This post was written by Nathan Gibson. His article was “Formation of Aromatic Structures during the Pyrolysis of Bio-oil”.
During the age of fossil fuels, finding means for using anything to make efficient energy has become a priority. Biomass has become a key material that scientists have been working with to create energy. Biomass has come to be known as organic material used as fuel. Biomass can be converted into bio-oil through a process known as Pyrolysis. Through this process, bio-oil becomes a “complex mixture of chemical compounds that can be used in many ways.” The bio-oil can be upgraded into a liquid or used as a feedstock for boilers or gasifiers. When used in this manner, the bio-oil is burned with coal.
Many problems can arise with not knowing the chemicals released from the burning of bio-oil. The research by Yi Wang, Xiang Li, Daniel Mourant, Richard Gunawan, Shu Zhang, and Chun-Zhu Li shows that the burning of bio-oil yields the formation of various aromatic structures. These structures can lead to the development of tar. The studies done by these scientists help to show the appropriate temperature at which burning bio-oil will yield less tar and be efficient in creating energy.
The paper written by Yi Wang, Xiang Li, Daniel Mourant, Richard Gunawan, Shu Zhang, and Chun-Zhu Li was published in the September 14, 2011 edition of the Energy and Fuels Journal for the American Chemical Society. It is titled: Formation of Aromatic Structures during the Pyrolysis of Bio-oil. The scientists used gas chromatography-mass spectrometry and ultraviolent fluorescence spectroscopy in order to trace the development of aromatic ring structures during pyrolysis. With the results, they were able to determine the efficient temperature for pyrolysis of bio-oil as well as the source that yielded the highest aromatic structures concentrations.
Here is a summary of their work.
Introduction:
Bio-oil when further heated is drastically reactive and undergoes many changes. The formation of large complex aromatic ring systems develops. Understanding the formation of these structures becomes important. With the formation of aromatic structures problems arise such as the creation of tar and coke. Tar becomes a prominent problem when bio-oil is used within gasifiers and boilers. Tar gums up the equipment and causes buildup. Coke is a substance created by the burning of coal and other materials to form a solid material that is not soluble or able to be broken down.
Bio-oil can be broken down into different cellular components when woody biomass is used: lignin, cellulose, and hemicellulose. By testing each component for the production of aromatic structures, bio-oil can further be understood as a whole.
This study sought out to investigate the creation of aromatic structures through the pyrolysis of bio-oil between the temperatures of 350 and 850 ◦C.
Methods:
The bio-oil used within this study was made from the pyrolysis of mallee eucalypt wood at fast heating rates in a fluidized-bed reactor at 500 ◦C. To test the cellular components within the bio-oil, the bio-oil was separated into water-soluble and water insoluble fractions. The water-insoluble was further separated by producing Methylene Chloride soluble and insoluble fractions. The lignin-derived oligomers within the bio-oil are primarily the water-insoluble/Methylene Chloride-soluble fraction. The cellulose powder was a component with the water-soluble components. The reasoning behind separating bio-oil was to determine which cellular component created the highest concentration of aromatic structures during pyrolysis.
To determine the presence of aromatic structures during the increase in temperature of pyrolysis, the process was carried out in a novel two-stage fluidized-bed/fixed-bed quartz reactor. Sand was placed in the bottom stage and fluidized with argon. The bio-oil was fed into the reactor through an injection probe at a constant rate. The stages on the reactor were able to be the same temperature of 500 ◦C, or with the bottom being 500 ◦C and top ranging to 850 ◦C. Three tar traps were placed at the end of the reactor to catch any tar that was produced via aromatic structure formations. Tar was also present within the sand of the bottom stage. The tar was totaled and studied further based on formation per temperature. An image of the reactor can be viewed on figure 1 of the article.
The tar of each sample was tested using UV fluorescence spectroscopy to show the relative size and concentration of aromatic rings. To determine larger aromatic structures, the spectroscopy was used, however for smaller aromatic structures, gas chromatography was used. The gas chromatography determines the mass of compounds that flow through a capillary column. A full description of the process of UV florescence spectroscopy and gas chromatography can be seen within the text.
These methods were used to determine the size and concentrations of aromatic structures created when pyrolysis of bio-oil was done. Based on the different temperatures of the second stage of the reactor resulted in higher levels of aromatic structures resulting in production of tar.
Results:
To begin, the authors created a figure (figure 2) showing a UV florescence spectroscopy of bio-oil and the soluble/insoluble components. The figure illustrated that the lignin-derived materials contained larger aromatic structures than cellulose after pyrolysis. Figure 3 demonstrates raw bio-oil as well as the water-insoluble/methylene chloride soluble fraction that went through the process of gas chromatography. The graph shows that in raw oil, there are multiple aromatic structures present but at low concentrations. Similar aromatic structures were present in the water-insoluble/methylene chloride soluble fraction, but these compounds were at much greater concentrations.
Next, Cellulose was experimented on and explained within figures. Figure 4, represented the amount of tar yield from the pyrolysis of cellulose between the temperature ranges of 500-850 ◦C. At 500 ◦C, the greatest amount of tar was created. As the temperature increased, the tar yield decreased to nearly nothing. Figure 5 shows the tar from cellulose pyrolysis undergo a UV florescence spectroscopy. The results of the graph show the creation of aromatic structures being greatest at the higher temperatures of 800 and 850 ◦C. Figure 6, represents the tar from cellulose pyrolysis at 700, 800, and 850 ◦C using gas chromatography. The results show that at higher temperatures, more complex aromatic structures were made.
Next, lignin-derived oligomers in the bio-oil were tested to show the levels and complexity of aromatic structures when pyrolysis occurs. Figure 7 represents the tar yield from the pyrolysis of lignin. The total tar yield and trapped tar yield were recorded. At 350 ◦C, the trapped tar was low, but the overall tar yield was at the highest concentration. It was not until 500 ◦C that the total tar yield began to decrease. In figure 8, the tar from the lignin-derived oligomers from temperatures of 350-800 ◦C were put into a UV fluorescence spectroscopy and tested for the presence of aromatic structures. Within the lignin tar, larger aromatic structures were created as the temperatures increased.
Lastly, bio-oil as a whole underwent pyrolysis to determine to total tar yield and presence of aromatic structures. Figure 9, represents the total tar yield from pyrolysis of bio-oil from temperatures 350-850 ◦C. Again, the tar concentration was higher at a low temperature and shows a decrease at higher temperatures. When the tar from bio-oil was tested for aromatic structures using UV florescence spectroscopy, the levels showed that new aromatic structures were created through the pyrolysis of bio-oil. These results can be seen within figure 10.
Discussion:
Cellulose and lignin are components within bio-oil. By testing these two components separately, scientists can understand where the larger and more complex aromatic structures are created. By separating the components and testing them separately, at constant energies, the controls for the experiment were made. By understanding the complexity of aromatic structures that are made during cellulose and lignin pyrolysis, bio-oil as a whole can be assessed. Both cellulose and lignin created little tar as the temperature of pyrolysis became greater than 500 ◦C. When pyrolysis of lignin was done, at higher temperatures aromatic structures resulted in the most complex and large aromatic structures. This can be seen within figure 8. However, at higher temperatures with more complex aromatic structures, few of the compounds could actually be made. With lower tar production at higher temperatures, there is little available material for the aromatic structures to be made in.
Based on the results of testing bio-oil for aromatic structures, some conclusions could be made. No matter how high the temperature for pyrolysis of bio-oil is, aromatic structures are going to be created. But, at higher temperatures, available energy is the greatest with little yield to aromatic structures. With the studies done on cellulose and lignin individually, one can understand this concept. The same concept of pyrolysis at higher temperatures resulted in less tar and aromatic structures and more available energy, when the scientists tested bio-oil.
Using the methods of UV fluorescence spectroscopy and gas mass chromatography was an effective way to determine the complexity and concentrations of aromatic compounds. The UV fluorescence spectroscopy was far more effective in showing the larger aromatic compounds that were present in lignin, while the gas mass chromatography was effective in displaying the smaller compounds found within cellulose pyrolysis.
Bio-oil as a fuel is an effective means to produce energy. Understanding the effects of burning bio-oil has led to the use of this fuel in a wide-scale way. Using this fuel within boilers and gasifiers allow for industrial companies to replace pricy heating oils. Understanding bio-oil as a fuel source has opened the door for many ways in which to access energy.
Thoughts:
I found this article to be interesting. When using fuel to operate any type of machinery, I never thought that someone had to determine what the byproducts of burning the fuel were, nor what the best temperature to burn the fuel efficiently was. By learning how scientists must confront the effects of burning fuels, I have come to question what could be done to other fuel sources to gain a better use of fossil fuels. New developments with recycled materials for fuels are a much more effective way to create energy than discarding the organic material to rot. The understanding of bio-oil is just a stepping stone into the discovery of other means of energy around us.
