Aitziber Erkiaga’s research while affiliated with University of the Basque Country and other places

Steam gasification (900 °C) of high density polyethylene (HDPE) in a conical spouted bed reactor (CSBR) followed by steam reforming in a fixed bed reactor (Ni commercial catalyst) has been carried out. The effect reforming temperature in the 600-700 °C range has on product yields and gas composition as well as on the amount and nature of the coke deposited on the catalyst has been studied. The reforming leads to a full transformation of C2+ hydrocarbons and tar. The maximum H2 yield is obtained at 700 °C (36 wt % by mass unit of HDPE in the feed, which accounts for 83 wt % of the maximum H2 yield according to stoichiometry), with CH4 conversion being 94% and the coke amount deposited on the Ni catalyst being minimum (3.3 wt % by mass unit of HDPE in the feed). The syngas obtained by reforming at 700 °C has a H2/CO volumetric ratio of 6 and is suitable for hydrogenation reactions and further valorization to produce H2.

Steam pyrolysis (500 °C) of high density polyethylene (HDPE) in a conical spouted bed reactor followed by a steam reforming (700 °C) in a fixed bed reactor (Ni commercial catalyst) has been carried out for obtaining a syngas stream rich in H2. The product yields obtained in the pyrolysis of HDPE using steam are very similar to the previous ones with N2 as fluidizing agent. A comparison of the results obtained here with those for gasification-reforming strategy reveals that tar and C2+ hydrocarbons are not fully reformed using the pyrolysis-reforming strategy. Besides, although a high H2 yield is obtained (81.5% the stoichiometric value), it is slightly lower than the yield obtained in gasification-reforming. Coke yield is high (4.43 wt.%), which causes fixed bed blockage and hinders a correct flow of the volatiles through the bed. SEM, TEM and TPO results show that two types of coke are deposited on the catalyst, with amorphous coke prevailing in the pyrolysis-reforming strategy and filamentous coke in gasification-reforming.

The steam gasification of different mixtures of biomass and high density polyethylene (0 wt%, 25%, 50% and 100% of HDPE) has been carried out in continuous mode in a conical spouted bed gasifier, and the results have been compared with those obtained feeding pure biomass and HDPE. The experiments have been carried out at 900 °C and using a steam/(biomass + plastic) mass ratio of 1. Olivine has been used as primary catalyst in order to minimize tar formation. The cofeeding of HDPE has given way to an interesting synergetic effect involving the reduction of both tar and char yields. Thus, the tar content in the gaseous product was reduced from 58.23 g Nm−3 in the gasification of pure biomass to only 9.74 g Nm−3 when a mixture made up of 50% of each feedstock was in the feed. These promising results evidence the interest of the cogasification strategy apart from contributing to increasing feedstock availability.

Hydrogen production from pyrolysis-catalytic steam reforming of pine sawdust has been investigated using two subsequent reactors: i) a conical spouted bed reactor for biomass pyrolysis at 500 °C, and ii) a fluidized bed reactor for catalytic reforming of volatiles from the pyrolysis step. A commercial Ni reforming catalyst has been used for the reforming step (Reformax® 330). 99.7 % conversion and a H2 yield of 93.45 % are achieved at 600 °C, 0.28 gcatalyst h gbiomass-1 and S/C ratio of 8.2, producing 11.2 g of hydrogen per 100 g of biomass fed into the process. Increasing reaction time, higher coke contents are deposited on the catalyst due to secondary reactions. This is the main cause of catalyst activity decrease, although deactivation is attenuated by the good performance of the fluidized bed reactor and the excess of steam in the reaction medium (high S/C ratio).

The steam gasification of high density polyethylene in continuous mode has been carried out in a conical spouted bed reactor. The effect of temperature (in the 800–900 °C range) and steam/plastic mass ratio (between 0 and 2) on the distribution of products (gas and tar) and their composition has been studied. In order to reduce tar formation, two catalysts have been used in situ, namely, olivine and γ-Al2O3. The spouted bed reactor has an excellent performance between 850 and 900 °C, and an increase in the steam/plastic ratio from 1 to 2 only improves slightly both carbon conversion efficiency (to 93.6% with steam/plastic ratio = 2) and hydrogen concentration (61.6%). The use of olivine and γ-Al2O3 instead of sand gives way to a moderate reduction in the tar formation, whose yield is 4.8% with olivine. The syngas obtained has a H2/CO ratio of 2.2, with a low tar content whose composition (monoaromatics, mainly benzene) augurs well for the use of the syngas in DME synthesis.

Pinewood sawdust has been gasified by using steam as gasifying agent in a conical spouted bed reactor operating in continuous regime. A study has been made of the effect of temperature (in the 800–900 °C range), steam/biomass ratio (between 0 and 2) and sawdust particle size (0.3–1, 1–2 and 2–4 mm ranges) on the distribution of products (gas, tar and char) and their composition has been studied. Temperature has a positive effect on the gas composition by increasing H2 content and reducing that of CO. Furthermore, the tar and char yields are reduced as gasification temperature is higher. An increase in steam/biomass ratio from 0 to 1 has a positive effect on hydrogen concentration, char gasification and tar reforming, and a further increase in this parameter from 1 to 2 gives way to only a limited improvement in the results. Sawdust particle diameter plays a minor role in the gasification process in the temperature range studied, which must be attributed to the high heat transfer rate in the bed. Thus, this technology allows operating with coarse sawdust particles without reducing the syngas yield. However, the reduced gas residence time gives way to a high tar yield.

Olivine and gamma-alumina have been used as primary catalysts for tar elimination in the continuous steam gasification of pine wood sawdust in a bench-scale plant provided with a conical spouted bed reactor. A comparison of the performance of each catalyst with that observed for a bed made up of inert silica sand shows that both catalysts have a significant activity for tar cracking/reforming, given that the amount of tar obtained by operating with beds of inert sand is reduced by 79% and 84% when olivine and -gamma-alumina are used, respectively. The tar cracking reduces selectively the content of light and heavy PAHs, giving way to an increase in the concentration of light aromatics. Furthermore, both catalysts cause a positive effect on the gas composition by slightly enhancing the water-gas shift and reforming reactions.

The production of activated carbons derived from waste tires has been carried out using steam and carbon dioxide as activation agents. The effect of temperature and activation time over the carbons prepared has been studied. The material subjected to activation has been char or residual carbon black obtained by continuous flash pyrolysis of waste tires carried out in a conical spouted bed reactor at 500 degrees C. The activation has been carried out at 850 and 900 degrees C using steam and only at 900 degrees C using carbon dioxide due to the slower kinetics of this process. The activation runs have been conducted in a fixed bed reactor. An essentially mesoporous structure is developed in the process, with BET surface areas above 500 m(2)/g for the carbons obtained with steam and above 370 m(2)/g in those obtained using carbon dioxide.

A commercial FCC catalyst based on a zeolite active phase has been used to perform the valorisation of waste tires by in situ catalytic pyrolysis. The process has been carried out in a conical spouted bed reactor provided by a continuous feeding system. The catalyst has been subjected to a steaming treatment in order to maximize diesel fraction. High diesel (50.5 wt %) and char (35 wt %) yields are obtained, together with low gas (0.6 wt %) and gasoline (13.9 wt %) fractions. The obtained liquid is highly aromatic, which is formed at the expense of the non-aromatic gasoline fraction.

The thermal and catalytic cracking of plastic waste has been regarded as one of the most feasible plastic recycling method at industrial scale. Three catalysts based on HZSM-5 zeolites with a ratio of SiO2/Al2O3 = 30, 80, and 280 were prepared and agglomerated by wet extrusion with bentonite (30 wt %) and with inert alumina (45 wt %). Continuous pyrolysis of HDPE was carried out in a conical spouted bed reactor at 500°C. The pyrolysis volatiles formed (mainly waxes) were fed on line to a catalytic fixed bed reactor. The product fraction yields were divided into light olefins (C2-C4), light alkanes (C1-C4), single-ring aromatics, non-aromatic C5-C11 compounds and C12-C21 hydrocarbons. Aan increase in the SiO2/Al2O3 ratio gave way to a decrease in the yields of C2-C4 olefins, light alkanes, and single-ring aromatics. Consequently, there was a increase in the yields of C5-C11 non-aromatic compounds and C12-C21 hydrocarbons. This is an abstract of a paper presented at the CHISA 2012 - 20th International Congress of Chemical and Process Engineering and PRES 2012 - 15th Conference PRES (Prague, Czech Republic 8/25-29/2012).