Calcium Doping Facilitates Water Dissociation in Magnesium Oxide

Abstract

is the most ubiquitously used form of energy, including in electricity generation, power plants, or industry. [5] In this context, in 2011, the International Energy Agency estimated in a report entitled "Solutions for a low-carbon energy future" that global energy loss through waste heat accounts for two-third of the overall energy production. [6] Therefore, methods suitable for reduction of waste heat or reutilization thereof could contribute significantly to a sustainable energy management. [7][8][9] Among the different approaches to this challenge, thermochemical energy storage (TCES) is a highly appealing concept, since by means of a reversible chemical reaction, a suitable material energy is converted from thermal to chemical energy. The charged material may be stored for any period of time without further losses or additional infrastructure (such as insulation), until, by addition of the proper reactant, the discharging reaction, releasing the stored energy in form of heat, is initiated. [2,[10][11][12] The economic benefit of implementing TCES cycles in the energy management was demonstrated by several economic feasibility studies, illustrating that, e.g., with the waste heat from Lenzing AG, [13] Calcium doping of magnesium oxide results in significantly increased water dissociation rates, thus enhancing both hydration rate and reaction completeness of hydration compared to pure MgO. For a series of mixed magnesiumcalcium oxides (Mg x Ca 1−x O) with varying Ca contents between 0% and 40%, the material of a composition Mg 0.9 Ca 0.1 O shows the fastest rehydration, transforming completely within 80 min to the mixed hydroxide. In consecutive dehydration/rehydration cycles, reasonable cycle stability is found. A "regeneration" of the aged material (reactivity reduced by excessive cycling) in liquid water re-establishes the initial rehydration reactivity. Density functional theory calculations support the experimental findings, confirming that calcium doping can reduce the energy of the rate limiting water dissociation reaction, exploiting both electronic and steric (size) effects. Three major requirements for future sustainable energy management have been established including reduction of greenhouse gas emissions, promotion and increased use of renewable energy sources, and enhancing the efficiency of energy use. [1,2] Whereas the first two issues are widely discussed and the subject of international treaties such as the Kyoto protocol [3] and the Paris agreement, [4] awareness of the poor overall efficiency of today's energy economy is rather limited. Heat

The Potential Use of Fly Ash from the Pulp and Paper Industry as Thermochemical Energy and CO2 Storage Material

“…In general, raw materials have drawbacks, such as a low cycle stability [11], a low conversion rate [12], a high sintering effect [13], and a high cost. To overcome these drawbacks, either doped TCES materials or mixture of a TCES material with other suitable inert or TCES materials have been created [14][15][16][17][18]. For instance, the cycling stability of doped-CaCO 3 with SiO 2 was improved as reported by Chen et al [14], and doping MgO by 10% CaO has enhanced the hydration rate of MgO [15].…”

Comparing Fly Ash Samples from Different Types of Incinerators for Their Potential as Storage Materials for Thermochemical Energy and CO2

“…Nonetheless, the screening of TCES materials has focused only on raw or doped materials. Overall, raw TCES materials possess drawbacks, such as sintering [21,22,23], slow conversion rates [22], low cycling stability [17], low thermal conductivity [17], and high costs [17,24], and researchers have attempted to overcome these drawbacks by doping or mixing raw TCES materials with other TCES/sensible heat storage materials [24,25,26,27,28]. For instance, the cycling stability of a calcium oxide (CaO)/CaCO 3 system decreased with the number of cycles because the carbonation of CaO to form CaCO 3 was significantly slow and sintering of material occurred at high temperatures [29].…”

Synthesis of Me Doped Mg(OH)2 Materials for Thermochemical Heat Storage

“…Junichi et al developed a 6.8 wt.% LiCl/Mg(OH) 2 system that drops the dehydration temperature of magnesium hydroxide, from 277 °C to 233 °C, being able to store 816 MJ/m 3 volumetric heat storage capacity [ 19 ]. Muller et al found that calcium doping of magnesium oxide results in significantly increased water dissociation rates, thus enhancing both hydration rate and reaction completeness of hydration compared to pure MgO [ 20 ]. Zamengo et al prepared a Mg(OH) 2 /MgO system supported on expanded graphite.…”