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Study on co-combustion characteristics of hydrochar and anthracite coal
Journal of the Energy Institute xxx (xxxx) xxx
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Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Study on co-combustion characteristics of hydrochar and anthracite coal Nan Zhang a, b, Guangwei Wang a, b, *, Jianliang Zhang b, Xiaojun Ning b, **, Yanjiang Li b, Wang Liang b, Chuan Wang c, d a
State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, China School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, China Swerim AB, SE-971 25, Luleå, Sweden d Thermal and Flow Engineering Laboratory, Åbo Akademi University, Åbo, FI-20500, Finland b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 June 2019 Received in revised form 17 October 2019 Accepted 21 October 2019 Available online xxx
Non-isothermal thermogravimetric analysis was employed to study the combustion reaction behavior of three different hydrochars and one anthracite in the paper. The particle size distribution analysis, scanning electron microscopy, specific surface area and Raman spectroscopy were also used to observe and characterize physiochemical properties. The Coats-Redfern kinetic model method was used to calculate the activation energy of different combustion reactions. The results showed that the combustion of fiber reject had a more effective combustion property than the other two hydrochars. The main reason was that fiber reject had higher specific surface area and lower carbon microcrystalline structure. Meanwhile, the co-combustion of different hydrochars with coal had similar combustion trends, and with the increase of hydrochar proportion, the combustion rate was higher. The results of the kinetic calculation correspond to this conclusion. It can be found that as the proportion of the three hydrochar increased, the activation energy of two stages gradually decreased. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Hydrochar Pulverized coal Combustion Kinetic
1. Introduction In recent years, the global economy has developed rapidly combined with the pace of social industrialization further accelerated. The rapid development of the economy is based on the transitional dependence on energy consumption. However, the consumption of resources will be accompanied by some new problems, including the sharp increase in the amount of waste, such as municipal waste, paper sludge, sewage sludge, etc., which are generated during urbanization [1e4]. Although the degree of mechanization of agriculture has increased the utilization rate of biomass resources, it has also caused an obvious problem in the amount of biomass sludge and other various organic wastes, but these wastes are difficult to further process prepared as materials that can be directly used by other industries [5,6]. At the same time, it also has a serious impact on environmental protection due to their own pollution problems. Many methods have been proposed for the rational use of these waste resources to achieve their recycling. Zhang et al. [7] detailedly study the change of gas production in anaerobic digestion reaction by changing the time and temperature conditions based on the principle of thermal hydrolysis. Zhou et al. [8] investigated the ignition characteristics and combustion characteristics under different oxygen concentrations and found that the volatility, the ignition and the burning out time of the carbon are reduced. Meanwhile, hydrothermal carbonization technology is considered to have broad applications. The core of this technology is based on the principle of carbonizing the raw material with low carbon content by subjecting the raw material to a series of complex chemical reactions in a high temperature and high pressure closed environment. This technology has significant advantages: a wide range of raw materials which include a variety of waste that difficult to treat, clean and efficient, convenient and feasible, and especially sustainable [9]. As for production (hydrochar), it has the characteristics of high carbon content, reduced volatile matter and ash content compared with the original substance, increased energy density, and a certain degree of
* Corresponding author. State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, China. ** Corresponding author. E-mail addresses: [email protected] (G. Wang), [email protected] (X. Ning). https://doi.org/10.1016/j.joei.2019.10.006 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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moisture content removal [10]. However, there are still some disadvantages in industrial utilization. Although carbonization process increases its carbon content in a certain degree, its thermal value is still not high in general. And because of high ash content, its energy density is still lower than traditional primary energy sources such as coal. As a result, its use as fuel is not in large scale nowadays. Some works have been done in the field of hydrochar. Issac et al. [11] investigated the mixed combustion process of biomass and coal. The results showed that the addition of wood chips had a significant effect on the combustion of the mixture. Nakason et al. [12] compared the effects of different reaction conditions on hydrochar products and found that different temperatures had a greater influence on the hydrogen, carbon and oxygen elements of products, which is attributed to the condensation or polymerization of HTC. Heidari et al. [13] comprehensively introduced the current status of hydrochar development technology and the dilemmas it faced, and found that there are still some problems in the construction of models and data supplementation. However, there are relatively few studies on the co-combustion of hydrochar and coal at present, so it is necessary to conduct an in-depth study on the common reaction behavior between them. This will provide a reference for improving the rational utilization of hydrochar and saving primary energy. In this study, three kinds of hydrochar samples and one anthracite have been chosed to conduct the experiment. The thermogravimetric analysis method and the advanced technologies such as structural characterization were used to investigate the reaction characteristics of different proportions of hydrochar mixed with anthracite. Also, the conclusions obtained can have a certain reference significance for technology of hydrochar injection into blast furnace. 2. Experimental 2.1. Preparation of raw materials Three kinds of hydrochar samples selected in this paper were biosludge, sewage sludge and fiber reject waste sludge, which were labeled as BS, SS and FR respectively. The coal sample used in this study is a kind of anthracite used in the blast furnace injection in a steel company, which was collected and recorded as AC. Before performing all kinds of other tests apart from proximate analysis, the sample was dried in a forced air oven at 105 C for 24 h to remove the free water and then ground and sieved to a particle size of less than 74 mm using a standard sieve to meet the experimental requirements. The proximate and ultimate analysis results were shown in Table 1 and the results of ash composition of all samples were shown in Table 2. Particle size distribution test were constructed on Mastersizer 3000 analyzer. Scanning electron microscopy (SEM, FEI Quanta-450) was used to observe the microscopic morphology of the samples with experimental conditions of 15 kV voltage. The magnifications were set as 2000 and 4000 times respectively. Pore structure tests were performed using Quadrasorb SI gas analyzer. The Raman spectra of all samples were determined by LabRam HR Evolution spectrometer so as to analyze the carbon structure of different materials. The experiment process includes: The sample was placed on a glass plate and irradiated with excitation light at 532 nm to generate scattering. The light spectra whose wavelength was shifted can be recorded, and the characteristics of some specific molecules in the sample were analyzed qualitatively or quantitatively. 2.2. Thermogravimetric analysis Thermogravimetric experiments were carried out using the HCT-4 microcomputer differential thermal balance produced by Beijing Henven Scientific Instrument Factory. Its structural diagram is shown in Fig. 1. Before the experiment, the sample of 5 mg weighing with high precision balance was taken, and then the sample was placed in a platinum crucible and reacted in an air environment. In the reaction process, the temperature rising rate was set to 20 C/min by means of program temperature control. The sample was heated from room temperature to 900 C, and a 10-min holding time was set up to ensure that the furnace temperature and sample temperature can be kept consistent. 2.3. Description of kinetic model The main purpose of kinetic analysis is to explore the mechanism of energy change rule during the reaction process and reveal the difficulty of different substances to overcome the energy barrier when the reaction occurs by comparing the activation energies of different chemical reactions, that is, whether the chemical reaction is easy to carry out. Here, the method to calculate the activation energy of coal and hydrochar combustion reaction is Coats-Redfern model. The specific calculation principle of this method is shown as follows: the initial mass of reactants is m0, and the mass of the reaction up to a certain time is denoted as mt. The reaction rate formula is expressed as:
da = dt ¼ kf ðaÞ
(1)
Table 1 Proximate and ultimate analysis of different samples (wt%). Sample
AC BS SS FR a
Proximate analysis
Ultimate analysis
Mad
FCada
Aad
Vad
Cd
Hd
Oda
Nd
Sd
0.53 4.06 1.81 1.51
79.33 6.72 7.18 28.48
10.60 46.58 38.76 23.28
9.54 42.64 52.25 46.72
80.78 28.13 33.76 50.96
3.25 2.36 3.77 4.50
3.31 17.63 20.58 15.06
1.04 0.93 0.96 3.01
0.49 0.31 0.36 1.67
Calculated by difference. FC, fixed carbon; A, ash; V, volatile matter; d, dry basis.
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Table 2 Ash composition of different samples (wt%). Sample
SiO2
Fe2O3
Al2O3
K2O
CaO
MgO
ZnO
SO3
AC BS SS FR
47.69 14.79 19.57 11.78
4.36 4.29 8.28 22.99
36.44 8.07 11.39 5.37
0.76 0.28 0.45 0.57
3.93 62.53 48.34 22.07
0.59 2.78 3.01 6.46
0.01 0.08 0.13 0.22
4.12 1.49 1.61 8.39
Fig. 1. Schematic diagram of TG analyzer.
k ¼ A exp½E = ðRTÞ
(2)
f ðaÞ ¼ ð1 aÞn
(3)
a ¼ ðmo mt Þ = ðmo m∞ Þ
(4)
here, A is the pre-exponential factor, min1; R is the gas constant, 8.314 103, kJ/(mol$k); t is reaction time, minutes; T is the temperature, K; a is the conversion rate of the sample; mo is the initial mass of the sample, mg; mt is the instantaneous mass of the sample at time t, mg; m∞ is the final mass after the reaction, mg; n represents the reaction order. Substituting the heating rate parameter b ¼ dT=dt into the above formula for integral calculation:
da A ¼ ð1 aÞn exp½E = ðRTÞ dT b
(5)
We assume the combustion reaction as a first-order reaction, and the following formula can be derived:
lnð1 aÞ 2RT E ¼ ln AR 1 ln E RT bE T2
(6)
For activation energy calculation of most first-order reaction integral equations, the first term on the right side of Eq. (6) is approximately equal to a constant. Based on this, a straight line can be obtained by solving the parameters of independent variables and dependent variables through this formula and conducting linear fitting, and the slope of the straight line is E=R, from which the apparent activation energy of the reaction will be solved and the intercept of the straight line can also be calculated. 3. Result and discussion
3.1. Physical and chemical characteristics 3.1.1. Particle size distribution analysis The particle size distribution results of four samples were shown in the Fig. 2. The meaning of each coordinate in the figure can be explained as: the abscissa represents the particle size distribution interval (mm) and the right vertical axis represents the columnar distribution diagram of particle size distribution content in the corresponding particle size interval. The left ordinate is the cumulative sum of the values of the histogram corresponding to different particle size intervals, i.e., the sum of the contents of all samples less than this particle size within a certain particle size interval. In general, when the sample content in the lower particle size range is higher, its combustion performance may be better. To some extent, particle size distribution can affect the specific surface area of samples. The specific surface area Please cite this article as: N. Zhang et al., Study on co-combustion characteristics of hydrochar and anthracite coal, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.10.006
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Fig. 2. Particles size distribution of different samples.
of the sample is positively correlated with its pore size or particle size distribution. Larger pore structure and higher content in the lower particle size distribution area are closely related to the specific surface area of the sample [14e18]. Here, the particle size values when the sample content reached 10% (X10), 50% (X50) and 90% (X90) were used to compare the changes of particle size of different samples. Results were shown in the Table 3. We can see that the average within the range of X10 and X50, the particle size of three hydrochar samples BS, SS and FR was significantly smaller than that of coal sample AC. In the range of X10, the particle size distribution of three hydrochars can be ranked as FR > BS > SS from fine to coarse. When the content accumulated to 50%, it was still the smallest average particle size of the FR sample, while the particle size of BS and SS samples was close to each other. In the range of X90, the particle size of both FR and BS samples was smaller than that of SS. It can also be observed from the distribution diagram that the FR sample had a peak value below 10 mm, indicating that its particle size was better than the other two samples [11,19e21]. 3.1.2. Micromorphology analysis From size results, different hydrochars had a different size distribution, based on this, it is necessary to pay attention to microstructure. SEM images of four samples at different magnification ratios were shown in Fig. 3. Due to the small particle size of the selected sample, the magnification was set as 2000 times (corresponding to 50 mm scale) and 4000 times (corresponding to 20 mm scale) respectively during the experiment process in order to better observe the micro-morphological characteristics of different samples. It can be found from the pictures that the microscopic morphology of samples with different types and structures varied greatly. For the coal sample AC, there were some small irregular blocks on the surface and the surface of the rest part was relatively flat without any indented pores or slits. The microstructure of three hydrochars was consistent with their original sample. Among them, the surface of BS sample at 2000 times was distributed with a lot of micro-convex particles arranged in disorderly, where the pores between irregular convex particles were not very obvious. As the magnification was increased to 4000 times, the irregular convex blocks became closely arranged and distributed with each other. Although the particles had different shapes, there were no significant concave pores between them. As for the SS sample at 2000 times, there were many snowflake particles of different sizes on its surface and among these particles were embedded many hollow pores, whose size and depth varied with the size of snowflake particles. After the magnification was turned to 4000 times, it can be clearly observed that there were pore structures during the convex blocks. The intuitive expression in the picture was that the area of the black shaded part of the sample increased. For the FR sample, many small uneven particles can be observed on the surface at a magnification of
Table 3 Particle size analysis results of different samples (%). Sample
X10
X50
X90
AC BS SS FR
9.83 3.55 5.92 1.88
48.81 31.12 51.82 27.45
88.59 111.23 272.31 163.44
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Fig. 3. SEM graphs of selected samples at different magnifications: AC-(a, b); BS-(c, d); SS-(e, f); FR-(g, h).
2000 times, and micro-pores of similar size and depth were distributed among them. In the picture of 4000 times, the amount of micro particles on its surface was more, but the size was smaller than that of SS samples. Furthermore, the number of pore structure between surface particles of SS was more than that of the first two samples. Anthracite is a highly ordered structure with benzene ring as the core unit. The macroscopic performance is that its stable structure smooth and smooth surface. From the proximate analysis, it can be seen that
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the hydrochar has significantly higher volatile content, which indicates few stable frameworks in the structure. The escape of a large number of unstable components during hydrothermal treatment makes a huge contribution to a significantly more developed pore structure than anthracite [22e24]. 3.1.3. Specific surface area analysis In order to quantitatively analyze the development degree of pore structure, the specific surface area was studied. The Fig. 4 showed the graph of the specific surface area and pore diameter distribution of the samples. The adsorption curve types of the four samples were close to the anti-s type, for which the adsorption process was changed from single-layer adsorption of micropores to multi-layer adsorption of mesopores or even macropores, the adsorption hysteresis phenomenon also existed in all the curves [25]. According to the curves, the pore systems of four samples were continuous and completed with the pore size distribution ranging from micropores to mesopores. The specific surface area results of four samples were shown in Table 4. As we can see, the coal sample was the smallest of all, followed by BS, SS. The largest was the FR sample. This phenomenon was consistent with the results observed by SEM. It indicated that there were many inherent hollow pore structures on the surface of FR samples, and the pore volume and pore radius of the porous solid of SS and BS samples were both smaller compared with FR. The microscopic morphology of the coal sample and the three hydrochars made a big difference, which can be contributed to the less developed pore structure and undeveloped pores. From the pore size distribution results, it can be concluded that the peak of the pore size distribution of the coal sample was in the microporous portion, and the pore sizes of the three hydrochars were similar, mainly concentrated in the mesopores, but some differences still exist. The FR sample had the highest pore volume, which was two orders of magnitude higher than BS and SS samples. 3.1.4. Raman analysis The carbon microcrystalline structure was considered to have a great correlation with the activity of the material. It was also used here to analyze the microcrystalline structure characteristics of different hydrochars. The Raman analysis results of different samples were shown in the Fig. 5. According to the method of Sadezky et al. [26], four lorentz-shaped wave bands were found near the wave number of 1580 cm1,1350 cm1,1620 cm1 and 1200 cm1 in the Raman spectrum of carbonaceous materials, which were respectively expressed as G, D1, D2 and D4. Another gaussian shaped wave band was located at the position of 1500 cm1, which was denoted as D3 shaped band. The single peak at 1580 cm1 corresponded to E2g mode vibration, while the peak at 1360 cm1 was usually attributed to the atomic vibration mode of the disordered carbon structure [27e29]. Due to the increased contribution of disorder to this peak, the peak at 1350 cm1 can be stronger [30]. The ratio of the D peak caused by the disordered carbon to the G peak of the E2g mode vibration R ¼ ID/IG can sensitively and quantitatively characterize the disorder degree of carbonaceous materials. Since there was no disorder characteristic, the pure graphite sample only showed a G peak. However, if the crystal suffered a long-term periodic damage its disorder degree was deepened, the D peak
Fig. 4. Pore structure measurement results of different samples: (a) Adsorption and desorption curves; (b) Pore size distribution curves.
Table 4 Specific surface area results of different samples. Sample
AC
BS
SS
FR
BET Surface Area
0.468 m2/g
6.748 m2/g
8.964 m2/g
44.086 m2/g
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Fig. 5. Raman fitting results of different samples.
appeared and its strength increased accordingly. Comparing three kinds of hydrochar spectra available, their spectral types were similar to that of amorphous carbon, which belonged to the hybrid unstructured mixture of the SP3 hybridization and SP2 hybridization carbon, R value of four types of sample were shown in the Fig. 6, Among the three hydrochar samples, the disorder degree of FR sample was the highest, and the lowest was BS samples. This reflected the difference of their chemical reaction performance. 3.2. Thermogravimetric analysis Three different kinds of hydrochar and coal powder were heated at a heating rate of 20 C/min for combustion reaction. The combustion conversion curves and conversion rate curves were shown in Fig. 7. In the figure, for three hydrochar mixtures, the combustion process can be divided into two stages: the first stage was the combustion of volatile substances. The corresponding temperature range was between 300 C and 500 C; the second stage was the combustion of carbonaceous materials, and the reaction temperature at this stage was between 500 C and 800 C. After the combustion reaction was completed, the remaining non-combustible material mainly consisted of ash. The combustion characteristics of different samples were shown in the Table 5. For hydrochars, the initial combustion temperature of FR was the lowest, and then in turn were BS and SS. By comparing their conversion rate in the first stage, the conversion rate of FR sample reached the fastest which was 0.00391 s1 and its weight loss ratio reached nearly 80% during this period. In the case of SS sample, its conversion rate was lower than FR as 0.00285 s1, and its weight loss ratio in the first stage was 65%, which was also smaller than that of FR sample. The rate
Fig. 6. Raman structure parameters of different samples.
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Fig. 7. Conversion curves and conversion rate curves of different samples at 20 C/min.
Table 5 Combustion characteristics of different samples. Sample
BS SS FR AC
First stage
Second stage
Tmax/ C
DTGmax/s1
Tmax/ C
DTGmax/s1
369.3 325.5 288.3 e
0.00159 0.00285 0.00391 e
748.8 727.5 424.4 595.9
0.00149 0.000799 0.000371 0.00353
peak value of BS was lower than the former two, which was at 0.00159 s1. According to the proximate analysis results, the FR sample had the lowest ash content, so it was the least affected by ash inhibition during combustion reaction, while the BS sample had the highest ash content, and the corresponding combustion reaction rate was the slowest. Ash can be heated to a high temperature along with combustible materials in which process it will consume a certain amount of heat. The higher the ash content, the lower the combustion efficiency. In the second stage of mixture combustion, the conversion and conversion rate had the opposite trend compared with the first stage. It can be seen from the figure that the combustion of hydrocahr started at a lower temperature stage, indicating that the combustion performance of hydrochar was significantly better than that of coal. During three hydrochars, FR had a better combustion behavior compared with the rest. Combined with previous research, specific surface area of FR was larger than SS and BS, which might make a promotion in the combustion because of more area to contact air. With microcrystalline structure results, FR had a higher degree of carbon disorder, this indicated it was more reactive. 3.3. Effect of different ratio on combustion To analyze the co-combustion behavior, three hydrochars were mixed with pulverized coal at ratios of 5%, 10%, 15%, 20%, 30% and 50% respectively with heating rate of 20 C/min. The conversion and conversion rate results were shown in Fig. 8. The combustion characteristics of four samples in different reaction stages were shown in Table 6. The results showed that with the increase of the proportion of hydrochar, the conversion rate of the mixture in the first stage was gradually increased, indicating that the addition of hydrochar can make a promotion effect on mixture combustion. Comparing three kinds of mixture, it can be investigated that the FR had the best combustion-promoting effect, which was consistent with the results of the previous analysis. In the first stage of reaction, with the addition ratio of FR increased from 5% to 10%, the weight loss ratio of the mixture was not obvious, while the proportion increased to 15%, the weight loss variation of the sample increased significantly. When the percentage increased from 15% to 20%, the increase in the weightlessness of the mixture was similar to the change of 5%e10%. The weight loss ratio increased markedly when the FR addition was at 30%, while the change of weight loss ratio also had an increasing trend at 50% ratio of FR, which was yet not as drastic as that of 30% ratio. Because of difference of volatile matter in hydrochar and coal, the volatilization of hydrochar released very quickly, as a result, the combustion of mixture in the first stage was obvious. The effect of SS on promoting the combustion was slightly worse than FR. It can be concluded from the corresponding image that when the adding proportion of SS increased from 10% to 15%, there was also an obvious promotion phenomenon. This indicated that the combustion effect of the hydrochar and coal will be much better when the mixing proportion reached 15%. Considering of curves with 30% and 50% proportion, there was an apparent surge in weightlessness at 30% ratio, it was not only because of the proportion rising larger, but also the reason that when the mixture ratio of hydrochar increased, the reaction condition of the mixture was improved due to its developed pore structure and more disordered carbon microcrystalline structure, which made the total combustion reaction proceed faster. Though with the increase of the proportion, the overall trend of combustion reaction also showed an enhanced trend, BS sample had the worst effect of promoting combustion of these three. Through the contrast analysis of conversion rate curve in the first stage, the combustion rate in descending order can be ranked as FR mixture > SS mixture > BS mixture, and it can be found the temperature corresponding to the rate peak in the burning rate curve of FR mixture and SS mixture was significantly lower than the BS mixture. The faster the burning rate, the greater the amount of reaction at the same time and the higher the conversion rate, the more consistent the x value is with the weight loss curve. Moreover, the temperature corresponding to the peak value of FR mixture and SS mixture combustion rate curve was Please cite this article as: N. Zhang et al., Study on co-combustion characteristics of hydrochar and anthracite coal, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.10.006
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Fig. 8. Conversion curves and conversion rate curves for blended sample (20 C/min): (a, b) BS and AC; (c, d) SS and AC; (e, f) FR and AC.
significantly lower than that of BS mixture. The fact was that the combustion rate was faster, the larger the reaction amount within the same time will be done, thus the value of conversion rate x increased even more [31e33], which was consistent with the results of the weightlessness curve. From above analysis, we can find that during the co-combustion of hydrochar and anthracite, the main characteristic is that it can be divided into two main stages and the combustion rate presents two obvious characteristic peaks. With the increase of hydrocahr content, the combustion process is closer to the combustion behavior of hydrochar. Secondly, the difference in the combustion rate of different mixtures is obvious, which is related to the complex reaction between coal and hydrochar. FR and coal mixture has the largest combustion rate and its combustion promotion effect is the best. 3.4. Kinetic analysis Comparing the combustion curves of three kinds of mixed samples, it can be found that there are obvious weight losses in the temperature range of 300e500 Cand 500e700 C. Based on this and for uniform comparison, these two intervals are defined as the first phase and the second phase, respectively, and the activation energy of the chemical reactions in these two phases were solved. Select the representative fitting results with hydrochar mixing ratio of 10%, 30% and 50% of as shown in Fig. 9 and Fig. 10. The Coats-Redfern kinetic model was used to calculate the activation energy of the chemical reaction of the sample at different stages. Kinetic parameters in the two stages of all samples were shown in Table 7 and Table 8. From the data in the table, the value of the parameter R2 indicating the fitting effect was above 0.9, which was a first-order reaction with a good fitting result. Comparing the results of the first stage, it showed that the reaction activation energy of three mixtures in the corresponding temperature range decreased with the increase Please cite this article as: N. Zhang et al., Study on co-combustion characteristics of hydrochar and anthracite coal, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.10.006
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Table 6 Combustion characteristics of blended samples in two reaction stages. Sample (wt %)
BS-5% BS-10% BS-15% BS-20% BS-30% BS-50% SS-5% SS-10% SS-15% SS-20% SS-30% SS-50% FR-5% FR-10% FR-15% FR-20% FR-30% FR-50%
First stage
Second stage
Tmax/ C
DTGmax/s1
Tmax/ C
DTGmax/s1
388.8 396.8 399.4 396.8 379.7 382.5 353.9 342.5 344.8 340.1 338.7 334.8 331.5 336.5 338.5 333.6 311.9 303.1
3.11E-5 9.26E-5 1.43E-4 2.07E-4 3.49E-4 5.68E-4 3.43E-5 1.11E-4 2.14E-4 3.09E-4 6.58E-4 0.00113 7.82E-5 2.35E-4 3.91E-4 5.22E-4 0.00112 0.00136
597.6 597.3 600.2 597.7 595.8 603.5 599.9 601.9 600.2 600.6 605.9 601.5 596.1 596.2 594.1 592.1 593.9 585.4
0.00358 0.00348 0.00341 0.00334 0.00295 0.00247 0.00344 0.00337 0.00328 0.00322 0.00264 0.00225 0.00344 0.00336 0.00331 0.00321 0.00233 0.00175
Fig. 9. Experimental and model profiles for the samples at heating rate of 20 C/min in the first stage.
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Fig. 10. Experimental and model profiles for the samples at heating rate of 20 C/min in the second stage.
Table 7 Kinetic parameters of blended samples in the first stage. Sample (wt%)
T/ C
Slope
E/kJ$mol1
R2
BS-5% BS-10% BS-15% BS-20% BS-30% BS-50% SS-5% SS-10% SS-15% SS-20% SS-30% SS-50% FR-5% FR-10% FR-15% FR-20% FR-30% FR-50%
350e480 350e480 350e480 350e480 350e480 350e480 320e490 320e490 320e490 320e490 320e490 320e490 300e420 300e420 300e420 300e420 300e420 300e420
5596.8 2638.6 1714.9 1859.8 1287.1 1252.8 3393.1 1710.2 1043.3 865.6 913.1 848.6 3240.8 2309.3 2295.7 2012.9 1148.5 943.3
46.53 21.94 14.26 15.46 10.70 10.42 28.21 14.22 8.67 7.20 7.59 7.06 26.94 19.20 19.09 16.74 9.55 7.84
0.9976 0.9909 0.9912 0.9821 0.9758 0.9779 0.9815 0.9701 0.9873 0.9894 0.8984 0.8998 0.9619 0.9823 0.9882 0.9961 0.9691 0.9454
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N. Zhang et al. / Journal of the Energy Institute xxx (xxxx) xxx Table 8 Kinetic parameters of blended samples in the second stage. Sample (wt%)
T/ C
Slope
E/kJ$mol1
R2
BS-5% BS-10% BS-15% BS-20% BS-30% BS-50% SS-5% SS-10% SS-15% SS-20% SS-30% SS-50% FR-5% FR-10% FR-15% FR-20% FR-30% FR-50%
520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650
9867.1 9276.8 8629.6 8455.8 7069.4 5176.4 9483.7 8808.7 8200.4 7526.2 5732.8 4540.1 9247.2 8762.2 7996.9 7580.2 5174.4 5037.5
82.04 77.13 71.75 70.30 58.77 43.04 78.85 73.24 68.18 62.57 47.66 37.75 76.88 72.85 66.49 63.02 43.02 41.88
0.9995 0.9986 0.9953 0.9947 0.9883 0.9623 0.9986 0.9971 0.9928 0.9892 0.9682 0.9510 0.9984 0.9972 0.9861 0.9851 0.9511 0.9603
of the proportion of hydrochar, indicating that the addition of hydrochar can improve the reaction of mixed samples and made the reaction easier to be produced. The microstructure and pore structure of the three hydrochars were different, which was also the reason for the different combustion performance of their mixture with coal. According to Raman result, the different degree of disorder of hydrochar will affect their reactivity, resulting in the different combustion promotion effects. The activation energy of the FR and AC samples was the lowest at 26.94 kJ/mol. While the addition ratio was increased from 5% to 50%, its value was still the lowest in all samples of 7.06 kJ/mol. For SS and AC mixture. The activation energy was slightly higher than that of FR. As the ratio increased, it decreased from 28.21 kJ/mol to 7.06 kJ/ mol. At last was the BS sample which was decreased from 46.53 kJ/mol to 10.42 kJ/mol. In the second stage of reaction, the change rule of activation energy was similar to that in the first stage. The order from high to low can be ranked as BS > SS > FR, which was consistent with the result of previous structure analysis. The activation energy of the three hydrochar mixtures was reduced from 82.04 kJ/mol, 78.85 kJ/mol and 76.88 kJ/mol to 43.04 kJ/mol, 37.75 kJ/mol, and 41.88 kJ/mol. All three kinds of hydrochar can promote the combustion reaction blended with coal to a certain extent, but due to the different structure and properties of them, the promotion effect was not the same. 4. Conclusions In this paper, the characteristics of three different hydrochars and one coal were investigated. The microstructure and specific surface area of three hydrochars were studied, which showed that FR had relatively mature pore structure and its specific surface was more abundant, followed by SS and BS samples. At the same time, the stability of carbon microcrystal structure of FR was lower than the other two, which was the reason that it had better combustion performance. The kinetic analysis of the combustion reaction of the mixture verified this point. The results showed that as the proportion of hydrochar increased from 5% to 50%, the activation energies of the three mixtures of BS, SS and FR decreased from 46.53 kJ/mol, 28.21 kJ/mol and 26.94 kJ/mol to 10.42 kJ/mol, 7.06 kJ/mol and 7.84 kJ/mol respectively in the first stage. In the second temperature range, the activation energy decreased from 82.04 kJ/mol, 78.85 kJ/mol and 76.88 kJ/mol to 43.04 kJ/mol, 37.75 kJ/mol and 41.88 kJ/mol, respectively. Acknowledgements This work was supported by the Yong Elite Scientists Sponsorship Program By CAST (2017QNRC001) and the Natural Science Foundation for Young Scientists of China (No. 51804026) . Nomenclature
BS SS FR AC SEM m0 mt m∞
a
t K A E
biosludge sewage sludge fiber reject anthracite coal scanning electron microscopy the initial mass of reactants the mass of the reaction up to a certain time the final mass of the sample in reaction the conversion rate of the sample the combustion time reaction rate constant the frequency factor the activation energy
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R T
b X10 X50 X90 G D1 D2 D3 D4 ID/IG SP2 SP3 R2
13
universal gas constant the absolute temperature heating rate of combustion particle size value when the sample content reached 10% particle size value when the sample content reached 50% particle size value when the sample content reached 90% wave band near the wave number of 1580 cm1 wave band near the wave number of 1350 cm1 wave band near the wave number of 1620 cm1 wave band near the wave number of 1500 cm1 wave band near the wave number of 1200 cm1 the disorder degree of carbonaceous materials a molecular orbital hybridization a molecular orbital hybridization squared pearson's correlation coefficient
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Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Study on co-combustion characteristics of hydrochar and anthracite coal Nan Zhang a, b, Guangwei Wang a, b, *, Jianliang Zhang b, Xiaojun Ning b, **, Yanjiang Li b, Wang Liang b, Chuan Wang c, d a
State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, China School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, China Swerim AB, SE-971 25, Luleå, Sweden d Thermal and Flow Engineering Laboratory, Åbo Akademi University, Åbo, FI-20500, Finland b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 June 2019 Received in revised form 17 October 2019 Accepted 21 October 2019 Available online xxx
Non-isothermal thermogravimetric analysis was employed to study the combustion reaction behavior of three different hydrochars and one anthracite in the paper. The particle size distribution analysis, scanning electron microscopy, specific surface area and Raman spectroscopy were also used to observe and characterize physiochemical properties. The Coats-Redfern kinetic model method was used to calculate the activation energy of different combustion reactions. The results showed that the combustion of fiber reject had a more effective combustion property than the other two hydrochars. The main reason was that fiber reject had higher specific surface area and lower carbon microcrystalline structure. Meanwhile, the co-combustion of different hydrochars with coal had similar combustion trends, and with the increase of hydrochar proportion, the combustion rate was higher. The results of the kinetic calculation correspond to this conclusion. It can be found that as the proportion of the three hydrochar increased, the activation energy of two stages gradually decreased. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Hydrochar Pulverized coal Combustion Kinetic
1. Introduction In recent years, the global economy has developed rapidly combined with the pace of social industrialization further accelerated. The rapid development of the economy is based on the transitional dependence on energy consumption. However, the consumption of resources will be accompanied by some new problems, including the sharp increase in the amount of waste, such as municipal waste, paper sludge, sewage sludge, etc., which are generated during urbanization [1e4]. Although the degree of mechanization of agriculture has increased the utilization rate of biomass resources, it has also caused an obvious problem in the amount of biomass sludge and other various organic wastes, but these wastes are difficult to further process prepared as materials that can be directly used by other industries [5,6]. At the same time, it also has a serious impact on environmental protection due to their own pollution problems. Many methods have been proposed for the rational use of these waste resources to achieve their recycling. Zhang et al. [7] detailedly study the change of gas production in anaerobic digestion reaction by changing the time and temperature conditions based on the principle of thermal hydrolysis. Zhou et al. [8] investigated the ignition characteristics and combustion characteristics under different oxygen concentrations and found that the volatility, the ignition and the burning out time of the carbon are reduced. Meanwhile, hydrothermal carbonization technology is considered to have broad applications. The core of this technology is based on the principle of carbonizing the raw material with low carbon content by subjecting the raw material to a series of complex chemical reactions in a high temperature and high pressure closed environment. This technology has significant advantages: a wide range of raw materials which include a variety of waste that difficult to treat, clean and efficient, convenient and feasible, and especially sustainable [9]. As for production (hydrochar), it has the characteristics of high carbon content, reduced volatile matter and ash content compared with the original substance, increased energy density, and a certain degree of
* Corresponding author. State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, China. ** Corresponding author. E-mail addresses: [email protected] (G. Wang), [email protected] (X. Ning). https://doi.org/10.1016/j.joei.2019.10.006 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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moisture content removal [10]. However, there are still some disadvantages in industrial utilization. Although carbonization process increases its carbon content in a certain degree, its thermal value is still not high in general. And because of high ash content, its energy density is still lower than traditional primary energy sources such as coal. As a result, its use as fuel is not in large scale nowadays. Some works have been done in the field of hydrochar. Issac et al. [11] investigated the mixed combustion process of biomass and coal. The results showed that the addition of wood chips had a significant effect on the combustion of the mixture. Nakason et al. [12] compared the effects of different reaction conditions on hydrochar products and found that different temperatures had a greater influence on the hydrogen, carbon and oxygen elements of products, which is attributed to the condensation or polymerization of HTC. Heidari et al. [13] comprehensively introduced the current status of hydrochar development technology and the dilemmas it faced, and found that there are still some problems in the construction of models and data supplementation. However, there are relatively few studies on the co-combustion of hydrochar and coal at present, so it is necessary to conduct an in-depth study on the common reaction behavior between them. This will provide a reference for improving the rational utilization of hydrochar and saving primary energy. In this study, three kinds of hydrochar samples and one anthracite have been chosed to conduct the experiment. The thermogravimetric analysis method and the advanced technologies such as structural characterization were used to investigate the reaction characteristics of different proportions of hydrochar mixed with anthracite. Also, the conclusions obtained can have a certain reference significance for technology of hydrochar injection into blast furnace. 2. Experimental 2.1. Preparation of raw materials Three kinds of hydrochar samples selected in this paper were biosludge, sewage sludge and fiber reject waste sludge, which were labeled as BS, SS and FR respectively. The coal sample used in this study is a kind of anthracite used in the blast furnace injection in a steel company, which was collected and recorded as AC. Before performing all kinds of other tests apart from proximate analysis, the sample was dried in a forced air oven at 105 C for 24 h to remove the free water and then ground and sieved to a particle size of less than 74 mm using a standard sieve to meet the experimental requirements. The proximate and ultimate analysis results were shown in Table 1 and the results of ash composition of all samples were shown in Table 2. Particle size distribution test were constructed on Mastersizer 3000 analyzer. Scanning electron microscopy (SEM, FEI Quanta-450) was used to observe the microscopic morphology of the samples with experimental conditions of 15 kV voltage. The magnifications were set as 2000 and 4000 times respectively. Pore structure tests were performed using Quadrasorb SI gas analyzer. The Raman spectra of all samples were determined by LabRam HR Evolution spectrometer so as to analyze the carbon structure of different materials. The experiment process includes: The sample was placed on a glass plate and irradiated with excitation light at 532 nm to generate scattering. The light spectra whose wavelength was shifted can be recorded, and the characteristics of some specific molecules in the sample were analyzed qualitatively or quantitatively. 2.2. Thermogravimetric analysis Thermogravimetric experiments were carried out using the HCT-4 microcomputer differential thermal balance produced by Beijing Henven Scientific Instrument Factory. Its structural diagram is shown in Fig. 1. Before the experiment, the sample of 5 mg weighing with high precision balance was taken, and then the sample was placed in a platinum crucible and reacted in an air environment. In the reaction process, the temperature rising rate was set to 20 C/min by means of program temperature control. The sample was heated from room temperature to 900 C, and a 10-min holding time was set up to ensure that the furnace temperature and sample temperature can be kept consistent. 2.3. Description of kinetic model The main purpose of kinetic analysis is to explore the mechanism of energy change rule during the reaction process and reveal the difficulty of different substances to overcome the energy barrier when the reaction occurs by comparing the activation energies of different chemical reactions, that is, whether the chemical reaction is easy to carry out. Here, the method to calculate the activation energy of coal and hydrochar combustion reaction is Coats-Redfern model. The specific calculation principle of this method is shown as follows: the initial mass of reactants is m0, and the mass of the reaction up to a certain time is denoted as mt. The reaction rate formula is expressed as:
da = dt ¼ kf ðaÞ
(1)
Table 1 Proximate and ultimate analysis of different samples (wt%). Sample
AC BS SS FR a
Proximate analysis
Ultimate analysis
Mad
FCada
Aad
Vad
Cd
Hd
Oda
Nd
Sd
0.53 4.06 1.81 1.51
79.33 6.72 7.18 28.48
10.60 46.58 38.76 23.28
9.54 42.64 52.25 46.72
80.78 28.13 33.76 50.96
3.25 2.36 3.77 4.50
3.31 17.63 20.58 15.06
1.04 0.93 0.96 3.01
0.49 0.31 0.36 1.67
Calculated by difference. FC, fixed carbon; A, ash; V, volatile matter; d, dry basis.
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Table 2 Ash composition of different samples (wt%). Sample
SiO2
Fe2O3
Al2O3
K2O
CaO
MgO
ZnO
SO3
AC BS SS FR
47.69 14.79 19.57 11.78
4.36 4.29 8.28 22.99
36.44 8.07 11.39 5.37
0.76 0.28 0.45 0.57
3.93 62.53 48.34 22.07
0.59 2.78 3.01 6.46
0.01 0.08 0.13 0.22
4.12 1.49 1.61 8.39
Fig. 1. Schematic diagram of TG analyzer.
k ¼ A exp½E = ðRTÞ
(2)
f ðaÞ ¼ ð1 aÞn
(3)
a ¼ ðmo mt Þ = ðmo m∞ Þ
(4)
here, A is the pre-exponential factor, min1; R is the gas constant, 8.314 103, kJ/(mol$k); t is reaction time, minutes; T is the temperature, K; a is the conversion rate of the sample; mo is the initial mass of the sample, mg; mt is the instantaneous mass of the sample at time t, mg; m∞ is the final mass after the reaction, mg; n represents the reaction order. Substituting the heating rate parameter b ¼ dT=dt into the above formula for integral calculation:
da A ¼ ð1 aÞn exp½E = ðRTÞ dT b
(5)
We assume the combustion reaction as a first-order reaction, and the following formula can be derived:
lnð1 aÞ 2RT E ¼ ln AR 1 ln E RT bE T2
(6)
For activation energy calculation of most first-order reaction integral equations, the first term on the right side of Eq. (6) is approximately equal to a constant. Based on this, a straight line can be obtained by solving the parameters of independent variables and dependent variables through this formula and conducting linear fitting, and the slope of the straight line is E=R, from which the apparent activation energy of the reaction will be solved and the intercept of the straight line can also be calculated. 3. Result and discussion
3.1. Physical and chemical characteristics 3.1.1. Particle size distribution analysis The particle size distribution results of four samples were shown in the Fig. 2. The meaning of each coordinate in the figure can be explained as: the abscissa represents the particle size distribution interval (mm) and the right vertical axis represents the columnar distribution diagram of particle size distribution content in the corresponding particle size interval. The left ordinate is the cumulative sum of the values of the histogram corresponding to different particle size intervals, i.e., the sum of the contents of all samples less than this particle size within a certain particle size interval. In general, when the sample content in the lower particle size range is higher, its combustion performance may be better. To some extent, particle size distribution can affect the specific surface area of samples. The specific surface area Please cite this article as: N. Zhang et al., Study on co-combustion characteristics of hydrochar and anthracite coal, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.10.006
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Fig. 2. Particles size distribution of different samples.
of the sample is positively correlated with its pore size or particle size distribution. Larger pore structure and higher content in the lower particle size distribution area are closely related to the specific surface area of the sample [14e18]. Here, the particle size values when the sample content reached 10% (X10), 50% (X50) and 90% (X90) were used to compare the changes of particle size of different samples. Results were shown in the Table 3. We can see that the average within the range of X10 and X50, the particle size of three hydrochar samples BS, SS and FR was significantly smaller than that of coal sample AC. In the range of X10, the particle size distribution of three hydrochars can be ranked as FR > BS > SS from fine to coarse. When the content accumulated to 50%, it was still the smallest average particle size of the FR sample, while the particle size of BS and SS samples was close to each other. In the range of X90, the particle size of both FR and BS samples was smaller than that of SS. It can also be observed from the distribution diagram that the FR sample had a peak value below 10 mm, indicating that its particle size was better than the other two samples [11,19e21]. 3.1.2. Micromorphology analysis From size results, different hydrochars had a different size distribution, based on this, it is necessary to pay attention to microstructure. SEM images of four samples at different magnification ratios were shown in Fig. 3. Due to the small particle size of the selected sample, the magnification was set as 2000 times (corresponding to 50 mm scale) and 4000 times (corresponding to 20 mm scale) respectively during the experiment process in order to better observe the micro-morphological characteristics of different samples. It can be found from the pictures that the microscopic morphology of samples with different types and structures varied greatly. For the coal sample AC, there were some small irregular blocks on the surface and the surface of the rest part was relatively flat without any indented pores or slits. The microstructure of three hydrochars was consistent with their original sample. Among them, the surface of BS sample at 2000 times was distributed with a lot of micro-convex particles arranged in disorderly, where the pores between irregular convex particles were not very obvious. As the magnification was increased to 4000 times, the irregular convex blocks became closely arranged and distributed with each other. Although the particles had different shapes, there were no significant concave pores between them. As for the SS sample at 2000 times, there were many snowflake particles of different sizes on its surface and among these particles were embedded many hollow pores, whose size and depth varied with the size of snowflake particles. After the magnification was turned to 4000 times, it can be clearly observed that there were pore structures during the convex blocks. The intuitive expression in the picture was that the area of the black shaded part of the sample increased. For the FR sample, many small uneven particles can be observed on the surface at a magnification of
Table 3 Particle size analysis results of different samples (%). Sample
X10
X50
X90
AC BS SS FR
9.83 3.55 5.92 1.88
48.81 31.12 51.82 27.45
88.59 111.23 272.31 163.44
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Fig. 3. SEM graphs of selected samples at different magnifications: AC-(a, b); BS-(c, d); SS-(e, f); FR-(g, h).
2000 times, and micro-pores of similar size and depth were distributed among them. In the picture of 4000 times, the amount of micro particles on its surface was more, but the size was smaller than that of SS samples. Furthermore, the number of pore structure between surface particles of SS was more than that of the first two samples. Anthracite is a highly ordered structure with benzene ring as the core unit. The macroscopic performance is that its stable structure smooth and smooth surface. From the proximate analysis, it can be seen that
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the hydrochar has significantly higher volatile content, which indicates few stable frameworks in the structure. The escape of a large number of unstable components during hydrothermal treatment makes a huge contribution to a significantly more developed pore structure than anthracite [22e24]. 3.1.3. Specific surface area analysis In order to quantitatively analyze the development degree of pore structure, the specific surface area was studied. The Fig. 4 showed the graph of the specific surface area and pore diameter distribution of the samples. The adsorption curve types of the four samples were close to the anti-s type, for which the adsorption process was changed from single-layer adsorption of micropores to multi-layer adsorption of mesopores or even macropores, the adsorption hysteresis phenomenon also existed in all the curves [25]. According to the curves, the pore systems of four samples were continuous and completed with the pore size distribution ranging from micropores to mesopores. The specific surface area results of four samples were shown in Table 4. As we can see, the coal sample was the smallest of all, followed by BS, SS. The largest was the FR sample. This phenomenon was consistent with the results observed by SEM. It indicated that there were many inherent hollow pore structures on the surface of FR samples, and the pore volume and pore radius of the porous solid of SS and BS samples were both smaller compared with FR. The microscopic morphology of the coal sample and the three hydrochars made a big difference, which can be contributed to the less developed pore structure and undeveloped pores. From the pore size distribution results, it can be concluded that the peak of the pore size distribution of the coal sample was in the microporous portion, and the pore sizes of the three hydrochars were similar, mainly concentrated in the mesopores, but some differences still exist. The FR sample had the highest pore volume, which was two orders of magnitude higher than BS and SS samples. 3.1.4. Raman analysis The carbon microcrystalline structure was considered to have a great correlation with the activity of the material. It was also used here to analyze the microcrystalline structure characteristics of different hydrochars. The Raman analysis results of different samples were shown in the Fig. 5. According to the method of Sadezky et al. [26], four lorentz-shaped wave bands were found near the wave number of 1580 cm1,1350 cm1,1620 cm1 and 1200 cm1 in the Raman spectrum of carbonaceous materials, which were respectively expressed as G, D1, D2 and D4. Another gaussian shaped wave band was located at the position of 1500 cm1, which was denoted as D3 shaped band. The single peak at 1580 cm1 corresponded to E2g mode vibration, while the peak at 1360 cm1 was usually attributed to the atomic vibration mode of the disordered carbon structure [27e29]. Due to the increased contribution of disorder to this peak, the peak at 1350 cm1 can be stronger [30]. The ratio of the D peak caused by the disordered carbon to the G peak of the E2g mode vibration R ¼ ID/IG can sensitively and quantitatively characterize the disorder degree of carbonaceous materials. Since there was no disorder characteristic, the pure graphite sample only showed a G peak. However, if the crystal suffered a long-term periodic damage its disorder degree was deepened, the D peak
Fig. 4. Pore structure measurement results of different samples: (a) Adsorption and desorption curves; (b) Pore size distribution curves.
Table 4 Specific surface area results of different samples. Sample
AC
BS
SS
FR
BET Surface Area
0.468 m2/g
6.748 m2/g
8.964 m2/g
44.086 m2/g
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Fig. 5. Raman fitting results of different samples.
appeared and its strength increased accordingly. Comparing three kinds of hydrochar spectra available, their spectral types were similar to that of amorphous carbon, which belonged to the hybrid unstructured mixture of the SP3 hybridization and SP2 hybridization carbon, R value of four types of sample were shown in the Fig. 6, Among the three hydrochar samples, the disorder degree of FR sample was the highest, and the lowest was BS samples. This reflected the difference of their chemical reaction performance. 3.2. Thermogravimetric analysis Three different kinds of hydrochar and coal powder were heated at a heating rate of 20 C/min for combustion reaction. The combustion conversion curves and conversion rate curves were shown in Fig. 7. In the figure, for three hydrochar mixtures, the combustion process can be divided into two stages: the first stage was the combustion of volatile substances. The corresponding temperature range was between 300 C and 500 C; the second stage was the combustion of carbonaceous materials, and the reaction temperature at this stage was between 500 C and 800 C. After the combustion reaction was completed, the remaining non-combustible material mainly consisted of ash. The combustion characteristics of different samples were shown in the Table 5. For hydrochars, the initial combustion temperature of FR was the lowest, and then in turn were BS and SS. By comparing their conversion rate in the first stage, the conversion rate of FR sample reached the fastest which was 0.00391 s1 and its weight loss ratio reached nearly 80% during this period. In the case of SS sample, its conversion rate was lower than FR as 0.00285 s1, and its weight loss ratio in the first stage was 65%, which was also smaller than that of FR sample. The rate
Fig. 6. Raman structure parameters of different samples.
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Fig. 7. Conversion curves and conversion rate curves of different samples at 20 C/min.
Table 5 Combustion characteristics of different samples. Sample
BS SS FR AC
First stage
Second stage
Tmax/ C
DTGmax/s1
Tmax/ C
DTGmax/s1
369.3 325.5 288.3 e
0.00159 0.00285 0.00391 e
748.8 727.5 424.4 595.9
0.00149 0.000799 0.000371 0.00353
peak value of BS was lower than the former two, which was at 0.00159 s1. According to the proximate analysis results, the FR sample had the lowest ash content, so it was the least affected by ash inhibition during combustion reaction, while the BS sample had the highest ash content, and the corresponding combustion reaction rate was the slowest. Ash can be heated to a high temperature along with combustible materials in which process it will consume a certain amount of heat. The higher the ash content, the lower the combustion efficiency. In the second stage of mixture combustion, the conversion and conversion rate had the opposite trend compared with the first stage. It can be seen from the figure that the combustion of hydrocahr started at a lower temperature stage, indicating that the combustion performance of hydrochar was significantly better than that of coal. During three hydrochars, FR had a better combustion behavior compared with the rest. Combined with previous research, specific surface area of FR was larger than SS and BS, which might make a promotion in the combustion because of more area to contact air. With microcrystalline structure results, FR had a higher degree of carbon disorder, this indicated it was more reactive. 3.3. Effect of different ratio on combustion To analyze the co-combustion behavior, three hydrochars were mixed with pulverized coal at ratios of 5%, 10%, 15%, 20%, 30% and 50% respectively with heating rate of 20 C/min. The conversion and conversion rate results were shown in Fig. 8. The combustion characteristics of four samples in different reaction stages were shown in Table 6. The results showed that with the increase of the proportion of hydrochar, the conversion rate of the mixture in the first stage was gradually increased, indicating that the addition of hydrochar can make a promotion effect on mixture combustion. Comparing three kinds of mixture, it can be investigated that the FR had the best combustion-promoting effect, which was consistent with the results of the previous analysis. In the first stage of reaction, with the addition ratio of FR increased from 5% to 10%, the weight loss ratio of the mixture was not obvious, while the proportion increased to 15%, the weight loss variation of the sample increased significantly. When the percentage increased from 15% to 20%, the increase in the weightlessness of the mixture was similar to the change of 5%e10%. The weight loss ratio increased markedly when the FR addition was at 30%, while the change of weight loss ratio also had an increasing trend at 50% ratio of FR, which was yet not as drastic as that of 30% ratio. Because of difference of volatile matter in hydrochar and coal, the volatilization of hydrochar released very quickly, as a result, the combustion of mixture in the first stage was obvious. The effect of SS on promoting the combustion was slightly worse than FR. It can be concluded from the corresponding image that when the adding proportion of SS increased from 10% to 15%, there was also an obvious promotion phenomenon. This indicated that the combustion effect of the hydrochar and coal will be much better when the mixing proportion reached 15%. Considering of curves with 30% and 50% proportion, there was an apparent surge in weightlessness at 30% ratio, it was not only because of the proportion rising larger, but also the reason that when the mixture ratio of hydrochar increased, the reaction condition of the mixture was improved due to its developed pore structure and more disordered carbon microcrystalline structure, which made the total combustion reaction proceed faster. Though with the increase of the proportion, the overall trend of combustion reaction also showed an enhanced trend, BS sample had the worst effect of promoting combustion of these three. Through the contrast analysis of conversion rate curve in the first stage, the combustion rate in descending order can be ranked as FR mixture > SS mixture > BS mixture, and it can be found the temperature corresponding to the rate peak in the burning rate curve of FR mixture and SS mixture was significantly lower than the BS mixture. The faster the burning rate, the greater the amount of reaction at the same time and the higher the conversion rate, the more consistent the x value is with the weight loss curve. Moreover, the temperature corresponding to the peak value of FR mixture and SS mixture combustion rate curve was Please cite this article as: N. Zhang et al., Study on co-combustion characteristics of hydrochar and anthracite coal, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.10.006
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Fig. 8. Conversion curves and conversion rate curves for blended sample (20 C/min): (a, b) BS and AC; (c, d) SS and AC; (e, f) FR and AC.
significantly lower than that of BS mixture. The fact was that the combustion rate was faster, the larger the reaction amount within the same time will be done, thus the value of conversion rate x increased even more [31e33], which was consistent with the results of the weightlessness curve. From above analysis, we can find that during the co-combustion of hydrochar and anthracite, the main characteristic is that it can be divided into two main stages and the combustion rate presents two obvious characteristic peaks. With the increase of hydrocahr content, the combustion process is closer to the combustion behavior of hydrochar. Secondly, the difference in the combustion rate of different mixtures is obvious, which is related to the complex reaction between coal and hydrochar. FR and coal mixture has the largest combustion rate and its combustion promotion effect is the best. 3.4. Kinetic analysis Comparing the combustion curves of three kinds of mixed samples, it can be found that there are obvious weight losses in the temperature range of 300e500 Cand 500e700 C. Based on this and for uniform comparison, these two intervals are defined as the first phase and the second phase, respectively, and the activation energy of the chemical reactions in these two phases were solved. Select the representative fitting results with hydrochar mixing ratio of 10%, 30% and 50% of as shown in Fig. 9 and Fig. 10. The Coats-Redfern kinetic model was used to calculate the activation energy of the chemical reaction of the sample at different stages. Kinetic parameters in the two stages of all samples were shown in Table 7 and Table 8. From the data in the table, the value of the parameter R2 indicating the fitting effect was above 0.9, which was a first-order reaction with a good fitting result. Comparing the results of the first stage, it showed that the reaction activation energy of three mixtures in the corresponding temperature range decreased with the increase Please cite this article as: N. Zhang et al., Study on co-combustion characteristics of hydrochar and anthracite coal, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.10.006
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Table 6 Combustion characteristics of blended samples in two reaction stages. Sample (wt %)
BS-5% BS-10% BS-15% BS-20% BS-30% BS-50% SS-5% SS-10% SS-15% SS-20% SS-30% SS-50% FR-5% FR-10% FR-15% FR-20% FR-30% FR-50%
First stage
Second stage
Tmax/ C
DTGmax/s1
Tmax/ C
DTGmax/s1
388.8 396.8 399.4 396.8 379.7 382.5 353.9 342.5 344.8 340.1 338.7 334.8 331.5 336.5 338.5 333.6 311.9 303.1
3.11E-5 9.26E-5 1.43E-4 2.07E-4 3.49E-4 5.68E-4 3.43E-5 1.11E-4 2.14E-4 3.09E-4 6.58E-4 0.00113 7.82E-5 2.35E-4 3.91E-4 5.22E-4 0.00112 0.00136
597.6 597.3 600.2 597.7 595.8 603.5 599.9 601.9 600.2 600.6 605.9 601.5 596.1 596.2 594.1 592.1 593.9 585.4
0.00358 0.00348 0.00341 0.00334 0.00295 0.00247 0.00344 0.00337 0.00328 0.00322 0.00264 0.00225 0.00344 0.00336 0.00331 0.00321 0.00233 0.00175
Fig. 9. Experimental and model profiles for the samples at heating rate of 20 C/min in the first stage.
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Fig. 10. Experimental and model profiles for the samples at heating rate of 20 C/min in the second stage.
Table 7 Kinetic parameters of blended samples in the first stage. Sample (wt%)
T/ C
Slope
E/kJ$mol1
R2
BS-5% BS-10% BS-15% BS-20% BS-30% BS-50% SS-5% SS-10% SS-15% SS-20% SS-30% SS-50% FR-5% FR-10% FR-15% FR-20% FR-30% FR-50%
350e480 350e480 350e480 350e480 350e480 350e480 320e490 320e490 320e490 320e490 320e490 320e490 300e420 300e420 300e420 300e420 300e420 300e420
5596.8 2638.6 1714.9 1859.8 1287.1 1252.8 3393.1 1710.2 1043.3 865.6 913.1 848.6 3240.8 2309.3 2295.7 2012.9 1148.5 943.3
46.53 21.94 14.26 15.46 10.70 10.42 28.21 14.22 8.67 7.20 7.59 7.06 26.94 19.20 19.09 16.74 9.55 7.84
0.9976 0.9909 0.9912 0.9821 0.9758 0.9779 0.9815 0.9701 0.9873 0.9894 0.8984 0.8998 0.9619 0.9823 0.9882 0.9961 0.9691 0.9454
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N. Zhang et al. / Journal of the Energy Institute xxx (xxxx) xxx Table 8 Kinetic parameters of blended samples in the second stage. Sample (wt%)
T/ C
Slope
E/kJ$mol1
R2
BS-5% BS-10% BS-15% BS-20% BS-30% BS-50% SS-5% SS-10% SS-15% SS-20% SS-30% SS-50% FR-5% FR-10% FR-15% FR-20% FR-30% FR-50%
520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650 520e650
9867.1 9276.8 8629.6 8455.8 7069.4 5176.4 9483.7 8808.7 8200.4 7526.2 5732.8 4540.1 9247.2 8762.2 7996.9 7580.2 5174.4 5037.5
82.04 77.13 71.75 70.30 58.77 43.04 78.85 73.24 68.18 62.57 47.66 37.75 76.88 72.85 66.49 63.02 43.02 41.88
0.9995 0.9986 0.9953 0.9947 0.9883 0.9623 0.9986 0.9971 0.9928 0.9892 0.9682 0.9510 0.9984 0.9972 0.9861 0.9851 0.9511 0.9603
of the proportion of hydrochar, indicating that the addition of hydrochar can improve the reaction of mixed samples and made the reaction easier to be produced. The microstructure and pore structure of the three hydrochars were different, which was also the reason for the different combustion performance of their mixture with coal. According to Raman result, the different degree of disorder of hydrochar will affect their reactivity, resulting in the different combustion promotion effects. The activation energy of the FR and AC samples was the lowest at 26.94 kJ/mol. While the addition ratio was increased from 5% to 50%, its value was still the lowest in all samples of 7.06 kJ/mol. For SS and AC mixture. The activation energy was slightly higher than that of FR. As the ratio increased, it decreased from 28.21 kJ/mol to 7.06 kJ/ mol. At last was the BS sample which was decreased from 46.53 kJ/mol to 10.42 kJ/mol. In the second stage of reaction, the change rule of activation energy was similar to that in the first stage. The order from high to low can be ranked as BS > SS > FR, which was consistent with the result of previous structure analysis. The activation energy of the three hydrochar mixtures was reduced from 82.04 kJ/mol, 78.85 kJ/mol and 76.88 kJ/mol to 43.04 kJ/mol, 37.75 kJ/mol, and 41.88 kJ/mol. All three kinds of hydrochar can promote the combustion reaction blended with coal to a certain extent, but due to the different structure and properties of them, the promotion effect was not the same. 4. Conclusions In this paper, the characteristics of three different hydrochars and one coal were investigated. The microstructure and specific surface area of three hydrochars were studied, which showed that FR had relatively mature pore structure and its specific surface was more abundant, followed by SS and BS samples. At the same time, the stability of carbon microcrystal structure of FR was lower than the other two, which was the reason that it had better combustion performance. The kinetic analysis of the combustion reaction of the mixture verified this point. The results showed that as the proportion of hydrochar increased from 5% to 50%, the activation energies of the three mixtures of BS, SS and FR decreased from 46.53 kJ/mol, 28.21 kJ/mol and 26.94 kJ/mol to 10.42 kJ/mol, 7.06 kJ/mol and 7.84 kJ/mol respectively in the first stage. In the second temperature range, the activation energy decreased from 82.04 kJ/mol, 78.85 kJ/mol and 76.88 kJ/mol to 43.04 kJ/mol, 37.75 kJ/mol and 41.88 kJ/mol, respectively. Acknowledgements This work was supported by the Yong Elite Scientists Sponsorship Program By CAST (2017QNRC001) and the Natural Science Foundation for Young Scientists of China (No. 51804026) . Nomenclature
BS SS FR AC SEM m0 mt m∞
a
t K A E
biosludge sewage sludge fiber reject anthracite coal scanning electron microscopy the initial mass of reactants the mass of the reaction up to a certain time the final mass of the sample in reaction the conversion rate of the sample the combustion time reaction rate constant the frequency factor the activation energy
Please cite this article as: N. Zhang et al., Study on co-combustion characteristics of hydrochar and anthracite coal, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.10.006
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R T
b X10 X50 X90 G D1 D2 D3 D4 ID/IG SP2 SP3 R2
13
universal gas constant the absolute temperature heating rate of combustion particle size value when the sample content reached 10% particle size value when the sample content reached 50% particle size value when the sample content reached 90% wave band near the wave number of 1580 cm1 wave band near the wave number of 1350 cm1 wave band near the wave number of 1620 cm1 wave band near the wave number of 1500 cm1 wave band near the wave number of 1200 cm1 the disorder degree of carbonaceous materials a molecular orbital hybridization a molecular orbital hybridization squared pearson's correlation coefficient
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Please cite this article as: N. Zhang et al., Study on co-combustion characteristics of hydrochar and anthracite coal, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.10.006