## 1 Introduction  
Plant biomass is a source of renewable energy, along with solar, wind, and geothermal energy. In agriculture, vegetable remains in the form of straw, corn stalks, sunflower, flax, castor and hemp, husks of sunflower seeds, rice seeds, the kernels of various fruits, etc., can become an additional energy source for households [1]. Among these cheap leftovers, straw represents one of the most important quantitative resources. Wheat straw usually contains about 12-18% moisture at the time of harvesting, so it does not require drying before briquetting, as the residual 12-15% water evaporates during burning. As with any lignocellulosic resource, straw dry matter consists of carbon (45.6%), hydrogen (5.8%), and oxygen (42.4%), as well as small amounts of oxides of nitrogen, sulfur, alkali silicon, chloride, and others (6.2%) that are usually found in the ash [2]. The ash content of straw can vary between 2 and 12%, with the average being 6-7%. Compared to this straw biomass, beech wood biomass (with bark) has 47.9% carbon, 6.2% hydrogen, 43.3% oxygen, and 2.6% ash [2]. Straw from crops that have been grown on sandy soils normally has the lowest ash content, while straw from lowland soils usually has the highest ash content. The calorific value of European beech biomass is usually 18.5 MJ/kg, and the ash content is lower than that of straw [2]. Globally, a huge amount of straw is produced [3]. The amount of heat obtained by burning straw as an alternative energy source [4, 5] has a maximum of 22.2 MJ/m\({}^{2}\) of agricultural land optimal for the cultivation of straw [6]. The average grain wheat production in the European Union is 6.5 t/ha, which means that, if we take into account a grain/straw ratio of 1:0.8, straw production of 5.2 t/ha is obtained annually. Corn with 1:1.3, sunflower with 1:4.1, and rapeseed with 1:2.9 have better ratios between the agricultural product and lignocellulosic residues [7]. The calorific value of the remains from agricultural crops is different from one crop to another, with the best being wheat straw at 17.21 MJ/kg and energetic willow at 17.28 MJ/kg, compared to wood pellets at 18.11 MJ/kg. A bale of parallelepiped straw with dimensions of 1.2 \(\times\) 1.2 \(\times\) 2.5 m (with a mass of about 500 kg) will have a calorific value of 4.7 kWh/kg, which means a total amount of energy of 2 MWh and represents the equivalent of 200 l of oil or 300 kg of coal [7]. During the natural or artificial drying of straw in order to obtain briquettes with a moisture content of 8-10%, for every 10% decrease in the moisture content of the straw, an increase in calorific value of 2.16 MJ/kg (0.6 kWh/kg) can be obtained. The main advantages of using straw in combustion are the large quantities obtained per hectare, the year-to-year cyclic availability of renewable biomass [8; 9], the easy ignition and rapid burning, as well as the fact that at harvest the moisture content is below 15% (it does not require additional drying). Briquettes are combustible products obtained from compressed lignocellulosic material (CEN/TC 335:2004), with densities of over 850 kg/m\({}^{3}\). Typical values of 900-1000 kg/m\({}^{3}\) of briquette density can be found in the specialized literature [10], depending on the production process, with lower values for hydraulic briquetting machines and higher values for mechanical conveyor machines with a helical snack or connecting rod-crank mechanisms. The great advantage of these briquettes is that they have a high density beyond the density of the wood biomass from which they were obtained, and therefore, have a higher calorific density than the wood from which they came. Straw briquettes are among the cheapest because the raw material can be obtained at a low cost, following the wheat harvest. When making straw briquettes, high pressing forces must be used because a large amount of silicon in straw creates adhesion problems for the chips [3].