The Williams Group’s collaboration with colleagues at Politecnico di Milano (primarily Stefano Consonni, Emanuele Martelli, and Giulia Fiorese) included finalizing an Illinois case study for the co-production of synfuels and electricity, exploration of novel approaches to syngas humidification, and modeling the production of substitute natural gas.
Illinois case study
The analysis reported in last year’s CMI Annual Report of a hypothetical Illinois case study of the coproduction of synfuels and electricity with carbon capture and storage from coal plus biomass (in the form of corn stover and mixed prairie grasses grown on degraded lands) was completed. An article based on that analysis was just published in Energy and Environmental Science.
Two novel methods of syngas humidification for CIGCCC-CCS via Shell gasifier
Numerous studies indicate that bituminous coal-based electric power with CCS is significantly less costly using integrated gasifier combined cycles (IGCC) instead of standard pulverized coal (PC) steam electric plants. However, for lower rank sub-bituminous coals and lignites which account for about half of world coal reserves, the advantages of gasification are much less clear.
The Shell coal gasification process (SCGP) is of particular interest because, having a dry coal feeder system, it is practical to convert coals of all rank into electricity and other energy carriers. However, the “standard” SCGP typically employs high-temperature heat exchangers to cool the raw syngas by generating high pressure steam prior to syngas cleaning and further processing. In traditional plants that vent CO2, this steam is used to generate more electricity, offsetting the cost of these “syngas coolers.” These dry feeding systems and syngas coolers are less well suited for plants that capture CO2, which requires a relatively moist syngas to promote the water-gas shift (WGS) reaction needed to achieve high levels of carbon capture. In a standard Shell CIGCC-CCS design, most of the steam generated in the syngas coolers is diverted to the WGS unit where it is used for syngas humidification. The Williams Group is investigating two novel plant designs which substantially reduce this parasitic loss.
The first approach investigated with Martelli and Consonni and presented at the 9th International Conference on Greenhouse Gas Technologies, is known as partial water quench cooling, an idea recently patented by Shell. The partial water quench cools the hot raw syngas by direct water injection, both humidifying the syngas and eliminating the costly syngas coolers. The second approach incorporates a novel design for the WGS section developed by visiting researcher Michiel Carbo (of ECN in the Netherlands). The team’s most recent work will be presented at the 2010 ASME Turbo Expo Conference in Glasgow, UK (June 14-18). A manuscript has been submitted to the peer-reviewed Journal of Engineering for Gas Turbines and Power that compares the thermodynamic performance and economics of three different Shell-based IGCC+CCS plants: 1) the “standard” configuration with high-temperature syngas coolers and traditional WGS unit design, 2) the partial water quench + traditional WGS, and 3) syngas coolers + the novel ECN WGS configuration. Our research indicates that, relative to the “standard” IGCC-CCS, the plant employing the partial water quench is simpler and less costly to build, but also less efficient; however, it offers the promise of significant economic benefits (0-7% lower cost of electricity). In contrast, the advanced ECN WGS offers increased efficiency, significantly reduced WGS-steam cycle integration, and slightly improved economics (1.5% lower cost of electricity).
Conversion of coal and/or biomass to substitute natural gas (SNG)
As an extension of their detailed analysis of the production of low-carbon synthetic liquid transportation fuels, the Williams Group has been investigating with Martelli and Consonni the prospects for low-carbon SNG that might be used for heating in buildings and industry as well as for NGCC power plants.
The team has designed in Aspen Plus a detailed Shell gasifier-based coal- and/or biomass-to-SNG conversion facility that utilizes Haldor-Topsoe’s TREMP methanation technology to produce pipeline-quality SNG that has a (highly constrained) Wobbe index and heating value that makes it suitable for fueling a natural gas combined cycle (NGCC) unit.
Plans are to compare the performances and economics for this “reference” SNG plant with one that employs a more novel approach: catalytic hydrogasification. Originally developed by Exxon in the 1970s and currently promoted by GreatPoint Energy, this approach involves the catalytic gasification of solids (especially low-rank coal and biomass) at a relatively low temperature (700°C) in a fluidized bed in the absence of O2, conditions that favor methane production.