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Release and Transformation Behaviors of Sodium, Calcium, and Iron during Oxy-fuel Combustion of Zhundong Coals Chang'an Wang, Lei Zhao, Tao Han, Wufeng Chen, Yu Yan, Xi Jin, and Defu Che Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03200 • Publication Date (Web): 24 Dec 2017 Downloaded from http://pubs.acs.org on January 5, 2018
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Release and Transformation Behaviors of Sodium, Calcium, and Iron during Oxy-fuel Combustion of Zhundong Coals Chang’an Wang, Lei Zhao, Tao Han, Wufeng Chen, Yu Yan, Xi Jin, and Defu Che*
State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
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Abstract Zhundong coal has attracted an increasing concern due to its super-huge reserve but high content of alkali metals. Oxy-fuel combustion of Zhundong coal benefits the near-zero emission of pollutants in coal-fired power plants and promotes the large-scale utilization of high-alkali coal. However, few efforts if any have been conducted on oxy-fuel combustion of Zhundong coal. The previous studies related to Zhundong coal were mainly focused on sodium behaviors but little work has been performed on calcium and iron, while calcium and iron are very likely to generate significant influences on fouling problems in combustion of Zhundong coal. The present study aimed to elucidate the release and transformation behaviors of sodium, calcium, and iron in oxy-fuel combustion of Zhundong coal using a fixed-bed reactor. Experimental results indicated that calcium in Zhundong coal was mainly present as ammonium acetate-soluble form, while the iron existed in forms of hydrochloric acid-soluble and insoluble. With the increasing combustion temperature, the ash yields of Zhundong coals decreased and the volatilization ratio of sodium increased, while the temperature had a weak influence on ash yield and the release of water-soluble sodium between 800-1000 oC. The variations of total calcium with combustion temperature were not significant, but transformations among various chemical forms occurred. The decreased iron of hydrochloric acid-soluble form was transformed into insoluble form and discharged into gaseous phase. Compared to air case, oxy-fuel combustion with 21% oxygen led to more sodium and iron retained in residual ash, while it promoted the release of calcium. The mineral transformation and ash formation were susceptible to the high content of carbon dioxide under oxy-fuel condition and were strongly associated with the chemical forms of
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sodium, calcium, and iron within Zhundong coals. The crystalline mineral species in Zhundong ash were obviously influenced by the combustion temperature and partly affected by the atmosphere. The differences of mineral species of Zhundong ash between air and oxy-fuel cases were mainly present in range of 800-1000 oC, which was closely related to the decomposition of calcite and transformation of calcium. The oxygen content dependency of transformation behaviors of sodium, calcium, and iron was greatly different during oxy-fuel combustion. This work possibly offered an improved understanding of the functional mechanisms of sodium, calcium, and iron on fouling issues.
Keywords Zhundong coal; Alkali and alkaline earth metals (AAEMs); Iron; Oxy-fuel combustion; Occurrence mode
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1 Introduction Coal has an indispensable position in chemical industry and electricity generation worldwide [1-3], while it is also the principal fossil fuel at present and in future of China. Zhundong coalfield is a super-huge coalfield recently discovered in Junggar Basin of China with an estimated reserve amount of approximate 1.64×1011 ton [4, 5]. Because of the low-cost exploitation, super-huge reserve, and high quality [5-7], Zhundong coal is attracting an ever-increasing concern and will play a significant role in energy production in the coming future. However, the utilization of Zhundong coal in power plants has brought about serious slagging and deposit problems mainly due to its high contents of alkali and alkaline earth metals (AAEMs) [5, 8-12]. These ash-related issues can give rise to heat transfer losses, corrosion in tubes, unscheduled shut-down, and an increasing operation cost. The insufficient understanding of ash deposition and transformation of basic metals during combustion of high-AAEM coal hinders the efficient and secure utilization of Zhundong coal in large-scale. On the other hand, CO2 emission from human activities should be mainly responsible for the intensification of greenhouse effect. Coal combustion in power plants has given rise to many environmental pollutants, such as sulphur oxides (SOx), nitrogen oxides (NOx), particulate matters (PMs), and mercury (Hg), which is also one of the largest contributors to anthropogenic emission of CO2. The CO2 emission per unit of electricity from coal-fired power plants is significantly higher than those from other fossil fuels. Therefore, CO2 reduction from coal-fired power plants is supposed to draw great attention. Several approaches have been developed to reduce CO2 emission from coal-fired power plants,
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including the introduction of combined cycles, the improvement of thermal efficiency, the replacement of coal with renewable resources, the CO2 capture and storage (CCS), etc. [13, 14] Oxy-fuel combustion technology is a definitely promising choice for CO2 capture in coal-fired power plants, which can
lower NOx emission and is considered to be
economically competitive and technically feasible [13-19]. Compared with traditional air combustion, the combustion medium air is replaced by O2 with recycled flue gas (mainly CO2 and H2O depending on the recycle mode) to control the flame temperature in oxy-fuel combustion. Coal utilization remains one principal cause of environmental pollutants in China, and it is greatly important to perform research on coal-fired electricity generation technology of high-efficiency and safe. Oxy-fuel combustion of high-AAEM coal is beneficial for near-zero emission of pollutants in coal-fired power plants in China, which can also further promote the large-scale utilization of Zhundong coal. The AAEMs are present as various chemical forms (also known as occurrence modes) in coal matrix and ash composition. The chemical extraction method of successive leaching with H2O, CH3COONH4 and HCl solutions was extensively employed to quantify the chemical forms of AAEMs in solid samples, sequentially named in terms of water-soluble (also denoted as H2O-soluble) form, CH3COONH4-soluble (also denoted as NH4Ac-soluble) form, HCl-soluble form, and insoluble form [5, 20-23]. The occurrence modes of AAEMs significantly affect their transformation behaviors and the fouling issues during combustion of solid fuels. Only AAEMs of part occurrence modes are inclined to cause severe ash-related issues, while water-soluble and organic species present the greatest harm. Hence, numerous studies have been conducted to elucidate the transformation among various occurrence modes
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of AAEMs during gasification [21, 24-26], pyrolysis [5, 23, 27], and combustion [28], with the majority of research focused on alkali metals. Whereas, little research has been performed on behaviors of AAEMs during oxy-fuel combustion of coal. To date, many studies have been conducted on Zhundong coal due to its super-reserve and the increasing concern, including the catalytic effect of sodium, the release behaviors of sodium during pyrolysis, combustion, and gasification, the sintering, fusion characteristics, and deposition of ash, the effect of additive on fouling issue, the transformation behaviors of AAEMs during pyrolysis, chemical looping combustion (CLC), supercritical gasification, liquefaction, etc. Zhang et al. [8] probed the effects of water-washing and HCl-washing on combustion behaviors of Zhundong coal, and observed the reduced combustion rate and prolonged burnout time due to the removal of catalytic sodium. The sodium could also promote Zhundong coal gasification in the fluidized bed and the formation of NaAlSiO4 inhibited the release of sodium and agglomeration [29]. Wang et al. [30] and He et al. [31] concluded that the char burnout stage featured the highest peak sodium content, while the ash stage had the longest duration of sodium release using a non-resonant Planar laser-induced fluorescence and a calibrated laser-induced breakdown spectroscopy, respectively. The sodium retention efficiency of additives was greatest in the ashing stage [31]. The chemical forms of sodium in fly ash differs remarkably from that in bottom ash during gasification of Zhundong coal in the circulating fluidized bed [10], while the sodium in coal underwent different transformation processes through combustion (oxidizing atmosphere) and gasification (reducing atmosphere) [32]. Li et al. [33] found out that the ash sintering of Zhundong lignite was due to Na- and Cl-bearing minerals, while its fusion was attributed to
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the eutectics formed among aluminum (Al), iron (Fe), calcium (Ca) and sulphur (S) using a muffle furnace. From the point of ash deposition, Li et al. [12] believed that the fine ash particles with abundant Na, Ca, S, and Mg from condensation and thermophoresis gave rise to the severe ash deposition in combustion of Zhundong coal. Two mechanism of AAEM species possibly enhanced contribution to the ash deposit, including the forming of a sticky inner layer and the sticky coating surface of bulk ash particles [34]. Small particles of
