Kinetic Studies of a Novel CO2 Gasification Method Using Coal from

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Kinetic studies of a novel CO2 gasification method using coal from deep unmineable seams Rico Silbermann, Arturo Gomez, Ian Gates, and Nader Mahinpey Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie401918e • Publication Date (Web): 20 Sep 2013 Downloaded from http://pubs.acs.org on September 23, 2013

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Industrial & Engineering Chemistry Research

Kinetic studies of a novel CO2 gasification method using coal from deep unmineable seams Rico Silbermann1, Arturo Gomez2, Ian Gates3, Nader Mahinpey1* Department of Chemical & Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, T2N 1N4, Canada

*Author for correspondence: Dr. Nader Mahinpey Dept. of Chemical and Petroleum Engineering Schulich School of Engineering The University of Calgary 2500 University Drive NW Calgary, AB T2N 1N4 Canada Phone: (403) 210-6503 Fax: (403) 284-4852 E-mail: [email protected]

1 ACS Paragon Plus Environment


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Abstract Seven coal samples taken from cores drilled in the Cretaceous Mannville Group were used for investigation of coal properties and carbon dioxide (CO2) gasification. The depths of the cores ranged between 700 and 800 meters below the surface in the Western Canadian Sedimentary Basin. A new method has been developed with an average heating rate of 200 K/min using CO2 as the gasifying agent from the experiment’s beginning until its end. The coal properties of the seven coals from these deep coal seams showed certain similarities and variations. There is an obvious relationship between the reactivity and the material properties determined in the study. In particular, the specific surface area calculated relative to the carbon content measured in the ultimate analysis showed a correlation with the reactivity. The ash content and composition also appeared to influence char reactivity. The gasification behaviors of the in-situ coals were compared to those of two surface-mined coals. The new method of coal gasification showed a significant difference to those that were heated up in an inert gas, such as nitrogen, to the target temperature. A maximum rate of reaction did not exist when the new method was used; and, the integrated core model gave better results than the commonly used random pore model in terms of kinetic modeling.

Keywords

Underground Coal; New Gasification Method; TGA Experiments; Char Reactivity; Maximum Reaction Rate; Kinetic Modeling


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1. Introduction More than 65 percent of Canada’s coal is produced in the Western provinces of Alberta and Saskatchewan1,2. This coal is easily accessible through open surface mining and is primarily used for electricity generation through combustion. Canada also has a huge amount of coal that is non-recoverable3 under current economic and ecological conditions. These thin layers, often less than 5 meters thick, are several hundred meters underground. In-situ gasification has the potential to make this coal useable4. Coal gasification technologies are used to produce syngas that contains mainly carbon monoxide and hydrogen. Both gases can be used for different continuative processes, such as Fischer-Tropsch synthesis. Surface-mined coals from different Canadian areas have been extensively investigated over the past few decades and have been used worldwide for several gasification studies5,6. The properties of coal from deeper Canadian coal seams and the behavior due gasification with different gasification agents have not been largely investigated. Currently, there are no kinetic studies in the literature about coal from deeper coal seams, because drilling cores are very rare and usually used for geological studies. This paper describes the properties of seven different coals from deeper coal seams, all in the range between 700 and 800 meters below the surface and originating from the Cretaceous Mannville Group in the Western Canadian Sedimentary Basin. The gasification behavior of the in-situ coal is compared to those of surface-mined coals. To get a general understanding of the gasification behavior, the first experiments were performed in an atmospheric thermogravimetric analyzer (TGA) with carbon dioxide (CO2) as the gasifying agent.



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In typical practice, the procedure used for TGA experiments is the heating up to a certain temperature with an inert gas, such as nitrogen (N2), which at a specific point of time is switched over to CO2. The previously reported heating rates in literature are in the range of 20-50 K/min, which are not comparable with those of continuously fed industrial gasification processes, where particles heat up quickly. In this paper, we describe a new procedure where CO2 was used from the start of the experiment until its ending. The experimental data used for the final kinetic studies in this paper were produced with an average heating rate of 200 K/min. However, the CO2 experiment exhibited significantly different kinetics from that of the procedure that uses inert gases with low heating rates.

2. Experimental 2.1 Coal Samples and Characterization The seven coal samples from the deep coal seams were provided by the Saskatchewan Geological Survey of the Saskatchewan Ministry of the Economy. These samples were taken from cored wells that were drilled in the Cretaceous Mannville Group within Alberta between 1969 and 1990. Figure 1 shows the location of origin of seven coal samples used in this study. The core samples were stored at atmospheric conditions in core boxes. The coal seams from which the core samples were taken are, on average, about 750 meters below the surface. From examination of the extracted core, the thickness of the coal intervals ranges from 0.5 to 2.0 meters. The locations (depths) of the core samples taken for analysis were taken at roughly the midpoint of the coal intervals.


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In addition to the in-situ coal samples, the experiments were also performed with two Canadian surface-mined coals, which were sub-bituminous coals that originated from two different Genesee mines in Alberta. Prior to TGA experiments, all coal samples were manually pre-ground to particles that were less than about 90 μm in diameter and then dried at 105°C in an air atmosphere for 24 hours. After drying, the coal was ground again in a ball mill and sieved to particles less than 90 µm in size. A Perkin Elmer CHNS/O 2400 elemental analyzer was used to measure the sulfur, carbon, hydrogen and nitrogen content. To determine the amounts of volatiles, fixed carbon and ash, an atmospheric TGA (NETZSCH TG 209 Libra F1) was used. The characterization procedure followed the ASTM D5142 standard for coal and coke. 2.2 Experimental Gasification Procedures The gasification experiments were performed in the same TGA that was used for the proximate analysis. Three different methods were used and compared with each other. In all experiments, the sample weight was equal to 10 mg (± 0.5 mg). The flow rate of gases was fixed at 25 ml/min. For Method 1, only CO2 was used from the beginning until the end of the experiments. No other gas was involved in this method. In Method 2, N2 was used from the start of the experiment until the sample temperature was equal to the final isothermal gasification temperature. The gas was then switched over to CO2. In Method 3, N2 was used to heat up the sample to the gasification temperature, but the sample was held for 60 more minutes in the N2 atmosphere before the gas was changed to CO2. Table 1 lists the protocols for the three methods.



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To get a stable weight reading, the procedure started with a five-minute isothermal period at 25°C. The TGA automatically heated up to a target temperature with an average heating rate equal to 200 K/min. All experiments were performed at three different target temperatures: 800, 850 and 900°C. The weight loss was recorded during the entire experiment. After the weight stopped decreasing over a period of 10 minutes, the experiment was stopped by cooling down the TGA. A blank run was also conducted with an empty crucible. The weight losses that were caused from the buoyancy effect were subtracted from the weight loss at the target temperature. During each experimental run, the weight of the sample and the time were saved automatically in a data log file. A data point was taken every 12 seconds. After subtracting the blank run from the sample run, the conversion (X) was calculated as: 𝑋=

𝑚0 − 𝑚𝑡 𝑚0 − 𝑚𝑎

(1)

where m0 is the initial sample mass at the time when the furnace was heated up and stabilized at the target temperature for Method 1 or when the gas was changed to CO2 for Methods 2 and 3. The sample mass at the time t is represented by mt, and ma is the mass of the ash. For the mass of ash, an average of the weight at one hundred percent conversion was used as indicated when the sample weight stopped changing. The rate of reaction r [mg/(mg·min)] is defined as the change of conversion versus reaction time: 𝑟=

Δ𝑋 1 𝑚𝑡 − 𝑚𝑡+𝑑𝑡 = Δ𝑡 𝑚0 𝑑𝑡

where m0 is the initial mass, mt the mass at time t, and mt+dt the mass at time t+ dt.


(2)

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During each gasification experiment, the TGA recorded several thousand data points. For analysis, a smooth function was fitted to the data and then 200 data points were generated from the smoothed data set over equal intervals from X=0 to X=1.

3. Gasification Kinetic Models Different kinetic models have been used to describe solid-gas reactions. Some of these models have been further developed with additional parameters that describe the form and structure of the solid or the behavior of the gas. However, the requirement for each model is supposed to be a simple equation with a strong physical background and the least number of parameters. For this reason, the following kinetic models were chosen to estimate the kinetic data. The simplest model that describes a gas-solid reaction is the volumetric model (VM)7. In this model, the reaction is considered to take place everywhere within the volume of the particle. Inside and outside of the particle are equivalently involved. The volumetric model is given by: 𝑑𝑋 = 𝑘𝑉𝑀 (1 − 𝑋) 𝑑𝑡

(3)

where kVM is the rate coefficient. A static particle whose structure does not change during the gasification is assumed. The shrinking core model (SCM)7 assumes that the reaction occurs only on the surface area of a shrinking carbon core. At the start, the particle is surrounded by a gas. With progressive conversion, an increasing ash layer surrounds the continuously shrinking internal core of unconverted material. This means that the reaction moves from the surface towards the interior of the particle. The external radius of the particle remains unchanged 7 ACS Paragon Plus Environment


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during the entire reaction. The gas film, the ash layer and the chemical reaction on the surface are three resistances that are most likely rate-controlled5. Different studies have shown that gas diffusion through the ash layer and reactions on the surface are the main rate-controlling steps8,9. The model is described by: 𝑑𝑋 = 𝑘𝑆𝐶𝑀 (1 − 𝑋)2/3 𝑑𝑡

(4)

where kSCM is the rate constant.

The integrated core model (ICM)10,11 improved the shrinking core model with a second parameter (n), which can be simultaneously adjust with the rate coefficient, kICM. dX = 𝑘𝐼𝐶𝑀 (1 − 𝑋)𝑛 dt

(5)

Parameter n is also described as reaction order or can be interpreted as a form factor for different geometries of the particle, for spheres, n=2/3, for cylinders, n=1/2, and for flat plates, n=0 10. The above models are not able to describe a maximum value of rate of reaction after zero conversion. The random pore model (RPM) mainly considers two competing effects of structural changes during the reaction procedure. There is the growth of accessible pores in the initial state of gasification and the coalescence or overlapping of neighboring pores’ surfaces, which reduces the area available for reaction12,13. Due to the high weighting effect of the growing pore surfaces that mainly happens at the beginning of the reaction, the model is able to predict a maximum in the rate of reaction. The overall reaction rate is given by:

𝑑𝑋 = 𝑘𝑅𝑃𝑀 (1 − 𝑋)�1 − 𝜓ln(1 − 𝑋) 𝑑𝑡 8 ACS Paragon Plus Environment

(6)

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where kRPM is the rate coefficient and ψ represent a structural parameter that describes the internal structure of the non-converted char. The definition of ψ is defined as: 𝜓=

4Π𝐿0 (1 − 𝜀0 ) 𝑆02

(7)

where S0 is the initial pore surface area per unit volume, L0 is the total pore length per unit volume, and ε0 is the initial solid porosity.

ψ is calculated using a reduced quantity, such as t/tX0.510,14: 𝑡

𝑡X0.5

=

�1 − 𝜓ln(1 − 𝑋) − 1

�1 − 𝜓 ln(1 − 0.5) − 1

(8)

Following this procedure, all the calculated data points are related to one specific chosen value of conversion. This method to estimate ψ is not representative if the data for r over X are not linear or show a maximum. Zou et al.11 determined ψ by zeroing the derivative term. For a maximum, it applies dr/dX=0, and the derivation of Eq. ( 6 ) is given as: 𝑑𝑟 𝜓𝑘𝑅𝑃𝑀 =0= − 𝑘𝑅𝑃𝑀 �1 − 𝜓ln(1 − X𝑚 ) 𝑑𝑋 2�1 − 𝜓ln(1 − 𝑋m )

(9)

where Xm is the conversion at the maximum reaction rate. Solving Eq. ( 9 ) yields an estimate of parameter ψ: 𝜓 = 2 (1 − 𝜓 ln(1 − 𝑋𝑚 ))

( 10 )

Assuming that rm is given at Xm=0, the value of ψ in Eq. ( 9 ) is equal to 2. The normal distribution function (NDF) is able to describe the gasification rate, even if the maximum is at X=0. Parameters are estimated by using nonlinear regression instead of a determined conversion assumption, which makes it easier to use this model. 9 ACS Paragon Plus Environment


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The rate of reaction can be expressed as a function of the intrinsic rate of reaction multiplied by a normal probability density: 𝑟=

𝑑𝑋 −(𝑋 − 𝑋𝑚 )2 = 𝑘𝑁𝐷𝐹 ∙ exp � � 𝑑𝑡 𝛼

( 11 )

where α is a constant and kNDF is the equivalent of the intrinsic rate of reaction under the conditions assumed.

4. Results and Discussion 4.1 Coal Properties Table 2 shows the properties of the seven in-situ coals and two surface-mined coals. The results for the surface-mined coals were in the predicted range and similar to the values in the literature15-17. However, the properties of the in situ coals were different. The results listed in Table 2 reveal that the ash content varied widely. For this reason, the in-situ coals can be classified as coals with ash contents of 7-10% or 44–80%. The values determined for Coals 3, 5, 6, and 7, through both proximate and ultimate analyses, were in similar ranges. Coals 1, 2 and 4 were the in-situ coals with ash contents larger than 40%. They exhibited different properties for the content of fixed carbon and volatiles. Coal 2 showed the highest ash content and the lowest carbon content. The sulfur contents of all the in-situ coals were significantly higher than those of the two surfacemined coals. Nitrogen and hydrogen contents were in the predicted normal range for all coals. All coals were characterized using a Micrometrics ASAPTM 2020 analyser to obtain surface area. As expected, the coal did not contain any macroporous structures, which is why the BET surface area results using nitrogen at 77°K are not given in Table 2. Using CO2 adsorption at 273°K (Dubinin-Radushkevich micropore surface area) yielded 10 ACS Paragon Plus Environment

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micropore surface areas between 49 to 135 m2/g for all coals. The last column in Table 2 shows the surface area relative to the carbon content measured at ultimate analysis.

4.2 Comparison of Gasification Methods Three different methods were used to heat up and gasify the coal samples. Methods 2 and 3 both used an inert gas (N2) to heat the sample to the target isothermal gasification temperature. Method 1 used CO2 from the beginning of the experiment. Figure 2 displays the results of the three methods for a target temperature of 900°C. Most of the gasification rate profiles in the literature

11,18,19

demonstrated curves

with maximum rates of reaction in the range between 0 and 0.25. Method 1 was able to improve CO2 gasification and identify that the maximum rate of reaction reported for conversion higher than zero may be mainly caused by mixing effects when N2 is switched to CO2 instead of the often-asserted intrinsic kinetics. There seems to be an inhibited condition at the beginning of the reaction if N2 or another inert gas is used to heat up the coal sample to a certain temperature before changing to CO2. The replacement of the inert gas with the reaction gas takes a certain time, especially if the sample is highly porous. In fact, the deep coal behavior was found to be the same as that of the surface-mined coal, but the new procedure avoids the overlapping of gasification kinetics and gas changing. This is a new contribution for kinetics analysis and gives a new tool to choose better kinetic models. Figure 3 shows the comparison of the experimental rates of reaction of the Genesee 1 coal for Methods 1, 2, and 3. When the reactivity of the coal was too low (e.g., deep coal 7), there was no evidence of a maximum rate of reaction, compared with highly reactive



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coal. The reason can be attributed to the slower kinetics compared with the faster mixtures and changing procedure of the gas in the TGA furnace. If the coal is highly reactive, such as the Genesee 1, the rates were faster than the mixture process itself. For high-reactivity coal, the occurrences of a maximum after X=0 is inescapable. The same conclusion applies for low temperature compared with high temperature. The rate of reaction for Method 1 was faster than that of the methods with inert gas in the beginning: presumably there was a surface change in the coal, which is consistent with the results of coke formation in slow and fast pyrolysis published by Umemoto et al.20, Kajitani et al.21 and Bruun et al.22. The reactivity of the char during gasification depends on the carbon surface that is generated during the pyrolysis. In order to obtain the kinetics, which is more representative of an industrial process where particles drop down in a continuously running gasifier or in-situ gasification pyrolysis, which is supposed to occur in the same ambient gas as the following gasification, all further analyses were based on Method 1.

4.3 Gasification Results All of the coal samples, both in-situ and surface-mined, can be classified with an ash content either larger than 25 percent or less than 10 percent. Figure 4 shows the results of all gasification experiments for three different temperatures following Method 1. As expected, the rate of gasification was faster at higher temperatures. Figure 5 and 6 show all coals in the order of time at the three different temperatures as they reached 50% of conversion (X=0.5) and 80% of conversion (X=0.8) during the gasification experiment. The surface-mined coals were the fastest at all three temperatures. Coal 2 was the coal with the highest ash content and exhibited the highest rate of reaction 12 ACS Paragon Plus Environment

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of the seven in-situ coals. Coals 3, 5, 6, and 7 had similar material properties, but did not show the same behavior during gasification. For example, Coal 5 reacted five to eight times faster than Coal 7, depending on the temperature. Similarly, Coals 1, 2 and 4 had similar material properties, but had different gasification rates. The order of coals also changed when they reached conversions of 50 and 80% at a certain temperature. For instance, Coal 1 was faster than Coals 3, 4 and 5 at 800°C, but was slower than all of the other coal samples at 850°C and 900°C. This strongly indicates different energies of activation even for coals with similar material properties. At the first glance, it seemed that there was no obvious correlation between the raw material properties and the gasification behavior. Assuming that active surface area is the one related to carbon content and not the one occupied by the ash content of the coal, the effect of the ash on the active surface area can be isolated using surface area associated only with carbon content. Therefore, a solid relationship exists between the reactivity and the carbon-based surface area (m2/gCarbon). The order of coal reactivity presented in Figures 6 and 7 reflects the results of the carbon-based surface areas listed in Table 2. Coals 7 and 6, which represented the lowest reactivity, showed the lowest carbon-based surface area (189 and 201 m²/gCarbon). Coals 1, 3, 4 and 5, which had almost the same carbon-based surface area (between 204 and 215 m²/gCarbon), were in the same medium range of gasification rates. Genesee 1, Genesee 2 and Coal 2 had the highest carbon-based surface area (256 to 329 m²/gCarbon) and also had the fastest gasification. These results suggest that the carbon-based surface area and reaction rate are correlated. They also indicate that the greater the ash content, the higher the reaction rate. Probably the mineral compositions in the ash, especially the alkali content, influence gasification behavior as suggested by Hattingh et al.25. 13 ACS Paragon Plus Environment


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Figure 7 shows the rates of reaction at 850°C. The experimental data points followed a logarithmic function for 800, 850 and 900°C. With increasing temperature, all curves changed slightly to a linear function. The surface-mined coal had a nearly linear profile at 900°C.

4.4 Modeling The experimental data were fitted by using the models described in Section 3. Due to the increasing uncertainty of measurement at lighter coal samples in higher conversion, only the rate of reaction data between 0 ≤ X≤ 0.8 has been used for parameter estimation, as practiced by other researchers11,19. The calculated rate of reaction was compared with the experimental data. The coefficient of determination (correlation coefficient, R2) was used to measure the goodness of fit between the model and experimental data. Figure 8 shows the comparison between the experimental data points and the calculated results using the different models for one of the surface-mined coals. Figure 9 shows the same comparison for the in-situ Coal 4. As stated above, the RPM (random pore model) was able to represent the maximum reaction rate. The RPM has been shown to provide good matches to experimental results in the literature10,11,18,19,23,24, where an inert gas was used to heat up the sample. Most of these studies determined geometric parameter ψ simultaneously with the rate coefficient. The present modeling follows the strict mathematical way to get ψ by zeroing the derivative term. Therefore, this parameter is not simultaneously determined but rather related to the most characteristic data point, i.e. the maximum rate of reaction. For this reason, the RPM was not able to describe the characteristics of gasification rate as exhibited in the results documented here. 14 ACS Paragon Plus Environment

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The best match was given by the ICM (integrated core model), which includes two adaptable parameters that are independent of each other. The normal distribution model (NDM) also has two adaptable parameters and provided a reasonable match, similar to that of the ICM. Both matches from the ICM and NDM resulted in similar average correlation coefficients (Fig. 10). The match between the VM (volumetric model) and the experimental results depended on the reactivity of the coal sample itself. The data show that the R2 of the VM was better and closer to the R2 of the ICM if the gasification was fast. This means the VM provided similar results to that of the ICM for coals with high reactivity, such as the two surface-mined coals. On the other hand, the R2 of the VM for all the coals with low reactivity, e.g., Coals 1, 4, 6, and 7, was significantly different from the R2 of the ICM, but closer to that of the SCM (shrinking core model). The coals with average reactivity, e.g., Coals 2, 3 and 5, matched the VM and SCM, between the high and low reactivity coals. The R2 for the VM and SCM were close to each other and closer to the R2 of the ICM. Figure 10 demonstrates this behavior for the temperature of 850°C. The results for 800 and 900°C were similar. These results are reasonable, due to the similar mathematical structure of the ICM, VM and SCM. Furthermore, based on figure 10, R2 is higher at higher temperatures regardless of the model. It is due to the higher reactivity, as the profile tends to be linear or just slightly curvilinear when the temperature increases. The reaction order decreases (e.g. Parameter “n” of ICM in Table S1) and the models fit more closely with experimental data. At higher temperature, the controlling step is pore diffusion and not chemical reaction26, 27.



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A summary of the modeling parameters and the matches with the experimental data as measured by the R2 correlation coefficient are listed in Table S1 of the Supporting Information.

4.5 Activation energy Based on the Arrhenius law and estimated rate coefficient, the activation energy and the frequency factor can be determined using the Arrhenius plot (listed in Table 3). The Arrhenius rate law is given by: ln(𝑘) = ln(𝐴)

−𝐸𝐴 𝑅∙𝑇

( 12 )

where A the pre-exponential factor, EA activation energy, T temperature and R the universal gas constant is. The coefficient of determination of the linear regression in the Arrhenius plot was 0.99 or higher. The determined value for the activation energy for each coal was in the expected range for coal.

5. Conclusions A new method that uses only carbon dioxide (CO2) and high heating rates (200 K/min) was successfully developed and used to evaluate nine different coals at three different temperatures. In typical practice, an inert gas (usually nitrogen) is used to heat the sample prior to gasification with carbon dioxide. In this study, two methods using inert gas and CO2 were investigated: one replaced the N2 with CO2 after the sample achieved the


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target temperature, and the other maintained the sample at the target temperature for 60 minutes with the inert gas prior to the switch to CO2. The new method used CO2 from the very beginning to the end of the experiment: no inert gas was used. The experiments showed significant differences in the determined rate of reaction for the three different methods used in the thermogravimetric analysis. The data analysis of the experiments, following the new method, showed no maximum rate of reaction. This new method helps to understand the gasification behavior in industrial processes, such as in situ gasification or gasification performed in a continuously fed reactor. There is an obvious relationship between the reactivity and the material properties determined in the study. In particular, the specific surface area calculated based on the carbon content measured in the ultimate analysis showed a correlation with the reactivity. The ash content and composition also appeared to influence char reactivity. The experimental data were used to evaluate five kinetic models: random pore, shrinking core, integrated core, volumetric and normal distribution models. However, the best matched model as described in the literature, i.e., the random pore model, did not fit the data presented in this study, due to the non-existing maximum after conversion zero. The best match was achieved by the integrated core and normal distribution models. The distribution of the activation energy determined based on the kinetic data was within the expected range for each model and coal. Acknowledgements The authors would like to express their appreciation for the financial support of the Business-Led Networks of Centres of Excellence (NCE) of Canada, Sustainable Technologies for Energy Production Systems (STEPS), through the Petroleum Technology



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Research Centre (PTRC). In addition, the authors would like to thanks Saskatchewan Geological Survey of the Saskatchewan Ministry of the Economy to facilitate the provisions of coal samples.

Associate Content Supporting Information A summary of the modeling parameters and the matches with the experimental data as measured by the R2 correlation coefficient are listed in Supporting Information for Publication Table S1. This information is available free of charge via the Internet at http://pubs.acs.org/.

Author Information Corresponding Author * (Nader Mahinpey) E-mail: [email protected]

Notes The Author declares no competing financial interest


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Nomenclature A EA k L m n r R R2 S T X

Pre-exponential factor [1·min-1] Energy of activation [kJ·mol-1] Rate coefficient [mg·(mg·min)-1] Pore surface area per unit volume [m·m-3] Mass [mg] Adjustable parameter integrated core model Reaction rate [mg·(mg·min)-1] Universal gas constant [J·(K·mol)−1] Correlation coefficient Total pore length per unit volume [m2·m-3] Temperature [K] Conversion

Greek Letters α Regression coefficient Δ Delta ε Solid porosity ψ Structural parameter Indices 0 a ICM m NDF RPM SCM t t+dt VM X0.5

Initial Ash Integrated core model Maximum Normal distribution function Random Pore Model Shrinking core model Time [min] Time plus time different [min] Volumetric Model Conversion of X=0.5



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REFERENCES

(1)

Coal and Mineral Development in Alberta, 2012 Year in Review; Alberta Energy, Resource Revenue and Operations Division Coal and Mineral Development Branch: Edmonton, 2013.

(2)

Saskatchewan Exploration and development Highlights 2012: Saskatchewan Ministry of the Economy: Regina, 2012, 1-2.

(3)

Richardson R. J. Alberta’s 2 Trillion Tonnes of Unrecognized Coal, Alberta Innovates - Energy and Environment Solutions, Alberta, 2010, 5-6.

(4)

Richardson, R. J.; Singh, S. Prospects for underground coal gasification in Alberta, Canada. P I Civil Eng-Energ 2012, 165 (3), 125-136.

(5)

Fung, D. P. C.; Sang K. D. Chemical reactivity of Canadian coal-derived chars. Fuel 1984, 63 (9), 1197-1201.

(6)

Furimskya, E.; Palmera, A. D.; Kalkreutha, W.D.; Camerona, A.R.; Kovacika, G. Prediction of coal reactivity during combustion and gasification by using petrographic data. Fuel Process. Technol. 1990, 25 (2), 135-151.

(7)

Levenspiel, O. Chemical Reaction Engineering 3nd ed.; John Wiley & Sons: New York, 1999, 566-586.

(8)

Schmal, M.; Monteiro, J. L. F.; Castellan, J. L. Kinetics of coal gasification. Ind Eng Chem Proc DD 1982, 21 (2), 256-266.

(9)

Chan, E. M.; Papic, M. M. Gasification of hat creek coal. Can J Chem Eng 1976, 54 (6), 645-646.


Page 20 of 37

Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10)

Xu, S.; Zhou, Z.; Gao, X.; Yu, G.; Gong, X. The gasification reactivity of unburned carbon present in gasification slag from entrained-flow gasifier. Fuel Processing Technology 2009, 90 (9), 1062-1070.

(11)

Zou, J. H.; Zhou, Z. J.; Wang, F. C.; Zang, W.; Dai, Z. H.; Liu, H. F.; Yu, Z. H. Modeling reaction kinetics of petroleum coke gasification with CO2. Chem Eng Process 2007, 46 (7), 630-636.

(12)

Bhatia, S. K.; Perlmutter, D. D. A random pore model for fluid−solid reactions: I. Isothermal and kinetic control. AICHE J 2004, 26 (3), 379-386.

(13)

Fermoso, J.; Arias, B.; Pevida, C.; Plaza, M. G.; Rubiera, F.; Pis, J. J. Kinetic models comparison for steam gasification of different nature fuel chars. J Therm Anal Calorim 2008, 91 (3), 779-786.

(14)

Everson, R. C.; Neomagusa, H.W.J.P.; Kaitanoa, R.; Falconb, R.; du Cannc, V. M. Properties of high ash coal-char particles derived from inertinite-rich coal: II. Gasification kinetics with carbon dioxide. Fuel 2008, 87 (15-16), 3403-3408.

(15)

Pollock, S.M., Goodarzi, F; Riediger, C.L Mineralogical and elemental variation of coal from Alberta, Canada: an example from the No. 2 seam, Genesee Mine. Int J Coal Geol 2000, 43 (1), 259-286.

(16)

Cousins, A.; McCalden, D. J.; Hughes, R. W.; Lu, D.Y.; Anthony, E. J. Entrained-flow gasifier fuel blending studies at pilot scale. Can J Chem Eng 2008, 86 (3), 335-346.

(17)

Gao, C.; Vejahati, F.; Katalambula, H.; Gupta, R. Co-gasification of Biomass with Coal and Oil Sand Coke in a Drop Tube Furnace. Energ Fuel 2009, 24 (1), 232-240.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(18)

Mandapati, R. N.; Daggupati, S.; Mahajani, S. M.; Aghalayam, P.; Sapru, R. K.; Rakesh, K. S.; Ganesh, A. Experiments and Kinetic Modeling for CO2 Gasification of Indian Coal Chars in the Context of Underground Coal Gasification. Ind Eng Chem Res 2012, 51 (46), 15041-15052.

(19)

Malekshahian, M.; Hill, J. Kinetic analysis of CO2 gasification of petroleum coke at high pressure. Energ Fuel 2011, 25 (9), 4043-4048.

(20)

Umemoto, S; Kajitani, S.; Hara, S. Modeling of coal char gasification in coexistence of CO2 and H2O considering sharing of active sites. Fuel 2013, 103, 14-21.

(21)

Kajitani, S.; Suzuki, N.; Ashizawa, M.; Hara, S. CO2 gasification rate analysis of coal char in entrained flow coal gasifier. Fuel 2006, 85 (2), 163-169.

(22)

Bruun, E. W.; Ambus, P.; Egsgaard, H.; Hauggaard-Nielsen, H. Effects of slow and fast pyrolysis biochar on soil C and N turnover dynamics. Soil Biol Biochem 2012, 46 (0), 73-79.

(23)

Cousins, A.; Hughes, R. W.; McCalden, D. J.; Lu, D. Y.; Anthony, E. J. Waste Classification of Slag Generated in a Pilot-Scale Entrained-Flow Gasifier. Coal Combustion and Gasification Products 2011, 4, 37-44.

(24)

Hui, S.; Song, B.-H.; Jang, Y.-W.; Kim, S.-D.; Li, H.; Chang, J. The characteristics of steam gasification of biomass and waste filter carbon. Korean J Chem Eng 2007, 24 (2), 341-346.

(25)

Hattingh,B. B.; Everson, R. C.; Neomagus, H. W. J. P.; Bunt, J. R. Assessing the catalytic effect of coal ash constituents on the CO2 gasification rate of high ash, South African coal. Fuel Process Technol 2011, 11, 2048-2054.


Page 22 of 37

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26)

Ahmed, I.I.; Gupta; A.K. Kinetics of woodchips char gasification with steam and carbon dioxid. Appl Energ 2011, 88, 1613-1619.

(27)

Tremel, A.; Haselsteiner, T.; Kunze, C.; Spliethoff, H. Experimental investigation of high temperature and high pressure coal gasification. Appl Energ 2012, 92, 279-285.

List of Tables

Table 1: Comparison of TGA methods Table 2: Proximate and ultimate analyses and Dubinin-Radushkevich surface area (micropore area using CO2 at 273K) Table 3: Comparison of the energies of activation for the different coal samples and different models



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List of Figures

Figure 1: Sample location Figure 2: Comparison of the three methods Figure 3: Comparison of rate of reaction for Methods 1, 2 and 3 Figure 4: Comparison of conversions and times for different coals at different temperatures Figure 5: Comparison of time to reach 50% of conversion Figure 6: Comparison of time to reach 80% of conversion Figure 7: Rate of reaction (dX/dt) vs. conversion (X) at 850°C Figure 8: Comparison of experimental data for the surface-mined Genesee 2 coal with different models at 850°C. Figure 9: Comparison of experimental data for in-situ Coal 4 with different models at 850°C Figure 10: Coefficient of determination (R2) for the matches between the models and experimental data at 850°C


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Table 1: Comparison of TGA methods State

Characteristic

General

Sample weight

10 ± 0.5 mg

10 ± 0.5 mg

10 ± 0.5 mg

Gas flow rates

50 ml/min

50 ml/min

50 ml/min

Start Temp.

25 °C

25 °C

25 °C

Isoth. time

5 min

5 min

5 min

Gas

CO2

N2

N2

Heating rate

200 K/min

200 K/min

200 K/min

Gas

CO2

N2

N2

At final temperature

Gas change after

no change

0 min

60 min

Isothermal gasification

Gas

CO2

CO2

CO2

Finish

Weight reading

constant

constant

constant

Start

Heat up

Method 1

Method 2


Method 3


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Table 2: Proximate and ultimate analyses and Dubinin-Radushkevich surface area (micropore area using CO2 at 273K) Proximate Analysis Volatiles

Fix Carbon

Ultimate Analysis Ash

C

H

Surface area Dubinin-Radushkevich N

S

Coal based 2

m

/gCoal

Carbon based 2

m

/gCarbon

wt%dry

wt%dry

wt%dry

wt%dry

wt%dry

wt%dry

wt%dry

Coal 1

24.1

18.7

57.2

22.6

2.0

Kinetic Studies of a Novel CO2 Gasification Method Using Coal from

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