## Abstract

Discarded cigarette butts contain polymers, biomass, and a variety of toxins that cause an adverse effect to the human health and environment for years. The cigarette residuals are not recyclable and often get mixed with other kinds of wastes so that much of this waste ends up in landfills. This study investigates the safe disposal of cigarette butts by the thermochemical pathways using pyrolysis and gasification. Mass loss during its thermal decomposition was examined first using a thermogravimetric analyzer. The effect of temperature on the pyrolysis and CO2-assisted gasification was then conducted using a semi-batch reactor with a focus on the flowrate of total syngas and its gas components. Syngas yield, energy recovery, as well as energy efficiency were calculated and compared. The effect of temperature on the CO2 consumption during the gasification process was also examined. The thermal decomposition of cellulose acetate, tar, and wrapping paper were the main contributors during the pyrolysis of cigarette butt. However, the gasification process mainly consisted of the pyrolysis, cracking, and reforming reactions in the gas phase and gasification of char derived from wrapping paper. An increase in temperature enhanced the syngas flowrate, syngas yield, and gas efficiency while decreasing the char yield and reaction time for both the processes. Energy recovery from gasification was higher than pyrolysis due to added CO generation. The maximum syngas energy of 13.0 kJ/g under the gasification condition at 1223 K was 67.2% higher as compared with the pyrolysis. High temperature strongly affected the gasification reaction, while it was negligible at a temperature lower than 1023 K. Complete conversion occurred during gasification at 1223 K that provided only ash residue. The CO2 gasification of cigarette butts provided an effective pathway to utilize 0.5 g CO2/g feedstock at 1223 K to form valuable CO by the Boudouard reaction. Compared with the gasification of other solid wastes, syngas energy yield from cigarette butts was found to be higher than syngas from polystyrene and polyethylene terephthalate. These results support the effectiveness of thermochemical pathways in the rapid conversion of cigarette butts to valuable syngas along with CO2 utilization.

## 1 Introduction

Cigarette butts are the most common garbage lying in streets, restaurants, bus stops, and other public places [1]. Although cigarette butts are small, they are considered harmful to the society and the environment. Cigarette butts cause pollution as they are difficult to recycle, and often discarded so that their collection and clean-up are difficult [2]. Cigarette butts are estimated to be the most discarded item in the world [3]. There are about 1 billion smokers worldwide, accounting for more than 6 trillion cigarettes a year, and more than 75% of cigarette butts are discarded in open environments [4]. China accounts for more than 40% of the world’s smokers and has the world’s largest production of cigarettes and packaging wastes. More than 150 billion cigarette butts are discarded in the United States each year, and the amount of such wastes and associated pollution issues poses a serious environmental threat [5]. Discarded cigarette butts retain carcinogenic and toxic substances when littered in the open environment that leach out to pose a threat via the contaminated soil, water, and biota [6]. Cigarette butts are also the most common form of plastic waste found along ocean coasts, threatening the quality of seawater and the survival of marine life. Therefore, the development of technology for the safe conversion of cigarette butts into usable energy offers significant benefits for both environmental protection and energy production.

Cigarette butts are residuals remaining at the conclusion of smoldering phase combustion following smoking [7]. Cigarette butts consist of filter tips, wrapping paper, and residual (unburned) tobacco. The filter mainly consists of cellulose acetate polymer, which is kind of plastic and is hardly biodegradable. The used filter in cigarette contains large amounts of tar, which is produced from the incomplete burning of tobacco. Tar from cigarettes was reported to contain more than 7000 toxic chemicals, including up to 50 carcinogens, such as nicotine, nitroamine, acetaldehyde, phenol, formaldehyde, ammonia, pyridine, acetone, pyrene, heavy metals, and polycyclic aromatic hydrocarbons (PAHs) [8]. These toxins, after getting leached out from cigarette butts, are released into water, soil, and air so that they slowly find their path to come into direct contact with humans, animals, and vegetation, to cause a direct impact on the environment and human health. The literature reveals that the mass degradation of cigarette butts takes some 720 days in grassland soil and somewhat less in dune soil [9]. Single cigarette butt can contaminate 1000 l of water and can bio-accumulate in wildlife and human food chains [10]. Furthermore, cigarette butts are very easily consumed by marine creatures to disrupt their ecological system [11].

An inappropriate disposal of cigarette butts and their irreversible damage to the surrounding environment have motivated more cities to adopt legislation and also setup collection vessels to manage the safe disposal of cigarette butts [12]. The recovery and utilization of cigarette butt is mainly aimed at the acetate polymer fiber in filter tip, nicotine [(S)-3-(1-methyl-2-pyrrolidinyl) pyridine], and solanesol (polyisoprenoid alcohol). Cigarette butts were directly pyrolyzed under different activation conditions to produce cigarette-derived functional carbon (CDFC) materials, which have dual functional applications in supercapacitors and water pollution removal [13]. Nitrogen-doped cigarette butt derived carbon (N-CBDC) was prepared by continuous carbonization, activation, and subsequent hydrothermal method and applied to electrode materials [14]. The production of fired clay bricks has also been examined in efforts to solve the problem of world’s cigarette butts [15]. Although these examinations can provide some ideas for cigarette butts utilization, the technology is not mature and the amount of cigarette butts collected from a given area is very limited so that most of the recovered cigarette butts are often mixed with other municipal solid wastes and used for direct combustion to generate power and electricity. In view of environmental and public health concerns, direct combustion is discouraged due to its low energy efficiency and emission of various gaseous pollutants such as PAHs, dioxins, SOx, and NOx as well as particulate matter during combustion.

Pyrolysis and gasification are alternative thermochemical technologies that can produce valuable chemicals by the thermal conversion of organic matter, and are considered as a commercially viable solution to achieve sustainable and renewable energy while avoiding pollutants formation and emission from combustion [1618]. Pyrolysis is mainly conducted in an inert atmosphere to produce syngas, tar, and char; their quality and yield can vary that depends on the operating parameters [19,20]. In general, fast pyrolysis with rapid heating rates and low residence time at low temperature is favorable to produce liquid fuels, while high-temperature pyrolysis results in the generation of syngas [21]. The pyrolysis of solid waste has been studied widely to produce syngas as well as unwanted tar and char during the conversion of wastes. The pyrolysis of polystyrene (PS) was conducted in a continuous two-stage process at different reactor temperatures to produce pyrolysis oil rich in benzene, toluene, ethylbenzene, and xylene [22]. An increase in temperature promoted the pyrolysis of pyrolytic oil to form light gases rich in hydrocarbons [23]. Gases generation from the pyrolysis of five different kind of common plastics at temperatures of 500, 700, and 900 °C was also investigated; the results showed the highest energy yield from the pyrolysis of polypropylene (PP) at 900 °C [24]. Apart from the pyrolysis process, gasification is also a good pathway to solve the problem of solid wastes. Gasification mainly produces desired syngas in the presence of some gasifying agent, such as air, oxygen, steam, or carbon dioxide. Pyrolysis, cracking, reforming reactions, and subsequent char gasification are all inclusive during the gasification process [25]. While there is little or no residue left over from gasification, it produces more syngas and conversion than pyrolysis. Ahmed and Gupta studied the characteristics of syngas produced during steam gasification of rubber and found that the syngas is mainly composed of hydrogen due to the char steam reforming reaction [26]. Burra et al. reported the syngas evolution from the air-assisted gasification of chicken manure; their results showed that higher O2 concentration resulted in higher CO2 generation at low temperatures, while at high temperatures, it enhanced the CO yield from the Boudouard reaction [27]. Compared with other gasifying agents, CO2-assisted gasification provided a direct pathway to utilize this greenhouse gas. The gasification of waste tires was investigated using CO2, and the results showed a high mole fraction of CO in the syngas that increased at high temperatures [28]. Singh et al. found that CO2-assisted gasification produced syngas with a mass-specific heating value similar to that of natural gas, suggesting that CO2 could be used for clean and efficient energy production [29]. Soreanu et al. examined the effects of temperature, pressure, and gas concentration on the CO2 gasification process and showed that the increase of temperature and pressure had a positive effect on the reactivity and gas rate, while the effect of CO2 concentration was negligible [30]. Gasification and pyrolysis technologies have been widely used in the management of biomass and solid wastes to result in valuable syngas yields. To the best our knowledge, no studies are reported in the literature on the pyrolysis and CO2-assisted gasification of cigarette butts. In the present work, a semi-batch reactor was used for the pyrolysis and CO2-assisted gasification of cigarette butts in a temperature of 1023–1223 K in steps of 50 K. Thermal decomposition kinetics, syngas evolution behavior, energy recovery, as well as the energy efficiency were investigated and reported here. The main focus was on converting cigarette butts into clean syngas by the thermochemical process demonstrating a robust pathway to simultaneously utilize cigarette butts and manage CO2 greenhouse gas.

## 2 Experimental Material and Facility

### 2.1 Material Preparation.

The cigarette butts examined here were collected locally. Note that cigarette butts are heterogeneous material containing mixtures of cellulose acetate, wrapping paper, and tar. The collected cigarette butts were then dried for 48 h at a temperature of 378 K to minimize the moisture content. They were fully mixed and milled and examined using thermogravimetric analysis as well as proximate and ultimate analysis. The samples were also used for pyrolysis and gasification studies using a small-scale reactor. Proximate and ultimate analysis as well as low heating value (LHV) of cigarette butts was carried out on the basis of China’s National Standard GB/T 212 [31], GB/T 31391 [32], and GB/T 213 [33], respectively. The results are presented in Table 1.

Table 1

Proximate and ultimate analysis of the cigarette butt sample

Proximate analysis (%)Ultimate analysis (%)
MoistureAshVolatile matterFixed carbonOxygenCarbonHydrogenNitrogenSulfurLHV (MJ/kg)
5.914.7775.2814.0442.6848.806.461.800.2517.12
Proximate analysis (%)Ultimate analysis (%)
MoistureAshVolatile matterFixed carbonOxygenCarbonHydrogenNitrogenSulfurLHV (MJ/kg)
5.914.7775.2814.0442.6848.806.461.800.2517.12

### 2.2 Thermogravimetric Analysis.

To understand the thermal decomposition behavior of cigarette butts, thermogravimetric analysis was performed using a thermogravimetric analyzer (TGA, TA Instruments, Wilmington, DE, model STD-Q600) at a heating rate of 5 K/min. Argon (99.998% pure, Airgas LLC, Hyattsville, MD) was used as the purge gas at a flowrate of 100 ml/min. About 3 mg of the crushed and evenly mixed cigarette butt sample was placed in the alumina crucible. The sample was heated to 373 K and kept for 10 min to minimize the effect of moisture content on the mass change. The sample was then heated to 1173 K at a heating rate of 5 K. The variation of sample mass with temperature was recorded during the heating process.

### 2.3 Pyrolysis and Gasification Experiments

#### 2.3.1 Semi-Batch Reactor.

A semi-batch reactor was used for the pyrolysis and CO2-assisted gasification of the cigarette butts. The test facility consisted of gas supply, reactor, gas cleaning and storage, and gas analysis sections. The gases used were N2 (99.999% pure, Airgas LLC), CO2 (99.999% pure, Airgas LLC), and Ar (99.999% pure, Airgas LLC). N2 was used for pyrolysis and tracer gas, CO2 as a gasifying agent, and Ar to purge the sampling gas tank so that it was free of air or syngas from the previous test run. The gas velocity was adjusted through the orifice flowmeter and its input pressure. The reaction section consisted of two tube furnaces, and the first furnace with an inner diameter size of Ф 3 × 30.5 cm was used to preheat the gas. The main reactor for pyrolysis and gasification studies had a tube size of Ф 5 × 45.7 cm that was enclosed in a furnace (Lindberg/Blue M 1200 °C Split Hinge Furnace). The sample was placed into the main reactor via a fast connect located at the end of the main reactor. The gaseous products evolved from the thermal decomposition reactor were passed through three ice-bath bottles, followed by a filter to remove moisture and tar. The gas line was divided into two paths after the ice-bath bottles after which most of the gaseous products exhausted to the environment line. Only a small amount of gas entered the sampling bottle using a pump that created a negative pressure. Five sampling bottles were used only when short sampling intervals of 0.5, 1, 2, 3, 4 min were needed. The sixth bottle was used as a passage way to introduce the syngas directly for online gas analysis for sampling intervals of 2.66 min or more. Micro-GC (Agilent 3000A) equipped with three channels for gas separation and a thermal conductivity detector (TCD) was used to measure the volumetric composition of stable gaseous products, such as H2, N2, CO, CO2, CH4, C2H6, C2H4, C2H2, and C3H8. By comparing with the known N2 flowrate, the corresponding flowrates of the individual component of gas species can be calculated using Eq. (1). The yield of individual gas species can be obtained from the integral of mass flowrate with time, see Eq. (2):
$Mi=XiXN2×VN2×ρi$
(1)
$Yi=∫0tMidt$
(2)
where Mi stands for the mass flowrate of the desired gas species, Xi is the mole fraction of the gas species measured from the GC and ρi is the density of the gas species at standard temperature and pressure; XN2 the mole fraction of N2; VN2 the volumetric flowrate of N2 in the sample which is equal to the flowrate at the inlet of the reactor; Yi the mass yield of the desired gas species, and t the time during the process.

#### 2.3.2 Operating Condition.

The effect of temperatures on the pyrolysis and gasification process of cigarette butts was investigated at temperatures of 1023, 1073, 1123, 1173, and 1223 K. The sample container is a cylindrical hollow glass tube with an inner diameter size of Ф 4 × 20 cm. A minimum temperature of 1023 K was chosen because CO2 gasification is generally considered to be negligible below 1000 K [34]. For the pyrolysis cases, the flowrate of N2 was fixed at 2.1 l/min and the reaction time was 19 min after which no syngas evolved. For the gasification case, CO2 at a flowrate of 1.58 l/min was used, while N2 trace gas flowrate was 0.52 l/min for a reaction time of 39 min. This is because the syngas continued to evolve up to this time. Thus, two different times were chosen for pyrolysis and gasification to ensure that no further gas evolved beyond the time examined. The detailed operating conditions set for the pyrolysis and CO2-assisted gasification are listed in Table 2.

Table 2

Operating conditions for pyrolysis and gasification in the semi-batch reactor

ConditionsThermal conversion process
PyrolysisGasification
Sample mass25 g25 g
Gas mediumPyrolytic gas N2CO2 for the gasifying agent and N2 for tracer gas
Gas flowrate2.1 l/min1.58 l/min of CO2 and 0.52 l/min of N2
Reactor temperature1023, 1073, 1123, 1173, and 1223 K1023, 1073, 1123, 1173, and 1223 K
Operating pressureatmosphericatmospheric
No. of times experiments repeated≥3≥3
Total (isothermal) reaction time19 min39 min
ConditionsThermal conversion process
PyrolysisGasification
Sample mass25 g25 g
Gas mediumPyrolytic gas N2CO2 for the gasifying agent and N2 for tracer gas
Gas flowrate2.1 l/min1.58 l/min of CO2 and 0.52 l/min of N2
Reactor temperature1023, 1073, 1123, 1173, and 1223 K1023, 1073, 1123, 1173, and 1223 K
Operating pressureatmosphericatmospheric
No. of times experiments repeated≥3≥3
Total (isothermal) reaction time19 min39 min

## 3 Results and Discussion

The thermal decomposition of cigarette butts is first presented here. The effect of reactor temperature on the syngas characteristics in terms of syngas composition, gas products flowrate, CO2 consumption, syngas yield, and energy recovery from pyrolysis and CO2-assisted gasification are then evaluated using the experimental facility shown in Fig. 1. The syngas yield and syngas energy evolved from cigarette butts were then compared with that evolved from other common plastic wastes such as PS, PP, and polyethylene terephthalate (PET) to evaluate the feasibility of cigarette butt disposal.

Fig. 1
Fig. 1
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### 3.1 Thermogravimetric Analysis.

The decomposition behavior of cigarette butts was obtained using thermogravimetric analysis at a heating rate of 5 K/min. The results obtained on the extent of conversion α and /dT as a function of temperature are shown in Fig. 2. The extent of conversion was calculated according to the equation α = (m0mT)/(m0m1), and /dT was the derivative of the extent of conversion with respect to the independent variable temperature, where m0 is the mass of the sample at a temperature of 373 K, m1 is the residual mass of the sample at a temperature of 1173 K, and mT is the sample mass at temperature (T). The results show that the extent of conversion experienced two rapid increases and one slow increase with an increase in temperature, see Fig. 2. Among them, the second phase of growth accounted for more than 80% of the conversion. The variation of conversion can be analyzed from the parameter /dT and that the thermal decomposition process can be grouped into three temperature ranges of 373–493 K, 493–673 K, and >673 K. Cigarette butt consists of cellulose acetate, remained tar, and wrapping paper. Note that the cigarette butt mixture was heated to 373 K for 10 min to minimize the moisture content. Therefore, the first peak in the temperature range of 373–493 K was due to the devolatilization of residual cigarette tar present in cigarette butt. The maximum weight loss at the second peak mainly corresponds to the decomposition of cellulose acetate and wrapping paper, which are the main component in cigarette butt. The third stage showed a slow change of sample mass with temperature, and this was mainly from the decomposition of char. The thermal decomposition of cigarette butt at a low heating rate results in slow pyrolysis [32]. Therefore, the pyrolysis of cigarette butt at high temperature mainly consists of the decomposition of tar, cellulose acetate, and wrapping paper.

Fig. 2
Fig. 2
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### 3.2 Pyrolysis and Gasification

#### 3.2.1 Effect of Reaction Temperature on the Mass and Morphology of Solid Residue.

The results on the mass of solid residue remaining of cigarette butts from the pyrolysis and gasification at different temperatures are shown in Fig. 3. A mass of 25 g of cigarette butts was used, and after the pyrolysis and gasification, most of the cigarette butts were decomposed to volatiles leaving behind only a small amounts of char which was attributed to be from the pyrolysis of wrapping paper. This phenomenon also indicates a relatively low decomposition temperature of tar and cellulose acetate and that it gets devolatilized at high temperatures. The pyrolysis of cigarette butts mainly consists of the decomposition of tar, cellulose acetate, and wrapping paper. The increase of pyrolysis temperature decreased the char yield. The char yield from pyrolysis decreased from 5.93 g at 1023 K to 4.04 g at 1223 K and is mainly attributed to the further decomposition of char and the condensed cyclization of benzene ring [33,34]. The CO2-assisted gasification of cigarette butts mainly consists of pyrolysis, reforming, and char gasification. An increase in gasification temperature showed a significant decrease in char yield. Char yield decreased from 3.72 g at 1023 K to 1.06 g at 1223 K during gasification. From the morphology of solid residue, it also displayed that all the charring content had already been consumed, with only ash remaining at 1223 K temperature [3537]. The Boudouard reaction between CO2 and char is the main reason accounting for the char consumption and mass loss [37,38]. The char yield from gasification was significantly lower than that from pyrolysis, indicating that gasification is beneficial for the treatment of cigarette butts and energy recovery.

Fig. 3
Fig. 3
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#### 3.2.2 Effect of Reaction Temperature on Gaseous Product Flowrate.

The results on the temporal evolution of syngas mass flowrate as well as major stable gaseous component flowrate during the pyrolysis and gasification of cigarette butts at different temperatures are shown in Figs. 48.

Fig. 4
Fig. 4
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Fig. 5
Fig. 5
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Fig. 6
Fig. 6
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Fig. 7
Fig. 7
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Fig. 8
Fig. 8
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Figure 4 shows the effect of temperature on the syngas flowrate during the pyrolysis and gasification. The results show that an increase in reactor temperature shifted the peak value toward shorter residence time, which is caused by the increase of temperature ramp rate and thermal decomposition rate under conditions of higher reactor temperatures [39]. The peak value also showed an increasing trend with an increase in reactor temperature. This can be seen from an increase in pyrolysis temperature from 1173 K to 1223 K. However, the syngas flowrate showed a decreasing trend with reaction time. Similar results were reported by other researchers from their experimental works on waste plastics. It is believed that coking and aromatization have negative effects on the syngas yield at high temperature [29,40]. Syngas flowrate with time from gasification was very different from that of the pyrolysis. The duration of syngas generation in gasification was much longer than that in pyrolysis.

The CO mass flowrates with time from the pyrolysis and gasification were similar to that of syngas flowrates, see Fig. 5. CO is the main gas product from pyrolysis and CO2-assisted gasification as this represented majority of the gas in syngas. At the same temperature, the syngas flowrate with time from the pyrolysis was very different than gasification. During the pyrolysis process, only one peak was found, mainly due to the decomposition of carbonyl and ether bond structures in biomass [41,42]. While during the gasification, the CO mass flowrate can be grouped into three stages of the pyrolysis peak followed by the stabilization and reduction stages. The reason for the first peak is due to the decomposition of cigarette butts, while the other stages were attributed to the Boudouard reaction between CO2 and cigarette char [43]. According to the CO flowrate of the stable segment, high temperature was beneficial to accelerate the Boudouard reaction rate. With the consumption of char, CO mass flowrate also decreased with time so that the syngas flowrate ended after the char was fully consumed at 1223 K. This explanation is also supported from the remaining mass of solid residue.

Figure 6 depicts the effect of temperature on the H2 mass flowrate, which shows that an increase in reactor temperature enhanced the mass flowrate and shifted the peak value toward shorter residence time for both pyrolysis and gasification. H2 formed from the condensed cyclization and dehydrogenation of benzene ring during the thermochemical conversion [44,45], is favored by high temperatures. In addition, it can also be found that H2 mass flowrate from the gasification was slightly less than that from pyrolysis at the same condition. The main reason for this phenomenon is attributed to reverse the water–gas shift reaction [46], and the consumption of the char which inhibits the condensed cyclization of benzene ring, and thus decreased H2 formation. Similar results were also reported by other researchers who investigated the H2 flowrate from the pyrolysis and CO2-assisted gasification of waste tires [28,47]. The mass flowrate of CH4 and other hydrocarbons C2 (C2H2, C2H4, C2H6) are shown in Figs. 7 and 8. An increase in the reactor temperature enhanced their mass flowrate and reduced the reaction time. The mass flowrate from the gasification was lower than pyrolysis, mainly due to the CO2 reforming reaction [48].

#### 3.2.3 Effect of Temperature on Gaseous Products Yield and Distribution.

Total syngas yields as well as different gaseous components yield are presented in Fig. 9. As shown in Fig. 9(a), apart from the pyrolysis of cigarette butts at 1223 K, an increase in temperature slightly increased the syngas yield. Syngas yield increased from 6.0 to 7.8 g/g, possibly due to the very low decomposition temperature of the cigarette butts. Besides, the syngas yield from gasification was higher than pyrolysis, and this is mainly attributed to the Boudouard reaction and CO generation. The syngas yield of 23.2 g at 1223 K showed its highest value from CO2-assisted gasification, while the highest syngas yield from pyrolysis was 7.8 g at 1173 K. The major contributor in the syngas yield was CO and its yield was 21.2 g at 1223 K during CO2-assisted gasification, which is almost 4.5 times higher than that from pyrolysis. Besides the CO and syngas yield, other gases (such as H2 and hydrocarbons) from CO2-assisted gasification were lower compared with pyrolysis at the same temperature. Chemical reactions such as reverse water–gas shift reaction and CO2 reforming reaction are attributed to the main reasons accounting for this phenomenon [49,50].

Fig. 9
Fig. 9
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Figure 10 shows the mole fraction of H2, CO, CH4, and other C2 gases (C2H2, C2H4, C2H6) at different reaction temperatures from the pyrolysis and gasification of cigarette butts. The results on syngas composition during pyrolysis shown in Fig. 10(a) reveal that an increase in reaction temperature significantly increased the mole fraction CH4 and C2 and decreased the CO mole fraction. This is mainly attributed to the relatively rapid growth of light hydrocarbons with temperature. Note that cellulose acetate provides a major proportion in the cigarette butt and pyrolysis is conducive to the conversion of cellulose acetate into light hydrocarbons rather than tar at high temperatures. As shown in Fig. 10(b), syngas composition from the gasification is different than that from pyrolysis. During the gasification, CO accounts for more than half of the syngas and its molar fraction increased with an increase in temperature. The mole fraction of CO increased from 69.0% at 1023 K to 81.5% at a temperature of 1223 K. The significant increase of CO resulted in the decreased proportion of other gases (hydrogen decreased from 11.5% to 7.2%, and CH4 decreased from 15.8% to 9.2%). After gasification at a temperature of 1223 K when all the cigarette butts had been consumed, the final molar fractions of H2, CO, CH4, and C2 were 7.2%, 81.5%, 9.2%, and 2.0%, respectively.

Fig. 10
Fig. 10
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#### 3.2.4 Effect of Temperature on Energy Output and Energy Composition.

The results on the evolutionary behavior of output power with time from the pyrolysis and gasification of cigarette butts are shown in Figs. 11(a) and 11(b). The results showed that an increase in reactor temperature enhanced the output power and shifted the power peak to shorter reaction times. The output power was completed in approximately 13 min during pyrolysis, with majority generated during the first 5 min. However, during gasification, a continuous evolution of output power can be seen until all the char was consumed. Most of the energy also yielded during the first 5 min, after which, the energy output was from the char gasification and CO generation. Figure 11(c) shows the cumulative energy yield of cigarette butts from pyrolysis and gasification. The syngas energy here was represented as a percentage of the initial mass of the cigarette butts sample given by total energy yield =Cumulative energy output/Initial mass of cigarette butts. An increase in temperature enhanced the syngas energy; the highest syngas energy was 8.2 kJ/g from pyrolysis at 1173 K and 13.0 kJ/g from gasification at 1223 K. The reason for a decrease in energy yield from pyrolysis at 1223 K is attributed to the reduction of CO and CH4 yields. Also, the complete consumption of feedstock during the gasification at 1223 K indicated that total energy recovery from the cigarette butts is 13.0 kJ/g.

Fig. 11
Fig. 11
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The percent energy contribution from gases can reveal the contribution of each gas to the overall energy yield that can directly help improve the energy yield. The energy distribution of syngas at different reactor temperatures from the pyrolysis and gasification process is shown in Fig. 12. From the pyrolysis results, it can be seen that the energy contribution of gas products is in the sequence of CH4 > CO > C2 > H2. Although the yield of CH4 was lower than that of CO, higher LHVs resulted from its higher energy composition gases. Besides, although hydrogen is low in mass, it contributed to more than 10% of the energy due to its high mass-energy density. In the CO2-assisted gasification process, CO contributed to the majority of energy from the Boudouard reaction. With an increase in reaction temperature that increased the CO yield, the contribution of CO to energy also increased and reached to 66.3% at a temperature of 1223 K.

Fig. 12
Fig. 12
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#### 3.2.5 Effect of Temperature on Energy Efficiency.

The effect of temperature on the cold gas efficiency (CGE) and overall energy efficiency (OEE) is illustrated in Fig. 13. Overall energy efficiency versus time at different reactor temperatures are also showed in Fig. 14. The cold gas efficiency and overall energy efficiency were calculated from Eqs. (3) and (4):
$Coldgasefficency(CGE)=msyngas×LHVsyngasmcb×LHVcb$
(3)
$Overallenergyefficency(OEE)=msyngas×LHVsyngasEnergyinput+mcb×LHVcb$
(4)
where mcb is the mass of cigarette butts, and LHVcb is lower heating value of cigarette butts. The energy input in the equation was obtained from the electrical energy consumption from the electrical furnaces after the pyrolysis and gasification. A low heating value of 17.1 MJ/kg was used here for the cigarette butts. The cold gas efficiency from the pyrolysis and gasification process showed a similar trend as that of energy yield. The highest cold gas efficiency was 48.0% from the pyrolysis at 1173 K and 75.9% from gasification at 1223 K, respectively. As to the overall energy efficiency, it can be seen from Fig. 14 that the overall energy efficiency showed a rapid increase followed by a decrease with reaction time. A decrease in overall energy efficiency was mainly because of energy production from cigarette butts was lower than electrical energy consumption. Besides, apart from the pyrolysis of cigarette butts at 1223 K, overall efficiency increased with an increase in temperature due to an increase in syngas yields. The efficiency peak reached 37.0% from pyrolysis at 1173 K and 32.8% from gasification at 1223 K, respectively. A similar trend was also found by Wang et al. during the CO2-assisted gasification of pine bark [51]. The overall energy efficiency from the pyrolysis and gasification indicates the good potential of using both the pyrolysis and CO2-assisted gasification of cigarette butts for useful sustained energy production while simultaneously mitigating environmental and health issues from landfilling of cigarette buts.
Fig. 13
Fig. 13
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Fig. 14
Fig. 14
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### 3.3 CO2 Consumption From Gasification.

CO2-assisted gasification not only converts waste into usable gaseous fuels but also considered as an important pathway to decrease the greenhouse gas emission. Therefore, it is of great value to calculate the CO2 consumption during the gasification of examined wastes. The CO2 consumed during the gasification process was obtained by the difference from the CO2 flowrate fed into the reactor and CO2 flowrate measured in the product gases. At the initial first few minutes, the negative peak was found due to the decomposition of cigarette butts which leads to CO2 formation. An increase in the temperature also shifted the negative peak toward shorter times. After that, a positive peak was formed due to the char gasification, and higher reaction temperature resulted in higher mass flowrate. The gradual reduction in CO2 mass flowrate with time was attributed to char consumption. After all the char was consumed (after about 35 min at a temperature of 1223 K), the CO2 consumption was evaluated to be zero. The cumulative CO2 consumption is shown in Fig. 15(b), which shows very few amounts of CO2 consumed at 1023 K. An increase in the reaction temperature enhanced the CO2 consumption, and highest CO2 consumption was 0.5 g/g at 1223 K temperature. The CO2 consumption from cigarette butts is lower compared with 0.8 g/g from pine wood as reported by Adnan and Hossain [46], and the difference between these two wastes is mainly from the char yield. Therefore, the CO2-assisted gasification is not only beneficial to the waste disposal but also offers a potential path for the utilization of this greenhouse gas.

Fig. 15
Fig. 15
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### 3.4 Comparison of Energy Recovery From Different Solid Wastes.

To compare the energy recovery of cigarette butts with other solid wastes, the CO2-assisted gasification of other common solid wastes such as PS, PP, and PET were conducted at a temperature of 1173 K. The gas yield and energy recovery from the gasification of different solid wastes were compared and are presented in Table 3. It can be seen that the main components in syngas yield from cigarette butts were different than that from other solid wastes. The H2 yield of cigarette butts was lowest compared with other solid wastes, the CO yield of cigarette butts was higher than that of PS and PET, while the CH4 yield was the highest as compared with other plastics. Differences in gaseous yield were caused mainly by molecular structure, H/C and O/C ratios [52]. Besides, the total syngas and energy yield of cigarette butts were higher than PS and PET, but lower than that of PP. The CO2-assisted gasification of solid wastes such as PS, PP, and PET has been studied by several researchers, and their results showed good potential to convert the solid waste into more favorable syngas [29,48]. Therefore, by comparing the gasification energy of other solid wastes, it can be concluded that CO2-assisted gasification can also be used as a potential pathway for the safe disposal of cigarette butts to syngas.

Table 3

Gas yield and energy recovery from the gasification of different solid wastes

Cigarette butt (CB)PSPPPET
H2 (g/g)0.00490.00910.02000.0126
CO (g/g)0.74730.67640.98120.7395
CH4 (g/g)0.05310.00560.05210.0267
Syngas (g/g)0.82560.70271.12590.7879
Energy (kJ/g)11.78688.790418.398210.7904
Cigarette butt (CB)PSPPPET
H2 (g/g)0.00490.00910.02000.0126
CO (g/g)0.74730.67640.98120.7395
CH4 (g/g)0.05310.00560.05210.0267
Syngas (g/g)0.82560.70271.12590.7879
Energy (kJ/g)11.78688.790418.398210.7904

## 4 Conclusions

Thermochemical pathways were investigated to convert waste cigarette butts into clean syngas. The pyrolysis and CO2-assisted gasification of cigarette butts were conducted in a semi-batch reactor at a temperature of 1023–1223 K in steps of 50 K. The decomposition of three main components, cellulose acetate, wrapping paper, and residual tar mainly occurred during the pyrolysis of cigarette butts. Pyrolysis was followed by CO2 reforming and char gasification during the gasification process. The effects of temperature on the gases flowrate, syngas component yield, syngas energy, and energy efficiency were investigated. An increase in the temperature enhanced the gas flowrate and syngas energy for both the pyrolysis and gasification processes. Energy recovery from the gasification was higher than from pyrolysis due to the Boudouard reaction. The syngas energy was found to be 5.9 kJ/g for pyrolysis and 6.0 kJ/g for gasification at 1023 K. While at 1223 K, it was increased to 7.8 kJ/g for pyrolysis and 13.0 kJ/g for gasification. Overall energy efficiency increased followed by a decrease with time due to changes in energy generation and electricity consumption. Higher temperature resulted in higher overall efficiency during gasification and reached maximum value of 32.8% at 1223 K. The consumption of greenhouse gas CO2 at different reaction temperatures was also determined during the gasification process. Almost no CO2 was consumed at 1023 K; however, with an increase in temperature, the CO2 consumption was evaluated to be 0.5 g/g at 1223 K temperature. In addition, energy recovery from the gasification of cigarette butts was compared with other three different plastic wastes. The energy yield from cigarette butts was found to be higher than polystyrene and polyethylene terephthalate. Therefore, high-temperature CO2-assisted gasification provides a promising pathway for the management of cigarette butts to clean energy generation while simultaneously using CO2 greenhouse gas.

## Acknowledgment

This research was supported by the US Office of Naval Research (ONR) and is gratefully acknowledged. The support provided to JL, ZW, and XL by National Natural Science Foundation of China (51974301) and China Scholarship Council (CSC) is also gratefully acknowledged. The authors also acknowledge Mengju Zhang from Henan Key Lab of Biomass Energy, Energy Research Institute Co. Ltd., Henan Academy of Sciences for providing the ultimate analysis of the feedstock samples.

## Conflict of Interest

There are no conflicts of interest.

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