**4. Conclusions**

In the paper was investigated the possibility to use hydrocarbons (pure fluids or mixtures) in Rankine cycles recovering heat from biomass furnaces. As working hypothesis, we assume a standard biomass oil furnace and, as a reference thermodynamic cycle, a saturated Rankine one with MDM as working fluid. Super-critical and sub-critical thermodynamic cycles were investigated.

Notwithstanding the adopted simplifications, from the previous presented results the following conclusions can be drawn:


Among the many conceivable different mixtures of hydrocarbons we identifyied here mixtures of iso-octane and n-octane. These mixtures have modest temperatures "glides" at every composition and the corresponding mixture critical temperatures result always greater than the maximum assumed working temperature (260 ◦C). So, saturated cycles are always possible for every composition.

An interesting result is that, varying the composition the thermodynamic efficiency results in practice constant (at least starting from an n-octane molar fraction greater than 0.5). Whereas, fixed the minimum and the maximum temperatures, because of the condensation and the evaporation pressures variation, the turbine power (fixed the number of revolution) change noticeably with the

composition. For example, at 3000 rpm, a two optimised stage turbine giving about 1000 kW with MDM, could produce about 4200 kW and 5600 kW with n-octane and with iso-octane respectively. The mixture composition results so an additional degree of freedom in the design of Rankine engines using organic fluids.

As for the dimension of the heat exchangers, we evaluated some results of *UA* per unit of produced power. Without the values of the overall heat transfer coefficients it is impossible determine the true total area *A*. Anyway, due to the smaller molecular complexity of the hydrocarbons compared to that of MDM, the regenerator should results with less surface.

In short, hydrocarbons as working fluids (pure or in mixtures), at least for the specific considered application, from the thermodynamic point of view, seem a good option.

**Author Contributions:** Conceptualization, C.M.I.; methodology, C.M.I.; software, A.A, C.M.I.; investigation, A.A., C.M.I.; writing–original draft preparation, A.A., C.M.I,; revision and final editing, G.D.M., P.I.

**Funding:** This research received no external funding

**Conflicts of Interest:** The authors declare no conflict of interest.
