Coal as a Raw Material for Making Carbon Nanotubes (CNT)

Coal is a cheap and abundant natural resource. Coal is generally used as a fuel for steam power plants (PLTU). Total world coal reserves in 2017 reached 1.035 trillion tons consisting of 718.3 billion tons of anthracite and bituminous coal types and 316.7 billion tons of sub-bituminous and lignite coal [1] . America, Russia, China and Australia are the countries that have the most coal reserves compared to other countries. However, Russian coal production is smaller than countries that have less coal reserves such as India and Indonesia as shown in Table 1.

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Table 1. Countries with the largest coal reserves and production in the world in 2017 [1]

Coal is a sedimentary rock which mainly contains carbon. The high carbon content in coal makes it possible to produce carbon nano materials, especially carbon nanotubes (CNT). The first CNT was made by Pang and Wilson in 1993 using the arc discharge method. Arc discharge is a method of gas decomposition using electricity to form plasma which produces temperatures around 4000 – 6000 K. The method is run with a DC current of 200 A at a voltage of 20 V [2] . The resulting CNT diameter is less than 5 nm [3] .

In 1999, William et al produced CNT using the same method but using a Ni-Y catalyst. The use of these catalysts increases CNT yield . The CNT diameter produced from the William, et al. Study was 1.2 – 1.7 nm [4] . The SEM results are shown in Figure 1 (a). However, coal has mineral content and non-catalytic elements such as sulfur, oxygen and nitrogen which can affect the structure of CNT. In addition, it can also make the quality of CNT products difficult to control. Wang et al have identified the causes of branched formationCNT (BCNT). The BCNT production process from anthracite coal uses the arc discharge method with the help of a copper catalyst. The process of BCNT formation is caused by the presence of sulfur in coal [5] . The results of the TEM BCNT are shown in Figure 1 (b).

Figure 1. (a) SEM CNT results produced using Ni-Y catalyst [4] (b) TEM BCNT results [5]

The content of fixed carbon and volatile matter in coal determines the yield of CNT produced. In 2002, Chinese researchers produced CNT from 10 bituminous coal from China and 1 bituminous coal from New Zealand using the arc discharge method . Proximate analysis of each sample is shown in Table 2.

Table 2. Proximate analysis of coal samples [6]

CNT yields from 10 Chinese coal samples are in the range of 23.5 – 60.4%. While the CNT yield of New Zealand coal was 62.4% [6] . Broadly speaking, the higher the fixed carbon content and the lower the volatile matter content , the higher the CNT yield . The high carbon content indicates more and more carbon ions or carbon clusters in the gas phase in the plasma zone so that it can be a precursor in the CNT growth process. In addition, volatile matter which contains hydrogen element in it can inhibit CNT growth process so that the expected volatile matter content is very low. The relationship between fixed carbon andvolatile matter to CNT yield is shown in Figure 2.

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Figure 2. Relationship between fixed carbon and volatile matter on CNT yields [6]

Coal is an option as a carbon precursor in the manufacture of nano materials. That is because the price of coal is much cheaper compared to graphite and the availability of coal is more abundant than graphite. In fact, the use of coal can reduce the cost of raw materials up to 10 times. If CNT can be produced from coal, can coal be the raw material in the production of other nano carbon materials such as graphene? Stay tuned for the next article, friend of warstek

Reference

[1] BP. 2018. BP statistical review of world energy. British Petroleoum. Accessed from:   https://www.bp.com/content/dam/bp/en/corporate/pdf/energy-economics/statistical-review/bp-stats-review-2018-coal.pdf on November 23, 2018

[2] Arora N., Sharma NN 2014. Arc discharge synthesis of carbon nanotubes: comprehensive review , Diamond & Related Materials, 50, 135-150

[3] Pang LSK, Wilson A. 1993. Nanotubes from coal . Energy & Fuels, 7, 436-437

[4] Williams KA, Tachibana M., Allen JL, Grigorian L., Cheng SC, Fang SL, Sumanasekera GU, Loper AL, Williams JH, Eklund PC 1999. Single-wall carbon nanotubes from coal . Chemical Physics Letters, 310, 31–37

[5] Wang Z., Zhao Z., Qiu J. 2006. Synthesis of branched carbon nanotubes from coal . Carbon, 44, 1321–4

[7] Qiu JS., Zhang F., Zhou Y., Han HM., Hu DS, Tsang SC, Harris PJF 2002, Carbon nanomaterials from eleven caking coals . Fuel, 81, 1509-1514.

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