China shows the way
Chinese researchers are pioneering new ways to turn coal into valuable liquid fuels

Coal liquefaction is a technology of the future, as the world faces up to declining reserves of oil and gas. And it is likely to become increasingly important in China, a country with huge stocks
of coal and dramatically increasing demand for energy. Steel belts from Sandvik Process Systems have shown their worth in a new process being developed by Chinese engineers.
Coal liquefaction (also known as coal-to-liquids, or CTL) describes the conversion of solid coal into liquid fuels. It is carried out via two main processes, of which the more commonly used is the indirect Fischer-Tropsch process. Here the coal is first converted into synthesis gas by heating it with steam and air, after which the synthesis gas is converted into hydrocarbons. Less common is the Bergius-Pier direct liquefaction process, in which coal is hydrogenated in a single step. In both cases the final products are gasoline, diesel, fuel oil, and raw materials for the chemical industry.

China started to develop direct coal liquefaction (DCL) technology on an industrial scale as long ago as the 1970s. More recently, the China Coal Research Institute (CCRI) installed a pilot plant to test the process on different types of coal. The result was a decision to start work in the coal fields of Inner Mongolia, a project which will involve financial investment believed to be second only to the Yangtze dam. The development of the coalfield and operation of the entire plant will be handled by the Shenhua Group, China’s largest producer of coal, with a total production of more than 200 million tonnes in 2006.

Solidifying coal slurry

The DCL process used in China produces a liquid residue containing hydrocarbon liquids mixed with solid matter. Its high heating value makes this coal slurry useful, but the fact that it is produced as a viscous liquid at temperatures up to 310 °C makes it difficult to transport. Pilot-scale trials showed that the slurry could be solidified on a steel belt cooler, producing flakes that are easy to handle. Steel belt cooling, developed by Sandvik Process Systems, has the advantages of being well-proven, reliable and environment-friendly.

The coal slurry is fed by means of an overflow weir onto a continuously-running,
1.5 m-wide steel belt whose underside is cooled by a water spray. The electrically-heated feeder delivers a uniform layer across the full width of the belt, while side retainers prevent the slurry from flowing over the belt edges. A release agent is sprayed over the belt just ahead of the feeder to prevent sticking and ease discharge at the end of the belt.

The discharge temperature of the product is approx. 90 °C, well below its melting point of 150 °C. The solidified product, now in the form of a uniform layer, can have varying physical properties, from brittle to tough and sticky, depending on the qualities of the coal and the process settings. Brittle material is broken up by mechanical fingers, and sticky material is removed by a knife.

A hood is installed over the weir feeder, the full length of the steel belt and the discharge device. Product fumes are extracted via an exhaust air cleaning system. Each solidification unit can deliver 15,000 kg/h of coal slurry flakes.

Project management

Sandvik Process Systems received the order for twelve steel belt cooling systems from the Chinese engineering company responsible for the entire project. The systems were engineered by Sandvik’s engineering department in Germany, based on experience with similar products and processes, after which detail engineering, manufacturing and preassembly were handled by Sandvik’s Shanghai plant. Final on-site assembly, commissioning and startup will be performed by Sandvik specialists.

The design of the units needed to take into account the very harsh climatic conditions of the site in Inner Mongolia, including ambient temperatures ranging from
–30 °C to 40 °C and the likelihood of heavy sandstorms. This challenge was further compounded by the requirement that systems need to be installed in the open air, without any shelter. This too required special consideration at the design stage.