Under its Master Plan for the power sector, the Vietnamese government is promoting the construction of new coal fired power plants. The first of these will be the Cao Ngan CFB plant, which is also one of the first coal fired plants in Vietnam to be developed as an IPP project.
Frank Kluger, Joachim Seeber, DinhCuong Tran, Alstom
To meet the rapidly increasing demand for electric power while utilizing mainly domestic energy resources, the Vietnamese government is promoting the construction of new clean, coal fired power plants. Apart from hydropower and combined cycle plants, more than ten coal fired power plants are planned with a combined total capacity of 2100 MW. These projects are outlined in the Master Plan, published by the Ministry of Industry in Vietnam in 2001. Cao Ngan is one of the first coal fired plants going into operation during the first step of the Master Plan (2001-2005).
The Cao Ngan project is also one of the first coal fired plants in Vietnam to be developed as an IPP project. It has been developed and is owned by Vietnam National Coal Corporation (Vinacoal). The project has been approved as a reference project to promote the use of CFB as a clean coal technology in Vietnam.
The majority of Vietnam’s coal fields are located in the northern part of the country in the Quang Ninh province and consist of anthracite, semi-anthracite and lean coal, i.e. fuels with low volatile matter content. The fuels are characterized by low reactivity and moderate ash and sulphur contents. The Cao Ngan coal, which comes from the Thai Nguyen province, shows the highest sulphur values and can be classified as semi-anthracite.
To select the most suitable combustion technology for the Cao Ngan project, the following main objectives were considered:
- Reliability of steam supply and power generation
- Complying with strict environmental regulations – 300 mg/Nm3 for SO2
- Fuel character and fuel flexibility including the ability to combust low reactive fuels
- Operation mode, e.g. base load operation between 40 per cent and 100 per cent
- High efficiency
- Balanced investment and operation costs.
The feasibility study for the Cao Ngan project led Vinacoal to select advanced CFB technology. Due to special fuel characteristics, the use of most efficient cyclone separators was a must for such conditions, as it allows wider ranges for fuel and limestone preparation than for systems with less efficient cyclones.
The Cao Ngan power station will be located 70 km north of Hanoi. Vinacoal will supply the coal from the two nearby mines Nui Hong and Khanh Hoa. The electricity is sold to EVN. Financing of the plant is performed by the Chinese export bank, based on a long-term loan. Vinacoal awarded the contract for the turnkey supply of the complete plant to HPE of China. The notice to proceed was given in November 2002 and the power plant is scheduled to be commissioned in autumn 2004. Alstom is supplying the two steam generators including the solids handling systems inside the boiler scope.
Figure 1. The Cao Ngan power station is located 70 km north of Hanoi in Vietnam
In accordance with contractual requirements and to check the combustion behaviour and the emission characteristics of the low volatile, high sulphur coal, combustion tests were carried out by Alstom at the University of Bochum, Germany, in a pilot CFB combustor. In the two week test campaign a mixture of the two different coals was burned at different temperature levels from 830-890°C, varying also the limestone feed rate for two types of Vietnamese limestone. The CO, SO2 and NOx emissions as well as the temperature and pressure profile in the combustor were measured and recorded. Furthermore experimental analysis determined the ignition characteristics of the fuel under CFB conditions and the part load behaviour.
The Cao Ngan plant
Each of the two CFB boilers has a maximum capacity of 237.6 t/h, supplying steam at 538°C and 98 bar to the two steam turbines. The maximum generator output is 64 MW resulting in a 55 MW net output to the grid. The boiler consists of a fully suspended furnace and backpass, designed as evaporator surface. One refractory lined cyclone is used to separate the circulating ash from the flue gases and send them back to the lower furnace. The backpass is also fully suspended but fabricated as steel casing without cooling.
Fuel feed system: Properly crushed fuel is extracted from one fuel silo per boiler at variable rates by two chain conveyors. It is then transported via two inclined chain feeders to the two coal injection chutes downstream of the seal pot. From there it enters the lower furnace together with the hot ashes from the cyclone.
Limestone injection system: The raw limestone is unloaded by trucks and prepared on site by three preparation lines consisting of vibrating three-tube-mills. It is then transported to the limestone day silos from where the limestone is pneumatically transported and injected together with the fuel into the lower furnace. In total four limestone injection points are foreseen. The limestone feed rate is controlled via two rotary feeders. The feed rate is determined by the boiler load and SO2 emissions at the stack outlet.
Circulating Fluidized Bed Furnace: The furnace consists of gas-tight waterwalls that are lined with refractory to a height of about 6 m above the grate. An uncooled primary air plenum is provided below the furnace, and feeds primary air to the fluidizing grate.
Figure 2. Cao Ngan steam generator – side view
Fluidizing nozzles are located in the floor of the furnace. They are heavy duty cast-type bubble cap nozzles, safely avoiding back-sifting of the hot ashes into the primary air box. Openings are provided in the refractory lined lower furnace hopper for solids return from the seal pots including fuel and limestone feed, for secondary air injection, for start-up burners, for the ash cooler vent and for instrumentation. Evaporator panels, vertically arranged in the upper furnace rear wall area opposite from the cyclone inlet openings, perform evaporator duty. In the upper furnace superheater surfaces are arranged as horizontal platen superheaters. They are required to keep the furnace temperature at the desired level and to get a good balance between radiation heat transfer and convective heat transfer in the furnace. The panels are manufactured from “double omega tubes” thus resulting in a very smooth surface of the complete panel. By this type of superheater technology, erosion on these panels is safely avoided.
Solids recycle system: Flue gases with entrained solids enter the high efficiency recycle cyclone, and the collected solids flow downward into a standpipe, ending in refractory lined, air fluidized siphon seal pots. The siphon seal pot arrangement splits the ashes into two equal streams and returns the solids through two inclined, refractory lined ducts to the lower furnace section. Onto these returning ashes the coal and the limestone are fed because they are then equally distributed within the furnace.
Convective backpass: Flue gas enters the convective backpass after exiting from the recycle cyclones, which minimizes the solids carry-over to the convective section. Heat is transferred from the flue gas to the steam and water circuits, which are made up of two superheater stages on top of the furnace and two economizer bundles in the backpass. Below the economizer bundles the tubular air heater is arranged, consisting of separate compartments for primary air and secondary air.
Air and gas system: The combustion air is supplied to the CFB furnace in two main streams: primary air and secondary air. Primary air is introduced through a plenum and grate assembly located at the bottom of the lower refractory furnace section. The secondary air is injected into the lower furnace through various ports in the front and rear walls. Fluidizing air for siphon seals is provided by the fluidizing air blowers and the air required for fluidization of the ash cooler is supplied by an ash cooler blower. The gas flow through the system is generated by the induced draft fan, which is located downstream of the electrostatic precipitator. One forced draft fan, one primary air fan and one induced draft fan are supplied. Two fluidizing air blowers, including a spare blower are provided and one ash cooler blower for each unit.
Burner system: The CFB burner start-up system is composed of oil fired start-up burners and oil fired lances which are used for preheating the unit until coal may be introduced. The two jet-type oil burners and two oil lances are installed in the lower furnace sidewalls.
Air pollution control system: Due to the stringent emissions limits, NOx is controlled by the furnace air staging and adequate furnace temperatures. SO2 control is achieved to a certain extent by the inherent desulphurization due to the lime in the fuel ashes, but also by limestone injection in the furnace.
Bottom ash cooling system: Coarse bottom ashes are drained from the lower furnace via a motor controlled “spiess” valve. The cooling of the ashes to 150°C is performed in an air fluidized bed ash cooler, containing water cooled tube bundles.
Ash re-injection system: A bed ash screening and re-injection system is provided for each unit. The re-injection of fine screened bottom ashes takes advantage of unused sorbent present in the ash. Also bed ash can be injected to provide the furnace with a bed for start-up or to reload the furnace when the bed pressure needs to be raised.
High efficiency cyclones: The separation efficiency is a crucial parameter for the proper functioning of the whole CFB system because the cyclone efficiency has a major impact on carbon burnout, limestone consumption and temperature profile. These factors are of importance for low reactive fuels as the better the cyclone efficiency the longer the char particles are kept in the furnace and the less furnace inventory is lost via the cyclone.
The increase in cyclone efficiency enhances the solid circulation rate thus ensuring a constantly high heat transfer in the furnace. The even heat transfer in the furnace is an important factor to avoid excessive temperatures in the convective heating surfaces downstream of the cyclone. Thus high flue gas temperatures at the inlet of the second pass, causing serious problems with slagging and excessive metal temperatures, are safely avoided.
The latest cyclone modification includes an optimized arrangement and shape of the cyclone inlet duct and the advanced vortex finder design. The positive results on the operating behaviour have been proven in a number of CFB plants: the particle size of the inventory became finer, reducing the d50 from 180 µm to less than 100 µm. The benefit of the smaller particle inventory was an improved carbon burnout, less limestone usage and less erosion inside the furnace due to smaller ash particles in the furnace. It is known that conventional cyclones or alternative separation systems cannot achieve such a high degree of separation and lack the high circulation of ashes that is required and thus they are not so flexible with respect to low volatile fuels or coal slurry.
Figure 3. High efficient cyclone separator
The design of the Cao Ngan plant is based on Alstom’s approach to standardize and modularize CFB plants. One main feature of CFB boilers is the constant furnace plane area heat release rate being independent from the plant size. This phenomenon is ensured by the fact that the fluidizing velocity in the furnace is kept constant with the increasing boiler capacity. In addition, the required residence time in the furnace and thus the height of the furnace does not depend on the plant size. These two facts enable a linear adjustment of the furnace dimensions to the plant capacity by varying the furnace width and keeping the furnace depth and height constant.
The same principle can be applied for the convective superheater surface arranged above the furnace in the cross pass where the height and depth of the cross pass are constant. This concept allows the main components of the steam generator to be standardized.
The evaporator side walls as well as the superheater coils and the economizer coils are very similar for all plant capacities and require only little adjustment for a new contract. The steam generator steel structure can also be standardized to a certain extent due to its constant height.
The linear adjustment of the furnace cross section, however, does not result in a linear increase of the evaporator surfaces in the furnace. Furthermore it is to be considered that an arrangement of superheaters in the furnace area is necessary due to heat balance reasons. If platen surfaces are used for the superheaters in a configuration, the superheater surface is automatically adjusted with increasing plant size by the proportional width enlargement.
The final adjustment is made through additional vertical evaporator surfaces, so-called wing walls, as well as from a certain plant size on, by an evaporator partition wall.
Figure 4. Emissions from combustion tests in the test rig
An advantage of standardization and modularization is the considerable shortening of the plant’s construction time due to pre-engineering. It also means that as the boiler main components are not designed for a specific fuel, a wide range of fuels can be combusted.
Standardization also entails the use of a modular system, so that individual components or modules can be combined, interchanged and adapted in number to suit the design needed. The following techniques are applied to the standardization of components for a CFB plant:
- Variation of the number of modules with a constant module size/type
- Variation of the module size/type with a constant number of modules
- Use of parametric design to allow rapid adjustment of modules.
This means that an existing 3D-model can be used that contains all the parametric configurations including direct links to detail design drawing. When the main dimensions and other boundary conditions have been adjusted in the model, all assembly drawings and also all the detailed drawings can be produced in a short time frame. The parametric design also ensures that fewer clashes between adjacent components occur, as the parametric modules can be loaded into a common 3D-model. By the use of such modern design tools as parametric design, it is ensured that all required design flexibility is kept to adjust the plant design to the relevant boundary conditions, as fuel type, boiler size and customer preferences.