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.