## 3 Results and Discussion
### Profitability Assessment and Economic Impact  
This section aims to demonstrate the economic viability of gasification technology in the agrifood sector. The economic feasibility analysis was developed for a 125 kW\({}_{\text{e}}\) gasification plant fueled with exhausted olive pomace pellets and applied to a real olive oil mill in Spain. The economic analysis involved the calculation of an objective function known as profitability index (PI) [5], defined as the ratio between the net present value (NPV) and the initial investment (INV):  
\[\text{PI}=\frac{\text{NPV}}{\text{INV}}. \tag{7}\]  
The net present value (NPV) is a standard method for the financial appraisal of investments. It involves discounting the net cash flows and making the sum over the plant lifespan period as follows:  
\[\text{NPV}=-\text{INV}+\sum_{l=1}^{n}\text{NCF}\ (1+i)^{-n}, \tag{8}\]  
where \(n\) is the total number of periods of one year that make up the useful lifespan of the gasification plant, \(i\) is the discount rate (real interest rate) and NCF are the net cash flows in each period. The internal rate of return (IRR) is the discount rate for which the NPV is equal to zero. Its calculation was done iteratively by variation of the discount rate using the Newton-Raphson method. Before showing the results of the economic feasibility assessment, it is essential to consider the following assumptions, which are conveniently summarized in Table 10. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Biochar Parameter** & **Value** & **Unit** \\ \hline Gasification temperature & \(>\)800 & \({}^{\circ}\)C \\ Bulk density & 550 & kg/m\({}^{3}\) \\ Particle size & 1–20 & mm \\ Higher heating value & 24.93 & MJ/kg \\ \hline
**Proximate analysis** & & wt. \% (dry basis) \\ Moisture & 39.11 & \\ Ash & 25.61 & \\ Volatile matter & 6.13 & \\ Fixed carbon & 68.26 & \\ \hline
**Ash composition** & & wt. \% (dry basis) \\ Ca & 51.20 & \\ O & 30.32 & \\ K & 10.30 & \\ Mg & 5.35 & \\ P & 2.83 & \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Physicochemical properties and sample of biochar from exhausted olive pomace gasification. * The gasification plant, in addition to electricity, generates waste heat in the form of hot water at 90 \({}^{\circ}\)C and combustion gases at 350 \({}^{\circ}\)C. This source of thermal energy is used to produce the hot water required for the milling and malaxing stages of the virgin olive oil extraction process, avoiding the burning of a substantial part of olive pits from the oil mill, which can later be sold at about 660/t. * Currently, there are non-refundable subsidies from the Regional Government of Andalusia (in Spanish, _Junta de Andalucia_) for sustainable small-medium size enterprises, which can amount up to a 40% of the total investment [40]. In this study, a 35% non-refundable subsidy was considered. * Annual discount rate equal to a real interest rate of 1%. * The gasification plant was installed in self-consumption mode, according to Royal Decree 244/2019 [41], largely reducing the variable term of the olive oil mill's cost of electricity. * The gasification plant operates for 4.5 months in total, during the entire production season from October to March. The plant requires two stops per month for maintenance works. * The operation and maintenance of the gasification plant may require the creation of a permanent part-time technical job. A skilled labor cost of 615,000/year has been considered. * Finally, the cost of the gasification technology was consulted directly with the manufacturer (62500/kW\({}_{\text{e}}\)). Therefore, for a 125 kW\({}_{\text{e}}\) gasification plant with a 35% non-refundable subsidy, the capital expenditures were determined as INV \(=(1-0.35)\times 125\) kW\({}_{\text{e}}\times\)62500/kW\({}_{\text{e}}\) = 6203,125. The initial investment (INV) takes into account both the fixed costs of the whole gasification plant and the installation costs (civil, electrical and mechanical works).