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The effect of halloysite additive on operation of boilers firing agricultural biomass
Fuel Processing Technology 92 (2011) 845–855
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
The effect of halloysite additive on operation of boilers firing agricultural biomass Kazimierz Mroczek a, Sylwester Kalisz a,⁎, Marek Pronobis a, Józef Sołtys b a b
Institute of Power Engineering and Turbomachinery, Silesian University of Technology, Konarskiego 20, 44-100 Gliwice, Poland PTH Intermark, ul. Św. Marka 9/7, 44-102 Gliwice, Poland
a r t i c l e
i n f o
Article history: Received 15 August 2010 Received in revised form 19 November 2010 Accepted 22 November 2010 Available online 6 January 2011 Keywords: Halloysite Biomass Additives Boiler Ash fusibility
a b s t r a c t The paper presents results of investigations on using halloysite as an additive in biomass-fired boilers. It has been shown that in the case of a few different agricultural biomasses the halloysite addition increased the ash sintering temperature to the values noted for coals. This is an effect of bonding sodium and potassium in the form of chlorides and other compounds. In practical terms the halloysite additive may reduce slagging and fouling of boiler heating surfaces as well as deteriorate the agglomeration processes in fluidized beds. Moreover, addition of halloysite reduces the amount of KCl and NaCl present in ash (and therefore in ash deposits) thus decreasing the rate of high temperature corrosion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The growing need for lowering CO2 emissions has led to increased biomass combustion in boilers which are primarily designed for firing solid fossil fuels. More and more often the annual crops and biowastes of various origins are utilized despite the number of negative side effects they introduce in boiler operation. As a rule such biomass fuels contain more potassium, chlorine and phosphorus than coal thus creating in-furnace conditions promoting corrosion, slagging and fouling of heating surfaces as well as bed agglomeration in circulating fluidized bed (CFB) boilers. Generally, their properties are inconvenient from the standpoint of boiler technology. Fig. 1 shows biomass ash particles stuck together with glass-like K–Ca–Si low-melting eutectics. During a relatively short period of time such substances may cause strong slagging of furnace walls in stoker and pulverized fuel (PF) boilers and agglomeration of CFBs. Additionally the coating of fuel particles with glass-like layer increases unburned carbon in ash. The problems associated with biomass combustion may be solved using both design (protection cladding and protection air installations) and operational (share and properties of biomass and excess air number) means. The third possibility is to add properly chosen substances binding K in structures of higher melting temperature and lower corrosion propensities. Positive results have been obtained while adding kaolinite to the combustion process. In the presence of kaolinite, the high-temperature melting (tB N 1400 °C) alkali-aluminum-silicates
⁎ Corresponding author. Tel.: + 48 32 2371163; fax: + 48 32 2372193. E-mail address: [email protected] (S. Kalisz). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.11.020
are formed, eliminating liquid-phase reactions. In result, the hightemperature corrosion rate is strongly decreased [1,2]. The imminent disadvantage of using kaolinite is its relatively small specific surface (5– 10 m2/g). Therefore, the tests of another clay mineral – halloysite – as an alternative additive replacing kaolinite have been performed in the Institute of Power Engineering and Turbomachinery of the Silesian University of Technology. Halloysite (Al4(OH)8/Si4O10 × 10H2O) is a rather rare aluminosilicate clay mineral of low hardness (1–2 in Mohs scale), high specific surface (approximately 70–85 m2/g) resulting from the nanotubular structure — Fig. 2, and high temperature resistance. Its high reactivity is a consequence of phase changes occurring above 550 °C allowing for the formation of high melting compounds with alkali metals. The chemical constitution of halloysite is similar to that of kaolinite. The basic difference is visible in its mineralogical structure. A single plate of halloysite, consisting of Si tetrahedra and Al octahedra, is separated from the neighbouring layer with a space in which cations of K [3–5] may be intercalated — Fig. 3. Such structure is characterized by significantly higher reactivity than the structure of kaolinite for which only the outer surfaces of crystals may participate in reactions and sorption. In the structure of halloysite there are two hydroxyl groups: an internal group located between tetrahedral and octahedral layers and a superficial group situated on the octahedral layer. It brings about that the bonds between ions may occur not only on the surface of layers (like in kaolinite's fast bonded structure), but also inside the crystals. In Poland there exists one of the three halloysite mines in operation worldwide (Dunino mine). Halloysite is available in amounts sufficient for many years of use as an additive in the power generation industry. The mean composition of the Dunino halloysite is as follows: Al2O3 — 26 wt.%, SiO2 — 32 wt.%, Fe2O3 — 22 wt.%, and TiO2 — 4 wt.%. The admixtures of Fe
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2. Laboratory investigations of halloysite additive The following biomass fuels have been chosen for investigations with halloysite additive: • • • • •
Fig. 1. Microphotograph of biomass ash particles stuck together with glass-like K–Ca–Si low-melting eutectics.
and Ti occur in the form of separated particles of magnetite, hematite and ilmenite. If necessary, the impurities may be eliminated by means of a simple hydro-metallurgical process. Since halloysite is an aluminosilicate containing mainly Si, Al and admixtures of Fe and Ti, its introduction to the furnace should decrease the slagging and fouling tendencies. The ability of bonding K, being a compound found in many biomasses, should suppress the formation of KCl deposits on waterwall and superheater tubes, thus protecting heating surfaces against high-temperature corrosion. Bonding of K also increases the melting temperatures of eutectics which are typical for biomass ashes. Moreover, the addition of halloysite suppresses the formation of mycotoxins [6] which are important for storage and handling of biomass. In [7], bio-fuels are classified in three categories in order to distinguish between their agglomeration, fouling and corrosion propensities — Table 1. Important are the proportions of Ca, K and Si in ash as well as the relative contents of S, Cl and P in biomass. The Bio-1 (for example wood and bark) can be fired without major problems in CFBs and other fossil fuel fired boilers. Bio-2 (wheat straw described further as SA, rice husk and similar), according to [7], must be co-fired or requires the use of additives, while Bio-3 (sunflower seed and rapeseed residue) needs very strong countermeasures against the unwanted phenomena mentioned above.
wheat straw (SA), mixture of rape and cereal straw in a proportion of 80/20 wt.% (SB), rape straw (SC), pelletized sunflower husk (PS), willow (W).
The contents of moisture and ash as well as the elementary analyses of investigated biomasses are presented in Table 2. The analysis of metals in biomass samples was made using Atomic Absorption Spectrometry (AAS). In order to obtain ash for further investigations, the abovementioned biomasses were ground to the size of dp b 5 mm. Depending on the moisture and ash contents, the mass of a sample was kept between 500 and 1500 g, which was necessary for obtaining about 50 g of ash from each biomass fuel. The fraction of halloysite in relation to the mass of the sample has been set mainly in the range of 1–2 wt.%. Only in the case of rape straw SC the mass fractions of 3 and 4 wt.% have been tested. At first, the relatively coarse halloysite powder denoted as H was used whose particle size distribution is shown in Table 3. Since problems with homogenization of a relatively small amount of additive with a big amount of biomass occurred, it was decided to use powders of finer particle size distribution — see Ha–Hd in Table 3. The homogenized samples with addition of halloysite were then combusted at temperatures of 450–550 °C. After primary incineration and significant reduction of sample volume, the ash was heated and held for 1 h at 800 °C in the laboratory furnace. The following analyses were performed for biomass samples and corresponding ashes (after heating with various mass fractions of halloysite): • proximate and elementary analysis of biomass • elementary analysis of ash • sintering (tS), softening (tA) and melting (tB) temperatures of ash according to PN-G-04535:1982 • microphotograph and EDS (Electron Dispersed Spectrum) analyses. Further on, after [8], the molar ratio X indicating the degree to which the alkali metals are bonded with chlorine was calculated for all ashes X = ðNa + KÞ = Cl:
ð1Þ
This ratio represents the shift of alkali from alkali chlorides to larger molecular compounds. If ratio X = 1 then monovalent alkali metals are in chloride form. Values X N 1 indicate that K and Na are bonded with other compounds (for example aluminosilicates). Next, the base-to-acid (B/A) ratio indicating the fouling/slagging tendency was also determined B= A =
Fig. 2. Nanotubular structure of halloysite.
Fe2 O3 + CaO + MgO + Na2 O + K2 O + P2 O5 : SiO2 + Al2 O3 + TiO2
ð2Þ
The results of the B/A index must however be treated with an appropriate care. Halloysite containing high fractions of Fe may shift the B/A index towards higher values indicating higher slagging propensities. Table 4 shows the properties of ashes resulting from incineration of wheat — SA, mixture of rape and cereal straw — SB, rape straw — SC, pelletized sunflower husk — PS and willow — W. Willow is meant to be a reference biomass of relatively well understood combustion properties. For the sake of comparison, the additional values of Cl(b)
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Fig. 3. Halloysite structure with the possible location of absorbed ions.
and S(b) have been shown in Table 4. They represent data obtained for raw biomass after recalculation into the corresponding mass unit of ash. The comparison between Cl and S contents in ash with Cl(b) and S(b) makes it possible to evaluate the amounts of both elements which remain bonded in ash after heating. The following conclusions may be drawn from the data included in Table 4: Wheat straw SA contains average contents of Cl and low contents of S. The heating at 800 °C brings about the decrease of Cl content in ash of about 20 times compared with its percentage in SA biomass. Nevertheless, a small part of chlorine remains in the ash. Nearly all sulfur passes to the flue gas. In [9] it has been shown that by incineration of straw at 800 °C the main part of potassium is represented as low melting K2Si4O9, arising from chloride in the presence of a significant amount of Si. It explains the low concentration of Cl in ash. The remaining part of potassium is bonded in KCl(g) and K2SO4(s). By 800 °C the percentage of sulfates may be small which explains the lowering of S contents between raw biomass and its ash (S reacts to SO2). By adding halloysite containing Al2O3, part of KCl is transformed into high melting potassium aluminosilicates, for example KAlSiO4, locking K in the ash. The HCl formed as a by-product is admixed to the flue gas. This explanation is confirmed by a certain increase of the X ratio and an increase of sintering (tS) and softening (tA) temperatures of ash (Table 4 — SA). The mixture of rape and cereal straw SB contains very high contents of Cl and S, with the similar proportion of both elements as it is in the case of SA. The content of silicon in ash (Si ≅ 10%) is more than 2 times lower than in SA, while the Ca content is more
than four times higher — Table 4. Although approximately half of chlorine is emitted with the flue gas, the chlorine remaining in ash is Cl = 6.8%. A prevalent part of sulfur remains in ash. By the high content of calcium (Ca = 15%), part of chlorine may be bonded in CaCl2. According to chemical equilibrium calculations [9], in the range of incineration temperatures and with percentages of chlorine exceeding those of sulfur, it may be stated that the phenomena occurring by combustion of SB are similar to those of SA, with the difference that they occur by higher levels of chlorides and sulfates. In line with the high Ca-content the low melting Ca–Si-compounds prone to agglomerate the ash particles may
Table 2 Characteristics of investigated biomass fuels (a: air-dried basis). SA
SB
SC
Proximate and elementary analysis, wt.% Wa 5.51 8.71 Aa 3.89 7.61 Ca 44.6 42.1 a H 6.13 5.23 Na 0.34 0.44 a S 0.04 0.23 Cla 0.15 0.91 Metals, ppm Al 196 Fe 473 K 4741 Na 49 Mg 653 Ca 1772 Mn 60
383 1371 19,703 415 1043 8667 41
PS 9.77 7.38 42.4 5.75 0.65 0.09 0.90
340 1725 19,678 405 985 6783 23
W 6.45 6.89 44.6 5.52 0.67 0.02 0.14
5.75 2.25 45.8 6.20 0.46 0.02 0.03
267 1098 9837 47 1696 2584 12
204 433 1857 45 612 3515 86
Table 1 Categories of biomass according to their agglomeration, fouling and corrosion propensities [7]. Biomass Biomass example category
Fractions Relative content in the in ash fuel
Bio-1
Wood, bark
Ca, K N Si
Bio-2
Straw, rice husk
Ca, K b Si
Bio-3
Sunflower seed, rapeseed residue
Ca, K N Si
S
Cl
P
Low b 0.03% Low b 0.04% High N 0.2%
Low b 0.01% High 0.1% Low/ high
Low b 0.1% Low b 0.2% High N 0.8%
Table 3 Particle size distribution (shown as residue on sieve, wt.%) of halloysite used for investigations. Mesh size mm
Residue wt.%
Halloysite fractions H
Ha
Hb
Hc
Hd
0.15 0.088 0.041 0.02
R0.15 R0.09 R0.04 R0.02
22.3 45.7 – –
0.0 2.30 44.0 –
0.0 0.0 8.5 –
0.0 0.0 0.0 –
0.0 0.0 0.0 0.0
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Table 4 Properties of ash from wheat straw (SA), mixture of rape and cereal straw (SB), rape straw (SC), pelletized sunflower husk (PS) and willow (W) incinerated with various fractions of halloysite. SA
Percentage of halloysitea
0% H
C H N Cl S Cl(b) S(b) Al Fe K Na Mg Ca Mn Si X = (K + Na)/Cl [mol/mol] B/A tS [°C] tA [°C] tB [°C]
0.40 b 0.01 0.04 0.17 b 0.01 3.86 1.03 0.66 2.20 11.5 0.10 1.54 4.16 0.12 27.1 78 0.44 840 960 1180
a b
SB 1% H 0.26 b 0.01 b 0.01 0.13 b 0.01
3.31 8.73 9.67 0.11 1.33 3.19 0.14 23.8 86 0.54 850 1080 1240
Nomenclature according to Table 3. Willow sample other than that in 2% H mixture.
0% Ha 0.29 b 0.01 0.06 6.82 1.94 12.0 3.00 0.66 2.46 24.7 0.58 1.40 15.0 0.07 8.89 4.3 2.90 770 1250 1330
SC 2% Ha
4% Ha
2% Hc
0.28 b 0.01 0.12 5.73 3.01
0.19 b 0.05 0.11 4.21 1.36
0.23 b 0.05 0.18 4.65 1.60
3.00 7.82 19.7 0.51 1.21 11.3 0.10 9.58 4.1 2.08 790 1210 1230
4.53 12.7 18.3 0.52 1.03 9.74 0.12 9.96 5.1 1.91 1100 1210 1240
3.04 8.85 19.5 0.49 1.21 12.0 0.10 9.54 5.0 2.17 780 1150 1170
0% Ha 0.27 0.05 0.21 6.40 1.15 12.2 1.22 0.70 7.28 20.7 0.48 1.25 12.5 0.08 10.7 3.8 2.33 610 1170 1190
PS 2% Ha 0.21 b 0.05 0.16 5.81 1.19
2.94 11.1 20.3 0.50 1.18 11.0 0.11 11.0 4.1 2.04 790 1150 1170
3% Ha
1% Hc
0.16 0.01 0.12 5.20 1.34
760 1150 1190
790 1200 1240
2% Hc
1% Hd
2% Hd
0.25 b0.05 0.19 5.48 1.26
0.25 b0.05 0.16 4.13 1.22
2.88 12.67 20.5 0.51 1.24 10.4 0.12 11.0 4.4 2.11 900 1090 1170
3.33 14.5 17.5 0.48 1.07 10.4 0.15 11.0 5.0 2.00 800 1090 1140
790 1030 1100
0% Hc 0.67 0.14 0.17 0.49 0.25 2.03 0.29 1.21 2.40 17.3 0.18 2.74 3.35 0.03 26.2 41 0.59 810 900 1110
W 1% Hc
2% Hc 0.24 0.03 0.15 0.49 0.20
900 1070 1210
3.61 7.63 16.1 0.23 2.60 2.86 0.07 22.8 38 0.72 1030 1090 1160
2% Hd 0.36 b 0.05 0.23 0.37 0.22
1120 1210 1240
0% H 0.48 b0.01 b0.01 0.08 0.70 1.33 0.89 1.29 4.18 7.70 0.18 3.15 13.7 0.43 14.5 109 1.21 690b 1500b 1500b
2% H 0.71 b0.01 b0.01 0.07 0.41
8.30 15.1 5.32 0.20 1.46 6.18 0.39 13.3 95 0.88 1100 1190 1330
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Biomass
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Fig. 4. The influence of fraction and fineness of halloysite on the sintering temperature tS. ○ — SA and □ — SB.
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Fig. 6. The influence of halloysite additive on the temperatures tA and tB of ash for SA and PS.
arise. The tests of co-firing biomass in CFB boilers [7,8] indicate that K and Ca easily migrate to the bed material (consisting mainly of SiO2) forming low melting coating layers gluing the particles together. The melting temperature of K–Ca-silicates depends strongly on the fraction of potassium. The combustion tests of SB straw without halloysite confirmed the strong agglomeration and the local melting tendency of ash. The addition of 2 wt.% of halloysite weakened this tendency significantly, although the sintering temperature increased only a little. Considerably stronger was the effect of halloysite by its 4 wt.% addition. Relatively low values of X suggest that the predominant part of light metals is bonded in chlorides (X ≅ 4 for the straw without halloysite). By increasing the percentage of the additive the proportion of chlorides decreases in favour of aluminosilicates. Considering the fractions of K, Ca, Si, Cl and partly S, the constitution of rape straw SC is similar to that of SB, while the composition of pelletized sunflower husk PS is similar to that of SA. Therefore the phenomena occurring during their combustion should also be similar. In fact it is not exactly so, because in the case of PS all sulfur remains in ash, while SA ashes contain only traces of sulfur.
The investigations show an overall positive effect of halloysite addition on sintering and softening temperatures of ash and this effect is influenced by the fineness of the additive. In the use of coarse halloysite (with the exception of one test for SB) and its amount ranging to 2 wt.%, the effect was insignificant — Fig. 4. On the contrary, by addition of 4 wt.% of halloysite the tS for SB increased by 300 K. The decrease of B/A and therefore the reduction of ash slagging and sintering tendency were also noticed [10]. It has to be mentioned that for a relatively coarse halloysite particle size distribution it is practically impossible to homogenize 1 wt.% of additive (density of about 3 g/cm3) with the base biomass material (density of approximately 0.2 g/cm3). It explains why the expected positive effects remained insignificant even by addition of 2 wt.% of halloysite. Using more fine-grained halloysite (SC — Fig. 5) facilitated the homogenization thus increasing its efficiency as a sorbent. In the case of rape straw, which is an agro-biomass with extremely high contents of chlorine and sulfur, the addition of 2 wt.% of halloysite (Hc, d — Table 3) increased the sintering temperature of ash by 150–200 K. An even higher increase of tS was noticed by incineration of PS — Fig. 5. The effect of halloysite additive on softening (tA) and melting (tB) temperatures of ash is not unambiguous — see Figs. 6 and 7. In this case we take into account all the tests with 2% share of halloysite, regardless of its granularity. For biomasses SA and PS, whose chemical compositions are similar to the Bio-2 type of biomass (Table 1), addition of halloysite caused an increase in the characteristic ash
Fig. 5. The influence of fraction and fineness of halloysite on the sintering temperature tS. ■ — SC (Ha), □ — SC (Hc,d), and ● — PS (Hc,d).
Fig. 7. The influence of halloysite additive on the temperatures tA and tB of ash for SB and SC.
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Fig. 8. Microphotograph of the rape straw SC ash after incineration without halloysite.
fusion temperatures tA and tB — Fig. 6. On the contrary, in the case of biomasses SB and SC (rape straw), characterized by very high contents of K, Cl and S, the opposite phenomenon was observed — Fig. 7. The case of the relative lowering of the characteristic temperatures of SB and SC while halloysite is added needs further explanation. It is likely that a significant release of light metals takes place during incineration of pure SB and SC materials. Nevertheless, it is evident that the potassium contents in both ashes — pure SB and SC, and SB and SC blended with halloysite, are similar despite the fact that about 30 wt.% of their ash is composed of sorbent material. Thus a part of volatile light metals is trapped in the sorbent material in comparison to the case of incineration of pure biomasses. On the other hand, the tA and tB temperatures, as noted for pure SB and SC biomasses, are already sufficiently high (1250 °C and 1170 °C respectively) for practical applications and the influence of halloysite is of minor importance. It also has to be outlined here that the results presented concern ashes from ‘static’ incineration, i.e. biomass and halloysite were contacted only during the blending process. In a full-scale combustion reality the additional two-phase or one-phase processes would enhance contacting biomass with halloysite. Therefore the undisturbed picture of the influence of additive is to be obtained only in full-scale trials. The visible effect of adding halloysite is a significant diminution of the ash grain size, resulting from the weakening of high-temperatureagglomeration processes commonly observed in boilers. Figs. 8 and 9 from the optical microscope show the difference of the ash grain size after halloysite addition. The influence of halloysite on sunflower husks (Table 5) incinerated at temperatures around 1000 °C is illustrated in Figs. 10 and 11. Ash from the husks without halloysite is agglomerated and locally melted (Fig. 10), while it contains considerable amounts of S, K, Ca and Mg. Its composition is strongly heterogeneous. Potassium sulfates of low melt temperature are visible in a point marked with letter A, whereas strong agglomeration occurred by the presence of a liquid phase (or partial melting) in point B. ‘Low viscous melts’ may occur when alkali (Na and K) and earth alkali (Ca and Mg) salt mixtures are present in ash [12]. It seems that such agglomeration type predominated in point B. After addition of 2 wt.% of halloysite the ash is fine-grained, without agglomeration — see Fig. 11. Absorption of alkali metals by halloysite and production of high melting point alkali compounds (e.g. potassium aluminosilicates) bond these metals in ash. 3. Full-scale investigations of halloysite additive The results of laboratory investigations on the use of halloysite as an additive in biomass combustion were very promising. Blending halloysite with several biomass types resulted in the increase of ash
Fig. 9. Microphotograph of the rape straw SC ash after incineration with halloysite. a) 2 wt.% halloysite added and b) 4 wt.% halloysite added.
softening temperature to a level noted for coals, which means that this additive can reduce slagging and fouling in boilers. Bonding of sodium and potassium from the chlorides of these metals was also observed. This in turn leads to the reduction of the share of KCl and NaCl in the ash deposits, and thus to the reduction of high temperature corrosion intensity. In order to confirm the suitability of halloysite as an additive for the processes of biomass combustion, the tests in real conditions were
Table 5 Chemical composition of biomass used in full-scale tests in the ESW power plant. Winter wheat Straw-briquettes Sunflower Wood straw seed husks chips O 64 C 27 Si 4.96 Al 0.08 K 2.57 Na 0.14 Mg 0.2 Fe 0.1 P 0.13 Ca 0.81 S 0.15 Cl 0.47
60.3 38 0.79 0.04 0.6 0 0.03 0.01 0.05 0.17 0.04 0.17
51.3 46 0.43 0.32 0.96 0.06 0.22 0.11 0.06 0.37 0.13 –
61.9 37 0.18 0.04 0.29 0 0.05 0.02 0.03 0.41 0.04 0.08
Calculated average composition 61.7 35.8 0.92 0.06 0.69 0.02 0.08 0.04 0.05 0.47 0.06 0.14
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Fig. 10. Ash from the sunflower husk PS incinerated without halloysite at approximately 1000 °C.
Fig. 11. Ash from the sunflower husk PS incinerated with 2 wt.% of halloysite at approximately 1000 °C.
carried out. In standard fossil-fuel boilers the two technologies may be considered for blending halloysite with the base fuel: • addition of halloysite during the processing of fuels (e.g. during manufacturing of briquettes or pellets from biomass or coal and biomass), • addition of halloysite in the form of fine powder in the biomass feeding system.
Both methods have been tested in practice. The PTH Intermark, the owner of Dunino halloysite mine, in co-operation with the Silesian University of Technology and the Institute for Chemical Processing of Coal has developed and patented the technology of manufacturing the ‘compact-BC’ fuel. The product of this kind is made from coal and agricultural biomass in the form of briquettes, pellets or granulated matter and may include halloysite. For small boilers (e.g. underfed burner furnace) as well as industrial grate boilers pellets or briquettes
Fig. 12. Various forms of mixed compact-BC fuels.
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Fig. 13. Integration scheme of precombustor and main PF boiler in the ESW power plant and location of the halloysite feeding port.
can be used — Fig. 12a and b. The most suitable form of the premixed fuel for the PF and CFB combustion is the granular fuel (GBC — Fig. 12c) [11]. For the manufacturing of compact-BC fuel any kind of biomass and coal can be used. In practical terms, using relatively cheap finely granulated coal sorts and biomasses not used to date in boilers (agricultural wastes, etc.), the total price of compact-BC fuel may be lower than the price of fuels actually used for heating purposes and in power plants. On top of that the halloysite additive will significantly reduce the negative phenomena caused by the alkali content of biomass. The full-scale tests were carried out in the Stalowa Wola power plant (ESW) in the OP 150 type boiler equipped with a precombustor
consisting of a rotary kiln and a CFB furnace. The precombustor is fed with the system of belt conveyors. The biomass dried and partly gasified in the rotary kiln is passed to the CFB part, where the combustion is completed. The resulting flue gas is further transferred through the connecting duct to the lower part of the furnace of the main PF boiler. The precombustor can burn up to 15 t/h of biomass which equals approximately to 40% of the energy processed by the boiler — Fig. 13. The precombustion installation is highly integrated with the main PF boiler. The heat recovery system of the precombustor is connected with the evaporator of the main OP 150 boiler. At the outlet of the CFB an evaporator in the form of a tube palisade is placed (Figs. 15–17) which is a critical element in the whole system due to its susceptibility
Fig. 14. Halloysite feeding port in full-scale trials.
Fig. 16. The state of the tube palisade after 6 days of firing of the agricultural biomass without halloysite. Approximately 90% of the palisade passage is clogged.
Fig. 15. The state of the tube palisade after 36 days of firing of wood without halloysite. Approximately 30% of the palisade passage is clogged.
Fig. 17. The state of the tube palisade after 15 days of firing of the agricultural biomass with halloysite. Approximately 8% of the palisade passage is clogged.
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Fig. 18. SEM image and EDS analysis of deposited ash from combustion of mixed biomass without halloysite in CFB precombustor.
to ash deposition. Clogging of the palisade and the connecting duct is the major reason of outages in boiler operation. The halloysite material was fed on the conveyor belt automatically by means of a screw feeder — Fig. 14. The feeding rate was constant, not depending on the instantaneous stream of biomass, its humidity or kind. The halloysite-to-biomass ratio was kept approximately at the level of 1–2 wt.%. The feeding system consisting of a sequence of belt conveyors ensured preliminary mixing of halloysite powder with biomass in an industrial scale. These feeding conditions were meant to resemble those maintained during the laboratory tests. The full-scale tests were conducted by burning the mixture containing in average 77 wt.% of woody biomass and 23 wt.% of agricultural biomass. Agricultural biomass was composed of approximately 70 wt.% of winter wheat straw and the residual part was composed of straw and sunflower seed husks. The moisture contents averaged 30 wt.% and 12 wt.% for the woody and agricultural biomasses accordingly. The mean LHV amounted to 12 MJ/kg and the ash content averaged 4 wt.%. The bio-fuel mixture was characterized by high chlorine concentration and considerable shares of potassium, silicon, calcium and sulfur. The mean chemical composition of the biomass fired in full-scale tests is shown in Table 5. The operational experiences from the use of precombustor in the ESW power plant indicate that it is the actual state of the palisade (Fig. 15) which determines the overall availability of the integrated
installation. Moreover, investigations have shown that palisade fouling and deposit composition depend to a large extent on the chemical composition of the fired biomass. When the wood biomass was fired the palisade fouling was moderate (Fig. 15). However, the addition of agricultural biomass caused, after only several days, a total clogging of the flue gas pathways and the outage of the installation (Fig. 16). On the other hand, observations carried out during combustion of the agricultural biomass mixed with the halloysite additive have shown significant reduction of the fouling tendency of the palisade and therefore the minor clogging of the flue gas pathways (Fig. 17). Figs. 18–20 show a microscopic view of deposited ash collected from the tube palisade by firing biomass in the CFB precombustor of the OP 150 boiler at 850–1000 °C. Basically, the evaporated light metals cause the formation of deposits and chlorine facilitates their release into the gas phase [13]. They form alkali silicates with low melting points. A sticky deposit layer may be formed in the process of cooling the flue gases when the volatile alkali chlorides or hydroxides condense in the form of aerosols on the surfaces of heat exchangers, boiler walls or on fly ash particles. Deposited material contains particles of fly ash which are inertially retained by the presence of a sticky phase on the surfaces of ash particles or already built-up deposit. The studied deposits contained relatively large shares of Si, K and Ca, something already reported in the literature [13].
Fig. 19. SEM image and EDS analysis of deposit ash obtained from combustion of mixed biomass in CFB precombustor with addition of halloysite.
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Fig. 20. SEM image and EDS analysis of deposit ash obtained from combustion of mixed biomass in CFB precombustor with addition of halloysite (the table shows the chemical composition in point H where the halloysite grain is saturated with potassium and in point L where the low melting temperature, multicomponent eutectic is located).
Addition of halloysite decreased the tendency for formation of deposits. The halloysite grains retained their structure even though they contain up to 28 wt.% of potassium (Fig. 19 — point M). Fig. 19 depicts finer deposit structure in comparison with Fig. 18, something already reported in laboratory tests (Figs. 9–11). Investigations of bed agglomeration by combustion of wood and rice straw blend (qualitatively similar to the biomass mixture examined in this study) have shown an adhesive mechanism for the agglomeration of bed (mullite sand) by formation of the cement structure with the following components: Si, Ca, K, Mg and P [14]. The mechanism of this phenomenon is manifested by the initial reaction between bed particles and potassium compounds (of flue gas or aerosols) in surface-zone around bed particles. Potassium is the dominating element to be found in this zone (rim) as measurements proved. The adhesive rim is composed from submicron particles of ash and eroded bed. It was suggested that melting of locally accumulated aerosol mixture occurs as the composition of rim indicated. The presence of adhesive cement may result in agglomeration processes. To some extent the similar agglomeration mechanism is believed to play a role in agglomerated structures seen in Fig. 21. If the amount of the absorbed potassium, which is the main factor decreasing the ash sintering and melting temperatures, is high, then the quantity of agglomerated matter and deposits is smaller and
eventually the time after bed undergoes defluidization is prolonged. This effect is illustrated in Fig. 22 which shows the grain size distribution of the fluidized bed material. In comparison with the initial state, the growth of the grain size can be noticed for all the cases. However, in the case of halloysite addition, the level of growth is significantly smaller and the suppression of large grain growth is even more pronounced. This is an overall positive behavior since the large particles are the subject of defluidization in the first instance. Despite of the high chlorine content in the original fuel, there were moderate Cl amounts found in the palisade deposits (max. 0.3 wt.%) and a maximum of 0.5 wt.% on the surface of the bed agglomerates. Analysis of ash from the electrostatic precipitators of the whole boiler system (precombustor and PF boiler) showed 0.45 wt.% of chlorine. Unburned particles present in the fly ash contained higher concentrations of chlorine. Since the estimated share of biomass ash is about 20 wt.% of the whole ash collected, over 60 wt.% of chlorine was retained in the biomass ashes. Thus less than 40 wt.% of Cl is estimated to escape with flue gas. The presence of chlorine in the deposits may result from uneven temperature distribution in the precombustor furnace (differences amounting to 200 °C), for example due to condensing on cooler precombustor parts. By adding halloysite to the bio-fuel the concentration of chlorine decreased by 40 wt.% in the fly ash collected from the electrostatic
Fig. 21. Exemplary SEM image and EDS analysis of agglomerated bed material.
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Acknowledgments Investigations presented in this work were performed within the frame of the Polish Strategic Research Programme on Advanced Technologies for Power Generation (contract No.SP/E/1/67484/10). References
Fig. 22. Grain size distribution of CFB without and with halloysite additive. Rx — residue on sieve, wt.%.
precipitator. Moreover, the trace concentrations (max. 0.1 wt.%) of chlorine were observed both in analyzed deposits collected from the palisade and in bed agglomerates. 4. Conclusions The laboratory and full-scale investigations reported in this work demonstrate the ability of halloysite to enhance the operating features of furnaces firing various kinds of biomass. This includes also the agricultural biomasses whose chemical composition enhances the unwanted deposition and agglomeration processes. The use of halloysite as an additive will therefore significantly contribute to the extension of operational time and availability of biomass fired boilers. In particular, the following conclusions can be drawn from the present study: • the use of halloysite additive increases the level of ash sintering temperature for all agricultural biomasses tested in this work • a proper mixing of an additive with base fuel must be secured by using halloysite of fine particle size distribution and is especially important for pulverized fuel boilers; grinding of halloysite mineral thanks to its brittleness is neither difficult nor costly • adding of an iron containing halloysite influences the B/A index towards higher values therefore an appropriate index interpretation must be secured.
[1] F. Wigley, J. Williamson, G. Riley, The effect of mineral additions on coal ash deposition, Fuel Processing Technology 88 (11–12) (December 2007) 1010–1016. [2] M. Öhman, D. Boström, A. Nordin, H. Hedman, Effect of kaolin and limestone addition on slag formation during combustion of wood pellet in small scale pellet appliances, Energy & Fuels 18 (5) (2004) 1370–1376. [3] A. Bolewski, Mineralogia szczegółowa (Detailed mineralogy), in Polish, Wydawnictwa Geologiczne, 3rd edition, 19828 Warsaw. [4] S.B. Hendrick, M.E. Jefferson, Structure of Kaolin and Talc-Pyrophyllite Hydrates and Their Bearing on Water Sorption of Clay, American Mineralogist 23 (1938) 863–875. [5] Nopadol Chaikum, Inorganic intercalation complexes of halloysite: an infrared study, Journal of the Science Society of Thailand (www.scienceasia.org) 3 (1977) 189–200. [6] M. Kulok, R. Kolacz, Z. Dobrzanski, I. Wolska, The influence of halloysite on the content of bacteria, fungi and mycotoxins in feed mixtures, XIIth International Congress on Animal Hygiene ISAH, 4–8 September, Warsaw, Poland, vol. 2, 2005. [7] E. Coda Zabetta, V. Barišić, K. Peltola, A. Hotta, Foster Wheeler experience with biomass and waste in CFBs, 33rd Clearwater Conference, Clearwater, Florida USA, June 1–5, 2008. [8] B. Coda, Studies on ash behaviour during co-combustion of paper sludge in fluidized bed boilers — PhD thesis, Universität Stuttgart, 2004. [9] X. Wei, U. Schnell, K.R.G. Hein, Behaviour of gaseous chlorine and alkali metals during biomass thermal utilization, Fuel 84 (7–8) (May 2005) 841–848. [10] M. Pronobis, Evaluation of the influence of biomass co-combustion on boiler furnace slagging by means of fusibility correlations, Biomass & Bioenergy 28 (4) (April 2005) 375–383. [11] M. Pronobis, J. Sołtys, K. Jóźwiak, Negatywne skutki spalania biomasy i możliwości ich redukcji przy stosowaniu mieszanin paliw wtórnych (Negative effects of biomass combustion and their mitigation with use of secondary fuel mixtures, in Polish), Clean Energy–Clean Environment Conference, Kraków, 2008. [12] B.-J. Skrifvars, R. Backman, M. Hupa, Characterization of the sintering tendency of ten biomass ashes in FBC conditions by a laboratory test and by phase equilibrium calculations, Fuel Processing Technology 56 (1998) 55–67. [13] A.A. Khan, W. de Jong, J.P. Jansen, H. Spliethoff, Biomass combustion in fluidized bed boilers: potential problems and remedies, Fuel Processing Technology 90 (2009) 21–50. [14] P. Thy, R.B. Jenkins, C.E. Lesher, R.R. Bakker, Bed agglomeration in fluidized combustor fueled by wood and rice straw blends, Fuel Processing Technology 91 (2010) 1464–1485.
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
The effect of halloysite additive on operation of boilers firing agricultural biomass Kazimierz Mroczek a, Sylwester Kalisz a,⁎, Marek Pronobis a, Józef Sołtys b a b
Institute of Power Engineering and Turbomachinery, Silesian University of Technology, Konarskiego 20, 44-100 Gliwice, Poland PTH Intermark, ul. Św. Marka 9/7, 44-102 Gliwice, Poland
a r t i c l e
i n f o
Article history: Received 15 August 2010 Received in revised form 19 November 2010 Accepted 22 November 2010 Available online 6 January 2011 Keywords: Halloysite Biomass Additives Boiler Ash fusibility
a b s t r a c t The paper presents results of investigations on using halloysite as an additive in biomass-fired boilers. It has been shown that in the case of a few different agricultural biomasses the halloysite addition increased the ash sintering temperature to the values noted for coals. This is an effect of bonding sodium and potassium in the form of chlorides and other compounds. In practical terms the halloysite additive may reduce slagging and fouling of boiler heating surfaces as well as deteriorate the agglomeration processes in fluidized beds. Moreover, addition of halloysite reduces the amount of KCl and NaCl present in ash (and therefore in ash deposits) thus decreasing the rate of high temperature corrosion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The growing need for lowering CO2 emissions has led to increased biomass combustion in boilers which are primarily designed for firing solid fossil fuels. More and more often the annual crops and biowastes of various origins are utilized despite the number of negative side effects they introduce in boiler operation. As a rule such biomass fuels contain more potassium, chlorine and phosphorus than coal thus creating in-furnace conditions promoting corrosion, slagging and fouling of heating surfaces as well as bed agglomeration in circulating fluidized bed (CFB) boilers. Generally, their properties are inconvenient from the standpoint of boiler technology. Fig. 1 shows biomass ash particles stuck together with glass-like K–Ca–Si low-melting eutectics. During a relatively short period of time such substances may cause strong slagging of furnace walls in stoker and pulverized fuel (PF) boilers and agglomeration of CFBs. Additionally the coating of fuel particles with glass-like layer increases unburned carbon in ash. The problems associated with biomass combustion may be solved using both design (protection cladding and protection air installations) and operational (share and properties of biomass and excess air number) means. The third possibility is to add properly chosen substances binding K in structures of higher melting temperature and lower corrosion propensities. Positive results have been obtained while adding kaolinite to the combustion process. In the presence of kaolinite, the high-temperature melting (tB N 1400 °C) alkali-aluminum-silicates
⁎ Corresponding author. Tel.: + 48 32 2371163; fax: + 48 32 2372193. E-mail address: [email protected] (S. Kalisz). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.11.020
are formed, eliminating liquid-phase reactions. In result, the hightemperature corrosion rate is strongly decreased [1,2]. The imminent disadvantage of using kaolinite is its relatively small specific surface (5– 10 m2/g). Therefore, the tests of another clay mineral – halloysite – as an alternative additive replacing kaolinite have been performed in the Institute of Power Engineering and Turbomachinery of the Silesian University of Technology. Halloysite (Al4(OH)8/Si4O10 × 10H2O) is a rather rare aluminosilicate clay mineral of low hardness (1–2 in Mohs scale), high specific surface (approximately 70–85 m2/g) resulting from the nanotubular structure — Fig. 2, and high temperature resistance. Its high reactivity is a consequence of phase changes occurring above 550 °C allowing for the formation of high melting compounds with alkali metals. The chemical constitution of halloysite is similar to that of kaolinite. The basic difference is visible in its mineralogical structure. A single plate of halloysite, consisting of Si tetrahedra and Al octahedra, is separated from the neighbouring layer with a space in which cations of K [3–5] may be intercalated — Fig. 3. Such structure is characterized by significantly higher reactivity than the structure of kaolinite for which only the outer surfaces of crystals may participate in reactions and sorption. In the structure of halloysite there are two hydroxyl groups: an internal group located between tetrahedral and octahedral layers and a superficial group situated on the octahedral layer. It brings about that the bonds between ions may occur not only on the surface of layers (like in kaolinite's fast bonded structure), but also inside the crystals. In Poland there exists one of the three halloysite mines in operation worldwide (Dunino mine). Halloysite is available in amounts sufficient for many years of use as an additive in the power generation industry. The mean composition of the Dunino halloysite is as follows: Al2O3 — 26 wt.%, SiO2 — 32 wt.%, Fe2O3 — 22 wt.%, and TiO2 — 4 wt.%. The admixtures of Fe
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2. Laboratory investigations of halloysite additive The following biomass fuels have been chosen for investigations with halloysite additive: • • • • •
Fig. 1. Microphotograph of biomass ash particles stuck together with glass-like K–Ca–Si low-melting eutectics.
and Ti occur in the form of separated particles of magnetite, hematite and ilmenite. If necessary, the impurities may be eliminated by means of a simple hydro-metallurgical process. Since halloysite is an aluminosilicate containing mainly Si, Al and admixtures of Fe and Ti, its introduction to the furnace should decrease the slagging and fouling tendencies. The ability of bonding K, being a compound found in many biomasses, should suppress the formation of KCl deposits on waterwall and superheater tubes, thus protecting heating surfaces against high-temperature corrosion. Bonding of K also increases the melting temperatures of eutectics which are typical for biomass ashes. Moreover, the addition of halloysite suppresses the formation of mycotoxins [6] which are important for storage and handling of biomass. In [7], bio-fuels are classified in three categories in order to distinguish between their agglomeration, fouling and corrosion propensities — Table 1. Important are the proportions of Ca, K and Si in ash as well as the relative contents of S, Cl and P in biomass. The Bio-1 (for example wood and bark) can be fired without major problems in CFBs and other fossil fuel fired boilers. Bio-2 (wheat straw described further as SA, rice husk and similar), according to [7], must be co-fired or requires the use of additives, while Bio-3 (sunflower seed and rapeseed residue) needs very strong countermeasures against the unwanted phenomena mentioned above.
wheat straw (SA), mixture of rape and cereal straw in a proportion of 80/20 wt.% (SB), rape straw (SC), pelletized sunflower husk (PS), willow (W).
The contents of moisture and ash as well as the elementary analyses of investigated biomasses are presented in Table 2. The analysis of metals in biomass samples was made using Atomic Absorption Spectrometry (AAS). In order to obtain ash for further investigations, the abovementioned biomasses were ground to the size of dp b 5 mm. Depending on the moisture and ash contents, the mass of a sample was kept between 500 and 1500 g, which was necessary for obtaining about 50 g of ash from each biomass fuel. The fraction of halloysite in relation to the mass of the sample has been set mainly in the range of 1–2 wt.%. Only in the case of rape straw SC the mass fractions of 3 and 4 wt.% have been tested. At first, the relatively coarse halloysite powder denoted as H was used whose particle size distribution is shown in Table 3. Since problems with homogenization of a relatively small amount of additive with a big amount of biomass occurred, it was decided to use powders of finer particle size distribution — see Ha–Hd in Table 3. The homogenized samples with addition of halloysite were then combusted at temperatures of 450–550 °C. After primary incineration and significant reduction of sample volume, the ash was heated and held for 1 h at 800 °C in the laboratory furnace. The following analyses were performed for biomass samples and corresponding ashes (after heating with various mass fractions of halloysite): • proximate and elementary analysis of biomass • elementary analysis of ash • sintering (tS), softening (tA) and melting (tB) temperatures of ash according to PN-G-04535:1982 • microphotograph and EDS (Electron Dispersed Spectrum) analyses. Further on, after [8], the molar ratio X indicating the degree to which the alkali metals are bonded with chlorine was calculated for all ashes X = ðNa + KÞ = Cl:
ð1Þ
This ratio represents the shift of alkali from alkali chlorides to larger molecular compounds. If ratio X = 1 then monovalent alkali metals are in chloride form. Values X N 1 indicate that K and Na are bonded with other compounds (for example aluminosilicates). Next, the base-to-acid (B/A) ratio indicating the fouling/slagging tendency was also determined B= A =
Fig. 2. Nanotubular structure of halloysite.
Fe2 O3 + CaO + MgO + Na2 O + K2 O + P2 O5 : SiO2 + Al2 O3 + TiO2
ð2Þ
The results of the B/A index must however be treated with an appropriate care. Halloysite containing high fractions of Fe may shift the B/A index towards higher values indicating higher slagging propensities. Table 4 shows the properties of ashes resulting from incineration of wheat — SA, mixture of rape and cereal straw — SB, rape straw — SC, pelletized sunflower husk — PS and willow — W. Willow is meant to be a reference biomass of relatively well understood combustion properties. For the sake of comparison, the additional values of Cl(b)
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Fig. 3. Halloysite structure with the possible location of absorbed ions.
and S(b) have been shown in Table 4. They represent data obtained for raw biomass after recalculation into the corresponding mass unit of ash. The comparison between Cl and S contents in ash with Cl(b) and S(b) makes it possible to evaluate the amounts of both elements which remain bonded in ash after heating. The following conclusions may be drawn from the data included in Table 4: Wheat straw SA contains average contents of Cl and low contents of S. The heating at 800 °C brings about the decrease of Cl content in ash of about 20 times compared with its percentage in SA biomass. Nevertheless, a small part of chlorine remains in the ash. Nearly all sulfur passes to the flue gas. In [9] it has been shown that by incineration of straw at 800 °C the main part of potassium is represented as low melting K2Si4O9, arising from chloride in the presence of a significant amount of Si. It explains the low concentration of Cl in ash. The remaining part of potassium is bonded in KCl(g) and K2SO4(s). By 800 °C the percentage of sulfates may be small which explains the lowering of S contents between raw biomass and its ash (S reacts to SO2). By adding halloysite containing Al2O3, part of KCl is transformed into high melting potassium aluminosilicates, for example KAlSiO4, locking K in the ash. The HCl formed as a by-product is admixed to the flue gas. This explanation is confirmed by a certain increase of the X ratio and an increase of sintering (tS) and softening (tA) temperatures of ash (Table 4 — SA). The mixture of rape and cereal straw SB contains very high contents of Cl and S, with the similar proportion of both elements as it is in the case of SA. The content of silicon in ash (Si ≅ 10%) is more than 2 times lower than in SA, while the Ca content is more
than four times higher — Table 4. Although approximately half of chlorine is emitted with the flue gas, the chlorine remaining in ash is Cl = 6.8%. A prevalent part of sulfur remains in ash. By the high content of calcium (Ca = 15%), part of chlorine may be bonded in CaCl2. According to chemical equilibrium calculations [9], in the range of incineration temperatures and with percentages of chlorine exceeding those of sulfur, it may be stated that the phenomena occurring by combustion of SB are similar to those of SA, with the difference that they occur by higher levels of chlorides and sulfates. In line with the high Ca-content the low melting Ca–Si-compounds prone to agglomerate the ash particles may
Table 2 Characteristics of investigated biomass fuels (a: air-dried basis). SA
SB
SC
Proximate and elementary analysis, wt.% Wa 5.51 8.71 Aa 3.89 7.61 Ca 44.6 42.1 a H 6.13 5.23 Na 0.34 0.44 a S 0.04 0.23 Cla 0.15 0.91 Metals, ppm Al 196 Fe 473 K 4741 Na 49 Mg 653 Ca 1772 Mn 60
383 1371 19,703 415 1043 8667 41
PS 9.77 7.38 42.4 5.75 0.65 0.09 0.90
340 1725 19,678 405 985 6783 23
W 6.45 6.89 44.6 5.52 0.67 0.02 0.14
5.75 2.25 45.8 6.20 0.46 0.02 0.03
267 1098 9837 47 1696 2584 12
204 433 1857 45 612 3515 86
Table 1 Categories of biomass according to their agglomeration, fouling and corrosion propensities [7]. Biomass Biomass example category
Fractions Relative content in the in ash fuel
Bio-1
Wood, bark
Ca, K N Si
Bio-2
Straw, rice husk
Ca, K b Si
Bio-3
Sunflower seed, rapeseed residue
Ca, K N Si
S
Cl
P
Low b 0.03% Low b 0.04% High N 0.2%
Low b 0.01% High 0.1% Low/ high
Low b 0.1% Low b 0.2% High N 0.8%
Table 3 Particle size distribution (shown as residue on sieve, wt.%) of halloysite used for investigations. Mesh size mm
Residue wt.%
Halloysite fractions H
Ha
Hb
Hc
Hd
0.15 0.088 0.041 0.02
R0.15 R0.09 R0.04 R0.02
22.3 45.7 – –
0.0 2.30 44.0 –
0.0 0.0 8.5 –
0.0 0.0 0.0 –
0.0 0.0 0.0 0.0
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Table 4 Properties of ash from wheat straw (SA), mixture of rape and cereal straw (SB), rape straw (SC), pelletized sunflower husk (PS) and willow (W) incinerated with various fractions of halloysite. SA
Percentage of halloysitea
0% H
C H N Cl S Cl(b) S(b) Al Fe K Na Mg Ca Mn Si X = (K + Na)/Cl [mol/mol] B/A tS [°C] tA [°C] tB [°C]
0.40 b 0.01 0.04 0.17 b 0.01 3.86 1.03 0.66 2.20 11.5 0.10 1.54 4.16 0.12 27.1 78 0.44 840 960 1180
a b
SB 1% H 0.26 b 0.01 b 0.01 0.13 b 0.01
3.31 8.73 9.67 0.11 1.33 3.19 0.14 23.8 86 0.54 850 1080 1240
Nomenclature according to Table 3. Willow sample other than that in 2% H mixture.
0% Ha 0.29 b 0.01 0.06 6.82 1.94 12.0 3.00 0.66 2.46 24.7 0.58 1.40 15.0 0.07 8.89 4.3 2.90 770 1250 1330
SC 2% Ha
4% Ha
2% Hc
0.28 b 0.01 0.12 5.73 3.01
0.19 b 0.05 0.11 4.21 1.36
0.23 b 0.05 0.18 4.65 1.60
3.00 7.82 19.7 0.51 1.21 11.3 0.10 9.58 4.1 2.08 790 1210 1230
4.53 12.7 18.3 0.52 1.03 9.74 0.12 9.96 5.1 1.91 1100 1210 1240
3.04 8.85 19.5 0.49 1.21 12.0 0.10 9.54 5.0 2.17 780 1150 1170
0% Ha 0.27 0.05 0.21 6.40 1.15 12.2 1.22 0.70 7.28 20.7 0.48 1.25 12.5 0.08 10.7 3.8 2.33 610 1170 1190
PS 2% Ha 0.21 b 0.05 0.16 5.81 1.19
2.94 11.1 20.3 0.50 1.18 11.0 0.11 11.0 4.1 2.04 790 1150 1170
3% Ha
1% Hc
0.16 0.01 0.12 5.20 1.34
760 1150 1190
790 1200 1240
2% Hc
1% Hd
2% Hd
0.25 b0.05 0.19 5.48 1.26
0.25 b0.05 0.16 4.13 1.22
2.88 12.67 20.5 0.51 1.24 10.4 0.12 11.0 4.4 2.11 900 1090 1170
3.33 14.5 17.5 0.48 1.07 10.4 0.15 11.0 5.0 2.00 800 1090 1140
790 1030 1100
0% Hc 0.67 0.14 0.17 0.49 0.25 2.03 0.29 1.21 2.40 17.3 0.18 2.74 3.35 0.03 26.2 41 0.59 810 900 1110
W 1% Hc
2% Hc 0.24 0.03 0.15 0.49 0.20
900 1070 1210
3.61 7.63 16.1 0.23 2.60 2.86 0.07 22.8 38 0.72 1030 1090 1160
2% Hd 0.36 b 0.05 0.23 0.37 0.22
1120 1210 1240
0% H 0.48 b0.01 b0.01 0.08 0.70 1.33 0.89 1.29 4.18 7.70 0.18 3.15 13.7 0.43 14.5 109 1.21 690b 1500b 1500b
2% H 0.71 b0.01 b0.01 0.07 0.41
8.30 15.1 5.32 0.20 1.46 6.18 0.39 13.3 95 0.88 1100 1190 1330
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Fig. 4. The influence of fraction and fineness of halloysite on the sintering temperature tS. ○ — SA and □ — SB.
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Fig. 6. The influence of halloysite additive on the temperatures tA and tB of ash for SA and PS.
arise. The tests of co-firing biomass in CFB boilers [7,8] indicate that K and Ca easily migrate to the bed material (consisting mainly of SiO2) forming low melting coating layers gluing the particles together. The melting temperature of K–Ca-silicates depends strongly on the fraction of potassium. The combustion tests of SB straw without halloysite confirmed the strong agglomeration and the local melting tendency of ash. The addition of 2 wt.% of halloysite weakened this tendency significantly, although the sintering temperature increased only a little. Considerably stronger was the effect of halloysite by its 4 wt.% addition. Relatively low values of X suggest that the predominant part of light metals is bonded in chlorides (X ≅ 4 for the straw without halloysite). By increasing the percentage of the additive the proportion of chlorides decreases in favour of aluminosilicates. Considering the fractions of K, Ca, Si, Cl and partly S, the constitution of rape straw SC is similar to that of SB, while the composition of pelletized sunflower husk PS is similar to that of SA. Therefore the phenomena occurring during their combustion should also be similar. In fact it is not exactly so, because in the case of PS all sulfur remains in ash, while SA ashes contain only traces of sulfur.
The investigations show an overall positive effect of halloysite addition on sintering and softening temperatures of ash and this effect is influenced by the fineness of the additive. In the use of coarse halloysite (with the exception of one test for SB) and its amount ranging to 2 wt.%, the effect was insignificant — Fig. 4. On the contrary, by addition of 4 wt.% of halloysite the tS for SB increased by 300 K. The decrease of B/A and therefore the reduction of ash slagging and sintering tendency were also noticed [10]. It has to be mentioned that for a relatively coarse halloysite particle size distribution it is practically impossible to homogenize 1 wt.% of additive (density of about 3 g/cm3) with the base biomass material (density of approximately 0.2 g/cm3). It explains why the expected positive effects remained insignificant even by addition of 2 wt.% of halloysite. Using more fine-grained halloysite (SC — Fig. 5) facilitated the homogenization thus increasing its efficiency as a sorbent. In the case of rape straw, which is an agro-biomass with extremely high contents of chlorine and sulfur, the addition of 2 wt.% of halloysite (Hc, d — Table 3) increased the sintering temperature of ash by 150–200 K. An even higher increase of tS was noticed by incineration of PS — Fig. 5. The effect of halloysite additive on softening (tA) and melting (tB) temperatures of ash is not unambiguous — see Figs. 6 and 7. In this case we take into account all the tests with 2% share of halloysite, regardless of its granularity. For biomasses SA and PS, whose chemical compositions are similar to the Bio-2 type of biomass (Table 1), addition of halloysite caused an increase in the characteristic ash
Fig. 5. The influence of fraction and fineness of halloysite on the sintering temperature tS. ■ — SC (Ha), □ — SC (Hc,d), and ● — PS (Hc,d).
Fig. 7. The influence of halloysite additive on the temperatures tA and tB of ash for SB and SC.
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Fig. 8. Microphotograph of the rape straw SC ash after incineration without halloysite.
fusion temperatures tA and tB — Fig. 6. On the contrary, in the case of biomasses SB and SC (rape straw), characterized by very high contents of K, Cl and S, the opposite phenomenon was observed — Fig. 7. The case of the relative lowering of the characteristic temperatures of SB and SC while halloysite is added needs further explanation. It is likely that a significant release of light metals takes place during incineration of pure SB and SC materials. Nevertheless, it is evident that the potassium contents in both ashes — pure SB and SC, and SB and SC blended with halloysite, are similar despite the fact that about 30 wt.% of their ash is composed of sorbent material. Thus a part of volatile light metals is trapped in the sorbent material in comparison to the case of incineration of pure biomasses. On the other hand, the tA and tB temperatures, as noted for pure SB and SC biomasses, are already sufficiently high (1250 °C and 1170 °C respectively) for practical applications and the influence of halloysite is of minor importance. It also has to be outlined here that the results presented concern ashes from ‘static’ incineration, i.e. biomass and halloysite were contacted only during the blending process. In a full-scale combustion reality the additional two-phase or one-phase processes would enhance contacting biomass with halloysite. Therefore the undisturbed picture of the influence of additive is to be obtained only in full-scale trials. The visible effect of adding halloysite is a significant diminution of the ash grain size, resulting from the weakening of high-temperatureagglomeration processes commonly observed in boilers. Figs. 8 and 9 from the optical microscope show the difference of the ash grain size after halloysite addition. The influence of halloysite on sunflower husks (Table 5) incinerated at temperatures around 1000 °C is illustrated in Figs. 10 and 11. Ash from the husks without halloysite is agglomerated and locally melted (Fig. 10), while it contains considerable amounts of S, K, Ca and Mg. Its composition is strongly heterogeneous. Potassium sulfates of low melt temperature are visible in a point marked with letter A, whereas strong agglomeration occurred by the presence of a liquid phase (or partial melting) in point B. ‘Low viscous melts’ may occur when alkali (Na and K) and earth alkali (Ca and Mg) salt mixtures are present in ash [12]. It seems that such agglomeration type predominated in point B. After addition of 2 wt.% of halloysite the ash is fine-grained, without agglomeration — see Fig. 11. Absorption of alkali metals by halloysite and production of high melting point alkali compounds (e.g. potassium aluminosilicates) bond these metals in ash. 3. Full-scale investigations of halloysite additive The results of laboratory investigations on the use of halloysite as an additive in biomass combustion were very promising. Blending halloysite with several biomass types resulted in the increase of ash
Fig. 9. Microphotograph of the rape straw SC ash after incineration with halloysite. a) 2 wt.% halloysite added and b) 4 wt.% halloysite added.
softening temperature to a level noted for coals, which means that this additive can reduce slagging and fouling in boilers. Bonding of sodium and potassium from the chlorides of these metals was also observed. This in turn leads to the reduction of the share of KCl and NaCl in the ash deposits, and thus to the reduction of high temperature corrosion intensity. In order to confirm the suitability of halloysite as an additive for the processes of biomass combustion, the tests in real conditions were
Table 5 Chemical composition of biomass used in full-scale tests in the ESW power plant. Winter wheat Straw-briquettes Sunflower Wood straw seed husks chips O 64 C 27 Si 4.96 Al 0.08 K 2.57 Na 0.14 Mg 0.2 Fe 0.1 P 0.13 Ca 0.81 S 0.15 Cl 0.47
60.3 38 0.79 0.04 0.6 0 0.03 0.01 0.05 0.17 0.04 0.17
51.3 46 0.43 0.32 0.96 0.06 0.22 0.11 0.06 0.37 0.13 –
61.9 37 0.18 0.04 0.29 0 0.05 0.02 0.03 0.41 0.04 0.08
Calculated average composition 61.7 35.8 0.92 0.06 0.69 0.02 0.08 0.04 0.05 0.47 0.06 0.14
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Fig. 10. Ash from the sunflower husk PS incinerated without halloysite at approximately 1000 °C.
Fig. 11. Ash from the sunflower husk PS incinerated with 2 wt.% of halloysite at approximately 1000 °C.
carried out. In standard fossil-fuel boilers the two technologies may be considered for blending halloysite with the base fuel: • addition of halloysite during the processing of fuels (e.g. during manufacturing of briquettes or pellets from biomass or coal and biomass), • addition of halloysite in the form of fine powder in the biomass feeding system.
Both methods have been tested in practice. The PTH Intermark, the owner of Dunino halloysite mine, in co-operation with the Silesian University of Technology and the Institute for Chemical Processing of Coal has developed and patented the technology of manufacturing the ‘compact-BC’ fuel. The product of this kind is made from coal and agricultural biomass in the form of briquettes, pellets or granulated matter and may include halloysite. For small boilers (e.g. underfed burner furnace) as well as industrial grate boilers pellets or briquettes
Fig. 12. Various forms of mixed compact-BC fuels.
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Fig. 13. Integration scheme of precombustor and main PF boiler in the ESW power plant and location of the halloysite feeding port.
can be used — Fig. 12a and b. The most suitable form of the premixed fuel for the PF and CFB combustion is the granular fuel (GBC — Fig. 12c) [11]. For the manufacturing of compact-BC fuel any kind of biomass and coal can be used. In practical terms, using relatively cheap finely granulated coal sorts and biomasses not used to date in boilers (agricultural wastes, etc.), the total price of compact-BC fuel may be lower than the price of fuels actually used for heating purposes and in power plants. On top of that the halloysite additive will significantly reduce the negative phenomena caused by the alkali content of biomass. The full-scale tests were carried out in the Stalowa Wola power plant (ESW) in the OP 150 type boiler equipped with a precombustor
consisting of a rotary kiln and a CFB furnace. The precombustor is fed with the system of belt conveyors. The biomass dried and partly gasified in the rotary kiln is passed to the CFB part, where the combustion is completed. The resulting flue gas is further transferred through the connecting duct to the lower part of the furnace of the main PF boiler. The precombustor can burn up to 15 t/h of biomass which equals approximately to 40% of the energy processed by the boiler — Fig. 13. The precombustion installation is highly integrated with the main PF boiler. The heat recovery system of the precombustor is connected with the evaporator of the main OP 150 boiler. At the outlet of the CFB an evaporator in the form of a tube palisade is placed (Figs. 15–17) which is a critical element in the whole system due to its susceptibility
Fig. 14. Halloysite feeding port in full-scale trials.
Fig. 16. The state of the tube palisade after 6 days of firing of the agricultural biomass without halloysite. Approximately 90% of the palisade passage is clogged.
Fig. 15. The state of the tube palisade after 36 days of firing of wood without halloysite. Approximately 30% of the palisade passage is clogged.
Fig. 17. The state of the tube palisade after 15 days of firing of the agricultural biomass with halloysite. Approximately 8% of the palisade passage is clogged.
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Fig. 18. SEM image and EDS analysis of deposited ash from combustion of mixed biomass without halloysite in CFB precombustor.
to ash deposition. Clogging of the palisade and the connecting duct is the major reason of outages in boiler operation. The halloysite material was fed on the conveyor belt automatically by means of a screw feeder — Fig. 14. The feeding rate was constant, not depending on the instantaneous stream of biomass, its humidity or kind. The halloysite-to-biomass ratio was kept approximately at the level of 1–2 wt.%. The feeding system consisting of a sequence of belt conveyors ensured preliminary mixing of halloysite powder with biomass in an industrial scale. These feeding conditions were meant to resemble those maintained during the laboratory tests. The full-scale tests were conducted by burning the mixture containing in average 77 wt.% of woody biomass and 23 wt.% of agricultural biomass. Agricultural biomass was composed of approximately 70 wt.% of winter wheat straw and the residual part was composed of straw and sunflower seed husks. The moisture contents averaged 30 wt.% and 12 wt.% for the woody and agricultural biomasses accordingly. The mean LHV amounted to 12 MJ/kg and the ash content averaged 4 wt.%. The bio-fuel mixture was characterized by high chlorine concentration and considerable shares of potassium, silicon, calcium and sulfur. The mean chemical composition of the biomass fired in full-scale tests is shown in Table 5. The operational experiences from the use of precombustor in the ESW power plant indicate that it is the actual state of the palisade (Fig. 15) which determines the overall availability of the integrated
installation. Moreover, investigations have shown that palisade fouling and deposit composition depend to a large extent on the chemical composition of the fired biomass. When the wood biomass was fired the palisade fouling was moderate (Fig. 15). However, the addition of agricultural biomass caused, after only several days, a total clogging of the flue gas pathways and the outage of the installation (Fig. 16). On the other hand, observations carried out during combustion of the agricultural biomass mixed with the halloysite additive have shown significant reduction of the fouling tendency of the palisade and therefore the minor clogging of the flue gas pathways (Fig. 17). Figs. 18–20 show a microscopic view of deposited ash collected from the tube palisade by firing biomass in the CFB precombustor of the OP 150 boiler at 850–1000 °C. Basically, the evaporated light metals cause the formation of deposits and chlorine facilitates their release into the gas phase [13]. They form alkali silicates with low melting points. A sticky deposit layer may be formed in the process of cooling the flue gases when the volatile alkali chlorides or hydroxides condense in the form of aerosols on the surfaces of heat exchangers, boiler walls or on fly ash particles. Deposited material contains particles of fly ash which are inertially retained by the presence of a sticky phase on the surfaces of ash particles or already built-up deposit. The studied deposits contained relatively large shares of Si, K and Ca, something already reported in the literature [13].
Fig. 19. SEM image and EDS analysis of deposit ash obtained from combustion of mixed biomass in CFB precombustor with addition of halloysite.
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Fig. 20. SEM image and EDS analysis of deposit ash obtained from combustion of mixed biomass in CFB precombustor with addition of halloysite (the table shows the chemical composition in point H where the halloysite grain is saturated with potassium and in point L where the low melting temperature, multicomponent eutectic is located).
Addition of halloysite decreased the tendency for formation of deposits. The halloysite grains retained their structure even though they contain up to 28 wt.% of potassium (Fig. 19 — point M). Fig. 19 depicts finer deposit structure in comparison with Fig. 18, something already reported in laboratory tests (Figs. 9–11). Investigations of bed agglomeration by combustion of wood and rice straw blend (qualitatively similar to the biomass mixture examined in this study) have shown an adhesive mechanism for the agglomeration of bed (mullite sand) by formation of the cement structure with the following components: Si, Ca, K, Mg and P [14]. The mechanism of this phenomenon is manifested by the initial reaction between bed particles and potassium compounds (of flue gas or aerosols) in surface-zone around bed particles. Potassium is the dominating element to be found in this zone (rim) as measurements proved. The adhesive rim is composed from submicron particles of ash and eroded bed. It was suggested that melting of locally accumulated aerosol mixture occurs as the composition of rim indicated. The presence of adhesive cement may result in agglomeration processes. To some extent the similar agglomeration mechanism is believed to play a role in agglomerated structures seen in Fig. 21. If the amount of the absorbed potassium, which is the main factor decreasing the ash sintering and melting temperatures, is high, then the quantity of agglomerated matter and deposits is smaller and
eventually the time after bed undergoes defluidization is prolonged. This effect is illustrated in Fig. 22 which shows the grain size distribution of the fluidized bed material. In comparison with the initial state, the growth of the grain size can be noticed for all the cases. However, in the case of halloysite addition, the level of growth is significantly smaller and the suppression of large grain growth is even more pronounced. This is an overall positive behavior since the large particles are the subject of defluidization in the first instance. Despite of the high chlorine content in the original fuel, there were moderate Cl amounts found in the palisade deposits (max. 0.3 wt.%) and a maximum of 0.5 wt.% on the surface of the bed agglomerates. Analysis of ash from the electrostatic precipitators of the whole boiler system (precombustor and PF boiler) showed 0.45 wt.% of chlorine. Unburned particles present in the fly ash contained higher concentrations of chlorine. Since the estimated share of biomass ash is about 20 wt.% of the whole ash collected, over 60 wt.% of chlorine was retained in the biomass ashes. Thus less than 40 wt.% of Cl is estimated to escape with flue gas. The presence of chlorine in the deposits may result from uneven temperature distribution in the precombustor furnace (differences amounting to 200 °C), for example due to condensing on cooler precombustor parts. By adding halloysite to the bio-fuel the concentration of chlorine decreased by 40 wt.% in the fly ash collected from the electrostatic
Fig. 21. Exemplary SEM image and EDS analysis of agglomerated bed material.
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Acknowledgments Investigations presented in this work were performed within the frame of the Polish Strategic Research Programme on Advanced Technologies for Power Generation (contract No.SP/E/1/67484/10). References
Fig. 22. Grain size distribution of CFB without and with halloysite additive. Rx — residue on sieve, wt.%.
precipitator. Moreover, the trace concentrations (max. 0.1 wt.%) of chlorine were observed both in analyzed deposits collected from the palisade and in bed agglomerates. 4. Conclusions The laboratory and full-scale investigations reported in this work demonstrate the ability of halloysite to enhance the operating features of furnaces firing various kinds of biomass. This includes also the agricultural biomasses whose chemical composition enhances the unwanted deposition and agglomeration processes. The use of halloysite as an additive will therefore significantly contribute to the extension of operational time and availability of biomass fired boilers. In particular, the following conclusions can be drawn from the present study: • the use of halloysite additive increases the level of ash sintering temperature for all agricultural biomasses tested in this work • a proper mixing of an additive with base fuel must be secured by using halloysite of fine particle size distribution and is especially important for pulverized fuel boilers; grinding of halloysite mineral thanks to its brittleness is neither difficult nor costly • adding of an iron containing halloysite influences the B/A index towards higher values therefore an appropriate index interpretation must be secured.
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