(103a) Modular Warm-Temperature Syngas Cleanup Technology – Development and Path Forward

Gasification is a mature technology that converts desired solid or liquid raw materials into synthesis gas (syngas) – a mixture of H₂, CO, CO₂, hydrocarbons, and several contaminants. The synthesis gas can be used as a building block and can be converted into several value-added products such as methanol, hydrogen, liquid fuels, power, or synthetic natural gas. The sulfur present in the feedstock is converted to a mixture of hydrogen sulfide (H₂S), carbon disulfide (CS₂) and carbonyl sulfide (COS) in the gasification environment and must be removed to avoid corrosion to downstream equipment and process units, and to meet emissions standards. Several technologies have been developed and commercialized to remove the sulfur contaminants. The most mature of these acid gas removal (AGR) processes – Rectisol, Selexol, aMDEA – operate efficiently at or below ambient temperature while almost all of the value-addition processes mentioned earlier operate at higher temperatures requiring the syngas to be first cooled for contaminant removal and then reheated for downstream processing resulting in lower energy efficiency and increased capital cost. Additionally, these AGR processes are most economical at large scale (downstream of gasifiers of 750 to 1,000 MWth capacity) and become expensive for small-scale applications such as gasification of opportunity feedstocks (biomass, waste, etc.), or electrification of remote locations (equivalent to 10 to 50 MWth capacity).

RTI’s Warm-syngas Desulfurization Process (WDP) is a temperature swing adsorption process that operates at temperatures in the range of 250-600°C and can be advantageously integrated with the gasifier and downstream processes without the need for syngas cooling and reheating which improves overall thermal efficiencies by up to 10%. The transport-reactor based design is ideal for both large- and small-scale applications as it results in a small-footprint system that is economic because it is constructed primarily of pipes. Successful pre-commercial demonstration of RTI’s transport-reactor based WDP design was achieved using a 50-MWe slipstream of syngas from the gasifier at Tampa Electric Company’s Polk Power Station. This technology is commercially offered by Casale SA.

Currently, RTI is developing a fixed-bed WDP process tailored for small-scale modular systems (1- to 5-MWe capacity), especially with low sulfur concentrations (<5000 ppmv). Many U.S. coals, and in particular western coals that account for over half of domestic coal production, have considerably lower total sulfur levels and such low-sulfur coals will represent an important potential market for modular systems. We anticipate similar cost benefits and efficiency improvement for the modular processes, as observed from the integration of WDP with the gasifier and downstream processes on large scale. At the envisioned small scale, fixed-bed processes become viable alternatives and will have many of the same simplified design aspects as our WDP transport-bed process. This presentation will summarize results from the fixed-bed sorbent extrudate synthesis and performance testing, and fixed-bed process development.

The goal of the sorbent extrudate development was to take the extremely effective sorbent chemistry of the fluidized form of WDP sorbent and create a formulation that is optimized for application in a fixed-bed system. Because the basic chemistry is initiated in the coprecipitation process, the wet cake obtained from the coprecipitation step is the starting point for the development of the fixed-bed sorbent. Parameters influencing extrudate physicochemical properties such as slurry preparation, spray drying conditions, additives, paste mixing, extrusion conditions, extrudate drying, and calcination conditions were investigated to achieve a balance between sorbent crush strength and porosity. A successful recipe of making extrudable paste and the process for making extrudates from wet cake was also developed. The extrudates synthesized using this method have shown crush strength ranging from 50 to 80 N/mm, which is comparable to the crush strength of commercial steam reforming catalyst and much higher than the crush strength of commercially available desulfurization sorbent extrudates (10 N/mm) and pellets (5.7 N/mm). Initial characterization data revealed that the prepared extrudates maintained the physicochemical properties of RTI’s fluidizable sorbent in terms of porosity, surface area, and sulfur pickup. Extensive parametric testing in microreactor systems using 1-5 g of sorbent has been conducted to develop the data set required to design and scale-up the fixed bed process. Currently, sorbent performance is being studied at a scale of 100-250 g in a bench-scale system over the range of process conditions such as gas velocity, process temperature, sulfur partial pressure etc. Using Aspen Adsorption modeling, sorbent performance data will be used to help develop and optimize the fixed-bed process, especially for low-sulfur syngas.