Biochar recovery, Quality parameters, Pyrolysis temperature, Simarouba biomass-waste, Nutrient content.

Simarouba (Simarouba glauca DC.) belonging to family Simaroubaceae, is a multipurpose evergreen tree. The tree thrives in warm, humid tropical climates with temperatures ranging from 10 to 40 C and annual rainfall ranging from 500 to 2200 mm. It can grow at elevations from mean sea level to 1000 m above mean sea level in all types of well-drained soil with pH range between 5.5-8.0 and minimum soil depth of 1 m. The tree can grow well even in the degraded soils.  The tree grows to a height of 12 to 15 m tall and has a crown spread of 7.6 to 9.1 m wide. The average yield of fruit from a hectare of a 10-year-old plantation of Simarouba is about 6000 to 8000 kg (Joshi and Hiremath 2001). Simarouba seed consists of 71 per cent woody seed coat and 29 per cent kernel. The seed kernel has higher non-edible oil content (61.04%) which makes it an appropriate candidate for biodiesel production (Dash et al., 2008). After recovery of kernels for biodiesel production, seed shells (containing no oil) are obtained in huge quantity which are considered as waste. Eco-friendly technologies like combustion, torrefaction, gasification, and pyrolysis, convert crop residues into biochar (Basu, 2018). The most popular, easy-to-use, and efficient method among the several treatments is pyrolysis that produces three products: solid-biochar, liquid-bio-oil, and gas-syngas.

Biochar is a carbon-rich substance produced by pyrolysis of agricultural or forest biomass characterized by its porous structure, higher stability, and capacity to enhance soil quality and sequester carbon. By using biochar, agriculture can shift from being a net carbon emitter to a way of restoring carbon stocks in the soil. This is because biochar has a durable and robust structure that makes it more resistant to biochemical processes. Consequently, it can persist in the soil for extended durations, spanning for several years. Biochar improves soil pH, available nutrients, and soil organic matter content. Due to greater cation exchange capacity (CEC), biochar efficiently prevents nutrient leaching and retains most of the essential nutrients. Biochar itself contains various nutrients and trace elements essential for crop growth. Biochar due to its physio-chemical properties stimulates root development of plants, leading to improved nutrient and water uptake, which ultimately contributes to improved crop growth and yield.

Quality of the biochar with respect to its physical parameters and nutrient content depends on many factors such as feedstock type, feedstock moisture content, pyrolysis temperature, pyrolysis time, heating rate, particle size and other pyrolysis process parameters which play important role in deciding recovery and quality of biochar. Temperature during pyrolysis was found to have a substantial impact on stability and physicochemical properties of biochar, however heating duration during biochar synthesis did not have any significant effect on the properties of biochar (Zhang et al., 2015). Changes in the structure and physicochemical characteristics of biochar are closely linked to the temperature at which pyrolysis takes place (Jindo et al., 2014). Pyrolysis temperature can decide a wide range of pH, electrical conductivity, maximum water holding capacity, bulk density, available nutrients, and total carbon content of biochar (Chan et al., 2008). So, the study aimed to understand the relationship between pyrolysis temperature and biochar quality with respect to its agricultural application which is essential for optimizing biochar production processes. 

Feed-stock collection and preparation. Simarouba seed coat i.e., feedstock for pyrolysis was collected from Biofuel Park, Agricultural Research Station, Madenur, Hassan, Karnataka, India. The feedstock was shade dried until moisture content of the coats was reduced up to 6.28 per cent (i.e., below 10 per cent, is ideal for pyrolysis). Then the seed coats were shredded to 0.2-2 mm size using mechanical biomass shredder and used for biochar production.

Biochar production. In a fixed-bed (batch) slow pyrolysis reactor, slow pyrolysis of Simarouba seed coat was performed at three different pyrolysis temperatures ranging from 300 to 500°C with a constant heating rate, 10°C min-1. The residence time was fixed for four hours and the material was run six times at three different temperatures that made up to a total of 18 runs. The treatment details were given by producing biochars at three different temperatures- 300, 400 and 500 °C and named as BC300, BC400, and BC500 respectively.

Biochar recovery analysis. The biochars obtained were weighed using an electronic weighing scale and their yield percentages were calculated using the formula (Sahoo et al., 2021), 

Biochar yield (%) = Mass of biochar (kg) / Mass of the raw material (kg) × 100

Bio-oil yield was measured in terms of litres and converted into kilograms by multiplying the value by its density (1.2 kg m-3). The syngas yield was calculated by using formula (Irfan et al., 2016)

Syngas yield (kg) = 100 – [Biochar yield (kg) + bio-oil yield (kg)] 

Biochar quality analysis. Biochars produced at different temperatures were ground and passed through a 0.2 mm sieve and used to analyse their physiochemical properties, including bulk density (BD), maximum water holding capacity (MWHC), pH, electrical conductivity (EC), and nutrient analysis. Standard procedures used for the analyses are given below.  

Proximate analysis of biochars. Proximate analyses of biochars for estimation of moisture content, volatile matter, ash content and fixed carbon was carried out as per ASTM D1762-84 method suggested by International Biochar Initiative (IBI), (Igalavithana et al., 2017).

Physiochemical analysis of biochars. For determination of pH and EC of biochar samples, method suggested by IBI was employed (Igalavithana et al., 2017). While for determination of biochar bulk density and maximum water holding capacity, Keen's cup method (Piper, 1966) was used.

Nutrient analysis of biochars. To determine the total carbon content of the biochars, the dry combustion method (CHNS, LECO) was used (Page et al., 1982). The biochar samples were tested for nitrogen (N) content by using Kjeldahl digestion & distillation method, for phosphorus (P) content by following diacid digestion and Vanadomolybdate phosphoric yellow colour method and for potassium (K) content by diacid digestion and flame photometer method (Jackson, 1973).

The Versenate titration method, as outlined by Jackson (1973), was used to determine the calcium (Ca) and magnesium (Mg) content of the di-acid digested biochar samples. As described by Piper (1966), the turbidometry method was used to detect the sulphur (S) level at a wavelength of 420 nm using a spectrophotometer.

Statistical analysis. The data on various parameters recorded during the investigation were tabulated and subjected to statistical analysis using one-way ANOVA. The test of significance (‘F’ test) and critical difference (CD) were read at 0.05 probability (Panse and Sukhatme, 1967). Wherever ‘F’ test was found significant, the ‘t’ test was performed to estimate critical differences among different treatments.

Biochar recovery. The feedstock was slow pyrolyzed at three different temperatures (300, 400, and 500ºC) in a fixed-bed (batch) pyrolysis reactor. The biomass underwent pyrolysis, producing three distinct products: solid biochar, liquid bio-oil and a gas called syngas. The change in pyrolysis temperature showed a significant impact on yield of all the three pyrolysis products. The yield of these three products were measured in terms of kilograms (kg) and the data pertaining to biochar recovery is given in Table 1. By taking reference of obtained quantities of these products and feedstock used for pyrolysis cycle, assessed recovery percentage are given in  Fig. 1.

The yield of biochar was significantly reduced as the pyrolysis temperature increased. Biochar production through slow pyrolysis yielded 10.16 kg of biochar at 300 °C from 13 kg of feedstock (Table 1), which was equivalent to 78.15 per cent of the feedstock weight (Fig. 1). At temperature 400°C, the biochar yield was decreased to 7.54 kg (58%), and at 500°C, it further decreased to 5.10 kg (39.23%). The results of current findings were in accordance with Elnour et al. (2019), reported that the yield of biochar from date palms decreased from 43 to 32 per cent with rise in pyrolysis temperature from 300°C to 700°C. Similar trend was reported by Suliman et al. (2016); Kim et al. (2012).

Table 1: Quantity of pyrolysis products obtained from Simarouba seed coats at different temperatures through slow pyrolysis.

Fig. 1. Recovery percentage of pyrolysis products obtained from Simarouba seed coats at different temperatures.

Unlike the biochar yield, there was a substantial increase observed in yield of bio-oil and syngas. Specifically, at 300 °C, 1.71 kg of bio-oil, equivalent to 13.15 per cent (by weight), was obtained. This yield increased to 4.00 kg (30.77%) at 400 °C and further to 6.00 kg (46.15%) at 500 °C. While the syngas yield was 1.13 kg (8.7%) at 300 °C, 1.46 kg (11.23%) at 400°C, and 1.90 kg (14.62%) at 500 °C, showing a significant rise with increasing temperature. The thermal breakdown of cellulose, hemicellulose, and lignin into syngas and bio-oil at higher pyrolysis temperatures results in the reduction of biochar yield and increase in bio-oil and syngas yield (Yue et al., 2016). Al Arni (2018) pyrolyzed sugarcane bagasse at three different temperatures (480, 580 and 680 °C) and concluded that, by increasing temperature, yield of syngas increased for slow pyrolysis. 

Proximate analysis of biochars.   Proximate analysis includes determining the amount of moisture, volatile matter, fixed carbon, and ash in biochar. The proximate analysis found all these parameters statistically significant for the three biochars (Table 2). 

Moisture content in biochars ranged from 1.24 to 4.14 per cent, least of which was observed in BC500 (1.24%) and maximum in BC300 (4.14%). Further, Higher volatile matter content was observed in BC300, reaching 23.06 per cent. Conversely, lower volatile matter content was noted in BC500, measuring 8.64 per cent. There was a consistent decrease in volatile matter content with temperature increase, suggesting a gradual loss of volatile matter during the charring process. Zhang et al. (2015), reported that as pyrolysis temperature increased, biochar yield and volatile matter decreased.

Higher ash content (8.65%) was recorded in the BC500. In contrast, lower ash content (5.12%) was recorded in BC300. While in BC400, the ash content was intermediate (6.34%) to the two temperatures. The ash content of biochar serves as an indicator for the components of biochar that are neither volatile nor combustible, as reported by Angin and Sensoz (2014). With the elevation in temperature, it can be expected that the mineral concentration would rise, and there would be an increase in the detrimental volatilization of lignocellulosic substances. Consequently, this would lead to a higher ash content, as suggested by Tsaia et al. (2012). 

As the pyrolysis temperature rose, the fixed carbon content of the biochar increased significantly. It varied from 67.68 to 81.47 per cent. Higher fixed carbon was found in BC500 (81.47%), intermediate in BC400 (77.16%) and lower in BC300 (67.68%). Noor et al. (2019) found that when the temperature was elevated from 350°C to 600°C at a constant heating rate of 5°C min-1, the fixed carbon content increased from 45.20 to 79.09 per cent.

Table 2: Proximate analysis of biochars produced at different temperatures through slow pyrolysis.

Physicochemical analysis of biochars. pH. Table 3 shows that there was a significant increase in pH of biochar with rise in pyrolysis temperature. The pH of the biochar samples ranged from acidic to alkaline (6.64 to 8.67). BC300 showed lower and acidic pH of 6.64. In contrast, BC500 showed higher and alkaline pH of 8.67. While BC400 had nearly neutral pH (7.27). The alkalinity of biochar is primarily caused by rising pyrolysis temperatures that decreases acidic functional groups and increases alkali salts and functional groups (Mukherjee et al., 2011). Wan et al. (2014) showed that pH, carbonate content, base cation, and biochar alkalinity increased with temperature rise in pyrolysis. Biochars elevated pH was due to hydrolysis of the Ca, Mg, and K salts. An additional element leading to the elevation of pH with rise in pyrolysis temperatures was the proportional rise in ash content present within the biochar. Several studies have demonstrated a correlation between increased ash content and higher pH levels as the temperature of pyrolysis rises (Nwajiaku et al., 2018; Wakamiya et al., 2022; Novak et al., 2009). 

Electrical conductivity (EC). With rise in pyrolysis temperature, EC of biochar increased significantly from 0.97 (BC300) to 1.53 dS m-1 (BC500) (Table 3). Higher EC was seen in BC500 (1.53 dS m-1). The increase in EC of biochar was attributed to greater production of ash with rise in temperature and higher salt concentration of the produced ash (Nwajiaku et al., 2018). Similarly, Al-wabel et al. (2013) noted that the EC of biochar rose with an increase in pyrolysis temperature, as evidenced by EC values of 0.76 dS m-1 and 1.34 dS m-1 for biochar produced at 200 ºC and 400 ºC, respectively. 

Bulk Density (BD). Pyrolysis temperature showed a significant influence on biochar bulk density (Table 3). With rise in temperature, there was a significant reduction in bulk density indicating increased porosity of biochar. The MWHC of low-temperature pyrolyzed biochar types was reduced because of their lower pores, lower interconnectivity pores, and residual tar components that clog the biochar pores (Useviciute and Baltrenaite 2021). BC300 showed highest bulk density of 0.36 Mg m-3, BC400 showed lesser bulk density of 0.30 Mg m-3 and the lowest bulk density of 0.28 Mg m-3 was found in BC500. Dhar et al. (2022) observed that the bulk density exhibited a decline from 0.55 to 0.39 Mg m-3 with rise in pyrolysis temperatures from 350 °C to 600 °C.

Maximum water holding capacity (MWHC). All the three biochars produced at different temperatures showed higher value of MWHC ranging from 192.67 to 270.79 per cent (Table 3). There was a significant increase in water holding capacity of biochar with rise in pyrolysis temperature. Higher water holding capacity was seen in BC500 (270.79%) and lowest in BC300 (192.67%). Useviciute and Baltrenaite (2021); Yabi Gadi et al. (2023) reported that, biochars produced at lower temperature showed lower pores, inter-connectivity pores and more residual tar components blocking biochar pores which ultimately reduces its MWHC. 

Table 3: Physicochemical properties of biochars produced at different temperatures.

Nutrient analysis of biochars 

Total carbon (%). There was a significant increase in total carbon content of biochar with rise in pyrolysis temperature (Table 4). It ranged from 52.39 to 70.46 per cent. The BC500 showed higher carbon content (70.46%), followed by BC400 (62.82%), and lower in BC300 (50.39%). Zheng et al. (2013), noted that, when the temperature rose from 300 °C to 500 °C, the total carbon content of the biochar made from giant seeds increased from 65.26 to 78.61 per cent. This increase was attributed to increase in degree of structural modification and carbon stability due to carbonization reactions in biochar with increasing pyrolysis temperature.

Primary nutrient content of biochars. The biochar primary nutrient content showed significant variation with increase in pyrolysis temperature (Table 4). Biochar N content reduced from 1.25 (BC300) to 1.12 per cent (BC500) with rise in pyrolysis temperature. In BC400 the N content was 1.14 per cent. This might be because nitrogen starts to volatilize at relatively low temperatures, around 200 °C due to its association with many organic molecules. (DeLuca et al., 2015). Similar results were found by many studies (Zheng et al., 2013; Zhang et al., 2015; Yuan et al., 2016; Elnour et al., 2019;  Zhang et al., 2020). Biochar P content showed no significant variation among the three biochars, in contrast to N. 

Biochar potassium (K) content increased significantly with rise in pyrolysis temperature. The K content of biochar was 3.61, 3.81 and 4.58 per cent at temperature 300, 400 and 500 °C, respectively. These results of present study are in accordance with DeLuca et al. (2015); Manish et al. (2022). Who observed that, K concentration of the biochar rose as the pyrolysis temperature rose, and this could be because K is associated with various inorganic minerals, which require higher temperature (700 °C to 800 °C) to get volatilized. Further, Zheng et al. (2013) also reported that, K content of giant seeds biochar increased significantly as the temperature rose from 300 to 600 °C.

Table 4: Total carbon and primary nutrient analysis of biochars produced at different temperatures.

Table 5: Secondary nutrient analysis of biochars produced at different temperatures.

Secondary nutrient content of biochars. With rise in pyrolysis temperature, the calcium content increased from 0.95 (BC300) to 1.33 (BC500) percent (Table 5). Zheng et al. (2013) reported that, as the temperature rose from 300 to 600 °C, the Ca content of the biochar made from giant seeds significantly increased since Ca needs a very high temperature (above 1000 °C) to volatilize (DeLuca et al., 2015). However, as the pyrolysis temperature increased, the magnesium (Mg) and sulphur (S) content of the biochar did not change significantly.

Many studies have reported that with increase in pyrolysis temperature, plant nutrients in biochar become less available to plants (Tag et al., 2016; Yuan et. al., 2016 Zornoza et al., 2016). This could be the result of dehydration and decarboxylation decreasing the amount of ion exchange functional groups (Glaser et al., 2002; Shenbagavalli et al., 2023). Therefore, when compared to biochars produced at higher temperatures, biochars produced at lower pyrolysis temperatures can more effectively improve crop growth and yield.

Present study demonstrate to convert bulk agri-biomass waste as a resource. Generally these agri biomass was burnt in rural area causing environmental pollution and also resulted in reduced incorporation of organic carbon to soil. Pyrolysis technology converts biomass into biochar in the absence of oxygen with bi-products of bio-oil and producer gas as renewable eco-friendly energy sources. Biochar has wide application in soil stabilization by providing congenial condition in the soil for biochemical reaction to elevate soil health for better microbial activities with better soil physical conditioning for higher productive system.

Aditya K.T., Raghu H.B., Umashankar N., M.N. Thimmegowda and  M.  Mahadevamurthy (2024). Exploring the Quality of Biochar derived from Simarouba Seed Coat. Biological Forum – An International Journal, 16(5): 126-131.