Hydrogen Evolution during Devolatilization To Predict Coking

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Hydrogen evolution during devolatilization to predict coking potential of metallurgical coals Pinakpani Biswas, Jeetendra Nath Panda, Debjani Nag, Nikhil Chougale, Vimal Kumar Chandaliya, Goutam Ghosh, Pratik Swarup Dash, and B. C. Meikap Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01704 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016

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Graphical Abstract

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Hydrogen evolution during devolatilization to predict coking potential of metallurgical coals Pinakpani Biswas a, Jeetendra Nath Panda b, Debjani Nag a, N Chougale c, V K Chandaliya a, G Ghosha, Pratik Swarup Dasha, B. C. Meikap d,e a

Research and Development, Tata Steel, Jamshedpur, India – 831007 b

Indian Institute of Technology Kharagpur, India-721302 c

d

VIT University, Vellore, India - 632014

Department of Chemical Engineering, School of Engineering, Howard College, e

University of Kwazulu-Natal, Durban, South Africa

*Corresponding Author. R&D Tata steel, India, Jamshedpur 831005 Tel: +91 657 2148972; Fax: +91 657 2345405 E-mail Address: [email protected]

Abstract Evolution of hydrogen during the plastic state is a key parameter in coke making for the production of quality coke. Based on this phenomenon, a new devolatilization method has been developed for accessing the coking property of coals using the evolution of H2 during thermogravimetric-Mass spectrometer (TG-MS) analysis. Four coals having different coking potentials, commonly used in steel industries, were considered. The investigation was performed at temperatures up to 1100 0C in an argon atmosphere under a constant heating rate of 3 0C/min to simulate the coking environment. Non-isothermal kinetics is considered as a good indication of different reaction regime during pyrolysis, which is generally done through TGA alone. In this study evolution of H2 is used to develop non-isothermal kinetics through a new quantification approach. .Finally, a definite and more generic correlation was established, based on H2 enrichment in volatiles around specific temperature zones, for measuring the exact coking behavior of coal through TG-MS analysis. This method is very robust and has the ability to predict the exact coking behavior of coal. The results are also compared with the blended coal used in plants which is in very good agreement with developed H2 enrichment correlation. 2 ACS Paragon Plus Environment

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Keywords: TG-MS, Coking potential, Coal pyrolysis, Devolatilazation, coke making, Coking characteristics


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1. Introduction Coking or rheological properties are the essential parameters in coke making process as these strongly influence the coke quality. Traditionally, Giesler Plastometer, Free swelling index, and Dilalotometer tests have been used to anticipate the coking properties. These methods are not very informative regarding coke making mechanism and behavior. However, these methods are utilized by the steel industry for coal selection as those properties gives a tentative idea on coking quality. For coal having marginal coking property or blended coals, these methods sometimes are not informative enough. For example, some coals may exhibit poor response towards these tests but in reality a minute addition of some modifier or additive may enhance its coking behavior. As a consequence, sometimes applicable coals may be unnecessarily rejected but with an improved approach these optimizations can be utilized and hence low cost coal can be used in steel industry. Thus, knowledge of coke making mechanism would help to produce good quality coke. In this paper, efforts are being made to understand the coking mechanism by the proposed characterization techniques so that non-coking or weakly coking coals could become coking coals. Researchers [1-3] have also tried to improve coking property of coal by different chemical beneficiation route. The coke oven gas composition provides a clue in developing such generic methods. The coke oven by-product plant is an integral part of the by-product coke making process. The volatiles leaves the coke oven chambers as hot, raw coke oven gas. After leaving the coke oven chambers, the raw coke oven gas is cooled which results in a liquid condensate stream, and a gas stream (Fig. 1). The functions of the by-product plant are to take these two streams from the coke ovens, to process them to recover by-product coal chemicals and to condition the gas so that it can be used as a fuel gas. The main emphasis of a modern coke by-product plant is to treat the coke oven gas sufficiently so that it can be used as a clean fuel. The coal by-product is the secondary product (apart from coke formed) when coal is heated up to 700-1100 oC at a slow heating rate in the absence of air. By-products of the coking process, including the coke oven gas and tar, represent about 20−25 wt. % of the parent coal.

The coke oven gas, which accounts for ∼15−20 wt. % of the coking process, is mainly composed of H2 (derived from aromatic condensation) and CH4 (from dealkylation reactions)

and, to a lesser extent, CO, CO2, and light hydrocarbons. Tar, another by-product of the coking activity (3−5 wt. %) is composed of hundreds of polycyclic aromatic hydrocarbons


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[4]. As H2 is the major component of the coke oven gas (Table 1), correlating the liberating H2 with that of the coking property may prove useful. Investigation of the co-pyrolytic behaviors of different plastics (high density polyethylene, low density polyethylene and polypropylene), low volatile bituminous coal and their blends using a thermogravimetric analyzer was performed by Cai et al. [5]. Liu et al. [6] investigated the effect of mineral matter in coal on reactivity and kinetic characteristics of coal pyrolysis through thermogravimetric method. Some authors also conducted experiments using varied particle size, heating rate and pressure with a high volatile bituminous coal to determine the possible effects of the experimental parameters on coal pyrolysis [7]. Extrapolation of kinetic parameters obtained through three different experimental techniques was done by Wiktorsson and Wanzl [8]. A mathematical model was proposed by Merrick et al. [9] to study the chemical changes during the thermal decomposition of coal. The model described the kinetics of the release of the volatile constituents, thereby permitting the changes in the mass and composition of the solid residue by elemental balances. A comparison of the non-isothermal and isothermal techniques was done by Lazaro et al. [10] in order to evaluate the kinetic parameters of coal pyrolysis. Solomon et al. [11] reviewed the progress on kinetics, the formation of volatile products, network models, cross-linking, rank effects, and the ‘twocomponent’ model of coal structure. Solomon and Hamblen [12] focused on coal pyrolysis kinetics by considering the rate of thermal decomposition of individual functional groups and the evolution of individual species (tars and gases). Some authors also revealed that coal pyrolysis was a two stage process and the activation energy for the primary pyrolysis stage was considerably higher than that for the secondary pyrolysis stage [13]. Brown and Phillpotts [14] discussed the mathematical method for determining the kinetic parameters of non-isothermal reactions (for example pyrolysis of solid fuels). Wang et al. [15] used a TG-MS device to investigate the process and characteristics of the gas generation of HCs (hydrocarbons) and non-HCs (non-hydrocarbons) during the pyrolysis process of coal and its late gas generation potential. Methane was the most prominent gas among all the gases and around 17% of the methane was generated at the late stage of coal pyrolysis i.e. after 600 0C. Study of simultaneous TG-MS of pyrolytic decomposition of mixtures of different plastic wastes and coking coals was done by Espina SM et al. [16]. A synergistic effect between coal and individual plastics was found and the maximum interaction occurred at temperatures close to the maximum evolution of the volatiles of the plastic waste. 5 ACS Paragon Plus Environment

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Arenillas et al. [17] used simultaneous thermogravimetry – mass spectroscopy to study the pyrolysis behavior of different rank coals. A normalization method, which permitted a semiquantitative comparison among the volatile species of the coals, was also developed. Arenillas et al. [18] made a comparison of different methods for predicting coal devolatilization kinetics. FG-DVC (Functional Group – De-polymerization, Vaporization and Crosslinking) computer code was used as the network model and the predicted evolution of volatiles was compared with the experimental results. Kandasamy et al. [19] pyrolysed high ash coal at different heating rates to analyze its char structure, kinetics and evolved species. Van Heek and Hodek [20] showed the possibilities and limitations of experimental and theoretical methods of non-isothermal reaction kinetics applied to the investigation of the interaction of coal structure and pyrolysis behavior. An attempt was also made to identify single reactions in coal pyrolysis by comparison with model substances. Coats and Redfern [21] developed a mathematical model for finding out the kinetic parameters of coal pyrolysis from the thermogravimetric data for both nth and 1st order reaction. Evolution characteristics of gases during pyrolysis of maceral concentrates of coking coals were performed by Das [22]. The reactions of formation of selected gas products like hydrogen and methane evolved during coal pyrolysis [23]. The kinetic parameters for the reactions as well as their yields were also calculated. Holstein et al. [24] explored the kinetics of methane and tar evolution during the pyrolysis reaction and the possible relationship between them. Experimental and theoretical investigation focusing on determination of species evolved during from single coal particles during pyrolysis along with their rates of mass evolution was done by Blair et al. [25]. Merceds et al. [26] also have shown us of 1H in situ NMR analytic technique to quantify the coal plasticity during carbonization. They have also shown [27] that the H2 donor ability is one of the important factors during coal carbonization and carbonization and In situ hightemperature 1H nuclear magnetic resonance spectroscopy (NMR) can be used to quantify the enhancement of fluidity by additives. Snape et al. [28] also shown that hydrogen donor ability has been assigned for stabilization of the plastic phase during coal making. In situ high temp 1

H NMR is used to quantify the interaction of low-volatile, non-coking and coal tar pitch with

a hydrogen donor pitch. The above research activities motivated us to do more fundamental study on carbonization process which directly relates to coal selection for the steel making process. Our intention is


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to simply simulate the carbonizing environment in the TG-MS apparatus and the information gathered from that will help us to devise a new methodology for predicting coking properties of coal. As H2 generation during carbonization is crucial step, which can be interpreted from the coke oven gas analysis (Table 1), processing of H2 provides new information on the coking mechanism. A new estimation approach is also developed to obtain an integral behavior of H2 evolution throughout the pyrolysis process. This will replace the conventional non-isothermal kinetics approach from TG analysis into H2 analysis from MS signal. Nonisothermal kinetics also revealed some interesting features on coke making mechanism. Finally, a quantification of H2 signal, predominantly during the plastic phase, gives a new approach which can directly correlates between coking property and generation of H2 during carbonization.

2. Experimental 2.1. Coal samples Four raw coals, O, M, W and N were used. The results are also verified with help of blended coal (BL) used in the coke making process. Coal O is an imported prime hard coking coal. This coal will pass through a fluid phase during heating in absence of air. Coal M is a low ash yield imported coal. This coal is mainly used for pulverized coal injection (PCI) purpose. Coal W is a captive medium coking coal of Indian origin. Coal N is an imported weakly coking coal. Though it has good amount of vitrinites but due to lower vitrinite reflectance this coal lacks appropriate coking properties. To compare the results with the actual coke, coal BL is taken into consideration which is actually a blended coal used in coke plant. Coal BL mainly comprises of 70% medium grade captive coal (Coal W), 20% imported prime coking coal (Coal O). It also consists of 10% of inferior coal (Coal N) to complete the blending proportion. Coal M is not used in coke plant as it is used directly in blast furnace as a PCI coal. Coal samples were initially dried at 105 0C for 24 hours and were ground to -200 mesh. The proximate analysis [29], ultimate analysis [30] and the CSN (crucible swelling number) [31] of the coal samples are listed in Table 2 and Table 3. CSN is a parameter which basically determines the swelling or caking capacity of the coals and hence is a measure of the coking property. It is often referred as free swelling index (FSI).

2.2. Pyrolysis 7 ACS Paragon Plus Environment

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TG-DSC-MS Thermogravimetric Analysis (TGA) is one of the most common techniques used to investigate thermal events and kinetics during pyrolysis of solid raw materials such as coal. It provides a measurement of weight loss of the sample as a function of time and/or temperature. Usually Arrhenius equation is applied to determine the kinetics of these thermal events corresponding to the separate slopes of constant mass degradation. Thermogravimetric analysis of the individual coal samples were performed using a TG apparatus. Fifteen mg of the coal sample was placed on a platinum cell. Then, the electric furnace was closed and purged with Ar with a flowrate of 100 ml/min and a pressure of 0.5 bar. The furnace was heated from ambient temperature to 1100 °C with a heating rate of 3 0

C/min. A MS, coupled with the TG (Fig. 2), which is a Quadrupole mass spectrometer with

heated capillary inlet system was used for the analysis of gases and gaseous decomposition products. QMS-430-D Aelos gas analysis system was used for the qualitative determination of gaseous components, which were emitted during the thermogravimetric analysis. TG coupled with mass spectroscopy is another great technique to identify the unknowns as well as portray the trajectory of known compounds over a period of time and temperature. The system consisted of the oil free vacuum system, Quadrupole Mass Spectrometer (QMA-200), range 0-300 amu and controlled heating transfer line with changeable capillary and gas inlet unit.

Non Isothermal Kinetics The kinetic parameters, activation energy and pre-exponential factor of coal pyrolysis were determined by the integral method. It is assumed that solid fuel pyrolysis is a first order reaction [5]. So the coal pyrolysis reaction equation may simply be expressed as following equation:

dx E = Aexp(− )(1 − x) dt RT where A is pre-exponential factor; E is activation energy; T is temperature; t is time; x is

(1)

calculated by Eq. (2). For a constant heating rate H during pyrolysis, H=dT/dt, rearranging and integrating Eq. (1) gives the following equation:

− ln(1 − x) AR 2RT E ln = ln 1 − − T HE E RT


(2)

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It was observed that for given temperature span, most values of E in the expression

ln

!1 −

"

#$ in Eq. (2) is essentially remains constant. If the left side of Eq. (2) is plotted

versus 1/T, a straight line may be obtained if the process can be assumed as a first order reaction. From the slope, −E/R, the activation energy E can be determined and by taking the temperature at which x% =

&' (&)

in the place of T in the intercept term of Eq. (2), the pre-

exponential factor ‘A’ can also be determined.

Mass spectroscopy Analysis (MS) MS is very sophisticated equipment but repeatability of the signal sometimes plays a big role in order to develop any quantified technique. Fig. 5 shows the MS response for H2 evolution over the entire temperature zone. The original signal need to be normalized for the desire quantification procedure. The sample mass and the flowrate of Argon were kept constant in both the runs. Other authors have used argon intensity as the internal reference to normalize the MS signals. Arenillas et al. [17, 18] attempted to normalize the MS signal with the maximum value of the total intensity registered in the MS. It was used as the normalization factor. They assumed that the sensitivity of the MS detector did not vary during the experiment, as each run was of short duration. Each signal registered was also normalized with the corresponding sample mass used. The TG-MS instrument, apart from it’s normal scanning mode, can be run in multiple ion detection mode (MID). In this mode instrument is operated on cognizable compounds scanning mode over the period of temperature and the results are delivered in relative abundance of the specific amu (atomic mass unit). A total number of 60 channel is able to track 60 predefined mass throughout the experimentation. The measured intensities can be continuously displayed as function of time, Hence the nature of the profile can be associated with the rate of release of specific compound in the entire temperature pathway.

3. Results and discussion 3.1. Thermogravimetric Analysis (TGA) Fig. 3 shows the TG-DTG yields graph of all the individual coal samples taken for the analysis. As these coals have been selected based on their different coke making ability, a variation in TG and DTG curve is observed which is tabulated in Table 5. 9 ACS Paragon Plus Environment

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Table 5 depicts the onset (Tonset), peak (Tmax) and end (Tend) temperatures of the DTG distribution curve. Tonset and Tend represents the temperature at which the pyrolytic conversions are significant whereas Tmax represents the temperature at which the rate of weight loss is maximum (DTGmax). The decomposition temperature range (∆T) is defined by the difference between Tonset and Tend.

The onset, peak and end temperature is more

prominent in Coal O with respect to others. Weight loss started early in coal O and reached the maximum at around 480 oC and completed significantly at very late stage (677 oC) than other coals. Coal BL uses 70 % of coal W and thus it has shown some similarities to coal W (onset 388 oC and end 545.5 0C). Coal M showed the most delayed response but reached its maximum similar to Coal W and settled down at the farthest temperature as compared to others. Coal W started changeover at 382 oC and reached its maximum at 461 oC and completed at 530 oC. Coal N showed an early start than coal W, but it might be expected high volatile yield, and settled down very quickly. The maximum point on DTG curve coincides with that of coal W. Coal N had the narrowest distribution which could be correlated with the predominance of cracking of low molecular weight matrix structure. The high yield of Volatiles N may have supported that fact. Based on the maximum peak temperature, end point and onset DTG figure can be represented as normalized DTG analysis (Fig. 4). In this method DTG signal gets converted into standard normalized signal having a span of 0-100 (Fig. 4). The normalized DTG signal shows much more competitive and the area under normalized signal follows the order Coal O> BL> M> W> N, which is in good agreement with the peak distribution area, but may not be valid as per the exact coking potential. So this study basically signified the more flexible DTG distribution curve which leads to effective devolatilization (Fig. 4). The concept of (Rmax/WL) developed by of Kidena et al. (1998) [32] based on TG-DTG analysis was not always very effective to predict the coking behaviour of different coals.

There are opportunities for improved approach and it is necessary to develop some new technique for predicting the exact coking behavior of coal. Thus, Mass Spectroscopy (MS) coupled with TG was carried out to develop a new method for prediction of coking propensity.

3.2. Quantification of Mass Spectroscopy Analysis (MS):


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In proposed work, a different approach has been developed. The whole MS signal has been normalized in the interval [0-100]. Fig. 5 shows the transformation of the original H2 intensity signal into normalized signal mode.

A different method is developed to quantify the rate of release a particular component. As pyrolysis of coal begins, H2 started releasing after a certain temperature and it will continue throughout the temperature zone. In this approach, the following correlation was developed with respect to H2 liberation. ∑./01 -.

*+ = ∑4

(3)

501 23

Where xi is the conversion of H2, Ni is the normalized ion intensity at each instant, n is the total number of intensity recorded by MS. This formula converts the normalized MS signal into fraction of H2 liberated in the entire zone of operation. Fig. 6 shows the transformation of normalized MS signal (Fig. 5) into conversion of H2. Table 2 represents the ultimate analysis of the selected coals and as per H2 content those can be arranged as N> W> BL> M> O. The area under the curve is calculated from Fig. 6. Based on the area value coals can be arranged as N> W> BL> M> O. This value signifies that the amount of H2 liberated during entire pyrolysis, , which is also in very good agreement with the total H2 content of the coal with respect to ultimate analysis result (Table 2). Thus, it demonstrates the proposed quantification technique is accurate and can be applicable for other components also.

3.3. Non-isothermal kinetics through H2 liberation Kinetic analysis of the coal pyrolysis is one of the major research field in order to predict the structural behavior of coal. The objective of introducing notion of non-isothermal kinetics is to develop different reaction regimes of coal pyrolysis rather than finding out the process kinetics information’s like order of reaction etc. Activation energy (E) in each zone and variation of that from coal to coal may also trigger out some clues regarding structural morphology and coking ability. The analysis adopted to find out the reaction zones based on non-isothermal kinetics was developed by Coats-Redfern [21]. A detail discussion on nonisothermal kinetics of coal based on TG data is available [18,24]. In their work, it has been shown very methodically how TG data can be translated into single step integral and subsequently multistep integral method. A detail discussion on identification of different reaction regime was also carried out. The same principal is also applied in our experimentations and it was in very good agreement as shown by them. This method is 11 ACS Paragon Plus Environment

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further modified to develop same non isothermal kinetics from H2 liberation. In this method Equation (3) is used here to convert normalized H2 signal into pyrolytic conversion (x). Fig. 7 shows the plots of ln [−ln (1−x)/T2] versus 1/T. The nature of the plot is showing clearly a multiple deviation from the straight line behaviour which signifies multiple reaction zones during pyrolysis. A thorough investigation revealed that in most of the cases 4 reaction zones are observed except coal N. As coal N produces

the highest volatile yields, there is

significant weight loss occurred prior to the conventional initial zone temperature. Thus for coal N five consecutive first order reactions happened whereas in other cases the pyrolysis can be explained with the help of four consecutive first order reactions.

Each reaction zone conversion (x) is again recalculated to frame the data set within 0 to 1[5, 6]. By plotting ln [-ln (1-x)/T2] against 1/T for each zone revealed the activation energy and pre-exponential factor for each reaction regime. These individual plots can be framed in a single plot with the progress of temperature (Fig. 8). The kinetic parameters are calculated from the slope of the individual lines (Table 7). So based on the kinetic parameters, devolatilization of coal consists of 4 essential reaction zones instead of 3 reaction regimes as shown in [5, 6]. The reaction zones occurred around the temperature ranges of (300-400 oC), (400-550 oC), (550-800 oC) and (800 to 1050 oC). The extent of reaction can be informed with the predicted activation energy.

Zone 1 Activation energy levels of coal samples are computed from TG and MS approaches tabulated in Table 7. Zone 1 primarily involves dissociation of mobile hydrocarbons in the macromolecular matrix. It was observed the primary devolatilization of coal occurred in the range of 300-400 oC. The activation energy in zone 1 followed the particular trend of Coal M> BL> W> O> N which is not in accordance with the order of coking behaviour based on the CSN value (Table 3). On comparison of the DTG values of coal with the activation energy, it was found that the high mass loss rate accompanying high activation energy could indicate faster cracking, while low mass loss rate accompanying high activation energy could indicate the presence of high macromolecular network structure in the parent coal. Coal M (266 kJ/mole) is example of such behaviour where high activation energy indicates dissociation of larger macromolecular network. In such cases, radical generated during pyrolysis not being fed with the liberated hydrogen from the condensation and aromatization reaction. The absence of proper condensation reaction results in ineffective clustering of the 12 ACS Paragon Plus Environment

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large molecular networks resulting in formation of low quality coke. Coal W and BL possessing equal activation energy of 190 kJ/mole. Coal BL contains 70% of coal W and thus shows almost similar trend of coal W. Coal W is classified as medium coking coal and Coal O known as prime coking coal. High activation energy signified the release of high molecular weight substances compared to other coal samples. A low value of activation energy in coal O can be explained as presence of significant amount of lower molecular weight hydrocarbons than coal W. Coal N possess the lowest activation energy due to low molecular hydrocarbon present in macromolecular structure. This explanation is strengthened by the results of volatiles, coke making test, fluidity test, and effective devolatilization explained from DTG analysis

Zone 2 Zone 2 is the most important zone in coke making process as it captures the thermoplastic behaviour of coal between 400 and 550 oC. The activation energy order through MS could be arranged as: Coal W> O> BL> M> N. As discussed earlier, the competitive reactions between the cracking and the condensation reaction begins and gets completed around the end of the zone 2. In this zone, Coal W carries the highest activation energy (287 kJ/mol). Coal W is a medium coking coal and is placed just behind the prime coking coal (234 kJ/mol) (coal O). The extent of reaction should be highest for coal O in this zone. Slight deviation in coal O energy level might be due to the extent of cracking that is predominant in coal W. Coal W probably contains more aliphatic linkage than coal O. In both the coals i.e. coal W and coal O, the thermoplastic nature was very smooth which could be conceived from the fluidity data (Table 3). Metaplast formation is very rapid in this zone which imparts solvolysis in rest of the coal mass. As coal BL is a blending of Coal W (70%) and Coal O (20%), it inherits average property (214 kJ/mol). A marginal higher activation energy value for coal M (203 kJ/mol) can be again defended as rupture of bigger larger macromolecular network. Coal N (139 kJ/mol) possess lowest activation energy as it contains low molecular weight hydrocarbons and also it is insensitive towards cracking condensation reaction. In the dominating environment of polyaromatic hydrocarbons, some aliphatic linkage is required which facilitates both cracking and condensation. This phenomenon brought faster metaplast formation in coal W, O and BL than other coals. A slight deviation of coke making order cannot be fully understood from this zone analysis.

Zone 3 13 ACS Paragon Plus Environment

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In this zone activation energy requirement is least in comparison to other zones and this feature was maintained for all rank of coals. The mechanism of this zone could be explained by the theory of fusion i.e. by formation of a 'metaplast' which causes a kind of solvolysis of the large condensed molecules. This metaplast progressively gets transformed into coke and volatiles by pyrolysis thereby putting an end to the plastic state. It was believed that the cracking reactions slow down when the temperature increases, through a deficiency of hydrogen and disappearance of the weakest C-C bonds, while the condensation reactions continue to progress. The molecular mass therefore increases very rapidly through the continuous development of network of chemical bonds, mainly aromatic. In case of coking coals, crosslinking coincides roughly with the disappearance of plastic phase and thus the requirement of activation energy is low. For non-coking coals, the cracking reaction may still exist, which is not desirable in that zone and thus may not be able to form large molecular network in comparison to coking coals. This phenomenon is in very good agreement with the order of activation level as: Coal M> N> W> O> BL, which is almost reverse of the experimental observations on the coking quality.

Zone 4 Zone 4 can be thought of as pregraphitization stage, which essentially involves elimination of H2 from the large condensed aromatic network thereby forming coke by large C-C network. As coking coal forms large molecular network, transformation into coke is predominant in this stage. This concept can be well explained by the activation energy order of the selected coals which was found during experimentation as: Coal W> BL> O> N> M. The unexpected high value of Coal W and Coal BL cannot be realized fully and it may involve cracking of mineral carbonates at that high temperature

It was found that this methodology can be applied for certain grade of coal but this may not be a good method for development of generic model for coal grading based on their coking property. Weight loss of coals varies with the rank or volatiles of the coal. Comparison of the activation energy values of different coals in different zones is very difficult as it does not always give similarities between the values. Although this study shows some interesting features, it may not be very useful among the different ranks of coals used for coke making


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3.4. Development of generic model based on H2 evolution All the previous studies shows some relevant information during pyrolysis but not able to predict the exact coking property directly. As per the current coke making knowledge, there is no single parameter exists for coal which is able to quantify the exact coke quality. The estimated kinetics from both the approaches coincided in few cases but in some cases it differed significantly.

So, further investigation is required to develop an approach to

determine the coking potential irrespective of the origin and maturity of coals. Although FSI is a good indicator of coke making property, in some cases like coal BL, this conventional method is not essentially the key property for producing quality coke. It is observed that coal BL, which is a blended coal used in coke plant, gets converted into high quality coke. Coal BL comprises of 70% medium grade coking coal (coal W) and 20% prime coking coal (coal O). The FSI value of BL is around 7.5 whereas coal O has the highest FSI value of 9. It was experimentally found that good quality coke can be formed using above proportion rather than 100% use of coking coal. So in this case, although FSI indicates it has good coking ability, the rank of coking quality can not be derived directly. Hence a direct relationship of coal for producing good quality coke is derived from the estimation of H2.As mentioned earlier, plastic zone is the most crucial zone in the coke making process. Good coking coal always exhibits appropriate thermoplastic behavior. As per the previous discussion, cracking and condensation reactions compete with each other and finally crosslinking of macromolecules takes place. In this process H2 should evolve in sufficient amount to act as feed for the cracked molecules. Simultaneously H2 evolution is also a measure of the aromatization process and it reaches its maximum at the peak fluidity temperature. Thus tracking of H2 gas in the zone of 300 to 700 oC showed a very good correlation for coke making perspective. The very basic step of the developed analysis is normalization of MS signal. The normalized peak obtained in that specified temperature zone is considered for quantitative analysis. A baseline treatment is done on the peak to convert all the peaks into a common platform. After baseline subtraction, the subtracted peak was separated and compared for each coal. The area under this subtracted peak is mentioned in Fig. 9 and could be directly compared for different coals. The area under the curve can be used directly to quantify the coking behavior. It has been observed repeatedly coking coal gives much more area value than the inferior one. In true sense, the curve area is a measure of liberated H2 and thus quantifies the coking behavior accurately. Based on the area obtained by the normalized curve of Fig 9 the H2 generation follows the order Coal BL>O>W>N>M, which does not


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follow the FSI sequence of the respective coals but it matches with the quality of coke produced. Therefore, it can be inferred that the H2 evolution is more for any coking coal in the thermoplastic zone and it follows the exact coking quality of coal. This technique can be used to realize coking property of any coal irrespective of the coal rank or volatiles. This technique may have the potential which can be used in addition of any binder or determination of synergic effect of coal with other organic material.

4. Conclusions This paper presents the characterization of the four coals having different coking behavior. The proposed method is also supported with blended coal used in plant. A good correlation was observed for predicting the appropriate quality coke from H2 signal generated from MS instrument which enabled in accessing reaction regimes during pyrolysis of coal. A thorough kinetic analysis is also developed through the quantified approach of MS signal which shows a very good agreement with the conventional TG technique. A detailed investigation revealed that carbonization occurred essentially in four reaction regimes. The most important finding in this paper is to develop a novel correlation with the H2 liberation during pyrolysis. It was observed that the liberation of H2 is almost getting doubled with respect to non coking coal during plastic state. From different characterization studies, it is clear that the coking coal has better thermoplastic behavior, effective devolatilization but may not be correlated directly to coking property of coal which can be done by estimated with H2 liberation. This technique could be more effective for selection of binder and other organic substances with coal.

5. References

[1]. Dash PS, Lingam RK. Kumar SS, Suresh A, Banerjee PK, Ganguly S. Effect of elevated temperature and pressure on the leaching characteristics of Indian coals; Fuel 140 (2015), pp. 302-308. [2]. Chandaliya VK, Biswas PP, Dash PS. Organo-refining of high ash Indian coals at bench scale. Fuel 165 (2016), pp. 425-431. [3]. Nag D, Biswas P, Dash P, Chandaliya V, Sahoo P, Saxena V, Chandra S. Characterization and Utilization of Organo-Refined Extract in Metallurgical Coke Making; International Journal of Coal Preparation and Utilization (2016), pp, 1-11.


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Energy & Fuels

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TG–MS and its late gas generation potential; Fuel 156 (2015), pp. 243–253. [16].

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pyrolysis by thermal analysis–mass spectrometry; Fuel Processing Technology 137 (2015), pp. 351–358. 17 ACS Paragon Plus Environment

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methane and tar evolution during coal pyrolysis; Proceedings of the Combustion Institute 30 (2005), pp. 2177-2185. [25].

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Energy & Fuels

[29].

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of the plasticity and a TGA-Derived Parameter; Energy & Fuels 12 (1998), pp. 728787.

Coke Oven Gas

Coal

Coke Oven Battery (Carbonization) 1000-1100 0C

By-products 24 %

Coke 76 %

Coal Tar

Fig. 1 – Process flow diagram of coke making process.


Ammonia Naphthalene

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Fig. 2 – TG – DSC – MS Equipment.

Fig. 3 - TG – DTG curves of Coal samples: (a) Coal O (b) Coal M (c) Coal W (d) Coal N (e) Coal BL.


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Fig. 4 - Normalized DTG vs Temperature of coal samples.


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Fig. 5 - Normalized H2 Intensity vs Temperature of coal samples.


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Fig. 6 - H2 Conversion vs Temperature of coal samples.


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Fig. 7 - Plots of ln [−ln (1−x)/T2] vs 1/T calculated by one-step integral method from H2 of Coal samples: (a) Coal O (b) Coal M (c) Coal W (d) Coal N (e) Coal BL


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Fig. 8 - Plots of ln [−ln (1−x)/T2] vs 1/T calculated by multistep integral method from H2 of Coal samples: (a) Coal O (b) Coal M (c) Coal W (d) Coal N (e) Coal BL.


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Fig. 9 - Area of the thermoplastic region vs Temperature of coal samples.


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Energy & Fuels

Table 1 – Composition of coke oven gas Major Components

Volume (%)

Hydrogen

48-55

Methane

28-30

Carbon dioxide

1.5-2.5

Carbon monoxide

5.0-7.5

Nitrogen

1.0-3.0

Oxygen

0.0-0.5

High paraffin and unsaturated hydrocarbons

2.5-4.0

Table 2 – Analysis of coal samples Coal

Proximate Analysis (wt. %)

Ultimate Analysis (wt. %) for elemental composition of coal

Moisture

Ash

Volatile

yield

Matter

C

H

N

S

O

yield O

2.07

11.93

23.23

74.86

4.049

3.960

1.306

3.895

M

2.30

11.94

18.09

79.24

4.195

3.650

0.815

0.160

W

1.41

16.70

25.40

71.01

4.310

3.700

0.726

3.554

N

2.15

6.89

32.57

76.25

4.830

0.747

1.320

9.963

BL

1.91

12.6

23.1

75.96

4.21

1.77

0.65

5.03


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Table 3 - Crucible swelling values of Coal Coal

CSN (Crucible Swelling Number)

BL

7.5

M

1

N

3

O

9

W

5

Table 4 – Test Parameters Common Coke making Coal

Softening Temp. (0C)

Resolidification Temp. (0C)

Maximum Fluidity (ddpm)

BL

413

495

250

M

-

-

-

N

400

459

124

O

405

493

2128

W

410

481

4015


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Energy & Fuels

Table 5 – Characteristic temperatures and residual mass determined by TGA Coal

Tonset (oC)

Tmax (oC)

Tend (oC)

∆T (oC)

Residual Mass (%)

BL

388

461.5

545.5

157.5

77.17

M

424.5

480.3

570.9

146.4

82.24

N

372.3

452.9

529.7

157.4

68.36

O

347.4

480.7

677.3

329.9

77.8

W

382.1

461.5

530.1

148

76.36

Table 6 – Area under the normalized DTG curve Coal

Normalized Area

BL

8637

M

8430

N

7650

O

9870

W

8010

Table 7 – Kinetic parameters for pyrolysis of coals estimated through H2 Coal Reaction Temperature Zone Range (0C) O

Activation Energy(kJ/mol)

Pre-exponential Factor (min-1)

1

310 - 410

136.18

2.05×10

2

410 - 530

234.47

1.4×10

3

530 - 825

54.30

40.13


10

15

R2 0.97 0.99 0.98

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M

W

N

BL

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6

0.97

14

0.93

4

825 - 1015

181.33

3.34×10

1

360 – 415

190.53

1.8×10

2

415 - 530

203.00

7.87×10

3

530 – 735

80.30

4739.79

0.97

4

735 - 1025

83.70

182.21

0.98

1

330 - 385

191.88

1.86×10

2

385 - 510

287.60

4.16×10

3

510 - 865

59.20

4

865 - 1000

381.83

3.62×10

1

210 - 285

82.95

31.33×10

2

285 - 375

139.00

1.6×10

3

375 - 515

179.44

7.01×10

4

515 - 780

73.06

773.21

0.98

5

780 - 1020

121.00

11159.38

0.98

1

300 - 360

185.17

7.83×10

13

0.96

2

360 - 490

214.89

2.54×10

7

0.95

3

490 - 850

44.56

19.47

4

850 - 1000

233.15

6.58×10


12

0.98

15

0.92

19

0.98 0.98

65.6 15

0.95

6

0.93

11 11

0.98 0.99

0.97 8

0.91

Hydrogen Evolution during Devolatilization To Predict Coking

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