| Heat recovery steam generators


after installation will be challenging due to space constraints. Thus, in anticipation of future H2


burning, a larger spool duct needs to be considered in the design of any new build installations. Furthermore, many existing power plants have supplementary firing systems installed either in the inlet duct of the HRSG or between the high-pressure superheater modules. Increasing H2


fuel of such burners will also likely increase the NOx


emissions and impact SCR performance. H2 fuel fraction (%vol) Above: A blend of natural gas with 30% H2 delivers about 11% CO2 reduction.


Hydrogen has high mass-related heating value (LHV): 120 MJ/kg (2.4 times higher than methane). Very low density, 8 times lower than methane. (Source: NEM Energy)


turbine exhaust gas, impacting the size and cost of the selective catalytic reduction (SCR) system required.


Possible higher gas turbine exhaust volume flow and exhaust gas inlet temperature. Increased water content in the gas turbine exhaust gas, leading to higher risk of water condensation in the cold end. This increase in water dew point is however minimal up to 50% vol hydrogen content.


Impact on HRSG performance and gas side pressure drop.


Burner system related challenges for HRSGs employing supplementary H2


co-firing. Safety aspects


Safety concerns relating to hydrogen firing and HRSGs specifically arise in the event of gas turbine or duct burner trip. Potential accumulation of hydrogen in the ‘attic’ of an HRSG in such circumstances is a particular


consideration for horizontal-exhaust-flow HRSGs. Design evaluation of the HRSG casing and attic and additional measures for optimal venting, can be applied as risk mitigation actions in accordance with NFPA and other applicable guidelines of local authorities.


Higher NOx exhaust gas


emissions in the GT


During combustion, the local flame temperature and flame speed of hydrogen are contributing factors to NOx


temperatures favour NOx Combustion of H2


formation. Higher flame production.


may lead to higher flame


temperatures than natural gas due to the higher heat of combustion of H2


. Current tests show


that gas turbines running on 100% hydrogen will produce significantly more NOx


running on natural gas. The higher NOx


than those emissions directly affect the sizing of the SCR system. Any SCR adaptations


Higher exhaust volume flow and increased exhaust gas inlet temperature


Firing hydrogen can potentially also add extra volume to the exhaust gas flow compared to firing natural gas, depending on the gas turbine conditions. CH4


has an LHV of 49 895 kJ/kg = 798.3 kJ/


has an LHV of 120 087 kJ/kg = 240.2 kJ/mol. Hydrogen has a higher energy density per unit mass but a much lower energy density per mol. Since the compressor of the GT will suck in the same volume flow of air, practically independent of the type of fuel that is fired, and the same amount of energy needs to be added, it means that 3.324 times the amount of CH4 mols) need to be added in the form of H2 of 100% H2


mol and H2 (in in case firing. Typically, for a modern GT, about 4% of the molar flow of air is added as CH4


This will then increase to 13.3% in case of 100% H2


firing. The molar mass of the flue gas drops from 28.3 g/mol (100% CH4


to 27.2 g/mol (100% H2 firing to 100% H2 firing, 60% RH @ ISO) firing, 60% RH @ ISO). A


curious phenomenon now occurs: switching from 100% CH4


firing, the mass flow


of flue gas decreases by 1.3% while the volume flow of flue gas increases by 2.5%.


For new HRSG units, design parameters such as the sizing of the duct and casing, heating surfaces, internal gas flow distribution within


.


ratios in the combustion


Above: Safety aspects. Hydrogen has wider explosion range and lower ignition energy compared with natural gas (NG), but similar auto ignition temperature to NG. The same accumulation pattern as NG, eg, in the event of gas turbine trip, is expected, since the density of both gases is much lower than that of air. Potential accumulation of hydrogen in the ‘attic’ of an HRSG in such circumstances is a particular consideration for horizontal-exhaust-flow HRSGs. (Source: NEM Energy)


Above: Hydrogen firing is likely to result in higher NOx


than natural gas. When no SCR


system is installed, an empty spool duct could reserve space for installation at a later stage. When SCR is installed, potentially more space may be needed to accommodate a future hydrogen-ready SCR system. (Source: NEM Energy)


www.modernpowersystems.com | October 2023 | 13


CO2


emissions (%vol)


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