Recent Progress in Biomass Pyrolysis and High Value Utilisation of Pyrolytic Carbon

1. Introduction

The modern world is facing an unprecedented challenge of energy transition, which requires a radical reorientation of how energy is produced and used. In this context, biomass pyrolysis technologies are gaining importance as a key element of decarbonisation strategies, offering the possibility of converting organic waste into high-value energy products. The increasing interest in these technologies arises not only from the necessity to discover sustainable alternatives to fossil fuels but also from the imperative to efficiently manage the escalating quantities of biomass waste. Pyrolysis, as a process of thermochemical conversion of biomass under limited oxygen conditions, enables the production of pyrolytic carbon with high energy value and a wide range of applications. This process occurs at temperatures between several hundred and over a thousand degrees Celsius, resulting in a substantial alteration of the chemical structure of biomass, producing a material with a markedly higher energy density than the feedstock. In line with the European Union’s ambitious climate policy, which aims to reduce greenhouse gas emissions by at least 55% by 2030, biomass technologies are playing an increasingly key role in the energy transition. Biochar serves as a particularly important substitute for conventional carbon fuels in heavy industry, which accounts for about 7% of global CO₂ emissions and is one of the most difficult sectors to decarbonise [1]. This article provides a summary of research conducted by scientists who have contributed new and complementary knowledge in the field of biomass pyrolysis and biochar applications, combining the technological perspective with the environmental and economic aspects of this promising technology.

2. A Short Review of the Contributions in This Special Issue

The utilisation of biochar has a history spanning millennia; however, modern investigations into the pyrolytic processing of biomass signify a fundamentally novel phase in the evolution of this technology. Rejdak et al. (1), in their research on bio-coke production, undertook a systematic analysis of the effect of biocomponent additives on the coking properties of coal mixtures. The work of these scientists is an example of the evolution of thinking from simple biomass combustion to its use as a component in advanced metallurgical processes. Researchers looked at different types of biomass, such as wood materials, including beech and alder chips, and sawmill waste, including pine sawdust, because the way these materials shrink during the plastic phase can be quite different. Different types of biomass undergo different shrinkage during pyrolysis [2], which causes the formation of empty spaces; therefore, adequate expansion potential is necessary [3,4]. A part of that is that they also consider food industry by-products such as hazelnut shells and olive pits. The diversity of raw materials exemplifies a pragmatic approach to utilising mainly locally available biomass resources, which is crucial for the sustainable development of pyrolysis technologies. The Rejdak’s team showed that the addition of up to 20% of biocomponents causes a systematic decrease in the Gieseler fluidity, dilatation, and Roga index of coal mixtures. However, the researchers did not stop at observing this phenomenon but developed a mathematical model describing the effect of the additive’s content and its properties on the decrease in fluidity. This modelling approach opens up the prospect of the predictive design of bio-coke mixtures with specified properties.

Building upon these foundational studies of biomass properties, researchers have simultaneously focused their efforts on developing more sophisticated processing technologies [5,6,7,8]. In parallel with research into product properties, scientists are focusing on optimising the pyrolysis processes themselves. Illankoon et al. have developed an innovative two-chamber pyrolysis reactor that represents a significant step forward in the democratisation of pyrolysis technology (2). Their design, aimed at producing biochar from rice husks in domestic and small agricultural contexts, fulfils the requirement for technology accessible to local communities in developing nations. The two-chamber reactor design, consisting of an outer cylinder for energy generation and an inner cylinder for pyrolysis, together with a synthesis gas circulation system, represents an elegant solution to the problem of energy self-sufficiency of the process. The parameters achieved by the team, e.g., a biochar yield of 49% with an average production rate of 1.8 kg/h, prove the practical feasibility of this concept. Particularly impressive is the achievement of a biochar specific surface area of 182 m²/g, which indicates a high product quality comparable to commercial activated carbons. The energy value of the pyrolysis gases, amounting to 23.3 MJ/m³, further confirms the energy efficiency of the entire process.

The transition from laboratory-scale innovations to real-world industrial implementation represents a critical phase in the development of pyrolysis technologies. This evolution is particularly evident in the steel industry, where revolutionary changes are taking place (3) [9,10,11]. Kieush and Schenk’s research on the use of biochar in electric arc furnace processes is an example of translating basic research into heavy industry (3,4). Their systematic approach to analysing the phenomenon of slag foaming using various carbon sources—from traditional anthracite and calcined coke, through high-temperature coke, to biochar as a single component and mixtures of biochar with coke—represents a comprehensive approach to the issue of replacing conventional fossil fuels. The team’s significant finding was that biochar can facilitate stable slag foaming at alkalinity levels between 1.2 and 3.4, which enables its potential application in the steel industry (4). Moreover, it was equally significant to discover that a 1:1 blend of biochar and coke demonstrates superior foaming properties compared to biochar alone, suggesting synergistic interactions among various carbon forms. The XRD analysis conducted by researchers showed that there are differences in the crystalline phases between the upper and lower sections of the foamed slag (3). The presence of maghemite in the upper section, indicative of oxidation processes, alongside metallic iron in the lower section, offers critical insights into the mechanisms underlying the foaming of slag with biochar.

Beyond metallurgical applications, pyrolysis technologies are finding significant applications in waste management and energy production sectors [12,13,14,15]. Carotenuto et al. undertook the ambitious task of optimising the sewage sludge gasification process for combined heat and power production [15]. The methodology employed by these scientists, utilising sophisticated modelling inside the Aspen Plus framework, exemplifies a contemporary approach to the design of biomass-derived energy processes. The researchers identified the optimal process conditions—an air heating temperature of 150 °C and an equivalence factor of 0.2—which allow an energy production potential of 2.54 kW per kilogram of dry sludge to be achieved. Equally important is the achievement of a CO₂ emission reduction of 0.59 kg per kilogram of dry matter compared to natural gas, confirming the environmental benefits of this technology. As pyrolysis technologies mature, researchers are exploring increasingly diverse and innovative applications that extend far beyond traditional energy production. The wine industry has emerged as an unexpected source of valuable biomass materials [16]. D’Eusanio et al. have initiated a novel phase in biochar research, concentrating on waste from the wine industry as a precursor for the synthesis of materials with electrical properties (5). Their research on grape seeds, defatted grape seeds, wooden stems, and whole grape seeds under pyrolysis conditions at 300 °C for 3 to 24 h represents a systematic approach to characterising the electrical properties of biochar. The results indicating the best thermal stability and higher electrical conductivity open up prospects for the use of this material in electronics and composite materials. The relationship between the development of carbonaceous structures and electrical conductivity offers significant insights into the mechanisms underlying electrical property formation during pyrolysis. Complementing these developments in electrical applications, alternative processing methods are being explored to maximise the potential of various biomass waste streams [17]. Wądrzyk et al. introduced an innovative approach to the conversion of coffee grounds through a hydrothermal process, which is an alternative to traditional pyrolysis (6). Their research on the influence of process temperature, retention time, and the use of a binary solvent on the yield and quality of products represents a holistic approach to process optimisation. Achieving a maximum hydrochar yield of 39% and a bio-oil yield of 46%, while obtaining bio-oils with a carbon content of 71–74% and an energy value of approximately 35 MJ/kg, and hydrochar with a carbon content of up to 73.8% and an energy value of 30 MJ/kg, confirms the potential of this technology as an alternative to conventional biomass processing methods.

The exploration of synergistic effects represents another frontier in advancing pyrolysis technologies [18,19]. Kudelytė et al. have investigated the synergistic effects of co-pyrolysis of plastic waste and lignin; they highlighted new opportunities for enhancing hydrocarbon recovery [19]. Their systematic investigations of polypropylene and low- and high-density polyethylene mixtures with lignin in diverse mass ratios exemplify an interdisciplinary methodology that integrates polymer chemistry with biomass technologies. The discovery that the best results are obtained for lignin–LDPE/HDPE mixtures at 600 °C in a mass ratio of 1:3, together with the finding that plastic radicals promote lignin depolymerisation, provides valuable information on synergistic mechanisms in co-pyrolysis processes. The increased recovery of high value-added hydrocarbons confirms the potential of this technology in the circular economy.

The development of high-temperature pyrolysis technologies represents the cutting edge of current research efforts, particularly in applications requiring high-performance characteristics [20,21]. Deutsch et al. have undertaken to determine the lowest thermal treatment temperature for wood that allows a biogenic reducing agent to be obtained that is suitable for use in existing carbon injection systems in blast furnaces (7). Their research under various temperature conditions ranging from 285 to 340 °C is an example of a meticulous strategy to improve chemical and mechanical properties. At the same time, they have retained the necessary transportability in a conventional pneumatic dense-phase transport system. Researchers have indicated that achieving an 18–40% reduction in CO₂ emissions in blast furnaces while maintaining transport properties similar to those of coal represents significant progress in the decarbonisation of the steel sector. This technology is a practical interim solution in the energy transition of heavy industry, offering the possibility of gradually replacing conventional fossil fuels.

An analysis of the presented studies reveals a fascinating mosaic of approaches to the use of biomass, which, despite the diversity of raw materials and applications, shows common themes. The first of these is the pursuit of process parameter optimisation to maximise product yields and quality. All research teams, regardless of the specific nature of their projects, focus precisely on determining the optimal conditions for temperature, retention time, and process atmosphere.

The second common theme is a pragmatic approach to the use of locally available waste raw materials. From rice husks in Sri Lanka to coffee grounds and wine production waste, scientists are constantly looking for ways to transform waste management problems into energy opportunities by using such diverse biomass waste streams that are locally available and respond to local challenges. This method demonstrates a profound comprehension of the tenets of the circular economy and sustainable development. The third key theme is the evolution from simple energy applications of biochar towards advanced industrial applications. Research on the use of biochar in the steel industry, composite materials, and electronics indicates a growing understanding of the potential of this material as a multifunctional component of modern technologies.

Despite promising results, the analysed studies also reveal significant technological and economic challenges. The main problem remains the deterioration of some technological properties when replacing conventional fuels with biochar. Rejdak’s research showed a systematic decrease in coking properties, while Kieush and Schenk identified problems with process stability when using biochar alone. These challenges culminate in the assertion that the most advantageous strategy is to employ hybrid mixtures that integrate biochar with conventional fuels. This method facilitates the incremental implementation of sustainable solutions while preserving essential technological attributes, which is vital for industrial acceptance. Another challenge is the economic scaling of processes from the laboratory to the industrial level. Most of the studies presented were conducted on a pilot scale, and the transition to full-scale production requires solving problems related to raw material logistics, product quality control, and cost optimisation.

The analysed studies provide valuable guidance for energy policy and decarbonisation strategies. The results point to the need for an integrated approach to the development of pyrolysis technologies that considers not only technical aspects but also economic and social aspects. Supporting the development of local biomass-based value chains is vital for fostering regional development and mitigating greenhouse gas emissions. Technologies such as the reactor developed by Illankoon et al. illustrate that advanced technological solutions can be customised to meet the specific needs of local communities. Establishing a suitable regulatory framework and quality standards is equally essential for promoting the commercialisation of pyrolysis technologies. The variety of raw materials and applications necessitates a flexible standardisation approach that considers specific local conditions and requirements.

3. Conclusions and Outlook

The presented analysis of the latest research on the pyrolytic use of biomass reveals a dynamically developing field of science and technology that offers real opportunities to accelerate the energy transition. The variety of research methodologies, spanning from fundamental investigations of material properties to practical industrial applications, illustrates the advancement of this field and its preparedness for broader implementation. The primary conclusion of the analysis is that pyrolytic technologies represent not a singular solution but rather a collection of complementary technologies that can be tailored to various feedstocks, local conditions, and end applications. This adaptability represents both the paramount strength and the principal challenge for this domain. The potential for CO₂ emission reductions of 18–59%, depending on the application [5,8], combined with the possibility of organic waste recovery, makes pyrolysis technologies a key element of decarbonisation strategies. However, realising this potential requires further research on process optimisation, the development of quality standards, and the creation of appropriate support mechanisms. The future of pyrolysis technologies is likely to be characterised by further specialisation and integration with other sustainable technologies. Investigations into composite materials incorporating biochar [4], the synergistic effects of co-pyrolysis [7], and hydrothermal alternatives to conventional pyrolysis [6] indicate the trajectory of this advancement. The success of pyrolysis technologies will ultimately hinge on the integration of scientific rigour with the practical requirements of industry and society. The studies analysed provide a solid scientific basis for this transformation, but its implementation will require further cooperation between the scientific community, industry, and policymakers.