Study on mercury emission during underground coal gasification of lignite
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Résumé
underground coal gasification (UCG) is a process in which underground coal, without mining and transportation, is directly converted into combustible gas. The regularity of mercury emission during UCG of lignite was studied with aims of prevention and control of mercury emission released from UCG. Mercury form in lignite is analyzed by sequential chemical extraction. It was found that the average mercury content of the test lignite is 0.3986?μg·g~(-1) and mercury in lignite mainly occurs in bound-sulfide and residue, taking up 77.8% and 19.1%, respectively. Based on UCG simulation test of lignite, the change of mercury content in UCG gas varying with time and temperature and the influence of gasification technology were investigated. Elemental mercury, Hg~0(g), and divalent mercury, Hg~(2+)(g), in UCG gas was captured by Ontario Hydro Method (recommended by EPA in USA) and detected by atomic fluorescence spectrometer. In addition, the mechanism of mercury transformation and emission during UCG process was analyzed. Mercury exists mainly in the form of Hg~0(g) in UCG gas. Small amount of Hg~(2+)(g) is produced through chemical reaction of Hg~0(g) and the product gas in the gasification tunnel, and it easily decomposes into Hg~0(g) when the temperature is above 700K. The content of mercury in UCG gas changes with gasification time. At the initial stage of UCG, the content of Hg~0(g) is lower and Hg~(2+)(g) can not be detected. The content of both Hg~0(g) and Hg~(2+)(g) increases in the stable gasification process and decreases at the end of UCG due to different amounts of gasified coal in the different periods of UCG. The average content of Hg~0(g) is 50～60μg·m~(-3) and that of Hg~(2+)(g) is 0.30～0.35?μg·m~(-3). The highest content of Hg~0(g) is found to be 160 times as much as that of Hg~(2+)(g). Changes of Hg~0(g) and Hg~(2+)(g) content in UCG gas with temperature of gasification zone was obtained based on the results of temperature field. The content of Hg~0(g) and Hg~(2+)(g) increases with elevated temperature of gasification zone. The content of Hg~0(g) reaches saturation when the temperature is higher than 600K, similar to the results of surface gasification. Gaseous mercury contents in UCG gas under different technologies were compared. It indicates that UCG gas contains more Hg~(2+)(g) under oxygen gasification and less Hg~(2+)(g) under oxygen-steam gasification since steam slows down the oxidation tendency of Hg~0(g) to Hg~(2+)(g). Analysis of mercury balance in UCG process shows that gaseous mercury takes up less than 20% of the total mercury in UCG products, lower than that from surface gasification and coal burning, and solid mercury takes up more than 60%. Since mercury content in UCG ash is lower, the majority of solid mercury is particulate mercury absorbed by char and ash when UCG gas flows through the long air stream tunnel. UCG gas from simulation test contains higher percentage of Hg~0(g) than that from field test, since the tunnel length is shorter in simulation gasification.
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Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,001 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
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Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découle