The first natural gas CHP plant in the Czech Republic’s Tábor and Sezimovo Ústí region is equipped with four generator sets based on medium-speed gas engines. They deliver almost 37 MW of electricity and heat for local companies and homes. Nina Felicitas Kunzi and Tomas Rocek detail how the project was successfully carried out.
Since January 2015 four four-stroke 20-cylinder engines have supported the new Planá nad Luznici cogen plant in Tábor and Sezimovo Ústí – roughly 100 km southeast of the Czech capital, Prague – in its official operation. The partial renovation of the existing plant, originally coal-fired, began in 2012. Supported by the EU, the project’s goals are long-term operation of the heating plant, reduction of its environmental impact, and reduction of operating costs. The outcome secures an ongoing supply of heat energy to its customers at an affordable and favourable price, and also improves its capability to produce electric energy flexibly, while providing higher capability to change its power output and, at the same time, increase its efficiency.
This project makes it possible for the heating plant to use both lignite and gas to produce power and heat. Strategically, it is able to react better to the fluctuating prices of primary fuels, changing legislation and energy production regulations, and political decisions in this area. Once completed, the heating plant’s SO2 and NOx emissions will be reduced by 50% compared to current emission levels. These overall aims of the CZK1.4 billion (ca $55 million) reconstruction are reached by using more efficient, ecological and economical equipment. When it came to the decision for the propulsion of the renewed plant, the tender selection of the main project stakeholders was generator sets based on medium-speed gas engines from Rolls-Royce. They have been selected because of their main parameters such as rated efficiency, startup speed and production flexibility.
|Delivery of the generator sets
Before the renovation, Planá was a 100% coal-fired heating plant which burned the coal in three 65-tonne boilers. The steam was fed in a condensation turbine with an installed capacity of 47 MWe, and the heat was delivered to local companies and the inhabitants of Sezimovo Ústí. With the renovation of the plant, operator C-Energy Bohemia decided on a combined technology solution to make it possible to produce 100% of the heat supply from either coal or gas.
‘The flexibility of using either coal or gas protects us against the fluctuation of ever-changing resource prices and allows us to fulfil current and future emissions legislation,’ said Libor Doležal, C-Energy’s CEO.
To increase the efficiency of the plant, smaller boilers have been installed and the steam turbine has been renewed. C-Energy Bohemia furthermore opted for the conversion of two-bar steam supply into hot water supply. Next to flue gas boilers, a spare gas boiler and four B35:40 Rolls-Royce cogeneration units with ABB electric generators has been installed, with the supplementary aim to evacuate the electrical output parallel to the local grid.
The construction preparatory phase began in July 2012. The application for a planning and building permit was filed in December 2012, and became legally effective in mid-February 2013. In parallel with the first preparatory stage, documentation and subsidy application from the EU’s Operational Programme Environment fund were prepared, and 30% of eligible costs were granted in October 2013 prior to construction start.
|The engines’ quick-start capability means they can ramp up to their rated maximum power output within five minutes|
In December 2012, notice of this tender was published in the EU Journal in compliance with the Public Procurement Act, and qualification and tender documents were prepared. The construction contractor was selected in the first half of 2013. Of the nine candidates, only three met the qualification requirements. Tendering ended in June 2013 with the signing of a contract for work with a consortium of two experienced Czech contractors, PSG International and Invelt Servis. The main construction site was handed over to the contractors at the end of August 2013.
During subsequent preparatory works and implementation of individual project stages, maximum emphasis was placed on timely completion of the basic design that was executed by the Czech firm H&D Engineering. ‘This was done in order to assess and verify the fulfilment of the contractual requirements, and also on gradual performance and approval of detailed design so that smooth progression of the construction is not impaired,’ explained Doležal.
Production of the gas generator sets based on the gas engines took five and a half months from the signing of the contract in August 2013 to the dispatch of the engines in January 2014. On 29 January a cargo ship moored directly at the Rolls-Royce docking facility in Bergen, Norway. Then the four 20-cylinder B35:40 generator sets were shipped to the Planá plant. Each of the four generator sets has a power output of 9.3 kWe. They are 13.2 metres long, 4.7 metres high, weigh 137 tonnes, and consist of around 3500 parts. The medium speed engines reach an electrical efficiency of up to 48% and are flexibly designed for different operating modes. They can be used to generate baseload as well as peak power, or can operate in cogeneration cycle.
|The Planá nad Luznici cogeneration plant
‘Our engines enable C-Energy to operate the plant efficiently, both in terms of cost and time,’ explains Tomas Rocek, Head of Customer Business CEE & Russia for Rolls-Royce Bergen Engines. Their quick-start capability means the engines can ramp up to their rated maximum power output within five minutes, giving the plant access to the exact amount of power and heat needed within just a short space of time.
For the provision of this quick-start service, C-Energy receives capacity payment from local grid operator CEPS. ‘This service requires constant monitoring of the systems. The equipment needs to be 100% reliable and the temperature of the water and oil needs to be at the correct levels at all times. Especially at the beginning of each project, the learning curve of how to calibrate the maintenance is quite high,’ says Rocek.
The engines are started by means of compressed air at 30 bar (g), which is provided by a compressor station for all four units. The gas engines are to be operated in cogeneration mode and will primarily serve the electric grid. In standby mode, conditions for reliable engine start and startup to rated power within the prescribed time must be fulfilled. These conditions are primarily limited by ensuring fast engine startup in the performance of MZ5 services. Thus, in standby mode, the engine jacket water is preheated to 70°C and the temperature in the lubricating oil circuit is maintained at 50°C. Moreover, the prescribed temperature and air exchange within the engine space is controlled by ventilation and air conditioning. Last but not least, the engine exhaust gases must be regularly ventilated. Conversely, during operation of the combustion engine units, a certain amount of heat is radiated into the surroundings. This amount of heat is removed from the engine space so that the room temperature does not exceed 45°C to 50°C. Turbocharger inlet air temperature (each engine has two parallel turbochargers delivering combustion air into the engine – one for each row of pistons) must be kept between 50°C and 35°C. Temperature in the vicinity of the generator must not exceed 40°C.
|Key parameters of the gas generator unit|
The machine room containing the gas engines, the 10.5 kV substation and the oil management are equipped with firefighting equipment (SHZ) using low-pressure water mist. This water mist, in the form of small droplets, is generated from water and inert gas. The extinguishing capability of this mist is based on several mechanisms such as rapid cooling, oxygen concentration reduction, absorption of radiation heat and prevention of imperfect combustion by means of volumetric extinguishing. The water mist’s maximum efficiency depends on the size of water droplets generated by mist nozzles. The finer the droplets, the greater the area of heat exchange and thus the higher the cooling effect. The droplet size does not exceed 50 µm. From one litre of water there are 15 billion droplets generated, with a specific surface of 100 m2. Rapid generation of this mist contributes to oxygen reduction within the fire area. This system replaces the originally planned SHZ system based on CO2.
Each gas engine generator unit is equipped with its own gas line for final pressure control at the combustion chamber inlet to 4.2 bar (g), which is supplied by Roll-Royce, and with a thermal module. This line is used for temperature control of the individual engine circuits, heat removal (oil cooling, jacket water cooling), and also provides additional support functions such as electrical preheating of jacket water/lubricating oil, oil filtration, emergency cooling, expansion system, etc. This is a separate module with its own base frame where individual elements and equipment are placed and interconnected – such as pumps, heat exchangers, valves, filters, expansion vessels, preheaters, etc. Apart from the above systems – such as the starting and control air system, natural gas supply system, ventilation system for the engine space, engine cooling and combustion air, there are other support systems such as lubricating oil management and urea management (which are necessary for flue gas scrubbing) and removal of the gas engines’ heat.
The electric power output of the gas engine generator sets is conducted through power cables into the newly constructed 10.5 kV substation, which is an ABB control cabinet equipped with vacuum switches with two busbar systems. From this substation, power from the generator sets is conducted through power cable bundles (six cables per phase) to two triple-wound transformers of 110/10.5/6.3 kV, 65/65/25 MW. From these transformers, the heating plant’s electric power is conducted into the existing 110 kV substation. The on-site power demand is met from these transformers with 6.3 kV winding. The 46.5 MW power of the existing condensing extraction turbine (TG3) is currently conducted at a 10.5 kV voltage level (encapsulated wires) to one of these transformers. Following refurbishing of the steam turbine, the TG3 power (approximately 20 MW) will also be conducted by means of power cables into the 10.5 kV substation, while the encapsulated wire outlets will be removed. The steam and heat energy are conducted through the renewed steam pipes from boilers and steam generators and from TG3 regulated power output, or using the new RS/RCHS systems at 20/10/2 bars, possibly through the hot water heat exchanger station heated by steam at two bars.
|The engines were selected because of their rated efficiency, startup speed and production flexibility
Combustion steam generator
The heat from engine cooling and flue gas is used to generate steam in combustion steam generators (Heat Recovery Steam Generators – HRSG) and also for preheating hot water. The HRSG is designed as a steam generator with natural circulation. It consists of several main sub-assemblies such as drum, steam generator block which contains an outlet superheater, inlet preheater with injection temperature control of superheated steam, evaporator bundle and economizer. These are installed on a common structure and interconnected through flue gas sheet-metal ducts. Due to high flue gas temperature, the superheater and evaporator bundle are equipped with internal insulation.
Steam generator parameters
In order to increase the overall circulation efficiency, flue gas exiting the steam generator is cooled down by heating the circulating water. Heat transfer is realised through a tubular heat exchanger, which removes heat from the flue gas and thus cools it from approximately 189°C down to 108°C. The circulating water is heated to 92°C and, ideally, is used as return water in the new hot water station. There are four steam generators installed in the HRSG building; each is individually connected with one gas engine. The steam generators were supplied by Invelt Service. Flue gases from each HRSG (possibly using bypass directly from a FGT catalytic reactor) are fed into a separate 30 metre-high chimney. The chimneys are self-supporting double-walled structures with built-in mufflers.
Flue gas scrubbing
Exhaust gases from the gas engines are scrubbed before they enter the combustion steam generators or chimneys so that the NOx and CO concentrations are in compliance with required limits. The gas engine flue gas technology is based on selective catalytic reduction (SCR) – the most effective and proven NOx reduction process. The harmful nitrogen oxides are reduced to harmless nitrogen and water using urea and the SINOx catalyst. The system is installed in the exhaust duct, downstream of each engine, in the direction of flow. It consists of static mixers, catalytic reactor, catalyst and common control unit based on either an open or closed loop, and also the common Quattro dosing units. The control unit ensures optimal and safe operation and controls the injection of the educing agent depending on actual engine performance. The basis of the SCR catalyst is a ceramic honeycomb. The carrier material is TiO2, and the active elements are V2O5 and WO3.
For purposes of CO reduction, part of the reactor is filled with OXI catalysts (a metallic version using modern platinum technology). The reactor consists of a sheet-metal reinforced cladding in which individual catalyst modules are stacked. It is designed in a horizontal arrangement to allow manual insertion and removal of the catalyst from the side. It consists of a single layer of SCR and OXI inserts and a single empty layer for each type, for possible subsequent insertion. Scrubbed flue gases are conducted into the HRSG steam generators in order to use the residual flue gas heat to generate steam.
Part of the process
During the whole development phase Rolls-Royce was part of the process. ‘After the engines arrived at the site in February 2014, we commissioned them. Our engines had to pass a series of certification tests to enable the customer to export the power to the grid. And they did it successfully,’ says Rocek. Since January 2015 the plant has met the requirements of the contract C-Energy signed with the grid operator. A service agreement between C-Energy and Rolls-Royce was concluded as well, covering scheduled maintenance, spare parts and technical support.
Nina Felicitas Kunzi is External Communication Advisor, and Tomas Rocek is Head of Customer Business CEE & Russia, at Rolls-Royce Power Systems – Bergen Engines