| As a main route for clean and high-efficiency utilization of carbon fuels, gasification technology has been widely used to produce industrial fuel gas. At present, for industrial fuel gas production in China, it mainly adopts the atmospheric fixed bed gasifiers, mostly the kind of single stage and two-stage fixed bed gasifier, which always have limited capacity, treat only lump coal, poor applicability of pieces coal, high carbon content in ash, and suffer from problems related to coal tar that would cause much troubles for the downstream process, such as environmental pollution(like phenol-containing wastewater) and so on. In recent years, some kinds of fluidized bed gasification process have been developed successfully for fuel gas production. However, summarizing these typical fluidized bed gasification process, one can see that the limited heating value of fuel gas and high tar content in it are still the great challenges. On the other hand, the coal mining based on mechanical tools produces large amount of crushed coal. This part of coal can not be utilized reasonablely and efficiently in the long time, which not only wastes resource, but also causes environmental pollution. So, the development of new two-stage gasification technology to gasify crushed coal and produce the clean fuel gas with low tar content is thus significant in economy, environment and society.In this context, a new fluidized bed two-stage gasification process for low rank coal has been proposed, which mainly consists of a fluidized bed pre-oxidation reactor and transport fluidized bed(TFB) gasifier. Firstly, coal is pyrolyzed autothermally or oxidized partially in the FB pre-oxidation reactor. Then, all the products from the FB reactor, including gas, tar, and char, are blown into the downstream transport fluidized bed gasifier to conduct char gasification and pyrolysis gas upgrading. In gasifier, by thermal cracking and especial the catalytic reforming from hot char bed layer, the tar will be removed as far as possible during passing through the hot char bed layer. Due to integrating the advantages of FB and TFB reactors, decoupling the gasification process of coal gasification into coal pyrolysis/pre-oxidation and char gasification, and utilizing the catalytic reforming of the produced char bed on tar, the new fluidized bed two stage gasification process is not only applicable to powder feedstock(below 10 mm), but also allows rather large treatment capacity and low tar content, which is very suitable for industrial fuel gas production.This study is on the basis of fundamental studies and is devoted to verifying this fluidized bed two-stage gasification technology and its technical features through performing pilot-scale tests by gasifying a kind of Chinese lignite. The fundamental studies, including coal pyrolysis and partial oxidation in atmosphere containing oxygen and steam, char gasification and ash sintering behavior in a series of atmosphere, a reaction analysis method for in-situ char gasification will be proposed and verified on a novel micro fluidized bed reaction analyzer(MFBRA). Moreover, tar catalytic removal by hot bed char, will be carried out on a lab fluidized bed reactor, a high temperature tube furnace, and a lab fluidized bed two-stage reactor, respectively. On the basis of fundamentals, an autothermal pilot system with a coal treatment capacity of 100 kg/h will be designed and built. The results from typical stable operation fully verify the feasibility of fluidized bed two-stage gasification. Both of the fundamentals and pilot test would provide technical support in the scale-up of this technology.The following summarizes the major research contents and their corresponding results.(1) Fundamentals of coal pyrolysis and partial oxidation in FB reactorA laboratory FB reactor was used to test the influence of pyrolysis reaction temperature, particle residence time in reactor, and reaction atmosphere(N2, O2/steam-containing) on product distribution and their property. The results showed that, in N2 atmosphere, with increasing the temperature in experimental temperature range, H2 and CO had the dominant increase in yields, while adding O2 into the N2 atmosphere mainly promoted the productions of CO and CO2; the presence of steam in O2-containing led to more H2 and CO2 but less CO, showing the effect from enhancing the water-gas shift reaction between CO and H2 O. Raising reaction temperature in FB reactor, prolonging particle residence time and introducing oxygen and steam in atmosphere, can promote the release of volatile matter, formation of pores in char, and the production of effective gas components in fuel gas. However, high temperature and long residence time, such as at 900 oC and 3 min, perhaps reduced the gasification activity of the produced char. In comparison to the effect of the pure N2 atmosphere on the char structure, the presence of O2 and steam was the most beneficial to the creation of micropores, although more mesopores were also formed. However, too high of a steam fraction destroyed some micropores to decrease the char surface area and also removed some alkali elements from char.From the view of adapting to the FB two-stage gasification process, the viable operating conditions for the upstream FB reactor would be at 800 oC and with O2 and steam in the reaction atmosphere according to T=800 oC, RTD=3min, ER = 0.14,and S/C = 0.12, respectively.(2) Study of char gasification in FB and ash sintering behaviorThe char prepared under the optimized conditions was gasified in a electricity-heating fluidized bed reactor. The influencing parameter, such as gasification reaction temperature, char particle residence time in reactor, the kind and content of gasification agent in atmosphere, were systematically examined. With the increase of reaction temperature and ER, the extension of reaction time and the introduction ofsteam in atmosphere, gasification reaction and carbon conversion will be promoted sharply. However, higher gasification reaction temperature(1050 oC) and excessive oxygen(ER=0.31) will cause ash sintering strongly and make more production of CO2. By examining the ash sintering behavior in high temperature tube reactor, it can also find that temperature and atmosphere strongly affected the ash sintering in reactor. Comparing with inert atmosphere, reduction atmosphere and steam atmosphere can reduce the sintering temperature obviously, while the oxidation atmosphere made it higher sintering temperature.From the view of adapting to the FB two-stage gasification process, the suitable operating conditions for char gasification in the downstream gasifier was temperature of 1000 oC with an ER of 0.27 and S/C of 0.1.(3) Analysis method of in-situ coal char gasificationIn this study, the implementation of gasification of in situ coal char with CO2 in the so-called MFBRA through clearly decoupling coal pyrolysis and char gasification. For comparison, the other two kinds of ex-situ chars(named ex-situ chars 1 and 2) commonly adopted are also tested in the MFBRA. With the minimum limitations of heat/mass transfer, the use of the MFBRA clarifies the gasification behavior and its corresponding reaction kinetics of in-situ and ex-situ chars. The results demonstrate that the in-situ coal char had the highest reactivity and lowest activation energy of char gasification reaction with CO2, whereas the ex-situ coal char 2 had the lowest reactivity and highest activation energy. These differences between the in-situ coal char and ex-situ coal chars fully show the great effect of the pyrolysis atmosphere and annealing(cooling) on the char gasification behavior and kinetics. The work also verified the feasibility of using the MFBRA to realize the reactions for in situ char.(4) Catalytic reforming of tar by char bed layer in FBIn this study, tar catalytic removal by hot char layer in FB reactror was conducted on a FB two-stage apparatus. Compared to thermal cracking, catalytic reforming of tar by char was more effective in tar removal, certain selectivity in the removed tarry chemical species, upgrading the content of fuel gas and inhibiting carbon deposition effectively. The property of char, such as the specific surface area, pore volume, and containing metal oxides, significantly affected its activity for tar removal and gas upgrading, especially the effective gas components. Comparatively, characterizing the fresh and spent chars clarified that both micro- and meso-pores contributed greatly to the performance of the char for tar removal. The higher the specific surface area, the lower the tar content in the gas after catalytic reforming. The spent char had a much decreased specific the micro-pore surface area and volume. Impregnating the oxides of Na, Mg, Ca, and Fe into the demineralized char all increased the activity of the char for tar removal, but the realized activities followed an order(from low to high) of Na2 O, Fe2O3, Ca O, Mg O. This reveals essentially that the catalytic tar removal by char was due to not only the facilitated adsorption of tar on the char surface, but also the catalytic effects from the inherent metal oxides inside the char. By activating the char by steam after usage in tar removal, it can find that steam atmosphere can improve the char property in pore structure and reaction activity.Considering the fundamentals and FB two-stage gasification process, the viable operating conditions for tar removal over a char bed were at 1000 oC, and the required gas residence time was above 0.9 s.(5) Pilot test of FB two-stage gasification processOn the basis of the fundamental studies mentioned above, an autothermal pilot plant has been designed and built to gasify a kind of lignite with a treating capacity of 100 kg/h for fuel gas production with low tar content. The obtained typical results from the continuous steady operations of the pilot plant shows that adopting the FB two-stage gasification process can obviously promote the production of effective components in fuel gas and reduce the production of tar. By comparing the typical operation case, it can find that under the temperatures of pre-oxidation reactor and gasifier at about 840 oC and 1000 oC, the tar content in the generated fuel gas can be lowered to a level of 365 mg/Nm3, the higher heating value of the product gas can reach about 1050 kcal/Nm3, the content and component in tar obviously decreased. These results agreed with the fundamental results very well. Both of the fundamentals and the obtained typical results from the continuous steady operations of the industrial demonstration plant fully verified the technology feasibility of the devised new two-stage gasification process, and also verified the technical features proposed.Although the tested small pilot plant had higher heat loss and the used gasification agent(air) in the tests was not preheated, the generated in the fuel gas was lower, and the lower heating value(LHV) of the product gas still need to be improved. Consequently, rather improved performance would be expected for the industrial two-stage gasification process, which will have definitely the lower tar content in the fuel gas and the higher LHV. |
PDF Full Text Request