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Modern gas turbines operate with large amounts of excess air for cooling and dilution of the combustion gases, in order to maintain gas turbine blade integrity. Selective recycling of CO2 into the gas turbine compressor inlet, also referred as Selective Exhaust Gas Recirculation (SEGR), can reduce the large volumetric flow rate through a CO2 capture system caused by the gas turbine excess air requirements, by 70 - 77%. It also increases CO2 concentrations to 14-18 vol% from 3-4% vol, increasing the driving force for post-combustion capture systems. This paper provides a comprehensive assessment of the concept and presents research outcomes from the UK-funded SELECT project, including power plant and process modelling, techno-economic assessments, pilot-scale gas turbine experimental work and experimental combustion tests on a representative combustor. Using an integrated model of turbomachinery, power cycles and a generic post-combustion CO2 capture technology with a 30%wt MEA solvent, we show that a reduction of up to 50% of the absorber of the capture plant – the most capital intensive part of the process – can be achieved. The compressor and gas turbine operate without any significant deviation from their design point, and a marginal increase of 0.5% point in the net electrical efficiency can be achieved. Pilot-scale testing - conducted at the Pilot Scale Advanced Capture Technologies (PACT) facilities at the University of Sheffield - show that CO2 injection at the compressor inlet of a 100 kW micro gas turbine (mGT) connected to a 1 tonne per day CO2 capture plant reduces net electrical efficiency by 1-2 %point. This is caused by lower flame temperatures, and, unlike in larger gas turbines, the control system of the micro gas turbine. Combustion tests at Cardiff University’s Gas Turbine Research Centre (GTRC) in a pilot-scale high-pressure generic premixed swirl burner, representative of modern dry-low emissions (DLR) gas turbine burners, show the effect of CO2 as diluent on the operational premixed CH4/air flame stability, chemical kinetics and measured exhaust gas composition. CO2 acts as a combustor inhibitor, causing downstream migration of the premixed flame zone, leading to eventual blow-off, instability and extinction, requiring a change in the operation equivalence ratio. The effect of adding CO2 leads to a reduction in the adiabatic flame temperature due to thermal quenching, which results in higher CO emissions and smaller thermal NOx emissions. Increasing pressure has a significant reducing effect on CO emissions, yet it results in higher NOx production, which may require mitigation if this trend is found to continue towards pressures approaching that of the F-class gas turbine. Finally, a conceptual design assessment of a regenerative adsorption wheel with structured adsorbents is proposed for the selective recycling of CO2 in combined cycle gas turbine (CCGT) power plants. It has the advantage of a relatively small pressure drop to reduce the derating of the gas turbine compared to selective CO2 membrane systems. An equilibrium model of a rotary adsorber with commercially available activated carbon adsorbents shows that four rotary wheels of 24 m diameter and 2 m length would be required in an 820MW CCGT plant. A reduction of 50% in the mass of adsorbent would be possible with an adsorbent with a higher capacity, such as Zeolite X13, with upstream dehydration of the flue gases.

The combustion of coal is responsible for nearly 40% of the world's electricity production, and char combustion accounts for about half of that amount. Clearly, an understanding of the combustion mechanism of carbon is of great importance not only because of its industrial significance but because it is a model heterogeneous reaction. A number of recent studies have been concerned with ab initio molecular orbital calculations on graphite including model chemistry and the reactions with molecular oxygen. This study is concerned with oxidation steps involving the attachment of oxygen to a graphene layer at high temperature leading to the formation of carbon monoxide, and particular attention is paid to the subsequent oxidation reactions. In addition, the reaction of oxygen with carbon catalyzed by metals inherent within the char matrix and the reaction of molecular oxygen with the analogous biomass char are investigated and their reaction paths are discussed.

Abstract Several investigations have shown that the differences between deposits obtained in oxy-firing and air-firing of coal mainly are due to differences in the flame temperature. Consequently, deposit rate predictions not taking the in-flight history into account are unlikely to be successful. In this paper, a model for predicting the deposit formation propensity of pulverized coal in oxy-fuel and air combustion due to the inertial impaction mechanism is developed and tested. The model builds on the use of viscosity as an indicator of the sticking probability. The composition and amount of the amorphous slag phase in the coal ash are calculated assuming thermodynamic equilibrium. Further, it is assumed that the maximum temperature the ash particle has experienced will control the composition and amount of the amorphous slag phase. As the ash particle impacts the probability to stick is estimated using the viscosity of this melt composition, but with the temperature of particle temperature at the moment of impaction. In the equilibrium calculation no material exchange with the gas phase is assumed. This assumption is based on X-ray diffraction (XRD) investigations of coal ash samples produced in a lab-scale burner simulating oxy-fuel and air combustion. The XRD showed that there was no significant impact on the mineralogy of the coal ash caused by the gas atmosphere. The probability of an ash particle to stick as a function of maximum experienced temperature and impact temperature was evaluated for three coals. For one of the coals a CFD study on particle deposit is done for a 300 kWth test facility.

Abstract The aim of this paper is to numerically investigate the effects of CO2 dilution on the operation of an industrial micro gas turbine combustor in order to assess the possible application of exhaust gas recirculation (EGR) for post-combustion CO2 capture. A complete 3D model of the combustion chamber has been developed, taking into account the conjugate heat transfer (CHT) and radiation effects, and a detailed chemical mechanism has been employed in the framework of the Flamelet Generated Manifolds approach to model the combustion process. The importance of including the effects of conjugate heat transfer in the model has been demonstrated for both air-fired and EGR conditions. Also, combustion with EGR resulted in lower temperature levels with respect to the air-fired case and thus in reduced NOx production. Further, the increased presence of carbon dioxide has been observed to have an impact on both the flame speed and the flame stabilization mechanism. According to the numerical results, EGR can be a viable way to increase the CO2 content in the flue gas of dry low-emissions (DLE) combustors, and therefore enhance the efficiency of post-combustion carbon separation. At the same time, due to the reduced temperature levels within the combustion chamber, it is possible to attain lower NOx emissions without compromising the combustion efficiency under the considered EGR levels.

The deployment of post-combustion CO2 capture on large-scale gas-fired power plants is\ud currently progressing, hence the integration of the power and capture plants requires a\ud good understanding of operational requirements and limitations to support this effort. This\ud article aims to assist research in this area, by studying a micro gas turbine (MGT) integrated\ud with an amine-based post-combustion CO2 capture unit. Both processes were simulated\ud using two different software tools – IPSEpro and Aspen Hysys, and validated against\ud experimental tests. The two MGT models were benchmarked at the nominal condition, and\ud then extended to part-loads (50 and 80 kWe), prior to their integration with the capture\ud plant at flue gas CO2 concentrations between 5 and 10 mol%. Further, the performance of\ud the MGT and capture plant when gas turbine exhaust gases were recirculated was assessed.\ud Exhaust gas recirculation increases the CO2 concentration, and reduces the exhaust gas\ud flowrate and specific reboiler duty. The benchmarking of the two models revealed that the\ud IPSEpro model can be easily adapted to new MGT cycle modifications since turbine\ud temperatures and rotational speeds respond to reaching temperature limits; whilst a\ud detailed rate-based approach for the capture plant in Hysys resulted in closely aligned\ud simulation results with experimental data.\ud

Abstract Gas diffusion layers (GDLs) are one of the main components in proton exchange membrane (PEM) fuel cells. In this paper, the effect of anisotropic thermal conductivity of the GDL is numerically investigated under different operating temperatures. Furthermore, the sensitivity of the PEM fuel cell performance to the thermal conductivity of the GDL is investigated for both in-plane and through-plane directions and the temperature distributions between the different GDL thermal conductivities are compared. The results show that increasing the in-plane and through-plane thermal conductivity of the GDL increases the power density of PEM fuel cells significantly. Moreover, the temperature gradients show a greater sensitivity to the in-plane thermal conductivity of the GDL as opposed to the through-plane thermal conductivity.

The dynamic stall phenomenon in Vertical Axis Wind Turbines (VAWTs) appears under some operating conditions that have not been very well established. Some studies have focused on describing the topology of the dynamic stall but little attention has been paid to understand how all the operating VAWT parameters influence the moment of stall inception. This paper focuses on analysing the influence of the tip speed ratio, pitch angle, reduced frequency, relative velocity and Reynolds number on the stall-onset angle of VAWTs. CFD simulations with an oscillating NACA0015 describing the angle of attack and relative velocity in VAWTs were employed. The results have revealed that an increase in the stall-onset occurs anytime the operating parameters increase the value of the non-dimensional pitch rate and the Reynolds number at the moment the angle of attack approaches to the static stall angle. The stall-onset angle showed a linear increase with the non-dimensional pitch rate in the range of Reynolds number tested, namely 0.8–3.3 x 10^5. This paper has elucidated how the several parameters governing VAWTs operation affect the stall-onset angle and therefore has contributed to a much better understanding of the causes that induce the stall in these devices.

The design of low emission combustion chambers using low NOx strategies involving staged burning or stratified combustion requires a detailed understanding of the combustion processes of the fuel volatiles and char burning. In this paper some aspects of the combustion of coal-volatiles and char are considered. The extreme cases of volatile combustion, namely premixed and diffusive burning are examined in order to consider the range of NOx reduction options available to the combustion chamber designer. A similar set of situations is examined for char burning and the release of the fuel-nitrogen to form NO. The implications of the processes are considered in two practical applications, those of the high temperature combustion found in pulverised coal burning and in a lower temperature regime of the conditions under fluidised bed combustion. In the case of pulverised coal flame the degree of mixedness of the volatiles played a dominant part in determining the extent of NO formation whilst the role of char-nitrogen is only to form NO and NO reduction is limited because of the short residence time and low char concentrations at the end of the reaction zone. In a circulating fluidised bed combustor it was concluded that a different situation can arise. If the bed is sufficiently large enough to give a residence time of several seconds, then the NO initially formed in the fluidised bed is reduced by the carbon in the top of the bed and the riser under steady state conditions and its concentration at the exit can be estimated by equilibrium calculations.

A general inversion procedure for determining the optimum rate coefficients for chemical kinetic schemes based upon limited net species production data is presented. The objective of the optimisation process is to derive rate parameters such that the given net species production rates at various conditions are simultaneously achieved by searching the parameter space of the rate coefficients in the generalised Arrhenius form of the reaction rate mechanisms. Thus, the goal is to both match the given net species production rates and subsequently ensure the accurate prediction of net species production rates over a wide range of conditions. We have retrieved the reaction rate data using an inversion technique whose minimisation process is based on the Darwinian principle of survival of the fittest which has inspired a class of algorithms known as genetic algorithms. The excellent results presented here from our initial study are based upon the recovery of reaction rate coefficients for hydrogen/nitrogen/oxygen flames. The successful identification of the reaction rate parameters which correspond to product species measurement data from a sequence of such experiments clearly suggests that the progression onto other chemical kinetic schemes and the optimisation of higher-order hydrocarbon schemes can now be realised. The results of this study therefore demonstrate that the genetic algorithm inversion process promises the ability to assess combustion behaviour for fuels where the reaction rate coefficients are not known with any confidence and, subsequently, accurately predict emission characteristics, stable species concentrations and flame characterisation. Such predictive capabilities are of paramount importance in a wide variety of industries.