Gasification of Agricultural Biomass Residues for Sustainable Development of Mediterranean Europe Regions: Modelling and Simulation in Aspen Plus

Academic Editor: Marcin D˛ebowski Received: 7 July 2025 Revised: 7 August 2025 Accepted: 9 August 2025 Published: 12 August 2025 Citation: López-García, E.; Rodriguez-Pastor, D.A.; Chacartegui, R.; Rouboa, A.; Monteiro, E. Gasification of Agricultural Biomass Residues for Sustainable Development of Mediterranean Europe Regions: Modelling and Simulation in Aspen Plus. Energies 2025,18, 4298. https:// doi.org/10.3390/en18164298 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Gasification of Agricultural Biomass Residues for Sustainable Development of Mediterranean Europe Regions: Modelling and Simulation in Aspen Plus Elisa López-García 1, Diego Antonio Rodriguez-Pastor 1, Ricardo Chacartegui 1, Abel Rouboa 2 and Eliseu Monteiro 2,* 1 Energy Engineering Department, University of Seville, Avda de los Descubrimientos s/n, 41092 Seville, Spain; [email protected] (D.A.R.-P.); [email protected] (R.C.) 2Associate Laboratory of Energy, Transport and Aerospace (LAETA), Faculty of Engineering, University of Porto, Rua Doutor Roberto Frias, 4200-465 Porto, Portugal; [email protected] *Correspondence: emonteir[email protected] Abstract The utilisation of agricultural residues for power generation is an opportunity to reduce fossil fuel usage and foster a sustainable circular economy in Mediterranean European regions. This can be achieved by resorting to the gasification process, which faces challenges such as optimising its operation parameters on real-world applications and lowering operational costs. This work studies the gasification process of a set of agricultural biomasses widely available in the Mediterranean Europe regions through modelling and simulation in Aspen Plus. The selected biomasses are olive stone, grapevine waste, and wheat straw. The effect of temperature, equivalence ratio, and steam-to-biomass ratio on gasifier performance and their effect on gas composition was assessed. The results indicate that olive stone and wheat straw performed best in terms of syngas composition and cold gas efficiency. The analyses show good gasification performance for temperatures above 750 ◦ C, equivalence ratios ranging from 0.1 to 0.3, depending on the raw material and steam-to-biomass ratios below 0.1. The obtained values show the validity and the potential of a downdraft gasification reactor to be used with these abundant agricultural biomasses in the Mediterranean European region. Its integration with a reciprocating engine is a rational choice for distributed power generation. Keywords: mediterranean residues; gasification; downdraft reactor; modelling; syngas 1. Introduction In the current scenario of growing demand for energy and environmental awareness, interest in alternatives to fossil fuels is increasing due to high oil prices, limited energy resources, and control of global greenhouse gas emissions [ 1 ]. According to life cycle analysis studies [ 2 ], biomass has a lower environmental impact and is widely available, making it a renewable substitute for fossil fuels. Additionally, it is a local fuel resource promoting domestic rural economies. Biomass accounts for 9% to 14% of total energy consumption in industrialised countries and 35% to 40% in developing countries [3]. An important source of biomass is the agricultural sector, including wastes from crop cultivation activity and secondary residues from processing products such as fruits, vegetables, sugarcane, rice, cereals, etc. Agriculture occupies half of the European Union land area, and the production of agricultural biomass has slightly increased over the last Energies 2025,18, 4298 https://doi.org/10.3390/en18164298 Energies 2025,18, 4298 2 of 18 two decades. The total annual agricultural biomass production in the European Union for the reference period (2016–2020) is estimated at 956 million tonnes per year, where 54% is economic production and 46% is residues [4]. In Mediterranean Europe, agricultural biomass residues come mainly from the dominant crops and farming systems shaped by the region’s warm, dry summers and mild, wet winters. The main agricultural biomass residues in this region typically include olive residues (olive stone and olive pomace), mainly in Spain, Italy, Greece, and Portugal; vineyard residues (vine pruning and grape pomace) in Italy, Spain, France, Greece, and Portugal; and cereal straw and stubble (straw, chaff, and husks) in widespread, especially inland areas of Spain, Italy, and France [4]. Biomass can be converted to a gaseous energy resource by biological or thermochemical processes. Anaerobic digestion stands out as a well-established biological process. However, it is relatively slow, taking weeks or even months to complete, which may require significant storage capacity for the feedstock and digestate [ 5 ]. Furthermore, the complex makeup of lignocellulosic biomass, in which cellulose fibre is firmly bound to hemicellulose and lignin, limits their biodegradability and, consequently, their use as the only substrate for anaerobic digestion [ 6 ]. Recent studies point out the need for lignocellulosic biomass to be co-digested with manure [ 7 ]. Among the available thermochemical technologies, combustion, pyrolysis, and gasification are the most important methods. The main advantages of biomass gasification over combustion are higher thermal efficiencies (85–90%), greater versatility in using various energy conversion technologies, and better environmental performance, particularly lower NO x and SO 2 emissions [ 8 , 9 ]. Pyrolysis involves the decomposition of lignocellulosic biomass at temperatures in the range of 300–650 ◦ C in the absence of oxygen [9]. It is also the first step in combustion and gasification processes, where it is followed by total or partial oxidation of the primary products, respectively. Pyrolysis is particularly suitable for producing bio-oil and biochar. However, when the goal is the production of a combustible gas, it provides less yield and efficiency than gasification [ 10 ]. Gasification is the thermochemical conversion of solids into synthesis gas, or syngas, which consists of a mix of gaseous compounds, with main components CO, CO 2 , H 2 , CH 4 , and N 2 [ 11 ]. The resulting gaseous fuel can be used in conventional power systems [12] such as gas turbines [13], combined cycles [14], reciprocating engines [15], as well as in emerging technologies as fuel cells [ 16 ]. Besides power production, using the gaseous products in high-efficiency power systems reduces the impact of the wastes used as raw material and generates a circular economy [17]. The exploitation of local agricultural residues by gasification is of high interest for circular economy development, allowing for the use of a huge amount of agricultural residue and reductions in CO 2 emissions [ 18 ]. It requires the characterisation of raw materials and gasification products to study the effect of their properties on thermochemical and physical conversion technologies. In this context, Mendoza et al. [ 19 ] extensively characterised residual biomass from the coffee production chain. They concluded that the coffee chain could be a suitable feedstock for the thermochemical conversion process due to its low moisture content and volatile matter content. Huang et al. [ 20 ] established an economic model and a sustainability evaluation model to study the effects of key variables on the performance of a biomass gasification power generation plant. They concluded that 5 MW was the optimal power level for distributed generation based on gasification. Syngas obtained by gasification can be used for synthesis processes after being conditioned. This option was studied by Motta et al. [ 21 ], who investigated the influence of operating parameters in the sugarcane bagasse gasification. Temperature was a key factor in syngas composition, enhancing CO and reducing CO 2 , while the steam-to-biomass ratio (SBR) is essential for adjusting the H 2 /CO ratio. Schweitzer et al. [ 22 ] compared Energies 2025,18, 4298 3 of 18 syngas compositions and impurities for different waste materials such as sewage sludge and manure. Based on the study, they correlated the fuel composition with impurities concentration in the product gas. The equivalence ratio (ER) plays a crucial role in gasification, as it directly influences the efficiency of the process, and the quality of the syngas produced. The ER is the ratio of the actual amount of oxygen supplied to the stoichiometric amount required for complete combustion. When the ER is low (sub-stoichiometric conditions), the gasification process tends to favour the production of combustible gases like carbon monoxide, methane, and hydrogen, enhancing the energy content of the syngas. However, a high ER can lead to more complete combustion, reducing the yield of valuable gases and producing more carbon dioxide and water vapour [ 23 ]. Thus, controlling the ER is essential to boost gasification, as it impacts both the calorific value of the syngas and the carbon footprint of the process. The impact of the equivalence ratio on gasification has been explored in various studies. For instance, Liu et al. [ 24 ] highlighted the role of the equivalence ratio in controlling tar formation during the gasification of agricultural residues, with lower ERs leading to reduced tar content in the syngas. Similarly, Zhang et al. [ 25 ] found that optimising the ER can enhance the carbon conversion efficiency and increase hydrogen production, a key element for clean energy generation. Furthermore, Cohen et al. [ 26 ] emphasised the balance between syngas yield and carbon dioxide emissions, pointing out that fine-tuning the ER is essential for minimising the environmental impact of agricultural residue gasification. These studies underscore the importance of the equivalence ratio in both improving syngas quality and advancing sustainable biomass conversion technologies. One of the biggest challenges for gasification systems is demonstrating their viability [ 11 , 27 ]. The use of fuels with high availability and low cost allows for a decrease in some operating costs [ 28 ]. As a result, this study examines the numerical performance of a biomass downdraft gasifier in the context of its use in rural areas near Mediterranean biomass sources in order to reduce the transportation costs. The literature analysis revealed a lack of knowledge in defining the key running parameters of a downdraft gasifier employing the most abundant biomasses in Mediterranean Europe. As a result, this study will provide key parameters for biomass gasification in Mediterranean Europe resorting to modelling and simulation in Aspen Plus. The methodology followed in this work starts by selecting widely available biomass feedstocks from the Mediterranean regions. A gasifier model is described and calibrated in Aspen Plus to assess its feasibility and accuracy by comparing results with those available in the literature. Following, the model is applied to the study of gasification of the selected Mediterranean biomasses and the results obtained are compared. The effect of temperature, equivalence ratio, and steam-to-biomass ratio on gasification performance, the produced gas composition, lower heating value, and cold gas efficiency is evaluated. A discussion on the integration of the gasifier with reciprocating engines and gas turbines for distributed power generation is provided. 2. Materials and Methods 2.1. Mediterranean Europe Agricultural Residues This study is orientated to the Mediterranean Europe region, which has characteristic climate conditions and agro-industrial activities such as wine, olive oil, and wheat production. The selected agricultural residues are olive stones, grapevine waste, and wheat straw. Figure 1shows the annual olive production, where the south of Spain stands out with over 800,000 tonnes per year [ 29 ]. In 2021, the olive oil production was 1,389,000 tonnes in Spain, 275,000 tonnes in Greece, 274,000 tonnes in Italy, and 100,000 tonnes in Portugal [ 30 ]. Energies 2025,18, 4298 4 of 18 The production of 1 tonne of olive oil needs 20 tonnes of olives; the olive stones represent 18–22% of the olive weight [31]. Figure 1. Olive production in the Mediterranean Europe region. In 2024, the European Union produced almost 144 million hectolitres of wine. Most of this production was shared between Italy (28.5%), Spain (26.5%), and France (26%). The area dedicated to harvesting grapevines in the Mediterranean area can be seen in Figure 2[ 32 ]. The production of 750 mL of wine needs 1.17 kg of grapes; 20% of a grape is waste [33]. Figure 2. Grapevine production surface in the Mediterranean Europe region. The most cultivated cereals in Europe are barley and wheat, with France and Spain the largest producers in the Mediterranean region. Figure 3shows the average wheat production in the Mediterranean region from 2010 to 2014 [ 34 ]. In 2024, France produced more than 25 million tonnes, Spain produced more than 6 million tonnes, and Italy produced more than 3 million [ 35 ]. The ratio of straw/grain in wheat varies between 1.3 and 1.4 [ 36 ]. Therefore, an average of 1 tonne of wheat grain can produce 1.35 tonnes of wheat straw. <1000 1000 – 2000 2000 – 3000 3000 – 4000 >4000 Wheat production (1000 ton) Figure 3. Wheat production in the Mediterranean Europe regions. The presented figures show the high availability of the selected agricultural residues corresponding to the main agro-industrial activities in the Mediterranean Europe region. Energies 2025,18, 4298 5 of 18 Moreover, there is an increased interest in exploiting the agricultural biomass residues to improve the circularity and sustainability of this economic sector. 2.2. Gasification Model The use of selected Mediterranean Europe biomasses near to the source of the raw material directs our research towards small-scale gasifiers. A downdraft fixed-bed gasifier, as shown in Figure 4, was chosen because it produces high-quality syngas with low tar content, making it the most successful unit utilised for small-scale power generation [ 11 , 37 ]. Figure 4. Downdraft fixed bed reactor. It comprises four stages: (a) drying, (b) pyrolysis, (c) oxidation, and (d) reduction zone [38]; the main gasification reactions are summarised in Table 1. Table 1. Gasification main reactions [39]. Reaction Enthalpy of Reaction Reaction Name Reaction Number C+O2→CO −111 MJ/kmol Char partial combustion R1 C+2H2→CH4−75 MJ/kmol Methanation reaction R2 CO + 0.5 O2→CO2−283 MJ/kmol CO partial combustion R3 H2+ 0.5 O2→H2O−242 MJ/kmol H2partial combustion R4 C + CO2↔2 CO +172 MJ/kmol Boudouard reaction R5 C+H2O↔CO + H2+131 MJ/kmol Water-gas reaction R6 CO + H2O↔CO2+ H2−41 MJ/kmol Water-gas shift reaction R7 CH4+ H2O↔CO + 3H2+206 MJ/kmol Steam methane reforming R8 The whole gasification system is schematised in Figure 5. Three reactor blocks and two separator units are used to simulate the downdraft fixed bed gasifier. The model was developed using Aspen Plus V12.2 [ 40 ]. The non-stoichiometric equilibrium modelling approach, which involves minimising the system’s Gibbs free energy, is the foundation of the model developed in Aspen Plus V12.2 [ 23 ]. Because only the global gasification reaction and the biomass’s elemental composition are needed, this modelling approach is especially appropriate for the gasification process [ 23 ]. The following assumptions are used in our gasification model [41,42]: •Reactions reach the equilibrium state. •Char consists just of carbon. •Syngas is composed of: CO, H2, CO2, CH4, N2, H2O, H2S, and NH3. •The production of tars is disregarded. •Ash is considered as an inert material. •The reactor runs at constant pressure and temperature. •The reactor is adiabatic. Energies 2025,18, 4298 6 of 18 Figure 5. Aspen Plus gasification model of the downdraft reactor. The “FEED” stream defines the biomass by its ultimate and proximate analyses [ 43 ]. An RStoic reactor “DRIER” is used to dry the biomass. Moisture content is controlled by a FORTRAN block calculator, while a separator unit removes water “H 2 O SEPARATOR”. Biomass decomposition into the different elements (C, H 2 , H 2 O, S, Cl 2 , O 2 , N 2 , and ash) is simulated through an RYield reactor “DECOMPOSER”. Mass concentration control is carried out using a Fortran calculator using each biomass’s proximate and ultimate analysis. Gasification and partial oxidation are modelled in an RGibbs reactor using Gibbs free energy minimisation to calculate syngas composition. An air stream and a steam stream are introduced into this reactor. A solid separator unit represents ash elimination. 2.3. Model Validation The model was validated by comparing model results obtained with the experimental results of Wei et al. [ 44 ] for hardwood chips gasification and with the results of Jayah et al. [45] for rubber wood gasification. The accuracy and feasibility of using the model to analyse and design gasification processes of similar biomass and urban residues can be assessed in this way. Proximate analysis with moisture content (M), fixed carbon (FC), volatile material (VM), and ash content (Ash) and ultimate analysis of the feedstock selected for model validation are shown in Table 2. Table 2. Proximate and ultimate analysis of the studied biomass and residue. Biomass Proximate Analysis (wt.% d.b. *) Ultimate Analysis (wt. % d.b.*) M FC VM Ash C H N S O Hardwood chips [44] 13.89 19.03 79.85 1.12 49.82 5.56 0.078 0.005 43.417 Rubber wood [45] 16 19.2 80.1 0.7 50.6 6.5 0.2 0.0 42.0 * d.b. stands for dry basis. The gasification parameters used for the simulations of each case are presented in Table 3. Energies 2025,18, 4298 7 of 18 Table 3. Gasification parameters. Hardwood Chips Rubber-Wood Biomass Flow (kg/h) 23.81 Flow (kg/h) 10 Air ER 0.37 Flow (kg/h) 22 T (◦C) 25 T (◦C) 25 Gasifier P (bar) 1 P (bar) 1 T (◦C) 790.35 T (◦C) 827 Simulation results are presented in Table 4, and the obtained syngas composition is compared with the results reported in the literature [ 44 , 45 ] (the relative deviation in the estimation of compositions is also specified). Table 4. Syngas composition and relative deviation between the present model and literature data. Gas Composition (%) H2CO CO2 Hardwood chips Wei et al. [44] 17.56 24.16 11.34 Our model 19.11 22.02 11.08 Deviation (%) 8.85 −8.84 −2.30 Rubber wood Jayah et al. [ 45 ] 18.3 20.2 9.7 Our model 17.9 19.65 10.7 Deviation (%) −2.18 −2.74 10.42 Results in Table 4show that the model satisfactorily predicts the composition of the generated syngas from the gasification of the raw materials. The model relative deviations with respect to the literature experimental results are below 10.42% for the three main gas species. These findings give the confidence to use the developed model for additional research and enable the demonstration of its good prediction accuracy. The relative deviation from the experimental results of Wei et al. [ 44 ] and Jayah et al. [ 45 ] may be explained by the differences between a real gasification scenario and the model assumptions made in this work. In a real gasification process, several complex phenomena occur, including chemical reactions, mass and heat transfer, and phase changes. These processes are influenced by factors like temperature gradients, pressure variations, the presence of impurities such as tar and unconverted carbon, and kinetic effects that are neglected in thermodynamic modelling [ 23 ]. Moreover, in a real gasification process, the presence of metals in the feedstock, which ultimately ends up in the ash, can significantly affect the gasification reactions in several ways [ 9 ]. First, alkali metals (sodium, potassium) and alkaline earth metals (calcium, magnesium) can catalyse or hinder certain chemical reactions. Alkali metals can promote the formation of tar, a byproduct that can decrease the efficiency of syngas production and require additional treatment for removal [ 46 ]. On the contrary, calcium and magnesium can participate in reactions that help capture sulphur and other impurities, improving syngas quality [46]. 3. Results In this section, a gasifier reference case is presented with fixed parameters, and then the effects of the variation of different parameters are analysed to identify how they affect the gasification performance. The validated developed model is used to study the gasification performance of agricultural biomass residues widely available in the Mediterranean Europe region, namely, olive stone, grapevine waste, and wheat straw. The Energies 2025,18, 4298 8 of 18 elemental and proximate composition of the main Mediterranean European agricultural biomass residues are depicted in Table 5. Table 5. Ultimate and proximate analysis for the different biomasses analysed in our work [3]. Proximate Analysis (% wt.) Ultimate Analysis (%, d.a.f. *) M FC VM Ash C H N S O Olive stone 11.00 20.35 78.30 1.35 46.55 6.33 1.81 0.11 45.20 Grapevine waste 11.16 13.70 73.00 13.30 35.74 5.95 1.35 0.30 56.67 Wheat straw 7.70 18.19 76.00 5.81 45.58 6.04 1.18 0.59 46.60 * d.a.f. stands for dry and ash free basis. 3.1. Analysis of Gasification Design Parameters The reference case was defined based on the operation characteristics of a downdraft fixed-bed gasifier [ 47 , 48 ]. A temperature of 800 ◦ C is recommended because it enhances the hydrogen yield and syngas heating value while decreasing the tar content [ 11 , 47 ]. In the case of the equivalence ratio (ER), one of the most important gasification parameters, the optimal values vary from 0.2 to 0.4 [ 49 ]. Another important gasification parameter is the steam-to-biomass ratio (SBR), representing the steam mass supplied to the process per unit of biomass mass. Using steam as a gasifying agent favours the water–gas, water–gas shift, and steam reforming reactions, ultimately leading to increased H 2 production [ 50 , 51 ]. Salient SBR range is dependent on the feedstock, on the use of other gasifying agents mixed with steam, and on the objective for the syngas (e.g., increase the amount of hydrogen, increase the syngas heating value, etc.). Generally, in an optimised biomass gasification process, the SBR ranges from 0.5 to 1.0 [ 52 ]. This range has been shown to yield salient cold gas efficiency (CGE) and lower heating value (LHV) of the syngas [ 53 , 54 ]. A steamto-biomass ratio above 1.0 may even reverse the char gasification reaction (R6) [ 52 ]. On the other hand, the production of steam is an energy-intensive and expensive process. Therefore, our reference case scenario establishes an SBR equal to zero. All the gasification parameters used for the simulation of the reference case are shown in Table 6. Table 6. Gasification conditions. Wet feed input 10 kg/h Gasifier temperature 800 ◦C Equivalence ratio 0.2 Steam-to-biomass ratio 0.0 Under these conditions, the estimated gas compositions for each feedstock output are plotted in Figure 6. (a) (b) 0 0.2 0.4 0.6 0.8 1 Olive stone Grapevine waste Wheat straw Syngas yield (kmol/h) CO CO2 CH4 H2 N2 0 20 40 60 80 100 Olive stone Grapevine waste Wheat straw Syngas molar composition (%) CO CO2 CH4 H2 N2 CO2 CH4 H2 N2 N2 H2 CH4 CO2 Figure 6. Comparison of syngas composition: (a) molar yield and (b) molar percentage. Energies 2025,18, 4298 9 of 18 Figure 7shows the syngas LHV and the CGE obtained for the Mediterranean biomasses. 5 6 7 8 70 75 80 85 90 95 Olive stone Grapevine waste Wheat straw LHV (MJ/Nm3) CGE (%) CGE Figure 7. Syngas LHV and CGE of the gasification process. According to Figures 6and 7, various conclusions can be obtained for each feedstock: • Olive stone presents the highest CO, CH 4 , and H 2 yield because it has the highest carbon and hydrogen contents. Due to its low ash content and especially high volatile material content, the produced syngas yield is the highest among the feedstocks studied. Given that the CGE is also the highest among the feedstocks under study, olive stone is an excellent candidate for biomass gasification plants. • Wheat straw presents the highest syngas heating value mainly because of its lower moisture content. However, due to its intermediate syngas yield, the CGE also falls between the maximum of olive stone and the minimum of grapevine wastes. This feedstock also shows a very similar syngas composition to that of the olive stone feedstock. These results are due to the similarity of the feedstocks’ ultimate composition, as shown in Table 5. • Grapevine waste gasification presents the lowest syngas yield, LHV, and CGE performance. These results found explanation mainly in the lower carbon content of the grapevine waste, as shown in Table 5. An in-depth analysis of the grapevine waste results may point out that the lower syngas yield is mainly due to the lowest volatile matter of this feedstock. The lower LHV is mainly due to the lowest carbon content and the highest ash and moisture contents. The lower CGE results from both the lowest LHV and syngas yields. 3.2. Sensitivity Analysis The following sections present a sensitivity analysis to study the performance of the gasification process. Temperature, equivalence ratio, and steam-to-biomass ratio were selected to study their effect on the syngas composition, lower heating value (LHV), and CGE. 3.2.1. Effect of Gasification Temperature In order to analyse the effect of the gasifier temperature on syngas composition, the gasifier temperature is varied from 600 ◦ C to 1000 ◦ C, while keeping ER and SBR constant at their operating values of the reference case. Figure 8shows the influence of temperature on gas composition. The molar fraction of CO, CO2, CH4, and H2of dry syngas is analysed. As the temperature is increased, CO increases because four reactions shift to CO formation: CO partial combustion (R3) and water–gas shift reaction (R7) are exothermic reactions; water–gas reaction (R6) and steam methane reforming (R8) are endothermic reactions. The maximum is reached at 750 ◦ C in the case of olive stone and wheat straw; the CO concentration from grapevine waste gasification is lower at any temperature. 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