Thomas Monz

Inverted Brayton Cycle With Exhaust Gas Recirculation—A Numerical Investigation

Microgas turbine (MGT) based combined heat and power (CHP) units provide a highly efficient, low-pollutant technology to supply heat and electrical power from fossil and renewable energy sources; however, pressurized MGT systems in an electrical power range from 1 to 5 kWel utilize very small turbocharger components. These components suffer from higher losses, like seal and tip leakages, resulting in a reduced electrical efficiency. This drawback is avoided by an inverted Brayton cycle (IBC) based system. In an IBC hot gas is produced in a combustion chamber at atmospheric pressure. Subsequently, the exhaust gas is expanded in a turbine from an atmospheric to a subatmospheric pressure level. In order to increase electrical efficiency, heat from the turbine exhaust gas is recuperated to the combustion air. After recuperation, the gas is compressed to atmospheric pressure and is discharged from the cycle. To decrease the power demand of the compressor, and thereby increasing the electrical cycle efficiency, it is crucial to further extract residual thermal power from the gas before compression. Coolant flows provided by heating applications can use this heat supply combined with heat from the discharged exhaust gas. The low pressure levels of the IBC result in high volumetric gas flows, enabling the use of large, highly efficient turbocharger components. Because of this efficiency benefit and the described cooling demand, micro-CHP applications provide an ideal field for utilization of the IBC. To further increase the total efficiency, discharged exhaust gas can be partially recirculated to the air inlet of the cycle. In the present paper a steady state analysis of an IBC with exhaust gas recirculation (EGR) is shown, and compared to the performance of a conventional Brayton cycle with equivalent component properties. Using EGR, it could be found that the sensitivity of the electrical cycle efficiency to the coolant temperature further increases. The sequent discussion focuses on the trade-off between total efficiency and electrical efficiency, depending on coolant temperature and EGR rate. The results show that EGR can increase the total efficiency by 10% to 15% points, while electrical efficiency decreases by 0.5% to 1% point. If the coolant temperature is below 35 °C, condensation of water vapor in the exhaust gas leads to a further increase of heat recovery efficiency. A validated in-house simulation tool based on turbocharger maps has been used for the calculations.

Validation of a T100 Micro Gas Turbine Steady-State Simulation Tool

Micro gas turbines (MGT) provide a highly efficient, low-pollutant way to generate power and heat on-site. MGTs have also proven to be a versatile technology platform for recent developments like utilization of fuels with low specific heating values and solar thermal electricity generation. Moreover, they are the foundation to build novel cycles like the inverted Brayton cycle or fuel cell hybrid power plants.Numerical simulations of steady operation points are beneficial in various phases of MGT cycle development. They are used to determine and analyze the future potentials of innovative cycles for example by predicting the electrical efficiency and they support the thermodynamic design process (by providing mass flow, pressure and temperature data). Numerical Simulation allows to approximate off-design performance of known cycles e.g. power output at different ambient conditions. Additionally, numerical simulation is used to support cycle optimization efforts by analyzing the sensitivity of component performance on cycle performance. Numerical models of the MGT components have to be tuned and validated based on experimental data from MGT test rigs.At DLR institute of combustion technology a MGT steady-state cycle simulation tool has been used to analyze a variety of cycles and has been revised for several years. In this paper, the validation process is discussed in detail. Comparing simulation data with measurement data from the DLR Turbec T100 test rig has led to extensions of the numeric models, on the one hand, and to modifications of the test rig on the other. Newly implemented numerical models account for the generator heat release to the inlet air and the power electronic limitations. The test rig was modified to improve the temperature measurement at positions with uneven spatial temperature distribution such as the turbine outlet. Analyzing these temperature distributions also yields a possible explanation for the apparent strong recuperator efficiency drop at high load levels, which was also observed by other T100 users before.Copyright

Experimental Investigation of the Combustion Characteristics of a Double-Staged FLOX?-Based Combustor on an Atmospheric and a Micro Gas Turbine Test Rig

To establish micro gas turbine (MGT) systems in a wide field of CHP applications, innovative combustion concepts are needed to meet the demands for low exhaust gas emissions, high efficiency and reliability as well as high fuel flexibility. A promising technology for future MGT combustion is the FLOX® concept. The goal of the presented work is to prove the feasibility of a double–staged, FLOX®–based MGT combustion system on a MGT test rig. The paper reports a reliable operating behavior of a Turbec T100 MGT in combination with the new FLOX®–based combustion chamber utilizing natural gas. The measured exhaust gas emissions are compared for different configurations of the combustion chamber and the standard Turbec system. It is shown that the carbon monoxide emissions are reduced whereas the nitrogen oxide emissions exceed the emission levels of the standard MGT burner. However, they still fall far below the German legal limits. For helping to interpret the results of the MGT combustion system, the double–staged combustor is compared to a single–staged FLOX®burner on basis of atmospheric measurements. Here, it is shown that the margin to lean blow–off is substantially increased by the fuel staging. Moreover, it is demonstrated that the exhaust gas emissions of the double–staged combustor could be kept at a similar very low level by applying the staging. Additionally, the overall reaction regions are reported by OH* chemiluminescence imaging as a function of burner air number. Based on this atmospheric study the transfer to MGT conditions is made and appropriate measures are derived to optimize the exhaust gas emissions of the MGT FLOX® combustion system.© 2015 ASME

Introduction of a New Numerical Simulation Tool to Analyze Micro Gas Turbine Cycle Dynamics

Micro gas turbine (MGT) technology is evolving towards a large variety of novel applications, like weak gas electrification, inverted Brayton cycles and fuel cell hybrid cycles; however, many of these systems show very different dynamic behaviors compared to conventional MGTs. In addition, some applications impose more stringent requirements on transient maneuvers, e.g. to limit temperature and pressure gradients in a fuel cell hybrid cycle. Besides providing operational safety, optimizing system dynamics to meet the variable power demand of modern energy markets is also of increasing significance. Numerical cycle simulation programs are crucial tools to analyze these dynamics without endangering the machines, and to meet the challenges of automatic control design. For these tasks, complete cycle simulations of transient maneuvers lasting several minutes need to be calculated. Moreover, sensitivity analysis and optimization of dynamic properties like automatic control systems require many simulation runs. To perform these calculations in an acceptable timeframe, simplified component models based on lumped volume or one-dimensional discretization schemes are necessary. The accuracy of these models can be further improved by parameter identification, as most novel applications are modifications of well-known MGT systems and rely on proven, characterized components. This paper introduces a modular in-house simulation tool written in Fortran to simulate the dynamic behavior of conventional and novel gas turbine cycles. Thermodynamics, gas composition, heat transfer to the casing and GTP-16-1386 / Henke 1

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-staged MGT combustor [1, 2], where the pilot stage 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 pilot and main stage 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 images. Emission measurements for NOx, CO and UHC emissions were carried out. At a global air number of λ= 2, a fuel split variation was carried out from 0 (only pilot-stage) to 1 (only main stage). The modification of the geometry lead to a decrease in NOx and CO emissions throughout the fuel split variation in comparison with the previous design. Regarding CO emissions, the pilot stage operations is 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 pilot stage 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 RANS simulations were carried out for both geometries at fuel splits of 0.93 and 0.78, respectively, using the DLR in-house code THETA with the k-w SST turbulence model and the DRM22 [3] detailed reaction mechanism. The numerical results showed a strong influence of the recirculation zones on the pilot stage reaction zone. Nomenclature Latin Letters ∗Address all correspondence to this author. ©2017 by ASME. This manuscript version is made available under the CC-BY 4.0 license http://creativecommons.org/licenses/by/4.0 1 The original publication is available at http://dx.doi.org/10.1115/1.4037749 a, b, c, d, e location a, b, c, d, e C Emission concentration (Vol. % or ppm) ṁ Mass flow rate (g/s) S Mass flow split between main and pilot stage u,v,w velocity in x-, y-, z-direction y+ dimensionless wall distance Greek Letters λ Air number (-) θ Circumferential velocity, yz-Plane (m/s) Subscripts amb ambient corr corrected exp experiment f fuel g global m main meas measured p pilot pre preheat recirc recirculated ref reference rel relative sim simulation Abbreviations Q̇ Thermal Power (kW) R Recirculation rate (-) FL Flame Length (mm) HAB Height Above Burner (mm) IRZ Inner Recirculation Zone LBO Lean Blowout LHV Lower Heating Value (MJ/kg) MGT Micro Gas Turbine MS Main Stage OH-CL OH∗ chemiluminescence ORZ Outer Recirculation Zone PRZ Primary Reaction Zone PS Pilot Stage

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.Copyright

Experimental characterization of a swirl stabilized MGT combustor

A swirl stabilized MGT combustor (Turbec T100) was operated with natural gas and was experimentally characterized in two test rigs, a pressurized and optically accessible MGT test rig and an atmospheric combustor test rig. For the detailed characterization of the combustion processes, planar OH-PLIF and simultaneous 3D-stereo PIV measurements were performed in the atmospheric combustor test rig. Flow fields, reaction zones and exhaust gas emissions are reported for a range of pressure scaled MGT load points. Parameter studies on combustor inlet conditions (e.g. air preheating temperature, air and fuel mass flow rates and fuel split) were conducted in the atmospheric combustor test rig. From the parameters studies the fuel split between the pilot and the main stage and the air preheating temperature were found to have the biggest impact on the flame shape, flame stabilization and exhaust gas emissions. The measurements of the ATM test rig are compared with measurements of the pressurized MGT test rig with and without an optically accessible combustion chamber. Opened and closed conical flame and flow pattern were found in both test rigs. Reasons for the two flame and flow pattern are supposed to be the interaction of pilot stage combustion and flow field and the interaction of the dilution air with the combustion and the flow field. The results are discussed and compared with repect to a transferability of combustion characteristics from the ATM test rig to the MGT test rigs.Copyright

Inverted Brayton Cycle With Exhaust Gas Recirculation: A Numerical Investigation

Micro gas turbine (MGT) based CHP units provide a highly efficient, low-pollutant technology to supply heat and electrical power from fossil and renewable energy sources; however, pressurized MGT systems in an electrical power range from 1 to 5 kWel utilize very small turbocharger components. These components suffer from higher losses, like seal and tip leakages, resulting in a reduced electrical efficiency.This drawback is avoided by an Inverted Brayton Cycle (IBC) based system. In an IBC hot gas is produced in a combustion chamber at atmospheric pressure. Subsequently, the exhaust gas is expanded in a turbine from atmospheric to sub-atmospheric pressure level. In order to increase electrical efficiency, heat from the turbine exhaust gas is recuperated to the combustion air. After recuperation, the gas is compressed to atmospheric pressure and is discharged from the cycle. To decrease the power demand of the compressor, and thereby increasing the electrical cycle efficiency, it is crucial to further extract residual thermal power from the gas before compression. Coolant flows provided by heating applications can use this heat supply combined with heat from the discharged exhaust gas. The low pressure levels of the IBC result in high volumetric gas flows, enabling the use of large, highly efficient turbocharger components. Because of this efficiency benefit and the described cooling demand, micro-CHP applications provide an ideal field for utilization of the IBC. To further increase the total efficiency, discharged exhaust gas can be partially recirculated to the air inlet of the cycle.In the present paper a steady state analysis of an IBC with exhaust gas recirculation (EGR) is shown, and compared to the performance of a conventional Brayton Cycle with equivalent component properties. Using EGR, it could be found that the sensitivity of the electrical cycle efficiency to the coolant temperature further increases. The sequent discussion focuses on the trade-off between total efficiency and electrical efficiency, depending on coolant temperature and EGR rate. The results show that EGR can increase the total efficiency by 10 to 15 %-points, while electrical efficiency decreases by 0.5 to 1 %-point. If the coolant temperature is below 35 °C, condensation of water vapor in the exhaust gas leads to a further increase of heat recovery efficiency.A validated in-house simulation tool based on turbocharger maps has been used for the calculations.© 2013 ASME