(98d) Decentralized Production of Hydrogen for Residential PEM Fuel Cells From Piped Natural Gas by Low Temperature Steam-Methane Reforming Using Sorption Enhanced Reaction Concept

Decentralized residential power generation employing a H₂  PEM fuel cell requires that essentially CO free H₂ be produced on site by catalytic steam reforming of piped natural gas and then purifying the product H₂ (removal of bulk CO₂ and dilute or trace CO impurities). Currently, it may be achieved by subjecting the reformed gas to water gas shift reaction followed by (a) removal of all CO_x impurities by a PSA process or (b) selective oxidation in a catalytic PROX reactor to reduce only the CO impurity below ~ 10 ppm for use in the fuel cell. The latter approach assumes that the detrimental effect of CO₂ on the performance of the fuel cell is negligible - an assumption which may not be valid.

A recently developed thermal swing sorption enhanced reaction (TSSER) process scheme can be used to combine natural gas reformation, shifting, and purification of reaction products in a compact, single unit operation for direct production of fuel-cell grade H₂. The process circumvents the thermodynamic limitations (H₂ conversion and purity) of the SMR reaction and allows direct production of fuel cell grade H₂ with high recovery, while carrying out the SMR reaction at a much lower temperature (~ 590 C) than that required by   a conventional reformation process (~850 C).  The concept uses an admixture of a reversible CO₂ chemisorbent  (potassium  carbonate promoted hydrotalcite) and a SMR catalyst in a packed-bed sorber- reactor which is periodically regenerated by using the principles of thermal swing chemisorption using super-heated steam as a purge gas. 

The CO₂ sorption and desorption characteristics of the chemisorbent  are reviewed and  simulated performance of the TSSER process for supplying H₂ to a 250 KW fuel cell unit is described. A first pass process design and cost are reported.