Article pubs.acs.org/EF
Polymeric Consolidation in Briquetted Biofuel as Compared to Raw Biomaterial: A TG-Vision for Pigeon Pea Stalks Sandip Gangil* Agricultural Energy and Power Division, Central Institute of Agricultural Engineering, Bhopal 462038, Madhya Pradesh India S Supporting Information *
ABSTRACT: The consolidation of different polymeric bioconstituents was clearly observed in briquetted biofuel produced from pigeon pea crop residues. Critical thermogravimetric diagnosis showed that the scattering of tail end segments of thermograms taken at different heating rates was remarkably low in briquetted biofuel as compared to the scattering in raw material. The consolidation of polymeric cellulose and lignin was discussed and explained. Kinetics was evaluated using isoconversional integral Ozawa−Flynn−Wall method. The activation energy profile of briquetted biofuel dominated over the raw pigeon pea stalks showing higher thermal stability, which was evidence, positive toward the consolidation of intrinsic biopolymers due to briquetting process.
1. INTRODUCTION The utility of crop residues (pigeon pea stalks) as fuel can be increased by converting the crop residues in briquettes.1−16 The open field burning of crop residues (CR) is one of the major environmental problems in developing countries.17 The CR being burned are basically surplus, which is not of any use for the farmers and can be used for the generation of thermal and electrical energy. Therefore, the energy security in the rural areas of developing countries can be ensured by converting the locally available energy resources (crop residues) in the form of briquetted biofuel. In general, the crop residues are not fit for energy generation due to their low bulk densities leading to difficulties during the handling and storage of crop residues.1−6 The energy and power generation systems operating on crop residues or biomass need biofuel of uniform size for better fuel flow in their bioreactors to avoid the chocking and blockages. Due to light weight and irregular shape and size, the crop residues show poor flow property in bioenergy reactors. The CR can be converted in briquettes and can be used as substitute biofuel against wood chips in the biomass based power generation systems and combustion devices. Densities on the order of 1000−1200 kg/m3 can be obtained by briquetting.9 Briquettes are solid compact uniformed sized biofuel that can be used as a substitute for firewood. In India, briquettes are mostly produced from sawdust, bagasse, mustard stalk, ground nut shell, cotton stalk, etc. The lignocellulosic materials can be briquetted without binder because the lignin present in these materials can itself act as binding agent. However, binderless briquetting needs high pressure and temperature as compared to the briquetting with binder. Commercial briquetting machines can be grouped into three types: (i) Screw-press type, (ii) piston-press (die-punch) type, and (iii) Rotary dies and roller type.1−3,5,8−13 All of them put compressive stresses on the raw CR. A pressing unit always exists in all systems that puts stress in confined and semiconfined environments on the raw CR. In piston-press type systems, the raw biomaterial is pressed by the piston through a die. Due to the exerted pressure, the raw material passing through the die is heated. © 2014 American Chemical Society
Due to the heating, the inner biomatrix of biomaterial, under briquetting, experiences internal changes and briquettes are formed. In a screw-press type briquetting machine, a screw is used to exert consistent heavy pressure on biomaterial through a die. Piston-press exerts pressure in regular pulses, whereas in screw-press the pressure is exerted continuously. The screwpress type system consumes more energy per unit output. The size reduction of the CR is essential for briquetting. As an engineering requirement, the raw CR obtained from the fields is required to be brought in the size desirable for the briquetting machines. The approach of making the briquettes from crop residues and then using the briquetted fuel for energy or power generation is beneficial in several ways. The farmers producing the crop residues will get financial benefits. Also, the use of crop residues for energy will reduce the pressure on the forest reserves by saving the fuel wood. This approach also reduces the environmental pollution that occurs when the crop residues are burned in the fields. A complete market chain from the farmers to biomass management agencies to briquette consumer is needed in this approach, which will also create the generation of the employment opportunities in the rural sector (Dubey et al., 2009)6 of developing countries. The financial aspects of briquetting are discussed by other researchers.5,18,19 The polymeric bioconstituents of lignocellulosic biomass and crop residues are cellulose, hemicellulose, and lignin. During the briquetting, it is expected that, due to the high pressure exerted by briquetting machine, the temperature of the raw crop residues increases at the die-region. The combined effects of pressure and temperature create the internal stresses in the raw material and the configurations of biomaterial change. Due to high temperature, the moisture in the loose CR evaporates. Received: February 19, 2014 Revised: April 26, 2014 Published: April 28, 2014 3248
dx.doi.org/10.1021/ef5004304 | Energy Fuels 2014, 28, 3248−3254
Energy & Fuels
Article
thermal stability of briquetted biofuel over raw crop residues as established by kinetics.
Further, hemicellulose, cellulose, and lignin undergo specific positive changes toward thermal stability in the briquetted fuel. When biomaterial is briquetted (pressure ≥100 MPa and temperatures ≥200 °C), the lignin becomes soft. This is an accepted fact, in the case of a binderless briquetting process, which states that the lignin in the CR melts during the briquetting and acts as glue to bind the different biomolecules to form the solid densified briquette.2,3 Almost all lignocellulosic crop residues can be converted to briquette by optimizing the process parameters of the briquetting.2,3,20−23 To understand the changes occurring in the biocomponents due to briquetting of crop residues, reliable and in-depth investigation can be performed using thermogravimetric analysis (TGA) wherein a biomass is heated in precisely monitored and controlled chamber. Using TGA, thermal degradation of different biocomponents can be understood in respect of a specific biomaterial. There are several stages in the thermal degradation of biomaterial:24 the moisture release, the hemicellulose degradation, the cellulose degradation, and the lignin degradation. As stated by Caballero et al.,25 the thermal degradation of lignocellulosic biomaterials could be explained as the addition of the independent degradations of their main components. López-González et al.26 also stated that the thermal decomposition profile could be understood to be the sum of the corresponding individual component contributions. Ledakowicz and Stolarek27 stated that the evolution of every component could be assumed as a single first order reaction considering that the various components of biomaterial were evolved by independent parallel reactions. Sanchez-Silva et al.28 mentioned that hemicellulose had a random and amorphous weak structure, whereas cellulose had a crystalline and strong structure. They also mentioned that lignin was the highest thermally stable component and corresponding thermogravimetric peaks originated in the wide range of temperatures studied (200−700 °C) and the lignin showed the flattest DTG profile. Also, the degradation of lignin was stated as very difficult because lignin is a heavily cross-linked highly branched complex polymer.28 Diagnosis of thermogravimetric signals can give the information about the hardening or softening of specific constituents in two or more biomaterials. The popular methods for determination of kinetics of thermal degradation are Friedman method, Coats−Redfern, Kissinger, Kissinger− Akahira−Sunose (KAS), and Ozawa−Flynn−Wall (OFW).29−31 Mathematically, Coats−Redfern, KAS, and OFW are integral isoconversional methods, whereas the Friedman method is a differential isoconversional method.32 Vyazovkin and Sibrrazzuoli30 stated that in nonisothermal conditions wherein the temperature of samples under thermogravimetry is increased at constant heating rate, the most popular isoconversional method of kinetics is OFW. This method uses the most important independent variables (reaction temperature and heating rate) for kinetics computations, and the calculations are relatively simple and reliable. The paper is intended to present the scientific support (using a TG-technique) in terms of increased activation energy levels obtained in briquettes, responsible for polymeric consolidation. The article is a first attempt to highlight the reduced scattering of thermogram in briquetted biofuel as compared to raw biomaterial showing the consolidation of polymeric bioconstituents of the biomaterial (pigeon pea stalk). The consolidations of polymeric bioconstituents lead to higher
2. METHEDOLOGY The stalks of the pigeon pea crop (Cajanus cajan) were collected from several fields of farmers in the Bhopal-Raisen area of Madhya Pradesh State of India. The leaves of the pigeon pea crop were not included while drawing the samples for study. The stalks brought from the fields of several farmers were powdered, and the particle distribution was particle size >0.2 mm (6%), 0.2−0.4 mm (6%), 0.4−0.7 (20%), 0.70− 1.4 mm (20%), 1.4−1.7 mm (21%), and
