Pre-combustion Conditions Research Articles

Combustion characteristics for a hydrogen-fuelled low-pressure-ratio split-cycle engine

Investigation of hydrogen-air combustion characteristics for low-pressure-ratio split-cycle engines is performed for the first time. Ignition delay and combustion duration data are presented for low initial pressures below 5 bars and temperatures from ambient level till 550 °C. A simplified constant-volume combustion chamber experiment with glow-plug ignition and timed intake and exhaust valves is constructed in order to simulate the real operation. Ignition delay is found to be inversely proportional to the temperature, and unexpectedly directly proportional with pressure. Equivalence ratio has a weak influence on the ignition delay. Combustion duration exhibits the same behaviour as the ignition delay and represents an average 40% portion of the total combustion delay. Ignition delay results are bounded to the selected glow plug surface temperature range and complements with data in literature. Based on the results, the engine rotational speed would be limited to a maximum of 4500 RPM for an intake duration of 30° at 550 °C and 3.5 bars pre-combustion conditions.

Flexible nanoporous activated carbon cloth for achieving high H2, CH4, and CO2 storage capacities and selective CO2/CH4 separation

Innovative tools are needed for the synthesis of smart, new, efficient, and safe nanoporous carbon materials for energy gas storage. Here, a flexible nanoporous activated carbon cloth was developed. Polypyrrole nanoparticles were polymerized in dispersed form on commercial viscose rayon cloth fiber surfaces. Then, the material was carbonized and activated by physical and chemical activation methods applied individually. Chemical activation conditions were varied and optimized. This produced a high porosity flexible nanoporous carbon textile with a surface area of ~2000 m2 g−1, total pore volume of 0.85 cm3 g−1, and high nitrogen content. The new flexible nanoporous carbon cloth achieved superior H2 and CH4 storage capacities and CO2 capture compared to so-far-reported activated carbon cloth, and values were comparable to or higher than those reported for powder activated carbons. Excess H2 uptake values were 4.0 and 0.173 wt% at 77 K and 298 K at 20 bar, respectively, and CH4 storage amounted to 7.5 mmol g−1 at 20 bar and 298 K, which is among the highest reported values for porous carbon materials. CO2 uptake values were 4.2 and 14.3 mmol g−1 at 1 and 20 bar at 298 K, respectively, which are values superior to those previously reported for activated carbon cloth. Hence, the flexible nanoporous activated carbon cloth is effective for greenhouse gas (i.e., CO2) uptake during post- and pre-combustion conditions. Separation selectivity for CO2/CH4 binary mixtures was evaluated based on the ideal adsorbed solution theory (IAST) model and a high value of 15.9 was achieved.

Thermal and mutual diffusivities of fuel-related binary liquid mixtures under pre-combustion conditions

In modelling heating and evaporation of fuel droplets, intra-droplet heat and mass transfer has to be considered. This is only possible with the help of reliable data for thermal and mutual diffusivity. At present, research activities in connection with the modelling of the evaporation of fuel droplets are based on theoretical diffusivity data. For their check and validation, there is, however, a lack of reliable experimental data. In this study, a dynamic light scattering apparatus which was especially developed for the simultaneous determination of both thermal and mutual diffusivity for fuel-related mixtures is presented. In the apparatus, fuel-related mixtures can be investigated under defined conditions in the compressed liquid phase close to saturation conditions in macroscopic thermodynamic equilibrium. Model binary mixtures composed of five different substances representative for fuel-related compounds were investigated at temperatures up to 523 K and close to their bubble point line. In detail, the mixtures investigated in the present study include 9 of the 10 possible binary combinations of isopentane, isooctane, toluene, n-decane, and ethanol. The results document that even under extreme conditions, thermal and mutual diffusivities are accessible with average expanded uncertainties (k = 2) of 8% and 6%. For all systems, concentrations, and temperatures, the thermal diffusivity changes only slightly. In contrast, the mutual diffusivity varies by more than two orders of magnitude from about 10−10 to 10−8 m2·s−1. For mixtures of isooctane and toluene, the concentration dependency of the mutual diffusivity reveals a nonideal behavior which becomes more pronounced with increasing temperature. For all binary mixtures, the temperature dependency of the mutual diffusivity can be correlated well with the vapor pressure of the individual components. The presence of the biofuel ethanol strongly slows down the molecular diffusion process at low temperatures.

Winery wastes as precursors of sustainable porous carbons for environmental applications

Abstract An efficient alternative for the management and valorization of the huge amount of seasonal wastes generated by winery industries is shown. Sustainable porous carbons were successfully prepared by one-pot activation of grape bagasse, stalks and oil free-seeds. Benefiting from the high moisture of these residues, a prior hydrothermal carbonization increases the activation yield and generates carbons with less oxygen content and inorganic impurities. The mild physical or chemical activation of winery wastes resulted in highly microporous carbons with specific surface SBET up to 2053 m2/g and great potential for environmental protection and electrical energy storage. They are excellent sorbents for CO2 capture under post- and pre-combustion conditions (16 and 69 wt%, respectively) and achieve superior electrochemical capacitance of nearly 300 F/g in aqueous electrolyte and 180 F/g in ionic liquid based-medium.

Ultra-high surface area mesoporous carbons for colossal pre combustion CO2 capture and storage as materials for hydrogen purification

Mesoporous carbons (with up to 95% of pore volume from mesopores) with surface area and pore volume of ∼4000 m2 g−1 and ∼3.6 cm3 g−1, respectively, are excellent CO2 absorbers under pre combustion conditions and can store 55 mmol g−1 (i.e., 2.42 g g−1) or 930 g l−1 at 25 °C and 50 bar.

Ab initio Thermodynamic Study of the CO2 Capture Properties of M2CO3 (M = Na, K)- and CaCO3-Promoted MgO Sorbents Towards Forming Double Salts

The CO2 capture properties of M2CO3 (M = Na, K)-promoted and CaCO3-promoted MgO sorbents are investigated by first-principles density functional theory complemented with lattice phonon calculations. The calculated thermodynamic properties indicate that by forming double salts (M2Mg(CO3)2 and CaMg(CO3)2), compared to pure MgO, the maximum allowable CO2 capture temperatures of the M2CO3- and CaCO3- modified MgO sorbents are shifted to higher temperature ranges. Under pre-combustion conditions with PCO2 = 10 bar, the Na2CO3-promoted and CaCO3-promoted MgO sorbents can capture CO2 at temperatures as high as 915 K and 740 K respectively. While under post-combustion conditions with PCO2 = 0.1 bar, their maximum allowable CO2 capture temperatures are 710 K and 600 K respectively. However, when adding K2CO3 into MgO, under both pre- and post-combustion conditions, its maximum CO2 capture temperatures only increased about 10 K relative to pure MgO. These results indicate that by mixing another solid into MgO, it is possible to shift its CO2 capture temperature to fit practical industrial needs.

Performance of a hybrid system sorbent–catalyst–membrane for CO2 capture and H2 production under pre-combustion operating conditions

This paper presents experimental results obtained for different approaches used for the simultaneous capture of CO2 and production of H2 under precombustion conditions. A novel hybrid system adsorbent–catalyst–membrane has been designed at Ciemat which combines a sorption enhanced WGS process (which consists of a mixture of adsorbent and catalyst) with a hydrogen selective membrane. First, the individual performance of the membrane and the binary adsorbent–catalyst system is evaluated in terms of H2 permeability, carbon monoxide conversion and CO2 capture and best operating conditions are determined for both approaches. The crucial role of temperature and steam on the overall performance of the system is especially discussed. Finally, first experimental results obtained for the hybrid adsorbent–catalyst–membrane system are presented and the advantages and challenges of adding a H2 selective membrane to the sorption-enhanced process are analysed.

First principles study on the adsorption of CO2 and H2O on the K2CO3 (001) surface

Potassium carbonate (K2CO3) is of interest as a CO2 absorbent. The molecular structure and properties of co-adsorption of CO2 and H2O on the K2CO3 (001) surface were investigated by density functional theory (DFT) methods. The DFT surface energies of K2CO3 low index planes were determined, and the (001) surface has the lowest surface energy. The calculated adsorption energies and Gibb's free energies indicate that H2O adsorbs much stronger than CO2, and their co-adsorption is repulsive on the K2CO3 (001) surface. The ab initio thermodynamics calculated coverage of CO2 under pre-combustion conditions is vanishingly small such that the net rate of bicarbonate formation is insignificant. This suggests that promoting bicarbonate formation requires a surface with stronger adsorption of CO2. This provides an indication that CO2 adsorption on the surface will be difficult in a wet stream, limiting the rate of conversion to the bicarbonate due to a low CO2 coverage. This is also in agreement with experimental results in the literature indicating that dry K2CO3 uptakes CO2 slowly, and that conversion to the bicarbonate proceeds initially through a hydrated structure.

Production of nanoporous carbons from wood processing wastes and their use in supercapacitors and CO2 capture

Highly porous carbons were obtained from solid wastes generated in the chemical and the mechanical processing of birch wood (substandard kraft cellulose, hydrolysis lignin, chips and bark). NaOH-chemical activation of these residues at 575–800 °C resulted in an efficient process to produce carbons with specific surface areas well above 1000 m2 g−1 and average pore widths of 1–1.7 nm.Comparative evaluations have shown the potentiality of wood wastes-based carbons in applications related to environmental protection. Activated carbons derived from chips- and bark-birch wood displayed specific capacitances as high as 308 F g−1 in the H2SO4 aqueous electrolyte and 200 F g−1 in the (C2H5)4NBF4/acetonitrile organic medium. Moreover, their capacitive performance at high current density competed well with that found for commercial carbons used in supercapacitors.Wood-derived carbons also proved to be highly promising for CO2 capture in power stations, achieving uptakes under post- and pre-combustion conditions of 11–16 wt.% and 49–91 wt.%, respectively.

On the limits of CO 2 capture capacity of carbons

This study shows that standard techniques used for carbons characterization, such as physical adsorption of CO 2 at 273 K and N 2 at 77 K, can be used to assess, with a good accuracy, the maximum capacity of carbons to capture CO 2 under post- and pre-combustion conditions. The analysis of the corresponding adsorption isotherms, within the general theoretical framework of Dubinin's theory, leads to the values of the micropore volume, W o, and the characteristic energy, E o, of the carbons, which provide direct information on the equilibrium CO 2 uptake of the carbons at different pressures and temperatures. Although in special cases slightly higher values can be obtained, an overall assessment of the textural parameters for a large variety of activated carbons, indicates that a CO 2 uptake upper-bound around 10–11 wt% seems to be realistic for standard activated carbons under post-combustion conditions. In the case of pre-combustion conditions, this limit would not exceed 60–70 wt%.