Investigation of biomass (pine wood) gasification: Experiments and Aspen Plus simulation

Biomass gasification is currently a hot research topic. To achieve a high hydrogen content in the product gas, the gasification feedstock used in this study is air‐dried pine woodchips. Experiments are performed in a downdraft gasifier by varying the operation parameters of the particle size (60 mesh, 80 mesh, 100 mesh), temperature point (700, 750, 800, 850, 900°C), and steam‐to‐biomass mass (S/B) ratio (0, 0.7, 1.4, 2.1, 2.8). The main effects of particle size, temperature, and S/B ratio on the composition of the product gas are analyzed to predict the optimal operation parameters of biomass feedstock. For pine woodchip gasification, the optimal particle size is 80 mesh or 0.17 mm, the preferred temperature is 850°C, and the optimal S/B ratio is 1.4. Although there is an error between the experiment and the simulation, the difference is not significant. The Aspen Plus model can provide guidance for the gasification of pine woodchips and can be extended to the gasification of other kinds of biomass.


| INTRODUCTION
The competition for traditional energy sources exists at the international level, and the trend of the use of renewable sources to replace petroleum products has attracted worldwide attention. [1][2][3] Biomass is defined as a green energy source due to its renewability. Biomass resources are widely distributed in nature, and they can be processed directly or indirectly through conversion into gas, liquid fuel, and solid fuel. 4,5 All types of products have a variety of uses, depending on their properties and the technical means. 6,7 There are many types of biomass feedstocks that show a variety of characteristics, and the current standard for selecting a feedstock varies according to product needs. 8 Biomass resources can be converted into multiple forms of energy through two types of process: (a) biochemical and (b) thermochemical. 9,10 A biochemical process decomposes the biomass into flammable gas rich in CH 4 and H 2 by the action of microorganisms under specific conditions. Nevertheless, the inefficiency of the biochemical process does not meet industrial requirements. 10 In the thermochemical conversion process, the biomass products include syngas, which is similar to natural gas; biodiesel, which is a replacement for petrochemical diesel; and a carbon product that is used for fuel and adsorption processes. 5 In general, gasification and pyrolysis are the main ways to produce the product gas. Gasification is a promising way to obtain hydrogen-rich gas. In gasification, different feedstocks and different operation conditions and reactions are interrelated owing to the complexity of gasification. [11][12][13][14] To optimize the operating conditions to produce high-quality product gas, some published literature has already described the impacts of the operating parameters of biomass gasification. 10,15 | 1179 HUANG ANd JIN For a selected biomass feedstock, different operation parameters can lead to various gasification results. 14,[16][17][18] The product gas mainly is made up of CO and H 2 , which can be used directly as a fuel for generating electricity and conveyance and as a raw material for chemical production. 19 Many studies have suggested that future biorefineries will incorporate hydrothermal gasification and use the internally produced hydrogen. 20 Gasification using steam is an appropriate route for a polygeneration mode conducted to produce hydrogen and electrical energy. 21 A previous study showed that the hydrogen yield can be upgraded by introducing steam directly in the reduction reaction. 10 According to the work of G. Mirmoshtaghi et al, 17 a steam-to-biomass (S/B) ratio of 0.7, a particle size of 3 mm, and a moisture capacity of 9 wt% resulted in the optimal product gas. Aydin et al 22 suggested that an operating temperature range of 800-820°C and a S/B ratio of 1.3 were the optimal operating conditions. Xiang et al showed that the calorific value of product gas per hour was the greatest under the conditions of T 1 = 800°C, steam/coal = 2, and coal/biomass = 0.25. The product gas that is considered a chemical material mostly demands high H 2 content; the parameters are listed as follows: coal/biomass = 0.25, steam/ coal = 2, and T 1 = 980°C. 23 The calorific value of the product gas when using O 2 and saturated steam as the gasification agent is higher than that using air and saturated steam as the gasification agent. 24 Shayan et al 15 reported that a rise in the gasification temperature led to a reduction in the hydrogen yield. However, Refs. 23,25-29 indicated that an increase in the gasification temperature led to an increase in the hydrogen yield. Introducing steam into gasification is more favorable for the hydrogen yield than pure air. 22,25,26 To accurately predict the gasification results, many scholars have developed models to obtain predictions. The CFD-DEM model has been used in the simulation of glucose gasification, showing that a higher wall temperature and a lower flow rate are favorable for gasification. 30 The Aspen Plus model has been used in gasification, adopting steam as the gasifying agent and showing the effects of temperature, S/B ratio, and temperature shift on the gas ingredients. 27,31 A mathematical model adopting the COMMENT code has been used to analyze the effects of multifarious operating parameters. 28 In the event that the existing models and formulas can be further modified by adding additional experimental data, the improved models will predict the product composition more accurately. 32,33 The advantages and disadvantages of certain types of gasifiers are listed in the study by Samiran. 19 The self-moisture gasification of fresh biomass is an innovative method of biomass application that is different from conventional gasification. 34 Steam gasification has drawn a great deal of attention to realize a higher H 2 /CO ratio and hydrogen yield. A substantial amount of experimental work has been performed on a downdraft gasifier with steam. 21,24,25,30,32 Numerous simulation studies have been compared with experimental results. 23,27,30,31 However, there is no unified conclusion. In the present study, experimental work is performed on pine woodchip gasification in downdraft gasifiers, and an Aspen Plus model is used to simulate the gasification process. This paper seeks the optimal gasification operating conditions of pine woodchips by changing the operating parameters. Comparing the experimental data with the data of the Aspen Plus model can reflect the reliability of the experiment and verify the adaptability of the Aspen Plus model to the gasification process. The conclusions in the present paper are complementary to those of previous studies.

| Experimental setup
The entire experimental setup is displayed in Figure 1, which contains five pieces of equipment: an inert atmosphere F I G U R E 1 Experimental setup for biomass gasification. 1: Nitrogen bottle; 2, 4: Switching valves; 3: Steam generator; 5: Electric heater; 6: Screw feeder; 7: Quartz tube gasifier; 8: Filter cartridge; 9: PID temperature control cabinet; 10: Scrubbing bottle; 11: Gas drying bottle; 12: Sucking pump; 13: Infrared gas analyzer; 14: Gasbag device, a steam generator, a downdraft biomass gasifier, a gas filtering system, and a gas collection and analysis system. The inert atmosphere device is composed of a nitrogen bottle and a switching valve. The steam generator includes a steam generator, a switching valve, and an electric heater. The downdraft biomass gasifier is composed of a screw feeder, a quartz tube gasifier (inner diameter: 0.5 m; height: 1.3 m), and a PID temperature control system. A filter cartridge, a scrubbing bottle, and a gas drying bottle constitute the gas filtering system. The gas collection and analysis system contains a sucking pump, an infrared gas analyzer, and a gasbag. Many resistance wires are used to heat the quartz tube gasifier and keep the wall temperature at a specific constant temperature, providing an external heat source as required.

| Experimental procedure
The experimental flowchart is shown in Figure 2.
The main experimental steps are as follows: 1. Do not connect the infrared gas analyzer. All switching valves are closed. Turn on the power switch and heat the quartz tube gasifier to a specified temperature by operating the PID temperature control cabinet.

2.
To form an inert atmosphere, open valve 2, and sweep the entire system with N 2 . Then, put the biomass into the gasifier. If steam is introduced as a gasification agent, close valve 2, and open valve 4. The mass flow rate of the steam introduced into the gasifier is determined according to the S/B ratio. Switch on the sucking pump and connect the infrared gas analyzer. When collecting the product gas, the gasbag will be connected to the outlet of the infrared gas analyzer. 3. During the gasification process, the product gas from the gasifier goes through the scrubbing bottle and the gas drying bottle. Then, it enters the infrared gas analyzer under negative pressure from the sucking pump and is collected in the gasbag.

| Test methods for important parameters
Pine woodchips as the gasification feedstock are processed into micro-sized pieces of biomass using a small blade mill and then sieved through 60 mesh, 80 mesh, and 100 mesh filters. The biomass properties are the key factors affecting the gasification results. A proximate analysis and an ultimate analysis are performed using a MAC-3000A automatic analyzer and a Vario Micro cube ultimate analyzer. The product gas is analyzed using an infrared gas analyzer (Gasboard-3100) that can calculate the volume fraction quickly and effectively.

| Description of all parameters in the experiments
The results of the proximate analysis and the ultimate analysis of the pine woodchips are listed in Table 1.
The other parameters used in the experiment are listed in Table 2.

| Index parameters
The product gas mainly consists of CO, H 2 , CH 4 , CO 2 , N 2 , and H 2 O. The gas production and the calorific value of the product gas are mutual restraint parameters. CO 2 has no contribution to the gas calorific value, CH 4 has little contribution to the gas production, N 2 is an inactive gas, and H 2 O is absorbed during the filtering phase. Thus, the main measurable indicators are the contents of H 2 and CO. Compared to CO, H 2 is an ecofriendly gas fuel with a high calorific value, which makes it more desirable. The ratio of H 2 /CO is thus the most appropriate evaluation indicator 10,25,31 and reflects volume changes as well as changes in the quality of the product gas.

| Aspen Plus simulation
The Aspen Plus flowchart for the gasification of pine woodchips is shown in Figure 3. Creating an Aspen Plus model involves the following steps: appointing the setup; appointing all system components and classifying nonconventional F I G U R E 2 Experimental flowchart and conventional components; selecting the method for describing the properties, especially the nonconventional properties, and specifying the stream class; establishing the main flowchart (using unit operation blocks and material streams); appointing the feed streams that contain flow rates (mass or mole), compositions, and thermodynamic conditions; and specifying the unit operation blocks that contain thermodynamic conditions, calculation options, and chemical reactions. Then, the model is debugged and run.

| Fundamental assumptions
The assumptions of the model are listed below: 1. The mass flow rate of the biomass is 3 g/min at 1 bar and 25°C, and steam is supplied at 200°C.

2.
The gasification process is steady-state and isothermal. 15,25 3. The internal pressure of the gasifier is kept at approximately 1.1 bar. 31 4. The formation of tar is ignored, and all sulfur (S) is formed by H 2 S. 31

5.
Char is the product of the preliminary drying and deashing step. Char is mostly made of carbon, and it is assumed to be pure carbon. 25,27 6. All carbon is transformed. 27 7. All gases are in an ideal state. 15 In the RYIELD block, the product component and basis yield are given according to the total mass balance. Due to the limitation of the residence time in the gasifier, the actual gasification process is extremely complicated and has difficulty achieving chemical equilibrium. Using the Gibbs equilibrium model results in considerable differences in the product gas composition between the simulation and the experiment. 34 Using the restricted equilibrium method in the model can achieve better agreement between the simulation and the experiment. 31 In this model, the RGIBBS block makes a choice of the calculation option "Restrict Chemical Equilibrium-Specify Temperature Method or Reactions," 33 and the zero temperature approach specification is used in individual reactions. Table 3 lists chemical reactions R1-R8 that are considered in the gasification process. However, in the present model, aspects of hydrodynamics, the restricted heat transfer in the reactor, the reactor size, the tar formation reaction, the catalyst deactivation side reaction, and other reactions are not considered. The Peng-Robinson (Peng-Rob) equation of state with the Boston-Mathias (BM) modification is used to calculate the thermodynamic properties of the conventional components in this model. PR-BM is chosen for the chemical industry.

| Chemical reactions
Biomass gasification is an extremely complicated process involving many chemical reactions. R1-R9, listed in Table 3, are considered for the gasification process in this study. However, in this study, the reactions containing nitrogen and sulfur are not the main reactions. R1, R2, R3, and R5 are endothermic reactions, meaning that a high temperature is favorable for the positive reaction. R4 and R6 are exothermic reactions, meaning that a high temperature is favorable for the reverse reaction. Although the process is isolated from air, the biomass itself contains oxygen. In the simulation, R1-R8 are considered.

Plus flowsheet
The flowchart for biomass gasification is displayed in Figure 3. The BIOMASS stream is designated as a nonconventional stream, and the mass flow rate of the feedstock is 3 g/min. In light of the ultimate and proximate analyses of the biomass feedstock, the component attributes of the BIOMASS stream can be defined. The BIOMASS stream goes to the yield reactor RYIELD block, generating conventional components such as carbon (C), H 2 , O 2 , N 2 , S, steam (H 2 O), and ash. The ultimate analysis of the feedstock determines the yield distribution to the RYIELD reactor block. The yields of each component are determined by the yield distribution, which determines the mass flow of each component in the RYIELD block export stream ELEM1. Then, as the only nonconventional component, the ash yield is specified as 100% by the ultimate and proximate analyses. The export stream ELEM1 out of the RYIELD block passes to the separation unit ASHSEP. The separation unit ASHSEP isolates ash as a stream ASH from the other components, such as C, H 2  In the RGIBBS block, the gasification temperature varies between 700 and 900°C, and the stream STEAM mass flow rate varies depending on the steam-to-biomass (S/B) ratio (0, 0.7, 1.4, 2.1, and 2.8). The gasification reactions R1-R8 are specific to the RGIBBS block.
Descriptions of the blocks used in the flowsheet are given in Table 4.

| Effect of particle size on the product gas
The gasifier is maintained at 900°C and 1.1 bar. The mass flow rate of pine woodchips is 3 g/min, and 60 mesh, 80 mesh, and 100 mesh filtered pine woodchips are the gasification feedstock. The experimental results are shown in Table 5 and Figure 4A,B. Table 5 and Figure 4A display the volume fraction profile of the main product gases (H 2 , CO, CO 2 , and CH 4 ). As the particle size changes from 60 mesh to 100 mesh, the proportions of CO and CO 2 decrease by 2.25% and 4.84%, respectively, and the proportions of H 2 and CH 4 increase by 6.85% and 0.49%, respectively. The volume ratio of each component gas does not change substantially. Similar trends were reported in Refs. 17,25. The increase in H 2 is greater than the increase in CO, leading to growth in the H 2 /CO ratio.
When the pine woodchips are crushed, the cellulose structure is effectively destroyed at the microcosmic level. As crystallinity decreases, the porosity of the particles increases, and the reduction in the particle size increases the specific surface area of the biomass particles that can react with the surrounding atmosphere, which benefits heat exchange between the pine woodchips and the surrounding atmosphere. R1 and R5 are shifted toward the positive direction, and the yield of H 2 increases. However, the yield of CO decreases.
CO is mainly generated by R1 and R3. R1 and R3 benefit from the high temperature, but the reactions are only slightly shifted in the positive direction. In R1, the fluctuation in the equilibrium constant k 1 , which is mainly affected by temperature, is not obvious, and H 2 O is derived from the internal water contained in the biomass itself. The concentration of H 2 O does not greatly change, and the concentration of CO decreases. CO 2 is mainly generated by R2. In R2, k 2 experiences little or no change and the change in the concentration of CO 2 has the same trend as that of CO. CH 4 is mainly generated by R4, and the proportion of CH 4 increases with that of H 2.
In Figure 4B, the H 2 /CO ratio increases almost linearly as the particle size decreases, and an inflection point is observed when the particle size is 80 mesh. As the particle size increases from 60 mesh to 80 mesh, the H 2 /CO ratio increases by 11.41% compared to a 3.75% increase when the particle size changes from 80 mesh to 100 mesh. Considering the cost of broken pine woodchips, 80 mesh may be the optimal particle size for the gasification of pine woodchips.

| Effect of temperature on the product gas
The pine woodchips are maintained at a mass flow of 3 g/ min at 25°C and 1.1 bar. The gasification temperature varies between 700 and 900°C, and the particle size of the pine woodchips is 80 mesh. Figure 5A,B shows the changes in the gas composition and the H 2 /CO ratio with the temperature. Figure 5A suggests that the content of CO decreases by 4.17% from 700 to 900°C, and the content of H 2 increases by 5.43%. Similar conclusions were reported in Refs. 23,[25][26][27]29. The fraction of CH 4 decreases by 6.95% as the fraction of CO 2 increases by 5.69%. Similar trends were reported in Ref. 23.
As the temperature increases, R1, R2, R4, and R5 move toward H 2 production, R1 and R3 move toward CO production, R4 and R5 move toward a decrease in CH 4 , and R2 moves toward an increase in CO 2 and a decrease in CO. The equilibrium constant k in the reactions is mainly affected by the temperature, and k 2 is significantly more sensitive to temperature than k 1 or k 3 . Moreover, when the temperature is lower than 850°C, the rate of the reverse reaction in R3 is higher than that of the positive reaction, in contrast to the case at temperatures above 850°C. Based on the above analysis, the observed experimental phenomena have a theoretical basis. According to R1-R5, as the concentration of H 2 grows, in a similar way, the proportion of CO 2 increases, and the fractions of CO and CH 4 decrease. As shown in Figure 5B, the ratio of H 2 /CO increases from 700 to 850°C and decreases from 850 to 900°C, and thus, 850°C is the optimal temperature for the gasification of pine woodchips. Figure 5C reveals that the differences in the H 2 /CO ratio at different temperatures between the experiment and the simulation are −14.1%, −9.6%, −0.24%, and −0.35%, and the lowest value is −0.24% at 850°C. The experimental conclusion agrees well with the simulation data. 850°C is thus the optimal temperature for the gasification of the pine woodchips in this study.

| Effect of gasifying agent dosage gasifying agent dosage
In this experiment, steam is chosen as the gasifying agent. Pine woodchips with a particle size of 80 mesh are the gasification feedstock. The gasifier temperature is maintained at 850°C, the mass flow rate of pine woodchips is 3 g/min, and the S/B ratio is 0.7, 1.4, 2.1, or 2.8. The experimental results are shown in Figure 6A,B. Figure 6A clearly shows that the content of H 2 increases by 6.78% as the S/B ratio increases from 0.7 to 2.8. In Refs. 25,27,31, similar conclusions were obtained. The content of CO decreases by 19.52%, the content of CH 4 decreases by 1.9%, and the content of CO 2 increases by 13.74%. Pala et al 31 and Kaushal and Tyagi 27 made similar arguments. After introducing steam as a gasification agent, the composition of the product gas varies greatly, especially the concentrations of CO and CO 2 .
The introduction of steam is conducive to the formation of H 2 . An increase in the H 2 O concentration promotes the forward reactions of R1, R2, and R5, resulting in more conversion of CO into CO 2 . The fractions of CO and CH 4 decrease. However, the steam temperature is lower than the ambient temperature, leading to heat loss, which impacts heat and mass transfer. As the S/B ratio increases from 1.4 to 2.8, the fraction of H 2 does not greatly increase, and thus, excess steam may inhibit the generation of H 2 .
The introduction of steam during the gasification process promotes the further pyrolysis and gasification of byproducts such as tar and coke to enhance the gasification reaction and improve the quality of the product gas. As shown in Figure 6B, the H 2 /CO ratio is the highest when the S/B ratio is 2.8 in the simulation. However, the degree of increase of the H 2 /CO ratio in the experiment is 1.073 when the S/B ratio increases from 0.7 to 1.4. The greatest growth rate with the different S/B ratios is 1.073. The amplitude of the CO/CO 2 ratio distinctly decreases as the S/B ratio increases from 0.7 to 1.4. The CO/CO 2 ratio is less than one when the S/B ratio is >1.4, and the calorific value of the product gas declines.
The difference in the H 2 /CO ratio between the simulation and experiment first decreases, an inflection point is observed when the S/B ratio is 1.4, and then, the difference gradually increases, although the difference is not obvious. The simulation results are obtained under ideal conditions, making the results much better than they truly are.
Steam gasification is a good option for hydrogen production. However, the use of additional high-temperature steam requires additional external heat sources. This study finds that the S/B ratio of 1.4 is optimal, which is in agreement with the results in Ref. 25.