**1. Introduction**

Charcoal (**Figure 1**) is a solid porous pyrolysis product of plant biomass (thermal decomposition without oxygen access), which is formed along with liquid products (resins) and combustible gases. Charcoal is used as a high quality renewable solid fuel (calorific value 30–35 MJ/kg), as well as in industry, for example, in the production of technical silicon [1].

Currently, the use of charcoal in the production of carbon sorbents is expanding, which are used in various technologies of adsorption purification and separation [2, 3].

The density of wood has a significant effect on the strength of coal, which can vary significantly both from breed to breed, and in different parts of the wood. **Table 1** shows the average physical characteristics of oak and birch wood, which are most often used in Belarus for the production of high-quality charcoal.

The yield of charcoal during pyrolysis increases as the size of the pieces of wood increases, while the yield of almost all liquid products decreases. Depending on the size of the wood raw material, it is possible to achieve different densities of its packing in the working chamber. Depending on the size of the wood raw material, it is possible to achieve different densities of its packing in the working chamber.

### **Figure 1.**

*General view (a) and micrograph (b) of charcoal.*


### **Table 1.**

*Physical parameters of raw materials for the production of charcoal [2].*


### **Table 2.**

*Coefficients of full wood content [1].*

The full wood ratio determines the volume fraction of wood in a densely folded woodpile. The values of the fullness coefficient for firewood of different sizes are presented in **Table 2**. Assuming that birch chopped firewood 0.5 m long is used for coal production, we will use the value of the wood content factor 0.72 in the calculations.

The process of wood pyrolysis essentially depends on the initial moisture content. This value varies in very wide ranges: from 60 to 65% for freshly sawn raw materials to 15% for air-dry wood. The optimal level of relative humidity of wood intended for pyrolysis is in the range of 15–25%. With a higher humidity, in addition to an increase in fuel consumption and a decrease in the productivity of devices, the mechanical strength of coal decreases. In this regard, the raw material must be pre-dried. With natural drying of wood, a 12-month supply of raw materials is technically justified. With artificial drying, the supply of wood must be at least 3 months [1].

Depending on the calcination temperature and its duration, the content of non-volatile carbon in the coal varies. Tables 2.3 and 2.4 show examples of the distribution of elements in the original wood and decomposition products for the calcination depths, as a result of which the content of non-volatile carbon in coal is 84 and 94%. The data are given in terms of absolutely dry wood. As follows from these tables, an increase in the content of non-volatile carbon leads to a decrease in the yield of coal and an increase in the duration of the process. Thus, the calcination of the coal should only be carried out to the extent required by the customer. For domestic purposes, coal with a non-volatile carbon content of 77–82% is optimal.

Traditionally, charcoal is produced in the process of slow pyrolysis at heating rates of 5–7 K/min and temperatures of 623–873 K. The duration of the pyrolysis process is 1–14 days, depending on the unit productivity. The mass yield of coal is 20–33%. In the world, such equipment produces from 26 to 100 million tons of charcoal per year with a growth trend of about 3% per year [1]. A consequence of the high duration and low efficiency of the process is a significantly higher cost of charcoal in comparison with fossil coal.

Usually, when charring, either direct heating with a hot gas stream flowing through the bed and washing the surface of individual pieces is used, or indirect heating through the wall of the apparatus.

**Table 3** shows the classification of charcoal burning equipment according to various criteria affecting the thermal regimes and the speed of the pyrolysis process [2].

**Figure 2** shows various types of equipment for the production of charcoal: (a) Combustion furnace VMR (USA) (Duty cycle: 48 hours. Chamber volume: 14 m3 . Loading: 10 m3 . Temperature ~ 450°C. Coal output: 1000 kg (≈30%)); (b) experimental reactor Antala (University of Hawaii, USA) (max. Coal yield ≈ 40%. cycle no more than 1.5 hours. Heating of raw materials in a sealed reactor leads to volatility and pressure rise to 7–10 bar at 350°C); (c) UVP-5US mobile incinerator (CIS) (Working cycle: 8–12 hours. Retort volume: 4.5 m3 . Coal output: 30–32%.); (d) charcoal kiln unitary enterprise "ECO-CARBON" (Ukraine) (Working cycle: 18–34 hours. Consumption of firewood for the furnace up to 0.2 m3 /day).

The main stages of the charcoal production process are: drying, endothermic pyrolysis, exothermic pyrolysis, calcination and cooling.

During the drying process, the wood is heated to a temperature of 100–120°C and the water evaporates. Due to the fact that with excessively intensive drying, vapors tear wood, the temperature of the heat carrier in known technologies does not exceed 200–220°C (average temperature in the chamber). At the same time, while the wood is not dry, its temperature (at normal pressure) remains about 100–120°C. The composition of the wood is practically unchanged. Drying is usually divided into two stages. At the first stage (heating and actual drying), the relative humidity is brought to 5%. The second stage – drying is carried out already in the pyrolysis process.

The stage of endothermic pyrolysis includes heating wood before the decomposition of hemicelluloses, removal of bound moisture, decomposition of hemicelluloses and individual lignin fragments, heating to the temperature of exothermic pyrolysis.


### **Table 3.**

*Systematization of charcoal burning equipment [2].*

Exothermic decomposition of wood occurs in the temperature range from 275 to 450°C. At this stage, there is an intensive decomposition of cellulose and lignin, secondary polymerization reactions occur, and resins are formed. The free heat of exothermic pyrolysis is 1000–1150 kJ per 1 kg of wood.

The beginning of intensive decomposition of cellulose and wood corresponds to a temperature of 280–290°C, lignin decomposes at a temperature of about 350–450°C.

*Features of Pyrolysis of Plant Biomass at Excessive Pressure DOI: http://dx.doi.org/10.5772/intechopen.99468*

**Figure 2.**

*Equipment for the production of charcoal: (a) combustion furnace VMR (USA); (b) experimental reactor Antala (University of Hawaii, USA); (c) UVP-5US mobile incinerator (CIS); (d) charcoal kiln unitary enterprise "EKO-CARBON" (Ukraine).*

At this stage, it is necessary to provide intensive heat removal to prevent the possibility of an uncontrolled process. The gases generated by pyrolysis can be used for combustion in the combustion chamber. In the event of an excessive temperature rise, intensive removal of the temperature is required. The completion of wood pyrolysis is most often determined by the absence of active combustion of pyrolysis gas in the combustion chamber.

Calcination is necessary to remove from the carbon skeleton the residues of volatile substances retained by adsorption. At the same time, the content of non-volatile carbon in coal increases and the mass yield of charcoal decreases. In the course of calcination, the structure of coal also changes – functional groups containing oxygen and hydrogen are split off from it, and the bonds between carbon atoms change.

In the course of calcination, endothermic and exothermic reactions proceed in parallel, the total balance of the stage is endothermic. However, with a high degree of accuracy, it can be assumed that the heat consumption during the calcination process is necessary only for heating the charcoal.

During the cooling process, the temperature of the charcoal is lowered to 120°C. This process is most efficiently carried out using steam. Lower temperatures can be

achieved using cold, non-condensable pyrolysis gases. After coal is unloaded from the reactor, it cools completely without oxygen.

Of the many factors that determine the yield and composition of biomass pyrolysis products, the main ones are the final process temperature and pressure in the apparatus, the heating rate of the material, the type of heat carrier used, the method of heat exchange and the presence of chemical additives, as well as the type and quality of raw materials.

It is known that the process of pyrolysis of plant biomass is influenced by various parameters, such as temperature, composition and structure of raw materials, moisture content of raw materials, environmental pressure, etc.

The influence of pressure on the process of wood pyrolysis has been studied for quite a long time [4–8].

Klason et al. [4, 5] more than 100 years ago, studying the pyrolysis of cellulose and woody biomass at a temperature of 400°C, found that the formation of charcoal is due to the occurrence of primary and secondary chemical processes. According to these authors, as a result of the course of primary processes, the formation of charcoal and a large amount of various hydrocarbons occurs, which then decompose with the formation of charcoal and volatile components CO2, CO, H2, etc. pyrolysis, the yield of charcoal is increased.

The results of Klason et al. [4, 5] were later confirmed in studies of pyrolysis of various types of biomass and various conditions of this process [6–8].

Despite extensive studies of the influence of pressure and other factors on the formation of charcoal in the process of pyrolysis of biomass, the chemical mechanism of the influence of pressure has not yet been clarified, which necessitates a study of this effect when developing an effective technology for the production of charcoal from biomass.

A laboratory reactor for the thermochemical conversion of wood was developed and constructed to work out the modes of the process of obtaining charcoal. The diagram of the reactor is shown in **Figure 3**.

**Figure 3.** *Schematic of a laboratory reactor.*

*Features of Pyrolysis of Plant Biomass at Excessive Pressure DOI: http://dx.doi.org/10.5772/intechopen.99468*

### **Figure 4.** *Laboratory bench for the production of charcoal.*

The main elements of the laboratory reactor are housing 1, cover 2, thermal insulation 3, casing 4, fittings 5–6, electric heater 7, monometer 8, thermocouples 9–10, support legs 11.

The general view of the reactor is shown in **Figure 4**.

The raw material used was birch wood in the form of firewood and chips.

The design of the reactor allows pyrolysis of biomass both at atmospheric and overpressure. For this, fitting 5 was plugged, and an adjustable safety valve was attached to fitting 6 instead of thermocouple 10, which was triggered at a pressure of 0.3–0.7 MPa.
