSSER-4 was developed by a Japanese steel company that claims it offers good resistance to chlorine corrosion.
The stainless steel NSSER-4 exhibited very high corrosion resistance and little effect of metal temperature in the range of 500 0C to 630 0C. The steel alloy SA213-T11 exhibited acceptable corrosion resistance at the metal temperature of 5400C, but its performance degraded dramatically when the metal temperature was above 5400C. The carbon steel SA178A had the highest corrosion rate among all of the metals. The corrosion rates measured in this laboratory test were quite close to the numbers reported from WTE facilities. Confirmation of the lab test results with actual corrosion rates observed out in the field gave greater confidence in the usefulness of the experimental apparatus and the testing procedure in forecasting long term effects of corrosion factors on superheater tube life.
The elemental analysis of NSSER-4 showed that the elemental concentrations in the corrosion products did not change significantly as a result of varying the metal temperature. The cross-sectional elemental analysis verified the statement that chloride (or chlorine) penetrated the oxide scale through cracks in the scale due to activity driven by chemical potential and temperature gradients. This resulted in deposition that led to accumulation in the metal/oxide scale. The elemental analysis of the other two metals, consisting mostly of iron, showed that more iron was oxidized under higher metal temperatures.
The other effort of this research is to investigate the feasible methods to reduce HCl concentration in the flue gas of WTE combustion chambers. An invention of HCl/Cl2 sequestration in the combustion gases has been proposed by injecting, through a system of pneumatic or pressure nozzles located at a level above the combustion grate of the WTE or in front of the superheater, where metal is subjected to most severe corrosion, the slurry of calcium hydroxide that reacts with HCl to form calcium chloride. The experiment has been conducting by utilizing a laboratory scale of injection system to inject the calcium hydroxide slurry into a tube furnace, together with the synthetic flue gas. The same alloys used in previous corrosion tests will be placed in the tube furnace. Due to the non-availability of HCl gas analyzer, the SO2 gas concentration in the exit gases will be measured instead of HCl gas by a SO2 gas analyzer; the concentration difference, between inlet and outlet flows, will be used as an indicator of the corrosive agent removal efficiency of the chemical injection system. In addition, the post-test analysis of the samples will include observations of surface morphology and elemental composition analysis of corrosion products by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The corrosion rates will be acquired by measuring the mass loss of samples after the test