Fortuna: demonstrating a role for flexible CCGT with cogeneration

Stadtwerke Düsseldorf's Siemens- supplied H class Fortuna unit - already the holder of three world combined cycle power plant performance records, for efficiency, electrical output and district heating output (see text box below) - is now in the process of establishing new benchmarks in the all-important area of operational flexibility, a major preoccupation for owners/operators/developers of large combined cycle plants looking to find new revenue streams to enable them to survive in Europe's current market conditions.

In so doing it is showing how there can still be a role for such combined cycle plants in the European power market, where CCGT has been, to say the least, struggling in recent years.

“Why are we still talking about large units in an era of distributed generation?", said Willibald Fischer, who spearheaded development and commercial introduction of Siemens H class technology and is now the company's director of GT product management, at a press briefing on the Fortuna project, Düsseldorf, 12 April. Physics combined with economies of scale dictate that large combined cycle plants are inherently able to deliver levels of efficiency and per-MW costs not achievable by small, distributed units, he pointed out. They are also able to provide the large rotating masses that a stable grid requires.

But until relatively recently large CCGTs have been perceived as inflexible and only suited to baseload operation. Siemens'
successful introduction of its all-air-cooled H class gas turbine technology (with the commercial operation of Irsching 4 in 2010) has made a major contribution to changing all that.

Flexibility enhancements

Further innovations introduced at Fortuna are making it something of a showcase for demonstrating what can be achieved by large-scale combined cycle technology when it comes to operational flexibility.

Flexibility enhancements implemented at Fortuna include the following:

• Co-Start and Quick-Stop. Building on the well established FACY concept introduced several years ago, Co-Start, which is essentially a control logic modification rather than a hardware change, reduces the time to full power from a hot start (ie after a shutdown period of eight hours or less) to less than 25 minutes, enabling the plant to start up rapidly in response to changing market conditions (eg, a rise in power prices or a call for back-up).

The basic idea is to start the gas and steam turbines simultaneously, with elimination of the waiting periods and hold points typical of conventional start-up practice. As in most H class installations to date, Fortuna employs a single-shaft configuration, in which the gas turbine and steam turbine both drive a single generator (located between them), with the steam turbine connected via a SSS clutch.

Plot 1 summarises the basic concept of Co-Start (from hot) and Plot 2 shows the results of tests carried out at Fortuna. These confirmed the expected benefits, including significant fuel savings, reduced fatigue stressing of the steam turbine HP section, and higher dispatch rate (resulting in improved load factor and plant economics).

Co-Start can also be applied to warm starts (ie, restart after a shutdown of up to 48 h, eg a weekend), reducing start-up time from the conventional 90 minutes to 45 minutes (Plot 3), and again tests at Fortuna have demonstrated benefits such as significant fuel savings and increased load factor due to greater dispatch rate.

According to Siemens, despite the acceleration in start-up processes "component life is not adversely impacted to any impermissible degree."

Tests at Fortuna have also demonstrated accelerated shutdown, allowing the operator to respond quickly to deteriorating market conditions (low, even negative, electricity prices) and reduced demand. Called Quick- Stop, this uses similar principles to Co- Start. The gas turbine is shut down as quickly as possible, independently of the steam turbine and without the conventional waiting periods and hold points. In addition I&C changes enable shut down of the steam turbine to start earlier and the complete power train to be brought to a full standstill very quickly by using a modified operating procedure.

The Fortuna trials have shown that a shutdown time of 25 minutes is achievable, compared with around 45 minutes for a conventional shut down (see Plot 4), resulting in significant fuel savings.

• Flex-Ramp. Again a control logic modification requiring no hardware alterations, Flex-Ramp increases achievable combined cycle ramp rates by fast ramping the steam turbine in addition to the gas turbine. For the ramp-up, steam stored in the HRSG is routed to the steam turbine IP section. For the ramp-down steam from the steam turbine IP section is routed to the condenser.

The result is an increase in load change gradient of 20 MW per minute over above that achievable by the gas turbine alone.

Fortuna is thus able to ramp up and down at a rate of 55 MW per minute (see Plot 5).

Once again the benefits are increased dispatch rate, because the unit becomes more valuable to the grid operator, and increased load factors. Such ramp rates also better enable the plant to participate in grid ancillary services markets, and to earn additional revenues from secondary frequency services, for example.

• Clean-Range. Fortuna is also host to the first power plant application of a new technology that Siemens has been working on for the past five years designed to significantly increase the time an H class plant can operate at 35% turndown while still remaining in CO and NOx emissions compliance, a particular challenge being to achieve the required flame stability.

Called Clean-Range (see Plot 6), it employs combustion system modifications that have been trialled in an SGT6-8000H gas turbine operating at Siemens' Berlin test facility.

Clean-Range is attracting "huge interest" says Willibald Fischer, and the potential for lowering the emissions compliant turndown level even further is currently being investigated.

Among the benefits are increased load range and the potential to reduce fuel consumption by operating at lower load.

Of special interest to a CHP unit such as Fortuna is that Clean-Range increases the ability of the plant to remain in operation at times of reduced power demand and therefore to meet district heating needs even when there is little requirement for electricity.

Flexibility and CHP in Germany

For combined cycle plants wishing to operate in the current German power market, flexibility is of particular importance, indeed crucial, a point emphasised at the 12 April briefing by Lothar Balling, executive VP global project management, Siemens.

Combined cycle units are presently well down the German merit order (see panel, right), after must-run renewables and low- marginal-cost nuclear, but also below coal and lignite. In the absence of a properly functioning CO2 emissions allowances market - with CO2 certificates trading at very low cost largely because of lack of demand due to a decline in economic activity - the current marginal-cost merit order model somewhat conflicts with the emissions merit order, he observed. Nevertheless, for the most flexible combined cycle plants there are opportunities to "jump in" and participate in the market, he believes, for example to compensate for a sudden fall in wind or solar generation.

Fortuna, as well as being well placed to take up these opportunities thanks to its flexibility improvements, also has the distinct advantage of being a cogeneration facility, located at Stadtwerke Düsseldorf's Lausward power plant site, close to the centre of Düsseldorf, within the harbour region.

Under Germany's amended CHP legislation of 2016, support is extended (subject to EU state aid approval) to cogeneration plants of over 2 MW and applies up to a plant's 30 000 th hour of full-load operation. Previously the subsidy duration had been defined as a period of time rather than in terms of operating hours. The change is intended to help incentivise investment in combined cycle CHP plants, such as Fortuna, despite their reduced operating hours in current market conditions, with the German government hoping to see about a quarter of the country's net controllable power generation coming from CHP plants by 2020.

Implementation of a major project to expand the Düsseldorf district heating network on the west bank of the Rhine started in 2011, with installation of a piping network in the districts of Heerdt and Lörick, including heat supply to a large state of the art office complex, and further expansions to follow. 2012 saw installation of a culvert line across the Rhine linking the new network directly to the Lausward plant site. The heating network on west bank of the Rhine has also been expanded and upgraded.

Further increasing its scope for flexible operation, by allowing temporary decoupling of power generation from heat supply, Stadtwerke Düsseldorf is also in the process of building a 35 000 m3 hot water storage tank at Lausward, with a design temp of 98°C, storage capacity of 1480 MWh max, and mass flow of 5000 m3/h (200 MW). This will enable the Fortuna combined cycle plant to increase its heat output. It will also allow heat to be supplied from the site when the combined cycle unit is not running, eg when electricity prices are so low that CCGT operation is not worthwhile.

The storage tank (see diagram, above left), with a diameter of 29 m (without insulation) and height of 54 m, is expected to be completed by the end of 2016.

Building Fortuna

Stadtwerke Düsseldorf is 150 years old and its Lausward site, where electricity has been generated since 1957, can be considered a privileged location for a power project, with good connections to the natural gas grid, river water cooling from the Rhine (which helps increase efficiency), close proximity to heat and power users, existing grid connection and power plant infrastructure, together with a high a level of public acceptance and local authority support.

The Lausward facility initially consisted of four coal units (block A (100 MW), block B (130 MW), block C (130 MW) and block D (150 MW)), plus a 420 MW gas fired unit, block E. Blocks B, C and D ceased generation in 2003, and block A was converted to gas. In 2006 a project was initiated to replace block C/D with a new coal unit, but this was halted in December 2008 in the face of opposition.

In February 2011 a new project was launched, initially designated block F, subsequently called block Fortuna, envisaged as a single-shaft natural gas fuelled combined cycle plant to be housed in new buildings.

To make space for the new combined cycle plant the flue gas scrubber towers of the obsolete coal units were demolished in March 2012.

The contract between Stadtwerke Düsseldorf and Siemens for supply of the Fortuna unit was signed in May 2012, with handover of the construction site to Siemens in July 2013.

The gas turbine (SGT5-8000H) for Fortuna was manufactured at Siemens' Berlin manufacturing plant while the steam turbine (SST5-5000, with extractions for heat supply), rated at 215 MW, was manufactured at Siemens' Mülheim manufacturing plant. The steam turbine consists of a combined high and intermediate pressure turbine section and two low pressure turbine sections. Exhaust area is 12.5 m2 and steam conditions are: HP, 170 bar/600°C; IP, 35 bar/600°C; LP, 5 bar/300°C.

The three stage LP steam extraction concept to provide heat for the district heating system is shown schematically, right.

The steam is used to heat the circulating district heating water to about 97°C.

The heat recovery steam generator (HRSG), delivering the 600°C/170 bar HP steam, was designed by Siemens Austria (formerly Balcke Dürr Austria, which was acquired by Siemens in 2007). The HRSG, measuring 35 x 26 m at its base and 40 m tall, weighs some 6000 tons and contains 400 km of tubing. The design, specially tailored to deal with advanced steam conditions and flexible operation, was first used at the Irsching H class combined cycle plant.

The Fortuna HRSG is "SCR-ready" should that be required in the future, although that seems unlikely. Plant NOx emissions are 20 ppm, and CO is 10 ppm (well within allowable NOx emissions of 75 mg/m3 (average per day) and 50 mg/m3 (average per year), and allowable CO, which is 100 mg/m3 (average per day)).

The generator (SGen5-3000W) is water cooled, a further contributor to maximising efficiency.

First firing of the Fortuna gas turbine took place in April 2015. Commercial handover was on 22 January 2016, 19 days ahead of the contractually scheduled date.

In total, over two million man-hours of work were performed on the site with "zero- harm", with no single reportable work-place accident. At the peak some 850 people were working simultaneously on the site.

To address the challenge of integrating this large industrial facility into the city considerable emphasis was placed on attractive architecture and minimal emissions, including noise, with allowable noise levels being 35 dB (A) during the day and 25 dB (A) during the night at a hospital on the other side of the river Rhine.

The Aachen-based architectural firm kadawittfeldarchitektur designed the striking façade of the new power plant, with embedded LEDs, and the steel and glass office-block-like building at one end that surrounds the 65 m high exhaust stack and contains what is called the "City Window", providing views of Dusseldorf city centre from an enclosed 45m high viewing platform accessible by lift. To provide the best view of the city this building is rotated a few degrees relative to the rest of the plant.

High energy density

The space required for the new combined cycle plant amounts to about the size of one soccer field, says Siemens, and estimates that generating the same quantity of electricity, ie, some 2 TWh per annum (assuming 3800 operating hours annually for the combined cycle plant) from solar or wind would require areas totalling 5300 and 10000 soccer fields, respectively, such is the energy density achieved by H class technology.