Intensifying Syngas Production: Experimental Investigations and Process Analysis for Production of Inherently Separated Syngas Streams

The continuing growth in world population along with a rapidly rising standard of living in the developing world is fueling a continuously growing demand for energy and bulk chemicals. At the same time, ever increasing concerns regarding climate change urge a transformation in how we produce this energy and chemicals without dramatically increasing the energy and environmental footprint of both industries. In this context, the recent expansion of economically exploitable natural gas reserves—the “shale gas boom”—is both a huge opportunity and a major concern unless handled responsibly.

One emerging “clean energy” technology that enables fossil fuel utilization with minimal carbon footprint is chemical looping combustion (CLC). In CLC, an oxide—typically an oxidized transition metal—is used as an oxygen source for fuel combustion in a “reducer” reactor, generating a stream of CO₂ and H₂O, which can be easily separated via condensation to generate a sequestration-ready CO₂ stream. After combustion, the reduced metal is then regenerated with air in an “oxidizer” reactor and recycled back to the reducer, completing the materials “loop.” Thus, CLC bypasses most of the problems of conventional fuel combustion, including NO_x formation and CO₂ separation from dilute gaseous streams, and is currently emerging as a favored solution for large-scale clean combustion applications.

However, “chemical looping” is in fact a rather flexible platform technology for oxidation reactions well beyond combustion, including CO₂ utilization and natural gas upgrading to value-added chemicals [1-4]. Our own group has demonstrated the use of chemical looping for partial oxidation and reforming of methane to synthesis gas (a mixture of CO & H₂), the most complex and costly step in the (indirect) upgrading of methane to valuable chemicals. We have demonstrated that pure H₂ streams can be produced by replacing air as oxidant with H₂O, resulting in chemical looping steam reforming of methane (CLSR) [5], or with CO₂, enabling CO₂ activation via reduction to CO [6, 7]. Furthermore, by carefully adjusting the fuel conversion in the “reducer” reactor, syngas is possible via chemical looping partial oxidation of methane (CLPOM)[8,9] or chemical looping dry reforming (CLDR) [10], i.e. the chemical looping variants of the conventional methane partial oxidation and dry reforming processes.

The attractiveness of these chemical looping syngas production processes could be further increased by making the syngas ratio (the H₂:CO ratio in the product stream) adjustable, thus adding flexibility to the downstream use of the produced syngas—such as high purity H₂ for fuel cells or ammonia production, or pure CO streams for carbonylation reactions. We are demonstrating that such a flexible syngas production is indeed achievable via a modification of these looping processes that results in the production of fully separated H₂ and CO effluent streams. In this redesigned two-step process, CH₄ is first converted to a pure hydrogen stream and solid carbon via thermocatalytic cracking over a metal catalyst in a “cracker” reactor. The deposited carbon is then selectively oxidized in an oxidizer reactor using CO₂ as oxidant, resulting in the production of high-purity CO streams and regenerating the metal which is recycled back to the cracker reactor. The metal is thus periodically cycled between the two reactors carrying carbon between the two half steps, i.e. the “oxygen carrier” that characterizes chemical looping processes effectively acts like a “carbon” carrier between the two reaction half steps.

This process configuration thus combines thermocatalytic methane cracking, which has been studied extensively over the past several decades [11-13], with CO₂ activation, resulting in the formation of inherently separated syngas streams, while at the same time addressing some of the key shortcomings of conventional methane cracking, such as low CH₄ conversion, rapid catalyst deactivation, and large temperature excursions during carbon burn-off in the oxidation half cycle. The required cracking step can be carried out efficiently using ceria-supported Ni carriers. Utilizing a reducible support with strong metal support interactions (i.e. ceria) allows efficient production of carbon (and hydrogen) at elevated temperatures without rapid catalyst deactivation and thus high fuel conversion at high throughput, making the process well suited for scalable, decentralized application. Similarly, the deposited carbon can be effectively removed by reaction with either O₂ or CO₂ to regenerate the active Ni species. While the use of oxygen as oxidant is favorable from an energetic point of view, it limits the selectivity of the oxidation reaction and yields a mixed CO/CO₂ product stream. In contrast, using CO₂ as a “soft” oxidant not only allows for CO₂ activation via reduction to CO (enhancing the total CO yield from the process), but also improves the overall process selectivity by selectively oxidizing carbon while keeping Ni in its metallic state, which is active and selective for catalytic cracking. However, it also renders the process overall endothermal and hence requires the use of an additional heat source, either via combustion of additional methane or combustion of part of the product stream.

Based on our experimental results, we are finally evaluating various process configurations for this intensified, flexible syngas process using a systems-level analysis. The overall system is designed to enable a desired syngas production rate while maximizing yield and minimizing overall energy consumption and CO₂ emissions. The model involves a detailed chemical looping reactor model, incorporated into the systems-level process model [14]. Reactor-level mass and energy balance calculations are performed to calculate the effect of operating conditions such as flow rates, solids conversion, and temperature on the final product composition, selectivity and yields and solids inventory. The systems-level model includes thermodynamic calculations of individual process components such as blowers, compressors, and heat recovery equipment. Using sensitivity analysis on the process parameters, we will identify operating conditions to optimize process performance and compare results to conventional alternatives such as SMR and POX processes.

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