Stabilization of pyrolysis oil derived from wood through Reactive Chromatography

Charaterisation of wood Pyrolysis oil

This part describes an analytical approach to determine the physico-chemical composition of bio-oil.

With declining petroleum resource and more concerns on environment and climate, the development for renewable energy is getting more necessary. Substantial research is being carried out within the field of energy in order to find alternative fuels to replace fossil fuels. The optimal solution would be renewable energy resource which is equivalent to the fuel which is sustainable and will decrease the CO₂ emission.

Biomass derived fuels could be the prospective fuels of tomorrow as these can be produced within a relatively short cycle and are considered benign for the environment. Biomass derived fuel is pyrolysis oil which is renewable liquid fuel which can be directly used for burning in boilers, readily stored, transported, retrofitting, and flexibility in production and marketing of chemicals.

It is important to characterize the bio-oil as every bio-oil has different composition, depending on its source and pyrolysis conditions. We have used various characterization techniques to characterize the bio-oil. Physical characterization was done by the measurement of viscosity, density, higher heating value, moisture content and pH. GC-MS was used to identify the different chemical composition. Fourier Transform Infra-Red Spectroscopy was used to identify the functional groups. 

Bio-oil obtained from vacuum pyrolysis of wood at 773 K at heating rate of 30 K/min is usually dark brown free-flowing liquid having a distinctive smoky odor. The physical properties of the bio-oil are resultant of chemical composition of the liquid which is significantly different from petroleum-derived oil. Bio-oil is a complex mixture of more than 300 compounds resulting from the depolymerization of biomass building blocks cellulose, hemi-cellulose and lignin. Bio-oil is differ from petroleum based fuels both is physical and chemical composition. Bio-oil is highly polar containing about 40-50 wt% oxygen resulting in low calorific value. This liquid is acidic in nature and unstable when heated, especially in air tends to polymerize i.e increases viscosity. Bio-oil typically contains high moisture and micron size char particles which insoluble with petroleum based fuels.

The chemical composition of bio-oils is very complex, mainly composed of water, organics and a small amount of ash. It is globally represented as: around 20 % water, around 40 % GC-detectable compounds, around 15 % non-volatile HPLC detectable compounds and around 15 % high molar mass non-detectable compounds. A complete analysis of bio-oils requires the combined use of more than one analytical technique. A precise description of bio-oil composition has not yet been achieved. The accuracy of some of these analytical techniques has been highlighted in Round Robin tests conducted by different laboratories.

Table 1. Describes the physical properties of crude bio-oil obtained from waste wood.

Physical Properties	Values
Moisture content (wt %)	26.36
pH	2.80
Density (kg m-3)	1.08
Ash (wt %)	0.03
HHV (MJ kg-1)	22.20
Viscosity (cP) at T=313K	73.62
Elemental composition (wt %)
Carbon	50.92
Hydrogen	8.27
Oxygen (by difference)	38.57
Nitrogen	2.23

The chemical characterization of crude bio-oil includes GC/MS procedure followed to obtained bio-oil fractions using column chromatography eluted using different polarities of solvents.

n-Hexane Fractionation

Phenol, 4-methyl

2-Pyridinemethanol

Phenol, 2,4-dimethyl

Phenol,2-methoxy-4-methyl

Phenol- 4-ethyl-2-methoxy-

Phenol, 2,6-dimethoxy

1,2,4-trimethoxybenzene

Phenol, 2-methoxy-4-(1-propenyl)-

5-tert-butylpyrogallol or 5-tert-Butyl-1,2,3-trihydroxybenzene

Phenol,2,6-dimthoxy-4-(2-propenyl)-

DCM Fractionation compounds

2-cyclopenten-1-one, 2-hydroxy-3-methyl

phenol, 2-methoxy-

phenol, 2-methoxy-4-methyl-

phenol, 4-ethyl-2-methoxy

phenol, 2,6-dimethoxy-

1,2,4-trimethoxybenzene

5-tert-butylpyrogallol or 5-tert-Butyl-1,2,3-trihydroxybenzene

phenol,2,6-dimethoxy-4-(2-propenyl)-

Desaspidinol or 1-(2,6-Dihydroxy-4-methoxyphenyl)-1-butanone

1,3-Benzodioxol-5-yl-1-oxo-2,4-pentadienyl-piperidine

Benzene Fractionation

2-cyclopenten-1-one, 2-hydroxy-3-methyl

Phenol, 4-methyl

Phenol, 2-methoxy-

Phenol, 2-methoxy-4-methyl

Phenol, 4-ethyl-2-methoxy-

Phenol, 2,6-dimethoxy

1,2,4-trimethoxybenzene

5-tert-butylpyrogallol or 5-tert-Butyl-1,2,3-trihydroxybenzene

Phenol,2,6-dimthoxy-4-(2-propenyl)-

1,3-Benzodioxol-5-yl-1-oxo-2,4-pentadienyl-piperidine

Ethyl Acetate Fractionation

Phenol, 2-methoxy-4-methyl

1,2-Benzenediol, 3-methoxy-

Phenol, 4-ethyl-2-methoxy-

Phenol, 2,6-dimethoxy

Dehydroacetic acid

5-tert-Butylpyrogallol

Phenol, 2,6-dimethoxy-4-(2-propenyl)-

Ethanone, 1-(4-hydroxy-3,5-dimethoxyphenyl)-

Desaspidinol or 1-(2,6-Dihydroxy-4-methoxyphenyl)-1-butanone

Methanol Fractionation

Benzoic acid

1,2-Benzenediol

Phenol 2,6-dimethoxy

2-propenooic acid, 1,7,7-trimethylbicyclo[2.2.1]hept-2-yl ester,exo

1,2,3-Trimethoxybenzene

1,6-anhydro-beta-d-glucopyranose

5-tert-Butylpyrogallol

Phenol 2,6-dimethoxy-4-(2-propenyl)-

Ethanone 1-(4-hydroxy-3,5-dimethoxyphenyl)-

Desapodinol

4H-1-Benzopyran-4-one,2-(3,4-dimethoxyphenyl)-7-hydroxy-3-methoxy-

Benzaldehye, 4-hydroxy-3,5-dimethoxy-

10,11-dihydro-10-hydroxy-2,3,6-trimethoxydibenz(b,f)oxepin

Benzene,1,1',1'',1'''-(1,6-hexanediylidene)tetrakis- (9CI)

Determination of functional groups of pyrolysis oil

The pyrolysis oil of wood obtained was analysed for its functional group composition using Fourier Transform Infra-Red Spectroscopy (FTIR). The system used was a Bunker`s Tensor 27 series with an on-line pen plotter to produce the IR-spectra of the derived liquid. It provides the absorbance spectra units along the wave number 4000 to 500 cm^-1. 

Figure 1 below shows the absorbance unit vs. IR frequency of crude bio-oil.

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