Solar-driven production of lime for ordinary Portland cement formulation

Cement production is an energy-intensive manufacturing process with potentially large environmental burdens. Among the others, it is one of the largest industrial sources of CO2 emission. Limestone calcination is the stage responsible for most of CO2 emissions and energy requirement. This article aims at supporting the use of solar energy as non-carbogenic renewable source to sustain limestone calcination, with advantages on both the economic and environmental aspects of the process. A directly irradiated Fluidised Bed (FB) reactor was used as limestone precalciner for clinker production. Concentrated solar radiation was simulated with an array of three short-arc Xe-lamps of 4 kWel each, coupled with elliptical reflectors, capable of producing a peak flux of about 3 MW m−2 at the centre of the reactor. The total irradiated power is of approximately 3.2 kWth. Thermocouples and an IR camera were used for the analysis of the FB thermal profiles. Calcination was carried out at a nominal bulk bed temperature of 950 °C, in an atmosphere containing about 70% CO2. The reactivity of lime generated by the solar-driven calcination process has been characterised. Lime produced by the solar-driven process was used together with commercial clay as kiln feed components for the formulation of Portland cement samples. A binary mixture composed by fresh limestone and the same clay as above was employed as a reference. The key focus of the investigation was the assessment of the reactivity of the solar-generated lime toward the main clay components in the clinker production process, as compared to lime from ordinary calcination. An aspect that is specifically scrutinised is whether the different, and possibly more severe, thermal history to which limestone particles undergo during solar-driven calcination in directly irradiated FB reactors may compromise lime reactivity. Portland clinkers were produced by burning the raw meals at 1500 °C for 15 min. Clinkers were mixed with 5% natural gypsum to prepare the related Portland cements, which were then paste hydrated for times ranging from 2 to 28 days (water/cement mass ratio = 0.5, 20 °C, 95% relative humidity). Parameters as lime saturation factor, burnability, phase composition of clinkers and hydration behaviour of cement pastes were taken into consideration. Techniques as X-ray fluorescence and diffraction, and simultaneous differential thermal–thermogravimetry were used to study the materials.