Ash-Related Issues in Fluidized-Bed Combustion of Biomasses

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Ash-Related Issues in Fluidized-Bed Combustion of Biomasses: Recent Research Highlights Mikko Hupa* Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FI-20500 Åbo, Finland ABSTRACT: Finland and Sweden are leaders in the use of biomass fuels in large-scale boilers. In these countries, the dominating large-scale combustion technology for biomass fuels is fluidized-bed combustion (FBC). Biomass fuels differ in many ways from the standard fossil fuels used in FBC, such as coal. They often have high moisture contents, lower heating values, and a variety of impurities, such as chlorine, sulfur, phosphorus, nitrogen, and a variety of ash-forming metals. FBC of biomass fuels is often connected with operational challenges, which are related to the fuel chemistry and fuel properties. Bed sintering, superheater fouling, and high-temperature corrosion are crucial factors to take into account when fuels are selected for FBC. It is of vital interest to find ways of predicting the degree of these kinds of ash-related problems for various fuels or fuel mixtures. This paper reviews some of the recent progress in our understanding of the fate and behavior of ash-forming matter in FBC. The following topic areas are discussed: fuel characterization, release of the ash-forming matter during combustion, interaction of the ash and bed material, fly ash formation, fly ash properties, ash deposits, and fouling and corrosion.

’ INTRODUCTION Fluidized-bed combustion (FBC) is considered a flexible technology that makes it possible to burn fuels of widely varying quality. Besides different coal qualities, including high-sulfur or high-ash coals, also low-grade fuels, such as various types of biomasses and waste-derived fuels, have become highly interesting as feed stocks to FBC. Many installations also burn mixtures of different fuels. Circulating fluidized-bed combustion (CFBC) is preferred in the largest unit sizes and for coal firing. For biomass firing, the more simple bubbling fluidized-bed combustion (BFBC) technology has become increasingly popular. However, the use of biofuels and waste-derived fuels in FBC may cause several operational problems because of the fuel impurities. Bed sintering is a problem with high alkali fuels. Sticky fly ash and problems of fouling and corrosion of the superheater tubes is another common phenomenon when using fuels containing chlorine, etc. Fuel ashes from different fuels burnt together may also interact with each other in a surprising way, thus sometimes leading to excessive fouling or bed sintering problems. This paper reviews some of the recent progress in our understanding of the fate and behavior of the ash-forming matter in FBC of biomasses. The following topic areas are discussed: fuel characterization, release of the ash-forming matter during combustion, interaction of the ash and bed material, fly ash formation, fly ash properties, ash deposits, and fouling and corrosion. The main interest is in the biomass and waste-derived fuels, but some comparisons to coals are also made. This is not intended to be a comprehensive review of all research in the field. Rather, the intention is to give an overview of the most significant areas where clear progress in understanding in the chemical details of the fate of the fuel impurities has taken place in the last 10 years or so. Also areas where there is an obvious lack of information and where more research work is needed will be identified and discussed. r 2011 American Chemical Society

Most of the examples in this review are taken from the research at Åbo Akademi University in Turku, Finland, but the general discussion is based on the lively recent research at the many laboratories active in the field, especially Chalmers University of Technology, Technical University of Denmark, CANMET in
University of Canada, University of Graz, University of Umea, Stuttgart, ECN in The Netherlands, and VTT in Finland. The FBC boiler manufacturing companies have contributed with relevant information also in the literature.

’ ASH-FORMING MATTER IN THE FUELS A number of excellent compilations of biomass fuel property data, including analyses of the ash-forming matter, are available in the literature 13 and in the form of net-based data banks. 4,5 Further, a number of recent papers deal with detailed characteristics of ashes from individual particular biomass-based fuels, such as agricultural wastes, rice husk, straw, rapeseed meal or cake, corn stalk, palm kernel, olive stones, sewage sludge, forest residue, or recovered “demolition” wood.620 Table 1 shows the typical elemental composition of ashforming matter in selected biomass-based fuels, including the sewage sludge. At the bottom of the table examples of peat and coal analyses are included for comparison. The table gives the elemental composition of the ash-forming matter after ashing the fuel samples according to a standard ashing procedure. It is common practice to give the percentage of each of the elements analyzed as if they were present as their most common oxides. It is important to note that the elements very seldom are present in Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 1, 2011 Revised: October 12, 2011 Published: October 13, 2011 4

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Table 1. Elemental Analysis of the Laboratory Ash of a Variety of Biomass-Based Fuelsa fuel sawdust

Al2O3

Fe2O3

TiO2

MnO

CaO

MgO

P2O5

Na2O

K2O

sum (%)

2.05

0.25

0.09

0.00

4.21

68.76

7.13

3.01

0.27

16.78

102.54

bark

10.74

3.20

4.96

0.14

1.77

60.16

5.82

5.24

0.67

8.69

101.38

forest residue

36.75

5.81

1.91

0.22

1.46

37.14

2.93

3.17

0.20

7.72

97.32

0.01

0.14

0.07

0.00

4.21

77.43

3.12

1.95

0.36

16.07

103.38

straw

58.49

0.39

0.33

0.03

0.00

21.10

2.13

3.53

0.25

13.59

99.84

rice straw

69.88

0.28

0.24

0.01

0.57

6.16

1.55

1.53

0.40

15.26

95.88

rice husk

95.41

0.10

0.05

0.00

0.12

0.74

0.28

0.53

0.01

1.84

99.09

bagasse grape seeds

72.96 3.44

4.97 0.57

2.53 0.56

0.29 0.03

0.19 0.05

11.00 52.97

2.06 3.12

0.96 11.81

0.34 0.18

3.86 17.52

99.15 90.24

almond shells

6.22

0.98

0.66

0.06

0.05

36.32

3.01

1.60

0.38

30.66

79.95

olive residues

19.75

1.74

2.69

0.08

0.06

36.50

12.12

2.73

0.11

17.61

93.40

sewage sludge

17.86

9.89

36.65

0.75

0.08

13.28

1.12

19.61

0.53

0.82

100.59

peat

20.20

23.09

26.21

0.44

0.22

19.42

2.07

4.10

0.15

0.64

96.56

coal

46.48

24.60

8.43

0.98

0.16

6.83

2.62

0.48

1.36

2.34

94.28

eucalyptus bark

a

SiO2

The numbers give weight percent of the elements expressed as their most common oxides (Åbo Akademi data).

the ash as the oxides indicated, for instance, K2O, CaO, or P2O5, but normally as some more complex compounds, such as K2SiO3 or Ca3(PO4)2. Giving the elemental composition as oxides has, however, the advantage that one obtains a good estimation of the amount of oxygen in the ash. Most of the elements in their real compounds possess the same amount of oxygen as in these simple oxides; Ca3(PO4)2 contains the same amount of oxygen as the oxides CaO and P2O5 combined (3CaO + P2O5). Consequently, adding up all of the oxide weight percents should be very close to 100% if all relevant elements have been included in the analysis. This helps in controlling if the ash analysis contains all of the important elements. However, some ashes may also contain non-oxygen-containing salts, such as chlorides or sulfides. Sometimes, also, the metals in the ash may not be fully oxidized to their most stable oxide; for instance, iron may be FeO instead of Fe2O3, or lead may be in the metallic form, Pb, rather than as the oxide PbO. Further, some ashes may contain carbonates, typically alkali carbonates or, if the ashing is performed at a lower temperature, also calcium carbonate. To obtain the full 100% in the total oxides, this carbonate has to be included in the ash analysis and presented as CO2. Most often, carbonate is not included (like in Table 1). This will show as a lower total sum of oxides. Grape seeds, almond shells, and olive residues in Table 1 all have less than 100% total oxides mostly because of the missing carbonate analysis. Potassium in these ashes is in the form of carbonate (K2CO3). The elemental ash analysis is the starting point in establishing the suitability of a given fuel to be burned in a fluidized bed from the ash behavior point of view. Biomass ashes are characterized by high contents of the metals K (sometimes also Na), Ca, and Mg. Some biomasses also contain large amounts of Si and P. Peat and coal differ from biomasses in many ways. Their proximate volatile content is much lower than most biomasses; their ashes have high contents of Al, Si, and Fe but low contents of K. This type of elemental analysis tells nothing about the chemical and physical form of the ash-forming matter in the fuels. To know more about the form of the ash-forming matter is quite important when trying to predict the behavior of the ash in a combustion system. More information about the ash-forming matter can be obtained if the elemental analysis is performed by analyzing

Figure 1. Chemical fractionation by leaching of the fuel sample in aqueous solutions of increasing leaching efficiency.23

the fuel itself without any ashing procedure. The chemical fractionation of the fuel sample implies a procedure where the fuel is leached in water solutions of increasing “aggressiveness” (Figure 1). This technique was originally introduced to ashforming matter in coals by Benson and Holm.21 Later, it has been applied to biomasses and even to waste-derived fuels.2224 Recently, the technique was extended to study also the anionic species in woody biomasses.25 Figure 2 shows examples of chemical fractionation of four interesting byproducts or wastes presently being planned to be used as fuels: a byproduct from bioethanol processing (distillers grains), two types of residues from vegetable oil production (rapeseed and palm kernel cakes), and sewage sludge.16 Selective leaching of the fuel in the three different solutions gives useful information of the chemical form of the ash-forming matter in the fuel. The water-soluble elements are mostly in the form of 5

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Figure 2. Chemical fractionation of the ash-forming matter in four biomass materials planned to be used as fuels in the FBC.16

Figure 3. Scanning electron microscopy (SEM) image of rice husk ash produced by cautious oxidation in a laboratory furnace at 500 °C. X-ray analysis of the particle shown in the middle of the right hand image gave a silica (SiO2) content of more than 95%.7 Magnification: true height of the images is 600 μm (left) and 300 μm (right).

0.10.3 mm size silica “skeletons” of the rice husk are clearly visible.7 Because of this shape and size, the rice husk ash has very special properties in the FBC. These properties cannot be predicted on the basis of the chemical composition only.

simple inorganic salts, such as alkali chlorides or sulfates. Ammonium acetate leaching releases the organically bound cations by ion exchange. The final stage, leaching in hydrochloric acid, dissolves most of the scarcely soluble inorganic salts, such as calcium carbonate or sulfate. The remaining, insoluble ash-forming elements are often bound in silicates. Also, sulfur, chlorine, or phosphorus covalently bound to the organic fuel matrix may stay in this insoluble leaching residue.22,26 Further information of the character of the ash-forming matter can be obtained by removal (oxidation) of the organic matter at temperatures where the ash being formed or exposed does not sinter or melt into lumps but remains as separate particles of various kinds. Figure 3 shows the ash formed under gentle oxidation of the organic matter from rice husk particles. The characteristic,

’ RELEASE OF THE ASH-FORMING MATTER IN THE FURNACE Any prediction or modeling of the formation and behavior of the fly ash requires some fundamental understanding of the initial release of the ash-forming matter during the burnout of the fuel particles. One essential question is to what extent the various ash-forming elements are released as gases during the burning process and to what extent they stay in the solid ash 6

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residue remaining after the burnout of the particle, respectively. The further reactions and fate of the ash-forming elements are very different depending upon this initial split of the elements into the gases versus the solid ash residue.2729 Frandsen et al.30 give an interesting overview of the methods and challenges in any systematic prediction of the release of ash-forming matter in combustion. Their overview focuses on conditions relevant to grate firing and pulverized fuel firing. Fuel particles burn in three stages: drying, devolatilization, and char oxidation. It is of vital interest to know in detail how the ashforming matter reacts during these stages. Drying takes place when the particle temperature reaches the temperature of around 100 °C. Very little ash-forming matter is expected to be released during the drying stage of the fuel particle after entering the furnace, even if very little experimental information to confirm this is available.31 During devolatilization, the dry fuel particle is heated to a higher temperature under a heavy release of organic vapors and gases because of thermal decomposition, pyrolysis, of the organic matrix of the fuel. For most of the biomasses, the majority, 7090% of the combustible matter, will be released as vapors during this devolatilization stage. Some ash-forming elements are released during this stage. During the following char burning, the particle temperature in the fluidized bed can reach 8501000 °C. During the char burnout, the easily volatilized ash-forming metals will form gaseous compounds, such as chlorides or hydroxides. The extent of this release into gases is heavily dependent upon the burning conditions (temperature and oxygen concentration). Dayton et al.32 made direct mass spectrometer measurements of the alkali and chlorine products released during combined pyrolysis and char combustion. The main gas-phase products detected containing chlorine or potassium were HCl during the volatile combustion phase and KCl and KOH during the char combustion phase. J€aglid et al.33 and Olsson et al.34 investigated alkali release from biomass pyrolysis by measurements of the total alkali in the gas phase as a function of the temperature. They found that 0.10.2% of the alkaline metal content in straw was released below 500 °C. A larger part of the alkali metal release took place at a higher temperature (>500 °C). The presence of chlorine was found to enhance the alkali metal emission at the higher temperature, while the alkali metal release in the lower temperature region could not be correlated with the chlorine content.34 Later, Jensen et al.35 pointed out that some minor amounts of inorganic matter may be lost during pyrolysis because of the convective transport of small particles caused by the liberated gases, and this may be the reason for this low-temperature alkali release reported by Olsson et al.34 Bj€ orkman et al.36 studied the release of chlorine during pyrolysis of different types of biomasses, applying a heating rate of 50 °C/min. Below 200 °C, no significant release of chlorine was observed. However, already at 400 °C, between 20 and 50% of the chlorine was released, and at 900 °C, between 40 and 70% of the chlorine was released. A recent laboratory study by Wu et al.37 on the combustion of bran showed that sulfur was almost fully vaporized during fixedbed pyrolysis below 700 °C. A total of 6070% of the K and P in bran were released during combustion, in the temperature range of 9001100 °C. They assumed that the release of K and P was attributed to the vaporization of gaseous KPO3 generated from the thermal decomposition of inositol phosphates, which were considered to be a major source of P and K in bran. Quite little fundamental information is available of the release of the various ash-forming elements of biomass fuels strictly

Figure 4. Thermodynamic equilibrium calculation of the composition of the ash in the flue gases from combustion of the fast growing willow tree Salix.44

under conditions relevant to the FBC. Advanced aerosol measurements in the flue gases of full-scale FBC boilers have shed some light to the total share of vaporized material in FBC conditions.27,28 It is clear that all ash-forming elements have their characteristic tendency to react during combustion, depending upon their chemical form in the fuel. One expects some connection between the advanced fuel characterization, e.g., chemical fractionation, and the behavior of the element in combustion. The ash-forming matter, which is present as non-soluble silicates, will probably have a low tendency to release volatile species during combustion. On the other hand, soluble alkali compounds in the fuel may easily be released as vaporized compounds.22,26,38 More light needs to be shed on the more quantitative connection of advanced fuel characterization and laboratory analyses, on one hand, and the release and formation of the initial ash particles during the burnout of the particle at FBC conditions, on the other (see the next paragraph).

’ FORMATION OF FLY ASH A typical fly ash particle size distribution from CFBC of biofuels is a bimodal curve, with the smaller maximum at around 0.20.5 μm and another maximum at a particle size of several micrometers.27,28,39 This type of curve, in principle similar to the ones for ashes from pulverized coal combustion, is a result of the two different routes of ash formation. The sub-micrometer particle fraction originates in condensation of volatilized metal compounds, and it is typically highly enriched in sulfates, carbonates, or chlorides of alkali metals.27,28 Also, zinc and lead salts may be enriched in the sub-micrometer fraction, when fuels such as demolition wood are used.40,41 There is a complicated interaction between the formation and composition of the sub-micrometer fly ash and the content of sulfur (SO2) and chlorine (HCl) in the furnace.31,39,42,43 In simple terms, it is a question of competition of the various reaction routes of the volatilized potassium. Dependent upon the detailed conditions, the released potassium can be converted into chloride, carbonate, or sulfate or it can react with the bed material to silicates (see the next paragraph). Thermodynamic equilibrium calculations have been shown to be useful when discussing the chemical composition of the fly ash. Figure 4 is a first thermodynamic prediction of the composition of the ash after combustion of the fast growing willow tree 7

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Salix as function of the temperature.44 In this calculation, the input was simply the total amount of ash-forming elements being considered together with the combustible matter from the fuel and a stoichiometric excess of air of 10%. The figure gives the main stable solid phases of the ash and also indicates the presence of a liquid phase at temperatures above about 900 °C. This liquid phase is a mixture of potassium sulfate and potassium carbonate. In the calculations behind Figure 4, all ash-forming matter in the fuel has been taken into consideration. This kind of calculation should, however, be complemented with information of the “reactivity” of the elements in the fuel. All ash-forming matter in the fuel is not equally reactive or “available” to the chemical conversions. Some ash constituents are present in such stable or inert compounds in the fuel that they do not react further in combustion, and, consequently, these constituents should be excluded from such equilibrium analyses. One attempt to do this would be to exclude the non-soluble metals based on chemical fractionation. These metals are most often bound together with silicon as practically “non-reactive” silicates of various kinds. In CFBC, only part of the ash will follow the flue gases after the cyclone. For these conditions, it may be of interest to study the composition of the fine, sub-micrometer fly ash only. Here, the equilibrium assumption appears acceptable; the gas chemistry including the condensation of the aerosol particles is quite rapid and seems to reach chemical equilibrium within the conditions of a CFBC furnace. However, these thermodynamic calculations require information of the percentage of the ash-forming matter that is released as gaseous species. The release information is needed for the ash-forming metals but also for sulfur and chlorine because these are the key reactants transforming the fly ash metals into their various compounds. This is the weak point of such predictions; these kinds of release percentages need to be established empirically (see above). Advanced fuel characterization (e.g., chemical fractionation) could be used as one starting point to such empirical information. Similar to that mentioned above, some promising generic laboratory studies have produced first empirical release data for some biomasses at grate or pulverized fuel conditions,30,38 but much more work is required, especially under FBC conditions. In CFBC applications, often limestone is added for sulfur capture. This is another complication to any predictions of the fly ash chemistry. The fly ash composition is strongly dependent upon the presence of SO2 available for the fly ash metals. If the SO2 level is reduced by adding limestone, this will naturally influence the fly ash chemistry.

The detailed mechanisms of the silicate formation are still not fully understood. The solidgas reaction between quartz and alkali chloride vapors is one probable route. SiO2 ðsÞ þ 2KClðgÞ þ H2 OðgÞ ¼ K 2 SiO3 ðlÞ þ 2HClðgÞ A similar reaction could also take place with other alkali vapors, such as hydroxides, which may be the dominant gaseous alkali compounds when there is no chlorine available for the alkalis. SiO2 ðsÞ þ 2KOHðgÞ ¼ K 2 SiO3 ðlÞ þ H2 OðgÞ Reactions resulting in calcium silicate in the outer layer of sand particles may take place via a calcium chloride intermediate.54 Rapid bed sintering has recently been reported also in boilers using biomass fuels with high phosphorus content, such a rapeseed cake or sewage sludge.1315,18,55 The composition of typical sewage sludge was shown in Figure 2. Today, there is an increased interest in better understanding the chemistry of phosphorus in biomass combustion.15,16,18,37,56 The formation of alkali phosphates appears to be another important initiator of the bed sintering. Calcium phosphates seem not to be reactive in the same way.15,18 Another mechanism for bed sintering is due to reactions of the lime in the bed with carbon dioxide and sulfur oxides.46 CaOðsÞ þ CO2 ðgÞ ¼ CaCO3 ðsÞ

2CaOðsÞ þ 2SO2 ðgÞ þ O2 ¼ 2CaSO4 ðsÞ These reactions may cause the lime particles to agglomerate by a process referred to as “reaction-induced sintering”.45,46 The recarbonation reaction takes place at temperatures below about 800 °C, and sintering because of this reaction may take place if the bed temperature is lower than normal for some reason (partial load). Also, this sintering is known to initiate plugging of the return leg at some applications with high lime use.46,57 This recarbonation-induced sintering may be further reinforced by the sulfation reaction.57 The bed chemistry may become very complex when several fuels are burned simultaneously. Figure 5 shows bed material from 550 MWth CFBC using mixtures of peat, forest residue, bark, and coal in different proportions and sequences.58 After some weeks of operation, the bed contained particles originating from each of the fuels but also agglomerates of particles from different fuels and also particles from the limestone being added in some sequences. The different particle identities in Figure 5 were deduced on the basis of the elemental composition and particle shape and size.58 It is obvious that the potential sintering of such a multi-component bed is difficult to foresee or theoretically predict. Besides quartz, also many other refractory materials have been suggested to be used as bed materials. These alternative bed materials include a variety of other natural minerals,59 porous alumina,60 blast furnace slag,43,61 olivine sand,43 etc. With these materials, the formation of the molten, glassy silicates may be eliminated. Because of the extra cost of most of the alternative bed materials, quartz is still today the dominant bed material even with the high alkali biofuels, and the bed sintering is kept under control by continuously replacing a sufficient part of bed material with fresh sand.59

’ INTERACTION BETWEEN ASH-FORMING MATTER AND THE BED MATERIAL A highly unwanted property of some biomass fuels is their tendency to cause bed agglomeration. Bed agglomeration has been studied since the early days of the FBC technology. Bed sintering is caused by the change of the bed particle chemistry because of the interaction with ash-forming matter. A number of papers have been published on the chemical mechanisms of the bed sintering.6,4553 The most used bed material is quartz or silica sand. With silica bed particles, the most common particle surface compounds when burning biomass fuels are silicates of potassium and calcium. These alkaline silicates can form a molten viscous glassy phase on the particle surfaces, thus making the particles sticky.45,4951 8

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Figure 5. Cross-section SEM images of bed particles from 550 MWth CFBC using mixtures of peat, forest residue, bark, and coal.58 Magnification: true height of the images is 100 μm.

installations using biofuels are significantly lower than in similar boilers using coal. The stickiness and corrosion properties of the fly ash are connected to its melting properties. Salt mixtures, such as the mixtures in the fly ash, melt in stages when heated. Figure 6 shows an example of a melting curve for a salt consisting of a mixture of sulfates and chlorides of potassium and sodium. The figure is based on the Åbo Akademi thermodynamic multicomponent melt model, by which one can calculate the percentage of the liquid phase as a function of the temperature for a given mixture of typical inorganic salts in biomass ashes.6365 Backman et al. originally defined four characteristic temperatures based on such theoretical melting curves:64 (1) T0 is the temperature at which the first molten phase appears, also called the first melting temperature or solidus temperature. This temperature may be of interest when discussing superheater corrosion. If the superheater tube metal temperature is higher than the T0 of the fly ash particles deposited on the tube, the tube metal will be exposed to some liquid phase of the salt deposit. (2) T15 is the temperature at which 15% of the mixture is molten, also called sticky temperature. By the time when this was introduced, it was assumed that roughly 15% of the molten phase is required to make the particles sticky. (3) T70 is the flow temperature, at which a sample body made of the mixture will be so heavily molten that it loses its shape and collapses (similar to the “radical deformation point” sometimes referenced). (4) T100 is the temperature at which the last crystalline phase is dissolved in the liquid phase, the temperature of complete melting. These melting curves and characteristic temperatures have been shown to be useful, and many boiler designers are using them to obtain a first impression of the quality of the fly ash. In particular, the sticky temperature has been shown to clearly indicate the extent and location of severe fouling in the flue gas duct. The stickiness criterion of the 15% molten phase has been tested experimentally for particles of the alkali salt mixture. Tran et al.66 used an entrained flow reactor and synthetic alkali salt mixtures. Figure 7 shows the deposition rate of synthetic ash particles on a cooled steel probe (500 °C surface temperature) as function of the percentage of the liquid phase in each of the fly

Figure 6. Calculated percentage liquid phase versus temperature curves for a salt consisting of a given mixture of sulfate and chloride of sodium and potassium, with the four characteristic melting temperatures indicated.56,64

’ FOULING AND CORROSION PROPERTIES OF FLY ASH In CFBC, the cyclone separates the coarse particles from the flue gases and only the finer fly ash passes to the flue gas duct. Dependent upon the properties, this fly ash may form disturbing ash deposits on tube surfaces in the superheater or economizer of the flue gas duct. Also, condensing alkali vapors may contribute to the fouling of the cooled heat-exchanger surfaces.27,28 These deposits may deteriorate the heat transfer to the steam tubes and disturb or even plug the flow of the flue gases through the heat-exchanger packages. They may also cause corrosion of the heat-exchanger tube metal. Fouling and corrosion caused by biomass fuel ashes has been a topic of active research during the last 1015 years. Frandsen62 summarized this research, which continues actively at the present moment. High-temperature corrosion caused by the fly ash is the main reason why the steam temperature and the power production efficiency in FBC 9

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deposition rate jumps from the baseline (indicating that the particles become sticky) when the share of the liquid phase in the particles exceeds 1520%. At liquid contents higher than 20%, practically all particles hitting the probe stay on the surface. The deposition rate at those conditions is simply limited by the amount of particles hitting the surface. The first melting temperature T0 has been suggested as an important parameter when discussing high-temperature corrosion of boilers. If the tube surface temperature is higher than the T0 of the deposited salt, the heat-exchanger steel surface is directly exposed to a molten salt phase. This contact results in a heavy increase of the corrosion rate. Figure 8 shows laboratory corrosion test results by Skrifvars et al. 67 In these tests, coupons of six steel qualities were exposed to alkali salt mixtures at different temperatures. After an exposure of 1 week, the oxide layers formed between the salt layer and metal were measured as indications of the corrosion rate. The steels 1 and 2 were carbon steels; the steels 3 and 4 were stainless steels, and the steels 5 and 6 were high nickel alloys (see details by Skrifvars et al.67). Within the range of temperatures tested, pure alkali sulfate (top in Figure 8) did not cause any measurable corrosion for any of the steels, except for the carbon steel number 1. However, even small additions of alkali chloride in the salt dramatically triggered corrosion (middle and bottom in the figure). The T0 for the sulfate salt in the test (mixture of Na2SO4 and K2SO4) was around 620 °C. The addition of alkali chlorides decreased the T0 to around 525 °C. As expected, above some 500525 °C, there was a clear increase in the measured corrosion rates in the tests with chloride present. This was true for the carbon steel but also for the austenitic stainless steel tested. However, additional tests have shown that salt mixtures containing chlorides can cause corrosion at temperatures clearly below the T0.68 Alkali-chloride-induced high-temperature corrosion has been studied already for more than 30 years. In fact, this corrosion is the main reason for the significantly lower superheated steam temperatures that can be applied in boilers burning biomasses. Since the excellent review by Nielsen et al.,69 a lot of research has been carried out. However, still many open questions remain concerning the details of the mechanisms of this corrosion, and the research continues actively.70,71

Figure 7. Deposition rate of alkali salt particles of a variety of compositions as a function of their percentage of the liquid phase at the temperature of deposition. The deposit probe temperature is 500 °C, and the entrained flow reactor temperature is 7001000 °C.66

’ FUEL MIXTURES One of the advantages of the FBC technology is that it makes it possible to burn different types of solid fuels simultaneously in a simple way. In fact, most of the new FBC installations in Europe are today designed for mixed fuel operation. However, mixtures of fuels may cause surprises. In particular, the ashes from the different fuels may interact with each other, thus resulting in fly ashes with completely different behavior than any of the component fly ashes.29 Consequently, the interaction of fuel ashes from two or more fuels being burned in FBC has become a very important research topic. During the last 5 years, more than 20 papers per year have been published on the issue of fuel mixtures in the FBC. Several recent studies, including pilot- and full-scale FBC tests have shown that both chemical and physical interactions may take place.8,13,14,7275 Chemical interaction is most often connected to the fate of the alkalis and chlorine.39,76,77 Alkali chlorides formed with one fuel can be converted to less harmful compounds by introducing a

Figure 8. Thickness of corrosion product oxide layers on six steel qualities after exposure for 1 week to alkali salt mixtures with increasing chloride content (see the text): (top) only alkali sulfates, (middle) alkali sulfate with 0.3% Cl added as alkali chloride, and (bottom) alkali sulfate with 1.3% Cl added as alkali chloride.67

ash mixtures tested. The particles were entrained by hot combustion gases through a reactor with a temperature of 7001000 °C before hitting on the cooled probe. The figure shows that the 10

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and practically no chlorine, and the concentration of gaseous KCl is 12 ppm. At 62 min, the feeding of straw pellets starts, which increases the input of both potassium and chlorine and the concentration of KCl increases to about 20 ppm. At 96 min, the PVC is added, increasing the chlorine input by a factor of almost 5. This does not introduce any more potassium. However, the PVC addition raises the concentration of KCl to about 70 ppm. Here, the added chlorine seems to strongly facilitate the volatilization of potassium. Figure 10 shows an example of the fouling tendency of fly ashes from eucalyptus bark and rice husk, respectively. The results come from entrained flow reactor tests using an air-cooled sampling tube of about 25 mm in diameter (similar to the tests in Figure 7). Eucalyptus bark alone showed a high degree of fouling, while rice husk alone appeared non-fouling. The high fouling rate of the fly ash from the eucalyptus bark was steeply reduced by the introduction of the rice husk as the secondary fuel. This effect was interpreted as being more physical than chemical. The coarse rice husk ash particles (see Figure 3) are not only non-fouling themselves, but they also seem to be able to keep the tube surface clean from the more sticky bark ash.8 Later, a similar “cleaning interaction” has been found with some other fuel pairs as well.73

Figure 9. In situ measurement of gaseous KCl at the exit from the cyclone during firing of (left) wood alone, (middle) wood together with straw, and (right) wood and straw together with PVC.42

’ CONCLUSION AND FUTURE RESEARCH NEEDS Bed sintering, superheater fouling, and high-temperature corrosion are crucial factors to take into account when fuels are selected for FBC, especially when biomass or waste-derived fuels are considered. It is of great interest to find ways of predicting the degree of ash-related problems for various fuels or fuel mixtures. Fuel characterization techniques have advanced significantly. Chemical fractionation and microscopic analysis of ash samples produced by gentle (low-temperature) ashing give deeper a understanding of the presence of the ash-forming matter in fuels. This may be one basis to better estimate the release and behavior of the ash-forming elements during combustion. Quantitative prediction of the release of the ash-forming matter in combustion is not possible just based on laboratory analysis of the fuel samples. Much more laboratory work is needed to establish the connection between the detailed composition of the fuels and the release of their ash-forming elements under various FBC conditions. Bed sintering is mostly connected to formations of molten or glassy alkaline silicates on the surfaces of the quartz particles used as bed material. The reactions between alkali or alkaline earth metals and silica in the quartz are influenced by several competing reactions, thus making the prediction of bed sintering quite demanding. Phosphorus-induced sintering has recently been reported when high-phosphorus biomasses are burnt. The details of the high-temperature chemistry of phosphorus are still unclear and require more fundamental studies. Alternative bed materials that do not contain quartz have become very interesting. More systematic work of the materials in connection with the different critical ash-forming matter is needed. Fouling and corrosion properties of the fly ash are strongly connected to the melting behavior of the ash. Thermodynamicphase calculations allow for quite accurate prediction of the melting curves of ash mixtures containing various alkali salts typical for biomass ashes. The melting curves can be used to

Figure 10. Deposit formation on a cooled sampling probe exposed to fly ash from combustion of rice husk together with two types of eucalyptus bark in different proportions. Entrained flow reactor tests. The reactor temperature is 1000 °C, and the probe surface temperature is 500 °C.8

second fuel with suitable other reactive compounds.76,78,79 The introduction of sulfur will convert the alkali chlorides into sulfates, thus releasing hydrochloric acid. Alkali chlorides may also react with silicates or aluminum silicates and kaolin in the other ashes, thus forming alkali silicates of various kinds.49,76,80,81 The reactions are similar to the reactions discussed above in the context of the bed sintering. Further, calcium in some ashes (or as a sulfur-capture additive) will also interact with the alkali chloride chemistry by competing by reactions of its own with sulfur oxides (forming calcium sulfate) or the silicates (forming calcium silicates). Both of these reactions may decrease the extent to which the alkali chlorides react with the same reactants. Recent studies using an online KCl analyzer at the exit from the cyclone have given valuable information of the ash and flue gas chemistry in CFBC during burning of fuel mixtures of various kinds.42 Figure 9 shows an example of a test campaign burning wood, straw, and polyvinyl chloride (PVC) waste introduced stepwise. The boiler is first fired with wood, containing little alkali 11

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estimate stickiness and corrosion properties of the fly ash. However, more work is needed to establish the connection of the presence of a molten phase in the ash deposit and hightemperature corrosion. Ash behavior in boilers using more than one fuel simultaneously may cause additional challenges. Ashes from different fuels may interact with each other. This can lead to surprising nonlinear behavior of the ash behavior as a function of the fuel mixture. Chemical interactions of this kind include reactions where alkali compounds from one fuel react with sulfur or maybe aluminum silicates from the other, thus leading to a major change in the alkali-chloride-induced problems, such as bed sintering or superheater corrosion. A general understanding of the ash chemistry in many fuel mixtures has increased significantly during the last 5 years or so. However, the interactions of the ashes from different biomasses or waste fuels with each other or with ashes from fossil fuels may be extremely complex. To make progress in the understanding of these interactions, ingenious systematic studies would be very welcome. Theoretical predictions of ash-related problems based on fuel characterization and modeling are under development. However, for completely unknown fuels or fuel mixtures, pilot- or full-scale FBC tests are still necessary before safe operation can be guaranteed.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: mikko.hupa@abo.fi.

’ ACKNOWLEDGMENT I thank all of my present colleagues and Ph.D. students at Åbo Akademi for the inspiring and productive collaboration in the many research activities behind this review. Further acknowledgements go to my many important colleagues outside of Åbo Akademi: Bo Leckner, Kim Dam-Johansen, Johan Hustad, Flemming Frandsen, Honghi Tran, Rainer Backman, Benco Skrifvars, Mischa Theis, Jatta Partanen, Vesna Barisic, and Edgardo Coda Zabetta. Åbo Akademi combustion research is supported by the National Technology Agency (Tekes) and the industrial consortium of the companies Andritz Oy, Metso Power Oy, Oy Mets€a-Botnia Ab, Foster Wheeler Energia Oy, UPM-Kymmene Oyj, Clyde Bergemann GmbH, International Paper, Inc., and Top Analytica Oy Ab. Their support is warmly acknowledged. This work is part of the activities of the Åbo Akademi Process Chemistry Centre within the Centres of Excellence Program by the Academy of Finland. ’ REFERENCES (1) Nordin, A. Chemical elemental characteristics of biomass fuels. Biomass Bioenergy 1994, 6 (5), 339–347. (2) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Combustion properties of biomass. Fuel Process. Technol. 1998, 54 (13), 17–46. (3) Obernberger, I.; Thek, G. Physical characterisation and chemical composition of densified biomass fuels with regard to their combustion behavior. Biomass Bioenergy 2004, 27 (6), 653–669. (4) BIOBIB, Database for Biofuels; Institute of Chemical Engineering, Fuel and Environmental Technology, University of Technology: Vienna, Austria; http://www.vt.tuwien.ac.at (accessed Sept 2011). (5) PHYLLIS, Database for Biomass and Waste; Netherlands Energy Research Foundation ECN: Petten, The Netherlands; http://www.ecn. nl/phyllis (accessed Sept 2011). 12

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