|Sirawasith Ruksathamcharoen||Department of Transdisciplinary Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan|
|Muhammad W Ajiwibowo||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Teerapong Chuenyam||Department of Transdisciplinary Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan|
|Adi Surjosatyo||-Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Kunio Yoshikawa||Department of Transdisciplinary Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan|
Hydrothermal treatment (HTT) is recognized as one of the promising thermochemical pre-treatments for biomass energy. It provides a low energy requirement and an elimination of pre-drying. Furthermore, a water washing method for the treated biomass is employed. One of the most promising biofuel candidates is empty fruit bunch (EFB) due to the large yearly production in Indonesia. In this study, the quality of hydrochars derived from EFB was assessed for its quality as a solid fuel. Raw EFB is thermochemically treated by employing HTT, and its characteristics in regard to grindability are studied. The study suggested that commercial-scale HTT produces a solid fuel with a high heating value (HHV) with a value similar to Indonesian coal. In addition, it has lower ash content than the parent biomass, which reduces the probability of slagging and fouling in boilers. Furthermore, the particle size distribution profile suggested better characteristics than the parent biomass and some types of coal.
Biomass energy; Empty fruit bunch; Grindability; Hydrothermal treatment
Sustainability in energy has been an important issue in recent years and has gained importance since the recent signing of the Paris agreement (Sansaniwal et al., 2017). Use of fossil fuels worldwide has contributed significantly to the increase of the earth’s temperature (McGlade & Ekins, 2015). The utilization of renewable energy sources and alternative technologies to meet the 2°C target are urgently needed, and there have been various research and development efforts in regard to clean and sustainable energy production (Surjosatyo et al., 2017; Zaini et al., 2017; Darmawan et al., 2018).
Bioenergy derived from biomass fuels are seen as a promising alternative energy source (Darmawan et al., 2018). The life cycles of biomass fuels are proven to be better than those of fossil fuels, and energy from biomass leaves no carbon footprint, as the generated CO2 will be processed again by plants for photosynthesis. In addition, the use of biomass wastes for fuel could potentially solve the waste problem that exists in developing countries (Prawisudha et al., 2012).
One of the biggest contributors to the biomass waste produced in Indonesia is the palm oil industry. Indonesia produces around 80% of the world’s demand for palm oil. In the year 2014, palm mills in Indonesia produced as much as 33.5 million tons of waste biomass, and 22% of it is empty fruit bunches (EFB) that are left after the oil processing, which indicates a strong potential for waste biomass to be used as fuel.
The bulky and fibrous nature of EFB makes it hard to handle (Zaini et al., 2017), and in its untreated state, EFB has a minimal high heating value (HHV) of about 16 MJ/kg and low energy density due to the fibrous composition. Therefore, pre-treatment of EFB is necessary before combustion. Among the various pre-treatments for solid fuel production, hydrothermal treatment (HTT) with water washing is one of the most promising (Novianti et al., 2015). With this method, the biomass is treated by pressurized sub-critical water at a temperature of 150 to 350°C and a pressure of around 2 MPa (Jin, 2014). The advantage of this particular treatment is the elimination of a pre-drying process.
The treated biomass, which is referred to as hydrochar, has significantly higher energy density and generally better quality. However, the main reactions present in the HTT involve some degradation and decomposition of the biomass (Prawisudha et al., 2012), which will cause volume and mass reduction as the volatile matter of the biomass decreases (He et al., 2013).
The possibility of co-combustion of the produced hydrochar with coal in commercial boilers is one of the main interests in this research topic (Liu et al., 2012). In this regard, the milling process of solid fuels for pulverized solid fuel boilers is also a matter of significance (Bridgeman et al., 2010). The particle size distribution of the fuel affects important characteristics such as combustion efficiency, the amount of unburned carbon in the ash, and combustion stability (Sarroza et al., 2017). In addition, the operation of the pulverizing unit is critical for the successful reduction of NO? emissions in boilers. A typical pulverization process includes drying, grinding, classification (sizing), and selection of a system for feeding the fuel to the burner. Each of these stages is influenced by the physical properties and quality of the fuel.
The most common grindability test for coals is the Hardgrove grindability index (HGI), which was chosen for this study to investigate the grindability of the hydrochar. This method is used to predict the capabilities, performance, and energy requirements of the mill as well as the typical particle size distribution after milling. This is regarded as an important parameter for fuel quality and is used in coal contract specification. The test, as described in the British Standard 1016–112:1995, involves grinding 50 g of air-dried coal with a fixed particle size distribution between 600 µm and 1.18 mm for 60 revolutions in a machine designed for Hardgrove grindability testing. The ground samples are then sieved through a 75 µm sieve.
For this study, first, the calibration graph was plotted using four standard reference sample coals with known HGI values. Once this graph was obtained, the proportion of the sample material passing through the 75 µm sieve was measured and plotted on the calibration curve, from which the HGI was determined. Generally, higher HGI values mean that the fuel is easier to grind, so it has lower grinding power requirements and produces a higher output of fuel in the mill and through to the boiler.
An experimental study regarding the upgrading of EFB as a sustainable solid fuel was successfully carried out, and a thorough analysis was done to assess the characteristics of the thermochemically treated biomass as solid fuel. The results showed that the hydrochars produced from the HTT could potentially be a substitute for coal or, as in this case, it has the possibility of being co-combusted with coal without any significant modification to the boiler. It was found that commercial-scale hydrothermally treated and water-washed hydrochar from EFB had a higher HHV (23.269 MJ/Kg) which was equal to Indonesian coal (23.997 MJ/Kg), but it also came with higher sulfur content. Moreover, HTT offered lower ash content than Indonesian coal and lower chlorine content than raw EFB, which promotes less fouling of combustion boilers.
A modified HGI test was also successfully employed, and the grindability test further suggested that the hydrochar from HTT had significantly better HGI than the parent biomass. It was also found that the HGI value of the treated EFB was to some degree higher than the highest HGI for coal. Moreover, this method requires approximately 10 times less work than with the parent biomass, and the particle distribution profile was significantly improved by employing HTT. This indicates that HTT has a strong potential as a method to produce renewable solid fuel from biomass wastes, especially EFB. Furthermore, this research could also promote future works in the area of thermochemical treatments for producing sustainable solid fuels to potentially replace coal.
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