## 2 Literature Review
Biochar is characterized by properties similar to brittle substances, which means it can be mixed with coal in amounts as high as 30% [17]. Pyrolysis is a thermochemical process of biomass transformation. It occurs under anaerobic conditions, or in a small amount of oxygen sufficient for combustion. This process involves the thermal decomposition of a substance rich in organic carbon. The pyrolysis process is classified by many factors, one of which is the temperature at which the process is carried out. In this case, a distinction is made between low-temperature pyrolysis, which is carried out in the temperature range from 450 \({}^{\circ}\)C to 700 \({}^{\circ}\)C, and high-temperature pyrolysis, or so-called coking, in the temperature range from 900 \({}^{\circ}\)C to 1200 \({}^{\circ}\)C. Another division of the pyrolysis process concerns its speed. In this case, a distinction is made between slow and fast pyrolysis. To distinguish between them, two basic parameters are used, namely temperature and time. Most often, the process is carried out at a temperature of 400 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C, and the time the raw material spends in the reactor ranges from a few minutes for slow pyrolysis to up to 0.5 s for very fast pyrolysis. Running the process at a low temperature, as well as slowly heating the raw material, are more effective in terms of solid product formation, while to obtain more liquid raw material, higher temperatures are used. Using very fast pyrolysis at 900 \({}^{\circ}\)C, we can obtain gaseous products. The conditions under which the pyrolysis process is carried out are chosen depending on the type of raw material; in this way, one can obtain the desired product. Pyrolysis with a slow increase in temperature was used for thousands of years, and the main product was charcoal. During the slow pyrolysis process, the biomass is heated to 500 \({}^{\circ}\)C, and the time it stays in the reactor is between 5 and 30 min. The residence of the particles in the chamber for a long time causes the additional transformation of the originally formed products. The components of the gas phase react with each other to form a solid phase in the form of carbon and a liquid phase. The material processed by pyrolysis can be kept at a constant temperature or gradually heated, and the emitted gases can be regularly removed during the process. Fast pyrolysis is a process carried out at high temperatures. The biomass is heated at a high rate and there is no access to air. The products of this process are gases, aerosols and a charred residue. If the gas phase and aerosols condense, a brown liquid is obtained, which has half of the heating value of traditionally obtained diesel fuel. The advantage of fast pyrolysis is that it generates no waste. Bio-oil and charred products can be used as fuel, and gases can be recycled back into the process. In this process, an important aspect is the rate of heat transfer, which is facilitated by the finely ground material. A feature of fast pyrolysis is also the residence time of the vapors in the reactor, which is very short (less than 2 s). Rapid cooling followed by the condensation of vapors and aerosols helps to produce bio-oil. During the process of rapid pyrolysis, i.e., at temperatures between 500 \({}^{\circ}\)C and 700 \({}^{\circ}\)C and a short residence time of primary products in the conversion zone, the content of individual products is as follows: liquid fraction--75%, biochar--12% and gas--13%. Comparing this to slow pyrolysis, carried out at temperatures below 550 \({}^{\circ}\)C and a long residence time of primary products in the conversion zone, the content of each product is distributed differently: liquid fraction--30%, biochar--35% and gas--35%. Knowing what material is to be subjected to the pyrolysis process and what products are to be obtained, the type of pyrolysis to be carried out should be selected accordingly. Thermal conversion of biomass opens up prospects for creating a more sustainable and environmentally friendly energy system.