Design and thermodynamic analysis of a novel methanol, hydrogen, and power trigeneration system based on renewable energy and flue gas carbon dioxide

Highlights

• A novel methanol, hydrogen and power trigeneration system with carbon capture is proposed.

• Performance of the system is analyzed from energy and exergy viewpoints.

• Energy and exergy efficiencies of 66.84% and 55.10% are achievable for the system.

• The highest exergy destruction occurs in water electrolyzer system.

• The inlet temperature of methanol synthesis reactor has a great effect on its performance.

Abstract

In this paper, a new trigeneration system is proposed to decrease atmospheric carbon dioxide emission and produce methanol, hydrogen, and power. The system is composed of an organic Rankine cycle, a direct methanol fuel cell, a carbon capture unit, a proton exchange membrane electrolyzer, and a methanol synthesis unit. A flue gas stream with a defined composition, solar energy, and the atmospheric air are the system's inlets. In the design step, special attention is paid to heat and mass integration between different components so that its waste can be lowered as much as possible. Then, mass balance law, energy conservation principle, exergy relations, and auxiliary equations are applied for each subsystem to investigate the system's thermodynamic performance. Also, the effect of changing operating parameters on the performance of each subsystem is studied. The obtained results show that the proposed system has the energy and exergy efficiencies of 66.84% and 55.10%, respectively. Furthermore, 94% of the total exergy destruction rate belongs to the water electrolyzer, while the contribution of the organic Rankine cycle is negligible. The performance of the methanol synthesis reactor depends strongly on its inlet temperature. Maximum equilibrium methanol concentration and carbon dioxide conversion are achieved at the inlet temperature of 210 °C. The parametric studies reveal that there is an optimum fuel cell current density in which its produced power density is maximized.

Introduction

The increase in world population, economic development, human welfare demands, and production rate has led to a rise in global energy consumption, mainly supplied by fossil fuel resources. It is estimated that fossil fuels provide nearly 85% of worldwide energy needs [1]. However, the combustion of fossil fuels in power plants produces a large amount of carbon dioxide which is a greenhouse gas (GHG) and main contributor to environmental problems such as global warming, ice melting, deforestation, floods, and climate change. It is reported that nearly 7 Gt of carbon dioxide is generated each year, and the rate is expected to increase in the coming years [2]. Therefore, it is crucial to decrease carbon dioxide emissions to the environment. Among various suggestions, carbon capture and storage (CCS) and its conversion to different products have gained particular attention. In carbon capture and utilization (CCU) process, an economic benefit through the generation of a value-added material for use in other chemical processes is achieved besides the decrease of carbon dioxide release. Various components, such as methanol (CH₃OH), dimethyl ether (CH₃OCH₃), and methane (CH₄), can be produced from emitted carbon dioxide [3].

Methanol is a product of a catalytic regenerative conversion reaction between carbon dioxide and hydrogen. It has several advantages like high energy density, easy storage and transportation, low toxicity, and little environmental pollution [4]. Also, it can be used as raw material for producing several essential chemicals like acid acetic (CH₃COOH), formaldehyde (CH₂O), methyl tertiary-butyl ether (MTBE), and dimethyl ether (DME) [5]. Furthermore, it can be used as an octane number booster for gasoline, and as a valuable fuel for different fuel cells [6]. So, it is concluded that carbon to methanol conversion is an excellent solution for lowering carbon dioxide emissions from power plants. Until now, many researchers have evaluated various designs for methanol production under different operating conditions [7]. However, carbon to methanol reaction needs a large amount of hydrogen, which is not available as a free component in the surroundings. The required hydrogen can be generated from the solar-powered water-splitting reaction, which is a clean process without environmental pollution. Additionally, even if all of the produced hydrogens are not consumed in the hydrogenation of carbon dioxide, surplus hydrogen may be considered as an energy storage option of solar energy for utilization during cloudy or night times. Hydrogen can also be used in fuel cells or sold as a byproduct. Although renewable energy sources cannot replace fossil fuel power plants in the near future, their integration can compensate for the adverse effects of carbon capture and utilization systems such as power consumption and power plant efficiency deterioration.

On the other hand, fossil fuel resources are limited, and their price has an increasing trend. So, it seems crucial to design more efficient energy conversion systems that can consume a significant part of fuel's chemical energy. In this regard, combining two or more different thermodynamic systems for simultaneous production of various demands like power, cooling, heating, and others from one energy source looks reasonable. These co/tri/multi-systems enjoy advantages like fair energy usage, higher efficiency, reliability, and safety in addition to lower cost and emission rates [8].

Migrand et al. [9] studied the effect of using different waste heat sources on a methanol production system from captured carbon dioxide. They concluded that the power production efficiency of renewable sources is as high as 59%. However, fossil fuel should be used to supply nearly 3.6% of system energy demand. Boretti [10] analyzed a methanol production system from the flue gas of an oxy-fuel combustion plant and hydrogen feedstock. He concluded that methanol has a higher conversion efficiency than gasoline. Since methanol resists knock in directly injected and turbocharged engines very well, it can be a suitable choice for high power concentration. Sayah et al. [11] designed a flue gas-based methanol production and wind energy based hydrogen generation system. Their obtained results showed that the employment of their system in Iran could lead to an increase in renewable energy integration, natural gas consumption, and emitted CO₂ rate. Esmaili et al. [12] investigated a solar-based hydrogen and methanol generation system. By studying the effect of varying operating parameters, they proved that sunlight intensity affects system efficiency. Leonzio et al. [13] considered three different configurations of methanol reactor at equilibrium condition: a once-through reactor, a reactor with the recycling of unconverted gases, and a reactor equipped with a membrane permeable to water. The feed flows were pure hydrogen and carbon dioxide. They showed that if a reactor with the recycle of unconverted gases is employed for methanol production, the highest carbon conversion of 69% is achievable. Atsonios et al. [14] evaluated the effect of different design and operating criteria on the carbon-to-methanol conversion system. They concluded that hydrogen production cost is the most significant parameter. Rivarolo et al. [15] studied the thermoeconomic performance of high-pressure reactors for methanol production when different renewable energy resources are employed. Their results depicted that biogas-based plant has the best economic performance, and purchasing carbon dioxide leads to lower capital investment. Nami et al. [16] investigated the performance of a power, methanol, and hydrogen production unit from thermodynamic, thermoeconomic, and environmental viewpoints. Their proposed design consisted of a geothermal driven organic Rankine cycle, a proton exchange membrane electrolyzer (PEME), an oxy-fuel combustion S-Graz cycle, and a methanol synthesis unit. They pointed out that the most crucial part of the system for investment was the S-Graz cycle, and the product unit cost was estimated at around 24.88 $/GJ. Kiatphuengporn et al. [17] studied the effect of an external magnetic field on the performance of a packed reactor in which methanol was produced through carbon dioxide hydrogenation. The reactor was filled with copper-iron-supported catalysts. Their results showed that applying a magnetic field enhanced reactor efficiency, carbon dioxide conversion, and methanol generation. Luu et al. [18] evaluated a post-combustion carbon capture system for enhanced gas recovery and methanol production from the flue gas of a coal-fired power plant. They claimed that their proposed design could operate well based on natural gas with high carbon dioxide concentration. Charoensuppanimit et al. [19] analyzed the possibility of using hydrogen from the Sodium methoxide (NaOCH₃) generation process in the carbon dioxide hydrogenation reaction. They reported that an adiabatic packed bed reactor was the best option. Ghosh et al. [20] evaluated three process schemes for methanol production from the generated carbon dioxide in a biogas plant. They demonstrated that all designs could provide their electricity demand and the two-reactor plan with fibrous catalyst has the highest efficiency and methanol yield. Gao et al. [21] proposed two options to set syngas ratio in the methanol generation process from landfill gas; providing additional hydrogen or separating extra carbon dioxide in landfill gas. They concluded that the first option is energy efficient, while the second one is more economical. Alsayegh et al. [22] investigated a new methanol production scheme in which captured carbon dioxide safely dilutes the produced hydrogen from photovoltaic water splitting and facilitates the hydrogenation reaction. Their economic evaluation revealed that generated methanol, in this case, is more expensive than the conventional ones. Matzen et al. [23] simulated a methanol and dimethyl ether production unit from wind-based electrolytic hydrogen and separated carbon dioxide from an ethanol (C₂H₅OH) fermentation system. They reported that although the environmental effect of methanol generation is less than dimethyl ether, its combustion offsets these benefits.

From the literature review in the previous paragraph, it can be concluded that many researchers have evaluated different schemes for methanol production from flue gas carbon dioxide. However, it seems more study and investigation are necessary in this field to identify proper applicable designs for various conditions. In this paper, a new methanol, hydrogen, and power trigeneration system is proposed and analyzed from the thermodynamic viewpoint. As it is well known, flue gas from refinery furnaces contains a considerable amount of carbon dioxide. So, a CO₂-to-methanol conversion system is considered to prevent carbon release into the surrounding atmosphere. The water-splitting reaction in the proton exchange membrane electrolyzer provides the necessary hydrogen for carbon hydrogenation. A solar energy collecting system is integrated to supply PEME power requirements. A part of the produced hydrogen is used in the methanol generation process, and the remainder is stored to be used in cloudy or night times or sold as a valuable byproduct. The main improvements of the proposed system can be highlighted as follow:

• An organic Rankine cycle is used to produce power from the flue gas energy content and adjust its temperature for the carbon capture unit.

• A part of the produced methanol is used in a direct methanol fuel cell to generate a stable and accessible electrical power for nearby buildings. Hence, it is not necessary to store and transport methanol to remote places for further processing.

• The electrochemical and thermodynamic performances of direct methanol fuel cell are simulated based on Aspen HYSYS available components and blocks. To the authors’ knowledge, this has not been done so far.

• Special attention is paid to heat and stream integration among different components of the system. Hence, the external heat requirements will be as low as possible. Also, depleted stream decreases significantly.

• The produced flue gas in the methanol synthesis unit contains carbon dioxide. So, it is returned to the starting point of the system. Thus, no carbon dioxide is emitted to the environment.

By applying the first and second laws of thermodynamics and necessary supplementary equations for each subsystem, the energy and exergy operation of the proposed system is simulated in Aspen HYSYS environment and Engineering Equation Solver (EES) software, respectively. Then, the performance of each subsystem is studied under various operating conditions.