The industrial processes underpinning modern civilisation are complex and diverse. But they share one key input: they require great amounts of heat, which takes staggering amounts of fuel to produce. Heat is critical to industrial operations, but it is also an overlooked and growing source of greenhouse gas emissions.
What if we could use heat from the sun instead?
The EU-funded SOLPART project is a good example of how solar thermal energy opens up new applications outside the technology’s core area of electricity generation or water heating. Solar thermal energy can also provide carbon-free heat for a wide variety of industrial processes. Researchers successfully designed two different solar reactors each operating at 750-950 °C for processing several industrially useful raw materials such as limestone, phosphate and cement raw meal. These solar reactors were a rotary kiln and a fluidised bed (something like a meat grinder for rocks). SOLPART also commissioned a pilot-scale reactor with power of between 40 kW and 60 kW able to treat 20 kg/h calcium carbonate (CaCO3). The decomposition on heating to high temperatures (calcination) of CaCO3 into lime (CaO) and CO2 is the first step towards cement production. The quality of the solar lime produced by CaCO3 calcination at pilot scale matched industrial quality standards. Furthermore, for the first time, researchers demonstrated the successful calcination of Moroccan phosphate at pilot scale using a fluidised bed reactor, with conversion rates exceeding 99 %.
Solar heat could replace fossil fuels in cement and lime production
“SOLPART uses solar energy as a substitute for energy from fossil fuels in processing industrial materials. Heat supply accounts for 40 % of CO2 emissions released by CaCO3 calcination, which can be fully avoided by substituting fossil fuels by solar heat,” says project coordinator Gilles Flamant. The concept behind solar thermal technology is deceptively simple. Sunlight is captured and focused through mirrors into a thermal receiver. Project researchers skipped the electricity-producing step that involves heating a fluid. Instead, they used solar heat to power a rotary kiln reactor and a fluidised bed directly (or indirectly using an absorbing wall). Conventional cement production involves heating a mixture of limestone (CaCO3) and other ingredients to temperatures up to 1 500 °C using vast amounts of carbonaceous fuels. “Such high temperatures may not be easy to achieve for now using solar thermal energy (concentrated solar power). But the first step in the cement-making process (the decomposition of CaCO3) requires lower temperatures of around 900 °C. And it is in this stage that most CO2 emissions occur,” explains Flamant. For now, SOLPART’s pilot system remains an impressive experimental validation of a futuristic concept. Powering cement production with sunlight requires some kind of energy storage so that it does not rely on sunshine availability. It would also require heating much more limestone, scaling it up from a few kilogrammes piloted in this project to several thousand per day.
SOLPART, cement, lime, calcination, solar heat, CO2, fluidised bed, rotary kiln, limestone