Adsorption of Water Vapor from Ambient Atmosphere onto Coal Fines

Dec 10, 2015 - This work assumed that rapid spontaneous combustion of coal in stockpile will start from the heat up of coal fines on the surface of th...
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Adsorption of Water Vapor from Ambient Atmosphere onto Coal Fines Leading to Spontaneous Heating of Coal Stockpile Kouichi Miura* Institute of Advanced Energy, Kyoto University Gokasho, Uji, Kyoto 611-0011, Japan ABSTRACT: Low rank coals are susceptible to spontaneous combustion, because oxidation rate of low rank coals are higher than that of high rank coals. Even low rank coals, however, must be heated over a critical temperature before spontaneous combustion starts. This work assumed that rapid spontaneous combustion of coal in stockpile will start from the heat up of coal fines on the surface of the stockpile by the heat supplied from the sun and moist air. Then direct measurement of possible temperature increase by the adsorption of water vapor on coal from ambient atmosphere was undertaken by using three kinds of brown coals/lignites and a bituminous coal. When about 100 mg of each brown coal pre-dried at 80 °C and cooled to 28 °C were exposed to an ambient and stationary atmosphere of 28 °C with 67−77% relative humidity, the coal temperatures increased up to 40−43 °C in a minute or so. When the brown coals pre-dried and cooled to 38 °C were exposed to a stagnant saturated air at 38 °C, the coal temperatures increased up to over 60 °C in a minute or so. This was found to occur by rapid adsorption of water vapor from the ambient atmosphere onto the coal. Next, a simple model based on mass and enthalpy balances was successfully formulated to simulate the water adsorption and temperature change. The model simulation then clarified that the amount of heat generated by the adsorption of water vapor are much larger than the heat generated by initial coal oxidation and that most of heat generated is transferred to the surrounding to be utilized to heat up the surroundings. Thus, the important role of the adsorption of water vapor from the ambient atmosphere on the spontaneous heating of coal fines in stockpile was clarified.

1. INTRODUCTION Spontaneous heating or self-heating can be defined as the phenomenon of a temperature rise in a material under ambient conditions, where the heating results from some chemical and/ or physical process occurring within the material.1 The spontaneous heating may lead to combustion or an explosion if the temperature exceeds a critical temperature. In the coal industry the spontaneous heating has been one serious problem associated with mining, transportation, and storage especially for low-rank coals. Fires due to spontaneous heating of coal may occur in the high wall of surface mines, on trains, or in storage piles, and present a potentially fatal hazard in underground mines.2 Spontaneous heating, therefore, has been the subject of research for more than 100 years, and a huge number of works have been published. Since the full survey of the works is beyond the author’s ability, some literatures related to the spontaneous heating of coal stockpiles will be examined. In 1979, Kim2 smartly summarized the factors affecting spontaneous heating, indices of combustibility, and experimental methods with 45 pertinent references. Rank and change in moisture content are judged to be the most important factors affecting the spontaneous heating of coal. Temperature, air flow rate, particle size, and pyrite concentrations are also the factors to be considered. It was said that two processes, oxidation and wetting, contribute to spontaneous heating. The amount of energy released by oxidation and/or wetting of a particular coal determines the spontaneous heating hazard. In 1994, Carras and Young1 published an excellent review on models, application and test methods on spontaneous heating of coal and related materials in stockpile. First, they summarized the scientific basis of spontaneous heating of carbonaceous materials and the state of knowledge of numerical © 2015 American Chemical Society

modeling of the physical and chemical processes responsible for spontaneous heating. Second, they described the “tests” currently in use or potentially useful for industry for coal stockpiles. Figure 1 shows a schematic diagram of a coal

Figure 1. Schematic diagram of heat, oxygen, and moisture exchange around coal stockpile (adapted from Carras and Young1).

stockpile and illustrates some of the main features of coal stockpile spontaneous heating. They described that spontaneous heating of coal stockpile occurs when the rate of heat generation within the stockpile is greater than the rate at which heat can be transported to and dissipated in the external environment. Air transported into the stockpile provides Received: October 3, 2015 Revised: December 9, 2015 Published: December 10, 2015 219

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This mechanism is afforded by repeated observation that stockpiled coal most often tends to heat up when exposed to rain after a period of dry, sunny weather. In 1960, Stott7 examined the influence of moisture on the spontaneous heating of a crushed sub-bituminous coal using 3 kg of sample dried in vacuum of 0.05 mmHg at 105 °C. The coal was contained in an aluminum foil cylinder, 12 cm i.d. and 24 cm high, with an open top. Either a saturated air, pure oxygen, or a saturated oxygen was passed through the coal layer at 1 L/min of flow rate. Pure oxygen saturated with moisture at 22 °C caused ignition of coal layer of the same temperature in 30 min and saturated air at 18 °C caused ignition of coal layer of the same temperature in 12 h, whereas pure oxygen took 12 h to ignite the coal layers. Stott explained these phenomena by saying, “Transfer of heat by evaporation of moisture can be over-riding importance in spontaneous heating although this factor has previously been neglected.” Nordon et al.8 pointed out that the heat liberated (or absorbed) by absorption or desorption of moisture is equal to or greater than the latent heat of change of phase of water vapor and is appreciable. In the second paper, Nordon et al.9 measured the rates of adsorption and desorption of water vapor on a Yallourn-Briquette char. Surprisingly as large as 150 L/min of air was supplied for 4 h to 15 g of the char layer to measure the adsorption/desorption rates. In a practical situation it is unlikely that such a small amount of coal is exposed to a huge amount of air, but it was necessary for coal to adsorb water vapor by several % in an hour or so. Then Nordon decided to assess the effect of sorption of water vapor using model simulation, and a model including the sorption of water vapor was formulated for the self-heating reaction of coal and char.10 Actual calculation, however, excluded the effect because of difficulty in differential equations and a prohibitive computing effort required. Based on these examinations Nordon and Bainbridge,11 in 1983, concluded that adsorption from the vapor phase is complicated by transport limitation (diffusion, convection) and the close coupling which occurs between mass and heat transfer, resulting, in practice, in extremely long times for completion of the process. Ren and the University of Nottingham group12 standardized the adiabatic oxidation technique to provide an opportunity to compare the oxidation potential of different coals. The standard test procedure is as follows: 150 g of coal sample is charged into reaction vessel and a 100 mL/min of oxygen-free nitrogen is passed through the sample for at least 24 h to stabilize the coal at 40 °C. Once the system attains the desired test condition, the nitrogen flow was cut off and a 200 mL/min of saturated air is allowed to pass through the coal sample. Based on a series of tests, they summarized the effect of humidity of air flow as follows: The rate of heat generation in the coal is finely balanced with the rate of heat loss. The humidity of the air is an important factor in deciding whether a heating will progress rapidly or not. When coal fines in stockpiles contacted the moist air, the coal fines will adsorb water and release heat, which may provide the initial energy for a “spot” heating. Allardice et al.13 stated that moisture can be a contributing factor in the spontaneous combustion of briquettes (and dried brown coal) in storage. Heat is generated when moisture is readsorbed onto briquettes. This can occur if there is an increase in humidity or even light rain, if storage under low humidity conditions has partially dried the briquettes. The resulting increase in briquette temperature can accelerate oxidation by air to the point where spontaneous combustion occurs.

oxygen for the oxidation of coal. The heat liberated is transported into and away from the stockpile. Water vapor is transported either into the stockpile or away from the stockpile depending on the relative humidity within the coal. Outdoor stockpiles are also affected by the weather through wind, rain, and solar radiation. To develop a general quantitative model for spontaneous heating of stockpile, quantitative description of each of the heat generation and dissipation processes is essential. As summary they concluded that no single property can be employed to assess spontaneous heating behavior. The conditions under which that property is measured are normally too conf ining and fail to match the real world. Nonetheless, measuring oxygen sorption of a series of coals, at simulated and typical stockpile atmospheric conditions does provide a most useful and relative indicator of self-heating propensity. In 2005, Nelson and Chen3 published a comprehensive review of 53 pages with pertinent 200 literatures, which examined experimental works covering the period of 1996− 2005. They summarized the essence of the self-heating and spontaneous combustion in stockpiles very clearly: Stock piles are safe in the two extremities of sufficiently low air circulation and sufficiently high air circulation. In the former, the oxidation rate is limited by the supply of oxygen and only a minor amount of self-heating occursclearly, if there is no air-flow through the stockpile, then once the oxygen within the pile is consumed, the oxidation rate will be zero. In the latter case, heat is removed quicker than it is generated, and the temperature of the stockpile approaches the air temperaturethis is called ventilated pile. Between these two extremes, there are two scenarios. In the first case, self-heating inevitably leads to spontaneous combustion. In the second case, there are two possibilities: either limited selfheating or spontaneous combustion occurs. Which possibility happens depends upon the initial conditions of the problem. The initial conditions include many factors which can be divided into two main types: properties of coal (intrinsic factors) and the environment/storage conditions (extrinsic factors). This summary is, for example, well supported by model calculations performed by Schmal et al.4 They estimated the temperature rise in a 10 m long coal stockpile by using a simple model where dried coal was contacted with dry air flow. This is the situation where largest temperature rise is expected. At high air velocity of v = 10−4−10−3 m s−1, where enough oxygen is supplied for the oxidation, it took “only” 0.5 year to reach a maximum temperature of 30 °C higher than the initial temperature of 20 °C, while at v = 0 it took 2 years. Oxygen−coal interactions are responsible for the self-heating and spontaneous combustion in stockpiles as summarized above. The processes are in general very slow and will take from several days to several months to cause significant selfheating or spontaneous combustion. On the other hand, it has been well recognized that humidity of air will affect the selfheating of coal in stockpiles and the event will take place much faster than the oxygen-coal interaction as many researchers have expressed in various ways. In 1929, Miyagawa et al.5 reported the adiabatic temperature increasing curves starting from 100 °C for 75 coals. The record of discussion at the end of the report states that coals in storage show high propensity to spontaneous combustion especially when the coals are faced to fine weather after rain. Berkowitz and Schein,6 in 1951, reported that a more general trigger of sustained, self-accelerating coal oxidation has been seen in heat releases that accompany wetting of partly dry coal. 220

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Energy & Fuels Smith and Glasser14 designed an elaborate batch reaction system to measure oxidation rate of coal at ambient temperature in which 10 g of coal was charged in a beaker. Its volume was not given but may be less than 200 mL, judging from Figure 1 in their publication. They also tried to measure the rates of adsorption of moisture from the atmosphere using the reaction system and concluded that the rate of adsorption or desorption of moisture is not as easily measured as the rate of oxygen, since the moisture content of saturated air at 23 °C is only 3.8%. Because of this low concentration, the sorption rates are very low. The chemisorption of oxygen evolves an order of magnitude more heat than the physical adsorption of water. It is therefore likely that adsorption of water is more important in catalyzing the oxidation reaction. They, on the other hand, mentioned, Although “heat generation” by adsorption of water vapor may be neglected at ambient temperatures, it is nevertheless important to consider sorption of water as a risk factor in spontaneous heating. This is because wetting, as might occur in a stockpile during a rainstorm, involves water at much higher concentrations than were studied here. A dry, unreactive stockpile could be moistened in a very short time, and rendered more reactive towards oxygen. Nelson and Chen in the review mentioned above3 stated, without specific citations, The heat released by water adsorption is particularly significant for dry coals. Practical experience has revealed that in stockpiles, hot spots occur frequently after rain. For very dry coals, the problems are very serious. Moisture from saturated air condenses not only onto the external surface of the coal but throughout its internal pore structure. This can rapidly release a tremendous amount of heat: very dry coals can ignite by water sorption. As introduced above, the effect of moisture in air on the selfheating of coal stockpiles is not completely understood. Qualitatively most of the reports state that the moisture in air will affect the self-heating of coal in stockpiles by stressing the role of rain. However, no reports definitely state how fast and how big the effect of moisture in air are. This is probably because the results reported are normally too confining and fail to match the real world as Carras and Young1 pointed out. Actual self-heating by water adsorption takes place in air atmosphere where almost no convectional flow of air exists. The experiments performed by Smith and Glasser, for example, well simulated the coal inside the stockpile. Then their conclusion was, “The sorption rates are very low. The chemisorption of oxygen evolves an order of magnitude more heat than the physical adsorption of water.” as stated above. The amount of water vapor in the air contacting with coal inside the stockpile is too small for self- heating to proceed under the conditions of no convectional flow of air. Nordon et al., recognizing this fact, supplied as large as 150 L/min of air for 4 h to 15 g of the char layer to measure the adsorption/ desorption rates as stated above. The experimental conditions unfortunately failed to match the real world of coal stockpile. These discussions suggest that the conclusions deduced from experiments in flowing air/oxygen will not be directly applied to actual spontaneous heating of stockpiles. Focusing on the role of moisture in air, it is necessary to examine how the moisture will contribute to spontaneous heating of coal in stockpile. One of biggest questions is if coal in the stockpile can adsorb water vapor from the ambient atmosphere at appreciable rate. Fine coal particles at the surface of stockpile

(Figure 1), if they exist, will surely have highest possibility to adsorb water vapor from the ambient atmosphere, because they are exposed to large volume of ambient atmosphere and their moisture adsorption rate will be much faster than that of large coal particles. Our research group has been investigating the coal oxygen interaction at temperatures over 60 °C.15,16 The author has measured the oxidation rate of 10 mg coal fines in a dry air stream at 60−80 °C using a sensitive thermobalance. After the experiment the coal sample was cooled down to room temperature in the stream of dry air and exposed to ambient atmosphere by lowering the furnace. Then the author by chance found that the weight of coal increased very rapidly by 10% or so in 5 min. The weight increase was judged to come from the rapid adsorption of water vapor from the ambient atmosphere, because the coal particles have been already oxidized at 60−80 °C and the rate of oxygen adsorption at the room temperature is very small. Since the heat of adsorption of water vapor is around 44 kJ/mol-H2O, 10% of water adsorption in 5 min were large enough to generate heat to heat up the coal particles and the surrounding atmosphere in appreciable time duration. This experience stimulated the author to examine the water vapor adsorption phenomenon in more detail and quantitatively. Then the purposes of this work are to measure the adsorption rate of water vapor and temperature change of coal directly under naturally occurring conditions during coal storage and transportation by extending a preliminary work,17 to formulate a simulation model that express the moisture adsorption phenomenon, and to examine the role of moisture adsorption on spontaneous heating based on the experiment and simulation.

2. EXPERIMENTAL SECTION 2.1. Coal Samples Used. Table 1 lists the names, abbreviations, and the properties of four coals used. LY is a Victorian brown coal

Table 1. Analyses of Coals Used ultimate analysis (wt%, dafa) coal Loy Yang (LY) Pendopo (PD) Powder River Basin (PRB) Alabama (ABM) a

proximate analysis (wt%, dbb)

C

H

N

O+S (diff)

VM

FC

ash

66.7 68.5 73.2

4.7 5.0 5.0

0.9 1.0 1.0

27.8 25.5 20.7

51.5 53.8 38.9

47.0 37.5 58.0

1.5 8.7 3.1

81.4

5.7

1.6

11.3

40.4

55.9

3.8

b

Dry, ash-free. Dry basis.

from Australia, PD is a brown coal from Indonesia, PRB is a Powder River Basin coal from USA, and ABM is a subbituminous coal from USA. Since main concern of this work is the behavior of low rank coals, the adsorption isotherms of water vapor at 30 °C were measured for LY, PD, and PRB using a volumetric adsorption apparatus (BELSORP-max, BEL Japan, Inc.), and they are shown in Figure 2. The coals were dried overnight in vacuo at 80 °C before the isotherm measurement. Each isotherm was approximated by four linear relationships drawn on the data for the simulation purpose as described below. The adsorption isotherms of PD and PRB were so close to each other, although the elemental compositions of the two coals were substantially different. Coal particles less than 150 μm in diameter were used for most of experiments. One of important variables is the particle size. Small lump coals that could be obtained only for PRB were used for comparison purpose. 221

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effect can be estimated by comparing the water vapor adsorption experiments performed in stationary air atmosphere and in a stationary inert gas atmosphere. It is, however, very difficult to measure the weight change of the dried coal on its exposure to the stationary inert atmosphere containing water vapor. As a compromise means the water vapor adsorption experiments were performed using the thermobalance in flowing gas streams containing a fixed amount of water vapor. About 13 mg of LY coal particles dried at 80 °C by following the procedure stated above but in a 100 mL/min of He stream were cooled to around 33 °C in the He stream. Then the He stream was changed to either a simulated air (a mixed gas consisting of 22% O2 and 78% He) stream containing water vapor by 2.3 vol% (= saturated water vapor at 20 °C, RH = 41% at 33 °C) or a He stream containing the same amount of water vapor to measure the adsorption rate of water vapor. The adsorption rate of oxygen was also measured by exposing the dried coal to the simulated air flow at the same temperature. 2.3. Measurement of Temperature Change of Coal through the Adsorption of Water Vapor from Stationary Atmosphere. Figure 3 schematically shows the setup used to measure the temperature change of coal particles on exposure to ambient atmosphere. Two series of experiments were performed, as described below: Experiment I: Expose the coals pre-dried and at room temperature (∼28 °C) to the ambient and stationary atmosphere of 67−77% relative humidity (RH) at room temperature. Experiment II: Expose the coals pre-dried and at an initial temperature (∼28, ∼38, ∼42, or ∼45 °C) to the stationary atmosphere of 100% H (saturated air) at a fixed temperature (∼35, ∼38, ∼42, or ∼45 °C). Experiment I was performed under the conditions corresponding to the steam adsorption experiment. Experiment II was performed by simulating the situations where dried coals at room temperature or at a slightly high temperature are exposed to saturated air at a same or a higher temperature. This simulates the situation where coals dried by facing to fine weather after rain are exposed to the atmosphere still in high humidity due to the rain. For all experiments except using the lump coal for PRB, about 100 mg of coal particles were wrapped by the stainless steel mesh used as the basket in the steam adsorption experiment. A K-type thermocouple of 0.5 mm diameter was inserted among the coal particles and the mesh wire with the coal particles was fixed to the thermocouple as shown in Figure 4. The coal particles were dried in a 100 mL/min of N2 stream under the same heating profile as for the steam adsorption experiments. For Experiment I, the wire mesh with the coal particles was just taken out from the drier to be exposed to the ambient and stationary atmosphere at room temperature when the temperature of the coal particles was decreased to the room

Figure 2. Adsorption isotherms of water vapor on LY, PD, and PRB coals measured at 30 °C (points) and approximated linear relationships for simulation purposes. 2.2. Measurement of Adsorption Rate of Water Vapor. To simulate how dried coals adsorb water when they are exposed to ambient and stationary atmosphere, the adsorption rate of water vapor was measured by the following procedure. Either about 10 or 100 mg of coal particles placed in a mesh basket (400 mesh opening) made of a stainless steel were heated using a thermobalance (TG50H, Shimadzu) in a N2 atmosphere at the rate of 10 °C/min up to 80 °C at which they were kept for 15 min. Then the basket with the coal particles was cooled to room temperature in the N2 atmosphere. When it was confirmed that the coal particles reached the room temperature, the electric furnace was lowered to expose the coal particles to the ambient atmosphere. Unfortunately the thermobalance controlling system is automatically changed from “measuring mode” to “idling mode” as soon as the switch to lower the electric furnace is turned on. The weight and temperature in the “idling mode” are shown only on CRT. Then the temperature change after exposing to the ambient atmosphere was estimated by taking photos of CRT and reading temperatures from the photos. One experiment using about 100 mg of coal lump was performed for PRB in a similar way. Around ambient temperature, the oxidation rate of coal is very small as compared to the adsorption rate of water vapor as discussed in the Introduction. However, even the small rate of coal oxidation may affect the rate of the adsorption of water vapor. Then it will be necessary to estimate the effect of the oxidation on the water vapor adsorption. The

Figure 3. Schematic of experimental setup used for the measurement of temperature change of coal on exposure to stationary air atmosphere. 222

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mainly from the difference in the mass transfer rates surrounding the aggregation of coal fine particles. The results of supplemental experiments performed to estimate the effect of oxidation on the water vapor adsorption for LY coal particles are shown in Figure 5. The weight of dried

Figure 4. Adsorption rate of water vapor on dried coals on exposure to ambient atmosphere. temperature. For Experiment II, the wire mesh with the coal particles was taken out from the drier to be exposed to the saturated air atmosphere prepared in a 10 L volume of desiccator when the temperature of the coal particles was decreased to a predetermined temperature. The change of temperature during the experiment was recorded on a data logger. About 200 and 400 mg of lump PRB coals were subjected to Experiment II exposing the coal dried and cooled to 38 °C to a stagnant saturated air at 38 °C in a similar way as for the coal particles. The thermocouple was embedded in the coal lump through a drilled hole to measure the temperature at the center of the lump.

Figure 5. Weight-change profiles of dried LY coal particles on exposure to a simulated air stream containing water vapor, a He stream containing water vapor, and a simulated air stream.

coal is shown in the bracket for each experiment. The relative weight-change profiles are very close in the simulated air stream containing water vapor and in the He stream containing water vapor. The relative weights rapidly increased and reached a same value of 1.12 in 10 min or so. On the other hand, the relative weight increased very slowly in the simulated air stream and it was as small as 1.003, even at 30 min. These results clearly indicate that the rapid weight increase in the simulated air stream containing water vapor is almost totally due to the adsorption of water vapor and that the effect of oxidation on the water vapor adsorption is very small at ambient temperature. The supplement experiments support that Figure 4 really shows the adsorption rate of water vapor from stagnant ambient atmosphere. It should be stressed again here that the coal temperatures in the supplemental experiments in the gas stream containing water vapor are kept almost constant because the heat generated by the water adsorption is rapidly dissipated to the surrounding by the gas stream flowing at high velocity. 3.2. Temperature Change of Coal through the Adsorption of Water Vapor. Next concern is how temperature of coal will change on such adsorption of water vapor as shown in Figure 4. Experiment I is the temperature measurement experiment corresponding to the larger coal weight in Figure 4. The results of Experiment I for the four coals are shown in Figure 6. In the bracket of each tag the weight of dried coal and the temperature of coal just before the exposure to the ambient atmosphere, the initial temperature, are shown. Since the weight of dried coal cannot be measured, the weight was calculated by using the value measured in the steam adsorption experiment. The actual initial temperature is shown because the initial temperature could not be exactly set to be 28 °C and because the initial temperature affected the temperature-change profile. For LY coal, three duplicate runs were performed under similar coal weights. The temperaturechange profiles were slightly different among the three runs. This is mainly because the humidity of ambient atmosphere

3. RESULTS AND DISCUSSION 3.1. Steam Adsorption Rate. Figure 4 shows the amounts of water vapor adsorbed, q, against time, t, for the four coals measured at two different coal weight levels. The weight value shown in the bracket for each experiment is the weight of dried coal. The results for the PRB lump of 116.7 mg are also shown. The experimental q values are all shown as the keys because all of the values were read in certain time intervals from the photos as stated above. For the smaller coal weight, the adsorption rates of water vapor were very rapid and the q values reached as large as 0.12 kg/kg-coal in 10 min for LY, PD, and PRB. This means that 293 kJ/kg-coal (= 0.12 kg/kg-coal/(0.018 kg-water/ mol) × (44 kJ/mol)) of heat was generated in 10 min for these coals if we assume that the heat of adsorption of water vapor is equal to the heat of condensation of water (∼44 kJ/mol). For ABM coal, the q value at 10 min is over 0.02 kg/kg-coal which will generate 49 kJ/kg-coal of heat. These simple calculations show that huge amount of heat is generated in short period by just exposing the dried coals to ambient atmosphere. The heat will surely be utilized to heat up the coal particles themselves and surroundings. For the larger coal weight, the adsorption rates of water vapor were smaller than those of the smaller coal samples, but the q values reached as large as 0.08−0.1 kg/kg-coal for LY, PD, and PRB and 0.02 kg/kg-coal for ABM in 10 min. These q values are also large enough to generate large amounts of heat in short period. For the PRB lump coal of 116.7 mg, the adsorption rate is much smaller than that of 98.0 mg of PRB fine particles. This difference is judged to come from the large intraparticle mass transfer resistance of the lump coal. The differences in the adsorption rates between the larger and the smaller coal weights of fine coal particles are judged to come 223

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showed almost same temperature-change profiles, showing good reproducibility of Experiment II. This was realized by controlling the temperature of saturated air accurately. The temperatures of all coal particles increased rapidly as soon as the coals were exposed to the saturated air, and they reached a peak of as high as 60.0−61.5 °C for LY, PD, and PRB and 46.5 °C for ABM in 1−1.5 min. The temperatures decreased very gradually to the saturated air temperature in more than 30 min. The maximum temperature increases of 22.0−23.5 °C for LY, PD, and PRB were larger than those observed in Experiment I. This is judged to be because the vapor pressure of the saturated air was much higher than that of the ambient atmosphere in Experiment I. The temperature-change profiles of the PRB coal lumps were significantly different from the temperature-change profile of the PRB coal fine particles. The temperatures increased gradually to reach a peak of 46.5 °C in 3 min for 254 mg of lump and a peak of 46.8 °C in 6 min for 413 mg of lump. The temperatures were kept at the temperatures close to the peak temperature levels for several minutes and decreased very gradually to stay at 42.6 °C for 254 mg of lump and 44.0 °C for 413 mg of lump even at 30 min. The temperature-change profiles shown in Figures 6 and 7 are judged to be well related to the water vapor adsorption behaviors. The temperature increased rapidly with rapid adsorption of water vapor for aggregate of coal particles and reached a maximum where the heat generation rate by water vapor adsorption and the heat dissipation rate are balanced, then the temperature decreases gradually because the heat dissipation rate exceeds the heat generation rate. For the PRB lump coal, the temperature increased gradually to reach rather plateau and lower peak temperature and then the temperature decreased very slowly. This is because the heat generation rate is small due to the slow water vapor adsorption rate and because the heat dissipation rate is also small due to low coal temperature reached. 3.3. Formulation of Differential Equations for Simulating Temperature Change of Coal Particles through the Adsorption of Water. The temperature increase of coal particles accompanying the water vapor adsorption was found to be very large for the coal particles of low rank coals, LY, PD, and PRB. Then it was intended to simulate the temperature change and water vapor adsorption behavior. To do so, about 100 mg of coal particles of less than 150 μm in diameter wrapped by the stainless steel mesh were approximated by a single spherical porous particle of radius R as shown in Figure 8. The spherical particle was assumed to be placed in an infinite

Figure 6. Temperature changes of dried coals on exposure to ambient atmosphere.

changes day to day, although the initial temperature and the coal weights were rather close to each other. The temperatures of all coal samples increased rapidly as soon as the coals were exposed to the ambient atmosphere, and they reached a peak of as high as 40.5−43.5 °C for LY, PD, and PRB and 33.5 °C for ABM in 1−1.5 min. The temperatures then decreased very gradually to return to the room temperature in more than 30 min. The maximum temperature increases were 12−15 °C for LY, PD, and PRB. The rapid initial temperature increases are well associated with the rapid water vapor adsorption shown in Figure 4. Experiment II was performed by simulating the situations where dried coals at an initial temperature of room temperature or a slightly higher temperature are exposed to a saturated air at the same or at a higher temperature. Of several initial coal temperature and saturated air temperature combinations, the results of the experiments with ∼38 °C initial coal temperature and 38 °C saturated air temperature are shown in Figure 7 for the four coals using fine particles and PRB lump coals. The exact initial coal temperature and the weight of dried coal are given for each experiment. Duplicate experimental runs for LY

Figure 8. Assumed configuration of coal fine particles and SUS mesh and distributions of temperature T, pressure of water vapor p, and amount of water vapor adsorbed q within and outside of the coal particles wrapped by the wire mesh.

Figure 7. Temperature changes of dried coals on exposure to saturated air at 38 °C. 224

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Energy & Fuels volume of stationary air of TEnv of temperature and pEnv of water vapor pressure. The temperature T, the water vapor pressure p, and the amount of water vapor adsorbed q per unit weight of dried coal were assumed to be uniform throughout the spherical particle, and q and p were assumed to be in equilibrium. The mass balance of water vapor around the spherical particle is given by ⎛ ε ⎞ WCoal d⎜⎜q + C ⎟⎟ /dt = kcA(C Env − C) ρb ⎠ ⎝

HH2O(l)

+ HH2O(l)(T ) =

HAir =

= hA(TEnv − T ) + kcA(C Env − C)HH 2O(g)(TEnv )

ln(ps /Pa) = 23.1964 −

(2)

C Env =

R gTEnv

,

CAir =

kc(2R )/DH2O − Dry Air ≅ 2

pT R gT

h(2R )/λAir ≅ 2

(3)

⎛ ∂q dp ⎞ ⎛ ∂q ε ⎞⎟ dp ε ⎟ dT s ⎜ ⎜⎜ + + − ⎜ ∂p dT ρb R gT ⎟⎠ dt ρb R gT 2 ⎟⎠ dt ⎝ ∂p ⎝ s kcA p⎞ 3 ⎛ pEnv − ⎟ ⎜ WCoalR g Rρb ⎝ TEnv T⎠

3816.44 T /K − 46.13

(8)

(9) (10)

where DH2O−Dry Air is the molecular diffusivity of water vapor− dry air system, and λAir is the thermal conductivity of humid air. They both can be estimated using chemical engineers’ handbooks to be D H2O−Dry Air/(m2 s−1) = 1.8 × 10−7 × T/K − 2.7 × 10−5 and λAir/(W m−1 K−1) = 9.0 × 10−5 × T/K in the range of T = 273−373 K. Now all physical properties and rate coefficients in eqs 4 and 5 are given as the functions of p and T. The simultaneous differential equations, eqs 4 and 5, can then be solved for given experimental conditions and initial conditions. 3.4. Comparison of Experimental Temperature Change of Coal Particles through the Adsorption of Water with the Simulation Calculation. Simulation calculations were performed with the physical properties and the rate parameters estimated above and ε = 0.60 and ρ b = 400 kg/m3 for LY and PD and ε = 0.50 and ρb = 500 kg/m3 for

where Rg is the gas constant and pT is the total pressure. The q value is related to the relative vapor pressure of water p/ps, and the saturated vapor pressure of water ps is a function of only temperature T. The thickness of the stainless steel mesh was assumed to be negligibly small as compared with R. Taking these relationships into consideration, eqs 1 and 2 are rewritten as simultaneous differential equations representing the changes of p and T as functions of t as follows:

=

(7)

The q vs p/ps equilibrium relationships were measured for LY, PD, and PRB coals as shown in Figure 2. To use the adsorption isotherms in eqs 4 and 5, their derivatives with p and ps are needed. To facilitate the derivative calculations, the adsorption isotherms were approximated by four linear relationships as shown in Figure 2. Remaining properties to be estimated are the mass transfer coefficient kc and the heat transfer coefficient h. They are estimated from the following relationships for the spherical particle in infinite volume of stagnant atmosphere:18

where WMesh is the weight of the stainless steel mesh surrounding the spherical particle, CAir is the total concentration of humid air in the spherical particle, hCoal and hMesh are respectively the enthalpies of the dried coal and the stainless steel mesh per unit weight, HH2O(l) and HH2O(g) are respectively the enthalpies of liquid water and water vapor, and h is the heat transfer coefficient in the gas film around the spherical particle. By assuming the ideal gas law for all gas species, the concentrations C, CEnv, and CAir are related to the pressures p, pEnv, and pAir and temperatures T and TEnv as follows: pEnv

p −p p HH2O(g) + T HDry Air pT pT

where WCoal, WMesh, R, ε, and ρb are the experimental variables, and cp,Coal, cp,Mesh, cp,H2O(l), Cp,Air, Cp,H2O(g), and Cp,Dry Air, HH2O(g), HDry Air, and ps are all given as functions of only T from physical chemistry handbooks. The saturated vapor pressure ps/Pa is, for example, given against T/K by the following the Antoine equation:

⎞ ε CAirHAir(T )⎟⎟ /dt ρb ⎠

p , C= R gT

⎞ kc ⎛ pEnv p⎞ 3 ⎛⎜ ⎟ ( ) ( ) − + − h T T H T ⎜ ⎟ Env H O(g ) Env ⎟ Rρb ⎜⎝ R g ⎝ TEnv T⎠ 2 ⎠

where cp,Coal, cp,Mesh, and cp,H2O(l) are respectively the heat capacities of dried coal, wire mesh, and liquid water. The molar heat capacity of the humid air, Cp,Air, and the molar enthalpy of the humid air, HAir, are respectively represented by the following equations: p −p p Cp,Air = Cp,H2O(g) + T Cp,Dry Air pT pT (6)

(1)

⎛ W d⎜⎜hCoal + Mesh hMesh + qHH2O(l)(T ) WCoal ⎝ +

εpT ⎛ ∂q dps H (T ) ⎞ ⎤ d T + ⎜Cp,Air − Air ⎟⎥ ∂ps dT ρ b R gT ⎝ T ⎠⎥⎦ dt

(5)

where WCoal is the weight of dried coal, C and CEnv are respectively the concentrations of water vapor in the spherical particle and in the ambient atmosphere, ε is the void fraction of the spherical particle including the pore volumes of coal, ρ b is the apparent density of the spherical particle, t is the time, kc is the mass transfer coefficient of water vapor in the gas film around the spherical particle, and A is the outer surface area of the spherical particle. By assuming that adsorbed water is normal liquid water, the enthalpy balance around the spherical particle can be written as WCoal

⎡ ∂q dp W + ⎢c p,Coal + Mesh c p,Mesh + qc p,H2O(l) ∂p dt WCoal ⎢⎣

(4) 225

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relationships, because only T vs t relationships could be measured. The calculated T vs t relationships well fitted the experimental T vs t relationships, showing the validity of the simulation strategy. The temperature measurement experiments were all performed by wrapping the coal particles with the stainless steel mesh to hold the coal particles. To simulate the coal particles at the surface of coal stockpile, for example, we want to know the temperature change of the coal particles without the stainless steel mesh. Then the calculations by just setting WMesh = 0 in eq 5 were made under all of the conditions, and the temperature-change profiles obtained are shown by the blue lines in Figure 9. The peak temperatures of the blue lines are higher by 2−10 °C than those of red lines, suggesting that the coal particles in coal stockpile may be heated to higher temperatures than those observed in this work. Similar comparisons between experimental data and calculated values were made for PD and PRB as shown respectively in Figures 10 and 11. Temperature measurement

PRB with other experimental conditions. One uncertain experimental variable was the weight of stainless steel mesh WMesh which must be the weight that will be heated or cooled with the coal particles. The weight of the stainless steel mesh used were 360 mg in total but only a part of the mesh intimately wraps the coal particles. Then it was arbitrarily assumed that one-third of the mesh will be heated or cooled with the coal particles, and hence WMesh was set to be 120 mg for the simulation. Now the changes in the temperature of coal T, p, and q could be calculated for the given initial conditions and the weight of dried coal WCoal. To fit the experimental data of T and q with the experimental data, however, the product of heat transfer coefficient and the outer surface area, which was originally equated to be hA = h × 4πR2, must be multiplied by ∼3. This may be because the sphere approximation of the aggregate of coal particles is not valid for the heat transfer. The SUS mesh and the irregularity of the outer surface of aggregated coal fines increase the effective surface area for the heat transfer over the geometric outer surface area of the single particle, A. The heat transfer coefficient h may also be affected slightly by the SUS mesh and the irregularity of the outer surface of aggregated coal fines. Due to these effects the heat transfer rate might be much faster than that calculated by assuming smooth outer surface of the spherical particle. Then all the simulations were performed by setting hA = 3h × 4πR2. This is the only modification of the estimated properties. Black lines in Figure 9 show the temperature-change profiles obtained experimentally under four different conditions for LY.

Figure 10. Comparison of experimental and calculated temperature changes under different exposure conditions for PD coal. In parentheses are shown (initial coal temp → temp and RH of exposed atmosphere).

experiments were made under three conditions. Under all of the conditions the experimental and calculated T vs t relationships showed fairly good agreements. The experimental and calculated q vs t relationships corresponding to Experiment I, (28 °C → 28 °C, RH ≈ 75%), also showed fairly good agreements for the both coals as shown as the red broken and dotted lines in Figure 4. The peak temperatures of the blue lines are higher by 2−5 °C than those of red lines for these coals under the conditions examined. 3.5. What Can We Learn from the Simulation? It was now found that the presented calculation method can simulate the changes of temperature and amount of water vapor adsorbed fairly well. Then we can now estimate the experimentally unmeasurable properties using the simulation calculation. First concern is how large is the heat generated by the adsorption of water vapor and how the generated heat is utilized. Figure 12 shows the calculated changes of temperature T (a), pressure of water vapor p and amount of adsorbed water q (b), and the rates and accumulated amounts of heat generation and heat dissipation (c) for LY under three

Figure 9. Comparison of experimental and calculated temperature changes under different exposure conditions for LY coal. In parentheses are shown (initial coal temp → temp and RH of exposed atmosphere).

The initial coal temperature and the temperature and relative humidity of the atmosphere exposed are shown for the four conditions in parentheses as (initial coal temp → temp, RH of atmosphere). Duplicate experimental runs are presented for two conditions as already shown in Figures 6 and 7. The calculated temperature-change profiles are shown by red lines. The simulated q vs t relationship corresponding to Experiment I, (28 °C→ 28 °C, RH ≈ 75%), is shown as the red solid line in Figure 4. For this condition, the experimental and calculated relationships of both T vs t and q vs t well coincided. For the other conditions, comparisons were made for only T vs t 226

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Figure 11. Comparison of experimental and calculated temperature changes under different exposure conditions for PRB coal. In parentheses are shown (initial coal temp → temp and RH of exposed atmosphere).

conditions without stainless steel mesh. The temperatures increase rapidly because of the rapid adsorption of water vapor. The adsorption rate of water vapor is mainly controlled by the driving force of mass transfer, pEnv − p. Saturated air at high temperatures gives large pEnv, which will give larger masstransfer rates. The enthalpyin other words, heat transferred to the coal particles from the water vaporof TEnv and pEnv is determined by the mass-transfer rate and the values of TEnv and pEnv. High TEnv and pEnv increase the enthalpy transferred from both mass transfer rate and the enthalpy level. The rate of heat dissipation increases with the increase of coal temperature because the driving force of heat dissipation, T − TEnv, increases with the increase of the coal temperature. The coal temperature reaches a peak temperature in 1−2 min when the rates of heat generation and heat dissipation are balanced. Then the coal temperature decreases gradually because the rate of heat dissipation exceeds the rate of heat generation as clearly shown in Figure 12a,c. The difference of the accumulated amount of heat generation and the amount of heat dissipation gives the heat used to heat up the coal temperature. The ratios of the heat used to heat up the coal to the accumulated amount of heat generated at the peak temperatures are 26% for the condition (A), 40% for the condition (B), and 49% for the condition (C). The ratios decrease with time and finally reach null when the coal temperatures are decreased to the ambient temperature. This result means that most of heat generated by the adsorption of water vapor are transferred to the surrounding. Many researchers have endeavored to measure the heat generated when coal is exposed to air at ambient temperature for estimating the temperature increase caused by the adsorption of oxygen and/or the coal oxidation. Miyakoshi et al.19 measured the amount of heat generated during initial 16 min exposure of coal to pure oxygen for 23 coals at 30, 40, and 45 °C by using a sensitive calorimeter. The largest value observed was 682 cal/kg-coal for a coal at 45 °C. This is converted 2851 J/kg-coal and the average heat generation rate of 3.0 W/kg-coal. Taraba and Pavelek20 reported the values named q30, which is the average heat generation rate of initial 30 min at 30 °C after the gas stream was changed from inert atmosphere to a pure oxygen atmosphere, for more than 300

Figure 12. Comparison of temperature T (a), water vaper pressure p and amount of water vapor adsorbed q (b), and heat generation and heat dissipation (c) under different exposure conditions for LY coal.

raw coals and weathered coals. Pure oxygen was used to enhance the rate of coal oxygen interaction. The maximum q30 value of 10.4 W/kg-coal was observed for a wet coal. The q30 values for raw dried coals were all less than 1 W/kg-coal. Shamsi et al.21 measured oxygen consumption rates of several upgraded coals by the ENCOL process with the raw coal at 38 °C. They have reported a maximum oxygen consumption of 0.4 μmol/(g min) for an upgraded coal, which can be converted to 2.3 W/kg-coal of heat generation rate by assuming ΔHOxi = −350 kJ/mol-O2. Similar magnitude of oxygen consumption rate continued for 10 h for the upgraded coal, which generates less than 100 000 J/kg-coal of heat in 10 h. The same amount of heat is generated in less than 3 min even for the condition (A) as shown in Figure 12c. The maximum heat generation rates reported by the three groups were rather close to each other, but the values are smaller by the order of 2 than the condition (A) shown in Figure 12c. These comparisons clearly show that the heat generated by water vapor adsorption is much larger than the heat generated by coal-oxygen interaction, 227

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those observed by oxidation of coal as discussed just above. These examinations indicate again that water vapor adsorption is a possible main cause increasing coal temperature of stockpiles in ambient atmosphere.

indicating that water vapor adsorption can be a main factor increasing coal temperature in ambient atmosphere. All the experiments were performed using dried coals due to difficulty in preparing partly dried coals with definite water contents as the starting samples. Then, the effect of the initial water content of coal, q0, on changing behaviors of water adsorption, temperature, and heat generation and dissipation was examined through simulation calculation for the case where 100 mg of dried LY coal at different water adsorption levels at 38 °C are exposed to the saturated air at 38 °C. Figure 13

4. CONCLUSION Direct measurement of possible temperature increase by the adsorption of water vapor on coal from ambient atmosphere was undertaken for three brown coals/lignites and a bituminous coal. When about 100 mg of each brown coal pre-dried at 80 °C and cooled to 28 °C were exposed to an ambient and stagnant atmosphere of 28 °C with 67−77% RH, the coal temperatures increased up to 40−43 °C in a minute or so with rapid adsorption of water vapor. The amount of water vapor adsorbed reached 0.08−0.1 kg/kg-coal in 10 min. When the brown coals pre-dried and cooled to 38 °C were exposed to a stagnant saturated air at 38 °C, the coal temperatures increased up to over 60 °C in a minute or so. Next, a simple model based on mass and enthalpy balances was successfully formulated to simulate the water adsorption and temperature change. The model simulation clarified that the rate and the amount of heat generated by the water vapor adsorption are much larger than the heat generated by initial coal oxidation and that most of heat generated is transferred to the surrounding to be utilized to heat up the surroundings. Thus, the important role of water vapor adsorption from the ambient atmosphere on the spontaneous heating of coal fine particles in stockpile was clarified.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-774-38-3420. Fax: +81-774-38-3426. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author is grateful to Prof. Jun’ichi Hayashi from Kansai University for his measurement of the adsorption isotherms shown in Figure 2. Prof. Hideaki Ohgaki from the Institute of Advanced Energy, Kyoto University, is acknowledged for his providing the author the opportunity to continue research on coal at Kyoto University.

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REFERENCES

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Figure 13. Effect of initial water content of coal on the temperature change, heat generation, and heat dissipation for LY coal.

shows the calculated results for four q0 levels of 0, 0.02, 0.05, and 0.10 kg/kg-coal. The water adsorption rate decreases with the increasing q0 value, which determines the heat generation rate and hence determines the peak temperature. The peak temperature was 66.5 °C for the dried coal, but it decreases to 61.6 °C at q0 = 0.02 kg/kg-coal, 54.2 °C at q0 = 0.05 kg/kg-coal, and 46.3 °C at q0 = 0.10 kg/kg-coal. However, the heat generation rate at the peak was over 800 W/kg-coal and the amount of heat generated by 5 min was 100 000 J/kg-coal even at q0 = 0.10 kg/kg-coal. These values are still much larger than 228

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