A new vigour in Riga

Latvia’s expanding and increasingly influential role as a member-state of 21st century Europe contrasts sharply with the Latvia of a little more than a decade ago, when this newly-independent eastern Baltic seaboard nation faced the enormous task of rebuilding its economy. Today, Latvia is making tremendous advances in many key sectors, among which energy has emerged as a leading driver of the country’s modernisation. Tangible evidence of such advances can be found at a rapidly-developing construction site on the outskirts of Riga, the Latvian capital, where power packaging specialist Turbomach is creating a new gas turbine based CC combined heat and power plant that promises to deliver world-leading standards of efficiency and flexibility when it commences operation next year.

Commissioned by energy developer Rigas Siltums, this r25 million investment will provide the Latvian capital with a single high-efficiency plant that will meet growing demand for heat and electrical power well into the future. The 48 MWe plant, in the Imanta district of Riga, is designed to meet the base load requirements of the city’s district heating network throughout the winter.

Turbomach is to supply all of the power and heat plant and strategic services within a schedule of just 60 weeks, co-operating widely with eastern European manufacturers to maximise economic benefits. With work already well underway, the project represents one of the largest yet undertaken by Turbomach and provides an opportunity for the Switzerland based specialist to demonstrate its capabilities throughout the CHP market in eastern Europe.

Work on the engineering phase of the plant began earlier this year and delivery of equipment is well underway in time for production of heat and power during the winter heating season in January 2006. Turbomach’s responsibilities include not only the supply of the plant but also supervision of installation, testing and commissioning. The company is also contracted to undertake maintenance at the plant under a nine-year service agreement. A time frame of 15 years has been set to evaluate the economic performance of the investment, to be based on 200 000 operating hours.

Packaging of all rotating machinery is being undertaken by Turbomach, in-house, drawing on the proven qualities of Rolls-Royce gas turbine technology and a steam turbine manufactured by B+V Industrietechniek of Germany.

Main equipment

Gas turbine generator package

At the heart of the plant is a TBM-RB211-T gas turbine generator, an aero-derivative system comprising a Rolls-Royce RB211-6761 gas generator matched to an RT-61 power turbine driving a four-pole generator through a speed reduction gearbox supplied by Allen Gears.

Heat recovery steam generator

The HSRG is fired by exhaust from the gas turbine; steam generation is supplemented by an in-duct burner, situated in front of the boiler inlet, to allow for daily variation in the heat demanded by the city’s district heating system. The exhaust gas path includes a closed district heating loop to transfer the final stage of available heat to the city’s network.

Steam turbine generator package

Steam produced in the HRSG is led to a MARC 4 turbine with one bleed, designed to provide optimum efficiency in both the unfired and supplementary-fired modes of the HRSG.

Exhaust from the steam turbine is used in the district heating condenser to bring cold end, ie return, district heating water up to the required temperature level. The resulting condensate is sent to the feed water storage tank, which is equipped with a de-aerator, then fed back into the HRSG to be re-used for steam production. The steam turbine package is equipped with a steam bypass system, which connects the live steam with the district heating condenser during start-up, shut-down, load rejection and trip of the steam turbine.

Electrical subsystem

The plant’s electrical system comprises high- medium- and low-voltage sections. Two HV/MV step-up transformers are installed, connected to the 110 kV network of Latvernergo, the state power utility. The GT and generator system is directly connected to the step-up transformer.

Distributed control system

Safe and reliable operation, control and supervision of the cogeneration power unit is ensured by a distributed control system, or DCS. This provides functions including signal conditioning, annunciation and recording; operation, monitoring and supervision; open and closed loop control; and data communication with subsystems fitted with local programmable logic controllers.

Plant performance

Performance of the plant, measured at 15 °C ambient temperature, 1.013 bar pressure and 60 % air relative humidity, is designed to achieve net electrical power output at a summer base load of 38.1 MWe combined with a thermal power output of 32.2 MWth, at a net electrical efficiency of 46.2 % and overall efficiency of 86.7 %.

The thermal output is designed to operate flexibly between 15 and 45 MWth in summer mode, and full load in the heating season, with total monthly efficiency above 80 %. Peak electrical efficiencies above 46.5 % are anticipated.

In service, the driving parameter for the plant’s load control system is demand from the district heating network. While meeting this requirement, production of electricity remains at maximum possible efficiency at the relevant load point. This capability is achieved by providing as much heat input as possible to the HRSG from the gas turbine, while switching on the duct burners at times of higher heat demand. The load point defined as maximum district heating load in winter is –24.5 °C with a flow temperature (ie of water sent to the district heating system) of 120 °C and return temperature of 63 °C. To meet these requirements the gas turbine and supplementary duct burners would all be running at full load.

In part-load operation, highest electrical efficiency at all times is guaranteed by the steam turbine’s operating in sliding pressure mode at loads in the range above 50 %. To maximise electrical output in part-load operation the district heating coil can be turned off.

With the local grid operator contracted to buy all power produced by CHP plants with minimum average efficiencies of 80 % – a level exceeded by Turbomach’s Imanta plant – future profitability of the new plant appears assured, and at a significant level. With 354 GWh/y of electricity to be sold to the state power utility and a further 321 GWh/y of thermal energy supplied to local business and domestic customers, Imanta is set to play a vital role not only in the development of the Latvian capital but also in the progress of this newly ambitious nation for many years to come.