The question of what to do with ageing coal-fired thermal power plants is a common one in the electricity industry. One cannot simply abandon the site or close down a facility which has much of its operational aspects intact. The solution often chosen is short-term life extension by refurbishment. This often provides the opportunity to extend the plant’s capabilities and add flexible operation to an otherwise inflexible plant.
With the increasing penetration variable renewable energy (VRE) plant operating under priority dispatch rules, there is an increasing need for flexible operation of other plant to match the varying output of the VRE plant. For this, high flexibility is needed, in terms of possessing resilience to frequent start-ups, meeting major and rapid load changes, providing frequency control duties, and low load operation .
Flexible generation is required to respond in real time to rises and falls in VRE output, and this is often characterised by very steep ramp rates, which thermal plant designed for baseload or mid-merit operation cannot follow. Adding flexibility often increases the wear and tear on the plant, but for short term life extension this may not be a problem. Many of the coal-fired power plants (CFPP) considered for life extension are very old, which makes the conversion process more difficult, but can also have advantages in greater resilience of components.
The question is whether many of the components that would require replacement or rework for life extension can be replaced by components that offer greater flexible operation or are more resistance to the effects of flexible operation These upgrades will be plant specific but are likely to include options such as changing to sliding pressure operation, variable speed drives for main cycle and auxiliary equipment, and boiler mill and burner control schemes, as well as more comprehensive control systems, and exploitation of the inherent properties of existing plant configurations.
Flexible generation is generally taken to mean the ability to ramp output up and down in a rapid manner without affecting the reliability of the plant or the critical parameters, and equally important, the ability to continue operating at low load levels. Characteristic features are mainly fast load change rates and operation in the lowest load range. Keeping the system operating at low load levels avoids the need for frequent shutdowns and restarts, which are costly, and result in additional stress on plant.
Modifications to improve flexibility
Changes in the following key component areas can improve flexibility without much physical modification:
Actions other than those designed to reduce damage and wear on components can be applied to increase flexibility. Changing the operating mode of the power plant can result in increased flexibility without replacing parts, and should be done in addition to part replacement.
The fuel feed system
This consists of the hopper, coal mill, and air delivery system. The response of the plant is inherently bound to the inertia of the coal supply system. Flexibility of the plant can be improved by using the coal mill as a short term storage system. Fuel flow to the mill can be changed in response to demand changes, but there is a time lag involved. The flow of ground coal out of the mill will depend on the particle size and the classifier. Coal too large to pass through the classifier is returned to the grinder and a reserve of unground coal remains in the mill. The size of coal particles depends on the pressure on the grinder wheel, and where this is controlled hydraulically, the flow of coal to the burner can be controlled. Increasing the pressure results in the flow of coal to the burner rapidly increasing, while the amount of stored in the mill is decreased.
This provides a means of rapidly increasing the amount of fuel available for a short period. A similar process applies when demand drops, and the grinder pressure is reduced. Altering the grinder pressure bridges the inherent delays in response to a change in demand, but requires close control and monitoring of operation to be effective.
Indirect firing systems
Flexibility can be increased by the use of indirect firing where the output of the mill is fed to a storage bunker (Fig. 1). This system disconnects the combustion rate from the mill rate. Indirect firing has the advantage that the mill does not have to follow rapid changes in load but can operate at an average rate.
Indirect firing has been retrofitted successfully to pulverised coal-fired units to enable increased ramp rates and better part load efficiency . Separation of milling rate and burner feed rate results in a considerable reduction in the inertia of the firing system, allowing a rate of change of firing of up to 10%/min) . When installed in conjunction with flexible burners, indirect firing can allow the minimum firing rate to be lowered to below 10% of MCR. Efficiency is also improved at low load as mills can continue to operate in their range of optimum power .
A disadvantage of indirect firing is that it could require significant changes and modifications to the fuel supply system, including installation of a pulverised coal hopper between the mill and burner, together with additional pipework and valves.
Boiler flexibility is determined by the inertia of the firing system and the low load operation capability. The inertia of the firing system is the rate at which coal combustion can be altered to accommodate increased or decreased load, or the delay in responding to increased or decreased output requests. The boiler inertia includes the response time of the boiler to changes in firing rate, i.e. delays in steam production after changes in firing rate.
The load dynamics of a coal-fired power plant unit are naturally restricted by the sluggish response of the steam generator, with its large iron mass and the large boiler drum and pipe volumes. Several minutes will pass between a step increase in fuel flow to the coal mills and the steam generator’s response. Fixed times are required to pulverize the coal, transport it to the furnace, and then burn the coal. An increased firing rate will gradually produce an increased flow of heat to the steam generator and hence the water and steam .
Low load operation: Burner and mill arrangement
The ability to operate at low loads is as important as the ramp rate. CFPP are fitted with multiple mills and burners. A tangentially fired boiler requires burners at each corner of the boiler for each burner level, and one mill will usually be associated with the four burners in a burner row. All mills are operational at full load and the number of mills and burners is reduced as the load decreases. The minimum number of mill/burners that can be operated determines the minimum load, and is determined by the requirement to maintain stable furnace combustion.
Many stations in service can only operate down to 40% of the maximum continuous rating (MCR). Consider an arrangement of four mills, feeding sixteen burners in two burner levels, each with two burner rows. Fig. 2 illustrates the possible operational ranges. At full load all mills and burners are operational. On four mills the output can be taken down to. The minimum load is normally restricted to one level of burners, and two mills in operation. this limits the lower output to around 40% of the MCR. That is far from the performance that will be needed. However, it is possible through changes to mill size and burner operating range to achieve 25% load with two out of four mills operating .
Where four mills are supplying tangential corner-fired systems, the minimum load can be further reduced. It has been reported that single-mill operation can be realised stably with a single row of burners down to level of 15 to 20% of the MCR, with close control of the mill operation and air flow. If indirect firing is used on the low level operational mill, it is reported to be possible to operate at a level as low as 10% of the MCR . In single-mill operation, only the highest burner stage is operated to release heat “higher” in the boiler.
Flexibility can be achieved by varying the position of the flame ball in the furnace. This is done with the use of tilted burners. The tilting of the burner controls the vertical position of the fireball and also the size of the fireball to produce a uniform temperature distribution to all boiler tubes. The position of the fireball can also be used to control the generation of steam in the boiler. Variation in the height of the fireball can influence steam generation. Lowering of the fireball causes more heat to be transferred to the evaporator. In the case of a once-through steam generator, the feedwater mass flow must also be increased so as to maintain the required steam enthalpy at the evaporator outlet at a constant level. If so, the increased feedwater mass flow immediately results in an increase in main steam mass flow.
This can be used for a rapid increase in the steam generating capacity of the boiler. Temporarily lowering the fireball at the start of or during a load increase will result in an overall reduction of the boiler inertia or time constants. With drum-type boilers, lowering of the fireball results in increased steam production and lowering of the drum level. Similarly, the boiler delay of drum-type boilers can therefore also be positively influenced using the centre-of-fire control method .
Not all boilers are equipped with tilting burners. If several mills supply burners on different levels, the position of the fireball can be influenced by trimming the outputs of the various mills. Similarly, level trimming can also be employed to influence the boiler time constant. Operation of the coal mills is staggered by the centre-of-fire control module during load ramps such that the mills on the lower levels are operated first, followed by those on the higher levels during an increase in load. This initially causes the fireball to be pulled downward so as to accelerate the increase in steam development. When the load increase stops, the fireball returns to its normal position .
Flame monitors can assist in control of the fireball and can ensure stability of the flame and combustion.
Steam and water system
Feedwater heating is achieved by bleeding steam off both the high pressure (HP) and low pressure sections of the turbine steam path. This removes steam from the system which could be used for generating power. If the steam supply to the feedwater heater is turned off or diverted , the steam in the system becomes available for generation, and the output power of the turbine can be increased rapidly for a short period to bridge the thermal inertia of the boiler system. The result of this action is a lowering of the feedwater temperature, which can have negative impacts on the boiler operation. This is overcome by the use of a thermal storage system for feedwater, where a reserve of hot water is maintained to allow the steam to be diverted. Fig. 3 shows a typical system When the FH is closed off, hot water is drawn from the storage and the normal operation parameters are retained.
The amount of steam fed to the turbine, and hence the power generated by the turbine, can be controlled by one of several methods such as throttling and sliding pressure operation.
The most common method of using stored energy is turbine valve throttling. Steam throttling is required when a defined unit load or steam mass flow is reached and the steam pressure upstream of the turbine is increased. If a fast increase in turbine output is required, the appropriate quantity of steam can be discharged by opening the turbine valve, thus immediately increasing generator power.
Steam pressure also decreases quickly in the event of a rapid load increase as the stored steam is quickly removed from the boiler system. The fuel mass flow rate must therefore be increased disproportionately – first to counteract the rapid decrease in pressure and second to raise the pressure back to its setpoint or original value. The change in pressure and the resultant over-firing also put additional thermal stress on the boiler system.
Sliding pressure operation (SP)
In this operation the steam pressure entering the turbine is the same as in the boiler and is not controlled by the throttle valve. Pressure in the system is changed by changing boiler pressure, although some throttling is applied to allow rapid changes in output. SP allows a closer link between steam temperature and turbine temperature more rapid output changes can be achieved using sliding pressure.
Effects of flexible operation of CFPP
Flexible operation, which involves plant cycling (on/off and variable output operation) has the potential to affect virtually every area of the installation. High temperature and high pressure components are particularly affected . Reduced efficiency may result in ancillary and emissions reduction plant.
Cycling damage causes increased risks such as boiler tube leaks, power piping failures, pressure vessel failures, pulveriser explosions, coal silo fires, turbine blade failures and transformer fires. These types of events are often categorized as low probability-high impact events but they can cause serious injury, fatalities and substantial property damage .
Upgrades and life extension will need to include components that can withstand cycling operation. boiler inertia reduction may involve thinner tubes which will require using new steels to allow reduced metal thicknesses.
Control systems for improved flexibility
Many of the above mentioned changes in operation are dependent on improved monitoring and control systems. It is generally worthwhile replacing control and instrumentation systems in older plant to increase efficiency and flexibility. Such retrofits can give faster ramp rates and lower minimum loads. Modern control system retrofits can also give:
Control systems designed to improve flexibility and improve low load operation are viable and have been successfully implemented at a number of sites. More advanced self-learning predictive systems are being developed which can optimise whole plant performance under different operating conditions .
Effect of felexible operation on pollution reduction and post combustion CO2 capture
Ramping the output of CFPP can affect the performance and efficiency of flue gas cleaning and CO2 capture systems. Changing the output of the plant changes the flue gas rate as well as the flue gas composition and this may require additional modifications to FG treatment systems.
 C Henderson: “Increasing the flexibility of coal-fired power plants”, IEA clean coal centre, September 2014.
 Dr L Sloss: “Levelling the intermittency of renewables with coal”, IEA clean coal centre, July 2016.
 D Gray and V Viswanathan: “Damage to Power Plants Due to Cycling” EPRI, July 2001.
 M Rech, et al: “Innovative control strategies improve boiler dynamic response”, Power, July 2008.
 H Bruggeman: “Conventional Firing Systems for Future Power Plants”, International conference power plants, 2012.
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