Ind. Eng. Chem. Res. 2002, 41, 3557-3565
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Hydrogasification of Almond Shell Chars. Influence of Operating Variables and Kinetic Study Juan F. Gonza´ lez,* Antonio Ramiro, Eduardo Sabio, Jose´ M. Encinar, and Carmen M. Gonza´ lez Departamento de Ingenierı´a Quı´mica y Energe´ tica, Universidad de Extremadura, 06071 Badajoz, Spain
The catalyzed hydrogasification of pyrolyzed almond shells was studied with the main objective of obtaining a gas with a high heating value (synthetic natural gas), as the major component of the gas is CH4 (with small amounts of CO). A kinetic study was also carried out using ideal models. Five experimental series were performed by varying particle size (0.4-2-mm diameter), initial sample weight (0.15-1 g), gas flow rate (150-300 cm3 min-1), temperature (700-900 °C), and hydrogen partial pressure (0.02-0.1 MPa). The range of particle size used in this work exerted little influence on the CH4 production. The hydrogasification rate was independent of the initial sample mass and hydrogen flow rate for values 250 cm3 min-1, respectively. The particle size exerted a slight negative influence on the hydrogasification rate, whereas temperature and hydrogen partial pressure exerted positive effects. The optimum temperature for the process was found to lie in the range 800-850 °C, giving a production of gas of 1.86 Nm3/kg of converted char (where Nm3 indicates m3 at normal conditions, T ) 0 °C and P ) 1 atm) with a high heating value of 74 170 kJ/kg of converted char. The activation energy and reaction order with respect to hydrogen were found to be 103.1 kJ mol-1 and 1, respectively. Introduction Natural gas and petroleum-derived hydrocarbons are currently used as raw materials in the manufacture of synthesis gas for the production of valuable chemicals (such as ammonia, methanol, methane, hydrocarbons, and alcohol) and energy fuels. However, because of the progressive depletion of deposits and increase in price of natural gas and crude oil, the technology for the conversion of renewable resources into valuable chemicals or fuels is becoming ever more important. Indeed, during the past few decades, wood and other biomass resources have begun to be utilized in the extraction of fuels and chemicals. The properties that make biomass interesting as a possible source of fuel are its renewability, its low sulfur and ash contents, and its excellent reactivity in thermal processes such as pyrolysis, gasification, and combustion. The hydrogasification of biomass has not been studied as extensively as steam or CO2 gasification, because the reaction occurs much more slowly than gasification in an oxidizing atmosphere. It requires higher hydrogen pressures to overcome thermodynamic limitations, and the reaction equilibrium is further limited as the temperature is increased.1 The utilization of a catalyst to increase the reaction rate allows the process to take place at atmospheric pressure, thus diminishing the required reaction temperature. However, some researchers have observed that the hydrogasification rate decreases sharply with increasing conversion of chars or coals1-4 and, therefore, that higher reaction temperatures are required to achieve complete conversion. These workers used temperatures in the range 600-900 °C and hydrogen pressures up to 3.3 MPa. Some researchers1 have concluded that the decline in rate is probably * To whom correspondence should be addressed. Phone: (34)924-289619. Fax: (34) 924-289601. E-mail: jfelixgg@ unex.es.
due to the strong adsorption of inactive hydrogen, which blocks active sites, as well as to the development of an unreactive surface structure. Therefore, the action of a catalyst and the surface of the reactive carbon species are two important factors in hydrogasification processes. Transition metals are very active hydrogasification catalysts, particularly Fe and Ni.5-7 Other researchers have utilized alkali carbonates in the hydrogasification of chars and coals, establishing that these materials are also very good catalysts for this reaction.1,3,8 The method used for the addition of the catalyst to the carbon can influence the rate of the process. Thus, ion-exchanged catalysts are known to act more effectively than impregnated catalysts in coal hydrogasification because of their better dispersion.9 The presence of oxygen functional groups seems to favor the gasification reaction. Some authors10 have concluded that the formation of methane is directly correlated to the desorption of oxygen complexes as CO. However, the usual woods and biomass contain few carboxyl groups as ion-exchangeable sites. The effectiveness of HNO3 oxidation and carboxymethylation as methods of increasing the carboxyl group content of wood has been described in the literature.6 Determination of the kinetics is one of the main objectives in studying biomass hydrogasification. Knowledge of kinetic parameters can be useful in mathematical modeling of the reactor and optimization of the process conditions. The design and construction of biomass gasifiers is dependent on this type of study. Although many papers related to coal reactivity have been published, few describe detailed studies on the kinetics.11-13 However, some authors have recently reported kinetic studies on the gasification process, using CO2, H2, steam, and a mixture of steam and hydrogen as gasification agents.14,15 The uncatalyzed carbon-hydrogen reaction has been extensively studied, and several postulated mechanisms and rate expres-
10.1021/ie010625l CCC: $22.00 © 2002 American Chemical Society Published on Web 06/22/2002
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Ind. Eng. Chem. Res., Vol. 41, No. 15, 2002
sions have been presented.2,16,17 However, this reaction is very slow at atmospheric pressure. Although the participation of a catalyst increases the reaction rate, as mentioned above, the study of the carbon-hydrogen reaction in the presence of a catalyst is more complicated. From a kinetic viewpoint, it is a triphase system (gas-solid reaction in the presence of a catalyst) in which the activity of the catalyst depends on numerous parameters such as the amount of catalyst; the manner of its addition or activation; and its active form, which can vary with the course of the reaction, the gasification conditions, the type of carbon species, etc. The hydrogasification process is interesting when a cheap source of hydrogen can be utilized. In the long term, the hydrogen required for this process can be produced by water electrolysis with electricity obtained from renewable sources. In the short term, the hydrogen can be obtained from hydrogen-rich gases that are byproducts of industrial processes. Some authors18 have reported than the hydrogen required in such processes can be produced within the process, e.g., by gasification of residual char from the hydrogasifier. Within this context, the present paper describes the influence of five variables (particle size, initial mass of sample, hydrogen flow rate, temperature, and hydrogen partial pressure) on the hydrogasification rate of almond shell chars catalyzed with a Ni salt [Ni(NO3)2‚6H2O], after the samples had previously been demineralized, HNO3 oxidized or carboxymethylated, and carbonized at 900 °C for 1 h. Almond shell residue is an abundant resource in the Extremadura Community (southwest Spain). A kinetic study of the process, applying ideal models, was also conducted to determine the activation energy and reaction order with respect to hydrogen. Experimental Section Sample Preparation. Almond shells were used as the raw material. Demineralization was performed by washing with 0.1 N HCl. Then, HNO3 oxidation or carboxymethylation was carried out to increase the ionexchange capacity of the sample. The oxidized sample was obtained by treatment with 6 N HNO3 at 40 °C for 1 h. The carboxymethylated sample was prepared as described by Nakano et al.19 (NaOH and ClCH2COOH dissolved in 80% of C2H5OH, heated for 2 h at 60 °C). The sample preparation included the following steps: Approximately 11 g of oxidized or carboxymethylated almond shells was placed in a glass column (length 50 cm, inner diameter 3 cm). Then, the following solutions were added consecutively: 50 mL of 0.1 N HCl, 300 mL of distilled water, 200 mL of 0.2 M Ni(NO3)2‚6H2O, and 50 mL of distilled water. The sample was dried at 106 °C for 24 h. Finally, the sample was pyrolyzed for 1 h at 900 °C, under a flow of 200 cm3 min-1 of nitrogen as the inert gas. The amount of Ni in the almond shell chars (after the pyrolysis at 900 °C with N2 gas and before the hydrogasification) was 2.36 wt %. Equipment and Analysis. The experimental setup was very similar to that used in previous works.20,21 Basically, it consisted of a cylindrical refractory stainless steel reactor with a heating system, an inlet for hydrogen or a hydrogen/nitrogen mixture, and an outlet for generated gases. The sample was placed in a basket that allows the gasificant agent to pass around the sample and was positioned in the heating zone of the reactor. All experiments were carried out isothermally. Initially, the sample was heated in an inert atmosphere (nitro-
Table 1. Characteristics of Almond Shells and Almond Shell Char proximate analysis (wt %) fixed volatile residue carbon matter ash almond 22.25 char 95.21
76.02 2.43
elemental analysis (wt %) C H N S
1.73 47.11 6.04 0.05 0.02 2.36 93.79 0.49 0.25 0.04
HHV (MJ kg-1) 18.1 27.8
gen), and once the desired temperature was reached, the nitrogen was replaced by hydrogen or a hydrogen/ nitrogen mixture to obtain the desired hydrogen partial pressure. This time was taken as the initial time of the reaction. The course of the reaction was followed by gas chromatography of the effluent gas every 15 min, using a 4000 HRGC KONIK gas chromatograph equipped with a thermal conductivity detector, a Carboxen-1000 column (15 ft in length, 1/8 in. in diameter), and an automatic injection valve. The duration of the reaction was 613 min. After this time, the hydrogen was replaced by nitrogen until the sample basket reached room temperature. The sample was then weighed to establish the mass balance with the generated gases. The carbon content in the sample basket after hydrogasification was exhausted in most of the experiments. The Ni in the ash was assayed by atomic absorption spectrometry at 232 nm. The adsorption isotherms of the chars with N2 at 77 K were obtained with an Autosorb-1 model Quantachrome analyzer, and FT-IR analysis of the chars was performed with a Perkin-Elemer 1720 FT-IR spectrometer using samples dispersed in KBr pellets. Results and Discussion Preliminary Observations. The proximate and elemental analysis results and higher heating values (HHVs) of the almond shells and almond shell char are given in Table 1. One sees that almond shells have a high content of volatile matter but low contents of ash, nitrogen, and sulfur. The char has a high content of fixed carbon; low contents of volatile matter, ash, nitrogen, and sulfur; and a longer HHV than the almond shells. A char was used in this work is because of our interest in studying the hydrogen-carbon reaction to produce CH4 and in establishing the mass balance to determine the carbon conversion. The main difference between the use of biomass or char as the raw material lies in the volatile matter of the biomass and the absence of pyrolytic gases in the case of char. In addition to hydrogen, steam and CO2 will react with the biomass char, resulting in faster reactions and higher carbon conversions. Using char as the raw material will most likely produce relatively more CH4. Hydrogasification of biomass will produce relatively less CH4 and more CO and CO2. The almond shell char hydrogasification was carried out in five experimental series with varying particle size (0.4-2-mm diameter), initial sample weight (0.15-1 g), gas flow rate (150-300 cm3 min-1), temperature (700900 °C), and hydrogen partial pressure (0.02-0.1 MPa). The studies of the first three variables allowed for the determination of the operating conditions for which the process is controlled by the chemical reaction. The studies of the temperature and hydrogen partial pressure allow one to obtain the activation energy of the hydrogasification process and the reaction order with respect to hydrogen, respectively.
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The Ni assays of all of the ash samples demonstrated that the ash contained only the Ni catalyst. Therefore, the initial demineralization process was adequate, and the catalytic effect was due only to the presence of Ni and not to any original mineral components of the sample. Some workers1 have also observed that the slight increase in reactivity of demineralized char might result from an additional surface area of char exposed by the removal of minerals or by surface oxidation with HNO3. Others22,23 have observed deactivation of Ni and Fe in the presence of silica or silicates (the main components of ashes of lignocellulosic materials) in studies of the hydrogasification of wood and bark chars. Two hydrogasification experiments were performed at 900 °C to study the influence of the oxidation method. The results showed that HNO3 oxidation was more effective than carboxymethylation. Also, a hydrogasification experiment was performed with uncatalyzed oxidized demineralized char at a temperature of 900 °C and a hydrogen partial pressure of 0.1 MPa with a flow rate of 250 cm3 min-1, but the char conversion obtained was zero. Therefore, the hydrogasification of almond shell chars at atmospheric pressure must be catalyzed. All of the samples were prepared in a similar form, as mentioned above. The catalyst loading was optimized in previous experiments by varying the volume (100300 mL) of the solution of 0.2 M Ni(NO3)2‚6H2O utilized to add the catalyst to the sample. The Ni loading was varied in the range of approximately 1.17-3.52 wt %. The best results were obtained for a volume of 200 mL, which corresponds to a Ni loading of 2.36 wt %. Methane was the main product formed during the process. The conversion of carbon into methane agreed with the conversion measured by the weight loss of the specimen to within 2%. Small amounts of CO were detected during the experiments and could be ascribed to decarboxylations produced in the char. The reaction involved, therefore, was
C + 2H2 S CH4
∆H° ) -74.77 kJ mol-1
(1)
The conversion of carbon can be defined as
X)
m m0 - m C
(2)
where m is the mass of carbon converted to CH4 and CO at time t, m0 is the initial mass of carbon, and mC is the mass of the ash. Three series of experiments, with varying particle size, initial mass of sample, and flow rate of hydrogen, were performed at 900 °C (maximum experimental temperature), as mentioned above, to determine whether heat or mass transport affected the hydrogasification rate and to delimit the range of operating conditions under which the reaction is the rate-controlling step. Only under such conditions can the measured rate be directly used to study the hydrogasification kinetics.24,25 Specifically, intraparticle transport is affected by particle size, particle-to-fluid transport depends on the hydrogen flow rate and particle size, and interparticle transport depends on the number of layers of particles in the sample basket. For a fixed particle size and for the basket that we used, the last process (interparticle transport) can be studied by varying initial sample weight.26
Table 2. Characteristic Parameters of ProcesssInfluence of Initial Mass of Samplea initial mass of sample (g) parameter final conversion, % Nm3 of CH4/kg of converted char Nm3 of CO/kg of converted char HHV of gases,kJ/kg of converted char
0.15
0.25
0.5
0.75
1
94 1.71
95 1.73
81 1.58
64 1.46
58 1.31
0.17
0.16
0.14
0.11
0.10
70 241 70 652 64 836 59 507 53 422
a General conditions of experiments: particle size ) 0.63-1 mm, hydrogen flow rate ) 200 cm3 min-1, temperature ) 900 °C, partial pressure of H2 ) 0.1 MPa, added Ni ) 2.36 wt %.
Effect of Particle Size. The particle size exerted little influence on the hydrogasification rate in the range that we examined. The evolution of the conversion and accumulated number of moles of CH4 for this experimental series were essentially the same for the four particle sizes used. The differences might be due to the uneven dispersion of Ni on the carbon or to experimental errors, as the observed trend did not appear to satisfy any logical criterion. Other characteristic parameters of the process, such as the final conversion, CH4 production [in units of meter cubed of CH4 at normal conditions (T ) 0 °C, p ) 1 atm) per kilogram of converted char] and HHV of the gases were also very similar for the four particle sizes used. The particle sizes were small, so that the possible temperature gradients in the particles would also be very small. Limitations of intraparticle transport must therefore have been negligible. The particle size chosen to study the rest of the variables was 0.63-1 mm. Other researchers have utilized particle sizes of 0.25-1-mm diameter to study this type of process.3,14,15,27,28 Effect of Initial Sample Mass. The initial sample mass had certain influence on the hydrogasification rate. A decrease in the initial sample mass exerted a positive effect on the conversion up to a point at which the conversion became independent of any further decrease in initial sample mass. An initial mass
