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Article

Experimental Study on Single-Particle Combustion Characteristics of Large-Sized Wheat Straw in a Drop Tube Furnace

by
Haoteng Zhang
,
Lihui Yu
,
Cuina Qin
,
Shuo Jiang
and
Chunjiang Yu
*
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310058, China
*
Author to whom correspondence should be addressed.
Energies 2025, 18(15), 3968; https://doi.org/10.3390/en18153968
Submission received: 27 June 2025 / Revised: 21 July 2025 / Accepted: 22 July 2025 / Published: 24 July 2025
(This article belongs to the Section A4: Bio-Energy)
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Abstract

Co-firing large-sized straw biomass in pulverized coal boilers is a potential pathway for carbon emission reduction in China’s thermal power plants. However, experimental data on large-sized straw combustion under pulverized coal boiler combustion conditions are critically lacking. This study selected typical large-sized wheat straw particles. Employing a two-mode experimental setup in a drop tube furnace (DTF) system simulating pulverized coal boiler conditions, we systematically investigated the combustion behavior and alkali metal release characteristics of this large-sized straw biomass, with combustion processes summarized for diverse particle types. The findings reveal asynchronous combustion progression across particle surfaces due to heterogeneous mass transfer and gas diffusion; unique behaviors distinct from denser woody biomass, including bending deformation, fiber branching, and fragmentation, occur; significant and morphology-specific deformations occur during devolatilization; fragmentation universally produces particles of varied shapes (needle-like, flaky, blocky, semi-tubular) during char combustion; and potassium release exceeds 35% after complete devolatilization and surpasses 50% at a burnout degree exceeding 80%. This work provides essential experimental data on the fundamental combustion characteristics and alkali metal release of large-sized wheat straw particles under pulverized coal boiler combustion conditions, offering engineering application guidance for the direct co-firing of large-sized flexible straw biomass in pulverized coal boilers.

1. Introduction

Under the robust impetus of China’s “Achieving carbon neutrality by 2030” strategic goals, the transformation of China’s energy structure is accelerating. As coal-fired power generation remains the dominant source of electricity supply, there is an urgent need to develop practical and effective low-carbon technological pathways. Among these, the direct co-firing of biomass fuels within existing mature large-scale pulverized coal boiler systems has emerged as a key technological focus for both industry and academia, owing to its potential for rapid and substantial reduction in fossil fuel carbon emissions.
China’s biomass resources are predominantly agricultural residues such as crop straws (mainly rice, wheat, and corn straws), endowing this category of biomass with immense potential for energy utilization [1]. Since such biomass is predominantly herbaceous in nature, it possesses a flexible fibrous texture. The coal mills (utilizing steel ball impact grinding) in existing coal-fired power plants are only suitable for processing rigid fuels (like coal and various minerals). In contrast, hammer mill shredders (employing gear-tearing mechanisms) are better suited for processing such flexible fibrous biomass. However, regardless of the crushing method used, energy consumption increases sharply as the target particle size decreases [2,3,4]. Biomass like rice straw, wheat straw, and corn straw features a porous and hollow structure. It also has high combustion reactivity and a thin-walled structure. Therefore, within the combustion chamber of a coal-fired boiler, larger-sized biomass of these types may achieve combustion efficiency comparable to that of micron-sized coal powder.
Currently, experimental data on the combustion characteristics of large-sized straw biomass remain relatively scarce. Existing research predominantly focuses on micron- or millimeter-sized powdered biomass fuels and woody biomass fuels. For instance, Saastamoinen et al. [5] investigated the combustion of 180–315 μm wood powder under pulverized coal boiler conditions using an entrained flow reactor and numerical simulations. Their results indicate that wood powder burns significantly faster than coal particles of the same size, suggesting the feasibility of utilizing larger wood particles in engineering applications. Riaza et al. [6] tested four types of 75–150 μm biomass powders in a drop tube furnace at 1400 K using optical techniques, demonstrating minimal differences in combustion rates among different biomass powder types at identical sizes. Zhang et al. [7] numerically modeled the combustion process of spherical and cylindrical wood particles. Their computational results reveal that increasing the particle diameter from 0.2 mm to 9.6 mm prolongs the volatile release time from 1.5 s to 40 s, with larger particle sizes reducing the volatile release rate per unit mass. Collectively, these studies demonstrate that particle size and biomass feedstock type have minimal impact on the combustion process at the micrometer scale. However, as particle size increases, the size effect exerts a substantially greater influence on the biomass conversion rate. As particle size increases further, the physicochemical reaction processes during fuel particle combustion undergo significant changes. Compared to powdered biomass fuels, larger biomass particles exhibit pronounced internal heat and mass transfer gradients, leading to marked differences in their heating process, pyrolysis volatile release pathways, and char formation pathways [8,9]. Simultaneously, the dominant factor controlling the char combustion rate of larger particles may shift from chemical reaction control (typical for powders) to diffusion control [10,11]. This size effect not only alters the reaction mechanisms of pyrolysis and char combustion (e.g., volatile flame structure, char combustion mode) but also correspondingly influences alkali metal release behavior [12].
Fixed-bed reactors or flame furnaces are commonly used to study the thermochemical conversion characteristics of larger-sized fuel particles. Yu et al. [9] studied the thermochemical conversion process of 0.5–5 mm pine wood particles in a fixed-bed reactor, demonstrating that larger particle sizes reduce carbon combustion reactivity. However, fixed beds cannot simulate the gas–solid contact conditions arising from the relative motion between particles and gas flow in pulverized coal boiler furnaces. Mason [13] investigated the combustion of thirteen types of near-spherical 0.5–5 mm wood particles in a methane flame furnace at 1600 K, revealing that the combustion rates differed by up to a factor of three among different wood particle types. Flame furnaces facilitate the direct observation of particle ignition, flame propagation, and char combustion phenomena. Yet, they rely on fuel combustion (e.g., methane) for heat, making precise and independent control of the particle combustion atmosphere difficult. Furthermore, flame furnaces also fail to simulate the gas–solid contact conditions created by particle–gas relative motion. In contrast, drop tube furnaces (DTFs) are widely recognized as experimental facilities that can more comprehensively simulate pulverized coal boiler combustion conditions [14,15]. DTFs offer precise atmosphere control and can replicate the high temperatures (>1200 °C), high heating rates (>1000 °C/s), and gas–solid contact conditions of pulverized coal boiler furnaces.
However, DTFs are typically designed for powdered particles. Large-sized biomass particles have long burnout times and settle rapidly under gravity. Within the conventional furnace height and residence time, they often exit the furnace before achieving complete combustion, making it difficult to capture and study their full combustion process. For larger-sized fuel particles, some researchers have employed methods like suspending them using wire or placing them in wire mesh baskets within the DTF for thermochemical conversion [16,17]. However, these methods also suffer from the drawback of not fully simulating the gas–solid contact conditions resulting from particle–gas relative motion in pulverized coal boiler furnaces. Therefore, in existing research, experimental data on the combustion of large-sized biomass fuels under the high-temperature, high heating rate, and gas–solid contact conditions remain insufficient, particularly for the flexible straw biomass that will be studied in this paper.
Co-firing larger-sized straw biomass in pulverized coal boilers is a potential technological pathway for carbon emission reduction in China’s thermal power plants. However, existing research exhibits a severe lack of data on large-sized straw combustion under pulverized coal boiler combustion conditions. Therefore, this study selected typical large-sized (centimeter-scale) wheat straw particles that had undergone preliminary crushing and employed a two-mode experimental setup (sequential sampling during combustion and decoupled combustion experiments) in a DTF experimental system (simulating the high-temperature, high heating rate, and gas–solid contact conditions of a pulverized coal boiler furnace) to systematically investigate the combustion behavior and alkali metal release characteristics of this type of large-size straw biomass. The key parameters tracked include particle shape evolution, fragmentation, size reduction, dynamic changes in moisture/volatile matter/fixed carbon/ash content, burnout progression, and alkali metal (K) release. Based on detailed experimental data, the combustion progression of typical large-sized wheat straw particles in the DTF was summarized. This study obtained fundamental combustion characteristics and alkali metal release data for large-sized wheat straw particles under drop tube furnace (DTF), providing valuable experimental data and engineering application guidance for the direct co-firing of large-sized flexible straw biomass in pulverized coal boilers.

2. Experimental Section

2.1. Experimental Materials

Figure 1a shows wheat straw particles after preliminary crushing, sourced from a coal-fired power plant in China planning a co-firing project. The target size of the crushing system is 5 cm. Therefore, apart from millimeter-scale debris particles incidentally produced during the crushing process, most large-sized particles are close to but do not exceed 5 cm in length. As particles derived from different parts of wheat straw exhibit distinct shapes (flaky, tubular, cylindrical), their combustible mass per unit length, internal component transport conditions, and average gas–solid contact conditions on particle surfaces during flow processes all differ. Therefore, this study classified these particles based on their shape differences. Typical large-sized wheat straw particle types include approximately rectangular flake-like particles representing leaves or stems (designated WSF1, shown in Figure 1b), approximately single-layer tubular particles representing stems (WSF2, Figure 1c), approximately multi-layer tubular particles representing leaves wrapping stems (WSF3, Figure 1d), approximately cylindrical–tubular particles representing stems connected to nodes (WSF4, Figure 1e), and approximately cylindrical particles representing nodes (WSF5, Figure 1f). Among these, WSF1-4 are approximately 1–5 cm long, while WSF5 is approximately 0.5–1 cm long. Additionally, WSF4 can be regarded as an elongated WSF5. Distinguishing it from other millimeter-scale debris particles, WSF5 possesses higher combustible content, and greater mass, so it is also classified as a large-sized particle in this study.
This study comprises two experimental components: a sequential sampling experiment during combustion and decoupled combustion experiments. For the sequential sampling experiment, experimental samples included 5 cm long WSF1 particles, 1 cm long WSF2, WSF3, and WSF4 particles, and 0.5 cm long WSF5 particles, as shown in Figure 1g. The particle size selection was based on the following considerations: The preliminary experiments revealed that lighter particles with less combustible material (e.g., WSF1) burnout over shorter settling distances, while heavier particles rich in combustible material (e.g., WSF2, WSF3, WSF4, WSF5) remain in the early stages of combustion even upon reaching the bottom of the furnace. Thus, within the 1–5 cm range, the shortest length (1 cm) was selected for WSF2-4, while the longest length (5 cm) was chosen for WSF1. Additionally, to distinguish WSF5 from the 1 cm WSF4 particles, the tubular structures at both ends of WSF5 were deliberately removed, retaining only the central cylindrical node section with a length of approximately 0.5 cm.
However, regardless of the particle size adjustments, the sequential sampling experiment could only capture partial combustion processes for each type of large-sized wheat straw particle. For example, for fast-settling, slow-burning particles like WSF4 and WSF5, even the relatively shorter size made it difficult to capture samples from their late combustion stages. Conversely, for slow-settling, fast-burning particles like WSF1, capturing their early combustion state was challenging even with the relatively longer size. Therefore, supplementary decoupled combustion experiments were necessary. This approach, which separates the pyrolysis and char combustion processes, enables the acquisition of a more complete combustion process for each type of large-sized wheat straw particle. Since the decoupled combustion experiments are not constrained by factors like particle settling velocity and burnout time, experimental samples for the decoupled combustion experiments were 5 cm long WSF1, WSF2, WSF3, and WSF4 particles and 0.5 cm long WSF5 particles, as shown in Figure 1h.
In all experiments, all wheat straw particle types were cut into experimental samples with standardized dimensions. To minimize experimental error, raw materials with similar diameters were selected for cutting, particles showing significant weight deviations were discarded, and each individual particle was weighed and measured prior to the experiment.
The fundamental fuel properties and potassium (K) content of the wheat straw particle types are presented in Table 1. Notably, the potassium content varied across the particle types. This variation originates from the non-uniform distribution of potassium within the plant structure, which correlates with organ-specific metabolic activity: different plant organs exhibit differential metabolic intensities, resulting in corresponding variations in the accumulated potassium content [18].

2.2. Experimental Systems and Methods

This study was conducted using a drop tube furnace (DTF) experimental system. The schematic diagram and photographs of the experimental system are presented in Figure 2(a1,b1) and Figure 2(a2,b2), respectively. Key equipment, namely drop tube furnace (Figure 2c), single-particle feeding device (Figure 2d), gas distribution plate with glass tube (Figure 2e), and sampling device with sampling slide block (Figure 2f), are illustrated in the respective figures. The DTF employs a three-stage independent temperature control system, with a furnace tube made of silicon carbide, an inner diameter of 85 mm, a heating zone length of 2 m, and the ability to operate continuously at 1300 °C. All experiments in this study were performed at 1250 °C. The single-particle feeding device, equipped with water cooling and sealing mechanisms, was maintained at room temperature within the feeding chamber. The sampling device, consisting of a double-walled water-cooled sleeve and an integrated nitrogen injection tube, enabled the sampling of particles during the combustion process by adjusting the insertion height into the furnace chamber. The nitrogen nozzle velocity (3.9 m/s) ensured particle quenching and cooling within 0.1 s.
The sequential sampling experiment was used in a static air atmosphere within the furnace, as shown in Figure 2a. Prior to the experiment, the nitrogen flow rate in the sampling device was set to 3 L/min, matching the flow rate of the vacuum pump. The sampling device was inserted into the furnace according to the target settling height and secured using a slide block. A pre-weighed and measured single-particle experimental sample was loaded into the feeding device. At the start of the experiment, the feeding valve was opened. Once the particle fell into the sampling cylinder, the sample was collected, and its weight, dimensions, and shape were recorded. Samples were then categorized by settling height and stored for subsequent analysis. To ensure statistical reliability and provide sufficient sample mass for instrumental analysis, each wheat straw particle type was tested at least 20 times at the same settling height. The calculations confirmed that the static air volume in the furnace tube could support the complete combustion of 1.37 g of wheat straw. Since the mass of individual particles for all types was below 0.05 g, the oxygen supply in the furnace did not limit the combustion of a single particle. The particle sampling drop height range was set at 1–2 m, primarily based on considerations of residence time and combustion progression. During the initial combustion phase (0–1 m), particles possess greater mass, settle rapidly, and consequently have a shorter residence time and combustion duration. Conversely, in the latter combustion phase (1–2 m), particle mass decreases significantly, settling slows down, and residence time increases. Sampling within this latter phase (1–2 m) is therefore more effective for capturing the progression of particle combustion.
The decoupled combustion experiment employed a gas flow method to control the particle residence time, as shown in Figure 2b. This method alters only the particle’s absolute velocity, preserving the gas–solid velocity difference and the magnitude/direction of forces acting on the particle. The decoupled combustion experiments proceeded as follows: First, the experiments were performed under a nitrogen atmosphere. The particle residence time within the furnace was controlled by adjusting the flow rate and direction (co-flow or counter-flow) of the nitrogen. This continued until the residual mass ratio of the obtained samples stabilized (i.e., showed no significant further change). This state was defined as the point where volatile matter release was just complete, yielding pyrolyzed char samples. Subsequently, these pyrolyzed char samples were reintroduced into the drop tube furnace, which was then switched to an air atmosphere. Once again, by controlling the flow rate and direction (co-flow or counter-flow) of the air, samples representing the char combustion stage were obtained. Before decoupled combustion experiments could be carried out, the gas line configuration (counter-flow or co-flow) was set accordingly (e.g., blower line for counter-flow, vacuum pump line for co-flow). The pre-weighed and measured single experimental particle was loaded into the feeding device. At the start of the experiment, the feeding valve was opened simultaneously as the stopwatch was started. Upon visual confirmation of the sample exiting the bottom of the furnace through the glass pipe, the stopwatch was stopped and low nitrogen flow was immediately initiated to quench and cool the sample on the gas distribution plate. After the collection of the sample, its weight, dimensions, and shape were recorded, categorized, and stored for subsequent analysis. To ensure statistical reliability and meet the sample quantity requirements for further testing, at least 20 valid experiments were performed for each wheat straw particle type (or its char).

2.3. Analysis and Test Methods

Upon obtaining each biomass single-particle sample, the following parameters were recorded: shape change, fragmentation mode and probability, size reduction ratio, and residual mass ratio. In this study, a sample is defined as having undergone fracture if it contains two or more particles with lengths greater than 3 mm. For the fractured sample, the particle size is defined as the size of the largest particle, and the residual weight is the total weight of all of the fractured particles. The axial and radial dimensions of the sample are measured using a combination of coordinate paper and an optical microscope. The length of bent samples is estimated using the arc length formula. The length or diameter of the samples with broken branches or unequal diameters is estimated using the average value method. The burnout degree was calculated using Equation (1), where φ is the combustion conversion rate, %; A0 is the ash content of the raw material, %; and Af is the ash content of the obtained sample, %.
φ = 1 − A 0 ( 100 − A f ) A f ( 100 − A 0 )
After grinding the original raw samples and the samples obtained from the experiment, proximate analysis and analysis tests for different potassium forms were conducted separately. The specific treatment and detection methods for each potassium form are shown in Figure 3. The absolute mass fractions of various types of potassium and proximate analysis components (moisture, volatile matter, fixed carbon, ash) in the samples were obtained by multiplying the measured relative mass fraction by the corresponding sample’s residual mass ratio.

3. Results and Discussion

3.1. Combustion Characteristics

The typical shape changes, size changes, fragmentation probability, residual weight rate, and proximate analysis of the combustion process of five types of wheat straw particles are shown in Figure 4, Figure 5 and Figure 6, respectively.
During combustion, WSF1 gradually bended, and its bundled fibers separated from each other, causing branching. As the settling height increased, it eventually formed into several needle-like particles. Fragmentation typically involved the radial fracture of these bundled fibers or their axial separation, producing shorter or narrower curved fragments or needle-like particles. WSF1 exhibited significantly higher size reduction, exceeding 60% in both axial and radial dimensions, compared to other particle types. This pronounced reduction likely resulted from the pre-separation of parallel fibers along the circumference of the originally tubular stem, reducing constraining forces. Due to its slow settling velocity and lower combustible content, moisture and volatiles were completely released by a 1 m settling height. Consequently, data for the early combustion stage were missing and will be obtained through decoupled combustion experiments.
For WSF2 and the inner tubular layer of WSF3, the circumferentially parallel-aligned bundled fibers in the inner tubular layer gradually separated or fractured, eventually forming needle-like and flake-like particles at greater heights. A key distinction was observed in WSF3, where the detachment of the outer leaf layer occurred before the volatile release from the inner tubular layer. This is likely because the outer leaf layer releases volatiles and contracts first, while the inner tubular layer has not yet initiated volatile release or begun contracting. Consequently, the outer leaf becomes mechanically constrained by the inner tubular layer during contraction, resulting in its fragmentation. Therefore, WSF3 fragmentation consisted of outer leaf detachment or breakage in the early combustion stage, while its inner tubular layer fragmented similarly to WSF2 in the later combustion stage. Volatile release was nearly complete for WSF2 at 1.5 m and for WSF3 at 1.75 m. The 1 cm size provided suitable settling velocity and combustible mass, enabling the capture of samples representing approximately 40% of the combustion process, primarily during the volatile release phase. Data for the remaining combustion information will be supplemented by decoupled combustion experiments.
WSF4 and WSF5, characterized by faster settling velocity and higher combustible mass, yielded only early-stage samples, corresponding to approximately 20% mass loss. Notably, 25–35% of volatiles remained unreleased in both types even at the 2 m height. During this captured stage, their overall shape remained largely unchanged without significant fragmentation, though the minor spalling of needle-like char particles occurred occasionally. WSF4 showed less than 10% contraction in both axial and radial dimensions, while WSF5 developed concave end faces. The internal region of both WSF4 and WSF5 samples retrieved at all heights appeared yellow. Information regarding their later combustion stages will be acquired via decoupled combustion experiments.
Furthermore, apart from the inability to obtain early-stage samples for WSF1, an uneven yellow distribution was observed on the surfaces of all wheat straw particle types during the volatile release stage. This is because of the differential mass transfer and gas diffusion conditions on distinct surface regions during their settling, such as the windward side, leeward side, and inner walls of tubular particles, leading to non-uniform volatile release rates. This phenomenon demonstrates that during the combustion of large-sized biomass particles, the uneven gas–solid contact conditions on the surface result in asynchronous combustion processes at different locations on the surface.

3.2. Decoupling Combustion Characteristics

The typical shape changes in the five types of wheat straw particles after volatile release are shown in Figure 7a. It can be observed that under a nitrogen atmosphere, after complete volatile release, WSF1 transformed into twisted narrow flakes; WSF2 became curved tubes with uneven diameters; WSF3 separated into its outer leaf layer and inner tubular layer, both bending, with the outer leaf layer exhibiting a higher length reduction rate than the inner tubular layer; WSF4 developed bent stems at both ends and its bundled fibers splayed apart; and some WSF5 particles exhibited concave end surfaces while others developed perforations through both ends.
The typical shape changes in the five types of wheat straw particles at the char combustion stage are shown in Figure 7b. It can be observed that under an air atmosphere, at the char combustion stage, WSF1, WSF2, and WSF3 typically fragmented into several needle-like particles, indicating that the final combustion stage of centimeter-scale wheat straw particles involves the sustained burning of the carbon skeleton structure formed by bundled fibers. The stems at both ends of WSF4 usually fractured away from the central node, ultimately forming flake-like, needle-like, or blocky particles. This occurs because the central node exhibits a solid structure, while the leaf-wrapped stems at both ends are tubular with lower mechanical strength. Particularly after volatile release, the ends develop branched needle-like structures with significantly weakened mechanical strength, making them highly prone to fragmentation; the concentric carbon layers of WSF5 typically undergo layer-by-layer peeling or fragmentation, ultimately forming semi-tubular, flake-like, or blocky particles. This behavior originates from the nodular section of the raw sample, a solid cylinder formed by circumferential layer-by-layer stacking, causing the resulting char to easily delaminate circumferentially. Consequently, it can be inferred that the carbon combustion process of WSF4’s central node resembles that of WSF5. This indicates that none of the five types of wheat straw particles can maintain their original shape during char combustion. Fragmentation into needle-like, flake-like, or blocky particles dominates this stage. This finding differs from the observations made by Riaza et al. [19] on the shape and size evolution of millimeter-sized woody biomass during combustion. Rod-like or needle-like millimeter-sized woody biomass particles tend to transform towards a more equidimensional (near-spherical) shape, whereas larger centimeter-scale straw biomass is more prone to bending, fiber bundle splaying, and fragmentation.
Notably, the char samples obtained after complete volatile release under nitrogen atmosphere remained intact, with no observed fragmentation or shedding of fine carbon particles. This contrasts with the observations made in the sequential sampling experiment, where wheat straw particles shed fine carbon particles or fragment during the volatile release stage. This difference in shedding/fragmentation behavior is likely related to the localized temperature increase caused by volatile combustion in the sequential sampling experiment, combined with the asynchronous combustion progress made at different locations on the particle surface (e.g., the windward side typically enters the char combustion stage earlier).
The residual weight and burnout degrees of the five types of wheat straw particles after volatile release and at the char combustion stage are shown in Figure 8a. Here, WSF-V and WSF-C represent the samples after volatile release and at the char combustion stage, respectively, obtained from the decoupled combustion experiment. The char of WSF4 and WSF5 has larger masses and required higher counter-flow gas rates to extend their residence time in the furnace. However, high counter-flow rates tended to blow samples at the late stage of char combustion back into the furnace, so only samples at the mid-stage of char combustion could be collected.
The size reduction ratios of the five types of wheat straw particles after volatile release are shown in Figure 8b. Compared to the size reduction ratios observed in the sequential sampling experiment (where volatiles were nearly fully released for WSF2 and WSF3 at drop heights of 1.5 m and 1.75 m, respectively), the size reduction ratios under nitrogen atmosphere were significantly higher. This indicated that the size reduction results under nitrogen atmosphere cannot reflect the size reduction behavior of wheat straw particles under actual combustion conditions. This discrepancy is likely because the particle residence time in the sequential sampling experiment was shorter, while the decoupled combustion experiment (nitrogen atmosphere) provided ample time for complete volatile release and size reduction. Particle size reduction during the char combustion stage primarily results from carbon consumption, whereas reduction during the volatile combustion stage is mainly caused by the release of internal matter. This study observed significantly lower reduction rates (5–65%) during pyrolysis for all wheat straw particle types compared to the 60–90% reported by Holmgren et al. [20] for micrometer-scale near-spherical wheat straw particles. This contrast suggests that larger dimensions may constrain particle reduction through morphological and structural effects. Multiple studies on the reduction characteristics of large-sized (>1 cm) woody biomass particles during pyrolysis [citation needed] similarly demonstrate that biomass particle shrinkage is governed by particle size, geometric shape, and heating rate [21,22,23,24].

3.3. Release Characteristics of Alkali Metals (Potassium)

The potassium content during the combustion process of five types of wheat straw pellets is shown in Figure 9. Specifically, the potassium content of samples obtained from the sequential sampling experiment is shown in Figure 9a–e, while the potassium content of samples obtained from the decoupled combustion experiment is shown in Figure 9f.
As the settling height increases, potassium was gradually released from all types of wheat straw particles. Among the released components, water-soluble potassium and weak acid-soluble potassium were the predominant fractions. In contrast, strong acid-soluble potassium and insoluble potassium were present at lower levels and exhibited relatively stable behavior during combustion. Water-soluble and weak acid-soluble potassium in biomass typically existed in organic/inorganic water-soluble or exchangeable ionic forms, whereas strong acid-soluble and insoluble potassium mostly belong to mineral salt potassium. Critically, water-soluble potassium released as KCl vapor crystallizes on high-temperature heat-transfer surfaces (e.g., boiler waterwalls and platen superheaters). These deposits induce the high-temperature corrosion of metal surfaces [25,26]. Meanwhile, the strong acid-soluble potassium and insoluble potassium retained in biomass ash, such as potassium silicates and aluminosilicates, typically serve as key contributors to adhesion, deposition, and slagging formation on convection pass heating surfaces in boilers [27,28].
Analysis combining the potassium content of the samples (Figure 9f) and the burnout degree (Figure 8a) revealed that after the complete release of volatiles from all types of wheat straw particles, the potassium release ratios exceeded 35%, and when the burnout degree exceeded 80%, the potassium release rate surpassed 50%. This aligns with the findings of Liu et al. [29], who investigated the potassium release process during the combustion of compressed poplar wood and corn straw particles using a flame furnace and optical measurement techniques. Their results indicated that while the primary potassium release peak occurs during the volatile release stage, a smaller secondary peak is also observed during the char combustion stage. They attributed the potassium release peak in the char combustion stage to volatile–char interactions, char consumption, and temperature increase, which primarily results from char combustion. Previous studies have shown that a portion of potassium is retained in the ash during biomass combustion, and the potassium release rate is typically dependent on combustion conditions and biomass types [30,31,32]. Given that wheat straw has a high potassium content of 1.9%, its potassium release rate can reach up to 50% under the simulated high-temperature, high-heating-rate and gas–solid contact conditions in this study. Consequently, when co-firing large-sized straw biomass fuels in pulverized coal boilers, significant attention must be paid to the risks of the high/low-temperature heating surface corrosion, slagging, and fouling caused by the substantial release of potassium from straw biomass fuels.

3.4. Combustion Process

Integrating the results from the sequential sampling experiments and the decoupled combustion experiments, the complete combustion process of five types of large-sized wheat straw particles can be summarized. In the original samples, most WSF1-4 particles were approximately 5 cm in length, while the cylindrical nodules in WSF4 or WSF5 measured about 0.5 cm. Thus, the 5 cm long WSF1-4 particles and the 0.5 cm long WSF5 nodules are taken as examples for the analysis.
WSF1 first undergoes approximately 0.9 s to complete the release of moisture and volatiles. During this stage, the particle experiences significant axial and radial size reduction (both >50%), forming bent, bifurcated slender flake coke particles, and releases about 42% of its potassium. Subsequently, the coke particle continues to burn for about 1.2 s to reach a 98% burnout degree. In this phase, the particle fragments into several shorter flakes or needle-like particles, achieving a potassium release ratio of 56%.
WSF2 first undergoes approximately 1.1 s to complete the release of moisture and volatiles. During this stage, the particle undergoes minor axial (~10%) and radial (>20%) size reduction, forming bent, bifurcated tubular coke particles, with a probability (<50%) of fragmenting into sub-particles larger than 3 mm, while releasing about 37% of its potassium. Subsequently, the coke particle burns for about 1.3 s to reach a 97% burnout degree. In this phase, the probability of fragmentation increases significantly (>50% and increasing as combustion progresses), primarily through circumferential separation or fracture of parallel-bundle fibers, forming flake or needle-like particles, with the potassium release ratio reaching 56%.
WSF3 first undergoes approximately 1.4 s to complete the release of moisture and volatiles. During this stage, the outer leaf layer burns and detaches or fragments first while the inner tubular layer undergoes minor axial and radial size reduction (both ~20%) and releases about 40% of its potassium. Subsequently, the coke particle burns for about 1.3 s to reach a 95% burnout degree. Due to the immediate detachment or fragmentation of the outer leaf layer upon entering the furnace, combined with fragmentation of the inner tubular layer, at least two particles burn simultaneously throughout the entire combustion process of WSF3. At a 95% burnout degree, the potassium release ratio reaches 55%.
WSF4 first undergoes approximately 3.1 s to complete the release of moisture and volatiles. During this stage, the particle undergoes minor axial and radial size reduction (<10%), forming coke particles with bent and bifurcated stems at both ends, with a low probability (<10%) of fragmenting, and releases about 44% of its potassium. Subsequently, the coke particle burns for about 3.5 s to reach an 85% burnout degree. In this phase, the stems at both ends fragment into needle-like or flake particles. Based on the combustion behavior of WSF5, the central node is inferred to undergo further fragmentation and burnout through circumferential layer-by-layer carbon exfoliation. At an 85% burnout degree, the potassium release ratio reaches 53%.
WSF5 first undergoes approximately 1.8 s to complete the release of moisture and volatiles. During this stage, the particle shape and dimensions remain largely stable, with a low probability (<6%) of exfoliation, yielding particles larger than 3 mm, but concavities or penetrations form at both end faces, and about 48% of its potassium is released. Subsequently, the coke particle burns for about 2.8 s to reach a 90% burnout degree. In this phase, the carbon layer around the circumference exfoliates in a layer-by-layer manner or breaks, forming semi-tubular, flake-like, or blocky particles, with the potassium release ratio reaching 59%.

4. Conclusions

  • During the volatile release stage, WSF1 particles transform into twisted narrow flake structures, while WSF2 particles form curved tubular structures. For WSF3 particles, the outer leaf layer burns and detaches first from the inner tubular layer, with the outer leaf exhibiting a significantly higher size reduction ratio than the inner tubular layer. WSF4 particles undergo stem bending and branching of bundled fibers at both ends. Regarding WSF5 particles, concavities form on both end faces in some samples, whereas penetrations develop in others.
  • At the char combustion stage, fragmentation occurs universally across all particle types. WSF1, WSF2, and WSF3 particles primarily fragment into needle-like or flake-like particles. Tubular stems at both ends of WSF4 particles typically fracture from the central node, generating flake-like, needle-like, or blocky particles. WSF5 particles experience layer-by-layer exfoliation or breakage of circumferential carbon layers, forming semi-tubular, flake-like, or blocky particles.
  • The combustion of large-sized biomass particles exhibits asynchronous progression across different surface regions due to heterogeneous mass transfer and gas diffusion conditions.
  • Distinct from relatively denser woody biomass, large-sized straw biomass combustion features unique behaviors, including bending deformation, the branching of bundled fibers, and fragmentation.
  • For all large-sized wheat straw particles combusted in the drop tube furnace, potassium release exceeds 35% after complete volatile release and surpasses 50% when the burnout degree exceeds 80%.
  • Despite innovating a two-mode method for comprehensive particle combustion data, it still differs from actual large-particle combustion (e.g., segregated volatile/fixed carbon stages, no char heating by volatiles, cooling/reheating from re-entrainment). Future drop tube furnace tests could extend the heating zone or use co/counter-flow gases for control particle acceleration/deceleration to enhance the replication of boiler conditions.

Author Contributions

Writing—original draft preparation, visualization, validation, investigation, H.Z.; investigation, L.Y.; investigation, C.Q.; investigation, S.J.; writing—review and editing, conceptualization, supervision, funding acquisition, resources, C.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Key Technologies Research and Development Program of China [Grant No. 2022YFB4202002].

Data Availability Statement

Dataset available on request from the authors. The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
DTFDrop tube furnace
WSFsWheat straw fragments
WSF-VSamples after volatile release
WSF-CSamples at the char combustion stage

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Figure 1. Crushed wheat straw particles: (a) preliminary crushing wheat straw particles; (b) rectangular flake-like particles; (c) single-layer tubular particles; (d) multi-layer tubular particles; (e) cylindrical–tubular particles; (f) cylindrical particles; (g) experimental particles of sequential sampling experiments; (h) experimental particles of decoupled combustion experiments.
Figure 1. Crushed wheat straw particles: (a) preliminary crushing wheat straw particles; (b) rectangular flake-like particles; (c) single-layer tubular particles; (d) multi-layer tubular particles; (e) cylindrical–tubular particles; (f) cylindrical particles; (g) experimental particles of sequential sampling experiments; (h) experimental particles of decoupled combustion experiments.
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Figure 2. The experimental systems and key equipment. (a) Sequential sampling experiment system; (b) decoupled combustion experiment system; (c) drop tube furnace; (d) single-particle feeding device; (e) gas distribution plate with glass tube; (f) sampling device with sampling slide block. 1. Drop tube furnace; 2. water cooler; 3. single-particle feeding device; 4. sampling device; 5. sampling slide block; 6. sampling cylinder; 7. flow meters; 8. vacuum pump; 9. nitrogen gas; 10. single large-sized particle; 11. blower; 12. gas switching valve; 13. gas distribution plate; 14. stopwatch; 15. glass pipe; and 16. cross pipe.
Figure 2. The experimental systems and key equipment. (a) Sequential sampling experiment system; (b) decoupled combustion experiment system; (c) drop tube furnace; (d) single-particle feeding device; (e) gas distribution plate with glass tube; (f) sampling device with sampling slide block. 1. Drop tube furnace; 2. water cooler; 3. single-particle feeding device; 4. sampling device; 5. sampling slide block; 6. sampling cylinder; 7. flow meters; 8. vacuum pump; 9. nitrogen gas; 10. single large-sized particle; 11. blower; 12. gas switching valve; 13. gas distribution plate; 14. stopwatch; 15. glass pipe; and 16. cross pipe.
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Figure 3. Schematic diagram of the leaching and detection process for potassium.
Figure 3. Schematic diagram of the leaching and detection process for potassium.
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Figure 4. Typical shape changes during large-sized wheat straw particles’ combustion: (a) WSF1; (b) WSF2; (c) WSF3; (d) WSF4; (e) WSF5.
Figure 4. Typical shape changes during large-sized wheat straw particles’ combustion: (a) WSF1; (b) WSF2; (c) WSF3; (d) WSF4; (e) WSF5.
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Figure 5. Size reduction rates and fragmentation probabilities during large-sized wheat straw particles’ combustion.
Figure 5. Size reduction rates and fragmentation probabilities during large-sized wheat straw particles’ combustion.
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Figure 6. Residual weight and proximate analysis during large-sized wheat straw particles’ combustion.
Figure 6. Residual weight and proximate analysis during large-sized wheat straw particles’ combustion.
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Figure 7. Typical shape changes during large-sized wheat straw particles’ decoupled combustion: (a) typical shape changes after volatile release; (b) typical shape changes at char combustion stage.
Figure 7. Typical shape changes during large-sized wheat straw particles’ decoupled combustion: (a) typical shape changes after volatile release; (b) typical shape changes at char combustion stage.
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Figure 8. Residual weight/burnout degree and size reduction ratios during large-sized wheat straw particles’ decoupled combustion.
Figure 8. Residual weight/burnout degree and size reduction ratios during large-sized wheat straw particles’ decoupled combustion.
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Figure 9. Release process of K during large-sized wheat straw particles’ combustion.
Figure 9. Release process of K during large-sized wheat straw particles’ combustion.
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Table 1. Basic fuel property analysis and potassium (K) content of wheat straw particles.
Table 1. Basic fuel property analysis and potassium (K) content of wheat straw particles.
Car
(%)		Har
(%)		Oar
(%)		Nar
(%)		Sar
(%)		Mar
(%)		Aar
(%)		Var
(%)		FCar
(%)		Qnet,ar
(J/g)
WSF40.514.7335.440.720.8612.275.4765.5716.6914,115
Kwater (%)Kweak acid (%)Kstrong acid (%)Kinsoluble (%)Ktotal (%)
WSF1.5080.3070.030.0461.891
WSF11.1510.2230.0220.0261.422
WSF21.1380.3380.020.0291.525
WSF31.4860.3230.0390.0381.886
WSF41.7050.3970.0380.0472.187
WSF51.3320.2840.0320.0351.683
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Zhang, H.; Yu, L.; Qin, C.; Jiang, S.; Yu, C. Experimental Study on Single-Particle Combustion Characteristics of Large-Sized Wheat Straw in a Drop Tube Furnace. Energies 2025, 18, 3968. https://doi.org/10.3390/en18153968

AMA Style

Zhang H, Yu L, Qin C, Jiang S, Yu C. Experimental Study on Single-Particle Combustion Characteristics of Large-Sized Wheat Straw in a Drop Tube Furnace. Energies. 2025; 18(15):3968. https://doi.org/10.3390/en18153968

Chicago/Turabian Style

Zhang, Haoteng, Lihui Yu, Cuina Qin, Shuo Jiang, and Chunjiang Yu. 2025. "Experimental Study on Single-Particle Combustion Characteristics of Large-Sized Wheat Straw in a Drop Tube Furnace" Energies 18, no. 15: 3968. https://doi.org/10.3390/en18153968

APA Style

Zhang, H., Yu, L., Qin, C., Jiang, S., & Yu, C. (2025). Experimental Study on Single-Particle Combustion Characteristics of Large-Sized Wheat Straw in a Drop Tube Furnace. Energies, 18(15), 3968. https://doi.org/10.3390/en18153968

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