(695d) Fast Pyrolysis of Four Different Microalgae: Apparent Kinetic Parameters Evaluation and Product Analysis

Fast
Pyrolysis of Four Different Microalgae: Apparent Kinetic Parameters Evaluation
and Product Analysis

Ribhu
Gautam^*, R. Vinu

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

            Economic
growth and development is resulting in fast depletion of fossil fuel reserves.
Presently, the mitigation of environmental impacts caused due to the excessive
use of fossil fuels has gained importance. Over the past few years, research
for replacement of conventional fossil fuels is gaining importance with focus
on biomass as it has got high potential and is an abundant energy source. Compared
to terrestrial biomass, marine biomass has advantages such as high growth rate,
high per hectare yield and high photosynthetic efficiency. Marine biomasses
have flexibility to grow in sea water, fresh water ponds, industrial effluents
and waste water. Microalgae, one of the most prominent marine biomass, are
abundantly distributed water plants. They consist of lipids, proteins and
carbohydrates and these lipids are converted into biodiesel. Some microalgae
have lipid content as high as 80 wt.%, and are hence considered as potential
feedstocks for biodiesel and biofuel production.

Fast pyrolysis is a
promising thermochemical processing technique to convert microalgae into
liquid, gas and solid products in the absence of oxygen at moderate
temperatures. High heating rates (>1000°C s^-1) involved in fast
pyrolysis leads to faster completion of reactions within seconds. This results
in high yields of liquid bio-oil as compared to slow pyrolysis. The existing
kinetic models of algae pyrolysis are all based on thermogravimetric mass loss
data obtained at slow heating conditions. The use of these data limits the
application of these models for fast pyrolysis. Therefore there is an immense
need to obtain kinetic parameters under fast pyrolysis conditions and validate
them with the existing rate parameters.

This study reports a new
experimental technique to evaluate the mass loss profiles of microalgae under
fast pyrolysis conditions using an analytical pyrolyzer. Fast pyrolysis of four
different microalgae (Chlorella vulgaris (C. vulgaris), Nannochlorpsis
oculata (N. oculata), Schizochytrium limacinum (S. limacinum) and Arthrospira
platensis (A. platensis)) was studied in the temperature range of 400-700°C
and sample residence times in the range of 2-60 s. The apparent kinetic
parameters of fast pyrolysis of the algae were evaluated using first order, one
dimensional (1D), 2D and 3D diffusion models. Prior to the fast pyrolysis
experiments the dried microalgae samples were characterized by TGA, proximate
analyser, CHNS analyser, bomb calorimeter for thermal stability, proximate analysis,
ultimate analysis and heating value respectively. Fast pyrolysis experiments
were carried out in an analytical pyrolyzer with 4.5±0.2 mg of sample in a
quartz tube. The mass loss data of the microalgae samples was generated in
isothermal conditions via gravimetry. The most conventional rate loss equation
was considered for the estimation of rate constants.

Where f(a)
is the dependence of conversion on the reaction model, k(T) is
dependence of rate on temperature. ‘a’
termed as normalized conversion can be calculated with the formula a
= (m₀-m_t)/(m₀-m_∞), where m₀,
m_t and m_∞ are initial sample mass, sample mass at
time t and final mass obtained respectively. As the reaction proceeds
from initial to completion, a increases from
0 to 1. This cannot be linked to specific reactions during the process, so a
is the degree of overall completion of reaction. Now,

            The
function of g(a) has been
approximated as  –ln(1-a),
a²,
[(1-a)ln(1-a)+a]
and [1-(1-a)^1/3]²
for first order, 1D, 2D and 3D diffusion respectively for solid state reactions.
The rate constants for each reaction temperature was obtained from the slope of
the g(a) v/s t graph. The
estimation of apparent activation energy was done by fitting temperature and
rate constant data in Arrhenius Equation represented below

First
order kinetic model fitted the experimental data well with very high R²
for all the four microalgae samples. The apparent activation energies (E_a)
and pre-exponential factors (A) for fast pyrolysis of the algae varied in the
range of 4-24 kJmol^-1 and 0.03–1.7 s^-1 respectively. The
obtained rate parameters were validated by constructing a kinetic compensation
plot, which showed that Ln(A)=0.19E_a–0.43 (R²=0.97)
justifying the kinetic compensation effect. The pyrolysates obtained from fast
pyrolysis in an analytical pyrolyzer at 500°C were characterized using gas
chromatograph/mass spectrometry for all the four microalgae samples. The pyrolysates
were characterized in a GC/MS coupled with pyrolyzer. Fast pyrolysis S.
limacinum yielded highest aliphatic and aromatic hydrocarbons owing to its
high lipid content. A detailed kinetic analysis comprising all the kinetic
models and the presence of various compounds observed in the pyrolysates will
be discussed during the presentation.