Converting Lignite To Binder In Subcritical H₂O-CO System And Its Application In Preparation And Carbonization Mechanism Of Briquette

The non-caking coal mainly includes two categories. One is high-rank coal represented by anthracite. It has a highly condensed aromatic structure(number of aromatic rings R ≈ 12~40, aromaticity fa ≈ 1, aromatic lamellas distance d002 ≈ 0.34 nm) and trends toward graphitization, which exhibits good chemical stability. The other is low-rank coal represented by lignite and long flame coal. It contains abundant three-dimension crosslink structures(R≈2,fa≈0.7,d002>0.38 nm)and the orientations of aromatic lamellas are all multi-directed and arbitrary, leading to the poor mobility of basic units. Both of coal are the non-caking coal which could not bond under the heating condition of carbonization.Using high-rank pulverized coal and non-caking pulverized coal as raw materials to produce carbonized briquettes for foundry and gasification(also known as foundry coke and gasify coke) is an important technique, which could provide the important raw materials for high-end casting and modern coal chemical industry. However, it is essential to develop a low-cost and efficient binder, perform in-depth investigations on the pyrolysis behavior of briquettes and prepare high quality carbonized briquettes. In this paper, the idea —converting lignite to binder by hydro-modification in a subcritical H2O-CO system and using the binder to prepare carbonized briquettes for foundry or gasification, was proposed. Based on the idea, the following areas of study were carried out: ① Fe catalytic hydro-modification to improve the caking property of lignite in a subcritical water-CO system; ② Mechanism of caking improvement by coal hydro-modification; ③ Carbonization mechanism of briquette prepared with modified coal binder; ④ Preparation of high quality carbonized briquettes. Main conclusions are as follows:1. Fe S catalyst has a significant catalytic performance on the development of caking property of lignite. The major function of Fe S catalysts on the hydro-modification is to synergistically promote the WGSR, coal pyrolysis and hydrogenation. The Fe S catalyst improves the CO conversion and reduces the coal pyrolysis activation energy so as to produce more active hydrogen(H?) and free radicals, which allows the aromatic rings to rearrange and then are stabilized by H? to form more caking materials. CO provideds active hydrogen H? to the reaction system through WGSR. Improving CO initial pressure increase the hydrogen partial pressure so as to promote active hydrogen H? to diffuse into the internal structure of coal and stabilize the free radicals. In addition, adsorbed CO could attract the electron cloud of near atoms, resulting in the corresponding bonds become weaker and then are disrupt. Subcritical H2 O is a media that coud provide active hydrogen H? for the coal hydro-modification. Furthermore, it promote the removal of oxygen-containing functional groups in lignite and weaken the diffusion resistance of inter/inner-phase. The caking index of modified coal reaches above 90 under the synergistic effect of the three materials, which could meet the requirements for the preparation of carbonized briquette.2. By means of the modulating effect of Fe-based catalytic subcritical H2O-CO system on the coal structure, the relationship among hydrogen supply amount, coal structural and caking propery was investigated. The compatibility between the amount of hydrogen donors and the degree of coal pyrolysis has an obvious influence on the composition of the modified coal. Moderate hydrogenation and rearrangement of generated free radicals benefit the generation of caking material such as asphalt which has a higher degree of molecular association and is rich in alkyl and aromatic structures. The caking index(GRI) of modified coal reaches 88~96 when the CO conversion(XCO) is in the range of 36.48%~43.48%.3. The coal hydro-modification of lignite based on the Fe S catalytic subcritical H2O-CO system lignite modification contains three reaction process. The first is generation of active hydrogen H?. Fe S promote WGSR to generate active hydrogen H?. The second is pyrolysis of lignite and generation of free radicals. Under the synergistic effect of adsorbed CO, hydrogen H? and Fe S catalytic pyrolysis, the bridge bond and side chains are disrupt so as to generate a large amount of free radicals. The last is rearrangement of free radical and generate of caking material. The aromatic rings are rearranged to generate larger aromatic free radicals that could react with H? to form caking materials, such as preasphaltene, thereby significantly improving the caking property of modified coal.4. The structural characteristic of carbonized briquette results indicates that little change in pore structure and surface morphology is observed at the pyrolysis temperature of 300 oC. With increased temperature, the pyrolysis of briquette become intense and pore structure of the carbonized briquette is much more developed due to the devolatilization. The original oriented anthracite crystallites are decomposed due to the cleavage of crosslinks, which opens up the coal structure and results in the microcrystalline parameter(La) underwent a remarkable decrease from 21.25 to 9.03. When the temperature rise to 900 oC, the microstructure collapses and the surface morphology become more compact. Higher pyrolysis temperature and a more open structure of the briquette promoted the repolymerization of free radicals produced by the decompositionof anthracite and binder. As a result, the value of La increass from 9.03 to 14.60.5. The evolution of free radical during the carbonization of briquettes indicates that: ①Briquettes enters into the polycondensation stage about 700 oC and more polycondensed free radicals are generated, which increases the exchange frequency of free electron and transform the EPR linetype from Gaussian into Lorentzian. ②The induced effect of free radical existed in the process of co-pyrolysis between binder and anthracite, consequently, the free radical concentration Ng of briquettes producing in the pyrolysis stage is far higher than that of anthracite and binder carbonized alone. ③The free radicals produced by pyrolysis interact with each other strongly at 700 oC, leading to a significant decrease on the free radicals concentration(Ng) of carbonized briquettes obtained at 900 oC.6. Based on the Raman and EPR analysis, the correlation of carbonized briquette strength and change in Ng and La was established. The ratio of Ng between carbonized birquettes obtained at 500 oC and 900 oC(Ng500/Ng900) and the change in La value above 500 oC(△La= La900-La500) are linearly associated with the strength of carbonized briquette(Ng500/Ng900 ∝△La= La900-La500∝M40). During the decomposition stage of carbonized briquette, the binder can promote free radicals and maintain the coal structure open. At the same time, free radicals produced from binder pyrolysis can fully react with the macromolecular free radicals in anthracite so as to promote the polymerization of smaller aromatic dense rings and rearrangement of the microcrystalline structure. These effects would make the non-caking coal to form a strong bond, thus improving the strength of carbonized briquette.7. The pyrolysis of anthracite briquette can be divided into four processes. The first is the drying process(20～300 oC), in which the briquette was dried and some of the lighter components started to decompose. The second stage was the beginning of decomposition(300~500 oC). The original crosslinks and other bonds were deposed, which opened up the coal structure and resulted in the generation of free radicals. The third stage was bonding-repolymerization(500~700 oC). The interactions between free radicals produced by the decomposition of anthracite and binder the AS binder and anthracite became more intense, which could generate carbom-carbon aggregated structure so as to achieve the bond of pulverized coal. With the increase of temperature, the pyrolysis of briquette entered solidify and shrikage stage. In a further heating and repolymerization process, the ordering degree and condensation of molecular structures were developed. As a result, the carbonized briquette was composed of polyaromatics.8. High quality carbonized briquettes for foundry and gasification were manufactured based on the developed binder. The shatter index(SI450) of foundry coke reaches 98.95%, which could meet the requirements for the smelting of raw iron. Meanwhile, the hot metal temperature rise to 1450-1470 oC. The gasify coke has high mechanical strength(SI425 > 95%), good thermal stability(TS+6 > 95%) and optimum reactive(CRI = 45%~66%). The gasify coke exhibts a good adaptability on the 3.0MPa fixed-bed gasifier. The yields of raw gas and syngas(CO+H2) are 2770~2810 Nm3/t and 1887~1946 Nm3/t respectively, which is superior to the gasification performance of lump coal. In additon, the pollutants amount of waste water produced during gasification dramatically decreases by replacing lump coal with carbonized briquette, indicating that carbonized briquette is a clean raw material for fixed-bed gasification. On the basis of this study, industrial experiments of foundry coke and gasify coke have been achieved and the technologies have also been adopt by companies.