The implications of one-mill operation for the operating regime and control of a coal plant are outlined by Hans Christian Schröder

Power Engineering International May 2017

In running existing power plants, operators want to achieve greater load flexibility.

The objective is to operate the installations down into their lower load range. Until now, these new operating conditions have existed or been common only during startup operation.

In relation to this new operating regime for older plant, it should be noted that load conditions for coal-only operation were often defined as a standard that lay above approximately 40 per cent.

Of fundamental importance here was the view that coal-only operation without supplementary firing presumes a minimum necessary thermal input for safe operation. This means that the plants were initially fired up to a predetermined minimum level of thermal input using oil or gas burners. Only then were individual coal mills successively switched on in a defined operating mode.

Credit: Schröder-GKM
Credit: Schröder-GKM

After switching on the first coal mill, the thermal input from the supplementary firing is reduced. When the second mill is switched on and a defined thermal input from coal is achieved (e.g., 40 per cent), supplementary firing is switched off. With a minimum thermal input being specified up to that time, coal-only running was often only possible or customary with two-mill operation.

This operating state is also the basis of a plant-specific combustion management program that is saved in the operation and process-control systems, often with corresponding free-load computers. It is based on existing written procedures as well as the associated process-control and safety requirements. These operating conditions also have a definitive role in the licensing and acceptance processes, and in periodic inspections.

The supporting documents must also be adapted to the changed operating regimes. Along with the procedures, manuals and process-control requirements, these include in particular combustion management programs and safety requirements and concepts. Based on the German Betriebssicherheitsverordnung (Ordinance on Industrial Safety and Health), a plant-specific risk assessment to evaluate potential hazards is usually undertaken and then adapted to the new operational requirements.

A German municipal utility operates four hard coal-fired steam boilers with downstream steam turbines in a city in the state of Baden-Württemberg. It conducts ongoing reviews of measures to increase efficiency and enhance operating regimes that feature load flexibility. Since its conversion from gas to coal firing in 2002, one block has been run with one-mill operation. This block has a rating of 280 MW gross and was originally equipped with three coal mills. Because one-mill operation was reliable from the start, more blocks were considered.

In the two other blocks, the minimum thermal input was previously between 30 and 40 per cent. The supplementary fuel was oil, with two firing levels: starting the first coal mill and switching on the second coal mill. Operation without supplementary firing took place with at least two coal mills. The first preliminary tests showed that only one of the two blocks was suitable for one-mill operation. For this block, the first of the ongoing measures were implemented in 2016.

The plant was commissioned in 1993. The steam boiler is a conventional design with tangential corner firing and four bowl mills. The steam boiler was designed for a rated thermal input (RTI) of 1140 MWth and for generating an electrical output of 475 MWe, and includes simple intermediate superheating.

Tangential firing systems typically employ jet burners. There are four burner levels in the steam boiler, with two burners in each corner and level. This steam boiler therefore has 32 pulverized-coal burners. These are arranged in such a way that they create a central flame cyclone in the combustion chamber. Put simply, it can be said that the combustion chamber is the actual burner.

High combustion quality

Pulverized coal and combustion air are mixed directly in the combustion chamber and swirled around in an optimal manner by an array of parallel top, bottom and intermediate nozzles. With the burner nozzles being symmetrically arranged in each plane, the heat is evenly distributed over the cross section of the evaporator. This facilitates very good burnout and high combustion quality. Other advantages include optimal control of fuel and air, with a low fuel-air ratio.

High load flexibility down to the lower control range is therefore achievable with one coal mill. In the previous two-mill operation, a recommended lower load of around 30 to 40 per cent was applied. This load range was also the recommended connection parameter for the steam turbine when synchronized with the power grid. The plants can attain even lower minimum loads, however. This has been demonstrated by test runs in recent years. This is because older systems were often designed with high levels of reserve, which today means more scope in load flexibility.

Previous operation with hard coal

The basis for the approval of the previous operation was the requirements of the German Technische Regeln für Dampferzeuger – TRD (Technical Regulations for Steam Boilers) and the supplier’s procedures and operating instructions. The associated requirements for firing are described in the TRDs:

• TRD 411: Oil firing for steam boilers;

• TRD 412: Gas firing for steam boilers;

• TRD 413: Pulverized-coal firing for steam boilers.

The primary safety objective was to operate the combustion processes safely. It was necessary to achieve minimum starting conditions for the safe introduction into the combustion chamber of coal via feeder, coal mill, and classifier. The pulverized coal had to be optimally distributed in the combustion chamber and be safely ignited by the supplementary firing. It was also necessary to ensure an optimum fuel/air ratio (Lambda). Any possibility of detonation-combustion should be prevented. Reliable detection of the combustion signals was also important. This is done with the flame detectors used in the individual burners. The minimum starting condition for the first mill was to be achieved with the existing ignition firing and supplementary firing at around 30 to 40 per cent of the rated thermal input.

The suppliers have created the associated operating manuals and instructions on the basis of the abovementioned regulations. The boiler operators run and maintain the plants using these manuals and instructions. The documents should include more than just a) an arrangement diagram of the coal feeding, pulverizing and combustion plant, b) the description of the process and functions, as well as c) an explanation of the operational concept.

A test procedure for the flame-monitoring systems was also required, as were maintenance instructions. Instructions for putting the coal feeding, pulverizing and pulverized coal-firing plant into and out of service were important, likewise a description of the actions that should be taken in the event of faults or dangers. The abovementioned documents, which were also part of the approval procedure, were reviewed on the basis of, among other factors, the TRD 520 guide to the procedure for permission to set up and operate steam boiler plants and the procedure for submitting them for approval.

Future operation

The future aim is to be able to run the coal block down to the lowest possible load points. The objective: further optimization of operating costs and customized load management under more flexible operating conditions.

But what does a flexible operating regime mean for reducing the existing minimum loads – and also in relation to higher load gradients? In future, numerous issues will affect optimization of startup and operation. Changed operating conditions also mean possible deviations from the previously existing operating manuals, the combustion management programs, regulatory requirements, and approval documents.

Altered response mechanisms can also affect the downstream heating surfaces and the operating parameters that are to be attained. A reduced thermal input and frequent starting and stopping can also influence the unburned fractions in the flue gas. Some other questions were: What is the impact of any changes to the existing instrumentation on the detection of emissions? How can their ability to be calibrated for these new load conditions be checked? What is the influence of the water chemistry in the steam-boiler circuit?

Also to be clarified was whether the usual pre-purging concepts needed be examined. Is a post-purging concept possible, i.e., after firing has been switched off? In this case, a qualified assessment is needed of whether any fuel can reach the combustion chamber in an uncontrolled manner during the stoppage. This measure will facilitate a judgment on the matter of whether another pre-purging is necessary before the start of firing. Last but not least is the question of which areas of the plant may possibly be subject to greater wear.

Conventional vs more flexible

Until now the plant has been operated conservatively. The design limits and operating manuals have been adhered to on the basis of the original planning. The future goal, however, is not just a precision run-up to those design limits. The efficiency of the process control should also be enhanced by means of a plant-specific process model. The objective is a look-ahead operating regime, which co-ordinates several influencing parameters and can be controlled online in real time. In addition, any feasible modern tools such as trend-quality monitoring and lifetime monitoring (especially fatigue-related) are to be applied.

All of this requires plant knowledge at the whole-system level. New threshold parameters need to be specified. The risks associated with the future plant operation and its effect on operation, wear and maintenance must be reliably assessed. This requires appropriate amendments to existing operating manuals and operating instructions to be made, and needs a knowledge of possible deviations, faults and damage. A major objective is to achieve the lowest load point for steam production from the boiler. Various test runs should demonstrate whether it is possible to bring the current minimum load down to a level of 15 to 20 per cent.

The steam boiler’s current technical minimum load of 380 t/h was to be reduced to 270 t/h, which corresponds to 20 per cent steam-boiler load in one-mill operation. It was necessary to control the exhaust steam temperatures in the high-pressure region (HP) after the topping turbine and to increase, if applicable, the HP steam pressure from 80 bar to 100 bar or 120 bar.

The maximum allowable HP exhaust steam temperatures were 420°C, because the cold intermediate superheating line between the topping machine and the boiler is designed for this temperature, whereas the HP exhaust steam turbine casing is designed for 450°C.

To compensate for differences, an interruptible connection for the intermediate superheaters of the various blocks was created, so that all intermediate superheater heating surfaces have adequate flows and are adequately cooled. Also to be evaluated was the minimum required flue gas temperature as an entry condition for the flue gas de-nitrification plant.

Disassembled grinding roller yokes
Disassembled grinding roller yokes
Source: Schröder-GKM

Test run results

One-mill operation affects the wet ash quality as well as the fly ash quality. The classifier had to be fully extended in the tests to meet the fly ash requirement of 5 per cent loss on ignition. The emission levels showed no critical values under these operating conditions. Over the entire test period, the flue gas entry temperatures at the catalytic converter were in the desired range. Apart from some minor fluctuations, the flame signals were stable.

The reason is that operation with one coal mill took place with a higher loading of pulverized coal, and the specific thermal input was therefore higher. In two-mill operation, the thermal input of each mill is significantly lower.

The experiments with one-mill operation produced no increased risks or hazards in any operating condition. Based on the above results, a certain mill level has been selected. The new requirements of future one-mill operation are already defined in the combustion management program.

The requirements for low-load operation with two mills that were previously implemented in the control program and in the boiler safety arrangements have now been adapted for one-mill operation. It was possible to eliminate one signal without any replacement. Nothing has changed in the actual boiler safety and combustion safety areas. Operationally, the combustion management program changes.

The new operating regime involves no increased operational risk compared with the usual operating conditions. In general, the lowest possible residual risk is achieved through the interaction of design provisions, the functional safety control system (protective functions) and organizational measures.

The process requirements and the combustion management program were adapted for the new operating conditions. No design measures were necessary. Fundamental to the organizational measures is the new combustion management program and the associated training for employees and operators. In this case, the existing plant-specific risk assessment was updated.

TÜV SÜD engineers reviewed the modifications made, using the accompanying tests as the basis, and compared the changed operating conditions. They produced a customized process description of the future plant operation based on the existing operating instructions and the additional technical documentation. The process-control changes were then checked and evaluated, likewise the updated risk assessment as per the Ordinance on Industrial Safety and Health and the current process description. The data compilation was based on the documentation available to the operator and to TÜV SÜD Industrie Service.

Since August 2016, one further power plant has operated successfully with one mill. The increased flexibility was implemented purely by process control measures via a new combustion program. The existing infrastructure of the plant remained unchanged. It was not necessary to raise the HP pressure to lower the HP exhaust steam temperature of the topping turbine. The values attained were always below 400 °C. Existing safety requirements had to be adjusted only slightly.

Hans Christian Schröder is Senior Expert, Power Plants at TÜV SÜD Industrie Service in Mannheim, Germany.