In addition to the Illinois case study described above, the Williams Group collaboration with colleagues at Politecnico di Milano has involved an investigation of the new Shell dry-feed partial water quench gasifier in an IGCC-CCS application and development of a new methodology for optimizing heat integration for power generation via steam cycles that is likely to be especially important in evolving optimal designs for polygeneration plants.
Shell IGCC+CCS: Partial Water Quench vs. Standard Syngas Cooling
Numerous studies indicate that bituminous coal-based electric power with CCS is significantly less expensive using integrated gasification combined cycles (IGCC) instead of standard pulverized coal (PC) steam electric plants. However, for lower rank sub-bituminous coals and lignites which comprise fully half of the world’s coal reserves, the advantages of gasification are much less clear. This year Tom Kreutz, in collaboration with Emanuele Martelli and Stefano Consonni at Politecnico di Milano, completed a techno-economic analysis of an idea recently patented by Shell: combining a dry-feed gasifier with a partial water quench of the raw synthesis gas.
The Shell coal gasification process (SCGP) is of particular interest because, as opposed to coal-water slurry gasifiers, it is able to convert coals of all rank into electricity and other energy carriers. However, the SCGP typically employs costly high-temperature heat exchangers to cool down the raw syngas by generating high-pressure steam prior to syngas cleaning and chemical processing. The SCGP offers significantly increased plant efficiency in traditional plants that vent CO2, but syngas coolers are not well matched to plants with carbon capture. A relatively moist syngas is required to promote the water-gas shift (WGS) reaction and achieve high levels of carbon capture; in a standard Shell IGCC+CCS design, most of the steam generated in the costly syngas coolers is needed for syngas humidification. Shell’s new partial water quench design cools the hot raw syngas by direct water injection, both humidifying the syngas and eliminating the costly syngas coolers.
In order to quantify the tradeoff between reduced efficiency and lower capital cost associated with the partial water quench, Kreutz and Politecnico di Milano colleagues completed a techno-economic comparison in the context of bituminous coal-fed Shell IGCC+CCS. Their detailed thermodynamic modeling indicates that the efficiency penalty caused by the partial water quench is relatively small, ~1 percentage point in LHV efficiency. On the other hand, the plant capital cost falls by 4-14%, and the levelized cost of electricity drops between 2.5 and 9%. In short, the partial water quench appears to be a promising strategy for use of dry feed gasifiers in plants with CO2 capture.
Optimization of Integrated Steam Cycles
In a gasification system with co-production of fuels and electricity, the steam cycle is fed by a large number of inputs (heat recovery from various sources at different temperatures) and may supply a large number of auxiliary processes. As part of his Ph.D. thesis at Politecnico di Milano, colleague Emanuele Martelli has developed a new methodology to optimize the configuration and the operating parameters of complex steam cycles. The novel and most relevant feature of the method is its capability to identify the optimal configuration of the heat exchanger network as part of the solution of the optimization algorithm, rather than pre-determining the configuration ahead of optimizing the operating parameters. The original linear model, created in Microsoft Excel, has been extended to allow for automatic optimization of the steam cycle pressure levels, requiring significantly more complex non-linear optimization techniques; the code has been re-written in both MATLAB and a dedicated package for solving sets of non-linear equations. The methodology has been applied to a number of studies, both at Politecnico and at Princeton by Tom Kreutz, and has proven to be a robust design tool, especially in complex polygeneration plants with high levels of heat integration.
Separation of CO2 from Syngas via Phase Change
The Williams Group has also continued to assess the performance (in terms of power consumption and fraction of CO2 captured) of cryogenic pre-combustion capture in IGCCs, deepening the analysis on the “external refrigeration scheme” developed in the CMI framework during 2007. Following interactions with BP Alternative Energy group, new schemes with propane/butane as the working fluid have been considered as substitutes for the ammonia chiller originally proposed. Also, an ammonia refrigeration scheme without de-superheating at compressor discharge has been investigated. Preliminary results show that the external refrigeration scheme already considered in 2007 offers slight margins for improvements.