Nigel Dong, energy specialist at the IEA Clean Coal Centre, discusses the history and reviews the types of coal drying technologies in Europe.
The development of coal drying technologies in Europe has been concentrated largely in Germany and is closely associated with the establishment and expansion of the brown coal briquetting industry. In 1855, the first drying of brown coal started at the von der Heydt mine in Ammendorf in central Germany, where a "clean coal" was produced through drying and pressing without a binder. The first registration of a patent for a pipe dryer for use in drying brown coal was made three decades later in 1884.
The drivers behind coal drying
There are two major drivers. Firstly, is the need to drive down CO2 emissions from the lignite-fired power plants or heat production plants. Burning dried lignite, even just a fraction of total fuel input, in conventional lignite-fired power or heat production plants can reduce the amount of energy wasted to evaporate the moisture in raw lignite. As such, more energy can be used to produce electricity or heat so that the overall energy efficiency will increase, reducing CO2 emissions.
Secondly, the need to upgrade lignite into a stable fuel of increased heating value so it can can be transported some distance away from the mine. Drying is an indispensable component of any upgrading processes. Without upgrading, lignite can usually only be used in mine-mouth power plants or, in some cases, to make briquettes for use in nearby industrial and household applications. Upgrading thus provides for new market opportunities to the otherwise low value fuel used only locally.
Types of coal drying: hot gas drying
A range of technologies has been developed to dry coal, including: hot gas drying, steam drying hydrothermal dewatering and mechanical thermal expression.
Hot flue gas has been used in two drying processes: the rotary drum dryer and the flash mill pre-drying system.
In a rotary drum dryer, the crushed coal is fed into a rotating kiln fitted with baffles and flushed with hot flue gas. This type of process was examined in the 1920s in Germany, but was eventually prohibited for drying brown coal for briquetting because of the high risk of fire and coal dust explosion.
Flash mill pre-drying emerged in the first half of the 20th Century and has since become the standard equipment in conventional large-scale lignite-fired power plants. This involves the simultaneous milling and drying of raw coal immediately before combustion in a beater-type mill, aerated by the hot flue gas drawn off at 900 – 1000°C from the upper levels of the furnace chamber. Such pre-drying requires a boiler substantially larger to cope with the large quantity of evaporated water. Most of the latent heat contained in the vapour is not recovered, penalising efficiency.
Moreover, to handle the additional volume of water vapour, the fan power consumption is higher, which results in higher auxiliary power requirements and further reduces efficiency. This has led to the development of modern pre-drying processes where low grade or waste heat is used and the removed moisture does not reach the boiler.
There also exist processes that use hot air instead of hot flue gas to dry coals. The hot air is used at high velocity and, in some cases, under increased pressures, so as to shatter coal particles to destroy the microstructure and release the moisture. LFP (Europe) Ltd provides such a dryer, LamiFloTM, which can be used to dry coal ash, bituminous coals, anthracite, carbon pellet and other low cost materials.
Types of coal drying: steam drying
Steam can be used to dry low rank coal, using either direct contact or indirect contact. Indirect steam drying involves steam-heated plate driers and was first used in Germany in 1874 for drying brown coal for briquetting. This method eventually gave way to the more efficient rotary drum and tube steam driers, which became the preferred and the most commonly used method for drying brown coal for briquetting. The rotary drum and tube steam drying systems have been refined over the years to minimise the variation in moisture content of the dried products.
In addition, fluidised bed driers were under development in the 1980s and two variants have emerged: the bed of coal can be fluidised by air or inert gas and be heated by a steam tube heat exchanger immersed in the bed; or superheated steam may be used both to fluidise and heat the bed of coal. A two-stage design with improved drying efficiency and reduced energy consumption was also proposed, where the superheated steam from the first fluidised bed is condensed in a heat exchanger in a second bed. A single stage superheated steam fluidised bed pilot plant was built in Leipzig, Germany and has been scaled up to 100 tpd capacity.
RWE and Vattenfall subsequently built on the experience to develop their proprietary fluidised bed pre-drying technologies for use at large-scale power plants. Following successful pilot-scale tests at Frechen, Germany, RWE's WTA process has been successfully demonstrated at full scale at the 1000 MW BoA ultra supercritical unit K of the lignite-fired Niederaussem power plant. This commercial prototype was designed to produce 110 tph dried product of 12% moisture content from 210 tph of 50 – 55% moisture lignite, which corresponds to 30% of the fuel requirement for the unit at maximum continuous rating. The dryer achieved 94% availability during continuous operations in 2011 and has reached 83% of the original design throughput to date. The technology is ready and is being licensed to three suppliers.
Vattenfall's PFBD process is similar to the WTA process but operates at higher steam pressure. However, this process is still at the pilot stage with some potential challenges to surmount.
Steam drying in direct contact has long been used to dry biomass and agricultural products, but is less commonly applied to coal. The UK-based GEA Process Engineering Ltd is offering a closed-loop pneumatic dryer, SSDTM, that can be used to dry peat, wood chips and many other materials.
Nigel Dong concludes his review of coal drying technologies and discusses the potential for their implementation here.
Written by Nigel Dong, IEA Clean Coal Centre, UK.
Edited by Jonathan Rowland
Read the article online at: https://www.worldcoal.com/power/21012014/nigel_dong_the_history_and_types_of_coal_drying_technologies_in_europe_power01a/