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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.

The utilization of blended coals has become popular in northwestern China. In this paper, investigations on coal properties and combustion characteristics of blended coals have been carried out to provide guidance for blended-coal-fired plants. Experimental results show that proximate, ultimate, and calorific value analyses of blended coals can be calculated by mass-weighted average of individual coals. However, grindability, ash composition, ash fusion temperatures, and some characteristics related to combustion performance of blended coals cannot be predicted accurately from those of individual coals and the mixture ratio. Blending a coal of high fusion temperatures with a coal of low fusion temperatures can significantly increase the ash fusion temperatures of blended coals. Thermogravimetric experimental results indicate that the intense combustion stage of blended coals is quite different from those of individual coals. Thermogravimetric curves of blended coals lie between those of individual coals but show nonadditive behavior. Furthermore, combustion characteristics of blended coals improve significantly with increased oxygen concentration, and combustion performance of low-reactivity coal is less sensitive to changes in oxygen concentration. Kinetic analysis indicates that the apparent activation energy of coal combustion is not constant at different conversion fractions. The combustion process of blended coals is charactered by segmentation and the apparent activation energy in low-temperature zone is higher than that in high-temperature zone. The mean activation energy of blended coals shows nonadditivity performance in both KissingerÀAkahiraÀSunose (KAS) and CoatsÀRedfern methods due to the interaction between individual coals. The KAS method is more adequate than the CoatsÀRedfern method for analyzing the kinetics of blended coal combustion because the CoatsÀRedfern method requires prior assumption of reaction mechanism and its analysis results sometimes differ clearly from experimental results.

A three-dimensional numerical investigation is presented on the coal combustion and NO x emission in a supercritical 600 MW wall fired utility boiler fed with lean coal. The distributions of velocity, temperature, and species were obtained using a finite volume method and some validated submodels. The influence of air staging condition, i.e., overfire air (OFA) ratio and OFA port position, on combustion and especially on NO x emission was studied. The temperature of the burner zone and the carbon content in fly ash increased with increasing OFA ratio, while the NO x emission decreased with increasing OFA ratio. An increase of the OFA port position resulted in a decrease of NO x emission and an increase of carbon content in fly ash. In addition, high burnout rate of pulverized coal and low NO x emission can be achieved in the present boiler, in which OFA was injected from Port 3 with a ratio of 34.2%. This study provides new insights into the physical and chemical processes in a wall fired utility boiler fed with lean coal and also illustrates a method to reduce NO x emission.

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