Microwave Pyrolysis Gasification Characteristics And Mechanism Study Of Lignite Based On The Multi-physics Field

China’s proved lignite reserves have reached 130 billion tons, accounting for about 13% of its total coal reserves. As a typical low-level type of coal, lignite is characterized by its high moisture and ash contents, low calorific value and inflammability and mainly used for direct combustion power generation, causing problems such as low efficiency, serious carbon dioxide and pollutant emissions, and ineffective use of its higher hydrogen content. Lignite pyrolysis using the clean and efficient microwave heating technology can produce quality pyrolysis gas rich in H2, CO and CH4, as well as tar and semi-coke, which help achieve the requirements for clean and efficient use of coal resources.Given the insufficient understanding of the properties of the gaseous products produced from lignite microwave pyrolysis, and the lack of in-depth study on the interaction mechanism between the microwave electromagnetic field and lignite pyrolysis reactor system, this paper carried out experiments and theoretical studies on China’s typical ligniteEastern Inner Mongolia lignite from the perspectives of microwave and electromagnetic properties of lignite. A multi-physical-field coupling numerical computation model was built that incorporates lignite complex dielectric coefficient function of temperature and lignite microwave pyrolysis process. Armed with this model, an interaction mechanism between microwave and lignite pyrolysis reaction system was proposed and the fundamental factors were identified that have an effect on lignite microwave pyrolysis gas production process. With this information known, the complex dielectric coefficient of the lignite and the microwave electric field intensity uniformity were optimized accordingly such that lignite microwave pyrolysis gas production efficiency and active principle H2 and CO contents were improved, thereby providing basis for industrial application of lignite microwave pyrolysis gas production.The lignite was identified and analyzed for its physical and chemical structure features by FT-IR, 13C-NMR, SEM, XRD, solvent extraction-GC/MS, and TG-DTG techniques. The results demonstrate that: Mengdong lignite is a typical low-rank coal, coming with a high degree of carbon atom disordering and a low degree of aromatic nucleus condensation. The lignite molecular structure contains a great number of oxygen-bearing functional groups, long-chain branch structures, and associated hydrogen bonds, forming a crosslinked macromolecular mesh structure. The lignite shows apparent flaky morphology and has a dense structure within the flakes, but inter-flake clefts abound. 350~750oC is the major temperature range at which the lignite pyrolysis reaction takes place.Compared with conventional pyrolysis, lignite microwave pyrolysis possesses quite some peculiar features in temperature rising and in generation of major gaseous products: In microwave pyrolysis process, the coal sample displays non-uniform temperature field distribution, namely a feature of large temperature difference(~500oC). Temperature rise is slow before the lignite reaches the temperature Tini at which the pyrolysis reaction takes place; the temperature surges after Tini; the time of temperature surge becomes earlier as the microwave input power grows. As a result, major gaseous products are generated abruptly and in group after 46 minutes, but the output is obviously lower than conventional pyrolysis. Active principles H2 and CO increased from 40.04% and 20.17% to 51.51% and 30.76% in content, but CO2 content drops by 10.73%.Therefore a multi-physical-field coupling numerical computation model was built, from the aspect of microwave and of lignite molecular structure electromagnetic characteristics and using the theories and tools of microwave engineering, that incorporates lignite complex dielectric coefficient function of temperature as well as lignite microwave pyrolysis process. This model was employed in analyzing the mechanism of the interaction between microwave and lignite pyrolysis reaction system and in exploring the fundamental factors that influence lignite microwave pyrolysis process. The results suggest: The lignite complex dielectric coefficient is a function of temperature, and the structural change of lignite molecules in the pyrolysis process is the intrinsic mechanism responsible for nonlinear change of the complex dielectric coefficient with the changing temperature. Lignite microwave pyrolysis being a process of interaction between microwave multiple physical fields and lignite pyrolysis reaction system, its temperature rising process is related to lignite complex dielectric coefficient, microwave electric field intensity & distribution, and coal sample process temperature field distribution. A higher initial complex dielectric coefficient and a stronger microwave electric field intensity are effective in increasing lignite temperature rise speed, accelerating the temperature surge, and improving the generation rate of major gaseous products. The microwave electric field being distributed non-uniformly in the form of stationary wave is the reason that the lignite temperature field is not uniformly distributed;In order to have a higher gas production efficiency and higher active principle contents and in consideration of the conclusion drawn from analyzing the interaction between microwave and lignite pyrolysis reaction system, the lignite microwave pyrolysis process was optimized and improved by using heterogeneous filler material(semicoke) to increase the equivalent complex dielectric coefficient of the coal sample and by including a microwave mode stirrer to establish a uniform microwave condition, which helps increase the microwave electric field intensity and distribution uniformity. The results indicate:Semicoke as a filler allows for a higher pre-Tini temperature rising speed and earlier temperature surge with the result that the generation of major gaseous products begins at 5min after microwave heating and the pyrolysis reaction is completed within 40min; and with an ameliorated temperature field distribution uniformity, the coal sample temperature difference narrows to ~100oC. With a filling ratio of 0~20%, the coal sample does not change much in equivalent complex dielectric coefficient, the filler phase features "being firstly heated", the generation rate of major gaseous products are characterized notably by two stages, with H2 being generated at the early gas production stage. With a filling ratio of 30%, the equivalent complex dielectric coefficient of the coal sample reaches a critical value that gives rise to abrupt change in both microwave electric field intensity and distribution, reflected mainly by a quick temperature rise of the coal sample as a whole, with the result that the generation rate of major gaseous products rockets to the peak suddenly, CO2 being the first principle generated. Compared with the original microwave pyrolysis conditions, with a semicoke filling ratio of 20%, good results are achievable, the output of major gaseous products becomes 2.7-fold and the energy consumption per unit volume of gas drops by 83%, the content of active principles H2 and CO grows to 51.18% and 28.19%. Hence, on the strength of equivalent complex dielectric coefficient theory and by managing the filling ratio it is not only possible to increase drastically the efficiency of lignite microwave pyrolysis gas production but some guidance may be provided as well for the benefit of production process control of major gaseous products.In the uniform field microwave pyrolysis process, the lignite as a whole has a quick temperature rise and experiences a short duration of temperature surge, the peak value is ~830oC, the process temperature field distribution is uniform, and for this reason, the entire coal sample has the same pyrolysis degree and is complete in reaction, and therefore it may be regarded as an "isothermal pyrolysis" process with a temperature surge. Relative to the original microwave pyrolysis condition, the total output of major gaseous products becomes 3.5-fold, the energy consumption per unit volume drops 90%. The content of active principles H2 and CO grows substantially to 52.93% and 34.52% respectively. Reaction kinetics study suggests that for a uniform field microwave pyrolysis process of lignite the major gaseous product of CO2 corresponds to an order of generation reaction of ~0.8, and in the case of CO, CH4 and H2 the order of generation reaction is all ~0.6. It then follows that improving microwave electric field intensity distribution uniformity is effective in rising substantially the output of major gaseous products, increasing the content of active principles H2 and CO, and its benefits are found to be superior to using semicoke filler.The multi-physical-field coupling numerical computation model, the microwave electric field intensity & its distribution uniformity, and the important influence on lignite microwave pyrolysis gas production process were verified using Australian lignite, a typical soft lignite quite dissimilar to Mengdong lignite in properties. The results suggest: The multi-physical-field coupling numerical model of lignite microwave pyrolysis is reliably built and may serve to predict and analyze microwave pyrolysis of different kinds of lignite. Microwave electric field intensity and its distribution indeed have significant influence on lignite microwave pyrolysis gas production process. At a given input power, improving the field intensity and its distribution uniformity increases substantially the efficiency of microwave pyrolysis gas production and raises the content of active principles H2 and CO.