Corrosion data on superheater alloys for waste and biomass boilers: Specific effects of H2O and deposits
Several boiler tube materials including 2.25Cr-1Mo, X10,X20, AC 66, Alloy 625, Esshete 1250, TP347h and San 28 were tested for 360 hours under various corrosive atmospheres (N2-O2-HCl-H2O-CO2)at 500 - 600°C with and without filter and cyclone ashes. Thereby, specific mass gains of samples, chemical changes of ashes and the corrosion products were investigated resp. determined. High temperature exposure tests showed that the mass gains for the samples covered with cyclone ashes were higher than for those covered with filter ashes and for uncovered samples. In the case of HCl addition, mass gains were increased by increasing the water content and mass gains with covering of cyclone ash were higher than those with filter ash. Nickel-based alloys exhibit most corrosion resistance. Moreover, the formation of gaseous chlorine by the reaction of alkali-chlorides with metal oxides was thermodynamically calculated by the FactSage-program and further calculations were to conduct in System (Alkalis- HCl- H2O- O2- CO2). Local reactions of KCl particles with bare metal surfaces and oxide scales were studied after deposition of KCl particles by thermophoretic migration. It was found that adhesion of particles on the bare metal surface is much better than on a preoxidized material. Furthermore local reaction (melt formation) between the metallic surface of pure iron and the KCl particles could be observed after short time reaction of a covered sample at 300°C.
Superheater corrosion in a fluidised boiler burning biomass with a steam temp of 550 deg C is reduced by (a) use of a suitable additive (ChlorOut) and (b) use of a suitable material (austenitic stainless steel) One of the major drawbacks to the combustion of 100% wood and waste wood fuels in power station boilers is the increase in the fouling and corrosion of superheaters, which increases operation and maintenance costs. Wood fuels have a high content of potassium and chlorine, but they contain very little sulphur compared to fossil fuels. To combat corrosion, boiler tests have been performed with ammonium sulphate, which is sprayed into the flue gases after combustion but before the superheaters and effectively converts gaseous KCl into potassium sulphate, K2SO4. This is much less corrosive than KCl. Long-term probe measurements showed that the corrosion rates were reduced by 50%. Deposit growth rates were also reduced by about 50%. This concept, together with an instrument for measuring alkali chloride levels in the flue gas has been patented and is known as “ChlorOut”. Probe tests with different steels showed that superheater corrosion can also be greatly reduced by the choice of the right material. The stainless steel Esshete 1250, which contains only 15% Cr, performed very well in all the boiler tests. A complete set of superheaters for a 100MW combined heat and power boiler costs about 1.5 MEuro. The durability of superheaters is thus an important factor in determining the long-term production costs. Unplanned outages due to leaking superheaters are also very expensive. As well as causing corrosion problems, the build-up of deposits reduces the heat uptake to the superheaters, which leads to lower efficiency. The use of ammonium sulphate additive, which approximately halves the corrosion and fouling rates in wood-fired boilers, is thus expected to greatly reduce the cost of producing electricity from biomass. This means that it becomes more technically viable and the amount of electricity produced from biofuels will therefore increase.
Fuel and process optimisation is needed when substituting ordinary fuels with solid biomass fuels. As fuel blending changes the behaviour of fuels during combustion, optimisation of the proportion of fuels in the fuel blend is required. Several approaches are available for the optimisation work. Pilot-scale reactors have been applied in studying the interaction of sulphur -chlorine- aluminium silicate species in order to find optimal shares of fuels in terms of safe and efficient combustion. Pilot-scale tests have been supported by studies where biomass combustion and its effect on boiler fouling and corrosion has been monitored with temperature controlled probes at large-scale power plants. With the aid of the measurements, analysis and modelling methods, the influence of fuel blend and fuel-ash properties on deposit formation, corrosion and heat transfer has been identified.
Corrosion data on superheater alloys for waste and biomass boilers: Specific effects of CO2 and deposits
Corrosion data collected from tests on superheater materials (X10, X20, 2.25Cr1Mo, AC66, Sanicro28, Esshette 1250 TP347H) in simulated waste/biomass environments in the vicinity of superheaters have shown a significant influence of the concentration of CO2 in the environment on corrosion behaviour. The tasks of establishing the individual influences of CO2 and H2O were divided between 2 CORBI partners (MPIE addresses the effect of H2O). Using a gas composition based on biomass combustion conditions containing N2+22%H2O+5%O2+xCO2, where the concentration of CO2 was varied in the range 0 to 25 vol. %. The experiments were carried out at isothermal temperature ¿ 535C for the samples without deposit and with (Cl-containing filter/cyclone ash deposits). The results showed that: - Corrosion rate, for the alloys without the deposit, increase with increasing CO2 content, especially for the ferritic steels. - Corrosion rate for samples with the deposit increase significantly and in this case increased internal oxidation was observed. - Various carbides of metallic alloying elements become less stable at the oxide scale-metallic alloy phase boundary with increasing partial pressure of carbon dioxide. - Carbides and oxides of various alloying elements (Fe, Cr, Mo, Mn...) are unstable in their mixtures with alkali metal carbonates or model ash deposits. - However, some of the mixed alkali oxide-transition metal oxides that are predicted to be formed as reaction products, may dissociate under atmospheres with appreciable carbon dioxide and/or water vapour content.
To explain excessive steel tube wastage occurring at metal temperatures below the first melting point of the deposits, it has been suggested that accelerated corrosion may be due that alkali chlorides react with metal oxides and oxygen to form complex alkali-transition metla oxides, such as alkali ferrites, and chlorine. The main objective of the present study was to evaluate in detail to which extent this postulation may apply in biomass and waste combustion. Modelling experiments were performed exposing metallic specimens to synthetic alkali chlorides, carbonates and sulphates or to their mixtures, or to ashes from biomass combustion in different atmospheres. Laboratory studies were also performed using cooled probes allowing condensation and subsequent evaporation of the salts. Thermodynamic and corrosion mechanistic analyses were performed to evaluate the role of alkalis and chlorine in the corrosion. - Themodynamic analysis performed predicted that alkali chloride-iron oxide scale reaction to form alkali ferrites (NaFeO2, KFeO2), as is commonly postulated when applying "active oxidation" theory, may not occur and do not explain the accelerated corrosion phenomena in the presence of alkali chorides. - Instead, complex mixture phases of alkali alkali oxides and iron oxides may play an important role in the corrosion mechanism. Accordingly, compositional features of experimentally produced corrosion products on the low alloy steel specimens in the presence of salts of alkali metals (Na, K) could be crossly explained using available experimental phase diagrams and other published material data for Na-Fe-O and K-Fe-O systems in support to theoretic thermodynamic calculations performed. - High alkali metal activity together with the presence of chloride in the superheater deposits, as may be the case in biomass and refuse co-combustion, is especially harmful. Calsium chemistry may play a special role in "active oxidation" of heat transfer surfaces at the superheater area in waste and biomass combustion when water vapour concentration in flue gases is high and that of sulphur dioxide low.
Corrosion of superheater tubes in power plant boilers was performed with air-cooled probe, where material temperatures can be adjusted and monitored. Specimens of different materials can be attached to the probe and their corrosion behaviour and deposit formation can be analysed. Analysis is done to carefully prepared metallurgical cross-sections by SEM/EDX. Deposits can be analysed separately. This method is applicable especially to material screening and material testing for difficult and changing boiler conditions, such as biofuel fired boilers and waste incinerators.
The development of a post-processing tool of CFD code aimed at simulating alkali-salt deposition in pf-boiler has been completed. The code includes well-assessed mechanisms available in literature: alkali–chlorine reactions, models for alkali adsorption from silicates and alkali salt deposition mechanism on heat transfer surfaces. The predictive capability of the code was assessed by full-scale measurement and the tool was used to foreseen deposition rate and deposit composition on the heat-exchangers (boiler, SH, RH) during coal combustion as well as waste co-combustion. As the industrial and economic relevance of the utilisation of biomass and waste in energy production is growing, the assessment of a comprehensive mechanism of alkali-salt deposition can be a powerful tool to evaluate operation flexibility of existing power plant to burn different fuel mix (power generation utilities) and to design and select boiler materials with respect to corrosion problems (manufacturing).
ENEL objective for this project is to assess the feasibility of RDF-coal co-combustion in pf-boiler without affecting thermal performance or damaging the existing components and respecting, at the same time, the emission limits fixed. Deposit measurements and its characterisation have been carried out during baseline and co-firing tests in pilot-scale unit and full-scale power plant at different thermal load and fuel blends. Full-scale test (320MWe pf-boiler) highlighted the increase of ash deposition on tubes of SH and the chemical fractionation analysis of the deposit showed the increase of chlorides content. This may produce the increase of corrosion potential of the deposit; long term corrosion test were not performed because of feeding and supply problems of RDF and the short term test did not shows any deterioration of the boiler. The full-scale data were used to assess the predictive capability of the alkali salt deposition models developed within the project.
Fine particle formation mechanisms were determined during combustion of bark and sludge in circulating fluidised beds. The fine particle formation was studied both in full-scale combustion, and in a pilot-scale reactor. The particles were found to form by the same mechanisms in the two systems. In addition, the effect of increasing the amount of sulfur and chlorine in the two systems on fine particle formation was determined.