Abstract
Coal is at present a major fuel source for power generation worldwide and will remain as such in the near future. The most important property of coal that determines its price is its calorific value. However, volatiles, ash, and moisture content are also very important properties needed for the quality control (QC) of the coal used to maintain an optimal operation of coal combustion in a boiler. The determination of these properties is carried out via well-established ASTM/DIN methods, which are slow and time-consuming. This study uses combined thermogravimetric analysis (TGA)/differential thermal calorimetry (DSC) instrumentation as a tool to evaluate the reactivity patterns of the aliphatic versus aromatic content of coals, which is correlated to the volatile content of coals. Two coals, bituminous (American Baily Pittsburgh No. 6) used in Israeli utilities and lignite (brown coal Hambach) used in German power plants, have been investigated in this study. The results show that the combined TG/DSC method can provide a much better understanding of the chemical reactivity of coals in the combustion process.
1. Introduction
Coal is an organic sedimentary rock that has been an important source of energy around the world for over 200 years and will stay as a major source for the next decades.
The process of coal formation takes about 10–350 million years7,10 and also depends on physical conditions. The carbon content increases with the release of carbon oxides and water from organic matter, and the higher the rank of the coal, the higher are its carbon content, aromatic character, and higher calorific value. Also, besides the mineral matter of plants, during the process, some inorganic constituents are trapped within the organic matter.
Coals are divided into several types according to the coalification age, carbon content, volatile matter, total moisture, oxygen or hydrogen content, and calorific values. The main types are low-rank coals: lignite (sometimes defined as brown coal), bituminous coal (BA, sometimes defined as steam coal), and anthracite. The main difference between the three types of coal macromolecules is the fact that lignite contains a large percentage of aliphatic C–H bonds and is low in the aromatic nature and hydrogen and oxygen contents as well as the water content is high; some water is bonded strongly to the coal macromolecule via hydrogen bonds19 (defined as intrinsic water3,8,20). Bituminous coal contains a much higher aromatic C–H character and no intrinsic water, and anthracite contains only aromatic carbon with very low hydrogen and water contents.
Thus, we will study the lignite coal (denoted Hambach, HA coal) and bituminous coal (denoted BA coal).
The aromatic and aliphatic nature of a coal can be evaluated by solid NMR spectroscopy, which was applied to the two coals in this study: the (HA/BA)Aliphatic and (HA/BA)Aromatic ratios were found to be 1.52 and 0.65, respectively.5
The main utilization of coal is for power generation (steam coal) and also to a lower extent in the metallurgical industry (cracking coal and PCI (pulverized coal injection) to the furnace using a stream of air), and it will remain as a major fuel source for the next decades. Two coal types are used for power generation: lignite and bituminous coals. Depending on the coal type, steam coals are ground to the micrometer size (∼10–70 μm) to achieve efficient combustion. Also, the grinding process occurs at higher temperatures to reduce the moisture content in lignite from up to 65 wt % to lower than 30 wt % and in bituminous coal from up to 10 wt % to lower than 2 wt %. After drying, the combustion process is composed of two main stages: (i) pyrolysis of the coal particle that involves emission and burning of volatiles (mainly methane and low-molecular-weight organic gases and molecular hydrogen) and char formation and (ii) combustion of char at high temperatures (1600–1700 °C).1−3,8,9
1.1. Coal Analysis and Combustion Behavior
Coal supply to the utility occurs by ship/train/truck transport and the coal is unloaded via a conveyer belt. Usually, coal samples for analysis (the sampling itself is also standardized to guarantee a representative sample, e.g., by subsampling) are taken directly from the moving belt. If coal is stored in the yard of the power station, coal samples are taken from the depth of the coal pile (as the surface coal undergoes low-temperature aerial oxidation and does not represent the coal stored in the pile4).
The next step prior to analysis of the coal is to grind it to the same size range as that of the pulverized coal, which is fired in a boiler (the necessary sample preparation is defined in analysis standards, mostly independent of coal utilization <0.2 mm). In addition, the samples must be analyzed and separately measured for each property (moisture, volatiles, ash content, and calorific value) according to the selected measurement method.
As mentioned, coal quality is characterized (inter alia) by determination of several properties.4−6
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1.
Moisture content: It affects the efficiency of coal combustion. The higher the moisture content, the harder it is to burn the coal.
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2.
Volatiles: It is expressed via mass loss via pyrolysis due to the release of gases upon heating (such as hydrogen, carbon monoxide, and organics). The percentage of volatile materials determines the coal quality (and hence its price), the calorific value, and the combustion profile.
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3.
Ashes: It is the inorganic residue post combustion. A high ash content increases the cost of power production and postcombustion treatment of ash to avoid environmental problems.
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4.
Calorific value: It is a direct indication of the energetic value of coal. The calorific value is expressed by the energy produced by the combustion process.
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5.
Chemical reactivity: When coal undergoes mass reduction that is dependent on temperature, the source can undergo moisture evaporation (usually, it terminates at >120 °C) or chemical reactions like pyrolysis, oxidation, etc. Thus, one can correlate the chemical reaction to the chemical nature of coal (e.g., aliphatic vs aromatic C–H bonds). It should be noted that the main processes observed are aliphatic and aromatic functional group decomposition (pyrolysis) or oxidation (combustion) reactions.
The analysis is carried out usually using well-established classical methods (e.g., NIST or DIN methods), though some thermal methods have been adopted as standards.
The combustion/pyrolysis behavior of coal is dependent on the actual boiler that is used, the residence time of the coal particle, the temperature profile in the boiler, and also very much on the chemical nature of the coal macromolecule. Namely, on the aliphatic and aromatic contents of the coal, as the C–H aliphatic groups are more active chemically to oxidation than the C–H aromatic groups.
Thus, the thermal methods of combined thermogravimetric analysis/differential thermal calorimetry (TGA/DSC) can help in both determination of coal properties and shedding light on the profile of the oxidation/combustion behavior of the coal used as fuel in utility.
1.2. TGA/DSC
A TGA/DSC device is a tool for diagnosing and analyzing processes occurring in a material under different temperature conditions. The thermal gravimetric analysis (TGA) technique determines the change in the sample mass as a function of temperature or time under different gas atmosphere conditions.
When the sample is exposed to a variable/constant temperature under a certain gas environment, it might undergo chemical/physical processes (such as oxidation, adsorption, drying, decomposition, etc.), which are accompanied by a mass change.
Differential scanning calorimetry (DSC) is the method that can measure in parallel to the TGA analysis the amount of heat involved in endothermic/exothermic processes of the sample tested during the temperature heating process and the parameter measured is the enthalpy change.
The advantage of a combined TG/DSC analysis is that it can obtain information on all properties of the coals to be determined in one measurement procedure. This is a much superior analytical process compared to classic ASTM/DIN methods used for coal analysis, in which every property is measured by a separate analytical procedure. Thus, if the combined TGA/DSC analysis is used, it reduces the time and cost of coal analysis prior to coal utilization.
Indeed, currently, thermal methods are used for coal analysis and also as standard methods for analysis in industrial analytical instruments.
2. Results and Discussion
2.1. TGA Experiments
2.1.1. Effect of the Fraction Size of Treated Coals
As mentioned (see above), each treated coal has been sieved into three different fractions size. The studied fractions of BA coal are >350 μm, 250 μm < BA < 350 μm, and 74 μm < BA < 250 μm and those of HA coal are >500 μm, 200 μm < HA < 500 μm, and HA < 200 μm.
The sieving units available in the German and Israeli laboratories have different ranges, which led to the fraction sizes that have been chosen for the study. To evaluate the chemical behavior and the moisture, volatile, and ash contents via the combined TG/DSC method, 5 mg of the sample of an assigned size coal was heated at a rate of 5–10 °C/min in a temperature range of 30–1000 °C under different atmospheres: air, oxygen, and nitrogen.
Experiments are also performed to determine the effect (if there is any) of the particle size.
The results of BA and HA coals are shown in Figures 1and 2, respectively.
Figure 1.
TGA analysis of the treated BA fraction size in (a) O2 atmosphere, (b) air atmosphere, and (c) N2 atmosphere.
Figure 2.
TGA analysis of the treated HA fraction size in (a) air atmosphere, (b) O2 atmosphere, and (c) N2 atmosphere.
As can be clearly seen, there is no appreciable effect of the particle size in both an oxidative environment (air or oxygen) and an inert atmosphere (nitrogen gas). This is expected, as solid/gas reactions take place at the surface of the macropores of coal, which is much larger (3.92–5.45 m2/g for BA coal and 2.35 m2/g for HA coal21) than the external surface of coal particles. The combustion of BA coal starts at ∼370–560 °C and is terminated at 510 °C in oxygen and at a somewhat higher temperature of ∼560 °C in air in the TGA instrument. When the more reactive HA coal is analyzed, the combustion process starts as expected at a lower temperature of ∼280 °C (same for an oxygen or air atmosphere) and terminates at 520 °C. However, prior to the combustion of BA coal, no water release is observed as expected (the treatment of coal is carried out in an oven at 60 °C under vacuum for 24 h, and during this process, all moisture has been evacuated). But when HA coal is studied, an appreciable weight reduction starts at 40 °C but the main process occurs in the 80–120 °C temperature region, which is observed due to intrinsic water vaporization.
Intrinsic water is composed of the water molecules that are strongly chemisorbly attached to the lignite coal macromolecule surface (probably via strong hydrogen bonds20) and are not evacuated during the pretreatment process.
It should be noted that the combustion process in an oxidative environment (air or oxygen atmosphere) is accompanied by the emission of low-molecular-weight hydrocarbons and molecular hydrogen—a pyrolysis reaction. However, these gaseous products are oxidized immediately to the final oxidation products (carbon dioxide and water). The same weight reduction in the TGA experiment with BA and HA coals is observed when inert N2 is used. In the inert gas atmosphere, the only second process that occurs is pyrolysis.
It is interesting to note that in the air or oxygen atmosphere, only the oxidation reaction occurs. In the combustion process of BA coal, only one process is observed, while when the younger HA coal is measured, two distinct oxidation processes are observed (Figure 2), the first one in a temperature range of ∼280–400 °C and the second in a range of ∼400–520 °C. Probably, the first process is the combustion reaction of active aliphatic C–H groups in the coal macromolecules and the second one (at higher temperatures) is the combustion reaction of aromatic C–H groups, which are more stable and thus a higher temperature for activation is needed.
When the TGA experiment is carried out in the inert atmosphere, no combustion occurs but rather emission of the volatile matter via pyrolysis of coal takes place. Also, under an inert atmosphere (nitrogen gas), the pyrolysis step in the two different coals, bituminous and lignite, is not affected by the fraction size of the treated coals, and the three fractions of the coals, BA and HA, show similar results for each coal studied.
2.1.2. Treated BA versus HA
To compare the behavior of the two coal ranks (lignite and bituminous coals), the results of the TGA experiment of the mid-size fraction from each type of the treated coal (250–350 μm of BA coal and the 200–500 μm of HA coal) in the different gas atmospheres are shown in Figure 3.
Figure 3.
TGA analysis of BA versus HA fraction size in (a) air atmosphere, (b) O2 atmosphere, and (c) N2 atmosphere.
Prior to the oxidation step (under the air or oxygen atmosphere, Figure 3a,b), intrinsic water release is observed in lignite coal, which begins at ∼80 °C and is also observed in the N2 inert atmosphere (Figure 3c). The results clearly indicate that in the oxidative environment (air or O2 atmosphere), the higher rank coal with larger aromatic nature BA is oxidized at higher temperature ranges of ∼350/390 and 520/580 °C (in O2 and air atmospheres, respectively), whereas more reactive lignite coal (but with a lower calorific value) begins the oxidation process at ∼220 °C and the process terminates at the same final temperature as in the bituminous coal (∼520/580 °C). Also, it is clear that the oxidation process of low-rank lignite consists of two separate steps. In the low-temperature region (∼220–400 °C), in the first step, most of the coal content undergoes oxidation, probably due to the more reactive aliphatic C–H content, and at higher temperatures (∼420 to 520/580 °C), in the second step, aromatic carbon C–H/–C=C–C=C–C is oxidized. The weight loss in the aliphatic oxidation process is ∼80% and that for the aromatic oxidation process is ∼7%. The pyrolysis step that is measured in the nitrogen atmosphere experiment is much larger in size in HA coal, ∼49% mass reduction compared to that in BA coal (∼33%). Also, of course, any gaseous pyrolysis product (see above) will be oxidized to carbon dioxide and water vapor as the final products.
Moreover, the intrinsic water of the treated HA coal (see above) is released at ∼80–130 °C, whereas in treated bituminous coal, there is no release of any detectable water, which is reasonable, as the treatment process evaporated all moisture from bituminous coal that contains no intrinsic water.
The results of the TGA experiment under the N2 atmosphere corroborate the former results. Namely, lignite coal contains a large percentage of aliphatic C–H bonds, which under heating in the inert atmosphere decompose to yield low-molecular-weight organic gases, the pyrolysis step. Thus, HA coal has a much higher volatile content (∼49%) than the BA coal (∼33%).
Furthermore, the pyrolysis process of HA coal starts at a much lower temperature of ∼220 °C compared to BA coal (∼380 °C).
2.1.3. Fresh (Untreated) Coal
To evaluate the moisture, volatile, and ash contents, 30–35 mg of a fresh coal sample was heated at a rate of 5–10 °C/min in a temperature range of 30–1000 °C in different atmospheres: air, oxygen, and nitrogen. The results of BA and HA coals are given in Figures 4 and 5, respectively.
Figure 4.
TGA analysis of fresh BA in (a) air atmosphere, (b) O2 atmosphere, and (c) N2 atmosphere.
Figure 5.
TGA analysis of fresh HA in (a) air atmosphere and (b) O2 atmosphere.
The results of BA coal show that the same behavior is observed in the oxygen or air atmosphere, a weight loss of ∼89% is measured due to the combustion of the organic content of BA coal in a temperature range of 300–600 °C. Also, a small weight loss of ∼1–3% due to moisture evaporation is observed in a temperature range of 50–110 °C. Under the nitrogen atmosphere, the same weight loss of moisture is observed and volatile emission via pyrolysis accounting to ∼21% is measured in a temperature range of 200–900 °C.
The oxidation (combustion) process of the fresh coals shows more or less the same behavior as that of the treated coals with one different result: the fresh coals do contain moisture (which has been evaporated during the treatment process) in addition to the presence of intrinsic water in HA coal. The results of HA coal show that the combustion process consists of two well-separated steps, the first one is probably of the aliphatic content in a temperature range of 200–400 °C and consumes ∼50% of coal, and the second step is probably of the aromatic content in a temperature range of 420–520 °C and consumes ∼6% of coal. The weight loss due to moisture evaporation at 60–120 °C was determined in the N2 atmosphere and is much higher than that measured for BA coal (∼21%), whereas the volatile content at 130–590 °C is approximately 51%.
The calculated values of the moisture, ash, and volatile contents in the fresh and treated coals are given in Table 1.
Table 1. Calculated Values of the Moisture, Ash, and Volatile Contents Obtained from the TG Experiments with Treated and Fresh Coalsa.
| ash DF [%] | volatile gases DF [%] | moisture [%] | coal type |
|---|---|---|---|
| 4.08 | 34.15 | 2.86 | BA 350X |
| 5.83 | 33.72 | 1.86 | BA 250X350 |
| 6.48 | 33.42 | 2.23 | BA 74X250 |
| 5.63 | 48.74 | 11.27 | HA 500X |
| 5.51 | 49.32 | 10.94 | HA 200X500 |
| 5.94 | 48.75 | 9.74 | HA X200 |
| 8.09 | 21.14 | 3.13 | BA fresh |
| 7.46 | 50.79 | 43.6 | HA fresh |
TGA results (wt % should be used as a unit).
It is evident from the results given in Table 1 that the results in the different samples of the treated coals are not affected by the particle size: moisture contents are 2.32 ± 0.51 and 10.65 ± 0.81 wt %; volatile contents are 33.76 ± 0.37 and 48.94 ± 0.33 wt %; and ash contents (DF) are 5.46 ± 1.24 and 5.69 ± 0.22 wt % for BA and HA coals, respectively.
2.2. Combined TG/DSC Analysis Experiments
To obtain a good accurate analysis of coal using thermal methods, 5 mg of the sample to be analyzed should be homogeneous. However, this cannot be achieved with the fresh untreated coal due to the large distribution of particle size but is available with the treated sieved coal samples. Thus, the results reported in this section have been measured only with the treated coals.
DSC analysis occurs simultaneously with the TG measurement, namely, the two types of analyses are carried out simultaneously; thus, the heat release—negative ΔH (calorific value), the heat absorption—positive ΔH (heat absorbed for moisture evaporation and volatile release in N2 atmosphere experiments), and the mass loss can be displayed on the same figure via the combined experiment.
To obtain a good accurate result using thermal methods, 5 mg of the homogeneous coal sample was taken for the analysis.
As stated above, it was decided to measure only the treated coals to refrain from extra moisture and the homogeneity problem expected in fresh coal, which stems from the small sample size of 5 mg.
2.2.1. Fraction Size of Treated Coals
The combined TG/DSC analysis of the different fractions of BA coal under air and oxygen atmospheres is given in Figures 6 and 7.
Figure 6.
BA coal in an air atmosphere with fraction sizes of (a) 74 < BA < 250 μm, (b) 250 < BA < 350 μm, and (c) >350 μm.
Figure 7.
BA coal in an O2 atmosphere with fraction sizes of (a) 74 < BA < 250 μm, (b) 250 < BA < 350 μm, and (c) >350 μm.
The results of the combustion process (under the oxygen or air atmosphere; Figures 6 and 7) in the combined TGA/DSC experiment indeed show that the conclusion reached in the above TG experiments, the combustion/oxidation process of HA coal, that two exothermic oxidation processes do occur is also true for BA coal, though the TG experiment does not indicate it! The simultaneous calorimetric experiment (Figures 6 and 7) show two separated exothermic processes: the first one (which is much smaller in size) is probably of the aliphatic content of the coal in a temperature range of 240–480 °C, and the second (much larger effect) is probably of the aromatic content of the coal in a temperature range of 480–600 °C. However, the TG experiment cannot indicate the first one because of the low aliphatic content in BA coal and does not resolve between the two processes! Also, the calorific value of the coal (assuming that the ΔH enthalpy change is that of the calorific value) is the same for the three fractions measured (18–19 000 kJ/kg). The combined TG/DSC analysis of the different fractions under the nitrogen atmosphere is given in Figure 8.
Figure 8.
BA coal in the N2 atmosphere with fraction sizes of (a) 74 < BA < 250 μm, (b) 250 < BA < 350 μm, and (c) >350 μm.
The combined TG/DSC experiments of BA coal under the nitrogen atmosphere of the different fraction sizes (Figure 11) give an extra added value of the ΔH enthalpy of the pyrolysis step in which emission of volatiles is observed in a temperature range of 140–900 °C and is very interesting from the point of view of the size effect, and the measured values for 74–250, 250–350, and >350 μm are 4224, 6878, and 5880 kJ/kg, respectively. The combined TG/DSC analysis of the different fractions of HA coal under air and oxygen atmospheres is given in Figures 9 and 10, respectively.
Figure 11.
HA coal in the N2 atmosphere with fraction sizes of (a) <200 μm, (b) 200 < HA < 500 μm, and (c) >500 μm.
Figure 9.
HA coal in the air atmosphere with fraction sizes of (a) <200 μm, (b) 200< HA < 500 μm, and (c) >500 μm.
Figure 10.
HA coal in the O2 atmosphere with fraction sizes of (a) <200 μm, (b) 200< HA < 500 μm, and (c) >500 μm.
The results of the combustion process (under the oxygen or air atmosphere; Figures 9 and 10) for HA coal show that two exothermic oxidation processes do occur (as observed for BA coal), but in this case, the two processes are observed both by the TG and DSC experiments: the first one is probably of the aliphatic content of the coal in a temperature range of 200–460 °C and the second one is probably of the aromatic content of the coal in a temperature range of 450–600 °C. It is clear that the aliphatic oxidation process accounts to a much larger percentage of the coal weight (∼66%) compared with the aromatic content, which accounts to ∼6%. Also, it is clear that the first combustion step is composed of two different aliphatic sites at 200–350 °C and the second one separated them at 360–460 °C. The fact that the aliphatic combustion accounts to a much larger effect is in accord with the fact that the aliphatic content in lignite coal is much higher than the aromatic content in this coal. Also, the calorific value of the coal (assuming that the total ΔH enthalpy change is that of the calorific value) is the same for the two fractions measured (16 000 kJ/kg).
Also, in the case with HA coal, the combined TG/DSC experiments under the nitrogen atmosphere of the different fraction sizes (Figure 11) give an extra added value of the ΔH enthalpy of the pyrolysis step in which emission of volatiles is observed in a temperature range of 140–900 °C and is very interesting from the point of view of the size effect; the measured values for 200–500 and >500 μm are 4492 and 4022 kJ/kg, respectively.
It is interesting to note that for each coal measured under a certain atmosphere, the J/g area of each relevant fraction is changed, but the quantitative result of the calorific value is the same.
2.3. Coal Properties
The validity of the results of the measured thermal methods (using 5 mg of the sample for the analysis) has been compared to classical DIN methods in which 1 g of the sample has been used for the analysis (see Section 2.2). The results are given in Tables 2 and 3 for the moisture, ash, and volatile contents and in Table 4 for the calorific value.
Table 2. TGA/DSC Results (the Lab/DIN Results are in Parentheses).
| calorific value [kJ/g] | ash DF [%] | volatile gases DF [%] | moisture [%] | coal type |
|---|---|---|---|---|
| 18.06 (30.0–31.1) | 4.08 (6.87) | 34.15 (34.55) | 2.86 (2.68) | BA 350X |
| 18.03 (30.2–31.3) | 5.83 (6.20) | 33.72 (34.46) | 1.86 (2.67) | BA 250X350 |
| 18.98 (30.1–31.2) | 6.48 (5.90) | 33.42 (34.49) | 2.23 (2.76) | BA 74X250 |
| 15.91 (23.8–24.8) | 5.63 (4.66) | 48.74 (50.33) | 11.27 (11.91) | HA 500X |
| 15.94 (23.8–24.8) | 5.51 (4.86) | 49.32 (50.12) | 10.94 (12.06) | HA 200X500 |
| 16.11 (23.7–24.7) | 5.94 (5.07) | 48.75 (49.62) | 9.74 (12.10) | HA X200 |
| 16.96 (29.1–29.8) | 8.09 (8.01) | 21.14 (21.37) | 3.13 (2.84) | BA fresh |
| 8.8 (23.7–24.7) | 7.46 (6.12) | 50.79 (51.92) | 43.6 (46.57) | HA fresh |
Table 3. TGA/DSC Results (Literary Information is in Parentheses).
| ash DF [%] | volatile gases DF [%] | moisture [%] | coal type |
|---|---|---|---|
| 8.09 (7.7–13.8) | 21.14 (14–37.2) | 3.13 (1,2–5.9) | BA fresh |
| 7.46 (4.0–5.1) | 50.79 (50.5–52.4) | 43.6 (30–70) | HA fresh |
Table 4. Comparison between the Calorific Values Achieved from DSC, the Lab, and the Literature.
| DSC CV [kJ/g] | lab CV [kJ/g] | literature CV [kJ/g] | coal type |
|---|---|---|---|
| 16.96 | 29.1–29.8 | 19–33 | BA fresh |
| 8.8 | 23.7–24.7 | 6.7–25.3 | HA fresh |
| 18.06 | 30.0–31.1 | BA 350X | |
| 18.03 | 30.2–31.3 | BA 250X350 | |
| 18.98 | 30.1–31.2 | BA 74X250 | |
| 15.91 | 23.8–24.8 | HA 500X | |
| 15.94 | 23.8–24.8 | HA 200X500 | |
| 16.11 | 23.7–24.7 | HA X200 |
2.3.1. Moisture, Volatile, and Ash Contents
To determine these properties, the following procedures have been utilized with 5 mg of the coal samples.
Moisture content: It is determined as mass depletion in the TG experiment in a temperature range of 30–130 °C carried out under oxygen, air, and nitrogen atmospheres.
Ash content: It is determined as the residual mass left (at 950 °C) at the end of the TG experiment carried out under the oxygen or air atmosphere.
Volatile matter is determined as the mass depletion in the TG experiment in a temperature range of 130–950 °C carried out under the nitrogen atmosphere.
The results of the TG experiments are given in Table 2.
To obtain a good estimate of the validity of the results, classical DIN methods (see Section 2.2) were used to measure the coals with 1 g of coal samples, and the results of DIN QC analysis are also displayed in parentheses in Table 2.
Also, the typical ranges that are reported for bituminous and lignite coals as cited in the literature are given in parentheses in Table 3.
As can be clearly seen, there is a very good correlation between the TG results and the classical DIN methods. This result indicates that the TG method can be used to replace the old classical time-consuming expensive methods.
2.4. Calorific Value
During the combustion process of the coal, the amount of heat released (ΔH values) from the sample is measured and can be extrapolated as the calorific value of the coal, and the results are given in Table 4.10−19
Although we see a similarity between the lab and literature results, we see a large difference (even if there is still a correlation) between the DSC and lab/literature results. The reason for this deviation is that the lab DIN or ASTM measurements are carried out in a Bomb calorimeter (which during the analysis absorbs all of the heat released from the coal sample); in the DSC used for the analysis, some of the heat evolved in exothermic processes is lost due to the external piping of the DSC unit. Thus, the enthalpy of the exothermic process measured is lower. To overcome this discrepancy, restructuring and upgrading of the DSC are needed. It is estimated that with a new DSC/TG unit in which future experiments will be carried out, this problem will be solved.
3. Conclusions and Further Direction for the Future
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1.
The combined TG/DSC method adds a new dimension, shedding light on the different chemical processes involved during the combustion process of the coal (e.g., oxidation of aromatic carbon vs aliphatic carbon). It gives a new approach to operators in utilities on the different chemical oxidation steps occurring during the combustion of the pulverized coal in the boiler of the utility. Also, the energy loss involved in the pyrolysis step (which occurs during the combustion process) can be estimated including the heat needed for the moisture release from the coals prior to the combustion step.
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The combined TGA/DSC offers a new faster and more efficient tool for determination of coal properties to be used as fuel for power generation.
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3.
There is no effect of the fraction size of each coal in a specific gas environment
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4.
The TG analysis is much faster and cheaper than classical ASTM/DIN methods and can replace them, leading to a faster and more efficient method for the QC of coals.
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5.
There is a difference between the DSC results and the classic ASTM/DIN lab results, which originates from the DSC structure that has to be solved technically. A factor/equation may bridge this gap. More work is needed to be done in this regard.
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6.
There is still a need for working with MS, FTIR, and GC to obtain a more complete picture of the composition of coal and materials participating in atmospheric oxidation at low temperatures (30–150 °C).
4. Materials and Methods
4.1. Materials
The following two coals were examined
Bituminous/subbituminous American Bailey (BA) coal was used as fuel in pulverized coal power plants in Israel.
German Hambach lignite (HA) was used in power plants in Germany.
Each coal was treated via grinding and sieving to three sizes and the coal was evacuated under a primary vacuum pump in an oven at 60 °C for 24 h and after cooling to room temperature; the sieved coal was kept prior to any analysis under a nitrogen atmosphere in sealed polyethylene vials. Also, the untreated coal sample (as is) was used for analysis.
The macropore surface areas [m2/g] of the two types of coals were 0.96 (BA) and 1.41 (HA) while the micropore surface areas [m2/g] were 39.5 (BA) and 110 (HA).5
4.2. Methods
To characterize the properties of coals, each coal sample was studied by the combined TG/DSC analysis under three different gas environments: air, oxygen, and nitrogen. From the combined TG/DSC analysis, the ash, volatile, and moisture contents were determined as well as the calorific value. The TG/DSC analysis started at 30 °C and the sample was heated up to 1000 °C at a rate of 5–10 °C/min. The moisture content could be determined (see the results below) in a relevant temperature range (30–130 °C). The ash content as the residual mass was measured at 950 °C in the experiments carried out under the O2 or air atmosphere. The volatile matter was determined as the mass depletion in a temperature range of 130–950 °C during pyrolysis under the N2 atmosphere. The calorific value was determined from the heat released during the burning process under O2 and air atmospheres. All of the measurements were performed using a Netzsch TGA/DSC, model STA 449C “Jupiter”. To compare the results obtained by the TG/DSC analysis to those of the standard NIST/DIN classical methods, the following measurements were carried out
Moisture content: according to DIN 51 718, the sample (fraction size <1 mm) was dried at 135 °C (for lignite) or 106 °C (for bituminous hard coals) in the nitrogen atmosphere until a constant weight was reached.
Ash content: according to DIN 51 719, the sample (1.0 g, <200 μm) was ashed at 815 °C (heating to 500 °C for 60 min, further heating to 815 °C, holding at 815 °C for at least 60 min, weighing the sample, and repeating ashing until weight constancy was reached) under air.
Volatile matter: according to DIN 51 720, definition by convention, the end temperature and heating rate should be set accurately. The analytical wet sample (1.0 g, <200 μm) was heated in a closed crucible (the lid and the crucible were made of quartz glass) within 7 min to 900 °C (fast heating!). The volatile matter was determined by the mass loss, corrected by the released water (according to DIN 51718).
Calorific value: according to DIN 51 900-21, the sample (1.0 g, <250 μm) was heated until complete combustion in a bomb calorimeter under the air environment was achieved. The amount of heat release (with regard to its mass and water content) was the calorific value.
Different parameters obtained from the TG results were calculated using the following formulas
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Acknowledgments
The authors thank Henry Lohse from the TU Bergakademie Freiberg Institute for his help in executing the lab measurement and Alon Khabra from Ariel University for his technical assistance.
Supporting Information Available
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c05296.
TGA/DTG analysis graphs of the treated BA and HA coals in air; proximate analysis; ultimate analysis; and calorific analysis tables of the coals (PDF)
The authors declare no competing financial interest.
Supplementary Material
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