*3.5. Composition of Biochars*

Biochar is a form of stable carbon and is a viable option for minimizing climate change and carbon footprint reduction. The pyrolysis process significantly increases the carbon content in the feedstock and converts it into stable aromatic compounds, increasing their number and density. The main characteristics of biochars obtained from AR are presented in Table 6. The highest amount of fixed carbon is found in samples from inflorescences, and the lowest from leaves. The ash content of the studied biochars varies from 31.9 to 46.8%. Leaves are characterized by the maximum ash content, and inflorescences are characterized by the minimum. The higher calorific value naturally decreases with the growth of the ash residue. The comparison of biochars obtained from inflorescences of AR weed and cultivated plants [70] showed that they have a similar composition; they contain a lot of calcium and potassium.

The study of the mineral part of biochar is a top priority when considering it as a fuel for power plants. Since most of the chemical elements contained in the samples have high melting and boiling points, after the pyrolysis process, they are concentrated in a solid carbonaceous residue. In the case of intensive slag formation processes, there is a high probability of not only unscheduled equipment shutdowns but also its failure due to the accumulation of uncontrolled amounts or forms of deposits. To predict the slag-forming and polluting properties of biochars, we determined the values of *B/A, SR, and Fu.*

The optimal value of *B/A*, at which the fuel will not be slagged, should be less than 0.6. The value of this parameter indicates that the biochar ash of AR will be highly prone to slagging. The slag viscosity index *SR* (>72) corresponds to a high viscosity and therefore a low tendency to slagging. The optimal value of the fouling index *Fu* should be less than 0.6. The parameters obtained indicate extreme slag formation in power equipment when using AR biochars.


**Table 6.** Results of the proximate analysis, oxide compositions of biochars, and slagging parameters.

A promising solution to the problem is the use of composite fuel from a mixture of coal and biochar [71]. When using Krasnogorskiy coal (south of west Siberia) [72] in a mixture with biochars, one can obtain high-quality fuel with optimal *B/A* and *SR* values (Figure 8).

**Figure 8.** Composite fuel composition.

If biochar is planned to be used without mixing with high-quality fuel, then it is advisable to use it as a soil additive. Adding biochar to the soil not only captures carbon, but also reduces greenhouse gas emissions from the soil, is a promising option for managing agricultural waste, increases soil resilience, reduces the need for fertilizers, and also has a number of other environmental benefits [73]. Thus, the use of biochar promotes carbon sequestration [74]. Mild conditions of thermochemical conversion (at temperatures below 600 ◦C) make it possible to avoid the melting of ash and retain nutrients in a form accessible to microorganisms and plants [75]. The removal of biomass from the biocenosis should not lead to the disruption of the biogeochemical cycle of substances, so it is advisable to use

biochars as soil additives. This use of biochar can also improve soil fertility by increasing its cation exchange and water retention capacity, as well as microbial activity [73,76,77].

A study [78] showed that biochar increased soil pH and improved its electrical conductivity, aggregate stability, water retention, and micronutrient content. In this case, it is necessary to take into account: soil type, seasonality, application rates, and characteristics of cultivated crops. In the study [76], it was noted that, depending on the feedstock and pyrolysis conditions, biochars have the most favorable effect on acidic and neutral soils. The authors of [79,80] proved that the introduction of biochar is of great importance for clay, sandy, and shale soils. The high content of calcium, potassium, magnesium, and phosphorus suggests that AR-derived biochars can not only improve soil structure, but also provide plants with essential nutrients. Thus, AR weed has a promising potential as a raw material for the production of soil additives.
