**1. Introduction**

The CE has emerged as an alternative model to the prevailing "take–make–waste" approach to production and consumption in contemporary economic systems, which is an unsustainable path leading to resource depletion and severe environmental problems, such as climate change, air and water pollution, and biodiversity loss [1, 2]. In the linear economy, resources are extracted from nature, transformed into products that are then consumed within the human economic system until they are finally disposed of as waste back to nature [2]. By contrast, the CE model fosters the responsible and cyclical use of resources to maintain their value within the economy, while

minimizing pressures on the environment [3, 4]. It operates at three system levels; the micro level (products, consumers, companies), the meso level (eco-industrial parks), and the macro level (cities, regions, countries), with the ultimate aim to achieve sustainable development [5].

The transition toward the CE requires, among others, the development of new technologies [6, 7]. An emerging technology that could promote the operationalization of the CE model is biochar systems. These are multifunctional systems that can produce bioenergy and biochar through the thermochemical conversion of different types of biomass feedstocks (e.g., wood, agricultural residues, and wastewater sludge) in an oxygen-limited environment [8, 9]. Biochar is a porous solid carbonaceous material with versatile physicochemical properties that has a multitude of applications, including its use for amendment of agricultural soils, water purification and wastewater treatment, concrete and steel production, and remediation of contaminated soils [10]. The application of biochar to soils is probably its most prominent application, as, apart from improving soil quality, it sequesters atmospheric CO2, thereby contributing to climate change mitigation [11]. The multi-functionality of the biochar systems offers opportunities for developing integrated systems for valorizing different waste streams [12, 13], which is vital for the implementation of the CE model.

In this chapter, biochar systems, for valorizing wood waste and contaminated soils, are presented, and the potential role of these systems in the CE is explored. The rest of the chapter is structured as follows: Section 2 provides an overview of the CE concept and its principles to set the context of the study; Section 3 provides a brief description of different biochar systems; Section 4 focuses on biochar systems for valorizing wood waste and contaminated soils, and describes a case study, where the environmental performance of such systems is assessed; Section 5. discusses the role of biochar systems in the CE; and Section 6 summarizes the conclusion of the study.
