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CO2 and H2O have great influence on pyrolysis char structure and its reactivity. Particularly, the effect of CO2 and H2O on pyrolysis char kinetic parameters are unrevealed. Char structure and the corresponding combustion kinetic parameters play a very important role for designing and modeling oxy-fuel combustion. This work focuses on the effects of CO2 and H2O on pyrolysis gas, coal burnout degree, char structure, and corresponding combustion kinetic parameters. The volatile compositions were measured by gas chromatography. Coal burnout degree, char structure, and reactivity were measured by a drop tube furnace, Raman spectroscopy, scanning electron microscopy and thermogravimetric anaylsis, respectively. Our results indicate that a H2O-containing atmosphere results in decreasing char yield, developed and disordered char structure, and high reaction rate constant. All of these features are more obvious in a H2O-containing atmosphere than in a CO2-containing atmosphere. Moreover, a H2O-containing atmosphere reduces char–O2 reaction rate, as well as coal burnout degree in a low oxygen atmosphere at early and middle combustion stages (i.e., burnout degree <80%), but increases at the end stage of combustion (i.e., burnout degree >80%). Our investigation will give basic kinetic parameters and features of char properties obtained in CO2 and H2O atmospheres.

The physical and chemical properties of H2O (oxy-steam combustion) and N2 (air combustion) are different, which has influence on the char properties in oxy-steam combustion. Measuring in situ char (without any cooling) kinetic parameters is very important, especially at oxy-steam atmosphere. Both in situ and ex situ char properties were investigated in this paper. Ex situ char was obtained at different cooling rates. Char properties were measured by X-ray diffraction, H/C ratio, Raman spectrum, and specific surface area. The effect of steam on in situ and ex situ char combustion kinetics were determined. We found that the combustion reaction rates of different chars are in order of in situ > rapid cooling (103–104 K/s) > medium cooling (10–100 K/s) > slow cooling char (0.1 K/s). The rapid cooling char structure is disordered and has a high active surface area, which results in the high reactivity. In situ char has a shorter burn-out time, a faster combustion rate, and a higher reaction constant. For example, at 903 K, it appears the reaction rate of 0.048 s–1 for ex situ char (Vientiane coal char) in oxy-steam atmosphere (30% O2 + 15% H2O), while that of in situ char is 0.07 s–1. The reactivity of in situ char is higher than that of ex situ char. The activation energy of in situ char (e.g., 20.9 kJ/mol) is much lower than that of ex situ char (e.g., 82.24 kJ/mol) in oxy-steam atmosphere (e.g., 30% O2 + 10% H2O). The reaction reactivity of both ex situ and in situ char increase in oxy-steam atmosphere when compared with those from O2/CO2 combustion.

CaO-based sorbents are usually used for high-temperature postcombustion CO2 capture through cyclic carbonation–calcination reactions. However, the rapid decay of sorbent reactivity limits their application. In this study, three types of CO2 sorbent precursors, namely Ca­(CH3COO)2, Ca­(OH)2, and CaCO3, are used to produce initial CaO-based sorbents. Then, the initial sorbents are mixed with aluminate cement and organic fibers at an appropriate ratio to synthesize high-efficiency sorbents. Our results show that the cyclic performance of the sorbent that decomposed from Ca­(CH3COO)2 is better than that decomposed from Ca­(OH)2 and CaCO3. It is mainly because the sorbent pore structure becomes much developed after Ca­(CH3COO)2 decomposition. Furthermore, when mixed with aluminate cement, the CaO-based sorbent achieves a good performance for CO2 capture because of the formation of the sorbent skeleton Ca12Al14O33. For example, when the mass ratio of Ca­(OH)2 to aluminate cement is set as 1:1, CaO conversion achieves 56% even after 20 cycles. To further improve the composite sorbent pore structure and its performance, pore-forming method is also put forward by mixing an organic fiber (cotton fiber, palm fiber, or microcrystalline cellulose) with the composite sorbent before decomposition. The result shows that the sorbent pore-formed by cotton fiber achieves a good performance with CaO conversion of 80% after 20 cycles, whereas the CaO conversion of the sorbent pore-formed by palm fiber shows only a little enhancement. On the contrary, the reactivity and stability of the sorbent pore-formed by microcrystalline cellulose show a marked decline.

As one of the significant properties in char combustion, char reactivity has been widely investigated. However, the previous deactivation kinetic models are unable to predict the char reactivity for different kinds of coals because of (i) the complex components of different coals; (ii) the additional holding process during heat treatment. In this study, the char reactivity under different heating rates was studied by a self-developed two-step fluidized bed reactor. The char deactivation model is proposed and applied in Carbon Burnout Kinetics (CBK). The results show that char tends to a higher reactivity under a high heating rate, and increasing holding time at the terminal temperature would further decrease char reactivity. Predicting results of char deactivation model are close to experiment results, indicating that this model has a good predicting ability. After applying this model to CBK, modified-CBK (M-CBK) has a better predicting performance for different kinds of coals in char combustion.