Prior work has established the thermodynamic viability of the CHAMP-SORB concept to achieve 90% fuel conversion at 400 °C and 2:1 steam to carbon ratio, as well as demonstrated the performance enhancements enabled by incorporation of CO₂ and H₂ removal in the absence of transport-limitations using the bench-scale reactor prototype. However, as the reactor is scaled up to a realistic production size, consideration of heat and mass transfer effects become necessary to develop the strategy for a practical design of the CHAMP-SORB reactor.

In this paper we present a comprehensive transport-reaction model, which carefully considers all relevant heat and mass transfer processes and their interplay with reactions and separation in application to the CHAMP-SORB for low-temperature steam reforming of methane. The model, which includes thermal effects of reaction and sorption and incorporates the Maxwell-Stefan description of diffusion for multispecies mass transfer, is applied to the most important, H₂ producing, compression step of the CHAMP-SORB cycle.

—Anderson et al. (2017)

The CHAMP-SORB device operates at temperatures much lower than conventional steam reforming processes, consumes substantially less water and could also operate on other fuels such as methanol or bio-derived feedstock. It also captures and concentrates carbon dioxide emissions, a by-product that now lacks a secondary use—although that could change in the future.

The reactor operates at only a few cycles per minute—or more slowly—depending on the reactor scale and required rate of hydrogen production.

Key to the reaction process is the variable volume provided by the piston rising and falling in a cylinder. As with a conventional engine, a valve controls the flow of gases into and out of the reactor as the piston moves up and down. The four-stroke system works like this:

• Natural gas (methane) and steam are drawn into the reaction cylinder through a valve as the piston inside is lowered. The valve closes once the piston reaches the bottom of the cylinder.

• The piston rises into the cylinder, compressing the steam and methane as the reactor is heated. Once it reaches approximately 400 ˚C, catalytic reactions take place inside the reactor, forming hydrogen and carbon dioxide. The hydrogen exits through a selective membrane, and the pressurized carbon dioxide is adsorbed by the sorbent material, which is mixed with the catalyst.

• Once the hydrogen has exited the reactor and carbon dioxide is tied up in the sorbent, the piston is lowered, reducing the volume (and pressure) in the cylinder. The carbon dioxide is released from the sorbent into the cylinder.

• The piston is again moved up into the chamber and the valve opens, expelling the concentrated carbon dioxide and clearing the reactor for the start of a new cycle.

Schematic of CHAMP-SORB reactor cycle. The reactor utilizes four strokes per cycle: (a) retracting piston to fill the reactor, (b) extending piston to produce H2 via SMR at constant pressure and then opening valve to exhaust products, (c) retracting piston to desorb CO2, and (d) extending piston to desorb and produce a purified CO2 as the final product. Credit: ACS, Anderson et al. (2017). Click to enlarge.

All of the pieces of the puzzle have come together. The challenges ahead are primarily economic in nature. Our next step would be to build a pilot-scale CHAMP reactor.

—Andrei G. Fedorov, a professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering

Fedorov’s lab first carried out thermodynamic calculations suggesting that the four-stroke process could be modified to produce hydrogen in relatively small amounts where it would be used. The goals of the research were to create a modular reforming process that could operate at between 400 and 500 degrees Celsius, use just two molecules of water for every molecule of methane to produce four hydrogen molecules, be able to scale down to meet the specific needs, and capture the resulting carbon dioxide for potential utilization or sequestration.

We wanted to completely rethink how we designed reactor systems. To gain the kind of efficiency we needed, we realized we’d need to dynamically change the volume of the reactor vessel. We looked at existing mechanical systems that could do this, and realized that this capability could be found in a system that has had more than a century of improvements: the internal combustion engine.

—Andrei G. Fedorov

The CHAMP system could be scaled up or down to produce the hundreds of kilograms of hydrogen per day required for a typical automotive refueling station, or a few kilograms for an individual vehicle or residential fuel cell, Fedorov said. The volume and piston speed in the CHAMP reactor can be adjusted to meet hydrogen demands while matching the requirements for the carbon dioxide sorbent regeneration and separation efficiency of the hydrogen membrane. In practical use, multiple reactors would likely be operated together to produce a continuous stream of hydrogen at a desired production level.

We took the conventional chemical processing plant and created an analog using the magnificent machinery of the internal combustion engine. The reactor is scalable and modular, so you could have one module or a hundred of modules depending on how much hydrogen you needed. The processes for reforming fuel, purifying hydrogen and capturing carbon dioxide emission are all combined into one compact system.

—Andrei G. Fedorov

This publication is based on work supported by the National Science Foundation (NSF) CBET award 0928716, which was funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5), and by award 61220 of the U.S. Civilian Research & Development Foundation (CRDF Global) and by the National Science Foundation under Cooperative Agreement OISE- 9531011. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of NSF or CRDF Global. Graduate work of David M. Anderson, the first author on the paper, was conducted with government support under an award by the DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a.

Resources

• David M. Anderson, Thomas M. Yun, Peter A. Kottke and Andrei G. Fedorov (2017) “Comprehensive Analysis of Sorption Enhanced Steam Methane Reforming in a Variable Volume Membrane Reactor,” Industrial & Engineering Chemistry Research doi: 10.1021/acs.iecr.6b04392

• Anderson, D. M.; Kottke, P. A.; Fedorov, A. G. (2014) “Thermodynamic Analysis of Hydrogen Production Via Sorption-Enhanced Steam Methane Reforming in a New Class of Variable Volume Batch-Membrane Reactor” Int. J. Hydrogen Energy 39, 17985 doi: 10.1016/j.ijhydene.2014.03.127