1280
Energy & Fuels 2000, 14, 1280-1285
Experimental Investigation of the Transformation and Release to Gas Phase of Potassium and Chlorine during Straw Pyrolysis P. A. Jensen,*,† F. J. Frandsen,† K. Dam-Johansen,† and B. Sander‡,§ CHEC Research center, Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark, and ELSAMPROJEKT A/S, Kraftværksvej 53, DK-7000 Fredericia, Denmark Received May 22, 2000. Revised Manuscript Received August 7, 2000
When straw undergoes thermal treatment the initial process is a pyrolysis at which some K and Cl can be volatilized, and this may result in problems with deposit formation and corrosion of the reactor containment. A laboratory batch reactor was applied to study the release and transformation of K and Cl as a function of temperature, at an initial heating rate of approximately 50 °C/s. To facilitate the interpretation of the batch reactor experiments thermodynamic equilibrium calculations at reducing condition were conducted, and SEM (scanning electron microscopy) and leaching investigations were carried out on straw and char samples. The experiments showed that chlorine was released in two steps, about 60% was released when the temperature increased from 200 to 400 °C and most of the residual chlorine was released between 700 and 900 °C. Below 700 °C no significant potassium release was observe; above that temperature it increased progressively until about 25% potassium release at 1050 °C. During pyrolysis most K was released from the original binding sites, and the part that was not transformed to gas phase existed as redeposited discrete particles of KCl and K2CO3, as potassium silicates, or bound to the organic matrix. The initial release of potassium to the gas phase at approximately 700 °C was caused by evaporation of deposited KCl particles. The release of Cl to the gas phase was strongly affected by heating rate and sample size.
1. Introduction Annual crops such as straw often contain large amounts of chlorine and potassium. Levels of 0.2 to 1.9 wt % of potassium and 0.1 to 1.1 wt % of chlorine are typically found in Danish straw.1 When straw undergoes thermal treatment in combustion, gasification, and pyrolysis reactors, the initial process is a pyrolysis of the straw at which the organic fraction is partly destroyed and large amounts of gas and tar are released. Depending on process conditions, alkali metals and chlorine may be released to the gas phase, which may subsequently cause problems with deposition and corrosion in thermal fuel conversion systems.2-4 The emission of chlorine as HCl also contributes to the acidification of the environment.5 The release of chlorine and * Author to whom correspondence should be addressed. Phone: 45 25 28 00. Fax: +45 45 88 22 58. E-mail:
[email protected]. † Technical University of Denmark. ‡ ELSAMPROJEKT A/S. § Phone: +45 79 23 33 33. Fax: +45 75 56 44 77. E-mail:
[email protected]. (1) Sander, B. Properties of Danish Biofuels and the Requirements for Power Production. Biomass and Bioenergy 1997, 12 (3), 177-183. (2) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr; Bryers, R. W.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. The Behavior of Inorganic Material in Biomass-fired Power Boilers: Field and Laboratory experiences. Fuel Process. Technol. 1998, 54 (1), 47-80. (3) Jensen, P. A.; Stenholm, M.; Hald, P. Deposition Investigation in Straw Fired Boilers. Energy Fuels 1997, 11, 1048-1055,. (4) Nielsen, H. P.; Larsen, O. H.; Frandsen, F. J.; Dam-Johansen, K. Deposition and High-Temperature Corrosion in a 10 MWth Strawfired Grate. Fuel Process. Technol. 1998, 54, 95-108.
potassium during pyrolysis and gasification can to some extent be avoided by limiting the maximum temperature in the system. The objective of this study was to quantify the influence of the pyrolysis temperature on the potassium and chlorine release, and to improve the understanding of potassium and chlorine transformation during pyrolysis. Ash- forming elements in straw can appear as discrete inorganic particles, as simple salts or organically associated. At present it is not clear how potassium and chlorine are bound in biomass, but both elements may to a large degree be extracted from the straw by leaching with water. Potassium probably exists as ions bound onto oxygen-containing functional groups in the organic matrix6,7 or as discrete KCl particles. When straw is pyrolyzed, the organic matrix is partly destroyed and relatively large amounts of H and O are released to the gas phase (as CO, CO2, H2O, and hydrocarbons).6 Even at moderate temperatures of 400 °C, only 30-35% char (wt % of dry straw) is left after pyrolysis at heating rates of 1-80 °C/min.8 It has been demonstrated that the presence of potassium in the straw modifies the final (5) Bjo¨rkman, E.; Stro¨mberg, B. Release of Chlorine from Biomass at Pyrolysis and Gasification Conditions. Energy Fuels 1997, 11, 10261032. (6) Wornat, M. J.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Structural and Compositional Transformation of Biomass Chars during Combustion. Combust. Flame 1995, 100, 131-143. (7) French, R. J.; Dayton, D. C.; Milne, T. A. The Direct Observation of Alkaline Vapor Species in Biomass Combustion and Gasification. Report: NREL/TP-430-5597, 1994.
10.1021/ef000104v CCC: $19.00 © 2000 American Chemical Society Published on Web 09/15/2000
Transformation/Release of K and Cl in Straw Pyrolysis
distribution of the evolved gas, tar, and char,9 showing that potassium strongly interacts with the organic matrix. During pyrolysis the original binding sites of K may be destroyed and K may be released to the gas phase, bound to mineral particles or bound to the organic matrix of the char. The binding of potassium to the organic matrix after initial pyrolysis could be to oxygen functionalities or as intercalates. Boem10,11 has demonstrated that many different oxygen-containing functional groups such as carboxylic, lactone, carbonyl, and phenoxide groups may exist on the surface of carbons. Also basic surface groups exist, and they seem to be more thermally stable than the acidic groups.10 Minkova et al.12 have pyrolyzed straw in argon at 750 °C for 2 h, and analysis of the generated char showed that the surface contained some phenolic groups but was depleted of carboxylic and lactonic groups. Alkali metals are capable of forming intercalation compounds with carbon and coal,13,14 and the intercalates are stable up to a temperature of at least 830 °C in an inert atmosphere.14 By intercalation, atoms of K can penetrate between the carbon layers and be inserted into a graphite structure. It is possible that K binds to nonvolatile minerals during the pyrolysis process. Wilberley and Wall15 have experimentally demonstrated that sodium silicate is generated at combustion conditions by reaction between NaCl or Na2CO3 and H2O and SiO2. Steenari and Lindquist16 have demonstrated that K can be bound to aluminosilicates at 700-800 °C. However, the low amount of Al in straw severely limits the amount of K that can be bound in this way. Both the binding of the K to the char matrix and the reaction of K with SiO2 may prevent the release to the gas phase at low temperatures. A few experimental investigations of alkali release during biomass pyrolysis have been published. Direct measurements with mass spectrometry of the alkali and chlorine products released during combined pyrolysis and char combustion have been carried out by Dayton et al.17,18 One of the investigated fuel samples consisted of switch grass with a mineral composition similar to that of wheat straw. The main gas-phase products (8) Stenseng, M.; Jensen, A.; Dam-Johansen, K.; Grønli, M. Experimental Investigation and Kinetic Modeling of Biomass Pyrolysis. Proceedings of the 2nd Olle Lindstro¨ m symposium on Renewable Energy-Bioenergy 1999, Royal Institute of Technology, Sweden. (9) Jensen, A.; Dam-Johansen, K.; Wo´jtowicz, M. A.; Serio, M. A. TG-FTIR Study of the Influence of Potassium Chloride on Wheat Straw Pyrolysis. Energy Fuels 1998, 12 (5), 929-938. (10) Boehm, H. P. Some Aspects of the Surface Chemistry of Carbon Blacks and other Carbons. Carbon 1994, 32 (5), 759-769. (11) Boehm, H. P. Surface Oxides on Carbon. High Temp.-High Pressures 1990, 22, 275-288. (12) Minkova, V.; Marinov, S.; Budinova, T.; Stefanova, M.; Zanzi, R.; Bjo¨rnbom, E.; Lakov, L. Pyrolysis of Byproducts from Biomass in Stream of Water Vapor and Carbon Dioxide. Proceedings of the 2nd Olle Lindstro¨ m symposium on Renewable Energy-Bioenergy. 1999, Royal Institute of Technology, Sweden. (13) Brinsmead, K. H.; Kear, R. W. Behavior of Sodium Chloride during the Combustion of Carbon. Fuel 1956, 35, 84-93. (14) Kapteijn, F.; Juriaans, J.; Moulijn, J. A. Formation of Intercalatelike Structures by heat Treatment of K2CO3-carbon in an Inert Atmosphere. Fuel 1983, 62, 249-251. (15) Wiberley, L. J.; Wall T. F. Alkali-ash Reactions and Deposit Formation in Pulverized-Coal-fired Boilers: Experimental Aspects of Sodium Silicate Formation and the Formation of Deposits. Fuel 1982, 61, 93. (16) Steenari, B. M.; Lindquist O. High-Temperature Reactions of Straw Ash and the Antisintering Additives Kaolin and Dolomite. Biomass and Bioenergy 1998, 14 (1), 67-76.
Energy & Fuels, Vol. 14, No. 6, 2000 1281
detected containing chlorine or potassium were HCl during the volatile combustion phase, and KCl and KOH during the char combustion phase. Also K+ was detected by the mass spectrometer, possibly originating from release of K, KOH, or K2SO4. Another significant potassium-containing component (KOCN) was detected during combustion of other biomasses (almond hulls and alfalfa stems).18 No quantification of the total release of potassium was conducted in these experiments, but the relative release of KCl (compared to the total K content in the switch grass) was measured to be approximately 23% at a combustion temperature of 1100 °C. Olsson et al.19 investigated alkaline release from biomass pyrolysis by measurements of the total alkali in the gas phase as a function of temperature. They found that 0.1 to 0.2% of the alkaline content in straw were released below 500 °C. However, no quantification of the alkaline release at higher temperatures was made. Some minor amounts of inorganic matter may be lost during pyrolysis due to convective transport of small particles caused by the liberated gases. This may be the reason for the alkali release below 500 °C. A quantification of potassium release from wood ash as a function of temperature have been carried out by Misra et al.20 The ash was prepared by oxidation at 500 °C. Most potassium was released from the ash between 800 and 1300 °C. However the wood ash has a composition very different from straw ash. Bjo¨rkmann et al.5 investigated the release of chlorine during pyrolysis of different types of biomasses applying a heating rate of 50 °C/min. Below 200 °C no significant release of chlorine was observed. At 400 °C between 20% and 50% of the chlorine was released, and at 900 °C between 40% and 70% of the chlorine was released. Zintl et al.21 have proposed that the initial low-temperature release of chlorine is caused by a reaction between carboxylic groups and KCl. It has been demonstrated that when KCl is mixed with a chlorine-free biomass as wood or cellulose a significant amount (30 to 50%) of the chlorine is released below 400 °C.5,21 2. Experimental Quantization of K and Cl Release Wheat straw applied for the laboratory pyrolysis quantification experiments was first ground in a cutting mill mounted with a 4 mm screen, and the fines were then removed by using a 0.5 mm screen. The generated straw particles consisted of flakes with a length up to 10 mm, a width of approximately 2 mm, and a thickness of approximately 0.1 mm. This straw preparation procedure was applied to ensure that representative sampling of small amounts of straw could be performed, (17) Dayton, D. C.; French, R. J.; Milne, T. A. Direct Observation of Alkali Vapor Release during Biomass Combustion and Gasification. 1. Application of Molecular Beam/Mass Spectrometry to Switch Grass Combustion. Energy Fuels 1995, 9, 855-865. (18) Dayton, D. C.; Milne, T. A. Mechanisms of Alkaline Release during Biomass Combustion. Pre-prints of papers presented at the 210th ACS National Meeting. Chicago, IL 1995. 40 (3), 758. (19) Olsson, J. O.; Ja¨glid, U.; Pettersson, J. B. C.; Hald, P. Alkali Metal Emission during Pyrolysis of Biomass. Energy Fuels 1997, 11, 779-784. (20) Misra, M. K.; Ragland, K. W.; Baker, A. J. Wood Ash Composition as a Function of Furnace Temperature. Biomass Bioenergy 1993, 4 (2), 103-116. (21) Zintl, F.; Stro¨mberg, B.; Bjo¨rkman, E. Release of Chlorine from Biomass at Gasification Conditions. Biomass for energy and Industry, 10th European conference and technology exhibition, 1998. Wurzburg, Germany 1608.
1282
Energy & Fuels, Vol. 14, No. 6, 2000
Jensen et al.
Figure 2. Release to gas phase of total volatiles, chlorine, and potassium as a function of pyrolysis temperature. Table 2. Fraction of Potassium in Char (compared to the straw K content) That Is Not Soluble in Sulfuric Acid
Figure 1. Sketch of batch pyrolysis reactor.
pyrolysis temperature
Table 1. Concentration of the Main Ash-Forming Elements in the Straw Applied to the Pyrolysis Experiments (wt % in dry straw)
fraction not dissolved in sulfuric acid
Si
Fe
Ca
Mg
Na
K
Cl
1.88
0.005
0.39
0.051
0.006
1.23
0.41
while maintaining particle properties approximately similar to that of hammer-milled straw particles. Repeated chemical analysis of the K and Cl content of 0.5 g straw samples confirmed that representative sampling was obtained. The laboratory batch reactor shown in Figure 1 was applied to investigate wheat straw pyrolysis at medium heating rates (initial heating rate of approximately 50 °C/s and a total residence time of 15 min). The ash content of the dry wheat straw was 7.06 wt %, and the content of the ash-forming elements in the straw is shown in Table 1. The composition of the ash is typical for wheat straw, and the molar ratio of K to Cl is 2.6 and of K to Si is 0.47. Prior to an experiment, the reactor was purged by a nitrogen flow of 1.36 ΝL/min, and heated to a specified temperature. A sample of approximately 0.5 g straw was inserted into the reactor and pyrolyzed. The Cl content of the generated char was found by dissolving the char with sulfuric acid, and measuring the concentration using ion chromatography. The K content was found by dissolving the char with hydrofluoric acid and measuring the concentration by ICP-AES (inductive coupled plasma-atomic emission spectroscopy). The total yields of volatiles from dry straw and the release of potassium and chlorine as a function of temperature found by the laboratory quantification experiments are shown in Figure 2. The relative release R of K or Cl was found by applying a simple mass balance as shown in eq 1, and by applying Si as a tracer as shown in eq 2:
R ) (1 - (mc‚Cc,K)/(ms‚Cs,K)) × 100%
(1)
R ) (1 - (Cc,K/Cc,Si)/(Cs,K/Cs,Si)) × 100%
(2)
It is seen in Figure 2 that with the exception of the Cl release data at 700 °C, the Cl measurements could be reproduced within a few percent, while the K data deviated with as much as 15% at a single temperature. Most of the volatiles was released between 200 and 400 °C. Totally, approximately 70 wt % volatiles was released at 400 °C and 78 wt % at 900 °C. The chlorine was released in two steps, about 60% was released when the temperature increased from 200 to 400 °C and most of the residual chlorine was released between 700 and 900 °C. Below 700 °C no significant potassium release was
700 °C
800 °C
900 °C
0%
21%
20%
observed; above that temperature potassium release increased progressively reaching 25% at 1050 °C. A few additional pyrolysis experiments were performed in another larger reactor applying 70 g of straw, using an initial heating rate of only 30 °C/min and a total residence time of 1 h. In this reactor a K release of 3% and a Cl release of 42% were observed at 500 °C. Additional gas-phase K and Cl measurements were performed on a continuous pyrolysis plant21 with a heating rate of about 25 °C/min up to 500 °C and an additional residence time of 5 min at 500 °C. A mass balance showed a K release of 0.0-0.3% and a Cl release of 12-14%. These experiments indicate that applying a low heating rate or a large sample size results in a significantly reduced Cl release. To investigate the possible binding of potassium to silicates, some char samples from experiments with a heating rate of 50 °C/s were dissolved in sulfuric acid and the potassium content in the leachate was determined by flame photometry. The fractions of K in the char that were not dissolved by the sulfuric acid are shown in Table 2. Up to a pyrolysis temperature of 700 °C all potassium could be dissolved. We believe that potassium silicate is the only potassium-containing component present in the straw char that does not dissolve in sulfuric acid, so by this method the amount of potassium silicates can be estimated. The numbers shown in Table 2 indicate that a significant amount of potassium silicates was generated between 700 and 800 °C. The concentration of all ash-forming elements was determined in char pyrolyzed at 1050 °C. Besides the release of K and Cl, only Na was released in significant amounts. Seventy percent of Na in the straw was released at 1050 °C. This agrees well with the work of Wornat et al.6 who demonstrated that during biomass combustion a larger fraction of Na than K is released to gas phase.
3. Scanning Electron Microscopy Investigation To gain more detailed information on the fate of K and Cl during pyrolysis, SEM (scanning electron microscopy) pictures and EDX (energy dispersive X-ray) spot analyses were performed on both loose and cast straw and char particles on a Leo 435 VP SEM equipment. Great care was taken during the polishing of the cast samples to ensure that no K and Cl were removed,
Transformation/Release of K and Cl in Straw Pyrolysis
Energy & Fuels, Vol. 14, No. 6, 2000 1283 Table 3. Molar Composition of Straw (in %) as Represented in the Equilibrium Calculations H
C
N
S
O
K
Cl
Si
49.4
26.4
0.17
0.03
23.4
0.209
0.08
0.44
Table 4. Relative Distribution of Potassium at 500 °C Predicted by Equilibrium Calculation as a Function of Straw Molar Composition case K,% Cl,% Si,%
Figure 3. Sectional view of a cast straw flake particle. Length of picture ) 180 µm.
1 2
0.209 0.08 0.444 0.209 0.08 0.05
3 4
0.209 0.08 0 0.209 0 0
relative molar distribution of potassium (condensed phase) 38% as KCl, and 62% as K2O.SiO2 38% as KCl, 48% as K2O.SiO2, and 14% as K2CO3 38% as KCl, and 62% as K2CO3 100% as K2CO3
are probably KCl and K2CO3. The EDX analyses performed on the organic matrix of the straw and the char indicated that all K in the investigated straw is bound to the organic matrix, while in the char a significant amount of K is bound both to the organic matrix and in the deposited particles. 4. Equilibrium Calculations
Figure 4. The inner side of a loose char flake particle pyrolyzed at 300 °C. Length of picture ) 415 µm.
and that the samples were not at any point subjected to water. In Figures 3 and 4, SEM pictures of a cast straw and a loose char flake particles are shown. The char was generated by pyrolysis up to 300 °C, applying an initial heating rate of 50 °C/s. The morphology of the straw is maintained during the pyrolysis process. On Figure 3 of the straw, a light outer Si-rich surface is present (in the bottom of the picture), as well as small discrete mineral particles appearing as bright spots. The black areas are pores that have not been filled with casting mass. The organic matrix is seen as a light gray pattern on the dark gray background of the casting mass. EDX analyses of the small bright mineral particles in the straw type applied to the quantifying experiments showed that they consisted of nearly pure SiO2. However, we have seen deposited KCl particles on other straws. SEM pictures of the chars revealed very variable amounts of small bright deposited particles on the surfaces and in the pores of the char particles. In Figure 4 is shown the inner surface of a char particle with many small deposited particles. Compared to the straw there was generally observed a large increase in the amount of deposited particles. EDX analyses showed that only a few of these particles consisted of SiO2. Most deposited particles contained K together with Cl, or together with C and O. A few particles containing K and S were also observed. Most of the K-containing particles
To facilitate interpretation of the experimental results, thermodynamic equilibrium calculations on straw at reducing conditions were performed with the MINGTSYS program.23-25 By the calculation the stable chemical species and physical phases of the elements included are determined as a function of the temperature, pressure, and total composition of the system considered. The calculations are performed by minimization of the total Gibb’s free energy for the system under a mass balance constraint. The calculations had some important limitations. Equilibrium calculations are based on the assumption that all elements are available for reaction and kinetic limitations are ignored, also the gas phase is considered ideal and all condensed phases are assumed to be pure. No organic association of K was included in the calculation. The cases shown in Table 4 were calculated by changing the K, Cl, and Si concentration while the concentrations of all other included elements were maintained at the levels shown in Table 3. In Table 5 the gas and condensed-phase components included in the calculation are listed. The calculated equilibrium distribution of potassiumcontaining components as a function of temperature is shown in Figure 5 (corresponding to case 2 in Table 4). It is seen that K is initially released to the gas phase by evaporation of KCl, while at higher temperatures, K is released to the gas phase by decomposition of K2CO3 and K2O.SiO2. Potassium silicates were only represented by K2O.SiO2 in this calculation; however, including K2O.2SiO2 and K2O.4SiO2 in the calculation only results in very minor changes. As seen in Table 4 potassium will preferentially appear as KCl in condensed phase at 500 °C, then as (22) Hansen. J. Alkaline Measurements on Haslev Pyrolysis Plant (In Danish). CHEC Rep. No. 9820, DTU, Denmark, 1998. (23) Frandsen, F. J.; Dam-Johansen, K.; Rasmussen, P. Trace Elements from Combustion and Gasification of Coal-An Equilibrium Approach. Prog. Energy Comb. Sci. 1994. 20 (2), 115-138. (24) Frandsen, F. J.; Dam-Johansen, K.; Rasmussen, P. GFEDBASE-A Pure Substances Trace Element Thermochemical Database. CALPHAD 1996, 20 (2), 175-230. (25) Michelsen, M. L. Calculation of Multiphase Ideal Solution Chemical Equilibrium. Fluid Phase Equilib. 1989, 53, 73-80.
1284
Energy & Fuels, Vol. 14, No. 6, 2000
Jensen et al.
Table 5. Components Included in the Equilibrium Calculation elements included: C, O, H, S, N, Cl, Si, K gas-phase components included: CCl, CCl2, CCl3, CCl4, CH4, CO, CO2, COS, C2Cl4, C2Cl6, C2H2, C2H4, Cl, ClO, Cl2, Cl2O, HCN, HCl, HNO, HNO2, HNO3, HS, H2, H2O, H2S, H2S2, H2SO4, NH, NH2, NH3, NO, NOCl, NO2, NO2Cl, N2, N2H4, N2O, N2O3, N2O4, N2O5, O, OH, O2, O3, S, SCl, SCl2, SN, SO, SOCl2, SO2, SO2Cl2, SO3, S2, S2Cl, S2Cl2, S2O, S3, S4, S5, S6, S7, S8, Si, SiCl, SiCl2, SiCl2H2, SiCl3, SiCl3H, SiCl4, SiH, SiH4, SiO, SiO2, Si2, Si3, K, KCl, KOH, (KCl)2, (KOH)2, K2SO4 condensed-phase components included: C, Si, SiO2, KCl, K2CO3, K2SiO3, K2SO4
Figure 6. Leaching of char and straw in 80 °C water. Relative residual amount of potassium in solid as a function of time.
Figure 5. Results of equilibrium calculation of case 2 in Table 4. Distribution of potassium-containing components as a function of temperature. (c) condensed phase, (g) gas phase. Table 6. Release to Gas Phase of Potassium Predicted by Equilibrium Calculationa Potassium appearing as KCl at 500 °C is released to gas phase between 680 and 830 °C Potassium appearing as K2O.SiO2 at 500 °C is released to gas phase between 1080 and 1230 °C Potassium appearing as K2CO3 at 500 °C is released to gas phase between 830 and 980 °C a The temperature ranges at which significant release took place in all performed calculations are shown.
K2SiO3, and if no Si and Cl are present, as K2CO3. In the gas phase, potassium is predominately found as KCl and K with only minor amounts of KCN and KOH. As seen in Table 6 the transfer of potassium to gas phase was predicted to be strongly dependent on which components contained the potassium. Initial significant KCl evaporation is predicted to happen at 680 °C, while some condensed-phase potassium silicate (K2O.SiO2) can exist up to a temperature of 1200 °C. In all calculated cases listed in Table 4, the molar ratio of K to Cl was greater than one and all Cl was predicted to appear as KCl. By comparison of Table 2 and Figure 5 it is obvious that potassium silicate is generated at a higher temperature than predicted by the calculation. This may indicate that potassium silicate generation is kinetically limited or that it is gas-phase KCl that reacts with SiO2. 5. Leaching of Straw and Char Information about the chemical form of an element in a fuel can be obtained by determining the watesoluble fraction of the element. A leaching experiment was performed by heating 400 mL of distilled water to 80 °C and adding 0.5 g of straw char or straw. During an experiment, samples of the solution were withdrawn through a filter with a syringe, and the concentration
of potassium and chlorine in the water sample was later analyzed. By this method the release of the elements could be determined as a function of time. The char applied for this experiment was pyrolyzed at 500 °C with an initial heating rate of 30 °C/min. The result of the washing experiment is shown in Figure 6. It is seen that most of the potassium in the straw could be removed within 30 min. An initial fast release of approximately 50% of the potassium from the char occurred during the first few minutes, followed by a much slower secondary release. The initial fast release of potassium is KCl and K2CO3 that are dissolved and the slow secondary release is probably potassium release from the organic char matrix. Nearly all chlorine was released from both straw and char within a few minutes. More detailed information about the components released during the washing process was obtained by analyzing leachate from char washed for 1 h at 80 °C. The ionic concentrations of the dissolve components are shown in Table 7. The charge balance was closed indicating that all important ionic components were detected. The leachate composition confirms that large amounts of KCl and K2CO3 were released. The relatively slow secondary wash-out of K from char, seen in Figure 6, indicates that this process is transport limited. It could be diffusion of K atoms in the char matrix or it could be migration and chemical reactions of oxygen-containing surface complexes. The counterions to potassium such as HCO3-, acetate, and formiate is probably released during the char matrix release phase. 6. Discussion and Conclusions The release and transformation of K and Cl during straw pyrolysis were studied by applying a lab-scale batch reactor. Generated chars were investigated by determining chemical composition, by SEM investigations, and by leaching with water. The experimental results were compared with thermodynamic equilibrium calculations on straw at reducing conditions. The pyrolysis experiments at a heating rate of 50 °C/s showed that chlorine was released in two steps: about 60% was released when the temperature increased from 200 to 400 °C and most of the residual chlorine was released between 700 and 900 °C. Below 700 °C no significant potassium release was observed; above that tempera-
Transformation/Release of K and Cl in Straw Pyrolysis
Energy & Fuels, Vol. 14, No. 6, 2000 1285
Table 7. Main Constituents in Char Washing Water positive ions concentrations: mmol/g(char) negative ions oncentrations: mmol/g(char)
K+ 0.655 Cl0.238
Na+ 0.004 CO320.194
ture potassium release increased progressively until 25% at 1050 °C. During the pyrolysis process small discrete particles of KCl and K2CO3 were generated in the pores and on the surface of the char. This shows that these components migrate during the pyrolysis process (see Figure 4). The migration indicates that during pyrolysis from 200 to 400 °C a liquid phase is present. It is an open question why Cl is released at relatively low temperatures. It has clearly been shown5,21 that chlorine can be released by a reaction between KCl and char. However, we do not think that this is the main source of initial chlorine release for the investigated straw at 200-400 °C. It has been observed that by applying a large sample or a low heating rate a reduced amount of chlorine is released between 200 and 400 °C. If the chlorine and potassium are bound to separate sites the influence of sample size and heating rate can be explained. Initially, chlorine is probably released from its solid-phase bounding sites, and is then present in a liquid-phase tar. If a long residence time is available or a large sample is present, secondary reactions will more likely occur. The secondary reactions can be reaction with potassium to generate KCl or reactions with relatively stable basic functionalities on the char surface. That some chlorine is bound to stable basic functionalities on the char may also explain why all the chlorine was not released from char pyrolyzed at 900 °C with a low heating rate (as shown by Bjo¨rkmann et al.5). Based on our data and the work of other researchers, the transformation of K and Cl as a function of pyrolysis temperature could be summarized as follows: A. At a temperature of 200 to 400 °C, much of the original organic straw matrix is destructed, and probably Cl and K are released from the original binding sites and transferred to a liquid tar phase. Chlorine is further released to the gas phase as HCl, or reacts with K or basic functionalities on the char surface. Potassium is bound in the condensed phase as KCl and K2CO3 deposited particles, and to char-matrix functional groups such as carboxylics and phenoxides. Some further Cl release is caused by reaction of KCl with char oxygencontaining functional groups, whereby HCl is released, and K is bound to the char matrix.
Mg2+ 0.002 HCO30.142
Ca2+ 0.015 SO420.061
PO430.004
acetate 0.007
formiate 0.028
B. At 400-700 °C, no significant amounts of K or Cl are released to the gas phase. Since carboxylic groups are not stable at high temperatures, the char-matrixbound potassium must exist as phenoxides or being intercalated at a temperature of 700 °C. C. From 700 to 830 °C, all KCl evaporates, and thereby most chlorine in a char heated at 50 °C/s is released. The release of chlorine in this temperature range corresponds to a release of 12% of the total amount of potassium in the investigated straw. In this temperature range K also reacts with silicon and potassium silicates are generated. D. From 830 to 1000 °C, K2CO3 is decomposed and potassium is released as KOH or as free K atoms. Possibly, potassium can also be released from the char matrix. E. Above 1000 °C, potassium may be released to the gas phase from the char matrix and from potassium silicates. This study shows that by limiting the temperature in a pyrolysis reactor to 700 °C a significant release of potassium could be prevented. Release of some chlorine cannot be prevented. However, by using a low heating rate and a large reactor the emission of HCl can be minimize. Acknowledgment. This work has been funded by the European Union under contract JOR3-CT95-0057, by Elsam and by the Danish Energy Research Program (EFP). The work was conducted in a collaboration between the CHEC research program (Combustion and Harmful Emission Control) and ELSAMPROJEKT (power station engineering consultants). The Danish Technological Institute performed the SEM and EDX analysis under the Center for Surface Microscopy, Micro-, and Image Analysis. Nomenclature Cc,K ) concentration of potassium in dry char [wt %] Cc,Si ) concentration of silicon in dry char [wt %] Cs,Si ) concentration of silicon in wet wheat straw [wt %] Cs,K ) concentration of potassium in wet wheat straw [wt %] mc ) mass of char [g] ms ) mass of straw [g] R ) relative release to gas phase of K or Cl [%] EF000104V
