Production of advanced biofuels: Co-processing of upgraded pyrolysis oil in standard refinery units
Introduction
First generation biofuels (bioethanol and biodiesel) are currently being used in many countries. Their utilisation can contribute to secure the supply of fuels and to the reduction of green-house-gas emissions, although the net energy value of some of them has been strongly questioned [1]. Advanced biofuels not only have the same advantages as the previously mentioned fuels, but also they do not compete with the food chain and they can be produced from a wider range of ligno-cellulosic biomass, including agricultural waste, wood, forest residues, etc. Several options are under development to produce advanced biofuels. Biomass can be gasified to produce synthesis gas followed by e.g. a Fischer Tropsch process. The process proposed in this work consists of co-processing upgraded pyrolysis oil from ligno-cellulosic biomass in standard refinery units. The advantages of this process are:
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The use of decentralised pyrolysis plants that can be near the biomass production site. This means that only the oil is transported, reducing transportation costs due to the increase of the volumetric energy of the oil compared to the original biomass.
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After pyrolysis, large part of the minerals from biomass is not transferred to the oil but remain as ash. Thus, pyrolysis oil contains less inorganic material that could poison subsequent catalytic processes. Moreover, the ash can be returned to the soil as fertiliser.
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As the upgrading plant would be next to (or inside) the refinery, all the necessary utilities would be already available and the product obtained after co-processing could use the existing distribution network.
During fast (or flash) pyrolysis, dry solid biomass is rapidly heated to temperatures around 400–500 °C in the absence of oxygen, converting it into a liquid oil with yields up to 70–80 wt.% [2]. Char (∼5–10 wt.%) and gas (∼20–30 wt.%) are also produced. Pyrolysis oil (or bio-oil) is a mixture of oxygenated compounds formed during the decomposition of lignin and (hemi-)cellulose and water (generated during the process and from the initial moisture content of the biomass). The oxygen content is typically 45–50 wt.% and the water content 15–30 wt.% [2]. Because of this, the heating value of pyrolysis oil (HHV ∼ 17 MJ/kg [2]) is low compared to fossil fuels (HHV ∼ 45 MJ/kg). All these properties make the direct co-processing of pyrolysis oil itself in standard refinery units problematic. Several pyrolysis process modifications are currently being studied (hot-gas vapour filtration [3], catalysis [4]) to obtain an oil with better properties. In this study, pyrolysis oil from standard flash pyrolysis has been used as feed for upgrading.
Various upgrading routes have been studied until now: hydrodeoxygenation (HDO) to remove the oxygen as water under high pressures of hydrogen and in the presence of a catalyst [5]; catalytic cracking using zeolites [6]; and high pressure thermal treatment (HPTT), in which pyrolysis oil is thermally treated to obtain an oil with a higher energy density [7]. Previous research on HDO suggested that a two-stage process is preferred to prevent excessive coke formation during HDO [8]. In the first step at relatively low temperature (∼150–175 °C), pyrolysis oil is stabilised and in a second step at higher temperature (∼350–380 °C) deep deoxygenation (>95%) could be achieved [9]. The main concern about this process is the high hydrogen consumption (>800 NL/kg feed). After catalytic cracking of crude pyrolysis oil, gasoline range products were obtained [10]. However, at low temperatures (370 °C) the amount of oxygenated compounds was high and at higher temperatures (410 °C), the production of coke and gas increased at the expense of the gasoline yield [11]. During HPTT of pyrolysis oil, phase separation occurs producing an aqueous phase (15–35 wt.% dry basis), an oil phase (55–65 wt.% dry basis), gas (0–10 wt.% dry basis) and water (5–15 wt.% dry basis). Experimental results showed that, with increasing temperatures, the release of gases (mainly CO2) and the production of water increased, reducing the oxygen content (from 40.1 to 20.0 wt.%, on dry basis) of the oil phase and hence increasing the energy content (from 14.1 to 28.4 MJ/kg). This increase of energy density was due to the transfer of organic components from the aqueous phase to the oil phase. However, an increase of the average molecular weight of the oil, probably caused by the polymerisation of the sugar constituents of the oil, was also detected [7].
Baldauf et al. [12] proposed the use of hydrotreated pyrolysis oil in standard refineries. From the properties of the hydrotreated pyrolysis oil, they concluded that it should be sent to the distillation tower where the fractions could be diluted in different refinery cuts and be sent for further processing. Co-processing of upgraded pyrolysis oil was studied by Samolada et al. [13]. In their research, a heavy and a light fraction were obtained by thermal hydrotreatment of flash pyrolysis oil (Union Fenosa, Spain). The heavy fraction, with and oxygen content of 4.9 wt.% (wet basis), was catalytically cracked in a MAT reactor, with a dilution ratio of 15/85 heavy fraction/LCO (in weight basis). They obtained gasoline yields between 20 and 25 wt.% and coke yields between 0.8 and 1.4 wt.%. Lappas et al. [14] co-processed the same heavy fraction as Samolada et al. but diluting it in LCO and VGO. The product yields of co-processing were approximately 1 wt.% higher for gasoline and LCO and 0.5 wt.% higher for coke, compared to the yields obtained after catalytic cracking of pure VGO. UOP LLC [15] patented a process for the hydrotreatment of the pyrolysis oil lignin fraction and the subsequent hydrocracking of the organic phase product (oxygen content of 5.9 wt.%), obtaining a gasoline yield of 30 wt.% (from the original lignin fraction).
In this paper, the results of the batch wise hydrodeoxygenation of pyrolysis oil (experiments by University of Twente) and the subsequent co-processing in a lab scale FCC fluidised bed reactor (experiments by Shell Global Solutions) are presented and discussed.4 In the first part, the effect of different experimental HDO conditions on the product yield and properties are shown. In the second part, the co-processing of the HDO oils with Long Residue oil in a lab scale catalytic cracking reactor is evaluated. In this manner, a link between the HDO step (process conditions and oil product properties) and the catalytic cracking product yields can be established.
Section snippets
Materials
The pyrolysis oil used in the present work was produced in a 20 kg/h process development unit from VTT, Finland [17]. The feedstock used to produce it was forest residue. More details about this feedstock can be found elsewhere [18]. i-Propanol (2 wt.%) was added to the freshly prepared oil to increase homogeneity. A top phase (10.6 wt.%) including most of the extractives was separated. A specification of the remaining phase, as used in the HDO upgrading experiments, is given in Table 1.
The
Hydrodeoxygenation of pyrolysis oil
A series of experiments with maximum reaction temperatures between 230 and 340 °C was carried out. The total pressure was kept constant at around 290 bar. Because of the vapour pressure of the components present, especially water, and the gas production, the hydrogen partial pressure is expected to decrease with experimental temperature and in the course of an experiment.
As already indicated, after HDO, a product with either two or three phases (depending on the exact process conditions) was
Discussion
The decrease of oxygen content of pyrolysis oil can be achieved by HDO. This already known fact [5] was, until now, considered to be the goal of HDO and the remaining level of oxygen the parameter that determines the quality of the HDO oil with respect to further use like FCC. Low oxygen levels (<10 wt.%) were targeted in most of the available literature [13], [14], [32]. However, this work has shown that high remaining levels of oxygen can be allowed in upgraded HDO oil (up to 28 wt.%) without
Conclusions
Co-processing hydrodeoxygenated pyrolysis oils having a dry oxygen content up to 28 wt.% under standard lab scale FCC conditions gives gasoline and LCO range bio-hydrocarbons from a ligno-cellulosic feed source with similar product yields as that obtained from the base FCC feed.
After the HDO step, pyrolysis oil underwent phase separation into an aqueous phase and one or two oil phases. An increase in the process temperature led to an oil with lower oxygen content and to the transfer of organic
Acknowledgements
The authors would like to acknowledge the EU for partial funding of the work through the BIOCOUP project within the 6th Framework Program (contract number: 518312) and the CORAF project of Senter Novem (project number EOSLT04018). We also would like to thank the partners of the BIOCOUP project, especially VTT (Finland) for performing part of the analyses and R.H. Venderbosch (BTG) and H.J. Heeres (Rijks Universiteit Groningen) for the discussions on the phenomena during HDO upgrading. The MSc
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In memory of Michiel Groeneveld.
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