Characterisation of biochar from various carbon sources

. The purpose of this study was to characterise biochar produced from various carbon sources. This study was conducted using a Nested De sign with three replications. Three carbon sources, rice husk (RH), corn co bs (CC), and bagasse sugarcane (BS) were pyrolysed for 2 hours at temper atures of 400, 500 and 600°C. The three types of biochar were then analyse d on their moisture content, ash content, fixed carbon content, volatile matt er, calorific value, particle size and elemental analysis results. The results o f this study were that the temperature of pyrolysis at 600 °C can optimally produce biochar with the lowest moisture content, ash content and volatile matter value, and highest fixed carbon and calorific values. The particle siz e analysis shows that biochar produced using this optimum condition has t he smallest average particle size distribution. The elemental analysis condu cted through Scanning electron microscopy with energy dispersive X-ray s pectroscopy (SEM-EDX) shows various elements in each biochar produce d from the three different carbon sources.

Abstract.The purpose of this study was to characterise biochar produced from various carbon sources.This study was conducted using a Nested De sign with three replications.Three carbon sources, rice husk (RH), corn co bs (CC), and bagasse sugarcane (BS) were pyrolysed for 2 hours at temper atures of 400, 500 and 600°C.The three types of biochar were then analyse d on their moisture content, ash content, fixed carbon content, volatile matt er, calorific value, particle size and elemental analysis results.The results o f this study were that the temperature of pyrolysis at 600 °C can optimally produce biochar with the lowest moisture content, ash content and volatile matter value, and highest fixed carbon and calorific values.The particle siz e analysis shows that biochar produced using this optimum condition has t he smallest average particle size distribution.The elemental analysis condu cted through Scanning electron microscopy with energy dispersive X-ray s pectroscopy (SEM-EDX) shows various elements in each biochar produce d from the three different carbon sources.

Introduction
Biochar, a carbon-rich product derived from the pyrolysis of organic materials in the absenc e of oxygen, has garnered considerable interest for its potential role in carbon sequestration, soil amendment, and as a renewable energy source.The process of biochar production and its resultant properties are influenced by the type of feedstock used and the conditions unde r which pyrolysis is conducted.Recognizing this, the present study aims to provide a compr ehensive characterisation of biochar produced from a variety of carbon sources.
Pyrolysis, a thermochemical decomposition process, is conducted in an oxygen-depleted environment at high temperatures, leading to irreversible changes in the physical state and chemical composition of organic materials.This process diverges from perfect combustion by producing distinct compounds due to the lack of oxygen.Typically, pyrolysis yields syn gas (including hydrogen and carbon monoxide), methane, short-chain hydrocarbons, carbon dioxide in the gas phase, aromatic bio-oil in the liquid phase, and biochar as a solid [5].Th e process aims to rapidly apply heat to disrupt the chemical bonds in polymers from cellulo se, hemicellulose, and lignin within the organic material [6].
Pyrolysis has emerged as an effective technology for managing waste, particularly agric ultural and agro-industrial residues.By converting these wastes into biochar and liquid smo ke, pyrolysis addresses environmental issues while producing valuable by-products for use i n energy, food, agriculture, and plantation sectors [7].Embodying biorefinery and zero was te principles, this method ensures complete raw material conversion without generating was te [8].Though generally conducted at temperatures above 300°C for several hours, specific conditions may vary based on the method and materials used [7].Lignocellulosic compone nts in biomass, such as cellulose and hemicellulose, decompose at 180-350°C, while lignin requires 300-500°C.Water content decreases at temperatures over 200°C, affecting the ove rall pyrolysis process [9].
Pyrolysis temperature plays an essential role in influencing the characteristics of the acti vated biochar produced.The surface area and porosity characteristics of biochar are vital thi ngs that are influenced by the temperature used in pyrolysis [10].Other research by [11] als o revealed that pyrolysis temperature is the main factor that very dominantly influences the stability of biochar compared to the chemical and mineral composition of biomass.Apart fr om influencing the characteristics of biochar, the pyrolysis temperature also determines ene rgy requirements, which will impact production costs.In the pyrolysis process, the use of th e right temperature must be carefully taken into consideration [12].Therefore, using three d ifferent carbon sources in this study, the characteristics of biochar produced in various temp eratures must be investigated.

Materials and methods
The rice husk (RH), corn cobs (CC), and bagasse sugarcane (BS) were obtained from Mala ng, East Java, Indonesia.A pyrolysis unit and a charcoal milling unit are among the tools ut ilised.The materials were dried using a tunnel dryer at 50°C for 4 hours before pyrolysis.Fi g. 1 shows the schematic pyrolysis unit.500 g of each material was added to the pyrolysis a pparatus.The slow pyrolysis procedure considered the best way to produce biochar, was car ried out at three different temperatures (400 °C, 500 °C, and 600 °C) for two hours.Biochar and pyrolysis oil were the products of each pyrolysis phase.Each sample of the biochar ma de at various temperatures was then characterised by the moisture content, ash content, fixe d carbon content, volatile matter, calorific value, particle size and elemental analysis result s.

Moisture content
The moisture content of each type of biochar produced at different temperatures is shown in Fig. 2. Generally, the moisture content of all biochar was under 10%.The moisture content decreased as the temperatures increased.According to the Indonesian National Standard, th e maximum moisture content of biochar is 10%.Therefore, the moisture content of all bioc har produced was within the national standard for moisture content.
Increasing pyrolysis temperatures leads to a reduction in moisture content due to several t hermal and chemical processes [13].At higher temperatures, water within the biomass rapi dly evaporates as the heat energy supplied exceeds the binding energy of the water molecul es, turning them into steam.This process is enhanced as temperatures rise, facilitating a qui cker and more complete evaporation.Additionally, the chemical structure of the biomass un dergoes decomposition at elevated temperatures.This breakdown involves cleaving chemic al bonds, such as hydroxyl groups, which can release water as a by-product [14].Alongside evaporation and chemical decomposition, the volatilization of various other compounds als o occurs.These volatile substances, which can include water, are released as gases into the atmosphere.Furthermore, the changes in the physical structure of the biomass at higher tem peratures can decrease its ability to hold onto water, as it may become more hydrophobic [1 5].This inherent change in the material's properties thus contributes to the overall decrease in moisture content.

Ash content
The ash content of each type of biochar produced at different temperatures is shown in Fig. 3. Generally, the ash content of all biochar was in the range of 16.00 -21.17%.The highest ash content was observed in biochar made from RH.The effect of temperature on ash cont ent is a complex interaction of thermal decomposition, volatilization, mineral transformatio n, and selective retention of constituents [16].The exact outcome will depend on the specifi c types of biomass and their inherent mineral makeup.BS, RH and CC chemical compositi on determine how much the ash content produced.For example, rice husk contains a high le vel of silica, which may not volatilize at the same rate as other minerals.This could explain why the reduction in ash content with increasing temperature may not be as significant for r ice husk as it might be for other substrates with different mineral compositions.

Fixed carbon content
The fixed carbon content of each type of biochar produced at different temperatures is sh own in Fig. 4. Generally, the fixed carbon content of all biochar was in the range of 49.14 -61.08%.The highest fixed carbon content was observed in biochar made from BS.As the p yrolysis temperature increases, the fixed carbon content in the biomass samples are also inc reases.This is due to the enhanced thermal decomposition that occurs at elevated temperatu res, where the heat breaks down the biomass more thoroughly.As volatile substances, inclu ding water vapor, carbon dioxide, and various organic gases, are driven off, the proportion of carbon in the residue becomes more concentrated.This process, known as devolatilizatio n, leads to an increase in the fixed carbon content [17].Concurrently, the higher temperatur es promote carbonization, which is the conversion of the remaining organic material into a char that is rich in fixed carbon.
The structure of the carbon also changes during this process, becoming more ordered.Th is transition, moving from amorphous carbon towards more aromatic and graphitic structure s, results in a stable and high fixed carbon residue [18].The variations in the increase acros s different biomasses-RH, CC, and BS-can be attributed to their unique compositions an d structures.For instance, rice husks have high silica content, which can affect the carbon y ield, whereas the fibrous nature of Bagasse might facilitate a more complete carbonization.Fig 4 clearly shows that, irrespective of the initial differences in their composition, all three types of biomass tend to yield higher percentages of fixed carbon as the pyrolysis temperatu re is ramped up from 400°C to 600°C.

Volatile matter
The volatile matter of each type of biochar produced at different temperatures is shown in F ig. 5. Generally, the volatile matter of all biochar was in the range of 18.40 -23.94%.The h ighest volatile matter was observed in biochar made from RH at all temperatures tested.For all three biomass sources, there is a general trend of decreasing volatile matter with increas ing temperature.This is because higher temperatures cause more extensive thermal decomp osition of the organic components within the biomass, leading to greater devolatilization [1 7].As a result, a larger fraction of the biomass is converted into gas and vapor, which is the n released, leaving behind a char with a higher proportion of fixed carbon and ash.
The rice husk (RH) starts with the highest volatile matter content at 400°C, which signif icantly decreases as the temperature rises.This suggests that rice husk has a high proportion of components that are susceptible to thermal degradation.In contrast, the corncob (CC) sh ows a more moderate reduction in volatile matter across the temperature range, indicating t hat it may contain more thermally stable components or a different composition of volatile substances.Bagasse sugarcane (BS) exhibits a behavior similar to RH, with a notable decre ase in volatile matter as temperature increases, again reflecting its composition and the reac tivity of its volatile components.
It is also noteworthy that the impact of temperature on the reduction of volatile matter a ppears to be more pronounced between 400°C and 500°C than between 500°C and 600°C f or all substrates.This could imply that a significant portion of the volatile matter is released at the lower end of the temperature spectrum, and that the remaining material is more ther mally stable [19].

Fig 5.
Volatile matter of biochar from RH, CC and BS produced at 400, 500 and 600°C.

Calorific value
The calorific value of each type of biochar produced at different temperatures is shown in F ig. 6.Generally, the volatile matter of all biochar was in the range of 4415.2 -7727.8cal.T he caloric values for biochar from rice husk (RH), corncob (CC), and Bagasse sugarcane (B S) vary significantly across different pyrolysis temperatures (400°C, 500°C, and 600°C).In itially, at 400°C, the caloric values for all three biomass sources are relatively lower, with R H showing the lowest energy content.This might be due to incomplete pyrolysis at this tem perature, leaving behind a higher proportion of volatile matter which generally has a lower energy content compared to fixed carbon.As the temperature increases to 500°C and further to 600°C, the caloric values for the bi ochars generally increase.This trend suggests that higher pyrolysis temperatures lead to a h igher degree of thermal decomposition, which reduces the volatile matter and increases the fixed carbon content [20].Since fixed carbon has a higher caloric value than volatile compo nents, this results in a higher energy content in the biochar.
The biochar from CC shows the highest increase in caloric value as the temperature rise s, indicating that the constituents of corncob may be particularly amenable to forming high- energy compounds at elevated temperatures.Meanwhile, the caloric value of BS biochar als o increases with temperature but seems to plateau between 500°C and 600°C, suggesting th at the optimal temperature for maximizing its energy content might be around 500°C.
The caloric value of RH biochar shows an interesting trend: it starts the lowest at 400°C, surpasses CC at 500°C, and remains relatively stable at 600°C.This could be attributed to t he specific composition of rice husks, which may undergo significant structural changes lea ding to a high caloric value at an intermediate temperature but does not benefit as much fro m further temperature increases.

Particle size
The particle size of each type of biochar produced at different temperatures is shown in Fig. 7. Generally, the particle size of all biochar was 24.24 -53.39%.The smallest particle size was observed in biochar produced at 600•C.Rice husk (RH) biochar has the largest particle size at 400°C, which significantly decreases as the pyrolysis temperature is increased to 60 0°C.This decrease may be due to the breaking down of the physical structure of the rice hu sk under the influence of higher temperatures, causing a reduction in the size of the particle s.Corncob (CC) also follows a similar trend, starting with larger particles at 400°C that bec ome smaller at elevated temperatures.This indicates that the structural integrity of the corn cob is also compromised at higher temperatures, leading to smaller particles.Bagasse sugarcane (BS), on the other hand, shows a less pronounced decrease in particl e size with an increase in temperature.It starts with a smaller particle size compared to RH and CC at 400°C and decreases slightly at 500°C, but then shows a more noticeable reducti on at 600°C.The initial smaller size could be due to the fibrous nature of bagasse, which m ay already be in a form that lends itself to smaller particles post-pyrolysis.However, it still exhibits a reduction in size at the highest temperature, likely due to further thermal degradat ion of the material.
Elevated pyrolysis temperatures generally result in smaller biochar particle sizes across different biomass types [21].This trend could be attributed to the intensification of thermal degradation processes at higher temperatures, which leads to a breakdown of the larger bio mass particles into smaller fragments.The specific rates of size reduction and the initial siz es vary depending on the structure and composition of the original biomass material [22].T hese changes in particle size can impact the surface area and porosity of the biochar, where smaller particles might be preferred for their greater surface area and higher reactivity.

Elemental analysis results
The elemental analysis results of each type of biochar produced at different temperatures ar e shown in Table 1-3.The general trend of carbon content increasing with temperature up t o a point before decreasing or plateauing, which corresponds with the removal of volatiles a nd increased carbonization.Oxygen generally decreases as pyrolysis temperature increases, indicating the loss of oxygenated compounds.The presence and variation of mineral eleme nts suggest changes in the inorganic component of the biochar, which can affect its properti es and potential applications.The deviations indicate the nature of an EDX analysis based o n the selected spots, which can be a general guideline of the actual contents.

Fig 3 .
Fig 3. Ash content of biochar from RH, CC and BS produced at 400, 500 and 600°C.

Fig 4 .
Fig 4. Fixed carbon content of biochar from RH, CC and BS produced at 400, 500 and 600°C.

Fig 7 .
Fig 7. Particle size of biochar from RH, CC and BS produced at 400, 500 and 600°C.

Table 1 .
Results of elemental analysis of biochar from rice husk produced at 400, 500 and 600°C.

Table 2 .
Results of elemental analysis of biochar from corn cob produced at 400, 500 and 600°C.

Table 3 .
Results of elemental analysis of biochar from bagasse sugarcane produced at 400, 500 and 6 00°C.