De-oiled Pongamia pinnata seed residue: a non-edible resource for sustainable biofuels production
Creators: name: Muktham, Radhakumari
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 300405
subject: Crop and pasture biomass and bioproducts
classificationCode: 310605
subject: Industrial microbiology (incl. biofeedstocks)
classificationCode: 400499
subject: Chemical engineering not elsewhere classified

description: Depleting reserves of natural resources and the deleterious effects of fossil fuel burning on the environment have led to great interest in developing renewable fuels. Biomass represents a promising renewable material for biofuels, electricity, heat, chemicals and biochar production. Consequently research has focused on aspects of biomass conversion that currently act as bottle necks to commercial development including: (i) feedstock development, (ii) selection of biomass conversion technologies, and (iii) development of large scale reactor systems. Presently, the production of biofuels from waste biomass, wood chips, forestry and agricultural residues through thermochemical and/or biochemical routes are the leading process strategies in many countries. Significant improvements in process can therefore be expected, thus making large-scale biofuels production from lignocellulosic biomass substrates possible.<br><br> Currently, non-edible bioresources including non-edible oil seeds are emerging as a potential feed-stock for the biofuel industry, as both oil and seed residues can be exploited to produce biofuels. Non-edible resources have the advantage of not interfering with food supply and food pricing, or impacting on land use for feedstock supply to achieve sustainable biofuels production.<br><br>Pongamia pinnata, a non-edible, drought resistant tree is capable of adaptation to different climatic conditions. It belongs to the legume family and is distributed in Asia, Australia and the Pacific Islands. P. pinnata seeds are composed of 30-35% oil which represents a precursor for biodiesel production and has been widely studied as a potential renewable feedstock. The de-oiled seed residue is of current interest in terms of bioethanol, bio-oil and biochar production. The annual yield of the seeds reached 200,000 metric tons from India alone. P. pinnata seeds costs around $0.06 kg-1 and the selling price of oil extracted from the seeds is $0.06 L-1 ($0.065 kg-1) which makes the cost of the seed residue almost insignificant in the process, making the seed residue an economical feed-stock. In this study a range of issues in the conversion of the seed residue to biofuels were investigated. The value addition of P. pinnata de-oiled seed residue was carried out using chemical (acid hydrolysis), enzymatic (enzymatic hydrolysis) and thermal (pyrolysis) treatments.<br><br>In the first part of this study, chemical hydrolysis of the seed residue was carried out in the presence of sulphuric acid together with analysis of the composition of the seed residue in terms of ash (6.7%), lignin (29%), protein (22.3%) and total carbohydrates (42%). The main objective of this study was aimed at testing the seed residue for the production of glucose by acid hydrolysis in the presence of sulphuric acid and to prove the potential of the seed residue as a promising feedstock for biofuels production. The Taguchi robust design method with L9 orthogonal array was applied to optimize hydrolysis reaction conditions in order to maximize the glucose yield. The effect of temperature (80, 100 and 120 ⁰C), acid concentration (2.5, 5 and 7.5%), and liquid to seed residue weight ratio (10, 15 and 20) were considered as the main influencing factors which affects the formation of glucose. From the study it was observed that acid concentration and temperature had a principal effect on the amount of glucose formed when compared to that of the liquid to solid ratio. The maximum glucose formed was 245 g/kg seed residue. This work demonstrated the effects of different operating conditions on glucose production and the potential of the seed residue for sugar production for further application in biofuel production.<br><br> The second part of this study explores the effect of different acids including hydrochloric acid (HCl), sulphuric acid (H2SO4) and phosphoric acid (H3PO4) on sugars yield from hydrolysis of de-oiled P. pinnata seed residue and the application of sugars obtained for ethanol production in a three step process: acid treatment; neutralization and fermentation. The Taguchi robust design of experiments method was employed to study the effects of the parameters for biomass pretreatment, including acid type (H2SO4, HCl, H3PO4), acid concentration (2-6%), and temperature (80-100 ⁰C) on the formation of sugars during acid hydrolysis. Acid concentration and temperature showed a positive effect on sugar release from the biomass with hydrochloric acid the best catalyst for acid hydrolysis. The energy required for this pretreatment process using HCl was estimated, to get an insight into the process energy demand (1080-1110 KJ kg-1). Downstream processing before fermentation for ethanol production included neutralization of the hydrolyzate to pH 4.8. Addition of alkali in the neutralization step influenced sugar concentration in terms of g sugar per litre of neutralized product together with the salts formed and the time required for complete conversion of sugars to ethanol during fermentation. The results indicated that fermentation of 2%, 4% and 6% HCl treatment followed by neutralization gave 67.52, 74.98 and 88.62 g ethanol kg-1 dry seed cake, respectively, corresponding to ~31.45%, 34.92% and 41.28% of the theoretical ethanol formation (214 g kg-1). The work demonstrated the selection of the best catalyst, determined the energy requirement of the acid hydrolysis process, and confirmed the potential of the seed residue as a feed-stock for ethanol production.<br><br>In the third part of this study, the enzymatic hydrolysis method was tested for sugar production from P. pinnata seed residue for further application in ethanol production. Currently the costs associated with lignocellulases represent a key limiting factor in the development of biomass enzymatic conversion processes. The main aim of this work was to exploit the seed residue for both lignocellulase enzymes and ethanol production. Spingomonas echinoides (isolated from P. pinnata seed residue) and Iprex lacteus (obtained from RMIT culture collection) were selected as novel sources of lignocellulases during solid state fermentation. Both organisms produced an array of lignocellulases including exoglucanase, endoglucanase, xylanase and laccase with activities of 3.9, 2.7, 0.8, 0.116 U ml-1 min-1 and 5.2, 8.2, 2.7, 0.129 U mL-1 min-1 for S. echinoides and I. lacteus, respectively. P. pinnata seed residue (5%, w/v) in sodium citrate buffer (pH 4.8) was pretreated at 121 ⁰C for 15 min. Enzymatic hydrolysis of the pretreated seed residue was carried out at 50 ⁰C with mixing at 150 rpm. Subsequent hydrolysis of the pretreated seed residue using the crude enzyme preparation from S. echinoides and I. lacteus reported appreciable sugars yields of 233 and 302 mg g-1 seed residue respectively, at 10 U g-1 enzyme concentrations. The yields from crude enzyme treatment were further compared with that from commercial cellulase from Aspergillus niger (330 mg g-1 seed residue). The three sugar-containing liquid products from the enzymatic hydrolysis of P. pinnata seed residue using crude enzyme prepared from S. echinoides (SE), I. lacteus (IL) and cellulase from A. niger (AN) were fermented using Saccharomyces cerevisiae under anaerobic conditions at 35 ⁰C. Ethanol yields of 81.5, 104.5 and 157.6 mg g-1 and final ethanol concentrations of 4.0, 5.3 and 7.9 mg mL-1 were observed from SE, IL and AN fermentations respectively, which correspond to 38%, 49% and 73% of the theoretical ethanol yield, respectively. The study demonstrated the feasibility of using the seed residue for enzyme preparation for application in hydrolysis reactions and the potential of using the hydrolysis product for ethanol production.<br><br>In the fourth part of this study thermal treatment of the seed residue was carried out under pyrolysis conditions. The aim of the work was to obtain kinetic parameters in relation to technological parameters in the conversion of non-edible seed residue biomass to bio-oil and bio-char by pyrolysis. The effects of heating rate on seed residue pyrolysis and kinetic parameters were investigated at heating rates of 5, 10, and 20 °C/min using thermogravimetric analysis. Thermogravimetric experiments showed that the onset and offset temperatures of the devolatilization step shifted towards the high-temperature range, and the activation energy values increased with increasing heating rate. In the present study iso-conversional methods (model free and model fitting methods) were applied in estimating the activation energies (118-124 KJ mol-1) and the pre-exponential factors using progressive conversion, obtained by thermogravimetric analysis. The model free methods, Kissenger-Akahira-Sunose (KAS) and Ozawa-Flynn-Wall (OFW), and the model fitting Coats and Redfern method (CRF) gave reliable activation energies and pre-exponential factors with a third order reaction model for the seed residue pyrolysis reaction. The pyrolysis reactions in the thermogravimetric analyzer at constant heating rate revealed a temperature of 450-500 °C was sufficient for carbonization of the biomass. Thermal treatment of the seed residue under pyrolysis conditions was performed in a fixed bed reactor at different temperatures including 350, 400, 450 and 500 ⁰C. Proximate-ultimate analyses, energy value, surface structure and Fourier transformed infrared spectra of the processed biomass (biochar) were reported. Pyrolysis resulted in an increase in the heating value of the seed residue biomass from 18.1 to 24.5 MJ kg-1 with the heating value of biooil determined as 33.38 MJ kg-1; importantly biochar exhibited high carbon and low oxygen content. The approach represents a novel method for upgrading the seed residue resulting in a biochar which has significant commercial potential as a biosorbent for industrial effluent treatment.<br><br>The results obtained from the study have made an important contribution in value addition of the inedible and low cost bio-resource, de-oiled Pongamia pinnata seed residue. The scientific knowledge obtained from the present study can be useful in solving the technical barriers involved in biofuel preparation from the seed residue.