Steam injection and methane steam reforming to enhance performance of existing natural gas combined cycles

The growth in worldwide energy demand is currently facing a lot of difficulty for installing new power generation facilities due to the limited funding and implementation of stringent environmental regulations. As such, it is important to assess the energy and economic feasibility of upgrading such systems’ capacity for them to cope with future energy requirements. Basically, electricity generation can be boosted through the installation of new power plants or upgrading of existing facilities, with the latter being the best option with reference to the subject matter.

The number of natural gas combined cycle (NGCC) installations is increasing over time, credit to impressive technological advances. As such, an increase in plant efficiency and a concurrent reduction in investment cost has been reported. The vital role being to ensure power grid stability in the future.

Recently, Professor Roberto Carapellucci from the University of L’Aquila together with Dr. Lorena Giordano of the Italian National Agency for New Technologies, Energy and Environment proposed a new concept for upgrading existing NGCCs by integrating an additional unit based on gas turbine. This additional unit allows the production of steam to be injected into the existing NGCC combustor (Option A) or syngas from methane steam reforming. The syngas is used to partially replace the natural gas at the inlet of the existing gas turbine combustor (Option B), or even to feed the supplemental gas turbine (Option C). Irrespective of waste heat recovery route, the power output augmentation arises from the capacity of the additional gas turbine and the increased output of the existing NGCC. The latter comprises the additional power production of the gas cycle due to steam or fuel injection, and that of bottoming steam cycle, resulting from the greater exhaust gas flow rate entering the heat recovery steam generator. Their work is currently published in the research journal Energy.

Briefly, the study evaluated the feasibility to improve both the efficiency and power output of NGCCs, by comparing the upgrading options envisaged from the energy and economic points of view. The reference study case was represented by a single shaft NGCC, based on a heavy-duty gas turbine and a three pressure and reheat steam cycle. A preliminary sensitivity analysis was used to assess the influence of operating conditions of the additional unit on the energy and economic performances of marginal power production. Considering commercial gas turbines, further investigations address the design of upgrading options and the comparison of their techno-economic performances referred to the additional or the overall power productions. Finally, focusing on Option A, the part-load operation of the additional gas turbine is examined to evaluate penalties on marginal efficiency and marginal cost of electricity.

The researchers observed that, in Option A, the maximum power increase states at around 45%, due to constrains on the maximum allowable condenser overload (+15%). Conversely, options based on methane steam reforming (Options B, C) allow higher marginal performance of the repowered NGCC, because of the combined chemical and thermal heat recovery.

In summary, Carapellucci-Giordano study concluded that the Option A provides the best economic performance combined with the highest operational simplicity, while the Option C is the preferred technical solution for a greater increase in plant capacity. Furthermore, in option A, by applying a part-load strategy based on inlet guide vane angle and fuel control to the gas turbine of the additional unit, the repowered power plant undergoes a slight decrease in marginal efficiency but it still outperforms, from an economic point of view, the existing NGCC in the whole range of part-load operation.