Search

Abstract: In this work several Li4SiO4-based sorbents from fly ashes for CO2 capture at high temperatures have been developed. Three fly ash samples were collected and subjected to calcination at 950°C in the presence of Li2CO3. Both pure Li4SiO4 and fly ash-based sorbents were characterised and tested for CO2 sorption at different temperatures between 400 and 650°C and adding different amounts of K2CO3 (0–40mol%). To examine the sorbents performance, multiple CO2 sorption/desorption cycles were carried out. The temperature and the presence of K2CO3 strongly affect the CO2 sorption capacity for the sorbents prepared from fly ashes. When the sorption temperature increases by up to 600°C both the CO2 sorption capacity and the sorption rate increase significantly. Moreover when the amount of K2CO3 increases, the CO2 sorption capacity also increases. At optimal experimental conditions (600°C and 40mol% K2CO3), the maximum CO2 sorption capacity for the sorbent derived from fly ash was 107mgCO2/gsorbent. The Li4SiO4-based sorbents can maintain its original capacity during 10 cycle processes and reach the plateau of maximum capture capacity in less than 15min, while pure Li4SiO4 presents a continual upward tendency for the 15min of the capture step and attains no equilibrium capacity. [Copyright &y& Elsevier]

Abstract: Increasing awareness of the influence of greenhouse gases on global climate change has led to recent efforts to develop strategies for the reduction of carbon dioxide (CO2) emissions. The strategy that is receiving the most attention involves the capture of CO2 from large point sources, such as fossil fuel-fired power plants, and long-term storage underground or in the ocean. CO2 capture can be achieved either by post or pre-combustion capture at ambient or high pressure respectively. Aqueous solutions of amines have long been used by industry as absorbents for acid gas (CO2,H2S) removal, but have a large number of short comings. As an alternative, adsorption is one of the more promising technologies for capturing CO2 from flue gases, potentially avoiding the shortcomings of aqueous amine systems. In this paper the development of solid adsorbents for post-combustion capture will be described. The generation and testing of a range of adsorbents for post-combustion capture of CO2 at ambient pressure has demonstrated the need for chemical type adsorbents. A range of chemical type adsorbents have been generated by two methods: [•] Modification of the surface chemistry of both low cost carbons and mesoporous silica by impregnation with a basic nitrogen-containing polymers (i.e., polyethylenimine). [•] Generation of a range of high nitrogen content carbon matrix adsorbents by carbonisation and subsequent thermal or chemical activation of a range of nitrogen compounds. CO2 adsorption capacities at equilibrium and under dynamic conditions using a simulated flue gas will be described at different temperatures and gas residence times. Regeneration using simulated temperature, pressure swing cycles and steam stripping and the effect of the processes on the lifetime of the adsorbent will be discussed. [Copyright &y& Elsevier]

Abstract: Adsorption is considered to be one of the more promising technologies for capturing CO2 from flue gases. The efficient adsorption of CO2 at low partial pressures, associated with post-combustion capture, require chemical type adsorbents containing basic amine functional groups. It has been demonstrated that amine polymers, for example polyethylenimine (PEI), immobilised on various porous substrates, silica, zeolites and fly ash, are effective adsorbents for CO2. When considering the use of adsorption for large scale CO2 capture, the ease of regeneration and the lifetime of the adsorbents are critical factors in determining their efficiency, cost and therefore feasibility for use. In this paper two approaches, thermal swing adsorption (TSA) cycles over a range of temperatures and time in an atmosphere of CO2 and thermally assisted pressure swing desorption, are explored for the regeneration of the PEI based adsorbents. The reactions occurring during the TSA regeneration of PEI based adsorbents in an atmosphere of CO2, especially the formation of a thermostable complex between PEI and CO2 above 130 ∘C are described. Identification of the complex by FTIR, XPS and 13C NMR as a urea linkage will be described. Overall, the results from this research have implications for the selection of regeneration strategies of all amine based CO2 adsorbents. [Copyright &y& Elsevier]

Abstract: Amine solvents have long been used by industry as absorbents for acid gas (CO2, H2S) removal and are the current technology of choice for post-combustion carbon capture from fossil fuel power plants. However, these technologies are energy intensive and have a number of short comings. In an effort to reduce the capital cost and energy penalty of carbon capture alternative technologies are being explored, of which solid sorbents have shown good potential. Adsorbents containing basic nitrogen functional groups to increase adsorbent / adsorbate interaction have been demonstrated to be effective for post-combustion capture. These materials are usually synthesised by the impregnation of basic amine polymers or bonding of amine groups to the surface of inorganic porous substrates. In this work the development of a range of novel high nitrogen content activated carbon adsorbents will be described. The aim of this research is to introduce basic nitrogen directly into the matrix of activated carbon to yield high capacity adsorbents with high thermal stability in terms of volatile and thermal loss of nitrogen. Novel nitrogen enriched activated carbons have been synthesised by a templating or nanocasting technique by which a removable inorganic template is used to generate porous polymers with high surface area. A range of adsorbents have been synthesised by templating of melamine–formaldehyde resin with silica followed by activation over a range of temperatures from 400–700 ∘C. The resultant adsorbents have been characterised in terms of their textural properties, elemental composition and surface chemistry, with materials containing up to 42 wt.% nitrogen and 880 m g surface area generated. Adsorption capacities up to 2.25 mmol g of CO2 at 25 ∘C were measured using thermogravimetric analysis and will be discussed in terms of the textural and surface chemical properties of the carbons as determined by X-ray Photoelectron Spectroscopy (XPS). Both texture and surface chemistry influence the CO2 capture performance of the adsorbents. The activation temperature used during the synthesis step controls the nitrogen functional groups present, as determined by XPS, with the loss of triazine nitrogen with increasing activation temperature proposed to account for the decreased CO2 affinity. Finally the stability and regeneration of the carbons over numerous thermal swing adsorption cycles will be described. [Copyright &y& Elsevier]

Abstract: Adsorption on porous solids is an emerging alternative for CO2 capture that seeks to reduce the costs associated to the capture step. The enhancement of a specific adsorption capacity may be carried out by increasing the affinity of the adsorbent surface to CO2. Nitrogen enrichment is reported to be effective in introducing basic functionalties that enhances the specific adsorbent–adsorbate interaction for CO2. In this work a templating technique was used to produce highly porous nitrogen enriched carbons from melamine–formaldehyde resins. Nitrogen incorporated into the polymer matrix results in the greater stability of the adsorbents in terms of volatile and thermal loss of nitrogen. CO2 capture performances were evaluated between 25°C and 75°C in a thermobalance. CO2 adsorption capacities up to 2.25mmolg−1 of CO2 at 25°C were achieved. Both texture and surface chemistry influence the CO2 capture performance of the adsorbents. The carbonisation temperature used during the synthesis step controls the nitrogen functional groups present, as determined by XPS, with the loss of triazine nitrogen with increasing carbonisation temperature proposed to account for the decreased CO2 affinity. [Copyright &y& Elsevier]

Abstract: The nitrogen enrichment of active carbons is reported to be effective in enhancing the specific adsorbate–adsorbent interactions for CO2. In this work, nitrogen-enriched carbons were prepared by co-pyrolysis of sugar and a series of nitrogen compounds with different nitrogen functionalities. The results show that although the amount of nitrogen incorporated to the final adsorbent is important, the N-functionality seems to be more relevant for increasing CO2 uptake. Thus, the adsorbent obtained from urea co-pyrolysis presents the highest nitrogen content but the lowest CO2 adsorption capacity. However, the adsorbent obtained from carbazole co-pyrolysis, despite the lower amount of N incorporated, shows high CO2 uptake, up to 9wt.%, probably because the presence of more basic functionalities as determined by XPS analysis. [Copyright &y& Elsevier]

Abstract: 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 4.5wt%) 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. [Copyright &y& Elsevier]

Abstract: The effect of oxidation upon the CO2 capture performance has been studied taking a phenolic resin carbon as the base material. Oxygen surface groups were introduced through liquid and gas phase oxidation treatments, using ammonium persulfate, nitric acid and air, respectively. The surface chemistry of the final carbon is strongly affected by the type of oxidation treatment: liquid phase oxidation introduces a greater amount of oxygen, mostly as carboxylic groups; these are absent in the gas phase oxidised sample that contains mainly ether and carbonyl functionalities. The porous texture of the samples is also affected by the oxidation treatment: through liquid phase oxidation the pore volume is somewhat reduced, while this is slightly developed by air treatment at 693K. Despite the reduction in the porous volume and the acidic surface, liquid-phase oxidised samples present greater CO2 adsorption capacity than the starting carbon due to Lewis acid–base interactions with the CO2 molecule. Moreover: oxidised samples are easily regenerated, and observed heats of adsorption are typical from physisorption processes, which will facilitate the adsorbent regeneration in cyclic adsorption processes. Oxidation is therefore proposed as a plausible modification technique for developing easy-to-regenerate carbon adsorbents with enhanced CO2 capture performance. [Copyright &y& Elsevier]