Transformations of Coal-Derived Soot at Elevated Temperature

52

Energy & Fuels 2001, 15, 52-59

Transformations of Coal-Derived Soot at Elevated Temperature J. Rigby,†,‡ J. Ma,§ B. W. Webb,*,† and T. H. Fletcher§ Brigham Young University, Provo, Utah 84602 Received May 30, 2000. Revised Manuscript Received September 29, 2000

Coal pyrolysis experiments were performed in the post-flame region of a CH4/H2/air flat-flame burner operating under fuel-rich conditions, where the temperature and gas compositions were similar to those found in the near-burner region of an industrial pulverized coal-fired furnace. Volatiles released from the coal particles formed a cloud of soot particles surrounding a centrally fed coal/char particle stream. Soot samples were collected from the cloud at different residence times using a water-cooled, nitrogen-quenched probe. The soot samples were then analyzed for their elemental compositions of carbon, hydrogen, nitrogen, sulfur, and (by difference) oxygen plus inorganic matter. Soot from three parent coals (Pittsburgh #8, Illinois #6, and Utah Hiawatha) and two gaseous hydrocarbon fuels (propane and acetylene) were investigated at temperatures of 1650, 1800, and 1900 K. The results reveal that the yield of coal-derived soot decreases with increasing reactor temperature, even though the total volatiles yield increased only slightly with temperature. The coal-derived soot yield at each reactor temperature condition also increased slightly with residence time. The carbon content in the coal-derived soot decreased with increasing particle residence time (at a given reactor temperature) and with increasing reactor temperature (at a given residence time) for all three coals. Carbon content remained constant with residence time for the gaseous hydrocarbon-fuel-derived soot. It is suggested that the observed decrease in coal-derived soot yield with increasing temperature is due to reactions of radical species from the flame with the soot precursors (i.e., the tar molecules). The slight increase in coal-derived soot yield with increasing residence time is due to attachment of light gas species such as acetylene which are richer in hydrogen than the local soot particles. The different behavior of soot from coal and the gaseous hydrocarbon fuels is explained in terms of their different chemical structures; coal-derived soot molecules have more aliphatic attachments and heteroatoms than soot from acetylene or propane. Carbon/hydrogen ratios in the soot samples were observed to be significantly different for the different soot types depending on parent fuel.

Introduction All hydrocarbon-based fuels have the potential to form soot during combustion or devolatilization reactions. Soot as an intermediate product of combustion is the main contributor to luminosity in flames, and it can therefore strongly affect the radiation transfer from the flame near the burner. Enhanced radiation transfer impacts the local combustion temperatures, which in turn affect the formation of pollutants (NOx, SOx, etc.). In addition, soot from coal pyrolysis contains nitrogen, and the generation of soot therefore represents an additional mechanism for fuel nitrogen evolution. Finally, soot has been identified as one of the sources of unburned carbon in ash samples from industrial boilers.1 These considerations underline the importance of understanding the mechanisms for soot formation in coal combustion systems. Soot formation mechanisms will not be covered in detail in this paper, but a short discussion of the * Corresponding author. † Department of Mechanical Engineering, 435 CTB. ‡ Currently with Exxon Production Research, Houston, TX. § Department of Chemical Engineering, 350 CB. (1) Veranth, J. M.; Fletcher, T. H.; Pershing, D. W.; Sarofim, A. F. Measurement of Soot and Char in Pulverized Coal Fly Ash. Fuel, in press.

differences between the formation of coal-derived soot and soot from gaseous hydrocarbons will be presented. Recent reviews of this material are found elsewhere.2,3 Soot from coal is formed in the fuel-rich near-burner region of pulverized coal reactors. Pyrolysis is the initial reaction step that occurs as the volatiles are released from the coal particle in an oxygen-depleted environment. Products of pyrolysis include light gases, char, and tar. It is believed that tar is a precursor to soot in coal systems, and that the secondary pyrolysis reactions of tar result in the formation of coal-derived soot.3-6 The large molecular weight tar molecules are thought to form the main building blocks of soot particles as they react in the early stages of pyrolysis. Some addition of mass from secondary reactions of light gases, such as (2) Fletcher, T. H.; Ma, J.; Rigby, J. R.; Brown, A. L.; Webb, B. W. Soot in Coal Combustion Systems. Prog. Energy Comb. Sci. 1997, 23, 283-301. (3) Ma, J. Soot Formation During Coal Pyrolysis. Ph.D. Dissertation, 1996, Brigham Young University, Provo, Utah. (4) Ma, J.; Fletcher, T. H.; Webb, B. W. Conversion of Coal Tar to Soot During Coal Pyrolysis in a Post-Flame Environment. TwentySixth Symposium (Int’l) on Combustion; The Combustion Institute, Pittsburgh, 1996; pp 3161-3167. (5) McLean, W. J.; Hardesty, D. R.; Pohl, J. H. Direct Observations of Devolatilizing Pulverized Coal Particles in a Combustion Environment. Eighteenth Symposium (Int’l) on Combustion; The Combustion Institute, Pittsburgh, 1981; pp 1239-1248.

10.1021/ef000111j CCC: $20.00 © 2001 American Chemical Society Published on Web 11/10/2000

Transformations of Coal-Derived Soot

Energy & Fuels, Vol. 15, No. 1, 2001 53

C2H2, seems to occur after initial soot inception from tar reactions.7 Soot formation from simple gaseous hydrocarbon fuels occurs in a different manner than from the reaction of high molecular weight tar molecules seen in coal pyrolysis. Simple aromatic hydrocarbon fuels are thought to decompose by combustion or pyrolysis into acetylene and polycyclic aromatic hydrocarbons.8 If no aromatic hydrocarbons exist in the parent fuel, light gas precursors undergo cyclization to create aromatic rings.9 When the aromatic substances reach a large enough size they condense into nuclei and soot particle inception occurs. The soot then grows due to attachment of gas-phase species to the young soot particle. The pathway for soot in gaseous hydrocarbon flames goes from lower molecular weight substances to higher molecular weight substances. By contrast, the pathway for soot formation in coal pyrolysis originates with very high molecular weight tar without breaking down to acetylene. Soot formation, agglomeration, and combustion have been studied extensively in gaseous hydrocarbon flames.10 However, relatively little research has been conducted on soot derived from coal. Related work was reviewed recently2. The purpose of this work is to report on measurements of the chemical composition of coalderived soot from the high-temperature pyrolysis of soot in an oxygen-depleted environment. The measurements reported here complement experimental work characterizing the spectral radiative properties of coal-derived soot.11 Experimental Apparatus and Procedure Soot was generated in a flat-flame reactor from the pyrolysis of three different coals, and for comparison with gaseous hydrocarbon fuels, from the pyrolysis of propane and acetylene. A suction probe was designed and constructed to separate the soot particles from the char particles and combustion gases when coal was injected, and from the combustion gases when gaseous hydrocarbon fuels were injected. Analysis of the chemical composition of the soot was performed using a CHNS analyzer. The experimental apparatus, soot collection system, and instrumentation will now be described. Flat-Flame Reactor. Soot was generated for this study in a burner designed and fabricated by Research Technologies. It is similar to that used by McLean et al.5 The reactor generates a flat flame and runs primarily on natural gas augmented by some hydrogen flow to regulate combustion gas temperatures. The reactor consists of a 51 mm by 51 mm square diffusion burner with a center fuel tube. Both coal and gaseous fuels can be introduced through the central feed tube. The burner surrounding the central feed tube consists of 750 hexagonally arranged capillary tubes evenly distributed on the (6) Seeker, W. R.; Samuelson, G. S.; Heap, J. P.; Trolinger, J. D. The Thermal Decomposition of Pulverized Coal Particles. Eighteenth Symposium (Int’l) on Combustion; The Combustion Institute, Pittsburgh, 1981; pp 1213-1226. (7) Chen, J. C.; Castagnoli, C.; Niksa, S. Coal Devolatilization During Rapid Transient Heating. Part 2. Secondary Pyrolysis. Energy Fuels 1992, 6, 264-271. (8) Haynes, B. S.; Wagner, H. G. Soot Formation, Prog. Energy Comb. Sci. 1981, 7, 229-273. (9) Glassman, I. Twenty-Second Symposium (Int’l) on Combustion; The Combustion Institute, Pittsburgh,1988; pp 295-311. (10) Haynes, B. S. In Fossil Fuel CombustionsA Source Book; Bartok, W., Sarofim, A. F., Eds.; Wiley: New York, 1991; pp 261326. (11) Rigby, J. R. Experimentally Determined Optical Properties and Chemical Compositions of Coal-Derived Soot. Ph.D. Dissertation, 1996, Brigham Young University, Provo, Utah.

Figure 1. Photograph of the flat-flame reactor showing the soot cloud surrounding the central flow of coal/char particles. burner surface. Each of these smaller burners has a fuel tube surrounded by oxidizer tubes. Upon ignition, laminar diffusion blue flamelets are formed on the burner surface 2 mm above the burner surface. Separate flow controls allowed stoichiometric adjustment to the fuel/oxidizer ratio. A 30-cm-high quartz tower of square cross-section with the same dimensions as the flat-flame burner was used to confine the post-flame gases. When a coal particle introduced through the central feed tube was heated by the hot post-flame gases, volatiles released from the coal particles expanded radially, while the flowing char particles remained in a thin stream along the reactor centerline. A luminous volatiles (soot) cloud extended radially from the reactor centerline to a maximum radius of approximately 1.5 cm, as illustrated in Figure 1. The flat-flame reactor was designed and operated to produce conditions similar to those found in the near-burner region of a pulverized-coal-fired furnace. Three different equivalence ratios, defined on the basis of fuel flow to the flat-flame burner but excluding the fuel fed through the center tube, were used. They were φ ) 1.26, 1.30, and 1.48 (fuel-rich). The fuel fed through the center tube provided less than 1% of the heating value of the natural gas/hydrogen in the surrounding flat flame. The post-flame combustion gases of the reactor consisted mainly of CO, CO2, H2O, and unburned CH4, which are similar to the gases found in the fuel-rich cloud in the nearburner region of a coal-fired furnace. As confirmed by measurements, the concentration of O2 was negligible above the flat flame. The particle heating rate for pyrolysis in the reactor was on the order of 105 K/s, similar to that found in industrialscale furnaces. The gas temperatures in the flat-flame reactor

54

Energy & Fuels, Vol. 15, No. 1, 2001

Rigby et al.

Figure 2. Radial gas temperature profiles (corrected) at different axial positions above the flat-flame burner, and (inset) variation of centerline gas temperature with height. were also similar to a full-scale combustion environment. The resulting thermal/chemical environment would not oxidize the soot directly but could possibly result in other chemical transformations at the higher temperature environment studied. Differences between this laboratory reactor and an industrial-scale furnace may include the turbulence and the particle interaction effects in a large devolatilizing coal cloud that are present in full-scale environments. The temperature of the post-flame gases was essentially uniform across the cross-section of the reactor at each height, as shown by the profiles shown in Figure 2 for the φ ) 1.26 experimental condition, which were measured using a type B thermocouple and corrected for radiation. Exhaust gas concentration profiles (not shown) also exhibited similar uniformity across the reactor cross-section. The slightly lower centerline temperatures seen in Figure 2 were caused by the nitrogen carrier gas used for transport of the coal particles through the central fuel tube. The inset panel of the figure illustrates the reactor centerline temperature profiles for the three different equivalence ratios tested. The peak temperatures for the three cases studied are nominally 1650 K (φ ) 1.48), 1800 K (φ ) 1.30), and 1900 K (φ ) 1.26). The gases increase in temperature over the first 3 cm of height above the burner, then decrease due to convective and radiation losses. For convenience, the three cases investigated will be characterized by their respective peak temperatures in the presentation of the data (i.e., 1650, 1800, and 1900 K). Coal particles were fed into the reactor using a syringe feeder system to entrain the flow in a passing nitrogen stream flowing at 0.04 slpm. The feeder consisted of a 250 mL glass funnel that had been modified such that a single stream exits through the base. The coal was introduced to the feeder using a 2.5 mL syringe that held approximately 2 g of coal. Coal was forced from the syringe as a stepper motor operating at 2 Hz compressed the syringe piston. A vibrator helped to maintain the feed rate uniform as it was entrained by the passing nitrogen stream. The coal-entrained nitrogen flow entered a 1 mm i.d. Tygon tube at the bottom of the funnel, which was connected to the 0.75 mm stainless steel tube introducing the fuel flow into the reactor. When soot from gaseous hydrocarbon fuels (acetylene and propane) was generated in the reactor, the syringe feeder was replaced by a tube delivering gaseous fuel at the desired flow rate. Soot Collection System. A water-cooled suction probe was designed and constructed to extract soot from the combustion environment at different heights (corresponding to different residence times) above the flat flame in the reactor. The probe extracted combustion gases and particles from the burner by the use of vacuum pumps, and is shown schematically in

Figure 3. Schematic of flat-flame burner and soot collection system. Figure 3. Nitrogen quenching via twelve radial jets just inside the mouth of the sampling probe quickly cooled the extracted combustion gases and particles. Additional nitrogen quench gas transpired through a sintered stainless steel liner with pore size of 5 µm. This flow further cooled the combustion gases and helped prevent particle deposition on the suction probe walls. A particle separation system downstream of the sampling probe was designed to aerodynamically classify the extracted particles into two groups. These groups were the large particles consisting of char and large soot agglomerates, and the small soot particles. A virtual impactor and cyclone system separated these groups. The virtual impactor was designed to allow small particles to pass through the sidearm (termed the soot leg) of the separation system. Due to inertial forces, the char particles continued to flow upward and were collected in the char leg of the separation system using the cyclone. Both the virtual impactor and the cyclone were designed to yield a cut-point diameter of 5 µm. Two polycarbonate filters with pore size of 1 µm were used to collect small soot particles. Deposition tests showed that 90% of the soot smaller than 5 µm passed through to the filters in the legs of the probe.3,4 Uncertainty in both total soot yield and total volatiles yield from the sampling measurements was estimated to be 2% absolute.3,4 A complete description of the probe and separation system with specifications and separation efficiencies are given by Ma.3 Particle Residence Time. The variation of soot chemistry with particle residence time gives information regarding the chemical history of soot formation. Each sampling height in the reactor corresponds to a different particle residence time. To obtain an accurate residence time of a coal particle as a function of sampling height, a high-speed camera (Kodak EktaPro Imager) was used to record the trajectory of a luminescent coal particle. Images were recorded at a rate of 500 frames per second. The images were then analyzed to determine the particle residence time corresponding to each sampling height in the reactor. A correction was made to this value for the time required for initiation of particle luminescence (typically 10-15 mm of height) by modeling the particle acceleration over this distance. The real residence time at a given collection height was the summation of the calculated time for the particle to begin luminescence and the time recorded from high-speed imaging thereafter. The calculated particle trajectory distance versus residence time was then fit



Table 1. Proximate and Ultimate Analyses of Coals Used in This Study wt %

wt % (dry)

wt % (daf)

coal type

moisturea

volatilesa

asha

C

H

N

S

O

PSOC 1451D Pittsburgh No. 8 hva bit. PSOC 1493D Illinois No. 6 hvb bit. PSOC 1502D Utah Hiawatha hvc bit.

1.87

37.10

4.11

84.70

5.40

1.71

0.92

7.26

6.94

38.69

15.13

76.65

4.93

1.47

6.93

10.01

7.58

38.78

9.14

80.53

5.96

1.33

0.47

11.71

a

ASTM analysis.

to a third-order polynomial. The difference in residence time calculated for a given sampling height for the different coals studied was within the uncertainty of the residence time estimates. Therefore, a single time was used to characterize the residence time for all coals at a given sample height. Further, the negligible hydrodynamic effect of a single 60 µm particle on the flowing post-flame gases suggests the use of the residence time calculated for the particulate fuel at a given sampling height for the gaseous hydrocarbon fuels as well. Char Analysis. The amount of mass released as volatiles was determined using the inherent titanium (Ti) in the coal as a tracer. The mass fractions of Ti in the parent coal, chars, and ash samples were measured using an inductively coupled plasma (ICP) method according to ASTM Procedure D 3682. Additional details of the tracer technique are given by Ma.3 Soot Elemental Analysis. The carbon, hydrogen, nitrogen, and sulfur content of the soot samples were measured on a Leco CHNS 932 analyzer. The CHNS results were then used to calculate the percentage of oxygen plus inorganic matter by difference from a mass balance, assuming no other species were present in the soot. The CHNS instrument manufacturer reports standard deviations for a nominal 2 mg sample at 0.3 relative percent for carbon, 0.01 absolute percent for hydrogen and nitrogen, and 0.02 absolute percent for sulfur. The samples used in this research were smaller than the nominal samples due to (i) difficulty in loading soot into the testing microcapsules, and (ii) the limited amount of soot available. The test capsules are approximately 4 mm in height with a 3 mm diameter opening. Because of the low packing density of the soot (fluffiness), it was difficult to place more than 0.5 mg into a capsule. An estimate of uncertainty for CHNS measurements in this study revealed a 2σ confidence level for elemental analysis of an average sample size of 0.5 mg to be 0.7% relative for carbon, 0.08% absolute for hydrogen and nitrogen, and 0.16% absolute for sulfur.11 Most of the samples had average masses equal to or higher than this value. A complete list of sample masses, compositions, and standard deviations may be found elsewhere.3,11 Carbon, hydrogen, nitrogen, sulfur, and oxygen + inorganics percentage compositions are presented for the coal-derived soots from three coal types, propanederived soot, and two sets of acetylene-derived soot. Between two and five samples were analyzed and averaged for each sample location in the reactor. Multiple samples collected at a given sampling height in the reactor showed very good repeatability. Typical standard deviations in measured data at a given sampling height were 0.5-7.0 relative percent of mean carbon, hydrogen, nitrogen, and sulfur compositions. Deviations (2σ) of multiple samples almost always fell within the error bars shown on the figures. The limited exceptions occurred when the sample masses were all small or when there were slight inhomogeneities in the sample. Coals Investigated. Soot formed during the devolatilization in the flat-flame reactor using Pittsburgh No. 8, Illinois No. 6, and Utah Hiawatha high volatile bituminous coals was collected with the suction probe described in the foregoing section. The chemical compositions of the coals are shown in

Figure 4. Measured total volatiles yields as a function of temperature and residence time for the three coals. Table 1. Size-classified particles between 63 and 74 µm were used for each coal type. The feed rates of the coal and gaseous fuels were approximately 1.5 and 2.5 g/hr, respectively. These feed rates produced comparable amounts of soot per unit time. At the highest reactor temperature (1900 K), soot was sampled for CHNS analysis at six different heights in the reactor (2.5, 5.1, 7.6, 10.2, 12.7, and 15.2 cm) for the three coals and the two gaseous hydrocarbon fuels. Additionally, soot was collected at two sampling heights (2.5 and 10.2 cm) for the three coals at the three different reactor temperatures (1650, 1800, and 1900 K). This test matrix permits exploration of the influence of both residence time and temperature on the soot composition.

Results and Discussion Soot was formed at very early residence times in the flat flame reactor. Soot samples taken at the lowest sampling height (2.5 cm) showed little or no extractable material3. Total volatiles yields for the three coals (see Figure 4) did not increase with residence time after the earliest sampled residence time (13 ms), indicating that the pyrolysis process was completed before this time. The total volatiles yield observed in Figure 4 is nearly constant with residence time for all three reactor temperatures. The measured soot yields from coal at

56


Rigby et al.

Figure 7. Measured carbon content as a function of residence time for the coal- and gaseous hydrocarbon-derived soot at 1900 K.

Figure 5. Soot yields for the three coals as a function of residence time and reactor temperature.

Figure 6. Measured soot yields for the three coals as a function of residence time at the 1900 K gas condition.

different residence times are shown as a function of temperature in Figure 5. The general trend in the soot yield data is decreasing yields of coal-derived soot as temperature increases. However, at any given reactor temperature condition, soot yields increase as a function of residence time, as illustrated in Figure 6 for the 1900 K condition. The decrease in soot yield with increasing gas temperature (Figure 5) was reported earlier by Ma et al.,4 and explained on the basis of the reaction of radical species from the CH4 flame (such as OH) on soot precursors near the flame. The higher temperatures increase the rate of reaction of these radical species. The slight increase in soot yield with residence time (Figure 6) can be explained by addition of light gas species, such as acetylene, to the soot particles. The fact that no additional devolatilization is occurring (see Figure 4) helps in the interpretation of the data. There is no evidence for soot gasification by the post-flame gases (H2O and CO2).

The measured carbon contents for the soot samples collected at the 1900 K reactor temperature from the coals and from the light hydrocarbon gases are shown in Figure 7. At the earliest measured residence time all coal-derived soots were comprised of approximately 96% carbon. Thereafter, the data for coal-derived soot reveals trends of decreasing carbon percentage with increasing residence time. This decrease in carbon content was seen for all three coal-derived soots at the 1900 K condition. The decrease in carbon percentage with residence time was greatest for the Utah Hiawatha coal, followed by the Illinois No. 6 and the Pittsburgh No. 8 coals. In contrast to the coal-derived soot, the data in Figure 7 for propane and acetylene soot showed a relatively constant carbon content at around 97%. This is appreciably higher in carbon percentage than that observed for the coal-derived soot. The carbon contents of the two gaseous hydrocarbon-derived soots are essentially indistinguishable. The acetylene-derived soot was removed from the filter on the soot leg of the sampling probe, denoted “Acetylene (soot leg),” and from the soot filter on the char leg of the sampling probe, denoted “Acetylene (char leg).” This was done to determine if different legs of the sampling system chemically classified the soot. The agreement between the carbon contents in acetylene soot samples from different legs of the sampling system shown in the lower panel of Figure 7, and the overlapping data for the other elements indicates that the two legs did not separate the soot with respect to chemical composition. Figure 8 shows the measured change in carbon composition between the 10.2 and 2.5 cm sampling heights (C10.2 - C2.5, in absolute %) for the three coals at the three reactor temperatures. The decrease in carbon with increasing sampling height (residence time) seen in the figure was not expected, and was observed only at the highest reactor temperature. With increasing height in a hot, oxygen-depleted environment it might be expected that carbon content would increase, as the more volatile hydrogen and sulfur in the soot were released into the combustion gases. Since the soot yields for any temperature condition increase slightly with residence time instead of decrease, soot gasification does



Figure 8. Measured change in carbon composition between the 10.2 and 2.5 cm sampling heights (C10.2 - C2.5, in absolute %) for the three coals at the three different reactor temperatures.

Figure 11. Measured nitrogen content as a function of residence time for the coal- and gaseous hydrocarbon-derived soot at 1900 K.

Figure 9. Measured hydrogen content as a function of residence time for the coal- and gaseous hydrocarbon-derived soot at 1900 K.

Figure 10. Measured change in hydrogen composition between the 10.2 and 2.5 cm sampling heights (H10.2 - H2.5, in absolute %) for the three coals at the three different reactor temperatures.

not seem to be a plausible explanation for the decrease in soot carbon concentration. The addition of light gas species such as C2H2 to the soot, hypothesized above as the reason for slight increases in soot yield versus time, is consistent with the decrease in carbon content of the soot. These light gas species are likely products of secondary reactions of coal volatiles, hence no change is seen in the carbon contents of light hydrocarbonderived soot. The light gas products of secondary coal pyrolysis (C2H2, CH4, H2) exhibit higher hydrogen content than the soot, hence the slight decrease in carbon and a corresponding slight increase in hydrogen (Figures 9,10). The hydrogen change between sampling heights of 10.2 and 2.5 cm (H10.2 - H2.5, in absolute %) shown in Figure 10 illustrates how the soot hydrogen fraction changes with residence time are affected by reactor temperature. Generally speaking, the hydrogen change increases with temperature for the three coals,

supporting the idea that the addition of light gas species increases with increased temperature. In contrast to the data from the coal-derived soot, the hydrogen content from propane and acetylene soot seen in Figure 9 remains essentially constant at a lower percentage, averaging 0.5% over the full range of residence time for which measurements were made. The acetylene and propane fed to the reactor are converted to soot very early in the reactor, and hence no additional soot growth occurs at longer residence times. The nitrogen compositions for the different soots shown in Figure 11 exhibit no clear trend with residence time; the nitrogen concentration was essentially constant. There was a slight difference in composition with coal type. The coal with highest nitrogen content in the parent fuel, Pittsburgh No. 8 (1.71% N), yielded the soot with the highest nitrogen percentage. The other coals with slightly lower nitrogen content exhibited lower nitrogen contents in their soot. The stability of nitrogen in coal-derived soot has been shown previously.7,12 The propane and acetylene soots exhibit very low nitrogen content, presumably because there is no nitrogen in the fuel. The mechanism for nitrogen incorporation into these soot samples from light hydrocarbon gases is not clear. A possible route for nitrogen entering the soot is through NOx reacting with the surface of the soot particle. In the present fuel-rich experiment the formation of NOx would probably be through the prompt NOx mechanism, which is prevalent in fuel-rich hydrocarbon flames.13 The hydrocarbon fragments formed during the pyrolysis of propane and acetylene would attack molecular nitrogen to form HCN, which could lead to NOx formation or even attach to the soot. NOx is reactive with the soot, whereas molecular nitrogen (N2) is not. Although the nitrogen content is low for the gaseous hydrocarbon-derived soot, there is a statistically significant content especially at lower residence times. The existence of nitrogen in soot from these types of gaseous fuels was not expected, but measurable values were present. (12) Perry, S.; Hambly, E. M.; Fletcher, T. H.; Solum, M. S.; Pugmire, R. J. Solid-State 13C NMR Characterization of Matched Tars and Chars From Rapid Coal Devolatilization. Proc. Combust. Institute, in press. (13) Smoot, L. D.; Smith, P. J. Coal Combustion and Gasification; Plenum Press: New York, 1985.

58


Figure 12. Measured sulfur content as a function of residence time for the coal- and gaseous hydrocarbon-derived soot at 1900 K.

The measured sulfur content of the soot samples is shown in Figure 12. An increase in sulfur content for Illinois No. 6 and Utah Hiawatha coal-derived soot was seen with increasing residence time. The soot from Illinois No. 6, as expected from the 6.93% sulfur composition of the parent coal, was the highest sulfurcontaining soot. However, the surprising data were from the Utah Hiawatha coal. This coal, with only 0.47% sulfur in the parent fuel, yielded soot with sulfur content nearly as high as the Illinois No. 6 coal-derived soot for the later residence times. The sulfur fraction in the soot at high residence time was actually higher than in the parent coal. The soot from the Pittsburgh No. 8 coal showed a constant composition for sulfur at nearly 0.35%. Figure 12 shows no measurable sulfur for the propane and acetylene soot. This is expected since the only sulfur in the system originates with the trace amounts added to natural gas for leak detection. Sulfur percentage uncertainty was the largest uncertainty of any of the elements. Even with the large error bars, there was significantly higher sulfur content in the soot samples at every residence time from Illinois No. 6 coal than seen in soot samples from other coals. Assuming no other species besides carbon, hydrogen, nitrogen, and sulfur present in the soot, the mass which remains is a combination of oxygen and inorganic material, which was calculated by difference from the CHNS data. While no mineral matter characterizations were carried out for the soot samples of this study, more recent preliminary work has suggested that inorganic material may amount to as much as 8% of the soot mass.14 Further, the recent data suggest that the fraction of soot composed of inorganics is only this high for low-rank coals. For most of the different kinds of soot in this study, the oxygen + inorganics content (or uncharacterized content) was less than 5%, as seen in the data of Figure 13. The fraction of oxygen + inorganics increased more dramatically, however, for the Illinois No. 6 coal from 2.5% at the early residence time to 7% at 55 ms. The change in oxygen + inorganics content in the soot from the Utah Hiawatha coal was from 2.5% to over 13% over the same time, although (14) Haifeng, Z., Secondary Coal Pyrolysis of Nitrogen Species. Ph.D. Dissertation (in preparation), Brigham Young University, Provo, Utah.

Rigby et al.

Figure 13. Oxygen + inorganic material content (by mass balance difference from CHNS data) as a function of residence time for the coal- and gaseous hydrocarbon-derived soot at 1900 K.

Figure 14. C/H ratio as a function of residence time for the coal- and gaseous hydrocarbon-derived soot at 1900 K.

only one data point (at the highest residence time) seems to indicate a significantly higher amount. The addition of oxygenated species to soot is not viewed as a likely reaction. Without mineral matter characterizations for these soots, the oxygen fraction cannot be definitively determined. The carbon-to-hydrogen (C/H) ratio in soot has been seen to influence the optical properties of soot.15-18 The C/H ratios of the coal-derived soot are plotted in Figure 14. There is a decreasing trend with increasing residence time for soot from all three coals. Values of the C/H ratio for the Pittsburgh coal-derived soot range between 10 and 7 at residence times of 14 and 40 ms. These data from the Pittsburgh No. 8 soot mirror the previously published data of Chen,19 where experiments were performed at a radiant wall temperature of 1750 (15) Millikan, R. C. Optical Properties of Soot. J. Opt. Soc. Am. 1961, 51, 698-699 (16) Dalzell, W. H.; Sarofim, A. F. Optical Constants of Soot and Their Application to Heat-Flux Calculations. J. Heat Transfer 1969, February, 100-104. (17) Ben Hamadi, M.; Vervisch, P.; Coppalle, A. Radiation Properties of Soot from Premixed Flat Flame. Comb. Flame 1987, 68, 57-67. (18) Habib, Z. G.; Vervisch, P. Soot Formation in Combustion Processes. Comb. Sci. Technol. 1988, 59, 261-274.


K in an argon environment, C/H ratios between 8.5 and 11 were measured. Both Ma and Chen found that temperature also had an effect on C/H ratios. Lower pyrolysis temperatures yielded soot with lower C/H ratios for the coal-derived soot. The C/H ratio data of Figure 14 reveal that the soot from propane and acetylene exhibit higher C/H ratios than the soot derived from coal. C/H ratios for propane averaged near 15 and acetylene near 20. The data were relatively constant for soot from both coal and gaseous fuels over the full range of residence time. Dalzell and Sarofim16 reported C/H values of 14.7 for acetylene soot and 4.6 for propane soot. The wide variety of soot compositions from this and previous studies seems to result from different residence time and temperature histories for the soot. The current tests show no correlating trend of C/H ratios for either the propane or acetylene soot. Further study and analysis are needed in this area. Summary and Conclusions Coal pyrolysis experiments were performed in the post-flame region of a CH4/H2/air flat-flame burner operating under conditions which simulate the environment found in the near-burner region of an industrial pulverized coal-fired furnace. Volatiles released from the coal particles formed a cloud of soot particles surrounding a centrally fed coal/char particle stream. Soot (19) Chen, J. C. Effects of Secondary Reactions on Product Distribution and Nitrogen Evolution from Rapid Coal Pyrolysis; HTGL Report No. T-280, 1991, High-Temperature Gasdynamics Laboratory, Mechanical Engineering Department, Stanford University.


samples were collected after complete devolatilization, and showed no evidence of tar. Soot yields from coal decreased with increasing temperature, and increased slightly with increasing residence time. The decrease in soot yield with increasing temperature is likely due to reactions of radical species from the flame with the soot precursors. The increase in soot yield with residence time is likely due to addition of light gases from secondary coal pyrolysis, such as acetylene. The carbon content in the coal-derived soot was observed to decrease both with increasing particle residence time (at a given reactor temperature) and with increasing reactor temperature (at a given residence time) for all three coals. Carbon content remained constant with residence time for the soot generated from pyrolysis of the acetylene and propane. The increase in carbon content in the coal-derived soot is consistent with the addition of light hydrocarbon species from secondary coal pyrolysis (such as C2H2), since these species are generally more rich in hydrogen than the local soot particles. No such light hydrocarbon gas addition seemed to occur in these conditions for the acetylene and propane experiments. For the coal-derived soot, hydrogen and sulfur fractions were seen to increase slightly with residence time, whereas the nitrogen content was nearly constant. Hydrogen, nitrogen, and sulfur fractions for soot generated from the two gaseous hydrocarbon fuels exhibited no significant change with residence time. The calculated carbon/hydrogen ratios are significantly different for the different soot types depending on parent fuel. EF000111J

Transformations of Coal-Derived Soot at Elevated Temperature

Recommend Documents