Article pubs.acs.org/EF
Effect of Upgraded Lignite Product Water Content on the Propensity for Spontaneous Ignition Kai Zhang and Changfu You* Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: The spontaneous ignition that occurs with upgraded lignite products is a concern for its storage, handling, and transportation. The Basket testing method and Oxidation Kinetics testing method were used to identify the propensity for upgraded lignite products to spontaneously ignite. The evaluation index for the Basket testing method was spontaneous ignition temperatures and duration. The fundamental evaluation index (I) for the Oxidation Kinetics testing method was used to evaluate the propensity for spontaneous ignition. For both testing methods, all three of the lignite samples (Huolinhe lignite, Hailaer lignite, and Indonesia lignite) showed that the propensity for spontaneous ignition decreased linearly with the increase in water content of the upgraded lignite product. Commonly used Chinese bituminous coals, which are not prone to spontaneous ignition, were used as a benchmark for determining the recommended water content for upgraded lignite products. For Huolinhe lignite, Hailaer lignite, and Indonesia lignite, the recommended water contents of the upgraded products are 6.53%, 4.28%, and 7.83%, respectively. The method that was developed to determine the recommended water content could be used to guide engineering design and operations when using upgraded lignite that is highly susceptible to spontaneous ignition.
1. INTRODUCTION Lignite, as a low rank coal, will become increasingly important in the supply of energy because of its abundance, accessibility, and low mining cost. At the end of 2005, worldwide lignite reserves were 207.4 billion tons and accounted for 17.7% of total coal.1 Lignite is cheap and low in sulfur but is also characterized by 25−65% water content and low energy output relative to other coals.2,3 The moisture increases the transportation costs per thermal unit of coal and leads to a lower coal price due to its lower heating value. The direct use of lignite for power generation may not be energy efficient if the latent heat of vaporization is not recovered. Using dried coal can increase power plant efficiency, decrease ash disposal requirements and decrease power plant emissions. Therefore, the removal of water from lignite is an important process for its effective use. Also, lignite must be dried for many other applications, including briquetting, coking, gasification, lowtemperature carbonization, and liquid fuel synthesis. However, the spontaneous ignition that occurs with upgraded lignite products is a concern for its storage, handling, and transportation.4−6 In certain areas of a power plant, especially in silos, the self-heating of pulverized dried lignite deposits can cause damage to components and can potentially be the ignition source of an explosion.7 Spontaneous ignition of coal is a naturally occurring process caused by low temperature oxidation. The self-heating of coal is dependent on a number of controllable factors. These controllable factors include power plant management, transport management, and stockpile storage in silos, bunkers, and mills. Moreover, self-heating also relates to controllable features of the coal itself, in particular, the water content of the upgraded lignite product.8 To date, there are few reports on the recommended water content for upgraded lignite products. Therefore, there is still considerable interest in the propensity for spontaneous ignition © 2012 American Chemical Society
of upgraded lignites products with different moisture contents and also the recommended water content for upgraded lignite products. The effect of moisture in the coal on the propensity for spontaneous ignition is complex. Investigations conducted to better understand the processes involved have produced conflicting results. For example, Sondreal and Ellman,9 Chen and Stott,10 and Vance et al.11 reported that coal oxidation rates reached a maximum when water content was intermediate. In contrast to these findings, Li and Skinner12 reported that, for raw wet subbituminous coal prepared under nitrogen, the relative reactivity was highest for coal with no water content and then progressively decreased as the coal water content increased. Bhat and Agarwal13 questioned the validity of the intermediate water content concept for maximum oxidation rates because they considered that the tests used to obtain the results might have altered the coal. Various physical and chemical analytical techniques have been applied to identify and quantify the spontaneous ignition phenomena. Differential thermal analysis (DTA),14,15 thermogravimetric analysis (TGA),16−18 and differential scanning calorimetry (DSC)19,20 are most commonly used in studying the propensity for spontaneous ignition. These techniques are suitable for the preliminary evaluation of thermal stability. However, more reliable information is required to evaluate the propensity for spontaneous ignition of upgraded lignite products with different water contents. The Basket testing method and Oxidation Kinetics testing method present as better methods for assessing the propensity of spontaneous ignition of coal and are therefore used for this purpose.21−23 Although requiring longer testing times, the Basket testing Received: June 21, 2012 Published: December 11, 2012 20
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Table 1. Ultimate and Proximate Analyses of the Raw Lignite ultimate analysis (%)
proximate analysis (%)
samples
Cd
Hd
Od
Nd
Sd
Vd
FCd
Mar
Ad
Huolinhe lignite Hailaer lignite Indonesia lignite
55.45 50.97 66.67
3.83 2.98 4.09
15.25 13.88 19.68
0.93 0.82 0.40
0.57 0.62 0.34
38.46 35.72 35.51
37.57 33.55 55.67
31.93 28.87 62.95
23.97 30.73 8.82
method approximates the actual conditions for spontaneous ignition. Zhang and Sujanti24 studied the effect of inorganic additives, when applied into a coal by ion-exchange, on low temperature oxidation and self-heating of a Victorian brown coal, and low temperature oxidation kinetics were estimated by a transient energy balance. They found the magnesium acetate, calcium chloride, and sodium chloride inhibited low temperature oxidation and copper acetate, potassium acetate, and sodium acetate promoted the reaction. The Oxidation Kinetics testing method is the current Chinese national standard for evaluating the propensity for spontaneous ignition. The results predicted by this method are consistent with the Adiabatic Oxidation method.22 In this research, the Basket testing method and Oxidation Kinetics testing method were used to identify the propensity for spontaneous ignition of upgraded lignite products. The purpose of this research is to provide practical guidance for the drying process of upgraded lignite. Three representative lignites were used in this study: Huolinhe lignite, Hailaer lignite, and Indonesia lignite. A method was developed to obtain the recommended water content for the upgraded lignite product.
Table 2. Lignite Type and Water Content for Each Experiment lignite type
water content (%)
upgraded Hailaer lignite 1 upgraded Hailaer lignite 2 upgraded Hailaer lignite 3 upgraded Hailaer lignite 4 raw Hailaer lignite upgraded Huolinhe lignite 1 upgraded Huolinhe lignite 2 upgraded Huolinhe lignite 3 upgraded Huolinhe lignite 4 raw Huolinhe lignite upgraded Indonesia lignite 1 upgraded Indonesia lignite 2 upgraded Indonesia lignite 3 upgraded Indonesia lignite 4 raw Indonesia lignite
0 7.09 14.07 21.26 28.31 0 7.69 15.31 23.19 30.84 0 15.90 30.48 47.25 62.95
2. EXPERIMENT 2.1. Sample Preparation. Table 1 lists the results of ultimate and proximate analyses of the raw lignite (the data are obtained by the China national standard GB/T 1574-2007). Lignite fines (smaller than 500 μm) were used to study the propensity for spontaneous ignition. For raw lignite, the maximum water content is approximately 65%. Therefore, to ensure that the results addressed a range of applications, Indonesia lignite (water content of 62.95%) was used in addition to the Huolinhe and Hailaer lignites. Coal samples of varying water content were obtained to study the effect of water content on spontaneous ignition. These samples were obtained by varying the drying time for each sample under a hot-air oven. Table 2 lists the lignite types and the water contents of each sample for each experiment. 2.2. Basket Testing Method. Figure 1 is a schematic diagram of the experimental system using the Basket testing method. The apparatus for the experiment was (1) a heating oven with a temperature range from room temperature to 300 °C; (2) a stainless steel wire mesh cube container with an open top surface, 50 mm sides, mesh opening of 0.05 mm; (3) K-type thermocouples of 1 mm diameterone placed in the center of the sample and another in the oven. To avoid the effects of air circulation, the container was installed in two cage holders made out of stainless steel mesh (0.6 mm grid size). The first cage (100 × 100 × 150 mm in size) was slightly larger than the sample container, and the second cage (150 × 150 × 250 mm in size) was slightly larger than the first cage. About 50 g sample with original particle size was used in each test. The procedures for this test were as follows: (1) a coal sample was charged in a preshaped basket and then placed in an oven set to a fixed temperature; (2) a thermocouple was inserted into the center of the coal sample to monitor the development of a runaway reaction; (3) if there was no sign of a runaway reaction, the experiment was repeated with a new sample using a higher oven temperature; (4) if spontaneous ignition occurred during the first run, the experiment was repeated using a lower oven temperature; (5) the minimum
Figure 1. Schematic diagram of the experimental system using the Basket testing method.
ambient temperature (the spontaneous ignition temperature) for spontaneous ignition was then determined for the samples; (6) a series of experiments were completed to determine the spontaneous ignition temperature (SIT). For the experiment, the coal sample was kept in a cube container, which was placed in a hot-air convection oven located in an environment with a constant temperature. At intervals of 5 °C, indications of self-heating and the potential for spontaneous ignition were measured. Measurements were taken over a 10-h period at each temperature interval. 2.3. Oxidation Kinetics Testing Method. Figure 2 is a schematic diagram of the experimental system using the Oxidation Kinetics testing method. The apparatus for the experiment was (1) a dry compressed air bottle, (2) a gas preheating copper tube, (3) a coal sample container, (4) a temperature-controlled oven, (5) a gas chromatograph, (6) a data acquisition unit. For the experiment, the valve pressure at the outlet of the dry air bottle was adjusted to 0.5 MPa. The gas preheating tube was made of pure copper (length of 50 m, internal diameter of 1 mm, external diameter of 2 mm). The temperature-controlled oven allowed for a temperature range from room temperature to 300 °C. The gas chromatograph provided gas collection and analysis. The temperature at the center of the coal sample was logged continuously using a data acquisition unit. The pure copper coal sample container had good 21
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The procedures for this test were as follows: (1) the oven was set to operate at a constant temperature of 40 °C; (2) dry air (oxygen concentration is 20.96%) was released through the experimental system at a flow rate of 96 mL/min. The flow rate of dry air was adjusted to 8 mL/min once the center of the coal sample reached a temperature of 35 °C; (3) the oven was set to heat at a rate of 0.8 °C/ min once the center of the coal sample reached a temperature of 40 °C. Both the coal temperature and ambient temperature were measured; (4) when the center of the coal sample reached a temperature of 70 °C, the oxygen concentration at the outlet of the sample container was measured; (5) the flow rate of dry air was adjusted to 96 mL/min. The oven continued to heat at a rate of 0.8 °C/min; (6) the test was completed once the center of the coal sample reached a temperature 5 °C higher than the ambient temperature. The fundamental parameters used by the Oxidation Kinetics testing method to identify the propensity of spontaneous ignition in coal were
Figure 2. Schematic diagram of the experimental system using the Oxidation Kinetics testing method.
thermal conductivity. K-type thermocouples were used at the center of the sample and at the top and bottom of both the gas inlet and outlet.
Figure 3. Sample center temperature curves for the upgraded Huolinhe lignite products with different water contents and for the raw Huolinhe lignite at two constant oven temperatures. 22
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Figure 4. Spontaneous ignition temperature of raw lignite and upgraded lignite products with different water contents. the crossing point temperature and the oxygen concentration value when the center of the coal sample reached a temperature of 70 °C. The comprehensive evaluation index is calculated with the following formula.22 Increasing the comprehensive evaluation index corresponds to decreasing the spontaneous ignition propensity.
constant oven temperatures, spontaneous ignition did not occur. Rather, the temperature at the center of the sample first reached a maximum temperature and then decreased slowly to equal the surrounding oven temperature. This may be caused by low temperature oxidation and heat accumulation. If the temperature at the center of the sample rises sharply at a specific point in time and is at least 60 °C higher than the oven temperature, then spontaneous ignition is assumed to have occurred. However, this paper assumes that spontaneous ignition does not occur if, after a prolonged period of time, the maximum temperature at the center of the sample remains close to the surrounding oven temperature. Similarly, this paper assumes that spontaneous ignition does not occur if the temperature at the center of the sample does not exceed the oven temperature by more than 60 °C. Comparison of the five charts (Figure 3) shows that the spontaneous ignition temperature curve is delayed by an increase in water content. Figure 4 shows the relationship between the spontaneous ignition temperature of raw lignite and upgraded lignite products with different water contents. The results show that for lignite samples with different water contents the spontaneous ignition temperature is almost identical (i.e., the water content does not affect the spontaneous ignition temperature for these three kinds of lignite). However, Fei et al.26 compared the spontaneous combustion of two Victorian brown coals and a Pakistan lignite that have been dried, mainly by the mechanical thermal expression technique in which aqueous slurries of the materials are heated (125 or 200 °C) and pressed (5−15 MPa). The basket testing method was used to characterize the propensity of raw coals and dried coal products to spontaneous ignition. It was found that the amount of residual moisture in the dried samples had the largest effect on the critical ignition temperature. For the Latrobe Valley coal, for which the diameter was less than 0.5 mm and water contents were from 57.1% to 19.3%, the critical ignition
I = 40 × (0.6IO2 + 0.4ICPT) − 300 Here
IO2 = 100 ×
CO2 − 15.5
ICPT = 100 ×
15.5 CPT − 140 140
where CO2 represents the oxygen concentration of the reactor outlet gas (%) when the center of the coal sample is 70 °C and CPT represents the crossing point temperature (°C).25
3. EFFECT OF WATER CONTENT ON SPONTANEOUS IGNITION 3.1. Results of the Basket Testing Method. The Basket testing method was used to conduct numerous experiments. A new sample was used for each test until the oven temperature was high enough to cause ignition of the sample. The oven temperature was increased by intervals of 5 °C. Figure 3 shows the sample center temperature curves for the upgraded Huolinhe lignite products with different water contents and for the raw Huolinhe lignite at two constant oven temperatures. The Hailaer lignite and Indonesia lignite sample center temperature curves are similar to those of the Huolinhe lignite. Furthermore, at higher constant oven temperatures, the temperature at the center of the sample increased sharply at a specific point in time and reached a maximum temperature much higher than the surrounding oven temperature. This maximum temperature also indicated the point at which spontaneous ignition occurred. At lower 23
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Figure 5. Spontaneous ignition time of raw lignite and upgraded lignite products with different water contents.
Figure 6. Comprehensive evaluation index (I) of raw lignite and upgraded lignite products with different water contents.
temperature was changing in the range from 170 to 160 °C. Opposed to the Latrobe Valley coal, the spontaneous ignition temperatures of the Pakistan Sonda coal with water contents in the range 6.2−14% are almost identical. The trends in the spontaneous ignition temperature can not only be explained in terms of the effects of water content of the samples but also in terms of the effects of other physical and chemical properties,
such as the inorganic matter24 and chemical structure.27−30 Although there are some trials to model the spontaneous combustion temperature, the assumption should be used due to the complexity of the kinetic processes.31 Figure 5 shows the relationship between spontaneous ignition time of raw lignite and upgraded lignite products with different water contents. The results show that the 24
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raw bituminous coals are between those of the completely dried lignite product and those of the raw lignite. Based on the values of the propensity difference of the bituminous coals (Figure 7), the averaged propensity value for the spontaneous ignition of bituminous coals is used as the benchmark value. Hence, the propensity for spontaneous ignition of the lignite product with the recommended water content is equal to the averaged value of the selected bituminous coals. Therefore, Table 4 provides the values of
spontaneous ignition time increased linearly with the increase of water content. The spontaneous ignition time increased because the water evaporation consumed a significant amount of heat and restricted the temperature. This indicates that the spontaneous ignition propensity decreased linearly with the increase of the water content of the coal sample. The spontaneous ignition time was defined as the period of time between the beginning of the process and the temperature at which spontaneous ignition occurred. 3.2. Results of Oxidation Kinetics Testing Method. Figure 6 shows the relationship between the comprehensive evaluation index (I) of raw lignite and upgraded lignite products with different water contents. For each of the three kinds of lignite the comprehensive evaluation index (I) increased linearly with the increase of water content. This indicates that the propensity for spontaneous ignition of upgraded lignite products decreased linearly with the increase of water content. The spontaneous ignition propensity found using the Oxidation Kinetics testing method is consistent with the results of the Basket testing method with the spontaneous ignition time as the evaluation index. 3.3. Recommended Water Content for Upgraded Lignite Product. The Oxidation Kinetics testing method is used to quantitatively evaluate the coal’s propensity for spontaneous ignition because the Basket testing method does not have a comprehensive index. Figure 7 shows the
Table 4. Recommended Water Contents for the Upgraded Lignite Products recommended water content (%) 6.53 4.28 7.83
the recommended water contents for the upgraded lignite products. For the Hailaer lignite, Huolinhe lignite, and Indonesia lignite, the recommended water contents are 4.28%, 6.53%, and 7.83%, respectively. These results are used to determine the target water content in the three upgraded lignite products. The method that was developed may be used to guide engineering design and operations when using highly susceptible upgraded lignite and enable much safer stockpiling.
4. CONCLUSION In this research, the Basket testing method and Oxidation Kinetics testing method were used to identify the propensity for spontaneous ignition for upgraded lignite products. The evaluation index for the Basket testing method was spontaneous ignition temperature and duration. The fundamental evaluation index (I) for the Oxidation Kinetics testing method evaluated the propensity for spontaneous ignition. For both testing methods, all three of the lignite samples (Huolinhe lignite, Hailaer lignite, and Indonesia lignite) showed that the propensity for spontaneous ignition decreased linearly with the increase in water content of the upgraded lignite product. Commonly used Chinese bituminous coals, which are not prone to spontaneous ignition, acted as a benchmark for determining the recommended water content for upgraded lignite products. For Huolinhe lignite, Hailaer lignite, and Indonesia lignite, the recommended water contents of the upgraded products were 6.53%, 4.28%, and 7.83%, respectively. The method that was developed to determine the recommended water content could be used to guide engineering design and operations when using upgraded lignite that is highly susceptible to spontaneous ignition.
Figure 7. Comprehensive evaluation index (I) for different coal types and water content.
comprehensive evaluation index (I) for different coal types and water content. To compare the propensity for spontaneous ignition of lignites and commonly used bituminous coals (which are less prone to spontaneous ignition), the Chinese coals Yankuang, Wennan, Shenhua 1, and Shenhua 2 were selected as references. Table 3 shows the proximate analyses of these coals. Figure 7 shows that the propensity values of the
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Table 3. Proximate Analyses of Bituminous Coals
AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-10-62785669. Fax: +86-10-62770209. E-mail:
[email protected].
received basis
dry basis
lignite type Huolinhe lignite Hailaer lignite Indonesia lignite
Notes
bituminous coal type
volatiles (%)
fixed carbon (%)
ash (%)
water (%)
Yankuang coal Wennan coal Shenhua 1 coal Shenhua 2 coal
35.51 42.48 34.55 24.11
55.67 51.25 54.09 55.52
8.82 6.27 11.36 20.37
4.78 4.35 6.48 7.12
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (No. 51076083). 25
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