Retrofit of a combined heat and power plant with gas and steam turbines to hydrogen with special consideration of the balance of plant

In 2020, around 44% of natural gas in Germany was used in combined heat and power as well as in combined cycle gas turbines plants. As district heating will play an important role in future heating planning, the retrofit of these plants to hydrogen is a viable option. This paper analyzes a typical combined cycle power plant, including its balance of plant under consideration of different hydrogen blends. We show that retrofits are limited by the gas turbines in many cases. For instance, the preheater in the fuel gas system must be dimensioned higher than with natural gas. While pressure losses are very low, materials could be a problem due to higher volume flows. Additionally, the higher combustion temperatures in the gas turbine can compensate for possible efficiency losses in the Heat Recovery System Generator (HRSG) and steam turbine making this a suitable approach for electricity-led plants. However, for heat-led plants this leads to a reduction in the district heating output. Therefore, the performance of the HRSG must be considered as a limiting factor for heat-driven plants and the change in flue gas must be analyzed. Currently, hydrogen blends of 20–40 vol.-% appear feasible without major adjustments. The water content in the exhaust gas can also lead to problems in the HRSG and flue gas aftertreatment due to changes in the dew point. For higher hydrogen blends, a plant specific analysis is recommended.