Modelling of a fluidized bed carbonator reactor to capture CO2 from a combustion flue gas

In recent years several processes incorporating a carbonation-calcination loop in an interconnected fluidized bed reactor have been proposed as a way to capture CO₂ from flue gases. This paper is a first approximation to the modelling of a fluidized bed carbonator reactor. In this reactor the flue gas comes into contact with an active bed composed of particles with very different activities, depending on their residence time in the bed and in the carbonation-calcination loop. The model combines the residence time distribution functions with existing knowledge about sorbent deactivation rates and sorbent reactivity. The fluid dynamics of the solids (CSTR) and gases (PF) in the carbonator are based on simple assumptions. The carbonation rates are modelled defining a characteristic time for the transition between a fast reaction regime to a regime with a zero reaction rate. On the basis of these assumptions the model is able to predict the CO₂ capture efficiency for the flue gas depending on the operating and design conditions. Operating windows with high capture efficiencies are discussed, as well as those conditions where only modest capture efficiencies are possible.     

Experimental study on CO2 capture conditions of a fluidized bed limestone decomposition reactor

In this study, the decomposition conditions of limestone particles (0.25-0.50 mm) for CO₂ capture in a steam dilution atmosphere (20-100% steam in CO₂) were investigated by using a continuously operating fluidized bed reactor. The results show that the decomposition conversion of limestone increased with the steam dilution percentage in the CO₂ supply gas. At a bed temperature of 920 °C, the conversions were 72% without steam dilution and 98% with 60% steam dilution. The conversion was 99% with 100% steam dilution at 850 °C of the bed temperature. Steam dilution can decrease not only the decomposition temperature of limestone, but also the residence time required for nearly complete decomposition of CaCO₃. The hydration and carbonation reactivities of the CaO produced were also tested and the results show that both the reactivities increased with the steam dilution percentage for decomposing limestone.     

A model on an entrained bed-bubbling bed process for CO2 capture from flue gas

A simplified model has been developed to investigate effects of important operating parameters on performance of an entrained-bed absorber and bubbling-bed regenerator system collecting CO₂ from flue gas. The particle population balance was considered together with chemical reaction to determine the extent of conversion in both absorber and regenerator. The calculated CO₂ capture efficiency agreed with the measured value reasonably well. Effects of absorber parameters — temperature, gas velocity, static bed height, moisture content of feed gas on CO₂ capture efficiency — have been investigated in a laboratory scale process. The CO₂ capture efficiency decreased as temperature or gas velocity increased. However, it increased with static bed height or moisture concentration. The CO₂ capture efficiency was exponentially proportional to each parameter. Based on the absolute value of exponent of the parameter, the effect of gas velocity, static bed height, and moisture content was one-half, one-third, and one-fourth as strong as that of temperature, respectively.     

Preliminary analysis of process flow sheet modifications for energy efficient CO2 capture from flue gases using chemical absorption

The energy penalty associated with solvent based capture of CO₂ from power station flue gases can be reduced by incorporating process flow sheet modifications into the standard process. A review of modifications suggested in the open and patent literature identified several options, primarily intended for use in the gas processing industry. It was not immediately clear whether these options would have the same benefits when applied to CO₂ capture from near atmospheric pressure combustion flue gases. Process flow sheet modifications, including split flow, rich split, vapour recompression, and inter-stage cooling, were therefore modelled using a commercial rate-based simulation package. The models were completed for a Queensland (Australia) based pilot plant running on 30% MEA as the solvent. The preliminary modelling results showed considerable benefits in reducing the energy penalty of capturing CO₂ from combustion flue gases. Further work will focus on optimising and validating the most relevant process flow sheet modifications in a pilot plant.     

A simplified model of SO2 capture by limestone in 71 MWe pressurized fluidized bed combustor

A mathematical model of SO₂ capture by uncalcined limestone particles with solid attrition under pressurized fluidized bed combustion conditions was developed based on the shrinking unreacted-core model. Since the thickness of the product layer is sufficiently much smaller than the particle size, a flat surface model was employed. The difference in SO₂ capture behavior between continuous solid attrition and intermittent attrition was investigated. The reaction rate for intermittent solid attrition was found to be lower than that for continuous attrition mode under low SO₂ concentration conditions. A simple mathematical expression to calculate reaction rate of SO₂ capture per unit external surface area of limestone is proposed.The present simplified mathematical model of SO₂ capture by single limestone particle under periodical attrition conditions was applied to the analysis of a large-scale pressurized fluidized bed combustor. By giving the period of attrition as a parameter, the experimental results agreed well with the model results. From the vertical concentration profile of SO₂ concentration, the emission of SO₂ was found to be governed by the balance between SO₂ formation rate from char and SO₂ capture by limestone at the upper surface of the dense bed. A simplified expression to estimate SO₂ emission from pressurized fluidized bed combustors was proposed.     

Pith based spherical activated carbon for CO2 removal from flue gases

Serials of pitch based spherical activated carbons (PSACs) were prepared and used as adsorbent for CO₂ adsorption from flue gases. The results indicate that the ultrafine micropores (<1 nm) are effective pores for CO₂ adsorption, and the equilibrium adsorption capacity of CO₂ has a linear relationship with the specific surface area of ultrafine micropores (S_{<1 nm}). The adsorption capacity of CO₂ can obtain 1.12 mmol/g at 15 kPa and 30 °C on one of PSAC sample due to its high S_{<1 nm} (845 m²_/g). Because the molecular CO₂ can be polarized into polar molecules and has four kinds of adsorption configuration, the adsorption selectivities of CO₂ vs. N₂ and O₂ are 86.99% and 69.91%, respectively. When the combined Electric Swing Adsorption and Vacuum Swing Adsorption were applied for CO₂ desorption, about 100% desorption efficiency can be obtained, the desorption rate is twice of that with Temperature Swing Adsorption (TSA) and the energy consumption is only 69% of that with TSA.     

Separation of CO2 from flue gas using electrochemical cells

Past research with high temperature molten carbonate electrochemical cells has shown that carbon dioxide can be separated from flue gas streams produced by pulverized coal combustion for power generation. However, the presence of trace contaminants, i.e., sulfur dioxide and nitric oxides, will impact the electrolyte within the cell. If a lower temperature cell could be devised that would utilize the benefits of commercially-available, upstream desulfurization and denitrification in the power plant, then this CO₂ separation technique can approach more viability in the carbon sequestration area. Recent work has led to the assembly and successful operation of a low temperature electrochemical cell. In the proof-of-concept testing with this cell, an anion exchange membrane was sandwiched between gas-diffusion electrodes consisting of nickel-based anode electrocatalysts on carbon paper. When a potential was applied across the cell and a mixture of oxygen and carbon dioxide was flowed over the wetted electrolyte on the cathode side, a stream of CO₂ to O₂ was produced on the anode side, suggesting that carbonate/bicarbonate ions are the CO₂ carrier in the membrane. Since a mixture of CO₂ and O₂ is produced, the possibility exists to use this stream in oxy-firing of additional fuel.From this research, a novel concept for efficiently producing a carbon dioxide rich effluent from combustion of a fossil fuel was proposed. Carbon dioxide and oxygen are captured from the flue gas of a fossil-fuel combustor by one or more electrochemical cells or cell stacks. The separated stream is then transferred to an oxy-fired combustor which uses the gas stream for ancillary combustion, ultimately resulting in an effluent rich in carbon dioxide. A portion of the resulting flow produced by the oxy-fired combustor may be continuously recycled back into the oxy-fired combustor for temperature control and an optimal carbon dioxide rich effluent.     

Sorbent attrition in a carbonation/calcination pilot plant for capturing CO2 from flue gases

There is increasing interest in CO₂ looping cycles that involve the repeated calcination and carbonation of the sorbent as a way to capture CO₂ from flue gases during the carbonation step and the generation of a pure stream of CO₂ in the oxyfired calcination step. In particular, attrition of the material in these interconnected fluidized bed reactors is a problem of general concern. Attrition of limestone derived materials has been studied in fluidized bed systems by numerous authors. In this work, we have investigated the attrition of two limestones used in a system of two interconnected circulating fluidized bed reactors operating in continuous mode as carbonation and calciner reactors. We observed a rapid initial attrition of both limestones during the calcination step which was then followed by a highly stable period (up to 140 h of added circulation for one of the limestones) during which particle size changes were negligible. This is consistent with previous observations of attrition in other systems that employ these materials. However, a comparison of the attrition model constants with the data reported in the literature showed the two limestones to be particularly fragile during the initial calcination and the first few hours of circulation. Thus, a careful choice of limestone based on its attrition properties must be taken into account in designing future carbonate looping systems.     

Comparison among attrition-reaction models of SO2 capture by uncalcined limestone under pressurized fluidized bed combustion conditions

Mathematical models of SO₂ capture by uncalcined limestone (CaCO₃) particles with solid attrition were compared under pressurized fluidized bed combustion conditions. For reaction, we used: (1) a shrinking core model with a distinct border between the product (CaSO₄) layer with a conversion of unity and unreacted core with a conversion of zero, and (2) a distributed reaction model with smooth transition from the unreacted part to the product part with conversion between zero and unity. Continuous attrition and intermittent attrition were compared for attrition. Apparent conversion of the solid was overestimated regardless of the reaction model for continuous attrition. Attrition model plays an important role in determining limestone utilization efficiency, whereas the reaction model played only a minor role.     

Syngas production by biomass gasification using Rh/CeO2/SiO2 catalysts and fluidized bed reactor

We have been developed novel catalysts for gasification of biomass with much higher energy efficiency than conventional methods (non-catalyst, dolomite, commercial steam reforming Ni catalyst). From the result of the gasification of cellulose over novel Rh/CeO₂/SiO₂ catalysts, it is found that the gasification process consists of the reforming of tar and the combustion of solid carbon. We also tested novel Rh/CeO₂/SiO₂ in the gasification with air, pyrogasification, and steam reforming of cedar wood. As a result, Rh/CeO₂/SiO₂ gave higher yield of syngas than the conventional steam reforming Ni catalyst. Furthermore, we compared the performance between single and dual bed reactors. Single bed reactor was effective in the gasification of cedar, however, it was not suitable for the gasification of rice straw since a rapid deactivation was observed. Gasification of rice straw, jute stick, baggase using the fluidized dual-bed reactor and Rh/CeO₂/SiO₂ was also investigated. Especially, the catalyst stability in the gasification of rice straw clearly was enhanced by using the fluidized dual bed reactor.     

Design and operation of pilot plant for CO2 capture from IGCC flue gases by combined cryogenic and hydrate method

This project is a trial conducted under contract with CO₂CRC, Australia of a new CO₂ capture technology that can be applied to integrated gasification combined cycle power plants and other industrial gasification facilities. The technology is based on combination of two low temperature processes, namely cryogenic condensation and the formation of hydrates, to remove CO₂ from the gas stream. The first stage of this technology is condensation at −55 °C where CO₂ concentration is expected to be reduced by up to 75 mol%. Remaining CO₂ is captured in the form of solid hydrate at about 1 °C reducing CO₂ concentration down to 7 mol% using hydrate promoters. This integrated cryogenic condensation and CO₂ hydrate capture technology hold promise for greater reduction of CO₂ emissions at lower cost and energy demand. Overall, the process produced gas with a hydrogen content better than 90 mol%. The concentrated CO₂ stream was produced with 95-97 mol% purity in liquid form at high pressure and is available for re-use or sequestration. The enhancement of carbon dioxide hydrate formation and separation in the presence of new hydrate promoter is also discussed. A laboratory scale flow system for the continuous production of condensed CO₂ and carbon dioxide hydrates is also described and operational details are identified.     

Spatial distributions of O2, CO2 and SO2 in a pilot-scale fluidized bed combustor operated with different coals

Axial concentration profiles of O₂, CO₂ and SO₂ have been measured at different radial positions in a coal-fired fluidized bed combustor. The profiles show some spatial inhomogeneity in the bed, the degree of which depends on fluidizing velocity and amount of excess air. Comparison of SO₂- with CO₂-concentration profiles reveals that SO₂ is formed in proportion to CO₂ only with anthracite as fuel. With bituminous coals, SO₂ is preferentially formed near the coal feed point. These results are discussed with regard to characteristic times of mixing and chemical reaction in fluidized beds, and SO₂-formation characteristics of the employed coals obtained in a thermobalance.     

CO2 capture using some fly ash-derived carbon materials

Adsorption is considered to be one of the more promising technologies for capturing CO₂ from flue gases. For post-combustion capture, the success of such an approach is however dependent on the development of an adsorbent that can operate competitively at relatively high temperatures. In this work, low cost carbon materials derived from fly ash, are presented as effective CO₂ sorbents through impregnation these with organic bases, for example, polyethylenimine aided by polyethylene glycol. The results show that for samples derived from a fly ash carbon concentrate, the CO₂ adsorption capacities were relatively high (up to 4.5 wt%) especially at high temperatures (75 °C), where commercial active carbons relying on physi-sorption have low capacities. The addition of PEG improves the adsorption capacity and reduces the time taken for the sample to reach the equilibrium. No CO₂ seems to remain after desorption, suggesting that the process is fully reversible.     

CO2 capture using membrane contactors: a systematic literature review

With fossil fuel being the major source of energy, CO₂ emission levels need to be reduced to a minimal amount namely from anthropogenic sources. Energy consumption is expected to rise by 48% in the next 30 years, and global warming is becoming an alarming issue which needs to be addressed on a thorough technical basis. Nonetheless, exploring CO₂ capture using membrane contactor technology has shown great potential to be applied and utilised by industry to deal with post- and pre-combustion of CO₂. A systematic review of the literature has been conducted to analyse and assess CO₂ removal using membrane contactors for capturing techniques in industrial processes. The review began with a total of 2650 papers, which were obtained from three major databases, and then were excluded down to a final number of 525 papers following a defined set of criteria. The results showed that the use of hollow fibre membranes have demonstrated popularity, as well as the use of amine solvents for CO₂ removal. This current systematic review in CO₂ removal and capture is an important milestone in the synthesis of up to date research with the potential to serve as a benchmark databank for further research in similar areas of work. This study provides the first systematic enquiry in the evidence to research further sustainable methods to capture and separate CO₂.     

Photooxidation of anionic surfactant (sodium lauryl sulfate) in a three-phase fluidized bed reactor using TiO2/SiO2 photocatalyst

The photooxidation of sodium lauryl sulfate (=sodium dodecyl sulfate) in two different types of three-phase fluidized bed reactors was investigated. A low concentration of sodium lauryl sulfate (0.1–0.6 mM) in aqueous solution was photocatalytically decomposed by a TiO₂ photocatalyst immobilized on a porous SiO₂ support. In order to determine the optimum operating conditions in the fluidized beds, the effects of the air flow rate, amount of catalyst, initial concentration of surfactant, light source power, and pH on the photooxidation rate were investigated. From the experimental results, it was observed that the superficial air velocity was an important parameter in determining the reaction rate for both reactors. The photooxidation reaction rate increased with increasing UV lamp power and the experimentally obtained reaction rates showed good agreement with the Langmuir adsorption model. Also, a higher reaction rate was observed when the aqueous solution was acidic.     

Modelling of CO2, CO, SO2, O2 and NOx emissions from the oxy-fuel combustion in a circulating fluidized bed

The paper presents a model of coal combustion in air and oxygen-enriched CFB environment. A computer program to calculate the CO₂, CO, SO₂, NO_x and O₂ emissions from the combustion of solid fuels in a circulating fluidized bed boiler was created. The validity of this program was verified by measurements on a 0.1MW_th OxyFuel-CFB Test Rig.The calculations have been carried out for air and so-called oxy-fuel conditions, i.e. when combustion runs in a gas mixture based on O₂ and N₂, with various fractions of oxygen.The comparison between measured and predicted by model CO, SO₂, NO_x and O₂ emissions is shown in this paper. The results of the calculation showed, that the kinetic equations of some reaction have to be modified. Authors propose to use the reaction surface area instead of the specific internal surface area of char in rate constant formulas as the combustion nature changes from internal-kinetic to external-diffusion controlling regime.     

CH4-CO2 reforming over Ni/Al2O3 aerogel catalysts in a fluidized bed reactor

Ni/Al₂O₃ aerogel catalysts were synthesized by a sol-gel method combined with a supercritical drying route. The catalytic performances of the catalysts in methane reforming with CO₂ were investigated in a quartz micro-reactor. The results indicated that the aerogel catalyst showed higher specific surface area and higher dispersivity of nickel species than those of impregnation catalyst. The excellent catalytic performances and stabilities were achieved over the aerogel catalysts in the fluidized bed reactor. Comprehensive characterization with TG, XRD and FESEM revealed that the aerogel catalyst in the fluidized bed had much lower carbon deposition than that in the fixed bed. The fluidization of the aerogel catalyst greatly improved the contact efficiency of gas-solid phase, which accelerated the gasification of the deposited carbon. In contrast, the deactivation of the aerogel catalyst was caused by the carbon deposition due to the catalyst without moving in the fixed bed. Moreover, decreasing activity of the impregnation catalyst in the fluidized bed resulted from the poor fluidization state of catalyst particles and low effective active sites on surface of catalyst.     

Ca-based sorbent looping combustion for CO2 capture in pilot-scale dual fluidized beds

To demonstrate process feasibility of in situ CO₂ capture from combustion of fossil fuels using Ca-based sorbent looping technology, a flexible atmospheric dual fluidized bed combustion system has been constructed. Both reactors have an ID of 100 mm and can be operated at up to 1000 °C at atmospheric pressure. This paper presents preliminary results for a variety of operating conditions, including sorbent looping rate, flue gas stream volume, CaO/CO₂ ratio and combustion mode for supplying heat to the sorbent regenerator, including oxy-fuel combustion of biomass and coal with flue gas recirculation to achieve high-concentration CO₂ in the off-gas. It is the authors' belief that this study is the first demonstration of this technology using a pilot-scale dual fluidized bed system, with continuous sorbent looping for in situ CO₂ capture, albeit at atmospheric pressure. A multi-cycle test was conducted and a high CO₂ capture efficiency (> 90%) was achieved for the first several cycles, which decreased to a still acceptable level (> 75%) even after more than 25 cycles. The cyclic sorbent was sampled on-line and showed general agreement with the features observed using a lab-scale thermogravimetric analysis (TGA) apparatus. CO₂ capture efficiency decreased with increasing number of sorbent looping cycles as expected, and sorbent attrition was found to be another significant factor to be limiting sorbent performance.     

Atomic layer deposition of TiO2 films on particles in a fluidized bed reactor

Atomic layer deposition (ALD) of controlled-thickness TiO₂ films was carried out on particle substrates in a fluidized bed reactor for the first time. Films were deposited on 550 nm SiO₂ spheres and 65 nm ZnO nanoparticles for enhanced optical properties. Nanoparticles were fluidized with the assistance of a magnetically-coupled stirring unit. The metalorganic precursor titanium tetraisopropoxide was used here followed by either H₂O or H₂O₂ to deposit TiO₂ at various substrate temperatures. Growth rates of 0.01 nm/cycle and 0.04 nm/cycle were achieved when using H₂O and H₂O₂ as the oxidizer, respectively. These conformal TiO₂ films were verified using HRTEM, ICP-AES, XPS and UV absorbance measurements. The specific surface area changed appropriately after the particle size increased by the deposition of films with a given density, which showed that primary particles were not agglomerated together due to the coating process. In situ mass spectrometry was used to monitor reaction progress throughout each ALD reaction cycle. Bulk quantities of powder were successfully functionalized by TiO₂ nanofilms without wasting excess precursor.