More flexibility in power grids

On 24 January 2017, the power demand in Germany amounted to 80 GW of electricity. However, the wind turbines and solar power plants failed to feed even three gigawatts into the grid. On this day, conventional power plants were more in demand than ever. Although solar and wind can cover electricity demand, the power they feed into the grid is subject to fluctuations. For as long as this problem persists, conventional power plants must also be able to be operated flexibly. This means they must be able to increase or decrease their electricity production on a daily basis, and plant operation at part loads must also be possible over longer periods.

When the plants are run at lower part loads – a mode of operation which in the past was characteristic of start-up operation – auxiliary firing fuelled by oil or gas provides for a minimum thermal output in the range of between 30 and 40%. On reaching this thermal output the first coal mill and subsequently further coal mills are activated. It is only once the defined thermal output of between 30 to 40% has been achieved that operations can continue exclusively fired by coal and the auxiliary firing can be gradually cycled down. In new and modern power plants, power plant operations exclusively fuelled by coal is generally only possible with multiple coal mills and the operations control system is designed accordingly.

Figure 1. View of a coal burner. Source: Hans Christian Schröder.

Single-mill operation without auxiliary firing

In the meantime, tests have demonstrated that some power plants can be run in single-mill operation under certain conditions. In this mode of operation, the minimum load may even be reduced. More modern power stations were to some extent designed for operation at low part loads. However, so far, there has been no dedicated approach to pursuing single-mill operation. As these new power stations were designed for high output, they cannot fulfil the required live steam parameters at the lowest part load, which poses a problem. Another issue concerns major imbalances in temperature distributions in the individual circuits. Tests have shown that the coal mill itself can be run safely and in a stable manner even at lower part loads. In other words, the flame signals can be clearly detected. The fact that the mill was designed for a certain minimum filling level of material to be ground in the grinding bowl and that this filling level acts as shut-off signal may also present a problem. The question arises whether this filling level can be reduced depending on the quality of the coal without leading to increased wear of the grinding roller yokes. Another possible consequence are fluctuations in the discharge of ground material into the combustion chamber. Plant suppliers and plant managers must address these issues in a constructive and collaborative dialogue.

Older plants offer the option of adjusting the operational systems. When doing so it becomes clear that – compared to more modern plants – many of the existing plants were ideally designed in terms of process engineering and operational equipment. The reason behind this is that these plants were designed with large safety margins to allow for more load flexibility.

Figure 2. Disassembled grinding roller yokes. Source: Schröder-GKM.

Thus, older power plants are particularly suitable for switching the mode of operation to single-mill without auxiliary firing. They can be run at lower part loads – provided the design of both the combustion chamber and the implemented firing system is such that the firing will run at a stable manner even at partial load. Reliable ignition of the pulverised coal supplied to the system must be possible at all times even if an additional coal-mill is added in the event of a possible increase in load. This ensures reliable detection of the firing signals by means of a flame guard. An example of single-mill operation without auxiliary firing can be seen in a large-scale power station in Southern Germany. Single-mill operation was ‘established’ in one of the power station's generating units in 2015. Up to that date, the generating units had been run at a part-load of around 30% of full load in two-mill operation. Since the switch to single-mill operation, the minimum load has been reduced to the lowest level possible from the perspective of process engineering. The steam capacity achieved ranges between 15 and 20%.

Combustion chamber with tangential corner-fired burners

In our example at hand, single-mill operation is sufficient as the combustion chamber, the implemented firing system and the coal used meet the relevant requirements. The steam generator, for example, is a classic model with tangential corner-fired burners and four bowl mills. Thermal output was designed for 1140 MWth at a production of 475 MWel of electricity. In addition, the steam generator was designed for intermediate superheating. The tangential corner-fired burners permit ideal firing throughout the stage of steam generation. The steal burners in the corners blow the pulverised coal directly and horizontally into the combustion chamber where it is optimally mixed with the volume of air fed into the chamber. The burner nozzles are symmetrically arranged at each of the four burner levels, ensuring even distribution of heat across the cross section of the evaporator and excellent burnout of the pulverised coal. This type of firing solution enables a high level of flexibility in terms of load even at lower part loads as heat spreads ideally in the combustion chambers and the downstream heating surfaces. In simple terms, the combustion chamber is the actual burner.

The coal used is of major importance in single-mill operation. Its composition impacts on the amount of combustible components in fly ash. Single-mill operation thus can significantly impact the quality of fly and wet ash. Quality must be constantly monitored to ensure compliance with the required limit of < 5% of non-combustible components in fly ash.

Safety assessment

TÜV SÜD supported the change of the generating unit from the perspective of power and safety engineering. A significant parameter was the firing control programme which is aligned to the design of the combustion chamber in question and can be used to influence the operational regime of the coal mills.

Figure 3. Coal mill. Source: Schröder-GKM.

The transition to single-mill operation without auxiliary firing does not constitute a major change as defined in the Steam Boiler Regulation (DampfkV), which applies to older power stations commissioned before 1 January 2003. In the present case, the change did not affect the validity of the operational permit. Issue of a new permit by the competent authority was not necessary. Neither does single-mill operation require any change in the implemented safety-related functions. Operation of the power station continues in an environmentally compatible manner within the legal limits.

Conclusion

Single-mill operation makes firing safer and more reliable. The reason is that plants in single-mill operation can be run at higher load conditions than plants in two-mill operation. Given this single-mill operation offers higher operational flexibility.

Written by Hans Christian Schröder, TÜV SÜD Industrie Service, Germany.

Read the article online at: https://www.worldcoal.com/special-reports/04092017/more-flexibility-in-power-grids/