Biofuels (solid, liquid or gaseous fuels produced from biomass) have emerged as one of the most promising source of energy for the foreseeable future, being the most sustainable way to limit greenhouse gas emissions.
Besides the advantages, the production and use of biofuels have several problems to be solved. The production costs and the energy consumption are too high to obtain a final product that can act as a substitute to fossil fuels, because of the low energy content of the biomass and the low efficiency of the biofuel production processes.
The goal of this research project is to produce a high grade bio-oil from hydrothermal liquefaction (HTL) of oak wood using metal catalysts. We expect to obtain high yield of high quality bio oil, with low oxygen content. Liquid biofuels produced by termochemical processes usually present high oxygen content which implies low calorific value, low chemical stability and high viscosity. It is therefore of fundamental importance to overcome this issue; one of the most pursued way is to perform an up-grading of the produced bio-oil. The up-grading processes are very expensive in terms of energy consumption, high hydrogen pressure are required. Hydrothermal liquefaction is a termochemical process operating at medium temperature (280-400°C) and high pressure (100-200 bar) in which biomass decomposes in presence of water into char, bio-oil, gas, and water soluble oirganic compounds.
In our process we expect to perform a partial up-grading of the bio-oil in the production step using suitable catalysts. The development of heterogerogeneous metal catalysts will be one of the scope of this proposal. We will test metal, such as Fe, Co and Ni, which in HTL condition should be oxidized and produce hydrogen in the reacting mass and which are recognized to be hydrogenation catalysts. Furthermore, these metals present magnetical properties leading to an easy recovery and thus to a subsequent re-use.
The use of heterogeneous catalysts is still unexplored in biomass hydrothermal liquefaction processes. In fact, homogeneous catalysts such as the one commonly used have great effect on bio-oil yield. The increase of bio-oil using homogeneous catalysts can be considerable, in some case the increase can reach 100 % with respect to the tests made without catalyst. Despite the advantages of the use of these catalysts, the main issue is related with the bio-oil composition, these catalysts do not produce notable effect on the bio-oil quality. The oxygen amount is still high (15-25 %) and so also the oil viscosity. The use of soluble salts containing metals that can be used to increase also the oil quality, such as salt of Ni and Co, is not feasible since at the end of the process huge amount of water containing these toxic metals should be treated before the discharge of the water in the environment.
The use of heterogeneous catalysts can overcome most of the limitations related to the use of the homogeneous ones. The composition of heterogeneous catalysts can be tuned in order to give them a double function: the maximization of the oil amount produced from HTL and the contemporary up-grading. Furthermore, the heterogeneous catalysts can be revìcovered easily. In the proposed project we intend to develop catalysts having magnetical properties making thus their recover even more easy. We intend to apply a magnetic field to separate the metals from the solid biomass residue. We intend also to reuse them after a regeneration if necessary.
The development of this kind of catalysts can open new prospectives for the continuous HTL plants that for now are using only homogeneous catalysts, such as alkali compounds, in order to avoid the problem of their recovery. All these plants thus, after the production unit, have an up-grading section which is high energy demanding, since it works with high hydrogen pressures (100-120 bars). The up-grading section has, thus, a great impact on the overall costs of the plants. Using the catalysts we propose could reduce significantly the costs related to the up-grading section of the plant since lower hydrogen pressure are required due to the lower oxygen content of the produced bio-oil. The plant should include a section for the recovery of the catalysts.