World firsts for Yaomeng with vertical-tube low- mass-flow Benson unit5 July 2002
Refurbishment of the Yaomeng coal fired plant in China has included fitting of a low mass flow vertical tube Benson boiler - the first installation of this type on a major utility boiler to enter commercial operation. It has also required production scale manufacture of the internally ribbed tubing that this type of boiler uses - another world first. Bob Brundle, Mitsui Babcock, Shanghai, China
On 9 November 2000, Mitsui Babcock signed a contract to upgrade the furnace and refurbish other parts of unit 1 of Yaomeng power plant in Henan Province, People's Republic of China (PRC). The contract was part of a full refurbishment programme at this 300 MWe unit, including the turbine, coal systems, DCS and new precipitators. At the time of writing, the unit has synchronised, completed its formal 168 hour full load reliability run and continues to generate to meet the owner's requirements at up to 327 MWe.
The Yaomeng power plant is situated at the edge of Pingdingshan City. The city's name means "flat top of the mountain", reflecting the local topography. The city is in the central southern area of Henan Province and is about 175 km south of the capital Zhengzhou. The city itself has developed since the mid nineteen fifties, driven by the expansion of the coal mining industry in the area, which now ranks in the top three coal producing areas of the PRC. The Pingdingshan area has a population of approximately 6 million and an urban population of over 1 million.
The Yaomeng power plant consists of four 300 MW coal fired units. The first two units entered service in the mid-1970s and the second two were in operation by the mid-1980s. Units 1 and 2 were the first domestically designed 300 MW coal fired units to enter service in the PRC.
The original boiler was a once through boiler with a double furnace and indirect tangential firing designed for base load operation. Towards the end of the 1990s, it had been recognised that high exit temperatures, uneven fuel distribution and poor fineness were impairing the boiler efficiency. Unit operation at full load was restricted and operation at part loads was also problematic due to furnace metal temperatures. Reheater spray flow was high, increasing turbine heat rate. Allied to these operational difficulties was the impending introduction of new NOx and particulate emission requirements, which would severely restrict the use of the unit.
Yaomeng Power Generation Limited (YPGL) therefore took the decision to refurbish unit 1 to bring it up to the most modern day operating performance levels and to extend operational life for a further 20 years. The refurbishment has included the boiler furnace and burners, steam turbine, control systems, pulverised fuel systems, draft systems and a new electrostatic precipitator.
Mitsui Babcock offered to replace the existing high mass flow vertical tube furnace with a low mass flow design having a positive flow characteristic. The new boiler uses modern Benson technology, a concept recognised by Mitsui Babcock in 1983 as being especially suitable for supercritical pressure operation. The technology has been developed and validated in collaboration with Siemens Power Generation over the last fifteen years.
The contract was awarded in November 2000, against very strong competition, to Mitsui Babcock for the design and supply of components to replace the furnace, burners and other heating surfaces, supports and restraint systems and to add the necessary start-up vessels, pumps and piping.
A very aggressive programme for completion of the design and erection work was set of 18 months from release of contracts to achievement of commercial operation. In order to achieve this programme a close collaboration was established with YPGL, China Power Complete Equipment Company Ltd and the Henan Provincial Electric Power Design Institute.
Mitsui Babcock engineers first visited the Yaomeng site in early 1999, when it was ascertained how the existing boiler was performing, what YPGL's requirements were for improving the unit and the likely scope of work to refurbish the unit. A number of subsequent visits were made during the early part of 2000 to refine this information and develop an offer.
The original bid was made in April 2000. There then followed a series of meetings, lasting up until August, during which the scope and requirements were finalised.
An important feature of the project is that it is the first time that low mass flow, vertical tube Benson boiler technology has been put into commercial operation on a major utility boiler anywhere in the world.
The manufacture on a production scale of the special internally ribbed tubing that this boiler design uses also constitutes a world first.
A number of presentations and detailed discussions therefore took place, involving experienced Chinese boiler experts, YPGL, design institutes and other technical authorities.
The concept of the new furnace design is to use a low water mass flow to give every tube in the furnace a positive flow response to an increased heat supply. This is particularly important in coal fired boilers where unpredictable ash deposition can take place, a characteristic not suited to furnaces with preset flow distributors. Not only does flow now change positively in tubes requiring more cooling, but overall pressure losses have been cut dramatically and full advantage can be taken of sliding pressure operation to reduce feed pump power and increase part load cycle efficiency. The required low-pressure operation and the pre-existing configuration of the Yaomeng boiler required thorough checks of both static and dynamic stability.
The two graphs, left, show the relative static and dynamic pressure losses in the original design and in the upgraded design of furnace wall tubes. This shows the principles on which the low mass flow concept is based.
In all furnaces an increase in heat supply to any individual tube reduces the static component of pressure loss. Whereas in the original boiler this was overwhelmed by an increase in dynamic losses which reduced flow and increased steam and metal temperatures, the low dynamic component of pressure loss in the upgraded furnace results in an increase in flow and gives the low mass flow design its positive flow characteristic.
A comprehensive flow model was used to ensure that the different heat release patterns experienced in daily operation were all accommodated in the new design and that flows were stable under all conditions, including those experienced during commissioning.
A critical aspect of heat transfer in a furnace wall is that the high heat transfer rates associated with nucleate boiling should be maintained in all regions of high heat flux. If it is not, tubes will overheat, leading to failure. Good ribbed tubing maintains safe nucleate boiling heat transfer in high heat flux regions at low water mass flows. The low mass flows give a positive flow response to heat input, the positive response and heat transfer keep metal temperatures in furnace walls with vertical tubes within acceptable limits over the whole load range.
The chosen ribbed tubing, made with a carefully optimised lead angle and rib profile, both of which differ significantly from conventional internally ribbed tubes, was cold drawn at a tube works in the United Kingdom under precisely controlled conditions. These features give high heat transfer coefficients at high steam qualities in combination with low water mass flows. The ribs rotate the water flow and thereby replace steam at generation sites by saturated water to maintain nucleate boiling and safe metal temperatures.
The low mass flow furnace has a significantly lower water/steam side pressure loss and at Yaomeng this has enabled the boiler output to be increased to 1000 tonnes/h to meet peak generation demands in the summer months. Alternatively the same evaporation could have been maintained at reduced feed pump power.
Other advantages are reduced metal temperature variation in the furnace circuits and improved flexibility of operation.
Also, construction and support of a vertically tubed furnace is considerably simplified when compared with the alternative, spiral wound, designs.
Among the requirements stipulated for the Yaomeng refurbishment was that the new furnace had to fit within the existing envelope, since much of the support structure was concrete, and in addition the overall weight of the boiler was only to increase by a small amount.
Boiler efficiency will be at least 91.25 per cent at 100 per cent BMCR and a steam output of 950 tonnes/h. The main steam temperature will be 545°C in the range 50-100 per cent BMCR and reheat temperature will be no less than 520°C at 40 per cent BMCR. Overall, there will be no increase in specific fuel consumption.
The boiler is capable of turndown without oil support to 40 per cent BMCR and has much better load following capability than the original unit. It was also agreed that the boiler should be capable of producing a maximum steam flow of 1000 tonnes per hour for periods during the very hot summers to enable maximum unit generation.
As with other recent contracts in China, Babcock (Shanghai) Trading Limited, a wholly owned subsidiary of Mitsui Babcock, took on the management role for Yaomeng, including engineering, planning, project management and procurement in the PRC. The engineering strategy was that conceptual engineering would be undertaken in the UK and the detail engineering and project management would be undertaken in the PRC. Mitsui Babcock expanded their office in Shanghai to undertake all the project functions under the control of a Project Director based in Shanghai for the duration of the project.
The contract, as well as being at the forefront of technology, had a number of commercial and organisational features that Mitsui Babcock had to tackle.
For example, the contract language and law is Chinese and all documentation to and from YPGL has been in Chinese. In order to maximise local content major areas of supply were subcontracted in the PRC, including the manufacture and fabrication of the pressure parts, burners, furnace framing and slings.
A major task was the manufacture for the first time on a commercial scale of the advanced ribbed tubing for the furnace and division walls. The desired geometry of the internal ribs and the tolerances required presented a considerable challenge both in respect of tool design and to achieve consistent drawing performance. Several trial pulls of each tube size were undertaken before full-scale production took place with the full participation of Mitsui Babcock and the UK tube supplier and his toolmaker to discover the optimum drawing parameters. This ensured that the tubes produced were of consistently high quality and within the dimensional tolerances necessary to the achievement of the essential pressure loss behaviour and heat transfer performance.
Other offshore supplies have included the start-up circulating pump, safety valves, control valves and the steel plate gilled economiser tubing.
As mentioned before, the intention from the outset was to maximise the work content in the PRC. To this end, design institutes and subcontractors in the PRC undertook major areas of detailed design. The two major orders let in the PRC were for the manufacture and fabrication of the pressure parts and the supply of the eight burner assemblies. Further orders for framing, slings, pipework and supports, flame monitors, actuators, valves and casings were placed in the PRC.
Mitsui Babcock undertook the pressure part conceptual design in the UK. The local boiler works then undertook the manufacturing detailing to suit their in-house processes. Mitsui Babcock supplied the ribbed tubing for the membrane walls. The local boiler works supplied the material for the remaining pressure parts (start-up system, partial supply of superheater and reheater). The pressure part manufacturing progressed well and the majority of the manufacturing activities took under five months to complete.
Both the original furnace, as already noted, and the replacement furnace are tangentially fired. But to ensure that the required NOx levels and efficiencies can be reached the burners were extensively redesigned, which in turn had to be reflected into the membrane wall design and the supporting structures.
Significant design changes had to be made to the boiler slings as the weight distribution had changed and, because of deterioration, the lower level of slings was replaced in total. The boiler framing was re-designed entirely to meet new requirements. The conceptual design for both these areas was completed in the UK and the detailing and supply completed by subcontractors in the PRC.
The first deliveries of equipment took place in August 2001 and the majority of major equipment deliveries were complete by the end of 2001.
The major outage for the dismantling and erection began in September 2001. As well as the dismantling of the furnace, YPGL also undertook the removal of the ID fans and the bag filter system to provide space for the erection of the electrostatic precipitator plus major refurbishment of the deaerator area. The control room and cabinet room were also being stripped out to enable the new DCS system to be installed. At the same time work to replace the turbine rotor and refurbish the feed systems was underway.
The majority of the dismantling sufficient to enable the start of erection of the pressure parts was completed by the middle of November. However, before this time and as soon as the main structure was free of loads, reinforcement of the structure by welding on plates to existing girders was undertaken.
Pressure part erection began on the November 2001 and continued through both the Christmas and Chinese New Year periods. The hydraulic testing of the boiler had taken place successfully by the middle of March 2002 and the boiler acid clean was completed a week later.
The client began cold commissioning of the boiler whilst still completing the erection of the insulation, turbine, electrostatic precipitator, control systems and replacing the galleries and ladders. During this period significant testing of the air systems and air flows in the furnace, burners and flue and ducts were completed. Most significantly the operation of the circulating pump was proven.
Hot commissioning of the unit progressed very quickly and successfully. First firing of the boiler took place in the middle of April and the steam purge of all systems completed by the end of April. Re-instatement of the boiler and the various systems took approximately one week. This then enabled the safety valves to be floated in the beginning of May and synchronisation within the first week of May.
The unit has undergone extensive optimisation of the control systems, during which senior engineers from Mitsui Babcock and Siemens have been based at site to ensure that all the control and logic functions operate as planned.
The full load 168 hour test was successfully completed on 26 May 2002 and since then YPGL has continued with operation, including peak output, to meet their generation requirements at loads up to 327MWe. Operation has been stable at all loads and under 40 per cent load is being achieved without oil support.
The successful completion of the refurbishment of No 1 boiler at Yaomeng is a major milestone in the development of the technology of once through low mass flow vertical tube boilers.
Wherever once through boilers are suffering load restrictions, inability to load change, and high metal temperatures, the technology described here offers a proven solution.
In the UK, operators are considering using this technology, which along with changing the HP piping and turbine, will allow them to move from a sub-critical to supercritical steam cycle at state of the art steam conditions.
The technology, now proven on a major utility boiler, is a major step forward for the future development of the large supercritical once through, low mass flow, vertical tube Benson boiler and refurbishment of sub-critical boilers. Both will benefit from the low mass flow vertical tube design by virtue of the low feed pump power requirements, ease of construction and reduced metal temperature variation.
We believe that this technology will become a standard for fossil fired boilers of the future.