**2. Description of experiments**

To reveal the effect of excess pressure on the yield of solid pyrolysis products (charcoal), experimental studies of the pyrolysis of wood chips with a particle size of 17x8x6 mm were carried out at atmospheric and excess pressure, at various temperatures and pyrolysis times. The moisture content of the wood is 14.2%. The density of the wood is 506.4 kg/m3 , the ash content is 0.23%. The moisture content was determined using a FAUNA-M moisture meter (manufactured by OOO Lenta, RF). **Table 4** shows the elemental composition of the wood raw material used. Measurement error of elemental composition 0.1%.

During pyrolysis without overpressure, raw wood was placed in a reactor. Then thermocouple 10 was placed in the wood layer (**Figure 3**). After that, the electric heater 7 was turned on. The temperature inside the wood was determined using a thermocouple 10. At the moment when the temperature exceeds 110°C, it can be concluded that the drying process is complete. Thus, the drying time of the chips was determined.

After that, the temperature in the reactor increased to 400°C. The thermocouple 10 was used to measure the temperature inside the reactor.


**Table 4.**

*Elemental composition of used wood raw materials.*

The process of cooling the obtained charcoal was carried out in a natural way (with the heater turned off) and with the supply of water through the choke 5. In this case, the change in temperature inside the coal layer was recorded and the time of their cooling was determined.

When pyrolysis of wood with excess pressure increases with temperature inside the reactor, the release of gaseous products begins, which accumulate in the reactor, creating excess pressure. When the required pressure is reached, the valve is activated, and the excess of gaseous pyrolysis products leaves the reactor, which allows maintaining a constant operating pressure in the reactor.

Experiments were carried out at a pressure inside the reactor of 0.1, 0.3, 0.5, 0.7 MPa. The amount of energy consumed for the coal production process was the same for all experiments and amounted to 34.89 MJ.

**Figure 5** shows a general view of the resulting charcoal.

There was no excess pressure in the reactor (pyrolysis gases were freely leaving the reactor). The total pyrolysis time was 600 minutes (10 hours). The mass yield of coal was 25.1% (29.2% on dry weight of wood). The calorific value of coal is 34.86 MJ/kg. The calorific value was determined on a V-08MA "K" calorimeter.

In the course of the experiments, it was revealed that with the same consumed energy, the temperature inside the reactor during pyrolysis with excess pressure is higher than at atmospheric pressure. This is due to the fact that the thermal energy released during the operation of the heater accumulates inside the reactor until it begins to be consumed in the drying process or endothermic pyrolysis. However, under excess pressure, these reactions proceed at higher temperatures. For example, at a pressure of 0.1 MPa, an active process of moisture evaporation was observed at a temperature of 104 C, and at a pressure of 0.7 MPa - 185 C. This agrees with the reference data on the dependence of the temperature of water evaporation on pressure.

The mass yield of coal was 27.8% (32.4% per dry weight of wood with a thermodynamically equilibrium solid carbon yield of 41–42% in the pressure range

**Figure 5.** *General view of the resulting charcoal.*

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


### **Table 5.**

*Dependence of the mass yield of charcoal on the pressure in the reactor.*

**Figure 6.** *Graph of the dependence of the mass yield of charcoal on the pressure in the reactor.*

of 0.12–0.7 MPa). This is 1.12 times more than in the experiment without excess pressure. The calorific value of coal is 30.23 MJ/kg.

The operating time in all experiments is the same, the voltage on the heater was also constant, which means that the amount of energy expended on coal production is the same (34.9 MJ).

**Table 5** and **Figure 6** shows the dependence of the mass yield of charcoal to dry mass on the pressure in the reactor.

The mass yield of charcoal to dry matter was determined by the expression

$$X\_c = \frac{M\_c \cdot 100}{M\_0 \cdot \left(1 - \frac{W}{100}\right)},$$

where *МС* – mass of coal, g;

*M*0 – mass of the feedstock, g;

*W* – moisture content of the feedstock, %.

In **Figure 6**, the dark symbols are the data obtained by the authors, and the light symbol is the data obtained at the University of Hawaii (USA) [9] at a pressure of 1 MPa.

An increase in pressure leads to an increase in the residence time of gaseous pyrolysis products in the reactor and their contact with the feedstock and coal, i.e. brings the conditions in the reactor to equilibrium.

**Table 6** shows the values of the calorific value of the obtained charcoal at various pressures.

**Table 6** shows that with increasing pressure in the reactor, the calorific value of the resulting charcoal decreases. This can be explained by the fact that coal contains tar, the calorific value is higher than that of pure coal. We have determined that the


**Table 6.**

*Dependence of the calorific value of charcoal on the pressure in the reactor.*


### **Table 7.**

*Data of elemental composition for charcoal obtained at various pressures. Measurement error of elemental composition 0.1%.*

calorific value of pure coal (without tar) is 27.01 MJ/kg. It is close to the value of the calorific value of the coal obtained by us at a pressure of 0.7 MPa. High purity activated carbon was taken as a reference. At the same time, pyrolysis resins have a higher calorific value. Therefore, the presence of resins increases the calorific value of the resulting product, but decreases its quality. Based on this, we can conclude that with increasing pressure, the quality of coal increases.

This is also confirmed by the results of the elemental analysis of the obtained products. **Table 7** shows the elemental composition data for charcoal obtained at various pressures. The elemental composition was determined by X-ray spectral microanalysis using an Oxford Instruments X-MaxN energy dispersive analyzer operating in conjunction with a LEO1455VP scanning electron microscope with a sensitivity of 0.1 at.%.

**Figure 7** shows a micrograph of the used wood and a graph of the elemental composition.

**Figure 8** shows micrographs (left) and graphs of the elemental composition (right) of charcoal obtained at a pressure of 0.1 MPa (a, b), 0.3 MPa (c, d), 0.7 MPa (e, f).

As can be seen from **Figure 8**, the charcoal composition obtained at a pressure of 0.1 MPa contains amorphous resinous inclusions (**Figure 8a**). At a pressure of 0.3 MPa, inclusions are absent (**Figure 8c**). At a pressure of 0.7 MPa, the general view of coal under a microscope coincides with wood (**Figures 7a** and **8e**). This indicates the complete preservation of the cellular structure of wood during its pyrolysis under a pressure of 0.7 MPa.

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

**Figure 7.** *Micrograph (a) of used wood and graph of elemental composition (b).*

### **Figure 8.**

*Micrographs (left) and graphs of the elemental composition (right) of charcoal obtained at a pressure of 0.1 MPa (a, b), 0.3 MPa (c, d), 0.7 MPa (e, f).*

As noted above, in the case of pyrolysis of birch chips at higher pressures, the temperature in the chip layer reaches the plateau-like area at higher temperatures. An increase in temperature in the layer of chips promotes the process of desorption of oxygen-containing components from the pores of the resulting charcoal and to an increase in the carbon content in it.

**Table 7** shows that with an increase in pressure from 0.1 MPa to 0.7 MPa, the oxygen content in charcoal decreases from 8.4% by weight. up to 2.9% weight. In this case, the carbon content increases from 89.1% by weight. up to 96.4% weight.

According to the authors of [4–8], such a change in the yield of charcoal and the carbon content in it is due to the occurrence of secondary chemical reactions, in which the decomposition of the primary pyrolysis products occurs with the formation of charcoal and volatile components. The positive effect of pressure in this case is due to the difficulty of the diffusion release of the components formed during the decomposition of the wood pulp. This hypothesis was confirmed by us through experimental studies of the influence of the difficulty in the yield of gaseous pyrolysis products on the quantitative yield and composition of the obtained coals.
