# Slow Pyrolysis as a Method of Treating Household Biowaste for Biochar Production
## 1 Introduction
Although this method is more energy-efficient, it requires well-dried and finely ground biomass, as well as precise process control, which limits its practical use for moistkitchen waste. Hydrothermal pyrolysis, on the other hand, occurs in the presence of water, at temperatures of 200-350 \({}^{\circ}\)C and under high pressure. It is distinguished by its ability to process high-moisture biomass without the need for drying, making it a promising technology for food waste and other wet organic waste [7]. The final product is a liquid bio-oil and a small amount of biochar. However, this technology is more complex and requires sealed high-pressure reactors, which currently restricts its application in larger industrial facilities [14; 15]. Given these distinctions, slow pyrolysis was selected for further consideration due to its operational simplicity and the high yield of stable biochar it provides. Its lower temperature and longer reaction time result in a carbon-rich product well suited for applications such as soil improvement or carbon sequestration. Unlike fast pyrolysis, it does not demand strict control parameters or finely processed input, making it more adaptable to various types of biowaste [12]. Although hydrothermal methods offer the advantage of processing wet materials without pre-drying [7], they involve significantly more complex and costly infrastructure due to the need for high-pressure equipment [14; 15]. The yield of pyrolysis products depends on the reactor design, feedstock properties, and operational parameters such as temperature, residence time, heating rate, pressure, ambient gas composition, and presence of mineral catalysts [16; 17; 18]. At low temperatures (<300 \({}^{\circ}\)C), heavy tars and higher biochar yields form due to limited biomass bond breakdown, while high temperatures (>550 \({}^{\circ}\)C) cause extensive fragmentation, increasing non-condensable gas yield and reducing biochar [16; 19]. Bio-oil yield peaks at 500-550 \({}^{\circ}\)C and decreases above 550 \({}^{\circ}\)C [20]. Longer residence time and slow heating promote biochar formation via secondary reactions, but reduce bio-oil yield by enhancing tar cracking to gas [16; 21; 22]. The heating rate influences product distribution: slow heating at 400-600 \({}^{\circ}\)C favors biochar, while fast heating increases bio-oil yield [21; 22]. Particle size affects heating and product yield--smaller particles enhance bio-oil production due to the larger surface area and less secondary reaction, while larger particles increase biochar by trapping volatiles and promoting re-polymerization [16; 23]. Particular attention should be given to biochar, a stable, porous, and carbon-rich by-product of biomass pyrolysis [16]. Biochar is typically produced under slow pyrolysis conditions--characterized by low heating rates and moderate temperatures, generally within the range of 400-700 \({}^{\circ}\)C [24]. Due to its unique physicochemical properties, biochar has been investigated for a variety of applications. Its most extensively studied uses include soil amendment [25] and water purification [26]. Additionally, biochar can serve as a solid fuel [10] or as a material for energy storage systems [27]. The production and application of biochar derived from organic waste also offer environmental benefits, including the mitigation of greenhouse gas emissions [28], climate change impacts, and the promotion of long-term carbon sequestration [25; 26]. Slow pyrolysis has been widely studied for various types of biowaste, including agricultural residues, forestry biomass, energy crops, algal biomass, food industry waste, and municipal solid waste (MSW) [29]. [PERSON] et al. [30] analyzed an integrated pyrolysis-CHP system converting the organic fraction of MSW in Leicester, UK, achieving nearly 60% cogeneration efficiency at 600 \({}^{\circ}\)C. [PERSON] et al. [31] pyrolyzed ten MSW fractions from Mumbai, determining an optimal temperature range of 170-520 \({}^{\circ}\)C for mixed waste. [PERSON] et al.