**1. Introduction**

Recycling biomass and various types of organic waste is a way of increasing the share of renewable sources in energy production [1]. The Sustainable Development Goals (SDGs) set out by the United Nations highlight renewable energy as key to the success of Agenda 2030 [1]. Possible sources of bioenergy include energy crops, biomass residues and organic wastes. However, direct use of biomass as a heat source may be inefficient and difficult. Complications may arise even at the storage stage, when high humidity is associated with microbiological biomass degradation. The co-combustion of biomass and coal can raise technical and economic issues. Wet biomass may cause instability in the combustion process itself. Incomplete combustion reduces the efficiency of the whole process and leads to energy losses. Incomplete combustion may also make it impossible to maintain the required emission parameters. Given the limited possibilities for using unprocessed biomass, pre-treatment is necessary to improve its energy properties. Various methods of initial biomass preparation are described in the literature, which enable co-combustion with coal in power boilers [2,3]. The process of thermal conversion of biomass to biofuel may include combustion, gasification, biocarbonization, torrefaction, dry distillation and pyrolysis [4–8].

Pyrolysis is a thermochemical treatment, involving the extensive thermal decomposition of organic material under oxygen-limited conditions or in an atmosphere of inert gases. The gas phase contains two kinds of compounds. The first are volatiles that condense and form a dark brown, viscous liquid phase. The second are volatile compounds with low molecular weight (e.g., CO, CO2, H2, CH4 and light hydrocarbons). These non-condensable gases remain in the gas phase. The physical process and chemical reactions that occur in pyrolysis are highly complex, and both the conditions of pyrolysis and the organic matrix (which may originate from different sources) affect the quality of the biochar and its eventual properties. These parameters may be helpful when ranking waste materials as potential sources of biocarbon, and for assessing their suitability for co-firing. These parameters may also be used to evaluate the possibility of using condensing and non-condensing gas products for energy generation.

The aim of this research was to convert biomass into a biofuel with properties similar to those of coal. We used waste from the agricultural industry and municipal management as feedstock. The properties of biochars obtained by biomass carbonization were determined in single-factor experiments. We also characterized the main products of the gas products and condensates. The results could contribute to the development of strategies for biomass treatment and the reduction of emissions, improving the sustainability of biomass conversion processes at an industrial scale.

## **2. Materials and Methods**

### *2.1. Materials*

Six agricultural waste biomass materials were used in the study: flavored spirits production waste (FSW) (lime, grapefruit and lemon), apple pomace (A.pomace), beetroot pulp (B.pulp), brewer's spent grain (BSG), bark (pine bark) and municipal solid waste (bark, off-cuts, wood chips, sawdust (MSW)). The analyzed biomasses were pre-prepared by drying and grinding.

#### *2.2. Volatile Component Analysis*

Volatile component analysis was carried out with use of a GCMS (Gas Chromatography–Mass Spectrometry) (Termo Science Trace GC Ultra) and an RTX—1.60 m × 0.25 mm × 0.25 μm capillary column (Restek, Saunderton, UK), combined with a DSQ-II (Dual-Stage Quadrupole) detector (Thermo Scientific, Austin, TX, USA). All the samples were analyzed in duplicate at a pyrolysis temperature of 550 ◦C with a heating rate of 20 ◦C ms−1. The samples were collected in Tedlar bags (Merck, Darmstadt, Germany Tedlar® PLV- Push Lock Valve Gas Sampling Bag).

#### *2.3. Moisture and Ash Content, Chemical Composition*

The total moisture content in the tested biomass was determined using the thermogravimetric approach. The materials were dried at 110 ◦C until the complete removal of moisture. The ash content was determined using the slow ashing method, in which combustion and annealing of the analytical sample occurs in two stages, differing in temperature and duration. Dry ashing was performed in open inert crucibles in a muffle furnace. The samples (1 ± 0.1 g of biomass powdered by a broom mill) were placed in the cold furnace and combustion was performed with a constant increase in temperature up to 500 ◦C for 30 min. The temperature was gradually increased to 815 ◦C over 30 min. Complete ashing was achieved after thermal decomposition for 90 min.

The chemical composition (content of carbon, sulfur, nitrogen) was determined using an elementary analyzer (CE Instruments, Milan, Italy.) and Eager 200 software (amlyzer type 2500), using methionine or BBOT (2,5-(bis(5-tert-butyl-2-benzo-oxazol-2-yl) thiophene) as the reference material.

#### *2.4. Combustion Heat and Net Calorific Values, Energy Yield*

The calorific value was determined using a 6100 compensated jacket calorimeter (Parr Instruments GmbH, Moline, IL, USA). Net calorific values were calculated on the basis of the amount of heat generated during the complete combustion of a mass unit (1 g) of biomass in an oxygen atmosphere using the formula

$$\text{LHV} = 2.326 \times \text{(HHV} - 91.23 \times \text{C}\_{\text{H}}) \tag{1}$$

where LHV is the net calorific value (Lower Heating Value) (J/g), HHV is the combustion heat and CH is the hydrogen content of the sample (%) [9]. The net calorific value (LHV) of biomass is calculated as the heat of combustion reduced by the heat of water evaporation, obtained from the fuel in the process of combustion and from hygroscopic moisture. The energy densification ratio describes the ratio of the HHV of the dry product to the dry raw material.

$$\text{Energy density} = \text{HHV}\_{\text{biochar}} / \text{HHV}\_{\text{biombas}} \tag{2}$$

The energy yields were calculated using the equation

$$\text{Energy yield} = \text{(mass}\_{\text{biochar}} \text{(mass}\_{\text{biombass}}) \times \text{(HHV}\_{\text{biochar}} \text{/HHV}\_{\text{biombass}}) \times 100\% \tag{3}$$

Mass yields were calculated using the equation

$$\text{Mass Yield} = \text{mass}\_{\text{biochar}} / \text{mass}\_{\text{biohmas}} \tag{4}$$
