Study of Corrosion in a Biomass Boiler
Biomass plants, apart from producing energy, help to reduce CO2(g) emissions. One of the biggest problems for their development is superheater corrosion due to fuel corrosivity, especially of the straw. This limits both the temperature of the vapour and also the effectiveness of the plant. In order to know more about the reactions which happen inside the boiler of biomass, thermodynamic calculations using software (HSC Chemistry) have been carried out. Field tests have been carried out in the Sangüesa Biomass Plant in Navarra (Spain): determination of the types of oxides and the deposits formed on the superheaters tubes as well as a program to measure temperatures. Finally, the global results are discussed.
Currently, much attention has been drawn towards the burning of biomass for power production. One of the main reasons for this is the concern regarding the global warming caused by carbon dioxide (CO2) emissions. Biomass that is produced at the rate it is consumed is considered CO2 neutral because during growth it accumulates the same amount of CO2(g) by photosynthesis as it releases during combustion.
Biomass includes a large variety of different fuels with different chemical composition and combustion characteristics. Wheat straw contains large amounts of potassium and chlorine, which are very undesirable due to their high corrosion power, so straw combustion is associated with some corrosion problems which are not encountered in coal-fire plants. The burned wheat straw studied in this paper contains typically 1.0 wt% K, 0.4 wt% Cl, and 0.15 wt% S. Its sulphur content influences the corrosion mechanism.
During combustion, potassium chloride and SO2(g) are released in the flue gas and through the condensation and deposition processes they will result in the formation of superheater deposits rich in potassium chloride and potassium sulphate.
In actual power stations, it proved to be difficult to relate specific parameters to corrosion observation from test superheaters, as so many of the parameters, such as temperature, ash deposit composition, and gas composition, were continually fluctuating.
There are many field and laboratory studies [1–16] which investigate the mechanics of corrosion due to straw burning, and some corrosion models based on chlorination have been suggested. Also, the use of chemical additives has been proposed to minimize this problem [17–21] in combination with mathematical models used to simulate the deposition behavior based on computational fluids dynamics .
To understand the corrosion mechanism of corrosion that superheaters tubes suffer it is necessary to know more about chemical reactions that happen in the boiler. To do so, a valid tool is the realization of thermodynamic calculations [23–25].
The focus of this work is the realization of thermodynamic calculations starting from the reactions that take place among the metals of the superheaters with the combustion atmosphere and the deposits that are formed on the tubes.
We have also carried out field tests. The Sangüesa Power Station belongs to Acciona Energia and is located in Navarra (Spain). It has a grate-fire boiler with a power of 25 MWth.
The most corrosive conditions in the plant happen in the third superheater which is in the third pass and has an outlet steam temperature of 540°C. The temperature of the metal is calculated to be at 570°C. The gas temperature at this point was 850–900°C. The fuel used in the power station was only straw; straws studied will cause high temperature superheater corrosion when fired alone in power plants (with steam temperature higher than 420°C). Fly ash from straw firing on grate was rich in the volatile elements K, Cl, and S .
To complete the thermodynamic calculations, the deposits formed on the superheaters tubes as well as the oxides formed on the tubes have been characterized. The final objective of this work is to contribute to the knowledge of the corrosion that superheaters tubes suffer in the biomass plants and, in this way, contribute to the development of the biomass as industrial fuel for obtaining energy.
2. Experimental Procedures
2.1. Thermodynamic Calculations
The program “HSC Chemistry for Windows”  has been employed to calculate equilibrium compositions in several sets of systems related to hot corrosion reactions.
To calculate an equilibrium composition the HSC program redistributed the input elements among the various species such that the total Gibbs energy of the system was minimized, subject to the constraints that all interacting species were in equilibrium with each other and that the mass of the elements was conserved. Thermodynamically, this corresponded to a closed system calculation.
It is important to bear in mind the limitations of these calculations obtained with this program. As a simplification, activity coefficients were taken as 1.0. On the other hand, there is no presumption about whether the salt phase is liquid or solid. Obviously, the issue of whether a liquid salt phase exists at the given temperature is important to the kinetics of corrosion, but not to the calculation of an equilibrium composition.
Also the solubility in the solid state has not been considered. For example, placing Fe and Cr into the “metal” phase is computationally equivalent to assuming they are soluble and exist as a single phase.
The first step in the equilibrium calculation is the definition of the system. To do so, the calculations have been carried out on a small scale although the results are considered valid for bigger scales. The chemical composition of the alloy AISI 347 CG has been used to introduce the values of the metallic elements (Table 1).
The composition of the gasses has been based on the representative composition of the atmosphere of biomass combustion (Table 2).
The selected salt has been KCl(s) because it is the one found in the greatest quantities, closest to the surface of the tube. The quantity of salt introduced in the calculus has been the following one: KCl(s): 1 g/74,5 g = 1,34 × 10−5 k moles. The species obtained in the calculations were grouped into phases: gas, metal, oxide(s), and salts.
In the calculations, the effect of the variation of the temperature and of the quantity of HCl(g) and Cl2(g) on the different chemical compounds has been proved. These factors are considered the most influential in relation to the corrosion mechanism.
The first variable is the temperature; in this way the stable compounds are observed in the range of temperatures between 300°C and 900°C (with increments of 86°C) analyzing especially the temperature of 557°C. The second and third variables are the increments of HCl(g) and Cl2(g) (maintaining constant the temperature of 600°C) that are related, since both suppose a contribution of chlorine; the quantity of HCl(g) and Cl2(g) has been increased in 8 steps of the same magnitude (3.1 × 10−8 k moles) until reaching a percentage in the gassy mixture of 4,7%; in this way, the effect of the possible variations of the composition of the biomass and in consequence of the combustion gasses has been proved.
2.2. Deposits Characterization
Deposits formed on the superheater were studied as composition can yield interesting information about the corrosion mechanism. Deposits were collected from the boiler superheaters during one plant stop.
Their chemical composition by scanning electron microscopy combined with energy dispersive X-ray analyses (SEM-EDX) and bulk chemical analyses (ICP-AES) was performed. The melting point of the inner layer was determined by differential scanning calorimetry (DSC).
2.3. Oxides Characterization
The structure of the oxides formed on the superheater tubes was determined by means of X-ray diffraction.
2.4. Temperature Measurement
Due to the influence of the temperature in the corrosion mechanism a program of the temperatures. Measurement was carried out. Type K of NiCr-Cr thermopars were used and they were protected with SiC powder.
3. Results and Discussion
3.1. Thermodynamic Calculations
3.1.1. Influence of the Temperature (300°C–900°C)
The results of the thermodynamical calculations of the solid and gaseous species appear in Table 3 versus the influence of the temperature.
Gaseous Species. Three of the gaseous species introduced in the calculations (N2(g), CO2(g), and H2O(g)) are stable in the whole range of temperatures, while O2(g) and SO2(g) are consumed in different reactions with the rest of elements to form different compounds. The forming of metallic chlorides increases as the temperature increases. FeCl2(g) and CrCl2(g) present a quantity of 7.0 × 10−18 k moles and 3.1 × 10−19 k moles, respectively, at 557°C while the formation of NiCl2(g) is very low (7.9 × 10−19 k moles).
Finally, it is should be noted that while Cl2(g) wastes react completely in the different reactions, the quantity of HCl(g) is increased with the increasing temperature.
Solids Species. The metallic chlorides are formed in low quantities; at 557°C CrCl2(s) presents 4.3 × 10−11 k moles, FeCl2(s) 6.0 × 10−14 k moles, and only a negligible quantity of NiCl2(s) is formed. Among the metallic oxides, Cr2O3(s) is the most stable with a great difference with respect to the test. At 557°C, 1.6 × 10−7 k moles are formed and it stays constant in the whole range of temperatures. The FeO(s) is the oxide of more stable iron presenting 4.0 × 10−13 k moles at 557°C, and their formation is favored with the temperature (3.0 × 10−11 k moles to 900°C). NiO(s) is formed in a very small quantity of 6.7 × 10−17 k moles. Finally, Cr2FeO4(s) is an oxide that it is formed in a considerable quantity of 9.3 × 10−10 k moles.
The quantity of metallic elements remains constant in the whole range of temperatures. The preference of the nickel to form NiS(s) is worth highlighting, since it forms 8.2 × 10−10 k moles. The quantity of KCl(s) remains constant in the whole range of temperatures.
3.1.2. Influence of the Quantity of HCl(g) (5.0 × 10−9–1.8 × 10−7 k moles) (T = 600°C)
The results of the thermodynamical calculations of the solid and gaseous species appear in Table 4 versus the influence of the quantity of HCl(g).
Gaseous Species. Three of the gaseous species of departure (N2(g), CO2(g), and H2O(g)) are stable. Metallic chlorides are present in low quantities, but their formation is favored with the increment of HCl(g). FeCl2(g) and CrCl2(g) present a quantity of 1.5 × 10−14 k moles and 7.1 × 10−16 k moles, respectively. Finally, it should be noted that while Cl2(g) is consumed almost in its entirety in the different reactions, the quantity of HCl(g) remains constant.
Solids Species. The metallic chlorides are present in low quantities, but their formation is favored with the increment of HCl(g); CrCl2(s) presents a maximum quantity of 1.6 × 10−8 k moles, FeCl2(s) 3.3 × 10−11 k moles, and NiCl2(s) 1.6 × 10−15 k moles. The formation of metallic oxides is not affected by the variation of HCl(g); Cr2O3(s) is the most present with 1.6 × 10−7 k moles; the FeO(s) is the most stable iron oxide of more stable iron with 8.8 × 10−13 k moles and NiO(s) is formed in a minimum quantity of 1.8 × 10−16 k moles. Finally, Cr2FeO4(s) is a mixed oxide with an appreciable quantity of 1.3 × 10−9 k moles. The quantity of metallic elements remains constant in the whole range of concentration of Hcl(g). The preference of the nickel to form NiS(s) is worth highlighting since it forms 8.2 × 10−10 k moles. The quantity of KCl(s) remains constant in the whole range of temperatures.
3.1.3. Influences of the Quantity of Cl2(g) (5 × 10−9–1.8 × 10−7 k moles) (T = 600°C)
Gaseous Species. Three of the gaseous species introduced in the calculations (N2(g), CO2(g), and H2O(g)) are stable in the whole range of temperatures, while O2(g) and SO2(g) react completely with the rest of the elements, to form different compounds. The formation of metallic chlorides increases with the increment of Cl2(g). FeCl2(g) and CrCl2(g) present a maximum quantity of 1.5 × 10−14 k moles and 7.1 × 10−16 k moles, respectively. The formation of NiCl2(g) is negligible. Finally, it is should be noted that while Cl2(g) is consumed almost in its entirety in the different reactions, the quantity of HCl(g) is increased with the concentration of Cl2(g).
Solids Species. The metallic chlorides are present in low quantities, but their formation is favored with the increment of Cl2(g); CrCl2(s) presents 1.6 × 10−8 k moles, FeCl2(s) 3.3 × 10−11 k moles, and NiCl2(s) 1.6 × 10−15 k moles.
The formation of metallic oxides is not affected by the variation of Cl2(g); Cr2O3(s) is the most present with 1.6 × 10−7 k moles while the FeO(s) is most stable iron oxide of more stable iron presenting 8.8 × 10−13 k moles. NiO(s) is formed in a minimum quantity of 1.8 × 10−16 k moles. Finally, Cr2FeO4 is a mixed oxide with an appreciable quantity of 13.10−9 k moles. The quantity of metallic elements remains constant in the whole range of temperatures. The preference of the neither to form NiS(s) is worth noting since it forms 8.2 × 10−10 k moles. The quantity of KCl(s) remains constant in the whole range of temperatures.
3.2. Deposits Characterization
In the figures one can observe the appearance of the deposits after 15000 h of operation. The superheaters tubes were completely covered by a 2–12 cm-thick layer of dark brown deposit. The deposit contained three distinct main layers: a thin dark brown outer layer, a thick white porous intermediate layer, and a complex inner thin layer (Figures 1 and 2). These three layers were studied separately (Table 5).
From the above data the three layers can be characterized as follows. The inner layer contains mainly potassium chloride (Figure 3(a)). There is a sublayer of iron oxides which come from the protective iron oxide scale of the superheater. There is no potassium sulphate. If sulphur is present in the deposit, it may react with potassium chloride, releasing chlorine .
In the inner layer, although potassium chloride is the principal component, we also find some potassium sulphate (Figure 3(b)). Fe-rich layers were observed indicating spallation of the iron oxide scale. At the outer part of this layer, the silicates composition increases. This fact agrees with similar studies carrying on oil-palm empty fruit bunch (biomass similar to straw related to content in Cl and K), where deposits were a thin inner white layer of KCl that condensate covering the entire probe surface and an outer grey layer consisting of KCl and Si-rich particles .
The outer layer is quite dense, which indicates that the layer has been molten. It is a compound of two phases (Figure 3(c)).
3.3. Studies of the Oxides
No peaks that can be associated with FeO(S), Cr2o3(s) nor to NiO(s) exist. The information on the diffraction of the phases has been obtained starting from the JCSP records (Figure 4).
The largest oxide is Fe2O3(s) and in smaller quantities Fe3O4(s). No peaks that can be associated with FeO(s) exist, Cr2O3(s) nor to NiO(s). Finally, the characteristic peaks of KCl(s) are observed.
3.4. Studies of the Temperature
Regarding the temperatures measured in superheaters 3 and 4, the great difference of temperatures is worth highlighting according to the localization of the thermopar. In any case, the temperatures (435–515°C) are lower than that expected, except for the thermopar number 12 (540–545°C).
4. Results Analysis
The results of the analysis of the corrosion of the superheaters tubes showed that the mechanism of more probable corrosion is that of active oxidation, which agrees with the literature.
As a final summary of the thermodynamic calculations carried out, one can affirm that of the three studied factors, Cl2(g) is the one that most favors the formation of solid and gaseous compounds with chlore. After accelerated oxidation is initiated by the presence of potassium, the presence of chloride is essential for it to continue [1, 27]. Chlorine is a small molecule which is likely to be able to diffuse through a deposit layer, rendering the protective oxide layer on the tube surface ineffective . A higher chlorine concentration in the feed would result in higher tendency of deposition .
The quantity of KCl(s) has remained constant under all the conditions, from which it is deduced that it has not reacted with any element .
The most stable iron oxide of the plant has been Fe2O3 and Fe3O4. However, regarding the mechanism of active oxidation, the rate of corrosion has been observed to be low for stainless steel AISI 347 CG (17). Also, hot corrosion is not observed because the salts do not melt at these low temperatures (450–500°C). On the other hand, it is necessary to bear in mind the metallurgists factors since the martensític steel X20CrMoV suffered a high speed of corrosion  and was changed at 15.000 h while the rest of austenitic steels haven been a good behaviour.
Another consideration is that excess alkali of biomass-based fuels may derive to KOH (ads.). Presence of KOH (ads.) on the heat transfer surfaces may be an important factor in the corrosion mechanisms in biomass-fired boilers .
Cl2(g) is the main factor that favors the mechanism of active corrosion. The deposits formed on the superheaters tubes are formed by three layers of different chemical composition. The nearest to the tubes is mainly a compound of KCl(s) and it has a melting point of coalition of 575°C.
At current work temperatures of the plant the presence of KCl(s) on the tubes is not dangerous from the point of view of the corrosion.
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