By Xu Shisen
China Huaneng Clean Energy Research Institute, President
China Huaneng Clean Energy Research Institute, Deputy Director
Carbon capture, use, and sequestration (CCUS) technology can potentially reduce greenhouse gas emissions on a large scale, and represents an important technological option for slowing carbon dioxide (CO2) emissions in the future. According to studies by the International Energy Agency, application of CCUS technology is a crucial emissions-reducing measure together with improving energy efficiency and employing nuclear energy and renewable energies. By 2050, emissions reductions realized through CCUS are anticipated to account for 17% of total emissions reductions.1–3 China’s energy structure is dominated by coal; development of CCUS technology will be an important measure to effectively control greenhouse emissions. Meanwhile, it will help promote the transformation and upgrade of the power industry.
China’s power system features centralized emissions sources and produces large quantities of CO2 emissions. The most challenging aspect of CCUS technology is capturing CO2 at power plants (see Figure 1). However, CCUS is also one of the most efficient ways to reduce carbon emissions. CO2 capture technologies are divided into three categories: post-combustion capture, pre-combustion capture, and oxygen-enriched combustion.4 Significant R&D progress has been made on CO2 capture technologies worldwide. However, the high cost and high energy consumption of CO2 capture remain obstacles. This is currently where major breakthroughs are being made in R&D technology. Demonstration tests solve various problems in the development of this technology through practice, but also create a path for its scaled and commercial application, so as to realize its full potential for reducing carbon emissions.
FIGURE 1. Technical approaches for CO2 capture
China’s largest power generation enterprise, China Huaneng Group, is interested in developing CO2 capture technologies suitable for power plant conditions, including post-combustion and pre-combustion capture. The company has built and put into operation several CO2 capture test and demonstration facilities, and is using the knowledge and information from these tests to undertake long-term operational experiments as well as the verification and evaluation of new technologies. Meanwhile, by combining fundamental application studies in experiments and engineering design reviews, mature technologies will be further expanded to adapt to future requirements on emissions reduction technologies in terms of energy consumption, scale, and reliability.
Pre-combustion CO2 capture unit in Tianjin IGCC plant
Post-combustion capture refers to capturing or separating CO2 from flue gas behind the combustion equipment. Technical approaches to post-combustion capture include chemical absorption, adsorption, and membrane separation methods. Chemical absorption, the method used most extensively, takes advantage of the acidic properties of CO2; it normally uses alkaline solutions to absorb CO2. Regeneration of the absorbent then takes place by means of a reverse reaction.5–7 Figure 2 shows a typical post-combustion CO2 capture process. Through the absorbent’s absorbing process in the absorbing tower and the absorbent’s regeneration process in the regeneration tower, the CO2 becomes concentrated.
FIGURE 2. Conventional post-combustion CO2 capture process using chemical absorption
Development and Verification of a New Compound Amine Absorbent
In terms of large-scale CO2 emission reductions in coal-fired power plants, high energy consumption, easy degradation, and large loss of traditional absorbents are factors in the high application costs of CO2 capture technology. Huaneng Group is targeting these problems by conducting independent R&D of new absorbents, such as organic amine molecules to which Huaneng researchers are applying design evaluations on molecular structure and functional groups. The evaluations explore the impact of such factors as carbon chain length, hydroxy group location, types and positions of substituents, as well as the steric hindrance effect on the performance of absorbents. By using theoretical simulation and high-flux selection evaluation on compound formulas and pilot optimization, and by combining the evaluation and selection of the performance of absorbents, Huaneng has managed to develop a new type of energy-conserving, highly efficient absorbent with properties that feature high circulating efficiency, high absorbing load, and low energy consumption for regeneration and low steam pressure, oxidization resistance, and low corrosion.
Progressing from the experimental stage to the pilot stage, Huaneng has developed the HNC-1–HNC-5 series of absorbents, suited for use in power plants with different flue gas conditions. The HNC-2 absorbent had a three-month trial run in Beijing Thermal Power Plant’s capture device, starting in September 2011. During the pilot stage, only a slight adjustment to the operating conditions of the capture system was required. The CO2 absorption speed increased 30% and the usage life of the absorbents improved 50% compared with the original absorbents, thus greatly reducing the cost of CO2 capture.
In 2015, the HNC-5 absorbent was run continuously for over 4000 hours at a capture facility in Shidongkou Second Power Plant with a 120,000-t/yr capacity, and compared with MEA absorbent under the same conditions. The results showed that, under the same operating conditions, the solvent consumption can be reduced to 1kg/t CO2 and the energy consumption for CO2 capture was below 3.0GJ/t, 20% lower than MEA’s energy consumption for CO2 capture. In addition, degraded products were produced at a speed of 50% compared to MEA. This absorbent can reduce approximately 20% of the overall operational costs of capture, and this system can operate consistently in the long term.
Development of Slurry CO2 Absorbent
With traditional chemical-absorbing methods, a high percentage of water in the absorbent is one of the main reasons for high energy consumption for CO2 capture. Thus, increases in temperature and volatilization of water in the high-temperature desorption process will consume a large amount of energy. To reduce water involvement in the regeneration process, Huaneng has developed a slurry CO2 absorbent based on potassium carbonate solution (see Figure 3). Taking advantage of the difference between K2CO3 and KHCO3 in solubility, by precipitating KHCO3 through the crystallization process and by regenerating high-concentration KHCO3 slurry, water involvement in the regeneration process can be reduced and full use can be made of steam heat to reduce energy consumption in CO2 capture. Scaled technical tests in the laboratory have shown that the potassium carbonate-based slurry CO2 capture technology’s energy consumption reaches 2.6GJ/t CO2, 20% lower than MEA. In addition, the cost of loss also decreases by 22–50% compared to MEA.
FIGURE 3. CO2 capture process (left) and pilot plant using slurry absorbent (right)
(In the schematic: 1–4, absorber; 7, crystallizer; 10, concentrator; 12, mixing tank; 17, regenerator; 21, reboiler; 5, 9, 18, 19. pump; 6, 8, 11, 13, 15, 16, valve; 14, 20, heat exchanger)
Development of Extraction and Phase-Change CO2 Absorbent
To decrease water usage in the regeneration process, extraction concentration technology and CO2 capture research have been combined to develop a CO2 absorbent that can achieve self-concentration extraction phase separation. Without the need for additional energy consumption, this type of absorbent, upon loading CO2, can automatically be divided into liquid-liquid phases, and achieve redistribution of CO2 in these two phases (see Figure 4). CO2 is concentrated in the phase-rich layer with a redistribution degree of more than 95%. The phase-poor layer has virtually no CO2 load, effectively concentrating CO2 in the rich phase with a concentration rate of 60%. Moreover, the extraction agent has limited influence on the organic amine’s speed of and capacity for CO2 absorption. The real thermal flow heat measuring method shows that, compared to direct desorption, phase-rich desorption after layer separation can significantly reduce regeneration energy consumption by 20–30%.
FIGURE 4. Solvent phase separation upon CO2 absorption after 2 min, 4 min, and 10 min
Beijing Thermal Power Plant Factory’s CO2 Capture Device (3000 t/yr)
In July 2008, Huaneng Beijing Thermal Power Plant established China’s first CO2 capture test demonstration device with a capacity of 3000 t/yr.8 Since becoming operational, this CO2 capture plant has achieved continuous and stable performance. A series of studies has targeted problems such as solution consumption, steam consumption, and system corrosion in the operation process. The system and equipment are optimized through such measures as anti-corrosion treatment, capacity expansion of the circulation cooling water system, and restructuring and recycling discharged steam water from the reboiler for reuse. In this process, the specific solution consumption and loss at each consumption point is analyzed. Corrosion types can be analyzed by taking samples and performing long-term clip-on tests. Using new types of absorbent, the capture performance has been significantly improved and the capture cost has been greatly reduced.
Huaneng Shanghai Shidongkou Second Power Plant’s CO2 Capture Device (120,000 t/yr)
To verify the operational stability and the technical and economic parameters of a larger scale CO2 capture system, Huaneng built and put into operation China’s largest coal-fired power plant CO2 capture demonstration project, Huaneng Shanghai Shidongkou Second Power Plant’s CO2 capture device with 120,000-ton/yr capacity, at the end of 2009.
Since it began production, a series of experiments and studies have been carried out during different seasons to test and perfect the operation optimization over a full year. Studies on device corrosion, safe treatment of waste liquid of the absorbent, and system reconstruction also have been conducted to ensure stable operation of the device. Meanwhile, to address the problem of large absorbent consumption by every unit of CO2, the integration of decarbonized flue gas pre-treatment technologies with the main unit desulfurization system is being discussed and developed, and flue gas pre-treatment devices have been installed. After using the new type of absorbing solvent, the device’s heat consumption for capture has been reduced to less than 3.0GJ/t CO2 and power consumption to less than 60kWh/t CO2.
Changchun Thermal Power Plant’s CO2 Capture Device (1000 t/yr)
To test the adaptability of the technology of post-combustion power plant flue gas capture to the extreme cold in Changchun (northeast China), Huaneng Changchun Thermal Power Plant built and tested a CO2 capture device.9 Completed in early 2014, this pilot device has undergone a 1000-hour continuous test on multiple types of solutions, including MEA, over the past two years, verifying the operational status of the carbon capture system in extremely cold weather, and analyzing the CO2 absorption-desorption features and stability of various new solutions.
This capture device’s absorption tower uses medium-cooling technology that effectively increases the CO2 absorption rate of the solution and reduces the amount of solution in circulation. The regeneration tower uses mechanic vaporization recompression (MVR) technology to effectively recycle and reuse the residual heat at the bottom of the regeneration tower, thus increasing the system’s heat regeneration efficiency while reducing its energy consumption. The impact of important operational parameters (such as the liquid-gas ratio, volume fraction of CO2, and regeneration pressure) on the capture system’s regeneration power consumption was systematically studied. In addition to studying each solution’s corrosion on the system, corrosion-measuring tags were hung at the bottom of the absorption tower (rich solution), inside the disk of the middle cooler (half-rich solution), and at the bottom of the regeneration tower (hot lean liquid) to provide reliable evidence for choosing construction materials for a full-size design.
Gas Turbine Flue Gas CO2 Capture Test Demonstration Device
Currently, in addition to the demonstration projects mentioned above, many post-combustion capture projects have been put into use in coal-fired power plants across China, with a level of technical research in line with international standards.10 However, R&D on CO2 capture technologies for the gas turbine are still in the initial stages.
In recent years, with increasingly strict environmental standards, more and more power generation units globally have been using natural gas combined-cycle (NGCC) power generation. The promotion of R&D and the industrialization of CO2 capture technologies has also become a new topic of interest. Compared to flue gases in coal-fired power plants, the concentration of CO2 in gases during the NGCC power generation process is lower (approximately 3% compared to a coal gas CO2 concentration of 12–15%) and the oxygen concentration is higher (13–18% vs. 5% in coal gas).
Based on the characteristics of flue gas in gas turbines, and having learned from experience with carbon capture in coal-fired power plants, Huaneng independently developed China’s first pilot device for the capture of CO2 from gas turbine flue gas (see Figure 5). There are plans to use the device for further R&D testing. This device is designed to capture CO2 in NGCC flue gas, with a processing capacity of 1000 tons of CO2/year. The main part of the system is similar to a coal-fired power plant’s capture system and adds new types of energy-conserving units such as medium cooling and mechanical compressing units. To study the problem of secondary pollution from emissions, an online system and a test device with comprehensive functions were added and continuous follow-up and sampling inspection are being conducted on the flue gas discharge.
FIGURE 5. Pilot plant for CO2 capture from flue gas of natural gas burner
This project is part of a first-stage technical verification at a CO2 capture project in Mongstad, Norway, with a capacity of 1.2 million t/yr. The project is being operated in strict accordance with EU standards and management models. Under the precondition of guaranteeing a 84–91% capture rate, this device has continuously operated for over 3000 hours, with highly stable system functions and each parameter meeting the designated targets. It features low emissions of pollutants in tail gases of the absorption tower and low consumption of solvents; emission of solvents in tail gases was <0.17 ppmv and discharge of nitrite amine was <3μg/m3. No amine was identified. The discharge performance meets the environmental requirements of northern Europe.
PRE-COMBUSTION CAPTURE TECHNOLOGY
Pre-combustion capture technology refers to transferring the chemical energy from carbon before the combustion of carbon-based fuel and separating the carbon from other substances carrying energy, thus achieving carbon capture prior to fuel combustion. Integrated gasification combined-cycle (IGCC) technology is commonly used for pre-combustion carbon capture. IGCC combines gasification and a gas-steam combined cycle, wherein fossil fuels will gasify and transform to synthetic gas (with the main contents being CO and H2). Then, using the water-coal gas transformation reaction, the CO2 concentration is increased. Hydrogen-rich gas after CO2 capture can be used for combustion and power generation, and the separated CO2 can be compressed, purified, and then utilized or sequestered.
IGCC integrates many advanced technologies to achieve higher thermal efficiency and extremely low discharge of pollutants. It is receiving more and more attention from major power companies worldwide. Because of the high pressure and low flow volume of synthetic gases in the IGCC power generation process, the concentration of CO2 is very high after the transformation reaction. Choosing pre-combustion capture technology will effectively reduce energy consumption and allow for a decrease in equipment size. The IGCC-based pre-combustion CO2 capture technology is an important technical category in large-scale carbon capture demonstration projects in today’s power generation field. The CCUS plans for the Hypogen (EU), ZeroGen (Australia), and New Sunshine (Japan) projects are all based on IGCC and pre-combustion CO2 capture.11
In 2004, Huaneng became the first power enterprise in China to create a “GreenGen” plan for near-zero carbon emissions.12 This plan studied, developed, demonstrated, and promoted an IGCC-based, coal-gasified hydrogen generation, hydrogen gas turbine combined-cycle power generation, and fuel battery power generation-focused coal base energy system, which would also facilitate CO2 separation and treatment. This plan will significantly improve the efficiency of coal-fired power generation and realize near-zero emissions of CO2 and other pollutants in coal-fired power generation.
In 2012, the first stage of “GreenGen” was completed when the Tianjin IGCC demonstration power station began operation. With an installed capacity of 265 MW, the station features the world’s first two-sectioned, dry coal powder pressure, pure oxygen combustion gasification furnace. This technology is Huaneng’s independently developed intellectual property. After a long cycle of demonstrated operation, the emissions performance of the power station has proven significantly superior to conventional coal-fired power generation units. Its major emissions parameters—dust, 0.6 mg/m3; SO2, 0.9 mg/m3; NOx, 50 mg/m3— indicate that the IGCC station has reached the emissions level of a gas turbine power generation unit.
During the design stage of the IGCC power station, Huaneng also began R&D and demonstration of a pre-combustion CO2 capture system. The technological design model of an IGCC-based CO2 capture system was established through technical comparison and selection. The technical approach chosen used a sulfur-tolerant shift, MDEA decarbonization and purification, compression, and liquefaction of CO2. The energy and material balance of the system were calculated using a model; the optimization of the fundamental design plan took into account the characteristics of the site.13 The transformation technology of this project adopts a low water-vapor ratio sulfur-tolerant shift and makes full use of the low water content in the feed gas of the two-sectioned furnace by regulating the inlet temperature of the first section of the furnace and water-vapor ratio. The transformation furnace’s reaction depth can be controlled, achieving partial transformation of high-concentration CO, and reduces vapor consumption and increases output. The MDEA desulfurization and decarbonization device uses the technology of sectioned absorption with lean solution and semi-lean solution. The regeneration process combines regeneration of a normal-pressure absorption tower with regeneration of the stripping tower, fully utilizing the physical and chemical absorption properties of the solutions to lower energy consumption.
This device began operation in July 2016. The CO content at the outlet of the transformation section is approximately 1%. The system has been running consistently. Calculations based on on-site operation data indicates the following: the device’s CO2 capture rate is more than 85%; the system’s energy consumption is lower than 2.5GJ/t CO2; and the CO2 capture capacity is 60,000–100,000 t/yr. After the compression and liquefaction of CO2, the next step is to conduct experiments on increasing the oil recovery rate and applying geo-sequestration. The separated hydrogen-rich gases will be compressed and sent into the gas turbine for mixed combustion. Relevant geological evaluation and research into CO2 injection is still underway. This demonstration system, upon completion, will become a pre-combustion CCUS system with the largest capacity internationally. It will be able to conduct various experiments under different loads and operating conditions, accumulating experience for exploration of CCUS technologies with low energy consumption and high recycling rate.
Dealing with climate change is receiving increased attention worldwide, but the sustainable development of traditional fossil fuel power generation technologies are facing a bottleneck. CO2 capture technology provides a new approach for power enterprises’ carbon emissions. Huaneng Group was the first in China to carry out research into capture technologies for coal-fired power plants. They are executing near-zero emissions projects with pre-combustion capture technologies and have carried out industrial demonstration of post-combustion capture in power plants. Focusing on the critical issue of reducing energy consumption and cost, Huaneng conducts application experiments on new technology and continuous operation demonstration projects of various sizes. Relevant technologies have reached an advanced standard worldwide, laying a solid foundation for Chinese power plants to use the technologies in the future.
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