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Main component of cement, clinker production can be very polluting

Have you ever heard of clinker? This name may not sound familiar, but be aware that it is much more common than you think. Buildings, houses, sidewalks, stages and, basically, any civil construction works rely on cement as one of their basic materials... And clinker is the main component present in the composition of cement.

Clinker is a granular and rigid material, as can be seen in the image below. In general, it can be said that clinker is considered a homogeneous powder (flour), made of different ground and mixed raw materials, which, when subjected to extremely high temperatures, turns into rock. The production process of these substances is far from simple and can cause considerable environmental impacts.

Production process

Clinker or Portland clinker, as it is also known, is obtained from the burning of ground raw materials in a rotary kiln at temperatures up to 1450 °C. The main raw material to manufacture clinker is limestone and, in addition to that, clay and iron and aluminum oxides are also used, to a lesser extent.

So, the first step is to extract and "refine" these raw materials. Limestone rocks undergo a crushing and crushing process after being extracted, until a fine powder is obtained. Then, a homogeneous mixture is carried out, with all the necessary raw materials. This mixture also refers to a fine powder and is called "flour" or "raw".

This material is then introduced into a rotary kiln where it is heated to a temperature of 1450 °C, at which time clinkerization occurs.

The fuels that feed the ovens are, in most cases, from non-renewable sources, such as oil and coal, which contributes negatively with the risks and impacts on the environment. Among the most used fuels, some solids stand out, such as petroleum coke and gasoline, and some gaseous ones, such as natural gas. Among these, petroleum coke is the main fuel used for the manufacture of clinker, and this is due to its high calorific value associated with its low acquisition cost. In addition to these traditional fuels, industrial and biomass residues and rejects, charcoal and agricultural residues can also be used to feed the ovens.

After passing through the oven, this material is abruptly cooled by air blasts to stabilize its structure and recover heat. This is how clinker is produced, the basic material needed for the manufacture of cement. After this process, the material obtained (clinker) is mixed with gypsum (gypsum) and other additions (such as limestone, pozzolan or slag) giving rise to different types of cement.

It is important to emphasize that, during the high temperatures reached inside the rotary kilns, the limestone calcination chemical reaction occurs. This process refers to the moment when limestone (CaCO3) is transformed into quicklime (CaO), releasing large amounts of CO2 gas.

Environmental impacts

Therefore, it is considered that the clinker manufacturing process has a high polluting potential, being responsible for significant environmental impacts.

The process, as a whole, requires a high consumption of energy, both in the form of thermal energy, by burning fuels to heat the rotary kilns, and in the form of electrical energy, consumed in the entire industrial process to move machines and make turn the ovens. Most of this consumption, however, is related to the expenditure of thermal energy during the use of fuels.

Although the manufacturing process of this material does not directly produce solid waste, since the ash from burning fuels in the rotary kiln is normally incorporated into the clinker itself, there is a high emission of gaseous pollutants and particulate material throughout the clinker manufacture.

The burning of fuels in furnaces, mostly from non-renewable sources, emits different polluting gases, such as carbon dioxide, sulfur oxide, nitrogen oxide, carbon monoxide, lead compounds and particulate matter, all of which are pollutants.

And, in addition to this known emission source, as previously reported, the calcination of limestone rocks is also one of the main factors responsible for the emission of carbon dioxide in the clinker manufacturing process, and consequently of cement, because for every 1,000 kg of Calcined calcite (CaCO3) generates 560 kg of CaO and 440 kg of CO2, according to a study. The chemical calcination reaction accounts for approximately half of the CO2 emissions in this process, while energy consumption in the form of heat (burning fuel) accounts for the rest.

To produce a ton of clinker, it is estimated that the cement industry emits between 800 and a thousand kilos of carbon dioxide, including the CO2 generated by the decomposition of limestone and the burning of fossil fuel to keep the kilns in operation.

In addition, during the first stage of extracting raw materials, physical impacts may also occur, such as landslides in limestone quarries and erosion due to vibrations produced in the ground. And the extraction of clay in rivers can deepen these watercourses, reducing the amount of water in the beds and disturbing the existing habitats, which reduces the biodiversity of several regions.

In Brazil, based on data from the United States Geological Survey (USGS - United States Geological Survey, in Portuguese), and the US Energy Information Administration (EIA - US Energy Information Administration), it is estimated that cement manufacturing is responsible for up to 7.7% of national CO2 emissions generated from the burning of fossil fuels, with clinker production being the largest source of these emissions.

Alternatives

Co-processing

An alternative that seeks to minimize the impacts generated in this production process is co-processing. Co-processing emerged as a strategy to improve the economic performance (lower energy consumption) of the cement industry. This technique consists of feeding waste from other industries to the rotary kiln, using less and less fossil fuels and also reducing waste production.

Previously selected materials are used, which are not capable of being recycled (ie, rejects), which have a high calorific value and which must be totally eliminated. According to some national companies, in this process, neither liquid nor solid effluents are created, as the ash that would previously be sent to landfills is now incorporated into the clinker without changing its priorities.

Thus, various materials can be co-processed, such as tires, grease, used oils, sawdust, vegetable waste, contaminated soil and packaging. Hospital, domestic, radioactive, explosives and pesticides are not used. Specifically on tires and rice husks, researchers Miguel Afonso Sellitto, Nelson Kadel Jr., Miriam Borchardt, Giancarlo Medeiros Pereira and Jeferson Domingues, from Unisinos, published an article in Ambiente & Sociedade magazine (read the full article here) about the reuse of these materials in cement production;

It is important to highlight that the National Environmental Council (Conama) recommends that, for the burning of waste in clinker kilns, the cement factory must have all the technical and environmental conditions to meet the required emission standards. In this sense, it must have: modern production line, stable, regulated and optimized manufacturing process; highly efficient devices for retaining particulate matter and washing gases generated in combustion; and burners specially designed for different types of fuels.

Change in clinker formulation

Another alternative found to help reduce CO2 emissions during clinker production would be to create a new clinker “recipe” . So that there is less CO2 spent during its composition, the cement industries have begun to replace part of this material with blast furnace slag – a waste from the steel industry – and also with fly ash – waste from coal-fired thermoelectric plants.

The biggest problem related to this alternative is the fact that the steel industry – also highly emitting CO2 – and the generation of fly ash do not grow at the same speed as cement plants, making long-term strategies unfeasible.

Due to this limitation, the cement industry has also been using for some decades another material that is a candidate to partially replace clinker in its formulation: limestone powder or 'crude limestone filler'. Filler is a raw material that does not require heat treatment (calcination) - a process that demands high energy consumption and is responsible for most of the CO2 emissions in the cement industries.

carbon dioxide capture

Carbon capture and storage techniques are extremely important to reduce emissions of this greenhouse gas into the atmosphere. These techniques use physicochemical mechanisms to separate this pollutant and other compression techniques to geologically store the carbon dioxide emitted by fixed sources, such as the clinker kiln used in cement production.

In this framework, carbon dioxide should be captured and stored before being emitted into the atmosphere. For this, industries would have to invest in new technologies and adaptation of their plants, which would demand a large investment, with a consequent increase in the price of the final product.