Hydrogen, the burning issue for OEMs14 April 2020
A new report from the European Turbine Network (ETN Global), Hydrogen gas turbines – the path towards a zero-carbon gas turbine (https://etn.global/wp-content/uploads/2020/02/ ETN-Hydrogen-Gas-Turbines-report.pdf), includes a very valuable overview of where the key gas turbine OEMs have got to in terms of employing hydrogen as a fuel.
The ultimate R&D target, the report notes, is achieving state-of-the-art low NOx emissions (< 25 ppm) with fuel gas mixtures containing increasing amounts of (green) hydrogen (from electrolysis) up to 100% H2.
Thus far, the main focus, according to the report, has been new/modified combustion technologies based on current dry low emission (DLE) combustion techniques (lean premixed combustion without dilution and/or water injection).
With such adapted DLE combustion systems OEMs (Ansaldo Energia, Baker Hughes, GE, MAN Energy Solutions, Mitsubishi Hitachi Power Systems, Siemens, and Solar Turbines) “report successful testing of frontrunner gas turbine products operated with fuel gas mixtures with up to 20% vol H2 (or even 30% vol H2).”
In some of these cases a de-rating of the gas turbine engine is still required (accomplished by reduced flame temperature). Combustor developments with other combustion concepts (eg, micro mixing and Constant Pressure Sequential Combustion) are also being pursued and “have shown promising results on gas turbine test bench installations.”
The report provides an overview for each OEM as follows:
Ansaldo Energia currently offers: 0-50% vol H2 in natural gas for the GT36 H class gas turbine; 0-30 or 0-45% vol H2 in natural gas depending on the respective GT26 F class engine rating, up to 25% vol H2 in natural gas for the AE94.3A F class machine.
In addition, the following retrofit solutions are offered: up to 35% vol H2 in natural gas for GE 6B/7E/9E machines fitted with lean premixed combustors; 0-40% vol H2 for engine retrofits with FlameSheet combustor (available for existing GE, Siemens and MHI, E and F class machines).
The advantage of the GT26 with reheat (sequential combustion) technology is an additional degree of freedom balancing the power of the two combustion chambers. A variation of flame temperature in the first burner is an effective parameter to maintain low NOx emissions as well as offsetting the impact of fuel reactivity on the auto-ignition delay time of the downstream reheat burner.
Extensive single burner high pressure tests at full scale have been performed for existing GT26 standard premix and reheat burners with 15 to 60% vol H2 in natural gas. These confirmed that the latest rating (2011) can cope with contents of up to 30% vol H2 with no changes in hardware and without performance penalty. With further validation and minimal de-rating, this limit can be extended to 45% vol H2.
The additional degree of freedom provided by sequential combustion is exploited to an even larger extent in the Ansaldo Energia GT36.
Since this can-annular engine does not have a high pressure turbine separating the two combustors the system is referred to as Constant Pressure Sequential Combustion (CPSC). Due to this, no efficiency or power penalty is incurred when lowering the temperature between the two combustion stages. The GT36 is today offered for commercial operation with hydrogen contents of up to 50% vol.
Further validation is ongoing, including full scale, high pressure tests. With the hardware as is, operation with up to 70% vol was demonstrated to be feasible with minimal or no de-rating, without dilution or SCR. With further optimisation this level is expected to be further extended. Since the CPSC system is a can-combustor, retrofits to other can engines are in principle possible. Recently, Ansaldo Energia and Equinor announced collaboration on development of a 100% H2 gas turbine combustor.
The AE94.3A has acquired experience with H2 operation in a commercial power plant, operating with hydrogen in natural gas with concentrations up to 25% vol, with several hundred thousand EOH on two units employing various hydrogen/ natural gas blends.
A retrofit of three 9E gas turbines demonstrated the capability to operate on H2 to 25% vol, with 35% vol successfully tested at sub 9 ppm NOx emissions. The combustor upgrade combined with automated tuning (AutoTune system) has demonstrated robust operation with varying natural gas/H2 mixtures over the past two years of commercial operation.
The FlameSheet combustor, specifically developed and proven as a commercial solution for retrofit to achieve low emissions and high fuel flexibility is available for commercial implementation in E and F class GTs with H2 capability of up to 40% vol. Blending of H2 up to 80% vol has been demonstrated on combustion rig tests.
A Dutch government subsidised programme with several industry/academic partners is in progress (2019/20) to demonstrate 0-100% hydrogen capability with sub 9 ppm NOx emissions in a combustion rig.
FlameSheet is already in commercial operation on seven F-class GE machines and is a simple retrofit for existing GE, Siemens and MHI, E and F-class machines.
Baker Hughes has been an independent gas turbine OEM since October 2019, after separation from GE Oil & Gas, with GE reducing its ownership stake below 50%. Baker Hughes has inherited ownership of the GE heavy duty gas turbine portfolio below 40 MW, while the aeroderivative fleet (25-100 MW) is managed via a joint venture with GE.
Baker Hughes has recently introduced a light industrial GT family (NovaLT) to target the power range up to 20 MW.
The maximum allowable H2 concentration in lean premixed combustors varies significantly across the Baker Hughes gas turbine fleet, the company says, as different combustion technologies are employed. Fuels with significant hydrogen content are carefully evaluated, and the feasibility is assessed case by case considering the peculiarities of each specific project.
Standard and lean head end combustors (for heavy duty gas turbines) or single annular combustors (for aeroderivatives) have been tested and employed in the past to burn very large H2 concentrations, with diluent injection for NOx emission abatement. 100% H2 capability was demonstrated on a GE10-1 with steam injection at Enel’s Fusina combined cycle power plant.
As far as the application of lean premixed combustors is concerned, capabilities are consistent with limits specified by GE for DLE and DLN1/DLN2 combustors (see below).
The NovaLT family of engines is equipped with piloted premixed burners arranged in annular combustors, capable of modulating the fuel split between pilot and premix lines along the operating range and based on fuel composition. This flexibility allows the engines to burn up to 100% H2, with variable fuel gas mixtures, with and without diluent injection, and with consequently variable NOx emission levels (demonstrated on full annular rig tests, currently ongoing for the NovaLT-16).
Baker Hughes has been working for several years on the development of novel burner technologies to allow the reliable application of lean premixed systems with hydrogen-rich fuels, with atmospheric rig tests at Enel laboratories.
Results indicate that the most mature solution consists of a cluster of partially premixed burners. This concept exhibited emissions consistent with the natural gas BAT (best available technology) and was found to be strongly resistant to flashback.
GE reports that it has been providing gas turbines for operation on fuels with hydrogen for over 30 years. More than 75 GE gas turbines have operated on fuels containing hydrogen, accumulating more than 5 million operating hours. These units have operated on fuels with a range of hydrogen content, from 5% (by volume) up to 100%. This experience includes both heavy- duty and aeroderivative gas turbines in a range of applications.
GE offers diffusion, dry low emissions, and dry low NOx combustion systems for aeroderivative and heavy-duty gas turbines, providing a range of options for hydrogen and similar low heating value fuels, for new units as well as retrofit.
Increased hydrogen capabilities are anticipated in the future. Combustion testing has demonstrated that the AEV combustor of GE’s GT13E2 gas turbine, an E class machine, is capable of operating on hydrogen/natural gas blends of up to 60% vol hydrogen without dilution and with NOx emissions less than 15 ppm.
GE’s newest combustion system, the DLN 2.6e, includes an advanced premixer developed as part of the US DOE’s High Hydrogen Turbine programme. The advanced premixer, unlike the DLN 2.6+, uses miniaturised tubes functioning as “fast” mixers.
This miniaturisation enables premixed combustion to be employed for gaseous fuels with higher reactivity (such as hydrogen).
The DLN 2.6e combustor with the advanced premixer has demonstrated the capability to operate on a 50% (by volume) blend of hydrogen and natural gas. A roadmap has been developed, mapping out the steps to reach 100% hydrogen.
MAN Energy Solutions
THM gas turbines (9-12 MW) are available with a standard diffusion combustion system. This allows up to 60% vol H2 content in a mixture with natural gas, but requires exhaust gas treatment when low NOx emissions are a requirement.
Some THM models are also offered with a dry low emission combustion system designated Advanced Can Combustion (ACC). These systems achieve very low emissions without water injection or exhaust treatment, and can also handle up to 20% vol hydrogen.
The MGT range of gas turbines (6-9 MW) is also equipped with ACC, again allowing up to 20% vol hydrogen in natural gas.
ACC systems for both THM and MGT gas turbines have been thoroughly tested in high pressure facilities at DLR, Cologne.
These tests showed that, with minor modifications, 20% vol hydrogen was handled without any problems, and with some additional modifications, stable and low emission combustion was achieved up to 50% vol hydrogen.
Both the THM and the MGT range of gas turbines feature externally mounted can combustion systems. The THM has two combustors, while the MGT has six in a circumferential arrangement.
This means that modified designs having greater fuel flexibility can easily be retrofitted to existing machines without disassembly of the core engine. In addition, testing is more easily carried out on a single can.
Theoretical and experimental studies have been carried out since 2010 with DLR Stuttgart, with the aim of reaching 100% vol hydrogen capability with low NOx emissions.
MHPS has three types of combustor for hydrogen fuelled gas tubines, see diagram, p28.
The multi-nozzle combustor is newly developed for hydrogen co-firing. It is based on conventional DLN combustor technology, with the aim of preventing flashback. The air supplied from the compressor to the inside of the combustor passes through a swirler and forms a swirling flow. Fuel is supplied from a small hole on the swirler’s wing surface and is mixed rapidly with the surrounding air thanks to the swirling flow effect. Combustion tests were performed successfully with a 30% vol hydrogen mix in natural gas, at J-Series firing conditions. The next step is to reach up to 100% hydrogen, which will probably involve more development of the cluster combustor.
The swirling flow used by the DLN combustor to mix fuel and air requires a relatively large space and increases the risk of flashback.
The multi-cluster combustor is a promising alternative which uses a greater number of nozzles than the eight fuel supply nozzles of the MHPS DLN combustor.
In the multi-cluster combustor, the nozzles are smaller and the amounts of fuel passing through each nozzle less. The air and hydrogen mixing is achieved at smaller scale without the need for swirling flow, which is more compatible with flashback resistance and low NOx.
Rig testing has confirmed the combustion characteristics of the multi-cluster combustor at 80% vol hydrogen.
MHPS has extensive hydrogen firing experience with diffusion combustors, up to 90% vol hydrogen content, going back nearly 50 years, in gas turbines located at such locations as refineries and coke oven facilities, running on such fuels as syngas, coke oven gas, etc.
Compared with premixed combustion, a region with high flame temperature is likely to be formed in the diffusion combustor, increasing the amount of NOx generated and therefore requiring steam or water injection. On the positive side, the stable combustion range is relatively wide, and the allowable range of fuel properties is also wide.
MHPS has been involved in a feasibility study looking at conversion of one of the diffusion- combustor equipped gas turbines in Vattenfall’s Magnum CCGT plant to 100% hydrogen.
New Siemens gas turbines are available with different levels of hydrogen admixing capability, depending on the type:
- Aeroderivatives, up to 100% vol H2 in diffusion combustion mode with NOx abatement using water. With DLE technology, up to 15% vol H2 is possible for the SGT-A65 and SGT-A35.
- Utility gas turbines, up to 30% vol hydrogen admixtures with DLE combustors.
- Medium-size industrial gas turbines (SGT-600 to SGT-800), admixtures of up to 60% vol H2 possible.
- Small industrial turbines, SGT-100 and SGT- 300, up to 30% vol hydrogen, SGT-400 up to 10% vol H2.
Choosing diffusion technology with unabated NOx increases hydrogen capability to 65% vol H2.
To reach these hydrogen capabilities with existing units, upgrades to the control system and hardware may be needed and are available for many GT types. For example, for Siemens 2000E and 4000F machines the H2DeCarb package is available to increase H2 capability, enabling the 2000E to operate with 30% vol H2 and the 4000F with 15% vol H2.
The standard capability for existing Siemens industrial gas turbines is up to 10% vol H2, and up to 15% vol H2 for new units.
Today’s Siemens industrial gas turbines with 3rd generation DLE technology (standard for SGT-700 and SGT-800 and an option for SGT- 600) have a high hydrogen capabilities, up to 50-60% vol H2.
Siemens aeroderivative gas turbines equipped with Wet Low Emissions (WLE) systems are usually good hydrogen capabilities.
Solar Turbines reports that it has experience of many applications involving significant concentrations of.H2 In the past decade many of these have entailed the running of Titan 130 and Taurus 60 gensets on coke oven gas (COG). COG is a waste gas typically generated in the process of creating coke for steel production. The typical gas turbine fuel created with COG has 55 to 60% vol. hydrogen, 25 to 30% vol. methane, 5 to 10% vol. CO, and 5 to 10% vol. diluents (N2+CO2).
Most of the applications have been in China where over 40 gas turbine packages have been installed. These units have operated with few problems and cumulatively have operated for more than 1.4 million operating hours, Solar says.
Many of these units have gone through multiple overhaul cycles. Where issues have occurred, the root cause was determined to be fuel and air contaminants unique to these applications. Once the appropriate filtration was installed, the units have operated without incident.
The initial assessment at Solar Turbines is that using existing SoLoNOx gas turbines with the latest combustion system technology with pipeline gas mixed with 5 to 20% vol hydrogen will not require significant modification.
The ability of earlier generations of SoLoNOx combustion systems to use these levels of hydrogen is under investigation.
SoLoNOx experience with associated and raw natural gases has become very extensive. These gases are quite comparable, in terms of flame speed and flame temperature, to mixtures of H2 and natural gas in the range of 5 to 20% vol.
Direct experience of natural gas/hydrogen mixtures with the SoLoNOx platform is currently limited to a refinery generator set application where a Titan 130S has operated with natural gas mixed with up to 9% vol hydrogen.
Qualification and combustor performance mapping were completed and the unit demonstrated 15 ppm NOx and no operational issues.
The unit was started on 100% natural gas and the package was updated to be compliant with the requirements for applications greater than 4% vol H2. However, due to operator requirements, the time running on the 9% vol hydrogen fuel mix has been short.
Package shipments to customers with high and medium Wobbe Index associated and raw natural gases are much more extensive. These units have few modifications from the standard configurations dsigned to operate on pipeline gas. The earliest shipments have been in operation for a number of years, with many of these packages reaching the standard overhaul operating hour interval. Typically, these SoLoNOx engines run on associated gases in much the same way as they operate on pipeline natural gas. For applications with fuels having higher adiabatic flame temperatures, the NOx emissions are slightly higher. As with all DLE gas turbines, fuel quality with adequate fuel treatment is a pre- requisite for trouble free operation.