Liquid hydrocarbons production by the steam gasification of used tires: energy characteristics and environmental sustainability

The technical characteristics of liquid hydrocarbons obtained by steam gasification of the recycled automobile tires were studied as well as the features of their combustion process. Steam gasification of grinded tires was carried out in a tubular reactor using superheated steam with 500 °C temperature and 5 kg/h mass flow rate. The obtained characteristics were compared with a traditional liquid fuel oil. The technical characteristics were determined via the standard analytical methods. An ignition and a combustion of liquid hydrocarbon samples were carried out in the combustion chamber at different temperatures of heating medium T g = 450-700 °C. Gas-phase combustion products were analyzed using a flow-through gas analyzer. It was found that liquid hydrocarbons were characterized by a low pour point (-43 °C – (-52) °C) and comparable to calorific value of traditional fuel oil (40.56 – 43.3 MJ/kg). In addition, the studied samples of liquid hydrocarbons were characterized by a lower ignition delay time (by 56 %, on average) and flame burning time (by 30 %, on average). Combustion process of the studied samples (including traditional fuel oil) was accompanied by the formation of micro-explosions.

Seven stages of thermal transformation during bio-oil droplet combustion were identified into [25]. In particular, the stages of surface and bulk microbursts (stages 1 and 2, respectively), evaporation (stage 3), expansion in form of an additional microburst (stage 4) caused by the rapid increase in pressure inside the droplet [26], combustion (stage 5), smoldering (stage 6) and attenuation (stage 7) were identified. In [27][28], the appearance of microbursts during the combustion of liquid hydrocarbons is explained by the formation of a groups of small bubbles inside droplets. They open channels for the release of light volatile compounds during vaporization and initiate the microburst effect. Intensity of microbursts was increased with the temperature of the heating medium, which was also reported in some other articles [29][30][31].
Currently, the majority of the existing research in the field of liquid hydrocarbon droplets combustion is dedicated to bio-oils derived from lignocellulosic products. The pyrolysis oil obtained in the result of steam gasification of used tires have a high energetic potential, and can provide a partial replacement of traditional energy raw materials (fuel oil, in particular) within the program of waste rubber recycling, which will allow to reduce the expanses of households and businesses on energy resources. This paper presents results of the experimental research on the combustion process of liquid hydrocarbons obtained via steam gasification of used automobile tires of summer type which were used for passenger cars.

Initial samples of used automobile tires
Four samples of used tires of passenger cars were used as initial material. In the current study, the summer type of tires was chosen due to their wide distribution worldwide and the possibility of all-season application (in countries with positive air temperatures throughout the year).
Tires were cut into chips via cutting tool with simultaneous removal of the metal cord. The size of the obtained chipped samples studied (width × height × length) was 7×1×7 cm. The size variation was less than 1 cm for both sample width and length. Technical characteristics and elemental composition of the initial samples of used automobile tires are presented in table 1.

Experimental installation of steam gasification
The schematic diagram of experimental setup for steam gasification of used automobile tires is presented in Figure 1. A 0.5 kg of used automobile tire sample was loaded into a tubular steam gasification reactor, in particular, in the container made of metal mesh tightly fixed into the inner volume of the reactor chamber. After that, the reactor was closed and tightly sealed. Then a regulating device was opened for constant steam flow through the chamber. The released during decomposition of the used automobile tires gases were removed together with steam and condensed in the tubular condenser.
Obtained liquid products (mixture of water and liquid hydrocarbons) were drained into a storage tank. Saturated vapor was generated in a steam generator which was fed with chemically treated water from the water storage tank by pump. The steam flow rate was adjusted by a needle-type valve at the reactor inlet and regulated by a liquid flow meter installed before the steam generator.
Then the heated to 120 °C steam consecutively passed through the first (electrical) and second (heated by hot air) stages of superheater, in which its temperature was increased to required value.
Temperature control inside the reactor and intertubular gap (to maintain constant temperature by the air heater) was performed continuously using installed thermocouples (T1 and T2) connected to a multi-channel thermocouple recorder.
Steam gasification of the studied samples of used automobile tires was carried out at 500 °C and 5 kg/h flow rate for 1 hour. The sample weight was 0.5 kg.

Characterization of materials
Before the analysis, the obtained condensed products of used automobile tires gasification were separated into liquid hydrocarbons and water using separating funnel. The liquid products later will be referred to as S1-S4 according to the designations of automobile tire samples used.
A sample of standard fuel oil was used to perform a comparative assessment of the obtained liquid hydrocarbon products. Such fuel oil is widely used in the energy complex [32][33][34]. This sample later will be referred to as S5. Thermal analysis of the liquid hydrocarbon samples was carried out using a differential thermal analyzer Netzsch STA 449 F3 Jupiter (Netzsch, Germany) in argon atmosphere (160 ml/min) at 10 °C/min heating rate in the temperature range between 25 and 600 °C. The sample weight was ~ 20 mg.

Droplet ignition and combustion of liquid hydrocarbon samples
Study of ignition and subsequent combustion of single hydrocarbon droplets was carried out using experimental setup, schematic diagram of which is presented in Figure 2. The fuel ignition process was considered to be stable if a well-visible flame was observed and the droplet was burned completely. At least 10 repetitive measurements were performed for each sample at each temperature of heating medium.

Characteristics of liquid hydrocarbons obtained by steam gasification of used automobile tires
The amount of liquid hydrocarbons (expressed in percent of the initial mass of the automobile tire sample) obtained after steam gasification was following: S1 -54.7; S2 -57.1; S3 -53.9 and S4 -57.4 wt%.
The determined technical characteristics and elemental composition of studied liquid hydrocarbon samples are presented in Table 2.  Table 2 data indicate that kinematic and dynamic viscosities of samples S1-S4 at 40 °C which were by 40 times smaller compared to standard fuel oil sample S5. According to [13], the difference in the obtained viscosity values may be due to the presence of heavier compounds in the S5 sample, in contrast to the S1-S4 samples. Smaller viscosity of liquid hydrocarbons S1-S4 allows to simplify their transportation and spraying into energy equipment thus reducing energy spent on pretreatment of such fuel before application.
The S1-S4 samples were also characterized by lower pour point (Tpp = -43 -(-52) °C) compared to standard fuel oil (Tpp = 10 °C). This could be relevant to energy enterprises operated into cold climate conditions. The significant difference in Tpp value for S1-S4 samples could be connected to the significant content of light fractions, paraffin naphtene and aromatic hydrocarbons as well as to the low content of n-paraffin which freezes at positive temperatures [36]. Additionally, such samples had lower flash point temperature (Tfp = 73-85 °C) compared to standard fuel oil (Tfp = 175 °C). This indicates more complex fractional composition of liquid hydrocarbons obtained via used automobile tires steam gasification compared to other fuel oil [37].
As it could be seen from the data presented in table 2, the sulfur content in S1-S4 samples is an average by 2 times lower compared to S5 fuel oil. This feature is undoubtedly important advantage of such fuel because during their combustion less sulfur oxide will be formed which, apart from anthropogenic effect, is causing intense corrosion of metal surfaces of tubes and other elements of boilers.
The S5 sample was characterized by a higher carbon content (87.78 wt%) and lower hydrogen and oxygen content (9.88 and 0.31 wt%, respectively), which was also correlated with the data in [26]. This was likely due to the presence of a smaller amount of light hydrocarbons in the composition of this sample. It should also be noted that S1-S4 samples had higher values of H/C and O/C ratios (on average by 16% and 3%, respectively) in contrast to S5.  hydrocarbons in the S1-S4 samples, which was also confirmed by the authors of other works [16,25].
According to Figure 3a, the mass loss of S1-S4 samples in the temperature range of 25-associated with the boiling of residual moisture and some petroleum fraction [38], for which characteristic temperature range of boiling was 40-100 °C. For S1-S4 samples at 200 °C the mass loss was 24.8-36.8 wt% which was associated with boiling and conversion of light gasoline fractions [39]. For S5 sample with presumably no light fraction it was 2.8 wt%. The mass loss for the studied S1-S4 samples was 38.0-46.9 wt% in the 200-300 °C temperature range corresponding to the boiling of the hydrocarbons fraction with intermediate molar mass (kerosene, in particular) [39]; for S5 sample -16.4 wt%. Finally, at a temperatures above 300 °C, which referred to the boiling of heavy hydrocarbons [39], the mass loss of the studied S1-S4 samples was 18.1-25.6 wt%, while for S5 sample it was 76.3 wt%.

Droplet ignition and combustion of liquid hydrocarbons
Dependences of the ignition delay time of the studied hydrocarbon droplets on the heating medium temperature in range from 450 to 700 °C are presented in Figure 4. For all samples the ignition delay time τi was found to exponentially decrease with the increasing of heating medium temperature which was in good correspondence with earlier published articles [29][30][31]. The average ignition delay time of the studied samples was reduced by 19.2 times with increase in the temperature from 450 °C to 700 °C. Significant deviation in the character of ignition delay time τi dependence on temperature (in comparison with samples S1-S4) was observed for standard fuel oil (sample S5) according to Figure 4. This difference could be explained by significantly different elemental composition (Table 2) as well as by the presence of light hydrocarbons in the composition of samples S1-S4.   The process of ignition and combustion of the studied hydrocarbons could be divided into several stages as it was done in [40]. The first stage was the inert heating of a single droplet. At the second stage, the processes of simultaneous intense evaporation, formation of volatile substances and a mixture of flammable gases were observed. The third stage was characterized by intense chemical interaction between fuel and oxidizer in a high-temperature gas environment.
The fourth stage was connected to heterogeneous combustion of the coke residue.
Based on the obtained video frames, it is possible to formulate a physical model of the ignition and subsequent combustion of single droplets of hydrocarbons. Thus, the intense heating of sample with the characteristic initial size (D=1.5 mm) begins after its introduction to the thermoregulated furnace. Then the components of hydrocarbons [40] start to evaporate from the near-surface layers of the droplet. The transformation of the droplet surface, change in its size along the longitudinal and transverse coordinates as well as change of its appearance (transition from "glossy" to "matte") identify this stage. These processes are more or less pronounced for the different samples of hydrocarbons, which are connected to the water content in their composition ( Figure 3). As a result, a vapor cloud [16] with combustible gases [41] is formed near the surface of the droplet. The gas-phase ignition is realized when the minimal required concentrations and temperature of hydrocarbon-air mixture are reached (Figure 5a). Formation of gradually developing flame around the droplet is observed after gas-phase ignition, without pronounced change in its surface. This may indicate that the primary evaporation of fuel's light hydrocarbons occurs at the surface of the droplet while the heavier components are subsequently concentrated in the char residue making the shell of the droplet more viscous [42].
Duration of microburst formation period depended significantly on the sample of liquid hydrocarbons studied. Thus, this effect had continuous character for S1-S4 samples and was observed throughout the combustion. It is also important to note that the intensity of microbursts, which manifested itself into the number of sparks formed, frequency of their occurrence and remoteness from the droplet surface was found to depend on the viscosity and composition of the studied fuels (Table 2). Thus, the stronger tendency to form microbursts could be referred to fuels with a high content of water and light hydrocarbons, as well as a lower viscosity (S1-S4 samples).
The latter affected the tenacity of the surface film, through which the volumetric accumulations of formed flammable vapors were released. According to [43], the breaking point temperature of weak chemical bonds of heavy oil was close to 352 °C and the thermal cracking did not occur until the 427 °C temperature was reached. Thus, the formation of microbursts at the early stage of studied samples combustion (for S1-S4 samples this was more relevant) was associated with evaporation of light fuel components, and not with thermal decomposition of the fuel.
It is important to note that there was an intensive expansion of the combustion front at an early stage of flame burning stage for S1-S4 samples, the size of which exceeded the initial characteristic size of the droplet by several times (Figure 5b) [44].  It could be seen that there was a decrease in the concentration of CO in the composition of the gas-phase combustion products (on average by 64 %) with the increase in the heating medium temperature, due to intensification of the combustion process of liquid hydrocarbons at higher temperatures. In turn, there was an increase in the concentration of CO2 in gas-phase combustion products on average from 900 to 2070 ppm with the growth of Tg for all studied samples. In this case, the dependence of the maximum CO2 concentration on the heating medium temperature Tg was linear. Increase in CO2 concentration was most likely caused by the more intensive oxidation of the formed CO at a relatively constant value of total combustion time of the liquid hydrocarbons ( Figure 6) at high temperatures of the heating medium (600-700 °C).

Analysis of gas-phase combustion products
Lower values of the concentration peaks of CO and CO2 (on average by 16 % and 39 %, respectively) were observed for S1-S4 samples compared to the S5 sample, which correlated with their elemental composition (Table 2) and the intensity of microbursts observed at an early stage of combustion. These microbursts also contributed to a more intensive oxidation of the carbon monoxide formed due to the intensification of interactions between combustible fuel components and oxidizing environment. In particular, this was due to the fact that water in micro-droplets evaporates first during liquid fuel heating, since its boiling temperature is smaller compared to liquid hydrocarbons. This phenomena occurs at a heating medium temperature that is much higher than the boiling point of water. The dispersion of fuel droplets promotes for their mixing with air, which, in turn, boosts the air-fuel interactions. Thus, this results into increasing in the secondary atomization of the fuel during its combustion with formation of flammable gases such as CO and H2.
In contrast to the studied liquid hydrocarbons (S1-S4), large values of concentration maxima of SO2 and NOx were observed for the S5 sample (on average by 51 % and 27 %, respectively), which were caused by the higher content of sulfur and nitrogen in the composition of this sample itself (table 2). As the Tg temperature increases, the concentration maxima of sulfur and nitrogen oxides release increase (on average by 5 and 4 times, respectively), which is also in good agreement with the results of other authors [45].
According to [45], formation of NOx in the result of the oxidation of fuel nitrogen is the predominant mechanism for the considered liquid hydrocarbons, since the formation of prompt and thermal NOx occurs at much higher temperatures (temperatures above 1300 °C). Thus, decrease in the total nitrogen content in samples of liquid hydrocarbons obtained as a result of steam gasification (S1-S4) is the one of the main reasons for lower NOx concentration maximum compared to traditional fuel oil. It is also worth noting that the presence of water in S1-S4 samples contributed to the reduction in NOx emissions, due to the formation of additional H-and OHradicals, which can reduce NO and SO2 [46][47].
Formation of intensive microbursts during combustion of S1-S4 samples could also contribute to the reduction in the amount of SO2 and NOx released. This is due to the presence of water vapor formed during combustion of water-containing fuels (samples S1-S4), which affects the physical and chemical kinetics of combustion in the result of the decrease in the flame temperature assisted by water evaporation [48]. Thus, the lower temperature had an effect on reducing the rate of NOx and SO2 release, since this process strongly depends on temperature [49].
Additionally, since the fuel was sprayed into smaller droplets (spark-like satellites) during microbursts, the reaction contact surface between the formed fuel droplets with CO and H2 increases, resulting in promotion of the reduction reaction of sulfur and nitrogen oxide: SO2 + 3CO → COS + 2CO2 [50], NO + CO → N + CO2 [51].

The comparative analysis
The differences between both physical and combustion characteristics of obtained liquid hydrocarbons and classical fuel oil were very significant. In order to consider the wide range of fuel characteristics, the simple comparison analysis methodic presented in [45,52]  While the viscosity of samples changes with temperature following the exponential law, the viscosity factor was determined using natural logarithm of corresponding values. The modified complex indicator was calculated according to following formula: The S5 fuel oil sample was considered as the reference sample. Calculated using equations (1) and (4) the basic and modified complex indicator values are presented in Table 3. The modified complex criteria didn't reveal significant difference into samples evaluation compared to basic criteria but illustrated that application of S1-S4 oils will be even more advantageous due to lower viscosity and pour point temperature. The only exception was S1  6) According to complex indicators calculated the combustion of obtained samples for energy production was much more advantageous compared to reference sample S5. The application of these fuels was found to be preferred at lower temperatures while the increasing temperature resulted into decline in complex indicator values by up to 25 % and 19 % for initial and modified complex criteria values, respectively.
Thus, steam gasification can be considered to be the effective method of recycling of used automobile tires to produce liquid hydrocarbon fuel, which, in turn, could be used as boiler fuel.
The studied hydrocarbons have good practical prospects, since their combustion in the chambers of steam and water boilers will not require significant resources for their implementation at existing and projected heat power facilities. In addition, it is possible to obtain stable technical characteristics of liquid hydrocarbons when using a group of tires of the summer type, which are more widely used in the world, which is a very important technological factor for their application in the burners of power boilers.