Retrofitting lignite plants: considering the options11 June 2016
In a number of countries, elderly lignite fuelled power plants remain an essential source of cheap power, but to continue operation they must be upgraded and their emissions reduced. A new report* for the IEA Clean Coal Centre by Dr Ian Reid, briefly summarised here, examines the options.
The global lignite power industry faces the unprecedented challenge of generating electricity efficiently and cleanly against a background of tightening emissions legislation. Modern high efficiency low emission (HELE) lignite power plants can approach the performance of hard coal fired power stations. However, much of the existing low cost lignite power plant fleet uses raw lignite feed and subcritical steam boiler technology. The average efficiency is only about 28% (compared to 40%+ for HELE plants) and some of the stations in the lignite fleet are 60 years old or more. Nevertheless, lignite is an essential source of power in many countries and so outdated plants must be upgraded to meet current and future standards.
New emission limits for acid gases present a significant challenge to 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 is now on the list of regulated flue gas components and new stringent limits have been set for fine particulates. To comply with the USA Clean Power Plan, a network of generators must lower emissions and show higher efficiency, achieved either by plant upgrades or by closure of coal generation.
New lignite plants are designed with the recognised 'best available technologies', including: SCR; particulate baghouses; carbon injection; and FGD. It may not be appropriate to retrofit these adaptations to older plants due to a lack of space, the high cost and plant outage duration. A potential efficiency upgrade from sub- to super- or ultra- supercritical steam systems is also unlikely to be implemented for the same reasons.
Suitable retrofit technologies seek to extend the operating lifespan by 10-15 years through improved performance and reduced emissions. Typical plant 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 short outage time for installation.
A US power plant review concluded that plant alterations with the most impact on overall efficiency are lignite beneficiation and steam turbine replacement. Plant operation can also be improved by using: modern control systems and improved wireless instrumentation; smart antifouling methods; and replacement of aged pumps and ID fans. The optimum upgrade package depends on national regulations and varies for individual plants.
Lignite beneficiation has been extensively investigated and, currently, fluidised bed pre-drying utilising low-grade heat streams is the most promising technique. Established plants in Germany (RWE/WTA) and the USA (GRE/DryFiningTM) show efficiency gains of several percent dependent on the degree of dryer integration. The two technologies differ in that WTA is designed for wet lignite (>60% H2O) while DryFiningTM (~35% H2O) also possesses a segregation step that partially removes dense minerals containing sulphur and mercury, alleviating downstream treatment.
Steam turbines demonstrate energy losses of 3 to 4% over a period of 20 years, while new designs incorporating contoured and extended blades offer improved durability and higher efficiency. Steam turbine replacement can now utilise existing casings, reducing the installation time, and has become the most common retrofit efficiency improvement.
Alternatives to mainstream flue gas treatment options combine a set of technologies to match SCR/FGD performance and include hybrid and multi-component systems to reduce the emission of acid gases and other pollutants. Suitable retrofit technologies may have 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 unit. Originally conceived as an ammonia slip trap, 'compact SCR', at one quarter scale, can match full SCR performance but fit into existing piping. An ozoniser, forming part of a hybrid system, which 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. Two examples of technologies new to lignite power generation are AirborneTM and Castle Light's Clean Combustion System.
AirborneTM 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.
The use of more efficient technologies reduces the carbon footprint of lignite stations and cofiring with lower carbon fuels such as natural gas and biomass can also help.
Due to their low fuel costs, lignite plants have been favoured as test facilities for carbon capture, eg as currently applied at the Boundary Dam project in Canada, where an old 115 MW subcritical plant has been adapted to incorporate an amine CO2 capture plant, with the CO2 used for enhanced oil recovery.
Where renewables are widely used, lignite plants will need to respond to variable load ranges to cope with the intermittency of wind and solar. The primary aim of flexibility measures is to keep components such as boilers 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.
*Retrofitting lignite plants to improve efficiency and performance, by Dr Ian Reid, IEA Clean Coal Centre, www.iea-coal.org