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Ind. Eng. Chem. Res. 2002, 41, 5640-5649
Black Liquor Gasification Characteristics. 1. Formation and Conversion of Carbon-Containing Product Gases Viboon Sricharoenchaikul Environmental Technology Program, Sirindhorn International Institute of Technology, Thammasat University, Pathumthani 12121, Thailand
Wm. James Frederick, Jr.* Department of Chemical Engineering and Environmental Science, Chalmers University of Technology, SE-41296 Gothenburg, Sweden
Pradeep Agrawal School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
Experimental measurements of the carbon-containing gases produced during gasification and pyrolysis of dry solids from two black liquors were made at high heating rates, at 700-1100 °C. The product gases were analyzed for carbon-containing species by Fourier transform infrared spectrometry. The 12 most abundant carbon-containing light product gases were quantified. Devolatilization of black liquor proceeded rapidly and was complete within the shortest sampling interval (0.3 s) in all of the laminar entrained-flow reactor experiments. Less than 15% of the carbon in the black liquor solids was converted to light gases during devolatilization. In a nitrogen atmosphere, additional carbonaceous material from tar was converted to light gases via secondary reactions after devolatilization was complete, and fixed carbon was gasified by reduction of Na2SO4 and Na2CO3. Neither the presence of water vapor nor the composition of the black liquor solids had a large effect on the transformation of organic matter to light gases during devolatilization. These parameters also had little impact on the evolution of light gases during secondary cracking and reforming reactions at low temperatures (700-800 °C). However, at higher temperatures (900-1100 °C), these variables had a greater impact. The water gas shift reaction apparently played a major role in the distribution of carbon between CO and CO2 during secondary reactions at longer residence times and higher reactor temperatures. Introduction Gasification of biomass-containing wastes from industrial, agricultural, and municipal sources has great potential to reduce fossil fuel demand for electrical power generation, automotive fuels, and chemical feedstocks. Integrated gasification-combined cycle power plants can produce electrical energy at about twice the efficiency as conventional steam-cycle power plants. Hydrogen or syngas for methanol or dimethyl ether production can be produced by coupling gasification plants with water gas shift reactors and pressure-swing adsorbers. Development of gasification technologies for these biomass-containing wastes requires an understanding of the conversion processes and efficiency of biomass to CO, CO2, hydrogen, and other light gases. Black liquor is a major biomass-containing waste produced in pulp and paper manufacturing regions worldwide. While there has been extensive research on the thermal conversion of coal and many biomass fuels, consistent data on the transformation of the organic matter in black liquor to light gases, condensable organic matter (tar), and char residue during gasification are very limited. The thermal conversion of solid and liquid fuels has been characterized as occurring in several, often over* To whom correspondence should be addressed. Phone: +46 31 772 2996. Fax: +46 31 772 2995. E-mail: jimfred@ sikt.chalmers.se.
lapping stages: drying, devolatilization, char reactions, and inorganic residue reactions. Devolatilization is the first step in the conversion of organic matter, producing light gases, condensable organic matter ranging from alcohols and light aromatics to semivolatile compounds (tar), and char.1 During char gasification, the char phase reacts with oxygen donor gases (CO2 and water vapor), which convert its fixed carbon to gases. Tar is a primary volatile product released during devolatilization. However, tar compounds undergo secondary reactions that produce lighter gases, lighter tar species, and soot. It is important to clarify the terms pyrolysis, devolatilization, and gasification when discussing thermochemical reaction data. In this paper, the term pyrolysis means the thermal degradation of organic matter in an inert (e.g., nitrogen) atmosphere. The term devolatilization means the evolution of volatile organic matter and light gases during thermal degradation in a gas environment that may include water vapor or CO2. The term gasification refers to the overall process of thermochemical conversion of a carbonaceous material to CO, CO2, etc., in the presence of an oxidizing gas such as water vapor or CO2. Carbon gasification refers to the conversion of fixed carbon to CO via oxidation with water vapor, CO2, etc. With these definitions, pyrolysis and devolatilization overlap but are different phenomena, and devolatilization is a process that occurs during gasification of a fuel that contains volatile matter. We
10.1021/ie020207w CCC: $22.00 © 2002 American Chemical Society Published on Web 10/19/2002
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 5641 Table 1. Typical Data Reported for Light Gases Yields from Cellulose and Wood study
Hajaligol et al.2
Scott et al.3
Fraga et al.4
Scott et al.5
Nunn et al.6
material particle size, µm temperature, °C heating rate, °C/s residence time, s light gases yields, % dry ash-free basis
cellulose 100 400-1000 1000 30 9-47
cellulose 100 400-750 104-105 0.5 12-48
silver birch 100-150 400-900 1 1000 30 30 33-41 33-43
Eastern red maple 100 400-800 104-105 0.5 5-60
sweet gum 45-88 327-1127 1000 0.3-1.1 0.6-28
Table 2. Gas Yields Reported from Pyrolysis of Kraft Black Liquors study experimental method nominal initial particle size, mm dry solids content, % temperature, °C heating rate, °C/s residence time, s light gas yields, % dry ash-free basis
Feuerstein et al.15
Bhattacharya et al.16
batch pyrolysis
batch pyrolysis 0.4 100 620-740 ∼10 900 12-18
65 400-975 ∼0.1 ∼7000 5-16
describe the gas atmosphere in which these processes occur by the composition of the gases prior to the onset of thermochemical reactions. Data reported for pyrolysis of biomass materials show that the yield of light gases depends on the material processed, the process conditions, and the collection method.2-5 The data in Table 1 show the variation in light gas yields typical of pyrolysis experiments conducted with different lignocellulosic materials at different pyrolysis conditions. They are representative of the limited data available on the detailed chemical composition for product gases from biomass pyrolysis. When compared with biomass data, there are very few data on the production of light gases from pyrolysis of kraft black liquor. Most of the available product gas data are from studies that were concerned with the volatilization of sulfur-containing gases. The data that are available have been obtained at relatively low heating rates and with limited analysis of the product species. The data on total light gas yields from three studies are summarized in Table 2. These data suggest that light gas yields increase with increasing heating rate. For transient measurements of gas species, the product collection technique is crucial to the reliability of the data obtained. This is because continued reactions change the yields of gas species when the pyrolysis products are not quenched rapidly. For example, Gairns et al.7 reported that the residence time of the pyrolysis gases between their point of formation and the condenser in their experiments was 0.3 s. Because pyrolysis might already be complete within that time frame,8 the data obtained may have reflected changes in tar and light gases between the reaction zone and condenser. There were no data reported on the effect of gas residence time and the presence of oxidizing gases on product gas composition in any of these studies. In the present study, measurements of the formation and transformation of light gas species during pyrolysis and gasification of black liquor were made at relatively short residence times. The products were quenched rapidly to ensure well-defined gas and particle residence times in the reaction zone. As a result, the changes in light gas yields with time, during secondary gas-phase reactions, could be measured accurately. The results reported here are the first reported experimental measurements of the carbon-containing light gas products from pyrolysis and gasification reactions at particle and gas residence times of the same order of magnitude as in industrial-scale, entrained-flow gasifiers.
Gairns et al.7 suspended single droplets 2.4 70 400-800 ∼100 15-150 40-55
Figure 1. Diagram of the LEFR and gas conditioning system used in this study.
Experimental Section The pyrolysis and gasification experiments were conducted in a laboratory-scale laminar entrained-flow reactor (LEFR). This type of reactor has two features that are important when obtaining fundamental pyrolysis and combustion data: it provides very rapid heating, and it uses particles that are small enough so that temperature differences within the particles are small. A schematic of the LEFR used in the experiments reported here is shown in Figure 1. The black liquor feeding system consisted of a cylindrical fuel particle container (test tube) driven slowly upward by a small motor and screw drive as shown in Figure 2. A much smaller diameter feed tube with a stationary plate was fixed in position coaxially inside the black liquor container. A primary gas flow (N2, 0.15 L/min at room temperature) entered the fuel particle container near its top and exited through the feed tube. As the fuel particle container moved upward relative to the feed tube, fine particles were entrained from the surface of the fuel particles and carried into the feed tube by the gas. The feed rate of fuel particles was controlled by adjusting the upward displacement rate of the rotating fuel particle container relative to the
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Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 Table 3. Elemental Analysis of the Dried Black Liquor Solids Used in This Work weight % liquor A
liquor B
element
liquor A
liquor B
carbon hydrogen oxygen sodium
31.94 3.65 38.59 18.60
34.90 3.05 35.10a 22.65
potassium chlorine nitrogen sulfur
1.76 0.55 0.10 5.26
0.62 0.67 0.11 2.90
a
Figure 2. Black liquor particle feeder.
fixed feed tube. The feed rates achieved in this work were from 0.3 to 1.5 g of black liquor solids/min. The LEFR consisted of a reactor core and a ceramic tube, 7 cm i.d. by 1 m inside a three-zone furnace, with a total heated length of 0.83 m. The particles and primary gas from the feeder were introduced to the top of the reaction zone through a water-cooled injector. A much larger, secondary gas flow (N2, 15-20 L/min at room temperature) entered the bottom of the annular region and was preheated as it passed through the annulus. It then turned 180°, passed through a ceramic flow straightener at the top of the reaction zone, and entered the inner tube coaxially with the primary gas. The particles and gas flowed downward, with the particles remaining in a narrow column at the center of the cylindrical reactor. The particles were heated rapidly (at rates estimated to be 6300-30 000 °C/s at the conditions employed in this work) and underwent reactions until they entered the product collector. The products exiting the reaction zone were quenched in a water-cooled, nitrogen-purged collection probe. This movable collector was made with a porous inner wall, through which cold nitrogen gas is passed to quench the reaction products and to prevent thermophoretic deposition of the fine particles (small char fragments and condensation aerosols) on the probe wall. About half of the nitrogen entered through the first 2 cm of the collection probe, providing a rapid quench to terminate the reaction. At the exit of the collector, the product temperature had been reduced to 200-300 °C. The gas and particles were separated at the exit of the product collector by a cyclone with a 3 µm cutoff. The product gases flowed continuously through the gas cell of an online Fourier transform infrared spectrometer (FTIR), with which gas analysis was performed. Two different FTIR spectrometers were employed: a Bomem model MB-100 FT-IR with a 7 m path length, 1.5 L gas cell and a Nicolet Nexus 470 FT-IR equipped with an ultrasmall 6 m path length, 325 mL heated gas cell. The detection limit of the FTIR was approximately 1 ppm(v) for CO, CO2, and hydrocarbons and about 20 ppm(v) for sulfur-containing gases. The reproducibility of the gas measurements in duplicate runs was within (10% of the measured value for gas species accounting for more than 10% of the total carbon input and within (15% of the measured value for gases accounting for less than 10% of the total carbon input. A more detailed description of the LEFR reactor and the experimental procedures are available from Sricharoenchaikul.1
weight %
element
By difference.
The ranges of experimental conditions in the LEFR were 700-1100 °C and 0.3-1.7 s particle residence time. At these conditions, the small particles of black liquor solids were heated uniformly, with temperature differences estimated as less than 10 °C from the center surface to the outer surface.9 The experiments were carried out in gas atmospheres of 100% nitrogen, 5% water vapor in nitrogen, and 12% water vapor in nitrogen, all at 1 bar of total pressure. Dry solids from two southern pine kraft black liquors were used in this study. Dry solids from liquor A, a southern pine black liquor, were produced by drying the liquor at a low temperature in a pulse combustor dryer. Because of the very hygroscopic nature of dried black liquor, the pulse combustor-dried liquor solids were reheated and sieved to a particle size of 45-63 µm before feeding into the reactor. The dry solids from liquor B, an oxidized, southern pine kraft liquor, were prepared by oven-drying the liquor at 110 °C, grinding, and sieving the resulting dry solids. The oven-dried black liquor solids were sieved through a No. 140 mesh screen and retained on a No. 170 mesh screen, yielding particles in the size range of 88-125 µm. Table 3 shows the elemental analysis of the dry solids from the two liquors. Results and Discussion When biomass is gasified, a wide range of light gases and organic compounds are volatilized and char is produced. The volatile organic matter is often classified as either light (noncondensable) gases or tar. For black liquor, in which the organic matter is composed mainly of chemically degraded lignin and carboxylic acids, the formation of these volatile species is mainly due to decarboxylation, decarbonylation, dealkylation, and aromatic ring rupture. The gasifier product gases produced in the experiments reported here were very dilute because of the relatively large volumetric flow rate of the secondary and quench streams. As a result, some semivolatile products with relatively low vapor pressures, such as methanol, acetone, formaldehyde, and acetaldehyde, did not condense at the collection temperature and are considered as light gases in this study. Transformation of Light Gases in Nitrogen. Figure 3 displays the total yield of carbon-containing gases versus residence time at all four experimental temperatures. The total yield of carbon-containing gases was calculated as the total moles of carbon in the carbon-containing gas species divided by the total carbon input to the reactor. The total carbon-containing gases at equilibrium for black liquor pyrolyzed in nitrogen were calculated to contain 33.6%, 46.2%, 75.4%, and 75.5% of carbon input for reactor temperatures of 700, 900, 1000, and 1100 °C, respectively.1 In general, the total carbon content of the light gases increased with reaction time and temperature. The
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 5643
Figure 3. Total light gases produced versus residence time and reactor temperature (liquor B, in N2).
Figure 5. CO2 produced versus residence time and reactor temperature (liquor B, in N2).
Figure 6. Methanol produced versus residence time and reactor temperature (liquor B, in N2). Figure 4. CO produced versus residence time and reactor temperature (liquor B, in N2).
reactions proceed much faster at the higher temperatures, 1000 and 1100 °C. Only for residence times longer than 1.3 s at 1100 °C are the total carbon gases comparable to the level predicted by equilibrium calculations. The gas formation reactions are relatively slow below 1000 °C. Secondary reactions further converted carbon-containing species to CO, CO2, CH4, and other light hydrocarbons. Because of the limited availability of oxygen atoms in a nitrogen environment, the product gases are formed mainly by the dissociation of a variety of functional groups in black liquor. Additional light gas formation may also come from the decomposition of tar formed during pyrolysis. The formation and disappearance of specific compounds or groups of compounds are described below, with the amounts given in units of carbon as the percent of carbon originally in the black liquor solids. CO was the major product at high temperatures (Figure 4). It increased continuously with reaction time. Relatively little CO was generated at 700 °C, while CO accounted for almost 75% of the carbon-containing gases after 1.5 s at 1100 °C (off-scale in Figure 4). The increase in the CO concentration at higher temperatures suggests a possible path for its formation from the ether linkages in lignin because they are one of the strongest types of bonds found in the organic matter in black liquor. A large amount of CO is expected because ether
links are the predominant bonds that connect the phenylpropane monomer units within the lignin structure. CO is also produced by reduction of Na2SO4 and Na2CO3 by char carbon. The reduction reactions, eqs 1 and 2, are very temperature sensitive, both having activation energies of about 250 kJ/mol.10,11 They would be expected to account for 5-10% of the CO produced at 1000-1100 °C.
Na2SO4 + 4C w Na2S + 4CO
(1)
Na2CO3 + 2C w 2Na(v) + 3CO
(2)
The carboxyl bond is the weakest and leads to early formation of CO2 even at low reactor temperatures. As seen in Figure 5, CO2 increased continuously with time at 700-1000 °C, with 6-10% of the carbon input as CO2 after 1.5 s. With time, the carboxyl groups become depleted and hence the CO2 concentration leveled off at longer residence times. At 1100 °C, the CO2 content reached a maximum of nearly 16% at 0.5 s and then decreased to about 5% after 1.5 s. The results suggest that the CO2 transformation process at 1100 °C is more complex than that at lower temperatures. At this high temperature, more alkali salts in black liquor may vaporize, react, and be involved in the gas transformation processes. Also, the influence of the water gas shift reaction may be significant at these temperatures. This will be discussed further when a comparison between pyrolysis and water vapor gasification is made, in the next section.
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Figure 7. Oxygenated gas species produced versus residence time at 900 °C (liquor B, in N2).
Figure 9. Light hydrocarbons produced versus residence time at 1000 °C (liquor B, in N2).
Figure 8. Methane produced versus residence time and reactor temperature (liquor B, in N2).
Oxygen-containing gases (methanol, acetone, acetaldehyde, and formaldehyde) formed within 0.3 s at 900 °C and above and then disappeared within 1-2 s. At 700 °C, these oxygen-containing compounds formed more slowly and had not begun to disappear by 1.5 s. Figure 6 shows how methanol was formed at all four temperatures and quickly consumed at 900-1100 °C. Acetone, acetaldehyde, and formaldehyde were formed and then disappeared in the same time frame as methanol. This is illustrated in Figure 7 for a reactor temperature of 900 °C. Most of these oxygenated compounds are not very stable; they react and disappear at longer residence times and higher temperatures. Note that except for formaldehyde, which is present in a trace amount at equilibrium, the oxygenated gases do not exist at equilibrium at any of the experimental conditions. The formation of the light hydrocarbons (methane, ethylene, and acetylene) proceeds more slowly than those of the oxygenated organic species. Methane increased continuously at 700-1000 °C and leveled off at about 7% of the carbon in black liquor at 1100 °C (Figure 8). At 1000-1100 °C, ethylene and acetylene reached maximum concentrations in 0.5-1.4 s and then stayed constant (Figure 9). At lower temperatures, they increased continuously up to 1.7 s (not shown). Traces of other light hydrocarbons through C4’s were observed, but they were not present in high enough concentrations to be quantified. Generally, alkyl groups, which have intermediate bond strengths, are the main source of a range of these hydrocarbon gases from black liquor. They are formed
Figure 10. COS and CS2 produced versus residence time and reactor temperature (liquor B, in N2). Table 4. Carbon Recovered as Light Gases and Char in the LEFR Experiments with Liquor B in Nitrogen temp, °C
carbon recovered as light gases and char, % of carbon input
standard deviation, % of carbon input
700 900 1000 1100
66.1 52.4 67.3 87.0
8.4 4.2 10.0 14.0
through a variety of dealkylation processes. These hydrocarbon evolution processes are kinetically controlled as evidenced by a comparison to their predicted equilibrium concentrations. Carbon- and sulfur-containing compounds were also identified but at very low levels. These gases included CS2, COS, mercaptans, and organic sulfides and disulfides. The transformations of COS and CS2 were very sensitive to changes in the temperature and residence time (Figure 10). They increased slowly with residence time at 700 °C. At higher temperatures, they quickly
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 5645 Table 5. Carbon-Containing Gaseous Products at 700 and 900 °C, Reported in Four Different Pyrolysis Studies with Kraft Black Liquors gaseous products, wt % carbon in BLS temp, °C heating rate, °C/s CO CO2 CH4 C2H6 C2H4 C2H2 methanol formaldehyde acetaldehyde acetone organosulfur gasesb CS2 COS
liquor A, this study (1.5 s) 700 6300 3.6 7.7 1.2 n.d.a 0.25 0.02 0.48 0.13 0.86 0.38 0.59 0.00 0.01
900 14 000 8.2 9.7 3.9 n.d. 1.6 0.25 0.00 0.10 0.03 0.06 0.20 0.00 0.01
Feuerstein et al.15 (∼120 min) 700
≈0.1
900
3.0 1.9 3.0 1.5
5.8 2.7 3.0 1.3
0.25
0.25
1.7
1.1
Gairns et al.7 (15 min) 700
≈100
900
6.4
24
0.50 0.00
0.18 0.05
Bhattacharya et al.16 (10 min) 700 ≈10 7.2 7.4 0.77
a Not detected; the detection limit for C H was approximately 0.01% of carbon input. b Measured collectively by FT-IR as the equivalent 2 6 methyl mercaptan concentration in this study; measured separately as CH3SH, CH3SCH3, and CH3S2CH3 by Feuerstein et al.15 and 7 Gairns et al.
increased to maximum values and then rapidly disappeared. In total, all of these carbon- and sulfur-containing species never accounted for more than 4% of the carbon input in black liquor solids. Carbon Mass Balance Closure. Table 4 contains the sum of the carbon measured as light gases and char in the LEFR experiments with liquor B in nitrogen. The char carbon data are reported in another paper.12 The tar species produced in these experiments were not measured. These data show that a large percentage of the carbon was not recovered in these experiments. The unrecovered carbon was most likely in the form of heavy tar compounds and soot. The carbon recovery as light gases and char carbon increased with residence times beyond 1 s at 1000 and 1100 °C, reaching 100% at the longest residence time (1.6 s) at 1100 °C, conditions where tar species would be most likely to decompose into light gases. These results are similar to those obtained earlier with the same experimental equipment.13 In other measurements with black liquor, we have consistently achieved a carbon balance closure of 95% or more by oxidizing all of the volatile carboncontaining species in the product gas to CO2 and measuring it.14 Comparison with Other Black Liquor Pyrolysis Experiments. Direct comparison of the present data with other pyrolysis data for black liquor is difficult because of the very different experimental methods employed and the limited carbon species data reported by others. Table 5 compares the gas species identified at 700 and 900 °C in this study with those reported at the same temperatures in three earlier studies. These temperatures were chosen for comparison because data from all four studies were available at 700 °C and from three of the four at 900 °C. Note the considerable differences between the heating rates and residence times used in these experiments and those performed by others. Where data are available, the values for each chemical species in Table 5, as a percent of carbon in black liquor solids, fall within an order of magnitude range at each temperature and often agree more closely. Most of the carbon was released during pyrolysis as CO and CO2, with CH4 being the predominant hydrocarbon species. Ethane was also reported at above 1% of the carbon in black liquor solids by Feuerstein et al.15 at
Figure 11. CO produced during different gasification conditions at 700 °C.
very low heating rates. It was not detected at any temperature in this study. There was no trend in species yields with heating rates. Gairns et al.7 reported only CO2 and sulfur species yields for gases. At 700 °C, their CO2 yields were in the same range as those in this study: 4-10% versus 6-8%. However, at 900 °C, they reported higher CO2 yields, 10-28% versus 9-11%. These values are all as a fraction of the carbon in black liquor solids converted to CO2. Bhattacharya et al.16 reported data on product gases composition for CO2, CO, H2, methane, and H2S in volume percent, not on the basis of carbon input. The CO/CO2 ratios in their product gases were 0.67-1.3 in the temperature range of 590-740 °C, with higher CO/ CO2 ratios at higher temperatures and longer residence times. This is slightly higher than the data from this study at the longest residence times and comparable temperatures. Their methane/CO ratios were 0.05-0.17, while ratios of 0.3-0.4 at the longest residence times and comparable temperatures were obtained from our measurements. Effect of the Liquor Composition and Gas Environment. A number of LEFR experiments with different black liquor solids and gas environments were conducted to study the impact of these variables on the transformation of product gases. The changes in CO formation using both liquors A and B are shown in
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Figure 12. CO produced during different gasification conditions at 800 °C.
Figures 11-14 for reactor temperatures of 700-1000 °C, respectively. The effect of 5 and 12 mol % water vapor of the secondary gas can also be seen from the same plots. At 700 and 800 °C, the gas environment had no detectable impact on the amount of CO produced (Figures 11 and 12). Production of CO was relatively slow at these temperatures, accounting for less than 5% of the carbon in black liquor in 1.7 and 1.3 s, respectively. At 900 °C (Figure 13), the CO produced increased more rapidly, to 18% of the carbon in black liquor by 1.7 s for the liquor B in N2. CO production was apparently faster initially (
