Catalysis 2022 issue

However, in addition to increas- ing gas yields, deoxygenation will influence yields of high-value prod - ucts, depending on which pathway is dominant. Producing more CO/ CO 2 will consume carbon that oth- erwise could be used to make fuels and chemicals. Producing more H 2 O will consume hydrogen, reducing the effective H/C, and potentially increase the tendency to form coke. New metal contaminants will also create challenges when processing RCOs. Plants are rich in alkali met- als that will be carried over into biomass based pyrolysis oils and plant based oils. Similar to metals contained in conventional oils, the alkali metals will accumulate on the surface of the FCC catalyst, requir- ing new metal passivation strategies to be incorporated into the design of the catalyst. The extent of alkali metal contaminants will also vary with the source of the RCO. On one end of the spectrum will be plas- tic-derived oils with little or no con - taminant metals. On the other end of the spectrum will be oils derived from biomass pyrolysis that will potentially have a significant con - tent of alkali metals such as Ca, K, Mg, and Na. Understanding the process of pyrolysis is important in under- standing the nature of different RCOs and how they will behave during co-processing in a refinery. Pyrolysis is a process involving the thermal decomposition of materials at elevated temperatures in an inert atmosphere. Pyrolysis results in the formation of a broad spectrum of hydrocarbon fragments that can be categorised by condensation tem- perature: waxes comprise products condensing at or above 25ᵒC, liq - uids comprise products condensing between -15ᵒC to 25ᵒC, and gases are products condensing below -15ᵒC. A fourth product category is char, which consists of a highly carbonaceous non-volatile residue forming in the pyrolysis reactor. The yields of gases, liquids, wax, and char will vary based on the design of the pyrolyser, pyrolysis conditions, and the material being Pyrolysis of renewable and recyclable materials

100%

80%

40%

60%

16%

79%

77%

40%

21%

51%

20%

LDPE 500C

HDPE 500C

HDPE 400C

Poplar wood 400C

Poplar wood 500C

Char

Wax

Liquid

Gas

Figure 2 Results from lab-scale pyrolysis illustrating the influence source and pyrolysis conditions will have on yields from pyrolysis

ysis, the large hydrocarbon poly- mers are thermally cracked into a very broad distribution of smaller hydrocarbon fragments. Using the lab-scale pyrolysis oils shown in Figure 2 as an example, the pyrol- ysis of low density polypropylene (LDPE) yields a range of hydro- carbons from methane to C 34 size hydrocarbons. In this experiment, the gas fraction contained hydro- carbons from methane to pentane ( Figure 3 ). The liquid and wax frac- tions were comprised of hydrocar- bons ranging from light naphtha to C 34 size hydrocarbons, with the wax generally containing a larger portion of the heavier hydrocarbons ( Figure 4 ). Certainly, the heavier portions of the liquid and wax fractions can be easily co-processed with con- ventional gasoil in FCC. While significantly lighter than conven -

pyrolysed. As one example of this variation, Figure 2 shows pyroly- sis yields from a lab-scale pyroly- sis unit for varying materials and pyrolysis temperatures. In general, the amount of upgradable products from plastics will be significantly more than that obtained from bio- mass. 1 In the lab-scale results illus- trated in Figure 2 , plastic pyrolysis yielded 80-90% upgradable prod- ucts (liquids and wax) compared to biomass pyrolysis yielding only 15-20% upgradable product (liquid). Plastic pyrolysis oils While any plastic can be pyro- lysed to produce an oil, polypro- pylene and polyethylene are more commonly used as feedstock for commercial production of plastic- derived pyrolysis oil, as they pro- duce oil well suited for upgrading. When these plastics undergo pyrol-

LDPE 500 C

CH C H C H C H C H C H C H C H C H

Figure 3 Composition of gas products from lab-scale pyrolysis of LDPE

Catalysis 2022 51