Compression And Purification Unit Research Articles

Energy efficiency analysis and optimization of a pressurized oxy-fuel circulating fluidized bed combustion system

The commercial application of oxy-fuel combustion technology is limited by energy deduction which is caused by addition of air separation unit (ASU) and carbon dioxide compression and purification unit (CPU). Pressurized circulating fluidized bed (CFB) oxy-fuel combustion technology is one option to improve the net electric generation efficiency. In this research, a series of simulations and optimizations were carried out to analyze the energy efficiency of a pressurized oxy-fuel combustion system. The boiler structure and heat exchange surface arrangement under different pressure were calculated. The cross-sectional area of the furnace and tail flue were both reduced up to 10 % of the original size and the floor space of the boiler was considered to be the lowest at 1.1 MPa. The energy efficiency and exergy analysis were used to explore the influence of pressure on the net efficiency and the optimization potential of system. The net electric generation efficiency reached a maximum value of 24.16 % at 1.1 MPa, which is 5.4 % higher than that at atmospheric pressure. Finally, four parameters, i.e. preheat temperature, oxygen concentration, ASU outlet oxygen purity and CPU compressor outlet pressure, were optimized to improve the system net efficiency to 26.67 %, 29.04 %, 25.87 % and 24.36 %, respectively.

System Analysis on Supercritical CO2 Power Cycle with Circulating Fluidized Bed Oxy-Coal Combustion

Supercritical carbon dioxide (S-CO2) Brayton power cycle is a competitive technology to achieve high efficiency in a variety of applications. However, in coal power applications, the CO2 generated from coal combustion still discharges into the atmosphere causing a series of environment problems. In this work, an 300 MWe S-CO2 power cycle with circulating fluidized bed (CFB) oxy-coal combustion was established including air separation unit (ASU), CFB boiler, recuperator system and carbon dioxide compression and purification unit (CPU). Based on the material and energy conservation, the cycle efficiency of S-CO2 (620°C, 25 MPa) Brayton power cycle with CFB oxy-coal combustion is evaluated compared to the oxy-coal combustion steam Rankine cycle and S-CO2 Brayton power cycle with the 31.65 kg/s coal supply. After that, the influence of several factors, e.g., exhaust flue gas temperature, split ratio in recuperator system and the oxygen supply on the cycle efficiency was investigated and analyzed. The results show that the net efficiency of S-CO2 power cycle with CFB oxy-coal combustion (32.67%) is much higher than the steam Rankine cycle utilizing CFB with 17.5 Mpa, 540°C steam (27.3%), and 25 Mpa, 620°C steam (30.15%) under the same exhaust flue gas temperature. In addition, lower exhaust flue gas temperature and higher split ratio are preferred to achieve higher cycle efficiency. Lower oxygen supply can reduce the energy consumption of ASU and CPU, further increasing the system net efficiency. However, the energy losses of ASU and CPU are still very large in oxy-coal combustion and need to be improved in further work.

Energy efficiency analysis of pressurized oxy-coal combustion system utilizing circulating fluidized bed

Oxy-fuel combustion with circulating fluidized bed (CFB) is one of the most important technology for carbon dioxide capture and sequestration. However, its development and application are seriously hindered by the low net efficiency mainly resulting from the use of air separation unit (ASU) and carbon dioxide compression and purification unit (CPU). In this work, a pressurized oxy-coal CFB combustion cycle was established mainly including ASU, CFB combustion unit, heat exchangers, power island and CPU. Based on the material and energy conservation, the detailed energy penalties of ASU, CPU, and recirculating compressor were investigated under the varying combustion pressures from 0.105 MPa to 3 MPa. Results revealed that with the increase of combustion pressure, the energy consumption of ASU increases while that of CPU decreases. Compared with PC boiler, the influence of the recirculating compression work on CFB system is negligible in lower pressures. Overall, Higher combustion pressure is favorable to the oxy-coal CFB combustion. By minimizing the penalties and recover more latent heat, the net efficiency rises up from 27.2% to 30.5% in this work while the optimum pressure being about 1.1 MPa. Meanwhile the mole fraction of carbon dioxide can reach 92% which is satisfied with the demands.

Thermodynamic Analysis and Optimization of an Oxyfuel Fluidized Bed Combustion Power Plant for CO2 Capture

In this paper, a 600 MW supercritical oxyfuel fluidized bed combustion power plant is integrated with an air separation unit (ASU) and a flue gas compression and purification unit (CPU). The plant performance is investigated, and two flue gas recirculation modes (i.e., wet and dry modes) are compared under different operating conditions. In the initial analysis, the dry-mode plant has a net power efficiency of 31.6%, while the wet-mode plant has a net power efficiency of 31.5%. In the sensitivity analysis, the air ingress has the most significant influence on the net power efficiency of the plant. The furnace combustion temperature also affects the net power efficiency. The plant is optimized by recovering heat from the ASU, CPU, and acid condenser and utilizing the heat to preheat the feedwater. Although the dry-mode plant has a higher efficiency before optimization, the wet-mode plant has a higher efficiency after optimization; that is, the net power efficiency increases by 2.7% for the wet mode compare...

An operational approach for the designing of an energy integrated oxy-fuel CFB power plant

In order to increase the overall net electric efficiency and consequently to decrease the energy penalty in oxy-fired systems, heat integration configurations must be proposed. Coupling among the Air Separation Unit (ASU), the Compression and Purification Unit (CPU) and flue gases leaving the boiler becomes critical to obtain good efficiency figures. Many solutions have tried to show outstanding efficiency results but practical proposals are necessary to develop the technology. The use of flue gases waste energy to recycle flue gases heating up, oxygen preheating and increasing temperature of feedwater to steam cycle has been proposed to surpass the efficiency reduction. Nevertheless, care should be taken as potential problems would appear if only theoretical analysis is carried out.This work deals with a suitable and flexible design to increase the overall efficiency of an oxy-fuel combustion power plant working with high O2 concentration. Waste energy has been integrated avoiding any potential risk/damage into a new designed steam cycle. Applied solutions try to use lower cost proven materials in heat exchangers and simple equipment designs avoiding gas-gas heaters. Novel arrangements are presented, such as indirect heat exchangers, plastic heaters or different configurations integrating high pressure feedwater and low pressure condensate mass flows. Finally, results are compared with a previously optimized power plant design without operational restrictions and just a slight reduction in power plant net efficiency (less than 1%) was observed between both concepts.

Sensitivity analysis and multi-objective optimization of CO2CPU process using response surface methodology

Compression and purification unit (CPU) is a common industrial process for capturing CO2 from oxy-fuel combustion where high energy requirement is one of its disadvantages. This study focuses on analyzing of the sensitivity and optimizing multi-objectively the operating conditions of CPU, using response surface methodology (RSM). The main objective was to increase the efficiency of CO2 removal from the oxy-fuel combustion power plant. Statistical analysis reveals that reducing the first separator temperature, not only, plays a major role in CO2 separation, but also, it decreases the total work and heat duty of the process. It was found that the optimal multi-stage CO2 compressor discharge pressure was 25.34 bar while regular pressure for this process was reported as 30 bar. Furthermore, the optimal flue gas temperature before, between and after compression, and the first and second flash separator temperatures were 20 °C, 20 °C, 20.6 °C, −38.2 °C, and −55 °C, respectively. with the previous works carried out in a constant amount of CO2 separation, the proposed process leads to lower pressure, and therefore lower operating and capital costs.

Minimization of CO2 capture energy penalty in second generation oxy-fuel power plants

Oxy-combustion is one of the most promising technologies to reduce CO2 emissions from coal-fired power plants. Nevertheless, as CO2 capture system there is an important energy penalty and efficiency of the overall power plant substantially decreases. It is well accepted that the application of first generation post-combustion and oxy-fuel combustion technologies reduce the power plant efficiency in 10–12 efficiency points.Air separation unit (ASU) and compression and purification unit (CPU) are the main energy consumers in the oxy-fuel process. Moreover, the oxidant flow, which is a mixture of O2 and recirculated flue gases (RFG), requires a high heating demand in order to preheat it before the boiler inlet.This paper presents a methodology for the minimization of the energy penalty in oxy-fuel power plants that also includes ASU and CPU optimized designs with lower energy consumption, a boiler working with a high oxygen concentration (up to 40%v) in oxidant and waste energy integrated with a new designed steam cycle. Results show an important increase in power plant net efficiency (36.42%, LHV basis) regarding oxy-fuel reference power plant (32.91%). As a consequence, energy penalty can be reduced from original 10.5 points to 7.3 points.

Optimization and control for CO2 compression and purification unit in oxy-combustion power plants

High CO2 purity products can be obtained from oxy-combustion power plants through a CO2 CPU (compression and purification unit) based on phase separation method. To ensure that CPU (with double flash separators) can be operated under optimal conditions, this paper focuses on single variable analysis, multi-variable optimization, dynamic simulation and control system design for CPU in oxy-combustion power plants. It is found that optimal operating conditions are 30 bar, 30.42 °C, −24.64 °C, and −55 °C for multi-stage CO2 compressor discharge pressure, flue gas temperature after compression, first flash separator temperature, and second flash separator temperature, respectively. The designed double temperature control structure based on a systematic top-down analysis and bottom-up design method is more suitable than the single temperature control structure under different operating scenarios (load change and flue gas composition ramp change). To bear operating disturbances, operating strategies like elevating temperature, manipulating valves and adjusting setpoints are proposed. Influence of SOx would be more obvious than that of NOx, whilst the kij mixing parameters in Peng–Robinson property method affects little on process optimization and control system design. Comprehensive dynamic model with specified control system provides possibility to integrate CPU with full-train oxy-combustion power plants.

Performance assessment of first generation oxy-coal power plants through an exergy-based process integration methodology

In this study, an exergy-based system level methodology is conducted to assess the energy penalty reduction potential of different configurations of first generation oxy-fired pulverized coal power plants. Once the process improvement potential is identified by exergy analysis on an oxy-fired plant with minimal integration, a heat integration methodology minimizing the exergy losses is carried out. Exergy analysis is employed as a guideline weighted by technological and operational constraints.This methodology has been applied to study the conventional cold flue gas recycle scheme and three alternative flue gas recirculation options including the widely studied warm recycle scheme. The integration potential of advanced cryogenic ASUs (air separation units) has also been assessed. Finally, the energy penalty associated to the capture when a high-purity CO2 flow is required has been determined.Among the different cases considered in this study, recirculation of the flue gas before the regenerative heater with an advanced ASU and a double-column CPU (compression and purification unit) leads to the highest NPE (net plant efficiency). For this case, the NPE is 39.1 % based on the lower heating value (7 %-pts energy penalty), which corresponds to a 6.5% improvement compared to the base-case plant. When high-purity CO2 is desired, the energy penalty is increased by 0.2 %-pts for the same configuration.

CO2 Balance in a Compression and Purification Unit (CPU)

The next step for many CO2 capture technologies is to move to commercial scale [1]. Without the experience that can only be gained through pilot plants, CCS will not become a commercially viable proposition due to unresolved technical challenges and uncertain cost estimations. Individual demonstration projects need to be at a scale that is sufficiently large to be representative of commercial operation.One of these individual projects was carried out by Fundación Ciudad de la Energía (CIUDEN), who successfully completed the full CO2 capture process in its Technology Development Centre for CO2 Capture and Transport using oxycombustion in Circulating Fluidized Bed (CFB) boiler and a cryogenic CO2 Compression and Purification Unit (CPU).Focused on the CIUDEŃs CPU, this paper describes the unit designed by Air Liquide, their sub-units in which the CO2 composition is increased to achieve the design value of >99.0% v in the liquefied product stream and a final case study based on an empirical mass balance; considering this data, in addition to the CO2 purity, a mass balance provides the recovery rate value; this last parameter was lower than the expected value. Studying the specific behaviour of each subunit, it is concluded that the warm-part of the CPU presents a recovery rate higher than 99% whereas the cold part of the unit (expansion and distillation columns to remove NOx and incondensable gases) obtained an average value of 60%, generated mainly because the CO2 composition in the inlet of the CPU was lower than 70% v w/b (design value). This was due to higher N2 content in the flue gas.Considering these results, the conclusions are very clear that air infiltration must be minimised. It also demonstrates the necessity of increasing the quantity of operating hours on demonstration plants such as CIUDEN in order to further optimise the technology with the end goal of improving the business case for CCS.

Oxy-combustion CPU – From Pilots Towards Industrial-scale Demonstration

The CPU (Compression and Purification Unit) is used to capture, purify and compress the CO2 contained in oxy-combustion boiler flue gas. Therefore, it plays a key role in the oxy-combustion route for carbon capture and storage (CCS). Over the last ten years Air Liquide has been developing this technology from a conceptual stage towards industrial maturity.Since the last GHGT conference in 2012 [1], significant progress has been made. The objective of this paper is to present an overview of the progress made in developing the technology with a particular focus on results from pilot plants, and the intended application of key technology developments to the FutureGen 2.0 project.

Evaluating CO2 Capture Technologies for Retrofit in Cement Plant

The retrofitting of two different CO 2 capture technologies to an existing cement plant has been evaluated technically. The two technologies considered are amine based post-combustion and oxy-combustion. These two technologies are chosen because amine based capture is considered to be closest to commercialization, while oxy- combustion is considered to be a possible future path for the cement industry. Amine based post-combustion CO 2 capture is an energy demanding process, therefore different energy supply options were considered. This included utilization of waste heat from the cement exhaust gas and the addition of a dedicated natural gas fired energy plant. The energy plant generated more CO 2 which also had to be captured resulting in a larger CO 2 capture plant compared to one where only the CO 2 from the cement is captured. Evaluations of the electricity consumption of oxy-combustion technologies identified the CO 2 compression and purification unit (CPU) as a major consumer. The amount of air in-leakage becomes an important factor for the CPU performance. Air in-leakage should be kept to a minimum to ensure effective CO 2 capture when retrofitting the cement plant with oxy-combustion CO 2 capture.

Study of Hg and SO 3 behavior in flue gas of oxy-fuel combustion system

Oxy-fuel combustion systems have been under development to reduce CO 2 emissions from coal-fired power plants. In oxy-fuel combustion system, Hg in the flue gas causes corrosion in CO 2 purification and compression units. Also, SO 3 in the flue gas corrodes the equipment and ducts of oxy-fuel combustion system. Therefore, Hg and SO 3 need to be removed. Babcock-Hitachi conducted tests using a 1.5 MWth Combustion & Air Quality Control System (AQCS) test facility which consists of oxygen supply unit, furnace, Selective Catalytic Reduction (SCR) catalyst, Clean Energy Recuperator (CER), Dry Electrostatic Precipitator (DESP), flue gas recirculation system, Wet Flue Gas Desulfurization (WFGD), and CO 2 Compression and Purification Unit (CPU). In both cases of air and oxy-fuel combustion, the Hg removal across the DESP could be improved, and SO 3 concentration at the DESP outlet could be reduced to less than 1 ppm by installing a CER upstream of the DESP and reducing the gas temperature at the DESP inlet. Hg was not dissolved in the drain recovered from CO 2 compressor, and may be adsorbed at an inner part of CO 2 compressor. This indicated that Hg needs to be removed at a location upstream of the CO 2 compressor to prevent corrosion of the compressor.