Characterization of liquid and hydrochars
Appearance characteristics of liquid
Temperature is the most important parameter which affects the moisture removal efficiency of oily scum during the HTC process. Therefore, its impacts on the present moisture removal process were studied on the four different HTC temperature levels (120, 150, 210 and 240 oC), and the illustrative results of these investigations are shown in Fig. 1.
As shown in Fig. 1, the liquid product obtained from the HTC process of oily scum at the different temperatures showed different shades of color. The increase of HTC temperature caused a significant color shift of liquid from light yellow to strong yellow, indicating an increase of dissolved organic compounds in produced liquid. At elevated temperatures, water solvent properties were significantly enhanced and some of organic compounds were dissolved in the liquid phase after oily scum HTC. This was because that the dehydration, decarboxylation, polymerization and aromatization reactions of petroleum hydrocarbons in oily scum occurred during the HTC process. In the subcritical conditions of HTC, the presence of water generally enhanced ion chemistry and suppressed free radical reactions. It was obvious that reaction extent of oily scum HTC was governed by temperature to a large extent. Higher HTC temperature generally increases reaction intensity. The liquid phase had a high load of organic compounds, many of which showed potentially valuable chemicals. This promoted bond cleavage of primarily hydrogen bonds, especially hydrolysis. Therefore, HTC temperature had an impact on the liquid components during the process of oily scum, especially produced dissolved organic compounds.
Photos analysis of hydrochars
After the HTC treatment of oily scum, the photographs of dried hydrochars obtained from the different temperatures are shown in Fig. 2.
From Fig. 2, the hydrochar products produced from oily scum through HTC at the different temperatures all displayed a dark brown color and appeared in a thick slimy morphology. HTC could mainly eliminate hydroxyl and carboxyl group of petroleum hydrocarbons in oily scum, resulting in solid product which had a lower moisture content and hydrophilicity than raw oily scum. So, this effect of reducing the amount of moisture and functional groups was effectively utilized via HTC treatment. However, the slimy appearance indicated that these hydrochar products might not be completely carbonized less than 240 oC in this study and still remained some parts of the original structure of oily scum. If the higher carbonized hydrochar products are needed, HTC is performed at more than 240 oC.
Observations at high magnification
The black and white as well as color display of hydrochars at high magnification are presented in Fig. 3 and 4, respectively.
In Fig. 3 (a), the surface of hydrochar-120 oC is granular, and the structure is dense. There are a large amount of carbon particles that formed on the surface. Fig. 3 (b) shows a granular structure of hydrochar-150 oC, and the amount of the carbon particles reduced due to the stronger hydrolysis and decarboxylation reactions. Also, there are more dissolved organic compounds in the liquid product in Fig. 2 and less carbon particles in the solid product at a relatively low HTC temperature in Fig. 3. From Fig. 3 (c), after the HTC treatment at 210 oC, there are less carbon particles. As seen in Fig. 3 (d), there are much smaller particles in the hydrochar-240. Meantime, there are some much bigger particles are located on the surface of hydrocha-240.
The hydrolysis reaction in the HTC process of oily scum was dominant at a relatively low HTC temperature (less than 220 oC), while the competition between hydrolysis and repolymerization was strong at 220-300 oC [18]. With the continuous increase in HTC temperature, repolymerization was predominant compared to hydrolysis [19]. Thus, the water-soluble products were decomposed and repolymerized into oily products. Meanwhile, the repolymerization occurred. These caused hydrochar-240 oC to obtain some much smaller particles and bigger polymer ones. The aggregates formed by partially deposited particles were finally detached from hydrochars, and the remaining structure was carbon microspheres. Those results indicated that higher HTC temperature facilitated the accumulation of carbon microspheres and other small molecule components. Besides, with the HTC conditions severer, the extent of the decomposition reactions of oily scum was increased, and the tendency of forming aggregates became more prominent. With the increase of HTC temperature, the aggregate structure became bigger, which might be that carbon microspheres and oil could be further aggregated on the surface of clusters simultaneously.
Analysis of the components in Liquid
Filtration performance
The experimental curves about the liquid percentage versus filtration time at the different HTC temperatures are shown in Fig. 5.
According to each curve in Fig. 5, the liquid percentage produced from oily scum HTC increases significantly with the filtration time. Liquid was removed in the first 15 min of filtration time with a high increasing rate, and then the liquid percentage increased slightly after 15 min, reaching the maximum at 30 min in this study. Additionally, as the HTC temperature increases from 120 to 240 oC, the maximum value of the liquid percentage gradually increases from 58.28 to 80.69% at 30 min filtration. This increase was attributed to the increase on the extent of dehydration, decarboxylation, and volatile matter decomposition reactions when the higher HTC temperatures were applied. Therefore, the liquid uptake of hydrochar significantly increased with the increase of HTC temperature. This suggested that the hydrochars with higher HTC temperature exhibited higher filtration ability and dewaterability. The hydrochar-120 oC presented the most hygroscopic behavior and lowest moisture uptake in this study. However, the liquid percentage at 240 oC is more than that at any other temperature. The higher liquid uptake of hydrochars produced at higher temperature was mainly ascribed to the bigger pore structure and higher surface area in comparison with hydrochars obtained at lower temperature. The dewatering and thermal decomposition of oily scum played a key role in causing the yield of liquid to increase with an increase in HTC temperature. Hence, HTC temperature was an important factor during the HTC treatment of oily scum when the liquid phase was recycled.
Compositions analysis
The water-soluble chemical components in the liquid product derived from the HTC process of oily scum were detected by GC-MS. Due to species and complexities of chemical components, the first hundred peaks with larger areas were applied to analysis the liquid product. The GC-MS curves and distributions of chemical components in the liquid product are shown in Fig. 6 (a) and (b), respectively.
The fifteen major peaks were observed in the different GC-MS curves in Fig. 6 (a), indicating that the fifteen main chemical components existed in the liquid product. As the HTC temperature increases, the value of Peak 1 (isovaleric acid, C5H10O2), 3 (aniline, C6H7N), 5 (2,4,6-collidine, C8H11N), 6 (o-cresol, C7H8O ), 7 (p-cresol, C7H8O) or 14 ((3S,6S)-3-Butyl-6-methylpiperazine-2,5-dione, C9H16N2O2) increases, while the value of Peak 8 (trans-2,5-Dimethyltetrahydro-4-thiopyrone, C7H12OS), 9 (2,4,6-trimethylaniline, C9H13N) or 10 (1-hydroxy-2,3-dimethylbenzene, C8H10O) decreases, and the value of Peak 11 (2,3,4,5,6-Pentamethylpyridine, C10H15N), 12 (2-(acetylthio) norbornane, C9H14OS) or 13 (3,5-di-tert-butylphenol, C14H22O) reduces or even disappears. In addition, the value of Peak 2 (2-methyl butyric acid, C5H10O2) or 4 (phenol, C6H6O) first increases and then decreases with the increase of HTC temperature. During the HTC process of oily scum, a large quantity of dehydrated carbohydrate compounds were formed by bond breaking of petroleum hydrocarbons, which was further dehydrated and polymerized to form important intermediate substances and small-molecule organic compounds. Especially, these intermediates mainly underwent different reaction pathways to form different contents of gases, oil compounds and microsphere carbons.
According to the analysis of GC-MS curves, the main chemical compounds produced from the HTC process of oily scum were divided into the following six functional groups: ketone and aldehyde, ester and acid, heterocyclic compounds, phenyl amines, phenols, and others. Correspondingly, the relative proportions of the six functional groups were obtained on the basis of the area of each peak, and the results are presented in Fig. 6 (b). The liquid product from the HTC process at 120 oC contained higher amounts of phenyl amines than those from at other temperatures. There were a large amount of intermediate or final products produced in the course of HTC based on the diversity of process conditions and the reaction extent of the relevant mechanisms involved. Phenols increases as the increase of HTC reaction through the dehydration, decarboxylation and decarbonylation reactions which occurred during the HTC treatment of oily scum [20]. Many chemical reactions might appear during the HTC process of oily scum, including dehydration, hydrolysis, decarboxylation, condensation polymerization, and aromatization [21]. However, the HTC process was mainly governed by dehydration and decarboxylation, which meaned that it was exothermal. Simultaneously, functional groups were being eliminated to some extent. Dehydration during the HTC process of oily scum could cover both chemical reaction and physical processes. Chemical dehydration mainly significantly carbonized oily scum by lowering the O/C and H/C ratios, while physical dehydration removed water from oily scum without changing its chemical constitution [21]. Dehydration was generally explained by elimination of hydroxyl groups. Main decarboxylation only appeared after that specific amount of water was formed, but dehydration could be achieved without significant decarboxylation. Main decarboxylation appeared in the HTC process and caused a partial elimination of carboxyl groups. Carbonyl and carboxyl groups rapidly decomposed at more than 150 oC and produced CO and CO2, respectively [22].
After oily scum HTC at 120 oC, the area percentage of phenyl amines in the liquid product accounts for 27.77%, followed by other chemical components (24.95%) and heterocyclic compounds (16.47%). However, when the HTC temperature was more than 120 oC, phenols accounts for a large proportion in the liquid. The change of the distribution of chemical components in the liquid product was mainly caused by the change of the chemical structure of hydrochars during the HTC process of oily scum. At 120 oC, the petroleum hydrocarbons of oily scum were transformed along the dehydration and even oxidation directions. The long-chain hydrocarbons or oil were decomposed and hydrolyzed, thereby improving the dewaterability. This was proved by the chemical components of the HTC liquid, which included many long chain organic species, such as 2,3,4,5,6-Pentamethylpyridine (C10H15N) and 3,5-di-tert-butylphenol (C14H22O).
SEM images before and after hydrochar combustion
SEM images before hydrochar combustion
The microscopic morphologies and microstructures of hydrochars derived from the HTC process of oily scum are measured by SEM images shown in Fig. 7. There was a very rough and heterogeneous surface structure presented on hydrochars. Moreover, the diameter of hydrochars with higher temperature (240 oC) was bigger than that of hydrochars with less than 240 oC (120, 150 and 210 oC). It was observed from Fig. 7 (a) that the surface of the hydrochar-120 oC displayed compact and dense in structure or arrangement. Additively, it exhibited an irregular crack structure. This was because that HTC produced myriad of tiny bubbles and enabled more vapor either enter the liquid phase or react on the gas-liquid/gas-solid interface. Meanwhile, the pit structure was also continually corroded and the residual organic structure in hydrochars was destroyed via decomposition reactions such as hydrothermal cracking and hydrolysis [23]. Therefore, with the effect of reaction temperature, petroleum hydrocarbons in oily scum took place a variety of reactions such as hydrothermal cracking, hydrolysis, depolymerization, oxidation, and rearrangement [24]. Finally, a pit structure formed which followed the previous research [13]. As the reaction temperature increased, the pit structure in hydrochars gradually became bigger.
Hydrochar formation was a series of very complex reactions under the hydrothermal conditions of oily scum, and many reactions including hydrolysis, dehydrations, condensations and polymerizations occurred at the same time. At 120 oC, the petroleum hydrocarbons in oily scum primitively decomposed, resulting in a rough surface of hydrochar matrix. Additionally, hydrochar-150, 210 and 240 oC all had the structure of hydrochar matrix. From Fig. 7 (a)~(d), the microsphere carbon particles were uniformly inserted into the hydrochar matrix whereas those particles were dispersed on the surface of the hydrochar matrix and prone to agglomerate. This was due to the fact that subcritical water raised vapor pressure, decreased surface tension and increased ionization constant, making the solid product formed in a sphere shape and distributed homogeneously [25]. As the HTC temperature increased, the size of the agglomerate became bigger. It may be attributed to the fact that severe hydrothermal conditions facilitated the formation of the inner-sphere surface complexes, while wet particles favored the appearance of outer-sphere surface complexes. Furthermore, as shown in Fig. 7 (d), the pore and crack structures of hydrochar-240 oC were more developed than those of hydrochar with less than 240 oC. These phenomenons showed that the HTC treatment under higher temperatures was a more effective way for the preparation of porous hydrochars. The porous of hydrochars was due to the decomposition and release of petroleum hydrocarbons in oily scum, which caused the blockage of many pores. The bigger pores and more irregular matrix of the hydrochar products demonstrated that the increase of HTC temperature could efficiently accelerate the dewaterability and decomposition process of oily scum. Hence, the hydrochar samples with higher degree of carbonization could be obtained.
Due to the improved hydrophobicity, the hydrochar products showed strong resistance against water immersion. Hydrochar-240 oC was consisted of scattered pieces combined with small particles, suggesting that a majority of petroleum hydrocarbons in oily scum was degraded. The pit structure of the hydrochars increased gradually with the increasing of HTC temperature, indicating that oily scum could be carbonized via HTC process. The increase of HTC temperature enhanced significantly the dehydration and decomposition of oily scum. However, since oily scum with high content of moisture was used directed as the initial reaction material during the HTC process, the larger quantity of moisture inside oily scum and the light organic molecules were separated from the oil-moisture structure and reacted with other matters. More sample particles with narrow dimensions were generated when higher temperature existed in the HTC condition, further evidencing that the chain rupture and degradation of petroleum hydrocarbons.
SEM images after hydrochar combustion
(1) SEM analysis
SEM images of the different ash samples after hydrochar combustion are presented in Fig. 8. From Fig. 8 (a)~(d), the microscopic characteristic of ashes after hydrochar combustion exhibited a comparatively rough and uneven surface with a large quantity of particles. After hydrochar-120 oC was oxidized in the air condition, the surface of ash-120 oC yielded large amounts of ash and slag particles with different sizes, but it still remained the hydrochar matrix before hydrochar combustion. When the HTC temperature increased to 150 oC, the surface of ash-150 oC began to appear fusion and slagging, which were spread over the surface in a sheet. When the HTC temperature continued to rise to 210 or 240 oC, the surface of ash-210 oC or ash-240 oC clumped and agglomerated to some extent. However, the fusion and slagging phenomena of hydrochar-150 oC were the most serious in this study. It could be seen that the degree of ash fusion and slagging after hydrochar combustion increased in varying degree with the extension of HTC temperature.
After HTC, hydrochars may have lower ash content than oily scum because the inorganic elements were released and dissolved in the liquid phase. Hydrochars with lower ash content would burn more cleanly and efficiently since the decrease of inorganic matters such as K, Na, S, Si, Cl and Ca caused easily fouling, slagging, and corrosion in combustors [26,27].
(2) EDX analysis
The slagging and fouling characteristics of solid fuels are important problems due the directly relation to the combustion performance in power plant boilers. To characterize hydrochars derived from different HTC temperatures in terms of their tendency to cause slagging and fouling on boiler heating surface, the ash composition after hydrochar combustion were analyzed. The EDX results of different ash samples after hydrochar combustion are shown in Fig. 9.
From Fig. 9, the ash compositions could be significantly changed after hydrochar combustion. The results of EDX analysis in Fig. 9 illustrated that the main elements of ashes were C (7.945~37.015%) and O (48.48~68.215%) after hydrochar combustion. There also were Na, Mg, Al, Si, Ca, Fe and P elements in ashes. It could be seen that the carbon content of ashes firstly decreased and then increased as the HTC temperature increased from 120 to 240 oC, reaching the minimum of 7.95% at ash-210 oC. However, the oxygen content was the opposite of the carbon content. On the whole, the increase of HTC temperature significantly changed the microstructures and compositions of hydrochars.
Combustion characteristics of hydrochars
Combustion behaviors
Thermogravimetric analysis data can be used to investigate the thermal degradation of different materials and kinetic parameters [28]. The TG and DTG curves from the combustion of hydrochars derived from the different HTC temperatures of oily scum are shown in Fig. 10. Also, the combustion characteristic parameters of hydrochars are listed in Table 1, 2 and 3. From Fig. 10, on the whole, the TG curve of hydrochar combustion shows an initial weight loss attributed to waster evaporation before 150 oC, followed by a flat region up to about 210 oC. Next, the three or four continue branches on each TG curve extend up to about 640 oC where the major weight loss rate was obtained. Last, the TG curve got flat until the combustion temperature reached 900 oC.
According to the weight loss rate in the DTC curve, the combustion process of hydrochars could be divided into four stages: (1) the evaporation stage of the moisture and low boiling point organic matters; (2) the devolatilization and combustion stage of volatile matters; (3) the burnout stage of fixed carbon; (4) the oxidation stage of inorganic compounds. The different temperature stages are shown in Table 1. Compared to hydrochar-120 oC and hydrochar-150 oC, the thermal decomposition temperature of hydrochar-210 oC and hydrochar-240 oC added a typical process of devolatilization in the temperature range of 360-410 oC and 370-410 oC, respectively. For hydrochar-120 oC and hydrochar-150 oC, the third weight loss rate peak was noticeable higher than the former two weight loss rate peaks because the volatile matters of oily scum were partly decomposed at a relatively low HTC temperature and were converted to thermally stable tar and hydrochars. However, for hydrochar-210 oC and hydrochar-240 oC, the first weight loss rate peak was higher than other three weight loss rate peaks. As expected, at a relative high HTC temperature most of the volatile matters in oily scum were decomposed with the increase of hydrothermal reactivity, and they were mostly in the form of easily crackable volatile and partly in the form of char. This caused the increase of volatile matters during the primary decomposition period of hydrochar combustion, bringing the increase of the first weight-loss rate peak. Also, the low proportion of organic matters and high proportion of ash decreased the fourth weight-loss rate peak of hydrochar-210 oC and hydrochar-240 oC.
Table 1 Combustion stages and characteristic temperatures of hydrochars
Samples
|
Evaporation stage
|
Weight loss at first stage/%
|
Devolatilization and combustion stage/oC
|
Weight loss at second stage/%
|
Burnout stage/oC
|
Weight loss at third stage/%
|
Oxidation stage
|
Weight loss at fourth stage/%
|
Phase 1
|
Phase 2
|
Phase 3
|
Hydrochar-120 oC
|
~185
|
1.58
|
185-370
|
-
|
370-498
|
-69.94
|
498-620
|
-24.62
|
620-900
|
0.07
|
Hydrochar-150 oC
|
~170
|
1.28
|
170-370
|
-
|
370-488
|
-69.66
|
488-610
|
-25.12
|
610-900
|
-0.54
|
Hydrochar-210 oC
|
~170
|
-0.31
|
170-360
|
360-410
|
410-502
|
-77.82
|
502-620
|
-23.80
|
620-900
|
-0.18
|
Hydrochar-240 oC
|
~170
|
0.05
|
170-370
|
370-410
|
410-495
|
-70.97
|
495-640
|
-27.00
|
640-900
|
-0.18
|
Table 2 summarized the characteristic temperatures and peak points of the combustion process of hydrochars. Ti and Tb of hydrochars first decreased and then increased with the increase of HTC temperature, both reaching the minimums at 150 oC. This was possible caused by the decomposition and repolymerization of organic matters in subcritical water. At 150 oC the decomposition reactions of the organic matters in hydrochars were dominant, causing Ti and Tb of hydrochars to the minimums. At 210 and 240 oC, the organic matters in hydrochars mainly occurred the repolymerization, leading Ti and Tb of hydrochars to increase [29]. So, HTC temperature is an important factor affecting the combustion characteristics of hydrochars.
Table 2 Combustion characteristic temperatures and indexes of hydrochars
Samples
|
Ti/oC
|
Tpeak1/oC
|
Tpeak2/oC
|
Tpeak3/oC
|
Tpeak4/oC
|
Tb/oC
|
Tmax/oC
|
S/
10-7oC-3·min-2
|
Rw/
-(%·min-1·oC-2)
|
Cr
-(10-5oC-2·min-1)
|
Hydrochar-120 oC
|
260
|
317
|
466
|
533
|
-
|
618
|
533
|
8.34
|
6.29
|
22.51
|
Hydrochar-150 oC
|
248
|
303
|
461
|
527
|
-
|
610
|
527
|
6.78
|
4.74
|
17.59
|
Hydrochar-210 oC
|
262
|
328
|
401
|
459
|
546
|
619
|
328
|
7.40
|
8.37
|
18.30
|
Hydrochar-240 oC
|
273
|
327
|
400
|
460
|
562
|
639
|
327
|
5.76
|
7.32
|
15.32
|
Table 3 Combustion characteristic parameters of hydrochars
Samples
|
DTGpeak1/
%·min-1
|
DTGpeak2/
%·min-1
|
DTGpeak3/
%·min-1
|
DTGpeak4/
%·min-1
|
DTGmax/
%·min-1
|
Mf/%
|
Hydrochar-120 oC
|
-9.51
|
-6.76
|
-15.22
|
-
|
-15.22
|
5.15
|
Hydrochar-150 oC
|
-9.98
|
-6.88
|
-10.82
|
-
|
-10.82
|
4.56
|
Hydrochar-210 oC
|
-12.56
|
-4.05
|
-9.03
|
-8.49
|
-12.56
|
-2.02
|
Hydrochar-240 oC
|
-11.42
|
-8.27
|
-8.62
|
-7.52
|
-11.42
|
1.22
|
Notes: Ti, Tpeaki and Tb are the ignition temperature of each sample, the temperature according to the max combustion rate at the i peak, and the burnout temperature of each sample, respectively; DTGmaxi is the maximum mass loss rate at the i peak; Mf is residual mass at 900 oC.
As shown in Table 2, Tmax was gradually shifted to a lower temperature as the HTC temperature increased. The hydrochar-120 oC or hydrochar-150 oC had higher Tmax value as compared to hydrochar-210 oC or 240 oC, indicating that the increase of HTC temperature improved fuel quality of subsequently produced hydrochars. This improvement was mainly attributed to the decreased volatile matter content and increased fixed-carbon content in hydrochars with an increase of HTC temperature. From Table 2, the Ti value for hydrochar-120 oC or hydrochar-150 oC was lower than that of hydrochar-210 oC or 240 oC, indicating that hydrochar-120 oC or hydrochar-150 oC ignited easily. This was because that the HTC process of oily scum removed volatile matter to a certain extent, and hydrochars from low HTC temperature had more volatile matter than that from high HTC temperature. Hence, hydrochar-120 oC or hydrochar-150 oC with more volatile matters resulted in lower Ti value and had the performance of easy ignition and combustion. However, hydrochar-240 oC with the highest burnout temperature is safer than hydrochar-120 oC, hydrochar-150 oC and hydrochar-210 oC during handling, storage and transportation. Higher Tb value indicates thermally stable fuel with more prolonged combustion phase. Hydrochar-240 oC had the highest Tb value and thus took longer time to burnout, showing its thermal stability as fuel.
In order to further evaluate the combustion performance of the hydrochars as solid fuel, S, Rw and Cr were calculated according to Eq (1), (2) and (3), respectively. The results in Table 2 showed that as the HTC temperature increased from 120 to 240 oC, S, Rw and Cr decreased initially, followed by an increase, but then again decreased. The change trend of S, Rw or Cr was according with that of DTGmax. This was because that the increase in HTC temperature caused a higher combustion reactive of hydrochars; therefore, there was a higher combustion rate for organic matters, and at the same combustion stage, the reaction time was shorter and combustion temperature was higher when the hydrochars were decomposed and oxidized. Additionally, S and Cr both reached the maximum values at hydrochar-120 oC, while Rw got the maximum value at hydrochar-210 oC. This was mainly due to the removal of a large amount of inorganic metals from oily scum with an increase in HTC temperature, resulting in the decrease of the catalytic effect of inorganic metals on hydrochars [30,31] and the reduction of S, Rw and Cr. S, Rw or Cr at hydrochar-210 oC began slightly to increase because a higher HTC temperature could accelerate the degradation of petroleum hydrocarbons in oily scum and the formation of hydrochars with high carbon content, thereby improving the combustion performance of hydrochars and obtaining better S value. These observations may indicate that the combustion characteristics of hydrochar-120 oC were better than those at the other HTC temperatures.
The S value of hydrochar-120 oC was higher than that of other hydrochar samples. Hydrochar-120 oC performed the best S value with 8.34×10-7 oC-3·min-2, followed by 7.40-, 6.78- and 5.76×10-7 oC-3·min-2 for hydrochar-210 oC, hydrochar-150 oC and hydrochar-240 oC, respectively. This suggested that the hydrochars had an optimal parameter to improve the combustion performance under appropriate HTC reaction temperature. The volatile matters of hydrochars decreased and the carbon content increased after the HTC treatment of oily scum, thus the combustion of hydrochars may be less violent and the flame was more stable. It could be found that the HTC reaction temperature had significant influences on combustion behaviors and characteristics of hydrochars. Generally, HTC could convert oily scum into solid fuel with an improved combustion performance.
Kinetic parameters
To date, there have been relatively few reports on evaluating the activation energy (E) change of hydrochars from the HTC process of oily scum. In order to confirm the combustion mechanism and E of hydrochars produced from the HTC treatment of oily scum, the kinetic parameters based on Arrhenius equation were applied for all hydrochar products in this study [32]. The first-order kinetic reaction model is typically used for the combustion process of solid fuel [33]. Therefore, the pre-exponential factor (A), E and correlation coefficient (R) associated with the combustion process of hydrochars were calculated according to the first-order reaction model equation [34]. The calculated results were shown in Table 4.
Table 4 Kinetic parameters of the different hydrochar samples
Samples
|
Combustion kinetic parameters
|
E/kJ•mol-1
|
A/min-1
|
R2
|
Hydrochar-120 oC
|
40.03
|
178.59
|
0.8982
|
Hydrochar-150 oC
|
37.62
|
119.48
|
0.8797
|
Hydrochar-210 oC
|
34.58
|
58.49
|
0.9410
|
Hydrochar-240 oC
|
32.15
|
27.00
|
0.9372
|
A high correlation coefficient indicated that the first-order reaction model fitted well to explain the combustion process of hydrochars produced from oily scum. E provides important information in the minimum amount of energy needed to initiate a reaction. From Table 4, the E value of hydrochars varied with the HTC temperature of oily scum, indicating that temperature affected the thermal reaction. Also, as the HTC temperature increased, E showed a decreasing trend and reached its minimum value (32.15 kJ/mol) at hydrochar-240 oC. A presented the same downward trend as E upon increasing the HTC temperature. The hydrochar-120 oC showed relative higher E (40.03 kJ/mol) compared with other hydrochars. The decrease of activation energies of hydrochars at higher HTC temperatures (150, 210 and 240 oC) was mainly caused by the decomposition and destruction of complicated petroleum hydrocarbons structure of oily scum in HTC process. The combustion of hydrochars-240 oC is suggested to be easier because of its lower activation energy and pre-exponential factor.
According to the results above, it can be seen that HTC temperature has significant effects on appearance characteristics and combustion behaviors of hydrochars derived from oily scum. Table 4 presents the whole kinetics of weight loss occurred during the thermal reactions of hydrochars in TGA. The combustion characteristics and kinetic parameters obtained in this study could be used to predict the combustion behaviors of hydrochars during combustion in a fluidized bed boiler or other boilers. Overall, it is observed that HTC temperature of oily scum improves the properties of hydrochars. Therefore, the effective recovery and rational utilization of petroleum hydrocarbons of oily scum waste using HTC and combustion technology gives us a new opportunity.