Article pubs.acs.org/IECR
NiO/Dolomite Catalyzed Steam/O2 Gasification of Different Plastics and Their Mixtures Prangneth Friengfung,†,‡ Ekaporn Jamkrajang,†,‡ Sasithorn Sunphorka,†,‡ Prapan Kuchonthara,*,†,‡ and Lursuang Mekasut†,‡ †
Fuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Bangkok 10330, Thailand ‡ Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University Research Building (7th Floor), Chulalongkorn University, Phayathai Road, Pathumwan, Bangkok 10330, Thailand ABSTRACT: The catalytic gasification of different individual plastics and their mixtures with a dolomite supported Ni catalyst was evaluated in a drop-tube fixed-bed reactor. The influence of the Ni loading level and the interactions of mixed plastics during steam gasification were both examined. A 5 wt % NiO/dolomite catalyst gave the highest degree of carbon conversion for all evaluated plastics and mixtures. A mixture of the four types of plastic that matched the estimated weight ratio of real plastic waste revealed a lower gas yield than predicted, suggesting their interaction during gasification. The mixtures which contain highdensity polyethylene or low-density polyethylene at 0.75 wt/wt led to a reduced gas production, while polystyrene increased it, especially in the polystyrene−polypropylene binary mixture with a markedly higher H2, CO, and CO2 production level than that expected. Thus, it would be beneficial to screen plastic types prior to the generation of suitable gas products. Ni−Mg−Al catalyst,17 the presence of steam favored gas production and decreased the liquid yield and coke deposition on the catalyst.17 However, the gas yield and composition obtained from the plastic mixtures were not obviously different from the pure plastics. This was probably due to the relatively low heating rate, a limitation of this reactor type, which results in the complete decomposition of raw materials. In contrast, an interaction among plastic mixtures was observed in other studies,18,19 where the experimental results were significantly different from the calculated results. Thus, the different types of plastic affected the product distribution. However, most of the literature reports on the interaction of plastics during their pyrolysis and liquefaction, while information on their interaction during gasification, especially in a high heating rate gasifier, is still scarce. Thus, the aim of this work was to reveal any advantage in the catalytic gasification of various types of plastics and their mixtures. Low-density polyethylene (LDPE), HDPE, PP, and PS, and their mixtures were used since they represent the major types of waste plastic. The catalytic effects of calcined dolomite and NiO/dolomite on the gasification of various types of plastic were examined in a drop-tube fixed-bed reactor, which achieves a relatively higher heating rate than in conventional fixed-bed reactors. The effect of the amount of Ni loaded onto the dolomite was also examined. In addition, the mixture of four types of plastic, at a weight percent composition ratio expected to represent that of actual plastic waste in Thailand, was evaluated as was that for paired mixtures of the plastics to reveal
1. INTRODUCTION The increasing amount of plastic waste that is being generated from daily activities is of increasing concern. Although some kinds of plastic can be recycled, most of them are discarded by means of landfill which requires plenty of space. Some thermochemical processes, including combustion, pyrolysis, and gasification, have been considered as alternative ways to reduce the landfill volume and to recover energy from the plastic waste. Gasification is a promising technology for converting solid masses into more valuable, gaseous fuels. Synthetic gas (syngas) derived from the gasification is an important feedstock for several chemical and manufacturing processes. Plastic wastes are sources of carbon, but their gasification is not straightforward compared to coal and biomass, which is related to their different thermal decomposition behaviors. Polymers or plastics are volatile materials, resulting in a relatively low gas yield under typical gasification conditions. Accordingly, research and development have focused upon pyrolysis or cracking of plastics to produce liquid products or pyrolytic oil as the major product.1−13 Very little literature has reported on the gasification of pure plastics.14−16 The air gasification of polypropylene (PP) in a fluidized-bed gasifier without any additives as in-bed materials for tar elimination was shown to potentially be performable and generated a fuel gas with a calorific value of 5.2−11.4 MJ/(N m3) and a low tar content.15 The influence of dolomite and olivine, as in-gasifier additives, on the gas quality and tar content was evaluated, and dolomite was found to be more active for tar elimination than olivine.16 The effect of mixing plastics has been reported previously.17−19 With respect to the influence of plastic types, including PP, polystyrene (PS), high-density polyethylene (HDPE), and their mixtures, in a two-stage reactor with a © 2014 American Chemical Society
Received: Revised: Accepted: Published: 1909
June 15, 2013 November 7, 2013 January 10, 2014 January 10, 2014 dx.doi.org/10.1021/ie401893s | Ind. Eng. Chem. Res. 2014, 53, 1909−1915
Industrial & Engineering Chemistry Research
Article
which type of plastic might account for the interaction during the gasification processes.
2. MATERIALS AND METHODS 2.1. Materials. The proximate and ultimate analyses of the LDPE, HDPE, PP, and PS plastics used in this study are summarized in Table 1. The actual mixed plastic waste was Table 1. Proximate and Ultimate Analyses of the Four Types of Plastics Used in This Study composition
PS
Proximate Analysis (wt volatile matter 99.8 fixed carbon 0.2 ash 99.9 >99.9
