Decarbonizing Cement Industry: Sustainable & Energy-Efficient Measures

Dr. L R Manjunatha (VP), Manoj Rustagi (Chief Sustainability & Innovation Officer), Gayatri Joshi (ASM), and Monika Shrivastava (Head of Sustainability) at JSW Cement Limited, discuss new approaches for Decarbonizing the Cement Industry through sustainability measures and energy-efficient resources to reduce carbon emission during cement production.


Concrete is the second most widely used material with an average consumption of 14 billion m3 per year across the world. Some of the key reasons for it being so widely used for construction, is the cost efficacy and the versatility it offers. Cement, being the binder, forms a major part of the concrete manufacturing process. 4.2 billion tonnes of cement is produced globally. China is the largest and India is the second largest producer of cement.

Production of Portland Cement accounts for about 7% of global CO2 emissions. Cement and concrete are used extensively in infrastructure development. Undoubtedly, infrastructure development is the driving force for the country’s economic growth, but construction should not be at the cost of disturbing the ecology and environment. There is therefore an urgent need to adopt sustainable approaches in the manufacturing of cement to cut down the carbon emissions.

Introduction

Cement is a key ingredient in construction of structures worldwide. Right from tall skyscrapers to deep sea marine structures, from small IHBs to a huge highway network and runways, cement finds a place of prime importance. The concrete and cement industry accounts for 13% of global GDP and with rapid expansion and development, cement demand is anticipated to rise.

cement and steel are major contributors to the pollution as a result of carbon dioxide and green house gases emissions. Details are attached in Table 1.

Cement manufacturing is an energy intensive process that majorly involves- crushing and grinding the raw materials, blending the materials in correct proportions, burning the prepared mix in a kiln, and grinding the clinker. Cement production accounts for 83% of total energy use in the production of minerals and 94% of CO2 emissions. The process is detailed in the Fig. 2 demonstrates the data from raw material extraction and handling to a finished product with the energy consumed and CO2 emissions.

It can be noted that Calcination process and fuel combustion process, which are crucial and difficult to subside, contributes to about 86% of total carbon dioxide emissions in cement manufacturing process. It is important to explore, adopt and align the methods to reduce carbon emissions in the cement production process.

Sustainable Development Background

Cutting down on carbon emissions with increasing cement demand will be challenging. We need to focus on the various levers which shall help shrink the overall carbon emissions in the process. Some of the strategies that can help in cutting down the carbon emissions are improving energy efficiency, switching to bio-fuels or low carbon fuels, reducing clinker-to-cement ratio, encouraging use of renewable energy, promoting alternative products for cement and promoting circular economy approach to maximum possible extent. Deployment of modern and innovative technologies like CCUS (Carbon Capture, Usage and Storage), Co-processing, and Waste Heat Recovery Systems (WHRS) shall be extremely beneficial for achieving “Net Zero” carbon emissions. According to the IEA tracking report of June 2020, global direct CO2 intensity of cement in the Sustainable Development Scenario (SDS), 2014-2030, was 530 kg CO2 /t of cement in 2014, 540kg CO2 /t of cement in 2018 and should be 480kg CO2 /t of cement by 2030.

Technical Details

This section highlights various methods and ways to combat carbon emissions and achieve net zero emissions.

Clinker-to-Cement Ratio

Clinker is the primary ingredient in cement which directly contributes directly to the CO2 emissions, because of decomposition of limestone and combustion of fuels. With the world average clinker-to-cement ratio being about 0.7, the global clinker-to-cement ratio is estimated to have increased at approximately 1.6% per year from 2015 to 2020. To achieve the motive of ‘Net Zero” emissions, the ratio should fall about 0.3% every year by 2030 to reach a global average of 0.66. Every 1% drop in clinker factor can reduce CO2 emission by 8-9 kg/tonne. China has the lowest clinker-to-cement ratio of 0.58 with highest 0.72 being shared by Eurasia and other Asia Pacific region. The reduction in clinker usage can be done by blending clinker with various alternatives also known as Supplementary Cementitious Materials (SCM), which are naturally available and industrial by-products. Some of them include:

1. Blast Furnace Slag or GGBS
2. Flyash/ PFA (Pulverised Fuel Ash)
3. Steel Slag

These additions lead to a new category of Cement as Blended Cements. These not only provide the reduction in carbon emissions but also help in enhancing the properties of cement and thus the concrete. SCMs improve the durability by refining the particle packing, increasing the volume of C-S-H gel formation thus making the microstructure denser. The SCMs work on the principle of secondary reaction as follows:

Hydration of Cement or Primary reaction:

Cement + H2O → C-S-H gel + Ca(OH)2

The Calcium hydroxide formed is a by-product and is susceptible to chemical attacks, which on addition of PFA or GGBS is utilized for production of C-S-H gel in the secondary reaction.

Secondary reaction:

Ca(OH)2 + SCM → C-S-H gel

The CSH gel formed in secondary reaction has lower C/S or Ca/Si ratio which helps in more compressive strength.

The mixes made with blended cements may have lower early age strength compared to pure OPC mixes, but is at par or exceeds the strength of pure OPC mixes at latter stages.

The majorly available Blended cements in India can be classified as:

1. PSC (Portland Slag Cement)- Utilizes maximum of 70% GGBS/ Slag
2. PPC (Portland Pozzolona Cement)- Utilizes maximum of 35% Flyash
3. Composite Cement (CC) - Utilizes Flyash in the range of 15%-35% and GGBS in the range of 20% -50% with Clinker percentage being between 35% and 65%.

The percentage of Blended Cement is currently 73% while Pure OPC is at 27%. Further reduction in OPC percentage shall help us cut the carbon emissions.

The carbon emissions of blended cement type is as shown in Fig.4. It can be concluded that PSC has the lowest CO2 emissions per tonne of cement produced.


Limestone can be used as mineral constituent upto 35% in manufacture of Portland Limestone Cement (PLC). Recently, Dolomite limestone, marble waste as mineral additives in manufacturing of Portland Dolomite Cement (PDC). Cement can be produced by using magnesium oxide (MgO) powder and a concentrated solution of magnesium chloride. This cement is environmental friendly and carbon neutral cement.

Carbon Capture, Utilisation and Storage (CCUS)

CCUS is a process of capturing carbon dioxide emissions and either using it to make materials or permanently storing it thousands of feet underground. The extent to which CCUS will be effective in achieving net zero emissions, as per the Paris agreement, depends on the efficiency of technology used. CCUS process flow is explained in Fig. 5.

Different technologies that can be used for Carbon Capture are:

1. Amine Scrubbing
2. Full Oxy-Fuel Combustion
3. Partial Oxy-Fuel Combustion
4. Calcium Looping
5. Direct Capture

Amine Scrubbing involves capture of flue gases or post combustion CO2. Amines are known for their reversible reactions with CO2, thereby, helping separation from other CO2 containing gases, including flue gas. Amine Scrubbing needs a very high thermal energy close to 2GJ/t CO2. Full Oxy-Fuel Combustion uses an oxygen separated from air and recycled CO2 as the combustion gas. This technology is presumed by some to be best technology for a new-build cement plant, however for old and existing plants, development and modifications are required in traditional cement manufacturing process. Thus, this is slightly on higher side when it comes to finances. Partial Oxy-Fuel Combustion eases the process of Full Oxy-Fuel combustion as it only involves redesigning of preheaters and precalciner as they get oxy-fueled compared to traditional process. Calcium Looping comprises chemical reactions between CO2 and calcium oxide sorbent in a pair of circulating fluidized beds. Direct Capture involves capturing of emissions due to calcination of limestone in a vertical shell-and-tube exchanger called as direct capture unit (DCU). As per reports, Direct Capture and Calcium looping is expected to grow fast compared to others.

Retrofitting is attachment of carbon capture facilities to existing Cement plants. As per IEA, retrofitting is necessary from 2020 to existing plants to achieve our targets. We need to focus on the time and effectiveness of different technologies to make our plant “Carbon–Capture Ready”. The storage capacity of CO2 is vast and only 3% of total global storage capacity is being used. Thus, it is high time we need to brace up for the major shift.

Waste Heat Recovery Systems (WHRS)

Basic principle for Waste Heat Recovery (WHR) is converting thermal energy produced in rotary kiln and after quenching cooler (AQC) processes to power. Approximately 30%-40% of heat produced in rotary kiln and AQC is wasted without proper utilization and thus implementation of WHRS is a major breakthrough for all. The hot gases generate steam in boilers which are then further used to generate electricity through steam turbo generator. Usually, power generated through WHRS shall cater to 50% of Cement plant’s requirement which will have a considerable impact on overall cost of production.

As per the reports of 2013, China tops in the WHRS unit installation globally in cement plants followed by India and Japan. Figure 6.

Key three primary WHRS available are as follows:

1. Steam Rankine Cycle(SRC)
2. Organic Rankine Cycle(ORC)
3. Kalina-based system

Steam Rankine Cycle is the most commonly used system. It uses water as a working fluid and steam is generated in waste heat boiler which is then used for driving steam turbine for power generation. These are economical preferably, where source heat temperature exceeds 300oC. Organic Rankine Cycle (ORC) use working fluids which have better generation efficiencies at lower heat source temperatures. ORC has two heat transfer stages- first, heat transfer from intermediate heat transfer fluid and second, from intermediate heat transfer fluid to organic working fluid. Kalina Cycle has working fluids as a binary mixture of ammonia and water. This allows system to be efficient in variable and lower temperature heat sources. It is 15-25% more efficient compared to ORCs at the same temperature level.

In India, SRC is widely used WHRS for cement industry. As per the reports of Global Cement, the total new WHRS unit capacity of Indian Cement producers was expected to reach 175MW by March’22. WHRS is a crucial step which needs to be adopted by all the countries to fast-track our march towards the destination of Net Zero emissions.

Co-processing

Co-processing can be defined as the use of suitable waste as a raw material, or as a resource for energy, or both to replace the natural and limited resources and fossil fuels like coal, gas and petroleum in industrial manufacturing processes. Various wastes that can be used as alternative Fuels to increase the thermal substitution rate(TSR) in Cement manufacturing are- used tyres, industrial plastic waste, biomass, RDF from Municipal Solid Waste, slaughter house waste, poultry litter, dried sewage sludge etc. We have also discussed prior about the various mineral admixtures or SCMs than can be used to reduce the clinker-to-cement ratio. Some of the hazardous wastes can also be used as partial fuel in cement kiln, after proper inspection. Biomethanation is an aspiring solution for the alternative fuels in cement industry for various processes.

Use of Renewable Energy

Renewable energy like solar, wind, biomass can be utilized for power generation at cement plants. This is one of the best decisions and opportunity to incline towards the reduction of Carbon emissions by cutting down the power generations from fossil fuels.

Conclusions

After Iron & Steel, Cement is the major contributor of carbon emissions and thus we need to prioritize this second highest CO2 contributor and consciously make an effort to strategize and consciously reduce the carbon emissions. Sustainable approach towards the processes should not be a brief goal but it should be the way of running our industry.

Numerous methods to bring down the carbon emissions have been focused which in turn are related to each other and with appropriate integration of all the processes, net zero is manageable. In addition to reduction in carbon emissions, reducing clinker-to-cement ratio and using blended cements will improve the quality of concrete. Improvement of concrete and structural durability leads to less repair works and renovation/ new construction which saves cement and construction raw materials along with other related limited resources. CCUS, WHRS and Co-processing form a part of major technology oriented approach and open doors for further optimized innovations that help the unwanted or polluting products generated during the cement manufacturing process or from any other industry to be recycled and reused.

Circular economy should be encouraged across the globe to reduce wastages and carbon emissions. No product is a waste and can be put to right use after dead-on research. To boost the reduction in carbon emissions, certain policies can be adopted by the all government authorities. “Polluter to Pay Policy” where penalties or additional taxes can be charged. Carbon pricing can be adopted globally, where approach is to attach a price to carbon which shall create a financial incentive to lower the carbon emissions.

“Net Zero” by 2050 mission thus needs 360 degree study and approach to help our environment go greener.

Acknowledgement

Authors acknowledge inputs from the study at IIT Bombay by JSW Cement Limited, authors of GCCA Handbook. Author are also grateful to the authors of the handbook named Waste Heat Recovery for the Cement Sector.

References

1. Manjunatha, L.R., Anvekar, S.R. and M.V Yogananda (2015) ‘Recent Developments in the Indian Ready Mixed Concrete Industry: Effects on Sustainable Construction’, CE&CR, Vol., No., 5 pp.
2. Nayak, N.V., Behare, S.A. and Manjunatha, L.R. (2022) ‘Innovative, Economic, Durable and Sustainable Concrete to last for +100 years of service life’, Concrete Construction, Mar.-Apr., pp.2
3. Thomas Czigler, Sebastian Reiter, Patrick Schulze, and Ken Somer(2020) ‘Laying the foundation for zero-carbon cement’
4. GCP Applied Technologies (2019) ‘Reducing CO2 through clinker replacement’
5. Wolfgang Kunther, Sergio Ferreirob, Jørgen Skibsted(2017), ‘Influence of the Ca/Si ratio on the compressive strength of cementitious calcium–silicate–hydrate binders’
6. David Hodgson, Paul Hugues, Tiffany Vass (2022), ‘IEA Report Cement.’