Characterization-, energy potential-, pyrolysis kinetics-, and non-isothermal thermogravimetric – study of raw and torrefied Moringa husk

ABSTRACT

About 0.384 million tonnes per year of unutilized Moringa husk (MH) is generated worldwide, which can produce a substantial amount of py-oil through pyrolysis. Based on the above fact, the present has been planned to study the chemical characterization, exergy analysis, and pyrolysis kinetics of raw and torrefied samples of MH. The characterization of raw and torrefied-MH in terms of proximate ‒ ultimate analysis and compositions of hemicellulose, cellulose, and lignin, was carried out. Further, exergy analysis of MH before and after torrefaction ensured the improvement of the heating value of MH. Torrefaction increased the heating value and carbon content of torrefied-MH by 21.5 % and 25.67%, respectively. For the kinetic study, non-isothermal TGA ‒ DTG thermograms were recorded for raw- and torrefied samples of MH at four different heating rates ranging from 5 to 30°C/min. Kinetic predictions were then made using multiple linear regression method (MLRM) as well as regular integral isoconversional methods. MLRM shows that reaction order, as well as activation energy and pre-exponential factor, varies during MH pyrolysis. Further, regular integral isoconversional methods predicted that for 5 ‒ 70% of fractional conversion, the average activation energy ranged from 210.96 to 212.07 kJ/mol for raw-MH and 99.08 to 104.38 kJ/mol for torrefied-MH.

KEYWORDS:

• Moringa husk
• torrefaction
• characterization
• exergy analysis
• non ‒ isothermal TGA
• pyrolysis kinetics

Nomenclature

α	=	conversion/reaction progress, dimensionless
β	=	heating rate, °C/min
Ea	=	apparent activation energy, kJ/mol
${\mathrm{E}}_{\alpha }$	=	activation energy at conversion α, kJ/mol
A	=	pre-exponential factor, min–1
n	=	order of reaction, dimensionless
R	=	universal gas constant, J/mol ‒ K
T	=	Temperature, °C or K
f(α)	=	differential form of reaction model, dimensionless
f(E)	=	normalized distribution curve of activation energy
g(α)	=	integral form of reaction model, dimensionless
${\mathrm{R}}^2$	=	correlation coefficient, dimensionless
${\mathrm{T}}_{\mathrm{m}}$	=	temperature at maximum peak in DTG curve, °C or K
${\mathrm{T}}_{\mathrm{f}}$	=	final temperature of the pyrolysis zone, °C or K
${\mathrm{T}}_{\alpha }$	=	temperature at α degree of conversion, °C or K
Ex	=	exergy, MJ/kg
γ	=	quality of fuel, dimensionless
$\mathrm{H}{\mathrm{c}}_{\mathrm{f}}$	=	higher heating value of torrefied biomass (MH250), MJ/kg
$\mathrm{H}{\mathrm{c}}_{\mathrm{S}}$	=	higher heating value of orignal biomass (raw-MH), MJ/kg

Abbreviations

Py-oil	=	pyrolysis oil
MH	=	Moringa husk
raw-MH	=	Moringa husk without any treatment
MH250	=	torrefied-MH (at T= 250°C and t=40 minutes)
TGA	=	thermogravimetric analysis/thermogravimetry
DTG	=	differential thermogravimetric analysis
ML	=	mass loss
MLRM	=	multiple linear regression method
DAEM	=	distributed activation energy model
OFW	=	Ozawa ‒ Flynn ‒ Wall
ASTM	=	American society for testing and materials
LHV	=	low heating value
HHV	=	high heating value
RDW	=	recycled distilled water
g	=	gram
mg	=	milligram
ml	=	milliliter
h	=	hour
EY	=	energy yield
DOE	=	design of experiments

Acknowledgments

The current research work in the Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, India, has been financially supported by the Ministry of Education (MoE), Government of India, New Delhi.