Introducing a Novel Rice Husk Combustion Technology for Maximizing Energy and Amorphous Silica Production Using a Prototype Hybrid Rice Husk Burner to Minimize Environmental Impacts and Health Risk

Abstract

Rice husk is the main by-product of the postharvest stage in rice production, which causes environmental impacts due to improper management as a solid waste. However, potential economic applications of rice husk combustion have been identified for energy generation and amorphous silica production in several industries. To minimize hazardous gaseous emissions and crystalline silica availability, rice husk combustion conditions should be properly controlled which also effect for efficient heat production. This study was conducted under different conditions of temperature, airflow, combustion time, and bulk density of rice husk in the combustion process using an experimental prototype hybrid rice husk burner with a fluidized bed. The availability of crystalline silica in rice husk charcoal and the CO and O₂ compositions in the exhaust gas were analyzed using XRD analysis and gas analysis, respectively. Furthermore, elemental and thermogravimetric analyses were conducted to find the most efficient combustion parameter for the optimum conditions of rice husk combustion using the experimental rice husk burner. Therefore, the most efficient heat generation was achieved with the observation of the lowest CO emission, the nonavailability of crystalline silica in rice husk charcoal, at a low temperature and air flow rate (430 °C; 0.8 ms⁻¹), high bulk density (175 kgm⁻³ and 225 kgm⁻³) and short combustion time (30 s).

1. Introduction

The potential of rice husk as a renewable energy source has been identified by researchers worldwide. As a process-based by-product in the rice processing industry, rice husk is collected automatically under a very low level of moisture content, which is an additional advantage compared to other biomass types. Due to the wide availability and uniformity of rice husk, it can be identified as a major biomass material that could be utilized for energy generation in many rice-producing countries [1]. Rice husk accounts for approximately 20% of paddy rice, which has been estimated at approximately 100 million tons of annual production in developing countries [2]. Presently, rice husk, when correctly combusted, is one of the most successful and profitable renewable energy sources available [3]. As rice husk contains a high amount of carbon, rice husk charcoal generated in the combustion process contains a considerable amount of carbon if it did not completely combust into the ash. As a by-product of rice husk combustion, rice husk charcoal was also identified as a valuable material due to the vast range of application possibilities, such as carbon sequestration, soil amelioration and a source of amorphous silica [4].

Carbon sequestration by depositing biochar in soil has become a promising solution for reducing the emission of carbon dioxide (CO₂) into the atmosphere. Previous studies conducted on rice husk charcoal application for carbon sequestration have confirmed the possibility of carbon deposition under certain conditions [5,6,7]. Additionally, the application of rice husk charcoal to agricultural fields improves texture, structure, and soil fertility. As a long-term practice in most rice-producing countries, charcoal from the combustion of rice husk is applied to rice cultivation as a soil ameliorant [8].

Rice is one of the major silica accumulator crops that absorbs 150–300 kg/ha of soluble silica from the soil [9]. According to the solubility and the physical structure, silica can be categorized as soluble, insoluble, and crystalline, amorphous (plant-available) respectively [10]. After confirming that silica was an essential element for the healthy growth of paddy plants, it was authorized for use as a fertilizer in Japan. Usually, silica is applied to the soil as a synthetic fertilizer, which results in a high cost of production in paddy cultivation [11]. Even though few advanced methods for the extraction of amorphous silica from rice husk are available, many of them are not popular in industry, as many feasibility issues are associated with them. Rice husk is converted into rice husk charcoal during the combustion process, which is identified as a by-product in rice husk combustion and contains 85–95% silica [12]. As rice husk contains a high amount of amorphous silica, there is a substantial application potential of the amorphous silica available in rice husk charcoal if it is combusted under controlled conditions to avoid silica crystallization [3]. The application of amorphous silica by rice husk charcoal has the potential to supply the required amount of silica for the healthy growth of rice plants to secure yield in rice cultivation [6,7].

Although rice husk combustion has a vast range of benefits, it is also associated with several problems (Figure 1), which emphasizes the importance of improving the technologies available for use in the combustion of rice husk.

The disposal of rice husk has become a greater issue when it is not used for processes such as combustion for generating heat, production of bedding materials for livestock, and use as raw materials in construction technology [10,11]. Combustion of rice husk in the field is a common practice among farmers and is already legally prohibited in most countries due to the generation of residual ash containing crystalline silica, emission of hazardous gasses, and the addition of a high amount of particulate matter (PM_2.5) into the atmosphere [13].

In some countries, rice husk is converted into rice husk ash/charcoal through open burning for more than 1 h and applied to fields as a soil conditioner [10]. Depending on the type of thermal treatment, the presence of amorphous silica, crystalline silica, or both can be observed in rice husk ash. From these two forms, only amorphous silica can be absorbed by plants, which are mostly stored in aquas form in plant tissues. Silicosis is a reported health issue, especially for those who are directly exposed to the emission of rice husk burning, as crystalline silica can be generated during rice husk combustion [14]. By confirming the nonavailability of crystalline silica in rice husk ash, rice husk ash/charcoal can be directly utilized as a valuable fertilizer in agricultural practices that can be handled without any risks to human health.

Carbon monoxide (CO) is one of the hazardous gas emissions in rice husk combustion and has important impacts on climate change and human health. CO influences the oxidation capacity of the atmosphere and the chemical cycles of greenhouse gases, such as CO₂ and methane (CH₄). CO is an indirect greenhouse gas, one of the precursors of surface ozone (O₃), can participate in photochemical reaction processes and is one of the causes of photochemical smog pollution [15]. Due to its high attraction to haemoglobin (production of carboxyhaemoglobin), CO is a toxic gas for human health. Long-term exposure to CO should not exceed 50 ppm in 4 h, and the lethal concentration is 650–700 ppm. CO is also a precursor of ground-level ozone, which can trigger serious respiratory problems [16]. Additionally, considering the energy generation by rice husk combustion, the production of CO causes a reduction in the efficiency of the system compared to the production of CO₂ due to the incomplete combustion of carbon [17].

Particulate matter emissions during rice husk combustion are categorized as toxic pollutants and carcinogens. The formation of particulate matter depends on combustion conditions such as temperature and time duration, where low temperature and long-term combustion produce higher levels of particulate matter according to previous studies. Additionally, inefficient rice husk combustion leads to the emission of more particulate matter during the combustion process [18].

According to the previous studies, among the technologies available for energy generation from rice husk, combustion is the most feasible and economical method [19]. However, its continued use and/or legality is uncertain because of the problems associated with efficiency in energy generation, residual ash management and other hazardous emissions. Most of the studies have been conducted to increase the efficiency in energy generation [3,20], while some researchers have focused on finding the solutions for environmental and health risks in available rice husk combustion plants by introducing waste management technologies [18,21,22,23]. Additionally, studies have been conducted to develop systems that control either silica crystallization or CO emission in rice husk combustion considering the combustion temperature, air-to-fuel ratio, and quality of the feedstock [24,25,26]. Furthermore, most of the experiments regarding rice husk combustion were performed using an electric furnace or small-scale prototype burners under laboratory conditions, applying the relevant experimental outcomes to practical applications is difficult.

Considering that the overall impacts of rice husk combustion have not yet been achieved, the purpose of this research is to investigate the characteristics of generated rice husk charcoal and CO emissions from rice husk combustion under different combustion conditions; to maximize heat generation and amorphous silica production while minimizing environmental impacts and human health risks.

2. Materials and Methods

2.1. Sample Collection

Rice husk (outer cover of the seeds of Oryza sativa species) obtained from a standard-sized (5 mm) Japonica rice variety (Koshihikari) cultivated in the Tsukuba-Plant Innovation Research Centre (T-PIRC) at the University of Tsukuba, Japan, was used for the experiment. Samples of 50 g, 70 g, and 90 g were measured using an electric balance with an accuracy of 0.001 g. Measured samples (rice husk with 5.9% wet basis moisture content) were inserted into airtight plastic bags and labelled according to the treatment categories.

2.2. Experimental Setup

2.2.1. Specifications of the Prototype Hybrid Rice Husk Burner with a Fluidized Bed

A prototype hybrid rice husk burner (PHRB) with a fluidized bed was used for this experiment (Figure 2). This PHRB was designed as a batch type prototype of a continuous type PHRB with the fluidized bed (Appendix B). This burner facilitated combustion under a vast range of conditions (

Table 1. Specifications of prototype hybrid rice husk burner (PHRB) with a fluidized bed.

Specification/Detail	Range	Controlling Factor
Kerosene consumption (Lh−1)	2.20–9.00	Temperature from 400 °C to 700 °C
Blower rotation (rpm)	2295–6023	Airflow from 0.5 ms−1 to 2.6 ms−1
Capacity of the combustion chamber (m3)	0.04	Sample density from 125 kgm−3 to 225 kgm−3

). The initial heat supply for the rice husk combustion was supplied by kerosene combustion. The robustness of kerosene promoted the consistency of temperature during rice husk combustion. In this system, temperature and airflow were set at specific values under defined burner levels (from burner level 1 to 101). The temperature was ascending with the burner level, and the corresponding airflow was determined by the set rpm value of the blower rotation.

Continuous monitoring of the temperature of important locations, such as inside the rice husk combustion chamber, inside and outside of the kerosene burning unit, near the outlet of the rice husk burning chamber and vertical outlet for the hot air and rice husk combustion chamber, were facilitated with temperature sensors. An exhaust fan was fixed above the exhaust outlet for the combustion stage to remove the gas emission during combustion.

2.2.2. Combustion Chamber of the Prototype Hybrid Rice Husk Burner (PHRB) with a Fluidized Bed

The combustion of rice husk was designed to take place in the rice husk combustion chamber. It was a compartment with two pieces made with an iron frame and iron wire mesh, where the bulk density of the sample was determined by the amount of rice husk put into the rice husk combustion chamber (Figure 3). The volume of the rice husk combustion chamber was 4 × 10⁻⁴ m³. The required burning temperature for rice husk was provided by burning kerosene and transferred to the ambient airflow delivered by the blower, which blows through the rice husk combustion chamber. The results were obtained as optimum conditions for rice husk combustion (temperature, airflow rate, time duration, and sample bulk density) using the batch-type rice husk combustion chamber as an improved design of a continuous-type PHRB with a fluidized bed.

2.2.3. Initial Heat Supply and Combustion Process of the Prototype Hybrid Rice Husk Burner (PHRB) with a Fluidized Bed

A kerosene tank provides the required kerosene to the injectors of the burner, and the blower forces air into the kerosene burning unit according to the set values (burner level), which were provided to start the burning. The heating air was diverted from the hot air preparation unit into the exhaust outlet for the hot air preparation stage until the air flow reached the target temperature. This was controlled by closing the gate towards the rice husk combustion chamber and opening the vertical outlet, allowing hot air to move out through the exhaust outlet for the hot air preparation stage. All the temperature values of the system could be monitored in real time in the display unit. After achieving the required temperature of the hot air, the gate towards the rice husk combustion chamber was opened using the hot air releasing lever and allowed the hot air to blow through the rice husk sample in the rice husk combustion chamber. Gaseous emissions were generated by rice husk combustion, and they moved out through the exhaust outlet at the combustion stage. At the same time, the exhaust fan was operated to remove the combusted gases. The operation was shut down manually after the predetermined combustion time.

2.3. Sample Preparation

Experiments were conducted under three temperature treatments above the initiation of the carbon combustion temperature. According to the results obtained by the thermogravimetric analysis of previously conducted research, carbon combustion commenced before 430 °C [27]. This result was confirmed by conducting a thermogravimetric analysis of the rice husk samples used for this experiment. Two temperature treatments were selected, 531 °C and 646 °C, which are lower than the silica crystallization temperature (800 °C) according to the literature [14], and XRD analysis was conducted on the rice husk charcoal to investigate the conditions for silica crystallization. Rice husk charcoal samples were prepared under treatment conditions as follows: 430 °C; 0.8 ms⁻¹, 531 °C; 1.6 ms⁻¹ and 646 °C; 2.5 ms⁻¹ for 125 kgm⁻³ (50 g), 175 kgm⁻³ (70 g) and 225 kgm⁻³ (90 g) sample bulk densities. Air flow rates were increased automatically by the system with increasing temperature to ensure the supply of the required amount of oxygen for the combustion process. The combustion time was maintained at 30 s for all samples (supplementary experiments were conducted for 10 min and 30 min combustion times as mentioned in Appendix A), and all experiments were repeated to improve the accuracy of the results.

The measured rice husk sample was inserted into the rice husk combustion chamber and spread to ensure an even distribution inside the rice husk combustion chamber. It was covered by the other flap and fixed together using a hand vice. Then, it was placed in the combustion outlet and fixed to the main body of the PHRB using two hand vices. Temperature and air flow were set using the displaying and controlling unit, and the rice husk burner was ignited. When the air temperature reached the required value (after 30 s), the heated air flow was diverted towards the rice husk combustion chamber by operating the hot air and releasing the lever. The peak temperature inside the rice husk sample during combustion (30 s) was monitored. After the rice husk was combusted for the required duration, the burner was switched to the cooling mode, and when the system reached room temperature, the rice husk combustion chamber was removed from the main body, and rice husk charcoal samples were collected.

2.4. Elemental Analysis of Rice Husk and Rice Husk Charcoal

Elemental analysis was conducted on rice husk and rice husk charcoal obtained under different treatment conditions using a Unicube elemental model device (Elementar, Langenselbold, Germany) to measure the composition of carbon (C), hydrogen (H), nitrogen (N), and sulphur (S) on a weight basis as a percentage of the total weight. A direct temperature programmed desorption technique was used in this instrument to obtain highly reliable results. Elemental ratios up to 120,000:1 for C:N and C:S are achievable using this technology. The measurable homogeneous substance sample size varies from 0.1 mg to 1 g, and the maximum measurable carbon content is 14 mg.

2.5. Differential Thermogravimetric Analysis of Rice Husk

A differential thermal balance system (TG/DTA7300 Seiko, Chiba, Japan) was used to simultaneously measure thermogravimetry (TG) and differential thermal analysis (DTA) in the rice husk sample as a function of temperature or time while changing the temperature of the sample and the reference material according to a pre-set program. In this analysis, the detection of chemical changes, such as dehydration, decomposition, oxidation, and reduction, and physical changes involving a change in mass, such as melting, heat generation, sublimation, evaporation, and desorption, at high sensitivity can be observed. In the TG/DTA7300 model, there is an autosampler that allows continuous measurement. The weight of the sample was 15.180 mg, which was placed in a deep aluminum pan, and the oven chamber was heated from 50 °C to 500 °C at a rate of 10 °C/min for 50 min under an air supply of 200 mL/min.

2.6. X-ray Powder Diffraction (XRD) Analysis of Rice Husk Charcoal

XRD analysis is an analytical technique used for phase identification of crystalline materials. Max Von Laue discovered that crystalline substances act as three-dimensional diffraction gratings for X-ray wavelengths similar to the spacing of planes in a crystal lattice. This technology is based on the constructive interference of monochromatic X-rays and a crystalline sample. X-rays are produced in a cathode ray tube, filtered to produce monochromatic radiation, concentrated by collimation, and directed towards the sample to be tested. The interaction between incident rays and the sample creates a constructive interference and a diffracted ray when conditions satisfy Bragg’s Law.

When a crystal is impinged by X-rays of a fixed wavelength, which is similar to the spacing of the atomic-scale crystal lattice planes and at certain incident angles, intense reflected X-rays are produced. If the travel path differences and integer multiples of the wavelength are equal, constructive interference will occur. At this moment, a diffracted beam of X-rays will leave the crystal at an angle equal to that of the incident beam. The general relationship between the wavelength of the incident X-rays, angle of incidence, and spacing between the crystal lattice planes of atoms is known as Bragg’s law, expressed as

$$n\lambda =2d\sin \theta$$

(1)

where $n$ (an integer) is the order of reflection, $\lambda$ is the wavelength of the incident X-rays, d is the interplanar spacing of the crystal, and $\theta$ is the angle of incidence [28].

The diffracted X-rays exhibit constructive interference when the distance between paths ABC and A’B’C’ differs by an integer number of wavelengths ($\lambda$), as represented in Figure 4. These diffracted rays are detected, processed, and counted. All possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material by scanning the sample through a range of $2\theta$ angles. Conversion of the diffraction peaks to d-spacing allows the identification of minerals because each mineral has unique spacings, which is achieved by comparison with standard reference patterns.

The presence of crystalline silica in rice husk charcoal under different treatment conditions was tested by using XRD analysis. Charcoal samples prepared using the rice husk burner were tested for crystalline silica by conducting the XRD analysis (qualitative analysis) in the facility available at the University of Tsukuba. It was performed on 0.5 mm thick layers of rice husk charcoal samples on glass plate sample holders (sample size of 20.0 mm × 20.0 mm × 0.5 mm). Rice husk charcoal samples were completely powdered using a mortar and pestle, and the XRD analysis was conducted using a Bruker X-ray Diffractometer D8 ADVANCE/TSM (40 kV, 40 mA) using a Cu X-ray tube with 1.5418 (A°).

2.7. Flue Gas Analysis of the Rice Husk Combustion

The stoichiometric combustion of rice husk was monitored by 3% availability of oxygen (O₂) in the exhaust gas to ensure the complete combustion of burners at the industrial level. Additionally, the presence of CO in the exhaust gas indicates incomplete combustion, while the presence of CO₂ indicates the complete combustion of carbon. The same procedure was used in this experiment to monitor the stoichiometric combustion of rice husk. Gas emissions were measured by a Testo 350 flue gas analyzer with a gas analyzer probe placed at the center of the exhaust outlet for the combustion stage. Concentrations of O₂ (%), CO₂ (%), and CO (ppm) were measured in the combustion gas emissions in this experiment. CO₂ (%) and CO (ppm) emissions were converted into CO₂ and CO (mgm⁻³) using the following equation [29] to calculate the total CO₂ and CO (mg) emissions considering the volume of the exhaust gas chamber (0.032 m³). Controlled experiments were conducted to measure the CO₂ (%) and CO (ppm) emissions only from kerosene combustion, and the balance emissions which were emitted only by rice husk combustion were used for calculations.

$${\mathrm{CO}\mathrm{or}\mathrm{CO}}_2\left({\mathrm{mgm}}^{-3}\right)={\mathrm{CO}\mathrm{or}\mathrm{CO}}_2\left(\mathrm{ppm}\right)\times \frac{M}{22.4}\times \frac{273}{273+T}\times \frac{P}{1013}$$

(2)

where $M$ is the molecular weight of CO or CO₂ (gmol⁻¹), $T$ is the combustion temperature (°C), and $P$ is the pressure at the point of measurement (hPa).

2.8. Calculation of the Combustion Efficiency and Thermal Efficiency

CO and CO₂ are the most important indicators of combustion efficiency [17]. The generation of CO₂ in a complete combustion process produces 393.5 kJ mol⁻¹ by converting all the carbon content into CO₂, while the generation of CO only produces 110.53 kJ mol⁻¹, which causes less heat production in incomplete combustion [30]. Combustion efficiency was calculated using the following equation considering the heat generated by the amount of CO and CO₂ production during the combustion process.

$$\alpha =\frac{\left(P_{CO}\times E_{CO}\right)+\left(P_{C{O}_2}\times E_{C{O}_2}\right)}{\left(PC{C}_{C{O}_2}\times E_{C{O}_2}\right)}$$

(3)

where $\alpha$ is the combustion efficiency, $P_{CO}$ is the production of CO (mol), $E_{CO}$ is the formation enthalpy of CO (110.53 kJ mol⁻¹), $P_{C{O}_2}$ is the production of CO₂ (mol), $E_{C{O}_2}$ is the formation enthalpy of CO₂ (393.5 kJ mol⁻¹), and $PC{C}_{C{O}_2}$ is the production of CO₂ (mol) under complete combustion.

Kerosene was the fuel used for heat generation in the initiation of combustion in the experiment. The thermal efficiency was calculated by the ratio between the heat produced by rice husk combustion and the heat supplied by kerosene combustion (the calorific value of kerosene was considered to be 36.5 MJL⁻¹) during 30 s of combustion [31].

$$\beta =\frac{\left(P_{CO}\times E_{CO}\right)+(P_{C{O}_2}\times E_{C{O}_2})}{\left(C_K\times t_K\times H_K\right)}$$

(4)

where $\beta$ is the thermal efficiency, $C_K$ is the kerosene consumption (Lh⁻¹), $t_K$ is the duration of kerosene combustion (h), and $H_K$ is the calorific value of kerosene (36.5 MJL⁻¹).

3. Results and Discussion

3.1. Elemental Analysis of Rice Husk

The amount of C available in rice husk contributed to the production of CO₂, CO, and charcoal during the combustion process (

Table 2. Elemental analysis results of rice husk (dry basis wt%).

Carbon (C)	Hydrogen (H)	Nitrogen (N)	Sulfur (S)	Others *
35.21	5.05	0.42	0.14	59.18

* Oxygen, silica, phosphorus, potassium, calcium, etc. [3].

). The higher carbon content in rice husk led to more heat generation and carbon emissions by oxidation during the combustion process.

Rice husk contains a high amount of cellulose, hemicellulose and lignin which contribute to the higher C and H levels. Other than C, H, N and S, rice husk contains a high amount of elemental oxygen and silica, according to the previous studies [3,10]. The results obtained for elemental analysis can be supported by the previous research conducted using a similar rice husk variety by Abah et al., 2021 [23].

3.2. Differential Thermogravimetric Analysis of Rice Husk

According to the thermogravimetric analysis, changes in the weight of rice husk with the thermal treatment were represented at the different phases (Figure 5).

Thermogravimetric analysis was conducted from 50 °C at a rate of 10 °C/min, and at 138.8 °C, the moisture removal (drying) process was ended. During this time, the moisture removal percentage was 5.9%. From 260.3 °C to 419.3 °C, the volatile decomposition phase was identified, where 63.1% of the weight was reduced. In the next phase of carbon combustion, the total weight loss was 10.9%. Finally, the ash content in the rice husk sample was measured as 20.1% (

Table 3. Thermogravimetric analysis results of rice husk (wet basis wt%).

Moisture Removal	Volatile Decomposition	Carbon Combustion	Total Weight Loss	Ash Content
5.9	63.1	10.9	79.9	20.1

).

Considering the results obtained by both elemental and thermogravimetric analyses, it was identified that the fixed carbon content in rice husk was 10.9% (wet basis) from the total carbon content of 35.21% (dry basis). The balance carbon amount was removed during volatile decomposition. According to the thermogravimetric results, carbon combustion of rice husk commenced at 419.3 °C for the rice husk sample used for the experiment. To identify the effect of temperature on rice husk combustion, the experiments were conducted under three different temperatures, 430 °C, 531 °C, and 646 °C, which were higher than the carbon combustion temperature (419.3 °C), according to the thermogravimetric analysis.

3.3. X-ray Powder Diffraction Analysis of Rice Husk Charcoal

Although there were no peak formations in the XRD analysis results for ash samples for the treatment conditions of 430 °C; 0.8 ms⁻¹, and 531 °C; 1.6 ms⁻¹ for long-time combustion (30 min), there was a clear peak formation under 646 °C; 2.5 ms⁻¹ treatment (Figure 5). The broad smooth hump from 15° to 30° ($2\theta$) in the graph indicates the presence of an amorphous form of silica in the samples (Figure 6). Throughout the increasing temperatures of the rice husk combustion process, only a slight increase in this pattern was identified, and sharp peaks that represent the crystallization of silica were not observed even in the highest temperature treatment of 646 °C; 2.5 ms⁻¹ for 10 min combustion time. According to the XRD analysis results, high-temperature combustion for a long time caused crystallization in silica. Ankyu et al., 2017, reported that the amount of eluted soluble silica (amorphous silica) in rice husk charcoal decreased after the combustion temperature of 600 °C for 60 min time duration which supports the results obtained by this study [10]. The risk of silica crystallization can be avoided by avoiding either high temperature or long-term combustion even though there is a possibility of the bond arrangements changing while remaining in the amorphous form.

3.4. Flue Gas Analysis of Rice Husk Combustion

Combustion experiments were conducted to determine the trend of CO emissions with the combustion time under different treatment conditions as follows: 430 °C; 0.8 ms⁻¹, 531 °C; 1.6 ms⁻¹, and 646 °C; 2.5 ms⁻¹. According to the results (Figure 7), under 531 °C; 1.6 ms⁻¹, and 646 °C; 2.5 ms⁻¹ treatment conditions, CO emissions exceeded the maximum measurable limit of the gas analyzer (40,000 ppm) when the combustion duration reached 30 s (Figure 7). Under the 430 °C; 0.8 ms⁻¹ treatment condition, CO emission reached its peak (37,022 ppm) at 50 s of combustion duration and gradually decreased. A sudden increase in CO emissions under higher temperature treatments (531 °C and 646 °C) and a gradual increase under low temperature (430 °C) treatment were observed during the experiments.

The CO and O₂ contents in exhaust gas emissions were measured during 9 experiments conducted under different treatment conditions based on combustion temperature, airflow, and sample bulk density (Figure 8). The charcoal samples were collected during each experiment and tested for remaining unburnt carbon content by CHNS analysis. According to the results of the experiments (Figure 8), there is a clear relationship between the rice husk combustion temperature and the amount of CO produced. With increasing combustion temperature and air supply, CO generation increased rapidly under all sample densities. In Figure 8a–c, the respective treatment conditions show an almost similar trend in CO emissions concerning combustion time. CO emissions exceeded 50 ppm (which is the safe limit recommended for CO emissions for a duration of 9 h) within 15 s to 20 s in these three samples [32].

The treatment conditions in Figure 8d–f show a similar trend for CO emissions until 20 s of combustion duration, and after 20 s, the CO emissions increased rapidly and peaked (31,398 ppm at 125 kgm⁻³, 34,477 ppm at 175 kgm⁻³ and 30,862 ppm at 225 kgm⁻³). In the 646 °C; 2.5 ms⁻¹ (Figure 8g–i) treatment conditions, there was a clear change in CO emissions according to the sample bulk density, as the highest emission (34,811 ppm) was recorded by the 225 kgm⁻³ bulk density sample, while the 125 kgm⁻³ bulk density sample recorded the lowest emission (22,767 ppm). CO emissions were similar under lower temperature combustions (430 °C) with different bulk densities of rice husk samples.

The generation of CO in combustion reactions occurred basically due to the lack of enough O₂ in the combustion chamber. During the combustion process of these 9 experiments, the exhaust gas contained at least 2.7% O₂, which was the lowest recorded in the treatment condition (Figure 8i). Therefore, the CO generation during the experiment could not be due to the lack of oxygen, which causes incomplete combustion.

3.5. Elemental Analysis of Rice Husk Charcoal

According to the results of the CHNS analysis of rice husk charcoal, the carbon content of charcoal under all 9 treatment conditions was between 4.09% and 21.78% (

Table 4. Summary of the main combustion experiment results.

Combustion Conditions (Temperature; Air Flow Rate; Bulk Density) for 30 s Combustion Time Duration	Maximum Sample Temperature (°C)	Silica Crystallization	Carbon Content in Charcoal (Dry wt%)	Combustion Efficiency $(\mathit{\alpha })$	Thermal Efficiency $\left(\mathit{\beta }\right)$
430 °C; 0.8 ms−1;
125 kgm−3	565.4	Not detected	4.60	0.90	0.59
175 kgm−3	745.7	Not detected	9.64	0.99 a	0.87
225 kgm−3	749.4	Not detected	21.78	0.92	0.94 b
531 °C; 1.6 ms−1;
125 kgm−3	754.9	Not detected	4.09	0.90	0.34
175 kgm−3	761.1	Not detected	6.29	0.89	0.47
225 kgm−3	813.1	Not detected	6.13	0.87	0.59
646 °C; 2.5 ms−1;
125 kgm−3	768.2	Not detected	6.88	0.87	0.19 d
175 kgm−3	818.6	Not detected	7.53	0.87	0.26
225 kgm−3	830.3	Not detected	5.66	0.86 c	0.34

The superscripts imply: a = Highest combustion efficiency, b = Highest thermal efficiency, c = lowest combustion efficiency, and d = lowest thermal efficiency.

). These results show that there was a significant difference in carbon combustion in the 430 °C; 0.8 ms⁻¹ treatment compared to the other two temperature treatments. Carbon combustion in rice husk was reduced with the increased bulk density of the sample as 125 kgm⁻³, 175 kgm⁻³ and 225 kgm⁻³ of bulk densities resulted in 4.60%, 9.64%, and 21.78%, respectively, under the 430 °C; 0.8 ms⁻¹ treatment condition. While considering the total CO emission under each treatment (Figure 9), the 430 °C; 0.8 ms⁻¹ treatment also shows a clear difference compared to the other two temperature treatments, as the 430 °C; 0.8 ms⁻¹ treatment shows very low CO emissions (less than 50 mg/g of rice husk).

3.6. Combustion Efficiency and Thermal Efficiency

Considering the combustion efficiency of the treatments (Table 4), 430 °C; 0.8 ms⁻¹; 175 kgm⁻³ shows 0.99, which was the highest due to the lower amount of CO emission and the higher amount of carbon combustion. The treatment, 646 °C; 2.5 ms⁻¹; 225 kgm⁻³, shows the lowest combustion efficiency of 0.86 due to the higher CO emission. While considering the thermal efficiency, the highest of 0.94 resulted from 430 °C; 0.8 ms⁻¹; 225 kgm⁻³ followed by 0.87 which resulted from 430 °C; 0.8 ms⁻¹; 175 kgm⁻³ due to the lowest kerosene consumption and higher heat generation by the lesser amount of CO emission and high amount of rice husk available for combustion. The lowest thermal efficiency was 0.19 resulting from 646 °C; 2.5 ms⁻¹; 125 kgm⁻³ treatment, where a high amount of kerosene was consumed to generate a lower amount of heat due to higher CO emissions and a low amount of rice husk available for heat generation. The sample reached the peak temperature from the lowest, 565.4 °C, to the highest, 830.3 °C, for the treatment conditions of 430 °C, 0.8 ms⁻¹, 125 kgm⁻³, and 646 °C, 2.5 ms⁻¹, 225 kgm⁻³, respectively. Considering the difference between the peak sample temperature and temperature provided by the burner, the 430 °C; 0.8 ms⁻¹; 225 kgm⁻³ treatment conditions showed the highest value (319.4 °C), followed by the 430 °C; 0.8 ms⁻¹; 175 kgm⁻³ treatment conditions (315.4 °C). These results also demonstrated that there was higher efficiency in heat generation under the above two treatment conditions. The major reason for higher heat generation under these conditions was identified as a lesser amount of CO emissions, which avoided incomplete combustion. These findings can be supported by Madhiyanon et al., 2010 where increasing fluidized bed velocity and higher CO emissions caused lower combustion efficiency and reduced bed temperature [20].

3.7. CO Emissions and Charcoal Production

According to the Ellingham diagrams, the oxidation of carbon into CO₂ is dominated by the generation of CO when the temperature reaches approximately 750 °C [33]. As the peak temperatures of the samples was 531 °C, 1.6 ms⁻¹ and 646 °C, the 2.5 ms⁻¹ treatment conditions exceeded 750 °C during combustion, which could accelerate the generation of higher CO emissions. Generated CO could be flushed out from the combustion chamber before converting into CO₂, as the residence time for this conversion is larger than the retention time of CO inside the combustion zone because the velocity of airflow rises with increasing temperature [20,34]. As the production of CO generates less heat than the heat generated by CO₂ production in carbon oxidation, higher amounts of CO generated during the combustion process reduce the efficiency of the system [17,30].

Charcoal generated during short-term (30 s) rice husk combustion contains the amorphous form of silica and a considerable amount of carbon, which can be utilized for both agricultural and industrial purposes. As charcoal can be used as a valuable material for another industry, there are no issues here regarding waste, such as the management of residual ash. Attempts to measure the particle matter emissions during this experiment were not successful due to the difficulties of setting the particulate matter analyzer at the exhaust side of the rice husk combustion burner. Future experiments will be conducted to measure particulate matter emissions with a new arrangement for setting the particulate matter analyzer.

4. Conclusions

Rice husk is considered as an economical renewable energy source due to its wide availability as a by-product of the rice processing industry worldwide. Combustion is the most feasible and cost-effective method for heat generation by utilizing rice husks as fuel. The experiment was conducted to overcome the problems associated with rice husk combustion, such as silica crystallization, emission of hazardous gases, and management of generated residual ash, using a prototype hybrid rice husk burner consisting of a fluidized bed. Experimental results demonstrated that low temperature (430 °C; 0.8 ms⁻¹), high bulk density (175 kgm⁻³ and 225 kgm⁻³) and short time (30 s) combustion could ensure higher combustion efficiency (0.99) and thermal efficiency (0.94) with minimum environmental impacts and human health risk. As lower temperature combustion requires less kerosene (2.81 L/h) compared to the higher temperature conditions, the cost of combustion operations can also be reduced. As the risk of silica crystallization was completely avoided, the generated rice husk charcoal can be utilized for purposes such as the extraction of amorphous silica, soil amelioration, and carbon sequestration.