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Experimental Study on Partial Oxidation Reforming of Pyrolysis Tar over Sewage Sludge Char Haimiao Yu, Yangyang Xu, Geng Chen, and De Zhen Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01037 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 27, 2016
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Experimental Study on Partial Oxidation Reforming of Pyrolysis Tar over Sewage Sludge Char Haimiao Yu,* Yangyang Xu, Geng Chen, and Dezhen Chen
Institute of Thermal and Environmental Engineering, Tongji University, No. 1239 Siping Road, Shanghai, P. R. China.
ABSTRACT: In the study, pine pyrolysis gases containing rich tar passed through the bed of sewage sludge char prepared at high temperature to explore the partial oxidation reforming characteristics of pyrolysis tar. Under the varying conditions of equivalence ratio (ER), residence time of 1.25, 2.5, and 3.75 s in the char bed, and reforming temperature of 700, 800, and 900 °C, we investigated the variations of gas composition, tar conversion ratio, and components in reforming products. After reforming over the high-temperature char bed, tar conversion ratio reached 95%. The suitable ER for the reforming process was about 0.05 and allowed the highest contents of H2 and CO and the tar conversion ratio of 98.34%. The increase in the reforming temperature increased the percentages of H2 and CO in syngas to some degree and significantly decreased the tar concentration of in syngas. At 900 °C, tar conversion ratio reached 99.19% and the obtained tar mainly belonged to 2-3 rings PAH compounts. In addition, at 800 °C, with the increase in the residence time, tar concentration could not be further decreased and the concentration of light aromatics was gradually increased. Sewage sludge char showed specific application advantages in catalytic reforming of tar. Keywords: sewage sludge char, catalysis, tar reforming, GC-MS. 1. INTRODUCTION Biomass gasification technology is an environmentally friendly and energy-efficient thermo-chemical treatment and has wide application prospects. However, tar removal remains a longstanding challenge in the development of biomass gasification technology. In biomass gasification process, tar is involved in 1
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generation reactions and secondary reactions, exists in the syngas, and may clog and corrode downstream equipment.1 Gasification conditions, such as temperature, gasification agent, residence time, and catalyst utilization way and other factors, significantly affect tar yields and the syngas composition.2 Compared with mechanical treatment methods, tar catalysis removal methods shows the better performance and can not only achieve effective tar removal but also promote the full conversion of energy.3 The methods for tar catalyst removal consist of treatments inside the gasifier (the primary method) and the hot gas cleaning processes after the gasifier (the secondary method).4 Zhang et al.5 found that alkali metals in straw char played an important role in the catalytic cracking process of tar and that the catalysis role of straw char promoted the generation of alkyl monoaromatics and suppressed the generation of PAHs (polycyclic aromatic hydrocarbons) from primary tar. Activated carbon had been applied in the catalytic cracking of tar and showed the better performance. El-Rub et al.6 also pointed out that the attractiveness of char as a catalyst originated from its low price and natural production mode inside the gasifier. Therefore, the char has the wide application prospect in the field of tar reforming. Bhandari et al.7 indicated that biochar with a high specific surface area could effectively remove toluene. Generally, the higher preparation temperature allowed the higher specific area and catalytic activity of the generated activated carbon. Moreover, Klinghoffer et al.8 found that although the contents of inorganic components (such as Ca, K, Na, P, Si, and Mg) in char was no more than 2%, inorganic components played an important role in the catalytic process of char. From the above, the characteristics of the activated carbon which is suitable for catalytic reforming of tar can be predicted. Sewage sludge is a special solid waste generated in the urbanization process. In recent years, sludge production from wastewater treatment plants has been increased dramatically in developed and developing countries and the rational utilization ways of sludge should be developed.9 Sludge char is an important product of the sludge pyrolysis process. Sludge char yield of the sewage sludge pyrolysis varies with 2
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pyrolysis temperature and the char production usually accounts for 35%-80% of the mass of dry sludge.10 Sludge char prepared at high temperature is characterized porous structures and the high specific surface area.11 Sludge char prepared at high temperature has the general characteristics of tar cracking catalysts: the higher content of inorganic composition and the larger specific surface area. Therefore, it might be useful to catalytic reforming of tar. Fuentes-Cano et al.12 indicated that two model tars (toluene and naphthalene) were completely converted within the sludge char bed at the temperature above 850°C. However, the application of sewage sludge char in catalytic reforming of real tar was seldom reported. Chen et al.13 found that without the catalyst of biochar, oxygen could reduce the tar concentration and that treatment with the biochar bed could further decrease the tar concentration. In addition, Wang et al.14 compared the reforming products of tar in two different atmospheres (air and water vapor) and found that air allowed the less residual tar. Therefore, the study on the application of sewage sludge char in partial oxidation reforming of tar is of practical significance. This paper focused on partial oxidation reforming characteristics of pyrolysis tar over sewage sludge char. We mainly investigated the main factors of the reforming process: temperature, residence time, and ER (equivalence ratio). We analyzed the gas compositions obtained under different conditions, tar conversion ratio, and tar composition and obtained the data of the partial oxidation reforming process over the sewage sludge char bed to provide the basis for gasification tar removal, sewage sludge char reduction, and resource utilization in the future. 2. EXPERIMENTAL 2.1. Preparation of Catalysts Sewage sludge was from a certain wastewater treatment plant in Jiading District in Shanghai. Sewage sludge was firstly dried in ventilated drier. Then, sludge char samples were prepared at various temperatures in the tubular furnace at 700, 800 and 900 °C and N2 was used as carrier gas. The results of proximate and 3
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ultimate analyses of sludge char samples are shown in Table 1. LHV was tested with an XRY-1 Bomb-Calorimeter and the ultimate analysis was performed with a Vario EL III Elemental Analyzer. The proximate analysis was carried out according to the Chinese Standard GB/T 212-2008.15 The particle size distribution of sludge char samples are listed in Table 2. Table 1. Proximate and ultimate analyses of sludge char samples. Samples
Proximate analysis (% dry basis) Volatile Fixed carbon
Ultimate analysis (% dry basis)
Ash
LHV (MJ/kg)
C
H
N
O
S
700°C Char
9.91
18.01
72.07
7.50
18.38
0.87
1.01
4.66
3.01
800°C Char
6.65
17.61
75.74
6.91
17.91
0.42
1.02
1.88
3.03
900°C Char
6.34
17.06
76.60
6.54
17.11
0.37
1.26
1.56
3.10
Table 2. Particle size distribution of sludge char samples. Particle size (mm) Percentages (%)
9.5
