studies on thermal decomposition of calcium carbonate /calcium hydroxide for energy storage

Abstract

Alternate energy sources are essential to meet the present?day energy demands. Non?conventional energy resources may substitute for natural sources to some extent and they alleviate discomfortable thoughts about the impending energy crisis. To use this substitute to its full potential, an efficient and economical energy storage system is desirable. For long?term energy storage, thermochemical energy systems look very prospective with their high energy densities. Thermochemical energy storage is based on the principle of reciprocal repetition of endothermic and exothermic chemical reactions. Hydroxides and carbonates of alkaline earth metals appear to be promising with their property of non?toxicity and bulk availability at low costs. Nevertheless, the decomposition temperatures of these materials are high and hence heat losses to surroundings could be considerable. Reducing the decomposition temperatures is therefore very essential. To utilize these materials effectively for energy storage, understanding of the kinetics becomes very important and in this study thermal decomposition of calcium carbonate and hydration of calcium oxide have been investigated in some detail. The effect of particle size distribution on the kinetics of calcium carbonate has also been studied. But investigations regarding the energy storage are looked at from a different perspective in the case of calcium hydroxide. After decomposition, recycling has been done with the same pellet and it is found that the hydration percentage decreases continuously with each cycle. Further, additives have been used with the parent material and remarkably once again, additives considerably enhance the percentage of hydration that otherwise flags with each cycle, which also accounts for the changes in diffusivity, conductivity and porosity. Model equations have been solved using orthogonal collocation, first with pseudo?steady?state conditions and then with more rigorous unsteady?state equations. Experimental results have been presented for calcium carbonate decomposition for comparison with the developed model. Hydration phenomena have been modelled without size distribution and solved in a similar fashion using orthogonal collocation method. Using a simple two?resistance model, kinetics of decomposition of calcium carbonate powder has been studied in the temperature range of 750°C to 870°C. Into this simple model also, particle size distribution has been included. Investigations on the pellets have also been carried out on calcium carbonate to see the effect of particle size. Nucleation and growth models have been tested as the formation of a solid product layer proceeded through nucleation phenomenon. The induction periods associated with the nucleation of product phase and further growth of these nuclei have been discussed.