(534e) Microwave Assisted Pyrolysis of Coals and Coal Blends of Different Origin for Liquid and Solid Fuel Production

Coal has been integral to the growth story of several Asian-Pacific countries in the past 25 years. Coal production has increased 175% to about 2750 MT oil equivalent and consumption has also increased by 190% to about 2800 MT oil equivalent in the region. In India, the coal consumption in 2015 went up by 4.8% compared to that in 2014 to about 400 MT oil equivalent, which is about 10% of the world’s share of coal, only second to China among all countries. Coal-to-liquid technologies have regained interest recently for the production of transportation fuels. Although coal gasification followed by Fischer-Tropsch (F-T) synthesis of the syngas to produce hydrocarbons is a well-known technology, the search for novel techniques to convert coal to hydrocarbon fuels in a single step is underway. Liquefaction can be achieved via direct or indirect routes. In direct liquefaction, coals are used to produce high yield of energy rich oil fraction whereas in the case of indirect liquefaction, coal is gasified to syngas which is in turn converted to liquid hydrocarbons under pressure. Pyrolysis of coal is a technique which produces oil, gas and char.

Microwave assisted pyrolysis is a novel technique involving volumetric heating of the material via the generation of tiny micro-plasma spots throughout the reaction mixture. In microwave assisted pyrolysis, better control of the process and selective production of compounds is possible due to the uniform temperature attained throughout the reactant mixture. Moreover, it controls the occurrence of secondary cracking, which in turn results in better liquid yield. Microwave irradiation penetrates into the solid matrix and causes the inner heating thereby leading to the faster escape of volatile matter. Microwave assisted pyrolysis of a number of feedstocks such as lignocellulosic biomass, waste plastics, waste tires, municipal solid wastes, waste engine oils and coals are reported in the literature.

The available studies on microwave pyrolysis of coals are not comprehensive in terms of the effects of particle size of coal, microwave power, ash composition and different ratios of susceptor: coal on oil, gas and char yields, and their quality. Importantly, the quality of coal oil from Indian coal is not known. The objectives of this study are two-fold. Firstly, to understand the pyrolysis behavior of different coals such as medium and high ash Indian coals, Indonesian coal and high rank Australian coal. Secondly, to understand the pyrolysis behavior of unique blends of the above coal varieties.

In this study, microwave pyrolysis experiments were conducted in a bench scale unit, and the effects of various parameters such as microwave power, coal particle size, coal: susceptor ratio and coal blend ratio on product yields, oil composition and char composition were thoroughly evaluated. The oil composition was semi-quantitatively analyzed using gas chromatograph–mass spectrometry. When the coals were individually pyrolyzed, oil yield was not significantly affected by microwave power and initial particle size of coal. The oil yields were 23±2 wt.% and 35±3 wt.% for Indian and Indonesian coals and 7±2 wt.% and 16 ± 2 wt.% for Australian and high ash Indian coal, respectively, on dry ash free basis. The use of graphite as susceptor enhanced the heating rate although the oil and gas yields were not significantly affected. Indian coal oil contained a mixture of monoaromatics, naphthalenes, polyaromatics and phenolic compounds, while Indonesian coal oil was rich in only phenolic compounds. The addition of graphite promoted the formation of long chain alkanes and alkenes in C12–C20 range for both the coal varieties. Hydrogen gas constituted major fraction of the non-condensable gases. Higher heating values of coal oils were estimated to be 33–39 MJ kg^-1 , while that of coal chars were in the range of 19–22 MJ kg^−1 [1].

Char obtained from individual coal pyrolysis was used as susceptor to pyrolyze coal blends. Two different susceptor ratios and five different blends were chosen for the experiments. Char was the dominant product when high ash Indian coal and Australian coals were pyrolyzed with 80-85 wt.% yield. Coal oil was a mixture of monoaromatics, naphthalenes, polyaromatics, phenolic compounds and aliphatics wherein aromatics and phenols were the major fractions. Char was highly porous and flaky. Carbon content in the char was 87-93%, and oxygen content was 5-10%, on dry ash free basis, which implies that char is valuable product. HHV of the chars were 19 – 25 MJ kg^-1, while that of oils were 35-40 MJ kg^-1. Hydrogen gas was the major fraction in all the blends and CO₂ gas was minimum. More interesting results on coal blend pyrolysis will be discussed during the presentation.

References

[1] Reddy B R, Vinu R. Microwave assisted pyrolysis of Indian and Indonesian coals and product characterization. Fuel Process Technol. 154(2016), 96-103.