lignin conversion


Lignin is produced in large quantities, approximately 250 billion pounds per year in the U.S., as by-products of the paper and pulp industry. Lignins are complex amorphous phenolic polymers, not sugar based, and hence cannot be fermented into ethanol. Lignin is a random polymer made up of phenyl propane units, where the phenol unit may be either a guaiacyl or syringyl unit (Figure 11.13). These units are bonded together in many ways, the most common of which are a- or ゚-ether linkages. A variety of C砲 linkages are also present, but are less common. The distribution of linkage in lignin is random because lignin formation is a free radical reaction that is not under enzymatic control. Lignin is highly resistant to chemical, enzymatic, or microbial hydrolysis, owing to extensive cross-linking. Therefore, lignin is frequently removed simply to gain access to cellulose.


Lignin monomer units are similar to gasoline, which has a high octane number; thus, breaking the lignin molecules into monomers and removing the oxygen makes them useful as liquid fuels. The process for lignin conversion consists of mild hydrotreating to produce a mixture of phenolic and hydrocarbon materials, followed by reaction with methanol to produce methyl aryl ether. The first step usually consists of two parts:


(1) hydrodeoxygenation (removal of oxygen and oxygen-containing groups from the phenol ring)


(2) dealkylation (removal of ethyl or large side chains from the rings).


One must be careful in carrying out these reactions to remove the unwanted chains without carrying the reaction too far, which would lead to excessive consumption of hydrogen and produce saturated hydrocarbons, which are not as good octane enhancers as the aromatic compounds. Catalysts that carry out these reactions have dual functions. Metals such as molybdenum and molybdenum/nickel catalyze the deoxygenation, whereas the acidic alumina support promotes the carbon膨arbon bond cleavage.


Although lignin chemicals have many applications such as in drilling muds, as binders for animal feed, and as the base for artificial vanilla, they have not been previously used as surfactants for oil recovery. According to Naae,30 lignin chemicals can be used in two ways in chemical floods for enhanced oil recovery. In one method, lignosulfonates are blended with tallow amines and conventional petroleum sulfonates to form a unique mixture that costs about 40% less to use than chemicals made solely from petroleum or petroleum-based products. In the second method, lignin is reacted with hydrogen or carbon monoxide to form a new class of chemicals called lignin phenols. These phenols, because they are soluble in organic solvents, but not in water, are good candidates for further conversion to chemicals useful in enhanced oil recovery.


None of the current facilities is yet operating on anything like the scale that will be needed to offset a big portion of the nation痴 oil imports. That era, if it ever comes, is still years away. In fact, no company in the United States even claims to be making commercial sales yet at any significant scale. (Iogen, a Canadian firm backed by Shell, makes ethanol from wheat straw and supplied a Shell station in Ottawa for a month last summer.)


Some companies use steam, chemicals or enzymes to break down the cellulose so that sugars locked inside can be freed and fed to yeasts to make alcohol. Others break the cellulose into simpler components, like hydrogen and carbon monoxide. These can be re-formed into hydrocarbon molecules that can be used in place of gasoline or diesel.


Coskata uses a plasma torch, which shoots 8,000-degree jets of air at twice the speed of sound, to blast wood chips into hydrogen and carbon monoxide. Those gases are pumped into a tank of bacteria that feed on them and excrete ethanol. For each ton of pine chips, the pilot plant produces about 100 gallons of ethanol. Many people in the industry say they believe the economics should work at that yield.


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