Kinetics of Isothermal and Non-Isothermal Fast Pyrolysis of Alkali Lignin

Kinetics of Isothermal and Non-isothermal Fast Pyrolysis of Alkali Lignin

Deepak Kumar Ojha and R. Vinu

Department of Chemical Engineering and National Center for Combustion Research and Development, Indian Institute of Technology Madras, Chennai- 600036, India

E-mail: deepakojha86@gmail.com, vinu@iitm.ac.in

Abstract

Fast pyrolysis has received enormous attention for the conversion of biomass and waste plastics into usable intermediates and valuable chemicals. The involvement of hundreds of chemical reactions makes the process extremely complex. The operating parameters such as pyrolysis temperature and vapor residence time play a decisive role in determining the product spectrum. High temperatures lead to enhanced conversion but also promote the formation of condensed ring aromatics that can be undesired for various reasons, while long vapor residence times lead to more vapor phase interactions, thereby promoting secondary reactions that will often lead to more char and undesired product formation (<C4 oxygenates and non-condensable gases). The knowledge of apparent kinetic parameters, viz. activation energy and frequency factor, and optimal reaction time can be very useful in designing a sophisticated fast pyrolysis reactor.

The objectives of this work are two fold: firstly, the apparent kinetic parameters for isothermal and non-isothermal fast pyrolysis of alkali lignin were evaluated. Conventionally, the kinetic parameters are reported only for slow or medium heating rate pyrolysis via thermogravimetry experiments. However, in this work, kinetics of fast pyrolysis is determined at heating rates greater than 1000 ^oC/s. Secondly, the time evolution of various organic functional groups formed in the vapor phase during fast pyrolysis of alkali lignin was studied.  The fast pyrolysis experiments are conducted in a Pyroprobe^® 5200 pyrolyzer (CDS Analytical Inc.) and the generated vapors were characterized by Fourier transform infrared spectrometer (FT-IR, Cary 660, Agilent Technologies) which is equipped with a high sensitivity MCT detector. The condensable and non-condensable products were thoroughly characterized using 2D-GC/MS (Agilent Technologies) and a gas analyzer (Bhoomi Analyzers, India).

Figure 1 depicts the FT-IR spectra of the major functional groups evolved during fast pyrolysis of alkali lignin at 500 ^oC.  The peak at 3585 cm^-1 indicates the formation of non-hydrogen-bonded phenolic and alcoholic moieties. Interestingly alkali lignin does not exhibit significant vibration above 3500 cm^-1 but it shows a vibration below 3500 cm^-1 indicating that most of the OH groups present in lignin are hydrogen bonded in lignin matrix. The aliphatic vibrations in the pyrolysis vapor are because of C-H bonds present in propyl chain of lignin matrix whereas aromatic vibrations are as a result of vibration of C-H bonds in benzene ring. The intensity of C-H aliphatic bonds was predominant over C-H aromatic vibration at moderate temperatures (400 ^oC). But with the increase in pyrolysis temperature, the intensity of the C-H aromatic vibration was found to dominate the C-H aliphatic vibration. This is because of the cleavage of propyl subunits of lignin into the smaller fragment such as CH₄, CO and CO₂. The formation of these non condensable gases was confirmed by FT-IR spectra and gas analyzer. The time required for maximum production of products at 400 ^oC was 12 s and it reduced to 8 s at 800 ^oC. The first order apparent activation energies for isothermal and non-isothermal degradation were calculated to be 14.29 kJ/mol and 25.53 kJ/mol, respectively.

Figure 1: Time evolution of major functional groups during fast pyrolysis of alkali lignin at 500 ^oC.