Introduction

Coal is a vital global energy resource, with studies indicating that worldwide coal reserves amount to 1.07 trillion tons1. China ranks third in global coal reserves and stands as the world’s largest coal producer, accounting for approximately 47.5% of the global total. As the dominant energy source in China, coal consumption reached 4.98 billion tons in 2024, representing 55.8% of global consumption2,3,4. During coal mining operations, China alone generates over 200 million tons of surplus coal fines annually5. The accumulation of such massive quantities not only occupies substantial land resources but also poses risks including dust pollution and spontaneous combustion. Given the urgent need for coal resource recycling and efficient utilization, the rational management of surplus coal fines has emerged as a critical challenge for coal mining enterprises6,7,8.

Emulsion explosives are widely employed in various blasting engineering applications due to their simple manufacturing process and excellent explosive performance. The incorporation of metal or non-metal powders into emulsion explosives represents a prevalent method for modifying their explosive power, with aluminum powder being the most commonly used metallic additive. The addition of small amounts of aluminum powder can enhance the reaction temperature during detonation as well as improve explosive and combustion effects. However, excessive aluminum powder can not only cause misfires but also increase the mechanical sensitivity of the explosive, posing potential safety hazards9. Non-metallic additives such as coal-based solid waste floating beads are primarily introduced as inert agents to regulate detonation velocity and charge density parameters10. Some researchers have incorporated energetic glass microspheres or boron powder to enhance the detonation performance of emulsion explosives, though studies investigating the introduction of coal powder remain limited11,12.

As a non-metallic material, coal powder inherently contains substantial energy potential. Our research group has conducted preliminary investigations into coal-containing industrial explosives. Tu Changchang et al.13 observed poor compatibility and safety concerns when adding coal powder to ammonium nitrate fuel oil (ANFO). Wang Quan et al.14 addressed these compatibility issues by incorporating micronized coal powder aqueous suspensions into ANFO, simultaneously achieving improved detonation velocity. In this study, coal powder water slurry (CPWS) was prepared and introduced into site-mixed emulsion explosives. Through detonation velocity measurements, brisance testing, and TG-DTG analysis, we systematically investigated the effects of varying CPWS on both explosive performance and thermal decomposition processes of site-mixed emulsion explosives. This research aims to provide theoretical foundations for the recycling of surplus coal powder generated during coal mining operations.

Experiment

Raw materials and instruments

Reagents: Ammonium nitrate (AN): Industrial grade, purity ≥ 98.5%, Anhui Shuntai Chemical Co., Ltd.

Diesel oil No. 0: Standard commercial grade, Sinopec Corp.

Emulsifier (T-154): Chemical pure, Aladdin Reagent Co., Ltd.

Engine oil: commercial grade, Sinopec Corp.

Sodium nitrite (SN): Analytical grade, purity ≥ 99.0%, Shanghai Macklin Biochemical Co., Ltd.

Coal Powder Water Slurry (CPWS): Prepared in-house from pulverized bituminous coal. The solid content of the slurry was 46 wt%. The coal powder had a median particle size of 2.3 μm, with a particle size distribution ranging from 0.399 to 28.23 µ.

Instruments: MCR 102e rotational rheometer, BT-9300ST laser particle size analyzer, XSP-86 series infinity-corrected biological microscope, BSW-3 A intelligent five-stage detonation velocity tester, DSCQ2000 thermogravimetric differential scanning calorimeter(TG-DSC).

Sample preparation

According to the formulation presented in Table 1, accurately weigh the components of the aqueous and oil phases. Heat these mixtures on a heating plate until they become clear and transparent. Subsequently, introduce the pre-weighed CPWS into the aqueous phase. Using an emulsifying disperser set at 1200 rpm, gradually add the aqueous phase into the oil phase over a period of 45 s under continuous agitation. Continue stirring for an additional 3 min to produce the on-site mixed emulsion matrix sample containing the CPWS. Finally, sensitize the sample by adding a predetermined amount of 33% sodium nitrite solution, thereby obtaining the final on-site mixed emulsion explosive specimen incorporating the CPWS.

Table 1 On-site mixing emulsified matrix formula for CPWS.
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Physical property testing of samples

  1. (1)

    Viscosity testing

The variation in viscosity of the matrix sample was evaluated under precisely regulated temperature conditions spanning from 25 °C to 85 °C, utilizing a heating rate of 2 °C per minute and a constant shear rate of 0.5 s− 1. Measurements were conducted using a parallel plate rotor setup, with a fixed gap of 1 mm between the rotor and the plate sensor. The viscosity-temperature relationship was determined by systematically recording viscosity values at successive temperature increments.

  1. (2)

    Microstructural analysis

For microstructural analysis, approximately 1.0 g of the freshly prepared emulsion matrix sample was evenly smeared onto a clean glass slide. To ensure adequate dispersion and minimize droplet coalescence during observation, the sample was gently diluted and dispersed using two drops (approximately 0.1 mL) of 0# diesel oil (identical to the continuous oil phase) and then covered with a coverslip. Microstructural observation was carried out using an optical microscope. Images were captured under bright-field illumination at magnifications of 40× and 400×. For each sample, at least five different fields of view were examined to ensure representativeness. The acquired images were analyzed to characterize the distribution, size, and morphology of the internal aqueous phase droplets, as well as to identify the presence of any ammonium nitrate crystals or coal powder agglomerates.

  1. (3)

    Particle size distribution testing

A laser particle size analyzer was utilized to examine the particle size distribution of emulsified matrices containing different proportions of CPWS. A 0.01 g aliquot of the prepared sample was introduced into a beaker with 100 mL of diesel oil, mixed using a glass rod, and subsequently allowed to rest for one hour. Following this, the test sample was carefully transferred into the instrument’s sample cell employing a rubber-tipped dropper. Relevant data, including the Sauter mean diameter D [3,2], were then recorded.

Explosive performance testing

  1. (1)

    Detonation velocity test

The detonation velocity was determined employing the timing method. The charge cartridge consisted of a polyvinyl chloride (PVC) tube measuring 300 mm in length, with an inner diameter of 36 mm and a wall thickness of 2 mm. Three pairs of holes were precisely drilled at distances of 70 mm, 120 mm, and 170 mm from one end of the tube. Enamel-coated copper wires with a diameter of 0.15 mm were inserted through these holes to serve as probes. Initiation was conducted using a No. 8 detonator in conjunction with 50 g of emulsion explosive. The detonation velocity was subsequently calculated according to Eq. (1):

$${V_{\text{d}}}=\frac{l}{t}$$
(1)
  1. (2)

    Brisance test

The brisance (shattering power) was evaluated quantitatively using the standard Hess lead block compression test. A cylindrical lead block (ø 40 mm × 60 mm, purity ≥ 99.9%) was placed on a solid steel base. A hardened steel crusher (ø 41 mm × 10 mm) was centered on top of the lead block. A steel tube (inner diameter 36 mm, height 60 mm, wall thickness 2 mm) was placed upright on the crusher. For each test, 45 g of the CPWS-containing emulsion explosive under investigation was loaded into the steel tube and lightly pressed to a consistent density. To ensure reliable initiation, a 5 g booster charge of a standard, high-sensitivity emulsion explosive was placed on top of the test charge. The assembly was initiated using a No. 8 industrial electric detonator inserted into the booster. After detonation, the steel tube and crusher were removed, and the height of the compressed lead block (H) was measured precisely using a vernier caliper (accuracy ± 0.02 mm). The compression value (Δh), representing the brisance, was calculated as the difference between the initial height (h0) and the final height (H) of the lead block. Δh = h0 - H. Each formulation was tested three times, and the average compression value is reported.

Thermal decomposition performance testing of specimens

The thermal decomposition behavior of emulsified matrices containing different ratios of CPWS was investigated utilizing thermogravimetric and derivative thermogravimetric (TG-DTG) analysis. Experiments were performed in an air atmosphere with a flow rate of 20 mL·min− 1, over a temperature interval from 25 °C to 400 °C, and at heating rates of 5, 10, 15, and 20 °C·min− 1. Alumina crucibles were used as the sample holders during the measurements.

Results and discussion

Analysis of the influence of coal-based slurry on the physical properties of matrix

  1. (1)

    Effect of CPWS on the viscosity of emulsified matrix

Figure 1 presents the viscosity-temperature profiles for the five samples. The data indicate a progressive decline in the viscosity of the matrix as temperature increases, which corresponds with an improvement in the fluidity of the emulsified matrix samples. This behavior is attributable to the intensified thermal motion of the internal molecules within the emulsified matrix at higher temperatures, leading to a reduction in intermolecular forces among particles. As a result, the viscosity of the emulsified matrix samples demonstrates a decreasing trend with rising temperature.

Fig. 1
figure 1

Viscosity-temperature curves of five matrix samples.

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During transportation to the blasting site via the charging truck, the on-site mixed emulsion matrix must be maintained at approximately 50 °C. As illustrated in Fig. 1, the viscosities of five samples measured near 50 °C are 174,070, 179,060, 167,435, 162,761, and 159,719 mPa·s, respectively. The viscosity of the matrices exhibits an initial increase followed by a decrease as the CPWS content rises. This behavior can be explained by the following mechanisms: at low CPWS content (e.g., 2#), active groups within the slurry interact with emulsifier molecules through adsorption, resulting in a denser oil film and consequently higher viscosity. However, with further increases in CPWS content, excessive adsorption of emulsifier molecules by these active groups reduces the effective emulsifier concentration at the oil-water interface. This reduction weakens the oil film strength, promotes crystallization of the aqueous phase, leads to emulsion breakdown, and ultimately decreases matrix stability. The interplay of these factors accounts for the observed transition in viscosity from an increasing to a decreasing trend.

Throughout the transportation and charging stages, the viscosity of the on-site prepared emulsion matrices is required to be maintained within the range of 150,000 to 300,000 mPa·s15. As illustrated in Fig. 1, all five samples satisfy this criterion at a temperature of 50 °C. It is noteworthy that 5# approaches the lower boundary of the specified viscosity range, indicating that the mass fraction of CPWS should be limited to a maximum of 9% to achieve optimal performance.

  1. (2)

    Effect of CPWS on the microstructure of emulsified matrix

The micro-structural changes in the matrix samples are illustrated in Fig. 2. As illustrated in Fig. 2.

Fig. 2
figure 2

Microscopic morphology diagrams of five matrix samples.

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As illustrated in Fig. 2, the on-site mixed emulsified matrix devoid of CPWS demonstrates a wide distribution of internal phase droplet sizes, ranging approximately from 5 to 50 μm, including the presence of notably large and small particles. The irregularly shaped entities observed correspond to ammonium nitrate crystals. This phenomenon can be attributed to the elevated water content and reduced viscosity of the oil phase in the on-site mixed matrix relative to conventional emulsified explosives, which facilitates ongoing coalescence of internal droplets post-preparation, ultimately resulting in the precipitation of ammonium nitrate crystal. The addition of CPWS at varying proportions results in a reduction of internal droplet sizes to differing degrees, alongside enhanced uniformity and dispersion. This effect suppresses the coalescence of smaller droplets into larger ones, thereby mitigating the crystallization of ammonium nitrate. In the matrix sample containing 3% CPWS, larger droplets ranging from 2 to 25 μm remain within the internal phase, characterized by a dense distribution of droplets and the presence of minor irregular ammonium nitrate crystals. Nonetheless, the extent of crystallization in this sample is less pronounced relative to the group without slurry (0%).

In samples containing 6% and 9% CPWS, the internal particles exhibit a uniform distribution and consistent size variation ranging from 1 to 10 μm. The majority of the coal powder is homogeneously dispersed within the droplets, with only a minor portion aggregating outside the droplets, and negligible ammonium nitrate crystallization is detected. Conversely, the 12% CPWS sample is characterized by larger droplet sizes, pronounced ammonium nitrate crystallization, poor particle uniformity, and a broad size distribution, all indicative of suboptimal performance. When considered alongside viscosity measurements, the formulations containing 6% and 9% coal powder demonstrate superior overall performance.

  1. (3)

    Effect of CPWS on internal phase particle size of emulsified matrix

The relationship between particle size and volume percentage of on-site mixed emulsified matrix with varying CPWS content, as measured by laser particle size analyzer, is shown in Fig. 3.

Fig. 3
figure 3

Laser particle size test diagram of matrix sample.

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Table 2 Parameters related to particle size distribution of matrix samples.
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Table 2 summarizes the particle size measurements for five distinct sample groups. The Sauter mean diameter, denoted as D [3,2] and determined according to Eq. (2), represents the ratio of droplet volume to total surface area. The size range provides an indication of the dimensional distribution of the droplet particles. The Polydispersity Index (PDI), defined as the ratio of the size range to D [3,2], serves as a metric for the breadth of the particle size distribution. Additionally, the span parameter, calculated as span = [d(90) - d(10)] / d(50), further characterizes the distribution width. Lower values of both PDI and span correspond to greater uniformity among the sample particles16. The d(50) value denotes the median particle size, indicating that 50% of the particles are equal to or smaller than this size, while d(10) and d(90) represent the particle sizes below which 10% and 90% of the sample fall, respectively.

The calculation formula for D [3,2] distributed in n particle size intervals is17,18:

$$D[3,2]=100/({f_1}/{D_1}+{f_2}/{D_2}+ \ldots +{f_i}/{D_i}+ \ldots +{f_{\text{n}}}/{D_n})$$
(2)

In Eq. (2), D [3,2] denotes the Sauter mean diameter. Here, fi represents the percentage content of the i-th particle size interval, Di corresponds to the geometric of the i-th size class, and the index i varies from 1 to n.

As shown in Fig. 3, relative to 1#, which lacks CPWS, the particle size distribution curves of Samples 2 through 5, containing varying proportions of CPWS, exhibit leftward shifts of differing extents. Notably, 5# displays the most significant leftward displacement, alongside a progressive decrease in internal droplet size from 2# to 5#. This trend corresponds to an increase in the number of smaller droplets and a reduction in the quantity of larger droplets, findings that are corroborated by the droplet size variations observed in the matrix micro-structure images.

The analysis of the data presented in Table 2 indicates a progressive decrease in the values of D[3,2], d[0.1], d[0.5], and d[0.9] from 1# through 5#, corresponding with an increasing content of CPWS. This trend quantitatively substantiates a reduction in the internal droplet size within the matrix specimens. The highest observed d[0.9] value of 8.600 μm among all samples suggests a gradual attenuation of ammonium nitrate crystallization.

Although the polydispersity index (PDI) increased from 1.563 to 2.450 and the span values rose from 0.763 to 1.141 with higher slurry content, these variations imply only moderate effects on particle size uniformity and distribution consistency. Importantly, the incorporation of CPWS did not provoke significant ammonium nitrate crystallization or demulsification phenomena.

These findings demonstrate that the addition of CPWS to site-mixed emulsified explosives exerts minimal influence on their microstructural characteristics. Subsequent to formulation development, further investigations should prioritize assessments of explosive performance and thermal stability to ensure the intrinsic safety of CPWS emulsified explosives.

Effects of CPWS on explosive performance of emulsion explosive

  1. (1)

    Brisance test

The brisance test results of site-mixed emulsion explosives containing varying proportions of CPWS are presented in Table 3.

Table 3 Results of Brisance Test.
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  1. (2)

    Detonation velocity test

The detonation velocity test results are summarized in Table 4.

Table 4 Results of detonation velocity Test.
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The data presented in Tables 3 and 4 from the five sample groups demonstrate that increasing the CPWS content from 0% to 6% results in enhanced brisance and detonation velocity.

However, when the slurry content surpasses 6%, these performance metrics exhibit a gradual decline. This analysis suggests that a moderate incorporation of CPWS (less than 6%) optimizes the oxidizer-to-fuel ratio, thereby aligning the explosive composition closer to a zero oxygen balance. Furthermore, the presence of micron-sized coal particles within the slurry functions as a physical sensitizer, which, in conjunction with bubbles generated by sodium nitrite, facilitates the formation of “hot spots” within the explosive matrix. This synergistic effect improves detonation reactivity and overall explosive performance. In contrast, an excessive amount of CPWS (greater than 6%) results in a negative oxygen balance, causing incomplete chemical reactions. Moreover, the surplus slurry disrupts the structural integrity of the emulsion matrix, leading to localized demulsification and destabilization of detonation wave propagation, thereby impairing the explosive’s performance. The findings regarding detonation velocity indicate that incorporating CPWS at concentrations of up to 6% improves explosive performance. However, additions exceeding this threshold (greater than 6%) lead to a decrease in both detonation velocity and brisance, thereby reducing the overall explosive effectiveness of the samples.

Influence of CPWS on thermal decomposition characteristics of emulsion matrix

Based on data from micro-structure analysis, particle size testing, and strength testing, the thermal decomposition processes of Samples 1 to 5 were analyzed using TG-DTG. The TG and DTG curves are shown in Figs. 4 and 5, respectively.

Fig. 4
figure 4

TG curve of the matrix sample.

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Fig. 5
figure 5

DTG curve of the matrix sample.

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As demonstrated in Figs. 4 and 5, the five samples display two characteristic thermal weight loss phases. Nevertheless, the incorporation of CPWS modified specific stages within the matrix samples. In general, the thermal decomposition process can be categorized into four distinct stages: In the TG analysis, the initial stage, occurring between approximately 25 °C and 100 °C, is characterized by a gradual decrease in mass as observed on the TG curve. Concurrently, the DTG curve exhibits a small, broad peak, which is attributed to the evaporation of free water present within the matrix as well as water droplets destabilized by phase separation. As the temperature nears 100 °C, the rate of evaporation intensifies, corresponding to a more pronounced yet narrower peak on the DTG curve. The mass loss percentages recorded for the five samples during this stage were 9.63%, 9.46%, 9.50%, 10.22%, and 11.67%, respectively.

The observed increase in mass loss with higher CPWS content is explained by the adsorption of micron-sized coal particles at the oil-water interface, which destabilizes smaller droplets and thereby facilitates the evaporation of the water phase.The second stage, spanning from 100 °C to 210 °C, exhibits minimal mass loss as indicated by the TG curve, with no notable variations observed in the derivative DTG curve. The third stage, occurring between 210 °C and 300 °C, is characterized by a pronounced decline in the TG curve, accompanied by a distinct and sharp peak in the DTG curve.

An analysis of the TG-DTG curves for the five samples reveals that the TG curves exhibit a leftward shift toward elevated temperatures as the heating rate increases, although this shift diminishes in magnitude at a heating rate of 15 K·min− 1. For each sample, the DTG peak associated with rapid reaction processes occurs earlier in time with increasing heating rates, however, the peak temperature demonstrates a slight decrease when subjected to the same heating rate.

As presented in Tables 5 and 6, both the initial decomposition temperature (Tonset) and the DTG peak temperature (TP) of the samples exhibit a decreasing trend with increasing CPWS content, indicating a shift toward lower thermal thresholds.This observation suggests that higher proportions of CPWS facilitate an accelerated thermal decomposition rate during Stage III, thereby diminishing the thermal stability of the material. More specifically, the incorporation of CPWS appears to promote an earlier onset of Stage III decomposition, enhancing the overall decomposition process. These results imply that excessive inclusion of CPWS in field-mixed emulsion explosive matrices may adversely affect thermal stability by expediting the decomposition reaction.

Table 5 DTG peak temperatures of the five samples.
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Table 6 Tonset of the samples in the third stage.
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Calculation of kinetic parameters

The Kissinger method19,20,21 was applied to perform differential analysis on the third stage with the maximum mass loss rate for five samples under different heating rates. The differential equation is expressed as follows:

$$\ln \left(\frac{\beta }{{T_{p}^{2}}}\right)=\ln \left(\frac{{AR}}{E}\right) - \frac{E}{{\mathcal{R}{T_p}}}$$
(3)

Tp denotes the DTG peak temperature, K, β denotes the heating rate, °C·min− 1, E denotes the activation energy, kJ·mol− 1, R denotes the gas constant, 8.314 J·mol− 1.

Using Eq. (3), the \(\ln (\frac{\beta }{{T_{p}^{2}}})\)-\((\frac{{{{10}^3}}}{{{T_p}}})\) plots were constructed and subjected to fitting analysis. The fitting results for the five substrate samples are illustrated in Fig. 6, and the calculated parameters are summarized in Table 7.

Fig. 6
figure 6

\(\mathcal{ln(}\frac{\mathcal{\beta }}{{\mathcal{T}_{\mathcal{p}}^{\mathcal{2}}}}\mathcal{)}\)-\(\mathcal{(}\frac{{\mathcal{1}{\mathcal{0}^\mathcal{3}}}}{{{\mathcal{T}_\mathcal{p}}}}\mathcal{)}\) plots obtained by the Kissinger method for samples 1–5.

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Table 7 Kinetic parameters of kissinger thermal decomposition of the samples.
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As shown in Fig. 6 and detailed in Table 7, the five samples demonstrated a strong fitting correlation within the conversion fraction (α) range of 20% to 90%. The activation energies calculated using the Kissinger method for these samples were 145.01 kJ·mol− 1, 131.64 kJ·mol− 1, 118.67 kJ·mol− 1, 138.53 kJ·mol− 1, and 172.46 kJ·mol− 1, respectively. Notably, the incorporation of CPWS resulted in an initial reduction in activation energy values, followed by a subsequent increase.

Comparative kinetic analysis using the Flynn-Wall-Ozawa method

To enhance the reliability of the kinetic analysis and address potential model-dependency concerns, the apparent activation energy was also calculated using the model-free Flynn-Wall-Ozawa (FWO) isoconversional method, as recommended in recent studies on the thermal analysis of energetic materials22,23. The FWO method is expressed as:

$$\lg \left( \beta \right)=\lg \frac{{AE}}{{Rg(\alpha )}} - 2.315 - 0.4567\frac{1}{{RT}}$$
(4)

Where g(α) is the integral form of the reaction model. For a fixed conversion degree α, plotting lg(β) against 1/T yields a straight line with a slope of 0.4567Ea/R, from which Ea can be calculated.

The activation energies were calculated at various conversion degrees (α = 0.5) for the five samples. The Ea values obtained from the FWO method are summarized in Table 8 alongside the values from the Kissinger method for direct comparison.

Table 8 Comparison of Ea calculated by the kissinger and FWO methods.
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The results show a very close agreement between the activation energy values derived from the Kissinger and FWO methods across all samples, with relative deviations consistently below 5%. This strong consistency between two fundamentally different kinetic approaches—one being a peak method (Kissinger) and the other an isoconversional method (FWO)—robustly validates the reliability and non-randomness of the calculated activation energy trends reported in this study. It confirms the characteristic trend of an initial decrease followed by an increase in Ea with increasing CPWS content.

Preliminary analysis of mechanisms

Fig. 7
figure 7

Schematic model illustrating the proposed mechanism of CPWS interaction within the emulsified matrix.

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To synthesize the proposed mechanisms linking microstructure to macroscopic properties, a conceptual model is presented in Fig. 7. CPWS, in essence, can be considered as a highly concentrated, ultrafine coal-water slurry. Its primary solid component is coal powder, which is predominantly composed of carbonaceous molecules. The surface properties of these coal particles exhibit an overall lipophilic character with localized hydrophilic sites. Upon addition to the aqueous phase of the emulsion matrix, most CPWS particles migrate and accumulate at the oil-water interface. The predominant lipophilic groups on the coal surface are attracted to and associate with the oil phase, while the minority hydrophilic groups couple tightly with the internal aqueous droplets.

This configuration enhances the encapsulating capability of the oil film around the droplets, leading to the observed initial increase in viscosity. With excessive CPWS addition, the surplus of lipophilic groups disrupts the integrity of the oil film, promoting demulsification and the crystallization of ammonium nitrate (AN). This corresponds to the subsequent decrease in viscosity and the reduction in sensitized explosive performance, which aligns perfectly with our experimental trends.

For 1# (0%), the activation energy was determined to be 145.01 kJ·mol− 1. In 2# (3%), the activation energy exhibited a slight decrease to 131.64 kJ·mol− 1. This reduction indicates that the incorporation of a small quantity of CPWS moderately reduced the activation energy of the matrix. 3# (6%) demonstrated the lowest activation energy, measured at 118.67 kJ·mol− 1. This observation can be explained by two primary factors:

Firstly, the solid constituents within the CPWS predominantly comprise macromolecular carbon chains characterized by high thermal conductivity. These components function as heat transfer agents, facilitating accelerated heat propagation and thereby enabling the aqueous-phase particles to attain the necessary energy more rapidly. Secondly, functional groups such as hydroxyl (-OH) and carboxyl (-COOH) present on the surface of the CPWS molecules engage in redox reactions with thermal decomposition products of ammonium nitrate, specifically NH3 and HNO3, within the matrix. This interaction effectively lowers the activation energy barrier of the reaction. Furthermore, the micron-sized solid particles in the CPWS exhibit a high specific surface area, which enhances the contact interface with aqueous-phase particles and generates additional active sites, collectively contributing to a further reduction in the energy required for thermal decomposition.

4# (9%) exhibited an activation energy of 138.53 kJ·mol− 1. Although this value is lower than that observed for 1#, it represents a deviation from the trend noted in 3#. In the case of 5# (12%), the activation energy further increased to 172.46 kJ·mol− 1, surpassing that of 1#. These findings suggest that higher slurry concentrations exert an inhibitory influence on the thermal decomposition process.

The observed inhibition can be attributed to several factors. Firstly, the excessive addition of slurry may disrupt the stability of the matrix. Additionally, micron-sized coal particles located at the oil-water interface may promote partial demulsification and crystallization within the emulsified matrix. These unstable aqueous-phase particles tend to react prematurely, whereas the solid particles facilitate the dissipation of accumulated heat, thereby limiting the buildup of thermal energy. Furthermore, residual coal powder present in the slurry contains trace amounts of SiO₂ and Al₂O₃, which, at elevated temperatures, form inert surface coatings that impede the advancement of the reaction.

Coal particles engage in coal-oxygen composite reactions24,25, wherein reactive sites on the particle surface interact with oxygen to form coal-oxygen complexes, resulting in the release of heat26. As the temperature increases, these complexes undergo decomposition, generating gaseous products such as CO and CO2, while simultaneously regenerating the reactive sites. Additionally, the thermal decomposition of the emulsified matrix is enhanced by the presence of these regenerated active sites and the heat produced from the coal-oxygen reactions. The reductive CO gas further facilitates this decomposition process.

The intrinsic porous architecture of coal particles functions as localized hotspots. Upon heating, the evaporation of moisture and the release of gases contribute to the further development of the pore network, thereby expanding the channels for coal-oxygen interactions and increasing the concentration of reactive sites. Together, these processes facilitate the enhanced thermal decomposition of the emulsified matrix when it is modified with a CPWS.

Conclusion

  1. (1)

    As the proportion of CPWS increases from 0% to 12%, the viscosity of the on-site mixed emulsified matrix exhibits an initial increase followed by a subsequent decrease. At a temperature of 50 °C, all five groups of on-site mixed emulsified matrix samples incorporating CPWS satisfy the viscosity criteria necessary for practical engineering applications.

  2. (2)

    The detonation velocity and brisance of the on-site mixed emulsion explosive initially increase and then decrease as the content of CPWS rises. The optimal explosive performance is observed at a CPWS concentration of 6%.

  3. (3)

    TG-DTG analyses indicate that the activation energy exhibits a non-linear trend in response to varying CPWS ratios, initially decreasing and subsequently increasing. The measured activation energies are 145.01, 131.64, 118.67, 138.53, and 172.46 kJ·mol− 1, respectively. Notably, the 6% CPWS formulation shows an 18.16% reduction in activation energy relative to the baseline sample.

In summary, the comprehensive analysis identifies the 6% CPWS formulation as offering the best balance of physicochemical and explosive properties for application in bulk emulsion explosives. Furthermore, the observed enhancement in thermal decomposition reactivity at optimal CPWS content is attributed to the synergistic effects facilitated by the incorporated coal powder, as discussed in Sect. "Comparative kinetic analysis using the Flynn-Wall-Ozawa method".