Effect of the non-uniform combustion core shape on the biochar production characteristics of the household biomass gasifier stove

. The global demand for biochar in agricultural and carbon sequestration applications is increasing; nevertheless, biochar production using the 50-liter household biomass gasifier stove (50L-HBGS) in Thailand found major issues that need to be improved. The objective of this study was to study the effects of the airflow in the non-uniform combustion core shape (NCCS) on the biochar production characteristic of the 50L-HBGS. The new design of the NCCS was constructed and studied to replace the existing combustion core shape (ECCS) at Mahasarakham University. The height, air inlet, and air outlet diameters of the NCCS were designed at 45, 24, and 11.4 cm, respectively. The NCCS with 21 holes of the pyrolysis gas outlet, a diameter of 4 mm for each, was integrated into the 50L-HBGS and performed comparative tests to the ECCS using 9 kg of bamboo wood chunks in three consecutive experiments. The airflow and the combustion behavior were studied through the stove temperature profiles, which were recorded every 5 minutes using a digital data logger. The biochar products were studied using the scanning electron microscope (SEM) with the energy dispersive x-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), and the proximate analysis technique. The study indicated that the 50L-HBGS with the NCCS made significantly improved the airflow rates in the combustion core, resulting in better continuous burning during the ignition state than with the ECCS. Moreover, the pyrolysis temperatures were significantly improved, it was provided temperatures during the pyrolysis process reached higher than 500 o C, resulting in the liquid tar being removed and no unburned wood chunks remaining at the end. The characterization result demonstrated that the 50L-HBGS with the NCCS had created biochar within a range of micropore and macrospore sizes and high fixed carbon content, which could be advantageously used for different agricultural and carbon sequestration applications.


Introduction
With the world moving toward carbon neutrality, biochar application is expanding beyond uses in agriculture and home heating (Nair et al., 2023;Kurniawan et al., 2023).Biochar also has potential usage as an adsorption material for carbon sequestration techniques (Gou et al., 2022;Rustamaji et al., 2022;Abdullha et al., 2023).According to Elkhlifi et al. (2023) and Qian et al. (2023), biochar is intentionally thermal converted organic material that is added to soil to enhance soil characteristics and increase carbon storage over the long term.Biochar can be produced from several sources of organic material feedstock, such as food waste, agricultural waste, macroalgae, etc. (Idris et al., 2021;Ibitoye et al., 2022;Chen et al., 2022), with variant techniques, for instance, pyrolysis, gasification, torrefaction, hydrothermal carbonization, etc. (Gabhane et al., 2020;Chen et al., 2020;Onokwai et al., 2022).However, the biochar converting method is commonly classified into two main methods, i.e., hydrothermal carbonization method and pyrolysis method (Liu et al., 2021).Whereas hydrothermally carbonization is suitable for moist feedstocks, pyrolysis is best for dry biomass (You et al., 2023;Chen et al., 2023).Even though pyrolysis is more complicated in production with the temperature range of 300-700 o C or above, and the process is gone under oxygen-limited conditions (Tomczyk et al., 2020), it provides biochar with a higher surface area, larger pore size, lower toxicity and more stable (high carbon-rich) than hydrothermal technique (Jian et al., 2018;Liu et al., 2021) which benefit for soil treatment and application as adsorption material (Muzyka et al., 2023;Kkaledi et al., 2023).The slow pyrolysis in the range of 350-600 o C is produces more biochar, while at temperatures higher than 700 o C (fast pyrolysis), the yield of the liquid and gas fuel is increased (Panyoyai et al., 2019;Petchaihan et al., 2020;Gabhane et al., 2020).At the commodity level, small pyrolysis stoves, such as the Anila stove, are popularly used to convert feedstocks into biochar (Pradana & Prasetya, 2017).The Anila stove has a couple with a small inner cylinder as the combustion core and a larger outer cylinder, a pyrolysis section, with tiny holes connecting the two sections.The stove can produce both heat for domestic cooking and biochar as a byproduct, with low emission dissipation (Smebye et al., 2017;Zahida et al., 2017), and has a CO2 sequestration ability of over 2,500 kg per year by estimates (Pradana & Prasetya, 2017).The Anila stove has been widely used on a household scale in India and Southeast Asia (Boateng et al., 2015;Carter & Shackley, 2011).Recently in Thailand, although the number of people using the 50L-HBGS (modified Anila stove) has increased, users always claim that it still has problems with stove production characteristics, which reflects the productivity of biochar (Maneekhat & Khamdaeng, 2022;Srisophon et al., 2020).The stove production characteristic problems concern; noncontinuous burning (Sittioad et al., 2022), needing more tendering during operation, and remaining tar and unburned biomass at the end, which is a similar problem to the Anila stove in Africa (Mosisa et al., 2019) that needs to be improved.
Recently, the development of the 50L-HBGS has focused on the appropriated combustion core puncher sizes, numbers, and bore positions in the core with the variant sizes of stoves and feedstocks (Petchaihan et al., 2020;Maneekhat & Khamdaeng, 2022;Srisophon et al., 2020;Panyoyai et al., 2019;Intagun et al., 2018).Even if previous studies revealed the relation of core puncher sizes and positions to the temperature distributions in biochar productions, however, no further method information points out methods for solving problems as stated above.
Since the studies only used the straight combustion core shape in the 50L-HBGS, the current research still lacks available information options of other combustion core types, which would be moderated by the problems above.Therefore, this research proposes to study application of the NCCS to the 50L-HBGS, which would be provide higher airflow rates, result in better continuous burning, higher pyrolysis temperatures, and better quality of biochar products.
For the reasons above, this research aims to improve airflow in the combustion core of the 50L-HBGS by using the nonuniform combustion core shape (NCCS) to replace the existing combustion core shape (ECCS), the most found combustion core type in the local market.The comparative test of both cases in the burning behaviors, pyrolysis processing temperatures, and quality of biochar products was done by using the ANOVA statistical technique, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and the proximate analysis technique in order to indicate the improvement of the biochar production characteristics by the 50L-HBGS.

Design of the NCCS
In order to make the 50L-HBGS user-friendly and to achieve higher biochar qualities, the NCCS in the 50-L HBGS was designed to increase the volume of the air inlet and the total heat transfer area, which should result in sufficient airflow for continuing combustion in the core.Also, it must be provided that the bottom stove's temperature reaches 500 o C, which is the optimum temperature for biochar production (Li et al., 2023;Ippolito et al., 2020) and sufficient to remove volatile matter and develop more pore structure (Balmuk et al., 2023;Basinas et al, 2023).Therefore, the new combustion core, the NCCS (Figure 1b) was made as a non-uniform tube, which had a larger air inlet diameter and a smaller air outlet diameter (number 2 in Figure 1c).It was made of an iron tube with a thickness of 4 mm and a height of 45 cm.The diameter of the air inlet section is 24 cm and then slopes downward to the same size as the air outlet section, which is 11.4 cm (number 3 in Figure 1c) at a length of 15 cm from the bottom of the core.It was bored with 21 points of 4 mm core puncture diameter.With this design, the overall volume of the core is 0.0098 m 3 , an increase of 114% compared to the ECCS.Furthermore, the overall surface area of the NCCS had expanded by 103.5% to 0.328 m 2 compared to the ECCS.
The ECCS was made of a straight iron tube 4 mm in thick, had a bottom bore with 16 points of 6 mm core puncture diameter (Figure 1a), was 45 cm high, and had an outer diameter of 11.4 cm.The outer walls of both cases were covered by ceramic sheet insulation.The NCCS and the ECCS were integrated into the 50L-HBGS and performed comparative experiments on the biochar production performance.

Stove Performance Analysis
In general cooking stoves, the water boiling test is applied to test the performance of the stoves.As indicated in the standard water boiling test (Chen et al., 2023;Ajieh et al., 2023), the stove process must be stopped after the water boiling.Since the 50L-HBGS process is different from a general combustion stove, it can not stop the stove process after water boiling.Therefore, the water boiling test was not applicable to this study.And, because the 50L-HBGS can produce both biochar and pyrolysis gas, the percent yield of biochar (%BC) and the percent of produced gas (%PG) were used to compare the difference of stove performance when using the NCCS and the ECCS.The following equation was used to determine biochar's yield from the stove.

%𝐵𝐶 = 𝑊
The %BC, WBC, WIB and WNC in Equation (1) are the percent yield of biochar, the weight of biochar, the weight of input biomass and the weight of non-carbonized biomass, respectively (Kongnine et al., 2020).The ideal charcoal yield during the pyrolysis process would be 35% of the weight of the original raw material (Meyer, 2009).The pyrolysis gas produced by the 50L-HBGS inferred by the loss of total weight after the pyrolysis process, which was estimated by Equation (2), i.e.,

%𝑃𝐺 = 𝑊
where %PG and WSY are the percent of produced gas and the weight of total solids yields, respectively (Intagun et al., 2018).Biochar production can use a variety of organic materials or waste feedstocks, such as citrus peel fruit (Selvarajoo et al., 2022), sewage sludge, and pine needles (Fatima et al. 2022), even microalgae, etc. (Chen et al., 2022).However, bamboo is one of the fastest growing plants in the world (Jeffery et al., 2023) and has covered a large area in many countries around the world (Sawarkar et al., 2023;Adeniyi et al., 2023), especially in Asia such as China, India, even Thailand, etc. (Sawarkar et al., 2020).The main components of bamboo wood are hemicelluloses, cellulose, and lignin (Rusch et al., 2023), which those are significant for biochar production by pyrolysis stove (Tomczyk et al., 2020).According to Chen et al. (2010), bamboo biochar has a surface area and micropore size that are approximately 4 and 10 times larger than those in regular biochar.Moreover, with the fact that biochar from bamboo is more stable than from grasses, sludge, and husks (Cely et al., 2014;Hilscher et al., 2009;Zimmerman, 2010) and has quality in the range that can create value-added healthcare products, such as applications for skin treatments, medical products (Sawarkar et al., 2023;Kumar et al., 2023), or even the electrical electrode, etc. (Sayed et al., 2022), bamboo was used as the feedstock for this research.In the study, bamboo wood chunks with a moisture content of 9.4% on a dry basis, a length of 10 cm, and a diameter of 3.5 cm were prepared for experiments in the 50L-HBGS.

Statistical Analysis Method
The stove temperature data of the NCCS case and the ECCS case that had been tested in the 50L-HBGS were analyzed by the statistical technique, so-called one-way analysis of variance (ANOVA).The ANOVA had been applied in biochar research, such as by Ippolito et al. (2020), Muzyka et al. (2023), andBonanomi et al. (2023) to define the differences in groups of variables.The comparison of the airflow rate, and heat rate between the NCCS and ECCS was done using the paired sample t-test due to a single paired comparison (Baldoni et al., 2023;Prakongkep et al., 2020;Bonanomi et al., 2023).The analysis in this study was performed by SPSS program, at a significant level of P < 0.05 to compare differences and justify the effects of the combustion core type on an improvement of stove characteristics.

Biochar Stove Testing
The three experiments of biochar production were performed in order to collecting information of airflows, temperatures and biochar products which gained by uses the NCCS and ECCS in the 50L-HBGS.After the NCCS and ECCS were integrated into the 50L-HBGS, 9 kg of bamboo wood chunks (WIB) were added to the pyrolysis section, and 3 kg added in to the combustion core of each case, which called as the NCCS case and the ECCS case for this study, respectively.Afterward, six K-type temperature sensors were then integrated into each point (Figure 2c).All temperature sensors were connected to the PCE multichannel data logger (Figure 2b), which automatically records every 5 minutes.Since the heats for pyrolysis is generated by fuel wood in the combustion core; then, the 3 kg of bamboo firewood was prepared inside the cores.The NCCS has a larger core volume; it contained all 3 kg in a fill.Conversely, the ECCS has a smaller core volume, and it took more than one filling in the core while the stove was processing, which makes it uncomfortable to use.After everything was prepared, the stoves started to ignite, and data monitoring was started.The experiment was conducted until no more pyrolysis gas is emitted from the stove.In the meantime, the total amount of firewood used in the core were recorded.When the kiln had cooled down, the biochar and the total solid weight in the stove were measured.
In the test of the differences in air flow, due to the very high gas flow rate in the core of both cases during pyrolysis gas formation, the airflow was recorded in the initial interval of 45 minutes after the stoves were lit only.The differences in the air flow rate of the combustion cores was classified by using the measured data, which obtained with the Fluke 922 air flow meter (Figure 2a).

Biochar Characterization
After the experiments, the quality of bamboo biochar was characterized by using Fourier transform infrared spectroscopy (FTIR), model Frontier and Spotlight 200i from Perkin Elmer, USA, which runs in the range 600-7,800 cm -1 .In the beginning, the biochar samples were examined for electrical resistivity, then the biochar sample in each group with the lowest resistivity was chosen to be placed on the ATR diamond crystal plate, coated with powder, and then characterized using the Frontier and Spotlight 200i.The relation between wavelength and percent transmittance obtained from the biochar sample can reflect the different of chemical components of the biochar.
After the test, the differences in surface functional groups were defined.This method also was examined to see how the pyrolysis temperature affected the components remaining in Biochar.Then, the pore size and element content of the biochar were determined by using the scanning electron microscopy (SEM), model TM4000Plus, by Hitachi, and energy dispersive X-ray spectroscopy (EDS).SEM and EDS were used to examine the surface morphology and chemical composition of the bamboo biomass sample from the NCCS case, which is important to define whether it is in the range for application as an adsorption material or not.Finally, proximate analysis was also performed for further explanation of the distinction between the biochar from the NCCS case and the ECCS case.The study was performed according to the standards set by ASTM D1037 (1991), ASTM D3172, ASTM D2017 (1998), and ASTM E1755, respectively.The parameters studied in the proximate analysis were the moisture content, volatile matter content (VOC), ash content, and fixed carbon content.

Combustion Core Air Flow
In the experiment, stoves were ignited at the top of the combustion cores, and then fuel wood was burned down to the bottom section.The heat was transferred to the bamboo wood in the pyrolysis section (outer cylinder), creating pyrolysis gas and biochar.The results of the average air flow rate from three tests, obtained by using the Fluke 922 air flow meter and measured after the stoves were lit, are shown in Figure 3.The average airflow, from 48 data points, before the continuous pyrolysis was started was 57.25 m 3 /h for the NCCS case (Table 1), which on average is 62.24% higher than the ECCS case.The result after the paired sample t-test of the airflows was performed (Table 2) indicates that the airflow rate in the combustion core of the NCCS case is significantly different from the ECCS case (P value = 0.000).The higher airflow in the NCCS relates to the physical shape of the core, which has a large lower air inlet volume (Figures 1b and 1c), which is important for better low-temperature air inlet flow into the core during hightemperature combustion.It is clarified that the application of the NCCS to the 50L-HBGS has significantly improved the passive airflow rate in the combustion core of the stove.

Pyrolysis Temperature Profiles
The average temperature data after stoves were three-time experiments are shown in Figures 4 and 5.The profiles Tl (green), Tm (red), and Tu (blue) represent to temperature at the lower, middle, and upper pyrolysis sections of the stove, respectively.And Tf (orange) is the flame temperature of the stove.The information from Figure 4 shows that temperature profiles in the ECCS case has more high fluctuation, especially, it clearly exposed in Ft.The high fluctuations, during the 75 minutes after the stove was lit, were caused by the physical design of the core, which has a small air inlet section (Figure 1a), causing it to deliver a low airflow rate, which is around 25 m 3 /h (Figure 3), combined with the ash blockage in the core, which results in a mismatch of air in combustion and misfires in the combustion core.Moreover, with a small volume of the core for contain the fuel wood, the stove had to be operated by the user to increase the air flow and enhance the flame, which made it uncomfortable to use and prolonged the time to produce biochar.The syngas burns continuously in the core at 75 minutes after ignition, and then it takes 65 minutes to finish the gas discharge (Figure 4).On average, the stove took an overall 140 minutes to finish ejecting syngas.From Table 3, the maximum average of flame temperature (Tf, ECCS) in this case is 890.23 o C. At the pyrolysis zone temperatures, the Tu, ECCS and Tm, ECCS have a maximum average value over 500 o C for approximately 30 minutes, which implies that bamboo chunks at the middle and upper zones of the stove have converted to biochar (Lee et al., 2013).However, the lower part temperature (Tl, ECCS) of the stove has a maximum average temperature of 365.73 o C, which is not enough to remove the tar and convert all bamboo chunks in this zone to biochar.
In the NCCS case (Figure 5), due to an improvement in airflow rates in the combustion core and a sufficient containment volume of fuel wood in the core (one-time filled), the growth of the pyrolysis temperature profiles in the stove is more stable than in the ECCS case.Even if the flame temperature decreased 35 minutes after lighting due to the ash blockage, the large air inlet volume of the core (Figure 1b) and sufficient fuel wood contained inside could create high airflow pressure, resulting in continued flaming throughout the process without the need for user intervention.Implies that the production method of the 50L-HBGS has been changed.The pyrolysis starts 45 minutes after lighting and ends 75 minutes later.The total duration from start ignition to end of pyrolysis gas discharge is on average 120 minutes, which implies that it took a shorter biochar production time than the ECCS case.The descriptive data of temperature from three experiments in the NCCS case is shown in Table 3.The maximum average flame temperature (Tf, NCCS) in this case is 909.93 o C. The maximum average of pyrolysis temperatures Tl, NCCS, Tm, NCCS, and Tu, NCCS are 560.5, 655.1, and 788.7 o C, respectively, which are all higher than in the ECCS case.The pyrolysis temperature in this case    reaches over 500 o C, which is in the range of optimum biochar production (Brady et al., 2008;Klüpfel et al., 2014;Lehmann, 2015) and sufficient to remove volatile matter and develop pore structure (Lee et al., 2013).
Comparative of the NCCS and ECCS by used the ANOVA statistics gives descriptive data of an average temperature in each point as shown in Table 3.In which the maximum average of Tl, NCCS is 602.00 o C, which is 64.6% higher than the maximum average of Tl, ECCS (Figure 6).The Tl, NCCS reached over 500 o C for 20 minutes during peak of pyrolysis is (Figure 5), which is important to produce high amount of fixed carbon, as well as reduces of volatile matter contain in biochar (Lee et al., 2013).Test of the ANOVA result (Table 4) indicated that all paired of Tl, Tm, Tu and Tf have significantly difference (P < 0.05).The largest different of mean value found in the lower zone temperature (Tl), with highest F value of 115.443.The 2 nd , and 3 rd difference found in Tf and Tu, respectively.Whereas Tm is smallest in difference.Moreover, the bar chart in Figure 6 shows   clearly that the major improvement is in the lower zone temperature (Tl) of the pyrolysis chamber.These results indicate that the pyrolysis temperatures and the profile characteristics of the 50L-HBGS have been improved by using the NCCS.

Heat Rate Analysis
Figures 7 and 8 show the heat rates at any position along the 50L-HBGS in both cases.The upper, middle, and lower sections of the 50L-HBGS are indicated by the red, blue, and green lines, respectively.The data in Figures 7 and 8 reveals that there are two key distinctions between the ECCS and NCCS cases: the first is the value of the heat rates, and the second is the profile pattern.The descriptive data in Table 5 indicates that the heats rate in both cases is less than 10 o C/min, implies that the test was carried out in the range of the slow-pyrolysis process, which is favorable for produces biochar (Wang et al., 2018;Zhao et al., 2019;Shen & Wu, 2023;Abdullha et al., 2023).In both cases, there has discernible variation in the heat rate at the upper area of the stove (@TU), which is confirmed by the ANOVA test data in Table 6 (P = 0.048).Moreover, there is a substantial difference between the heat rates at the middle (P = 0.002) and lower sections (P = 0.000).The heat rate at lower section of the NCCS case is higher than the ECCS case, however, the heat rate is lesser in the middle and upper section.
The heat rate characteristics above are because the size of the core puncher in the ECCS case is bigger than in the NCCS case, which results in a higher volume of pyrolysis gas dissipating to burn in the combustion core, and results to a higher heat rate transfer through the wall of the middle section (red) and upper section (blue) of the stove, as shown in Figure 7.However, with the high gas discharge, which causes insufficient time for organic vapor to react with the  carbonaceous material (Antal & Grønli, 2003;Crombie & Mašek, 2015) at the lower part of the stove, and the straight combustion core design, gives the ECCS lacks of surface area and time to transport heat in the flame to the lower portion of the pyrolysis chamber, then the heat rate at the lower pyrolysis zone is not sufficient (green line in Figure 7) for the decomposition of the wood chunk, resulting in the remaining of tar and unburned bamboo at the end (Figure 9a).In the NCCS case, the new design of the combustion core has a larger volume and surface area at the lower section (Figure 1b).The sensible heat from fuel wood combustion at the wall of the lower core causes an increased heat rate through the large wall of the lower core to the lower pyrolysis zone, as shown by the green lines in Figure 8.Moreover, large air inlet section of the core provides sufficient airflow for burning gas and wood fuel in the combustion core, therefore the heat rate profile pattern is less fluctuation than in the ECCS case.However, it has smaller size of the core puncher at lower core, effects to the lower rate of pyrolysis gas discharge to the combustion core.The lower pyrolysis gas discharge during start of pyrolysis (at 50-75 minutes in Figure 5) allows organic vapor to have more time to react with the carbonaceous material, forming more H2 and CO with exothermic heat, result to the higher in maximum temperature and pressure at the final pyrolysis process (Antal & Grønli, 2003;Crombie & Mašek, 2015).

Biochar Yield and Pyrolysis Gas Yield
From the stove testing, it was found that the weight of total solid yield (WSY) and the weight of biochar yield (WBC) in the NCCS case are 2.50 and 2.45 kg, respectively, which is slightly lower than the ECCS case (Table 7).The ECCS case seems to be more dominant than the NCCS case in this point; however, the physical details of biochar from the ECCS case in Figure 9a shows the contaminate of tar and part of unburned bamboo wood chunk (0.35 kg). Figure 9b simply indicates that the biochar products from the NCCS case are better than those from the ECCS case, where all bamboo wood chunks and tar are converted to biochar and pyrolysis gas.With the %BG yields by the NCCS is 27.37%, shown in Table 7, indicates that production of biochar by the NCCS case is in the rage of %BG gained by the 50L-HBGS, as reference to Maneekhat and Khamdaeng (2022), Sittioad et al. (2022) and Panyoyai et al. (2019).However, it was slightly lower due to the improvement in pyrolysis temperature in the stove and the difference in feedstock used in the experiment.
Nonetheless, the results of higher %PG and lower %BG in the NCCS case than the ECCS case can be explained by the fact that the modified combustion chamber in the NCCS case effects an increasing pyrolysis temperature (esp.at the lower section), which is sufficient to decompose all of the bamboo wood chunks in the pyrolysis chamber and convert tar to pyrolysis gas and char (Ojolo et al., 2013).This concludes that apply the NCCS in the stove results to increase the %PG but decrease the %BC, compared to the ECCS.

Biochar Surface Function Group
FTIR analysis revealed that the pyrolysis temperature had a remarkable impact on the functional groups of biochar.FTIR spectra of the bamboo biochar from the 50L-HBGS are shown in Figure 10.The BB-C1 is a spectra line of biochar from the ECCS case.And BB-C2 is the spectra line of biochar from the NCCS case.Several peaks of Both BB-C1 and BB-C2 biochar disappeared during the pyrolysis process (from 200 o C) due to hemicellulose beginning to demonstrate pyrolysis, as denoted by the characteristic's bands at 3335-3483 cm −1 (O-H stretching vibrations of lignin, hemicellulose, and cellulose), 1711 cm -1 (C=O stretching of lignin), and 1610 cm -1 (C=C asymmetric stretching of lignin) which were caused by H2O and CO2.The absorption peak at 1378 cm -1 (C-O stretching of hemicellulose) suggests the ether contains in the biochar.Absorption bands at 785 cm -1 may have been caused by the bending vibrations of the C-H atom in polycyclic or substituted aromatic groups (Zhao et al., 2019).All absorption peaks displayed a rising pattern at 350 °C, alongside the breakdown of hemicellulose, cellulose, and lignin.The absorption bands at 2362 cm -1 (C-C asymmetric stretching of hemicellulose), 2850 cm -1 (CH2 stretching vibrations for aliphatic CH3 and CH2 groups), and 2923 cm -1 (CO and CH4) were brought on by CO and CH4, primarily from broken C-O bonds and C=O.
The transmittance at 1317 cm −1 is due to the occurrence of aromatics with C-C stretching (ester and phenol) (Abdullah et al., 2020;Grover et al., 2002;Armynah et al., 2018).And transmittance at 1097 cm −1 is due to symmetric C-O stretching and C-OH bending of hemicellulose, cellulose, and lignin (Armynah et al., 2019).The change in temperature in biochar products affect the decomposition of cellulose, hemicellulose, and lignin.The volatile matter emission from biochar increases with pyrolysis temperature (Enders et al., 2012;Crombie et al., 2013).The compounds containing C-C and C-H stretching or bending functional groups were prevalent in each bamboo subfamily.When the temperature reached 600 °C, the CO2 absorption peaks were almost at their maximum absorbance (i.e., at the lignin decomposition stage), whereas the other component's absorption strength decreased, this suggested that the reaction was complete (Zhao et al., 2019).In the case of the BB-C2, the spectral bands at 2923-2951, 2850, 2362, 1711, 1610, and 1378 cm −1 clearly decreased as the maximum temperature of the pyrolysis zone increased.This result indicates that the NCCS effects an increase in the 50L-HBGS temperature, which improves the carbon content of biochar (Sahoo et al., 2021).

The surface morphologies and chemical composition
The BB-C2 sample has a less rough or cracked outside surface, as seen in Figures 11(a) and 11(a-1), due to less fluctuation of temperature while slow low heating rates occurred (Ong et al., 2021;Zhang et al., 2020) during the pyrolysis process of the stove, and heat is likely to transfer through the longitudinal direction rather than in the radius direction of bamboo wood chunks (Kalderis et al., 2023;Pinisakul et al., 2023).However, increasing pyrolysis temperature results in improved total surface and pore volumes of biochar (Binh et al., 2022;Hettithanthri et al., 2023;Faraji et al., 2023), but a thinner wall of the pore was found due to the decomposition of hemicellulose, cellulose, and lignin with increased temperature (Figures 11b  and 11b-1).The distribution of porosity is larger than 50 nm, which implies that pore size is in the macropore (Chen et al., 2023), which relates to an increase in the calcination temperature of the NCCS case.Moreover, according to Yang et al. (2016), Viglaová et al. (2018), and Ji et al. (2022), the pore of biochar in the NCCS case is in the range of micropore and macropore size, indicating that it is suitable for adsorption applications (Xu et al., 2017).While the BB-C1 and BB-C2 samples' elemental compositions, as determined by EDS analysis, indicate that carbon, oxygen, silicon, and potassium are the main components, as seen in Figure 12.The nitrogen concentration vanishes from the graphs in both situations due to the prolonged pyrolysis process (Armynah et al., 2019).The biochar created by the 50L-HBGS with the NCCS is more suited for soil treatment than the biochar produced by the ECCS case, according to the dominating potassium observed in the biochar from the NCCS case (Oram et al., 2014;Wang et al., 2018).

Proximate Analysis
Table 8 shows the proximate analysis results.The results from the test show that the dry bamboo wood chunk is appropriate for the experiment; it has a 9.4 ± 0.11% moisture content.The VOC contained in the bamboo wood chunk is 76.0 ± 0.76%.Ash and Fixed Carbon found in the test are 1.5 ± 0.13% and 13.1 ± 0.7%, respectively.The biochar obtained from the NCCS case (sample BB-C2) has a fixed carbon content of 74.2 ± 3.6%, which is approximately 50% higher than the biochar obtained from the ECCS case (sample BB-C1).The improvement of fixed carbon content indicates that the NCCS is beneficial for improving the carbon sequential characteristic of the biochar obtained from the stove (Ogawa et al., 2006;Yablonovitch and Deckman, 2023).Moreover, 9.5 ± 0.75% of the VOC in the BB-C2, which is lower than in the BB-C1, indicated that the purity of biochar can be improved by using the NCCS instead of the ECCS in the 50L-HBGS.

Limitation of the study
Even though the study found that the 50L-HBGS with the NCCS served continuous burning and deployed of remining tar and unburned bamboo wood at the end, however, there still have  limitation of the study due to the type, size and humidity of fuel wood was used in the specific range.Further study should be concerning to the varying in the other range of bamboo wood size and humidity, as well as different type of fuel wood uses in the study.Since the water boiling Test method is not suitable for application in the study, an option of the thermal performance investigation is a key that needs to be focused on.

Conclusion
The study of the development of the combustion core of the 50L-HBGS was to address the problems of the existing 50L-HBGS in Thailand, which are discontinuous combustion during the ignition process, wood pieces remaining at the end of combustion, and the problem of liquid tar contamination of biochar products.The combustion characteristic result indicated that the NCCS case provided continues to burn better than the ECCS case during the ignition process because the airflow rate in the combustion core was improved.In addition, the stove's temperature was higher than 500 o C during the pyrolysis process, which resulted in the removal of the liquid tar and left no unburned wood pieces at the end.The characterization result showed that the NCCS case produced biochar in the range of micropore and macropore sizes with a higher purity and fixed carbon content than the ECCS case.And, because of the way the volatile organic compounds (VOCs) in the biochar derived from the NCCS case were lower than that of the ECCS case, the biochar obtained from the NCCS case had better characteristics for adsorption, soil treatment, and other agricultural uses.The percentage improvement of fixed carbon contained in the biochar from the NCCS case is also advantageous for future carbon sequestration applications.This study shows that applying the NCCS to the 5L-HBGS results improves both the stove's combustion characteristics and the quality of bamboo biochar production.This could be beneficial for the promotion of the stove under the concept of the United Nations Sustainable Development Group (UNSDG), such as growing more food in poor soils by using biochar to reduce hunger, replacing chemicals in agriculture with biochar for good health and well-being, low-cost filtration medium by biochar for clean water and sanitation, etc.

Fig. 1
Fig. 1 The combustion core of the 50L-HBGS; (a) the existing combustion core shape (ECCS) (b) the non-uniform combustion core shape (NCCS) (c) components of the 50L HBG with the NCCS

Fig. 2
Fig. 2 Monitoring of the 50L-HBGS: (a) the air flow meter (b) Temperature data recorder (c) position of sensors during experiment

Fig. 3
Fig.3The average air flow rate at outlet section of the combustion core from three experiments

Fig. 4
Fig. 4 Average temperature profiles of the ECCS case from three experiments

Fig. 5
Fig. 5 Average temperature profiles of the NCCS case from three experiments

Fig. 6
Fig.6The percent difference of maximum average temperature between the NCCS case and the ECCS case

Fig. 7
Fig.7The average heat rate of the 50L-HBGS with the ECCS Fig.8The average heat rate of the 50L-HBGS with the NCCS

Fig. 12
Fig. 12 The elements in BB-C1 and BB-C2 by the EDS technique

Table 1
Sample statistics of the airflow

Table 2
Paired sample t-test of the airflow of the NCCS and ECCS

Table 3
Descriptive data of average temperature in each position between the NCCS case and EECCS case

Table 5
Descriptive data of stove's average heat rate in each position

Table 7
Weights parameter of biochar (three average data)