Cement Manufacturing: Ways to Reduce CO2 Emissions

Mario HonrubiaSustainability

Cement is the largest man-made material in the world. With the global population set to hit 8 billion soon, the demand for cement is about to skyrocket to unprecedented levels.

However, cement manufacturing is linked inexorably to the ongoing phenomenon of climate change. Greenhouse gases like CO2 trap the sun’s heat and cause the average temperature to increase in the world. For the last one million years, the total CO2 concentration in the earth’s atmosphere averaged between 100 and 300 ppm (parts per million).

This equilibrium changed during the industrial revolution when the CO2 concentration started rising rapidly due to the increasing use of coal as a heating source. In the second half of the 19th century, the CO2 growth was exponential and as recently as September 2019, the total CO2 concentration in the world crossed 410 ppm, a value which is extraordinary in the history of the Earth.

Cement Manufacturing Process: Environmental Issues

By Arun Appadurai

In terms of weight, roughly 900 g of CO2 is produced as a by-product, for every 1 kg of cement produced. This is the uniqueness of the cement process, wherein CO2 is produced in substantial quantity, along with the main product (cement). 

In cement manufacturing, there are two major sources of CO2 emissions:

  • The primary source (60% of total emissions) is the calcination of limestone (CaCO3). Limestone, when heated to above 900 ºC, converts to quick lime (CaO), releasing CO2.
  • The secondary source (40% of total emissions) is the burning of coal/fuel to provide the heat required for the calcination and clinkering process.

In the past, cement producers have targeted specific fuel consumption as a means of both improving the economy of operation as well as reducing CO2 emissions. Over the last thirty years, the specific fuel consumption of cement manufacturing has decreased by 40%, which directly reduces the CO2 emission by the same magnitude. Furthermore, coal, which is conventionally used for combustion, is increasingly being replaced by alternative fuels like Municipal solid waste (MSW), rubber tires, dried sewage sludge, etc.

In fact, it is the industry’s best-kept secret that cement kilns are the last and best resort for recycling almost any waste produced in human societies. Since the kiln combustion happens at 1500 C, almost anything which has volatile matter could be burnt as an alternative fuel, and the burnt ash is a beneficial additive for the cement end product. Single-use plastics, which are becoming a pressing issue for the environment lately, can be a very good candidate for recycling in cement manufacturing. Additional research is needed to work out the intricate details of such plastic recycling.

Sustainable alternatives fuels and Governmental Incentives

Substituting conventional coal with alternative fuel achieves twin benefits by removing harmful waste from the environment as well as reducing process CO2 emissions in the cement kilns. The biggest stumbling block for wider usage of alternate fuels is turning out to be the cost of transportation. Cement manufacturing is a low margin process which cannot justify the added cost of transporting waste over long distances. It is, in fact, not economically viable to transport waste over 200 km for burning in cement kilns, assuming cement is priced normally as is done now.

Typically, governments have alleviated this issue by providing incentives to cement plants that process waste as fuel. The incentives vary from a straightforward payment per ton of waste burnt to the provisioning of carbon credits, which could be utilized towards the mandated emission norms. Perhaps an additional way that should be looked at by the governments is to encourage more private players in waste processing.

Private players could unlock more value in the waste streams by recovering useful minerals and transporting the remaining in an efficient way to the cement plants. For cement plants, this would ensure a stable and predictable supply of processed waste which is beneficial for their operation.

CO2 Capture: Benefits and Barriers

Nevertheless, in order to have a true zero-emission cement plant, more work needs to be done. As mentioned earlier, cement produces CO2 as a by-product, so, unless the CO2 is captured, stored or utilized, it is not possible to drastically reduce the emissions from the cement plant, CO2 capture being the easiest part of the process.

There are ready solutions available that can capture the emitted CO2 from the process: Oxyfuel combustion, chemical looping, all-electric process heating, etc. are some of the technologies that are in various stages of development for carbon(CO2) capture.

Storage of the captured CO2 is slightly more complicated and, presently, the most viable option seems to be the pumping of CO2 into used oil wells and other geological formations. The utilization of captured CO2 into other beneficial minerals is still in its early stages.

However, installing these technologies in a process like cement is not viable in today’s economy. The average cost of production of cement is 58 €/tonne.  With a limited profit margin, investment costs and limited potential for realizing carbon costs, the currently viable selling price for cement is 78 €/tonne.

Unfortunately, the operation of CO2 reduction using technologies available today costs roughly 60 €/tonne of cement produced. This is comparable to the production cost of cement itself. In other words, a cement manufacturer might roughly spend the same cost for preventing CO2 emission as he spends on producing cement. Therefore, it is unviable in the present economic scenario for a sustainable cement manufacturer to realize a reasonable return on his investment.

This is because, at present, only the end product (Cement) is priced and sold by the manufacturer. The environmental cost of production is borne by society as a whole. This pricing structure needs to be inverted for the sustainable manufacturing of cement. The price of cement should include both the “cost of cement manufacturing” as well as the “cost of not emitting CO2”.  

Towards a Zero CO2 emission process

Establishing a market for CO2 is the most efficient way of calculating its cost. Initial steps along this line are already been taken in the form of carbon credits in the EU. This needs to be made more universal with strong regulation and covering all sources of carbon emission, both industrial and non-industrial. And this market should become global with all countries partnering and becoming part of it.

Alternatively, another localized solution is possible if the cement manufacturer is allowed to realize its manufacturing price including the cost of preventing CO2 emission. This could work by establishing a “green cement” similar to organic vegetables, priced higher compared to the normal variety:

  • The manufacturer can market their “green cement” by highlighting the fact that "Zero CO2” is emitted during its production, verified by a third-party.
  • This can be bought by environmentally conscious consumers who are willing to pay a higher price.
  • Suitable incentives from the government in the form of tax concessions/incentives towards the construction of “greenhouses” can also go a long way in establishing the “green cement market”.

Either way, the challenge of sustainable cement manufacturing is not technological but economic. The solution would be to re-align the economy by rewarding environmentally sustainable products which will ensure that cement production becomes more sustainable in the long run.

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