| Many researches on low rank coal pyrolysis have been done in recent years, but tar yield of some low rank coal still remains low. Fundamentally, pyrolysis technology is not suitable for some certain kinds of low rank coal to improve quality. In coal geology, coal is classified into three types according to its low temperature tar yield level. That with tar yield of less than 7% at low temperature is called tar-containing coal, and that with tar yield higher than 7% but less than or equal to 12% is called tar-rich coal, and that with tar yield more than 12% is called the high-tar coal. Previous study has found that tar yield of coal in China’s western part at low temperature is high, generally between 7% to12%, which belongs to the "tar-rich coal". However, the tar has a higher content of heavy component when boiling point is over 360 ℃. This not only reduces the tar value in use, but also make the condensation point get higher, easier to condense. It is prone to block the pipeline, thus affects the stability of the system operation and hinders the application of pyrolysis technology to a certain extent. Therefore, how to extract the tar by an appropriate pyrolysis technology, and to obtain a higher yield of tar by regulating the pyrolysis process, and to upgrade tar, will undoubtedly play an important role in promoting the application of tar-rich coal pyrolysis technology. In this paper, the effects of inherent mineral and added iron ore on the pyrolysis characteristics of four kinds of tar-rich coal from western China were investigated by means of a fixed bed reactor, a thermogravimetric analyzer(TGA) and a gas chromatography-mass spectrometer(GC-MS). The recovery condition of magnetic separation of iron ore was explored with a magnetic separation equipment. The main conclusions of this paper are as follows:(1) The results of basic properties analysis showed that the average volatile yield of the four kinds of tar-rich coal was higher than 39.73%. All the coal ash generally contained alkaline earth metals and transition metal oxides. HM2 R coal sample had a high content of moisture, volatile, oxygen but low ash content. YLR coal sample was mainly transformed from the gelation of plant xylem and leaf tissue. ZC3 R maceral was composed of exinite, mainly large sporophyte. The metamorphic degree of the four coal samples increased in an order of HM2R<HM4R<YLR<ZC3R.(2) Thermogravimetric analysis showed that the weight loss of the pyrolysis of four kinds of tar-rich coal ranged from 400℃ to 600℃, with maximum weight loss rates between 450℃ and 470℃. The maximum weight loss rate of HM2 R coal sample was significantly higher than that of the other three kinds of coal samples. Weight loss of YLR and ZC3 R samples got to a small peak at 640℃ and 745℃, respectively. YLR weight loss was due to the decomposition of the pyrite in coal, and ZC3 R weight loss was because of the decomposition of the carbonate minerals in coal. When the pyrolysis temperature reached 800℃, the weight loss of HM2 R, HM4 R, YLR and ZC3 R was 43.82%, 31.54%, 31.47% and 31.33%, respectively. Kinetic calculation showed that the activation energy of HM2 R was the lowest at the pyrolysis maximum weight loss rates. Four coal samples could be described using second order reaction within the range of pyrolysis maximum weight loss rate in the second stage of pyrolysis reaction. HM4 R and ZC3 R had to absorb more energy from the surrounding environment in the pyrolysis reaction, while the molecules in HM2 R and YLR got activated to react more easily from normal state under the same pyrolysis conditions.(3) The fixed-bed pyrolysis result indicated that the liquid product yield peaked when the pyrolysis temperature was 600℃. At that time, tar yield of HM2 R, HM4 R, YLR and ZC3 R reached its highest level to 10.79%, 8.81%, 8.42% and 10.03% respectively. Gas phase products of coal pyrolysis were produced at 550℃~600℃, which had a higher yield at 600℃. The main product of pyrolysis gas phase was oxygen-containing gas such as CO2 and CO when the final pyrolysis temperature was lower than 550℃, while it was low carbon hydrocarbons such as H2 and CH4 when final temperature was greater than 600℃. The yield of volatile matter, the atomic ratio of hydrogen and carbon, the product of the hydrogen content multiplied by the yield of volatile matter, was generally in direct proportion with tar yield at low temperature. It was more accurate to predict tar yield at low temperature using the product of volatile yield and hydrogen content.(4) As the atomic ratio of hydrogen and carbon decreased gradually in the samples, the relative content of aliphatic compounds with less than 10 or 10~20 carbon atoms on the main chain decreased while the content of those with more than 20 carbon atoms increased. HM2 R, ZC3 R and HM4 R coal tar had a relatively high content of C1 alkylbenzenes, 2.21%, 1.72% and 1.55% respectively. The relative content of CO and C1 alkylbenzenes benzene series in the tar of the four samples kept decreasing. The relative content of ethyl(C2) and trimethyl(C3) phenols in the coal tar was the highest, followed by that of methyl(C1) and tetramethyl(C4) phenols. The relative content of aliphatic hydrocarbons in HM2 R, ZC3 R, HM4 R and YLR coal tar were 25.77%, 26.86%, 28.36% and 30.46%, respectively. The content of phenolic compounds was lower, and that of benzene series compounds and polycyclic aromatic hydrocarbons was the lowest.(5) Thermogravimetric analysis showed that the weight loss of demineralized coal was always greater than the raw coal since the inherent minerals in coal had an inhibition effect on coal pyrolysis, leading to a greater difference between the weight loss values than that calculated by proximate analysis. When the pyrolysis final temperature reached 800℃, the weight loss of HM2 D, HM4 D, ZC3 D and YLD was 47.18%, 35.27%, 35.05% and 34.36%, respectively. When the pyrolysis temperature was lower, the release characteristics of H2 and CH4 produced by step-by-step demineralized coal samples pyrolysis didn’t differ much. When the temperature reached 700 ℃, the release concentration of H2 reached its maximum, in an order of HM2D>HM2H>HM2A>HM2R. The release concentration of CO first increased, then decreased, and then increased with the increasing of the pyrolysis temperature.(6) Compared with raw coal, the four demineralized coal samples had higher tar yield but lower light tar yield and smaller light tar mass. The samples contained more carbon and hydrogen in the tar, especially hydrogen, so the ratio of hydrogen to carbon was bigger. Nitrogen, sulfur and oxygen in the tar was less than in the demineralized coal tar.The demineralized coal tar had a higher oxygen content and a lower content of carbon and hydrogen than raw coal tar. The content of nitrogen and sulfur in demineralized coal tar did not show obvious changing trend. Compared with the tar of HM2 R, HM4 R, YLR and ZC3 R, the content of heavy component in the demineralized coal tar increased by 11.84%, 8.99%, 8.85% and 8.87%, respectively. The content of light oil(< 170 ℃) in light component with boiling point lower than 360 ℃ in demineralized coal tar was higher than that of raw coal tar, while the content of other light component was lower than raw coal tar.(7) During the pyrolysis process, Fe2O3 was reduced to Fe3O4 and FeO, etc. The peak intensity of Fe3O4 in the char of hematite is larger than that of the specularite char, but the peak intensity of FeO is smaller than that of the specularite char. The standard Gibbs free energy(△r Gθm) of the reaction of Fe3O4 and CO was more than zero, so chemical reaction could not conduct spontaneously. Compared with the dissemination size of Fe3O4 in specularite, the dissemination characteristics of Fe3O4 in hematite spurred the reduction to produce FeO and the standard Gibbs free energy was greater than that of specularite. Therefore, the dissemination characteristics of Fe3O4 in specularite was more advantageous to the reduction reaction, resulting in a larger intensity of FeO peak.(8) When at low temperature, the inherent minerals restrained coal pyrolysis, but the catalytic effect of iron ore was much greater. The catalytic effect of hematite and specularite increased gradually with the temperature, and the inhibition effect of the inherent minerals decreased. The active energy of adding iron ore decreased greatly in the second pyrolysis stage because of the inhibition effects of some minerals in raw coal decreased. And the catalytic effect of iron ore and minerals decreased as follows: specularite > hematite > inherent minerals. With addition of specularite which had a mass of 20% of the coal sample, the pyrolysis conversion of HM2 RS, HM4 RS, YLRS and ZC3 RS increased by 5.78%, 5.24%, 4.33% and 5.46%, respectively. The fraction of heavy component with boiling point higher than 360℃ reduced by over 9.85% in average. When the amount of iron ore added to the sample was relatively small, the catalytic cracking effect of iron ore on the tar was weak. In this case, the iron ore mainly promoted the coal sample to pyrolyse and produce gas phase products. The effect of iron ore on the catalytic cracking of tar was enhanced with the increase of the proportion of iron ore. The yield of liquid product of the four samples was reduced, as the main liquid was tar and the tar yield decreased. The pyrolysis water yield of remained generally unchanged when different proportions of iron ore were added. Based on the comprehensive consideration of energy consumption and tar yield, it was suitable to add the two kinds of iron ore with a mass of 20% of the coal.(9) The specularite was more effective for removing substituents in benzene in tar than hematite. The phenolic compounds in tar increased when iron ore was added to the coal samples. And there were more phenolic compounds in tar when specularite was added than hematite. Both kinds of iron ore could significantly increase benzene series content and polycyclic aromatic hydrocarbons content in tar. When iron ore was added, phenol and naphthalene oil yield of the coal tar increased while washing oil yield of the coal tar decreased, and the tar of the four samples had higher H/C ratio and lower O/C ratio. The catalytic effect of iron ore reduced the content of hydroxyl group in alcohols, phenols and carboxylic acids in tar, resulting in the decrease of oxygen content in the tar.(10) The iron concentrate recovered was measured by magnetic separation method. The iron concentrate grade was 50.49% with hematite added in coal when the residence time of ultimate pyrolysis temperature was 25 min and the magnetic field strength was 96.48kA/m while it reached 57.62% with specularite added when residence time was 35 min and magnetic field strength was 109.27 k A/m, both with a char grinding fineness of less than 0.074mm(ground char accounting for approximately 80% of total char). And iron concentrate recovery rate was 84.72% when hematite was added and 77.38% when specularite was added. Iron atoms gained or lost electrons during the reduction process of the iron ore, changing the chemical valence state of iron, but the dissemination characteristics and the particle composition remained unchanged because of the inheritance of the iron minerals to the iron ore.Although the total iron content and iron concentrate recovery rate of hematite was higher, the grade of iron concentrate was lower than that of the specularite, as a result of the finer characteristics of particle and dissemination of the minerals in hematite. |
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