Other Liquid Feedstocks
Orimulsion. Orimulsion is the trade name for an emulsified bitumen-water mixture produced from the bitumen fields in the Orinoco belt of Venezuela. Orimulsion consists of about 70% bitumen and 30% water and contains about 1 % surfactants. The emulsifying technology converts the bitumen into a transportable fuel with a pour point of 3°C and a viscosity of about 200 cP at 30°C. It has a lower heating value of 27.8 MJ/kg (Marrufo, Sarmiento, and Alcala 2001).
Technically, Orimulsion is a petroleum product; it contains sulfur, vanadium, and nickel in substantial quantities, and these behave just as in a conventional residue. The sodium content is low (12mg/kg), assuming that there is no contamination in transport. The differences of Orimulsion when compared to a refinery residue are in the water content and the surfactant used. The water content causes a considerable loss of efficiency since energy is used in its heating and evaporation. The resulting syngas has a C02 content of about 10% compared with around 3% in unquenched syngas generated from a conventional residue. Early formulations of Orimulsion contained considerable quantities of magnesium, which could have presented problems similar to those caused by calcium. The newest formulation, Orimulsion 400, has a magnesium content of 6 mg/kg, which is sufficiently low to avoid these.
When designing for or operating with Orimulsion, it is necessary to take specific precautions (e. g., reduced preheat temperatures) to avoid the emulsion breaking. A handbook of suitable handling guidelines can be obtained from the suppliers.
Orimulsion has been tested as a gasification feedstock in Texaco’s Montebello, California, pilot facility in 1989 with apparent success. The producer of Orimulsion, Bitumenes Orinoco S. A. (Bitor), claims to be able to supply the material at a price to allow competitive production in a gasification plant but there is no recorded commercial application at this time (2002).
Tar Sands Residues. Tar sands are deposits of heavy hydrocarbons located in a sandstone matrix that are not amenable to conventional pumping technology. The largest and most well-known deposits are in northern Alberta, Canada. Other deposits exist (in approximate order of size) in Venezuela, the United States (Utah, Texas, California, Kentucky), Russia (Olenek), Madagascar, and Albania, as well as in other locations in Canada (Melville island). A typical analysis is included in Table 4-10.
Tar sands represent a major hydrocarbon resource with an estimated 450 billion barrels of recoverable reserves (Speight 1998, p. 117), but the difficulties and cost of extraction have limited commercial exploitation. At present, there are only two commercially operating plants, both in the Athabasca River basin of northern Alberta. A number of pilot operations for the development of improved extraction techniques exist, also mostly in this area, and a number of commercial projects are currently under development (Parkinson 2002).
The processing of tar sands can be described in three principle steps:
1. Extraction, for which there are two fundamentally different approaches. One is to mine the bitumen-laden sandstone and transport it to a central extraction plant, where the sandstone and bitumen are separated by a hot-water extraction process (HWEP). The commercial operations of Syncrude and Suncor are both based on this method.
Alternative methods have been developed for in situ extraction and separation, such as steam-assisted gravity drainage (SAGD). There are pilot plants in operation that demonstrate the possibilities of this approach, and at least one current project is based on it.
2. Primary conversion, for which conventional or modified coking, cracking, or solvent de-asphalting processes are applied.
3. Secondary conversion, which is essentially a hydrotreating step.
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Table 4-11 Bitumen Solids Yields and Metals Analysis for BS Free Tar Sand Asphalt
Source: Zhao et al. 2001 with permission from Elsevier |
There is potential to use the residue from the primary conversion as gasifier feedstock to provide hydrogen for the secondary conversion. Existing operations do not do this; they generate their hydrogen by steam reforming of natural gas. But where or when natural gas availability is critical, gasification could become a serious option. A first project of this sort has recently been announced (Arnold etal. 2002).
Work has been performed on the characterization of residues from tar sands (Zhao etal. 2001).
From the point of view of gasification, these investigations have highlighted a number of important and interesting aspects (Table 4-11). The extremely high vanadium and nickel contents are a feature of the Canadian material. These values exceed current long-term experience for fresh feed in gasifier operation. In particular it would be important to avoid a recycle configuration for the carbon management system so as to avoid metals build-up in the circuit. The second interesting feature is the quantity of bitumen solids (BS) observed in the mined material, which is absent in that recovered by SAGD. These solids are typically ultra-fine aluminosilicate particles originating from clay inclusions in the sandstone structures that are brought into the processing plant by the inherently nonselective mining processes. Experience with gasification of conventional residues containing catalyst fines from an FCC unit has shown a tendency for such material to deposit in both the gasifier and the syngas cooler. The bitumen produced by the SAGD process is practically free of ultrafine solids, which makes it far more suited as a gasifier feedstock.
Liquid Organic Residues. Some gasifiers process organic residues from petrochemical processing, such as the manufacture of oxo-alcohols, and have done so successfully for many years. The only important consideration is that such residues may contain catalyst fines. Depending on the catalyst and/or carrier, this may have an abrasive effect on critical equipment or cause fouling or plugging as described above for refinery residues.
Coal Tar. The MPG process (see Section 5.4.3) was originally developed for coal — based tars generated in a plant using Lurgi fixed-bed gasifiers to gasify lignite. It has been in successful operation in such service since 1969 (Hirschfelder, Buttker, and Steiner 1997; Liebner 1998).
Other oil-processing gasifiers have taken in coal tar in order to reduce feedstock costs. Such attempts at mixing coal tar and petroleum-derived residues have not generally been successful. The principle difficulty is the incompatibility of the different types of ash, which tend to form eutectica. The result is plugging of either the throat area in a quench reactor or of the tube bundle in a syngas cooler.
Spent Lubricating Oil. Spent lubricating oil is included here as a potential gasifier fuel more to warn against it than to encourage its use. Used lubricating oil can contain typically 1500mg/kg each of lead and zinc. The lead content can be as much as 10,000mg/kg. Lead and zinc sulfides solidify at temperatures of 700-800°C and will block syngas coolers and the throats of quench reactors. Soyez (1988) reports that “some 100 ppm was sufficient to plug the waste heat boilers completely within only seven days.” Other similar cases are also known. The only sound advice concerning gasification of spent lubrication oil is: don’t.