The second and fundamentally different route to producing liquid products as oil substitutes or chemicals from coal is direct liquefaction. The only process category developed to large industrial scale is slurry-phase hydrogenation of coal.
Suitable feedstock range
Whereas almost every coal can be used for indirect liquefaction if a suitable gasification technology is applied for syngas production, direct coal liquefaction requires rather reactive coals with high volatile content. In addition, a high hydrogen content is advantageous. Further requirements include being able to achieve a low moisture content of ≤1% after drying, as well as low oxygen, sulphur, nitrogen and chlorine contents. Higher water contents cause problems with oil-water separation, requiring the use of density phase separation or distillation. Hetero-atoms are problematic regarding the refining of the light and middle product oils to achieve the quality requirements for transportation fuels. In contrast to indirect liquefaction, with intermediate gas cleaning stages for the raw gas removing all catalyst poisoning or other harmful contaminants, hetero atoms become part of the product phases during direct conversion. They are typically removed from the products by using hydrogen and applying similar processes and catalysts as applied to oil refining (Krzack and Schmalfeld, 2008).
Description of underlying process principle
Direct coal liquefaction relies on catalytic conversion of coal into liquid hydrocarbons as the major product, with solid residues and gaseous side products. It is not the major coal conversion route, today, with only one commercial plant operating in Erdos, Inner Mongolia, China. This has an annual capacity of 1,080,000 t liquid products. It comprises a slurry-phase reactor where coal is suspended in oil, mixed with a powdery catalyst and split into hydrocarbons, thereby consuming additionally provided hydrogen. The reactions occurring during that conversion process are mainly exothermic. The complex coal molecule is split into shorter and lower weight molecules of liquid hydrocarbons. Hydrogen needs to be provided to the process to saturate split C-C bonds and to hydrogenate, isomerise and refine the products. Common operating temperature ranges between 450°C and 500°C, with the pressure for modern applications typically in the range 14–19 MPa.
Major influencing factors on the product yield are the catalyst, temperature, total pressure, specifics of the oil used for suspension of the coal, residence time, partial pressures and reaction principle (slurry or gas phase hydrogenation). Products obtained from the process include a heavy oil fraction mainly composed of asphaltenes (molar weight of >500 kg/kmol) and a lighter fraction as the major product fraction (~250 kg/kmol) (Krzack and Schmalfeld, 2008).
Significant differences compared to early technology variants are the improved separation processes for segregation of solid residues from the liquid product phases, the staged refining of the product oil, enabling higher oil yield and quality, and gasification of the spent catalyst with the carbon containing residue by applying modern solids gasification technology (entrained-flow gasification) working at high pressure (up to 4–8 MPa) to satisfy the hydrogen demand of the process (Wanzl and Schmalfeld, 2008).
Within this process, the first stage is the slurry-phase conversion where the powder-grained coal (<0.1 nm) is mixed with a disposable catalyst and oil forming the slurry with a solids content of 40–45 wt%. The oil serves both as a suspension agent and a means for improved exchange and transport of hydrogen. It mainly consists of the heavier product fraction yielded during hydrogenation, which is then recycled for slurry preparation after separation of the solids. The amount of hydrogen added to the process is in the range of 7–10 wt% compared to the input of coal on a dry- and ash-free basis. The applied catalyst must be resistant towards sulphur and low cost because it cannot be recovered from the solid phase that also consists of residual coal and ash. Hence it will be discharged from the process with the other solids and fed into the gasification stage, where the residual carbon is used for hydrogen production. About 2–3% of catalyst is mixed into the slurry compared to the input of coal on a dry- and ash-free basis. Typical catalysts for slurry-phase hydrogenation are mixtures of iron oxides (Wanzl and Schmalfeld, 2008; Krzack and Schmalfeld, 2008).
The second stage is the separation of light and heavy oil phases and separation of solids from the heavy oil phase. The solids contain residual, unconverted coal, ash and the spent catalyst. Because of mixing with the catalyst, the coal ash content should not exceed a certain limit to reduce the mineral matter content fed into the gasifier. For example, a hard coal was mechanically separated to achieve ash contents not higher than 5 wt%.
The third stage is the adjustment of hydrocarbon composition, for example, iso-alkanes and aromatics content, and removal of hetero-atoms from the liquid products. This comprises gas-phase hydrogenation and refining of the product oil, which will become gaseous if the pressure is reduced but the temperature is kept high. The catalyst is a solid material, either arranged in a fixed bed or as monolithic component. In contrast to the disposable, rather inexpensive, catalyst applied to the slurryphase stage, higher-quality catalysts are used during refining, often consisting of molybdenum or tungsten sulphide. (Krzack and Schmalfeld, 2008).
A summary of different process developments is provided in Table 16, while a process layout of a direct coal liquefaction plant according to the ‘Deutsche Technologie’ approach is presented in Figure 29.
Figure 29 Common process schematic of a direct coal liquefaction plant according to the ‘Deutsche Technologie’ approach (Wanzl and Schmalfeld, 2008)
Major effort has been put on the development of advanced product treatment processes. For example, the DT – German Technology is characterised by a comprehensive refining initiated by cooling of the reactor product (to preheat the inlet stream). The cold product stream is sent to the cold separator where the syncrude is obtained. A warm side stream from the cooler is passed to a first refining reactor and the heavy residue after separation is sent to the gasifier whereas the light liquids are further refined in a second reactor and the heavier liquids from the first refining stage are recycled to the slurry-phase for suspension of the feed coal and the catalyst. The light products are sent to the cooling stage and recovered in the syncrude stream. Figure 30 shows an example process chain for a product treatment.
Figure 30 Example flow scheme of a direct coal liquefaction product treatment section (Wanzl and Schmalfeld, 2008)
Efficiency and environmental performance
As noted previously, major criteria for evaluating the environmental and energetic performance include specific product yields and energetic efficiency, CO2 emission, water consumption, emission or handling/treatment of gaseous, solid and liquid pollutants. A comprehensive review of performance data and comparison of different coal liquefaction routes was performed by Couch (2008).
The energetic efficiency of direct coal liquefaction is some 57–58%, with significantly lower carbon emissions reported compared to indirect liquefaction routes. Total carbon emissions including CO2 and other minor carbon losses along the process chain (for example, residual carbon in the slag or with purge gases) are reported at 23–25 kg/GJ product. An important parameter is the specific water consumption. For direct liquefaction, the vast majority of the fresh water (about 70%) is used as makeup for losses from cooling towers. About 8% can be assigned to boiler feed water while the remainder is mainly used as process water for providing the hydrogen by gasification and gas conditioning. The minimum consumption for a subbituminous coal is about 6.1 litres of water per litre of oil product. Other process emissions like waste water, off-gases etc can be controlled by application of suitable environmental technologies.
The synthetic fuel has superior combustion and emissions characteristics compared to conventional oil-based fuels.
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
