Coal-mill optimisation in coal-fired power stations aids flexibility

January 23rd, 2018, Published in Articles: Energize

Many of the existing pulverised coal-fired (PCF) power stations are operated at a fixed steady load (i.e. baseload operation). There is however a growing requirement for load following or flexible operation, which requires flexibility in most of the components of the power station. Flexibility and efficiency can be improved by close control of the coal milling plant.

As with any high performance piece of thermal machinery, the achievement of peak performance depends on the quality of the fuel fed into the plant. With PCF it is not only the heat value of the pulverised coal but the size and shape of the particles which affect performance. So does the rate at which coal is blown into the furnace. This is all controlled by the coal mill or pulveriser, and its associated feeder equipment. Operation of the coal mill can affect the ramp rate of the power station and its ability to handle rapid changes in output. For coal-fired power plants, the response time of the coal mills is critical for the overall reaction time to changing demand.

Flexible operation or load following requires that the output of the plant can be made to vary in accordance with demand and in accordance with allowable ramp rates. Varying the electrical output can be achieved by varying the mechanical output of the steam turbine. There are various ways of doing this but all are dependant upon the ability of the fuel supply and the combustion system to respond to demand changes.

Fig.1: Mill operating window [2].

Typical PCF plant can operate  down to levels of 40 to 50% of maximum capacity, but new flexibility requirements require operation down to 20% or lower. The limiting factor can be the coal mills themselves. This is often solved in multi-mill operation by shutting down some of the mills and burners, but this often requires modifications to the furnace burner arrangement to ensure that the flame can be sustained at low loads. Modifications and closer control of coal mills allows operation at lower load levels.

Combustion system requirements

The fineness of the coal powder and the uniformity of the coal flow sent to the burners are crucial parameters to achieve an effective combustion in coal-fired power plants. Coal pulverisers or coal mills are the heart of a PCF boiler. Often, the root causes of non-optimised combustion lie with the pulverisers. Capacity, reliability, and environmental issues such as slagging, fouling, and higher-than-desired CO or NO emissions; overheated superheater and reheater tube metals, and cinder fouling of selective catalytic reduction catalyst and air heaters have all, at times, been linked to poor pulveriser performance [3].

Fig. 2: Indirect firing system [8].

The key to improving the load following capability is the control of the fuel supply to the burner, which is provided by the coal mill. Coal mills play a critical part in the flexible operation or load following operation of PCF plants. Critical components which need to be controlled in coal supply operation are coal-feed rate, air-feed rate, and particle size.

Many problems experienced with ramp rates and flexible operation  of PCPP can be traced to coal mills, both the mill itself and the operation and control system. Poor pulverised fuel (PF) distribution to burners has a significant, negative effect on combustion efficiency, wear of equipment and emissions, not to mention economics. Coal mills can only operate within a range of capacities (t/hr) depending on the design and control system used.

Mill operating window

Mill operating parameters determine the mill operating window which provides the limits in which a mill best operates, and outside of which constraints are experienced. The mill operating window is plotted using the mill operating parameters, such as drying limit, erosion limit, milling capacity limit, tampering limit, flame stability limit and pulverised coal transport. Fig. 1 shows a typical mill operating window.

Fig. 3: Rossin-Rammier plot for a typical power station [2].

Indirect firing

Flexibility can be increased by the use of indirect firing where the output of the mill is fed directly to the boiler, and the mill system must be capable of responding to changes in load rapidly. In the indirect system the output of the mill is fed to a storage bunker (Fig. 2). Indirect firing has the advantage that the mill does not have to follow rapid changes in load but can operate at an average rate. The air/fuel ratios are controlled by the bunker system.

Pulverised fuel requirements

Efficient boiler operation requires close control of the pulverised coal particle sizes. Coarse coal particles do not burn as quickly, easily, or cleanly as finer particles. Because they take longer to burn, coarse coal particles raise a boiler’s average NO emissions. They also foster agglomeration and deposition of slag, making boilers and heat-recovery boilers more vulnerable to fouling. Coarse coal can even poison the catalyst of a selective catalytic reduction (SCR) system. If enough coarse coal passes through a boiler without being burned completely, its flyash may have too much unburned carbon (UBC) for commercial use [4].

Fig. 4: Fuel feed control system components [3].

Particle size range fuel fineness

The ideal situation would be to have coal particles all of the same size. This is not possible for various reasons and so a range of sizes is adopted. The ideal size is taken as 74 μm or smaller, but standards allow a range of larger particles, the amount decreasing with size until the maximum size acceptable is reached. The standard test applied to ground coal particles measures the percentage of a sample that passes through a series of sieves of decreasing size. This profile is plotted in what is known as a Rossin-Rammier size distribution curve. Fig. 3 shows an example of the plot for a power station [4].

Fineness is expressed as the percentage pass through a 200‐mesh screen (74 μm). Coarseness is expressed as the percentage retained on a 50‐mesh screen (297 μm). Typical recommended value of pulverised fuel fineness through 200 mesh sieve is 70% and 1% retention on 50 mesh sieve. The standard method of testing is to withdraw a sample from the pipes feeding the burner and run the sample through a series of sieves. Instruments have been developed that allow in line testing or particle size distribution.

Fig. 5a: Typical spindle mill [3].

Mill operation

The mill in operation carries out four functions simultaneously namely, grinding, drying, classification and transportation of product to the burners. The coal mill is not the only component involved in the combustion path which must be controlled. In addition there is heated combustion air fed to the mill to transport the pulverised coal, and the secondary air supply as shown in Fig. 4.

Grinding

The grinding function reduces coal fed into the mill into a dust with particles of the size required for combustion. There are two main types of mill in common use: the spindle type and the drum type. A typical spindle mill is shown in Fig. 5.

Fig. 5b: Simplified typical spindle mill diagram [3].

The vertical spindle mill crushes coal by feeding it between a grinding roller and either a bowl, table or ring at the bottom. Coal enters via the coal feed and falls into the grinding zone, where it is crushed by the grinding wheels. The coal particles are spun outwards by the rotation of the grinding table, and  transported by the incoming heated air feed to the classifier, which separates particles based on size. Particles which are too large are returned to the crusher, while those which are within the required size limits are fed via the output to the burner. The grinding operation is controlled by varying both speed of rotation of the grinding table and pressure on the rollers.

Drying

Immediate contact with the hot primary air feed evaporates any moisture contained in the ground coal particles, so that the desired mill outlet temperature is achieved.

Fig. 6: Static classifier [9].

Classification

The classification process ensures that coal particles of the correct size are fed to the boiler and larger particles are returned to the crushing table. There are two types of classifier: static and dynamic. Both use centrifugal force to carry out the classification function. In the static classifier the coal particles are passed through an angled vane structure. Heavier particles collide with the vanes, lose energy and fall back into the grinder. The particle size is controlled by varying the angle of the vanes. Fig. 6 shows a typical static classifier.

A dynamic classifier includes a set of rotating vanes in addition to the static vanes. The speed of rotation can be varied to alter the performance of the classifier. Particles passing through the static vane stage encounter the moving vanes which remove medium-sized particles, giving a two stage classification function. Only the finer particles make it to the outlet.

Fig. 7: Improvement from fitting dynamic classifier [5].

Significant performance improvement has been claimed for dynamic classifiers, including increased throughput and finer control of particle size [5]. Fig. 7 shows the results obtained by retrofitting dynamic classifiers to existing plant.

Tranfer of product to the burners

Pulverised coal that passes through the classifier is carried to the burners by the primary air flow. Along the way it is mixed with secondary air to achieve the required coal/air ratio. The configuration of the feed pipes for each burner can differ, resulting in different air flow rates and different coal feed rates, and balancing off the feed rates is required.

Fig. 8: Diagram of a dynamic classifier (B&W).

Control of coal-mill operation

It is fairly obvious that a great deal of instrumentation and measurement points, as well as an advanced control system, are required to optimise the performance of the combustion system.

Proportional-integral-ID systems are commonly used to control the operation of the combustion system, but are known to have drawbacks, and model predictive control (MPC) systems have been considered as an alternative [6]. All systems require accurate assessment of the parameters governing the operation of the system. Control systems measure the value of an output parameter, and compare with either a set point or a system model and adjust input values accordingly. The operation of the mill is complex and in many cases the output can be affected by more than one factor, necessitating complex modelling procedures [7].

Mill load lines

A mill is normally controlled to operate according to a mill load line, which maintains a relationship between the air and fuel mass flow. There are a few reasons for this form of control [2]:

  • To ensure that at mill start up and low load (throughput) operation, there is sufficient air flow to maintain the minimum required velocity to prevent particle settling in the PF pipes.
  • To ensure that as the fuel flow is increased the air flow is increased accordingly so as to prevent mill choking by evacuating the pulverised fuel from the mill.

A typical mill load line is shown in Fig. 9.

Fig. 9: Mill load line [2].

The operating range is primarily determined by the air flow rate and the mill capacity. Below the lower operating level there is difficulty in maintaining the flame in the furnace. Minimum operating load is defined as the lowest safe and reliable power plant operation mode without the use of supplementary firing. Low load operation is characterised by worse relative emissions and efficiency, which both reflect negatively on the marginal cost of production.

Air to fuel ratios

Pulveriser capacity is not simply a measure of coal throughput. Capacity refers to a certain coal throughput at a given fineness, raw coal sizing, HGI (Hardgrove grindability index), and moisture. Often, if the desired coal throughput or load response is not achieved, the primary airflow will be elevated to higher flow rates than are best for capacity. However, increased throughput achieved in this way sacrifices fuel fineness.

When the primary airflow is higher than optimum, it creates entrainment of larger-than-desired coal particles leaving the mills, promotes poor fuel distribution, lengthens flames, and impairs low-NO burner performance.

Conclusion

Much effort has been put into gaining a few percentage points of efficiency by the use of supercritical and ultracritical technologies, but the performance of a modern PCF power station still depends on the efficient operation of a mechanical coal grinder. Proper maintenance and control of pulverisers is essential for efficient performance.

References

[1] M Wiatros Motyka: “Optimising fuel flow in pulverised coal and biomass-fired boilers”, IEA Clean Coal Centre, CCC/263, January 2016.
[2] H Archary:  “Condition monitoring and performance optimisation of pulverised fuel vertical spindle type mills”,  MSc Thesis: EPPEI specialisation centre in combustion engineering, University of the Witwatersrand, Johannesburg.
[3] SR Cheteya: “Performance optimisation of vertical spindle coal pulverisers”, MSc Thesis, North West University, November 2016.
[4] R Storm: “Coal pulveriser maintenance improves boiler combustion”, Powermag, January 2015.
[5] R Summerland and K Dugdale: “Dynamic classifiers improve pulveriser performance and more”, Powermag, July 2015.
[6] P Niemczyk: “Improved coal grinding and fuel flow control in thermal power plants”, 18th IFAC World Congress, Milano, 28 August to 2 September 2011.
[7] P Pradheepa, et al: “Modeling and Control of Coal Mill”, Tenth IFAC International Symposium on Dynamics and Control of Process Systems, Mumbai, 18 to 20 December 2013.
[8] C Henderson: “Increasing the flexibility of coal-fired power plants”, IEA Clean Coal Centre,  September 2014.
[9] D Storm, et al: “Performance driven maintenance of coal pulverisers: Importance of mill performance testing” Storm technologies.

Send your comments to energize@ee.co.za