The Mawan power station is a 6 x 300 MW coal fired power station owned by Shenzhen West Power Company, a subsidiary of Shenzhen Energy Group. The plant is the site of a large and important environmental project for China – not only is it the first flue gas desulphurization (FGD) system in Guangdong Province but is also the first power plant in China to use seawater FGD.
Junior Isles, Editorial Director
China is making serious efforts to combat pollution through the reduction of acid rain emissions from its coal fired power stations. The country took an important step forward in 1996 when it selected Alstom Environmental Control Systems to supply the first flue gas desulphurization (FGD) system for the populous and energy intensive region of Guangdong Province. When it began operation in 1999, the FGD system installed at Shenzhen West Power Company’s Mawan unit 4 coal fired plant, was also a technology breakthrough for China. It was the first use of seawater flue gas desulphurization (SWFGD) in the country. The success of the project led to repeat installations on Mawan units 5 and 6, which began operation in 2002 and 2003, respectively.
China has a huge need for new generating capacity. With GDP growing at an average annual rate of seven per cent, the country has one of the world’s fastest growing economies. Population growth also ensures continued pressure on energy demand.
In 2003, China had an installed capacity of 385 GW and generated 1908 TWh. Electricity demand was rising at 15 per cent in 2003 and power shortages were experienced in many parts of the country. To combat power shortages, the State Council is implementing a policy to fast track power stations already under construction. It has stipulated, however, that accelerated construction should not be at the expense of quality.
Figure 1. The Mawan coal fired power station is the site of China’s first seawater FGD project
Equally importantly, the government is undertaking measures to reduce emissions of pollutants such as sulphur dioxide (SO2) and nitrous oxide (NOx), through improved pollution controls on power plants as well as through policies designed to increase the share of natural gas in the country’s fuel mix – especially around metropolitan areas.
While China is rich with indigenous energy sources, coal makes up the bulk – 64 per cent – of its primary energy consumption. Although a cheap fuel for China, such a reliance on coal has its drawbacks. The country’s heavy use of unwashed coals leads to large emissions of SO2 and particulate matter.
According to a report by the World Health Organization (WHO), seven of the world’s ten most polluted cities are in China. Statistics also show that China will soon overtake the USA in becoming the largest emitter of carbon dioxide (CO2) by 2015. However, while the country is concerned with environmental problems such as global warming, it is currently more concerned with specific local pollution problems.
Shenzhen West Power Company’s (SWPC) SWFGD system at the Mawan power station is a prime example of local efforts to combat pollution. SWPC is a subsidiary of Shenzhen Energy Group Co. Ltd. (SEC), a state-owned enterprise. Since its establishment in June 1991, the company has pursued a policy of “concentrating on power and developing accessory business” in order to meet the need for electricity for the economic development of the ‘Special Economic Zone’.
According to SWPC, its goal is “to be innovative in the field of power construction and enterprise management”. Indeed the utility has been successful thus far. It has brought Shenzhen – a city once stricken by severe power supply problems where shortages four days within a week were the norm – to be among the top cities in terms of consumption per capita.
Figure 2. Artist’s drawing of the Shenzhen site
Currently, SEC has a total installed capacity of 3850 MW accounting for 80 per cent of the total power capacity of Shenzhen. The main power plants include Mawan (6 x 300 MW), Shajiao B (2 x 350 MW), Yueliangwan power plant (330 MW), Nanshan (720 MW), and the Tongling power plant in Anhui Province (1 x 330 MW). As a key enterprise supported by both the Shenzhen municipal government and Guangdong provincial government, today SEC ranks first among state-owned enterprises in Shenzhen and 255th in the 500 top enterprises in China in 2003.
More notably, SEC has become a pioneer with the environmental project it undertook in cooperation with Alstom Environmental Control Systems of Norway. The SWFGD system project for unit 4 achieved breakthrough status as the first flue gas desulphurization project in Guangdong Province. These are also the first power plants in China to use SWFGD. The FGD project at Mawan has been listed in China’s “Cross Century Green Project Plan” and under the “Blue Sky Project” programme by the government of Guangdong Province and is held as a model by Chinese National Environment Protection Bureau for other SWFGD projects.
Compared to other desulphurization techniques, SWFGD has lower operating costs while demonstrating a sulphur removal efficiency in excess of 95 per cent. The installation at Mawan will reduce SO2 emissions from the plant by 7000 t/year per FGD system.
Alstom Environmental Control Systems’ SWFGD process is suitable for oil and coal fired power stations and treats flue gases containing 20-6500 ppm of SO2. The process has been recognized and approved by several environmental agencies around the world. In the working draft of the European IPPC Bureau’s Council Directive 96/61/EC on Integrated Pollution Prevention and Control, Alstom’s SWFGD is included as a BAT (Best Available Technique) and described under “126.96.36.199 Seawater Scrubbing”.
Alstom’s first plant started operation in 1968 and since then more than 30 of these units have been installed around the world.
Figure 3. Outline of Alstom’s seawater FGD process
The first power generation application was a pilot plant at an oil fired utility boiler for Guam Power Authority. The first commercial scale plant was installed in 1988 at Tata Electric’s Trombay Unit 5 in Mumbai, India. The SWFGD unit was installed downstream of a 125 MWe coal fired boiler and handled a gas flow of
445 000 Nm3/h. In Europe, SWFGD facilities have been installed at oil fired power plants operated by Unelco in Tenerife, Spain and Gran Canaria, Spain.
Most of the installations in recent years, however, have been in Asia. In addition to Mawan, other installations in Asia include the Paiton power plant in Indonesia. Here, SWFGD systems were installed on two 670 MW coal fired boilers (Paiton Units 7 and 8) in 1999. Another system was installed in 2002 in Malaysia – on three 700 MW coal fired units at the Manjung power station. The SWFGD systems at Manjung are the largest that Alstom has supplied to date. Each unit handles a gas flow of 2 611 000 Nm3/h.
Today this process removes SO2 from flue gas flows corresponding to a total capacity of 6500 MWe.
Other units are also planned for the future. In Europe, a system for Electricity Authority of Cyprus will be installed at the 130 MWe Vasilikos unit 3 oil fired power plant. This SWFGD unit, with a gas flow handling capacity of 384 000 Nm3/h, is scheduled to begin operation in 2005.
Looking further ahead, Alstom will begin installation of three units at the Tanjung Bin coal fired plant in Malaysia in 2006. When they begin operation, they will be Alstom’s largest units to date – each SWFGD system will handle 2 650 000 Nm3/h from each 700 MW boiler. Another system is also to be installed in China at Qingdao power station (300 MW) units 1 and 2.
With moderate investment cost and low operating cost, Alstom’s SWFGD is one of the most cost-effective technologies for control of SO2 emissions. The performance range is defined by the acid neutralising capacity available as seawater coolant at the power plant.
The process uses the spent seawater coolant from the condensers to absorb and neutralize SO2 from the flue gas. The SO2 in the seawater is converted by oxygen in the ambient air to sulphate ion (SO42-) before it is discharged to the sea.
The key benefits of the technology are:
- It uses only seawater and air
- No chemicals addition
- No by-product handling
- No harmful effects on sea water.
Alstom’s SWFGD system is a fully automated plant that requires no operator assistance during normal operation. The process can be fully monitored from the control room and has few pieces of rotating equipment, resulting in low power consumption and low maintenance cost.
System components exposed to corrosive media are made of corrosion resistant materials or protected by corrosion resistant lining. The chemical conditions in the absorber do not allow scale formation. These factors all contribute to low maintenance cost. Usually, the FGD plant maintenance can be carried out by the power station maintenance staff.
The process is also environmentally sound, as it recycles the sulphur released by the fuel back to its origin in the sea in its original form of sulphate ions. Design precautions are taken to carefully control the seawater discharge quality to avoid any negative effect on the marine environment. At all its installations, Alstom’s design team focuses on the quality of the discharged seawater to: keep sulphate (SO42-) concentration within natural fluctuations found in the sea; achieve acidity (pH) within natural variations; achieve a satisfactory content of dissolved oxygen – necessary for marine life; and ensure a level of unoxidized sulphite below existing standards and deemed harmless to the marine environment.
Most fly ash entering the SWFGD system is washed down by seawater in the absorber and ultimately ends up in the discharge water. A high efficiency dust filter is installed upstream of the SWFGD system to reduce the dust load on the marine environment.
Coal used at Mawan power station comes from the north of SanXi province. It is soft coal (Table 1 shows the coal composition). The coal is first transported to Qinghuangdao port by train, and then by ship to the site. On arrival it is unloaded to the coal square and the bunkers by an unloading system. It is then milled and fed to the boilers continuously.
The boilers operate at a temperature of 540°C and a pressure of 17.4 MPa. The boiler water inlet temperature is 260°C and steam flow is 1025 t/h. The steam turbines are of a reheat design and operate with a main steam temperature of 538°C and main steam pressure of 16.7 MPa. The reheated conditions are 3.3 MPa at 538°C. Steam is discharged at a pressure of 5.5 KPa. The power blocks are controlled by a distributed control system supplied by Shanghai Xinhua Control Co. Ltd.
The SWFGD process consists of two major systems, the SO2 absorption system and the seawater treatment plant (SWTP). Figure 3 shows the gas and sea water flows in one 300 MW FGD unit.
SO2 absorption system: The SO2 absorption system receives flue gas from two ID fans. An axial FGD booster fan is installed to overcome the pressure loss through the FGD system. Then the flue gas passes a gas-gas reheater (GGH) before entering the SO2 absorber. The GGH removes thermal energy from the hot, uncleaned flue gas and adds it to the cleaned flue gas from the absorber up to the specified level, prior to atmospheric release.
The absorber is a packed tower design and ensures very efficient contact between flue gas and liquid, resulting in highly efficient SO2 absorption. Depending on the required overall SO2 removal efficiency this facilitates in many cases bypass of part of the flue gas that is used for further reheat of the cleaned flue gas from the absorber.
A portion of the cooling water (CW) from the condenser is pumped to the top of the absorber. The seawater passes through the absorber in a ‘once through’ mode.
The seawater fed to the absorber also cools the flue gas before it exits the absorber, thus water condensation from the flue gas takes place inside the packed section. This eliminates the possibility of scaling or clogging in the absorbers.
Only seawater is used in the absorption process; no additional alkali or fresh water is required. The process requires a certain amount of available seawater.
Figure 4. Flow diagram of the seawater FGD system at Shenzhen
The acidic absorber effluent flows through the force of gravity from the absorber to the SWTP. The effluent is processed and mixed with a major part of the remaining spent CW and the rest of the CW is mixed with the treated CW from the SWTP to make it safe prior to final discharge to sea.
A calculated excess aeration provides the additional oxygen required to obtain an environmentally acceptable pH and dissolved oxygen before the final discharge.
The SWFGD at Shenzhen was required to achieve 90 per cent removal of SO2 since low sulphur coal is used. However, the system has a removal efficiency which is much higher than the required 90 per cent.
Installation of an SWFGD system can typically take 28-36 months from project start to hand over of the last unit, although much shorter installation times are often possible. For example, the installation at Shenzhen unit 5 took just 20 months.
Erection start of the individual absorber systems have a time lag of four months. According to Alstom, this has proven to be effective according to earlier project experience. Alstom also notes that boiler shut down periods for tie-ins has to be evaluated before signing of contract. The typical shut down period required for tie-in for each unit could be as low as 1 week.
The contract for Shenzhen unit 4 was awarded in September 1996 and the installation began operation on March 18, 1999. The second phase saw the construction of systems for units 5 and 6. This contract was awarded in January 2001 and the SWFGD systems began operation on August 20, 2002 and July 8, 2003, respectively.
As China’s first SWFGD system, the experience gained at Shenzhen will be crucial as China continues its drive to reduce pollution in its populated coastal cities.