1. Introduction
The reasonable price and rich resources of biomass attracts researchers’ interest. Biomass, being a rich resource of bio-based compounds, is used in many applications. It may be used as a substratum for the production of chemicals [
1] and polymers, including bio-based polyesters and epoxy resins, fuels for transportation, and carbon materials [
2].
Sewage sludge is a by-product of wastewater treatment plants. The highly valuable organic and inorganic content enables its usage as a biomass feedstock for energy and resource recovery [
3]. Depending on the season, technology applied in treatment plants, and specification of the source area, the composition of sewage sludge may vary. After drying, it may contain 50–70% of organic matter, 30–50% of mineral components, including carbon (from 1% to 4%), 3.4–4.0% of nitrogen, and 0.5–2.5% of phosphorus, and some other nutrients [
4].
The water content of raw sludge may be up to 99%; thus, dewatering and drying are essential in order to prepare it for further usage and utilization [
5]. However, the storage and transport of dry sludge may pose a risk of self-ignition. The reduction in water content to 10% or less adversely affects fire safety [
6].
The increase in interest in biomass utilization has attracted attention to the economic and environmental concerns that arise with biomass storage. The disposal of sewage sludge may pose problems due to the emission of odor emission, possible risk of pathogenic organisms [
7], and hazardous and toxic substances content, including dioxins or heavy metals [
8]. The levels of environmental contamination vary geographically. However, the release of wastewater residues into rivers is one of the most important routes of the contamination of aquatic resources by As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, and Zn [
9]. Heavy metal removal in terms of electrokinetic processes, supercritical fluid extraction, ion-exchange treatment [
10], adsorption [
11], or other treatments needs to be provided for the safe further processing of sewage sludge.
Several sewage sludge management strategies include its application in agriculture, wet oxidation, pyrolysis, incineration, and landfilling [
7]. Thickening is essential in some processes, including agriculture applications, wet oxidation, or pyrolysis [
12]. Although sewage sludge exhibits comparatively low calorific value, the co-pelletizing and, therefore, co-combustion with other fuels enhances its performance [
13]. The pelleting process is widely used in biomass densification by compression under high pressure. The life-cycle assessment of the pelleting process reveals that its decentralization and the usage of renewable energy resources reduce the environmental impact in all studied impact categories [
14].
The self-heating processes and dust explosions of biomass are of particular concern due to the necessity of developing strategies for the safe storage of waste biomass [
15]. In the presence of oxygen and/or an ignition source, a higher potential for fire or explosion incidents occurs [
16]. Biomass, while stockpiled and stored, maintains contact with air during, in which slow or intensive oxidation may take place, resulting in fires and explosions [
17].
Previous studies on fire and explosion characteristics revealed that thermally dried sewage sludge dust poses an explosion hazard, as is classified in the St 1 dust explosion class, according to EN 14034-1 Determination of explosion characteristics of dust clouds Part 1: Determination of the maximum explosion pressure p
max of dust clouds [
18] and EN 14034-2 Part 2: Determination of the maximum rate of explosion pressure rise (dp/dt)
max of dust clouds [
19]. The results showed that the maximum ignition temperature for the 5 mm thick layer of sludge dust and the minimum ignition temperature of the cloud, according to ISO/IEC 80079-20-2 Explosive atmospheres Part 20-2: Material characteristics: Combustible dusts test methods [
20] were 270 °C and 490 °C, respectively. The minimum explosible concentration according to EN 14034-3 Part 3: Determination of the lower explosion limit LEL of dust clouds [
21] was found to be 60 g/m
3. A limiting oxygen concentration of 20% and a minimum ignition energy of more than 1000 mJ were obtained according to EN 14034-4 Part 4: Determination of the limiting oxygen concentration LOC of dust clouds [
22] and ISO/IEC 80079-20-2 Explosive atmospheres Part 20-2: Material characteristics: Combustible dusts test methods [
20], respectively. The achieved results suggest that studied sludge dust may pose a risk when stored in insufficient conditions.
Dust explosions pose a significant threat to industrial processes. There are various methods of preventing explosions of dust–air mixtures, including methods involving partial inerting with inert gases, such as nitrogen or carbon dioxide, which can be used to mitigate the effects of an explosion inside the equipment [
23,
24].
According to reports [
25,
26], 137 combustible dust incidents occurred worldwide in 2021, including 57 fires and 80 explosions. The latest sludge dust fires and explosions were reported in wastewater treatment plants in, i.a., San Francisco (USA, 2021), Bristol (UK, 2020), Hamilton (Canada, 2020), and Koziegłowy (Poland, 2019).
Spontaneous ignition is a complex phenomenon, which occurs as a result of exothermic reactions of material taking place without external heat or other sources of ignition [
27]. It is a possible cause of fires and explosions. The self-heating of powders, including coal, metals, biomass, and waste [
28], may occur during processing, transportation, or storage [
29].
A process safety culture is essential for incidents and accident prevention. In addition to conducting interviews, drawing up questionnaires, controlling behavior, and preparing documents, extending the knowledge of the occurring process significantly affects the level of safety culture [
30]. Risk assessments in industrial plants should be based on the characteristics of produced and stored materials in order to achieve a complete scenario of hazard [
28]. The risk of spontaneous ignition may be evaluated under adiabatic, isothermal, or isoperibolic conditions. The latter is used in the European standard EN 15188, describing the determination of the self-ignition temperature of dust accumulation or granular materials dependent on their volume. The extrapolation of results is performed afterward, in order to assess the safe storage conditions [
31].
A proper understanding of the self-heating behavior of the thermally dried sewage sludge may prevent fires and explosions that occur in wastewater treatment plants. An awareness of all potential fire hazards will enable us to effectively perform risk assessments and fire safety plans. Therefore, this study aims to assess the propensity to spontaneously combust of thermally dried sludge dust and pellets from a municipal wastewater treatment plant in order to determine the safe storage and transport conditions. Moreover, the role of the moisture content, bulk density, elemental composition, and particle size distribution in the self-ignition behavior of thermally dried sludge was investigated.
4. Conclusions
The higher induction times and higher self-ignition temperatures of sludge pellets compared to sludge dust confirms that sludge after pelletization is safer for storage for long periods. The highest self-ignition temperatures were obtained for the smallest basket volume. For the sludge pellets, the value of 186 °C was achieved, whereas for the sludge dust, it was 26 °C lower. The obtained TSI values decrease with the reduction in material volume and are the lowest for the highest basket volume (160 °C and 142 °C for the sludge pellets and sludge dust, respectively).
The moisture content of both dust and pellets is low. However, slightly higher results were obtained for the sludge pellets (9.974 ± 0.414%) compared to the sludge dust (6.067 ± 0.193%). The low moisture content and the small difference in these values indicate its negligible impact in consideration of both materials. Sludge pellets have a higher bulk density (0.621 ± 0.005 g/cm3) compared to sludge dust (0.427 ± 0.008 g/cm3). Apart from the positive influence on increasing the critical temperature, the higher bulk density of materials affects their storage and handling. The elemental analysis showed that the sludge dust has a higher hydrogen content (5.235 ± 0.053%) compared to sludge dust (4.575 ± 0.008%), which may result in a faster appearance of the initial exothermic reactions. The most frequent particle sizes of sludge dust are in the range of 100–150 µm, which are significantly lower than pellet size. Lower particle sizes have been shown to reduce self-ignition temperatures.
The obtained information on safe storage volumes, corresponding temperatures, and times revealed that, despite the sludge pellets being easier in terms storage and handling issues and having a more favorable particle size and bulk density, sludge dust is more favorable for the management of studied sewage sludge in terms of fire risk.
Further work on the impact of the pellet size, as well as ash content, would be of interest. Future research into the hazards of silo fires and explosions is needed to better understand the process of self-heating of dried sewage sludge. Silo fire prevention and management should also be a following area of research.