Investigating the use of concentrated solar energy to thermally decompose limestone.

Abstract

The objectives of this research investigation are to answer fundamental questions regarding the effectiveness of using concentrated solar energy as the sole heating source for the thermo-chemical decomposition of limestone-marble, supplied by Penrice, Angaston. Specifically, scientific analyses are used to investigate the energy requirements for the efficient manufacture of quicklime using solar thermal energy. To achieve these aims, the energy requirements for an industrial scale solar lime manufacturing system were first evaluated. The main conclusion from this analysis is that the thermal efficiency of a solar energy supplied lime manufacture system compares favourably with the best fossil fuelled system. A good heat recovery system as well as a comprehensive preheating system is recommended to minimise the energy losses from the system. A zero dimensional model was then used to determine that the most energy efficient shape for a travelling grate solar furnace is a triangular cross section. This shape maximise the exposure of the limestone to the radiant energy while minimising structural heat losses. This analytical evaluation also identified that the open area of entrance and exit openings, which allow the process materials to flow through the kiln and for the exhaust gases to escape the kiln, should be minimised. Thirty three times more heat flux is lost through these openings than through the kiln structure. Minimising the openings area therefore improves kiln thermal efficiency. This investigation then evaluated the maximum bed thickness for the limestone when using a grate bed system within the proposed solar furnace. Due to the nature of radiation it is recommended that the limestone layer be no thicker than 2.5 times the nominal diameter of the limestone in use. This thickness optimises the exposure of the stone to the direct radiation and increases the heat transfer to the stones lower within the bed and allows for the unrestricted diffusion of CO2 away from these stones. The investigation then experimentally quantified the effects of radiant heat flux intensity on the calcination kinetics of the Penrice, Angaston marble as a function of stone size. This experimental investigation involved comparing results from an electric muffle furnace, an atmospherically open solar radiation furnace, and an enclosed triangular shaped solar radiation furnace. The muffle furnace provided a baseline values to which the solar calcination rates could be compared. The open system solar calcination experiments showed that the preheating time of the stone is directly proportional to the illuminated surface area of the stone and the intensity of the heat flux to which it is exposed. Additionally, the reaction rate is directly proportional to the radiant heat flux, and is independent of the stone size for heat fluxes greater than 430kW/m2. The enclosed solar furnace experiments identified a 45% improvement in decomposition time could be achieved by using the triangular shaped solar furnace compared to the open solar system calcination. This benefit to the calcination time is best for the more intense heat fluxes and for the larger stone sizes. The measured calcination times were similar to those found for a conventional rotary kiln. This demonstrates the practicalities of using solar radiation technology for interchange with, or as a supplementary heating source to, a combustion driven lime manufacturing industrial plant. A multi-zone two dimensional mathematical model was then used to evaluate the radiant heat exchange within the triangular solar furnace. The developed mathematical scheme provides a comprehensive package with a validated base model for future evaluations of solar furnace designs. A modified shrinking core calcination model was then developed, which uses an energy balance approach to calculate the preheating times and calcination rates for the Penrice marble exposed to various intensities of radiant heat flux. This version of the heat transfer based shrinking core model was used after considering the one sided heating of the stone from the point source radiation.