Inorganic Phase Transformation in Miscanthus Ash

Abstract

The aim of this work is to describe the inorganic phase transformation of Miscanthus × giganteus ash at a high temperature taking into account the effect of oxidizing/reducing atmospheres and different harvest periods. A double approach was used: thermodynamic calculations and experimental validation in laboratory devices. The samples were characterized by scanning electron microscopy−energy-dispersive X-ray spectroscopy (SEM−EDX) and X-ray diffraction (XRD). Below 800 °C, the inorganics are mainly present as salts [KCl, K 2 SO 4 , and Ca 3 (PO 4 ) 2 ], carbonates (CaCO 3 ), and SiO 2 , while at higher temperatures (>900 °C), the solid phase is enriched in CaSiO 3 and Ca 3 (PO 4 ) 2 . The ash starts to melt around 750 °C, and the liquid phase is composed of SiO 2 , K 2 O, and CaO. The main difference between oxidizing and reducing atmospheres is the form of sulfur. In an oxidizing atmosphere, K 2 SO 4 is formed, while in a reducing atmosphere, only traces of organically attached S were detected by SEM−EDX and most of the S has volatilized. The thermodynamic calculations were performed using the software package FactSage 6.4 with two different oxide databases: FToxid and GToxid. GToxid database predicts a higher solidus temperature (T s > 900 °C) but greater liquid/solid ratio than FToxid (T s ≈ 740 °C). Both databases predict the formation of K silicates or K−Ca (Mg) silicates below 800 °C, while the main solid phase observed by XRD was SiO 2 . Reasons are linked to uncertainties in the initial mass balance of ashes, database uncertainties, and limitations of the diffusion and chemical reaction of elements. The composition of the liquid phase is well-described in both databases, although GToxid gives somewhat closer results to the SEM−EDX analysis with the incorporation of Ca and Mg into the liquid phase.

Physicochemical characterization of miscanthus and its application in heavy metals removal from wastewaters

“…Figure 2b shows the XRD pattern of MA with diffraction line at 2h 5 21.6 that correspond to K 2 SO 4 . Also, the K 4 H 2 (CO 3 30.1, and 43.4, respectively [41]. It is obvious from Figure 2b that potassium hydrogen disilicate (KHSi 2 O 5 ) and potassium chloride (KCl) are most dominant phases in the MA sample [42].…”

“…2 was 77.1% which confirms the presence of the larger fraction of crystalline cellulose in the miscanthus sample. Also, the K 4 H 2 (CO 3 30.1, and 43.4, respectively [41]. The crystallite size calculated by Scherrer equation (Eq.…”

On the relevance of thermodynamics to predict the behaviour of inorganics during CO 2 gasification of willow wood

“…As for the solutions phases, two liquid solutions phases were considered: (i) SALTF, which describes liquid salts solutions (Na, K//F, Cl, CO3) with low melting points, and represents fly ashes (ii) SLAGA, which describes liquid silicates solutions (Si, K, Ca // O, S) with high melting points, and represents bottom ashes. As pointed out by Ma et al (2015) and Kaknics et al (2015), in the reducing atmosphere conditions of the gasification, most of the sulfur form a stable H2S gaseous compound, which leads to the absence of any liquid sulfate solution. To sum up, the final data selection includes 373 gaseous, 247 liquid and 1115 solid species, as well as 2 liquid solution phases.…”

“…A selection of the databases available within the FactSage package was made, using a combination of SGPS (for gas only), FACT PS, FToxid and FTsalt databases. Even if Konttinen et al (2013) or Kaknics et al (2015) used larger database sources, the review of Linberg et al (2013) gives support to this selection. As thousands of compounds and phases exist in the multicomponent system consisting of willow wood, a preliminary selection of the compounds which might form in gasification conditions was required: software limitations, computing and post processing times impose to take into account a maximum of 3,000 compounds.…”

“…Thousands of compounds exist in the multicomponent system composed of contaminated biomass. As evidenced by recent works (Froment et al, 2013, Konttinen et al, 2013, Kaknics et al, 2015, Ma et al, 2015, thermodynamic calculations in such systems face several challenges: database limitations (Lindberg et al, 2013), software capacities, large deviation from equilibrium. Several modeling options and databases sets are considered here, using the Gibbs energy minimization method.…”