Numerical and experimental study of ethanol combustion in an industrial gas turbine

The application of ethanol as a biomass-derived fuel in OPRA’s 2 MWe class OP16 radial gas turbine has been studied both numerically and experimentally. The main purpose of this work is to validate the numerical model for future work on biofuel combustion. For the experimental investigation a modified OP16 gas turbine combustor has been used. This reverse-flow tubular combustor is a diffusion type combustor that has been adjusted to be suitable for numerical validation. Two series of ethanol burning experiments have been conducted at atmospheric pressure with a thermal input ranging from 16 to 72 kW. Exhaust gas temperature and emissions (CO, CO2, O2, NOx) were measured at various fuel flow rates while keeping the air flow rate and air temperature constant. In addition, the temperature profile of the combustor liner has been determined by applying thermochromic paint. CFD simulations have been performed in Ansys Fluent for four different operating conditions considered in the experiments. The simulations are based on a 3D RANS code. Fuel droplets representing the fuel spray are tracked throughout the domain while they interact with the gas phase. A temperature profile based on measurements has been prescribed on the liner to account for heat transfer through the flame tube wall. Detailed combustion chemistry is included by using the steady laminar flamelet model. The predicted levels of CO2 and O2 in the exhaust gas are in good agreement with the experimental results. The calculated and measured exhaust gas temperatures show a close match for the low power condition, but more significant deviations are observed in the higher load cases. Also, the comparison pointed out that the CFD model needs to be improved regarding the prediction of the pollutants CO and NOx. Chemiluminescence of CH radicals in the flame front indicated that the flame extends up to the liner, suggesting the presence of fuel near the surface. However, this result was not confirmed by liner temperature measurements using thermochromic paint.</jats:p

On the atomization and combustion of liquid biofuels in gas turbines: towards the application of biomass-derived pyrolysis oil

The combustion of liquid biofuels in gas turbines is an efficient way of generating heat and power from biomass. Gas turbines play a major role in the global energy supply and are suitable for a wide range of applications. However, biofuels generally have different properties compared to conventional fossil fuels. This can lead to various problems in case biofuels are directly used in existing installations. This thesis aims to provide better insight into the combustion of biomass-derived pyrolysis oil in gas turbines. Pyrolysis oil is a combustible liquid that can be produced from non-edible biomass via the fast pyrolysis process. Two research objectives have been formulated to support the development of pyrolysis oil combustion technology. The first objective is to describe the evaporation and flame characteristics of pyrolysis oil. This objective has been addressed by developing a CFD model based on the Euler-Lagrange method in ANSYS Fluent. The second objective is to determine the effect of fuel viscosity on the atomization and combustion in a gas turbine. Given that pyrolysis oil is an unstable fuel, reducing viscosity via preheating is only possible up to a limited temperature. Therefore, pyrolysis oil is more viscous than standard gas turbine fuels such as diesel, which can deteriorate the performance of the atomizer. This problem has been studied by measuring the fuel spray characteristics, and relating these to burning tests in a micro gas turbine. The present research provides more fundamental knowledge, mathematical models, experimental setups and practical guidelines that may speed up further developments in combustion technology for pyrolysis oil

Characterization of viscous biofuel sprays using digital imaging in the near field region

The atomization of biodiesel, vegetable oil and glycerin has been studied in an atmospheric spray rig by using digital imaging (PDIA). Images of the spray were captured in the near field, just 18 mm downstream of the atomizer, and processed to automatically determine the size of both ligaments and droplets. The effect of the spray structure in this region is of major interest for the combustion of biofuels in gas turbines. The sprays were produced by a pressure-swirl atomizer that originates from the multifuel micro gas turbine (MMGT) setup. Various injection conditions have been tested to investigate the influence of viscosity on the spray characteristics and to assess the overall performance of the atomizer. The spray measurements have been compared to combustion experiments with biodiesel and vegetable oil in the micro gas turbine at similar injection conditions. The results show that the primary breakup process rapidly deteriorates when the viscosity is increased. A higher viscosity increases the breakup length, which becomes visible at the measurement location in the form of ligaments. This effect leads to an unacceptable spray quality once the viscosity slightly exceeds the typical range for conventional gas turbine fuels. The SMD in the investigated spray region was not significantly affected by viscosity, but mainly influenced by injection pressure. The data furthermore indicate an increase in SMD with surface tension. It was found that the penetration depth of ligaments can have major impact on the combustion process, and that the droplet size is not always the critical factor responsible for efficient combustion. The measured delay in primary breakup at increased viscosity shows that pressure-swirl atomization is unsuitable for the application of pure pyrolysis oil in an unmodified gas turbine engin

The impact of spray quality on the combustion of a viscous biofuel in a micro gas turbine

The relation between spray quality and combustion performance in a micro gas turbine has been studied by burning a viscous biofuel at different fuel injection conditions. Emissions from the combustion of a viscous mixture of straight vegetable oils have been compared to reference measurements with diesel No. 2. The effect of fuel viscosity on pollutant emissions is determined by adjusting the injection temperature. The measurements confirm that a reduction in fuel viscosity improves the spray quality, resulting in faster droplet evaporation and more complete combustion. CO emission levels were observed to decrease linearly with viscosity in the tested range. For the pressure-swirl nozzle used in the tests, the upper viscosity limit is found to be 9 cP. Above this value, droplet evaporation seems to be incomplete as the exhaust gas contains a considerable amount of unburned fuel. Additionally, the influence of increased injection pressure and combustor temperature is evaluated by varying the load. Adding more load resulted in improved combustion when burning diesel. In case of vegetable oil, however, this trend is less consistent as the decrease in CO emissions is not observed over the full load range. The outcome of this study gives directions for the application of pyrolysis oil in gas turbines, a more advanced biofuel with high viscosit

Numerical study of pyrolysis oil combustion in an industrial gas turbine

The growing demand for the use of biofuels for decentralized power generation initiates new research in gas turbine technology. However, development of new combustors for low calorific fuels is costly in terms of time and money. To give momentum to biofuels application for power generation robust numerical models for multicomponent biofuels must be developed. This paper discusses the use of CFD techniques for modeling the combustion of pyrolysis oil in a new burner geometry from OPRA Turbines. Pyrolysis oil contains many different compounds, which are represented by a discrete fuel model consisting of seven components. The components and their initial fractions approximate the volatility, water content, elemental composition and heating value of a typical fast pyrolysis oil. Simulations have been carried out for both the multicomponent pyrolysis oil and, as a reference, ethanol, a single-component biofuel with a higher volatility. Comparative simulations have been performed to examine the influence of the initial droplet size and to evaluate different combustion models. The results were compared to available experimental data for pyrolysis oil and ethanol combustion. A qualitatively good agreement was achieved

One-dimensional model for heat transfer to a supercritical water flow in a tube

Heat transfer in water at supercritical pressures has been investigated numerically using a one-dimensional modeling approach. A 1D plug flow model has been developed in order to make fast predictions of the bulk-fluid temperature in a tubular flow. The chosen geometry is a vertical tube with an inner diameter of 10 mm and a heated length of 4.0 m.\ud \ud The simulations concern a heated upward flow with an imposed wall temperature profile. Viscous effects, internal conduction and enthalpy changes due to a pressure gradient have been neglected after evaluation of the governing equations in dimensionless form. The resulting set of equations is closed using Nusselt correlations found in literature and solved using an explicit Euler scheme to simulate heat transfer in a supercritical water flow.\ud \ud The results for three different cases show that the model is able to accurately predict the bulk temperature based on heat transfer rates provided by a suitable Nusselt correlation. However, there is also reason to assume that these correlations are very specific for the flow conditions, since boiling effects occurring at certain conditions can highly influence the heat transfer rate. As a consequence, the model may be unable to describe supercritical heat transfer over a broad range of configurations when only using one correlation. The agreement of these results with the two-dimensional simulations will be investigated in a separate article.\ud \ud The description of the model is preceded by a mathematical description of supercritical water flows and by an overview of the supercritical heat transfer phenomena as observed in earlier studies\u

Heat transfer characteristics of supercritical water in a tube: Application for 2D and an experimental validation

\ud \ud \ud Heat transfer to water at supercritical pressures has been numerically investigated using a two-dimensional modeling approach. The simulations in a two-dimensional domain have been performed using the low-Reynolds k–ϵ turbulence model, and the IAPWS-IF97 formulation to describe the properties of water at different conditions.\ud \ud The accuracy of the model is validated using an experimental setup at supercritical pressures. The experimental dataset was obtained in supercritical water flowing upward in a 0.4 m long vertical bare tube with 10 mm ID. The temperature data were collected at multiple heights in the tube and at pressures of about 24 MPa, an inlet temperature of 300 °C, values of mass flux ranged from 6.6 to 10 kg/m2 s and an outer wall temperature of 300 °C resulting in bulk-fluid temperatures exceeding the pseudo-critical temperature. The comparison of the temperature results shows a good agreement for low mass fluxes between the experimental and numerical data. At these low flow conditions, the 2D model predicts recirculation zones near the inlet which results in a more complex simulation. The accuracy of the 2D model for higher fluxes cannot be properly assessed on basis of the experimental data because of practical limitation of the setup. But the accuracy of the 2D model for the higher mass flow cases is expected to be even more accurate, due to less complexity in the flow calculation because of smaller buoyancy effects.\ud \ud Finally simulation results of the two-dimensional model at higher mass flows are compared with several frequently used one-dimensional correlations from literature for heat transfer at supercritical pressures\u

Heat transfer characteristics of supercritical water in a tube: Application for 2D and an experimental validation

Heat transfer to water at supercritical pressures has been numerically investigated using a two-dimensional modeling approach. The simulations in a two-dimensional domain have been performed using the low-Reynolds k–ϵ turbulence model, and the IAPWS-IF97 formulation to describe the properties of water at different conditions. The accuracy of the model is validated using an experimental setup at supercritical pressures. The experimental dataset was obtained in supercritical water flowing upward in a 0.4 m long vertical bare tube with 10 mm ID. The temperature data were collected at multiple heights in the tube and at pressures of about 24 MPa, an inlet temperature of 300 °C, values of mass flux ranged from 6.6 to 10 kg/m2 s and an outer wall temperature of 300 °C resulting in bulk-fluid temperatures exceeding the pseudo-critical temperature. The comparison of the temperature results shows a good agreement for low mass fluxes between the experimental and numerical data. At these low flow conditions, the 2D model predicts recirculation zones near the inlet which results in a more complex simulation. The accuracy of the 2D model for higher fluxes cannot be properly assessed on basis of the experimental data because of practical limitation of the setup. But the accuracy of the 2D model for the higher mass flow cases is expected to be even more accurate, due to less complexity in the flow calculation because of smaller buoyancy effects. Finally simulation results of the two-dimensional model at higher mass flows are compared with several frequently used one-dimensional correlations from literature for heat transfer at supercritical pressure

Bioethanol combustion in an industrial gas turbine combustor: simulations and experiments

Combustion tests with bioethanol and diesel as a reference have been performed in OPRA's 2 MWe class OP16 gas turbine combustor. The main purposes of this work are to investigate the combustion quality of ethanol with respect to diesel and to validate the developed CFD model for ethanol spray combustion. The experimental investigation has been conducted in a modified OP16 gas turbine combustor, which is a reverse-flow tubular combustor of the diffusion type. Bioethanol and diesel burning experiments have been performed at atmospheric pressure with a thermal input ranging from 29 to 59 kW. Exhaust gas temperature and emissions (CO, CO2, O2, NOx) were measured at various fuel flow rates while keeping the air flow rate and air temperature constant. In addition, the temperature profile of the combustor liner has been determined by applying thermochromic paint. CFD simulations have been performed with ethanol for five different operating conditions using ANSYS FLUENT. The simulations are based on a 3D RANS code. Fuel droplets representing the fuel spray are tracked throughout the domain while they interact with the gas phase. A liner temperature measurement has been used to account for heat transfer through the flame tube wall. Detailed combustion chemistry is included by using the steady laminar flamelet model. Comparison between diesel and bioethanol burning tests show similar CO emissions, but NOx concentrations are lower for bioethanol. The CFD results for CO2 and O2 are in good agreement, proving the overall integrity of the model. NOx concentrations were found to be in fair agreement, but the model failed to predict CO levels in the exhaust gas. Simulations of the fuel spray suggest that some liner wetting might have occurred. However, this finding could not be clearly confirmed by the test dat