High Iron and Calcium Coal Ash As the Oxygen Carrier for Chemical Looping Combustion

Abstract

The potential of Zhundong’s coal ash as a novel oxygen carrier (OC) in chemical-looping combustion (CLC) was evaluated in a thermogravimetric analysis (TGA) under both the CO/Ar and air atmospheres. A single reduction–oxidation cycle was carried out in the TGA reactor to determine the optimum reaction temperature, which was 1123 K. The cyclic stability and agglomeration tendency of ash also was evaluated by consecutive redox cycles. On the basis of the kinetic analysis, the apparent activation energy of ash was about 26–29 kJ/mol and the reduction process of ash was divided into three stages. The reaction control steps for these stages were gas diffusion, diffusion and kinetics, and kinetics, respectively. Besides, the reactions in the first and second stages corresponded to the reduction of Fe2O3 to Fe and CaSO4 to CaS. In the third stage, the reaction occurred between the CaS and CaSO4.The products of the equilibrium reaction between CO and ash were obtained using Factsage (software), while the transition products from the reduction of ash were determined by X-ray diffraction (XRD). The main reactants in ash were Fe2O3 and CaSO4. Because of sintering and particle agglomeration, the actual percentage of oxygen release (22%) was less than the theoretical value (26%). In addition, after some reactions CaSO4 was converted to MgOCa3O3Si2O4.

Kinetics of Coal Char Gasification with Fe-Based Oxygen Carriers under Pressured Conditions

“…Riley et al , successfully simulated the gasification process of coal char and the multistep reduction process of CaFe 2 O 4 and CuO–Fe 2 O 3 –Al 2 O 3 with its product syngas using the shrinking core model and the dual-interface particle model, which was limited by kinetic control. Lin et al used high iron and calcium coal ash as the OC to study the kinetics at different degrees of the reduction process. The effects of gas diffusion and kinetic control on the apparent reaction of the OC at different reduction stages were fitted using different kinetic mechanism models.…”

“…There are many factors that can affect the coal CLG process, such as catalysis, the pore structure of the solid reactant, the concentration of gasification agent, and the diffusion of the gas-phase product. ,,, Therefore, it is necessary to select appropriate basic models or modify them according to the specific investigation. According to existing literature reports, the Langmuir–Hinshelwood (L–H) model and n th-order exponential model ( n th model) are used to describe the influence of the pressure during the gasification process. ,− However, as a result of the heterogeneity of coal char, the gasification rate varies with the carbon conversion and the relationship between it and pressure can only be determined under a specific carbon conversion, which cannot express the influence of the pressure change on the whole gasification process .…”

“…There are many factors that can affect the coal CLG process, such as catalysis, the pore structure of the solid reactant, the concentration of gasification agent, and the diffusion of the gasphase product. 12,20,23,28 Therefore, it is necessary to select appropriate basic models or modify them according to the specific investigation. According to existing literature reports, the Langmuir−Hinshelwood (L−H) model and nth-order exponential model (nth model) are used to describe the influence of the pressure during the gasification process.…”

Study of NH3 Removal Based on Chemical-Looping Combustion

“…In the oxidation reactions between the reductive OCs and air, the reductive OCs are regenerated to the oxidative OCs with O 2 . 26 The significant difference between CLC and traditional combustion is that in CLC, the oxidation of fuel can be performed with oxidative OCs rather than molecular oxygen; thus, it is endowed with the characteristics of inherent CO 2 capture with almost zero energy-consumption, low NO x emissions, and high energy utilization efficiency. 27,28 In addition to CH 4 , CO, and H 2 , NH 3 , can also play the role of fuels in the reduction reaction.…”

“…There are two separate reactions in chemical-looping combustion (CLC). , In the reduction reactions between oxidative oxygen carriers (OCs) and hydrocarbon fuels, oxidative OCs are reduced to reductive OCs and hydrocarbon fuels are oxidized to H 2 O and CO 2 . In the oxidation reactions between the reductive OCs and air, the reductive OCs are regenerated to the oxidative OCs with O 2 . The significant difference between CLC and traditional combustion is that in CLC, the oxidation of fuel can be performed with oxidative OCs rather than molecular oxygen; thus, it is endowed with the characteristics of inherent CO 2 capture with almost zero energy-consumption, low NO x emissions, and high energy utilization efficiency. , …”