Environ. Sci. Technol. 1997, 31, 3148-3153
Reduction of Thermal Coals Environmental Impact by Nitric Precombustion Desulfurization RAMO Ä N AÄ L V A R E Z , * CARMEN CLEMENTE, AND DULCE GO Ä MEZ-LIMO Ä N Universidad Polite´cnica de Madrid, Escuela Te´cnica Superior de Ingenieros de Minas C./Rı´os Rosas 21, 28003 Madrid, Spain
Sulfur content in coal is converted into sulfur oxides during combustion which, among other effects, causes acid rain. To reduce this environmental impact, precombustion coal desulfurization by means of nitric leaching has been researched. Two Spanish high-rank thermal coals have been treated at laboratory scale using nitric acid concentrations of 20 and 30 wt %, at 90 °C, and with residence times ranging from 5 min to 2 h. The tests have been carried out at atmospheric pressure. The evolution of total sulfur, pyritic, sulfate, and organic forms of sulfur and also the modification of ashes, nitrogen, and calorific value have been determined by applying European and ASTM standard methods. Results demonstrate that pyritic and sulfate sulfur are practically eliminated but organic sulfur is affected very little by the treatment. Ash is reduced, mainly by dissolving cabonates and sulfides, while the nitrification produced is moderate whenever the times of treatment are controlled. The calorific value expressed per unit of organic carbon practically does not vary. The coal energy recovery has been calculated using the experimental data for mass recovery and calorific value and demonstrated that, in this precombustion desulfurization process, very small losses of the total coal energy are produced.
Introduction Environmental protection makes it necessary to reduce the emissions of sulfur oxides into the atmosphere which, among other effects, causes acid rain. Sulfur in coal, except for small quantities present as elemental sulfur, can be considered as divided into three large groups: pyritic sulfur contained in inorganic sulfides, mainly iron; sulfate sulfur that is composed of calcium sulfate, iron sulfate, etc.; and organic sulfur that is present as sulfurated or heterocyclic organic compounds in the matter of coal. During combustion, the sulfur content in coal (especially the pyritic and organic sulfurs) is transformed into sulfur oxides that, except for the part that may be held up in the ashes (either in a natural way or by adding absorbents during combustion), are emitted together with the combustion gases. One of the ways to reduce the emission of sulfur oxides is to decrease the sulfur content in coal before combustion, for which many physical processes have already been investigated, such as the gravimetric ones (1-3), froth flotation (4, 5), selective floculation, and sedimentation (6, 7). Research has also been carried out with biological processes (8-11), but the ones most intensively studied perhaps have been the different chemical processes that show variable reductions of the different types of sulfur compounds in coal. In this way, compressed air (12), oxygen (13, 14), ferric sulfate (15,
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16), chlorine (17), copper sulfate (18), hydrogen peroxide (1921) and peroxyacetic acid (22, 23) were taken into account. The KVB process (24) utilizes nitrogen dioxide for the oxidation of sulfur, although a treatment with sodium hydroxide is required afterward to extract part of the organic sulfur. Hayatsu et al. (25) used nitric acid as an oxidizing reagent for the organic matter in coal in order to determine its nature. The ASTM D2492-90 (26) standard for the determination of the forms of sulfur makes use of nitric acid to determine pyritic sulfur. The reduction of sulfur was studied by means of nitric leaching in Slovakian lignites (3) and in subbituminous Spanish coal from Puertollano (27), showing the possibilities of this process. In this work, the behavior of two kinds of typical, thermal, high-rank coals that are used in Spain is studied, and the results are compared with the previous data on thermal coal from Puertollano. These types of coal are important in their respective regions from the socioeconomic point of view. The authors have also investigated another low-rank coal with very high sulfur content that cannot be considered as thermal coal but in which a notable reduction of organic sulfur and a preferential attack on the coal molecules that contain sulfur are shown so that the Sorg/Corg relation is reduced continuously.
Experimental Section The coal types studied are a Fabero semianthracite produced by flotation as well as a La Robla low-volatile bituminous coal also obtained by flotation. Table 1 presents the main features of these two kinds of coal plus the one from Puertollano with which they are compared as well as their ultimate analysis on dry base. For this ultimate analysis, Leco SC32 and Leco CHN600 instruments have been used. The oxygen has been calculated by difference. According to the X-ray diffractograms, the mineral matter of the semianthracite is made up of minerals of mica and illite groups, some quartz, ankerite, and in which the peaks of pyrite were also detected. The mineral matter of bituminous coal is made up of minerals of kaolin and mica groups, quartz, calcite, some ankerite as well as siderite, and in which the peaks of pyrite were also detected. Calcite, ankerite, and siderite are easily dissolved by nitric acid. The tests have been carried out with 50 g of coal and 500 mL of acid, using a three-mouth flask in a thermostated bath, previously described (27), while the gases emitted were collected in water washing bottles.
Results and Discussion Behavior of Sulfur. As the previous works show that the temperature has a preponderant effect on the sulfur attack, the temperature of 90 °C was considered appropriate and used to achieve a quick sulfur reduction. Figure 1 shows the results obtained, representing the total sulfur content in accordance with time for coal from Fabero and La Robla, which are expressed on dry base and using nitric acid concentrations of 20 and 30 wt %. In the case of the coal from Fabero, a fast sulfur reduction is observed during the first 5 min, reaching a value that is practically equal to the initial organic sulfur content, and then it remains practically constant and unaffected by the concentration of the leaching agent. This decrease is somewhat more abrupt than observed in the coal from Puertollano (27), and it is not affected by the concentration of acid, as in the case of Puertollano coal. The influence of the different grain sizes is unsignificant. The fraction of less than 0.25 mm is 84.06% of the total weight.
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1997 American Chemical Society
TABLE 1. Characteristics of Three Spanish Thermal Coals (Dry Base)
ashes % (db) volatile matter % (db) fixed carbon % (db) calorific value, mj/kg (db) pyritic sulfur % (db) sulfate sulfur % (db) organic sulfur % (db) CO2 content % (db) % less than 0.25 mm C % (db) H % (db) N % (db) S % (db) O % (db) (diff.)
Fabero coal
La Robla coal
Puertollano coal
24.7 6.6 68.7 24.9 0.85 0.24 0.69 0.37 84.06 69.72 1.86 0.8 1.78 1.14
17.0 12.0 71.0 29.7 1.39 0.10 0.85 1.54 50.48 75.20 2.99 1.8 2.34 0.67
40.6 21.2 38.2 18.8 0.73 0.18 0.27 2.35 100 47.08 2.58 0.7 1.18 7.86
FIGURE 2. Variation of the total sulfur content with time 90 °C (db), 20% nitric acid: La Robla (O) total sample; (9) 0.25, 0.5, 1 mm fraction. Fabero (2) total sample, (1) 0.25 mm fraction.
TABLE 3. Reduction Percentages of Sulfate and Pyritic Sulfur for Fabero Coal (%)
FIGURE 1. Variation of the total sulfur content with time at 90 °C (db): (O) La Robla 20% nitric acid, (b) La Robla 30% nitric acid, (2) Fabero 20% nitric acid, (1) Fabero 30% nitric acid.
TABLE 2. Characteristics of Different Grain Size Fractions of La Robla and Fabero Samples (Dry Base) size (mm)
weight (%)
ash (%)
Stotal (%)
N (%)
La Robla -2.3; +1.0 La Robla -1.0 ; +0.5 La Robla -0.5; +0.25 La Robla -0.25 La Robla, total Fabero -0.5; +0.25 Fabero -0.25 Fabero, total
12.62 17.48 19.42 50.48 100.00 15.94 84.06 100.00
15.27 13.29 15.11 19.42 17.00 22.86 24.95 24.70
2.48 2.27 2.27 2.45 2.34 1.88 1.76 1.78
1.8 1.8 1.8 1.7 1.8 0.7 0.8 0.8
This fraction and the total sample have very similar characteristics, as shown in Table 2. Figure 2 shows the total sulfur content over time for the fractions of more than 0.25 mm, less than 0.25 mm, and the total sample. The evolution of the less than 0.25 mm fraction and the total sample is practically the same with the same curve. The evolution of the more than 0.25 mm fraction is close to this curve. After 5 min, the sulfur reduction in the coal from La Robla is significant, but this reduction continues for quite a few more minutes until reaching practically constant values at a range close to organic sulfur content. The reduction is slower than in the case of Puertollano (27), where it is also affected in the same way by the acid concentration. In principle, the slower desulfurization of coal from La Robla could be attributed to its thicker grain size. The main characteristics of the different grain size fractions are given in Table 2. The behavior of these fractions has been represented in Figure 2, wherein it can be observed how those fractions of
20% nitric acid at 90 °C
30% nitric acid at 90 °C
time (min)
pyritic S
sulfate S
pyritic S
sulfate S
5 10 15 30 60 120
96.2 96.9 96.2 96.9 96.9 97.0
83.2 87.0 86.6 92.9 93.3 88.7
96.3 97.0 97.0 97.7 97.6 97.7
88.7 91.2 92.9 95.8 96.2 97.1
less than 0.25 mm already show a behavior that is more similar to the case of Puertollano (27), although the sulfur decrease is still slower, specially in thick fractions. In order to get to know the behavior of the different forms of sulfur, Table 3 shows the reduction percentages of sulfate and pyritic sulfur at Fabero, and Table 4 shows the same data for coal from La Robla expressed on dry base. In the case of Fabero, it is observed that the pyritic sulfur is reduced 96% during the first 5 min, while the sulfate sulfur goes down more slowly, but also undergoes important reductions of about 83% during the first 5 min. The sulfate sulfur reduction is more erratic than the pyritic sulfur reduction due to the low sulfate sulfur content and the more difficult solubilization of calcium sulfate in nitric acid. At La Robla, the reduction is slower for both kinds of sulfur, especially for sulfate; the desulfurization is faster in smaller sizes, and therefore the desulfurization velocity is higher in the fraction of less than 0.25 mm and gets close to the one of Fabero. These data tally with the fast reduction of inorganic sulfur in coal from Puertollano (27). Figure 3 represents the variation of organic sulfur in these coals expressed on dry base and free of ash, allowing a better view of the sulfur content in the organic matter. In the case of La Robla and using a nitric acid concentration of 20 wt %, a very small and gentle decrease of the curve is observed; in the case of 30 wt %, there is an initial decrease, and then the curve shows the same tendency as in the previous case. In the case of Fabero and using a nitric acid concentration of 20 wt %, there is a drop at the beginning, and then the curve continues decreasing; in the case of 30 wt %, the curve is practically the same as in the 20 wt % case. This variation of acid concentration has no influence. Anyway, the decreases after 120 min are 14% for Fabero and 5% for La Robla with 20% nitric acid. These decreases are less important than the one shown in the data on Puertollano, for which 18% is indicated (27).
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TABLE 4. Reduction Percentages of Sulfate and Pyritic Sulfur for La Robla Coal (%) total sample < 2.3 mm 20% nitric acid at 90 °C
