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Title: Liquid CO{sub 2}/Coal Slurry for Feeding Low Rank Coal to Gasifiers

This study investigates the practicality of using a liquid CO{sub 2}/coal slurry preparation and feed system for the E-Gas™ gasifier in an integrated gasification combined cycle (IGCC) electric power generation plant configuration. Liquid CO{sub 2} has several property differences from water that make it attractive for the coal slurries used in coal gasification-based power plants. First, the viscosity of liquid CO{sub 2} is much lower than water. This means it should take less energy to pump liquid CO{sub 2} through a pipe compared to water. This also means that a higher solids concentration can be fed to the gasifier, which should decrease the heat requirement needed to vaporize the slurry. Second, the heat of vaporization of liquid CO{sub 2} is about 80% lower than water. This means that less heat from the gasification reactions is needed to vaporize the slurry. This should result in less oxygen needed to achieve a given gasifier temperature. And third, the surface tension of liquid CO{sub 2} is about 2 orders of magnitude lower than water, which should result in finer atomization of the liquid CO{sub 2} slurry, faster reaction times between the oxygen and coal particles, and better carbon conversion at the same gasifiermore » temperature. EPRI and others have recognized the potential that liquid CO{sub 2} has in improving the performance of an IGCC plant and have previously conducted systemslevel analyses to evaluate this concept. These past studies have shown that a significant increase in IGCC performance can be achieved with liquid CO{sub 2} over water with certain gasifiers. Although these previous analyses had produced some positive results, they were still based on various assumptions for liquid CO{sub 2}/coal slurry properties. This low-rank coal study extends the existing knowledge base to evaluate the liquid CO{sub 2}/coal slurry concept on an E-Gas™-based IGCC plant with full 90% CO{sub 2} capture. The overall objective is to determine if this technology could be used to reduce the cost and improve the efficiency of IGCC plants. The study goes beyond the systems-level analyses and initial lab work that formed the bases of previous studies and includes the following tasks: performing laboratory tests to quantify slurry properties; developing an engineering design of a liquid CO{sub 2} slurry preparation and feed system; conducting a full IGCC plant techno-economic analysis for Powder River Basin (PRB) coal and North Dakota lignite in both water and liquid CO{sub 2} slurries; and identifying a technology development plan to continue the due diligence to conduct a comprehensive evaluation of this technology. The initial task included rheology tests and slurry data analyses that would increase the knowledge and understanding of maximum solids loading capability for both PRB and lignite. Higher coal concentrations have been verified in liquid CO{sub 2} over water slurries, and a coal concentration of 75% by weight in liquid CO{sub 2} has been estimated to be achievable in a commercial application. In addition, lower slurry viscosities have been verified in liquid CO{sub 2} at the same solids loading, where the liquid CO{sub 2}/coal slurry viscosity has been measured to be about a factor of 10 lower than the comparable water slurry and estimated to be less than 100 centipoise in a commercial application. In the following task, an engineering design of a liquid CO{sub 2}/coal slurry preparation and mixing system has been developed for both a batch and continuous system. The capital cost of the design has also been estimated so that it could be used in the economic analysis. An industry search and survey has been conducted to determine if essential components required to construct the feed system are available from commercial sources or if targeted R&D efforts are required. The search and survey concluded that commercial sources are available for selected components that comprise both the batch and continuous type systems. During normal operation, the fuel exits the bottom of the coal silo and is fed to a rod mill for grinding to the desired particle size. From the rod mill, the coal is transported in a dense phase pneumatic transport system to the top of a solids heat exchanger, wherein the ground coal is chilled to a low temperature (in the range of -23.3°C (-10°F)) prior to mixing with liquid CO{sub 2}. This temperature was selected based on evaluating trade-offs between refrigeration work and the cost of the system pressure boundary at various combinations of pressure and temperature that correspond to the gas/liquid phase boundary for CO{sub 2}. Electrical loads to drive the equipment comprising the liquid CO{sub 2} feed system are significantly greater than those for a water slurry system, and this effect has been captured in the technical performance analysis. In the next task, a plant-wide techno-economic analysis has been conducted for PRB coal and lignite in both liquid CO{sub 2} and water slurry feed. The IGCC cases using a liquid CO{sub 2} slurry system show reduced plant output and higher heat rate for PRB coal and for ND lignite at 90% CO{sub 2} capture. Some of these performance differences can be attributed to the higher requirement for steam for the liquid CO{sub 2} slurry cases to drive the water-gas shift reaction, thereby reducing steam turbine power generation. Other factors contributing to the calculated performance differences are the increase in parasitic loads attributable to refrigeration to produce liquid CO{sub 2} and chilled coal and the reduction in enthalpy of the inlet streams to the gasifier associated with the low temperature liquid CO{sub 2} slurry feed. The capital costs for the complete plant are slightly higher for the liquid CO{sub 2} slurry cases for PRB coal but somewhat reduced for ND lignite relative to the corresponding water slurry cases. Differences in dollar/kWe costs are higher for both coals due to the reduction in net output. The cost of electricity computed for the liquid CO{sub 2}/coal slurry cases is greater for both PRB and ND Lignite coals. It does not appear that there is any benefit to using liquid CO{sub 2}/coal slurries for feeding low rank coals to the E-Gas™ gasifier. Any incidental benefits in improved cold gas efficiency are more than compensated for in higher overall plant costs, increased complexity, and reduced power output and efficiency. The results of the study are compared with previous published analyses, and the differences in model assumptions, approach and basis are summarized. It has been concluded that the use of liquid CO{sub 2} may still prove to have a significant advantage in a different type of gasifier, i.e., single-stage entrained flow with radiant quench section, but some key questions remain unanswered that can validate the potential improvement of gasifier performance using liquid CO{sub 2} slurries. In order to provide a path to answering these questions, a technology development roadmap has been developed to resolve fundamental issues and to better define the operation aspects of using liquid CO{sub 2}/coal slurries. The fundamental issues could be resolved by conducting additional laboratory analyses consisting of: • A rheological test program to quantitatively evaluate slurry preparation and handling for liquid CO{sub 2} including experiments to evaluate preparation systems. • An experimental program on CO{sub 2}-assisted gasification in order to obtain the most relevant experimental data from drop tube furnace studies to aid in verifying the potential advantages of direct feed of liquid CO{sub 2}/coal as gasifier feedstocks. Quantifying the operational aspects of liquid CO{sub 2} slurries can best be achieved with: • An experimental program using a flow test loop to evaluate equipment performance and handling properties of liquid CO{sub 2}/coal slurries for gasifier feedstocks on a scale sufficient to predict full scale operating parameters. • Spray atomization studies necessary to evaluate the effect of atomization properties of liquid CO{sub 2}/coal slurries that could be significantly different than those of water/coal slurries.« less
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Resource Type:
Technical Report
Research Org:
Electric Power Research Institute, Incorporated
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Country of Publication:
United States