These are oily sludge ( Figure 1 ) and various equipment used for the waste characterization and the realization of pyrolysis on a sludge sample.

At the Maria Gléta thermal power plant, liquid waste (water, oily sludge) is stored in two tanks, where the denser water settles at the bottom. The samples were taken in tank n˚2, after purging the water until the first oily sludge appeared. Two samples taken at different dates made it possible to obtain a better chemical characterization of the sludge, which is essential for its recovery. The quantity of produced sludge was evaluated from January to May 2023 and is estimated at 170 m³ for 5575 m³ of heavy fuel oil used.

The parameters analyzed in this study are summarized in Table 2 :

$\begin{array}{c}T=\frac{m_1-m_2}{m_1}\times 100\end{array}$ (1)

where $m_1$ is the mass (g) of the sample before drying, $m_2$ the mass (g) of the sample after drying, and $T$ the moisture content (%).

So, viscosity can be determined by the theorem of the center of inertia applied to an inclined plane (see Figure 2 ):

The viscosity force F is determined by the following formula [17] :

$\begin{array}{c}F=\mu \cdot S\cdot \frac{V_f-V_i}{e}\end{array}$(2)

where S is the surface (m²) of the cube in contact with the mud; $V_f$ and $V_i$ are respectively the final and initial velocities (m/s) of the cube and e is the thickness (m) of the layer of mud whitewashed on the inclined plane.

$\begin{array}{c}\mu =\frac{m_c\cdot e\left(g\cos \alpha -a\right)}{S\cdot V_f}\end{array}$(3)

$\begin{array}{c}\nu =\frac{\mu }{{\rho }_b}\end{array}$(4)

where $\nu$ is the kinematic viscosity of the slurry in cSt and $\rho$ is the density of the slurry in kg/m³.

Three tests were carried out and the average was taken.

The experiments were carried out at room temperature (31˚C).

The results obtained after each method of analysis of the different parameters are presented in Table 3 :

The sludge from the Maria Gléta thermal power plant has a water content of 25%, lower than the values in the literature (30% to 90%) [18] , which nevertheless requires pre-treatment to remove residual water and avoid contamination. Their sulphur content is about 0.3%, well below the 1% to 2% reported in the literature [19] , but treatment is required to reduce its environmental impact. The concentration of heavy metals is relatively low (<0.5 g/kg) with a predominance of Zn (208.925 mg/kg) and a low presence of Cd (0.3 mg/kg). Although these levels are lower than existing benchmarks [19] , further reduction during pyrolysis is recommended. Finally, with a viscosity of 2446 cSt, the sludge is very viscous and requires heating to facilitate its handling.

In conclusion, although these sludges contain fewer toxic substances than those studied in the literature, pre-treatment remains crucial before pyrolysis. An environmental study was carried out, based on the available data, in order to define the optimal pyrolysis conditions for effective recovery.

Oily sludge contains pollutants such as heavy metals, sulfur which are harmful to the environment and human health. Their environmental impacts are grouped in Table 4 [14] [17] :

Heavy metals like Cu, Zn, Cr, and Ni are essential for plants and animals but at low levels, while Cd and Pb are non-essential [20] . Heavy metal concentrations below 1 mg/kg do not have a significant impact on sludge recovery, unlike those above 110 mg/kg [19] .

During pyrolysis, heavy metals mainly migrate to pyrolysis coal, requiring a reduction in its production to limit their environmental impact. Sulphur also migrates during pyrolysis, degrading the quality of the obtained products and making reduction essential. The use of a catalyst would reduce heavy metal and sulphur contents, improve the quality of the final products and reduce gaseous emissions [18] . A thorough study is required to identify a catalyst that optimizes oil and syngas production.

Influence of pyrolysis conditions on the products obtained

Studies on the pyrolysis of oily sludge indicate an optimal temperature range between 500˚C and 700˚C with an ideal temperature of 500˚C [18] . However, due to the specificities of the plant’s sludge, several experiments were conducted, starting with 500˚C, to identify the optimal temperature (see Figure 4 ).

1^st experience:

In the first experiment, carried out at 500˚C for 1 hour, the product obtained was only ash, indicating an excessive conversion of organic matter. This suggests that the temperature and duration of pyrolysis were too high for the formation of coal. Additional tests were carried out to refine these parameters by adjusting the pyrolysis time.

2^nd experience:

In the second experiment, carried out at 500˚C for 30 minutes, pyrolysis also produced only ash. These two experiments lead to the conclusion that the temperature of 500˚C, regardless of the duration, is too high to promote the complete conversion of organic matter into coal. Thus, the ideal pyrolysis temperature for the oily sludge from the plant seems to be below 500˚C, in contrast to the temperature range of 500˚C to 700˚C mentioned in the literature. A third experiment was therefore carried out at a temperature below 500˚C.

3^rd experience:

The third experiment, carried out at 400˚C for 30 minutes on 20.051g of sludge, showed that the reduction of the pyrolysis temperature favored the production of coal. The pyrolysis time of 30 minutes was found to be sufficient for the conversion of organic matter to charcoal without promoting ash formation. This experiment demonstrates that pyrolysis conditions at 400˚C for 30 minutes are more suitable for the plant’s sludge than those at 500˚C (see Figure 5 ).

4^th experience:

During this fourth experiment, carried out at 400˚C for 1 hour on 20.003 g of mud, coal was also obtained. This confirms that the temperature of 400˚C is suitable for the pyrolysis of the plant’s sludge (see Figure 6 ).

The choice of pyrolysis duration aims to minimize coal production while maximizing gas and oil production, due to the migration of heavy metals to coal.

Table 5 is for the mass balance showing the mass of the initial sample and the percentages of coal, oil, gas and water for each experiment.

Effects of pyrolysis duration on pyrolysis product yields

Figure 7 Highlights the influence of pyrolysis duration on coal rate.

The results show a decrease in the coal rate with increasing pyrolysis time. Indeed, the yield has decreased from 6% at 30 minutes to 5.6% at 1 hour, suggesting that longer pyrolysis leads to increased degradation of organic compounds, producing more gas and oil at the expense of coal. Although the extension of pyrolysis slightly increases gas and oil production, this improvement of only 0.4% does not justify a longer duration. Thus, for economic and ecological reasons, it is preferable to carry out pyrolysis at 400˚C for 30 minutes.

Figure 8 shows that the pyrolysis of the oily sludge from Maria Gléta would give a yield of 70% in oil and pyrolysis gas, 5% in coal on the stock of 170 m³ for 5575 m³ of heavy fuel oil used.

Environmental and economic implications of scaling up

Finally, in our study, no detailed characterization of gaseous or liquid effluents was carried out to check their composition or verify compliance with standards (Air Liquide, IED directive, etc.). To this end, we propose that a series of analyses (GC–MS, FTIR for PAHs, SOₓ/NOₓ assay, etc.) be carried out in a pilot phase to ensure compliance.