Cyclic CO2 Capture Research Articles

Simulation and Optimization of Evaporative Gas Turbine with Chemical Absorption for Carbon Dioxide Capture

This article studied the integration of an evaporative gas turbine (EvGT) cycle with chemical absorption for CO2 capture. Two systems of EvGT cycle without CO2 capture and EvGT cycle with CO2 capture were simulated and optimized. The impacts of key parameters such as the water/air ratio (W/A), the stripper pressure, and the flue-gas condensing temperature were studied regarding the electrical efficiency and CO2 reduction. Simulation results show that (1) there always exists an optimum point of W/A for both EvGT and EvGT combined with CCS; (2) although lowering the stripper pressure would lower the heat quality requirement of reboiler, it increases the quantity more obviously. Therefore increasing the operating pressure of stripper would help to increase the total electrical efficiency; but the efficiency improvement becomes smaller if stripper pressure is high; (3) adding a flue-gas condenser to condense out the excessive water is another method to increase the total electrical efficiency. There is also an optimum point of condensing temperature considering the concentration of mono ethanol amine (MEA) and inlet temperature of stripper; and (4) comparatively the combined cycle has a higher gross electricity generation and electrical efficiency than the EvGT cycle no matter if combined with CO2 capture or not.

CaO-Based Pellets Supported by Calcium Aluminate Cements for High-Temperature CO2 Capture

The development of highly efficient CaO-based pellet sorbents, using inexpensive raw materials (limestones) or the spent sorbent from CO2 capture cycles, and commercially available calcium aluminate cements (CA-14, CA-25, Secar 51, and Secar 80), is described here. The pellets were prepared using untreated powdered limestones or their corresponding hydrated limes and were tested for their CO2 capture carrying capacities for 30 carbonation/calcination cycles in a thermogravimetric analyzer (TGA). Their morphology was also investigated by scanning electron microscopy (SEM) and their compositions before and after carbonation/calcination cycleswere determined by X-ray diffraction (XRD). Pellets prepared in this manner showed superior behavior during CO2 capture cycles compared to natural sorbents, with the highest conversions being > 50% after 30 cycles. This improved performance was attributed to the resulting substructure of the sorbent particles, i.e., a porous structure with nanoparticles incorporated. During carbonation/calcination cycles mayenite (Ca12Al14O33) was formed, which is believed to be responsible for the favorable performance of synthetic CaO-based sorbents doped with alumina compounds. An added advantage of the pellets produced here is their superior strength, offering the possibility of using them in fluidized bed combustion (FBC) systems with minimal sorbent loss due to attrition.

Experiment and Modeling of CO2 Capture from Flue Gases at High Temperature in a Fluidized Bed Reactor with Ca-Based Sorbents

The cyclic CO2 capture and CaCO3 regeneration characteristics in a small fluidized bed reactor were experimentally investigated with limestone and dolomite sorbents. Kinetic rate constants for carbonation and calcination were determined using thermogravimetric analysis (TGA) data. Mathematical models developed to model the Ca-based sorbent multiple cycles of CO2 capture and calcination in the bubbling fluidized bed reactor agreed with the experimental data. The experimental and simulated results showed that the CO2 in flue gases could be absorbed efficiently by limestone and dolomite. The time for high-efficiency CO2 capture decreased with an increasing number of cycles because of the loss of sorbent activity, and the final CO2 capture efficiency remained nearly constant as the sorbent reached its final residual capture capacity. In a continuous carbonation and calcination system, corresponding to the sorbent activity loss, the carbonation kinetic rates of sorbent undergoing various cycles are different, ...

CO2 Looping Cycle Performance of a High-Purity Limestone after Thermal Activation/Doping

The influence of thermal pretreatment on the performance of a high-purity limestone (La Blanca) during CO2 capture cycles is investigated in this paper. This limestone was chosen for more detailed investigation because, in earlier research, it failed to show any favorable effect as a result of thermal pretreatment. Here, the original sample, with a particle size of 0.4−0.6 mm, and ground samples were thermally pretreated at 1000−1200 °C, for 6−24 h, and then subjected to several carbonation/calcination cycles in a thermogravimetric analyzer (TGA). This work shows that thermal pretreatment failed to produce a significant self-reactivation effect during CO2 cycles, despite the use of a wide range of conditions during pretreatment (grinding, temperature, and pretreatment duration) as well as during cycling (CO2 concentration and duration of the carbonation stage). Additional doping experiments showed that both high Na content and lack of Al in La Blanca limestone cause poor self-reactivation performance afte...

Qualitative and Quantitative Comparison of Two Promising Oxy-Fuel Power Cycles for CO2 Capture

Since the Kyoto conference, there is a broad consensus that the human emission of greenhouse gases, mainly CO2, has to be reduced. In the power generation sector, there are three main alternatives that are currently studied worldwide. Among them oxy-fuel cycles with internal combustion with pure oxygen are a very promising technology. Within the European project ENCAP (enhanced CO2 capture) the benchmarking of a number of novel power cycles with CO2 capture was carried out. Within the category oxy-fuel cycles, the Graz Cycle and the semiclosed oxy-fuel combustion combined cycle (SCOC-CC) both achieved a net efficiency of nearly 50%. In a second step, a qualitative comparison of the critical components was performed according to their technical maturity. In contrast to the Graz Cycle, the study authors claimed that no major technical barriers would exist for the SCOC-CC. In this work, the ENCAP study is repeated for the SCOC-CC and for a modified Graz Cycle variant as presented at the ASME IGTI Conference 2006. Both oxy-fuel cycles are thermodynamically investigated based on common assumptions agreed upon with the industry in previous work. The calculations showed that the high-temperature turbine of the SCOC-CC plant needs a much higher cooling flow supply due to the less favorable properties of the working fluid. A layout of the main components of both cycles is further presented, which shows that both cycles rely on the new designs of the high-temperature turbine and the compressors. The SCOC-CC compressor needs more stages due to a lower rotational speed but has a more favorable operating temperature. In general, all turbomachines of both cycles show similar technical challenges and are regarded as feasible.

Cycle Development and Design for CO2Capture from Flue Gas by Vacuum Swing Adsorption

CO2 capture and storage is an important component in the development of clean power generation processes. One CO2 capture technology is gas-phase adsorption, specifically pressure (or vacuum) swing adsorption. The complexity of these processes makes evaluation and assessment of new adsorbents difficult and time-consuming. In this study, we have developed a simple model specifically targeted at CO2 capture by pressure swing adsorption and validated our model by comparison with data from a fully instrumented pilot-scale pressure swing adsorption process. The model captures nonisothermal effects as well as nonlinear adsorption and nitrogen coadsorption. Using the model and our apparatus, we have designed and studied a large number of cycles for CO2 capture. We demonstrate that by careful management of adsorption fronts and assembly of cycles based on understanding of the roles of individual steps, we are able to quickly assess the effect of adsorbents and process parameters on capture performance and identify optimal operating regimes and cycles. We recommend this approach in contrast to exhaustive parametric studies which tend to depend on specifics of the chosen cycle and adsorbent. We show that appropriate combinations of process steps can yield excellent process performance and demonstrate how the pressure drop, and heat loss, etc. affect process performance through their effect on adsorption fronts and profiles. Finally, cyclic temperature profiles along the adsorption column can be readily used to infer concentration profiles-this has proved to be a very useful tool in cyclic function definition. Our research reveals excellent promise for the application of pressure/vacuum swing adsorption technology in the arena of CO2 capture from flue gases.

SO2 Retention by Reactivated CaO-Based Sorbent from Multiple CO2 Capture Cycles

This paper examines the reactivation of spent sorbent, produced from multiple CO2 capture cycles, for use in SO2 capture. CaO-based sorbent samples were obtained from Kelly Rock limestone using three particle size ranges, each containing different impurities levels. Using a thermogravimetric analyzer (TGA), the sulfation behavior of partially sulfated and unsulfated samples obtained after multiple calcination-carbonation cycles in a tube furnace (TF), following steam reactivation in a pressurized reactor, is examined. In addition, samples calcined/sintered under different conditions after hydration are also examined. The results show that suitably treated spent sorbent has better sulfation characteristics than that of the original sorbent. Thus for example, after 2 h sulfation, > 80% of the CaO was sulfated. In addition, the sorbent showed significant activity even after 4 h when > 95% CaO was sulfated. The results were confirmed by X-ray diffraction (XRD) analysis, which showed that, by the end of the sulfation process, samples contained CaSO4 with only traces of unreacted CaO. The superior behavior of spent reactivated sorbent appears to be due to swelling of the sorbent particles during steam hydration. This enables the development of a more suitable pore surface area and pore volume distribution for sulfation, and this has been confirmed by N2 adsorption-desorption isotherms and the Barrett-Joyner-Halenda (BJH) method. The surface area morphology of sorbent after reactivation was examined by scanning electron microscopy (SEM). Ca(OH)2 crystals were seen, which displayed their regular shape, and their elemental composition was confirmed by energy-dispersive X-ray (EDX) analysis. The improved characteristics of spent reactivated sorbent in comparison to the original and to the sorbent calcined under different conditions and hydrated indicate the beneficial effect of CO2 cycles on sorbent reactivation and subsequent sulfation. These results allow us to propose a new process for the use of CaO-based sorbent in fluidized bed combustion (FBC) systems, which incorporates CO2 capture, sorbent reactivation, and SO2 retention.

Sequential Capture of CO2 and SO2 in a Pressurized TGA Simulating FBC Conditions

Four FBC-based processes were investigated as possible means of sequentially capturing SO2 and CO2. Sorbent performance is the key to their technical feasibility. Two sorbents (a limestone and a dolomite) were tested in a pressurized thermogravimetric analyzer (PTGA). The sorbent behaviors were explained based on complex interaction between carbonation, sulfation, and direct sulfation. The best option involved using limestone or dolomite as a SO2-sorbent in a FBC combustor following cyclic CO2 capture. Highly sintered limestone is a good sorbent for SO2 because of the generation of macropores during calcination/carbonation cycling.

Combined cycles for CO<SUB align=right>2-capture with high efficiency

The presented paper gives an overview on combined cycles for CO2-capture with high efficiency focussed on processes using a Solid Oxide Fuel Cell (SOFC), because of the air separating ability of the SOFC-membrane. The SOFC-membrane is only permeable for oxygen ions, so the cathode exhaust gas is oxygen depleted air, which should be discharged to the atmosphere after recycling or using the heat via a heat exchanger. Using fuels consisting of hydrocarbons like gasified fossil fuels and biomass or reformed (bio)gas, the anode exhaust gas consists of CO2, water vapour and not used gaseous fuel. If the not used gaseous fuel is separated and recycled instead of being burnt by mixing with the cathode exhaust gas as usual today, CO2 is left as only anode exhaust gas by simply condensing the water vapour. Similar results can be achieved by a mixed conducting membrane and chemical (metal?metal oxide) looping.

Removal of CO2by Calcium-Based Sorbents in the Presence of SO2

In an investigation of in situ CO2 removal for fluidized-bed combustion processes, seven calcium-based sorbents were tested for simultaneous CO2 and SO2 removal using both an atmospheric thermogravimetric reactor and a pressurized thermogravimetric analyzer. SO2 was found to impede cyclic CO2 capture because of pore blockage by sulfate products, resulting primarily from direct sulfation during the later stage of each cycle. The sorbents showed similar patterns during cocapture. Loss in sorbent reversibility could not be prevented but was improved by higher CO2 partial pressures.

Use of the castner reaction for hydrogen fuel production

The predicted increase in global temperatures requires that greenhouse gas emissions, especially carbon dioxide, are reduced. This may necessitate the development of alternative methods to produce fuel for electricity generation and transport. Adaptation of the Castner process provides a method by which hydrogen can be generated in industrial quantities from all forms of fossil and other carbon based fuels from any source, i.e. natural, biomass and waste. The amount of carbon dioxide produced by this method is less than that presently generated by fossil fuel burning power stations. Also, the carbon dioxide produced is in a form suitable for capture and disposal by the Weldon process. The component processes of the Castner–Weldon cycle for hydrogen generation and CO2 capture are well established, despite the fact that these processes have not been in use for about 100 years. The Castner–Weldon process can be operated in the UK from materials available within the UK and is sustainable as long as any form of carbon based fuel is available. Initially development of this method only requires information concerning the efficiency and cost compared to existing methods. The costs involved for this stage are unlikely to exceed £1 000 000. It is envisaged that most of the present coal fired power stations would not have to be rebuilt immediately but merely adapted, thus reducing the cost of conversion to hydrogen use. The cost of conversion cannot be in excess of the present construction cost of these power stations at £0.5 billion and is considerably less than the construction cost of equivalent nuclear power station at £1.5 billion.

06/01718 Particle-metal interactions during combustion of pulp and paper biomass in a fluidized bed combustor: Eldabbagh, F. et al. Combustion and Flame, 2005, 142, (3), 249–257

Sulphation and carbonation have been performed on hydrated spent residues from a 75 kWth dual fluidized bed combustion (FBC) pilot plant operating as a CO2 looping cycle unit. The sulphation and carbonation tests were done in an atmospheric pressure thermogravimetric analyzer (TGA), with the sulphation performed using synthetic flue gas (0.45% SO2, 3% O2, 15% CO2 and N2 balance). Additional tests were carried out in a tube furnace (TF) with a higher SO2 concentration (1%) and conversions were determined by quantitative X-ray diffraction (QXRD) analyses. The morphology of the sulphated samples from the TF was examined by scanning electron microscopy (SEM). Sulphation tests were performed at 850 °C for 150 min and carbonation tests at 750 °C, 10 cycles for 15 min (7.5 min calcination + 7.5 min carbonation). Sulphation conversions obtained for the hydrated samples depended on sample type: in the TGA, they were ∼75–85% (higher values were obtained for samples from the carbonator); and in the TF, values around 90% and 70% for sample from carbonator and calciner, respectively, were achieved, in comparison to the 40% conversion seen with the original sample. The SEM analyses showed significant residual porosity that can increase total conversion with longer sulphation time. The carbonation tests showed a smaller influence of the sample type and typical conversions after 10 cycles were 50% – about 10% higher than that for the original sample. The influence of hydration duration, in the range of 15–60 min, is not apparent, indicating that samples are ready for use for either SO2 retention, or further CO2 capture after at most 15 min using saturated steam. The present results show that, upon hydration, spent residues from FBC CO2 capture cycles are good sorbents for both SO2 retention and additional CO2 capture.

Effect of Preparation Temperature on Cyclic CO2 Capture and Multiple Carbonation−Calcination Cycles for a New Ca-Based CO2 Sorbent

The cyclic CO2 capture, transient phases change, and microstructure appearance of a new kind of Ca-based regenerable CO2 sorbent, CaO/Ca12Al14O33, obtained by the integration of CaO as solid reactant with a composite metal oxide of Ca12Al14O33 as a binder, were investigated by thermogravimetric analysis, XRD, and SEM at different preparation calcination temperatures. When the calcination temperature in the preparation stage is higher than 1000 °C, the cyclic CO2 capture of this new sorbent declines. The lowered CO2 capture may mainly be attributed to the formation of Ca3Al2O6, which decreases the ratio of CaO to binder in sorbent, and the severe sintering of sorbent occurs when calcined at such high temperatures in the preparation processes. These results suggest that the calcination temperature for this new sorbent should not be higher than 1000 °C in order to obtain its high reactivity. The performance of the new sorbent over 50 cycles was evaluated under mild and severe regeneration conditions, respectively. CaO/Ca12Al14O33 attained 41 wt % CO2 capture after 50 carbonation−calcination cycles under mild calcination conditions (850 °C, 100% N2), and the results obtained here indicate that the new sorbent, CaO/Ca12Al14O33, has significantly improved CO2 capture and cyclic reaction stability over multiple carbonation−calcination cycles compared with limestone and dolomite under mild calcination conditions. When more severe calcination conditions (980 °C, 100% CO2) were used, the capture of CaO/Ca12Al14O33 decreased from 52 wt % in the first cycle to about 22 wt % in the 56th cycle; however, the capture of CaO/Ca12Al14O33 sorbent over 56 cycles is still higher than that of dolomite and limestone under the same severe calcination conditions.

New Pressure Swing Adsorption Cycles for Carbon Dioxide Sequestration

A rigorous pressure swing adsorption (PSA) process simulator was used to study a new, high temperature PSA cycle, based on the use of a K-promoted HTlc adsorbent and a simple, 4-step, Skarstrom-type, vacuum swing cycle designed to process a typical stack gas effluent at 575 K containing (in vol%) 15% CO2, 75% N2 and 10% H2O. The effects of the purge-to-feed ratio (γ), cycle step time (t s ) (with all four steps of equal time), and pressure ratio (π T ) on the process performance was studied in terms of the CO2 recovery (R) and enrichment (E) at a constant throughput θ of 14.4 L STP/hr/ kg. R increased with increasing γ and π T and decreasing t s , while E increased with increasing t s and π T and decreasing γ. The highest E of 3.9 was obtained at R = 87% and π T = 12, whereas at R = 100% the highest E of 2.6 was obtained at π T = 12. These results are very encouraging and show the potential of a high temperature PSA cycle for CO2 capture.

Control Issues in the Design of a Gas Turbine Cycle for CO2 Capture

This article is concerned with control issues related to the design of a semi-closed O 2/CO 2 gas turbine cycle for CO 2 capture. Some control strategies and their interaction with the process design are discussed. One control structure is implemented on a dynamic simulation model using a predictive controller, and simulations assess the performance and compare its merits with a conventional PI structure. The results indicate that it can be advantageous for operability to allow a varying (as opposed to fixed) compressor inlet pressure, at the cost of a more expensive design. Furthermore, the results show that a predictive controller has some advantages with respect to the simpler conventional PI control structure, in particular in terms of constraint handling.

Modeling and Control of a O2/CO2 Gas Turbine Cycle for CO2 Capture

This article is concerned with the dynamics and control of a semi-closed O2/CO2 gas turbine cycle for CO2 capture. In the first part the process is described and a model is developed. Thereafter, a control system structure is proposed, and an output feedback model predictive control algorithm is implemented and simulated on the process.

00/01518 The effects of additive on limestone capturing sulfur during coal combustion

The effects of manganese salts including Mn(NO3)2 and MnCO3 on CO2 capture performance of calcium-based sorbent during cyclic calcination/carbonation reactions were investigated. Mn(NO3)2 and MnCO3 were added by wet impregnation method. The cyclic CO2 capture capacities of Mn(NO3)2-doped CaCO3, MnCO3-doped CaCO3 and original CaCO3 were studied in a twin fixed-bed reactor and a thermo-gravimetric analyzer (TGA), respectively. The results show that the addition of manganese salts improves the cyclic carbonation conversions of CaCO3 except the previous cycles. When the Mn/Ca molar ratios are 1/100 for Mn(NO3)2-doped CaCO3 and 1.5/100 for MnCO3-doped CaCO3, the highest carbonation conversions are achieved respectively. The carbonation temperature of 700–720 °C is beneficial to CO2 capture of Mn-doped CaCO3. The residual carbonation conversions of Mn(NO3)2-doped and MnCO3-doped CaCO3 are 0.27 and 0.24 respectively after 100 cycles, compared with the conversion of 0.16 for original one after the same number of cycles. Compared with calcined original CaCO3, better pore structure is kept for calcined Mn-doped CaCO3 during calcium looping cycle. The pore volume of calcined MnCO3-doped CaCO3 is 2.4 times as high as that of calcined original CaCO3 after 20 cycles. The pores of calcined MnCO3-doped CaCO3 in the pore size range of 27–142 nm are more abundant relative to clacined original one. That is why modification by manganese salts can improve cyclic CO2 capture capacity of CaCO3.