The environment in waste incineration plants contains high concentrations of chloride, alkalis, sulfur, and low temperature melting species, This corrosive environment along with a need to fulfill the pressure vessel code requirement, calls for a dual purpose material in the waterfalls and superheaters of the boiler. Composite tubes consisting of a corrosion resistant cladding metallurgically bonded to the load-bearing core, make it possible to optimize both of the aforementioned requirements while meeting the demands for higher efficiency in waste-to-energy plants, During the last years several field tests of composite tubes have been performed. The experience of these tests will be the subject of this paper.
Alloy 825 mod (UNS N08825 mod), Alloy 28 (UNS N08028), Alloy 65 (UNS N06625 mod) and Alloy 625 (UNS N06625) were evaluated as superheater tubes, Among these the Alloy 625 showed the best performance and it was estimated that the corrosion rate is about 10 - 20 times lower when compared to carbon steel. The superior corrosion resistance of Alloy 625 was due to its high Mo content in combination with either a low Fe content or a high Nb content. All tested alloys suffered from corrosion neat sootblowers, However Alloy 625 corroded but at a considerably lower rate than the other materials, At a material temperature of about 540º C (1000° F)the wastage of Alloy 625 was 0.2 µm/h (7.8 mils/1000 h).
Alloy 28 composite tube in waterfalls showed a corrosion rate that is about 8 times lower than carbon steels and a factor 2 to 3 better than type 304L (UNS-S30403). Composite Alloy 825 as screen tubes exhibited a similar corrosion rate as weld overlay Alloy 625.
When experiencing short operating lifes and high maintenance costs, due to corrosion, in waste incineration boilers, composite tubes provide an interesting alternative to other corrosion protection methods.
Waste incineration
Waste incineration plants suffer from metal wastage due to corrosion, erosion and a combined effect of these two processes [1]. The environment in a waste incinerator plant is harsh due to varying concentrations of chlorine, but alkalis, sulfur, and metallic species such as lead, zinc and tin, are also important. These elements originating from the waste can lead to condensation and formation of compounds that melt at the boiler tube temperatures [2]. The environment and wastage rate vary considerably from one plant to another depending on the type of waste, boiler design and operating conditions giving different metal temperatures [2,3]. Operators strive to continuously improve the cost efficiency of their boilers, requiring higher steam pressure and temperature. Clearly, this need for high pressure and temperature will generate a need for more corrosion resistant materials, Earlier field tests have shown that stainless steels and nickel base alloys offer greater corrosion resistance compared to carbon and low alloy steel. In monobloc or composite tube these alloys increase the service lifetime of the various tube components within the boiler [4-7].
