Article pubs.acs.org/EF
CO2 Regeneration of Used Alkali Carbonates for High-Temperature Desulfurization in Gasification Slamet Raharjo,*,† Ivan Nedjalkov,‡ Yasuaki Ueki,§ Ryo Yoshiie,‡ and Ichiro Naruse§ †
Department of Environmental Engineering, Faculty of Engineering, Andalas University, Kampus Limau Manis, Padang, West Sumatera 25163, Indonesia ‡ Department of Mechanical Science & Engineering, Graduate School of Engineering and §Institute of Materials and Systems for Sustainability, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ABSTRACT: Gasification systems are expected to be the next-generation, highly efficient, and environmentally friendly energy conversion technology of carbonaceous resources. However, those systems are still ongoing development. Our previous studies suggested that molten alkali carbonates can be used to enhance gasification reactions and absorb H2S and COS at high temperatures. Furthermore, sulfurized alkali carbonates can be regenerated using steam and CO2. However, employing only waste-gas CO2 would further decrease releases of this greenhouse gas. Therefore, regeneration characteristics of used alkali carbonates using only CO2 as the regeneration agent were studied in this paper. Chemical equilibrium simulation of the regeneration process was carried out using Factsage 6.2 software for selecting the optimum condition for experiments. An electrically heated quartz tube was used for conducting the experiments. Na2S·9H2O and K2S were placed inside the tube, and CO2 was introduced with the hourly space velocity of 59.72 h−1. Experimental results showed that CO2 flow into the used alkali carbonates at 900 K resulted in around 92% conversion from alkali sulfides into alkali carbonates. Thermogravimetric analysis at 650, 750, 800, 873, and 900 K gave rate constants (k) of 0.2022, 0.2171, 0.2588, 0.3885, and 0.3926 min−1, respectively.
1. INTRODUCTION The worldwide energy issues in recent decades have pressed many researchers to put more efforts on developing highly efficient and clean energy conversion technologies for carbonaceous resources. Integrated gasification fuel cell (IGFC) combined cycle is one of those technologies that have been developed in recent years.1,2 It features a triple combined cycle power generation system consisting of a gas turbine, a steam turbine, and high-temperature fuel cells, such as molten carbonate fuel cells (MCFCs). To meet the high standard of gas quality required for the application in MCFCs, IGFC must be equipped with a precise gas purification system.3,4 Usually it is a wet-type gas purification system. To protect the solvents used in the wet-type system, some of those units must be operated at lower temperatures.5 For high-temperature product gas, lowering the gas temperature means loss of energy, which leads to significant loss of the overall thermal efficiency of the system. Therefore, it is necessary to develop a system that reduces H2S and COS to sufficiently low levels at a relatively high temperature. A system using both gasification and gas purification using molten alkali carbonates was proposed in our previous studies.6 Eutectic salt with the composition of 43 mol % Na2CO3−57 mol % K2CO3 was employed. As a result, the molten alkali carbonate gasifier can gasify carbonaceous materials and remove gaseous sulfur, even at high temperatures.6,7 Regeneration of the used alkali carbonates using steam and CO2 was also studied. It suggested that introducing a mixture of steam and CO2 at 773 K could regenerate the used alkali carbonates to around 95%.8 Steam and CO2 contained in the gasified gas can be reused as the regeneration agents. However, employing only waste-gas CO2 would further decrease releases of this greenhouse gas. Therefore, this paper © XXXX American Chemical Society
explains regeneration characteristics of used alkali carbonates using pure CO2 as a regeneration agent.
2. EXPERIMENTAL SECTION 2.1. Used Molten Alkali Carbonates. Our previous studies suggested that, during the gasification process, molten alkali carbonates (43 mol % Na2CO3−57 mol % of K2CO3) absorb H2S and COS at 1173 K to form alkali sulfides (the used molten alkali carbonates) according to the following reactions:7
M 2CO3(l) + H 2S(g) → M 2S(c) + H 2O(g) + CO2 (g)
(1)
M 2CO3(l) + COS(g) → M 2S(c) + 2CO2 (g)
(2)
where M is Na and K, l is liquid, g is gas, and c is condensed. During gasification and desulfurization processes, catalytic and gaseous sulfur sorbent capabilities would be deactivated as a result of the formation of alkali sulfides. Therefore, the fully loaded sorbent should be regenerated, so that it can be reused many times. These regeneration experiments used a chemical compound of Na2S·9H2O and K2S as the raw M2S sample, crushed and sieved to a particle size of less than 500 μm.9 The raw simulated M2S sample contains moisture and crystal water that must be removed before introducing the regeneration agent. A mixture of Na2S and K2S with a mole ratio of 1:1 was set for the regeneration experiments. Moisture and crystal water composition in the raw M2S sample was investigated by a thermogravimetric analyzer (TGA). It suggested that the weight decrease as a result of the evolution of the moisture and crystal water contents in the raw M2S sample was around 50%.8 This value is very important to estimate the net weight of the raw sample because the electrically heated quartz tube cannot record the weight loss. Received: March 29, 2016 Revised: August 27, 2016
A
DOI: 10.1021/acs.energyfuels.6b00722 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels 2.2. Chemical Equilibrium Calculations. Prior to experiments, the regeneration process was simulated using FactSage 6.2 software. The optimum regeneration temperature and CO2/M2S mole ratio were determined from these simulations. Simulation conditions are given in Table 1. The Equilib module of FactSage was selected to
calculated by subtracting the moisture and crystal water content from the raw M2S sample, obtained from the thermogravimetric analysis.
Table 1. Simulation Conditions for Chemical Equilibrium Calculation
2.3.2. Kinetic Parameters. Thermogravimetric analysis was used to predict kinetic parameters by applying the Arrhenius equation. The raw M2S sample was placed inside a platinum plate, which was then positioned in the TGA. At first, nitrogen was supplied to the system to maintain an inert atmosphere. The temperature of the furnace was raised at rate of 0.33 K/s, until 383 K, where it was kept for 8 min, before being raised again, to the regeneration temperatures (650, 750, 800, 873, and 900 K). Nitrogen was switched to CO2 at the regeneration temperatures. Weight changes of the sample during experiments were continuously recorded. The Arrhenius equation can be expressed as
⎤ ⎡⎛ W (g) ⎞ R s (mass %) = ⎢⎜ sr ⎟ × 100%⎥ ⎥⎦ ⎢⎣⎝ Ws0 (g) ⎠
mole ratio Na2S K2S regeneration agent (CO2) temperature range (K)
run 1
run 2
1 1 4
1 1 50 470−1010
conduct the chemical equilibrium simulations, with FTPulp as the solution database and FACT53 as the solid and gaseous database. The mole ratio of Na2S/K2S was set to 1:1 in both runs, while the fraction of CO2 was increased in the second run. 2.3. Regeneration Experiments. 2.3.1. Regeneration Experiments in an Electrically Heated Quartz Tube. Figure 1 shows the
(3)
k = Ae−Ea / RT
(4) −1
where k is the rate constant (time ), A is the pre-exponential factor/ frequency factor (time−1), e is Euler’s number (2.72), Ea is the activation energy (J/mol), R is the gas constant (8.31 J mol−1 K−1), and T is the temperature (K). Equation 4 can be rewritten as follows:
ln k = ln A −
Ea RT
(5)
First-order kinetics was used to analyze thermogravimetric data, as illustrated in Figure 2, to determine the kinetic parameters in eq 4.
Figure 1. Experimental apparatus.
quartz tube used for the regeneration experiment. It measures 800 mm in length with an inner diameter of 40 mm. The tube was heated by electric ceramic heaters. At first, the tube was purged using nitrogen gas for at least 10 min with a flow rate of 16.67 cm3/s before inserting the sample. N2 continued to be supplied until switching to the regeneration agent. Around 1 g of alkali sulfides was placed in a ceramic boat and put into the quartz tube. The temperature of the tube was increased at a rate of 0.17 K/s, held at 383 K for 8 min, and finally raised to the desired regeneration temperature (650, 773, 873, 900, and 925 K). Afterward, the temperature was kept constant for 30 min under a nitrogen atmosphere before switching to the regeneration agent. CO2 was then supplied for 60 min with a flow rate of 16.67 cm3/s, giving an gas hourly space velocity of 59.72 h−1. Plastic bags were used to collect the emitted gas at 15, 30, 45, and 60 min during the regeneration process. The composition of gaseous sulfur in the exhaust pipe during experiments was analyzed using a gas chromatograph with a flame photometric detector (GC−FPD). During the reaction, a small fraction of sulfur may fail to react with the regeneration agent and remain in the sample (solid residue). The fraction of the remaining sulfur in the solid residue can be used to calculate the conversion efficiency from alkali sulfides into alkali carbonates. Equation 3 calculates the percentage of remaining sulfur in the solid after regeneration experiments (Rs). An EMIA-120 Horiba apparatus was used to analyze the mass of sulfur in the solid residue (Wsr). The total amount of sulfur in the raw M2S sample (Ws0) was
Figure 2. Typical TGA results of the regeneration process. Figure 2 shows typical thermogravimetric data obtained from regeneration experiments, where the blue line represents the weight of the sample, the orange curve shows the temperature profile, and the x axis is the time. When the desired regeneration temperature was reached, the atmosphere was switched to CO2, causing the solid sample to react with CO2 and increase its weight (W2 − W1).
3. RESULTS AND DISCUSSION 3.1. Chemical Equilibrium Calculations. Figure 3 shows the results of the FactSage simulation. Panels a and b of Figure 3 show production fractions of sodium carbonate and potassium carbonate, respectively. These graphs suggest that the regeneration efficiency of sodium carbonate and potassium carbonate would be 100 and 50%, respectively. The simulation results suggest that the amount of carbon dioxide has a minor effect on the production fractions of sodium carbonate, while a relatively higher amount of carbon dioxide would be favorable for potassium carbonate. In addition, the simulations suggested that the optimum regeneration temperature for both cases would be between 600 and 900 K. B
DOI: 10.1021/acs.energyfuels.6b00722 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 4 shows that 900 K is the optimum regeneration temperature at which the remaining sulfur is only 8% of the total sulfur content in the raw sample. This result suggests that around 92% of the sulfur was vaporized during regeneration. The optimum regeneration temperature is higher than the previous result using steam and CO2 as the regeneration agents.8 Therefore, the regeneration process with the addition of steam contributed to enhancing the regeneration process at a lower temperature compared to that without steam addition.10 A low regeneration temperature is more favorable to improve the overall efficiency in the gasification system. Figure 5 shows
Figure 5. Gaseous sulfur concentration in exhaust gas during regeneration experiments.
the average concentration of gaseous sulfur in the exhaust pipe during regeneration experiments at 650, 773, 873, and 973 K. Carbonyl sulfide was the dominant gaseous sulfur, with the highest concentration of around 900 K. It also suggests that the regeneration process reaches its peak performance, as explained in Figure 4. On the basis of those results, the expected regeneration reaction scheme can be proposed as follows: Figure 3. Chemical equilibrium calculation of the regeneration process.
M 2S(s) + 2CO2 (g) → M 2CO3(s) + COS(g)
(6)
where M is Na and K, s is solid, and g is gas. Meanwhile, the addition of steam results in the formation of hydrogen sulfide instead of carbonyl sulfide.8 3.2.2. Kinetic Parameters. Figure 6a shows the weight changes of the M2S sample during the regeneration process at 900 K from the TGA experiments. The decreasing weight under a nitrogen atmosphere shows the weight loss of moisture and crystal water content. As carbon dioxide was introduced, the weight of the M2S sample increased. This is due to the formation of a heavier compound, which is expected to be M2CO3. Thermogravimetric data from t1 to t2 in Figure 6a can be rearranged as displayed in Figure 6b. It shows the decreasing total weight change over time [TWC]t. Figure 6c shows a graph of ln[TWC]t with respect to time (t) for first-order kinetics. The slope of this graph gives a rate constant (k) of 0.3926 min−1. Other TGA experiments at 650, 750, 800, and 873 K gave rate constants of 0.2022, 0.2171, 0.2588, and 0.3885 min−1, respectively. Finally, the relation between ln k and 1/T of each regeneration experiment was displayed in Figure 7. The graph gives the straight-line equation y = −1.6709x + 0.8635. The slope (m) and intercept (c) of the fitted model give the activation energy (Ea) of 13.88 kJ/mol and pre-exponential factor (A) of 2.37/min. However, Figure 7 also suggests that the apparent activation energy is not constant with temperature
3.2. Regeneration Experiments. 3.2.1. Electrically Heated Quartz Tube. These experiments provided the weight fraction of sulfur in the solid residue. Figure 4 displays the remaining sulfur in solid after regeneration experiments (Rs).
Figure 4. Results of the quartz tube experiment with CO2 as the regeneration agent. C
DOI: 10.1021/acs.energyfuels.6b00722 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 7. Relation between ln k and 1/T.
area of the solid may be the reason for reduced regeneration efficiency above 900 K, as displayed in Figures 4 and 5. Changes in the surface area of the solid have significant implications on the amount of heat transfer in the gas−solid phase reaction.
4. CONCLUSION Chemical equilibrium calculations suggested that carbon dioxide can regenerate the used alkali carbonates in the optimum temperature range of 600−900 K. Regeneration experiments using an electrically heated quartz tube demonstrated that the used alkali carbonates can be regenerated by introducing carbon dioxide in a temperature range of 650−925 K. The regeneration temperature of 900 K displayed the optimum result at which around 92% of the alkali sulfides was regenerated. Thermogravimetric analysis at 650, 750, 800, 873, and 900 K gave rate constants (k) of 0.2022, 0.2171, 0.2588, 0.3885, and 0.3926 min−1, respectively. However, the apparent activation energy is not constant with temperature coverage. A sintering problem that leads to a decrease of the surface area of the solid may be the reason for reduced regeneration efficiency above 900 K.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +62-751-72497. Fax +62-751-72566. E-mail:
[email protected] and/or
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The corresponding author expresses his sincere gratitude to the Faculty of Engineering, Andalas University, West Sumatera, for giving him advice, support, and guidance in preparing this manuscript under a program of International Publication Acceleration 022/UN16/PL/AKS/2015.
Figure 6. TGA data analysis from the regeneration experiment at 900 K.
coverage. It is likely that the regeneration process in not purely a kinetic phenomenon. It may also include a contribution from the thermodynamics of the process. The gas−solid phase reaction relies on the surface area of the solid. In the current regeneration experiments, 900 K is the optimum contact between alkali sulfides and carbon dioxide. Davim et al. observed that the surface area of calcium phosphate glass increases up to 740 °C. However, the reduction of the surface area as a result of the sintering process was observed above 740 °C.11 A sintering problem that leads to a decrease of the surface
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DOI: 10.1021/acs.energyfuels.6b00722 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.6b00722 Energy Fuels XXXX, XXX, XXX−XXX
