The effect of pyrolysis temperature and ball-milling duration on characteristics of micro bio-char derived from oil palm empty fruit bunches

ABSTRACT Oil palm empty fruit bunch is a solid waste that accounts for 22% of palm oil production, but its utilization is still limited, so it needs further processing. One alternative for processing oil palm empty fruit bunch waste is using pyrolysis to produce biochar and syngas. The present research worked on producing micro biochar to increase the surface area by making smaller particles. Oil palm empty fruit can be converted into micro biochar with a ball mill. This study aimed to determine the effect of pyrolysis temperature and ball mill duration on the proximate characteristics and particle size distribution of oil palm empty fruit bunch micro-biochar. The research design was arranged using a Randomised Block Design with two factors: pyrolysis temperature (400°C, 500°C, and 600°C) and ball mill time (24 hours, 48 hours, and 72 hours). The micro biochar characterisation includes moisture content, ash content, bound carbon content (bound carbon), volatile matter, calorific value, and particle size distribution. The results showed that pyrolysis temperature and ball mill duration had an insignificant effect on water content. The heating value increased with increasing pyrolysis temperature. The carbon content slightly increased with pyrolysis temperature and ball mill duration. The temperature and duration of the ball milling influenced the particle size distribution. The higher the pyrolysis temperature and the ball mill duration, the smaller the average particle size distribution. The smallest particle size was 3.04 µm, pyrolyzed at 600°C and had a ball milling duration of 72 hours.


Introduction
Oil palm (Elaeis guineensis.) is an annual plant that grows in Indonesia and Malaysia.Oil palm is one of the largest plantation commodities in Indonesia.In 2017-2020 l, oil palm production in Indonesia continued to increase.In 2020, the total national output was 48.2 million tons, and the plantation area reached 14.8 million hectares (ha).The size of the oil palm plantation commodity has made Indonesia the largest producer and exporter of palm oil (CPO) in the world since 2006.This will also impact the increasing waste generated from the manufacture of palm oil (Ruml et al., 2022).
Processing of oil palm fruits produces 22% palm oil (CPO) and 5% kernel with solid waste, 6% shell, 11% fibre, and 22% oil palm empty fruit bunches (OPEFB; Yustinah Hidayat et al., 2019).The most significant production of solid waste from the palm oil industry is the OPEFB.The use of OPEFB is still limited and yet optimal, resulting in the biomass accumulation of OPEFB as waste.Therefore, it is necessary to process this waste as a functional product, one of which is biochar.The OPEFB can be pyrolyzed into biochar in a low-oxygen environment.
Pyrolysis is a thermal decomposition method used to extract liquid smoke from biomaterials, such as agroindustrial bio-wastes (Fernandez et al., 2022), quince waste (Torres-Sciancalepore, Fernandez et al., 2022), and rosehip seed waste (Torres-Sciancalepore, Asensio et al., 2022).Pyrolysis of biomass produces biochar and syngas that can be condensed into liquid.Biochar is more environmentally friendly because it has a lower sulphur content when compared to coal (Leng et al., 2022).However, biochar application is limited because the surface area is small, making the adsorption process less optimal.Therefore, the biochar's surface area can be increased by reducing the particle size into microbiochar.Micro-biochar has a larger surface area and absorption particles and smaller pore size than biochar.According to (Xiao et al., 2020) manufacture of micro-biochar uses a milling machine.
The present study used a ball mill to reduce the biochar particle size.The use of ball mills is preferred and effectively utilised (Amusat et al., 2021).(Naghdi et al., 2017) said that the manufacture of bio-char using a ball mill with a sonification process could reach a particle size of up to 60 nm with a speed parameter of 575 rpm for 1.6 hours, a ball ratio of 4.5 g/g and a temperature condition of 80°C.Biochar has a higher adsorption capacity than raw biochar, so that it can compete with commercial activated carbon.However, this sonication process is energy intensive.
The factors influencing the process of making micro biochar using a ball mill are rotational speed, ball mill time, the ball-to-powder mass ratio, product, and temperature.Several studies on manufacturing micro biochar, especially its characteristics, are still limited in the literature.Based on this, it is necessary to conduct further research on the characteristics of micro biochar from the pyrolysis of OPEFB waste for further utilization based on several factors.Therefore, in this study, a study was conducted on the effect of the pyrolysis temperature and time of ball mill for making micro biochar using a ball mill on the characteristics of micro bio-char of OPEFB.This study aims to determine the effect of pyrolysis temperature and ball mill time on proximate characteristics and particle size distribution of OPEFB micro-biochar and determine the best treatment for OPEFB micro-biochar.

Experimental
The empty oil palm fruit bunches (OPEFB) were obtained from Bandungrejo village, Bantur district, Malang, East Java, Indonesia.The equipment used includes a pyrolysis unit and a biochar milling unit.Before the pyrolysis process, the OPEFB fibres were dried using a tunnel dryer at 50°C for 4 hours.The schematic pyrolysis unit is presented in Figure 1.A 500 g dried OPEFB was put into the pyrolysis unit.The slow pyrolysis process, known as the best method for generating biochar, was carried out at 400°C, 500°C, and 600°C, each for 2 hours and repeated three times.Each pyrolysis process produced biochar and pyrolysis oil.Each sample of the biochar obtained from the different temperatures was put into a ball mill at a speed of 250 rpm for 24 hours, 48 hours, and 72 hours.Each treatment was repeated three times.After the ballmilling, the biochar was then characterized.OPEFB biochar characterisation includes moisture, ash, volatile matter, calorific value, carbon content, and particle size distribution.

Results and discussion
The properties of the selected biomass are displayed in Table 1.The OPEFB is composed mainly of cellulose.Compared to the other biomass, the cellulose content of OPEFB is the highest.The biomass's composition is reflected in the calorific value.OPEFB had the highest calorific value because it included the most cellulose and  the least ash and moisture.The moisture content of biomass is another factor that might impact the pyrolysis process.The carbon content is generally similar, except for rice husk.
The moisture content of dried OPEFB fibre before pyrolysis was 10-15%.The biochar's visual characteristics of pyrolyzed OPEFB are glossy black and do not look significantly different with different pyrolysis temperatures.However, the texture of the biochar pyrolyzed at 600°C was more brittle than the other temperatures.After the ball milling process, the colour of biochar pyrolyzed at 600°C was slightly darker.However, there was no significant difference between the texture and colour of the 400°C and 500°C.Good quality biochar has a strong black colour, which contains more carbon elements than its brown counterpart (Kane & Ryan, 2022).
Figure 2 shows the yield of biochar produced during the pyrolysis process.The biochar yield was calculated by comparing the mass of biochar obtained with the raw materials used.The yield of bio-char tends to decrease with increasing pyrolysis temperature.Figure 2 shows that at 400°C, the yield was nearly half of the OPEFB weight, but it gradually decreased to approximately 30% at 600°C.This is because material degraded better at a higher temperature.The lower yield occurs because the high temperature and longer pyrolysis time cause more significant weight loss (Fu & Yao, 2022).According to Yogalakshmi et al. (2022), the yield was reduced with increasing temperature because the compounds in OPEFB decomposed more completely into simpler compounds.At higher temperatures, biochar yield decreases due to the quick and complete degradation of lignocellulosic components, but at lower temperatures, the biochar yield is high due to incomplete degradation of the biochar (Selvarajoo & Oochit, 2020).
The effect of pyrolysis temperature and ball mill duration on biochar characteristics (water, ash, carbon, and volatile matter) is presented in Figure 3.The X-axis shows the pyrolysis temperature (400-600°C).The Y axis is the biochar characteristics.The symbols inside the figure show the ball mill duration for 24 hours, 48 hours and 72 hours.Figure 3 (a) shows that the water content was slightly decreased with increasing pyrolysis temperature.The ultimate and proximate analyses of char generally demonstrate that temperature has little impact on water content since the pyrolysis temperature is above the water boiling point (Lee et al., 2019).The effect of ball-mill duration was also insignificant.The physical process, such as milling, generally has little to no effect on water content.Figure 3(b) shows that the temperature and ball-mill duration do not affect ash content.The ash content of raw palm fibre is relatively low, around 2-3% (Selvarajoo & Oochit, 2020).Ash primarily consists of carbon, with trace amounts of other elements such as calcium, magnesium, potassium, and phosphorus that were not degraded during fuel burning or physical milling.Therefore, the various pyrolysis temperature or milling process does not affect the ash content.
Figure 3(c) shows that volatile matter slightly decreases with temperature.Aromatic hydrocarbons, short-and long-chain hydrocarbons, as well as a small amount of sulphur and chlorine, make up the most volatile matter.Tar condenses easily and can cause serious operational issues by clogging the piping and fouling the surfaces of downstream components if the gas temperature drops below the condensation point.Tar is created when volatile matter with longer hydrocarbon chains and higher boiling temperatures is present (T.Wang, 2017).Determination of volatile matter content aims to determine the compound content of volatile micro biochar.The water content of the material can also influence the levels of volatile matter, where the higher the water content, the higher the volatile matter content.According to a study using coconut pith, as the temperature increases from 300 °C to 900 °C, the yield, nitrogen and oxygen content, and volatile matter decrease (Yogalakshmi et al., 2022).In the pyrolysis stage, temperatures exceeding 600 °C caused the release of gases such as carbon monoxide, carbon dioxide, and methane as well as high molecular weight volatiles due to the breakdown of the highly bonded methyl group, CH 2 (S.Wang et al., 2009).
The total energy released as heat by the complete combustion of a biomass sample is known as the calorific value, typically expressed in terms of heat of combustion.Figure 3(d) shows that the calorific value is affected by pyrolysis temperature but not ball-milling duration.The higher the pyrolysis temperature, the higher the calorific value.The calorific value of biochar can be predicted using a proximate analysis of solid fuels, including moisture (M), volatile matter (VM), ash (Ash), and fixed carbon (FC; Qian et al., 2020).According to Jiang et al. (2018), the difference in the calorific value test results is mainly due to the carbon content of biochar.The amount of carbon atoms will affect the calorific value of biochar.
Figure 3 (d) shows the carbon content of the biochar.It was found that the carbon content increased with temperature.This explained the increase in the calorific value of biochar.The carbon content determines the value of pure carbon contained in the product.The higher the temperature, the higher the heat content value produced.The proportion of carbon content is also influenced by the ash content and volatile matter content contained in the material.Samer (2017) mentioned that the pyrolysis temperature affects the carbon concentration in biochar because its lignin content is decomposed more completely.The content of cellulose and lignin in the raw material affects the value of the carbon produced (Wu et al., 2022).The higher the ash content and volatile matter, reduce the level of carbon content and conversely, the lower the content of ash and volatile matter will increase the carbon content of a product.
Figure 3(e) shows that the particle size decrease with temperature and ball mill duration.The higher the pyrolysis temperature, the wider the surface area of the particles.Particle size is influenced by the duration of milling (Kamal et al., 2022).The longer the ball-milling time, the smaller the particle size produced, as shown in the 72 hours of the ball-milling result.The more milling time applied, the more the granules split into smaller sizes.However, the ball mill may reduce particle size but has difficulty producing a uniform particle size.The size uniformity can occur because of the friction intensity obtained by each particle.The number of balls affects the non-uniformity of the particle size distribution (Lopez-Tenllado et al., 2021).The more the number of balls used, the higher the intensity of the collision and the distribution of the particle size becomes more uniform (Lopez-Tenllado et al., 2021).However, apart from the number of pounding balls, the size of the ball also has a significant influence.The larger the size of the ball, the higher the grinding quality because the contact between the ball and the powder will be greater, but it will also accelerate the temperature rise in the ball mill because the energy generated is large.On the other hand, if the ball used is small, then the alloying process runs more optimally, and the energy produced is relatively small, so to maximize the milling process, it is necessary to use different balls (Rivera Madrid et al., 2014).
Figure 4 shows the effect of pyrolysis temperature on biochar after ball-milled for 48 hours at 400-600°C.Overall, the biochar surface looks more porous with increasing pyrolysis temperature.In the magnification of 1000x, it can be seen that there was a large chunk of biochar at lower temperatures, and the structure reduced with increasing temperature.In the magnification of 5000x, the pore size of biochar increases with increasing pyrolysis temperature.This is relevant to the pyrolysis of rice straw, as the increasing temperature increased the pore size, resulting in a smoother biochar surface (Torres-Sciancalepore, Asensio et al., 2022), At high-temperature heating, the biochar pores will open and cause the pore diameter to increase so that complex compounds, especially those inside the pores, are released (Yang et al., 2021).

Conclusions
The effect of pyrolysis temperature and ball mill duration on OPEFB micro biochar characteristics has been investigated.The effect of pyrolysis temperature was insignificant on the water content.The higher the pyrolysis temperature, the higher the heating value and carbon content from OPEFB produced.The ball mill duration did not significantly affect the proximate characteristics of the micro-biochar product.The temperature and duration of the ball milling influence the particle size distribution.The higher the pyrolysis temperature and the ball mill duration, the smaller the average particle size distribution due to the porous structure of the biomass.Future work on utilising the high surface area biochar is required, such as slurry fuel and soil amendment material.

3.
The effect of pyrolysis temperature and ball mill duration on (a) water content, (b) ash content, (c) volatile matter, (d) calorific value, (e) carbon content, and (f) particle size.

Figure 4 .
Figure 4.The effect of pyrolysis temperature on the surface of the biochar ball-milled for 48 hours.

Table 1 .
The physical and chemical characteristics of the OPEFB compared to the other biomasses