Lignite-fired power plants are in operation in Asia,many parts of Europe and in Canada and the USA. Manyof the existing units are old and some are approaching theend of their service life. There is a need for these units tobe upgraded, repowered or replaced by new power plants.The power producers will need to assess the best availabletechnologies and select the options most suited to theirpreferred coal types, unit sizes, local conditions andnational compliance requirements. This report reviews therecent technical innovations and improvements in systemand engineering designs of lignite combustion processes.

Lignite drying technologiesThe high moisture content of lignite is a major issue in

its commercial utilisation. In conventional pulverisedlignite fired power plants, a significant amount of theenergy in the coal is absorbed as heat to evaporate thewater present before any useful energy can be obtained andconverted to electricity. This leads to low thermalefficiency, high CO2 emissions per unit of energy outputand high capital costs of a plant. Other technicaldifficulties that arise from high moisture content includefuel handling problems, difficulty in achieving ignition,and larger boiler size required due to the increased flue gasvolumetric flow. Therefore, drying of coal prior tocombustion is important to improve thermal efficiency andconsequently reduce CO2 emissions.

Extensive research and investigations have been carriedout worldwide to develop energy efficient and costeffective coal drying processes. A number of approachesare taken to dry lignite and other low rank coals. Thesetechnologies broadly fall into two categories: evaporativedrying and non-evaporative dewatering processes.Recently, several advanced coal drying technologies havebeen developed and are offered to the commercial market,whilst many more are under development. Among these,the WTA developed by RWE, Germany and theDryFining developed in the USA by a team led by GreatRiver Energy (GRE), have been successfully demonstratedon commercial-scale lignite-fired power generating units.This is an area that is developing rapidly.

Pulverised lignite firingThe major challenge facing the power generation

industry over the coming decades will be to increase theefficiencies of fossil-fuelled power plants while alsomeeting more stringent environmental goals. In particular,there is a need to reduce the emissions of CO2 to theatmosphere, with near-to-zero CO2 emissions being theultimate goal. At the same time, plant reliability,availability, maintainability and operational costs, as wellas the cost of electricity (COE), must not be compromised.

Today, most lignite-fired power plants around the worldin operation use pulverised coal combustion (PCC)technology. The majority of existing pulverised lignitefired power plants are based on the conventional singlereheat thermal cycle with subcritical steam pressure andtemperature. Recent advances in boiler and turbinematerials have led to the installation of high efficiencysupercritical (SC) or ultra-supercritical (USC) PCC steamgenerators. In modern lignite-fired power plants, steampressures up to 27.5 MPa and main/reheat steamtemperatures as high as 600C/605C have been applied,and a net plant efficiency of >43% (LHV) has beenachieved. In addition to the use of advanced steam cycles,technical innovations in system and process engineering,larger unit size, improved waste heat recovery from theflue gas, and optimisation of auxiliary power needs haveall contributed to the plant efficiency increases. Othertechnical advances in system and process engineeringinclude: improved design of low-NOx burners; an optimised heat cycle for the regenerative preheat

unit; the reduction of condenser pressure; the use of efficiency-improved bladings at the steam

turbines; increased use of fibre-glass reinforced plastics and

flue gas draw off through the cooling tower; the introduction of highly efficient after-burning

grates with residues of lower than 1% of unburntcarbon;

improved combustion chamber cleaning with the helpof water jet sprayers.

ProfilesNo 12/8 July 2012

Update on lignite firing

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 contractingparties and sponsors from: Australia, Austria, Brazil, Canada, China, Denmark, the European Commission, GermanyIndia, Italy, Japan, Republic of South Korea, the Netherlands, New Zealand, Poland, Russia, South Africa, Sweden,

Thailand, the UK and the USA.

Improvements in process design and engineering notonly increase the total plant efficiencies, but also improvethe availability, flexibility and environmental performancesof the plants and reduce capital and operating costs.

Due to the considerable variation in the quality oflignite coals, careful selections of technologies and properprocess and engineering designs are required to ensurelignite-fired plants achieve high efficiency, reliability andavailability, good environmental performance as well ascost effectiveness. Solutions are available for particularproblems relating to certain coal qualities. The experienceof operating lignite-fired power plants in Germany hasshown that lignite can be burned efficiently to producepower at competitive prices. It has been proven thatlignite-fired power plants can achieve high efficiency andhave good environmental performance. The high ligniteplant efficiency is achieved by application of advancedtechnologies and improved engineering designs to all partsof the power plant.

PCC versus CFBThere are currently two competing technologies for

lignite firing: pulverised coal combustion (PCC) andcirculating fluidised bed (CFB) combustion. PCC utilises aproven technology with a very high reliability level. PCCboilers have the advantage of being able to accommodateup to 1300 MWe. While the vast majority of existing PCCboilers use subcritical steam conditions, newly-constructedsupercritical PCC boilers are currently being designed toapply SC or USC steam conditions. To date, severalultra-supercritical (USC) PCC boilers are in operation inChina, Europe, Japan, South Korea and USA. Theeconomies of scale and advanced steam cycle can result ina significant reduction in total costs as well as lower CO2emissions, and therefore large SC and USC PCC boilerswill be the preferred choice of technology to subcriticalPCC boilers in future.

Compared to conventional PCC, CFBC is a relativelyyoung technology. However, CFBC is commerciallyproven and has been in reliable electric utility service forthe last two to three decades. Commercial CFBC boilersoffer greater fuel diversity than PCC units, operate atcompetitive efficiencies, and when coupled with apolishing SO2 scrubber and SNCR system, can meet theincreasingly stringent emissions standards.

There have been continued advances in CFBCtechnology. Recently, significant developments have beenmade in scaling-up of CFBC units and in implementing

super-/ultra-supercritical steam conditions. Today, SC/USCCFBC units with size up to 800 MWe for bituminous coalor up to 600 MWe for lignite are commercially available.The 600800 MWe class CFB boilers will haveperformance and economics that are comparable tocorresponding PCC boilers while offering greater fuelflexibility, especially the ability to burn low heating valueopportunity fuels. The capability to burn fuels withcharacteristics unacceptable, or barely acceptable, to PCCboilers is becoming a key issue for CFB technologyselection. CFB technology will play a key role incoal-based power generation in the future, especially inlow quality coal and cofiring applications.

A range of advanced combustion technologies has beendeveloped to improve the efficiency of lignite-fired powergeneration. Todays state-of-the-art lignite-fired powerplants in operation in Germany have achieved energyefficiencies as high as 43%, putting lignite plants in asimilar position as modern hard coal-based power plants.With modern technologies it is now possible to produceelectricity economically from lignite while addressingenvironmental concerns.

Each issue of Profiles is based on a detailed study undertaken by IEA CleanCoal Centre, the full report of which is available separately. This particularissue of Profiles is based on the report:Update on lignite firingQian ZhuCCC/201, ISBN 978-92-9029-521-1, 75 pp, June 2012, 255*/85/42.50* non-member countries member countries educational establishments within member countries

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