1. The key to achieving China’s dual carbon goals
As pointed out in the
CO2 Emissions in 2023 report released by the International Energy Agency, global carbon dioxide (CO
2) emissions reached 37.4 billion tonnes in 2023 [
1], setting a new record high. The increase in CO
2 emissions has exacerbated global warming and led to a series of global climate problems. China is a major emitter of CO
2, accounting for approximately one third of the world’s total CO
2 emissions [
2]. Therefore, at the 75th session of the United Nations General Assembly in 2020, China solemnly promised to achieve a carbon peak by 2030 and carbon neutrality by 2060. Thus, there is a clear and urgent need for CO
2 reduction and resource utilization technologies.
CO
2 is not only a greenhouse gas but also a valuable resource that can be captured from industrial processes through separation and purification technologies and applied to refrigeration [
3], gas fertilization [
4], oil displacement [
5], oil recovery [
6], and chemical product production [
7]. However, the current cost of CO
2 capture and purification technology from emissions is 250–450 CNY∙t
−1 [
8], meaning that it is only possible to utilize CO
2 on a large scale when the utilization benefit exceeds this cost. Thus, developing a low-cost CO
2 capture and high-return utilization technology is significant to achieving China’s dual carbon goals.
2. CO2 application in agriculture: A green route for CO2 resource utilization
Plant photosynthesis requires a large amount of CO
2, and the concentration of CO
2 in the air is far from sufficient to achieve optimal crop growth [
9],[
10]. In addition, “carbon thirst” in crops brings a series of adverse consequences, such as poor microbial reproduction, fertilizer failure, soil salinization and hardening, and root system damage, reducing crop yield and quality [
11]. Appropriately enriching the CO
2 in the crop growth environment (from air or soil) can significantly increase crop yields, enhance crop disease resistance, improve crop quality, extend the harvest period, and improve soil quality [
12]. Hence, the resource utilization of CO
2 for agriculture is a natural and green way to achieve large-scale CO
2 reduction.
In addition, China has about 99 million hectares of saline-alkali soil (∼1/3 of which can be improved), mainly distributed in the northwest and northeast of China, north China [
13], with the area of saline-alkali soil in Xinjiang accounting for one-third of the total in China. Soil salinization is a key factor restricting plant growth and development, severely restricting the development of agricultural productivity. Currently, the annual crop production losses related to soil salinization range from 18% to 43% around the world [
14]. It is predicted that about 50% of the world’s arable land will be salinized by 2050, posing a serious threat to regional agricultural production and food security [
15]. Therefore, there is an urgent need to develop new technologies for improving saline-alkali soil and increasing agricultural production.
3. CO2-rich irrigation water: An innovative idea for CO2 agricultural utilization
CO
2-rich irrigation water (CRI-water), which is prepared by dissolving CO
2 into irrigation water for crop irrigation, can neutralize the alkaline components in the soil and reduce the pH value. Simultaneously, excessive CO
2 will react with carbonate to form bicarbonate, which easily dissolves in water and penetrates downwards into the soil, thereby reducing the salt content in the soil and achieving the goal of improving saline-alkali soil [
16]. In addition, the generated carbonate or bicarbonate ions can be absorbed by crop roots and transported to the leaves for photosynthesis, which is beneficial for crop growth and development [
17],[
18]. However, due to the low solubility of CO
2 in water, the dissolved CO
2 is easily desorbed into the air. Therefore, the question of how to quickly and stably dissolve CO
2 into irrigation water and avoid CO
2 desorption during transportation is the key to the agricultural utilization of CO
2.
More than 10 years ago, we conducted research on CO
2 capture and purification and their application during the period of the 11th Five-Year Plan, developing high-gravity CO
2 capture and purification equipment and technology [
19],[
20]. In response to the significant demand for agricultural CO
2 utilization and soil improvement and considering the advantages of high-gravity technology in enhancing mass transfer and dispersion, we invented a novel high-gravity technology for continuous CO
2 dissolution and transportation technology (HCDT). This technology is used to absorb and disperse CO
2 (pure CO
2 or gases containing CO
2) into water (including irrigation water, seawater, etc.) to prepare CRI-water. Simultaneously, an HCDT device can pressurize the water as a pump to transport CRI-water for agricultural irrigation. Based on these technologies, we propose a novel CO
2 utilization route: the use of CO
2 capture technology to capture and purify industrial CO
2, followed by the adoption of HCDT to prepare CRI-water for agriculture, mariculture, and soil improvement, as shown in
Fig. 1.
4. Implementation effects
4.1. Performance of HCDT
Systematic research on the CO
2 dissolution and dispersion process using HCDT has been carried out. In every run of the laboratory-scale experiment, the dissolving and dispersing process was continuously operated. We found that obvious scattering occurs in the prepared CRI-water after laser irradiation (as shown in
Fig. 2(a)), indicating that a large number of nano-bubbles are generated during the process [
21]. The results of a laser particle size analysis showed that the dispersed CO
2 bubbles have an average size of about 230 nm and generally fall within the range of 150–400 nm (
Fig. 2(b)). After sealed storage of the prepared CRI-water for 24 h, nano-bubbles with an average size of about 171 nm (mainly in the range of 120–250 nm) were still found to exist in the CRI-water. It is clear that the CO
2 nano-bubbles can stably disperse in the water for a long time and slowly dissolve into the water to maintain its CO
2 concentration and pH.
By optimizing the operating conditions, a CO
2 utilization rate of almost 100% can be achieved in this process. As shown in
Fig. 3, the pH of the prepared CRI-water can be decreased to around 5.2 and can be regulated by the operating conditions, including the CO
2 concentration in the gas, rotation speed, gas–liquid flux ratio, salt concentration of the irrigation water, and so forth. Even when the gas used has a low CO
2 concentration, the pH of the prepared CRI-water can be maintained at below 6, which can meet the demand for agricultural irrigation. Thus, HCDT can be adopted for the preparation of CRI-water using industrial gases with a low CO
2 concentration, such as industrial flue gas and tail gas.
To test the stability of the CRI-water, long-term stability experiments were carried out on both open and sealed CRI-water. Under experimental condition (rotation speed = 1200 r∙min−1; gas–liquid flux ratio = 1.5; gas flux = 12.5 L∙min−1; CO2 concentration in gas = 100%; salt concentration of irrigation water = 2 g∙L−1; temperature = 298 K; pH before storage = 4.98), it was found that the pH stabilizes at below 6 after 12 days (5.88) of open storage or after 14 days (5.76) of sealed storage. When using the prepared CRI-water for saline-alkali soil irrigation, the pH at the HCDT outlet and the pH at the irrigation point remain basically unchanged, which can be attributed to the fast CO2 dissolution rate and the high concentration of the dispersed CO2 nano-bubbles caused by the high mass transfer and dispersion effects generated by the HCDT device.
4.2. HCDT for agricultural irrigation
We also conducted a study on the effects of the CRI-water prepared via HCDT on soil and crops. Except for the differences in the irrigation water, all other conditions were kept consistent. It was found that both the pH and the soil conductivity of the exudate after irrigation with the prepared CRI-water (using well water) were lower than those after irrigation with well water (CO2-free). These phenomena can be attributed to the transformation of carbonates with lower solubility in water to bicarbonates, which have a higher solubility in water, and the loss of partial bicarbonates to the flowing irrigation water underground. As a result, the total salt content in the soil decreased by 9.6% after irrigation with the CRI-water prepared for one crop, indicating that this technology can clearly improve soil quality. In addition, during the irrigation of CRI-water, the CO2 content in the crop growth environment increased by 150–180 ppm, which is beneficial for plant growth.
When HCDT was applied to crops research, it was found that the growth rate of the crops was significantly improved under similar sowing and growth conditions; for example, the total length, average diameter, surface area, and volume of rice root tips irrigated with CRI-water (using well water) were all higher than those irrigated by well water (CO
2-free), with increases of 29.31%, 10.29%, 29.72%, and 17.24%, respectively. Similar phenomena were also observed in other crops. Benefitting from the improvement of the soil environment and the increased CO
2/CO
32–/HCO
3– in the crop growth environment after using the CRI-water prepared by HCDT, the crop yield was significantly increased relative to that irrigated with well water. For example, the yields of tomato, rice, corn, and celery increased by 27%, 21%, 31%, and 40%, respectively (
Fig. 4). In addition, the energy consumption of HCDT (excluding the cost of CO
2 transportation) is lower than 6 CNY per tonne of CO
2, which is less than 1/40 that of the CO
2 capture and purification process. Furthermore, the total cost of this technology is equal to less than 1/10 of the average benefits from the increase in crop yields.
5. Conclusion and outlook
In summary, HCDT is an innovative technology for achieving low-cost CO2 capture and resource utilization. It can be used to prepare stable CRI-water as a fertilizer to increase crop yields and improve the soil, thereby providing important technical support for China and the world in achieving a carbon peak and carbon neutralization. If the novel HCDT is promoted and applied nationwide, it will improve about 33 million hectares of saline-alkali soil in China. Calculated on the basis of a 20% increase in the crop yield of common crops in this improved soil, the annual economic benefits will exceed 0.4 trillion CNY, and 150 million tonnes of CO2 can be utilized as resources (i.e., CO2 reduction) per year. In the future, HCDT can also be promoted and applied to mariculture, facility agriculture, and other fields, thereby achieving resource utilization of the CO2 generated by human activities, improving the human living environment, and generating huge economic benefits as a green and economic path to achieve China’s dual carbon goals.
Acknowledgments
This work was supported by the National Key Research and Development Program of China (2022YFC3901103), the National Natural Science Foundation of China (22288102), and the Science and Technology Plan Project of the Xinjiang Production and Construction Crops (XPCC) (2023AB017-01).
PDF
(1324KB)
