Hydrothermal liquefaction of biomass delivers high valuable bio crude which can be used for burner firing. Upgrading of the HTL crude is required to reach con- ventional diesel or jet-fuel standards. Most research done on bio-oils upgrading focuses on pyrolysis bio-oils, upgrading of bio-oils from hydrothermal liquefaction (HTL) has not yet been extensively studied.
The liquefaction process produces a crude oil replacement, which has an important key difference from conventional crude oil: the oxygen and water content in the crude are significantly higher, typically 9–25 and 6–25 wt. %, respectively in HTL bio-oils versus\1 % in conventional petroleum [92]. In Table9.1unwanted characteristics of bio-oil and their effects are listed.
In regard to other thermochemical biomass conversion processes HTL delivers raw oil, with a comparable low oxygen and water content. In Table9.2property ranges of bio-crude obtained by HTL and in comparison to pyrolysis of wood are shown.
Compared to pyrolysis oil though HTL oils have almost half of the oxygen content, which makes upgrading of HTL much easier and less expensive.
When considering upgrading of HTL bio-crude, it is advantageous to look into standard conventional refinery procedures, to be able to use existing refinery infrastructure and knowledge.
Hydroprocessing is one way of upgrading crude oils. As it can be seen in the Van-Krevelen diagram in Fig.9.16, the oxygen content in the oil has to be decreased and the hydrogen content increased (red arrow). Hydroprocessing is a generic term that describes the use of hydrogen and an appropriate catalyst to remove undesired components form refinery streams. In refinery, hydroprocessing is used to remove reactive compounds like olefins, sulfur, and nitrogen com- pounds. At higher temperatures and pressures, aromatic rings can be saturated and all sulfur and nitrogen are removed. Hydroprocessing at higher temperatures and pressures is referred to as hydrocracking. Large molecules are broken down and saturated with hydrogen. In standard refinery, this process is used for cracking of vacuum gas oil (VGO). Hydrocracking in standard refineries yields a high percentage of products in the diesel and kerosene boiling range.
With regard to bio-crude, the high oxygen content is the major issue. Hydro- processing can be used to deoxygenate the crude and subsequently improve the Table 9.1 Unwanted characteristics of bio-oil and their effects collected from [93,94]
Characteristic Effect
Low pH value Corrosion problems
High viscosity Handling and pumping problems
Instability and temperature sensitivity Storage problems Phase separation
Decomposition and gum formation Viscosity increase
Char and solids content Combustions problems Equipment blockage Erosion
Alkali metals Depositions of solids in boilers, engines, and turbines Water content Complex effect on heating value, viscosity, pH,
homogeneity and other characteristics Oxygen content Higher viscosity and lower heating value
Table 9.2 Property ranges of bio-crudes
HTL Pyrolysis Fossil oil
Carbon (wt. %) 68–81 56–66 83.0–87.0
Sulfur + Nitrogen (wt. %) 0.1 0.1 0.01–5
Oxygen (wt. %) 9–25 27–38 0.05–1.5
Water (wt. %) 6–25 24–52 \1
Density (kg L-1) 1.10–1.14 1.11–1.23 0.75–1.0
Reprinted from Ref. [95], Copyright 2013, with permission for Elsevier
properties of the bio-crude. This deoxygenating process is called hydrodeoxy- genation (HDO). The oil is treated at temperatures of 300–400C under a hydrogen atmosphere of 150–300 bar. The presence of water is crucial since it prevents from char formation during the reaction. Therefore, high pressures of 150–300 bar are necessary to prevent from char formation [93].
During the reaction oxygen from oxygen containing compounds (like phenols, ketones, and fatty acids) is partially or totally eliminated in the form of water in the presence of hydrogen and a suitable catalyst such as Ruthenium on Carbon, Cobalt–
Molybdenum (CoMo /cAl2O3) or Nickel-Molybdenum (NiMo /cAl2O3) sulphides catalyst [93–95,97]. Studies on upgrading of HTL bio-oil crude have not yet been published and upgrading work mainly includes upgrading of fast pyrolysis bio-oils [95] Since water is formed during HDO standard hydrodesulphirisation (HDS), and/or hydrodenitrification (HDN) catalyst can’t be used because water affects catalyst performance negatively [96].
During hydrodeoxygenation, oxygen is removed from the biocrude through the formation of water. The overall reaction stochiometrie is shown in reaction (9.17).
ðCH2Oị ỵH2! ðCH2ị ỵH2O ð9:17ị During HDO dehydration, through which oxygen is released in the form of water, decarboxylation reaction, through which oxygen is released in form of CO2, hydrogenation reaction, in which hydrogen reacts with unsaturated carbon bounds of bio-oil compounds. Through hydrogenolysis, C–O bonds are broken up and oxygen is removed in the form of water. Also hydrocracking reactions may occur Fig. 9.16 Van Krevelen diagram [96]
in which big molecules are broken down into smaller ones and saturated with hydrogen.
For upgrading of the HTL crude fuel, it could be advantageous to upgrade before separating the oil and aqueous phase. By this most polar components can be converted to hydrocarbons and the aqueous phase can be easily separated from the hydrocarbon phase. Research is being done on upgrading of biocrudes in the aqueous phase in either sub-critical or super critical conditions and a recent review by Furimsky has been published addressing also upgrading of biocrudes in aqueous phases [95].