A small-size gasification unit is improved through process optimization to simulate industrial United Gas Improvement Company gasification. It finds that the reaction temperature has important impacts on semicoke catalyzed methane gas mixture. The addition of water vapor can enhance the catalytic activity of reforming, which is due to the fact that addition of water vapor not only removes carbon deposit produced in the reforming and gasification reaction processes, but also participates in gasification reaction with semicoke to generate some active oxygen-containing functional groups. The active oxygen-containing functional groups provide active sites for carbon dioxide reforming of methane, promoting the reforming reaction. It also finds that the addition of different proportions of methane-rich gas can yield synthesis gas with different H2/CO ratio. The kinetics study shows that the semicoke can reduce the activation energy of the reforming reaction and promote the occurrence of the reforming reaction. The kinetics model of methane reforming under the conditions of steam gasification over semicoke is as follows:
China is one of the largest countries of coke production and consumption in the world. At the same time, a great amount of coke oven gas is produced. Part of the coke oven gas is used to maintain furnace temperature, but about 85 billion cubic meters are wasted without using [
However, the average bond energy of C–H is 4 × 105 J/mol, and CH3–H bond dissociation energy also reaches 4.35 × 105 J/mol. The methane molecules have strong stability and can be converted or decomposed only under high temperature or catalytic action. In recent years, researches on the catalyst for methane reforming are constantly emerging. In addition to precious metals and transition metals, carbon material is also found to possess a significant catalytic action to methane reforming and, therefore, has preferable anticarbon deposition property. Zhang et al. found that, under the catalytic action of carbon, the conversion rate of CH4 and CO2 was higher in the initial phase and then declined to the stable level [
Carbon material used in the research was provided by Town Star Industry Co., Ltd., in Linfen, Shanxi. The results of industrial analysis and elemental analysis are shown in Table
The proximate analysis and ultimate analysis of carbon catalyst.
Sample | Proximate analysis, wt, %, ad | Ultimate analysis, wt, %, daf | ||||||
---|---|---|---|---|---|---|---|---|
Moisture | Ash | Volatile matter | C | H | N | S | O (diff) | |
Semicoke | 0.54 | 15.89 | 2.09 | 81.29 | 0.61 | 0.60 | 1.33 | 0.28 |
ad: air dried; daf: dry ash free; diff: difference.
Composition of methane-rich gas.
Composition | O2 | N2 | CH4 | CO | CO2 | CmHn | H2 |
---|---|---|---|---|---|---|---|
Content, % | 0.91 | 5.63 | 53.88 | 19.31 | 8.05 | 6.63 | 5.59 |
The reactions over the catalyst were carried out at normal pressure in a continuous-flow quartz reactor (i.d. 20 mm) packed with particle size of 2 mm,
After the catalyst had served the reaction for a specified period of time, the reaction feed was switched to inert nitrogen (high purity), followed by cooling in nitrogen flow of the reactor to room temperature at which the used catalyst was unloaded for various characterizations.
All samples were degassed at 573 K for 1 h before measurements to remove impurities of the catalyst surface. The specific surface area of the catalysts was determined by nitrogen adsorption-desorption measurement at −196°C in Tristar gas adsorption system.
Scanning electron microscopy (SEM) observations of the catalyst samples were performed using a JSM-4800 (Japan, JEOL Ltd.).
Under the conditions of atmospheric pressure, flow rate of 363 mL/min, and CO2/CH4 = 1, the influence of semicoke temperature on methane reforming is inspected, with the results shown in Figure
Effect of temperature on conversion of methane and carbon dioxide.
Under the conditions of atmospheric pressure, flow rate of 363 mL/min, and CO2/CH4 = 1, the influence of semicoke particle size on methane reforming is inspected, with the results shown in Figure
Effect of particle size on conversion of methane and carbon dioxide.
Effect of water vapor on conversion of methane.
Under the conditions of atmospheric pressure and flow rate of 240 mL/min, CO2/CH4 = 1, and H2O/CH4 = 0.9, methane-rich gas mixture and water vapor are engaged in reforming reaction in the presence of semicoke. The methane conversion is illustrated in Figure
Surface area, pore volume, and pore size of semicoke.
Sample | Surface area/m2 g−1 | Pore volume/cm3 g−1 | Pore size/nm |
---|---|---|---|
Before reaction | 15.192 | 0.009 | 8.735 |
After reaction | 18.418 | 0.013 | 7.116 |
SEM of catalysts.
Under atmospheric pressure, flow rate of methane-rich gas mixture 343 mL/min, and temperature 1100°C, the influence of different CH4/CO2 ratios on H2/CO ratio is inspected, with the results shown in Figure
Effect of the proportion of CH4 on H2/CO ration in the syngas.
During the water vapor gasification of carbon, the methane-rich gas mixture is mainly involved in the following reactions:
Due to the multiplicity of reactions in this system, the following presumptions are made for the model to study methane conversion: methane reforming reaction is the main reaction and only the influences of temperature and initial concentration of reactants on conversion rate are considered, regardless of the intermediate products [
When water vapor is introduced, the methane consumption rate is expressed as the function of reaction rate and reaction time. Whether the maximum rate occurs or not is not considered. The reaction rate is expressed by
Reaction rate
The integration of formula (
The comparison of formula (
By establishing simultaneous equations with formula (
By whole-process integration of formula (
The relations between average rate constant and partial pressure of gas component
A logarithm is taken on both sides of the equation:
Under the conditions of atmospheric pressure and inlet gas flow rate 343 mL/min, methane conversion is analyzed under different temperatures. At methane partial pressure of 0.44 atm, CO2 partial pressure of 0.08 atm, and hydrogen partial pressure of 0.28 atm, the methane conversions are shown in Figure
Kinetic parameters.
|
|
|
|
---|---|---|---|
|
0.71 | 0.26 | 74.2 |
The effect of different proportions of methane on CH4 conversion.
The apparent activation energy for methane consumption in this system is 74.2 kJ·mol−1, but Meng et al. calculated the value to be 94.5 kJ·mol−1 by theoretical analysis and simulation of reforming of methane-rich gas conversion to synthesis gas in the presence of semicoke [
Based on United Gas Improvement Company coal gasification technology, a small-size gasification unit is improved through process optimization to simulate industrial United Gas Improvement Company gasification. The results show that the reaction temperature has important impacts on semicoke catalyzed methane gas mixture. The addition of water vapor can enhance the catalytic activity of reforming, which is due to the fact that addition of water vapor not only removes carbon deposit produced in the reforming and gasification reaction processes, but also participates in gasification reaction with semicoke to generate some active oxygen-containing functional groups. The active oxygen-containing functional groups provide active sites for carbon dioxide reforming of methane, promoting the reforming reaction. It also finds that the addition of different proportions of methane-rich gas can yield synthesis gas with different H2/CO ratio. The kinetics study shows that the semicoke can reduce the activation energy of the reforming reaction and promote the occurrence of the reforming reaction. The kinetics model of methane reforming under the conditions of steam gasification over semicoke is as follows:
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the National Science & Technology Pillar Program (Grant No. 2012BAA04B03), Natural Science Foundation of China (Grant Nos. 21006066 and 51274147), Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi and Shanxi Provincial Natural Science Foundation (Grant No. 2010011014-1).