Sensitivity Analysis of Biomass Gasification Reactors Using Aspen Plus

Sensitivity Analysis of Biomass Gasification Reactors Using Aspen Plus

¹Jaja Zina, ²Gwarah Bright Gogoro

¹Rivers State University, Faculty of Engineering, Department of Chemical/Petrochemical Engineering, Port Harcourt, Rivers State, Nigeria

²Kenule Beeson Saro-Wiwa Polytechnic, Bori, School of Engineering, Department of Chemical Engineering, Port Harcourt, Rivers State, Nigeria

DOI: https://doi.org/10.51584/IJRIAS.2025.10020010

Received: 17 January 2025; Accepted: 24 January 2025; Published: 04 March 2025

ABSTRACT

Municipal solid waste (MSW) management has become a critical issue in rapidly growing urban centers like Port Harcourt, Nigeria, due to increasing waste generation from population growth and industrial activities. Traditional waste disposal methods, such as landfilling and open burning, present significant environmental challenges, including greenhouse gas emissions, land degradation, and air pollution. Gasification, a thermal conversion technology, offers a sustainable waste-to-energy solution by transforming MSW into syngas (a mixture of hydrogen, carbon monoxide, methane, and other gases), which can be utilized for power generation or as a chemical feedstock. This study presents the simulation of a gasification plant designed for the management of MSW in Port Harcourt City using Aspen Plus, a process simulation tool. The heterogeneous nature of MSW, composed of organic matter, and inorganics, was modeled based on proximate and ultimate analysis, with key components such as cellulose, hemicellulose, and lignin representing the organic fraction. The simulation was carried out using an equilibrium-based approach in Aspen Plus, where the MSW undergoes drying, pyrolysis, gasification, and syngas cleaning stages. Key gasification reactions, including partial oxidation, the water-gas shift reaction, and methanation, were considered. The sensitivity analysis results from the simulation showed the effect of that the effect of biomass, steam and air flow rate on yield of products composition in the oxidation and reduction reactors. gasification process achieves a high conversion rate of approximately 85%.

Keywords: Biomass, Gasification, oxidation, reduction, plug flow reactor, sensitivity analysis, Aspen Plus and Municipal solid waste.

INTRODUCTION

The term “Municipal solid waste” refers to non-liquid waste that Socio-economic settlements generate. Poor waste management ranging from ineffective collection systems to disposal causes air, water, and soil contamination leading to habitat destruction, biodiversity loss, and climate changes (Neira et al. 2016). A study by (Khan, S., et al. 2019) shows that open and unsanitary landfills contribute to the contamination of drinking water and can cause infection and transmit diseases. Furthermore, a study by (Leão, R et al. 2019) analyzed trends in waste generation, emphasizing how increasing urbanization and economic growth have led to higher levels of waste production. It showed that as cities expand and populations grow more affluent, the quantity and complexity of waste materials increase, posing significant challenges for existing waste management infrastructure. Another study by (Hoornweg et al. 2013) shows that the dispersal of debris pollutes ecosystems and dangerous substances from industrial garbage strain the health of urban dwellers and the environment, and as the global population grows, there is a corresponding increase in waste generation.

Recent estimates suggest that the Municipal Solid Waste (MSW) generation globally exceeds 2 billion tons per year, which is a potential threat to environmental dilapidation (Karak et al., 2012) and according to the World Bank’s report, approximately 13.5% of the world’s municipal solid waste (MSW) is being properly managed through recycling, composting, or controlled disposal in landfills or waste-to-energy facilities (World Bank, 2018). Various works in the field of waste-to-energy have explored different technologies and approaches for converting waste into valuable energy resources. Studies such as those by (Wang et al. 2018) and (Smith & Jones, 2020) have investigated the efficiency and environmental impact of incineration processes. Anaerobic digestion is another widely studied technology for organic waste treatment and energy recovery.

In their study, (Lu et al., 2018) delved deeply into the process of thermochemical conversion, which involves converting waste biomass into energy-rich syngas through gasification. The researchers investigated different types of waste biomass, such as agricultural residues, forestry waste, and municipal solid waste, to determine their suitability for gasification. They analyzed the thermochemical reactions occurring during gasification and explored methods to optimize process parameters, such as temperature, pressure, and residence time, to maximize syngas yield and quality. (Arena et al., 2019) conducted a comprehensive examination of anaerobic digestion as a process for the treatment of organic waste and the production of biogas. Arena and colleagues investigated various aspects of anaerobic digestion, focusing on its efficiency, applicability, and environmental benefits. In their study, (Ferrari et al., 2020) conducted a comprehensive life cycle assessment (LCA) to evaluate the environmental impacts of various waste-to-energy (WtE) pathways.

MATERIAL AND METHODS

Simulation Model

A kinetic equilibrium model based on rigorous Gibbs phase and chemical equilibrium   approach has been developed for the air and steam gasifier of waste biomasses by using ASPEN Plus version 14. In this study, the developed Aspen Plus model involves the following steps: specification of stream class, selection of property method, determination of the system component from databank, specification of the conventional and non-conventional components, Specifying the process flowsheet by using unit operation blocks and connecting material and energy streams, defining feed streams (flow rate, composition, and thermodynamic condition) and Specifying unit operation blocks (thermodynamic condition, chemical reactions.).

Assumptions

The following assumptions are employed in the simulations of waste biomass gasification.

1) The model is at steady state, kinetic free and isothermal.

2) All gases are ideal gases, including hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), steam (H₂O), nitrogen (N₂) and methane (CH₄).

3) Char contains only carbon and ash in solid phase.

4) Tar and other heavy hydrocarbons are not considered.

5) Operation at atmospheric pressure (~1 bar).

6) No heat and pressure losses occur in the gasifier.

7) Simulation is based on stoichiometric approach and by considering reactions of R1, R2, R3, R4, R5 and R6 as shown in Table 1.

Physical Property Method

According to our conducted review, about 30% of studies employed Peng Robinson as the physical property method for simulation of biomass gasification (Table 2). However, approximately 44% of researches have not talked about the applied physical property method. Hence in our work, Penge Robinson equation of state has been used to estimate all physical properties of the conventional components in the gasification process. This method is suitable for the nonpolar or mildly polar mixtures such as hydrocarbons and light gases and the parameter alpha in this property package is a temperature dependent variable that could be helpful for the correlation of the pure component vapor pressure when temperature is quite high. Moreover, the enthalpy and density model selected for both biomass and ash which are non-conventional components are HCOALGEN and DCOALIGT. MCINCPSD stream containing three sub streams comprising MIXED, CIPSD and NCPSD class, was also used to define the structure of simulation streams for the components of biomass and ash which are not available in the standard Aspen Plus component database.

Table 1. Main gasification reactions

Table 2. Physical property methods selected by various researches.

Model Description

Figure 1 shows the process flow diagram of waste biomass gasification simulation by using ASPEN Plus based on the stoichiometric approach and Table 3 gives the brief descriptions of the unit operations of the blocks used in the simulation. The BIOMSS stream was defined as a nonconventional stream and it was created by inputting of elemental and gross compositions of wastes feedstocks obtained from proximate and elemental analyses. The information used to describe the feedstocks is given in Table 4. In pyrolysis/decomposition stage, the feedstock was transformed from a non-conventional solid to volatile materials and char. The volatiles included carbon, hydrogen, oxygen, nitrogen, ash and carbon, by specifying the product distribution based on the proximate and ultimate analysis of the waste biomasses. The yield of volatiles was equal to the volatile content in the fuel according to the proximate analysis. The products from the pyrolysis reactor (B1) were separated into gas and solid streams after which the solid stream was further separated into ash and carbon. The air, steam, and gas streams were mixed inside a mixer and sent to the oxidation reactor which was modelled as a plug flow reactor (PFR 1), the first two reactions occurred here. The products from the oxidation reactor were further sent to a reduction reactor (PFR 2) where the last four reactions occurred. The products from the reduction reactor were heated and sent to separator to separate out hydrogen gas from bottoms products.

Table 2. Reaction Kinetic Parameter Data

Table 3. Description of ASPEN Plus unit operation blocks used in model.

Table 4. Ultimate, proximate and Sulphur analysis feedstocks.

Figure 1. Process Flow Diagram of waste biomass gasification simulation using Aspen Plus.

RESULTS AND DISCURSION

Sensitivity Analysis

Sensitivity analysis for the biomass gasification plant was carried out to study the effect of biomass flow rate, steam flow rate and air flow rate on the yield of products component compositions. The result obtained from the sensitivity analysis are presented in figures 4.1 to 4.12

Effect of Biomass Flow rate on Yield of Product Composition in Oxidation Reactor (PFR 1)

The effect of biomass flow rate on the yield of product components of oxidation reactor are presented as follows:

Effect of Biomass Flow Rate on Yield of Moisture

Figure 2: Moisture Composition versus Biomass Flow Rate

Figure 2 shows the effect of biomass flow rate on yield of moisture (H2O). The flow rate of biomass was varied from 5000kg/h to 8000kg/h to see the effect it will have on moisture component and a it is crystal clear from figure 2 that an increase in the flow rate of biomass lead to decrease in the composition of moisture from 0.1092 to 0.1028. There are several reasons for this behavior (a) By increasing the flow rate of biomass, the overall composition of the feed shifts more towards the solid components, effectively diluting the moisture concentration in the feed. As a result, the reactor’s output will contain a lower proportion of moisture relative to other yield products.(b) When you increase the biomass feed rate without altering the reactor’s operational parameters (such as residence time or energy input), the biomass spends less time in the reactor. This reduced residence time can result in incomplete reactions or less effective breakdown of the biomass, including the conversion of moisture to vapor or other products. Since moisture might not fully separate or be processed as efficiently in a shorter time, its proportion in the yield might decrease. (c) As more biomass is added, a higher portion of solid matter is processed. This solid matter can retain moisture, effectively trapping water within the biomass structure. If the reactor’s process does not allow sufficient time or conditions for the water to be fully liberated from the biomass, more water will remain in the solid residue, reducing the moisture content in the reactor’s yield.

Effect of Biomass Flow Rate on Yield of CO

Figure 3: CO Composition versus Biomass Flow Rate

Figure 3 shows the effect of biomass flow rate on yield of CO. The flow rate of biomass was varied from 5000kg/h to 8000kg/h to see the effect it will have on CO component and a it is crystal clear from figure 3 that an increase in the flow rate of biomass lead to a corresponding increase in the composition of CO from 0.1092 to 0.2667. As more biomass is fed into the reactor, more carbon is available for these conversion processes. This leads to a higher production of gases, since the gas-producing reactions (like oxidation, reduction, and cracking) are carbon-driven. When you increase the biomass flow rate, more organic material is available for thermochemical reactions, such as pyrolysis, gasification, or combustion.

Effect of Biomass Flow Rate on Yield of CO₂

Figure 4: CO₂ Composition versus Biomass Flow Rate

Figure 4 shows the effect of biomass flow rate on yield of CO₂. The flow rate of biomass was varied from 5000kg/h to 8000kg/h to see the effect it will have on CO₂ component and a it is crystal clear from figure 4 that an increase in the flow rate of biomass lead to a corresponding increase in the composition of CO₂ from 0.07585 to 0.07758. As more biomass is fed into the reactor, more carbon is available for these conversion processes. This leads to a higher production of gases, since the gas-producing reactions (like oxidation, reduction, and cracking) are carbon-driven. When you increase the biomass flow rate, more organic material is available for thermochemical reactions, such as pyrolysis, gasification, or combustion.

Effect of Biomass Flow Rate on Yield of H₂

Figure 5: H₂ Composition versus Biomass Flow Rate

Figure 4.4 shows the effect of biomass flow rate on yield of H₂. The flow rate of biomass was varied from 5000kg/h to 8000kg/h to see the effect it will have on H₂ component and a it is crystal clear from figure 5 that an increase in the flow rate of biomass lead to a corresponding increase in the composition of H₂ from 0.4837 to 0.4933. As more biomass is fed into the reactor, more carbon is available for these conversion processes. This leads to a higher production of gases, since the gas-producing reactions (like oxidation, reduction, and cracking) are carbon-driven. When you increase the biomass flow rate, more organic material is available for thermochemical reactions, such as pyrolysis, gasification, or combustion.

Effect of Steam Flow rate on Yield of Product Composition in Oxidation Reactor (PFR 1)

The effect of steam flow  rate on the yield of product components of oxidation reactor are presented as follows:

Effect of Steam Flow rate on Yield of Moisture

Figure 6: Moisture Composition versus Steam Flow Rate

Figure 6 shows the effect of steam flow rate on yield of moisture. The flow rate of steam was varied from 100kg/h to 700kg/h to see the effect it will have on moisture component composition and a it is crystal clear from figure 6 that an increase in the flow rate of steam lead to a corresponding decrease in the composition of moisture from 0.1092 to 0.2192. The reason for this behavior are as follows: (a) As you increase the flow rate of steam, more water is consumed in these reactions to produce more gases, effectively reducing the amount of moisture left as a product. The higher the steam flow rate, the more reactant is available for these gas-producing reactions, thus decreasing the amount of residual moisture (water vapor) in the output. (b) Introducing more steam can raise the overall temperature in the reactor, especially if the steam is superheated. Higher temperatures can shift the equilibrium of reactions that produce gaseous products over liquid or vapor-phase water. (c) If the steam is being used in a gasification or pyrolysis reactor, the increased flow of steam can enhance the breakdown of biomass. Gasification with steam can lead to more complete conversion of biomass into gaseous products, which reduces the amount of moisture in the product stream because steam is consumed in the formation of gases like H₂ and CO.

Effect of Steam Flow rate on Yield of CO₂

Figure 7: CO₂ Composition versus Steam Flow Rate

Figure 7 shows the effect of steam flow rate on yield of CO₂. The flow rate of steam was varied from 100kg/h to 700kg/h to see the effect it will have on CO₂ component composition and a it is crystal clear from figure 7 that an increase in the flow rate of steam lead to a corresponding decrease in the composition of CO2 from 0.07585 to 0.06647. The reason for this behavior is due to the fact that as more steam is added to the reactor, the total volume of gases (including the steam itself) increases significantly. However, the increase in the production of gaseous products (like CO, CO₂, H₂) may not keep pace with the added steam.

4.2.2 c Effect of Steam Flow rate on Yield of CO

Figure 8: CO Composition versus Steam Flow Rate

Figure 8 shows the effect of steam flow rate on yield of CO. The flow rate of steam was varied from 100kg/h to 700kg/h to see the effect it will have on CO₂ component composition and a it is crystal clear from figure 8 that an increase in the flow rate of steam lead to a corresponding decrease in the composition of CO2 from 0.2618 to 0.2295.The reason for this is same as explained in figure 8

Effect of Steam Flow rate on Yield of H₂

Figure 9: H₂ Composition versus Steam Flow Rate

Figure 9 shows the effect of steam flow rate on yield of H₂. The flow rate of steam was varied from 100kg/h to 700kg/h to see the effect it will have on H₂ component composition and a it is crystal clear from figure 9 that an increase in the flow rate of steam lead to a corresponding decrease in the composition of H₂ from 0.4837 to 0.4240.The reason for this is same as explained in figure 9.

Effect of Air Flow rate on Yield of Product Composition in Reduction Reactor (PFR 2)

The effect of Air flow  rate on the yield of product components of reduction reactor are presented as follows:

Effect of Air Flow rate on Yield of Moisture

Figure 10: Moisture Composition versus Air Flow Rate

Figure 10 shows the effect of air flow rate on yield of H₂. The flow rate of air was varied from 100kg/h to 700kg/h to see the effect it will have on moisture component composition and a it is crystal clear from figure 10 that an increase in the flow rate of steam lead to a corresponding decrease in the composition of moisture from 0.4837 to 0.4240.The reason for this is as follows:  (a) As you increase the flow rate of air, more water is consumed in these reactions to produce more gases, effectively reducing the amount of moisture left as a product. The higher the air flow rate, the more reactant is available for these gas-producing reactions, thus decreasing the amount of residual moisture (water vapor) in the output. (b) Introducing more steam can raise the overall temperature in the reactor, especially if the steam is superheated. Higher temperatures can shift the equilibrium of reactions that produce gaseous products over liquid or vapor-phase water. (c) If the air is being used in a gasification or pyrolysis reactor, the increased flow of air can enhance the breakdown of biomass. Gasification with steam can lead to more complete conversion of biomass into gaseous products, which reduces the amount of moisture in the product stream because steam is consumed in the formation of gases like H₂ and CO.

Effect of Air Flow rate on Yield of CO₂

Figure 11: CO₂ Composition versus Air Flow Rate

Figure 11 shows the effect of air flow rate on yield of CO₂. The flow rate of air was varied from 100kg/h to 700kg/h to see the effect it will have on CO₂ component composition and a it is crystal clear from figure 4.10 that an increase in the flow rate of steam lead to a corresponding decrease in the composition of CO₂ from 0.07585 to 0.7020.The reason for this is as follows:  (a) As you increase the flow rate of air, more water is consumed in these reactions to produce more gases, effectively reducing the amount of moisture left as a product. The higher the air flow rate, the more reactant is available for these gas-producing reactions, thus decreasing the amount of residual moisture (water vapor) in the output. (b) Introducing more steam can raise the overall temperature in the reactor, especially if the steam is superheated. Higher temperatures can shift the equilibrium of reactions that produce gaseous products over liquid or vapor-phase water. (c) If the air is being used in a gasification or pyrolysis reactor, the increased flow of air can enhance the breakdown of biomass. Gasification with steam can lead to more complete conversion of biomass into gaseous products, which reduces the amount of moisture in the product stream because steam is consumed in the formation of gases like H₂ and CO.

Effect of Air Flow rate on Yield of CO

Figure 12: CO Composition versus Air Flow Rate

Figure 12 shows the effect of air flow rate on yield of CO. The flow rate of air was varied from 100kg/h to 700kg/h to see the effect it will have on CO component composition and a it is crystal clear from figure 12 that an increase in the flow rate of steam lead to a corresponding decrease in the composition of CO from 0.2618 to 0.2422.The reason for this is as follows:  (a) As you increase the flow rate of air, more water is consumed in these reactions to produce more gases, effectively reducing the amount of moisture left as a product. The higher the air flow rate, the more reactant is available for these gas-producing reactions, thus decreasing the amount of residual moisture (water vapor) in the output. (b) Introducing more steam can raise the overall temperature in the reactor, especially if the steam is superheated. Higher temperatures can shift the equilibrium of reactions that produce gaseous products over liquid or vapor-phase water. (c) If the air is being used in a gasification or pyrolysis reactor, the increased flow of air can enhance the breakdown of biomass. Gasification with steam can lead to more complete conversion of biomass into gaseous products, which reduces the amount of moisture in the product stream because steam is consumed in the formation of gases like H₂ and CO.

Effect of Air Flow rate on Yield of H₂

Figure 13:  H₂ Composition versus Air Flow Rate

Figure 13 shows the effect of air flow rate on yield of H₂. The flow rate of air was varied from 100kg/h to 700kg/h to see the effect it will have on H₂ component composition and a it is crystal clear from figure 13 that an increase in the flow rate of steam lead to a corresponding decrease in the composition of H₂ from 0.4837 to 0.4475.The reason for this is as follows:  (a) As you increase the flow rate of air, more water is consumed in these reactions to produce more gases, effectively reducing the amount of moisture left as a product. The higher the air flow rate, the more reactant is available for these gas-producing reactions, thus decreasing the amount of residual moisture (water vapor) in the output. (b) Introducing more steam can raise the overall temperature in the reactor, especially if the steam is superheated. Higher temperatures can shift the equilibrium of reactions that produce gaseous products over liquid or vapor-phase water. (c) If the air is being used in a gasification or pyrolysis reactor, the increased flow of air can enhance the breakdown of biomass. Gasification with steam can lead to more complete conversion of biomass into gaseous products, which reduces the amount of moisture in the product stream because steam is consumed in the formation of gases like H₂ and CO.

CONCLUSION

The simulation of a gasification plant for municipal solid waste (MSW) management in Port Harcourt City using Aspen Plus demonstrated the potential of gasification as an effective waste-to-energy technology. By converting MSW into syngas, the process achieves high thermal efficiency and waste conversion rates, while minimizing harmful emissions compared to traditional methods like incineration. The syngas produced, primarily composed of hydrogen and carbon monoxide, is suitable for energy generation or as a feedstock for synthetic fuel production. The simulation results suggest that gasification could serve as a sustainable and economically viable solution to address the growing waste management challenges in Port Harcourt. Finally sensitivity analysis was carried out to study the effect of varying the flow rate of biomass, steam and air on product component yield for the oxidation reactor.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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