Catalytic cracking of other oxygenated feedstocks (lignin, glycerol and sugars)

Lignin, which consists of polyaromatic oxygenated compounds, represents a major fraction of biomass (10-30%) and is currently used as a low-grade fuel to provide heat in the pulp and paper industry, but it would be highly desirable to produce value-added products from lignin. Lignin can be converted into a transportation fuel by dehydroxygenation or zeolite upgrading. These are the same methods used to upgrade bio-oils, which contain a large fraction of lignin-derived products. Thring et al. (2000) studied zeolite upgrading of lignin with HZSM-5 zeolite as a catalyst in a fixed bed reactor operating at an atmospheric pressure, over a temperature range of 500-650°C and weight hourly space velocities of 2.5-7.5/ hour. The liquid product fraction, which consisted of mostly aromatic hydrocarbons (mainly benzene, toluene and xylene — with toluene dominating), was maximized at a temperature of 500°C and a space velocity of 5/hour. On the other hand, the gas product consisted of olefins, light hydrocarbon gases, CO and CO2 and was produced at the highest yield at a temperature of 650°C and a space velocity of 5/hour. Among the light hydrocarbon gases produced from the lignin, ethylene and propylene were the olefins produced in the highest quantities. Coke and char formation was particularly high at the low reaction temperatures employed in this work but decreased rather drastically with increasing temperature. For instance, at a space velocity of 5/hour, 50 wt.% of the lignin was converted to coke and char when a reaction temperature of 500°C was used compared to only 21 wt.% at 650°C. Small FCC pilot tests were run to determine the crackability of pyrolysis oil and pyrolytic lignin blended with VGO (Holmgren et al., 2007). In the blends, the VGO serves as a hydrogen donor. Compared to VGO, the pyrolysis oil and pyrolytic lignin tend to form high levels of coke. For the blends of VGO with pyrolysis oil or pyrolytic lignin, the acid bio-oils appeared to increase the crackability of the VGO and shift VGO yields towards increased light ends and lower LCO and clarified slurry oil (CSO), which is an economically attractive outcome. Nevertheless, the high levels of coke obtained with both blends (7% and 9%, respectively) would be unacceptable for most FCC units.

Glycerol is produced from biomass through fermentation of sugars and mainly by transesterification of vegetable oils during biodiesel production. The glycerol

market is currently undergoing radical changes, driven by very large supplies of glycerol arising from biodiesel production. Glycerol is currently too expensive to be used as a fuel; however, as biodiesel production increases, the price of glycerol will decrease. Corma et al. (2007) studied the catalytic cracking of aqueous glycerol and its mixture with VGO in a microactivity test (MAT) reactor at 500- 700°C with six different catalysts. Products from this reaction include olefins (ethylene, propylene and butanes), aromatics, light paraffins (methane, ethane, propane), CO, CO2, H2 and coke. The ZSM-5 catalyst had the highest level of olefins and aromatics and the lowest level of coke (< 20%) in the catalytic cracking of glycerol, whereas the other catalysts had high coke yields (30-50%). When glycerol is fed together with VGO, interactions between the hydrocarbon components and the glycerol reaction intermediates occur, resulting in final selectivities better than those calculated by considering a simple additive effect. These experiments showed that mixtures of VGO with biomass-derived feedstocks can help to transfer hydrogen from the VGO to the biomass molecules. One option for further improving the olefin and aromatic yields for co-feeding of glycerol and petroleum-derived feedstocks into an FCC reactor might involve adding ZSM-5 to the FCC catalyst because ZSM-5 produced more olefins and less coke than FCC catalyst.

Sugars can be used as feedstock for fuels production by different processes. Chen (1976) discussed the conversion of carbohydrate materials to petroleum — type hydrocarbons. The process is composed of microbial conversion of agricultural carbohydrate materials to alcohols followed by direct conversion of the oxygenated microbial reaction product to a hydrocarbon product comprising a substantial highly aromatic fraction. This latter conversion was carried out in the presence of a ZSM-5 zeolite at about 260-540°C. Later, Chen and co-workers (Chen and Koening, 1990; Chen et al., 1986) passed concentrated sugars, including glucose, xylose, starch and sucrose, over ZSM-5 at a temperature from 300°C to 650°C and observed hydrocarbon, CO, CO2, coke and water as products. The addition of methanol to the feed decreased the amount of coke and increased the hydrocarbon products. The hydrocarbon products consisted of gaseous alkanes (methane, ethane, propane), liquid alkenes and alkanes (butane, pentene, hexane) and aromatics (benzene, toluene, C8-C10 aromatics). One of the problems of this reaction is that when methanol is not used, 40-65% of the carbon is converted into coke.