Article pubs.acs.org/EF
Cite This: Energy Fuels XXXX, XXX, XXX−XXX
Hydrogen-Rich Gas Production from Steam Gasification of Lignite Integrated with CO2 Capture Using Dual Calcium-Based Catalysts: An Experimental and Catalytic Kinetic Study Long Jiang,† Song Hu,*,†,‡ Syed Shatir A. Syed-Hassan,§ Kai Xu,*,† Chao Shuai,† Yi Wang,†,‡ Sheng Su,†,‡ and Jun Xiang†,‡ †
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China ‡ China-EU Institute for Clean and Renewable Energy at Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China § Faculty of Chemical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia ABSTRACT: As an eco-friendly, cheap and readily available material, calcium-based oxides could play the dual roles of both tar reforming catalyst and CO2 sorbent, thereby producing hydrogen-rich gas with low tar concentration and low carbon emission during steam gasification of coal/biomass. This study aimed to experimentally investigate effects of different parameters on hydrogen-rich gas production during steam gasification of lignite using dual calcium-based catalysts. Furthermore, in view of the importance of the water−gas shift reaction to hydrogen-rich gas production, its catalytic kinetics with the homemade calcium sorbent was also studied. The results show that the addition of active calcium oxide remarkably decreased tar yield, leading to a significant increase of noncondensable gas yield. Increase of loading amount of homemade sorbent enhanced CO2 absorption, thereby breaking the water−gas shift reaction equilibrium and shifting it, resulting in a distinct increase of H2 yield and a decrease of CO yield. The highest H2 yield of 1.24 N m3/kg-coal was obtained in current study. Finally, the catalytic kinetics of the water− gas shift reaction with homemade sorbent were obtained using a newly developed and more precise kinetic model, and from there the conversion of the WGS reaction was well predicted by the new kinetic model.
C + H 2O → CO + H 2
1. INTRODUCTION Nowadays in China, coal resource fulfills a very large portion of the total energy demands, and it will still remain a major energy source for a long time in the future.1 However, the extensive and unadvanced utilization of coal leads to many problems such as excessive greenhouse gas emission, resource waste, and serious air and water pollutions.2,3 Thus, there is an immediate need to develop clean and efficient advanced utilization technologies of coal. Hydrogen is expected to be the most important energy carrier due to its clean feature and high energy density, and it can be widely used in fuel cells, engines, and turbines with high energy efficiency.4−6 Steam gasification of coal has emerged as one of the most promising technologies, wherein high reactivity gasification agent of steam reacts with solid coal and also accelerates secondary reforming reactions (such as water−gas shift (WGS) and hydrocarbons reforming) to produce hydrogen-rich gas.7 The principal reactions are as follows8
C + 2H 2O → CO2 + 2H 2
ΔHR > 0
ΔHR = −41.2 kJ/mol
ΔHR > 0
→ nCO2 + (2n + m /2)H 2
(5)
ΔHR > 0
(6)
where CnHm represents light noncondensable gases such CH4, C2H4, C2H6, etc. Boudouard reaction: C + CO2 → 2CO ΔHR = +172 kJ/mol
(7)
In addition, lignite, as an abundant reserve of low rank coal, is believed to be particularly suitable for the above technology due to its high content of moisture/volatile matter and high gasification reactivity.9,10 The current commercialized coal gasification processes are commonly operated at extremely high temperatures (1200− 1600 °C of entrained-flow bed and 900−1050 °C of fluidized bed11). However, this high temperature considerably increases
(1)
(2)
Received: October 19, 2017 Revised: December 10, 2017
where CxHy represents tars. Char gasification: © XXXX American Chemical Society
(4)
Hydrocarbon reforming: CnHm + 2nH 2O
Steam‐tars reforming: Cx Hy + 2x H 2O → (2x + y /2)H2 + xCO2
ΔHR = +90.1 kJ/mol
(3)
WGS reaction: CO + H 2O ↔ CO2 + H 2
Pyrolysis: Coal → H 2 + CO + CO2 + CH4 + CnH m + Steam + Tars ···
ΔHR = +131.5 kJ/mol
A
DOI: 10.1021/acs.energyfuels.7b03213 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Proximate and Ultimate Analysis of the Lignites ultimate analysis (wt %, air-dry basis)
a
proximate analysis (wt %, air-dry basis)
samples
C
H
N
S
Oa
moisture
ash
volatile
fixed carbon
HLH HLHAW XL XLAW
42.34 45.79 57.47 61.43
2.52 2.66 4.14 4.38
0.90 0.84 0.75 0.65
0.41 0.22 1.34 0.85
20.78 23.01 16.22 18.42
10.34 10.62 6.88 7.04
22.71 16.86 13.19 7.23
37.12 40.68 34.73 37.91
29.83 31.84 45.20 47.82
By difference; AW represents acid wash.
In addition, with respect to CO2 capture capacity of the calcium-based sorbent, our another previous work systematically evaluated sorbent performance derived from different calcium precursors by using different modifications to improve sintering and attrition resistance of CaO sorbents and obtained a high performance of Al2O3-supported CaO sorbent that was derived from calcium acetate with incorporation of calcium aluminate cement. It possesses an absorption capacity of 0.61 mol-CO2/mol-sorbent and an attrition loss of 7.4 wt % over 48 cycles of carbonation/calcination.29 Based on these fundamental works, the current study aimed to develop a highly efficient gasification approach of lignite using dual calcium-based catalysts to hydrogen-rich gas production with low tar yield integrated with CO2 capture. Effects of different parameters (absorption/gasification temperature and loading amount of catalyst and sorbent) on hydrogen-rich gas production during steam gasification of lignite using dual calcium-based catalysts were experimentally investigated. Moreover, in view of the importance of the WGS reaction for hydrogen-rich gas production and lack of intensive study, the catalytic kinetics of the WGS reaction with calcium-based sorbent was also studied, and from there a more precise kinetic model for the WGS reaction was proposed.
the capital and operation costs of the process. Over recent decades, gasification with specific catalysts (salts and oxides of alkali metals and alkaline earth metals, natural mineral catalyst, transition/noble metals, etc.) to lower operating temperature and increase reaction rate has been intensively investigated.12−15 Calcium-based catalysts, as a species of eco-friendly, cheap, and readily available catalysts, have been widely employed for coal gasification to produce hydrogen-rich gas.16−19 It is well-known that they could lower gasification temperature as well as promote volatile/tars thermal cracking, resulting in not only reducing tar yield but also enhancing noncondensable gas production, thus increasing gasification efficiency.20−22 Ohtsuka et al.20 investigated the influence of Ca(OH)2 on the reactivity of 16 coals during steam gasification by a thermogravimetry (TG) method and found that the calcium catalyst significantly promoted gasification rate and decreased gasification temperature by 110−150 °C for the lowrank coals. Jordan et al.22 observed that the addition of calcium oxide could decrease tar concentration in syngas ranging from 44 to 80% and increase syngas yield by 17−37% during gasification of fuel cane bagasse. On the other hand, they also act as in situ sorbent to absorb CO2, resulting in the shift of chemical equilibrium of the WGS reaction and catalyzing the WGS reaction to enhance hydrogen production while at the same time providing CO2 capture capability23−25 through the following reactions:
2. EXPERIMENTAL SECTION 2.1. Materials. Inner Mongolia accounts for nearly 3/4 of lignite reserves of China. The lignite samples were collected from two of the biggest Inner Mongolia coal fields (Huolinhe (HLH) and Xilinguole League (XL)). The two samples were crushed, ground, and sieved. The particle size fraction of 150−250 μm was dried in an oven at 105 °C for 24 h before being used in the experiment. The proximate and ultimate analysis of the lignites (measured by TGA-2000 (Las Navas Instruments, Spain) and EL-2 analyzer (Vario Company, Germany), respectively) are listed in Table 1, which had been presented elsewhere.27 2.2. Sample Preparation. To avoid interference of active minerals of the lignite, the samples were washed by hydrochloric acid whose procedure has also been described in the previous published work.27 Briefly, coal powders were immerged into the HCl solution (6 M, 150 mL solution per 10 g coal) and stirred at temperature of 50 °C for 24 h. The slurry was filtered, flushed with deionized water several times until the filtrate was neutral, and then air-dried at 105 °C for 48 h. Then Ca(OH)2 (equal to 10 wt % of calcium element, analytically pure, purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd.) was mixed into the acid-washed samples in deionized water and then dried at 105 °C for 24 h, which is called wet-mixing method. The corresponding wet-mixed samples were abbreviated as HLH-Ca and XL-Ca, respectively. The content of calcium after wet-mixing in HLHCa and XL-Ca, as measured by inductively coupled plasma-mass spectroscopy (ICP-MS, ELAN DRC-e, PerkinElmer), was 10.71 and 10.14 wt %, respectively. Our another previously published work29 systematically investigated the performance of CaO sorbent produced from different calcium precursors, and the results indicate that the best performance of CaO sorbent was derived from calcium acetate with incorporation of calcium aluminate cement, possessing an absorption capacity of 0.48 g-
CaO carbonation: CaO + CO2 → CaCO3 ΔHR = −178 kJ/mol
(8)
Therefore, the overall steam gasification reaction with CO2 sorbent is C + CaO + 2H 2O → CaCO3 + 2H 2 ΔHR = −87.5 kJ/mol
(9)
24
Kong et al. achieved over 97% CO conversion to H2 by the WGS reaction as well as over 96% CO2 conversion to CaCO3 by the carbonation reaction using CaO core-in-shell pellets. Li et al.26 found that addition of calcined dolomite promoted H2 yield significantly due to in situ CO2 absorption and the catalytic effects of the CaO on the WGS reaction during steam gasification of corn stalk. They obtained gas production with a maximum H2 concentration over 85 vol % and negligible amounts of C2+ and CO2 using calcined dolomite integrated with NiO/γ-Al2O3. Our previous works27,28 systematically investigated the synergistic effect of catalysis and CO2 absorption by calcium-based catalysts on enriching H2 concentration during steam gasification of coal and found that Ca(OH)2 is a highly active catalyst that not only accelerates tar decomposition to increase total gas yield but also promotes H2 concentration due to additional effect of CO2 absorption. B
DOI: 10.1021/acs.energyfuels.7b03213 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Schematic of the laboratory-scale two-stage quartz reactor system. coil heated by the furnace at temperature of 450 °C (ca. 250 mL/min steam)). At the same time, the coal particles were fed into the fluidized reactor at 150 ± 10 mg/min by the gas carrier feeder via the injection probe which was water cooled to ensure that coal does not react before entering the fluidized reactor. The sample feeding time lasted 10 min (total amount of the feeding coal was 1.5 g). The steam gasification of coal started, and the gas product passed through the upper CO2 absorbing bed. After that, the resulting gas successively passed through the tar traps to capture the tar. Tar collection method and tar yield determination were formerly described in detail elsewhere.8 In brief, four tar traps were respectively filled with 0, 50, 50, and 30 mL of a mixture of HPLC-grade chloroform and methanol (80:20, vol, purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd.). The first two traps were cooled by ice salt water (0 °C) to mainly condense the excess steam and parts of heavier tar. The other two traps were cooled by dry ice (−78.5 °C) to capture the tar completely. The tar-free syngas was dried by anhydrous calcium chloride filter, and its composition was monitored by the gas analyzer (Gasboard-3100, Wuhan Cubic Optoelectronics Co., Ltd., China). The analyzer outlet was sampled by a gas sampling bag, and its composition was determined by gas chromatography (GC, Agilent micro-GC 3000A, USA). The gas content was calculated by the following equation which was described elsewhere:8
CO2/g-sorbent (0.61 mol-CO2/mol-sorbent) and an attrition loss of 7.4 wt % over 48 cycles of carbonation/calcination. Thus, this sorbent aged by 48 carbonation/calcination cycles (CA sorbent) was used as CO2 sorbent in this study. The procedures of sorbent production and carbonation/calcination aging can be referred to in the previously published work.29 2.3. Coal Gasification. Steam gasification of lignite to produce hydrogen-rich gas integrated with in situ CO2 capture was carried out in a laboratory-scale two-stage quartz reactor (shown in Figure 1) which was previously used for pyrolysis/catalytic reforming of coal/ biomass in other studies.30,31 The experimental system consists of three parts: (1) sample and gas feeding system; (2) reaction system; (3) tar trap and gas sampling system. The sample and gas feeding system is composed of a gas mass flowmeter controller (N2/CO/CO2/H2), a steam generator, and a gas carrier feeder whose feeding speed was controlled through controlling the distance of concentric tube and coal powder layer by a stepping motor. The reaction system is composed of a two-stage temperatureprogrammed control furnace and a two-stage quartz reactor in which the bottom and upper reactors acted as a fluidized-bed gasifier and a fixed-bed CO2 absorber, respectively, in this study. These two reactors were separated by four quartz sieves with diameter of 100 μm. Furthermore, a thin layer of high-purity quartz wool (purity >99.99%, ultimate temperature 1600 °C) was placed on the upper throat of absorption reactor to prevent fine particulates escaping from the dualstage reactor. Approximately 40 g of the acid-washed and calcined silica sands (particle size between 83 and 150 μm) was fluidized in the bottom reactor which was heated in the temperature range of 700− 900 °C. The upper reactor was loaded with homemade CA sorbent (in the range of 0−10 g), and it was heated in the temperature range of 550−650 °C. Tar trap and gas sampling system is composed of a tar trap system, a gas filter, and a gas analyzer. Before each experiment, high-purity nitrogen (purity >99.999%) was first introduced into the reactor to remove impurities. The nitrogen flowing into the reactor was divided into two parts (Figure 1). One part was used as a steam carrier (flow rate of 0.3 L/min), while the other part, which was installed with a pressure meter to ensure the safety operation of the system (pressure
