# Biochar from Agro-Forest Residue: Application Perspective Based on Decision Support Analysis
## 1 Introduction  
The average annual biomass produced in the land-based sectors (agriculture and forestry) of the European Union (EU) is 1466 Mt dry matter between 2006 and 2015 (956 Mt agriculture and 510 Mt forestry, respectively). Some of the biomass should stay in the field to maintain the carbon sink and the other ecosystem services [1]. Currently, biomass residues from agriculture, forest, and food industry are used for different purposes. Portugal is a huge producer of wine. The wine industry generates a significant amount of grape pomace--20-25% of the grape used in wine production [2]. The grape pomace consists of seeds, skins, and stems in different proportions. Bulgaria is well known for having long-term traditions in cultivating agricultural produce, which is usually used for human and animal food as well as raw materials for biofuel manufacturing [3, 4]. This Member State is one of the biggest European sunflower producers [5], mainly processing it into sunflower oil. The remaining sunflower husks have pronounced energy potential. In addition, the fruit-growing sector (cherries, peaches, plums, etc.) plays a remarkable role in the Bulgarian industry, as well as the cultivation of wheat and colza [6]. The production of cherries and peaches has increased during the last years [7]. The fruit stones remaining after fruits' processing, together with other types of biomass residue generated by the food industry, are foreseen as possible energy carriers by the Ministry of Energy of Bulgaria [8]. These residues could be converted into solid biofuel, or some of them can be directly burnt, such as fruit stones. Different technologies of biomass conversion are applied depending on the targeted products. The agriculture biomass can be used in several thermochemical and biochemical conversion processes (gasification, pyrolysis, anaerobic digestion, etc. ), obtaining valuable products [9; 10; 11]. Recent study proposes biochar (from maple leaf) for the removal of anionic and cationic dyes [12]; others obtain good adsorptive characteristics of biochar of diverse origin [13; 14; 15]. The process of carbonization is often used for producing biochar from different feedbacks (e.g., coal, biomass, refuse-derived fuels, sludge, and others). The resulting products typically show increased carbon content and higher calorific value [16; 17]. Biomass carbonization at low temperature improves not only its energy content but also facilitates its handling, transportation, and storage. Additionally, carbonized biomass is easy to be ground and, thus, it requires less energy for pulverization [2; 18]. [PERSON] et al. [2] studied the hydrothermal carbonization of grape pomace at 175-275 \({}^{\circ}\)C and torrefraction (under nitrogen atmosphere at 250 and 300 \({}^{\circ}\)C), and obtained mass yield biochar ranging between 47 and 78%. [PERSON] et al. [19] investigate biochar from different feedstocks after slow pyrolysis and carbonization. The used biomass was carbonized at four temperatures (300, 400, 600, and 750 \({}^{\circ}\)C) and two residence times, 10 min and 60 min. It is observed that the biochar yield for wood, straw, green waste, and algae is 20-25% in the temperature range of 450-750 \({}^{\circ}\)C. The authors point out that the biochar yield depends mainly on the residence time, especially at temperatures above 400 \({}^{\circ}\)C. [PERSON] et al. [20] report similar temperature dependence during carbonization of palm fiber, where the biochar yield changes from 24 to 18% between 400 and 700 \({}^{\circ}\)C. [PERSON] et al. [21] produce biochar from sunflower husks at different temperatures (480, 530, and 580 \({}^{\circ}\)C), heating time (65 and 120 min), and retention time (10 and 60 min). The authors suggested that the obtained biochar can be used as source of chemical elements, which are important for the plants' growth (Ca, B, Mg, and P). [PERSON] et al. [22] studied the sunflower shells' utilization in a captive sample reactor at temperatures between 300 and 600 \({}^{\circ}\)C and in inert atmosphere (helium).