Ind. Eng. Chem. Res. 2008, 47, 1005-1010
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Catalytic Air Gasification of Plastic Waste (Polypropylene) in Fluidized Bed. Part I: Use of in-Gasifier Bed Additives Jesu´ s A. Sancho, Marı´a P. Aznar,* and Jose´ M. Toledo Department of Chemical Engineering and EnVironmental Technology, C.P.S., UniVersity of Zaragoza, st. Marı´a de Luna 3, 50018 Zaragoza, Spain
Gasification of 100 wt % polypropylene waste was carried out for this work. The main objective of this study was to compare the effectiveness of tar elimination by some additives in the gasifier bed with an inert bed. Dolomite and olivine were used as in-gasifier bed additives. Dolomite was more active than olivine when using the same amount in the gasifier bed (30 wt %). Tar content was reduced by 92% when dolomite was used, whereas the reduction of tar with olivine was 40% when compared with an inert bed. However, dolomite caused a few problems by plugging the gas cleaning devices because of the high amount of particulates; this did not happen with olivine. Therefore, olivine was used as the preferred gasifier bed material. The bed was then tested with 100 wt % olivine, which is a hard material. The results achieved were good and promising, obtaining from the gasifier a tar content of 2 g/nm3 at the exit, with an L.H.V. of 6 MJ/Nm3 and a gas yield of 6 Nm3/kgdaf. Introduction The consumption of plastics in developed countries has increased a lot in recent years. According to Shell Briefing,1 this consumption, in the 1970s, was around 13 million ton/year, and in 2007 the demand for plastics will exceed 70 million ton/ year. This increase is generating a high amount of plastic waste, such as high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and polyethylene terephthalate (PET). From the many plastic waste treatments, the most used is dumping as it is also the cheapest, but it is the worst when considering the environment. Another possibility is the direct use of plastics for energy, for example, combustion, due to the high energy content of the raw material. However, more interesting alternatives are mechanical and chemical recycling. Mechanical recycling is the most interesting because it enables plastic to be used as a raw material in other plastic products. However, this type of recycling is only feasible for products formed by one kind of plastic or by special mixtures, so a good separation and identification of each component is necessary. When mechanical recycling is not possible, chemical recycling technologies can be used and plastic waste is converted into different products through several chemical processes to break it down. In general, the goal is to obtain hydrogen or light hydrocarbons, which can be used as energy products or raw materials. There are different chemical recycling processes such as pyrolysis, hydrogenation, and gasification. Zevenhoven et al.2 studied the behavior of the most-common plastics (PE, PP, PS, PVC) in combustion and gasification processes and compared them with conventional fuels such as coal, peat, and wood. They found that co-firing with plastic-derived fuels significantly * To whom correspondence should be addressed. Tel.: +34+ 976762391. Fax: +34-976761879. E-mail:
[email protected].
increased the amount of volatiles in the freeboard of a bubbling fluidized bed. The pyrolysis of different plastic waste has also been widely studied. Mastral et al.3 studied thermal degradation (pyrolysis and gasification) of HDPE at the bench scale. Kodera et al.4 developed a novel process using a moving bed reactor for the pyrolysis of PP. Marcilla et al.5 studied kinetics in the catalytic pyrolysis of PP. Some research was carried out on the thermal and catalytic decomposition of PVC by Ali et al.6 Pyrolysis of other plastic waste such as polymethyl methacrylate was also studied.7 Gasification is appearing as an interesting solution for the utilization of plastic wastes, although there has not been much work done at the pilot scale. Most of these studies were for cogasification in a fluidized bed of plastics and other solids such as biomass, coal, and waste tyre. Pinto et al.8,9 studied cogasification with air/steam of PE mixed with coal and biomass. Aznar et al.10 also worked with cogasification of ternary mixtures of coal-biomass-plastic waste with air in a fluidized bed at the small pilot plant scale. In this case, plastic waste was composed of a mixture of PE and PP (50 wt %) from the car industry. Pohorely et al.11 carried out some research on the cogasification of coal and PET, using nitrogen as a gasifying agent with 10 vol % O2 in bulk. Ponzio et al.12 developed the technology of high temperature air/steam gasification of pellets made from waste materials of wood and plastic origins. There are also some studies on the gasification of wastes containing PVC with steam13 and with [steam + O2].14 Besides, other technologies are appearing in this field such as the cogasification of waste tires and PET using the solar thermochemical process.15 From the literature that the authors found, only one article addressed air gasification with 100 wt % plastic wastes (polypropylene),16 which is also the aim of this work. However, the study by Xiao et al.16 was done without using additives for tar elimination as in-bed materials. The use of these additives
10.1021/ie071023q CCC: $40.75 © 2008 American Chemical Society Published on Web 01/19/2008
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Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008
Figure 1. Small pilot plant for advanced plastic-waste gasification used at the University of Zaragoza.
significantly improves the quality of the gas produced in the gasification process, as several authors have already demonstrated.9,17-26 Dolomite is a mineral commonly used as an in-gasifier bed material additive. Prof. Corella et al. at the Universities of Zaragoza and Complutense of Madrid have done extensive work on this subject: comparing dolomites with other solids such as calcite (CaO) and magnesite (MgO) in biomass gasification with steam,17,18 testing different dolomites in biomass gasification with air,19 and determining the best location of the dolomite in the gasification process.20-22,24 Nevertheless, dolomite is very soft and therefore it erodes, producing a high amount of particles at the gasifier exit. To avoid this problem, olivine [(Mg, Fe)2 SiO4] was used as the in-gasifier bed material. The particle content at the gasifier exit decreased between 4- and 6-fold using this kind of solid,23 because olivine is highly attrition-resistant.25 Nevertheless, the activity of this mineral for tar elimination was lower than the one obtained for dolomite.23 All of this work was done in biomass gasification but never for plastic gasification. In the work presented here, dolomite and olivine were compared as in-gasifier bed materials for the gasification of plastic waste with air in a fluidized bed. Furthermore, the influence of these additives on gas composition, tar content, gas yield, and lower heating value were also analyzed. Facility Used. An illustration of the facility used in this study is shown in Figure 1. The gasifier is based on a bubbling fluidized bed reactor divided in two zones: the bed zone is 9.2 cm i.d. and 1 m high, with 4 thermocouples along the height which makes it easy to know when a good fluidization takes place, and the freeboard is 15.4 cm i.d. and 1 m high with 2 thermocouples along the height. At the bottom of the gasifier, in the bed zone, there is a bed of silica sand, which was mixed with dolomite or olivine in different ratios depending on the test run. The feedstock for this study, polypropylene, was
continuously fed near the bottom of the bed. The feeding system has one hopper where solids are stored and two screwfeeders. The first screwfeeder is the dosing device, and it is used to control the feedstock flow rate by changing its speed. This screwfeeder unloads solids into the second one, which has the function of delivering solids in the bed at a very fast speed to avoid feedstock pyrolysis within it (with subsequent plugging). The feeding pipe has a heat exchanger near the gasifier to avoid feedstock pyrolysis before entering into the gasifier. The flow rate of plastic waste fed to the gasifier was 1 kga.r./h. Air was used as the gasifying agent. It was introduced to the gasifier at two points: at the bottom of the gasifier through a distributor plate, as primary air, and at the bottom of the freeboard as secondary air. The temperature in the reactor was controlled by two electric ovens, which make it possible to regulate independently the bed temperature and the freeboard temperature. After the gasifier, there are two high-efficiency cyclones connected in series where most of the particles were separated. The temperature at this point was around 400-500 °C to avoid tar condensation. The gas coming from the cyclones was cooled in a heat exchanger where the condensates, water and tar, were separated. After the heat exchanger, the cooled gas was passed through a filter to avoid damaging in the measuring devices downstream. Finally, the cooled exit gas was measured for its flow rate and then burned before being expelled. The plant has an advanced control system for recording and controlling the most important parameters of the process. Sampling and Analysis. The sampling point was located after the cyclones and before the heat exchanger. Between four and five gas samples were periodically collected at different timeson-stream and analyzed by gas chromatography for major components (H2, CO, CO2, light hydrocarbons, N2). The tar sampling system is based on cold tramps and was developed by Narvaez et al.26 It consists of four impinger flasks
Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008 1007 Table 1. Chemical Composition of the Feedstock (Polypropylene) Used in This Work proximate analysis (wt %) moisture ash volatiles fixed carbon
0.94 99.0
Ultimate Analysis (wt %) C H N S O
85.25 14.71 0.04
Table 2. Chemical Composition (wt %) of the Olivine Used component
wt %
MgO SiO2 Fe2O3 Al2O3+ Cr2O3+ Mg3O4 CaO NiO
48.0-50.0 39.0-42.0 8.0-10.5 0.8
