Indirect coal to liquid technologies PDF

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Applied Catalysis A: General 476 (2014) 158–174 Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata Review Indirect coal to liquid technologies Erlei Jin a , Yulong Zhang b , Leilei He a , H. Gordon Harris a , Botao Teng a , Maohong...

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Applied Catalysis A: General 476 (2014) 158–174

Contents lists available at ScienceDirect

Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Review

Indirect coal to liquid technologies Erlei Jin a , Yulong Zhang b , Leilei He a , H. Gordon Harris a , Botao Teng a , Maohong Fan a,c,∗ a

Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, United States Western Research Institute, Laramie, WY 82070, United States c School of Energy Resources, University of Wyoming, Laramie, WY 82071, United States

b

a r t i c l e

i n f o

Article history: Received 13 December 2013 Received in revised form 24 February 2014 Accepted 26 February 2014 Available online 6 March 2014 Keywords: Coal liquefaction Fischer–Tropsch MTO Ethylene glycol Syngas

a b s t r a c t 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. © 2014 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4. 5. 6.

7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The development of catalysts for the Fischer–Tropsch process (FT process) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. 2.3. Deactivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methanol to olefin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Syngas to methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Syngas to dimethyl ether . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of ethylene glycol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of dialkyl oxalate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. 6.2. DMO hydrogenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. 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 [1–7]. To overcome the environmental challenges as associated with the conventional utilization approaches, people are

∗ Corresponding author at: Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, United States. Tel.: +1 3077665633. E-mail address: [email protected] (M. Fan). http://dx.doi.org/10.1016/j.apcata.2014.02.035 0926-860X/© 2014 Elsevier B.V. All rights reserved.

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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

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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 [8–12], syngas to EG [13–25], syngas to DME [18,26–28] and methanol [29–32], as well as methanol to olefins (MTO) [33–38], 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.

2. 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.

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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 water–gas shift reaction (WGS). The products from FTS have many outstanding, advantageous properties. FTS produces sulfur free, nitrogen free and few aromatic hydrocarbon components, and thus is which are environmentally friendly. FTS is also an important path for industrial materials manufacture, due to its production of chemicals with high value, especially waxes and light olefins [39]. FTS has a long history. After it was firstly invented by Fischer and Tropsch in the 1920s, researchers working on FTS have made great strides in adjusting and refining the process. Many prominent large-scale coal to liquid companies have been established, including Sasol South Africa plant (the world’s largest oil-from-coal plant) and Sasol Qatar plant [40]. The development of FTS has been greatly influenced as a result of the fluctuation in the price of global crude oil. In recently decades, due to global the energy-deficiency and the demand of green energy, FTS has been widely recognized as an alternative path to liquid fuels. Currently there are two FT operating modes. The high-temperature (HTFT, 300–350 ◦ C) process with iron-based catalysts is used for the production of gasoline and linear low molecular mass olefins [41]. The low-temperature (LTFT, 200–240 ◦ C) process, with either iron or cobalt catalysts, is used for the production of high molecular mass linear waxes. Only the metals Fe, Ni, Co and Ru have the required FT activity for commercial application. On a relative basis, taking the price of scrap iron as 1.0, the approximate cost of Ni is 250, of Co is 1000 and of Ru is 50,000 [42]. However, Ni produces too much CH4 while Ru has really high price and the amount available is insufficient for large-scale application. So, only Fe and Co can be used as viable catalysts. Various types of reactors have been developed for the FTS process, such as fixed-bed (FBR), slurry bubble column reactor (SBCR, or CSTR in bench scale testing) and fluidized-bed reactor [43]. However, the selection of type of product in FTS is still the one of the most important issues. Chain growth in FTS follows the principles of stepwise polymerization and the product distribution of hydrocarbons follows an Anderson–Schulz–Flory distribution [44]. But the selectivity for methane and heavy hydrocarbon is higher than the selectivity for gasoline and diesel (C5–11 and C12–20 ). For maximum gasoline production the best option is high capacity fixed fluidized bed (FFB) reactors operating at about 340 ◦ C, with an iron catalyst. This produces about 40% straight run gasoline. Twenty percent of the FT product is propene and butene [45]. These can be oligomerized to gasoline and because the oligomers are highly branched it has a high octane value. The straight run gasoline, however, has a low octane value because of its high linearity and low aromatic content. The C5 /C6 cut needs to be hydrogenated and isomerized and the C7 –C10 cut requires severe platinum reforming to increase the octane value of these two cuts [46]. Mild hydrocracking of wax was investigated at the Sasol R&D division during the 1970s [47]. The product heavier than diesel was recycled to extinction. The overall yields were about 80% diesel, 15% naphtha and 5% C1 –C4 gas. When the decision to construct the third Sasol plant was made, the wax hydrocracking proposal was rejected because at that time making gasoline was the more economic option, and the straight duplication of the second plant resulted in huge savings in time and capital. Also at that stage, the FT slurry reactors had not yet been developed. About 20 years later the same concept of wax hydrocracking was implemented at the Shell Bintulu plant where multi-tubular FT reactors are used and currently Sasol/Chevron are designing a slurry FT plant with wax hydrocracking in Nigeria [48]. The high-temperature fluidized bed FT reactors with iron catalyst are ideal for the production of large amounts of linear-olefins. As petrochemicals they sell at much higher prices than fuels. The olefin content of the C3 , C5 –C12 and C13 –C18 cuts are typically

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Fig. 2. The reaction energy diagram of CO insertion into adsorbed CH2 and subsequent CO dissociation [11].

Fig. 1. FT stepwise growth process. Note that no specific chemical mechanism is implied in the sequence presented [48].

85%, 70% and 60%, respectively [49]. Ethylene goes to the production of polyethylene, polyvinylchloride, etc. and propylene to polypropylene, acrylonitrile, etc. The extracted and purified C5 –C8 linear-olefins are used as co-monomers in polyethylene production. The longer chain olefins can be converted to linear alcohols by hydroformylation [50]. The only required purification of the narrow feed cuts is the removal of the acids. The hydroformylation was investigated at the Sasol R&D laboratories in the early 1990s. The alcohols are used in the production of biodegradable detergents. Their selling prices are about six times higher than that of fuel. The LTFT processes produce predominantly longer chain linear paraffins. After mild hydro-treatment to convert olefins and oxygenates to paraffins the linear oils and various grades of linear waxes are sold at high prices [51]. 2.1. Selectivity FT synthesis always produces a wide range of olefins, paraffins and oxygenated products such as alcohols, aldehydes, acids and ketones regardless of reaction conditions. Variables such as temperature, feed gas composition, pressure, catalyst type and promoters will influence the selectivity of products. Fig. 1 illustrates the relationship between the CH4 selectivity and that of some selected hydrocarbon cuts [48]. The explanation of these interrelationships lies in the stepwise growth process occurring on the catalyst surface. The CH2 is formed firstly by the hydrogenation of CO and plays the role a monomer in a stepwise chain propagation reaction. At each stage of growth the adsorbed hydrocarbon species have two possibilities of, (i) being hydrogenated to form the products or, (ii) adding another monomer to continue the chain growth. If the chain growth (˛) is independent of the chain length then it is really easy to calculate the product distribution with various values of ˛.

However, the stepwise growth process illustrated in Fig. 1 might not represent the actual FT mechanism. A lot of other mechanisms have been proposed and this matter still remains controversial [52–55]. Zhuo et al. [11,56] have computationally studied reaction intermediates and activation energies of the corresponding elementary reaction steps on the surface of Co catalyst (Fig. 2). Fig. 2 shows that formyl intermediate (HCO) is really unstable and the chain growth step may occur through CO insertion. More possibilities have been researched includes: (i) whether the CO molecule first dissociate into C and O be the hydrogenation of C to CH2 monomers or not, (ii) whether CO hydrogenated to “CHO” or “HCOH” then insert into the growing chain or not and (iii) whether CO hydrogenated after it inserted into the chain or not [52]. To improve the selectivity of FT process, better catalyst and optimized process conditions are the two important aspects. Generally, increasing of the operating temperature results in a shift in selectivity toward lower carbon number products and to more methane. At the same time, the degree of branching increases, and the amount of secondary products formed such as ketones and aromatics, also increases [42]. These shifts are proportional with thermodynamic expectations and the relative stability of the products. For Co catalyst, the CH4 selectivity rises more rapidly with increasing temperature than it does with Fe catalysts due to Co is a more active hydrogenating catalyst. Promoters also play important roles in varying the selectivity. The main function of alkali metals is through an electronic effect, which means these promoters may change the electronic properties of Fe-based catalysts, as well as modifying the adsorption pattern of reactants (H2 and CO) on the active sites. Alkali on iron catalysts could increase 1-alkene selectivity, reaction rate, growth probability of hydrocarbon chains and also decrease the yield of CH4 . Alkali promoters in iron-based catalysts could also enhance CO conversion with addition of external water, and cause an increase of both the average carbon number of synthesized hydrocarbons and the 1-alkene selectivity [57,58]. Many researchers have found the impact of Group I alkali metals (Li, Na, K, Rb and Cs) modified iron catalysts through different synthesis conditions and at the same carbon monoxide conversion levels [59,60]. Promoters have fewer effects on Co catalysts. The most used promoters for Co are noble metals, transition metals and rare earth oxides. Many studies indicate that introducing of proper promoters many enhance the adsorption of CO as well as the chain growth [61–63].

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2.2. Catalysts Compared with other cataly...