Tyres to wires
Environmental concerns have often resulted in the burning of some fairly unusual fuels. The beginning of this year saw Taiwan Cogeneration Corporation begin construction of a power plant which in addition to burning coal, will also run on sludge and crushed tyres.
Taiwan has an installed cogeneration capacity of 2500 MW, a figure which is forecasted to grow to 4500 MW i.e. some 12 per cent of electricity demand. Almost all of this cogeneration capacity is for the industrial sector with the heat being used for process steam and electricity exported to the grid. The Taiwan government does in fact have a policy to build a power plant for every industrial park or independent factory.
One company established under this policy is Taiwan Cogeneration Corporation (TCC), a company specifically set up in 1992 to develop industrial power projects. TCC, which is owned by Taipower, currently has one pulverized coal boiler and about 10 MW of diesel fuelled plant. An interesting project recently undertaken by TCC is the Kuan Tien cogeneration plant.
A $13 million contract for the project was awarded at the end of 1998, under which Foster Wheeler Corporation will design and manufacture a circulating fluidized bed boiler with a capacity of 138 MWth as well as the auxiliary equipment. The balance of plant supplier is Formosa Heavy Industries (FHI), which will also be in charge of installing equipment supplied by Foster Wheeler.
The boiler will be sited at the Kuan Tien industrial park near Tainan in the south of the country. Its main purpose is to supply process steam to TTET Union Corporation, for its food oil manufacturing plant. The boiler is being designed to supply 45-60 t of process steam to the plant, with all the electricity being sold to the utility company.
This boiler is only the third CFB unit ordered in the country. The first, also a Foster Wheeler unit, has been in operation for ten years at the Yuen Foong Yu paper manufacturing company.
The plant will be fuelled primarily by sub-bituminous coal and old car tyres, but will also be designed to burn bituminous coal, sludge, tea leaves and waste oil. The ability to burn this combination of fuels will allow the lowest cost production of power.
About 100 000 t/y of waste tyres is produced in Taiwan and treating them is a big problem. Accordingly, the government decided to provide incentives for using waste tyres as fuel for power plants. Under the programme, operators of these plants receive NT3200/t ($87/t) for tyre chips burned.
The Kuan Tien power plant project receives waste tyres from four or five existing tyre shredding plants in the country. TCC would also like to invest in a tyre shredding plant to ensure an uninterruptible supply, although to date there have been no problems with tyre chip delivery and preparation.
Crushed tyres are expected to represent about 30 per cent of the CFB`s fuel once the plant is operational. The designed fuel feed for the boiler is 3.9 kg/s sub-bituminous coal and a maximum of 1.4 kg/s of crushed tyres.
Environmental performance combined with fuel flexibility were the main reasons for choosing CFB technology. They are ideal for handling difficult fuels like waste tyres and sludge which can have high dioxin content.
In the CFB, fuel is fed into the fluidized bed via screw feeders. Combustion takes place in the bed at about 870-890 degreesC. The bed material is formed from the fuel ash, limestone and sand. Due to the large heat capacity of the bed, combustion is stable and no supporting fuels are required.
Fuel entering through feeding screws enters the bed close to the grid. This ensures good mixing of the fuel and longer residence time in the boiler. The flow of fuel is controlled according to the steam demand (pressure) signal.
There is a system of oil burners, located about two metres above the grid, for boiler startup and possibly during upset conditions. The main purpose of the burners is to heat the bed material to a temperature at which the fuel ignites so that fuel feeding can start.
The burners are normal steam atomizing oil burners complete with a burner management control and safety system. When not in use, a small quantity of air is blown through the burners to prevent accumulation of bed material in the burner.
In addition to startup burners, bed lances are located around the periphery of the combustion chamber at about 0.5 m above the grid. Their main purpose is to carry load when the main fuel is not available.
The CFB is ideal for meeting requirements for SO2 emissions. By feeding limestone into the bed, a high sulphur retention rate can be achieved. Sulphur capture is most efficient at a bed temperature of 850 degreesC. SOx emissions from the Kuan Tien plant will be less than 80 ppm (6 per cent O2).
Due to the low combustion temperature, thermal NOx formation is negligible. Staged combustion using overfire air is introduced to complete the combustion. By injecting ammonia into the combustion chamber or cyclone further reduction of NOx levels is possible. Guaranteed NOx levels at Kuan Tien are below 80 ppm (at 6 per cent O2).
CO emissions are low due to turbulent mixing in the bed and mixing in the cyclone.
Turbulent mixing in the bed and longer residence time in the boiler also minimizes hydrocarbons, CxHy, and residual unburned carbon.
The boiler operates on the circulating fluidizing bed principle. At high fluidizing velocities (about 5 m/s) part of the bed material becomes entrained and is carried through the combustion chamber with the flue gases.
Coarser entrained particles are separated in a water-cooled separator and returned via return leg to the bed. Fine fly ash is carried through the boiler convection sections and separated from the flue gases in an electrostatic precipitator (ESP) after the boiler. Separation levels of >99.5 per cent are expected to be achieved. The ESP, to be manufactured by Rothemüle, will keep dust levels to 50 mg/m3.
Boiler steam system
A high degree of heat transfer is obtained through the circulating material which is approximately proportional to load. This means the boiler has good response over a wide load range with a relatively small excess air factor.
Heat is recovered from the circulated material and flue gases in the water-cooled combustion chamber. After the water-cooled separator, the flue gases are further cooled in the superheaters, economizer and air heater areas before removal of the fly ash in the ESP.
Boiler feedwater enters the boiler via the economizer, where it is preheated before entering the steam drum. Here it is mixed with the boiler water. The boiler is a natural circulation boiler and the tubes are arranged so that as the water is evaporated to steam, it is free to rise up into the drum.
Unheated downcomer pipes take the water from the drum to the combustion chamber inlet headers. The combustion chamber wall tubes are heated by the flue gases and the water is partly evaporated to steam. Water and steam rise through these tubes back to the steam drum where the steam is separated from the water in cyclone separators and the water is returned to the circulating system.
After leaving the drum the saturated steam is led to SH1, which serves to dry and superheat the saturated steam. After the SH1 (superheater1), steam is led to SH2 which is located in the furnace of the boiler. Final superheating takes place in superheater 3, which is similar to superheater 1 and is located in the convection pass.
Superheated steam has to be cooled at higher loads so that full superheat can be obtained at lower loads. This is done by desuperheaters located in the interconnecting pipes between the superheaters. Feedwater is sprayed into the steam to cool it. Following superheater 3 the steam enters the main steam pipeline. The final steam temperature is regulated by desuperheater 2 and is kept constant over the range from 100 per cent MCR to typically 60 – 80 per cent MCR.
Steam from the boiler is fed to a condensing steam turbine. This machine is being manufactured by Fuji Electric Co. Ltd., of Japan. At the turbine inlet, the steam has a temperature of 538 degreesC and pressure of 123.6 bar. The steam turbine has a maximum extraction capacity of 100 t/hr. Typically the steam extraction is 45 t/hr at a pressure of 12 bar and 246 degrees C.
The designed maximum power output of the steam turbine is 49.6 MW. Under typical conditions the power output is expected to be 46.9 MW.
The steam turbine will be directly coupled to a generator operating at 3600 r/min. The generator will have a voltage output of 11.4 kV. This will be increased to 69 kV through the 50 MVA main transformer before being fed to the power grid. The 11.4 kV busbar is connected to a 10 MVA transformer to provide a 3.3 kV supply for high voltage motors. There is also a 3 MVA transformer to reduce the 11.4 kV to 440 V for supplying low voltage motors.
Civil works at the site is now well underway. Shipping of the first pressure parts is scheduled for December this year with pressure testing in April 2000. The installation will be completed in July 2000 and first steam fed into the turbine in September 2000. Commercial operation of the plant will begin in November 2000.
Figure 1. The Kuan Tien plant is located near Tainan