Elsevier

Applied Catalysis A: General

Volume 476, 22 April 2014, Pages 158-174
Applied Catalysis A: General

Review
Indirect coal to liquid technologies

https://doi.org/10.1016/j.apcata.2014.02.035Get rights and content

Highlights

  • The latest progress of indirect coal liquefaction catalysts was summarized.

  • Syngas to methanol, dimethyl ether, olefins and ethyl glycol was summarized.

  • Novel catalysts and mechanisms of Fischer–Tropsch were summarized.

  • Future prospects of indirect coal liquefaction technologies were discussed.

Abstract

Indirect coal liquefaction has enormous potential applications. Increasingly, new synthetic technologies have been concentrating in this area, and a number of new large-scale indirect coal liquefaction plants have been set up during very recent years. Further, a large volume of papers on indirect coal liquefaction have been published over the last two decades, including those on Fischer–Tropsch synthesis, syngas to ethylene glycol, syngas to methanol, dimethyl ether as well as methanol to olefins. In this review, the recent literature of indirect liquefaction, including Fischer–Tropsch and syngas to chemicals, are summarized, with an emphasis on the reaction mechanisms, conditions and novel catalysts.

Introduction

Coal is a material that some people like because of their needs while others hate due to the various emissions resulting from its combustion [1], [2], [3], [4], [5], [6], [7]. To overcome the environmental challenges as associated with the conventional utilization approaches, people are increasingly interested in using alternative approaches including gasification and liquefaction. Liquefaction can be direct or indirect. Indirect coal liquefaction (ICL) processes mainly include two important steps. In the first step, the coal is gasified and converted into hydrogen and carbon monoxide, also called as syngas. In the second step, the syngas is further synthesized into liquid fuel. Coal is the most abundant energy reserve in the world. According to statistics of the International Energy Agency (IEA), of the top 10 coal producers in 2011, China has the highest coal production – 3576 metric tons (Mt) (46%), whereas the United States produces 1004 Mt (13%). Meanwhile, world crude oil demand in 2012 was approximately 92.0 million barrels per day (mmb/d), slightly higher than the 2011 demand of 91.9 mmb/d; global crude oil production in 2013 (from January to May) was 75.87 mmb/d. Global demand for crude oil continues to rise, which means that improvements in technologies to produce liquid fuels from other sources would be highly beneficial. In addition, because of the abundance and low-price of coal, many countries still use it in large tonnages in traditional way. However, it should not be ignored that emissions of SOX/NOX, Hg, CO2 from coal combustion cause environmental problems. Researchers have been making great effort in recent years to mitigate these environmental concerns, and great progress has been achieved worldwide. Coal energy resources have been developed and commercialized through alternative utilization technologies, such as pyrolysis, gasification and liquefaction. Among these technologies, indirect liquefaction promises to be one of the most effective approaches to convert coal to fuel liquid. Synthetic fuels derived via indirect liquefaction can outperform fuels directly derived from crude oil or from direct liquefaction, with regard to air pollution, and greenhouse gas emissions and other environmental constraints.

In contrast to direct liquefaction, two steps have to be developed in order to make indirect liquefaction possible. The first step is to break down the carbon-based raw material to form syngas. The second step is to catalytically produce hydrocarbon fuels and/or chemicals from syngas. Indirect liquefaction can be classified into two principal areas: (1) conversion of syngas to light hydrocarbon fuels via Fischer–Tropsch synthesis (FTS) and (2) conversion of syngas to oxygenates such as methanol, dimethyl ether (DME), ethylene glycol (EG) and so on. FTS, a gas to liquid technology, is one of the most important processes, which produces synthetic fuel and lubrication oil, mainly from coal, natural gas or biomass resources. Following its invention by Fischer and Tropsch in the 1920s, research has made great strides in adjusting and refining the process. The development of FTS has been greatly influenced by fluctuations in the price of global crude oil. In recent decade, due to global energy-deficiency and the demand for green energy, FTS has received wide recognition. Based on coal gasification to syngas technology, the integrated gasification combined cycle (IGCC) process has also attracted extensive attention, due to its high efficiency and favorable environmental performance. As important chemical intermediates and peak shaving fuels, methanol and DME are the top-priority products of the IGCC process. As a primary part of indirect liquefaction, coal to EG has also been attracting extensive attention in both academic and business circles in the past decades.

Since indirect liquefaction has enormous potential applications, more and more new synthetic technologies have been concentrated in this area, and research on FTS [8], [9], [10], [11], [12], syngas to EG [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], syngas to DME [18], [26], [27], [28] and methanol [29], [30], [31], [32], as well as methanol to olefins (MTO) [33], [34], [35], [36], [37], [38], have resulted in a large number of publications during the past couple of decades. However, most of the papers and reviews mainly focused on one specific detail of the range of subjects relevant to indirect liquefaction. In this review, the recent developments of indirect liquefaction, including FTS, syngas to EG, DME, methanol and MTO are summarized, with an emphasis on the reaction mechanisms, conditions and novel catalysts.

Section snippets

The development of catalysts for the Fischer–Tropsch process (FT process)

FTS is one of the most important synthetic processes. This process, which provides an effective gas to liquid technology, produces a broad range of hydrocarbon products, which are converted to synthetic lubrication oil and synthetic fuel in subsequent refining process, mainly from coal, natural gas resources or biomass. FTS is a collection of chemical reactions, and mainly produces synthetic linear hydrocarbons (alkane and alkene). It also comes with production of oxygenates and utilizes the

Methanol to olefin

The conversion of methanol to olefins tends to be an interesting and promising way of converting methane to chemicals. Solid acids can effectively catalyze this reaction to form hydrocarbons. However, the selective production of light olefins, especially ethylene and propylene, is a challenge to catalysis. Before the year of 1990, most of the literatures discussed the zeolite ZSM-5 as an appropriate catalyst, which possesses a ten-member ring, interconnected channel system and yields branched

Syngas to methanol

The production of clean liquid fuels from coal-based syngas is one of the key steps for future clean coal utilization. Based on coal gasification to syngas technology, the integrated gasification combined cycle (IGCC) process attracts extensive attention due to its high efficiency and friendly environmental performance [30], [33], [111], [112], [113], [114]. As an important chemical intermediate a fuel, methanol is the top-priority product of the IGCC process [115], [116]. Originally methanol

Syngas to dimethyl ether

DME is gaining increasing worldwide interests due to its extensive application such as chemical feedstock, cooking fuel, spray-can propellant and diesel fuel oil [28], [134], [135], [136], [137], [138]. DME can be manufactured in large quantities from coal, natural gas, biomass and municipal solid waste. DME synthesis is generally considered to proceed through three consecutive reactions: (i) methanol synthesis, (ii) methanol dehydration, and (iii) water–gas shift reaction. Presently there are

Synthesis of dialkyl oxalate

EG is a crucial chemical raw material with a global demand of around 25 million tons each year, which is mostly produced ethylene through traditional petrochemical technology. The cost of this production is high due to the continuous increasing price of natural gas and crude oil, and dwindling sources of petroleum. Coal to ethylene glycol, as a potentially more green and economic technology, has been attracting extensive attention in both academic and business circles in the past decades.

Conclusion

ICL technology provides a sustainable way to use our limited energy sources. In China, this technology is approaching maturity and is poised to play important roles in the growing markets. For example, large-scale plants to produce EG and diesel via ICL technology have been setup in recent years. In consideration of the continuous increasing price of crude oil and ever-dwindling sources of petroleum, ICL is a promising alternative source of liquid fuels, especially for countries with abundant

Acknowledgements

The research was supported by the U.S. Department of Energy and the Wyoming Clean Coal Program.

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