Waste Heat Recovery from Iron Production by Using Magnesium Oxide/Water Chemical Heat Pump as Thermal Energy Storage

Abstract

A heat recovery system based on thermal energy storage from the iron-making process at medium temperature range (200–300°C) is presented. For an efficient waste heat recovery system the selection of suitable thermal energy storage material is essential. Accordingly, a new candidate for a chemical heat storage material used in a magnesium oxide/water chemical heat pump at medium-temperature was developed in this study. The new composite, named EML, was fabricated by mixing pure magnesium hydroxide with lithium bromide and expanded graphite, which are employed as reactivity and heat transfer enhancers, respectively. The effects of mass mixing ratios w of EG to Mg(OH)₂ on dehydration and hydration were investigated by a thermogravimetric (TG) method, with the result that the w of 0.83 was the optimal mass mixing ratio for the EML composite. Thereby the heat output capacities of the EML composite (w = 0.83) were evaluated with varying reaction vapor pressure and hydration temperature. Heat output capacity per unit initial weight of the EML composite (w = 0.83) was calculated as 1168.7 kJ kg_EML^–1 at a hydration temperature of 110°C and reaction vapor pressure of 57.8 kPa. This value was 1.2 times higher than the corresponding heat output capacity of pure Mg(OH)₂ powder (958.5 kJ kg_Mg(OH)2^-1). This result showed that the EML composite has sufficient heat output capacity and mold-ability provided by EG for practical use in a heat exchange reactor. Thus, this composite could potentially be used as chemical heat storage materials for thermal heat storage.