Article pubs.acs.org/EF
Sequential Pyrolysis and Potassium-Catalyzed Steam−Oxygen Gasification of Woody Biomass in a Continuous Two-Stage Reactor Tomoyuki Oike,† Shinji Kudo,† Hua Yang,† Junya Tahara,† Hyun-Seok Kim,‡ Ryo Koto,† Koyo Norinaga,† and Jun-ichiro Hayashi*,†,‡ †
Institute for Materials Chemistry and Engineering and ‡Research and Education Center of Carbon Resources, Kyushu University, 6-1, Kasuga Koen, Kasuga 816-8580, Japan S Supporting Information *
ABSTRACT: A recently proposed type of gasification (Type IV) has a potential to produce syngas from biomass or lignite at chemical energy recovery over 96% on an LHV basis with H2O/C and O2/C molar ratios as small as 0.5 and 0.2, respectively. This type of gasification converts biomass or lignite in a sequence of pyrolysis, gas-phase partial oxidation of volatiles, and simultaneous steam gasification of char and steam reforming of the volatiles under catalysis of potassium. The present authors experimentally simulated a Type IV gasification of woody biomass loaded with 1.5 wt % K in a laboratory-scale continuous reactor system. The pyrolysis at 550 °C in a screw conveyer reactor and the subsequent steam gasification/reforming at 700−720 °C in the bed of K-loaded char demonstrated steady-state carbon conversion over 99%, residual tar concentration in the syngas well below 100 mg/m3N-dry, and CH4 concentration below 3 vol %-dry simultaneously. The potassium retained by the char catalyzed the steam reforming/gasification, consuming externally added steam and in situ formed one from the pyrolysis and partial oxidation at consumption more than 0.7 mol H2O/mol C. The composition of syngas reached near-equilibrium among H2, steam, CO, and CO2 at 700−720 °C. This was caused by fast reactions within the bed of the K-loaded char; steam reforming/gasification forming CO2/H2 and reverse water−gas shift reaction.
1. INTRODUCTION Gasification of carbon resources is a key technology in future industrial systems, because of its particular function; that is, integration of different types of chemical energy and also heat into the chemical energy of syngas that plays a role of platform substance common between energy and chemical industries. The present authors1 recently reviewed previous studies on the gasification of low-rank carbon resources such as biomass and lignite, classifying it into five different types (Type I to Type V) considering operating temperature, consumption of O2 (or air) and steam, type of feedstock, reactor configuration, and chemical recuperation of heat. These are factors crucial to the recovery of chemical energy (CER is defined as the ratio of chemical energy of syngas to that of dry feedstock) and tar emission. Among the different types, Type IV is a newly proposed one by the present authors, and it has a potential to achieve CER over 95% with O2 consumption and steam/ carbon ratio (S/C), as small as 0.2 mol O2/mol C and 0.5 mol H2O/mol C, respectively. However, its performance has not been demonstrated experimentally. Figure 1 schematically shows a Type IV gasifier that consists of pyrolyzer and gasifier in series. The feedstock is fed into the pyrolyzer together with steam at S/C of ∼0.5, and pyrolyzed with a peak temperature of 550−600 °C. The pyrolysates, i.e., char and volatiles, are introduced into the gasifier. The char particles fall onto the preformed fixed or moving bed while the volatiles undergo partial oxidation with O2 or air in the headspace forming hot gas and then enter the bed. A particular feature of the char bed is catalysis of potassium. The potassium retained by the char catalyzes steam reforming of the volatiles and gasification of the char simultaneously. The char is thus a © XXXX American Chemical Society
Figure 1. Schematic diagram of a Type IV gasifier.
solid that is to be gasified and is also the support of the potassium catalyst.1,2 A highly mobile and volatile nature of potassium maintains its dispersion on the char surface and also allows interparticle transfer, respectively. The potassium can be recycled by continuous or intermittent discharge of the potassium-loaded char and return to the gasifier (the top of char bed) or to the pyrolyzer. It is necessary to make up a greater or lesser amount of potassium due to irreversible deactivation of the potassium by its reaction with ash Received: July 7, 2014 Revised: August 10, 2014
A
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components, such as silica.3,4 A reasonable way is to feed potassium together with the feedstock as well as to employ a low-ash feedstock. Potassium can easily be loaded to the feedstock by spraying aqueous solution of potassium salt (e.g., K2CO3).2 Some recent studies have demonstrated the effectiveness of K2CO3 as an excellent precursor of the potassium catalyst for steam or CO2 gasification of chars from ash-free coals.5−9 According to the authors’ calculation,1 the endothermic steam reforming and gasification within the char bed, if the char/volatiles are fully converted to syngas, cools the hot gas to temperatures of ∼700 °C (Tref) at the exit of the gasifier. The syngas is further cooled to temperatures even below 300 °C while transferring its sensible heat to the pyrolyzer in which endothermic reactions take place over a wide temperature range from ca. 150 °C to 550−600 °C, and finally reaches the exit of the reactor system at temperature Texit. A portion of heat of the syngas is thus recuperated into the chemical energy of the pyrolysates. Such a low exit temperature of the syngas enables high CER, as mentioned above. The heat of the syngas at Texit is not enough to generate steam, but an exothermic process at downstream such as power generation, water−gas shift, methanation, liquid synthesis, can supply sufficient quantity of heat for the steam generation. Figure 2 shows a result of numerical process simulation of Type IV gasification.
The simulation predicts CER values of >96% if the biomass is completely converted to syngas with S/C ≈ 0.5 and Tref < 740 °C. The present authors investigated Type IV gasification of woody biomass with a laboratory-scale continuous reactor system. This paper reports the experimental results and discusses mainly on the variation in the performance of the gasification in terms of conversion of the biomass, tar emission, and gas composition. The catalytic mechanism of the steam reforming and gasification is also discussed.
2. EXPERIMENTAL SECTION 2.1. Preparation of Biomass and Char Samples. Chipped Japanese cedar with rectangular shapes and sizes of ∼10 mm × 10 mm × 2 mm was used as the original biomass sample. The contents of carbon, hydrogen, oxygen, nitrogen, and ash were 48.9, 6.0, 45.0 (by difference), 0.1, and 0.05 wt % on a dry basis, respectively. The heating value of the cedar was 18.25 MJ-HHV/kg. The cedar was dried in air at 110 °C for 24 h prior to use, and then loaded with a catalyst precursor, potassium carbonate (K2CO3, a reagent grade). Details of the loading were reported elsewhere.2 In brief, an aqueous solution of K2CO3 was sprayed onto the predried cedar chips that were agitated in a bucket until the potassium content reached 1.54 wt %-dry-cedar. The K2CO3-loaded chips were then dried at ambient temperature under reduced pressure to adjust the moisture content. The mass ratio of moisture to the dry cedar was within a range from 0.34 g/g to 0.74 g/g. The use of moistened and K2CO3-loaded cedar samples enabled us to generate steam in the pyrolyzer with the prescribed S/C ratio within a range of 0.46−1.01 mol H2O/mol C. The K2CO3loaded cedar samples are hereafter referred to as K-loaded ones. Dried K-loaded cedar samples were pyrolyzed in an atmospheric screw-conveyer reactor.2,10,11 The peak temperature and average heating rate were 550 °C and ca. 6 °C/s, respectively. Details of the procedure have been reported elsewhere.2,10,11 The pyrolysis runs were performed for the preparation of K-loaded char samples that were used as the initial bed material of the gasifier, and for investigating the distribution of the pyrolysis products (see Table 1). The definition of the light and heavy tars will be mentioned in the following section. The yield of H2O is that of the water formed by the pyrolysis. The conditions for the pyrolysis such as peak temperature, average heating rate, feeding rate of the K-loaded cedar, and flow rate of the carrier gas (N2) were the same as those for the pyrolysis in the gasification runs with the twostage reactor. It was thus assumed that the composition of the pyrolysis products at the entrance of the gasifier of the twostage reactor was the same as that shown in Table 1. The H/C and O/C atomic ratios of the K-loaded char were 0.35 and 0.12, respectively. The potassium content in the char was determined as 3.2 wt %-dry-char according to a method reported previously.2,12 2.2. Gasification of Potassium-Loaded Cedar. The runs of gasification were carried out with an atmospheric two-stage reactor system that is illustrated schematically in Figure 3. A prescribed amount of the K-loaded char (109−122 g of C) was charged into the gasifier so that the bed height was ∼350 mm. After both the pyrolyzer and gasifier had been heated to the prescribed temperatures, the wet K-loaded cedar was fed continuously into the horizontal screw-conveyer pyrolyzer (inner diameter = 50 mm) at a constant rate in a range of 2.0− 2.3 g/min together with N2 (purity of >99.9995 vol %). The
Figure 2. CER, O2/C ratio, and Texit for Type IV gasification, each as a function of Tref. The following assumptions were made in the simulation: (1) elemental composition of biomass (C100H145.4O69.8); (2) heating value of biomass (18.5 MJ-LHV/kg); (3) carbon conversion to gas (100%); (4) temperature of char and volatiles at the exit of the pyrolyzer (inlet of gasifier), 550 °C; (5) components of syngas (H2, CO, CO2, CH4, and H2O); (6) CH4 yield (4% on biomass carbon basis); (7) chemical equilibration among H2, CO, CO2, and H2O at the gasifier exit; and (8) the reactor system is adiabatic. More details of the calculation are reported in ref 1. B
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transferred without cooling to the vertical gasifier (inner diameter = 50 mm) that consisted of the bed of K-loaded char and headspace. The volatiles and steam entering the gasifier was mixed with N2 or a N2/O2 mixture in the headspace. The feeding rate of O2 corresponded to an O2/biomass-carbon molar ratio (O2/C) of 0.12, or ∼0.2 mol O2/mol C. It was believed that the major portion of the O2 was consumed for oxidative reforming (partial oxidation) of the volatiles. 11,13 The char was continuously dropped onto the top surface of the preformed char bed. The volatiles were forced to pass through the char bed while converted to syngas. The K-loaded char was not discharged from the gasifier during the run. In preliminary experiments, trials were made toward continuous discharge of the char and recycling to the gasifier, which was very difficult, because of the very small inner diameter of the gasifier tube. The amount of the char in the bed varied with time or reached steady during the run depending on the net rate of gasification of the char, which will be reported in detail in the Results and Discussion section. Table 2 lists the conditions for the nine gasification runs. The gasifier was heated in either of two different temperature modes: isothermal and gradient ones. In the runs except R3 and R9, an isothermal zone at 630, 670, 700, or 720 °C was provided through the char bed and headspace. Two thermocouples were inserted vertically into the char bed along with the axis. One was used for controlling the temperature at a fixed point while the other was moved for monitoring the temperature distribution. In Runs R3 and R9, the same gradients of temperature were formed. The temperature distribution was as follows: height from the bed bottom and temperature: 0 mm/600 °C, 100 mm/700 °C, 200 mm/800 °C, and 300 mm/920 °C and 350 mm (top)/850 °C. The product gas and steam were cooled in the gasifier downstream sequentially to 170 °C, 3 °C, −40 °C, and then −70 °C using the aerosol filter, the first to third condensers, respectively.2,10,11 The aerosol filter captured a major portion of organic compounds with boiling point temperature above that of anthracene (∼340 °C). It was also confirmed that water and the condensable organic compounds (carbon number >5) were condensed quantitatively in the three condensers. The organic matter deposited on the aerosol filter was recovered by dissolution in tetrahydrofuran and its evaporative removal. The recovered solid was a portion of heavy tar that was defined as the organic compounds with boiling point higher than 340 °C. The organic compounds condensed in the condensers were
Table 1. Product Yields from Pyrolysis of K-Loaded Cedar at 550 °Ca Yield product
mol C/100 mol C-bm
wt % dry bm
H2 H2O CO CO2 CH4 C2H4 C2H6 C3H6 C3H8 CH3OH CH3CHO tar (total) light tar heavy tar char
1.25b 32.1c 7.92 8.25 2.87 0.68 0.68 0.61 0.21 0.02 0.45 32.0 13.7 18.2 46.3
0.103 23.6 9.05 14.8 1.88 0.39 0.42 0.35 0.13 0.025 0.41 21.6 11.0 10.6 27.1
a
The units for H2 and H2O yields are mol/100 mol C-biomass, and that for the other products is mol C/100 mol C-biomass. bGiven in units of mol H2/100 mol C-bm. cGiven in units of mol H2O/100 mol C-bm.
Figure 3. Schematic diagram of experimental apparatus for continuous and sequential pyrolysis and gasification.
feeding period was in the range of 60−180 min. The K-loaded cedar was pyrolyzed with the peak temperature of 550 °C. The moisture of the K-loaded cedar was evaporated in the early period of heating. The nascent char and volatiles were then Table 2. Conditions for Gasification Runs Run
mode
Tref (°C)
feeding period (min)
feeding rate (mol C/min)
S/C ratio (mol H2O/mol C)
O2/C ratio (mol O2/mol C)
tra (s)
R1 R2 R3 R4 R5 R6 R7 R8 R9
isothermal isothermal gradient isothermal isothermal isothermal isothermal isothermal gradient
670 720 600−920 720 630 670 700 720 600−920
60 60 60 60 180 180 180 180 70
0.0817 0.0968 0.0866 0.0816 0.0911 0.0946 0.0846 0.0848 0.0847
0.923 0.863 0.980 1.008 0.495 0.514 0.477 0.519 0.462
0 0 0 0.120 0.197 0.176 0.206 0.217 0.196
2.37 1.86 1.77 1.95 2.91 2.51 2.45 2.10 2.28
Gas residence time within the char bed was calculated from the flow rate of gas (N2, steam, and the product gases) at the gasifier exit and based on empty bed volume. The height of char bed was assumed 350 mm. For the isothermal runs, the gas temperature was assumed the same as the bed temperature. For the nonisothermal runs, the average gas temperature was assumed as 760 °C. a
C
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dissolved in acetone, methanol, or methylene chloride, and the solution was analyzed by gas chromatography/mass spectrometry (GC/MS). Details of the analysis are reported elsewhere.2,10,14 It was found that the organic matter collected in the condensers consisted of monoaromatic hydrocarbons (benzene, toluene, xylenes, ethylbenzene, and styrene), phenols (phenol, cresol, and other alkylated phenols), benzofurans (benzofuran and methyl-benzofurans), indene, naphthalenes (naphthalene and alkylnaphthalenes), other diaromatics (biphenyl and acenaphthylene), triaromatics (anthracenes and phenanthrenes), and tetra-aromatics (pyrene and fluoranthene). Among them, the tetra-aromatics were categorized into the heavy tar. The lighter aromatics (except the monoaromatic hydrocarbons) were defined as the components of light tar, and these were distinguished from the monoaromatic hydrocarbons (MAHC). The tar thus consisted of MAHC, light tar, and heavy tar. Water collected in the condensers was quantified by a general Karl Fischer titrimetry. The dry gas that had passed through the condensers was analyzed every 3−5 min via gas chromatography with an Agilent 490 Micro GC for determining the flow rates of H2, CO, CO2, CH4, C2H4, and C2H6. A bulk analysis of the product gas detected C3 hydrocarbons, but their yields were below 0.01% on the biomass carbon basis.
3. RESULTS AND DISCUSSION 3.1. Total Carbon Conversion to Syngas. Figure 4 shows three examples of the cumulative amount of product gas on a carbon basis (mcg), as a function of time, together with the amount of K-loaded cedar (mcf). It is seen in a certain period (t1 − t2) of every run that mcg increased in a linear manner with time (correlation coefficient r2 = 0.99−0.999), where the flow rate of the product gas was steady. It was thus reasonable to define dynamic conversion to gas on the basis of carbon (Xc) by Xc at t1 − t 2 =
flow rate of carbon‐containing gas downstream feeding rate of K‐loaded cedar
The Xc values for Runs R1−R9 are compared in Figure 5. Xc values higher and lower than 100% represent a net consumption and accumulation of K-loaded char in the gasifier, respectively, at t1 − t2. As reported later, the reforming of tar was almost complete for all of the runs. For Runs R5−R8 with S/C ≈ 0.5 and O2/C ≈ 0.2, Xc increases from 65% to 125% as the bed temperature (Tref) increases from 630 °C to 720 °C. It seems that Tref values of 700 °C or higher enable complete or near-complete conversion of the K-loaded cedar, even with a limited supply of steam and O2, which is a requirement of a proof-of-concept of Type IV gasification. The rate of gas formation in Run R8 increases gradually with time, up to ∼100 min. Although not indicated in Figure 4, if values of t1 = 30 min and t2 = 60 min are given, Xc = 106%. It is also seen that mcg surpasses mcf at ∼100 min, after which the net decrease of the amount of char occurred in the gasifier. Such fast gasification of the char was probably due to the accumulation of K catalyst in the char bed. Reactivity of the spent K-char from Run R8 was investigated by thermogravimetry (details are shown in the Supporting Information) with temperature and steam concentration of 650 °C and 10 vol %, respectively. The steam gasification of the spent K-char was, in fact, more rapid than that of the fresh K-char under the same conditions as above. A pyrolysis-gasification run was performed employing the same conditions as those for Run R8 while the feeding time was shortened to 60 min. The reactivity of the
Figure 4. Cumulative amount of syngas on carbon basis as a function of time for Runs R6, R7, and R8.
Figure 5. Xc for Runs R1−R9. The black bars indicate the Xc values for runs in isothermal modes with O2/C ≈ 0.2 and S/C ≈ 0.5 (Runs R5−R8), whereas the white bars indicate the Xc values for Runs R1−R4 and R9.
spent K-char was between the above-mentioned two chars. It was thus confirmed that the reactivity of the K-char was promoted by extending the feeding period. D
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ways. The net and gross steam consumptions are defined as follows:
Isothermal runs of Runs R2, R4, and R8 were performed with Tref = 720 °C, and these runs gave Xc values of 95%, 102%, and 125%, respectively. The O2/C ratio for R2, R4, and R8 were 0, 0.120, and 0.217, respectively. It was clear that that the supply of O2 had a positive impact on Xc. In comparison of S/C and O2/C between R4 and R8, it is noted that the increase in the O2/C from 0.120 to 0.217 caused a significant increase of Xc in the face of reduction of the S/C ratio from 1.01 to 0.51 as well as the strong volatile-char interaction15,16 that inhibits the char gasification. More details of the effectiveness of O2 on Xc are discussed in the following section. Runs R3 and R9 were the runs in the same temperature gradient modes. In a practical gasifier for Type IV gasification, a temperature gradient will occur across the K-loaded char bed, because of exothermic partial oxidation of volatiles with O2 or air in the headspace and subsequent endothermic steam reforming/gasification in the char bed. The temperature gradients for Runs R3 and R9 were qualitative simulations of those in such a practical gasifier, while it was difficult to predict the gradient quantitatively. The Xc value for Run R9 was 100%, and this demonstrated that the temperature gradient (temperature range = 920−600 °C, Tref = 600 °C) was as effective as the isothermal condition for Run R7 with Tref = 700 °C, in terms of Xc. 3.2. Effect of O2 on Dynamic Conversion and Steam Consumption. Figures 6a and 6b plot Xc against the gross consumptions of steam that were calculated by two different
net steam consumption = (amount of steam fed into the pyrolyzer) − (amount of steam collected at the gasifier downstream) gross steam consumption (1) = (amount of steam fed into the pyrolyzer) + (amount of steam formed by pyrolysis) − (amount of steam collected at the gasifier downstream) gross steam consumption (2) = (amount of steam fed into the pyrolyzer) + (amount of steam formed by pyrolysis) + (amount of steam formed by partial oxidation) − (amount of steam collected at the gasifier downstream)
The flow rate of steam at the gasifier exit was not directly measured but calculated from H and O balances between the gasifier entrance and exit in the period of t1 − t2. Two assumptions were made in the calculation. The first assumption was complete conversion of the volatiles from the pyrolysis, which was reasonable because the overall carbon conversion to gas was higher than that of the cedar into the volatiles by the pyrolysis. The second assumption was that the elemental composition of the gasified char was the same as that of the char just after the pyrolysis with H/C and O/C atomic ratios of 0.351 and 0.125, respectively. The second assumption might cause some error if the Xc value is far away from 100%. The total H and O flow rates at the gasifier inlet were given from the total flow rates of the volatiles, added steam, added O2 and char that was gasified. The flow rate of steam at the gasifier exit was then calculated so that the H and O flow rates there were the closest to those at the inlet. The differences in the H and O flow rates between the inlet and exit were as small as 0−1 mol/ 100 mol C-bm (“bm” denotes biomass) for the runs except R1 and R5. The differences for these two runs were 3−5 mol/100 mol C-bm. The net steam consumption is often employed for evaluating the extent of steam reforming and gasification, but it is not necessarily reasonable, because it is different from the steam consumption in the K-loaded char bed. The pyrolysis inevitably forms steam (i.e., pyrolytic water). The pyrolytic water yield from the K-loaded cedar was substantial, as shown in Table 1. Gross steam consumption (1) considers the pyrolytic water. This mode of steam consumption was still insufficient in Runs R4−R9, because the partial oxidation of the volatiles undoubtedly generated steam. Hosokai et al.13 investigated gas-phase O2 oxidation of nascent volatiles from the pyrolysis of pulverized cedar with O2 at 700 °C. They found that the consumption of 1 mol of O2 resulted in increase in the yields of H2O, CO, and CO2 by 0.98 mol ( f H2O), 0.60 mol (f CO), and 0.32 mol ( f CO2), respectively, unless the molar ratio of consumed O2 to carbon involved in the initial volatiles exceeded 0.35 mol O2/mol C-volatiles. They also found that the partial oxidation decreased the tar yield selectively while those of the gaseous products such as H2 and light hydrocarbon gases were maintained. According to the report by Hosokai et
Figure 6. Relationship between (a) Xc and gross steam consumption (1) and Xc and gross steam consumption (2). E
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al.,13 gross steam consumption (2) was calculated assuming that the increase in the O2/C ratio caused the additional formation of steam at a rate of 0.98 mol H2O/mol O2. Figure 6a shows that the isothermal reforming/gasification with O2/C ≈ 0.2 (Runs R5−R8) required smaller gross steam consumption (1) than that without O2 (Runs R1 and R2) by ca. 42 mol H2O/100 mol C-bm. Part of such a difference is attributed to the formation of steam by the partial oxidation, and its contribution is corrected by considering gross steam consumption (2). Figure 6b shows that Xc by the gasification with O2/C ≈ 0.2 is higher by ca. 18 mol C/100 mol C-bm than that without O2. This difference is in good agreement with the amount of CO and CO2 formed by the partial oxidation, which is calculated as (O2 /C)(fCO + fCO ) × 100 = 0.2 × (0.60 + 0.32) × 100 2
≈ 18 mol/100 mol C
according to the report of Hosokai et al.13 The Xc value for Run R4 with O2/C = 0.12 is between those for Runs R5−R8 and Runs R1 and R2, and this is reasonably accepted. Thus, the positive effect of O2 supply on the increase in Xc is explained quantitatively by the partial oxidation of volatiles that formed H2O, CO, and CO2. In other words, the role of O2 is not only to cause the exothermic reaction but also to help the steam reforming/gasification by reducing the burden of carbon conversion on the steam reforming/gasification. Gross steam consumption (2) for Run R7, which gave an Xc value of 99%, was 73 mol H2O/100 mol C-bm, and it was much greater than the net steam consumption, 47.7, which is the same as the S/C ratio. 3.3. Steam Reforming of Tar. The yields of light and heavy tars were 13.7% C and 18.2% C, respectively, at the exit of the pyrolyzer, and both yields were reduced tremendously in the gasifier. The elimination rates of the heavy and light tars were in ranges of 99.925%−99.997% and 89.0%−99.98% on the carbon basis, respectively, as shown in Table 3. The
Figure 7. Yields of MAHC, light tar, and heavy tar.
Table 3. Elimination Rates of Light and Heavy Tars on Carbon Basisa Run
heavy tar (%)
light tar (%)
R1 R2 R3 R4 R5 R6 R7 R8 R9
99.925 99.930 99.991 99.978 99.924 99.922 99.994 99.997 99.997
92.82 97.34 99.91 99.29 89.03 98.94 99.85 99.98 99.94
and 0.014% C−0.0006% C, respectively. These yields decrease as Tref becomes higher. The MAHC yield decreases with Tref in a trend similar to those of the light and heavy tars. Comparison of the MAHC, light tar, and heavy tar yields between Runs R5 and R7 demonstrates a positive effect of increasing Tref (from 630 °C to 700 °C) on the reduction of the tar and MAHC yields by 1−2 orders of magnitude. Comparison among Runs R2, R4, and R8 with Tref = 720 °C shows that the introduction of O2 enhanced the reforming. Concentrations of light and heavy tars in the product gas are shown in Figure 8. Among the runs with O2/C ≈ 0.2 and S/C ≈ 0.5, Runs R7 and R8 with Tref ≥ 700 °C give total light/heavy tar concentrations well below 100 mg/Nm3, which is a threshold concentration for direct use of the syngas as a fuel for internal combustion engines.19 Among the vapor-phase thermal cracking, the vapor-phase partial oxidation, and steam reforming over the K-loaded char, the last one was the most crucial to very high elimination rates of tar. According to recent reports of steam reforming over charcoal,11,18,20−22 the tar is converted to syngas by thermal decomposition forming carbon deposition on the char surface and by the subsequent K-catalyzed steam gasification of the deposited carbon. Another pathway for the catalytic reforming, that is, direct steam reforming of the tar, should also be taken into consideration. It is not easy to distinguish direct steam reforming from the carbon deposition/gasification sequence.
a
The rates were determined from the ratio of respective tars at the downstream of gasifier to those from the pyrolysis (see Table 1).
catalytic effect of K was clear when this result was compared with a previous report4 that investigated in situ steam and steam-O2 reforming of tar from the pyrolysis of a cedar over charcoal without catalyst loading. Results of some previous reports11,17,18 in fact showed that the reforming of tar over char was not so fast as to eliminate the tar at temperatures of ≤800 °C, in the absence of extraneous catalyst. As seen in Figure 7, Runs R5−R8 with O2/C ≈ 0.2 and S/C ≈ 0.5 give light and heavy tar yields in the ranges of 1.5−0.003 F
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to 720 °C (i.e., changing the conditions from Run R5 to Run R8) decreased the yields of MAHC, phenols and naphthalenes by factors of 170, 810 and 90, respectively. Such significant progress of the steam reforming may not be explained only by the carbon-deposition/carbon-gasification sequence. The present results of the reforming of MAHC and light tar strongly suggest direct and K-catalyzed steam reforming. The authors investigated steam reforming of benzene over the K-char and also char from the original cedar (without K loading) in a fixed bed reactor. The result is described here in brief. The temperature, gas residence time (on an empty bed basis), steam concentration, and initial concentration of benzene were 700 °C, 0.4 s, 10 vol % and 50 g/Nm3, respectively. The particle sizes of the chars were within a range of 1.0−2.0 mm. The conversion of benzene over the char without K loading was lower than 1% on the carbon basis, but was >20% over the K-char. This result supports the pathway of the direct steam reforming, as well as insignificant carbon deposition at temperature as low as 700 °C. 3.4. Composition of Tar. Figure 10 displays compositions of the light tar and MAHC. Phenols and naphthalenes were
Figure 8. Concentration of light and heavy tars in dry and N2-free syngas.
Figure 9 exhibits the relationships among the yields of MAHC, light tar, and heavy tar. The dashed lines are drawn
Figure 9. Yields of light and heavy tars, each as a function of MAHC yield.
assuming linear relationships between MAHC and light tar yields (YMAHC and YLT, respectively) and between MAHC and heavy tar yields (YMAHC and YHT, respectively). The slope of the lower dashed line means that the heavy tar yield decreases by only 0.055 mol of C while the MAHC yield does by 1 mol of C, and that the heavy tar is not an important precursor of MAHC. The slope of the upper dashed line shows that the decrease in the light tar yield is ∼1.7 times that of MAHC. This appears to be consistent with that the light tar is the precursor of MAHC. However, such conversion is not necessarily reasonable. Hosokai et al.20 investigated the decomposition of phenol, naphthalene, phenanthrene, and pyrene over charcoal from woody biomass, and found no formation of MAHC such as benzene and toluene from those aromatics. This fact supports independent reforming of light tar and MAHC. Thus, the decrease in the MAHC yield is mainly due to its steam reforming over the K-loaded char rather than elimination of the light tar. Abu El-Rub et al.21,22 investigated decomposition of aromatics over commercial charcoal in atmospheres that simulated those for biomass gasification. They reported that phenol and naphthalene were decomposed at 700 °C with conversions as high as 80%. The decomposition of the aromatics was probably and mainly due to deposition of carbon in/on mesopores and micropores.13,18 Under the present experimental conditions, increasing Tref from 630 °C
Figure 10. Composition of light tar (top) and MAHC (bottom). Diaromatics: biphenyl and acenaphthylene; triaromatics: anthracenes and phenanthrenes.
always major components of the light tar. The share of benzene in MAHC increases with the severity of the reforming in terms of Tref, temperature mode, and/or O2/ ratio. Figure 11 shows the yields of specific types of aromatic compounds as functions of the light tar yield. The yields of phenols and diaromatics/ triaromatics decrease in ways very similar to that of the light tar (total) over the range from 3 × 10−3 up to 4 × 10−1 % C. Thus, phenols and diaromatics/triaromatics had similar reactivities in the K-catalyzed steam reforming. However, at the light tar yield, >4 × 10−1 % C (in other words, for low severity reforming), the phenols yield decreases more steeply than the light tar yield G
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Figure 12. Composition of syngas (top graph) and yields of components (bottom graph). The gross chemical energy recovery (CER) is indicated on an LHV basis for each run in the bottom graph. Figure 11. Relationships of light tar yield with those of phenols, diaromatics and triaromatics, benzene, and alkylbenzenes.
combined effects of Tref, S/C ratio, and O2/C ratio on the yield of CH4 are considered (note that CH4 was the major component of the gaseous hydrocarbons). Chemical equilibrium of the reactions with the following stoichiometry is important in consideration of decomposition and formation of CH4.
while that of the diaromatics/triaromatics slightly increases. Precursors of the diaromatics/triaromatics are unknown, but the phenols were probably the most important candidates.23,24 The benzene and alkylbenzenes yields decrease more rapidly than those of the light tar components when the severity of reforming is relatively low. This trend is consistent with previous reports that showed the less reactivity of benzene with char surface than greater aromatic hydrocarbons.11,18,20 On the other hand, with higher severity of reforming corresponding to the light tar yield below 2 × 10−2 % C, the yields of benzene and alkylbenzenes decrease as rapidly as or more rapidly than that of the light tar. This trend may be attributed to the catalysis of K on the char surface, details of which presently are not clear. 3.5. Analysis of Syngas Composition. Figure 12 shows the composition and yield (at t1 − t2) of syngas. In this section, the composition of syngas is analyzed considering the Kcatalyzed steam reforming of tar and gaseous hydrocarbons, Kcatalyzed steam gasification of char, water−gas shift reaction (WGSR) and reverse water−gas shift reaction (RWGSR). The figure also shows the gross chemical energy recovery (CER) for each run. The CER values were calculated as the ratio of the total chemical energy of the syngas, relative to that of the Kloaded cedar on an LHV basis and Xc. Steam Reforming of Gaseous Hydrocarbons. The total molar fraction in the syngas and yield of C1−C2 gaseous hydrocarbons vary, depending on the reforming/gasification conditions within the ranges of 1.8−5.2 vol % on a dry and N2free basis and 3.2% C−10.0% C, respectively. Here, the
CH4 + H 2O = 3H 2 + CO (forward, steam reforming; backward, hydrogenation)
Figure 13 compares the experimental reaction quotient for the above reaction (RQsrm) with its equilibrium constant (Ksrm). RQsrm is defined by
Figure 13. Comparison of reaction quotient for steam reforming of methane (CH4 + H2O = CO + 3H2) with equilibrium constant. H
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Table 4. Measured RQwgsr, Optimized Δrwgsr, and Recalculated (Reproduced) RQwgsr Valuesa
pCO (pH )3 2
pCH pH O 4
Run
Tref [°C]
Kwgsr
measured RQwgsr
optimized Δrwgsr [mol/100 mol C-bm]
recalculated RQwgsr
R1 R2 R4 R5 R6 R7 R8
670 720 720 630 670 700 720
1.80 1.44 1.44 2.20 1.80 1.57 1.44
5.93 3.84 1.05 3.11 1.25 1.86 1.28
11.9 11.1 −4.5 2.9 −3.9 1.8 −1.1
1.80 1.44 1.44 2.20 1.80 1.57 1.44
2
pi represents the partial pressure of species i. The partial pressures were calculated from the flow rates of the individual product gases, H2O and N2, at the gasifier exit. RQsrm is smaller than the corresponding Ksrm, regardless of the conditions. The trend of RQsrm values for the runs with O2/C ≈ 0.2 and S/C ≈ 0.5 (Runs R5−R8) is indicated by the dashed line. RQsrm approaches Ksrm as Tref increases, and this is mainly due to the increase in the partial pressure of H2 rather than the conversion of CH4. The increase in Tref from 630 °C (Run R5) to 720 °C (Run R8) resulted in a decrease in the CH4 yield from 4.7% C to 3.8% C, which was not so significant as to greatly contribute to the increase in RQsrm. Thus, the progress of steam reforming of CH4, if any, was slower than that of tar, as well as the steam gasification of K-loaded char. Mechanism of K-Catalyzed Reactions over the Char. The comparison between RQ and K was also made on WGSR and RWGSR:
a
The measured and recalculated RQwgsr are defined by the following equations: measure RQwgsr = YH2YCO2/(YH2OYCO) and calculated RQwgsr = (YH2 − Δrwgsr)(YCO2 − Δrwgsr)/[(YH2O + Δrwgsr)(YCO + Δrwgsr)], where Yi represents the yield of product i (in units of mol/100 mol C-bm).
corresponding optimized Δrwgsr values are 11−12 mol/100 mol C-bm, and beyond experimental errors. The trends of RQwgsr for Runs R1 and R2 are important, because RQwgsr > Kwgsr is possible only if a particular type of steam gasification/reforming takes place. The following reactions are the candidates for the formation or consumption of H2, H2O, CO, and CO2 in the gasifier. • Steam gasification/reforming producing CO2 and H2 (Type A G/R) • Steam gasification/reforming producing CO and H2 (Type B G/R) • CO2 gasification/reforming (CO2 G/R) • WGSR • RWGSR Both Type A G/R25−29 and Type B G/R30−32 have been proposed for K- or Na-catalyzed gasification of carbon or char. The explanation of RQwgsr for Runs R1 and R2 requires the progress of Type A G/R at least to a substantial degree, because the RQwgsr of the volatiles from the pyrolysis (before the reforming) was only 0.10−0.15 and much smaller than Kwgsr. The composition of syngas is often discussed assuming the combination of Type B G/R and WGSR, but RQwgsr never exceeds Kwgsr if the initial RQwgsr is smaller than Kwgsr. It is also believed that the contribution of the CO2 G/R to the gasification and reforming was much less than Type B G/R. Hashimoto et al.29 investigated K- and Na-catalyzed steam gasification of carbon at 600−835 °C. The detailed kinetic analysis revealed that Type A gasification was responsible for the carbon conversion while the contribution of Type B was negligible. In the analysis of the gas composition under the present conditions, the assumption of Type A G/R seems to be more reasonable than assuming both Type A G/R and Type B G/R. This is straightforwardly followed by another assumption of net progress of RWGSR, because that of WGSR fails to explain the gas compositions for Runs R1 and R2. The yield and composition of CO, CO2, H2, and H2O were analyzed by assuming a sequence of the pyrolysis, gas-phase O2 oxidation of the volatiles, Type A G/R, and then RWGSR. Details of the analysis are shown in the Appendix. Figure 15 exhibits the relationship between the extent of char gasification and that of RWGSR. The extent of RWGSR is linearly related to that of the char gasification. It is known that the char surface is very active toward both WGSR and RWGSR32−34 and the rates of these reactions are much faster than that of steam gasification.32 It was also reported that alkali
CO + H 2O = CO2 + H 2 (forward, WGSR; backward, RWGSR)
Hereafter, RQwgsr and Kwgsr represent the reaction quotient and equilibrium constant for WGSR, respectively. Figure 14 compares RQwgsr with Kwgsr for the runs in the isothermal modes. RQwgsr is much greater than Kwgsr for Runs
Figure 14. Comparison between experimental reaction quotient for water−gas shift reaction (RQwgsr) and equilibrium constant (Kwgsr).
R1 and R2, while they are more or less similar to each other for Run R4 and Runs R6−R8. RQwgsr was very sensitive to the concentrations of H2, H2O, CO, and CO2. Table 4 shows a result of the sensitivity analysis, in which the yields of H2, CO, and CO2 and amount of H2O (units of mol/100 mol C-bm) were increased or decreased, assuming more or less progress of RWGSR. The extent of assumed RWGSR, Δrwgsr, was defined by the amount of H2O and CO formed by this reaction, which is equivalent to the consumption of CO2 and H2. Δrwgsr was optimized so that the recalculated RQwgsr agreed with Kwgsr. The optimized Δrwgsr values for Runs R4−R8 are in a range from −4.5 mol/100 mol C-bm to +2.9 mol/100 mol C-bm, and are sufficiently small, compared with the measured yields of H2, CO, and CO2. Thus, the measured RQwgsr and Kwgsr values are close to each other, in terms of the extent of WGSR or RWGSR. On the other hand, the RQwgsr values for Runs R1 and R2 are clearly greater than the Kwgsr values, and the I
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Figure 16. Difference between the total enthalpy of pyrolysates, steam, and O2 at the inlet of gasifier and that of syngas at the exit.
Figure 15. Relationship between the extents of progress of steam gasification of char (Type A G/R) and RWGSR.
4. CONCLUSIONS This study was carried out aiming at experimental proof of the concept of Type IV gasification of biomass. The two-staged conversion of the K-loaded cedar established the following simultaneously at Tref ≥ 700 °C, O2/C ≈ 0.2 mol O2/mol Cbm, and S/C ≈ 0.5 mol H2O/mol C-bm: • Conversion of gas, based on carbon (Xc) of ≥99%; • Total concentration of light and heavy tars
