Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Effects of Alkali and Alkaline Earth Metallic Species and Chemical Structure on Nascent Char−O2 Reactivity Lei Zhang, Tingting Li, Shuai Wang, Li Dong, and Chun-Zhu Li* Fuels and Energy Technology Institute, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia ABSTRACT: Factors influencing the nascent char−O2 reactivity was investigated in this study from the aspects of alkali and alkaline earth metallic species and char chemical structure. Char samples were prepared from the fast pyrolysis of two low-rank coals (Loy Yang brown coal and Collie sub-bituminous coal) in a wire-mesh reactor at temperature ranging from 600 to 1200 °C with holding time up to 5 s. The concentrations of alkali and alkaline earth metallic species in char were determined by inductively coupled plasma-optical emission spectroscopy. The char chemical structure was characterized by Fourier transformRaman spectroscopy. Our results show that the char−O2 reactivity is greatly related to the Raman sensitive O-containing functional groups at the initial stage of char formation (short residence time at 600 °C during pyrolysis). As char pyrolysis proceeded to higher temperature, the loss of O-containing functional groups and the condensation of aromatic ring systems play a key role in the decreasing magnitude of char−O2 reactivity. The chemical structure of 1200 °C pyrolysis char is relatively stable, thereby posing limited effect on the changes in char−O2 reactivity. For Loy Yang brown coal, the volatilization of alkali and alkaline earth metallic species during pyrolysis at 1200 °C became the dominant reason for the observed decreases in char−O2 reactivity.
1. INTRODUCTION The behavior of alkali and alkaline earth metallic (AAEM) species is one of the central topics about the utilization of biomass1−3 and coals4−6 during gasification and combustion. On one hand, AAEM species are notorious for causing ashrelated problems such as fouling and slagging in the pulverizedfuel combustion systems and erosion/corrosion of turbine blades. On the other hand, AAEM species can behave as a good catalyst, which enhances the gasification/combustion reactivity of char. Therefore, it is essential to investigate the volatilization of AAEM species and the catalytic effect of AAEM species on gasification reactivity. The gasification of coal is a complicated process. When coal particles are fed into a gasifier, they undergo dewatering and pyrolysis first. In the step of pyrolysis, primary volatiles are immediately formed and explosively released, leaving the remaining solid residue as nascent char. The nascent char still keeps high reactivity7 and determines the overall gasification reactivity. However, the following reactions, such as the interaction/interparticle reaction with volatiles (especially with H radicals8), the condensation of aromatic ring systems, and the selective consumption of active portion of char by gasification agents,9,10 would bring about the rapid deactivation of the nascent char and slow down the gasification rate. It is well-known that the rate-limiting step of gasification is char gasification (especially “aged” char).11 Previous studies have mainly focused on the factors influencing the reactivity of the char that underwent volatile−char interaction and extended (several minutes) gasification,5,12−17 little efforts have been put into the investigation on the factors influencing the char reactivity during the formation and transformation of nascent char. “Nascent char” is defined as the char that experienced relatively low extent of secondary cracking or other reactions in this study. © XXXX American Chemical Society
The evolution of char structure during the gasification of two nascent chars (produced at 600 °C with 0 s holding time) in O2 was investigated in our previous work.18 The reasons, responsible for the high reactivity of nascent char during gasification in O2, were discussed. Our findings in ref 18 suggested that there were cooperating effects between AAEM species and nascent char structure on char−O2 reactivity. For Loy Yang coal char, the reactive nascent char structures played a vital role in the initial stage of char−O2 reaction. With increasing residence time in O2, AAEM species played a dominant role in the char−O2 reaction due to the consumption of reactive char structures and the accumulation of AAEM species. For Collie coal char, AAEM species showed the negligible effect on the nascent char−O2 reactivity for the char prepared at 600 °C. Nevertheless, with the fast transformation of nascent char structure during pyrolysis from 600 to 1200 °C,19,20 the roles of AAEM species and char structure on char− O2 reactivity is still unclear, which arouse the motivation of this study. Wire-mesh reactor (WMR) is well-known in that it can achieve very short holding time (increments of 10 ms), fast heating rate (≥1000 °C s−1), and minimized volatile−char interaction, which is very suitable for nascent char preparation.21−26 In this study, two low-rank coals were fast pyrolyzed in a WMR at temperatures ranging from 600 to 1200 °C. The concentrations of AAEM species, the chemical structure, and the specific reactivity of char samples were measured. The effects of AAEM species concentrations and char structural changes on the nascent char−O2 reactivity during the fast pyrolysis at different temperature will be discussed in this study. Received: October 9, 2017 Revised: November 26, 2017
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DOI: 10.1021/acs.energyfuels.7b03022 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Our results show that the factor, which determine the char−O2 reactivity, would significantly change during the formation and transformation of nascent char structures within the temperature range investigated in this study.
2. EXPERIMENTAL SECTION 2.1. Coal Samples and Pyrolysis Experiments. Western Australia Collie sub-bituminous coal and Victorian Loy Yang brown coal were used in this study. The preparation and the properties of coal samples have been described in ref 19. The nascent chars, which are prepared for the measurement of AAEM concentration, chemical structure, and char−O2 reactivity, were produced under the same experimental conditions as those in ref 20. Briefly, the heating rate was chosen as 1000 K s−1 to achieve fast pyrolysis. Pyrolysis temperature was selected as 600, 800, 1000, and 1200 °C. The holding time at peak temperature varied from 0 to 5 s. All the experiments were carried out under atmospheric pressure. 2.2. Quantification of AAEM Species in Coal/Char Samples. The quantification of AAEM species mainly followed the method in ref 27. The procedure consisted of three steps: ashing, digestion, and quantification. All chars produced from one experiment, ∼4 mg for Loy Yang coal char or ∼6 mg for Collie coal char, was put in a platinum crucible. The crucible with char sample was placed in a muffle furnace in air, and the heating rate was very low to minimize the possibility of ignition. The heating program was set as follows. First, coal/char sample was heated from room temperature to 300 °C at the heating rate of 5 °C min−1. Second, the coal/char sample was further heated from 300 to 375 °C at 1 °C min−1 followed by 10 min holding at 375 °C, the temperature range in which a high possibility of nascent char ignition existed. Third, the temperature was increased to 415 at 5 °C min−1 with 10 min holding at 415 °C. Finally, the sample was heated to 600 °C at 5 °C min−1 with 30 min holding at 600 °C. Subsequently, the ash was naturally cooled down to room temperature. The platinum crucible together with the ash was placed in a Teflon beaker. A mixed solution of HF and HNO3 (1:1 vol %) was transferred into the beaker. Six milliliters of the acid mixture was used to guarantee that the crucible was fully submerged in the acid. The covered Teflon beaker was then placed on a hot plate at ∼80 °C for 16 h. After that, the lid on the beaker was removed to start the evaporation of acids. The residue was washed by 2% HNO3 and transitioned to a 10 mL vial. The AAEM concentration in the solution was then quantified by a PerkinElmer Optima 7300 DV inductively coupled plasma optical emission spectrometer (ICP-OES). As the concentration of K in Loy Yang raw coal is too low to be quantified accurately,27 the concentrations of Na, Mg, and Ca in Loy Yang coal and the concentrations of Na, Mg, Ca, and K in Collie coal will be used herein for the discussion on the concentration of AAEM species and their effects on nascent char−O2 reactivity. 2.3. Characterization of Char−O 2 Reactivity. Char−O 2 reactivity was measured in air by a PerkinElmer Pyris 1 thermogravimetric analyzer (TGA). The peak temperature was chosen as 370 °C in TGA for the char pyrolyzed at 600/800 °C in WMR and 400 °C in TGA for the char pyrolyzed at 1000/1200 °C in WMR. The selection of reactivity measurement temperature is mainly considered from two aspects, limiting the reactivity measurement within a reasonable period of time and avoiding any ignition. 400 °C is a typical temperature for reactivity measurement that we use in our studies, which is suitable in this study for the chars produced at ≥1000 °C. However, ignition was observed at 400 °C in TGA for the nascent chars produced at the temperature ≤800 °C; hence, 370 °C was selected for reactivity measurement for these chars. Since nascent char is very reactive, the char was sealed and stored in the fridge until required for further analysis. The oxidation of the char has been minimized. The repeatability of nascent char−O2 reactivity with different lengths of storage time was also investigated and is shown in Figure 1. It is clearly seen that, for the char sample that was picked at random, the char−O2 reactivity with different storage times almost overlapped together. The results indicate that there is little effect of the
Figure 1. Effect of residence time in ambient air on the nascent char reactivity. Char was produced by the pyrolysis of Loy Yang coal at 600 °C with 1 s holding. Char was prepared on the first day shown in the figure. storage time on nascent char−O2 reactivity. Further details can be found in ref 28. 2.4. Characterization of Char Structure. Fourier transform (FT)-Raman spectroscopy was applied in this study to characterize the chemical structure of nascent char samples; details are available elsewhere20 and some data are replotted here. The effects of the changes in aromatic ring systems (represented by I(Gr+Gl+Vr)/ID) and the O-containing functional groups (represented by total Raman area) on char−O2 reactivity were investigated.
3. RESULTS AND DISCUSSION 3.1. Concentrations of AAEM Species as Functions of Char Yield for Two Coals. The concentrations of AAEM species for Loy Yang coal and Collie coal as functions of char yield at temperatures from 600 to 1200 °C and holding time from 0 to 5 s are illustrated in Figures 2 and 3, respectively. For Loy Yang coal (shown in Figure 2), with increasing pyrolysis temperature from 600 to 1000 °C, the concentrations of AAEM species in char gradually increased. With further heating up to 1200 °C, a large decrease in the concentration of Na or Mg was observed within 5 s holding. As pyrolysis proceeded, the decrease in the AAEM concentrations indicated that a large amount of Na and Mg volatilized, which is in agreement with previous work.27 In comparison, no obvious decrease in the Ca concentration was noticed, reflecting a more stable chemical form of Ca than that of Na/Mg. It is worth noting that the AAEM concentrations increased obviously from 600 to 1000 °C while changing little with residence time (up to 5 s) at each temperature. Figure 3 displays the changes in AAEM concentrations for Collie coal at different pyrolysis temperatures. The concentrations of Na, K, Ca, and Mg all increased with increasing pyrolysis temperature. Similar to Loy Yang coal, the AAEM concentrations increased more relative to the increase in pyrolysis temperature rather than the residence time (up to 5 s) at each temperature. 3.2. Effects of AAEM Concentration and Char Structure on Nascent Char−O2 Reactivity. 3.2.1. Char− O2 Reactivity of Loy Yang Coal. According to a previous study,27 minimal amounts of AAEM species would volatilize during the slow oxidation of coal/char in O2 at temperature ≤600 °C. Therefore, no volatilization of AAEM species would take place in the process of char−O2 reactivity measurement, where the temperature is only at ≤400 °C in this study. As the amount of AAEM species is constant, the concentration of B
DOI: 10.1021/acs.energyfuels.7b03022 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
the Raman data that represent the char structure were shown in Figure 5. The figure was redrawn based on the data shown in ref 20. As displayed in Figure 4a, the specific reactivity changed little within 5 s holding at 600 °C, while a large evolution of aromatic ring systems was observed during the initial 5 s holding at 600 °C (I(Gr+Vl+Vr)/ID decreased from ∼3.1 to ∼2.6 in Figure 5). This decrease in the ratio of small to large aromatic rings can be caused by the condensation of aromatic rings and/or the decomposition of relatively small aromatic rings. As there was ∼5% decrease in char yield20 for 5 s holding at 600 °C, this reduction of the ratio was more likely due to the loss of relatively small aromatic rings. This implies that the changes in aromatic ring systems during pyrolysis at 600 °C show negligible effect on the char−O2 reactivity determined at 370 °C. The O-containing functional groups, as the other important parameter reflecting the char structural evolution that was detected by FT-Raman spectroscopy, are very reactive structures. In Figure 5, little changes in O-containing functional groups (in terms of total Raman area) were observed within 5 s holding at 600 °C. This is in agreement with little changes in the specific reactivity (shown in Figure 4). Moreover, the negligible changes in char−O2 reactivity also reveal that the short residence time at 600 °C does not change the dispersion of AAEM species and the chemical nature of AAEM−char matrix remarkably.5 In Figure 4b, at 800 °C, a large decrease in char−O2 reactivity occurred during the initial 1 s holding at the same AAEM concentration level (Figure 2). This reveals the crucial effect of char structure on the specific reactivity. In Figure 5, Loy Yang coal started to have significant aromatic ring condensation and great loss of O-containing functional groups at 800 °C within 1 s, leading to a notable effect on the char−O2 reactivity. The increase in char−O2 reactivity at the char conversion levels ranging from 40% to 80% was almost linear with the total AAEM concentration in char, indicating that the gasification rate was largely associated with the concentration of AAEM species in char within this conversion range. However, the decrease in the magnitude of char−O2 reactivity at the same AAEM concentration with increasing holding time reveals the important role of char structure. It is believed that with the increasing stability of char matrix, the bond between AAEM− char matrix turned out to be more stable and correspondingly decreased the gasification rate. Figure 4c shows the changes in char−O2 reactivity during holding at 1000 °C. As mentioned in section 2.3, a lower temperature (370 °C) was chosen for the reactivity measurement for the nascent chars produced at 600/800 °C in TGA, as the char would ignite at 400 °C. While no ignition of chars produced at 1000 °C was observed during the reactivity measurement at 400 °C in TGA, it is fair to say the magnitude of char−O2 reactivity of the chars produced at 1000 °C must be lower than that of the chars produced at 600/800 °C. The significant loss of O-containing functional groups and the condensation of aromatic ring systems during heating from 800 to 1000 °C (shown in Figure 5) would be the main reasons responsible for this obvious deactivation in char−O2 reactivity. During 5 s holding at 1000 °C, the char−O2 reactivity changed little within initial 60% char conversion level. Compared to 600 and 800 °C chars, the initial high reactivity at
