Compact CFBs scaled up for Tha Toom

20 July 1998

Thailand's National Power Supply Co is building a circulating fluidized bed cogeneration plant at Tha Toom in Prachinburi province, comprising two 150 MWe units. Located within Industrial Park 304, north-east of Bangkok, it will sell 60 per cent of its power output to EGAT under Thailand's small power producer programme. The process steam and the remainder of the power will be sold to local industrial customers and to the nearby Advance Agro pulp and paper mill. In March, CMS Energy Corporation of the USA acquired a 50 per cent stake in the project, which had slowed as a result of the economic downturn in the region.

Rapid growth in Thailand led to an average increase in power consumption of around 10 per cent per year during the early 1990s, although it fell last year as a result of the economic downturn throughout the region. At the end of 1997, the country had total installed capacity of 16 980 MW, an increase of 8 per cent compared with 1996. Electricity Generating Authority of Thailand (EGAT) operated 14 687 MW, or 87 per cent of the total, with private power producers operating the remaining 2293 MW. Of that 2293 MW, 2056 MW was owned by Electricity Generating Public Co (EGCO) and 224 MW by small power producers (SPPs), with the final 13 MW accounted for by the Department of Energy Development and Promotion's Kiridhan hydropower plant.

EGAT's 1997 annual report indicated that it was working on 16 new power projects of its own, with a combined capacity of 7381 MW. During the year, 1108 MW of this capacity entered service. In addition, power purchase agreements were signed with seven independent power producers (IPPs) to buy an aggregate power capacity of 5943 MW from 1999 onwards, and with 45 SPPs for the purchase of 1810 MW.

The Tha Toom project was launched in 1995 by Thai Power Supply Co, which is a subsidiary of Soon Hua Seng Group, a diversified trading and services company whose major interests are in the agricultural and pulp and paper sectors. Initially, TPS studied the feasibility of building an IPP power plant at the Tha Toom site, adjacent to Soon Hua Seng's Advance Agro pulp and paper mill, but it soon became obvious that the decision-making process associated with an IPP project would take too long and that there might be an unwieldy number of bidders. Development therefore focused on a smaller cogeneration power plant producing electricity, as well as steam for the mill and for local industrial customers.

Cogeneration was made more attractive by regulatory changes in the Thai electricity market. An industrial power plant can sell electricity to the national grid at a guaranteed price under long term contracts, according to conditions set out in the Small Power Producer programme, which was introduced to help meet the country's growing demand for electricity by attracting more operators into the market. A further objective was to increase decentralization of power generation, enabling better utilization of biofuels from the forestry and agricultural sectors.

In September 1995 Ekono Energy of Finland was given the job of preparing a viability study and developing a power plant concept that would match the proposed plant to the requirements of the industrial complex and would comply with the SPP regulations. Shortly afterwards, preparation of the environmental impact assessment (EIA) was started by Jaakko Poyry, Thailand.

The plant concept is based upon two identical units, each comprising a Foster Wheeler Compact CFB (circulating fluidized bed) boiler producing steam to drive an Alstom Energie extraction-condensing steam turbine. Each unit will produce 150 MWe of electricity, 50 kg/s of steam at 6 bar and 6 kg/s of steam at 13 bar.

Tender documents for the main equipment were prepared after the plant concept had been established, and contracts for the boilers, steam turbines and generators were signed in June 1996. Site excavation started in August 1996 and civil works in January 1997.

The 1997 economic problems prompted EGAT and Soon Hua Seng to review their power capacity development plans. This led to some doubts over the future of Tha Toom and gave CMS the opportunity to embark upon its second venture in Thailand, the first being the operation and maintenance of a 170 MW gas-fired cogeneration plant owned by Amata-EGCO Power at the Bang Pakong II Industrial Park near Bangkok. That deal was put in place in July 1997.

For an investment of $60 million in equity, CMS Energy's independent power unit CMS Generation Co acquired half ownership of the Tha Toom project from Soon Hua Seng via a shareholding in National Power Supply, the dedicated company that was set up by TPS to develop the project. When the deal was announced in March, Rodney Boulanger, CMS Generation's president, commented, "This state of the art plant is a long term asset that allows us to expand our presence in Thailand. These generating units will provide power to fuel Thailand's growth for many years to come." David Weaver, vice president and managing director of the CMS regional office in Singapore, added, "Thailand's economy is recovering and we wish to support that recovery. Thailand's current energy market offers an attractive opportunity to make a solid, long-term investment such as this."

Commissioning of the Tha Toom plant is scheduled to start in August and the first unit will enter commercial operation at the end of this year, followed by the second unit three months later.

Plant location

The site is located in 304 Industrial Park, 140 km north-east of Bangkok, 15 km from the town of Kabinburi. Inside the park, the plant site is 400 m north of the Advance Agro paper mill, so only short connections were needed to link it to the steam consumers and to the 115 kV grid. The plant site itself has an area of around 20 ha, making it large enough for future extension with two additional units to the north. In addition to the 20 ha plant site, there is a 5 ha coal storage area 500 m to the east, connected by conveyors to the plant's fuel silos.

Plant capacity

Plant capacity was determined by two main criteria. First, it should be sufficient to cover anticipated steam and power demand from the nearby offtakers, 304 Industrial Park and Advance Agro's pulp and paper mill. Second, NPS wanted to operate the plant under the SPP scheme, whereby a certain amount of electricity would be sold to the national grid at a guaranteed price. In addition, it was a requirement that 10 per cent of the combined total electricity and heat produced should be in the form of heat energy.

The SPP contract capacity was set to 2 x 90 MWe. Anticipated future capacity needs in 304 Industrial Park and in Advance Agro's pulp and paper mill amount to a total of around 120 MWe. High availability requirements therefore led to a plant design comprising two identical and independent units, each of 150 MWe capacity.

Fuel supply

Coal was selected as the main fuel because its price has been stable, and low compared with fuel oil. This situation is expected to continue and there are several potential coal suppliers in the south-east Asian region.

In addition, the plant can co-fire a variety of biofuels, including wood residues and rice husk. To obtain the lowest fuel price and generate power as profitably as possible, a requirement was that the plant should be able to burn a wide range of fuel qualities. This requirement formed an essential design pre-requisite for the combustion system, leading to the selection of bituminous coal, anthracite and biofuels as the design fuels for the plant.

The coal transport chain starts with 100 000 t Cape-size vessels for ocean transport to the port of Ko Si Chang, where the coal is unloaded either to a floating terminal or directly onto barges for onward shipment to Bang Pakong terminal. There, it is loaded onto trucks and transported by road the final 100 km to the plant.

Compact CFB concept

According to project manager Ari Asikainen of Ekono Energy, it was obvious at an early stage of the project that circulating fluidized bed combustion (CFBC) was the only combustion method capable of meeting the fuel flexibility requirements. The client had considerable experience of bubbling-type boilers, so was familiar with the technology. Ekono Energy had been involved as a designer and project manager in several CFB projects and was able to specify the boiler and steam process so that experience from other projects would be directly applicable.

After the comprehensive technical and commercial evaluation, the contract was signed with Foster Wheeler Energia of Finland. FWE's latest Compact CFB system was selected because of its water cooled solid separators integrated into the upper part of the furnace. This resulted in more straightforward construction.

The Compact CFB concept is a second-generation circulating fluidized bed boiler design originally developed by Ahlstrom of Finland before the company was purchased by Foster Wheeler. The main difference between this and earlier designs is that the round cyclone of the traditional CFB boiler is replaced by a solid separator. This has a casing of rectangular cross section, containing "vortex finders" in place of the earlier cyclone. The square cross section means that the unit can be joined to the furnace without expansion joints.

Fuel and bed material are fed into the lower portion of the combustion chamber in the presence of fluidizing air, which causes the fuel, ash and bed material to circulate and rise through the combustion chamber, finally entering the solid separator. The separator captures most of the circulating solids, including unburned fuel, and returns them to the combustor. This continuous circulation increases fuel residence time and results in very efficient combustion, while a relatively low combustion temperature of around 870°C and the introduction of combustion air at various levels limits the formation of NOx.

Although the Compact CFB concept is relatively new, operating experience was available for a full-size unit in Finland at the time the decision was taken to use Compact CFB boilers at Tha Toom. Even so, there was considerable discussion of possible scale-up problems because the first Compact units were much smaller than those planned for Tha Toom. The scale-up risk was reduced by using vortex finders of similar size to those fitted to smaller units but to increase the number. Existing units use one solid separator fitted with two vortex finders, and mounted on the front of the boiler. For Tha Toom, it was decided that two separators would be used per boiler, each fitted with four vortex finders. Instead of being mounted at the front, one separator is mounted on each side of the boiler.

The boilers themselves are of natural circulation type, equipped with omega-type superheaters and reheaters located in the furnaces. Main and reheat steam parameters are 161/35 bar and 542/542°C, and two 100 per cent feed water pumps supply water to each boiler.

Because of the large amount of circulating hot fluidized material, the CFBC system is a very effective method of burning various grades of coal on their own, or mixed with low grade biofuels. Low SO2 and NOx emissions are relatively easy and inexpensive to achieve. SO2 levels depend largely upon the sulphur content of the fuel, and can be reduced if necessary by injecting limestone into the bed, while low combustion temperatures mean that NOx emissions can be kept low without any costly secondary reduction measures.

A four-field electrostatic precipitator, installed before the stack, is used for fly ash removal, and the ash it collects is transported by truck to an ash yard. The common 120 m-high concrete stack has two flues, one for each unit.

Steam turbines

The steam turbine/generators and the feed water heating plant, including the feed water pumps, were supplied by Alstom Energy, of Nürnberg, Germany. The steam turbines are of the single-shaft extraction-condensing type, with a single flow low pressure module and common high pressure/intermediate pressure module.

The main steam and the reheated steam are admitted in the central part of the HP/IP module. The main steam flows along the axis of the machine towards the HP exhaust (cold reheat line) in one direction, while the reheated steam flows towards the IP exhaust in the other direction. A large labyrinth-type steam seal separates the two flows.

The main characteristic of this design is that there is only one hot point in the machine, at the point where the main and reheated steam are admitted. Heat resistant materials are used in this area but are not required in other parts of the turbine, where the materials are subjected to lower temperatures. From the IP exhaust, a crossover pipe leads the steam to the LP section, where there is one flow, in the direction of the generator towards the LP exhaust.

The length of the last stage blade is 885 mm.

The steam/water cycle of the power plant is optimized to minimize fuel consumption and to give the highest possible power yield. The calculations required to achieve this were carried out using Ekono Energy's Tursim software.

Tursim is a modelling program designed for steady state simulation of power plants. It is based on physical process quantities and component models, and has a graphical user interface. The program comprises a series of modules that enable the user to form a flow sheet on the screen. Parameters are then entered, and the results can be read directly from the flow sheet.

The program also generates detailed text printouts showing process parameters, component-specific parameters and heat balance summaries. Transfer of data to spreadsheet programs is also possible. An additional feature is Tursim's optimization function, whereby it chooses optimum values for free variables, if the model contains degrees of freedom.

In the case of Tha Toom, there are six preheating stages in the steam cycle: two high pressure, a feed water tank and three low pressure stages. The process steam is taken from a point between the IP and LP casings, and the steam pressure is controlled by LP casing admission steam control valves. The main cooling system is based on forced draught wet cooling towers supplied by Hamon-Grimm. Design wet bulb temperature is 28.2°C. In full condensing mode, producing only electricity, the net efficiency of the plant is 37.6 per cent, which increases to 52.2 per cent in steam extraction, or combined heat and power mode.

The plant is designed so that when a unit is shut down for maintenance, the remaining one can continue to function as normal. Also, under those conditions the design enables the operating unit to satisfy all of the steam demand.

Energy balance

The power plant design is based on both units operating at their maximum capacity throughout the year. Process steam and electricity will be delivered to the Advance Agro pulp and paper mill and to the industrial park to meet customer demand, with the remainder of the electricity sold to EGAT under the Small Power Producer contract. This assumes that the electricity requirements of non-EGAT customers will be no greater than 120 MWe, as the Small Power Producer contract is for 90 MWe from each of the 150 MWe units.

Annual energy balances have been calculated using Ekono Energy's Epsilon software, a package developed specifically for combined heat and power generation calculations. Energy production has been simulated in three-hour periods, based on anticipated energy demand variation and the plant performance data.

Control system

The power plant is equipped with an on-line programmable distributed control system (DCS) supplied by ABB Industry, Singapore. It is based on ABB's Advant Controller 450, with five AC450 stations installed in separate DCS control rooms in various parts of the plant.

The AC450 units are assigned as follows:

  • two for the boilers

  • two for the turbogenerators, cooling towers and power distribution system

  • one for the demineralization plant, fuel supply and compressor station.

    The four units that control the boilers and turbogenerators are equipped with double CPUs for redundancy. If a failure occurs in the first CPU, the redundant one will be switched on automatically and an operator alarm raised. The main bus interface, an ABB Master bus, is also doubled.

    There are operator stations in the various local control rooms, with a further 10 stations – five for each of the two power plant units – in the main control room. There are also two alarm and event printers in the main control room, along with terminals for the shift engineer and for software maintenance, as well as two large 1.7 m video screens for the display of DCS graphics to large groups of people.

    The boilers are controlled by two AC450 controllers, with one dedicated to each boiler. The CPUs and their input/output cards are installed in cabinets in the Auxiliary Building DCS room, and there are double CPUs for redundancy.

    All the boiler functions are programmed into these controllers, including:

  • motor starting and stopping sequences

  • analogue control loops

  • control of remotely controlled valves

  • the boiler purge program

  • burner management programs

  • sootblowing sequences

  • sequences for fuel, sand and limestone supply

  • ash removal sequences.

    All the boiler functions can be operated using the control room displays, either manually or with automatic sequences and control loops. The boiler safety interlocking system is based on a separate Siemens Simatic 115F double failsafe programmable logic controller (PLC). This is connected to the AC450 system using a bus interface for status feedback information, and with hard wiring for all safety relevant functions.

    The two AC450 controllers assigned to the turbogenerators, cooling towers and power distribution system are located in the Control Building DCS room, and once again have double CPUs for redundancy. In the case of the turbines, the most essential control functions are programmed in the vendor's own control system, in this case a Mauell ME4012. All the other functions are programmed using the AC450s. These include:

  • turbine automatic start and stop sequences

  • condensate group controls

  • lubricant and control oil supply

  • gland steam supply

  • auxiliary analogue control loops

  • feedwater supply and monitoring of feedwater pumps

    These AC450s also handle the process steam and cooling tower control functions, as well as the power distribution system, including the 11, 22 and 115 kV substations. Most of the circuit breakers can be controlled using the DCS but those items requiring synchronization are only selected using the DCS, with the synchronization itself performed by a special synchronization panel connected to the DCS using separate inputs and outputs. All safety interlocks are hard wired in each substation's control system, giving only feedback information to DCS screens and printers in the event of an alarm or trip.

    A separate energy management system (EMS) is built on top of the automation hierarchy. It comprises a network of personal computers located in the power plant offices and connected to the DCS system bus, enabling process data to be gathered. This data, plus manually entered information such as prices, costs and heat values, enables a range of management reports to be compiled.

    Power distribution

    Each of the two Tha Toom power generating units is connected to a 190 MVA unit transformer via a 13.5 kV air insulated bus duct. Connection to the 115 kV substation is then via 115 kV cables installed in a cable gallery between the power plant and the substation, which is equipped with a double busbar system and has its own control room. Power will be distributed locally at 22 kV via 80 MVA transformers at the substation, with distribution around the pulp and paper mill via four 22 kV feeders.

    Two overhead lines, each with a capacity of 300 MVA, will carry the contracted power export of 90 MWe per unit to the Srimahapote switching station, which is 1 km from the power plant. The switching station will be connected to two existing overhead lines and will be operated by EGAT. The transfer capacity of the lines is limited, so upgrading of the existing lines or installation of new ones will be necessary to maintain redundancy in the area grid. The closest 230/115 kV substation is located 12 km from the switching station.

    Environmental performance

    It is anticipated that the plant's emissions will be well within the Thai regulatory limits. It is forecast that SO2 levels will be less than 400 ppm, compared with a limit of 450 ppm, while NOx emissions should be below 180 ppm, compared to a limit of 350 ppm. Particulates will be below the regulatory limit of 120 mg/Nm3.

    The SO2 level of 400 ppm will be achievable without any special emission control measures because the design fuels have a low sulphur content. If tighter limits are introduced in the future, it would be possible to reduce SO2 emissions to around 150 - 200 ppm by adding limestone directly to the fluidized bed. Emission monitoring will be via instrumentation within the single stack that will handle the flue gases from both boilers.

    Waste water from the power plant will be fed into the paper mill's water treatment system, so there will be no local waste water emissions at the power plant.

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