Behavior of such oils in hydroprocessing reactions
Compared to classical fossil derived energy carriers, pyrolysis oils have a low hydrogen content and an excess of carbon and oxygen. Upgrading techniques should enable the removal of carbon as carbon dioxide (decarboxylation) or/and the removal of oxygen in the form of water (hydroprocessing). An is catalytic cracking, similar to FCC like processing with known catalysts, but this is known to yield considerable amounts of coke. The forced, non catalytic stabilization of the oil by treating it at elevated temperatures and pressures and in the presence of the water yields less char but product properties appear rather disadvantageous for further processing. The best option so far is by treating the oil with hydrogen over active catalysts to reduce the various functionalities present in the pyrolysis liquids. At temperatures from 150 up to 400oC in the presence of hydrogen and a catalyst, pyrolysis liquids are indeed hydro(deoxy)genated. At the lower temperatures (< 200oC) mainly hydrogenation of the carbohydrates occurs, whereas in the mid temperature range (200 – 350oC) dehydration and hydro-deoxgenation are the main reactions. At the high temperature regime mainly hydrocracking reactions predominate.
The mild hydrogenation below 200oC results in the formation of two liquid phases, an organic and a water rich phase. Here the hydrogen consumption is limited to < 100 Nm3 H2/tPL. The organic phase higher in carbon and hydrogen content compared to untreated pyrolysis liquid; the H/C ratio is significantly lower as some carbon (and oxygen) is transferred to the aqueous phase while the O/C is only marginally reduced. A typical Van Krevelen plot for the various products at different processing temperatures is presented in the Figure below.
Undesired repolymerization reactions can be suppressed by selection of the right catalyst, by reducing the effective reactant’s concentrations, and by selection of temperature and residence time. Analysis of the organic phase indicate that in this temperature regime hydrogenation of the aldehyde/ketone units of sugars to the corresponding alcohols indeed reduces the number of reactive oxygen functionalities.
Only at the higher temperatures, above 250oC, the oxygen can be removed to a larger extent. Depending on the temperature and weight hourly space time a two-phase oil is obtained with an organic product either heavier or lighter than water. In this region high hydrogen consumptions can be noted, up to 500 to 600 NL/kgPL.
In the mild and more severe treatment step high pressures (> 150 bar) are required, to keep the water in a liquid state (and to reduce charring reactions), but moreover to enhance the hydrogenation reactions by securing high solubility of the hydrogen in water. Active catalysts are required to accelerate/enable the reactions, otherwise thermal reactions will cause polymerization at the expense of the preferred reactions thus include dehydration, hydrogenation (hydrogenolysis), hydrodeoxygenation, and hydrocracking.
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 carbonization processes, charcoal). To obtain a liquid product with a higher H/C ratio, additional hydrogen is thus required. This path is shown in Figure below and includes the mild hydroprocessing step, at around 175 oC (no phase separation) and 225 oC (phase separation), followed by further hydrodeoxygenation (and hydrocracking).