Article pubs.acs.org/IECR
Instantaneous Ignition of Activated Carbon Leonard C. Buettner,† Charles A. LeDuc,†,§ and T. Grant Glover*,‡ †
U.S. Army Edgewood Chemical Biological Center, 5183 Blackhawk Road, Aberdeen Proving Ground, Maryland 21010-5423, United States ‡ Department of Chemical and Biomolecular Engineering, University of South Alabama, 150 Jaguar Dr. SH4136, Mobile, Alabama 36688, United States ABSTRACT: The spontaneous ignition temperature as defined using the American Society for Testing and Materials (ASTM) methods may not accurately determine the temperature at which carbon will combust when a step change in bed temperature occurs; therefore, an instantaneous ignition temperature is defined. Seven different activated carbons have been heated at various heating rates in three different bed configurations, and the data show that the instantaneous ignition temperature can be significantly lower than the spontaneous ignition temperature. In some cases, the carbon ignited at temperatures 100 °C lower than the spontaneous ignition temperature. The results show that ignition temperatures are dependent not only on the carbon source and bed dimensions but also on the rate of heating used in the experiment. The results emphasize the importance of evaluating carbon combustion with experiments that closely resemble the operational conditions of the fixed-bed.
1.0. INTRODUCTION Activated carbon is the most widely used adsorbent material and is utilized in a variety of industrial situations.1 It is critical that all operations utilizing activated carbon maintain the adsorbent temperature below the thermal stability of the carbon to prevent fires, and therefore, care must be taken to properly identify the ignition temperature of the carbon.2 Thermal stability of activated carbons is currently evaluated using two metrics, the point of initial oxidation (PIO) where the temperature causes the carbon bed to start oxidizing, and the spontaneous ignition temperature (SIT) where the temperature is high enough to cause the bed to combust in a self-sustaining manner. However, it has been shown previously that these tests can produce significantly different ignition temperature values and these values are not exclusively intrinsic properties of the carbon.4,5 In addition, it has been shown that carbons containing organic impregnates, such as triethylenediamine (TEDA), have significantly different PIO and SIT values than their unimpregnated analogs and that proper use of SIT and PIO in context to the operational environment of the carbon is critical to ensure nonhazardous operation of carbon filters.4,5 It has been documented that carbons containing metal impregnates, such as Cr, Cu, Ag, Mo, Zn, or Co, can have significantly reduced PIO and SIT values.4−6 Dhan et al. showed that impregnation of carbon with potassium carbonate also accelerates carbon oxidation.7,8 In addition to preadsorbed organic species and metal impregnates, the ignition process is also sensitive to oxygenated surface groups, the presence oxygen concentration in the combustion air, the physical structure of the carbon, as well as gas stream humidity, and carbon age.2,9−11 It has also been shown that particular carbon sources and activation methods produce unique activated carbon surface chemistries and that the oxygen content and surface chemistry impacts the ignition temperature of carbon.12−14 © 2014 American Chemical Society
Absent from these studies is an examination of near instantaneous heating of carbon and the impact on carbon filter ignition temperature. In the circumstances where carbon filters can undergo a step change in the inlet temperature, great caution must be taken to ensure that the carbon bed does not ignite. The American Society for Testing and Materials International (ASTM) defines the SIT based on a fixed-bed 25 mm deep and 25 mm in diameter that is heated at a rate of 2 or 3 °C/min.3 However, there are many practical circumstances where carbon filters undergo an instantaneous step change in the inlet temperature that heats the carbon at a rate much higher than 3 °C/min. In this case, the temperatures at which a filter fire can occur may not be accurately reflected using the SIT and PIO methods. An example of practical importance is the use of carbon to filter hot-air exhaust from a turbine. In this example, the air stream to be filtered is passed through a cooling apparatus prior to filtration to reduce the turbine air temperature, but, upon failure of the cooling apparatus, an almost instantaneous increase in the inlet air temperature to the carbon filter can occur resulting in a filter fire. The purpose of this work is to measure the instantaneous ignition temperature (IIT) of BPL, MaxSorb, and coconut shell carbons, as well as BPL carbon impregnated with metal oxides and TEDA. As shown previously, the ignition temperatures of carbons depend on the configuration of the fixed-bed and therefore three different fixed-bed configurations will be considered. Received: Revised: Accepted: Published: 15793
June 10, 2014 September 19, 2014 September 19, 2014 September 19, 2014 dx.doi.org/10.1021/ie502343y | Ind. Eng. Chem. Res. 2014, 53, 15793−15797
Industrial & Engineering Chemistry Research
Article
Table 1. Activated Carbon Properties type
precursor
ash (%)
particle type
size
bulk density (g/cm3)
surface area (m2/g)
BPL MaxSorb coconut shell
bituminous coal petroleum coke coconut shells
8 30 4
granular cylinder granular
12 × 30 mesh 1.5 mm, 3 mm 8 × 16 mesh
0.48 0.37 0.48
>1100 2060 1200
Table 2. Composition of Impregnates in Activated Carbons Percent by Weight type
copper
chromium oxide
silver
zinc
molybdenum
ASC ASC-T ASZM ASZM-T
8 8 4−6 4−6
3 3
≈0.1 ≈0.1 ≈0.1 ≈0.1
4−6 4−6
1−3 1−3
TEDA 2−4 2−4
2.0. METHODS AND MATERIALS 2.1. Activated Carbons and Impregnation. The unimpregnated carbons investigated were BPL (Calgon, Pittsburgh, PA), MaxSorb (Tokyo Zairyo, White Plains, NY), and coconut shell (Sorb-Tech, Woodlands, TX). The physical properties of these carbons, according to suppliers’ literature, are listed in Table 1. All impregnated carbons were prepared by the manufacturer (Calgon) from the base BPL activated carbon, and the composition of these impregnants is summarized in Table 2. 2.2. Carbon Bed Parameters. To determine the influence of bed sizing parameters three different fixed carbon beds were examined: a fixed-bed operating at ASTM conditions, a fixedbed consistent with a current industrial filter (CF), and a fixedbed consistent with a proposed industrial filter (PF). The parameters of each of these configurations are summarized in Table 3. Table 3. Carbon-Bed Operational Parameters
ASTM conditions current filter (CF) proposed filter (PF)
bed diam. (mm)
bed length (mm)
superficial velocity (cm/s)
25 25 50
25 52 25
50 22.9 10.4
Figure 1. Representative schematic of the apparatus used to measure instantaneous ignition temperatures.
a 15 °C/min heating rate. The tubing and oven were allowed to equilibrate at the set point temperature for at least 15 min to ensure the entire mass of copper reached the same temperature as the oven. Air flow was then diverted through the oven heat exchanger and into the base of the carbon bed. The carbon bed inlet and outlet air temperatures were monitored. If the outlet temperature exceeded 450 °C, the bed was considered to have ignited and the air flow was shut off. If the bed did not ignite, a new carbon bed was prepared and the experiment was repeated with the inlet air temperature increased by 5 °C from the temperature of the previous experiment. For example, if, after completing the first IIT experiment at 10 °C below the SIT, ignition did not occur, then the next IIT experiment would be conducted at 5 °C below the SIT. Continuing this approach, if ignition failed to occur, the next IIT experiment would use inlet air at the SIT temperature. Subsequent experiments would continue to operate at increasing inlet air temperatures until ignition occurred. The instantaneous ignition temperature (IIT) was defined as the lowest inlet temperature to cause ignition. For clarity, we note the key difference between the SIT and IIT is the near step change in temperature of inlet air to the carbon bed during an IIT measurement versus a controlled heating rate in the SIT. The IIT identifies the ignition
2.3. Determination of Spontaneous Ignition Temperature. SIT values were determined for ASTM and CF bed configurations. Using a temperature controlled oven and gas heat exchanger, the temperature of both the carbon bed and the inlet air is held at 40 °C until the exit temperature had equilibrated. After equilibration at this initial temperature, the temperature of the inlet air and bed was increased at a constant rate of either 0.5, 2, 5, 10, or 15 °C/min to the point at which the exit temperature of the carbon bed reached 450 °C. The inlet, outlet, and oven temperatures were recorded at one second intervals. 2.4. Determination of Instantaneous Ignition Temperature. IIT values were determined for the ASTM, CF, and PF bed designs. The apparatus used previously to measure the SIT was modified to allow “instantaneous” heating of the carbon bed, as shown in Figure 1. The air inlet stream traveled through a three way valve that allowed flow through either a 50 foot coil of 0.25 in. copper tubing inside an oven and into the inlet of the carbon bed, or the flow could also be directed to bypass the bed and exit the system as exhaust. The insulated carbon bed was positioned directly on top of the oven. The temperature of the air was set at a temperature 10 °C less than the SIT found from 15794
dx.doi.org/10.1021/ie502343y | Ind. Eng. Chem. Res. 2014, 53, 15793−15797
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configuration and the results are shown in Figure 3. The 0.5, 2, and 5 °C/min experiments show exothermic events in the
temperature of a carbon bed when the bed undergoes a near instantaneous increase inlet air temperature.
3.0. RESULTS AND DISCUSSION SIT data was gathered prior to the IIT experiment, and SIT data for coconut and ASC carbons in the ASTM fixed-bed configuration is shown in Figure 2. In this figure, the difference
Figure 3. Effect of heating rate on ASZM-T carbon ignition temperature. Beds that conform to the ASTM method in sizing and flow rates (25 mm diameter, 25 mm deep, superficial velocity of 50 cm/s) were heated along with the inlet air at various rates (0.5, 2, 5, 10, and 15 °C/min) and inlet and outlet temperatures were monitored. The TEDA is oxidized slowly enough when the temperature is raised at 0.5 °C/min that there is a very small exotherm at 175 °C, when the temperature is increased more rapidly, the TEDA oxidation can become significant enough to ignite the bed at 175 °C.
Figure 2. Spontaneous ignition temperature determination for coconut shell carbon and ASC carbon. Beds that conform to the ASTM method in sizing and flow rates (25 mm diameter, 25 mm deep, superficial velocity of 50 cm/s) were heated along with the inlet air at a rate of 2 °C/min and the temperature of the outlet and inlet air was monitored.
bed consistent with the desorption and oxidation of TEDA occurring between 170 and 210 °C.4 At 0.5 °C/min, the TEDA is heated slowly enough to prevent rapid oxidation of the TEDA and as a result bed combustion occurs at 311 °C. The exothermic events beginning at 170 °C are more pronounced when the carbon is heated at 2, 5, or 10 °C/min, but these still result in a SIT greater than 250 °C. At a heating rate of 15 °C/ min, however, the presence of TEDA on the carbon results in a 190 °C SIT, which is lower than the SITs found at all other heating rates. It is likely that, unlike lower heating rate SITs where TEDA is either desorbed or decomposed slowly, at the 15 °C/min rate, TEDA is more rapidly oxidized which promotes carbon ignition. In all cases, as the heating rate increased the SIT decreased. The data shown in Figure 3 are significant because they illustrate that the heating rate has a significant influence on the temperature of carbon combustion and that utilizing the 2 °C heating rate recommended in the ASTM method does not reflect the much lower carbon ignition temperature that results from a rapid heating event for ASZM-T carbon. Additionally, carbons impregnated with an organic species that is not readily desorbed, such as TEDA, may be more sensitive to the rate of heating used to determine SIT. To illustrate the impact of different bed configurations, data gathered for coconut carbon and ASZM carbon in both the ASTM configuration and the CF configuration are compared in Figure 4. In general, the larger the bed diameter the smaller the wall heat loss effects, and with a lower superficial velocity, less
between the outlet and the inlet temperature provides a convenient scale to display the increase in effluent gas temperature as a result of combustion. Eventually, the combustion of the carbon provides a significant increase in the outlet temperature of the bed, which produces the vertical increases in temperature shown in the figure. The SIT can be determined from this figure by drawing a line from the x-axis parallel to the y-axis that passes through the linear portion of the vertical rise in temperature. The value of the SIT is then determined by reading the point on the x-axis that this line intersects. This method and the SIT data are consistent with the determination of the SIT that was presented in earlier papers from our laboratory; however, the 350 °C SIT of the coconut carbon is slightly lower than the 365 °C SIT reported previously.4,5 Figure 2 illustrates that for the ASC carbon the transition to combustion is a relatively smooth function of increasing inlet temperature, and although steep, the combustion process moves smoothly from approximately 200 to 265 °C. The coconut carbon, however, shows a more abrupt combustion transition beginning at 350 °C, with additional exothermic events occurring between 265 and 310 °C. The nonuniform profile of the coconut carbon and near instantaneous ignition at 350 °C indicates that coconut carbon may be more sensitive to abrupt temperature changes as compared to the ASC carbon. The influence of heating rate on the SIT was also investigated for ASZM-T carbon using the ASTM bed 15795
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to 75 °C. Using this approach several IIT experiments might be required before the temperature that causes instantaneous bed ignition is determined. However, the number of experiments can be reduced by having some knowledge of the SIT of the carbon. As shown in Figure 3, the SIT is reduced with increasing heating rate. Therefore, for a step change heating rate, ignition of the carbon should occur at temperatures less than the SIT determined using the 15 °C/min rate. Thus, the starting point for the IIT experiments was selected as 10 °C below the SIT determined from a 15 °C/min heating rate. The IIT for the carbons has been determined and the results are summarized in Table 4. Comparing the ASTM bed configuration SIT and the IIT provides a comparison of the effect of heating rate on the ignition of the carbon while maintaining the same bed dimensions and flow rate. The results show that for the ASTM beds the IIT is lower than, or that same as, the SIT for all carbon studied. In the ASTM configuration, the most dramatic temperature differences between the SIT and IIT are observed for coconut carbon and ASZM-T carbon. The 105 °C temperature difference between the SIT and IIT of ASZM-T in the ASTM configuration is consistent with the exothermic decomposition of the TEDA contributing to the heating of the carbon as illustrated in Figure 3 and demonstrates the critical importance of completing carbon ignition testing in the same bed configuration that will be used in a given application. As with SIT, the IIT reflects the accumulation of heat in the system and the IIT must be determined in bed configurations that closely match the operational conditions of the bed. Similar to the SIT data, the IITs are lower in the CF configuration. The PF configuration shows similar IITs as the CF configuration, and the bed configuration impacts all the carbons examined. For BPL, the addition of metals, or TEDA, to form ASC, ASZM, and ASZM-T carbon reduces the IIT in both configurations. The IIT reflects the sensitivity of the carbon upon the addition of TEDA with a 95 °C IIT difference between ASZM and ASZM-T carbon for the CF configuration and a 105 °C difference in the ASTM configuration. The SIT data do not reflect the impact of the TEDA on the ignition of the carbon as a result of the lower heating rate used in the SIT method. It is important to note that the IIT data are complementary to the SIT data and that knowledge of each should dictate the operational temperature range for the carbon bed. The IIT does not replace the SIT. The SIT and IIT answer two distinct questions about carbon ignition with the SIT identifying the temperature of ignition when a carbon bed is heated at a controlled rate and the IIT identifying the temperature of ignition when the inlet air undergoes a near step change in temperature. The IIT can be significantly lower than the SIT. An operational hazard could be present when utilizing a carbon bed if the IIT is not known.
Figure 4. Spontaneous ignition temperature determination for coconut shell carbon and ASZM carbon in both the ASTM sized bed and the current filter. Beds that conform to either the ASTM method in sizing and flow rates (25 mm diameter, 25 mm deep, superficial velocity of 50 cm/s) or the current filter (31 mm in diameter, 52 mm in depth, and a superficial velocity of 22.9 cm/s) were heated along with the inlet air at a rate of 2 °C/min and the temperature of the outlet and inlet air was monitored.
heat is exhausted from the bed. The CF configuration, which has both a larger bed diameter and a lower superficial velocity than the ASTM configuration, generated lower SIT values than the ASTM configuration for both carbons. The results show that coconut carbon has several exothermic transitions prior to ignition in both configurations and that the ASZM carbon has a relatively smooth transition to ignition. Data for ASC, ASZMT, MaxSorb, and BPL in these configurations are summarized in Table 4, and when compared to the ASTM configuration, lower SIT values for the CF configuration were observed for all of the carbons. Table 4. Ignition Temperatures (°C) of Various Carbons carbon
ASTM SIT
CF SIT
ASTM IIT
CF IIT
PF IIT
ASC ASC-T ASZM ASZM-T coconut shell MaxSorb BPL
265
230
300 292 350 >370 >370
260 275 315 340 >370
250 203 300 195 285 >363 >370
230 175 270 175 260 345 >370
230 184 275 175 260 355 >370
4.0. CONCLUSIONS The rate of heating of carbon beds significantly impacts the temperature of ignition. The SIT measured using ASTM methods may not capture the lower temperatures at which carbon will combust when the bed is heated rapidly. The IIT is a function of not only the fixed-bed dimensions and superficial velocity but also a function of the type of carbon used. Impregnation of BPL carbon with metals reduced both the SIT and the IIT. The addition of TEDA to the metal impregnated BPL carbon reduced the IIT by approximately 100 °C. This
SIT data were gathered first to provide a starting temperature for IIT experiments, but knowledge of the SIT is not necessary to determine the IIT. Specifically, the IIT experiments are seeking the temperature that ignites the carbon bed when a near step change in inlet air temperature occurs. Thus, a reasonable set of IIT experiments would include examining a range of inlet air step changes. For example, for a carbon bed at 25 °C a step change of inlet air temperature from 25 to 50 °C could be examined. Then, when ignition did not occur, a larger step change could be examined, such as a step change from 25 15796
dx.doi.org/10.1021/ie502343y | Ind. Eng. Chem. Res. 2014, 53, 15793−15797
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underscores the importance of evaluating carbon combustion with experiments that closely match the operational conditions of the carbon bed, which should include selecting an operationally relevant worst case scenario rate at which the carbon is heated.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 251-460-7462. Email:
[email protected]. Present Address
§ Division of Molecular Genetics Department of Pediatrics, Columbia University, Russ Berrie Pavilion, 1150 St. Nicholas Avenue, Room 620, New York, NY 10032, United States.
Notes
The authors declare no competing financial interest.
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REFERENCES
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