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Drying lignite – how and why?

World Coal,

Lignite is an important and affordable fuel for power generation. The problem is its high moisture content. Hot flue gas can be used to dry the wet incoming lignite in conventional lignite-fired power plants but this reduces thermal efficiency and so increases CO2 emissions. Moisture vapour in the flue gas also means a larger boiler furnace is needed which increases the capital costs.

Drying lignite before combustion in the boiler is an effective way to increase the thermal efficiency of lignite-fired power plants. The challenge is to avoid using hot flue gas and carrying the moisture through the entire power plant.

What technology is available?

Modern pre-drying technologies have been developed in Germany and the US. RWE’s WTA dryer and GRE’s DryFining™ system have been demonstrated successfully at commercial scale, while Vattenfall’s PFBD dryer has reached pilot scale.

The technical details of these modern lignite pre-dyers are well documented, but there is limited techno-economic information. However, Dr Nigel Dong of the IEA Clean Coal Centre has some insights into the impact of pre-drying on power plant economics for both retrofit and new build situations.

The WTA (Wirbelschicht-Trocknung mit interner Abwämenutzung) system was developed by RWE Rheinbraun in Germany. WTA uses recycled steam from coal moisture to fluidise the wet lignite in the dryer. The energy for drying comes mainly from the steam bled from the low-pressure turbine via a heat exchanger immersed in the fluidised bed.

In 2007, a commercial prototype WTA dryer was commissioned in the supercritical Unit K of RWE’s Niederaussem power plant. The dryer can reduce the lignite moisture from 50 – 55% to approximately 12%. Pre-dried lignite can form up to 30% of the fuel on an energy basis, which can lead to 1% gain in the plant thermal efficiency and a 2.5% reduction in CO2 emissions. RWE reported a total investment of € 50 million for erection and operation of this WTA dryer.

For a supercritical lignite-fired power plant firing 100% pre-dried lignite (BoA Plus), RWE considered a specific investment cost of € 70/kW for the open cycle variant of WTA, which included costs of modifying the burners and boiler combustion chambers. The overall investment costs of a BoA Plus are similar to a BoA unit, but the thermal efficiency could increase by 4 – 5%.

Vattenfall’s Pressurised Fluidised Bed Dryer (PFBD) is similar in principle to the WTA, but it operates at a higher fluidised bed pressure. There is very limited cost information available.

RWE and Vattenfall have different views on 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 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, US, developed the DryFining 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.

The DryFining dryer can also beneficiate the wet coal by gravitationally segregating mineral-rich materials from the coal and removing them from the first stage of the dryer.

The technology is in commercial-scale operation in both Unit 1 and Unit 2 at GRE’s Coal Creek plant. Coal moisture can be reduced from 38% to 29%. There are fuel savings of about 4% and a 4% gain in the overall plant thermal efficiency.

What advantages?

The US Department of Energy reported a US$ 13 million total investment into retrofitting 4 DryFining dryers into Unit 2 of Coal Creek power plant. According to GRE, the first commercial installation incurred investment costs of US$ 240/kWnet, but these costs could be reduced toUS$ 80 – 100/kWnet for future retrofit installations. The O&M costs for the dryers at Coal Creek Station were estimated at US$ 0.35/wet t of lignite processed or US$ 350 000/year for a 125 short tph dryer. But pre-drying of coal at both units resulted in more than US$ 20 million/year savings in fuel, auxiliary power consumption and other O&M costs.

Modern lignite pre-drying is included in a number of new technology concepts. For ultra-supercritical lignite-fired power plants using 700°C advanced steam parameters, the plant thermal efficiency gain could reach 7 – 8% if pre-drying is incorporated.

Techno-economic information on modern pre-drying processes is scarce and incomplete in the public literature. Their capital costs 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% (LHV) gain in the plant thermal efficiency. An increase of 4 – 5% (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.

The report, Techno-economics of modern pre-drying technologies for lignite-fired power plants, CCC/241 by Nigel Dong, 46 pp, September 2014) is available for download from the IEA Clean Coal Centre Bookshop.

Edited from various sources by Sam Dodson

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