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Farzaneh, A., Richards, T., Sklavounos, E., and van Heiningen, A. (2014). "A kinetic study of CO2 and steam gasification of char from lignin produced in the SEW process," BioRes. 9(2), 3052-3063.

Abstract

The reaction kinetics of gasification are important for the design of gasifiers using biomass feedstocks, such as lignin, produced in biorefinery processes. Condensed and uncondensed lignin samples used in the present study were prepared using the SEW (SO2-ethanol-water) fractionation process applied to spruce wood chips: the dissolved lignin is precipitated during the recovery of SO2 and ethanol from the spent fractionation liquor. The gasification of char made from condensed and uncondensed SEW lignin was investigated using thermogravimetric analysis (TGA) at atmospheric pressure using either CO2 or steam. The main aim of this study was to quantify the reaction rate during the gasification process, which was found to be best described as zero-order. All experiments were performed at constant temperatures between 700 and 1050 °C to obtain the necessary information for describing the reaction rate equation in an Arrhenius form; the heating rate was 20 °C/min for both samples. The experiments led to almost similar results for both samples. The activation energies of CO2 gasification were approximately 160 kJ/mol and 170 kJ/mol for uncondensed and condensed lignin char, respectively. The activation energies of steam gasification were approximately 90 kJ/mol and 100 kJ/mol for uncondensed and condensed lignin char, respectively.


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A Kinetic Study of CO2 and Steam Gasification of Char from Lignin Produced in the SEW Process

Amirfarrokh Farzaneh,a Tobias Richards,a,* Evangelos Sklavounos,band Adriaan van Heiningen b

The reaction kinetics of gasification are important for the design of gasifiers using biomass feedstocks, such as lignin, produced in biorefinery processes. Condensed and uncondensed lignin samples used in the present study were prepared using the SEW (SO2-ethanol-water) fractionation process applied to spruce wood chips: the dissolved lignin is precipitated during the recovery of SO2 and ethanol from the spent fractionation liquor. The gasification of char made from condensed and uncondensed SEW lignin was investigated using thermogravimetric analysis (TGA) at atmospheric pressure using either CO2 or steam. The main aim of this study was to quantify the reaction rate during the gasification process, which was found to be best described as zero-order. All experiments were performed at constant temperatures between 700 and 1050 °C to obtain the necessary information for describing the reaction rate equation in an Arrhenius form; the heating rate was 20 °C/min for both samples. The experiments led to almost similar results for both samples. The activation energies of CO2 gasification were approximately 160 kJ/mol and 170 kJ/mol for uncondensed and condensed lignin char, respectively. The activation energies of steam gasification were approximately 90 kJ/mol and 100 kJ/mol for uncondensed and condensed lignin char, respectively.

Keywords: Sulfur dioxide-ethanol-water (SEW) fractionation; Kinetics; CO2 and steam gasification; Lignin; Thermogravimetric analysis

Contact information: a: School of Engineering, University of Borås, SE 501 90 Borås, Sweden; b: School of Chemical Technology, Aalto University, FI-02150, Espoo, Finland;

* Corresponding author: tobias.richards@hb.se

INTRODUCTION

The continuing increase in the world’s population and the subsequent increase in energy consumption per capita have resulted in a dramatic increase in the consumption of fossil fuel during the past few decades. Unfortunately, this has an adverse effect on the environment, with global climate change being of particular concern (Lv et al.2010). Woody biomass, a globally available feedstock, holds great potential as an important raw material for biofuels and renewable chemicals in an integrated forest products biorefinery (IFBR) (van Heiningen 2006). Lignocellulosic biomass is renewable and does not directly compete with food production, so its sustainable utilization would reduce the emission of greenhouse gases and the depletion of natural resources (Di Blasi 2009).

This has resulted in extensive research being undertaken to develop economically feasible methods of producing renewable energy and biofuels from biomass feedstock. Forest biomass is one of the most viable resources for producing biofuels. One promising process developed as part of the “Bioforest Project” at Aalto University in Finland uses an SO2-ethanol-water (SEW) fractionation method to produce cellulosic fibers and spent liquor with a high concentration of hemicellulose mono-sugars. The objective of this project is to produce butanol from the hemicellulose mono-sugars when they are combined with the glucose obtained by enzymatic hydrolysis of the fibers using ABE (acetone-butanol-ethanol) fermentation. The SEW liquor is subjected to a conditioning process to remove compounds that inhibit ABE fermentation (Sklavounos et al. 2011) and to recover the fractionation chemicals (SO2 and ethanol). The ABE process utilizes Clostridia bacteria to ferment all of the dissolved wood sugars to produce a mixture of acetone/isopropanol, butanol, and ethanol (Iakovlev et al. 2011; Rakkolainen et al. 2010).

The conditioning process consists of a number of steps (Iakovlev et al. 2011; Sklavounos et al. 2011) during which lignin is precipitated from the SEW liquor in two different forms, condensed and uncondensed, depending on the fractionation conditions. This paper investigates the reaction kinetics of these lignin fractions during CO2and steam gasification at atmospheric pressure.

Understanding the kinetics of gasification is necessary for selecting the process and the design of the gasifiers (Mani et al. 2011; Zhang et al. 2010). Several studies dealing with the kinetics of CO2 and steam gasification have been reported for chars derived from biomass materials (Khalil et al. 2008; Mani et al. 2011; Zhang et al. 2010).

The gasification of carbonized char from biomass produces “syngas,” a valuable raw material used in several chemical industries; however, the use of this feedstock has not yet been implemented on an industrial scale (Ollero et al. 2002). In this study, the reaction kinetics of CO2 and steam gasification of condensed and uncondensed lignin, derived from spruce wood chips using the SEW process, were investigated using a thermogravimetric apparatus (TGA).

CO2 Gasification

Mechanism

Several mechanisms have been described for the Boudouard reaction. The Ergun mechanism describes the reaction as a two-step process (Hargrave et al. 1988):

where Cf and C(O) represent a free carbon active site and carbon-oxygen complex, respectively. Initially, carbon dioxide is reduced to carbon monoxide, forming an oxidized surface complex. In the next step, the C(O) complex is released as carbon monoxide and a new free carbon active site is formed. Assuming that the sum of Cf and C(O) is constant during the reaction, a steady state is reached, and the CO desorption reaction is rate-determining (Ergun 1956), the following reaction kinetic expression can be derived:

 (1)

where rc,CO2 is the carbon converted from the solid to the gas phase, k2 is the forward reaction rate in Step 2, Ct is the total number of active sites (free and occupied), K1 is the equilibrium constant of Step 1 (equal to k1/k-1, where k1 and k-1 are the reaction rates of the forward and backward reactions in Step 1, respectively), and p is the partial pressure of either CO2 or CO.

Steam Gasification

Mechanism

The mechanism of the steam gasification reaction is basically similar to the Boudouard reaction. This reaction also proceeds in two steps:

In the first step, hydrogen is released and a carbon-oxygen complex is formed by a water molecule attacking a free active carbon site. In the next step, similar to the Ergun mechanism of the Boudouard reaction, the C(O) complex decomposes to produce carbon monoxide and a new free carbon active site.

Step 2, the decomposition of the carbon-oxygen complex, is the rate-limiting step of the overall reaction, although the effect of this step on the reaction rate may be neglected at high temperatures. In addition, the concentration of product gases is small, and the inhibiting effect is negligible. Using the same assumptions as for CO2 gasification, the following reaction kinetic expression can be derived:

 (2)

where rc,H2O is the carbon converted from the solid to the gas phase, k4 is the forward reaction rate in Step 2, Ct is the total number of active sites (free and occupied), K3 is the equilibrium constant of Step 1 (equal to k3/k-3, where k3 and k-3 are the reaction rates of the forward and backward reactions in Step 1, respectively), and p is the partial pressure of either H2O or H2.

EXPERIMENTAL

Raw Material

Both lignin samples were produced at Aalto University in Helsinki, Finland. Air-dried screened spruce chips were used as the raw material for SEW fractionation. A HAATO 43427 silicon oil bath containing six 220-mL bombs, each of which was filled with 25 g of spruce chips (oven dry), was used for SEW fractionation. The fresh liquor was prepared by bubbling SO2 through a 55% (v/v) ethanol-water solution cooled in an ice bath at a low flow rate to dissolve the set amount of either 3% (w/w) or 12% (w/w) SO2, measured continuously on an electronic balance.

Two different pulping conditions were used, depending on the SO2charge in the fresh liquor. For the 3% (w/w) SO2 concentration, the pulping conditions used were as follows: a liquor-to-wood ratio of 6 L kg-1, a pulping temperature of 150 oC (±1 oC), and a total pulping time of 120 min, which included the heating-up period. For the 12% (w/w) SO2 concentration, the pulping conditions employed were as follows: a liquor-to-wood ratio of 3 L kg-1, a pulping temperature of 150 oC (±1 oC), and a total pulping time of only 30 min, which included the heating-up period. SEW spent liquor was collected by squeezing the pulp suspension contents in a washing sock. In the case of 12% (w/w) SO2 spent liquor, the pulp, after removal of the SEW spent liquor, was washed twice with 40% ethanol-water at 60 oC and three times with deionized water at room temperature. The wash liquor was then added to the SEW liquor (hereafter known as the MSEW liquor). The 3% (w/w) SO2-SEW spent liquor and the MSEW liquor were then treated in a rotovap evaporator to remove ethanol and SO2. The temperature of the water bath was 95 oC, and the vacuum pressure was 300 mbar.

About 65% of the original weight of the liquors was evaporated. The liquor treated in the rotovap (hereafter known as EVAP) contained condensed lignin (a char-like solid) when the level of SO2 in the cooking liquor was set to 3% (w/w). However, uncondensed lignin was formed in the EVAP liquor when the SO2 in the cooking liquor was set to 12% (w/w). The latter lignin had the form of a precipitate as a result of its higher degree of sulfonation, which was removed from the EVAP liquor by centrifugation. The values of ultimate and proximate analyses for the two samples of lignin are given in Table 1.

Table 1. Ultimate and Proximate Analyses of Samples

The higher carbon content of the condensed lignin is indicative of more carbon-carbon bonds being formed by condensation reactions.

Raw Material Preparation

A condensed lignin sample with an average dry matter content of 58.6% and an uncondensed lignin sample with an average dry matter content of 39.2% were used to prepare the char. After drying for 24 h at 105 oC, the lignin samples were ground to obtain a fine powder composed of homogeneous particles. The samples were ground separately to omit the effect of particle size on the reactivity of the material (Di Blasi 2009). The samples were thereafter dried once more for 24 h at 105 oC to remove the moisture absorbed during the grinding process.

Experimental Equipment

The gravimetric measurement was performed in a DynTHERM HP analyzer manufactured by Rubotherm GmbH. A sample was loaded in a small ceramic crucible (diameter and height of 9 mm) on a balance with a resolution of 1.0 μg in an environment with a controlled temperature. All of the gasification experiments were performed isothermally. The sample was heated up to the particular temperature of the reaction in a nitrogen atmosphere before the inert gas was replaced by the gasifying media. The mass loss of the sample was recorded during the reaction time. Only a small amount of sample (0.1 g) was used (Higman and van der Burgt 2008) to avoid significant differences in temperature between the TGA set point and the sample caused by the high endothermic reaction heat of CO2gasification (+172 kJ/mol) and steam gasification (+131 kJ/mol).

Pyrolysis

Pyrolysis of the samples was conducted using the TGA. In each test 0.10+0.01 g of dried powder was placed into a small ceramic crucible inside the reactor under a flowing N2 atmosphere (100 mL/min) and was heated up to the reaction temperature at a heating rate of 20 °C/min at atmospheric pressure. This relatively slow rate of heating was adopted to eliminate any atypical heat transfer problems (Khalil et al. 2008). Each sample was kept at the particular temperature for 20 min to ensure complete devolatilization (Zou et al. 2007). The final pyrolysis temperature was the same as for the isothermal gasification temperature in question for all the experiments and exceeded 700 °C.

CO2 Gasification Process

CO2 gasification of the pyrolyzed samples was performed isothermally at three different temperatures, ranging from 850 to 1050 °C (lower temperatures gave very slow reaction rates and were excluded). Carbon dioxide was used as the gasifying media at a flow rate of 50 mL/min and was mixed with the same flow rate of nitrogen gas. Samples from each experiment were kept at the final temperature for 50 min; all experiments were duplicated.

Steam Gasification Process

Almost the same procedure was employed for steam gasification. The steam gasification process was performed isothermally at temperatures between 700 °C and 1000 °C for both of the pyrolyzed samples, with the steam being supplied from an evaporator operating at 200 °C. Distilled water at a flow rate of 0.06 mL/min was injected into the evaporator by an HPLC water-pump; the steam produced was mixed with 50 mL/min nitrogen gas before being fed into the reactor.

RESULTS AND DISCUSSION

Gasification Kinetics

Rate of reaction

Ignoring the effect of desorption of the carbon-oxygen complex, both reactions can be assumed to be zero-order reactions, according to the experimental data for carbon conversion (Figs. 1 through 4), which shows a constant relationship between time and the rate of carbon conversion (Barkia et al. 2004; Lee 1993; Rao and Jalan 1972). This is consistent with the kinetic expressions derived (Eqs. 1 and 2) when the partial pressure of the gasifying media (CO2 or H2O) is much higher than that of the intermediate products (CO or H2). In this particular case, the gasifying media are being constantly supplied to the reaction chamber and the gases that are produced are being constantly removed.

Fig. 1. The amount of remaining char produced from condensed lignin at various temperatures during CO2 gasification

Fig. 2. The amount of remaining char produced from uncondensed lignin at various temperatures during CO2 gasification

Fig. 3. The amount of remaining char produced from condensed lignin at various temperatures during steam gasification

Fig. 4. The amount of remaining char produced from uncondensed lignin at various temperatures during steam gasification. A line representing the average mass loss rate is added at each temperature.

For both reactions, the reaction rate can be described by the rate of char conversion (Eq. 3), where c is the reactive fraction of char:

In Eq. 3, m is the mass and subscript c is char, i is inert (not reacting mainly inorganics), and 0 is initial weight. By neglecting the effect of concentration of products in relation to the concentration of the reacting gases (either CO2 or H2O), the reaction rate is constant.

The number of sites, CT, is related to the surface area (sites accessible for the gas) and can, in this particular case, be assumed constant (which was used to derive Eqs. 1 and 2). The reason for the assumption is the fact that the pyrolyzed char has the shape of a cylinder, where the top is directed towards the free gas and the sides are facing the walls of the sample basket. It is therefore likely that most of the reaction will occur from the top and downwards with equal cross sectional area all the time. This is also consistent with the constant reaction rate found in the experiments. Dependency of k on temperature can, at atmospheric pressure, be described in the Arrhenius form according to Eq. 5:

where Ac is the cross sectional area of the char, η is number of active sites per area unit which depends on the previous conditions, A is the pre-exponential factor, E is the activation energy, and R is the universal gas constant. The equation can be expressed alternatively as:

Based on Eq. (6), the reactivities of chars derived from condensed and uncondensed lignin were plotted against 1/T in Fig. 5 for CO2gasification and in Fig. 6 for steam gasification. These figures confirm that the relationship between k and T follows the Arrhenius expression, thereby allowing the activation energy (E) and the apparent pre-exponential factor (A’=A·η) to be determined for both materials. Reactivity parameters for the gasification step for pyrolyzed char from both lignins are shown in Table 2.

Table 2. The Activation Energy (E) and Apparent Pre-exponential Factor (A’) of CO2 and Steam Gasification

Fig. 5. An Arrhenius plot for CO2 gasification of char derived from condensed and uncondensed lignin

Fig. 6. An Arrhenius plot for steam gasification of char derived from condensed and uncondensed lignin

With respect to the char conversion, the reaction order was found to be zero, which is similar to results calculated by previous studies on other biomass chars (Di Blasi 2009; Mani et al. 2011; Wang and Kinoshita 1993). The activation energies for CO2 gasification (94 to 112 kJ/mol) and steam gasification (86 to 100 kJ/mol) were, on the other hand, relatively low in comparison with other biomass chars. For instance, the activation energy of COgasification of biomass materials has been reported to be in the range of 200 to 300 kJ/mol, and that of steam gasification has been reported to be between 170 and 250 kJ/mol (Di Blasi 2009; Klose and Wölki 2005; Wang and Kinoshita 1993). However, the values obtained are close to the activation energy of the lignin materials: Iwaki et al. (2004), for example, reported activation energies of 100 to 120 kJ/mol for steam gasification. The lower activation energy is probably due to the large amount of alkali metals present in the materials, which catalyze the gasification process (Li and van Heiningen 1990).

Effect of temperature

Temperature affects both the rate constants and the duration of the gasification reaction when biomass char is gasified. Figures 2 through 5 show the carbon conversion obtained at temperatures between 850 and 1050 °C for condensed and uncondensed lignin material. 70% was converted during 50 min at 850 °C, and at 950 and 1050 °C, complete conversion was achieved when CO2 was used as gasifying media. For steam, the conversion, especially at the lower temperatures, was faster and there was a conversion of between 20 and 30% already at 700 °C (for CO2 gasification this number was below 15% at 750 °C).

CONCLUSIONS

  1. The CO2 and steam gasification reactions followed a zero-order reaction rate, based on the carbon conversion and constant partial pressure of the gasifying media. Taking the Ergun mechanism into account, the reaction rate equation was determined to be an Arrhenius expression.
  2. The maximum char conversion was achieved for both samples at the highest temperature (1050 and 1000 °C for condensed and uncondensed lignin, respectively). Activation energies in the range of 85 to 115 kJ/mol were determined.
  3. The values calculated in this study are comparable to reaction rates determined by other researchers for the gasification of biomass chars and lignin materials at similar temperatures.

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Article submitted: September 12, 2013; Peer review completed: November 24, 2013; Revised version received and accepted: April 4, 2014; Published: April 10, 2014.