Characteristics of syngas from pyrolysis and CO₂-assisted gasification of waste tires

Highlights

• Studied waste tire pyrolysis and CO₂-gasification in semi-batch reactor for syngas.

• Investigated the effect of temperature (973–1273 K) on syngas quality and yield.

• 0.75 g CO₂ was consumed per gram of waste tire for CO yield enhanced by 3.3 times.

• Energy efficiency of 30% and 62% cold-gas efficiency from waste-tire gasification.

Abstract

The growing problem of waste tire generation worldwide can be converted from a major environmental issue to a valuable energy source using the thermochemical conversion processes. CO₂ gasification can offer a prominent position in the tire waste to energy panorama since it offers high quality syngas production and direct mitigation pathway for greenhouse gas emissions. An evaluative study of syngas yield and quality between pyrolysis and CO₂ assisted gasification has been carried out in a laboratory scale fixed bed reactor in this work. Pyrolysis was performed in the temperature range of 673–1173 K and gasification at temperatures of 973–1273 K in steps of 100 K. Flow rates of syngas and its major gaseous components (CO, H₂, CH₄) for both the processes and on CO₂ consumption during gasification were reported. The results provided direct comparison between pyrolysis and gasification and also on cold gas efficiency. Results showed that gasification temperature strongly affects the syngas yield, quality, and energy content. Gasification reactions below 973 K were negligible. Char reactivity even at higher temperature was found to be low. Gasification resulted in 3.3 times increase in CO yield at 1073 K and 2.8 times increase at 1173 K as compared to pyrolysis. The increase in gasification temperature from 1173 to 1273 K enhanced CO yield by 1.5 times. While pyrolysis provided higher efficiency from a merely energetic point of view, gasification still presented high cold gas efficiency of 62.6% at 1273 K and an overall efficiency greater than 30%. In addition, CO₂ assisted gasification of waste tire provided a direct pathway to utilize green-house gas that showed carbon dioxide consumption of 0.75 g/gram of scrap tire gasified at 1273 K, and produced significant amounts of valuable CO, which offers good value for both energy production and fuel, and to value-added products with further synthesis.

Introduction

The continuous increase in energy demand, rising global warming level, great dependence of society on fossil sources for both fuels and raw materials have led to a growing interest in renewable sources and waste valorization processes. Scrap tires are a great example for waste valorization. This is due to high heating value in terms of lower-heating-value (LHV), second only to natural gas, and its wide availability. It was estimated that, worldwide, around 2.7 billion automotive tires were produced in 2017, and around 1 billion units were disposed [1], of which approximately 290 million were from the US alone [2]. In addition, the world demand for tires continue to grow, which is primarilyfrom the continuous increase in the number of vehicles. Furthermore, the average annual vehicle mileage travelled worldwide, is expected to increase by 4.1% per year, reaching 3.2 billion tires by 2022 [3]. Therefore, development in tire waste valorization, especially in the energy sector, is of pinnacle importance to restrain the environmental hazards linked to landfilling practice.

In 2017, around 40% of the scrap tires generated in the USA were combusted for energy in paper mills and other industries, while 25% used as ground rubber for landscaping and around 8% in other civil engineering applications [4]. Despite direct combustion being useful for both power and thermal energy generation, the pollutant emissions, such as polyaromatic hydrocarbons (PAH), dioxins, particulate matter, and SO_x produced during tire combustion discourage the use of this process in view of environmental and public health concerns [5]. On the other hand, pyrolysis and gasification are advanced thermochemical conversion techniques that offer favorable waste management options, especially for complex materials, such as waste tires, that cannot be easily remolded or recycled with conventional methods. Pyrolysis consists of thermal decomposition of the carbonaceous volatile fraction of the sample (rubber polymers in this case), carried out under inert atmosphere using gas, such as nitrogen, argon or helium to produce hydrocarbon-rich synthetic gas, oils and char whose quality and yields can be varied using operational parameters. In the gasification process, the feedstock to syngas conversion takes place under mild oxidizing conditions. Air, steam, CO₂ and their mixtures are used as gasifying agents depending on the desired syngas quality [1]. Both processes lead to the production of a solid residue (char), a fraction of highly viscous condensable vapors (tars) and gases.

Char from pyrolysis is rich in fixed carbon and retains the inorganic matter initially present in the tire, and it can be used either as low grade carbon black, coal supplement or as a precursor to manufacture activated carbon or for catalytic applications as a catalyst especially for tar cracking reactions [6], [7], [8]. During gasification, the oxidative environment leads to a lower yield of solid residue but higher porosity than pyrolytic one [3], [9]. The liquid phase obtained from pyrolysis consists of a complex mixture of heavy hydrocarbons (5–20 carbons) including aliphatic and aromatic hydrocarbons as the major compounds [6], [10]. It can be exploited either as a fuel with high HHV of approximatively 40 MJ/kg [10], [11] or in petroleum refinery applications as a source of refined chemicals such as limonene, benzene, toluene and xylene [6], [10]. Product gas obtained from pyrolysis and gasification mainly consists of H₂, CO, CO₂ and light hydrocarbons such as CH₄, C₂H₄, C₃H₆, C₄H₈ [6], [12], [13]. The gases can be used, after removing minor components such as particulates, tars, and contaminants, to produce electrical energy using gas turbines, gas engines, or fuel cells after extracting H₂ from the syngas (at high H₂/CO ratio using steam gasification) [14].

Although many studies on tire pyrolysis have been conducted on both laboratory and industrial scale in the past decades [15], [16], the results reported by different authors are often significantly different and sometimes contradictory, making the comparison among them difficult. This is because pyrolysis product yields and their percentage composition over the investigated temperature range depends on a broad variety of parameters, such as feedstock composition, operating temperature and pressure, gasifying agent, heating rate, apparent vapor residence time, reaction time and flow dynamics and transfer process phenomena. The heating rate depends on the temperature distribution, convective and radiative heat transfer conditions inside the reactor, as well as mass, size and shape of the feedstock particles.

Conesa et al. [17] performed pyrolysis of scrap tire (~4 mm) in a pilot scale fixed bed reactor over isothermal range of 723–1273 K at atmospheric pressure with vapors residence time estimated to exceed 10 min. Char yield increased from 35 to 37%, while oil yield decreased from 38 to almost 0, and gas yield increased from 27 to 62 wt% as temperature increased from 723 to 1273 K. High operating temperatures, heating rates with long residence time and slow quenching of the products promoted the production of syngas and enhanced the percentages of H₂, CO, CO₂ and light hydrocarbons in the gas phase [10]. This is from favored secondary reactions (i.e., cracking reactions) of the primary vapors formed in the pyrolysis process [18]. Significant decrease in oil yield and increase in gas has been confirmed by other studies [10], [19]. Zabaniotou et al. [20] reported pyrolysis of scrap tire in a wire-mesh micro-reactor at atmospheric pressure, at 70–90 K/s heating rate, and a vapor residence time of approximately 15 min. They reported a high gas yield of about 73% at temperature of approximately 1103 K. On the other hand, it is known that rather low heating rates (5–15 K/min), short residence times and fast vapors quenching benefits the oil yield. Under these conditions, a greater yield of heavier hydrocarbons was reported in the gas fraction, even at high temperatures [6], [12]. At low heating rate and short residence time, most of the thermal decomposition and gases generated were at pyrolytic temperatures of 673–773 K. Cunliffe et al. [18] investigated pyrolysis of waste tire at 723–873 K in a static bed batch reactor at a heating rate of ~5 °C/min and the vapors residence time of ~2 min. They reported a maximum oil yield of 58.2 wt% at 748 K along with gas yield of 4.5 wt% and no significant changes in liquid, char and gas yields with increase in temperature. Gas yields in the range 2–10 wt% with non-significant influence of reactor temperature on the relative composition of the product yields have also been reported [6], [7], [12]. Williams et al. [21] reported a decrease in char yield at short residence times, increase in temperature and heating rate. They attributed this to enhanced devolatilization of solid hydrocarbons present in the char. In contrast it is known that longer residence times of tars in hot zone of the reactor can result in the formation of coke-like carbonaceous materials due to secondary repolymerization and carbonization reactions [12], [18].

Gasification reactions, preceded by pyrolytic breakdown of the polymers, include the reaction of formed hydrocarbon species with the gasifying agent [22]. Several studies are available on waste tire gasification in the literature, ranging from production of syngas or high purity hydrogen gas to activated carbon and carbon nanotubes generation [1]. Temperature effects on syngas yield and composition using air gasification can be found in the literature [17], [23]. Steam gasification of tires in rotary kiln and fixed bed reactor can also be found [24], [25]. Effect of varying gasifying agent addition to air including steam + air, CO₂ + air and effect of temperature and feedstock particle size have been carried out in bubbling fluidized bed gasifier to understand their effect on syngas [14]. Activated chars were produced using steam and CO₂ from waste tire chars [26].

While these studies on gasification of waste tires exist, utilization of CO₂ as gasifying agent for waste tire gasification is lacking in the literature. Exploiting carbon dioxide in fuel making processes, with an estimated ability to sequester hundreds of millions of tons of carbon dioxide every year [27], could offer an interesting supplement to CO₂ capture and storage technologies (CCS). Recent research results from CO₂-assisted gasification of municipal solid waste and biomass have shown promising results. Couto et al. [28] showed that increase in CO₂ content in the air-CO₂ mixture used as gasifying agent in MSW gasification improved carbon conversion, carbon dioxide conversion and process efficiency. Moreover, CO₂ is reported to behave as a catalyst in the gasification process, enhancing thermal cracking and thus reducing tar formation [28], [29]. Similar trends were found using CO₂ for coal and biomass gasification. Butterman and Castaldi [27] showed enhanced feedstock conversion and production of a more porous char structure when CO₂ was added to steam in biomass gasification, as compared to steam gasification. However, to the authors’ knowledge, no studies on CO₂ assisted gasification of waste tire in a fixed bed reactor are available in the literature that focus on syngas production. Furthermore, most of the research work related to tire pyrolysis are focused either on the production and analysis of oils or in the properties of the solid residues and theirs subsequent upgrading to activated carbon; only few studies examine the kinetics of product gases evolution with temperature [6], [10], [12]. This provided the motivation for the present work to gain insight into the syngas production from waste tires via gasification. The relative uniformity of syngas makes it a better product for energy extraction compared to the extensively studied oil production from waste tires, which contains significant number of chemical components that are relatively difficult to extract or reform. Understanding CO₂ gasification of waste tires to syngas not only provides insight into gaseous product production capability but also examines the pathway of making this process less carbon positive via utilization of waste CO₂.

In the present work, the influence of reactor temperature on syngas yield, quality and energy content, product gases evolution kinetics, and on CO₂ consumption in CO₂ assisted gasification of waste tires is investigated. A comparison between syngas obtained from pyrolysis and CO₂-assisted gasification for the same feedstock mass and size, operating temperature and apparent vapor residence time is also presented. A comparison between cold-gas efficiency of both these processes is also provided. The use of a sampling time significantly shorter than the one adopted in most of previous studies, using sampling cylinders and an inline gas analysis with a micro GC, allowed for a detailed analysis of syngas flow rate and composition as a function of time. Most available studies on waste tire pyrolysis and gasification have used non-specified rubber mixtures obtained from recycling plants or steel and fiber free rubber crumbs as feedstock. In this work samples representative of a whole specific commercial tire were adopted without removing its reinforcing textile fibers to replicate the characteristic industrial feedstock material available for energy production. The samples examined contained textile fibers but no steel wires. This work provides insight into the ability of CO₂ assisted gasification for the disposal of tire wastes as clean and energy efficient pathway for our quest to provide sustainable energy as the product yield.