Article pubs.acs.org/EF
Experimental and Kinetic Investigations of CO2 Gasification of Fine Chars Separated from a Pilot-Scale Fluidized-Bed Gasifier Xuliang Jing,†,‡ Zhiqing Wang,† Zhongliang Yu,†,‡ Qian Zhang,†,‡ Chunyu Li,† and Yitian Fang*,† †
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: The CO2 gasification behaviors of two fine chars separated from a pilot-scale fluidized-bed gasifier were studied in a thermogravimetric analyzer (TGA) within the temperature range of 1000−1300 °C. The physical properties of fine chars were examined by scanning electron microscopy (SEM), N2 adsorption, and X-ray diffraction (XRD). The differences in gasification reactivity and related properties between the fine chars and the corresponding experimental-produced coal chars were also compared. The results show that the fine chars have higher ash content, larger Brunauer−Emmett−Teller (BET) surface area, and better gasification reactivity than the corresponding coal chars. The gasification reactivity of fine chars was promoted by the catalytic alkali and alkaline earth metals (AAEMs) but inhibited by the enrichment of the ash layer in a higher carbon conversion range or the ash melting at a higher temperature. In addition to the AAEMs, the reactivity of different fine chars is mainly influenced by their pore and carbon crystalline structures. The kinetic investigation reveals that the modified random pore model (MRPM) and shifted-modified random pore model (S-MRPM) perform more reasonably than the random pore model (RPM) in some special conditions. Moreover, the reactivity of fine chars increases, and the reaction shifts from chemical reaction control to gas diffusion control as the gasification temperature increases. relatively lower surface area and smooth surface morphology.8,9 The gasification behaviors of pyrolysis coal chars have also been widely studied.10−13 These chars are pyrolyzed under an inert atmosphere, while the gasifier fine chars have been partially gasified, oxidized, and activated by steam. These factors make the properties of fine chars markedly different from the fly ashes and coal chars. For example, the anthracite fine char from the fluidized-bed gasifier has a larger surface area and higher carbon crystalline structure,14 while the entrained flow gasifier fine char exhibits a lower crystalline structure and higher CO2 gasification reactivity than the rapid pyrolysis coal char.15 Earlier work by Xu et al.16 identified the crystalline structure and inherent minerals as the determinants for the gasification reactivity of entrained flow gasifier fine char. The gasification kinetics of fine chars from the AFB has been studied by ICC,5 but the adopted regasification temperature is lower than 1000 °C, which could not match with the previously mentioned entrained flow gasifier. The gasification kinetics is essential for the efficient and reliable design of the coal gasification system. Because of the simplicity of design and ease of operation, the thermogravimetric analyzer (TGA) is widely used to obtain the gasification kinetics. In this paper, the CO2 gasification behaviors of fine chars separated from the pilot-scale AFB were studied by a TGA at temperatures of 1000−1300 °C. The property and reactivity differences between the fine chars and the corresponding coal chars were evaluated. Moreover, the gasification processes were described by the random pore
1. INTRODUCTION Fluidized-bed gasification has been considered as a promising syngas-producing technology because of its wide coal adaptability, effectiveness in the control of pollutant emissions, and lower operational cost.1,2 However, massive carbon-rich fine chars are entrained out by the syngas, resulting in a lower total carbon conversion (∼90%). Thus, the typical fluidized-bed gasifiers, such as the HTW, KRW, U-gas, and transport gasifiers, recirculate the fine chars to the gasifier and reuse them as a feedstock.3,4 Nevertheless, the fine chars have been partially gasified, and the regasification reactivity is lower. On the other hand, the fine chars have undergone a shorter residence time in the gasifier because of the smaller particle sizes. These factors make the fine chars difficult to be converted in the fluidized-bed gasifier at lower temperatures (900−1000 °C), which has already been verified by the Institute of Coal Chemistry (ICC), Chinese Academy of Sciences (CAS).5 One solution to this problem is to regasify the fine chars at a higher temperature. The entrained flow gasifier, for example, operates at 1250− 1600 °C with a residence time of a few seconds, the carbon conversion of which could be up to 99%.6 Consequently, a new approach integrating the ash agglomerate fluidized-bed gasifier (AFB) with an entrained flow gasifier was proposed, by which the fine chars from AFB were directly transferred to an entrained flow gasifier and gasified in the temperature range of 1200−1300 °C.7 Hence, understanding of the gasification behavior of fine chars in this temperature range is essential for actualizing this integration. However, few attempts have been made to examine the gasification of fine chars derived from a fluidized-bed gasifier at elevated temperatures. It has been investigated that the fly ashes derived from a circulating fluidized-bed combustor have a © 2013 American Chemical Society
Received: November 9, 2012 Revised: April 13, 2013 Published: April 15, 2013 2422
dx.doi.org/10.1021/ef4002296 | Energy Fuels 2013, 27, 2422−2430
Energy & Fuels
Article
Table 1. Proximate and Ultimate Analyses of Samples proximate analysis (wt %, air-dry basis)
a
ultimate analysis (wt %, dry and ash-free basis)
sample
V
M
A
FC
C
H
Oa
N
St
XY coal YC coal XYCC YCCC XYFC YCFC DXYFC DYCFC
8.3 7.6 1.1 2.1 1.2 1.3 2.2 2.4
1.6 1.2 0.6 1.1 2.3 2.1 2.2 2.0
19.4 22.2 19.7 17.9 25.9 49.1 1.2 1.3
70.7 69.1 78.6 78.9 70.6 47.5 94.5 94.2
90.2 89.5 95.1 94.7 93.5 92.4 93.5 93.2
4.2 3.0 1.3 1.3 1.4 1.6 1.1 1.0
3.9 5.7 2.2 2.0 3.7 4.9 4.2 4.5
1.2 1.2 1.0 1.1 1.0 0.8 0.8 0.8
0.5 0.5 0.4 1.1 0.4 0.3 0.5 0.4
By difference. 2.3. Measurements of CO2 Gasification Reactivity. The isothermal CO2 gasification experiments were performed with a Setaram Setsys TGA under atmospheric pressure at the temperatures of 1000−1300 °C. In each experiment, an approximate 5 mg sample was placed in an alumina crucible (8 mm inner diameter and 5 mm height) and heated to the desired temperatures under a N2 atmosphere (120 mL/min), with a heating rate of 30 °C/min. After being held under the desired temperature for 3 min, the temperature and weight of samples were approximately unchanged. Then, the gas was switched to CO2 with the same flow rate, and the gasification reaction began. The weight loss of the sample was recorded every 5 s, and the repeatability was found to be excellent, with a relative standard deviation of less than 1.5%. The carbon conversion X and gasification reaction rate r of samples were calculated by the following equations:
model (RPM) and two modified RPMs, and the kinetic parameters were obtained. The aim is to investigate the gasification characteristics of fine chars and provide more useful information for designing and operating the integrated AFBentrained flow gasifier.
2. EXPERIMENTAL SECTION 2.1. Raw Materials and Sample Preparation. Two kinds of fine chars separated from the secondary cyclone of a pilot-scale AFB were selected in this study: Xiangyuan bituminous fine char (XYFC) and Yangcheng anthracite fine char (YCFC). The operational temperature for these fine chars was about 1000 °C, while XYFC derived from a lower pressure (0.6 MPa) than YCFC (2.1 MPa). The gasification mediums of fine chars were H2O/O2, while more operation parameters can be consulted elsewhere.17 The fine chars sieved to fractions of a particle diameter 80 wt %) were used as the experimental samples. To evaluate the influences of ash/inorganic elements on the gasification reactivity, two demineralized fine char samples were acid-washed by HF and HCl according to the criterion of GB/T 7560-2001 (Chinese Standard) and denoted as DXYFC and DYCFC, respectively. Two rapid pyrolysis coal chars corresponding to these two coals were prepared in a fixed-bed reactor.18 In brief, about 10 g of coal sample (1500 >1500
>1500 >1500
DT, deformation temperature; ST, soften temperature; and FT, flow temperature.
the AAEMs are removed. Consequently, the influences of ash/ AAEMs on the gasification reactivity of demineralized fine chars could be neglected.21,22 The SEM micrographs could provide surface morphology of fine char and coal char samples. As seen from Figure 1, two fine chars agglomerate to form larger spherical-like particles, which may be the result of the higher temperature in the central tube zone of the AFB.23 The agglomeration can also be accelerated by the higher operating pressure of the gasifier.15 In contrast, no agglomeration can be found for the coal chars deriving from a lower operating temperature and pressure. Besides, the fine chars exhibit more pores than the irregular and sharp-edged coal chars, which will be quantitatively discussed on the basis of N2 adsorption. The pore structure analyses of fine chars and coal chars are listed in Table 3, where the BET surface area, micropore area, total pore volume, and micropore volume were obtained by N2 adsorption.24,25 The surface area and pore volume reveal that the fine chars are more porous than the coal chars. For example, YCFC has the largest BET surface area and total pore volume (274 m2/g and 0.152 cm3/g, respectively), which are much larger than those of YCCC (13 m2/g and 0.012 cm3/g, respectively). The high porosity of fine chars could be explained
Table 3. Pore Structure Analyses of Fine Chars and Coal Chars sample
BET surface area (m2/g)
micropore area (m2/g)
total pore volume (cm3/g)
micropore volume (cm3/g)
XYCC YCCC XYFC YCFC
