Experimental investigation on combustion characteristics and kinetics during Co-Firing bituminous coal with ultra-low volatile carbon-based solid fuels
Introduction
The low-rank coal reserve of China has been proved approximately 500 million tons, which accounts for about 42% of coal reserves of China [1,2]. As the rapid enlargement of low-rank coal chemistry industry, the production of ultra-low volatile carbon-based solid fuels (abbreviate as LVCF, hereafter), including pyrolyzed and gasified semi-cokes, has been maintaining an ever-increasing trend. The parts of granular pyrolyzed semi-coke (SC) could be effectively consumed in chemical industry, while the SC powder which obtained from industrial pyrolysis furnace with ultra-low volatile content could not be efficiently used in chemical industry [3,4]. Gasified semi-coke (GS) is also an ultra-low volatile by-product of circulating fluidized bed [5]. The semi-cokes could not be efficiently fired by itself due to their ultra-low volatile contents, and their combustion could generate high NOx emission [6]. Some scholars put forward that the semi-cokes could be utilized in circulating fluidized bed boiler (CFB) [[7], [8], [9]], and other researchers indicated that the preheated combustion technology could be applied on utilization of semi-cokes [[10], [11], [12]]. However, the applications of these technologies always need new equipment or reconstruction, which causes the increase of cost. Hence, the strategy of co-firing semi-cokes with bituminous coal (BC) in utility boiler was proposed by some scholars [6,[13], [14], [15]], which was believed as one clean and efficient approach to consume semi-cokes in large scale [16].
The semi-cokes usually feature poor ignition and burnout performances due to their ultra-low volatile contents. Therefore, the high-volatile bituminous coal with superior combustion characteristics could be blended with these LVCFs to improve the firing performance of semi-cokes. Hence, the firing features and chemical properties of semi-cokes and co-firing features of semi-cokes and other fuels were investigated by some scholars [13,[17], [18], [19], [20]]. Zhang et al. [13] focused on the firing characteristics of semi-coke, and believed that the firing conditions of semi-coke were associated with the pyrolysis temperature, residence time and particle size. Peng et al. [21] conducted the thermogravimetric analysis to investigate the combustion and structure features of semi-coke. The flammability of semi-coke was lessened with the rise of temperature, which was mainly affected by chemical structure of the semi-coke. These scholars both studied the firing features of semi-cokes and indicated that the semi-cokes had poor combustion behaviors. Therefore, the co-firing of semi-coke with coal was evaluated by other investigators [[22], [23], [24], [25]], which could enhance the firing features of blend by co-firing BC. Zhang et al. [25] concluded that the co-firing behaviors of blends were promoted with the proportion of BC. Yao et al. [26] investigated the semi-coke and coal blends in O2/CO2. The results showed that the ignition behavior of blended fuel was weakened and the burnout performance was improved in O2/CO2 compared with combustion in air. According to the previous study of our research group, the burnout ratio of bituminous coal and LVCFs was lessened with the fractions of LVCFs raised [3]. The previous research mainly focused on the binary blending (bituminous coal blended with individual pyrolyzed or gasified semi-coke), especially the NOx generation characteristics of blends. Few studies concentrated on firing features of ternary blending (bituminous coal blended with both pyrolyzed and gasified semi-cokes). Therefore, the combustion and kinetics features of co-firing bituminous coal with LVCFs are still unclear. Furthermore, the effects of mass fractions of LVCFs on co-combustion characteristics are vague, which is important for the realization of co-firing semi-cokes with bituminous coal in utility boiler. The interactions between various carbon-based solid fuels on blends are also valuable for utilization of semi-cokes. In addition, the kinetics of ternary blending need to be further evaluated.
Here, the thermogravimetric analyzer was conducted to clarify the firing characteristics of LVCFs and BC blends. The effects of fractions of semi-cokes on co-firing features were further discussed. The influences of heating rate and the interactions between bituminous coal and semi-cokes on co-combustion characteristics were also investigated. In addition, the Flynn-Wall-Ozawa (FWO) method was applied to further elucidate the kinetics of ternary blending. The present study is beneficial for the development of the low-rank coal chemistry industry, together with the clean and efficient utilization of pyrolyzed and gasified semi-cokes in utility boiler.
Section snippets
Samples and experimental system
Here, the samples selected in experiments are Zaoquan bituminous coal (abbreviate as ZB, hereafter) from Zaoquan coalfield of Ningxia Province, pyrolyzed semi-coke (SC) from an industrial fixed-bed pyrolysis furnace of Shaanxi Province, and gasified semi-coke (GS) from a CFB gasifier. As shown in Table 1, the low-volatile carbon-based solid fuels (LVCF), including GS and SC, contain low volatile contents, and the volatile content of GS is below 10% of volatile content of ZB. In addition, the
Reactivity comparison between bituminous coal and semi-cokes
The repeated tests of Zaoquan bituminous coal combustion in air condition have been performed, which is depicted in Fig. 3. It could be concluded that the reproducibility of thermogravimetric analysis is acceptable. As depicted in Fig. 4, the ignition temperature of ZB is obviously lower than those of semi-cokes. The ignition temperature of GS is the highest, because the volatile content of GS is ultra-low compared with those of ZB and SC. In addition, the combustion parameters of individual
Conclusion
Here, the co-firing characteristics of ultra-low volatile carbon-based solid fuels and bituminous coal blends were investigated by thermogravimetric experiments. The effects of blending ratio of gasified semi-coke (GS) or pyrolyzed semi-coke (SC) and sum of proportions of GS and SC were illustrated. The thermal interactions between various fuels were discussed. Moreover, the Flynn-Wall-Ozawa (FWO) method was employed to analyze the kinetics of blends. The main conclusion are as follows:
The
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors acknowledge financial support from the National Key R&D Program of China (2017YFB0602003).
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