The catalytic pyrolysis of food waste by microwave heating

Bioresource Technology 166 (2014) 45–50

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The catalytic pyrolysis of food waste by microwave heating Haili Liu a,b,⇑, Xiaoqian Ma a, Longjun Li a, ZhiFeng Hu a, Pingsheng Guo c, Yuhui Jiang b a

Electric Power College, South China University of Technology, Guangzhou 510640, China Department of Mechanical and Electrical Engineering, Hunan Institute of Humanities, Science and Technology, Loudi 417000, China c Physics Science and Technology Institute, Guangxi Normal University, Guilin 541004, China b

h i g h l i g h t s  Microwave pyrolysis with catalysts was a feasible method to handle food waste.  MgO, Fe2O3, MnO2, CuCl2 and NaCl can lower bio-oil yields and enhance gas yields.  The optimal microwave power to maximize bio-oil yield during pyrolysis was 400 W.  Chloride salts promoted formation of acidic compounds in the bio-oil’s lower layer.  CuCl2 was the most effective among the tested catalysts, followed by MnO2.

a r t i c l e

i n f o

Article history: Received 18 March 2014 Received in revised form 8 May 2014 Accepted 10 May 2014 Available online 19 May 2014 Keywords: Microwave pyrolysis Food waste Catalyst Metal oxides Chloride salts

a b s t r a c t This study describes a series of experiments that tested the use of microwave pyrolysis for treating food waste. Characteristics including rise in temperature, and the three-phase products, were analyzed at different microwave power levels, after adding 5% (mass basis) metal oxides and chloride salts to the food waste. Results indicated that, the metal oxides MgO, Fe2O3 and MnO2 and the chloride salts CuCl2 and NaCl can lower the yield of bio-oil and enhance the yield of gas. Meanwhile, the metal oxides MgO and MnO2 can also lower the low heating value (LHV) of solid residues and increase the pH values of the lower layer bio-oils. However, the chloride salts CuCl2 and NaCl had the opposite effects. The optimal microwave power for treating food waste was 400 W; among the tested catalysts, CuCl2 was the best catalyst and had the largest energy ratio of production to consumption (ERPC), followed by MnO2. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction With the parallel rise in population and living standards that is occurring in China, the generation of municipal solid waste (MSW) is also increasing rapidly, at an annual rate of 8–10%. In 2007, the total national amount of MSW reached 152 million tons (Lai et al., 2011). MSW is composed mainly of food waste, paper, slag, ceramics, plastics, glass, metals, textiles and wood – among these components, food waste currently represents the greatest share at more than 40% of the weight of solid waste (Luo et al., 2010; Zhao et al., 2009). Appropriate handling of this great amount of food waste has become a serious social and environmental issue. Especially, the gutter oil, the concomitant of food waste, which is illegally used to cook food in many restaurants in China, causing serious health problems. In light of these problems, the disposal ⇑ Corresponding author at: School of Electric Power, South China University of Technology, No. 381, Wushan Road, Tianhe District, Guangzhou 510640, China. Tel.: +86 20 87110232; fax: +86 20 87110613. E-mail address: [email protected] (H. Liu). http://dx.doi.org/10.1016/j.biortech.2014.05.020 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

of food waste has been receiving more and more public and research attention. Today, the main methods of disposing food waste are landfill, incineration and composting. The dumping of food waste in landfill sites causes some environmental impacts, including the generation of landfill leachate, methane gas, and offensive odours (Ahmed and Gupta, 2010). When food waste is incinerated together with other combustible wastes, dioxins may be produced because of the high moisture content (Vavouraki et al., 2013). Composting is a popular solution to treat food waste (Caton et al., 2010) because it produced nutritious fertilizer, yet it can discharge harmful byproducts such as leachate, NH3, and greenhouse gases, creating secondary environmental pollution (Yang et al., 2013). Therefore, there is a strong need for effective methods and treatment processes for food waste that have fewer environmental impacts. Caton et al. (2010) found that dried food waste contained more energy than wood, which is a more typical biomass fuel. Tanaka et al. (2008) studied the basic characteristics of steam gasification of food waste, and found it to be an effective method that not only reduced the final amount of waste, but also produced gas that was

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H. Liu et al. / Bioresource Technology 166 (2014) 45–50

useful as fuel. Ahmed and Gupta (2010) argued that waste food offers a good potential feedstock for power generation via gasification. In spite of the similarity between pyrolysis and gasification, the research on pyrolysis of food waste has been limited. Pyrolysis is one of the promising methods for application of biomass-to-energy, and it has been widely employed in the disposal of municipal solid waste (Velghe et al., 2011; Nurul Islam et al., 2005). So far, pyrolysis mainly includes microwave pyrolysis and conventional pyrolysis. Compared to the conventional pyrolysis, microwave pyrolysis has some advantages, such as rapid heating, generation of fewer hazardous compounds, prevention of undesired secondary reactions, and higher heating efficiency (Domínguez et al., 2003; Miura et al., 2004). It has been used to treat biomass (Huang et al., 2010; Miura et al., 2004; Zhao et al., 2013), and sewage sludge (Menéndez et al., 2005; Tian et al., 2011). However, not all materials have the high absorption capacity that is necessary for effective microwave pyrolysis, so in order to improve the efficacy of this method, many researchers have experimented with adding various microwave absorbers or catalysts to the raw materials, such as metal oxides (Hu et al., 2012; Li et al., 2013; Wan et al., 2009), salts (Li et al., 2013; Wan et al., 2009), activated carbon (Bu et al., 2011; Hu et al., 2012), and acids (Wan et al., 2009). Our objective was to investigate the effect of microwave power levels and catalysts on the effectiveness of food waste pyrolysis. Temperature profiles, three-phase products characteristics, and the energy ratio of production to consumption (ERPC) were analyzed to determine the optimal conditions for this waste treatment process. 2. Methods 2.1. Sampling and preparation of materials Food waste was collected from a residential quarter in Guangzhou, Guangdong Province, China. Once fruits, plastic and shells were removed from the raw food waste, the remaining, three main components were white rice, vegetable leaves, and meat/ bones, with proportions of 32.69%, 44.23% and 23.08%, respectively, according to a previously published survey of domestic food residues (Ma et al., 2009). The samples were dried at 105 °C for 24 h, and then broken, ground and screened. The sizes of all resulting particles were less than 4 mm. The ultimate analysis, proximate analysis and lower heating value (LHV) of food waste (dry basis) were analyzed, and the results were shown as follows: C, 42.34 wt.%; H, 7.54 wt.%; O, 33.50 wt.%; N, 3.68 wt.%; S, 0.31 wt.%; volatile matter, 81.63 wt.%; fixed carbon, 5.74 wt.%; ash, 12.63 wt.% and LHV, 18.92 MJ/kg. Metal oxides (CaO, MgO, CuO, Fe2O3 and MnO2) and chloride salts (CuCl2, NaCl and MgCl2) were added to the ground food waste as catalysts. Catalysts were first dried in the oven at 105 °C for 24 h, then milled and sieved to obtain uniform particles smaller than 250 lm. The amount of food waste used in each experiment was 30 g, and the content of catalysts for each addition was 5% (mass basis).

Fig. 1. The diagram of pyrolysis system: (1) nitrogen bottle; (2) pressure reducing valve; (3) float flowmeter; (4) quartz reactor; (5) thermocouple; (6) cotton thread; (7) condenser; (8) gas collecting bag; (9) liquid collecting bottle; (10) materials; (11) microwave oven; (12) touch screen; (13) electronic energy meter.

MT6070iH touch screen (Weinview Science Stock Co., Ltd., Guangdong Province, China), and the oven was then started. During the experiment, the sample temperature was measured with a thermocouple, and the data were saved automatically on the touch screen, while electric energy consumption was recorded with an electronic energy meter. The condensable volatiles emitted during treatment were collected using a condenser filled with flowing, cooling water, and the non-condensable gases were collected in a gasbag; the process was left to continue until no further volatiles were observed, at which point the oven’s power was stopped. After the sample had cooled to ambient temperature, the liquid collecting bottle and quartz reactor were removed, and the mass of bio-oil and solid residue was measured. The weight of gas production was calculated based on the difference, using the mass balance. All experiments were repeated three times and mean results are reported, in order to the accuracy of the experimental results. 2.3. Evaluation method To evaluate the cost-effectiveness of microwave pyrolysis and to determine the optimal conditions for the procedure, we calculated an energy ratio, the ERPC, which we defined as:

ERPC ¼

Production energy Energy consumption

ð1Þ

The main cause of energy consumption during the process was the use of electric energy for pyrolysis. The energy produced included the energy of solid residue, bio-oil and gas production, whereas the gas proved difficult to use as fuel because it contained a large amount of carrier gas (N2). Therefore, Eq. (1) was refined as follows:

ERPC ¼

LHVsolid

 Msolid residue þ LHVbiooil  Mbiooil Electric energy consumption

residue

ð2Þ

where LHVsolid residue is the lower heating value of solid residue (MJ/kg); Msolid residue is the mass of solid residue (kg); LHVbio-oil is the lower heating value of bio-oil (MJ/kg); Mbio-oil is the mass of bio-oil (kg).

2.2. Experimental procedure 3. Results and discussion A sketch of the experimental apparatus used for microwave pyrolysis of food waste is shown in Fig 1. The microwave oven had a frequency of 2450 MHz and the power could be regulated continuously, up to 4000 W. In order to maintain anoxic conditions, N2 flow rate was kept at 500 mL/min for 20 min before the experiment, and then reduced to 300 mL/min during the experiment. An appropriate microwave power was entered on the

3.1. Characteristics of pyrolysis under different microwave powers 3.1.1. Temperature rise The rise in temperature is one of the most important characteristics of microwave pyrolysis (Huang et al., 2010), as it may affect the loss of mass and the distribution of products (Zhao et al., 2010).

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H. Liu et al. / Bioresource Technology 166 (2014) 45–50

3.1.2. Products of pyrolysis The distribution of product yields under different microwave powers is presented in Fig. 2b. The yield of solid residue declined sequentially, however, the gas yield increased continuously, and the bio-oil yield first increased, and then decreased when the microwave power increased from 300 to 600 W. Those results are similar to what has been reported for other studies of pyrolysis (Hu et al., 2012; Li et al., 2013; Tian et al., 2011). When microwave power was 300 W, the maximum yield of solid residue reached up to 81.63%, however, the bio-oil yield was 11.90% and the gas yield reached just 6.47%. Moreover, the solid residue produced at 300 W had a dark yellow color, unlike the black residues produced at greater powers, which showed that most of the materials had not been pyrolysed at the lower powers. Under a microwave power of 400 W, the highest bio-oil yield of 35.73% was obtained. The bio-oil separated into two parts when stored at room temperature about for 5 min: an upper layer was a black, thick viscous liquid, while a lower layer was a reddish brown liquid of low viscosity. When the microwave power was 400 W, 500 W and 600 W, the upper layer was roughly half the weight of the bio-oil, the yield of which was 16.79%, 14.84% and 12.87%, respectively. Nevertheless, the upper layer amounted to only 12.23% the weight of the bio-oil at 300 W. This result might indicate that higher microwave intensity and temperature favor generation of more of the upper layer type of bio-oil. The LHV of solid residue and of the upper layer bio-oil at different microwave powers were analyzed using a WR-3 calorimeter (Changsha Bente Instruments Co. Ltd, Hunan Province, China). The pH value of the lower layer of bio-oil was measured with a PHS-3E pH meter (Shanghai Leici Instrument Plant, Shanghai Municipality, China). The LHV of solid residue decreased gradually, when the microwave power increased from 300 to 600 W (Table 1).

80

Yield(wt.%)

Therefore, measurement of the temperature profiles of food waste under different microwave powers was a key aim of our work. There were three stages for each profile (Fig. 2a) namely slow warming, fast heating and finally slow temperature decrease; this differed from the typical temperature curves observed during the pyrolysis of microalgae (Hu et al., 2012) and bales of corn stalk (Zhao et al., 2010). Furthermore, the stronger the microwave power that we used in pyrolysis, the higher was the maximum achieved temperature and average heating rate (from the start to the maximum temperature). When the microwave power climbed from 300 to 600 W, the maximum value and average heating rate increase rapidly from 240 to 845 °C and from 0.35 to 3.18 °C/s, respectively. This may have been because the stronger microwave power made the polar molecules in food waste move more quickly, and thus generated more heat.

300W 400W 500W 600W

60

40

20

0 Solid residue

Bio-oil

Gas

Fig. 2b. The distribution of product yields under different microwave powers.

This result differed from another report (Huang et al., 2008), suggesting that the volatile matter in food waste has high calorific value. At 300–600 W, the LHV of the bio-oil’s upper layer was 31.82–33.39 MJ/kg, almost equal to that of bio-oil derived from coffee hulls (Domínguez et al., 2007), while higher than that of distillers dried grain with solubles (Lei et al., 2011). This might indicate the existence of high hydrocarbon contents (CxHy) in the upper layer of the bio-oil (Lei et al., 2011). The lower layer of the bio-oil was acidic (pH = 4.9) at 300 W, however, it was alkaline (pH > 8) at 400–600 W (Table 1). This may indicate that the breakdown of food waste produces acidic compounds under lower microwave intensities and temperatures, which can be converted easily to alkali compounds under higher microwave intensities and temperatures during pyrolysis. 3.1.3. Energy produced and consumed during pyrolysis The measured values of energy production, consumption and the ERPC are presented in Table 2. The electric energy consumption was at a minimum at a microwave power of 300 W (0.07 kW h), when the ERPC reached a maximum (2.49). However, as most food waste does not pyrolyse successfully under these conditions (as described in Section 3.1.2), which cannot be regarded as the optimal conditions for pyrolysis. At a microwave power of 400 W, electric energy consumption peaked (0.11 kW h), as did also production maximum energy (360.14 kJ), so the resulting ERPC was at its maximum (0.91). Therefore, 400 W was chosen as the best power level for food waste pyrolysis, and accordingly was also used in the following experiments. Given that the ERPC was only 0.91 under the best condition at 400 W, this means that the electric energy consumption was about 10% more than the amount of energy produced, and consequently about 10% of energy was wasted during the pyrolysis process. This made it necessary to add catalysts to food waste in order to save energy and maximize efficiency during the pyrolysis process.

800

Temperature

3.2. Pyrolysis characteristics in the presence of metal oxides

300W 400W 500W 600W

600

3.2.1. Temperature rise with metal oxides Both MgO and MnO2 provided good catalytic effects when added to food waste; compared to the results of pyrolysis without

400

Table 1 The LHV of solid residue, the LHV of the upper layer bio-oil and the pH values of the lower layer bio-oil under different microwave powers.

200

0 0

200

400 Time(s)

600

800

Fig. 2a. The temperature profiles under different microwave powers.

The LHV of solid residue (MJ/kg) The LHV of the upper layer bio-oil (MJ/kg) The pH value of the lower layer bio-oil

300 W

400 W

500 W

600 W

25.05 31.82

18.23 32.91

17.46 33.39

12.20 32.62

4.90

8.38

8.77

8.51

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H. Liu et al. / Bioresource Technology 166 (2014) 45–50

Table 2 The ERPC under different microwave powers. Microwave power (W)

Production energy (kJ)

300 400 500 600

solid residue

Bio-oil

613.47 194.33 176.70 117.61

13.89 165.81 148.66 125.95

Electric energy consumption (kW h)

ERPC

0.07 0.11 0.11 0.08

2.49 0.91 0.82 0.85

catalysts, the maximum temperature increased by 33.84% and 19.58%, respectively, upon addition of the metal oxides (Fig. 3a). At the same time, the average heating rate had risen by 53.92% and 335.29%, respectively. However, the other metal oxides tested, namely CaO, CuO and Fe2O3, had an entirely opposite, negative effect on pyrolysis. Compared to results achieved without catalysts, they caused the maximum temperature to decrease significantly by up to 51.75% and 56.90% in the presence of CaO and CuO, and the maximum temperature decreased slightly in the presence of Fe2O3. Moreover, the average heating rate had also decreased markedly by 52.94%, 53.92% and 22.55% with the addition of CaO, CuO and Fe2O3, respectively. These may be attributed to two factors. First, various metal oxides have different microwave absorbency. Previous studies (Salema et al., 2013; Zuo et al., 2011) found that the microwave absorbency of material was determined mainly by its dielectric properties in microwave field. The larger of material dielectric loss in microwave field, the better the microwave absorbency. Moreover, material dielectric loss depends greatly on its crystal structure – when there is a structural defect in the crystals, the dielectric loss will increase (Alford and Penn, 1996; Tamura, 2006). Structural defects have been demonstrated in MgO (Alfè and Gillan, 2005; Mackrodt, 1988) and MnO2 (Julien et al., 2002; Sayle et al., 2005), and consequently, their dielectric losses may be larger than CaO, CuO and Fe2O3 as well as have the stronger microwave absorbency. Second, the introduction of various metal oxides would have caused different changes in the chemical composition and microscopic physical structure of food waste materials. It is possible that the changes induced by adding MgO and MnO2 accelerated the chemical reactions involved (Li et al., 2013), while the changes induced by adding CaO, CuO and Fe2O3 may have had adverse impacts on the chemical reactions. 3.2.2. Distribution of product yields in the presence of metal oxides When CaO and CuO were added to food waste, the yield of solid residue rose to 78.87% and 79.47%, respectively, while the bio-oil yield decreased to 12.80% and 12.97%, and the gas yield decreased

Cn Hm ! C þ Cx Hy þ gas

80

600 400 200

CaO MgO CuO Fe2O3 MnO2 Without catalyst

60 Yield(wt.%)

800

ð3Þ

where CnHm denotes hydrocarbons in bio-oil vapors, and CxHy represents the lighter hydrocarbons. When MgO, Fe2O3 and MnO2 were added, the yield of the upper layer of bio-oil was 13.84, 13.91 and 11.56%, which was approximately half the weight of the bio-oil. These results were similar to what was achieved at 400 W without a catalyst. Moreover, the yield of the upper layer of bio-oil was only 1.26% and 1.43% with the additives CaO and CuO, similar to results at 300 W without catalysts. These findings further demonstrated that high temperature promoted formation of the upper layer of bio-oil. The measures of LHV and pH values are presented in Table 3. Except for CaO and CuO, the addition of metal oxides lowered the LHV of solid residue. Compared to results achieved without catalysts, MgO, Fe2O3 and MnO2 caused the LHV of solid residue to decrease by 15.25%, 3.84% and 23.75%, respectively. This may have been due to a lower calorific value of metal oxides, or to a stronger pyrolysis reaction with the addition of metal oxides, leading to an increased release of volatile products (which have a high calorific value) and ultimately to a lower LHV of solid residue. The metal oxides appeared to have less effect on the LHV of the upper layer of bio-oil, which measured 31.96–33.25 MJ/kg. However, pH values of the lower layer of bio-oil changed significantly in the presence of metal oxides. Generally, the higher the maximum temperature and average heating rate, the higher the pH values that

CaO MgO CuO Fe2O3 MnO2 Without catalyst

1000

Temperature

to 8.33% and 7.57% (Fig. 3b). There results were similar to what was achieved at 300 W without the use of a catalyst, and the vast majority of materials were not pyrolysed because of the lower temperature of materials. Also, the solid residue caked in the presence of CaO, while this phenomenon was not observed either when CuO was added, or at 300 W without a catalyst. A possible cause was a chemical reaction between CaO and the pyrolysis products of food waste, leading to formation of a hard substance. For example, CaO reacted with CO2 to form CaCO3, which could not decompose because of the low temperature during the experiment (Zhao et al., 2013). This might be another factor that hampered temperature rises in the presence of CaO. Other additives had no dramatic effect on the yield of solid residue, while causing a marked decreased in the yield of liquid, and a significant increase in gas yield. Compared to results with no additive, the addition of MgO, Fe2O3 and CuO caused liquid yield to decrease by 18.18, 19.02 and 28.16%, respectively, while gas yield increased by 25.42, 16.72 and 37.84%, respectively. The opposite impact on gas and bio-oil yield might be attributed to the secondary cracking of hydrocarbons in bio-oil vapors (Eq. (3)) upon the addition of metal oxides (Abu El-Rub et al., 2004):

40

20

0 0

200

400 600 Time(s)

800

Fig. 3a. The temperature profiles in the presence of metal oxides.

0 Solid residue

Bio-oil

Gas

Fig. 3b. The distribution of product yields in the presence of metal oxides.

49

H. Liu et al. / Bioresource Technology 166 (2014) 45–50 Table 3 The LHV of solid residue, the LHV of the upper layer bio-oil and the pH values of the lower layer bio-oil in the presence of metal oxides. MgO

CuO

Fe2O3

MnO2

Without catalyst

20.58

15.45

21.40

17.53

13.90

18.23

32.16

32.83

31.96

33.25

32.46

32.91

7.45

9.06

5.32

8.16

9.28

8.38

600 Temperature

The LHV of solid residue (MJ/kg) The LHV of the upper layer bio-oil (MJ/kg) The pH value of the lower layer bio-oil

CaO

800

CuCl2 NaCl MgCl2 Without catalyst

400

200

0

resulted. This was consistent with the results reported in Section 3.1.2.

3.3. Pyrolysis characteristics in the presence of chloride salts 3.3.1. Temperature rise with chloride salts CuCl2 was a very effective catalyst: compared to the results achieved without a catalyst, the maximum temperature and average heating rate dramatically increased by 18.51% and 233.33%, respectively (Fig. 4a). However, the complete opposite result was observed with MgCl2, more similar to what occurred with the addition of either CaO or CuO. That is, the maximum temperature and the average heating rate were only 224 °C and 0.32 °C/s, respectively. Also, NaCl had little effect on the heating process: the maximum temperature and average heating rate differed by only 2 °C and 0.01 °C/s, respectively, compared to results achieved without it. 3.3.2. Distribution product yields in the presence of chloride salts The yields of solid residue increased more or less in the presence of chloride salts (Fig. 4b). Curiously, adding CuCl2 at a higher maximum temperature, however, whose yield of solid residue was 3.64% higher than the yield without catalysts. This was perhaps because chloride salts influence the basic reactions of pyrolysis, leading to the formation of more semi-coke (Chen et al., 2008), and hence an increased yield of solids. Also, the addition of chloride salts (except MgCl2) caused higher gas yield from food waste, while the bio-oil yield decreased. This phenomenon was similar to the addition of metal oxides (except CaO and CuO), which can be explained by the fact that the formation of gas and bio-oil are competing processes. When CuCl2 or NaCl was added, the yield of the upper layer of oil was 9.86% and 13.20%, respectively. However, that yield was only 1.04% in the presence of MgCl2. This situation was similar

200

400 Time(s)

600

800

Fig. 4a. The temperature profiles in the presence of chloride salts.

80 CuCl2 NaCl MgCl2 Without catalyst

60 Yield(wt.%)

3.2.3. The ERPC in the presence of metal oxides In addition to the cases where CaO or CuO was added (when most food waste was not properly pyrolysed), the ERPC was greater than 1 when either MgO or MnO2 was used as additives (Table 4), which suggested that they have an energy saving effect. In particular, MnO2 was the best catalyst among the tested metal oxides, resulting in an ERPC of up to 1.93.

0

40

20

0 Solid residue

Bio-oil

Gas

Fig. 4b. The distribution of product yields in the presence of chloride salts.

the result of using CaO or CuO as additives, and can be ascribed to the low temperature that materials reached under those conditions. The chloride salts increased the LHV of solid residue to some extent (Table 5). This was opposite to the effect of adding metal oxides, possibly because chloride salts suppressed some of the release of volatiles, which have a high calorific value, and hence the yield of solid residue and its LHV would have increased accordingly. The chloride salts had a lesser influence on the LHV of the upper layer of bio-oil, which was 32.48–33.05 MJ/kg, similar to that of metal oxides. Moreover, the chloride salts lowered the pH values of the lower layer of bio-oil, whose value was lower than 7 (i.e., acidic). This influence was opposite to the effect of adding metal oxides, perhaps because chloride salts promoted production of furfural (Wan et al., 2009), an acidic liquid aldehyde.

3.3.3. The ERPC in the presence of chloride salts The chloride salts CuCl2 and NaCl increased the ERPC of pyrolysis (Table 6), the addition of either salt resulted in an ERPC >1. This

Table 4 The ERPC in the presence of metal oxides. Metal oxides

CaO MgO CuO Fe2O3 MnO2 Without catalyst

Production energy (kJ) Solid residue

Bio-oil

517.79 184.16 542.28 223.68 165.69 194.33

12.13 136.30 13.71 138.71 112.55 165.81

Electric energy consumption (kW h)

ERPC

0.11 0.08 0.11 0.12 0.04 0.11

1.34 1.11 1.40 0.84 1.93 0.91

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H. Liu et al. / Bioresource Technology 166 (2014) 45–50

Table 5 The LHV of solid residue, the LHV of the upper layer bio-oil and the pH values of the lower layer bio-oil in the presence of chloride salts.

The LHV of solid residue (MJ/kg) The LHV of the upper layer bio-oil (MJ/kg) The pH value of the lower layer bio-oil

CuCl2

NaCl

MgCl2

Without catalyst

20.38 32.84

18.48 33.05

22.43 32.48

18.23 32.91

3.90

6.15

3.58

8.38

Table 6 The ERPC in the presence of chloride salts. Metal chlorides

CuCl2 NaCl MgCl2 Without catalyst

Production energy (kJ) Solid residue

Bio-oil

270.04 255.95 560.53 194.33

97.12 130.90 10.11 165.81

Electric energy consumption (kWh)

ERPC

0.05 0.10 0.12 0.11

2.04 1.07 1.32 0.91

was especially true for CuCl2, which led to an ERPC of up to 2.04. This means that the production energy was more than twice as much as the electric energy consumption, indicating that CuCl2 was an effective catalyst. When this result is considered in light of the discussion in Sections 3.2 and 3.3, we draw the conclusion that, in terms of ERPC, CuCl2 was indeed the best of the tested catalysts.

4. Conclusions Food waste can be converted into useful forms of energy such as solid residue and bio-oil, using microwave irradiation. 400 W was the optimal level of power for pyrolysis, based on the maximum achieved ERPC of 0.91. The additives CuO, CaO and MgCl2 had a negative effect on the pyrolysis of food waste, and Fe2O3 or NaCl had little effect, while MgO, MnO2 and CuCl2 had a significant catalytic effect on the process. Metal oxides and chloride salts had different impacts on the three phases of products. CuCl2 is the best catalyst, followed by MnO2 based on the ERPC. Acknowledgements This work was supported by the National Basic Research Program of China (973 program) (2011CB201500), the Science and Technology Project of Hunan (2013SK3175, 2013SK3176) and the Science and Technology Project of Loudi (2011ZD13). References Abu El-Rub, Z., Bramer, E.A., Brem, G., 2004. Review of catalysts for tar elimination in biomass gasification processes. Ind. Eng. Chem. Res. 43 (22), 6911–6919. Ahmed, I.I., Gupta, A.K., 2010. Pyrolysis and gasification of food waste: syngas characteristics and char gasification kinetics. Appl. Energy 87 (1), 101–108. Alfè, D., Gillan, M., 2005. Schottky defect formation energy in MgO calculated by diffusion Monte Carlo. Phys. Rev. B 71 (22), 1–4. Alford, N.M., Penn, S.J., 1996. Sintered alumina with low dielectric loss. J. Appl. Phys. 80 (10), 5895–5898. Bu, Q., Lei, H., Ren, S., Wang, L., Holladay, J., Zhang, Q., Tang, J., Ruan, R., 2011. Phenol and phenolics from lignocellulosic biomass by catalytic microwave pyrolysis. Bioresour. Technol. 102 (13), 7004–7007.

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