Gissi Plant Gets First Commercial Installation of LLOC
The first application of Low Load Operation Concept (LLOC) technology was at Abruzzoenergia SpA’s 800-MW Gissi combined-cycle plant located in the Abruzzo region of central Italy. Alstom was contracted to design, supply, install, and commission the entire power plant. The fast-track project was given the green light in January 2006 and the plant entered commercial service in early 2008 (Figure 7).
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| 7. Italian plant design. The first commercial application of Alstom’s Low Load Operation Capability (LLOC) was with the two KA26-1 units installed at Abruzzoenergia’s Gissi plant. “KA26-1” designates a single-shaft driveline combined-cycle plant based on Alstom’s GT26 combustion turbine. Each unit is rated at a nominal 400 MW. The LLOC design feature allows Abruzzoenergia to “park” the plant during low power demand periods at about 20% (~80 MW) combined-cycle load. CO and NOx emissions are kept below baseload permit limits while the plant is parked. Courtesy: Alstom |
The Gissi plant consists of two GT26 sequential combustion turbines installed in a 2 x 410-MW single-shaft combined-cycle arrangement. Each power train comprises one GT26 gas turbine, a triple-pressure reheat, horizontal heat-recovery steam generator, an STF15C steam turbine using 565C/140 bar high-pressure steam, and a shared hydrogen-cooled TOGAS turbogenerator, all manufactured by Alstom.
Initially, Abruzzoenergia planned to shut down the Gissi plant overnight when electricity demand was low. The Gissi plant design assumed two different operating modes: operate about 8,000 hours per year at baseload or about 3,000 hours per year with daily start/stop operation. However, the possibility of low-load operation using LLOC presented Abruzzoenergia with a financial opportunity. The ability to keep the plant online overnight meant Abruzzoenergia could provide power for grid support as well as rapid peak load response. Grid support at night is especially valuable in Italy, which suffered a massive blackout in 2003 caused by a lack of spinning reserve when it was critically required.
However, combined-cycle power plants in this region of Italy usually do not operate at night because emissions regulations historically have prevented combustion turbines from operating at less than 60% of rated load.
Shutting down a combustion turbine and steam bottoming plant overnight, only to restart them the next morning, is a less-than-desirable operating scheme. Daily cycling causes thermal stresses that will shorten the life of major plant components and increase operating costs over the plant’s lifetime as every start-stop operation avoided will save 10 equivalent operating hours. The extended time necessary to start up the plant can also result in lost revenue in a competitive merchant power market.
Alstom’s LLOC allowed Abruzzoenergia to park each unit at less than 100 MW during off-peak hours as an alternative to daily cycling. Each GT26 combustion turbine met the baseload plant emissions requirements while parked.
The units are required to achieve 30 mg/m3 for NOx and CO in all ambient conditions down to 30% load.
In addition, when parking the plant at around 20% combined-cycle load (made possible by shutting down the second or Sequential EnVironmental combustor of the GT26 at low loads, while keeping the first or EnVironmental combustor operating at nominal conditions), emissions are about the same as at full load.
Plant tests have confirmed that the combustion turbine can reach baseload operation and the plant can achieve over 95% of combined-cycle baseload output in less than 20 minutes from the “parked” condition of 20% load.
At a gas price of €0.26/m3 and an off-peak electricity price of €28/MWh (during low-load operation), plant owners expect to save between €10 million and €18 million over the plant’s lifetime. Additional revenue is expected from providing valuable grid frequency support and spinning reserve capacity.
After the simulation phase, the LLOC design was next tested in the GT26 Test Power Plant. During this test and validation phase, data across all possible loads in full combined-cycle mode were recorded in order to verify the plant’s low-load and response capabilities.
Actual results confirmed that the power plant can be loaded from a “parked” condition (about 20% of combined-cycle load) to baseload (100% load) in less than 25 minutes with standard load gradients, whereas normal start-up times for hot start of a combined-cycle plant after an 8-hour shutdown are typically above 40 minutes. In addition, the overall combined-cycle efficiency was approximately 60% of the baseload plant thermal efficiency.
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