Bench-Scale Gasification of Olive Cake in a Bubbling Fluidized Bed Reactor

Citation: Blázquez, G.; Calero, M.; Gálvez-Pérez, Á.; Martín-Lara, M.Á.; Pérez, A. Bench-Scale Gasification of Olive Cake in a Bubbling Fluidized Bed Reactor. Appl. Sci. 2024,14, 7282. https://doi.org/10.3390/app14167282 Academic Editors: Dae Sung Lee and Demis Pandelidis Received: 4 July 2024 Revised: 31 July 2024 Accepted: 18 August 2024 Published: 19 August 2024 Copyright: © 2024 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/). applied sciences Article Bench-Scale Gasification of Olive Cake in a Bubbling Fluidized Bed Reactor Gabriel Blázquez , Mónica Calero, Ángela Gálvez-Pérez, MaríaÁngeles Martín-Lara * and Antonio Pérez Department of Chemical Engineering, University of Granada, 18071 Granada, Spain; [email protected] (G.B.); [email protected] (M.C.); [email protected] (Á.G.-P.); [email protected] (A.P.) *Correspondence: [email protected] Abstract: The gasification of olive cake is a promising method for converting this material into valuable energy. This work offers interesting results about the effect of equivalence ratio and temperature on the composition and quality of the produced gas obtained during olive cake gasification in a fluidized bed plant with air as a gasification agent. Additionally, the efficiency of the gasification process was evaluated. The results show that, for a specific temperature, an equivalence ratio of 0.3 showed a higher cold gas efficiency. For example, at 850 ◦ C and an equivalence ratio of 0.1, the cold gas efficiency was 22.7%; however, at the same temperature but at an equivalence ratio of 0.3, the cold gas efficiency was increased to 61.2%. In addition, for a constant equivalence ratio, by increasing the operating temperature, there was no significant increase in the lower heating value of the exit gas, and the gas flow was practically constant with temperature, but it varied substantially with the equivalence ratio, reaching values in the range of 3.44–14.89 NL/min (825.6–3573.6 NL/kg feed). Finally, the production of CO, H 2 , and CH 4 is estimated to be higher for tests conducted with an equivalence ratio of 0.3. Keywords: fluidized bed gasifier; gasification; olive cake; olive oil industry; bench-scale plant 1. Introduction Olive oil production is a significant industry in many Mediterranean countries, with Spain, Italy, and Greece being the largest producers [ 1 ]. This region’s unique climate and soil conditions are ideal for cultivating olive trees, making the olive oil industry a cornerstone of their agricultural sector. The process of producing olive oil involves several stages, including the crushing and pressing of olives to extract the oil, which results in a byproduct known as olive cake. In modern agricultural and industrial contexts, olive cake is increasingly viewed as a valuable byproduct rather than waste. It contains a substantial amount of lignocellulosic material, making it a valuable resource for generating renewable energy. One of the promising methods for utilizing this byproduct is through gasification technology. The gasification process is a thermochemical conversion method that transforms solid biomass and residues at high temperatures (for common gasification technologies, including fixed bed gasifiers and fluidized bed gasifiers, this is 800–1000 ◦ C) in the presence of a gasifying agent. This process converts the solid material into a versatile product gas, which can be utilized for direct heat and power generation or further upgraded into green gases or liquid fuels. Gasification is particularly appealing because it offers a sustainable way to manage agricultural waste while producing clean energy [ 2 – 5 ]. Other thermochemical processes, such as pyrolysis and combustion, also play significant roles in biomass conversion. Pyrolysis involves heating biomass in the absence of oxygen to produce bio-oil, biochar, and syngas, while combustion directly burns biomass to generate heat and power. Each process has its own advantages and limitations, but gasification is particularly appealing for its efficiency in producing high-value products and its potential to reduce environmental impact. Appl. Sci. 2024,14, 7282. https://doi.org/10.3390/app14167282 https://www.mdpi.com/journal/applsci Appl. Sci. 2024,14, 7282 2 of 13 Several gasification technologies have been developed for processing olive cake, including fixed bed gasification, fluidized bed gasification, and entrained flow gasification [ 3 , 4 , 6 – 8 ]. Each of these technologies has its own set of advantages and challenges, varying in terms of cost, scalability, and efficiency. Fixed bed gasification is known for its simplicity and lower capital costs but can suffer from lower efficiency and scalability issues. Fluidized bed gasification offers better mixing and heat transfer, making it more efficient, though it is more complex and costly due to the high capital investment required for equipment and the significant operating costs associated with maintenance and energy consumption. Entrained flow gasification, on the other hand, operates at higher temperatures and can handle a wider range of feedstocks but requires significant investment and precise control. Experimental studies on the gasification of olive cake have yielded promising results, indicating that gasification is a sustainable and economically viable method for energy conversion. These studies highlight the potential of olive cake as a renewable energy source, contributing to energy sustainability in the olive oil industry. Moreover, the adoption of gasification technologies can help mitigate environmental impacts by reducing the reliance on fossil fuels and lowering greenhouse gas emissions [ 3 , 6 , 8 – 15 ]. For example, Gálvez- Pérez et al. [ 3 ] focused on the effects of torrefaction and prior hydrothermal treatment on the air gasification of olive cake at low temperatures (between 625 and 700 ◦ C), finding that higher CO, CH 4 , and H 2 yields and cold gas efficiencies were achieved at specific conditions, with notable differences between raw, hydrolyzed, and torrefied samples. Dogru and Erdem [ 6 ], utilizing a 500 kg/h autothermal modified updraft gasifier, found that the syngas generation rate is approximately 2.5 Nm 3 per kg of olive pomace, with a calorific value ranging from 5.0 to 7.0 MJ/Nm 3 , and that the gasification system converted over 85% of the carbon in the pomace into syngas. Tezer et al. [ 8 ] also investigated the gasification of dried olive pomace using various fixed bed gasifier systems. Syngas produced with dry air had H 2 contents of 48 mol % and 45 mol % in updraft and downdraft gasifiers, respectively, with a heating value of around 12.4 MJ/Nm 3 . This paper presents these findings as part of a larger research project focused on hydrogen production from fixed bed reactors. On the other hand, Tamosiunas et al. [ 9 ] explored olive byproducts for energy recovery through thermal arc plasma gasification. Experiments varied parameters such as biomass flow, water vapor, and plasma power. The resulting syngas had CO ( 41.17 mol % ), H 2 (13.06 mol %), and CO 2 (13.48 mol %), with a heating value of 6.09 MJ/Nm 3 . The work of Vera et al. [ 10 ] evaluated a pilot plant that converts these byproducts into electrical and thermal power. The plant, featuring a downdraft gasifier and a modified engine, achieved a cold gas efficiency of 70.7–75.5% and calorific values of 4.8–5.4 MJ/kg, with a payback period of 5–6 years , highlighting the potential for effective energy recovery from olive waste. Another study [ 11 ] performed the tecno-economic assessment of a combined heat and power (CHP) plant using syngas from dry olive pomace. The work modeled the plant in ASPEN Plus, using real data for calibration and performance metrics. Borello et al. [ 12 ] also presented a thermodynamic model of a combined heat and power (CHP) plant using syngas from dry olive pomace gasification. ChemCad software modeled the gasification process, with experimental data informing the model. The study of Puig-Gamero et al. [ 13 ] assessed how dolomite affects the co-gasification of coal, petcoke, and olive pomace. Results showed that dolomite improves reactivity, reduces weight loss, and enhances the H 2 /CO ratio while lowering sulfur and nitrogen emissions. The study of Sert et al. [ 14 ] explored hydrogen production from olive pomace using a batch autoclave at temperatures between 300 ◦ C and 600 ◦ C and pressures ranging from 200 to 425 atm. The effects of different catalysts were also examined. Hydrogen production increased with temperature and decreased with pressure. The highest hydrogen yield of 16.80 mol/kg biomass was achieved at 600 ◦ C with a KOH catalyst. Finally, Cardoso et al. [ 15 ] presented a 2-D simulation of olive pomace gasification in a bubbling fluidized bed reactor, validated with experimental data. The authors found that the process has a low cold gas efficiency (~20%), and it is more suitable for small-scale domestic cogeneration rather than large facilities. Appl. Sci. 2024,14, 7282 3 of 13 The purpose of this work is to explore the impact of equivalence ratio and temperature on the gasification process of olive cake in a fluidized bed plant, using air as the gasifying agent. By conducting experiments in a bench-scale plant, this research aims to simulate a more realistic pre-industrial scale, providing valuable insights into the optimization of gasification conditions. The focus on olive cake contributes to the development of sustainable practices within the olive oil industry. Through this work, we hope to advance the understanding and application of gasification technology, promoting a circular economy and enhancing the sustainability of Mediterranean agriculture. 2. Materials and Methods 2.1. Olive Cake A local olive oil factory sited in Jaén, Spain, provided the olive cake used in this investigation, with an average particle diameter of 0.96 mm. A summary of the main properties of the olive cake used in this study is reported in Table 1. Table 1. Main properties of olive cake used in this study. Data from Gálvez-Pérez et al. [ 3 ] and Quesada et al. [16]. Elemental analysis (dry basis) C (wt., %) 44.0 H (wt., %) 7.0 N (wt., %) 1.1 S (wt., %) ND Proximate analysis Moisture (wt., %) 6.6 Volatile (wt., %) 60.8 Fixed carbon (wt., %) 22.2 Ashes (wt., %) 10.4 2.2. Brief Description of the Bench-Scale Plant The equipment used consists of a biomass gasification plant based on atmospheric pressure bubbling fluidized bed technology and built of AISI 304 stainless steel for high temperatures, as shown in Figure 1. The bench-scale plant can be divided into five large interconnected modules, which are the following: (i) the feed system of the agent or gasification agents; (ii) the solid feed system; (iii) the gasification reactor and electric oven; (iv) the gas purification system; and (v) the monitoring and control systems. Sand particles were used as bed material during the gasification experiments, with a mean size of 200 µ m. The gasifier consists of a cylindrical reactor with two bodies. The lower body, with an inner diameter of 50 mm and a height of 750 mm, is connected to the upper body, which has an inner diameter of 100 mm and a height of 250 mm, by a truncated conical intermediate section. The total height of the reactor is 1150 mm. The biomass to be treated is stored in a 1 L capacity hopper with a lid and is introduced into the gasifier through the lower part of it, through an endless screw designed to feed until 0.5 kg/h. The feed screw is attached to the lower body of the gasifier through flanges located at 3 different heights so that it is possible to feed the reactor at different heights. In our work, the feed was introduced at 0.5 m from the bottom of the cylindrical reactor. The plant was also equipped with a cyclone, condenser system, and a filter for removing particulate matter and condensing tar and other condensable gases from the producer gas, thereby ensuring cleaner gas output and protecting downstream equipment. Appl. Sci. 2024,14, 7282 4 of 13 Appl. Sci. 2024, 14, x FOR PEER REVIEW 4 of 13 Figure 1. Schematic representation of the biomass gasification plant. 2.3. Description of the Operational Procedure Before the experimental tests began, several checking and switching tasks were performed. Once the leak check was done, the plant started to be heated. For this, an air stream was circulated, the resistors of the reactor and the air preheater (350 °C) were activated, and the cooling apparatus (4 °C), prior to the final filter, was connected. When the reactor temperature was 50 °C below the standard temperature (750, 800, 850, or 900 °C), the hose feeder (250 g/h) that had been previously inserted into the feeding pipe was O2 N2 T ASHES T T T T SAMPLING T T CYCLONE H2O T P Q Q Q 1.3 m Figure 1. Schematic representation of the biomass gasification plant. 2.3. Description of the Operational Procedure Before the experimental tests began, several checking and switching tasks were performed. Once the leak check was done, the plant started to be heated. For this, an air stream was circulated, the resistors of the reactor and the air preheater (350 ◦ C) were activated, and the cooling apparatus (4 ◦ C), prior to the final filter, was connected. When the reactor temperature was 50 ◦ C below the standard temperature (750, 800, 850, or 900 ◦ C), Appl. Sci. 2024,14, 7282 5 of 13 the hose feeder (250 g/h) that had been previously inserted into the feeding pipe was triggered. Once the temperature and gas composition were stable (measured firstly with a portable gas analyzer, MRU-Vario Luxx, MRU Instruments, Inc., Humble, TX, USA), the determination of the composition of the exhaust gas was performed with an Agilent 990 Micro GC (Agilent, Santa Clara, CA, USA). Three measurements were made at intervals of 5 min, and the values were recorded for the subsequent calculation of the mean values. For each temperature, three experiments were carried out, keeping the flow rate constant, and varying the air rate to three levels of equivalence ratio (0.1, 0.3, and 0.5). The equivalence ratio (ER) defines the ratio of the mass of the oxidant to the mass of fuel divided by the oxidant stoichiometrically required per unit mass of dry feedstock. It expresses the quantity of air used as a proportion of the stoichiometric air requirements used to gasify a given unit of fuel. In this work, to establish the chosen equivalence ratios of 0.1, 0.3, and 0.5, air flows were varied between 0.13 Nm 3 /h and 0.67 Nm 3 /h for a flow of 0.25 kg/h of olive cake. Cold gas gasification efficiency, defined as the ability of the reaction system to harness the energy available in the biomass and convert it into energy available in the gas, was calculated according to the Equation (1) as the ratio of output gas energy to the input feed energy: ηCG = Vg·LHVgas mfuel ·LHVfuel (1) where ηCG is the cold gas efficiency, V g the volume flow of exit gas (Nm 3 /h), LHV gas the lower heating value of the exit gas (MJ/Nm 3 ), LHV fuel the lower heating value of the solid fuel (MJ/kg), and mfuel the mass flow of solid fuel (kg/h). The fluidization process was achieved using quartz sand with a mean particle size of 200 µ m. The gas flow was introduced through a perforated plate located at the base of the reactor to ensure uniform distribution and stable fluidization. Liquid residues were sampled directly from the gasification system at designated collection points. Specifically, the sampling was conducted from the condensate collection unit, which is positioned downstream of the gasifier where the syngas cools and condenses. This unit is equipped with sampling ports to ensure a representative collection of the liquid phase. The solid residues were collected from three key locations within the gasification system: (1) the fluidized bed reactor, (2) the cyclone, and (3) the gas filter unit. The fluidized bed reactor includes an ash collection system at the bottom to collect the ashes generated during the gasification process. After each experiment, a manual ash removal mechanism was executed. Also, the ash was recovered from the cyclone and gas filter unit sited at the end of the installation. 3. Results 3.1. Influence of Temperature and ER on Cold Gas Efficiency Figure 2a shows the effect of temperature on cold gas efficiency for the three ER levels studied. The results show values ranging from 22.7% (700 ◦ C; ER 0.1) to 61.2% (850 ◦ C; ER 0.3). The relatively low efficiency values may be because the light hydrocarbon content of the resulting gas was not considered for the LHV gas calculation. On the other hand, generally, for a specific ER, a temperature increase shows an increase in the efficiency of cold gas. However, for a 0.3 ER, the variations between the cold gas efficiency values were barely affected by the temperature, even showing a slight decrease when the temperature increased from 850 to 900 ◦ C. If you analyze the three ER levels in more detail, you can see how the lowest efficiency values were found for a 0.1 ER. Appl. Sci. 2024,14, 7282 6 of 13 Appl. Sci. 2024, 14, x FOR PEER REVIEW 6 of 13 Temperature, ºC 700 750 800 850 900 950 Cold gas efficiency, % 0 20 40 60 RE 0.1 RE 0.3 RE 0.5 (a) ER 0.00.10.20.30.40.50.6 Cold gas efficiency, % 0 20 40 60 750 ºC 800 ºC 850 ºC 900 ºC (b) Figure 2. Effect of temperature (a) and ER (b) on cold gas efficiency. These trends are consistent with the results obtained in previous works reported by other authors and other materials, such as Abdelrahim et al. [17], who evaluated sludges at a fixed temperature of 780 °C and with an ER between 0.1 and 0.5 and achieved a simulated efficiency of between 30 and 35%. Also, Hernández et al. [18] found that the efficiency of cold gas increased from 15 to 28% by increasing the temperature from 750 to 1050 °C for a mixture of biomass and coal. On the other hand, Niu et al. [19] found that when the ER increased to 0.28, the gas yield increased by approximately 20% for the gasification of urban solid waste. While higher temperatures in a gasification process require more energy input, cold gas efficiency was perhaps improved by enhancing reaction rates, reducing tar formation, and better converting it into valuable gases. At low temperatures, the gasification process is less efficient as it hinders the conversion of biomass into useful gases. This is because the chemical reactions involved in gasification are favored at higher temperatures. Figure 2. Effect of temperature (a) and ER (b) on cold gas efficiency. These trends are consistent with the results obtained in previous works reported by other authors and other materials, such as Abdelrahim et al. [ 17 ], who evaluated sludges at a fixed temperature of 780 ◦ C and with an ER between 0.1 and 0.5 and achieved a simulated efficiency of between 30 and 35%. Also, Hernández et al. [ 18 ] found that the efficiency of cold gas increased from 15 to 28% by increasing the temperature from 750 to 1050 ◦ C for a mixture of biomass and coal. On the other hand, Niu et al. [ 19 ] found that when the ER increased to 0.28, the gas yield increased by approximately 20% for the gasification of urban solid waste. While higher temperatures in a gasification process require more energy input, cold gas efficiency was perhaps improved by enhancing reaction rates, reducing tar formation, and better converting it into valuable gases. At low temperatures, the gasification process is less efficient as it hinders the conversion of biomass into useful gases. This is because the chemical reactions involved in gasification are favored at higher temperatures. Therefore, at low temperatures, the biomass conversion is lower, and a low cold gas efficiency was obtained [ 20 ]. It is also important to indicate that electric heating was utilized to maintain the Appl. Sci. 2024,14, 7282 7 of 13 required operating temperatures for the gasifier. However, this input was not accounted for in the cold gas efficiency calculations. Consequently, while the cold gas efficiency generally improves with increasing temperature—reflecting enhanced gasification reactions—it does not capture the complete energy dynamics of the gasifier. This discrepancy indicates that the measured efficiency improvements may not fully align with the broader energy balance considerations. Figure 2b shows the effect of the equivalence ratio on cold gas efficiency for the four temperature levels analyzed. It is observed that the ER of 0.3 shows a higher cold gas efficiency for all the temperatures studied. However, the increase in temperature in the operating range studied hardly showed improvements in the efficiency of cold gas, ranging, for example, from 22.67% (750 ◦ C) to 27.29% (900 ◦ C) for the 0.1 ER; from 56.59% (750 ◦ C) to 59.55% (900 ◦C) for the 0.3 ER; and from 46.16% (750 ◦C) to 53.38% (900 ◦C) for the 0.5 ER. Maintaining a high equivalence ratio during gasification has negative effects on cold gas efficiency. When there is an excessive amount of oxygen compared to the biomass, it can lead to over-oxidation of the fuel, resulting in a decrease in the overall energy conversion efficiency. Additionally, the high equivalence ratio can also lead to an increase in the formation of nitrogen compounds, which are harmful pollutants that contribute to air pollution and climate change [ 21 ]. Therefore, finding the right balance in the equivalence ratio is essential for maximizing cold gas efficiency and minimizing environmental impacts. 3.2. Influence of Temperature and ER on Gas Composition Figure 3shows the effect of temperature on gas composition for the different ER levels analyzed. Of the three components that provide energy to the gas, and which were possible to quantify, the most contributing is CO, followed by H 2 and CH 4 . The percentage of CO in the gas was in the range of 6.9–20.2 mol %, while H 2 presented a proportion of 2.3–8.6 mol %. For its part, CH 4 showed gas concentration variations between 1.7 mol % and 5.8 mol %. Regarding CO concentrations, higher temperatures generally increased the CO concentration due to the enhanced breakdown of olive cake and increased reactions involving CO formation due to the Boudouard reaction, which is favored at higher temperatures. On the other hand, the data do not reveal a significant relationship between changes in temperature and the concentrations of H 2 or CH 4 in the gas product. At an ER of 0.1, the concentrations of H 2 and CH 4 were relatively higher compared to ERs of 0.3 and 0.5 and exhibited less variation with changes in temperature. At an ER of 0.3, the concentrations of hydrogen (H 2 ) and methane (CH 4 ) demonstrated less consistent behavior across different temperatures, with distinct peaks observed. At an ER of 0.5, the H 2 and CH 4 concentrations were relatively lower than at ERs of 0.1 and 0.3 and showed less variation with temperature changes. The results are consistent with previous works [ 22 , 23 ]. The increase in CO concentration with the rise in temperature can be explained by the increase in the rate of both water gas and Boudouard reactions [ 24 ]. Regarding the slight change in CH 4 concentrations, it has been reported that this is mainly due to the simultaneous formation and consumption of CH 4 in exothermic reactions at low and high temperatures, respectively [25]. In relation to the molar percentage of N 2 , this was in the range 45.1–61.7 mol %, increasing as the ER used was raised, but it was practically constant with the temperature. A deep analysis suggests that both the H 2 and CH 4 percentages decrease as the ER increases. This is because increasing the ER introduces more air in the gasifier, improves the oxidation reaction rate more than reforming and cracking reactions [ 26 ], and eventually enhances the formation of more CO 2 and H 2 O. However, the percentage of CO increased as the ER increased from 0.1 to 0.3, and then it decreased when the ER was increased to 0.5. Then, lower ER values resulted in higher CH 4 and H 2 concentrations, but CO concentrations depended on the range of ER analyzed. This different effect on CO concentrations was also observed by Karatas et al. [ 27 ] in their experimental study of the gasification of waste tires with air in a bubbling fluidized bed gasifier. The results shown are also like those obtained Appl. Sci. 2024,14, 7282 8 of 13 by Arena [ 28 ], Lee et al. [ 29 ], and Niu et al. [ 19 ], in which a high ER turned into low yields of CH4, CO, and H2in gaseous products. Appl. Sci. 2024, 14, x FOR PEER REVIEW 8 of 13 750 ºC 800 ºC 850 ºC 900 ºC molar % 0 20 40 60 80 100 CO H2 CH4 CO 2 O2 N2 Temperature, ºC 700 750 800 850 900 950 % CO (molar) 6 8 10 12 14 16 18 20 22 ER 0.1 ER 0.3 ER 0.5 (a) ER 0.1 (b) CO 750 ºC 800 ºC 850 ºC 900 ºC molar % 0 20 40 60 80 100 CO H2 CH4 CO2 O2 N2 Temperature, ºC 700 750 800 850 900 950 % H2 (molar) 2 3 4 5 6 7 8 9 ER 0.1 ER 0.3 ER 0.5 (c) ER 0.3 (d) H2 750 ºC 800 ºC 850 ºC 900 ºC molar % 0 20 40 60 80 100 CO H2 CH4 CO 2 O2 N2 Temperature, ºC 700750800850900950 % CH 4 (molar) 1 2 3 4 5 6 7 ER 0.1 ER 0.3 ER 0.5 (e) ER 0.5 (f) CH4 Figure 3. Effect of temperature on the molar composition of the gas for the three levels of ER studied (blue is presented in (a) (800 °C and 900 °C) and also in (c) (900 °C)). In relation to the molar percentage of N2, this was in the range 45.1–61.7 mol %, increasing as the ER used was raised, but it was practically constant with the temperature. A deep analysis suggests that both the H2 and CH4 percentages decrease as the ER increases. This is because increasing the ER introduces more air in the gasifier, improves the oxidation reaction rate more than reforming and cracking reactions [26], and eventually enhances the formation of more CO2 and H2O. However, the percentage of CO increased as the ER increased from 0.1 to 0.3, and then it decreased when the ER was increased to 0.5. Then, lower ER values resulted in higher CH4 and H2 concentrations, but CO concentrations depended on the range of ER analyzed. This different effect on CO concentrations was also observed by Karatas et al. [27] in their experimental study of the gasification of Figure 3. Effect of temperature on the molar composition of the gas for the three levels of ER studied (blue is presented in (a) (800 ◦C and 900 ◦C) and also in (c) (900 ◦C)). 3.3. Influence of Temperature and ER on the Lower Heating Value of the Gas and the Gas Flow Table 2reports the lower heating value of the raw gas as well as the gas flow obtained (measured under normal conditions), depending on temperature and ER. It should be noted that in the calculation of the LHV, only the energy contents of carbon monoxide, hydrogen, and methane were considered since the rest of the fuel compounds, such as light hydrocarbons with two or more carbon atoms, were not quantified. Appl. Sci. 2024,14, 7282 9 of 13 Table 2. Effect of temperature on LHVgas and gas flow for the three ER levels studied. Temperature, ◦C ER LHVgas, MJ/Nm3Gas Flow (0 ◦C, 1 atm), L/min Gas Flow (0 ◦ C, 1 atm), L/kg Feed CGE, % 750 0.1 4.09 3.95 948.0 22.67 0.3 4.08 9.88 2371.2 56.59 0.5 2.21 14.89 3573.6 46.16 800 0.1 5.11 3.44 825.6 24.70 0.3 4.24 9.72 2332.8 57.83 0.5 2.50 13.89 3333.6 48.73 850 0.1 4.77 4.01 962.4 26.90 0.3 4.43 9.84 2361.6 61.19 0.5 2.79 13.39 3213.6 52.39 900 0.1 5.01 3.88 931.2 27.29 0.3 4.28 9.92 2380.8 59.55 0.5 2.89 13.15 3156.0 53.38 Furthermore, the LHV includes the effect of nitrogen dilution because the gas was generated by the operation of the gasifier using air. This made the LHV gas low, between 2.21 and 5.11 MJ/Nm 3 , compared to other gasification studies with other agents, in which LHV gas reached values of up to 11.3 MJ/m 3 , as in Saebea et al. [ 30 ]. Thus, for example, Puig-Arnavat et al. [ 31 ], Boerrigter [ 32 ], and Habibollahzade et al. [ 33 ] found LHV values of 3 to 6 MJ/m 3 and explained the nitrogen dilution in their works. More recently, Njuguna et al. [ 34 ] also obtained an LHV between 3.2 and 4.8 MJ/Nm 3 in their experimental investigation of the gasification of macadamia nutshells. It can be observed that, in general, by increasing the operating temperature, there is no significant increase in the LHV of the gas. However, Table 2shows that the ER affects the LHV of the product gas because LHV is dependent on the concentrations of CO, H 2 , and CH 4 , and H 2 concentrations decrease greatly as the ER becomes higher. At a higher ER, oxidation reactions are more favorable due to the increased availability of oxygen. This resulted in a higher production of CO2and a reduced concentration of combustible gases, ultimately causing a negative effect on the syngas higher heating value [ 34 ]. Instead, the gas flow was practically constant with the temperature, reaching values in the range of 3.44–14.89 L/min (825.6–3573.6 L/kg feed), and it varied substantially with the ER, with the highest value observed at 0.5 and the lowest value at 0.1. 3.4. Influence of Temperature and ER on the Production of CO, H2, and CH4 Table 3reports the CO, H 2 , and CH 4 production evaluated in g of CO, H 2 , or CH 4 per kg of olive cake. For the four selected temperatures, the production of CO, H 2 , and CH 4 was estimated to be higher for tests conducted with an ER of 0.3, with averages of 511.2 g of CO per kg of olive cake, 11.9 g of H 2 per kg, and 68.7 g of CH 4 per kg. Furthermore, the results confirm that the production values of CO, H 2 , and CH 4 obtained are like those of other researchers using comparable operating conditions [ 35 , 36 ]. Thus, for example, those studies found that the production of CO and H 2 decreased significantly as the ER increased. Furthermore, the production of CH 4 varied considerably, from 50 to 80 g/kg of biomass in the range of the operating conditions analyzed.