A New Method for Capturing CO₂ from Effluent Gases Using a Rice-Based Product

Abstract

In 2013, UAE imported around 772 million kilograms of rice, making it one of the largest consumers of this popular grain in the world. However, 40% of rice available in the market is discarded, contributing to the country’s CO₂ footprint. Given that CO₂ emissions are recognized as a significant contributor to climate change and efforts aimed at their reduction are proving insufficient for combatting the global increase in temperature, various approaches aimed at its removal from the atmosphere have been proposed. The goal of this study is to contribute to this initiative by proposing a new method for CO₂ removal based on a special gas contact device filled with buffered puffed rice cakes obtained by heating in a purposely designed sealed chamber at high pressure to obtain layers with 9−12 mm thickness. The resulting cakes are subsequently immersed in a sodium hydroxide liquor (0.25−2.5 M) to increase the moisture content to 5% and pH to >11.0. In the experiments, different rice structures (stacked layers, rice grains, and multi-spaced layers) were tested, varying the CO₂ percentage in the simulated effluent gas (1−15%). The highest CO₂ uptake value (7.52 × 10⁻³ mole CO₂/cm² rice cake surface area) was achieved using 10% CO₂ and a 500 mL/min flow rate with rice cakes of 80 mm diameter, comprising 12 mm thick layers that occupied 20% of the device volume. These results indicate that the proposed design exhibits high CO₂ removal efficiency and should be further optimized in future investigations.

1. Introduction

In 2019, the Food and Agriculture Organization (FAO) of the United Nations released a report on the greatest contributors to food waste at the global level, indicating that about one-third of food produced each year is wasted or lost [1]. Given that food insecurity is a serious issue in many parts of the world, these findings are alarming. However, as all elements of the food chain are also significant contributors to CO₂ emissions, such waste also has adverse environmental impacts. Consequently, the 2030 agenda for sustainable development calls for a reduction in global food waste by half in both per capita terms and by market share [2,3]. Food production and disposal requires considerable resources, including land, soil, energy, and water. For example, according to the OLIO website, food waste contributes to 25% of global freshwater consumption, and its disposal in landfills generates significant amounts of methane (CH₄), which is 23 times more harmful than CO₂ [2]. It is also worth noting that according to the United Nations Environment Programme report issued in 2021, the percentage of food that is discarded is rapidly increasing in most countries, irrespective of their income level [4]. Similarly, the UN Food and Agriculture Organization estimated that wasted food generates about 3.5 billion tons of CO₂ annually, exceeding the emissions of all industrial countries except the US and China. Another UN report also highlights the adverse environmental impact of rice production, which has increased considerably in recent decades in response to the massive growth in population, especially in developing countries [5]. Yet, even though rice is considered a staple food in most parts of Asia, it is wasted in large quantities, releasing around 610 million tons of CO₂ into the atmosphere each year [1]. Although the United Arab Emirates is a highly developed country, it is still one of the highest per capita consumers of rice, which is mostly imported, and about 40% of the quantity available in the market is wasted each year [6].

Rice is a versatile crop and can be used in many forms, including rice cakes or puffed rice layers, which are obtained by heating raw rice grains in a sealed chamber under high applied pressure, allowing them to expand, taking the shape of the mold used [7]. The resulting cakes are acid-balanced, have a smooth surface, and exhibit high porosity. Currently, available fabrication techniques result in layers with 9−12 mm thickness [8].

As noted previously, all elements of the food chain produce significant amounts of greenhouse gasses (carbon dioxide, nitrous oxide, and methane in particular) that contribute to global warming [9,10]. Given that CO₂ is emitted in the atmosphere in the highest amounts and about half of this quantity will dissolve in the oceans, thus increasing saltwater acidity, it will have a detrimental effect on aquatic life [11]. Although fossil fuels in power generation plants, industrial facilities, buildings, and transportation account for a significant percentage of CO₂ emissions, given that a large percentage of the landmass is designated for food production, it is still one of the greatest contributors to greenhouse emissions [12,13,14,15]. In recognition of this issue and to limit the increase in global temperatures to less than 2 °C, governments of many countries have committed to halving CO₂ emissions by 2050.

One of the measures that can be adopted for reducing CO₂ emissions is based on its sequestration from power generation plants, which are estimated to generate around 40% of the overall CO₂ emissions [16,17]. Currently, four main approaches for CO₂ sequestration are being explored: pre-combustion, post-combustion, oxyfuel, and chemical looping combustion [18]. However, none of these methods is suitable for large-scale industrial applications [19]. Post-combustion methods could be classified into physical, chemical, and biochemical methods. The physical methods are based on physical absorption, cryogenic condensation, and membrane separation technology [20]; chemical methods include chemical adsorption, chemical absorption, and chemical-looping combustion [21]; and biological methods involve biological fixation by terrestrial vegetation and marine or freshwater microalgae [22]. CO₂ sequestration can also be achieved by scrubbing with reactive solvents [23]. This chemical absorption process, whereby CO₂ is removed using amine as a solvent, is highly efficient, as it can be applied to significant quantities of exhaust gas [24]. However, it is very expensive due to the high energy requirements for regenerating amine solutions. Moreover, it may result in operational problems, such as corrosion, solvent loss, and solvent degradation [25]. As an alternative, CO₂ can be captured using highly alkaline solutions, as this strategy has been in use as a pre-treatment step for cryogenic air separation since 1943 [26]. As a part of this process, CO₂ is absorbed into sodium hydroxide solution and forms an aqueous solution of sodium hydroxide and sodium carbonate.

Available evidence also indicates that increasing the contact between effluent gases and stripping media improves CO₂ absorption and sequestration, which can be achieved via packed scrubbing and convective towers [27,28,29]. The contact efficiency in these systems can be further improved by applying a fine spray of sodium hydroxide solution [30]. However, CO₂ sequestration using sodium hydroxide produces sodium carbonate solution, which needs to be regenerated to sodium hydroxide solution again to be reused in this process, making it more economically viable but also less energy efficient. For this purpose, “causticization” or “caustic recovery” is typically applied, although it is inefficient and consumes a significant amount of energy [30].

Membrane technology can thus be considered as an alternative, as it can be employed as a gas separation process in natural gas sweetening, air separation, and hydrogen production [31]. For CO₂ sequestration, microporous organic polymers [32], carbon molecular sieve membranes [33], inorganic membranes [34], fixed-site-carrier membranes [35], and mixed-matrix membranes [36] are typically employed, depending on the effluent gas composition, process conditions, and separation requirements. However, efficient CO₂ separation requires membranes characterized by high selectivity and permeance. Moreover, to make such membranes commercially feasible and competitive with amine absorption, they should be tolerant to SO₂, NO_x, and other impurities commonly present in emitted gases and should have a long lifetime and long-term stability [22]. If these aims can be achieved, membrane technology has the potential to reduce installation and operating costs [37,38,39,40].

From the presented literature review, it can be concluded that CO₂ removal is a vital step for any sustainable and environmental approach to managing the increase in greenhouse gas emissions into the atmosphere. Moreover, there is a significant potential to reuse rice waste in an environmentally friendly and cost-effective manner. Accordingly, in this work, a novel method for CO₂ removal based on passing effluent gas through a gas contact device filled with puffed rice cakes is proposed. The goal of this design is to reduce the cost and energy consumption associated with CO₂ removal compared to the conventional methods and to ensure high removal efficiency. Based on our knowledge and extensive literature search, this method is new and has never been tested before. In general, the motivation for this new approach is overcome the main limitations of the traditional methods, such as high energy input, cost, and limited efficiencies. This research work is expected to develop an efficient and sustainable method for CO₂ capturing from effluent gases using low cost, available, and environmentally friendly material, namely rice waste material. In this work, we propose a new technology that benefits the development of many industries in food waste management sectors, as well as gas emissions controlling agencies.

2. Methodology and Experimental Procedure

2.1. Rice Cake and Grain Fabrication

To obtain the raw material required for the proposed design, raw rice grains were soaked in water for approximately 12 h to reach the desired 10−15% moisture content as, well as to remove as much of the starch present as possible in order to achieve maximum expansion and improve puffing ratio. After soaking, grains were heated to 260–265 °C to eliminate any excess moisture and ensure that the remaining starch inside the grains attained rubbery consistency. The dried samples were placed into a mold and were subjected to high temperature and pressure (chosen based on a detailed parametric investigation to determine the optimal conditions), causing the grains to expand, as shown in Figure 1. Figure 2 depicts a sample of a final layer of 8 cm diameter and 9−12 mm thickness. During the parametric investigation, different parameters were tested to optimize the sample thickness and porosity, whereby the moisture content in the raw material was varied from 12% to 18% and the temperature inside the mold ranged from 200 to 260 °C. The pressure inside the mold was increased to reach around 40 bar. These conditions were applied to identify the most optimal form of rice samples. In addition to producing layered cakes, rice grains were also puffed at 260 °C, and mositure content varied from 12% to 18% at atmospheric pressure (without the mold) to obtain separate puffed grains [41].

2.2. Rice Cake Puffering Process

To adjust the pH level of puffed rice cakes and grains, 500 mL of aqueous sodium hydroxide solution of 0.25, 0.5, 1, 1.25, 2, and 2.5 M concentrations was prepared by dissolving granular NaOH in distilled water. After rice cakes of 80 and 12 mm thickness were dipped in each solution, as shown in Figure 3, individual samples absorbed around 18−22 mL of solution. The soaked rice cakes were left for 24 h to decrease the moisture percentage through water evaporation at room temperature.

2.3. Fresh and Buffered Puffed Rice Cake Characterization

Figure 4 shows images of fresh and NaOH-treated puffed rice samples after 24 h of drying at room temperature. These samples were subjected to scanning electron microscopy (SEM), XRD analysis, TGA analysis, and Fourier transform infrared (FTIR) spectroscopy to elucidate their physical structure, morphological properties, and composition, as described below.

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XRD analysis: The structural properties of samples were determined using an X-ray diffractometer by exposing them to Cu Kα radiation (λ = 1.54 Å) while applying a 30 mA tube current and a 40 kV target voltage. A 2θ scanning range of 5−70° with a scan speed of 2°/min was adopted to capture all significant diffraction peaks.

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SEM analysis: The surface morphology, texture, and shape of both fresh and treated puffed rice samples were characterized using a scanning electron microscope. Samples were first coated with a 300 Å thick gold layer before subjecting three different areas to analysis.

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FTIR analysis: The rice samples were subjected to FTIR analysis using an IRTrace-100 FTIR spectrophotometer (Shimadzu, Kyoto, Japan) to investigate the presence of effective functional groups. The spectral outcomes were documented within a 500−4000 cm⁻¹ wavelength range using 4 cm⁻¹ spectral resolution and 34 scans.

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TGA analysis: TGA analysis was conducted in a thermogravimetric analyzer (Q500 series, TA Instruments) using samples of 2.0−5.0 mg weight while incrementing the temperature at a 15 °C/min rate. Nitrogen gas with a flow rate of 20 mL/min was used as a carrier gas.

2.4. Gas Contact System

Once all the required layers had been prepared, they were inserted into a glass cylindrical contactor system with an internal diameter of 80 mm and 450 mm height, equipped with a 2 mm diameter gas inlet at the center of its horizontal base, as shown in Figure 5. During the experiments, the gas flow through the contactor system was controlled by a gas flow controller connected to the SCADA station to provide digital monitoring and control. For this purpose, a gas effluent tube of 5 mm diameter connected to the contactor system was passed through a moisture trap, then to a CO₂ gas analyzer (Model 600 series, Non-Dispersive Infrared NDIR Analyzers), as shown in Figure 6.

2.5. CO₂ Capturing Process

The developed system’s ability to capture CO₂ was evaluated under different operational conditions. In each scenario, once the treated puffed rice structures (stacked layers, rice grains (SPRC), or multi-spaced layers (MSPRC) were placed inside the contactor system, a gas mixture (10% CO₂ and 90% air (synthesized gas mixture provided by Abu Dhabi Oxygen Company, UAE, simulating effluent gas from refineries and industrial applications) or 3% CO₂ and 97% air) was passed through the system, while varying the flow rate between 350 and 750 mL/min [42]. The contactor system was operated at atmospheric pressure and room temperature. The time required to reach full CO₂ saturation was recorded for each experiment to determine the maximum loading per surface area of the treated rice cake structure. For testing the performance of the stacked-layer design, a sample comprising buffered (using 0.5 M NaOH liquor) puffed rice cakes of 80 mm diameter and 12 mm thickness was placed into the gas contactor system (with 20% total reactor volume). At time zero, the simulated effluent gas (10% CO₂) was injected at a 500 mL/min flow rate, and the CO₂ percentage in the treated gas was recorded continuously at 15 min intervals. As can be seen from Figure 7, as a result of this process, gas bubbles gradually became trapped inside the rice structure. The experiment was terminated once full CO₂ loading (zero CO₂ capture) was achieved [43,44,45,46], as this was considered to signify a complete reaction. The number of moles of CO₂ absorbed and trapped by the rice cake layers was calculated according to the following equation [45,46]:

$$\mathrm{Moles}\mathrm{of}{\mathrm{CO}}_2\mathrm{captured}=\frac{{\int }_0^t{\mathrm{volume}\mathrm{of}\mathrm{CO}}_2\mathrm{captured}\left(\frac{1}{\min }\right)}{{\mathrm{Molar}\mathrm{volume}\mathrm{of}\mathrm{CO}}_2\left(\frac{1}{\mathrm{mol}}\right)}·dt$$

(1)

3. Results and Discussion

3.1. Puffed Rice Cake

To identify the most optimal production process characteristics for obtaining puffed rice cakes with a thickness and porosity that would maximize CO₂ removal, extensive optimization experiments were conducted, revealing that 18% moisture content and 260 °C temperature are the most optimal, as they result in the greatest internal porosity and maximum sample thickness (12 mm), as shown in

Table 1. Proposed gas separation material with different moisture content and temperature.

Moisture [%]	Temperature [°C]	Thickness [mm]
18	260	12
16	240	9
14	220	8
12	200	7

.

3.2. Fresh and Treated Rice Cake Characterization

3.2.1. XRD Analysis of Fresh and Treated Puffed Rice Samples

The structural properties for fresh and treated puffed rice samples were examined using an X-ray diffractometer at a Cu Kα radiation wavelength of 1.54 Å (Figure 8). The fresh samples showed an A-type diffraction pattern with one intense peak at 2θ = 20.3°. In contrast, samples treated with sodium hydroxide exhibited a V-type pattern, with two peaks at 2θ values of 17.2° and 20.3°. This change in pattern type is related to the formation of a hydroxide bond in the treated puffed rice [47].

3.2.2. Scanning Electron Microscopy (SEM)

SEM micrographs of fresh and treated sample cross sections displayed clear morphological differences, as shown in Figure 9. The internal structure of fresh samples was highly porous with several cavities of different sizes [47,48]. The analysis also revealed the presence of voids and walls within the rice structure, as indicated by black and white colors in the micrographs, respectively. Available evidence shows that these black channels, which consist of voids and gaps, can be altered by physical pre-treatment processing to optimize gas permeation [49]. On the other hand, although voids and cavities were still present in the structure of the treated samples, their size markedly increased. These findings indicate that chemical treatment is beneficial for CO₂ removal through adsorption and absorption processes.

3.2.3. Fourier Transform Infrared (FTIR) Spectroscopy

The fresh and treated samples were also subjected to FTIR analysis to investigate the presence of effective functional groups, and the findings are presented in Figure 10. As can be seen from the spectra pertaining to both samples, at λ = 1250−1500 cm⁻¹ and 1500−1700 cm⁻¹, peaks that represent aromatic asymmetric stretching (C=C) and C=O acetyl group were produced, respectively, but were of lower intensity for the treated sample. On the other hand, no significant effect of the chemical treatment with sodium hydroxide on the absorption band at λ = 910−1010 cm⁻¹ is evident. Finally, the peak within the 3250−3500 cm⁻¹ range shows that NaOH treatment increases the sample absorption range compared to the fresh puffed rice sample. This peak represents hydrogen-bonded stretching absorption of the OH functional group in cellulose [50]. Accordingly, these results indicate that NaOH treatment is successful in exposing puffed rice to the hydroxyl group, which is the base in the CO₂ absorption/adsorption process and can improve its reactivity with CO₂ molecules.

3.2.4. Thermogravimetric Analysis

Thermogravimetric analysis of the fresh and treated rice samples was conducted using a Q500 series thermogravimetric analyzer from TA Instruments. Throughout the thermodecomposition process, sample mass was continuously recorded as a function of time and temperature. Derivative thermogravimetry (DTG) curves were used to describe the weight loss of each sample per unit of time with respect to temperature [51]. As can be seen in Figure 11, as the temperature increased from 20 °C to approximately 311 °C, sample mass declined by 2.88% as a result of moisture removal and degradation of light organic compounds (which occurs at around 243.52 °C and results in mass loss of around 1.613%). TGA findings further revealed that the decomposition rate was markedly reduced due to sodium hydroxide treatment, suggesting that the thermal stability of treated puffed rice was slightly reduced [51]. Moreover, at temperatures above 243.52 °C, mass losses declined by about 30%. These results confirm that treated puffed rice is suitable for CO₂ capture, especially at temperatures below 200 °C.

In addition, puffed rice cakes are economically and technologically viable CO₂ absorption media due to their high mass transfer area and strong thermal and chemical resistance. Moreover, as rice cakes can be modified and functionalized, their permeability for free CO₂ gas, as well as selectivity for CO₂ gas molecules, can be increased. When used as a permeable barrier, different components can penetrate the cake structure at different rates or be absorbed completely and thus separated from the gas flow. In particular, the large voids and cavities in the treated puffed rice cake structure would allow CO₂ to dissolve, completely eliminating it from flue gas. In addition, CO₂ solubility can be modified by changing the temperature and pressure used in the rice cake manufacturing process. Finally, as rice cakes are cheap to produce, they are a viable option for large-scale commercial applications.

3.2.5. CO₂ Capturing Efficiency

The CO₂ removal percentage was calculated and plotted for each experiment at different gas flow rates and NaOH molar concentrations. To calculate the moles of CO₂ removed by the rice cake layers, integration for the area under the curve (CO₂ parentage in the treated gas versus flow time data) was calculated by graph software to provide the total volume of CO₂ loaded to the contactor media within the specific contact time, as shown in Figure 12.

When stacked layers of puffed rice cakes (SPRC) of 80 mm diameter and 12 mm thickness buffered with 0.5 M NaOH were placed inside the gas contactor system (20% of the reactor volume) while applying a 500 mL/min effluent gas flow rate, 0.27 mole of CO₂ (38% of the total that entered the reactor) was removed. Accordingly, connecting three such units in a series would ensure 100% removal, whereby standby units could be considered for the regeneration step.

Comparing these results to the total CO₂ capture efficiency for buffered multi-spaced puffed rice cakes (MSPRC) (80 mm diameter, 12 mm thickness, and 30 mm space in between layers), rice cakes buffered with 0.5 M NaOH were placed inside the gas contactor system (20% of the reactor volume) while applying 500 mL/min effluent gas flow rate, and 0.24 mole of CO₂ (34% of the total that entered the reactor) was removed. With puffed rice grains (PRG) at the same reactor volume ratio, NaOH molarity, and gas flow rate, 0.26 mole of CO₂ (37% of the total that entered the reactor) was removed.

For the stacked layers of puffed rice cakes (SPRC), the CO₂ removal percentage is related to the rice cake surface area, which is equivalent to 7.52×10⁻³ mole CO₂/cm² or 0.0168 L CO₂/cm² or 0.152 L of effluent gas/cm² for 10% CO₂. These results indicate that treated rice cakes can be adopted in CO₂ removal.

As a part of this investigation, the effect of gas flow rate and NaOH solution concentration on the CO₂ removal percentage was also studied. The 3D plot shown in Figure 13 represents the effect of these factors on the CO₂ moles loaded per puffed rice cake unit surface area when the contactor volume is increased from the 20% used in the experiments to 100%. These results indicate that an NaOH molar ratio of 1.5 M ensures maximum CO₂ loading, which can be explained by the negative effect of a low NaOH molar ratio (0.5 M) on the buffering level of the rice medium. On the other hand, a high NaOH molar ratio (2.5 M) decreases the pore size and thus the total contact surface area with the flue gas. The analyses further indicate that the gas flow rate should be reduced to enhance CO₂ loading, as this would increase contact time.

These results should be examined through further optimization studies, as they indicate that puffed rice cakes have the potential for use in CO₂ capturing from effluent gases at low cost while being environmentally friendly. Such a technology would greatly benefit the development of the food waste management sector in the UAE while reducing greenhouse gas emissions. Moreover, by achieving high CO₂ removal efficiency, the energy consumption of this process can also be reduced. As the contact device design described here has significant operational flexibility, it can be adapted to changes in applied gas pressure. Moreover, as rice samples developed as part of this work have a large mass transfer area and exhibit strong thermal and chemical resistance, they can be further modified and functionalized to increase permeability and selectivity, thus meeting the needs of a variety of industrial applications. Therefore, it is envisaged that the proposed method will be used in waste management (which would be of particular interest for companies that deal with food waste from hotels, restaurants, hospitals, and homes), gas purification systems manufacturing (benefitting companies that fabricate systems for treating different industrial gas effluent sources), and power generation (such as plants that discharge effluent gases with high CO₂ content), as well as in natural gas purification and gas separation in the petrochemical industry.

4. Conclusions and Future Work

We proposed a new process for CO₂ removal using a treated rice-based product in a special gas-contact media structure and design. Rice products were fabricated and puffed, then buffered using NaOH solution. The CO₂ removal process was applied under different conditions of gas flow rate and NaOH molarity. The most optimal condition was observed by using stacked layers of puffed rice cakes (of 80 mm diameter and 12 mm thickness) buffered with 0.5 M NaOH and at 500 mL/min effluent gas flow rate, which reduced the CO₂ content in a gas effluent by 38%, corresponding to CO₂ loading of 7.52 × 10⁻³ mole CO₂/cm² of rice cake. To further improve this approach, additional investigations into the regeneration process are needed, and the use of lime as a regeneration medium with different calcium oxide solution concentrations (e.g., 1−5 wt %) should be considered. Different methods of regeneration should also be explored, such as those based on solutions prepared with alkaline solid waste. In general, traditional methods of CO₂ removal are restricted by significant energy consumption and limited effectiveness. According to experimental observations of the traditional processes, the proposed process has a good chance to overcome the mentioned limitations. The approach aims to reduce cost and energy consumption and ensure high CO₂ removal efficiency. However, some limitations may be considered, such as the feasibility of collecting waste rice material and its dependence on geographical and statistical parameters. Other limitations are related to the cost of chemical treatment. However, this cost can be reduced using another buffering waste material as alternative. In general, the advantages of the proposed method and system are highly encouraging for more investigations and statistical optimization to reach the maximum efficiency with minimal operational cost.