Comparison of moroccan argain nut shell and olive cake combustion to determine the best combustible for CHP system and for the thermodynamic cycle

Abstract

The current research aims to valorize Moroccan agricultural waste by using it as a combustible fuel. The physicochemical properties of argan cake were determined and the results were compared with other studies of argain nut shell and olive cake. A comparison of argain nuts shell, argain cake, and olive cake was carried out in order to determine the best combustible in terms of energy, emissions and thermal efficiency cycle. The CFD modeling of their combustion was presented using Ansys fluent software, The Reynolds averaged Navier-Stokes (RANS) method is the foundation of the numerical approach, which models the turbulent flow using realizable.

k-ε, A non-premixed combustion model is used for the gas phase, and a Lagrangian approach is chosen for the discrete second phase, there was a good agreement between the numerical and experimental data, as well as Wolfram Mathematica 13.1 is used to predict the mechanical work generated by the Stirling engine, and the result encourages the use of studied biomasses as combustible in order to generate heat and power.

1. Introduction

Nowadays, in the field of energy All communities share the goal of producing sustainable energy at the lowest feasible cost investment, maintenance [1] and having the minimum possible impact on the environment. From 2005 to 2050, the world plans to reduce CO2 emissions in half, hence many studies and research are aimed at reducing greenhouse CO2 emissions by introducing revolutionary technology [2]. Certainly, fossil fuels have a higher energy density [3], but their effects on the environment humans and animals force us to stop using them immediately and look for other combustibles. Agricultural residue is an alternative solution might be used to generate energy from a natural resource with low emission Using the thermochemical conversion process [4].

Morocco is one of the countries that encourages the use of renewable energy, as evidenced by the NOOR project, which uses solar energy in the south of the country, as well as eolian parks and hydroelectric dams, also it plans to produce 3900 MW of electricity from renewable resources by 2025, However, despite the vast amount of biomass available, the use of this natural resource for energy generation remains limited, with the exception of a few rural areas that use it for heating, as in the case of cold cities such as Ifran, or to prepare food in some rural areas, an other hand, the production of power from fossil fuels increases by 10% during the summer due to agricultural operations for pumping water required for irrigation and starting air conditioners, therefore gas emissions during this time of year can contribute to global warming, according to the most recent governmental statistical the production of argan nut increased from 3460 ton in 2012–5640 ton in 2019 and the Moroccan vision for the horizon 2030 is to plant 50,000 ha of Argan tree and 1.220.000 ha of olive tree by 2022 as part of the strategy 'Green generation, and actually, about 282,000 ton/year of Argan waste and 1.9 million ton/year waste are present in the area, also the olive tree surface area has increased from 720.000 ha in 2007 to more than 1.200.000 ha during the 2021–2022 campaign.

The increasing demand for energy and the growing trend of these agricultural resource makes it more intriguing to evaluate and valorize this natural resource [5]. Similar recent efforts, studies and experiments in this field attempted to valorize olive and Argan waste and use them as combustible fuels. In this area, in 2019 (Elorf et al., 2019) conducted a combustion experiment with olive cake inside a vertical furnace to assess the effect of particle size, air mass flow, and bulk density on the temperature, also (Rahib et al., 2019), presents a recent experimental study of argan nut shell (ANS) combustion in a lab-scale furnace, the highest heat value of the biomass measured was 23.2 Mj/kg, and the maximum temperature obtained during combustion was 1150 K, An-other study suggests that argan pulp can be used to produce bioethanol transforming natural resource to biogas [8], also Another experiment was carried out to investigate the possibility of using argan as a bio fuel, and it was discovered that the argan nut shell has a higher ignition temperature and higher thermal conductivity, the same study shows that pretreatment of biomass is an important step in improving the chemical energy of biomass [9], As a result, it has been demonstrated numerous times that argan can be used as a natural resource to generate energy.

From these encouraging results, the Producing heat and mechanical energy from biomass is an alternative solution by combustion process, as well as CHP systems that produce heat and power is an innovative and efficient new technology that is compatible with the use of agricultural waste as combustible [10]. Combining a biomass furnace and a Stirling engine is one option for building this kind of system [11], that could be seen as a feasible way to meet increased energy demand while reducing polluting emissions. The Stirling Engine is a closed-cycle heat engine that converts thermal energy to mechanical work, and it is known as external combustion engine and often used to generate mechanical work or electricity from the thermal energy of combustion gas [4], its main advantages for an industrial application are that it operates at external combustion, has minimal vibration and noise during operation, and has a low maintenance cost.

The recent tests confirmed that the combination of Stirling engine with biomass inside a furnace produces good results. For example, in 2021, KRAMENS studied the energy yielded from 1 kg of wood, and as a result, this quantity was able to create 2.23 kwh of thermal energy and 7.84 Wh of electricity, and the electrical power generated at 600 °C was 590 Wh and it was discovered that there was a direct relationship between the temperature and the electrical power generated [12], also, Chin-Hsiang Cheng (2021) developed a new CHPC system technique, combining a 500-w Stirling engine to produce mechanical power, water heating, and a cooler Stirling of 90-k for refrigeration applications, the overall efficiency has been approximately 91%, the system's heat source has been solar energy [13], but biomass might be another alternative combustible. Also Ferrera et al. (2020) simulate the performance of a stirling engine using two different types of energy, biomass and solar energy, and discover that when using biomass, the performance of the Stirling engine is 87.5%, but only 46.76% when using solar energy [14].

Consequently, this project is an application of a CHP system that uses biomass as a combustible at the external combustion process using a furnace and a Stirling Engine, and present an approach that integrates olive cake, argan nut shell, and argan cake (waste of argan fruit) to assess their combustion. The central topic focus to compare between the combustion of argain nut shell and olive cake to determine the biomass most powerful and most efficient for the thermodynamic cycle during their combustion. The cost of the engine is high and its efficiency is low compared to other types of engines, while the availability of biomass is restricted in some areas, these factors are the main barriers to the use of biomass with Stirling engines. Additionally, before being used in a stirling engine, biomass fuel may need to undergo additional conditioning procedures.

The first part of the studies is devoted to discover the possibility of argain cake to be seen as combustible and the result obtained will be compared with previous results of olive cake (Elorf et al., 2022) and argan nut shell [7]. The sample of argan cake will be prepared in the laboratory in order to discover their chemical structure with XRF and evaluate their HHV with a calorimeter. The simulation of combustion using Ansys Fluent 19.2 is the focus of the other portion of the studies.

The CFD modeling may aid in comprehension of the biomass combustion process [15], and provide insight into the combustion of biomass particles and the factors influencing temperature and emissions, to the best of our knowledge, no previous CFD studies have examined the combustion of argain nut shell and his impact on Stirling engine. Therefore, the combustion of the Argain nut shell will be simulated until the result converges, applying multiphase flow model and discret particle model. These results are then put through simulation to determine the temperature and emissions produced, generally, the CFD modeling of biomass combustion inside the furnace to describe their behavior produced a good result. A previous study was conducted in order to validate an olive cake experience through simulation using Ansys fluent, as well as a good agreement between experimental and numerical data [16], the argan nut shell will be simulated in the same manner and using the same validated model.

The contour of temperature and mass fraction CO2 as a result of the simulation can give an idea about the thermal energy and pollutant emissions of the used biomass and their effect on the simple Stirling engine, The results will also be compared to determine which biomass is most suitable for a CHP system and for the thermodynamic cycle. The idea was inspired by another comparative study of two types of biomasses that have about the same properties and combustion quality [17].

The use of argan nut shell, argan cake, or olive cake as combustibles for Stirling engines and generally for CHP has many benefits [10], including the creation of clean energy from agricultural waste, particularly in rural areas, the reduction of solid waste, and the contribution to the reduction of CO2 emissions. Additionally, this kind of combination with a Stirling engine can produce two types of energy (heat and power) from the same system [18], but another side the system has faces three distinct challenges, the first is an environmental issue related to recycling waste and ash emissions; the second is related to the lower high heat value (HHV) content of the biomass; and third is related to the availability through the year Furthermore, the olive and argan are only available between October and November.

The study aims to valorize Moroccan agricultural biomass and waste, with the end result encouraging the use of olive cake, argan nut shell, and the waste argain cake as combustibles, as well as combining these biomasses with a Stirling engine as an alternative solution for producing heat, mechanical work, and electricity.

2. Materials and methods

As depicted in Fig. 1, The schema is divided into three sections, the first of which is dedicated to the treatment of combustibles, including the bunker, the mill and the separator, the second to the furnace and its associated equipment, which provides combustion conditions such as the valve control that regulates the flow of mass injected and the pulverized that aids in pulverizing the biomass to a small particle before injecting it into the furnace, and the third is related to the management of combustion waste in order to protect the environment such as the cyclone and a filter before the chimney. The indicators are installed before and after each installation to indicate their performance, A delta T at the exchanger indicates the state of the thermal change, while a deta P indicates a blockage at the installation. The Bunker holds the Argan cake or other biomasses. It goes through the mill and the coarse separator which separates the heavy particles from the fine particles and then sucks the fine particles with a blower and sends them to a feeder. The flow regulator controls the amount of the combustible that is injected into the furnace. The adjustable compressor aspires ambient air, regulates the input flow, and sends it to the furnace via the exchanger to raise the temperature. In the circuit smoke, the combustion gases pass through a Stirling thermal engine, which converts the heat from the gases into electricity to power a motor, the gas then passes through the air/gas exchanger after passing through the heater to increase the temperature of the water. At the end of the cycle, the gas passes through a cyclone, which traps unburned and volatile particles and prevents them from being released into the atmosphere. Different parameters of the circuit can be checked using pressure gauges and thermometers connected to the installation, as well as an analyzer installed in the chimney in order to report the percentage of O2, CO, CO2, NO, NO2, and fly ash emissions to atmosphere. The temperature difference before and after the heat exchanger indicates clogging caused by combustion ash; the clogging automatically affects gas and air exchange, lowering the temperature of the exchanger outlet air heat. The proposed schematic includes installations that can use argan cake, argan nut shell and olive cake as combustible at the inlet fuel.

Fig. 1.

Proposed CHP system.

2.1. Sample preparation

Argan cake is a by-product of the pressing of Argan kernels; and it is regarded as an agricultural resource capable of being used as a biofuel; it is passed by three steps as illustrated at Fig. 1. 1 KG Argan nut yields 800 g of waste, accounting for 80% of the total mass. Argan cake storage is easy and does not require any special equipment; it can be stored at room temperature. In May 2022, a sample was collected from a local association in Essaouira City, prepared, and analyzed at a certified laboratory dried for 5 h at 101° Celsius, pulverized, and sieved through a 0.15 mm sieve before being collected for analysis and measurement.

2.2. Determination of chemical reaction and gas combustion behaviors

Generally, the chemical reaction of the biomass for a stochiometric combustion is illustrated as follow [9].

C_aH_bO_cN_dS_X+ a₁(3.76N₂+ O2) +K.H₂O → b₁CO + c₁H2O + d₁N2+ f1CO2+ g1CH4+ h1H2 + Ash (1)

where a. b, c, d: biomass components, K moisture content and a₁. b₁, c₁, d₁,f₁,g₁. h₁: molar coefficients that can be calculated using mass balance for C, H, N, O, When this formula is applied to olive cake and argan nut shell, the following equations are obtained.

• •olive cake chemical reaction gas phase [6].

C_1.10H_3.64O_0.79N_0.03+ 1.06O2 → 1.01CO + 1.82H₂O + 0.01N₂+ + Ash (2)

• •Argan nut shell chemical reaction gas phase [7].

C_0.88H_3.53O_0.96N_{0.02 +}0.84O₂→ 0.88 CO + 1.76 H2O + 0.01N2+ Ash (3)

also, the following chemical reaction describes the unburnt particles caused by a lack of oxygen.

CO+0.5O2→CO2 (4)

$$\mathrm{a}\mathrm{n}\mathrm{d}\text{CO}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2+41.1\text{Kj}/\text{mol}$$ (5)

in terms of energy, the standard enthalpy change is expressed as [19].

$$\Delta \mathrm{H}=\Delta {\mathrm{H}}_{\mathrm{P}\mathrm{R}\mathrm{O}\mathrm{D}\mathrm{U}\mathrm{C}\mathrm{T}}-\Delta {\mathrm{H}}_{\mathrm{R}\mathrm{E}\mathrm{A}\mathrm{C}\mathrm{T}\mathrm{A}\mathrm{N}\mathrm{T}}=-882\mathrm{K}\mathrm{j}/\mathrm{m}\mathrm{o}\mathrm{l}$$ (6)

The equations that govern two-dimensional fluid flow through an axisymmetric combustor are listed below [19].

• -continuity equation for steady state:

$$\nabla u\left(\mathrm{\rho }.{\mathrm{{\rm E}}}_{\mathrm{f}+}P\right)=\nabla \left[C_{\text{ef}}\nabla \mathrm{T}-\sum h_{\mathrm{s}.}\times {\mathrm{D}}_s+\left({\tau }_{\text{ef}}\times \mathrm{u}\right)\right]+{\mathrm{S}}_c$$ (7)

C_ef is the effective conductivity, Ds: diffusion flux of species s, The other term represents the sum of energy transfer due to conduction, species diffusion, and viscous dissipation. The heat of chemical reaction is included in Sc.

• -momentum equation:

$$\nabla \left(\mathrm{\rho }.\overrightarrow{\mathrm{\nu }}.\overrightarrow{\mathrm{\nu }}\right)+\mathrm{\rho }\left(\frac{\partial \overrightarrow{\nu }}{\partial t}\right)=\nabla \left(\mathrm{p}+\mathrm{\theta }\right)$$ (8)

• -combustion product energy equation:

$$\nabla \left(\mathrm{\rho }.\overrightarrow{\mathrm{\nu }}{Y}_i\right)+\frac{\partial }{\partial t}\left(\mathrm{\rho }.{\mathrm{Y}}_I\right)=-\nabla {\overrightarrow{J}}_i+{\mathrm{R}}_I+{\mathrm{S}}_I$$ (9)

where $\mathrm{Y}$ is the local mass fraction of the local species i, Where R_i is the total current rate of species production due to the chemical reaction. S_i signifies the presence of additional sources such as emissions, soot, or particulate matter.

• -DPM approach:

By integrating the force balance on the particle, which is expressed in a Lagrangian reference frame, ANSYS FLUENT can predict the trajectory of a discrete phase particle [20].

$$\frac{d{\overrightarrow{u}}_p}{dt}=F_d\left(\overrightarrow{u}+{\overrightarrow{u}}_p\right)+\frac{\overrightarrow{k}\left({\rho }_p-\rho \right)}{{\rho }_p}$$ (10)

$F_d\left(\overrightarrow{u}+{\overrightarrow{u}}_p\right)$ is the drag force per unit particle mass.

2.3. Combining stirling engine and biomass to build CHP system

As illustrated at Fig. 3, It is suggested that the Stirling engine be coupled inside the furnace and installed on the furnace's lateral to convert the thermal energy of gas combustion to mechanical work or electricity. Other studies have been conducted to evaluate the solution's feasibility, and positive results have been discovered [11], with the added benefit of burning biomass fuel and managing energy in a sustainable manner. The Stirling engine, as a thermal engine, is recommended for use in the design of a CHP system to generate both heat and mechanical energy (work). The engine heater is exposed to the combustion chamber gas, and the mechanic work or electrical energy generated is proportional to the temperature of the gas, increasing the power generated as well as the engine's efficiency. Sadi Carnot discovered an ideal cycle around 1824, which states that the exchanges of the hot source towards the cold source allow mechanical work to be performed. The ideal Stirling cycle has four processes: an external heat reservoir raises the working fluid temperature in an isochoric process, the piston then undergoes an isothermal expansion process, and the cycle concludes with an isothermal contraction process. In the second isochoric process, the cooled down working fluid undergoes isothermal compression and the cycle ends after exchanging heat with the regenerator and rejecting heat to the cold part. The power output of Stirling engine can be calculated as [18].

$$W_{\text{SE}}={\mathrm{\eta }}_{\text{pc}}\left(Q_{\text{High}}-{\mathrm{Q}}_{\text{rej}}\right)$$ (11)

where ${\eta }_{\text{pc}}$ polytropic efficient, $Q_{\text{high}}$ is heat gas absorbed by the Stirling Engine and $Q_{\text{rej}}$ is rejected heat.

Fig. 3.

Olive cake and argan nut shell ultimate analysis comparison.

The heat transfer rate at the heater and the exchanger [21].

$$\frac{\text{dQ}}{\text{dt}}=\frac{\mathrm{d}{\mathrm{m}}_{\text{hotgas}}}{\text{dt}}\left(h_{\text{inlet}}-{\mathrm{h}}_{\text{outlet}}\right)=\frac{\mathrm{d}{\mathrm{m}}_{\text{hotgas}}}{\text{dt}}{C}_p\Delta \mathrm{T}$$ (12)

as depicted in Fig. 1, Before the gas reaches the chimney, it will also pass through the water heater and the exchanger, which will reduce his energy. Generally, the efficiency of exchanger heat achieves 45%.

When the gas expands in the cylinder of Stirling engine from volume v₁ to volume v₂, the pressure of the gas changes as the relation $\text{PV}=\text{nRT}$ and this variation results in a mechanic work as illustrated in the following formula as

$$\mathrm{W}=\underset{\mathrm{V}1}{\overset{\mathrm{V}2}{\int }}\mathrm{P}.\mathrm{d}\mathrm{V}$$ (13)

The total work is the sum of the two works generated during the expansion and compression phases [22].

$$W_{\text{tot}}={\mathrm{W}}_{\exp }+{\mathrm{W}}_{\text{comp}}=\oint P_{\exp }.\mathrm{d}{\mathrm{V}}_{\exp }+{\oint P_{\text{comp}}.\text{dV}}_{\text{comp}}$$ (14)

The analysis of energy and exergy can be expressed as follows [23]:

$$\mathrm{Q}+\sum m_{\text{in}}{h}_{\text{in}}=\mathrm{W}+\sum m_{\text{out}}{h}_{\text{out}}$$ (15)

$$\mathrm{\eta }=\frac{W_{\text{ou}}}{Q_{\text{in}}}$$ (16)

Stirling engine efficiency is given as where Qin is the amount of heat absorbed at the heater, and W_ou is the work provided at the outlet. The ideal efficiency of the Stirling engine is the Carnot efficiency is expressed as [25].

$${\eta }_{\mathrm{c}\mathrm{a}\mathrm{r}}=1-\frac{T_2}{T_1}$$ (17)

where T₁ and T₂ are the temperature successively in the heater and the cooler.

The efficiency of the cycle is given as the product between the efficiency of the furnace ɳ (furnace) and Stirling ɳ (Stirling engine) as expressed at the formula [24].

$${\eta }_{\text{cycle}}={\mathrm{\eta }}_{\text{Stirling}}\times {\mathrm{\eta }}_{\text{furnace}}=\frac{W_{\text{out}}}{Q_{\text{in}}}\times \frac{Q_2}{Q_1}$$ (18)

where W_out is the mechanical work given by the piston of Stirling engine, Q_in is the amount of heat absorbed by the heater, Q₁ is the amount of heat storage at the biomass and Q₂ is the heat generated by the furnace.

The electrical efficiency is a performance indicator used to analyze a system, it is the product of the efficiency furnace ɳ_furnace and S.E efficiency S_tirling. The thermal input Q_He of S.E and thermal energy Q_fuel generated by fuel is also necessary to be taken into account [25].

ɳ_electric = ɳ_furnace x ɳ_Stirling = Q_He/Q_fuel x P_ele/Q_He (19)

resulting in

ɳ_electric = P_electric/Q_fuel (20)

CHP system cogenerate heat and power, ɳ_f include the global output power, while Q_ther thermal energy the heater of S.E, and Q_c at the cold part [25].

ɳ_f= (P_ele + Q_the)/ Q_fuel = (P_elec + Q_c) / (m_fuel + H_l) (21)

the rate ɳ_f is usually exceed 80% for small CHP system. Efficiency greater than 90% is possible, moreover it need higher specific investment costs [26].

2.4. Statistical analysis

This project's objective is to contrast the olive cake and argan nut shells combustion in order to evaluate how they burn. The combustion of the two biomasses has been the subject of previous experiments (Rahib et al., 2019a), and (Elorf et al., 2022), but these results were insufficient for determining how the temperature, emissions, and other factors varied throughout the furnace. As shown in Table 1, Table 3, the biomasses were caracreized in terms of ultimate and proximate analysis and energy content (HHV). The temperature of the flue gas from the two experiments has been measured using thermocouples installed at various levels of the furnace and a gas analyzer installed at the furnace's output to measure the gas emissions, It equipped with an infrared bench for CO2 and CH4 measurements as well as an electrochemical cell to measure O2, CO, NO, NO2, and SO2.

Table 1.

Quantity of biomasses produced by year 2019 and 2021, their specifics and their equivalent on energy.

Parameter	Biomass amount (ton/year)		Biomass specific		Equivalent energy (Mwh)
	2019	2021	HHV(Mj/kg)	%Moisture (As received)	2019	2021
Argan nut shell	15,800	17,600	23.5	10.5%	103	114.8
Olive cake	27,500	30,700**	21.5	6.5%	164	183.4
Argan cake	560	780	16.8	8.7%	2.6	3.6
Total	43,860	49,080	20.6*	8.56%*	269.6	301.8

* Average value.

Table 3.

Comparison of the three biomasses between the Results of ultimate and proximate analysis and density.

Parameter	Max temperature(K)			CO2 emissions (mass fraction)			Thermodynamic cycle
	experimental	Simulation	Error	experimental	simulation	Error	work (j/cycle)
Argan nut shell	1270	1155	−11.5%	0.32	0.16	50%	145
Olive cake	1380	1473	9.3%	*	0.59	*-*	160
Argan cake	*	1130	*	*	0.43	*	136

* Average value.

** The three-biomass mixture is supposed to be homogeneous and contain the same amount of each biomass.

To contrast the simulations and the experiment, this study only simulates the temperature and CO2 emission of the argan nut shell. The results of this simulation are then compared with those from earlier simulation studies of olive cake (AlShwawra and Asfar 2018). AnSYS Fluent 19.2 was used for all calculations in order to simulate the combustion of the biomasses inside the fluidized bed. The models were validated by simulating the species transport model of olive cake and argan nut shell combustion in the presence of oxygen and comparing the results to Elorf et al. [6] and Rahib [27]. The transport equations are solved using the finite volume method in this simulation program. The pressure-based Navier-Stokes algorithm is used to compute a solution. The SIMPLE algorithm handles the pressure-velocity coupling, and the flow is considered STEADY, incompressible, and turbulent. On the other hand the use of the radiation model is prompted by the radiation that the flame emits, the P1 model was used to resolve these issues because of the high optical layer combustion thickness and the radiation and absorption between the furnace walls and the gaseous molecules. While the discrete second phase is modeled using a Lagrangian method. For turbulence, a workable k-epsilon model was selected.. Because the air inlet and the fuel inlet are separate, and the fuel and oxidizer do not combine upon entering the inner space of the chamber, the reaction in the current simulation is categorized as non-premixed for the gas phase. A 20 mm-diameter tube is used to inject the biomass. The primary and secondary air temperatures are 300 K at the inlet. As a result, an unburned particle and an ash are created. The two-step process is activated to simulate the soot. Fig. 2a depicts the furnace in three dimensions, while Fig. 2b depicts it in two dimensions, taking advantage of the furnace's symmetry to hasten and facilitate the convergence of the results. A 750-mm height, 450-mm length, and 124-mm width are the dimensions of the furnace. The numerical model was used in the study while maintaining the same geometry and the same combustion conditions. The furnace's geometry has no impact on combustion, so it can be used in simulations without affecting the results of temperature distribution and gas emissions. The heater of the Stirling engine is housed in the furnace's cylinder.

Fig. 2.

3d(a), 2d (b) geometry of the furnace and median plan meshing(c).

To produce as many "hexahedral" elements as possible, the assembly meshing with Cutcell strategy was chosen. The higher resolution air inlet, particulate inlet, and outlet components total 851,786 and 728,397 hexahedral elements, respectively. The problem converged after mesh refinement and average inlet temperature changes. In order to correct this, zones near the inlet were repeatedly refined using the edge sizing method until the temperature stabilization value was reached.

Rahib identified the argain nut shell's chemical characteristics (Rahib et al., 2019a). Secondary air moves at 0.35 g/s and primary air at 0.7 g/s in mass flow, respectively. Argan nut shell particles have a speed of 0.2 g/s. Comparisons between the numerical and experimental results were used to validate the used numerical model (N. Rassai et al., 2018). the results were more comparable between simulation and experience, with a 9.3% difference between the first comparison and a 11.5% difference between the second (argan nut shell combustion).

In addition, A simple Stirling engine is modeled using Wolfram Mathematica software to determine the difference transformation phase as well as the cycle's pressure, temperature, and energy utilizing the commercial code [28]. The presented model is used only to assess the energy extracted from the used biomasses; it does not provide precise and in-depth results. Although the information provided by the commercial code is not entirely accurate, it does provide significant information and approximations that can help the user get a sense of how much mechanical energy can be extracted from biomass based on the temperature at which the biomass is burned at its highest.

Fig. 6, Fig. 7 depict a Stirling engine model. In these figures, the heater is seen in contact with the combustion gas. Only at their highest temperature are the two biomasses, olive cake and argan nut shell, taken into consideration. Depending on the type of combustible, the maximum temperature can range between 882 °C for an argan nut shell and 1200 °C for an olive cake. The working gas is assumed to be ideal, there are no losses within the cycle, isothermal compression and expansion, perfect heat exchange, and regeneration are all assumed as a result of these conditions, and the work energy of gas for each cycle will be determined, ranging from 160 j/cycle for olive cake to 145 j/cycle for argan nut shell. These assumptions are being made to facilitate the studies.

Fig. 6.

The thermodynamic cycle obtained for the different transformations when coupling the Stirling engine to olive cake combustion [29].

Fig. 7.

The thermodynamic cycle obtained for the different transformations, when coupling the Stirling engine to argan nut shell combustion [27].

3. Results and discussions

The modeling of thermal and mechanical energy extracted from biomass and Stirling engine during the combustion process are the focus of previous section. This section is a comparison of the three biomasses, about calculating the energy of olive cake, argan cake and argan nut shell combustion, as well as validating the two combustion experiments and evaluating their temperatures and emissions and quantity of olive cake, argan nut shell and argan cake available in Morocco in the years 2019 and 2021 from official statistics, and their characteristics such as high heat value, moisture as received and density. (Table 1), (Table 2) of this section compares the maximum temperature and emissions yielded during the combustion of the three biomasses and defines the error between the simulation result and the experiment, as well as the mechanical work generated for all cases. The ultimate and proximate analyses, HHV, maximum temperature produced during combustion, and density of olive cake and argan nut shell particles are shown in (Table 3).

Table 2.

Comparison between the max temperature and CO2 emitted during the combustion of the three biomasses, the work generated and the efficiency of the thermodynamic cycle.

Parameter	Ultimate analysis				Proximate analysis				Density (Kg/m3)
	C%	H%	N%	O%	Ash%	VM%	FC%	MC%	Average
Argan nut shell [9]	51.3	6.32	0.005	42.34	1.5	67.5	21.5	9.5	321
Olive cake (Elorf et al., 2019)	59	8.5	1.5	31	6.5	64	23.2	6.3	1260
Argain cake	49	5.5	0.5	45	8	60	26.5	5.5	1130
Biomass mixture	53.1	6.8	0.6	39.5	5.33	63.84	23.73	7.1	904

3.1. Average comparison

The presence of a high percentage of carbon and hydrogen raises the combustible's HHV, which then raises the temperature and thermal energy, while oxygen lowers it (Table 3). The presence of nitrogen has a negative impact on pollutant emissions because NOX has a negative impact on human health and the environment. The presence of sulfur can improve the temperature and give energy during combustion but has a negative impact on the plant because it is a corrosive element. After seeing the results shown at Table 3, one can conclude that Argan nut shell is preferable to olive cake for reducing slag formation after combustion because it contains less ash. However, because they contain a higher amount of volatile matter, it is not recommended to avoid the formation of ash fly. However, it is useful for other industrial cemeteries applications, also the argan nut shell contains more moisture than the olive cake, which can have a negative impact on combustion and burn more fuel to dewater the H2O particles, the lower fixed carbon values of argan nut shell indicate their easiness of combustion and ignition.

3.1.1. Effects of C, H, N, O, S and density on HHV biomass

as illustrated in Fig. 3 Argan nut shell yields 11.49% less co2 than olive cake. Olive cake yields 13% more H2O than argan nut shell. Also, olive cake yields three time No emissions more than argan nut shell and no emissions suffer for the three biomasses, because suffer traces are not detected during elemental analyses, and this is a good advantage because suffer is a corrosive element that can destroy the iron-containing plant.

3.1.2. Effect of pollutant content on combustion emissions

Because samples high in volatiles are more likely to spontaneously combust, volatile matter is a serious safety and health concern. The volatile matter content of an argan nut shell is 67.5%, compared to 64% for an olive cake. Argan nut shell will be more reactive and produce less char because it contains more volatile matter. Also, By deducting the percentages of moisture, volatile matter, and ash from the sample's initial mass, the fixed carbon content is calculated; a lower value indicates that ignition and combustion are easier, and argan nut shell has the lowest value of the three biomasses by 21.5%.

During combustion, ash content is responsible for the formation of slag inside the furnace. The biomass contains the most ash, which is the most pollutant of combustion, and it is the case of argain cake by 8%, while argan nut shell is the biomass the most clean that contains only 1.5%.

3.1.3. Effect of moisture content on HHV

The HHV is the main parameter indicating a combustible's thermal energy, and it is reduced by high moisture or vice versa because it slows the burning rate. As shown in Table 2, Table 3, the HHV of the three biomass ranges from 16.8 Mj/kg to 23.2 Mj/kg and can be improved by drying to reduce moisture content. And their pretreatment before to incorporation into the combustion process can also improve their efficacy.

3.1.4. Effects of temperature on energy

Table 2 shows that the mechanical work generated by the Stirling thermal engine had a significant impact on the temperature generated by the combustion gas. In the case of olive cake combustion, the work generated was 160 j/cycle, whereas only 145 j/cycle for argan nut shell, correspond to 1473 K and 1155 K on temperature respectively, and the work generated can be converted to electrical energy by coupling an alternator in the output of the Stirling engine, and the electricity generated will also be impacted the temperature combustion in this case.

3.1.5. Comparison CO2 emissions contour of argan nut shell and olive cake

Fig. 5 depicts the CO2 gas combustion concentration of the argan nut and olive cake, with the maximum value of 0.16 for the argan nut and 0.59 for the olive cake. The maximum values were near the burnout location. At the furnace exhaust, the Co2 is reduced to 0.065 and 0.32 for argan nut and olive cake, respectively. And the min values were at the lower point of the furnace in both cases, explaining why the co2 particles followed the trajectories from the furnace to the atmosphere. For the two biomasses, the CO2 concentration is concentrated at the bed floor, which is also where the density of the combustion flame and the highest temperature values are concentrated.

Fig. 5.

Co2 contour of argan nut A) and olive cake B) [29].

3.1.6. Comparison temperature contour of argan nut and olive cake

Fig. 4 depict the temperature distribution of gas combustion inside the furnace and show the simulation results of argan nut and olive cake combustion contour temperature. The maximum temperature for olive cake combustion during the experiment was 1380k and 1270k for argan nut shell, but the maximum temperature for olive cake combustion during the simulation was 1473k and 1155 K for argan nut. Cendre is principally concentrated in this area where must be strength mechanically and thermally. In both cases, there was good agreement between the simulation and the experiment, with a 9.3% error between experiment and simulation in the first case and a −11.5% error in the second.

Fig. 4.

Temperature contour through into the furnace of argan nut shell A) and olive cake B).

3.1.7. Comparison energy cycle for olive cake and argan nut shell combustion

The Stirling engine works by transforming a working gas (hydrogen) several times as mentioned in Fig. 6, Fig. 7 and operates on a thermodynamic cycle that converts heat into mechanical work by taking benefit of a temperature difference between his heater and the cold part of the engine. The simulation is carried out with the Wolfram Mathematica software, with the parameter compression ratio set to 1.64, the maximum volume set to 1600 L, and the cold end temperature set to 29° Celsius, the time varied to determine the phase of the thermal cycle. The maximum temperature is adjustable based on the type of combustible, 882 °C for argan nut shell and 1200 °C for olive cake. To facilitate the studies, the following assumptions are being considered, the working gas is assumed to be ideal, there are no losses within the cycle, isothermal compression and expansion, perfect heat exchange, and regeneration are all assumed as a result of these conditions, the work energy of gas for each cycle were determined, 160j/cycle for olive cake and 145j/cycle for argan nut shell, and with the same mass of biomass, olive cake is more energetic than argan nut shell at the combustion process, and this power can be used especially for agricultural purposes, such as pumping water for irrigation, which will reduce the amount of electricity consumed for this mission. A similar analysis study is carried out to examine the effect of a Stirling engine on a system, but in the case of gasification biomass rather than combustion processes [30].

4. Conclusion

The study aims to valorize Moroccan agricultural biomass and waste and help to promote regional development, with the end result encouraging the use of olive cake, argan nut shell as combustibles, as well as the combustion of the two biomasses was investigated in a fixed bed combustor. From this study the following conclusions were obtained.

• •The CFD modeling of the combustion of the argan nut shell and olive cake inside the furnace to describe their behavior produced a good result; the realizable K- was selected for turbulence, non-premixed combustion was used to model the gas phase, and the DPM model was used to model the particle phase. The CFD results were validated with compared the simulation results and literature, and it is shown that the CFD modeling is a powerful tool to predict the results and treat the problem related to biomass combustion. The contour of temperature and mass fraction CO2 can give an idea about the thermal energy and pollutant emissions of the two biomasses.
• •In this study, the thermo-physical properties of agricultural wastes from argan fruit and olive during combustion were investigated. The suitability of Moroccan waste argan nut shell and olive cake for use in applications was also determined. According to a comparison of the emission properties and temperature during combustion, Olive is more suitable for use as a combustible for thermo-chemical conversion in energy production and for CHP systems because it generates higher temperatures, is more efficient for the thermodynamic cycle, and emits less CO2 while still being environmentally friendly.
• •The three biomasses have a similar physicochemical property; the difference is in the proportion of their elements; thus, their combustion results in the same ignition, energy, and emission. A possible alternative for creating a new biomass combustible is to mix them.
• •Ansys Fluent simulation may help to prevent some problems related to depositing ash in Stirling engines, which can lower the cost of maintenance. Ash management is a crucial consideration when studying combustion parameter and has a significant impact on emissions, operating plant efficiency, and recycling issues, additionally, combining these biomasses with a Stirling engine as an alternative solution for producing heat, mechanical work, and electricity, and this power can be used specifically for agricultural purposes, such as pumping water for irrigation, which can reduce the amount of electricity consumed for this mission
• •The energy produced from the biomass mentioned is still insignificant compared to energy from fossil fuels, but using this agricultural waste reduces CO2 emissions by 0.2 g for every kilowatt-hour produced from fossil fuels.
• •The need for a consistent and high-quality biomass fuel is the main barrier to using argan nut shell and olive cake in a Stirling engine. Additionally, the Stirling engine may need additional installation in order to accept the biomass fuel. This may increase the system's cost and complexity.

Author contribution statement

Ayoub Najah ELidrissi: Conceived and designed the experiment, Contributed reagents, materials, analysis tools or data; Analyzed and interpreted the data; Wrote the paper.

Mohammed Benbrahim: Contributed reagents, materials, analysis tools or data; Wrote the paper.

Nadia Rassai: Performed the experiments, contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

No data was used for the research described in the article.

Declaration of interest’s statement

The authors declare no competing interests.

Nomenclature

m_s

mass sample (kg)

h_o

output enthalpy (kJ/kg)

H_i

input enthalpy (kJ/kg)

m^˙_da

mass of dry air (kg/s)

ρ_b

biomass density (kg/m3)

vf

velocity of the fluid (m/s)

^A_dc

^{heat transfer surface area (m2)}

Ta

ambient temperature (^◦ C)

CHP

Combined heat and power

T

ambient temperature (^◦ C)

HHV

high heat value (kJ/kg)

W

work (kj/cycle)

CFD

Computational fluid dynamics

M_C

moisture content (%)

VM

Volatile matter (%)

FC

fixed carbon (%)

C

Carbon

H

Hydrogen

N

Nitrogen

S.E

Stirling Engine.

$\eta$

^efficiency

Q_high

thermal flux high temperature (kj)

Q_rej

thermal flux rejected (Kj)

L

length (m)

K

thermal conductivity coefficient (w/m^◦ C)