Local Reactions Between Nacl and Kcl Particles and Metal Surfaces

Local reactions between NaCl and KCl particles and metal surfaces

Sung Chul Cha and Michael Spiegel

Max-Planck-Institut für Eisenforschung GmbH (MPIE), Max-Planck-Str. 1, D-40237 Düsseldorf, Germany, [email protected]

In bio-fuel combustion, the efficiency in electricity production is limited through corrosion of the superheater tubes induced by alkali chlorides in the deposits. Alkali chloride particles are formed during biomass combustion and transported via aerosols or in the vapor phase within the combustion gas, subsequently depositing on the metallic surface or on the already formed oxide layer. By reaction with the metal or the oxide layer the oxidation process is accelerated and metal is heavily consumed. In this paper, local reactions of NaCl particles with pure iron surfaces have been investigated in order to compare with results of former studies on the local reactions of KCl particles. Prior to short term oxidation at different temperatures in N2-20 O2-0.05 vol. % HCl atmospheres, bare iron surfaces and pre-oxidized samples were deposited with micro-sized NaCl particles by thermophoresis. The reacted samples were subsequently investigated with FE-SEM and XPS analysis. Especially, NaCl particles react with iron surface at 300°C, depending on the gas phase composition. In N2-O2 atmospheres very local reactions take place, whereas the addition of 500 vppm HCl leads to complete coverage of iron surface with chloride. In addition, the reactions of KCl particles on nickel surfaces at 500°C were investigated for comparison with KCl- and NaCl particles on iron surfaces at 300°C.

Keywords: Alkali chloride, Biomass, Kinetics, Oxidation, Thermophoresis

1. Introduction

Super heater tubes used for incinerators of municipal waste or advanced biomass-fired plants tend to suffer from rapid tube thinning and early failure when the metal temperature is above 300°C. These fuels include a large variety of chemical compositions and combustion characteristics. During combustion, gas-corrosion between materials (metal, ceramics) and gas-phases (O2, HCl, SO2), deposit-corrosion between materials and deposits (chloride, sulfates, silicates) and deposit-gas-reactions between gas-phase and deposits can be occurred. In following, rapid corrosion results from the complex chemical reactions between materials, gaseous species and deposits [1-4].

Biomass-fired electric generating stations are assuming increasingly prominent positions in energy and environmental issues [5-6]. The targets for energy production in the EU are to increase the total amount of energy originating from renewable sources up to 12 % until 2010 and, to increase the efficiency of power plants. On this way, biomass can be expected to have a 20 % share of the current primary energy supply in the long term [7]. In bio-fuel combustion corrosion of the superheater tubes induced by alkali chlorides in the deposits limits the efficiency in electricity production. The corrosion depends on e.g. the compositions of the flue gas and deposits as well as the amount of molten phase in the deposit. The most severe corrosion generally occurs in the vicinity of the edge of the main deposit crest [8]. During biomass combustion alkali chloride particles are formed and transported via aerosols or in the vapor phase within the combustion gas, subsequently depositing on the metallic surface or on the already formed oxide layer. By reaction with the metal or the oxide layer the oxidation process is accelerated and metal is heavily consumed.

Potassium chloride is one of the aggressive species in the deposits in boilers burning biomass. Alkali chloride can form e.g. alkali chromate thus consuming a chromia layer [4]. Biomass has a high content of alkali metals and chlorine, and most biomass fuels contain very little sulfur. The majority of potassium is released into the gas phase and is present in the gas phase mainly as potassium chloride and potassium hydroxide during combustion. In biomass combustion, potassium chloride is the main chloride. The chlorides in the deposit may form low-temperature melting eutectics, which may flux the oxide layer. And the formation of gaseous chlorine species could originate from sulphate formation from alkali chlorides or by reaction between the chlorides in the deposit and the metal scale [9].

The chlorides in the deposits react with the scale at 500°C, form chlorine that enters the scale and causes accelerated oxidation due to the formation of FeCl2 (s) at the scale/metal interface, its evaporation as FeCl2 (g) and its oxidation to Fe2O3 at the scale surface followed by chlorine partially returning into the scale. This leads to the formation of a porous unprotective scale by a process called ‘active oxidation’ catalyzed by chlorine [3]. During combustion of biomass, the chlorine stems from the reaction of KCl, Fe2O3 and O2 giving K2Fe2O4 (Eq. 1) and from the reaction with NaCl, giving Na2Fe2O4 (Eq. 2).

2KCl + Fe2O3 + 0.5O2 = K2Fe2O4 + Cl2 (1)

2NaCl + Fe2O3 + 0.5O2 = Na2Fe2O4 + Cl2 (2)

Table 1 shows the melting temperatures of different species and binary salt systems. Potassium chloride has a melting point of 772°C, but can form low-temperature eutectics together with several other substances. Sodium chloride can melt at 801°C, but can also form

low melting eutectics at 156°C with FeCl3, and at 370°C with FeCl2 [10-12]. Molten phases increase the corrosion rate, and the presence of a liquid phase on the surface of the metal leads to accelerated corrosion.

Table 1. Eutectic temperatures [°C] of different species and binary salt systems

KCl-system NaCl-system KCl- FeCl2 340 NaCl- FeCl2 370 KCl- FeCl3 202 NaCl- FeCl3 156 KCl- NiCl2 508

The local reactions of KCl particles on iron surfaces were studied earlier [7]. Iron samples with deposits of KCl were exposed to N2-20 vol. % O2 and N2-20 O2-0.05 vol. % HCl atmospheres for short times at 300°C. In an oxidizing atmosphere, deformation and local spreading of the KCl particles was observed, probably by melt formation in contact with the metal. Oxidation with an HCl addition led to a significant increase of the chlorine and oxygen content of the KCl-deposited sample surfaces. The increased chlorine potential of the gas resulted in an increase in FeCl3 formation, thus leading to the formation of low-melting eutectics in the KCl-FeCl3 system. A chlorine-rich scale was formed on top of the metal surface and the chloride particles are embedded in the scale. The results showed that especially in the presence of HCl in the combustion gas, KCl particles lead to the formation of a chlorine-rich reaction layer, which would promote the adhesion of further KCl particles from the flue gas. In this paper, the local reactions of NaCl particles on iron surfaces were investigated for comparison with those of KCl particles. Therefore, same procedure by KCl reactions, e.g. pre-oxidation, thermophoretic deposition and oxidation, are conducted for NaCl particles. Afterwards, the local reactions of Ni-surfaces with KCl deposition at 500°C were compared to those Fe-surfaces with NaCl deposition in HCl containing atmospheres.

2. Experimental Technique

The deposition of the homogenous and micro-sized alkali chloride particles onto the metal surface was carried out by impactor and thermophoresis. Particle deposition can be divided into four steps, i.e. atomization, drying, impacting and thermophoretic deposition. Nitrogen as a carrier gas flows through the filter, the regulator and the dryer and picks up droplets from the salt solution reservoir through the atomizer. The droplets are carried into a heater in order to increase the efficiency of the drying process, and then to the diffusion dryer. The dried and heated particles then move to the cascade impactor and are separated into different size fractions between >1.5 and 0.05 µm. Subsequently, the particles flow to the thermophoretic depositor through the heater. This has a temperature gradient of 45°C and the small salt particles deposit on the sample surface. In this temperature gradient the particles move in the direction of decreasing temperature and thus to the cooled sample surface. The alkali chloride deposition equipment is illustrated in Fig. 1.

Fig. 1. Equipment of alkali chloride deposition [7].

Before deposition of alkali chloride particles, pure iron and nickel specimens of size 10 × 10 × 2 mm were ground by 600 grit SiC-paper and polished to 3 µm. The constant parameters in KCl and NaCl deposition were a salt molarity of 0.1 mol/l (KCl: 7.455, NaCl: 5.844 g/l), 4 impactor steps, a temperature gradient of 45°C, the use of 1 l of water, a pressure of 3.3 bar and a deposition angle of 90°. After the tests, the samples were investigated by various techniques, e.g. FEM, SNMS and XPS.

The characteristics of samples prepared in this work are shown, as described in the following. The oxidation of samples No. I-II was done at 100°C with 10 min. in the atmosphere of N2- 20 vol.-% O2, Table 2. At first, samples No. I were deposited on each iron surface and subsequently oxidized at 100°C for 10 min. In addition, samples No. II were primarily pre- oxidized for 1 h at 300°C in N2- 20 vol.-% O2, particles were deposited, and subsequently oxidized in order to indicate the effect of pre-oxidation.

Table 2. Samples No. I-II for comparison between KCl and NaCl particles

Oxidation Sample Pre- Deposition time temp. atmosphere No. oxidation [min.] [°C] [vol.-%] a KCl I - 10 100 b NaCl N - 20 O a KCl 2 2 II 1 h, 300°C 10 100 b NaCl

To identify the effect of time, sample No. III were oxidized for 20 min. and sample No. IV for 40 min. at 300°C after KCl and NaCl deposition, Table 3.

Table 3. Characteristic of samples No. III-IV for identification of the time effect

Oxidation Sample Deposition time temp. atmosphere No. [min.] [°C] [vol.-%] a KCl III 20 300 b NaCl N - 20 O a KCl 2 2 IV 40 300 b NaCl

In addition, the oxidation of samples No. V was carried out at 300°C in an atmosphere with HCl addition to investigate the effect of HCl with and without NaCl particles. The untreated iron sample (No. Va) was oxidized for 10 min. without any deposition and Vb was reacted for 20 min. with deposited NaCl particles, Table 4.

Table 4. Characteristic of samples No. V

Oxidation Sample Deposition time temp. atmosphere No. [min.] [°C] [vol.-%] a no deposits on Fe-surface 10 V 300 N - 20 O - 0.05 HCl b NaCl on Fe-surface 20 2 2

Finally, sample No. VI-VII were oxidized for 10 (VI) and 40 min. (VII) in oxidizing atmosphere with HCl addition. The NaCl deposited iron samples (VI-VIIa) were oxidized at 300°C and the KCl deposited nickel samples (VI-VIIb) were oxidized at 500°C, Table 5. Pre-investigation of local reactions of KCl particles on Ni-surfaces at 300°C showed no visible reactions and oxidations of Ni-surfaces by FE-SEM. Therefore, the reaction temperature for Ni-surfaces are selected up to 500°C.

Table 5. Characteristic of samples No. VI-VII

Oxidation Sample Deposition time temp. atmosphere No. [min.] [°C] [vol.-%] VI a NaCl on Fe-surface 300 10 b KCl on Ni-surface 500 N - 20 O - 0.05 HCl VII a NaCl on Fe-surface 300 2 2 40 b KCl on Ni-surface 500

3. Results and Discussion

The SEM images of sample No. I (Fig. 2a.) show the relatively homogeneous deposition of KCl and NaCl particles with a size distribution between 0.05 and 0.3 µm, but KCl particles are in square form and NaCl particles in round form. The 10 min. oxidation at 100°C has no significant influence on the particle behaviors, e.g. size, form, K-, Na- and Cl-content. In comparison of oxygen content of sample surfaces between non-oxidised and oxidised sample at 100°C for 10 min. showed no differences on both surfaces (O-content of <0.5 wt. %). SEM images of Fig. 2b show the surfaces of pre-oxidised sample. The particles appeared to be smaller (10-100 nm) and no reaction zone was observed. Additionally, the adhesion of KCl

and NaCl particles was more effective on the metal surface, proved by chemical and EDX- analysis. The relative similar behavior of KCl- and NaCl particles are found after reaction at 100°C on non pre-oxidised and oxidised surfaces.

a) No. I (depos.- oxid. (100°C, 10 min.), KCl (l.), NaCl (r.)

b) No. II (pre-oxid.- depos.- oxid. (100°C, 10 min.), KCl (l.), NaCl (r.)

Fig. 2. SEM images of sample No. I-II

The surfaces after reaction at 300°C in N2-20 vol. % O2 showed deformed KCl particles in round and NaCl particles in square form, spreading on the metal surface (Fig. 3a). This localized reaction was more visible after longer reaction time (Fig. 3b). It was observed Cl- content did not increase significantly in comparison with the deposited but not reacted sample. However, more oxide has formed in the vicinity of the chloride particles. The Cl- contents from KCl and NaCl deposited surfaces are relatively constant, but the Na-contents of NaCl deposited samples were higher than K-contents of KCl deposited samples, probably concerning their different eutectic temperatures.

a) No. III (depos.- oxid. (300°C, 20 min.): KCl (l.), NaCl (r.)

b) No. IV (depos.- oxid. (300°C, 40 min.): KCl (l.), NaCl (r.)

Fig. 3. SEM images of sample No. III-IV

Fig. 4 shows the reacted surface without any deposition after oxidation for 10 min. and the the reacted surface after NaCl deposition and oxidation for 20 min. in N2-20 vol. % O2- 500 vppm HCl. On the NaCl-deposited and reacted sample surface, the Cl-amount was much higher than on the sample surface reacted in HCl-containing gas only. This effect was already observed on the KCl deposited and reacted iron surface [7].

Fig. 4. SEM images of samples No. V: l. (no deposits- oxid. with HCl (300°C, 10 min.), r. (NaCl depos.- oxid. with HCl (300°C, 20 min.)

The reacted sample surfaces with HCl gas after NaCl deposition on iron surfaces and after KCl deposition on nickel surfaces are shown in Fig. 5. After 10 min. oxidation (Fig. 5a), on the NaCl-deposited and reacted Fe-surface chlorine rich particles are locally formed, but on the Ni-surface with KCl deposition chlorine rich layers are locally formed. EDX-analysis by 550 x magnification showed Cl-content of KCl deposited Ni- surfaces (20 wt. %) are higher than that of NaCl on Fe-surfaces (3 wt. %) by approx. 10 wt. % of Oxygen content. Fig. 5b shows the sample surfaces at higher magnification and Fig. 5c at lower magnification after reaction in N2-20 vol. % O2- 500 vppm HCl for 40 min. The Fe-surface was covered with a thick scale, containing iron and chlorine. The Cl-content of Fe-surfaces increased by increasing of reaction time, e.g. Cl-content of about 10 wt. % after 40 min. reaction. Especially, it was found that the chlorine rich area on NaCl deposited sample was formed partly on the surface. The reactions on Ni-surface at 500°C are more visible after 40 min. oxidation. The O-content is increased from 10 to 40 min. reaction, but especially Cl-content decreased significantly (< 1 wt. %). The nickel oxide is mainly formed on the reacted surface with trace of chlorine and without trace of potassium, and has the thickness of 1-2 µm, proved in the line scan diagrams, Fig. 6.

a) No. VI (depos.- oxid. with HCl (10 min.): NaCl-Fe-300°C (l.), KCl-Ni-500°C (r.)

b) No. VII (depos.- oxid. with HCl ( 40 min.): NaCl-Fe-300°C (l.), KCl-Ni-500°C (r.)

c) No. VII (depos.- oxid. with HCl (40 min.): NaCl-Fe-300°C (l.), KCl-Ni-500°C (r.)

Fig. 5. SEM images of samples No. VI and VII

a) 10 min. oxidation

b) 40 min. oxidation

Fig. 6. Line scan of the Ni-samples (KCl-deposition- oxid. with HCl)

4. Conclusions

The KCl and NaCl particles show very similar behaviors by reactions with iron. Micro-sized KCl and NaCl particles can be deposited homogenously on iron surfaces through thermophoresis. By pre-oxidation of surfaces both particles appeared to be smaller, no reaction zone was observed at 100°C. After reaction at 300°C in N2-20 vol. % O2, NaCl particles were deformed in square, but KCl in round form, spreading on the metal surface, probably by melt formation in contact with the iron. This localized reactions of both particles were more visible after longer reaction time. Concerning different eutectic temperatures of KCl and NaCl, Na-contents of NaCl deposited samples were higher than K-contents of KCl deposited samples. By adding of HCl, a NaCl deposited scale had formed to the cracked chlorine rich particles, compared to smooth forming KCl-particle embedded in the scale. After longer oxidation, the surface was covered with a thick scale, containing iron and chlorine. The colonization of chlorine rich area on NaCl deposited sample was more intensive than that on KCl deposited surfaces. In addition, on the NaCl-deposited and reacted sample surface, the amount of chlorine was much higher than on the sample surface reacted in HCl-

containing gas only. The size of the chlorine rich particles by NaCl deposition was smaller than those on the non pre-deposited iron surface. This effect was also observed by the reaction with KCl particles.

In addition, the reactions on nickel surfaces with HCl gas after KCl deposition leads to primarily the local formation of chlorine rich nickel oxide, but after longer reaction time, wide nickel oxide layer are formed on the surface with trace of chlorine. In conclusion, the alkali chloride particles react locally with iron surface at 300°C, showing spreading and enhanced local oxidation. But the reactions of alkali chloride particles with nickel surface do not seem to lead to enhanced local oxidation.

5. Acknowledgements

This work is a part of EU-project CORBI. The financial support from the European Commission for the project CORBI is gratefully acknowledged.

6. References

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