Article pubs.acs.org/EF
Low-Temperature Corrosion in Biomass Boilers Fired with Chemically Untreated Wood Chips and Bark Stefan Retschitzegger,*,† Thomas Brunner,†,‡,§ and Ingwald Obernberger‡,§ †
BIOENERGY 2020+ GmbH, Inffeldgasse 21b, 8010 Graz, Austria Institute for Process and Particle Engineering, Graz University of Technology, Inffeldgasse 13, 8010 Graz, Austria § BIOS BIOENERGIESYSTEME GmbH, Inffeldgasse 21b, 8010 Graz, Austria ‡
ABSTRACT: Low-temperature corrosion often causes failures of cold-end parts (economizers, air pre-heaters, and fire tubes of hot water boilers) in biomass boilers firing chemically untreated wood chips and bark. The most relevant mechanisms causing low-temperature corrosion are condensation of acids and hygroscopic salts in the deposits on heat-exchanger surfaces. This article offers a short review on acid condensation and presents a detailed study on the formation of hygroscopic salts, which may absorb moisture from the flue gas to such an extent that they are dissolved and form highly concentrated salt solutions. Typical fuel compositions of wood chips and bark in combination with typical operating conditions of biomass grate furnaces have been used to estimate the compositions of ash deposits on heat-exchangers. Based on these compositions, thermodynamic equilibrium calculations have been conducted in the range of 70−400 °C, which have revealed that the hygroscopic salts K2ZnCl4, KCaCl3, and CaCl2 can be formed at these conditions. Finally, recommendations are given on how to minimize the risk for lowtemperature corrosion.
1. INTRODUCTION AND OBJECTIVES Besides high-temperature corrosion of evaporators and superheaters, low-temperature corrosion of economizers and air preheaters as well as fire tubes in fire tube boilers can also cause significant problems in biomass combustion plants. In the past, low-temperature corrosion was a problem typically occurring through the utilization of coal with high sulfur content (up to 6 wt% dry basis1) in combustion units.2 The high S-content of the fuels results in high concentrations of H2SO4 acid in the flue gas. The acid condenses preferably on cooled surfaces, where it causes corrosion which is known as low-temperature corrosion. H2SO4 can, depending on its concentration in the flue gas, condense at temperatures up to 160 °C.3 For high-S coals this topic has already been intensively investigated.4−6 Sulfuric acid was also found to cause low-temperature corrosion in waste incinerators.7 By decreasing the return temperatures of the water (which results in decreased heat-exchanger surface temperatures) in order to increase the efficiency of combustion plants, plants firing fuels with low sulfur content also started to have problems with low-temperature corrosion.8,9 An investigation of corrosion in Austrian fire tube boilers firing chemically untreated wood fuels revealed that mainly toohigh moisture content in the flue gas and too-low water return temperatures were responsible for corrosion, but no corrosion mechanism has been proposed.10 Examinations of lowtemperature corrosion mechanisms in German steam boilers firing chemically untreated wood as well as waste wood showed that the condensation of sulfuric acid is responsible for corrosion.9 Lindau and Goldschmidt11 examined low-temperature corrosion in Swedish fire tube boilers firing chemically untreated wood fuels and showed that ash deposits on heatexchanger surfaces can contain hygroscopic salts which may be responsible for the corrosion attack. Hygroscopic salts absorb © XXXX American Chemical Society
moisture from the surrounding atmosphere. Salts such as ZnCl2 and CaCl2 are so hygroscopic that they dissolve in the water they absorb and form a highly concentrated salt solution: this property is called deliquescence. The process can occur at temperatures significantly above the water dew point, depending on the hygroscopic characteristics of the salt. Hygroscopic salts were also found to be responsible for low-temperature corrosion in plants firing municipal solid waste and waste wood.3 The objective of the work presented here was to evaluate low-temperature corrosion in grate-fired steam and hot water boilers using chemically untreated wood chips and bark. Sections investigated are economizers and air pre-heaters of steam boilers and fire tubes of hot water boilers. The aim is to give a short review on acid condensation and focus on the formation of hygroscopic salts. Their formation is evaluated on the basis of thermodynamic equilibrium calculations (TECs). Furthermore, sensitivity analyses regarding influencing parameters such as variations of the fuel compositions are considered.
2. BACKGROUND 2.1. Acid Condensation. Flue gases from combustion processes can contain acids which condense at temperatures significantly above the dew point of water. In biomass combustion processes, the acids HCl, HNO3, H2SO3, and H2SO4 can be formed.4−6 Their dew points can be calculated using the following empirical correlations: Received: February 15, 2015 Revised: April 10, 2015
A
DOI: 10.1021/acs.energyfuels.5b00365 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels HCl:
− 0.0326 ln(pHCl ) + 0.00269 ln(pH O ) ln(pHCl ) 2
HNO3 :
2
2
2
(2)
1000 = 3.9526 − 0.1863 ln(pH O ) 2 TDP
+ 0.000867 ln(pSO ) − 0.00091 ln(pH O ) ln(pSO ) 3
H 2SO4 :
(1)
1000 = 3.664 − 0.1446 ln(pH O ) 2 TDP
− 0.0827 ln(pNO ) + 0.00756 ln(pH O ) ln(pNO )
H 2SO3 :
measured yet. But even for SO3 concentrations below 1 × 10−4 vol%, acid dew points up to 123 °C can arise, which underlines the relevance of H2SO4 condensation. In order to estimate H2SO4 dew points for the biomass fuels considered, SO3 concentrations can be estimated using measurement data of the sum parameter SOx, consisting of SO2 and SO3. SOx concentrations measured by BIOENERGY 2020+ and BIOS BIOENERGIESYSTEME in the flue gases of real-scale as well as pilot-scale plants firing chemically untreated wood chips and bark are in the range of 0.2 × 10−4−6.1 × 10−4 vol%, relative to dry flue gas at 8 vol% O2. According to the literature, the share of SO3 in SOx ranges from 1 to 5 vol%.4,18 Therefore, a maximum concentration of SO3 is expected to be on the level of 0.3 × 10−4 vol%. Using this concentration and 20 vol% H2O in the flue gas, a calculated H2SO4 dew point of 118 °C results. Due to the assumptions made, this calculation has to be considered as a first approximation. However, the result illustrates that H2SO4 condensation in biomass combustion plants cannot be excluded as a reason for low-temperature corrosion, not even for chemically untreated wood fuels. 2.2. Hygroscopic Salts. The formation of highly concentrated salt solutions due to hygroscopic salts in ash deposits is explained in detail in a report by Lindau and Goldschmidt.11 Between a liquid surface and a gas phase, the vapor pressure of the liquid will adjust in the gas phase, which is called the equilibrium partial pressure or equilibrium vapor pressure. This equilibrium partial pressure depends only on the temperature. For condensation to occur, the partial pressure of a certain substance in the gas phase has to exceed the equilibrium partial pressure. Above aqueous salt solutions, the partial pressure of water in the gas phase will adjust to a value higher than the one above pure water. The principle is commonly known from the elevation of the boiling point of aqueous salt solutions. For condensation processes, the principle applies in the way that condensation on a salt particle will occur at temperatures above the water dew point and create an aqueous salt solution. To describe the condensation of water on salt particles, the concept of deliquescent relative humidity (DRH)11 is used. The relative humidity (RH) of water in a gas phase is given by
1000 = 3.7368 − 0.1591 ln(pH O ) 2 TDP
2
3
(3)
1000 = 2.276 − 0.02943 ln(pH O ) 2 TDP
− 0.0858 ln(pSO ) + 0.0062 ln(pH O ) ln(pSO ) 3
2
3
(4)
where TDP is the dew point [K] and pi is the partial pressure of gaseous compound i [mmHg]; eqs 1 and 3 are from ref 12, eq 2 is from ref 13, and eq 4 is from ref 14. Figure 1 shows calculated dew points of the acids considered as a fuction of their concentration in the flue gas, according to
Figure 1. Dew points of acids in biomass combustion flue gases calculated according to eqs 1−4. Flue gas moisture content = 20 vol%; total pressure ptot = 101 325 Pa.
eqs 1−4. For the calculations, a moisture content of 20 vol% in the wet flue gas was used, which results from a fuel moisture content of 50 wt% (wet basis, w.b.) with an oxygen content of 8 vol% (dry basis, d.b.) in the flue gas. Typical moisture content for the biomass fuels considered ranges from 30 to 60 wt% (w.b.). Return temperatures of the water in biomass boilers in district heating, process heating, or CHP systems are typically between 75 and 110 °C.10,11 The surface temperature of heatexchangers is mainly determined by the water temperature, since the heat transfer coefficients on the water side typically exceed the ones on the flue gas side by a factor of 102−103. Considering these return temperatures and the dew points shown in Figure 1, only H2SO4 is relevant for acid condensation. To determine specific H 2 SO 4 dew points, the SO 3 concentration in the flue gas must be known. With state-ofthe-art measurement methods, the detection limit for SO3 is 1 × 10−4 vol%.15−17 Since the SO3 concentrations in the flue gases of biomass combustion plants firing low-S fuels such as wood chips and/or bark are below this limit, they cannot be
RH =
pW p0 (T )
(5)
with pW being the partial pressure of water in the gas phase and p0(T) being the saturation pressure of pure water at temperature T. This value is commonly known as air humidity. The equilibrium partial pressure pS(T,c) of water above a salt solution is a function of the temperature T and the concentration c of the salt in the solution, with lowest values of pS(T,c) for saturated solutions. When the temperature of the gas phase is reduced, at first a saturated solution will form as condensate. Due to this characteristic, only the formation of saturated solutions is of relevance and will be discussed in the following. The DRH gives the ratio of the equilibrium partial pressure of water above a salt solution, pS(T), to the pressure of pure water, p0(T), according to DRH = B
pS (T ) p0 (T )
(6) DOI: 10.1021/acs.energyfuels.5b00365 Energy Fuels XXXX, XXX, XXX−XXX
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determined. For this purpose, stoichiometric salt mixtures from the single salts (KCl, CaCl2, and ZnCl2) were produced and dissolved in water at 25 °C until a saturated solution was achieved. The solubility of KCaCl3 was determined to be 40 g/ 100 g H2O, and the solubility of K2ZnCl4 is 177 g/100 g H2O. Comparison with the solubilities and dew points arising from the single salts shows that the dew point of KCaCl3 will be higher than that of KCl but significantly lower than those of CaCl2 and ZnCl2. The solubility of K2ZnCl4, on the other hand, lies between those of CaCl2 and ZnCl2; hence, the dew point of its salt solution will also lie in the range of the values of these two salts. Therefore, K2ZnCl4 has to be considered as highly relevant for low-temperature corrosion. In the following, the probability of the appearance of both double salts in economizer deposits is evaluated.
For condensation to occur, the partial pressure of water in the gas phase has to exceed the equilibrium partial pressure above the salt solution, meaning
pW > pS (T )
(7)
resulting in the terms RH > DRH
(8)
for condensation and RH = DRH
(9)
for the determination of the dew point. To identify salts which are of relevance in biomass combustion plants, the following considerations have been made: (a) the salts have to occur in the ash deposits on heatexchangers at the cold end of the plants (since these are the affected parts), and (b) the salts have to be highly soluble in water to cause an increase of the dew point. With these criteria, especially chlorides of Na, K, Zn, and Ca come into consideration. Figure 2 shows trends of RH for relevant moisture contents in the flue gas and curves of DRH for hygroscopic salts. For a
3. METHODOLOGY In order to investigate the formation of hygroscopic salts in deposits in the low-temperature section of biomass combustion plants, TECs have been performed. TECs can be used to estimate the composition of these deposits. It is assumed that the residence time of the deposits on the heat-exchanger surfaces is long enough to reach equilibrium;, therefore, TECs can be applied. It has to be mentioned that particles rich in Si, Ca, and Mg (especially oxides and sulfates) will most likely experience no conversions at the temperatures considered (see section 3.1). If hygroscopic salts containing Si, Ca, and Mg are formed according to equilibrium, this fact has to be considered in order to use TECs as an appropriate approximation. 3.1. Thermodynamic Equilibrium Calculations. TECs provide the opportunity to predict multi-phase equilibria. These calculations are conducted for a multi-component thermodynamic system in a predetermined gas phase. In this work, TECs have been applied for the identification and quantification of phases of interest for lowtemperature corrosion using the thermochemical software package FactSage 6.2. The accessed databases consist of pure component databases containing data for over 4500 stoichiometric components as well as solution databases for a wide range of solution phases. In this work, the component database Fact 53 and the solution databases FToxid, FTsalt, and FTpulp have been applied. The elements considered in the calculations were H, C, N, O, Si, Ca, Mg, K, Na, S, Cl, Al, Fe, P, Mn, Zn, Cu, Pb, and Cd. TECs have been performed for temperatures ranging from 70 to 400 °C. 3.2. Definition of Input Parameters for TEC. To perform TECs, the compositions of the flue gas and the ash forming particles have to be defined as input parameters. Regarding ash formation in grate-fired biomass combustion plants, three major ash fractions have to be considered: grate ash, coarse fly ashes, and aerosols. During combustion, semi-volatile (K, Na, Zn, Pb) as well as easily volatile (S, Cl) elements are released to the gas phase to a certain extent and form aerosols. From the remaining ash, a certain amount is entrained from the grate and forms coarse fly ashes. What remains after the entrainment is considered as grate ash. The formation of different ash fractions is presented schematically in Figure 3. Typical fuel compositions for wood chips with bark as well as for pure bark (spruce from Austria) have been used to calculate the compositions of the ash fractions. The fuel compositions originate from the databases of BIOS and BIOENERGY 2020+. The release rates of these ash-forming elements have been obtained within fuel characterization tests in a specially designed laboratory-scale reactor.21 The fuel compositions and release rates are given in Table 1. Furthermore, the oxide forms of the elements are given, which were used to determine the ash compositions. From the remaining ash, 30 wt% is assumed to be entrained from the grate and form coarse fly ash, which is a typical value for biomass grate furnaces.22 The composition of the coarse fly ash can be assumed to be identical to that of the grate ash as a first approximation. The ash compositions have been calculated for the aerosol fractions according
Figure 2. Trends of RH at 15 and 25 vol% H2O and DRH of salts, which can occur in ash deposits of biomass combustion plants. ptot = 101 325 Pa; RH, relative humidity; DRH, deliquescent relative humidity; ●, dew points.
flue gas containing 15 vol% moisture, the following dew points arise: H2O, 54 °C; KCl, 58 °C; NaCl, 60 °C; (K,Na)Cl, 62 °C; CaCl2, 95 °C; ZnCl2, 127 °C. Condensation will occur if the flue gas temperature falls below the specific dew point of the saturated solution of one of the salts present in the deposits or if the water dew point is reached. It is known that, for the magnitude of dew point increase, only the concentration of salt in the saturated solution is decisive and not individual characteristics of the salts.19 A comparison of the solubilities in water (100 g H2O, T = 25 °C) demonstrates this effect:20 KCl, 35.5 g; NaCl, 36.0; CaCl2, 81.3 g; ZnCl2, 408 g. The higher the solubility, the higher is the dew point of a salt solution. Return temperatures of water in biomass boilers are usually between 75 and 110 °C.10,11 Based on the data provided by Lindau and Goldschmidt,11 only CaCl2 and ZnCl2 are able to cause condensation due to hygroscopic salts above these temperatures. Hence, only chlorides of Ca and Zn have been further examined in this work. These salts include the double salts KCaCl3 and K2ZnCl4. No data for the DRH of saturated solutions of these salts were found in the literature. Therefore, the resulting dew points cannot be calculated. To estimate the relevance of the two salts, the solubility in water was C
DOI: 10.1021/acs.energyfuels.5b00365 Energy Fuels XXXX, XXX, XXX−XXX
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Table 1. Fuel Compositions and Release Rates of Semivolatile as Well as Easily Volatile Ash Forming Elements and Oxide Forms of Elementsa wood chips with bark composition
bark
release [wt%]
composition
release [wt%]
oxide form of element
a.c.
wt% d.b.
1.00
4.41
C H N
wt% d.b. wt% d.b. wt% d.b.
49.80 6.64 0.17
100 100 100
49.76 5.72 0.19
100 100 100
S Cl
mg/kg d.b. mg/kg d.b.
163 81
80 99
300 172
70 90
SO3 Cl
Ca Si Mg Al Na K Fe P Mn
mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg
d.b. d.b. d.b. d.b. d.b. d.b. d.b. d.b. d.b.
3226 939 296 155 61 1146 110 173 108
0.5 0 0.5 0 20 25 0 0 0
12472 3538 1268 795 131 2480 574 484 653
0.5 0 0.5 0 15 15 0 0 0
CaO SiO2 MgO Al2O3 Na2O K2O Fe2O3 P2O5 MnO
Cu Zn Pb Cd
mg/kg mg/kg mg/kg mg/kg
d.b. d.b. d.b. d.b.
2.26 16.38 0.82 0.20
0 90 99.5 0
5.04 124.18 1.56 0.44
0 70 90.0 0
CuO ZnO PbO CdO
a
Explanations: a.c., ash content; d.b., dry basis; source of fuel compositions, databases of BIOS and BIOENERGY 2020+; source of release rates, Brunner et al.21
Table 2. Compositions of Ash Fractions wood chips with bark
Figure 3. Ash formation and differentiation between ash fractions. to eq 10 and for coarse fly ashes according to eq 11. The resulting ash compositions are presented in Table 2.
wi ,aerosol = [wi ,fuel · releasei)]/∑[wi ,fuel ·releasei · Mi ,oxide /(Mini)] (10) where wi,aerosol is the mass fraction of element i in the aerosols [wt%], wi,fuel is the mass fraction of element i in the fuel according to Table 1 [wt% d.b.], releasei is the release of an element according to Table 1 [wt%], Mi,oxide is the molecular weight of the oxide form of element i according to Table 1 [kg/kmol], Mi is the molecular weight of element i [kg/kmol], and ni is the number of moles of element i per mole of oxide [mol/mol].
aerosols
coarse fly ash
aerosols
S Cl
wt% wt%
0.36 0.01
16.42 10.10
0.26 0.05
15.84 11.65
Ca Si Mg Al Na K Fe P Mn
wt% wt% wt% wt% wt% wt% wt% wt% wt%
35.03 10.25 3.21 1.70 0.53 9.38 1.20 1.89 1.18
2.03 0.00 0.19 0.00 1.53 36.15 0.00 0.00 0.00
36.20 10.32 3.68 2.32 0.33 6.15 1.67 1.41 1.90
4.70 0.00 0.48 0.00 1.49 28.05 0.00 0.00 0.00
Cu Zn Pb Cd
wt% wt% wt% wt%
0.02 0.02 0.00 0.002
0.00 1.86 0.10 0.00
0.01 0.11 0.000 0.001
0.00 6.56 0.11 0.00
O
wt%
35.23
31.62
35.58
31.14
The work presented here focuses on grate furnaces combined with a steam boiler or a hot water fire tube boiler. Depending on the type and design of the plant, the deposits present on heat-exchanging surfaces can be formed by different types of ashes: • Aerosols and coarse fly ashes are relevant for economizers which are located downstream of the boiler. • Only aerosols will be relevant for deposit formation for economizers which are located downstream of a flue gas cleaning unit, since coarse fly ashes are almost completely
wi ,coarse fly ash = wi ,fuel · [(1 − releasei)]/∑ [wi ,fuel ·(1 − releasei)· Mi ,oxide /(Mi · ni)]
bark
coarse fly ash
(11)
where wi,coarse fly ash is the mass fraction of element i in the coarse fly ashes [wt%], and the other terms are as defined for eq 10. D
DOI: 10.1021/acs.energyfuels.5b00365 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 3. Overview of Case Studies aerosols + coarse fly ashes
aerosols wood chips + bark moisture content in fuel [wt% w.b.]a case no. a
30 I
bark
60 II
30 III
wood chips + bark 60 IV
30 V
60 VI
bark 30 VII
60 VIII
w.b., wet basis.
removed from the flue gas by electrostatic precipitators (ESPs) or baghouse filters. • Both ash fractions are relevant in the case of fire tube boilers. High temperature gradients between the flue gas and the cooled surfaces additionally cause direct condensation of aerosol-forming species on heat-exchanger surfaces, but deposition of coarse fly ashes and aerosols also occurs. Additionally, for the definition of the ash-forming elements, the fuel moisture content and the residual oxygen content in the flue gas need to be specified in order to calculate the flue gas composition. Therefore, fuel moisture contents of 30 and 60 wt% w.b. and a residual oxygen content in the flue gas of 8 vol% d.b. have been considered. 3.3. Case Studies. With the considerations presented in section 3.2, case studies have been performed, which are summarized in Table 3. They represent typical biomass combustion plants with grate-fired steam or hot water fire tube boilers. Furthermore, for consideration of aerosols and coarse fly ashes (cases V−VIII), sensitivity analyses regarding the Cl concentrations in the fuels have been performed. For this purpose, the original Cl concentrations were increased stepwise until the formation of hygroscopic salts occurred. 3.4. SO3 Formation in the Flue Gas. According to TECs, for temperatures below 200 °C, H2SO4 is formed in large quantities in equilibrium. Since the formation of H2SO4 results from the reaction of gaseous SO3 with H2O in the flue gas,14 the availability of SO3 determines the amount of H2SO4 formed. SO3 in the flue gas originates from the oxidation of SO2.23 The equilibrium of this reaction favors the formation of SO2 at temperatures above 800 °C but is shifted toward SO3 with decreasing temperatures. However, the formation of SO3 from SO2 is limited because the reaction rate significantly decreases at temperatures below 800 °C. Simulations reported by Christensen23 showed that the kinetic limitation of the SO2 oxidation caused the SO2 concentration to become practically constant at temperatures below 800 °C. Hence, SO3 is mainly formed in the furnace and the convective section of a boiler. Investigations on the formation of gaseous alkali sulfates reported by Glarborg et al.24 and Kassman et al.25 have shown that SO3 reacts rapidly with alkali chlorides and alkali hydroxides at temperatures above 800 °C and forms sulfates. Therefore, it can be assumed that SO3 which is formed at high temperatures is consumed almost instantly, and only small concentrations will exist in the flue gas at temperatures relevant for low-temperature corrosion. As a consequence for the TECs, the formation of H2SO4 has not been considered. Small amounts which can occur in reality are considered not to be relevant for the formation of hygroscopic salts.
Figure 4. Results of TECs for wood chips with bark, case I. Fuel moisture content, 30 wt% w.b.; only aerosols as well as HCl and Cl2 are considered.
Figure 5. Results of TECs for bark, case III. Fuel moisture content, 30 wt% w.b.; only aerosols as well as HCl and Cl2 are considered.
Plots of the Zn distribution versus temperature (Figure 6) show that, for wood chips with bark, all of the Zn available is bound in K2ZnCl4 at temperatures below 220 °C, while for bark alone, ZnSO4 is also formed at temperatures below 190 °C. Due to the higher Zn content in the fuel, the concentration of K2ZnCl4 in aerosols is higher for bark (Figure 7), although besides K2ZnCl4, also ZnSO4·4H2O is formed. For both fuels, significant concentrations of K2ZnCl4 are formed in the ashes. These concentrations can cause the condensation of water and the formation of highly corrosive salt solutions. 4.2. Aerosols and Coarse Fly Ashes Considered for Deposit Formation (Cases V and VII). In Figure 8, the results of TECs are shown for wood chips with bark, considering the whole fly ash in the flue gas before dust precipitators, consisting of aerosols and coarse fly ash particles. Since the general information obtained from the TECs is the same for wood chips with bark and for pure bark, only the results for wood chips with bark are presented. TECs show that no chlorides of Zn or Ca are formed. Cl present in the ash is
4. RESULTS AND DISCUSSION The TECs show that the fuel moisture content influences the compositions of the ashes only to a minor extent, since it mainly dilutes the flue gas. Therefore, only the results for a fuel moisture content of 30 wt% w.b. are presented. 4.1. Aerosols Considered for Deposit Formation (Cases I and III). Figures 4 and 5 show the results of TECs for both fuels when considering only the aerosol fraction for deposit formation. The most relevant temperatures for lowtemperature corrosion are in the range between 70 and 130 °C. In this range, mainly sulfates of K, Na, and Ca prevail. According to the calculations, K2ZnCl4 is formed in both cases. E
DOI: 10.1021/acs.energyfuels.5b00365 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 6. Zn distribution in aerosol deposits versus temperature according to TECs.
of Ca are also present as carbonates, which are considered to be reactive at low temperatures. Therefore, non-reacting Ca compounds such as oxides and sulfates do not prevent the formation of the hygroscopic chlorides. The formation of these compounds as a function of the temperature is shown in Figure 9. The original Cl concentrations in the fuel have been increased by the factors given in the diagrams. TECs show that, in the temperature range from 70 to 130 °C, Cl is primarily bound as KCl. If a surplus of Cl is available for temperatures ranging from 70 to 100 °C, K2ZnCl4 is formed. However, formation of K2ZnCl4 is limited by the availability of Zn. If at the lowest temperature all the Zn is bound in K2ZnCl4 and still Cl is available, also CaCl2·6H2O is formed. For higher temperatures, the share of K2ZnCl4 decreases. Ca, which is bound at 70 °C in CaCl2·6H2O, forms KCaCl3 at temperatures exceeding 100 °C. The concentration of all relevant hygroscopic salts in the ash decreases with increasing temperatures, as shown in Figure 10. As expected, the concentrations increase with increasing Cl concentrations in the fuel. TECs show comparable results for both fuels regarding the magnitude of the Cl increase. The Cl concentration needs to increase by a factor of 6.0 (wood chips with bark) or 5.5 (bark) for the formation of salts which are relevant for lowtemperature corrosion. Since Cl mainly forms KCl at temperatures between 70 and 130 °C, these results can be explained with comparable molar K/Cl ratios in the fuels. These ratios are 12.9 for wood chips with bark and 13.1 in the
Figure 7. Concentration of K2ZnCl4 in aerosol deposits according to TECs for cases I and III.
bound with K as KCl. Zn mainly forms oxides with Al, Fe, and Mg. Ca is bound as sulfate, carbonate, phosphate, and silicate. With 15 vol% moisture in the flue gas, the dew points of the saturated solutions of these salts are clearly below the minimum flue gas temperatures occurring in biomass combustion plants. Therefore, these salts will not contribute to condensation due to hygroscopic salts. Additionally, sensitivity analyses regarding the Cl concentrations in both biomass fuels have been performed. The Cl concentrations were increased stepwise until the formation of hygroscopic salts occurred. The calculations show that, for both fuels, different salts which are relevant for low-temperature corrosion are formed when more Cl is available. These are K2ZnCl4, KCaCl3, and CaCl2·6H2O. In these cases, large shares
Figure 8. Results of TECs for wood chips with bark, case V. Fuel moisture content, 30 wt% w.b.; aerosols and coarse fly ashes are considered. F
DOI: 10.1021/acs.energyfuels.5b00365 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 9. Concentrations of relevant chlorides in the ash, as functions of the temperature and the amount of Cl in the fuel according to TECs. Fuel moisture content, 30 wt% w.b.
Figure 10. Concentrations of all hygroscopic salts in the ashes according to TECs.
case of bark alone. Fuel Cl contents originating from the databases of BIOS and BIOENERGY 2020+ are in the range of 81 ± 31 mg/kg d.b. for wood chips with bark and 172 ± 52 mg/kg d.b. for bark alone (see Table 1). The minimum Cl concentrations in the fuels necessary for the formation of the relevant hygroscopic salts are calculated to be 445 and 860 mg/ kg d.b., respectively. These concentrations are usually not expected in wood chips but can arise from increased shares of tree needles or from waste wood in the fuel. Furthermore, an increase in Cl can result from the addition of salt in the winter to prevent icing on loading areas. 4.3. Design and Operational Recommendations. From the results presented, different design and operational measures can be recommended to prevent low-temperature corrosion:
1. Boiler water return temperatures should be kept as high as possible to prevent the formation of highly concentrated salt solutions. This can be achieved by pre-heating the return with the feed water. For the surface temperature of heat-exchangers, mainly the water temperature is relevant, because the heat-transfer coefficients on the water side typically exceed the ones on the flue gas side by a factor of 102−103. 2. Moreover, fire tube boilers should be designed in such a way that the water flows co-current to the flue gas. Thereby, the cold end of the fire tubes (with respect to the flue gas) faces the highest water temperatures, and condensation of corrosive species is reduced. An even better solution would be pre-heating the return in a large fire tube (radiative section) before entering the tube bundles. G
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ACKNOWLEDGMENTS This article is the result of a project carried out within the framework of the Austrian COMET program, which is funded by the Republic of Austria as well as the federal provinces of Styria, Lower Austria, and Burgenland.
3. The fuel quality should be monitored so that the Cl content in the fuel does not exceed 400 mg/kg d.b., to prevent the formation of hygroscopic salts on economizers located upstream in the flue gas cleaning system and in fire tube boilers. 4. Economizers should generally be placed upstream in the flue gas cleaning system. 5. Economizers which are located downstream of an ESP should be cleaned with water at regular intervals. Thereby, corrosive species are regularly removed from the economizer tubes. If these steps are not sufficient or applicable, economizer tubes can be coated with corrosion-resistant materials (e.g., thermally sprayed high-velocity oxy-fuel coatings).26
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5. SUMMARY The formation of hygroscopic salts in deposits and acid condensation on heat-exchangers/heat-exchanging surfaces have been evaluated regarding their relevance for lowtemperature corrosion in grate-fired boilers that use chemically untreated wood (bark and wood chips) as fuel. Regarding acid condensation, only H2SO4 is of relevance when considering typical boiler water return temperatures in the range of 75−110 °C. However, a reliable determination of the acid dew point is not yet possible due to the low concentrations of H2SO4 typically occurring in the flue gas of these fuels. The main work presented here focused on the formation of hygroscopic salts, which can form highly concentrated salt solutions and cause corrosion. To evaluate their formation in deposits, ash and flue gas compositions have been calculated on the basis of typical compositions of wood chips with bark as well as pure bark and typical combustion conditions for grate furnaces. Furthermore, different ash fractions (aerosols and coarse fly ashes) have been considered for deposit formation. With these data, TECs have been performed for temperatures ranging from 70 to 400 °C to investigate the possible occurrence of relevant hygroscopic salts. The results showed that the hygroscopic salts K2ZnCl4, KCaCl3, and CaCl2 can be formed, especially if the deposits are mainly formed by aerosols and not by coarse fly ashes. This is relevant for economizers located downstream of dust precipitators. Hence, a high corrosion risk prevails in this case. Deposits on economizers located upstream of dust precipitators are formed by aerosols and coarse fly ashes. Here, less hygroscopic salts are formed when considering typical fuel compositions of wood chips and bark. However, elevated Cl concentrations in the fuel may also result in the formation of hygroscopic salts in these deposits. For hot water fire tube boilers, both cases are relevant since high-temperature gradients between the flue gas and the cooled surfaces cause direct condensation of aerosol-forming species on heat-exchanger surfaces but also deposition of coarse fly ashes occurs. From these results, design and operational recommendations have been drawn to prevent low-temperature corrosion.
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DOI: 10.1021/acs.energyfuels.5b00365 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.5b00365 Energy Fuels XXXX, XXX, XXX−XXX
