## 1 Introduction  
The energy system transition to \(\mathrm{CO_{2}}\)-neutral requires the change and diversification of energy sources. One of the possible ways is to apply biomass-derived fuels [1]. Within different options, liquid biofuels are of special interest due to their similarities with fuels obtained using crude oil processing [2; 3]. According to the definition presented by [PERSON] et al. [4] \"liquid biofuels constitute a fuel category, which includes substances made of raw materials of organic origin-- biomass or biodegradable fractions of waste\" [4]. One considered option is the application of animal waste (e.g., fats) as a feedstock to produce valuable fuels [5]. Another motivation to use this waste as an energy feedstock is the large amount of this material produced per year [6; 7]. According to the Statistics Poland Statistical Yearbook of Agriculture [8], in 2020, the production of meat from slaughtered animals was 337,180 thousand tonnes and 5246 thousand tonnes in the world and Poland, respectively. In Poland, the production of fats from slaughter was 403 thousand tonnes in 2021, whereas the consumption of fats from slaughter was 204 thousand tonnes in 2021 (p. 305 in [8]). The production of animal fat not intended for human consumption, AFN, was 1,351,955.3 in EU 27 and 102,279.9 in Poland [9]. It was estimated that biodiesel from animal fat achieves nearly 80% fossil CO\({}_{2}\) reduction compared with 30% for soya [7]. Animal fats are considered a feedstock for biodiesel production [5; 7] and different chemical products [6; 7]. [PERSON] et al. [6] listed six groups of potential applications that are related to the specific treatment processes. They mentioned (1) biodiesel production using transesterification, (2) paraffinic fuel production using catalytic cracking, hydrogenation, and isomerization, (3) production of hydrocarbon N-/O-heterocycles using pyrolysis, (4) emulsified biofuel production using microemulsion, (5) blended biofuel production using direct mixing of animal fats, and (6) bioliquid production using direct use and purification process. [PERSON] and colleagues [10] presented a review paper on biodiesel synthesis from various mixed oils. They discussed (inter alia) different types of reactors for biodiesel synthesis. Animal fat can be used in the process of biodiesel production followed by the distillation process as a key step [5]. There is a common idea that any organic material, specifically carbohydrates, proteins, and fats, could be converted into crude oil [11]. It should be underscored that within animal fat production, some parts of this material cannot be used for food production. They are included in the waste group if a reasonable way of application is not presented. In the literature, this part of the animal part is called the Animal By-products Not Intended for Human Consumption, ABNIHC [12], or animal fat not intended for human consumption, AFN [7]. These materials can be used as a feedstock for fuel production. In such a way, this step is desired in the waste management hierarchy [13]. Namely, the direct application of fat waste as a fuel or its utilization as feedstock in the production of fuels and/or valuable chemicals increases the waste management level from the disposal level to the recovery or recycling level. There are different strategies for waste treatment to obtain valuable products. One of the possible ways is thermo-chemical treatment, including such processes as hydrothermal treatment [11; 14], distillation [5], and thermal treatment [3; 15]. There are investigations on the application of animal fats as a fuel in stationary combustion chambers. [PERSON] et al. [9] presented the combustion AFN and co-combustion of AFN with other liquid fuels in a 26.7 kW boiler (fuel consumption 1.86 kg/h). [PERSON] and [PERSON] [16] presented the combustion of animal fat using a rotary cup-type burner in a 4 MW\({}_{\text{th}}\) boiler. They compared the results with the combustion of heavy fuel oil.