*2.5. Carbonization Process*

Thermal carbonization (pyrolysis) was performed in a CZYLOK reactor (Jastrzebie-Zdroj, Poland; model FCF 2R) modified in the laboratory to enable the collection of pyrolytic gases. An accurately weighed sample was placed in the oven at room temperature. Thermal decomposition was then performed under conditions of limited oxygen access. The temperature in the reactor was gradually increased to 850 ◦C over 30 min. Pyrolysis was continued for 60 min at a constant temperature. Subsequently, the sample was left in the oven until the temperature fell. The solid residue after the process was weighed and stored for further analysis. The experiments were carried out in triplicate with seven gas collection points (450, 515, 585, 650, 715, 785 and 850 ◦C).

#### **3. Results**

### *3.1. Physico-Chemical Properties and Chemical Composition of the Biomass*

The results from proximate and elemental analysis of all the waste samples are given in Table 1. The moisture contents of the tested biomasses in the working state were from 9.65% to 16.54%. The lowest water content in the biomass was observed in waste from vodka production, which can be explained by the fact that these wastes had been dried in the factory before being delivered for analysis. The difference in the moisture content (6.02%) between the bark samples and municipal waste was due to the weather conditions under which the biomasses had been processed and then stored.

The content of mineral substances was similar in most of the analyzed biomasses, ranging from 2.57% to 4.37%. An exception was apple pomace, which contained only 1.05% of inorganic components. This value was in line with the data presented in the literature [10,11]. The ash content of MSW was in line with the average values described in the literature for wood biomass, in the range of 0.3–7.4% [12,13]. These results also indicate that the samples were not contaminated with soil. Our values are much lower than those presented in the literature for coal, i.e., 5–45% (8.5–10.5% on average) [13]. The chlorine content in the analyzed biomass was in the range of 0.02–0.20% and such values are consistent with the literature data for woody biomass, the chlorine content of which ranges from <0.005% to 0.057%. In cultivated plants, the chlorine content may even be >1.00% [2,14]. High chlorine content in crops is often associated with the use of potassium fertilizer.

The total sulfur content was highest for agricultural crops, B.pulp 0.75%, which probably results from the application of fertilizers for agrotechnical treatments. The nitrogen content was similar in all the analyzed biomasses (1.35% ± 0.26%) and did not exceed 1.50%, with the exception of BSG for which it was almost four-fold higher. However, this result is in accordance with the literature data, according to which the nitrogen content in crops can be up to 6.45%. The high content of nitrogen in crops is correlated to their high content of proteins, which can represent up to 40% of the dry mass [2].

The bulk density of the analyzed biomasses varied widely, from 130 to 307 kg/m3. This was lower than that of coal, which is on average in the range of 800–1000 kg/m3. Low bulk density is uneconomical from the point of view of storage and transport. Therefore, biomass pre-treatment such as grinding, pressing or palletization should be considered.

#### *3.2. Characterization of Biochars*

#### 3.2.1. Morphology

Applying pyrolysis to the waste biomass resulted in carbonization. Figure 1 presents graphical images of the biomass and the carbonized material. Carbonization occurred in the whole mass, and was as high on the surface as at the core. The transformations throughout the whole volume were probably a consequence of biomass fractionation, generated either by the production process (A.pomace, B.pulp, BSG) or by the grinding applied in this study (MSW, bark, FSW). Carbonization was not followed by major changes in the structure and mass density. The values for most of the carbonizates were similar to those determined for the biomass (Tables 1 and 2). An increase in mass density occurred only in two samples, apple pomace and MSW. These results are a consequence of pre-treatment, including grinding.

### 3.2.2. Material and Energy Balance

The material balances after pyrolysis were investigated using the gravimetric method. The results are presented in Figure 2. The solid residue (i.e., biochars) represented from 26.65% to 40.85% of the initial weight of the organic substance. However, this wide variation in yield was caused mainly by one sample, the bark. When the highest value is excluded, the values were very close, with a mean value of 27.98% ± 2.08%. The decrease in mass can be attributed to two causes. The first is moisture loss and the second is organic matter decomposition, with the formation of volatile products such as CO, CO2, CH4 and many others. During the process of carbonization, variable amounts of liquid and gas products were formed, in proportions that depended on the biomass. Volatile matter, including water and both gas and oil fractions, composed up to 73% of the initial mass of the samples, with the exception of bark, in which these fractions comprised 59.15%. These differences were probably due to the high content of hemicellulose, cellulose and lignin in bark.

The liquid fraction collected by condensation was composed of oil and water. Based on the mass of the liquid fraction, the oil fraction was calculated by diminishing the weight of the condensate by the moisture content. In general, the content of water was lower in the biomass from fruit waste (9.65% FSW, 12.56% A.pomace). The highest value was obtained for biomass from bark (16.54%). The greatest changes in the amounts of volatile compounds were recorded during heating up to 450 ◦C.

Table 2 presents the results of proximate and elemental analysis of the biochars obtained from pyrolysis. The carbonization process increased the carbon content on average 1.7-fold, from an average percentage of 46.09% ± 3.65% for biomass to an average percentage of 74.72% ± 5.36% for carbonizates. As a consequence, the C/H ratio also increased, reaching a value seven times higher.

**Figure 1.** Photographs of the raw and carbonized materials. Thermal decomposition performed under conditions of limited oxygen access with gradually increased temperature up to 850 ◦C over 30 min and continued for 60 min at a constant temperature.


*Materials* **2020** , *13*, 4971

B.pulp

BSG

Bark

MSW

 30.18

 26.92

 40.85

 30.26

 16.95

 14.43

 9.35

 8.51

 30.311 ± 215

 26.775 ± 146

 27.840 ± 26

 29.188 ± 26

 25.572 ± 139

 26.465 ± 25

 28.142 ± 26

 28.974 ± 206

 120.9 ± 10.9

 240.3 ± 4.8

 138.2 ± 3.4

 236.0 ± 3.4

 1.635

 1.372

 1.495

 1.617

 49.4

 36.9

 61.1

 48.9

 79.06

 1.62

 1.55

 79.34

 1.58

 0.52

 69.75

 1.63

 6.42

 68.54

 1.19

 1.87

**Figure 2.** Solid, liquid and gaseous product distribution (wt.%), expressed as average values, obtained with carbonization (850 ◦C).

Ash content reached values from 5.38% for A.pomace to 16.95% for B.pulp. Relative to the ash content in the raw materials, these values represent increments of 4.33% and 14.02%, respectively. Similar results from the same thermochemical processes have been described by other authors, using municipal solid waste and biomass [15,16]. High ash biochars are of limited use as fuels in the combustion process, since they can cause excessive ash deposition or slag and contamination phenomena, leading to operational difficulties.

Differences in the elemental compositions of the biomasses and biochars resulting from carbonization clearly illustrate the effect of fuel refining, which involves the removal of water and the elimination of oxygen in the form of oxidized volatile compounds through decarboxylation, decarbonization and dehydration reactions.

To determine the energetic yield of the analyzed process, the energy densification ratio and energy yield were calculated (Equations (2) and (3)). The energy densification ratio, indicating the elevation in HHV during carbonization, differed in a narrow range from 1.372 to 1.724, with a mean value of 1.5615 ± 0.124. However, energy densification does not indicate true changes in the energy value of the product, because it does not take into account the reduction in mass that occurs as a result of the process. When mass loss is considered, greater differences between the biomasses are revealed. The energy yield varied from 36.9% to 61.1% for BSG and bark, respectively. The mean value of the energy yield was 46.97% ± 8.572%. The biochars resulting from carbonization had a net calorific value of between 27 and 32 MJ/kg. These values are comparable or even higher than good-quality milled coal (eco pea coal 24–26 MJ/kg, grain diameter 5–25 mm, produced from specially selected hard coal species to obtain fuel with low contents of sulfur and ash).

#### 3.2.3. Composition Analysis of Non-Condensable Gases

Gaseous products are released during pyrolysis by the evaporation or thermal decomposition of the raw material. The amounts of emissions produced during thermal decomposition depend on the composition of the raw material, the heating rate, the temperature and the residence time. Table 3 shows the variations in the content and composition of volatile fractions. Figure 3 shows the variations in the content of hydrocarbons. The major gases produced from biomass carbonization were carbon dioxide and carbon monoxide. The content of CO2 decreased gradually, whereas the

content of carbon oxide increased with the pyrolysis temperature (i.e., the time of the process), which is assumed to be the result of thermal decomposition in an oxygen-poor atmosphere. The high content of carbohydrates, cellulose, hemicellulose and lignin in the tested materials implies the formation of carbon dioxide, carbon monoxide and water, as a result of decarboxylation, decarbonization and dehydration reactions during thermal decomposition. More CO is produced at elevated temperatures and with longer residence time. The formation of CO is strongly affected by secondary reactions of low molecular weight products (especially aldehyde-type compounds) and CO2 is presumably produced in the early stage of cellulose pyrolysis, primarily in decarboxylation reactions [17–19].

**Table 3.** Chemical composition of non-condensable gases produced during the pyrolysis of biomass, expressed as average values. Content of compounds expressed as relative peak area (%) of gases found in the fraction. CxHy—hydrocarbons, FSW—flavored spirits production waste (lime, grapefruit and lemon), B.pulp—beetroot pulp, A.pomace—apple pomace, BSG—brewer's spent grain, bark, MSW—municipal solid waste.


The yields of N2 and CH4 were much lower than those for CO and CO2. Production of N2 was the highest in the initial part of the process of carbonization. At temperatures from 515 ◦C, only slight fluctuations were observed. There was also a small amount of methane, the concentration of which practically did not change at temperatures above 515 ◦C. Methane may be formed by methanation (the reaction of carbon with hydrogen oxide to obtain methane and water) at higher temperatures [20].

Based on the chemical compositions of the non-condensing gases, the values for combustion heat and net calorific value were calculated (Table 4). The LHV of the biomass increased with rising pyrolysis temperature. The most intense increase of LHV was observed in the first phase of the process, when the temperature reached 515 ◦C. Bark and BSG were exceptions, in that a steady increase in LHV was observed up to 650 ◦C. Further temperature changes up to 715 ◦C did not have a major effect on LHV, which remained stable with some fluctuations. This was due to the pyrolysis reaction that occurred at higher temperatures. Following this stage of relative stabilization, LHV increased slightly again. These changes were the least pronounced in the case of bark and BGS. In the process of biomass thermal treatment, the major energy loss due to the release of volatile products took place in the torrefaction phase (up to 450 ◦C). The results for LHV show that non-condensing volatile products can be a valuable source of energy.

**Figure 3.** *Cont.*

**Figure 3.** Volatile fractions produced during biomass pyrolysis as a function of temperature. FSW—flavored spirits production waste (lime, grapefruit and lemon), B.pulp—beetroot pulp, A.pomace—apple pomace, BSG—brewer's spent grain, bark, MSW—municipal solid waste.


**Table 4.** Net calorific value (Lower Heating Value) (LHV) of non-condensable gases from pyrolysis of the analyzed biomasses, expressed as average values. CxHy—hydrocarbons, FSW—flavored spirits production waste (lime, grapefruit and lemon), B.pulp—beetroot pulp, A.pomace—apple pomace, BSG—brewer's spent grain, bark, MSW—municipal solid waste.

3.2.4. Composition Analysis of Condensable Gases—Liquid Products

The color and consistency of the condensates varied depending on the biomass. Images of the fractions are shown in the Supplementary Data (Figure S1). The condensates collected after A.pomace carbonization were distinctive. Their color was lighter and they did not look oily. Liquid phases collected from FSW and MSW were clearly darker than the others. They were oily and with a thicker consistency. As shown in Figure 2, the yield of condensates varied greatly, from 9.7% to 25.2% for MSW and BSG, respectively. The composition of the condensate fractions from the thermal decomposition of biomass appeared to be very complex. Gas chromatography (GC) analysis revealed the presence of over 250 organic compounds (Figure S2, Supplementary Data). The main compounds were cresols, phenols, aromatic hydrocarbons (toluene, benzene, xylene), nitrous aromatic hydrocarbons, aliphatic ketones, furan derivatives and aromatic polycyclic hydrocarbons. The cresol and phenol contents were similar, ranging from 5.2% to 9.2% and from 5.3% to 8.7%, respectively. The total content of benzene, toluene and xylenes ranged from 2.4% to 7.1%. None of the organic compounds from the remaining groups occurred in quantities greater than 1%. Polycyclic aromatic hydrocarbons were probably the product of the condensation of aromatic compounds, forming polycyclic structures [21]. The chemical pathways for cellulose pyrolysis and decomposition have been studied extensively [19,22,23]. According to the systematic review presented by Shen and Gu [19], furan and its furan derivatives are formed as a result of direct ring-opening and rearrangement reactions in cellulose molecules. As our results show, the condensates were composed of different organic compounds that can be recirculated and used as additives for other fuels, producing a positive impact on the environment. The elemental chemical composition values for bio-oils have been extensively reported in the literature [24]. The typical ranges for the products of fast pyrolysis are 50–60% for carbon, 6–9% for hydrogen, 30–40% for oxygen, <0.5% for nitrogen and <0.05% for sulfur [25].

#### **4. Conclusions**

Carbonization of agricultural waste biomass can be used to generate products with the composition and properties of alternative fuels. In this study, the thermochemical decomposition of waste biomaterials during rapid pyrolysis resulted in homogeneous biochar with yields of up to nearly 41% of the original mass. The biochars had much higher combustion heat and calorific values compared to the biomasses from which they were made. This was due to the fact they no longer contained water, which is physically and chemically bound to these biomasses and alternates in its chemical composition into products with higher carbon content. The biomass transformation process applied in this study enabled the moisture content to be lowered to such an extent that the addition of products no longer had a negative impact on the stability of the combustion process or the total combustion. As a consequence, the efficiency of the entire process was not affected. Both the non-condensable and condensate gaseous

products were composed of various organic compounds with sufficiently high combustion heat and net calorific values that they could be recycled and used as additives in other fuels.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/13/21/4971/s1, Figure S1: Photograph of the condensable gases—liquid products collected after pyrolysis of agricultural waste biomass. FSW—flavored spirits production waste (lime, grapefruit and lemon); B.pulp—beetroot pulp; A.pomace—Applepomace; BSG—brewer's spent grain; bark; MSW—municipal solid waste., Figure S2: GC–MS separation of organic compounds extracted to chloroform from the condensates obtained by carbonization of biomass. FSW—flavored spirits production waste (lime, grapefruit and lemon); B.pulp—beetroot pulp; A.pomace—Apple pomace; BSG—brewer's spent grain; bark; MSW—municipal solid waste.

**Author Contributions:** Conceptualization W.K. and P.D.; methodology, P.D. and M.A.B.; formal analysis M.A.B. and P.D.; investigation, M.A.B. and P.D.; resources, M.A.B.; data curation M.A.B.; writing—original draft preparation, M.A.B.; writing—review and editing, M.A.B. and P.D.; project administration W.K. and P.D.; funding acquisition, W.K. and P.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was financially supported by a grant from NCBiR (Polish National Centre for Research and Development) implemented within the project POIR.01.01.01-00-0374/17; Smart Growth Operational Programme -POIR in 2017; Priority axis I: Increased R&D activity of enterprises (Badania przemysłowe i prace rozwojowe realizowane przez przedsi ˛ebiorstwa; Konkurs nr 2/1.1.1/2017). Titled: Innowacyjny system zwi ˛ekszenia wykorzystania potencjału energetycznego paliwa oparty o układ skojarzonej gospodarki cieplnej i elektrycznej. zagospodarowuj ˛acy energi ˛e odpadow ˛a niezb ˛edn ˛a do zasilenia instalacji odbiorczych o róznych ˙ stanach energetycznych.

**Conflicts of Interest:** The authors declare no conflict of interest.
