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«Biomass Gasifier “Tars”: Their Nature, Formation, and Conversion T.A. Milne and R.J. Evans National Renewable Energy Laboratory N. Abatzoglou ...»

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November 1998 NREL/TP-570-25357

Biomass Gasifier “Tars”:

Their Nature, Formation,

and Conversion

T.A. Milne and R.J. Evans

National Renewable Energy Laboratory

N. Abatzoglou

Kemestrie, Inc.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401-3393

A national laboratory of the U.S. Department of Energy

Managed by Midwest Research Institute

for the U.S. Department of Energy

under contract No. DE-AC36-83CH10093


Biomass Gasifier “Tars”:

Their Nature, Formation, and Conversion T.A. Milne and R.J. Evans National Renewable Energy Laboratory N. Abatzoglou Kemestrie, Inc.

National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 A national laboratory of the U.S. Department of Energy Managed by Midwest Research Institute for the U.S. Department of Energy under contract No. DE-AC36-83CH10093 Prepared under Task No. BP811010 November 1998


This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Available to DOE and DOE contractors from:

Office of Scientific and Technical Information (OSTI) P.O. Box 62 Oak Ridge, TN 37831 Prices available by calling 423-576-8401

Available to the public from:

National Technical Information Service (NTIS) U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 703-605-6000 or 800-553-6847 or DOE Information Bridge http://www.doe.gov/bridge/home.html Printed on paper containing at least 50% wastepaper, including 10% postconsumer waste PREFACE This literature study was commissioned by the IEA Bioenergy, Biomass Utilization Task XIII, “Thermal Gasification of Biomass” activity. At the invitation of Dr. Suresh Babu, Institute of Gas Technology, activity leader, the authors agreed to compile the following information pertinent to one of the persistent problems in coupling gasifiers to energy conversion devices—the presence and modification or removal of the organics historically called “tars.” The authors are grateful for the support of the IEA; CANMET, Canada, Bioenergy R&D, Mr. Ed Hogan, MRN Quebec, Energy from Biomass, Mr. Georges B.B. Le; the University of Sherbrooke and Kemestrie Inc. (NA); the U.S. Department of Energy Biomass Power Program, Dr. Richard Bain, and Dr. Helena Chum at NREL (RE and TM), Ms. Stefanie Woodward (editor) NREL, and the Library and Word Processing staff at NREL.

This report will be issued under the auspices of the IEA Biomass Utilization Activity and will be published as an NREL technical report and appear on the NREL/DOE Power Home Page.

*“When I use a word,” Humpty Dumpty said in a rather scornful tone, “it means just what I choose it to mean—neither more nor less.” Through the Looking Glass, Lewis Carroll.

–  –  –

References (See annotated bibliography in Appendix IV) Appendix I: A List of Major Compounds Found in “Primary Organics”................A-2 Appendix II: A List of Major Compounds Found in “Secondary Organics”.............A-6

–  –  –

iv SUMMARY 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.

v 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 warrantees.

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); hightemperature 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.

–  –  –

The characterization of “tars” as primary, secondary, and tertiary is a first step in classifying these materials and relating the composition of “tars” with formation conditions. Some gasifiers show the presence of primary and tertiary “tar” constituents in the same “tar” sample, and this raises the question of the importance of process upsets and large, residence-time distributions that could cause this occurrence. This could have important implications in the design and operation of gasifiers to ensure adequate control of reaction conditions. These “tar” constituents can be used as indicators of overall reactor performance and design (Brage et al. 1997b).

Although past work has shown the systematic nature of “tar” composition as a function of reaction temperature, more detailed study is needed to characterize the product at a higher level of detail.

Some primary products will likely be more refractory to secondary thermal and oxidative cracking reactions than others, so an accepted method of characterizing the compound classes in each major group is desirable and a method of rapidly screening for this information is needed. For example, it may be possible to “train” spectroscopic techniques to provide the necessary analysis based on correlation with more detailed work on a test system with GC/MS and other techniques that give highly specific information, but are expensive to perform. Another approach is to identify “marker compounds,” or predominant constituents, which are indicators of overall chemical composition and to use methods to monitor these representaitve indicators of overall “tar” composition (Brage et al.


This chemical characterization could be correlated with key physical property data and process operations, such as performance of wet scrubbing systems or catalytic cracking units. Primary, secondary, and tertiary classes are a starting point, but more detail is needed about the conversion of specific compound classes such as organic acids, which seem to persist beyond other primary products.

Kinetics and reaction pathways for primary to secondary and tertiary processes should be known so they can be included in the design of gasifiers and cleanup systems. The qualitative and quantitative effects of oxygen and steam on product distributions should also be better known. More quantitative studies are needed of primary, secondary, and tertiary products in fluid beds where residence time distribution affecting reaction severity must be considered.

Alternative feedstocks, such as herbaceous crops with high nitrogen content, raise questions about nitrogen-containing constituents. Analysis of these materials warrants more study.

Finally, the pathways to soot and particulates, from tertiary products, require quantitative study to better ascertain the importance of these processes. This may be critical in hot-gas cleanup technology.

–  –  –

Once “tar” collection protocols are established, compound-class analysis methods and the analysis of predominant constituents should be established as standard procedures. Kinetic modeling of these groups should be attempted to help gasifier designers systematically address the relative importance of process upsets and residence-time distributions in accounting for mixed product slates.

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