Medupi and Kusile boilers build on operating experience with the South African coal-fired fleet

1 October 2009

The design of the boilers being supplied to Eskom’s new generation of large coal fired plants aims to fully address the issues that have arisen from previous operating experience in South Africa, including dealing with the high ash content of the fuel.

aOver the last five decades the power plant boilers built in South Africa were either of the natural circulation (drum boiler) or the once-through (Benson boiler) design. Due to increased operational demands and higher efficiencies, the once-through design became the leading technology. But prior to Medupi and Kusile the latest coal plant design implemented in South Africa was that for the once-through units at Majuba some 25 years ago. It was therefore imperative to ensure that the lessons of previous operating experience and the latest technologies were taken fully on board in the new plants. This was achieved by Eskom through careful research prior to issuing the tender specifications and a strict adjudication and examination process for the submitted designs.

For Eskom’s two most recent coal plants, Medupi and Kusile (see MPS, April 2009), Hitachi was selected as boiler supplier, with an extensive scope of supply, including auxiliaries such as coal mills, bunkers, feeders, PF piping, conveyors, fans, gas-to-air heat exchangers, boiler bottom ash scraper conveyor and outlet conveyors, fly ash collecting plant (fabric filter), and sootblowers and water cannons.

To meet the comprehensive technical requirements laid out by Eskom and address the properties of the coal specified for the two stations, the tower type once-through supercritical boiler was selected as being the most suitable technology.

Both Medupi and Kusile consist of six 794 MWe units and Eskom’s fleet concept has resulted in a number of significant savings and advantages. The design is done only once, except for some site specific items arising from the different altitude and coal specifications (fan dimensions, economiser surface area, foundation design). Medupi (altitude, 900 m, design pressure 913.3 mbar, design ambient temperature, 25°C) is at Lephalele in Limpopo Province, adjacent to the existing Matimba plant, while Kusile (altitude, 1460 m, design pressure, 849 mbar, design ambient temperature, 17.3°C) is located in Mpumalanga Province close to the existing Kendal plant.

Because of the relatively high ash content of the coals, a design which suffers least from erosion was deemed to be imperative. The once-through design satisfies this requirement as the flue gas changes direction only after it has left the last pressure part.

The main parameters for each boiler can be summarised as follows:

• Once-through tower type with Benson start-up system.

• Water/steam parameters: feedwater, 269°C/295 bar a; live steam, 2288 t/h/564°C/258 bar a; reheat steam, 2010 t/h/572°C/53.2 bar a.

• Pressure part design according to DIN EN 12952.

• Spiral furnace membrane walls in evaporator.

• Vertical furnace membrane walls in first stage superheater.

• 38.8% BMCR Benson load.

• SH control range from 38.8% to 100% BMCR.

• SH and RH spray water attemperation.

• BMCR with n-1 mills in operation.

• 30 low NOx coal burners with staged combustion.

• Curtain and overfire air for protection against slagging.

• Emissions (referred to 6% O2 content in dry flue gas): NOx < 650 mg/Nm3; particulates < 50 mg/Nm3.

• Mass of structural steel, about 14000 t per unit.

Meeting Eskom’s requirements

The model used for furnace design takes into consideration the experience gained with the operation of some 20 000 MW of installed capacity in South Africa.

The furnace is dimensioned such that the calculated burnout time downstream of even the upper burners still amounts to over 3 seconds which is more than sufficient to realize optimum combustion efficiency with minimum unburned carbon in ash. The furnace exit gas design temperature is 1200°C, well below the ash melting point, thus minimising fouling and slagging in the superheaters.

Furthermore the furnace heat release rates are very conservative and well within the designer’s experience and Eskom’s specification requirements. The heat release values at BMCR are: furnace cross section, 5.60 MW/m2; burner zone, 1.32 MW/m2; and furnace volume, 92 kW/m2.

A particular concern has been to take full account of the ash properties in considering burner dimensions and general burner layout, to minimise the possibility of slagging. Conservative furnace heat release rates are achieved through the use of generously dimensioned furnace and burner arrangements (pitch, wall clearances, etc).

In addition, the furnace and heating surfaces are provided with water lances and steam sootblowers to maintain the optimum heat exchange performance. These are controlled by a system that monitors the heat pick-up of the combustion zones and activates the soot-blowing system when ash build-up retards heat transfer.

The tube banks are spaced at large distances apart, in accordance with Eskom’s specification. This prevents any slag formed on the elements from bridging across to neighbouring elements.

Based on CFD modelling and actual previous operating performance the furnace geometry for the Medupi and Kusile boilers has been refined, with fine tuning of the precise burner and OFA arrangement.

The burners are of the staged air swirl DS type, well established in Europe as a low NOx design, with an initial, lean air, ignition zone, the remaining air being admitted at later stages of combustion. The consequent low temperatures mean that only fuel NOx is produced.

Careful consideration has been given to minimising the gas side attrition rates within the furnace. Past experience has shown that this requires attention and must be taken into account during the initial planning phase because the correct selection of burner and furnace dimensions is critical to achieving the required performance and long term reliability.

The following arrangement is employed: six burners per mill; individual PF pipes are connected directly to the mill without splitter boxes; five burner rows.

Each row of six burners (connected to one mill) is vertically staggered with three rows at the front wall and two rows at the rear wall.

This arrangement aims to achieve optimised flame configuration and wall atmosphere, with the shortest possible PF pipes overall.

One row of six OFA double nozzles is provided in both the front and rear walls, with the aim of ensuring optimum OFA distribution and penetration into vertical flue gas flow. Adjacent to each outer burner a row of curtain-wall air nozzles admits air to the side of the burner at the wall to prevent recirculation of combustion products which could cause slagging or corrosion in these areas.

In the case of design fuel the expected NOx emissions at BMCR are approximately 420 mg/Nm3, which is well below the required level specified by Eskom of less than 650 mg/Nm3.

Abrasion countermeasures

In view of the nature of the coals to be fired, special attention was given to abrasion-resistant design of the furnace, boiler heating surfaces and firing system.

The flue gas velocity within the boiler heating surfaces is very moderate and does not exceed 12 m/s at BMCR firing worst coal.

With the vertically staggered firing system the velocity profile and particle concentration are more even, with less peaks, than a system of burner rows opposing each other with large gaps between the vertical rows. Further the tower type boiler does not incorporate any aerodynamic features within the boiler that might cause a change in flue gas flow direction (eg, no furnace arch, no horizontal pass and no second pass), where high local velocities and particle concentrations can be expected.

In addition, the boiler design includes the comprehensive standard erosion protection measures that have been developed based on worldwide experience. This incorporates wear resistant materials such as special PF pipe elbows, hard-face protection at burner PF pipes and inserts, gas deflection screens between the boiler heating surfaces, protection of tubes against sootblower erosion, etc.

Material choices

In the original specification P91 and T91 were specified for the high temperature components. However recent international experience has shown that the use of T91 (9% chromium) for the heated components of the final superheater and final reheater heating surfaces (above 540°C steam temperature) has led to significant problems, eventually resulting in tube failures due to extensive growing of the inner magnetite layer, which leads sooner or later to overheating of the tubes. Consequently, the final design is based on the use of TP347HFG material with 18% chromium for the final superheater and final reheater heating surfaces.

This is an example of how Eskom has taken international experience fully into consideration right up to contract finalisation and at the same time was able to optimise the thermal cycle by increasing steam parameters further.

The evaporator water walls are constructed of 13CrMo45 material (1% chrome), an old favourite which has proved itself many times in numerous boilers, while the superheater and reheater tube materials range from 13CrMo45 to TP347HFG (18% chrome) – the latter material being satisfactory for temperatures over 600°C, well above those to be experienced at Medupi or Kusile.

The auxiliaries

For the coal milling plant five vertical spindle mills (MPS 265) with static classifiers are installed for each boiler. In comparison to the previously used tube mills, the vertical spindle mills provides significant savings in operating and maintenance costs, and the investment costs are also significantly lower.

Furthermore the bunker outlet and mill feeder arrangement is simplified as only one feeder per mill is required instead of two, which results in further savings in investment costs and operating and maintenance costs.

The MPS mills are preferred not only in terms of economics but also because of the simplified operation and control.

Each mill is supplied with coal from its coal bunker, of capacity 1275 m3, sufficient for 12 hours operation. The bunkers are lined with erosion resistant steel 12 mm thick.

The mills are driven by electric motors through a gearbox situated below the mill.

Each mill has three rollers running on a grooved table, loaded so that they exert pressure on the coal lying on the table. Hydraulic cylinders exert the required force on the rollers.

The design is such that each roller can be swung out of the mill housing for maintenance or replacement, the roller tyres being replaceable in a weekend.

Six pulverised fuel outlet pipes are fitted to the top of the mill, one pipe to each burner. The pipe bends are lined with erosion resistant ceramic liners.

Leaving the boiler, the flue gas flows to the regenerative airheaters, where the gas is cooled by giving up its heat to the combustion air. This raises the efficiency of the complete cycle as the last of the available heat is extracted from the waste gases and is transferred to the combustion process. The ducts are sized generously so that velocities are low, to reduce erosion.

In the downstream gas cleaning system the flue gases are cleaned of almost all of their dust burden (> 99.9%) by a pulse jet fabric filter before being routed to the stack via two induced draft fans. The maximum dust burden at filter outlet will not exceed 50 mg/Nm3, so no dust plume will be visible.

A wet deashing system will provided for the removal of bottom ash from the furnace. This system consists of the submerged scraper conveyor, the discharge chute with grizzly screen and a transfer chute to the coarse the ash conveyor.

Advantages of using a submerged scraper conveyor are: high availability proven by extensive experience from previous plants; low water consumption; low energy requirement; and compactness.

The draught plant employs fixed speed fans in combination with variable blade pitch control for the axial-type FD & ID fans and with variable inlet vane control for the radial-type PA fans. Criteria for fan selection were: safe and reliable operation, based on experience gained from previous units; low capital cost; and low power consumption.

Finally the air heating system was designed with special attention to the avoidance of blocking, excessive leakage and erosion, to allow the required maintenance outage intervals to be achieved.

The air heating system consists of two regenerative four-sector airheaters for the primary and secondary air, and two steam airheaters downstream of the secondary air fans, after the primary air fan take-off, for keeping temperatures above sulphuric acid dew-point at start-up and low loads.

The four-sector airheaters have two secondary air sections.

This avoids a layout that puts the high pressure primary air sector next to the negative pressure flue gas sector, and results in reduced leakage of air to flue gas.

The arrangement of combined primary and secondary airheaters is a Hitachi Power standard solution and has been used in all recent plant designs, with extremely low leakage rates – less than 6%.

The benefits can be summarised as follows: low investment and maintenance costs; simplified ductwork system; avoidance of control dampers in the flue gas ducts, simplifying operation and minimising corrosion problems.

Boiler auxiliary subcontractors

Local subcontractors to Hitachi in the supply of boiler auxiliaries include: Murray and Roberts, for manufacture of structural steel and ducting and erection of plant; SPX-DB Thermal for supply of regenerative airheaters and steam airheaters as well as supply & construction of fabric filters and manufacture of boiler pressure parts; Alstom SA for supply of MV motors; and Steinmueller Africa for manufacture of pressure parts. International subcontractors include: Vallourec & Mannesmann, for supply of pressure part materials; Welland & Tuxhorn AG for supply of valves; TLT-Turbo for supply of fans; KSB for supply of pumps; and Clyde Bergeman for supply of soot blowers.

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