By Weng Li
Director, National Institute of Clean and Low-Carbon Energy
Professorate Senior Engineer, National Institute of Clean and Low-Carbon Energy
Senior Engineer, National Institute of Clean and Low-Carbon Energy
Indirect coal-to-liquids (ICTL) technology consists of a two-step process using coal as a feedstock that is first gasified to produce synthesis gas (CO+H2). The syngas is then converted into hydrocarbon compounds and other products via Fischer–Tropsch (F-T) synthesis.1-3 The liquid fuels produced through ICTL are environmentally friendly. For example, ICTL fuels contain less sulfur and fewer aromatic hydrocarbons and the cetane number of ICTL diesel can be as high as 70. (The higher the cetane number the better for combustion performance; premium diesel fuels usually have cetane numbers of approximately 60.) The advantage of such a high-quality diesel is that it can be used when there are strict constraints regarding automobile exhaust gases or it can be used as a blending stock to upgrade lower quality diesel. ICTL can also produce higher value products such as wax and lubricating oil.2,3 Today, the wide array of products that can be made through ICTL is an extremely active R&D area. The core technology of ICTL is undoubtedly the F-T processes. A brief history of its development follows.3-5
F-T synthesis (FTS) was first invented by German scientists F. Fischer and H. Tropsch in 1923. F-T synthesis later became the basis of, and principal step in, ICTL technology. The first commercial-scale ICTL plant was built by Ruhrchemie, a German company, in 1934 and was put into operation in 1936. In 1955, Sasol successfully commercialized the first ICTL plant using a fixed bed reactor in South Africa. After some 50 years of continuous development, Sasol has become the largest producer of ICTL fuels today. The company is able to produce nearly eight million tons of liquid fuels every year. Moreover, Sasol has advanced their technical capabilities for all areas within its ICTL process. One particular achievement worthy of mention is the use of cobalt-based catalysts at commercial scale in F-T technology with natural gas as feedstock. This has inspired worldwide interest in ICTL. Since 1980, China and a few other countries have boosted development and accelerated industrialization of ICTL. International energy companies such as Exxon, Rentech, and Shell all have their own programs to advance the technology toward commercialization. Since the turn of this century, these international companies have mainly focused on cobalt F-T catalysts and gas-to-liquids (GTL) technologies. Taking a different approach, companies in China have focused primarily on iron-based catalysts, resulting in breakthroughs in many areas that have advanced ICTL technologies.
Current Status of ICTL Technology in China
ICTL Development in China
ICTL development in China can be divided into three stages2: the first stage was before 1980. During this time, there was little ICTL-related activity in China. The second stage was between 1980 and 2000. During this period, the focus was on the fundamental research necessary to accumulate knowledge and to play catch-up. Only Shanxi Institute of Coal Chemistry (SXICC) of the Chinese Academy of Sciences developed a two-stage process with two different configurations. The first configuration was a modified Fischer–Tropsch (MFT), in which two fixed beds in series were utilized to increase the overall conversion efficiency. The second configuration was a slurry modified Fischer–Tropsch (SMFT), in which a slurry reactor and a fixed bed in series were developed. Both reactor types were tested at the pilot-scale. The third stage of ICTL development is the time since the turn of the 21st century. ICTL in China is experiencing a period of fast-paced development, which has made China a global leader in the field of ICTL technologies and their applications.
China’s Shenhua Group, a bellwether in China’s coal industry, pioneered its modern coal-to-chemicals technologies with a direct coal liquefaction technology as its first project. Following this success, Shenhua Group extended its effort to include most modern coal-to-chemicals technologies, including ICTL. A specialized ICTL research department was formed and a demonstration ICTL plant was successfully operated in 2010. Yankuang Group also founded a research department specifically focused on ICTL and operated a 10-ktpa (kilo tonnes per annum) scale plant in 2004, based on which a megaton process development package (PDP) was developed. SXICC, another major force in the coal industry in China, developed a successful catalyst for ICTL and also was able to mass produce this catalyst. SXICC also formed a partnership with Yitai Group, which is the parent company of Synfuels China. Synfuels was commissioned to conduct research and engineering design on ICTL technology. Sinopec, a petroleum giant in China, is also developing a natural-gas-to-liquids (GTL) technology. Shehua’s Ningxia Coal Industry Group, Yankuang Group, Yitai Group, Lu’an Group, and others are either constructing or are planning to construct megaton-scale ICTL plants. The ICTL projects are listed in Table 1. In addition to these major players, some smaller technology companies and institutes in China, such as Shaanxi Gold Nest, are also working on the development of ICTL applications.
Key Features of ICTL Technologies Developed in China
A typical flowchart of an ICTL process is illustrated in Figure 1. It can be seen from Figure 1 that the F-T step is the centerpiece of ICTL. In the F-T process, the most important aspects are the catalyst and the reactor type. Catalysts used in the F-T process are developed around transition metal elements such as Fe, Co, Ni, and Ru (iron, cobalt, nickel, and ruthenium, respectively), which have the property of ionizing the CO molecules. In addition, they also exhibit the ability to catalyze the hydrogenation process to extend the length of the hydrocarbons. Among these catalytic metals, Ru exhibits a very high catalytic activity, but its cost is exceptionally high. Ru deposits are rare and for this reason the metal is unlikely to be used as a catalyst in large-scale applications. When Ni-based catalysts are used, they demonstrate a very high selectivity for producing methane as an end product. Ni-based catalysts are more often used in the hydrogenation and methanation processes. The catalysts used in modern F-T processes can be divided into Fe-based and Co-based. Fe-based catalysts are less expensive; metal iron is readily available. They also have a wide operating temperature range for their applications, 200–340oC. Therefore, these catalysts can be made to suit either a high-temperature or a low-temperature F-T process. In addition, they can be manipulated to yield different products. On the other hand, Co-based catalysts exhibit high catalytic activities in the F-T process, but they are more applicable in processes with narrow temperature fluctuations in the 220–230oC range. Cobalt catalytic F-T processes can produce hydrocarbons with a wide range of carbon numbers without producing CO2. Products include a high percentage of saturated hydrocarbons and wax.2-5
The reactors used in F-T processes can be grouped into the following categories: fixed bed (FB), circulating fluidized bed (CFB), fixed fluidized bed (FFB), and slurry bubble column reactor (SBCR). The advantages of FB are the ease of collection of the liquid products and easy separation of the heavy hydrocarbons from the catalysts. Its disadvantages are the non-uniform temperature distribution both axially and radially. In addition, construction of FB reactors can be complex, costs are high, and loading and unloading of the catalysts is difficult. A CFB is better suited to high-temperature F-T, leading to products with lower carbon numbers. Turbulence mixes the catalysts and reactant gases in the reactors, which results in effective heat transfer, in turn leading to a more homogeneous temperature profile inside. The uniform temperature distribution allows the control of reaction selectivity (i.e., close control of products). The effective heat exchange also offers the benefit of a smaller heat transfer area, which means a higher capacity with a similar-sized reactor. Its disadvantages include high cost, complicated operations, expensive repairs, and difficult scale-up. The advantages of a FFB are the uniformity in the temperature profile of the same bed, ease of control of reaction selectivity, low equipment cost, and production-targeted fuel products. Sasol’s projects in South Africa that were finished in the 1980s all used FFB reactors. The advantages of a SBCR are the homogeneous reactants, homogeneous temperature profiles, low pressure drop (one-fourth of that in a FB), high yields per reactor volume, flexible operations, low operation costs, and the ability to exchange the catalysts while on-line. The shortcoming of the SBCR is the stringent requirements in the separation of the liquid products from the solids.4-6
The F-T process can be classified into LTFT (low-temperature F-T) and HTFT (high-temperature F-T) according to the operating temperatures. An LTFT operates in the 200–270oC range and HTFT operates in the 300–340oC range. The F-T process can also be described according to its number of stages. In general, F-T processes have one or two stages. Some researchers believe that a two-stage process can increase the overall conversion and product capacity. However, two-stage design makes the process more complicated and more difficult to operate, so initial investment may also be higher. Different temperature-based processes lead to different product distributions. These in turn have led to several technologies such as Sasol Slurry Phase Distillate (SSPD), Sasol Advanced Synthol (SAS), and Shell Middle Distillate Synthesis (SMDS)2. The technical details are listed in Table 2.
Because Fe-based catalysts have a greater resistance to sulfur, whereas Co-based catalysts are prone to sulfur poisoning, most Chinese ICTL researchers prefer Fe-based catalysts. A few LTFT processes with SBCR are progressing toward commercialization.
As shown in Figure 1, the following processes are involved in an ICTL technology: coal gasification, F-T product refining, and other technologies. In addition, the composition of the inlet gases and the partial pressure of the effective gas (CO+H2) can influence overall syngas conversions and product yield. For example, the lower the inert gas partial pressure in the fresh syngas, the better for the Texaco gasification technology in an ICTL process. The refining technology for FTS preliminary products is selected to meet the final products requirement. In general, hydrofining is used to deoxidize and to remove olefins and hydrocracking is used to produce diesel.
China’s ICTL Projects
In China, most ICTL technology operators have experience based on technology progression from bench-scale to pilot-scale to large-scale demonstrations. Yanhuang Group achieved a 10-ktpa demonstration plant in 2004. Shenhua, Yitai, and Lu’an constructed a demonstration plant with 160–180-ktpa capacity mainly based on Synfuels China ICTL technology during 2006 to 2009. Also, during the 2006–2009 period, Shenhua independently started and operated a demo plant using its own catalyst and completed a modified ICTL process. Lu’an and Yitai operated their own demo plants using Synfuel China’s technology, including catalyst and technical support. The Yitai Group plant has been operating the longest. With these experiences, China has accumulated abundant experience in the key aspects of ICTL technologies such as F-T catalyst scale-up, commercialized reactors, and various F-T processes. In the meantime, China has also had first-hand experience in industrial scale-up. Table 3 lists some of the technical parameters for the three demo plants.7
Based on the experiences of operating the demonstration plants, especially the long-term operation by the Yitai Group, the Chinese government approved a four-million-tons-per-year ICTL plant based on homegrown, rather than international, ICTL technologies. The approval allowed Shenhua Ningxia Coal Industry Group to begin construction of the ICTL project in Ningxia Province.
Resource, Environment, and Industrial Policy Challenges
Generally for ICTL processes, when one ton of synthetic fuel is produced, four to five tons of coal and four to eight tons of water are consumed. Clearly, ICTL has a high demand for coal and water. Therefore, attention must be paid to the availability of coal and water in different regions when planning ICTL projects. China’s coal and water resources are unevenly distributed. The coal reserves are centered in the north or mid-west regions such as Shanxi, Inner Mongolia, Ningxia, and Xinjiang provinces, where water resources are less abundant. Some areas have even less water; in these areas, industrial water uses even rely on underground water. Consequently, any ICTL project developer must pay close attention to the efficiency of water use, how much water can be recycled, and the reduction and control of water pollution. At the state level, the central government should carry out the overall planning with a step-by-step, hierarchical coordination strategy to formulate industrial policies to ensure a balance between sustainable development and resource conservation. ICTL also requires a large initial investment and, therefore, necessitates a great deal of support. The investment for a one-million-tpa plant is approximately US$2.5 billion. China’s ICTL industry is still in the early stages of engineering development. Although the technology has gone through solid research, development, and industrial demonstration, its further commercialization still faces hurdles due to the upfront investment required. Gasification accounts for over 50% of the total investment. Breakthroughs in this technology will most definitely reduce the financial burden. Improving catalyst performance, optimizing the process parameters, and selectively choosing the materials of construction will undoubtedly reduce the investment required.
Sasol has acquired a tremendous amount of ICTL experience based on research, scale-up, and operations. Even so, some serious problems arose when Sasol started up the 1400-ktpa capacity Qatar Oryx GTL plant; these problems were resolved by 2009. The take-home message for China’s ICTL industry is that engineering scale-up problems are real possibilities. In addition to the accumulation of engineering experience and actual operational experience, simulation is also a powerful tool to predict the possible problems in reactors and throughout the processes.6,8 The National Institute of Clean and Low-Carbon Energy (NICE) is cooperating with Pittsburgh University and other research institutions in the areas of industrial hydrodynamics, reaction kinetics to improve reactor capabilities, and process simulations to support the megaton ICTL PDP for Shenhua Group.
Water produced in LTFT process contains about 5 wt% organic chemicals; for HTFT, this value is even higher. The separation and purification of these organic chemicals for higher value products can improve the overall project economics. Water cleanup is also important for environmental protection. As a result, optimizing the water treatment technologies currently used is critical. Presently, only Yankuang Group has completed comprehensive research in this area; they were able to remove the organic acids using a technology that combined extraction and distillation. However, the largest scale of F-T water treatment in China is approximately 0.2 million tpa, and the project economics are still problematic unless the megaton process is begun in the next few years.2
Currently, Co-based catalysts are mainly used for GTL processes. The question remains as to whether they can be used in ICTL. In GTL, the feedstock is natural gas (mainly CH4). The syngas from natural gas has a H2-to-CO molar ratio near two with very low sulfur content, which makes it suitable for Co-based catalysts. On the other hand, the syngas from ICTL has a H2-to-CO molar ratio of less than two with higher sulfur content, even after cleanup, which makes it suitable to Fe-based catalysts. Fe-based catalysts also have an advantage of lower preparation cost. However, Fe-based catalysts require a longer preparation cycle, consume a large amount of water, and can change structure during reactions, which leads to attrition, shorter life, and difficulty in reactivation. Co-based catalysts may be expensive initially, but are more resistant to attrition, have better longevity, trigger simple reactions, and can be reactivated. Co-based catalysts have received greater attention for further research and development efforts.2,5
ICTL in China has entered into an industrial-scale era and China has become a global leader of ICTL technology developments and commercialization. Megaton-scale industrial demonstration plants will be constructed and operated in China over the next few years, which will allow the industry in China to continue expanding its experience in ICTL technology. As breakthroughs occur related to Co-based catalysts and problems associated with water constraints and environmental issues are solved, ICTL will find more applications and play an even bigger role in the modern coal-to-chemicals industry.
- Yuzhuo Zhang, Transform High-carbon Energy to Low-carbon Energy Prospect of Clean Coal Conversion, Energy of China, 2008, 30 (4), 20–23. (in Chinese)
- Qiwen Sun, Indirect Coal Liquefaction. Beijing: Chemical Industry Press, 2012. (in Chinese)
- M.E. Dry, Fischer–Tropsch Synthesis-Industrial, Encyclopedia of Catalysis. New York: John Wiley & Sons, 2010.
- M.E. Dry, The Fischer–Tropsch Process: 1950–2000, Catalysis Today, 2002, 71, 227–241.
- A. Steynberg, M.E. Dry, Fischer–Tropsch Technology, Studies in Surface Science and Catalysis, 2004, 152.
- R. Krishna, S.T. Sie, Design and Scale-up of the Fischer–Tropsch Bubble Column Slurry Reactor, Fuel Processing Technology, 2000, 64, 73–105.
- Xiuzhang Wu, Jiming Zhang, Yulin Shi, Zhuowu Men, Industrial Application of SFT418 Catalyst for Fischer–Tropsch Synthesis, Petroleum Processing and Petrochemicals, 2011, 42 (6), 45–49. (in Chinese)
- M.T. Dhotre, B.L. Smith, CFD Simulation of Large-scale Bubble Columns: Comparisons against Experiments, Chemical Engineering Science, 2007, 62 (23), 6615–6630.
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