Article pubs.acs.org/EF
Steam Gasification of Thermally Extracted Ash-Free Coals: Reactivity Effects Due to Parent Raw Coals and Extraction Solvents Lia Priscilla,†,‡ Yongjin Kong,† Jiho Yoo,*,† Hokyung Choi,† Youngjoon Rhim,† Jeonghwan Lim,† Sangdo Kim,† Donghyuk Chun,† Sihyun Lee,† and Youngwoo Rhee*,‡ †
Clean Fuel Laboratory, Korea Institute of Energy Research, Daejeon 305-343, South Korea Graduate School of Energy Science & Technology, Chungnam National University, Daejeon 305-764, South Korea
‡
ABSTRACT: Utilization of coal is currently limited to coal-fired power plants and iron smelting, partly because of the incombustible ash in coal and the inability of existing technologies to modify its properties. This work investigates whether the gasification behavior of ash-free coals (AFCs) can be modified. Sixteen different AFCs were prepared using variously ranked coals (Eco, Cyprus, Drayton, and Hail Creek) and extraction solvents with different polarities (N-methyl-2-pyrrolidone, ethylenediamine, 1-methylnaphthalene, and tetralin). Next, the reactivities of the AFCs in steam gasification at 800 °C were tested, and the results are discussed taking into consideration their compositional differences. A combination of low-rank coals and polar solvents produces reactive AFCs that can be catalytically gasified under mild conditions. In contrast, AFCs extracted using nonpolar solvents are less reactive and possibly applicable as a carbon electrode precursor. In short, the properties of AFCs can be changed to some extent using the appropriate combination of coals and extraction solvents.
1. INTRODUCTION Coal is one of the most important energy sources, currently accounting for ∼25% of worldwide energy consumption. Because of its abundance and economic advantages, the use of coal is expected to increase.1 Coal combustion emits ∼31% of the world greenhouse gas (GHG) and has been continuously implicated as the main cause of global warming.2 The integrated gasification combined cycle (IGCC) has been proposed as one of the most realistic measures to minimize coal’s adverse environmental impact because of its greater efficiency compared with conventional combustion.3 Inorganic ashes in coal decrease the power efficiency and cause air pollution when discharged.4,5 They are also problematic when coal is utilized as a carbon source in materials chemistry. Therefore, many groups have tried to develop efficient processes to remove ashes from coal.6−9 Two such technologies have been developed: (1) The ultraclean coal (UCC) process removes ashes using a strong acid or base solution, resulting in a coal product with relatively high ash content (∼0.5%), which usually behaves like its original raw coal.4 (2) Thermal extraction using organic solvents dissolves the carbonaceous components in coal and produces ash-free coals (AFCs), typically with 1400 °C and 20−70 atm), inflicting a heavy burden with respect to economy and safety.24 Catalytic coal gasification under mild conditions has not yet been realized because of deactivation of the catalysts via irreversible interactions with the ashes in coal. AFCs were produced as a means of making repeated use of catalysts. The present work deals with changing the properties of AFCs to enhance their gasification kinetics. Extensive experimentation was conducted to determine the effect of the extraction conditions on the reactivity of AFCs. Sixteen different AFCs that were produced using combinations of four different raw Received: May 25, 2017 Revised: September 7, 2017 Published: September 12, 2017 A
DOI: 10.1021/acs.energyfuels.7b01502 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Proximate and Ultimate Analyses and Calorific Values of the Raw Coals proximate analysis (wt %, wb)a
a
c
sample
M
Eco Cyp Dra Hai
11.1 13.2 2.9 1.3
ultimate analysis (wt %, dafb)b
d
e
VM
ash
FC
47.5 39.3 32.6 21.0
3.7 5.3 12.3 9.7
37.7 42.2 52.2 68.0
C
H
N
O
S
calorific value (MJ/kg)
70.4 76.4 84.6 86.2
5.2 6.3 5.8 4.1
0.9 1.3 1.8 1.5
23.4 15.7 7.2 1.1
0.1 0.3 0.6 7.1
24.60 28.03 27.32 35.96
wb = wet basis. bdafb = dry, ash-free basis. cM = moisture. dVM = volatile matter. eFC = fixed carbon. stainless steel filtering unit to separate it from the undissolved solids, including ashes. The solvent was evaporated in a 300 °C oven for 1 h, and the AFC was obtained. A total of 16 AFCs (a matrix of four raw coals and four solvents) were produced and named according to the combination of raw coal and solvent. For example, the Eco AFC extracted using NMP solvent is called Eco-NMP. 2.2. Steam Gasification in a Fixed-Bed Reactor. Steam gasification of the coals was performed in a fixed-bed reactor.26 The temperature was increased at a ramp rate of 30 °C/min to 800 °C, where the coals were gasified using steam. The reactor was made of a quartz tube and installed in a vertical furnace. A quartz frit (∼3 mm thick) was placed in the middle of a quartz tube (i.d. = 10 mm). Quartz wool was put on the frit, and 0.1 g of sample was loaded on top of the wool. Water (30 vol % steam) was injected into the reactor using a syringe pump, vaporized in a heated (150 °C) stainless steel tubing, and delivered to the reactor by 100 cc/min N2. Side products, water, and tar were removed by two cold traps (2 °C) and an oil filter. During steam gasification, gaseous products passed through the frit and solid coal remained. The produced gases (H2, CH4, CO, and CO2) were quantified by gas chromatography (GC) (Agilent 6890). 2.3. Instrumental Analysis. On the basis of the ASTM D3172 standard, the proximate analyses were performed using a thermogravimetric analyzer (TGA-701, LECO Co., St. Joseph, MO, USA). Elemental compositions (C, H, N, O, and S) were determined using a CHN-2000 elemental analyzer (LECO), and the calorific values were obtained using a Parr 1261 calorimeter (Parr Instrument Co., Moline, IL, USA). X-ray diffraction (XRD) patterns were taken using a Rigaku DMax 2500PC diffractometer (40 kV, 150 mA). The chemical functional groups were detected by fourier transform infrared (FT-IR) spectroscopy (Nicolet 6700 spectrometer, Thermo Electron Co.). Solid-state Carbon-13 nuclear magnetic resonance (13C NMR) spectra were measured using a cross-polarization/magnetic-angle-spinning (CP/MAS) 13C NMR spectrometer (Bruker Avance II, 400 MHz).
coals and four different solvents were compared in terms of their composition differences and gasification behaviors.
2. EXPERIMENTAL SECTION 2.1. Preparation of Ash-Free Coals. The soluble carbonaceous components in four raw coals of different ranks were extracted using organic solvents. The raw coals used were Eco (Eco) and Cyprus (Cyp) as LRCs and Drayton (Dra) and Hail Creek (Hai) as high-rank coals (HRCs). The proximate and ultimate analyses as well as the calorific values of the parent raw coals (Raws) are shown in Table 1. Two polar solvents (N-methyl-2-pyrrolidone (NMP) and ethylenediamine (EDA)) and two nonpolar solvents (1-methylnaphthalene (1-MN) and tetralin (TTL)) were chosen for the thermal extraction. The properties of the solvents are summarized in Table 2.
Table 2. Physical Properties of the Extraction Solvents
solvent
density (g/cm3, at 25 °C)
melting point (°C, at 1 atm)
boiling point (°C, at 1 atm)
dielectric const. (at 20 °C)
purity (%)
maker
NMP EDA 1-MN TTL
1.028 0.9 1.001 0.97
−24 8.5 −22 −36
202 116 240 206
32.6 14.2 2.7 2.8
99 99 96 98
Junsei Duksan Alfa-Aesar Kanto
The AFCs were produced via extraction, filtration, and a drying step.14,25 The raw coal was ground, meshed to
