Waste Management: Valorisation Is the Way

3.1. Data Collection

The Core Collection of the Web of Science [18] database was used in the present study, which searches leading academic journals and proceedings from around the world. It includes the Science Citation Index Expanded (SCIE), Social Sciences Citation Index (SSCI), Arts & Humanities Citation Index (AHCI), Emerging Sources Citation Index (ESCI), Conference Proceedings Citation Index (CPCI), Book Citation Index (BKCI) and Current Chemical Reactions and Index Chemicus. The objective was to analyse results to identify trends and publication patterns. It was interesting to know how the concept of incineration has evolved towards that of valorisation. In this work, the search was carried out on 11 March 2021, for the investigation period, which ran from January 1987 to 11 March 2021 and thus included all documents related to the study of ‘incineration’ and ‘valorisation’ treated simultaneously. To do this, we searched for ‘incineration’ AND ‘valorisation’ in the title, abstract, author keywords, and keywords plus. A total of 214 documents were obtained from 889 authors. Figure 3 shows the number of documents obtained in each year.

3.2. Methodology

The open-source software SciMAT (Science Mapping Analysis Software Tool) was used to perform a longitudinal analysis of scientific mapping. In this study, there were three periods: period 1, 1987–2010, 39 documents; period 2, 2011–2015, 55 documents; period 3, 2016–2021, 120 documents. Table 1 reports the number of documents per period.

To carry out the analysis, 10 steps have been followed:

1. Periods: 1987–2010; 2011–2015; 2016–2021;

2. Unit of analysis: words (author’s words, source’s words and added words);

3. Data reduction: not necessary in this analysis;

4. Kind of matrix: co-occurrence;

5. Network reduction: not necessary in this analysis;

6. Normalisation: equivalence index;

7. Clustering algorithm: simple centres algorithm; maximum network size: 12, and minimum network size: 3;

8. Document mapper: core mapper;

9. Quality measures: h-index, average citations, sum citations;

10. Longitudinal: evolution map: inclusion index; overlapping map: Jaccard’s index.

A co-word analysis was carried out. In a co-word analysis, clusters are obtained that are considered ‘themes’. Each research topic is characterised following two parameters, namely, density and centrality, obtaining a classification of four groups, one in each quadrant, as follows (Figure 4a):

• ‘Motor themes’ are located in the upper right quadrant. They are well developed and important topics for structuring a field of research;

• Basic and transversal themes are located in the lower-right quadrant. Cross-cutting and basic themes are located there;

• Highly developed and isolated themes are located in the upper-left quadrant. They are subjects of marginal importance for the field, because they present external links without importance, although they maintain well-developed internal links;

• Emerging or declining themes are in the lower left quadrant. They are marginal, underdeveloped, emerging, or missing issues.

Thematic networks are formed from the keywords and their interconnections by drawing a network graph. The network is tagged with the name of the most central keyword of the topic (Figure 4b). The volume of the spheres is proportional to the number of documents of each keyword, and the width of each link between two spheres is proportional to the equivalence index.

In Figure 5a, two thematic areas framed in a rectangle are presented: one thematic area composed of Theme A¹ and Theme A², and another thematic area composed of Theme B¹, Theme B² and Theme C². The thematic evolution map (Figure 5a) is the longitudinal representation of the work, and it is composed of as many columns as periods which have been chosen in the analysis: in our case, three. Each column contains the most relevant topics of that period (in the example there are two thematic lines). The first thematic line (theme A, that does not change its name from period 1 to 2). The second thematic line, theme B, begins in period one (theme B¹), but nevertheless, in period 2, it becomes two themes (a theme that preserves the old name, B, and a new thematic line, C, opens). The lines that connect with the themes correspond to the evolution of these themes over time.

In Figure 5b, the stability between two consecutive periods is shown. Each circle represents the number of keywords for that period. The horizontal arrow shows the number of keywords shared in the two periods with the index of similitude between parentheses. The entry arrow represents the number of new keywords, and the exit arrow represents the number of keywords that are no longer present in the current period but were present in the previous period.

4.1. Longitudinal Map

To analyse the development of the present field of research, incineration and valorisation, it was useful to represent how these have evolved over time in different periods. Figure 7 shows a graph with circles that represent the periods described and the number of keywords of interest in those years. In the first period, from 1987 to 2010, there were 52 keywords. In the second period, there were 85 words, of which 43 words were from the previous period (9 words were no longer used), and 42 new words appeared. In the third period, there were 112 words, of which all but one came from the previous period, and 28 new words were incorporated. The stability index between the first two periods was 0.46. This stability index increased between the second and third period to 0.74. The increase in the stability index shows that the concepts treated were gradually consolidating, although they were still developing.

Regarding the evolution of the topics by periods, as seen in Figure 8, it was found that the concept of ‘valorisation’ was consolidated over time. In the period 1987–2010, the interest was in the recovery of resources. However, the ‘residues’ concept was gaining more prominence compared to the ‘waste’ concept.

In the period 2011–2015, the concept of incineration appeared strongly as a transformation from the previous period of the more generic concepts ‘waste’ and ‘municipal solid waste’. The concern in these years was that the recovery of waste from the previous period had not been enough, and that too much waste was still going to landfill, which is why the fundamental concern was in the concept ‘anaerobic digestion’ [25] as a transformation of the concepts ‘resource recovery’ and ‘waste’. Incineration represented a priority in these years. The generic concept ‘waste’ from the previous period was transformed into the concept ‘incineration’. In this second period, the waste treatment methods began to be concerned with their benefits and their costs. That is why studies on ‘pyrolysis’ and ‘fast pyrolysis’ were increasing. It was in this second period where the concept of ‘environmental impact’ took hold. In the third period, the main concern was valorisation.

The concept of ‘valorisation’ converged from the previous period of the concepts ‘anaerobic digestion’, ‘incineration’, ‘waste’ and ‘environmental impact’. In this third period, a concept also of interest was ‘ash’. On the one hand, the ‘incineration’ of the second period was called valorisation and the concern was ash. This concern for ash from incineration was especially relevant in ‘construction’, which is why the concepts of the Second Period of incineration and construction converged to the concept of ash as a current concern in scientific studies.

In the period 2016–2021, the main concept was valorisation, which linked the concepts of the previous period: incineration, waste and anaerobic digestion [17,33,34]. This is why it follows that the concept of valorisation is a more generic and friendlier concept; it did not produce rejection as the concept of incineration did. Researchers preferred the concept of ‘valorisation’, and yet in this period, 2016–2021, they attempted to avoid the concept of ‘incineration’ because it had a significant negative connotation with the general population, perhaps because this concept is associated with that of health risks.

4.2. Strategic Map and Thematic Network

Longitudinal analysis made it possible to know the evolution of the concepts between periods. In this section, the importance of each subject in the research field for each of the periods is analysed.

Table 4 shows the cluster properties, namely, the clusters for each period, indicating the score in terms of centrality, centrality range, density and density range. Likewise, Table 4 also shows the number of documents that used specific keywords as well as the h-index, average citations and the sum of citations.

5.1. Theoretical Implications

This article presents a significant theoretical contribution in the field of research on waste management, organising the knowledge generated in this area to date and detecting the driving themes in which the research will be structured in the coming years. From a theoretical point of view, all the articles from the Web of Science (WoS) related to incineration and valorisation were analysed, which provides clarity due to the thematic evolution maps that help us to better understand the longitudinal progression of the research field.

This study offers an important theoretical contribution to the field of research on the valorisation of waste, by expanding the existing knowledge. The use of co-word analysis allowed us to conduct a study of the valorisation and incineration literature to examine this field, its structures, patterns of influence, and propose future research directions. Furthermore, the study presents SciMAT as a good tool for performing longitudinal bibliometric analyses.

In addition, the longitudinal bibliometric analysis is presented as very suitable to carry out this type of analysis, which has not been performed in this field to date.

5.2. Practical Implications

From a practical point of view, the findings of this research are of substantial interest to different stakeholders such as policymakers, consumers and companies, among others. The benefits in this field for most stakeholders represent a considerable benefit to humanity.

This article reflects growing environmental sensitivity and pressure for proper waste management.

The work shows that the research topics at present and in the near future will be focused around some motor topics. They are well developed and important topics to structure the research field. The study shows that the most important motor theme is valorisation. Appreciation is the way. Around this theme, other important themes will revolve such as waste management, incineration, municipal solid waste, environmental impact, among others.

5.3. Limitations and Future Lines of Research

This work has some limitations. On the one hand, we have worked with articles included in the WoS Core Collection database. Although it is true that this database is very complete and relevant, without a doubt, other collections could be used in future bibliometric investigations.

Another limitation comes from the management of the software itself, where the researcher must guide the study, through filters in each of the steps, to achieve the necessary precision.

A future line of research could include a co-citation analysis, which would complement this study.