Energy Recovery of Expired Pistachios From Pyrolysis and CO₂-Assisted Gasification

According to the Food and Agriculture Organization of the United Nations (FAO), food waste is defined as “food that is removed from the food supply chain” [1]. People waste a large amount of food every year, about 13 tons per year. Improper disposal of large quantities of expired food products can cause great harm to both the ecological environment and human health [2]. A recent survey indicates that the food waste accounts for about 15% of solid municipal waste in the United States, and only a small amount of expired food is recycled. Therefore, the safe disposal of expired foods deserves further consideration and research [3,4]. Currently, the treatment choices for food waste often consist of anaerobic digestion, incineration, composting, and landfill, but all of these treatment methods are greatly limited by their own challenges and technological shortcomings. Anaerobic digestion technology is capable of breaking down organic matter into renewable energy sources such as CH₄ and has a low environmental impact. However, it takes too long to fully support the plight of expired food disposal. Landfills are the most expensive way to dispose of the wastes and are also limited by land resources. Incineration is a traditional thermochemical method that can be used to generate electricity, but the toxic and harmful gases produced by the power generation process seriously impact the environment [5]. Therefore, a sustainable technology is urgently wanted to handle the huge amounts of expired foods generated every year.

The main components of nuts are cellulose, hemicellulose, lignin, and fat [6,7]. Expired nuts still contain the same amounts of oil, sucrose, and reduced sugar as those still within their shelf life [8]. Among nuts, pistachios are higher in energy and are high in unsaturated fatty acids, having about 87% of their total content [9]. When pistachios are out of date (expired), a large number of unsaturated fatty acids will produce acid decay and breed aflatoxin, which is harmful to both humans and animals, so it cannot be treated as fertilizer [10]. Treating expired pistachios by direct disposal cannot achieve energy recovery and by direct incineration it can cause more environmental pollution and also causes much waste of energy [11]. In general, long-time degradation, low utilization, and significant environmental damage to the environment are the amongst the challenges in energy recovery from expired pistachios [12,13]. Consequently, the development of technologies for the safe and efficient conversion of expired pistachios into usable clean energy is of great importance for reducing food waste and environmental pollution, increasing economic cycles, and clean energy production [13].

With the limited resources of fossil energy, biomass not only has the potential of compensating for the limited resources of fossil energy but also plays an important role in reducing fossil energy consumption and also reduction of CO₂ emission [14–16]. Pyrolysis and gasification technologies have a great potential to convert biomass into energy and can be considered as a sustainable strategy for the treatment of waste foods. Pyrolysis is the thermal degradation of substances in inert gases that produces energy in the form of bio-oils, biochar, and gaseous products. Different pyrolysis techniques, such as pyrolysis temperature and ramp-up rate as well as various reactors configurations, have been shown to have an impact on the reaction products [17,18]. The pyrolysis of solid waste materials has been extensively researched. Pyrolysis technologies such as conventional heating, microwave or plasma-assisted and hydrothermal methods have been proposed and used to produce liquid, gaseous fuels, and char [15]. Biomass gasification is a thermochemical conversion process that takes place at high temperatures in the presence of some gasifying agent [19]. Hydrocarbon feedstocks can produce valuable syngas at high temperatures of gasification using different gasifying agents (air, CO₂, O₂, water vapor) [20]. Extensive studies are available in the literature that reports on the energy recovery and reuse of biomass, such as the production of H₂, alkanes, and olefin through pyrolysis and gasification [21,22]. Researchers generally agree that thermal conversions by pyrolysis and gasification are effective ways to reuse biomass and solid wastes, converting them into syngas that can be used to generate electricity and heat [23]. The positive effect of higher temperature and pressure on the reaction was confirmed in a microalgal gasification study by Soreanu et al. [24]. Lin et al. [25] investigated the different performance features of hemicellulose, lignin, and cellulose during pyrolysis by using a thermogravimetric analyzer (TGA). It is shown that there are four intervals of the whole process of pyrolysis as follows: moisture evolution below 220 °C, mainly hemicellulose decomposition from 220 to 315 °C, cellulose decomposition from 315 to 400 °C, and lignin decomposition above 400 °C [25]. As biomass pyrolysis produces undesirable amounts of tar, adding gasifying agents can provide a new idea for biomass conversion. Using different gasification agents, Pinto et al. [26] studied the effect of rice waste mixture production on co-gasification and found that using CO₂ as a gasification agent reduces the tar content in syngas by 45% and increases syngas production by about 70%. Butterman and Castaldi [27] carried out gasification experiments on mixed substances such as bark, needle-shaped leaves, and grass in a mixed atmosphere of CO₂, and steam of different qualities and found that CO production increased with the increase in CO₂ amounts at a higher gasification temperature. In addition, the researchers gasified biomass in both CO₂-containing and CO₂-free gases and found that the CO₂-free group resulted in a large residue, while the CO₂-containing group produced only a small amount of light mineral residue [28]. Pistachio is a common household food that belongs to the biomass category, which can generate a large amount of clean gas energy during pyrolysis and CO₂-assisted gasification, and thus offers a good resource utilization potential. Based on the composition of pistachios, the approximate products of pyrolytic gasification of expired pistachios can be evaluated from previous studies as H₂, CO, CH₄, and some organic substances (mixtures of acids, aldehydes, alkanes, and ethers) [29,30]. However, further studies are needed for specific product yields and energies to help support the effects of reusing expired foods.

In this study, we used a semi-batch reactor to pyrolyze expired pistachios at temperatures of 823, 973, 1073, 1173, and 1223 K. Since the gasification below 973 K is normally considered as insignificant, experimental temperatures of 973, 1073, 1173, and 1223 K were selected for CO₂-assisted gasification. The syngas production, energy production, and consumption of CO₂ during gasification were examined under the above conditions. This study aims to reduce food waste and environmental pollution by converting expired (hydrocarbon) pistachios into clean usable syngas and also reducing CO₂ emissions through thermochemical methods.

The pistachios selected for the experiment were expired foods that had been sitting for a long period of time. Whole pistachios including shells and kernels were selected for the study. Pistachios were dried at 378 K under a negative pressure of 0.08 bar for 10 h to drain the water away from the pistachios. Dried samples were then subjected to pyrolysis and CO₂-assisted gasification for further analysis.

M_i represents the mass flowrate of the ideal gas and X_i represents the molar fraction of the gas measured by gas chromatography. ρ_i indicates the gas density at standard temperature and pressure. $XN2$ represents the molar fraction of N₂ when $VN2$ is the N₂ volume flowrate in the sample. Y_i is the yield of each gas.

Temperature effects on the pyrolysis process of expired pistachios were investigated at 823, 973, 1073, 1173, and 1223 K. The gasification of expired pistachios was examined at temperatures of 973, 1073, 1173, and 1223 K. Through a quick elbow on the experimental platform, the 35 g dried (expired) pistachio sample was quickly inserted into the center of the reactor. A brief detail on the operating conditions examined under the pyrolysis and gasification conditions performed in this study is shown in Table 1.

The pyrolysis and CO₂-assisted gasification of expired pistachios were performed using an experimental setup as shown in Fig. 1. The effects of different reaction temperatures on the evolution of syngas during pyrolysis and CO₂-assisted gasification experiments were investigated here including the composition of syngas, the mass flowrate of each gas, and syngas mass flowrates and energy. The differences between pyrolysis and CO₂-assisted gasification reactions at the same temperature were compared, and the CO₂ consumption for CO₂-assisted gasification was calculated. Furthermore, the experiments evaluated the effect of pyrolysis and gasification reactions on the conversion of expired pistachios to valuable clean syngas and the reduction of CO₂ emissions.

A comparison of the solid residue left after the pyrolysis and CO₂-assisted gasification reactions of expired pistachios at different temperatures is shown in Fig. 2. The findings indicated that the higher the reaction temperature, the less the solid residue. Previous data showed that the hemicellulose, cellulose, and lignin in expired pistachios and shells can be pyrolyzed at 550 °C to produce a large amount of volatile substances [31]. The experimental temperatures examined in this study were all above this temperature. There were a large number of unsaturated fatty acids esters present in pistachio nuts, which were mostly C_16∼30 compounds with an even carbon number. Unsaturated fatty acids are less stable at high temperatures, and that large amounts of saturated hydrocarbons were precipitated. Compared to pyrolysis, the solid residue remaining of expired pistachios from the CO₂-assisted gasification reaction was less, which was as expected since the CO₂-assisted gasification reaction of expired pistachios included the rapid pyrolysis of the carbon-based material and the subsequent gasification of char [32,33]. The Boudouard reaction between CO₂ and char allowed for the pistachios to be consumed more completely [34]. At a CO₂-assisted gasification temperature of 1223 K, only the ash remained, indicating that the carbon in the feedstock was completely consumed at this temperature.

The volume fraction distribution of different components in the produced syngas at different examined temperatures is shown in Fig. 3. The main components of syngas produced by pyrolysis and CO₂-assisted gasification reactions were H₂, CO, CH₄, and C₂ (C₂H₆, C₂H₄, and C₂H₂). Figure 3(a) demonstrates that the volume fraction of H₂ produced from the pyrolysis of expired pistachios increased with an increase in temperature, indicating that higher temperature facilitates the dehydrogenation reaction. The rise in reaction temperature from 823 K to 1073 K increased CH₄ and C₂ volume fractions and decreased CO, mainly because of the faster increase in the production of hydrocarbons with an increase in temperature. After 1073 K temperature, the volume fraction of CO and hydrocarbons was essentially stable. The pyrolysis of expired pistachios at 1223 K showed that the volume fractions of H₂, CO, CH₄, and C₂ were 29.88%, 28.84%, 26.61%, and 14.68%, respectively. Figure 3(b) illustrates that the volume fraction of CO in the syngas produced by CO₂-assisted gasification at the experimentally selected temperatures was all above 45%. At increased temperatures, the percentage of CO volume fraction increased, reaching 74.67% at 1223 K. At 1223 K gasification temperature, expired pistachios were completely consumed, and the volume fractions of H₂, CO, CH₄, and C₂ were 8.82%, 74.67%, 10.44%, and 6.07%, respectively.

The evolutionary behavior of the mass flowrate of syngas and the mass flowrate of each component during pyrolysis and gasification of expired pistachios with different temperatures are shown in Figs. 4–8. The gaseous yields were quantified using Eqs. (1) and (2).

Figure 4 shows the temporal variation in syngas mass flowrate and syngas yield during pyrolysis and CO₂-assisted gasification of expired pistachios at different temperatures. An increase in temperature led to a significant increase in both syngas yield and reaction rate. This is because the increase in temperature accelerates a range of reactions including cracking reactions, in both pyrolysis and gasification. Similar results were demonstrated in energy recovery from cigarette butts during pyrolysis and gasification [34]. As shown in Fig. 4(a), the yield of syngas from pyrolysis increased from 0.06 g/g to 0.41 g/g when the temperature was increased from 823 K to 1223 K. The syngas yield from CO₂-assisted gasification syngas increased from 0.20 g/g to 1.01 g/g with the increase in temperature from 973 K to 1223 K, see Fig. 4(b).

Figure 5 shows that the increase in temperature shifted the peak value in mass flowrate of H₂ earlier from the beginning of the experiment and that the mass flowrate of the gas gradually decreased so that the pyrolysis and gasification reactions completed sooner at increased temperatures. The higher the reaction temperature, the faster the evolution of H₂ mass flowrate which resulted in a larger peak value. The higher temperatures examined promoted the secondary cleavage of macromolecules that improved the efficiency of the pyrolysis and gasification processes [20,35]. Comparing Figs. 5(a) and 5(b), the H₂ production from pyrolysis at the same temperature is greater than that from the CO₂-assisted gasification reaction due to the reverse water-gas shift reaction (CO + H₂O↔CO₂ + H₂) [36].

Figure 6 shows that the maximum mass flow and yield of CO are proportional to the reaction temperature, and the increase in temperature leads to a shorter peak at the residence time. As the reaction proceeds, the CO mass flowrate gradually decreases. After ten minutes from the start of the pyrolysis experiment, the mass flowrate of CO was essentially negligible, see Fig. 6(a). Figure 6(b) shows the evolutionary behavior of the mass flowrate of CO during the CO₂-assisted gasification process. The gasification results on the evolution of CO mass flowrate with time are different from the pyrolysis results, wherein the CO₂ participated in the Boudouard reaction and continued to generate CO during the gasification process. This resulted in high cumulative CO yield in gasification than that in pyrolysis. At all temperatures examined (973, 1073, 1173, and 1223 K), the peak value of CO mass flow under gasification conditions is lower than that under pyrolysis reaction conditions. We conjecture that the participation of CO₂ in the Boudouard reaction makes the gasification process absorb more heat which decreases the temperature of the gas in the reactor, leading to a decrease in the reaction rate.

Figures 7 and 8 show the evolutionary behavior of the mass flowrate of CH₄ and C₂ (C₂H₆, C₂H₄, and C₂H₂) and their yield, respectively. With the increase of pyrolysis temperature, the yield of CH₄ and C₂ gases increases, and the peak mass flowrate of the individual gases increases. At increased temperatures, larger molecules of heavy hydrocarbons crack into smaller molecules of light hydrocarbons. However, at the highest temperature of 1223 K, the C₂ yield from pyrolysis showed a slight decrease, which may be related to the CO₂ reforming reaction. In the CO₂-assisted gasification process, the decrease in C₂ yield was more pronounced at 1223 K, which is attributed to the enhanced conversion of hydrocarbons to CO from the reverse water-gas shift reaction.

The experimental results showed that both pyrolysis and CO₂-assisted gasification reactions produced more syngas at increased reaction temperatures. The output power of the syngas was calculated based on the mass and low calorific value of each component in the syngas. The energy of the syngas was obtained from the integration of the syngas output power over time, see Fig. 9 [37]. An increase in temperature resulted in increased energy of the syngas, which suggests that temperature alone can effectively improve the energy conversion efficiency of expired pistachios. Comparing Figs. 9(a) and 9(b) shows that CO₂-assisted gasification offers greater capacity than pyrolysis, with syngas energies of 13.62 kJ/g and 17.27 kJ/g at 1223 K, respectively. The maximum energy produced from CO₂-assisted gasification of expired pistachios at 1223 K was found to be 17.27 kJ/g.

Gasification offered a higher energy recovery rate and a more complete reaction, as compared to pyrolysis, so that gasification can be considered better for the thermal treatment of organic solid waste [38,39]. In this study, CO₂ was selected as the gasification agent. Experiments were conducted on expired pistachios under pyrolysis and CO₂-assisted gasification conditions. The results showed that CO₂-assisted gasification provided less residue. It was found that the syngas from gasification was more energetic, indicating that CO₂ promoted energy recovery of the examined biomass. Note that CO₂ emission is an important contributor to the greenhouse effect. Experiments were conducted at a fixed CO₂ flowrate. The CO₂ consumption was calculated from the difference between the CO₂ flowrate into the reactor and the amount of CO₂ in the syngas.

The overall trend of CO₂ consumption is at first a negative peak, then rapidly increased to a positive peak, and finally, it slowly decreased. In the early stage of the reaction, the reaction is mainly from the pyrolysis of expired pistachios. The pyrolysis produced more CO₂, so that the CO₂ flowrate in the syngas was greater than that into the reactor to result in a negative value of CO₂ consumption. As time proceeded, the main reaction changed from pyrolysis to the gas–solid reaction of CO₂ and char (C + CO₂→2CO), which is an endothermic reaction, generally occurring above 700 °C. The increase in temperature promoted the reaction to accelerate and produce more CO [32]. Figure 10(a) displays that the increase in temperature resulted in more quickly achieving a positive CO₂ consumption and that at a higher temperature, the CO₂ consumption is higher. Figure 10(b) shows the CO₂ consumption of the gasification reaction at different temperatures. The CO₂ consumption is only 0.17 g at a reaction temperature of 973 K, while it reached a value of 23.49 g at 1223 K. Thus, CO₂ has a positive role as gasification in increasing the energy production and reducing greenhouse gas emission.

The paper investigated the generation of syngas from expired pistachios during pyrolysis and CO₂-assisted gasification. The evolutionary behavior of syngas yield and energy of each syngas component and total syngas at different temperatures were analyzed. The examined temperature ranged from 823 K to 1223 K for pyrolysis and from 973 K to 1223 K for the gasification experiments. The results showed that an increase in temperature can significantly promote the pyrolysis and gasification reactions and that the gas mass flowrate peaked faster and higher with the increase in temperature. The volume fraction of H₂, CO, CH₄, and C₂ during pyrolysis in expired pistachio syngas at 1223 K temperature was 29.88%, 28.84%, 26.61%, and 14.68%, respectively. However, in gasification at 1223 K temperature, the volume fraction of H₂, CO, CH₄, and C₂ in expired pistachio syngas was found to be 8.82%, 74.67%, 10.44%, and 6.07%, respectively. Under pyrolysis conditions, the syngas yield and energy yield were 0.06 g/g and 1.41 kJ/g, at 823 K, and 0.41 g/g and 13.62 kJ/g, respectively, at 1223 K. However, under gasification conditions, the syngas yield was 0.20 g/g and energy yield was 4.07 kJ/g at 973 K, which shows an increase to 1.01 g/g and 17.27 kJ/g at 1223 K. At 1223 K, the syngas yield and energy yield from the gasification reaction process were 2.43 and 1.3 times higher than that from pyrolysis, respectively. The CO₂ consumption during CO₂-assisted gasification also increased with an increase in temperature showing CO₂ consumption to increase from 0.0025 g/g to 0.67 g/g with an increase in temperature from 973 K to 1223 K. The CO₂-assisted gasification process offered a significantly higher capacity than pyrolysis that provided a promising way to use greenhouse gas for energy production. The CO₂-assisted gasification offers a promising way to treat expired food waste and generate energy.

This research was supported by the US Office of Naval Research (ONR) and is gratefully acknowledged. The support provided to Jinhu Li from “National Postdoctoral Program for Innovative Talents (No. BX2021001)” and “National Natural Science Foundation of China (No. 52104178)” is also gratefully acknowledged.

There are no conflicts of interest.

The authors attest that all data for this study are included in the paper.