Boilers

Advancements in Duct Firing Technology

Advancements in gas-fired generation are allowing high efficiency and flexible operation of duct-fired plants for the first time. 

By Mat Palmer, Product Line Manager at Siemens Power and Gas

Advancements in gas-fired generation are allowing high efficiency and flexible operation of duct-fired plants for the first time. Conventional duct-firing in combined-cycle power plants is slow to respond to fast load changes. For highly duct-fired plants, conventional technology can reduce unfired efficiency. New technologies now enable fast starting and load changing capability for duct-fired, combined-cycle plants, and the ability to achieve high unfired efficiency even in highly duct-fired plants. These capabilities are achieved by combining new technologies, including a fast duct firing system, with modifications to the steam cycle that result in increased efficiency during non-duct fired operation. A plant utilizing these technologies can increase revenue by being flexible enough to support rapid load changes and efficient enough to save fuel and reduce emissions. Technologies such as these also provide other technical advantages that increase plant dispatch rate.

Flexibility

These technologies can make a duct-fired power plant flexible for the first time ever. Plants with conventional duct firing are limited to a ramp rate of about 3 MW/min. New technologires provide an integrated plant system that allows the entire output of the plant to change load at the full ramp rate of the gas turbine, ramping at about 40 MW/minute per gas turbine. This performance is achieved through an integrated design and control approach in which equipment is sized and rated for transient operation, and the control logic is programmed to keep equipment within design limits while ramping rapidly. New available technology has been enabled by experts that evaluated and designed active components together as a system, instead of using a traditional approach of attaching independently designed parts. The resulting flexibility opens the door to using duct firing to support the rapid load swings that are occurring with increasing frequency in every region of the country.

Modified Steam Cycle for Higher Efficiency and Lower Environmental Impact

Duct firing has been used as a low-cost option to add capacity to combined-cycle power plants for nearly as long as combined-cycle gas turbines have existed. However, many plants equipped with duct firing only utilize the duct burners about 20 percent of the year.

Depending on the level of duct firing, when the duct burners aren’t dispatched the plant has unused steam turbine capacity. Use of a steam turbine that is not size-optimized to plant flow can reduce the operating efficiency of the plant. To improve plant unfired efficiency, Siemens modifies the way the steam is routed through the steam turbine in the unfired mode, enabling a more optimized flow path both fired and unfired, and high unfired efficiency even in highly duct-fired plants.

Technical Advantages Return Economic Advantages

The fast duct firing system and the modified steam cycle enable higher revenue in today’s market. Higher unfired efficiency means that more power can be generated, while consuming less fuel, which reduces operating costs. The reduction in fuel consumption also means fewer emissions from the power plant. This reduces greenhouse gas production and potentially avoids costs for emissions credits.

The fast duct firing system and higher unfired efficiency can also result in higher plant dispatch. In a conventional power plant, high duct firing can result in higher revenue in the form of increased capacity payments, or by enabling higher power production during times of high demand.

The price of this additional capacity used to be a lower un-duct-fired heat rate. This new cycle design improves unfired efficiency, which can improve the dispatch order of the plant during times of lower demand.

This flexibility also provides an additional dispatch benefit; plants are able to operate to the full base load, instead of being forced to reserve or hold back plant megawatts.

Some power plant operators in different regions are forced by either grid rules or operational requirements to operate the plant at lower than base load, in order to have 2 to 10 percent of their power available at all times. Because the power needs to be rapidly available, standard duct firing will not be able to meet this need. With the addition of a fast duct firing system, the operator is able to dispatch the entire base load of the plant and support the real-time changes in the grid with duct-fired capacity.

hese additional megawatts have been evaluated by a Siemens customer employing this technology to increase revenue upwards of 5 percent per day.

 

Tags