# Biochar as a Multifunctional Component of the Environment--A Review
## 1 Introduction  
In recent years, biochar and the wide range of its possible applications have been extensively investigated by researchers worldwide. In accordance with the definition specified by the International Biochar Initiative (IBI), biochar is a fine-grained product of carbonization, characterized by a high content of organic carbon and low susceptibility to degradation, which is obtained through the pyrolysis of biomass and biodegradable waste [1]. It is produced from organic matter as a result of pyrolysis, a process which is carried out in the absence of oxygen. Biochar can be utilized for energy-related purposes associated with environmental conservation and agriculture. The wide range of biochar applications is continuously expanding, mainly in such areas as industry, agriculture and operations related to the natural environment. It can be used as a soil additive, or added to fodder and silage, or applied in water treatment [2; 3]. Biochar can also be used for the immobilization of contaminants from soil, and in sewage treatment; it can be applied as a supplementary material in composting and in methane fermentation processes [4; 5; 6; 7; 8; 9]. Biochar application can be used as a filterfor tar reduction in pyrolysis and gasification, as a fuel when pelletized, and also used as a substrate to produce hydrogen [10; 11; 12]. One of the ways to convert biomass is the gasification process, a state-of-the-art method of energetic use of biomass. The advantages of this process in relation to other methods are the possibilities of multi-directional use of the obtained product, namely the gas. Modern gasification methods make it possible to obtain two products: synthesis gas (syngas) being a mixture of gases (H\({}_{2}\), CO, CO\({}_{2}\), CH\({}_{4}\)) and residues in the form of ash. When classifying gasification processes, they can be divided using different criteria depending on reactor type, gasification factor, thermal relations, gasification process parameters (pressure, temperature). The first stage of the gasification process is drying the material, which can have up to 50% water content, at a temperature of 100-200 \({}^{\circ}\)C. The next step is pyrolysis at increased temperatures (200-600 \({}^{\circ}\)C) using an anaerobic atmosphere to release the volatile parts contained in biomass. Products of this stage are solid substances such as charcoal, liquid substances (tars, oils and gas water), flammable gas and aromatic hydrocarbons such as benzene, toluene. The last stage is gasification (temperature above 750 \({}^{\circ}\)C) of solid and liquid substances produced at pyrolysis. This is a series of exo- and endothermal reactions that result in the production of flammable gas components [13; 14; 15]. Thermal processes of biomass (mainly the ones of lignocellulosic origin) processing also include torrefaction. The process is carried out in an anaerobic atmosphere at a temperature of 200-300 \({}^{\circ}\)C and the rate of temperature increase is in the range of 10-100 \({}^{\circ}\)C min\({}^{-1}\). Given the long duration of the process and relatively low temperatures, this process is also called roasting or mild pyrolysis [16]. Biomass processed as a result of the torrefaction process acquires new physicochemical properties, especially important when used as a fuel for the power industry. Products of torrefaction are characterized by increased milling susceptibility and energy density and their properties resemble low calorific coals. A typical torrefaction process is characterized by loss of mass and chemical energy of the raw material used. When analyzing the mass/energy ratio, an increased concentration of chemical energy of the fuel obtained in relation to the raw material can be observed [17]. When analyzing environmental aspects, biochemical conversions have an important role. The products (fuels) produced by these technologies are biogas, thio-alcohols and biodiesel. The use of biochemical processes seems reasonable when biomass contains large amounts of water.