Once-through Steam Generators Power Remote Sites

Issue 6 and Volume 102.

Once-through Steam Generators Power Remote Sites


By Michael F. Brady, Innovative Steam Technologies

A Highly Competitive Marketplace has Placed Greater Demands on Developers and Independent

power producers to build facilities that are cost effective and technologically advanced within a reduced time schedule. Many of these demands have been transferred to the shoulders of the original equipment manufacturers. If an equipment manufacturer wishes to be profitable and to participate in construction or upgrade projects, it must design products that are economically attractive and have suitable operating and capital cost benefits. Innovative Steam Technologies` once-through steam generator (OTSG) offers a cost-effective heat recovery system useful in a variety of applications and viable in extreme conditions. IST has delivered 39 OTSGs in its 12-year history.

OTSG Basics

Though not a new technology, once-through steam generation has not gained widespread application. Unlike conventional heat recovery steam generators (HRSGs), OTSGs do not have defined economizer, evaporator or superheater sections. The point at which the steam-water interface exists is free to move through the horizontal tube bank depending on the heat input and the mass flow rate and pressure of the water. The single point of control for the OTSG is the feedwater control valve; valve actuation depends on predefined operating conditions that are set through the distributed control system (DCS). The DCS is connected to a feedforward and a feedback control loop, which monitor the transients in gas turbine exhaust load and outlet steam conditions, respectively. If a transient in gas turbine load is monitored, the feedforward control sets the feedwater flow to a predicted value based on turbine exhaust temperature, producing steady state superheated steam conditions.

Also unlike conventional HRSGs, OTSGs do not have steam drums, mud drums or blowdown systems. Water volume is typically one-eighth to one-tenth that of a drum-type HRSG. The absence of a blowdown system limits the steam generator`s thermal losses and lowers the makeup water requirements to less than 0.1 percent of total cycle flow rate, thereby permitting a smaller makeup water treatment plant. Water quality is maintained using conventional deionization and polishing exchange systems, which eliminate deposition and carryover into the tube bundle. Deionized water treatment systems are not unique to OTSGs; they are being used with increased frequency on traditional drum-type HRSGs and are favored for any installation where low life-cycle costs, high reliability, and/or high-purity steam is desired.

OTSGs configured for combined-cycle or cogeneration operation are typically arranged as vertical flow/horizontal tube systems. The horizontal tube configuration results in a smaller footprint, pushing the units vertically rather than horizontally. OTSGs configured for steam injection applications are much smaller, often containing less than six rows of tubes, enabling a horizontal flow/vertical tube arrangement.

Conventional HRSGs use carbon steel as the tube material. Carbon steel loses strength at elevated temperatures, however, making bypass stacks and diverter valves necessary to prevent the hot exhaust gas from damaging the tubes during dry running conditions. The use of high-nickel Incoloy 800 and 825 alloy tube material–which maintains a substantial fraction of its strength and resistance at high temperatures–permits full dry running without the need for a bypass stack and diverter valve. Incoloy tube material also limits the OTSG`s oxygen sensitivity, avoiding the need for active chemical water treatment. The elimination of the bypass stack and the diverter valve, together with the system`s modular design, causes the OTSG to be up to 60 percent smaller and lighter than a comparable HRSG, making the OTSG suitable for projects that have size and weight restrictions, such as power barges and marine applications.

If necessary, OTSGs can accommodate supplementary firing at up to 1,500 F. Viable fuels include natural gas, naphtha, fuel gas, No. 1 oil and No. 2 oil. When dirtier fuels such as oil are used, the number of fins per inch on the heat exchanger tubes is reduced to prevent clogging. The duct burners are situated between the inlet duct and plenum sections of the OTSG. If NOx control is required, steam injection or low- or high-temperature selective catalytic reduction (SCR) are available options. Low-temperature SCR is viable when dry running is not required and/or duct burning is included; the SCR catalyst system is installed within the heat transfer section at the appropriate temperature level. High-temperature SCR is applicable when dry running capabilities are needed; the SCR system is installed above the plenum and below the heat transfer module.

Pipeline Power

TransCanada PipeLines (TCPL) is a natural gas transmission company that operates 57 compressor stations along the company`s 8,671-mile pipeline system.1 TCPL recognized the potential for waste heat recovery from many of its gas turbine compressor sets. By building a base combined-cycle power plant and supplementing it with heat recovered from the exhaust of a gas turbine compressor set, an “enhanced combined cycle” is created.

TCPL first applied OTSG technology to its Nipigon, Ontario compressor station and then later to the North Bay and Kapuskasing stations (Table 1). The Nipigon 40 MW power plant acted as a test bed and training site for the other projects. IST delivered two OTSGs to Nipigon in 1992, one for a General Electric LM2500 gas turbine generator set and one for a Rolls Royce RB211 compressor drive, then four more OTSGs for two Pratt & Whitney FT8 gas turbine generator sets and two RB211 compressor drives to North Bay and Kapuskasing in late 1996. The units were similar with respect to physical size and steam production, which minimized engineering costs and simplified manufacturing. The units produce 67,000 lb/hr to 113,000 lb/hr steam at 600 psi to 750 psi and 760 F to 840 F for the high-pressure circuits and 12,000 lb/hr to 19,500 lb/hr steam at 50 psi to 60 psi and 315 F to 360 F for the low-pressure circuits.

The 40 MW TransCanada Power power plants are in northern Ontario, where winter temperatures can fall to -40 F. The OTSG is capable of continuous full load operation in subzero weather without a building enclosure. Pressure parts are contained within the module casing, where it is always hot during gas turbine operation. If there is a gas turbine trip and the steam generator needs to shut down, the system can be boiled dry with the residual heat within a few minutes, regardless of the ambient temperature. In addition, the potential hazard from the large volume of high-pressure water normally contained in drums is not a concern.

The RB211 compressor drive at Nipigon is operated independently of the power plant. Natural gas transmission has first priority, power production is secondary. The OTSG`s ability to run without water, therefore, is critical to plant operation. Compressor drive operation is not compromised if there is a power plant shutdown. TCPL has received permission from the Ontario provincial Technical Safety Standards Association to run the Nipigon plant remotely attended, except during the normal eight-hour shift (provided that for the remaining hours the plant is continuously monitored either locally or remotely). This ability to run remotely has strategic importance to TCPL and it expects to operate several plants on such a semi-attended basis.1

Furthermore, TCPL recognized the potential savings if installation costs could be lowered. IST designs OTSGs in five modules: inlet duct, plenum, steam generator module, hood and stack. Each of the five modules is shop-fabricated and can be delivered to the site by rail and/or tractor trailer. The modular design and manufacturing facilitates rapid construction and minimizes crane and work-hour requirements at plant sites. The OTSGs can be set in position within one day following the placement of the plenum module. Once the plenum is set, the steam generator module, hood and stack are simply placed on top of each other and then seal welded. Additional time is required for completing the module joints for external piping and commissioning.

TCPL is currently expanding the Nipigon and Kapuskasing gas transmission and power plant stations. An additional RB211 gas turbine compressor drive coupled with one OTSG per station will be added. The additional steam production will increase each plant`s output and increase the enhanced combined-cycle efficiencies to approximately 67 percent. The two OTSGs are identical, which again will minimize the engineering, manufacturing, erection, and commissioning costs. All power generated is sold to the Ontario Hydro grid.

Oil Sand Boost

Syncrude Canada Limited operates the largest oil sand production facility in the world. The Athabasca Oil Sand deposit satisfies more than 16 percent of Canada`s total petroleum needs and produces 21 percent of all oil sand worldwide. Syncrude mines oil sand from a surface mine, extracts the raw oil, or bitumen, from the sand using steam and hot water, and upgrades it into crude oil by fluid coking, hydroprocessing, hydrotreating and reblending.2

Syncrude has large steam generation requirements for internal power production and oil sand product extraction. Syncrude Canada, via CoSyn Technology, contracted with IST to deliver two 26 MW GE Frame 5-sized OTSGs producing 466,000 lb/hr of high pressure steam, increasing the total plant`s steam production 11 percent, to 4,200,000 lb/hr. Three fuel gas-fired utility boilers and two carbon monoxide-fired boilers satisfied the facility`s steam requirements prior to commissioning of the OTSGs. Fuel gas is an oil sand mining byproduct, with a composition similar to that of natural gas. IST incorporated the fuel gas byproduct into the OTSG design as the supplementary firing fuel. The supplementary firing at 1,455 F raises steam temperature and pressure to 1,008 F and 930 psi, respectively.

The teamwork and coordination between IST and CoSyn Technology made for a successful and timely project. To meet Syncrude`s accelerated schedule, both units had to be completed and delivered within seven months of award. IST received the contract award on May 1st, 1997, engineering commenced on the 12th, manufacturing began on the 14th, and both units were manufactured in parallel, with two shifts, seven days a week for the project duration. The engineering and manufacturing schedules overlapped, which left little room for deviation. The accelerated schedules lowered the project cost and enabled Syncrude to increase their production sooner. As a result, both units were completed, shipped, commissioned, and producing steam by early December 1997. p

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The Nipigon compressor station and power plant in northern Ontario relies on two once-through steam generators, one tied to a GE LM2500 turbine and one tied to a Rolls Royce RB211 turbine.

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The five sections of the once-through steam generator are evident in this photo at the SynCrude Canada oil sand production facility: inlet duct, plenum, boiler section, hood and stack.

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A crane sets the boiler section in place during construction of the once-through steam generator at the North Bay compressor station.

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1 Johns, W.D., “Enhan -ced Combined Cycle Technology UsedBy TransCanada Pipe Lines For Power Generation,” Eleventh Symposium on Industrial Applications of Gas Turbines, October 1995.

2 http://www.syncrude.com.


Michael F. Brady isa sales engineerwith Innovative Steam Technologies. He is responsible for project consultation and performance/cost modeling of the once-through steam generator. Brady holds a bachelor`s degree in structural engineering from the University of Western Ontario.