The catalytic hydrotreatment reactions were carried out at three process severity levels, a mild hydrogenation at either 175 or 225 oC, a mild hydrodeoxygenation (HDO) at 225 — 275 oC and a deep hydrodeoxygenation. For the latter, samples from the mild HDO were first allowed to phase separate completely, after which the organic fraction (containing about 3 wt.% water) was treated at temperatures ranging from 350 oC in the first two reactor segments, to 400 oC in the last two.

1.1.1.2 Visual appearances of liquid phase after reaction

The catalytic hydrotreatment reaction at 175 oC resulted in a single phase oil with a visual appearance close to that of the original feed. Thus, at this temperature, phase separation does not occur. This may be related to the limited production of water at this temperature. The product has a considerable sweeter smell/odor than the original pyrolysis oil. The mild hydrogenation at 225 oC gives two liquid phases, an organic and a water rich phase. The water phase has a higher density than the aqueous phase. A similar situation was observed for experiments at higher process severities (mild HDO), see Figure 5 for details. The second stage HDO product oil has even a lower density than the aqueous phase.

The organic product yields for the various process severities are given in Figure 6. Here, the severity is expressed in terms of hydrogen consumption, and high severity is associated with high hydrogen consumption. The yield is a clear function of the temperature. A drop in the yield to about 40% is observed at about 200 oC due to the occurrence of phase separation and transfer of part of the carbon and oxygen to the aqueous phase. A further slight reduction in yield is observed at higher severities, presumably due to gasification reactions and further net transfer of components from the organic to aqueous phase.

Oxygen contents of the product oils are a function of the process severity, see Figure 6 for details. Phase separation between 175 and 225 oC results in a dramatic drop in the oxygen content. This is due to the loss of water and the transfer of very polar highly oxygenated components to the aqueous phase. At the highest severity, the oxygen content is about 15%, compared to about 40% for the original pyrolysis oil.

The hydrogen consumption ranges between 65 and 250 Nm3/1 pyrolysis oil. Higher process severities lead to higher hydrogen uptakes (Figure 6).

A useful representation to assess the changes in the elemental composition of the product oils at various process severities is a van Krevelen diagram. Here, the ratio between O/C and H/ C of the products are plotted together in a single diagram. In Figure 7, a typical plot is provided for selected literature data on pyrolysis oil hydroprocessing (Elliott, 2007; Venderbosch et al., 2010) and our results with Ru/ C at different severities. Presented here are data points from e. g.:

— wood and pyrolysis oil, and for the four cases referred to in this paper (HPTT, hydroprocessing at 175 and 225 oC, Mild HDO and 2nd stage HDO);

— A selection of data points derived from literature studies (Baldauf et al. 2007; Churin et al., 1988; Conti, 1997; Diebold, 2002; Kaiser 1997; Samolada et al., 1998). Some of these data are derived from various oils from a variety of resources and processed in different reactors, different catalysts and at different conditions.

The plot also contains curves to represent the changes taking place in elemental composition during hydroprocessing, a theoretical curve for the dehydration of pyrolysis oil, and trend lines for the thermal (HPTT) route and hydroprocessing routes based upon the experimental data points.

Based on our work on the Ru/C catalysts and supported by the literature points in Figure 7, several reaction pathways can be distinguished:

a. Essentially repolymerisation of the pyrolysis oil (no catalyst, no hydrogen, ‘HPTT’);

b. Merely hydrogenation of the pyrolysis oil at mild conditions (up to 250oC, with catalyst and hydrogen, referred to as mild hydrogenation),

c. Dehydration of the oil at temperatures near 250-275 oC, and

d. Hydroprocessing of pyrolysis oil at temperatures up to 400 oC

Upon thermal treatment, the principal reactions are rejection of oxygen as water. Some CO2 and CO is released as well, which shifts the trend line to slightly higher H/ C ratios (but decarboxylation / decarbonylation is limited to approx. 10 wt. % of the feed). A high conversion (i. e. at high temperatures and residence times) eventually leads to a hydrogen — depleted solid material (and probably similar to conventional carbonisation processes, charcoal).

О

To obtain a liquid product with a higher H/C ratio, additional hydrogen is thus required. This path is shown in Figure 7 and includes the mild hydroprocessing step, at around 175 oC (no phase separation) and 225 oC (phase separation), followed by further hydrodeoxygenation (and hydrocracking).