Boilers, Coal

Modular HRSG Design Saves Time and Construction Costs at the Jobsite

Issue 6 and Volume 120.

By Jeroen van de Garde, Thermal & Conceptual Design Engineer at NEM Energy B.V.

In the competitive business of engineering power plants, the market-driven demand for cost-efficient heat recovery steam generators (HRSG) has sparked the creation of an innovative design and construction method by NEM Energy, a Siemens subsidiary. This method incurs shorter lead times than conventional HRSG design.

Suspended between two cranes, a boiler module is ready to be positioned next to the already-placed module behind the red crane (on the left-hand side). Photo courtesy: NEM Energy B.V.

Work done onsite during the construction of a gas-fired power plant with a conventional HRSG can become a significant factor in a project’s cost. Highly modularized HRSG design allows a boiler to be shop-fabricated as very large pieces, called modules, which are then shipped to the construction site. These modules consist of the heat exchanging harps and the boiler casing (walls, floor, and ceiling, including insulation and liner plating). Flue gas baffles are also installed in the modules. The size of the modules is kept as large as possible, within the constraints of allowable shipping dimensions.

Using so-called G-fast design, a boiler’s main steel components (referred to as goal posts) are erected first. Next, the boiler modules are positioned between the goal posts and secured in place.

Due to the high level of modularization, the interconnecting piping is already made to penetrate the casing, so onsite welding in tight spaces is very limited. In fact, the G-fast design requires fewer field welds altogether.

Drums leave the fabrication workshop with all internals installed and can be placed on the module roofs after the modules have been set and securely bolted into place. Once the drums are set, the platforms and walkways can be installed.

Ducts are shipped in maximum sizes (typically that of a truck) and then welded to support beams using seal welds (external) and stitch welds (internal). The standard stack consists of three sections, each spanning 120 degrees.

Compared to moving only the separate harps, G-fast HRSG design requires less transportation steel. Some transportation steel is still required for module stiffness, but most of these trusses can remain in place after module placement. Only in the high-temperature modules like the superheater and reheater boxes is removal of transportation steel required. In the other modules, simply removing some bolts and nuts suffices, since this will allow free thermal expansion without causing problems such as additional material stresses.

The HRSGs for Panda Power’s Stonewall project in Leesburg, VA were delivered and built using the G-fast method. The turnkey project of Siemens Energy Inc., ain consortium with Bechtel, is a 778-MW combined-cycle power plant consisting of two combustion turbine generators, two HRSGs, and one steam turbine generator. The combustion turbines are natural gas-fueled, as are the duct burners in the HRSGs. To lower plant emissions, the HRSGs are equipped with selective catalytic reduction (SCR) and a CO catalyst.

As an additional advantage in certain projects, modules can be fabricated with even the goal posts already attached, if transportation dimensions allow. This is called C-fast design and leads to larger modules with even less onsite construction work.