Modeling and simulation for the methane steam reforming enhanced by in situ CO2 removal utilizing the CaO carbonation for H2 production

The transient behavior of catalytic methane steam reforming (MSR) coupled with simultaneous carbon dioxide removal by carbonation of CaO pellets in a packed bed reactor for hydrogen production has been analyzed through a mathematical model with reaction experiments for model verification. A dynamic model has been developed to describe both the MSR reaction and the CaO carbonation-enhanced MSR reaction at non-isothermal, non-adiabatic, and non-isobaric operating conditions assuming that the rate of the CaO carbonation in a local zone of the packed bed is governed by kinetic limitation or by mass transfer limitation of the reactant CO₂. Apparent carbonation kinetics of the CaO pellet prepared has been determined using the TGA carbonation experiments at various temperatures, and incorporated into the model. The resulting model is shown to successfully depict the transient behavior of the in situ CaO carbonation-enhanced MSR reaction. The effects of major operating parameters on the transient behavior of the CaO carbonation-enhanced MSR have been investigated using the model. The bed temperature is the most important parameter for determining the amount of CO₂ removed by carbonation of CaO, and at temperatures of 600°C, 650°C, 700°C and 750°C, the CO₂ uptake is 1.43, 2.29, 3.5 and -CO₂/kg-CaO, respectively. Simultaneously with the increase in CO₂ uptake with increasing temperature, the corresponding amounts of hydrogen produced are 1.56, 2.54, 3.91 and -H₂/kg-CaO, at the same temperatures as above. Operation at high pressure, high steam to methane feed ratio, and the decreased feed rate at a given temperature are favorable for increasing the degree of the overall utilization of CaO pellets in the reactor bed, and for lowering the CO concentration in the product.     

Properties of char particles obtained under O2/N2 and O2/CO2 combustion environments

Pulverized coal combustion in O₂/N₂ and O₂/CO₂ environments was investigated with a drop tube furnace. Results present that the reaction rate and burn-out degree of O₂/CO₂ chars (obtained in O₂/CO₂ environments) are lower than that of O₂/N₂ chars (obtained in O₂/N₂ environments) under the same experimental condition. It indicates that a higher O₂ concentration in O₂/CO₂ environment is needed to achieve the similar combustion characteristic to that in O₂/N₂ environment. The main differences between O₂/N₂ and O₂/CO₂ chars rely on the pore structure determined by N₂ adsorption and chemical structure measured by FT-IR. For O₂/CO₂ char, the surface is thick and the pores are compact which contribute to the fragmentation reduction of particles burning in O₂/CO₂ environment. The organic functional group elimination rate from the surface of O₂/CO₂ chars is slower or delayed. The present research results might have important implications for further understanding the intrinsic kinetics of pulverized coal combustion in O₂/CO₂ environment.     

Calcination and sintering characteristics of limestone under O2/CO2 combustion atmosphere

The O₂/CO₂ coal combustion technology is an innovative combustion technology that can control CO₂, SO₂ and NO_x emissions simultaneously. Calcination and sintering characteristics of limestone under O₂/CO₂ atmosphere were investigated in this paper. The pore size, the specific pore volume and the specific surface area of CaO calcined were measured by N₂ adsorption method. The grain size of CaO calcined was determined by XRD analysis. The specific pore volume and the specific surface area of CaO calcined in O₂/CO₂ atmosphere are less than that of CaO calcined in air at the same temperature. And the pore diameter of CaO calcined in O₂/CO₂ atmosphere is larger than that in air. The specific pore volume and the specific surface area of CaO calcined in O₂/CO₂ atmosphere increase initially with temperature, and then decline as temperature exceeds 1000 °C. The peaks of the specific pore volume and the specific surface area appear at 1000 °C. The specific surface area decreases with increase in the grain size of CaO calcined. The correlations of the grain size with the specific surface area and the specific pore volume can be expressed as L = 744.67 + 464.64 lg(1 / S) and L = − 608.5 + 1342.42 lg(1 / ε), respectively. Sintering has influence on the pore structure of CaO calcined by means of influencing the grain size of CaO.     

Simultaneous easy CO2 recovery and drastic reduction of SOx and NOx in O2/CO2 coal combustion with heat recirculation

Coal combustion with O₂/CO₂ is promising because of its easy CO₂ recovery, extremely low NO_x emission and high desulfurization efficiency. Based on our own fundamental experimental data combined with a sophisticated data analysis, its characteristics were investigated. It was revealed that the conversion ratio from fuel-N to exhausted NO in O₂/CO₂ pulverized coal combustion was only about one fourth of conventional pulverized coal combustion. To decrease exhausted NO further and realize simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x, a new scheme, i.e. O₂/CO₂ coal combustion with heat recirculation, was proposed. It was clarified that in O₂/CO₂ coal combustion, with about 40% of heat recirculation, the same coal combustion intensity as that of coal combustion in air could be realized even at an O₂ concentration of as low as 15%. Thus exhausted NO could be decreased further into only one seventh of conventional coal combustion. Simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x could be realized with this new scheme.     

Reactivity of CaO derived from nano-sized CaCO3 particles through multiple CO2 capture-and-release cycles

The carbonation characteristics of pure CaO derived from nano-sized CaCO₃ were investigated as part of a multi-cycle performance study which showed potential for exploiting the properties of nano-sized CaO sorbents in a continuous CO₂ capture-and-release process. To help understand the approach to the decay asymptote, which is established through multiple capture-and-release cycles, a qualitative model was proposed. The rate of approach and residual conversion defined by the decay asymptote represents the establishment of an equilibrium between the pore volume and surface area loss during thermal sintering; and the pore volume and surface area regeneration as a consequence of a solid-state diffusion mechanism, and the subsequent release of CO₂ in the next calcination cycle. This qualitative explanation is valid for all CaO derived CO₂ sorbents.     

Thermodynamic performance of O2/CO2 gas turbine cycle with chemical recuperation by CO2-reforming of methane

This paper proposes a novel power cycle system composed of chemical recuperative cycle with CO₂–NG (natural gas) reforming and an ammonia absorption refrigeration cycle. In which, the heat is recovered from the turbine exhaust to drive CO₂–NG reformer firstly, and then lower temperature heat from the turbine exhaust is provided with the ammonia absorption refrigeration system to generate chilled media, which is used to cool the turbine inlet gas except export. In this paper, a detailed thermodynamic analysis is carried out to reveal the performance of the proposed cycle and the influence of key parameters on performance is discussed. Based on 1 kg s⁻¹ of methane feedstock and the turbine inlet temperature of 1573 K, the simulation results shown that the optimized net power generation efficiency of the cycle rises up to 49.6% on the low-heating value and the exergy efficiency 47.9%, the new cycle system reached the net electric-power production 24.799 MW, the export chilled load 0.609 MW and 2.743 kg s⁻¹ liquid CO₂ was captured, achieved the goal of CO₂ and NO_x zero-emission.     

Study on coal chars combustion under O2/CO2 atmosphere with fractal random pore model

When predicting the variation of pore structure during O₂/CO₂ combustion of coal chars using the random pore model (RPM), it is impossible to calculate exactly the structure parameter ψ from the pore characteristics. The values of structure parameter ψ, which were calculated based on its fractal feature at various carbon conversions, should be almost constant. However, this investigation exhibited a drastic increase of ψ at the end of combustion reaction. In this work, structure parameter ψ of the RPM was modified according to the experimental analysis and a new model, fractal random pore model (FRPM), was constructed. Compared with other models such as RPM, discrete random pore model (DRPM), the Struis model (Model I) and the Liu model (Model II), it was found that fractal random pore model was more accurate to describe coal chars combustion, especially at higher conversions. Using the FRPM, O₂/CO₂ isotherm combustion of coal chars were analyzed at different temperatures.     

High-pressure sorption isotherms and sorption kinetics of CH4 and CO2 on coals

Using a manometric experimental setup, high-pressure sorption measurements with CH₄ and CO₂ were performed on three Chinese coal samples of different rank (VR_r = 0.53%, 1.20%, and 3.86%). The experiments were conducted at 35, 45, and 55 °C with pressures up to 25 MPa on the 0.354-1 mm particle fraction in the dry state. The objective of this study was to explore the accuracy and reproducibility of the manometric method in the pressure and temperature range relevant for potential coalbed methane (CBM) and CO₂-enhanced CBM (CO₂-ECBM) activities (P > 8 MPa, T > 35 °C). Maximum experimental errors were estimated using the Gauss error propagation theorem, and reproducibility tests of the high-pressure sorption measurements for CH₄ and CO₂ were performed. Further, the experimental data presented here was used to explicitly study the CO₂ sorption behaviour of Chinese coal samples in the elevated pressure range (up to 25 MPa) and the effects of temperature on supercritical CO₂ sorption isotherms.The experiments provided characteristic excess sorption isotherms which, in the case of CO₂ exhibit a maximum around the critical pressure and then decline and level out towards a constant value. The results of these manometric tests are consistent with those of previous gravimetric sorption studies and corroborate a crossover of the 35, 45, and 55 °C CO₂ excess sorption isotherms in the high-pressure range. The measurement range could be extended, however, to significantly higher pressures. The excess sorption isotherms tend to converge, indicating that the temperature dependence of CO₂ excess sorption on coals at high-pressures (>20 MPa) becomes marginal. Further, all CO₂ high-pressure isotherms measured in this study were approximated by a three-parameter excess sorption function with special consideration of the density ratio of the “free” phase and the sorbed phase. This function provided a good representation of the experimental data.The maximum excess sorption capacity of the three coal samples for methane ranged from 0.8 to 1.6 mmol/g (dry, ash-free) and increased from medium volatile bituminous to subbituminous to anthracite. The medium volatile bituminous coal also exhibited the lowest overall excess sorption capacity for CO₂. However, the subbituminous coal was found to have the highest CO₂ sorption capacity of the three samples. The mass fraction of adsorbed substance as a function of time recorded during the first pressure step was used to analyze the kinetics of CH₄ and CO₂ sorption on the coal samples. CO₂ sorption proceeds more rapidly than CH₄ sorption on the anthracite and the medium volatile bituminous coal. For the subbituminous coal, methane sorption is initially faster, but during the final stage of the measurement CO₂ sorption approaches the equilibrium value more rapidly than methane.     

Impact of SO2 concentration on the corrosion rate of X70 steel and iron in water-saturated supercritical CO2 mixed with SO2

The corrosion behavior of X70 steel and iron in water-saturated supercritical CO₂ mixed with SO₂ was investigated using weight-loss measurements. As a comparison, the instantaneous corrosion rate in the early stages for iron in the same corrosion environment was measured by resistance relaxation method. Surface analyzes using SEM/EDS, XRD and XPS were applied to study the morphology and chemical composition of the corroded sample surface. Weight-loss method results showed that the corrosion rate of X70 steel samples increased with SO₂ concentration, while the corrosion rate increased before decreasing with SO₂ concentration for iron sample. Comparing resistance relaxation method results with weight-loss method results, it is found that the instantaneous corrosion rate of iron is much higher than the uniform corrosion rate of the iron tablet specimens which are covered with thick corrosion product films after a long period of corrosion. The corrosion product films were mainly composed of FeSO₄ and FeSO₃ hydrates. The possible reaction mechanism under such environment was also analyzed, and the electrochemical reaction between the dissolved SO₂ in the condensed water film with iron is the critical reaction step.     

Experimental and modeling of CO2 capture by dry sodium hydroxide carbonation

In this work, CO₂ capture from the air using dry NaOH sorbents has been studied. The influences of the main operating parameters such as temperature, air humidity, and NaOH loading on the CO₂ removal rate have been experimentally investigated using Taguchi method. The results revealed that the appropriate value of the temperature to maximize the rate was in the range of 35–45 °C. A multilayer artificial neural network (ANN) was also used to model the process in order to find the optimal conditions. A procedure reported in the literature was modified and applied to design the ANN model. The model predictions were validated by conducting some more experiments. The experimental results proved the accuracy of the model to predict the optimal conditions. The effects of NaOH particle size and multiple carbonation cycles have also been investigated.     

Partial O2-fired coal power plant with post-combustion CO2 capture: A retrofitting option for CO2 capture ready plants

In the work presented in this paper, an alternative process concept that can be applied as retrofitting option in coal-fired power plants for CO₂ capture is examined. The proposed concept is based on the combination of two fundamental CO₂ capture technologies, the partial oxyfuel mode in the furnace and the post-combustion solvent scrubbing. A 330 MW_el Greek lignite-fired power plant and a typical 600 MW_el hard coal plant have been examined for the process simulations. In a retrofit application of the ECO-Scrub technology, the existing power plant modifications are dominated by techno-economic restrictions regarding the boiler and the steam turbine islands. Heat integration from processes (air separation, CO₂ compression and purification and the flue gas treatment) can result in reduced energy and efficiency penalties. In the context of this work, heat integration options are illustrated and main results from thermodynamic simulations dealing with the most important features of the power plant with CO₂ capture are presented for both reference and retrofit case, providing a comparative view on the power plant net efficiency and energy consumptions for CO₂ capture. The operational characteristics as well as the main figures and diagrams of the plant’s heat balances are included.     

Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming

The production of hydrogen from methane via two chemical looping reforming (CLR) processes was simulated and thermodynamically analysed, one process being the conventional CLR process, the other being a CO₂ sorption enhanced process. The aim of the work was to identify suitable operating conditions for obtaining an optimum hydrogen gas purity and yield, whilst operating auto-thermally, at atmospheric pressure and with no carbon formation. In both simulations, the reactors were simulated using the Gibbs minimisation technique. NiO was used as the oxygen storing species, whilst CaO was used as the CO₂ adsorbent.     

Effect of TiO2 particle size on the photocatalytic reduction of CO2

Pure TiO₂ anatase particles with a crystallite diameters ranging from 4.5 to 29 nm were prepared by precipitation and sol–gel method, characterized by X-ray diffraction (XRD), BET surface area measurement, UV–vis and scanning electron microscopy (SEM) and tested in CO₂ photocatalytic reduction. Methane and methanol were the main reduction products. The optimum particle size corresponding to the highest yields of both products was 14 nm. The observed optimum particle size is a result of competing effects of specific surface area, charge–carrier dynamics and light absorption efficiency.     

Theoretical study on interaction between CO2 and carbonyl compounds: Influence of CO2 on infrared spectroscopy and activity of CO

The interactions between CO₂ and carbonyl compounds at different CO₂ pressures have been studied both experimentally and theoretically. In situ high-pressure FTIR on carbonyl compounds, i.e., acetaldehyde, acetone, and crotonaldehyde, in supercritical CO₂ have been measured at various CO₂ pressures varying from 6 to 22 MPa. In order to get insights into the mechanism, theoretical study has been conducted concerning the effect of CO₂ on frequency shift of CO in acetaldehyde, acetone, benzaldehyde, crotonaldehyde and cinnamaldehyde at different CO₂ pressures. It has been shown that the experimental frequency shifts can be well simulated by the theoretical model calculations using particular structures, in which a carbonyl compound interacts with a few CO₂ molecules, depending on the carbonyl compounds examined, except for acetaldehyde.The interaction energies between CO₂ and those carbonyl compounds are also given. In addition, the effect of CO₂ on hydrogenation of crotonaldehyde and benzaldehyde has been discussed by means of the local softness (s⁺) calculated at CO₂ pressures of 0-22 MPa, which can explain the reactivity difference in the crotonaldehyde and benzaldehyde hydrogenations in supercritical CO₂.     

Influence of calcination conditions on carrying capacity of CaO-based sorbent in CO2 looping cycles

This study examines the loss of sorbent activity caused by sintering under realistic CO₂ capture cycle conditions. The samples tested here included two limestones: Havelock limestone from Canada (New Brunswick) and a Polish (Upper Silesia) limestone (Katowice). Samples were prepared both in a thermogravimetric analyzer (TGA) and a tube furnace (TF). Two calcination conditions were employed: in N₂ at lower temperature; and in CO₂ at high temperature. The samples obtained were observed with a scanning electron microscope (SEM) and surface compositions of the resulting materials were analyzed by the energy dispersive X-ray (EDX) method. The quantitative influence of calcination conditions was examined by nitrogen adsorption/desorption tests, gas displacement pycnometry and powder displacement pycnometry; BET surface areas, BJH pore volume distributions, skeletal densities and envelope densities were determined. The SEM images showed noticeably larger CaO sub-grains were produced by calcination in CO₂ during numerous cycles than those seen with calcination in nitrogen. The EDX elemental analyses showed a strong influence of impurities on local melting at the sorbent particle surface, which became more pronounced at higher temperature. Results of BET/BJH testing clearly support these findings on the effect of calcination/cycling conditions on sorbent morphology. Envelope density measurements showed that particles displayed densification upon cycling and that particles calcined under CO₂ showed greater densification than those calcined under N₂. Interestingly, the Katowice limestone calcined/cycled at higher temperature in CO₂ showed an increase of activity for cycles involving calcination under N₂ in the TGA. These results clearly demonstrate that, in future development of CaO-based CO₂ looping cycle technology, more attention should be paid to loss of sorbent activity caused by realistic calcination conditions and the presence of impurities originating from fuel ash and/or limestone.     

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.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

An economic feasibility study of O2/CO2 recycle combustion technology based on existing coal-fired power plants in China

In order to reduce the CO₂ emission from the coal-fired power plants, O₂/CO₂ recycle combustion (Oxy-combustion) technique has been proposed through combining a conventional combustion process with a cryogenic air separation process. The technique is capable of enriching CO₂ concentration and then allowing CO₂ sequestration in an efficient and energy-saving way. Taking into account the CO₂ taxation and CO₂ sale, the paper evaluates the economic feasibility of Oxy-combustion plants retrofitted from two typical existing conventional coal-fired power plants (with capacities of 2 × 300 MW and 2 × 600 MW, respectively) with Chinese data. The cost of electricity (COE) and the CO₂ avoidance cost (CAC) are also considered in the evaluation. The COE of the retrofitted Oxy-combustion plant is nearly the same as that of the corresponding conventional plant if the unit price of CO₂ sale reaches 17-22 $/t (different cases). The CAC of the retrofitted 2 × 300 MW Oxy-combustion plant is 1-3 $/t bigger than that of the retrofitted 2 × 600 MW Oxy-combustion plant. Supercritical plants are more economical and appropriate for Oxy-combustion retrofit. The result indicates that Oxy-combustion technique is not only feasible for CO₂ emission control based on existing power plants but is also cost-effective.