Article pubs.acs.org/EF
Experimental Investigation into the Spontaneous Ignition Behavior of Upgraded Coal Products Haiming Wang and Changfu You* Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: The propensity for spontaneous ignition of nine coal samples and their upgraded products were experimentally investigated. The basket method was used to study the effects of the variation of mineral matter and volatile content on the spontaneous ignition of coal. The removal of mineral matter by chemical leaching caused the spontaneous ignition temperature (SIT) to decrease, which indicated a higher propensity for ignition. As the mineral matter increased, SIT increased approximately linearly. The results indicated that the SIT of coal char first decreased with a decreasing volatile content and then rapidly increased as the volatile content continued to decline, because of the deepening of the devolatilization. This paper proposes a prediction model based on the oxygen and carbon ratio (O/C) parameter in raw coal, which can be used to predict its SIT.
1. INTRODUCTION The use of low-rank coals (such as lignite and sub-bituminous coal) as a primary energy source has become increasingly important for energy supplies (especially in China) because of their abundance, accessibility, and low mining costs. However, despite these benefits, the low heating value, high water content, and high ash content of low-rank coal results in low efficiency and high pollution in practical applications. To increase the efficiency of low-rank coals, upgrading technologies have been used to improve the performance of the coal,1 including drying, chemical leaching, pyrolysis, and gasification. Nevertheless, the spontaneous ignition of coal is a global concern for its storage, transportation, and handling.2 Not only does the self-heating of coal cause huge economic loss and personal casualty, it may also lead to massive environmental pollution. As such, it is essential to assess the susceptibility of coal to spontaneously ignite and also to study the effects of the upgrading process on the propensity for spontaneous ignition. It is well-known that the self-heating of coal is mainly attributed to the complex process of low-temperature oxidation. Extensive research and studies have been carried out to understand this process.2−5 The spontaneous ignition of coal is affected both intrinsically and extrinsically. The present research focuses on the influences of some intrinsic parameters, including the water content, mineral matter content, and volatile content, which affect the self-heating process of coal significantly.6 The effect of the upgraded products of lignite with different water contents on spontaneous ignition of coal and the recommended water content for upgraded lignite products have been discussed in detail in the previous study.7,8 This paper examined the effects of changes in the mineral matter content and volatile content of coal caused by the upgrading process on spontaneous ignition. The mineral matter content (defined as the ash content) in coal is an important factor that affects the self-heating propensity of coal.9 The upgrading technology, chemical leaching, aims to remove mineral matter in low-rank coal to boost the ignition efficiency and reduce environment contamination, which also changes the propensity of coal to spontaneously ignite. However, limited research has been © 2014 American Chemical Society
published about the effect of the mineral matter content. Humphreys et al.10 tested a mineral-matter-free correlation for the self-heating rate by adding different amounts of ash to the same coal sample. Beamish et al.9,11 proposed a significant negative correlation between the ash content and the R70 selfheating rate. Zhang and Sujanti12,13 used HCl- and HSO4washed coal samples to remove the majority of inorganic matter. They found that the removal of inorganic matter decreased the propensity of coal to spontaneously ignite. The volatile content in coal represents the reactive component. It is considered to be one of the most statistically significant factors in determining the self-heating propensity of coal.14 Nevertheless, insufficient research exists about the influence of the volatile content on the self-ignition of coal. The pyrolysis or gasification of coal may significantly change the volatile content. The effect of the variation of the volatile content caused by these processes is still unknown. This paper used the basket method to study the effects of the variation of mineral matter and volatile content on the spontaneous ignition of coal. The basket method was first developed by Bowes3,15 based on the Frank−Kamenetskii model. It provides a full temperature history of the self-heating process of a coal sample from room temperature to thermal runaway.16,17 The test index to assess the propensity for spontaneous ignition is the spontaneous ignition temperature (SIT). In addition to low-rank coals, two kinds of high-rank coals (bituminous and anthracite) were used in the experiments to determine a method for predicating the approximate SIT of coal.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Nine coal samples of different ranks were used in this investigation. Table 1 lists the proximate and ultimate analyses of each sample. S1, S2, S4, and S5 are sub-bituminous coals from Indonesia; S3, S8, and S9 are lignite coals from China; S6 is Received: December 30, 2013 Revised: February 20, 2014 Published: February 20, 2014 2267
dx.doi.org/10.1021/ef402569s | Energy Fuels 2014, 28, 2267−2271
Energy & Fuels
Article
Table 1. Ultimate and Proximate Analyses of Nine Coal Samples ultimate analysis (wt %)
a
proximate analysis (wt %)
samples
Cd
Hd
Od
Nd
Sd
volatile matterd
ashd
fixed carbond
moisturead
S1 S2 S3 S4 S5 S6 S7 S8 S9
64.67 58.19 64.18 60.76 58.26 45.24 52.50 55.45 50.97
4.37 3.88 3.92 4.04 4.04 2.85 2.60 3.83 2.98
12.74 17.00 24.53 15.24 14.09 5.87 1.57 15.25 13.88
1.56 0.92 0.88 1.36 1.34 1.28 0.95 0.93 0.82
0.72 0.23 0.17 0.52 0.65 0.31 0.38 0.57 0.62
38.73 41.61 49.01 39.59 38.57 21.08 4.53 38.46 35.72
15.94 19.78 6.32 18.09 21.61 44.46 41.99 23.97 30.73
45.33 38.61 44.67 42.32 39.82 34.47 53.47 37.57 33.55
3.58 5.73 9.20 5.31 4.27 1.79 2.70 30.84a 28.31a
Moisture content as received.
bituminous coal with high ash from India; and S7 is anthracite from China. Prior to experimentation, all of the coal samples were stored under seal at room temperature to minimize the pre-oxidation effect. In the laboratory, each sample was carefully crushed, ground, and then sieved to a certain size range (90−335 μm) for the experiments just prior to testing. 2.2. Experimental System. The basket test was used to determine the SIT and record the temperature history of coal samples. Figure 1 is
chemical methods. A chemical method was chosen in this paper to clean high-ash coal. It is generally accepted that the process of chemical leaching is the only way to remove mineral matter for ultra-clean coal products.18,19 The present research used a two-stage leaching sequence to remove mineral matter, described in detail by Wu and Steel.20 Sample S6 was used to determine the effect of the mineral matter content on the SIT because of its high ash content, approximately 44.46%. After chemical treatment, the ash content of S6 decreased to 1.69% by weight, indicating that most of the mineral matter was washed off by the two-stage leaching sequence. The acidwashed S6 was then mixed with the raw sample in different proportions to obtain varying mineral matter samples (Table 2). The blended coal samples were used to research the effects of the mineral matter on the susceptibility of coal to self-ignite. Table 2. Ash Content and SIT of Different Coal Samples
Figure 1. Diagram of the experimental system. a schematic diagram of the experimental system using the basket method. Coal samples were filled in a cubic container made of a stainless-steel net. The container had a mesh opening of 50 μm, and the side was 50 mm; the top surface was open. The cubic container was then put into a heat oven at a preset temperature. Two K-type thermocouples of 0.5 mm diameter were used to record the temperature history. One thermocouple was inserted into the center of the coal sample and the other was installed between the container and the oven for oven temperature measurements. To avoid the effect of air circulation in the oven, the container was installed in two stainless-steel cages made from a net. The mesh size was 0.6 mm. The first cage (100 × 100 × 150 mm) was slightly larger than the sample container, and the second cage (150 × 150 × 250 mm) was slightly larger than the first cage. Spontaneous ignition was considered to have occurred if the sample temperature rose rapidly and was at least 60 °C higher than the oven temperature. If there was no sign of a runaway reaction inside the coal sample, then the experiment was repeated with fresh coal samples using a higher oven temperature at an interval of 5 °C. If the spontaneous ignition occurred, the experiment would be repeated at a lower oven temperature. A series of experiments was carried out until the critical ambient temperature (SIT) was determined.
samples
mass of acid-washed coal (g)
mass of raw coal (g)
ash content (%, dry)
SIT (°C)
S6-1 S6-2 S6-3 S6-4 S6-5
400 300 200 100 0
0 100 200 300 400
1.69 12.38 23.08 33.77 44.46
128 133.1 142.6 152.3 157.6
Figure 2 shows the SIT of S6 samples with different ash contents. SIT increased approximately linearly with an increasing ash content. The existence of ash in the coal inhibited the process of self-heating, which led to higher SITs. Beamish et al.11 used the adiabatic method and the index of R70
3. RESULTS AND DISCUSSION 3.1. Effect of the Mineral Matter Content. The direct use of low-grade coals with a higher mineral matter content results in lower efficiency and higher costs to deal with the emissions. Several upgrading technologies can be used to remove the mineral matter in the coal, including physical and
Figure 2. Plot of SIT against different ash contents. 2268
dx.doi.org/10.1021/ef402569s | Energy Fuels 2014, 28, 2267−2271
Energy & Fuels
Article
Victorian brown coal used by Zhang et al. were alkalis and alkaline-earth metal. The presence of some inorganic matter, such as pyrite,2,4,5 may accelerate the rate of oxidation of coal.12,22 As a result, the removal of these mineral matter by acid-washing slowed the spontaneous ignition process, which increased the SIT. The mineral matter in S6 largely consisted of aluminosilicate compounds and silica, which suppressed the low-temperature oxidation of coal instead of prohibiting the self-heating process. Consequently, the removal of mineral matter in S6 made it easier to spontaneously ignite. Even though some inorganic matter, such as pyrite, can accelerate the oxidation of coal, the effect of them is insignificant in comparison to that of the massive aluminum and silicon oxides, which eventually caused SIT of S6 to decrease after chemical treatment. 3.2. Effect of Devolatilization. S2, S3, and S4 are low-rank coals with comparatively high volatile content, suitable for experimental usage after pyrolysis pretreatment. Low-temperature pyrolysis in the fixed bed was used to partially remove volatile content from the coal samples to produce semi-coke samples with different volatile contents. Raw coal samples were used to fill a stainless-steel tube. Nitrogen gas was then introduced from the bottom of the pipe at a flow rate of 1 L/ min. Different pyrolysis temperatures ranging from 400 to 700 °C and duration times ranging from 30 to 120 min were set to obtain coal samples with different volatile contents (Table 4).
to study the effect of the ash content and proposed a relationship between ash and R70 as follows: R 70 = 0.0029ash2 − 0.4889ash + 20.644
(1)
When the index R70 was high, the propensity of coal to spontaneously ignite was high. This equation was applied to the S6 samples. Figure 3 shows the relationship between the R70
Figure 3. Relationship between the R70 and ash content of S6 samples.
self-heating rate and the ash content. The R70 decreased approximately linearly as the ash content increased, which is in agreement with the results of this paper. The mineral matter in coal acted as a heat sink10 that absorbed heat during the selfheating process, similar to the water content in coal. In addition to the heat sink effect, the ash in the coal also covered the coal surface, thereby inhibiting oxygen from reaching the reactive sites on the surface. This slowed the self-heating process. The SIT of the S6 raw sample (S6-5) was 157.6 °C. After the two chemical leaching sequences, the ash content was reduced to 1.69% and the SIT was reduced to 128 °C, indicating that the removal of mineral matter in the coal increased the propensity of coal to spontaneously ignite. However, Zhang et al.12,21 used the basket method to study the effect of inherent inorganic matter on the self-heating process of coal and found that the removal of inorganic matter in coal suppressed the selfignition process, which is incompatible with the results obtained in this paper. This may be caused by the different methods of acid-washing used to remove the mineral matter. In the Zhang et al. research, the coal sample was mixed with HCl or H2SO4 acid solution, which could only remove some of the mineral matter in the coal (excluding aluminosilicate and silica). In the present study, the coal samples were first washed by HF and then washed by Fe3+, which could remove almost all of the mineral matter in the coal. This chemical leaching sequence caused no change in the calorific value of coal, indicating that no oxidation happened to the coal matrix.20 The oxidation of the coal structure may lead to the loss of some reactive components, which can react with O2 and release heat. That is to say, the oxidation caused by the chemical treatment will suppress the self-heating process of coal and cause SIT to increase. In addition, the coal samples used in the Zhang et al. research was Victorian brown coal with 3 wt % ash content, and the component of the inorganic matter differed with the sample used in this paper (Table 3). Most inorganic components in
Table 4. Volatile Content and SIT of Different Coal Samples samples
volatile content (%, dry)
SIT (±2.5) (°C)
S2 S2-1 S2-2 S2-3 S2-4 S3 S3-1 S3-2 S3-3 S3-4 S4 S4-1 S4-2 S4-3
41.61 30.36 18.68 9.51 1.56 49.01 40.70 37.31 21.25 7.85 39.59 33.59 13.25 6.10
123.3 95.5 95.1 144.1 239.1 119.9 87.4 99.9 73.3 128.4 111.6 99.6 125.2 193.4
Figure 4 shows the relationship between the SIT and semicoke with different volatile contents. For all three samples, the coal sample SIT underwent two stages as the volatile content decreased. In the first stage, the SIT decreased slowly with a decreasing volatile content. When the volatile content decreased to approximately 25%, the SIT obtained its lowest value, after which it increased rapidly with a decreasing volatile content. This result indicated that the susceptibility to spontaneous ignition of coal with high volatile content increased with the pyrolysis process. In the second stage, the susceptibility decreased with the deepening of the devolatiliza-
Table 3. Main Components of the Mineral Matter in Coal (wt %) samples
Si
Al
Ca
Fe
K
Mg
Na
Zhang et al.12,21 S6
