Lignite-fired Niederaußem K aims for efficiency of 45 per cent and more

The lignite fired BoA unit

The steam generator, shown in Figures 3a, b and c, is characterised by a tower-type design, ie all pressure parts except for the flue gas heat recovery system are suspended from the boiler supporting grid. The grid itself is supported by the four main columns arranged in a square shape. The tower-type design has been selected in view of the high steam parameters, particularly since:

This type of construction, with which good experience has been gained since the construction of the first lignite-fired 300 MWe plants in the sixties, has the advantage of fully drainable heating surfaces as well as flue gas tight enclosing walls.

Coal storage and preparation

The coal bunkers are arranged at the side of the steam generator. This type of construction was preferred over the otherwise usual front arrangement, as it results in less complex and shorter routes for the feeding of the milling plants. The coal bunkers are designed to accommodate seven hours’ worth of full-load steam generator operation when firing the worst coal.

At full-load operation, nearly 20 000 t of lignite will be fired every day in the steam generator and seven out of the eight mills are needed. The maximum coal throughput per mill is 142.5 t/h. Beater wheel mills with overhung shafts are arranged evenly around the steam generator in order to supply the burners of the tangential firing system in the most direct possible way.

The mills are not fitted with classifiers because the fired coal has a high tendency to cause wear of the equipment. In order to achieve the necessary fineness without the classifier, the mills are provided with primary beaters at the side of the coal inlet. A multiple-circuit variable-speed gear is installed for reducing the power consumption to a minimum over the whole speed range.

The combustion system

The air and flue gas system as well the design and arrangement of the firing system can be seen from the simplified process flow diagram, Figure 4. The steam generator is characterised by a double-flow design on the air side. The primary air is taken in by means of the two FD fans either via the external intake system (41.9 m) or the internal intake system (157.4 m) underneath the boiler top casing. The air is led to the hot water air preheaters of the flue gas heat recovery system through concrete shafts in the stair towers and is then supplied to the furnace via the two rotary flue gas air heaters.

The plant uses an advanced version of tangential firing, with reburning burners, so that the NO_x limit value of 200 mg/m³ (stp, 6 per cent O₂) is complied with by combustion measures alone. Based on extensive experience with conversions to low NO_x firing in the Rhenish and Lausitz lignite mining districts, the Niederaußem firing system is characterised by the following features:

Three-dimensional computational fluid dynamics models were used to validate the optimisations and the design of the firing system and combustion chamber.

A post-combustion grate is arranged underneath the furnace hopper for minimization of the combustion loss. A dry lignite firing system is provided for start up. This is designed for up to 35 per cent of full steam generator capacity.

Integrated flue gas heat recovery

At full load the flue gases produced by the coal combustion leave the furnace at a temperature of around 1050°C. In order to minimise wear at the high-heat-release surfaces due to flue gas particle erosion and to reduce the draught loss over the heating surfaces, the maximum flue gas velocity in the convective part above the furnace was limited to 9 m/s.

After having passed the convection heating surfaces arranged above the furnace the flue gases are conveyed in the downward open pass to the two parallel rotary air heaters and to the air heater bypass economiser (lubeco (=LUVO bypass economizer)) which is also arranged parallel to these two rotary air heaters. The flue gases are joined underneath the two rotary air heaters or at the side underneath the lubeco and then conveyed to the two electrostatic precipitators, the two flue gas coolers and then to the FGD system.

A flue gas cooler is arranged upstream of the FGD plant for flue gas heat recovery. The flue gas cooler reduces the temperature from 160°C to the minimum flue gas temperature of 100°C for an effluent-free operation of the FGD plant and keeps this temperature constant over all operating conditions. For the first time, an arrangement is being used with which this heat can be introduced into the cycle process with extremely favourable energy efficiency.

This is achieved by feeding the approximately 80 MJ/s of heat which is transferred in the flue gas cooler to the process of steam generation. In the first stage the heat is transferred via a water cycle in a feedwater/air heater to the cold combustion air. As the quantity of heat flux to be absorbed by the combustion air in the rotary air heater in the second preheating stage is now reduced, a part of the flue gas heat can be utilised for the preheating of feedwater and condensate. About one-third of the flue gases, with a temperature of around 350°C, is led through the lubeco which is arranged in bypass to the rotary air heaters, where they dissipate the heat in the first section to a partial stream of the feedwater in parallel with the HP heaters.

In the second section, steam is generated by natural circulation for heating the last but one LP heater. The flue gas temperature downstream of air heater bypass is 160°C. Thanks to this arrangement, less extraction steam from the turbine is needed for the condensate and feedwater preheating.

The flue gas cooler requires special material properties. To avoid low temperature corrosion due to condensate containing sulphuric acid, it is planned to manufacture the flue gas cooler tubes and the heat exchanger wall coating from plastics, namely PFA, a fluor polymer. Onerous demands are placed on new materials here as well as in other parts of the plant. To help with the selection of corrosion-resistant materials a large scale test programme was done at Frimmersdorf power station.

Steam generator system

The feedwater, which is preheated with a steam extraction from the HP turbine part and as a substream in the lubeco, flows then through the individual heating surfaces and pressure section components, which are shown in the simplified water–steam diagram. The flow time of the water is about 7 minutes.

Economizer – The feedwater enters the two-part feedwater preheater at a high terminal temperature of 295°C. In the upper part, which is passed first, the working fluid is led in counterflow to the flue gas, and in the lower part, which is passed second, in parallel flow to the flue gas.

Evaporator – After leaving the economizer, the water at the bottom part of the furnace hopper is led to the evaporator which is made up of the enclosing walls of the steam generator. The evaporator consists of helical-pattern wall tubes in the furnace area, which are provided with a cross-sectional expansion of the tubes in order to improve the distribution of the working fluid to the parallel evaporator tubes at the upper hopper edge and underneath the flue gas ducts.

In the area above the furnace, the evaporator consists of vertical tubing. In the upper area of the helical-pattern evaporator, complex mixing of the supercritical working fluid ensures that the temperature asymmetries caused by the asymmetric operation of the firing system are compensated on the fluid side. Particularly because of the high steam temperatures in the evaporator at supercritical pressures, this measure is required to reduce the thermal stresses in the enclosing wall which is exposed to high thermal stresses.

Superheater – The fluid leaving the evaporator is then led to the separation system consisting of six separators and one level vessel, where water and steam are separated in part-load operation below 40 per cent load. The filled level vessel is maintained hot in supercritical steam condition. From there, the steam flows to superheater 1 made up of the supporting tubes and the platen support tube.

The superheater comprises four other stages. Stages 2 and 4 are arranged in counterflow and stage 5 in parallel flow to the flue gas in order to avoid any excess temperatures of the materials. The 3rd SH stage integrated into the triflux heat exchanger does not absorb any heat from the flue gas, but dissipates heat in counterflow to the RH2.

The SH outlet temperature is controlled by means of spray attemperators, which are arranged in the connecting pipes between the individual SH stages. For the minimisation of the auxiliary power requirement of the feed pump, the pressure drop on the SH side was reduced to 36 bar. The superheated steam leaves the steam generator at 275 bar and 580°C in the direction of the HP part of the turbine and is led back to the reheater at full load at 60 bar and 345°C.

Reheater and triflux – The reheater is made up of three parts. The final stage is also arranged in parallel flow to the flue gas. The outlet temperature of the steam is 600°C. The reheater is run in steady-state operation of the steam generator without spray attemperation. The RH temperature is controlled with reheater 2 (RH2) which is designed as triflux heat exchanger.

The annular cross-section of the triflux heat exchanger which is made up of two concentric tubes is passed by the reheat steam. Superheated steam flows through the internal tube in counterflow and dissipates heat to the reheater on account of a higher temperature level. The RH steam absorbs heat both from the flue gas and from the superheated steam. The amount of the heat absorption can be influenced on the SH side via the SH-side triflux injection and the SH-side bypass flow. Apart from transient operating conditions no efficiency-reducing RH spray attemperation is thus necessary for the control of the RH outlet temperature in spite of the wide calorific value range and possible differences of fouling of the SH and RH heating surfaces.

Steam generator materials

The total heating surface of the SH and RH parts of the steam generator comprises around 150 000 m². The tube length is approximately 1100 km. The enclosing walls account for approximately 170 km, the superheater accounts for 540 km and the reheater for 370 km. The heating surfaces for the flue gas heat recovery system amount to a surface area of another 150 000 m². This is equivalent to an additional tube length of approximately 1300 km. The flue gas cooler accounts for approximately 1000 km.

The processing of such large tube lengths, as well as the advanced steam parameters, require particular attention as far as materials are concerned. The material concept had to be redesigned particularly for the following components:

The highest alloy material available for the design conditions, ie 13 CrMo 44, is used for the steam generator walls. This material requires no post-weld heat treatment.

For the separators and the level vessel, material X10 CrMoVNb (P 91) is used. This highest-alloy material is suitable for the strain-induced corrosion conditions caused by high temperatures.

The convection heating surfaces, SH2, SH4, SH5 and RH3 are manufactured from the austenitic steel X 3 CrNiMoN 17-13 with 17 per cent chromium and 12 per cent nickel for resistance against high-temperature corrosion. This material has been subject to a large-scale testing phase in the form of a test tube in the Weisweiler power station at 600°C since 1994. The experience gathered so far is very positive.

In order to avoid any thick-walled ferritic-martensitic welded joint between the martensitic connecting piping between SH or RH outlet and turbine, the SH and RH outlet headers are also made from a martensitic material.

For the first time in a large steam generator, the material E 911 (X 11 CrMoWVNb 9-1-1), which belongs to the group of the tungsten-alloy 9-12 per cent chromium steels is used for this purpose. Compared with material P 91, this leads to a reduction of the wall thickness from 100 mm to 75 mm for the HP outlet headers. For the other thick-walled headers and connecting pipes the usually applied material X 20 CrMoV 12-1 is completely replaced by P91 which is less difficult to process.

The ferritic/martensitic compounds between the austenitic heating surfaces and the martensitic headers are arranged outside the steam generator enclosing wall in the unheated area. Compared to a ferritic/martensitic compound in the header/piping area these compounds can be manufactured with clearly smaller wall thicknesses and checked without any problems.

A total of almost 60 000 t of material is used for the complete steam generator plant. The complete steam generator plant is housed in the steam generator building. The steam generator building has a floor surface area of 86.5 m x 90 m and a roof parapet height of 167.5 m.

The operational management of the steam generator has been optimised and largely automated through use of a number of new components including:

Emissions control

Particulates – Fly gases leaving the steam generator are be passed through two parallel duct arrangements. Thus, without further cross connections, the gases are passed via air heaters, electrostatic precipitators and ID fans towards the downstream desulphurization system. NO_x formation is controlled by primary low NO_x corner fired burners.

The electrostatic precipitator plant for Niederaußem K has been designed by Apparatebau Rothemühle Brandt+Kritzler with two parallel precipitator casings which for gas distribution reasons have been provided with two inlet and outlet nozzles. To minimize corrosion by high dew point flue gases during low boiler load operating conditions, the casings have been provided with heat/noise insulation layers.

Each casing carries some 7900 m³ of electrode arrangement, split into parallel electrical bus sections and three electrical downstream fields. This amounts to 40 000 m² of projected collection surface per casing with 400 mm lane spacing. Each casing carries a gas flow of 1 950 000 Nm³/h at 149 – 184°C.

Electrical supply to the fields is provided by eight T/R sets per casing designed for 111 kV peak and 1750 as well as 3750 mA (arithmetic) output.

The outlet gas flow dust loading is guaranteed to be below 50 mg/Nm³ (6% 02) under all conditions according to GFAVO definitions. This requires a 99.9 per cent removal efficiency, ie up to 1380 tons of fly ash each day per casing.

Flue gas desulphurization – The characteristics of AE’s IFO (in-situ forced oxidation) limestone scrubbing process are especially aimed at large and medium sized power station units:

Precleaned flue gas flows through a spray tower from the bottom upwards, passing through a washing train consisting of several spraying levels and a mist eliminator.

Absorber slurry taken from the sump is sprayed into the flue gas in a counter current and distributed in the gas chamber in the form of droplets. As a result, the acids contained in the gas – HCl, HF, SO₃, and above all, SO₂ – react with the washing slurry. In particular, the SO₂ forms calcium sulphite which is partially oxidised into gypsum by the oxygen in the flue gas and partly by the oxygen in the air blown into the spray tower sump.

As gypsum is continually produced, a partial flow must be constantly sluiced out of the sump. The CaCO₃ contained in this flow which has been spent during the chemical reaction, is steadily replaced by fresh absorption medium.

Largest ever cooling tower

The 200m high natural draft wet cooling tower supplied by Balcke-Dürr Energietechnik GmbH is claimed to be the world’s highest ever. It has an integrated cleaned flue gas discharge system.

The hot water coming from the condenser flows to the cooling tower via a supply line and is conveyed to the height of the water distribution channels via two risers. The lower distribution pipes with sprayers finely distribute the water to be cooled over the plastic cooling fill packings. The cooling air is drawn through the cooling flow in counterflow to the water trickling downwards.

Contactors with electric actuators are installed in the riser heads to permit fully automatic winter operation. Under extremely low ambient air temperatures and low thermodynamic load, some of the tower sections can be disconnected and/or the deicing pipe loaded.

At each riser there is also a bypass downcomer which is designed as an overflow bypass and can be loaded with the nominal water flow rate under any mode of operation irrespective of the position of the cooling tower contactors without causing damage to the water circuit or to the cooling tower fill.

High grade plastics and stainless steels were required for the cooling tower components due to the integrated cleaned gas discharge system. Noise emission protection screens made of corrosion resistant aluminium alloy some 12 m high surround the base periphery of the tower. The main technical data are listed in Table 3.

Type HMN supercritical turbine

Nearly 2 per cent of the efficiency gain over recently built lignite fired power plants in this size range comes from advances in steam turbine design. Fully three dimensional, variable reaction blading with compound lean is now a main ingredient in optimising HP and IP turbine efficiencies.

This novel approach, which will be used for the first time in a Siemens KWU Type HMN steam turbine in the Nideraußem K turbine, combines the benefits of both multi-stage reaction blading and low reaction impulse blading. The main difference is the choice of stage reaction ie the split of pressure drop and velocity increase between stationary blades and moving blades.

Stage reaction is the ratio of the enthalpy drop in the rotating row to the enthalpy drop of a whole stage. A reaction turbine is characterised by a stage reaction of 50 per cent, and the enthalpy drop is equally divided across stator and rotor rows.

The symmetry in enthalpy drop entails a symmetry in flow relative to stator and rotor flows and allows the same profile to be used for both blade rows. In this case, flow velocities and flow directions along the blade path are moderate and profile and secondary losses are quite low. Since half of the enthalpy drop occurs in the rotor row, the pressure differential across the rotor blades is rather high and exerts a large axial thrust on the rotor.

In order to compensate this axial thrust, a dummy balance piston is required in single flow designs. The leakage flow across this dummy piston reduces the turbine cylinder efficiency.

In impulse turbines with zero stage reaction the total stage enthalpy is converted into kinetic energy in the stator row, while the rotor row merely deflects the steam without further acceleration. In this case different profiles must be used for stator and rotor blades because their flows are asymmetrical. Flow velocities and flow deflection along the blade path are significantly larger than the equivalent reaction stages incurring higher profile and secondary losses. Since the pressure differential across the rotor is much less than that for reaction turbines, a much smaller dummy balance piston with lower losses can be used.

Three dimensional blades have the potential to exploit the best of both worlds. By the use of computer controlled CAE – CAD – CAM – five axis milling machines, any three dimensional blade that the design engineer can invent can now be manufactured.

For the first stages of HP and IP turbines, a special three dimensional blade with compound lean has been developed by Siemens. In these stages secondary losses are significant due to the low volume flow rates and short blade lengths. Compound lean and twist both serve to reduce the secondary losses at the root and the tip of blade. The reaction of each stage is set individually and may vary between 10 and 60 per cent.

Compound lean and twist both serve to reduce the secondary losses at the root and tip and substantial control is possible. Extensive measurements were performed on a four-stage test turbine to confirm efficiency improvements of up to 2 per cent compared with conventional cylindrical blading. In other words, in order to achieve the highest possible turbine efficiency and performance, all turbine blades employ a three dimensional profile design. If a three-dimensional blade has to be designed for each individual stage and each particular application, the purpose of employing either impulse or reaction blading has ultimately lost its benefit.

Siemens has adopted this paradigm and has developed blading where, in addition to the three-dimensional blade shape, the reaction of each stage is set individually and may vary between 10 and 60 per cent. In addition, therefore, to the use of advanced design practice, many new degrees of freedom are obtained and these allow a further improvement in efficiency. In fact, the new 3DV blading is the consequent and logical next step in the development activities that led to the earlier 3DS blading and combines a fully three-dimensional blade shape and a variable degree of stage reaction.

When designing blading, a multitude of parameters has to be specified for each stage of blading such as, for example, stage loading, stage reaction, profile shape blade height, blade twist, bow and taper, and inner and outer flow contours.

Any design and any parameter selection is, however, restricted by constraints. These are derived from considering the strength of blade and root, the critical frequency of the rotor and the blade rows. the maximum allowable axial thrust, the length of the cylinder and other geometrical and mechanical constraints. Due to the complexity of the problem and the large number of parameters, manual optimisation will lead to an accepted design that, in all likelihood, is not the true optimum design but rather a compromise.

Direct numerical optimization

New, up-to-date computer codes and design tools allow rapid direct numerical optimisation of the blading. In a numerical optimisation process, the objective function – in this case blading or cylinder efficiency – is calculated for a certain set of parameters. Then, data constraining the design are determined, such as values for strength and stresses, natural frequencies and geometrical constraints.

If one of these constraints is violated or if the maximum efficiency is not yet attained, the optimisation algorithm progresses by selecting an improved set of parameters and repeating the calculation. This iteration continues until the maximum blading efficiency is obtained for the given set of constraints. A typical blading design will involve more than 40 design parameters subject to over 100 constraints.

A numerical optimisation algorithm is used to design the 3DV blading. Inner and outer wall dimensions, blade height, stage reaction and stage loading are determined by the algorithm for each individual stage. The total enthalpy drop is divided across the individual stages such that high loading at low reaction and low loading at high reaction is appropriately applied to each stage to achieve the highest efficiency possible for a particular application.

The blade profiles are also designed with the aid of numerical optimisation techniques. In this case. the two-dimensional flow around the profile is calculated and the shape of the profile is altered by the algorithm until the desired flow quality is obtained. The optimised profile shape has less diffusion on the suction side and this results in lower losses. Profiles optimised in such a way will not only have a higher efficiency, they are also less prone to fouling and to deposits on the suction surface.

In LP turbines, the transonic flow induces shock waves on the suction side of the blades and this leads to large losses. In this case, the optimisation determines a profile shape that significantly reduces the strength of these shock waves and hence the losses. Numerical optimisation algorithms drastically reduce the design cycle time and insure that all constraints are satisfied.

These optimisation systems were used in conjunction with the latest computer programmes to calculate three-dimensional viscous flows and experimental data derived from extensive wind tunnel and model turbine tests. The excellent agreement indicates that these methods are well capable of describing three dimensional flows both qualitatively and quantitatively.

Instrumentation and control

Unit K will be controlled by the Siemens Teleperm XP power plant automation system, which aims to provide economic benefits such as maximum efficiency, highest availability, optimal fuel utilization and low operating costs.

Every task required, for example, fail safe automation or turbine control, is carried out by specially designed and approved subsystems which are fully integrated in the power plant automation system. To optimize diagnostics and maintenance the process field bus Profibus-DP is part of the automation system for interoperation of the actuators, while transmitters are realized using the HART-protocol.

The operation and management system is based on large area displays and carries out process control, process information and process management. Safe operation is ensured by supporting the operator with an efficient alarm system and fault analysis. Dynamic function diagrams support the operator with immediate identification of stepping criteria. The management system comprises plant performance enhancement I&C modules such as integrated condensate throttling control and load margin prediction to improve environmental protection.

The engineering system is an integrated part of Teleperm XP used to configure the subsystems, its function modules, the communication systems and the software for operating, monitoring and management. The engineering system is used from the beginning of the engineering over the period of commissioning up to modification or updates of the I&C sytem during power plant lifetime.

Commissioning

Construction of unit K of the Niederaußem power station started on 3 August, 1998. On 1 November, 2002 the unit is due to start trial operation. The new plant is one of the largest, most modern and pioneering units in the world for lignite-based power generation, setting the standards for future stations. It will contribute to environmentally compatible power generation based on conservation of resources.

Tables

Table 1 Main technical parameters of Niederaußem K power plant under BoA specifications
Table 2 Comparison of data for Niederaußem H and K
Table 3
Cooling tower technical data