Advances in Environmental Studies

Pilot Plant Model for Sequential CO₂ Fixation and Storage

Abstract

In the previously developed method using low NaOH and CaCl₂ concentrations, the atmospheric CO₂ is simply fixed to CaCO₃ and NaCl, which exist as coal or limestone, and a large chamber comprising spray nozzles to capture CO₂ efficiently by mists or droplets of NaOH solution has been designed. In the present study, a sequential CO₂ fixation and storage system was designed to efficiently capture a large amount of CO₂ from the atmosphere and exhausted gases. Using NaOH and CaCl₂, CO₂ was converted to CaCO₃, which is harmless and stable. This system with NaCl electrolysis can be applied to seawater instead of artificial NaCl solution and automatically fix CO₂ without adding NaOH or CaCl₂. Additionally, the process could potentially be integrated with existing generator systems based on atomic, thermal, solar, wind, hydro or wave power. This pilot plant is definitely practical and economical for direct air capture without environmental concerns. Thus, this system is consistent with the sustainable development goals (SDGs).

Introduction

It is scientific evidence that atmospheric CO₂ concentration on the earth has increased since the Industrial Revolution which was started about 200 years ago, inventing the steam engines using fossil coal as fuel, and the internal combustion engines using oil, and that our present developed civilization has owed to these technical inventions. However, we have not paid a little attention to the effect of increase in atmospheric CO₂ concentration for a long time, while the young generation represented by a Swedish High School student, Greta Thunberg, led climate change activities "Friday for Future" events as worldwide movements. Unfortunately, this movement would not expand all of peoples worldwide. One of reasons is due to our wide tolerance toward CO₂ concentration in daily life. In fact, the atmospheric CO₂ concentration at the house room is about 400 ppm, while the concentration easily reaches twice in the presence of several persons in the same room without ventilation. Even under high CO₂ concentration around 1,000 ppm for certain time in the limited space, our life doesn't feel an abnormal symptom. Eventually, many people would be apparently insensitive to a small increase in atmospheric CO₂ concentration, although this small change has clearly induced climate change crises on the earth. Indeed, certain many people have insisted that climate change is a myth. Based on scientific evidence for the relationship between global temperature, atmospheric CO₂ increases and hydroclimate changes [1-3], the intergovernmental panel on climate change concluded on August 9^th, 2021, that climate change has been caused by human activities that have produced carbon dioxide (CO₂) since the Industrial Revolution [4].

Although Earth has undergone many periods of significant environmental change over time, the planet's environment has been unusually stable for the past 10,000 years [5]. During this time, various natural systems regulated the Earth's climate and maintained the conditions that enabled human development. However, these regulatory systems have been greatly disturbed, and the planet may be nearing a threshold beyond which unpredictable environmental changes may occur, such as increases in the mean global temperature [6]. To reduce atmospheric CO₂ concentrations as a means of mitigating such effects, the so-called Paris Agreement was reached at the United Nations Climate Change Conference (COP20) in 2015. This agreement was based on the requirement to keep the increase in the mean global temperature below 2 °C relative to the temperature prior to the Industrial Revolution, and preferably less than 1.5 °C. At present, this goal is challenging based solely on the development of carbon-neutral energy systems.

The 7th G summit was held in Cornwall, England, on June 12-13, 2021, with climate change being one of the main themes. Electric vehicles have been developed and used instead of ordinary automobiles using gasoline, to reduce atmospheric CO₂ concentrations. While electric vehicles do not exhaust CO₂ directly into the atmosphere, the current electricity generated by renewable energy is insufficient to power electric vehicles. However, hydrogen vehicles have been developed, although the cost is expensive. Indeed, hydrogen usage does not exhaust CO₂; however, its production process using brown coal and high-temperature water produces a significant amount of CO₂, except for the electrolysis of water. Additionally, because nuclear power plants are one of energy sources that do not emit CO₂ into the atmosphere, they can contribute to reducing the atmospheric CO₂ concentration. However, nuclear plants cannot directly capture the accumulated CO₂ in the present atmosphere. In other words, nuclear power plants cannot immediately contribute to improving climate change at present. Furthermore, the outbreak of the Ukrainian crisis has shown that nuclear power plants are under serious military threat.

CO₂ can be captured from the atmosphere or from flue gas via several techniques, including absorption [7], adsorption [8-14], and membrane gas separation [9,15]. Absorption with amines is currently the dominant technology, while membrane and adsorption processes are still in the developmental stages with the construction of primary pilot plants anticipated in the future. Contrarily, the method based on amines is not widely used because of the toxicity of organic solvents. Synthetic membranes are useful for desalination, dialysis, sterile filtration, food processing, dehydration of air, and other industrial, medical, and environmental applications because of their energy requirements, compact design, and mechanical simplicity. In addition, biopolymer cellulose membrane can be used instead of synthetic membranes because they have similar characteristics [15-17]. Recently, we developed a novel method for CO₂ fixation and storage [18]. This method is based on simple chemical reactions involving NaOH and CaCl₂. Using low concentrations of these chemicals prevented the formation of Ca (OH)2 in the absence of CO₂, but resulted in CaCO₃ formation in the presence of CO₂ bubbling. Additionally, a polytunnel-based improvement method for CO₂ fixation with NaOH mist was reported as an "artificial forest" model [19,20].

One crucial factor that determines the chemical reaction rate is substrate concentration. In the CO₂ fixation reaction, CO₂ gas molecules must first be efficiently absorbed by the NaOH solution. Therefore, the exhaust gas or atmosphere is formed as micro bubbles to increase the surface area of gas bubbles in contact with NaOH solution (Figure 1). Moreover, stirring the NaOH solution increases the probability of CO₂ bubbles coming into contact with the NaOH solution. The high gas pressure increases the solubility of CO₂ molecules in the NaOH solution, resulting in a higher reaction yield of CO₂ with NaOH. It has also been found that deepening NaOH solution increases the contact time of CO₂ bubbles with the solution. Hence, the liquid pressure increases with the liquid depth, showing a higher CO₂ fixation. When multiple reaction towers are constructed, sequential CO₂ fixation can be achieved.

The amount of CaCl₂ that reacts with Na2CO₃ can be determined from the measured concentration of Na2CO₃ generated from CO₂ and NaOH at the sensor. Furthermore, as the formation of Ca(OH)2 strongly inhibits the formation of CaCO₃, the NaOH concentration in the reaction solution should also be measured to prevent Ca(OH)2 precipitation at high pH (> 0.2 N). Finally, the products, CaCO₃ and NaCl, are separated by filtration or decantation.

NaOH is practically produced from the artificial NaCl solution by electrolysis to generate H₂ and Cl₂. Therefore, it would be possible to use a filtrate containing NaCl produced by CO₂ fixation instead of an artificial NaCl solution (Figure 2), indicating that NaOH can be generated in a cyclic process without adding NaOH. Furthermore, if seawater is used instead of artificial NaCl solution, CaCl₂ can be supplied by seawater, eliminating the need for NaOH and CaCl₂ [21]. This system has been patented already by our company (JP Patent#6906111). Furthermore, this process can be integrated with existing power generation systems such as nuclear, thermal, solar, wind, hydro, and wave power. Ultimately, the integrated system can capture CO₂ and produce CaCO₃ without external materials other than electricity.

A pilot plant was fabricated, as shown in Figure 3. Air was supplied into a 0.1-N NaOH solution through a Teflon microbubble former. When air with a CO₂ concentration of approximately500 ppm was supplied to ~33-LNaOH solution at a flow rate of 30 L/min, the CO₂ concentration in the ~43 L upper air space of the reaction tank reached 0 ppm within 2 minutes. When a stirrer was used, the CO₂ disappeared more rapidly in the air space. This indicates that stirring is effective in fixing CO₂ in a solution.

CO₂ storage, geo-sequestration by injecting CO₂ into underground geological formations, such as oil fields, gas fields, and saline formations, has been suggested [22,23], although these systems are still projects for the future. However, the proposed method can achieve both CO₂ fixation and storage [18] simultaneously. The combination of large scale polytunnels that spontaneously absorb CO₂ from atmosphere could be imitated like an "artificial forest" using simple and economical technology.

Our present civilization has been obviously created through the use of fossil fuels including coal, oil and natural gas, and this means that our daily lives are supported by a large cohort of fossil fuel workers. To make real change to renewable energy from fossil energy while maintaining life style and redeploying these workers. The smooth energy source transfer from fossil fuel to renewable energy source requires renewable energy development. It contributes not only to reduction of atmospheric CO₂ concentration in the future, but also serves to reserve valuable natural energy sources for future generation. The drastic social changes imposed, however, may induce not only significant social concerns but also a slowdown of the accumulated atmospheric CO₂ reduction.

Readers in the United States of America, the People's Republic of China, India and Japan declared that their countries would achieve the carbon-neutral society by 2050, 2060, 2070 and 2050, respectively. These achievements apparently would be effective to prevent a global climate crisis. However, we should recognize the fact that the present climate change, which has been caused by the present accumulated atmospheric CO₂ exhausted not only from our daily activities including industrial activities but also from natural phenomena even before the Industrial Revolution. Thus, accumulated atmospheric CO₂ should be reduced immediately to prevent the present ongoing rate of climate change. The concept of the carbon-neutral society by 2050 seems to be far too late.

Renewable energy production systems, such as solar radiation and wind power, can really reduce CO₂ emission into the atmosphere but cannot reduce the present accumulated atmospheric CO₂, which has induced the current climate change. Thus, using the method, which can capture most efficiently the atmospheric CO₂ not only from the present atmosphere and exhaust gases but also from the future CO₂, which will be emitted by human activities, is extremely important and should be developed as soon as possible.

The ultimate human evolution has been achieved based on the Industrial Revolution [24] which has resulted in climate change. Thus, as we are responsible for this crisis, we have amoral duty to address the situation through global cooperation beyond just making changes to our economy. Everybody must recognize that both wealth and civilization would be valueless on a ruined Earth without human prosperity.

Acknowledgents

The author thanks Enago (https://www.enago.jp) for editing a draft of this manuscript.

Competing Financial Interests

The author declares that the present data have been used to support applications to the Japan Patent Office (PCT/JP2019/03400, PTC/JP2019/045389, PCT/JP2019/045390, PCT/2020/026989, PCT/JP2019/048178, PCT/2020/002064, PCT/2020/026990, PCT/JP2020/029505, PCT/JP2020/029504, JP2020/79418, JP2021-090928, JP2021-126892, JP Patent #6783436, 6788170, 6878666, 6788169, 6830564, 6788162, 6739680, 6817485, 6906111, 6864143).

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