**9. CFP feed pretreatment**

Two main strategies have been published to combat the challenges of pyrolysis: torrefaction and acid-washing of lignocellulose. Torrefaction is low temperature, slow pyrolysis of less than 60 minutes, and installed upstream to CFP. During torrefaction, removal of undesirable acetic acid and guaiacol from the feed improves the downstream CFP. Typical temperature of torrefaction ranges from 250 to 300 °C [112]. When torrefaction was tested on pinewood conversion at 250 °C, the resulting CFP step produced 30% more aromatics. Other feedstocks such as corn cobs produced a lower O/C ratio in the CFP product [134].

Acid-washing of finely divided lignocellulose can extract AAEMs from the feed. The wash step is conducted under very mild conditions, but it is necessary that thorough drying is conducted prior to CFP [51]. For example, rice husks are washed at 30 °C by acetic acid [84, 135]. Use of acids such as acetic acid or mineral acids can add significant variable costs to the process. One investigator recommended using acetic acid recovered from the pyrolysis process for the wash. Extreme aqueous washing must be avoided as the extraction can result in undesirable changes in hemicellulose and cellulose. Formation of sticky materials can coat catalyst bed particles and risk defluidization [136]. Aqueous HCl washing of the spent catalyst to remove deposited minerals is also necessary to maintain catalyst lifetime [118].

A comparison of both processes was studied to determine their effects on aromatics yield. Acid-leaching promoted the formation of levoglucosan. Torrefaction promoted the formation of catechols and guaicols. It was determined that acid leaching + CFP made less aromatics than torrefaction + CFP [137]. Both processes were conducted in series in the CFP of rice husk [135]. Lower levels of volatiles were made in the pyrolysis result in lower yields of oxygenates and higher yields of aromatics. However, severe torrefaction conditions due to longer residence

### *Catalytic Fast Pyrolysis of Lignocellulosic Biomass to Benzene, Toluene, and Xylenes DOI: http://dx.doi.org/10.5772/intechopen.99946*

time can result in cross-linking of lignocellulose, resulting in more charring and reduced formation of aromatics. A combination of both processes did result in higher yields of products [136].

One team of investigators described development of ultrasonic pretreatment [138]. This mechanical method excludes use of toxic chemicals. Ultrasound promotes cleaving of α-O-4 and β-O-4 linkages and overall decreased crystallinity of lignocellulose and increased yield of pyrolysis oils. CFP can accommodate a wide



### **Table 2.**

*Feedstocks used in CFP.*

variety of feedstocks. **Table 2** summarizes the feedstocks examined, which include woody plants of varying levels of lignin, cellulose, and hemicellulose. The most frequently studied woody biomass for pyrolysis was pinewood. Also included in this table are references to studies of model feeds.

Glucose is one of the intermediate products made in pyrolysis [169]. This monosaccharide is rapidly converted to anhydrosugars before they are dehydrated to furanics and subsequently to aromatics. Sophisticated isotopically labeled glucose experiments permitted an elucidation of the mechanism of glucose conversion [170]. Single ring aromatics contained 13C labels and later these labels began to appear in naphthalene rings, evidence for a ring growth mechanism from monoaromatics to polyaromatics. Other studies included a study of the conversion of other model feeds to aromatics under identical conditions [80, 121, 140–142, 147, 160, 161, 171]. Glycerol CFP is of interest because of its availability from the production of fatty acid methyl esters for bio-diesel [175, 176].
