Warm Gas Cleanup
In any gasification process, the production of clean synthesis gas (syngas)—free of contaminants such as particulates, sulfur, ammonia, chlorides, mercury and other trace metals, and possibly carbon dioxide—is crucial to final product quality, to protecting downstream units such as gas turbines, catalytic reactors, and fuel cells, and to ensuring low environmental emission levels. Therefore, gas cleanup steps are indispensible, but do have a sizeable impact on plant economics, as they can account for a substantial portion of the overall capital cost and operational costs. Although raw syngas leaving the gasifier is at high temperature, conventional gas cleaning is typically carried out at low temperature by scrubbing the syngas using chemical or physical solvents (these require cooling the gas to typically below 100°F). The cooling equipment required, and the need to reheat the syngas before making use of it in a combustion turbine or synthesis reactor, result in economic and thermodynamic penalties that decrease the efficiency of a gasification plant. Accordingly, gas cleanup that would operate at higher temperature while still removing multiple contaminants would provide a significant efficiency improvement in gasification-based processes. This is behind DOE's interest in warm gas cleanup.
Challenges of High-Temperature Gas Cleanup
The extremely heterogeneous nature of coal and other carbonaceous feedstocks used to produce syngas by gasification presents a complex and technically challenging situation for any comprehensive syngas cleaning system. These challenges include:
- Multiple contaminants at different concentrations
- Different product requirements for various syngas utilization processes
- Simultaneous removal of multiple contaminants including trace elements
- Process, materials and equipment handling issues
The focus of most high temperature syngas cleanup has been the removal of particulates, sulfur, chloride, and alkalis, of which particulate removal (via devices such as candle filters) and sulfur removal have been commercially demonstrated. Preliminary efforts of the so-called "hot gas" cleanup focused on a maximum operating temperature of about 1000°F so that the alkalis in the hot syngas could be condensed out on the particulates so as not to cause corrosion problems with downstream equipment. Substantial syngas cooling was still required. Although particulates, sulfur, and alkali compounds could be removed to very low levels, serious material durability problems were encountered. It was also difficult to remove the other contaminants, including ammonia, chlorides, and mercury. DOE recognized many of these issues and challenges, and the desire to overcome them by operating gas cleanup at a lower temperature of 700-800°F – so-called warm gas cleanup (WGCU) - to minimize equipment and material handling problems. A more comprehensive multi-component syngas cleanup system is also being developed.
Research Triangle Institute (RTI), under a cooperative agreement with DOE, is developing WGCU technology. The program has an objective of developing a warm multi-contaminant syngas cleaning system for operation between 300 and 700°F. This system will be composed of a bulk contaminant removal stage and a polishing removal stage. Specific goals are to:
- Reduce the hydrogen sulfide (H2S) and carbonyl sulfide (COS) to less than 5 parts per million by volume (ppmv) using a regenerable solid sorbent (e.g., RTI's RTI-3 sorbent) in a bulk removal stage
- Reduce the hydrochloric acid (HCl) to less than 5 ppmv with the use of disposable sodium bicarbonate (nahcolite) sorbent in a bulk stage
- Reduce Arsenic (As), selenium (Se) using the regenerable RTI-3 sorbent
- Reduce sulfur species and HCl to less than 50 ppbv and less than 800 ppbv, respectively, in the polishing stage
RTI has demonstrated that its attrition-resistant RTI-3 zinc oxide-based sorbent can reduce sulfur gas concentrations to less than 1 ppmv in syngas at 650°F and 300 to 600 psig. The RTI-3 sorbent is regenerable at 1230-1300°F for numerous absorption cycles.
The technology, along with RTI's proprietary direct sulfur recovery process, was pilot tested at Eastman Chemical Company's Kingsport, Tennessee coal gasification facility.
The project team performed preliminary techno-economic assessment of RTI's WGCU technology for IGCC application, and found that WGCU compared favorably against conventional acid gas removal (AGR) technology; WGCU plants were about 5% less expensive to build and reduced the cost of electricity produced by 5-10%.
In October 2012, RTI announced that a pre-commercial, 30 MW WGCU unit is under construction at the 250 MW IGCC unit at Polk Power/Tampa Electric in Florida; this is supported by a $170 million grant from the U.S. DOE for design, construction and operation of the WGCU technology at this much larger scale. The 30 MW unit is to be built by early 2014, with technology demonstration tests to be completed by July 2015. The tests aim to establish reliability, availability and maintainability (RAM) targets, and verify capital and operating costs. In this latest configuration, RTI is relying on BASF’s "aMDEA" amine capture to perform 90% carbon capture as well. In latest studies, RTI estimates that earlier assessments were conservative; current estimates put the RTI WGC coupled with CO2 capture at 15-30% cheaper than conventional syngas cleanup paired with CO2 capture.1
1. Hart Energy Gasification News, December 4, 2012 Volume XV Issue 242012.
Syngas Clean Up