Current advances and future prospects of in-situ desulfurization processes in oxy-fuel combustion reactors.

Oxy-fuel circulating fluidized bed combustion is known as one of the most potent fuel combustion technologies that capture ultra-low greenhouse gases and pollutant emissions. While many investigations have been conducted for carbon capturing, the associated in-situ desulfurization process using calcium-based sorbents should also be underlined. This paper critically reviews the effects of changes in the operating environment on in-situ desulfurization processes compared to conventional air combustion. A comprehensive understanding of the process, encompassing hydrodynamic, physical and chemical aspects can be a guideline for designing the oxy-fuel combustion process with effective sulfur removal, potentially eliminating the need of a flue gas desulfurization unit. Results from thermogravimetric analyzers and morphological changes of calcium-based materials were presented to offer an insight into the sulfation mechanisms involved in the oxy-fuel circulating fluidized beds. Recently findings suggested that in-situ direct desulfurization is influenced not only by the desulfurization kinetics but also by the fluidization characteristics of calcium-based materials. Therefore, a complex reaction analysis that incorporated oxy-combustion reactions, computational fluid dynamics modeling, in-situ desulfurization reaction models and particle behavior can provide a thorough understanding of desulfurization processes across the reactor. Meanwhile, machine learning as a robust tool to predict desulfurization efficiency and improve operational flexibility should be applied with consideration of environmental improvement and economic feasibility.

In-situ desulfurization using porous Ca-based materials for the oxy-CFB process: A computational study.

Within circulating fluidized bed (CFB) processes, gas and solid behaviors are mutually affected by operating conditions. Therefore, understanding the behaviors of gas and solid materials inside CFB processes is required for designing and operating those processes. In addition, in order to minimize the environmental impact, modeling to reduce pollutants such as SOx emitted from those processes is essential, and simulation reproduction is necessary for optimization, but little is known. In this study, the gas and solid behaviors in a pilot-scale circulating fluidized bed combustor were investigated by using computational particle fluid dynamics (CPFD) numerical simulation based on the multiphase particle-in-cell (MP-PIC) method under oxy-fuel combustion conditions. In particular, the combustion and in-situ desulfurization reactions simultaneously were considered in this CPFD model. Effect of fluidization number (ULS/Umf) was investigated through the comparison of particle circulation rates with regards to the loop seal flux plane and bed height in the standpipe. In addition, the effects of parameters (temperature, Ca/S molar ratio, and particle size distribution), sensitive indicators for the desulfurization efficiency of limestone, were confirmed. Based on the cycle of the thermodynamic equilibrium curve of limestone, it is suggested that direct and indirect desulfurization occur simultaneously under different operating conditions in CFB, creating an environment in which various reactions other than desulfurization can occur. Addition of the reaction equations (i.e., porosity, diffusion) to the established simple model minimizes uncertainty in the results. Furthermore, the model can be utilized to optimize in-situ desulfurization under oxy-CFB operating conditions.

Recent advances of hybrid solar - Biomass thermo-chemical conversion systems.

Solar biomass hybridization is a promising energy technique for efficient utilization while mitigating the disadvantages associated with both biomass and solar energy source. In conventional concentrating solar power (CSP) systems, the contribution of solar energy is relatively low, merely supplementing the system with low/medium temperature air/steam. This paper aimed to emphasize the improvement of solar heat share, particularly in the topping cycle of the hybrid system. The solar aided processes, either directly generating superheated air/steam or direct gasification are thermodynamically favorable at very high temperatures, in excess of 800 °C. Unfortunately, this temperature is unattainable in conventional CSP systems using molten salt. Accordingly, the integration of solar power tower (SPT) with solid particle fluidized system in a beam down configuration has been proposed for the hybrid solar-biomass systems. Studies of such integration system presented challenges in terms of operating temperature, continuous supply/syngas production and scaling of reactor, particularly for circulating fluidized bed (CFB). The selection of solid particle and gas flow rate are among the governing parameters for high operating temperature and effective utilization of solar heat. The development of high temperature hybrid solar-biomass system is anticipated for higher solar-to-fuel conversion efficiencies, minimizing the direct combustion of biomass and reduce the emission of greenhouse gas (GHG) emissions.