Effect of rice husk ash addition on CO2 capture behavior of calcium-based sorbent during calcium looping cycle

Rice husk ash/CaO was proposed as a CO₂ sorbent which was prepared by rice husk ash and CaO hydration together. The CO₂ capture behavior of rice husk ash/CaO sorbent was investigated in a twin fixed bed reactor system, and its apparent morphology, pore structure characteristics and phase variation during cyclic carbonation/calcination reactions were examined by SEM-EDX, N₂ adsorption and XRD, respectively. The optimum preparation conditions for rice husk ash/CaO sorbent are hydration temperature of 75 °C, hydration time of 8 h, and mole ratio of SiO₂ in rice husk ash to CaO of 1.0. The cyclic carbonation performances of rice husk ash/CaO at these preparation conditions were compared with those of hydrated CaO and original CaO. The temperature at 660 °C–710 °C is beneficial to CO₂ absorption of rice husk ash/CaO, and it exhibits higher carbonation conversions than hydrated CaO and original CaO during multiple cycles at the same reaction conditions. Rice husk ash/CaO possesses better anti-sintering behavior than the other sorbents. Rice husk ash exhibits better effect on improving cyclic carbonation conversion of CaO than pure SiO₂ and diatomite. Rice husk ash/CaO maintains higher surface area and more abundant pores after calcination during the multiple cycles; however, the other sorbents show a sharp decay at the same reaction conditions. Ca₂SiO₄ found by XRD detection after calcination of rice husk ash/CaO is possibly a key factor in determining the cyclic CO₂ capture behavior of rice husk ash/CaO.     

Determination of intrinsic rate constants of the CaO-CO2 reaction

The rate constant of the CaO-CO₂ reaction was studied for two sorbents using an atmospheric thermogravimetric analyzer (ATGA) and a pressurized thermogravimetric analyzer (PTGA). A grain model was used to determine the rate-controlling steps. The intrinsic rate was found to have a variable order with respect to CO₂ partial pressure, with a first-order reaction changing to zero-order dependence when the CO₂ partial pressure exceeded . No further rate enhancement was observed under pressurized conditions, which was explained by a two-step Langmuir mechanism. The activation energies were 29±4 and for the limestone and dolomite tested.     

Reduction and oxidation kinetics of Mn3O4/Mg-ZrO2 oxygen carrier particles for chemical-looping combustion

The kinetics of reduction with methane and oxidation with oxygen of Mn₃O₄ supported on Mg-ZrO₂ prepared by freeze granulation has been investigated. The reactivity experiments were performed in a thermogravimetric analyzer (TGA) using different reacting gas concentrations and temperatures in the range of 1073-1223 K. The oxygen carrier particles showed high reactivity during both reduction and oxidation at all investigated temperatures. An empirical reaction model, which assumes a linear relation between time and conversion, was used to determine the kinetic parameters for reduction and oxidation, with chemical reaction being the main resistance to the reaction. The order of reaction found was 1 with respect to CH₄ and 0.65 with respect to O₂. The activation energy for the reduction reaction was 119 and for the oxidation reaction. The reactivity data and kinetic parameters were used to estimate the solid inventory in the air and fuel reactor of a CLC system. The optimum solid inventory obtained was at a value of ΔX_s=0.4. At these conditions, the recirculation rate of oxygen carrier between air and fuel reactor was per MW of fuel, which could be accomplished in an industrial reactor. The high reactivity of the Mn₃O₄/Mg-ZrO₂ with both methane and oxygen showed that this is a very promising oxygen carrier for CLC.     

Mechanisms of aqueous wollastonite carbonation as a possible CO2 sequestration process

The mechanisms of aqueous wollastonite carbonation as a possible carbon dioxide sequestration process were investigated experimentally by systematic variation of the reaction temperature, CO₂ pressure, particle size, reaction time, liquid to solid ratio and agitation power. The carbonation reaction was observed to occur via the aqueous phase in two steps: (1) Ca leaching from the CaSiO₃ matrix and (2) CaCO₃ nucleation and growth. Leaching is hindered by a Ca-depleted silicate rim resulting from incongruent Ca-dissolution. Two temperature regimes were identified in the overall carbonation process. At temperatures below an optimum reaction temperature, the overall reaction rate is probably limited by the leaching rate of Ca. At higher temperatures, nucleation and growth of calcium carbonate are probably limiting the conversion, due to a reduced (bi)carbonate activity. The mechanisms for the aqueous carbonation of wollastonite were shown to be similar to those reported previously for an industrial residue and a Mg-silicate. The carbonation of wollastonite proceeds rapidly relative to Mg-silicates, with a maximum conversion in 15 min of 70% at , 20 bar CO₂ partial pressure and particle size of . The obtained insight in the reaction mechanisms enables the energetic and economic assessment of CO₂ sequestration by wollastonite carbonation, which forms an essential next step in its further development.     

The decrease of carbonation efficiency of CaO along calcination-carbonation cycles: Experiments and modelling

Successive calcination-carbonation cycles, using CaO as sorbent, have been performed either in a classical fixed bed reactor or using a thermogravimetric analyser. Significant differences in carbonation efficiencies were obtained, possibly due to different conditions prevailing for CaO sintering during the calcination stage. The effect of the presence of CO₂ on sintering was confirmed.A simple model of the decay of the carbonation capacity along cycles based on the specific surface area of non-sintered micrograins of CaO is able to predict the decrease of the extent of conversion obtained after 40 carbonations along calcination-carbonation cycles. The asymptotic extent of conversion is obtained when all the micrograins present within a grain are sintered. A detailed model of the carbonation shows that the voids present between the micrograins are filled up by carbonate when a critical thickness of the carbonate layer around each micrograin reaches 43 nm. Then, carbonation becomes controlled by diffusion at the scale of the whole grain, with the CO₂ diffusion coefficient decreasing (at ) from 2×10^-12 to as carbonation proceeds from 50% conversion to 76% (first cycle). This scale change for diffusion is responsible for the drastic decrease of the carbonation rate after the voids between micrograins are filled up.     

Preparation and electrochemical studies of metal–carbon composite catalysts for small-scale electrosynthesis of H2O2

Numerous transition metal–carbon composite catalysts (M = V, Zn, Ni, Sn, Ce, Ba, Fe, Cu) have been synthesized and tested for electroreduction of O₂ to H₂O₂, The activity and selectivity of all synthesized catalysts for electrosynthesis of H₂O₂ were determined by the rotating ring-disk electrode method in acidic and neutral electrolytes. The Co-based catalysts in general showed the highest activity towards H₂O₂ formation. Experiments with different loading contents of Co showed that the activation overpotential losses of oxygen reduction to H₂O₂ reduces as loading increases to about 4 wt% Co. Addition of Co beyond this level did not seem to impact the overpotential losses. The cobalt-based catalysts, were spray-coated onto 120 μm thick Toray^® graphite substrates, and were studied in bulk electrolysis cells for up to 100 h at potentiostatic conditions (0.25 V vs. RHE) in pH 0, 3, and 7 electrolytes. At (25 °C and 1 bar) with a catalysts loading of about and using dissolved O₂ in 0.5 M H₂SO₄, typical H₂O₂ electrosynthesis rates of about were reached with current efficiencies of about 85 ± 5% at 0.25 V (vs. RHE).     

Modified CaO-based sorbent looping cycle for CO2 mitigation

CaO-based sorbent looping cycle, i.e. cyclic calcination/carbonation, is one of the most interesting technologies for CO₂ capture during coal combustion and gasification processes. In order to improve the durability of limestone during the multiple calcination/carbonation cycles, modified limestone with acetic acid solution was proposed as an CO₂ sorbent. The cyclic carbonation conversions of modified limestone and original one were investigated in a twin fixed bed reactor system. The modified limestone shows the optimum carbonation conversion at the carbonation temperature of 650 °C and achieves a conversion of 0.5 after 20 cycles. The original limestone exhibits the maximum carbonation conversion of 0.15 after 20 cycles. Conversion of the modified limestone decreases slightly as the calcination temperature increases from 920 °C to 1100 °C with the number of cycles, while conversion of the original one displays a sharp decay at the same reaction conditions. The durability of the modified limestone is significantly better than the original one during the multiple cycles because mean grain size of CaO derived from the modified limestone is lower than that from the original one at the same reaction conditions. The calcined modified limestone shows higher surface area and pore volume than the calcined original one with the number of cycles, and pore size distribution of the modified limestone is superior to the original one after the same number of calcinations.     

Investigation of formic acid oxidation on Ti/IrO2 electrodes

A model has been proposed according to which the voltammetric charge involved in the Ti/IrO₂ electrodes is due to two contributions: a faradaic contribution due to surface redox activities at the IrO₂ coating and a non-faradaic contribution due to the charging of electrical double layer (). The later has been proposed as a tool for the estimation of the relative surface area of the Ti/IrO₂ electrodes.Differential electrochemical mass spectrometry (DEMS) measurements using H₂¹⁸O has demonstrated that we are dealing with an active electrode in which the surface redox couple IrO₃/IrO₂ acts as mediator in the oxidation of formic acid (FA).From the voltammetric measurements using different IrO₂ loading and FA concentrations, the kinetic parameters of FA oxidation via the surface redox couple IrO₃/IrO₂ have been determined.Finally a model has been proposed considering that FA oxidation at Ti/IrO₂ anodes is controlled by mass transfer. The good agreement between the experimental results and the model indicates that the surface reaction between FA and the electrogenerated IrO₃ is a fast reaction.     

Selective oxidation of methanol to dimethoxymethane over acid-modified V2O5/TiO2 catalysts

Selective oxidation of methanol to dimethoxymethane (DMM) was conducted in a fixed-bed reactor over an acid-modified V₂O₅/TiO₂ catalyst. The influence of the acid modification on its structure, redox and acidic properties, and catalytic performance for methanol oxidation were investigated. The results indicated that the content of vanadia in the catalyst exhibits a vital influence on the dispersion of vanadium species, while the acid modification can enhance its surface acidity. Proper amounts of the acid (W() = 15%) and V₂O₅ (W(V₂O₅) = 15%) components loaded in the acid-modified V₂O₅/TiO₂ catalyst are able to build a bi-functional circumstance that is favorable for the formation of DMM with high activity and selectivity. As a result, for the selective oxidation of methanol, the H₂SO₄-modified V₂O₅/TiO₂ catalyst gives a much higher DMM yield at 150 °C than the unmodified one.     

Characteristics and reactivities of Ca(OH)2/silica fume sorbents for low-temperature flue gas desulfurization

Ca(OH)₂/silica fume sorbents were prepared with various Ca(OH)₂/silica fume weight ratios and slurrying times at 65°C and a water/solid ratio of 10/1. Dry sorbents prepared were characterized, and their reactivities toward SO₂ were measured in a differential fixed-bed reactor at the conditions similar to those in the bag filters in the dry and semidry flue gas desulfurization (FGD) processes. The reaction between Ca(OH)₂ and silica fume in the slurry was very fast. The formation of calcium silicate hydrates, which were mainly C-S-H(I), resulted in sorbent particles with a highly porous structure that seemed compressible under high pressures. The sorbents were mesoporous, and their specific surface areas and pore volumes were much larger than those of Ca(OH)₂ alone. The utilization of Ca of sorbent increased with increasing silica fume content mainly due to the increase in the specific surface area of sorbent. The sorbent with Ca(OH)₂ had the maximum SO₂ capture. Sorbents with Ca(OH)₂ contents less than and greater than would have a SO₂ capture greater than that of Ca(OH)₂ alone. Both the utilization of Ca and SO₂ capture per unit specific surface area of sorbent decreased in general with increasing specific surface area. At the same Ca(OH)₂ content, the utilization of Ca or SO₂ capture of the Ca(OH)₂/silica fume sorbent was greater than that of the Ca(OH)₂/fly ash sorbent; however, the amount of SO₂ captured per unit surface area of the former sorbent was smaller than that of the latter sorbent. The results of this study are useful to the preparation of silica-enhanced sorbents for use in the dry and semidry FGD processes.     

Cyclic calcination/carbonation looping of dolomite modified with acetic acid for CO2 capture

The dolomite modified with acetic acid solution was proposed as a CO₂ sorbent for calcination/carbonation cycles. The carbonation conversions for modified and original dolomites in a twin fixed-bed reactor system with increasing the numbers of cycles were investigated. The carbonation temperature in the range of 630 °C–700 °C is beneficial to the carbonation reaction of modified dolomite. The carbonation conversion for modified dolomite is significantly higher than that for original sorbent at the same reaction conditions with increasing numbers of reaction cycles. The modified dolomite exhibits a carbonation conversion of 0.6 after 20 cycles, while the unmodified sorbent shows a conversion of 0.26 at the same reaction conditions, which is calcined at 920 °C and carbonated at 650 °C. At the high calcination temperature over 920 °C modified dolomite can maintain much higher conversion than unmodified sorbent. The mean grain size of CaO derived from modified dolomite is smaller than that from original sorbent with increasing numbers of reaction cycles. The calcined modified dolomite possesses greater surface area and pore volume than calcined original sorbent during the multiple cycles. The pore volume and pore area distributions for calcined modified dolomite are also superior to those for calcined unmodified sorbent during the looping cycle. The modified dolomite is proved as a new and promising type of regenerable CO₂ sorbent for industrial applications.     

Steam reforming of LPG over Ni and Rh supported on Gd-CeO2 and Al2O3: Effect of support and feed composition

The steam reforming of liquefied petroleum gas (LPG) over Ni- and Rh-based catalysts supported on Gd-CeO₂ (CGO) and Al₂O₃ was studied at 750-900 °C. The order of activity was found to be Rh/CGO > Ni/CGO ∼ Rh/Al₂O₃ > Ni/Al₂O₃; we indicated that the comparable activity of Ni/CGO to precious metal Rh/Al₂O₃ is due to the occurring of gas-solid reactions between hydrocarbons and lattice oxygen () on CGO surface along with the reaction taking place on the active site of Ni, which helps preventing the carbon deposition and promoting the steam reforming of LPG.The effects of O₂ (as oxidative steam reforming) and H₂ adding were further studied over Ni/CGO and Ni/Al₂O₃. It was found that the additional of these compounds significantly reduced the amount of carbon deposition and promoted the conversion of hydrocarbons (i.e., LPG as well as CH₄, C₂H₄ and C₂H₆ occurred from the thermal decomposition of LPG) to CO and H₂. Nevertheless, the addition of too high O₂ oppositely decreased H₂ yield due to the oxidizing of Ni particle and the possible combusting of H₂ generated from the reaction, while the addition of too high H₂ also negatively affect the catalyst activity due to the occurring of catalyst active site competition and the inhibition of gas-solid reactions between the gaseous hydrocarbon compounds and on the surface of CGO (for the case of Ni/CGO).     

Enhancement of reactivity in surfactant-modified sorbent for CO2 capture in pressurized carbonation

The calcination/carbonation loop of calcium-based (Ca-based) sorbents is considered as a viable technique for CO₂ capture from combustion gases. Recent attempts to improve the CO₂ uptake of Ca-based sorbents by adding calcium lignosulfonate (CLS) with hydration have succeeded in enhancing its effectiveness. The optimum mass ratio of CLS/CaO is 0.5 wt.%. The reduction in particle size and grain size of CaO appeared to be parts of the reasons for increase in CO₂ capture. The primary cause of increase in reactivity of the modified sorbents was the ability of the CLS to retard the sintering rate and thus to remain surface area and pore volume for reaction. The CO₂ uptake of the modified sorbents was also enhanced by elevating the carbonation pressure. Experimental results indicate that the optimal reaction condition of the modified sorbents is at 0.5 MPa and 700 °C and a high conversion of 0.7 is achieved after 10 cycles, by 30% higher than that of original limestone, at the same condition.     

CO2 uptake of modified calcium-based sorbents in a pressurized carbonation-calcination looping

This study focuses on enhancing CO₂ uptake by modifying limestone with acetate solutions under pressurized carbonation condition. The multicycle tests were carried out in an atmospheric calcination/pressurized carbonation reactor system at different temperatures and pressures. The pore structure characteristics (BET and BJH) were measured as a supplement to the reaction studies. Compared with the raw limestone, the modified sorbent showed a great improvement in CO₂ uptake at the same reaction condition. The highest CO₂ uptake was obtained at 700 °C and 0.5 MPa, by 88.5% increase over the limestone at 0.1 MPa after 10 cycles. The structure characteristics of the sorbents on N₂ absorption and SEM confirm that compared with the modified sorbent, the effective pores of limestone are greatly driven off by sintering, which hinders the easy access of CO₂ molecules to the unreacted-active sites of CaO. The morphological and structural properties of the modified sorbent did not reveal significant differences after multiple cycles. This would explain its superior performance of CO₂ uptake under pressurized carbonation. Even after 10 cycles, the modified sorbent still achieved a CO₂ uptake of 0.88.