Using fossil fuels has negative environmental impacts due to greenhouse gas emissions and air pollution problems. For this reason, in order to satisfy the continuous increase in energy demands and to reduce the environmental impact, the replacement of fossil fuels with renewable and sustainable resources is necessary. Biomass is considered an ideal energy source, thanks to its availability and its clean relationship with the environment [1
]. Environmental analysis demonstrated that the biomass conversion processes can give a good performance [3
]. A wide variety of biomass exists, e.g., food crops, energy crops, municipal solid wastes, green wastes and agricultural residues [5
]. The use of biomass wastes avoids the “biomass vs. food” contrast [7
] and solves the problem of waste disposal. In the Mediterranean areas of Europe, agro-industrial activities are very important, and a lot of residue is produced.
Among the major activities is the olive oil industry. Olive oil production generates a significant number of by-products, solids and liquids. It has been estimated [8
] that one hectare of olives produces about 5000 kg of olives and, from these, about 2250 kg of olive pomace can been obtained. The production process of olive oil typically brings an oily component, a solid residue and an aqueous component given by the water content of the olive pulp [9
Anaerobic fermentation of the aqueous component, which has a high biochemical oxygen demand (BOD), has been suggested as a mean of solving this problem [10
]. Although the solid residue does not present such a serious environmental problem, the option of producing a clean gaseous fuel composed of a residue that can be used as a fertiliser rich in nitrogen should be well thought out [11
]. The global annual olive pomace production has been estimated at around 400 million tons on a dry basis [8
For the management of mill solid wastes, some solutions have already been explored in the literature [10
]: animal feed, biogas production, extraction of useful materials and fertilisers. Pelletising residue olive oil to increase the density and energy was also investigated. However, this solution is affected by the high oil content of the pomace, which reduces the quality of the pellets [13
Biomass combustion is one of the most promising ways to reuse it, however it is necessary to consider the limits due to the low thermal efficiency of olive pomace [13
]. In the literature, several studies have been reported on the combustion of olive mill wastes for energy production, alone or in combination with other fuels. Miranda et al. [14
] investigated the combustion characteristics of solid mill waste (kernels, pulp and olive pomace) with different proportions of semi-solid waste (such as mill wastewater). Their results showed that the combustion of olive stones and olive-pomace gives a good efficiency and a reduced presence of non-combusted components, while lower combustion efficiencies were obtained in the case of pulp. Atimtay and Topal [15
] have estimated co-combustion of various blends of olive pomace with lignite coal using various excess air ratios. Their study showed that with an air ratio of 40%–50%, considerable amounts of CO and non-fuelled hydrocarbons are formed, and the combustion efficiency drops to 84%–87%. According to the results, the combustion efficiency increased with an increasing excess air ratio. The authors suggested the addition of secondary air within the freeboard to improve the efficiency of the combustion process. This solution was also considered by Varol and Atimtay [16
]. Combustion efficiencies in the range of 83.6%–90.1% were obtained from olive pomace.
The potential utilisation of solid olive mill wastes in combined heat and power (CHP) plants has been investigated by several authors [17
] who highlighted the economic viability of such plants. The organic Rankine cycle (ORC) is one of the preferred and most studied CHP systems because of its reliability, versatility and low maintenance needs [21
]. The ORC system can be considered a valuable application, also because the combustion is external with respect to the power generation system and unrelated to the features of the fuel.
The working fluid plays a key role in the ORC process. Organic fluids have higher pressures and lower boiling points compared to steam and since most of them are dry or isentropic fluids they do not require superheating before expansion [22
]. Many studies have been carried out to select the best working fluids. Liu et al. [23
] discovered that some working fluids at specific evaporations and condensation temperatures showed a similar thermal efficiency and the thermal efficiencies were found to increase with the critical temperature of the working fluid. Wang et al. [24
] analysed the performance of the ORC system with different working fluids and showed that R245fa and R245ca are the most environment-friendly working fluids. Cataldo et al. [25
] proposed to choose the fluid which has a low value of critical temperature and a high value of the latent heat of vaporisation. However, no single pure fluid has been found as optimal for the ORC due to the strong interdependence between the optimal working fluid, the working conditions and the cycle architecture [21
Another important aspect to take into account is the heat transfer efficiency of the evaporator [26
]. The pinch point temperature difference is often used to analyse the coupling heat transfer in the evaporator. Chen et al. [27
] suggested a method to optimise the operating parameters of an ORC with a constrained inlet temperature of the heat source and the pinch point temperature difference in the evaporator.
The growing energy consumption of the agricultural field requests a fast evolution of the technologies aimed to biomass waste energy conversion because the application of more sophisticated processes and technologies within the chain of the agricultural and food industries requests more and more resources [28
The aim of the present work is to evaluate the potential of olive pomace produced in a mill located in Central Italy as an energy resource, which represents the considered case study, whose combustion feeds an ORC unit combined with an absorption chiller. After the preliminary analysis of the biomass waste, the energy performance achieved by a small size CCHP ORC system powered by olive pomace combustion was investigated, considering various working fluids and different operating conditions.
In this work, the chemical composition and energy performance of the olive pomace has been analysed, showing a good value of LHV (around 22 MJ/Kg) and good values of the proximate and ultimate analysis. A trigeneration system composed of an ORC unit, powered by a biomass boiler, developed in Aspen Plus and coupled with an absorption chiller was investigated with different working conditions. Four dry/isentropic organic fluids were considered for their negligible ozone depletion potential, higher critical temperature and critical pressure, and a sensitivity study was carried out in order to determine the mechanical, thermal and electrical efficiency of the plant at varying operative conditions for each working fluid. The minimum and maximum level obtained whilst varying the organic fluid and the pressure of the pump were 13.1%–20.0%, 63.5%–70.3% and 12.7%–19.4% for mechanical, thermal and electrical efficiency, respectively. Furthermore, a heat recovery of the exhaust gas out of the evaporator was managed in order to heat up an amount of water (120 kg/h) to 85 °C temperature and feeding a 35 kWc absorption chiller. The present analysis deepened the coupling between an ORC CCHP system and olive pomace for the first time in the literature and lead to the conclusion that this biomass waste can be an effective and available by-product of the olive oil production process suitable for an energy-from-biomass-waste trigeneration system. In particular, the most important findings of the present work were:
At a national level, considering an olive pomace production ratio of 2250 kg/ha and about 1052000 ha of Italy being occupied with olive trees aimed at oil production in 2017 [42
], the energy potential of this energy system would be of 5.9 GWh, 31.1 GWh and 10.7 GWh, corresponding to a 0.286 Mtoe (Million Tonnes of Oil Equivalent) in terms of primary energy, which would give an important contribution to the overall primary energy national production.
The triple energy could be used within the agricultural chain, representing a virtuous case of distributed generation.
At a local level, the developed model shows that Cyclopentene is the most highly performing fluid in terms of electricity production, while R245fa is the least.
The increasing pressure entails an energy benefit in terms of power production and electric efficiency, but the increasing trend is slower at the highest pressures of the considered range.