From the article:
The main purpose of this review is to update the available information on gasification “tar.” “Tar” is the most cumbersome and problematic parameter in any gasification commercialization effort. For this reason the IEA Gasification Activity has commissioned this work, which aims to present to the community the scientific and practical aspects of (a) “tar” formation and (b) “tar” conversion or removal during gasification as a function of the various technological and technical parameters and variables.
Historically “tar” was an operationally defined parameter, based largely on organics from gasification that condensed under operating conditions of boilers, transfer lines, and internal combustion engine (ICE) inlet devices. Such a definition requires a more detailed chemical explanation in light of the greatly expanded uses proposed for both high- and low-energy gas from a variety of biomass and waste materials. At present the literature contains many data on the “destruction,” “conversion,” “removal,” etc., of “tars,” “condensibles,” “heavy hydrocarbons,” etc., without a consistent definition of these terms and a description of the sampling and analytical methods used for the organics of interest. Though the data presented are useful in the context of the system being studied, they are limited in their transfer to other systems because they are “apparatus dependent.”
It is not within the mandate of this work to propose a widely accepted definition of “tar,” but rather to report the varied use of the term. Hopefully this report will complement a recent effort of the IEA Gasification Task [BTG/UTWENTE 1998] to reach a consensus among its members regarding such an acceptable definition, as the first step in the adoption of a “tar” sampling protocol for the product from a variety of gasifiers, both high- and low-energy (producer) gas. Thus, within these limitations, this work suggests that “tar” is defined as follows:
“The organics, produced under thermal or partial-oxidation regimes (gasification) of any organic material, are called “tars” and are generally assumed to be largely aromatic.”
Although this definition does not allow for distinction between classes and families of compounds, to be presented comprehensively in Chapter II of this report, it is a useful starting definition for gasification “tar.”
Chapter III points out the main, and consequently the most practically important, differences in “tar” nature and quantities as a function of gasification conditions and applied technology. “Tar” nature also depends on gasified feedstock and degree of feedstock contamination. A summary of the known mechanisms of chemical formation and conversion during gasification regimes is presented and commented on in Chapter II.
Chapter IV undertakes a short presentation of “tar” sampling and analysis protocols used worldwide by workers and researchers in this field. A comprehensive report on this topic is available in the literature. Nevertheless, in this chapter the authors have undertaken a comparison of the technical details of a few of the sampling and analysis protocols, the aim being to relate facts with intrinsic difficulties and encountered errors, and thus provide an insight into the efforts to formulate widely accepted protocols for “tar” identification and quantitative measurement.
A very important, though not well studied, topic is the tolerance of gasifier gas end-use devices for “tar.” Data are available from R&D activities and from field experience, mainly coming from manufacturers. In Chapter V there is a presentation of the gasifier-gas applications for energy and chemicals production, followed by a report of gas specifications for these processes. The reader has access to a large amount of information regarding the content and nature of contaminant “tar” in fuel gases, but their impact on a variety of energy conversion and process applications is only beginning to be documented. One should contact manufacturers and involve them in the process leading to commercial application as well as performance warranties.
Chapters VI, VII, and VIII deal with raw-gas cleaning technologies. They focus on tar removal through physical processes (Chapter VI) and “tar” conversion through thermochemical and catalytic processes (Chapters VII and VIII). The physical processes are classified into wet and dry technologies depending on whether water is used. Cyclones, cooling towers/scrubbing columns, venturis, demisters/coalescers, cold and hot filters, baghouses, electrostatic precipitators, and wet-dry contactors/scrubbers are reported with sample literature coverage. Technologies available for treating wastewater coming from wet-scrubbing processes are also briefly presented. They concern organic solvent extraction, distillation, adsorption on activated carbon, incineration, biological treatment, and wet oxidation. The choice of cleaning train depends on the specific application and the results of technoeconomic evaluation that must be carried out before a process is selected.
The chemical “tar” conversion processes are divided into four generic categories: thermal, steam, partially oxidative, and catalytic processes. Because of their particular importance as well as the intensive R&D work dedicated to them, the catalytic processes are analyzed and reported separately in Chapter VIII. Among these processes, catalytic steam reforming using dolomites and, more efficiently, Ni-based catalysts seem of great importance and should lead to commercial applications in the near future, especially for gas use in gas turbines. It is widely accepted that physical cleaning technologies are suitable for gas use in boilers and ICEs (for downdraft gasifiers at least); high temperature chemical “tar” conversion schemes may be required for gas turbine or high-temperature fuel cell applications.
The review is complemented with a selected bibliography on biomass gasifier “tars,” with annotations relevant to formation, nature, analysis, removal, conversion, and end-use device tolerance. This bibliography is composed of some 400 publications. Comments/annotations are meant to help interested readers select papers for their specific needs.
In conclusion, we would like to reiterate our intention to provide the gasification community with an appropriately compiled resource regarding the important issue of “tar” presence in raw gas from the variety of gasifiers being developed.