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Tars: Composition and Characteristics

Page history last edited by bk 10 years, 4 months ago

 Great online database of Tar Composition and Characteristics, most notably, condensation temps:


Category Formation Temperature Constituents
Primary 400-600°C Mixed Oxygenates, 
Phenolic Ethers
Secondary 600-800°C Alkyl Phenolics, 
Heterocyclic Ethers
Tertiary 800-1000°C Polynucleic Aromatic

Table 4 - Categories of Tars

Source: Milne & Evans, 1998

"The primary tars are mixed oxygenates and are a product of pyrolysis. As gasification

takes over at higher temperatures, the primary products thermally decompose to lesser amounts

of secondary and tertiary products and a greater quantity of light gases.  Tertiary products are the

most stable and difficult to crack catalytically.  Provided that there is adequate gas mixing,

primary and tertiary tars are mutually exclusive in the product gas. Both lignin and cellulose in

the fuel result in the formation of tertiary tar compounds.  However, lignin rich fuels have been

shown to form heavier tertiary aromatics more quickly (Evans & Milne,1997)."



Table 1 - Classification of quantitatively analysed tar compounds according to Ref. [2]


primary tars- 200-500C

phenols, alcohols, keytones, aldehydes, carbon acids, monoaromatics

secondary tars- 500-800C

alkylated mono and diaromatics, pyridine, furans, thiophene, dioxin

tertiary tars >800C

benzene, naphtalene, phenanthrene, pyrene, benzopyrene, polynuclear aromatic hydrocarbons (PAHs)





Tar compound class Compound type Compound name Solubility Melting/Boiling/Flash Point [°C]  Notes: Use Peak Absorption Spectra [nm] Molecular Diagramtructure

Primary tar compounds



    Acetic acid          
    Propionic acid          
    Butyric acid          


    Acetol (1-hydroxy-2-propanone)            


Phenol limited solubility in water 8.3 g/100 ml   white crystalline powder, slightly acidic antiseptic properties

absorption peak at 270nm

fluorescence peak at 300nm

    2,4/2,5-Dimethylphenol a            


    Guaiacol Soluble in ethanol, ethyl ether, chloroform, triglycerides, fats, such as acetic acid mp:28 °C, bp:  204-206C colorless aromatic oil, samles darken with exposure to air and light flavoring, main component in the flavor of coffee    
    4-Methylguaiacol *see creosol, same compound*          


    Furfural organic molecule, high solubility in methanol 12.02 M, highly soluble in lower alcohols

mp: -36.5 °C

bp: 161.7 °C

colorless oil, when exposed to air quickly becomes yellow aromatic aldehyde, smells like almonds    
    Furfural alcohol highly soluble in most organic solvents, soluble but unstable in water

mp: 29C,

bp: 170C

clear amber liquid with faint burning odor under heat or acic  treatment , furfural alcohol can poylymerize into poly(furfural alcohol) 292-195 nm  

Secondary tar compounds



    Cresol slightly soluble in water, miscible in ethanol, ether, and benzene

mp: 5.5 °C

bp: 220 °C

colorless to yellowish aromatic liquid       

Monoaromatic Hydrocarbons

    Xylene insoluble in water

highly flammable fp: 17-30C

bp: 138-134C

mp: -38 - 14C

clear colorless sweet smelling liquid used as a solvent, also used as an inhalant for its intoxicating properties    

Secondary/tertiary tar b


Monoaromatic hydrocarbons


Miscellaneous hydrocarbons


Methyl derivatives of aromatics


Tertiary tar compounds (PAHs)







    pyrene low solubility in water 0.135 mg/l

melting point: 145-148 °C

boilinig point: 404 °C

colorless solid, yellow impurities are found at trace levels in many samples      






a - Compounds lumped together for analysis.

b - There are several compounds that appear in the second and in one of the

other two classes as well. This demonstrates the evolutionary development

and the somewhat arbitrary boundaries for the three tar classes [2].


Table 1 from Morf, et al.



-Tar reduction through partial combustion of fuel gas, Houben, 2005

One-ring aromatics and naphthalene account for 80% of GC-detectable tar mass from woody biomass in a downdraft gasifier. Naphthalene has a structure of two fused carbon rings, volitile at 78 C, and turns into an inflammable gas. Tertiary aromatics like naphthalene are predominate in downdraft gasifiers. Characterizes the mechanism of cracking, polymerization, or partial combustion of naphthalene in a down draft gasifier across air/fuel ratios and temperature. Minimum tar content was found to be at 900 C with an excess air ratio of 0.5. Naphthalene was also studied under varying concentrations of methane. Methane is used to introduce hydrogen free radicles into the system.  For hydrogen concentrations lower than 20% a strong increase in total tar concentration is found.    With higher hydrogen fractions, the free radical aromatic compound species formed durning cracking are neutralized with a hydrogen. In cases where hydrogen is scarce, these free radical compounds have the tendency to combine into higher aromatic complexes. This is a demonstration of the mechanism H-abstraction/C2H2-addition (HACA).


-Analysis of tar removal in a partial oxidation burner. Tar Thesis of Marja Petra Houben, 2004

The gasification configuration in which this study was conducted consists of an outer combustion chamber connected to the drying, pyrolysis, and reduction zones in the second chamber with a recycling system. In this paper two definitions of wood tar are stated: According to the Tar Guideline, a generic (unspecified) term for the entity of all organic components present in the producer gas excluding gaseous hydrocarbons (C1-C6), benzene is not included. Milne et al. denfines tars as The organics produced under thermal or partial oxidation regimes (gasification) of any organic material, they are generally assumed to be largely aromatic. Although tars may consist of over a hundred different components, in most cases only about 20 species are present in significant quantities. This paper has an interesting graph showing the gas-phase thermal cracking relation to temperature relating an exponential decrease of primary tars to secondary tars at 650-700 C and then another inflection point around 850-900 degrees from secondary to tertiary alkyl and tertiary condensed tars at a 0.3 second residence time.

Thermal cracking seems to take place when raising the temperature to 500-900 C. Adding oxygen above 700 C results in a considerable reduction of tar content. The amount of tar generated at 500 C is 12%. This is reduced 1.1% by raising the temperature to 900 C and adding oxygen. These results indicate that the tar, and not the carbon monoxide are being oxidized. Partial oxidation of the producer gas, to increase temperature cracks the tars into mainly carbon monoxide thus converting the tars to a better burnable permanant gas (in a state that will not recombine).


- Tar cracking from fast pyrolysis of large beech wood particles.  J. Rath, G. Steiner, M.G. Wolfinger, G. Staudinger. 2002

"Boroson carried out experiments with sweet gum hardwood and found that at temperatures of 650°C and residence times of 1 s of the volatiles considerable decomposition of tar occurs in the reactor. He also found that this cracking of tar leads to an increase of the yield of carbon monoxide, carbon dioxide, hydrogen and small fragments of hydrocarbons."



"Pakdel and Roy (1991) noted that the composition of the hydrocarbons obtained at

temperatures around 500°C (932°F) are highly branched in nature (see Table 1). Branched

PAHs are known to have lower environmental and toxicological impact compared with the tars

obtained at high temperature, which tend to be less oxygenated in structure.  Hydrocarbons

produced at higher temperatures (over 700°C (1292°F)) typical of gasification processes are

highly condensed polyaromatic hydrocarbons with a high level of mutagenic activity (see Table

2) (Elliott and Baker 1986a, 1986 b). Table 2 shows the quantification of the content of

polyaromatic hydrocarbons in gasification oils as reported by Pakdel and Roy (1991). "

Reactions, T. (n.d.). The Formation of Polyaromatic Hydrocarbons and Dioxins During Pyrolysis: A Review of the Literature with Descriptions of Biomass. Retrieved from



Other papers:

Characteristics of evolution of tar from wood pyrolysis in a fixed-bed reactor

     close correlation with the C2 compounds over the temperature range 700–900°C. Some characteristic compound ratios, namely indene to naphthalene (I/N), phenols to aromatics (P/A) and saturated to unsaturated C2 compounds, were identified. These reactor-specific correlations have obvious potential application for on-line continuous monitoring of non-gaseous products that so far are determined off-line.


Milne, T. A., Evans, R. J., & Abatzoglou, N. (1998). Biomass gasifier “tars”: their nature, formation, and conversion. November 1998. NREL/TP-570-25357. Retrieved from


  1. Klein, A. (2002). Gasification: an alternative process for energy recovery and disposal of municipal solid wastes. Columbia University. Retrieved from
  2. Morf, P., Hasler, P., & Nussbaumer, T. (2002). Mechanisms and kinetics of homogeneous secondary reactions of tar from continuous pyrolysis of wood chips. Fuel, 81(7), 843-853. Retrieved from

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