Performance analysis of the micro gas turbine Turbec T100 with a new FLOX-combustion system for low calorific fuels

doi:10.1016/j.apenergy.2015.08.075

Applied Energy

Volume 159, 1 December 2015, Pages 276-284

https://doi.org/10.1016/j.apenergy.2015.08.075 Get rights and content

Highlights

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A new FLOX-combustion system has been successfully tested in a micro gas turbine.
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The operating performance of the Turbec T100 with LCV fuels was characterized.
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Reliable start-up and steady-state operation from 50 to 100 kW_el was observed.
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Low emissions over the whole operating range: CO < 30 ppm, NO_x < 6 ppm, UHCs < 1 ppm .
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The pressure drop across the combustion system was below 4%.

Abstract

This paper presents the first combustion system, which has been designed for the use of biomass derived product gases in micro gas turbines. The operating performance of the combustion system and of the micro gas turbine Turbec T100 was analyzed experimentally with synthetically mixed fuel compositions. Reliable start-up procedures and steady-state operation were observed. The Turbec T100 reached an electrical power output of 50 to 100 kW_el with a lower heating value of 5.0 MJ/kg. Compared to natural gas, the electrical power output was noticeably higher at constant turbine speeds. Therefore, operation was limited by the power electronic at low speeds, while a second limitation was compressor surging at high speeds. To avoid surging, the turbine outlet temperature had to be reduced at turbine speeds between 64,400 rpm and its maximum of 70,000 rpm. The pressure losses across the FLOX-combustion chamber remained below 4%, which corresponds to a reduction of 30% compared to the Turbec combustion chamber fired with natural gas. Low pollutant emissions, i.e. CO < 30 ppm, NO_x < 6 ppm and unburnt hydrocarbons <1 ppm, were obtained over the whole operating range. Further optimization potential of the Turbec T100 was analyzed numerically. Neglecting compressor surging and the limitations of the power electronic, the numerical simulations predicted a maximum power output of 137 kW_el. The ability of the micro gas turbine to run with low calorific fuels is demonstrated and optimization potential is specified.

Graphical abstract

Introduction

For decentralized combined heat and power (CHP) production, micro gas turbines (MGT) constitute a promising technology [1]. The stationary combustion of MGTs enables low pollutant emissions without exhaust gas aftertreatment and it facilitates firing of alternative fuels. As MGTs are considered to be fuel-flexible, there is a growing interest to use them in combination with biomass derived fuels [2]. For instance, using product gases from biomass gasification in efficient MGT-CHP plants offers a reduction of CO₂-emissions and saves fossil resources. However, commercially available MGTs are designed for conventional fuels such as natural gas or diesel, which have a higher lower heating value (LHV). If fuels with lower LHVs are used, the fuel mass flow rate increases respectively. In case of a product gas with a LHV of 5.0 MJ/kg the fuel mass flow rate increases almost by the factor of ten compared to natural gas. Usually the fuel mass flow rate is limited by the size of the fuel valves, the flow cross-sections and the fuel pressure. Enlarging the fuel mass flow rate affects the impulse ratios inside the combustion chamber and as a consequence the flame stability. Furthermore, important combustion characteristics like flame speed and ignition delay time depend on the fuel composition. A completely new design of the combustion system is necessary if these characteristics are very different to the designated fuel. Nevertheless, only minor modifications of the original combustion system were conducted in all previous studies about the operating performance of MGTs with alternative fuels.

To analyze the impact of biogas on the operating performance of the 100 kW_el MGT Turbec T100, Nikpey et al. [3] diluted natural gas with carbon dioxide at various load points. They found that 15% CO₂ (in mole fraction) could be added until flame out occured at part load with a power output of 50 kW_el. The possible amount of CO₂ decreased to 10% at full load and 100 kW_el. The LHV in their study varied between 46 and 33 MJ/kg. In this range, no significant changes in performance were observed. D’Alessandro et al. [4] analyzed the part load performance (20–40 kW_el) of the 80 kW_el MGT from Elliot Energy systems with modified fuel nozzles. By diluting natural gas with nitrogen, the LHV was decreased down to 23 MJ/kg. No significant effect on the electrical efficiency was found, which is in accordance with [3]. Similar results were obtained by Kataoka et al. [5], who operated the Elliot MGT at full load with digester gas featuring a LHV of 17.5 MJ/kg. However, neither for LCV fuels with LHVs below 17.5 MJ/kg nor for fuels with a similar composition as product gases experimental data is available in literature. Some authors tried to predict the operating performance of MGTs for LCV fuels numerically. The simulations are based on models which are validated with experimental data obtained from natural gas operation. Prussi et al. [6] simulated the steady-state behavior of the Turbec T100 at full load for various blends of a representative biomass product gas and methane. The LHV ranged from 50 MJ/kg for pure methane down to less than 4 MJ/kg for pure product gas. Considering the energy for fuel compression, they received a sharp decrease of the electrical power output and the electrical efficiency for blends with a LHV less than 10 MJ/kg. It is noteworthy that the operating points of the turbomachinery components remained in the stable range, even for the pure product gas. Bohn and Lepers [7] investigated the impact of the biogas composition on the full load performance of a 80 kW_el MGT. Their results predict that the compressor remains inside the surge margin up to a methane content of only 15 vol%, i.e. a LHV of 3 MJ/kg. As there are no further restrictions known, the last two studies suggest that the mentioned MGTs would tolerate low calorific fuels with a LHV of only 3 or 4 MJ/kg.

To overcome the present limitation, the first LCV combustion system for MGTs has been developed in this work. It allowed an extensive characterization of the Turbec T100 with low calorific fuels featuring LHVs from 3.5 to 5.0 MJ/kg. These synthetically mixed fuels were similar in composition to product gases from fixed-bed gasifiers. In this way, further operational limitations of the MGT were identified. While compressor surging limited operation at full load, the power electronic turned out to be a restriction at part load. Additionally, a numerical model was validated with the experimental data obtained in this work. The model was used to analyze optimization potentials of the Turbec T100 for product gas operation. Finally, this work gives the first comprehensive investigation of operating the MGT Turbec T100 with product gases from biomass gasification.

The developed combustion system was successfully implemented and tested in the Turbec T100. The design of the combustion system is based on the concept of flameless oxidation (FLOX) [8]. This technique is already applied in industrial furnaces and similar approaches are also known as MILD combustion [9], colorless distributed combustion (CDC) [10] or High Temperature Air Combustion (HiTAC) [11]. The FLOX-concept is an efficient and fuel-flexible combustion concept with low emissions of hazardous pollutants like NO_x and CO [8], [9], [12], [13], [14], [15], [16]. It features a low risk of flashback as well as relative pressure losses across the combustor below 5% [15]. Concerning low calorific value gases, Danon et al. [17] obtained low pollutant emissions with a prototype FLOX combustor. For gas turbine application, these combustion concepts are still at the level of prototypes, which have been tested at combustor test rigs. Only Zanger et al. [18] reported the successful operation of a FLOX-based combustion system (designed for natural gas) in a MGT.

Section snippets

Micro gas turbine test rig

The FLOX-combustion system presented in this work has been investigated in a micro gas turbine Turbec T100PH series 3, which features a nominal electrical power output of 100 kW_el, an electrical efficiency of 30%, a maximum turbine speed of 70,000 rpm and a thermal power output of 150 kW_th. A schematic of the MGT test rig at DLR is illustrated in Fig. 1. The MGT itself consists of a compressor, a turbine, a generator, a combustion chamber and a recuperator. The radial compressor achieves a maximum

Numerical setup

At DLR a numerical simulation program was developed to analyze the steady-state performance of the Turbec T100 [22], [23], [24]. The program is based on models for each MGT component and on extensive experimental data collected at the DLR MGT test rig. Furthermore, the compressor map and the turbine map are embedded in the program. The limits of the power electronic are included and can be optionally turned on by the user. The input parameters are fuel composition and temperature, ambient air

Start-up procedure

The start-up procedure constitutes a critical maneuver because the MGT must be accelerated rapidly to avoid excitation of its resonant frequencies. As a consequence, the conditions in the combustion chamber change strongly and therefore, the risk of flame extinction is high. Fig. 5 illustrates the start-up procedure of the Turbec T100 fired with product gas PG1 from cold conditions. Turbine speed, TOT and electrical power output P_el are plotted against time. At the beginning, the generator

Numerical results

The operating performance of the MGT fired with product gases has been simulated with a Turbec T100 steady-state simulation tool for two cases. While in the first case the limitation of the power electronic is neglected, this restriction is considered in the second case. In both cases it is assumed that the compressor behaves according to the compressor map without being limited by surging. The results are presented in Fig. 10 and compared to the experimental results. Electrical power output

Conclusions

The performance of the micro gas turbine Turbec T100 has been characterized for the use of product gases from biomass gasification in a laboratory test rig. To operate the Turbec T100 with product gases, a new FLOX-combustion system has been successfully developed and integrated. Low pollutant emissions are achieved over the whole operation range, i.e. CO-emissions are less than 30 ppm and NO_x less than 6 ppm. Furthermore, no unburnt hydrocarbons have been detected. The FLOX-combustion system

Acknowledgements

The authors wish to thank J. Zanger, M. Stärk, J. Eichhorn and R. Bruhn for their support at the test rig as well as N. Klempp and M. Henke for their help with the numerical simulation. The German Federal Ministry for Economic Affairs and Energy is gratefully acknowledged for funding the project (Grant No. 03KB047A).

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Performance analysis of the micro gas turbine Turbec T100 with a new FLOX-combustion system for low calorific fuels

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Micro gas turbine test rig

Numerical setup

Start-up procedure

Numerical results

Conclusions

Acknowledgements

Appl Therm Eng

Renew Sustain Energy Rev

Appl Energy

Prog Energy Combust Sci

Prog Energy Combust Sci

Appl Energy

Appl Therm Eng

Appl Energy

High temperature air combustion-from energy conservation to pollution reduction

FLOX combustion at high pressure with different fuel compositions

J Eng Gas Turb Power