(720b) Multi-Fuel Scaled-Down Pure Hydrogen Generator: Design and Proof of Concept

Multi-fuel
scaled-down autonomous pure H₂ generator:

Design
and proof of concept

Michael
Patrascu*, Moshe Sheintuch

Technion-Israel Institute of Technology,
Department of Chemical Engineering, Haifa 32000, Israel

The world is showing an increasing interest in
H₂ production for PEM fuel cell owing to the breakthroughs in fuel
cell technology in the late 1990s. Conventionally H₂ is produced
mainly through methane steam reforming (MSR), conducted in packed bed tubes
filled with Ni based catalyst which are placed in a furnace supplying the heat
of reaction through the burning of fuel. Full conversion of MSR to syngas
(mixture of CO and H₂) is obtained at typical operation conditions
of ~25 bar and temperatures as high as 1000°C. This unit is followed by high and
low temperature water-gas-shift (WGS) to produce as much H₂ as
possible. These steps are followed by pressure swing adsorption (PSA) for CO₂
removal and methanation (reverse steam reforming) for
clearing any residual CO. Adopting this large scale production (~10⁵
Nm³/h) scheme as a H₂ source for electrical power
production in PEMFC holds significant complications concerning compression,
transportation and storage of H₂. Special infrastructure would be
required to supply pure H₂ to a small-scale end user, either
stationary or portable applications, with major safety and economic concern.
Thus alternate process schemes for local scaled-down production of H₂
from conventional, environmentally friendly and safe feedstocks
(e.g. natural gas, biofuels) are required. Alternative sources for H₂
production are desired, with lower (or zero) related carbon emissions, such as
biofuels; methanol, ethanol or glycerol (which is a
biodiesel production by-product) etc.

In this
paper we successfully demonstrate, for the first time to our best knowledge,
the feasibility of producing pure atmospheric pressure H₂, in a
compact integrated membrane reactor, from various fuels. Demonstrating pure
atmospheric H₂ production in one unit is challenging, and most
previous studies used either vacuum or sweep gas to facilitate H₂
permeation in membrane reactors. Here this is achieved by steam reforming
reaction using a commercial Ni catalyst and Pd-Ag
membranes for in situ separation of
the H₂ product. Feeds of methane, ethanol or glycerol and stoichiometric
steam are considered, with the heat being supplied by recycling the SR
effluents to an internal combustion reactor (Figure 1). The operating
temperature in this auto-thermal mode of operation is not controllable
independently, and is directly set by the feed flow rate. The production system's
performance is characterized in terms of thermal efficiency of conversion (up
to 25%) and power output (up to 0.15kW), Figure 2. Changing the fuel source
leads to similar qualitative behavior, with small quantitative differences,
mainly the operating temperature obtained. The feasibility of operating the
same unit with various fuel types holds great potential and offers flexibility
for technology developments based on H₂ as energy vector. Efficiencies
obtained with MSR were better than those with ethanol SR or glycerol SR when
compared at same maximal temperatures. Still these efficiencies are about 25%
at best. Noting that around 50% of the energy is lost to the surroundings, we
expect that increasing membrane area and applying better insulation will push
the efficiencies to ~60%. A mathematical model predicts the results and
operational trends well, while accounting on membrane permeance
inhibition, which is significant, and affects the results and efficiencies.

Of
the three feeds tested here methane is the best, having the largest H/C ratio,
but ethanol, glycerol and other biofuels can be delivered in liquid phase,
mixed with water. Other fuels like methanol can also be considered; methanol
will allow to reduce the operating temperature to around 300°C. The SR and WGS catalytic reactions are fast
and the power density is limited by the membrane permeance.
With thinner membrane and better permeance it
will be possible to improve the power density. Moreover, in many studies the permeance was found to be inhibited
by co-adsorption of CO and methane on the Pd-Ag
membrane. As we show here, accounting for such inhibition leads to better
prediction of the experimental results.

Figure 1: A detailed drawing of the reactor. The cross sections of the two
reactor zones are depicted above. The 11 outer and 6 inner thermocouples
(greed) and 2 of the 4 membranes (red in the colored version) are depicted in
the axial view. The recycled SR effluents are fed to the internal combustion
reactor through 2 axial locations using the branch tee (left) and an inner hasteloy tube.

Figure 2:  System's
performance for different fuels fed vs the inlet
fuel's equivalent power: (a) thermal efficiency and (b) equivalent power of H₂
product. Markers are experimental measurements and lines are model predictions.
Stoichiometric feed (S/M=2 S/E=S/G=3), pressure of 11 bar.