• Energy Research

Solid biofuels from energy crops are widely considered as an attractive alternative for power production in dedicated combustion or cofiring applications. The aim of this work is to present fuel and ash characterization along with thermogravimetric investigations of pyrolysis and combustion behavior for five crops (cardoon, Miscanthus, Paulownia, willow, and poplar) compared with Greek lignite. Devolatilization is well modeled by three independent parallel reactions and reaches higher rates than lignite; char reactivity of woody biomass and Miscanthus was high, whereas cardoon char resembled more closely that of lignite. High potassium concentration and low melting temperatures were found for all biomass ashes; therefore, a severe/high slagging or agglomeration potential is expected. Overall, cardoon appears to be the most problematic of the investigated biomass fuels, due to high ash content, low ash melting temperature, a slower char combustion rate, and risk of chlorine corrosion; cofiring with lignite...

In this study, six high-Ca limestones and one dolomite from Germany, Greece, Italy, and Poland were tested for their CO2 uptake capacity during carbonation-calcination experiments in a TGA apparatus, as well as in a lab-scale atmospheric bubbling FB reactor. The calcium looping experiments were carried out both in the presence and absence of sulfur in gas phase, to study its likely inhibitory role to the CO2 penetration into the CaO particles. The mineralogy, microstructure, specific surface area, and pore size distribution of the fresh, sulfated/carbonated, and non-sulfated/carbonated sorbents were comparatively evaluated by means of X-Ray Diffraction (XRD) Spectroscopy, Energy Dispersive-Scanning Electron Microscopy (EDS-SEM), and N2-porosimetry, respectively. All samples were examined after five cycles of carbonation-calcination. In most sulfated samples, a shell of anhydrite (CaSO4) has been identified peripherally to the CaO particles, preventing part of their core from further carbonating. The macro-porosity (%) of sulfated samples is increased, compared to the non-sulfated ones, suggesting less sintering in the former, a fact also supported by their BET area findings. On the other hand, micro-porosity shows no clear tendency with sulfation. The loss of microporosity, that was in particular cases observed in the sulfated samples, is attributed to a drop in the associated conversion during carbonation. Overall, this work contains an integrated comparative characterization of the tested limestones, accompanied with suggestions for their optimum utilization in Ca-looping.

This study focuses on the optimization of an electric heater design for molten salt pre-heating in a supercritical CO2–molten-salt loop. The scope of the investigation is to analyze typical designs of similar components for identifying possible malfunctions and defining proper modifications in the geometry and operating conditions to address such technical issues and optimize the attained thermal efficiency. By performing computational fluid dynamics simulations for reference designs of such components, two particularities pertinent to the temperature distribution are identified as the most likely ones: the development of hot spots and thermal stratification. As a further step, new designs and operating conditions are proposed and their effects on eliminating the hot spots and stratification development phenomena are evaluated. It is shown that the homogeneous distribution of heat flux density across the heating elements is the most favorable option for avoiding the development of hot spots, while the mitigation of thermal stratification is possible through the development of turbulent flow. The proposed design and operating conditions are expected to facilitate the optimization of molten-salt electric heater operation and promote the development of next-generation molten-salt–supercritical-CO2 concentrating solar power plants.