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 practi ce of us ing hot flue gas to dry the wet incoming lignite in conventional lignite

- fired power plants leads to reduced thermal efficiency and increased CO 2 emissions on an equivalent energy production basis. The presence of moisture vapour in the flue gas also neces sitates 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 CO 2 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 WT A dryer and GRE’s DryFiningTM system have been successfully demonstrated at commercial scale, while Vattenfall’s PFBD dryer has reached pilot scale. While t he technical details of these modern lignite pre

- dyers are well documented , their cost and techno

- eco nomic information is limited in the public domain. Nevertheless, some useful implications can still be extracted from the available information, w hich provide insights into impact of pre

- drying on the performance and costs of lignite

- fired power plants in both re trofit and new build situations.

The WTA ( Wirbelschicht

- Trocknung mit interner Abwämenutzung ; or known as fluidised bed drying with internal waste heat utili s ation) 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

- p ressure turbine via a heat exchanger immersed in the fluidi s ed 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 res ult in approximately 1 percentage point gain in the plant thermal efficiency and a 2.5% reduction in CO 2 emissions. No complete techno

- economic analyses are available, but RWE reported in 2009 that the prototype WTA dryer at Niederaussem incurred a total i nvestment of €50 mill ion 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 bo iler 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 Fluidis ed Bed Dryer ( PFBD ) is similar in principle to RWE’s WTA, though operating at a higher fluidised bed pressure. There i s very limited cost information available. RWE and Vattenfall have different views in respect of whether fluidised bed drying at higher pr essures 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 pr ovide 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 A frica, 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 www.iea

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- coal.orgNo 11/01 August 201 4

detailed thermodynamic configuration, such as whether or not the heat from the evaporated moisture is recovered and used f or drying.
Great River Energy (GRE) in Minnesota, USA developed the DryFiningTM technology, which is based on a t wo

- stage fluidi s ed 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 fluidi s ed 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 c oal by gravitationally segregating mineral

- rich ma terials from the coal and removing them from the first stage of the dryer.

WTA fine grain drying permits significan t 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 m oisture 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. T he US Department of Energy reported a $ 13 million tot al investment through its Clean Coal Power Initiative into retrofitting 4 DryFiningTM dr yers 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 $ / k W net , but sa id th ese costs could be reduced to 80−100 $ /kW net fo r 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 $0.3 5 /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 reduce d 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 par ameters ( 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 mor e 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 reas on 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 CO 2 capture. Nevertheless, abat ed lignite power plants could be cost

- comparable or even cheaper

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- coal.orgNo 11/01 August 201 4

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 pro cesses 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 t hermal 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 substitut ing 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 firin g 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 CO 2 capture.

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

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

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