Oxygen enriched burners, CO2 utilisation and mineralisation

Annual global cement production in 2019 was 4.2 gigatonnes, of which 55% was produced in China. The worldwide cement market is expected to grow by up to 25% until 2050.

It is regarded as a ‘hard-to-abate sector’, as much of its CO₂ emissions are intrinsically connected to the chemistry of cement production and cannot be mitigated using electrification with renewable power. At 900°C limestone is decomposed into lime and CO₂.

In addition to CO₂ emissions from the process, there are also emissions associated with the burner in the cement making kiln where natural gas, coal, wastes or petcoke are burned to make the heat and high temperature that are required to drive the chemical reactions.

Accounting for the mixing of CO₂ released from the mineral processing and combustion, the typical CO₂ concentration in the flue gas is up to 15%. Though 375 times higher than the CO₂ partial pressure in the atmosphere, it is still challenging to capture CO₂ from a flue gas with such a dilute concentration of CO₂.

Utilising oxygen-enriched combustion

To increase the CO₂ concentration in the flue gas, it is possible to use oxygen enriched combustion. This technology can be retrofitted to many existing cement plants. An additional benefit of this approach is that cement production will be increased by 5 to 10% due to process intensification. This can offset some of the costs of the oxygen supply and equipment modifications.

An increased CO₂ concentration in the flue gases eventually makes carbon capture much more cost effective. Furthermore, the mitigation of pollutant emissions such as NOx, SOx and particulates can be simplified using combustion with oxygen enriched air because the flue gas treatment equipment can be downsized.

Separation of CO₂ streams from process and combustion off-gases

An alternative approach is to use air to provide oxygen for the combustion process and separate the process gas stream from the combustion off-gas stream. Carbon capture can then be focused on the process gas stream which has a higher CO₂ concentration.

About 65% of the CO₂ emissions from cement making are associated with the process gas stream so there is good potential to make a significant impact on decarbonisation.

Within the European project Low Emissions Intensity Lime and Cement (LEILAC) a pilot plant has been built at HeidelbergCement’s premises in Lixhe, Belgium. The 60m tall pilot plant has a capture capacity of about 18,000 tonnes of CO₂ per year which results from 240 tonnes per day of raw meal feed for cement production or 190 tonnes per day of ground limestone feedstock.

Core to the process is the direct separator reactor (DSR) which has been developed by Calix. It acts as a large heat exchanger to indirectly heat the limestone, thereby enabling separation of the combustion and process gas emissions streams. The exhaust of the process heat tube consists mainly of CO₂ which can be captured and liquefied for further utilisation.

A similar technology is also employed by CarbonEngineering within its Direct Air Capture (DAC) process.

The follow-up project LEILAC2 started in 2020. Intended to be operational by 2023 at the Hannover plant of HeidelbergCement, it is designed to capture 100,000 tonnes of CO₂ per year. If natural gas is employed as the fuel to heat the limestone, conventional carbon capture technologies can be employed to separate the CO₂ from the rest of the flue gas.

During the LEILAC2 project alternative fuels based on biomass and the use of electrical heating using renewable power will be demonstrated to heat the calciner. Furthermore, the flexible combination of intermittent renewable electricity and fuel is to be validated within the demonstration project, so that additional dispatchable power consumption can be offered as a service to the electricity grid.

One important question remains. Even if all anthropogenic CO₂ emissions from cement making plants are captured, what eventually happens with this CO₂?

CO₂ utilisation with enhanced weathering

Weathering is a natural process where rocks, and minerals break down as they are exposed to water and atmospheric gases. When CO₂ from the atmosphere is dissolved in rain or river water, carbonic acid is formed. This aqueous solution causes ‘weathering’ of primary minerals into secondary carbonate minerals in a process referred to as ‘carbonation’.

The overall speed of the process is very slow, and depends on local temperature, pressure, permeability, and the relative strength of the original rock formation. Some primary minerals like olivine, a weak basalt rock with the chemical formula (Mg, Fe)₂SiO₄, ‘weather’ quite quickly under the right environmental conditions. Lessons from this natural weathering process can be used to capture CO₂ emissions through various mineralisation techniques.

To store CO₂ in a carbonate form, a raw material with a high metal oxide or metal hydroxide content is required. One example includes cured Portland cement which has a distinctive calcium hydroxide fraction. Other raw materials include fly ash (CaO 10-40%, MgO 0-10%) and steel slag (CaO 40-50%, MgO 5-10%). These materials also contain high concentrations of other metal oxides. For example, MgO can be utilised to sequester carbon dioxide by the formation of magnesium carbonates (MgO + CO₂  MgCO₃).

Growing underground rocks with CO₂

To implement this mineralisation carbon capture process, two options are available. Either transport the raw material to the point source of CO₂ emissions, or transport the CO₂ to suitable rock formations to store the gas securely.

The latter approach has been developed by CarbFix in Iceland since 2007. CO₂ captured from flue gases or directly from the air is mixed with water to form an aqueous solution of carbonic acid. This solution is pressurised and pumped underground where it heats in hot underground geological formations. As hot water can bind less CO₂ than cold water, the CO₂ is released underground, and carbonation of basalt rocks takes place in-situ.

Cement curing with CO₂

The Canadian-based company Carbon Cure takes advantage of the sequestration capability of concrete by injecting an additional 1.5kg of CO₂ per tonne of cement during concrete preparation. This increases the strength of the concrete as additional calcium carbonate is formed as the CO₂ reacts with the cement fraction of the concrete. The result is that 5% less cement is required to make the concrete, and a reduced amount of cement translates to a reduction in CO₂ emissions from a reduced requirement for cement making.

The technology is readily available to reduce the CO₂ emissions of concrete and can be scaled up immediately, as few changes are necessary in the concrete production process. Several reference projects exist in the US, with the Amazon HQ2 the largest project to date, achieving a net saving of 1,144 tonnes of CO₂.

Precast concrete made from CO₂

US-based company Solidia Technologies goes one step further and alters the cement production process itself. Instead of a 3:1 ratio of calcium carbonate to silica within Portland cement, the ratio has been adjusted to 1:1 and the burning temperature has been reduced to 1,250°C instead of 1,450°C. This creates synthetic calcium silicate (Wollastonite) instead of tricalcium silicate.

By utilising additional CO₂ which has been captured from another process, the Solidia concrete is cured at 60°C and ambient pressure with just a minimal amount of water, instead of up to 50% water required for Portland cement-based concrete. The additional CO₂ that is applied during the curing process reacts with the calcium silicate to form calcium carbonates. The process is favourable for precast concrete producers given the fact that about 50% of the CO₂ emissions can be mitigated. The technology is currently being offered by LafargeHolcim in the US.

CO₂ sequestered aggregates

To sequester anthropogenic CO₂ emissions, Blue Planet has developed a new mineralisation process. It uses recycled concrete as a base material. The crushed concrete contains aggregate and an old cement fraction. During the process, the aggregate is upcycled and can be reused as aggregate in new concrete. Whereas the old cement fractions are mineralised with CO₂ to form a new layer of calcium carbonate around the old cement parts, which act as incubation seeds for the mineralisation process.

The CO₂-sequestered aggregate is used in addition to the upcycled aggregate in newly mixed concrete. In a reference project in 2016, the aggregates were used at the Interim Boarding Area B at San Francisco International Airport.

Food-grade baking soda from flue gases

The company CarbonFree has developed a new process called SkyMine, where the CO₂ capture and sequestration can be connected to standard cement kilns. The flue gas is directed into an absorber column where CO₂ is stripped using an aqueous sodium hydroxide solution. In a second step the sodium hydroxide is reacted with additional flue gas to produce sodium hydrogen carbonate, commonly known as baking soda.

In a first demonstration of the SkyMine process, 90% of the CO₂ from a slipstream at a cement making plant in San Antonio is being captured. It has operated since 2016 and captures 75,000 tonnes of CO₂ annually. Food-grade baking soda from the process is supplied to the consumer market. Ultimately, the captured CO₂ is released into the atmosphere in ovens during baking.