Table of Contents
1 SUMMARY
Cement is the world’s top construction material but it also has a considerable impact on climate change, causing around 8% of global carbon dioxide emissions. To decarbonize cement manufacturing, carbon capture and sequestration is an essential technology, as up to 70% of carbon dioxide emissions are generated by the calcination of calcium carbonate. To combat anthropogenic climate change, there is a great need to reduce greenhouse gas emissions by the cement industry. It has been suggested in recent studies that, to achieve successful decarbonization, the specific emission levels of carbon dioxide will need to be maintained at around 350 to 410 kg per ton of cement. As per most NGO-based analysts, such as the IPCC and the IEA, carbon capture and sequestration is the most optimum technology to achieve emissions reduction targets. A variety of carbon-capture technologies having different readiness levels can be used in the cement industry. Among them, oxyfuel combustion, amine scrubbing, and calcium looping are considered to be the most developed methods. In this article, a concise review has been provided which covers carbon dioxide capture and storage technologies employed in the cement industry. In addition, some statistical facts and figures have also been provided relating to carbon dioxide emissions. Likewise, major challenges faced by the different capture and storage technologies have also been discussed.
2 Introduction
An increase in human activities, since the industrial revolution, has led to several challenges to climate change. At present, a variety of environmental threats are being faced by the human race such as ozone depletion, melting of glacier caps and so on. The first international agreement was Kyoto Protocol through which binding was made on the members to reduce harmful emissions. A significant burden is laid on the developed countries because their contribution towards greenhouse emissions is more significant due to an industrial activity of more than 150 years [1] [2].
A number of collaborative programs have also been initiated by developed countries such as the Global Climate Change Initiative (GCCI), the Intergovernmental Panel on Climate Change (IPCC), and the United Nations Framework Commission on Climate Change (UNFCC) to address the issue of climate change. The common theme among all of these initiatives is that they are all aimed at both capturing and sequestering greenhouse gases, especially carbon dioxide, to reduce global warming. The gaseous components having the largest contribution to air pollution include carbon dioxide, carbon monoxide, oxides of nitrogen and sulfur oxides and so on. Among these, carbon dioxide represents the major shhttps://epcmholdings.com/wp-admin/post.php?post=13490&action=edit#saveare of greenhouse emissions. The major sources of carbon dioxide are listed as follows,
- Fossil fuels used for power generation and transportation
- Industrial processes such as cement manufacturing or hydrogen production
- Combustion of biomass
- Multiple domestic and commercial activities
Generally, the fossil fuels such as natural gas, petroleum and coal have contributed around 85% to greenhouse emissions. Rests of the emissions are attributed to chemical processing and rapid deforestation [3]. Keeping in view these considerations the capturing and storage technology of carbon dioxide has gained increasing interest in recent years. Carbon dioxide capture and sequestration (CCS) is considered a potential means of controlling the changes in climate and environmental destruction.CCS is considerably important for the two main commercial sources of carbon dioxide, i.e., power generation and cement production. Power generation has the largest share in the production of commercial carbon dioxide from a stationary source. Carbon dioxide emission from the cement industry has been estimated to be around 7% of the total emissions from stationary sources [4, 5].
3 Background
In 1824, the relationship between quantities of carbon dioxide present in the earth’s atmosphere and its impact on climate temperature was first established by Jean-Baptiste-Joseph Fourier, a famous French mathematician. However, any statistical data in this regard was obtained first time in the 1950s which presented a considerable increase in the concentration of atmospheric carbon dioxide. The issue of changes in climate due to human activities first came into the limelight in the year 1970 and serious efforts were started to control the growing problem.
The first conference on this issue was held in 1972 by the United Nations in which degradation of air and water quality due to human activities was discussed and declared as a burning issue. In 1988, an intergovernmental panel (IPCC) was established with the approval of the United Nations General Assembly. The purpose of this panel was to analyze the existing scientific knowledge regarding climate change and present its assessment. Its first report surfaced in the year 1990 which proposed several possible reasons for climate change and rising global temperature. Many sessions held between 1991 and 1992 by the IPCC led to the development of the UNFCCC. This framework was signed by approximately 150 States at the Earth Summit held in 1992. The signatories agreed to reduce greenhouse gases emission to the level of 1990 by the year 2000 [1, 6].
3.1 Predictions of Climate Changes
Article 3 of the UNFCCC stresses the long-term threats that are either serious or a source of irreversible damage. Solomon et al. [7] used existing models such as the Atmosphere-Ocean General Circulation model and the Earth System Model of Intermediate Complexity for the projection of future climate concerning increasing carbon dioxide concentration. He based the results of the models on the peak carbon dioxide concentrations and showed the irreversible effects of these concentrations on atmospheric warming, precipitation changes, and sea-level rise. It was also identified that the increase in global temperature is not expected to seize completely even if the carbon dioxide emissions due to human activities reduce to zero.
The results obtained by the models are shown in figures 1 and 2. In figure 1, the trend reveals the average surface warming of the earth as shown below,
Figure 1. The predicted trend of surface warming for this century [7]
Figure 2. Expected thermal expansion trend of the ocean [7]
4 Role of THE Cement Industry
The main usage of cement is in concrete production which is a mixture of a variety of inert materials aggregated together such as sand, crushed stone, gravel and other materials. The production scale of both of these products gives an estimation of the general economic activity of a country since they are extensively used for construction purposes. The production of cement or concrete is not confined to any geographical location and, due to its critical importance, it is produced in almost all countries around the globe. It is also a fact that wide-scale production of cement has made its availability at a relatively lower price and its transportation between geological locations only adds to its cost [8].
4.1 Cement Production Method
Cement is commonly defined as a non-metallic substance made by a combination of a variety of inorganic compounds and has the properties of hydraulic binding. When water is added to form a paste, hydrates are formed and the material sets to a hard mass having considerable strength. There is a variety of methods which are used for making cement and a general method has been described here for understanding. The major steps involved in the production are [8],
- Preparation and mixing of material
- Clinker formation in the kiln
- Addition of gypsum and refinement to form the final product
A simplified diagram of the cement production process is shown in figure 3. Here the input of material and the output gases are based on the production of one ton of cement.
Figure 3. Conventional schematic diagram of cement production [3]
4.1.1 Preparation of Raw Material
In cement making around 30 raw materials are commonly used and their selection depends upon the type of application for which the cement is to be prepared. Clay, limestone, chalk, and gypsum are some of the commonly used materials. For producing one ton of cement clinker an estimated 1.70 tons of raw material is commonly needed. The different components of the raw materials are mixed and then crushed as well as ground to a fine powder. The chemical composition and the fineness of the material are adjusted as required and the material is then sent to the clinker production unit.
4.1.2 Production of Clinker
The production of clinker is commonly termed Pyro-processing. In this step, the raw material is processed at high temperatures and, through multiple steps of calcination and clinkerization, the final product is produced. This whole process is accomplished in a kiln which could be either a Rotary or Vertical shaft type. The second type is a primitive design and is only in use in underdeveloped countries.
Industrialized countries employ rotary kilns for the processing of raw materials. It is consisted of a long tunnel or tube having a diameter of about 6 meters placed at an angle of 3° or 4° and rotating at a speed of 1-4 revolutions per minute. The finely ground raw material is introduced from one end and then slowly moves down the tube towards the other end from where hot gases enter the tube. There is a variety of raw materials slurries depending upon their moisture content such as wet (38% moisture), semi-wet (20% moisture), and semi-dry (13% much). The wet method has been much preferred due to the ease of handling the process material.
4.1.3 Final Cement Product
The cement clinker formed through multiple treatments in the rotary kiln is then ground with a number of additives such as fly ash, blast furnace slag, and so on. The fineness of the final product is then adjusted as it is a determinant factor when it comes to the setting time and properties of the cement.
4.2 Carbon dioxide Emissions from Cement Processes
The contribution of the cement industry to the total anthropogenic carbon dioxide emissions is around 5% which makes this industrial sector one of the major sources of greenhouse gas emissions. The cement production process is also considered an energy-intensive process and the energy consumption by this industrial sector has been estimated to be almost 5% of the total industrial energy consumed globally. The historical trends are provided in figure 4 which clearly shows an increase in cement production on the global level.
Figure 4. Global estimates of cement production vs fossil energy production [3, 10]
- Limestone calcination produces 50% of the total carbon dioxide
- Fuels (coal, coal, waste oil, solvents et cetera) combustion in the kiln generates 40%
- Transportation sources emit around 5%
- Electricity consumed in production operations contributes to around 5%
Figure 5.Comparison of carbon dioxide emissions from cement and other industries [11, 12]
5 Conventional methods for reduction of carbon dioxide emissions
To reduce the carbon dioxide emissions in the cement industry several measures have been taken over the last few years such as [3, 13-15],
- Improvement in the efficiency of energy systems
- Use of mixed or alternative fuels
- Production of blended cement through clinker/cement ratio reduction
5.1 Improvement in Energy Efficiency
The improvement in the efficiency of the energy systems in the cement industry can not only fully decrease carbon dioxide emissions but also reduce the overall cost of cement. The rotary kiln stage in cement production represents the largest share of energy consumption and hence carbon dioxide emission. Improvements could be made by converting the wet process to a semi-dry or dry process. Similarly, the upgradation of process units such as clinker cooler optimisation, improved design of burners, and better process controls can lead to a significant reduction in energy and hence control of carbon dioxide emission.
5.2 Use of Alternative Fuels
For mitigation of carbon dioxide emissions, another route is to use fuels having a lower concentration of carbon or those obtained from waste sources. By using waste or renewable sources as fuel the use of conventional fossil fuels can be reduced. Studies have shown that the use of any such alternative fuel can also affect negatively the quality of cement and hence proper measures should be taken before choosing any alternative.
5.3 Production of Blended Cement
In cement manufacturing, the energy-intensive stage is the formation of clinker and it also represents the largest share of the whole carbon dioxide emission from a cement production unit. It has been proposed that the clinker-to-cement ratio can be reduced by replacing a particular percentage of clinker with compounds such as blast furnace slag or fly ash. The observed potential of reduction in carbon dioxide emission through this method has been estimated to range between 5 to 20%.
6 CCS Concept
The carbon dioxide capture and sequestration (CCS) schemes involve several technologies through which carbon dioxide can be captured from power plants or cement production units through various steps such as compression, transportation, and storage. Apart from the conventional strategies mentioned previously, the CCS presents a promising opportunity for mitigation of the carbon dioxide emissions from cement plants. The capturing of carbon dioxide can be performed through a number of techniques such as pre-or post-combustion or oxyfuel technologies as shown schematically in figure 6 [2].
In addition, many potential destinations can be selected for the permanent storage of captured carbon dioxide gas. These include porous geological structures, saline aquifers, and depleted oil or gas or coal seams. As described generally in previous sections the generation of carbon dioxide is from three different sources as mentioned below. The estimated values have been based on the production of one ton of cement clinker.
- The breakdown of limestone while processing in a rotary kiln produces around 525 kg of carbon dioxide
- Flue gases produced as a result of fuel combustion add up to around 335 kg of carbon dioxide
- Electricity consumption adds up to 50 kg of carbon dioxide
Figure 6. Combustion Techniques that are currently in practice [2]
7 CCS Techniques
A number of CCS technologies which are currently in practice or still in the research phase are discussed as follows [17-19],
7.1 Conventional Amine Scrubbing
This conventional method which is also termed wet scrubbing is an industrially mature technology and is being practised in industry for more than 50 years. Even though post-combustion wet scrubbing is a well-developed process but its application has been mostly studied with respect to power plants and only a limited number of studies have discussed its implementation in cement manufacturing units [3].
The scrubbing is brought about by using a primary amine termed methyl ethanolamine (MEA). In this process, the aqueous solution of an amine having a concentration of around 25 to 30 weight percent, is brought in contact with the flue gas containing carbon dioxide in a counter-current flow. This contact is brought about in a vertical column and the amine is introduced from the top of the tower while the flue gases are introduced from the bottom. The absorption takes place at a temperature of around 40°C and the reaction between amine and carbon dioxide occurs through a mechanism termed zwitterion and carbamates are formed.
The spent amine solution is then regenerated in a stripping column at a temperature ranging between 100 to 140°C around atmospheric pressures. Steam is used to strip out carbon dioxide from the amine solution and a considerable amount of energy is used for this regeneration step. The regenerated amine is then used back for further absorption cycles [2].
7.2 Oxy-firing Technique
The oxy-firing technique involves the use of almost pure oxygen obtained from a separation unit at the place of air used for commercial purposes. In this firing scheme, the flue gas rich in carbon dioxide is also recycled for the moderation of flame temperature. This technology requires some modification in the existing cement production plants and some equipment is required such as [4],
- An oxygen separation unit which will produce pure oxygen from the air
- Flue gas recycle loop for circulation of carbon dioxide from the exhaust to the pre-calcination burner
- Carbon dioxide compression and treatment facility where the captured carbon dioxide gas undergoes purification, compression and drying before transportation through pipelines.
There are many parameters which need consideration before the application of this technology to any existing cement manufacturing plant and they have been considered in the later sections. The significance of this technology for a cement manufacturing plant is the consumption of less quantity of oxygen. Generally, a ton of carbon dioxide can be captured by using only one-third of the oxygen in cement plants as compared to coal-fired plants [3].
7.3 Calcium Looping
In common practices, the capture of carbon dioxide reduces the overall economy of the manufacturing plant. Considering this, several studies have reported new cost-effective methods for the separation of carbon dioxide and it is a topic of global research at the moment. Bosoaga et al. have reported calcium oxide as a new solvent for carbon dioxide capture which can also be regenerated for use in multiple absorption cycles as shown in figure 7.
Figure 7.A schematic diagram of calcium looping
Calcium oxide combines with the carbon dioxide gas present in flue gases at a temperature of 650°C and produces calcium carbonate in a reactor operating on the same principle as a fluidized bed reactor. The particular calcium carbonate is transferred to the regeneration vessel after separating from the flue gases. The regenerated calcium oxide is sent back for further carbonation cycles and the pure carbon dioxide gas is stored. It is a well-known fact that with the increase in the number of cycles the solvent reactivity deteriorates and as a result of which a certain amount of deactivated absorbent is regularly purged from the system. This purged solvent could be used as a feedstock in the rotary kiln and is regularly replaced by fresh solvent.
This technology can be used in four different configurations in cement plants which are given as follows,
- This absorption facility can be used for supplying lime to the rotary kiln
- The purge material could be sintered at a temperature less than 1250°C (decomposition of calcium sulfate is prevented) and then added to the cement clinker
- If the purge material is sintered at temperatures greater than 1400°C then calcium sulfate decomposes and the desulphurization of flue gas could be carried out by using lime which leads to the reformation of gypsum.
In order to use the purged line as a feedstock in cement production the concentrations of ash and calcium sulphate need to be carefully controlled [3].
7.4 Emerging Materials and Methods
Over the years, several modifications have been introduced to the existing technologies to improve their performance efficiencies and cost. Likewise, several new methods and techniques have been developed to capture larger amounts of carbon dioxide gas and store it for useful purposes. Some of the key methods are discussed as follows [20-23],
7.4.1 Physical Solvents
In recent years, potential alternatives to chemical absorption by using MEA have been developed. These include physical solvents such as Selexol, a mixture of polyethylene glycol and dimethyl ethers, which can selectively combine with carbon dioxide at moderate temperatures and high pressures. Methanol has been used at cryogenic temperatures for many years for the sweetening of natural gas and synthesis gas. It is commercially called Rectisol and when used for carbon dioxide absorption it offers advantages such as less heat consumption since the regeneration could be carried out through simple flash distillation [24].
7.4.2 Ionic Liquids
An important class of physical solvents is the ionic liquids which are famous for their selective carbon dioxide absorption. These consist of smaller inorganic anions in combination with larger organic cations having considerable viscosity at room temperature. These solvents offer advantages such as very low vapor pressure, non-flammability, biocompatibility, and thermal stability.
7.4.3 Solid Absorbents
Solid adsorbents can be used for carbon dioxide adsorption by packing them in either packed bed vessels or by using them as fluidized beds. These adsorbents range from microporous to mesoporous and include a variety of materials. Carbon-based materials such as activated carbon, zeolites, and molecular sieves are commonly used as sorbents. Other materials include metal oxides such as magnesium oxide or calcium oxide and hydrotalcite compounds. These adsorbents offer advantages such as lower energy consumption and increased process economy as compared to chemical or physical absorption methods [2].
8 Outlook & Limitations of CCS Technologies
In order to fully assess the potential of any new given material or technology for CCS the associated economics and commercial viability must be considered (figure 8).
Figure 8.Technology status and development for CCS [2, 25]
8.1 Post Combustion Techniques
The post-combustion techniques such as chemical absorption used for capturing carbon dioxide are industrially developed and there retrofitting to any existing cement manufacturing unit is easy. However, such methods require a significant amount of energy for the regeneration of solvent and different inhibitors are used to control corrosion or oxidative degeneration caused by the excess oxygen present in the flue gases. In addition, the solvents can easily undergo degradation due to the presence of impurities such as sulfur oxides, nitrogen oxides, and dust particles. In order to avoid solvent degradation additional equipment is installed which negatively impacts the process economics.
In the case of physical absorbents, even though ionic liquids reduce the solvent loss during the absorbance stage but their higher viscosity limits the process dynamics and lower absorption rates are observed. However, a few studies have reported modifications in the ionic liquids for increasing the absorption capacities. The solid absorbents, on the other hand, suffer from lower heat transfer rates when working between the hot and cold cycles of the capture and regeneration process. The improvements in post-combustion techniques have been incorporated by using solvents having low heat of absorption, increasing the process dynamics, and using secondary amines which possess low regeneration temperatures [2, 4].
8.2 Oxy-firing Technique
Oxy-firing or oxy-combustion offers a significant advantage when implemented in cement manufacturing plants as compared to power production plants. But studies have shown that this technique causes substantial consumption of auxiliary power as compared to the normal operating cement plants having no CCS technique. Certain process modifications such as redesigning kiln burners and respective coolers might be needed. Further exploration in the implementation of this technique is needed for addressing some issues such as,
- Leakage of air into the kiln
- Cement cooling system next to the kiln
- The high partial pressure of carbon dioxide in the calcination process
- Emissions of carbon dioxide during the start-up and shutdown of the cement manufacturing unit
If the above-said issues could be addressed the applicability of the Oxy-combustion technique could be made practically feasible [4, 25].
9 Conclusion
In this project, the various carbon dioxide capture and storage/ utilisation techniques have been concisely reviewed. An analysis of the existing literature reveals that no unique solution exists for solving the problems associated with carbon dioxide capture and storage. Each method has its applications and limitations concerning cement manufacturing units. The post-combustion techniques can be retrofitted to the existing cement plants with certain modifications and are commonly considered reliable as compared to the emerging capture and storage solutions. The pre-combustion techniques are only preferred for power generation plants and they are not considered favorable for cement-producing units. The oxy-combustion technique which involves the use of pure oxygen is considered a potential technology for controlling carbon dioxide emissions. But still, there are certain limitations associated with this technology and further research is needed for optimizing the existing designs for making it a competitive option. Research efforts are in progress to find new and innovative for storing and using carbon dioxide further for useful purposes [5, 27-29] .
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