Coal-fired power plant is under pressure to operate with higher efficiency, cleaner emissions, and sufficient flexibility to provide rapid back-up to intermittent renewable energy sources. Recent advances in both the sensor hardware and control software available have made control system upgrades part of the solution.
Online sensors generate more accurate, real time data. More advanced algorithms can determine the optimum control response. But, sensors for combustion processes are still limited by their reliability in the harsh environments in which they operate.
High-efficiency coal power such as advanced ultra-supercritical (AUSC) and integrated gasification combined cycle (IGCC) plants require sensor technologies which are even more resilient to high temperatures and corrosive atmospheres. Research in this area is on-going.
For pulverised coal plant, new sensor and control technologies are being developed to optimise the combustion process. For example, furnaces are frequently operated with an unnecessarily high proportion of excess air to ensure complete coal combustion and maintain low CO levels at the expense of lower efficiencies and higher NOx emissions. Such practice is usually due to limited information available on the distribution of air, fuel, and combustion products in the furnace, so that localised regions of poor combustion can arise. Newer technologies such as tunable diode laser absorption spectroscopy and advanced acoustic pyrometry map out the temperature and composition of flue gas over a furnace cross-section. Meanwhile, online monitoring of the coal and air flow distribution to individual burners enables combustion stoichiometry to be precisely controlled in real time. Thus the control system can balance combustion and reduce excess air to optimum levels, while better control of furnace exit gas temperatures means improved steam temperature control and reduced slagging.
Even where sensor data is limited, coal plant can draw on a growing range of advanced process control software for maintaining an optimum state. These systems usually make use of complex algorithms such as neural networks which can be trained on operational data to build up an empirical model of the plant system. These model-based approaches are able to continuously identify the optimum combination of control actions for a given set of demands on the plant.
In his latest report for the IEA Clean Coal Centre, Advanced sensors and smart controls for coal-fired power plant, Toby Lockwood found that: State-of-the-art process control software can demonstrate efficiency improvements of up to 1%, around 20% lower NOx emissions, and improved load dynamics and steam temperatures – meaning rapid payback times for the plant. Together with intelligent soot blowing systems, which replace fixed interval boiler cleaning with more targeted actions, slagging and material damage are also minimised.
Advanced control systems can be particularly effective when combined with online sensor technologies which supply them with more useful data. Such systems may indeed be best-placed to make use of this growing influx of information.
Combustion sensors research has focused on developing more robust, miniaturised devices which can operate reliably in the challenging environments of furnaces, gas turbines, and gasifiers. Devices based on optical fibres can encode and transmit sensory data as some property of light, offering higher signal fidelity and better immunity to electromagnetic noise than electronic sensors. Although conventional silica fibres become unstable at temperatures much over 800°C, new techniques incorporate these sensors into sapphire fibres which are viable to nearly 2000°C. Optical fibres can also be embedded in other power plant components, giving the possibility of ‘smart parts’ which can report on their own condition.
Microelectronic devices have potential as low cost and robust sensing platforms. Following the model of the widespread zirconia-based oxygen sensor, related solid electrolyte sensors have also been designed for detection of NOx, CO, and SO2. Even simpler gas sensors can be based on chemi-resistive metal oxide films.
Conventional microelectronics based on silicon are not viable at temperatures greater than 350°C, but equivalent sensors based on higher temperature silicon alloys can be used, as well as providing integrated high-temperature electronics for signal processing or even miniaturised radio transceivers for wireless devices. The capability for wireless communication is itself a key technology for miniaturised sensors, avoiding the cost and vulnerability of wiring and granting access to previously inaccessible locations such as turbine blades. Self-powered devices based on dielectric resonators or surface acoustic wave sensors are amongst the most promising concepts.
The US Department of Energy calculated that incremental improvements in plant efficiency and reliability provided by control system upgrades could represent yearly savings of US$ 358 million and 14.4 MtCO2 if applied to the country’s entire coal fleet, and so is funding research in this field. As more complex, higher efficiency plant is deployed, potentially incorporating carbon capture systems, the application of these emerging technologies for optimising plant control is likely to become increasingly necessary.
The report Advanced sensors and smart controls for coal-fired power plant by Toby Lockwood CCC/251, ISBN 978–92–9029–573-0, 104 pp, June 2015 is available for download from the IEA Clean Coal Centre Bookshop http://bookshop.iea-coal.org.uk/site/uk/clean-coal-technology-research-reports. Residents of member countries and employees of sponsoring organisations can download the report at no charge after a one-off registration.