Bio-oil can be deoxygenated by two theoretical routes: the oxygen can be thermally cracked from the bio-oil in the form of water and carbon oxides, or it can be removed as water by addition of hydrogen. Both routes have been tested using different catalysts. Catalytic cracking has a capital cost advantage because of a lower cost reactor which operates at low pressure and higher throughput. Catalytic hydroprocessing uses more expensive equipment due to the higher pressure and slower throughput and also has a hydrogen reagent cost; however, it typically has a higher yield overall. The current process development efforts focus for the most part on hydroprocessing. Some of the work has used model compounds and has been directed at more fundamental chemistry questions . Other studies have investigated traditional sulfided catalysts while some have investigated non-sulfided precious metal catalysts reactors . Studies have determined that much, but not all, of the hydrogen requirement can be derived within the process from reformed gas byproducts.
Conventional hydroprocessing of petroleum to finished fuels is performed almost exclusively with fixed catalyst bed operation. Similarly, most of the bio-oil processing has also been performed in fixed bed reactors. The scale of such operations has been limited to bench-scale reactors. Scale-up of the process is underway and should be on line in 2014 .
The operation of the bio-oil catalytic hydrotreating process has similar concerns as that found with petroleum hydroprocessing. The oil feedstock needs to be relatively free of unreactive solids and preferably will be low in metals content. Distribution of the oil over the whole bed is a concern in order to maintain the highest efficiency of contacting the catalyst. The flow rate over the catalyst bed needs to be carefully controlled in relation to the process kinetics. The reported rates of hydrodeoxygenation of bio-oil are relatively low, typically about 0.15 volume of bio-oil per volume of catalyst bed per hour, in order to approach complete deoxygenation. The exothermic nature of these hydrodeoxygenation reactions, which form water, is severe. Temperature increases as much as 60°C have been reported in small fixed beds and are expected to be a significant design factor in scale-up of the system.
The catalysts used have included both conventional promoted molybdenum sulfides (CoMo and NiMo) and also precious metal catalysts. When using sulfided catalysts an important concern is maintaining the sulfided nature of the catalyst when the bio-oil may actually be deficient in sulfur. Alternatively, when using precious metal catalysts, the amount of sulfur in bio-oils, even from clean wood at 50 ppm or less, may still be sufficient to deactivate the catalyst in the long term.
The primary issue interfering with long-term operation has been the fouling of the catalyst bed by carbonaceous deposits. These deposits appear to be condensation reaction products forming at elevated temperature from the most reactive components in the bio-oil. Low temperature hydroprocessing of the bio-oil prior to conventional hydroprocessing appears to ameliorate this problem. Refinement of the processing conditions to allow long term operation remains at the forefront of the process R&D.
Dependent on the processing severity, a range of deoxygenated products can be produced from bio-oil catalytic hydroprocessing. Nearly deoxygenated hydrocarbons (<0.5 wt% oxygen) have been of most interest. These product mixtures are different from petroleum crudes in that they are high in cyclic hydrocarbon structures, both saturated (naphthenes) and aromatic. These cyclic structures are clearly derived from the cyclic structures in the bio-oil itself. With less severe processing, higher space velocity or lower temperature, more oxygen is left in the product mix. The most stable oxygenates, and the last to be eliminated, have been reported to be the phenolics .
The use of 13C nuclear magnetic resonance spectrometry has been found useful for identifying the functional groups residing in the process bio-oils. Although this method does not specify the actual chemical compounds in the product, it is helpful to distinguish the relative amounts of functional types of carbon in the hydroprocessed bio-oil. Through quantitative methods, carbon can be categorized into saturated and unsaturated carbon, as well as different oxygen-containing functional types, like carbonyls, phenolics, ethers, carboxylic acids, etc.
As for specific chemical components, alkylated benzenes and alkylated cyclohexanes are most commonly found by gas chromatography-mass spectrometry (GC-MS).Bicyclics, both saturated and aromatic, are also found by GC-MS. Through the use of other types of liquid and gas chromatography, other oxygen containing components can be identified in the products.
Since oxygen plays a big role in the physical properties of the chemical components in the hydroprocessed bio-oil, physical properties such as density viscosity and colour can be correlated to the oxygen content in the oil. Density measurement is a simple way to determine the state of the catalytic processing. Effective hydrodeoxygenation to remove oxygen to levels of less than 10% will reduce the density of the product to less than 1 g/ml from 1.2 g/ml for the starting bio-oil; and the trend continues downward with density nearing 0.9 g/ml with 5% oxygen remaining, and closing toward 0.82 g/ml as the oxygen is further reduced to levels approaching zero .
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