Fuel Flexible Rich Catalytic Lean Burn System for Low BTU Fuels

Limited fuel resources, increasing energy demand, and stringent emission regulations are drivers to evaluate process off-gases or process waste streams as fuels for power generation. Often these process waste streams have low energy content and their operability in gas turbines leads to issues such as unstable or incomplete combustion and changes in acoustic response. Due to above reasons, these fuels cannot be used directly without modifications or efficiency penalties in gas turbine engines. To enable the use of the wide variety of ultra-low and low Btu fuels in gas turbine engines, a rich catalytic lean burn (RCL®) combustion system was developed and tested in a subscale high pressure (10 atm.) rig. Previous work has shown promise with fuels such as blast furnace gas (BFG) with Lower Heating Value (LHV) of 3.1 MJ/Nm3 (85 Btu/scf). The current testing extends the limits of RCL® operability to other weak fuels by further modifying and improving the injector to achieve enhanced flame stability. Fuels containing low methane content such as weak natural gas with an LHV of 6.5 MJ/Nm3 (180 Btu/scf) to fuels containing higher methane content such as landfill gas with an LHV of 21.1 MJ/Nm3 (580 Btu/scf) were tested. These fuels demonstrated improved combustion stability with an extended turndown (defined as the difference between catalytic and non-catalytic lean blow out) of 140°C–170°C (280°F–340°F) with CO and NOx emissions lower than 5 ppm corrected to 15% O2.

N+3 and N+4 Generation Aeropropulsion Engine Combustors: Part 1 — Large Engines’ Emissions

A comprehensive assessment of emissions characteristics of the 1st, N and N+1 generation rich-dome combustion products has been done to identify the lowest emissions products. Focus of this paper is on the large rich-dome engines with its potential application for the (N+3) and (N+4) mixers with inspirational target takeoff NOxEI of 5 at 55 OPR. A total of ten engine models of the 1st generation were selected in addition to eight recently certified large engines. After evaluating several choices for conducting comparative assessment, the following three expressions were proposed for average takeoff NOxEI, idle COEI and HCEI entitlements, respectively: NOxEI L = 0.0288 × OPR 1.991 Idle COEI L = 815.36 Takeoff NOxEI L 1.159 Idle HCEI L = 0.15 × Idle COEI L - 2.0 In regard to application of the rich-dome technology to the (N+2) cycle based (N+3) mixers, the author tentatively gives it low probability of success barring success story stemming from Lee et al. [2012].

Experimental Study on a Double-Swirled Non-Premixed Humid Air/Syngas Burner

The development of integrated gasification combined cycle (IGCC) systems provides cost-effective and environmentally sound options for meeting future coal-utilizing power generation needs in the world. The combustion of gasified coal fuel significantly influences overall performance of IGCC power generation. Experimental measurements are carried out on a non-premixed model combustor, equipped with a double-swirled syngas burner. Planar laser-induced fluorescence (PLIF) of OH radical measurement is adopted to identify main reaction zones and burnt gas regions as well. Together with the temperature and emission measurements during the exhaust section, some important characteristics of the syngas flame are investigated overall. In this paper, the effects of the CO/H2 molar ratio consisting of syngas fuel are investigated under different humidity. With the increase of CO/H2 ratios, the concentration field of OH radicals is gradually away from the nozzle exit, and the nozzle exit almost no existence of OH radicals, forming a typical lifted flame. In addition, fluorescent signal strength of OH radicals pronounced weakening, the flame gradually appeared W type distribution and more and more obvious with the increased of humidification amount. At the same time the average exhaust temperature of combustor CO and NOx missions almost no change. The study can provide a reliable database for high moisture gas turbine combustor design and combustion numerical simulation.

Study of Fuel Composition, Burner Material and Tip Temperature Effects on Flashback of Enclosed Jet Flame

Flashback is a key challenge for low NOx premixed combustion of high hydrogen content fuels. Previous work has systematically investigated the impact of fuel composition on flashback propensity, and noted that burner tip temperature played an important role on flashback, yet did not quantify any specific effect. The present work further investigates the coupling of flashback with burner tip temperature and leads to models for flashback propensity as a function of parameters studied. To achieve this, a jet burner configuration with interchangeable burner materials was developed along with automated flashback detection and rim temperature monitoring. An inline heater provides preheated air up to 810 K. Key observations include that for a given condition, tip temperature of a quartz burner at flashback is higher than that of a stainless burner. As a reasult, the flashback propensity of a quartz tube is about double of that of a stainless tube. A polynomial model based on analysis of variance is presented and shows that, if the tip temperature is introduced as a parameter, better correlations result. A physical model is developed and illustates that the critical velocity gradient is proportional to the laminar flame speed computed using the measured tip temperature. Addition of multiple parameters further refined the prediction of the flashback propensity, and the effects of materials are discussed qualitatively using a simple heat transfer analysis.

Numerical Analysis of the Primary Breakup Under High-Altitude Relight Conditions Applying the Embedded DNS Approach to a Generic Prefilming Airblast Atomizer

The primary breakup under high-altitude relight conditions is investigated in this study where ambient pressure is as low as 0.4 bar and air, fuel and engine parts are as cold as 265 K. The primary breakup is crucial for the fuel atomization. As of today, the phenomena dictating the primary breakup are not fully understood. Direct Numerical Simulations (DNS) of liquid breakup under realistic conditions and geometries are hardly possible. The embedded DNS (eDNS) approach represents a reliable numerical tool to fill this gap. The concept consists of three steps: a geometry simplification, the generation of realistic boundary conditions for the DNS and the DNS of the breakup region. The realistic annular airblast atomizer geometry is simplified to a Y-shaped channel representing a planar geometry. Inside this domain the eDNS is located. The eDNS domain requires the generation of boundary conditions. A Large Eddy Simulation (LES) of the entire Y-shaped channel and a Reynolds-Averaged Navier-Stokes Simulation (RANS) of the liquid wall film are performed prior to the DNS. All parameters are stored transiently on all virtual DNS planes. These variables are then mapped to the DNS. Thus, high-quality boundary conditions are generated. The Volume-of-Fluid (VOF) method is used to solve for the two-phase flow. The results provide a qualitative insight into the primary breakup under realistic high-altitude relight conditions. Instantaneous snapshots in time illustrate the behavior of the liquid wall film along the prefilmer lip and illustrate the breakup process. It is seen that a slight variation of the surface tension force has a strong impact on the appearance of the primary breakup. Case 1 with the surface tension corresponding to kerosene at 293 K indicates large flow structures that are separated from the liquid sheet. By lowering the surface tension related to kerosene at 363 K, the breakup is dominated by numerous small structures and droplets. This study proves the applicability of the eDNS concept for investigating breakup processes as the transient nature of the phase interface behavior can be captured. At this time, the authors only present a qualitative insight which can be explained by the lack of quantitative data. The approach offers the potential of simulating realistic annular highly-swirled airblast atomizer geometries under realistic conditions.

Turbulent Consumption Speeds of High Hydrogen Content Fuels From 1–20 atm

This work describes measurements and analysis of the turbulent consumption speeds, ST,GC, of H2/CO fuel blends. We report measurements of ST,GC at pressures and normalized turbulence intensities, u′rms/SL,0 up to 20 atm and 1800, respectively for a variety of H2/CO mixtures and equivalence ratios. In addition, we present correlations of these data using laminar burning velocities of highly stretched flames, SL,max, derived from quasi-steady leading points models. These analyses show that SL,max can be used to correlate data over a broad range of fuel compositions, but do not capture the pressure sensitivity of ST,GC. We suggest that these pressure effects are more fundamentally a manifestation of non-quasi-steady behavior in the mass burning rate at the flame leading points.

Towards Low-NOx Operation in a Complex Burner: Optimization of an Annular Trapped Vortex Combustor

This paper focuses on optimizing an innovative annular Lean Premixed staged burner, following the Trapped Vortex Combustor concept. The latter consists of a lean main flame stabilized by passing past a rich cavity pilot flame. Unfortunately, this configuration is highly sensitive to combustion instabilities and the flame is not well stabilized. This work consists of adjusting aerodynamic variables, chemical parameters and burner geometry to reach a “low-NOx” operation while reducing other pollutants and getting a stable flame. Results show that stability is reached when mass transfers between main and cavity zones are reduced. Then, the main bulk velocity is increased to reduce the cavity thermal expansion, due to the hot gas expansion. In addition, the cavity flow rate is reduced to prevent from penetrating and disturbing the main flow. Re-arranging injections in the cavity also avoid local unsteady equivalence ratios, which creates an unsteady heat release and combustion with pulses. Regarding NOx, a leaner main flame combined with a sufficiently rich cavity mixture creates local stoichiometric zones at the interface between the cavity and the main zone. The latter point is found to be a good anchoring mechanism. Compared with the original configuration, a stable point of operation is found: acoustic energy is reduced by an order of 100, NOx level is less than 0.4 g/kgfuel, CO is cut by 93% with no more Unburned Hydro-Carbons.

N+3 and N+4 Generation Aeropropulsion Engine Combustors: Part 2 — Medium Size Rich-Dome Engines and Lean-Domes

Comprehensive assessment of the medium size rich-dome engines was conducted leading to the following emissions correlations: (1) LTO NOx = 1.129 × OPR 1.0899 with R 2 = 0.9248 Takeoff NOxEI given by (2) NOxEI = 0.0729 × OPR 1.7197 with R 2 = 0.9603 COEI idle = 396.42 NOxEI Takeoff 0.814 These correlations may be compared with the following for the CFM56 Tech Insertion: Takeoff NOxEI CFM_TI = 0.0744 × OPR 1.7151 Idle COEI CFM_TI = 396.42 Takeoff NOxEI 0.814 Idle HCEI CFM_TI = 0.1609 × Idle COEI - 3.1959 TALON II takeoff NOxEI data are reproduced well by: NOxEI TALON II = 0.0167 × OPR 2.1403 TALON II gives 10% lower NOx at 26 OPR and its NOx is comparable with the CFM_TI at 34 OPR. The CFM DAC technology is competitive with LEC’s for the low rated thrust engines. However, interaction between the two domes leads to early quenching with resultant higher idle COEI plateau. On the other hand, the 40 OPR lean DAC gave 25% higher NOx than LEC. Moreover, lean DAC (Gen-1) impacted fuel burn adversely making its likelihood to continue as product discouraging. The second generation lean dome technology initially kicked off under NASA sponsorship with significantly larger funding support from the CFMI and GE Aviation (GEA) led to successful introduction of TAPS into products (GEnx-1B and Gen-2B) with potential applications in other future GEA engines.

Study of a Rich/Lean Staged Combustion Concept for Hydrogen at Gas Turbine Relevant Conditions

The combustion of hydrogen-rich fuels (> 80 % vol. H2), relevant for gas turbine cycles with “pre-combustion” carbon capture, creates great challenges in the application of standard lean premix combustion technology. The significant higher flame speed and drastically reduced auto-ignition delay time of hydrogen compared to those of natural gas, which is normally burned in gas turbines, increase the risk of higher NOX emissions and material damage due to flashback. Combustion concepts for gas turbines operating on hydrogen fuel need to be adapted to assure safe and low-emission combustion. A rich/lean (R/L) combustion concept with integrated heat transfer that addresses the challenges of hydrogen combustion has been investigated. A sub-scale, staged burner with full optical access has been designed and tested at gas turbine relevant conditions (flame temperature of 1750 K, preheat temperature of 400 °C and a pressure of 8 bar). Results of the burner tests have confirmed the capability of the rich/lean staged concept to reduce the NOx emissions for undiluted hydrogen fuel. The NOx emissions were reduced from 165 ppm measured without staging (fuel pre-conversion) to 23 ppm for an R/L design having a fuel-rich hydrogen pre-conversion of 50 % at a constant power of 8.7 kW. In the realized R/L concept the products of the first rich stage, which is ignited by a Pt/Pd catalyst (under a laminar flow, Re ≈ 1900) are combusted in a diffusion-flame-like lean stage (turbulent flow Re ≈ 18500) without any flashback risk. The optical accessibility of the reactor has allowed insight into the combustion processes of both stages. Applying OH-LIF and OH*-chemiluminescence optical techniques, it was shown that mainly homogeneous reactions at rich conditions take place in the first stage, questioning the importance of a catalyst in the system, and opening a wide range of optimization possibilities. The promising results obtained in this study suggest that such a rich/lean staged burner with integrated heat transfer could help to develop a new generation of gas turbine burners for safe and clean combustion of H2-rich fuels.

Influence of Burner Material, Tip Temperature and Geometrical Flame Configuration on Flashback Propensity of H2-Air Jet Flames

Flashback is a key operability issue for low emission premixed combustion systems operated on high hydrogen content fuels. Previous work investigated fuel composition impacts on flashback propensity and found that burner tip temperature was important in correlating flashback data in premixed jet flames. An enclosure around the jet flame was found to enhance the flame-burner rim interaction. The present study further addresses these issues using a jet burner with various geometric configurations and interchangeable materials. Systematic studies addressing the quantitative influence of various parameters such as tip temperature, burner material, enclosure size, and burner diameter on flashback propensity were carried out. A comprehensive overview of the flashback limits for all conditions tested in the current study as well as those published previously is given. The collective results indicate that the burner materials, tip temperature and flame confinement play significant roles for flashback propensity and thus help explain previous scatter in flashback data. Furthermore, the present work indicates that the upstream flame propagation during flashback is affected by the burner material. The material with lower thermal conductivity yields larger flashback propensity but slower flame regression inside the tube. These observations can be potentially exploited to minimize the negative impacts of flashback in practical applications.