Impact of renewable energies on flexibility of coal fired power plants
Impact of renewable energies and full load hours
The development of the full load hours of conventional power plants in Germany is influenced by both the increasing part of renewable energy sources and the shutdown of nuclear power plants. Another influencing factor is the new situation of the coal power plants within the merit order as described in Section 2.2. The actual development of the full load hours of the German power plants is depicted in Fig. 1. Apparently, the shutdown of nuclear power plants is compensated partly by coal power plants. As a consequence, the full load hours of lignite and hard coal power plants still remain on a high level. Due to the influence of the increasing part of offshore wind power in Germany with more than 3000 full load hours, the operating time of coal power plant is expected to decrease in the near future.
Flexibility
The increasing share of renewable energies, especially of wind and photovoltaics, leads to sharper power gradients. The present situation and the estimated future development of these power gradients in 2023 and 2033 are displayed in Fig. 2. Today, the value of the hourly power gradient is about 5 GW. This value corresponds approximately to the power range of the biggest German lignite power plant Neurath with 4.4 GW.
The power gradients depicted in Fig. 2 have to be compensated in future by more flexible operating power plants as well as the usage of storages, demand side management, and European electricity exchange.
The flexibility capabilities of typical conventional power plants in Germany are listed in Table 1. From the values of the maximal power gradients, it may be concluded that no type of conventional power unit alone is capable of managing the expected power gradients from wind and solar.
Another future problem arises from the vanishing base load. With the increasing share of wind (especially off-shore wind energy) and solar energy forces, the base load in future at extended time periods tends to be zero. It is expected to have no base load in 2050 at about 3000–5000 h [
]. This fact is in a serious contradiction to the possible minimum power of conventional power plants depicted in Table 1 and longer shutdown periods will be necessary.
Impact of renewable energies on the economic situation of coal fired power plants
Development of coal fired power plants in Germany
The actual situation and the age structure of conventional power plants in Germany are depicted in Fig. 3. About 60% of the conventional power plants started its operation before 1990. Due to the decision of the Federal Parliament, the last nuclear power plants in Germany will cease their operation in 2022. The last German hard coal fired power plant Moorburg near Hamburg with a nominal power of 1700 MW was commissioned in 2015. According to the situation in Germany, with the pressure to reduce the carbon dioxide emissions, it is expected to be the last new conventional coal fired power plant in Germany.
Merit order change and impact on stock exchange prices
The increasing supply of renewable energies leads to a lowering of the electrical energy prices due to the so called merit order effect at the energy stock exchange market. For the explanation of this effect, a typical merit order curve from the European Energy Exchange (EEX) is shown in Fig. 4. This merit order curve contains the marginal costs of several types of conventional power plants corresponding to their bids at the stock market of the EEX. The power plants with the lowest operating costs like nuclear or lignite power plants are able to provide the lowest bids. Natural gas or even mineral oil burning power plants represent the highest marginal costs due to their elevated fuel costs at low full load hours. The overall power price at the energy exchange is determined by the highest bid from the power plant just needed to satisfy the load demand. To give an example from Fig. 4, a power demand of 60 GW leads to an overall power price of 40 €/MWh with contributions from nuclear, lignite, hard coal and natural gas power plants. Mineral oil power plants would not contribute in this case.
Electrical energy from renewable energy sources has to be sold on the energy exchange, too. Because of their very low operating costs, they force more expensive power plants to the right end of the merit order curve. Consequently, these expensive power plants are vulnerable to be pushed out of the market. Renewable energy sources such as photovoltaic installations and wind turbines reduce the overall power price. If an additional power supply of 40 GW of renewable energy were added to the merit order curve in Fig. 4, the overall power price would be reduced to about 20 €/MWh at a load of 60 GW.
Therefore, it is expected to have an increasing pressure on the energy exchange prices in future due to the expansion of renewable energy installations in Germany.
Impact on primary control power operation of power plants
Primary control power is an essential ancillary service to be delivered by modern conventional power plants to uphold the system balance between power generation and consumption, and consequently, to keep the power frequency exactly to 50 Hz. Due to the small portion of hydro power plants in Germany which are best suited for this purpose, the main sources for primary control power are still nuclear, and coal and gas power plants. The response time for the delivery of primary control power is 30 s. A primary control power source must be capable of supplying the maximum primary control power within this time interval. Corresponding to this rapid flexibility demand, a primary control power range of about± 5% of the nominal power is viable with modern lignite power plants. Besides the control power supply, the inertia and the kinetic energy of the rotating masses of the turbine-generator-arrangements of conventional power plants are still a significant property for the frequency stability of the electrical power system in the case of a sudden power deficit.
With the German “Energiewende (German for energy transition)”, intensified efforts have been made to deliver primary control power as well as artificial inertia by new renewable and decentralized energy sources. In recent years, several battery storages in the megawatt-range have been erected to provide primary control power. Actually, the installed primary control power by batteries in Germany already exceeds 30 MW. An additional power of 75 MW is announced to be installed in the next two years. Therefore, batteries will supply a sixth of Germany’s overall primary control power demand of about± 600 MW in the near future. The increasing concurrence in the primary control market, which is based on the principle of bids, has a serious impact on the market prices and hence on the revenues of conventional power plants in this market segment. The average capacity price for the weekly bid dropped e.g. from about 3700 €/MW in 2015 to about 2500 €/MW in 2016 [
].
Maintenance and corrosion problems with dynamic operation of coal fired power plants
The liberalization of the energy markets and the spread of renewable energy sources will lead to a high penetration of unpredictable energy sources such as wind and solar, with a significant impact on the electricity market. Flexibility, availability, and fast cycling have become fundamental concepts to be competitive in this new electricity market. For this reason, thermoelectric units need to switch from base-load to cycling operation: an operation mode characterized by fast load ramps, short start-up and shut-down time, which permits to enhance the power plant’s competitiveness and to maintain the grid stability. New operating requirements including two-shift operation, island operation, load-follow operation, black start capability, and severe start-up for fossil-fuel power plants arise in order to stabilize power grid dynamics and ensure economic electricity supply. This new kind of operation strategy determines a significant reduction in the lifetime of the most critical power plant devices which are subjected to thermo-mechanical fatigue, creep, and corrosion [
]. It has been seen that about 60%–80% of all plant failures are a result of cycling. Figure 5 shows the common equipment problems that are related to cycling [
].
The corrosion problems include wet corrosion (non-operating corrosion, normal operating and special procedures), high temperature corrosion, and stress corrosion crack [
]. Wet corrosion may occur in areas of the economizer. Cold water reduces the metal temperature (structure or tubing) below the acid dew point of the stack gas during low-load operation. When the stack gas is reduced below the acid dew point, the minute portions of sulphur that remain in the gas can combine with the condensed moisture and form dilute solutions of very corrosive sulfuric acid on tubes and structures [
].
High temperature corrosion describes the degradation of material surfaces in boilers by flue gases and (molten) coal ash containing elements such as sulphur and chlorine. The accumulation of coal ash and corrosion by-products on metal surfaces has two effects. Firstly, heat transfer rates are decreased, resulting in reduced thermal efficiency. Secondly, material strength is reduced through overheating and the gradual reduction of cross-sectional area. Generally, the rate of fireside corrosion increases with flue gas temperature. For coal combustion, fireside corrosion occurs in the high temperature gas phase, high temperature gas and liquid phase, and low temperature liquid phase, as explained subsequently [
]. High temperature corrosion and corrosion fatigue damages can be found in membrane waterwalls. Figure 6 shows tube damages resulted from cyclic operation [
,
].
The same corrosion type is detected in the steam-cooled wall of heat recovery areas. The steam-cooled sidewall has a damaged economizer header penetration (see Fig. 7). The cycling causes a differential thermal growth, and the penetration is badly damaged [
].
Stress corrosion crack (SCC) is generally found to occur in waterwall tubes. Particularly, it is detected at attachments such as corner tubes, wall box openings, etc. SCC in the waterwall tubes at the buckstay attachments used to resist the internal pressure of the furnace occurs by non-uniform thermal stresses cycles [
].
Actual research and development for flexibility of coal fired power plants
Advanced coal drying for lignite power plants
German raw lignite has a moisture content of approximately 55% depending on its origin. To increase the net efficiency of a coal fired power plant, a lignite pre-drying process is used. In the latest dryer optimization project, a special process for advanced lignite drying is developed: the pressurized steam fluidized bed drying (PSFBD). The unpressurized evaporation drying of lignite has been used in Germany for several years. For example, the company RWE has used a 170 t/h evaporation dryer since 2008 at the lignite power plant in Niederaußem, North Rhine-Westphalia. The new feature is to increase the pressure inside the dryer. One main effect is that the steam parameters of the evaporated coal water are delivered on a higher pressure and temperature level. This enlarges the possibilities for the further use of the steam (evaporated from the wet coal) in the water-steam-cycle of the power plant. Also, the heat transfer coefficient is rising when the pressure is rising, which leads to a better drying kinetics, resulting in a decrease of the size and thereby the investment costs of the dryer. Of course, there are some challenges which have to be managed, for example, the inflation of the dryer against the pressurized atmosphere with steam and fine coal dust. Besides, a lot of effects of pressurization have to be considered, for example, the higher saturation temperature, the change of the density of the fluidization steam, and the change in equilibrium moisture content of the lignite. The functionality of the pressurized steam fluidized bed (PSFB)-dryer is shown in Fig. 8. Lignite flows from the top to the bottom during the drying process. Water vapor is used as fluidization gas. The drying energy is mainly supplied through banks of steam-heated horizontal tubes in the fluidized bed. These tubes can be heated by any medium, although water steam is normally used for lignite drying. The requirements for the heating steam pressure depend on the drying pressure and the desired driving temperature difference. The PSFB-dryer component can increase the net efficiency of a coal fired power plant by 4%–5%. In context of the German “Energiewende”, another feature of the pre-drying of lignite gets more and more important. Because of the decoupling of the milling and drying process from the downstream firing process, a dry lignite-based ignition and firing support system can be used instead of the current oil-based systems, which are characterized by high operating costs. Through this change, the control range of the unit can be extended downwards and cost intensive startups and shutdowns can, therefore, be avoided.
In the PSFBD system, some selected research tasks include:
Task 1: Optimization of the geometry of the heating tube bundle: finding the best adjustment of tubes for maximal heat transfer for a tube bundle under consideration of the necessary conditions of flow for a stable fluidized bed and the required mechanical strength of the tubes depending on the tube diameter, length, thickness and material. In Ref. [
], a wide range of tube diameters and different horizontal and vertical distances have been investigated.
Task 2: Integration of heat and mass transfer into the actual CFD-model: Asegehegn [
] has modeled and simulated the flow conditions in a fluidized bed. Schreiber [
] has integrated the heat transfer to the CFD-model. In actual works, it is planned to integrate the mass transfer into the CFD-model. Therefore, the whole drying process can be simulated only in a small scale or 2-dimensional. Höhne [
] has investigated the influence of pressure on equilibrium moisture content as the boundary conditions for the drying simulation.
Modern ignition systems for coal power plants
For start-up or partial-load operations, oil fuel is primarily used to pre-heat the combustion chamber and ignite the main burner of coal power plants. This ignition system consists of several oil burners around the combustion chamber and primarily two ignition burners are supporting one coal burner level which is supplied by one coal mill. Furthermore, the oil burner is more often necessary for supporting the combustion, because of increasing partial-load operation time, new minimum load requirements with two or one mill operations, and increased speed of load-changed operations. Concerns about increasing economic costs of pulverized-coal-fired power stations arising from oil fuel consumed has spurred interest in developing oil-free ignition systems. Various investigators have reported studies of plasma-fuel systems [
] or microwave assisted burners for a thermo-chemical preparation of coal for burning [
]. However, among these ignition systems, the electric capacity of plasma systems is high and a frequent maintenance is necessary during operation.
Therefore, the focuses of oil-free ignitions systems for lignite power plants in Germany are on pre-drying of lignite and its usage for start-up swirl burners with a capacity of around 30 MW. The ignition of such burners is possible with central light-oil or gas burner, which are only needed for an initial ignition step. Furthermore, to avoid all oil or gas supply systems, a necessary initial ignition step is taken by a low capacity plasma system. A plasma system based on the microwave technology with a capacity of 3 kW has been investigated to ignite pre-dried lignite particles [
]. Moreover, one of the 815 t/h steam boilers of the power plant Jänschwalde in the federal state of Brandenburg has been converted with 8 start-up swirl burners equipped with initial ignition plasma systems [
]. These start-up burners are supplied with pre-dried lignite from one silo, which could be used to pre-heat the combustion chamber at extreme partial-load operation and as a booster fire system to compensate a fluctuating fuel quality of lignite. The pre-dried lignite from an industrial preparation process is used for the investigations, and the ignition of lignite from a pressurized steam fluidized bed drying process is found to be successful.
For power plants fired by hard or bituminous coal, the requirements for ignition in terms of high-moisture are not present. Therefore the start-up burner has almost a similar design, with burner air-staging, swirl assemblies, and a fuel nozzle ring where a part of coal particles lose their flow velocity for the time necessary to start the pyrolysis. Due to the burner’s flow pattern, hot flue gas is circulated between primary and secondary flow, which induces the heating of the fuel nozzle [
]. For the initial ignition step, the fuel nozzle ring is pre-heated to approximate 700°C. Primary air is set to a minimum flow during the heating phase. After turning-off the heating process, primary air flow is set to the nominal value prior to the fuel feeding starts with solid pulverised fuel. Opening a valve and injecting pulverised fuel into the primary air flow has to be done very fast in order to prevent the fuel nozzle from cooling down. This technology has been tested in a thermal power station with a 450 t/h steam boiler. In this case, the heating process is supplied by electric energy [
]. The possibility to ignite pulverized coal at the hot fuel nozzle surface has been safely demonstrated several times. With such start-up burners, a combined firing system comprising of a dust silo installed between mills and burners is of interest. This silo can be loaded at times when the plant operates at a low or minimum electric power load and discharged at times with a quick increase of power output or during start-up operations. The loading of the silo can be shifted timely so it is independent of the actual operation mode of the plant. The idea is to shift the auxiliary load needed for milling to operation times with a low load (e.g. at night).
Higher Education Press and Springer-Verlag Berlin Heidelberg
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