|Bio-oil-graded upgrading route based on molecular distillation and catalytic cracking. Credit: ACS, Wang et al. Click to enlarge.|
A team at Zhejiang University, China, has developed a novel cracking technology for the upgrading of bio-oil, produced by the fast pyrolysis of biomass, to biogasoline. In a paper published in the ACS journal Energy & Fuels, they report that the co-cracking of the distilled fraction (DF) from bio-oil molecular distillation and ethanol produced a well-defined gasoline phase, and that both increasing the reaction temperature and adopting pressurized cracking benefited the yield and quality of this gasoline phase.
Under optimum reaction temperature and pressure, co-cracking of the DF and ethanol, with different weight ratios, all generated high-quality gasoline phases. Under 400 °C and 2 MPa, co-cracking of DF and ethanol with a weight ratio of 2:3 produced a high gasoline phase yield of 25.9 wt %; the hydrocarbon content in this gasoline phase was 98.3%. CO2, CO, and C3H8 (propane) were the main gaseous products, and a low coke yield of 3.2 wt % was achieved.
Crude pyrolytic bio-oil cannot be directly used as a high-grade liquid fuel because of a number of characteristics, such as corrosivity, high viscosity, high oxygen and water contents, low heating value, and low pH value. Accordingly, developing effective upgrading processes is of great interest. (Earlier post.)
Different bio-oil upgrading techniques have been developed, including hydrodeoxygenation, catalytic cracking, esterification, emulsification, and steam reforming. Catalytic cracking is often adopted to reduce the oxygen content in bio-oil using zeolite catalysts. The HZSM-5 zeolite has been proven to be an effective catalyst for cracking, because it has sufficient Brønsted acid sites. These active sites can catalyze deoxygenation reactions, such as dehydration, decarbonylation, and decarboxylation, which can remove the oxygen in bio-oil in the form of CO, CO2, and H2O.
...After catalytic cracking, the upgraded bio-oil becomes richer in hydrocarbons. However, the composition of crude bio-oil is very complicated because it includes acids, ketones, alcohols, aldehydes, phenols, esters, sugars, etc. Different compounds in bio-oil have distinct cracking reactivity; therefore, it is difficult to crack bio-oil directly. A previous study showed that the direct cracking of crude bio-oil produced high coke deposition (up to 20 wt %), and the operation had to be terminated after 30 min. Alcohol, ketones, and acids were found to have good cracking performance, while phenols showed low reactivity. In addition, non-volatile and low-reactivity oligomers in bio-oil (such as sugars and pyrolytic lignin) can easily form coke during the cracking. This deactivates the catalysts and eventually leads to the blockage of the reactor. Therefore, proper bio-oil separation pretreatment to enrich those components suitable for cracking is important to ensure the stability of the cracking process.
...Molecular distillation is a special liquid−liquid separation technology based on the difference of mean free path for various substances. This technology can achieve high separation efficiency at a low temperature; therefore, it is very suitable for the separation of thermosensitive compounds in bio-oil...On the basis of this high-efficiency separation technology, an innovative bio-oil-graded upgrading route is proposed.—Wang et al.
In the new process, bio-oil is first separated into bio-oil into a distilled fraction and a residual fraction by molecular distillation. The DF has a high proportion of small-molecular-weight acids and ketones with good volatility and high reactivity—this fraction has better cracking behavior than crude bio-oil and can be co-cracked with ethanol for biogasoline production.
The residual fraction is rich in large molecular sugars and phenols, which can be more easily separated by solvents than crude bio-oil. The sugars can be fermented to produce ethanol, which could be used as the co-reactant in the cracking process.
The hydrocarbons in the gasoline phase were mainly C7−C9 aromatic hydrocarbons; therefore, this biogasoline is better to be blended with some other high-saturation hydrocarbon fuels (for example, the gasoline from Fischer−Tropsch synthesis) for direct application in engines. In addition, the biogasoline with a high content of aromatic hydrocarbons can also be used as a promising chemical.—Wang et al.
Shurong Wang, Qinjie Cai, Xiangyu Wang, Li Zhang, Yurong Wang, and Zhongyang Luo (2013) Biogasoline Production from the Co-cracking of the Distilled Fraction of Bio-oil and Ethanol. Energy & Fuels doi: 10.1021/ef4012615