Origin of coal conversion by-products
These by-products that represent a source for fuels production comprise the direct hydrogenation of coal, the synthesis of liquid hydrocarbons from coal-derived syngas, and tars obtained from carbonisation or moving-bed gasifiers. Tar originating from carbonisation can be obtained as the major product either in the case of coal pyrolysis or as a by-product of coke production from coal. Coal from moving-bed gasifiers, which is recovered from a tar-oil-dust-water mixture from the gas cooling process, represents a by-product. The composition and yield of tar and oil hydrocarbons depend strongly on the feedstock properties and process characteristics. Generally, the tar yield is reduced with increasing coal rank and elevated operating temperature.
Tar from moving-bed gasification processes
Multiple hydrocarbon streams of different quality can be collected from the gasification process during the gas cooling and water scrubbing of the raw gas exiting the moving bed gasifier (Gräbner, 2015). Because of the counter-current flow regime, between 15% and 25% of the original coal heating value can be bound in the hydrocarbons. Dependent on their density, solubility in water and pollution with solids, the hydrocarbon phases include oil (characterised by a density lower than water), gas liquor (mixture of water and dissolved hydrocarbons, for example, phenols, ammonia and organic acids) and dust-containing or dust-free tar (heavy hydrocarbons with a density higher than water). Besides these three hydrocarbon species being collected in the wash cooler, very light non-condensed hydrocarbons referred to as naphtha can be recovered during downstream low-temperature acid gas removal.
The different hydrocarbon phases are commonly recovered by a multi-stage process separating first dusty and clear tar and oil from the gas liquor before recovering phenol and separating acidic gases (for example, CO2, H2S, HCN) before reclaiming ammonia and other organics from the waste water stream that needs final sewage water treatment.
As an example, the hydrocarbon phases from a lignite fuelled moving bed gasifier include tar/oil with about 84% carbon, up to 10% hydrogen, up to 5% oxygen and up to 0.5% sulphur. The major compounds are BTX aromatics and to a lesser extent phenols and cresols. Only minor amounts of nitrogen or amine group-containing hydrocarbons are contained in that phase. In contrast, naphtha and crude phenol are mainly carbon and hydrogen, with the naphtha consisting of aromatic hydrocarbons and aliphatic hydrocarbons.
Hydrocarbons from carbonisation
The tar quality from coal pyrolysis for liquids or coke production differs with coal rank and pyrolysis carbonisation temperature. Up to 25% hydrocarbon yield (referred to as total product yield) can be expected for low-temperature (below 700°C) carbonisation compared to 8% for high-temperature carbonisation. The ratio of heavy tar hydrocarbons to liquor is approximately 0.6:1 for low temperature processes while it increases for high-temperature carbonisation to about 1:1 to 1.5:1. Both processes also yield minor amounts of light oils (naphtha).
In 2014, close to 1 billion tonnes of coal was used for coke production in the top ten coking coal producing countries worldwide, with 54% of that production occurring in China, resulting in close to 25 million tonnes of tar. In addition, over 4 million tonnes was produced under medium temperature conditions as found in moving-bed gasification units.
Tar upgrading technologies
There are specific processes and applications that use dedicated hydrocarbons or chemical compounds extracted from coal tar or gas liquor to produce fine chemicals. However, the major fraction of the coal tar is processed to either produce fuel oil for industrial furnaces or blended with other hydrocarbons to provide transportation fuels. Conventional blending results in comparatively low added value; consequently, recent utilisation routes for these coal-based tars and gas liquors often include a hydrogenation process to increase the quality of the hydrocarbon product by reducing potential emissions for fuel applications.
An alternative to refining processes is the gasification of the coal tar which is currently applied at the Sokolovska Uhelna integrated gasification combined cycle plant located in Vresova in the Czech Republic. This uses a Siemens gasifier to convert the coal tar produced by a number of fixed bed gasifiers into a fuel gas which is provided to the power block of the plant.
Besides recovering selected chemicals, hydrorefining and hydrocracking are frequently used to convert the tar into valuable products. The former aims for removal of sulphur, oxygen, nitrogen and other hetero atoms to improve the quality by hydrating unsaturated hydrocarbons. In contrast the latter aims for conversion of heavy hydrocarbons into lighter fractions with adjustment of the hydrocarbon range (for example, n-/cyclo-/iso-alkanes, aromatics) by cracking and saturation of bonds using hydrogen.
Many of those technologies are commercially applied in China including coal tar hydrorefining, coal tar fixed bed hydrocracking, coal tar delayed coking-hydrogenation, fluidised bed hydrogenation and homogeneous phase slurry-bed hydrocracking technology. In addition, there is heterogeneous phase slurry-bed hydrocracking, based on principles developed for direct coal liquefaction (BRICC, 2014). Some information about the indicated processes is summarised in Table 17.
The abovementioned technologies have limitations regarding applicability to heavy tars and light products yield; heterogeneous coal tar hydrogenation technology (derived from coal hydrogenation applications) is promising for the processing of a wide range of tar qualities at reduced coking and catalyst deactivation at simultaneously high yield of light oil fractions. Two technologies are currently commercially offered, namely Veba™ Combined Cracking (VCC) by KBR and the BRICC technology by the China Coal Research Institute.
The Veba™ process schematic is shown in Figure 32. Typical operating conditions are 21–23 MPa and 460–480°C. The technology is characterised by using a doped natural mineral catalyst with little coking activity.
Figure 32 Schematic of the VCC heterogeneous slurry-bed hydrogenation process (BRICC, 2014)
The BRICC technology for heterogeneous phase slurry-bed hydrocracking technology represents a further development of the direct coal liquefaction technology demonstrated at the 1,000,000 t/y coal liquefaction plant located in Ordos, Inner Mongolia, China. The technology dates back to 1979 and has been commercialised since 2011. Several process layouts were developed accounting for differences in the treatment of high-temperature and low-temperature coal tar. Schematics of the different process layouts are provided in Figure 33. For higher value-creation, the phenol extraction stage for low and medium-temperature tar processing can be extended to recover chemicals like naphthalene.
The first step is independent from tar quality and includes preparation of the tar slurry by mixing the raw tar with hydrocracking catalyst, sulphur and heavy oil recycled from downstream process units at a temperature range between 80°C and 200°C. Slurry bed hydrocracking of the raw tar and heavy oil compounds is performed at about 320–470°C and 12–19 MPa. Operated at a LHSV of 0.3–3 per hour the process uses 500–1000 g of hydrogen per litre of oil with about 0.1–4% of catalyst in the slurry. Light oil upgrading by fixed bed hydrorefining aims for the provision of naphtha, jet fuel, diesel, phenol and fine chemicals like ink solvents. The product ratio can be adjusted according to market requirements. The naphtha cut can be further converted to gasoline by catalytic reforming, allowing for additional aromatics extraction. Heterogeneous phase slurry-bed hydrogenation is characterised by very high conversion rates of heavy tars, in particular close to 100% conversion of asphalt, resulting in an increased light products yield. Some 78–85% can be recovered as light oil compounds from hightemperature tar, while for low or medium-temperature tar the number is 87–94%. The process is commercially marketed by the China Coal Research Institute and Luoyang Engineering Corporation of Sinopec. The first commercial application is a 500,000 t/y tar upgrading project for the Quinghua company.
Figure 33 Schematics of the BRICC process for treatment of low and high-temperature tars (BRICC, 2014)
Technical maturity and industrial applications
Most of the described processes are adapted from the petrochemical industry and thus are considered to be technologically mature.
The 10 largest coal producers and exporters in the Indonesia:
- Bumi Resouces
- Adaro Energy
- Indo Tambangraya Megah
- Berau Coal
- Bukit Asam
- Baramulti Sukses Sarana
- Harum Energy
- Mitrabara Adiperdana
- Samindo Resources
- United Tractors
Source: IEA Clean Coal Centre
