Fast Pyrolysis of Coffee Ground in a Tilted-Slide Reactor and Characteristics of Biocrude Oil PDF

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Fast Pyrolysis of Coffee Ground in a Tilted-Slide Reactor and Characteristics of Biocrude Oil Yeon Seok Choi,a,b Sang Kyu Choi,a,b Seock Joon Kim,a,b Yeon Woo Jeong,a Ramesh Soysa,b and Tawsif Rahmanb a Environmental and Energy Systems Research Division, Korea Institute of Machinery & Materials,...

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Fast Pyrolysis of Coffee Ground in a Tilted-Slide Reactor and Characteristics of Biocrude Oil Yeon Seok Choi,a,b Sang Kyu Choi,a,b Seock Joon Kim,a,b Yeon Woo Jeong,a Ramesh Soysa,b and Tawsif Rahmanb a Environmental and Energy Systems Research Division, Korea Institute of Machinery & Materials, Daejeon 34103, Republic of Korea; [email protected] (for correspondence) b Department of Environment and Energy Mechanical Engineering, University of Science and Technology, Daejeon 34113, Republic of Korea Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12585 Coffee grounds are considered to be a promising biomass resource due to the growth of coffee consumption and their high calorific value. In this study, we produced biocrude oil from coffee grounds in a tilted-slide reactor, which has been previously used for the fast pyrolysis of woody biomass. Various pyrolysis temperatures were tested and the maximum biocrude oil yield was 59% at the pyrolysis temperature of 5508C with a feeding rate of coffee grounds at 17.7 kg/h. In a multistage condenser, the biocrude oil yields and properties were different in each condenser stage. The higher heating value of the biocrude oil from the primary condenser was 7,157 kcal/kg, which was about 60% higher than that from woody biomass. A phase separation was observed when the biocrude oil was stored at the atmospheric condition, and the biocrude oil viscosity became quite larger than that from woody biomass, which could even impede its flow. The viscosity decreased with increasing temperature and was more sensitive to temperature variations at lower temperatures. Mixing of ethanol also significantly improved viscosity characC 2017 American Institute of Chemical Engineers Environ teristics. V Prog, 00: 000–000, 2017

Keywords: biocrude oil, coffee ground, fast pyrolysis, tilted-slide reactor, biocrude oil, coffee grounds, fast pyrolysis, tilted-slide reactor INTRODUCTION

Studies on renewable energy are widely performed these days due to the depletion of fossil fuels and global warming issues. Biomass is considered to be one of the sustainable energy sources that can reduce CO2 emissions because the amount of CO2 absorbed by photosynthesis is equivalent to that produced by the combustion process. There are various methods in utilizing biomass as an energy source, such as direct combustion, gasification, liquefaction, and torrefaction. Especially, converting biomass into liquid fuel has the advantages of storage and transportation. Fast pyrolysis is a thermochemical process to convert biomass into a liquid fuel, which is generally known as biocrude oil. In the conditions of relatively high temperatures around 5008C in an oxygenfree atmosphere, biomass particles are quickly decomposed into the volatiles, the noncondensable gases, and the char. C 2017 American Institute of Chemical Engineers V

Biocrude oil is produced by condensing the volatile which is produced via fast pyrolysis [1–4]. Biocrude oil can be applied to furnaces or boilers by direct combustion, and to engines and turbines by upgrading into high-quality hydrocarbon fuels and to high-value-added chemicals [4, 5]. There have been many studies on the fast pyrolysis of biomass in various pyrolysis reactor configurations, such as a bubbling fluidized bed reactor, a circulating fluidized bed reactor, an ablative reactor, a rotating cone reactor, a vacuum reactor, a conical spouted bed reactor, and a tilted-slide reactor [3, 6–11]. For achieving fast pyrolysis, woody biomass has been widely used along with herbaceous plants. Biocrude oils from these biomasses are known to have relatively smaller heating values due to having higher oxygen contents that originate from the feedstock characteristics. Coffee has a huge potential for biomass feedstock because it is one of the most popular beverages worldwide [12–15]. According to the U.S. Department of Agriculture, the world coffee production for 2015/16 is forecasted to be 9.2 million tons per year [16]. When coffee is brewed, only about 20% of coffee bean weight is reduced while 80% of it is wasted as coffee grounds [17]. Most of the coffee grounds are disposed of as waste, while a small amount is used as compost or in deodorant. Because coffee consumption is increasing continuously, coffee grounds could be a promising biomass resource to produce biocrude oil. The fast pyrolysis of coffee grounds was previously studied in a fluidized-bed reactor [18, 19] and it was found that the heating values of the biocrude oil from coffee grounds are generally higher than that of the biocrude oil derived from woody biomass. The optimum pyrolysis temperature was also determined for the maximum biocrude oil yield [19]. Previous studies were performed in lab-scale reactors, whereas the production of biocrude oil from coffee grounds in a pilot-scale pyrolyzer is rather limited. In this study, we produced biocrude oil via the fast pyrolysis of coffee grounds in a tilted-slide reactor of 20 kg/h capacity, which had been used for the fast pyrolysis of woody biomasses [11]. The operation stability of the plant was investigated at various pyrolysis temperatures, and the biocrude oil yields were compared. The characteristics of the produced biocrude oil were also analyzed in regards to its heating value and viscosity, and the possibility for fuel application was determined.

Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep

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Table 1. Proximate analysis of coffee ground and sawdust (dry basis). Physical properties

Coffee ground

Sawdust

Volatile (wt %) Fixed carbon (wt %) Ash (wt %) HHV (kcal/kg)

78.7 16.9 3.4 5538.2

83.6 15.5 0.9 4231.2

Table 2. Elemental analysis of coffee ground and sawdust (dry and ash-free basis). Element (wt %)

Figure 1. Dried coffee grounds. [Color figure can be viewed at wileyonlinelibrary.com]

C H O* N S

Coffee ground

Sawdust

53.62 6.97 37.07 2.34 0

43.7 5.49 50.78 0.03 0

*By difference.

sawdust, and this difference cannot only be explained by the result obtained from the proximate analysis. The elemental compositions were compared in Table 2. The element analysis was performed by the Flash EA 1112 series. The difference in the oxygen content was quite significant, where the oxygen content of coffee grounds was 13.7% lower than that of sawdust. The carbon and hydrogen content were 9.9% and 1.5% more than those of sawdust, respectively. These differences could lead to a larger heating value for coffee grounds.

Figure 2. Particle size distribution of coffee grounds.

MATERIALS AND METHODS

Feedstock for Fast Pyrolysis The coffee grounds were collected from coffee shops in Seoul, South Korea. Because the moisture content of these coffee grounds was over 50 wt %, it was dried in a rotary dryer so that it was lower than 10 wt % in moisture. The dried feedstock is shown in Figure 1. The particle size distribution of the dried coffee grounds was determined by ASTM mesh sieves ranging from 0.42 to 2 mm. As shown in Figure 2, the ratio of small particles below 0.42 mm is 60.4 wt %. We noted that a higher biomass heating rate could be achieved for smaller particles, which can increase the yield of biocrude oil [3]. The proximate analysis of dried coffee grounds was carried out and the result was compared with dried sawdust (Douglas fir), as shown in Table 1. Sawdust was selected as a reference material for comparison, because it is widely used as a feedstock material for fast pyrolysis. A proximate analysis was carried out using a TGA 701 LECO in accordance with ASTM D5142. A bomb calorimeter (LECO AC350) was employed to measure the higher heating value (HHV) in accordance with ASTM D2015. The volatile content of coffee grounds was 4.9% less than that of sawdust. Fixed carbon and ash content were 1.4% and 2.5% more than those of sawdust, respectively. But the HHV was 31% more than 2 Month 2017

Experimental Method The fast pyrolysis of coffee grounds was performed in a tilted-slide reactor. The schematic of the reactor system is shown Figure 3 [11]. The hot sand was fed via the sand hopper into the top of the tilted reactor. The coffee grounds were supplied from the biomass hopper to the top of the reactor and contacts with the hot sand, and were thermally decomposed into the volatiles, the char, and the noncondensable gases under oxygen-free condition. The volatiles were condensed into biocrude oil while passing through a direct contact condenser and then through shell-and-tube condensers. The details of the experimental setup have already been described [11]. The reaction temperature of the pyrolyzer was varied to find the optimum temperature for obtaining the maximum biocrude oil yield. The experimental conditions are summarized in Table 3. The pyrolysis temperatures were 520, 550, and 5808C with a biomass feeding rate of about 18 kg/h. The feeding time was maintained for at least 4 h to determine the stability of the operation of the plant. RESULTS AND DISCUSSION

The Operation Characteristics of the Plant The operation stability of the system was determined by monitoring the temperature variation during the fast pyrolysis. Figure 4 shows the temperature profiles for the sand hopper (unit 8 of the schematic diagram in Figure 2), air heat exchanger (unit 1), pyrolysis reactor (unit 10), spray condenser (unit 13), and the char combustor top (unit 2) with time. The temperatures for the pyrolysis reactor—which are denoted by thick red lines—were well controlled at the target pyrolysis temperature and showed few fluctuations.

Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep

Figure 3. Schematic diagram of the tilted-slide fast pyrolyzer [11].

Table 3. Experimental conditions. Temperature (8C) 520 550 580

Biomass feed (kg)

Feeding time (h)

Feeding rate (kg/h)

71.08 90.23 88.29

4 5.1 5

17.8 17.7 17.6

The temperatures for other parts also had small variations, which indicate stable system operation. The sand was heated by char combustion at the circulating fluidized bed combustor (unit 2), and then was separated at the cyclone (unit 7) to be stored in the sand hopper (unit 8) before being supplied to the pyrolysis reactor (unit 10). As shown in Figure 4, the temperature at the top of the char combustor was higher than the pyrolysis reactor for all of the pyrolysis temperature cases, which agrees with the sand flow direction. But in cases of the pyrolysis temperature being 550 and 5808C, the temperature for the sand hopper was comparable or even lower than the temperature for the pyrolysis reactor, even though it was expected to be higher than for the reactor. This trend might be related to the plant’s operation methods. During the fast pyrolysis at 5208C, the sand level in the sand hopper (unit 8) was maintained at a specific level to prevent air penetrating into the top of the pyrolysis reactor (unit 10). Note that the bottom of the reactor was blocked with sand in the screw conveyer (unit 11) to maintain oxygenfree conditions in the pyrolysis reactor. The sand level in the sand hopper was controlled by a slit installed in the bottom of the sand hopper. Usually, the sand level in the sand hopper was pretty much consistent and did not need to be adjusted that often. However, in the cases of reaction temperatures at 550 and 5808C, an unexpected variation in the sand level was observed and the slit had to be adjusted quite frequently. Therefore, a condition that the slit was fully opened was tested where the sand was not charged in the sand hopper but instead directly flowed from the cyclone (unit 7) down to the pyrolysis reactor. With this method, there could be a small amount of air that gets in through the slit and into the pyrolysis reactor, which might quickly oxidize the coffee grounds. If this oxidization occurs, the pressure in the pyrolysis reactor should suddenly

increase. However, this phenomenon was not observed when the slit was fully opened, which could indicate that the pyrolysis reactor being penetrated with air was negligible. Therefore, this condition was maintained during the plant operation at 550 and 5808C. Because the thermocouple for the sand hopper became exposed from the sand in this condition, the temperature for the sand hopper was lower than for the pyrolysis reactor. The temperature at the air heat exchanger (unit 1) was kept at approximately 3008C. The preheated air from the heat exchanger entered the char combustor, which can enhance the system’s energy efficiency. The temperature for the spray condenser was about 70–808C. The biocrude oil that was for being pumped into the spray condenser was stored in the container (unit 14) at the bottom of the spray condenser. This contained biocrude oil was cooled by a chiller (unit 15), where the coolant temperature was controlled at a temperature below 108C. The temperature for the spray condenser becomes higher than the controlling temperature. This is because the biocrude oil in the container was replaced by the newly produced one through the condensation of the hightemperature volatiles near the pyrolysis temperature. If the temperature of the biocrude oil in the container was lower, then the production of the biocrude oil through the condensation might increase due to the enhanced heat transfer for condensing the volatiles. However, an overcooling of the biocrude oil is not desirable because of the increase in viscosity, which can cause a pipe to clog during plant operation. The total energy balance was evaluated based on the thermal and electric energy into the pyrolysis system and the energy output by the production of biocrude oil. At the reaction temperature of 5508C, the total energy consumption of the pyrolysis system per hour is 56,281 kcal/h, including the thermal energy of the oil burner (50,436 kcal/h) and the electric energy of fans, pumps, heaters, and so forth (5,845 kcal/h). The energy production rate was calculated to be 54,791 kcal/h from the production rate and the heating value of the biocrude oil. The ratio of energy production to consumption leads to the overall efficiency of the production of biocrude oil at 97.4%. Biocrude Oil Yield and Properties The biocrude oil yield was determined by the weight ratio of the collected biocrude oil to the supplied feedstock. The

Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep

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Figure 4. Temperatures in the tilted-slide reactor during operation. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5. Product yields with pyrolysis temperature.

biocrude oil yields at various pyrolysis reaction temperatures are shown in Figure 5. The maximum yield was 59% at 5508C. This temperature is identical to the previous study in a lab-scale bubbling-fluidized-bed reactor of 0.4 kg/h, where the maximum yield was approximately 55% [19]. The temperature for the maximum yield was about 608C higher than the optimum value for the pyrolysis of woody biomass [10], which was 4908C for Douglas fir. The maximum biocrude oil yield from coffee grounds was about 5.7% lower than that from woody biomass, which was 64.7% in the same reactor [11]. The biocrude oil samples from each condenser at the pyrolysis temperature of 5508C are shown in Figure 6. The biocrude oil produced from coffee grounds is dark brown in color and has a distinctive, smoky, coffee smell. It is interesting to note that the biocrude oils produced via the second and third condensers have two different phases that are dark and light brown. At each condenser, the biocrude oil yield and heating value are different, as shown in Figure 7. The primary spray condenser produces 54% of the total biocrude oil, while 31% of the biocrude oil is from the second shell-and-tube condenser. The following three condensers produce relatively small portions of biocrude oil. The HHVs from the multistage condensers are compared in Figure 7. The HHV in the primary condenser is 7,157 kcal/kg, while that in the second condenser is 1,622 kcal/kg. The HHV increases in the following condensers and the HHV in the last condenser is 7,928 kcal/kg, which has the largest value, although its yield fraction of total biocrude oil produced is only 8%. It is worth mentioning that the fractionation of the biocrude oil occurs in the multistage condenser. A possible application of this fractionation could be 4 Month 2017

Figure 6. Sample biocrude oil collected from each condenser at the pyrolysis temperature of 5508C. [Color figure can be viewed at wileyonlinelibrary.com]

mixing fuel in a heavy oil power plant by selecting the biocrude oil fraction with a relatively larger HHV. Because the HHVs are different depending on condensers, the overall HHVs are determined by the yield fraction and HHV at each condenser as follows: HHVoverall 5

X

HHVi 3YFi

i

where, HHVi and YFi denote the HHV and yield fraction at the i-th condenser, as summarized in Figure 7, respectively. The overall HHV at each pyrolysis temperature is compared in Figure 8. The maximum overall HHV was 5,246 kcal/kg at the pyrolysis temperature of 5508C, which is the same temperature where the maximum biocrude oil yield was achieved. However, the temperatures for the minimum overall HHV (5808C) and for the minimum biocrude oil yield (5208C) are different. The reason that the overall HHV becomes lower than that of the primary condenser is due to the relatively small HHVs, particularly in the second and third condensers. At the pyrolysis temperature of 5508C, the HHVs in these condensers are 1,622 and 2,722 kcal/kg, respectively. The small heating values could be related to the different colors of biocrude oil produced in these condensers, as shown in Figure 6. Especially, the yield fraction in the second condenser is 31%, which is the main factor for reducing the overall HHV. This trend is similar to that of the previous result using woody biomass [11]. Therefore, it is important to maximize the yield fraction in the primary condenser while minimizing it in the second condenser.

Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep

Figure 7. Biocrude oil yield fraction and higher heating value at each condenser at the pyrolysis temperature of 5508C. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 8. Higher heating values with pyrolysis temperature.

Some approaches, such as controlling the spraying temperature of the biocrude oil in the primary condenser, controlling the cooling water temperatures for the shell-and-tube condensers, or adjusting the volatile inducing pressure from the pyrolysis reactor, might be possible. Further investigations are needed to control the yield fractions. The physical and chemical properties of the biocrude oil obtained from each condenser at the pyrolysis temperature of 5508C are summarized in Table 4. The water content was measured by a Karl-Fisher titrator (870 KF Titrono plus) following ASTM D1744, and the viscosity was measured by a viscometer (Brookfield, Model DV-II 1 Pro). The water contents of the biocrude oils obtained from the second and third condensers are relatively higher than others, which can reduce their heating values. It is worth noting that the biocrude oil from the primary condenser has a significantly large viscosity. The viscosity characteristics will be discussed in more detail in the next subsection. Table 5 shows the elemental compositions of biocrude oil from each condenser at the pyrolysis temperature of 5508C. The oxygen content from the second and third condensers are quite large in relation to their high water contents, which can reduce the heating values. The features of the water contents and elemental compositions are consistent with the HHV characteristics, which are shown in ...