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Cleaver-Brooks Engineers Custom-Designed Nebraska D-Type Superheated Steam Boiler for Alberta Power Plant

Case Study

  • Company: Alberta, Canada Power Plant
  • Industry: Energy & Petrochemical
  • Location: Alberta, Canada
  • Profile: The City of Medicine Hat is a Power Plant in Alberta, Canada that desired an upgrade to its power station to better service area residents.
  • Challenge: Devise a means to allow the plant to run solely on a single Combustion Gas Turbine/HRSG system at night.
  • Solution: An auxillary steam source. A Nebraska Custom D-Type Superheated boiler capable of generating 150,000 lb/hr of steam at 600 psi and 750 degrees.
  • Results: Exceeded contract guarantees on steam capacity and pressure, boiler efficiency, noise level, steam purity, NOx and CO emissions requirements.

The City of Medicine Hat Power Plant in Alberta, Canada, desired to upgrade its power station to better service area residents. The main focus of the project was to devise a means to allow the plant to solely run on a single Combustion Gas Turbine/HRSG system at night. This scenario produced the need for a reliable backup steam source should the single operating GT system trip unexpectedly, thereby causing an unwanted city-wide blackout. Plant executives tapped the services of Peter Stanley, PE, a senior boiler consultant at HRST, Inc., Eden Prairie, Minn., to develop the specs for an auxiliary D-type, superheated natural gas-fired boiler system.

The backup boiler had to meet two main criteria. First, it had to be maintained in a hot standby mode, so in the event of an unexpected turbine trip, the auxiliary boiler could be brought online in less than five minutes. Secondly, the boiler had to be able to meet future Alberta emissions regulations, projected to be 15 ppm NOx. According to Stanley, meeting these criteria was challenging to the six bidders. Only Cleaver-Brooks and one other bidder offered an affordable solution.

During the initial proposal review meeting, city engineers met with Cleaver-Brooks local representative Tundra Process Solutions, and Cleaver-Brooks Engineered Boiler Systems(EBS)Sales Manager Jason Jacobi. During the discussions, Jacobi proposed a custom boiler solution, and the meeting turned into a working session. The group began designing the boiler on a whiteboard and utilized Jacobi’s laptop computer, which was loaded with Cleaver-Brooks proprietary programs, to model the boiler performance under varying conditions. Due to this and multiple other reasons, Cleaver-Brooks was awarded the standby boiler project, nicknamed “future 15 ppm ready” by plant engineers.

Jacobi took the specs and design input back to the engineering team at Cleaver-Brooks EBS, who began working on a Nebraska Custom D-Type boiler capable of generating 150,000 lb/hr of steam at 600 psi and 750°F. Jacobi is quick to point out, “We have the finest engineering and manufacturing teams in the industry, between our Lincoln, Neb., and Montreal, Quebec, locations. I knew that they would not only succeed in executing this challenging project, but that they would exceed customer expectations while doing so.”

To achieve the first requirement of being hotstandby ready, Cleaver-Brooks engineers incorporated their proprietary Natcom burner, which includes a unique “Center Core” stabilizing gas injector that typically is used to improve flame stability and turndown. For this application, the Center Core doubles as a second, smaller burner during hot standby.

Heat input is approximately 5% MCR. This maintains the boiler at pressure so it can be ramped to full load quickly. To avoid damage to the superheater during this operating condition, it was nested safely in the convection bank of the boiler, behind multiple rows of screen tubes.

To meet the projected 15 ppm NOx limit, the engineers developed an optimum configuration, taking into consideration the gas pressure drop, economizer exit gas temperature and fan power consumption. Multiple rows of finned tubes of density 3 fins/in were used in the lowtemperature zone of the convection bank to optimize the energy transfer. This innovative tube finning strategy lowers boiler gas side pressure drop, so it can accommodate the future higher flue gas recirculation (FGR) required to achieve tougher NOx emission regulations. The Nebraska Custom D-Type boiler can be upgraded to 15 ppm NOx with minimal changes. In the future, the city only has to change the fan, burner nozzles and reprogram the controls. The existing boiler, burner and other equipment can remain intact, which will save the City of Medicine Hat significant costs down the road.

There are many other innovative features incorporated into this critical standby boiler system. An integral dual-stage superheater system with interstage spray attemporation was provided to maintain a constant steam temperature over the specified turndown. A sweetwater condenser was utilized to ensure the highest purity water is delivered to the interstage spray attemporator. This ensures that the power turbine receives pure, superheated steam, thus avoiding the potential for costly blade damage. Boiler tube wall thicknesses were upgraded beyond ASME code requirements and all tubes were seal-welded to the boiler drums to survive aggressive ramping and heavy cycling service. Cleaver-Brooks standard Nebraska Boiler design with refractory-free front and rear furnace walls and 100% welded gas seals also gave city engineers peace of mind that this boiler will operate safely and reliably for years to come.

Cleaver-Brooks shipped the custom boiler to the Medicine Hat Power Plant in May 2010, and it was commissioned in March 2011.

Today, the boiler is maintained in a hot standby mode (approx. 5% MCR). In the event of a system trip of the main HRSGs, the auxiliary boiler stands ready for immediate backup steam generation. The boiler operator can demand transfer of the standby burner (with a 10 hp fan) to the main burner (with 400 hp FD fan) and vice versa without a time-intensive NFPA shutdown-restart sequence. As a result, the backup boiler can achieve full load within five minutes.

Stanley with HRST conducted thorough performance tests on the boiler in March 2011.The Nebraska Custom D-Type Superheated boiler exceeded contract guarantees with regard to: steam capacity and pressure, steam temperature 50% to 100% MCR, boiler efficiency, noise level, steam purity, as well as NOx and CO emissions requirements, among other parameters.