Transformation of Alkali Metals during Pyrolysis and Gasification of a Lignite

Wei, Xiaofang; Huang, Jiejie; Liu, Tiefeng; Fang, Yitian

doi:10.1021/ef7007858

Cited by 73 publications

(65 citation statements)

References 15 publications

(20 reference statements)

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“…The slight difference between two coals could result in different characteristics of sodium release during pyrolysis. With the increase of pyrolysis temperature from 700 to 900°C, the decrease of CH 3 COONH 4 -soluble Na and K was also reported by Wei et al 25 Quyn et al 38 indicated that sodium in the form of NaCl was more easily volatilized than that in the form of carboxylate in coals, which is consistent with the present results.…”

Section: Resultssupporting

confidence: 92%

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Release and Transformation of Sodium during Pyrolysis of Zhundong Coals

Wang

et al. 2014

Energy Fuels

110

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Zhundong coalfield is one of the superhuge coalfields newly discovered in Xinjiang Autonomous Region of China. While many studies on alkali metals in coal/biomass have been reported previously, few efforts were conducted on Zhundong coals which are characterized by a high content of sodium. It is challenging to explore the ash deposition problems related to Zhundong coals directly using existing findings on the behavior of alkali metals. The occurrence mode and transformation of alkali metals during thermal conversion of Zhundong coals could be quite different. Specific investigation on sodium transformation during pyrolysis of Zhundong coals will make a contribution to our understanding on the fate and behavior of alkali metals involved in high-sodium coals. Here, the release and transformation fundamentals of sodium during pyrolysis of two Zhundong bituminous coals were investigated using an experimental approach of sequential chemical extraction. The sodium volatilization and evolution of occurrence modes in chars of Zhundong coal were probed in a laboratory-scale reactor. The occurrence of alkali metals in Zhundong coals greatly differs from those in other coals of China. Water-soluble sodium is the predominant chemical form in Zhundong coals. Most of the water-soluble sodium is released into gas phase as a volatile during pyrolysis and part of the remainder is converted into an insoluble form, but the release of sodium is not always synchronous with volatility. The release behavior of sodium demonstrates a nonmonotonic variation with the particle size of the coal while the effect of particle size includes many factors resulting in the challenge of demonstrating its individual contribution to sodium release. The duration time has an insignificant effect on volatile release and sodium transformation at low temperature, while it has an obvious influence at high temperature or in the initial stage of pyrolysis. With the extension of duration time at high temperature, no more sodium is obviously released from the residual chars of Zhundong coal but more water-soluble sodium will still transform into insoluble form. During the devolatilization of Zhundong coals under CO 2 atmosphere, the release of sodium and its conversion into insoluble form will be inhibited compared to that under N 2 condition.

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Section: Resultssupporting

confidence: 92%

“…Wei et al 25 and Sugawara et al 14 also observed the increase trend of sodium volatility with increasing pyrolysis temperature. The volatilization ratios of sodium are 37% and 35% at 800°C for ZJ coal and WCW coal, respectively.…”

Section: Resultsmentioning

confidence: 88%

Release and Transformation of Sodium during Pyrolysis of Zhundong Coals

Wang

et al. 2014

Energy Fuels

110

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“…This even more affirmed the possibilities of reactions R1 and R2 indicated above. Besides of the reaction R2 from CaO to Ca2Al2SiO7, CaSO4 was also proposed to react with SiO2 and Al2O3 to produce Ca2Al2SiO7 at around 800 o C [21].…”

Section: Figure 6 Residual Ash Morphologies At (A)-600 O C (B)-815 mentioning

confidence: 99%

The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: A study from ash evaporating to condensing

Wang

Wei

et al. 2015

Applied Thermal Engineering

258

138

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ABSTRACT:The high contents of sodium and calcium in Zhundong coal induce severe slagging and ash deposition in boilers. In this study, the ash deposition mechanism was investigated based on the results obtained from a full-scale boiler (350 MW) burning Zhundong coal, anda fixed bed reactor used for ash evaporating-condensing. In the full-scale boiler, the condensing and depositing of sodium and calcium sulfates play an important role on ash depositing on convection heating surfaces. Sulfates start to significantly condense and deposit at the flue gas temperature of about 850 o C (at Medium and High Temperature Reheater). Ash evaporating tests proved that, with the increasing in temperature from 400 o C to 1200 o C, the ash evaporating process is divided into three stages: 1) 400-800 o C, 80% of sodium, and 100% of chlorine are released; 2) 800-1000 o C, all the left sodium evaporates and sulfur starts to be released with the formation of partial aluminosilicates; 3) 1000-1200 o C, all the left sulfur is released through the decomposition of calcium sulfates and then calcium starts to evaporate, while silicon oxides disappears due to the formation of new complex silicates. Ash condensing tests further proved that, the sodium in Zhundong coal was released mainly in the forms of atom, oxide, and chloride, in which sodium chloride account for about 50%. When the evaporating temperature increased higher than 1000 o C, partial alkali and alkaline earth metals were released as gaseous sulfates, and afterwards condense and deposit on the heating surfaces. At last, a temperature-dependent ash deposition mechanism in Zhundong coal combustion was proposed.

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“…20,32−35 The AAEM species in biomass and low-rank coal mainly exist in four different chemical forms: H 2 O-soluble forms (e.g., soluble salts), CH 3 COONH 4 -soluble forms (e.g., carboxylates), acid-soluble forms (e.g., silicates and aluminates), and stable form (e.g., covered by carbon and silico-aluminates). 36,37 Generally, AAEM species contain more-soluble salts and carboxylates than the other two chemical forms. The catalytic effects of AAEM species largely depend on their chemical form.…”

Section: Introductionmentioning

confidence: 99%

Release and Transformation of Sodium in Kitchen Waste during Torrefaction

Liu

Qiao

et al. 2014

Energy Fuels

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The release and transformation of typical chemical forms of sodium (i.e., H 2 O-soluble salts and carboxylates) in kitchen waste during torrefaction at 200−300°C was investigated in this study. It was found that sodium release is negligible (<2%) at 200°C after torrefaction for 15 min, but the release increases to ∼10% when the temperature is increased to 300°C. Because of the high concentration of water-soluble sodium in raw noodles, the release of sodium during torrefaction is mainly in the form of NaHCO 3 or Na 2 CO 3 , as NaCl is relatively stable at torrefaction temperatures. The transformation of Na from a water-soluble form to a CH 3 COONH 4 -soluble form during torrefaction was found to be an important factor in the release of sodium from raw noodles, since sodium release occurs much more readily from a Na-exchanged sample during torrefaction. CH 3 COONH 4 -soluble sodium is also found to transform back into a water-soluble form during torrefaction. For example, ∼18% of the sodium in a Na-exchanged sample transforms to a water-soluble form (i.e., Na 2 CO 3 ) at 300°C, indicating the existence of an interconversion mechanism between the water-soluble and CH 3 COONH 4 -soluble forms. A small amount of water-soluble or CH 3 COONH 4 -soluble sodium could also transform to acid-soluble and stable forms during torrefaction at high temperatures (i.e., 300°C).

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Transformation of Alkali Metals during Pyrolysis and Gasification of a Lignite

Cited by 73 publications

References 15 publications

Release and Transformation of Sodium during Pyrolysis of Zhundong Coals

Release and Transformation of Sodium during Pyrolysis of Zhundong Coals

The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: A study from ash evaporating to condensing

Release and Transformation of Sodium in Kitchen Waste during Torrefaction

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