Experimental Investigation on the Gasification Kinetic Model of a Char

Article pubs.acs.org/EF

Experimental Investigation on the Gasification Kinetic Model of a Char Particle in Supercritical Water Hui Jin,* Xiao Zhao, Liejin Guo, Chao Zhu, and Wenwen Wei State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China ABSTRACT: Supercritical water gasification technology has bright prospects because it can convert biomass into hydrogen-rich gaseous products in an effective and clean way. The devolatilization process is relatively fast, and char conversion is the ratedetermining step in the whole gasification process. To discuss the kinetic model of char gasification in supercritical water, the quartz tube reactor was adopted as the reactor to omit the undesired catalytic effect of the reactor wall and bamboo charcoal was selected as typical char as a result of the low volatile content. The experiments were investigated within the operating range of temperatures of 700−900 °C and residence times of 5−20 min. The experimental results were analyzed within the framework of homogeneous, non-reacted core, and random pore models. Dependences of the carbon gasification rate, residence time, and temperature were described in an assumption of the first-order reaction and the Arrhenius dependence. The calculation results showed that the random pore model fit the char gasification process in supercritical water best and the activation energy was 125.43 kJ/mol. activation energies. Liu et al.30 clarified that the non-reacted core model was not appropriate and developed a modified RPM for a high reaction temperature. The contribution made by the above investigators took the porous properties into account; however, the conclusions were drawn based on traditional gasification environments. SCW has unique chemical and physical properties, especially the nearzero surface tension31,32 and obtains easier access inside the porous structure of the char; therefore, the gasification characteristics may be complex and special. Goto et al.33 described supercritical fluid extraction with a non-reacted core model and also considered the influence of the biomass nature upon the SCWG results.34 A novel gasification kinetic model focusing on the gas products was established on the basis of the lumped parameter method and homogeneous model; however, the content of volatile and fixed carbon were not considered.35 Lan et al.10 conducted gasification kinetics of the coal particle with a homogeneous model. Su et al.36 proposed a kinetic model for the Zhundong coal particle, which fit the experimental results well; however, the structural change of the coal particle was not well considered. Vostrikov et al.37 conducted investigations on homogeneous model, non-related core model, and RPM; however, the feedstock used was the coal particle, and it might not provide accurate information for char particle gasification, which is believed to be the ratedetermining step for biomass/coal gasification. To date, most of the previous works were performed with the homogeneous model to describe the kinetic mechanism. The experiments sometimes processed with thermogravimetric analysis (TGA), which could not illustrate the pore structure changes and gasification reaction under the real SCW

1. INTRODUCTION The rational use of biomass has attracted great interests, owing to the gradual depletion of fossil energy and the deterioration of the ecological environment worldwide.1,2 Biomass will definitely count a lot in the future energy market3 as a result of the large worldwide biomass resource potential and the optimistic long-term contribution.4,5 Biomass supercritical water gasification (SCWG) was frequently investigated because of a high hydrogen yield and high thermal efficiency.6,7 Supercritical water (SCW) is defined as the water beyond the critical point (374 °C and 22.1 MPa) and has unique physical and chemical properties,8,9 such as low viscosity, high diffusivity, and low dielectric constant.10,11 These properties provide a homogeneous and rapid condition for biomass gasification with a short residence time and high efficiency.12,13 For the past several years, considerable research has been performed and various biomasses were gasified in SCW, such as wood,14 glucose,15 cellulose,16 lignin,17 and algae.18,19 For biomass gasification in SCW, the kinetic model provides extremely important information for the reactor optimization and scaling up. In fact, the gasification of biomass in SCW consists of two main steps: the pyrolysis step and the char gasification step, within which the char gasification is the ratedetermining step and the char is an undesirable product of SCWG.20−24 Therefore, the elements affecting char gasification kinetics are important.25 The char particle has a porous structure, and its gasification characteristics in a traditional gasification process have attracted attention. Peterson26 first assumed that the reaction was based on the homogeneous pore system. Bhatia and Perlmutter27,28 developed the random pore model (RPM) for fluid−solid reactions. It was assumed that the reaction processed in a random pore structure system. Three pore parameters were used to describe the model: total pore length, total surface area, and pore volume. Su and Perlmutter29 used different chars to obtain pore structure parameters, intrinsic kinetics, and © 2015 American Chemical Society

Received: September 5, 2015 Revised: November 13, 2015 Published: November 13, 2015 8053

DOI: 10.1021/acs.energyfuels.5b02014 Energy Fuels 2015, 29, 8053−8057

Article

Energy & Fuels conditions. The experimental runs on SCWG of char are still limited. To obtain the information in the kinetic mechanism under SCW conditions, high carbon content feedstock bamboo char was gasified in a quartz tube reactor in SCW conditions. The parameters of homogeneous model, non-reacted core model, and RPM were separately calculated using carbon gasification efficiency (CE) data versus residence time. The activation energy and pre-exponential factor were also obtained on the basis of the Arrhenius equation. The best kinetic model describing the char gasification process in SCW was discussed.

Table 1. Elemental and Proximate Analyses of Bamboo Char test item (unit) elemental analysis (wt %)

proximate analysis (wt %)

2. EXPERIMENTAL SECTION heat value, Qb,ad (MJ/kg)

results carbon hydrogen sulfur nitrogen oxygena moisture ash volatile fixed carbon

79.00 3.37 0.48 0.49 7.52 5.03 4.11 15.84 75.02 34.48

2.1. Apparatus and Procedure. The micro quartz tube reactor is a cylinder with a length of 200 mm and an inside diameter of 1.5 mm. The designed maximum temperature and pressure are 1000 °C and 45 MPa, respectively.36,38 The bamboo char and deionized water were measured separately, mixed uniformly, and then loaded into the bottom of the reactor with one-end sealed. The other end of the quartz tube was melted and sealed with the help of a high temperature provided by a hydrogen flame. When the temperature was steady around the desired temperature, the tube reactor was placed into the furnace quickly (as seen in Figure 1). The heating time was within 30 s; therefore, the heating time can be ignored in comparison to the residence time.36,38

3. RESULTS AND DISCUSSION 3.1. Gasification Results. The influences of the residence time and temperature upon bamboo char gasification in SCW can be observed in Figure 2. It can be seen that, when the

Figure 1. Experimental system with a quartz tube reactor for SCWG.

Figure 2. Influences of the residence time and temperature upon bamboo char gasification in SCW (23−25 MPa gasification pressure and 10 wt % concentration): (a) 700 °C, (b) 800 °C, and (c) 900 °C.

a

By difference. gas yield = (molar amount of a certain component of the gaseous products)/(mass of the bamboo char) (mol/kg)

temperature was 700 °C, CO2 had the highest yield, while when the temperature was 800 or 900 °C, H2 had the highest yield. CO always had the lowest yield. As residence time increased from 5 to 20 min, the yield of all gas increased. The reaction temperature was the most important parameter that influences the SCWG process.42 Higher temperatures promoted the free-radical reactions, which led to a higher gas yield.43 It can be obviously seen that, as the temperature increased, the growth rate slowed. As the residence time increased from 5 to 20 min, the H2 yield increased from 2.86 to 6.89 mol/kg at 700 °C, while the H2 yield increased from 40.76 to 44.20 mol/kg at 900 °C. 3.2. Homogeneous Model. Three different kinetic models were used in this paper: homogeneous model, non-reacted core model, and RPM. Dependences of the CE, residence time, and

2.2. Materials and Analytical Methods. The bamboo char was used as feedstock, and the elemental and proximate analyses were listed in Table 1. Bamboo char particles were selected within the diameter range of 0.9−1.0 mm. The composition of the gaseous products was analyzed by gas chromatography (Agilent 7890A) with thermal conductivity detectors (TCDs). High-purity argon was used as a carrier gas. To obtain the pore structure parameter ψ, we used a thermogravimetric analyzer (Netzsch STA 449F3) with crucibles made of Al2O3. High-purity nitrogen with a flow rate of 250 mL/min was used as the carrier gas.39,40 2.3. Data Interpretation. CE and gas yield were used to evaluate the gasification characteristics, and the definitions were as follows:41 CE = (total carbon in the gaseous products) /(total carbon in the bamboo char) × 100 (%) 8054

DOI: 10.1021/acs.energyfuels.5b02014 Energy Fuels 2015, 29, 8053−8057

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Energy & Fuels temperature were described in an assumption of the first-order reaction and the Arrhenius dependence. The homogeneous model reduces the heterogeneous gas− solid reaction of char gasification to a homogeneous reaction by assuming that the gas reacts with char in all possible places.25 The homogeneous model is shown as eq 1, and the linear relationship was fitted by the experiment data, with X standing for the CE in this paper. The activation energy Ea and preexponential factor k0 were obtained through linear regression of the ln(k) versus 1/T plot and calculated to be 77.85 kJ/mol and 217.94 s−1, respectively, as seen in Figure 3. dX = k 0e−Ea / RT (1 − X ) dt

dX = k 0e−Ea / RT (1 − X ) [1 − ψ ln(1 − X )] dt

(3)

where ψ is a pore structure parameter related to the unreacted sample. Bhatia and Perlmutter have already integrated a formula to obtain ψ, which can be shown as eq 4.27,47 The structure parameter of the bamboo char ψ was 2.0 calculated from thermogravimetry (TG)−differential thermal analysis (DTA) data by eq 4. The first-order plot for char gasification in SCW by the random porous model was seen as Figure 5.

(1)

Figure 5. First-order plot for char gasification in SCW by RPM (700− 900 °C, 23−25 MPa, 5−20 min, and 10 wt %).

The vertical axis label 1/cos(w) was derived from the integral transformation from eq 3, and the definition of w is seen as eq 5. Ea and k0 were 125.43 kJ/mol and 60 114.12 s−1, respectively.

Figure 3. First-order plot for char gasification in SCW by the homogeneous model (700−900 °C, 23−25 MPa, 5−20 min, and 10 wt %).

3.3. Non-reacted Core Model. Non-reacted core model assumes that the reaction happens on the spherical surface and the core is shrinking as the reaction progresses.44 The typical non-reacted core model is shown as eq 2. The first-order plot for char gasification in SCW by the non-reacted core model was seen in Figure 4. The vertical axis label (1 − X)2/3 was derived

X m = 1 − exp[(2 − ψ )/2ψ ]

(4)

w = arctan −ψ ln(1 − X )

(5)

3.5. Comparison of the Models. Figure 6 presents the Arrhenius plot of the reaction rate constants at different

Figure 6. Arrhenius plot for char gasification in SCW by different models (700−900 °C, 23−25 MPa, 5−20 min, and 10 wt %). Figure 4. First-order plot for char gasification in SCW by the nonreacted core model (700−900 °C, 23−25 MPa, 5−20 min, and 10 wt %).

reaction temperatures. The activation energy Ea and preexponential factor k0 were obtained through linear regression of the ln(k) versus 1/T plot. Table 2 shows the comparison of the activation energy and pre-exponential factor calculated by different kinetic models, which were obtained in SCWG. It is necessary to decide which model is best for fitting the experimental results. Therefore, a confirmatory experiment was

from the integral transformation from eq 2. Ea and k0 were calculated to be 40.07 kJ/mol and 1.15 s−1, respectively. dX = k 0e−Ea / RT (1 − X )2/3 dt

(2)

Table 2. Calculation of the Arrhenius Equation with Different Kinetic Models

3.4. RPM. The RPM takes the structural changes during the gasification reaction into account. Bhatia and Perlmutter27,28 proposed that a RPM could be applied to the coal gasification reaction. They considered the random growing and overlapping of pore surfaces, which could increase first and then reduce the area available for reaction simultaneously, as illustrated in eq 340,45,46 8055

model

Ea (kJ/mol)

k0 (s−1)

RPM homogeneous model non-reacted core model

125.43 77.85 40.07

60114.12 217.94 1.15

DOI: 10.1021/acs.energyfuels.5b02014 Energy Fuels 2015, 29, 8053−8057

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Energy & Fuels

Figure 7. Comparison of the experimental result to three different models (23−25 MPa and 10 wt % bamboo char): (a) 850 °C and (b) 750 °C.

especially the elements with a catalytic effect, is not uniform, the RPM needs to be modified.39

conducted for comparison of different models, and the operating conditions of 850 °C (Figure 7a) and 750 °C (Figure 7b) were selected. The calculation of different models at four different residence times was seen in Figure 7. It can be obviously seen that RPM had the least disparity with the experimental data. Although there were still some errors between RPM expectations and experimental data, RPM fit the experimental data best among all three models. Apparently, the calculation of the homogeneous model and non-reacted core model had a remarkable difference with experimental data. For example, when the reaction temperature was 20 min, the calculation of the homogeneous model and non-reacted core model was 64.75 and 43.27%, respectively. However, there is still small deviation between the experimental data and the simulation results by RPM. The errors may be caused by the catalytic effect of the alkalis metal in biomass char. Moreover, RPM assumes that the diffusion time of the reaction products can be neglected in comparison to the chemical reaction time. However, in the real SCWG of the char particle, the polymerization reaction happens in the quartz tube, so that the simulated results overestimate the CE. It can be seen from panels a and b of Figure 7 that the CE was overestimated by RPM. The reasons why RPM fit the experimental result best are speculated as follows: The homogeneous model assumes that the reaction occurs from both inside and outside the biomass particle, and the non-reacted core model assumes that the reaction only advanced on the spherical surface of the biomass particle. However, after the volatile matter is released from the biomass particle, the porous structure is formed in the biomass particle.48 SCW has zero surface tension and becomes easy access to the inner volume of the porous structure. Moreover, the high diffusion of SCW may make the reaction between the water (both around the biomass particle and inside the porous structure of the biomass structure) and the surface of the porous particle become the rate-determining step. The RPM takes into account both the porous structure characteristics and the growth of the volume inside the particle as the reaction progresses; therefore, the RPM fit the experimental results with the most accuracy. Both the original content of the biomass and the char formation operation process may be different; therefore, the char particles investigated may be different. It may be arbitrary to conclude that the RPM fit all of the biomass char particle gasification processes best. It is speculated that the RPM may be predictive, given that the element content distribution is uniform. However, if the element content distribution,

4. CONCLUSION The char gasification process is the rate-determining step in biomass gasification in SCW. A bamboo char particle was selected as a typical biomass char to investigate the gasification kinetics in SCW. The experimental conditions covered the typical operating parameters: 700−900 °C, 23−25 MPa, and residence times of 5, 10, 15, and 20 min, and the typical gasification results were obtained. The homogeneous model, non-reacted core model, and RPM were implemented to fit the experimental results and dependences of the carbon gasification rate. The reaction time and temperature were described in an assumption of the first-order reaction and the Arrhenius dependence. The calculation results showed that RPM fit the char gasification process in SCW with the highest accuracy, and the activation energy and pre-exponential factor were 125.43 kJ/mol and 60 114.12 s−1, respectively.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-29-82660876. Fax: +86-29-82669033. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Contracts 51306145 and 51236007) and the National Basic Research Program of China (Contract 2012CB215303).

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b02014 Energy Fuels 2015, 29, 8053−8057