Getting ready to demo “warm” syngas cleanup at Polk IGCC

Among the priorities for IGCC (integrated gasification combined cycle) technology if it is ever to realise its potential as a clean coal technology must be to reduce costs and complexity. Warm syngas cleanup technology - shortly to be demonstrated on a 50 MWe equivalent syngas slipstream from the Polk 1 IGCC plant in the USA, under a $168.8 million US Department of Energy funded programme - could make a significant contribution.

The DOE explains that its interest in this project stems from recognition that in a gasification plant the gas cleanup steps have a big impact on plant economics. Raw syngas leaving the gasifier is at high temperature, but conventional gas cleaning technologies typically operate at low temperatures, scrubbing the syngas using chemical or physical solvents. (Rectisol and Selexol are examples of the latter, with operating temperatures of about -40 to -80°F and 30 to 100°F, respectively.)

The cooling equipment required and the need to reheat the syngas before its use in a combustion turbine result in economic and thermodynamic penalties that decrease the efficiency of a gasification plant, the DOE notes. Therefore 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 what lies behind its interest in warm gas cleanup, the DOE says.

The focus of most high temperature syngas cleanup has been the removal of particulates, sulphur, chloride, and alkalis, of which particulate removal (via devices such as candle filters) and sulphur removal have been commercially demonstrated, the DOE observes.

Early efforts to develop hot gas cleanup systems

Preliminary efforts to develop hot gas cleanup systems focused on a maximum operating temperature of about 1000°F in order 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, sulphur, and alkali compounds could be removed to very low levels, serious material durability problems were encountered and it was also difficult to remove the other contaminants, including ammonia, chlorides, and mercury, DOE notes.

DOE recognised many of these issues and challenges, and the potential for overcoming them by operating gas cleanup at a lower temperature, around 700-800°F or less - so-called warm gas cleanup (WGCU) - in order to minimise equipment and material handling problems.

In 2010 RTI (Research Triangle Institute) was selected by DOE to scale-up its warm syngas desulphurisation technology, which employs a ZnO based sorbent, for an installation in a demo facility at Tampa Electric's Polk IGCC plant (as part of a US national research effort to produce cleaner energy from domestic coal sources). This facility is now in the final stages of commissioning, with actual syngas feed from Polk going into it very soon.

RTI's technology removes sulphur and heavy metals at "warm" process temperatures, "eliminating the need for substantial syngas cooling and expensive heat recovery systems", which "would significantly increase the thermal efficiency and reduce the capital and operating costs of new gasification-based systems when compared to conventional process technologies", the company says, with perhaps 3% improvement in thermal efficiency (HHV).

The pre-commercial demonstration facility at Polk will use about 20% of the raw syngas from the 250 MW Polk IGCC plant, which is fuelled with coal and petcoke, as its input feed stream. It is expected to remove more than 99.9% of the sulphur contaminants from the raw syngas at gasifier pressure and warm process temperature, says RTI.

The RTI system also includes a water-gas shift reactor to enrich the hydrogen content of the cleaned syngas and will demonstrate an advanced activated amine process (employing BASF's "aMDEA" as scrubbing medium) for capture of more than 90% of the carbon dioxide from the syngas stream. Following clean-up, the hydrogen-enriched syngas will be re-introduced to the Polk plant and combusted in the existing syngas turbine.

In addition to the IGCC plant owner/operator Tampa Electric, RTI's technology partners in the project include BASF, Clariant Corp, and Eastman Chemical Company. The DOE funding is being provided under the American Recovery and Reinvestment Act.

The overall project is managed by RTI and the EPC contractor was AMEC Kamtech. CH2M Hill served as owner's engineer for RTI, while Clariant manufactured the sulphur sorbent and the catalysts needed for the project in its Louisville, KY, facility.

The demonstration facility is expected to be operated until the end of June 2015 under the existing DOE agreement, generating the data needed to reduce design and scale-up risks for a commercial-scale plant.

RTI's warm syngas desulphurisation technology was previously successfully tested at pilot scale on actual syngas for more than 3000 hours at Eastman Chemical Company's coal gasification facility in Kingsport, TN.

RTI is now in the process of identifying an industrial partner for commercialisation of the technology.

Programme objectives

DOE has summarised the specific objectives of its joint programme with RTI as follows:

• Development of a warm multi-contaminant syngas cleaning system for operation between 300 and 700°F, with bulk contaminant removal stage and a polishing removal stage.
• Reduce the hydrogen sulphide and carbonyl sulphide (COS) to less than 5 ppmv using a regenerable solid sorbent (eg, RTI's RTI-3 sorbent) in the bulk removal stage.
• Reduce the HCl to less than 5 ppmv with the use of disposable sodium bicarbonate (nahcolite) sorbent in the bulk stage.
• Reduce As and Se using the regenerable RTI-3 sorbent.
• Reduce S 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 ZnO-based sorbent can reduce sulphur 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, DOE reports.

Preliminary assessments of RTI's technology for IGCC applications suggest that it compares favourably with "acid gas removal (AGR) technology" (which is what the IGCC community tends to call conventional low temperature syngas desulphurisation systems), with RTI warm gas cleanup plants about 5% less expensive to build, leading to a reduction in cost of electricity produced of 5-10%, according to DOE.

Estimates have suggested that RTI warm syngas cleanup coupled with CO2 capture might be 15-30% cheaper than conventional syngas cleanup paired with CO2 capture, DOE says.