**1. Introduction**

Second generation biofuels are produced from lignocellulosic biomass derived from agricultural waste, forest waste, municipal and industrial waste, and grass and aquatic plants. All of these types of biomass have one common property; they are formed from nonfood resources [1]. *Amaranthus retroflexus* (AR), or redroot pigweed, is a second-generation lignocellulosic biomass. This plant is a fast growing herb of class C4, consisting of 60–70 species [2,3]. AR is a cosmopolitan plant capable of growing in any climatic zone, including the cryolitic zone [4]. The plant can reach 1.5–3 m in height, thus, it is possible to obtain huge biomass resources with little water and fertilizer consumption [2].

Pyrolysis is the most promising technology for thermal utilization, since it allows for the procurement of gaseous, liquid, and solid products, and is also the first stage of all thermochemical processes [5]. The quantity, properties, and application of these three mains products depend on the parameters of the feedstock, the type of reactor and the technological conditions for the implementation of the process (the heating rate, final pyrolysis temperature, pyrolysis atmosphere, etc.) [6].

From a practical point of view, bio-oil and biochar are of the greatest interest, since pyrolytic gas is most often used for its own technological needs. The rich chemical composition of bio-oil allows it to be used as renewable fuels and value-added chemicals [7,8].

**Citation:** Karaeva, J.; Timofeeva, S.; Gilfanov, M.; Slobozhaninova, M.; Sidorkina, O.; Luchkina, E.; Panchenko, V.; Bolshev, V. Exploring the Prospective of Weed *Amaranthus retroflexus* for Biofuel Production through Pyrolysis. *Agriculture* **2023**, *13*, 687. https://doi.org/10.3390/ agriculture13030687

Academic Editor: Vincenzo Alfano

Received: 3 February 2023 Revised: 13 March 2023 Accepted: 13 March 2023 Published: 15 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Biochar has the following applications: soil amendment, nutrient and microbial carrier, immobilizing agent for remediation of toxic metals and organic contaminants in soil and water, catalyst for industrial applications, porous material for mitigating greenhouse gas emissions and odorous compounds, and feed supplement to improve animal health and nutrient intake [9,10]. Due to its low thermal conductivity, it can also be mixed with soil and used as a thermal backfill [11]. In addition, biochar-derived activated carbon is suitable for batteries and electrode material for supercapacitor applications [12].

There are known studies on the pyrolysis of weeds *Ageratum conyzoides*(goat weed) [13], *Ageratina adenophora* (Crofton weed) [14], *Alternanthera philoxeroides* (alligator weed) [15], *Parthenium hysterophorus* [16], and *Cannabis sativa* [16] to obtain biochar, bio-oil, and biogas. It should be noted that there are studies on the pyrolysis of invasive plants, such as: *Acacia Holosericea* [17], water hyacinth [18], *Prosopis juliflora* [12], and *Eupatorium adenophorum* [19]. However, there are no reports on the utilization of AR as a feedstock for pyrolysis. Therefore, the use of pyrolysis for the use of weeds and invasive plants with the production of bio-oil and biochar can contribute to the formation of a circular economy and increase the profitability of the agricultural industry [20].

For a detailed study of the process of thermochemical conversion, the plant can be divided into fractions. For example, leaves and stems [21], seeds [22–24], flowers, leaves, and stems [25], leaves, hurds, and roots [26], pseudo-stems [27], stem, leaves, fiber, chaff, and seed husks [28], and inflorescences [29,30]. This will make it possible to understand the contribution of each fraction to the material balance of the pyrolysis process. The content of hemicellulose, cellulose, and lignin differs in all constituent parts of the plants, so the quantity and quality of the resulting gaseous, liquid, and solid products varies significantly.

The aim of the work is to study the possibilities of thermal utilization of the AR weed plant, to study in detail the prospects for using its aboveground biomass to obtain new liquid and solid products with high added value. In this research, the authors studied the pyrolytic process of the leaves, stems, and inflorescences. Conventional pyrolysis was performed in combination with a thermogravimetric analysis. Therefore, the present study aims to solve the following problems: (a) determine the material balance of the AR pyrolysis of leaves, inflorescences, and stems of AR; (b) analysis the characteristics of the features of thermal decomposition of leaves, inflorescences, and stems according to TGA data, at a heating rate of 10 ◦C/min in an inert atmosphere; (c) study of the composition and quality of the chemical composition of the oil fraction of the pyrolysis liquid; and (d) analyze the possibility of using biochar for combustion both as an independent fuel and as part of a mixed fuel, taking into account the parameters of slag formation.

#### **2. Materials and Methods**
