Sulfur-Looping Mechanism for the Two-Step Cyclic Process of

Subscriber access provided by UNIVERSITY OF LEEDS

Article

Sulfur-looping mechanism for the two-step cyclic process of fluidized bed CO2 capture and Phosphogysum thermal decomposition assisted by H2S Siqi Zhao, Liping Ma, Dongdong Wang, Jie Yang, Yuhui Peng, and Lichun Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02529 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

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

Energy & Fuels

Sulfur-looping Mechanism for the Two-step Cyclic Process of Fluidized Bed CO2 Capture and Phosphogysum Thermal Decomposition assisted by H2S Siqi Zhao, Liping Ma*, Dongdong Wang, Jie Yang, Yuhui Peng, Lichun Wang Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093, China HIGHLIGHTS:

PG decomposition used as a Ca-based absorbent to capture CO2.

Exhaust gas generated during carbonation process can be reused.

Two-step cyclic reaction system for CO2 capture.

Mechanism of sulfur-looping routes in reaction process investigated.

ABSTRACT: Anthropogenic greenhouse gas emissions are the main concern for global warming, and needs to be urgently addressed. Ca-based absorbent for the method is one of the CO2 capture technology, and can be combined with waste H2S to create a better capture capacity. Here, we use decomposed phosphogysum (PG) as a Ca-based absorbent to solve the problem of PG resource utilization along with CO2 capture. Furthermore, the H2S exhaust gas generated during the carbonation process is reused to further promote capture capacity of the absorbent. Different liquid-solid ratios (L/S) and thermal parameters are investigated to determine optimal conditions. A relatively low value of L/S (6:1) can better capture CO2. Meanwhile, the CO2 capture capacity and H2S concentration were measured to explore the mechanisms of the carbonation process. XRD and XRF analyzed the chemical components of both reactants and resultants. The morphology of PG and CaCO3 are rhombus and calcite shapes, respectively. Most importantly,

1

ACS Paragon Plus Environment

Energy & Fuels

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

sulfur

looping

(S-looping)

routes

and

reaction

Page 2 of 33

process

mechanism

of

this

decomposition/carbonation cyclic system are determined and outlined, indicating that sulfur-looping throughout the entire experiment. Keywords: S-looping; phosphogypsum decomposition; Ca-based absorbent; CO2 capture, reaction mechanisms

1. Introduction Global warming due to anthropogenic greenhouse gases needs to be urgently and applicably addressed [1]. With the acceleration of industrialization process, human activity is considered to be the main source of the CO2 greenhouse gas emissions [2]. It has been estimated that by the year 2050, to decrease the average global temperature by 2.0-2.4ºC compared to the year 2000, then the world needs to reduce nearly 50%-80% of CO2 emission levels [3]. According to the IEA's forecast, by 2050 the world can reduce about 19% of its greenhouse gas emission with CO2 Capture and Storage (CCS) energy-saving emission reduction techniques. Consequently, it is decisive to curtail the CO2 emission substantially in order to meet the target to limit global warming well below 2°C above the pre-industrial levels agreed upon in the 2015 Paris Agreement [1]. Carbon dioxide capture and storage (CCS) is an important measure to reduce emissions of greenhouse gases in the face of extreme energy demand growth [4, 5]. CO2 capture refers to the separation and enrichment process of CO2 produced from point sources such as metallurgical, cement, electricity and other industries that use fossil fuels. According to different characteristics, CO2 capture can be divided into pre-combustion, post-combustion and oxy-fuel combustion. Currently, CO2 storage technology mainly includes geological storage, marine storage and

2


Page 3 of 33

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

Energy & Fuels

industrial carbon sequestration. The so-called industrial carbon sequestration refers to the carbonation cycle reaction, or reacting CO2 with various materials to create a solid carbonate material (CO2 or CO3- based substance) [2]. Calcium-based absorbent (Ca-based absorbent) is one of the CO2 capture technologies currently widely implemented. The Ca-based absorbent is placed in conditions so that CO2 is absorbed to the material. Ca-based absorbent has the characteristics of high absorption capacity, good absorption effect, high temperature resistance and wide distribution in nature. In recent years, the use of CaO and other Ca-based materials for CO2 capture has been widely researched and developed. Based on the cyclic carbonation/calcination of CaO, Miranda-Pizarro, et al. has developed a potentially low cost technique for CO2 capture, although it consumes energy [6]. Steam hydration has been found increase the reactivity of CaO for CO2 capture and reactivity and friability of reactivated sorbent [7]. Table 1 shows the research status of Ca-based absorbent, which provides a theoretical basis and research direction. Table 1. Research Status of Ca-based Absorbents Researcher

Ca-based Absorbent

Joshuah K. Stolaroff etc.[8]

CaO/Ca(OH)2

Zhen-shan Li etc. [9]

CaO/Ca12Al14O33

V. Nikulshina etc. [10]

Ca(OH)2/CaO

Diego Alvarez etc. [11]

Ca/Mg-based absorbent

Ikuo YANASE etc.

CaOAluminosilicate foam

[12]

Research Content CaO and Ca(OH)2 components in steel slag and concrete were dissolved in an aqueous solution, and CO2 was introduced to produce CaCO3. The reaction time, speed and scale were tested and in line with industrial processes. CaO and Ca12Al14O33 were synthesized into CO2 absorbents. The adsorption capacity and the stability of the cycle reaction were significantly improved. CaCO3 was collected by Ca(OH)2 solution to capture CO2, and CaCO3 was then decomposed into CaO by a solar calcination kiln. Ca/Mg-based absorbers in different crystal forms were used to capture CO2, and it was found that limestone is able to better capture CO2 than the aragonite or dolomite. Ca(OH)2 was decomposed into CaO and combined with aluminosilicate to obtain CO2 absorbent. The adsorption rate was maintained at 90%. 3


Energy & Fuels

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

Christina S etc. [13]

CaO-Ca12Al14O33

Hong Lu etc. [14]

Zr doped CaO

Wenqiang Liu etc. [15]

CC-CaO, CH-CaO, CC70nm-CaO,CaO1 60nm-CaO,CA-CaO ,CL-CaO,CF-CaO, CG-CaO CaO/Ca(OH)2

Josefa Fernandez etc. [16]

Chang, PH etc. [17]

Ca-Al-O oxide

Martínez I etc. [18]

CaO/ water vapor

Huichao Chen etc. [19]

CLS/CaO/water vapor CaO

Alexander Charitos etc. [20] Huichao Chen etc. [21] J.R. Fernández etc.

CaSiO3/Ca2SiO4 CaO/Cu-CuO

[22]

Jose Manuel [23] Valverde etc. Min-Jung Hsu etc. [24]

CaO

Yingjie Li etc. [25]

CaO-Ca3Al2O6

Ze-Hua Li etc. [26]

CaO-H2O

CaO/Ca(OH)2

Ca(CH3COO)2 were used to prepare CaO-Ca12Al14O33, and the absorbent exhibited good capture capacity after 45 cycles. By adding Si, Ti, Cr, Co, Zr, Ce etc into CaO, it was found that Zr doped CaO has better capture capacity. Different CaO composites were studied, CG-CaO (Monohydrate D-gluconate multiple CaO) has better capture capacity.

CaO, Ca(OH)2 combined with MCM-41 and SBA-15 respectively, to prepare CO2 absorbent. Different experimental conditions were examined to obtain the best capture effect. Ca-Al-O oxide had a CO2 capture rate of 95.6% after 30 cycles. By studying the effect of water vapor on CO2 capture of CaO, a reasonable amount of water vapor was obtained. Lignin (CLS) was added to hydrate CaO to enhance CO2 capture efficiency. CaO was used to repeated cyclic capture in a circulating fluidized bed (CFB) to mineralize CO2 into CaCO3. The best capture capacity was found when Si/Ca was 0.2. The period of Cu-CuO regenerated into CaO-CaCO3 were used to provide energy for CO2 capture. Thermal and mechanical stability of the calcium-based absorbent was improved to increase collection efficiency. Trapping efficiency of Ca-based absorbents in CO2 cycle was studied at relatively high temperatures. Best capture capacity was found when CaO/Ca3Al2O6 was 73/27. Under high temperature N2 atmosphere, the addition of water vapor promoted CO2 capture efficiency of CaO.

Phosphogysum (PG) is a waste product of industrial hydrometallurgical phosphoric acid, whose main component is calcium sulfate dihydrate (CaSO4·2H2O). According to a rough estimate, production of 1 ton of phosphoric acid will produce 3.5 tons of phosphogypsum [27]. CaO and CaS are the main components of PG decomposition, and are able to adequately capture CO2. Several studies on PG decomposition have been carried out by our group, laying the foundation 4


Page 4 of 33

Page 5 of 33

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

Energy & Fuels

for subsequent experiments. Bei Y et al. [28, 29] has investigated Ca, S transformation and reaction mechanism with iron catalyst during the process of PG decomposition. Yan X et al. [30] explored the PG decomposition under multi-atmosphere composition of CO and air. Yang J et al. [31, 32] further studied the chemical thermodynamics of PG and lignite chemical looping gasification for syngas generation. Zhao S Q et al. [33] proposed a method of CO2 capture using PG decomposition slag. Based on these previous studies, this paper proposes a cyclic method of PG decomposition (Ca-based absorbent) to solve the problem of PG resource utilization. The exhaust gas generated during the carbonation process can be reasonably reused to enhance capture capacity. To determine the effectiveness as a Ca-based absorbent for CCS, different liquid-solid ratios (L/S) and thermal parameters were investigated to determine optimal CO2 capture capacity. Furthermore, the CO2 capture capacity and H2S concentration were measured to determine the mechanisms of the

carbonation

process.

Finally,

the

reaction

process

mechanisms

of

this

decomposition/carbonation system were investigated to determine the role of sulfur in the reactions.

2. Material and Experiment 2.1 Preparations PG and lignite samples received from Yunnan Nature Gas and Chemical Engineering Company (62.37% wt CaSO4), CO2 (20% wt) and N2 (99.99% wt) gas from Stone Man (Kunming, China) were used in the experiments. The electrical resistivity of ultrapure water is 18.25 ohms. To determine the entire process of H2S assisted decomposition analysis, PG and lignite samples were prepared following. First, the PG sample and lignite sample were dried in an oven at a

5


Energy & Fuels

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

constant temperature of 105°C for 2 h in air to remove surface moisture on the sample and sieved in a 120 mesh. Based on previous work [31], the optimal molar ratio for PG and lignite is 4:1, which produces the most CaS for subsequent experiments. PG and lignite was mixed at a 4:1 ration, stirred at room temperature for 24 h and dried once more at 105°C to remove any surface moisture potentially absorbed during the mixing process from the air.

2.2 Experimental Facilities The instrument used to perform the weight loss and endothermic/exothermic decomposition process was a thermogravimetric analyzer (XXWRT-2C) from Beijing Optician Plant. Chemical composition of these samples was determined by X-ray diffraction (D/max-3BPEX-P96, Japan) and X-ray fluorescence (ZSX100e) as shown in Table 2 and Table 3. Morphologies of carbonation products CaCO3 and PG were examined on scanning electron microscopy (SEM, Quanta FEG 250) with an accelerating voltage of 20.00 kV. Complex fuel gas analyzer (KM9106, KANE Company, British) and ultrapure water machine (UPT-I -5/10/20T) were used to analyze the gas components and provide ultrapure water.

2.3 Experimental Methods Two-step cyclic experiments were performed as follows and Fig. 1 shows the flow diagram of the cyclic system.

6


Page 6 of 33

Page 7 of 33

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

Energy & Fuels

Figure 1. Flow diagram of the two-step cyclic experiments. i. Tube furnace thermal decomposition experiments Thermal decomposition with different calcination temperatures (950 ºC, 1100 ºC and 1200 ºC) and molar ratios (PG/lignite: 1:1, 2:1, 3:1 and 4:1) were investigated within the tube furnace. Solid product was obtained at a proper temperature and molar ratio under a reducing atmosphere. In addition, H2S produced from step ii was investigated to be used as a reducing atmosphere. ii. Fluidized bed carbonation experiments CO2 capture was performed in a three-phase fluidized bed with varied ratios of L/S to determine the most effective ratio of CO2 absorption Ca-based absorbent was removed from the tube furnace and stored in a fluidized bed and hydrated was then investigated at different temperatures.

7


Energy & Fuels

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

Page 8 of 33

Figure 2. Schematic diagram of three-phase fluidized bed. 2.3.1. Tube Furnace Thermal Decomposition Experiments As previous work had been done on several variables [28-32], only two variables were investigated during these experiments: (i) decomposition temperatures: 950, 1100 and 1200 ºC; (ii) molar ratios of PG/lignite: 1/1, 2/1, 3/1 and 4/1. A D80 tube furnace from Nanjing University Experiment Factory was used to heat the samples to 950, 1100 or 1200 ºC in a N2 atmosphere with a flow rate of 100 ml/min to generate Ca-based absorbent. The samples were then cooled in situ before introducing three-phase fluidized bed to start the carbonation process. This entire process took 9-11 hours to complete, and needed to be done continuously. The conversion rate of main product CaS (XCaS(%)) to PG as then calculated:

100%

(E1)

8


Page 9 of 33

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

Energy & Fuels

in which MCaS (g) and MPG (g) are the mass qualities of CaS and PG, respectively. 2.3.2 Fluidized Bed Carbonation Experiments A carbonation set-point temperature at 25 ºC was used throughout the fluidized bed experiments. The carbonate setup is shown in Fig. 2, which fed ultrapure water at L/S ratio under CO2 atmosphere (flow rate of 1.5L/min). Toward the end section of the fluidized bed, there were two baffles of a three-phase reactor so that the gas was either vented to the atmosphere or mixed with the other phase at the base of the reactor bed. The heating programs were stimulated by Factsage 7.1 indicates a trend of relative low temperature. In the process of carbonation, the CO2 cylinder was switched on first in the fluidized bed. Second, the liquid phase and then powder were added, ensuring that the powder was not blown by the gas flow. Approximately 50 min later, a carbonation temperature of 25 ºC was achieved. The outlet gas concentration was measured at certain reaction time intervals. Upon achieving sufficient reaction, a conversion rate of the main solid product CaCO3 (XS1, XS2) was calculated by performing a mass balance and chemical component measure by XRF and XRD instrument:

100%

(E2)

100%

(E3)

in which XS1 and XS2 are the conversion rates of CaCO3 in CaS (decomposition process) and PG, respectively. MCaCO3 (g), MCaS (g) and MPG (g) are the mass qualities of CaCO3, CaS and PG, respectively. Table 2. Main components of PG (wt%) (dry basis) Component

Al

Ca

Si

P

S

K

Fe

As

Sr

Content (wt%)

0.630

57.5

10.00

0.462

30.3

0.450

0.577

0.0279

0.117

9


Energy & Fuels

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

Page 10 of 33

Table 3. Main components of carbonation solid product (wt%) Component

Si

P

S

K

Ca

Ti

Fe

Sr

Zr

Content (wt%)

10.7

0.891

1.05

0.269

85.1

0.317

1.52

0.133

0.0136

3.1 Theoretical Analysis 3.1.1

Thermodynamics Analysis

The two steps of the experiments (decomposition in the tube furnace and carbonation in the fluidized bed; Fig. 3) consists of the following reactions. The main reactions of thermal decomposition are labeled as R1-R7 and R10-R11. Furthermore, the carbonation process are identified as R12-R16. R8 and R9 during the thermal decomposition are the cyclic reducing decomposition reaction, which is the release of H2S which assists the thermal decomposition of phosphogypsum and lignite.

Figure 3. Sketch of cyclic two-step experiments. CaSO4·2H2O → CaSO4·1/2H2O + 3/2H2O

(R1)

CaSO4·1/2H2O → CaSO4 + 1/2H2O

(R2)

CaSO4 + 4C → CaS + 4CO

(R3)

10


Page 11 of 33

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

Energy & Fuels

CaSO4 + 4CO → CaS + 4CO2

(R4)

CaSO4 + CO → CaO + CO2 + SO2

(R5)

CaSO4 + 3CaS → 4CaO + 4S

(R6)

3CaSO4 + CaS → 4CaO + 4SO2

(R7)

CaSO4 + H2S → CaS + H2O + SO2

(R8)

CaSO4 + C + H2S → CaS + H2O + CO2 + SO2

(R9)

CaS + 2SO2 → CaSO4 + 2S

(R10)

4CaO + 4S → CaSO4 + 3CaS

(R11)

CaS + 2H2O → Ca(OH)2 + H2S

(R12)

CaO + H2O → Ca(OH)2

(R13)

Ca(OH)2 + CO2 → CaCO3 + H2O

(R14)

CaS + H2O + CO2 → CaCO3 + H2S

(R15)

CaCO3 → CaO + CO2

(R16)

FactSage software, which contains FACT-Win and Chem-Sage, can calculate the chemical thermodynamic equilibrium under the limitation of a variety of conditions [28]. In this study, three modules of FactSage 7.1 named Reaction, Equilib and Phase Diagram were used to investigate the thermodynamic reaction mechanisms. The change in Gibbs free energy, change in free enthalpy and phase diagrams were calculated with FactSage 7.1 with variable of temperature. Table 4 shows the thermodynamic parameters calculated by Factsage 7.1 of R3 to R16, which indicates the temperature at which the reaction begins. R13-R15, which belong to the carbonation process, may occur at room temperature, while R6 and R12 may not occur at all based on the theoretical data shown in Tab. 4.

11


Energy & Fuels

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

Page 12 of 33

Table 4. Thermodynamic parameters at the beginning of reactions. Reaction

Temperature(°C)

∆G(kJ/mol)

∆H(kJ/mol)

R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16

500 25 900 1200 800 600 25 25 25 25 25 900

-38.135 -175.966 -2.197 -30.996 -2.010 -1.565 -252.883 -312.128 -57.916 -73.422 -75.718 -0.949

513.247 -171.082 198.359 936.345 251.437 236.623 -367.214 -313.378 -65.165 -114.001 -62.727 166.0688

The R6 that occurs during the PG decomposition process has a positive Free Gibbs Energy (∆G) value from the beginning to 1300 °C (Fig 4a). When the temperature is less than 900 °C, ∆G value for R5, R6, R7 and R10 all have positive values, indicating that R5, R6, R7 and R10 are spontaneous reactions during that period to some extent. When the temperature ranges from 900-1100 °C, ∆G values of several reactions (in ascending order: R3, R11, R4, R9, R8, R5) are all negative, indicating nonspontaneous reactions to some extent. Lastly, when the temperature is greater than 1200°C, G3, G9, G11, G4, G8, G7 and G5, in ascending order of value, all have negative ∆G values, again indicating nonspontaneous reactions to some extent (Fig. 4a). These results confirm that it is beneficial to produce Ca-based absorbents at a relatively high temperature (1100 ⁰C), as relatively high temperatures will create a sufficient reaction. The ∆G value of R12 still has a positive value from 0 ⁰C to 1300 °C (Fig. 4b), in some way indicating that R12 is a spontaneous reaction throughout the entire calculated temperature range. Furthermore, R14 is a nonspontaneous reaction from 0 ⁰C to 1300 ⁰C because its ∆G is negative

12


Page 13 of 33

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

Energy & Fuels

throughout this temperature range. When temperature ranges from 700-900 °C, the Free Gibbs Energy values of R12, R15, R13, and R16, in ascending order, indicate that they all should be spontaneous reactions in this temperature range. Finally, R14 and R16 appear to be spontaneous reactions from 900-1100 ⁰C, according to their positive ∆G values. However, the Gibbs energy shown in our manuscript provides us a theoretical guidance to judge the reactions to be spontaneous or not and our understanding of reaction spontaneity mainly depend on the experiments as well. For example, Zhao S Q et al. [33] provided a heating temperature program during the decomposition period and explored the reaction that proceeded during the carbonation period at a given temperature. During the decomposition period, the actual temperature program was set as: Firstly, from 0 ºC to 950 ºC, a heating rate of 5 ºC/min. Secondly, maintained a temperature of 950 ºC for an hour. Thirdly, the temperature was increase from 950ºC to 1100 ºC at heating rate of 5 ºC/min. Finally, the temperature was maintained for an hour at 1100 ºC. The relatively low heating rate and heat preservation time were extended to realize the sufficient reaction. It has been found that, the carbonation period mostly was sustained for about 48min and with different trends from different period. From 10 to 26 min the reaction proceeded quickly and with the time increase, the reaction tended to be steady and slow.

13


Energy & Fuels

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

Figure 4. The change in Gibbs free energy (∆G) varying with temperature for (a) tube furnace decomposition reactions R1-11 and (b) fluidized bed carbonation reactions R12-16. Free enthalpy (∆H) is a thermodynamic parameter that combines internal energy and flow work. Changing curves of ∆H reflect whether the reaction is endothermic (negative ∆H) or exothermic (positive ∆H). Several reactions are endothermic (R16

Sulfur-Looping Mechanism for the Two-Step Cyclic Process of

Recommend Documents