99/01071 Catalytic fast pyrolysis of biomass : Salter, E. H. et al. IChemE Res. Event, Two-Day Symp., 1998, 89–96

Abstract

Utilization of biomass offers a potential to sustain the current petro-chemical economy for the production of chemicals and (transportation) fuels on basis of renewable resources. Crude bio-oil derived from fast pyrolysis of lignocellulosic biomass is a mixture of water (15-30 wt.%) and various oxygen containing organic compounds. The presence of oxygen in bio-oils (ca. 35–40 wt.%) is commonly believed to be the origin of problems caused by its high water content (15–30 %), corrosiveness (pH of 2–3), relatively low heating value compared to fossil fuels (ca. 17 MJ/kg), poor volatility, and high viscosity (35–1000 cP at 40 °C). However, not only the level of oxygen in the bio-oil is too high, but also the way it exists (functionality) is a part of the problem. Improving the quality of the bio-oils, whether or not in combination with a certain degree of oxygen removal, would include a selective transformation of certain oxygen functionalities such as acids and aldehydes into ‘desired’ or acceptable ones like alcohols, phenols, and ethers. Application of heterogeneous catalysis in fast pyrolysis (i.e. catalytic fast pyrolysis; CFP) may lead to a liquid product (i.e. catalytic fast pyrolysis oil, CFP-oil) with an improved quality compared to that of crude bio-oil. Here, the improvement in bio-oil quality refers to the production of either high yields of transportation fuel compounds (e.g. aromatics, olefins) and specialty chemicals (e.g. phenolics), or just a drop-in refinery feedstock to be blended with the feed streams of existing petroleum refineries. While the literature on catalytic fast pyrolysis of biomass -mainly focussed on catalyst screening- is rapidly expanding, there is an urgent need for the translation of laboratory results to viable process concepts and bench/pilot plant trials. Together with the development of efficient catalysts, the design and the intensification of the process with efficient heat integration are of significant importance in the catalytic conversion of lignocellulosic biomass to the targeted liquid product. The present thesis discusses the catalytic fast pyrolysis of lignocellulosic biomass in a process oriented way that may initiate a useful process technology development in the near future. The final goal is to come up with recommendations and suggestions on how to realize this technique at a commercial/industrial scale. That requires a better understanding of the precise effects of the essential process parameters (e.g. processing mode; in- or ex situ) and design elements (e.g. reactor type, catalyst type) on the one hand, and definitions and outcomes of possible obstacles (e.g. successive regeneration of the catalyst, effect of biomass ash) on the other. In this work, two types of continuously operated (catalytic) fast pyrolysis reactors were used, viz. an auger reactor and a mechanically stirred bed reactor. In all experiments performed in both setups, pine wood with a particle size range of 1 to 2 mm was pyrolyzed at a constant reactor temperature of 500 °C. In the auger reactor, first the effect of the operation mode on the product yields and compositions has been investigated while using a single type of heterogeneous ZSM-5 based acidic catalyst. Two operation modes were tested. In situ operation includes the mixing of biomass and catalyst inside a single reactor, while ex situ refers to catalytic treatment of the pyrolysis vapours in a secondary reactor. A second study was concerned with the screening of various heterogeneous catalysts (and their metal doped counterparts) in in situ operation. In all experiments, the presence of catalysts led to the production of additional water, coke and gases at the expense of the liquid organics and char. The overall performance of in situ catalysis in terms of oil quality was considerably better than that of ex situ catalysis; more aromatics and phenols were produced in the case of in situ operation. That may be caused by different vapour residence times and vapour-catalyst contact times. Among all eight catalysts tested, the acidic catalyst containing some redox active metal, the basic catalyst with a mixture of two metal oxides (calcined), and a metal oxide doped gamma-alumina catalyst (calcined) were found to be the best performing ones, based on both the deoxygenation requirements and the production of desirable compounds in high yields. In the mechanically stirred bed reactor, we studied i) the effect of a repeated catalyst regeneration (eight cycles in total), and ii) the effects of the pine wood ash on the yields and composition of the products. In all catalytic experiments, a single type of a ZSM-5 based catalyst was used in situ. Along the reaction/regeneration cycles, trends in pyrolysis product yields converging to that of non-catalytic levels were observed. This revealed that the activity, and thus the influence of the catalyst slowly declined, which was confirmed by a BET surface area reduction of 63 %. Ash concentrations as low as ca. 3 wt.% relative to the amount of pine wood fed, and ca. 0.002 wt.% relative to the amount of bed material, were found sufficient to affect the yield and composition of the CFP products unfavourably. Finally, the technical and operational barriers for the implementation of catalytic fast pyrolysis technology are discussed while focusing on the process modes and parameters, economical use of the primary and secondary products, and heat integration. Some process alternatives for an efficient CFP operation are suggested as well. Research has, until now, been focused mainly on screening and small-scale testing of various catalysts. One challenge in developing CFP of biomass is the design and large scale production of such catalysts to enable testing in continuously operated, bench and pilot scale installations. FCC type of catalysts are the only suitable ones commercially available. But they are developed especially for use in a riser reactor and short contact times (differing significantly from typical biomass devolatilization times). The main problem in CFP of biomass was found to be the presence of the biomass originated alkaline ash which eventually poisons any catalyst in case of direct contact. In a commercial process, a solution may be to separate the biomass fast pyrolysis from the catalytic treatment of the vapours (i.e. ex situ processing mode) where the physical contact between the biomass minerals and the catalyst is excluded. Even though this requires significant process adjustments, ex-situ processing allows the catalyst to be re-used in a much larger number of reaction/regeneration cycles than in case of in situ operation.