Credit: Bilfinger SE / Schlüter Fotografie Essen
Credit: Bilfinger SE / Schlüter Fotografie Essen


Given current market boundary conditions, existing boilers must be optimized and further reductions of the ‘stable’ minimum load must be achieved. While solutions vary, in all cases a detailed analysis of the existing system is the basis for success, write Christoph Nachtigall, Stefan Hamel and Christian Storm


The energy market is increasingly influenced by growing use of renewable energy and a resulting supply of fluctuating energy into the grid. As a challenge of the future, the interaction of different sectors – namely renewable energy sources, conventional power plants, energy storage and energy demand – must be optimized.

The main goals for a state-of-the art generation system will be the achievement of security of energy supplies, system stability and energy quality. In the past, conventional power plants were designed for meeting the demand of total load. In contrast, the demand of the future will be to provide a regulating measure for residual load.

Due to these changing conditions, new requirements for conventional power plants will appear. Issues of enhanced flexibility include the extension of load range, increase in load-change rate, optimization of startup and shutdown processes, improvement of efficiency factor and emissions minimization for part load and low load.

Increasing existing plants’ flexibility

Service companies in the energy sector mainly deal with modernization or adaptation of older power plant components, such as steam generators. As seen in Figure 1 (page 36), clients’ requests must be adapted to the existing plant’s boundary conditions. A service company’s activities cover the whole range, from the total system concept up to detail analysis of a single component.

Figure 1. General situation from the point of view of a service company
Figure 1. General situation from the point of view of a service company

In the case of a low load study for an existing steam generator, different steps are required (see Figure 2, page 36).

Figure 2. Typical procedures of a low load case study for existing steam generators
Figure 2. Typical procedures of a low load case study for existing steam generators

After thermal modelling of the steam generator and recalculation of the current operation, several thermal and stability calculations are conducted in order to determine the limitation of low load. Furthermore, depending on the problem definition, additional measurements are installed, for example at single tubes of the evaporator or other critical heating surfaces.

These measurements are required to evaluate the calculation results and, later, to monitor and supervise safe boiler operation at minimum load.

A low load study will give information about the current minimum technical boiler load, and will present an analysis of possible modifications in order to decrease it. After basic and detail engineering, Babcock Borsig Steinmüller will also deal with boiler conversion and commissioning of the new components (see Figure 3, right).

Figure 3. Service features of a low load case study for existing steam generators
Figure 3. Service features of a low load case study for existing steam generators

For a particular plant, the solution is always individual and will be elaborated iteratively in close collaboration with the client.

Case studies

Typical selected case studies illustrate the range of challenges and their individual solutions. Conversion measures are not required in any case.

For example, the first case study shows that a reduction of minimum technical load is possible without any modification of the installation, but only through modification of the operation procedures.

Low load optimization by operational modification

In this first case study, two different two-pass Benson boilers, manufactured in 1982 and 1985 by Deutsche Babcock AG, were analyzed concerning minimum possible load.

The steam generator of 2160 t/h HP live steam (see Figure 4, page 38), with HP parameters 186 bar/530°C and reheat parameters 36 bar/530°C, is equipped with an opposed-firing system for hard coal with 32 burners.

Figure 4. Two-pass Benson boiler for hard coal with opposed firing system
Figure 4. Two-pass Benson boiler for hard coal with opposed firing system

A low load recirculation loop covering the evaporator and economizer already exists.

Since the DeNOx retrofit in 1989, the lower limitation of the technical load of 40 per cent is dictated by the minimum required SCR catalyst operating temperature of 300°C. The target of the recent investigation was to optimize the block unit’s flexibility, especially at low load, down to 22 per cent. This means a reduction of the minimum load from 40 per cent to 22 per cent.

The analysis revealed that one possible solution was to apply the existing circulation system. Here the boiling water from the separator is led over a suction pipe, a circulation pump and a pressure pipe towards the feedwater inlet of the economizer.

As a main effect of this circulation system, the mass flow rate through the evaporator can be kept constant at a minimum required value for low load in order to ensure sufficient tube cooling.

In addition, the solution for controlling the flue gas temperature at SCR catalyst uses a side effect of this kind of circulation system. Due to mixing with circulated boiling water, the economizer’s feedwater inlet temperature is increased, resulting in an increased flue gas temperature upstream of the catalyst. The effect could be reinforced by increasing the circulation rate and the boiling temperature through an HP system pressure increase. With these operational modifications, no further conversion measures were necessary.

Reduction of minimum technical load by conversion

Another case study was elaborated for a two-pass drum boiler with forced circulation, manufactured in 1985. The steam generator is equipped with a four-corner tangential firing system with 20 tilting burners for hard coal. It produces 1700 t/h HP live steam of 178 bar/540°C and reheat steam of 42 bar/540°C.

Presently, after a low-NOx retrofit comprising burner modification and installation of an SCR catalyst, the minimum required flue gas temperature upstream of SCR is 320°C. In consequence, the operational minimum load is limited to approximately 50-60 per cent due to the required SCR inlet temperature.

Target was to reduce minimum boiler to 30 per cent load while maintaining the minimum SCR inlet temperature of 320°C. Accurate and reliable boiler operation must be maintained, and prevention of economizer steaming must be ensured.

The technical solution is illustrated in Figure 5 (left). With decreasing boiler load, first the economizer bypass system is opened. Part of the feed water bypasses the eco, resulting in a reduced heat absorption which means the flue gas temperature is higher than in the case of full feed through the eco. With decreasing load, the bypass flow increases in order to maintain the 320°C flue gas temperature. Unfortunately, this measure is limited by the eco-steaming, which occurs at too-low flow through the eco. In this case the second system is activated in parallel where hot water downstream of the eco is recirculated to the eco inlet. This increases mass flow through the eco, which prevents eco steaming. And, due to a higher mixing inlet temperature, heat absorption is further decreased – which, on the other hand, contributes to higher flue gas temperatures. The increase in economizer inlet temperature is covered by the design parameters of the economizer inlet header. Increased linear expansion of the header was provided for.

Figure 5. Solution for the increase of SCR operating temperature at low load
Figure 5. Solution for the increase of SCR operating temperature at low load

The average feedwater temperature downstream of the economizer is 7 K lower than the drum temperature. Nevertheless, there is a residual risk of steaming for single economizer tubes. But due to upward water flow, static instability of the economizer is impossible. Figure 6 (right) gives a representation of the required pipe routing.

Figure 6. New installation of economizer bypass and feedwater recirculation
Figure 6. New installation of economizer bypass and feedwater recirculation

Low-load stabilization of the pressure part

The target of the investigation was to identify the low load operation capabilities of two lignite-fired Benson boilers. The basic data were as follows:

  • Commissioning: 1973;
  • Boiler type: Benson tower boiler;
  • Steam rating of HP part: 1.805 t/h;
  • Steam parameters of HP part: 172 bar / 530°C;
  • Steam parameters of reheat part: 32 bar / 530°C;
  • Feedwater inlet temperature: 235°C;
  • Firing system: four-wall tangential firing;
  • Number of burners: eight;
  • Fuel: lignite.

    Discussion of the results is focused on the water/steam pressure part of the boilers. In order to determine the stable minimum operation point without any modifications, on the one hand, and to give recommendations for further reducing the minimum technical load through the application of different modifications on the other, a detailed stability analysis of evaporator and heating surfaces passed downstream is required. Therefore the procedure comprises the following steps:

    • Creation of thermal boiler model;
    • Analysis of operating data for full load and part load;
    • Development of characteristic curves for low load (boiler boundary conditions over load, e.g., parameters for superheated steam, cold reheat, feedwater etc);
    • Extrapolation of thermal calculation to load cases below the present minimum load (calculation of low load operation at 20 per cent);
    • Thermal modelling and geometrical segmentation of the spiral evaporator section;
    • Calculation of static and dynamic stability for the spiral evaporator section;
    • Calculation of static stability for all heating surfaces passed downstream;
    • Determination of the stable minimum load of the steam generator without modification;
    • Comparative investigation of potentials and limitations for different stabilization concepts.

    Evaluation of evaporator spiral

    Evaporator instability has been an issue for all boiler manufacturers for several decades.

    Typical specific design mass flow through evaporator tubes in case of lignite-fired units is in the range from 2000 kg/(m² s) at full load to approximately 800 kg/(m² s) at Benson minimum load. If the specific mass flow rate reaches values below this limit, a reliable cooling of all individual tubes cannot be ensured. Typically, no operational experience is available in that flow regime.

    For that reason a detailed geometrical and thermal analysis of the evaporator tubes will be carried out in order to identify their behaviour at lower specific mass flow rates.

    First, a detailed analysis of the geometry parameters must be carried out. The routing of all individual tubes has to be traced and compared in order to identify the shortest and longest tube, also taking into account individual flow resistance.

    Investigation of the static distribution of the medium into the parallel tube system of the evaporator walls may give a first indication of potential instabilities. During the present investigation, a more important analysis appeared to be a dynamic analysis of the evaporator wall system. The medium inside the tubes starts as liquid feedwater, evaporates during the flow through the tube, and leaves the tube as steam or, depending on the operating conditions, as a two-phase steam/liquid mixture. This means that, with regard to flow behaviour, the medium consists of incompressible liquid and compressible gaseous steam. Under certain conditions, the combination of compressible and incompressible amounts may lead to an oscillation, which may subsequently result in time-dependent variation in mass flow and thus in tube cooling.

    The evaporator tube reacts to a sudden change in heat input. Due to an incident, it is assumed that the heat input is increased for 10 seconds to 120 per cent of the normal heating rate of the tube. The oscillation of the inlet and outlet mass flow is a result of the suddenly increased heating rate. The system can be considered as ‘stable’ as long as the oscillations level off and reach the initial behaviour after a certain time.

    In contrast, with the same tube at different load with the same disturbance in heating rate, the system responds with an increase in oscillation amplitude. This is an example of an ‘unstable’ system clearly below the critical allowable load.

    This analysis allows us to identify the critical specific mass flow for a stable operation of the evaporator. In the present case, stable operation of the evaporator spiral is possible above 30 per cent load.

    Evaluation of tube banks

    Besides the detailed static and dynamic analysis of the evaporator, all other heat transfer surfaces are also examined with regard to potential instabilities. While all superheater and reheater bundles are uncritical, the analysis revealed potential unstable behaviour for the economizer below a certain boiler load.

    The reason is that the flow through the eco is directed downwards, which means pressure drop due to flow resistance and pressure increase due to geodetic height have opposite direction.

    At low mass flow this may result in the situation that, at the same overall pressure loss in the tube, two solutions are mathematically possible. One solution is characterized by a flow rate close to the nominal value, while the other is characterized by a very low flow rate (see Figure 7, page 42), meaning that, in a parallel and heated tube system, some tubes with high flow rate and some tubes with very low flow rate may exist simultaneously. Consequently, the low flow rate tubes may not be cooled enough and may be damaged due to material temperatures that are too high. The result is that the economizer is potentially unstable below 50 per cent load.

    Figure 7. Pressure drop of a single economizer tube vs relative mass flow as a function of boiler load
    Figure 7. Pressure drop of a single economizer tube vs relative mass flow as a function of boiler load

    As a result of the investigation several different options were derived in order to increase the stability of the eco and thus to decrease the minimum stable boiler load. These options are, for example:

    – Installation of additional eco-nozzles in order to increase pressure drop;
    – Different methods for establishing an eco circulation in order to increase specific flow rate and achieve stabilization;
    – Inverting the eco into an upward-directed flow.


    Minimum boiler load reduction is a challenge initiated by the present market boundary conditions. Optimization of existing boilers is requested, and further reduction of ‘stable’ minimum load is targeted.

    Solutions vary from optimization of operating procedures without any hardware modification to extensive modifications with additional hardware installations. But in all cases an individual detailed analysis of the existing system is the basis for successful modification towards lower load operation.

    Christoph Nachtigall is Process Engineer for Thermal Engineering; Dr Stefan Hamel is Head of Thermal Engineering; and Dr Christian Storm is Head of Process Engineering (Combustion Systems) at Babcock Borsig Steinmüller GmbH.

    This article is based on a Best Paper Awards winner at POWER-GEN Europe 2014.

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