Systems And BoP - Fuel Processing

Question 1

What is reforming?

Answer 1

Fuel processing for fuel cells transforms carbonaceous fuels into a hdrogen-rich fuel. This step
is called reforming. This can be used according to the current state of the art of methanol and natural gas, but also gasoline and diesel.

Question 2

What is water-gas shift reaction?

Answer 2

The carbonaceous gas generally contains CO and needs further treatment for low temperature FCs. The following gas clean-up stages are being done by a conversion of CO with water to CO2. This is called the water-gas shift reaction. That reaction is an equilibrium reaction; it is not complete. It is often carried out in two stages

Question 3

What is done with residual CO after water-gas shift reaction?

Answer 3

Finally, some fine clean-up by oxidation of the residual CO to CO2 or reduction to CH4 is needed.
These steps are called selective oxidation or preferential oxidation and methanation, respectively

Question 4

Why don‘t high temperature FCs require CO purification?

Answer 4

Because CO is suitable for them as a fuel gas. With low-temperature fuel cells, the CO compatibility drops with the operating temperature.

Question 5

What is an additional step required in Alkaline FCs (AFC) on top of CO purification?

Answer 5

For alkaline fuel cells, CO2 fine cleaning is also necessary because the carbon dioxide with the alkaline electrolyte, e.g., B. KOH, reacts with a carbonate and clogs the pores of the electrode with it. This type of fuel cell is very sensitive and so, in addition to the CO2 in the fuel gas and CO2 from the air, must be cleaned.

Question 6

What fuels is reforming carried out on?

Answer 6

Carbonaceous fuels. This can be used according to the current state of the art of methanol and natural gas, but also gasoline and diesel.

Question 7

In what FC can methanol directly be reacted at the electrode?

Answer 7

DMFC.

Question 8

What must be done to the fuel gas other than CO cleaning?

Answer 8

In addition to CO cleaning, the fuel gas must also be desulphurized. Liquid fuels much be first evaporated.

Question 9

What are the ways to reform?

Answer 9

In principle, there are two ways to reform, namely endothermic steam reforming and exothermic partial oxidation. Neither reaction is isothermal and they cause further heat exchange in the system. The steam reforming is more energy-efficient than the partial oxidation, as it can absorb waste heat from the system. The partial oxidation, however, already uses part of the enthalpy of the energy source for reforming.

Question 10

Why are the absolute thermal effects (reaction enthalpies) comparatively small for reforming against gas purification?

Answer 10

The gas purification reactions only apply to a small percentage of the gas, while the reforming reaction itself applies to all or a large part of the gas. The absolute thermal effects are therefore comparatively small in the gas purification for reforming.

Question 11

What are the types of shift reactions for CO purifications?

Answer 11

The shift reaction is divided into a high-temperature (HT) shift and a low-temperature (LT) shift.

Question 12

Compare HT Shift and LT Shift.

Answer 12

While the equilibrium is in an unfavorable position at high temperatures (more towards the reactants), the reaction proceeds very slowly at lower temperatures. The technical solution to this is to precede a high-temperature shift as the first reactor and carry out further purification via a shift reactor at a lower temperature.

Question 13

What is reaction for CH4 when going through steam reforming?

Answer 13

CH4 + H20 → CO + 3H2

Question 14

What is the reaction for CH4 when going through partial oxidation reforming?

Answer 14

CH4 + 1/2O2 → CO + 2H2

Question 15

What is Autothermal reforming?

Answer 15

When the steam reforming is suitably combined with partial oxidation, thermoneutrality can be achieved. This eliminates additional heat exchangers, which saves costs and leads to less space and system weight. A system that does not rely on heat exchange can be operated much more dynamically; a requirement in most systems. This would mean that the enthalpy change of the system would be around 0.

Question 16

Explain the principle of Le Chatelier.

Answer 16

The principle of Le Chatelier states that systems in equilibrium always assume the state of the lowest external constraint. So to shift the reaction towards the endothermic reaction (like steam reforming), pressure can be decreased (volume increased) and temperature can be increased. And to shift it in the exothermic direction (methanation), the pressure can be increased and temperature decreased.

The reactions, however, can be kinetically inhibited, so do not necessarily occur when a propellant is present. The principle of Le Chatelier cannot make any statement and the kinetic conditions must be estimated separately.

Question 17

What is the catalyst for the reforming reactions?

Answer 17

The steam reforming is always carried out with nickel; precious metals are the exception in pure steam reforming but are used in the combination with steam reforming with partial oxidation, i.e., autothermal reforming.

Question 18

Catalyst in methanol steam reforming.

Answer 18

Methanol steam reforming takes place on either copper/zinc oxide or on palladium catalysts.

Question 19

Catalyst for HT Shift reaction.

Answer 19

The high temperature shift reaction is carried out either with iron/chromium catalysts or with platinum.

Question 20

Catalyst for LT Shift reaction.

Answer 20

The low-temperature shift reaction takes place with copper/zinc oxide catalysts or with platinum.

Question 21

What is pre-reforming in high temp. FCS?

Answer 21

• MCFC & SOFC can disintegrate CH4 internally in the anode via steam reforming if water is provided.
• Since steam reforming is endothermic, this is benficial for heat removal from the cell.
• Yet, higher hydrocarbons or aromatic compounds spontaneously form soot, when disintegrating at higher temperature even in the presence of steam
• Natural gas contains 80-90 vol-% of CH4.
• So it can be reformed internally within the cell, if the higher hydrocarbons get removed
• This happens in a separate steam reformer at lower temperature – the pre-reformer at 500 C, with Ni as catalyst and 10-20% of CH4 is sacrificed

Question 22

How does H2 clean up in a Pd-Ag membrane reactor work?

Answer 22

At the end of the reformer’s side area, the CO-rich gas (retentate) is withdrawn and fed to the afterburner. The tubes consist of a porous material (ceramic or metal) and the actually active, gas-tight palladium/silver membrane. The membrane can only be overcome by diffusion of the hydrogen through the metallic material. The hydrogen absorbs on the surface and is broken down into its constituent atoms, which then diffuse through the material. At the end of the transport through the membrane, hydrogen molecules are formed again. Such reactors are used today as standards in the purification of hydrogen for chip production.

Question 23

Why are Pd-Ag membrane reactors not popular?

Answer 23

The diffusion coefficient of hydrogen is temperature-dependent, which is why the membranes are operated at about 300 ° C. The problem is that the membrane material at about 180 ° C undergoes a phase transformation, which goes hand in hand with an expansion. This considerably impedes the dynamic operation of the reactors and often leads to cracks in the supported materials, although they have high potential, especially when operating with liquid fuels.

Question 24

What process is PROX?

Answer 24

Comes after Shift to further remove CO.