Description of underlying process principle
In principle, the preferred gasification process for the production of methane (that is synthetic natural gas, SNG) is fixed bed gasification because of the high methane yield already obtained from the syngas exiting the gasifier. That said, the use of entrained flow systems is also favoured because of other process advantages. The synthesis unit itself consists of three to four sequentially aligned adiabatically operated reactors. The operating pressure depends on the pipeline pressure after synthesis and typically ranges between 3 and 5 MPa. The reactors are filled with nickel-based catalysts with the nickel content varying by reactor dependent on the maximum temperature of some 973 K (about 45% for the 1st reactor and up to 55% for subsequent reactor stages).
CO + 3 H2 ↔ CH4 + H2O -220 kJ/kmol (700 K) (22)
CO2 + 4 H2 ↔ CH4 + 2 H2O -183 kJ/kmol (700 K) (23)
The temperature decreases with each reactor because of the lower amount of syngas to be converted within each one. The maximum reactor temperature can be limited by the steam or methane content of the feed gas or by recycling partially unconverted syngas into the first reactor.
The current established synthesis technology variants are provided by Lurgi and Haldor Topsøe, as shown in Figure 31. Both are characterised by 95–98% methane yield and highly integrated heat recovery systems since some 20% of the chemical heat of the syngas is released as sensitive heat by the exothermic reactions. High standards are required for the reactor material and the heat exchanger piping material because of the potential threat of metal dusting in a hydrogen-rich gas atmosphere between 770 and 1170 K.
Figure 31 Comparison of Lurgi and Haldor Topsøe SNG synthesis (Haldor Topsøe, 2015; Weiss and others, 2008)
Suitable feedstock range
The coal quality requirements for pre-treatment (milling and drying) of coal are strongly determined by the gasification process providing the syngas, which are based on entrained flow and fixed bed systems. Advantages of entrained-flow gasification with respect to SNG production include high single unit raw gas capacity and lower impact for waste water treatment. In particular, the high single unit capacity is advantageous considering the need for large amounts of syngas for commercial-scale SNG plants. In contrast, fixed bed gasification features lower single unit capacity and much higher impact for waste water treatment, especially for recovery of tar compounds. However, it provides a raw gas that is significantly better suited for SNG synthesis. The reasons for this are a thermodynamically implied much higher methane content of the raw gas and a higher H2/CO ratio reducing the need for CO conversion during gas purification. The significantly higher methane content of the raw gas allows for simplification of the synthesis loop as the temperature increase occurring in the first reactor is reduced, eliminating the need for partial recirculation of the gas exiting the first reactor for cooling purposes. The increased methane content also results in a reduced overall size of the synthesis unit as less syngas needs to be catalytically converted to methane.
At present, the technology is far from established in China (see main text) and it remains to be seen whether fixed bed gasification will prove to be the commercial preference, which will ultimately depend on whether it can meet the stringent environmental constraints.
Efficiency and environmental performance
As inferred above, whereas the efficiency of the gasification island is the same as for typical indirect coal liquefaction routes, the effort for gas purification differs depending on the gasification process and the specific synthesis. Generally, the energetic losses of the gas purification process chain increase with increasing amounts of CO to be converted for H2 enrichment. A major influence on the overall process chain efficiency can be attributed to the synthesis block. Because of the highly exothermic heat release during methanation, the efficiency strongly depends on the efficiency of heat recovery and heat integration. For conventional Lurgi-type or Haldor Topsøe’s TREMP™ processes, there is a need for maximum heat recovery as approximately 20% of the chemical heat inventory is released as sensitive heat during reaction. The majority of commercial plants apply high-pressure steam generation with superheating of the generated steam. This makes it possible to recover up to 93% of the released heat. Because of the large amount of heat and consequently generated steam, an efficient use of the steam, that is for electricity generation, needs to be ensured as the steam generation typically exceeds steam consumption along the process chain (Haldor Topsøe, 2015; Foster Wheeler, 2015).
Technical maturity and industrial applications
The main text of this report notes that the current market for CTSNG plants is in China, although very few are as yet in commercial operation due to various technical/environmental, regulatory and management reasons. Consequently, most projects remain at the early stage of development.
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
