1. Introduction
For contemporary human civilization, increasing energy production and consumption of all kinds are characteristic [
1]. The majority of this energy is obtained from conventional sources such as coal [
2] or oil, and even though new reserves of these substances are still being discovered today, it is not sustainable to rely on them for the future [
3]. The term ‘sustainable development’ is increasingly used, aiming for energy, economic, and societal sustainability while considering environmental preservation for future generations [
4]. There are various ways to address this issue, such as the utilization of renewable energy sources like water, wind, geothermal energy, or solar power for heat and electricity generation [
5,
6].
Another problem related to the ever-increasing energy consumption is also a significant increase in the amount of waste produced [
7]. Some of it can be recycled, but a large portion of the waste still ends up in landfills without further utilization, negatively impacting its surroundings.
A sustainable economy of waste is a very important environmental aspect. In 2020, the EU adopted the circular economy action plan and aimed to ensure that the resources it uses remain in the EU economy for as long as possible, and that waste is prevented [
8].
“The transition to the circular economy will be systemic, deep and transformative, in the EU and beyond. It will require an alignment and cooperation of all stakeholders at all levels—EU, national, regional and local, and international” [
9].
According to [
10], 4815 kg of waste were generated per EU inhabitant in 2020 (39.2% of waste were recycled and 32.2% landfilled). For comparison, waste generation in Poland was 4492 kg per capita, and in Czech Republic, 3598 kg per capita.
In the EU, about 48% of municipal waste was recycled by material recycling and composting. For comparison, municipal waste generation in the EU was 500 kg per capita (in 2004) and 513 kg per capita (in 2022); in Poland, 256 kg per capita (in 2004) and 364 kg per capita (in 2022); and in the Czech Republic, 279 kg per capita (in 2004) and 570 kg per capita (in 2022) [
11].
In the EU, the quantity of waste recovered—in other words, recycled, used for backfilling or incinerated with energy recovery—increased from 870 million tonnes in 2004 to 1165 million tonnes in 2020. As a result, the share of such recovery in total waste treatment rose from 45.9% in 2004 to 59.1% in 2020 [
12].
In 2020, 39.9% of the total treated waste was recycled, 12.7% was backfilled, and from 6.5%, the energy was recovered. The remaining 40.9% of the waste was landfilled (32.2%), incinerated without energy recovery (0.5%) or disposed of otherwise (8.2%) [
13].
Research by various authors shows that recycling in any form helps reduce the environmental impact, prevents waste generation, and consumes fewer natural resources. One of the options for utilizing the energy potential of these waste materials is their thermal utilization. Thanks to today’s technologies, this method is relatively environmentally friendly, but it only involves the processing and disposal of waste, or the generation of electricity and heat [
14,
15,
16,
17,
18,
19,
20,
21,
22].
For example, coal mines, wanting to meet the expectations of coal quality, were forced to expand and modernize coal enrichment plants. This causes a continuous increase in waste in the form of coal sludge. The best disposal method for these wastes is their combustion or co-combustion with other fuels [
19].
The use of polymer waste in thermal processes, by combustion methods, pyrolysis, and gasification, has energetic purposes related to the thermal utilization of waste and energy recovery, ecological purposes related to reducing gas emissions that are harmful to the environment, and economic purposes related to the partial replacement of fuels, e.g., coal [
16,
18,
20,
21,
22]. It is also important to use different wastes in composites with plastics [
20,
21,
22].
However, the main goal should be not only waste disposal and the utilization of thermal energy, but also the acquisition of energy-rich substances for further use. This aspect is fulfilled by pyrolytic technologies, which belong to potentially advantageous methods of material and energy processing of waste. Due to the possibility of washing the output gases and the inert reaction environment, pyrolysis also leads to a significantly lower production of sulfur and nitrogen oxides compared to the conventional low-temperature incineration of municipal waste [
23,
24,
25,
26,
27,
28,
29,
30,
31,
32].
In general, it can be said that any modification and recycling will have an impact on sustainability, the economy, and the environment. That is why it is so important to consider energy strategies and the circular economy, including environmental aspects in waste management and energy engineering [
33,
34]. The profitability of sustainable technologies should also be kept in mind because financial viability drives long-term sustainability [
35,
36,
37,
38,
39]. The advanced technologies in modern manufacturing are crucial, especially because of automation in decision-making and different process optimization [
40,
41].
The aim of this paper is to present the research results of the pyrolysis of different waste in the context analysis of pyrolysis products, their quality and calorific value.
In the future, the authors plan to continue the thermal research of waste materials as potential fuel sources, in environmental aspects and energy efficiency.
4. Conclusions
This work was conducted with the aim of analyzing and evaluating the results of the pyrolysis gasification of selected waste materials, specifically waste that no longer has further utility. These materials include paper rejects—waste generated during paper recycling, plastic waste separated from municipal plastics, waste wood from wood recycling, and carpets and roofing from the automotive industry. Proximate analyses were conducted on all these materials, leading to the selection of three mixtures designated for pyrolysis. All mixtures underwent pyrolysis at a temperature of 600 °C. The pyrolysis tests were carried out without difficulties, with only a portion of the charge remaining unburned in Sample 6 (likely paper rejects). From the product weights of the pyrolysis, the yield of pyrolysis oil and gas was determined. When comparing the yield of bio-oil with existing technologies (where biomass pyrolysis achieves 50–75% w/w), the yield from the pyrolysis tests can be considered low (14–23%). Conversely, the yield of pyrolysis gas was high (39–50%). The highest yields were observed in Mixture 2 (plastic waste and wood waste), where the yield of bio-oil was 23.34%, and pyrolysis gas was 50.67%.
Pyrolysis gas and bio-oil were mainly evaluated for their energy potential, and in many technologies, after purification, the gas is used as a process gas instead of natural gas. Bio-oil can be further refined and used as a liquid fuel. Both products had calorific values that were average to above-average, with the best values achieved for the products from the mixture of separated municipal plastic and waste wood. In terms of the energy potential of waste materials and the products produced by their pyrolysis, the selected wastes and their mixtures can be recommended for this processing method. The calorific value of these materials is on par with readily available commercial fuels such as oil and natural gas. The pyrolysis gas of the mixture of separated municipal plastics and waste wood had the highest calorific value of 49.45 MJ/m3. Mixture 2 (plastic waste and wood waste) had the highest calorific value of pyrolysis condensate at 30.62 MJ/kg. Due to the use of biochar as sorbent, Mixture 3 had the highest iodine value at 90.01mg/g.
Pyrolysis can be a suitable method for material and chemical recycling and for reclaiming a high proportion of raw materials from waste. Given the nature of the waste, the pyrolysis of these materials should be integrated as a preliminary process within another facility, such as a pre-processing step in an energy source utilizing pyrolysis products. From an economic perspective, we can consider incorporating a portion of the energy obtained from these waste materials as renewable energy (green energy). However, given the current state and level of boilers, investment in specialized units would be necessary, especially concerning exhaust gas cleaning. In the long term, there is a lack of transparent legislation.