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Development of Continuous Fast Microwave Assisted Pyrolysis System for Sewage Sludge Conversion and Utilization

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Published: 10th Dec 2019

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Development and application of a truly continuous fast microwave assisted pyrolysis system for sewage sludge conversion and utilization


A continuous fast microwave-assisted pyrolysis system was designed and fabricated. The sewage sludge was used as the raw materials to investigate the new system. The effect of pyrolysis temperature (450, 500, 550, and 600 ℃) on the products yield, distribution and potentially recovery energy were investigated. The raw material (sewage sludge), bio-oil, char and gas samples were analyzed using elemental analyzer, GC-MS, Micro-GC, and ICP-OES and SEM to characterize the physical properties and chemical composition. Experimental results showed that the maximum bio-oil yield was 41.39 wt.% at 550 ℃. The optimal pyrolysis temperature is 500℃, at which the total Qrecovery is 18.42MJ/kg with energy value of bio-oil, gas and char are 7.16 MJ/kg, 8.69 MJ/kg and 2.57 MJ/kg, respectively. The higher HHV of gas products was obtained because of there is no carrier gas in this study, which would dilute the valuable gas and degrade the gas quality.



Continuous system

Fast Microwave-assisted pyrolysis

Sewage sludge

Energy recovery potential



The oil crisis, increasing greenhouse gas emissions and global warming issues have motivated researchers to discover new alternative and economic energy resources. Renewable energy is considered as one of promising energy and has attracted significant attention. As a renewable energy source, biomass (e.g., agricultural residues, forestry residues, industrial residues, animal residues, municipal solid waste, and others) offers a potential to sustain the current petrochemical economy for the production of chemicals and transportation fuels. Fast pyrolysis has become one of the most prevailing routes for converting biomass into liquid fuels (also known as bio-oil) with gas and char as valuable by-products within the past decades.

Microwave-assisted pyrolysis (MAP) is an alternative heating method and has been used to extract the energy from biomass. Compared to conventional electrical heating processes, microwave-assisted technology has many advantages, including in-core volumetric and uniform heating at molecular level, process flexibility and equipment portability, lower thermal inertia and faster response, energy saving and cleaner products because there is no carrier gas. Xie et al. (2015) conducted catalytic co-pyrolysis of microalgae and scum and obtained the maximum aromatic yield of 42 % at 550

. (Bu et al., 2012) investigated the catalytic microwave pyrolysis of douglas fir to obtain high quality bio-oil with high concentrations of phenol (38.9%) and phenolics (66.9%). (Fan et al., 2017a) studied catalytic co-pyrolysis of lignin and low-density polyethylene with HZSM-5 and MgO and found that LDPE significantly removed methoxyl groups from phenols. However, most of microwave pyrolysis units in these papers are all batch or semi-batch reactor. Some of them mixed the feedstocks and microwave absorbent before the experiments and heat them together to the desired temperature. Some of them heat the microwave absorbent first and then introduced the feedstocks onto the top of the microwave absorbent with mixing well. In strictly, they did not belong to truly fast pyrolysis. Lower heating rate and high residence time would influence the bio-oil yield and chemical composition. In addition, mixed solid residue and microwave absorbent are hard to be cleaned and collected. Without continuous feeding, separation and collection, this process is hard to scale up for pilot scale and commercial scale. In the present study, a continuous fast microwave-assisted pyrolysis (cfMAP) system was developed to increase the bio-oil yield and quality. With the continuous mixing feedstocks and microwave absorbent, the temperature was uniform and the feedstocks could be instantaneously heated to the set temperature. The liquid, gas and solid residues could be separated and collected easily.

Sewage sludge is the major waste generated from municipal and industrial wastewater treatment process, and attracts more attention recently, probably because of the hot topic, environment and health issues. Agricultural use, incineration, and landfill are the three main ways of disposing of sewage sludge (Fonts et al., 2012; Laturnus et al., 2007; Xie et al., 2014). However, all of them have limitation and drawbacks to some extent. The sewage sludge contains heavy metals, and some organic compounds, which could negatively affect the environment. In addition, the high cost for incineration and the land needed for landfill encourage scientists to find new alternative management technique(Houillon & Jolliet, 2005; Rio et al., 2006).  Moreover, sewage sludge has a similar calorific value to the low-grade coal. Inguanzo et al. (2002) concluded that pyrolysis technology could achieve 50% reduction in waste volume and both oils and gases produced showed high overall heating values. Furthermore, the application of microwave pyrolysis in bio-oil production from sewage sludge is rarely reported.

In this study, a continuous feeding and mixing microwave-assisted pyrolysis system was developed. Sewage sludge was used as the case study raw material in the new system to analyze the physical characterization of pyrolysis products such as bio-oil, gas and bio-char. The effects of pyrolysis temperature was investigated on product yield and bio-oil composition. Meanwhile, the potentially recovery energy of each product with pyrolysis temperature increased from 450

to 600

was also investigated. In addition, characterization of biochar was conducted using elemental analyzer, FE-SEM and ICP-OES multi-element determination.

2. Methods

2.1 Materials

The sewage sludge used in this study was obtained from the Metropolitan Wastewater Treatment Plant, Saint Paul, Minnesota, USA. The sewage sludge was a mixture of primary and secondary sludge with about 80wt.% moisture contents. Prior to experiments, the sewage sludge samples were dried at 105

for 24h and pulverized mechanically and sieved to less than 1mm before pyrolysis.  Table 1. shows the physicochemical characteristics of the sewage sludge.

Table 1. The physicochemical characteristics of the sewage sludge

Proximate analysis (wt.%) Elemental analysisa (wt.%) HHVc (MJ/kg)
Moisture Asha Volatile Fixed Carbonb C H N Ob
18.54 43.40 6.99 5.66 25.40 21.31
Mineral elementsa (mg/L)
Al As B Ba Be Ca Cd Co Cr
5753.51 9.06 96.41 522.45 <0.001 27052.79 0.77 4.61 75.45
Cu Fe K Li Mg Mn Mo Na Ni
388.69 8462.37 5876.18 1.63 5957.68 2786.20 12.86 1995.78 48.07
P Pb Rb S Si Sr Ti V Zn
23827.73 28.86 52.85 10067.31 1572.33 53.11 57.41 12.78 848.77

a Dry basis

b Calculated by difference, FC(%)= 100-A-V, O(%) = 100-C-H-N-A

c Higher heating value, calculated using the equation (Vallios et al., 2009) HHV


34.1 C


123.9 H

9.85 O


6.3 N


19.1 S

2.2 Design of a continuous fast microwave-assisted pyrolysis system

A lab-scale continuous fast microwave-assisted pyrolysis (cfMAP) system has been developed. Fig.1 shows a schematic diagram of the cfMAP system. It consists of a continuous feeding system, a microwave-assisted reactor with feedstock and microwave absorbent mixing system, a cooling system, and a collecting system. The continuous feeding system is mainly composed of an auger of adjustable rotating speed and a hopper made of vinyl. Special modification is made to ensure air tightness of the feeding system. By adjusting the auger rotating speed, feedstock can be fed in an accurately controlled rate into the microwave-assisted reactor, which is placed in a modified microwave oven (Mars 5, CEM Corporation, 1000 W, 2450 MHz).

The double layer microwave-assisted reactor is made of quartz and is composed of two vessels connected with ground joints for leak-free performance. The inner vessel is mainly used to contain the microwave absorbent while the outer vessel collects the char.

The bottom of the inner vessel is a 4mm-thick mesh plate, which has staggered rows of holes that are 4mm in diameter and separated by 15 mm (center of one hole to the center of the next hole) to make sure the char could fall through and be collected at the bottom of the outer vessel.

In order to achieve a fast heating rate, mixing is an essential step to promote heat exchange between the feedstock and the hot microwave absorbent. A lab stirrer (LR500B, Yamato) with adjustable speed was modified and incorporated into the system to mix the microwave absorbent within the inner vessel. Silicon carbide balls (6 mm in diameter) are used to as microwave absorbent in this system because 1) silicon carbide has excellent microwave absorbing ability and heat conductivity as demonstrated by our previous studies (Liu et al., 2016) and thus ensures efficient and uniform heating of the process, 2) the spherical shape of the balls makes the mixing process easy and energy-efficient, and 3) the gaps between the balls allow the char to fall through and get collected.  Process temperature is monitored by thermocouples (K-type) inserted in the SiC ball bed within the inner vessel.  The thermocouples are connected to a 4-channel thermocouple probe thermometer (Sper Scientific 800023) for temperature read-out. Prior to the pyrolysis, the reactor was vacuumed for 15 min and the vacuum was kept during the whole process at 100 mmHg to eliminate the influence of oxygen. The vacuum pump is also used to draw the pyrolysis vapor through the microwave-assisted reactor into the cooling system and collecting system. Through the cooling system, condensable vapors are separated from the non-condensable gases and collected in the collecting system, while the non-condensable gases can either be collected or flared up.

Different from traditional batch microwave-assisted reactor, continuous mixing ensures the temperature uniform and the feedstocks could be instantaneously heated to the set temperature. The three main products, bio-oil, char and gas could be separated easily and completely. This continuous feeding, continuous mixing and separation process makes commercial applications possible. As a result, the truly fast microwave-assisted pyrolysis conditions will be achieved.

For a typical run, 1000 g silicon carbide ball is used in the reactor as absorbent of the microwave. The feedstocks are first placed inside the hopper and sealed before the experiments. The agitator is on during the whole process to mixing the SiC and feedstocks. When the temperature of reactor reaches the set point, the sample is then fed onto the heated SiC bed through the auger feeding system. To maintain a stable temperature of microwave absorbent bed, the microwave oven was manually turned on or off. The biomass is pyrolyzed in the pyrolysis reactor and solid residues are separated through the mesh plate and collected at the bottom of the outer vessel. After the pyrolysis vapor goes through the condensers, its condensable components were condensed into liquid form, which is collected as bio-oil. The weight of char was determined by weight difference of quartz reactor before and after experiments. The bio-oil and solid residue yields were calculated using their actual weight, and the gas yield was computed using the following equation (Gas yield



Char yield

Bio-oil yield). Microwave leakage was monitored by a type MD-2000 microwave detector (Digital Readout) for safety purpose. The experiments were repeated twice.

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Fig. 1 The experimental apparatus is composed of: (1) hopper; (2) biomass feedstock; (3) feeding auger system; (4) oven; (5) control panel; (6) SiC bed; (7) quartz reactor; (8) agitator; (9) char; (10) mesh; (11) thermocouple (K-type); (12) outlet quartz connectors; (13) liquid fraction collectors; (14) bio-oil; (15) condenser; (16) connection for vacuum pump

2.3 Bio-oil characterization

Agilent 7890-5975C GC/MS with a HP-5 MS capillary column at 30 m


0.32 mm and 0.25 μm thickness was employed to detect chemical composition of the bio-oil samples. The initial oven temperature was kept at 50

for 2 min, followed by continuously heating it to 260

at 5

/ min, and maintained at 260

for 5 min. The injector temperature was 290

, and the injector size was 1 μL with a split ratio of 1:10.

The carrier gas flow (helium) was 1.2 mL/min. The chromatographic peaks were identified by comparing the mass spectra and library data of National Institute of Standards and Technology (NIST). The calibration was not carried out due to the large number of chemical compounds. A semi-quantitative method was used by calculating the peak area percentage of gas chromatogram to determine relative proportion of each compound.

The elemental composition of sewage sludge, bio-oil and char were also determined by an elemental analyzer (Type CE-440, Exerter Analytical Inc., MA).

2.4 Non-condensable gas characterization

A Varian Micro-GC (CP-4900) with a thermal conductivity detector (TCD) was used to determine the composition of non-condensable gas product. The two columns used were Molecular Sieve 5A and PoraPLOT Q with helium as carrier gas. The analyzed gas compounds included H2, CO, CO2, CH4 and some C2-C3 hydrocarbon gases (ethylene, ethane, propylene, and propane)

2.5 Biochar characterization

The micro morphology of sewage sludge and char were observed by a Field Emission Gun Scanning Electron Microscope (Hitachi SU8230, Japan). The magnifications of the SEM was selected as 1000


to compare pore developments in the sludge char. In addition, Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Rb, S, Si, Sr, Ti, V and Zn elemental analyses are performed by a ThermoFisher iCap 7600 Duo ICP-OES Analyzer.

3. Results and discussion

3.1 Effect of pyrolysis temperature

Fig. 2 shows the effect of pyrolysis temperature on the continuous fast microwave-assisted pyrolysis of sewage sludge with the constant feeding and mixing rate, at temperature ranging from 450 to 600

. As shown in Fig. 2 (a), the pyrolysis temperature plays an important role in the cracking reaction of sewage sludge. The bio-oil yield increased from 16.47 wt. % at 450

to 41.39 wt. % at 550

and then slightly decreased to 27.95 wt. % at 600

, and the maximum bio-oil yield was obtained at 550


Organic bonds in the sewage sludge that might have been broken when the temperature increased from 450 to 550

to form the bio-oil. This is the main possible reason for the initial increase in bio-oil yield. However, the bio-oil yield decreased when the temperature increased above 550

is believed to be a result of secondary thermal cracking reactions. More liquid product was converted to gas compounds.  Oil fraction decreased with an increase of gas yield can also explain it. In addition, the char yield decreased from 62.26 wt.% (450


to 32.98 wt.% (600

) with increasing pyrolysis temperature. As more volatiles are released, the char yields are expected to decrease. It is noteworthy that the char yield did not change too much around 550

. The possible reason is the high ash content in the char, and almost all volatiles have been extracted from the sewage sludge.

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Macintosh HD:Users:shiyuliu:Desktop:continuous sludge paper:manuscript:数据:图:fig.3.wmf


Fig. 2 Effect of pyrolysis temperature on the continuous fast microwave-assisted pyrolysis of sewage sludge: (a) Product distribution; (b) Bio-oil compositions.

The bio-oil was a complex mixture of chemical compounds, such as alkanes, alkenes, aromatics, acids, phenols, ketones, and nitrogen contained compounds. In this study, the relative components of bio-oil were classified into several groups, such as aliphatic hydrocarbons, aromatic hydrocarbons, oxygen-containing aliphatic compounds, oxygen-containing aromatic compounds, nitrogen-containing compounds and polycyclic aromatic hydrocarbons (PAHs). The effect of pyrolysis temperature on the bio-oil compositions is shown in Fig. 2 (b). With the increasing pyrolysis temperature, the proportion of aliphatic hydrocarbons, aromatic hydrocarbons and PAHs increased and reached its maximum of 31.2 % at 500

, 8.4 % at 550

and 1.6 % at 550

, respectively. From the results, aromatic hydrocarbons and PAHs favor higher reaction temperature (500 – 600

) because of the triggering of Diels-Alder reaction (Kim et al., 1997; Lopez et al., 2009). The lower aliphatic hydrocarbons were seen in the bio-oil obtained at higher pyrolysis temperature, which were converted into gas and oil fraction at higher reaction temperature. Besides, the proportion of oxygen-containing aliphatic compounds and oxygen-containing aromatic compounds decreased at first and then increased in the test temperature range. As could be seen, nitrogen-containing compounds were also the main components of the bio-oil, such as triacetonamine, pyridine, and pyridine, 2-methyl-. It is known that nitrogen-containing aromatic compounds and PAHs have carcinogenic and mutagenic issues (Shaw & Connell, 1994). Fonts et al. (2009) investigated the pyrolysis of three different kinds of sewage sludge in fluidized bed reactor for bio-oil production, and 2-12% of nitrogen-containing aromatic compounds were observed in the bio-oil fraction. Different kinds of sewage sludge could obtain different products. For this reason, the raw materials should be also taken into account that minimizes the production of these hazard compounds.

3.2 Energy recovery potential

Table 2 shows the HHVs of bio-oil and char under different pyrolysis temperature. The highest HHVs of bio-oil and char are 20.61 MJ/kg at 500

and 10.32 MJ/kg at 450

, respectively. The energy recovery potential (ERP) is defined as the energy could be potentially recovered from biofuel products, including bio-oil, biochar and gas (Mei et al., 2016) based on the estimated HHV of each of the pyrolysis products and its distributions. The recoverable energy (Qrecovery) was calculated as follows:



















HHVgas        (1)

where, Qrecovery is the potentially recovered energy per 1kg sewage sludge; Qbio-oil, Qbiochar, and Qgas are the maximum energy potentials of bio-oil, bio-char and gas per 1kg of sewage sludge; Ybio-oil, Ybiochar, and Ygas are the yield (wt.%) of bio-oil, bio-char and gas of sewage sludge; HHVbio-oil, HHVbiochar, and HHVgas are the higher heating value of bio-oil, bio-char and gas.

The HHV of bio-oil was calculated using equation (Friedl et al., 2005).

HHV (MJ/kg)
























10-3                                                                                                                   (2)

where C, H and N represents carbon, hydrogen and nitrogen contents of material, respectively.

The HHV of char was calculated using equation (Channiwala & Parikh, 2002).

HHV (MJ/kg)


0.3491 C


1.1783 H


0.1005 S

  0.1034 O

   0.0151 N

  0.0211A                                                                                                                       (3)

where C, H, O, N, S and A represents carbon, hydrogen, oxygen, nitrogen, sulphur and ash contents of material, respectively.

The HHV of gas was calculated using equation(Li et al., 2004).

HHVgas (MJ/Nm3)
















YC2-C4,mol)/100                                                                                             (4)

where YCO,mol, YH2,mol, YCH4,mol, and YC2-C4,mol are the corresponding molar fractions (molar ratio) and their heats of combustion in the gas. The molecular weight of ethylene was used for assuming ideal-gas behavior for the C2-C4 hydrocarbon gases.

HHVgas (MJ/kg)


HHVgas (MJ/Nm3)


Vgas (Nm3/kg)

) =

HHVgas (MJ/Nm3)

×  [(









22.4]/100        (5)

where Vgas is the volume of gas per 1 kg sewage sludge; MCO, MH2, MCH4, and MC2-C4 are the molar mass of CO, H2, CH4 and C2-C4 hydrocarbon gases (kg/mol), respectively.

The Qrecovery, including bio-oil, char and gas at different pyrolysis temperature are shown in Fig. 3, based on the HHV of each of the pyrolytic products and their distributions. As the pyrolysis temperature rose, the potential energy value of char decreased gradually from 5.42 MJ/kg at 450

to 2.14 MJ/kg at 600

. The high ash content in the sewage sludge led to the lower potential energy value of char. It is noticed that gas had a higher potential energy value from 5.58 to 9.43 MJ/kg over the rages of pyrolysis temperature studied, which was much higher than the result (1.81 MJ/kg) from (Li et al., 2014).  Because of there is no carrier gas in this study, which would dilute the valuable gas and degrade the gas quality. The potential energy value of bio-oil increasing with the increased pyrolysis temperature, and reached its maximum value of 7.65 MJ/kg at 550

. From the results, the optimal pyrolysis temperature is 500

, at which the total Qrecovery is 18.42 MJ/kg with energy value of bio-oil, gas and char are 7.16 MJ/kg, 8.69 MJ/kg and 2.57 MJ/kg, respectively.

Table 2. HHVs of bio-oil and char under different pyrolysis temperature

Bio-oil Biochar








C 36.21 47.61 44.20 40.12 23.47 21.94 22.01 20.73
H 11.00 10.62 9.02 10.23 2.19 1.50 1.44 1.09
N 6.01 6.16 6.83 4.91 3.78 3.50 3.60 3.21
Oa 46.78 35.61 39.95 44.74 6.44 7.32 4.27 4.95
Ash 64.12 65.74 68.68 70.02
HHV (MJ/kg) 13.50 20.61 18.47 15.85 8.70 7.23 7.44 6.48

a Calculated by difference, O (%)= 100





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Fig. 3. Potential energy values of Bio-oil, Char and Gas per kilogram sewage sludge obtained from a continuous fast microwave-assisted pyrolysis at different pyrolysis temperature.

3.3. Gases composition

Fig. 4 shows the effect of pyrolysis temperature on the major gases contents in a continuous fast microwave-assisted pyrolysis of sewage sludge. As the result, H2, CO, CO2, CH4, and some C2-C3 hydrocarbon gases (ethylene, ethane, propylene, and propane) were the main gaseous compounds of sewage sludge pyrolysis. H2S was also detected with micro GC, however, the maximum volume percent is 13022 ppm at 550

. It was not shown in the Fig. 4 because of the trend cannot be represented clearly. It was found that CH4 was the dominant gaseous species at the lower temperature (450- 500

). Increasing the pyrolysis temperature to 600

, the CO2 has the maximum yield of 31 mol %. The H2 was increasing with the increased temperature and reached maximum yield at 6.68 mol % at 600

. Besides, the CO was decreased slightly at the temperature range. One the other hand, the C2-C3 hydrocarbon gases showed a maximum in their yields at 500

for ethylene, ethane and propylene, and 600

for propane. Fan et al. (2017b) studied the pyrolysis of low-density polyethylene with MgO as the catalyst, and found that with the increasing pyrolysis temperature, the ethane and propylene increased first and then decreased because of the severe chain scission of C4+ hydrocarbons at higher pyrolysis temperatures.

Moreover, the effect of pyrolysis temperature on the HHV (MJ/Nm3) of the gases was shown in Fig. 5. The maximum heating value of 22.5 (MJ/Nm3) were obtained at 500

. Thus, pyrolysis temperature of 500

is the optimal temperature for higher heating value gas production.

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Fig. 4 Effect of pyrolysis temperature on the major gases contents in a continuous fast microwave-assisted pyrolysis of sewage sludge.

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Fig. 5 Effect of pyrolysis temperature on the HHV (MJ/Nm3) of the gases.

3.4. Biochar

Fig. 6 presents the comparison of the SEM micrographs to determine the pore characteristic of the char obtained from the pyrolysis. It is noticeable that the structure was different during the pyrolysis process. The sewage sludge had relatively smooth surface, while the char had rough and porous surface. The possible reason was that pores were developed from the evaporation of moisture content and volatile matter during pyrolysis process. Bagreev et al. (2001) proved that water released by dehydroxylation of the inorganic matters favor the pore structure formation and act as activation agent creating micropores in the carbon deposit.

Table 3 shows the mineral elements of biochar at pyrolysis temperature of 500

. The results showed that the contents of major plant nutrients (P, K, Ca and Mg) in biochar from sewage sludge pyrolysis were very high. When compared with the results from Table 1, the concentrations of heavy metals including Cd, Cr, Cu, Zn, Pb and Ti, etc. were increased in the biochar than those in sewage sludge, which showed that pyrolysis process retained the heavy metals in the biochar. However, the pore structure of the bio-char enhanced ability to immobilize heavy metal. DTPA extraction method was used to estimated the readily available concentration of heavy metals in biochar in Liu et al. (2014)  study, the results showed that the heavy metals were more stable in the biochar than those of sewage sludge. Therefore, biochar can be used to produce fertilizer or soil amendment, or adsorbents such as activated carbon for pollutant removal.

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Fig. 6 SEM micrographs of (a) sewage sludge and (b) char at pyrolysis temperature of 500


Table 3 Mineral elements of biochar at pyrolysis temperature of 500

Mineral elements (mg/L)
Al As B Ba Be Ca Cd Co Cr
20867.30 8.27 125.86 1047.94 0.03 87263.14 2.89 13.60 231.11
Cu Fe K Li Mg Mn Mo Na Ni
1394.80 26515.97 17962.99 5.69 19769.66 8900.25 44.93 4126.22 151.25
P Pb Rb S Si Sr Ti V Zn
75076.61 86.59 16.86 6715.70 899.86 163.30 262.01 27.48 2380.19

4. Conclusion

The sewage sludge was used as the raw materials to investigate the new continuous fast microwave-assisted pyrolysis system. The optimal pyrolysis temperature is 500

, at which the total Qrecovery is 18.42 MJ/kg with energy value of bio-oil, gas and char are 7.16 MJ/kg, 8.69 MJ/kg and 2.57 MJ/kg, respectively. Due to the high major plant nutrients contents and pore structure, char could be used to produce fertilizer or soil amendment, or adsorbents such as activated carbon for pollutant removal. High HHV of bio-oil (20.61 MJ/kg) and gas (22.5 MJ/Nm3) implied sewage sludge was a promising biomass resource in the future.


This project is supported in part by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative‐Citizen Commission on Minnesota Resources (LCCMR), MCES, and UMN MNDrives and Center for Biorefining.

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