No 11/01 August 2014

Techno-economics of modern 

pre-drying technologies for 

lignite-fired power plants

Lignite is an important and affordable fuel for power generation in many parts of the 

world. The high moisture content of lignite is often quoted as the major problem with 

the ongoing use of lignite for power generation. The practice of using hot flue gas to dry 

the wet incoming lignite in conventional lignite-fired power plants leads to reduced 

thermal efficiency and increased CO2 emissions on an equivalent energy production 

basis. The presence of moisture vapour in the flue gas also necessitates a larger boiler 

furnace, which increases the capital costs of lignite-fired power plants. 

Drying the lignite prior to combustion in the boiler is thus an effective way to increase 

the thermal efficiencies and reduce the CO2 emissions from lignite-fired power plants. 

The challenge is to avoid using hot flue gas and carrying the moisture through the entire 

power plant. Modern pre-drying technologies, based on this principle, have been 

developed in Germany and the USA. RWE’s WTA dryer and GRE’s DryFiningTM system 

have been successfully demonstrated at commercial scale, while Vattenfall’s PFBD dryer 

has reached pilot scale. While the technical details of these modern lignite pre-dyers are 

well documented, their cost and techno-economic information is limited in the public 

domain. Nevertheless, some useful implications can still be extracted from the available 

information, which provide insights into impact of pre-drying on the performance and 

costs of lignite-fired power plants in both retrofit and new build situations.

The WTA (Wirbelschicht-Trocknung mit interner Abwämenutzung; or known as fluidised 

bed drying with internal waste heat utilisation) was developed by RWE Rheinbraun in 

Germany. WTA uses recycled steam from coal moisture to fluidise the wet lignite in the 

dryer, and the energy for drying comes mainly from the steam bled from the 

low-pressure turbine via a heat exchanger immersed in the fluidised bed.

A commercial prototype WTA dryer was commissioned in Unit K of RWE’s supercritical 

Niederaussem power station (BoA) at the end of 2007. This prototype dryer can reduce 

the lignite moisture from 50‒55% to approximately 12%. The pre-dried lignite can 

substitute for up to 30% of the fuel on an energy basis, which can result in 

approximately 1 percentage point gain in the plant thermal efficiency and a 2.5% 

reduction in CO2 emissions. No complete techno-economic analyses are available, but 

RWE reported in 2009 that the prototype WTA dryer at Niederaussem incurred a total 

investment of €50 million for erection and operation. For a supercritical lignite-fired 

power plant firing 100% pre-dried lignite (BoA Plus), RWE assessed a specific investment 

cost of 70 €/kW for the open cycle variant of WTA, which included costs incurred to 

modify the burners and boiler combustion chambers. The overall investment costs of a 

BoA Plus are similar to a BoA unit of equivalent net power output, but the thermal 

efficiency could increase by 4−5 percentage points.

Vattenfall’s Pressurised Fluidised Bed Dryer (PFBD) is similar in principle to RWE’s WTA, 

though operating at a higher fluidised bed pressure. There is very limited cost 

information available. RWE and Vattenfall have different views in respect of whether 

fluidised bed drying at higher pressures delivers better economics. The actual 

cost/benefits balance will depend not only on the properties of lignite but also on the 

IEA Clean Coal Centre is a 

collaborative project of 

member countries of the 

International Energy 

Agency (IEA) to provide 

information about and 

analysis of coal 

technology, supply and 

use. IEA Clean Coal Centre 

has contracting parties and 

sponsors from: Australia, 

Austria, Canada, China, the 

European Commission, 

Germany, India, Italy, 

Japan, New Zealand, 

Russia, South Africa, 

Thailand, the UK 

and the USA.

Each issue of Profiles is 

based on a detailed study 

undertaken by IEA Clean

Coal Centre, the full report 

of which is available 

separately. This particular

issue of Profiles is based 

on the report:

Techno-economics of 

modern pre-drying 

technologies for 

lignite-fired power plants

CCC/241, ISBN 978-92-9029-562-4

46 pp, August 014

This report is free to 

organisations in member 

countries. £100 to 

organisations in 

non- member countries for 

six months after

publication, and free 

thereafter.

IEA Clean Coal Centre Park House 14 Northfields London SW18 6DD

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No 11/01 August 2014

detailed thermodynamic configuration, such as whether or not the heat from the 

evaporated moisture is recovered and used for drying.

Great River Energy (GRE) in Minnesota, USA developed the DryFiningTM technology, 

which is based on a two-stage fluidised bed system. It uses warm air, not steam as in 

WTA and PFBD, to fluidise and dry the wet coal. The warm air is heated with low-grade 

heat recovered from the flue gas and cooling water, which would otherwise be wasted. 

Additional heat is supplied by hot water flowing through a heat exchanger immersed in 

the fluidised bed. The hot water is also heated with low-grade heat recovered from the 

flue gas. In addition to drying, the DryFiningTM dryer can beneficiate the wet coal by 

gravitationally segregating mineral-rich materials from the coal and removing them 

from the first stage of the dryer. 

WTA fine grain drying permits significant reduction in the dryer size and investment costs

DryFiningTM is in commercial-scale operation in both Unit 1 and Unit 2 at GRE’s Coal 

Creek Station. Coal’s moisture can be reduced from 38% to 29%, and this pre-drying 

technology delivers approximately 4% fuel savings and 4% gain in the overall plant 

thermal efficiency. In addition, there are significant reductions in air pollutant emissions. 

The US Department of Energy reported a $13 million total investment through its Clean 

Coal Power Initiative into retrofitting 4 DryFiningTM dryers into Unit 2 of Coal Creek 

Power Station. GRE disclosed in a recent report that the first commercial installation 

incurred an investment costs of 240 $/kWnet, but said these costs could be reduced to 

80−100 $/kWnet for future retrofit installations. These need to be added with other 

site-specific costs and owners’ costs to calculate the total capital costs for future retrofit 

projects. The O&M costs for the dryers at Coal Creek Station were estimated at 

$0.35/wet tonne of lignite processed or $350,000 per year for a 125 short ton/h

(113 t/y) dryer. But pre-drying of coal at both units reduced costs by more than $20 

million annually in fuel, auxiliary power consumption and other O&M costs.

Modern lignite pre-drying was included in a number of new technology concepts. For 

ultra-supercritical lignite-fired power plants that use 700°C advanced steam parameters

(700°C AUSC technology), it was found that the plant thermal efficiency gain could 

reach 7-8 percentage points if pre-drying is incorporated. In the ZEP Carbon Capture 

and Storage cost study, it was found that that post-combustion capture was a more 

expensive option for lignite plants than for hard coal plants, due primarily to higher 

levelised CAPEX and levelised fixed O&M costs of the lignite plants. IGCC 

pre-combustion capture and oxyfuel combustion seemed attractive for lignite; part of 

the reason might be that lignite pre-drying was incorporated into these two capture 

technologies. Natural gas CCGT plants were a more expensive option for power 

generation than both hard coal plants and lignite plants, regardless of CO2 capture. 

Nevertheless, abated lignite power plants could be cost-comparable or even cheaper 

IEA Clean Coal Centre Park House 14 Northfields London SW18 6DD

www.iea-coal.org | +44 (0)20 8877 6280 | email: mail@iea-coal.org
No 11/01 August 2014

than unabated natural gas CCGT plants. The sole costs of pre-drying cannot be inferred 

from these concept studies though. 

In summary, the techno-economic information on modern pre-drying processes is 

scarce and incomplete in the public literature. Their capital costs, as indicated in the 

literature, are likely to be in the range of US$33−50 million (currency in the year of 

reporting). Such costs could be largely offset by the gains in plant thermal efficiencies 

and power savings due to reduced flue gas flows and fuel handling equipment. The 

actual cost level, however, depends both on the properties of the lignite in question and 

the operational parameters. Retrofitting pre-drying and substituting dried lignite for up 

to 30% of wet fuel feed can produce about 1 percentage point (LHV) gain in the plant 

thermal efficiency. An increase of 4−5 percentage points (LHV) in the plant thermal 

efficiency can be expected in supercritical power plants firing 100% dried lignite; a 

further 0−3 percentage points (LHV) efficiency improvement could be achieved if 700°C 

advanced steam parameters are adopted at dry lignite-fired power plants. These 

modern pre-drying processes will also benefit future lignite power plants with CO2

capture.

IEA Clean Coal Centre Park House 14 Northfields London SW18 6DD

www.iea-coal.org | +44 (0)20 8877 6280 | email: mail@iea-coal.org