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Effects of microwave irradiation on combustion and sodium-release characteristics of Zhundong lignite Zhi-hua Wang, Yingzu Liu, Yong He, Ronald Whiddon, Kaidi Wan, Jun Xia, and Jian-zhong Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01494 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016
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Effects of microwave irradiation on combustion and sodium-release characteristics of Zhundong lignite
3
Zhihua Wang†, Yingzu Liu†,‡, Yong He*,†, Ronald Whiddon†, Kaidi Wan†,‡, Jun Xia‡, Jianzhong
4
Liu†
1
5
†
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, P.R.China
6 7
‡
Department of Mechanical, Aerospace and Civil Engineering & Institute of Energy Futures, Brunel University London, Uxbridge UB8 3PH, UK
8 9
*
Corresponding author: Tel: +86-571-87952111, E-mail:
[email protected] (Y. He)
10
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ABSTRACT Lignite can be upgraded by using microwave irradiation (MI), which may change
12
physical and chemical properties of the coal, affecting its combustion characteristics. In this work,
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MI was used to upgrade coal samples and the changes in combustion characteristics of the
14
upgraded coal samples were analyzed by the thermal gravity analysis (TGA) and surface
15
temperature measurement. The method of sequential extraction was employed to investigate the
16
changes of various Na classes in different coal samples, and Laser-induced breakdown
17
spectroscopy (LIBS) was then used to measure the temporal sodium release during combustion of
18
the different coal samples. Results show that the MI process significantly reduces the moisture
19
concentration, thereby increasing the carbon content and calorific value of the coal. The decrease
20
of the oxygen to carbon atomic ratio (IO/C) indicates an improvement of the coal rank. The TGA
21
shows that the upgrading process will delay the combustion process towards higher temperatures.
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The comprehensive combustion parameter calculated from the TGA shows the combustion
23
performance of upgraded coal samples became worse, which is also seconded by the measured
24
surface temperatures.
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MI produces inter-conversion among different sodium classes, particularly between
26
NH4Ac-soluble sodium and water-soluble sodium. The sodium release characteristics are similar
27
to the surface temperature profiles, indicating that, despite the MI process, sodium release is
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predominantly controlled by the combustion process. However, the MI process is found to
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increase both the mass fraction of sodium in coal samples and the amount of sodium release
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during the combustion process.
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KEYWORDS: Lignite, Upgrading, Coal rank, LIBS, Sodium. 2 ACS Paragon Plus Environment
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1 INTRODUCTION
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Since the compositions of coal vary considerably with the environment at the time of
34
formation and the aging process, coal can be categorized into different ranks: from high-rank
35
anthracite with a high carbon-hydrogen ratio to low-rank lignite, which contains large proportions
36
of volatile matter and water. In China, low-rank coal takes up 41.18 % of the total coal reserves 1.
37
As the energy demand grows, depletion of high-rank coal has necessitated the exploitation of
38
low-rank coals. Identified major issues such as low burning efficiency and corrosion are
39
associated with the utilization of lignite due to its low calorific value, high proportions of mineral
40
matter and moisture. For example, Bosoaga A. et al.
41
combustion temperature and pollutant emissions in a 1 MW furnace when burning Romanian
42
lignite. They found the moisture in coal can make the temperature at furnace center obviously
43
lower and concluded that drying the moist lignite before combustion was required to enhance
44
power plant efficiency. Fernandez-Turiel et al.
45
lignite combustion is caused by thermal decomposition of calcite in the lignite.
3
2
investigated moisture effects on the
believed that the corrosion of high-calcium
46
Reduction in the calorific value of low-rank coals due to moisture can be mitigated by
47
upgrading procedures that remove moisture 1. Microwave irradiation (MI) has been widely used
48
for promoting the coal rank. Unlike traditional thermal dehydration methods, MI uses
49
electromagnetic radiation to quickly heat coal internally
50
procedure has many advantages, including: (1) non-contact heating, (2) energy transfer instead of
51
heat transfer, (3) rapid heating, (4) selective material heating, (5) volumetric heating, (6) rapid
4-6
. According to Kazi 7, the MI
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on/off transition, (7) heating from the interior of the material body, (8) a higher level of safety
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and automation. However, using MI dewatering to upgrade coal will not only dehydrate the coal
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but also lead to changes in coal’s physicochemical properties. Lester and Kingman
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the expansion of moisture within the coal matrix as the mechanism for causing cracks and
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breakage that may change the coal’s physicochemical properties. Recently, a number of
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investigations have been presented on various effects of MI upgrading, such as dewatering
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efficacy 1, ash reduction 9, sulfur species reduction 10 and improvement in grinding characteristics
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11
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combustion have not been reported.
6, 8
suggested
. However, to the authors’ best knowledge, effects of MI power and time on low-rank coal
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In addition to aforementioned identified disadvantages, another major issue of burning
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low-rank coals from specific regions of China is due to alkali species. These minor species can be
63
incorporated in coal as discrete mineral particles and ion-exchangeable cations 12. Alkali species
64
can be particularly problematic, as they lead to severe fouling and corrosion problems in coal
65
combustion applications
66
mineral particles and promote ash coalescence or agglomeration during combustion. As the
67
temperature in the exhaust plume reduces, the aerosolized alkali metal will condense on heat
68
transfer surfaces and lead to corrosion/erosion problems
69
reacting dynamics, much work has been done to measure the amount and release characteristics
70
of the alkali species during coal combustion, such as: (1) identifying different classes of sodium
71
in coal, (2) in-situ measurement of sodium release during coal combustion, (3) reaction
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mechanism of sodium in the gas phase. These investigations have shown that many alkali metals
13-16
. According to Miller
16
, alkalis may reduce the melting point of
14, 17
. In order to better understand their
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are readily released during coal combustion
. However, little work has been done on sodium
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release characteristics of upgraded low-rank coal, a better understanding of which is essential for
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mitigating fouling and corrosion of heat transfer surfaces within industrial coal-fired boilers and
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sustained use of low-rank coals.
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Due to the above motivations, another objective of the present study is to investigate the
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sodium release characteristics during combustion of Zhundong lignite upgraded by microwave
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irradiation. Our work therefore focuses on the influence of upgrading by microwave irradiation
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on characteristics of coal combustion and sodium release during the combustion. The reported
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results are useful for understanding the change in combustion of low-rank coals and alkali metal
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release of upgraded low-rank coals. In this study, microwave irradiation has been used to upgrade
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low-rank coal. The coal samples were exposed to MI with varying durations and power. The
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technique of laser-induced breakdown spectroscopy (LIBS) was employed to measure the sodium
85
release during the combustion of the upgraded coal. LIBS is a technique for in-situ species
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measurement of gas, liquids and solids 24. For coal/biomass combustion, LIBS is commonly used
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for coal sample analysis, ash analysis, and to measure sodium/potassium release 25-27. A benefit of
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the LIBS process is that species are broken into their elemental components by laser plasma,
89
meaning that the signal arises from all respective conformations of a species in the measurement
90
region. In addition, as the characteristic timescale of LIBS measurement is small, it can be used
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to monitor the time-resolved release of Na above a burning coal particle, which is not influenced
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by scattering, allowing proper measurement during all phases of coal combustion.
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2 EXPERIMENTAL METHODOLOGY
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2.1 COAL SAMPLES. The low-rank coal used in this study was from the Zhundong region,
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which contains coal reserves of 164 Giga-tons and is one of the largest coal-producing areas in
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China
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particles. The coal was then upgraded by MI using various combinations of irradiation energy,
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resulting in 4 sample groups: the raw coal and coal upgraded for 1 minute at 500 W
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(500W&1min), 5 minutes at 500 W (500W&5min) and 1 minute at 800 W (800W&1min). The
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sample groups were prepared for surface temperature and LIBS measurement by pressing 50 mg
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of the respective sample into a 4 mm diameter pellet. In the present study, each experiment was
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repeated three times. Error bars that were calculated from the three measurements have now been
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supplied in all the figures where applicable. The difference between the three measurements is
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generally minor.
28
. In this study, the raw coal samples were crushed and sieved to select sub 75 µm coal
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2.2 MICROWAVE IRRADIATION PROCESS. Microwave irradiation experiments were
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carried out in a microwave reaction workstation (MAS-II; Sineo Microwave Chemistry
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Technology Ltd., Shanghai, China) at atmospheric pressure using a microwave frequency of 2450
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MHz. Approximately 20 g of raw coal was spread over the bottom of a Pyrex flask under a
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nitrogen gas flow of 0.3 L/min to prohibit oxidation. The raw coal samples were irradiated for
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either 1 min. or 5 min. durations at a power of 500 W and for 1 min. at 800 W to achieve different
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levels of upgrading.
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2.3 THERMO-GRAVIMETRIC ANALYSIS (TGA). The combustion characteristics of
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coal samples were evaluated by the TGA using a TA-Q500 apparatus (TA Company, USA).
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Analysis was performed using 10 mg samples. The air flow rate was 30 ml/min and the heating
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rate was 10 K/min. The combustion-related parameters such as the ignition temperature (Ti), the
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burnout temperature (Tb), the temperature (Tmax) at which the mass-loss rate of the burning coal
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particle is maximum, the maximum (kmax) and average (kmean) mass loss rates can be obtained
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from the TGA and differential thermal analysis (DTA) curves
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comprehensive combustion parameter can be calculated to demonstrate the ignition, combustion,
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and burnout properties of coal 30-32. The comprehensive combustion parameter S is defined as: =
121 122
29
. Using these parameters, a
×
×
(1)
At the beginning of coal combustion, the reaction is controlled by kinetics. According to the Arrhenius equation, the rate k (mg/min) of a solid-state reaction can be described as follows: = × exp−/
(2)
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where A is the pre-exponential Arrhenius factor, min-1; E is the activation energy, kJ/mol; T is the
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temperature, K; R is the gas constant 8.314 J/(mol·K). Therefore at the ignition point, × = 1 × × × × = × × =
×
(3) (4)
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In Equation (4), R/E is the activation energy of coal combustion; dki/dTi defines the reaction rate
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at the ignition point, a higher value indicating a greater possibility of ignition; kmax/ki is the ratio
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of the highest burning speed to the ignition burning speed; kmean/Tb is the ratio of the average
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burning speed to the burnout temperature, a higher value indicating the coal is easier to burnout.
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2.4 IDENTIFICATION OF SODIUM CLASSES. The sequential extraction method 7 ACS Paragon Plus Environment
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was used to investigate the classes of Na in Zhundong coal before and after microwave upgrading.
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Sodium compounds were distinguished by their solubility by stepwise extraction using water, 1
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mol/L ammonium acetate and 1 mol/L hydrochloric acid. The detailed sequential extraction
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procedure was as follows: 1 g of the coal sample was added to 100 ml deionized water at 333 K
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and stored for 24 hours. It should be mentioned that due to the loss of mass by dehydration, the
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actual mass of the upgraded coal sample was slightly less than 1 g. The coal-water mixture was
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filtered to separate the filtrate that contains water-soluble Na from the coal sample. Next, the
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same coal sample was added to 100 ml of 1 mol/L NH4Ac and then HCl (1 mol/L, 100 ml) and
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processed in a similar manner to the water extraction. After the sequential extraction experiments,
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each 0.1 g insoluble sample was digested by a solution of HCl (12 mol/L, 2 ml), HNO3 (14.5
140
mol/L, 6 ml) and HF (33.3 mol/L, 2 ml). The digested solution was then diluted with deionized
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water to a volume of 100 ml for analysis. The solutions formed in each step were analyzed by
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inductively coupled plasma-atomic emission spectrometry (ICP-AES) to determine the amount of
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Na in each class.
144 145 146 147 148 149 150
From these extractions, it was found that the sodium compounds in coal could be divided into four classes 12, 33, 34: • Water-soluble: this class of sodium is water-soluble salt inside the pore structure, such as sodium chloride, sulfate etc. • NH4Ac-soluble: NH4Ac can leach the sodium, which will then appear as exchangeable ions that are organically connected with carboxyl groups. • HCl-soluble: this form of sodium is organically bonded with nitrogen- or oxygen-containing 8 ACS Paragon Plus Environment
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functional groups. • Insoluble: this class of sodium is believed to be stable silicate minerals.
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2.5 LIBS MEASUREMENT OF NA RELEASE. The coal pellet was suspended on two
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ceramic rods (d = 1 mm) at a height of 10 mm above a flat flame burner. The details of the burner
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can be found in Ref.
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equivalence ratio of 0.8 with 0.59 SL/min methane and 7.06 SL/min air. The flame temperature
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was calculated to be ~1892 K. The gas composition in the region of the coal pellet was estimated
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to be: 3.9% O2, 7.6% CO2, 15.4% H2O and 72.8% N2, based on CHEMKIN calculations.
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. The burner was operated with a premixed methane/air flame at an
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The LIBS system employed to measure the Na release during coal combustion is shown in
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Fig. 1. An Nd:YAG laser (Model RPO-250, Spectra Physics, USA) at the fundamental
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wavelength of 1064 nm and the repetition rate of 10 Hz was used. The pulse duration was 10 ns
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and the energy of each pulse was 300 mJ. The laser beam, the initial diameter of which is 10 mm,
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was focused by a quartz lens (f = 200 mm) into the gas plume at 10 mm above the coal pellet.
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A compact spectrometer (Ocean Optics, USB4000) was used to record the LIBS spectrum.
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The focal length and diameter of the collecting lens were 60 mm and 50 mm, respectively. The
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spectrometer was synchronized to the laser by a pulse generator (Stanford Research System,
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DG535). The optimal delay and exposure time were determined to be 2 µs and 4 ms, respectively;
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using these values, background signal from the continuum emission at the beginning of the
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plasma and spontaneous emission of the flame was reduced. The emission intensities of the
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sodium 3P3S doublet at 588.995 and 589.592 nm were recorded.
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2.6 LIBS CALIBRATION. A series of calibration experiments were carried out to allow 9 ACS Paragon Plus Environment
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quantitative measurement of sodium release using LIBS. An ultrasonic vaporizer was used to
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generate a fog of sodium chloride (NaCl) solution, which was carried by the feeding gas into the
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pilot flame through the vapor chamber. This calibration method was described in detail in Ref. 27.
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In this work, the average seeding rate of NaCl solution was 0.69 g/min, which was calculated
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based on the mass loss from the seeding solution. In order to obtain various concentrations of Na
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in the flame, different concentrations of the NaCl solution were used. The distribution of NaCl
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vapor in the flame was assumed to be uniform. For the experimental setup used here a response
179
function was measured as: ,
= 2430 × % = 0.96
(5)
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where ILIBS,Na is the LIBS signal, CNa is the seeding concentration of Na in the calibration flame,
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and 2430 is the calibration constant.
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2.7 SURFACE TEMPERATURE MEASUREMENT OF A COAL PELLET. The 18, 36
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two-color pyrometry technique
was employed to detect the surface temperature of a coal
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particle during combustion. The thermal emission from the burning pellet was collected with a
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gated intensified CCD camera (Princeton Instruments PIMAX3-1024i). Simultaneously 2D
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images of the coal particle were recorded using a bi-optic lens attachment (LAVISION VZ-image
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doubler). Filters of 1-nm bandwidth centered at 633 nm and 647 nm placed in alternative paths of
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the bi-optic provided the spectral discrimination needed for the two-color pyrometry technique.
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Images were collected at 0.5 Hz with a gate width of 800 ms throughout the duration of the coal
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pellet combustion. These combustion tests were repeated five times. Since the LIBS signal is
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vastly stronger than the thermal emission, it was not possible to conduct LIBS and thermal 10 ACS Paragon Plus Environment
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measurements simultaneously. The 2D images of thermal radiation of the coal pellet can be used
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to calculate the surface temperature by using Wien's equation to compare the intensity of two
194
wavelengths： 1 1 − . , , * = ,4 3 /0 1- + /0 1- + /0 31- + /0 -4 , 1 1 1 % × +
(6)
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where Tp is the surface temperature, C2 is the second Plack's constant, λ is the wavelength of
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thermal radiation, E is the emissive power, ε is the emissivity of the pellet surface, and S is the
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charge coupled device (CCD) spectral response. The value of S was taken from the camera
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manufacturer’s calibration. As the wavelengths are very close to each other (633 nm and 647 nm),
199
the value of ελ1 /ελ2 was approximated to 1. Since the Biot number of the coal pellet is very small
200
(0.02), Tp is also a good approximation of the temperature of the whole coal pellet.
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3 RESULTS AND DISCUSSION
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3.1 EFFECTS OF UPGRADING ON COAL COMPOSITIONS. The MI process excites
203
vibrational modes of water molecules in coal, thereby increasing internal energy of the molecules
204
and causing water to evaporate. As shown in Table 1, significant dehydration was achieved by the
205
MI process: the moisture concentration of the coal decreased from 16.06 wt.% to 8.64 wt.% after
206
1 minute with 500 W irradiation and to 6.95 wt.% after 5 minutes with the same irradiation power.
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Higher irradiation power leads to a stronger dehydration effect, as can be seen from the
208
comparison between the MI results at 500W&1min and 800W&1min. Weight based parameters
209
showed improvement due to water loss: both the calorific value and the carbon concentration 11 ACS Paragon Plus Environment
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increased. For combustible compounds (dry, ash-free basis), the volatile concentration (Vdaf)
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decreased while the fixed carbon concentration increased (FCdaf). The loss of volatile compounds
212
indicates that they maybe outgassed during the MI process. The coal rank therefore moves
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towards a higher rank. An additional effect of the MI process is that volatile compounds may be
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converted from unstable to stable forms 31. The major conversion of volatile compounds may be
215
due to those compounds that contain oxygen groups, substantiated by the increase of the
216
oxygen/hydrogen ratio after MI. The ratio (IO/C) of atomic oxygen to carbon can reflect the
217
aromaticity in the coal and therefore is closely related to the rank of the coal. Since the value of
218
IO/C was calculated from the oxygen and carbon content in dry-ash-free basis. So the evaporating
219
of moisture will not affect this value. According to Ge et al. 31, a lower value of IO/C indicates a
220
higher rank of coal. After MI, the IO/C values decreased from 0.204 (Raw) to 0.193 (500W&1min)
221
and 0.17 (500W&5min), respectively. Also seen in Table 1 is that the sodium concentration
222
followed the same trend of change as the concentration of fixed-carbon compounds after MI.
223
Consequently, the coal sample with the lowest moisture concentration had the highest sodium
224
concentration, i.e. 0.357 wt.% (500W&5min), while a higher-moisture-concentration coal sample
225
contained a lower concentration of sodium, e.g. 0.355 wt.% (500W&1min) and 0.313wt.%
226
(Raw).
227
3.2 EFFECTS OF UPGRADING ON THERMAL GRAVITY PROPERTIES OF
228
COAL. Changes in combustion properties of coal after MI are shown in Fig. 2. When the coal
229
rank increases, the TGA curve moved toward a higher temperature due to a higher ignition
230
temperature and a lower concentration of volatile hydrocarbons. A longer MI time period or 12 ACS Paragon Plus Environment
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higher MI power made the duration of the coal combustion become longer, as can be seen by
232
contrasting the 500W&1min curve with the 800W&1min and 500W&5min ones. Each TGA
233
curve showed two obvious dips. The first dip at the start of heating was caused by vaporization of
234
water in the coal samples. The second dip was caused by the release of volatile compounds just
235
prior to char combustion. Because of moisture removal from the MI processed coals, water
236
evaporation was shortened. At the location of the second dip, a higher ignition temperature was
237
evident.
238
To quantitatively describe the changes of combustion properties of different coal samples,
239
the results of the TGA and DTA curves are incorporated into the comprehensive combustion
240
parameter S, as shown in Table 2. Comparing the effect of the upgrading time as shown in Figs. 2
241
and 3, a longer MI period would make the TGA curve move towards a higher temperature region,
242
and the coal burning rate would be lower. This was quantified in Table 2: for example, the Ti
243
value increased from 506.3 K (Raw) to 534.7 K (500W&1min) and 546.9K (500W&5min),
244
respectively. The increase in Ti has been attributed to the decrease of volatile concentrations,
245
reduction of oxygen functional groups and changes in the coal’s pore structure 37, 38. Tb followed
246
the same trend as Ti, increasing from 771.7 K (Raw) to 801 K (500W&1min) and 848.8 K
247
(500W&5min), respectively. The average mass-loss rate kmean decreased from -0.3185 (Raw) to
248
-0.3033 (500W&1min) and -0.298 (500W&5min), respectively.
249
For the same MI duration, comparing the Raw case to the 500W&1min and 800W&1min,
250
change of irradiation power impacts significantly on thermal gravity properties of the coal,
251
similarly to the role played by the MI duration. For example, Tmax became higher when the 13 ACS Paragon Plus Environment
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irradiation power increased, changing from 654.9 K (Raw) to 658.9 K (500W&1min) and 668.9
253
K (800W&1min), respectively. The S index changed in an opposite trend to that of Tmax, from
254
2.561×10-9 (Raw) to 1.859×10-9 (500W&1min) and 1.452×10-9 (800W&1min), respectively.
255
For high-rank coal, Ti and Tb are usually higher than for low-rank coal, so MI makes the 31, 39
256
upgraded coal samples perform similar to high-rank coals in this respect
. However, all the
257
other parameters (Tmax, kmax, kmean and S) decreased after MI, a trend that strengthens with the MI
258
duration or power. Since the coal samples with a lower S index exhibit worse combustion
259
performance, the decreasing parameters suggest that MI changed the char and pore structures,
260
making the coal burn more slowly. This phenomenon also indicates the coal rank has been
261
upgraded.
262
3.3 EFFECTS OF UPGRADING ON THE SURFACE TEMPERATURE DURING
263
COAL COMBUSTION. The surface temperature versus time for the raw coal and upgraded
264
coal pellets is shown in Fig. 4. The surface temperature profiles can be analyzed according to the
265
three respective stages of coal combustion. The first temperature peak occurs during the pyrolysis
266
and volatile gas combustion stage shown in Fig. 1(a). In this stage, the coal pellet undergoes
267
pyrolysis and the volatile compounds burning around the coal pellet quickly heat the pellet. After
268
the volatile hydrocarbons are consumed, the char begins to burn and the heat exchange with the
269
surrounding gas contributes more directly to increasing the coal pellet temperature than burning
270
of gaseous volatile species. Through the second temperature peak, the char burns steadily and the
271
surface temperature reaches its maximum value (Fig. 1(b)). The majority of the chemical energy
272
of the coal is released before the pellet reaches the maximum temperature. After the char has been 14 ACS Paragon Plus Environment
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273
consumed, the chemical energy is exhausted so the pellet cools to the temperature at which
274
thermal equilibrium with the surrounding post-combustion gases is established (Fig. 1(c)).
275
The effect of MI on the surface temperature of the coal pellet is apparent. In the volatile
276
combustion stage, the time at which the peak surface temperature was reached is similar for all
277
coal samples, but the peak temperature was lower for upgraded coals. The duration of char
278
burnout increased from 313 s (Raw) to 357 s (500 W&1min) and 383 s (500W&5min),
279
respectively. The peak surface temperature decreased from 1596 K (Raw) to 1517 K (500
280
W&1min) and 1487 K (500W&5min), respectively. That is because MI reduces the amount of
281
volatile compounds through either outgassing or conversion to more stable carbon compounds,
282
resulting in the lower peak temperatures of the upgraded coal samples during the volatile
283
combustion stage. In the char combustion stage, the surface temperatures were lower and the
284
peak widths at half maximum were bigger for upgraded samples as compared to the raw sample.
285
In tandem with the wider peaks, the second peak temperature showed an increasing delay with
286
increasing MI power. As in section 3.2, the combustion of char of upgraded coal samples
287
occurred more slowly. As the heat transfer conditions were equivalent for the various coal
288
samples, the worse performance of combustion of the upgraded coal samples can be seen from
289
the delay in the onset of the char combustion stage and the slower burning speed characterized by
290
a lower peak temperature but a longer combustion duration.
291
3.4 EFFECTS OF UPGRADING ON SODIUM COMPOUND CLASSES IN COAL.
292
The mass fractions of the four sodium-compound classes in the raw and upgraded coal samples
293
are shown in Fig. 5 according to different durations and power of microwave irradiation. The 15 ACS Paragon Plus Environment
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294
amount of water-soluble Na was found to increase after MI; while the amount of NH4Ac-soluble
295
sodium decreased in the upgraded coal samples. The respective gain and loss of these two classes
296
increased with increasing MI durations. According to our previous Fourier transform infrared
297
spectroscopy (FTIR) results
298
methyl/methylene and aromatic/aliphatic ratios will be higher after MI treatment. All these
299
phenomena indicated the destruction of unstable components in the brown coal or the conversion
300
of unstable components to stable components, such as: MI treatment can dissociates and shifts
301
the oxygen-containing functional groups. As NH4Ac-soluble sodium can be organically
302
connected with carboxyl groups, the destruction of these functional groups would reduce the
303
amount of sodium in this class. Since the decrease of the mass of NH4Ac-soluble sodium is
304
similar to the increase of the mass of water-soluble sodium, it is reasonable to speculate that MI
305
has shifted some of the NH4Ac-soluble sodium to water-soluble sodium. The mass of
306
HCl-soluble or insoluble sodium did not show an obvious change after microwave upgrading. As
307
these sodium compounds are present in stable nitrogen- or oxygen-containing functional groups
308
or silicate minerals, these sodium classes are largely unaffected by microwave irradiation. Using
309
different irradiation power levels yielded a similar trend to using different MI durations; a greater
310
amount of input microwave energy enhances the conversion of NH4Ac-soluble Na to
311
water-soluble Na. Again, there is no recognizable change in the stable insoluble and HCl-soluble
312
sodium classes.
31
, the carbonyl/aromatic ratio will be lower but the values of
313
3.5 EFFECTS OF UPGRADING ON SODIUM RELEASE DURING COAL
314
COMBUSTION. The profiles of sodium release during combustion of the raw coal and 16 ACS Paragon Plus Environment
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upgraded coal samples are given in Fig. 6. Similar to the surface temperature curves, the sodium
316
release profiles also have two peaks. The first combustion stage, i.e., de-volatilization, is the
317
shortest, characterized by a modest peak shortly after ignition. In this stage, water in the coal
318
pores is heated and begins to vaporize, ejecting water-soluble sodium with the steam. High
319
vapor-pressure compounds are also released with some organic sodium
320
combustion stage features the global maximum of the sodium release profiles
321
char burns, causing the temperature of the pellet to reach the highest. Most organic sodium will
322
be released during this stage. Some inorganic sodium may also be decomposed and released due
323
to high temperatures. The third stage begins shortly after the peak signal, and is indicated by a
324
steep decline in the signal. At this stage, the majority of organic matter has been burnt away and
325
only ash remains. But, as the pellet remains in a high temperature plume, inorganic sodium can
326
still be released due to reaction with water in the flame 21.
12, 33
. The second
18, 27
. In this stage,
327
As seen in Fig. 6, MI was found to have a significant impact on sodium release during coal
328
combustion. Specific information from the profiles is shown in Table 3. In the table, tpeak,v and
329
tpeak,c are the time instants at which the peak concentrations are found during the first and second
330
stages, respectively; tb,A and te,A indicate the beginning and ending time of the ash stage,
331
respectively; IV and IC are the peak values of the Na release curve in the first and second stages,
332
respectively, and IA is the mean value of the third stage.
333
It is known that the peak values of the sodium concentration are closely related to the 27, 33
334
burning temperature and burning rate of the coal sample
. For the first stage, although the
335
upgraded samples burned at consistently lower temperatures, the peak sodium concentrations 17 ACS Paragon Plus Environment
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336
were almost unchanged. It is likely that changes in the structure of the coal pellet and conversion
337
of sodium among different classes, as discussed in section 3.3, enhance the sodium release during
338
this stage. During the char burning stage, the sodium release was delayed in the upgraded
339
samples. The appearance of the peak sodium concentrations during the second combustion stage
340
(tpeak,c) was delayed from 252 s (Raw) to 314 s (500W&1min) and 382 s (500W&5min),
341
respectively. For different MI powers, tb,A increased from 424 s (Raw) to 641 s (500W&1min)
342
and 656 s (800W&1min), respectively. Additionally, the peak concentration of sodium was
343
reduced after MI, which is similar to the trends for the surface temperature. The lower pellet
344
temperature during the char combustion stage of the upgraded coal sample decreased the sodium
345
release rate, resulting in the observed decrease of sodium concentrations.
346
In the ash stage, it is apparent that the factors influencing sodium release have changed.
347
Although the measured temperatures of each sample were similar, the sodium release of upgraded
348
coal samples was enhanced in comparison with that of the raw coal. This can be attributed to
349
changes in the ash structure caused by MI. For example, an increase in the porosity will cause
350
more active interactions of the mineral matter with H2O in the plume.
351
4 CONCLUSIONS
352
Microwave irradiation is an effective method for upgrading low-rank coals, which can
353
remove moisture of raw coal samples by more than 50% after a few minutes of irradiation. The
354
carbon content and calorific value are increased as a result of the water removal. But the volatile
355
compounds (dry; ash-free) and the IO/C parameter decrease after MI. Overall, the ultimate analysis 18 ACS Paragon Plus Environment
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356
Energy & Fuels
of upgraded low-rank coals show increased similarity to high-rank coal.
357
The TGA results indicate that after MI, the propensity for combustion decreases. The coal
358
ignition and burnout occur at higher temperatures, and the comprehensive combustion
359
characteristic index is reduced. These changes, caused by the MI process, will make upgraded
360
coal samples more difficult to ignite, sustain burning and burnout than the raw samples.
361
Increasing either the duration or irradiation power of MI will increase the extent of conversion.
362
MI transforms sodium among different classes as measured by sequential extraction. After
363
microwave upgrading, the amount of water-soluble sodium in the coal sample was found to
364
increase, while the amount of NH4Ac-soluble sodium decreased. The HCl-soluble sodium and the
365
insoluble sodium did not show an appreciable change.
366
The surface temperature of the burning coal pellet featured two peaks, occurring in the
367
volatile burning stage and the char burning stage. The surface-temperature peak of the upgraded
368
coal sample occurred at different times from that of the raw coal sample, and the upgraded coal
369
samples showed consistently lower temperatures while the pellet was actively burning. The lower
370
temperatures during the volatile burning stage are due to the loss or conversion of volatile
371
compounds. MI also leads to a slower burning of char, as indicated by a delayed peak
372
temperature and lower overall heat release.
373
The changes in coal compositions, combustion characteristics and sodium classes after MI
374
greatly influence the sodium release during combustion. For the de-volatilization stage, the
375
decrease of the pellet temperature is offset by the increase of less stable sodium species, resulting
376
in little changes in sodium release. However, the pellet temperature greatly influenced sodium 19 ACS Paragon Plus Environment
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Page 20 of 29
377
release during the char burning stage, i.e. the sodium concentration decreased for upgraded coal
378
samples. In the ash stage, more sodium was released in upgraded coal samples, likely due to
379
more active interactions of sodium mineral with H2O in the plume, which is facilitated by a more
380
porous ash structure.
381
ACKNOWLEDGEMENTS
382
This work is supported by the National Natural Science Foundation of China (Contact No.
383
51406178), National Basic Research Program of China (Contract No. 2012CB214906),
384
Specialized Research Fund for the Doctoral Program of Higher Education of China (Contract No.
385
20130101110095) and China Postdoctoral Science Foundation (Contract No. 2014M551732).
386
387
REFERENCE
388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403
(1) Rao, Z.; Zhao, Y.; Huang, C.; Duan, C.; He, J. Prog. Energy Combust. Sci. 2015, 46, 1-11. (2) Bosoaga, A.; Panoiu, N.; Mihaescu, L.; Backreedy, R.; Ma, L.; Pourkashanian, M.; Williams, A. Fuel 2006, 85 (10), 1591-1598. (3) Fernandez-Turiel, J.-L.; Georgakopoulos, A.; Gimeno, D.; Papastergios, G.; Kolovos, N. Energy Fuels 2004, 18 (5), 1512-1518. (4) Tahmasebi, A.; Yu, J.; Li, X.; Meesri, C. Fuel Process. Technol. 2011, 92 (10), 1821-1829. (5) Li, D.-L.; Liang, D.-Q.; Fan, S.-S.; Li, X.-S.; Tang, L.-G.; Huang, N.-S. Energy Conv. Manag. 2008, 49 (8), 2207-2213. (6) Lester, E.; Kingman, S. Fuel 2004, 83 (14), 1941-1947. (7) Haque, K. E. Int. J. Miner. Process. 1999, 57 (1), 1-24. (8) Lester, E.; Kingman, S. Energy Fuels 2004, 18 (1), 140-147. (9) Sönmez, Ö.; Giray, E. S. Fuel 2011, 90 (6), 2125-2131. (10) Mesroghli, S.; Yperman, J.; Jorjani, E.; Carleer, R.; Noaparast, M. Fuel Process. Technol. 2015, 131, 193-202. (11) Sahoo, B.; De, S.; Meikap, B. Fuel Process. Technol. 2011, 92 (10), 1920-1928. (12) Benson, S. A.; Holm, P. L. Ind. Eng. Chem. Res. 1985, 24 (1), 145-149. (13) Sandberg, J.; Karlsson, C.; Fdhila, R. B. Appl. Energy 2011, 88 (1), 99-110.
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(14) Bryers, R. W. Prog. Energy Combust. Sci. 1996, 22 (1), 29-120. (15) Neville, M.; Sarofim, A. Fuel 1985, 64 (3), 384-390. (16) Miller, S. F.; Schobert, H. H. Energy Fuels 1994, 8 (6), 1197-1207. (17) Toftegaard, M. B.; Brix, J.; Jensen, P. A.; Glarborg, P.; Jensen, A. D. Prog. Energy Combust. Sci. 2010, 36 (5), 581-625. (18) Van Eyk, P.; Ashman, P.; Alwahabi, Z.; Nathan, G. Proc. Combust. Inst. 2009, 32 (2), 2099-2106. (19) Kim, S. S.; Kang, Y. S.; Lee, H. D.; Kim, J. K.; Hong, S. C. J. Ind. Eng. Chem. 2012, 18 (6), 2199-2203. (20) Yuan, Y.; Li, S.; Yao, Q. Proc. Combust. Inst. 2015, 35 (2), 2339-2346. (21) Van Eyk, P. J.; Ashman, P. J.; Alwahabi, Z. T.; Nathan, G. J. Combust. Flame 2011, 158 (6), 1181-1192. (22) Van Eyk, P. J.; Ashman, P. J.; Nathan, G. J. Combust. Flame 2011, 158 (12), 2512-2523. (23) Glarborg, P.; Marshall, P. Combust. Flame 2005, 141 (1), 22-39. (24) Singh, J. P.; Almirall, J. R.; Sabsabi, M.; Miziolek, A. W. Anal. Bioanal. Chem. 2011, 400 (10), 3191-3192. (25) Musazzi, S.; Golinelli, E.; Perini, U.; Barberis, F.; Zanetta, G. Elemental analysis of coal by means of the Laser Induced Breakdown Spectroscopy (LIBS) technique, Sensors Applications Symposium (SAS), 2012 IEEE, 2012; IEEE: 2012; pp 1-3. (26) Gaft, M.; Dvir, E.; Modiano, H.; Schone, U. Spectroc. Acta Pt. B-Atom. Spectr. 2008, 63 (10), 1177-1182. (27) He, Y.; Zhu, J.; Li, B.; Wang, Z.; Li, Z.; Aldén, M.; Cen, K. Energy Fuels 2013, 27 (2), 1123-1130. (28) Zhou, J.; Zhuang, X.; Alastuey, A.; Querol, X.; Li, J. Int. J. Coal Geol. 2010, 82 (1), 51-67. (29) Vyazovkin, S.; Burnham, A. K.; Criado, J. M.; Pérez-Maqueda, L. A.; Popescu, C.; Sbirrazzuoli, N. Thermochim. Acta 2011, 520 (1), 1-19. (30) Boxiong, S.; Dechang, L.; Jidong, L. China Pet. Process. Petrochem. T. 2000, 31 (10), 60-64. (31) Ge, L.; Zhang, Y.; Wang, Z.; Zhou, J.; Cen, K. Energy Conv. Manag. 2013, 71, 84-91. (32) Li, Q.; Zhao, C.; Chen, X.; Wu, W.; Li, Y. J. Anal. Appl. Pyrolysis 2009, 85 (1), 521-528. (33) Zhang, J.; Han, C.-L.; Yan, Z.; Liu, K.; Xu, Y.; Sheng, C.-D.; Pan, W.-P. Energy Fuels 2001, 15 (4), 786-793. (34) Zevenhoven-Onderwater, M.; Blomquist, J.-P.; Skrifvars, B.-J.; Backman, R.; Hupa, M. Fuel 2000, 79 (11), 1353-1361. (35) He, Y.; Wang, Z.; Weng, W.; Zhu, Y.; Zhou, J.; Cen, K. Int. J. Hydrog. Energy 2014, 39 (17), 9534-9544. (36) Huang, Y.; Yan, Y.; Riley, G. Measurement 2000, 28 (3), 175-183. (37) Cheng, J.; Zhou, J.; Li, Y.; Liu, J.; Cen, K. Energy Fuels 2008, 22 (4), 2422-2428. (38) ZHOU, J.-h.; LI, Y.-c.; CHENG, J.; LI, S.-s.; ZHAO, X.-h.; LIU, J.-z.; CEN, K.-f. J. China Coal Soc. 2007, 6, 014. (39) Haykiri-Açma, H.; Ersoy-Meriçboyu, A.; Küçükbayrak, S. Energy Conv. Manag. 2002, 43 (4), 459-465.
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438 Table 1. MI effects on coal properties.
439
Proximate analysis (wt.%) Sample
Calorific value (J/g) Mr
Ar
Vr
FCr
Vdaf
FCdaf
Raw
16.06
3.55
26.64
53.75
33.14
66.86
22802
500W&1min.
8.64
4.04
28.38
58.94
32.50
67.50
24898
800W&1min.
7.14
4.13
28.61
60.12
32.24
67.76
25406
500W&5min.
6.95
4.17
28.56
60.32
32.13
67.87
25448
Ultimate analysis (wt.%, dry, ash-free basis)
Sodium Io/c
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygen
(wt.%,received basis)
Raw
78.45
4.15
0.87
0.54
15.99
0.313
0.204
500W&1min.
79.5
3.76
0.83
0.54
15.37
0.347
0.193
800W&1min.
79.91
4.86
1.03
0.58
13.62
0.355
0.170
500W&5min.
80.01
4.87
0.89
0.59
13.64
0.357
0.170
440 441 442 443 Table 2. MI effects on characteristic combustion parameters.
444
Ti
Tb
Tmax
kmax
kmean
Sample
S (K)
(K)
(K)
(%/K)
22 ACS Paragon Plus Environment
(%/K)
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Energy & Fuels
Raw
506.3
771.7
654.9
-1.59
-0.3185
2.561·10-09
500W&1min.
534.7
801
658.9
-1.38
-0.3084
1.859·10-09
800W&1min.
536.1
836
668.6
-1.15
-0.3033
1.452·10-09
500W&5min.
546.9
848.8
675.1
-0.88
-0.2980
1.045·10-09
445 446 447 448 449 Table 3. MI effects on sodium release.
450 Sample
tpeak,v (s)
tpeak,c (s)
tb,A (s)
te,A (s)
IV (mg/m3)
IC (mg/m3)
IA (mg/m3)
Raw
10
252
424
3192
1.411
3.539
0.346
500W&1min.
13
314
641
3930
1.404
3.377
0.382
800W&1min.
15
346
656
4000
1.401
3.368
0.426
500W&5min.
17
382
674
4000
1.394
3.365
0.422
451 452 453
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454 455
Figure 1.Experimental setup of LIBS measurement
456 457
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458 459
Figure 2.TGA curves of the raw and upgraded coals.
460
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461
462 463
Figure 3. DTA curves of the raw and upgraded coals.
464
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465
466 467
Figure 4. Surface temperature of the raw and upgraded coals.
468 469 470 471 472 473 474 475 476 477 478
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479
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(a) water-soluble sodium
(b) NH4Ac-soluble sodium
(c) HCl-soluble sodium
(d) insoluble sodium
Figure 5. MI effects on sodium classes.
480
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481 482
Figure 6. Sodium release during the burning of the raw and upgraded coals.
483 484 485
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