Torrefaction of Oil Palm Empty Fruit Bunches Pellets under Mild-to-Severe Temperature Conditions through Thermogravimetric Analysis

Malaysia generates significant quantities of lignocellulosic wastes through the production of crude palm oil (CPO). Over the years, the accumulation of the oil palm wastes (OPW) have become an environmental burden. These problems can be addressed by pretreatment and valorisation of OPW in bioenergy as envisioned in the National Biomass Strategy (NBS2020). However, current strategies for the OPW valorisation are inefficient and unsustainable resulting in increased environmental challenges. Therefore, this paper proposes the pelletization and torrefaction of oil palm empty fruit bunches (OPEFB). Furthermore, the thermal degradation behaviour and potential product yields from OPEFB pellet torrefaction will be examined. The results revealed that the mass yield (MY) decreased from 67.89% to 33.11%, whereas energy yield (EY) decreased from 88.29% to 49.18% as the torrefaction temperature increased from 250 °C to 350 °C. However, the energy density (DE) increased from 1.30 to 1.49 due to the increase in higher heating value (HHV) from 22.85 MJ/kg to 26.10 MJ/kg. Likewise, the severity factor (SF) increased from 5.89 to 8.84 with increasing torrefaction temperature. The results also revealed that effect of temperature on the torrefaction parameters; MY, EY, DE, and HHV are slightly reduced after 300 °C. Overall, the findings demonstrate that torrefaction improved the fuel properties and energy recovery potential of the OPEFB pellets.


Introduction
The oil palm industry in Malaysia accounts for 31% of global crude palm oil (CPO) [1].
However, the production of CPO generates significant wastes which currently pose waste management and environmental challenges.Consequently, the National Biomass Strategy (NBS-2020) was ratified by the government of Malaysia (GoM) to stimulate the cost-effective and efficient valorisation of 20 million tons of OPW annually through various thermochemical or biorefinery techniques [2].
Conversely, pretreatment technologies such as pelletization and torrefaction can improve the solid biofuel properties of OPWs thereby addressing the outlined operational challenges.
Currently, the most widely used technique is combined Torrefaction and Pelletization, or the TOP process [15].Studies have shown that the degradation of hemicellulose and cellulose during torrefaction disrupts the inter-particle bonding thereby resulting in brittle, friable and poor quality pellets [16].Alternatively, pelletization before torrefaction can potentially address the attendant problems of the TOP process.
Therefore, this paper seeks to examine the effect of torrefaction on pelletized Oil Palm Empty Fruit Bunches from mild to severe temperature conditions ranging from 250 °C to 350 °C.
It will examine reaction conditions, thermal degradation behaviour and potential product yield from the OPEFB pellet torrefaction through thermogravimetric analysis (TGA).The technique is a simple, timely and reliable approach widely applied to simulate and examine the thermochemical biomass conversion processes such as torrefaction and pyrolysis in the literature [17][18][19].

Experimental
The OPEFB pellets examined in this study were acquired from an oil palm mill in Johor, Malaysia.The physicochemical, thermal, kinetic and calorific fuel properties of the OPEFB pellets are presented in our previous study [20].In this study, torrefaction was performed by TGA under nitrogen gas flow of 50 mL/min.For each run, approximately 10 mg of the pulverised OPEFB pellets of particle size 250 µm was heated from room temperature to the selected torrefaction temperatures; 250 °C, 275°C, 300 °C, 325 °C and 350 °C for a holding time of 30 mins.In the end, the mass loss and derivative mass loss data were retrieved and plotted against time in Microsoft Excel (version 2013) to obtain TG and DTG plots, respectively.Next, the degree of torrefaction was examined based on the parameters; mass yield (MY), energy yield (EY), energy density (DE), higher heating value (HHV), and severity factor (SF) in equations 1 -5; ℎ , = 19.85+ 9.34 × ( , %) , = log × exp − 14.75 The terms; m, t, Th and Tr in Equations 1 -5; represent the mass of torrefied or raw biomass, time (minutes), reaction (torrefaction) temperature and reference temperature (100 °C), respectively.Consequently, the parameters; MY, EY, DE, HHV, and SF were estimated from the residual mass (RM) and mass loss (ML) during the TGA torrefaction of the pellets.

Results & Discussion
The TG and DTG plots for the torrefaction of OPEFB pellets are presented in Figures 1 and   2, respectively.The results revealed that mass loss increased with increasing temperature during the torrefaction of the OPEFB pellets.The temperature dependency of torrefaction is due to its effect on the decomposition of lignocellulosic biomass components.The decomposition of biomass during torrefaction can be ascribed to the degradation of hemicellulose, and partial decomposition of cellulose and extractives.As a result, the high mass loss (ML) and decrease in residual mass (RM) during the OPEFB pellets torrefaction can be ascribed to the loss of holocellulose and extractives.
Table 1 presents the residual mass (RM) and mass loss (ML) during the torrefaction of OPEFB pellets.As observed, the mass loss increased progressively whereas the residual mass decreased during torrefaction of OPEFB pellets from 250 °C to 350 °C.Since the residual mass (RM) is the mass of the fuel remaining after the process, it can be termed the solid product or mass yield (MY) of torrefaction.Likewise, the mass loss (ML, %) is an indication of liquid and gas products of torrefying the OPEFB pellets.Based on the results in Table 1, it can be surmised that torrefaction of the fuel above the 300 °C does not result in significant change in mass loss or residual mass.Therefore, the torrefaction of OPEFB pellets should be limited between 250 °C and 300 °C.
The DTG plots for the torrefaction of OPEFB pellets are presented in Figure 2. Similar to the trends observed in the TG plots, the size of the DTG plots increased with increase the severity of torrefaction temperature.As observed, the size of DTG curves increased proportionately from 250 °C to 300 °C with increase in the torrefaction temperature.However, the change in the size of the plots did not differ markedly after 300 °C.This may somewhat suggest that temperature did not significantly influence the process beyond 300 °C.However, this requires further tests particularly at holding times or torrefaction temperatures higher than 30 minutes and 350 °C, respectively.
Next, the mass yield (MY), energy yield (EY), energy density (DE), higher heating value (HHV), and severity factor (SF) were estimated from the residual mass (RM) and mass loss (ML) during the TGA torrefaction of the pellets as presented in Table 2.The heating values of the torrefied OPEFB pellets were in the range reported for subbituminous and bituminous coals.Hence, the torrefied fuels be termed bio-coal due to their similarities with various ranks of coals.Overall, the indicate torrefaction improved the fuel properties and energy recovery potential of the OPEFB pellets.

Conclusion
The study examined the torrefaction of OPEFB pellets through thermogravimetric analysis (TGA) from 250 °C to 350 °C or under mild to severe conditions.The results revealed that the mass yield (MY), and energy yield (EY), decreased whereas the energy density (DE), higher heating value (HHV), and severity factor (SF) increased at higher temperatures.However, the results showed that effect of temperature on the torrefaction parameters was somewhat diminished after 300 °C.Nonetheless, torrefaction improved the fuel properties and energy recovery potential of the fuel.

Table 1 :
Time Profiles and Residual Mass for OPEFB Pellet Torrefaction