Energy Fuels 2010, 24, 1980–1986 Published on Web 02/10/2010
: DOI:10.1021/ef9014982
Behavior of Alkali Metals As the Carbonate Compounds in the Biomass Char Obtained As a Byproduct of Gasification with Steam and Oxygen at 900-1000 °C Keigo Matsumoto,*,† Keiji Takeno,† Toshimitsu Ichinose,† Tomoko Ogi,‡ and Masakazu Nakanishi‡ †
Nagasaki Research and Developement Center, Mitsubishi Heavy Industries, Ltd. 5-717-1, Fukahori-Machi, Nagasaki, 851-0392, Japan, and ‡Biomass Technology Research Center, National Institute of Advanced Industrial Science and Technology. 16-1, Onogawa, Tsukuba-shi, Ibaraki, 305-8569, Japan Received August 20, 2009. Revised Manuscript Received January 7, 2010
When gasifying biomass in an entrained-flow type gasifier with steam and oxygen at 900-1000 °C, cold gas efficiency was high, and consequently, char, gasification byproduct, contained relatively large amounts of carbonated alkali and alkali earth metals. An elemental analysis of this sort of char resulted in substantial error, especially oxygen content, when applying a method developed for analyzing coal, because the carbonated alkali and alkali earth metals were oxidized through heating in air or ashing pretreatment at 815 °C. To clarify effects of the pretreatment, 23 kinds of biomass were ashed at 600 °C and 3 of them were ashed at temperature ranging from 600 to 900 °C. Various kinds of metallic compounds remained after ashing at 600 °C, however almost all of them were oxidized or recomposed to be more stable at above 800 °C. Potassium compounds vaporized more as ashing temperature rose. The temperature of the melting point dropped, especially under reducing atmosphere, as ashing temperature dropped and the potassium compounds increased.
through gasification, were measured by using a drop tube furnace (DTF),3 and the char recycling system designed based on its kinetics was developed and operated successfully.4 Results of the DTF measurements showed that both alkali metal and alkali earth metal accelerated the gasification reactions.3 In designing and operating a char recycling system, it is important to clarify elements contained in the char, on which its gasification kinetics is dependent. Ash contents and chemical properties of activated carbon, produced from biomass through pyrolysis in nitrogen or steam, were reported.5-7 According to these reports, carbonated alkali metals remained after pyrolyzed below 750 °C and enhanced adsorption ability and surface area of the activated carbon. In cases of gasifying biomass or biomass char, alkali and alkali earth metals accelerated gasification reaction rate, both when originated in the biomass feedstock3,8 and when added to the reaction.9,10 Alkali and alkali earth metals in feedstock biomass vaporized, when combusted above 1100 °C.11,12
1. Introduction Renewable sources of energy have been steadily introduced in many countries and the concern with biomass as a renewable source has been rather growing. To develop and establish advanced energy conversion technologies from nonedible biomass is urgently required in order to avoid socio-economic side effects such as the foods price rise. Among the promising technologies, gasification and liquid fuel synthesis through gasification have an advantage of rapid conversion from a large amount and various kinds of biomass to easily storable and transportable gas or liquid fuel. We developed a 2 ton/day-scale test plant under the auspices of a national project organized by the New Energy and Industrial Technology Development Organization (NEDO),1 which was a progress of a 240 kg/day-scale pilot plant developed under previous national project organized by the Ministry of Agriculture, Forestry and Fisheries of Japan (MAFF).2 We tested gasification performances of these plants by gasifying various kinds of woody and herbaceous biomass with steam and oxygen at 900-1000 °C. Although cold gas efficiency and methanol synthesis yield of the test plant were quite high (>65% and >20 wt %, respectively) and estimated performances of a commercial plant are considerably higher (cold gas efficiency >75% and methanol synthesis yield >40 wt %), both would be improved of approximately 5% by gasifying and recycling char, which is a byproduct of the gasification. Gasification kinetics of the char, obtained
(3) Matsumoto, K.; Takeno, K.; Ichinose, T.; Ogi, T.; Nakanishi, M. Fuel 2009, 88, 519–527. (4) Matsumoto, K.; Takeno, K.; Ichinose, T.; Nakanishi, M.; Ogi, T. Proc. 16th EBCE 2008, 877–882. (5) Guerrero, M.; Ruiz, M. P.; Millera, A. M.; Alzueta, U.; Bilbao, R. Energy Fuels 2008, 22, 1275–1284. (6) Savova, D.; Apak, E.; Ekinci, E.; Yadim, F.; Petrov, N.; Budinova, T.; Razvigorova, M.; Minkova, A. Biomass Bioenerg. 2001, 21, 133–142. (7) Arjmand, C.; Kaghazchi, T; Latifi., S.; Soleimani, M. Chem. Eng. Technol. 2006, 29, 986–991. (8) Li, Z.; Capart, R.; Gelus, M. Biomass Energy Ind. 1990, 2, 760– 764. (9) Azargohar, R.; Dalai, A. K. Microporous Mesoporous Mater. 2008, 110, 413–421. (10) Risnes, H.; Fjellerup, J.; Henriksen, U.; Moilanen, A.; Norby, P.; Papadakis, K.; Posselt, D.; Sorensen, L. Fuel 2003, 82, 641–651. (11) Dayton, D.; French, R.; Milne, T. Energy Fuels 1995, 9, 855–865. (12) Dayton, D.; Jenkins, B.; Turn, S.; Bakker, R.; Williams, R.; Belle-Oudry, D.; Hill, L. Energy Fuels 1999, 13, 860–870.
*To whom correspondence should be addressed. E-mail: keigo_
[email protected]. Telephone: þ81-95-834-2400. Fax: þ81-95834-2505. (1) Matsumoto, K.; Takeno, K; Ichinose, T.; Ishii, H.; Nishimura, K. Proc. 15th EBCE 2007, 1945–1950. (2) Nakagawa, H.; Harada, T.; Ichinose, T.; Takeno, K.; Matsumoto, S; Kobayashi. M; Sakai, M. Proc. of 3rd Symposium on Greenhouse Gases and Carbon Sequestration in Agriculture and Forestry 2005. r 2010 American Chemical Society
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pubs.acs.org/EF
Energy Fuels 2010, 24, 1980–1986
: DOI:10.1021/ef9014982
Matsumoto et al.
Both atmosphere and temperature of these reports were different from those of our plants, in which biomass were gasified with steam and oxygen at 900-1000 °C. Because of its high cold gas efficiency, relatively small amounts of carbon, hydrogen, and oxygen were contained in the char; consequently, relatively large amounts of alkali and alkali earth metals were contained. FT-IR analyses showed that these metals were carbonated. An elemental analysis of the char resulted in substantial error, especially in oxygen content, when applying a method developed for analyzing coal. According to the defined pretreatment of the elemental analysis, a sample under test is first heated in air or ashed at 815 °C to remove carbon, oxygen, hydrogen, and nitrogen, and the residue, consisting of oxidized metals, is defined as ash. Carbonated alkali and alkali earth metals are oxidized through this pretreatment; any oxidized metal is lighter than a carbonated one and this weight loss is counted as weight of oxygen. In order to clarify effects of the pretreatment, 23 kinds of biomass were ashed at 600 °C and 3 of them were ashed at temperatures ranging from 600 to 900 °C. The effects of the pretreatment became more clear when ashing biomass than ashing char because metals in biomass are a variety of compounds but those in char are mainly carbonated, and because a lot of potassium in the char already vaporized during gasification at 900-1000 °C. Additionally, it is necessary to distinguish carbonated compounds in the biomass char recombined with CO2 in the evolved gas from ones originally contained in the biomass. Various kinds of metallic compounds remained after ashing at 600 °C, but oxidized or recomposed to be more stable at above 800 °C. Potassium compounds vaporized more as the ashing temperature rose. Temperature of the melting point dropped, especially under reducing atmosphere, as the ashing temperature dropped and the potassium compounds increased.
Figure 1. Temperature profile on biomass ashing for chemical analysis. (a) 600 °C ashing, (b) 800 and 900 °C ashing according to JIS8815 except for the final temperature.
Functional groups of the chemical compounds in these char with and without acid treatment were qualitatively analyzed by using the FT-IR as explained in the subsection 2.2. 2.2. Experimental Apparatus. 2.2.1. Test Plant. The 2 ton/ day-scale test plant was used to gasify JC, JB, and MH.2 2.2.2. Electric Furnace for Ashing Pretreatment. An electric furnace, with an atmosphere of air, was used for the ashing pretreatment. 2.2.3. Fourier Transform Infrared Spectrometer. A Fourier transform infrared spectrometer (JASCO, FI/IR-4200) was used for FT-IR analysis. 2.2.4. X-ray Diffractometer. An X-ray Diffractometer (Rigaku, RINT-2500) was used for XRD analysis. 2.3. Analysis. 2.3.1. Ashing Pretreatment. By using the electric furnace described above, three kinds of biomass (JC, JB, or MH) were ashed at temperatures ranging from 600 to 900 °C. Temperature profile of the 600 °C pretreatment was controlled as shown in Figure.1a: a test sample was first dried at 200 °C, pyrolyzed at 300 °C, and completely ashed at 600 °C. Temperature profile of other pretreatments was controlled as shown in Figure.1b based on JIS-M8812. In the JIS-M8812, temperature of the ashing pretreatment is defined to be 815 °C, and the residue after the pretreatment, expected to be a mixture of oxidized metals, is defined as ash. Weight of the residue is defined as weight of ash, which is assumed to be equal to total weight of the metallic compounds in the test sample, because it is assumed in the JIS-M8812 that all metals in the sample are oxidized, not affected by the pretreatment and collected as their original compounds. Twenty other kinds of biomass were ashed at 600 °C, the temperature profile of which is shown in Figure 1a. Three kinds of biomass char (JC char, JB char, and MH char) were ashed at 815 °C, the temperature profile of which is shown in Figure 1b. 2.3.2. Elemental Analysis (Metal, Carbon, Hydrogen, Sulfur, Nitrogen, and Oxygen Contents Analysis). The ash, obtained after the pretreatment described above, is first dissolved into acid and each metal ion is quantitatively analyzed following methods defined in JIS-M8815. All metals in the ash are assumed to be oxidized, such as SiO2, Al2O3, Fe2O3, CaO, TiO2, MgO, SO3, P2O5, Na2O, K2O, and
2. Samples and Methods 2.1. Sample. 2.1.1. Biomass Sample. Japanese cedar (Cryptomeria Japonica) is the most widely available woody biomass in Japan, and it is usually divided into wood (JC) and bark (JB) in lumbermills. The mixture of hardwood (MH) consists of branches trimmed from trees lining streets, which is usually burnt as wastes. Hence, JC and JB were chosen as being typical of softwood and MH was chosen in comparison with Japanese cedar. These three kinds of biomass (JC, JB, and MH) were heated in air or ashed at temperatures ranging from 600 to 900 °C in an electric furnace. An elemental analysis, XRD analysis, and melting point analysis were applied to these ashes. Twenty other kinds of biomass including woody biomass, herbages, and agricultural wastes, were ashed at 600 °C. The elemental analysis was applied to these ashes. 2.1.2. Char Sample. Three kinds of biomass char, obtained as byproduct when gasifying JC, JB, and MH in the 2 ton/ day-scale test plant with steam and oxygen at 900-1000 °C, were chosen. A cyclone, installed at the exit of the reactor, was used to collect the char. The collected char dropped to a container, installed at the bottom of the cyclone, and cooled in it. The cooled char was then recovered every a couple hours after closing the valve located between the cyclone and the container and refilling nitrogen. Elements contained in these three kinds of char were quantitatively analyzed as explained in the subsection 2.3. 1981
Energy Fuels 2010, 24, 1980–1986
: DOI:10.1021/ef9014982
Matsumoto et al.
Table 1. Analyzed Ash Melting Temperature and Compositions for Test Samples (Dry Basis)a
JC JC JB JB MH MH MH a
ashing temp. (°C)
SiO2 (wt%)
Al2O3 (wt%)
Fe2O3 (wt%)
CaO (wt%)
TiO2 (wt%)
MgO (wt%)
SO3 (wt%)
P2O5 (wt%)
Na2O (wt%)
K2O (wt%)
MnO (wt%)
analysis residual (wt%)
600 800 600 800 600 800 900
4.0 10.3 31.7 24.2 1.2 2.2 2.9
0.9 3.3 8.0 7.6 0.8 1.1 1.2
2.9 3.6 3.5 3.9 0.4 0.5 0.6
36.0 46.4 25.2 33.0 36.1 46.8 51.2
0.3 0.3 0.4 0.4 0.1 0.1 0.1
5.5 7.0 3.6 4.4 5.7 7.9 8.8
3.5 4.7 3.2 5.6 5.0 6.8 9.7
3.2 4.1 2.4 3.6 2.1 2.9 3.2
1.8 1.7 2.0 2.9 0.6 1.1 1.3
13.4 6.8 7.0 8.6 17.7 9.0 8.2
0.2 0.2 1.6 0.4 1.6 2.2 2.5
28.4 11.6 11.4 5.4 28.7 19.4 10.3
Analysis residual was calculated by subtracting the sum of the analyzed values of ash compositions from total ash weight.
MgO, prior to this analysis. Weight of each metallic compound is defined as its oxidized one, for example, weight of K2O neither Kþ, KOH, K2CO3, nor K2SO4, although some oxidized metals, such as K2O and Na2O, are usually too active to exist stably in biomass feedstock because of its moisture content. Total weight of these oxidized metals is not equal to, usually lighter than, the weight of the ash defined in JIS-M8812, because some metals in the ash are carbonated, phosphated, or sulfated and because their weights are heavier than the oxidized compounds. This difference between the weight of the ash and the total weight of the oxidized metals, or the difference divided by the original weight of biomass, is defined as an analysis residual and is roughly proportional to weight of the carbonated, phosphated, and sulfated metals. Carbon and hydrogen contents in the test sample are quantitatively analyzed by the methods defined in JISM8819. Sulfur and nitrogen contents in the test sample are quantitatively analyzed by the methods defined in JISM8813. In JIS-M8813, oxygen content is defined as subtraction of the carbon, hydrogen, sulfur, nitrogen, and the ash contents from 100%, in which oxygen content in the oxidized metals is not included. 2.3.3. Acid Treatment. In the acid-treatment, char of about 1 g was steeped in 1N HCl solution of about 100 cm3 for about 3 h to separate CO2 existing as a carbonate from the char. Weights of the char and HCl were individually measured at first. Then, they were mixed in a beaker, the total weight of which was continuously measured by a balance during bubbling. We measured the final weight when the bubbling stopped. After that, this acid-treated char was washed with distilled water several times and then dried at 120 °C for 12 h. 2.3.4. Ash Melting Point Analysis. Ash melting points in oxidizing and reducing atmosphere were measured through procedures defined in DIN-51730 and ASTM1857, respectively.
amounts of potassium compounds remaining in the biomass char obtained after gasified at 900-1000 °C, although boiling points of many kinds of potassium compounds are lower than 900 °C. Potassium content, assumed as K2O, in the JC and MH ashes decreased with rising ashing temperature, due to vaporization. Increases of other contents were caused of condensation by vaporizing potassium compounds. However, the K2O content in the JB ash increased with rising ashing temperature. The SiO2 and the Al2O3 contents in the JB ash, which were much higher than those in the JC and MH ashes, decreased with rising ashing temperature, although they were stable and hardly decomposed or vaporized at temperatures ranging from 600 to 900 °C. Both SiO2 and Al2O3 were main compounds of soil and sand. We concluded that the SiO2 and the Al2O3 in the JB ash mainly originated from soil and sand, which adhered to the bark, because the bark was the outer side of the wood. When excluding the SiO2 and the Al2O3, recalculated K2O content in the JB 900 °C ash of 13.7% was lower than that in the JB 600 °C ash of 14.3%, the temperature dependence of which was the same as those of other ashes. As shown in the XRD analysis (Figure 2a), various kinds of metallic compounds were contained in the MH ash pretreated at 600 °C (MH 600 °C ash). Almost all of them were water-soluble salts supplied to and absorbed by the biomass as manure and fertilizer. These metallic compounds were oxidized or recomposed to become more stable, such as CaO, CaSO4, and apatite, at above 800 °C as shown in Figure 2b (MH 900 °C ash). Potassium must be included in the apatite by replacing some calcium. Ashing temperature of 600 °C is low and moderate enough to extract original metallic compounds, but 800 °C or higher is too high. Twenty-three kinds of biomass, including JC, JB, and MH, were ashed at 600 °C, the ashes of which were thought to be mixtures of original metallic compounds. Instead of the XRD analysis, metal contents of each ash were quantitatively analyzed because the elemental analysis was much easier and cheaper than the XRD analysis. Analysis residual, which is defined as the difference between the weight of the ash and the total weight of its metal contents, is roughly proportional to the weight of nonoxidized metals, such as carbonated, phosphated, and sulfated metals. As Figure 3a shows, the analysis residuals were roughly proportional to CaO contents, which suggested that almost all calcium in any biomass was carbonated, phosphated, or sulfated. As closed circles in Figure 3b indicate, the analysis residuals of woody biomass were roughly proportional to (Na2O þ K2O). Potassium and sodium in the woody biomass with small CaO content (CaO < 10%) were also carbonated, phosphated, or sulfated. The analysis residuals of herbages and agricultural wastes were almost independent of
3. Results and Discussion 3.1. Effects of the Ashing Pretreatment on Elements. In the elemental analysis, it is assumed that all metals in biomass or biomass char are oxidized, but not actually. It is very difficult, almost impossible, to identify the original metallic compounds, because they should be extracted from the sample for analyzing but are affected through the extracting treatment. To evaluate the effects of the ashing pretreatment, the three kinds of biomass (JC, JB, and MH) were ashed at temperatures ranging from 600 to 900 °C. Metals in these ashes were quantitatively analyzed and are listed in Table 1, in which the weight of each metal is expressed as its oxidized value. Small but certain amounts of potassium compounds remained after ashing at 900 °C, which supported some 1982
Energy Fuels 2010, 24, 1980–1986
: DOI:10.1021/ef9014982
Matsumoto et al.
Figure 2. XRD image of MH ash made at 600 and 900 °C.
(Na2O þ K2O), as open circles in Figure 3b indicate. Herbages and agricultural wastes inherently contain large amounts of calcium and silica, but silica in woody biomass mainly originates in soil adhering to their outside, especially to bark and root. Biomass with large CaO content (CaO > 10%), the inherent SiO2 content of which is also large, stably contained K2O and Na2O because oxidized potassium or sodium with oxidized silica (e.g., K2O-nSiO2) is stable as reported.13 3.2.2. Effect of Ashing Procedure to Ash Melting Points Analysis. Figure 4 shows ash melting points of JC, JB, and MH ashes, measured using both DIN and ASTM methods, the atmospheres of which were respectively oxidizing and reducing. In the DIN method, deformation temperature (EP), hemisphere temperature (SP), and flow temperature (FP) were defined as the ash melting proceeded. In the ASTM method, initial deformation temperature (IDT), softening temperature (ST), hemisphere temperature (HT), and flow temperature (FT) were also defined. The EP corresponded to the IDT, at which the edge became rounded; the SP to the HT, at which the sample form turned to hemispherical; and the FP to the FT, at which the sample fluidized. The temperature differences between these corresponding temperature pairs (EP, IDT), (SP, HT), and (FP, FT), were mainly caused by the difference in atmosphere (oxidizing or reducing). Although the initial shape of the sample was different, such as cubic in the DIN method and pyramidal in the ASTM method, the effect of this was negligible. The effect of the atmosphere became clear, especially in the JC 600 °C ash and the MH 600 °C ash, in which large amounts of
carbonated metals (about 30%) were contained. These carbonated metals were oxidized at relatively low temperature in the oxidizing atmosphere but were preserved below their decomposed temperature of 800-900 °C in the reducing atmosphere. The carbonated metals lowered the ash melting points, with the edges of the JC 600 °C ash melting at 815 °C and those of the MH 600 °C ash at 780 °C. Decomposition of these carbonated metals and vaporization of potassium compounds progressed as the temperature rose. HT and FT measured in a reducing atmosphere were higher that 1400 °C, nearly equivalent to the corresponding temperatures of SP and FT measured in oxidizing atmosphere. Nearly all the ashes oxidized at 800 °C (800 °C ashes) showed higher melting points than those at 600 °C (600 °C ashes) by a margin of approximately 50 °C, attributable to lower K2O content in the 800 °C ashes than in the 600 °C ashes, because potassium vaporized to a greater extent as the ashing temperature rose. We concluded that the 600 °C ashing pretreatment was more suitable for analyzing the correct melting points of biomass ash, since biomass usually contains high potassium as indicated in Table 1. All the char made from JC, JB, and MH were powdered and not melted or agglomerated, although the gasification temperature of 900-1000 °C was higher than IDT of the JC ash and than IDT and ST of the MH ash (Figure 5), all of which were measured in the reducing temperature. As previously reported,14 oxidization proceeds at first and then steam reform proceeds in an entrained-flow type gasifier with steam and oxygen, because biomass oxidization is a much quicker reaction than steam reforming. Because the atmosphere, in which char is produced, is oxidizing, the
(13) Lin, W.; Krusholm, G.; Dam-Johansen, K.; Musahl, E.; Bank, L. Proc. 14th Int. Conf. of FBC 1997, 831.
(14) Nakanishi M.; Ogi T.; Inoue S.; Kawamura A. Proc. ICCS&T 2005, 2005, CD-ROM.
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Energy Fuels 2010, 24, 1980–1986
: DOI:10.1021/ef9014982
Matsumoto et al.
Figure 3. Relationship between ash compositions and the analysis residual (subtracting the sum of the analyzed values of ash compositions from total ash weight) on 23 kinds of biomass ash made at 600 °C, including JC, JB, and MH (above: CaO, below: Na2O þ K2O).
char melting points have to be measured in the oxidizing atmosphere. Some entrained-flow type gasifiers use only steam as gasification agent, in which biomass combustors or furnaces surrounding the reactors generate heat and temperature for gasification. The atmosphere of this type of gasifier is reducing; therefore, melting points of char with large potassium content is lower than the gasification temperature of 900-1000 °C and the char is possibly melted or agglomerated and hardly extracted from the reactor in some cases. 3.2.3. Consideration of Oxygen Remaining in Char by Analyzing Ash Characteristics. To identify functional groups, we analyzed three kinds of biomass char and the acid treated ones using the FT-IR. The upper curve in Figure 6 indicates the FT-IR profile of the JC char. Like profiles of other kinds of char, it contained carbonate (1451 and 875 cm-1) and carbonyl (about 1600 cm-1) compounds. A broad peak of hydroxide compounds (3433 cm-1) was attributable to the moisture in the char. During the acid treatment, the JC char bubbled. After the acid treatment, the carbonate peaks in the FI-IR profile, the lower curve in Figure 6, disappeared and the carbonyl peak nearly disappeared, which confirmed that the alkali and alkali earth metals in the char were carbonated. A portion of the metals, mainly potassium and sodium, could dissolve,
Figure 4. Relationship between ashing temperature and ash melting points (DIN method in oxidizing atmosphere and ASTM method in reducing atmosphere).
corresponding to the well-known propensity for dissolution of metallic compounds through mild acid treatment.15 Through the acid treatment, the total weight of the JC char decreased by 19.6%, which was slightly more than the measured “oxygen content” of 10.5% or 15.7%.3 This suggested that CO2 was produced, although the bubble was not analyzed. The char, directly exposed to the gas evolved when cooling as explained above, reacted with CO2, which was one of the four main gas components. The specific surface areas of JC char, JB char, and MH char were, respectively, 184, 379, and 204 m2/g.3 Because of these porous structures, alkali and alkali earth metals existed at the surface of the carbon-rich (15) Bardet, M.; Hediger, S.; Gerbaud, G.; Gambarelli, S.; Jacquot, J. F.; Foray, M. F.; Gadelle, A. Fuel 2007, 86, 1966–1976.
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Energy Fuels 2010, 24, 1980–1986
: DOI:10.1021/ef9014982
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Figure 5. Pictures of measuring ash melting temperature of MH ash made at 600 °C (above: reducing atmosphere; below: oxidizing atmosphere).
Figure 6. FT-IR result of JC char captured by cyclone in the 2 ton/day-scale biomass gasification test plant. The difference of functional groups in the biomass char before and after the acid treatment.
char, although some metallic compounds were contained in aromatic structures formed from the wood nanocomposite structures that decompose when pyrolyzed16 or gasified. The metals, existing at the surface of char, reacted well with CO2 (e.g., CaO þ CO2 f CaCO3) when cooling because the carbonation reactions proceeded at moderate temperature. When these carbonated alkali and alkali earth metals were ashed at 815 °C, their weights were reduced (e.g., CaCO3 f CaO þ CO2). These weight changes or weight losses were added to the calculated oxygen content. The error of oxygen content was very large when analyzing biomass char obtained as a byproduct of entrained-flow gasification because of high cold gas efficiency and its relatively high contents of the carbonated alkali and alkali earth metals. As previously reported,3 we measured progress of gasification and changes of contents in these three kinds biomass char in H2O or CO2 atmosphere and their temperature dependences, using the DTF, in order to investigate their gasification kinetics. As the gasification reaction progressed, both carbon and hydrogen contents decreased as expected. However, it is curious that the oxygen content remained nearly constant, about 10-20 wt % and regardless of the kind of biomass, although it was reported that small amounts of oxygen remain in the char after H2O gasification5,6 and it was untenable that oxygen of more than 10 wt % remained in the char after CO2 or H2O gasification at such high temperature. This large oxygen content in the char was attributable to the error of the elemental analysis caused from the oxidization
of carbonated metals through the ashing pretreatment as discussed above. In our previous report of the DTF experiments,3 the measured gasification reaction rates both in H2O and in CO2 atmosphere (H2O gasification and CO2 gasification) linearly depended on 1/Tg (DTF internal temperature), known as the Arrhenius expression, and the H2O gasification showed higher temperature dependence than the CO2 gasification as shown in Figure 8 of ref 3. This higher temperature dependence of the H2O gasification was supported by a previous report, which reported that the effect of carbonated alkali and alkali earth metals were accelerated in H2O atmosphere.8 4. Conclusions Relatively large amounts of metallic compounds were contained in the biomass char, obtained as byproduct of entrained flow gasification with steam and oxygen, because of its high cold gas efficiency. The char was porous and its surface area was large, corresponding to the larger surface area of char after H2O gasification than N2 gasification9,17,18 or CO2 gasification.19 Alkali and alkali earth metals, existing at the porous surface, reacted with CO2 during cooling, when cooled in the gas evolved, primarily H2, CO, CO2, and CH4, because these carbonated alkali and alkali earth metals were (17) Klose, W.; Woelki, M. Fuel 2005, 84, 885–892. (18) Mitomo, A.; Sato, T.; Kobayashi, N.; Hatano, S.; Itaya, Y.; Mori, S. J. Chem. Eng. Jpn. 2003, 36, 1050–1056. (19) Pastor-Villegas, J.; Valenzuela-Calahorro, C.; Gomez-Serrano, V. Biomass Bioenerg. 1994, 6, 453–460.
(16) Paris, O.; Zollfrank, C.; Zickler, G. A. Carbon 2005, 43, 53–66.
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Energy Fuels 2010, 24, 1980–1986
: DOI:10.1021/ef9014982
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temperature rose. The 600 °C ashing pretreatment was more suitable for elemental analysis, especially for biomass and biomass char, due to the facts that potassium compounds hardly vaporized and that various kinds of salts of original forms remained in the ash, with the ash properties being strongly dependent on both these factors. Both potassium carbonate and potassium sulfate lowered the ash melting points. As we reported previously,3 H2O gasification of biomass char in the DTF showed higher temperature dependence than CO2 gasification, which was supported by the previous report about effects of the carbonated alkali and alkali earth metals on accelerating gasification.8
composed or decomposed at moderate temperature, much lower than the gasification temperature of 900-1000 °C. In designing and operating a char recycling system, it is important to clarify elements contained in the char because the served char containing carbonated alkali and alkali earth metals accelerates biomass gasification,20 due to the accelerative effects of CaCO3 and K2Ca(CO3)2.3,8 We concluded that an elemental analysis of the char resulted substantial error, especially oxygen content, when applying the method developed for analyzing coal, from the elemental and FT-IR analyses of the 3 kinds of biomass char and from properties of 23 kinds of biomass ashes. The carbonated alkali and alkali earth metals were oxidized during the defined ashing pretreatment at 815 °C. In addition, the potassium compounds of low boiling points, such as carbonated and sulfated potassium, vaporized at above 800 °C and the potassium compounds decreased as the ashing
Acknowledgment. The authors would like to express their sincere thanks to NEDO (the New Energy and Industrial Technology Development Organization) and the Ministry of Agriculture, Forestry and Fisheries of Japan (MAFF), who organized and fiscally supported this study.
(20) Ogi, T.; Nakanishi, M. Japanese patent No. 4085160, 2008.
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