Catalysis 2022 issue

char. 3 For biomass, formation of char during pyrolysis is inevitable and can represent a significant frac - tion of the overall pyrolysis yields. The composition of the biochar will be a mixture of inorganic materials originating from the plant material combined with highly carbonaceous residue with the appearance simi- lar to charcoal. The conditions used for pyrolysis will dictate the carbon content of the char, with higher pyrolysis temperature and longer time driving more carbon from char into liquids with the consequence of also increasing yield of gases – also illustrated in Figure 2 . While there are many different possible sources of biomass, all bio- mass will be comprised of lignin, cellulose, and hemicellulose that are rich in oxygen. Typical oxy- gen content for biomass wastes is 40-45%, which is roughly four times the oxygen content of plant based oils. Between the pyroly- sis process and co-processing the resulting liquids in an FCC unit, virtually all of the oxygen con- tained in biomass will undergo deoxygenation to form CO, CO 2 , and H 2 O. How much deoxygena- tion occurs during pyrolysis, which depends on the conditions used for pyrolysis, dictates how much oxy- gen will be contained in the RCO. During pyrolysis, biomass will ini- tially decompose to form smaller hydrocarbon fragments and oxy- genates. The oxygenates will then undergo deoxygenation, where higher pyrolysis temperature will generally result in deeper deoxy- genation. For example, in the lab- scale results illustrated in Figure 2 , the amount and diversity of oxy- genates is observed to decrease as the pyrolysis temperature is increased due to the higher reac- tion temperature driving deeper deoxygenation ( Figure 5 ). Ultimately, the liquids from the pyrolysis process will become the feedstock to be co-processed in the FCC unit. These RCOs will be a mixture of hydrocarbons and oxygenates. In the FCC riser, the deoxygenation process that started during pyrolysis will be completed as oxygen is converted to CO, CO 2 , and H 2 O through the same path-

1.0E+8

LDPE pyoil hydrocarbon distribution, T = 500C, t = 60 min

8.0E+7

C -C

6.0E+7

4.0E+7

2.0E+7

LDPE py-wax hydrocarbon distribution, T = 500C, t = 60 min

1.6E+7

1.2E+7

C -C

8.0E+6

4.0E+6

0.0E+0

Retention time (min)

Figure 4 Composition of liquid and wax products from lab-scale pyrolysis of LDPE

tional gasoil, it is possible that the lighter portions of the liquid and wax could be co-processed as well, assuming the impact of the lighter feed on product yields can be toler- ated. Both fractions can be readily mixed with gasoil without miscibil- ity issues. When co-feeding plastic pyrolysis oil in a lab-scale ACE unit, the impact of the lighter feedstock results in higher yields of LCO, gasoline, and LPG yields consistent with the percentage being co-pro- cessed. Otherwise, the co-process- ing of plastic pyrolysis oil does not negatively impact yields. Plant based oils Plant based oils such as corn oil and soybean oil are extracted directly from the plant. These oils consist of a mixture of triglycerides, which are large molecules comprised of glycerol and three long-chain fatty acids. Different oils will contain dif - ferent mixtures of triglycerides, but all plant oils will contain long-chain fatty acids and an overall carbon number in the order of 50. While containing a mixture of triglycer- ides, the distribution of different molecules in plant based oils will be significantly narrower than oils pro - duced by pyrolysis. As a feedstock, several processing options exist to upgrade vegetable oils into fuels and chemicals including co-process- ing in FCC. 2

The oxygen contained in the glyc- erol and fatty acid combined result in an overall oxygen content of roughly 10-15%. In the FCC riser, the majority of this oxygen will be converted to CO, CO 2 , and H 2 O through the deoxygenation path- ways discussed above. Depending on which pathway is followed, either carbon will be consumed, resulting in less carbon being avail- able for formation of desired prod- ucts, or hydrogen will be consumed, potentially increasing selectivity to coke. Which pathway is dominant is a function of the chemistry occur- ring on the surface of the catalyst. When co-processing plant oils in a lab-scale ACE unit, the yields of LPG, gasoline, or LCO will gener- ally decrease as the result of pro- ducing CO and CO 2 . Similar to plastic pyrolysis oils, plant oils also blend well with gasoil without sig- nificant miscibility issues. Biomass pyrolysis oil Similar to plastic wastes, biomass based wastes can also be pyrolysed to produce hydrocarbon based liquids as an alternative to con- ventional feedstocks. Examples of biomass waste include a vari- ety of different materials such as corn stover, wheat straw, or wood cuttings and residues. Similar to plastic, the products of pyrolysis will include gases, liquid/wax, and

52 Catalysis 2022