Production of phenolic compounds using waste coir pith: Estimation of kinetic and thermodynamic parameters
Graphical abstract
Introduction
Increase in distress concerning energy, fuel crisis, and environmental issues such as global warming and the greenhouse effect, have lead to a developing interest in the production of energy and chemical commodities via renewable sources. Lignocellulosic biomass is an inexpensive and plentiful resource of organic carbon which can be used for the production of liquid fuels, petrochemical feedstocks, and fine chemicals, via the biochemical or thermochemical routes. Agriculture wastes, such as coir pith are the underexploited lignocellulosic biomass available abundantly. Pyrolysis is one of the profitable technique that involves the thermal degradation of organic matter in the absence of oxygen which can provide not only liquid fuel but also a variety of valuable hydrocarbons.
Coir is the term given to the fibers constituting the husk/mesocarp of the coconut (Cocos nucifera), which are used for the manufacture of ropes, matting, and other hard-fiber based products. Coir pith is essentially a cementing material that binds coir fibers together and constitutes up to 70% of the husk (Namasivayam and Kavitha, 2002). It is a soft, spongy, highly hygroscopic, lignocellulosic biomass, which is resistant to easy microbial degradation because of high lignin content. Raveendran et al. (1995) reported 31.2% lignin, 28.6% cellulose and 15.3% hemicellulose in the coir pith. As it has no economic value, coir pith is often dumped outside the coir industry in massive quantities. Being low in density, it is blown away by the wind when dumped on roadsides, thus causing vehicular obstruction. Because of high ash content, it has poor combustion properties and emits high smoke when burnt. The high lignin content in coir pith makes it unfitting as raw organic manure for crops (Dhyani and Bhaskar, 2018a). The water-soluble phenolic compounds injure the growing tender roots of plants and inhibit growth. Dumping of coir pith not only consumes valuable land but also poses a threat to the environment, because of contamination of groundwater by percolation of phenolic compounds from these dumps (Gopal and Gupta, 2001). India is a leading producer of coir pith, marking an annual production of around 7.5 million tons per year (Namasivayam and Kavitha, 2002). In certain parts of India, coir pith finds its application in making lightweight bricks (Dan, 1992) and particle-boards (Viswanathan, 1998).
Much of the research done with coir pith, has been fixed around its use as an adsorbent in wastewater treatment, by chemical/physical treatment. Various researchers have studied the removal of contaminants such as dyes (Kavitha and Namasivayam, 2007, Namasivayam et al., 2001), phosphate (Krishnan and Haridas, 2008), transition metal ions (Namasivayam and Sureshkumar, 2008, Parab et al., 2006, Suksabye et al., 2009), etc. Studies have also been done in analyzing coir pith as a substitute for sphagnum or sedge peat in soil-less cultivation (Meerow, 1994). However little work has been done for energy generation or production of important phenolic compounds from coir pith. Phenolic compounds are important industrial chemicals, which find utility in the preparation of resins, dyes, explosives, lubricants, pesticides, and plastics. They are indirectly involved in the production of plywood. Phenol also works as an organic solvent to dissolve other alcohols, chloroform, and ethers. The phenolic compounds present in the bio-oil such as methyl-phenols (cresols), methoxy-phenols (guaiacol), methoxy-propenylphenol (isoeugenol), etc., have enormous economic potential, in food, pharmaceutical, and adhesive industries (Horne and Williams, 1996, Stoikos, 1991).
Production of phenolic compounds from coir pith not only solve the problem of coir pith waste but also provide a green route for the production of phenolic compounds, subsequently leading to efficient utilization of the waste. In the work presented in this paper, coir pith was used as feedstock for the production of valuable hydrocarbons (phenols) via pyrolysis, in a laboratory scale pyrolysis reactor, and the products obtained were analyzed both quantitatively and qualitatively. However, for implementation of a reaction on a substantial scale, it’s not sufficient to know the nature of products, but equally important to understand the rate at which the products are obtained. Kinetic analysis of the decomposition process taking place was performed using model-free isoconversional methods. The conversion data for implementing these methods was obtained using thermogravimetry. A thermodynamic translation of the kinetic information was also made, using the values of activation energy derived from Friedman’s method.
Section snippets
Feedstock
Coir pith was obtained from Coir Board, Kerala. The fibers were ground and sieved to ≤630-µm size, before conducting experiments. 10 g ground Coir pith was used for slow pyrolysis, while in the Thermogravimetric analysis (TGA), approximate 5 mg sample was used in each run. The moisture content in the powdered feedstock was 8.84%, while the volatile and ash contents were 45.91% and 35.56% respectively (9.96% fixed carbon by difference). The carbon, hydrogen, and oxygen content in the feed were
Thermogravimetric analysis
The results of TGA (Fig. 1(a)) show that the thermal decomposition of coir-pith took place in a wide range of temperature. The average weight-loss of 47.8% was observed from 25 to 900 °C, with the maximum weight-loss occulting in the range of 200–500 °C (∼30% of initial weight). The shoulder around 275 °C in the differential thermogravimetry (DTG) curve indicates the peak of hemicellulose decomposition. The de-convolution of − vs. curve (Fig. 1(b)) shows the presence of three distinctive
Conclusions
The maximum bio-oil yield from the pyrolysis of coir pith was obtained at the reaction temperature of 350 °C. Phenol was present as the most abundant compound. With the increase in reaction temperature, the product composition showed a shift towards alkyl-phenols. The average activation energy of 140 kJ/mol was observed for the decomposition process. A thermodynamic translation of kinetic data implied the conversion range of 0.6–0.7 as the most reactive and endothermic zone. Combining the yield
Acknowledgments
The authors would like to thank the Director, CSIR-Indian Institute of Petroleum (IIP) for support and encouragement and AcSIR for granting permission to conduct this research work at CSIR-IIP. They thank the Analytical Science Division (ASD) of CSIR-IIP for 1H NMR and FT-IR analyses. The authors also thank Centre for High Technology (CHT) for providing financial assistance in the form of the GAP-3220 project.
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Equal contributions.