Effects of Steam on the Release of Potassium, Chlorine, and Sulfur

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Effects of Steam on the Release of Potassium, Chlorine, and Sulfur during Char Conversion, Investigated under Dual-Fluidized-Bed Gasification Conditions Placid A. Tchoffor,*,†,‡ Kent O. Davidsson,† and Henrik Thunman‡ †

SP Technical Research Institute of Sweden, Box 857, SE-501 15 Borås, Sweden Chalmers University of Technology, Hörsalsvägen 7B, SE-412 96 Göteborg, Sweden

‡

ABSTRACT: The corrosion and fouling of heat-transfer surfaces and the agglomeration of bed materials in fluidized beds are some of the ash-related problems caused by the transformation and release to the gas phase of ash-forming elements from biomass during thermochemical conversion processes. The magnitudes of these problems are largely dependent upon the release of potassium (K), chlorine (Cl), and sulfur (S) from the biomass. We investigated the effects of steam on the release of K, Cl, and S during char conversion, under conditions relevant for dual-fluidized-bed gasification (DFBG). The study was carried out with wheat straw in a laboratory-scale bubbling fluidized-bed reactor in the temperature range of 800−900 °C. The release of K, Cl, and S from wheat straw during devolatilization, char gasification, and char combustion was quantified with a mass balance that linked the masses of these elements in the wheat straw to the mass of the solid residue obtained at the end of each experiment. To facilitate analyses of the experimental results, leaching and the Brunauer−Emmett−Teller surface area measurement of the wheat straw and some of the solid residues were carried out. The results show that, during devolatilization, the release of volatile salts, e.g., KCl, is significantly limited by intraparticle diffusion resistance, owing to a compact char matrix (i.e., negligible porosity). However, during char gasification, steam renders the char less compact by expanding and/or creating new pores in the char. As a result, intraparticle diffusion resistance decreases, thereby facilitating the evaporation of volatile salts of K and S from the char matrix. The conversion of the char is also conducive to the release of char-bound K and S, especially at 900 °C. At temperatures of >800 °C, the relative proportions of the elements released and char gasified indicate that the release of K can somewhat be decoupled from the release of S and Cl by maximizing the extent of char conversion in the gasification chamber. The results also show that, during char combustion, the proportions of the char that can be combusted and the extent of the release of the elements are influenced by the extent to which the char is gasified in the gasification chamber.

1. INTRODUCTION Biomass gasification in dual-fluidized-bed gasifiers is a promising technology for the production of renewable energy. The product gas produced by this process, which typically has a heating value of 15−20 MJ/Nm3,1 can be used for the production of electricity and heat as well as for the syntheses of transportation fuels and other high-value chemicals. Dual-fluidized-bed gasification (DFBG) is carried out in two interconnected fluidized beds.2−4 Figure 1 shows a simplified schematic of the process for DFBG of biomass. One of the fluidized beds is the gasification chamber, while the other is the combustion chamber. In the gasification chamber, biomass is devolatilized, after which the generated char is partially gasified with steam. Volatiles are released at a fast rate during devolatilization, which prevents the fluidization gas from penetrating into the fuel particle.5,6 Therefore, steam penetrates and reacts with the char mainly when the devolatilization process is completed. The unconverted char and bed material from the gasification chamber are circulated to the combustion chamber. In this chamber, the char is combusted to generate heat, which among others heats the bed material. The hot bed material is subsequently recirculated to the gasification chamber to supply the necessary heat to convert the incoming biomass. DFBG of biomass is susceptible to the ash-related problems observed during the combustion and gasification of biomass in fluidized beds. These problems include agglomeration of bed materials7−9 and the fouling10 and corrosion11−14 of © 2014 American Chemical Society

heat-transfer surfaces. Furthermore, the presence of sulfur species, such as H2S, in the product gas poisons catalysts used in some processes downstream of the gasifier.15−17 Associated with these problems are high maintenance costs, costly unscheduled shutdowns, and a decrease in overall efficiency. Ash-related problems during fuel conversion emanate from the release and transformation of inorganic species, which mainly contain the elements K, Cl, and S. Among the measures needed to mitigate effectively the ash-related problems is quantification of the release of K, Cl, and S under relevant operating conditions.18 The relevant operating conditions for DFBG include among others: (1) temperatures in the range of 700− 970 °C,19−21 (2) steam in the gasification chamber,21,22 (3) air in the combustion chamber,22,23 (4) short retention times for fuel particles (for example, 70−160 s22,23), and (5) high rates of heat transfer to fuel particles (the heat-transfer coefficient in fluidized beds can be as high as 1000 W m−2 K−1).24 The quantification of the release of K, Cl, and S from various biomass fuels during thermal conversion has been reported in the literature.25−31 These previous studies focused on the effects of the temperature and ash composition on the release of these elements under conditions relevant to fixed-bed pyrolysis and Received: July 14, 2014 Revised: October 1, 2014 Published: October 9, 2014 6953

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Figure 1. Laboratory-scale bubbling fluidized-bed reactor used in this work.

are mostly metal cations, which are bound to anionic groups of the organic fuel matrix.35 The inorganically associated elements in biomass may exist as soluble salts and/or minerals.35 The water-soluble fractions of the elements present in biomass can be somewhat separated from the other forms of ash-forming elements in biomass by leaching with water.37,38 A commonly used technique to separate the different fractions of ash-forming matter in biomass is chemical fractionation.35,39,40 In this technique, biomass is sequentially leached with solvents of increasing aggressiveness, i.e., water, 1 M ammonium acetate, and 1 M hydrochloric acid. The various ash-forming elements dissolve in these solvents based on solubility. The fractions of ash-forming elements that exist as soluble salts and/or that are organically associated are believed to be highly reactive during thermal conversion processes.40−43 The fraction of ash-forming matter that exists as minerals is less reactive during thermal conversion processes.39,43 Therefore, the forms in which the ashforming elements are present in biomass can give an indication of the extent to which they are released during thermal conversion. At temperatures below or close to 700 °C, organically associated K and S are released during biomass devolatilization.26,31 Although evaporation is negligible at this temperature, owing to its low vapor pressure,25,31 KCl reacts with oxygencontaining groups that are formed during biomass devolatilization, leading to the release of Cl in the form of HCl.29,44,45 This reaction appears to be enhanced by high heating rates.32 At temperatures of >700 °C, the vapor pressure of KCl increases25 and evaporation of KCl becomes the main pathway for the release of Cl.25,27 During devolatilization, the release of K and Cl through the evaporation of KCl from the fuel can be limited by intraparticle diffusional resistance, which is imposed by the still intact or compact fuel matrix.31 After devolatilization, the resulting char is oxidized, making it less compact.46 As a result, diffusional resistance to the evaporation of KCl from the

combustion. However, fewer studies have looked at the release of these elements under DFBG conditions.32 In particular, the effects of steam on the release of these elements during char conversion under DFBG conditions have received little attention. As mentioned above, some of the char that results from the devolatilization of biomass is partially gasified in the gasification chamber before being circulated to the combustion chamber. During the char gasification process, K, Cl, and S may be released to different extents from the char, and the release may be lower than, higher than, or equal in proportion to the char converted. Furthermore, the extent to which the elements are released during char combustion may be influenced by the extent to which the char has been gasified in the gasification chamber. Therefore, to control the transformation and release of ashforming elements and, thereby, the mitigation of ash-related problems during DFBG, it is important to know how steam affects the release of K, Cl, and S during char conversion. The overall aim of the present work was to quantitatively investigate the transformation and release of K, Cl, and S from biomass under conditions relevant to DFBG. More specifically, the effects of steam on the release of K, Cl, and S during char conversion were studied. The information obtained is useful in the optimization of operating conditions and the mitigation of ash-related problems during DFBG.

2. THEORY ON K, CL, AND S RELEASE PATHWAYS The transformation and release of K, Cl, and S during thermal conversion of biomass depend upon how these elements are associated in the biomass, the sizes of the fuel particles, the ash composition, the temperature and atmosphere in the reactor, the retention time, and the rate at which the fuel particles are heated.18,25,27,28,30,31,33,34 Ash-forming elements can be organically and/or inorganically associated in biomass.26,35,36 The organically associated elements 6954

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char decreases, and this leads to the release of K and Cl. In addition to the evaporation of KCl, K can be released from the char through the decomposition of K2CO3 and the release of char-bound K.25,27,34 At temperatures approaching 900 °C, K2CO3 decomposes, whereby K is released as KOH.25 The decomposition of K2CO3 is enhanced by steam.25,34 Char-bound K is either released or retained in the ash depending upon the extent of char conversion and the concentration of Si in the char. In the early stage of char conversion, the organic matrix hinders the formation of potassium silicates, and thus, the retention of K in the ash is limited.25 Consequently, for the portion of the organic matrix of the char converted, the char-bound K in both Si-rich char and Si-lean char are released,25 most likely as KOH.25,47 Toward the end of char conversion, where the organic matrix of the char is virtually completely converted, the formation of K silicates and, thus, the retention of K is favored.25,31 This situation is conducive to more K being released from Si-lean char than from Si-rich char.25 It has also been observed25 that, irrespective of the ash composition of the char, the release of K will be favored if a high amount of Cl is present in the char. The release of gaseous K2SO4 is negligible at temperatures of 50% of Cl (RCl,P3) in the wheat straw was released when the wheat straw was devolatilized (in the P3 experiment). RCl,P3 was almost constant with the temperature. The release of Cl during the char gasification steps of G3 and G6 (QCl,G3 and QCl,G3, respectively) is shown in Table 7. Because these data were calculated from the data (RCl,P3, RCl,G3, and RCl,G6) presented in 6958

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Table 6. Release (% in Dry Wheat Straw) of K, Cl, and S from the Wheat Straw during the P3, PC3, G3, GC3, G6, and GC6 Experiments release of Cl, K, and S from the wheat straw (R, % i in dry wheat straw) during P3, PC3, G3, GC3, G6, and GC6 (mean ± standard deviation) Cl

K

S

Cl

K

P3

S

PC3

temperature (°C)

RCl,P3

RK,P3

RS,P3

RCl,PC3

RK,PC3

RS,PC3

800 900

58.5 ± 0.4 61.8 ± 2.3

22.3 ± 1.1 31.0 ± 2.8

60.9 ± 4.1 67.1 ± 0.9

70.6 ± 1.3 77.0 ± 3.9

41.3 ± 3.0 49.4 ± 1.6 GC3

68.2 ± 2.1 71.3 ± 0.3

G3 800 900

800 900

RCl,G3

RK,G3

RS,G3

RCl,GC3

RK,GC3

RS,GC3

62.0 ± 1.0 75.6 ± 1.2

25.3 ± 1.1 38.2 ± 2.6 G6

72.4 ± 0.7 80.1 ± 2.2

80.2 ± 1.8 90.9 ± 2.4

52.1 ± 3.4 65.7 ± 2.9 GC6

79.9 ± 2.5 83.8 ± 2.1

RCl,G6

RK,G6

RS,G6

RCl,GC6

RK,GC6

RS,GC6

69.5 ± 2.6 92.2 ± 1.3

32.4 ± 4.0 61.1 ± 3.9

84.8 ± 1.6 90.0 ± 0.8

89.1 ± 2.5 98.4 ± 1.3

67.0 ± 0.1 86.0 ± 4.1

88.6 ± 0.1 93.1 ± 2.9

Table 7. Release (% in P3-Char) of K, Cl, and S during the Char Gasification Steps of G3 and G6 release of i from char during the char gasification steps of G3 and G6 (% of i in P3-char) G3

G6

temperature (°C)

QCl,G3

QK,G3

QS,G3

QCl,G6

QK,G6

QS,G6

800 900

8.4 ± 2.6 36.1 ± 7.1

3.9 ± 2.0 10.4 ± 5.5

29.4 ± 11.1 39.5 ± 7.3

26.5 ± 6.3 79.6 ± 8.4

13.0 ± 5.3 43.6 ± 7.2

61.1 ± 13.0 69.6 ± 4.1

The fractions RCl,PC3, RCl,GC3, and RCl,GC6 of Cl in the wheat straw released during PC3, GC3, and GC6, respectively, are shown in Table 6. At both 800 and 900 °C, RCl,GC3 was significantly higher than RCl,PC3, although the total retention times for GC3 and PC3 were the same (6 min). 4.2.2. Potassium. The standard deviations in the release of K during the experiments were determined similar to the release Cl. For the investigated temperature range, the fraction (RK,P3) of K released from the wheat straw when it was devolatilized in the P3 experiment was 50% of the S in the wheat straw was released (RS,P3) when it was devolatilized in the P3 experiment. In this temperature interval, RS,P3 increased marginally. The release of S (RS,G3 and RS,G6) during G3 and G6 increased as the temperature was increased from 800 to 900 °C. The release of S during the char gasification step of G3 and G6 (QS,G3 and QS,G6, respectively) is

Table 6, the standard deviations presented in Table 7 were determined by propagation of the standard deviations of RCl,P3, RCl,G3, and RCl,G6 presented in Table 6. Similar to char conversion (XG3) presented in the preceding section, QCl,G3 expresses the effect of steam on the release of Cl during char gasification. QCl,G3 increased significantly (∼6−43%) when the temperature was increased from 800 to 900 °C. When the retention time was increased from 3 to 6 min (G3−G6), the release of Cl during char gasification increased at least 2-fold (see QCl,G3 and QCl,G6 in Table 7). The fractions UCl,P3‑char, UCl,G3‑char, and UCl,G6‑char of Cl in the char samples P-char, G3-char, and G6-char, respectively, released during combustion as a function of the temperature are shown in Table 8. Because these data were calculated from the data (RCl,P3, Table 8. Release (% in Each Char) of K, Cl, and S from Char Samples during Combustion release of K, Cl, and S from P3-char, G3-char, and G6-char during combustion (% of i in each char) char type P3-char G3-char G6-char

temperature (°C)

UCl,P3‑char

UK,P3‑char

US,P3‑char

800 900 800 900 800 900

29.2 ± 3.3 39.8 ± 12.1 47.9 ± 5.6 62.7 ± 11.4 64.3 ± 13.0 79.5 ± 27.0

24.4 ± 4.1 26.7 ± 4.8 35.9 ± 4.7 44.5 ± 6.6 51.2 ± 6.6 64.0 ± 15.9

18.7 ± 11.9 12.8 ± 2.9 27.2 ± 9.4 18.6 ± 15.4 25.4 ± 10.9 31.0 ± 30.0

RCl,P3, RCl,G3, RCl,GC3, RCl,G6, and RCl,GC6) presented in Table 6, the standard deviations presented in Table 8 were determined by propagation of the standard deviations of RCl,P3, RCl,P3, RCl,G3, RCl,GC3, RCl,G6, and RCl,GC6 presented in Table 6. At both 800 and 900 °C, the release of Cl from G3-char was higher than that from P3-char, although both were combusted for 3 min. 6959

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5. DISCUSSION 5.1. Release of K, Cl, and S during Devolatilization and Char Gasification. 5.1.1. Chlorine. As mentioned in the Theory on K, Cl, and S Release Pathways, the forms of K, Cl, and S that are water-soluble exist as salts in biomass. Because almost all Cl (96%; see Table 10) in the wheat straw could be leached out with water, it can be inferred that Cl exists mainly as a salt in the wheat straw. This has also been observed in other studies.51,52 The most likely Cl salts in biomass fuels are KCl and NaCl. The concentration of K in the straw was far higher than that of Na (see Table 1), and >80% of K was leachable. Thus, Cl was present in the form of KCl in the wheat straw. Because the retention times during P3 and G3 were the same (3 min), the finding that more Cl was released during G3 than during P3 (see Table 6) indicates that steam has an effect on the release of Cl. The main difference between P3 and G3 was that the wheat straw was only devolatilized during P3, while during G3, the straw was devolatilized and part of the generated char was gasified with steam. Therefore, any substantial effect of steam on the release of Cl from the wheat straw should have occurred during char gasification. Table 5 shows that ∼3−28% of the char was converted as the temperature was increased from 800 to 900 °C during the G3 experiment. At these temperatures, the vapor pressure of KCl is highly significant for evaporation. As mentioned in the Introduction, the compactness of the char is one of the factors controlling the extent of evaporation of KCl from the char. The degree of compactness of each of the char samples obtained at the end of P3 (P3-char) and G3 (G3-char) is assessed in the present work in terms of pore volume and BET surface area. Table 9 shows that, for the entire investigated temperature interval, the BET surface area and pore volume of G3-char were significantly larger than those of P3-char. Therefore, G3-char was less compact than P3-char and, thus, posed less resistance to the diffusion of KCl through the char matrix. It can be inferred from this finding that the lower resistance to the diffusion of KCl posed by G3-char is what led to a greater release of Cl from G3-char, as compared to P3-char. It is well-known that steam increases the surface area and pore volume of char by removing carbon atoms from the char.53−56 The reaction of carbon with steam can be represented by reaction R1.

shown in Table 7. QS,G3 reflects the effect of steam on the release of S during char gasification. QS,G3 increased when the temperature was increased from 800 to 900 °C. At both temperatures, QS,G6 was significantly higher than QS,G3. Table 8 shows the fractions US,P3‑char, US,G3‑char, and US,G6‑char of S released in P3-char, G3-char, and G6-char, respectively, as a function of the temperature during char combustion. US,G3‑char was virtually the same as RS,P3‑char at 800 and 900 °C. Table 6 shows the fractions RS,PC3, RS,GC3, and RS,GC6 of S in the wheat straw released as a function of the temperature at the end of PC3, GC3, and GC6, respectively. Although the total retention time during GC3 and PC3 was the same (6 min), RS,GC3 was higher than RS,PC3 at 800 and 900 °C. 4.3. Total Pore Volume and BET Surface Area of the Char Samples Obtained from the Pyrolysis/Gasification of the Wheat Straw. The total pore volume and BET surface area as a function of the temperature for each of the char samples P3-char, G3-char, and G6-char obtained at the end of P3, G3, and G6, respectively, are presented in Table 9. Although the retention Table 9. Pore Characteristics of Some of the Char Samplesa P3-char

G3-char

G6-char

temperature (°C)

BET SA (m2/g)

total PV (cm3/g)

BET SA (m2/g)

total PV (cm3/g)

BET SA (m2/g)

total PV (cm3/g)

800 900

5.0 5.6

0.02 0.02

361.1 579.6

0.24 0.35

460.3 na

0.30 na

a

SA, surface area; PV, pore volume; and na, not analyzed.

times during P3 and G3 were the same (3 min), the pore volume in G3-char was 12−18-fold larger than that in P3-char. Furthermore, while the total pore volume in G3-char increased with the temperature, the total pore volume was almost constant in P3-char. A similar trend was observed for the BET surface area. Both the BET surface area and pore volume for G6-char were larger than for G3-char. This indicates that a longer retention time during gasification is conducive to an increased BET surface area and pore volume. 4.4. Chemical Forms of K, Cl, and S in the Wheat Straw and Char Residues. The water-soluble fractions of K, Cl, and S in the wheat straw and some selected char samples are shown in Table 10. More than 90% of Cl, 80% of K, and 60% of S in the

C(s) + H 2O(g) → CO(g) + H 2(g)

The more carbon atoms that are removed, the more the char is converted and the more porous the char becomes. When the temperature was increased from 800 to 900 °C during G3, ∼3−28% of the char resulting from the devolatilization of the wheat straw was gasified (see Table 5) and the BET surface area and pore volume of the char residue (G3-char) increased from 361 to 580 m2/g and from 0.24 to 0.35 cm3/g, respectively (see Table 9). Because a high extent of char conversion leads to high porosity56,57 and high char porosity decreases the resistance to the diffusion of KCl through the char matrix, it can be concluded that a high extent of char conversion is conducive to the increased release of Cl. The fraction of Cl in the straw released to the gas phase versus the fractions of the straw devolatilized in the P3 experiment is shown in Figure 4. The “parity line” in Figure 4 represents a hypothetical situation, in which the release of Cl is proportional to the fuel converted. Similarly, Figure 5 shows the fractions of Cl in the char released to the gas phase versus the fractions of the char converted during the char gasification steps of G3 and G6.

Table 10. Water-Soluble Fractions of K, Cl, and S in the Wheat Straw and Char Residues water-soluble fraction (%) fuel

K

Cl

S

wheat straw G3-char GC3-char

88 80 83

96 107 125

68 35 20

(R1)

wheat straw were water-soluble. After gasification of the wheat straw at 800 °C for 3 min (G3), the fraction of water-soluble S in the resulting char (G3-char sample) was decreased by almost half, while the corresponding fractions of water-soluble K and Cl remained virtually the same. When G3-char was combusted at the same temperature for an additional 3 min, the watersoluble fraction of S in the resulting char (GC3-char) decreased, while the water-soluble fractions of K and Cl remained essentially the same. 6960

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present in this fuel was in the chemical form of KCl. Other possible salts are K2CO3 and K2SO4. The fraction of K that was bound to the organic fuel matrix and/or was present as silicates is likely to be