Lignite is poorer quality coal, typically with a high moisture and ash content, which is cheap and abundant. In the face of tightening emissions legislation the lignite power industry has to find ways to generate electricity more efficiently and cleanly, while remaining economic. It is possible for modern high efficiency low emission (HELE) lignite power plants to approach the performance of hard coal electricity stations (40%+). But much of this low cost power generation uses raw lignite feed and subcritical steam boiler technology. This means the average efficiency of the lignite fleet is only about 28% and there are some 60 year old stations still running.
Despite these drawbacks, lignite is an essential fuel in many countries. This means that outdated plants must be upgraded or replaced to meet current and future standards.
In my new report for the IEA Clean Coal Centre, I identifiy two distinct trends in the lignite industry: construction of new stations in developing countries, where lignite is relatively cheap and energy demand is growing fast; while in mature economies lignite power competes against renewable energy sources and, as a result, underperforming stations are being re-assigned to reserve supply.
New emission limits for acid gases are a real challenge for existing lignite plants; typical current limits of 200 mg/m3 are under review. For example, China has set limits of 100 mg/m3 NOx for stations up to twelve years old. Mercury has been added to the list of regulated flue gas components and there are tough new stringent limits for fine particulates.
New lignite plants can be designed to include the recognised ‘best available technologies’ to reduce pollutant emissions: selective catalytic reduction (SCR), particulate bag houses, carbon injection and flue gas desulphurisation (FGD). But it may not be possible to retrofit these adaptations to older plants due to the cost, a lack of space, and duration of plant outage. The same reasons make a potential efficiency upgrade from sub to super or ultra-supercritical (USC) steam conditions unlikely.
However, there are some suitable retrofit technologies which can extend the operating lifespan by 10 – 15 years, improve performance and reduce emissions. Such modifications include: replacing aged equipment; selecting technologies that can be added to existing plant in a straightforward manner; and fitting treatment methods that are cheaper than mainstream technologies and require a shorter outage time for installation.
Lignite beneficiation and steam turbine replacement may have the most impact on overall efficiency. Using modern control systems and improved wireless instrumentation, smart antifouling methods, and replacing aged pumps and ID fans can also improve performance. The optimum upgrade package depends on national regulations and will vary for individual plants.
Lignite beneficiation has been extensively investigated and fluidised bed pre-drying utilising low-grade heat streams is currently the most promising technique. Established plants in Germany (RWE/WTA) and the USA (GRE/DryFining™) show efficiency gains of several percent dependent on the degree of dryer integration. The differences are that WTA is designed for wet lignite (>60% H2O) while DryFining™ (~35% H2O) also possesses a segregation step that partially removes dense minerals containing sulphur and mercury.
Over a period of 20 years, steam turbines typically demonstrate energy losses of 3 to 4%, while new designs using contoured and extended blades offer improved durability and higher efficiency. Steam turbine replacement can now utilise existing casings which reduces the installation time and has become the most common retrofit efficiency improvement.
Alternatives to mainstream effluent treatment combine a set of technologies to match SCR/FGD performance and include hybrid and multicomponent systems to reduce the emission of acid gases and other pollutants. Suitable retrofit technologies may have a relatively low initial investment cost but consume higher levels of reagents.
Hybrid systems incorporate low NOx burners (LNBs) and selective non-catalytic reduction (SNCR) technologies to match the efficiency of a new SCR NOx unit. Originally conceived as an ammonia slip trap ‘compact SCR’, at one quarter scale, it can match the full SCR performance but fit into existing piping. An ozoniser forming part of a hybrid system that oxidises rather than reduces NO can achieve lower levels of NOx than SCR, and can also oxidise mercury to soluble HgO.
Multi-component technologies remove several contaminants in a single device. Airborne™ utilises sodium bicarbonate (SBC) which reacts with both acid gases to outperform SCR+FGD. The SBC is then regenerated to produce fertiliser which helps to overcome economic barriers. The ‘Clean Combustion System’ is a gasifier ‘add-on’ hybrid reactor, which adapts an existing boiler to create reducing conditions, and so prevents the formation of NOx. Furthermore, sulphur forms molten sulphides which are removed prior to the boiler section, which is especially attractive for processing high ash fuels. This technology has the potential to essentially replace an effluent treatment plant.
Where renewable energy is widely used, lignite plants will need to respond to a variable load range to cope with the intermittency of wind and solar power. The primary aim of flexibility measures is to maintain the reactor in a ‘hot’ state to minimise the time needed to bring the plant fully on-stream.
In regions where natural gas is cheap, the combination of a gas turbine and lignite plant can maintain the lignite boiler at readiness (using waste heat), enhance overall capacity and allow early synchronisation to the grid. Alternatively, a hot water reservoir can be introduced to even out the effects of capacity load variation on the steam system.
The technologies outlined offer a range of options to improve plant efficiency, flexibility and emission. The significance of these technologies, specifically for plant retrofits has become increasingly relevant due to recent exacting legislative standards.
The report Retrofitting lignite plants to improve efficiency and performance, CCC/264, by Dr Ian Reid, April 2016, 97 pg, ISBN: 978-92-9029-587-7, 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.