Experimental Analysis of the Fuel Flexibility of a Jet-stabilized Micro Gas Turbine Combustor Designed for Low Calorific Gases

Fuel flexible burners are an important concept to aid in the advancement and implementation of renewable energy sources into existing infrastructure. As focus shifts from conventional to renewable fuel sources, designing gas turbines which meet both the technical and load requirements for fluctuating fuel compositions and heating values is imperative. The present work aims to study the stability and fuel flexibility of a two-stage burner, consisting of a jet-stabilized main stage and a swirl-stabilized pilot stage. Various fuel compositions, consisting of natural gas, hydrogen, carbon monoxide, carbon dioxide and nitrogen, with lower heating values ranging from 7MJ/kg to 49MJ/kg at an air preheat temperature of 873K were tested. Additionally, differing power loads (60kW to 100kW) and air-fuel equivalence ratio ranges (1.5-3.6) were examined. This study utilized OH* chemiluminescence measurements in conjunction with exhaust gas analysis of carbon monoxide and nitrogen oxide levels to assess the operation and reliability of the burner. Moreover, the experimental results are supported by steady state computational fluid dynamics simulations to provide explanation of the flame flow field characteristics and kinetics. The results indicate flame stability and low emissions levels for the majority of fuel compositions and thermal loads tested, therefore signifying high fuel flexibility of the burner. Additionally, optimal combustor operating points, which display emissions levels below the proposed German legal limits (German Federal Ministry for the Environment, 2016), were determined. Furthermore, the computational fluid dynamics simulation results indicate a good match to the experimental results, providing insight into the burner flow field characteristics and kinetics

Thermal Incineration of VOCs in a Jet-Stabilized Micro Gas Turbine Combustor

Using gas turbines for waste gas treatment is a promising technology compared to other thermal oxidizing systems in terms of total efficiency, due to the combined heat and power production. In this work, experiments have been carried out where an air stream containing volatile organic compounds (VOCs) is fed to a micro gas turbine combustor which is co-fired using natural gas. All experiments were conducted at atmospheric pressure. For lower preheat temperatures, the equivalence ratio of the pilot stage was step-wise increased at constant global air to fuel ratios to further extend the operation limit. The VOC destruction efficiency of the burner is analyzed for various oxygenated VOCs and concentrations with a maximum lower heating value of the VOC containing air stream of 0.25 MJ/Nm3. The stability range of the burner is presented for various equivalence ratios with a global volumetric heat release up to 33 MW/m3 and air preheat temperatures up to 650 °C. The flame is analyzed in terms of shape, length and lift-off height, using the OH* chemiluminescence signal detected by an ICCD-camera. At same air numbers, changes on the flame are insignificant to the different VOCs in most cases. Regarding the emissions of NOx, the VOC containing air is beneficial at all air numbers, while a decrease in CO is only observed for lower air numbers. A minimum of CO emissions for both preheat temperatures is obtained at an adiabatic flame temperature of 1671 K.</jats:p

Detailed examination of two-staged micro gas turbine combustor

Jet-stabilized combustion is a promising technology for fuel flexible, reliable, highly efficient combustion systems. In this work, experiments have been carried out on a two-staged combustor, with a jet-stabilized main stage and a swirl-stabilized pilot stage. Both stages have been run separately to allow a more detailed understanding of the flame stabilization within the combustor and its range of stable operation. All experiments were conducted at atmospheric pressure and preheating temperatures of 650 °C. The air was fed to both stages of the combustor for all experiments. The flame was analyzed in terms of shape, length and lift-off height, using the OH* chemiluminescence signal detected by an ICCD-camera. Emission measurements for NOx, CO and UHC emissions were carried out. The pilot stage was examined at a local air number between 0.14 and 1.43, which corresponds to a global air number of 2.0 to 20.7. For lowest air numbers, the combustor works with the RQL principle with lowest emissions in pilot stage only operation. This is because the remaining fuel fed to the pilot stage mixes rapidly with the air from the main stage and reacts under lean conditions. The optimum operating range of the main stage is at global air numbers between 3 and 3.2 with a blow-off limit beyond λg = 4.0. At a global air number of λg = 2, a fuel split variation was carried out from 0 (only pilot stage) to 1 (only main stage). In combined operation and at higher fuel splits, the NOx emissions are reduced compared to the main stage only operation, while the opposing effect on NOx emissions was observed for lower fuel splits. CFD simulations of the combustor test rig showed higher residence times in the pilot stage compared to the main stage which facilitates higher NOx formation rates in the pilot stage. This could be improved by a geometry optimization. The operation of the pilot stage was beneficial at fuel splits above 90 %, especially concerning an extended operating range to higher global air numbers. In addition, the capability of the combustor to operate at higher thermal power inputs was investigated. Originally designed for the Turbec T100 micro gas turbine, the combustor was operated at 160% of the original design point. At a constant air number, this led to a decrease in NOx and to an increase in CO emissions, caused by shorter residence times in the combustion chamber at higher power input. An operation strategy of constant pilot air number increases the envelope of a stable operation regime further.</jats:p

Detailed Examination of a Modified Two-Staged Micro Gas Turbine Combustor

Jet-stabilized combustion is a promising technology for fuel flexible, reliable, highly efficient combustion systems. The aim of this work is a reduction of NOx emissions of a previously published two-stage micro gas turbine (MGT) combustor (Zanger et al., 2015, “Experimental Investigation of the Combustion Characteristics of a Double-Staged FLOX-Based Combustor on an Atmospheric and a Micro Gas Turbine Test Rig,” ASME Paper No. GT2015-42313 and Schwärzle et al., 2016, “Detailed Examination of Two-Stage Micro Gas Turbine Combustor,” ASME Paper No. GT2016-57730), where the pilot stage (PS) of the combustor was identified as the main contributor to NOx emissions. The geometry optimization was carried out regarding the shape of the pilot dome and the interface between PS and main stage (MS) in order to prevent the formation of high-temperature recirculation zones. Both stages have been run separately to allow a detailed understanding of the flame stabilization within the combustor, its range of stable combustion, the interaction between both stages, and the influence of the modified geometry. All experiments were conducted at atmospheric pressure and an air preheat temperature of 650  °C. The flame was analyzed in terms of shape, length, and lift-off height, using OH* chemiluminescence (OH-CL) images. Emission measurements for NOx, CO, and unburned hydrocarbons (UHC) emissions were carried out. At a global air number of λ = 2, a fuel split variation was carried out from 0 (only PS) to 1 (only MS). The modification of the geometry leads to a decrease in NOx and CO emissions throughout the fuel split variation in comparison with the previous design. Regarding CO emissions, the PS operations are beneficial for a fuel split above 0.8. The local maximum in NOx emissions observed for the previous combustor design at a fuel split of 0.78 was not apparent for the modified design. NOx emissions were increasing, when the local air number of the PS was below the global air number. In order to evaluate the influence of the modified design on the flow field and identify the origin of the emission reduction compared to the previous design, unsteady Reynolds-averaged Navier–Stokes simulations were carried out for both geometries at fuel splits of 0.93 and 0.78, respectively, using the DLR (German Aerospace Center) in-house code turbulent heat release extension of the tau code (theta) with the k–ω shear stress transport turbulence model and the DRM22 (Kazakov and Frenklach, 1995, “DRM22,” University of California at Berkeley, Berkeley, CA, accessed Sept. 21, 2017, http://www.me.berkeley.edu/drm/) detailed reaction mechanism. The numerical results showed a strong influence of the recirculation zones on the PS reaction zone.</jats:p

Pore water chemistry of sediment core GeoB15101-7 and water chemistry of CTD casts GeoB15101-1 and GeoB15101-6

Submarine brine lakes feature sharp and persistent concentration gradients between seawater and brine, though these should be smoothed out by free diffusion in open ocean settings. The anoxic Urania basin of the Eastern Mediterranean contains an ultra sulfidic, hypersaline brine of Messinian origin above a thick layer of suspended sediments. With a dual modeling approach we reconstruct its contemporary stratification by geochemical solute transport fundamentals, and show that thermal convection is required to maintain mixing in the brine and mud layer. The origin of the Urania basin stratification was dated to 1650 years before present, which may be linked to a major earthquake in the region. The persistence of the chemoclines may be key to the development of diverse and specialized microbial communities. Ongoing thermal convection in the fluid mud layer may have important, yet unresolved consequences for sedimentological and geochemical processes, also in similar environments