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The paper is focused on coupling a small-scale power plant, based on a micro gas turbine (mGT) and a bottoming Organic Rankine Cycle (ORC), with a biomass gasifier. The aim of this study is to define the optimal strategies to maximize the benefits related to distributed generation and to promote the organic solid waste gasification, in terms of energy efficiency and renewable sources exploitation. In particular, they were investigated the energetic performances of the system when the micro gas turbine was fed with several fuel blends, made by specific volume concentration of syngas and biogas. The low heating value of both considered fuels implies the necessity of operating the mGT in peculiar conditions as determined by the performance maps of compressor and turbine. Then, the thermodynamic analyses of the whole energy system have been carried out to evaluate the performance for each fuel. The high hydrogen content of syngas and the different thermodynamic properties of the studied fuel blends required a deeper investigation of the combustion process. In order to analyze the combustion stability and the fluid dynamic aspects, an accurate investigation of combustion chamber has been performed through a CFD solver. Finally, a comparison of the plant performances for each fuel blend have been reported, along with opportunities and critical aspects related to power plant integration.

The use of biogas in micro gas turbines (MGTs) generally requires invasive structural modifications to fuel compression units, control valves and combustor. Costs associated with the modifications significantly reduce the benefits of a rational and sustainable exploitation of energy resources of lower value as the biogas. In order to overcome this aspect, in a recent paper the authors proposed a dual fuel approach, identifying a structurally non-invasive and cost-effective technical solution to increase the CO2 concentration in the fuel and to extend the operation domain of the MGT when low calorific value fuels are used. In that study, it has been evaluated, implemented and tested an innovative management strategy of fuel feeding and combustion. Experimental preliminary tests validated the idea of a new supply strategy for exceeding the current limits of biogas exploitation as MGT fuel. With this approach the MGT operating domain has been extended, increasing more than twice the CO2 concentration limit than experimentally validated limit available in literature. In this study the authors present a more extensive investigation concerning the energetic performance and the environmental impact of a 100 kWe MGT Turbec T100 when fuelled with a mixture of natural gas and carbon dioxide (synthesis biogas) in the dual fuel approach, by varying CO2 concentration in the fuel at part load condition. The data acquired by an extensive experimental campaign, in terms of thermodynamic parameters and gaseous exhaust emissions, are presented and compared with the results of numerical simulations. A 3D CFD analysis of the combustion process is also presented. Initial and boundary conditions of the numerical approach were obtained from a previous validated 0D thermodynamic matching model. The synergistic activity between numerical modeling and experimental work allows to analyze and explain the overall behavior of the micro gas turbine in the dual fuel mode with biogas fuel. The study shows that the micro gas turbine can stably operate in a dual fuel mode until the CO2 content rises above 25%.

International audience; h i g h l i g h t sWater-in-oil emulsion is produced using two micro-channels in series. Droplets size of the dispersed phase reaches 11 µm without surface active agent. A decrease of the average water droplet size by 40% compared to the use of single micro-system. Energy balance is performed. Flow visualization is given.a b s t r a c t Development of alternative fuels and improvement of their combustion is a necessity in a general context of scarcity of fossil energy resources and of environmental pollution. In particular, the use of water dispersed in biofuels is known to improve combustion quality of diesel engine and to reduce pollution in gas emissions. This work aims at contributing in the development of a compact continuous emulsifier that could be used to feed energy systems. In order to fulfil requests such as convenient size of water droplets, absence of surfactant and needed flow rates of biofuel, the association of two micro-systems in series is investigated and compared to the use of a single one. The comparison of the emulsions obtained by the different systems is made in terms of mean size and size distribution of the water droplets in the dispersion, and process energy consumption. In the absence of surfactant, depending on applied flow-rates and water fraction, the use of two micro-systems in series allows to reduce the mean size of droplets by a factor 2 to about 3. Flow phenomena which lead to water phase fragmentation are described thanks to high-speed visualization of the flows in the micro-channels.

In this study, research activity concerning the utilization of biogas into a commercial micro gas turbine (MGT) Ansaldo Turbec T100P is presented. The use of biogas in MGT has been already quite explored in literature. However, conflicting and not yet fully comprehensive results highlight the need for further scientific investigations. Although several authors have demonstrated, through numerical studies, the possibility of using biogas with amounts of CO2 up to 40–50% in volume in commercial MGT, experimental evidences of this condition are not yet present in literature. In fact, the CO2 concentration limit experimentally validated in literature, after which phenomena of detachment and instability of the flame happen, is considerably lower (∼12%). The utilization of biogas in micro gas turbines generally requires structural adaptations, often invasive and expensive, to the fuel compression unit and control valves. Not infrequently a partial redesign of the combustor is often necessary. Costs associated with the modifications significantly reduce the benefits of a rational and sustainable exploitation of energy resources of lower value as the biogas. Aim of this activity is to overcome these issues, by identifying a non-invasive and cost-effective technical solution to increase the CO2 concentration in the fuel and to extend the operation domain when low calorific value fuels are used. It has been studied, implemented and tested an innovative management strategy of fuel feeding and combustion. Experimental layout and preliminary results of this novel approach are shown and discussed.

The paper reports the experimental results concerning emissions from 30 kWe commercial micro gas turbine feed with blends of straight vegetable oil with fossil fuel. Both gaseous and particulate emissions were measured at full and partial load for blends 10% v/v and 20% v/v of rapeseed and sunflower oils with JET A1 kerosene. The variations of nitric oxide and carbon monoxide emissions were taken into account to evaluate the combustor behavior at different loads and blends. The effects of the fuel composition on the variations, in terms of concentrations and dimensions, of ultrafine – particulate matter were also evaluated. NOx and CO emissions are practically insensitive to the composition of the fuel being the differences within the experimental uncertainty of the instrumentation. This demonstrates a similar overall combustion at both partial and full load for the five fuels used (Jet A1 and four SVO blends). On the contrary, particle matter emission is three times for blends of rapeseed oil and more than fifty times greater for blends of sunflower oil with respect to pure Jet A1. The differences are ascribed to the chemical structure of the vegetable oil tested.

The off design behavior of a 100kW Turbec T100P micro gas turbine fuelled with methane is analyzed in the paper. The work is focused on the analysis of combustion efficiency, overall performance and gaseous emissions at different loads. A critical comparison between experimental data and results of a 3D CFD analysis of the combustor is discussed. The RANS RSM turbulence model is used in the numerical computations. The Zeldovich mechanism for NO formation evaluation is coupled to a methane-oxygen multistep chemical kinetic mechanism for predicting the concentration of O2, CO2, CO and NO at the exhaust. The experimental campaign showed no sensible variations in NOxemissions by varying MGT load; otherwise a great CO increasing is highlighted by load decreasing. The results of numerical analysis are in good agreement with experimental data as concerns CO emissions, while overestimate NO production in the diffusive flame region at 50% load.

The emulsification is well known for being considered as a solution for improving biodiesel combustion. In this extent, the size of the dispersed droplets is crucial to favour micro-explosions and optimising the combustion. The present mini-channel operates liquid/liquid dispersion at higher flow rates than with usual implementations of minichannels and micro-channels. Due to this particular condition of functioning, the flow enhanced in the mini channel is classified as a high-speed emulsifying one. In this study, the dispersion of water into sunflower oil as a water-in-oil (w/o) emulsion is tested. In such an in-line process, high-shear implies higher velocities at smaller scales, making an optical visualisation difficult. But the light scattering technique enables to detect micro-scaled droplets, at the required high velocities. The proper design and exploitation of a focalised laser beam through a transparent, hydrophobic mini-channel is explained. A focalised laser beam is crossing the mini-channel, dealing with the scattering cross sections of the water droplets. The measurement of the collected beam is successfully matched to the geometrical properties of the emulsion (water fraction and size of inner droplets) through the classical light scattering theoretical basement. The calibration of the collected laser light technique obtained in the straight mini-channel is followed by measurements approaching the high shear rate zone of the cross-flowing mini-channel

The paper deals with the development of a dynamic model of a commercial 100 kW Micro Gas Turbine (MGT) fuelled with mixtures of standard (i.e. natural gas or methane) and alternative fuels (i.e. hydrogen). The model consists of a first-order differential equation (ODE) describing the dominant dynamics of the MGT imposed by its own control system during production electrical power. The differential equation is coupled to a set of nonlinear maps derived numerically from a detailed 0D thermodynamic matching model of the MGT evaluated over a wide range of operating conditions (i.e. mechanical power, fraction of hydrogen and ambient temperature). The efficiency of the electrical machine with power inverter and power absorbed by auxiliary devices is also taken into account. The resulting model is experimentally validated for a sequence of power step responses of the MGT at different ambient conditions and with different fuel mixtures. The model is suited for simulation and control of hybrid energy grids (HEGs) which rely on advanced use of MGT and hydrogen as energy carrier. In this regard, the MGT model is used in the simulation of an HEG based on an appropriate mix of renewable (non-programmable) and non-renewable (programmable) energy sources with hydrogen storage and its reuse in the MGT. Here, the MGT is used as a programmable energy vector for compensating the deficits of renewable energies (such as solar and wind) with respect to user demand, while excess renewable energy is used to produce hydrogen via electrolysis of water. The simulated HEG comprises a solar PhotoVoltaic (PV) plant (300 kW), an MGT (100 kW) fuelled with natural gas and hydrogen blends, a water electrolyzer (WE) system (8 bar, 56 Nm3/h), a hydrogen tank (54 m3), and an Energy Management Control System (EMCS).

Aim of the paper is to evaluate the potentialities and critical issues of the micro-gas turbine Turbec T100 properly modified in order to be powered by liquid fuels. The main difference with respect to the version designed for gaseous fuels concerns the combustor, namely the combustion chamber and the fuel injection system. The numerical study was carried out using both a 0D matching analysis and a 3D CFD analysis of the combustor. The thermodynamic model, coupled with the operating maps of rotating components, was calibrated using the experimental data collected during operation of the micro-gas turbine with gaseous fuel. The operative parameters computed through the 0D model have been utilized to simulate the behavior of micro-gas turbine fed by liquid fuels, thus obtaining the initial and boundary conditions to be used in the CFD analysis. The combustion process of liquid fuels is analyzed; the main differences at varying loads are discussed in terms of temperature and velocity profiles of the air flows inside the combustion chamber and mean values of turbine inlet temperature and exhaust gaseous concentration. A preliminary study concerning the characteristics of the injection system is also reported.

In recent years an increasing interest is focused on the study of micro gas turbines (MGT) behavior at part load by varying fuel, in order to determine their versatility. The interest in using MGT is related to the possibility of feeding with a wide range of fuels and to realize efficient cogenerative cycles by recovering heat from exhaust gases at higher temperatures. In this context, the studies on micro gas turbines are focused on the analysis of the machine versatility and flexibility, when operating conditions and fuels are significantly varied. In line of principle, in case of gaseous fuels with similar Wobbe Index no modifications to the combustion chamber should be required. The adoption of fuels whose properties differ greatly from those of design can require relevant modifications of the combustor, besides the proper adaptation of the feeding system. Thus, at low loads or low calorific value fuels, the combustor becomes a critical component of the entire MGT, as regards stability and emissions of the combustion process. Focus of the paper is a 3D CFD analysis of the combustor behavior of a Turbec T100P fueled at different loads and fuels. Differences between combustors designed for natural gas and liquid fuels are also highlighted. In case of natural gas, inlet combustor temperature and pressure were taken from experimental data; in case of different fuels, such data were inferred by using a thermodynamic model which takes into account rotating components behavior through operating maps of compressor and turbine. Specific aim of the work is to underline potentialities and critical issues of the combustor under study in case of adoption of fuels far from the design one and to suggest possible solutions.Copyright © 2015 by ASME

The performance of a micro gas turbine in terms of global efficiency and exhaust emissions at different loads and methane-hydrogen blends has been experimentally studied. The load was varied from full load down to half load. A critical comparison between experimental data and results of the 3D CFD analysis of the combustor is discussed in order to study the effects of part load operation. Additional objective of the present study is to extend the numerical analysis of the combustor in order to describe the influence of a wider range of hydrogen concentrations in the fuel gas mixture. Numerical simulations were performed through the commercial code ANSYS CFX 14.5. Turbulence model adopted was the RSM RANS model, which ensures a good evaluation of effects of swirled turbulent flows in the combustion chamber. The oxidation of methane is simulated by using multistep kinetics models. They assure a better reproduction of CO emissions with load. Boundary and initial conditions were defined by using experimental data, when available, and results of numerical matching analysis, which assumes importance in case of absence of specific measurements at combustor inlet. Discussion of results is mainly focused on the variation in terms of exhaust gaseous emissions and temperature distributions in the combustor by varying load and fuel. A numerical evaluation of the MGT combustor behavior in critical conditions is also presented.Copyright © 2014 by ASME

The micro-explosion phenomenon is involved in emulsified fuel droplets placed in a hot atmosphere, such as spray combustion. Droplets of water-in-sunflower oil emulsion are used, since they are representative of a class of emulsions used in practical applications of biofuels. Once the micro-explosion is triggered after a short delay, the rapid (

The combustion efficiency and the gaseous emission of a 100 kWe MGT, designed for working with natural gas but fuelled with blends containing up to 10% of hydrogen is investigated. A critical comparison between experimental data and results of the CFD analysis of the combustor is discussed. The k-epsilon RANS turbulence model and the Finite Rate – Eddy Dissipation combustion model were used in the numerical computations. The chemical kinetic mechanisms embedded were the 2-step Westbrook and Dryer for methane oxidation, 1-step Westbrook and Dryer for hydrogen oxidation and the Zeldovich mechanism for NO formation. The experimental data and numerical computations are in agreement within the experimental accuracy for NO emissions. Regarding CO, there is a significant deviation between experimental and computational data due to the scarce predictive capability of the simple two steps kinetic mechanism was adopted. From a practical point of view, the possibility of using fuels with a similar Wobbe index was confirmed. In particular the addiction of 10 % of hydrogen to pure methane doesn’t affect the behavior of the micro gas turbine either in terms of NO or CO emissions.

In recent years, the interest in the research on energy production systems fed by biofuels has increased. Gaseous fuels obtained through biomass conversion processes such as gasification, pyrolysis and pyrogasification are generally defined as synthesis gas (syngas). The use of synthesis gas in small-size energy systems, such as those used for distributed micro-cogeneration, has not yet reached a level of technological maturity that could allow a large market diffusion. For this reason, further analyses (both experimental and numerical) have to be carried out to allow these technologies to achieve performance and reliability typical of established technologies based on traditional fuels. In this paper, a numerical analysis of a combustor of a 100-kW micro gas turbine fed by natural gas and biomass-derived synthesis gas is presented. The work has been developed in the framework of a collaboration between the Engineering Department of the University of Ferrara, the Istituto Motori - CNR (Napoli), and Turbec S.p A. of Corporeno di Cento (FE). The main features of the micro gas turbine Turbec T100, located at the Istituto Motori - CNR, are firstly described. A decompression and distribution system allows the feeding of the micro gas turbine with gaseous fuels characterized by different compositions. Moreover, a system of remote monitoring and control together with a data transfer system has been developed in order to set the operative parameters of the machine. The results of the tests performed under different operating conditions are then presented. Subsequently, the paper presents the numerical analysis of a model of the micro gas turbine combustor. The combustor model is validated against manufacturer performance data and experimental data with respect to steady state performance, i.e., average outlet temperature and emission levels. A sensitivity analysis on the model capability to simulate different operating conditions is then performed. The combustor model is used to simulate the combustion of a syngas, composed of different ratios of hydrogen, carbon monoxide, methane, carbon dioxide and water. The results in terms of flame displacement, temperature and emission distribution and values are analyzed and compared to the natural gas simulations. Finally, some simple modifications to the combustion chamber are proposed and simulated both with natural gas and syngas feeding.

A new recursive algorithm to calculate the internal and scattered fields of finely stratified inhomogeneous spheres has been developed. No restriction on the number of layers—the thicknesses of which can be arbitrarily small—is imposed by this method. The number of layers is restricted only by the computer’s capability: calculations with spheres with more than 10,000 layers were successfully performed with a HP work station. The new algorithm circumvents the limitations introduced by the numerical round-off errors encountered when using the previously developed recursive relations to calculate the ratios of Riccati-Bessel functions. Tests and calculations show that the method is stable and accurate for a large range of size parameters and optical properties. By employing the proposed algorithm, the problems encountered in analyzing the scattering by spheres with continuous-profile refractive indices can be solved with good accuracy.

A detailed study of rainbow thermometry and its application to droplets in reactive systems is presented. To this end the light-scattering history of a vaporizing droplet under unsteady conditions is discussed. Unlike in previous papers, the reduction of the droplet's diameter is also taken into account in addition to the variation of the refractive-index profile. A finely stratified sphere model with thousands of layers (i.e., 20,000) is used to compute the scattering patterns of a radially inhomogeneous evaporating droplet at different heating times and therefore with different diameters and refractive-index profiles. In the studied case the temperatures inferred from rainbow thermometry do not represent the actual temperatures inside the droplet. They do not represent an average internal temperature or even the surface or the core temperature. For droplets with a temperature that increases from the core to the surface, the inferred values are always lower than the minimum temperature inside the droplet. Therefore the rainbow technique should be applied with caution in all cases in which droplet inhomogeneities are suspected. In addition, a careful analysis of the scattering in the rainbow region is presented. Because of the physical structure of the rainbow, a marked uncertainty in the inferred temperatures also has to be considered in the case of homogenous droplets. For inhomogeneous spheres this intrinsic uncertainty has to be added to the effects caused by the internal profiles of the refractive index.