Intermediate pyrolysis of Acacia cincinnata and Acacia holosericea species for bio-oil and biochar production
Introduction
Pressure of ever increasing energy requirements on fossil resources coupled with the impact towards environment has motivated the research communities to look for alternative energy resources [1], [2], [3]. Burning of fossil fuels produce carbon dioxide and other flue gases which are considered as the main contributor for the global warming and environmental pollution [4], [5]. Biomass is an abundant energy resource which can be exploited to minimize the energy dependency on fossil fuels along with mitigation of environmental pollution issues [6]. Lignocellulosic biomass is considered as the most abundant biomass resource among different types of biomass, which is mainly composed of cellulose, hemicellulose and lignin components [7]. This work presents the intermediate pyrolysis of biomass Acacia cincinnata and Acacia holosericea species in a fixed bed pyrolysis reactor. The yields of pyrolysis products of bio-oil, biochar and non-condensable gases have been reported along with the characterisation of bio-oil and biochar produced from biomass collected from the trunk and phyllodes (leaves). Acacia cincinnata and Acacia holosericea belong to Acacia genus which are native to Queensland, Australia. They are fast-growing tree species and have the capability to accumulate large quantities of lignocellulosic biomass within a short span of time. They do not require major agricultural inputs, grow fast in poor soils and can be considered as an energy crop [8]. These fast growing tree species are a sustainable source of lignocellulosic biomass and can be used as feedstock to produce bioenergy [9]. Prior to this study, no significant work has been carried out to evaluate the potential of Acacia cincinnata and Acacia holosericea species as bioenergy resources. Different processing options have been investigated to produce energy from biomass feedstocks [10]. Thermochemical conversion processes are considered as the most feasible options to produce energy from biomass which include direct combustion, pyrolysis, gasification and liquefaction processes [9]. Pyrolysis process has some advantages over the other thermochemical processes including lower capital investment cost, comparatively lower heating temperature requirements, absence of oxygen in the process and flexibility of the process to produce more yields of desired product. Bio-oil is the major product of pyrolysis process which has the ease of storage, transportation and can be upgraded to engine grade oil [11], [12]. In pyrolysis process, biomass is heated at a certain high temperature in the absence of oxygen to decompose its basic components to produce vapours which are condensed to produce liquid bio-oil, leaving other products as solid biochar and non-condensable gases [13], [14]. Different types of pyrolysis include slow pyrolysis, intermediate pyrolysis and fast pyrolysis which are differentiated on the basis of process conditions. Intermediate pyrolysis of biomass is carried out between the process conditions of slow pyrolysis and fast pyrolysis having moderate pyrolysis temperature up to 500 °C, vapour residence time of few seconds and feedstock residence time between 0.5 and 25 min [15], [16].
Bio-oils are considered as a potential replacement of petroleum fuels, but their nature is significantly different from petroleum oil as the properties change if stored for longer periods of time. They have higher concentration of oxygenated compounds and strong acidic nature which make them unsuitable for direct use in engines; requiring pretreatment and upgrading by employing different bio-oil upgradation processes [17]. Various methods have been investigated by researchers to improve the quality and stability of bio-oil such as catalytic cracking, ash and char removal, adding solvents, catalytic hydrogenation, adding antioxidants and emulsification [18]. Biochar is a carbon enriched material which is found to have versatile physicochemical properties. The products obtained from intermediate pyrolysis can be used for energy production, as feedstock to produce different value-added products and chemicals and in many other useful applications [19], [20], [21].
Yang et al. 2014 have reported the intermediate pyrolysis of biomass energy pellets in a pyroformer intermediate pyrolysis reactor and Mahmood et al. 2013 have studied the intermediate pyrolysis and the steam reforming of brewers spent grain to produce bio-oil, biochar and gases [22], [23]. Various studies are reported on different biomass species including some of the Acacia species for consideration as biofuels resource [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. Ahmed et al. 2018 have reviewed the potential of various Acacia species to produce bioenergy in Brunei Darussalam [9]. In this paper, comprehensive thermochemical characterisation of the biomass samples was carried out including proximate analysis, ultimate analysis, thermogravimetric analysis and calorific value analysis to determine the bioenergy potential and to develop the suitable pyrolysis conditions. Thermogravimetric analysis (TGA and DTG) were carried out to understand the thermal degradation behaviour of biomass and to establish the temperature window suitable for the pyrolysis experiments. The properties of bio-oil and biochar produced in the study were analysed using various analytical techniques.
Section snippets
Samples collection and preparation
Biomass samples used in the study were obtained from the Acacia cincinnata and Acacia holosericea trees grown in the disturbed heath forest located near the Universiti Brunei Darussalam premises. Biomass was collected from the trunk and phyllodes parts of trees separately. Samples were packed in separate bags to ensure no cross mixing during all the stages of biomass drying, preparation and processes. The samples were abbreviated as ACP (Acacia cincinnata phyllodes), ACT (Acacia cincinnata
Characterisation of biomass
Understanding the physicochemical properties of biomass prior to its utilization as feedstock of pyrolysis process is very important. The properties of biomass samples used in the study are given in Table 1. Proximate analysis is a basic test to know the biomass properties and includes the determination of moisture content, volatile matter, ash content and fixed carbon content present in biomass samples. Moisture content play very important role in the selection of biomass feedstock for
Conclusion
Thermochemical characterisation and intermediate pyrolysis of Acacia cincinnata and Acacia holosericea biomass was carried out in this study. The yields of pyrolysis products including bio-oil, biochar and non-condensable gases were reported in the range of 41.24–52.95%, 31.16–38.79% and 14.64–21.92% respectively. GC–MS analysis of bio-oil samples showed the presence of aromatic hydrocarbons, phenols, furans, aldehydes, ketones, alcohols and organic acids compounds. Higher yields of bio-oil
Acknowledgements
This project was funded by the Brunei Research Council under project UBD/BRC/11 and was conducted with the collaboration of Faculty of Integrated Technologies (FIT), Institute of Biodiversity and Environmental Research (IBER) and Faculty of Science (FOS), Universiti Brunei Darussalam. First author is thankful to Universiti Brunei Darussalam for the award of Graduate Research Scholarship to pursue his PhD studies. We also acknowledge the support of the Brunei Forestry Department, Ministry of
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