Precise fuel flow control offers efficiency improvements for coal burners

A research project that seeks to prove a technique for improving the efficiency and environmental performance of coal-fired plants is set to take a major step forward, following laboratory trials witnessed by engineers from Drax, the largest power station in the UK. Led by specialist engineering company The Greenbank Group, the £500 000 Intelligent Flow Control System (IFCS) project, which sets out to exploit the potential of component integration to reduce emissions, has now completed its demonstration phase, with the support of Drax Power Ltd and research partners at the University of Nottingham and Argenta Ltd. The next phase will see the trials reproduced at Drax itself.

Legislative pressure

As coal remains the largest source of power across the globe a fresh approach to the way that coal fired power stations operate is needed to achieve the improvements in emissions performance required under new legislation. The High Efficiency, Low- Emissions Coal-Fired Power Generation technology roadmap set out by the International Energy Agency (IEA, 2012) outlined a path forward for the industry that embraces new technologies and the further development of existing ones to achieve this necessary goal.

Clearly, generating electricity at the present scale will require more-efficient coal- fired units to burn less fuel, emit less carbon, release fewer air pollutants, consume less water and have a smaller environmental footprint.

Since the IEA’s roadmap was established, more than half of new coal-fired power plants have adopted HELE technologies, predominantly in the form supercritical and ultra-supercritical (USC) pulverised coal combustion units.

However, despite the widespread adoption of such technology, about three- quarters of stations still in operation – and many new plants – rely on non-HELE, subcritical units. Furthermore, more than half of current capacity is over 25 years old and comprises units with a capacity of less than 300 MW. 

As a result, stations such as these will require huge expenditure on clean coal technology and other pollution-mitigation equipment or face closure, prompting a rethink in the way that existing technology can be refined and integrated to deliver improved efficiency and environmental performance.

Current measures

To date, improvements that have been made require manual intervention, particularly in relation to fuel and air delivery from mills to burners – a key factor in power station performance.

For example, pipes that feed to multiple burners are prone to mall distribution, causing inefficiency in combustion. There are fuel flow control products available that reduce these issues; however, it has been identified by plant performance engineers from across active UK power generation sites that existing fuel flow control products may not be wholly practical from a boiler operational point of view for the flexible load operations or changes in operational parameters needed to react rapidly to grid demands.

Furthermore, manual adjustments are most likely small, and performance improvements due to minor changes are not quantified, as boiler and mill modulation is frequent and adjustments cannot be made quick enough to make a difference.

Automation

Currently under development by Greenbank in partnership with Drax, the University of Nottingham and Argenta Ltd, the IFCS is designed to fully automate this process, utilising sophisticated monitoring systems – which have previously been used independently – to regulate the flow of fuel and air into the boiler dependent on demand or other factors that may impact on its performance.

Initial trials of the IFCS on a specially designed test rig at Greenbank’s manufacturing facility in Derbyshire have proved encouraging, and the system is soon to be further developed in a ‘real-world’ situation when it will be installed at Drax later this year.

By way of comparison, typical passive control systems are designed to a specific mill load or flow characteristic, with the allowance of some flow tolerances, reflected in performance guarantees. Usually, guarantees of a burner set from a single mill outlet pipe can be in the order of ±6% fuel mass flow to each burner from the mean value at design load and ±10% when at other loads. Therefore, combustion efficiency when a new mill load is used, the mill performance decreases, or the fuel type changes, can greatly affect the fuel distribution, leading to rich and lean combustion across all the burners on a unit. This highlights the need for boiler management systems that can continuously optimise efficiency to reduce costs and emissions.

In the trials – which also utilised technology such as Greenbank’s own particle size analysers, and flow velocity and mass flow monitors, combined with a gravimetric feeding system to control flow optimisation devices such as diverter valves and dampers – these tolerances have been reduced to ±1–2%, with improvements continuing to be made as the system is further refined ahead of full scale plant installation.

In fact, the laboratory tests are continuing to prove the limits of the system, and the researchers are confident that they will soon be able to see the results replicated in a ‘real-world’ situation.

Component integration

By adjusting flow control devices to suit mill load, power output, performance, fuel type and fuel quality, the system allows burners to achieve optimum stoichiometric conditions.

This would not be possible, of course, if the individual components within the system did not work together, and this is achieved by using intelligent learning algorithms and dynamic control systems that understand what adjustments are being made and feed information back through the monitoring loop to determine if further adjustments are necessary (Figures 2 and 3).

The system builds on previously developed monitoring and control systems, such as the online pulverised fuel-monitoring systems (PFMS®) and the online particle size analyser, working in combination with the fuel-handling and processing equipment. These devices are to be adapted to generate a novel active feedback loop, which will develop highly reliable, efficient flow control.

Alongside work on the test rig, other components that make up the IFCS have undergone extensive testing at a number of power plants in the UK and overseas.

This has helped to address a common problem within pulverised fuel pipes when material being transported accelerates around bends at different rates. Such ‘roping’ can itself have a negative impact on performance.

Engineers working on the IFCS project have helped to overcome these issues with the further development of the Variable Area Rope Breaker (VARB®) and control- gate technology. Proven plant evidence has established that existing systems are at their most effective when handling a specific load. However, when loads change, the modulation is too quick for an engineer to effectively make the adjustments to the control gates that are needed to maintain efficiency. The IFCS, however, automates this control and optimises fuel delivery to each burner, by means of equalising the mass flow of fuel across each bifurcator, trifurcator and quadrificator. 

Monitoring

The system also incorporates improvements in monitoring the throughput of the pulverised coal, which has traditionally been achieved by sporadic manual sampling or by inferring the mass flow from the burner performance. Both these methods provide only a limited view of the balance and are unsuitable for anything more involved than static control elements.

The PFMS allows both a direct, accurate measure of velocity and fuel mass flow rate in individual burner lines and detailed information on the effects of differing system conditions on flow. This allows for more-informed decisions to be made with regard to static balancing and enables the use of active systems.

During the development process, tests were performed at a power plant in the UK. The curves shown in Figure 4 demonstrates the results of balancing the fuel distribution using PFMS equipment.

Particle size has also been taken into consideration when developing the system. Previous research has shown that particle size distribution (PSD) has a large effect on the accuracy of velocity and mass measurements observed by electrostatic sensors; thus, by providing an online PSD to the PFMS, a mass flow and velocity measurement can be compensated by the observed particle size to provide a more accurate result. This work required careful analysis and a fresh approach to tackle the inaccuracy issue and has been subject to much research and development work.

Classifiers

So too has balancing the flow of pulverised coal and biomass fuel (PF) from multi- outlet mill classifiers, which is generally hindered by differing pressure drops across the multiple pipelines that convey PF to the burners. As each pipeline takes a different route to the boiler and connects to a different burner, it is inevitable that some pipelines will be longer than others and some will have more-complex changes in direction.

Extensive trials have been carried out on Greenbank’s own solution to this problem: its Coalflo Damper system. As an example, the Mill E Coalflo damper was adjusted on a five outlet mill located on a unit at another large Chinese power station. The data results are shown in Figure 5.

Looking ahead

Moving forward, the research to date and initial testing clearly illustrate how the performance of individual components relies on a statically configured system, while each can be used to provide suitable conditions for the optimal functionality of the other devices.

Interaction between the control- and measurement-based components would allow for the verification of suitable system conditions through the potential range of operating conditions and for informed corrections to be applied to both measurements and control elements.

By changing the static function of the control elements, rapid changes in load can be accounted for, providing maximum performance benefits without restricting the operating regime of individual mills.

Each of these individual components has a significant part to play in helping the industry to meet its environmental obligations while generating significant cost savings for operators, but it is the central ‘brain’ of the system that will have the greatest impact on the long-term future of the coal-reliant energy industry. 

Dr Neetin Lad PhD is head of research and development at Greenbank Group headquarters in Woodville, Derbyshire, UK