3.1. Steam methane reforming
Steam methane reforming (SMR) is a process in which the
natural gas or other methane stream, such as biogas or landfill gas
reacts with steam in the presence of a catalyst to produce
hydrogen and carbon dioxide. According to DOE of USA (2010),
SMR is approximately 72% efficient in hydrogen generation when
starting with natural gas. Conversely, on a lower heating value
basis [25] SMR produces a hydrogen rich gas that is typically on
the order of 70–75% hydrogen on a dry mass basis, along with
smaller amounts of methane (2–6%), carbon monoxide (7–10%),
and carbon dioxide (6–14%) [26].
It is worth mentioning that, natural gas reforming produces
about half of the global supply of hydrogen and remains the most
commonly used method for hydrogen production [11,27–29].
However, it is not an attractive production route for mature
hydrogen economy. This is due to the fact that order-of magnitude
increase in demand would deplete our finite reserves. Moreover,
the concentration of gas reserves in relatively few regions of the
world could lead to geopolitical tension and unstable supplies.
Environmental impact is also a major concern because reforming
natural gas to hydrogen produces as much pollution and CO2 as
burning the natural gas directly. Generation of hydrogen from
natural gas is a well established process. If sufficient quantities are
produced to power the world’s cars and light trucks it would strain
the world’s supply of conventional methane, making natural gas as
geopolitically sensitive as oil [30].