Overview of Contaminant Removal From Coal-Derived Syngas

Gasification is an important strategy for increasing the utilization of abundant domestic coal reserves. DOE envisions increased use of gasification in the United States during the next 20 years. As such, the DOE Gasification Technologies Program, including the FutureGen initiative, will strive to approach a near-zero emissions goal, with respect to multiple pollutants, such as sulfur, mercury, and nitrogen oxides. Since nearly one-third of anthropogenic carbon dioxide emissions are produced by coal-powered generation facilities, conventional coal-burning power plants, and advanced power generation plants, such as IGCC, present opportunities in which carbon can be removed and then permanently stored. Gas cleaning systems for IGCC power generation facilities have been effectively demonstrated and used in commercial operations for many years. These systems can reduce sulfur, mercury, and other contaminants in synthesis gas produced by gasifiers to the lowest level achievable in coal-based energy systems. Currently, DOE Fossil Energy’s goals set for 2010 direct completion of R&D for advanced gasification combined cycle technology to produce electricity from coal at 45–50% plant efficiency. By 2012, completion of R&D to integrate this technology with carbon dioxide separation, capture, and sequestration into a zero-emissions configuration is targeted with a goal to provide electricity with less than a 10% increase in cost of electricity. By 2020, goals are set to develop zero-emissions plants that are fuel-flexible and capable of multi-product output and thermal efficiencies of over 60% with coal. These objectives dictate that it is essential to not only reduce contaminant emissions into the generated synthesis gas, but also to increase the process or system operating temperature to that of humid gas cleaning criteria conditions (150 to 370 °C), thus reducing the energy penalties that currently exist as a result of lowering process temperatures (−40 to 38 °C) with subsequent reheat to the required higher temperatures. From a historical perspective, the evolution of advanced syngas cleaning systems applied in IGCC and chemical and fuel synthesis plants has followed a path of configuring a series of individual cleaning steps, one for each syngas contaminant, each step controlled to its individual temperature and sorbent and catalyst needs. As the number of syngas contaminants of interest has increased (particulates, hydrogen sulfide, carbonyl sulfide, halides such as hydrogen chloride, ammonia, hydrogen cyanide, alkali metals, metal carbonyls, mercury, arsenic, selenium, and cadmium) and the degree of syngas cleaning has become more severe, the potential feasibility of advanced humid gas cleaning has diminished. A focus on multi-contaminant syngas cleaning is needed to enhance the potential cost savings, and performance of humid gas cleaning will focus on multi-contaminant syngas cleaning. Groups of several syngas contaminants to be removed simultaneously need to be considered, resulting in significant gas cleaning system intensification. Intensified, multi-contaminant cleaning processes need to be devised and their potential performance characteristics understood through small-scale testing, conceptual design evaluation, and scale-up assessment with integration into the power generation system. Results of a 1-year study undertaken by DOE/NETL are presented to define improved power plant configurations and technology for advanced multi-contaminant cleanup options.