Effect of high temperature and separated combustion on SO2 release characteristics during coal combustion under O2/CO2 atmosphere
Introduction
Nowadays carbon capture and storage (CCS) technology is getting more concern and being developed due to the global climate issues. Among CCS technologies, oxy-fuel combustion technology is considered as one promising technology. In oxy-fuel combustion, the mixture of pure O2 produced by air separation unit (ASU) and recycled flue gas are utilized as oxidant instead of air during coal combustion. Flue gas recirculation technology is applied to avoid excessive combustion temperature [1,2]. With the absence of N2 and adoption of flue gas recirculation, CO2 concentration of flue gas generated in oxy-fuel combustion can reach 95% or higher [[3], [4], [5]] theoretically when moisture is removed from recycled flue gas.
In oxy-fuel combustion, sulphur (S) is introduced by coal in the form of organic sulphur compounds, inorganic sulphur compounds (mainly sulphides and sulphates), and traces of elemental sulphur [6]. Pyrite (FeS2) is usually the main component of sulphides in coal. During devolatilization, organic sulphur compounds is released to gas phase and sulphides start the first step of decomposition. While the second step of decomposition for sulphides (FeS produced in the first step) occurs in char combustion. Besides, sulphates are only released during char combustion, and generally present in coal with low content. Compared to conventional air-fuel combustion, pollutants emission characteristics in oxy-fuel combustion are different. As for SO2 emission, the most experiments show that SO2 emission concentration is lower in oxy-fuel combustion than that in air-fuel combustion. Due to the absence of diluting agent N2, SO2 concentration under oxy-fuel combustion conditions is about 3–6 times higher than that under air-fuel combustion conditions [[7], [8], [9], [10], [11]]. However, SO2 emission in mg/MJ is lower resulting from the higher conversion of SO2 to other sulphur species throughout the whole process [12]. The conversion of fuel-S to SO2 from oxy-fuel combustion is around 35% lower compared to air-fired conditions [1]. A higher concentration of SO2 can lead to a higher possibility of low temperature corrosion on heat exchangers surfaces where temperature is below the acid dew point (ADP) [12,13]. Therefore, a better understanding on fuel-S conversion mechanism and fuel-SO2 emission in oxy-fuel combustion are required due to these potential problems.
Many researches on SO2 emission have been conducted in oxy-fuel combustion. Tian et al. [14] used thermal gravimetric analyzer and Fourier transform infrared spectrometer (TG-FTIR) to study the sulphur behavior during coal combustion. They found that less SO2, more H2S and COS are released in oxy-fuel combustion compared with those in air-fuel combustion. Hu et al. [15] utilized a quasi-one dimensional, electrically heated combustor to research the pollutants emissions of coal combustion with different O2 concentration gases. They found that there was a small effect of temperature on SO2 emission no matter in air atmosphere, 50%O2/50%CO2 atmosphere or pure O2 atmosphere. In other researches, Courtemanchea et al. [16] measured the SO2 emission of bituminite combustion in a sealed, electrically heated drop-tube furnace. They found that with the increasing of temperature from 1300 K to 1600 K, conversion rate of fuel-S to SO2 decreased. Apart from it, the conversion rate of fuel-S into SO2 varied from 100% to 40% as equivalence ratios varied between 0.4 and 1.8, respectively. Atal et al. [17] and Levendis [18] researched the sulphur released during volatile combustion and char combustion under fuel-lean conditions. They proposed that almost all the sulphur released (90%–95%) could be oxidized to SO2 and a little part of sulphur was remained in the ash. About 57 wt % sulphur in the coal was released as volatile-SO2 during volatile combustion and about 33 wt % sulphur in the coal was released as char-SO2 during char combustion. In the experiments from Hu et al. [15], two peaks of SO2 emission was noticed in combustion process, so they proposed that there are two types of S in the coal: one is S contained in volatile and the other is S combined with fixed-C in char. Beer [19] and Chang et al. [20] considered that the decreasing of SO2 emission is caused by aluminosilicate components in ash. These components prefer sulphur retention. Therefore, the retention of sulphur in char or unburned coal may be another important reason for SO2 emission.
However, the experimental temperatures employed in many researches are not high enough to cover the real combustion environment temperature. The previous experiments and simulations about the oxy-fuel boilers (35 MW–820 MW) showed that the average gas temperature in the burner zone is about 1600 K–1800K, where the peak value even can reach 2000K [[21], [22], [23], [24], [25]]. The experiments focused on SO2 release at high combustion environment temperatures especially between 1573 K and 1873K have rarely been reported in current literatures. Additionally, the researches have confirmed that there are two types for SO2 release: volatile-SO2 and char-SO2. But the distribution of fuel-S in volatile and char is not confirmed. Whether the fractions of volatile-S and char-S are a constant or whether the fractions are affected by the combustion environment temperature needs to be determined. Because in SO2 simulation model from current commercial software [26], the fractions of fuel-S in volatile and char need to be specified artificially and they are supposed as different constant values at any temperature. If the relationship between volatile-S or char-S fraction and combustion temperature is established, a more practical and accurate simulation model of SO2 emission can be developed by considering a dynamical fraction distribution of volatile-S and char-S at different temperatures.
To improve the full understanding on SO2 release characteristics and contributions of volatile-S and char-S to fuel-SO2 release at different temperatures, it is necessary to study the effect of temperature on SO2 release characteristics and study the volatile combustion and char combustion individually. In this paper, the upper limit combustion environment temperature was expanded to 1873K and separated combustion mode was realized via a fixed-bed experimental platform. There are three objectives based on this expanded high temperature. One is to investigate the effect of high temperature on SO2 release characteristics in O2/CO2 atmosphere. Another one is to study the relative contribution of volatile-S and char-S to total fuel-SO2 release amount. At last, the effect of sulphur self-retention caused by ash on SO2 release at different temperature was studied by experiments and simulation.
Section snippets
Experimental setup and methods
All the experiments were conducted in a high-temperature fixed-bed setup. Based on this fixed-bed setup, many experiment researches have been conducted. Details about this setup can refer to our previous research work [27,28].
Two combustion modes were conducted in the experiments: separated combustion and coupled combustion of volatile and char in coal, as shown in Fig. 1. In coupled combustion, which is the same as conventional combustion mode, coal sample begins combustion in the O2/CO2
Effect of combustion environment temperature on SO2 release process
Fig. 2 shows SO2 release curves of coal sample with particle size of 60 μm at different combustion environment temperatures in coupled combustion. The SO2 release curves can demonstrate the whole SO2 release process from the beginning to the end of combustion. The SO2 release process at 1273 K presents a single-peak structure, in which the peak value is very low. As the temperature rises, this single-peak structure begins a transition to a double-peak structure. The second peak is distinct at
Conclusions
In this paper, the separated combustion and coupled combustion experiments were conducted in O2/CO2 atmosphere. The effects of combustion environment temperature and combustion mode on SO2 release were investigated. Eventually, the following conclusions are derived.
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At low temperatures, there is only one peak value caused by volatile combustion during SO2 release process. However, two peak values occur due to volatile combustion and char combustion at high temperatures. Peak values increase as
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgements
This work was supported by the National Key R&D Program of China (2018YFB0605301).
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