## 2 Current Situation of Olive Stones in Spain
### Energy Potential of the Ground Olive Stone in Spain  
In all olive mill production (olive oil, olive pomace, etc. ), and in a high percentage of table olives, the olive stone is a waste product generated in the different industrial processes. However, despite the fact that it is a waste product, in all olive-cultivating areas it is already valued as an energy resource of some importance given its possibility of being burned in biomass boilers to obtain thermal energy. Ground olive stones have a lower calorific value (LCV) of 4489 kWh/kg according to the document \"Lower calorific values of the main energy sources\" of the IDAE-Spanish Institute for Energy Diversification and Saving [31]. The average density of ground olive stones is 1200 kg/m\({}^{3}\), although its apparent density (due to air voids in storage) drops to about 600-650 kg/m\({}^{3}\). The percentage by weight of olive stone in the olive's total varies according to the variety, the type of olive tree and, above all, the percentage of humidity contained in the olives at the time of harvesting. A value between 10 and 20% of the weight (150 g per kilogram of olive) is a good approximation to quantify the average weight of the olive stone over the total. With these values and the production levels detailed above, it is possible to estimate a total annual production in Spain of about 1200 million kilograms of ground olive stone, with the contribution of the Andalusian Autonomous Community being fundamental, which contributes 975 million kilograms (81.27% of the total). Although, there are other possibilities for the evaluation of olive stones, such as combustion as the main destination, since it has an important performance as a source of thermal energy. In fact, a significant percentage of this olive mill waste in the production of olive oil and olive pomace does not leave the facilities themselves, as it is used for the internal generation of thermal energy and electrical energy (cogeneration). It is also used as a direct fuel for domestic and tertiary biomass boilers to meet the demand for heating and domestic hot water. In other cases, it can also be sent to pellet manufacturing plants where it is compacted in order to standardize its size and thermal properties. All these reasons make olive stone waste a good alternative to the more common and much more polluting fossil fuels (natural gas, diesel, propane, etc.). A conversion efficiency of 75% of the calorific value of the ground olive stones into useful energy during the combustion process in the boiler can be considered quite prudent, taking into consideration the losses due to unburned stones, as well as in the transport and distribution of heat through the pipes. With an approach of energy recovery of all the olive stones for combustion produced in Spain and with this yield of 75%, a total of 4040 GWh/year of calorific energy could be available, as shown in Table 1.  
In order to obtain a comparison model to objectively quantify these values of potential useful thermal energy feasible by the combustion of ground olive stone biomass, data from the Study of the distribution of residential energy consumption for heating in Spain are used. This was published by the Spanish Ministry of Public Works in 2017 [32], which in turn emanates from studies carried out by IDAE in the SECH-SPAHOUSEC project (2011) [33] and its subsequent evolution SECH-SPAHOUSEC II (2019) [34]. Figure 1: Total production in Spain (2010–2020) in thousands of tons of olives according to the Government’s Ministry of Agriculture, Fisheries and Food. They quantify a real total energy consumption for heating homes in Spain of 79,982 GWh/year, although this figure is reduced to 70,109 GWh/year when considering only homes that have heating consumption and are not in a situation of energy poverty. The purpose of this is to eliminate homes from the statistics that do not consume because they are uninhabited or because they do not have sufficient income to meet the thermal needs of the home.