Modeling the Performance and Emissions of Integrated Gasification Combined Cycle based Lurgi Ammonia Synthesis System
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2002-01-14
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To evaluate the risks and potential pay-offs of a new technology, the Integrated Gasification Combined Cycle (IGCC), a systematic approach for assessment needs to be developed. Characterization of the performance and emissions of the technology needs to be made comparable to conventional and other advanced alternatives. The current study deals with the development of models for estimating energy consumption and emission of a polygeneration IGCC based Lurgi ammonia synthesis process.Polygeneration IGCC is a multipurpose technology for waste control and co-production of energy and chemicals. The processes involved in IGCC include partial oxidation of carbonaceous materials to produce a synthesis gas (syngas) containing CO, H2 and hydrocarbons, for example, methane. After gas cooling and cleaning, the purified syngas can be used to produce chemicals such as methanol, ammonia, hydrogen etc., or drive a gas turbine to generate electric power after further gas saturation. The high temperature and pressure steam recovered from the system can also produce electric power, and the tailgas recover from desulfurization can be treated in Claus plant to produce sulfur. This research modeled the Lurgi ammonia synthesis process that will be integrated with a British Gas/Lurgi (BGL) Slagging gasifer-based IGCC system in ASPEN Plus, which is a powerful and versatile unit process simulation software. The Lurgi ammonia synthesis technology is based on partial oxidation of solid feedstock such as coal, or solid waste if incorporated with a more robust gasifier design. The liquid nitrogen wash, Rectisol process and recycle gas reforming and shifting make the Lurgi ammonia system capable to process different types of incoming syngas with significant amount of impurities. The ASPEN Plus performance model calculates mass and energy balances for the entire ammonia system. For validation, the model was calibrated to the best available reference. After setting up the design basis of Lurgi ammonia synthesis system, a case study was performed based on 1000 lbmol clean syngas input for ammonia synthesis. The clean syngas was produced from gasification on American Waste Fuel, which is an American 75/25 percent mixture of Refuse Derived Fuel (RDF) and Pittsburgh No. 8 Bituminous coal. Based on the results from the case study, prediction on the performance and emissions of the ammonia model integrated with the IGCC system was performed, and the total power consumption of the base system was compared with the reference data for model verification. Then, sensitivity analyses on properties of incoming syngas, hydrogen to nitrogen ratio, purge gas recycle ratio and flow rate of incoming syngas were performed on the calibrated base case model to identify the key variables affecting the system-wide performance and emissions significantly and how. These sensitivity analyses can also be used to test the robustness of the model, which is critical for integration with the IGCC system. Based on the pre-assessment of the performance of the base case ammonia model integrated with the IGCC system, an IGCC system with about 447429 lb/hr of American Waste Fuel input can at least support an ammonia plant with 31979 lb/hr of ammonia production. But the typical yield of ammonia that can be supported by a calibrated IGCC system firing 287775 lb/hr of Pittsburgh No. 8 coal and without methanol production is about 1700 short tons/day. Through the sensitivity analyses, Operation near the stoichiometric point of the ammonia synthesis reaction was found to be the best choice from the perspective of electric energy consumption. Higher purge gas recycle ratios can better the conversion of ammonia, and reduce electric power, net steam consumption and emissions. Different feedstock had no obvious effect on the total electric power consumption of this ammonia system, but the composition of CO, CH4 and sulfur influence the steam consumption and emissions substantially. In order to make the model converge faster and more normally at different flow rates of incoming syngas, some of the design specification boundaries were changed. In the future, when the ammonia model is integrated with the IGCC system in ASPEN Plus, the fate of steam, purge gas, ammonia and sulfur emissions should be considered. In addition, a conventional ammonia model based on steam reforming of natural gas needs to be developed for Life Cycle Assessment, and Life Cycle Indexes should be compared between the conventional and Lurgi's ammonia synthesis processes to assess any advantages and disadvantages. Probability analysis methods such as Monte Carlo method or Orthogonal Latin Square experiment design should be introduced to optimize the model performanc, to identify which model parameters most affect performance and to quantify the uncertainty and variability associated with the model/
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MS
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Civil Engineering
