Parametric study of bio-char production from the co-pyrolysis of tire and coffee wastes via fixed bed reactor

Co-pyrolysis has become an eye-catching process for obtaining fuel and biochar and is attracting attention from both the scientific and industrial communities. One of the reasons for this is the wide variety of raw materials for the process itself and the possibility of obtaining attractive and high-quality products. The influence of three distinctive parameters has been studied on the co-pyrolysis reaction. These variables were temperature, reaction time, and plastic ratio. The results were analysed using Response Surface Methodology (RSM), and it was found that the optimum conditions to produce biochar from the co-pyrolysis of coffee and tire wastes were 359.441 °C, 10 minutes of reaction duration, and a plastic ratio of 97.8528 wt.%.


Introduction
Due to the increase in fuel demands and prices, dwindling fossil fuel supplies, and negative environmental impact, there is an intention to replace these fuels with those obtained from renewable energy sources [1,2].There are various alternative energy sources: solar, wind, and hydro-energy.However, bioenergy has attracted much attention [3,4].The conversion of biomass into biofuels and other value-added renewable commodities is the subject of extensive research [5,6].Several types and processes of biomass-to-energy conversion are known, which can be biochemical, thermochemical, or physicochemical.Drying, torrefaction, combustion, gasification, and pyrolysis were utilized to convert biomass into usable products [7].Pyrolysis is the name given to the thermal decomposition of biomass in the absence of oxygen, and the outputs of this process are biochar, bio-oil, and syngas [8].The higher the heating rate, the higher the amount of biofuel would reach at moderate temperatures.In contrast, the low heating rate, besides normal temperature conditions, would increase biochar production [9].Finally, enhancing syngas production requires high temperatures and low heating rates.When biomass roasts in a closed container under inert conditions, biochar, a carbon-rich substance-forms.Technically, biochar is produced by what is known as the thermal decomposition of organic material at relatively low temperatures and with limited oxygen [10].Biochar usually contains multiple components: carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, potassium, calcium, magnesium, and sodium.The largest proportion, usually more than 60%, is carbon, followed by hydrogen and oxygen [11].In the production of biochar, co-pyrolysis is particularly interesting.The concept of co-pyrolysis is that two feedstocks are used at the beginning, and then the process is continued like conventional pyrolysis.The co-pyrolysis of different feedstocks has already been carried out and optimized many times [12].In the literature, there are a lot of articles on this topic, although the process of obtaining energy from biomass in this way is still being studied.Zanella et al. [13] studied the low-temperature co-pyrolysis of polypropylene and coffee waste.In their study, they investigated the effects of the composition of a sample and the pyrolysis temperatures.The process was monitored with a constant heating rate of 5 °C/min and a pyrolysis temperature between 360 and 420 °C.A higher polypropylene content leads to a decrease in the number of liquid products and an increase in the number of solid products.Sajdak et al. [14] performed co-pyrolysis of polypropylene waste and two different types of biomass: alder and pine wood.The variable parameters in this study were the temperature and the polymer concentration, both for five different values, and the analysis of variance (ANOVA) investigated the influence of the varied parameters on the properties of the obtained products.The addition of polypropylene can improve the quality of biochar and has a good effect on the properties of bio-oil and syngas.The temperature and the type of material used for co-pyrolysis also have a noticeable influence on the properties of the coal and oil fractions.Inayat et al. [15] studied the production of biochar from coffee waste by slow pyrolysis at different reaction temperatures (200 °C−300 °C) and residence times (20−40 minutes).More biochar is obtained at low reaction temperatures and long reaction times, while bio-oil production is highest at moderate time and temperature: 30 minutes and 250 °C.Hossain et al. [16] performed the co-pyrolysis process of tire waste and rice husk to obtain biofuels and chemicals.The main parameter was the tire content in the feedstock, while the temperature was constant at 450°C.As the percentage of tire waste in the blends decreased, the biochar and gas products increased, while the bio-oil yield decreased.A different study by Inayat et al. [17] showed the possibility of obtaining bio-oil, biochar, and syngas from the co-pyrolysis of plastic waste and date seeds.The bio-oil's highest yield occurred at 500 °C, and the highest biochar's yield was detected at 300 °C.The biochar amount increases while the plastic ratio increases and the temperature decreases.Raza et al. [18] simulated the effect of pyrolysis operating temperature on the products of the process in a temperature range from 300 °C to 500 °C.Mixtures of date seeds and coffee waste were used as feedstock.The simulation shows that biochar production decreases with increasing temperature, while gas and bio-oil production increases at 450 °C.Therefore, the bio-oil production rate diminished, while gases continued to enhance with temperature.Since there is a large amount of tire and coffee waste globally, it is a readily available and suitable feedstock for copyrolysis.In this work, the co-pyrolysis of tire and coffee waste was carried out to obtain biochar and to analyze and optimize biochar production using Response Surface Methodology (RSM).

Feedstocks
The co-pyrolysis reaction was carried out in a pyrolysis reactor (Weihai Borui Chemical Machinery Mfg.Co., Ltd., China, model number BFK-1L/12.5).Tyre waste was supplied from a nearby auto repair shop.Coffee waste (Americano and espresso) was collected from the cafeteria of the Faculty of Engineering, University of Sharjah, UAE.

Experimental section
The tire wastes have been chopped into small pieces to fit into the reactor, and the coffee wastes have been in the oven at 110 °C.Each coffee layer took 8−12 minutes to dry; the duration depends on its thickness.The reactor should be well closed after putting the waste inside.The reason for this is mass losses.Nitrogen was used as the inert media.After the reactor had sealed well, the temperature had settled to the required level.Response Surface Methodology (RSM) was the method to optimize the reaction sequences (Table 1).The variables examined in each run were as follows:

Results and discussion
The effect of the three parameters (reaction time, reaction temperature, and plastic ratio) on biochar production is shown in this section.Based on the literature data mentioned in the introduction of this paper, we saw that this reaction is usually tested at temperatures between 300°C and 500°C, and that at higher temperatures a larger amount of bio-oil is obtained, and at lower temperatures a larger proportion of bio-char is obtained.In order to obtain the largest possible share of both biochar and biooil, we decided to test our reaction at three temperatures (350, 400, and 450°C).Also considering that we wanted to check how the share of plastic affects the amount of biochar obtained, we decided on three values: 0%, 50%, and 100% share of plastic.Fig. 1a shows different amounts of biochar obtained as a function of diverse reaction times (10, 20, and 30 minutes) at a constant temperature of 400 °C and a plastic ratio of 50%.The highest amount of biochar is obtained at a reaction time of 20 minutes, while the lowest amount is obtained after 10 minutes.In conclusion, the amount of biochar increases with the duration of the reaction, but up to a specific time, it decreases.
Fig. 1b shows the effects of different reaction temperatures on biochar production at a constant reaction time of 20 minutes and a plastic content of 50%.The amount of biochar recorded the highest value at a temperature of 400˚C.The latter starts decreasing after the temperature crosses 400 ˚C.
The last parameter that has been studied is the plastic ratio.All reactions were performed at a reaction time of 20 minutes and a temperature of 400˚C but with a different plastic ratio (0, 50, and 100 wt.%).Fig. 1c shows that the biochar yield increases with the plastic ratio and that the most biochar (48.18%) was obtained from the experiment with 100 wt.% plastic and the least (26.18%) with no plastic waste at all.3D surface plots (Fig. 2a-c) show the combined effect of two process parameters on biochar production.The impact of reaction temperature and time is shown in Figure 2a.As can be seen from the greenyellow area on the graph, the amount of biochar produced is consistently close even when the temperature and time conditions are changed.The best values for time and temperature are 20 minutes and 370°C, respectively.The significance of the plastic ratio in the process is shown in Figs.2b and 2c.From the red area of the graphs shown, a higher plastic ratio results in higher biochar production.Fig. 2c shows that the optimum reaction time is 20 minutes since most biochar is produced at this time when the plastic content is highest.
Perturbation diagrams are prepared for biochar.Fig. 3 shows the influence of reaction time on the result.The temperature in Figs.3a and 3b is 375 °C, the plastic content is 50%, and the reaction times are 20 and 30 minutes.The letters A, B, and C represent the reaction temperature, reaction time, and plastic ratio, respectively.The plastic ratio impacts the amount of biochar the most, as noticed.
The optimized values (Fig. 5) to produce biochar from tire and coffee waste obtained by the RSM study were a temperature of 359.441 °C, a reaction time of 10 minutes, and 97.8528 wt% plastic.Under these conditions, 48.5293 wt% biochar and 24.4238 wt% bio-oil can be produced.
From all the obtained results, it can be seen that all the examined parameters affect the amount of biochar obtained to different extents.The biggest influence is the share of plastic, when during pyrolysis the largest part of biochar is obtained from plastic while the greater part of biooil is obtained primarily from coffee waste.

Conclusion
Tire and coffee wastes were used as feedstock for the co-pyrolysis reaction, and the results were shown and optimized by RSM.In the experiments, the variations of three parameters were monitored -temperature, plastic content, and reaction length.The highest biochar yield is exhibited at reaction conditions of 20 minutes, 400 °C, and 100% plastic content.In addition, RSM results show that the plastic ratio has the highest influence on biochar production.The optimum conditions for biochar production are 359.441°C, 10 minutes reaction time, and 97.8528 wt.% plastic.

Figure 1 .
Figure 1.Effect of different: a) reaction times in biochar production, b) reaction temperatures, and c) plastic to coffee waste ratio in biochar production.

Figure 3 .
Figure 3. Default perturbation graph for biochar results based on actual factors for: a) 375 °C, 50% plastic ratio and 20 minutes reaction time, and b) 375 °C, 50% plastic ratio and 30 minutes reaction time.

Fig. 4
Fig.4shows the dependence between the predicted and the actual value of the obtained biochar.Only two values do not agree, which indicates the validity of the model created.

Figure 5 .
Figure 5. Optimized values of bio-char production from the co-pyrolysis

Table 2 :
The changing parameters in pyrolysis.