Pre-Combustion Carbon Capture Technology

There are several technologies available for separation of CO2 from a mixture of gasses - the choice depends on the required products. Some of the most common application for CO2 capture encompasses removing impurities from natural gases and hydrogen production, ammonia as well as other chemicals, used in the industry. Additionally, CO2 has been captured in power plant through coal burning process. When discussed in the context of carbon capture, the methods are classified as pre-combustion, post-combustion, and oxy-combustion. In essence, it will be imperative to provide an overview of their application, conditions under which each is used, challenges, and future developments.

Pre-Combustion Carbon Capture Technology

Removal of carbon from fuel in firepower plant entails first converting coal into a form amenable to capture carbon. This is achieved through a reaction of coal, steam, and oxygen at high pressure and temperature - a step referred as gasification and partial oxidation. The later process allows heat leading to proper gasification reactions. Instead of burning, coal is broken down chemically by pressure and heat in gasifier chamber (Leung, Caramanna, and Maroto-Valer 2014, p.38). The resultant mixture comprises gaseous fuel, made of hydrogen and CO. This is a mixture called syngas or synthesis gas that can then be burned to generate electricity. After the syngas is generated, it is processed in a water gas shift reactor, where it is prepared for pre-combustion capture. The water gas shift reactor is characterized by a typical fixed-bed reactor that contains shift catalyst that converts the carbon monoxide into water and additional hydrogen as well as CO2. The generated CO2 and sulfur are separated from hydrogen sulfide and other gases, such as carbonyl sulfide (Leung, Caramanna, and Maroto-Valer 2014, p.38). Sulfur and CO2 can then be separated from the other products separately or simultaneously, based on the composition of the syngas and the prevailing conditions as well as end fuel gas specification. Further, certain impurities have to be removed from the syngas - a process that occurs through two stages that convert CO into CO2 as it is reacted with steam. Although some of the fuel conversion stages are elaborate, traditional pants that generate fuel from coal allow separation of CO2 easily and cheaply because of high pressure as well as CO2 concentration used. After CO2 has been removed, the remaining hydrogen gas is then combusted in turbine cycle to produce electricity.
Presently, four major pre-combustion technologies that apply physical absorption are being used for CO2 capture: selexol, rectisol, fluor, and purisol. Selexol technology is used for high partial pressure of carbon dioxide gas stream. It is capable of removing sulfur and CO2 simultaneously. On the other hand, rectisol is a process used for low-moderate partial pressure of the gas. Fluor is one of the most used processes for treatment of gas stream with high carbon dioxide partial pressure. Finally, purisol is used for high-pressure gas stream and uses N-methyl-pyrrolidine as a solvent, arising from high boiling point.

Problems and Challenges of Pre-Combustion

In most of the coal fire power plants, the temperature is often kept at 40oC and cooler, subjecting the syngas to this temperature level and condensing water from accompanies by a considerable loss of mass and energy. In other words, while loss of energy is lower as compared to post-combustion method, the loss is considerable. If the process can be achieved through higher temperatures, there would be little loss of energy and more mass of gas, directed to the turbine (Folger 2013, p.23). Currently, there has been the development of process for removing sulfur species from syngas at above 230oC from syngas, using zinc oxides based sorbent. However, for power plants to utilize this technology completely, there should be processes for removing the hot traces of contaminants, for example, chlorides. Further, the production of power from hydrogen remains a significant challenge, because the gas is highly combustible (Folger 2013, p.23). At the same time, if carbon dioxide generated a need to be vented, additional purification might be required. This presents a considerable challenge of capital.
In addition, certain power plants rely on transport membrane for low-cost oxygen production, while allowing the process to occur under high temperature and moderate pressure. However, the integration of this type of technology into combustion plants is a considerable challenge. Additionally, there has been an improvement on the shift reaction, which can now be performed by syngas before removal of sulfur (Folger 2013, p.23). Specifically, this is critical for dry coal fed gasifier with high CO content in the syngas. However, the catalysts used during the process decrease the steam turbine output, which in return impacts the plant efficiency and performance.

Possible Solutions in the Future

Loss of energy is the major problem that faces pre-combustion carbon capture. The current RD&D efforts are tailored toward ensuring a reduction in energy losses that result from the various steps of the process. However, the greatest improvement in the overall carbon capture will be the development of high fire temperature as well as larger gas turbines to enhance effectiveness (Folger 2013, p.23). This improvement is likely to occur regardless of the requirement for capturing carbon dioxide. Presently, there is an active program focused on RD&D elements to improve carbon capture efficiency. There will be multiple pathways for improvement, using distinct gasification technologies that will be used, based on particular locations, climate as well as coal types. Similarly, the process will require the use of coal in liquid carbon dioxide slurry, which will enhance efficiency of slurry fed gasifiers, specifically for low-rank coals (Folger 2013, p.23). Further, there will be a need to reduce firing temperature, which will require larger gasifier to offer hydrogen fuel for larger gas turbines.

Post-Combustion

The removal of carbon from coal-fired plants is achieved through the post-combustion flue gas, using regenerated solvents. The most commonly used solvent is monoethanolamine (MEA). The process is referred as solvent scrubbing and involves bringing the solvent absorber at the temperature between 40 and 60 degrees Celsius. Under these conditions, CO2 is bound in the solvent. Further, water is used to create a balance in the system and to remove droplets of solvent. It is also possible to lower concentration of CO2 in the exit gas to low values due to a chemical reaction in the solvent (Spigarelli and Kawatra 2013, p.56). The chemically bound CO2 is moved to the top of the stripper through a heat exchanger. Production of the solvent is accomplished in stripper unit at a high temperature between 100 and 140oC, and pressure is kept below atmospheric. Heat is the supplier to the boiler, aimed at maintaining production conditions. In return, this leads to thermal energy penalty as the solvent is heated, allowing the required temperature for desorption to enable removal of CO2, bound in the solvent. At the same time, this allows generation of steam that acts as stripping gas (Spigarelli and Kawatra 2013, p.56). It is then recovered in the condenser and moved back to the stripper. At this stages, MEA contains less carbon dioxide concentration, pushed back to the absorber cooler. The solvent is used, because it has several advantages, for example, high reactivity, high ability for absorbing the gas, reasonable thermal stability, low cost, and thermal degradation rate.

Problems and Challenge of Post-Combustion Carbon Capture

The major drawback of post-combustion carbon capture is the increase in cost because of high energy, required in the process. The use of MEA as solvent can lead to up to 20% reduction of power production. At present, the thermal efficacy of the power plants that use post-combustion technique is about 38.9%. However, if the retrofitted with a supercritical turbine or boiler technology is used, the efficiency can rise to 44.9%. The problem is exacerbated by any addition of amine solvent, as this leads to 9.4% reduction of power generation. Further, although retrofitting is one of the primary advantages in the process of capturing CO2, the difficulty involved in doing this cannot be estimated at all (Spigarelli and Kawatra 2013, p.56). In other words, the current amine technologies led to a net loss of power and reduction in effectiveness. When it comes to retrofit, this may call for replacement of power to make up for the loss.
In addition to the problem caused by amine solvents used in the post-combustion process, it is noteworthy that these products are often found in relatively small scale. In addition, most of the sorbents require pure flue gas, since impurities can lead to increase sorbent cost due to raised usage (Spigarelli and Kawatra 2013, p.56). Little amount of sulfur dioxide and nitrogen dioxide is allowed, based on specific sorbent used. At the same time, it is fundamental to note that extraction of steam for generation of solvent lowers flow that leads to the low-pressure turbine with considerable operational effect on its competence and turns down capacity.
Similarly, when it comes to transforming to commercial scale, lowering energy penalties and reducing cost are the primary challenges that most post-combustion carbon capture power plants face. Additionally, current literature indicates that the method will significantly increase the cost of production of electricity from traditional coal power plants, coupled with the fact that retrofitting can be a challenge due to site-specificity. It is also critical to indicate that post-combustion capture imposes considerable efficiency penalty, because the energy required for the solvent and compression of carbon dioxide can reduce the output of the plant by approximately 30% (Spigarelli and Kawatra 2013, p.56). As a result, the inefficiency can lead to increased coal use for an equivalent amount of electricity sold. At the same time, this can raise the plant cooling requirement, accompanied by significant adverse effect on plant water usage. Up to date, power plant technology developers have had little innovation for optimizing solvent and configure the process properly to mitigate if not all, then some of the challenges, mentioned above.
Further, degradation of the solvent results in higher material, disposal costs as well as energy demand for carbon dioxide capture. Most of the products that can be degraded cause corrosion in the solvents that occur for capture system. This impacts the process directly economically, because it leads to downtime, losses in production, and reduced equipment lifetime (Wu et al. 2014, p.93).

Solution of Post-Combustion Carbon Capture

The current research is focused on improving the existing technologies, for example, simplification and integration of the process as well as improvement of the solvent used. These approaches are important, since they will result in a reduction of current heat requirement. It has been indicated that when 15% cost decreases, the heat for post-combustion is also reduced by 25%. Focus on improving the solvent has been given on concentration, degradation and corrosion, and alternatives for MEA. Regarding solvent concentration, normally, the solution of MEA contains approximately 30wt% MEA (Wu et al. 2014, p.93). However, when the amine concentration is raised, its capacity increases, which in turn reduces circulation of solution. In return, this causes increased cost of operation in the plant. Currently, there are simulations studies that have shown that thermal energy required can be decreased substantially with MEA concentration.
To solve the problems associated with MEA, for example, degradation and corrosion, current R&D are focused on finding suitable alternatives for this sorbent. The essence is to find a requirement for regeneration of energy. This is part of the major priority for developing advanced solvent technology. The research is focused on the development of superior solvents. These are likely to perform better than current sorbent in several ways: lowering energy for regeneration, increase capacity for carbon dioxide capture, enhanced stability, high absorption, and less degradation and corrosiveness (Wu et al. 2014, p.93). The most viable examples of alternative include alkanolamines, sodium carbonate solutions, and amino acid salts. These are concepts, being tested in pilot plants. Moreover, there are additional solvent advances aimed at improving the design of the absorption contactors that will significantly contribute towards decreasing energy requirement for capture process (Wu et al. 2014, p.93). This is to facilitate contact and interaction of the liquid and gas phases that can be accomplished through packed columns membranes.
Other breakthrough technologies focus on other choices for capturing carbon dioxide from the flue gasses, for instance, low-temperature distillation, adsorption, and membranes. An alternatively novel approach is using high-temperature solid compounds with the ability to react with the gas for carbonate (Blomen, Hendriks, and Neele 2015, p.28). The low-temperature distillation is designed in a way that allows it to use freezing technique. It capitalizes on the difference between the freezing point of carbon dioxide and the other gasses in the flue. All approaches are at research phase. In addition, another promising project will aim at capturing CO2 using liquid absorbent, referred as ionic liquids (Blomen, Hendriks, and Neele 2015, p.28). This will ensure efficiency in post-combustion capture.

Oxy-Combustion Carbon Capture

Oxy-combustion is the newest technology for capturing CO2 from fire power plants. The process entails burning coal with pure oxygen instead of air. The resulting flue gas is recycled, as primary and secondary air flows in a furnace. Similarly, the process occurring involves three main components: oxygen production in air separation unit (ASU), fuel combustion unit (FCU), and carbon dioxide purification and compression. These components together are configured differently into distinct designs, leading to energetic and enhanced economic performance. During the process, coal moisture is carried as vapor at a relatively reduced temperature to avoid explosion and corrosion problems (Leung, Caramanna, and Maroto-Valer 2014, p.49). Further, the primary cycle stream is dried and recycled after all flue gas passes the cleaning unit. During the secondary recycle, it is imperative to ensure that oxygen concentration does not exceed 40 mol%. At the same time, the process involves protecting the downstream equipment, using an electrostatic precipitation, which is carried out at air preheater stage. Additionally, sulfur accumulation is controlled to prevent degradation and corrosion of the catalyst used. After flue gas is cleaned, it is sent to the condenser to lower water content. Lastly, 60% to 70% of the gas is recycled, using the primary recycle, and up to 40% is transported to carbon dioxide conditioning process (Leung, Caramanna, and Maroto-Valer 2014, p.49).

Problems Associated with Oxy-Combustion Carbon Capture Technology

One important problem relied on the development of sub-scale oxy-combustion power plants. However, this requires commitment of the entire power plant, which means it may be costly as compared to post and pre-combustion capture methods. There is auxiliary power related to compression of air in cryogenic air separation unit (ASU) and carbon dioxide compression in purification unit, which reduced net plant output by approximately 25% (Leung, Caramanna, and Maroto-Valer 2014, p.49). In addition, air-fired combustion is required when starting oxy-combustion in power plants. The process is associated with emission impurities, and CO2 purification cannot be accomplished during the start-up operations. At the same time, there is additional quality control requirement, which leads to capital cost. Moreover, ASU is associated with considerable adverse effect when it comes to effectiveness of power plant.

Future Solutions

A critical aspect in terms of cost saving in oxy-combustion process involves developing advanced production of oxygen. Presently, power producers use cryogenic distillation in oxygen generation. There are three primary areas of improvement: advancement of the current design of oxygen production, better design for the components of the plants, and integration of heat cycle of plants, fitted with oxygen-capture (Leung, Caramanna, and Maroto-Valer 2014, p.49). These are important improvements that can decrease effect of capture process. In addition, this technology will entail improving these processes and validating simulation of oxy-combustion carbon capture. On the other hand, it is imperative to note that improvement of ASU is less promising, because the technology has been tried for a long time, but there have only been few breakthroughs. Moreover, research will be focused on reducing as well as eliminating recycled flue gas. Similarly, other ways will aim at developing several innovation technologies for generation of oxygen and carbon dioxide capturing (Leung, Caramanna, and Maroto-Valer 2014, p.49). In essence, there are potential areas, where improvements will be made, for example, using a membrane and chemical looping technology as well as hydroxyl-fuel combustion.
Further, it is noteworthy that there have been critical advances in production of oxygen using mixed metallic oxide ceramic material due to its ability to diffuse pure oxygen are high flux rates and temperature, impacted by partial pressure of oxygen. Such materials are referred as oxygen transport membranes that could enable technologists to attain up to 99% carbon dioxide capture and 4% efficiency (Leung, Caramanna, and Maroto-Valer 2014, p.49). However, at present, oxygen transport membranes are being developed. Their composition and performance are being investigated in pilot studies. Preliminary findings show possibility of attaining above average performance. Other methods that will be used to manage problems created by air separation include using membranes operating at atmosphere temperature as well as solid zeolite material.
Chemical looping combustion is another fundamental concept, which is based on oxygen transference from combustion air to fuel through solid oxygen carrier. In return, this prevents direct contact between air and the fuel. However, the concept is still in initial stages of development. Such system is designed into two reactors - air and fuel, - and the quantity of heat evolved in them are the same as for normal combustion, but the significant advantage is that carbon dioxide is not diluted with nitrogen gas (Li, Duan, Luebke, and Morreale 2013, p.21). It has the potential for small energy penalty for the capture process.
Hydroxyl-fuel is another technology that can minimize or result in complete removal of necessity for flue gas recirculation. Conversely, it is being investigated for its feasibility as a variant next generation oxy-fuel system. Up to date, only prototype has been developed. A further solution for oxy-combustion is being investigated to find ways to moderate boiling temperature to enable conventional material to be applied in designing the system (Li et al. 2013, p.21). If a breakthrough is realized, this may lead to smaller boilers that will considerably reduce flue gas volume, which in turn will lower the size of flue gas equipment. As an outcome, it will be possible to decrease the capital as well as operating cost for the fired-power plant carbon dioxide capture.
Other studies are focused on developing advanced zero emission power plants (AZEP). The primary aim is to substitute the conventional chamber in a gas turbine, using mixed conducting membrane (MCM) reactor that will allow combination of oxygen generation from air, heat transfer, and fuel combustion (Li et al. 2013, p.21). It means that combustion heat will be transferred to oxygen depleted air. Another advantage of this technology is that it will be combined well with current systems, since it will require minimal adaptations that will provide 100% carbon capture. At the same time, it will help to reduced electricity cost by approximately 38% (Li et al. 2013, p.21). However, there are still several technical challenges that face development of this technology.

Conclusion

Indeed, pre-combustion, post-combustion, and oxy-combustion are some of the major technologies, used by fired power plants for production of electricity. It has been imperative to provide an overview of the current process underlying them coupled with major challenges, facing each as well as promising technologies that can be used to curb those problems. It has been noted that post-combustion uses solvent-scrubbing, while current technology in pre-combustion entails absorption of carbon dioxide. Further, it has been determined that focus is and will be paid on various breakthrough technologies with potential to increase power generation, cost reduction and enhancing efficiency in producing oxygen, and capturing carbon simultaneously. In essence, there exist opportunities devoted to developed technologies that are vital for mitigating emission of carbon dioxide from power plants. However, most of the technologies are at stage of development.

References

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