| 1. |
- Bu, C. S., et al.
(författare)
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Iron based oxygen-carrier-aided oxy-fuel combustion of coal char: Reactivity and oxygen transfer mechanism
- 2023
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 333
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Tidskriftsartikel (refereegranskat)abstract
- Adding an active oxygen carrier to the inert bed material may favor the oxygen distribution in fluidized bed (FB) combustors. This so-called method “Oxygen-carrier-aided combustion, OCAC” has been applied recently to air combustion in FB boilers, but could also be potentially interesting in oxy-fuel combustion. Here we analyze OCAC in oxy-fuel combustion by experimental determination of the reactivity of char from three coals (lignite, bituminous and anthracite coal) using three oxygen carriers (OC, analytical reagent Fe2O3, hematite, and steel slag). The tests were conducted in a micro thermo-gravimetric analyzer (micro-TGA) and the combustion kinetics was determined for the nine combinations of char and OC. Additional tests were conducted in a macro-TGA i.e., much sample in a big ceramic crucible, to evaluate the phase composition and crystal quality using X-ray diffraction (XRD) to the combustion residues collected at various degrees of carbon conversion (0.2, 0.5, and 0.8). The results show that the burnout temperature decreases with OCs as the combustion rate increases, while the ignition temperature hardly changes. The enhancement of the rate of char combustion by OCs is in the following order: steel slag < hematite < AR-Fe2O3. The lower the fuel rank, the stronger the effect of the oxygen carrier. The combustion kinetics of chars using OC has a higher activation energy and pre-exponential factor than pure char. The enlarged lattice constant of Fe2O3 measured by XRD during the char combustion indicates that the redox reactions of Fe2O3 accelerate the oxidization of char by enhancing the rate of oxygen transport to the carbon surface.
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| 2. |
- Bu, C. S., et al.
(författare)
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The effect of H2O on the oxy-fuel combustion of a bituminous coal char particle in a fluidized bed: Experiment and modeling
- 2020
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Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 218, s. 42-56
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Tidskriftsartikel (refereegranskat)abstract
- Oxy-fuel fluidized bed (FB) combustion is considered one of the promising ways to control CO2 emission from coal-fired power plants. The effect of H2O on the char conversion during wet flue-gas recycle, in which the H2O concentration could be around 40%, is still not well understood. To this end, experiments and modeling were performed in this work. Combustion tests with bituminous coal char were carried out in an electrically heated fluidized bed in O2/CO2, O2/H2O and O2/CO2/H2O for various O2, CO2 and H2O concentrations at the bed temperature of 850 °C. At the same time, the influence of the bed temperature and the char size on char combustion was investigated in O2/CO2/H2O atmosphere. A thermocouple was inserted into the center of the char particle to measure the particle temperature, from which the char combustion characteristics were determined and analyzed. The results indicate that the participation of H2O in the combustion atmosphere enhances the carbon conversion, and it also reduces the particle temperature. A transient char-particle conversion model, taking into account heat and mass transfer from the bed to the particle and heterogeneous combustion and gasification of char, was developed to quantitatively examine the role of H2O. The model shows a good ability to predict the measured char-temperature history. Simulations were carried out to establish the role of H2O in O2/H2O and O2/CO2/H2O as in the FB experiments. The model was used to analyze the peak temperature and the burnout time of a char particle, as well as the relative contributions to the consumption of the carbon in the char by O2 (combustion), and CO2 and H2O (gasification). The results indicate that the endothermic char-H2O reaction is the main reason for the prolongation of the burnout time of char and the decrease in the particle temperature in O2/CO2/H2O as compared in O2/CO2. During wet flue-gas recycle, char-O2 still accounts for a major part of the total carbon consumption, but the contribution of char-H2O to the overall carbon consumption increases with the H2O concentration and cannot be ignored (i.e. when the H2O concentration attains 30%, the contribution of the char-H2O reaction to the overall carbon consumption is 14%). However, the contribution of char-CO2 to the char conversion is limited.
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| 3. |
- Wang, Wenkang, et al.
(författare)
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O2/CO2 and O2/N2 combustion of bituminous char particles in a bubbling fluidized bed under simulated combustor conditions
- 2018
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 336, s. 74-81
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Tidskriftsartikel (refereegranskat)abstract
- Most of the time char particles in commercial FB units encounter considerably lower oxygen co ncentrations than that in the entrance, say between 2% and 10%, and the effects of the oxy-fuel atmosphere on the conversion history of char particles need to be clearly treated under these conditions. In the present work, an experimental study of combustion of bituminous coal char was carried out in a laboratory FB in O 2 /CO 2 and O 2 /N 2 atmospheres under simulated FB combustor conditions at O 2 concentrations of 4–10% v/v, bed temperatures of 800–900 °C and char particle sizes of 2–8 mm by continuously measuring the concentrations of O 2 and CO in the flue gas. The results indicate that the conversion of char is controlled by diffusion of O 2 in the boundary layer of the particle in O 2 /CO 2 and O 2 /N 2 environments and that the gasification of char in O 2 /CO 2 is limited by chemical kinetics. A char conversion model, taking into account the mass transfer from the bed to the particle and the gasification kinetics of char, was built, based on the experimental results and the intrinsic reactivity of char obtained in TGA tests. Simulations, carried out under the same conditions as in the FB experiments, give good agreement in terms of burnout time and instantaneous reactivity during the conversion of single particles. Simulated results prove that the low diffusivity of O 2 in CO 2 is the main reason for the decreased reaction rate of the char particle in O 2 /CO 2 compared with O 2 /N 2 . The contribution of gasification to the consumption of char is more notable at high bed temperature (900 °C) and coarse particles (8 mm), particularly at lower oxygen concentration (4–6% v/v).
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