China is investing heavily in new coal fired power stations and is looking for the most efficient and cleanest technology. Mitsui Babcock’s supercritical boiler at the Changshu station is the first in a new generation of boilers.
John Prosser, Mitsui Babcock, China
lectrical power production in China is growing rapidly, at roughly 15 per cent year-on-year. In 2003, in excess of 100 GW of pulverised coal fired power plant was ordered – equivalent to almost twice the total installed generating capacity within the UK. Roughly 75 per cent of power in China is generated from coal. It has been predicted that coal’s contribution to global energy consumption will remain at around 38 per cent by 2030 as developing countries such as China and India increase generation and draw on their huge reserves of coal.
The dual demands of meeting environmental and power supply challenges make China an interesting case study for the use of clean coal technologies. For new coal fired plants in China approximately 50 per cent of orders are for supercritical equipment.
Mitsui Babcock has been designing and supplying once-through boilers for nearly 50 years but it is only very recently that economic and environmental benefits have begun to drive the decision making process of plant owners. Previously the power market had access to indigenous coal with no incentive to be the most efficient or environmentally considerate. With China’s continuing economic development and the demand for power, the country has acted swiftly and decisively to put in place the necessary power growth.
On the face of it, coal fired powered generation may appear less attractive in environmental terms. The large existing coal fired power plants introduced in the UK in the 1960s each produce approximately 1 t of CO2 per megawatt hour of electricity (MWh). The latest combined cycle gas turbine (CCGT) power plants emit approximately 50 per cent of this. However, while 200 years of global coal reserves are available, reserves of gas and oil are far more limited, with gas reserves estimated at 50 years.
Clean coal technology provides power generators and governments with a sustainable solution. Continued use of coal is possible, while maintaining security of supply, achieving the longer-term emissions reduction in a cost effective solution.
New advanced supercritical coal fired plant, with 300 bar and 600°C steam conditions, can be designed with an overall efficiency of some 46 per cent, a 20 per cent improvement in efficiency compared with the sub-critical designs installed in the UK in the 1980s. These plants can also be designed to accommodate up to 20 per cent biomass firing, realising the possibility of up to 40 per cent reduction in CO2 emissions compared to the older subcritical designs.
The efficiency, and therefore the CO2 emission levels, of the older subcritical boiler plants in China can also be improved through the introduction of boiler and steam turbine efficiency improvements and the inclusion of biomass co-firing.
Optimised ribbed tubing for vertical tube furnace
The key element in this upgrade is the application of supercritical technology by upgrading the boiler and steam turbine plant from sub-critical conditions (e.g. 180 bar, 540°C final steam temperature) to supercritical conditions (e.g. 300 bar, 600°C final steam temperature). In doing so, plant efficiency will increase from ~36 per cent to ~43.5 per cent. For the same MW output this equates to a CO2 reduction of 20 per cent. For older, less efficient units, this saving in CO2 emissions can be greater, approaching 30 per cent.
The boiler improvements are achieved by replacing the current natural circulation boiler design with a supercritical design, resulting in a lighter overall boiler design capable of being retrofitted within the existing boiler house structure. Recent Mitsui Babcock advances in supercritical boiler design enables application of its own Posiflow system of internally ribbed boiler tube technology. This design of tubing allows a vertical furnace tube arrangement to be used, which is easier to retrofit, has a lower cost and is more efficient than the conventional spiral wound arrangement of tubing.
In addition, enhanced co-firing levels can then be achieved with minimal generation risk on a plant that is retrofitted with supercritical technology. The replacement furnace would be designed with this intent. Supercritical retrofit design, with an enhanced biomass co-firing system, has the potential to deliver up to 40 per cent reduction in CO2 emissions.
Mitsui Babcock has a technology transfer agreement with Harbin Boiler Company (HBC), China’s largest utility boiler manufacturer. This partnership is key to its work in the region, and has resulted in Mitsui Babcock/Harbin being the world’s largest supplier of new supercritical boiler plant in 2004, with over 7200 MW of plant.
With a well established domestic boiler manufacturing industry in China the desire was to keep the power growth within the China domestic market, with only limited opportunities for international bidding and import where overseas funding was available or required. The technology to be adopted had to reflect basic needs and be coal firing, economic to operate and environmentally superior.
This leads to the adoption of once-through supercritical boiler units, but there was no domestic experience with the later design of units capable of sliding pressure operation. The boiler makers needed to align themselves with a technology provider to get this experience. So when the Harbin Boiler Company had the opportunity to bid for Changshu it was agreed that Mitsui Babcock would offer technology support to Harbin Boiler Company for once-through supercritical boiler technology.
A new generation
China leads the drive for more CO2 efficient power plant firing coal in once-through type boilers operating at supercritical temperatures and pressures. The first of these under the Mitsui Babcock agreement with the Harbin Boiler Company was at Changshu. This is the first in the new generation of supercritical power plant boilers in China. In addition, the technology supplied to Changshu was delivered in a time scale unprecedented for this type of plant and is a compliment to the ongoing relationship of the two organizations.
The Changshu thermal power project is for 3 x 600 MW bituminous coal fired units. The owner is China Resources Electrical Power Changshu Company Limited and the station is located adjacent to an existing power plant at Changshu in Jiangshu province just a two hour drive from Shanghai.
Changsu is now a key element in the Shanghai power supply system
One of the concerns of the customer was the combustion system and furnace design in particular. Due to previous reported slagging problems on units by others with the specified fuel, the burner arrangement was made very generous and the supply of burners was specified as import only. Mitsui Babcock therefore designed and supplied 96 of their three core low NOx burners from their manufacturing facility in Renfrew, Scotland. These were all delivered to Shanghai port ahead of schedule to meet the programme. As one of the largest boiler companies in China and with its success in winning orders, Harbin Boiler Company was able to achieve time saving procurement of both raw materials and components to secure the ultimate schedule.
Unit 1 currently operates continuously, regularly generating more than the designed 600 MW with up to 670 MW being delivered to the China power system. Optimization of the combustion system and loop tuning of the controls is to be undertaken shortly although with the high demand for power Changshu is now a key element to the Shanghai supply system. Unit 2 has passed its hydraulic test, steam purge is complete and full load has been achieved.
Close working collaboration between each organization brought Changshu unit 1 from signing to hydrotest in under 22 months and reliability run in 27 months. The technology support is being extended to other once-through supercritical projects and covers some 11 x 600 MW units. Under a 15-year agreement, Mitsui Babcock has now transferred the technology to allow Harbin Boiler Company to undertake the design itself. In a little over two years, orders for a total of 29 units of 600 MW have been received. Further collaboration between the two organizations is set to bring the benefits of supercritical operation to the firing of anthracite coal, something that will again place China at the front of technology and will have environmental benefits over the existing power plants.
The boilers supplied to Changshu by Harbin Boiler Company with Mitsui Babcock are a small example of the power demand and supply in China as a whole. Large, very capable, domestic boiler makers need to provide vast amounts of power to the rapidly growing economy in the most economic and technically effective way possible. The economic advantages of the supercritical, once-through boiler and its lower emissions per MW had been recognised by the Chinese authorities and has resulted in the import of technology from the West.
The application of supercritical technology in China is a major milestone for Mitsui Babcock as China represents a major world market. With this reference point it is well positioned to carry out further contracts in the region. The ageing coal fleet in China could be replaced by new, efficient boilers which are kinder to the environment.