Design and Economical Evaluation of POLYGEN process to co-produce Synthetic Natural Gas (SNG) and Ammonia
Bor-Yih Yu and I-Lung Chien
Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
The steady state design and economical evaluation of poly-generation (POLYGEN) process to co-produce synthetic natural gas (SNG) and ammonia is studied in this work. POLYGEN has been a widely studied topic recently, in which several different chemicals could be produced parallel at the same time. One of the two products, SNG, holds very similar composition and heat value to typical natural gas, and can be used as a replacement in industrial and home usages. Another product, Ammonia, is one of the most important inorganic chemical over the world, which could be used as the precursor of various kinds of chemicals, as fertilizers, or as cleaning agent. In POLYGEN process, the relative production rates for different chemicals could be adjusted based on different market demands, daily usages, and also changing political strategies.
The items covered by this work are illustrated in the following. In coal-based POLYGEN processes, syngas is first generated from coal gasification, and the raw syngas is then passes the treating stage. In syngas treating section, the sulfur contents in raw syngas stream from gasification section is first removed. After that, the syngas is then split into two streams. One of them passes through water-gas-shift-reaction (WGSR) and CO2 removal section, and the other bypasses. After almost completely reaction, the one that passes WGSR and CO2 removal has high purity in H2. To meet the required H2/CO ratio for the feed into SNG plant, part of the syngas after treatment is mixed with the one that bypasses. After syngas treatment and composition adjustment, SNG and ammonia are produced individually by methanation and ammonia synthesis reaction, respectively. For ammonia synthesis, a novel Ru/C catalyst, which has higher activity than the traditional Fe-based catalyst at lower pressure, is used to catalyze the reaction. Besides, some waste heat could be recovered by generating steam, and the steam is used to drive turbine to produce electricity for usage inside the plant-wide process.
In our previous study (Yu and Chien, 2015)1, we illustrated that the SNG production price is lower than liquefied natural gas (LNG) importation price in Taiwan. The SNG production price is about 10.563 (USD/MMBTU) in a SNG-only plant. By POLYGEN process to co-produce SNG and ammonia, the SNG production cost could become even lower. If 20% of syngas is used to produce ammonia, the SNG production price will drop to 9.518 (USD/MMBTU), and if 40% for ammonia production, the SNG production price will further drop to 7.156 (USD/MMBTU). Thus, although the POLYGEN process leads to increasing total capital investment, it still has positive influences from the economic aspects. Besides, the flexibility of shifting the production rate of SNG or ammonia gives the odds for adapting the changes in market demand.
- Yu, B. Y; Chien, I. L. Design and Economic Evaluation of a Coal-to-Synthetic Natural Gas Process. Ind. Chem. Res. Res. 2015, 54, 2339-2352.