# Comparative Analysis of Partially Renewable Composite Fuels Based on Peat, Lignite and Plant Oil
## 3 Results and Discussion
### The Conversion Temperatures and Oxidation Heat  
It is well known that temperature is one of the decisive factors of fuel conversion [11]. The focused light flow quickly increases the temperature of the lit surface of fuel to the steady state value that is defined by both external heating and the chemical reaction heat inside the fuel layer. The trend in the average conversion temperature versus a wide range of oil fractions in the fuel composition is shown in Figure 3. One can see that an increase in the fraction of RO in the P + RO composition leads to a slight increase in the process temperatures followed by a decrease (after 45 wt% of RO). An equivalent dependence for the L + RO compositions shows the peak temperature at about 35 wt% of RO. The subsequent decrease in temperature (within the red zone) is accompanied by an increase in the conversion duration (for the fixed weight of fuel sample). It increases 2-3 times in the case of 0.35P + 0.65 RO compositions relative to the conversion time of pure peat. Similarly, the conversion of 0.6L + 0.4 RO proceeds 1.3 times longer than that of pure lignite. Thus, the red area shows the optimal oil concentration range where the overall heat production through conversion is close to maximum (the temperature decrease is compensated by increased duration of heat production). The green areas correspond to the regimes with excess CO\({}_{2}\) production. The gray areas correspond to the fuel with an evident excess of RO, where the intensity of gas production is very weak. This occurs because the external heating power is not enough to keep the temperature of around 600 \({}^{\circ}\)C during the rapid evaporation of the volatiles. Figure 4 shows the results of the calorimetric analysis (red diagram) and the peak temperatures (green) observed during the light-induced allothermal conversion of fuels (using the setup from Figure 1). One can see that the maximum temperature was recorded in the conversion of pure lignite (-1300-1400 \({}^{\circ}\)C). The lowest peak temperatures were recorded during the conversion of pure rapeseed oil and pure peat. However, the P + RO composition shows a higher temperature. This indicates that the heating is enhanced due to the interaction of the fuel components. The L + RO composition shows a certain decrease in the conversion temperature relative to the maximum possible one for lignite. At the same time, the L + RO mixture features higher conversion temperatures combined with a higher consumption of fossil fuel. Figure 4: Characteristic temperatures (green) recorded during the allothermal conversion of composite fuels and their components (normalized to the highest temperature) and high calorific values of the fuels (normalized to the highest value). Normalization factors: \(T_{\rm max}\) = 1350 \({}^{\circ}\)C, \(Q_{\rm max}\) = 38.5 MJ/kg. Figure 3: Average temperatures during conversion for P + RO (**a**) and L + RO (**b**) fuel compositions. The red zone corresponds to fuel conversion. The green and gray zones correspond to compositions with excess production of CO\({}_{2}\) and extremely low overall gas production. The P + RO mixture contains much more oil per unit weight than the lignite-based one does. However, their calorific values are quite similar. The peat oxidation appears to slowly smolder, whereas lignite oxidizes much faster. The high calorific value obtained for pure RO was 38.5 MJ/kg. All the compositions containing a considerable amount of RO featured quite high calorific values too. Pure peat and lignite have a much lower calorific value due to the presence of mineral matter and certain moisture. It is clear that the high calorific values measured for the composite fuels differed greatly from the estimations obtained using the common additive relations (0.6 and 0.76 for the L + RO and P + RO compositions, respectively). This is especially evident for the P + RO mixture. This composition has a very attractive heat of combustion with a very high fraction of renewables.