INVESTIGATIONS OF FOULING AND SLAGGING IN HIGH TEMPERATURE GASIFICATION AND USE OF THAT INFORMATION TO MINIMIZE THOSE TENDENCIES

摘要

High temperature slagging gasifiers are used to convert a carbon feedstock; typically coal, petcoke, and/or biomass; into the primary desired product of CO and H_2 (called synthesis gas or, shortened to syngas); creating process by-products of carbon char and organic-metallic or mineral impurities. Coal averages about 10 wt pct ash and petcoke about 1 wt pct ash, although values can be higher/lower depending on the source of the feedstock and, in the case of coal, consistency in its geology, mining, and beneficiation. Peak temperatures in slagging gasifiers can range from about 1350-1575°C, creating an environment where ash becomes molten and can remain as individual particles or coalesce to form liquid slag that flows down the gasifier refractory sidewall. Because of the short residence time during gasification (seconds vs minutes) and the physical size and particle size distribution of a carbon feedstock, the gasifier environment is considered as metastable or non-equilibrium. The process by design produces excess carbon as char (partially processed carbon feedstock). As ash particles melt and start to coalesce, they move toward a global thermodynamic equilibrium involving gas, solid, and liquid phases that is dependent on what non-carbon content material exists in the coal. The gas flow rate, feedstock chemistry, gasification temperature, and oxygen partial pressure determine the amount of each phase created. Minerals that contain chlorine or sulfur can also oxidize; forming gases, liquid phases, or particulate that can contribute to deposits or react with material surfaces. Regardless of the source, ash formed from carbon feedstock materials contribute to agglomeration, fouling, or slag issues, the most serious being: 1) poor slag flow cause by high viscosity slag (typically at the exist of the gasifier), or 2) downstream fouling in areas like syngas coolers caused by particulate buildup that require periodic cleaning and that impact overall system efficiency. Fouling and slagging issues are also important in new gasification system designs, such as the Radically Engineered Modular Systems under study by NETL, which will require accurate process control over a wide range of temperatures. In these gasification systems, the potential for agglomeration, fouling, or slag formation at many stages in not known. The causes of fouling and slag buildup can be broken down into areas such as the feedstock material, the processed carbon particle size and size distribution, additives made to the carbon feedstock, the gasifier type, how a carbon feedstock is introduced in a gasifier, the temperature and oxygen partial pressure of gasification, and how the syngas and by-products are removed from a gasifier. Several analytical tools are available to study fouling and slagging - information which can be used to control the gasification environment and minimize those tendencies. This paper will discuss how knowledge of ash chemistry; information from analytical tests and microstructure studies (ash fusion, high temperature controlled atmosphere confocal microscopy, high temperature viscosity, controll atmosphere high temperature thermogravimetric analysis, and thermodynamic calculations) can be used to minimize or control agglomeration, fouling, or disruptive slag tendencies.