In the first part of this article, Nigel Dong, IEA Clean Coal Centre, reviewed the history of coal drying technology in Europe and discussed hot gas and steam drying. In this part, he concludes by reviewing hydrothermal dewatering and mechanical thermal expression and discusses the potential future for coal drying in Europe.
Types of coal drying: hydrothermal dewatering
Non-evaporative thermal dewatering has been applied commercially to brown coal since the 1920s. The Fleissner process, developed for Austrian brown coals, involves steam treatment of brown coal in batch autoclaves under conditions wherein the bulk of the moisture in brown coal is removed in liquid form. The Fleissner process has been widely applied to European brown coals, while investigation tests have also been undertaken on North Dakota lignites and Victorian brown coals.
A number of non-evaporative dewatering processed based on the Fleissner process but with improved design and energy efficiencies, have evolved, several of which have progressed to pilot plant stage but not beyond. Notably, in the 1970s, the Evans-Siemon process was developed using pressurised hot water (>374°C) to replace the steam in the Fleissner process as a heat transfer medium. The next step would be to design a continuous or semi-continuous reactor system with a number of designs proposed.
Other processes have similar technical features to the Fleissner and Evans-Siemon processes. However, the difficulties of treating the wastewater from those processes, which is rich in both organic and inorganic contaminants, in conjunction with increased equipment costs due to using high pressure and temperature conditions, hinder their commercial applications.
Types of coal drying: mechanical thermal expression
This non-evaporative dewatering concept has been used in Europe for generations to mechanically compress peat to remove water to speed up the solar-drying of this fuel. However, commercial application of this concept to drying lignite has been limited by the high pressure and residence times (hours) required. In Germany, a quasi-continuous fully-automated process was developed and achieved 1.6 tph throughput of dried coal. RWE also demonstrated this process at a 15 tph of dried coal facility at the Niederaussem power plant, but the results were not promising.
In contrast to limited progress made in Europe, Australian developers have taken this process forward, with successful demonstrations by White Australia and Aquex Pty Ltd. However, as with hydrothermal dewatering, the waste effluent is difficult to treat and the process suffers from high capital requirements, hindering commercialisation.
The current status of coal drying technology in Europe
Coal drying development in Europe is focused on pre-drying processes in large-scale power plants, while upgrading processes development is limited. This is largely due to shift from coal in energy consumption by household and the industrial sectors. RWE's WTA process is the furthest developed pre-drying technology in the world and has been successfully demonstrated at a 1000 MWe power plant in Germany and is ready for the market. Vattenfall's PFBD process is at a lesser-developed stage but the potential is high.
The key challenges to development
The main challenge to developing drying technology in Europe is the current investment climate, which is shaped by governmental climate change and energy policies favouring lower carbon alternatives. Currently, it would be very difficult to build a new coal-fired power plant without CCS in Europe. At the same time, CCS is yet to be demonstrated at a commercial scale on coal-fired power plants. This restricts the market for modern pre-drying processes on new build plants.
The uncertainties surrounding coal also dampen investment in retrofitting modern pre-drying processes to existing plants. More stringent air emission standards have led to eventual shutdown of many old plants; those remaining operation need extra investment on top of those made to comply with tightened air emission standards, which could be difficult.
The outlook for coal drying technologies in Europe
In the short to medium term, there will be limited uptake of coal pre-drying technologies in Europe for the above-mentioned reasons. In the long term, given that pre-drying can increase plant efficiencies and reduce CO2 emissions, these technologies could add to the benefits gained from CCS. However, there remain many uncertainties over the role that coal drying technologies could play in a carbon constrained future.
Written by Nigel Dong, IEA Clean Coal Centre, UK.
Edited by Jonathan Rowland
Read the article online at: https://www.worldcoal.com/power/22012014/nigel_dong_the_types_of_coal_drying_and_their_future_use_in_europe_power01b/