A Solar ‘Breakthrough’ Won’t Solve Cement’s Carbon Problem

The Solpart team used the world’s largest solar furnace in southern France and tested two different cement reactor concepts. One was a rotary kiln and the other a bubbling fluidized bed, a kind of meat grinder for stones. The reactors were placed in a receiver the size of a ship’s container and then irradiated with 1 megawatt of concentrated sunlight, which was reflected by a segmented parabolic mirror the size of a 17-story building.

Both cement reactors worked, but the overall concept goes far behind a commercial plant, says Jan Baeyens, managing director of European Powder and Process Technology and member of the Solpart team. First, the system needs some kind of energy storage so that cement production does not depend on the current solar radiation. And a lot more rock has to be baked, from a few dozen kilograms to several thousand tons of cement per day.

Moumin says the latter challenge is one of the Solpart team’s most active research areas. A problem that Heliogen also has to solve as soon as it is connected to a commercial cement plant.

The differences between the two systems lie in how Heliogen and Solpart use their mirror arrangements to focus the sunlight. Heliogen uses machine vision to constantly optimize the positions of the mirrors so that they are always aligned in the most optimal spot. This means that it achieves roughly the same temperatures with significantly less energy and ultimately brings around 300 kilowatts to an area that is only slightly larger than a basketball hoop. “We have a black silicon carbide plate on which the light shines and which glows white hot from all the energy,” says Gross. “It’s insane.”

For now, both Solpart and Heliogen systems remain impressive experimental validations of a very futuristic concept. If they can manage to power cement production with sunlight, it would be an important step towards sustainability in an industry that is struggling to reduce its carbon footprint. “I think more cement plants would like to use alternative fuels,” said Jeremy Gregory, executive director of MIT’s Concrete Sustainability Hub.

However, the industry will not easily decarbonize. Solpart’s Baeyens says he “doubts” the prospect of a fully solar cement plant due to the variability of the available sunlight, making hybrid operations that combine solar power with alternative fuels like biomass more likely. Gross from Heliogen also recognizes the limits of solar energy. He estimates that only half of the world’s cement plants have enough land to house a Heliomax system on site, provided they want it.

But even if every single cement kiln were powered by photons, the industry would still be producing an unsustainable amount of carbon. The use of solar energy to operate the kilns only accounts for around 40 percent of cement’s carbon footprint. Most of its emissions come from the chemical processes that take place when the limestone is heated. Even with fully solar-powered ovens, global cement production would continue to emit around 1.2 billion tons of CO2 annually. In comparison, that is still 30 percent more than in the entire airline industry.

A more comprehensive solution would combine the solar stoves with new cement materials or carbon capture technologies. There are many ideas for old cement, but none have been widely adopted. For example, Democratic presidential candidate Andrew Yang has suggested that the federal government should invest in research into cement mixes that can trap carbon in the air. These types of cement already exist; they are simply prohibitively expensive to manufacture.

Carbon capture technologies are also struggling to gain ground due to a variety of technological and funding issues. Gross sees better opportunities for CO2 capture with Heliomax. His stoves, unlike the dirtier fumes from fossil fuel stoves, release pure CO2, which he believes is easier to capture. The company still has to prove that this is economically feasible.

Heliogen’s solar power system could solve a large part of the cement carbon problem, but the industry needs more concrete solutions.

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