The expansion of renewable energy resulted in new challenges for the operators of coal-fired power plants. Continuous full-load operation is a thing of the past. Operation must become more flexible to balance volatile feed-in. At the same time, they must remain profitable and safe. TÜV SÜD accompanied a project in Baden-Wuerttemberg, Germany, that shows how this can be achieved.

The share of renewable energy in German power supply is continuously growing. According to the German Federal Ministry for Economic Affairs and Energy, 29 percent of the total gross electricity production in 2016 came from renewable sources. But the residual load varies by up to 70 gigawatts per day. This is related to the consumption peak values and weather-induced factors. These variations are balanced by regulatable power plants. To react quickly on fluctuations during feed-in and, at the same time, to produce in a profitable and safe way, technical modifications or adjustments to operation are needed for thermal base-load power plants.

What are the new strategies?

Contrary to the original strategy to continuously run the plants with full load, more flexible operation is required now. Characteristic features are mainly fast load change rates and operation in the lowest load range. From an economic point of view, under specific circumstances it might be more profitable to slow down the plant to the lowest load point and to quickly power up when needed compared to temporarily stopping electricity production completely. A possible mode of operation that was tested in a coal-fired power plant and that might meet the new requirements is the so-called one-mill operation. Due to their solid design, in particular, older plants are well suited for this approach. They are often designed with an optimal reserve potential. New plants are, at least partially, designed for more flexible modes of operation, but cannot be scaled down to 50 to 100 megawatts due to their performance capacity. Furthermore, the firing system included in the steam generator is of vital importance.

It should also be noted that a modified mode of operation requires an adaption of related documents. Next to the process instructions and control requirements this includes fire control programs and implemented protection requirements and concepts. To assess possibly arising risks, in general, a plant-specific risk analysis is carried out based on the industrial safety regulation and is adjusted to the new operational requirements.

Initial situation for Baden-Wuerttemberg plant

When operating under full load, four coal mills were used to generate the required rated thermal input. To allow more load flexibility, it was determined a new regular mode of operation should be established – operation in the lowest load range. So far, this was only common during startup and was needed to synchronize the generator. Fifteen to 20 percent steam output was to be achieved. To do so, the coal dust inlet was to be clearly reduced. To realize this, one-mill operation without backup fire with oil or gas burners was used. The prevailing opinion was that backup fire was needed to achieve a minimum-rated thermal input. Only after this was achieved and another coal mill was connected, the backup fire was reduced or switched off. Thus, defined load conditions for sole coal operation were at 30 to 40 percent. Furthermore, at least two coal mills were common or planned for this mode of operation. This was also logged accordingly in the operating and process control technology.

Firing is what matters

In the Baden-Wuerttemberg  coal-fired power plant, it was the operating engineers’ task to specifically target the load limits of a power-generating unit. In the steam generator, a so-called tangential, corner-fired furnace was installed. For this firing concept, a total of 32 coal dust burners were located on four levels around the boiler. Coal dust was fed in the furnace chamber and created a fire cyclone when rising in spirals. This ensured even heat distribution at all times. Further advantages were, for example, optimal control of fuel and air with low fuel-air ratio. The system known as EVT concept is vital for the realization of the new mode of operation since it facilitates the one-mill operation without backup fire, thanks to its stable firing. Thus, high-load flexibility down to the lower control range is possible with only one coal mill.

Disassembled grinding roller yokes, Coal-fired power plants, TUV SUD,

Disassembled grinding roller yokes

Detecting consequences of the new mode of operation

When changing the operation of a plant numerous issues can effect startup and operating optimization. This is accompanied by possible deviations from currently existing operating manuals, the fire control programs, regulation requirements and approval documents. Changed reaction mechanisms might affect downstream heating surfaces and the operating parameters to be achieved. Reduced rated thermal input and frequent starting up and shutting down can also influence the unburnt parts in flue gas.

In the case of the power plant unit in Baden-Wuerttemberg, further questions had to be answered, such as:

  • Does the changed mode of operation influence the so-far common emission values?
  • Do the measuring devices require different calibration due to the new load conditions?
  • Do the changed operating conditions such as fast startup and shutdown possibly influence wear and tear of single components?

These questions were finally assessed positively and did not require fundamental modification.

In the field of pre-airing concepts, reasonable adjustments must be discussed. This involves the replacement of the pre-airing of the steam generator, which was common according to the set of rules, with a suitable post-airing concept after switching off the furnace. This would lead to higher startup availability without previous pre-airing the steam generator. Prerequisite would be an additional control measure to ensure that after switch-off, no fuel uncontrollably enters the furnace chamber through possibly defective isolating equipment. This calls for rethinking with regard to former interpretation of the set of rules and could affect the surveillance companies’ work.

During future operation, not only targeting of load limits will be possible. Efficiency of process control is to be further improved based on a plant-specific process model. The goal is forward-looking operation that coordinates several influencing parameters and that can be controlled online in real time. To make this become reality, profound knowledge of the plant and its system is crucial. It is essential to describe new limits and to assess in detail the risks and consequences of the modified operation and its effects. To do so, existing operating manuals and instructions need to be aligned and knowledge of possible deviations and their effects on possible errors and damages is required. The focus was furthermore placed on the verification that operation with a partial load reduced to 15 to 20 percent is possible.

Conclusion of the test runs

It turned out that the quality of the used coal can have a stronger effect on the characteristics of fly and wet ash than it had before. This should be taken into consideration by operators when purchasing coal. When measuring emissions, no significant thresholds were exceeded. The inlet temperature of flue gas in the catalyst was within an optimum temperature range above 300°C that is needed for the process – and was within this range during the complete testing period. The flame signals detected by the flame monitors installed at the coal burners were stable. The reason: Operation with only one coal mill is carried out with higher load of coal dust and results in higher firing quality and thus higher rated thermal input. This is different from two-mill operation. In this mode, the rated thermal input per mill is clearly lower.

The experts stated that during the tests with one-mill operation, increased risks or dangers were not detected in any operation mode. Based on these results, a specific mill level was selected. The new requirements of future one-mill operation are already described in the fire control program. Similarly, the control technology and boiler protection for light-load operation with two mills was adjusted to one-mill operation. In the course of this, one signal was cancelled entirely. With regard to actual boiler and furnace protection, nothing has changed. From an operation perspective, changes to the fire control program become necessary. Compared to the former operating condition, no increased risk could be detected.

Which measures were necessary, which weren’t?

Adjustments were realized with regard to process engineering and the fire control program. Constructive measures were not required. Fundamental for the organizational measures is the new fire control program and the related training of employees and operating staff. For this purpose, the already existing operation-specific risk assessment has been updated.

Afterward, TÜV SÜD engineers realized supportive tests and compared the changed operation. They created an adjusted process description of the future operation of the plant based on the existing operation manual and supplementing technical documents. This was followed by testing and assessing the changes to the control systems, the adjusted risk assessment according to the industrial safety regulation and the existing process description. This was based on the documentation available to the operator and TÜV SÜD.

New mode of operation since August 2016

Since 2016, the operator has been able to operate another power plant unit with only one mill. Flexibility was realized in terms of control systems with a new firing program. However, the plant’s infrastructure has not changed fundamentally and no decisive effect on the live steam parameters was detected. Existing safety requirements only had to be adjusted to a minor degree.

All realized, adjustments did not constitute fundamental modifications and did not require a permit from a competent authority. For coal-fired power plants requiring monitoring that were put into operation for the first time before Jan. 1, 2003, the then-valid regulations and set-of-rules requirements remain effective.

Hans Christian Schröder is senior expert, Power Plants, at TÜV SÜD Industrie Service, Mannheim. He may be reached at hanschristian.schroeder@tuev-sued.de.