• Energy Research

AbstractIncreasing 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.

The aim of this study was to investigate the influence of coal rank and operating conditions during coal pyrolysis on the resultant char texture properties, morphology and reactivity. A range of bituminous coals were pyrolysed in a fixed bed reactor at different heating rates. It was found that the higher the heating rate and the lower the coal rank, the more microporous chars were obtained. Isothermal (500 °C) gasification in 20% oxygen in argon of the chars was carried out using a differential thermogravimetric system (DTG). The results of this work indicated that the increase in the availability of char-active surface sites led to an increase in char reactivity, not only for oxygen but also for other reactive gases, in particular NO, diminishing emissions during the combustion process.

AbstractBACKGROUNDMicroalgae are one of the most promising biofuel sources that the world has to offer; nevertheless the conversion process is hampered by technical and economic problems that are mainly related to de‐watering and extraction. The efficiency of the process can be dramatically improved by means of non‐conventional techniques such as ultrasound (US) and microwaves (MW). Scaling‐up feasibility is strictly linked to reactor efficiency, energy consumption, environmental impact and overall cost. In the present work, the optimization of lipid extraction from Nannochloropsis gaditana microalga is investigated.RESULTSA series of selected solvent mixtures and procedures have been tested and compared. Conventional extraction procedures with chloroform/methanol mixtures and fast US‐ and MW‐assisted extractions with methanol gave comparable fatty acid (FA) w/w% from dried microalgae. The highest extraction yield and lowest energy consumption was found to occur under MW irradiation, especially at high temperatures and under pressure.CONCLUSIONThis study highlights the advantages of US‐ and MW‐assisted lipid extraction from microalgae, both in terms of efficiency and operational costs. © 2013 Society of Chemical Industry

Abstract Perhydrous coals are characterised by high H/C atomic ratios and so their chemical structure is substantially modified with respect to that of conventional coals. As a result, perhydrous coals show different physico-chemical properties to common coals (i.e. higher volatile matter content, enhancement of oil/tar potential, relatively lower porosity and higher fluidity during carbonisation). However, there is little information about thermal behaviour during the pyrolysis of this type of coal. In this work, six perhydrous coals (H/C ratio between 0.83 and 1.07) were pyrolysed and analysed by simultaneous thermogravimetry/mass spectrometry. The results of this work have revealed the influence of high H/C values on the thermal behaviour of the coals studied. During pyrolysis the perhydrous coals exhibit very well defined, symmetrical peaks in the mass loss rate profiles, while normal coals usually show a broader peak. The shape of such curves suggests that in perhydrous coals fragmentation processes prevailed over condensation reactions. The high hydrogen content of perhydrous coals may stabilise the free radicals formed during heat treatment, increasing the production of light components.

The mechanism of reaction between NO and two models of carbonaceous materials with active sites was investigated at the UB3LYP/6-31 + G(d) and UM06-2X theory levels. The small model is the anthracene radical and the large one is also a monoradical built with ten benzene rings. The mechanistic routes found with both models lead to a satisfactory justification of the experimental data and showed the important role of the temperature and the oxygen and nitrogen surface complexes, generated in the carbonaceous material at intermediate steps of the mechanism, in the global process. The computational results presented in this work revealed that, at low temperatures, the high Gibbs energy barrier that appears after N2 release from the (NO)2 dimer, initially chemisorbed on the char surface, prevents the subsequent evolution of the system with the result that CO2 emission does not take place. On the other hand, at high temperatures, the mean energy available to the reactants may be sufficient to overcome this energy barrier giving rise to the formation of N2 and CO2 as reduction products. The N2 may come from two sources depending on the approach of the NO molecule at different points of the reaction coordinate. The best description of the carbonaceous surface through a larger model confirms the absence of N2O release in the reduction of nitric oxide on carbon surfaces.

Hybrid xerogels RF/Si were synthesized by controlling the chemical variables involved in the polymerization process (i.e., molar ratios, dilution ratio, catalysts, etc.) and evaluated as insulator materials. Higher insulating performances were recorded for these hybrids compared with their counterparts made from only one of their components (i.e., RF or Si xerogels with similar porous characteristics). The analysis of chemical and structural features correlated with heat transfer methods was useful in understanding the sum of contributions involved in the thermal conductivity of RF/Si xerogels. Variables such as roughness and tortuosity can be used to improve the performance of xerogels from a different perspective. In this way, thermal conductivities of 25 mW/mK were achieved without lengthy process steps or special drying methods. Knowledge of material design and the use of microwave heating during the synthesis allowed us to approach a simple and cost-effective process. These results suggest that the hybrid materials developed in this work are a good starting point for the future of the massive production of insulation materials.

Abstract Coal blends are now widely used by the power generation industry and the general characteristics are well known. Attention is still directed to the emission of NOx, which is subject to more stringent regulation, and to the amount of carbon in ash. The latter is increased when low NOx burners are employed, which is the norm now. It is also increased as a result of additional air staging when over-fire air is added in furnaces, especially tangential fired systems. Such a furnace is studied here. Two approaches can be employed for prediction of NOx and unburned carbon. The first approach uses global models such as the ‘slice’ model which requires the combustor reaction conditions as an input but which has a detailed coal combustion mechanism. The second involves a computational fluid dynamic model that in principle can give detailed information about all aspects of combustion, but usually is restricted in the detail of the combustion model because of the heavy computational demands. The slice model approach can be seen to be complimentary to the CFD approach since the NOx and carbon burnout is computed using the slice model as a post-processor to the CFD model computation. The slice model that has been used previously by our group is applied to a commercial tangentially fired combustor operated in Spain and using a range of Spanish coals and imported coals, some of which are fired as blends. The computed results are compared with experimental measurements, and the accuracy of the approach assessed. The CFD model applied to this case is one of the commercial codes modified to use a number of coal combustion sub-models developed by our group. In particular it can use two independent streams of coal and as such it can be used for the combustion of coal blends. The results show that both model approaches can give good predictions of the NOx and carbon in ash despite the fact that certain parts of the coal combustion models are not exactly the same. However, if a detailed insight into the combustor behaviour is required then the CFD model must be used.

The effect of curing temperature on smokeless fuel briquettes has been studied by Fourier transform infrared spectroscopy (FT-IR), mass spectrometry (MS), and temperature programmed decomposition (TPD). These techniques help to predict the final properties of these briquettes which were prepared with a low-rank coal, sawdust, and olive stone as biomasses and humates as binder. The best mechanical properties are reached with both the mildest thermal curing at 95 °C and the cocarbonized at 600 °C of Maria coal (M2) and sawdust (S) due to the fibrous texture of sawdust. The temperature of curing causes the release of a certain amount of oxygenate structures and the decrease of the mechanical resistance.