Carbonation of lime-based materials under ambient conditions for direct air capture

Carbonation of lime-based materials at high temperatures has been extensively explored in the processes for decarbonisation of the power and industrial sectors. However, their capability to capture carbon dioxide from air at realistic ambient conditions in direct air capture technologies is less explored. In this work, lime and hydrated lime samples are exposed to ambient air for prolonged durations, as well as to calcination/ambient-carbonation cycles, to assess their carbonation performance. It is shown that the humidity plays a key role in carbonation of lime under ambient conditions. Furthermore, faster weathering and higher conversions are demonstrated by hydrated lime, showing a carbonation conversion of 70% after 300 h. Importantly, it was found that there was a negligible difference in the carbonation conversions during five calcination/ambient-carbonation cycles, which can be explained by simultaneous reactivation of cycled material by moist air. These findings indicated that lime-based materials are suitable for carbon dioxide capture from ambient air employing cyclic processes, in a practical time-scale, and that humidity of air plays a key role

Demonstration of a kW-scale solid oxide fuel cell-calciner for power generation and production of calcined materials

Carbonate looping (CaL) has been shown to be less energy-intensive when compared to mature carbon capture technologies. Further reduction in the efficiency penalties can be achieved by employing a more efficient source of heat for the calcination process, instead of oxy-fuel combustion. In this study, a kW-scale solid oxide fuel cell (SOFC)-integrated calciner was designed and developed to evaluate the technical feasibility of simultaneously generating power and driving the calcination process using the high-grade heat of the anode off-gas. Such a system can be integrated with CaL systems, or employed as a negative-emission technology, where the calcines are used to capture CO2 from the atmosphere. The demonstration unit consisted of a planar SOFC stack, operating at 750 °C, and a combined afterburner/calciner to combust hydrogen slip from the anode off-gas, and thermally decompose magnesite, dolomite, and limestone. The demonstrator generated up to 2 kWel,DC power, achieved a temperature in the range of 530–550 °C at the inlet of the afterburner, and up to 678 °C in the calciner, which was sufficient to demonstrate full calcination of magnesite, and partial calcination of dolomite. However, in order to achieve the temperature required for calcination of limestone, further scale-up and heat integration are needed. These results confirmed technical feasibility of the SOFC-calciner concept for production of calcined materials either for the market or for direct air capture (DAC)

Effect of impurities on ultra-pure hydrogen production by pressure vacuum swing adsorption

The most viable technology for production of ultra-pure hydrogen (>99.99%), required for fuel cells, is steam methane reforming (SMR) coupled with pressure vacuum swing adsorption (PVSA). A PVSA process with a two-layer bed of activated carbon (AC)/zeolite 5A for ultra-pure hydrogen production from syngas was developed and simulated with the aim of exploring the effect of impurities on energy intensity of the process. The simulated concentration profiles showed that CH4 was removed by first half of the AC layer, CO2 and CO were mostly removed by the end of that layer, but zeolite 5A (the second layer) could not completely remove the remaining N2. Further, the effect of the N2 on performance of the PVSA process was demonstrated by simulating purification of two feeds with 3.1 and 1.1 vol% N2, respectively. The 2% drop in N2 concentration in the syngas feed resulted in decreased energy consumption of the PVSA process from 940 kJ/kg to 430 kJ/kg H2, while H2 recovery increased from 47% to 55%. Therefore, the presence of N2 has a very large impact on recovery and energy intensity of the ultra-pure hydrogen production process, and development of adsorbents with better N2 removal performances is required

Modelling of sorption-enhanced steam methane reforming in a fixed bed reactor network integrated with fuel cell

In this study sorption-enhanced steam methane reforming (SE-SMR) in fixed beds is investigated by means of 1D numerical modelling, and the model is validated with the data reported in the literature. Isothermal conditions (973 K) are considered, and the equilibrium between the carbonation and calcination stages is shifted by a pressure swing: 3.5 · 106 Pa and 1013 Pa, respectively. The results showed that under these operating conditions at least 8 reactors in parallel are required to continuously produce a high-purity stream of H2, and a separated stream of concentrated CO2. The average H2 purity is 0.92, whilst the average H2 yield and selectivity are 2.9 molH2 molCH4−1 and 90%, respectively. A thermodynamic analysis was performed, which highlighted that, by using a portion of the produced H2 (about 0.4 molH2 molCH4−1), it is possible to fully cover heat and power demands of the process, making it completely energy self-sufficient. In the case when the proposed SE-SMR is integrated with a solid oxide fuel cell, net power generation at the scale of ∼950 kWel can be achieved with a net efficiency of the entire system of 51%, with the important feature that CO2 is concentrated

Nitrogen-rich hyper-crosslinked polymers for low-pressure CO2 capture

A series of poly[methacrylamide-co-(ethylene glycol dimethacrylate)] (poly(MAAM-co-EGDMA)) porous polymeric particles with high CO2-philicity, referred to as HCP-MAAMs, were synthesised for CO2 capture. The polymers with a MAAM-to-EGDMA molar ratio from 0.3 to 0.9 were inherently nitrogen-enriched and exhibited a high affinity towards selective CO2 capture at low pressures. A techno-economic model based on a 580 MWel supercritical coal-fired power plant scenario was developed to evaluate the performance of the synthesised adsorbents. The presence and density of NH2 moieties within the polymer network were determined using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The thermogravimetric analysis (TGA) showed that the polymers were thermally stable up to 515–532 K. The maximum CO2 adsorption capacity at 273 K was 1.56 mmol/g and the isosteric heat of adsorption was 28–35 kJ/mol. An increase in the density of amide groups within the polymer network resulted in a higher affinity towards CO2 at low pressure. At a CO2:N2 ratio of 15:85, CO2/N2 selectivity at 273 K was 52 at 1 bar and reached 104 at ultra-low CO2 partial pressure. The techno-economic analysis revealed that retrofitting a HCP-MAAM-based CO2 capture system led to a net energy penalty of 7.7–8.0%HHV points, which was noticeably lower than that reported for MEA or chilled ammonia scrubbing capture systems. The specific heat requirement was superior to the majority of conventional solvents such as MDEA-PZ and K2CO3. Importantly, the economic performance of the HCP-MAAM retrofit scenario was found to be competitive to chemical solvent scrubbing scenarios

Prediction and control of drop formation modes in microfluidic generation of double emulsions by single-step emulsification

Hypothesis Predicting formation mode of double emulsion drops in microfluidic emulsification is crucial for controlling the drop size and morphology. Experiments and modelling A three-phase Volume of Fluid-Continuum Surface Force (VOF–CSF) model was developed, validated with analytical solutions, and used to investigate drop formation in different regimes. Experimental investigations were done using a glue-free demountable glass capillary device with a true axisymmetric geometry, capable of readjusting the distance between the two inner capillaries during operation. Findings A non-dimensional parameter (ζ) for prediction of double emulsion formation mode as a function of the capillary numbers of all fluids and device geometry was developed and its critical values were determined using simulation and experimental data. At logζ > 5.7, drops were formed in dripping mode; the widening jetting occurred at 5 < logζ < 5.7; while the narrowing jetting was observed at logζ < 5. The ζ criterion was correlated with the ratio of the break-up length to drop diameter. The transition from widening to narrowing jetting was achieved by increasing the outer fluid flow rate at the high capillary number of the inner fluid. The drop size was reduced by reducing the distance between the two inner capillaries and the minimum drop size was achieved when the distance between the capillaries was zero

Temperatures of coal particle during devolatilization in fluidized bed combustion reactor

The purpose of this study was to investigate the thermal behavior of coal during devolatilization in fluidized bed. Temperatures in the center of single coal particle were measured by thermocouple. Two coals were tested ( brown coal Bogovina and lignite Kosovo), using dry coal particle, shaped into spherical form of diameters 7 and 10 mm, in temperature range from 300 to 850 degrees C. Unsteady behavior of coal particle during heating and devolatilization in fluidized bed was described by a model that takes into account heat transfer between bed and particle surface, heat transfer through particle and an endothermic chemical reaction of first-order. Based on the mathematical model analysis and compared with experimental results, values of heat conductivity (lambda(C)) and heat capacity (C-p) of coal were determined. The best agreement was obtained for constant thermal properties, for brown coal lambda(C) = 0.20 W/mK and C-p = 1200 J/kgK and for lignite lambda(C) = 0.17 W/mK and C-p = 1100 J/kgK

Temperatures of coal particle during devolatilization in fluidized bed combustion reactor

The purpose of this study was to investigate the thermal behavior of coal during devolatilization in fluidized bed. Temperatures in the center of single coal particle were measured by thermocouple. Two coals were tested ( brown coal Bogovina and lignite Kosovo), using dry coal particle, shaped into spherical form of diameters 7 and 10 mm, in temperature range from 300 to 850 degrees C. Unsteady behavior of coal particle during heating and devolatilization in fluidized bed was described by a model that takes into account heat transfer between bed and particle surface, heat transfer through particle and an endothermic chemical reaction of first-order. Based on the mathematical model analysis and compared with experimental results, values of heat conductivity (lambda(C)) and heat capacity (C-p) of coal were determined. The best agreement was obtained for constant thermal properties, for brown coal lambda(C) = 0.20 W/mK and C-p = 1200 J/kgK and for lignite lambda(C) = 0.17 W/mK and C-p = 1100 J/kgK

A model of coal particle drying in fluidized bed combustion reactor

Experimental and theoretical investigation on drying of a single coal particle in fluidized bed combustor is presented. Coal particle drying was considered via the moist shrinking core mechanism. The results of the drying test runs of low-rank Serbian coals were used for experimental verification of the model. The temperature of the coal particle center was measured, assuming that drying was completed when the temperature equalled 100 degrees C. The influence of different parameters ( thermal conductivity and specific heat capacity of coal, fluidized bed temperature, moisture content and superheating of steam) on drying time and temperature profile within the coal particle was analyzed by a parametric analysis. The experimentally obtained results confirmed that the moist shrinking core mechanism can be applied for the mathematical description of a coal particle drying, while dependence between drying time and coal particle radius, a square law relationship, implicates heat transfer control of the process and confirms the validity of assumptions used in modeling

Heat transfer between a coal particle and fluidized bed during heating and devolatilization

This paper presents an experimental and theoretical investigation of temperature in the coal particle during heating and devolatilization in hot fluidized bed (FB). A mathematical model, results of which are shown here, describes the unsteady behaviour of a coal particle during heating and devolatilization. The mathematical model is based on certain physical assumptions such as: presence of internal heat transfer resistance within the coal particle, variable thermal conductivity and heat capacity, and constant diameter of the coal particle. In order to obtain the values of parameters necessary for running the model, additional measurements were performed on released quantities of volatiles at definite temperatures. The measurements were carried out both in the FB and electrical oven, in inert (nitrogen) atmosphere, in temperature range 300-850°C. Devolatilization was isolated from combustion by conducting tests in nitrogen atmosphere with dried coal particles. Maximum discrepancy of FB reactor and oven results was 9%. The results were approximated (fitted) by a fourth-order polynomial of density vs. temperature. To test the model and to determine the unknown parameters, measurements of coal particle center temperature were conducted with a thermocouple. Two coals, a lignite and a brown coal, were tested using dried coal particles shaped into spheres of diameters 5, 7, 8 and 10 mm. FB temperature was in the range 590-710°C. On the basis of the performed experiments, a correlation for devolatilization duration vs. particle diameter was determined. The experimental and modeling results were compared. It was shown that in the first part of the process, during heating, temperature gradient within the particle was high, and later during devolatilization, it decreased