| Gasification of coal or biomass is one of the main techniques to achieve the clean raw material. The low rank coal, especially brown coal, is always thought to be a good raw material for gasification due to its high reactivity. There is plenty of brown coal resource in our country, which provided the guarantee for its development and utilization in a large scale. Research on structure and reactivity of brown coal char was the hot point for a long time. At present, study on brown coal char was usually to cool it after pyrolysis and then its reactivity was researched. Obviously, the cooling treatment of these chars means that they experience some extra process from cooling to reheating. A lot of researches showed that during the process the cold char was reheated, its reactivity would be changed and thus could not reflect the actual behavior of gasification process, which lost its property of in-situ as well. In order to investigate the effect of quenching effect on char reactivity which was ignored in current researches, in-situ gasification of brown coal char was studied in a newly-designed two-stage quartz reactor in this paper, and the effect of pre-treatment on char reactivity has also been explored. The stage of pre-treatment and in-situ gasification could be carried out respectively in different conditions(e.g. temperature and reaction atmosphere), while the two processes were continuous relatively. Different from other researches, it could be more easily to study the effect of pre-treatment on reactivity of in-situ gasification char by changing the pre-treatment condition in this paper.In this paper, the Shengli brown coal was selected as fedstock, effects of pre-treatment temperature, duration in the inert atmosphere or steam on the structure and reactivity of char were investigated. The evolution of structure and reactivity of in-situ gasification char, the influence of sudden injecting the steam of char on its structure and reactivity, and the cooling treatment on structure and reactivity of brown coal char were studied respectively, and the kinetic of chars were analyzed as well. The aromatic ring systems(ratio of small to big rings) and the O-containing functional groups were characterized by Raman spectroscope, the specific surface area and total pore volume were measured by specific surface area analyzer, the surface morphology was analyzed by scanning electron microscope(SEM) and the reactivity of char was examined by Thermogravimetric Analyzer(TGA). The main research contents and results were shown as follows:(1) In chapter 2, pyrolysis of Shengli brown coal and the in-situ gasification of “hot char” in steam were carried out in a newly-designed two-stage fixed bed reactor. Firstly, the brown coal was pyrolyzed from 5 s to 30 min at the temperature of 400, 600 or 800 ℃ with ultrapure argon in reactor at the pyrolysis stage. Then the reactor was shifted to the “gasification stage” immediately and the “hot char” obtained at the pyrolysis stage was gasified in-situ for 2 or 10 min under the temperature of 900 ℃ with a mixed gas containing 15 %H2O balanced with argon instead of argon itself, obtaining in-situ gasification char. Results show that heat treatment temperature had a great influence on the yield of char from in-situ gasification, the char yield decreased with enhancing the heat treatment temperature. It was also found that the holding time of heat treatment had little effect on char yield. Reactivity with air of in-situ gasification char changed with heat treatment time of the raw coal, when it was 5 s, the reactivity decreased with increasing the heat treatment temperature, however, when it was more than 5 min, char prepared from the heat treatment of 600 ℃ plus in-situ gasification showed the highest reactivity, followed by 400 ℃, and the 800 oC one had the lowest reactivity(the order of reactivity of char was: 600 ℃ >400 ℃ >800 ℃). The O-containing functional groups as well as small aromatic ring systems decreased with the increase of heat treatment temperature and in-situ gasification time. The structure of in-situ gasification chars tended to be consistent due to the combined influence of reactions between pyrolysis and in-situ gasification. The reactivity of in-situ gasification char had a certain correlation to its structure, the lower the ratio of small aromatic rings to big aromatic rings was, and the lower the reactivity was.(2) In chapter 3, experiments of pre-treatment in steam of brown coal char and the in-situ gasification of “hot” char was carried out, the effect of pre-treatment in steam on char reactivity was also examined. The brown coal was pre-treated in different temperatures and duration, subsequently, the obtained hot char was gasified in-situ under given condition(900 ℃,15%H2O,10 min). Results show that pre-treatment in steam and inert atmosphere had the similar effect on in-situ gasification char. In the short pre-treatment time(e.g.1 min), the char reactivity decreased with temperature; when it was longer than 5 min, however, the char derived from 600 ℃ had the highest reactivity, the largest specific surface area as well as the total pore volume, followed by the 400 ℃ one, and the 800 ℃ one was the lowest.The influence rule that treated in both steam and argon on in-situ gasification char was extreme similar, which confirmed it was a common phenomena that char treated at 600 ℃ for a long time + in-situ gasification had the highest reactivity.(3) In chapter 4, the evolution of reactivity and microstructure of char during in-situ gasification was investigated. Firstly, the brown coal was pyrolyzed for 5 s under the temperature of 400/600/800 ℃ with ultrapure argon in reactor at the pyrolysis stage. Then the reactor was shifted to the “gasification stage” immediately and the “hot char” obtained at the pyrolysis stage was gasified in-situ for 2-30 min at 900 ℃ with a 15 %H2O balanced with argon instead of argon itself, obtaining in-situ gasification char. Results show that yields of in-situ gasification char decreased significantly within the initial 10 min, and it decreased slowly with prolonging the holding time further(10-30 min). Plenty of O-containing functional groups were released in the first 2 min during gasification and thus little existed in char after gasification for 2 min. The ratio of small aromatic ring systems to big aromatic ring systems decreased dramatically within 2 min, and then it decreased slowly with the extension of holding time from 2 to 30 min. Microstructure of char has an effect on its reactivity: with gasification reacting, the amorphous carbon as well as small aromatic ring systems with high activity decreased, and char reactivity decreased consequently.(4) In chapter 5, the effect of sudden injecting steam on reactivity and microstructure of in-situ gasification was studied, experiments of drying and pyrolysis of brown coal as well as the “hot” char in-situ gasification was carried out. The raw coal was dried completely at 200 ℃ at the drying stage firstly, and then it was pyrolysed at the temperature range from 600 to 900 ℃ for 30 min at the pyrolysis stage, whereafter 15 vol.% H2 O was injected into the reactor immediately to conduct the gasification reaction and thus the in-situ gasification char was prepared. The results show that at relative low temperature(such as 600 ℃), the steam had little effect on the conversion, reactivity and microstructure of char. When it reached 700~900 ℃, there were different change trends before and after 2 min of reaction between char and steam: in the first 2 min, the reactivity, the ratio of small aromatic ring systems(3~5 rings) to the big fused rings(≥6 rings) and O-containing functional groups decreased dramatically although the char conversion was very little; after 2 min, however, char conversion increased gradually, char reactivity, the ratio of small to big aromatic ring systems and O-containing functional groups decreased slowly. In addition, it was also found that the pore structure of char followed almost the same change trend before and after 2 min. During the first 2 min of reaction, a sharp decrease of the ratio of small to big aromatic ring systems and O-containing functional groups was the important factor that led to the decrease of char reactivity, while after 2 min, the change of aromatic ring systems was the reason that resulted in the further decrease of char reactivity.(5) In chapter 6, the changes in structure and reactivity of chars with different quenching treatments for “hot” char derived from Shengli brown coal was examined. The “hot” chars prepared from the pyrolysis of the brown coal were remained in argon for 30 min in a newly-designed two-stage quartz reactor at 400, 600 or 800 °C, respectively. Subsequently, the “hot” chars were used to prepare the “cold” char by quenching at an ambient temperature, dry ice and liquid nitrogen. The “cold” chars were then employed for the ex-situ gasification in 15% steam atmosphere at 900 °C for 10 min. For comparison, the in-situ gasification of “hot” char derived from pyrolysis without quenching was conducted by changing the reaction atmosphere from argon to 15% steam(under the same condition as the ex-situ gasification). Results show that the yield of “hot” char in-situ gasification was lower than that of the “cold” char ex-situ gasification. Quenching approaches had a significant influence on the porous structure of subsequent char. The specific surface areas and porous volumes were reduced dramatically with increasing quenching rate, but it has no significant effect on the chemical structure of char(such as aromatic ring systems and O-containing functional groups). In addition, the reactivity of “cold” chars was decreased with increasing quenching rate. The irreversible destroy of char structure caused by the quenching treatment was contributed to the decrease of reactivity of chars.(6) In chapter 7, microstructure and reactivity of in-situ/ex-situ gasification chars mentioned in chapter 6 were characterized, and the reaction kinetics was compared and analyzed. Results indicate that small to big ring systems of in-situ gasification was lower than that of ex-situ gasification, which declared that the residual active components of the former one was less than the later one, therefore the reactivity of ex-situ gasification char was higher than the in-situ gasification one. Through the calculation of kinetic parameters of the chars, it was found that the apparent activation energy of in-situ gasification char was higher than that of the ex-situ gasification char. Besides, there was kinetic compensation effect between apparent activation energy and apparent pre-exponential factor, and dynamic compensation equations of different chars were obtained respectively. Result from the analysis of collision and transition stage theory indicated that the later one was more reasonable explanation for kinetic compensation effect of in-situ gasification char. |
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