1. Introduction
Using biomass as an energy source is considered to be one of the main options to supply renewable power and heat, thus reducing carbon dioxide emissions as well as consumption of imported fuels. Especially, the use of agricultural residues and other wastes from food industry is a good option to provide bioenergy with low additional carbon dioxide emissions. Biomass is a perfect investment, especially when hybridized with other unstable sources like wind and solar energy [
1]. From several types of biomass, we chose olive residues for this research due to the abundant availability in Mediterranean countries.
Olive-processing residues are considered as a promising source of biomass due to the energy utilization property shown based on its characteristics [
2]. For 2015/2016, the worldwide production is 2,742,500 tons, produced mainly by Mediterranean countries (approximately 97%) [
3,
4]. Olive oil-extracting technologies are mainly traditional pressing and the centrifugation method with either two- or three-phase schemes [
1,
5]. Solid residues (rich in water) are approximately 40% from pressing, and 30%–50% and 60%–70% for three and two phases, respectively [
6,
7]. Each olive tree can produce an average of 10–30 kg/year of pruning residues; these are mostly burned in the field with a small portion used for animals feeding [
5].
The produced water contains organic acids, lipids, alcohols, and phytotoxic materials that are dangerous for the environment; however, its organic matter and nutrients make it valuable for fertilizing soil [
8]. Roughly, solid dry residues can be estimated at 25% of the mass of the processed olive [
9], thus it can be estimated that 685,625 tons is available worldwide for 2015/2016, with 97% in Mediterranean countries.
The chemical composition of olive waste generally differs from one sample to another because it depends on several factors like olive mill processing pressure and temperature, but also olive type, soil, and growing conditions. When the process is handled in normal conditions, the fat percentage is between 8% and 16%; the nucleus presence is approximately 40%, which is rich in minerals like calcium and copper, and has a high percentage (up to 40%) of fibers [
10]. The variability in the chemical composition of olive waste for two prototype samples, a Spanish and a Jordanian sample, is shown in
Table 1. However, the effect of sample composition on electrical efficiency and plant operation will be investigated in a future study. For the present study, the Aspen modeling calculations were performed with the Jordanian sample. A noticeable difference is clear in fixed carbon and ash contents, in addition to the relatively high values for higher heating value (HHV).
The most common thermochemical energy conversion processes used to convert solid biomass combined heat and power are direct combustion, pyrolysis, and gasification of biomass [
12]. To achieve electricity from a combustion process, it is necessary to select a suitable power plant technology that meets biomass distribution due and countries’ conditions. For these reasons, small power plants are more practical than large ones, especially when concerned with developing distributed generation concepts.
Typical combined heat and power technologies in the range below 1 MW
el are Stirling engines, steam engines, microgas turbines, gas engines (combined with gasification), or fuel cells. For solid fuels, the most suitable systems are based on combustion in combination with Stirling and steam engines, or with organic Rankine cycle (ORC) turbine technology [
13]. Another suitable system is the so-called externally fired gas turbine (EFGT) as shown in
Figure 1, while gasification processes could also be used, as discussed in other research such as that conducted by Link et al. [
14].
Being able to process a wide variety of fuels, including those with suboptimal combustion properties, EFGT is considered to be a promising option for small- to medium-scale plant sizes [
15,
16]. The literature shows that technical and economic studies have been done for plants in the range of 30–100 kW
el, mainly for microscale. Typically, the discussion is about the use of EFGT in heat recovery, dealing with high-temperature heat exchangers, and thermal analysis for either biomass direct combustion or gasification [
15,
16,
17,
18,
19,
20,
21,
22]. Such technology has some advantages over the other systems; for example, greater flexibility in fuel type and lower cleaning and maintenance costs when compared with internal combustion gas turbines [
23,
24]. Additionally, there is no need for water as a medium or for cooling in the condenser, which can be an advantage in arid regions [
19]. There is no liquid working medium, as in steam turbines or engines; therefore, the startup, shutdown, and transient period of EFGT systems are mainly limited by the combustion process itself since the turbine has a relatively fast response [
25,
26]. Systems based on gasification (e.g., in combination with gas engines or fuel cells), as discussed in the past [
27,
28], are not considered within this study. This is because such existing systems are mostly designed for woody fuels and there are still unresolved questions regarding gas cleaning of the gas produced from agricultural residues like olive waste. Additionally, a comparison between EFGT and a gasification-based gas turbine system has been published for a 30 kW
el plant fueled by olive tree leaves and prunings [
29].
Typical electrical efficiencies for systems like ORC and EFGT are in the range of 10%–20%. Although higher efficiencies of 25% for EFGT have been reported [
30], for the relevant size within this study, we expect an electrical efficiency below 20% [
13,
15,
22,
30]. Actually, there is no exact definition for EFGT classes with respect to the plant size, so we defined microscale, based on other researches, as being up to 200 kW
el [
31,
32,
33,
34]. An overview of existing research on EFGT is given in
Table 2.
For analyzing the thermal operation of the gas turbine, Joule-Thomson or (as in other references) Brayton cycles are the best and ideal selections for calculating the efficiency of the gas turbine [
40], since the working fluid (air) is compressed and then expanded by the operation of the gas turbine after heat addition from the combustion process.
Turbines are generally defined as rotating machines dependent on the flow of liquid or gas materials at a certain pressure and temperature, resulting in rotation of a connected shaft with an electrical generator. The ideal thermal efficiency by the means of temperature or pressure is given by Equation (1) (see also [
41]):
The heat exchanger (HE) is a key component in an EFGT biofuel-based plant due to the high combustion temperature (combined with elevated pressure) and the aggressive behavior of the flow gases that cause oxidation, surface corrosion, and erosion [
42]. There are several classifications and types for the heat exchanger depending on the specific application. In this research, shell-and-tube HE was selected (as in [
22,
23]), since shell-and-tube type is the most suitable due to its ability to operate in high-temperature conditions, the design can withstand fouling caused by combustion, and its thermal stress reluctance [
38].
In a recent study on EFGT technology based on woody biomass [
43], the resultant electrical cost was found to be 0.33 euro/kWh at a utilization factor of 0.8 (7000 h/year) with an assumed power output of 60 kW
el. Another study was performed on the thermoeconomics of an EFGT cycle with two firing schemes—natural gas and biomass—with the plant designed to provide 100 kW
el with the possibility of adjusting percentages of energy sources (natural gas (NG) and biomass) from 0% to 100%. The capital cost was determined to be 471,000 euros with operating costs (including biomass supply, ash discharge, and maintenance) of 69,750 euro/year when the biomass quota is 100% [
21].
In [
44], an economic analysis of advanced and EFGT cycles was made by estimating the capital, operating, and maintenance specific costs. The specific capital cost of a power plant of 1 MW
el size was calculated to be 540 £ (ca. 733.5 euro) per kW
el.
While previous research on power production with EFGT technology concentrated on higher nominal power, as shown in
Table 2, the following study aims to evaluate the use of a very small system (15 kW
th) and provide suitable parameters for the operation of such a system. The expected efficiency values and parameter mappings for nominal operation are presented.
At the beginning of the Methodology section, the design of the proposed system is described, followed by the models for the combustion process and the thermodynamic cycle. In the Results and Discussion section, suitable parameters for an EFGT of this small size are specified for olive waste as an exemplary biomass. Finally, a cost estimation of the proposed system is given.
4. Conclusions
Combined heat and power (CHP) technology based on biomass in general is a promising way to reduce fuel import dependency, improve the electrical grid as a distributed generation technology, and reduce carbon dioxide emissions. Using biomass as a fuel has advantages compared to solar and wind power, since its availability is not dependent on weather conditions. Solid olive waste can be stored as briquettes or as pellets, so power can be produced on demand. Also, the wastes are already available at centralized places (olive mills) which reduce transport costs and emissions. Usage of adequate technology (e.g., EFGT) makes it possible to have higher fuel flexibility, which provides more options (e.g., under changing agricultural situations).
For the current study, an EFGT with a nominal thermal power of 15 kW has been investigated to map optimized pressure and turbine air-mass flow for several conditions. The plant electrical efficiency is in the expected range based on the literature. The results show that plant efficiency between 5% and 17% is realizable, taking into consideration that the nominal state is around 10%, which depends on temperature limits for the setups. Noting that, efficiency could be improved when using a more efficient compressor and gas turbine, in addition to the potential of waste heat recovery. Application of the CHP concept is possible for domestic heating; as a result, the heating from olive residues is applicable beyond electrical generation applications.
Turbine inlet temperature is a dominant factor in the electrical efficiency, so higher temperature values in combustion flue gas could provide a more efficient operation. To this point, it is important to develop high-temperature standing material for a pressurized heat exchanger and turbine, which is also required to get the maximum benefit.
It is also shown that the plant tends to be less efficient at low input power percentages when the back-work ratio is more than 70%, while moving to higher flue gas temperatures leads to higher efficiency.
Further research is currently running to describe a detailed technical operation including practical findings, controls, transient behavior, and results of testing further biomass types on a real combustion system combined with a hardware-in-the-loop (HIL) system.