• Energy Research

Abstract This paper presents a novel methodology for the optimisation of the preliminary design of heat-integrated multicomponent distillation sequences. This is achieved through use of a reduced process superstructure where the role of splitting each adjacent key component pair is assigned to an individual separation column, greatly reducing the size and complexity of the problem. This methodology uses information about other process streams on site with which heat can be exchanged within the initial design in order to find a plant-wide optimal separation configuration with the aim of reducing the cost of heating provided by utilities including those used for heating and cooling process streams. In order that this model can be formulated as a mixed-integer linear program, this model utilises a discretised temperature grid where stream temperatures are allowed to vary. This methodology was tested on an example of a mixed alkane feed stream, with the sequence changing dependent on the degree of process integration. The method was found to have the potential for significant cost reductions compared to a heuristic design, with the example exhibiting a cost saving of over 50% and a reduction in CO2 associated with process heating of almost 60%, though the magnitude of these savings is highly dependent on the specific example to which it is applied.

AbstractTo shift the world to a more sustainable future, it is necessary to phase out the use of fossil fuels and focus on the development of low‐carbon alternatives. However, this transition has been slow, so there is still a large dependence on fossil‐derived power, and therefore, carbon dioxide is released continuously. Owing to the potential for assimilating and utilizing carbon dioxide to generate carbon‐neutral products, such as biodiesel, the application of microalgae technology to capture CO2 from flue gases has gained significant attention over the past decade. Microalgae offer a more sustainable source of biomass, which can be converted into energy, over conventional fuel crops because they grow more quickly and do not adversely affect the food supply. This review focuses on the technical feasibility of combined carbon fixation and microalgae cultivation for carbon reuse. A range of different carbon metabolisms and the impact of flue gas compounds on microalgae are appraised. Fixation of flue gas carbon dioxide is dependent on the selected microalgae strain and on flue gas compounds/concentrations. Additionally, current pilot‐scale demonstrations of microalgae technology for carbon dioxide capture are assessed and its future prospects are discussed. Practical implementation of this technology at an industrial scale still requires significant research, which necessitates multidisciplinary research and development to demonstrate its viability for carbon dioxide capture from flue gases at the commercial level.

In recent years, Carbon Capture and Storage (Sequestration) (CCS) has been proposed as a potential method to allow the continued use of fossil-fuelled power stations whilst preventing emissions of CO2 from reaching the atmosphere. Gas, coal (and biomass)-fired power stations can respond to changes in demand more readily than many other sources of electricity production, hence the importance of retaining them as an option in the energy mix. Here, we review the leading CO2 capture technologies, available in the short and long term, and their technological maturity, before discussing CO2 transport and storage. Current pilot plants and demonstrations are highlighted, as is the importance of optimising the CCS system as a whole. Other topics briefly discussed include the viability of both the capture of CO2 from the air and CO2 reutilisation as climate change mitigation strategies. Finally, we discuss the economic and legal aspects of CCS.

Abstract The value of dispatchable, low carbon thermal power plants as a complement to intermittent renewable energy sources is becoming increasingly recognised. In this study, we evaluate the performance of post-combustion CO 2 capture using monoethanolamine (MEA) retrofitted to a 600 MW CCGT, with and without exhaust gas recycle (EGR). Our results suggest that the EGR ratio plays a primary role in the regeneration energy penalty of the process. We contrast a gas-CCS process with its coal counterpart and show that whilst CCGTs have a greater energy penalty per tonne of CO 2 captured than coal ( i.e. , G J t C O 2 G a s > G J t C O 2 C o a l ), owing to the high thermal efficiencies of CCGTs relative to coal-fired power plants, the energy penalty per MWh of low carbon energy generated is lower for gas than it is for coal ( i.e. , G J M W h G a s G J M W h C o a l ), making CCGT-CCS an attractive choice for low carbon electricity generation.

Abstract Flow patterns within a 3D bed of oil-containing seeds fluidised by nitrogen have been observed for the first time using magnetic resonance imaging (MRI). Attention was focused on the lower region of the bed, just above the multi-orifice distributor: the orifices were 1.0 or 1.5 mm in diameter with square or triangular layouts, of pitch 7–10 mm. Two sizes of seeds were used: 1.2 and 0.50 mm. Each MRI image was a time-average over ∼ 5 min and measured the local concentration of seeds. Values of U / U mf were in the range 0.0–3.6, where U is the superficial gas velocity and U = U mf at incipient fluidisation. The images revealed: (1) There was a substantial ‘jet’ above each orifice in the distributor, remarkably these ‘jets’ were found even when U ⩽ U mf . The length of a ‘jet’ increased with U / U mf . Because of the time-averaged nature of the measurements, a ‘jet’ could be: (a) a permanent void, (b) a stream of bubbles, or (c) a ‘jet’ followed by bubbles. (2) When U / U mf 1.0 , the particles surrounding each ‘jet’ were in motion. This was apparent, particularly as U / U mf approached 1.0, even though the bed was not fully fluidised at all points. (3) When U / U mf > 1.0 , the upper parts of the ‘jets’ merged with each other forming a central dilute core. For the first time, a time-averaged velocity map over a horizontal plane was obtained; it demonstrated that the central core was rising upwards and that the surrounding material was descending. (4) Between each pair of ‘jets’, there was a small region of motionless particles sitting on the upper surface of the distributor, forming a fixed dead zone. A criterion for the maximum pitch of the orifices, to minimise the volume of this dead zone between pairs of ‘jets’, has been derived. Simple correlations between dimensionless groups summarise the measurements well, giving the length and half angle of a ‘jet’ in terms of the gas velocity and other variables. These correlations are consistent with published results and include a dependence on the pitch of the orifices, which was found to be important.

The reaction 3NO + 2 Fe → F e 2 O 3 + 3 / 2 N 2 ( I ) is important for converting the pollutant NO into harmless N2 in combustion systems, particularly fluidized beds. Its initial rate has been measured thermogravimetrically at 1 bar and 500–900 °C for particles of Fe sufficiently small for the reaction to be kinetically controlled. A detailed analysis of the measured rates for a variety of [NO] and two different particle sizes indicates that the measured initial rate of disappearance of NO fits well to r = α 8 [ NO ( 1 + ( α 8 NO β 8 ) 1 / 2 ) 2 per unit surface area. The most likely mechanism involves dissociative adsorption of NO, followed by the desorption of N2 and also O2− ions diffusing into the particle. There is another rate-law, which is almost as good, with the measured r=α9[NO]0.68. This rate expression could originate from NO adsorbing according to the Freundlich isotherm, followed by surface reaction to give N2 and O2−. Kinetic parameters together with activation energies and so on are all measured. Reaction I has a second stage, whose rate is determined by the amount of oxide product around each particle.

Abstract Chemical-looping combustion (CLC) is a novel combustion techology offering the potential to provide uninterrupted and reliable heat and power production from fossil or bio-derived fuels with integrated, intrinsic CO2 capture and minimal energy penalty. Operation of CLC at elevated pressures provides the potential for integration with a combined cycle, which makes the use of solid fuels significantly more feasible. To date, only a few experimental studies investigating CLC processes and oxygen carrier performance under pressurised conditions have been reported in the open literature. This article reports findings from investigations into the effect of pressure, temperature and CO concentration on the intrinsic reaction kinetics of an Al2O3-supported Fe-based oxygen carrier. Our study employed an innovative pressurised fluidised-bed reactor, designed for operation at temperatures up to 1273 K and pressures up to 20 bara, to simulate ex-situ gasification of solid fuels at elevated pressures. An intrinsic reaction model was developed and pseudo-intrinsic rate constants were derived. Differences in the activation energies and pre-exponential factors of the Al2O3-supported Fe2O3 and a pure Fe2O3 oxygen carriers were observed, indicating a change in reaction mechanism when Al2O3 was present. Subsequently, an adapted random pore model was developed to describe the variation of reaction rate with solid conversion. The good agreement between the adapted random pore model and empirical measurements indicated that the change in mechanism was due to a significantly higher product layer diffusivity for the Al2O3-supported Fe2O3 oxygen carrier compared with the pure Fe2O3 material. When pressurised, the observed reaction order with respect to CO was slightly lower than 1. The model developed using atmospheric pressure measurements was successfully applied to predict reaction kinetics at elevated pressures up to 5 bara providing further validation of the model.

AbstractCalcium oxide has been proposed as a regenerable sorbent for separating CO2 from flue gas at high temperatures. It is well known that natural sorbents (i.e., CaO derived from natural limestone) lose their capture capacity as the number of the carbonation/calcination cycles increases. If the behaviour of the sorbent can be improved (i.e., the decay in reactivity of sorbent reduced or residual reactivity of sorbent increased), the viability of a CaO-based CO2 capture scheme could be improved. One potential method to achieve this is doping the sorbent with different salts. In this work, a simple wet impregnation method has been used to dope two different limestones using solutions of KCl and K2CO3 with different concentrations. Doped samples were then subjected to repeated carbonation/calcination cycles in both a Fluidized Bed Reactor (FBR) and a Thermogravimetric Analyser (TGA) in order to compare their reactivity in both cases. The results obtained show that samples doped with 0.5 M solution of KCl have a decreased reactivity over the initial cycles, but an increased long-term reactivity in both the FBR and the TGA, with improved results observed in the FBR. Sorbent doping could prove to be a relatively inexpensive method of improving the reactivity of sorbent for the calcium looping cycle for CO2 capture.