| Huge reserves of low-rank coal such as lignite, long flame coal and non-caking coal have been proved for more than56.1billion tons in China. Among these resources, the ascertained reserves of lignite were more than13billion tons. The technology which converted low rank coal into supplementary fuel of gasoline and diesel had received wide attention. As the core for clean, high-efficient and scientific grading utilization of low-rank coal, low-temperature carbonization technology has also attracted great attention, especially the low-temperature carbonization technology with efficient recovery of tar. Facing rich lignite and high pulverized coal rates of mechanization of coal mining in china, the molding and low-temperature carbonization technology of lignite briquette have been the key point of clean coal technology. The characteristics of producing tar from carbonization of Inner Mongolia lignite briquette were studied by gourd-shaped carbonization reactor which was developed and designed by author’s group in the paper. The carbonization kinetics was investigated using non-isothermal method TGA. Lignite briquette at different terminate temperature were characterized using methods of elemental analysis. The thesis’s main conclusions are as follows:1、Some factors significantly affect tar yield of lignite briquette, such as carbonization temperature, heating rate, entering temperature, and atmosphere. With the increase of carbonization temperature, the tar yield increase dramatically, and change smoothly about9%when above650℃.As the heating rate increases, the tar yield rise slowly before flattening steady. As the constant temperature time extends, the tar yield rise before remaining slow decrease when above60min.It shows that some secondary cracking of tar from carbonization is occurred. Compared with the same carbonization temperature, tar yield of entering temperature is higher, especially when above500℃, because extent of coal subjected to thermal shock of entering temperature is larger, which easily breaks chemical bonds into more free radicals and tar molecular. The tar yield of lignite briquette is highest under a H2atmosphere, about26%higher than under a N2atmosphere. With the increase of H2gas flow rate, the tar yield increase then descending slightly. The optimized technology parameters of the carbonization reaction are as follows:H2atmosphere,600℃carbonization temperature,30min constant temperature time,20℃/min heating rate,10-30mm particles,720ml/min H2gas flow rate.2、Thermo gravimetric of lignite briquette show that with the increase of heating rate, the curves of TG and DTG move to the high-temperature zone, at the same time, mass loss rate (dw/dt) max become larger. The pyrolysis process of Lignite briquette can be divided into four stages, which follow by predehydration, degassing, coal prolysis and char contraction. Each stage can be descriped by the first-order kinetics which is the equation of. Phased kinetic analysis show that, in5-20℃/min heating rate, phased activation energy(E) of low-temperature pyrolysis of lignite briquette change little, but in the same heating rate, phased activation energy have large changes. Ascending order of phased activation energy is as follows:degassing<predehydration<char contraction<coal prolysis.3、With the increase of carbonization temperature, the macromolecular structure of lignite briquette gradually crack and devolatilize, and then generate tar, semi-coke and gas. The process can decrease volatile content of semi-coke, increase fixed carbon and ash content gradually. With the carbonization temperature increasing, carbon and hydrogen content have a significant increasing trend, but the trend of oxygen content is just opposite. Nitrogen and sulfur content change little, by contrast. The carbonization process of lignite briquette is a carbon-rich, dehydrogenation, deoxygenation process.4、The carbonization process of lignite briquette can remove the aliphatic side chains carbon-oxygen-ether bond and the oxygen-containing functional groups. Compared with raw coal, the content of oxygen-containing functional groups of semi-coke decreases slightly. Methane peak in semi-coke is weaker, because the carbonization process can remove a large amount of alkanes and aromatics with side chain.5、With carbonization temperature increasing, the intensity of002diffraction peak of semi-coke in XRD enhanced gradually, which shape tended to be sharp. The intensity of100diffraction peak enhanced slightly. Arrangement of its aromatic sheets in space is clearer and neater, and the order has a significant growth, which aromatic microcrystalline structure size increases slightly.6、The adsorption-desorption isotherms of lignite briquette are typical type Ⅱ adsorption isotherm. With carbonization temperature increasing, specific surface area of semi-coke reduces gradually, but the average pore diameter and pore volume increase and then decrease. Due to semi-coke void structure blocked by tar in carbonization, specific surface area decreases, but new void structure which produced by devolatilization result in the increase of average pore diameter and pore volume with micro pores expanding and cross linking. When the temperature is550℃, aromatic carbon layer will tend to be more orderly with semi-coke polycondensation reaction. It also leads to the decrease of average pore diameter and pore volume. 7、With carbonization temperature increasing, the surface morphology of lignite briquette in carbonization which have obscure void structure, gradually develop into large pore structure. When the temperature is550℃, large pore structure will disappear. The changes among SEM are consistent with the test of BET.8、There are four forms carbon exists in coal surface in different terminate temperature, such as C-C and C-H, C-O, C=O and COO-, in which aromatic unit and alkyl (C-C and C-H)account for the largest proportion. The C-C and C-H content with temperature increasing have a significant increasing trend, but the trend of C-O、C=O and COO-is just opposite. Compared with the lignite briquette, the relative content of the C-C and C-H groups of semi-coke surface in650℃increase65%, and the relative content of the C-O,C=O and COO-groups respectively decrease55.57%,71.85%and57.17%. There are four forms of oxygen on the surface of coal, but during carbonization process, the main form of oxygen is aromatic hydrocarbons (C-O, C-H). This is because a stable structure can be formed between lone electron pair of oxygen in phenolic hydroxyl and aromatic by a conjugated way.9、Based on the ideas of quick export of prolysis gas, low-temperature carbonization furnace with efficient tar recovery and gas-guided wall are developed, which have the new combination structure of gas guide. The greatest feature of the furnace is that gas collector or gas-guided wall that has quick export of coke oven gas is sett up in the middle of furnace. The technology can solve a longer export trip of coke oven gas in carbonization of pulverized coal, the longer resident time and secondary cracking problems, which can improve tar yield significantly. |
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