Wolfgang Henselak, Jürgen Heufer, Siemens Power Generation Group (KWU), Erlangen, Germany
Throughout the 1990s, China expanded its generating capacity by some 20 GW per annum in order to meet the demand for power from its burgeoning economy. Coal fired steam power plants account for the major portion of this additional capacity.
One such power plant has been developed by the Shandong Rizhao Power Company (SRPC), which erected a coal fired dual-unit plant with a rated output of 2 x 350 MW at Rizhao in the province of Shandong. The contract to supply the boiler and turbine island was won by a consortium consisting of Siemens Power Generation Group KWU of Germany and Foster Wheeler of Spain. Siemens Project Ventures (SPV) is also involved in the project on an equity capital basis.
Construction of the 2 x 350 MW dual-unit plant at Rizhao started in September 1996. Both units have now commenced commercial operation. Construction was performed by the customer with personnel from the Siemens KWU-Foster Wheeler consortium available as consultants.
Low specific investment costs through the installation of a compact steam turbine, low operating costs thanks to easy-to-service components and plant design, and low-cost coal and high plant availabilities have all helped to make a substantial reduction in electricity generating costs at Rizhao.
Unusually for a coastal location previously a greenfield site coal is delivered to the plant by rail. Intermittent high air humidity in the Yellow Sea coastal region led to a specific requirement on the part of the customer that the switchgear be installed in a permanent building instead of in the customary outdoor switchyard.
The general arrangement of the plant includes a common turbine building for both turbine generators, an electrical equipment annex (switchgear building) located between the turbine building and bunker bay, and the two outdoor-type boilers. Two electrostatic precipitators are connected to the flue gas side of both boilers. On exit from the electrostatic precipitators the flue gases are discharged to a common stack.
The steam turbine generators are installed in the turbine building (main bay) with the boiler feed pumps (side bay). The turbine building has a partial cellar underneath the condenser (main bay) to accommodate the circulating water inlet and outlet pipes, the main condensate pumps, the drain system for the water/steam cycle and turbine systems, and the various components of the circulating water system. Three horizontal high pressure (HP) feed heaters are installed at the head end of the HP turbine and transverse to the turbine axis in the main bay.
The switchgear building, designed as an electrical equipment annex, houses electrical and instrumentation and control (I&C) components such as switchgear, batteries and I&C cabinets. The dual-unit control room is also located here at the same elevation as the turbine operating floor. Access to the control room is from the turbine building and from the switchgear building.
The subcritical two-pass drum-type boiler with single reheat operates by natural circulation. The combustion chamber is maintained at a slight vacuum (balanced draft) by forced-draft and induced-draft fan systems and is equipped with a dry ash removal system. The boiler is approximately 63 m high at the top of the structural steelwork.
The boilers were supplied by Foster Wheeler of Spain; both have front loading furnaces with 16 pulverized-coal burners arranged in four burner tiers on the bunker bay side of the boiler. The burners are supplied with pulverized coal from four bowl mill crushers. Each coal mill has its own coal bunker with sufficient storage capacity to keep the boiler in full-load operation for 12 hours and a coal feeder system.
Two rotary air heaters (trisector air heaters) and two steam coil air heaters are installed on the air/flue gas side. The forced-draft, induced-draft and primary-air fans are designed as two 50 per cent load units.
Main steam temperature control is done by means of a two-stage injection system which injects boiler feedwater from a kicker stage of the boiler feed pumps between the three superheater stages. The reheater outlet temperature is controlled by means of flue gas side dampers. A feedwater injection system installed upstream of the reheater and supplied from an intermediate extraction point on the boiler feed pumps is provided for emergency temperature control functions.
A turbine bypass with capacity for approximately 35 per cent of the full main steam mass flow is available for starting the boiler when the turbine is not ready for operation.
Key components of the dual-unit plant are the two turbine-generators with a two-casing condensing turbine. The HP and IP turbines are of single-flow design and are installed in a common casing. The low pressure (LP) turbine is of double-flow construction with an exhaust cross section of 2 x 10 m2. The steam turbine drives the two-pole generator together with brushless exciter. The rotor and stator have direct hydrogen cooling.
The steam turbine was designed with a control stage in response to a specific requirement of the customer. Main steam enters the HP turbine through four control valves with two stop valves. The IP turbine has two combined stop and control valves. Integral steam strainers in the stop valves protect the turbine from the ingress of foreign matter.
A standard lube oil system for the turbine has been supplemented by various components which are specific to China, such as storage tanks for clean and dirty oil, and emergency discharge tanks.
The two-way box-type condenser has split water boxes and titanium tubing. The heat exchanger tubes are roller-expanded and welded into the tubesheets. The cooling water system is open. The plant is cooled with seawater supplied by two 50 per cent main circulating water pumps.
Two 100 per cent vacuum pumps (liquid-ring pumps) are provided for extracting air from the steam/condensate side of the condenser and another two for the circulating water system. In addition, an on-load sponge rubber ball tube cleaning system is installed to maintain the heat transfer process in the condenser.
The duplex-type feedwater heater for the main condensate heating system is installed in the neck of the condenser. The exhaust steam from both boiler feed pump turbines is charged to the turbine condenser.
Water and steam cycle
The water/steam cycle has an eight-stage main condensate/feedwater heating system, redundantly configured 3 x 50 per cent boiler feed pumps and 2 x 100 per cent main condensate pumps as well as single-train steam piping for the main steam and hot and cold reheat systems.
The two operational boiler feed pumps are driven by an auxiliary turbine. The third pump in the group is an electric-powered start-up pump of the same size with a hydraulic variable-speed coupling. Exhaust steam from the auxiliary turbines is conducted straight to the turbine condenser and is not condensed in separate auxiliary condensers as is common practice in many other plants.
Figure 2. General arrangement of Rizhao power plant, Shandong province, China
The auxiliary turbines were built at Siemens’ Görlitz manufacturing plant and have a common steam admission chest for extraction steam and cold reheat steam. Each pump unit consists of a booster pump and a main pump. The main pumps are barrel pumps. The main steam temperature is controlled by increasing injection feedwater pressure via a kicker stage of the feedwater pumps to a pressure higher than that needed to supply the boiler drum. Injection water for the reheater is tapped from intermediate extraction points.
The main condensate pumps are designed as electric motor driven vertical can-type pumps.
The entire feedwater heating system is of single-train configuration consisting of a duplex-type heater installed in the neck of the condenser, horizontal LP feed heaters, a feedwater tank with externally mounted deaerator and three horizontal HP feed heaters. The feedwater-side HP feed heater bypasses are self-medium operated switch-over valves. To protect the turbine, all extraction steam lines are fitted with actuated and free-swinging dampers in accordance with VGB specification or in-house standards. Motor-operated isolation valves are installed upstream of the feed heaters. The extraction lines in the condenser do not have any valves fitted.
The main steam and hot reheat and cold reheat steam systems are all of a single-train design. The state-of-the-art material X10CrMoVNb9 (P91) was used for the main steam piping.
Figure 3. Installation of the 350 MW steam turbine in Unit 2 during the summer of 1999
An auxiliary boiler system has been provided for initial start-up during commissioning and in the event that both units are shut down. The two units normally supply each other with auxiliary steam for the start-up process. Feedwater is initially supplied by the electric motor-driven start-up pump. The process of preheating the steam lines with the turbine stop valves open begins as soon as the boiler is producing a sufficiently superheated steam flow equivalent to approximately ten per cent of the full load flow. The warm-up valves and the drains are opened during this process. Pressure is allowed to build up in the steam system by adjusting the boiler load, and the process is controlled via the two-stage main steam bypass station.
Once the appropriate steam parameters have been reached the turbine is warmed up before being run up to rated speed. The bypass station starts to close as steam is admitted to the turbine. Continued loading of the turbine to rated load and continued heating of the machine takes place by increasing boiler load with the pressure setpoint of the bypass station set to a high value.
The plant is normally operated in modified sliding-pressure mode, but constant-pressure operation is also envisaged. With modified sliding-pressure operation the plant operates between 50 per cent and 90 per cent load at variable pressure and between 90 per cent and 100 per cent load at constant pressure.
Unit control follows the power control concept with the turbine controller responsible for controlling the electrical generator output. The boiler load controller has the task of controlling main steam pressure during this process. Failure or non-availability of redundantly configured components such as boiler fans, coal mills or feedwater pumps is taken into account by appropriate adjustment of load capability factor at the unit coordination level by modification of settings or restriction on plant output.
The boiler is shut down by tripping the fuel supply (master fuel trip) in case of turbine trip or if the main circuit breaker is opened.
The automation system of the Rizhao power plant is the Teleperm XP process control system. Teleperm XP has been installed in about 300 plants around the world, including Fuzhou, Yangcheng and Hanfeng in China.
An I&C system overview of the Rizhao plant is shown in Figure 4. The diagram shows the configuration for Unit 1. Unit 2 has a similar I&C configuration. Plant equipment common to both units is assigned to Unit 1, but can also be operated from Unit 2. The automation system has fail-safe design features for boiler protection, turbine protection and burner control. Turbine control functions are handled by ‘Simadyn’ processor modules for fast-response control.
The customer specified the installation of a hardwired back up panel for a small range of defined plant sections. Seven operator terminals (VDU workstations) have been provided for plant operation and monitoring functions; these are all on the same hierarchical level. Two operators are generally sufficient for normal operation. During plant start-up the terminals are manned by turbine and boiler operators and other personnel if necessary.
The central control room for the dual-unit plant is housed in the electrical equipment annex.
Construction of the Rizhao coal-fired power plant marks the beginning of a whole series of orders for similar plants in the Chinese power generation market. While the plant concept and project management model of Rizhao are still heavily influenced by the sort of boundary conditions typically encountered in the mid-1990s, further advances in all aspects of large power plant construction have since led to fundamental changes.
For instance, a new integrated IT environment was available to assist with the planning of the Fuzhou coal-fired plant, where engineering started later than for the Rizhao plant. Plant and design concepts for other contracts within China and elsewhere have also been implemented on the basis of pre-engineered reference power plants.
State-of-the-art power plant concepts featuring high operating efficiencies, low fuel costs and therfore low electricity generating costs have now also started to make their mark on the Chinese market For instance, the Wai Gao Qiao II plant in Shanghai, at 2 x 900 MW, the largest hard coal fired plant in China is designed for supercritical steam parameters and is equipped with a once-through boiler in tower construction.