Microwave pyrolysis for valorisation of horse manure biowaste
Introduction
The main drivers for developing renewable and sustainable energy are due to the increasing demand for energy supply and the need to reduce reliance on fossil fuels. An energy study shows that the global production of renewable energy has increased significantly, from 500 TWh in 2008 to around 2500 TWh in 2018, mostly attributed to the growth in wind, solar and biomass-based energy sector [1]. Biomass is a type of renewable energy feedstock which can be sourced from forest residue, agriculture residue, animal manure, municipal waste or energy crops. The advantage of biomass energy is that biomass collection and utilisation are not constrained by geographical location, season nor weather, unlike those of solar and wind energy. In recent years, there is rising interest in using low value biowaste to generate bioenergy under the concept of circular economy. Animal manure has been identified as one of the potential candidates that can be recycled to maximise its value, rather than being disposed or landfilled.
The UN Food and Agriculture Organization (FAO) reported that the total animals farmed has increased by 75% between 1990 and 2014, given the increase in demand for meat consumption globally [2]. The growth in animal farming sector has inevitably led to the increase of faeces production, which is often used as fertilisers due to the high phosphorus and nitrogenous contents. However, large quantity of manure sometimes exceeds the utilisation rate. According to the report by FAO [3], only 25% of animal manure worldwide are treated in waste management system or reused as fertilisers. The improper management of the manure has resulted in the pollution of environment and endangered human health [4]. Furthermore, excessive manure left to decompose on an open field will release foul smell and methane gas that exacerbate the impact on climate change. Disposing manure into open water will induce the risk of water-borne pathogens, along with the growth of harmful algae [5].
Animal manures need to be treated and managed effectively to reduce the impact to the environment. One of the approaches is via pyrolysis, a thermo-chemical conversion process which can convert biomass into bioenergy and valuable chemical products. The process is known to emit lesser pollutants such as COx, SOx, and NOx when compared with combustion and gasification processes [6]. Pyrolysis can be carried out by using conventional furnace or microwave heating methods. The furnace heats the feedstock through conduction and convection with a thermal gradient directed towards the inner core of the feedstock. The microwave heating method utilises microwave irradiation to excite and vibrate the molecules within the feedstock, inducing a thermal gradient from the inner core of the feedstock to create a more efficient and uniform heating [7]. The latter is deemed superior as it provides volumetric heating and induces higher heating rate that leads to improved yield and quality of end products.
Valorisation of animal manure has been conducted by researchers using small-scale pyrolysis reactors. Pyrolysed cattle manure showed the degradation of three main constituents, namely hemicellulose, cellulose and lignin [6]. From kinetic studies, the thermal decomposition behaviour of dairy manure and swine manure were investigated by adopting a model-free method to derive the activation energy [8]. It was reported that swine manure has lower activation energy during carbonisation process as compared to lagoon sludge. The result shows the feasibility of using swine manure for solid fuel production due to the low energy requirement for the conversion process [9]. Biochar derived from poultry manure pyrolysis at 450–650 °C was observed to possess copper (II) adsorption ability after a series of pre-treatment [10]. Another study shows that pyrolysis of poultry manure under CO2 environment results in positive energy gain in end products [11].
The biochar produced from the pyrolysis of swine, dairy and poultry manures were reported to contain high level of phosphorus and potassium which are useful as soil improvement additives [12]. Further, pyrolysing swine manure with high cellulosic and alkali metal contents was reported to generate high levels of H2 and CH4 gases, which can be utilised as a source of bioenergy for power generation [13]. Yıldız et al. [14] showed that activated carbon with CO2 adsorption functionality can be produced by pyrolysing poultry manure at 400 °C in a pilot-scale pyrolysis reactor. The heating values of the biochar and gaseous products derived from manure mixed with swine solids are 21.2 MJ/kg and 29.5 MJ/m3, respectively, indicating the potential of the valorised products as biofuels. Recently, the pyrolytic behaviour of horse manure was investigated via a thermogravimetric analyser (TGA) [15]. The high level of volatile component (~70%) and relatively low decomposition temperature (190–400 °C) encourage the formation of bio-liquid and gaseous products at low energy levels. The biochar derived from the pyrolysis of horse manure at 450 °C has been reported to contain lower volatile, fixed carbon and aliphatic carbon as compared to corn, hardwood and Miscanthus feedstocks [16]. A techno-economic and life cycle analysis using tail gas recycling pyrolysis (TGRP) to upgrade bio-products from horse manure pyrolysis into bio-chemical phenol and bio-liquid fuel had also been conducted by Sorunmu et al. [17]. They suggested that biofuel produced from TGRP emits lower greenhouse gases (<10%) as compared to aviation jet fuel while phenol production from TGRP has been found to have lower cumulative exergy demand along with higher economic allocation as compared to petroleum-based cumene process. Thus, the reported literature suggests the potential of horse manure for bioenergy recovery at moderate pyrolysis temperature.
The present study aims to determine the yield and properties of end products obtained from pyrolysing horse manure. Horse manure is a potential bioenergy source due to its high volatile content [15]. Although the number of horses (58 mil) is numerically lower as compared to cow (1 billion) globally [18], the total amount of dry matter excretion produced by them are comparable. This is due to the fact that horse manure contains ~75 wt% of dry matter, while cow manure constitutes ~10 wt% of dry matter [19]. The relatively dried horse manure makes it easier to be managed for the purpose of drying, transporting and storage. In this study, pyrolysis of the horse manure is conducted via the use of microwave irradiation to derive the end products in solid, liquid and gaseous phases. The derived products are characterised thoroughly to assess their potential for use as bioenergy. It is to note that a parametric study is conducted to evaluate the yield and properties of end products derived from microwave pyrolysis of horse manure. Subsequently, a thermal decomposition pathway for the horse manure is established. Lastly, energy analysis is performed via energy profit to ascertain the degree of product valorisation.
Section snippets
Feedstock material and catalyst
The feedstock used in this experiment is horse manure (HM) obtained from the horse stable in Universiti Teknologi Malaysia, Skudai, Johor, Malaysia. The horse manure was collected and dried in an electric oven at 120 °C for 24 h before being ground into fine particles of 100–500 μm. The raw horse manure (freshly collected) is measured to have an initial moisture content of about 76 wt% ± 1.4. The physical and chemical properties of horse manure are shown in Table 1. It is to highlight here that
Product phase distribution
The product yield distribution obtained under the fixed N2 flow of 1 L/min and HM:AC ratio of 1:1 is shown in Fig. 3. It is to note that selective results in product yield were presented while the detailed yield distribution is tabulated in Supplemental Data Table A1. In general, the solid yield obtained from the parametric study is 12–79 wt%, whereas the liquid and the gas yields are 10–35 wt% and 5–42 wt%, respectively. The wide range of yield for the products shows the dependence on the
Conclusions
In this work, valorisation of horse manure is performed via microwave-induce pyrolysis. AC was added to the feedstock as catalyst to improve the dielectric properties of the mixture, thereby enabling the absorbance of microwave to elevate the pyrolysis temperature. The increase in the temperature and AC:HM ratio results in an increased gaseous yield at the expense of a reduction in solid yield. The residence time of the gaseous product is shown to be affected by the carrier gas flow rate, where
CRediT authorship contribution statement
Guo Ren Mong: Methodology, Investigation, Formal analysis, Data curation, Writing - original draft. Cheng Tung Chong: Conceptualization, Methodology, Investigation, Validation, Writing - review & editing, Funding acquisition, Resources, Supervision. Jo-Han Ng: Methodology, Investigation, Validation, Writing - review & editing. William Woei Fong Chong: Investigation, Validation, Project administration, Resources, Writing - review & editing, Supervision. Su Shiung Lam: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors acknowledge the financial support from the Malaysian Ministry of Education and Universiti Teknologi Malaysia (International Incentive Grant – vote no. 01M20).
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