Natural gas-hydrogen hybrid combustion retrofit method and practice for F-class heavy-duty combustion engines

In order to reduce carbon emissions, enhance the operational flexibility of gas turbine power plants, and fill the gap in practical engineering transformation of natural gas-hydrogen blended combustion in heavy-duty gas turbines, a hydrogen blending retrofit was conducted on an F-class heavy-duty gas turbine combined heat and power unit. This served to examine the problems of combustion chamber tempering, combustion pulsation, and NOx emission increase caused by direct hydrogen-doped combustion in the combustion chamber. In this paper, the gas turbine body and hydrogen mixing system were reformed respectively. Retrofit schemes were proposed that were suitable for two operating conditions: 5%–15% and 15%–30% hydrogen blending. Experimental tests were conducted as a means of evaluating the performance of the retrofitted gas turbine and its compatibility with the boiler and steam turbine. The results of the retrofit showed there to be stable combustion, and there was no significant increase in average burner temperatures or occurrence of flashback. The gas turbine power output mostly remained unchanged and NOx emissions met the regulatory standards. The waste heat boiler flue gas temperature was controlled within the range of 84.9–88.2 °C, meaning that the safe operation of the steam turbine was not affected. The hydrogen blending rate was 0.2 Vol%/s, which indicates a smooth and precise control of the hydrogen blending process. It was estimated that the annual reduction in CO2 emissions would be 11,000 tons and 28,400 tons following respective hydrogen blending at 15% and 30%. A reliable retrofit scheme for hydrogen blending in gas turbines based on practical engineering transformation is presented in this study, which has significant reference value.


Introduction
Due to the recent fossil fuel energy shortage and the emergence of the carbon neutrality concept, researchers have started focusing on renewable energy utilization efficiency while also reducing greenhouse gas emissions [1][2][3]. Hydrogen energy has advantages that include high energy density, large-scale compressibility and storage, and zero carbon emissions. The integration of hydrogen energy with renewable energy helps form an energy technology path that is known as hydrogen-electric coupling, which is an important means for the effective utilization of renewable energy [4,5]. Gas turbine power plants are currently facing a dual pressure of carbon reduction and high natural gas costs. In this context, the retrofitting of existing gas turbines with hydrogen blending satisfies the requirements of green and low-carbon emissions while also enhancing the operational flexibility of gas turbine power plants, which serves to improve their competitiveness in the market. This is of significant importance.
The presence of hydrogen in the fuel changes the characteristics of flame speed, ignition delay, and flammability limits [6]. Different hydrogen blending ratios in natural gas and different hydrogen/diluent fuel Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. mixtures exhibit significant differences in combustion, thermoacoustic, and emission performance in comparison to natural gas. In addition, hydrogen has unique properties, including high diffusivity and high chemical reactivity. A high proportion of hydrogen induces nonlinear effects on combustion chemistry and pressure dependence, which impacts free radical distribution and the competition of elementary reactions [7]. The strong transport, diffusion, and reactivity of hydrogen and hydrogen-rich fuels influence the characteristics of laminar flames, which include the distribution structure of species within the flame front and its intrinsic instability. This then affects the coupling between turbulence and combustion chemistry, resulting in significant deviations in turbulent combustion and heat release processes in the combustion chamber in comparison to traditional hydrocarbon fuels.
Relevant research results have indicated that the addition of hydrogen to natural gas can serve to enhance the combustion rate while also extending the stable lean flammability limit of natural gas [8]. However, the achievement of high blending ratios of hydrogen in the fuel mixture presents several technical challenges, such as flashback, combustion pressure fluctuations, and nitrogen oxide emissions. Overcoming these challenges requires significant time and effort. During the transition toward hydrogen-based energy systems, combustion chambers must adapt to rapid variations in hydrogen/natural gas mixtures and fuel compositions. In the medium term, flexible gas turbines with the capability to burn hydrogen/natural gas mixtures with a higher hydrogen content than is currently used must be developed. In the long term, gas turbines with full fuel flexibility (hydrogen and natural gas blending, in addition to pure hydrogen combustion) will need to be developed, which requires long-term research and innovation.
Several researchers have extensively examined the combustion and emission characteristics of hydrogen blending at constant fuel volumetric flow rates. Liu, Giménez et al [9,10] investigated the effects natural gashydrogen blended fuels have on the combustion performance and pollutant emissions of a can-type combustor. The results found that the addition of 10% hydrogen to natural gas reduced CO 2 and NO x emissions by approximately 60% and 14%. However, the decrease in average exhaust temperature caused a reduction in output power of the micro gas turbine. Sorgulu et al [11] found hydrogen blending to increase engine combustion temperature, improve thermal efficiency, reduce CO 2 emissions, and increase NO x emissions. A study by Xu et al [12] demonstrated that hydrogen blending increased flame temperature and decreased combustion chamber thermal load. Research by Ueda et al [13] demonstrated that hydrogen blending increased flame temperature, reduced flame size, decreased CO 2 emissions, and slightly increased NO x emissions. Hydrogen is a gas that has high reactivity, and the addition of it to natural gas can serve to expand the lean flammability limit. Assad, Karam Gheshlaghi et al [14,15] found engines that operated under lean conditions resulted in lower pollutant emissions.
Hydrogen blending modifications for gas turbines are currently limited mostly to theoretical research and numerical simulations [16], and no mature solutions available for practical engineering modifications are available. Major international gas turbine manufacturers including General Electric (GE) Power, Siemens Energy, Mitsubishi Hitachi Power Systems (MHPS), and Ansaldo Energia have an active involvement in the research and testing of high-power gas turbines for hydrogen blending and pure hydrogen combustion. However, the commercial operation of such units has yet to be achieved.
This study is focused on the key technical challenges that can potentially arise during the hydrogen blending combustion of an F-class heavy-duty gas turbine, including gas super-lean combustion, combustion chamber flashback, and increased NO x emissions. Two retrofit schemes for hydrogen blending in the F-class heavy-duty gas turbine cogeneration unit were developed to target hydrogen blending ratios of 5% to 15% and 15% to 30% for different operating conditions. A retrofit scheme for the hydrogen blending system was proposed for power plants that require external hydrogen supply. Calculations estimated that CO 2 emissions can be reduced by 11,000 tons and 28,400 tons per year with hydrogen blending at 15% and 30%. Engineering practices were performed for the retrofit scheme under the 5%-15% operating condition, and operational performance was analyzed. The results indicated that the proposed retrofit scheme will ensure stable hydrogen blending combustion without flashback, maintain turbine power output, and fulfill NO x emissions standards. The main steam temperature of the steam turbine did not exceed the allowable requirements, which ensured the safe operation of the turbine. In addition, the hydrogen blending process was smooth and controlled with high precision.

Key technical difficulties of hydrogen-doped combustion in F-class heavy-duty combustion engines
In this paper, as the proposed modification scheme is to purchase hydrogen externally and then transport it to the plant by tube bundle truck, a fuel blending ratio of 0 ∼ 30% was obtained by combining the calculations relating to fuel interchangeability by studying the F-class heavy-duty combustion engine. According to the combustion engine design calibration, when the hydrogen content exceeds 15 vol%, a new burner that is suitable for hydrogen must be installed. Therefore, the modification scheme this paper proposes is divided into two working conditions of 5%-15% and 15%-30%. The formula of hydrogen doping ratio is as follows: where H 2 a( )denotes the hydrogen doping ratio, and qv H 2 ( )and qv CH 4 ( )represent the volume flow of hydrogen and methane, respectively.
As the hydrogen blending ratio increases, the thermal load in the combustion chamber gradually decreases, while maintaining a constant fuel volume However, as the maximum combustion temperature in the combustion chamber T max may occur, this can cause damage to the turbine blades and other components. In comparison to natural gas, hydrogen possesses characteristics such as low ignition energy, short quenching distance, and fast flame propagation. Direct hydrogen doping combustion in the combustion chamber can result in the following problems: (1) The characteristics of fast flame propagation and short ignition delay time increase combustion chamber backfire risk.
(2) The amplitude and frequency of the combustion chamber thermoacoustic oscillations have changed, which increases combustion pulsation risk.
(3) The local flame temperature of the combustion chamber increases, which leads to the exit of the combustion chamber NO x emissions to increase.
In addition, hydrogen blending has high technical requirements for gas fuel systems: (1) The fluctuation control range of hydrogen content is small, and the maximum allowable variation limit of hydrogen content in gas is ±0.5 vol%/s for meeting the needs of combustion and control.
(2) In the interval of 5 ∼ 25% hydrogen content, the gas fuel temperature must not exceed 140°C, and when the hydrogen in the gas fuel exceeds 25 vol%, the gas fuel temperature must be lower than 55°C.
(3) When hydrogen content exceeds 5 vol%, the original infrared detector cannot detect hydrogen in the gas and modification is required.
The difficulties of hydrogen-doped combustion mainly involve the transformation of the combustion chamber and its auxiliary engine and the hydrogen doping system. On this basis, the transformation plan of the combustion engine body and the hydrogen doping system is proposed in this paper.

Overall transformation program
The hydrogen-doped combustion unit that is mentioned in this paper is a small F-class gas-steam combined cycle unit with combustion engine model SGT-800 gas turbine and rated power of 54 MW. The waste heat boiler is a double-pressure horizontal natural circulation boiler with evaporation capacity of 77t/h. The steam turbine is an extracted steam condensing unit, model number C23-7.8/1.3, with pure condensing rated power of 22.6 MW and rated heating power of 10.5 MW.
3.1. Reconstruction scheme and performance analysis of gas turbine body 3.1.1. Hydrogen blending reform of gas turbine body To adapt this small F-class gas turbine to hydrogen-doped combustion, several modifications are required to the gas turbine and its auxiliary systems. This mainly includes the combustion system, fuel system, cowling ventilation system, gas detection system, fire monitoring system, and air purge system modification, and involves combustion engine output power correction and hazardous area reclassification.
For the two operating conditions of 5%-15% and 15%-30% of hydrogen doping ratio, a modification plan was developed for the combustion engine with different hydrogen doping ratios. The ultimate goal of modifying the burner, fuel system, combustion engine hood hazardous gas detection system, flame detection system, and ventilation system is the achievement of 30 vol% hydrogen doping capacity of the combustion engine. The main modification items can be seen in table 1.

Combustion system modification (1) Installation of new burners
According to the design calibration of the combustion engine, when hydrogen content exceeds 15 vol%, a new burner that is suitable for hydrogen must be installed. This renovation replaced a total of 30 burners in the burner body. In order to prevent backfire, a backfire monitoring element was installed on each burner for monitoring, alarming, and backfire tripping processing. The control logic was also modified and upgraded.
(2) Gas check The maximum allowable hydrogen content variation in the gas was limited to ±0.5 vol%/s. Fuel mixing needed to be precisely controlled and monitored to ensure the instantaneous rate of change in the fuel composition did not exceed this maximum rate of change. Signal exchange was required between the fuel mixing unit and the SGT-800 control system. To ensure more accurate and faster feedback on the gas composition supplied to the combustion engine was provided, the monitoring data of the gas components was communicated by signal to the control system of the gas turbine.
3.1.1.2. Gas leak detection system modification If hydrogen content exceeded 5 vol%, a catalytic detector was required. This was due to the original infrared detector being unable to detect hydrogen in the gas. A new gas detection system with a catalytic gas detector was added to the system. The catalytic detector required more frequent calibration (every three months) than the infrared detector. CFD analysis of the ventilation in the combustion engine hood was performed as a means of determining the required change in location and number of gas detectors [17], and three additional detectors were added for the detection of gas leaks in the hood as a means of maintaining a safe level of gas detection.

Fire detection system modification
The standard infrared flame detector (FMX-3501) in the fire detection system could detect flames with a hydrogen content of up to 15 vol%. For combustion mixtures with hydrogen content above 15 vol%, the flame detector was replaced with a combined UV/IR detector type.

Hazardous area and ATEX classification
The introduction of more than 5 vol% H2 into the fuel affected the hazardous area classification, which ultimately affected the external gas fuel system and the interface flange with the gas fuel system vents, which need to comply with the hazardous area classification principles. When fuels with <30% hydrogen content are used, it is essential that equipment in the hazardous area complies with the relevant requirements.

Analysis of combustion engine performance following hydrogen doping modification
The performance parameters of the gas turbine for ISO conditions and 75% ISO conditions were calculated and three hydrogen blending fuel ratios were considered (0 vol%, 15 vol%, 30 vol%). The calculation process is referenced in [18] and the results can be seen in table 2. As table 2 demonstrates, the increase in hydrogen doping ratio reduced the power and efficiency of the combustion engine and the exhaust temperature. At the rated load of the combustion engine, following doping with hydrogen 15% and 30%, CO 2 emissions were reduced from 7.76 kg s −1 to 7.25 kg s −1 and 6.44 kg s −1 respectively, and according to the annual utilization hours of 6,000 h, the annual reduction of CO 2 emissions are anticipated to be approximately 11,000 tons and 28,400 tons respectively.

Hydrogen doping system modification program
The power plant involved in hydrogen doping and combustion transformation in this paper purchased hydrogen and transport it to the plant by hydrogen tube trailer and set up additional hydrogen unloading stations in the plant in order to realize hydrogen supply. To ensure a continuous and stable supply of hydrogen, 4 hydrogen unloading stations and 4 unloading columns were set up, and the long tube trailer and the unloading columns were considered to be 3 in use and 1 in reserve. Hydrogen entered the hydrogen regulating unit through the unloading column, and the hydrogen regulating unit was set up in parallel in three ways. Hydrogen pressure before regulating: 20 MPa(g) ∼4.0 MPa(g), hydrogen pressure after regulating: 3.5 MPa(g). The blending point was located behind the pressure reduction skid of the built natural gas regulator station. The natural gas was led from the pipeline after the regulator and entered the blending tank in two ways with the hydrogen pipeline for static premixing. The premixed fuel entered the hydrogen blending gas pipeline to the front module of the combustion engine.
High-precision flow ratio control valves and flow meters were installed in the hydrogen pipeline before the mixing tank and a high-precision flow meter was installed in the natural gas pipeline before the mixing tank. A closed-loop control system was created by linking the flow signals from the two pipelines with the hydrogen flow ratio control valve. Real-time measurement of natural gas flow rate was used for controlling the valve opening of hydrogen in a certain proportion to the measured natural gas flow rate and alarm thresholds were set.
The process flow of hydrogen doping modification this paper proposed can be seen in figure 1.

Unloading air column
The unloading column was special equipment for unloading hydrogen from the long tube trailer. Through the unloading column in the parking space, the hydrogen was unloaded from the long tube trailer before being transported to the hydrogen decompression skid for operation. The hydrogen unloading column was a 20 MPa (working pressure) dual-system unloading column, which included detection instrumentation, control system, self-protection interlock, and electrical system.

Hydrogen decompression skid
The hydrogen decompression skid decompressed the hydrogen from the long pipe trailer to 3.5 MPa before delivering it to the mixing unit for mixing. A three-way regulating system was used and an emergency cut-off  system set up as a means of ensuring the pressure range following decompression. In addition, a purge function was reserved for replacing the fuel and air in the piping system.

Mixing skid
The mixing device was installed in the gas mixing skid and hydrogen and natural gas were evenly mixed before being fed into the combustion engine. The equipment was also fitted with a proportional adjustment device with the ability to adjust the hydrogen flow rate based on the natural gas flow rate and regulated the hydrogen mixing ratio (up to 30%) of the natural gas load.
(1) The gas mixing unit adopted static mixer for the production of the cyclonic mixing effect by setting multidirectional spoiler in the mixing tank to ensure the full mixing of hydrogen and natural gas. The downstream long-distance fuel delivery pipeline was used as the supplement of gas mixing.
(2) Blending tank volume was 2m3 and mixture residence time was approximately 14 s. The blending tank was equipped with a multi-directional spoiler as a means of improving blending uniformity.
(3) The control valve in the proportional control device was chosen as a pneumatic control valve to ensure a flow control accuracy within 1% and satisfied the precision requirements of the hydrogen blending ratio.

Nitrogen sink
A nitrogen manifold was installed and nitrogen displacement pipelines were connected to the hydrogen offloading column, pressure relief valve, and pressure regulating valve. Before system operation, nitrogen was used to purge air from the original system and hydrogen was then used to displace the nitrogen in the system, which ensured the purity and safe operation of the hydrogen.

Control program
The hydrogen mixing station adopted the DCS control system, which was connected to the original host control system of the power plant as a remote node. The control system of the hydrogen mixing station provided comprehensive inspection, control, safety interlocking, and monitoring of the working conditions of the station equipment, including unloading column, pressure reducing skid, gas mixing skid, and safety facilities. It also measured, recorded, displayed, and stored the process parameters of the whole station. The hydrogen mixing control system adopted the flow ratio control method as a means of realizing the automatic mixing of natural gas and hydrogen according to the set ratio. Several hydrogen leak detection probes and fire alarm probes were installed in the hydrogen unloading, pressure reduction, and hydrogen mixing areas for the safe detection of the operation of equipment in each area and interlock protection was set to ensure safe and stable system operation.

Combined cycle adaptability analysis
In addition to the gas turbine, the other major pieces of equipment in the gas-steam combined cycle were the waste heat boiler and the steam turbine, which required analysis for adaptability. Under conditions of 15% and 30% hydrogen doping, the exhaust temperature of the waste heat boiler increased slightly (see table 3) and no corrosion effect on the original steel chimney was observed.
The heat of the flue gas that entered the waste heat boiler gradually decreased as hydrogen doping ratio increased. The flow and temperature trends of the high-pressure superheater and low-pressure superheater of the waste heat boiler in ISO conditions can be seen in figures 2 and 3. Figures 2 and 3 demonstrate that the flow rate and temperature of high-pressure main steam decreased, and the flow rate of low-pressure main steam increased as the proportion of hydrogen-doped combustion in the combustion engine of gas-steam combined cycle increased, while the temperature basically remained the same. The main steam temperature of the turbine did not exceed the permitted requirement and the operational safety of the turbine was not affected.

Modified operation effect
The operation diagram of hydrogen doping and mixing at the rated power of the combustion engine can be seen in figure 1. Among them, the hydrogen inside the long tube trailer is approximately 20 MPa. After passing through the hydrogen offloading column, it is reduced to 9.42 MPa. Then, it goes through the hydrogen pressure reduction valve and decreases to 3.45 MPa. After the flow meter and control valve, it further decreases to 3.18 MPa. It is mixed with natural gas at a pressure of 3.19 MPa, regulated by the pressure regulating station, inside the gas mixing device. The outlet pressure is 3.18 MPa, with a temperature of 18.5°C, and the hydrogen content at the outlet of the gas mixing device is 14.9%. The hydrogen ratio change curve can be seen in figure 4. One data was recorded every second, and the maximum change rate was 0.2% per second, which fulfilled the requirement of the maximum allowable change limit of ±0.5 vol%/s. This demonstrates that this hydrogen blending and mixing process is relatively smooth and high precision, and satisfies the requirement of control.
The operating conditions of the combustion engine at rated operating conditions with 15% hydrogen doping can be seen in table 4. According to the table, at a rated power output of the gas turbine (54 MW), 2555 Nm3 of hydrogen was required for a hydrogen blending ratio of 15%. The turbine inlet temperature,  combustion turbine exhaust average temperature, and burner average temperature were slightly reduced, and the burner average temperature was a new measurement point that was added after hydrogen doping, which was located near the nozzle to monitor the tempering situation. The burner average temperature did not increase significantly following hydrogen doping, meaning that the new burner could control the flame area well and there was no tempering phenomenon. The flue gas flow rate remained basically unchanged following hydrogen doping and the test results were consistent with the change pattern of the reformed combustion engine performance parameters that were calculated and analyzed in section 2.3. Following hydrogen doping, NO x emission was within the qualified range.

Conclusions
This study has investigated the hydrogen blending retrofit scheme for an F-class gas turbine with the aim of solving the key technical difficulties of gas supercombustion, combustor tempering, and NO x emission increase. Retrofit plans were developed for two operating conditions with hydrogen blending ratios of 5% to 15% and 15% to 30%. A retrofit plan for the hydrogen blending system was proposed for power plants that require external procurement of hydrogen gas, which is transported to them using hydrogen gas bundles. The transformation scheme under 5%-15% working conditions was conducted in engineering practice and the performance of the retrofitted gas turbine, boiler, and steam turbine was tested. The following conclusions were reached after the practical tests were performed: (1) Stable combustion was achieved following hydrogen blending, with no significant increase in average burner temperatures or flashback occurrence. The power output of the gas turbine essentially remained unchanged and emissions met the regulatory standards.
(2) The hydrogen blending control precision was high during the actual operation of the unit, with a maximum rate of change of 0.2 Vol%/s, satisfying the requirement of maximum allowable change rate of ±0.5 vol%/