1 Introduction

Biomass includes all materials that are directly or indirectly derived from photosynthesis reactions as fuel crops, agricultural and agro-industrial byproducts, and animal manure. Biofuels can be considered a renewable source of energy if they are harvested or produced in a suitable manner [1, 2]. The sources of biomass are grown energy crops whose energy is converted to heat, power, or chemical feedstock mainly by thermochemical conversion. Combustion of biomass fuel in a fixed bed involves complicated heat and mass transfer along with various chemical reactions. Conductive, convective, and radiative heat transfer take place (a) in solid phases, (b) between solid and gas within the combustor, and (c) between the pellet, the walls, and the flame.

Fuel properties and the combustion applications are important factors that affect the nature of the combustion process. The combustion processes can be divided into drying, pyrolysis, gasification, and combustion. The time required for each process depends on the fuel size, fuel properties, temperature, and combustion conditions. The combustion process can be continuous feed or batch, with air currents introduced being either forced or natural [3]. Over 90% of the energy generated from biomass worldwide is provided by direct combustion, such as wood-fired heating plants used for district heating and wood pellet burners [4]. A high-speed camera coupled to a single particle apparatus was optimized to reveal new data on the combustion behavior of El Cerrejón coal and various biomass under air and O2/CO2 atmospheres [5]. The combustion characteristics of single biomass pellets of Pinus bungeana and rice husk in air and O2/CO2 atmospheres containing different oxygen fractions in a vertical tube furnace and combustion visualization analysis were investigated [6]. An artificial neural networks model was applied to thermal data obtained from suspension combustion experiments with single peanut shells (PS, millimeter scale) pellets in an O2/CO2 atmosphere [7]. Two modes of biomass pellet ignition were observed: homogeneous ignition of volatiles and hetero-homogeneous ignition of volatiles and char simultaneously. The burning characteristics of single pellets made of wood and coal mixtures for co-firing in a laboratory reactor at rapid heating rates and under different oxygen concentrations were studied [8]. Flame and char combustion were recorded by means of a high-resolution camera. It was found that the char combustion time increased significantly compared with the volatile combustion time, which only varied a little, when the pellets contained coal in the mixture. An experimental study was conducted to examine the combustion behavior of single pine wood and empty fruit bunch pellets in an entrained-flow reactor in air and CO2/O2 [9]. Direct observation, optical pyrometer, and flue gas analysis are used to analyze the relationship between phenomenological events, particle temperature, and flue gas emission. The combustion characteristics of mixed rice straw and sewage sludge pellets in an air were investigated using a plasma combustion system [10]. A high-speed camera was used to record and observe the combustion processes. The shape and distribution of flame in hollow pellets are found to be better compared to solid pellets. A study was carried out to investigate the effects of pelletizing die temperature and moisture content on the combustion behavior of single wood pellets [11]. The time required for single pellet combustion increased with both the increase in pelletizing temperature and the moisture content. Bamboo pellets were combusted in a tube furnace individually at different air flow rates and at various temperatures, to investigate the dynamic emission characteristics during combustion processes [12].

Combustion behaviors and carbon monoxide emission characteristics of four kinds of coal and their blends pellets were investigated in a drop-tube furnace with various gas flow temperatures and oxygen concentrations [13]. The ignition of the bituminous pellet was found to be homogeneous, but it was heterogeneous for anthracite. The models of the flow, heat transfer, and mass transfer of single-particle biomass fuel during combustion were established and analogously calculated [14]. The results showed that in about 2 min, a 2-cm biomass pellet fuel was completely burnout, and the highest temperature reached was 1280 k. The effects of wood pellet particle size, environmental conditions, and blending ratio on the combustion characteristics of single fuels and blends using a TGA and a drop tube furnace were investigated [15]. The results indicate that the pellet showed a higher mass reduction in the devolatilization region and a faster reaction rate compared with coal. The combustion behaviors of single particles of Zhundong lignite in air at various temperatures in a horizontal tube furnace were investigated [16], aided with a charge-coupled device camera. A combination of high-speed and spectroscopic imaging and image processing techniques is used to investigate the combustion behaviors of single biomass pellets in a visual drop tube furnace under oxy-fuel combustion conditions [17]. Characteristic parameters of the burning pellet, such as flame size and temperature, are defined and computed based on the images obtained. The combustion properties of wood pellets, Japanese cedar, and larch were evaluated using a cone calorimeter [18]. The changes in heat release rate, weight decrease during the burning process, ignition time, flameout, and burnout time, as well as combustion heat, are determined. A new method for measuring the volume of a burning wood pellet was developed [19]. The volume of a burning wood pellet can be calculated by image processing and the direct measurement of a burning pellet in gasification mode. Surface temperature, volatile gas temperature, and polycyclic aromatic hydrocarbon (PAH) emission were reported over the entire burning history of individual biomass pellets in various combustion atmospheres [20]. PAHs are mainly released when the temperature of the pellet exceeds ∼600 K.

A thermochemical conversion process of a single wood pellet under low temperature isothermal heating was investigated [21]. The results showed that the wood decomposed primarily at 240 to 520 °C, with low-temperature ignition occurring at 293 °C. Single particles of eucalyptus, pine, and olive residue combustion behaviors were investigated by a transparent visual drop-tube furnace and a high-speed camera coupled with a long-distance microscope [22]. Due to differences in chemical compositions and properties of the three materials, their combustion behaviors were also somewhat inconsistent. Single particles of sugarcane bagasse, pine sawdust, torrefied pine sawdust, and olive residue were burned in a drop-tube furnace in both air and O2/CO2 atmospheres containing different oxygen fractions [23]. The results showed that the volatile flames of biomass particles were much less sooty than those of previously burned coal particles of analogous size, and the char combustion durations were briefer as well. With optical access that enables particle combustion characterization via a high-speed camera, a wire mesh system for rapid radiation heating and combustion of individual fuel particles was used [24]. Both fuels showed a sequential combustion of volatile matter followed by char combustion. The combustion characteristics of single biomass pellets in air of different temperatures and O2/CO2 containing various oxygen fractions were investigated [6]. Char and coal single particle combustion in different gas mixtures has been performed [25]. The interior temperature of the particles was examined, and the results showed that the particles burnt at greater temperatures in combinations with high water vapour concentration than in mixes with dry O2/CO2. The burning characteristics and combustion visualization analysis of single pellets made of wood and coal mixtures for co-firing were studied experimentally [8]. The results showed that the char combustion time increased significantly in comparison to the volatile combustion time, which only varied a little, when the pellets contained coal in the mixtures. Experimental and mathematical modeling approaches are employed to study the combustion characteristics of a single biomass particle [26]. Different processes such as moisture evaporation, devolatilization, tar cracking, gas-phase reactions, and char gasification are examined. The effect of volatile matter and fixed carbon content in sawdust; a mixture of pine and willow, corncob, and rice husk on single-particle combustion phases; and their luminous properties using a high-speed camera were investigated [27]. The results showed that the ignition mechanisms of volatile matter and fixed carbon corresponded to homogeneous and heterogeneous reactions, respectively. The combustion kinetic characteristics of wood powder and pellets were investigated using TGA and a tube furnace system [28]. The results showed that the combustion mechanisms of wood pellets differed significantly between the volatile and char combustion stages. The burning of corn straw and poplar biomass pellets and the release characteristics of potassium (K) were studied using laser induced breakdown spectroscopy [29].

Single pellets of sewage sludge, sunflower husks, wheat straw, and their mixtures were combusted in an electrically heated furnace at fluidized bed combustion process temperatures [30]. Over 90% of K and P were found to be captured within the residual ash, with 30–70% of P in crystalline K-bearing phosphates for mixtures with low amounts of sewage sludge. A cylindrical corn straw pellets were burned at elevated temperatures and different air flow velocities [31]. The pellets devolatilized and formed volatile envelope flames, upon extinction of which the chars experienced concurrent heterogeneous combustion and ash fusion, as was observed. The combustion characteristics of a single BIC fuel (Japanese knotweed) in high temperature air flow in a blast furnace were investigated [32]. Single particle (coal and biomass mixtures) combustion in a hot gas stream at a rapid heating rate and their flame characteristics were studied [33]. The impact of the high coal-blending ratio caused an increase in the flame size and intensity, and the ignition time was close to that of pure coal particles. Torrefied softwood, spruce bark, waste wood, miscanthus, and wheat straw pellets were combusted at 3% O2 and 100% N2 under different temperatures [34]. The effect of raw material composition on the combustion characteristics of single pellets produced from stem wood, branches, and bark of Norway spruce compared with commercial stem wood pellets made of Norway spruce and Scots pine sawdust, respectively, was examined [35]. The results showed that the char combustion time is of major importance for the total conversion time. The combustion process of a Korean wood pellet was studied through experiments and numerical analysis at 500 °C in an air-oxygen mixture [36]. The combustion process was divided into three categories based on their findings: gasification, flame burning, and char burning.

The combustion and mass degradation of a single biomass particle were investigated using a cone calorimeter [37]. The particle drying, volatile combustion, and char combustion, besides a superposition of homogeneous and heterogeneous combustion phases, were observed. The combustion behavior of a single biomass pellet was investigated for pine wood and bean husk in comparison to sub-bituminous coal [38]. The combustion behavior of single pulverized biomass (olive residue, almond shell, and hazelnut shell) and lignite coal particles under high temperature–high heating rate conditions was investigated [39]. The results showed that all biomass particles ignited homogeneously, forming large and circular volatile matter envelope flames, followed by distinct char combustion phases. The combustion behavior and images of single pulverized biomass particles from ignition to early stages of char oxidation in a confined region with hot combustion products produced by a flat-flame McKenna burner were investigated [40]. Single particle combustion behaviors of coal-biomass fuel mixtures were studied [41]. Because of higher ash and particle density, rice husk showed higher flaming time and char glowing time. Single particles were supported by a water-cooled cover and then exposed above a flame, simulating biomass combustion in a furnace [42]. Measurements of ignition delay, volatile burning, and char burn-out times were obtained using high-speed image capture. TGA was used to evaluate the combustion behavior of biomass pellets made from oil palm leaves and fronds, para-rubber leaf litter and branch, and their blends [43]. The results showed that the burnout temperature was improved and higher in all cases of mixed biomass pellets.

All practical applications, including packed and fluidized-bed combustion, as well as suspended and pulverized fuel combustion, need research into the behavior of a single biomass particle. The behavior and characteristics of biomass pellets (wheat and rice husk waste) in terms of combustion and mass loss % were explored in this study under various air temperatures, air flow rates, and new pellet orientation angles inside the combustor. Images of the burning process of biomass pellets were captured using high-speed photography. The ignition time, temperature, devolatilization duration time (volatile pyrolysis combustion duration time) and temperature, maximum temperature and time, char burnout temperature and time, and so on are determined. The independent parameters (operational circumstances) and the combustion characteristics parameters are provided and debated.

2 Materials and methodology

2.1 Material preparation and characterization

The raw biomass materials as shown in Fig. 1 were collected from Al-Sharkia Province farms in Egypt and dried in the sun and then kept at room conditions in tightly sealed plastic bags to prevent moisture pollution. Thereafter, they were ground by using grinder (Model MDY-2000, China). The physio-chemical composition and properties are measured using different measuring devices. The fixed carbon content was determined by difference during the proximate analysis utilizing a muffle furnace (Model: Ney Vulcan D-130, USA) to measure moisture, volatile, and ash percentages, the elemental composition using a CHNS elemental analyzer, Model Vario EL III, fibre composition, X-ray fluorescence using (lab- × 3500, Oxford, UK), scanning electron microscopy (SEM) using (Quanta 250 FEG; FEI, USA), and Higher Heating Values using Model 1341EE Plain Oxygen Bomb Calorimeter and the measurements are presented in Table 1.

Fig. 1
figure 1

Wheat and rice straw stalks from field, chopped stalks, materials powders, and produced pellets

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Table 1 Chemical compositions (proximate and ultimate) and elemental, structural, and XRF analyses of the biomass materials
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2.2 Material sieving and pellet preparation

The two materials were sieved, and the determined average mean particle diameters of the powdered rice straw and wheat straw, respectively, were 553.7 and 625.4 µm, as shown in Table 2. The sieving process was carried out with 100 gm of each biomass material using the ELE Sieve Shaker (code: 80–0200/0), which has a series of nested sieves. The American Society for Testing and Materials (ASTM) C136.39, the American Association of State Highway and Transportation Officials (AASHTO) T27, and ASTM D 422—Standard Test Method for Particle-Size Analysis were used to sieve a dry sample utilizing the mechanical sieving technique. For both Folk and Ward measures, the PSD statistical analysis and statistical parameters are performed using the GOLDISTAT tool. The bulk density of biomass, chemical reactions, and heat and mass transport mechanisms can all be affected by the particle’s PSD, aspect ratio, and sphericity. After grinding and screening, the requisite sieved sizes (300 to 800 µm) are pelletized using a hydraulic press with a capacity of 400 bar in a cylindrical shape of 15 mm in diameter and 30 mm in length under pressure of 30–40 bar. Rice straw pellets have an average mass of 5.1 gm and a bulk density of 270 kg/m3 based on triplicate measurements, while wheat straw pellets have values of 4.2 gm and 200 kg/m3 for average mass and bulk density, respectively.

Table 2 Statistical parameters for the biomass material powder using method of moments and Folk and Ward method
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2.3 Experimental setup

A laboratory fixed bed furnace was used in this study. This furnace was also equipped with an analytical balance (Brother (Jiangsu, China)), enabling it to be used as a macro-TGA. To measure and record the temperature at these locations, two stationary, calibrated thermocouples (type K) are fixed at the surface (at the midpoint) and the center of the pellet (at the midpoint on the center line). The temperatures at these two locations are recorded using a multichannel digital data logger thermometer (TM-747DU with accuracy (0.1 percent + 0.7 °C) and resolution (0.1 °C/1 °C)) that is connected to a computer. The pellet is located on the center line of the combustor to be far away from the velocity boundary layer inside the combustor tube. The combustor outer wall was insulated with a ceramic-fiber material 30 mm thick and externally covered by a galvanized steel sheet 1 mm thick. According to the heat balance equation, the relative error of the temperature measured by the thermocouple is about 3–8% the value, which is within the allowable range. The air flow is admitted through an electric blower as shown in figure. The blower is rotated using an electric motor of speed 3000 rpm and capacity of 3.0 hp, and the air inlet to blower is controlled through a gate at air intake. The heating section includes heaters of total capacity of 18 kW, and the hot air temperature is controlled through PID temperature controller (Toho TTM-J4) connected with thermocouple for this purpose. A vertical steel combustor pipe of 30 cm long and 10 cm in dimeter with an openings of 27 cm length and 5 cm width, on which a thermal glass is installed to depict the burning process. Also, the combustor tube is equipped with slots for inserting the thermocouples. The air flow rate is measured using a calibrated orifice-plate meter (accuracy =  ± 0.6%) and is controlled by a ball valve. Pressure difference across the orifice-plate meter was used to determine the velocity of the fluidizing air. The pressure difference was monitored by airflow PVM100 micro manometer (TSI Company, Shoreview, MN, USA) (accuracy ± 1% of reading ± 1 digit and pressure resolution of 1 Pa). A schematic representation of the experimental setup, including measurement instruments, is shown in Fig. 2. Prior to the trials, the furnace was heated to reaction temperature using PID controlled electrical heaters. A distribution perforated plate was used to continually supply pre-heated air to the furnace from the bottom. A steel wire was used to hang the sample in a mechanism connected to the analytical balance (± 1 mg). Before recording the readings, the bouncing effect is removed. The furnace had a front observation window where a video camera could record the pellet combustion sequences. The mass of the pellets was measured in real time (once every second). Combustion tests were carried out at temperatures of 350 °C, 400 °C, 450 °C, and 500 °C with different air flow rates. The pellet is secured in the required position (angle of inclination) using a mechanical mechanism devised and constructed for this purpose in the combustor tube at its centerline in the almost middle position after attaining the steady-state temperature of the incoming hot air (1–2 h).

Fig. 2
figure 2

Schematic diagram of the combustion setup

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3 Results and discussion

3.1 Material characteristics

It can be observed from the chemical composition presented in Table 1 that the volatile and fixed carbon contents of the two materials are comparable. In addition, wheat straw has more char to be combusted. Wheat straw has a low ash content compared to rice straw and similar HHVs. Their moisture content was < 10%, which is preferred by ISO/TC 238 solid biofuels [44]. There were no major differences in their elemental composition, with the exception of their nitrogen contents, where wheat straw is free of N2, so fuel NOx cannot exist. The low H2 content in both materials is preferable because the problem of increasing moisture in fuel during storage will be minimized.

Some deposition-predictive indices can be calculated using XRF data. Large melting point elements like Ca, Mg, or Si (calcium or magnesium silicates) reduce this occurrence, while high concentrations of low melting point elements like Na, K, S, or Cl (alkali sulfates or chlorides) augment it [45]. The XRF analysis of both materials is shown in Table 1. The dominating components in the two materials are clearly (K), (S), (Si), and m (Ca), with minor elements such as Fe, P, Zn, and others. K2O, SiO2, SO3, and CaO are the most common oxides found in both materials. Both minerals include high levels of K and Ca, which are significant in the reaction between silica (SiO2) and (S) to form alkali silicate and alkali sulfates, which can cause ash deposition in combustors and impact combustion rates and pollutant emissions. Wheat straw contains a higher potassium content than rice straw, which has a greater impact on the combustors’ life. Both materials are sodium and magnesium free, whereas iron and calcium concentrations drop during combustion, resulting in lower ash levels. Because phosphorus and phosphorus pentoxide (P2O5) have such low concentrations, they have no influence on the ash fusion temperature of the two materials.

A variety of deposition-predictive factors as slag and fouling indexes govern the utilization of biomass as a fuel in furnaces and power plants [46]. The base/acid ratio (Rb/a) as a slagging indication for forecasting the slagging impact can be given as [46]:

$${R}_{b/a}=\frac{{Fe}_{2}{O}_{3}+CaO+MgO+{K}_{2}O+{Na}_{2}O+{P}_{2}{O}_{5}}{{SiO}_{2}+{TiO}_{2}+{Al}_{2}{O}_{3}}$$
(1)

where the symbol of each oxide represents its weight percentage from the XRF data.

The Fouling index (Fu) can be evaluated by the following equation [46]

$${F}_{u}={R}_{b/a} ({K}_{2}O+ {Na}_{2}O)$$
(2)

According to the XRF data in Table 1, the slagging index (Rb/a) for rice straw and wheat straw is 0.98 and 2.48, respectively. The fouling index for rice straw and wheat straw, on the other hand, is 22.2 and 102, respectively. Rice straw has a medium deposition risk (0.5 < Rb/a < 1 and 0.6 < Fu < 40, medium deposition risk), but wheat straw has a high deposition risk (Rb/a > 1 and Fu > 40, high deposition risk) based on these values.

Hemicellulose is made up of a variety of saccharides, as seen in Table 1. It has multiple branches that, at low temperatures, easily breakdown into volatiles, particularly in wheat straw residue. Because both materials have a strong skeletal structure, their cellulose elements account for 35–40% of the cell wall material, where cellulose is made up of polymers of glucopyranose residues and decomposes at a greater temperature than hemicellulose. Although lignin is made up of aromatic rings with several branches that allow it to decompose across a large temperature range (100–900 °C), the lignin in the cell walls is not regarded highly branched because its percentages do not surpass 7%.

The morphology, as well as their pore diameters and distribution, were studied using SEM images as shown in Fig. 3. The pores in the samples are representative of the stalk’s skeletal structure. Wheat and rice straw grains have an elongated shape, and both materials exhibit irregularly produced (amorphous) fibers with no micro- or mesoporous microstructure. This indicates a dense network with a lot of polymeric hemicellulose, lignin, and cellulose components. The scanned images in Fig. 3 demonstrate a mix of particle sizes that produce the optimum pellet endurance due to higher inter-particle bonding and fewer interspaces. Lignin emerges as droplets on the surface depicted in Fig. 3.

Fig. 3
figure 3

Scanning electron microscope (SEM) for different biomass materials

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3.2 Mass loss histories of pellets

Because the pellet is colder than the oven, the sample temperature may drop or rise slowly in a short period of time, but it quickly starts heating and losing moisture, and some simple volatile matters first, and a significant amount of mass loss (10–20% of its original mass) occurs within a few minutes. Exothermic oxidation is characterized by pellet temperatures that are significantly higher than those of the incoming gas, with peaks approaching 800 °C for both materials. Slower rates of solid consumption (the slope of the weight loss curve, Fig. 4a–c) and temperature increase (250 < t < 650 s) are clearly associated with char oxidation, as shown in the figure, which correspond to results in [47].

Fig. 4
figure 4

a Temperature and mass loss histories of pellets at different air temperature. b Temperature and mass loss histories of pellets at different air flow. c Temperature and mass loss histories of pellets at different orientations

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As evidenced by the weight loss in connection with the pellet temperature in Fig. 4a–c, the mass decreased quickly in the volatile burning region and gradually in the char burning part. As the temperature of the hot air increases, the mass decrease appears to be much faster, as stated in [11]. For rice straw, the char burning time accounts for the majority of the overall conversion time (~ 200–250 s) compared to volatile combustion (~ 100–150 s) at all operating circumstances, while higher values are reported for wheat straw (Tables 4 and 5). Due to the material type and high air temperature employed, this matched to some extent with the published values in [36]. When 60–80% of the total weight has been lost, temperature peaks, showing the progress of the reaction from the pellet’s external surface to its center, where the center thermocouple reveals a greater temperature than the surface thermocouple. Within the diversity of the pellet composition, as indicated in Table 1, such a sequence of chemical and physical events is easily repeatable. The slow, heterogeneous char combustion continues, progressively slowing with a decrease in available surface area. As shown in Fig. 5a, b, the remnant solid char retains a nearly cylindrical shape after being combined with ash (approximately 2–5 percent of the initial weight). Despite the little mass, the original geometry has been kept, which explains how combustion occurs: the original porous material is consumed internally. On the other hand, depending on the material type, particle size of the raw materials, pellet porosity, and other factors, some pellets break down or crack near the end of combustion. As illustrated in Fig. 4a–c, there is an approximately linear relationship between pellet mass loss and time. It demonstrates that heat transport regulates the rate of mass loss during pyrolysis. Conduction occurs in both the porous pellet and the heated air. The mass loss and pellet temperature profiles at the surface and center allow the two main phases of degradation (pyrolysis and char oxidation) to be distinguished as an obvious shift in slope in Fig. 4a–c for both materials. When the pellet quickly enters a hot gas environment, a greater percentage of its mass is discharged as volatiles. Indeed, as the oven temperature rises, the char yield (measured by the change in slope in the mass loss profile) falls, from 20% at 350 °C to 10% at 500 °C. As a result, rather than the heterogeneous combustion of char depicted in Fig. 5a, b, the heat release must come from homogeneous, gas phase interactions of the volatiles. As soon as the volatiles begin their exothermic oxidation, the pellet temperature rises quickly, practically exponentially with the gas temperature, as predicted by the Arrhenius law, which states that the hotter the air temperature, the faster the homogenous processes occur. Because of the particle’s shrinking, as illustrated in [48], the pellet reflects the rise in temperature caused by interactions in the gas phase nearby. Char oxidation raises the pellet temperature even higher due to the sluggish reaction of heterogeneous combustion.

Fig. 5
figure 5figure 5

a Images of combustion processes of rice straw pellets under different operating conditions. b Images of combustion processes of wheat straw pellets under different operating conditions

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3.3 Temperature–time histories at surface and center of pellet

The temperatures (surface and center) and mass loss histories of wheat and rice straw pellets at various operating conditions are shown in Fig. 4a–c. Three regimes of single pellet combustion were identified: drying and heating, flameless pyrolysis, and char conversion. The duration of flameless pyrolysis was measured from the time the pellet was ignited after being introduced into the furnace until the luminosity around the pellet began to fade and small dark gray patches developed on the pellet surface. It demonstrated a greater and faster weight loss than the char combustion stage, accounting for approximately 70–80 percent of total weight loss at temperatures ranging from 200 to 450 °C [22] (Fig. 4a–c), attributed to hemicellulose and cellulose thermal degradation. The char conversion was measured from the time the pellet luminosity (brightness) began to fade and dark gray patches on the pellet surface developed until the char stopped glowing [11]. The char that remained after devolatilization of the pellets combusted at a temperature of roughly 450 to 650 °C, accounting for around 20% of the overall mass loss. In terms of mass loss histories under various operating parameters, the majority of mass was released from the pellet during flameless pyrolysis with gas-phase combustion of pyrolysis products (Fig. 5a and b). During char conversion, the char particle glowed less, indicating that combustion was taking place in the char particle (Fig. 5a and b). As expected, the diagrams indicate that lowering the air temperature in the combustion chamber results in a longer pellet combustion time (see Tables 4 and 5). The time measured between the moment of fixing the pellet inside the combustor and ignition is known as the ignition delay. At ignition delay, the mass loss is < 10%, and its values decrease with increasing hot air temperature and mass flow rate but fluctuate between high and low values according to the angle of inclination (Tables 4 and 5). The increase in oxidant and heat transfer boosted the igniting of biomass pellets due to the effects of high hot air flow rate and air turbulence. A higher stream temperature enhances heat transfer to the fuel, facilitates the chemical reaction, leads to a more significant heat release rate from pellet oxidation, and shortens the total combustion time (Tables 4 and 5).

Important heat transfer characteristic parameters can be used to clarify the heat transfer between the hot stream and the pellet. A circular cylinder in crossflow gives the average Nusselt number from reference [54] as follows: \({Nu}_{avg}=0.3+(0.62 {{Re}_{d}}^\frac{1}{2}{Pr}^\frac{1}{3}/[1.0+{(0.4/Pr)}^{2/3}\)]1/4)\({[1.0+{({Re}_{d}/282000)}^{5/8}]}^{4/5}\), where (ReD Pr > 2) and Biot number can be estimated from h = NuD k/db, Bi = h L/k, and thermal diffusivity α = k/ρ Cp, where h is the convective heat transfer coefficient, W/m2K; k is the thermal conductivity of the materials, W/mK; dp is the pellet diameter, m; Pr is the Prandtl number = 0.71; and L = db/2 is characteristics length, m [55]. The obtained Bi number was greater than 1.0, corresponding to a thermally thick item and showing that, during the initial stages of heating, there is always a temperature difference between the surface of the pellet and its interior as seen in Fig. 4a–c. The temperature gradient suggests that it is crucial to consider the thermal conduction barrier within the pellet and air. The exothermic reaction starts to release heat once the heat reaches a certain point and ignites the biomass pellets, which greatly boosts the temperature in the second stage before reaching a maximum. Additionally, when the pellet burns out, the recorded pellet center temperature always shows a very little decrease relative to the surface and then rises to a temperature almost equal to the ambient hot stream. The primary cause of this phenomena is the spreading reaction front entering the pellet center prior to extinction. As seen in Fig. 4a–c, the convective flow moves air to devour the char through a heterogeneous reaction from the windward edge surface to the leeward side center of the pellet as shown in photos. The char burns up quickly, and eventually ash is left as a residue. Because their chemical characteristics are quite similar, Table 3 demonstrates that both materials had Bi ranging between 7.5 and 9.5 with very little variation. According to the calculated thermal and physical parameters, rice straw has a thermal diffusivity of 5.89 × 10−7 m2/s, and wheat straw has a thermal diffusivity of 2.68 × 10−7 m2/s. The rate at which a surface heat flux spreads through a material is measured by thermal diffusivity. The surface temperature is more sensitive to variations in heat transfer on the surface the lower the material’s thermal diffusivity is. The Cp values are obtained from [58], and the k values are equal to 0.09 W/m K for wheat straw [56] and 0.191 for rice straw [57].

Table 3 The dimensionless parameters calculated for both material pellets at different hot air temperatures
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3.4 Combustion visualization video analysis

Images of the combustion process of biomass pellets were captured using high-speed photography. The video images were post-processed using standard software that permits frame-by-frame viewing and counting (and thus time) as well as image contrast and brightness adjustments. The photos show two types of ignition: (1) homogeneous ignition of volatile or heterogamous particles due to flameless combustion (no visible flames) and (2) heterogeneous ignition on the particles’ surface (char combustion). After ignition, the combustion was no longer a homogeneous combustion of volatiles. Imaging started when biomass pellets were positioned inside the furnace and stopped once a visible flame existed or brightness emerged. The time measured between both events is the ignition delay. Biomass pellets have a high bulk density; thus, the rate of devolatilization is slow, and ignition takes a long time, as shown in photos (Fig. 5a and b). To investigate the changes in combustion sequences under various situations, three variables were chosen: pellet orientation, hot air temperature, and air flow rate. The following phenomena were observed in the photos: (i) beginning with inserting the pellet into the combustor, where its color is light yellow (material nature), and progressing through ignition, where the color changes to gray. With the emittance of volatile and its combustion, the color turns into light brown and then light brown with a dark cap until it becomes a fully bright radish color at the end of volatile combustion. At the start of char combustion, the luminosity intensity (brightness) disappears slowly, and dark parts on the pellet surface appear. Until the char combustion is ended, the color turns dark gray or black. (ii) Because the rate of volatile release is too small to sustain and construct a sizing flame due to the low temperature of hot air, almost all biomass pellets ignite homogeneously as flameless combustion of volatiles with a white bright color, followed by char combustion phases in which the pellet becomes reddish. (iii) As the flow temperature dropped, the heating rate on the surface of the pellets dropped, increasing the ignition delay. (iv) In most pellets, the initial indicator of combustion is a bright spot on the pellet’s edge surface with a high brightness. After a few seconds, the brightness becomes brighter and extends throughout the entire pellet, and the color of the pellet changes depending on the conversion process. (v) With the fixed carbon to volatile matter mass ratio, the ratio between the evaluated char burnout time and volatile combustion time increases quadratically, demonstrating that char combustion is a significantly slower process than volatile combustion. (vi) At a high air temperature of 500 °C, wheat straw shows obvious combustion sequences with increased luminosity. (vii) Rice straw burnout time ranged between 280 and 480 s, while wheat straw burnout time ranged between 230 and 438 s, which is virtually comparable because their chemical analyses are almost identical. (viii) Although their volatile contents are the same, the volatile combustion time of wheat straw (17–258 s) is shorter than that of rice straw (20–300 s). This could be due to the fact that the particle sizes, shapes, and structural compositions of these materials differ.

3.5 Parameters of combustion properties in all operating and prescribed conditions

Tables 4 and 5 show the values of the various combustion characteristic parameters under various operating situations. Figures 6 and 7 depict selected pellet temperatures at the surface (Ts) and center (Tc), as well as mass loss histories and visual representations of the RS and WS pellet combustion processes under various operating conditions. Figure 6a compares the scenario of air temperatures of 350 and 450 °C at a specific air flow rate and a 0° angle pellet orientation. When the air temperature is 450 °C, the pellet (Tc) profiles have higher values than when the air temperature is 350 °C. This could be owing to the fast heat transfer from the pellet surface to the pellet center, which speeds up the reaction and raises the temperature (Tc). In terms of mass loss history, the mass loss profile shows a quick drop in the devolatilization zone at 400 °C, which concluded in a shorter time interval than at 350 °C for these particular conditions.

Table 4 Combustion characteristics of rice straw pellets
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Table 5 Combustion characteristics of wheat straw pellets
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Fig. 6
figure 6

Temperature and mass loss histories with visual images of the RS pellet combustion processes at certain conditions with uncertainty bars for the case of angle 45°

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Fig. 7
figure 7

Temperature and mass loss history with visual images of the WS pellet combustion processes at certain conditions

Full size image

Comparing the center and surface temperature histories for angle 45° and 90° at specified air temperature and two different air flow rate is shown in Fig. 6b. The figure shows that angle 90° has somewhat higher (Ts) values than angle 45°, but the (Tc) values are significantly greater. Because the strong turbulence at the pellet edges, separation zone breadth, and vortices strengths at 45° angle, let heat transfer from the outside to the inside of the pellet may be delayed. Figure also demonstrates that the rate of devolatilization is nearly same in both angles where the slope of the curves is nearly identical.

Figure 6c shows a comparison of the center and surface temperature histories for angle 0° at certain air temperature and two different air flow rates. It is shown that the (Ts) values at a 78 kg/h air flow rate are higher than those at a 71 kg/h air flow rate. Because the total heat transfer coefficient is larger at high air velocity (Re), the heat transported from outside the pellet to its inside is enhanced, and the chemical reaction is enhanced, allowing (Tc) to exhibit higher values than (Ts) at this specific circumstance. Due to their differing natures, these materials show almost identical behavior trends with varying values.

3.6 Effect of operating conditions on combustion characteristics

3.6.1 Effect of pellet orientation inside the combustor

The temperatures at the pellet surface and center, as well as the mass loss percentage histories for the two materials, are shown in Fig. 4c. In general, the combustion and mass loss characteristics of all pellet orientations inside the combustor are nearly identical. The temperature inside the pellet is lower than the surface temperature in some orientations at the start of heating and ignition, but as time goes on, the pellet center temperature rises in comparison to the pellet surface temperatures. This means that during the ignition, the temperature inside the pellet will be low, the temperature distribution inside the pellet will be nearly constant, and no reaction will occur, while the temperature outside the pellet is high. Although there is a certain temperature gradient between the particle surface and inside the pellet itself, their temperature change trends are similar and match with the results in [49,50,51]. When compared to other angles of pellet orientation, the pellet horizontal position (0°) exhibits reduced combustion and mass loss % time intervals. Because the total heat transfer surface area exposed to the hot gas is higher in the horizontal position, with more uniform flow stream lines surrounding the pellet and vortex formation in the wake zone behind the pellet, the total heat transfer surface area exposed to the hot gas is higher. On the other hand, this short period is insufficient to allow the complex volatile compound to crack into simple ones and be released from the pellet during pyrolysis, resulting in a lower maximum temperature than with other pellet orientations. The maximum temperature at the midpoint of the pellet centerline is higher for angle 90° than for angle 0°, but the maximum surface temperature deviations between the two orientations are small (see Tables 4 and 5) because the local heat transfer coefficient of angle 90° is strong for a tiny area near the front of the pellet in the upstream zone. The influence of the horse shoe vortex surrounding the cylinder near the heat transfer surface is responsible for this. The local heat transfer in the downstream zone, on the other hand, is higher and spans a larger area [52]. The order of increasing the maximum temperature at the pellet surface is 30° > 60° > 90° angles because the local heat transfer coefficient decreases as the cylinder inclination angle increases (= 30°, 45°, 60°, and 90°) in the upstream region and increases in the downstream zone. For cylinder inclination angles of 30° and 90°, respectively, the highest and minimum heat transfer coefficients were determined [52]. In contrast to the reasonable interpretation of the local heat transfer coefficients, the angle of 45° displayed unusual behavior. These various cylindrical pellet orientations produce varying vortex intensities along the trailing and leading edges, as well as in the wake region behind the pellet, which improves heat transfer from the hot gas to the pellet. The turbulent flow behind the pellet was aided by the separate flow, which disrupted the boundary layer. This will have an impact on the pellet’s ability to transfer heat. Because of the conduction internal heat transfer, porosity, and composition of the pellet, this behavior order differs in terms of the highest temperature at the pellet center.

3.6.2 Effect of hot air temperature

The effect of air temperature on the temperature and mass loss histories of the two materials is shown in Fig. 4a. The graphic depicts how increasing the temperature speeds up the degradation and combustion of pellets. It is a result of a pellet’s temperature rising faster, as well as the mass loss percentage, as seen in the diagram. The pyrolysis phase is nearly linear, with the slope increasing as the temperature of the initial air rises. Because radiative heat transfer is proportional to temperature to the fourth power and heat transfer is a strong exponential function of temperature, even a little increase in the beginning air temperature can have a significant impact on igniting time. Pellet heating, on the other hand, takes a long time at low air temperature (350 °C) compared to high air temperature (500 °C). As the temperature of the beginning air rises, the rate of chemical reactions rises, and the conversion rate of the combustion process rises. As the temperature of the initial air rises, the inner molecules gain enough kinetic energy to increase their velocity and break chemical bonds, which raises the frequency factor of chemical reaction rate [53], which raises the rate of collision and chemical reaction rate. According to the results, increasing the beginning air temperature from 350 to 500 °C reduces the devolatilization time, as indicated in Tables 4 and 5. When the initial air temperature rises, devolatilization accelerates due to a faster internal heat transfer that speeds up the rate of devolatilization reactions [49]. As shown in Tables 4 and 5, increasing the beginning air temperature from 350 to 500 °C reduces the char combustion time, while increasing the devolatilization temperature. This effect was expected since the hot environment increased heat transport and so accelerated the combustion reaction rate. The maximum combustion temperature rises as the initial air temperature rises, affecting cellulose, hemicelluloses, and lignin breakdown. During char combustion, ash has a significant impact on heat transmission to the fuel surface as well as oxygen diffusion to the fuel surface (Table 1). As in rice straw, a high ash content in pellets causes poor heat transfer and oxygen diffusion to the pellet surface. As the initial air temperature rises, the combustion rate also rises, releasing more energy and boosting combustion efficiency.

3.6.3 Effect of mass flow rates of hot air

The influence of air flow rates on the temperature and mass loss histories of the two materials is shown in Fig. 4b. Heat is transferred to the pellet surface by conduction within the pellet, convection, and radiation from the surrounding area. As a result, larger air flow rates improve pellet-air interactions, which improves heat and mass transfer and hence enables higher heat of combustion. The graphs depict how the mass left declines with a consistent slope during the ignition propagation period. As the air flow rate increased from 68 to 83 kg/h, the gradient of the curves increased based on the different air flow rates and hot air temperatures. Due to some differences in the physio-chemical composition of wheat and rice straw pellets, the influence of air flow rate on wheat straw pellets is more pronounced when compared to other factors. At the end of the ignition propagation stage (steady mass loss period), the mass loss slowed significantly, signifying the char oxidation stage.

Tables 4 and 5 demonstrate that when the air flow rate increases, the ignition time and temperature decrease, the mass loss percentage increases, and the combustion duration time reduces. The tables also show that as the air flow rate increases, the pellet maximum surface temperature rises as more heat is transferred from the hot gas to the pellet surface via convection, and radiation is transferred from the hot gas to the pellet surface in a shorter amount of time, especially for wheat straw pellets. Furthermore, due to internal conduction heat transfer and heat created by chemical reaction, the pellet’s maximum central temperature rises as the air flow rate rises, resulting in greater temperatures than the maximum surface temperature. On the other hand, as the air flow rate is increased, the mass loss drops because more volatiles are cracked and released due to the pyrolysis of the materials. The devolatilization time interval and mass loss % time rise as the air flow rate increases, as shown by the slope of the mass loss percent-time profiles in Fig. 4b. Devolatilization and char proceed less when the air flow rate increases, which may be referred to a more rapid internal heat transfer. Due to a few differences in their physio-chemical compositions, rice straw pellet burnout time increases and mass loss percentage falls, while wheat straw pellet burnout time increases and mass loss percentage drops (Table 1). Due to the opposing of oxygen penetration by emitted volatiles from inside the pellet and the porosity of it, the burnout temperature at the pellet surface reduces, while the burnout temperature at the pellet center rises.

4 Conclusions

The temperature, velocity, kind of biomass material, and orientation of the pellet with regard to the airflow are all factors that affect how quickly biomass pellets heated in hot air streams ignite. Investigating the ignition and burning of biomass pellets under the aforementioned operational circumstances is the goal of this study. Two different biomass pellet types were researched in a fixed bed tube furnace, and the histories of temperature and mass loss were measured and examined. Utilizing high-speed photography, the burning processes were seen and documented. The experiments led to the following discoveries:

  1. 1-

    Shorter char and volatile ignition and burnoff times result from higher hot air temperatures. Rice straw takes longer (17–258 s) for the volatile to burn than wheat straw (20–300 s). This might be as a result of the various particle sizes, shapes, and structural composition of different materials. Due to improved heat transfer by convective temperature conditions, which are essential for pre-processing and the early ignition phase of biomass, it also enhances the ignition temperature at the surface.

  2. 2-

    For rice straw pellets, the maximum volatile combustion time increased from 86 to 136 s as air temperature increased from 350 to 450 °C, decreased from 136 to 118 s as air flow rate increased from 64 to 83 kg/h, and increased from 158 to 138 s as pellet orientation angle decreased from 90 to 45 degrees. For wheat straw, the maximum volatile combustion time decreased from 108 to 45 s as air temperature increased from 350 to 500 °C.

  3. 3-

    The volatile burnout time for wheat straw decreases from 234 to 177 s as air temperature increases from 350 to 500 °C, from 231 to 130 s as air flow rate increases from 64 to 83 kg/h, and from 231 to 189 s as pellet orientation angle decreases from 90 to 0°, except for angles 45 and 60 the time increase. For char burnout time, it decreases from 274 to 231 s as air temperature increases from 350 to 500 °C, from 430 to 270 s as air flow rate increases from 71 to 83 kg/h, and from 390 to 274 s as pellet orientation angle decreases from 90 to 0°.

  4. 4-

    The weight loss evolution of pellet combustion occurred in three stages. The first is the drying stage, where moisture and very simple gases are released (< 10%). At temperatures above 450 °C, the second stage (mloss = 10%-85%) (devolatilization dominated) showed a faster weight loss rate than the third stage (char combustion dominated).

  5. 5-

    Regarding pellet orientation in the air flow, the volatile and char ignition times increase up to 30° and then drop up to 90° as the pellet orientation angle increases, with angle 45° giving the lowest value. The same pattern may be seen during volatile and char burnout times. When the pellet has lost 60–80% of its total weight, temperature peaks, indicating the movement of the reaction from the pellet’s external surface to its center, where the center thermocouple reveals a higher temperature than the surface thermocouple.

  6. 6-

    As char combustion progresses, the luminosity intensity (brightness) fades, and dark portions of the pellet surface emerge. The color turns dark gray or black after the char combustion is finished, as shown in the photos. At a high air temperature of 500 °C, wheat straw shows obvious combustion sequences with increased luminosity. Rice straw burnout time ranged between 280 and 480 s, while wheat straw burnout time ranged between 230 and 438 s, which is virtually comparable because their chemical analyses are almost identical.

  7. 7-

    Despite the fact that the volatile contents of the two materials are identical, the volatile combustion duration of wheat straw (17–258 s) is less than that of rice straw (20–300 s), which could be due to differences in particle sizes, shapes, and structural compositions.

  8. 8-

    The maximum combustion temperature for wheat straw pellet rises from 361.9 to 506.4 °C and 316 to 450 °C for rice straw pellet as the initial air temperature rises from 350 to 500 °C. During char combustion, ash has a significant impact on heat transmission to the fuel surface as well as oxygen diffusion to the fuel surface. As in rice straw, a high ash content in pellets causes poor heat transfer and oxygen diffusion to the pellet surface.

  9. 9-

    These study results can be useful for designing stoves that can be used for various purposes, such as water desalination and biochar production or as an alternative to oil and natural gas, as well as in household apparatuses for heating purposes.