Abstract
Carbon capture is an essential approach for reducing greenhouse gas emissions. Different plants utilize various technologies in reducing carbon dioxide emission. Some of the conventional techniques employed include oxy-fuel, pre, and post-combustion carbon capture. These approaches have a high potential for capturing carbon dioxide. However, their massive initial and implementation costs have been a challenge to plants globally. Additionally, most of them are yet to experience large scale applications. Therefore, it is essential for research and development activities to be conducted. This can ensure a reduction in the cost of production. The carbon capture technologies differ in costs. This has also been a challenge in implementing highly expensive technologies. Oxy-fuel combustion carbon capture is the most expensive. However, its cost can be reduced by modifications to the air supply and purification unit. Additionally, other methods require extensive research to modify. Governments, as well as world environmental leaders, should continue to encourage the utilization of environmentally friendly approaches.
Introduction
In the recent past, coal has become an attractive source of energy. It is commonly utilized in the production of electric power. This is due to its inexpensiveness compared to sources of fossil fuels. Additionally, coal's domestic reserves are larger than its competitors. However, burning coal releases certain dangerous substances such as carbon. It is regarded as the biggest human-made source of various greenhouse gasses (GHG) globally (Morgan, 2012, p. 3). Power plants fired by coal produce approximately fifty percent of total power in the United States. Besides, these plants release nearly thirty percent of carbon dioxide. Upon the establishment of a coal plant, it is essential to devise ways aimed at capturing released carbon. Companies use different methods in curbing the release of greenhouse gasses. This paper provides a critical review of methods used in coal-fired power plants (CFPP). Moreover, it discusses how the technologies fit, existing problems, challenges, and possible future solutions.
Carbon Capture Technologies
Anthropogenic emissions of carbon have become an environmental concern. The release of greenhouse gasses into the environment should be regulated to avoid pollution and other climatic changes. Therefore, it is essential that coal power plants consider the construction of flexible designs allowing for carbon emission regulation (Global CCS Institute, 2016). Besides, retrofitting of coal power plants has a potential of reducing anthropogenic emissions. However, the cost of original fittings can be expensive. Plants with ‘capture ready' designs can be considered as models. This paper analyses three technologies employed by various coal-fired power plants. They include post-combustion, pre-combustion, as well as oxy-fuel technologies.
Post-Combustion Carbon Capture
A solvent can be utilized in the removal of carbon dioxide from post combustion gas. The commonly used solution is monoethanolamine (MEA). The process begins by cooling of flue gas in a solvent. It is then brought into close contact with the solution. This occurs in the absorber at temperatures between forty and sixty degrees. Binding occurs between the solvent and carbon dioxide. It is essential to water wash the flue gas. This has a potential of removing remaining solvent vapors or droplets. These reactions reduce the amount of carbon dioxide in exiting gas.
The carbon dioxide rich solvent is pumped to a regeneration vessel through a heat exchanger. This leads to recovery of the fuel, which occurs at high temperatures. The Minimum temperature for this process lies in the ranges of 100-140 degree centigrade (Morgan, 2012, p. 15). Pressure should exceed the normal atmospheric pressure. Supply of heat to the reboiler is vital since it maintains regeneration conditions. Thermal energy created is utilized in removing the carbon dioxide bound solvent. Additionally, the recovered steam is used as a stripping gas. The steam is pumped into stripper chambers while carbon dioxide is cooled and stored.
In areas where retrofitting is needed, post-combustion carbon capture is essential. This allows plants to continue their operations during the fittings. It is suitable, as well as flexible approach for power generation. It can also be part of the solution to various problems facing carbon capture. Additionally, the utilization of MEA makes post-combustion capture an ideal method. The MEA has high reactivity, high capacity for absorbing carbon dioxide, and low cost. This makes post-combustion capture a better approach in resource constraint areas.
Post carbon capture methods have a challenge of being scaled up. Scaling these methods to handle carbon emitted from fossil fuels has been an enormous challenge. Solvent losses due to the use of MEA leads to degradation. This is as a result of the reaction between MEA and flue gas (Vortmeyer, et al., 2013, p. 7). These losses reduce the absorption of carbon dioxide. Additionally, the corrosive nature of MEA poses significant risks to compartments and vessels. Therefore, a high concentration of the solvent cannot be utilized. Besides, it requires additional construction materials to prevent corrosion of materials. Despite the challenges, MEA is still regarded as a primary solvent in post-combustion carbon capture technologies.
Improvement to the current post-combustion carbon capture (PCC) technology is essential in ensuring cost reduction. The cost of initial setup has been high due to various equipment needed. Additionally, it is critical to use a less corrosive solvent that has a high capacity for absorption of carbon dioxide. This can reduce the solvent and repair costs. The MEA corrodes equipment that must regularly be repaired. Moreover, losses of MEA in the absorption chamber wastes resources. Therefore, improvements should be made to avoid unnecessary loss.
Pre-Combustion Carbon Capture approach
Carbon can also be removed from the fuel before combustion. It is essential to convert the carbon in a manner that can be captured. Coal is reacted with steam, as well as oxygen at high temperatures and pressures (Muradov, 2014, p. 189). The process is commonly referred to as gasification or partial oxidation. This results in the formation of a gaseous fuel consisting of hydrogen and carbon monoxide. The mixture forms a syngas or synthesis gas. Particulate impurities are removed from the gas and then passed to a shift reactor. This converts carbon monoxide to carbon dioxide following a series of reactions with water. The resultant mixture consists of hydrogen and carbon dioxide gasses. An appropriate solvent is utilized in the absorption of carbon dioxide. This leaves a pure hydrogen that is burned in a cycle to produce electricity.
The approach has several processes that can be utilized in capturing carbon. Most of them differ in solvents and sub-processes involved. Physical absorption of carbon dioxide is the most common approach. This process is also utilized in the removal of sulfur products. Sharing of equipment used in carbon dioxide and sulfur removal leads to reduced air clean-up costs. However, plants must have separate gas streams for carbon dioxide and sulfur products. This process is almost similar to post-combustion carbon capture.
Another technology that is still under development is the hydrate based separation (HBS). A hydrate is utilized in trapping carbon dioxide molecules in water lattices. These hydrates are stable at low temperatures and high pressures. It begins with the formation of a hydrate. This is done by exposing gas containing carbon dioxide to a high-pressure water. The formation of a hydrate results in carbon dioxide capture. When the hydrate is dissociated and separated, carbon dioxide is released. The approach is estimated to have a lower energy penalty compared to post-combustion capture.
The utilization of integrated gasification combined cycle (IGCC) is costly, as well as complicated compared to traditional ways for coal combustion (Muradov, 2014, p. 198). However, the separation of carbon dioxide is cheaper and easier. This is due to high carbon dioxide concentration and operating pressures within the design (Smit, et al., 2014, p. 144). Additionally, this approach does not require a chemical reaction to take place between solvent and carbon dioxide. It leads to adsorption of gasses on the solution surface. When pressure drops, carbon dioxide is released in several stages.
The approach can also be employed to natural gas powered plants. When coal is used, reactions with oxygen and steam converts gaseous fuel to syngas. This process is referred to as reforming. It goes through a carbon dioxide separation and shift reactor. The result is hydrogen and concentrated carbon dioxide gas. This process is regarded as the best way of manufacturing hydrogen gas. Additionally, several engineers propose the use of natural gas in producing electricity.
The process losses less energy per unit compared to post-combustion carbon capture. However, energy losses have to be reduced. The utilization of pre-combustion methods assists industries in avoiding substantial energy penalties. Pre-combustion processes occur at a lower temperature below forty degrees Celsius. Water from syngas condenses when it is cooled to the recommended temperature. This is accompanied by losses in mass and energy. Carrying out the processes at low temperature can potentially reduce energy losses. The reduction has an influence on production costs.
The pre-combustion capture of carbon is a promising technology. Plants using this approach are often attractive. This is as a result of combined-cycle design. Additionally, the design is regarded as more efficient compared to traditional methods (Smit, et al., 2014, p. 146). Its ability to produce an enormous amount of hydrogen make it a vital economic process. The hydrogen produced can significantly be used in other essential operations.
High initial costs of pre-combustion carbon capture challenge its implementation in various areas globally. Costs of operation are higher than other standard plants because of energy penalties. However, these costs are lower compared to post-combustion capture approaches. It is estimated that continued use of pre-consumption approaches would raise electricity cost. It is expected to rise by approximately twenty-five percent. It is essential to invest significantly in research and development for production of cheaper pre-combustion capture approaches.
Oxy-fuel Combustion
This approach has a potential of producing an exhaust gas stream with high carbon dioxide concentration. This is because fuel is burned with pure oxygen rather than air. It is less chemically intensive and simpler than post-combustion capture (PCC). However, the process is highly expensive and immature technologically (Global CCS Institute, 2016). It offers a solution for removal of multiple pollutants. The oxygen-fired approach has a high initial cost with maximum penalties of approximately thirty-seven percent. In the contemporary energy sector, it is not viable leading to its under-development. However, researchers are currently conducting pilot studies.
Since pure oxygen is used, nitrogen and other air components are extracted from the air stream. The elimination of nitrogen, which forms the largest air component leads to increased carbon dioxide concentration. This process offers gas capture without further reactions. When oxygen is utilized in burning, combustion temperatures often rise. Therefore, it is essential to recycle produced carbon oxide. This is used in cooling the furnace to reduce combustion chamber temperature. The process is also referred to as carbon dioxide recycling approach.
The major advantage of this process is its ability to operate without chemicals. This is in contrast to post-combustion carbon capture that utilizes MEA as a solvent. Additionally, nitrogen oxides produced from air do not often occur in the final product. This is because nitrogen is removed before combustion begins. On the other hand, its main disadvantage lies on expected energy penalties. These are higher than most approaches used in carbon capture techniques. Besides, oxygen combustion's high initial cost is a barrier to its implementation. The cost is approximated to be two hundred and forty United States dollars per kilowatt.
Corrosion of boilers by sulfur dioxide is a challenge. Continuous carbon dioxide recycling causes the concentration of sulfur dioxide. Sulfur dioxide is introduced into the system by fuel. Therefore, its reduction is difficult to boiler operators. Capital costs are influenced by the end products large sizes (Smit, et al., 2014, p. 544). Using oxy-fuel combustion to produce electricity could lead to increased power costs by approximately one hundred and fifty percent. Therefore, development of cheaper oxy-fuel combustion approaches is essential.
Primary factors that lead to high costs of oxy-fuel combustion approach are carbon dioxide purification unit (CPU) and separation unit. It is essential to develop efficient components that can reduce implementation costs. The CPU should be developed in a way that it acts as a purifier and emission control unit (ECU). This can assist in eliminating selective catalytic reducer (SCR), FGD (Flue Gas Desulfurization), and Mercury control equipment. Removal of these devices can reduce initial capital. Additionally, it can improve the plant's efficiency. The flexibility of oxy-fuel approach allows for a reduction in processes. This is in contrast to post-combustion methods of carbon capture.
Oxygen storage also has a potential of improving oxy-fuel combustion carbon capture. This can be done by operating oxygen producing processes when electricity costs are lower (Morgan, 2012, p. 20). Additionally, producing large amounts of oxygen may lead to economies of scale. The generated oxygen is then stored for future uses. However, this requires high investment in oxygen storage devices. The combination of various approaches can potentially reduce oxy-fuel combustion costs. These may also reduce energy losses resulting in improved efficiency.
In conclusion, carbon capture is essential in reducing anthropogenic releases into the environment. This reduces climatic conditions caused by greenhouse gasses. Therefore, companies should employ approaches to reduce their carbon emission into the environment. Technologies such as post-combustion, oxy-fuel, pre-combustion carbon capture are essential in reducing accumulation of greenhouse gasses. However, these approaches should be improved to enhance their efficiency, as well as cost reduction.
References
Global CCS Institute, 2016. Advantages and disadvantages of major Carbon dioxide capture technologies. [Online] Available at: https://hub.globalccsinstitute.com/publications/technology-options-co2-capture/advantages-and-disadvantages-major-co2-capture[Accessed 19 January 2017].
Morgan, M. G 2012. Carbon Capture and Sequestration: Removing the Legal and Regulatory Barriers. New York: Routledge.
Muradov, N 2014. Liberating Energy from Carbon: Introduction to Decarbonization. Heidelberg, Germany: Springer Science & Business Media.
Smit, B., Reimer, J., Oldenburg, C. & Bourg, I. C 2014. Introduction to Carbon Capture and Sequestration. 1st ed. Singapore: World Scientific.
Vortmeyer, N., Schneider, R. & Hohe, S 2013. Post-combustion Carbon Capture-Leading Mature Technology for Decarbonization of Fossil Power Generation, Daegu, South Korea: World Energy Congress
