Article pubs.acs.org/EF
Properties of Upgraded Shengli Lignite and Its Behavior for Gasification Xianjun Yang, Cheng Zhang,* Peng Tan, Tao Yang, Qingyan Fang, and Gang Chen* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: It is hard to use lignite directly because of the problems in low thermal efficiency and difficulties in transportation. Thermal upgrading is a potential way for the use of lignite with the increasing consumption of coal in China at present. In this study, the pyrolysis characteristics of Chinese Shengli (SL) lignite were investigated by thermogravimetric analysis (TGA), and the upgraded lignite obtained by isothermal (250−750 °C) and non-isothermal (5, 12, and 20 °C/min) upgrading from the tube furnace was used to characterize the gasification properties of it. The structural information of upgraded lignite was identified by a nitrogen adsorption isotherm and Fourier transform infrared spectroscopy (FTIR), and the gasification reactivity of the upgraded lignite was determined by TGA in a self-designed thermobalance reactor. The results indicated that the temperature was the primary factor affected on the property of upgraded lignite. Isothermal treatment at 350 °C could be an optimal condition for SL lignite considering both the quality of products and the economic issues. The chemical and physical structural characteristics of lignite changed significantly after thermal upgrading, which influenced the gasification characteristics of upgraded coal. The decomposition of surface functional groups and ordering of the crystalline carbon structure resulted in the decrease of active sites and further reduction of the reactivity of upgraded lignite. The total surface area (TSA) could not be used to explain the change of reactivity of upgraded lignite obtained above 550 °C because of the coal particle collapse and the loss of active sites. The higher heating rates enhanced the porosity of upgraded lignite and led to the increase of reactivity, and the residual macromolecules remaining in upgraded lignite pores at a higher heating rate could release again at the initial gasification stage, which had a negligible effect on the gasification reactivity. The isothermal upgrading within 450−550 °C should be the optimal operation condition for the use of SL lignite in the process of gasification after thermal upgrading.
1. INTRODUCTION Low-rank coals (LRCs, i.e., brown coal and lignite) have an estimated proportion of 45% of the world’s coal reserves and constitute a significant resource for both energy and chemical feedstocks.1,2 However, despite their low market prices, only a limited amount of LRCs has been used because of their low calorific value, high moisture content, and low thermal efficiency, being rich in oxygen-containing groups and high spontaneous combustion tendency.3 Therefore, lignite upgrading provides a potential way to widen the range of lignite use.2 At present, methods used for upgrading mainly include thermal evaporative, mechanical, and non-evaporative dewatering upgrading (i.e., hydrothermal upgrading).1,3−6 Thermal evaporative drying upgrading is the most commonly applied upgrading method.7,8 Present studies on thermal upgrading of lignite are mainly focused on moisture removal, reducing the spontaneous combustion tendency, and the heat value improving.9 The upgraded lignite is now mainly used for electricity generation at power stations.10 However, as wellknown, the chemical activity of lignite is much better than bituminite or anthracite; thus, the upgraded lignite has the potential advantages for gasification use.11 In addition, coal combustion increases the emissions of greenhouse gases and further leads to global warming. Thus, the gasification of upgraded lignite could also be realized as one of the coal clean technologies (CCT), which provide a clean and economic way for the use of lignite. The thermal temperature and heating rate are the primary factors affecting lignite thermal upgrading.12 The thermal © 2013 American Chemical Society
upgrading process can be regarded as pyrolysis or carbonization, which may be accompanied by drastic changes in the pore structure, texture, and molecular structure of the coal. The products of lignite thermal upgrading consist of gases, tar components, and a carbon-rich solid residue, called char or upgraded lignite;13 therefore, the thermal upgrading efficiency and the characteristics of upgraded lignite were influenced by the pyrolysis characteristics of lignite seriously. Reactivity of coal is the main factor for gasification that determines the design of the gasifier and the operation conditions of gasification.11 The gasification rate of coal char in carbon dioxide is governed by its structure;14−17 therefore, the structural changes in the thermal upgrading process certainly affect the reactivity of upgraded lignite significantly. Feng and Bhatia studied the pore structure variation of coal during heat treatment and concluded that some pores closed and blocked by the presence of large molecules or groups, comprising disorganized carbon and cross-linking crystallites, and the closure of pores was a function of the heat treatment time and temperature.17 Many researchers studied the effects of pyrolysis conditions on coal char reactivities. They concluded that a higher charring temperature, slower heating rate, and Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 1, 2013 Revised: October 28, 2013 Published: October 29, 2013 264
dx.doi.org/10.1021/ef401497a | Energy Fuels 2014, 28, 264−274
Energy & Fuels
Article
Table 1. Proximate, Ultimate, and LHV Analyses of SL Lignite ultimate analysis (%)
a
proximate analysis (%)
coal samplea
C
H
N
S
Ob
M
V
A
FC
LHV (MJ kg−1)
SL (ad) SL (ar)
54.48 43.61
4.29 3.07
0.76 0.77
1.74 1.23
15.94 10.87
9.23 28.25
40.13 29.09
13.56 12.20
37.08 30.46
20.48 15.96
ad, air-dry basis; ar, as-received basis. bBy difference.
Figure 1. Schematic of the fixed-bed thermal upgrading reactor. calculated according to the sulfur, hydrogen, and moisture contents. The results of SL raw coal analysis are shown in Table 1. 2.2. Fixed-Bed Thermal Upgrading and Gasification. The preparation of upgraded lignite and the fixed-bed gasification experiments were performed in a tube furnace. The schematic diagram of the setup is shown in Figure 1. 2.2.1. Isothermal Upgrading Experiment. In the isothermal upgrading experiment, the tube was purged continuously with N2 and, when the desire temperature (250, 350, 450, 550, 650, and 750 °C) was reached, then a sample of 0.75 g in the crucible was pushed into the center of the tube for thermal upgrading at the desire temperature in nitrogen for 10 min. After that, the upgraded lignite was pushed to the end of the tube cooling in nitrogen for 10 min. After each experiment, the upgraded lignite were weighted and sealed for other determinations. In isothermal upgrading, the upgraded lignite was named as R250, R350, R450, R550, R650, and R750, respectively. 2.2.2. Non-isothermal Upgrading Experiment. A sample of 0.75 g in the crucible was inserted into the tube furnace and heated in a stream of nitrogen at a heat rate of 5, 12, and 20 °C/min. Thermal upgrading stopped when the temperature reached 500 °C, and after that, the sample was pushed to the cooling zone to cool, then weighted, and collected. In non-isothermal upgrading, the upgraded lignite was named as S5, S12, and S20, respectively. 2.2.3. Isothermal Fixed-Bed Gasification. In the isothermal gasification experiment, the tube was heated to 950 °C and purged continuously with N2 for 5 min. A sample of 1 g in the crucible was inserted to the tube. At the same time, N2 was switched to CO2 with a rate of 0.5 L/min, and the gas analyzer (Gasboard-3100P) started to record the production of gases. The experiment was finished when the yield of CO did not increase. 2.3. Upgraded Lignite Characterization. 2.3.1. FTIR Analysis. To characterize the effect of thermal upgrading on the change of the chemical structure for SL lignite, the upgraded sample were analyzed by a FTIR spectrometer (Bruker VERTEX70). The dried KBr and dried upgraded sample were ground at a ratio of about 100:1. The spectra were obtained with 32 scans at a resolution of 4 cm−1. Infrared (IR) spectra of the lignite sample for the 4000−400 cm−1 region were studied by curve-fitting analysis. 2.3.2. Nitrogen Adsorption Isotherm. The nitrogen adsorption isotherm experiment was performed to characterize the effect of thermal upgrading on the pore structures of SL lignite. Nitrogen adsorption at 77 K was performed for the upgraded lignite in an automated adsorption analyzer (ASAP2020, Micromeritics). The pore parameters were determined by Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) equations. The coal sample was dried at 105 °C for 24 h before the experiment. 2.4. TGA Experiments. 2.4.1. Isothermal Gasification. To characterize the reactivity of upgraded lignite, the fixed-bed isothermal
longer thermal treatment time caused lower reactivities of the char products in gasification.15,18,19 In addition, it has also been reported that the gasification rate of char is controlled by the intrinsic reactivity of coal, which is subject to the active sites in the coal matrix associated with carbon atoms bonded to heteroatoms and nascent sites.20 In general, severe thermal treatment conditions, such as a higher treatment temperature and a longer treatment time, lead to poorer reactivity of char. The structural changes, including the variation of the pore structure, the order of the crystalline carbon structure, and the loss of functional groups on the carbon surface under thermal treatment, have been recognized to be the main reasons for the decrease of reactivities of coal chars during gasification.21−24 In this study, the Shengli (SL) lignite from Inner Mongolia of China was used, which had a good activity and was suitable for chemical industry production. The SL coalfield is located in northeastern Inner Mongolia, China. It is the largest lignite coalfield in China, with a total area of 342 km2, and the retained reserves are around 16 billion tons. The aim of the work is to understand the variation behaviors of SL lignite in the thermal upgrading and gasification use process, including evaluating the thermal upgrading efficiency and the reactivity and gas yield capacity of SL upgraded lignite obtained by different upgrading conditions. Nitrogen adsorption isotherm and Fourier transform infrared spectroscopy (FTIR) analysis were used to identify the structural changes of SL lignite in thermal upgrading. The gasification characteristics of SL upgraded lignite was investigated by thermogravimetric analysis (TGA) in a self-designed thermobalance reactor to finally obtain the economic operation condition for the use of SL lignite in the process of thermal upgrading and gasification.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The coal sample was air-dried, milled, and then sieved into a particle size within 0−0.2 mm. Proximate analysis was performed using TGA2000 (Las Navas Corporation). The carbon, hydrogen, and nitrogen contents were measured using VARIO CHNMAX (Elementar Analysensysteme GmbH Corporation). The total sulfur content in coal was determined using RAPID CS CUBE (Elementar Analysensysteme GmbH Corporation). Three parallel tests were performed for each sample. The PARR 6200 oxygen bomb calorimeter was used to determine the oxygen bomb calorific value of samples (PARR Corporation), and the low heating value (LHV) was 265
dx.doi.org/10.1021/ef401497a | Energy Fuels 2014, 28, 264−274
Energy & Fuels
Article
Figure 2. Schematic of the thermobalance reactor. gasification experiments were conducted in a self-designed thermobalance reactor (Figure 2) for upgraded lignite samples. The thermobalance reactor has a maximum injection value at 0.5 g, which could increase the uniformity for lignite. In each experiment, the reactor was heated to a desired temperature (950 °C) under a CO2 flow of 2 L/min, and then the coal sample (about 0.25 g) was placed in a quartz basket suspended from an electronic balance and slowly declined into the reaction zone for gasification. The experiment was finished when the weight loss did not increase. 2.4.2. Temperature Programmed Pyrolysis. Pyrolysis experiments were conducted in TGA (NETZSCH STA 449F3), which was used to identify the pyrolysis characteristics of SL lignite. The experiments were performed under the following conditions: sample weight, 10 mg; atmosphere, N2; heating rate, 10, 30, and 50 °C/min; and terminal temperature, 1000 °C. All samples were dried at 105 °C for 3 h to remove moisture prior to the measurements.
3. RESULTS AND DISCUSSION 3.1. TGA of Pyrolysis for SL Lignite. Panels a and b of Figure 3 show the thermogravimetry (TG) and differential thermogravimetry (DTG) curves of SL raw coal pyrolysis at the heat rate of 10, 30, and 50 °C/min within the temperature range from room temperature to 1000 °C, respectively. Table 2 shows the data obtained from the TG and DTG curves. DTGmax and Tmax were defined as the maximum weight loss rate and the temperature at the maximum weight loss rate. As seen from Figure 3 and Table 2, the weight loss increased with the increase of the pyrolysis temperature, only about 5% weight loss was determined below 250 °C, and about 3% weight loss was obtained between 750 and 1000 °C. The major weight loss about 30% during pyrolysis happened between 250 and 750 °C. Therefore, the pyrolysis process can be separated into three stages. The first stage was the drying degasification stage (
