| Combustion of municipal solid waste(MSW)in waste-fired boilers has become one of the best available technologies to treat waste materials due to the advantages of recovering energy in the form electricity,heat,and steam for energy-consuming industries.Due to the specific composition and nature of MSW,incineration of wastes creates a very corrosive environment inside the combustion boiler,which causes high-temperature corrosion of the boiler parts such as the superheater.Increasing the steam temperature and pressure to increase boiler efficiency severely increases the corrosion rate of the boiler tube metals.This corrosion problem forces waste-to-energy(Wt E)plants to operate at a low steam temperature?420oC and pressure?40 bars,which is a compromise between energy-generating efficiency and corrosion rate,thereby resulting in low boiler efficiency of only 15-23%.High-temperature corrosion causes unplanned plant shutdowns for maintenance leading to shortages of electricity supply,loss of treatment of wastes,and eventually loss of jobs.The maintenance and replacement of the damaged boiler tubes are costly and make the operation of the WtE boilers expensive to run.Therefore,the solution to this problem is urgently needed for the Wt E industry to be economically viable.The overall objective of this Ph.D.research was to provide a solution to the high-temperature corrosion,which will also enable steam temperature and pressure to be increased,hence increase the boiler efficiency.In order to achieve this,the corrosion trends against changes in the steam temperature,pressure,and concentration of different corrosive species such as alkali chloride(KCl and Na Cl),SO2,and H2O vapour were investigated.The corrosion mechanism was explored by computational calculations using Gaussian 09 software,and then reaction pathway modelled using chemkin-Pro.The changes in corrosion rates with time as well as with changes in the corrosive environment containing alkali chloride in which Na Cl was used as a surrogate compound,moisture,and SO2 were also investigated using well-controlled lab-scale experiments.Commercially available alloy 310H and pure Fe were used as examples to study the corrosion kinetics and corrosion reaction pathway.After that,a new corrosion-resistant alloy that can withstand the severe corrosive environment when steam parameters are raised to 600oC and 100 bars were designed through thermodynamic equilibrium calculations(CALPHAD approach)using the computer software Factsage.The designed alloy was then fabricated,and its corrosion resistance ability at 600oC and 100 bars was compared with Inconel 625,Esshete 1250,304,and13CrMo4-5TS,which are commonly used alloys.Furthermore,the corrosion rate/cost analysis was carried out to compare the economic feasibility of the designed alloy against commercially used alloys.The results revealed that increasing the concentration of KCl accelerated the corrosion rate of Fe with a parabolic rate law.The concentration of SO2>500ppm increased the corrosion rate of Fe,concentrations between 250–500ppm reduced the corrosion rate,and the concentration<250ppm had less effects.Water vapour of?10vol.%was optimum to reduce the corrosion effects while the concentrations above,the corrosion rate was accelerated.Steam pressure had a greater influence on the corrosion rate of Fe-metal than steam temperature.The reaction series that replenished the Cl radicals,HCl,and KSO3Cl promoted the high corrosion rates,while reactions that formed sulphate suppressed the corrosion rate.The exposure environment containing NaCl+20vol.%H2O+8vol.%O2 caused a linear corrosion attack of alloy 310H for a prolonged time,causing this condition to be the most corrosive environment as it resulted in?400%mass gain increase of the alloy 310H than in the condition with Na Cl+20vol.%H2O+300ppm SO2+8vol.%O2.The environment containing NaCl+300ppm SO2+8vol.%O2 resulted in a breakaway corrosion attack within a short time frame(1 h),and a decrease in mass gain due to the suppression of the chloridation process by sulphation of Na Cl.In the absence of NaCl salt,the formation of adsorbed surface sulphate inhibited the vaporisation of chromic acid.The contribution of the corrosive species to the corrosion rate was in the order of NaCl>H2O vapour>SO2.The designed alloy(Ni-5W-6B-28Cr-13Al)showed a better corrosion resistance than13Cr Mo4-5TS and 304 alloys but exhibited similar corrosion rates with that of Inconel625 and Esshete 1250.The cost of the designed alloy was less by almost 36%than that of Inconel 625 and by 22.5%than that of Esshete 1250.This thesis recommended the Wt E industry to combine two corrosion mitigation aspects investigated in this study in order to effectively control the corrosion problem,(1)optimisation of flue gas components such as optimising the concentration of moisture by drying either in sun when its not summer or install the drying stage which will be used during summer season,the concentration of SO2 by using S-containing fuels,and(2)use the newly designed cost-effective corrosion-resistant alloy(Ni-5W-6B-28Cr-13Al).Nevertheless,the corrosion resistance of the newly designed alloy still needs to be tested over long periods particularly in real plant boiler. |
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