## 2 Materials and Methods
### Determination of Calorific Value  
Calorific power is the main thermal property of briquettes, which gives information about the amount of heat obtained when burning them (EN ISO 17225-1:2014) [35]. To test the calorific power, an adiabatic calorimeter (XRY-1C oxygen bomb calorimeter, Shanghai Geological Ltd., Shanghai, China) was used, which determined the value by obtaining the temperature increase inside a calorimeter bomb between the moment of ignition of the sample and the end of its combustion, based on the calorimetric coefficient of the calorimetric installation. Eight samples were used [36; 37], extracted from briquettes, with a mass of 0.8 \(\pm\) 0.2 g, dried to 2-3% moisture content, and weighed with an accuracy of 3 decimals with an analytical balance. This degree of moisture was chosen because, during the test, 3 mL of distilled water was put inside the calorimetric bomb for the absorption of nitric acids [38], which increased the moisture content of the samples up to 4% until the moment of ignition (about 5-6 min), regardless of whether the samples were absolutely dry or had a moisture content of 2-3%. For the determination, the Renault-Phander program was used, by using the following general relationship (Equation (7)):  
\[\text{HCV}=(\text{C}_{\text{c}}\cdot(\text{T}_{\text{f}}-\text{T}_{\text{i}} \ )-\text{Q}_{\text{w}}-\text{Q}_{\text{c}})/\text{m}\ [\text{MJ}/\text{kg}] \tag{7}\]  
where HCV is the higher calorific value in MJ/kg; C\({}_{\text{c}}\) is the coefficient of the calorimeter in MJ/\({}^{\circ}\)C; T\({}_{\text{f}}\) is the final temperature after complete combustion of the sample in \({}^{\circ}\)C; T\({}_{\text{i}}\) is the initial temperature before igniting the sample in \({}^{\circ}\)C; Q\({}_{\text{w}}\) is the amount of heat given off by the nickel wire in MJ; Q\({}_{\text{c}}\) is the amount of heat given off by the cotton thread in MJ and m is the mass of the sample extracted from the straw briquette in kg.  
The calorimeter software provided, at the end of the test, the high calorific value (HCV) and lower calorific value (LCV), as well as all the temperature values obtained during the tests and the total test duration for the samples of dry briquettes with a moisture content of 2-3%, which in the determination time became 4%.  
To calculate the calorific value of the briquettes for absolutely dry briquettes, with 0% moisture content, the following relationship was used [8]:  
\[\text{CV}_{0}=(\text{100}\cdot\text{CV}_{\text{MC}}-\text{2.44}\cdot\text{Mc })/(\text{100}\ -\text{Mc})\ [\text{kJ}/\text{kg}] \tag{8}\]  
where \(\text{CV}_{0}\) is the calorific value for 0% moisture content in kJ/kg; \(\text{CV}_{\text{MC}}\) is the calorific value for a certain moisture content in kJ/kg and Mc is the moisture content dry mass percentage.  
In order to find the influence of moisture content on the calorific value, based on the previous relationship (8), a coordinate point (0; CV) was found, which was arranged on a graph with the moisture content of the briquettes on the horizontal axis and the calorific value on the vertical axis. On the same graph, the two previously determined points were arranged, respectively (4; LCV) and (4; HCV), thus obtaining two lines that intersected the horizontal axis of moisture content in two limiting points.  
Caloric power per hectare took into account an average production of 5200 kg of straw/ha. The calorific value per hectare of wheat was calculated with the following formula (Equation (9)):  
\[\text{CV}_{\text{ha}}=\text{CV}\cdot\text{ms}_{\text{ha}}\ [\text{MJ}/\text{ha}] \tag{9}\]  
where \(\text{CV}_{\text{ha}}\) is the calorific value related to the crop hectares in MJ/ha; CV is the calorific value of wheat straw in MJ/kg and ms\({}_{\text{ha}}\) is the mass of the resulting straw per hectare in kg/ha.  
Modeling of the calorific power was undertaken depending on the carbon content of the biomass of the wheat straw mass [39].