Rick Gould looks at the options for decarbonising cement
Two centuries ago, Joseph Aspdin of Portland, England, an entrepreneurial bricklayer and stonemason, invented the forerunner to modern cement when he made a chemical binder for concrete by baking finely ground chalk and clay in a lime kiln.
Aspdin’s invention, now enhanced and known as Ordinary Portland Cement (OPC), was a quantum leap that improved on the Romans’ two millennia-old, lime/volcanic ash/seawater recipe for concrete. Now, more than 3,400 cement plants worldwide make four billion tonnes of cement annually.
Cement is a versatile and critically important building material, and one which, according to the journal Nature, will have a crucial role in climate-resilient construction and adaptation.
Conversely, cement production is carbon intensive and contributes around 8% of global CO2 emissions. Although the sector has made enormous progress in reducing its CO2 emissions per tonne of product since 1990, particularly in Europe and North America, the two biggest challenges lie in the fuels used and the chemistry of the process.
Two-pronged CO2 generator
OPC consists mainly of four types of compounds based on oxides of iron, aluminium and silicon, all bound to calcium oxide. This last compound is made by heating ground calcium carbonate from limestone in a rotary kiln with other materials such as silicates and alumina from fly ash, at very high temperatures. Coal, natural gas and waste-derived fuels typically provide the energy sources. The resulting ‘clinker’ is then ground into cement.
The problem is that CO2 is a by-product of calcination, a process where calcium carbonate is transformed into calcium oxide. Therefore, changing to carbon-free fuels such as hydrogen still means up to 60% of the CO2 emissions would remain, owing to the chemistry of limestone. In terms of numbers, making OPC creates an average of 700kg and as much as 900kg CO2 per tonne of cement produced, depending on the fuel used. So how can the sector decarbonise?
In its UK Concrete and Cement Industry Roadmap to Beyond Net Zero, the Mineral Products Association has identified seven technology levers for decarbonisation, while the biggest reductions can be achieved in the kiln processes. In simple terms, there are five options here: carbon-free energy sources, process changes, end-of-pipe solutions, alternative raw materials, or combinations of all of these.
Decarbonising the energy source
Green electricity and hydrogen have already shown enormous promise. For example, Hanson Cement, in the Heidelberg Cement Group, has trialled hydrogen in the main burner, in combination with other net-zero energy sources at its Ribblesdale works in Clitheroe. If used for the entire kiln system, it could prevent 180,000 tonnes of CO2 emissions per year. Elsewhere, this year the French manufacturer Vicat SA will be trialling a 330 megawatt electrolyser to make hydrogen from water, to provide fuel for its Montalieu-Vercieu plant. Yet regardless of the energy source, there is still the challenge of CO2 emissions from calcination. This requires a different solution, notably carbon capture, utilisation and storage (CCUS).
Capturing the carbon
There are three main methods for capturing CO2; one of the most promising is post-combustion amine scrubbing, already used at a few small power stations and planned for multiple other industries.
The waste-combustion gas is passed through a scrubbing solution where the CO2 binds chemically to an amine-based solvent. The resultant compound is then recycled, with the CO2 removed and compressed for permanent storage or use elsewhere.
Although there have been small-scale pilot plants, until now cement companies have not scaled up CCUS to process all of the waste gases from a cement plant. However, Heidelberg Materials is building the world’s first full-scale amine-based CCUS facility at its Brevik plant in Norway, which is scheduled for completion in 2024. The new plant will capture 400,000 tonnes of CO2 annually. The company also plans to build more CCUS units at its other cement plants, such as the Hanson Cement works at Padeswood in Wales. This will feed into the government-supported hub for CCUS and capture 800,000 tonnes of CO2. The project will dovetail with the HyNet North West industrial hydrogen and CCUS cluster.
Although the CCUS technique is effective, it is also operationally complex, expensive to build and requires lots of energy. Brimstone, a small start-up company in the US, has discovered how to make OPC that avoids CO2 as a by-product and, in the longer term, potentially gets round the need for CCUS.
Portland cement without the CO2
Brimstone in Oakland, California, uses basalt and other silicate rocks that contain calcium oxide, instead of limestone. These rocks are ground up and processed using a novel chemical technique to extract the calcium oxide. Moreover, the alternative raw materials also produce the supplementary materials instead of having to add them from other sources such as fly ash, while basalt and other silicates are much more abundant than limestone.
In terms of energy use, the process has similar demands to the traditional method for making OPC. However, because Brimstone’s process begins with a carbon-free feed rock, it eliminates up to 60% of emissions from the start. Additionally, the Brimstone process also produces magnesium oxide, which then reacts with CO2 in the air to produce magnesium carbonate. In other words, the process is carbon negative, and independent tests have verified this.
Brimstone was started by two friends: environmental chemist and engineer Dr Cody Finke, and fellow engineer Hugo Leandri, who wanted to find an alternative way of making OPC and decarbonising the process.
Materials used in the construction sector typically need testing and certification before their manufacturers and suppliers can sell them. This is not simply to assure performance, it’s also about safety, which is critical for cement, considering its uses. Recently, a third-party test laboratory evaluated and certified Brimstone’s OPC to the US ASTM C150 specification, which is the key standard for OPC.
ASTM C150 specifies the composition, safety and performance of OPC; this means that Brimstone’s product is the same as OPC made from limestone and waste-derived supplementary cementing materials and can be used for the same applications.
The challenge that Brimstone faces is the same as that for carbon-free fuels and CCUS – acceptance and scaling up the infrastructure and processes for the global cement sector. During this transformation, it is likely we will need all available decarbonisation techniques and, as Heidelberg’s full-scale CCUS plant and the UK’s HyNet North West hub have already demonstrated, governmental engagement and political will are crucial to make it happen.
key facts about cement
- We use more cement than any other human-made material.
- Currently, manufacturers produce four billion tonnes of cement annually and this is projected to rise by about 50% by 2050 because of increasing urbanisation.
- Portland cement is the most common type, which is difficult to decarbonise because at least half the CO2 emissions result from chemical reactions in the kiln.
- Depending on the energy source used, Portland cement production emits an average of about 700kg CO2 per tonne of cement and as much as 900kg CO2 per tonne of cement.
- Portland cement was invented in 1824 and the process of making it has hardly changed in the past 100 years.
- Cement production emits about 8% of global emissions of CO2 and around 5% of greenhouse gases annually.
- If cement production were a country, it would be the third-highest CO2 emitter after China and the US.
Rick Gould is an air-quality adviser at the Environment Agency. He writes in a personal capacity.
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