Characteristics Study On Pyrolysis And Gasification Of Low Rank Coals

The technology of coal gasification in fluidized bed is extensively concerned due to its mild gasification conditions and large gasification intensity. Although the process and structure of fluidized bed gasifier is improved continuously, the regulation mechanism is still unclear because of the complicated chemical reactions in the reactor. A series experiments needs to be carried out in order to solve the problems of high carbon content in fly ash and low gasification efficiency. It will take a lot of time and effort to conduct experiments directly in the fluidized bed reactors because of the complex operations, inadequate data and poor reproducibility. Therefore, it is theoretically and practically significant to study coal pyrolysis and gasification properties in the conditions closing to fluidized bed reactor with modern precision instruments technology. Then the kinetic data can be obtained, and the gasification reactivity of raw coal can be determined by influence factors analysis and dynamics analysis, which provides the important reference for optimizing operation parameters.In this dissertation, the fast pyrolysis characteristics of low rank coal and gasification reactivity of char at the conditions closing to fluidized bed reactors are investigated. The effects of coal characteristics and reaction conditions on gasification reactivity are analyzed, and the differences in structure and reactivity among the laboratory-prepared fast pyrolysis char, bottom slag and gasifer fly ash of the fluidized bed pyrolysis furnaces are compared.Characteristics of pyrolysis process and gas products during temperature-programmed pyrolysis of different coals are analyzed with thermogravimetric-mass spectrometry (TG-MS). Fast pyrolysis device is used to prepare fast pyrolysis char at the temperatures close to fluidized bed operating temperatures. The fast pyrolysis characteristics of coal and the char morphology under different pyrolysis conditions are studied.Contrastive study on the reactivity of the chars gasification with steam and CO2, respectively, is conducted by thermogravimetric technology. The effects of gasification temperature, pyrolysis temperature and pyrolysis rate on gasification reactivity of the chars are investigated. Under the experimental condition, the results show that pyrolysis rate greatly affects gasification reactivity, however the effects of pyrolysis temperature and residence time are relatively small. In the temperature conditions, gasification temperature is the most important factor influencing gasification rate, while the effect of pyrolysis temperature is weaker. The reactivity index and the maximum reaction rate changes linearly with the increase in gasification temperature. The activation energy of both char-CO2and char-steam gasification increases with increase in carbon conversion rate.Seven types of coals with different coal ranks are used for char preparation and gasification reactivity test, and some conclusions are obtained. It is found that the value of half reaction specific rate is approximate to the average specific rate under the same conditions. Ash behavior plays an important role in the char reactivity, and changes with gasification temperature and reaction degree due to the variation in the compositions and relative amount. The influence of pore structure is more noticeable during a relatively moderate reaction process, but the gasification reactivity of char has no immediate connection with the initial pore structure. The relative gasification reactivity ratio of char-steam to char-CO2generally decreases with the increasing temperature, indicating that high temperature can reduce the reactivity gap between the two reactions.The surface morphology and structure of the chars from fast pyrolysis in the horizontal tube furnace, a bubbling bed and a fluidized bed are compared, and its effect on gasification reactivity is discussed. Different indicators are used to evaluate the gasification reactivity of the chars, and it is found that there is a linear relationship between the indicators of the chars from different reactors. The more intense of the heat and mass transfer process is, the more orderly of the crystal lattice and the higher of char gasification reactivity will be.The fly ash from the outlet of the fluidized bed gasifier cyclone is chosen as the research sample. The structure and reactivity differences between fly ash and laboratory-prepared fast pyrolysis chars are compared. The XRD results demonstrate that the aromatic nucleus of laboratory-prepared fast pyrolysis chars is larger and more orderly than that of fly ash. N2and CO2are used as adsorbent for surface area test of char and fly ash, respectively, and it is found that the pores of fly ash are mainly macropores and intermediate pores, while the pores of fast pyrolysis chars are mainly micropores. The CO2gasification reactivity of fly ash is higher than that of fast pyrolysis chars, while its steam gasification reactivity is lower due to the pore structure difference.A method for coal gasification reactivity evaluation applicable to the fluidized bed gasifiers is established according to the results of this dissertation, including a series of standardized process involving sample preparation, fast pyrolysis char preparation, and char gasification, etc.