Gasification of Chlorella vulgaris for Syngas Production and Energy Generation Through Gas Turbine

Abstract

The increasing need for sustainable energy sources has driven research toward innovative solutions, including biomass gasification for syngas production, with applications in the chemical industry and energy generation. This study explores the application of Chlorella vulgaris in the gasification process to produce syngas intended for gas turbine operation. Using Aspen Plus V11 (academic version) simulations, the study evaluates optimal process conditions and syngas yields, focusing on operational parameters such as the S/B ratio and gasifier temperature. Results show that a higher S/B ratio increases H₂ and CO₂ concentrations while reducing CO and CH₄, with final syngas composition in dry conditions reaching 0.42 CO, 0.52 H₂, and 0.036 H₂O. Contaminants like H₂S and HCl were effectively reduced below critical thresholds, with H₂S levels under 20 ppm and HCl under 1 ppm to meet GT requirements. The system achieved a cold gas efficiency of 55% and an overall turbine cycle efficiency of 25%, with CO₂ emissions of 0.198 kg per kWh produced. In conclusion, the gasification of C. vulgaris offers a promising and sustainable solution for syngas production and energy generation, with reduced environmental impacts. However, economic feasibility and certain technical challenges will require further advancements to fully realize this technology’s potential.

1. Introduction

Feedstocks derived from algae, such as biomass from microalgae, are considered among the best sources of biofuels due to their rapid growth, ease of harvesting, and the fact that they do not require cultivated land. Algae cultivation can be carried out not only in open waters but also inside wastewater networks, allowing algae to serve as a foundation for bioenergy production without impacting other industrial sectors. Typically, the composition of biomass from algae differs from that of traditional biomass, given the high content of lipids and proteins that can be transformed into chemical compounds with high energy value.

The choice to use biomass gasification [1,2], particularly focusing on the Chlorella vulgaris species, for syngas production is motivated by several key benefits. Algae gasification offers a sustainable and environmentally favorable solution. Algae, such as C. vulgaris, grow rapidly, providing a renewable biomass source that is easy to cultivate. Additionally, C. vulgaris absorbs CO₂ during its growth cycle, helping to reduce GHG emissions. Its adaptability to various environments and frequent harvestability further enhance process efficiency [3].

The conversion of biofuels derived from algae—and, in general, third-generation biofuels—into energy can be classified based on three approaches: chemical, biochemical, and thermochemical [4,5]. The choice of the process is influenced by various factors, including feedstock availability, desired products, and overall production costs. The thermochemical approach is preferred as the process is both simple and efficient, and there is no need for separation and purification steps before the actual process. Additionally, some thermochemical processes do not require drying of algae biomass, reducing costs associated with water removal. This method thermally decomposes the organic compounds found in the feedstock, thus producing combustible products through direct combustion, liquefaction, pyrolysis, and gasification processes.

Combustion is not preferred due to its low efficiency, ranging between 20% and 40%, and because only a minimal portion of the biomass, approximately 15%, actually undergoes combustion. On the contrary, gasification is preferred for its versatility in obtaining products: it involves the partial oxidation of a carbonaceous feedstock, which is converted into a mixture of combustible gases or syngas, containing H₂, CO, CO₂, CH₄, H₂O, N₂, NH₃, H₂S, and HCl. In recent years, the gasification process has become a subject of great interest, as the gas obtained from it can be used in various applications based on its characteristics: it can be burned directly in fuel furnaces or GTs, or be utilized in the production of value-added chemical compounds. The main processes involved in gasification include the WGS reaction, as well as the Bouduard, methanation, and SMR reactions. These occur between 800 °C and 1000 °C [6] with the controlled introduction of gasifying agents such as air, steam, and oxygen.

The gasification of C. vulgaris yields a syngas rich in hydrogen and carbon monoxide, providing a versatile energy source for electricity generation or as a raw material for the synthesis of fuels and chemicals. This choice aims not only to diversify energy sources but also to contribute to climate change mitigation efforts by capturing CO₂ during the algae’s growth cycle. Ultimately, C. vulgaris gasification represents an innovative and sustainable solution for syngas production, offering significant environmental and energy-related benefits [7,8,9,10].

In this study, a detailed steady-state equilibrium simulation has been developed using Aspen Plus, in order to analyze and evaluate the efficiency of the gasification process of C. vulgaris. A downstream purification process was added to remove HCl through adsorption on ZnO and eliminate H₂S using NaHCO₃ as an adsorbent material. The effects of various operational parameters—such as gasification temperature [11] within the ranges of 700 °C, 800 °C, and 1000 °C, as well as the equivalence ratio—on syngas composition were examined using sensitivity analysis. From the analysis, it is evident that the concentration of H₂ and CO₂ increases with the increase in the S/B ratio, while the concentration of CO and CH₄ decreases. An increase in steam promotes WGS and SMR, leading to an increase in the concentration of H₂ and CO₂, in line with available literature data. The effect of temperature on H₂S and HCl removal has also been investigated. In the case of H₂S, an increase in temperature hinders adsorption, reaching a steady state at 700 °C. In the case of HCl, it could be possible to increase temperature; however, it is observed that even at 600 °C, a threshold value in terms of ppm of HCl is maintained below the desired level. Therefore, the reactor is operated at this temperature [12].

Attention has also been given to the impact of moisture on syngas composition. Dry syngas tends to have higher concentrations of desirable components such as CO and H₂, as the presence of water does not dilute the mixture. In contrast, wet syngas contains higher levels of CH₄ and CO₂. In terms of heating value, a noticeable difference exists between dry and wet syngas. Dry syngas has a higher heating value due to its greater concentration of combustible gases, while wet syngas exhibits lower values due to the presence of water.

Unlike previous studies, this paper provides a comprehensive simulation of C. vulgaris gasification integrated with a purification system optimized for gas turbine use, a novel approach not yet fully addressed in the literature. Additionally, this study examines the effects of varying process parameters on both contaminant removal and syngas composition, offering new insights into optimizing algae-based syngas for low-emission energy applications.

2. Microalgae Production and Potential in Europe

Microalgae generally follow a growth cycle characterized by five main phases:

• Lag phase: the initial period during which the cells adapt to the new environment, synthesizing the enzymes necessary for metabolism;
• Growth-acceleration phase: cells begin to multiply rapidly, with a progressive increase in growth rate;
• Exponential growth phase: the growth reaches the maximum specific rate and remains constant until the end of this phase;
• Stationary phase: growth slows or halts due to nutrient depletion or the accumulation of metabolites;
• Death phase: the number of live cells decreases due to nutrient scarcity or autolysis phenomena.

The study of Jacinto et al. [12] monitored the growth phases of Pediastrum boryanum and Desmodesmus subspicatus in different culture media. During these phases, PB reached a maximum growth of approximately $51\times {10}^5$ cells ${\mathrm{mL}}^{-1}$ on the seventh day, while DS reached approximately $52\times {10}^5$ cells ${\mathrm{mL}}^{-1}$ on the eleventh day, with a more prolonged stationary phase and no final decline.

In comparison, Chiranjeevi and Venkata [13] evaluated the productivity of C. vulgaris and Scenedesmus obliquus cultivated in fish viscera effluents and domestic wastes. C. vulgaris showed good overall yields, while Scenedesmus obliquus achieved cell growth of up to $120\times {10}^5$ cells ${\mathrm{mL}}^{-1}$ in viscera effluents and $250\times {10}^5$ cells ${\mathrm{mL}}^{-1}$ in domestic wastes, demonstrating high adaptability and efficient nutrient utilization in effluents.

In recent years, microalgae production in Europe, including Italy, has been growing, although it remains relatively small compared to more established biomass sources. However, the sector is gaining increasing attention due to its potential in sustainability, especially for biofuels, animal feed, cosmetics, and nutraceuticals.

In the European Union, microalgae production is expanding but still lags behind other major global producers. According to recent estimates, the EU produces approximately 1500–2000 tons of dry microalgae biomass annually. The leading countries for production within the EU are France, Spain, Portugal, and Italy, thanks to favorable climates, especially in the southern regions. Despite this, production in Europe is still modest compared to other agricultural biomass sources. Italy, in particular, is known for cultivating Spirulina and Chlorella, which are primarily used in food supplements and biofuel research [14]. In Italy, microalgae production is concentrated in regions such as Sardinia and Sicily, where the warm, sunny climate is ideal for algae cultivation. While specific annual production numbers for Italy are not well-documented, Italian companies are recognized for cultivating Spirulina and Chlorella, species highly valued for their nutritional benefits and use in biofuels [15]. Italy’s microalgae industry is focusing on nutraceuticals and renewable energy, although the scale of production remains limited compared to other regions.

Asia is the global leader in microalgae production, with China alone accounting for over 70% of global microalgae biomass. China produces an estimated 15,000–20,000 tons of dry microalgae annually, primarily for nutraceuticals and biofuels. Countries like Japan and Taiwan are also involved, but China dominates the market [16].

The United States is another significant player in the microalgae sector, with an estimated annual production of 5000–7000 tons. While the primary market is for nutraceuticals, there is increasing interest in biofuels and bioplastics. The U.S. government supports this industry through initiatives such as the Bioenergy Technologies Office of the Department of Energy, which funds research into microalgae-based biofuels [17].

In Brazil, microalgae production is smaller than in Asia or the U.S., with an estimated annual output of 500–1000 tons. However, Brazil has initiated significant projects for algae cultivation, especially to support its biodiesel industry and carbon sequestration efforts. The country’s established biodiesel infrastructure makes algae-based biofuels an attractive option [18].

The European microalgae sector faces several challenges, including high production costs, a lack of adequate infrastructure, and climatic constraints. Algae cultivation is more feasible in southern European countries like Italy, where warmer climates prevail. Despite these challenges, the European Union is investing in policies to support bio-based industries, including microalgae, through programs like Horizon Europe and the backing of the European Algae Biomass Association (EABA), which promotes innovation and research in algae cultivation and processing [19].

While the microalgae industry in Europe, including Italy, is still in a development phase, it holds significant potential for growth, particularly in biofuels, food supplements, and environmental applications. With ongoing research, policy support, and investments in infrastructure, Europe is well-positioned to increase its production of microalgae in the coming years, closing the gap with global leaders like China and the United States.

3. Materials and Methods

C. vulgaris is just one of the numerous species belonging to the genus Chlorella, appreciated for its versatility. The main properties of C. vulgaris are shown in

Table 1. Ultimate Analysis.

Name	Composition (wt%)
Carbon	56.18
Hydrogen	7.37
Nitrogen	1.78
Chlorine	0.33
Sulfur	0.64
Oxygen	33.70

and

Table 2. Proximate Analysis.

Name	Composition (wt%)
Moisture	10.63
FC	27.23
VM	55.00
Ash	7.14

[20].

As suggested in the literature, the Peng-Robinson-Boston-Mathias modelling approach is employed for the base method [21]. The Peng-Robinson thermodynamic model is widely used for industrial processes. It is an equation of state that describes the behavior of fluids, particularly for simulating gas and liquid phases, designing separation units, and modeling chemical reactions in industrial processes. The equation is commonly employed to estimate thermodynamic properties of fluids, such as pressure, temperature, and volume, in different process environments. The expression for the PR-BM equation of state is as follows:

$$P={\displaystyle \frac{RT}{v-b}}-{\displaystyle \frac{a\left(T\right)}{\left(v\right(v+b)+b(v-b\left)\right)}}$$

(1)

where:

• P is the pressure;
• T is the absolute temperature;
• v is the specific volume;
• R is the gas constant;
• a(T) and b are temperature-dependent parameters.

This equation of state accounts for the temperature and pressure effects on compressibility isotherms and incorporates coefficients a and b to represent the critical behavior of the fluid. It’s important to note that the Peng-Robinson equation of state is a simplification and may not be accurate under all conditions and for all types of fluids. However, it is widely used in industrial processes, along with other thermodynamic models, to provide reasonable predictions of fluid properties under various temperature and pressure conditions.

In

Table 3. Component Names, Types and Compounds.

Component Name	Type	Alias
Biomass	Nonconventional	-
Ash	Nonconventional	-
Carbon-graphite	Solid	C
Sodium-Bicarbonate	Solid	NaHCO3
Sodium-Chloride	Solid	NaCl
Zinc-oxide	Solid	ZnO
Zinc-sulfide-wurtzite	Solid	ZnS
Carbon-dioxide	Conventional	CO2
Carbon-monoxide	Conventional	CO
Water	Conventional	H2O
Nitrogen	Conventional	N2
Oxygen	Conventional	O2
Methane	Conventional	CH4
Hydrogen	Conventional	H2
Chlorine	Conventional	Cl2
Sulfur	Conventional	S
Hydrogen-sulfide	Conventional	H2S
Hydrogen-chloride	Conventional	HCl
Sulfur-dioxide	Conventional	SO2
Ammonia	Conventional	NH3

, the components involved in this process are defined:

In Figure 1 there is a simplified PFD of the biomass gasification process:

In addition, in Figure 2 there is the PFD simulated on Aspen Plus:

The main Aspen Plus blocks are reported in

Table 4. Reactors used in the simulation.

Aspen Plus Module	Name
DECOMP	RGibbs
GASSIFI	RGibbs
HCLREMOV	RGibbs
H2SREMOV	RGibbs
BURNER	RStoic

.

To define the incoming feed flow rate, the process begins by selecting the thermal size of the plant. For this type of plant, the thermal size typically ranges between 100 kW and 1 MW. In this case, a pilot plant with a thermal size of 1 MW is assumed. The LHV of the biomass is then determined to be:

$$\mathrm{LHV}=23.52{\displaystyle \frac{\mathrm{MJ}}{\mathrm{kg}}}$$

Thus, we can determine the feed flow rate to be supplied to our plant:

$$1\mathrm{MW}=154.8{\displaystyle \frac{\mathrm{kg}}{\mathrm{h}}}$$

The temperature is set to 700 °C, which is the grade at which pyrolysis occurs.

First of all, Aspen Plus has a database that relies on known chemistry components, both natural and synthetic. However, it does not have information on the chemical composition of biomass. Therefore, to function properly, it requires the proximate and ultimate reported in Table 1 and Table 2 for the analysis of biomass components. Aspen Plus, therefore, needs an RYield reactor that, through the calculation of yields for various elements in the ultimate analysis, informs us about how biomass breaks down into its conventional components [22]. Moving on to define the reactor, of course, the operating conditions are the same as those of the feed:

$$\mathrm{T}=700°\mathrm{C};\mathrm{P}=1\mathrm{atm}$$

Moving on to the actual gasification process, an RGibbs reactor is used. The choice of this reactor was made based on the involved reactions. Since these reactions reach equilibrium at different temperatures, using an RGibbs with a restricted chemical equilibrium at 1000 °C ensures that all reactions have reached equilibrium. The reactions involved are eight [20,23], specifically:

• $C+0.5O_2\to CO$, $\Delta H>0$; requires high temperatures. Typically occurs at temperatures above 1000 °C;
• $C+H_{2}O\to CO+H_2$, $\Delta H>0$; requires high temperatures. Occurs at temperatures above 700 °C;
• $H_2+0.5O_2\to H_{2}O$, $\Delta H<0$; occurs at high temperatures. Hydrogen combustion in air can occur around 2000 °C;
• $CO+H_{2}O\to C{O}_2+H_2$, $\Delta H<0$; occurs at moderate temperatures. Usually happens between 200 and 400 °C;
• $C{H}_4+H_{2}O\to CO+3H_2$, $\Delta H>0$; requires high temperatures. Occurs at temperatures above 700 °C;
• $C{l}_2+H_2\to 2HCl$, $\Delta H<0$; occurs at moderate temperatures. Can occur between 200 and 500 °C;
• $S+H_2\to H_{2}S$, $\Delta H<0$; occurs at moderate temperatures. Can occur between 200 and 400 °C;
• $0.5N_2+1.5H_2\to N{H}_3$, $\Delta H<0$; occurs at moderate temperatures. Usually happens between 300 and 500 °C.

Moving on the use of two heat exchangers, it was driven by the need to cool the stream exiting the gasifier in order to remove pollutants such as HCl and ${\mathrm{H}}_2$S through two dedicated reactors.

If the incoming feed flow rate were to be varied, then the steam flow rate entering the gasifier would automatically become inadequate. Therefore, a design specification is imposed where the steam/biomass ratio is set to 0.5 [24]. The incoming steam to the gasifier varies between a lower bound of 5 kg and an upper bound of 350 kg.

The steam, which mixes with the stream exiting the decomposer (T = 700 °C, pyrolysis temperature), is preferably maintained at a high temperature. Water is introduced at 20 °C into the exchanger and preheated through double thermal recovery, as previously described. Initially, the water is preheated in the exchanger located downstream of the HCl remover (from 600 °C to 550 °C), and subsequently in the exchanger downstream of the gasifier (from 1000 °C to 600 °C), both in countercurrent flow and with a shell and tube configuration.

Thermal recovery in a plant is carried out to harness the heat generated by specific processes or machinery and reuse it for other purposes within the same plant. This helps to increase the overall energy efficiency of the plant while simultaneously reducing energy costs. Additionally, thermal recovery can contribute to reducing the plant’s environmental impact by lowering overall energy consumption and greenhouse gas emissions.

Since HCl and ${\mathrm{H}}_2$S are responsible for corrosion and environmental issues, it is essential to include, within the gas cleaning chain, a step dedicated to reducing their levels below the desired limits, which vary depending on the specific application.

Table 5. Contaminants Limits (ppm) for Different Processes.

Process	${\mathrm{H}}_2$S	HCl
Biomass Waste Gasification	20–500	30–1000
Ammonia Production	<0.1	<1.5
Gas Turbine	<20	<1
SOFC	<1	<1
Methanol Synthesis	<0.5	<1

provides some contaminant limit values for different commercial applications [25]. A multifunctional reactor capable of simultaneously removing all contaminants would be ideal; however, it is impractical to use a single sorbent for the simultaneous removal of multiple contaminants in hot gas cleaning, as certain contaminants can interfere with the adsorption of others (e.g., HCl inhibits the adsorption of ${\mathrm{H}}_2$S in many metal oxides). For this reason, different steps have been developed to achieve the desired ppm level for the inorganic impurities considered in the simulation.

High-temperature sorbents for syngas applications operate through a solid-gas reaction between a metal oxide and the contaminant, as follows:

$${\mathrm{MO}}_{\mathrm{solid}}+{\mathrm{H}}_2{\mathrm{S}}_{\mathrm{gas}}\rightleftharpoons {\mathrm{MS}}_{\mathrm{solid}}+{\mathrm{H}}_2{\mathrm{O}}_{\mathrm{gas}}$$

The equilibrium constant is:

$$K_{eq}={\displaystyle \frac{\left[{\mathrm{H}}_2\mathrm{O}\right]}{\left[{\mathrm{H}}_2\mathrm{S}\right]}}$$

The equations presented illustrate the impact of high water concentration on the sulfidation level of a metal oxide, indicating that the reaction may only proceed if the sorbent possesses a very high equilibrium constant. As a result, the steam content in the syngas presents a notable challenge for material development. In this study, a sensitivity analysis of the S/B ratio has been performed, as detailed in the results section.

Another limitation of metal oxide sorbents, related to chemical and mechanical degradation in hot gas cleaning, is their thermal instability at the higher end of the temperature range, an aspect that has been rarely mentioned in the literature until now. The metal oxide sorbent must meet certain criteria: economic affordability, high equilibrium constant, high adsorption capacity, rapid adsorption kinetics, and fast and cost-effective regeneration capability. According to these criteria, zinc-based sorbents are currently the primary contenders for ${\mathrm{H}}_2$S removal, demonstrating almost complete removal of hydrogen sulfide at the operating temperature of 400 °C. In addition to zinc-based sorbents, the only others capable of removing ${\mathrm{H}}_2$S below 1 ppm are those based on cerium or copper.

Generally, the ${\mathrm{H}}_2$S cleaning capacity of different oxides in the mid- to high-temperature range follows this sequence: Sn < Ni < Fe < Mn < Mo < Co < Zn < Cu and Ce, as shown in Figure 3.

The properties of the metal oxides considered as sorbents in this study [9,25] are reported in

Table 6. Characteristics of Metal Oxides for Hydrogen Sulphide Removal.

Sorbent Type	Chemical Formula	Theoretical Sorption Capacity (g S/g Sorbent)	T Range (°C)	Price (USD/kg)
Cerium oxide	${\mathrm{Ce}}_2{\mathrm{O}}_3$	0.093	500–700	5.9
Copper oxide	${\mathrm{Cu}}_2$O	0.224	540–700	5.8
Zinc oxide	ZnO	0.395	450–650	2.6
Iron oxide	FeO	0.445	450–700	1.4
Manganese oxide	MnO	0.400	400–900	4.2

:

In the examined case, ZnO is used as a sorbent [9]. The flow rate entering the ${\mathrm{H}}_2$S remover is calculated from the following chemical reaction:

$$\mathrm{ZnO}+{\mathrm{H}}_2\mathrm{S}\to \mathrm{ZnS}+{\mathrm{H}}_2\mathrm{O}$$

(2)

The ratio between the two molar masses is 1.9, so approximately twice the amount of ZnO is required compared to ${\mathrm{H}}_2$S. Exiting the gasifier, there is 0.29 kg of ${\mathrm{H}}_2$S per hour, so a ZnO flow rate entering the ${\mathrm{H}}_2$S remover is required to be 0.55 kg/h. A design specification is then set, where the calculated ratio is fixed:

$$X={\displaystyle \frac{\mathrm{ZnO}}{{\mathrm{H}}_2\mathrm{S}}}=1.9$$

(3)

The most promising method for HCl removal at mid- and high-temperature involves alkali-based sorbents, mostly sodium and potassium compounds. Marcantonio et al. (2020) [25] showed nahcolite (NaHCO₃) as one of the best alkali-based sorbents, since it can remove HCl to concentrations lower than 1 ppm in the temperature range of 526–650 °C. Using nahcolite, the following reaction will take place:

$${\mathrm{NaHCO}}_3\left(\mathrm{s}\right)+\mathrm{HCl}\left(\mathrm{g}\right)\to \mathrm{NaCl}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)$$

(4)

However, it has to be taken into account that, when the temperature of nahcolite is raised up to 550 °C (reaction (1)), water molecules are separated. Then, nahcolite disintegrates to sodium carbonate, and HCl will react with Na₂CO₃ to form sodium chloride (reaction (2)).

$$2{\mathrm{NaHCO}}_3\left(\mathrm{s}\right)\to {\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)$$

(5)

$${\mathrm{Na}}_2{\mathrm{CO}}_3+2\mathrm{HCl}\left(\mathrm{g}\right)\to 2\mathrm{NaCl}\left(s\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)$$

(6)

Experimental results show that in order to reduce HCl ppm levels to 1 ppm, the nahcolite temperature has to be approximately 550 °C. In the Aspen Plus simulation, the removal of HCl is simulated by means of an RGibbs reactor, H2SREMOV, settled at 550 °C and 1 bar with the restricted chemical equilibrium option, using reactions (1) and (2).

In the examined case, NaHCO₃ is used as a sorbent. The flow rate ($\dot{m}$) entering the HCl remover is calculated from the following chemical reaction:

$${\mathrm{NaHCO}}_3\left(\mathrm{s}\right)+\mathrm{HCl}\left(\mathrm{g}\right)\to \mathrm{NaCl}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)$$

(7)

The ratio between the two molar masses is 2.3, so approximately twice the amount of NaHCO₃ is required compared to HCl.

Exiting the gasifier, there is 0.19 kg of HCl, so a NaHCO₃ flow rate entering the HCl remover is required to be 0.437 kg/h. A design specification is then set, where the calculated ratio is fixed:

$$X={\displaystyle \frac{{\mathrm{NaHCO}}_3}{\mathrm{HCl}}}=2.3$$

(8)

Moving on the GT reported in Figure 4, is fundamental to calculate the air excess to send in the compressor that is 355.23 kg/h.

4. Results and Discussion

The yield results, associated with Section 3, are presented in

Table 7. Component Yields Results.

Component	Basis	Yield
ASH	Mass	0.0638
C	Mass	0.4842
H2	Mass	0.0480
N2	Mass	0.0114
S	Mass	0.0018
O2	Mass	0.2833
H2O	Mass	0.1063
Cl2	Mass	0.0012

.

Table 8. Gasifier Outlet Stream Results.

Mass Fractions
BIOMASS	0.0000
ASH	0.0426
C	0.0000
CO2	0.1004
CO	0.6896
H2O	0.0907
N2	0.0076
O2	0.0000
CH4	0.0001
H2	0.0669
Cl2	0.0000
S	0.0000
H2S	0.0013
HCl	0.0008
NaHCO3	0.0000
NaCl	0.0000
SO2	0.0000
ZnO	0.0000
ZnS	0.0000
NH3	0.0000

shows the results for the gasifier outlet stream:

The composition of the “PD-CLEAN” syngas obtained is shown in

Table 9. Syngas Composition Results.

Mass Flows	Unit	kg/h
BIOMASS	kg/h	0.0000
ASH	kg/h	9.9876
C	kg/h	0.0000
CO2	kg/h	93.5996
CO	kg/h	19.0796
H2O	kg/h	55.9304
N2	kg/h	1.7850
O2	kg/h	0.0000
CH4	kg/h	17.9050
H2	kg/h	7.3384
Cl2	kg/h	0.0000
S	kg/h	0.0000
H2S	kg/h	0.0608
HCl	kg/h	0.0936
NaHCO3	kg/h	0.0000
NaCl	kg/h	0.0000
SO2	kg/h	0.0000
ZnO	kg/h	0.0000
ZnS	kg/h	0.0000
NH3	kg/h	0.0066

:

The results of the PD-CLEAN outlet stream from H2SREMOV are reported in

Table 10. Stream: PD-CLEAN—Results.

Mole Flows	Unit	kmol/h
C	kmol/h	0.0000
CO2	kmol/h	2.1268
CO	kmol/h	0.6812
H2O	kmol/h	3.1046
N2	kmol/h	0.0637
O2	kmol/h	0.0000
CH4	kmol/h	1.1161
H2	kmol/h	3.6403
Cl2	kmol/h	0.0000
S	kmol/h	0.0000
H2S	kmol/h	0.0018
HCl	kmol/h	0.0026
NaHCO3	kmol/h	0.0000
NaCl	kmol/h	0.0000
SO2	kmol/h	0.0000
ZnO	kmol/h	0.0000
ZnS	kmol/h	0.0000
NH3	kmol/h	0.0004

:

A GT is a type of internal combustion engine that converts thermal energy from the combustion of a fuel into rotary mechanical energy. This process occurs in three main phases: air compression, fuel combustion, and expansion of combustion gases through a series of turbine blades, which drive a connected shaft linked to a generator for electricity production [7,25,26]. The outgoing steam “VAPORE” stream with enthalpy:

$$Enthalpyflow=-622.8\mathrm{kW}\approx -623\mathrm{kW}$$

(9)

This section discusses the effects of key operating conditions, such as the S/B ratio and moisture, on syngas composition and ${\mathrm{H}}_2$S conversion. To facilitate this investigation, simplified assumptions were applied in the simulation.

Figure 5, Figure 6 and Figure 7 illustrate the impact of the S/B ratio on the syngas composition produced by the gasifier at temperatures of 1000 °C, 800 °C, and 700 °C. As the S/B ratio increases, the concentrations of H₂ and CO₂ rise, while those of CO and CH₄ decrease. The addition of steam promotes the WG, WGS, and SMR reactions, leading to an increase in H₂ and CO₂ levels. Similar trends have been reported in the literature [10].

To understand better the plot reported in Figure 8, results were reported in

Table 11. Sensitivity Results.

Row/Case	Status	VARY 1	CO	CH4	CO2	H2	H2O	% Moisture
1	OK	0.0000	0.4190	0.0002	0.0181	0.5204	0.0369	0.0000
2	OK	4.5000	0.4067	0.0001	0.0231	0.5168	0.0480	2.8769
3	OK	9.0000	0.3949	0.0001	0.0277	0.5132	0.0589	5.7537
4	OK	13.5000	0.3836	0.0001	0.0320	0.5096	0.0696	8.6306
5	OK	16.6381	0.3761	0.0001	0.0348	0.5071	0.0769	10.6368
6	OK	18.0000	0.3728	0.0001	0.0360	0.5059	0.0801	11.5075

:

These results are reported in Figure 9 and Figure 10:

As observed in the Figure 9 and Figure 10, as the moisture content (kg/h) varies, there is a noticeable change in the syngas composition. At a value of 0 on the x-axis, the syngas is dry, and as the values increase, the syngas becomes progressively wetter. The main differences between dry syngas and wet syngas are related to the presence or absence of H₂O in the gas produced during the gasification process.

The trend in Figure 11 of T on H2S in the outlet stream shows a decrease as the temperature increases, eventually reaching a point, especially around 700 °C, where the reactor can no longer absorb anything. The chosen temperature for the “H2SREMOV” reactor is 550 °C. This decision is made cautiously, considering that above 600 °C, the zinc devolatilization process may occur. Therefore, it is advisable to stay at the temperature of 550 °C [27].

One could consider further increasing the reactor temperature since it continues to absorb. However, upon reviewing the Table 5, it is observed that the HCl concentration must be kept below 2.5 ppm at 600 °C. Since we are already below this threshold, avoiding excessive temperature elevation to reach 700 °C for the “HCLREMOV” reactor seems to be a reasonable choice. Therefore, the decision is to stop at 600 °C as shown in Figure 12.

Based on the plant size of 1 MW, the amount of CO₂ emitted per kWh produced has been calculated to be 0.198 kg of CO₂/kWh produced. The cold gas efficiency of the process, calculated as [27]:

$$\eta ={\displaystyle \frac{{\dot{m}}_{\mathrm{syngas}}·{\mathrm{LHV}}_{\mathrm{syngas}}}{{\dot{m}}_{\mathrm{biomass}}·{\mathrm{LHV}}_{\mathrm{biomass}}}}$$

where ${\mathrm{LHV}}_{\mathrm{biomass}}=23.5\mathrm{MJ}/\mathrm{kg}$, is equal to 0.55, meaning the efficiency is 55%.

In this analysis, is assumed that the working fluid is an ideal gas, which is a common and justified assumption in many thermodynamic analyses, especially for gases at moderate pressures and high temperatures. Syngas exhibits behavior close to that of an ideal gas under these conditions due to the following reasons:

• Gas constant (R): The gas constant for the syngas mixture is assumed to be $R=0.287\mathrm{kJ}/\mathrm{kgK}$. This value is representative of typical gas mixtures like syngas.
• Specific heat at constant pressure ($C_p$): The specific heat at constant pressure is taken as $C_p=1\mathrm{kJ}/\mathrm{kgK}$. This value is typical for gas mixtures such as syngas, as it reflects the average thermal capacity of the gas components over the temperature range considered.
• Ratio of specific heats (k): The ratio of specific heats, defined as $k={\displaystyle \frac{C_p}{C_v}}$, is assumed to be $k=1.4$. This value corresponds to diatomic gases or mixtures dominated by diatomic components (e.g., ${\mathrm{H}}_2$ and $\mathrm{CO}$). It reflects the thermodynamic behavior of syngas under ideal gas conditions.

These assumptions simplify the calculations while providing a reasonable approximation of the thermodynamic properties of the syngas.

Additionally, for the GT cycle, reported in Figure 13, the compression efficiency ${\eta }_{\mathrm{comp}}$ is 0.93, the expansion efficiency ${\eta }_{\exp }$ is 0.75, and thus the overall cycle efficiency $\eta$ is calculated from temperatures reported in

Table 12. Temperatures and their respective values.

T	Value
T1	298 K
T2’	675 K
T3	1373 K
T4’	819 K

as:

$$\eta ={\displaystyle \frac{Q_1-Q_2}{Q_1}}={\displaystyle \frac{\mathrm{Useful}\mathrm{Work}}{Q_1}}=0.25(0.25\%)$$

(10)

5. Conclusions

In conclusion, the exploration of C. vulgaris for syngas production presents a promising avenue for sustainable energy generation, particularly in the context of GT feed. The comprehensive article underscores the potential of this approach, shedding light on optimal process conditions and gasification technologies considerations. By leveraging the rich composition of C. vulgaris, including proteins, vitamins, lipids, and carbohydrates, this study highlights a viable pathway towards renewable energy production. However, challenges persist, particularly concerning economic viability and environmental impact. Future advancements in technology and policy frameworks will be crucial in realizing the full potential of C. vulgaris-based syngas production for energy generation, paving the way for a greener and more sustainable energy future.

The investigation into the use of C. vulgaris for syngas production represents a promising approach to sustainable energy generation, particularly for feeding GTs. This study demonstrates that optimal conditions, such as maintaining a reactor temperature at 600 °C for HCl removal, allow for efficient syngas production while keeping contaminants below critical thresholds (e.g., HCl < 2.5 ppm). Calculations for a 1 MW plant show that CO₂ emissions can be limited to 0.198 kg per kWh of energy produced, a significant outcome in the context of low-emission energy systems. Additionally, the cold gas efficiency of 55% (based on biomass and syngas energy content) reflects the conversion efficiency under these conditions, with the overall cycle efficiency of 25% considering turbine performance factors (compression efficiency of 0.93 and expansion efficiency of 0.75).

While the syngas produced from C. vulgaris offers substantial potential, challenges related to economic feasibility and environmental impacts remain areas for future focus. Addressing these issues through technological advancements and supportive policies will be essential to enabling the broader adoption of microalgae-based renewable energy solutions.

The main practical advantages of the study’s findings on C. vulgaris gasification for syngas production include:

• Sustainable Energy Generation: C. vulgaris is a renewable biomass source that grows quickly and can absorb CO₂ during growth, reducing greenhouse gas emissions. Gasifying it for syngas production supports a low-emission energy pathway, contributing to carbon-neutral energy goals.
• Enhanced Syngas Quality for Versatile Use: The study’s optimization of steam-to-biomass ratios and temperature results in high H₂ and CO concentrations in the syngas. This clean, high-energy syngas can be effectively used for power generation in gas turbines, providing a reliable alternative to fossil fuels.
• Efficient Contaminant Removal: By achieving low levels of contaminants like H₂S and HCl, the syngas becomes suitable for direct use in sensitive applications like gas turbines, which require stringent gas quality. This minimizes equipment corrosion and prolongs turbine life, improving the viability of biomass-based syngas in energy production.
• Improved Cold Gas Efficiency: With a cold gas efficiency of 55%, the study’s process maximizes energy yield from biomass, making it more economically viable for commercial energy production. This higher efficiency reduces biomass input costs and improves the overall cost-effectiveness of algae-based syngas systems.
• Low Environmental Impact: The system produces low CO₂ emissions (0.198 kg CO₂/kWh), making it a more environmentally friendly energy source. This is particularly valuable for industries aiming to reduce their carbon footprint or comply with strict environmental regulations.

These advantages position C. vulgaris gasification as a promising technology for renewable energy production, particularly in applications requiring high-quality syngas with minimal environmental impact.

The authors highlight several main limitations of this study, along with possible strategies to address them, and suggest directions for future research. Firstly, there is the issue of economic feasibility. While C. vulgaris shows promising potential as a renewable biomass, the current costs of algae cultivation and processing remain high compared to other biomass sources, posing a barrier to large-scale commercial application. To mitigate this, the authors suggest exploring ways to reduce cultivation costs, such as integrating algae production with wastewater treatment facilities, where nutrient-rich effluents could support algae growth at a lower expense.

Another limitation is that the results are primarily derived from simulations in Aspen Plus, which, although highly detailed, may not fully capture the operational complexities of a real-world plant. To address this, it will be essential to conduct pilot-scale testing to validate the simulation results, allowing researchers to observe how actual variables, such as feedstock quality or environmental conditions, might influence system efficiency.

The complexity of the purification process poses a third limitation, as removing contaminants like H₂S and HCl requires multiple stages, potentially raising operational costs and complicating plant management. The authors propose that this issue could be alleviated through the development of multifunctional sorbents or integrated purification technologies, which could streamline the process and reduce both energy consumption and operational costs. Research into more efficient sorbents or catalysts may also make the process more economical and straightforward.

In terms of future directions, the authors propose building a pilot plant to evaluate system performance under real-world operating conditions, which would help refine operational parameters and improve process reliability. Additionally, a comprehensive cost-benefit analysis, incorporating algae cultivation, processing, and operational expenses, would provide greater insight into the system’s economic sustainability and help identify optimal configurations.

Further advancements could involve investigating other algal species or biomass blends to identify more efficient and sustainable biomass sources. In parallel, integrating gasification with carbon capture and utilization technologies could enhance the environmental benefits of the system, contributing further to CO₂ emissions reduction. To fully understand environmental impact, the authors also recommend conducting a LCA that quantifies emissions, resource consumption, and environmental benefits, enabling a comparison of this system with fossil fuel-based technologies.

By addressing these limitations and pursuing the suggested future directions, this research could advance C. vulgaris gasification as a practical and sustainable renewable energy solution.