Production of Liquid Fuels from Coal and Gas

CTL (coal-to-liquid) and GTL (gas-to-liquid) processes allow the conversion of coal and natural gas into liquid hydrocarbons such as gasoline, diesel, methanol, dimethyl ether (DME) and others. The CTL process includes direct coal liquefaction (DCL) and indirect coal liquefaction (ICL), both aimed at increasing the hydrogen/carbon (H/C) ratio of the final product. The first CTL processes, developed in Germany in 1913, involved the high-pressure hydrogenation of lignite. In 1925, Fischer and Tropsch developed the homonymous process (FT) which involves, after the gasification of coal, the production of a synthesis gas which, by catalytic hydrogenation of CO, is then converted into a wide range of hydrocarbons liquids (olefins, paraffin, alcohols, aldehydes, ketones, waxes) from which diesel, gasoline and other products are obtained by upgrading treatments. Both DCL and ICL processes were used extensively in Germany in the Second World War. Currently, the largest operator in the CTL sector is the Sasol company (South Africa) with ICL-FT technology.

In recent years, before the drop in oil prices, a renewed interest in these processes had developed with the involvement of numerous operators such as Shell, Rentech, Shenhua Energy (China) and the construction of several demonstration and industrial plants. The world's largest CTL production capacity is concentrated in Sasol plants (160 kbbl/d). Recently, China has invested in both DCL and ICL CTL plants. Shenhua Indirect CTL demonstration project is located in Majiata, Ejin Horo Banner, Ordos, Inner Mongolia. The FT synthesis technology is developed by Synfuels China and is owned by the Shenhua Group. The production capacity is 180 000 t/y of liquid fuels to be mixed with the Shenhua Group's petroleum products (1 Mt/y of products from direct liquefaction of coal) to improve the quality.

Among the several CTL plants and projects operating worldwide, the Sasol synthetic fuel plants in South Africa (Sasol I in Sasolburg, Sasol II and Sasol III in Secunda) represent the most important and commercialized examples of coal conversion into liquid synthetic products via Fischer-Tropsch. Sasol I, built and started up in 1955, operated 17 Sasol-Lurgi gasifiers to convert the syngas produced from sub-bituminous coal into a wide variety of liquid chemicals. In 2004, this plant opted for natural gas conversion into syngas through the autothermal reforming process. The Sasol II and Sasol III plants, established in 1974 and operational since 1979, are currently called Secunda CTL and include 80 gasifiers using Sasol-Lurgi gasification technology to convert the produced syngas into a wide variety of chemicals and liquid fuels (petrol and light olefins) [21].

Sasol Limited continues to support and expand its operations in South Africa with Project 2050. Several strategies are involved in this project, among which the construction of new coal mines is essential to ensure the supply of coal to the CTL plants up to the mid-century. Additionally, the company has set carbon reduction targets at 15% for existing CTL facilities by 2020 and 30% for all new CTL plants by 2030. It is also working as a member of the South African Center for carbon capture and storage developing a CCS demonstration facility operational by 2020 [22].

The 4 Mt/year Shenhua Ningxia Coal project plant is a CTL demonstration project located in the Ningdong Energy Coal Chemical Industry Park, in the Ningxia Hui Autonomous Region. This project employs the F-T synthesis technology developed by Synfuels China and is owned by Shenhua Ningxia Coal Industry Group Co., Ltd., with an investment of 55.0 billion RMB.

The project uses coal as raw material and converts 20.36 Mt/y of coal to produce 4.052 Mt/y of liquid fuels, of which 2.733 Mt/y blending diesel, 983 000 t/y naphtha, 336 000 t/y LPG, 128 000 t/y sulfur, 75 000 t/y mixed alcohols by-product and 107 000 t/y ammonium sulphate by-product.

Less mature is the MTG (methanol-to-gasoline) process which produces gasoline from methanol derived from coal or gas.

GTL production capacity is currently installed in Malaysia (12 500 bbl/d), Qatar (140 000 bbl/d), Indonesia, Mexico, New Zealand, South Africa for the production of diesel, petrol and methanol. GTL and CTL processes based on FT synthesis have high commercial maturity (TRL 9). However, these are technologies with niche market shares compared to liquid fossil fuels and exposed to risks of economic profitability linked to fluctuations in the oil and gas markets. It should be noted that the market target of GTL products is different from that of the more well-known LNG (liquid natural gas): the LNG is liquefied natural gas destined for transport by sea and for electro-generation or injection in the gas network, while GTL technology aims to produce liquid fuels to replace crude oil derivatives.

For both processes, CTL and GTL, the Italian reality does not have applications on an industrial scale but only pilot plants (ENI and Politecnico di Milano - GTL 20 bbl/day pilot plant at the San Nazzaro (PV) refinery) and research activity on the process and the catalytic aspects (ENI, Enea, Politecnico di Milano).