c Pleiades Publishing, Ltd., 2016. ISSN 0010-5082, Combustion, Explosion, and Shock Waves, 2016, Vol. 52, No. 3, pp. –. c M.B. Gonchikzhapov, A.A. Paletsky, A.G. Tereshchenko, I.K. Shundrina, L.V. Kuibida, A.G. Shmakov, O.P. Korobeinichev. Original Russian Text
Structure of UHMW Polyethylene–Air Counterflow Flame M. B. Gonchikzhapov a, b , A. A. Paletskya , A. G. Tereshchenkoa , I. K. Shundrinab, c , L. V. Kuibidaa, b , A. G. Shmakova, b , and O. P. Korobeinicheva, d
UDC 541.124
Published in Fizika Goreniya i Vzryva, Vol. 52, No. 3, pp. 8–22, May–June, 2016. Original article submitted April 1, 2015; revision submitted September 23, 2015.
Abstract: The combustion of ultrahigh molecular weight polyethylene (UHMWPE) in airflow perpendicular to the polyethylene surface (counterflow flame) was studied in detail. The burning rate of pressed samples of UHMWPE was measured. The structure of the UHMWPE–air counterflow flame was first determined by mass spectrometric sampling taking into account heavy products. The composition of the main pyrolysis products was investigated by mass spectrometry, and the composition of heavy hydrocarbons (C7 —C25 ) in products sampled from the flame at a distance of 0.8 mm from the UHMWPE surface was analyzed by gas-liquid chromatography mass-spectrometry. The temperature and concentration profiles of eight species (N2 , O2 , CO2 , CO, H2 O, C3 H6 , C4 H6 , and C6 H6 ) and a hypothetical species with an average molecular weight of 258.7 g/mol, which simulates more than 50 C7 —C25 hydrocarbons were measured. The structure of the diffusion flame of the model mixture of decomposition products of UHMWPE in air counterflow was simulated using the OPPDIFF code from the ChemKin II software package. The simulation results are in good agreement with experimental data on combustion of UHMWPE. Keywords: ultrahigh molecular weight polyethylene, flame structure, counterflow flame, heavy hydrocarbons, modeling. DOI: 10.1134/S0010508216030023
INTRODUCTION The wide use of polymers requires an increased level of their fire safety. One of the most common polymers in the world today is polyethylene. Ultrahigh molecular weight (UHMW) (MW ≈2.5 · 106) polyethylene (UHMWPE) is a promising structural polymer material with unique physical and mechanical properties suitable for use in a variety of fields, including extreme conditions [1]. a
Voevodsky Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences Novosibirsk, 630090 Russia;
[email protected]. b Novosibirsk State University, Novosibirsk, 630090 Russia. c Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia. d Far Eastern Federal University, Vladivostok, 690950 Russia.
To reduce the flammability of UHMWPE, it is very important to know its combustion mechanism. An effective method for this purpose is the study of the combustion of polymers exposed to oxidizer flow perpendicular to the surface of polyethylene (in oxidizer counterflow) [2–5]. One of the major advantages of this method is that the investigated diffusion flame is quasione-dimensional, which allows a numerical description of this flame using existing simple models. Holve and Sawyer [2] have studied the combustion of various polymers, including high molecular weight (MW ≈8 · 105–106 ) polyethylene (HMWPE) in air counterflow. The dependence of the linear burning rate of HMWPE on the oxidizer flow velocity in the range of 75–150 cm/s was obtained. The radiation-corrected maximum temperature of the flame was 1800◦C. In the measurements, a Pt–PtRh (13%) thermocouple was used.
c 2016 by Pleiades Publishing, Ltd. 0010-5082/16/5203-01
1
2 Pitz et al. [3] have studied the flame structure of low-density polyethylene (LDPE, MW ≈5 · 105 ) in counterflow with an oxygen/nitrogen mixture using a quartz microprobe with an orifice diameter of 75 µm. The flow velocity of the oxygen/nitrogen mixture at the nozzle exit of 4.25 cm diameter was 48 cm/s. Samples were prepared by hot pressing (p = 3.2 MPa, T = 180◦ C) in the form of a cylinder 12.7 mm in diameter and 30 mm long. The distance between the nozzle exit and the surface of the LDPE samples was 12.7 mm. Study [2] has shown that the plane of stagnation of the oxidizer and fuel flows is between the polymer surface and the flame plane. The composition of the combustion products was determined by a Hewlett Packard 5751 chromatograph. Scanning of the flame zone was performed at a distance of 2 and 4.25 mm from the axis of the sample. The oxygen concentration was varied (21.2, 23.2, 25.3%). At an oxygen concentration of 2.21%, the distance from the polymer surface to the luminous zone and the width of this zone were 1.36 and 0.7 mm, respectively (at 4.25 mm from the axis of the sample), and the maximum flame temperature was ≈1450◦C. The concentrations of H2 and H2 O were calculated using a mass balance. Only the summary concentration profiles of CO + CH4 and C2 H2 + C2 H4 were measured. The flame had a domelike shape (the flame thickness increased to the center) and actually was not one-dimensional (along the radius of the flame). The mass burning rate at an oxygen concentration of 21.2%, similar to the oxygen concentration in air, is not specified in [3], and at a concentration of 25.3%, it was 6.6 g/(m2 · s) (linear burning rate 7.3 µ/s). The temperature of the surface of the sample at 25.3% O2 was ≈630◦C and the oxygen consumption zone was ≈4 mm. The volume concentration of oxygen was estimated at ≈0.54% by calculating the oxygen concentration gradient (for a flame with 25.3% O2 ) at the surface of the sample. It has been suggested [3] that monomers are formed in the pyrolysis of polyethylene. Estimates have shown that the heat released at the surface in the complete oxidation of ethylene with oxygen is ≈20% of the heat needed for the pyrolysis of the fuel. As the oxygen concentration in the oxidizer flow is decreased and the extinction conditions are reached, the CO/CO2 ratio slightly increases, the oxygen concentration in the luminous flame zone reduces, and the flame temperature and the distance above the polymer surface at which the temperature is maximal decrease. It has been shown [3, 4] that at the flame extinction boundary, the CO/CO2 ratio reaches a critical, which at an oxygen concentration of 21.2% was 0.43. Richard et al. [5] have made a thermogravimetric analysis, studied the pyrolysis and flame structure of high-density polyethylene (HDPE), MW ≈3 · 105) and
Gonchikzhapov et al. low-density polyethylene (LDPE, MW ≈5 · 105). Pyrolysis was carried out in a vertical flow reactor with a gas and liquid sampling system (heavy products) samples. Gas samples were analyzed on a HewlettPackard 5751 chromatograph. The main gaseous products of HDPE pyrolysis at 700◦C are saturated hydrocarbons and hydrocarbons with one double bond: 7 (vol.)% ethylene, 5% methane, 3% propylene, 2% ethane, and 0.5% propane. Experiments on combustion of polymers in oxidizer counterflow have been performed by Richard et al. [5] using a setup similar to that used in [2]. The mass burning rate of LDPE was 4.6 g/(m2 · s), and that of HDPE, 6 g/(m2 · s). The maximum temperature in ˙ et al. [6] have studthe HDPE flame is 1 500◦COgami ied the pyrolysis kinetics of LDPE in counterflow of hot air diluted with water vapor or carbon dioxide. Dilution of the oxidizer was used to study the possibility of reducing the yield of NOx in the HiTAC commercial technology [7]. It has been shown that the kinetic parameters obtained by this method are very different from thermogravimetric analysis data (TGA). The following kinetic parameters of polyethylene decomposition in air are recommended: k0 = 1.5 · 104 and Ea = 40 kJ/mol. The measured surface temperature was 520◦ C. The kinetic parameters of polyethylene decomposition upon dilution with nitrogen, water vapor, or carbon dioxide, respectively, were calculated: k0 = 1.82 · 104, Ea = 50 kJ/mol, k0 = 1.09 · 105, Ea = 55 kJ/mol; k0 = 6.84 · 104, and Ea = 55 kJ/mol. The study of the flame structure of condensed materials requires information on the composition of their pyrolysis product in inert and oxidizing atmospheres. Onwudili et al. [8] have studied LDPE pyrolysis process (MW = 15 000) in a closed batch reactor under an inert atmosphere. It has been found that the gas pyrolysis products of C1 —C4 at temperatures of 400 and 425◦ C are dominated by alkanes. Liquid samples were found to contain C5 —C30 hydrocarbons, which accounted for 92% by weight of the sample. At a temperature of 425◦C they include 46% alkanes, 19% cyclic hydrocarbons with alkynes, 12% alkenes, 12% aromatics, and 2.7% naphthalenes. The proportion of aromatic compounds sharply increases with increasing pyrolysis ˙ a temperature of 400◦ C, the temperature to 500◦ CAt mass ratio of liquid/gas/carbonaceous residue in the products is 89.5/10/0.5%. Increasing the temperature increases the proportion of gaseous and solid products and reduces the proportion of liquid products. Gascoin et al. [9] have studied the effect of the heating rate on the composition of the pyrolysis products of HDPE (MW = 15 000) at low (1 and 100 and K/s) and high (20 000 K/s) heating rates. Decomposition of polyethylene at a heating rate of 100 and 20 000 K/s
Structure of UHMW Polyethylene–Air Counterflow Flame was performed using a CDS 5200 high-pressure pyrolysis reactor. The reactor was a thin-walled quartz tube heated by by a metallic platinum winding. Decomposition at a heating rate of 100 K/s was carried out at a pressure of 1 atm, and at 20 000 K/s, at a pressure of 30 atm. The pyrolysis products at a heating rate of 100 K/s, a pressure of 1 atm, and a temperature of 1000 K contain 24/68/7% alkanes/alkenes/alkadienes. The greatest concentration (in mole fractions) were detected for propylene (5.5%), propane (5%), nonene (9.5%) decene (11.8%), and undecene (6.5%). In flash pyrolysis, the amount of light alkenes was significantly increased: ethylene by 63%, propylene by 40%, and hexene by 20%. It has been shown [9] that the composition of the pyrolysis products strongly depends on the experimental conditions (pressure, temperature, heating rate, surrounding atmosphere). Thus, in the literature there are data on the burning rate, flame structure, and surface temperature for combustion of polyethylene with different molecular weights in air counterflow. In this combustion mode, the main decomposition product is ethylene or similar hydrocarbons. However, in some studies, a large proportion of heavy hydrocarbons has been detected in polyethylene decomposition products. Analysis of the literature shows that the composition of the products of polyethylene pyrolysis and combustion strongly depends on the experimental conditions and the type of polyethylene (HDPE, LDPE, HMWPE, UHMWPE). The flame structure was determined only under the assumption that the decomposition of the polyethylene through the monomer, the presence of heavy products of decomposition does not take into account. The aim of this work is a comprehensive study of UHMWPE–air counterflow flame taking into account heavy decomposition products using mass spectrometry, chromatography, elemental analysis and microthermocouples, including an analysis of the flame structure and measurements of the gas-phase temperature profile. EXPERIMENTAL Cylindrical samples of ultrahigh molecular weight polyethylene (UHMWPE, Institute of Catalysis Siberian Branch, Russian Academy of Sciences, MW ≈2.5 · 106, melting point 142◦ C) were investigated. The samples were prepared by hot pressing of a powder (granule size ≈60 µm) at a temperature of 140◦ C and a pressure of 100 atm. The samples were 14 mm in diameter and 30–40 mm long, and their density was 0.94 g/cm3 . The study was conducted on a specially designed burner with parameters similar to those in [2, 4, 5]. The
3
Fig. 1. Diagram of the experiment.
burner was fitted with a sample moving mechanism and a nozzle of special design to form an flow air directed onto the surface of the polymer sample. The experimental setup is shown schematically in Fig. 1. This burner used two independently operating stepper motors, one of which was responsible for the sample rotation around its axis, and the second for its linear movement along the axis. During combustion, the samples were rotated at a rate of ≈1 Hz in a temperature-controlled (70◦ C) metal cylinder. The rotation provided uniform heating of the sample during its ignition with a red-hot coil. The upper part of the sample (≈4 mm) was isolated from the walls of the metal cylinder by a PTFE ring, which reduced the cooling of the upper molten layer of UHMWPE during combustion. The distance between the nozzle and the sample surface was 14 mm in all experiments. The linear air velocity (under normal conditions) at the nozzle exit 43.9 cm/s (volume consumption 270 cm3 /s) was controlled with an MKS electronic gas flow meter (Type 247, dev-0.3%), which ensured consistency of the air velocity to within ±0.13 cm/s. After ignition of the samples with the heated coil, the flame was stabilized relative to the upper edge of the metal cylinder by moving the sample with a second stepper motor at a fixed speed equal to the burning rate. The sample movement rate was chosen in preliminary experiments. The error of stabilization of the surface of the burning sample was monitored visually using a cathetometer and was not more than 50 µm during the experiment (10–15 min). Gas was sampled by a quartz probe with an orifice diameter of 60 µm, a wall thickness of 140 µm, and an inner angle of 20◦ , which acted as a molecular leak-in. The minimum possible distance from the probe
Gonchikzhapov et al.
4
Fig. 2. Temperature profiles in the neat UHMWPE flame in the presence of the probe (open points) and without it (filled points) (l is the distance from the sample surface).
Fig. 3. Radial air flow velocity profile at a distance of 5 mm from the nozzle exit.
orifice to the sample surface was 700 µm. The position of the probe relative to the sample surface during the experiment was controlled using a three-axis scanning system and a cathetometer with an accuracy of ±10 µm. To minimize the flame perturbation by the probe, sampling was carried out at a distance of ≈5 mm from the axis of the sample, i.e., in the region where edge effects were visually absent. The volumetric flow rate of gas through the probe was 0.5 and 0.24 cm3 /s under normal conditions at 300 and 1400 K, respectively. A sample taken from the flame was delivered to the massspectrometer inlet system through a Teflon tube 1.5 m long and 4 mm in inner diameter at room temperature. Gaseous combustion products were analyzed on a Hiden HPR 60 mass spectrometric system based on a quadrupole mass spectrometer. The first vacuum stage of the system (probe-skimmer region) was modified and pumped with a VMN-500 turbomolecular pump go a pressure of 5 · 10−3 torr in the sampling mode. First experiments showed the accumulation of a significant amount condensed products (in the form of a white deposit, presumably heavy hydrocarbons) on the skimmer, which led to clogging of the skimmer orifice. In subsequent experiments, the sample was delivered into the mass spectrometer through the same tube, but with two fine filters. In this case after a series of experiments, the skimmer was virtually clean, i.e., all condensed materials were in the filter. The ionizing electron energy in the ion source was 70 eV. Temperature was measured with a Pt-PtRh (10%) thermocouple of 50 µm diameter. For gas-phase measurements, the thermocouple was coated with a SiO2 anticatalitic layer; the length of the thermocouple shoul-
ders was 8 mm. The thermocouple was placed at a distance of 350 µm from the probe inlet upstream of the air flow. Measurements in the absence of a probe (Fig. 2) showed that the width of the combustion zone of 4.3 mm and the radiation-corrected maximum temperature of 1380◦C were very close to the results of measurements in the presence of the probe. Thus the probe had no appreciable effect on the thermal structure of the flame. The signal from the thermocouple was measured using a E14-140-M ( L-Card) 14-bit analog-to-digital converter, which recorded DC voltage. The operation and power supply of the E14-140-M voltage converter were controlled from a personal computer through a standard USB interface. The device provides a measurement precision of ±5◦ C. To obtain a one-dimensional (flat) flame, it is necessary to produce counterflows of fuel and oxidizer with a uniform velocity distribution over the cross section (plug flow). Fuel flow from the sample surface can be considered uniform since the surface of the UHMWPE sample during combustion is virtually flat. The onedimensionality of the oxidizer flow was ensured by using a fixed-area convergent Witoszynski nozzle, which provided outlet flow with an U-shaped velocity profile. Airflow uniformity was checked by measuring the radial velocity profile in the absence a flame using an anemometer based on a 10 µm diameter platinum wire pre-calibrated at velocities of 0.1–2 m/s. The measurement error of the flow velocity was 1%. Figure 3 shows the cross-sectional velocity profile of the air flow at 5 mm from the nozzle exit. It can be seen that the air flow velocity at the location of the polymer sample is constant.
Structure of UHMW Polyethylene–Air Counterflow Flame
5
Fig. 4. Photograph of the UHMWPE flame.
The products near the burning surface of UHMWPE were identified by gas-liquid chromatography-mass spectrometry. A gas sample of volume ≈1 cm3 was taken from the blue flame zone at 0.8 mm from the sample surface using a syringe with a metal capillary of 0.7 mm diameter. After sampling, a white deposit appeared on the walls of the syringe—presumably, condensed products of polyethylene pyrolysis. The samples were analyze with anAgilent HP 6890N/5973N chromatography-mass spectrometer using a DB-1 column with a length of 30 µm, an inner diameter of 250 µm, and a coating (siloxane [OSi(CH3 )2 ]n ) thickness of 0.25 µm. Mass spectra were interpreted automatically using the standard software of the chromatography-mass spectrometer. Since the DB-1 column does not separate N2 , CO2 , and CO, a further analysis of the gas sample was performed on a Kristall 2000 chromatograph with zeolite and coal columns. A sample was taken from the flame at a distance of 0.8 mm from the sample surface using a quartz probe with an orifice diameter of 60 µm; the sample volume was 90–100 cm3 . The sample surface was analyzed for C, H, and O on an Eurovector EA 3000 analyzer before and after combustion. The determination error was 0.3 % for C, 0.05 wt.% for H, and 0.5 wt.% for O.
RESULTS AND DISCUSSION UHMWPE–Air Counterflow Flame. Visual Observations Figure 4 shows the UHMWPE–air counterflow flame. It is seen that the probe does not cause notice-
able perturbations of the flame during sampling. The flame has the shape of a disk which is slightly bent at the edges. The distance from the surface of the polyethylene sample to the middle of the luminous zone is 1.7 mm, and the width of the luminous zone is 1.3 mm. During combustion, flow of polyethylene melt droplets from the edge of the sample surface was observed. Decreasing the sample movement speed significantly reduced the flow. However, this decreased the flame diameter, and the flame was rapidly extinguished when the probe was introduced into it. This can explained by the fact that the heat released in the gas phase was not sufficient for pyrolysis of UHMWPE. The total mass burning rate was 14.4 ±1 g/(m2 · s), the mass burning rate minus the part flowing down the sample surface was 9.9 g/(m2 · s), and the linear burning rate was 18 ±2 µm/s. The obtained mass burning rate of UHMWPE is higher than that measured in [3, 5], due to the difference between the molecular weights of the starting polyethylenes. The linear burning rate of UHMWPE is close to that of high molecular weight polyethylene from [2], ≈15 µm/s, obtained by extrapolation to the flow velocity of 43 cm/s. The linear burning rate is calculated from the results of measuring the length of the burned sample. Elemental Analysis of Polyethylene Samples Table 1 shows the results of elemental composition of the starting UHMWPE, melt droplet flow, and the condensed products deposited on the fine filters in the sample delivery line to the mass spectrometer. The H/C ratio is ≈2 only in the starting material. In two other cases, this ratio is lower, indicating partial decomposition of the polymer with the formation of new decomposition products. Oxygen in the melt droplet flow and
Gonchikzhapov et al.
6 Table 1. Results of elemental analysis of the investigated materials Investigated material
C
H
H/C (on the number of atoms)
Initial sample, %
85.6
14.31
2.007
Flow of melt droplets, %
86
13.98
1.95
Deposit on the filter, %
86.2
13.85
1.93
the deposits on the filters was not found. This implies that the thermal expansion on the sample surface does not involve diffusion oxygen [10] or the resulting oxidation products have high volatility.
Fig. 5. Mass of condensed products of UHMWPE pyrolysis deposited on the fine filters of the sampling system (sampling time 600 s).
UHMWPE Flame Structure According to the results of [10], the main products of polyethylene decomposition are propylene (C3 H6 ), butadiene (C4 H6 ), and benzene (C6 H6 ). These species in the flame were identifies from parent peaks of masses 42, 54, and 78, which had the greatest intensities in the primary mass spectra among 40 measured peaks. The concentration of these species in the flame was determined using calibration coefficients. The sampling system did not allow measuring the concentration of H2 O. The H2 O concentration profile was constructed from the CO2 profile measured in the experiment. The ratio of the concentration of H2 O to CO2 was calculated at the temperature maximum point assuming thermodynamic equilibrium at this point during oxidation of the main pyrolysis products (propylene (C3 H6 ), butadiene (C4 H6 ), and benzene (C6 H6 )) by atmospheric oxygen. Concentration ratios for some species at a distance ≈0.8 mm from the sample surface that could not be determined by mass spectrometry were obtained by gas chromatography. In the selected sample, the volume concentrations of CO and CO2 were 3.2 and 9.7% respectively, and, hence, CO2 /CO ≈3. Volume concentrations were also measured for the following species: 0.68% f or H2 , 0.18% f or CH4 , and 0.92% for C2 H4 . In sampling from the flame, a white deposit appears on the fine filters in the sample delivery line to the mass spectrometer, presumably consisting of heavy hydrocarbons formed by thermal decomposition of polyethylene [8]. We measured the dependence of the mass of the condensed material passing through the probe on the distance to the surface of the sample l was mea-
sured. The difference between the masses of the filters before and after sampling from the flame for 10 min was measured. The corresponding dependence is presented in Fig. 5; it was approximated by an exponential function. The concentration of heavy hydrocarbons at a distance l = 1.2 mm from the sample surface is close to zero, which, according to visual observations, coincides with the width of the dark zone of the flame. It can be expected that at a distance of 0.7 mm to 0 (polymer surface), the mass of the condensed products (heavy hydrocarbons) will increased and approach 100% of the products coming from the surface of the polymer. Suppose that a volume of 0.24 cm3 (at T ≈ 1400 K) sampled by the probe for 1 s can be represented as a cube with a side of 0.62 cm. Then the polymer surface area from which UHMWPE pyrolysis products are formed is 0.38 cm2 . In this approximation, the mass of heavy hydrocarbons leaving the sample surface at a mass rate of 9.9 g/(m2 · s) for a sampling time of 600 s will be ≈0.23 g. Extrapolation of the exponential dependence of the mass of heavy hydrocarbons (see Fig. 5) to the sample surface gives a value of ≈0.16 g, close to the estimated value. Thus, the estimate does not contradict the hypothesis that most of the pyrolysis products of UHMWPE under combustion conditions comes from the surface in the form of a mixture of heavy hydrocarbon vapors. According to [8], in polyethylene pyrolysis at a temperature of 500◦C in a closed batch reactor, the mass fraction of heavy hydrocarbons (C5 —C30 ) is 38%. The insufficiently high value as compared with the assumption made above may be associated with pyrolysis of the polymer under isothermal conditions for a long time.
Structure of UHMW Polyethylene–Air Counterflow Flame
7
Fig. 7. The distribution of heavy hydrocarbons (C7 —C25 ) in combustion products taken from the flames at 0.8 mm from the UHMWPE surface according to the number of carbon atoms in the compound.
Composition of Heavy Hydrocarbons near the Surface of the UHMWPE Sample
Fig. 6. Flame structure (a) and mass balance (b) in the UHMWPE–air counterflow flame with water taken into account.
This is consistent with our data on a large amount of heavy hydrocarbons in the polyethylene decomposition products. This is a difference between our results and those of [3, 5], in which only CH1 —C4 hydrocarbons were identified. Accounting for heavy products will provide a more correct flame structure of UHMWPE. Figure 6 shows the experimentally measured structure of the UHMWPE– air counterflow flame excluding heavy decomposition products, and the mass balance. From the material balance, it follows that the ratio H/C ≈1.17 is far from the original ratio in UHMWPE (H/C = 2). It should also be noted that the mole fraction of CO2 increases when approaching the sample surface and does not decrease as would be expected. This indicates that not all of the flammable products in this system are taken into account. The white deposit collected on the fine filters during flame sampling probably consists of heavy hydrocarbons formed by thermal decomposition of polyethylene.
Table 2 shows the composition of heavy hydrocarbons (C7 —C25 ) in products sampled from the flame at a distance of 0.8 mm from the surface of the UHMWPE sample, as determined by gas-liquid chromatographymass spectrometry of gas and condensed sample. These data are averaged over three experiments. Precalibration was carried out for mixtures of alkanes (C7 — C40 ), and the times of their release and the sensitivity coefficients were obtained. Since the gas portion of the sample contained a large amount of nitrogen, the initial portion of the chromatogram was blurred, making it impossible to determine the contents of propylene, butadiene, and benzene in the sample. The products consist of a wide range of linear hydrocarbons from C7 to C25 . Hydrocarbons over C26 were not found. Ethylene and methane were also not detected. Similar results on the composition of the decomposition products are reported in [8, 9]. Figure 7 shows the distribution of heavy hydrocarbons (C7 —C25 ) in the products taken from the flame at 0.8 mm from the UHMWPE sample surface obtained from the data of Table 2. For each n (number of carbon atoms in a compound), the mass fractions of alkane, alkene, and alkadienes are given. Based on the distribution of the products in Fig. 7, the average molecular weight is 258.7 g/mol. Table 2 also presents the data of [9] on the composition of pyrolysis products of low-density polyethylene at 1000 K at a heating rate of 100 K/s. According to [9], in the pyrolysis products of UHMWPE, the alkane(Cn H2n+2 )/alkene(Cn H2n )/alkadiene(Cn H2n−2 )
Gonchikzhapov et al.
8
Table 2. Composition of heavy hydrocarbons (C7 —C25 ) in the products taken at a distance of 0.8 mm (T ≈ 1400 K) from the UHMWPE sample surface Name
Formula
Mass fraction, %
Heptadiene
C7 H12
0
Heptene
C7 H14
1.68
Heptane
C7 H16
0.76
Octadiene
C8 H14
0.23
Octene
C8 H16
1.14
Octane
C8 H18
0.53
Nonadiene
C9 H16
0.38
Nonene
C9 H18
1.91
Nonane
C9 H20
0.61
Decediene
C10 H18
0.37
Triplet (alkadiene/alkene/alkane)
Mass fraction, % [9] —
0/69/31
0.06
12/60/28
0.89
13/66/21
9.52
2.64 10/71/18
Decane
C10 H22
0.66
4.01
Undecediene
C11 H20
0.53
1.7
Undecene
C11 H22
1.76
Undecane
C11 H24
0.88
4.17
Docediene
C12 H20
0.42
1.25
Dodecene
C12 H24
1.25
Dodecane
C12 H26
0.83
Tridecediene
C13 H24
0.33
Tridecene
C13 H26
0.99
Tridecane
C13 H28
0.66
1.45
Tetradecediene
C14 H22
0.41
0.73
Tetradecene
C14 H28
1.22
Tetradecane
C14 H30
0.95
Pentadecediene
C15 H28
0.37
Pentadecene
C15 H30
1.48
Pentadecane
C15 H32
0.92
C16 H30
0 1.43
Hexadecane
C16 H34
2.85
Heptadecediene
C17 H32
1.08
Heptadecene
C17 H34
1.63
Heptadecane
C17 H36
0.87
0/71/29
3.93
2.56
C16 H32
not separated
0
C10 H20
Hexadecene
0/7/93
0.77
Decene
Hexadecediene
Ratio in the triplet [9]
17/56/28
17/50/33
11.8
6.47
2.91
14/64/22
14/52/34
17/40/43
3.08 1.02 17/50/33
16/47/37
1.81
1.52
24/42/34
32/68/0
0
13/53/33
3.06
not separated
0/33/67
2.81
not separated
30/45/24
2.44
not separated
Structure of UHMW Polyethylene–Air Counterflow Flame
9
Table 2 (Continued) Name
Formula
Mass fraction, %
Octadecediene
C18 H34
1.67
Octadecene
C18 H36
8.89
Octadecane
C18 H38
1.11
Nonadecediene
C19 H36
1.97
Nonadecene
C19 H38
3.94
Nonadecane
C19 H40
0
Eicosadiene
C20 H38
1.06
Eicosene
C20 H40
2.11
Eicosane
C20 H42
10.55
Heneicosadiene
C21 H40
0.74
Heneicosene
C21 H42
1.48
Heneicosane
C21 H44
2.95
Docosadiene
C22 H42
0.92
Dococene
C22 H44
1.83
Docosane
C22 H46
7.32
Tricosadiene
C23 H44
1.13
Tricosene
C23 H46
2.25
Tricosane
C23 H48
2.25
Tetracosadiene
C24 H46
0
Tetracosene
C24 H48
2.42
Tetracosane
C24 H50
6.05
Pentacosadiene
C25 H48
2.20
Pentacosene
C25 H50
2.20
Pentacosane
C25 H52
3.30
Triplet (alkadiene/alkene/alkane)
Mass fraction, % [9]
Ratio in the triplet [9]
14/76/10
2.43
not separated
33/67/0
2.24
not separated
8/15/77
3.07
14/29/57
3.46
not separated
9/18/73
2.82
not separated
20/40/40
2.12
not separated
0/29/71
0.16
not separated
29/29/43
0.3
not separated
triplet is observed. The total mass ratio in it is 42/45/13%. This corresponds to the mechanism of intermolecular transfer of H radicals in polyethylene chain scission [11, 12], which predicts the formation of products with high molecular weights (C18 —C25 ). It is evident from Table 2 that the relationship between the hydrocarbons in the triplet changes with increasing number of carbon atoms in the compound. In the range from C7 to C17 , the triplet is dominated by alkenes, but their concentration decreases with increasing number of C atoms, whereas the alkane concentration is gradually increasing. As a result, from C18 to C25 , the triplet is dominated by alkanes. A comparison of our data and [9] shows that in distribution of hydrocarbons in [9], the maximum is a at C10 —C11 , and in our data (see Fig. 8), it is at C18 —C20 . The shift of
the maximum can be explained by the fact that the pyrolysis temperature in [9] was higher than the surface temperature of the sample in our study. However, the ratio for C9 —C14 hydrocarbons in the triplet is in good agreement with our data. It is worth noting that the results of chromatographic analysis of the dissolved white deposit from the skimmer and the melt drops falling from the sample in hexane gave a similar molecular weight distribution of hydrocarbons in the flame at a distance of 0.8 mm from the sample surface. Hence it can be concluded that the composition of the gaseous products of polyethylene destruction originating from the sample surface is nearly identical to the composition measured in the gas phase.
Gonchikzhapov et al.
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Fig. 8. Structure of the UHMWPE–air counterflow flame: asterisk denotes gas chromatography data for samples taken at a distance of 0.8 mm.
UHMWPE Flame Structure Taking into Account Heavy Products of Combustion Figure 8a shows a diagram of the experimentally measured structure of the UHMWPE–air counterflow flame taking into account C7 C25 heavy hydrocarbons in the form of a single hypothetical species with a weight average molecular weight of 258 g/mol. The concentration profiles of light combustible products of UHMWPE pyrolysis having a low concentration are shown in a separate graph below. Temperature measurement at a distance of 0–0.7 mm was performed without probe. The maximum temperature in the flame zone was 1380◦C. The mole fraction profile of heavy hydrocarbons was obtained by the following conversion of the dependence shown in Fig. 5. The mass of condensed products was
converted into moles by dividing it by the weight average molecular weight. The mole fraction of gaseous products was calculated using the measured gas flow rate through the probe. When the mole fraction profile of heavy products was added to the flame structure obtained earlier without them (see Fig. 6a), the mole fractions were renormalized. The proportion of condensed products increases when approaching the sample surface. The zone of consumption of combustible decomposition products is 1.7–1.8 mm. The oxygen concentration falls to zero at a distance of 1.2 mm from the sample surface. The measurement error of the oxygen concentration is ±0.3%. Carbon dioxide is present up to a distance of 0.7 mm from the sample surface. The ratio of the maximum
Structure of UHMW Polyethylene–Air Counterflow Flame
11
Table 3. Product composition and temperature of the UHMWPE flame at a distance 0.8 mm from the surface of the sample Source
T , ◦C
N2
O2
CO2
CO
H2 O
H2
CH4
C2 H 4
C2 H 6
C3 H 6
C4 H 6
C6 H 6
X7 —C25
This work
1 180
0.58
0
0.14
0.04
0.2
0.009
0.002
0.012
—
0.003
0.006
0.0001
0.03
[3]
1 270
0.72
0.002
0.11
0.05
0.14
0.001
0.004
0.05∗
0.012
—
—
—
—
[5]
—
0.72
0.02
0.12
0.03
0.02
—
—
0.012
—
0.003
—
—
—
∗ Sum
of C2 H2 and C2 H4 (was not separated in the experiments).
concentrations of CO2 /H2 O is ≈1.33, which is close to the stoichiometric for the flame of the combustible mixture (0.35 · C3 H6 + 0.55 · C4H6 + 0.1 · C6 H6 ). It should be noted that the mole fractions of H2 O and CO2 change only slightly when approaching the sample surface. Accounting for heavy hydrocarbons had little effect on the mole fraction profiles of the species; however, it led to a marked decrease in the mass fractions of CO2 and of H2 O (Fig. 8b) when approaching the sample surface. The mass fraction of heavy hydrocarbons C7 —C25 at a distance of 0.7 mm from the surface was about 0.2. The large concentration gradient of heavy hydrocarbons suggests their high concentrations (up to 100%) near the polymer surface. Flame structure analysis showed that during conversions of heavy hydrocarbons into lighter ones (e.g., butadiene, propylene, and benzene), a significant increase in their concentrations as intermediates was not observed, but the concentrations of the final products combustion (CO2 and H2 O) were high. This may be due to the fact that the flow stagnation plane is closer to the fuel surface than the flame plane. Therefore, the products formed in the flame front diffuse to the sample surface. Below, it will be shown that the flow stagnation plane is between the flame front and the polymer surface. Thus, the presence of the final combustion products CO2 and H2 O near the sample surface is due to the diffusion of CO2 and H2 O to the sample surface, rather than to the chemical reaction on the sample surface. The obtained estimate of heavy hydrocarbons is a lower bound since the experiments have shown that even in the presence of filters in the sample delivery line to the mass spectrometer, a white deposit nevertheless appears on the skimmer, though in much smaller amounts than without the filters. This implies that the filters traps not all of the heavy decomposition products. In the fuel pyrolysis products and the products of their combustion, we determined 11 species, including water (determined by the mass balance) and a hypothetical species with a weight average molecular weight, which describes 50 hydrocarbons.
Table 3 shows data of the present work and [3, 5] on the composition of the products and the temperature of the UHMWPE flame at 0.8 mm from the sample surface. Concentrations of N2 , CH4 , and C2 H4 were measured by gas chromatography. The concentrations of CO2 , CO, H2 , CH4 , and C2 H4 obtained in the present work are close to those measured in [3]. However, in [3, 5], only light C1 —C2 hydrocarbons were detected. The CO2 and CO concentration profiles recorded by gas chromatography are very close to those measured by mass spectrometry at a distance of 0.8 mm. This confirms the accuracy of the measurement of the CO concentration profile in the flame. Increasing the molecular weight of the UHMWPE compared to the polyethylene described in the literature led to the appearance of new hydrocarbons C4 H6 and C6 H6 in the pyrolysis products. C3 H6 was detected in [5]. In [3], the C2 H4 concentration was not measured separately, but only in combination with C2 H2 . The C2 H4 concentration obtained in [5] is close to that measured in the present work. The dependence of the material balance on the distance to the sample surface presented in Fig. 9 shows that the ratio H/C ≈ 2, taking into account heavy decomposition products, holds in the combustion zone at a distance of 0.7–3 mm to the sample surface. When approaching the sample surface, the N/O ratio decreases and reaches a constant value in the flame zone. Simulation of Structure of the UHMWPE–Air Counterflow Flame To determine the reasons for the large number of combustion products (CO2 and H2 O) in the vicinity (at a distance of 0.7 mm) of the polymer surface, we performed held simulation of the UHMWPE–air counterflow flame which qualitatively described the real structure of the flame polymer. The mixture of the decomposition products of UHMWPE burning in air counterflow was simulated by a mixture of C3 H6 , C4 H6 , and C6 H6 combustible gases. The width of the zone in candle-like burning of UHMWPE [10] in a quiescent gas exceeds
Gonchikzhapov et al.
12
Fig. 9. Dependence of the element ratio in the UHMWPE combustion products at a distance to the sample surface taking into account changes in the number of moles (normalized to the maximum mass of 38.6 g).
the width of the zone of polymer combustion in forced air counterflow. However, the general mechanism of diffusion combustion of UHMWPE is the same: polymer pyrolysis followed by oxidation of the resulting products. In both types of diffusion flame near the polymer surface, the oxygen concentration is extremely low, which suggests the absence of (or a very small) contribution of oxidative destruction to the total pyrolysis process of the polymer during its combustion. The diffusion flame was simulated using the OPPDIFF code [13] from the ChemKin II software package [14]. The calculations were performed for the USC ver.2.0 (2007) mechanism [15]. The hydrocarbon proportions of 35% propylene, 55% butadiene, and 10% benzene were selected as the boundary condition on the basis of the experimental results [10]. The mass flow rate of the fuel equal to 9.9 g/(m2 · s), and the air velocity of 43.9 cm/s also correspond to the conditions of these experiments. The initial temperature of the mixture of hydrocarbons was 550◦ C, and the temperature was 20◦ C. The distance between the nozzles was 14 mm. Figure 10a shows the experimental data for the UHMWPE flame and the simulation data for the (C3 H6 , the C4 H6 , C6 H6 )/air flame. The calculated oxygen concentration and temperature profiles are in good agreement with the experimental data. The width of the zone for CO2 and H2 O is also in good agreement with the experiments, but the calculated maximum concentrations of CO2 and H2 O are about two times lower because heavy hydrocarbons were not taken into account in the calculations. According to the simulation results, the concentrations of nitrogen, carbon dioxide, and water do not fall to zero at the very surface of the sample. The presence
Fig. 10. Species mole fractions versus distance between the nozzles (a). Calculated dependence of the axial velocity of the oxidizer and fuel flows on the distance between the nozzles (b): points are the experimental data for the UHMWPE flame, curves are the simulations of the (C3 H6 , C4 H6 , and C6 H6 )/air flame.
of significant concentrations of final combustion products near the sample surface was not clear. However, the simulation results (Fig. 10b) show that the collision plane of the fuel and oxidizer counterflows does not coincide with the flame front, in which maximum temperature is reached, as is usually observed in counterflow flames of gases. The collision plane is much closer (≈400 µm) to the sample surface than the plane of the flame front at the temperature maximum point (≈1.5 mm from the sample surface), which the final combustion products are formed. Thus, after the passage of the flame, the resulting CO2 and H2 O continue to move to the sample surface to the collision plane, and then, when their axial velocity becomes zero, they diffuse to the sample surface, which is the cause of their high concentration at the sample surface.
Structure of UHMW Polyethylene–Air Counterflow Flame CONCLUSIONS The structure of the UHMW polyethylene–air counterflow diffusion flame was determined taking into account heavy products. It is shown that polymer decomposition on the sample surface does not involve oxygen since its concentration at the surface is close to zero. Numerical simulation of the flame structure of a model mixture of UHMWPE pyrolysis products (C3 H6 , C4 H6 , C6 H6 ) in air counterflow was performed using the USC 2.0 detailed mechanism. Satisfactory agreement between the experimental and calculated data on the flame structure was obtained, indicating that propylene, butadiene, and benzene are the most important intermediate products of UHMWPE pyrolysis. The composition of the products of heavy hydrocarbons at a distance of 0.8 mm from the sample surface was analyzed. The method and the results can be further used to study the pyrolysis kinetics of polyethylene and other polymers under combustion conditions and to study combustion mechanism of polymers, as well as the mechanism of action of flame retardants. We thank V. A. Zakharov for providing UHMWPE samples and M. M. Katasonov for help in anemometric measurements of air flow velocity profiles. This work was supported by the Ministry of Education and Science of the Russian Federation (Grant No. 14.Y26.31.0003).
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