(645e) Microwave Assisted Co-Pyrolysis of High Ash Indian Coal and Rice Husk and Detailed Product Characterization

With a share of over 60% in the commercial energy sector, coal has become India’s energy security. The relative abundance of coal in India compared to other fossil fuels makes it a natural choice as the primary source of fuel. The reserves to production ratio at the current rate of production for coal is around 230 years as against 15 and 23 years, respectively, for indigenous oil and natural gas. This makes domestic coal a very secure energy source for the country. India has got 192 billion tons of coal reserves but the majority contain high amount of ash [1]. Coal is a complex heterogeneous material which consists of a number of distinct organic (maceral) and inorganic (mineral) compounds. Although regained interest in the production of transportation fuels from coal is positive sign, it is not sufficient as liquefaction of coal yields relatively low liquid yield compared with biomass liquefaction.

Coal and biomass to liquid hydrocarbon fuels offer great hope in commercialization than other alternatives as it not only reduces the carbon footprint in the environment but also facilitates the utilization of locally available low-rank coals [2]. To improve the utilization of coal and to increase the carbon and energy density, co-pyrolysis, co-gasification and co-combustion processes have been developed. Among these, co-pyrolysis of coal and biomass is a promising strategy to produce liquid bio-oil that can be either combusted for power/heat or can be further catalytically upgraded to liquid transportation fuels [3]. Co-processing of coal and lignocellulosic biomass can provide a solution for the use of biomass on a commercial scale. Conventional pyrolysis involves heat transfer in the form of conduction, convection and radiation, while in microwave heating electromagnetic energy is converted to thermal energy from inside of the sample to the surface. Due to its uniform, volumetric, rapid, selective and efficient heating mechanism, microwave heating reduces the processing time and enables the selective production of chemicals [4]. Moreover, secondary cracking of primary pyrolysates can be controlled in microwave heating using different microwave absorbing materials/susceptors.

In this study, microwave assisted co-pyrolysis of high ash Indian coals and rice husk is investigated to evaluate the liquid fuel potential and to understand the interaction effect in the liquid yields and its composition. Microwave co-pyrolysis experiments were conducted in a bench scale unit, and the effects of various parameters such as microwave power, coal: rice husk ratio, feed: susceptor ratio on product yields and oil composition were thoroughly evaluated. The bio-oil composition was semi-quantitatively analyzed using gas chromatograph-mass spectrometry. Gas composition was analyzed using an online gas analyzer. The oil yield was not significantly affected by the microwave power and feed: susceptor ratio for a particular coal: rice husk ratio. Additive effect was observed with increase in rice husk ratio. The bio-oil yields were 11±2 wt.%, 15± 2 wt.%, 18 ± 2 wt.%, 21 ± 2 wt.% and 26 ± 2 wt.% for 100:0, 75:25, 50:50, 25:75 and 0:100 mass ratios of coal to rice husk. The use of graphite as susceptor improved the heating rate significantly owing to its high dielectric tangent value. High microwave powers increased the second cracking of condensables, which resulted in reduction of oil yield as compared to oil yield from low microwave power. Coal oil is a mixture of aromatics, aliphatic HC and phenolic compounds, while pure rice husk oil was a mixture of furans, aromatics, phenolic compounds, guaiacols, syringols and cyclooxygenates. The addition of coal to rice husk demoted the yield of phenolic compounds and promoted the aromatic content. HHV of bio-oil was 32-35 MJ kg^-1_, while HHV of char was 11-15 MJ kg^-1. Although char was highly porous, its HHV was low due to the high amount of ash that was present in both feedstocks (20% ash in rice husk and 33% ash in Indian coal). Hydrogen gas constituted a major fraction in the pyrolysis gases. Addition of rice husk promoted CO₂ evolution.

References

[1] Haque E. Indian Coal: Production and Ways to Increase Coal Supplies. Intl J Sci Res Publ. 2(2013), 1-3.

[2] Arvelakis S, Folkedahl B, Dam-Johansen K, Hurley J. Studying the melting behavior of coal, biomass, and coal/biomass ash using viscosity and heated stage XRD data. Energy Fuel 20(2006), 1329-1340.

[3] Guan Y, Ma Y, Zhang K, Chen H, Xu G, Liu W, Yang Y. Co-pyrolysis behaviors of energy grass and lignite. Energy Convers Manage. 93(2015), 132-140.

[4] Shra’ah A A, Helleur R. Microwave pyrolysis of cellulose at low temperature. J Anal Appl Pyrol. 105(2014), 91-99.