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Preliminary results on the ash behavior of peach stones during fluidized bed gasification: evaluation of fractionation and leaching as pre-treatments
ARTICLE IN PRESS
Biomass and Bioenergy 28 (2005) 331–338 www.elsevier.com/locate/biombioe
Preliminary results on the ash behavior of peach stones during fluidized bed gasification: evaluation of fractionation and leaching as pre-treatments Stelios Arvelakisa,, Hans Gehrmannb, Michael Beckmannc, Emmanuel G. Koukiosa a
Bioresource Technology Unit, Laboratory of Organic and Environmental Technologies, Department of Chemical Engineering, National Technical University of Athens, Zografou Campus, GR-15700, Athens, Greece b Clausthaler Umwelttechnik Institut GmbH, Leibnizstrasse 21+23, D-38678, Clausthal-Zellerfeld, Germany c Bauhaus Universitat Department of Process Engineering and Environment, Weimar Coudray Straı´e 13C, D-99423 Weimar, Germany Received 31 July 2003; received in revised form 21 June 2004; accepted 9 August 2004
Abstract Peach stones comprise a valuable agroindustrial by-product that is available in many countries of the World and especially in the Mediterranean region. A number of important advantages such as its high energy value, the low ash content in combination with the absence of transportation costs due to the fact that is produced in agro-industries, make peach stones an ideal fuel for energy production via gasification. Gasification tests were performed in a lab-scale fluidized bed gasifier in order to study the behavior of peach stones and especially its ash during the gasification process. Apart from the tests with the initial peach stone samples, gasification tests were performed using peach stones that had been pre-treated using two different methods fractionation and leaching. Pretreatments used in order to study their effect on the beneficiation of the materials ash and on the avoidance of ash-related problems such as deposition, agglomeration and corrosion during the gasification process. A water-cooled steel tube placed vertical to the flow of the gasification gases was used in order to collect samples of ash deposits that were analyzed using SEM-EDX analysis techniques in order to assess the effect of the pre-treatment techniques on the peach stones ash behavior. The produced results showed that peach stones can be used as gasification feedstock without significant ash problems. Fractionation resulted in a deterioration of the ash behavior of the material, increasing the amounts of alkali metals and chlorine included in its ash, while leaching showed a positive effect but to a moderate extent. r 2004 Elsevier Ltd. All rights reserved. Keywords: Peach stones; Ash; Gasification; Sintering; Deposition; Fractionation; Leaching; Prunus persica
Corresponding author. Fax: +353-61-234-169.
E-mail address: [email protected] (S. Arvelakis). 0961-9534/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2004.08.016
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1. Introduction Biomass fuels cover a wide range of materials such as wood, energy crops, energy plantations, agricultural and agro-industrial byproducts and wastes, i.e. straws, cobs, hulls and pits, that can be used for energy production using thermochemical conversion methods. Recently, the use of biomass fuels for energy production has gained increasing importance as a substitute for or being used in combination with fossil fuels. This development is mainly caused by the international concern regarding carbon dioxide (CO2) emissions. Biomass fuels are CO2-neutral and their use for energy production could substantially decrease the greenhouse effect at a global level [1,2]. However, the use of biomass fuels for energy production is restricted by the fact that they contain a large amount of elements with very reactive and problematic behavior. The ash-forming elements, Al, Ca, Fe, K, Mg, Na, and Si, occur in the biofuels as internal or external mineral grains, simple salts such as KCl and CaSO4 or associated with the organic parts of the fuel. Depending on the gas/particle temperature and the redox conditions during the reaction process, these elements may vaporize if they are in the forms of simple salts, while the mineral grains will approach each other and undergo phase transformations, forming fly ash particles. In specific, alkalis and alkali earth metals tend to react with silicon in the form of silica (SiO2), and create low melting point silicates. The reactions can take place either in the solid phase during the burn-out of the biomass particles char or most commonly in the gas phase where the fly ash particles have been formed [3–5]. Chlorine acts as a facilitator increasing the mobility of potassium since most of it is present as KCl. Potassium chloride is among the most stable high-temperature gas-phase alkali-containing species, while the amount of chlorine in the fuel often dictates the amount of the alkali that can be vaporized during combustion or gasification. Calcium also appears to react with sulfur to form sulfates, but the lower mobility of calcium in combination with the low amounts of sulfur in these biofuels does not make it a significant
problem. The produced alkali silicates and/or mixed alkali and/or calcium salts (chlorides/ sulfates) have very low melting points that may reach To700 1C and tend to deposit on the reactor walls or on the heat exchange surfaces in the case of the conventional grate-fired systems, increasing the deposition and corrosion problems observed. In the case of the fluidized bed reactors, they contribute significantly, especially KCl, to bed sintering and defluidization through the development of a sticky deposit layer on the surface of the bed particles [6–9]. The aim of this work is to study the effect of leaching, and fractionation pre-treatment techniques on the ash behavior of the peach stone material during gasification in a lab-scale gasifier. Leaching has been seen to beneficiate the ash of the treated materials by decreasing the amounts of alkali metals, chlorine, sulfur and also the total ash amount in variable levels depending on the material and the applied leaching conditions [10–12]. The results from various experimental trials varying from lab-to-commercial scale [13–16] clearly demonstrate that the combustion properties and the ash behavior of the leached biomass can be improved to an extent. On the other hand, fractionation affects positively mainly the total ash content of the treated material, while its effect on the chemical composition of the ash depends mainly on the specific characteristics of the material [10]. The produced results showed that there was a clear improvement regarding both the chemical composition and the amounts of ash in the case of leaching, while in the case of fractionation the results appeared to be encouraging regarding only the reduction of the ash amount contained in the material.
2. Experimental Greek peach stones (Prunus persica), were used as feedstock material during the gasification tests. Peach stones consist of kernels and pulp and they are produced as by-product from the production of stewed fruits and/or natural juices in agroindustrial factories. The material was supplied by a local juice company situated at the area of Argos,
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Peloponisos, Greece in 1998. The peach stone material originated from the northern part of Greece and the harvest time was the Summer of 1997. The peach stone material was first crushed using a laboratory hammer mill and a 4 mm screen. Two different pre-treatment techniques, fractionation and leaching, were studied in order to investigate their effect on the ash behavior of the specific residue during the gasification process. The leached peach stone sample used in the gasification tests was pre-treated with tap water in order to extract a part of its inorganic constituents, mainly alkali metals and chlorine, which are found in large quantities in its ash and are thought to cause ash-related problems during biomass gasification. In specific, leaching comprises the immersion of the treated material in tap water using specific water/mass ratios for a specific period of time in order to achieve the extraction of water-soluble inorganic elements (K,Na,Cl) of its ash. The water-to-mass ratio applied to the leached peach stone sample used in the gasification tests was 15 ml/g, while the leaching period was 8 h and the temperature 20 1C. The water used in the leaching process was obtained from a local spring and the main elements contained in it were calcium (155 mg/l), magnesium (53 mg/l), sodium (63 mg/l) and potassium (14 mg/l). At the end of the leaching process, the pH of the leachate was acidic, taking values between 6 and 6.5 approximately. On the other hand, fractionation divides the peach stone material into two different fractions, one with average particle size above 1 mm, which counts for almost 90% of the whole material and was selected for the gasification tests, and the other with average particle size below 1 mm that counts for the remaining 10% of the material. The untreated peach stone material was encoded as P1, while the fractionated and leached samples as PF1 and PL1, respectively, for further use in the paper. A detailed description regarding the leaching and fractionation pre-treatment of the peach stone samples is given by Arvelakis et al. [10]. Additional information regarding leaching pre-treatment technique and its effect on the ash behavior of the treated samples can be found elsewhere [11,12]. A lab-scale gasifier depicted in Fig. 1 situated at the CUTEC-GmbH institute in Germany was used
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Fig. 1. Lab-scale gasifier.
in conducting the gasification tests. The fluidized bed reactor consists of the following parts: fluidized bed, free board, hot gas cyclone, postcombustion chamber system and flue gas purification. A conveyor screw was used to feed the raw material to the reactor about 300 mm above the distribution plate of the fluidized bed. A maximum mass flow of about 20 kg/h can be attained, while the maximum fluidized bed height at steady height mode is about 300 mm. The heating of the fluidized bed is achieved with a controllable natural gas burner situated at about the same height as the input supply pipe connection. The bed inert material used in all tests was silica sand with particle diameter varying from 0.4 to 1.4 mm and a mean particle diameter of 0.82 mm. Sand particles had a theoretical specific surface of 29 cm2/g, in combination with an apparent density of 2.65 g/cm3 and a true density of 1.5 t/m3. Approximately 15 kg of silica sand was used as bed inert material during each gasification test. The gasification agent used in all tests was a mixture of air and nitrogen in order to have the appropriate equivalence ratio for the gasification of the specific bio-fuel, while all tests were conducted under atmospheric pressure. In order to study the deposition phenomena during the gasification of the various peach stone samples, a water-cooled steel tube with dimensions 30 2 cm was put vertically approximately 30 cm above the bed of the gasifier. The temperature of the tube surface was kept at the level of 500 1C. Samples of the produced fly ash material during the
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gasification process were deposited at the watercooled steel tube, which played the role of a heat transfer surface. The produced deposit was removed after the end of each test from the tube surface using a brush. The material was collected and examined using a JEOL 6300 scanning electron microscope in order to have a clear view regarding the effect of the different pre-treatment methods on the morphology and composition of the ash particles formed in these deposits. The examination was focused on analyzing the surface of the ash particles using the EDS microprobe of the microscope in order to calculate the concentrations of the main inorganic elements forming the surface of the ash particle, while photos of the analyzed surface areas helped in better understanding and evaluation of the results. Several analyses (15–20) were performed for each ‘‘class’’ of particles covering different magnifications and
Table 1 Analysis and characterization of the peach stone samples P1
PF1
PL1
8.53 0.65 81.3 18.1
13.28 0.38 80.54 19.08
11.98 0.43 81.61 17.96
0.79 51.95 5.76 o0.01 0.14 40.7 21.6
0.89 52.66 6.1 — 0.1 39.87 22.02
0.95 53.45 6.23 — 0.00 38.94 22.8
Proximate analysis (%d. basis) Moisture Ash Volatiles Fixed carbon Ultimate analysis (%d. basis) Nitrogen Carbon Hydrogen Sulfur Chlorine Oxygen GCV (MJ/kg)
surface areas in order to be sure for the repeatability of the produced results.
3. Results and discussion 3.1. Analysis and characterization of the raw materials Table 1 presents the effect of fractionation and leaching pre-treatment techniques on various properties of the different peach stone samples used in the gasification tests. As it is seen from Table 1, leaching has a positive effect on the main properties of the peach stone samples such as ash content, volatiles and gross calorific value, even though the effect is marginal in most cases, with the exception of ash content that appears to be reduced by approximately 40% w.b. compared with the initial P1 peach stone sample. Fractionation shows also to have the same effect and appears to improve significantly the ash content of the PF1 sample that is reduced by almost 50% w.b. compared to the untreated P1 peach stone sample. Ash elemental analysis of the peach stone samples depicted in Table 2 shows that the ash of the initial peach stone sample P1 appears to be highly reactive, containing high amounts of potassium that counts for almost 39% w.b. of the ash, while the amounts of the other elements appear to be rather low with concentrations being below 10% w.b. Furthermore, the ash analysis of the PF1 sample shows that fractionation leads to a deterioration of the ash composition of the specific peach stone sample. The amount of potassium in the ash increases significantly by almost 20% w.b.,
Table 2 Ash elemental analysis of the peach stone samples %w.b. Ash basis (%) P1 PF1 Difference (%) PL1 Difference (%)
K2O 38.45 45.78 19.07 21.24 44.74
Na2O 0.07 0.07 0 0.02 76
CaO 7.14 6.13 14.11 19.32 170.58
MgO 2.73 1.4 48.78 2.53 7.25
SiO2 5.97 5.03 15.92 10.47 75.15
Al2O3 0.47 0.03 93.17 0.02 95.98
Fe2O3 0.52 0.63 19.83 0.086 83.69
TiO2 0.15 0.08 51.61 0.04 72.04
SO3 5.24 5.09 2.88 3.16 39.71
Cl 0.06 0.03 50.81 0.02 60.65
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while the amount of chlorine appears to be reduced now by almost 50% w.b., and the amounts of sodium and sulfur remain almost constant. On the other hand, the amounts of alkali metals and chlorine contained in the ash of the leached sample PL1 are reduced by 44–76% w.b. regarding potassium and sodium content, and by 60% w.b. regarding chlorine. At the same time, the amounts of sulfur, iron and titanium in the ash of the PL1 sample are also reduced by almost 40%, 84% and 70% w.b., respectively, while the amounts of calcium, and silica are increased significantly. According to these data, the ash behavior of the PL1 material during the gasification tests is expected to improve, though the amounts of potassium, chlorine and sulfur in its ash fraction remain substantial. 3.2. Gasification tests with the P1 sample Two gasification tests were performed using the material P1. Each test lasted for approximately 90 min with a feeding rate of 10.0 kg/h. The gasifier was first heated up to 830 1C using the gas burner until good fluidization conditions were established. Then the gas burner was shut off and the feeding of the peach stones material in the gasifier was started. The reactor was observed to operate at the temperature range of 850–860 1C without problems at an equivalence ratio of 0.5 for about 90 min. After the end of the test the water-cooled tube that is placed vertically above the bed was removed and inspected for possible signs of deposition. As it is seen in Fig. 2a, the tube appears to be covered with a thick layer of ash particles mixed together with tars and unburned carbon. The deposition appears to take place both on the upper and lower parts, with the majority of the ash being captured on the upper part of the tube. The strength of the deposit is observed to be higher on the lower part of the tube and it is increased by the presence of the tars among the ash particles that act like glue. Ash particles appear to have mainly green color and to a lower extent white. The deposit is removed from the tube and is analyzed using SEM-EDX. Fig. 3a depicts the morphology of the ash particles that appear to be rough and irregular. As it is seen from Table 3, the
Fig. 2. Ash deposits on the water-cooled steel tube during the gasification of peach stones samples: (a) P1, (b) PF1, (c) PL1.
ash particles contain mainly potassium up to 40% w.b., while phosphorus, calcium and iron appear to be present in concentrations varying from 10% to 15% w.b. The concentration of iron appears to be higher in some cases, reaching 60–70% w.b., but this is attributed to the presence of material from the steel tube in the ash sample. The specific elemental analysis supports that the ash particles are composed of a silicate matrix with potassium, calcium and phosphorus as the main elements participating in the silicate structure. A part of these elements may have condensed later on the surface of the silicate particles in the form of salts and further reacted with it, forming new more eutectic compounds. The chlorine is observed to be absent from this mixture, apart from certain spots where it appears in very low concentrations below
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S. Arvelakis et al. / Biomass and Bioenergy 28 (2005) 331–338 Table 3 SEM–EDX analysis of various ash particles during the gasification tests with the peach stone samples %w.b. Samples
P1a
PF1b
PL1c
K2O Na2O CaO MgO SiO2 Al2O3 Fe2O3 P2O5 SO3 Cl
43.06 0.00 16.16 0.30 5.16 0.00 14.82 15.39 5.04 0.00
55.62 0.62 9.66 0.00 16.46 0.00 4.49 10.22 0.00 2.24
12.95 0.00 34.66 0.00 33.66 2.42 14.14 0.00 1.55 0.00
a
Gasification conditions: Temperature 850–860 1C, ER=0.5, Test duration=90 min. b Gasification conditions: Temperature 850–860 1C, ER=0.5, Test duration=85 min. c Gasification conditions: Temperature 870 1C, ER=0.5, Test duration=120 min.
mainly, in the gas phase, where they react further, substituting chlorine that remains in the gas phase. 3.3. Gasification tests with the PF1 sample
Fig. 3. SEM–EDX photos of ash particles during the gasification of various peach stone samples: (a) P1, (b) PF1, (c) PL1.
2% w.b., accompanied with potassium, and this is attributed to its main role as an intermediate during the transport of alkali metals, potassium
Two tests were performed using the material peach stone PF1 sample. The tests were performed under the same conditions used in the previous tests with the untreated P1 peach stone sample. The amount of material used in each of these tests was 14 kg and the operation time 85 min approximately. The gasifier appears to operate again under steady temperature conditions in the range 860–8701C, while no agglomeration problems were observed. After the end of each test, inspection of the water-cooled tube revealed the formation of an ash deposit layer that appears to be thicker and stronger compared with the former one from the tests with the P1 sample, as it is seen in Fig. 2b. The SEM–EDX analysis of the ash particles from the specific deposits revealed that now they appear to be more compact having smaller voids in their structure, while secondary deposition of smaller fly ash particles on the surface of larger particles is observed, as it is seen in Fig. 3b. The deposited ash particles appear to contain mainly potassium in concentrations above 55%
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w.b. and lower amounts of silicon, calcium and phosphorus in the range of 10–15% w.b. according to Table 3. These results are in good agreement with the ash elemental analysis of the specific sample where potassium appears to be the dominant element. The comparison of the produced results to the results from the tests with the P1 sample clearly show a significant deterioration of the ash particle chemistry. The composition of the ash particles is seen to be more reactive now favoring melting interactions among them in a larger extent compared to the former case. 3.4. Gasification tests with the PL1 sample Finally, the effect of leaching pre-treatment technique on the ash behavior of the peach stones material was studied through two gasification tests using the PL1 sample. The experiments were performed at the temperature of 870 1C, which is slightly higher than the operating temperature of the gasifier in the previous tests. In this case, the amount of the PL1 sample used in each of the tests was 19.5 kg and each test lasted for almost 2 h with a feeding rate of 10 kg/h. As in the cases with the previous peach stone samples, the gasification tests ended without any visual problem regarding agglomeration or defluidization of the bed. However, the removal of the water-cooled tube from the gasifier revealed that the formed ash deposit was in this case comparable in size with the deposits observed in the tests with the untreated material, even though the reactor operated for longer time and in higher temperature, as it is seen in Fig. 2c. The SEM–EDX analysis of the specific ash particles presented in Fig. 3c and Table 3 showed a significant change in the chemistry of the specific ash particles. The ash particles appear to have a more loose structure, while there are no signs of deposition of smaller ash particles on larger ones. Silicon and calcium, in concentrations that vary from 30% to 60% w.b., appear to be the main forming elements now, while potassium and iron are also present principally in concentrations below 10% w.b.. These results are in accordance with the results from the ash analysis of the PL1 sample, where it is seen that leaching has caused a
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significant extraction of alkali metals, chlorine and sulfur from the ash. The potassium content on the surface of the ash particles is low indicating that the melting tendency of the specific ash particles is anticipated to be low as it is also observed from the size and strength of the tube deposit.
4. Conclusions The results obtained from the gasification tests of peach stone samples showed that the ash behavior of the specific material could vary significantly depending on the pre-treatment technique applied. The ash from the untreated peach stone P1 sample is seen to be highly reactive, containing large amounts of alkali metals that could have a significant negative effect on the large-scale gasification process causing deposition, agglomeration and corrosion problems. The composition of the generated ash particles verified this observation. Fractionation pre-treatment technique shows to act positively regarding only the ash content of the treated material that is seen to be reduced significantly. At the same time, fractionation results in a deterioration of the elemental composition of the peach stones ash, increasing significantly the amount of potassium in the ash. The ash particles from the gasification tests with the PF1 sample had higher potassium concentrations compared to those from the tests with the P1 sample. In addition, signs of secondary deposition of small ash particles in the surface of bigger ones, and a significant increase in both the size and the strength of the generated deposit, were also observed. The chemistry of the secondary deposited particles is seen to be similar to the chemistry of the larger particles. On the contrary, the leaching pre-treatment technique showed a positive effect as far as it concerns the ash thermal behavior of the peach stones material. Leaching resulted, according to Table 2, in a significant removal of alkali metals and chlorine from the ash of the sample PL1. The ash particles generated from the tests with the PL1 peach stone sample were seen to have silicon and
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calcium as their main forming elements, while both the size of the created deposit and its strength were reduced significantly. These results are in good agreement with the results obtained from other research trials performed using various leached biomass materials worldwide [11,12,14–16], and clearly demonstrate the ability of the specific pretreatment to improve the ash behavior of a variety of different biomass materials. The low ash content of this particular material, less than 1% w.b. of the total mass as it is seen in Table 1, makes necessary the performance of very long test runs (424 h) in order to be able to study with accuracy the effect of the peach stone ash on the bed material agglomeration tendency.
Acknowledgements The research work was financed by the European Commission under the COPES research program, and thus its contribution is gratefully acknowledged by the authors.
References [1] Morris G. Environmental costs and benefits of biomass energy use in California. Berkeley, CA: Future Resources Associates, Inc. NREL/ SR-430-22765; 1997. [2] Miles Sr TR, Miles Jr TR. Environmental implications of increased biomass energy use. Research Report NREL/ TP-230-4633, 1, 1992. [3] Miles TRPE, Miles Jr TR, Baxter LL, Bryers RW, Jenkins BM, Oden LL. Alkali deposits found in biomass power plants, vols. I, II. Summary Report for the National Renewable Energy Laboratory. NREL Subcontract TZ-211226-1, 1995. [4] Real C, Alcala MD, Criado JM. Preparation of silica from rice husks. Journal of American Ceramic Society (ACERS, USA) 1996;79(8):2012. [5] Baxter LL. Ash deposition during biomass and coal combustion: a mechanistic approach. Biomass and Bioenergy 1993;4(2):85–105.
[6] Benson SA. Ash formation and behavior in utility boilers. Newsletter published by Microbeam Technologies Inc., Grand Forks, USA, www.microbeam.com, 1998. [7] Baxter LL, Jenkins BM, Miles TR, Miles Jr TR, Milne T, Dayton D, Bryers RW, Oden LL. Inorganic material deposition in biomass boilers. In: Proceedings of the Ninth European Bioenergy Conference, Oxford, UK: Pergamon; Copenhagen, Denmark. 1996. p. 1114. [8] Mann MD, Galbreath KC. The role of ash chemistry and operating parameters on ash agglomeration and deposition in FBC systems. In: Fifth Engineering Foundation Conference on ‘Inorganic Transformations and Ash Deposition During Combustion, ACERC, Palm Coast, FL, USA, 1991. p. 773. [9] Nordin A, Ohman M. Agglomeration and defluidization in FBC of biomass fuels—mechanisms and measures for prevention. In: Baxter LL, DeSollar RW, editors. Proceedings of the Symposium: Applications of Advanced Technology to Ash-Related Problems in Boilers, New York, USA, 1996. p. 353. [10] Arvelakis S, Koukios EG. Physicochemical upgrading of agroresidues as feedstocks for energy production, via thermochemical conversion methods. Biomass and Bioenergy 2002;22(5):331–48. [11] Turn SQ, Kinoshita CM, Ishimura DM. Removal of inorganic constituents of biomass feedstocks by mechanical dewatering and leaching. Biomass and Bioenergy 1997;12(4):241–52. [12] Jenkins BM, Bakker RR, Wei JB. On the properties of washed straw. Biomass and Bioenergy 1996;10(4):177–200. [13] Arvelakis S, Vourliotis P, Kakaras E, Koukios EG. Effect of leaching on the ash behavior of wheat straw and olive residue during fluidized bed combustion. Biomass and Bioenergy 2001;20(6):459–70. [14] Bakker RR, Jenkins BM, Williams RB, Blunk S, Yomosida DE, Carlson W, Duffy J, Bates R, Stucki K, Tiangco VM. Combustion of leached rice straw for power generation. In: Overend RP, Chornet E editors. Biomass: a growth opportunity in green energy and value-added products. Oxford, UK: Pergamon; 1999. p. 1357–63. [15] Bakker RR, Jenkins BM, Williams RB. Fluidized bed combustion of leached rice straw. Energy and Fuels (ACS, USA) 2002;16:356–65. [16] Bakker RR, Jenkins BM, Williams RB, Carlson W, Duffy J, Baxter LL, Tiangco VM. Boiler performance and furnace deposition during a full-scale test with leached biomass. In: Overend RP, Chornet E editors. In making a business from biomass in energy, environment, chemicals, fibers and materials. Oxford, UK: Pergamon; 1997. p. 497–510.
Biomass and Bioenergy 28 (2005) 331–338 www.elsevier.com/locate/biombioe
Preliminary results on the ash behavior of peach stones during fluidized bed gasification: evaluation of fractionation and leaching as pre-treatments Stelios Arvelakisa,, Hans Gehrmannb, Michael Beckmannc, Emmanuel G. Koukiosa a
Bioresource Technology Unit, Laboratory of Organic and Environmental Technologies, Department of Chemical Engineering, National Technical University of Athens, Zografou Campus, GR-15700, Athens, Greece b Clausthaler Umwelttechnik Institut GmbH, Leibnizstrasse 21+23, D-38678, Clausthal-Zellerfeld, Germany c Bauhaus Universitat Department of Process Engineering and Environment, Weimar Coudray Straı´e 13C, D-99423 Weimar, Germany Received 31 July 2003; received in revised form 21 June 2004; accepted 9 August 2004
Abstract Peach stones comprise a valuable agroindustrial by-product that is available in many countries of the World and especially in the Mediterranean region. A number of important advantages such as its high energy value, the low ash content in combination with the absence of transportation costs due to the fact that is produced in agro-industries, make peach stones an ideal fuel for energy production via gasification. Gasification tests were performed in a lab-scale fluidized bed gasifier in order to study the behavior of peach stones and especially its ash during the gasification process. Apart from the tests with the initial peach stone samples, gasification tests were performed using peach stones that had been pre-treated using two different methods fractionation and leaching. Pretreatments used in order to study their effect on the beneficiation of the materials ash and on the avoidance of ash-related problems such as deposition, agglomeration and corrosion during the gasification process. A water-cooled steel tube placed vertical to the flow of the gasification gases was used in order to collect samples of ash deposits that were analyzed using SEM-EDX analysis techniques in order to assess the effect of the pre-treatment techniques on the peach stones ash behavior. The produced results showed that peach stones can be used as gasification feedstock without significant ash problems. Fractionation resulted in a deterioration of the ash behavior of the material, increasing the amounts of alkali metals and chlorine included in its ash, while leaching showed a positive effect but to a moderate extent. r 2004 Elsevier Ltd. All rights reserved. Keywords: Peach stones; Ash; Gasification; Sintering; Deposition; Fractionation; Leaching; Prunus persica
Corresponding author. Fax: +353-61-234-169.
E-mail address: [email protected] (S. Arvelakis). 0961-9534/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2004.08.016
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1. Introduction Biomass fuels cover a wide range of materials such as wood, energy crops, energy plantations, agricultural and agro-industrial byproducts and wastes, i.e. straws, cobs, hulls and pits, that can be used for energy production using thermochemical conversion methods. Recently, the use of biomass fuels for energy production has gained increasing importance as a substitute for or being used in combination with fossil fuels. This development is mainly caused by the international concern regarding carbon dioxide (CO2) emissions. Biomass fuels are CO2-neutral and their use for energy production could substantially decrease the greenhouse effect at a global level [1,2]. However, the use of biomass fuels for energy production is restricted by the fact that they contain a large amount of elements with very reactive and problematic behavior. The ash-forming elements, Al, Ca, Fe, K, Mg, Na, and Si, occur in the biofuels as internal or external mineral grains, simple salts such as KCl and CaSO4 or associated with the organic parts of the fuel. Depending on the gas/particle temperature and the redox conditions during the reaction process, these elements may vaporize if they are in the forms of simple salts, while the mineral grains will approach each other and undergo phase transformations, forming fly ash particles. In specific, alkalis and alkali earth metals tend to react with silicon in the form of silica (SiO2), and create low melting point silicates. The reactions can take place either in the solid phase during the burn-out of the biomass particles char or most commonly in the gas phase where the fly ash particles have been formed [3–5]. Chlorine acts as a facilitator increasing the mobility of potassium since most of it is present as KCl. Potassium chloride is among the most stable high-temperature gas-phase alkali-containing species, while the amount of chlorine in the fuel often dictates the amount of the alkali that can be vaporized during combustion or gasification. Calcium also appears to react with sulfur to form sulfates, but the lower mobility of calcium in combination with the low amounts of sulfur in these biofuels does not make it a significant
problem. The produced alkali silicates and/or mixed alkali and/or calcium salts (chlorides/ sulfates) have very low melting points that may reach To700 1C and tend to deposit on the reactor walls or on the heat exchange surfaces in the case of the conventional grate-fired systems, increasing the deposition and corrosion problems observed. In the case of the fluidized bed reactors, they contribute significantly, especially KCl, to bed sintering and defluidization through the development of a sticky deposit layer on the surface of the bed particles [6–9]. The aim of this work is to study the effect of leaching, and fractionation pre-treatment techniques on the ash behavior of the peach stone material during gasification in a lab-scale gasifier. Leaching has been seen to beneficiate the ash of the treated materials by decreasing the amounts of alkali metals, chlorine, sulfur and also the total ash amount in variable levels depending on the material and the applied leaching conditions [10–12]. The results from various experimental trials varying from lab-to-commercial scale [13–16] clearly demonstrate that the combustion properties and the ash behavior of the leached biomass can be improved to an extent. On the other hand, fractionation affects positively mainly the total ash content of the treated material, while its effect on the chemical composition of the ash depends mainly on the specific characteristics of the material [10]. The produced results showed that there was a clear improvement regarding both the chemical composition and the amounts of ash in the case of leaching, while in the case of fractionation the results appeared to be encouraging regarding only the reduction of the ash amount contained in the material.
2. Experimental Greek peach stones (Prunus persica), were used as feedstock material during the gasification tests. Peach stones consist of kernels and pulp and they are produced as by-product from the production of stewed fruits and/or natural juices in agroindustrial factories. The material was supplied by a local juice company situated at the area of Argos,
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Peloponisos, Greece in 1998. The peach stone material originated from the northern part of Greece and the harvest time was the Summer of 1997. The peach stone material was first crushed using a laboratory hammer mill and a 4 mm screen. Two different pre-treatment techniques, fractionation and leaching, were studied in order to investigate their effect on the ash behavior of the specific residue during the gasification process. The leached peach stone sample used in the gasification tests was pre-treated with tap water in order to extract a part of its inorganic constituents, mainly alkali metals and chlorine, which are found in large quantities in its ash and are thought to cause ash-related problems during biomass gasification. In specific, leaching comprises the immersion of the treated material in tap water using specific water/mass ratios for a specific period of time in order to achieve the extraction of water-soluble inorganic elements (K,Na,Cl) of its ash. The water-to-mass ratio applied to the leached peach stone sample used in the gasification tests was 15 ml/g, while the leaching period was 8 h and the temperature 20 1C. The water used in the leaching process was obtained from a local spring and the main elements contained in it were calcium (155 mg/l), magnesium (53 mg/l), sodium (63 mg/l) and potassium (14 mg/l). At the end of the leaching process, the pH of the leachate was acidic, taking values between 6 and 6.5 approximately. On the other hand, fractionation divides the peach stone material into two different fractions, one with average particle size above 1 mm, which counts for almost 90% of the whole material and was selected for the gasification tests, and the other with average particle size below 1 mm that counts for the remaining 10% of the material. The untreated peach stone material was encoded as P1, while the fractionated and leached samples as PF1 and PL1, respectively, for further use in the paper. A detailed description regarding the leaching and fractionation pre-treatment of the peach stone samples is given by Arvelakis et al. [10]. Additional information regarding leaching pre-treatment technique and its effect on the ash behavior of the treated samples can be found elsewhere [11,12]. A lab-scale gasifier depicted in Fig. 1 situated at the CUTEC-GmbH institute in Germany was used
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Fig. 1. Lab-scale gasifier.
in conducting the gasification tests. The fluidized bed reactor consists of the following parts: fluidized bed, free board, hot gas cyclone, postcombustion chamber system and flue gas purification. A conveyor screw was used to feed the raw material to the reactor about 300 mm above the distribution plate of the fluidized bed. A maximum mass flow of about 20 kg/h can be attained, while the maximum fluidized bed height at steady height mode is about 300 mm. The heating of the fluidized bed is achieved with a controllable natural gas burner situated at about the same height as the input supply pipe connection. The bed inert material used in all tests was silica sand with particle diameter varying from 0.4 to 1.4 mm and a mean particle diameter of 0.82 mm. Sand particles had a theoretical specific surface of 29 cm2/g, in combination with an apparent density of 2.65 g/cm3 and a true density of 1.5 t/m3. Approximately 15 kg of silica sand was used as bed inert material during each gasification test. The gasification agent used in all tests was a mixture of air and nitrogen in order to have the appropriate equivalence ratio for the gasification of the specific bio-fuel, while all tests were conducted under atmospheric pressure. In order to study the deposition phenomena during the gasification of the various peach stone samples, a water-cooled steel tube with dimensions 30 2 cm was put vertically approximately 30 cm above the bed of the gasifier. The temperature of the tube surface was kept at the level of 500 1C. Samples of the produced fly ash material during the
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gasification process were deposited at the watercooled steel tube, which played the role of a heat transfer surface. The produced deposit was removed after the end of each test from the tube surface using a brush. The material was collected and examined using a JEOL 6300 scanning electron microscope in order to have a clear view regarding the effect of the different pre-treatment methods on the morphology and composition of the ash particles formed in these deposits. The examination was focused on analyzing the surface of the ash particles using the EDS microprobe of the microscope in order to calculate the concentrations of the main inorganic elements forming the surface of the ash particle, while photos of the analyzed surface areas helped in better understanding and evaluation of the results. Several analyses (15–20) were performed for each ‘‘class’’ of particles covering different magnifications and
Table 1 Analysis and characterization of the peach stone samples P1
PF1
PL1
8.53 0.65 81.3 18.1
13.28 0.38 80.54 19.08
11.98 0.43 81.61 17.96
0.79 51.95 5.76 o0.01 0.14 40.7 21.6
0.89 52.66 6.1 — 0.1 39.87 22.02
0.95 53.45 6.23 — 0.00 38.94 22.8
Proximate analysis (%d. basis) Moisture Ash Volatiles Fixed carbon Ultimate analysis (%d. basis) Nitrogen Carbon Hydrogen Sulfur Chlorine Oxygen GCV (MJ/kg)
surface areas in order to be sure for the repeatability of the produced results.
3. Results and discussion 3.1. Analysis and characterization of the raw materials Table 1 presents the effect of fractionation and leaching pre-treatment techniques on various properties of the different peach stone samples used in the gasification tests. As it is seen from Table 1, leaching has a positive effect on the main properties of the peach stone samples such as ash content, volatiles and gross calorific value, even though the effect is marginal in most cases, with the exception of ash content that appears to be reduced by approximately 40% w.b. compared with the initial P1 peach stone sample. Fractionation shows also to have the same effect and appears to improve significantly the ash content of the PF1 sample that is reduced by almost 50% w.b. compared to the untreated P1 peach stone sample. Ash elemental analysis of the peach stone samples depicted in Table 2 shows that the ash of the initial peach stone sample P1 appears to be highly reactive, containing high amounts of potassium that counts for almost 39% w.b. of the ash, while the amounts of the other elements appear to be rather low with concentrations being below 10% w.b. Furthermore, the ash analysis of the PF1 sample shows that fractionation leads to a deterioration of the ash composition of the specific peach stone sample. The amount of potassium in the ash increases significantly by almost 20% w.b.,
Table 2 Ash elemental analysis of the peach stone samples %w.b. Ash basis (%) P1 PF1 Difference (%) PL1 Difference (%)
K2O 38.45 45.78 19.07 21.24 44.74
Na2O 0.07 0.07 0 0.02 76
CaO 7.14 6.13 14.11 19.32 170.58
MgO 2.73 1.4 48.78 2.53 7.25
SiO2 5.97 5.03 15.92 10.47 75.15
Al2O3 0.47 0.03 93.17 0.02 95.98
Fe2O3 0.52 0.63 19.83 0.086 83.69
TiO2 0.15 0.08 51.61 0.04 72.04
SO3 5.24 5.09 2.88 3.16 39.71
Cl 0.06 0.03 50.81 0.02 60.65
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while the amount of chlorine appears to be reduced now by almost 50% w.b., and the amounts of sodium and sulfur remain almost constant. On the other hand, the amounts of alkali metals and chlorine contained in the ash of the leached sample PL1 are reduced by 44–76% w.b. regarding potassium and sodium content, and by 60% w.b. regarding chlorine. At the same time, the amounts of sulfur, iron and titanium in the ash of the PL1 sample are also reduced by almost 40%, 84% and 70% w.b., respectively, while the amounts of calcium, and silica are increased significantly. According to these data, the ash behavior of the PL1 material during the gasification tests is expected to improve, though the amounts of potassium, chlorine and sulfur in its ash fraction remain substantial. 3.2. Gasification tests with the P1 sample Two gasification tests were performed using the material P1. Each test lasted for approximately 90 min with a feeding rate of 10.0 kg/h. The gasifier was first heated up to 830 1C using the gas burner until good fluidization conditions were established. Then the gas burner was shut off and the feeding of the peach stones material in the gasifier was started. The reactor was observed to operate at the temperature range of 850–860 1C without problems at an equivalence ratio of 0.5 for about 90 min. After the end of the test the water-cooled tube that is placed vertically above the bed was removed and inspected for possible signs of deposition. As it is seen in Fig. 2a, the tube appears to be covered with a thick layer of ash particles mixed together with tars and unburned carbon. The deposition appears to take place both on the upper and lower parts, with the majority of the ash being captured on the upper part of the tube. The strength of the deposit is observed to be higher on the lower part of the tube and it is increased by the presence of the tars among the ash particles that act like glue. Ash particles appear to have mainly green color and to a lower extent white. The deposit is removed from the tube and is analyzed using SEM-EDX. Fig. 3a depicts the morphology of the ash particles that appear to be rough and irregular. As it is seen from Table 3, the
Fig. 2. Ash deposits on the water-cooled steel tube during the gasification of peach stones samples: (a) P1, (b) PF1, (c) PL1.
ash particles contain mainly potassium up to 40% w.b., while phosphorus, calcium and iron appear to be present in concentrations varying from 10% to 15% w.b. The concentration of iron appears to be higher in some cases, reaching 60–70% w.b., but this is attributed to the presence of material from the steel tube in the ash sample. The specific elemental analysis supports that the ash particles are composed of a silicate matrix with potassium, calcium and phosphorus as the main elements participating in the silicate structure. A part of these elements may have condensed later on the surface of the silicate particles in the form of salts and further reacted with it, forming new more eutectic compounds. The chlorine is observed to be absent from this mixture, apart from certain spots where it appears in very low concentrations below
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S. Arvelakis et al. / Biomass and Bioenergy 28 (2005) 331–338 Table 3 SEM–EDX analysis of various ash particles during the gasification tests with the peach stone samples %w.b. Samples
P1a
PF1b
PL1c
K2O Na2O CaO MgO SiO2 Al2O3 Fe2O3 P2O5 SO3 Cl
43.06 0.00 16.16 0.30 5.16 0.00 14.82 15.39 5.04 0.00
55.62 0.62 9.66 0.00 16.46 0.00 4.49 10.22 0.00 2.24
12.95 0.00 34.66 0.00 33.66 2.42 14.14 0.00 1.55 0.00
a
Gasification conditions: Temperature 850–860 1C, ER=0.5, Test duration=90 min. b Gasification conditions: Temperature 850–860 1C, ER=0.5, Test duration=85 min. c Gasification conditions: Temperature 870 1C, ER=0.5, Test duration=120 min.
mainly, in the gas phase, where they react further, substituting chlorine that remains in the gas phase. 3.3. Gasification tests with the PF1 sample
Fig. 3. SEM–EDX photos of ash particles during the gasification of various peach stone samples: (a) P1, (b) PF1, (c) PL1.
2% w.b., accompanied with potassium, and this is attributed to its main role as an intermediate during the transport of alkali metals, potassium
Two tests were performed using the material peach stone PF1 sample. The tests were performed under the same conditions used in the previous tests with the untreated P1 peach stone sample. The amount of material used in each of these tests was 14 kg and the operation time 85 min approximately. The gasifier appears to operate again under steady temperature conditions in the range 860–8701C, while no agglomeration problems were observed. After the end of each test, inspection of the water-cooled tube revealed the formation of an ash deposit layer that appears to be thicker and stronger compared with the former one from the tests with the P1 sample, as it is seen in Fig. 2b. The SEM–EDX analysis of the ash particles from the specific deposits revealed that now they appear to be more compact having smaller voids in their structure, while secondary deposition of smaller fly ash particles on the surface of larger particles is observed, as it is seen in Fig. 3b. The deposited ash particles appear to contain mainly potassium in concentrations above 55%
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w.b. and lower amounts of silicon, calcium and phosphorus in the range of 10–15% w.b. according to Table 3. These results are in good agreement with the ash elemental analysis of the specific sample where potassium appears to be the dominant element. The comparison of the produced results to the results from the tests with the P1 sample clearly show a significant deterioration of the ash particle chemistry. The composition of the ash particles is seen to be more reactive now favoring melting interactions among them in a larger extent compared to the former case. 3.4. Gasification tests with the PL1 sample Finally, the effect of leaching pre-treatment technique on the ash behavior of the peach stones material was studied through two gasification tests using the PL1 sample. The experiments were performed at the temperature of 870 1C, which is slightly higher than the operating temperature of the gasifier in the previous tests. In this case, the amount of the PL1 sample used in each of the tests was 19.5 kg and each test lasted for almost 2 h with a feeding rate of 10 kg/h. As in the cases with the previous peach stone samples, the gasification tests ended without any visual problem regarding agglomeration or defluidization of the bed. However, the removal of the water-cooled tube from the gasifier revealed that the formed ash deposit was in this case comparable in size with the deposits observed in the tests with the untreated material, even though the reactor operated for longer time and in higher temperature, as it is seen in Fig. 2c. The SEM–EDX analysis of the specific ash particles presented in Fig. 3c and Table 3 showed a significant change in the chemistry of the specific ash particles. The ash particles appear to have a more loose structure, while there are no signs of deposition of smaller ash particles on larger ones. Silicon and calcium, in concentrations that vary from 30% to 60% w.b., appear to be the main forming elements now, while potassium and iron are also present principally in concentrations below 10% w.b.. These results are in accordance with the results from the ash analysis of the PL1 sample, where it is seen that leaching has caused a
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significant extraction of alkali metals, chlorine and sulfur from the ash. The potassium content on the surface of the ash particles is low indicating that the melting tendency of the specific ash particles is anticipated to be low as it is also observed from the size and strength of the tube deposit.
4. Conclusions The results obtained from the gasification tests of peach stone samples showed that the ash behavior of the specific material could vary significantly depending on the pre-treatment technique applied. The ash from the untreated peach stone P1 sample is seen to be highly reactive, containing large amounts of alkali metals that could have a significant negative effect on the large-scale gasification process causing deposition, agglomeration and corrosion problems. The composition of the generated ash particles verified this observation. Fractionation pre-treatment technique shows to act positively regarding only the ash content of the treated material that is seen to be reduced significantly. At the same time, fractionation results in a deterioration of the elemental composition of the peach stones ash, increasing significantly the amount of potassium in the ash. The ash particles from the gasification tests with the PF1 sample had higher potassium concentrations compared to those from the tests with the P1 sample. In addition, signs of secondary deposition of small ash particles in the surface of bigger ones, and a significant increase in both the size and the strength of the generated deposit, were also observed. The chemistry of the secondary deposited particles is seen to be similar to the chemistry of the larger particles. On the contrary, the leaching pre-treatment technique showed a positive effect as far as it concerns the ash thermal behavior of the peach stones material. Leaching resulted, according to Table 2, in a significant removal of alkali metals and chlorine from the ash of the sample PL1. The ash particles generated from the tests with the PL1 peach stone sample were seen to have silicon and
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calcium as their main forming elements, while both the size of the created deposit and its strength were reduced significantly. These results are in good agreement with the results obtained from other research trials performed using various leached biomass materials worldwide [11,12,14–16], and clearly demonstrate the ability of the specific pretreatment to improve the ash behavior of a variety of different biomass materials. The low ash content of this particular material, less than 1% w.b. of the total mass as it is seen in Table 1, makes necessary the performance of very long test runs (424 h) in order to be able to study with accuracy the effect of the peach stone ash on the bed material agglomeration tendency.
Acknowledgements The research work was financed by the European Commission under the COPES research program, and thus its contribution is gratefully acknowledged by the authors.
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