An engineering study and cost estimate was conducted to assist a power plant in investigating repowering the existing boiler to reduce heat rate. The project involved the addition of one combustion turbine, whose exhaust will provide hot combustion air to the windbox, and a second combustion turbine to operate in a separate combined cycle mode, with the steam feeding the existing steam turbine and the exhaust going to a stack. The proposed arrangement will significantly improve the plant’s thermal efficiency and increase the power output. The exhaust from the combustion turbine that will repower the boiler will be diluted with about 30 percent fresh combustion air to increase the oxygen to the boiler in order to maintain flame stability at various operating scenarios. Oxygen levels in the hot windbox combustion after dilution are expected to be in the range of 15-16 percent. The second combustion turbine will operate in a separate combined cycle mode, with a single pressure HRSG generating high pressure superheated steam. This superheated steam will the tie into the main steam
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A ±25 percent grade estimate was performed for the supply and installation of this repowering project. The estimated capital cost of the project is expected to range between $235-$245 million in first quarter of 2008 pricing, with escalation until the end of 2010.
Process description
The power plant used for this study includes a heavy oil natural circulation boiler and steam turbine, generating 600 MW. The existing boiler was manufactured by B&W in the mid-1970s. The boiler maximum continuous rating (MCR) is 4 million lb/hr of main steam and 3.7 million lb/hr of reheat steam. Main steam to the turbine is at 2,400 psig and 1,000°F. Reheat steam is at 1,000°F. The boiler was modified in the mid-1990s to be capable of firing natural gas in addition to heavy oil.
The proposed project will include hot windbox repowering by one combustion turbine, with a second combustion turbine and HRSG operating in a combined cycle mode. The main goal of the proposed repowering evaluation is to improve the overall unit heat rate.
The conversion to hot windbox repowering will lower the furnace exit temperature, elongating the life of the superheater and reheater tube materials. Any particular tube modifications resulting from hot windbox repowering are not accounted for in this study.
Design basis
Since hot windbox repowering involves using the exhaust from the combustion turbine as combustion air for the boiler, the combustion turbine was selected based on its exhaust gas flow rate and how close it is to the original boiler requirements. Also, in order to minimize de-rating of the existing unit, the combustion turbine exhaust must be diluted with fresh combustion air. This is mainly due to the fact that the oxygen content in the combustion turbine exhaust is about 12 percent vs. 21 percent for normal combustion air. After dilution, the oxygen content in the combustion turbine exhaust will be in the range of 15-16 percent. The boiler can then be fired to about 1.5 percent O2 in the flue gas.
Several combustion turbines and combustion turbine combinations were investigated to provide the best match to the existing boiler requirements (Table 4). The comparison study indicated that repowering with a single combustion turbine in the 100 MW range does not offer a significant improvement in heat rate. Repowering with one or two combustion turbine(s) with HRSGs, both operating in combined cycle modes without any air dilution, exceed the flue gas requirements of the boiler and do not fully load the steam turbine. Repowering with two combustion turbines, one repowering the boiler windbox while the other exhausting into a HRSG and repowering the steam turbine, seemed to be the best approach in terms of matching the boiler and steam turbine requirements. This research study concluded that a combination of a Siemens SGT6-3000E (formerly W501D5A) and a Siemens SGT6-5000F4 (formerly W501F) have a slightly better match and performance for this project. The Siemens SGT6-3000E combustion turbine will be a good match for the hot windbox repowering portion of this project. The SGT6-3000E, with 30 percent dilution air, will match the existing boiler flue gas flow. A Siemens SGT6-5000F4 was selected for the combined cycle repowering portion of this project.
