New Market Opportunities for Methane Conversion to Carbon and Hydrogen

Other forms of carbon besides carbon black may be considered as products. Examples include graphite/graphene, carbon fiber precursors, CNTs, and needle coke. Hydrocarbons in the form of aromatics that can be produced from CH4 pyrolysis also are discussed in this section.

Comparison of the structure of (A) graphite, (B) graphene, and (C) a single- walled carbon nanotube (Tsai and Chen 2009)

FIGURE 7.13 Comparison of the structure of (A) graphite, (B) graphene, and (C) a single- walled carbon nanotube (Tsai and Chen 2009).

Graphite/Graphene

Graphite/graphene materials are nearly 100% carbon and differ primarily in topology (Figure 7.13). Graphite is a naturally occurring material. It has a layered, planar structure. The individual layered sheets are called graphene. In each layer, the carbon atoms are arranged in a honeycomb lattice with a separation of 0.142 nm, and the distance between planes is 0.335 nm (Grand View Research 2015). Atoms in the plane are bonded covalently, with only three of the four potential bonding sites being satisfied. The fourth electron is free to migrate in the plane, making graphite electrically conductive. However, it does not conduct in a direction at right angles to the plane. Weak van der Waals forces bond the layers, allowing layers to be easily separated or to slide past each other. A rolled layer is a CNT.

Demand for natural graphite is 1-1.2 million tons per year and consists of several different forms of graphite - flake, amorphous, and lump. Historical applications primarily use amorphous and lump graphite, while most newly emerging technologies and applications use flake graphite. Of the up to 1.2 million tons of graphite that are processed each year, just 40% is the flake form. Historically, China was responsible for the large decline in graphite prices in the 1990s as product was dumped on the market to earn foreign exchange. Much like the market for rare elements, this essentially killed the industry in the West, making the United States highly dependent on supply from China. It is unlikely China can repeat that practice. The majority of Chinese graphite mines are small, many are seasonal, and labor and environmental standards are poor. Easily mined surface oxide deposits are being depleted, and mining is now moving into deeper and higher-cost deposits.

Graphite is used in electronic applications such as for Li-ion batteries where the market in 2015 was 80,000 tons/yr (Benchmark Mineral Intelligence 2016). The U.S. Geological Survey states that large-scale fuel cell applications being developed today could consume as much graphite as all other uses combined.

But even if only half of the U.S. Geological Survey demand is realized, graphite use is going to explode just because of fuel cells, let alone other known demand drivers and new applications. Thus, the natural graphite market is currently small, but it possibly will be very important for new fuel cell applications and/or batteries.

Graphene is a special form of graphite that is 100% carbon and only one layer or at most a few layers thick. It potentially has all of the application space that graphite has but currently has little production. The global market for graphene reached $9 million by 2012, with most sales in the semiconductor, electronics, battery energy, and composites industries (Wikipedia 2017). The market is projected to grow from 80,000 metric tons produced in 2015 to 250,000 metric tons by 2020 (Benchmark Mineral Intelligence 2016). Generation as a co-product with H2 could be a potential benefit, but the current market would soon be saturated unless demand increases.

There is little reason to believe a priori that carbon produced from a pyrolysis process would be well-ordered. However, one study reported the formation of well- ordered pyrolytic graphite structures as well as fibrous carbon on transition metals. The pyrolytic carbon was reported to be of equal quality to recrystallized graphite normally produced at much higher temperatures. Results appeared to be repeatable (Robertson 1970). However, the value of the carbon will depend heavily on the treatment of the feed gas. Carbon black furnace processes operate by injecting a heated aromatic liquid hydrocarbon into the combustion zone of furnace fired by natural gas, where the hydrocarbon is decomposed to form carbon black at temperatures on the order of 1320-1540°C. Depending on the feed composition and the grade of black produced, process carbon black yields have ranged from 35-65% (U. S. Environmental Protection Agency 1983).

Carbon Fibers

Carbon fibers are polycrystalline, two-dimensional planar hexagonal networks of carbon containing between 92 and 100% carbon by weight formed by heating carbon-containing precursors at temperatures ranging from 1000 to 1500°C (Park and Lee 2015). If the fibers are heated above 2000°C, the hexagonal carbon network undergoes conversion to graphene with yields in excess of 99%. These fibers are referred to as “graphite fibers.”

Carbon fibers have a number of favorable mechanical and chemical properties, such as high tensile strength and stiffness, low density, dimensional stability, low coefficient of thermal expansion, fatigue resistance, and chemical inertness and biological compatibility (Park and Lee 2015). Carbon fibers are finding increasing use in a variety of applications such as aerospace, automobiles, sports equipment, the chemical industry, wind turbines, carbon-reinforced composite materials, textiles, etc. (Holmes 2014; Park and Lee 2015; Mazumdar 2016; Witten et al. 2016). The physical properties (primarily tensile strength and modulus, etc.) determine the proper use of carbon fibers (Milbrandt and Booth 2016).

Carbon fibers are manufactured from precursor fibers using a combination of heat and stretching treatments. The most common precursors are polyacrylonitrile (PAN) and pitch, which is a complex blend of polyaromatic and heterocylcic compounds. Other linear and cyclic precursors include phenolic polymers, polyacenephthalene, polyamide, polyphenylene, poly-p-phenylene, benzobisthiazole, polybenzoxazole, polybenzimidazole, polyvinyl alcohol, and polyvinylidene chloride, and polystyrene (Park and Heo 2015). Currently, PAN is used to produce 95% of the carbon fibers worldwide, and pitch is used for the remaining 5%. Precursors, namely PAN, account for ~51% of the manufacturing cost of carbon fibers, and their high price is one of the barriers to their widespread use (Milbrandt and Booth 2016). Additionally, the current methods for manufacturing carbon fibers are slow and energy-intensive. Thus, both alternative methods of manufacturing and use of cheaper precursors are under exploration.

Alternative precursors to PAN under investigation include biomass precursors such as lignin, glycerol, and lignocellulosic sugars (Milbrandt and Booth 2016). There has been a particular interest in using lignin as a precursor because of its availability, low cost relative to other precursors, and enhanced structural properties. However, no biomass-based carbon fiber has been developed with the necessary structural properties required for use in the major carbon fiber applications (e.g., aerospace, wind, and automotive). The report authored by Milbrandt and Samuel Broth provides both technical and market information about each bio-based carbon fiber precursor (Milbrandt and Booth 2016). The report authored by Baker and Rials (2013) provides a comprehensive review of carbon fiber manufacture specifically from lignin, which includes a cost comparison of potentially low-cost carbon fibers.

The process for producing carbon fibers depends on the precursor and the desired physical properties. For PAN, the process starts by polymerizing the PAN-based precursor, which is then spun into fibers. The fibers are treated using an air-based oxidation process at temperatures between 200 and 400°C to stabilize the fiber. The stabilized fibers are heat treated in the absence of oxygen at temperatures ranging from 800 to 1600°C to remove noncarbon impurities such as H2, oxygen, and nitrogen and to induce carbonization. Next, a surface treatment is used to improve the mechanical properties of the carbon fiber. Finally, the fiber is washed, dried, and sized (Park and Lee 2015; Park and Heo 2015). For pitch, the process starts by melting pitch so that it can be extruded and drawn into fibers. Similar to the PAN-based process, fibers are air-treated to stabilize the fiber; they then undergo a higher temperature heat treatment to induce carbonization (Park and Heo 2015). Pitch has the advantage of lower cost and producing a higher char yield than PAN, but the processing costs are higher to achieve carbon fibers of similar performance to PAN (Park and Heo 2015).

An alternative to the PAN- and pitch-based processes is the “vapor-grown” production process. The process involves exposing light-hydrocarbon gases, such as CH4, acetylene, or ethylene or coal-gas, to a solid catalyst, such as Co, Fe, or Ni, to form carbon filaments with diameters as small as 0.1 pm as precursors for carbon fiber growth (Tibbets 1983; Alig et al. 2000; Endo and Dresselhaus 2003; Park and Lee 2015). The filaments consist of graphitizable carbon that is transformed into larger diameter graphite fibers by treatment at temperatures above 250°C. Exposing these carbon filaments to subsequent chemical vapor deposition using the same carbonaceous gases causes the filaments to grow in diameter ranging from 60 to 200 nm and ~100 pm in length, yielding vapor-grown carbon fibers or gas-phase-grown carbon fibers (Park and Lee 2015). Compared to the PAN-based process, the manufacturing process for vapor-grown carbon fibers is simpler, faster, and cheaper and could provide an innovative approach for fabricating high-performance fibers at lower costs.

The global market demand for carbon fibers in 2016 was 70,000 metric tons (Mazumdar et al. 2017). The market is projected to grow at an annual growth rate of 10 to 13% through 2020 with the market demand expected to exceed 100,000 metric tons in 2020 (Witten et al. 2016). The global market value was $2.15B in 2015, which is projected to grow to $4.2B by 2022 (Witten et al. 2016). Combined, the aerospace industry including defense, the automotive industry, and wind turbine manufacturing in the energy sector accounted for between 35,000 to 45,000 metric tons, and continuing growth in these industries is expected to support the high projected annual growth rate. Although worldwide demand for carbon fibers has increased significantly over the past decade, high production costs have limited wider spread use of carbon fibers (Park and Lee 2015). Because of the complex and multistage manufacturing processes required to produce carbon fibers, only a limited number of companies are engaged in their mass production. These companies include Toray (including its purchase of Zoltek), Toho Tenax, Mitsubishi Chemical, Formosa Plastics, SGL, Hexcel, DOW, and Kemrock (Park and Lee 2015; Das et al. 2016).

Analysts believe there is an excellent potential market opportunity for the use of carbon fibers in construction (Holmes 2014; Mazumdar 2016; Mazumdar et al. 2017). An increasing demand for fiber-reinforced plastic bathtubs, doors, windows, and panels is being spurred on by the continual growth in the U.S. housing market (Mazumdar 2016). Another application is carbon fiber reinforced concrete (i.e., “carbon concrete”), which is increasingly being used to repair bridges and other aging structures (Holmes 2014). Using carbon concrete to repair bridges that are in unsatisfactory condition is considered a major market opportunity. For example, more than 150,000 of the 600,000 bridges in the United States are considered unsuitable for the current or projected traffic demands, primarily because of corrosion of the steel reinforcement (Kawahara et al. 2012). Germany is projected to invest up to €16B-€17B to repair or replace bridges by 2030 (Holmes 2014). Although the cost of carbon-reinforced concrete is higher than steel-reinforced concrete, the higher cost is counterbalanced by its high specific properties such as lower weight relative to a steel-reinforced concrete deck (which reduces the load demand on the supporting structure), better corrosion resistance, better seismic protection, and lower erection and maintenance costs (Kawahara et al. 2012). Worldwide demand for carbon concrete in 2013 was 2300 metric tons, representing revenues totaling $590M. An annual growth rate of 6-9% is projected through 2020, and revenues are projected to exceed $1B by 2022. Lowering the cost of carbon concrete is considered key to more rapidly increasing its use in the construction industry.

The Institute for Advanced Composites Manufacturing Innovation (2017) is working to advance the composite industry, developing new manufacturing techniques and identifying potential new markets. Advancements will increase scrap and material costs and applications within the automotive industry has been identified.

Carbon Nanotubes

CNTs, including single-walled carbon nanotubes (SWCNTs) and multi-walled nanotubes (MWCNTs), are used in polymers, electronics, plastics, and energy storage.

The major application for CNTs is in composite fibers in polymers to improve thermal, electrical, and mechanical properties. This application accounted for over 60% of the market share in 2014. The high molecular complexity of graphene in M WCNTs increases their tensile strength. MWCNTs are increasingly being used in engineered polymers including polyetherimide, polycarbonate, and polyetheretherketone. The growing demand for polymers for use in the automotive and construction industries, particularly in China, India, Brazil, and the Middle East, is expected to spur market growth (Grand View Research 2015). CNTs are increasingly being used in the production of lithium-ion batteries. The application growth in lithium-ion batteries for use in grid and renewable energy storage is expected to increase the demand for CNTs for these applications.

The global CNT market demand for CNTs was slightly over -5000 tons in 2014 and is projected to grow to over 20,000 tons by 2022 (Figure 7.14) (Grand View Research 2015). This is orders of magnitude lower than for other carbon products such as carbon black (12 million tons per year). Estimates of market value range from $3.4B (2020) to $5.6B in (2022) (Grand View Research 2015; Markets and Markets 2017a). MWCNTs are the largest production segment given that their manufacturing cost is significantly lower than that of SWCNTs. Selling prices range from $50/lb (MWCNT at Hyperion Catalysis) to $600 per gram (SWCNT in defense and niche markets) (Sherman 2007).

The Asia Pacific region is the fastest growing market due to increasing domestic demand coupled with lower manufacturing costs compared to the United States and Europe. Among the major manufacturers of CNTs are Arkema S.A. (France), Arry International Group LTD. (China), Carbon Solutions Inc. (United States), Cheap Tubes

Global CNT market estimates and forecast by application from 2012 through 2022. Volume is in tons (Grand View Research 2015). (Reproduced with permission from Grand View Research.)

FIGURE 7.14 Global CNT market estimates and forecast by application from 2012 through 2022. Volume is in tons (Grand View Research 2015). (Reproduced with permission from Grand View Research.)

Inc. (United States), CNano Technology LTD. (United States), CNT Company Ltd. (Korea), Continental Carbon Company (United States), Hanwha Chemical Co. Ltd. (Korea), Hyperion Catalysis international Inc. (United States), KLEAN CARBON Inc. (Canada), Kumho Petrochemical Company LTD. (South Korea), Nano-C Inc. (United States), Nanocyl S.A. (Belgium), Nanolntegris Inc. (United States), NanoLab, Inc. (United States), Nanoshel LLC (United States), Nanothinx S.A. (Greece), Showa Denko K.K. (Japan), Southwest NanoTechnologies Inc. (United States), Thomas Swan and Co. Ltd. (United Kingdom), and Toray Industries, Inc. (Japan).

Needle Coke

Needle coke is a premium-grade, high-value petroleum coke used in the manufacturing of graphite electrodes for electric arc furnaces in the steel industry. The main differences between needle coke and ordinary coke are their structural characteristics, coefficients of thermal expansion, electrical conductivity, and oxidizability. Needle coke has a high level of graphite resulting from its microcrystalline structure. A high level of anisotropy, large crystalline size, and large crystal areas must be achieved to obtain good quality needle coke (Halim et al. 2013). The term “needle” is used to describe the acicular morphology of the coke; it tends to form oriented needle-like structures that are visible to the naked eye. The coefficient of thermal expansion is one of the most important characteristics of petroleum coke in evaluating the feasibility of using a particular coke in the production of graphitized items that have a high resistance to shock. Carbon with its low coefficient of thermal expansion can dissipate thermal energy without cracking (Halim et al. 2013).

Needle coke is typically produced by delayed coking of the heavies remaining after catalytic cracking in a refinery. Delayed coking is a process for producing coke by transforming a complex mixture of aromatics to solid carbon (Halim et al. 2013). It provides thermal energy to form the mesophase of the precursor during carbonization. To achieve excellent quality needle coke, two major steps are needed: first, coalescence of the mesophase to its formation and second, rearrangement of the mesophase in the solidification stage. Different starting materials will have different chemical makeups, thus requiring different operating conditions. These conditions, particularly temperature and pressure, need to be optimized to achieve quality needle coke. Calcined needle coke typically is higher in carbon and lower in ash constituents, such as sulfur and metals, than standard calcined petroleum coke. A calcined form of needle coke is the raw material to produce graphite electrodes used in electric arc steel furnaces. The global market demand for needle coke is currently -1.5M tons/yr. It has been reported that demand has increased in recent years and that this trend will continue.

 
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