The efficiencies of steam power plants have been improved by 20 percent since 1985. Efficiencies of 46 percent are being achieved by steam power plants run on black coal nowadays. These efficiencies will also be possible in steam power plants that run on lignite as soon as lignite drying using waste heat at around 100 °C, which is currently at the development stage, is implemented on a large scale. When compared with coal-fired power plants built before 1985, coal-fired power plants built using current state-of-the-art technology emit around 30 percent less CO2 per unit power generated and achieve efficiencies of 46 percent.
Pre-drying of raw coal has the potential to increase efficiencies by up to 10% for plants that burn moist lignite with a water content of over 45%.
Two processes for external coal drying are currently being developed. The first is a mechanical-thermal drying process that mechanically dries raw coal that has been heated to around 140 to 200 °C.
The second process, steam fluidised-bed drying, is already at an advanced stage of development. In this process, raw coal is fluidised in a steam atmosphere in a stationary fluidised bed and is heated at the same time. The process can be operated at atmospheric pressure or under higher pressure. In the atmospheric variant, the steam released can be used within the process to heat the dryer.
A pre-commercial demonstration plant is currently being constructed at an existing power plant as a final step before the commercial implementation of a large-scale power plant fired using dried coal.
The extent to which these processes are suitable for lignite drying first has to be demonstrated in plants of an appropriate size. In addition, issues regarding combustion and pollution behaviour have to be investigated for dried lignite.
Gains in efficiency of 4 to 5 percentage points can be achieved using modern coal-drying processes such as fluidised-bed drying with integrated waste heat utilisation, pressurised steam fluidised-bed drying or mechanical/thermal drying. This is 1 to 2 percentage points more than the potential that can be realised by increasing the fresh steam parameters in a 700 °C power plant. For this reason, the next phase of measures to increase the efficiency of lignite-fired power plants should remain focussed on the integration of modern coal-drying processes. These should be pursued in a targeted manner, as the relevant processes are by now highly developed and have already proven themselves at a demonstration level.
Around one quarter of Germany's electricity requirement is met by lignite. It is mined at ten mines in four main coal-mining areas: the Rhenish area near Cologne, the Lusatian area in the east, the Central German area, and the Helmstedt area. Lignite is thus an important domestic energy resource.
As a result of its capillary structure, lignite has a significantly higher water content than black coal, which generally has less than 10% water. The amount of physically and chemically bound water in the capillaries and pores can lead to a water content of 60% by mass. While surface moisture can be removed relatively easily (e.g. by centrifuging), energy must be supplied to the system to overcome the binding forces that bind water to the coal structure. When moist raw lignite is burned, some of the fuel energy is used up in evaporating water.
In order to increase the efficiency of the power plant process, lignite has generally been simultaneously ground and dried using hot flue gas at 1,000 °C from the power plant boiler up to now. However, this process of grinding and drying has disadvantages from an energy viewpoint as the drying energy is drawn from the hot flue gas, thus impacting negatively on power generation. In addition, waste heat remains unused.
Alternative processes such as fluidised-bed drying with internal waste heat utilisation operate with significantly lower temperatures and use the vaporisation energy (energy taken up by the water from the coal during drying) for the drying or power plant processes. In this way, the power plant efficiency of the Niederaußem lignite-fired power plant with improved plant technology has been increased by around 4 percentage points by using fluidised-bed drying in place of grinding and drying. Efficiency gains of up to 5 percentage points are believed to be possible.
The application and integration of modern coal-drying processes has thus been identified by experts as an important development step on the way to significantly increasing the efficiency of lignite-fuelled power plants.
Fluidised-bed drying with internal waste heat utilisation has been in testing in demonstration plants (raw coal throughput of 27-170 t/h) in Frechen and Niederaußem since 1993. Since 2008, this technology has been integrated into a large-scale power plant for the first time at RWE's Niederaußem lignite-fired power plant with improved plant technology. The test plant (200 t/h) used here is to replace 20 to 30% of the lignite that would otherwise be ground and dried in the conventional manner for use in the power plant.
The change from grinding and drying to fluidised-bed drying with internal waste heat utilisation increases the net efficiency of the power plant process by around 10 to 12% (4 to 4.7 percentage points), depending on the residual water content of the dried lignite.
The steam fluidised-bed drying process that has been the subject of research since 1980 is comparable with fluidised-bed drying with internal waste heat utilisation. In the steam fluidised-bed drying process, drying also takes place in a pure steam atmosphere in a fluidised bed. The difference here is the manner in which the fluidised bed is heated; this is accomplished in the steam fluidised-bed drying process using steam taken from the low-pressure turbine. The gain in efficiency possible compared to the grinding-drying process depends on the use made of the vapours created, which cannot be used directly in this process – in contrast with the fluidised-bed drying concept with internal waste heat utilisation. The harnessing of the heat of vaporisation of the vaporised coal moisture in the power plant allows for a potential increase of 4 to 5 percentage points in the power plant's net efficiency. Fundamental research work was carried out at Brandenburg University of Technology Cottbus, in cooperation with Vattenfall, between 2001 and 2006 on pressurised steam fluidised-bed drying using a small test plant (0.5 t/h of raw lignite, 1.2-6.5 bar). The results obtained from the operation of another test plant (Schwarze Pumpe, 10 t/h, 2008-2010) should have been used in the design of a demonstration plant (Jänschwalde, 250 MW, from 2013). Commercial, large-scale use was planned from 2020 onwards. But those plans are restricted by legal and financial limitations.
2 current research projects
Development of design concept for a large-scale test plant for pressurised steam fluidised-bed drying of lignite
Organisations carrying out research:
Vattenfall Europe Generation Aktiengesellschaft
Brandenburgische Technische Universität Cottbus - Faculty Mechanical, Electrical and Industrial Engineering - Institute for power engineering - Chair of Power Plant Technology
Steinmüller-Instandsetzung Kraftwerke Gesellschaft für Energie- und Umwelttechnik mbH
Project numbers: 0327753A, 0327753F, 0327753K
Development of engineering concepts for a treatment plant for raw coal from the Central German coal-mining area
Organisation carrying out research: MIBRAG mbH - Bereich Bergbau/Veredlung, Anwendungstechnik
Project number: 0327753C