Thermal Stability of Calcium Oxalates from CO2 Sequestration for Storage Purposes: An In-Situ HT-XRPD and TGA Combined Study

Mineralization of SF6 and NF3 fluorinated compounds for greenhouse gas abatement by oxalates

Fluorinated compounds (FCs) such as sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3) have garnered attention due to their environmental impact. This study investigates the mineralization and removal of two potent FCs: SF6 and NF3. The results confirm that utilizing various oxalate salts leads to the formation of corresponding metallic fluorides: lithium fluoride (LiF), sodium fluoride (NaF), and potassium fluoride (KF), validating the occurrence of mineralization reactions. Among the oxalate salts, sodium oxalate demonstrates the highest mineralization efficiency in both SF6 and NF3 removal. Real-time Fourier transform infrared spectroscopy (FT-IR) gas-phase analysis confirms rapid and complete gas removal within a short reaction time using the selected oxalate salts. Meticulous mass balance calculations revealed that oxalates (LiF, NaF, and KF) yielded sulfur (S) at rates of 92.09%, 91.85%, and 84.98% following SF6 mineralization. Additionally, the conversion rates of oxalates to the corresponding metallic fluorides (LiF, NaF, and KF) after SF6 mineralization were 98.18%, 95.82%, and 95.21%, respectively. Similarly, after NF3 mineralization, these conversion rates stood at 92.18%, 90.67%, and 90.02%, respectively. The removal efficiencies for SF6 (1000 ppm) were 4.98, 12.01, and 7.23 L/g, while those for NF3 (1000 ppm) were 14.1, 12.6, and 11.7 L/g, respectively. Notably, sodium oxalate exhibits superior effectiveness, achieving 100% SF6 conversion within 30 min and 100% NF3 conversion within 50 min. This work underscores the potential of oxalate mineralization as a promising strategy for efficient and rapid removal of potent fluorinated compounds, paving the way for environmentally benign FC remediation techniques with broader implications for sustainable gas treatment technologies.

Complex Study of Straw Suitability for the Production of Nonindustrial Straw Pellets

Nonindustrial straw pellets should comply with limitations on the content of ash, chlorine, nitrogen, sulfur, and heavy metals, and have a high melting temperature of ash. To produce such pellets, the properties of straw can be improved by leaching. In known papers, the completion of chlorine washing-out was not controlled. Aims of the paper were to study ash solubility at leaching of straw until completion of chlorine removal and to make a conclusion on studied straw suitability for the production of nonindustrial pellets. Aims were achieved by straw soaking with heating to 100 °C and subsequent plug flow flashing with control of leaching completion by the absence of chlorine in leachate; studying the ash, chlorine, nitrogen, sulfur, and heavy metals content of straw; studying the thermal behavior of ash at heating; determining the initial deformation temperature (IDT) of ash; and comparing the properties of original and leached straw with the specification of straw pellets. Straw leaching until completion of chlorine washing-out provided decreasing chlorine, nitrogen, and sulfur contents below limitations, and the ash content decreased from 7.15 to 3.93% at water leaching to 4.29% at leaching with a 10% solution of acetic acid. In the ternary diagram, the composition of straw ash shifted from a zone of low melting eutectics to zones of high-melting tridymite and cristobalite. The IDT of the original straw ash was 847, 1250 °C after water leaching, and above 1275 °C after leaching with an acetic acid solution. Monitoring the absence of chlorine in the leaching liquid can be applied as a control parameter for straw leaching completion. The original straw was not suitable for the production of nonindustrial pellets because of the high contents of Cl, S, and Cr and the low IDT of ash. All indexes of straw were improved due to leaching, but the Cr content was above limitation. Producers of pellets need to assess straw suitability as to heavy metal content both in the original and leached states.