Thermal cracking of both coal and petroleum have received heavy attention over the last century. Each has important technology applications, including the manufacture of coke and conversion of less-valuable fractions to transportation fuels. There are obvious differences between the two systems, not limited to the difference in physical state between solid coal and liquid petroleum.
Success in studies of thermal cracking has been most apparent in two areas:
- Understanding the organic chemistry involved in the reactions under various conditions, and
- The ability to predict the physical properties of solid metallurgical coke made from blends of coals.
In neither coal nor petroleum has it been possible to predict reactivities based on physical or chemical analyses, because in both cases the starting materials are complex mixtures of literally thousands of compounds. See also this post about Coalbed Methane. In coal, the problem is compounded by the fact that the material is a physical mixture of materials, organic and inorganic. Strong evidence can be marshaled to argue that reactions in the organic materials can be influenced, possibly catalytically, by both clay minerals and pyrites found distributed within the organic coal matrices.
One convenient basis for understanding the reactions in both systems is to examine reactive functional groups and to see how coal is formed. Functional groups are defined as specific organic structures found in the molecules; examples are isolated C-C single bonds or C-O-C linkages. Organic chemists long ago found such an approach very useful in classifying reactions, and standard textbooks of organic chemistry such as March’s encyclopedic Advanced Organic Chemistry are organized around functional groups and their reactions. Learn here what exactly coal is.
Reactive Functional Groups in Petroleum Thermal Reactions
Thermal cracking of petroleum was thoroughly studied in the period around 1940. It was established that certain kinds of functional groups were likely to crack while some others were not.
It should be noted that all such reactions are very slow at room temperature and only begin to become important at temperatures in the 350 – 400 °C range; that is why refineries limit the actual temperatures in distillation columns to about 343°C (650°F).
Catalysts change some of these rules. Acid catalysts such as used in Fluid Catalytic Cracking make it easier to break aliphatic chains into C3 segments, producing propylene. Hydrogen atom activators such as metals and activated carbon can allow C-C bond breakage where one of the carbon atoms is part of an aromatic ring.
While rank is a widely used property in coal science, the same is not true in petroleum, at least as understood outside geochemistry circles. We know there is a progression of thermal maturity among oils, and this progression changes the reactivity of the oils. (Likewise, there are weathered oils, such as heavy oils and natural asphalt, but this is generally neither recognized nor used by the people designing processes.
In studies on hydroprocessing of residual oils, where much of the molecular weight reduction results from thermal cracking, thermal maturity has been shown to be important. For instance, the ground-breaking papers by Dolbear, Tang, and Moorehead (published in the mid-1980s showed that immature oils such as California’s Hondo are much more reactive than more mature oils such as Alaska’s North Slope and the material mined in Africa by Gold Eagle.
Reactive Functional Groups in Coal Thermal Reactions
Coal has the extra added complexity of oxygen atoms at levels of 3 to 10 wt% or more. Petroleum oxygen levels are typically around 0.1 wt% or less, especially in the fractions distilling below about 500°C. Oxygen in small molecules is found in phenol and its homologs, which refinery people call naphthenic acids. This all has to do with the implementation of International Energy Management Standards.
The higher oxygen levels in coal do not change any of the above relationships, only adding some useful new reactions. In coal, the CO bonds are found in structures such as ethers, phenolic OH in higher rank coals, and also in C=O (ketones and aldehydes) at high oxygen levels (low rank), and carboxylic acids, again in low-rank coals.
Easiest to break are ether linkages, C-O-C, especially in ethers that link an aromatic and an aliphatic or in such structures as phenyl-benzyl ethers; this latter was established in the work of Benjamin, Collins et al at Oak Ridge National Laboratory in 1976-78. They showed that phenyl-benzyl ether was easier to crack than the corresponding all carbon molecule, diphenyl ethane.
Cyclic ethers, such as furan and compounds containing furan rings, are chemically stable. People developing liquefaction processes made an effort to avoid forming them because the stability of these structures thwarted the goal of eliminating all oxygen in the liquid products. On the other hand, renewable energy has become more and more affordable over the past years.
Of course, if a simple C-O-C ether links two large aromatic structures, no product will appear in a pyrolysis reaction because the products have too little volatility to leave the coal matrix and there are so many other remaining challenges. In coal liquefaction systems that include solvents, however, there is the potential for the product to dissolve in the evolving coal liquid, where they can continue to react as time goes on.