Tar is a complex organic material with many possible origins. Environmental forensic investigators can misidentify a substance that is dark, gooey and aromatic as coal tar because it is one of the most familiar forms of tar. However, this description could refer to a material originating from one or more wood, coal, or petroleum sources. The purpose of this article is to summarize the general history of tar manufacture and illustrate some of the factors to be considered when identifying the origin of a tar unearthed during a site investigation.

Wood Tar

Wood tar was an important preservative in pre-18th century civilization. European colonization was predicated on a fleet of sea worthy war and merchant ships. Wood and rope decay posed as great a threat to maritime supremacy as war. In addition, European prosperity fostered urban growth and placed a premium value on wood as a building material and fuel resource. When used as a preservative, wood tar was the primary means for ensuring the longevity of wood and plant products (boats, buildings, vehicles and tools) while conserving the ever-dwindling European forests.

Although wood tar was primarily produced by Sweden, Finland and Russia, “Stockholm Tar” dominated the international market by virtue of its light color and high preservative quality. Concern about reliance on its European neighbors prompted Great Britain to encourage the production of wood tar in its North American colonies. Great Britain’s concern about European suppliers was prescient. In 1703, Russia assumed control over Scandinavia and raised the price by limiting the supply of European wood tar. Great Britain reacted by raising the production requirements of American wood tar dramatically until the American Revolution of 1776, after which it returned to the Scandinavian suppliers. Throughout the 1700’s and 1800’s, the American centers of wood tar production shifted from New England and the Carolinas to Georgia, Florida, Alabama, Mississippi, Louisiana and Texas as the population expanded on the East coast and reduced the local supply of desirable wood stocks. The overall production of wood tar declined between the 1860’s and 1900’s as the US navy and merchant marine shifted from wooden to iron fleets. While the amount of American wood tar was likely minor in relation to coal and petroleum tars, the amount could be significant when wood tar residues are discovered at a particular site.

There existed two general types of wood tar. In general, hard wood tar was derived from oak, beech and birch while soft wood tar was derived from resinous pine species. Both of these wood tar types were produced by the destructive distillation of split roots, tree stumps or cordage placed in earthen kilns or crude retorts. Therein, the wood was heated in the absence of air for approximately one week. The temperature ranged from 100°C to 1000°C depending on the time and location within the distillation chamber. The liquid product was a mixture of liquefied wood resins, steam distillates and decomposed wood. Consequently, environmental forensic investigators can often identify wood tar based on the abundance and distribution of oxygenated benzenes, monoterpenes, sesquiterpenes and diterpenes.

Coal Tar

The destructive distillation of coal generated three valuable products: manufactured gas, metallurgical coke and coal tar by-product. In the US, the manufactured gas technology was imported from Great Britain for the purpose of illuminating the urban centers. The construction of manufactured gas plants (MGPs) began in Baltimore (1816), Boston (1822), New York (1825) and spread to other American cities.

By comparison to tar derived from petroleum, coal tar may not have been the most abundant form of tar produced in the US. In this example, gas production is used as a rough estimate of tar production. It is acknowledged that this estimate is prone to some error because the quantity of tar generated from petroleum was generally less than coal per unit of gas produced. However, we do know that the production of coal tar outpaced other types of tar in regions like the Ohio River Valley. Prior to 1887, MGPs either burned or discarded the coal tar. After 1887, coal tar was commonly recovered and refined into marketable by-products.

Metallurgical coke was initially produced on an industrial scale in beehive ovens. This technology proliferated in the mid-to-late 1800’s due to the high demand for metallurgical coke and low cost of plant construction. Standard beehive ovens heated coal for two to three days, while venting or consuming almost all gases and coal tar. As the iron-dependent US industrial revolution grew, the demand for coke and profit set the stage for the byproduct coke ovens developed in Germany.

In the US, the first byproduct ovens were built near Syracuse, NY (1892) for the production of coke and ammonia for the creation of soda ash as an essential ingredient for the manufacture of soap and glass. The first byproduct oven constructed for metallurgical coke was built in Johnstown, PA (1894). The first byproduct oven constructed for city gas was built in Everett, MA (1989). The coke from this plant was largely sold to the Boston and Maine Railroad as a smokeless locomotive fuel and exemplified the range of different markets serviced by this technology. Revenue from the coal gas, coke and by-product encouraged the construction of byproduct ovens wherever the products could reach a viable market. Consequently, these ovens proliferated in the iron and coal districts of the Ohio River valley, especially during World War I.

Byproduct ovens carbonized coal in a sealed chamber by heating it through the oven walls to approximately 900°C. Coal gas and tar were vented from the chamber. The gas was stripped of high molecular weight compounds and undesirable impurities while traveling to a storage unit known as a gasholder prior to distribution. The coal tar was collected throughout the plant because no single purification step removed the gas impurities completely. After 1887, the coal tar was then refined on or off site into various intermediate and final products. Frequently, the tar collection equipment leaked to the subsurface and this fugitive tar was rarely recovered prior to the environmental regulations of the 1970’s and 1980’s.

Carbureted Water Gas Tar

Carbureted water gas (CWG) plants generated the majority of manufactured gas used for heating and lighting throughout the 1900’s in the US. The generation of CWG tar consisted of two basic steps. First, steam injected through an incandescent bed of coke or anthracite coal produced a mixture of primarily hydrogen and carbon monoxide, known as water gas. Second, the water gas was directed into a carburetor where it was enriched with light hydrocarbons generated by spraying gas oil on hot brick (~870°C). The light hydrocarbons from the carburetor raised the BTU content of the manufactured gas from 300 BTU/ft3 to about 530 BTU/ft3.

The CWG tar principally originated from the petroleum cracking step and not the water gas generation step. The gas oil type and the cracking conditions (temperature, residence time and equipment) governed the yield and properties of CWG tar. Between 1872 and 1910, naphtha was commonly cracked in the carburetor. The proliferation of the automobile elevated the price of naphtha and diminished its supply. In similar fashion, the supply and demand of petroleum products forced CWG plants to switch to straight run distillates, residual oils and heavy residuum throughout World War I, the Great Depression and World War II. The change in gas oil type typically required some degree of change in the plant operation or equipment. The increasing weight of the gas oil increased the yield and density of the tar. Consequently, most CWG plants produced many different types of tar over time.

Unlike coal tar, CWG contained little to no tar acids and bases. Other chemical signatures of CWG are site specific and may reside in the trace aliphatic residues of the gas oil mixed with the tar. These can include diagnostic distributions of biomarkers and saturated alkanes. Thermal signatures may also be evident in the relative abundances of chemically similar polycyclic aromatic hydrocarbons (PAH).

Oil Tar

The quantity of oil tar produced in the US was relatively low and most common on the West coast due to the abundant supply of local oil and limited supply of suitable coal. The oil gas process was basically a modification of the carburetion step used at CWG plants. The gas oil (usually crude or residual oil) was heated to improve fluidity and sprayed onto hot brick (~870°C). The cracked hydrocarbons were purified of lampblack, tar and light oil prior to distribution. The composition of oil tar resembled CWG tar in that the residues of the petroleum feedstock and the thermal signatures of the cracking and quenching processes produced site-specific signatures. Unlike coal tar, oil tar contained little to no tar acids or bases.

Tar Mixtures

Mixtures of tar from various origins can confound environmental forensic investigations. For example, many former MGPs supported multiple types of gas generating equipment at a given site over time. Many coal gas plants were converted to CWG plants and many CWG plants were converted to oil gas plants. At other facilities, CWG and oil gas supplemented the coal gas during peak production periods. In short, the extent to which coal and petroleum tars commingled is typically site specific.

Forensic investigations involving tar-processing plants also encounter mixtures of different tars.

While these plants were often sited near tar producing facilities, they often imported tar from other producers when the demand for tar products exceeded the local supply of crude tar. In addition, the performance specifications and efficient manufacture of some tar products necessitated the blending of multiple tars of different properties. For example, the chemical composition of creosote evolved over time from a straight run distillate (approximate boiling point range of 200°C to 400°C) to a reformulated product (approximate boiling point range of 220°C to 355°C) containing non-marketable tar by-product (pressed anthracene cake oil and phenanthrene), enhancement blends (heavy coal tar fractions improved permanence; selected petroleum and tar fractions improved penetration) and/or bulking agents (selected water gas or oil tars). In other words, the composition of creosote depended largely on the industrial practices at the site and varied at any given time across the US.

Tar Releases

Before approximately 1860, many facilities discarded tar as a useless waste product. After 1860, tar was discarded when it failed the specifications as a fuel or tar-processing feedstock due to the presence of emulsified water or other impurities. In addition, small plants, mostly CWG plants, produced too little tar for economical recovery. Disposal usually referred to the discharge of tar into a nearby water body, pit or lagoon. The environmental legacy of this practice persists in most urban coastal or river environments only to be rediscovered when the sediments and coastlines are altered.

In addition to the obvious release of unusable tar, many tar producing or processing plants experienced sudden or chronic releases of hydrocarbons from brakes in the plant pipelines and storage containers. Other tar-contaminated materials included waste sludge associated with the gas purification and refining processes. In addition, the practice of facility renovation often involved the burial of demolished building materials sometimes contaminated with tar. When found during an environmental investigation, samples from a tar handling facilities can contain gas oil (petroleum product of varying composition), light oil (recovered from gas stream), drip oils (recovered from gas transfer lines), wash oils (recovered from gas purifiers) and others. The environmental forensic investigator should not confuse these products with an off-site or modern source of hydrocarbon.

Analytical Approach

Tars of different origin are often difficult to distinguish when analyzed by generic fingerprinting methods, like EPA 8100 Modified. While wood tar is relatively distinct, coal and petroleum tars are quite similar. Few of the obvious differences among these tars remain as the respective tars weather.

The chemical data required to characterize the tar origin(s) is often site specific. These authors recommend starting with a high-resolution hydrocarbon fingerprint by a gas chromatograph equipped with a flame ionization detector (GC/FID). Sample extracts should also be analyzed on a gas chromatograph equipped with a mass spectrometer operated in scanning (GC/MS/Scan) and selected ion monitoring (GC/MS/SIM) modes, respectively. The GC/MS/Scan analysis will permit the identification of many organic source indicators (tar acids, tar bases, oxygenated benzenes, monoterpenes, sesquiterpenes and diterpenes), if present. The GC/MS/SIM analysis will permit a detailed characterization of PAH (parent and alkylated isomers) and biomarkers (steranes and terpanes). Other analyses can be employed if these data require supporting evidence. Some of the other analyses that help describe the bulk tar composition can include density, viscosity, average molecular weight, elemental composition, metals, isotopes, and particle characterization. When associated with an adequate sampling resolution, these data reveal a great deal of information about the type of tar and feed stocks at the site.

Summary

Tar can originate from wood, coal and petroleum. The likelihood is good that most tar producing or processing sites experienced acute and chronic releases over time. The identification of tar origin is often complicated by the presence of unused feedstock (coal and petroleum), tar purification materials and intermediate by-products that coexist and weather at the site. Consequently, the identification of a tar source requires a careful study of the site history and regional industrial development. However, the historical record is often incapable of completely reconstructing the total mass and character of the tar(s) released over time. Consequently, environmental forensic investigations focused on tar origin often require multiple measurements of numerous samples from several depths and a wide geographic area in order to generate a defensible understanding of the releases and nature of fugitive tars at the site.

References

Harkins, S.M., R.S. Truesdale, R. Hill, P. Hoffman, and S. Winters (1988). U.S. production of manufactured gases: assessment of past disposal practices. EPA/600/2-88/012. Hazardous Waste Engineering Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency. Cincinnati, OH. 388pp.

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