Macplus, il progetto per migliorare l’efficienza degli impianti a fonti fossili


 The future of fossil fuels-fired power plants in Europe will critically depend on the possibility of building new advanced and more efficient plants able to achieve the CO2 capturing targets. In addition those systems needs to reach CO2 purity level that meet the requirements for transportation and storage. In this view, next generation of power plants as Advanced-UltraSuperCritical (A-USC), oxy-fuel including eventually biomass co-firing, can be promising key enabling technologies for CCS. Their accomplishment relies on the capability of solving specific technological problems mainly related to the flexibility of working with a diverse mix of fuels in a reliable way. The introduction of CCS technologies needs the change of the plant working conditions, in term of higher service temperature (A-USC power plants) and atmosphere composition (in oxy- and co- ombustion plants). The main challenges for structural materials are: – the ability to predict and assess critical components life span (optimization of materials design and elaboration of behaviour models: creep, creep-fatigue. oxidation, ….) – the enhancement of materials properties and surface protective systems (coatings) under new operating conditions (A-USC, gas turbines, co- and oxy- combustion). – the verification of large components and welded joints manufacturing (A-USC) of currently state of art materials. – the improvement of monitoring methods. The Macplus Project is an European funded project started in 2011, coordinated by Centro Sviluppo Materiali and parteicpated by the main industrial European stakeholders (as Endesa, EON, RWE, Foster Wheeler, Alstom, Doosan Babcock), aiming to increasing the net efficiency of coal fired plants by improving the performance and reliability of the main critical components identified in: – refractory materials of the combustion chamber (especially for oxy-combustion application), – headers and pipe-works (avoidance of weld Type IV cracking phenomena, working temperature increase), – super-heaters (optimised creep performance, high temperature oxidation, hot corrosion environments), – coated pipes and boiler components able to withstand co-combustion conditions (high temperature oxidation/hot corrosion, erosion-adhesion and wear), – HP and IP steam turbine rotor components and turbine casing operating at very high temperature. During high-temperature operation the materials suffer time dependant degradation by processes such as creep, fatigue, steam oxidation and corrosion. The ability to understand and control the microstructure evolution by optimizing chemical composition, heat treatment and manufacturing processes becomes thus a crucial factor for improving the efficiency of the A-USC power plants. Physical based models of microstructure evolution, instead of statistical ones, may certainly help to control the microstructure of the materials itself but such models need to be validated against results obtained by investigations on long-term exposed materials. Macplus aims at developing and validating such models by microstructure investigation of materials samples after long-term creep tests or after exposure in test rigs. The developed models will be exploited to optimize chemical composition, heat treatment and manufacturing processes in order to attain the correct microstructure for critical components. Oxy-fuel combustion and co-firing of biomass are both attractive options for reducing the CO2 emission levels as an alternative to post combustion techniques. However, it is clear that the high temperature boiler components will face even more aggressive environment. This issue represents a true technical challenge, novel more performing materials solutions are needed. The environment generated the fossil fuel combustion is normally characterized by the presence of sulphates and sulphur, while the presence of HCl or Cl2, due to biomass co-combustion, accelerates the corrosion rate of super-heater alloys. Due to the higher steam temperature, alkali sulphates and coal ash deposits are the predominant origin of fireside corrosion of super-heaters in advanced combustion systems. Several factors, including sulphur, alkali and chlorine content of coal, excess air level and metal temperatures, determine the extent of hot corrosion of super-heater materials in coal-fired boilers. Ash corrosion usually leads to the formation of alkalis-iron-sulphates on the tube surface, with possible accumulation of alkali-chloride compounds produced during the combustion. In oxy-fuel combustion, fuel is combusted in pure oxygen rather than air. This technology recycles flue gas back into the furnace to control temperature and to makeup the volume of the missing N2 to ensure there is sufficient gas to maintain the temperature and heat flux profiles in the boiler. The oxy-fuel combustion generates more aggressive gas environment, CO2 from 15 to 59 vol%, H2O from 10 to 32%, and, for the same fuel, SO2 increases by a factor of 3-4 (0,3-0,9% as a function of the coal type). These changed environmental boundary conditions will affect corrosion life of the materials especially on the water walls and the heat exchanger surfaces. These working conditions constitute a big challenge for the refractory materials inside the combustion chamber, since the refractory thermal barriers need to counteract new extreme working conditions, namely: 1) very high temperature, beyond the resistance of current refractory, high concentrations of CO2 and locally O2, higher concentration of minor components deriving from coal like sulphur compounds; 2) large temperature gradients as a function of both space and time; 3) use of co-fuels (e.g. biomasses) and related different ash composition. Refractory materials According to the current literature, the degradation of refractory materials in combustion reactors can be attributed to two main classes of phenomena: thermo-physical phenomena and chemical corrosion. The refractory corrosion rate depends on the ability of the slag to penetrate in the refractory and to dissolve material’s components. Penetration and dissolution rates depend, on the chemical composition of the refractory components, grain size, porosity and pore size. The same material properties also affect the degradation due to thermal shock and spalling but, typically, in an opposite way. Today the only refractory, which are not likely to fail, are high alumina based materials, with minor amount of additives, like zirconia or chromium oxide, to increase the corrosion resistance However there are still open questions on the possibility of matching corrosion and thermal shock endurance. A solution could be represented by the capability to produce modified refractory bricks with enhanced surface properties against slag corrosion, while retaining the bulk properties. The final goal is the development and realisation of graded bricks, characterised by the surface able to resist against slag aggression at very high temperature (taking into account the more aggressive environment expected in co-combustion conditions), the bulk against violent thermal stresses without breakage. This objective could be achieved by means of a laser treatment applied to melt and re-solidify the brick surface to produce a smooth surface with reduced porosity. The laser treatment can also be extended to the laser cladding and alloying of a pre-positioned ceramic layer with specially formulated chemistry according to bulk composition and the service environment. The bulk properties, unaffected by the surface treatment are developed to withstand the thermal solicitations. Metallic components Looking at super-heater and re-heater components, the activities performed within Macplus project aim to develop new steel and nickel alloys and advanced surface engineered coatings. Modifications of commercial austenitic steels and Ni-based alloys will be achieved by fine tuning of their chemical compositions and optimisation of steel-making process and fabrication routes. These objectives will be reached through the understanding of metallurgical micro-mechanisms governing long-term high temperature behaviour, that includes also response to HT corrosion and oxidation, obtained by means of advanced investigation techniques on available exposed samples from test loops and laboratory tested samples. The achievement of optimized coating/base alloy system, (in terms of cost, operating life and maintainability) depends on the development of the best coating composition able to assure the environmental protection and compatible with the substrate. In particular, reliable fireside coatings with long operating lifetimes have to offer protection against corrosion and erosion. The functionality of the system is dependent on the coating being devoid of open porosity and being resistant to either cracking or delamination The emphasis is on the development of coatings deposited by HVOF (high velocity oxy–fuel) thermal spray technology. HVOF process parameters can be manipulated to deposit coatings offering the desired structure to perform in corrosive environments, i.e. low porosity, high hardness and yield stress coatings, with in conjunction with the CTE (coefficient of thermal expansion) mismatch with the substrate will determine the durability of the coating/substrate system. HVOF technology has also potential to regenerate worn components. Selection of coating systems for fireside corrosion protection have been performed on the base of current status coating systems performances, substrates requirements and operating environment. In particular, Al2O3 and Cr2O3 former coatings have been selected as the best candidates for fireside corrosion protection. Attention has been focused on evaluation of interdiffusion phenomena between coating and substrate. For systems supposed to operate at high temperature, depletion of coating/substrate system performances appear as a consequence of composition and microstructure modification generated by interdiffusion. In order to minimize migration of key elements between coating and substrate, its main driving force, i.e. composition differences between coating and substrate, have to be reduced. In this framework, both single layer and multilayer coatings have been proposed as innovative solutions for fireside corrosion protection. Single layer systems will be based on M(Co/Ni) CrAlY and on FeCr HVOF coatings. An Al based top layer will be adopted for the multilayer system development. Post deposition heat treatments have to be planned in order to avoid uncontrolled substrate microstructure modifications. Testing Facility Both refractory and superheater components will be tested at Ciuden testing facility in Ponferrada (Spain). The test plant is a CFB (Circulating Fluidized Bed) Boiler (Foster Wheeler Design) able to operate either under conventional combustion with a capacity of 15MWh or under oxy-combustion conditions reaching up to 30MWh, size sufficient to allow the scaling of the results to commercial scale. di Egidio Zanin, Project Leader energy & transport presso Centro Sviluppo Materiali

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