- Email: [email protected]
Quantitative imaging of potassium release from single burning pulverized biomass char particles
Fuel 264 (2020) 116866
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Quantitative imaging of potassium release from single burning pulverized biomass char particles
T
⁎
Wubin Wenga, , Shen Lia, Mário Costab, Zhongshan Lia a b
Division of Combustion Physics, Lund University, P.O. Box 118, S 221 00 Lund, Sweden IDMEC, Mechanical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser-induced photofragmentation fluorescence Potassium Biomass chars Combustion UV absorption spectroscopy
The release of potassium from single burning pulverized wheat straw char particles was quantitatively measured using laser-induced photofragmentation fluorescence (LIPF). The char particles were prepared in a drop tube furnace at 1000 °C from wheat straw particles with sizes in the range 224–250 µm. Subsequently, the char particles were injected upward into a hot flue gas flow produced by a premixed CH4/air flame anchored on a McKenna burner. The flue gas had a mean temperature of 1580 K and a mean O2 concentration of 6.5 vol%. The 193 nm laser beam from an ArF Excimer laser was formed into a collimated laser sheet to photodissociate potassium hydroxide (KOH) and potassium chloride (KCl) around the burning char particles, and the signal of the produced fluorescence was captured by a camera. The measurements were conducted for char particles during residence times in the flue gas between 10 and 70 ms. Quantitative data was obtained from a direct calibration of the LIPF signal in hot gas products doped with known amounts of KOH and KCl. The maximum potassium concentration measured surrounding the burning char particles was over 40 ppm. During the oxidation period until 70 ms, the measured potassium release rate remained almost constant at around 0.5 µg/s, with more than 60% of the potassium being released in the form of KOH. The results indicate that the LIPF imaging method can be used to study the potassium release from burning biomass fuels.
1. Introduction Biomass fuel utilization is continually increasing in the global energy consumption aiming at the CO2 emission reduction. Many biomass fuels contain high amounts of potassium, which can be released in different thermal conversion processes. Potassium increases the risk of slagging and corrosion in combustion equipment. A number of fundamental studies were performed to measure and model potassium release processes from single biomass particles [1–4]. However, most measurements were conducted using particles with sizes of several millimeters. In contrast, studies using pulverized biomass, which are also commonly used in combustion and gasification processes, are rarely reported. Most of the previous studies on the combustion behaviors of single pulverized biomass or char particles were focused on obtaining parameters such as ignition delay times and particle temperatures [5–12]. Recently, Weng et al. [11,12] investigated the potassium release from single burning pulverized biomass particles using the chemiluminescence of K*, which allowed for a qualitative understanding of its release trend. Different optical techniques have been developed for quantitative
⁎
measurement of different potassium species released during the burning of biomass particles. Laser-induced breakdown spectroscopy (LIBS) can provide a quantitative point measurement of the total potassium [1,4,13,14]. Tunable diode laser absorption spectroscopy (TDLAS) [15,16] and laser-induced fluorescence (LIF) [17] have been applied to the measurement of atomic potassium above burning biomass pellets. Collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) was developed by Sorvajärvi et al. [18] to measure quantitatively and simultaneously the release of K atoms, KCl and KOH from burning biomass fuels. In the present study, we focused on the quantitative measurement of potassium released from burning pulverized biomass char particles. This is motivated by the fact that over 80% of the potassium present in the raw biomass can remain in the char, despite the char representing generally not more than ~25% of the initial mass [2]. In this study, the challenges mainly came from the small size (~200 µm) of the pulverized particles and the movement in the combustion environment. Laserinduced photofragmentation fluorescence (LIPF) imaging was adopted here to measure the potassium release. Compared with the other aforementioned techniques, LIPF has the advantage of capturing the
Corresponding author. E-mail address: [email protected] (W. Weng).
https://doi.org/10.1016/j.fuel.2019.116866 Received 18 September 2019; Received in revised form 12 November 2019; Accepted 12 December 2019 Available online 20 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 264 (2020) 116866
W. Weng, et al.
As shown in Fig. 1, for the LIPF measurements, an ArF Excimer laser (Compex 102, Lambda Physik) was used to provide the 193 nm laser pulses with a pulse duration of 25 ns. The laser beam was transformed into a vertical laser sheet with a height of around 30 mm and a thickness of around 1 mm through a telescope system containing two cylindrical lenses of focal lengths f = 500 mm and f = -75 mm, respectively. The laser sheet passed through the hot gas products, right above the central hole of the burner. It was placed at different heights to photodissociate KOH/KCl released from the burning wheat straw char particles at different residence times. The photofragmentation fluorescence of excited potassium atom from KOH/KCl was captured by an ICCD camera (PI-MAX III, Princeton, 1024 × 1024 pixels) with an f = 50 mm objective (Nikkor f/1.4). The exposure time was set to 400 ns. An interference filter centered at the wavelength of 766 nm with a FWHM of 10 nm (Edmund Optics) was used to detect the fluorescence signal while suppressing the background radiation and the fluorescence from other species, such as sodium atoms. A synchronization system was used to manage the timing among each in-coming single particle, the laser pulse and the ICCD camera gate. In this synchronization system, a 532 nm continuous-wave laser (200 mW) was adopted to provide a laser beam having a diameter of around 1 mm. This laser passed through the hot gas flow close to the exit of the central tube of the burner. Scattering was detected by a photo diode (PDA100A2, Thorlabs) as the char particle moved through the laser beam. The photo diode triggered a pulse generator (DG535, Stanford Research Systems), which provided trigger signals for the ICCD camera and the Excimer laser. Fig. 1 shows a typical single shot image of the photofragmentation fluorescence around a wheat straw char particle at a residence time of 60 ms in the hot gas products. The particle was surrounded by an axisymmetric potassium cloud with a size close to 20 mm, which was much larger than the char particles with an initial size of ~0.2 mm. The wheat straw char particles were prepared in a drop tube furnace at 1000 °C. The drop tube furnace was fed with pulverized wheat straw particles, sieved to sizes in the range 224–250 µm, as measured by the Malvern 2600 Particle Size Analyzer, and N2. The residence time of the particles in the drop tube furnace was around 2 s. The shape of the resulting wheat straw char particles is shown in Fig. 1, and the properties of the raw wheat straw and respective char particles are given in Table 1. The proximate and ultimate analysis of the raw wheat straw and wheat straw char were carried out following the procedures specified in the standards ASTM-E-870, EN 14,918 and EN 14775. Each sample was analysed three times to assess the repeatability of the data,
instantaneous spatial distribution of KCl and KOH, which are the dominant species formed during the char oxidation period [18]. Leffler et al. [19,20] have applied LIPF for two-dimensional quantitative measurements of KCl/KOH distribution in flames. To obtain quantitative data, critical parameters, such as the absorption cross-sections of KOH and KCl at 193 nm, the distribution on different excited states of the dissociated K atoms, the quenching rate of the excited K atoms, and the signal collection efficiency have to be accurately determined. This can introduce substantial uncertainties in the measured data. Here, an in-situ calibration process was introduced to establish the correlation between the photofragmentation fluorescence signal and the concentration of KOH/KCl. The calibration was conducted in hot laminar gas flows with gas-phase KOH and KCl provided by a home-made burner, a multi-jet burner [21], with a homogenous distribution for temperatures ranging from 1000 to 2000 K. The concentrations of KOH and KCl were monitored using UV-absorption spectroscopy [22,23]. Based on the calibration, the KOH/KCl distribution surrounding the burning char particles were retrieved, and, indeed, the results revealed the potassium release process during the char oxidation period. The wheat straw char particles investigated here were prepared in a drop tube furnace at 1000 °C, with sizes of the original straw particles in the range 224–250 µm. To the best of our knowledge, this work presents the first attempt to quantify the potassium release in-situ from single burning pulverized biomass char particles. 2. Materials and methods Fig. 1 shows a schematic of the experimental setup used for the measurement of potassium released from single burning wheat straw char particles using LIPF imaging. The combustion system consists of a McKenna flat flame burner, a biomass feeding unit and a CH4/air supply system, which has been described in detail elsewhere [8,11]. A flat premixed CH4/air/O2 flame anchored on the surface of the McKenna burner with a diameter of 60 mm produced a laminar hot flue gas flow. The CH4/air/O2 supply system was controlled by mass flow controllers (Bronkhorst). The wheat straw char particles were stored in a 10 mL syringe and were fed into a N2 stream with a flow rate of 0.05 SL/min, and transported into the hot flue gas flow above the burner upwards through a 1.5 mm diameter tube with the opening located on the center of the burner. The hot flue gas flows were confined by a quartz tube with an inner diameter of 70 mm, height of 500 mm, and thickness of 2 mm. This tube was used to avoid the entrainment of cold ambient air, while providing optical access for the laser-based system.
Fig. 1. Experimental setup for the measurement of potassium released from single burning wheat straw char particles using the LIPF imaging system. Top-right corner: typical single shot LIPF image; bottom-right corner: char particles. 2
Fuel 264 (2020) 116866
W. Weng, et al.
tubes, with an inner diameter of 1.6 mm, to generate premixed flames, attached on each of the jet tubes, see Fig. 2(c). Each jet tube was surrounded evenly by the co-flow with a mixture of N2/air supplied by the co-flow chamber. Thus, after the mixing between the co-flow and the products of the premixed flames, a stream of homogenous hot gas products was established at the burner exit, with a size of 85 × 47 mm, and used for signal calibration. During the calibration, potassium was introduced into the hot gas flow by seeding potassium carbonate (K2CO3). A K2CO3 water solution was prepared in 0.5 mol/L, atomized by an ultrasonic fogger and transported into the CH4/air/O2 mixture using an air stream with a flow of 0.5 SL/min. As the small droplets of K2CO3 fog passed through the premixed flames above the jet tubes, K2CO3 was converted into KOH in the hot gas products. Chloroform (CHCl3) was used to introduce chlorine into the hot gas products. Then, KCl could be generated with potassium seeding. Liquid phase CHCl3 was stored in a bubbler with a temperature of 10 °C using a chilled bath (PolySicence). A N2 stream was used to carry the CHCl3 vapor after bubbling through the CHCl3 liquid. The concentration of the Cl seeded into the hot gas products was around 100 ppm, which was sufficient to convert all KOH in the hot gas products into KCl, with a potassium concentration of around 20 ppm [24]. Hence, the pure KOH was provided through seeding K2CO3, while the pure KCl was investigated by introducing additional 100 ppm of Cl. Table 2 summarizes the gas composition used in all flame cases of the multi-jet burner. The gases were controlled by mass flow controllers (Bronkhorst). The temperature of the hot gas products in each flame condition were measured using two-line atomic fluorescence thermometry with indium atoms with an accuracy of ~2.7%, as reported by Borggren et al. [25]. The temperature varied between 1120 K and 1750 K with an equivalence ratio around 0.7. During the calibration, the McKenna burner used in the LIPF setup (cf. Fig. 1) was replaced by the multi-jet burner – the layout is shown in Fig. 2(b). The signal of the fluorescence captured by the ICCD camera is shown in Fig. 3(a) with the same amounts of KOH or KCl seeded in the hot gas products at 1260 K. It can be seen that both the KOH and KCl
Table 1 Properties of the raw wheat straw and wheat straw char. Parameter
Raw wheat straw
Wheat straw char
Proximate analysis (wt.%, as received) Volatile matter Fixed carbon Moisture Ash
64.9 12.4 8.0 14.7
22.9 25.2 1.1 50.8
Ultimate analysis (wt.%, dry ash free) Carbon Hydrogen Nitrogen Sulfur Oxygen
41.1 5.3 0.7 < 0.02 52.6
77.7 1.5 0.9 < 0.02 19.9
which was found to be appropriate. The premixed flat flame anchored on the McKenna burner was fueled by a mixture of CH4 (1.3 SL/min), air (13.52 SL/min) and O2 (0.78 SL/min), with an equivalence ratio of 0.72. The mean temperature of the hot gas products from the flame, measured using a B-type thermocouple (OMEGA), was around 1580 K. The thermocouple junction bead, with a diameter of ~1.1 mm, was placed ~2 cm above the burner axis. The measured temperatures were corrected for radiation losses following the heat transfer theory [21], which yield an estimated uncertainty of ~70 K, in accordance with the temperatures measured using two-line atomic fluorescence thermometry in the work of Weng et al. [21]. The mean velocity of the hot gas products and its mean O2 concentration were around 0.6 m/s and 6.5 vol%, respectively. The LIPF signal from KOH/KCl was calibrated through a homemade burner, a multi-jet burner, which provided homogenous hot gas products containing known amounts of KOH and KCl. The multi-jet burner is described in detail elsewhere [21]. The basic structure of the burner is presented in Fig. 2(a). It consists of two chambers, namely the jet chamber and the co-flow chamber. The premixed CH4/air/O2 flow entered the jet chamber and was distributed evenly through 181 jet
Fig. 2. Schematics of the multi-jet burner (a), and laser setup (b), and a photograph of a typical flame stabilized on the burner (c). 3
Fuel 264 (2020) 116866
W. Weng, et al.
section spectrum of KOH and KCl provided by Weng et al. [23] with a concentration of 17 ppm based on the Beer-Lambert law. Through this process, the concentrations of KOH and KCl in the hot gas products can be determined with an uncertainty of ~5% [26]. In the hot gas products environment seeded with KOH and KCl, the concentration of K atoms should be considered even though it is negligible comparing to that of KOH and KCl. The K atoms can have strong absorption of the fluorescence produced by the photofragmentation process. This has also been discussed by He et al. [27] and Leffler et al. [19,20]. The correction of the obtained LIPF signal was needed. Hence, in the present study, the concentration of K atoms was measured together with the concentration of KOH and KCl. A TDLAS system with a 769.9 nm continuous-wave laser was used, as discussed by Weng et al. [16]. The laser had a power of about 3 mW and a beam size of about 1 mm2, and was provided by an external cavity diode laser (Toptica, DL100). The laser was controlled by an analog control package, which contained a temperature control module (DTC 110), a current control module (DCC 110), and a scan module (SC 110). It can scan the wavelength with a range over 35 GHz with a repetition rate of 100 Hz. After the passage of the hot gas products region containing K atoms, the laser light was absorbed with a pass length of 8.5 cm. Two photo diodes (PDA100A, Thorlabs) were used to monitor the power of the laser before and after the passage. Using the Beer-Lambert law, the concentration of K atoms was obtained, as described by Weng et al. [16], with an uncertainty of ~5% [26].
Table 2 Flame conditions. Gas flow rate (SL/min)
Flame case
Jet-flow
T1 T2 T3 T4 T5
Gas products temperature T (K)
Co-flow
CH4
Air
O2
N2
Air
2.66 2.47 2.28 2.09 1.71
16.88 11.77 11.43 10.44 8.45
1.89 2.58 2.26 2.07 1.69
10.83 18.97 22.69 26.50 26.92
7.74 8.90 9.83 10.66 10.25
1750 1550 1390 1260 1120
3. Results and discussion Following Leffler et al. [19], the fluorescence signal can be expressed as,
S= R·V ·Afi /(Afi + Q )·Ab·N ∗
(1)
where R is the efficiency of the signal collection and detection, V is the volume of the probe volume, Afi is the Einstein coefficient for spontaneous emission given as 3.8·107 s−1 [28], Q is the collisional quenching rate estimated to be 4.1·108 s−1 [19], Ab is the fluorescence losses due to the self-absorption by K atoms, and N ∗ is the potassium atoms in the excited state, which can be obtained from [19],
N ∗ = N ·η ·(1 − exp((−σE )/(hνA)))
(2)
where N is the number density of KOH or KCl, η is the yield of the excited fragmented K atoms, σ is the absorption cross-section of KOH or KCl, E and v are the laser energy and frequency, respectively, and A is the cross section area of the probe volume. From Eqs. (1) and (2), it can be seen that the signal might be strongly influenced by the power of the laser. Hence, in the first measurements, the power of the laser was varied to evaluate the correlation between the photofragmentation fluorescence signal and the laser fluence. Constant amounts of KOH and KCl were introduced into the hot gas products at 1260 K. The results (cf. Fig. 4a) show that the signal increased with the laser fluence, but the increase rate became smaller at higher laser fluence for both KOH and KCl. It can also be found that KCl produced stronger signal than KOH. The difference in the fluorescence signal between the KOH and KCl cases was attributed to the difference in the absorption cross section σ and yield of excited K atom η of KOH and KCl. The calculated photodissociate K* fraction is shown in Fig. 4(b). Since the Gaussian distribution of the laser was not considered in the calculation, KCl could reach a complete dissociation with a smaller fluence than that shown by the measurements. The laser used in the following measurements had a pulse energy of 15 mJ, i.e. 75 mJ/cm2 laser fluence, with a standard deviation of ~4%. The correlation between the signal of the photofragmentation fluorescence and the potassium concentration is essential for the calibration. To account for the effect of the absorption by K atoms present in the hot flow on the fluorescence signal, the concentration of K atoms in the hot gas products was measured using the TDLAS system
Fig. 3. Images of the LIPF signal with same amount of KOH and KCl in the hot gas products at 1260 K (a), and typical absorbance obtained in the UV absorption spectroscopy system together with the fitting curve based on the spectrally resolved absorption cross-section of KOH and KCl [23] (b).
were evenly distributed in the hot gas products, and that more fluorescence signal was produced for the KCl case than for the KOH case. To calibrate the LIPF signal, the concentrations of KOH and KCl in the hot gas products were measured using UV-absorption spectroscopy. Details of this technique can be found in Weng et al. [22]. A deuterium lamp was used to provide the broadband UV light, which was collimated and passed through the hot gas products. Five UV-enhanced aluminum mirrors were used to guide the light to pass above the burner six times to have a path length of 522 mm. After the passage, the light was collected by a spectrometer (Ocean optics 2000+). The absorption spectrum of KOH/KCl was obtained after the division between the intensity of the light passing through the hot gas products with potassium seeding and the one without potassium seeding. A natural logarithm of the ratios was determined as the absorbance. A typical absorbance is shown in Fig. 3(b), which can be well fitted by the absorption cross4
Fuel 264 (2020) 116866
W. Weng, et al.
simultaneously with the measurement of the concentration of KOH and KCl using UV-absorption spectroscopy. The measured concentration of K atoms as a function of the total amount of KOH/KCl seeded in the hot gas products is shown in Fig. 5(a), which increase almost linearly with the concentration of the total potassium. The KCl case produced much less K atoms than the KOH case. A similar characteristic has been reported for sodium by Schofield. [24] The self-absorption rate was retrieved using the Beer-Lambert law, as the concentration of K atoms, and it also increased with the total potassium concentration. The selfabsorption rate was used to correct the raw signal to obtain the true value of the fluorescence emitted from the probe volume. The fluorescence signal after the correction is shown in Fig. 5(b). The correlations between the fluorescence signal and the concentration of KOH and KCl were fitted as follows,
N = 1.437·1015·S 2 + 1.420·1017·S
(3)
for KOH, and
N = 2.596·1014 ·S 2 + 1.583·1016·S
(4)
for KCl. The signal after the correction did not increase linearly with the concentration of potassium for both KCl and KOH. Here, another selfabsorption, occurring in the probe volume, should be considered. The exited K atoms were produced by the photodissociation of KOH and KCl, which decayed to the ground state quickly through spontaneous transition and collision quenching. In this process, part of the spontaneous fluorescence emission was self-absorbed by the atoms already on the ground state. Combing the decay process and the absorption process that occurred in the probe volume using the Beer-Lambert law, a selfabsorption rate with a value up to 60% was obtained in the case with 20 ppm potassium seeding. After the correction, a good linear correlation between the fluorescence signal and the concentration of the potassium was built, as follows,
S = 5.286·10−18·N Fig. 4. Signal of the LIPF (a) and simulated photodissociated K* fraction (b) of KOH and KCl as a function of laser fluence.
(5)
for KOH, and
S = 1.416·10−17·N
(6)
Fig. 5. Concentration of K atoms in the hot gas products and the absorption of LIPF by K atoms as a function of KOH/KCl seeding (a) and environment temperature with the concentration of KOH and KCl fixed at around 1.0·1020 m−3 (c). Detected LIPF signal after self-absorption correction as a function of KOH/KCl seeding (b) and environment temperature (d). 5
Fuel 264 (2020) 116866
W. Weng, et al.
Fig. 6. Temporal image sequences of the average LIPF signal around a single burning pulverized wheat straw char particle in the hot gas products without Cl seeding (a) and with 320 ppm Cl seeding (b). Average of spatially integrated intensities of the LIPF signal versus residence time (c).
Fig. 7. Average mass of potassium released from burning wheat straw char particles and their average thermal emission signal and particle temperature as a function of the particle residence time in the hot gas products. (a) Concentration distribution of potassium surrounding the burning particle for a residence time of 60 ms. (b) Measured potassium release trend and its linear fitting. (c) Sequences of instantaneous image of a typical burning char particle recorded with the ICCD camera equipped with the 652 nm band-pass filter. (d) Average emission signal and particle temperature trend.
time. At 20 and 30 ms, they had the peak signal close to the particles, but at 60 and 70 ms, the signal at the same position almost dropped to zero. As 320 ppm of Cl was added into the hot gas products, all the potassium species in the potassium cloud was presumably converted into KCl [24], and its effect on the potassium release process was negligible since above 800 ℃ the biomass fuels release Cl fully [29]. Fig. 6(b) shows the photofragmentation fluorescence of KCl surrounding the char particles. They had the same signal area as the case without Cl seeding; however, the signal was stronger because more fluorescence was produced from KCl than from KOH, as described above. In addition, the dark holes observed in the case without Cl seeding did not appear anymore in the case with Cl seeding. It was considered that during the oxidation of the char particles, the potassium was released as potassium atoms at the very beginning [30], but they could be transformed into KCl very quickly as sufficient Cl existed. The average of the spatially integrated signals in Fig. 6(a) and (b) is presented in Fig. 6(c). For both cases, the signals increase almost linearly with the residence time. This indicates that potassium is released continuously from burning char particles during its oxidation. The signal from the case with Cl seeding was almost twice the case without Cl seeding. Since the fluorescence yield of KOH was about 37% of that of KCl, the results indicate that over 60% of potassium surrounding the
for KCl. According to the ratio between the slope of these two equations, it was determined that the fluorescence produced from KOH was about 37% of the one from the same amount of KCl. In addition, the effect of the gas temperature on the fluorescence signal was also investigated. In the study, the concentrations of KOH and KCl were fixed at around 1.0·1020 m−3. The concentration of K atoms was measured and is shown in Fig. 5(c), which shows significant increase with the temperature; the self-absorption by the K atoms in the hot gas products also increased up to 80% for KOH at 1750 K. However, after the self-absorption correction, the signal of fluorescence from the probe volume was almost independent of the gas temperature for both KOH and KCl, as shown in Fig. 5(d). Using the LIPF imaging technique with the calibration procedure, the distribution of potassium surrounding the single burning pulverized wheat straw char particles was quantitatively measured. Each measurement was conducted for 100 particles. The sequences of the images of the average fluorescence signal is shown in Fig. 6(a) as a function of the residence time in the hot gas products. The signal was not corrected with the self-absorption due to the short path length in the potassium cloud. The effect of the self-absorption on the calculation of the total amount of potassium was estimated to be less than 4%. Due to the free diffusion of potassium, the signal gradually decreased from the center of the images, and its area had a continuous increase with the residence 6
Fuel 264 (2020) 116866
W. Weng, et al.
4. Conclusions
burning char particles, originally in the form of KOH, was converted into KCl with Cl seeding. Using the correlations between the fluorescence signal and the concentrations of KCl and KOH, expressed by Eqs. (3) and (4), the LIPF signal around the particles in Fig. 6(a) and (b) was converted correspondingly into the distribution of the potassium concentration. In Fig. 7(a), a typical image with the average concentration distribution of potassium around a burning char particle for a residence time of 60 ms is shown. Using the results of the concentration distribution and considering that most potassium clouds have an axisymmetric structure, as shown in Fig. 7(a), regardless of the char particles’ shape, the total amount of potassium released from the char particles as a function of the residence time was obtained through a volume integral, cf. Fig. 7(b). The results were expressed in terms of the mass of the potassium element based on measurements from 100 particles. The uncertainty of the method was estimated to be ~7.5%, mainly due to uncertainties in the concentration measurements of the potassium species during the calibration and the fluctuation in the power of the laser pulses. It is seen that there was about 0.034 µg potassium released until the residence time of 70 ms, noting that the released potassium was not consumed in this flue gas environment. The results in Fig. 7(b) were linearly fitted, and the potassium release rate was estimated to be 0.5 µg/s based on the slope. Moreover, the particle temperature was determined using two-color pyrometry [31]. The ICCD camera was equipped with a stereoscope with 515 nm and 652 nm band-pass filters installed in each imaging channel, and the thermal radiation at corresponding wavelengths from the burning char particles was collected by the camera. The camera generated a burst of 16 exposures triggered by each in-coming particle. The time gap between each trigger pulse was 5 ms and the exposure time was 500 µs each. A sequence of instantaneous images of the emission at 652 nm of a typical burning char particle is shown in Fig. 7(c). The average emission signal at 652 nm and particle temperature from 100 particles is presented in Fig. 7(d). During the entire burning period, the particle temperature remained at a quasi-constant temperature of ~1600 K, slightly over the surrounding gas temperature, except at the very beginning, in the first 5 ms, when the particle was heated up from the room temperature with a weak thermal emission, which could barely be detected. The criterion of 15% of the peak of the emission signal was used to define the ignition delay for the present wheat straw char [8]. The char particles were ignited at a residence time of around 13 ms – see the red mark in Fig. 7(d). Since the particle temperature was almost constant, the increase in the thermal emission signal after the ignition was mainly caused by the expansion of the burning area, and after 30 ms; the slow decrease towards the end of the oxidation process was mainly produced by the char particle shrinking. The linear decrease indicates a constant char consumption rate after ~30 ms. Hence, the potassium release, which has almost a constant release rate, especially after 30 ms, should have a strong dependence on the consumption rate of the particle. In this work, the potassium release trend is different from those observed in previous studies for millimeter-sized biomass particles [2,3], where the potassium release intensity increased gradually until the end of the char oxidation process. This indicates that the size of the particle influences the behavior of the potassium release. Overall, this work demonstrates that the LIPF imaging technique can measure the potassium distribution and follow its release process during the oxidation of pulverized char particles. Combining the potassium release data with other burning behaviors of single biomass char particles, such as ignition delay time, burnout time and particle temperature, a better insight of the role of the potassium species in the combustion and gasification of biomass chars can be achieved for various atmospheres and biomass fuels.
For the first time, the release of potassium from single burning pulverized wheat straw char particles was quantitatively measured. Laser-induced photofragmentation fluorescence imaging was employed to measure the two-dimensional distribution of KOH/KCl surrounding moving single burning char particles. An Excimer laser at 193 nm was used as the light source to photodissociate KOH/KCl to produce fluorescence, and the fluorescence signal was calibrated using homogenous hot gas environments containing KOH/KCl, in which the concentration of KOH/KCl could be monitored using previously developed UV-absorption spectroscopy. The results show that the potassium release rate is almost constant during the char oxidation process from 10 to 70 ms, and that KOH represents over 60% of the total potassium. Using the quantitative potassium measurement technique developed in the present work, further studies on different biomass fuels and thermal conversion environments will be conducted – data from single moving biomass particles, without mutual interference, are critical not only to enhance the understanding of potassium release behavior in thermochemistry conversion processes of biomass particles, but also to support the development and validation of numerical models [32]. Credit authorship contribution statement Wubin Weng: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing. Shen Li: Data curation, Investigation, Methodology, Writing - review & editing. Mário Costa: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing. Zhongshan Li: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financed by the Swedish Energy Agency through CECOST, the Knut and Alice Wallenberg foundation, the Swedish Research Council, and by the Fundação para a Ciência e a Tecnologia (FCT) through IDMEC, under LAETA, project UID/EMS/50022/2013 and project PTDC/EMS-ENE/5710/2014. References [1] Fatehi H, He Y, Wang Z, Li ZS, Bai XS, Aldén M, et al. LIBS measurements and numerical studies of potassium release during biomass gasification. Proc Combust Inst 2015;35(2):2389–96. https://doi.org/10.1016/j.proci.2014.06.115. [2] Mason PE, Darvell LI, Jones JM, Williams A. Observations on the release of gasphase potassium during the combustion of single particles of biomass. Fuel 2016;182:110–7. https://doi.org/10.1016/j.fuel.2016.05.077. [3] Mason PE, Jones JM, Darvell LI, Williams A. Gas phase potassium release from a single particle of biomass during high temperature combustion. Proc Combust Inst 2017;36(2):2207–15. https://doi.org/10.1016/j.proci.2016.06.020. [4] Hsu L-J, Alwahabi ZT, Nathan GJ, Li Y, Li ZS, Aldén M. Sodium and potassium released from burning particles of brown coal and pine wood in a laminar premixed methane flame using quantitative laser-induced breakdown spectroscopy. Appl Spectrosc 2011;65(6):684–91. https://doi.org/10.1366/10-06108. [5] Riaza J, Khatami R, Levendis YA, Álvarez L, Gil MV, Pevida C, et al. Combustion of single biomass particles in air and in oxy-fuel conditions. Biomass Bioenergy 2014;64:162–74. https://doi.org/10.1016/j.biombioe.2014.03.018. [6] Mock C, Lee H, Choi S, Manovic V. Combustion behavior of relatively large pulverized biomass particles at rapid heating rates. Energy Fuels 2016;30(12):10809–22. https://doi.org/10.1021/acs.energyfuels.6b01457. [7] Mock C, Lee H, Choi S, Manovic V. Flame structures and ignition characteristics of torrefied and raw sewage sludge particles at rapid heating rates. Fuel 2017;200:467–80. https://doi.org/10.1016/j.fuel.2017.03.055.
7
Fuel 264 (2020) 116866
W. Weng, et al.
[21] Weng W, Borggren J, Li B, Aldén M, Li Z. A novel multi-jet burner for hot flue gases of wide range of temperatures and compositions for optical diagnostics of solid fuels gasification/combustion. Rev Sci Instrum 2017;88(4):045104https://doi.org/10. 1063/1.4979638. [22] Weng W, Leffler T, Brackmann C, Aldén M, Li Z. Spectrally resolved ultraviolet (UV) absorption cross-sections of alkali hydroxides and chlorides measured in hot flue gases. Appl Spectrosc 2018;72(9):1388–95. https://doi.org/10.1177/ 0003702818763819. [23] Weng W, Brackmann C, Leffler T, Aldén M, Li Z. Ultraviolet absorption cross sections of KOH and KCl for nonintrusive species-specific quantitative detection in hot flue gases. Anal Chem 2019;91(7):4719–26. https://doi.org/10.1021/acs. analchem.9b00203. [24] Schofield K. The chemical nature of combustion deposition and corrosion: The case of alkali chlorides. Combust Flame 2012;159(5):1987–96. https://doi.org/10. 1016/j.combustflame.2012.01.007. [25] Borggren J, Weng W, Hosseinnia A, Bengtsson P-E, Aldén M, Li Z. Diode laser-based thermometry using two-line atomic fluorescence of indium and gallium. Appl Phys B 2017;123(12):278. https://doi.org/10.1007/s00340-017-6855-z. [26] Weng W, Zhang Y, Wu H, Glarborg P, Li Z. Optical measurement of KOH, KCl and K for quantitative K-Cl chemistry in thermochemical conversion processes. Submitted to Fuel. [27] He Y, Zhu J, Li B, Wang Z, Li Z, Aldén M, et al. In-situ measurement of sodium and potassium release during oxy-fuel combustion of lignite using laser-induced breakdown spectroscopy: Effects of O2 and CO2 concentration. Energy Fuels 2013;27(2):1123–30. https://doi.org/10.1021/ef301750h. [28] Sansonetti JE. Wavelengths, transition probabilities, and energy levels for the spectra of potassium (KI through KXIX). J Phys Chem Ref Data 2008;37(1):7–96. https://doi.org/10.1063/1.2789451. [29] Knudsen JN, Jensen PA, Dam-Johansen K. Transformation and release to the gas phase of Cl, K, and S during combustion of annual biomass. Energy Fuels 2004;18(5):1385–99. https://doi.org/10.1021/ef049944q. [30] van Eyk PJ, Ashman PJ, Alwahabi ZT, Nathan GJ. Quantitative measurement of atomic sodium in the plume of a single burning coal particle. Combust Flame 2008;155(3):529–37. https://doi.org/10.1016/j.combustflame.2008.05.012. [31] Saw WL, Nathan GJ, Ashman PJ, Alwahabi ZT, Hupa M. Simultaneous measurement of the surface temperature and the release of atomic sodium from a burning black liquor droplet. Combust Flame 2010;157(4):769–77. https://doi.org/10. 1016/j.combustflame.2009.12.016. [32] Fatehi H, Weng W, Costa M, Li Z, Rabaçal M, Aldén M, et al. Numerical simulation of ignition mode and ignition delay time of pulverized biomass particles. Combust Flame 2019;206:400–10. https://doi.org/10.1016/j.combustflame.2019.05.020.
[8] Simões G, Magalhães D, Rabaçal M, Costa M. Effect of gas temperature and oxygen concentration on single particle ignition behavior of biomass fuels. Proc Combust Inst 2017;36(2):2235–42. https://doi.org/10.1016/j.proci.2016.06.102. [9] Carvalho A, Rabaçal M, Costa M, Alzueta MU, Abián M. Effects of potassium and calcium on the early stages of combustion of single biomass particles. Fuel 2017;209:787–94. https://doi.org/10.1016/j.fuel.2017.08.045. [10] Li T, Niu Y, Wang L, Shaddix C, Løvås T. High temperature gasification of high heating-rate chars using a flat-flame reactor. Appl Energy 2018;227:100–7. https:// doi.org/10.1016/j.apenergy.2017.08.075. [11] Weng W, Costa M, Li Z, Aldén M. Temporally and spectrally resolved images of single burning pulverized wheat straw particles. Fuel 2018;224:434–41. https:// doi.org/10.1016/j.fuel.2018.03.101. [12] Weng W, Costa M, Aldén M, Li Z. Single particle ignition and combustion of pulverized pine wood, wheat straw, rice husk and grape pomace. Proc Combust Inst 2019;37(3):2663–71. https://doi.org/10.1016/j.proci.2018.05.095. [13] Liu Y, He Y, Wang Z, Xia J, Wan K, Whiddon R, et al. Characteristics of alkali species release from a burning coal/biomass blend. Appl Energy 2018;215:523–31. https:// doi.org/10.1016/j.apenergy.2018.02.015. [14] Zhang Z-h, Song Q, Alwahabi ZT, Yao Q, Nathan GJ. Temporal release of potassium from pinewood particles during combustion. Combust Flame 2015;162(2):496–505. https://doi.org/10.1016/j.combustflame.2014.07.030. [15] Qu Z, Steinvall E, Ghorbani R, Schmidt FM. Tunable diode laser atomic absorption spectroscopy for detection of potassium under optically thick conditions. Anal Chem 2016;88(7):3754–60. https://doi.org/10.1021/acs.analchem.5b04610. [16] Weng W, Gao Q, Wang Z, Whiddon R, He Y, Li Z, et al. Quantitative measurement of atomic potassium in plumes over burning solid fuels using infrared-diode laser spectroscopy. Energy Fuels 2017;31(3):2831–7. https://doi.org/10.1021/acs. energyfuels.6b02638. [17] Liu Y, Wang Z, Xia J, Vervisch L, Wan K, He Y, et al. Measurement and kinetics of elemental and atomic potassium release from a burning biomass pellet. Proc Combust Inst 2019;37(3):2681–8. https://doi.org/10.1016/j.proci.2018.06.042. [18] Sorvajärvi T, DeMartini N, Rossi J, Toivonen J. In situ measurement technique for simultaneous detection of K, KCl, and KOH vapors released during combustion of solid biomass fuel in a single particle reactor. Appl Spectrosc 2014;68(2):179–84. https://doi.org/10.1366/13-07206. [19] Leffler T, Brackmann C, Aldén M, Li Z. Laser-induced photofragmentation fluorescence imaging of alkali compounds in flames. Appl Spectrosc 2016;71(6):1289–99. https://doi.org/10.1177/0003702816681010. [20] Leffler T, Brackmann C, Weng W, Gao Q, Aldén M, Li Z. Experimental investigations of potassium chemistry in premixed flames. Fuel 2017;203:802–10. https://doi. org/10.1016/j.fuel.2017.05.013.
8
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Quantitative imaging of potassium release from single burning pulverized biomass char particles
T
⁎
Wubin Wenga, , Shen Lia, Mário Costab, Zhongshan Lia a b
Division of Combustion Physics, Lund University, P.O. Box 118, S 221 00 Lund, Sweden IDMEC, Mechanical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser-induced photofragmentation fluorescence Potassium Biomass chars Combustion UV absorption spectroscopy
The release of potassium from single burning pulverized wheat straw char particles was quantitatively measured using laser-induced photofragmentation fluorescence (LIPF). The char particles were prepared in a drop tube furnace at 1000 °C from wheat straw particles with sizes in the range 224–250 µm. Subsequently, the char particles were injected upward into a hot flue gas flow produced by a premixed CH4/air flame anchored on a McKenna burner. The flue gas had a mean temperature of 1580 K and a mean O2 concentration of 6.5 vol%. The 193 nm laser beam from an ArF Excimer laser was formed into a collimated laser sheet to photodissociate potassium hydroxide (KOH) and potassium chloride (KCl) around the burning char particles, and the signal of the produced fluorescence was captured by a camera. The measurements were conducted for char particles during residence times in the flue gas between 10 and 70 ms. Quantitative data was obtained from a direct calibration of the LIPF signal in hot gas products doped with known amounts of KOH and KCl. The maximum potassium concentration measured surrounding the burning char particles was over 40 ppm. During the oxidation period until 70 ms, the measured potassium release rate remained almost constant at around 0.5 µg/s, with more than 60% of the potassium being released in the form of KOH. The results indicate that the LIPF imaging method can be used to study the potassium release from burning biomass fuels.
1. Introduction Biomass fuel utilization is continually increasing in the global energy consumption aiming at the CO2 emission reduction. Many biomass fuels contain high amounts of potassium, which can be released in different thermal conversion processes. Potassium increases the risk of slagging and corrosion in combustion equipment. A number of fundamental studies were performed to measure and model potassium release processes from single biomass particles [1–4]. However, most measurements were conducted using particles with sizes of several millimeters. In contrast, studies using pulverized biomass, which are also commonly used in combustion and gasification processes, are rarely reported. Most of the previous studies on the combustion behaviors of single pulverized biomass or char particles were focused on obtaining parameters such as ignition delay times and particle temperatures [5–12]. Recently, Weng et al. [11,12] investigated the potassium release from single burning pulverized biomass particles using the chemiluminescence of K*, which allowed for a qualitative understanding of its release trend. Different optical techniques have been developed for quantitative
⁎
measurement of different potassium species released during the burning of biomass particles. Laser-induced breakdown spectroscopy (LIBS) can provide a quantitative point measurement of the total potassium [1,4,13,14]. Tunable diode laser absorption spectroscopy (TDLAS) [15,16] and laser-induced fluorescence (LIF) [17] have been applied to the measurement of atomic potassium above burning biomass pellets. Collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) was developed by Sorvajärvi et al. [18] to measure quantitatively and simultaneously the release of K atoms, KCl and KOH from burning biomass fuels. In the present study, we focused on the quantitative measurement of potassium released from burning pulverized biomass char particles. This is motivated by the fact that over 80% of the potassium present in the raw biomass can remain in the char, despite the char representing generally not more than ~25% of the initial mass [2]. In this study, the challenges mainly came from the small size (~200 µm) of the pulverized particles and the movement in the combustion environment. Laserinduced photofragmentation fluorescence (LIPF) imaging was adopted here to measure the potassium release. Compared with the other aforementioned techniques, LIPF has the advantage of capturing the
Corresponding author. E-mail address: [email protected] (W. Weng).
https://doi.org/10.1016/j.fuel.2019.116866 Received 18 September 2019; Received in revised form 12 November 2019; Accepted 12 December 2019 Available online 20 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 264 (2020) 116866
W. Weng, et al.
As shown in Fig. 1, for the LIPF measurements, an ArF Excimer laser (Compex 102, Lambda Physik) was used to provide the 193 nm laser pulses with a pulse duration of 25 ns. The laser beam was transformed into a vertical laser sheet with a height of around 30 mm and a thickness of around 1 mm through a telescope system containing two cylindrical lenses of focal lengths f = 500 mm and f = -75 mm, respectively. The laser sheet passed through the hot gas products, right above the central hole of the burner. It was placed at different heights to photodissociate KOH/KCl released from the burning wheat straw char particles at different residence times. The photofragmentation fluorescence of excited potassium atom from KOH/KCl was captured by an ICCD camera (PI-MAX III, Princeton, 1024 × 1024 pixels) with an f = 50 mm objective (Nikkor f/1.4). The exposure time was set to 400 ns. An interference filter centered at the wavelength of 766 nm with a FWHM of 10 nm (Edmund Optics) was used to detect the fluorescence signal while suppressing the background radiation and the fluorescence from other species, such as sodium atoms. A synchronization system was used to manage the timing among each in-coming single particle, the laser pulse and the ICCD camera gate. In this synchronization system, a 532 nm continuous-wave laser (200 mW) was adopted to provide a laser beam having a diameter of around 1 mm. This laser passed through the hot gas flow close to the exit of the central tube of the burner. Scattering was detected by a photo diode (PDA100A2, Thorlabs) as the char particle moved through the laser beam. The photo diode triggered a pulse generator (DG535, Stanford Research Systems), which provided trigger signals for the ICCD camera and the Excimer laser. Fig. 1 shows a typical single shot image of the photofragmentation fluorescence around a wheat straw char particle at a residence time of 60 ms in the hot gas products. The particle was surrounded by an axisymmetric potassium cloud with a size close to 20 mm, which was much larger than the char particles with an initial size of ~0.2 mm. The wheat straw char particles were prepared in a drop tube furnace at 1000 °C. The drop tube furnace was fed with pulverized wheat straw particles, sieved to sizes in the range 224–250 µm, as measured by the Malvern 2600 Particle Size Analyzer, and N2. The residence time of the particles in the drop tube furnace was around 2 s. The shape of the resulting wheat straw char particles is shown in Fig. 1, and the properties of the raw wheat straw and respective char particles are given in Table 1. The proximate and ultimate analysis of the raw wheat straw and wheat straw char were carried out following the procedures specified in the standards ASTM-E-870, EN 14,918 and EN 14775. Each sample was analysed three times to assess the repeatability of the data,
instantaneous spatial distribution of KCl and KOH, which are the dominant species formed during the char oxidation period [18]. Leffler et al. [19,20] have applied LIPF for two-dimensional quantitative measurements of KCl/KOH distribution in flames. To obtain quantitative data, critical parameters, such as the absorption cross-sections of KOH and KCl at 193 nm, the distribution on different excited states of the dissociated K atoms, the quenching rate of the excited K atoms, and the signal collection efficiency have to be accurately determined. This can introduce substantial uncertainties in the measured data. Here, an in-situ calibration process was introduced to establish the correlation between the photofragmentation fluorescence signal and the concentration of KOH/KCl. The calibration was conducted in hot laminar gas flows with gas-phase KOH and KCl provided by a home-made burner, a multi-jet burner [21], with a homogenous distribution for temperatures ranging from 1000 to 2000 K. The concentrations of KOH and KCl were monitored using UV-absorption spectroscopy [22,23]. Based on the calibration, the KOH/KCl distribution surrounding the burning char particles were retrieved, and, indeed, the results revealed the potassium release process during the char oxidation period. The wheat straw char particles investigated here were prepared in a drop tube furnace at 1000 °C, with sizes of the original straw particles in the range 224–250 µm. To the best of our knowledge, this work presents the first attempt to quantify the potassium release in-situ from single burning pulverized biomass char particles. 2. Materials and methods Fig. 1 shows a schematic of the experimental setup used for the measurement of potassium released from single burning wheat straw char particles using LIPF imaging. The combustion system consists of a McKenna flat flame burner, a biomass feeding unit and a CH4/air supply system, which has been described in detail elsewhere [8,11]. A flat premixed CH4/air/O2 flame anchored on the surface of the McKenna burner with a diameter of 60 mm produced a laminar hot flue gas flow. The CH4/air/O2 supply system was controlled by mass flow controllers (Bronkhorst). The wheat straw char particles were stored in a 10 mL syringe and were fed into a N2 stream with a flow rate of 0.05 SL/min, and transported into the hot flue gas flow above the burner upwards through a 1.5 mm diameter tube with the opening located on the center of the burner. The hot flue gas flows were confined by a quartz tube with an inner diameter of 70 mm, height of 500 mm, and thickness of 2 mm. This tube was used to avoid the entrainment of cold ambient air, while providing optical access for the laser-based system.
Fig. 1. Experimental setup for the measurement of potassium released from single burning wheat straw char particles using the LIPF imaging system. Top-right corner: typical single shot LIPF image; bottom-right corner: char particles. 2
Fuel 264 (2020) 116866
W. Weng, et al.
tubes, with an inner diameter of 1.6 mm, to generate premixed flames, attached on each of the jet tubes, see Fig. 2(c). Each jet tube was surrounded evenly by the co-flow with a mixture of N2/air supplied by the co-flow chamber. Thus, after the mixing between the co-flow and the products of the premixed flames, a stream of homogenous hot gas products was established at the burner exit, with a size of 85 × 47 mm, and used for signal calibration. During the calibration, potassium was introduced into the hot gas flow by seeding potassium carbonate (K2CO3). A K2CO3 water solution was prepared in 0.5 mol/L, atomized by an ultrasonic fogger and transported into the CH4/air/O2 mixture using an air stream with a flow of 0.5 SL/min. As the small droplets of K2CO3 fog passed through the premixed flames above the jet tubes, K2CO3 was converted into KOH in the hot gas products. Chloroform (CHCl3) was used to introduce chlorine into the hot gas products. Then, KCl could be generated with potassium seeding. Liquid phase CHCl3 was stored in a bubbler with a temperature of 10 °C using a chilled bath (PolySicence). A N2 stream was used to carry the CHCl3 vapor after bubbling through the CHCl3 liquid. The concentration of the Cl seeded into the hot gas products was around 100 ppm, which was sufficient to convert all KOH in the hot gas products into KCl, with a potassium concentration of around 20 ppm [24]. Hence, the pure KOH was provided through seeding K2CO3, while the pure KCl was investigated by introducing additional 100 ppm of Cl. Table 2 summarizes the gas composition used in all flame cases of the multi-jet burner. The gases were controlled by mass flow controllers (Bronkhorst). The temperature of the hot gas products in each flame condition were measured using two-line atomic fluorescence thermometry with indium atoms with an accuracy of ~2.7%, as reported by Borggren et al. [25]. The temperature varied between 1120 K and 1750 K with an equivalence ratio around 0.7. During the calibration, the McKenna burner used in the LIPF setup (cf. Fig. 1) was replaced by the multi-jet burner – the layout is shown in Fig. 2(b). The signal of the fluorescence captured by the ICCD camera is shown in Fig. 3(a) with the same amounts of KOH or KCl seeded in the hot gas products at 1260 K. It can be seen that both the KOH and KCl
Table 1 Properties of the raw wheat straw and wheat straw char. Parameter
Raw wheat straw
Wheat straw char
Proximate analysis (wt.%, as received) Volatile matter Fixed carbon Moisture Ash
64.9 12.4 8.0 14.7
22.9 25.2 1.1 50.8
Ultimate analysis (wt.%, dry ash free) Carbon Hydrogen Nitrogen Sulfur Oxygen
41.1 5.3 0.7 < 0.02 52.6
77.7 1.5 0.9 < 0.02 19.9
which was found to be appropriate. The premixed flat flame anchored on the McKenna burner was fueled by a mixture of CH4 (1.3 SL/min), air (13.52 SL/min) and O2 (0.78 SL/min), with an equivalence ratio of 0.72. The mean temperature of the hot gas products from the flame, measured using a B-type thermocouple (OMEGA), was around 1580 K. The thermocouple junction bead, with a diameter of ~1.1 mm, was placed ~2 cm above the burner axis. The measured temperatures were corrected for radiation losses following the heat transfer theory [21], which yield an estimated uncertainty of ~70 K, in accordance with the temperatures measured using two-line atomic fluorescence thermometry in the work of Weng et al. [21]. The mean velocity of the hot gas products and its mean O2 concentration were around 0.6 m/s and 6.5 vol%, respectively. The LIPF signal from KOH/KCl was calibrated through a homemade burner, a multi-jet burner, which provided homogenous hot gas products containing known amounts of KOH and KCl. The multi-jet burner is described in detail elsewhere [21]. The basic structure of the burner is presented in Fig. 2(a). It consists of two chambers, namely the jet chamber and the co-flow chamber. The premixed CH4/air/O2 flow entered the jet chamber and was distributed evenly through 181 jet
Fig. 2. Schematics of the multi-jet burner (a), and laser setup (b), and a photograph of a typical flame stabilized on the burner (c). 3
Fuel 264 (2020) 116866
W. Weng, et al.
section spectrum of KOH and KCl provided by Weng et al. [23] with a concentration of 17 ppm based on the Beer-Lambert law. Through this process, the concentrations of KOH and KCl in the hot gas products can be determined with an uncertainty of ~5% [26]. In the hot gas products environment seeded with KOH and KCl, the concentration of K atoms should be considered even though it is negligible comparing to that of KOH and KCl. The K atoms can have strong absorption of the fluorescence produced by the photofragmentation process. This has also been discussed by He et al. [27] and Leffler et al. [19,20]. The correction of the obtained LIPF signal was needed. Hence, in the present study, the concentration of K atoms was measured together with the concentration of KOH and KCl. A TDLAS system with a 769.9 nm continuous-wave laser was used, as discussed by Weng et al. [16]. The laser had a power of about 3 mW and a beam size of about 1 mm2, and was provided by an external cavity diode laser (Toptica, DL100). The laser was controlled by an analog control package, which contained a temperature control module (DTC 110), a current control module (DCC 110), and a scan module (SC 110). It can scan the wavelength with a range over 35 GHz with a repetition rate of 100 Hz. After the passage of the hot gas products region containing K atoms, the laser light was absorbed with a pass length of 8.5 cm. Two photo diodes (PDA100A, Thorlabs) were used to monitor the power of the laser before and after the passage. Using the Beer-Lambert law, the concentration of K atoms was obtained, as described by Weng et al. [16], with an uncertainty of ~5% [26].
Table 2 Flame conditions. Gas flow rate (SL/min)
Flame case
Jet-flow
T1 T2 T3 T4 T5
Gas products temperature T (K)
Co-flow
CH4
Air
O2
N2
Air
2.66 2.47 2.28 2.09 1.71
16.88 11.77 11.43 10.44 8.45
1.89 2.58 2.26 2.07 1.69
10.83 18.97 22.69 26.50 26.92
7.74 8.90 9.83 10.66 10.25
1750 1550 1390 1260 1120
3. Results and discussion Following Leffler et al. [19], the fluorescence signal can be expressed as,
S= R·V ·Afi /(Afi + Q )·Ab·N ∗
(1)
where R is the efficiency of the signal collection and detection, V is the volume of the probe volume, Afi is the Einstein coefficient for spontaneous emission given as 3.8·107 s−1 [28], Q is the collisional quenching rate estimated to be 4.1·108 s−1 [19], Ab is the fluorescence losses due to the self-absorption by K atoms, and N ∗ is the potassium atoms in the excited state, which can be obtained from [19],
N ∗ = N ·η ·(1 − exp((−σE )/(hνA)))
(2)
where N is the number density of KOH or KCl, η is the yield of the excited fragmented K atoms, σ is the absorption cross-section of KOH or KCl, E and v are the laser energy and frequency, respectively, and A is the cross section area of the probe volume. From Eqs. (1) and (2), it can be seen that the signal might be strongly influenced by the power of the laser. Hence, in the first measurements, the power of the laser was varied to evaluate the correlation between the photofragmentation fluorescence signal and the laser fluence. Constant amounts of KOH and KCl were introduced into the hot gas products at 1260 K. The results (cf. Fig. 4a) show that the signal increased with the laser fluence, but the increase rate became smaller at higher laser fluence for both KOH and KCl. It can also be found that KCl produced stronger signal than KOH. The difference in the fluorescence signal between the KOH and KCl cases was attributed to the difference in the absorption cross section σ and yield of excited K atom η of KOH and KCl. The calculated photodissociate K* fraction is shown in Fig. 4(b). Since the Gaussian distribution of the laser was not considered in the calculation, KCl could reach a complete dissociation with a smaller fluence than that shown by the measurements. The laser used in the following measurements had a pulse energy of 15 mJ, i.e. 75 mJ/cm2 laser fluence, with a standard deviation of ~4%. The correlation between the signal of the photofragmentation fluorescence and the potassium concentration is essential for the calibration. To account for the effect of the absorption by K atoms present in the hot flow on the fluorescence signal, the concentration of K atoms in the hot gas products was measured using the TDLAS system
Fig. 3. Images of the LIPF signal with same amount of KOH and KCl in the hot gas products at 1260 K (a), and typical absorbance obtained in the UV absorption spectroscopy system together with the fitting curve based on the spectrally resolved absorption cross-section of KOH and KCl [23] (b).
were evenly distributed in the hot gas products, and that more fluorescence signal was produced for the KCl case than for the KOH case. To calibrate the LIPF signal, the concentrations of KOH and KCl in the hot gas products were measured using UV-absorption spectroscopy. Details of this technique can be found in Weng et al. [22]. A deuterium lamp was used to provide the broadband UV light, which was collimated and passed through the hot gas products. Five UV-enhanced aluminum mirrors were used to guide the light to pass above the burner six times to have a path length of 522 mm. After the passage, the light was collected by a spectrometer (Ocean optics 2000+). The absorption spectrum of KOH/KCl was obtained after the division between the intensity of the light passing through the hot gas products with potassium seeding and the one without potassium seeding. A natural logarithm of the ratios was determined as the absorbance. A typical absorbance is shown in Fig. 3(b), which can be well fitted by the absorption cross4
Fuel 264 (2020) 116866
W. Weng, et al.
simultaneously with the measurement of the concentration of KOH and KCl using UV-absorption spectroscopy. The measured concentration of K atoms as a function of the total amount of KOH/KCl seeded in the hot gas products is shown in Fig. 5(a), which increase almost linearly with the concentration of the total potassium. The KCl case produced much less K atoms than the KOH case. A similar characteristic has been reported for sodium by Schofield. [24] The self-absorption rate was retrieved using the Beer-Lambert law, as the concentration of K atoms, and it also increased with the total potassium concentration. The selfabsorption rate was used to correct the raw signal to obtain the true value of the fluorescence emitted from the probe volume. The fluorescence signal after the correction is shown in Fig. 5(b). The correlations between the fluorescence signal and the concentration of KOH and KCl were fitted as follows,
N = 1.437·1015·S 2 + 1.420·1017·S
(3)
for KOH, and
N = 2.596·1014 ·S 2 + 1.583·1016·S
(4)
for KCl. The signal after the correction did not increase linearly with the concentration of potassium for both KCl and KOH. Here, another selfabsorption, occurring in the probe volume, should be considered. The exited K atoms were produced by the photodissociation of KOH and KCl, which decayed to the ground state quickly through spontaneous transition and collision quenching. In this process, part of the spontaneous fluorescence emission was self-absorbed by the atoms already on the ground state. Combing the decay process and the absorption process that occurred in the probe volume using the Beer-Lambert law, a selfabsorption rate with a value up to 60% was obtained in the case with 20 ppm potassium seeding. After the correction, a good linear correlation between the fluorescence signal and the concentration of the potassium was built, as follows,
S = 5.286·10−18·N Fig. 4. Signal of the LIPF (a) and simulated photodissociated K* fraction (b) of KOH and KCl as a function of laser fluence.
(5)
for KOH, and
S = 1.416·10−17·N
(6)
Fig. 5. Concentration of K atoms in the hot gas products and the absorption of LIPF by K atoms as a function of KOH/KCl seeding (a) and environment temperature with the concentration of KOH and KCl fixed at around 1.0·1020 m−3 (c). Detected LIPF signal after self-absorption correction as a function of KOH/KCl seeding (b) and environment temperature (d). 5
Fuel 264 (2020) 116866
W. Weng, et al.
Fig. 6. Temporal image sequences of the average LIPF signal around a single burning pulverized wheat straw char particle in the hot gas products without Cl seeding (a) and with 320 ppm Cl seeding (b). Average of spatially integrated intensities of the LIPF signal versus residence time (c).
Fig. 7. Average mass of potassium released from burning wheat straw char particles and their average thermal emission signal and particle temperature as a function of the particle residence time in the hot gas products. (a) Concentration distribution of potassium surrounding the burning particle for a residence time of 60 ms. (b) Measured potassium release trend and its linear fitting. (c) Sequences of instantaneous image of a typical burning char particle recorded with the ICCD camera equipped with the 652 nm band-pass filter. (d) Average emission signal and particle temperature trend.
time. At 20 and 30 ms, they had the peak signal close to the particles, but at 60 and 70 ms, the signal at the same position almost dropped to zero. As 320 ppm of Cl was added into the hot gas products, all the potassium species in the potassium cloud was presumably converted into KCl [24], and its effect on the potassium release process was negligible since above 800 ℃ the biomass fuels release Cl fully [29]. Fig. 6(b) shows the photofragmentation fluorescence of KCl surrounding the char particles. They had the same signal area as the case without Cl seeding; however, the signal was stronger because more fluorescence was produced from KCl than from KOH, as described above. In addition, the dark holes observed in the case without Cl seeding did not appear anymore in the case with Cl seeding. It was considered that during the oxidation of the char particles, the potassium was released as potassium atoms at the very beginning [30], but they could be transformed into KCl very quickly as sufficient Cl existed. The average of the spatially integrated signals in Fig. 6(a) and (b) is presented in Fig. 6(c). For both cases, the signals increase almost linearly with the residence time. This indicates that potassium is released continuously from burning char particles during its oxidation. The signal from the case with Cl seeding was almost twice the case without Cl seeding. Since the fluorescence yield of KOH was about 37% of that of KCl, the results indicate that over 60% of potassium surrounding the
for KCl. According to the ratio between the slope of these two equations, it was determined that the fluorescence produced from KOH was about 37% of the one from the same amount of KCl. In addition, the effect of the gas temperature on the fluorescence signal was also investigated. In the study, the concentrations of KOH and KCl were fixed at around 1.0·1020 m−3. The concentration of K atoms was measured and is shown in Fig. 5(c), which shows significant increase with the temperature; the self-absorption by the K atoms in the hot gas products also increased up to 80% for KOH at 1750 K. However, after the self-absorption correction, the signal of fluorescence from the probe volume was almost independent of the gas temperature for both KOH and KCl, as shown in Fig. 5(d). Using the LIPF imaging technique with the calibration procedure, the distribution of potassium surrounding the single burning pulverized wheat straw char particles was quantitatively measured. Each measurement was conducted for 100 particles. The sequences of the images of the average fluorescence signal is shown in Fig. 6(a) as a function of the residence time in the hot gas products. The signal was not corrected with the self-absorption due to the short path length in the potassium cloud. The effect of the self-absorption on the calculation of the total amount of potassium was estimated to be less than 4%. Due to the free diffusion of potassium, the signal gradually decreased from the center of the images, and its area had a continuous increase with the residence 6
Fuel 264 (2020) 116866
W. Weng, et al.
4. Conclusions
burning char particles, originally in the form of KOH, was converted into KCl with Cl seeding. Using the correlations between the fluorescence signal and the concentrations of KCl and KOH, expressed by Eqs. (3) and (4), the LIPF signal around the particles in Fig. 6(a) and (b) was converted correspondingly into the distribution of the potassium concentration. In Fig. 7(a), a typical image with the average concentration distribution of potassium around a burning char particle for a residence time of 60 ms is shown. Using the results of the concentration distribution and considering that most potassium clouds have an axisymmetric structure, as shown in Fig. 7(a), regardless of the char particles’ shape, the total amount of potassium released from the char particles as a function of the residence time was obtained through a volume integral, cf. Fig. 7(b). The results were expressed in terms of the mass of the potassium element based on measurements from 100 particles. The uncertainty of the method was estimated to be ~7.5%, mainly due to uncertainties in the concentration measurements of the potassium species during the calibration and the fluctuation in the power of the laser pulses. It is seen that there was about 0.034 µg potassium released until the residence time of 70 ms, noting that the released potassium was not consumed in this flue gas environment. The results in Fig. 7(b) were linearly fitted, and the potassium release rate was estimated to be 0.5 µg/s based on the slope. Moreover, the particle temperature was determined using two-color pyrometry [31]. The ICCD camera was equipped with a stereoscope with 515 nm and 652 nm band-pass filters installed in each imaging channel, and the thermal radiation at corresponding wavelengths from the burning char particles was collected by the camera. The camera generated a burst of 16 exposures triggered by each in-coming particle. The time gap between each trigger pulse was 5 ms and the exposure time was 500 µs each. A sequence of instantaneous images of the emission at 652 nm of a typical burning char particle is shown in Fig. 7(c). The average emission signal at 652 nm and particle temperature from 100 particles is presented in Fig. 7(d). During the entire burning period, the particle temperature remained at a quasi-constant temperature of ~1600 K, slightly over the surrounding gas temperature, except at the very beginning, in the first 5 ms, when the particle was heated up from the room temperature with a weak thermal emission, which could barely be detected. The criterion of 15% of the peak of the emission signal was used to define the ignition delay for the present wheat straw char [8]. The char particles were ignited at a residence time of around 13 ms – see the red mark in Fig. 7(d). Since the particle temperature was almost constant, the increase in the thermal emission signal after the ignition was mainly caused by the expansion of the burning area, and after 30 ms; the slow decrease towards the end of the oxidation process was mainly produced by the char particle shrinking. The linear decrease indicates a constant char consumption rate after ~30 ms. Hence, the potassium release, which has almost a constant release rate, especially after 30 ms, should have a strong dependence on the consumption rate of the particle. In this work, the potassium release trend is different from those observed in previous studies for millimeter-sized biomass particles [2,3], where the potassium release intensity increased gradually until the end of the char oxidation process. This indicates that the size of the particle influences the behavior of the potassium release. Overall, this work demonstrates that the LIPF imaging technique can measure the potassium distribution and follow its release process during the oxidation of pulverized char particles. Combining the potassium release data with other burning behaviors of single biomass char particles, such as ignition delay time, burnout time and particle temperature, a better insight of the role of the potassium species in the combustion and gasification of biomass chars can be achieved for various atmospheres and biomass fuels.
For the first time, the release of potassium from single burning pulverized wheat straw char particles was quantitatively measured. Laser-induced photofragmentation fluorescence imaging was employed to measure the two-dimensional distribution of KOH/KCl surrounding moving single burning char particles. An Excimer laser at 193 nm was used as the light source to photodissociate KOH/KCl to produce fluorescence, and the fluorescence signal was calibrated using homogenous hot gas environments containing KOH/KCl, in which the concentration of KOH/KCl could be monitored using previously developed UV-absorption spectroscopy. The results show that the potassium release rate is almost constant during the char oxidation process from 10 to 70 ms, and that KOH represents over 60% of the total potassium. Using the quantitative potassium measurement technique developed in the present work, further studies on different biomass fuels and thermal conversion environments will be conducted – data from single moving biomass particles, without mutual interference, are critical not only to enhance the understanding of potassium release behavior in thermochemistry conversion processes of biomass particles, but also to support the development and validation of numerical models [32]. Credit authorship contribution statement Wubin Weng: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing. Shen Li: Data curation, Investigation, Methodology, Writing - review & editing. Mário Costa: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing. Zhongshan Li: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financed by the Swedish Energy Agency through CECOST, the Knut and Alice Wallenberg foundation, the Swedish Research Council, and by the Fundação para a Ciência e a Tecnologia (FCT) through IDMEC, under LAETA, project UID/EMS/50022/2013 and project PTDC/EMS-ENE/5710/2014. References [1] Fatehi H, He Y, Wang Z, Li ZS, Bai XS, Aldén M, et al. LIBS measurements and numerical studies of potassium release during biomass gasification. Proc Combust Inst 2015;35(2):2389–96. https://doi.org/10.1016/j.proci.2014.06.115. [2] Mason PE, Darvell LI, Jones JM, Williams A. Observations on the release of gasphase potassium during the combustion of single particles of biomass. Fuel 2016;182:110–7. https://doi.org/10.1016/j.fuel.2016.05.077. [3] Mason PE, Jones JM, Darvell LI, Williams A. Gas phase potassium release from a single particle of biomass during high temperature combustion. Proc Combust Inst 2017;36(2):2207–15. https://doi.org/10.1016/j.proci.2016.06.020. [4] Hsu L-J, Alwahabi ZT, Nathan GJ, Li Y, Li ZS, Aldén M. Sodium and potassium released from burning particles of brown coal and pine wood in a laminar premixed methane flame using quantitative laser-induced breakdown spectroscopy. Appl Spectrosc 2011;65(6):684–91. https://doi.org/10.1366/10-06108. [5] Riaza J, Khatami R, Levendis YA, Álvarez L, Gil MV, Pevida C, et al. Combustion of single biomass particles in air and in oxy-fuel conditions. Biomass Bioenergy 2014;64:162–74. https://doi.org/10.1016/j.biombioe.2014.03.018. [6] Mock C, Lee H, Choi S, Manovic V. Combustion behavior of relatively large pulverized biomass particles at rapid heating rates. Energy Fuels 2016;30(12):10809–22. https://doi.org/10.1021/acs.energyfuels.6b01457. [7] Mock C, Lee H, Choi S, Manovic V. Flame structures and ignition characteristics of torrefied and raw sewage sludge particles at rapid heating rates. Fuel 2017;200:467–80. https://doi.org/10.1016/j.fuel.2017.03.055.
7
Fuel 264 (2020) 116866
W. Weng, et al.
[21] Weng W, Borggren J, Li B, Aldén M, Li Z. A novel multi-jet burner for hot flue gases of wide range of temperatures and compositions for optical diagnostics of solid fuels gasification/combustion. Rev Sci Instrum 2017;88(4):045104https://doi.org/10. 1063/1.4979638. [22] Weng W, Leffler T, Brackmann C, Aldén M, Li Z. Spectrally resolved ultraviolet (UV) absorption cross-sections of alkali hydroxides and chlorides measured in hot flue gases. Appl Spectrosc 2018;72(9):1388–95. https://doi.org/10.1177/ 0003702818763819. [23] Weng W, Brackmann C, Leffler T, Aldén M, Li Z. Ultraviolet absorption cross sections of KOH and KCl for nonintrusive species-specific quantitative detection in hot flue gases. Anal Chem 2019;91(7):4719–26. https://doi.org/10.1021/acs. analchem.9b00203. [24] Schofield K. The chemical nature of combustion deposition and corrosion: The case of alkali chlorides. Combust Flame 2012;159(5):1987–96. https://doi.org/10. 1016/j.combustflame.2012.01.007. [25] Borggren J, Weng W, Hosseinnia A, Bengtsson P-E, Aldén M, Li Z. Diode laser-based thermometry using two-line atomic fluorescence of indium and gallium. Appl Phys B 2017;123(12):278. https://doi.org/10.1007/s00340-017-6855-z. [26] Weng W, Zhang Y, Wu H, Glarborg P, Li Z. Optical measurement of KOH, KCl and K for quantitative K-Cl chemistry in thermochemical conversion processes. Submitted to Fuel. [27] He Y, Zhu J, Li B, Wang Z, Li Z, Aldén M, et al. In-situ measurement of sodium and potassium release during oxy-fuel combustion of lignite using laser-induced breakdown spectroscopy: Effects of O2 and CO2 concentration. Energy Fuels 2013;27(2):1123–30. https://doi.org/10.1021/ef301750h. [28] Sansonetti JE. Wavelengths, transition probabilities, and energy levels for the spectra of potassium (KI through KXIX). J Phys Chem Ref Data 2008;37(1):7–96. https://doi.org/10.1063/1.2789451. [29] Knudsen JN, Jensen PA, Dam-Johansen K. Transformation and release to the gas phase of Cl, K, and S during combustion of annual biomass. Energy Fuels 2004;18(5):1385–99. https://doi.org/10.1021/ef049944q. [30] van Eyk PJ, Ashman PJ, Alwahabi ZT, Nathan GJ. Quantitative measurement of atomic sodium in the plume of a single burning coal particle. Combust Flame 2008;155(3):529–37. https://doi.org/10.1016/j.combustflame.2008.05.012. [31] Saw WL, Nathan GJ, Ashman PJ, Alwahabi ZT, Hupa M. Simultaneous measurement of the surface temperature and the release of atomic sodium from a burning black liquor droplet. Combust Flame 2010;157(4):769–77. https://doi.org/10. 1016/j.combustflame.2009.12.016. [32] Fatehi H, Weng W, Costa M, Li Z, Rabaçal M, Aldén M, et al. Numerical simulation of ignition mode and ignition delay time of pulverized biomass particles. Combust Flame 2019;206:400–10. https://doi.org/10.1016/j.combustflame.2019.05.020.
[8] Simões G, Magalhães D, Rabaçal M, Costa M. Effect of gas temperature and oxygen concentration on single particle ignition behavior of biomass fuels. Proc Combust Inst 2017;36(2):2235–42. https://doi.org/10.1016/j.proci.2016.06.102. [9] Carvalho A, Rabaçal M, Costa M, Alzueta MU, Abián M. Effects of potassium and calcium on the early stages of combustion of single biomass particles. Fuel 2017;209:787–94. https://doi.org/10.1016/j.fuel.2017.08.045. [10] Li T, Niu Y, Wang L, Shaddix C, Løvås T. High temperature gasification of high heating-rate chars using a flat-flame reactor. Appl Energy 2018;227:100–7. https:// doi.org/10.1016/j.apenergy.2017.08.075. [11] Weng W, Costa M, Li Z, Aldén M. Temporally and spectrally resolved images of single burning pulverized wheat straw particles. Fuel 2018;224:434–41. https:// doi.org/10.1016/j.fuel.2018.03.101. [12] Weng W, Costa M, Aldén M, Li Z. Single particle ignition and combustion of pulverized pine wood, wheat straw, rice husk and grape pomace. Proc Combust Inst 2019;37(3):2663–71. https://doi.org/10.1016/j.proci.2018.05.095. [13] Liu Y, He Y, Wang Z, Xia J, Wan K, Whiddon R, et al. Characteristics of alkali species release from a burning coal/biomass blend. Appl Energy 2018;215:523–31. https:// doi.org/10.1016/j.apenergy.2018.02.015. [14] Zhang Z-h, Song Q, Alwahabi ZT, Yao Q, Nathan GJ. Temporal release of potassium from pinewood particles during combustion. Combust Flame 2015;162(2):496–505. https://doi.org/10.1016/j.combustflame.2014.07.030. [15] Qu Z, Steinvall E, Ghorbani R, Schmidt FM. Tunable diode laser atomic absorption spectroscopy for detection of potassium under optically thick conditions. Anal Chem 2016;88(7):3754–60. https://doi.org/10.1021/acs.analchem.5b04610. [16] Weng W, Gao Q, Wang Z, Whiddon R, He Y, Li Z, et al. Quantitative measurement of atomic potassium in plumes over burning solid fuels using infrared-diode laser spectroscopy. Energy Fuels 2017;31(3):2831–7. https://doi.org/10.1021/acs. energyfuels.6b02638. [17] Liu Y, Wang Z, Xia J, Vervisch L, Wan K, He Y, et al. Measurement and kinetics of elemental and atomic potassium release from a burning biomass pellet. Proc Combust Inst 2019;37(3):2681–8. https://doi.org/10.1016/j.proci.2018.06.042. [18] Sorvajärvi T, DeMartini N, Rossi J, Toivonen J. In situ measurement technique for simultaneous detection of K, KCl, and KOH vapors released during combustion of solid biomass fuel in a single particle reactor. Appl Spectrosc 2014;68(2):179–84. https://doi.org/10.1366/13-07206. [19] Leffler T, Brackmann C, Aldén M, Li Z. Laser-induced photofragmentation fluorescence imaging of alkali compounds in flames. Appl Spectrosc 2016;71(6):1289–99. https://doi.org/10.1177/0003702816681010. [20] Leffler T, Brackmann C, Weng W, Gao Q, Aldén M, Li Z. Experimental investigations of potassium chemistry in premixed flames. Fuel 2017;203:802–10. https://doi. org/10.1016/j.fuel.2017.05.013.
8