31st European Biomass Conference and Exhibition, 5-8 June 2023, Bologna, Italy PREDICTION OF TOTAL PRODUCT COMPOSITION FROM PYROLYSIS AND GASIFICATION OF LIGNOCELLULOSIC BIOMASS: A MODEL FOR REACTOR DESIGN AND OPTIMIZATION Richard Ochieng1, Alejandro L. Ceron2, Alar Konist2 and Shiplu Sarker1 of Civil and Manufacturing Engineering, Faculty of Engineering, Norwegian University of Science and Technology, NTNU, Teknologivegen 22,2815 Gjøvik, Norway 2Department of Energy Technology, Tallin University of Technology, TalTech, Ehitajate tee 5,12616 Tallin, Estonia 1Department ABSTRACT: As far as thermochemical biorefineries are concerned, the development of a gasification process model capable of accurately predicting the total composition of non-condensable gases, char, and tar remains an unsolved problem. This paper presents a detailed kinetic mechanism for predicting the total product yield and composition of slow pyrolysis and steam gasification. The detailed kinetic mechanism in the study extends the previously proposed multi-step kinetic mechanism of Faravelli and co-authors to include extractives, secondary reactions for thermal cracking, and steam reformation of primary pyrolysis products. The model has been implemented in Aspen Plus® and validated using experimental data obtained from the slow pyrolysis and steam gasification (Steam/Biomass ~ 0.21) of pine wood in a fixed-bed batch reactor at temperatures of 750, 850, and 930 oC. A comparison of the gasification model with experimental data revealed average absolute errors of 17.8 percent for char yield and 17.3 percent for gas composition. While detailed experimental data is necessary to verify tar formation, the model can be useful for studying pyrolysis and gasification processes in terms of their operating parameters, especially temperatures and solid and vapor residence times. Keywords: Lignocellulosic biomass, pyrolysis, gasification, kinetic modeling, Aspen Plus® 1 INTRODUCTION Despite the necessity to understand the effect of reactor operating and design parameters on tar formation, numerous research studies have often neglected or ignored the effect of tar formation in gasification modeling. Instead, researchers assume the complete conversion of tar into light hydrocarbon components via non-conventional biomass simulations [5,11]. Furthermore, previous studies have also demonstrated that non-conventional biomass considerations usually lead to pyrolysis and/or gasification models with an unacceptable degree of accuracy [12]. At the same time, previous studies have identified kinetic modeling of pyrolysis and gasification processes as the most reliable and effective approach to the design and optimization of reactor performance [5,12,13]. Although kinetic analysis has the potential to offer valuable insight into the behavior of bio-oils and tars, the limited availability of relevant kinetic data, specifically for cracking tars, poses a challenge. According to some researchers, it is possible to predict the composition of tar components based on assumptions and lumped representations of the kinetics [14-16]. However, despite the model simplicity, validation of results can unrealistic due to the use a lumped set of kinetic parameters. Moreover, studies that combine that use kinetic models to predict both devolatilization and gasification of biomasses are not frequently published [17]. By using kinetic models to evaluate both the devolatilization and gasification steps, more realistic insight into the various factors that influence gasifier design and optimization can be gained [18]. In order to predict the detailed product distribution of biomass gasification, this work extends the multi-step kinetic mechanism of biomass devolatilization, which was first proposed by Faravelli and co-authors, to include the decomposition of extractives and thermal and steam cracking reactions of primary pyrolysis products. The kinetic model was implemented in Aspen Plus® and validated using experimental data obtained from the slow pyrolysis and steam gasification of pine wood in a fixedbed batch reactor at various temperatures. The results provide valuable insights into the effect of reactor operating conditions on product distribution. In the context of the circular economy, biorefineries present an attractive, and sustainable way to produce various renewable products from biomass. In the literature, a variety of biochemical and thermochemical methods have been investigated for the production of biofuels and bioproducts. In contrast to biochemical conversion techniques that use slow-acting microbes, thermochemical conversion technologies can quickly convert all types of dry biomass into valuable products [1]. As an intermediate process in thermochemical methods, pyrolysis stands out as the most significant step in biomass conversion via gasification and combustion processes. In light of this, the application of a two-stage approach involving biomass devolatilization and secondary cracking reactions is of primary importance since the pre-step can significantly influence the final product [2]. While significant progress has been made in biorefinery research, commercialization remains a challenge due to technological immaturity [3]. While process engineering techniques present the necessary tools to synthesize, optimize, and up-scale technologies in biorefineries, more research is still needed to fully understand and predict the complex mechanisms of biomass conversion at the reactor level [4]. According to Mutlu and Zeng [5], the absence of kinetic models to simulate tar formation is one of the drawbacks limiting the complete prediction of biomass gasification products in the reactor. Tar is considered a very complex material, whose composition depends on many parameters such as temperature, type of gasifying agent, and reactor design [6,7]. As part of gasification products, tar is produced by the decomposition, oxidation, and polymerization of different biomass devolatilization products at high temperatures [8, 9]. In addition to temperature, the yield and composition of tar in gasification products are also highly dependent on the nature of the biomass feedstock and other reactor parameters, such as the solid and vapor residence times of the reactor contents [10]. 959 31st European Biomass Conference and Exhibition, 5-8 June 2023, Bologna, Italy 2 MATERIAL AND METHODS the unreacted solid components of biomass, i.e., hemicellulose, cellulose, lignin, and extractives. While the batch-type reactor in Aspen Plus® has been recommended for a slow pyrolysis process [25], the continuous-type (RCSTR) has been used in this study due to constant temperature conditions during the experiments. For the thermodynamic properties of conventional components in the model, the Peng-Robinson Equation of State with Boston-Mathias (PR-BM) modification was used. The thermophysical properties of components including; 3-hydroxypropyl, para-coumaryl alcohol, sinapyl aldehyde, xylose, Lignin (Lig, Lig-C, Lig-CC, Lig-O, Lig-OH), extractives (Tannin and triglyceride), cellulose and activated cellulose, and hemicellulose components were estimated using an approach proposed by Gorensek et al. [26]. 2.1 Ultimate and proximate analysis In this study, pinewood elements and proximate compositions were analyzed. A description of the equipment used for elemental and proximate analysis can be found in ref. [19, 20]. The biochemical composition of pine wood considered in this study was obtained from the literature. 2.2 Experimental setup The slow pyrolysis and steam gasification experiments were performed in a laboratory-scale fixed bed reactor, described in more detail in ref. [19, 20]. The sweeping gas flow (argon) was maintained at 100 mL/min throughout the experiment, and a steam-tobiomass ratio of 0.21 was used to study steam gasification at all temperatures (750, 850, and 930 oC). To ensure reproducibility, the experiments were performed at least three times for each temperature. As shown in Fig. 1, an FTIR was used to determine the solid residence time, and 8 minutes was deemed sufficient to complete devolatilization. The vapor residence time was estimated from the carrier gas flow rate and the reactor volume. 3 RESULTS AND DISCUSSION 3.1 Biomass characterization The ultimate, proximate, and biochemical compositions of pine wood (Pinus sylvestris) used in this study are shown in Table I. The biochemical composition of pine wood (Pinus sylvestris) was obtained from the literature [27]. The composition of lignin components (Lig-C, Lig-H, and Lig-O), tannin (TANN), and triglycerides (TGL) was estimated from the lignin and extractive weight fraction using the approach proposed by Faravelli et al. [28], and Debiagi et al. [22], respectively. Using Debiagi's method to estimate the total biochemical composition was not possible due to the overestimation of the cellulose weight fraction. In Table II, we include the estimated composition of lignin and extractive components. Figure 1: Effect of solid residence time on the total organic carbon (TOC) of pine wood. Table I: Ultimate and proximate analysis of pine woods. To investigate the secondary reactions, the vapor residence time was obtained from the sweep gas flow and the volume of the quartz tube reactor (diameter 1 inch and length 24 inches). As the biomass sample was placed halfway inside the reactor tube, only the first half of the tube was considered as the secondary reaction zone. Therefore, at a sweep gas flow rate of 100 mL/min, the vapor residence time was estimated to be 40 seconds. The pyrolytic gas was collected in Tedlar bags as it exited the batch reactor. The volumetric concentration of combustible gases H2, CO, and CH4 was measured using gas chromatography with a thermoconductivity detector (GC-TCD) and a Gazohrom 3101 gas analyzer. Each experiment and/or gas analysis was repeated to ensure reproducibility of the results. Proximate analysis (wt.%) Ultimate analysis (wt.%) Biochemical analysis (wt.%) [27] Moisture Volatile matter Fixed carbon Ash C H N S O Cellulose Hemicellulose Lignin Extractives 8.5 85.2 14.5 0.3 50.1 6.6 0.19 n.d. 43.1 41.0 25.7 28.6 4.8 Table II: Lignin and extractives components (wt.%) Lig-C 16.60 2.3 Modeling in Aspen Plus® In this study, drying was modeled based on the kinetics of the devolatilization of water (moisture) loosely bound in the biomass material. A multistep kinetic scheme originally proposed by Faravelli et al. [21] was modified to accommodate extractives [22], and secondary reactions for tar cracking and steam gasification [23, 24] (see support information). In the kinetic model, char was described as carbon and ash. However, in this work, the composition of char after the reaction was assumed to consist of carbon, ash, and Lig-O 2.14 Lig- H 9.75 TANN 3.36 TGL 1.44 3.2 Slow pyrolysis Pyrolysis and gasification have three product distributions: char, non-condensable vapors, and liquid or condensable vapors (tar). The study was divided into two parts: i) a two-stage process to slow pyrolysis of wood biomass at a low sweep gas flow rate (100 mL/min), so secondary tar reactions could be investigated; ii) a twostage process to predict steam gasification product yields 960 31st European Biomass Conference and Exhibition, 5-8 June 2023, Bologna, Italy and composition, as shown in Fig.2. For investigating the slow pyrolysis and thermal cracking, the steam flow rate was zero. Due to the fact that secondary pyrolysis becomes more dominant at temperatures above 600 oC [29], the model considered a primary devolatilization temperature of 500 oC and a solid residence time of 8 minutes. The secondary reactions have been modeled in the second reactor at vapor residence times of 40 seconds. ethane as intermediate products from the thermal decomposition of the tar. B. Char yield As shown in Figure 4, the model's predicted char yield closely matched the experimental data. However, the char yield obtained from experimental data was slightly lower than that predicted by the model. A mean absolute error of 14.1% was obtained. The difference in Figure 2: Process flow scheme in Aspen Plus® char amounts could be attributed to losses during the removal of the char sample from the reactor. A. Gas composition As illustrated in Fig.3, the predicted hydrogen composition increased from 25 to 31%, while that of carbon monoxide slightly increased from 40 to 42.5% between 750 and 930 oC. There was a slight increase in CO due to the thermal cracking of carboxyl and carbonyl groups in cellulose, which decompose below 750 °C [30]. Figure 4: Comparison of the char yield (wt.%) from slow pyrolysis model with experimental data. C. Effect of temperature on product distribution From the simulation model, primary pyrolysis (noncracking) yields at 500 oC yielded 12.27 wt.% gas, 48.15 wt.% tar, 12.81 wt.% water, and 26.7 wt.% char. The primary pyrolysis liquid products consisted of aromatic and aliphatic compounds, aldehydes, alcohols, levoglucosan, phenols, other hydrocarbon compounds, and water [21]. Under thermal cracking conditions (750 to 930 oC), the char yield decreased as the gas yield increased, as shown in Table III. However, the tar yield changed only slightly. Perhaps, under the model conditions, the increase in gas yield was mainly due to the heterogeneous reaction of the pyrolytic products with char [23]. At lower temperatures (500–700 oC), previous studies have shown that the gas fraction increased at the expense of the liquid fraction [10]. At higher temperatures, further investigations have revealed that vapor residence time is the controlling Figure 3: Comparison of the char yield (wt.%) from slow pyrolysis model with experimental data. The model and experimental results obtained in this study were in close agreement with other studies in the literature [31]. The mean absolute error between the model and experimental results was 21.53%. The model predicted a slightly higher H2 composition (26.5 %) than the experiment (21 %) at 750 oC. The model prediction for CO concentration was about 14% lower than that observed in the experiment. The CO composition predicted by the model is within the range of that reported by Tao et al.[31]. However, the model estimated a low methane concentration at all temperatures when compared to the experimental data in this study and that of Tao et al. [31]. It is possible that the low methane yield in the model is caused by the inclusion of ethylene and 961 31st European Biomass Conference and Exhibition, 5-8 June 2023, Bologna, Italy factor during thermal cracking. A detailed analysis of the effect of vapor residence time was discussed in the subsequent subsection. thermal cracking is not a major contributor to char yield in the process. Furthermore, the simulation results revealed that within 1 second, the gas yield increased by over 112 percent at the expense of tar thermal cracking, as shown in Figures 5 and 6. Table III: Effect of temperature on the gas, tar, and char yield from the model. Weight percent 750 oC 850 oC 930 oC Gas 57.7 64.1 65.5 Tar 5.7 5.8 5.8 Char 22.3 15.7 14.2 Water 14.2 14.3 14.4 The total product composition after thermal cracking is shown in Table IV. Figure 5: Effect of vapor residence time on the gas yield Table IV: Product composition of slow pyrolysis. The tar decomposition reactions became even faster as the temperature increased from 750 to 930 oC. These findings indicated that the thermal cracking of unstable hydrocarbons in tar occurs rapidly at higher temperatures. The char yield increased by less than 1.0 % within 1 second of the vapor residence time, as shown in Figure 7. mg/g (biomass) 750 oC 850 oC 930 oC H2 14.5 17.69 18.77 CO 305.9 353.7 367.66 CO2 140.7 136.0 134.28 CH4 21.4 29.50 30.18 C2H4 88.6 94.71 94.67 Methanol 34.2 34.49 34.57 Formaldehyde 0.011 0.003 0.002 Acetaldehyde 12.19 12.41 12.50 5-Hydroxymethylfurfural 0.011 0.004 0.002 Benzene 1.41 2.14 2.22 Toluene 0.000 0.000 0.000 Naphthalene 6.16 9.36 9.74 Cyclopentadiene 0.001 0.011 0.048 H2O 142.45 143.47 143.84 Char 223.30 157.30 142.40 Figure 6: Effect of vapor residence time on the tar cracking. Although not experimentally validated, the model predicted the composition of heavy hydrocarbons such as benzene, toluene, levoglucosan, 1,3-cyclopentadiene, furan, and phenols in tar produced from biomass pyrolysis [10, 32]. D. Effect of vapor residence time To investigate the effect of vapor residence times on gas, tar, and char yields. As the temperature increased from 750 to 930 oC, the gas yield increased at the expense of tar thermal cracking (Figure 5). However, the char yield did not show any significant decrease in the temperature range of 750 to 930 oC. Even though soot formation occurred during tar thermal cracking, the increase did not have a significant impact on the overall yield of char in the process. This observation also agrees with previous literature studies [10]. The results suggest that the thermal cracking of tar occurs at a higher temperature than that of char. This indicates that tar Figure 7: Effect of vapor residence time on the char yield The slight increase in char yield due to secondary reactions has also been reported in previous studies [10]. The findings from this study illustrate that at higher 962 31st European Biomass Conference and Exhibition, 5-8 June 2023, Bologna, Italy temperatures, very low vapor residence times can be adequate to crack the thermally unstable tar components. For instance, Tao et al. [31] considered vapor residence times as low as 9 seconds for tar cracking at 700 oC. 3.3 Steam gasification A. Gas composition Figure 8 shows a comparison of the experimental and model compositions of hydrogen, carbon monoxide, and methane from steam gasification. At all temperatures with a steam-to-biomass ratio of 0.21, the results indicate that the model composition closely matches the experimental values. A mean absolute error of 17.4% results from the model's prediction. This suggests that the model can be used to predict the gas composition of steam gasification with a reasonable degree of accuracy. Figure 9: Comparison of the char yield (wt.%) from gasification model with experimental data. C. Effect of temperature on product distribution Table V shows the simulation results of the variation in gas, tar, char, and water yield with temperature. The gas yield increased from 76.4 wt% at 750 oC to 88.8 wt% at 930 oC. Table V: Effect of temperature on the gas, tar, and char yield from the gasification model. Weight percent oC Figure 8: Comparison of gas compositions from gasification model with experimental data. Gas Generally, hydrogen composition increased while carbon monoxide composition decreased with temperature. These trends could be attributed to the steam reforming and water gas shift reactions that subsequently occur to increase and reduce the concentration of H2 and CO, respectively [33]. As a result of methane reforming reactions, both experimental and simulation models predict only traces of methane gas. Similar observations have been reported in the literature [34]. Char Tar Water 750 76.4 850 oC 82.8 930 oC 88.8 4.79 4.8 4.8 11.3 7.8 3.54 7.5 4.54 2.7 Table VI: Product composition of the gasification mg/g (biomass) H2 B. Char yield As shown in Figure 9, the model predicted lower char yields when compared to the experimental results. The mean absolute error for the prediction was 17.1%. However, for both the experiment and simulation, the char yield significantly decreased between 750 and 930 oC. For instance, experimental findings indicate a decrease in the char yield from 13.2 wt% at 750 oC to 5 wt% at 930 oC. The decrease in char yield can be attributed to the heterogeneous and endothermic reaction of steam and carbon, which is favored by increasing temperatures [23]. The prediction of low char yield in the model can be attributed to the assumption that char is made up of only carbon, which makes its reaction with steam easier. Despite that, the model predicted the small amount of soot that remains during the steam reforming of biochar [34]. CO CO2 CH4 C2H4 Methanol Formaldehyde Acetaldehyde 5-Hydroxymethylfurfural Benzene Toluene Naphthalene Cyclopentadiene H2O Char 750 oC 36.24 850 oC 42.700 930 oC 47.94 471.35 485.303 513.40 184.67 220.304 241.29 2.42 2.490 2.40 67.71 75.189 79.76 28.83 29.066 29.14 0.144 0.047 0.022 10.28 10.464 10.53 0.152 0.050 0.024 0.365 0.579 0.820 0.000 0.000 0.000 1.60 2.533 3.59 0.000 0.003 0.018 75.10 45.414 27.86 113.0 78.0 35.40 Like pyrolysis, the increase in gas yield coincided with the decrease in char yield, signifying that the increase in gas yield could be attributed to the 963 31st European Biomass Conference and Exhibition, 5-8 June 2023, Bologna, Italy heterogeneous reaction of char and steam. There was no noticeable change in the tar yield over the temperature range, signifying that the gas yield was mainly obtained from char-steam reactions at higher temperatures and vapor residence time [32]. As shown in Table VI, the gasification products consisted mainly of aromatic compounds such as benzene, toluene, naphthalene, and cyclopentadienyl, which mainly exist as secondary compounds [[32]. The presence of aromatic compounds indicates that steam has only limited influence on the conversion of aromatics, even at high temperatures [34]. 4 [5] [6] [7] CONCLUSIONS [8] In this work, a rigorous pyrolysis and gasification model capable of predicting the thermal and steam cracking of pyrolytic products was proposed. The model was validated using experimental data from a laboratoryscale fixed-bed reactor. The proposed model can be applied to study the influence of reactor parameters such as temperature, pressure, residence times, and steam flow rate. Unlike the other models described in the literature, the key novelty in the proposed model was the ability to predict the effect of solid and vapor residence time on pyrolysis and gasification. At an overall error of 15.1 %, the proposed model can be useful in the design and optimization of thermochemical biorefineries, especially when the prediction of tar and pollutant composition such as benzene is required. Despite the advantages, more detailed experimental data that includes analysis of tar components is needed to further improve and optimize the kinetic data used in the model. [9] [10] [11] [12] 5 ACKNOWLEDGEMENTS The authors would like to acknowledge the Nordic Energy Agency for providing financial support via the BIOELEC project, grant no. 120006. [13] 6. SUPPORT INFORMATION [14] Support information will be made available on request. [15] 7 [1] 1] [2] [3] [4] REFERENCES This section should have the progressive number before the title, exactly as for the previous ones. 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